The solution processing of Cu­(In,Ga)­(S,Se)2 photovoltaics from colloidal nanoparticles has long suffered from deleterious carbonaceous residues originating from long chain native ligands. This impurity carbon has been observed to hinder grain formation during selenization and leave a discrete residue layer between the absorber layer and the back contact. In this work, organic and inorganic ligand exchanges were investigated to remove tightly bound native oleylamine ligands from Cu­(In,Ga)­S2 nanoparticles, thereby removing the source of carbon contamination. However, incomplete ligand removal, poor colloidal stability, and/or selective metal etching were observed for these methods. As such, an exhaustive hybrid organic/inorganic ligand exchange was developed to bypass the limitations of individual methods. A combination of microwave-assisted solvothermal pyridine ligand stripping followed by inorganic capping with diammonium sulfide was developed and yielded greater than 98% removal of native ligands via a rapid process. Despite the aggressive ligand removal, the nanoparticle stoichiometry remained largely unaffected when making use of the hybrid ligand exchange. Furthermore, highly stable colloidal ink formulations using nontoxic dimethyl sulfoxide were developed, supporting stable nanoparticle mass concentrations exceeding 200 mg/mL. Scalable blade coating of the ligand-exchanged nanoparticle inks yielded remarkably smooth and microcrack free films with an RMS roughness less than 7 nm. Selenization of ligand-exchanged nanoparticle films afforded substantially improved grain growth as compared to conventional nonligand-exchanged methods, yielding an absolute improvement in device efficiency of 2.8%. Hybrid ligand exchange nanoparticle-based devices reached total area power conversion efficiencies of 12.0%, demonstrating the feasibility and promise of ligand-exchanged colloidal nanoparticles for the solution processing of Cu­(In,Ga)­(S,Se)2 photovoltaics.

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Cu(In,Ga)(S,Se) 2 (CIGSe, CIGSSe) has emerged as an attractive thin-film solar cell absorber material owing to its high light absorption coefficient and tunable bandgap. In CIGSSe processing and fabrication, the use of alkali treatments has been implemented as sodium doping is considered a requirement for obtaining high-efficiency CIGSSe solar cell devices and has been used extensively. One of the more significant developments in recent years has been the discovery of the beneficial effects that potassium post-deposition treatments have on vacuum-processed CIGSSe solar cells as they are responsible for a major increase in CIGSSe solar cell performance. Here, we conduct a study of the effect of potassium treatments on solution-processed CIGSSe films grown from colloidal sulfide-based nanoparticle inks. By adding potassium through e-beam evaporation of potassium fluoride (KF) prior to selenization and grain growth, we find that the grain growth of CIGSSe is enhanced with potassium addition and that a larger-grained film results compared to the untreated selenized CIGSSe film, similar to that observed in sodium-treated films. We also observe through X-ray photoelectron spectroscopy (XPS) that films treated with K show the presence of the high-bandgap K−In−Se surface phase. Upon fabricating the devices, we find that films that have been subjected simultaneously to both sodium and potassium treatments have enhanced optoelectronic performance, mainly manifested in the higher open-circuit voltage and higher short-circuit current.

forest fires, and melting glaciers: it is evident that global over-reliance on fossil fuels must shift in favor of carbonfree energy sources to mitigate climate change. [1][2][3] Global energy consumption in 2020 was 162 Petawatt hours (PWh), out of which the electricity consumption was ≈30 PWh. [2,4] It is predicted that by 2050, an additional 30 TW will be required to meet mankind's increasing power demands. [5][6][7] The sun is an inexhaustible clean energy source transmitting nearly 120 000 TW to the earth's surface, far greater in magnitude than all other renewable energy sources combined. [8,9] While this power is abundantly available, harnessing it on terawatt scales with reliable and cost-efficient methods poses a tremendous challenge. [9] Photovoltaics (PVs) harvest the sun's energy by directly converting photons into electricity without the release of greenhouse gases (GHGs). PV systems are reliable, silent (no moving parts), and operate on a zero-cost energy source. These advantages coupled with the falling prices of energy storage systems suggest that PV systems can provide a continuous supply of electricity for residential and commercial applications. [8,9] The levelized cost of energy (LCOE) can be used to evaluate the cost-effectiveness of different energy sources. [10,11] For PV systems, LCOE denotes the discounted lifetime costs associated with the PV system installation divided by the discounted lifetime energy production of the system (typically ≈ 25 years). In sunny regions, the LCOE for utility-scale PV (29 $/MWh) now competes with electricity generated from conventional sources

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