High Performance, Earth Abundant and Non-Toxic Thin Film Solar Cells by Atomic Layer Deposition and Chemical Vapor Deposition

Abstract

Cuprous oxide (Cu2O) and tin sulfide (SnS) are two of the most promising absorber layers for thin film solar cells. Both photovoltaic absorber materials are non-toxic, Earth-abundant and low-cost semiconductor materials that demonstrate excellent surface morphology, crystal structure, phase and elemental purity, and opto-electronic properties for use in high performance solar cells. Cuprous oxide (Cu2O) thin films were grown by chemical vapor deposition (CVD) using precursors (N, N’- di-sec-butylacetamidinato)copper(I) and degassed water at low substrate temperatures of 125 to 225 °C. Despite being a widely studied material, vapor deposition of Cu2O faces numerous challenges in avoiding material agglomeration, in obtaining high phase purity, and in limiting the process temperature to below 200 °C for temperature sensitive applications. We present pinhole-free single-phase oxide films that exhibit Hall mobilities up to 17 cm2 V-1 s-1 and wide band gaps exceeding 2.6 eV that are free from contaminants such as nitrogen, carbon, and cupric oxide (CuO). With good control of growth parameters (source temperature, substrate temperature, flow rate of carrier gas, etc), the film morphologies could be tuned to achieve both smooth, pinhole-free coatings as well as highly crystalline thin films with rough surfaces that are highly advantageous for applications to solar cells. Thin film solar cells were fabricated using cuprous oxide (Cu2O) absorber layers grown by chemical vapor deposition (CVD) and gallium oxide (Ga2O3) buffer layers grown by atomic layer deposition (ALD) on the cuprous oxide CVD films. The in-situ formation of heterojunction in the same deposition system without exposure to oxygen-rich ambient was found to be effective in mitigating the creation of detrimental cupric oxide (CuO) at the interface, resulting in a pristine photovoltaic junction capable of delivering an enhanced open-circuit voltage of 1.78 V. Numerical device simulations of a novel two-layer absorber architecture (CVD-Cu2O on ECD-Cu2O) showed promising possibilities (theoretical 13.2 % efficiency) for a solar cell combining in-situ junction formation with electrochemical deposition of the absorber layer. Thin film solar cells fabricated using non-toxic, Earth-abundant tin sulfide (SnS) absorber layers have performance limiting mid-gap defects that necessitates effective defect mitigation strategies. High temperature annealing treatments that enable performance enhancements are investigated using photoluminescence techniques to probe their effectiveness under annealing ambient of various sulfur contents. Decoupling of charge recombination on the surface from the bulk, and in grain interior from grain boundaries were leveraged to elucidate the impact of annealing conditions on photovoltaic device performances, leading to an enhanced open-circuit voltage of 0.38 V and an optimized photo-conversion efficiency of 4.99%. Tin germanium oxide, (Sn,Ge)O2, films were prepared using atomic layer deposition and tailored to SnS absorber layer by incorporating various amounts of germanium into tin oxide to adjust band alignments at the interfaces of SnS/(Sn,Ge)O2 photovoltaic devices. Carrier concentrations of (Sn,Ge)O2 were suppressed from 1020 to 1018 cm-3 with germanium incorporation, with nitrogen doping of further reducing carrier concentrations by another order of magnitude. Excellent tunability of both band energy levels and carrier concentrations of (Sn,Ge)O2 allowed optimizing SnS-based solar cells. SnS/(Sn,Ge)O2:N devices were demonstrated, with an open-circuit voltage as high as 400 mV, due to effective mitigation of interfacial recombination of photogenerated carriers at the SnS/(Sn,Ge)O2:N absorber-buffer heterojunction interface.