Adjusting the energy of interfacial states in organic photovoltaics for maximum efficiency

Nicola Gasparini,Michael Salvador,Derya Baran,Tetsuhiko Nagahara,Andrej Classen,Andrew Wadsworth,Christos L Chochos,Franco V A Camargo,Vasilis G Gregoriou,Andreas Görling,Steffen Roland,Stefan Frühwald,Iain Mcculloch,Larry Lüer,Christoph J Brabec,Giulio Cerullo,Dieter Neher

doi:10.1038/s41467-021-22032-3

Abstract

A critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states. This requires small energetic offsets often detrimental for high external quantum efficiency (EQE). Here, we obtain OSC with both non-radiative voltage losses (0.24 V) and photocurrent losses (EQE > 80%) simultaneously minimised. The interfacial CT states separate into free carriers with ≈40-ps time constant. We combine device and spectroscopic data to model the thermodynamics of charge separation and extraction, revealing that the relatively high performance of the devices arises from an optimal adjustment of the CT state energy, which determines how the available overall driving force is efficiently used to maximize both exciton splitting and charge separation. The model proposed is universal for donor:acceptor (D:A) with low driving forces and predicts which D:A will benefit from a morphology optimization for highly efficient OSC.

Highlights

A critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states
We introduce a bulk heterojunction (BHJ) solar cell based on polymer:NFA26 blends that combine relatively high power conversion efficiency (PCE > 12%) with high EQEEL (10−4) and low non-radiative loss ΔVOC;nr 1⁄4 0:24 V
This shows that the absorption onset of WF3:OIDTBR blends can be assigned to small aggregates of O-IDTBR in contact with WF3; the absorption spectra show no evidence of an interfacial CT state (Supplementary Fig. 3)

Summary

Introduction

A critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states. For inorganic (GaAs) and hybrid (perovskites) solar cells, EQEEL close to 100% is observed, while in organic photovoltaic (OPV) devices, EQEEL is orders of magnitude lower, indicating the dominance of non-radiative pathways for charge recombination[3,4,5,6] These losses are mainly caused by recombination of the charge transfer (CT) states generated at the interface between the electron donor (D) and acceptor (A) materials[7,8,9,10,11]. We gain further insight into the electronic structure of the interfacial CT state through quantum chemical calculations, which we apply to model the population equilibrium between interfacial and bulk states from EL and EQE spectra We show that this equilibrium is decisively controlled by the degeneracy and energy of the interfacial CT states and that it attains a nearoptimum value in WF3:O-IDTBR, explaining its ability to produce a high open-circuit voltage (VOC) and EQE at the same time. We discuss known methods to adjust the CT energy in a given blend in light of this new functional relationship

Methods

Results

Conclusion