Atomic layer deposition of photovoltaics

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McCarthy, Melissa M.
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University College Cork
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In recent years organic-inorganic halide perovskites have gained significant interest due to their high power conversion efficiencies (PCEs). These high efficiencies can be attributed to the perovskites’ tuneable optical bandgap and long carrier diffusion length, with their low fabrication cost facilitating accelerated research. The first reported PCE using an organic-inorganic halide perovskite was 3.8 %, achieved in 2009 in conjunction with a liquid electrolyte. Since then, the perovskite field has been extensively studied and efficiencies now reach over 24.2 %. Despite these advances, long term stability and scalability have remained major challenges. Atomic layer deposition (ALD) is a low processing temperature technique capable of providing unrivalled thin-film conformality with uniform sub-nanometre thickness on large scale areas. Its use in depositing thin charge selective contacts as well as passivation layers in upscaled perovskite devices could further the field toward viable commercialisation. The work carried out in this thesis is primarily concerned with the control and modification of electron transport layers (ETLs) deposited by ALD for use in mesoporous and planar perovskite solar cells (PSCs). ALD TiO2 was compared to TiO2 deposited by the alternative methods of spin-coating and RF sputtering and incorporated into mesoporous devices using both commercial and atmospheric pressure chemical vapour deposited (APCVD) FTO coated glass. The ability of ALD to produce conformal and pinhole free layers with high thickness uniformity at < 200◦C was demonstrated and the optimisation of ALD TiO2 led to the fabrication of 1 cm2 mesoporous PSCs with efficiencies of 16.45 %, matching the performance of cells incorporating RF sputtered TiO2 and exceeding that of cells containing spin coated TiO2. Following on from this, alternative growth chemistries and laminate doping of ALD TiO2 were explored for use in mesoporous PSCs. For planar perovskite devices, ALD TiO2 was problematic. Impurities and non-stoichiometry, previously improved by an anneal step for mesoporous devices, hampered performance. To counter this, rapid thermal annealing and in-situ interface engineering were investigated. Improvements were observed but the performance of TiO2 in the mesoporous structure was not matched in the planar configuration. Attention was then turned to SnO2, a material with a superior conduction band alignment with perovskites. In the planar n-i-p configuration 18.3 % PCE was achieved on 0.09 cm2 and 15.9 % PCE was obtained on 0.7 cm2 demonstrating considerable promise for commercial processes. Finally, the p-i-n configuration of planar cell architecture was considered, where the ETL is grown directly on to the perovskite by ALD. Initial studies indicate that ALD chemistry can be tuned to prevent damage to either the bulk perovskite or its interface. The work presented in this thesis has demonstrated ALD to be an ETL deposition technique of great potential for efficient upscaled perovskite devices particularly in the reverse p-i-n architecture.
Perovskite solar cell , Thin film deposition , Surface passivation , Perovskite , Metal oxide , Energy , Atomic layer deposition
McCarthy, M. M. 2019. Atomic layer deposition of photovoltaics. PhD Thesis, University College Cork.
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