Basics: Dyes Sensitized Solar Cells; Energy Alignment Concept

Dye sensitized solar cells (DSSCs) have been shown to convert solar light into electricity with promising efficiencies and have attracted considerable interest in the fundamental aspect of their operation. At the heart of the device is a thin oxide film, composed of a network of nanocrystalline TiO2 particles, deposited on a transparent conducting substrate and sintered so as to establish an effective conduction path. This nanoporous structure is then sensitized with an organic dye molecule and infiltrated with an electrolyte, which, in turn, makes contact to a counter electrode. As the band gap of TiO2 is over 3 eV, only ultraviolet radiation directly produces electron-hole pairs in the native material. However, with the appropriate alignment of the electronic levels, sensitization by a chemisorbed dye molecule capable of harvesting photons of energy smaller than the TiO2 band gap, enables efficient absorption across a large fraction of the solar spectrum. Figure (a) schematically represents the ground-state energy alignment between the three main components of DSSCs: a wide band gap semiconductor substrate, a dye molecule chemisorbed onto this substrate, and an electrolyte in contact with the dye. Focusing on the dye/oxide interface, a light-harvesting dye molecule can absorb a photon from within the visible region of the solar spectrum, resulting in a photoexcitation that can, in the simplest terms, be thought of as the elevation of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the dye (Figure (a), step 1). If the LUMO is degenerate with the substrate conduction band, the excited electron can transfer to the substrate (Figure (a), step 2) and participate in current flow across the cell. The resulting singly occupied HOMO of the oxidized dye can then be filled via an appropriately chosen electrolyte (Figure (a), step 3). From this, it is clear that the performance of DSSCs depends strongly on the relative alignment of the dye molecular levels with respect to the substrate band edges. Using XPS, UPS and IPS, we can determine the HOMO-LUMO positions relative to the substrate band structure. The geometry of the dye molecule at the surface is also studied using scanning tunneling microscopy and NEXAFS spectroscopy.

       Recent Highlight:
       Energy Alignment of a ZnTPP-Ipa on the TiO2(110) and ZnO(11-20) surfaces

Metalloporphyrins play an essential role in the photosynthetic process and therefore, are attractive candidates for photoinduced electron-transfer mediators in DSSCs. Among metalloporphyrins, zinc tetraphenylporphyrin (ZnTPP) derivatives have been found to have similar electron injection and charge recombination properties as N3 dye, the standard ruthenium-containing dye used for DSSCs while exhibiting reasonable performances using either nanostructured TiO2 or ZnO as substrates. Nevertheless, many fundamental properties of the dye/metal oxide interface, that is essential for DSSC operation, such as the electronic structure of the adsorbed dyes, adsorption geometry and energy level alignment are not well studied and need careful consideration. Although in functional dye sensitized solar cells nanostructured metal oxide thin films are used as a substrate, to study electronic and geometric properties of the dye/oxide system in a controlled way, single crystal are used here. The occupied and unoccpied states of both the oxides surfaces, before and after sensitization with the ZnTPP-Ipa dye (shown in (b)) have been evaluated by means of UPS and IPS in a single UHV chamber. The measured spectra are shown in the Figure (c) and (d) for the ZnO(11-20) and TiO2 (110) surfaces. A comparison of the experimental molecular contribution to the electronic structure (e) with the calculated density of states (f) of the ZnTPP-Ipa dye, enable the direct determination of the alignment of the moleculat levels with respect to the substrates band egdes. An energy diagram representative of the dye/oxides interface (g) can be built. It is found that the HOMOs are located 1.6 eV above the valence band edge and that the LUMOs are situated 1.9 eV above the conduction band edge of the oxides.

       Recent Publications                                                                       (back to top)