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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. 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% is 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. 

The enargite phase of Cu3AsS4 (ENG) is an emerging photovoltaic material with a ∼1.4 eV bandgap and is composed of earth abundant elements with favorable defect properties arising from the differing ionic radii of the constituent elements. Unfortunately, ENG-based photovoltaic devices have experimentally been shown to have low power conversion efficiencies, possibly due to defects in the material. In this joint computational and experimental study, we explore the defect properties of ENG and employ synthesis approaches, such as silver alloying, to reduce the density of harmful defects. We show that shallow copper vacancies (VCu) are expected to be the primary defects in ENG and contribute to its p-type character. However, as shown through photoluminescence (PL) measurements of synthesized ENG, a large mid-bandgap PL peak is present at ∼0.87 eV from a band edge, potentially caused by a copper- or sulfur-related defect. To improve the properties of ENG films and mitigate the mid-bandgap PL, we employed an amine-thiol molecular precursor-based synthesis approach and utilized silver alloying of ENG films. While silver alloying did not affect the mid-bandgap PL peak, it increased grain size and lowered film porosity, improving device performance. In conclusion, we found that incorporating silver such that [Ag]/([Ag] + [Cu]) is 0.05 in the film using an amine-thiol based molecular precursor route with As2S3 as the arsenic source resulted in improved photovoltaic device performance with a champion device of efficiency 0.60%, the highest reported efficiency for an Cu3AsS4 (ENG)-based device to date. 

Solution processing of CuInSe 2 /CuInGaSe 2 (CISe/CIGSe) photovoltaic devices via non-hydrazine based routes has been studied for the past few years and a significant improvement in the device performance has been achieved for multiple solvent routes. However, none of these routes have ever reported the fabrication of absorbers with a thickness of above 1.2–1.3 microns which is almost half of what has been traditionally used in vacuum based high efficiency CIGSe devices. The main reason for this limitation is associated with the formation of a fine-grain layer in solution based systems. Here we manipulate the formation of such a fine-grain layer in an amine–thiol based solution route through surface modifications at the bottom Mo interface and achieve an active area efficiency of up to 14.1% for CIGSe devices. Furthermore, with a detailed analysis of the fine-grain layer, not just in the amine–thiol based film, but also in the film fabricated via the dimethylformamide-thiourea route, we identify the reason for the formation of such a fine-grain layer as the presence of the sulfide material and carbon impurity (if any) in the precursor film. We utilize the amine–thiol solvent system's ability for selenium and metal selenide dissolution to manipulate the ink formulations and demonstrate the reduction in the formation of sulfide materials as well as the extent of trapped carbon in the precursor film. With modified precursor films, we then successfully grow CISe/CIGSe thin films of 2-micron thickness with the complete absence of a fine-grain layer through a high temperature, thickness independent bulk growth mechanism making the film morphology similar to the one fabricated using a high efficiency hydrazine based route.