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 t o a class of self-organizing porphyrins. Since its LUMO lies above the cond uction band of titanium dioxide (TiO2) and its visible light absorption is very strong, sensitization of TiO2 with H2TOPP thin films is possible. Afte r 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 semico nductor, i.e., donor and acceptor densities, if present, compensate each ot her. Thin films of zinc 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (ZnT CPP) are also investigated and show profound p-type character. Moreover, th e LUMO of ZnTCPP is located about 0.4 eV above that of H2TOPP making an org anic 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 t he p-i-n structure is beneficial for solar-energy conversion.