Photo-induced unidirectional electron transfer in a porphyrin heterojunction

Organic heterojunctions made by deposition of two different types of porphyrins onto an ITO substrate by electro-polymerisation and spin-coating exhibit rectification upon illumination. The redoxpotentials for the various porphyrin D/A combinations can be varied by using different porphyrin metal centers. Different exothermicities for photo-induced electron-transfer in this way results in different values of the short circuit current. Depending on layer thickness and type of porphyrins, the open circuit voltage is ⩽ −0.4 V and the incident photon to current efficiency is 2.5% at 440 nm. Possible mechanisms for the observed recitification are dicussed.