Atmospherically Processed and Stable Cs-Pb Based Perovskite Solar Cells
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Atmospherically Processed and Stable Cs-Pb Based Perovskite Solar Cells Shubhra Bansal, Michelle Chiu Department of Mechanical Engineering, Center of Energy Research, University of Nevada Las Vegas, Las Vegas, Nevada 89154, U.S.A. ABSTRACT In this work, a planar heterojunction superstrate n-i-p device based on Zn(O,S) electron transport layer and CsPbI2Br absorber material at 1.93 eV bandgap is presented. The CsPbI2Br films are deposited using a 2-step atmospheric solution deposition process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis spectroscopy and photoluminescence (PL). Best device with an efficiency of 12.34 % and 11.94% in reverse and forward scans respectively and stabilized power output of 12.14 mW/cm2 has been demonstrated via atmospheric solution processing with minimal hysteresis between forward and reverse scans. The devices show voltage dependent current collection as well as light-dark crossover in forward bias. Light soaking tests at 65 °C and 1-sun at Voc, resulted in open-circuit voltage and fill-factor degradation. Electroluminescence (EL) after 100 hours of light soaking shows a reduction in overall EL intensity as well a shift in emission to lower wavelength. The devices exhibit a positive temperature coefficient of about 0.14 %/°C. It is found that Zn(O,S) is a viable alternative electron transport layer to replace TiO2. By replacing methylammonium cation with cesium and addition of Br has improved the stability of the perovskite phase. INTRODUCTION Since the introduction of methylammonium lead halide based solar cells by Kojima et al. in 2009 [1], organic-inorganic hybrid perovskites have gained significant attention as an absorber layer in thin film solar cells. The characteristics exhibited by these materials such as high absorption coefficient [2], excellent transport properties [3], tunable bandgap [4], absence of deep trap states within the bandgap [5-6], low temperature processing etc. [7-9], make the materials suitable for low cost photovoltaic applications. Consequently, the photo-conversion efficiency of devices based on these materials has improved dramatically from 3.8% to 22.1% in a relatively short span of time [10-11]. Despite several advantages of perovskites, the application of these materials in commercial PV modules is seriously hindered by their stability. Perovskites have been shown to degrade rapidly upon exposure to heat, moisture, air, and light [4,8,12-14]. Limited success has been achieved in improving device stability by encapsulating devices [15,16]. The formation energy of MAPbX3 has been reported to be very low, which makes these materials unstable at high temperatures and in humidity [17]. By replacing the organic cation with inorganic Cs, the intrinsic material stability can be improved [18]. Mixed A-cation [19-21] and mixed-halide systems have shown to be more structurally stable than MAPbI3 due to increased formation energy and tolerance factors [22]. Conventionally used ETL and HTL materials are not completely immun
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