Toward Improved Environmental Stability of Polymer:Fullerene and Polymer:Nonfullerene Organic Solar Cells: A Common Energetic Origin of Light- and Oxygen-Induced Degradation

Abstract

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.

Unraveling the Microstructure‐Related Device Stability for Polymer Solar Cells Based on Nonfullerene Small‐Molecular Acceptors

“…Compared to extrinsic instabilities, e.g., chemical reaction with oxygen (with or without light) [ 6 ] and water, [ 7 ] the intrinsic materials stability against light and the microstructure changes upon external stress are most relevant in the phase of new materials development. Most of the stability studies so far have been focused on fullerene‐based OSCs, [ 7–10 ] while for unstable NFA‐based OSCs, understanding of the degradation mechanism is still limited.…”

What is Killing Organic Photovoltaics: Light‐Induced Crosslinking as a General Degradation Pathway of Organic Conjugated Molecules

“…While the development of new materials is mainly driven by the goal to increase the device efficiency, using materials design to improve the operation stability of organic solar cells remains to date too little investigated. This is somehow surprising as multiple degradation pathways can occur in organic solar cells under realistic operation conditions that severely restrict their lifetime and, therefore, represent one of the key obstacles for commercialization of this technology . Generally, indeed, practically useful organic solar cells are encapsulated using appropriate barrier coatings to minimize the penetration of oxygen and moisture into the active layer of the cell, thus emphasizing the importance of intrinsic degradation pathways …”

Indoor application of emerging photovoltaics—progress, challenges and perspectives

“…The rapid degradation of device performance is typically triggered by a degradation of the donor and/or acceptor materials driven by the combined exposure to light and molecular oxygen, with the formation of singlet oxygen and superoxide ions, both identifying degradation mechanisms. [94][95][96] In contrast, the photochemical degradation of OPV devices under low light conditions remains signicantly underexplored. Yin et al compared the degradation kinetics of unencapsulated PCDTBT:PC 71 BM devices under outdoor (AM1.5G) and indoor (300 lx LED) conditions in air (Fig.…”