Potassium Treatments for Solution-Processed Cu(In,Ga)(S,Se)₂ Solar Cells

Abstract

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.

Present Status of Solution‐Processing Routes for Cu(In,Ga)(S,Se)2 Solar Cell Absorbers

“…While these two specific ink types are typically distinguished in literature (colloidal inks and true solutions), hybrid methods have been reported as well. [ 60 ] To deposit the precursor ink, several solution deposition techniques can be utilized, such as doctor blade, inkjet, [ 61 ] dip‐ and spin‐coating, [ 62,63 ] on a small scale while spray‐pyrolysis, [ 64 ] screen printing, curtain‐coating, slot‐, slit‐, and die‐ casting, [ 58,65 ] commonly finds application for larger area substrates (see Figure 1 ). Notably, the inherently colloidal‐free nature of molecular ink precursors allows for a wide choice of deposition methods minimizing potential challenges such as clogging of nozzle heads (e.g., inkjet, spray coating, etc.).…”

Extrinsic Doping of Ink‐Based Cu(In,Ga)(S,Se)2‐Absorbers for Photovoltaic Applications

“…The authors further reported thinner MoSe 2 layers with the addition of Na at the CIGSSe/Mo interface (reference device MoSe 2 thickness >1 µm) reducing interfacial resistance between the CIGSSe film and Mo substrate caused by denser absorber morphology delayed selenium diffusion. [ 209 ] However, as opposed to many previous studies for vacuum CIGSSe, [ 89,204,208,209 ] the majority carrier concentration decreased from 7.2 × 10 17 to 1.8 × 10 17 cm −3 with Na addition (photoelectrochemical measurements). Subsequently, total area (TA) efficiencies of 10.7% (with anti‐reflective coating (ARC)) were achieved for Na‐treated CIGSSe devices.…”

“…[ 40,85 ] Several ink coating techniques are available, including doctor blade, spin‐coating, screen printing, curtain‐coating, and die‐casting, amongst others. [ 40,86–89 ] Broadly, the liquid‐based (or non‐vacuum) deposition processes can be performed either by immersing the substrate in a precursor solution (e.g., chemical bath deposition (CBD)), [ 90 ] electrodeposition (ED) [ 91,92 ] ) or by coating the substrate with a precursor ink (IDR). [ 40 ] For recent comprehensive reviews of the different IDRs (both molecular ink (MI) and NP based), the reader is referred to.…”

“…For CIGSSe/CISSe PV technology, the extrinsic (or foreign atom) elements alter and tune the: i) free carrier concentration, charge carrier diffusion length and lifetime, [ 136,137 ] ii) absorber growth (e.g., Ga and Cu depletion at the surface and changes in the intermixing of gallium and indium), [ 34,138–140 ] and morphology (e.g., morphological changes resulting in CdS (buffer layer) nucleation sites, and changes in the grain size), [ 25,59,67,141 ] iii) grain interior (GI, e.g., changes in the surface potential) and grain boundary (GB, e.g., decreased tail state formation due to reduced band bending), [ 51,55,59,120,121,142 ] iv) concentration of traps or deep levels, [ 132–134,143–147 ] and v) absorber surface (e.g., surface bandgap of KF‐treated CIGSSe absorbers are larger than that of untreated reference absorber layers), [ 25,71,89,148,149 ] without altering the parent diamond‐like tetragonal crystal structure. The use of extrinsic elements combined with bandgap engineering and optically wider buffer layers (compared to CdS) has boosted PCEs beyond 21.0%.…”

“…In MI‐based routes, extrinsic impurities may be incorporated into the films by i) pre‐deposition treatments, for example dipping the film in an alkali‐containing solution, [ 134,135,188 ] ii) during the deposition itself, for example by adding the dopant compounds directly into the precursor ink, [ 133–135 ] and/or iii) post‐deposition treatments, for example annealing the as‐deposited precursor film with the dopant compound (often an ex situ process requiring exposure to ambient conditions) in Se vapor. [ 89 ] In the pre‐deposition treatments, the dopants are introduced prior to absorber deposition. This implies that dopants will be present during the liquid film deposition (e.g., when using Mo:Na substrates) and/or the entirety of the high temperature (450–600 °C) selenization and grain growth step.…”