Anatomy of the energetic driving force for charge generation in organic solar cells

Kyohei Nakano,Weining Han,Erjun Zhou,Jianming Huang,Bo Xiao,Yujiao Chen,Keisuke Tajima,Hiroyuki Yoshida

doi:10.1038/s41467-019-10434-3

Abstract

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; hence, it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. In this study, we systematically investigate 16 combinations of four donor polymers and four acceptors in planar heterojunction. The charge generation efficiency and its electric field dependence correlate with the energy difference between the singlet excited state and the interfacial charge transfer state. The threshold energy difference is 0.2 to 0.3 eV, below which the efficiency starts dropping and the charge generation becomes electric field-dependent. In contrast, the charge generation efficiency does not correlate with the energy difference between the charge transfer and the charge-separated states, indicating that the binding of the charge pairs in the charge transfer state is not the determining factor for the charge generation.

Highlights

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency
We observed a stricter requirement, that Egopt − ECT larger than 0.2 to 0.3 eV is necessary for efficient electric field-independent charge generation
Egopt at the interface becomes larger than that of the bulk due to the disorder, and ECT increases due to a deeper EHOMOD in the disordered donor or a shallower ELUMOA in the disordered acceptor (Supplementary Fig. 17)

Summary

Introduction

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. The high FF is associated with flat currentvoltage curves around the short-circuit condition, which indicates that the charge generation efficiency is independent of the electric field at the donor/acceptor (D/A) interfaces These observations have proved that efficient, electric field-independent photoelectric conversion using organic semiconductors is possible with a sufficiently large driving force of Egopt − ECS. This excess energy (Egopt − ECS) is wasted as heat, resulting in large overall energy loss in the form of low open-circuit voltage (VOC) This fundamental trade-off between the energetic driving force for charge generation and the cell voltage is a reason for the power conversion efficiency (PCE) of OSCs being limited to 15% to date[6]. The essential question in realizing efficient OSCs beyond the current limit is how much the excess energy can be reduced to increase VOC while maintaining efficient charge generation

Methods

Results

Conclusion