(536b) Copper Zinc Tin Sulfide Solar Cells

Although thin film solar cells based on copper indium gallium diselenide (CIGS) and CdTe have already achieved impressive power conversion efficiencies ranging between 15 and 20%, the semiconductor materials commonly used for their production are either toxic (e.g., cadmium) or rare in the earth’s crust (e.g., indium, tellurium). Copper zinc tin sulfide (Cu₂ZnSnS₄; commonly known as CZTS) is emerging as a photovoltaic material composed of elements that are nontoxic and abundant. CZTS has a band gap of ~1.4 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity, and a high absorption coefficient (> 10⁴ cm^-1 in the visible region of the electromagnetic spectrum).  We are exploring various methods for depositing thin CZTS films from liquid precursors (e.g., “inks”). These CZTS “inks” can be used in conjunction with high throughput roll-to-roll deposition techniques such as slot coating under atmospheric conditions and may offer alternative low-cost manufacturing routes to making thin film CZTS solar cells.

Specifically, we have developed a novel and facile synthesis method for making CZTS nanocrystals and stable colloidal nanocrystal dispersions. Our nanocrystals were synthesized from copper, zinc, and tin diethyl dithiocarbamate complexes dissolved in octadecene. Presence of oleylamine in octadecene reduces the decomposition temperature of these metal dithiocarbamate complexes to a narrow temperature range (170-220 ^oC). Thus, nucleation and growth of phase-pure CZTS nanocrystals can be initiated by rapidly injecting oleylamine into a hot solution of tin, zinc and copper metal complexes in octadecene. The nanocrystal size can be controlled between 2 to 7 nm by changing the synthesis temperature and time. We also used colloidal dispersions of these nanocrystals to cast  thin (0.5-2 micron) CZTS films. Heating these films in argon atmosphere to temperatures ranging from 500-700 ^oC forms continuous CZTS layers with larger grains than the starting nanocrystals. We found that the CZTS nanocrystals melt significantly below the bulk melting temperature. Larger grains grow upon recrystallization. This process of forming CZTS films is fast compared to other techniques, which require several hours of annealing for grain growth. Solar cells based on these CZTS films will be reported.