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Nina Vaidya1 Jing-Shun Huang1 Harry Atwater1

1, California Institute of Technology, Pasadena, California, United States

Perovskite solar cells have emerged as a prominent candidate in the field of photovoltaics. With their efficiency comparable to silicon solar cells and easy of fabrication involving low temperature solution processing and self-assembly, Perovskites are an attractive prospect to reduce costs and implement large scale solar energy adoption. However, the main hurdle in the way of success of Perovskites is their sensitivity to moisture. We present passivation schemes to protect organic–inorganic lead halide perovskite solar cells from degradation when exposed to moisture and also encapsulation schemes for vacuum environment like space applications. In specific, hydrophobic BAI (Butyl Ammonium Iodide) that has been used for passivation of 2D layered perovskite solar cells [1] improves perovskite stability and efficiency even in solution processed 3D perovskites [2, 3]. The device architecture in our experiments is ITO Indium tin oxide/ NiO Nickle Oxide/ MAPbI3 Methyl Ammonium Lead Iodide/ BAI/ PCBM [6, 6]-phenyl-C61-butyric acid methyl ester/Ag Silver. For fabrication of the passivated cells, a layer of BAI solution was span at 4000 rpm for 20 seconds followed by heat at 100°C for 10 minutes, over the perovskite layer. The concentrations of the BAI solutions varied from 2mg/ml to 10mg/ml. 2mg/ml gave the best results so far with efficiency of 12.1% after 18 days for passivated cells as compared to 12.3% for as-fabricated cells without passivation which degraded to about 9% efficiency after the 18 day interval (measured under solar simulator spectra of AM 1.5G). The suggested hypothesis of the mechanism of the BAI passivation, which will be studied in this paper, is that the long chains of the butyl group extend outwards from the perovskite layer blocking other molecules to react with the perovskite layer and in effect passivating the perovskite surface. In x-ray diffraction (XRD) analysis, there is a pronounced peak for PbI2 seen for 6-day old cells without passivation which is not present in the 6-day old passivated cells, further providing evidence for passivation using BAI. The absorption between 500 nm and 1000 nm wavelength for as-grown perovskite and 6-day old passivated perovskite agree with each other and the curves are almost identical. We further investigate the passivation scheme, structure, and potential improvements using x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and opto-electronic measurements.

Success of passivation schemes will propel the promising perovskite solar cell technology from research in inert glove boxes to industry-wide adoption.

References:
1) 10.1038/nature18306
2) 10.1021/nn5036476
3) 10.1038/nenergy.2017.135

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