Influence of molecular orientation on the photovoltaic properties of octithiophene

We investigate here the influence of molecular orientation of octithiophene (8T), the conjugated linear octamer of thiophene, on its photovoltaic properties in both Schottky junctions and pn heterojunctions. Slow vacuum evaporation of 8T on a heated ITO substrate induces vertical orientation. Simple rubbing of such a thin vertical pre-layer (5–10 nm) by using a stretched teflon roll leads to efficient horizontal 8T orientation (dichroic ratio=10). Thicker films of horizontally oriented 8T (40 nm and above) can then be easily deposited on top of this pre-layer due to quasi-epitaxy. Horizontal 8T orientation in photovoltaic sandwich cells improves both light absorption and charge transport, thus resulting in higher light conversion efficiencies.