Voids and compositional inhomogeneities in Cu(In,Ga)Se2 thin films: evolution during growth and impact on solar cell performance

• Main Authors: Enrico Avancini, Debora Keller, Romain Carron, Yadira Arroyo-Rojas Dasilva, Rolf Erni, Agnieszka Priebe, Simone Di Napoli, Martina Carrisi, Giovanna Sozzi, Roberto Menozzi, Fan Fu, Stephan Buecheler, Ayodhya N. Tiwari
• Format: Article
• Language: English
• Published: Taylor & Francis Group 2018-12-01
• Series: Science and Technology of Advanced Materials
• Subjects: Cu(In, Ga)2; multistage coevaporation; STEM/EDX
• Online Access: http://dx.doi.org/10.1080/14686996.2018.1536679

Structural defects such as voids and compositional inhomogeneities may affect the performance of Cu(In,Ga)Se2 (CIGS) solar cells. We analyzed the morphology and elemental distributions in co-evaporated CIGS thin films at the different stages of the CIGS growth by energy-dispersive x-ray spectroscopy in a transmission electron microscope. Accumulation of Cu-Se phases was found at crevices and at grain boundaries after the Cu-rich intermediate stage of the CIGS deposition sequence. It was found, that voids are caused by Cu out-diffusion from crevices and GBs during the final deposition stage. The Cu inhomogeneities lead to non-uniform diffusivities of In and Ga, resulting in lateral inhomogeneities of the In and Ga distribution. Two and three-dimensional simulations were used to investigate the impact of the inhomogeneities and voids on the solar cell performance. A significant impact of voids was found, indicating that the unpassivated voids reduce the open-circuit voltage and fill factor due to the introduction of free surfaces with high recombination velocities close to the CIGS/CdS junction. We thus suggest that voids, and possibly inhomogeneities, limit the efficiency of solar cells based on three-stage co-evaporated CIGS thin films. Passivation of the voids’ internal surface may reduce their detrimental effects.