(559d) Fabrication of High-Performance Solution-Processed AgInSe2 Semiconductor Thin-Films

AgInSe₂ is a promising direct bandgap thin-film material for photovoltaic devices. It has high defect formation energy, n-type conductivity, and a suitable bandgap of 1.24 eV.^1,2 AgInSe₂ is also thermodynamically stable.³ All these favorable properties of silver addition are enough to motivate researchers to study AgInSe₂ in detail. However, AgInSe₂ films have been prepared using costly vacuum processing techniques in most cases. These techniques require a high vacuum, have inefficient material utilization, and involve batch processing. This work utilizes a cost-effective solution-processing route and fabricates AgInSe₂ via the N-N Dimethylformamide (DMF)-Thiourea (TU)-Chlorides chemistry.⁴ However, the stoichiometric AgInSe₂ film thus fabricated had moderately poor optoelectronic properties with low photoluminescence (PL) yield (low theoretical V_oc of 600 mV) and a poor carrier lifetime of 300 ps. Moreover, slight improvements in PL yield and carrier lifetimes were observed for the Ag-poor (Ag/In<1) films, but the best carrier lifetime was still less than 500 ps.

This study hypothesized that increased selenium loss from the film at higher temperatures limited the performance. In other words, selenium vacancies were possibly formed in AgInSe₂ film by selenizing at higher temperatures in an insufficient selenium environment. Hence, different methods were used to incorporate increased selenium in the film during selenization. In the first method, rather than putting the substrate directly on the graphite surface of the selenization box, glass shims were used as support for the as-coated sulfide film in the selenization furnace, which led to the film surface being much cooler than the selenium vapor during the initial heating period, resulting in increased selenium condensation on the film surface. In other methods, selenium layers were coated, and selenium powder was spread on the as-coated AgInS₂ film respectively and selenized without shims.

Significant improvements in PL yield and lifetime are observed in all three cases. Also, the improvements are more significant for Ag-poor films. Modifying the synthesis route by taking these observations into account helped achieve a comparatively high minority-carrier lifetime of 2.22 ns for the solution-processed AgInSe₂ with Ag/In ~ 0.91 selenized at 515 ºC for 20 minutes. In order to further increase the selenium condensation, DMF-TU-chlorides ink was coated on an alumina-coated glass substrate instead of a molybdenum-coated glass substrate. The alumina has poor heat transfer properties than the molybdenum, which further slowed down the heat transfer to the film during the early phase of heating, driving higher selenium condensation on the film surface. A high carrier lifetime of 9.22 ns was achieved for the film selenized using alumina back contact. Using the photoluminescence technique, the film achieved a high estimated V_oc of around 740 mV compared to 600 mV achieved by the film selenized directly on the graphite surface. Also, improved AgInSe₂ film has a PLQY of 0.0128%, suggesting a high device efficiency of around 18% inferred from the efficiency vs. PLQY plot published elsewhere for different photovoltaic materials.⁵ The x-ray photoelectron spectroscopy analysis showed higher selenium content on the surface of film selenized on shims than the film selenized on graphite which supports the selenium hypothesis. These excellent optoelectronic properties of solution-processed AgInSe₂ are promising.

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