WEBVTT

1
330W: It's 3 years down.

2
330W: And this one, it looks good to me, that's good.

3
330W: So should I connect to Wi-fi? Yes.

4
330W: Okay.

5
330W: Hey, cut on the map.

6
330W: Thank you.

7
330W: Yes.

8
330W: No, there's this new one, right?

9
330W: I don't think you're wrong, I haven't figured it out yet. Yeah, if you haven't configured it, you could estimate, as well as secure. Okay. So you have.

10
330W: some time before they shut down. Basically, there's prohibition for police.

11
330W: Okay.

12
330W: Must be the same.

13
330W: That could be.

14
330W: You didn't need it.

15
330W: You just…

16
330W: Okay.

17
330W: Alright, so I need to… And… Some problems.

18
330W: So.

19
330W: As it looked fine before.

20
330W: I didn't connect this to this one. I connected to MMPO.

21
330W: Oh, okay, alright, so let's, go to our wireless space.

22
330W: 0 point 4 people.

23
330W: All right.

24
330W: These are the ones.

25
330W: Okay.

26
330W: Thanks.

27
330W: I think it's not… No.

28
330W: It's a treatment approach.

29
330W: Sorry, November.

30
330W: Yeah, what it is.

31
330W: Okay, so… And some of the ones that's finished.

32
330W: Okay.

33
330W: here.

34
330W: There we go.

35
330W: What about it?

36
330W: And you'd have to run the, that… Finance was just downloaded.

37
330W: I mean… Yeah.

38
330W: Show up here, and put Rachel next to the committee.

39
330W: Thank you, Father.

40
330W: And it's actually fun.

41
330W: Yeah.

42
330W: That makes total sense. I noticed there's a slightly better one in the abstract.

43
330W: But, I haven't reviewed it.

44
330W: Not utilized. Utilized. That's really hard to see.

45
330W: Yeah.

46
330W: I looked at, like… There it is.

47
330W: Thanks.

48
330W: But actually…

49
330W: So, I'm just… She kind of feels like she's not…

50
330W: Yeah, I mean, she has. Yeah, she does, like, collecting stuff.

51
330W: Right away, I was trying to… Oh.

52
330W: Okay, so… It's like a bar.

53
330W: Some calls.

54
330W: She said, I can't explain.

55
330W: And…

56
330W: Okay, but I guess the question is…

57
330W: By the way, for the Zoom, Where's the microphone?

58
330W: This thing, yeah.

59
330W: basically disable all audio on your laptop and everything.

60
330W: Okay.

61
330W: You're waiting for that, there's, like, a dialogue window, just waiting for it to pop up for you to enter your… Okay. Enter your net ID and password. Okay, here, right? Yeah.

62
330W: June.

63
330W: Maybe we say today.

64
330W: Yeah, the, speaking of, Okay.

65
330W: Oh, yeah.

66
330W: That doesn't change.

67
330W: Thank you.

68
330W: Yep, there you go.

69
330W: He's another stranger.

70
330W: I think we're done.

71
330W: Give it some boroughs, let's see if it's working.

72
330W: 1 and.

73
330W: Nice.

74
330W: Great.

75
330W: Yeah, like, good for you.

76
330W: So I want to open email to click on the Zoom link. Yeah.

77
330W: 2 dogs.

78
330W: It was one of those things that I signed up to.

79
330W: And they just saw that.

80
330W: Say, well, sure, sure.

81
330W: I think you can play with it in the existing room.

82
330W: I agree. Please, make sure that…

83
330W: breaking something that's working, right? Yeah, yeah, yeah.

84
330W: When you're ready…

85
330W: Sometimes only for these laptops when power is infinite, it becomes much slower. Oh, okay. That's why I'm checking, but it looks like the… I have this LED here… Yeah. …that indicates that actually power is coming, so I don't know why it's associated.

86
330W: Yeah, that's true. And it doesn't happen from the speed. That's true. Because I always write the caption as left panel, middle panel, right panel. So it actually…

87
330W: The only thing ABC Matters for is referring to the taxes. Okay. Which, yeah, we could try to do that. Not a big deal.

88
330W: Thank you. That's true, thank you.

89
330W: And also, I think we do need to have information on…

90
330W: because space is tight, I already really trimmed down. Yeah. But if you can find a place

91
330W: in the existing text to cite the ratio of POM. Yeah. That sounds good. I'm not sure we cited it. Okay. I think we do, but maybe we should change it. I can IDSA get red.

92
330W: Yeah, I wouldn't give her credit right now. Just add the text. Yeah, okay. If it stays on, come back to the main part. Yeah. With, like, one sentence. Stay. It is tight. Yeah.

93
330W: Rutgers Physics.

94
330W: Oh, there are other investigations happening.

95
330W: Thanks.

96
330W: Alright, see you.

97
330W: And Okay.

98
330W: Stone.

99
330W: Try to.

100
330W: This is my email. Yeah, that's the email here.

101
330W: Okay. Okay. That's if you needed an extra…

102
330W: Okay.

103
330W: Okay.

104
330W: Okay, so this is Bob Steven.

105
330W: Zoom link, so I guess it's… Yeah, you can also.

106
330W: I can close this one. Yeah, you can close that.

107
330W: It's doing something. No.

108
330W: For some reason, it's slow.

109
330W: Okay.

110
330W: Yeah, we could mute the stuff up. Yeah, you could join.

111
330W: So now… Good. Progress.

112
330W: And also your laptop's audio, because you can still hear the feedback. How do we… You can go to the microphone, or if there's a key press, you can try.

113
330W: Okay.

114
330W: Yeah. So the ones on the far right, the speaker.

115
330W: Yeah. Yes.

116
330W: you

117
330W: distracted.

118
330W: This one.

119
330W: Okay, it's muted now. Okay. Yeah, so usually we use the function keys and… Okay.

120
330W: function. Yeah, this one has 4 functions. Alright. So yeah, we can close.

121
330W: Okay.

122
330W: Okay.

123
330W: Everything. The meeting will be recorded. Okay. Yeah, no, you can share.

124
330W: talk.

125
330W: Hmm.

126
330W: Sure.

127
330W: I need to open it.

128
330W: I think so.

129
330W: I'm good. How are you?

130
330W: This one up and running.

131
330W: Class is okay. Yeah. Heading to my last recitation of the week. Okay. This is teaching… Oh, I agree.

132
330W: Goodbye.

133
330W: Okay.

134
330W: I do. A little bit of a, you know, but they're… I'm not used to teaching so many.

135
330W: Bye. Have a good day.

136
330W: 24.

137
330W: Yeah, doable, but different from small liberal arts college.

138
330W: That's good. I'm gonna test that, but…

139
330W: Yeah, it's nice to have a complimentary place.

140
330W: It's not hard for them to say that.

141
330W: Yeah, they're enthusiastic. How you doing? I'm okay.

142
330W: Did you change the term? Different statements.

143
330W: Okay.

144
330W: Why was the first year grant signed?

145
330W: Nice to meet you.

146
330W: It's very complicated system.

147
330W: including Zoom.

148
330W: Yes.

149
330W: This is strange, somehow, my computer.

150
330W: This laptop works very slowly. Like, I've never seen this in a picture. Usually just boom, everything opens instantly.

151
330W: Do not connect it to the, should you maybe close and open again? I guess. That is too bad, and I apologize to you, but I can sort of see the idea, because if you do it for the entire department, then there'd be an unused scheduled slot for physics recitations.

152
330W: Which probably is fine, because, you know, maybe… Are you connected?

153
330W: Yeah, it looks like it is, yeah.

154
330W: Click down there. Are you sure you don't know why? At least it looks like this time… Yeah, we just connected to… Okay, okay.

155
330W: I'm not sure what you said.

156
330W: Obviously, sharing this. Can you?

157
330W: I heard it's coming slowly. How many gigabytes is this presentation?

158
330W: It's actually a little bit bigger than, than usual, folks. It's, like, 100 megabytes, because I have some videos.

159
330W: I see you said you had videos?

160
330W: Yeah, actually, yesterday I connected to the here in the evening. I used the bus just through HD, and I ran through. Maybe we should switch to that format. Right now, we're just going strictly through. Okay, should I run the schedule?

161
330W: But I didn't ask because I realized that… It's almost like it's not responsive.

162
330W: I'm going to walk through my teaching. And I need to scoot out of their system of teaching.

163
330W: And the biggest problem about getting from there to here is any distance of College Ave campus you can drive through. So the parking deck is almost a 318 entrance.

164
330W: You come out of there, the College Avenue parking deck, it's the main parking lot for College Ave. It's up in that corner. And that's… You come out of there onto… you come out of there onto George Street.

165
330W: Yeah, there's there seems to always be a slower to open the first.st I have to go up 5 flights, and it takes quite a long time.

166
330W: No. No, that one has space, but it's… I don't think you can park there for free, and… And that up there, we can park. Yeah, so College Under Parking Direct… I haven't investigated this. Yeah, it's kind of like, if you need a safe place, if you need a place to park at a college, that's the place to go.

167
330W: There can be a lot of blocks for some parts. But from our teaching, five minutes, fine. And then I pop out of there, I take a left turn without any crosswalks for students, and I just get on 18 so I can get back here in five minutes.

168
330W: It's okay.

169
330W: activity.

170
330W: Well, I… Take a sucker.

171
330W: Share to the Zoom, let's see if we get the audio. Let me stick with it first. Let's not use such things. Right, for the students, for a late crying loud.

172
330W: Hmm.

173
330W: Oops.

174
330W: because Yes, sir.

175
330W: Yeah, did you put one in? Sorry, but did you put one of these in here? Oh, okay, good. Yeah, this is 8, so you can go up to 10, don't go higher than that. 8 is probably good. Okay, okay, thank you. Now, what's your question?

176
330W: Mmm.

177
330W: should be in metric.

178
330W: Then we'd all know, we could just look at this and know how many milliliters this is, right? As experimentalists? I remember. When I was still in the state.

179
330W: Gave out selection fears. One of these rumors. Rumors.

180
330W: And I was standing sort of somewhere at a desk, and this teacher just got into my face. I've been telling them for a year that no one uses inches. Only backward CUNYO geeks use inches, and then you go to university, and you just — I brought them all here.

181
330W: Okay.

182
330W: It's… And this has audio?

183
330W: This one has a video.

184
330W: Absolutely.

185
330W: out of July.

186
330W: Is that, like… But…

187
330W: There's no damage. It's just damage, like, infrastructure needs to be brought back up. We haven't ever had a snowstorm. It's completely ridiculous. They didn't talk about any damage that's supposed to go up your camera, so let's just get it going, and what they're really going to end up doing is restarting the survey.

188
330W: So, because this, like, pretty much was made after two weeks of data, Oh, yeah. It was yesterday.

189
330W: I don't understand, why is it so much, like… Oh, it's just… Right? So, so the… I just want the data… Actually, here, it's…

190
330W: You're gonna have a big hole in the middle of the sky. It's more like it should be like this sort of thing. I think we could…

191
330W: Let me see, it has audio, or is it just like a video? No audio. Okay.

192
330W: during the survey.

193
330W: Yeah, I can try clicking through a little bit.

194
330W: I thought we can try switching to HDMZ and see.

195
330W: and also switch up.

196
330W: Yeah, the area gets some… it's not those… it's the Atacama, but it's not, like, the high Atacama. No, yeah, they're not… so they got flash floods, this huge storm, they had, like, snow up to here at the summit.

197
330W: And the rooms.

198
330W: they basically pay.

199
330W: Yeah, so the first thing I have to do is get to the point where they can turn on the generator.

200
330W: which requires a fuel supply. So it's all you know, it's just like all your jurisdiction goes away.

201
330W: The cinematic mode. Get into the 21st century. These are old style.

202
330W: You're gonna go to cinema mode. You're gonna get present right? I will. I will present next week's speaker. Okay. But I can't. I'm trying to find it.

203
330W: It is, and it's very difficult to get to the local design pieces.

204
330W: Rutgers SAS Newark. Colloquium Physics Rutgers doesn't go to us anymore.

205
330W: in the old days, when we controlled our own Internet. Okay, now.

206
330W: And then it goes to fall 2025. I mean, what am I supposed to do?

207
330W: Of course.

208
330W: through 2026. I could be in charge of this, but the people who are doing the website…

209
330W: Why doesn't she get… Rutgers Yeah, can you…

210
330W: AI takes them. AI says that we're transitioning on the floor. Thank you very much. I like your cup. But the interesting thing is that you can condense my stem, and I'll show more.

211
330W: Then it goes to newer. It really is. She's next week.

212
330W: That's why I can't tell, because I can't get to the Colloquium website. Rutgers Physics Colloquium for 2026, New Brunswick.

213
330W: Sorry for this delay. Yes.

214
330W: It doesn't work. Next week is… Some of those figures look familiar.

215
330W: This is what it is. So this is the body, soft electronics, the alphanane of the universities.

216
330W: There's an idea, we're going to choose the transistors.

217
330W: It's his fault, though.

218
330W: No, I didn't even think about it.

219
330W: No, you can go there.

220
330W: Where is the event? Downtown.

221
330W: Oh, I know, I know it, right? Yes. Oh, wow. It's right next to the theater, right? Correct. Okay. Well, parking is in, so we're going with parking. Okay!

222
330W: Good afternoon, everyone.

223
330W: And welcome to this week's Digital Autonomy.

224
330W: Hello, Pierre. Before I hand over to Bob to introduce Vitaly this week, I want to draw your attention to two events next week, two special events next week. First of all, on

225
330W: Tuesday, in the evening, we have a special irons lecture that will be presented by Eva on the topic of her Kavli Prize. That would be a grand event. Please register for it. Please come next Tuesday evening.

226
330W: And then as a continuing part of that, on next Wednesday, we have Yael Hanain, who is coming to us from Tel Aviv University, who is going to give a talk on listening to the body, soft electronics.

227
330W: electrophysiology and the physics of human-machine interfaces. So it kind of connects up with this week's Colloquium, and please come to that too, and I'll hand over to Bob.

228
00:00:00.480 --> 00:00:02.110
330W: Thank you, Piers, it's,

229
00:00:02.390 --> 00:00:12.030
330W: It's my great pleasure to introduce a colleague and friend, Natali Potzerov, giving our — presenting our Colloquium today.

230
00:00:12.100 --> 00:00:26.600
330W: Vitaly, came to us in, the early 2000s, or late 1990s, I guess, coming from Moscow Institute of Physics and Technology, where he got his Master's of Science in 1995.

231
00:00:26.870 --> 00:00:39.790
330W: He joined… he got his PhD here in 2002, with Misha Gershenson, and then continued on here as a postdoc, through 2007, when he.

232
00:00:40.100 --> 00:00:51.219
330W: Yes, right. When he became assistant professor of physics here in the department, tenure track position, and then has climbed through the ranks, very successfully in the intervening years.

233
00:00:51.500 --> 00:00:52.900
330W: And,

234
00:00:53.140 --> 00:01:06.300
330W: He has received a number of distinctions over that time. Most recently, in 2022, he was appointed the David Jacobson Chair of Applied Physics.

235
00:01:06.510 --> 00:01:20.399
330W: And in 2025, a year ago, just about exactly a year ago, he was awarded for, elected fellow of the American History Society. And that, citation was for pioneering contributions to the understanding of char transport.

236
00:01:20.420 --> 00:01:33.430
330W: photoconductivity and exciton, crystalline semiconductors, layered inorganic semiconductors, and metal hydride perovskites, which I believe is going to be central to what he's discussing today.

237
00:01:33.590 --> 00:01:48.559
330W: So, what Vitaly has done is basically opened up new frontiers with new semiconductor materials, designing and fabricating transistors on these materials, and it's very appropriate, as you know.

238
00:01:48.560 --> 00:02:02.040
330W: The, first transistor, the first point contact transistor was developed at Bell Labs, down the road over here, in, I believe, December of 1947, by, Bardeen, and.

239
00:02:02.120 --> 00:02:05.050
330W: Shockley and, Bertet.

240
00:02:05.460 --> 00:02:07.010
330W: And,

241
00:02:07.490 --> 00:02:18.759
330W: Vitaly, will tell us about his single crystal rubring transistors, that he developed in 2003. So he's keeping the tradition alive.

242
00:02:18.920 --> 00:02:21.630
330W: But there's another tradition we hope he keeps alive.

243
00:02:21.990 --> 00:02:32.059
330W: Those three characters from Bell Labs, nine years later, won the Nobel Prize for the development of the

244
00:02:32.210 --> 00:02:34.299
330W: of the point contact transistor.

245
00:02:34.950 --> 00:02:45.599
330W: The tally… We're still waiting. Okay, Natalia, please. Welcome, Natalia Potzerov, and… Thank you very much for your detailed

246
00:02:45.710 --> 00:02:50.449
330W: introduction. I actually have a special slide about the Bell Labs story.

247
00:02:50.730 --> 00:02:54.290
330W: about the transistor invention. So.

248
00:02:54.460 --> 00:03:01.549
330W: I wanted just to give a brief overview of our research over the latest decade.

249
00:03:01.830 --> 00:03:10.000
330W: So we were primarily focusing on these two systems, organic semiconductors and metal highlight perovskites.

250
00:03:10.170 --> 00:03:21.249
330W: So here in this photograph you see a rubrin single crystal and fet structure wired on the sample holder. Rubrin is one of the best organic semiconductors still today.

251
00:03:21.540 --> 00:03:22.980
330W: And,

252
00:03:23.150 --> 00:03:31.679
330W: What's a unifying theme of these two classes of materials is that they're soft materials. They are really soft. You can cut them with a razor.

253
00:03:31.750 --> 00:03:43.900
330W: You cannot cut silicon wafer. And that comes from the softness of the lattices, the weakness of the bolts. And this has certain advantages and disadvantages.

254
00:03:44.450 --> 00:03:57.760
330W: In particular, application people like these materials because they're easily processable at low temperature. You can, deposit them from solution, you can easily melt them, crystallize from melts at low temperatures.

255
00:03:57.860 --> 00:04:08.740
330W: and so on. But also the disadvantage is that the softness of the lattices leads to strong dynamic disorder. The lattices are always vibrating.

256
00:04:08.840 --> 00:04:14.780
330W: Strongly at room temperature, and that leads to transient localization of carriers and lower mobility.

257
00:04:15.260 --> 00:04:19.870
330W: So, nevertheless, they are very interesting in terms of applications, so…

258
00:04:23.179 --> 00:04:29.170
330W: Okay, let me see… So there's a little bit of a lag.

259
00:04:29.389 --> 00:04:42.539
330W: So, first, let me, talk a little bit about organic semiconductors. So, the, one of the main things that, is important for charge carrier transport and mobilities in these, molecular crystals.

260
00:04:42.980 --> 00:04:49.109
330W: is the so-called transfer integral. It's electronic coupling between the neighboring molecules.

261
00:04:49.310 --> 00:05:05.289
330W: That, you know, when they calculate the, this, this electronic coupling or transfer integral, it has a, you know, it's very sensitive to the mutual position of the molecules. And, for instance, for the case of rubrine, this transfer integral

262
00:05:05.520 --> 00:05:22.820
330W: happens to be the maximum at the particular arrangement that we have with these crystals, right? So we're, in a sense, lucky that in the Rubin, you have a maximum transfer integral here, but transfer integral is a static equilibrium property. It's calculated when the molecules are

263
00:05:23.080 --> 00:05:26.709
330W: in the frozen… equilibrium average position.

264
00:05:27.060 --> 00:05:35.780
330W: So, Okay, so the another thing that needs to be considered is the

265
00:05:36.350 --> 00:05:45.259
330W: is the, fluctuations and dynamic disorder. So when molecules fluctuate, and they fluctuate strongly in these, soft crystals.

266
00:05:45.540 --> 00:05:56.650
330W: This causes large fluctuations in the transfer integrals and, leads to a strong off-diagonal dynamic disorder. And this is transiently localizing the charge here, so you…

267
00:05:57.100 --> 00:06:16.049
330W: Even if you could form a narrow band due to the transfer integral, this band will be likely destroyed, or you create a lot of electronic disorder. And this is bad for mobility, because it will definitely localize carriers and reduce the mobility.

268
00:06:18.450 --> 00:06:22.850
330W: Another important phenomenon that one needs to consider is that.

269
00:06:23.220 --> 00:06:31.850
330W: charge carriers in these soft lattices are frequently polar, so they have some sort of a polaronic character. When the charge moves through the lattice.

270
00:06:32.130 --> 00:06:39.970
330W: It polarizes the electronic clouds and forms this kind of, dressed charge, a polar… polaron.

271
00:06:40.100 --> 00:06:51.300
330W: Of course, this happens only when the charge is moving slow enough. If the motion is too quick, like in silicon, you do not form polarons, because the electrons don't have enough time to form.

272
00:06:51.640 --> 00:06:54.919
330W: But in molecular crystals, the polarons are frequently occurring.

273
00:06:55.320 --> 00:07:04.910
330W: So this kind of, polaronic effect is also bad for mobility, because it makes the charge heavier, the charge has to drag this polaronic cloud as it moves.

274
00:07:05.540 --> 00:07:16.989
330W: Right? So, as a result of all this, we have this kind of bird's-eye view of the different transport regimes in molecular… in, generally, in semiconductors.

275
00:07:17.270 --> 00:07:27.330
330W: One needs to compare the strength of the transfer integral with this lambda, which represents here either polaronic reorganization energy.

276
00:07:27.490 --> 00:07:41.630
330W: or the amplitude of fluctuations of the transfer integral. So if the transfer integral is much slower, smaller than this lambda energy, then you have what we know as charge carriers, and you can only conduct by hoping.

277
00:07:41.850 --> 00:07:51.230
330W: Opening regime, you have thermal activated transport, low mobilities, and, you know, these are typical situations in a small molecule.

278
00:07:51.630 --> 00:07:53.130
330W: the solar crystals.

279
00:07:54.220 --> 00:07:56.410
330W: Yes.

280
00:07:56.410 --> 00:08:16.669
330W: Since you're talking about disorder, can you tell us a little bit about the situation with static disorder? Because, of course, for devices, we want very… Right, right. Static disorder, is basically defects, dirt, impurities that I'll mention later. Okay. So far, I'm just covering intrinsic disorder, dynamic disorder. Okay, fair enough, but I just didn't get to it.

281
00:08:16.670 --> 00:08:17.410
330W: Okay.

282
00:08:17.560 --> 00:08:34.779
330W: So on the, the other side of the spectrum, we have, purely band transport. When the electrons are strongly delocalized, this happens when the bandwidth is much, stronger than any of this kind of local polarization of it.

283
00:08:35.100 --> 00:08:40.989
330W: energy scales, so you have fewer block waves that are moving fast.

284
00:08:41.380 --> 00:08:52.770
330W: And somewhere, we have here, intermediate regime. This happens for the charge carrier mobilities of the order of, like, 10 to maybe 50 centimeters squared per whole second.

285
00:08:53.140 --> 00:08:57.869
330W: This is the situation if we have good molecular crystals.

286
00:08:58.170 --> 00:09:05.030
330W: Where, these energies are of the same order of magnitude. So here, you have transient localization.

287
00:09:05.340 --> 00:09:10.680
330W: So what charges I see is a you know, Olivia Groff.

288
00:09:11.080 --> 00:09:16.429
330W: Potential landscape created by those slowly moving molecules.

289
00:09:16.720 --> 00:09:24.370
330W: So that kind of dynamic disorder is kind of static on the electronic timescales, but on a longer field, it's constantly fluctuating.

290
00:09:24.600 --> 00:09:32.109
330W: So the electrons are localized. in the next moment they see different potentials. So they kind of move and they diffuse slowly.

291
00:09:32.530 --> 00:09:42.810
330W: So this kind of transport regime is neither hopping nor banned. So it's a transient localization kind of scenario.

292
00:09:42.990 --> 00:09:46.429
330W: So that's where we are with molecular crystals.

293
00:09:46.530 --> 00:10:03.610
330W: And this represents a challenge. There's something counterintuitive here. Normally, increasing mu increases densities of carriers, increases the Fermi energy, and would make the system more delocalized and more quantum. You've got it becoming more Boltzmann-like, and I don't understand that.

294
00:10:03.720 --> 00:10:08.350
330W: But I don't have… I don't have a vestige of carriers here in this port.

295
00:10:08.750 --> 00:10:13.290
330W: You've got chemical potential at the bottom. No, no, no, it's mobility. This is the mobility.

296
00:10:13.750 --> 00:10:27.599
330W: notation. Okay, sorry, Mr. Mu being… Okay, got it, thank you. Dynamic disorder, basically, it's, when you have, high orbital overlap between static

297
00:10:27.980 --> 00:10:28.860
330W: Oh.

298
00:10:29.370 --> 00:10:34.540
330W: molecules when you consider them frozen at the position you have transferred through.

299
00:10:34.850 --> 00:10:36.780
330W: which is proportional to the bandwidth.

300
00:10:37.830 --> 00:10:38.840
330W: And then…

301
00:10:39.000 --> 00:10:46.320
330W: It's weak. It's just that in the strong correlation community, mu is reserved for Kevin's potential. That's the confusion.

302
00:10:46.530 --> 00:11:00.879
330W: Right, so this is, this regime is challenging for theorists to describe, because they cannot use simplifications like this or like that. You know, they have to factor in all this dynamic disorder, polaronic effects.

303
00:11:01.230 --> 00:11:04.190
330W: The, narrow bands, and so on.

304
00:11:04.600 --> 00:11:05.700
330W: Okay.

305
00:11:07.690 --> 00:11:09.220
330W: And, as…

306
00:11:09.410 --> 00:11:23.379
330W: there was a question, right, about the static disorder. On top of all these intrinsic factors that I mentioned that limit mobility, we have… we also have static disorder. This is the… a photograph of one of the best, polar crystalline

307
00:11:23.540 --> 00:11:37.620
330W: fantasy and thin films, right? You see all these grain boundaries and other defects, and they lead to limited, chest hair mobility, a limited exciton diffusion length. They mask intrinsic behavior when we do experiments on them.

308
00:11:37.850 --> 00:11:49.630
330W: And in many cases, it also leads to poor stability of these materials, because, you know, the elements penetrate through and may react with your semiconductor. And so for that… for this reason, we want to go from this to this.

309
00:11:50.420 --> 00:11:58.550
330W: large… you know… highly purified Single crystals with perfect surfaces.

310
00:11:58.720 --> 00:12:05.530
330W: And, make samples out of these, so that we can look at intrinsic, transport properties.

311
00:12:05.780 --> 00:12:14.570
330W: And so many molecular semiconductors can be grown in the form of these beautiful microscopic high-order crystals. There are some examples here.

312
00:12:15.140 --> 00:12:16.810
330W: And…

313
00:12:18.860 --> 00:12:31.450
330W: In our community, we have developed a number of techniques for fabricating high-quality single crystalline organic field effect transistors. This is all FETs.

314
00:12:31.590 --> 00:12:47.269
330W: Right? So some of them were actually invented here at Rutgers, particularly this freestanding devices based on paralleling conformal coating. Paralleling is a very good electric, as it turns out. Also, this type of… we call them air gap stems.

315
00:12:47.510 --> 00:12:56.630
330W: Or, vacuum gap stamps, where the crystal is laminated here on this kind of a prefabricated stamp, chooses the structure.

316
00:12:56.780 --> 00:13:12.130
330W: And by the way, we have developed this in collaboration with John Rogers' group at Bell Labs, but later on, much later, actually, the same idea was adopted in the graphene community. Suspended graphene. It's essentially the… thing.

317
00:13:12.310 --> 00:13:13.010
330W: Arch.

318
00:13:13.330 --> 00:13:14.140
330W: Yes.

319
00:13:14.280 --> 00:13:21.629
330W: Since we were talking about mobility before, what kind of mobilities do you have in this? This one has up to 20. Okay. At room temperature.

320
00:13:21.830 --> 00:13:25.230
330W: This one has mobilities of about 5.

321
00:13:26.100 --> 00:13:32.130
330W: And the reason is for the intrinsic reason, because the dielectric constant here is 1, it's just vacuum.

322
00:13:32.250 --> 00:13:36.699
330W: In this case, Parallel has a dielectric constant of about 2.4.

323
00:13:37.240 --> 00:13:44.920
330W: So the mobility of the interface is roughly inversely proportional to the FC. So it's an intrinsic difference.

324
00:13:45.170 --> 00:13:55.520
330W: Also, we have came up at some point with this vacuum-laminated giga-electric, where you get a crystal with contacts, and then you can attach your giga-electric by vacuum.

325
00:13:55.650 --> 00:13:57.889
330W: It actually has certain advantages.

326
00:13:59.400 --> 00:14:05.929
330W: Okay, right. Okay, so since a field fact transistor is such an important device, you know.

327
00:14:06.580 --> 00:14:22.489
330W: everywhere in our electronics. So let me briefly explain how it works. So, essentially, what happens here is that you are measuring the conductivity of your semiconductor, shown schematically in pink, using these two metallic contacts, the source and the drain.

328
00:14:22.650 --> 00:14:26.590
330W: And you can modulate this conductivity by the third electrode, the gate.

329
00:14:26.800 --> 00:14:31.369
330W: by applying voltage to it. So, essentially, this works like.

330
00:14:31.550 --> 00:14:35.670
330W: Like a capacitor. So the gate is isolated from the rest of the structure.

331
00:14:36.580 --> 00:14:40.099
330W: And, the gate and the channel.

332
00:14:40.500 --> 00:14:43.730
330W: A forming parallel plate capacitor, right?

333
00:14:48.020 --> 00:14:48.800
330W: Correct.

334
00:14:49.020 --> 00:15:07.009
330W: So when you apply the voltage, the field induces the… injects, charges from the contacts, and they are accumulating at the interface here, forming the opposite plate of the capacitor. So the only difference is that charges are flowing laterally in the second… in the lower plate of this capacitor.

335
00:15:07.820 --> 00:15:14.620
330W: And you can control the, concentration of these charges by the gate voltage. So essentially, it works like a valve.

336
00:15:14.840 --> 00:15:24.770
330W: You have flow in this direction from source to drain, and you can control it with the gate valve without drawing any current into the gate. That's important.

337
00:15:27.090 --> 00:15:39.720
330W: And, so we can simply write down this equation for the charge carrier density here at the interface. It's proportional to the, the voltage that you're applying. CCI is the capacitance per unit area.

338
00:15:39.850 --> 00:15:43.999
330W: So the conductivity becomes E and mu. Mu is charge element ability.

339
00:15:44.230 --> 00:15:50.490
330W: And then, if you combine these 2 sigma, the conductivity that we're measuring here in our experiments.

340
00:15:50.880 --> 00:16:00.939
330W: is going to be proportional to the need voltage, and the coefficient of proportionality is proportional to the mobility. So that's how you you actually can determine experimentally to measure the mobility.

341
00:16:01.080 --> 00:16:04.280
330W: by doing this Sigma versus Vg measurements.

342
00:16:04.580 --> 00:16:10.230
330W: quite simple, but, you know, the history of development of FETs, as Paul mentioned.

343
00:16:10.710 --> 00:16:22.690
330W: You know much of what happened at Bell Labs, and rather long and complicated. The initial idea was was suggested by Lilienfeld. He actually patented it in 1930.

344
00:16:23.000 --> 00:16:32.679
330W: And then, Shoplin and Bertin, at Bell Labs, around this time, they were trying to make practical devices, but they failed, they never worked.

345
00:16:32.780 --> 00:16:42.720
330W: And Bardeen was the one who suggested that the reason those guys failed is because when you have a surface of silicon, you have very high density of dangling boards.

346
00:16:42.990 --> 00:16:44.520
330W: And these are deep traps.

347
00:16:44.660 --> 00:16:47.430
330W: If you calculate the concentration is huge.

348
00:16:47.730 --> 00:16:53.009
330W: So, you simply are not able to reach that high field with the gauge to

349
00:16:53.670 --> 00:16:56.590
330W: Saturated formula to fuel all these traps.

350
00:16:56.940 --> 00:17:16.490
330W: And then it took another, about 20 years of development until two groups independently, Atala and Khan at Bell Labs, and Hyman and Holstein at RCA Sarnoff, they found a way to actually make practical working silicon field detections, okay? And this was a very important point, that essentially.

351
00:17:16.720 --> 00:17:20.060
330W: This is the birth of the information page.

352
00:17:20.290 --> 00:17:25.900
330W: Because FADs are so important for the… it's a main building block of all the electronics.

353
00:17:26.330 --> 00:17:31.059
330W: So, by the way, both companies, Bell Labs and RCS Arnoff.

354
00:17:31.280 --> 00:17:41.379
330W: We're located in New Jersey. So New Jersey is very big on field transitions. It's essentially the birthplace of the information age. Yeah. Right?

355
00:17:41.610 --> 00:17:45.599
330W: Question? Yeah. How did they fix the problem?

356
00:17:45.910 --> 00:17:51.960
330W: Okay. So they found a way to persuade persuade the venue.

357
00:17:52.070 --> 00:17:54.510
330W: So… 8.

358
00:17:55.140 --> 00:18:08.969
330W: Briefly speaking, if you grow thermal oxide that naturally grows, you kind of get rid of this. You might have a situation where you have clean interface from silicon that goes to the oxide.

359
00:18:10.510 --> 00:18:15.490
330W: It's a it's a problem persuading the the Probably not.

360
00:18:17.060 --> 00:18:30.430
330W: So these guys got a Nobel Prize in 1956, although we… this picture shows a point, it's a pin junction transistor, right? But the formulation is more broad, it's for their research in semiconductors.

361
00:18:30.860 --> 00:18:39.960
330W: Anyways, this slide shows how many of these fats we have in the modern process, if you… Ask.

362
00:18:40.300 --> 00:18:42.310
330W: Nowadays, we'll ask this.

363
00:18:42.660 --> 00:18:46.570
330W: Right? The, And you, you see…

364
00:18:46.720 --> 00:18:56.400
330W: The numbers are mind-boggling, so you… you look at these processors, and you have billions and tens of billions of transistors that are not in the same type of

365
00:18:56.830 --> 00:19:08.400
330W: morphology, as those guys were working on, these are usually FinFETs, they have different structure, but nevertheless, the main idea is the same, it's short of transistors. So that's how many of them we have.

366
00:19:08.580 --> 00:19:11.340
330W: in our chips. This is, amazing.

367
00:19:13.030 --> 00:19:15.719
330W: Okay, so very briefly.

368
00:19:15.910 --> 00:19:30.810
330W: the, no, the first shortly developed the model, how to describe field effect transistor, and it was improved by Hofstein and Hyman, but basically we, for simplicity, we call it a Shockley model.

369
00:19:30.810 --> 00:19:38.650
330W: So you write down an equation for the current density J that flows along X in the channel.

370
00:19:38.680 --> 00:19:46.890
330W: right? So this is Sigma E. Sigma is E and Mu E is the lateral electric field.

371
00:19:47.100 --> 00:20:03.499
330W: which can be expressed as a gradient of the potential V of X. V of X is the potential of the channel at a certain point X. And then, the charge carry density in the channel here can be expressed as the capacitance times the potential difference between the gate.

372
00:20:03.600 --> 00:20:23.190
330W: And this V of X point, so it's, essentially, this is VET is a threshold voltage. And then, simply by remembering that current densities should be the same at any cross-section of the channel, so it's a constant. You can easily integrate this over, you know, along the channel length from 0 to L.

373
00:20:23.520 --> 00:20:41.170
330W: Also remember that mu is assumed to be constant here, so by doing this, you easily get this famous equation. This equation, the blue equation, describes this linear, quasi-linear part of the IV curve, but then something new happens at this point. At this point.

374
00:20:41.620 --> 00:20:45.869
330W: The, near one of the contacts you form as a collision.

375
00:20:46.040 --> 00:20:47.680
330W: feature of origin.

376
00:20:48.030 --> 00:20:56.729
330W: And… There's a way to show that current becomes independent of the, sort of voltage in this regime, so this red flat curve.

377
00:20:57.360 --> 00:21:00.289
330W: is described by this quadratic equation.

378
00:21:00.840 --> 00:21:02.550
330W: So…

379
00:21:02.670 --> 00:21:12.549
330W: This is the Shockley model, these two equations, and it describes the whole family of current voltage characteristics of the transistor at different gate voltages.

380
00:21:16.250 --> 00:21:25.439
330W: And the Shopping model is universal, in a sense, right? It's materials independent. So it actually works for a number of semiconductors, and

381
00:21:25.550 --> 00:21:32.380
330W: Actually, here at Truckers, we, contributed to this field effect, transistor area by

382
00:21:32.430 --> 00:21:44.030
330W: demonstrating several types of FETs. So, for instance, this one is the, well, first, intrinsic single crystal organic transistor that we.

383
00:21:44.040 --> 00:21:52.849
330W: demonstrated here at Truckers. We also demonstrated the first, transistor based on TMDs, transitional metal dichalcogenides.

384
00:21:52.990 --> 00:22:01.030
330W: Here, and more recently, actually during the pandemics, we developed this first, really working, intrinsic, perovskite.

385
00:22:01.240 --> 00:22:07.129
330W: transistors. And actually, they're all well described by this model, so it's a very nice model.

386
00:22:08.730 --> 00:22:11.660
330W: So here's an example. This is one of the transistors.

387
00:22:11.840 --> 00:22:15.060
330W: Made out of Rubin single crystal.

388
00:22:15.130 --> 00:22:35.110
330W: This one is created with exactly that third technique I showed you. It's a vacuum laminated dielectrics. So you can see that in the linear regime, we have very nice linear transfer characteristics, as model prescribes. In the saturation regime, it's perfectly parabolic, again, as the model is prescribing.

389
00:22:35.190 --> 00:22:44.050
330W: Also, if you know how to read this and understand, you will see that the, this device has negligible conduct resistance.

390
00:22:44.260 --> 00:22:48.129
330W: Nearly zero stress of separation.

391
00:22:48.340 --> 00:23:00.660
330W: What else? Very small contact resistance, matching abilities in these two regimes, so very nice device, textbook looking, okay? By the way, this particular device was made by Hitec.

392
00:23:00.930 --> 00:23:05.659
330W: It's probably online. But anyway, so that's humanity devices.

393
00:23:05.790 --> 00:23:09.190
330W: This is one of the, you know, the highest performance.

394
00:23:09.300 --> 00:23:10.439
330W: new transistors.

395
00:23:10.610 --> 00:23:23.220
330W: Here's another example, it's also Rubri with a different type of interface. Here you have Cytop, it's a type of very good dielectric, combined with spiraling. Again, you see all those features, but on top of this, you see that

396
00:23:23.310 --> 00:23:38.919
330W: hole effect measurements in this device give the, carrying ability, consistent with, what you measure in the, field effect. What is your metric for high performance? Is it switching speed, or is… No, it's, primarily is the mobility. Oh, no, the catch carry mobility.

397
00:23:39.030 --> 00:23:41.989
330W: So for this device, for instance, mobility is about

398
00:23:42.300 --> 00:23:45.849
330W: Thanks. And what kind of switching speeds do you have here?

399
00:23:46.080 --> 00:24:05.530
330W: Well, we didn't really measure it, but let me tell you a little bit about switching speed. So, why people are interested at all, like, looking at the mobility is exactly what the reason that you mentioned is because of the switching. So basically, What's important in the semiconductor device, when you have

400
00:24:05.840 --> 00:24:07.149
330W: two contacts.

401
00:24:08.790 --> 00:24:10.629
330W: And you have charge flow.

402
00:24:13.300 --> 00:24:22.480
330W: in this direction, is the so-called transit time, the carrier transit time from source to drain. Let's say if you have L is your distance.

403
00:24:22.670 --> 00:24:25.530
330W: Then the, the, transit time.

404
00:24:25.860 --> 00:24:29.280
330W: would be basically L divided by the drift velocity.

405
00:24:29.920 --> 00:24:36.559
330W: And then drift velocity is mu times E. Mu is the carrying mobility, E is the driving electric field.

406
00:24:36.860 --> 00:24:41.320
330W: Right? And, well, in the electric field.

407
00:24:42.070 --> 00:24:45.189
330W: So L over mu, the electric field is

408
00:24:45.630 --> 00:24:48.090
330W: voltage that you apply V over L,

409
00:24:48.230 --> 00:24:53.950
330W: So you get V, L squared, right? And then… The frequency F

410
00:24:54.240 --> 00:24:58.809
330W: of the operation of this kind of transistor will be basically one over T.

411
00:25:00.660 --> 00:25:03.020
330W: So invert this.

412
00:25:03.600 --> 00:25:06.010
330W: So it's going to be proportional to

413
00:25:10.870 --> 00:25:11.880
330W: Mute.

414
00:25:12.400 --> 00:25:19.790
330W: over L squared, right? So the frequency of transistor operation is going to be mu over L squared.

415
00:25:19.810 --> 00:25:33.219
330W: proportional to that. So the higher the mu, the higher frequencies you can reach, and this also shows that, miniaturization or scaling down the transistor is important, because L reduces your

416
00:25:33.250 --> 00:25:39.579
330W: We've moved the… That's why people are, you know, interested in Mew.

417
00:25:39.820 --> 00:25:41.420
330W: and improving it.

418
00:25:42.750 --> 00:25:46.629
330W: Okay, so this is also very good textbook looking devices.

419
00:25:48.010 --> 00:25:54.530
330W: Okay, so these are devices that Vladimir made by Mercy. Here, he improved

420
00:25:54.700 --> 00:26:05.500
330W: our crystal growth, by some, you know, by introducing some innovations. So, supposing we now have even Pure crystals.

421
00:26:05.580 --> 00:26:21.150
330W: And he measured this whole family of transfer characteristics of these devices. So here, you see that these dots, small dots, are actually experimental measurements, actual measurements. And the thin solid lines is a global feed of these

422
00:26:21.150 --> 00:26:29.800
330W: this whole collection of curves with the Shockley model with these 2 equations. So the global feed, there's a way you can do this global feed in origin.

423
00:26:29.900 --> 00:26:36.299
330W: that Gives you this set of parameters that are common for all these family of groups.

424
00:26:37.170 --> 00:26:37.930
330W: All right.

425
00:26:38.130 --> 00:26:50.239
330W: So you can see that the model, actually, the shopping model, fits pretty nicely with the data. So this is also a device that is working as a textbook device. These are output characteristics, you can also feed them with this model, right?

426
00:26:50.500 --> 00:26:51.490
330W: Precisely.

427
00:26:52.610 --> 00:26:58.179
330W: By the way, I wanted to mention that all the materials growth, device fabrication.

428
00:26:58.420 --> 00:27:01.600
330W: and measurements are all done here. Now it's part of it.

429
00:27:02.000 --> 00:27:07.679
330W: And we have a lot of, detailed technical notes online, how we do this.

430
00:27:09.380 --> 00:27:28.830
330W: Okay, so now I wanted to switch gears a little bit and talk about perovskites. So these are metal halide perovskites. They have a general composition AB… ABX3, ABX3. So the way to think about the lattices is basically, imagine a cube, you have a cube, and the B side, B side.

431
00:27:29.060 --> 00:27:43.229
330W: is at the corners of this cube. At the center, you have a large space, which is filled by this A-side, A-side cation. It usually is something B, like cesium plus, or it could be an organic cation, like.

432
00:27:43.440 --> 00:27:45.350
330W: Methylammonium plus.

433
00:27:45.520 --> 00:27:47.899
330W: And then you have these ectahedras.

434
00:27:48.010 --> 00:27:53.149
330W: you know, formed around around the B side. They're consisting of these halots.

435
00:27:53.600 --> 00:28:00.839
330W: So the, important notion about these materials is that they're very good somehow in making source.

436
00:28:00.970 --> 00:28:08.579
330W: and from solution grown films, you can easily obtain about 25 20% efficiency.

437
00:28:09.260 --> 00:28:18.340
330W: And there are certain reasons and debates about why they perform so well, even though these materials are made by these cheap and dirty room temperature processes.

438
00:28:18.820 --> 00:28:23.069
330W: Can the lead be replaced by something without losing its properties?

439
00:28:23.520 --> 00:28:24.770
330W: Umm.

440
00:28:25.620 --> 00:28:29.820
330W: It's been hard, but there are some suggestions like you can replace it with team.

441
00:28:30.270 --> 00:28:38.900
330W: Tin is not so toxic as lead, but the tin perovskites, they do work, but they are much less stable chemically.

442
00:28:39.290 --> 00:28:40.070
330W: I see.

443
00:28:40.510 --> 00:28:59.930
330W: Yeah, so this… so far, lead… the lead ones are working the best in terms of the solar cell performance. So here's the, this NREL, the national lab that certifies solar cells efficiencies, and they maintain this interactive chart that includes everything, all the technologies up to, you know.

444
00:29:00.200 --> 00:29:01.120
330W: Today.

445
00:29:01.360 --> 00:29:15.779
330W: So, for simplicity, I removed all other relevant curves, and on the left, showing here, three curves. So, the blue one is our main, kind of, workhorse. This is the single junction, single crystalline silicon, okay?

446
00:29:15.870 --> 00:29:26.610
330W: The best performance is 26.68% source efficiency. So this is the organic single junction cell up to 19.

447
00:29:26.710 --> 00:29:31.599
330W: So, Taronska has already beaten the silicon here. It reaches 28%.

448
00:29:31.940 --> 00:29:32.760
330W: What?

449
00:29:33.050 --> 00:29:35.410
330W: I have to make a point here that

450
00:29:35.760 --> 00:29:44.830
330W: even though this is a certified value by this national lab. This is still very small area source. So it's

451
00:29:45.070 --> 00:29:47.500
330W: It turns out it's harder to scale than pop.

452
00:29:48.380 --> 00:30:03.489
330W: Right? When we're talking about single-crystal silicon, it would be foot by foot, essentially. And what's the theoretical maximum efficiency here? Well, it's, like, something like 32 or 33, shortly before… Pretty close. It's pretty close, yeah, so… so,

453
00:30:03.490 --> 00:30:14.319
330W: I don't know, we're probably reaching… so now the community is… the main point, the main effort is not to even increase further, because soon you'll hit this, shortly quasar limit.

454
00:30:14.510 --> 00:30:23.439
330W: But how to make them stable in terms of size, okay? Because these are not really stable, not as stable as silicon.

455
00:30:23.570 --> 00:30:31.449
330W: Silicon solar cells are essentially infinite. Does the spin-orbit coupling play a role in that efficiency?

456
00:30:34.320 --> 00:30:37.099
330W: Spin orbit coupling perovskite.

457
00:30:37.300 --> 00:30:52.589
330W: We modify band structure due to — Right. And doesn't that enable it, the electron hole pairs, to separate out more easily? There were some proposals for that. Yes, there was proposals for that, but not really confirmed.

458
00:30:53.010 --> 00:31:01.240
330W: As far as I understand, there's no such consensus. And yet, somehow, you need to replace lead with tin, which also has strong spin-orbit coupling.

459
00:31:01.450 --> 00:31:05.430
330W: So what's the role of the heavy atom if it's not spin orbit?

460
00:31:11.180 --> 00:31:15.120
330W: Why specifically heavy atoms work? Hmm, it's a good question.

461
00:31:20.400 --> 00:31:28.970
330W: It's probably has to do with — Recombination is affected by spin orbit. Is that probably the problem? Yes.

462
00:31:29.560 --> 00:31:39.560
330W: But they, but they essentially… now consensus is that they, most of them are working as direct band semiconductors, so this inner orbit that creates some indirectness.

463
00:31:39.810 --> 00:31:56.180
330W: Yes. The gap is not important. It's very small. That comes back to the question, why you need lead? Yeah, that's a good question. I think it's more of a chemistry question. The chemistry works in such a way that lead halide easily forms this crystal structure. I see.

464
00:31:57.870 --> 00:32:05.489
330W: Okay, so, so basically what is known in a kind of very brief way about these particular perovskites is that

465
00:32:05.530 --> 00:32:19.140
330W: they have soft lattices, right? You know, up to 10 times softer than silica. And you… there is a lot of evidence for that, particularly from Raman scattering, you see soft optical formal modes.

466
00:32:19.240 --> 00:32:32.740
330W: that leads to, a lot of dynamic disorder in these crystals. This A site is… has high dynamic disorder, so basically this cation here is rattling, like in a cage, right?

467
00:32:33.200 --> 00:32:39.220
330W: A lot. Also, you have, large displacements of halides, so these, these.

468
00:32:39.320 --> 00:32:44.940
330W: these halides, they're kind of, like, fluctuating a lot. So all these, like, the heaters are…

469
00:32:45.630 --> 00:32:55.610
330W: fluctuating significantly, and this leads to a very large dynamic disorder in the system. So kind of similar feature to the organic molecular crystals.

470
00:32:55.820 --> 00:33:02.519
330W: And… This… creates this wobble in Lexuses, right?

471
00:33:04.590 --> 00:33:19.720
330W: Okay, so as I mentioned, it has advantages and disadvantages, right? The disadvantage is that you… you would have lower carrying mobilities because of the dynamic disorder. Also, some instabilities, mechanical or chemical, you know, faster degradation.

472
00:33:19.830 --> 00:33:29.479
330W: But there are also advantages. So, you know, low temperature processing is possible. For instance, compared to silicon, silicon

473
00:33:29.700 --> 00:33:33.350
330W: To grow a crystal, you have to melt and crystallize the material.

474
00:33:33.560 --> 00:33:35.530
330W: What's the melting point of silver?

475
00:33:36.650 --> 00:33:49.930
330W: more than 1,400 1,400 something very high. These things can be melted at about 500. But you can also do solution. First, st you can dissolve them in solvents and deposit the film.

476
00:33:50.050 --> 00:33:55.139
330W: crystallized film at room temperature, and you get solar cells efficiencies that are comparable to silicon.

477
00:33:55.480 --> 00:34:02.459
330W: This is what's amazing. Also, there are such phenomena as defect tolerance, defect self-healing, and so on, all related to this.

478
00:34:02.930 --> 00:34:05.120
330W: A high dynamic disorder.

479
00:34:06.250 --> 00:34:21.989
330W: Okay, so, early in this field, we were basically asking some simple fundamental questions about the… what's the intrinsic mobilities in these materials, what's the fundamental charge transport mechanisms, looking at all these things.

480
00:34:22.100 --> 00:34:31.779
330W: We're trying to disentangle electron and hole mobilities, looking at this phenomenon of defect tolerance, self-healing of defects, and so on.

481
00:34:31.990 --> 00:34:33.050
330W: And…

482
00:34:33.960 --> 00:34:53.390
330W: Our earlier contributions were, you know, a few things were done here, like, first proposal of large polaronic… large polaronic effect that makes sense for these materials. Also, we, were the first to measure intrinsic mobilities in some of these hybrid systems, this temperature dependence.

483
00:34:53.440 --> 00:34:59.819
330W: And so, for this reason, these papers were pretty much well appreciated by the community.

484
00:35:00.610 --> 00:35:18.749
330W: But more recently than I wanted to tell you, a different story. We came up with another interesting effect here. So, initially, it was done in this paper. What we were trying to do here, we were trying to gate one of these bulky solution-grown perovskite crystals.

485
00:35:18.850 --> 00:35:25.900
330W: by using an iron gel technique. So what's iron gel is, potentially, like, an electrolytic gating.

486
00:35:26.030 --> 00:35:34.099
330W: An ion gel is a polymer matrix, like a two-phase semi-solid gel, which is infused with an ionic liquid.

487
00:35:34.250 --> 00:35:41.629
330W: Right? And then you apply gate voltage and can transmit the fields here at the interface.

488
00:35:41.740 --> 00:35:57.000
330W: But we didn't see any field effect in the sense of electrical current. It didn't work. And the reason is because the surface of these crystals are pretty ugly. They're pretty good in both, but the surface is really, really, has lots of defects, stabs, scratches, and so on.

489
00:35:57.440 --> 00:36:08.370
330W: But instead of the dark electric field effect, we saw something different. When you excite the crystal by shining laser through this semi-transparent, actually.

490
00:36:08.520 --> 00:36:18.209
330W: This one is completely transparent iron gel. You excite the crystal, and then you can collect the photoluminescence that is coming out of the crystal, and we saw that we can gate the photoluminescence.

491
00:36:18.390 --> 00:36:27.770
330W: So this is a PL spectrum. You can see that this is 0 VG, and for negative gate voltages, it grows, and then you can even suppress it at positive VG.

492
00:36:28.030 --> 00:36:34.229
330W: So this is a normalized integral intensity. You can go up and down, and you can modulate the PL

493
00:36:34.700 --> 00:36:38.449
330W: intensity with the gate voltage quite significant here. Right?

494
00:36:38.640 --> 00:36:42.640
330W: Now, the problem with this system here is that this interface

495
00:36:42.750 --> 00:36:46.169
330W: It has 2 possible 2 possibilities. You can

496
00:36:46.700 --> 00:36:55.639
330W: Accumulate charges due to the field effect at the interface, so basically holes will be injected and form some sort of accumulation layer.

497
00:36:55.890 --> 00:37:00.480
330W: But another possibility is that due to the local strong electric field.

498
00:37:01.470 --> 00:37:04.960
330W: or by this ions of the iron gel.

499
00:37:05.070 --> 00:37:10.970
330W: You may have some latency migration, some… basically some electrochemistry.

500
00:37:11.840 --> 00:37:14.190
330W: So we couldn't separate these two effects.

501
00:37:14.450 --> 00:37:16.840
330W: So both would play a role here in this effect.

502
00:37:17.390 --> 00:37:32.409
330W: And then, during the pandemics, we have developed our own growth of high-quality crystals, and actually, Vladimir sitting here has done this. So he grew this, we set up, actually, the CVD type actual growth.

503
00:37:32.780 --> 00:37:44.959
330W: where you can grow perovskites as high-quality single crystals along the substrate. So this is one of these devices, so this yellow film is a cesium lead bromide, it's one of these type of perovskites.

504
00:37:44.970 --> 00:37:54.550
330W: on the substrate, and we were able to make high-quality field effect transistors based on this, without using INGL, without ionic liquid gating.

505
00:37:54.640 --> 00:37:57.899
330W: So here the gate is based on the paralleling

506
00:37:57.980 --> 00:38:07.900
330W: Pyrolene conformal coating. Pyrolene is a very good, high-quality polymer dielectric. It's very solid, very inert, doesn't have any ions.

507
00:38:07.910 --> 00:38:18.499
330W: So it's a purely electrostatic radiated device. And they show very nice performance. This is the first intrinsic Perovskite field effect transistors. You can see that they

508
00:38:18.880 --> 00:38:29.800
330W: feature all the, you know, the textbook-like, textbook features of, sharp onset, linear, transfer characteristics.

509
00:38:30.149 --> 00:38:31.859
330W: Small hysteresis.

510
00:38:32.250 --> 00:38:44.329
330W: Field effect mobility and hole, hole mobility is, pretty close. Also, the temperature dependencies from the individual measurements and from gated hole are similar.

511
00:38:44.979 --> 00:38:49.129
330W: And it shows band-like behavior, so it's a band transport.

512
00:38:49.350 --> 00:38:52.309
330W: So we're pretty much sure that we're seeing

513
00:38:52.420 --> 00:38:57.580
330W: intrinsic, which means not dominated by static disorder behavior.

514
00:38:58.950 --> 00:39:10.969
330W: Also, we are very grateful to our surface science group, specifically Sylvie. She's done a lot of XPS measurements, also Layla, she's done a lot of RDS.

515
00:39:11.100 --> 00:39:22.189
330W: As well as from Angio, the Department of Chemical, who did the x-rays for us. Thanks to their work, we actually know what we are doing, this analysis.

516
00:39:23.050 --> 00:39:24.669
330W: for materials. People right?

517
00:39:24.770 --> 00:39:41.710
330W: So, these data show that we have very clean surfaces, well, relatively. We have correct stoichiometric composition, and we have, what we call incommensurate petexy in this case, so the crystal is kind of aligned at a certain angle to the substrate.

518
00:39:44.390 --> 00:39:55.950
330W: This is how the samples look like. This is, they're large. This is a mica substrate with, with cesium with bromide under UV flashlight with luminescence in green.

519
00:39:56.720 --> 00:40:06.200
330W: This is a magnified view, so you see some domain boundaries and crystallographically oriented cracks, but nevertheless, you have large areas.

520
00:40:06.310 --> 00:40:08.439
330W: Of, you know, very good

521
00:40:08.790 --> 00:40:11.420
330W: uniform surface with the single crystals.

522
00:40:13.490 --> 00:40:29.279
330W: Okay, so, once we have those nice devices where we have definitely electric field effect, seeing, we're measuring, right? And these, surfaces are very good. We, sort of repeated this measurement with the photoluminescence, so again, here's the device.

523
00:40:29.450 --> 00:40:32.709
330W: We have top gate, which is semi-transparent.

524
00:40:33.230 --> 00:40:47.360
330W: paralleling dielectric, and, this is, the cesium lead bromide film on the substrate. So we excite, we come in with, high-energy photons, like blue, let's say, for the excitation, and then we collect photoluminescence, right? And we see this.

525
00:40:48.210 --> 00:40:52.300
330W: The photoluminescence can be modulated with the gate voltage.

526
00:40:53.260 --> 00:41:02.889
330W: you know, a lot. The reason here you have much higher span of VG than in previous cases, because this is a real… this is not an equal to gating, this is a real big shadow.

527
00:41:03.450 --> 00:41:09.170
330W: But this is this is okay, because this is scalable. The the thickness of the data can scale up.

528
00:41:09.510 --> 00:41:19.990
330W: So you see very large modulation of the field effect with, of the, photoluminescence with the, gate voltage. We developed some theoretical model for this, how it works.

529
00:41:20.220 --> 00:41:26.580
330W: Well, and this is a fundamentally new device, new device. So this, this is because this is not an electrical in essence.

530
00:41:27.100 --> 00:41:41.300
330W: By themselves, these devices and the dark do not shine in light. There is no bipolar injection. It's just photoluminescence. What is photoluminescence? It's kind of similar to color, right? For instance.

531
00:41:41.690 --> 00:41:50.499
330W: The reason… these 2 pieces have red and blue color is because you have organic dimolecules.

532
00:41:50.900 --> 00:42:10.610
330W: of different sort that are mixed into these plastics, and so when the external photon comes in with high energy, like UV or blue, it excites the electron across the homogloma gap, and then when this electron relaxes, it emits that photon of that energy, right? And if you use different molecules, then you have different color of photoluminescence that comes up.

533
00:42:13.300 --> 00:42:16.160
330W: So,

534
00:42:19.670 --> 00:42:36.120
330W: So this is an example of how they work. This picture shows the experimental arrangement, so we basically use a photoluminescence microscope for these observations, so you come in with the blue, excite through the transparent gate, and collect photoluminescence back to the

535
00:42:36.220 --> 00:42:43.449
330W: digital camera. So this video is the top view of the device. You're looking through the semi transparent gate.

536
00:42:44.080 --> 00:42:47.410
330W: The source and drain contacts are here on top and the bottom.

537
00:42:47.780 --> 00:42:58.680
330W: This little blob here is actually the gate wire attached to the top transparent gate with the silver print. And you can see here that as the gate voltage is changing.

538
00:42:59.440 --> 00:43:02.770
330W: So when you go to positive, you can

539
00:43:02.920 --> 00:43:08.770
330W: quench here completely, but when you go to the… in the other direction, to negative.

540
00:43:09.030 --> 00:43:10.400
330W: It brightens up.

541
00:43:12.700 --> 00:43:22.369
330W: And by the way, this is… this is the room… near the room temperature measurement, so you can see that the modulation contrast is very significant. You can quench the EL.

542
00:43:22.480 --> 00:43:24.219
330W: Almost by 100%.

543
00:43:30.610 --> 00:43:37.310
330W: there's a lag between the… my clicking and the slideshow. So, this is another.

544
00:43:37.860 --> 00:43:43.200
330W: Another example. For some reason it's not Moving upwards.

545
00:43:48.060 --> 00:43:50.939
330W: I can see it here, but I'm okay.

546
00:43:51.160 --> 00:43:54.959
330W: There's some lag, or maybe some of the frames are not shown.

547
00:43:55.260 --> 00:43:56.060
330W: forget.

548
00:43:59.200 --> 00:44:00.440
330W: Hold on.

549
00:44:00.550 --> 00:44:04.719
330W: Maybe I can show you from my laptop.

550
00:44:06.470 --> 00:44:09.069
330W: I'm not sure if you can see this.

551
00:44:12.090 --> 00:44:12.800
330W: No.

552
00:44:13.850 --> 00:44:23.680
330W: Okay, so the the image is going bright and then dark, and then again bright and dark. That's because in this experiment Vladimir

553
00:44:23.900 --> 00:44:25.390
330W: Swept VG.

554
00:44:25.760 --> 00:44:30.720
330W: from 0 to positive 50, and then back to negative 50, and then back to 0.

555
00:44:31.080 --> 00:44:32.600
330W: Quickly. Twice.

556
00:44:32.890 --> 00:44:37.580
330W: So, the image goes… First goes dark, then brighter.

557
00:44:38.940 --> 00:44:41.099
330W: Then, back to zero.

558
00:44:41.290 --> 00:44:45.180
330W: couple of times. So anyway, this is a different device, the same geometry.

559
00:44:45.550 --> 00:44:51.309
330W: You see the same. The PL is responding significantly to the gate.

560
00:44:53.210 --> 00:44:59.559
330W: So what happens here? We think that this is our model that we developed.

561
00:45:00.880 --> 00:45:11.630
330W: Pierce, you probably would be interested in seeing more. So we simply assign… we excite… we're fully excitable carriers in this, active layer, which is

562
00:45:12.090 --> 00:45:13.780
330W: season one, right?

563
00:45:14.100 --> 00:45:25.569
330W: the yellow, and we model it by writing down a simple rate equation for the N. N is the photocarrier density. So you have a source, which is proportional to the incident photon flux F.

564
00:45:26.010 --> 00:45:37.109
330W: Then you have two channels of, of carrier… for the carrier decay. This last one is trap-limited carrier… is basically trapping, so tau is a trap-limited carrier lifetime.

565
00:45:37.570 --> 00:45:49.559
330W: This one is the bimolecular electron hole recombination. So this thing in parentheses, these are concentration of holes. N is the concentration of photogenerated electrons.

566
00:45:49.980 --> 00:46:02.119
330W: So these holes, N is photogenerated holes, and this additional NG, these are the gate-induced interfacial holes, okay? We apply gates, so we have, at the interface, we have dark

567
00:46:02.700 --> 00:46:07.180
330W: density of holes that are helping to increase this rate.

568
00:46:07.710 --> 00:46:11.460
330W: Okay, so this term this Ng.

569
00:46:11.970 --> 00:46:21.380
330W: follows, you know, known field effect relationship. So Ng is proportional to the gate voltage when you're below the threshold and 0 above the threshold.

570
00:46:21.920 --> 00:46:32.110
330W: And then… Then the total luminescence intensity, this IPL, intensity of PL, is proportional to this.

571
00:46:32.470 --> 00:46:40.020
330W: radiative, recombination term, right? To this term. And then it turns out you can solve this equation analytically.

572
00:46:40.440 --> 00:46:46.730
330W: And… This is the solution. After some averaging.

573
00:46:46.990 --> 00:46:51.700
330W: Yeah. The, this, this is the feat with, theory.

574
00:46:52.060 --> 00:47:00.879
330W: That's a very simple theoretical expression, but it actually captures correctly all our experimental trends that we see in this.

575
00:47:03.190 --> 00:47:09.820
330W: By the way, these are, these are, you know, particularly this measurement is, is done at some… what was the temperature?

576
00:47:10.930 --> 00:47:14.520
330W: I think… liquid nitrogen.

577
00:47:15.400 --> 00:47:18.099
330W: nearly 4. Oh, no, I we

578
00:47:18.290 --> 00:47:24.830
330W: Right. But it works also at room temperature. The video, as I was showing you, is From zero.

579
00:47:24.990 --> 00:47:29.719
330W: So how do you switch it off?

580
00:47:30.720 --> 00:47:42.810
330W: The current silicose is still finite. Yeah, you… in this case, it's finite, but in the… for instance, in the videos I showed, the extinction is almost complete.

581
00:47:42.910 --> 00:48:01.069
330W: You modulate almost by… what was our best? By 98%. So, but how does the model adjust to that case? I think this is just because, you have… the model does not adjust to this case. This is, this is the, the, remaining some, you know.

582
00:48:01.780 --> 00:48:05.160
330W: life coming from the bulk, from different bulk of the crystal.

583
00:48:06.360 --> 00:48:10.610
330W: I cannot fully explain why some samples have some residual.

584
00:48:11.890 --> 00:48:15.569
330W: feel even at the current. So the model predicts a residual.

585
00:48:16.740 --> 00:48:25.530
330W: Model, model, it's increasing because if you have like a density of traps there, then model kind of disables them.

586
00:48:25.750 --> 00:48:32.090
330W: It means that different samples, different temperatures have different effects of traps, so…

587
00:48:32.520 --> 00:48:36.130
330W: So can you adjust the parameters in the model to bring.

588
00:48:36.470 --> 00:48:42.210
330W: that current down to zero, or almost zero? Yeah, it's a… it's a basically true density.

589
00:48:42.330 --> 00:48:43.719
330W: I see, okay. Yeah.

590
00:48:44.500 --> 00:48:54.899
330W: It depends a lot on the trap composition of the sample. If you have some residual traps… by the way, that's an important point. For this effect to occur, you have to have some.

591
00:48:55.550 --> 00:49:06.519
330W: certain trap density in the sample. If the sample is absolutely perfect, no traps, then this modulation would not be there. The luminescence as a function of BG would be a flat line.

592
00:49:06.900 --> 00:49:11.129
330W: I see. So you are feeling traps with this effect with the gate induced.

593
00:49:11.280 --> 00:49:12.000
330W: points.

594
00:49:12.250 --> 00:49:15.729
330W: and that helps appeal quantum yield go up.

595
00:49:15.950 --> 00:49:16.720
330W: I see.

596
00:49:16.990 --> 00:49:23.870
330W: And so if you fully switch it off, the residual carriers are all trapped. Yes. I got you. All right.

597
00:49:25.730 --> 00:49:29.509
330W: Okay, so if I have a little bit more time, I wanted to.

598
00:49:30.740 --> 00:49:37.050
330W: very briefly mention some of our activities, also impact measurements, because these are important

599
00:49:37.820 --> 00:49:39.770
330W: I don't know, fraction of our work.

600
00:49:39.910 --> 00:49:52.019
330W: So, in organic semiconductors specifically. So, it started with this, the first measurement of the hole effect in organic semiconductors. This was done, in rubrine, in these crystals.

601
00:49:52.670 --> 00:50:01.340
330W: So this is, something maybe boring for most of the condensed matter people, because this is a simple band classical hole effect. So what happens here

602
00:50:01.670 --> 00:50:07.579
330W: is that you apply a magnetic field perpendicular to your channel. The gate is not shown here, for simplicity.

603
00:50:07.730 --> 00:50:22.400
330W: And your carriers are drifting along this direction, and because of the Lorentz force that is acting on the carrier, their trajectories are shifted toward one side, and this leads to the development of the whole voltage across the sample.

604
00:50:22.550 --> 00:50:24.480
330W: right? And that's what we'll measure here.

605
00:50:24.950 --> 00:50:37.130
330W: But this experiment was important for organic semiconductors, because it's only possible to have this effect if you… if your carriers are really localized, and they move like a…

606
00:50:37.190 --> 00:50:47.679
330W: semiconductor, right? Then, your Lorentz force would be non-zero, because the drift velocity V is non-zero, and you would have this effect.

607
00:50:47.970 --> 00:50:53.169
330W: If your transport were purely a hoping transport.

608
00:50:53.470 --> 00:50:57.100
330W: The Lawrence force in that case would be 0, because

609
00:50:57.310 --> 00:51:13.810
330W: In hopping regime, the charge is tunneling from molecule to molecule, you're hopping from molecule. This is kind of an incoherent hopping, and in this regime, you cannot define the velocity, even though you may have, on average, a certain charge flow in a certain direction.

610
00:51:13.820 --> 00:51:18.799
330W: So the the Lawrence force would be 0. So just observing this.

611
00:51:19.090 --> 00:51:26.640
330W: kind of like ended the 20 year old debate whether the transport can be done like or cannot be done like molecular sorts.

612
00:51:27.060 --> 00:51:33.270
330W: Also, you can use this measurement, the whole effect, to extract the intrinsic, band mobility, right?

613
00:51:33.650 --> 00:51:35.850
330W: This fact that you have untransported.

614
00:51:36.700 --> 00:51:46.190
330W: And the problem, though, with hall effect measurements in organic semiconductors is that they're very difficult to do because of the very poor signal-to-noise ratio.

615
00:51:46.650 --> 00:52:02.940
330W: You can see that the noise is very high in all these measurements, even though people are using high magnetic fields, up to 6 Tesla, 10 Tesla, 10 Tesla, 6 Tesla, still the noise signal-to-noise ratio is poor, and that's because the mobilities are intrinsically low.

616
00:52:03.120 --> 00:52:11.739
330W: And residual trapping creates this multiple trap and released processes that contribute to the noise. So we have, you know, this terrible situation.

617
00:52:11.990 --> 00:52:14.329
330W: And so, for that reason, we came up

618
00:52:15.530 --> 00:52:17.950
330W: At some point with this method.

619
00:52:19.010 --> 00:52:26.980
330W: Unfortunately, all the movies in my presentations are not really smoothly running, as they should.

620
00:52:28.150 --> 00:52:36.849
330W: on my laptop with… But anyway, so, here, in this new technique that we, developed, this AC hole effect.

621
00:52:37.030 --> 00:52:46.659
330W: Instead of applying static magnetic field, we oscillate B field at a certain low frequency, up and down, and we use a lock-in amplifier

622
00:52:46.660 --> 00:52:57.219
330W: which is tuned exactly into the frequency of this B field to measure the whole voltage. And that results in a massive boost in the signal-to-noise ratio. It's, like, improves this technique dramatically.

623
00:52:57.400 --> 00:53:03.769
330W: With this new method, we can measure all effect in practical all these long new systems.

624
00:53:04.230 --> 00:53:12.850
330W: So, in our particular implementation, we create AC magnetic field by mechanical rotation of this assembly of strong permanent magnets.

625
00:53:14.850 --> 00:53:26.219
330W: Right, so this sample, which is sitting here in this flat chamber, somewhere here, it's seeing past the radio magnetic field. So that's how large is the magnetic field oscillation.

626
00:53:26.340 --> 00:53:38.550
330W: What's the magnet? Here, it's basically a quarter of a Tesla, okay? So what we have here, these plates, these are huge, very strong neodymium magnets, the size of my phone.

627
00:53:39.070 --> 00:53:46.770
330W: So they're arranged in certain ways that when this thing rotates samples, which is here, you're seeing oscillating magnetic field.

628
00:53:47.020 --> 00:54:05.829
330W: So these magnets are very strong. If they come together, you'll need to go to a machine shop. It's inhuman force. So they're held at this distance, there's a huge force between them, but they're mechanically held by structure.

629
00:54:06.600 --> 00:54:10.279
330W: Okay, so By the way.

630
00:54:11.020 --> 00:54:19.539
330W: But yeah, I have to say that you can do… this kind of method is very important for all these low-mobility type of materials, including

631
00:54:19.720 --> 00:54:25.430
330W: small molecules, polymers, oxides, and so on. But certainly you can use it even for high-growth systems.

632
00:54:26.810 --> 00:54:29.299
330W: And you know who actually.

633
00:54:36.100 --> 00:54:36.930
330W: Okay.

634
00:54:37.110 --> 00:54:38.170
330W: Who's got it?

635
00:54:39.350 --> 00:54:50.289
330W: Robert Dick. He was a, astrophysicist in Princeton. He is the one turns out who invented working amplifiers.

636
00:54:50.420 --> 00:54:59.119
330W: I was amazed. He's one of those genius scientists who are kind of universal science. He had many contributions in various fields.

637
00:54:59.540 --> 00:55:04.769
330W: right? Not only in astrophysics. He was his students. He invented walking under.

638
00:55:05.280 --> 00:55:20.110
330W: And you know who was the person who recommended us to use this technique, and specifically to use rotating permanent magnets for this measurement? Because using AC electromagnets, it turns out to be much harder. It's easier to do this way.

639
00:55:24.790 --> 00:55:26.540
330W: Oh, come on, switch already!

640
00:55:27.200 --> 00:55:40.510
330W: But he also basically discovered micro background. Right, right. Misha was the one who told us to do it this way, and we were… me and Chen, my co-doc Chen.

641
00:55:40.670 --> 00:55:58.380
330W: We're working on this. So, basically, we're very grateful to these two guys for their, you know, all the suggestions and inventions, because once we installed the system, we had a huge, and we still have a huge string of very good results with this, so…

642
00:56:00.230 --> 00:56:07.910
330W: Okay, so just as an example, what kind of thing this whole measurement can, Explain to us.

643
00:56:08.190 --> 00:56:16.220
330W: It actually helped us to unravel some of the mysteries that existed in this field, field of qualified measurements in organic semiconductors, and there were many.

644
00:56:16.260 --> 00:56:29.629
330W: For instance, one of the results was coming from the University of Tokyo, where they created this Pentacin single crystal transistors, and they did Hall effect measurements. By the way, they were doing DC Hall measurements at a very strong field, like 14 Tesla.

645
00:56:29.990 --> 00:56:34.330
330W: So apparently they boiled up tons of helium, tons of helium, like…

646
00:56:34.610 --> 00:56:52.110
330W: But they're rich, so it doesn't matter for them. We can do this type of measurements now for free, essentially, with our machine. But the important thing is that they saw what is called underdeveloped hall effect, is when the hall carry density is much higher, in this case.

647
00:56:52.110 --> 00:56:57.240
330W: twice greater than the total electrostatic or induced period density. Right?

648
00:56:57.320 --> 00:57:03.479
330W: And this is kind of highly unusual result. Why would you have whole density higher than you…

649
00:57:03.520 --> 00:57:23.489
330W: have in the sample, right? And we realized that this is… the reason for this kind of inconsistency is because they're using this simple band semiconductor hole effect formula, where you have a voltage is proportional to the current you're going to do with flows… that flows in your device. Magnetic field, and you extract this carrier density.

650
00:57:23.550 --> 00:57:25.009
330W: By using this formula.

651
00:57:25.860 --> 00:57:41.720
330W: But this formula assumes that 100% of your carriers are bend-like. Then you can apply it. For example, imagine that your sample is purely a hopping conductor. If it's a purely hopping conductor, your current will be non-zero. This is longitudinal current.

652
00:57:42.550 --> 00:57:49.730
330W: but your whole voltage would be 0. Then, if you use this formula, you have an infinite carry density, which is nonsense. Right?

653
00:57:49.970 --> 00:58:04.810
330W: So, something like this is happening here. So, to kind of, remedy this situation, they introduced this, some kind of coherence factor, alpha, which is NFAT, this one, divided by N whole. And in this case, it's roughly 50%.

654
00:58:04.970 --> 00:58:07.999
330W: And then they say, okay, our carrier was…

655
00:58:08.420 --> 00:58:10.339
330W: Partial to here, they're not full.

656
00:58:11.010 --> 00:58:15.049
330W: And then we realized that the situation like this can occur.

657
00:58:15.580 --> 00:58:27.009
330W: if you have, a coexistence of band and hoping carriers in your sample. So this is the channel. These two dots represent a band-like call, which is moving

658
00:58:27.150 --> 00:58:35.560
330W: particular instant as a… as a band carrier in the… in the band, and this is also a hole, but it's, it's a hopping hole.

659
00:58:36.040 --> 00:58:44.669
330W: It's a hole that is localized on this tail state. And by the way, these tail states are common in this material for intrinsic reasons, for that dynamic disorder that I was talking about.

660
00:58:45.030 --> 00:58:54.120
330W: So you have a lot of tail states, and therefore your overall collection of carriers consists of the band carriers, and part of them are holding carriers.

661
00:58:54.870 --> 00:58:56.670
330W: And then,

662
00:58:57.370 --> 00:59:03.859
330W: The rain carriers experience the warrants force, and they create the whole op. But the the hobby

663
00:59:04.700 --> 00:59:07.540
330W: carriers. They do not feel any warrants for us.

664
00:59:07.720 --> 00:59:10.459
330W: Yet they, of course, feel the electric forces.

665
00:59:10.580 --> 00:59:18.500
330W: And they would be deflected in the opposite direction, because they're pushed by this, whole electric field that is developed between this cone.

666
00:59:18.630 --> 00:59:24.300
330W: So this hopping subsystem would partially screen your whole fold.

667
00:59:24.860 --> 00:59:26.690
330W: And that causes the,

668
00:59:27.270 --> 00:59:34.170
330W: You know, all those mysteries. And it turns out this model is pretty successful in explaining a number of phenomena that people are seeing.

669
00:59:34.290 --> 00:59:51.739
330W: So the, way to work it out in a kind of very simple binary way is to assume that, okay, you have band carriers, you have copy carriers, and they're characterized with their own densities and mobilities, and then you have to, you have to equilibrate the, transverse current, this J, right?

670
00:59:51.740 --> 00:59:58.480
330W: So one part of the current transverse current comes from the event carriers that experience both

671
00:59:58.710 --> 01:00:07.180
330W: electric force and the Lorentz force, and from the Hopi carriers, they only experienced the, this, this electric force, but no, no.

672
01:00:07.290 --> 01:00:25.109
330W: Lawrence force. And then, in the steady state, this current should be zero, it's completely equilibrated, and this leads to this master equation for the whole electric field, the electric field which is in this direction, okay? Which becomes a function of these two… these two, carrier populations.

673
01:00:26.350 --> 01:00:27.130
330W: Okay.

674
01:00:27.640 --> 01:00:40.470
330W: Then one can parameterize this, this situation with these two important parameters, gamma and beta. So, gamma is just a fraction of band carriers among the total population.

675
01:00:40.470 --> 01:00:54.069
330W: And beta is the ratio of hopping and mobility. And then you can express everything that you measure, whole mobility, whole carrier density, and field effect mobility through these gamma and beta parameters.

676
01:00:54.340 --> 01:01:03.190
330W: And you can even plot it as this… you can see it shows this nice, kind of surface.

677
01:01:03.710 --> 01:01:11.239
330W: And if you draw a unity plane, right, so pink is a plane corresponding to the… this ratio.

678
01:01:11.620 --> 01:01:29.529
330W: V1, and then you can see that it intercepts with this blue surface along this line. So, strictly speaking, the system would show perfectly in all FAT measurements only if you are along this intercept line, okay? If you are here, somewhere, or here.

679
01:01:29.540 --> 01:01:33.599
330W: It'll go up or down, so it's not going to be fully coherent for that.

680
01:01:33.990 --> 01:01:41.820
330W: And, for instance, for that practicing result, let's say if you are here at this point, where this ratio is 2,

681
01:01:42.410 --> 01:01:45.260
330W: This is corresponding to their situation, right?

682
01:01:45.970 --> 01:01:50.329
330W: Gamma that corresponds to the point is only 1%.

683
01:01:50.820 --> 01:01:54.520
330W: That means that only 1% of your carriers are banned carriers.

684
01:01:54.980 --> 01:01:57.839
330W: and all other 99% opioid carriers.

685
01:01:57.950 --> 01:02:02.269
330W: Yet, you would see Holocaust with that kind of, strange

686
01:02:02.390 --> 01:02:05.870
330W: result that showing you that whole

687
01:02:05.990 --> 01:02:08.339
330W: Density is twice greater than your total.

688
01:02:08.490 --> 01:02:10.150
330W: And that explains it, essentially.

689
01:02:11.470 --> 01:02:15.219
330W: And there are other phenomena that probably I should skip this.

690
01:02:16.870 --> 01:02:28.039
330W: you know, the, the majors, or maybe I'll explain this one. So, in our old measurements, we saw this, this type of transistor, Tetris and single-crystal transistors that have low mobility, below 1.

691
01:02:28.360 --> 01:02:31.809
330W: Yet we see here that hall effect measurements.

692
01:02:31.970 --> 01:02:37.820
330W: Give consistent result with the, with the, fat measurements for the mobility and carrier density.

693
01:02:37.940 --> 01:02:50.749
330W: So the question is, how can it be? Because apparently this is a too low mobility for the system to be accurate. Yet, this result apparently tells us that your 100% of the carriers are bank carriers. How can this be?

694
01:02:50.820 --> 01:03:09.939
330W: It turns out this model, also explained this, so if you, add two planes, one 20% above this unity plane, and another one 20% below, so you have this ratio, 1 plus minus 0.2, and look at these two intercepts, two additional intercepts.

695
01:03:10.550 --> 01:03:11.940
330W: It will look like this.

696
01:03:12.570 --> 01:03:28.409
330W: Okay? So, if you are in this parameter space, shown here by this, yellow and orange, if you're here somewhere, then your whole measurements would be, very close to your fat measurements, only the difference would be less than 20%.

697
01:03:29.200 --> 01:03:29.960
330W: Right?

698
01:03:30.150 --> 01:03:37.140
330W: So, for instance, if you are somewhere in this droplet in the parameter space here.

699
01:03:37.420 --> 01:03:45.170
330W: right? You would see this polyfact which is looking like nearly 100% here.

700
01:03:45.440 --> 01:04:00.939
330W: Yet, this corresponds to gamma at about 0.3, which means that in this droplet, only about 30% of your carriers are hopping, bankers, and all the rest are hopping carriers. So the system is very far from being secure.

701
01:04:01.270 --> 01:04:09.140
330W: Okay? Nevertheless, you will see the situation that polar fire gives, basically the same density and mobility.

702
01:04:09.670 --> 01:04:12.100
330W: We're just as if we are children.

703
01:04:12.600 --> 01:04:16.830
330W: So, one needs to understand all these two, kind of, you know.

704
01:04:17.090 --> 01:04:23.129
330W: Be careful with the Hall effect measurements in the systems that have a mixed OP and band.

705
01:04:24.730 --> 01:04:26.299
330W: transport regime.

706
01:04:27.360 --> 01:04:44.219
330W: So, and this model is, materials not specific, it's not material specific, so it can actually work with, the whole measurements in all these low mobility systems, where, open care is important. Molecular crystals, polymers, oxides, amorphous.

707
01:04:44.260 --> 01:04:47.420
330W: you know, nanocrystals and quantum dot arrays.

708
01:04:47.580 --> 01:04:53.679
330W: hybrid growth and so on. So, even though we published it in some obscure journal, it was pretty…

709
01:04:53.790 --> 01:04:56.259
330W: Well accepted by this community.

710
01:04:57.630 --> 01:05:07.080
330W: Okay, so I'm nearing the end. I think in the name of time, let me… Go to the conclusions.

711
01:05:07.300 --> 01:05:12.869
330W: We've done some intrinsic strain, effect measurements on the mobility.

712
01:05:14.350 --> 01:05:21.990
330W: For the whole effect, many other results. But let me conclude by saying that.

713
01:05:22.300 --> 01:05:33.480
330W: you know, I think that field effect transistors based on novel materials is a kind of very challenging research field, but it's also very fascinating when you see a device working, it's kind of exciting.

714
01:05:33.740 --> 01:05:47.040
330W: And specifically, the combination of longitudinal field effect transistor measurements and hole measurements is a great, great way to elucidate the intrinsic, charge transport parameters, like mobility and stuff like that.

715
01:05:47.430 --> 01:05:51.990
330W: intrinsic meaning that not affected too much by static disorder.

716
01:05:52.320 --> 01:05:58.719
330W: And particularly, we are very excited about this new type of device that we

717
01:05:58.940 --> 01:06:01.550
330W: came up with this photoluminescence transistor.

718
01:06:01.700 --> 01:06:09.109
330W: which can… it's a new concept, it's a very interesting device. You can envision a number of things with it, like

719
01:06:09.570 --> 01:06:15.430
330W: fabric that changes color, for instance. You can imagine that you can create They've got 3 different…

720
01:06:15.540 --> 01:06:25.949
330W: transistors that use different perovskites. Perovskites, you can easily change the band gap by changing halide, for instance, so you have, like.

721
01:06:26.100 --> 01:06:32.779
330W: A composite pixel that consists of three this kind of transistors together.

722
01:06:32.950 --> 01:06:42.600
330W: RGB, red, green, and blue, and you would have 3 gates that you can control independently, so you can change the RGB composition, so this pixel would change color.

723
01:06:43.820 --> 01:06:51.559
330W: That's one possibility. Also, you can use this effect for improving the PL quantum yield.

724
01:06:51.880 --> 01:06:57.389
330W: lasing and led applications by surface needing to. That's also possible.

725
01:06:59.480 --> 01:07:03.619
330W: Okay, so with this, let me thank all the people who contributed to this.

726
01:07:03.870 --> 01:07:06.660
330W: A research?

727
01:07:06.860 --> 01:07:22.190
330W: Vladimir Hitak, Yanzhen Chen, who is now a faculty member in China. Choi was another of my postdocs, he's now also faculty in South Korea. Sylvie, Layla, and others. Also, we have wonderful collaborators here.

728
01:07:22.600 --> 01:07:33.269
330W: Specifically, the surface science group, Tom Angie. You know, several of our, faculty members, and also, this is the list of our.

729
01:07:33.510 --> 01:07:44.649
330W: outside collaborators, these are kind of international and domestic, but many of them, we have some active projects going on. And of course, I'm very grateful to funding.

730
01:07:44.850 --> 01:07:49.469
330W: specifically us Department of Energy and Seth and Jacob's chair.

731
01:07:49.950 --> 01:07:51.659
330W: And thank you very much for your attention.

732
01:07:57.090 --> 01:08:09.679
330W: A number of questions during the presentation, but we have time for maybe a few more, right? Is the hopping electron density intrinsic to the material, or does it depend on defects?

733
01:08:09.760 --> 01:08:27.659
330W: in the material? It's actually, intrinsic to the material, because I'm talking about salt lattice materials, like molecular crystals, and even if you imagine an absolutely mathematically ideal molecular crystal, it would have failed states, one by the dynamic disorder. At finite temperature.

734
01:08:28.120 --> 01:08:36.219
330W: Molecules oscillate a lot, and these oscillations usually cause a large amplitude of,

735
01:08:36.370 --> 01:08:37.630
330W: and it's running disorder.

736
01:08:37.890 --> 01:08:41.609
330W: So it's a kind of like an intrinsic disorder. You cannot get rid of it.

737
01:08:41.930 --> 01:08:45.459
330W: The final temperature. And that creates failed states, realistically.

738
01:08:46.490 --> 01:08:50.509
330W: So, when you populate, when you pour in your carriers in the channel.

739
01:08:50.640 --> 01:08:57.820
330W: they would occupy tail states, and maybe if you push the density, some of them will spill.

740
01:08:58.220 --> 01:08:59.620
330W: Standard Band.

741
01:09:00.410 --> 01:09:03.259
330W: But you always will have 10 states that are.

742
01:09:03.990 --> 01:09:08.380
330W: Those carriers that occupy real space, they move by hoping.

743
01:09:09.640 --> 01:09:10.999
330W: those that are…

744
01:09:11.570 --> 01:09:20.730
330W: reaching the band, they move kind of, like, in a delocalized way, like a band carrier. So this coexistence is, intrinsic, I would say, in this soft light sentience.

745
01:09:23.350 --> 01:09:24.160
330W: Duck.

746
01:09:24.320 --> 01:09:29.270
330W: It seems, in the case you mentioned, with only 1% band carriers.

747
01:09:29.400 --> 01:09:31.100
330W: That seems pretty extreme.

748
01:09:31.350 --> 01:09:42.049
330W: And so I guess my question is, are there any other measurements you can imagine doing to be able to suss out those particular ratios of popping versus bank error?

749
01:09:43.650 --> 01:09:46.989
330W: Probably some photo conductivity measurements.

750
01:09:47.450 --> 01:09:53.499
330W: But I, I can't imagine… Anything that's so distinctly, so clearly distinct.

751
01:09:55.220 --> 01:09:58.580
330W: Brandon Holt, because See you.

752
01:09:59.280 --> 01:10:03.430
330W: Lawrence force. The the classical Lawrence force is very sensitive to you.

753
01:10:04.090 --> 01:10:07.320
330W: Most of the care. From Earth, the most sensitive way.

754
01:10:08.930 --> 01:10:15.920
330W: That that means 1% is just one possibility in that parameter space that would give the University of Tokyo result.

755
01:10:16.290 --> 01:10:22.360
330W: There are probably some other points, because it's surface. You can find other possibilities.

756
01:10:23.050 --> 01:10:29.270
330W: It's just, we'll speak as an example to show that you can theoretically have that situation.

757
01:10:29.570 --> 01:10:35.660
330W: even with a very low number, even with, yeah, even with a tiny number of Okay.

758
01:10:37.600 --> 01:10:47.109
330W: Remind everyone, anyone who's… anyone and everyone who's online that, if you have a question, raise your hand, or just, type something into the chat so I can see.

759
01:10:47.280 --> 01:10:49.059
330W: Haven't seen any yet.

760
01:10:49.720 --> 01:10:52.650
330W: Any other questions in the room here?

761
01:10:54.600 --> 01:11:00.159
330W: Okay, well, if not, nothing online. Let's thank Vitaly one more time.

762
01:11:07.440 --> 01:11:08.180
330W: Okay.

763
01:11:20.150 --> 01:11:20.830
330W: Okay.

764
01:11:23.270 --> 01:11:33.090
330W: It's amazing that you can measure both.

765
01:11:33.820 --> 01:11:49.290
330W: No, no, I yield.

766
01:11:50.590 --> 01:11:54.269
330W: Yeah, he's gonna be talking.

767
01:11:54.380 --> 01:12:04.380
330W: It's, like, so far away from us.

768
01:12:04.930 --> 01:12:16.220
330W: Yeah, yeah.

769
01:12:16.270 --> 01:12:25.149
330W: I think ultimately it's just like, I was humbled. Yeah, Shmaly and Fernandez. Yeah, yeah.

770
01:12:25.800 --> 01:12:50.699
330W: So this hopping… Hopping density. Yeah, no, I caught on that, I caught on that. Must be, yeah.

771
01:12:50.700 --> 01:13:13.960
330W: Actually, very strange results came out of another paper.

772
01:13:13.960 --> 01:13:19.020
330W: So you mentioned the situation when we use two systems.

773
01:13:19.020 --> 01:13:38.950
330W: In one of them, they see a color fiber, which is . Another one, they see similar to what the . They see a large disparity between .

774
01:13:38.950 --> 01:13:41.880
330W: Static or induced, which is strange.

775
01:13:41.990 --> 01:13:52.210
330W: When they measure the temperature dependence, the system, which seems to be very coherent, they see hobby. Activated.

776
01:13:52.210 --> 01:14:14.529
330W: Which is the engineering mentality. Which is strange. I know. But in the other system, which should be less coherent, they see a little bit of a friend like… I see. So this is a huge inconsistency in the papers of PRF. Everyone will discuss this. To understand it, then we explain… this model explains that you can have this kind of…

777
01:14:14.530 --> 01:14:15.550
330W: Subscribe.

778
01:14:15.560 --> 01:14:21.350
330W: When I was in high school, I liked biology, I liked chemistry.

779
01:14:21.480 --> 01:14:36.480
330W: And then we have to physics. This is great. I don't remember anything. That's right. Yeah. Four equations. Don't have to know 92 elements. Right, right. Or 1,000 organic elements.

780
01:14:36.960 --> 01:14:43.749
330W: Yes, I got to learn the chemistry when I saw organic chemistry. You turn on all the people. Yeah.

781
01:14:43.980 --> 01:14:46.450
330W: But what did their students do?

782
01:14:47.010 --> 01:14:50.280
330W: I don't know. I don't know if I want to ask.

783
01:14:50.750 --> 01:14:54.650
330W: Petali, that was very nice. Thank you.

784
01:14:56.710 --> 01:14:57.740
330W: Okay.

785
01:14:57.870 --> 01:15:04.129
330W: Chadburn? As a graduate student? Yeah, that's a good… Yeah, graduated 20.

786
01:15:04.220 --> 01:15:22.080
330W: 22. 22, yeah. 22, yeah. And so you came back for the storm, correct? Yeah, well, yeah. Oh, yeah. Oh, yeah. From, from, Oregon. Yeah, yeah. We're getting, intel in, Oregon now. Oh, nice. Yeah, I'm on a vacation now. It's in Hillsborough. It's, like, 10, 15 miles outside of Portland.

787
01:15:22.220 --> 01:15:25.490
330W: My kid lives in that. My kid lives in that.

788
01:15:26.020 --> 01:15:27.330
330W: Oh, really?

789
01:15:27.880 --> 01:15:28.700
330W: Yeah.

790
01:15:28.830 --> 01:15:35.460
330W: Yeah, I like it a lot. Yeah. It's pretty laid back. Yeah. Kind of far, though, but, you know.

791
01:15:36.140 --> 01:15:38.540
330W: From here, yes.

792
01:15:40.560 --> 01:15:41.390
330W: Cool.

793
01:15:41.900 --> 01:15:45.940
330W: At least in Portland, when we wander around, I think.

794
01:15:46.630 --> 01:15:57.720
330W: Everyone's just working remotely, sitting at a coffee shop. Did you get that? She's being jealous. Except that now they have to go back to work on-site, as you can tell.

795
01:15:58.360 --> 01:16:04.950
330W: Yeah, well, I got my… my daughter-in-law, she works at Nike, and so she's… she's gotta go back. Gotta go back to her, yeah.

796
01:16:06.620 --> 01:16:11.740
330W: We don't have as many highways over there as we do here, so there's the… there's one road that goes out to, like, all the…

797
01:16:12.010 --> 01:16:25.870
330W: the, like, companies that are outside the city, like, everyone collects on that way. Oh, is that where Nike is as well? Yeah, I think Nike's not quite as far, but it's, like, in the same direction. Yeah. What blew me away when I visited up there is that

798
01:16:26.030 --> 01:16:32.930
330W: Yeah, no matter the road, right? Yeah. Everybody drives 55 miles an hour. Yeah. It drove me crazy! Yeah.

799
01:16:33.190 --> 01:16:34.530
330W: I couldn't do it.

800
01:16:37.760 --> 01:16:38.900
330W: People tune us.

801
01:16:40.770 --> 01:16:42.399
330W: Alright, I'll head back.

802
01:16:42.630 --> 01:16:46.550
330W: Very good class tomorrow. All right, you're going back.

803
01:16:46.720 --> 01:16:49.160
330W: All right.

804
01:16:49.370 --> 01:17:03.210
330W: I think today… Yeah, I've been, like, my mom. I see, so it's sort of a… When my son comes back. Yes.

805
01:17:03.490 --> 01:17:16.190
330W: Yeah, Rutgers has, like, just… I know, Bill!

806
01:17:16.990 --> 01:17:19.699
330W: This does…