Vertical Phase Separated Cesium Fluoride Doping Organic Electron Transport Layer: A Facile and Efficient “Bridge” Linked Heterojunction for Perovskite Solar Cells

• Published in: Advanced functional materials Vol. 30; no. 27
• Main Authors: Xia, Jianxing, Luo, Junsheng, Yang, Hua, Zhao, Fengjiao, Wan, Zhongquan, Malik, Haseeb Ashraf, Shi, Yu, Han, Keli, Yao, Xiaojun, Jia, Chunyang
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01.07.2020
• Subjects: Carrier recombination; Cesium fluorides; Charge efficiency; Circuits; Dipoles; Doping; Efficiency; Electron transport; Energy conversion efficiency; Fermi surfaces; heterojunction; Heterojunctions; Interface stability; Interfaces; Materials science; Metal oxides; perovskite solar cells; Perovskites; Phase separation; Photovoltaic cells; Solar cells; trap states; vertical phase separation; Vertical separation
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202001418

In perovskite solar cells (PSCs), the interfaces of the halide perovskite/electron transport layer (ETL) and ETL/metal oxide electrode (MOE) always attract and trap free carriers via the surface electrostatic force, altering quasi‐Fermi level (EFq) splitting of contact interfaces, and significantly limit the charge extraction efficiency and intrinsic stability of devices. Herein, a graded “bridge” is first reported to link the MOE and perovskite interfaces by self vertical phase separation doping (PSD), diminishing the side effect of notorious ionic defects via both reinforced interface Ebi and the vacancies filling. Experimental and theoretical results prove that the inhomogeneous distribution of CsF in the bulk or surface of PC61BM would not only form metal–oxygen (M–O) dipole on MOE, reinforcing the interface Ebi, but also create a graded energy bridge to alleviate the disadvantage of band offset raised by the enhanced interface Ebi, which significantly avoid the carrier accumulation and recombination at defective interfaces. Employing PSD, the power conversion efficiency of the devices approaches 21% with a high open‐circuit voltage (1.148 V) and delivers a high stability of 89% after aging 60 days in atmosphere without encapsulation, which is the highest efficiency of organic electron transport layers for n–i–p PSCs. The undercoordinated ionic defects at heterojunction interfaces remain challenges that limit the performances and stability of perovskite photoelectric devices. A self‐phase separated doping strategy is developed to link multilayer heterojunction interfaces including both the energy level and trap states, paving a novel route for nonequilibrium distributed dopants to solve the key challenge of interface defects.