Illumination induced charge separation at tetraphenyl-porphyrin/metal oxide interfaces

Charge separation processes were studied at tetraphenyl-porphyrin (H2 TPP) /metal oxide interfaces by surface photovoltage spectroscopy. The thickness of the depleted Ti and Sn oxide layers was about 2 nm. Acceptorlike interface states play a crucial role in the steady-state light-induced negative charging of H2 TPP surface layers as well as in tunneling or light-induced discharging. The results indicate that steady-state negative charging of the H2 TPP layer is induced by preferential recombination of holes at the (H2 TPP) /metal oxide interface and depends only weakly on temperature. The modulated surface photovoltage is dominated by hole transport at low temperatures and by electron transport at higher temperatures. The activation energy of the latter is of the order of 0.4 eV.