Structural characterizations of C60-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 behaviors and charge-transfer process of their LB films on n-type Si substrates and their powders were studied by surface photovoltage spectra (SPS). For the electronic effect caused by different electron-withdrawing groups, they exhibited different photovoltaic behaviors, and the reverse response values of SPS were obtained when the C-60-derivatives powders contact with indium tin oxide substrates. The reason was that the relatively strong electron-withdrawing substituent shifted the molecular orbital levels to lower energy, which caused the opposite surface band bending. When deposited on n-Si substrates, the two kinds of C-60-derivative LB films both exhibited the same increment on the direct transition of n-Si. Furthermore, the structural characterization 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 of the C-60 moiety gave an actual cluster with the hydrophilic substituents sticking out.