Effect of alcohol treatment on the performance of PTB7:PC71BM bulk heterojunction solar cells

The effect of an environmentally friendly alcohol treatment on bulk heterojunction (BHJ) polymer solar cells using the low-bandgap copolymer based on thieno[3,4-b]thiophene-alt-benzodithiophene units and [6,6]-phenyl-C71-butyric acid methyl ester is systematically investigated. Different alcohols are tested, and besides the most commonly used methanol treatment, other alcohols such as ethanol, 2-propanol, and 1-butanol also improve the device performance to certain extents as compared to the untreated solar cells. Changes of the surface structure caused by the alcohol treatment are probed with atomic force microscopy, and the modification of inner film morphology is probed by time-of-flight-grazing incidence small-angle neutron scattering (TOF-GISANS). UV/vis measurements show that the thickness of all BHJ films remains unchanged by the different solvent treatments. Thus, the enhanced device performance induced by the alcohol treatments is correlated to the reconstruction of the inner film structures probed with TOF-GISANS and the modified energy levels at the interfaces between the BHJ layer and the aluminum electrodes, evident by the enhanced short-circuit current and open-circuit voltage of the I-V curves.