Complementary solvent additives tune the orientation of polymer lamellae, reduce the sizes of aggregated fullerene domains, and enhance the performance of bulk heterojunction solar cells

Abstract

In this study we employed 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) as binary additives exhibiting complementarily preferential solubility for processing the crystalline conjugated polymer poly[bis(dodecyl)thiophene-dodecyl-thieno[3,4-c]pyrrole-4,6-dione] (PBTC₁₂TPD) and the fullerene [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) in chloroform. Using synchrotron grazing-incidence small-/wide-angle X-ray scattering and transmission electron microscopy to analyse the structure of the PBTC₁₂TPD–PC₇₁BM blend films, we found that the binary additives with different volume ratios in the processing solvent allow us to tune the relative population of face-on to edge-on PBTC₁₂TPD lamellae and the size of PC₇₁BM clusters in the blend films; the sizes of the fractal-like PC₇₁BM clusters and the aggregated domains of PC₇₁BM clusters increased and decreased, respectively, upon increasing the amount of DIO, whereas the relative ratio of face-on to edge-on PBTC₁₂TPD lamellae increased upon increasing the amount of CN. When fabricating the photovoltaic devices, the short-circuit current density of the devices with the PBTC₁₂TPD–PC₇₁BM active layer having been processed with the binary additives is higher than that of the device incorporating an active layer processed without any additive. As a result, the power conversion efficiency of a device incorporating an active layer of PBTC₁₂TPD–PC₇₁BM (1 : 1.5, w/w) processed with binary additives of 0.5% DIO and 1% CN in chloroform increased to 6.8% from a value of 4.9%, a relative increase of 40%, for the corresponding device containing the same active layer but processed without any additive.