Photophysics and Charge Generation in Low Energy-offset Blends for Organic Solar Cells.

Rishi Shivari,Natalie Banerji

doi:10.2533/chimia.2021.862

Abstract

The power conversion efficiency of organic solar cells has seen a huge improvement in recent years with state-of-the-art solar cells showcasing efficiencies of ∼18.5 %, which is approaching the performance of inorganic and hybrid-perovskite solar cell technologies. This improvement can be mainly attributed to the discovery of highly efficient donor:acceptor blends with a near-zero energetic offset between the molecular orbital levels of the donor and the acceptor component. A distinctive feature of the high efficiency, low energy-offset blends is that they exhibit a concomitant increase in the short-circuit density and the open-circuit voltage of the solar cell. High open-circuit voltage results from the reduced photon energy loss in the exciton dissociation step, while a high short-circuit current density can be attributed to an efficient charge generation process. The reasons for the efficient exciton dissociation and subsequent separation of Coulomb bound electron-hole pair at negligible driving force is not well understood and, in this short review, we highlight recent results which shed light on the mechanism of charge generation in low energy-offset blends.

Highlights

IntroductionOrganic solar cells (OSCs) are typically composed of a mixture of an electron-donating material (typically a semiconducting polymer) and an electron-accepting compound (a fullerene or non-fullerene based small molecule)
Organic solar cells (OSCs) are typically composed of a mixture of an electron-donating material and an electron-accepting compound
Our work suggests that the driving force in itself is not a limiting factor for the electron transfer (ET)/hole transfer (HT) process, since the intrinsic ET/HT rate is in the sub-picosecond range irrespective of the energetic offset

Summary

Introduction

Organic solar cells (OSCs) are typically composed of a mixture of an electron-donating material (typically a semiconducting polymer) and an electron-accepting compound (a fullerene or non-fullerene based small molecule). The governing idea behind blending two or more materials is to create a heterojunction at the Earlier results suggest that a substantial offset (~300 meV) of the frontier energy levels at the interface (i.e. between the highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbital (LUMOs) of the D/A molecules) is necessary for efficient dissociation of excitons and to have a high yield of free charge carriers (Fig. 1a).[6,7] a majority of first-generation OSCs employed D:A combinations (typically polymer:fullerene blends) with large energy-offsets for efficient charge generation This came at a price of losing excess photon energy, defined as: Eloss = Eg – eVOC, where Eg is the optical-gap of the semiconductor and VOC is the open-circuit voltage of the solar cell. PCE values for first-generation OSCs were limited to around 9–10% in the best case.[8,9,10]

Objectives

Findings

Conclusion