Unravelling Heating and Light Induced Degradation Mechanisms in Fullerene and Nonfullerene Acceptor Based Organic Solar Cells
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Date
2025-06-12
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Addis Ababa University
Abstract
Organic solar cells (OSCs) have emerged as a promising alternative to expensive silicon-based solar cells. Over the past few years, these devices have achieved remarkable advancements, with power conversion efficiencies (PCE) surpassing 20 %, comparable to commercially available thin-film solar cells. However, under continuous illumination, OSCs are susceptible to thermal and photo-induced degradation. Addressing the challenges of thermal and photostability is essential for their industrial application. To enable widespread commercialization, a comprehensive understanding of the degradation mechanisms caused by exposure to light, heat, or humidity is crucial. This thesis examines how different acceptors influence the thermal degradation of thin-film OSCs under a controlled environmental condition.
Poly(3-hexylthiophene) (P3HT), a commonly utilized donor polymer, was used to fabricate the bulkheterojunction (BHJ) organic solar absorber. Changes in the optical and structural characteristics of the polymer film provided clear signs of degradation. Additionally, data collected from various characterization techniques demonstrated that the extent of degradation is heavily influenced by the type of acceptor molecules present in the polymer matrix. Notably, solar absorber films blended with PC60BM acceptors exhibited greater stability compared to those based on 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC). This difference was reflected in the measured device parameters, such as short-circuit current density (Jsc) and open circut voltage (VOC ), which showed poorer performance in P3HT:ITICbased devices. Furthermore, a 50% drop in open-circuit voltage was observed in ITIC-based solar cells within one hour of thermal degradation. Additionally, impact of non-electron-withdrawing unit copolymerization on the photostability of fullerene-based OSCs was examined using two donor polymers: benzodithiophene-4,8-dione (BDD) copolymerized with α-quaterthiophene-thiophene (4T) (PBDD4T) and P3HT. Optical and electrochemical characterizations revealed that PBDD4T had a deeper highest occupied molecular orbital (HOMO) and broader absorption compared to P3HT, attributed to BDD copolymerization. This enhanced its power conversion efficiency (PCE), as evidenced by higher VOC and JSC values. The photostability study showed that PBDD4T-based devices retained 86% of their initial PCE after seven hours of irradiation, whereas P3HT-based devices experienced a 48% reduction.
Further analysis of the differences in photostability suggested that BDD copolymerization suppressed photo-oxidation and recombination induced by irradiation in PBDD4T-based devices, leading to improved stability. Conversely, P3HT:PC71BM-based solar absorbers exhibited bimolecular recombination due to photo-aging, which adversely affected their stability. This reduction in device stability was evixvident in the diminished photogenerated current, attributed to decreased charge mobility and increased surface roughness.
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Unravelling Heating, Light Induced Degradation Mechanisms, Nonfullerene Acceptor Based Organic Solar Cells