A new model for the formation of heterojunctions in polycrystalline CuInSe2 thin films on the basis of surface analysis experiments is presented. In situ photoemission measurements of CuInSe2 clearly show the existence of an In-rich n-type surface layer on samples relevant for solar-cell devices. Furthermore, this layer has been identified as an ordered vacancy compound (OVC) with a band gap of about 1.3 eV. The previous model of the CuInSe2/CdS solar cell with a p-n heterojunction between p-type CuInSe2 and n-type CdS is replaced by the model of a chalcopyrite/defect chalcopyrite heterojunction between p-type bulk CuInSe2 and the In-rich n-type OVC. The existence of this junction was proven directly by evaporating an ohmic metal contact onto the surface n-type layer and measuring the spectral quantum efficiency and electron-beam-induced current of this device. The band offsets of CuInSe2-based devices have been determined.

This article presents a systematic study on the electronic transport mechanisms of CuGaSe2-based thin film solar cells. A variety of samples with different types of stoichiometry deviations, substrates and buffer layers is investigated. We propose two transport models, namely tunneling enhanced volume recombination and tunneling enhanced interface recombination, which allow to explain the observed features for all devices under consideration. The doping level of the absorber layer turns out to be the most decisive parameter for the electronic loss mechanism. The doping is influenced by the type 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 assumptions on the dominant loss mechanism in the different devices.

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CuGaSe 2 /CdS/ZnO heterostructures with different CuGaSe2 stoichiometry deviations, glass substrates with different Na content and varying CdS buffer deposition procedures are analyzed with admittance spectroscopy, deep level transient spectroscopy, and capacitance–voltage measurements. Cu-rich CuGaSe2 exhibits two acceptor-like bulk traps with activation energies of 240 and 375 meV. The density of both defect states is reduced by air annealing at 200 °C. Ga-rich CuGaSe2 material displays a tail-like energetic distribution of acceptor defects. The maximum of this distribution is at an energy of 250 meV. Defect densities and doping concentrations of Ga-rich material are considerably lower than in Cu-rich material. The different defect and doping densities found in the present investigation fully explain the efficiency gain which has recently been made by changing the material stoichiometry, the glass substrate and the CdS-deposition method for CuGaSe2-based thin film solar cells.