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The variation of the sodium content in low-temperature grown Cu(In,Ga)Se 2 (CIGSe) absorbers has a strong influence on the doping level and the photoelectrical properties of the bare CIGSe films and corresponding solar cells. The negative impact of excessive sodium contents is studied for samples from CIGSe processes with co-evaporation of NaF during absorber deposition. By a combined analysis of temperature dependent current–voltage, capacitance-voltage and quantum efficiency measurements we conclude that the dominating recombination takes place in the bulk of the absorber for all samples independent of the sodium content. The optimum sodium content with respect to solar cell performance results from a tradeoff between the beneficial influence of sodium on open circuit voltage ( V OC ) and the negative impact of high sodium contents on short circuit current ( I SC ) and fill factor ( FF ). The change of V OC , I SC and FF can be explained by a concurrent increase of net doping and deep defect density for increasing sodium content and the presence of barriers at the CIGSe/CdS and the CIGSe/Mo interface. [ABSTRACT FROM AUTHOR]

In this study strong evidence for an illumination-induced change in minority charge carrier diffusion length is given for Cu(In,Ga)Se 2 solar cells. After annealing under illumination (light soaking) the cells show the well-known metastable increase in open circuit voltage, but also a metastable reduction in current collection efficiency (which can be reversed by annealing in the dark). Partly, this can be attributed to an increase in doping density causing a reduced space charge region width as verified by capacitance-voltage profiling. Nevertheless, by using time-resolved photoluminescence and electron-beam-induced current measurements we found that the changes in doping density and space charge region width are not sufficient to describe the modification in current collection efficiency. Additionally there seems to be a reduction in minority carrier diffusion length and lifetime after white light soaking. This can be explained by a metastable change of electronic defects as found in temperature-dependent admittance spectroscopy. Device simulations confirm the impact of the found defects on the photocurrent. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se 2 thin film solar cells are known to show a metastable behavior of their open circuit voltage when exposed to light. Time resolved measurements of the photoluminescence decay after pulsed excitation (TRPL) reveal that also the photoluminescence decay (PL) changes with conditioning, yielding shorter effective lifetimes after annealing under illumination (light soaking) than after annealing in the dark. By using time resolved device simulations we applied a model that explains the TRPL metastable behavior. For this we used a model of capture and reemission of minority charge carriers via two characteristic trap states near the conduction band, while also accounting for changes in trap density and doping concentration caused by light soaking. To further understand the mechanisms behind PL decay we analyse the influence that the charge carrier dynamics play in TRPL decay by a variation of select simulation parameters. [ABSTRACT FROM AUTHOR]