Alkali Treatments for Solution-Processed Chalcopyrite Photovoltaics Fabricated from Colloidal Nanoparticle Inks

Abstract

Today, most of the worldwide energy demand is being met by the use of non-renewable carbon-based fossil fuels that are harmful to the environment and contribute to climate change. With rising populations and rising standards of living worldwide, the demand for energy is expected to rise, increasing the need for sustainable and environmentally-friendly energy sources. Photovoltaics has the most potential of all renewable energy sources in fulfilling the worldwide energy demand in the future.The worldwide photovoltaics market is dominated today by the use of silicon-based photovoltaics. In recent years, thin-film Cu(In, Ga)(S, Se)2 (CIGSe, CIGSSe) has attracted attention as a feasible solar cell absorber material due to its favorable properties such as a high light absorption coefficient and tunable bandgap. Today, CIGSe and CIGSSe solar cells are mainly fabricated through the use of costly and resource-intensive vacuum-based routes, limiting its potential for large-scale utilization. However, solution-based routes towards CIGSe and CIGSSe manufacturing have emerged as a potentially low-cost alternative to vacuum-based CIGSe and CIGSSe manufacturing.One of the strategies implemented in high-quality CIGSe film and device fabrication is alkali treatments. Sodium treatments are widely used in CIGSe and CIGSSe processing, and are required for high efficiency CIGSe and CIGSSe devices. Furthermore, the use of heavy alkali (K, Rb, Cs) post-deposition treatments in vacuum-based CIGSe fabrication have a resulted in a substantial increase in CIGSe device performance. Despite their beneficial effects, their use on solution-processed CIGSe (CIGSSe) films and devices remains limited. In this work, the effects of different alkalies on solution-processed nanoparticle-based CIGSSe films and devices are investigated and analyzed.Starting with potassium, introduction of K treatments to solution-processed CIGSSe films selenized from oleylamine-capped colloidal sulfide-based Cu(In, Ga)S2 nanoparticle inks resulted in enhancements in the selenization and grain growth of CIGSSe films. Furthermore, X-ray photoelectron spectroscopy (XPS) on the as-selenized films shows evidence of the presence of a high bandgap K-In-Se phase on CIGSSe films that were treated with potassium. Moreover, solar cell devices fabricated from CIGSSe films that were treated with both sodium and potassium showed significantly enhanced performance.Moving onto low carbon CIGSSe films that are selenized from sulfide-capped Cu(In, Ga)S2 (CIGS) nanoparticle films, significant growth resistance was observed for the sulfide-capped CIGS nanoparticles, resulting in selenized and annealed films that are characterized by a thin-coarsened layer and a significantly thicker fine grain layer that is mainly composed of metals that did not incorporate into the growth front. By introducing sodium alkali treatments to the sulfide-capped films, significant enhancements to the grain growth were observed, resulting in fully-grown low-carbon CIGSSe films. It was also found that the use of an alkali treatment, prior to selenization and growth, is a requirement for sufficient growth of sulfide-capped CIGS nanoparticle films into coarsened CIGSSe films needed for high quality devices. Furthermore, rubidium alkali treatments on sulfide-capped CIGS nanoparticle films were also found to be effective in their growth-assisting ability. Moreover, increased PL response was observed with CIGSSe films that were treated with Rb prior to growth. The results and observations presented in this work provide an avenue towards enhancing the performance of solution-processed nanoparticle-based Cu(In, Ga)(S, Se)2 solar cells.