Structural Order: The Dominant Factor for Nongeminate Recombination in Organic Photovoltaic Devices

Reducing nongeminate recombination (NGR) will provide a major boost to the performance of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. It is therefore critical to identify the dominant factor(s) that determines NGR. While substantial research effects have been made and various factors have been considered, the determining factor has not been ascertained. After comprehensively evaluating the impact of deep defect states, donor–acceptor interfacial area, and mobility, here we demonstrate that the structural order is the dominant factor that determines NGR in OPV devices. Intensity modulated photovoltage spectroscopy in conjunction with the charge extraction technique is applied to quantitatively measure bimolecular recombination coefficients. A model P3HT:PCBM system with varying composition and annealing temperature, which removes uncontrolled variables yet resembles the behaviors of different polymer donor OPV systems, enables us to reach the unambiguous conclusion. Our findings shed light on key strategies to reduce NGR and pave the road for future high performance OPV design and optimization.