Impact of Divalent Metal Additives on the Structural and Optoelectronic Properties of CH 3 NH 3 PbI 3 Perovskite Prepare
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Impact of Divalent Metal Additives on the Structural and Optoelectronic Properties of CH3NH3PbI3 Perovskite Prepared by the Two-Step Solution Process Suneth C. Watthage, Zhaoning Song, Niraj Shrestha, Adam B. Phillips, Geethika K. Liyanage, Paul J. Roland, Randy J. Ellingson, and Michael J. Heben Wright Center for Photovoltaics Innovation and Commercialization, School for Solar and Advanced Renewable Energy, Department of Physics and Astronomy, University of Toledo, Toledo, OH, USA 43606 ABSTRACT Here, we investigate the effect of divalent metal (Zn2+, Cd2+ and Hg2+) on the structural and optoelectronic properties of methylammonium lead iodide perovskite materials prepared by the two-step deposition process. The incorporation of Cd2+ significantly improved the grain size, crystallinity, and charge carrier lifetime of CH3NH3PbI3. The inclusion of Hg2+ and Zn2+ improved the grain size compare to the control sample but adversely affected the optoelectronic properties of perovskite films. The Hg- and Zn-based impurities were formed on the surface of the films, which increased the charge trap density and lead to high non-radiative recombination rate. Time resolved photoluminescence measurements indicated that the Cd and Zn point defects do not create deep-level trap states, but the Zn-modified film showed a low lifetime due to morphology changes in the film and particle segregation on the surface. INTRODUCTION Thin film solar cells based on organic-inorganic metal halide perovskites, especially methylammonium lead iodide (CH3NH3PbI3, MAPbI3), have become highly attractive in recent years due to the high power conversion efficiency (PCE) up to 22.1 % [1] and ease of fabrication [2, 3]. The perovskite materials possess excellent optoelectronic properties, such as high absorption coefficients, long charge carrier diffusion lengths, appropriate optical band gaps that fit the solar irradiation, and fast charge carriers separation [2], which make the materials an ideal candidate for the absorber of solar cells. In addition, the low manufacturing costs provide perovskite solar cells (PSCs) an edge over the competition with commercial PV technologies, such as crystalline silicon and CdTe. Typically, the crystallinity, grain size, and morphology of the perovskite film in PSCs are crucial to the optoelectronic properties of the films and the performance of the devices [4]. These factors determine the density of trap states (Nt), which causes non-radiative recombination of charge carriers and energy loss in the devices [5, 6]. To enhance solar cell performance, Nt has to be reduced. One effective way is to enhance the grain size of the perovskite films, which can greatly reduce the grain boundary and decrease Nt at boundaries. Furthermore, the diffusion length and mobility of carriers can be improved significantly in large grains [5]. In addition to the grain size and crystallinity, surface morphology of the MAPbI3 perovskite film is also crucial to achieve better performance in the PV devices. Poor surface coverage and rough surfa
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