Enhancing the optoelectronic properties of solution-processed AgInSe2 thin films for application in photovoltaics

Abstract

AgInSe₂ is a promising direct bandgap thin-film material with a rare n-type conductivity. Similar to thin film photovoltaic materials such as Cu(In,Ga)Se₂ (CIGSe), which have achieved efficiencies as high as ∼23%, AgInSe₂ also crystallizes in a chalcopyrite phase while also being more tolerant to antisite defects due to higher defect formation energies resulting from more significant variations in cation sizes. AgInSe₂ has a suitable bandgap of 1.24 eV, which lies in the high-efficiency region of the detailed balance limit. In this work, we have utilized a dimethyl formamide–thiourea–chloride-based solution-processed route to deposit a thin film of AgInS₂ which is converted into AgInSe₂ after a heat-treatment step in a selenium environment. We observed that AgInSe₂ optoelectronic properties depend on the Ag/In ratio and the selenium heat-treatment conditions. Significant improvements in photoluminescence yield and lifetime are observed for Ag-poor films in selenium-rich conditions. X-ray photoelectron spectroscopy (XPS) measurements confirm a higher amount of selenium on the surface of films with improved optoelectronic properties. Furthermore, a high minority carrier lifetime of 9.2 ns and a photoluminescence quantum yield (PLQY) of 0.013% are obtained without any passivating layer, which improved to 0.03% after CdS passivation. Hall effect measurements confirm that AgInSe₂ has n-type conductivity with a moderate carrier concentration (10⁻¹⁴ cm⁻³), more suitable for a p–i–n architecture. XPS has further confirmed the moderate n-type conductivity.