Deposition of methylammonium lead tri-iodide organometallic halide perovskites using close space vapor transport

Date
2016
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University of Delaware
Abstract
Organometallic halide perovskites (OHPs) are a class of materials that are currently being heavily investigated as an absorber material for thin-film photovoltaic devices due to their favorable optoelectronic properties and flexible fabrication options. The first report of an OHP PV device was published in 2009 with a conversion efficiency of only 3.9% (Kojima et al. 2009; Snaith 2013) and the current most efficient research cell, certified by National Renewable Energy Laboratory (NREL), has an efficiency greater than 22.1% (NREL 2016). However, the technology is not without its challenges. The material properties of films depend on the processing conditions and the deposition technique, compromising reproducibility (Du et al. 2015). Films deposited using solution-based techniques, are especially unreliable. Solution deposition, namely one-step spin coating, is the most prominent choice due to easy accessibility for laboratory scale experiments and operation at ambient conditions. However, challenges with reproducibility, control over film thickness and uniformity, manufacturing scalability, and the lack of environmental control make it a less desirable choice for commercialization (Petrović et al. 2015). Characterization of films spun cast in this investigation were consistent with these shortcomings, exhibiting poor surface coverage, non-uniform composition, and acute sensitivity to humidity. ☐ In contrast, vapor deposition techniques provide better control over film morphology and deposition environment, minimizing degradation of OHP films and ensuring reliable data on their material properties (Ono et al. 2016). Close space vapor transport (CSVT) is a novel, highly controllable vapor deposition technique for fabricating OHP films, originally developed for manufacturing of CdTe thin-film solar devices. The potential of CSVT as a scalable technique for OHP PV fabrication was demonstrated in this thesis through the development of a mass transport model for predicting the deposition flux of sublimated reactants methylammonium iodide (MAI) and PbI2 in an assembled pilot plant system. These two materials were sequentially deposited to fabricate CH3NH3PbI3 thin-films. Flux values calculated at various source temperature and system pressures overestimated actual values obtained under the same conditions by at least an order of magnitude. Some of the assumptions used to develop the model attributed to the large deviation from the actual flux, namely a constant temperature gradient and isolation of deposition to only the designated deposition region. Furthermore, more accurate diffusivities for MAI and PbI2 vapor are needed. Future research should focus on obtaining better estimates for reactant diffusivities through empirically instead of theoretically, quantifying the actual temperature gradient during system operation, and confirming if convective heat transport is in fact applicable to this system.
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