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Subodh Mhaisalkar1 Nripan Mathews1 Tze Chien Sum1 Cesare Soci1 Tim White1

1, Nanyang Technological University, Singapore, , Singapore

Since the first reports of metal halide perovskite solar cells, intense research focus has centered on CH3NH3PbI3, and related family of materials. Although the initial priority of research was on increasing solar cell efficiencies, focus has now shifted to improve the stability of these solar cells, especially for long-term atmospheric exposures.
The hydrophilicity and volatility of methylammonium cations (MA+) turn the archetypical metal halide perovskite vulnerable to degradation through humidity and thermal exposure. The prospects for advancing device stability are contingent upon exploring structural variants, including bilayer 2D/3D perovskites and surface passivation approaches which yield more environmentally stable PSCs. Through molecular design of organic A-site cations, we developed a series of Ruddlesden-Popper perovskites, (CHMA)2(MA)n-1PbnI3n+1, in which a thin lower dimensional perovskite was formed over the 3D perovskite layer in one-step deposition. The bilayer 2D/3D hybrid perovskites possess striking moisture resistance and displayed high ambient stability up to 65 days. In order to modulate the formation of bilayer 2D/3D perovskites for high efficiency PSCs, top surface of well-formed 3D perovskite is converted into 2D perovskite in two-steps deposition, resulting a better bilayer perovskite layer in term of perovskite quality, morphology and interfacial contacts. Not only the enhanced moisture tolerance from the hydrophobicity of long chain organic cations, this approach could also suppress surface defects and vacancies of solution-processed perovskite which in turn results in higher power conversion efficiency (PCE) with excellent moisture stability.
Surface passivation approach is another way to improve the stability of PSCs. Different from conventional organic cations, utilization of hydrophobic fluorinated organic salt to passivate highly efficient triple-cations perovskite without triggering the formation of 2D perovskite has been demonstrated. The passivated perovskite thin film displays narrower band gap (halide substitution), longer PL lifetime and a notable improvement in PCE. More importantly, the passivated PSCs show remarkable stability for more than 169 days under ambient conditions at an average relative humidity (RH) of 55% without any significant change in its initial PCE. These findings provide new insights into intrinsic bilayer perovskite formation and passivation through careful design of the organic components in the perovskite structure to achieve a more stable and highly efficient perovskite material for photovoltaics.
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