Doping, Defects And Solar Cell Performance Of Cu-rich Grown CuInSe2

Cu-rich grown CuInSe2 thin-film solar cells can be as efficient as Cu-poor ones. However record lab cells and commercial modules are grown exclusively under Cu-poor conditions. While the Cu-rich material’s bulk properties show advantages, e.g. higher minority carrier mobilities and quasi-Fermi level splitting - both indicating a superior performance - it also features some inherent problems that led to its widespread dismissal for solar cell use. Two major challenges can be identified that negatively impact the Curich’s performance: a too high doping density and recombination close to the interface. In this work electrical characterisation techniques were employed to investigate the mechanisms that cause the low performance. Capacitance measurements are especially well suited to probe the electrically active defects within the space-charge region. Under a variation of applied DC bias they give insights into the shallow doping density, while frequency and temperature dependent measurements are powerful in revealing deep levels within the bandgap.
CuInSe2 samples were produced via a thermal co-evaporation process and subsequently
characterized utilizing the aforementioned techniques. The results have been grouped into
two partial studies. First the influence of the Se overpressure during growth on the shallow
doping and deep defects is investigated and how this impacts solar cell performance.
The second study revolves around samples that feature a surface treatment to produce a
bilayer structure - a Cu-rich bulk and a Cu-poor interface.
It is shown that via a reduction of the Se flux during absorber preparation the doping
density can be reduced and while this certainly benefits solar cell efficiency, a high deficit
in open-circuit voltage still results in lower performance compared to the Cu-poor devices.
Supplementary measurements trace this back to recombination close to the interface. Furthermore a defect signature is identified, that is not present in Cu-poor material. These
two results are tied together via the investigation of the surface treated samples, which do
not show interface recombination and reach the same high voltage as the Cu-poor samples.
The defect signature, normally native to the Cu-rich material, however is not found in the surface treated samples. It is concluded that this deep trap acts as a recombination centre close to the interface. Shifting it towards the bulk via the treatment is then related to the observed increase in voltage. Within this thesis a conclusive picture is derived to unite all measurement results and show the mechanisms that work together and made it possible to produce a high efficient Cu-rich thin-film solar cell.