Michele Melchiorre

Potassium Fluoride Ex Situ Treatment on Both Cu-Rich and Cu-Poor CuInSe2 Thin Film Solar Cells

In this paper, we show that CuInSe2 (CIS) absorbers grown under Cu-excess have better collection efficiencies compared to Cu-poor ones. We also show that an ex situ potassium fluoride postdeposition treatment leads to an improvement in VOC for CIS absorbers grown under both Cu-excess and Cu-poor conditions. Additionally, for absorbers grown under Cu-excess, the junction breakdown, which is observed in reverse bias of untreated cells, is removed. This improvement is based mainly on improving the interface of the CIS absorber grown under Cu-excess to the cadmium sulphide buffer layer through moving the dominant recombination from the interface to the bulk. In contrast to observations in the literature, the treated surface is not completely Cu-free.

Doping mechanism in pure CuInSe2

We investigate the dopant concentration and majority carrier mobility in epitaxial CuInSe2thin films for different copper-to-indium ratios and selenium excess during growth. We find that all copper-poor samples are n-type, and that hopping conduction in a shallow donor state plays a significant role for carrier transport. Annealing in sodium ambient enhances gallium in-diffusion from the substrate wafer and changes the net doping of the previously n-type samples to p-type. We suggest that sodium incorporation from the glass might be responsible for the observed p-type doping in polycrystalline Cu-poor CuInSe2 solar cell absorbers.

Space-charge-limited currents in CIS-based solar cells

Non-linear shunts in Cu(In,Ga)Se2 solar cells have been well described mathematically using the model of a space-charge-limited current, but their physical origin remained unclear so far. We study space-charge-limited currents on Cu-rich CuInSe2 (CIS) devices, which represent a very suitable system: the devices always exhibit non-linear shunts with a very pronounced behavior. Here, we demonstrate a fundamental difference in the transport mechanism between the Cu-rich-based device and the conventional Cu-poor one. We discuss the location of a space-charge-limited current by comparing devices containing various component layers with Ohmic contacts. We confirm that Cu-rich CIS and cadmium sulfide layers alone do not create a non-linear shunt. Our experimental results demonstrate that the origin of the non-linear behavior is located at the interface between the absorber and buffer layers. Temperature dependent current-voltage measurements performed on Cu-rich-based CIS devices are discussed in agreement with ...

Understanding quaternary compound Cu2ZnSnSe4 synthesis by microscopic scale analyses at an identical location

The synthesis of multinary compound films from layered precursors is only partially understood. Identical location microscopy resolves the multi-step synthesis of Cu2ZnSnSe4 from metallic stacks on the micron-scale. Large scale metal alloying and the seemingly illogical observation that ZnSe segregates preferentially on locations previously poor of zinc are revealed.

Correction to “Potassium Fluoride ex-situ Treatment on both Cu-rich and Cu-poor CuInSe2 Thin Film Solar Cells” [Mar 17 684-689]

Presents corrections to the paper, “Potassium fluoride ex situ treatment on both Cu-Rich and Cu-Poor CuInSe2 thin film solar cells,” (Elanzeery, H., et al), IEEE J. Photovolt., vol. 7, no. 2, pp. 684–689, Mar. 2017.

Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers

Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.Sodium doping is necessary to achieve high performance in polycrystalline chalcopyrite solar cells, but retards gallium interdiffusion, and thus efficiency optimisation. Here, Colombara et al. show that in contrast to the polycrystalline case, sodium accelerates atomic interdiffusion in monocrystalline samples.