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

Si-Cheng Wu; Lisa T Strover; Xiang Yao; Xue-Qiang Chen; Wen-Jing Xiao; Li-Na Liu; Jiandong Wang; Iris Visoly-Fisher; Eugene A Katz; Wei-Shi Li

doi:10.1021/acsami.8b11506

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

ACS Appl Mater Interfaces. 2018 Oct 17;10(41):35430-35440. doi: 10.1021/acsami.8b11506. Epub 2018 Oct 3.

Authors

Affiliations

¹ Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry , Chinese Academy of Sciences , 345 Lingling Road , Shanghai 200032 , China.
² Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, The Jacob Blaustein Institutes for Desert Research (BIDR) , Ben-Gurion University of the Negev , Sede Boqer Campus , Midreshet Ben-Gurion 8499000 , Israel.
³ Engineering Research Center of Zhengzhou for High Performance Organic Functional Materials , Zhengzhou Institute of Technology , 6 Yingcai Street , Huiji District, Zhengzhou 450044 , China.

PMID: 30247021
DOI: 10.1021/acsami.8b11506

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)phenylene)- alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[ c]-[1,2,5]thiadiazole)] (PPDT2FBT), a high-performance photovoltaic donor-acceptor polymer, with different contents of terminal vinyl-appended 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-V₀, PPDT2FBT-V₅, and PPDT2FBT-V₁₀, respectively, in blending with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). 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-V₁₀ solar cells. UV-cross-linked PPDT2FBT-V₁₀ 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 PC₇₁BM 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-V₁₀/PC₇₁BM active layer can endure continuous light exposure to a solar simulator without deteriorating their performance.

Keywords: concentrated sunlight; crosslinkable D−A copolymers; organic photovoltaics; photostability; thermal stability.