(109c) Copper Zinc Tin Sulfide Thin Films for Solar Cells

The photovoltaic market is dominated by silicon solar cells but low absorption coefficient of silicon necessitates the use of relatively thick (0.1-0.3 mm), high electronic quality and expensive wafers, which increases the cost of solar-to-electric energy conversion. Use of solar cells made from thin semiconductor films is growing rapidly because they are typically a factor of 100 thinner than Si devices and cost less. Amongst the thin film solar cells, those based on the copper indium gallium selenide (CuIn_1-xGa_xSe₂ or CIGS) and CdTe absorbers have emerged as highest efficiency and lowest cost alternatives to silicon. However, terawatt-level production of these thin film solar cells may be limited by the scarcity of Te and In. Thus, an important problem in developing the next generation thin film solar cells is the synthesis of direct band gap semiconductors that are made of nontoxic and abundant elements. The rapid rise in power conversion efficiencies of thin-film solar cells based on Cu₂ZnSnS₄ (CZTS), Cu₂ZnSnSe₄ (CZTSe) and their alloys Cu₂ZnSn(S_1-xSe_x)₄ (CZTSSe) attests to their potential as low-cost, earth-abundant alternatives to CIGS and CdTe. To date, many thin CZTS and CZTSe film formation methods have been developed and solar cells with efficiencies exceeding 12% have been demonstrated. Increasing solar cell efficiencies relies heavily upon trial-and-error optimization of multiple film properties during the film’s synthesis. Ideally, a monolayer of single crystal grains with sizes on the order of the film thickness is required to reduce charge recombination at grain boundaries. This talk will focus on elucidating the factors that affect the microstructure and composition of thin CZTS and CZTSSe films formed by annealing, in a sulfur or selenium containing atmosphere, of thin films deposited from colloidal CZTS nanocrystal dispersions. In this method, 20 to 30 nm diameter CZTS nanocrystals are synthesized in hot (150-300 ˚C) oleylamine from copper, zinc, and tin diethyldithiocarbamates. Following, thin (2-3 mm) coatings are drop cast from colloidal dispersions of these nanocrystals, which are then annealed in a well-controlled closed system to form large grain microcrystalline films. Abnormal and normal grain growth compete with each other and lead to bimodal grain size distributions. The factors that affect the abnormal and normal grain growth rates include, the temperature, the substrate, and the sulfur (selenium) vapor pressures. Substrates containing alkali impurities lead to larger grains at lower temperatures. Annealing films in selenium leads to the formation of a layer of 2-5 µm size CZTSSe grains on top of a nanocrystalline layer that is rich in carbon: the carbon originates from the dispersion-stabilizing ligands on NC surfaces. Tendency of CSe₂ to polymerize leads to carbon segregation at the CZTSSe-substrate interface. In contrast, films annealed with sulfur do not show such distinct carbon-rich layers and most of the carbon volatilizes from the film during annealing. Careful balancing of these mechanisms is required to obtain films with microstructures suitable for solar cells.