Visible Light Sensitization of Titanium Dioxide with Self-Organized Porphyrins: Organic P−I−N Solar Cells

Free base 5,10,15,20-tetrakis(4- n -octylphenyl) porphyrin (H 2 TOPP) belongs to a class of self-organizing porphyrins. Since its LUMO lies above the conduction band of titanium dioxide (TiO 2 ) and its visible light absorption is very strong, sensitization of TiO 2 with H 2 TOPP thin films is possible. After spin-coating this porphyrin onto n-type TiO 2, the Fermi-level of H 2 TOPP is measured, from which it is established that it behaves as intrinsic semiconductor, i.e., donor and acceptor densities, if present, compensate each other. Thin films of zinc 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (ZnTCPP) are also investigated and show profound p-type character. Moreover, the LUMO of ZnTCPP is located about 0.4 eV above that of H 2 TOPP making an organic based heterostructure p−i−n solar cell possible. In this cell, H 2 TOPP is sandwiched between n-type TiO 2 and p-type ZnTCPP. The LUMO positions of these porphyrins is such that unidirectional energy transfer from ZnTCPP to H 2 TOPP occurs. By carefully comparing the photocurrent action spectra with the absorption spectra, it could be established that a built-in field in the p−i−n structure is beneficial for solar-energy conversion.