文摘
The molecular organization at interfaces of organic semiconducting materials plays a crucial role in the performance of organic photovoltaics and field effect transistors. Vibrational sum-frequency generation (VSFG) was used to characterize the molecular orientation at interfaces of regioregular poly-3-hexylthiophene (rrP3HT). Polarization-selected VSFG spectra of the C鈺怌 stretch of the thiophene ring yield the orientation of the conjugated backbone of P3HT, which is directly relevant to the electronic properties at the interface. The molecular orientation at buried polymer鈥搒ubstrate interfaces was compared for films spin-cast on SiO2 and AlOX substrates, before and after thermal annealing at 145 掳C. On SiO2, annealing results in the thiophene rings adopting a more edge-on orientation, tilting away from the surface plane by 螖胃 = +(3鈥?0)掳. In contrast, an opposite change is observed for films deposited on AlOx, 螖胃 = 鈭?3鈥?6)掳, where annealing leads to a more face-on orientation of the thiophene rings of the polymer. Although subtle, such orientational changes may significantly affect the electron transfer rates across interfaces and hence the overall photovoltaic efficiency.
