Microbiologically produced ZnS nanoparticles and functional thin films for photovoltaic applications

High efficiency solar cells and low-cost manufacturing techniques are critical to meet the future requirements of terawatt level power generation. Nontoxic inexpensive ZnS can replace CdS as the buffer layer containing toxic cadmium. We report biologically produced ZnS nanoparticles (NP) and photovoltaic applications. The main advantages of the microbiological process include low precursor cost, non-vacuum processing, low processing temperature, scalability, and high throughput. The ZnS NP synthesis proceeds with anaerobic metal-reducing Thermoanaerobacter species reducing partially oxidized sulfur sources to sulfides that extracellularly and thermodynamically incorporate Zn ions to produce sphalerite ZnS NP with ~5nm average crystallite size (ACS). A short incubation period resulted in a narrower ACS distribution and increased emission intensity. The up-scaling to 24L kept process yield (~5g/L/month), while the recovery remained efficient (>95%). Surface characterization using FT-IR and XPS revealed that the surface coatings facilitated keeping a nano-dimensioned ACS, but also increased particle aggregation. The present results for ZnS NP via microbial production clearly show that synthetic conditions play a key role in determining the resulting optical properties of the prepared ZnS NP. By mixing the material with nitrilotriacetic acid followed by sonication and sedimentation, ~25nm aggregates of ZnS were produced. The ZnS ink exhibited good wetting characteristics (contact angle - 34°) on a glass substrate at a deposition temperature of 100°C. Upon high temperature annealing, the resulting ZnS films showed a significantly improved surface roughness profile and increased density. An initial measurement of 15-layer-deposited functional thin film showed high uniformity of the film.