Electronic properties of CuGaSe2-based heterojunction solar cells. Part I.Transport analysis

This article presents a systematic study on the electronic transport mechan isms of CuGaSe2-based thin film solar cells. A variety of samples with diff erent types of stoichiometry deviations, substrates and buffer layers is in vestigated. We propose two transport models, namely tunneling enhanced volu me recombination and tunneling enhanced interface recombination, which allo w to explain the observed features for all devices under consideration. The doping level of the absorber layer turns out to be the most decisive param eter for the electronic loss mechanism. The doping is influenced by the typ e of stoichiometry deviation as well as by the Na content of the substrate. High doping levels result in tunnel assisted recombination. The best solar cells display the lowest tunneling rates. For these devices treatments of the absorber surface by air-annealing and/or the deposition temperature of the CdS buffer layer are decisive for the final device performance. We use the investigation of the open-circuit voltage relaxation to verify the assu mptions on the dominant loss mechanism in the different devices. (C) 2000 A merican Institute of Physics. [S0021-8979(00)05801-1].