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N-Type Alcohol-Soluble Small Molecules as an Interfacial Layer for Efficient and Stable Polymer Solar Cells

Research paper by Xiangfu Liu, Lie Chen, Lin Hu, Yiwang Chen

Indexed on: 10 Nov '15Published on: 02 Nov '15Published in: Journal of Physical Chemistry C



Abstract

Three novel n-type alcohol-soluble small molecule electrolytes (n-SMEs) based on the diketopyrrolopyrrole (DPP) backbone functionalized with different polar groups (diethylamino, diethanolamino, quaternary ammonium), called DPPN, DPPNOH, and DPPNBr, respectively, have been designed and synthesized, respectively. Thanks to the chemical interaction with zinc oxide (ZnO), the n-SMEs can be easily deposited on the ZnO surface, and then we modified ZnO defects, fine-tuned the work function of ZnO, and simultaneously improved the compatibility at the ZnO/active layer inorganic/organic interface. Moreover, the n-type feature of the backbone in the n-SMEs also facilitates the electron transfer. Consequently, these n-SME-modified ZnO can serve as an efficient electron-transporting layer (ETL) in inverted polymer solar cells (I-PSCs). Inspiringly, the device based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methyl ester (PC61BM) with ZnO/DPPNOH as ETL delivers a notable power conversion efficiency (PCE) of 4.0%, which is a 38% enhancement compared to the control device with pure ZnO as an ETL. The devices based on ZnO/DPPNBr and ZnO/DPPN as ETLs also display the improved PCE of 3.7% and 3.1%, respectively. The enhancement of the PCE at different degrees is mainly caused by the various interactions between ZnO and the diverse polar group in the n-SMEs. Notably, the device based on polythieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) with ZnO/DPPNOH as the ETL can further improve the PCE to 8.0%. In addition, ZnO/n-SME ETLs also ensure the devices have long-term stability. These findings indicate that self-assembled ZnO/n-SMEs ETLs provide a promising strategy for fabrication of I-PSCs with high-performance and long-term stability.

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