Structural characterizations of C-60-derivative Langmuir-Blodgett films and their photovoltaic behaviors

Langmuir and Langmuir-Blodgett (LB) films of two kinds of C-60 derivatives with the substituents which have methoxycarbonyl (C-60(I)) and a carboxylic group (C-60(II)) were investigated, respectively. The photovoltaic behavio rs and charge-transfer process of their LB films on n-type Si substrates an d their powders were studied by surface photovoltage spectra (SPS). For the electronic effect caused by different electron-withdrawing groups, they ex hibited different photovoltaic behaviors, and the reverse response values o f SPS were obtained when the C-60-derivatives powders contact with indium t in oxide substrates. The reason was that the relatively strong electron-wit hdrawing substituent shifted the molecular orbital levels to lower energy, which caused the opposite surface band bending. When deposited on n-Si subs trates, the two kinds of C-60-derivative LB films both exhibited the same i ncrement on the direct transition of n-Si. Furthermore, the structural char acterization of C-60(II) LB film was studied in detail by measuring the pi- A isotherm, FTIR, and small-angle X-ray diffraction. It revealed that this kind of C-60 derivative formed micellar aggregates. The hydrophobic cores o f the C-60 moiety gave an actual cluster with the hydrophilic substituents sticking out.