RbF Postdeposition Treatment Research Articles

Group III Elemental Composition Dependence of RbF Postdeposition Treatment Effects on Cu(In,Ga)Se2 Thin Films and Solar Cells

The effects of RbF postdeposition treatment (RbF-PDT) on Cu(In,Ga)Se2, CuInSe2, and CuGaSe2 thin films and solar cell devices are comparatively studied. Similar to the effect of the KF postdeposition treatment (KF-PDT), Cu(In,Ga)Se2 and CuInSe2 film surfaces show significant pore formation resulting in a rough surface morphology with RbF-PDT, whereas this is not the case for In-free CuGaSe2. The device properties of the In-containing and In-free Cu(In,Ga)Se2 solar cells also show contrasting results, namely, Cu(In,Ga)Se2 or CuInSe2 devices show an increase in the open circuit voltage (Voc) and fill factor (FF) values and almost constant or a slight decrease in the short-circuit current density (Jsc) values with RbF-PDT, whereas CuGaSe2 devices show no significant improvements in the Voc and FF values but a substantial increase in the Jsc values. These results suggest that the alkali effects on the Cu(In,Ga)Se2 film and device properties strongly depend on the group III elemental composition in the Cu(In,G...

Evaluation of recombination losses in thin film solar cells using an LED sun simulator − the effect of RbF post-deposition on CIGS solar cells

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses.

Injection Current Barrier Formation for RbF Postdeposition‐Treated Cu(In,Ga)Se2‐Based Solar Cells

AbstractAmong the thin‐film solar cell technologies, Cu(In,Ga)Se2‐based solar cells demonstrate the highest efficiencies, where the recent boost in efficiency is triggered by a KF postdeposition treatment (PDT). In this contribution, Cu(In,Ga)Se2‐based solar cells are fabricated using RbF PDTs after absorber layer growth with varying substrate and RbF source temperature. The electronic charge transport properties of the solar cell devices are investigated using temperature‐dependent current–voltage analysis and admittance spectroscopy. To investigate the observed transport barriers, a novel concept based on the differential series resistance is proposed. This approach is supported by simulations of current–voltage curves, which reproduce qualitatively experimental data. Experimentally, two parallel conduction paths are found, which act as barriers with different activation energies and impede the charge carrier transport. Both the thickness and height of these barriers increase with an increasing amount of incorporated Rb and can lead to losses in the fill factor and power conversion efficiency at room temperature. Etching in HCl prior to CdS buffer layer deposition reduces the barrier width and can recover these losses.

Impact of a RbF Postdeposition Treatment on the Electronic Structure of the CdS/Cu(In,Ga)Se2 Heterojunction in High-Efficiency Thin-Film Solar Cells

Recently, a world-record efficiency of 22.6% was achieved by applying a RbF postdeposition treatment (PDT) on a Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorber surface. Here, we study the impact of this RbF-PDT on the electronic structure of the CIGSe surface and the CdS/CIGSe interface using ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS), as well as inverse photoemission spectroscopy (IPES). After RbF-PDT, we find a small downward shift of the band edges, while the surface band gap value itself is not affected. In addition, a further downward band bending in the CIGSe absorber is observed upon formation of the RbF-PDT CdS/CIGSe interface. We derive a flat conduction band alignment between the RbF-PDT CIGSe absorber and the CdS buffer, commensurate with the high efficiencies of solar cell devices prepared with RbF-PDT.