Employing Si solar cell technology to increase efficiency of ultra-thin Cu(In,Ga)Se sub(2) solar cells

• Published in: Progress in photovoltaics Vol. 22; no. 10; pp. 1023 - 1029
• Main Authors: Vermang, Bart, Waetjen, Jorn Timo, Fjaellstrom, Viktor, Rostvall, Fredrik, Edoff, Marika, Kotipalli, Ratan, Henry, Frederic, Flandre, Denis
• Format: Journal Article
• Language: English
• Published: 01.10.2014
• Subjects: ALUMINUM OXIDE; CIGS; COPPER INDIUM SELENIDE; COPPER SELENIDE; MICROSTRUCTURES; Nanostructure; PASSIVATION; Photovoltaic cells; Point contact; Reflection; Solar cells; Surface chemistry
• ISSN: 1062-7995; 1099-159X
• DOI: 10.1002/pip.2527

Reducing absorber layer thickness below 500nm in regular Cu(In,Ga)Se sub(2) (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF sub(2) coated with a thin atomic layer deposited Al sub(2)O sub(3) layer, or direct current magnetron sputtering of Al sub(2)O sub(3) are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al sub(2)O sub(3)/CIGS rear interface. (MgF sub(2)/)Al sub(2)O sub(3) rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells. copyright 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd. Reducing absorber layer thickness below 500nm in regular Cu(In,Ga)Se sub(2) solar cells decreases cell efficiency considerably: both short-circuit current and open-circuit voltage are reduced, respectively due to incomplete absorption and high Mo/Cu(In,Ga)Se sub(2) rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contacts is used to enhance rear internal reflection and decrease rear surface recombination velocity significantly.