Ultrafast hole transfer mediated by polaron pairs in all-polymer photovoltaic blends

Rui Wang,Yindong Zhang,Xiaoyu Xie,Min Xiao,Yongfang Li,Yao Yao,Chunfeng Zhang,Zhi-Guo Zhang,Haibo Ma,Lingwei Xue,Xiaoyong Wang,Haijun Bin

doi:10.1038/s41467-019-08361-4

Abstract

The charge separation yield at a bulk heterojunction sets the upper efficiency limit of an organic solar cell. Ultrafast charge transfer processes in polymer/fullerene blends have been intensively studied but much less is known about these processes in all-polymer systems. Here, we show that interfacial charge separation can occur through a polaron pair-derived hole transfer process in all-polymer photovoltaic blends, which is a fundamentally different mechanism compared to the exciton-dominated pathway in the polymer/fullerene blends. By utilizing ultrafast optical measurements, we have clearly identified an ultrafast hole transfer process with a lifetime of about 3 ps mediated by photo-excited polaron pairs which has a markedly high quantum efficiency of about 97%. Spectroscopic data show that excitons act as spectators during the efficient hole transfer process. Our findings suggest an alternative route to improve the efficiency of all-polymer solar devices by manipulating polaron pairs.

Highlights

The charge separation yield at a bulk heterojunction sets the upper efficiency limit of an organic solar cell
Here we focus on the dynamics of charge separation in an all-polymer organic photovoltaic (OPV) blend consisting of a commercial acceptor of naphthalene diimide-bithiophene (N2200)[51], which has been most widely used in high-performance all-polymer devices[31,35,36,37,38,39,41], and a polymer donor of benzodithiophenealt-benzotriazole copolymers (J51) having absorption coverage commentary to that of N220041
Using transient absorption (TA) spectroscopy, we clearly identify an ultrafast hole transfer process with a quantum efficiency up to about 97% from the polymer acceptor to the polymer donor mediated by intra-moiety polaron pairs (iPPs), while the photoinduced EXs act as spectators during this efficient hole transfer process

Summary

Introduction

The charge separation yield at a bulk heterojunction sets the upper efficiency limit of an organic solar cell. In spite of a marked improvement of power-conversion efficiency (PCE) from 2 to 9% in the last few years[31,36,37,38,39,40,41], the performances of allpolymer devices still lag far behind the devices using smallmolecule acceptors (with PCE up to above 14%)[42,43,44,45,46] To narrow this PCE gap, it is essential to elucidate the mechanisms of charge generation and transport in all-polymer OPV blends, which have been much less investigated[47,48,49,50] in comparison to the intensive studies on polymer/fullerene blends. The iPP-mediated hole transfer process has been identified in other all-polymer blends with donor J51 and three different polymer acceptors, implying that it is a quite general pathway of charge generation in all-polymer OPV systems The identification of such an iPP-mediated charge separation pathway suggests that it is possible to further improve the efficiency of all-polymer OPV devices by manipulating the generation dynamics of iPPs

Methods

Results

Conclusion