Open-circuit-voltage shift of over 0.5 V in organic photovoltaic cells induced by a minor structural difference in alkyl substituents

Abstract

The recent surge in the efficiency of organic photovoltaic devices (OPVs) largely hinges on the reduction of energy loss (E_loss) that leads to improvements in open-circuit voltage (V_OC). However, there are still many unclarified factors regarding the relationship between the molecular structure and V_OC, hampering the establishment of widely applicable, effective principles for the design of active-layer materials. In this contribution, we examine the origin of the large V_OC shifts induced by minor structural differences in end-alkyl substituents on a series of anthracene-based p-type compounds. The examined p-type compounds are all highly crystalline, thereby enabling detailed investigation of the molecular packing with X-ray diffraction analysis. At the same time, they are strongly aggregating and hardly soluble; therefore, they are deposited with the aid of a photoprecursor approach which we have been employing for controlled deposition of insoluble acene-based organic semiconductors. The resultant OPVs afford the highest V_OC of 0.966 V when the end-alkyl groups are 2-ethylbutyl, and the lowest of 0.419 V when n-butyl is used in replacement of 2-ethylbutyl. X-ray diffraction analyses and density-functional-theory calculations indicate a critical impact of the non-slipped herringbone arrangement on the observed large loss in V_OC. This type of molecular arrangement is prohibited when branched alkyl chains are introduced at the ends of linear π-systems, which we consider an important factor contributing to the relatively high V_OC obtained with the 2-ethylbutyl derivative. These results may serve as a basis of useful molecular-design rules to avoid unnecessary losses in V_OC.