CuInSe₂ nanowires and earth-abundant nanocrystals for low-cost photovoltaics




Steinhagen, Chet Reuben

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Widespread commercialization of photovoltaics (PVs) requires both higher power conversion efficiencies and low-cost, high throughput manufacturing. High efficiencies have been achieved in devices made from materials such as CuIn[subscript x]Ga₁₋[subscript x]Se₂ (CIGS). However, processing of these solar cells still requires high temperature and vacuum, driving up cost. A reduction in manufacturing costs can be achieved by utilizing colloidal nanocrystals. Semiconductor nanocrystals can be dispersed in solvents and deposited via simple and scalable methods under ambient conditions to form the absorber layer in low-cost solar cells. Efficiencies of ~3% have been achieved with CIGS nanocrystal PVs, but this must be improved substantially for commercialization. These devices suffer from poor charge transport in the nanocrystal layer. Here, the synthesis of nanowires and their utilization in solar cells was explored as a way to improve charge transport. CuInSe₂ (CIS) nanowires were synthesized via the solution-liquid-solid method. PV devices were fabricated using the nanowires as the light absorbing layer, and were found to exhibit a measureable power output. Earth-abundant materials were also explored, motivated by the material availability concerns associated with CIGS. Pyrite FeS₂ nanocrystals were synthesized via an arrested precipitation reaction to produce phase-pure particles 15 nm in size. These nanocrystals were spray coated to form the active layer in several different common device architectures. These devices failed to produce any power output. The material was determined to be slightly sulfur deficient, leading to a high carrier concentration and metallic behavior in the thin films, with conductivities measured to be ~5 S/nm. A nanocrystal synthesis of Cu₂ZnSnS₄ (CZTS) was also developed to produce highly dispersible crystalline particles ~11 nm in size. These nanocrystals were spray coated onto glass substrates to form the absorber layer in test PV devices, and an efficiency of 0.23% was achieved without high-temperature or chemical post-processing. Additional studies included the synthesis of CZTS nanorods and their incorporation into functioning solar cells. The selenization of CZTS nanocrystal films was also studied as a way to improve solar cell performance. High temperature annealing in a Se atmosphere was found to produce CZTS(Se) layers, which could be used in working PV devices.



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