Vapor Deposition of Halide Perovskites and Hole- Transport Materials for Use in Thin-Film Photovoltaics
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AbstractThe drive toward a sustainable, low-carbon energy future has created a considerable impetus to reduce the cost per watt of electricity produced from solar cells. Thin-film photovoltaics, specifically those produced with perovskite absorber layers, demonstrate great potential in this regard, given the high power conversion efficiencies that can be achieved with low material usage. However, perovskite solar cells are typically deposited by solution processing methods, which may be difficult to scale to the industrial level, particularly if an inert atmosphere is required to prevent the decomposition of these air-sensitive materials. Additionally, the most efficient perovskite cells employ an expensive and resistive p-type organic hole-transport material and a lead-based absorber layer. Use of the former increases cost and likely limits performance and use of the latter raises toxicity concerns that could restrict widespread commercial deployment of these devices. Chemical vapor deposition and atomic layer deposition provide scalable alternatives to solution processing that allow for the growth of materials in an inert atmosphere. Methods for the vapor deposition of the lead-free absorber layers CH3NH3SnX3 (X = I, Br) and the p-type inorganic hole-transport materials CuX are presented and the resultant films are evaluated for their use in thin-film photovoltaics.
The chemical vapor deposition of halide materials is shown to be complicated by the formation of nonvolatile salts produced by the reaction between the strongly acidic HI and HBr precursors and the basic ligands of the metalorganic Sn and Cu precursors. The atomic layer deposition of these same materials is also precluded by the lack of surface reactive sites provided by the hydrogen halides. To circumvent these difficulties, two-step conversion processes are developed with the goal of producing the selected materials CH3NH3SnX3 and CuX indirectly. It is first demonstrated that a post-deposition exposure of the contaminated (H3cyc)xSnBr(2+x) produced by chemical vapor deposition to CH3NH2 produces a film with atomic composition approaching that of the desired CH3NH3SnBr3. It is then shown that the surface of Cu(2-x)S grown by pulsed-chemical vapor deposition can be transformed to crystalline γ-CuBr upon exposure to anhydrous HBr. The produced CuBr initially forms as a continuous film, but becomes discontinuous as the conversion front approaches the interface with the SiO2 substrate. Al2O3 is predicted by contact angle measurements to have surface properties compatible with those of CuBr and does in fact demonstrate improved wetting of the converted material. Finally, the introduction of HI to Cu(2-x)S and Cu2O results in the formation of thin films of crystalline γ-CuI, with the conversion from the oxide proceeding to completion in less than 2 hours at room temperature. Exposure to wetting agents thiodiglycol and ethylene glycol prior to and during conversion is shown to improve the surface coverage of converted films. The quantification of surface properties by contact angle measurements allows for an optimization of substrate and wetting agent to produce films of CuI with high surface coverages at thicknesses of only 60 nanometers. The converted CuI demonstrates an optical bandgap of 3.1 eV, with resistivity values as low as 7.5 × 10-2 Ω×cm and hole mobilities of up to 6 cm2/V×s.
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