Preventing electrode penetration and burn-in degradation in non-fullerene organic solar cells via pre-annealing: Insights from experimental and computational studies

Abstract

Understanding the thermal stability of organic solar cells (OSCs) is key to their commercial viability, as many high-performance non-fullerene acceptor (NFA)-based OSCs suffer from a drastic decline in power conversion efficiencies (PCEs) upon thermal stress within a short period. In this study, we investigate how NFA-OSCs based on the PBDB-T-2Cl:ITIC-4F active layer experience burn-in degradation when exposed to heat and propose a simple pre-treatment method to improve device stability. We utilize in-situ temperature-dependent time-of-flight secondary ion mass spectrometry to reveal that (i) the penetration of MoO3 and Ag is the primary factor contributing to burn-in degradation, and (ii) this penetration is significantly mitigated by a pre-annealing process at 140 °C. Devices without pre-treatment show a sharp drop in PCEs from 10.83 to 8.90% within 10 min under thermal stress at 140 °C, while pre-treated devices maintain their initial efficiency with a marginal drop from 10.71 to only 10.12%. We reveal that the pre-treatment reduces the free volume of active layers, leading to an 8% thickness decrease (from 128.5 to 118.1 nm), and the increased film density effectively blocks electrode penetration. Molecular dynamics simulations suggest that the density of active layers increases from 0.96 to 1.20 g cm−3 after the pre-treatment, which is due to acceptor–acceptor binding interaction between PBDB-T-2Cl and ITIC-4F. Our findings on the detailed mechanism and the potential solution for the burn-in degradation of NFA-based OSCs illuminate a promising pathway to enhance the thermal stability and overall durability of OSCs for practical applications. © 2024 Elsevier B.V.