Efficient Charge Transport Enables High Efficiency in Dilute Donor Organic Solar Cells.

Nannan Yao,Yuan Zhang,Qingzhen Bian,Zeng Chen,Fengling Zhang,Jianqi Zhang,Yuxin Xia,Jianqiu Wang,Rui Zhang,Leiqiang Qin,Haiming Zhu

doi:10.1021/acs.jpclett.1c01219

Abstract

The donor/acceptor weight ratio is crucial for photovoltaic performance of organic solar cells (OSCs). Here, we systematically investigate the photovoltaic behaviors of PM6:Y6 solar cells with different stoichiometries. It is found that the photovoltaic performance is tolerant to PM6 contents ranging from 10 to 60 wt %. Especially an impressive efficiency over 10% has been achieved in dilute donor solar cells with 10 wt % PM6 enabled by efficient charge generation, electron/hole transport, slow charge recombination, and field-insensitive extraction. This raises the question about the origin of efficient hole transport in such dilute donor structure. By investigating hole mobilities of PM6 diluted in Y6 and insulators, we find that effective hole transport pathway is mainly through PM6 phase in PM6:Y6 blends despite with low PM6 content. The results indicate that a low fraction of polymer donors combines with near-infrared nonfullerene acceptors could achieve high photovoltaic performance, which might be a candidate for semitransparent windows.

Highlights

The donor/acceptor weight ratio is crucial for photovoltaic performance of organic solar cells (OSCs)
Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have achieved rapid development in recent years due to the tunable energy levels and absorption spectra of NFAs.[1−6] The power conversion efficiency (PCE) of single-junction OSCs has reached over 18%.7−9 In NFA-OSCs, efficient charge separation and low voltage losses can be achieved simultaneously, yielding high short circuit current density (Jsc) and open-circuit voltage (Voc).[10−12] The third important parameter fill factor (FF) is strongly affected by the competition between charge transport and recombination
It has been demonstrated that efficient charge transport and suppressed recombination losses are largely related to balanced electron and hole mobility in OSCs.[13,14]

Summary

Introduction

The donor/acceptor weight ratio is crucial for photovoltaic performance of organic solar cells (OSCs). The unbalanced charge transport can induce a space-charge effect, causing more recombination loss and poor performance.[16] Interestingly, some previous studies have reported that fullerene based OSCs with very low donor contents could maintain efficient hole transport.[17,18] It was proposed that hole tunneling could occur between isolated donors,[19,20] another possible channel is that fullerene acceptors such as PCBM can act as an ambipolar conductor for both electrons and holes in dilute donor solar cells.[17,21,22]

Results

Conclusion