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The influence of accelerated stress testing on the electrical properties of Cu(In,Ga)(S,Se)2 thin-film solar cells is presented. Although encapsulated modules are stable, extended exposure of unencapsulated cells to damp heat (144 h, 264 h and 438 h) at 85°C and 85% humidity leads to a reduction of the fill factor and the open-circuit voltage. We further analyzed these changes by applying admittance spectroscopy, deep-level transient spectroscopy and current–voltage measurements. Damp heat stress reduces the net doping concentration of the absorber material and shifts a dominant defect state close to or at the buffer/absorber interface to higher activation energies. By comparing experimentally observed changes with the SCAPS device simulation program several possible causes for the observed changes in electrical properties are discussed: decrease of ZnO resistivity; decrease of the density of interface states; increase of shallow acceptor concentration and the introduction of deep acceptors in the Cu(In,Ga)(S,Se)2 absorber. [Copyright &y& Elsevier]

Abstract: Simulations of photovoltaic devices provide a promising tool for the exploration of internal loss mechanisms and assessment of optimization potentials. The knowledge of the internal wave propagation and local photon absorption rate is a fundamental prerequisite for modeling Cu(In,Ga)(Se,S)2 thin film solar cells by means of a sophisticated Technology Computer Aided Design (TCAD) device simulator. In a first step, we applied variable-angle spectroscopic ellipsometry to derive the optical constants of the involved layers, namely the doped and undoped zinc oxide, the In2S3 buffer, the MoSe2 and the molybdenum film. These data enter for a semi-coherent calculation of the wave propagation and of the local generation rate in the thin film stack. Scattering effects due to interface roughness were considered appropriately and the TCAD-simulated photogeneration and reflection spectra were compared with measured quantum efficiencies and reflection spectra, respectively. [Copyright &y& Elsevier]

Abstract: We investigated industrially produced chalcopyrite solar cells based on the absorber modifications Cu(In,Ga)Se2 and Cu(In,Ga)(S,Se)2 in order to study the nature of the experimentally verified efficiency improvement, mainly caused by an increased open circuit voltage. We show that the introduction of sulfur during the absorber formation via rapid thermal processing leads to a substantial lowering of the surface doping concentration and widening of the space charge region (SCR). Temperature dependent diode analysis revealed a reduction of the SCR recombination in (Se,S) devices which would lead to a larger splitting of quasi-Fermi levels and hence to an increased open circuit voltage as compared to neat Cu(In,Ga)Se2 devices. [Copyright &y& Elsevier]

We investigated the influence of different Cu and Na contents on the electrical properties of Cu(In,Ga)(S,Se)2 solar cells. The devices were characterized applying admittance spectroscopy and deep-level transient spectroscopy (DLTS). We observed four kinds of deep defect states in differing concentrations in the stoichiometry-varied samples: two acceptor-like deep-level states, one donor-like trap, and a midgap center. In order to characterize the properties of these defect states, we recorded the DLTS capacitance transients of a Na poor sample with different pulse widths (ranging from 100 μs to 1 s duration) at selected fixed temperatures. We discuss the properties of the defect states. [Copyright &y& Elsevier]