title: Observation of Enhanced Hole Extraction in Br Concentration Gradient Perovskite Materials.
abstract: Enhancing hole extraction inside the perovskite layer is the key factor for boosting photovoltaic performance. Realization of halide concentration gradient perovskite materials has been expected to exhibit rapid hole extraction due to the precise bandgap tuning. Moreover, a formation of Br-rich region on the tri-iodide perovskite layer is expected to enhance moisture stability without a loss of current density. However, conventional synthetic techniques of perovskite materials such as the solution process have not achieved the realization of halide concentration gradient perovskite materials. In this report, we demonstrate the fabrication of Br concentration gradient mixed halide perovskite materials using a novel and facile halide conversion method based on vaporized hydrobromic acid. Accelerated hole extraction and enhanced lifetime due to Br gradient was verified by observing photoluminescence properties. Through the combination of secondary ion mass spectroscopy and transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis, the diffusion behavior of Br ions in perovskite materials was investigated. The Br-gradient was found to be eventually converted into a homogeneous mixed halide layer after undergoing an intermixing process. Br-substituted perovskite solar cells exhibited a power conversion efficiency of 18.94% due to an increase in open circuit voltage from 1.08 to 1.11 V and an advance in fill-factor from 0.71 to 0.74. Long-term stability was also dramatically enhanced after the conversion process, i.e., the power conversion efficiency of the post-treated device has remained over 97% of the initial value under high humid conditions (40-90%) without any encapsulation for 4 weeks.
authors: Global Frontier Center for Multiscale Energy Systems , Seoul 151-742, Republic of Korea.|Department of Mechanical Engineering, Seoul National University , Seoul 151-742, Republic of Korea.
authors: Min-Cheol
authors: MC
authors: Kim
authors: School of Advanced Materials Science & Engineering, Sungkyunkwan University , Suwon, Gyeonggi-do 440-746, Republic of Korea.
authors: Byeong Jo
authors: BJ
authors: Kim
authors: School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University , Suwon, Gyeonggi-do 440-746, Republic of Korea.
authors: Dae-Yong
authors: DY
authors: Son
authors: School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University , Suwon, Gyeonggi-do 440-746, Republic of Korea.
authors: Nam-Gyu
authors: NG
authors: Park
authors: School of Advanced Materials Science & Engineering, Sungkyunkwan University , Suwon, Gyeonggi-do 440-746, Republic of Korea.
authors: Hyun Suk
authors: HS
authors: Jung
authors: Global Frontier Center for Multiscale Energy Systems , Seoul 151-742, Republic of Korea.|Department of Mechanical Engineering, Seoul National University , Seoul 151-742, Republic of Korea.
authors: Mansoo
authors: M
authors: Choi
keywords: Perovskite solar cell; diffusion; halide concentration gradient; halide substitution; photoluminescence; secondary ion mass spectroscopy