Aligned and Graded Type-II Ruddlesden-Popper Perovskite Films for Efficient Solar Cells
Jian Qing a, Xiao-Ke Liu b, Mingjie Li c, Chun-Sing Lee a, Tze Chien Sum c, Feng Gao b
a City University of Hong Kong, Tat Chee Avenue, 83, Hong Kong, Hong Kong
b Department of Physics Chemistry and Biology Linkoping University 58183 Linkoping , Sweden
c Nanyang Technological University (NTU), Singapore, Singapore
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Benidorm, Spain, 2018 May 28th - 31st
Organizers: Emilio Palomares and Rene Janssen
Poster, Xiao-Ke Liu, 220
Publication date: 21st February 2018

Organic-inorganic halide perovskites have emerged as promising light absorbers for solar cells due to their superior photophysical properties and promise for low-cost solution processability. Perovskite solar cells have experienced tremendous development in the past few years with power conversion efficiency (PCE) rapidly increased to over 20%. Besides efficiency, another critical factor for practical application of perovskite solar cells is stability.

Recently, Ruddlesden-Popper perovskites (RPPs) have been reported to have promising stability. The RPPs can be structurally derived from their 3D (three-dimensional) counterparts with alternating organic ammonium layers and perovskite layers, giving the general formula of (RNH3)2An-1MnX3n+1, where n represents the number of perovskite layers, RNH3 is the organic spacer, and the A (small cation), M (divalent metal cation) and X (halide anion) form the perovskite framework. The superior stability of RPPs can be attributed to the hydrophobic property of the organic spacer, which inhibits water molecules from penetrating and attacking the inorganic layers.

However, the efficiency of solar cells based on RPPs is much lower than that based on three-dimensional perovskites, mainly attributed to their poor charge transport. Herein, we report a simple and universal method for growing high-quality RPP films, which results in hysteresis-free solar cells with a high PCE over 12% and excellent reproducibility. These high-quality films are achieved by incorporating DMSO and CH3NH3Cl (MACl) additives into precursor solution, followed by one-step spin-coating and solvent annealing processes. We systematically study the synergistic effects of DMSO and MACl additives on controlling the crystallization of RPPs. We find that the additives can greatly improve the film quality of RPPs, leading to uniform morphology, enhanced crystallinity, and reduced energetic disorder. In addition, the resulting films show preferential orientation with their perovskite frameworks perpendicular to the substrate, which we assume is responsible for facilitating efficient carrier transport. Furthermore, they show graded distribution of multiple RPP phases with type-II band alignment, which is favourable for self-driven charge separation. Our method paves the way for further development of high-quality RPP films for efficient optoelectronic devices.

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