Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells

Abstract

Compared with their rigid counterparts, thin-film solar cells grown on flexible substrates usually display lower power-conversion efficiencies. Now, the application of a post-deposition alkaline treatment that modifies the chemical composition of the surfaces of Cu(In,Ga)Se2 thin films reduces optical losses in these flexible photovoltaic architectures. Furthermore, efficiencies comparable to solar cells based on polycrystalline silicon are achieved. Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20% (refs 1, 2). This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices3. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance4,5. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters6. Ion exchange processes, well known in other research areas7,8,9,10,11,12,13, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.