Single-layered organic photovoltaics with double cascading charge transport pathways: 18% efficiencies

Ming Zhang,Bryon W Larson,Zheng Tang,Thomas P Russell,Lei Zhu,Yongming Zhang,Guanqing Zhou,Zhe Zhao,Tianyu Hao,Haiming Zhu,Feng Liu,Qin Hu,Chaoqun Qiu,Wei Feng,Zaifei Ma

doi:10.1038/s41467-020-20580-8

Abstract

The chemical structure of donors and acceptors limit the power conversion efficiencies achievable with active layers of binary donor-acceptor mixtures. Here, using quaternary blends, double cascading energy level alignment in bulk heterojunction organic photovoltaic active layers are realized, enabling efficient carrier splitting and transport. Numerous avenues to optimize light absorption, carrier transport, and charge-transfer state energy levels are opened by the chemical constitution of the components. Record-breaking PCEs of 18.07% are achieved where, by electronic structure and morphology optimization, simultaneous improvements of the open-circuit voltage, short-circuit current and fill factor occur. The donor and acceptor chemical structures afford control over electronic structure and charge-transfer state energy levels, enabling manipulation of hole-transfer rates, carrier transport, and non-radiative recombination losses.

Highlights

Improving all characteristics simultaneously has been hard to achieve, due to morphological and electronic structure constraints, leading to performance trade-offs
These results demonstrate the importance of manipulating the electronic structure in BHJ thin films, while simultaneously manipulating the morphology, opening a new route to higher efficiency organic photovoltaics (OPVs) devices
The highest occupied molecular orbital (HOMO) levels are accurately measured by ultraviolet photoelectron spectroscopy (UPS)[22], coupled with the optical band gaps to estimate the lowest unoccupied molecular orbital (LUMO) levels (Fig. 1c, d)

Summary

Introduction

Improving all characteristics simultaneously has been hard to achieve, due to morphological and electronic structure constraints, leading to performance trade-offs. It is advantageous to establish a tiered energy level alignment, to form cascading charge hopping channels that mitigate the JSC loss by fine-tuning the charge splitting and allow manipulation of multiple chargetransfer energies to ensure a high VOC. This applies to both electron and hole transport pathways to maximize the energy gain. The double cascading quaternary blend strategy is implemented in other material systems, which is reflected in similar behavior of device operation, demonstrating the novelty in the approach for OPV device fabrication These results demonstrate the importance of manipulating the electronic structure in BHJ thin films, while simultaneously manipulating the morphology, opening a new route to higher efficiency OPV devices

Methods

Results

Conclusion