UV-Cross-linkable Donor–Acceptor Polymers Bearing a Photostable Conjugated Backbone for Efficient and Stable Organic Photovoltaics

Abstract

High-performance photovoltaic polymers bearing cross-linkable function together with a photorobust conjugated backbone are highly desirable for organic solar cells to achieve both high device efficiency and long-term stability. In this study, a family of such polymers is reported based on poly [(2,5-bis(2-hexyldecyloxy)), a high-performance photovoltaic donor− acceptor polymer, with different contents of terminal vinylappended side chains for cross-linking. The polymers were named PPDT2FBT-V x and prepared by varying the feeding ratio (x mol %, x = 0, 5, 10, and 15) of the vinyl-appended monomer in polymerization. It was found that the vinyl integration did not sacrifice the original high photovoltaic performance of the polymers, as evidenced by comparable average power conversion efficiencies (PCEs) (6.95, 7.02, and 7.63%) observed for optimized devices based on PPDT2FBT-V 0 , PPDT2FBT-V 5 , and PPDT2FBT-V 10 , respectively, in blending with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM). Unlike thermal cross-linking that greatly reduced device efficiency, UV cross-linking has proven to be an effective way to achieve both high device efficiency and thermostability for PPDT2FBT-V 10 solar cells. UV-cross-linked PPDT2FBT-V 10 solar cells displayed an initial average PCE of 5.28% and almost no decrease upon heat treatment at 120 °C for 40 h. Morphology studies revealed that UV-cross-linking did not only alter initial nanophase separation but also suppressed morphology evolution by aggregation in bulk heterojunction blend films. Photo-cross-linking requires material photostability. It is therefore worthwhile to note that these polymers and their blends with PC 71 BM were found to be extremely photostable, even upon continuous exposure to concentrated sunlight (up to 200 suns), and UV-cross-linking does not hamper this photostability. Further studies found that the devices fabricated with the UV-cross-linked PPDT2FBT-V 10 /PC 71 BM active layer can endure continuous light exposure to a solar simulator without deteriorating their performance.

Progress in Stability of Organic Solar Cells

“…[178] Li and co-worker proposed a similar strategy that adding UV cross-linking in the side chain of donor materials can enhance the stability of the device. [179] They designed and synthesized donor materials poly[ (2,5-…”

Challenges to the Stability of Active Layer Materials in Organic Solar Cells

“…Furthermore, the devices based on UV‐crosslinked polymers of PPDT2FBT‐V 10 displayed almost no performance decrease under continuous light irradiation for 16 h and upon heating at 120 °C for 40 h when compared to its initial PCEs value (as shown in Figure a). It is mainly attributed to the stable morphology of active layer originating from the UV‐crosslinked PPDT2FBT‐V 10 , which can suppress the nanophase separation and aggregation of the active layer …”

“…a) Graphic diagram of UV‐crosslinked donor–acceptor polymers bearing photostable conjugated backbone for efficient and stable organic photovoltaics. Reproduced with permission . Copyright 2018, ACS Publications.…”

Introducing neat fullerenes to improve the thermal stability of slot-die coated organic solar cells

“…49 Later in 2018, UV activated crosslinking of copolymers based on PPDT2FBT, having vinyl end groups on the side chains, were used to fix the morphology in the active layer, improving the thermal stability of the OPVs at 120 1C. 50 As for slot-die coated devices, Song et al briefly highlighted the impact that solvents and solvent additives could have on the thermal degradation at 25 1C & 120 1C as well as photodegradation under ambient conditions. The conclusion of this work was, that the addition of a high boiling point -low vapour pressure solvent additive reduced the overall stability of PPDT2FBT:PC 71 BM devices.…”