Impact of compositional grading and overall Cu deficiency on the near‐infrared response in Cu(In, Ga)Se2 solar cells. (9th December 2016)

• Record Type: Journal Article
• Authors: Avancini, Enrico; Carron, Romain; Bissig, Benjamin; Reinhard, Patrick; Menozzi, Roberto; Sozzi, Giovanna; Di Napoli, Simone; Feurer, Thomas; Nishiwaki, Shiro; Buecheler, Stephan; Tiwari, Ayodhya N.
• Is Part Of: Progress in photovoltaics. Volume 25:Number 3(2017)
• Page Start: 233
• Page End: 241
• Publication Date: 2016-12-09
• Subjects: Cu(In, Ga)Se2 solar cells -- NIR optical response -- Cu deficiency in CIGS; Solar cells -- Periodicals; Photovoltaic cells -- Periodicals; Solar power plants -- Periodicals; 621.31245
• DOI: 10.1002/pip.2850
• Languages: English
• ISSNs: 1062-7995

Abstract

Abstract: Highly efficient thin film solar cells based on co‐evaporated Cu(In, Ga)Se2 (CIGS) absorbers are typically grown with a [Ga]/([Ga] + [In]) (GGI) gradient across the thickness and a Cu‐poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased absorption in the near‐infrared (NIR), diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the NIR response. In this contribution [Cu]/([In] + [Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometric point. In order to obtain comparable Ga contents at the interface for proper band alignment, the front GGI gradings were actively modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. By characterizing the modified absorber properties by reflection‐transmission spectroscopy, we attribute the observed behavior to changes in the optical properties rather than to improved carrier collection. Cu‐dependent modifications of the NIR‐absorption coefficients are likely to be responsible for the variations in the optical properties, which is supported by device simulations. Adequate re‐adjustments of the co‐evaporation process and of the alkali‐fluorides post‐deposition treatments allow maintaining Voc and FF values, yielding an overall increase of efficiency as compared to a reference baseline. © 2016 The Authors. Progress in Photovoltaics : Research and Applications published by John Wiley & Sons Ltd. Abstract : An improved response in the NIR‐EQE of CIGS solar cells is observed upon increasing Cu concentration in the Ga‐graded CIGS absorber. This is found to be because of changes in the NIR absorption coefficient upon varying composition, rather than to an improved collection function. The improved optical properties can be exploited to increase the short‐circuit current density and therefore the efficiency.