Cross-Linkable Molecular Hole-Transporting Semiconductor for Solid-State Dye-Sensitized Solar Cells

In this study, we investigate the use of a cross-linkable organosilane semiconductor, 4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi2), as a hole-transporting material (HTM) for solid-state dye-sensitized solar cells (ssDSSCs) using the standard amphiphilic Z907 dye which is compatible with organic HTM deposition. The properties and performance of the resulting cells are then compared and contrasted with the ones based on poly(3-hexylthiophene) (P3HT), a conventional polymeric HTM, but with rather limited pore-filling capacity. When processed under N-2, TPDSi2 exhibits excellent infiltration into the mesoporous TiO2 layer and thus enables the fabrication of relatively thick devices (similar to 5 mu m) for efficient photon harvesting. When exposed to ambient atmosphere (RHamb similar to 20%), TPDSi2 readily undergoes cross-linking to afford a rigid, thermally stable hole-transporting layer. In addition, the effect of tertbutylpyridine (TBP) and lithium bis(trifluoromethylsulfonyl)imide salt (Li-TFSI) additives on the electrochemical properties of these HTMs is studied via a combination of cyclic voltammetry (CV) and ultraviolet photoemission spectroscopy (UPS) measurements. The results demonstrate that the additives significantly enhance the space charge limited current (SCLC) mobilities for both the P3HT and TPDSi2 HTMs and induce a shift in the TPDSi2 Fermi level, likely a p-doping effect. These combined effects of improved charge transport characteristics for the TPDSi2 devices enhance the power conversion efficiency (PCE) by more than 2-fold for ssDSSCs.