Control of the molecular orientation in small molecule-based organic photovoltaics

The continuously improved power conversion efficiency (PCE) of organic photovoltaics in the past few years is largely ascribed to the synthesis of new molecular donors and acceptors as well as the subsequent optimization of morphology of the active layers. As one crucial morphological parameter, the interfacial molecular orientation of organic semiconductors not only determines the charge separation and transport, but also influences the device energy loss process. The control of the molecular orientation of photoactive materials via tuning process conditions, especially for molecular semiconductors, is becoming vitally important and is full of challenges. Here, we systematically summarize previous work on molecular orientation control in small molecule (SM)-based thin films and discuss the dependency of molecular orientation on three critical aspects, including substrate properties, processing methods, and solution systems. More importantly, the impact of the molecular packing configuration on the device performance in terms of generation of photocurrent and photovoltage in SM-based devices is comprehensively analyzed. Finally, we propose future potential directions for further promoting research on molecular orientation control in SM-based OPVs.