Steady-state spectroscopic and photovoltage studies of hemin and hemin/n-octadecylamine Langmuir-Blodgett films

Hemin (iron(III) protoporphyrin IX chloride) and mixed hemin/n-octadecylami ne (ODA) with the molar ratio of 1:2 were used to investigate the Langmuir monolayer behavior and to prepare the Langmuir-Blodgett (LB) films. It show ed that well-defined Langmuir monolayers were formed at the air-water inter face and could be successfully transferred onto solid substrates. UV-vis sp ectra of hemin indicated the formation of H-aggregate in a LB matrix for th e blue shift of the Soret band from 400 nm in solution to 370 nm in LB film . The inlay of ODA molecules partly reduced the aggregation of hemin molecu les. Both the small-angle X-ray diffraction (XRD) pattern of hemin and mixe d hemin/ODA LB multilayers exhibited an ordered layer structure in an LB ma trix with the d-value of bilayer hemin equal to 4.8 nm. Surface photovoltag e spectroscopy (SPS) was used to study the photoinduced interfacial charge- transfer process between a monolayer or multilayer of hemin and mixed hemin /ODA LB films on n- and p-type Si substrates. The results showed that the h emin LB monolayer film had a photosensitization effect on n-type Si. Partic ularly, the photovoltaic response value at the region of 400 nm which corre sponds to the Soret band absorption of hemin molecules was increased 6-fold . The mixed hemin/ODA LB monolayer and multilayer films increased the photo voltaic conversion efficiency by separating the hemin molecules into indivi dual ones with about a 15-fold and 7-fold increment of response value at th e Soret band, respectively. But the SPS response value of the hemin LB mult ilayer film was decreased almost 20-fold and the photovoltaic conversion ef ficiency was largely reduced.