Reaction engineering of CVD methylammonium bismuth iodide layers for photovoltaic applications
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Reaction engineering of CVD methylammonium bismuth iodide layers for photovoltaic applications Dominik Stümmler1,a), Simon Sanders1, Felix Gerstenberger1, Pascal Pfeiffer1, Gintautas Simkus2, Peter K. Baumann3, Michael Heuken2, Andrei Vescan1, Holger Kalisch1 1
Compound Semiconductor Technology, RWTH Aachen University, Aachen 52074, Germany Compound Semiconductor Technology, RWTH Aachen University, Aachen 52074, Germany; and AIXTRON SE, Herzogenrath 52134, Germany 3 APEVA SE, Herzogenrath 52134, Germany a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 23 October 2018; accepted: 14 December 2018
In the past years, numerous alternative cations to replace Pb21 in perovskite solar cells have been investigated. In terms of toxicity and chemical stability, methylammonium bismuth iodide [(CH3NH3)3Bi2I9 or MBI] containing the Bi31 cation has been considered as a promising material. However, fabrication of coherent MBI films remains challenging. Recently, significant progress has been achieved by using vapor deposition processes. Compared with solution-processed ones, vapor-deposited MBI solar cells show higher fill factors and efficiencies. In this work, chemical vapor deposition (CVD) of MBI is investigated. Employing nitrogen as carrier gas, the precursors bismuth iodide (BiI3) and methylammonium iodide (MAI) are deposited sequentially over several cycles and form MBI during the process. In order to form dense and coherent layers, the lengths of the deposition cycles as well as the substrate temperature have been optimized. Scanning electron microscopy reveals the strong influence of both parameters on growth and crystal properties. Optimized films of MBI integrated into solar cells show that CVD of MBI is a promising method for fabricating large-area solar cells.
Introduction Benchmark perovskite photovoltaic devices already achieve power conversion efficiency (PCE) figures over 20%, which is comparable to those of established solar cell technologies, such as silicon solar cells [1]. Although the performance of perovskite solar cells is competitive, the toxicity of the widely used lead compounds and their solvents used for solution processing is still problematic [2]. To tackle the first aspect, less toxic alternative systems, like Sn- or Bi-based perovskites, have been considered [3]. Sn-based solar cells reach PCE up to 6% but suffer from a pronounced instability under ambient conditions [4]. In contrast to Sn-based perovskites, Bi-based compounds are stable over days under such conditions [5]. However, due to the poor coverage of solution-processed Bi-based perovskites, PCEs remain rather low, rarely overcoming the threshold of 1%. Recently, Zhang et al. demonstrated a significant performance boost of Bi-based perovskite solar cells by employing vapor deposition of the precursors methylammonium iodide (MAI) and bismuth iodide (BiI3). By using a two-step method, high-density layers with enhanced coverage and relatively large
ª Materials Research Society 2019
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