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 (H2TOPP) belongs to a class of self-organizing porphyrins. Since its LUMO lies above the conduction band of titanium dioxide (TiO2) and its visible light absorption is very strong, sensitization of TiO2 with H2TOPP thin films is possible. After spin-coating this porphyrin onto n-type TiO2 the Fermi-level of H2TOPP 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 H2TOPP making an organic based heterostructure p-i-n solar cell possible. In this cell, H2TOPP is sandwiched between n-type TiO2 and p-type ZnTCPP. The LUMO positions of these porphyrins is such that unidirectional energy transfer from ZnTCPP to H2TOPP 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.