Alkali-Templated Surface Nanopatterning of Chalcogenide Thin Films: A Novel Approach Toward Solar Cells with Enhanced Efficiency

Concepts of localized contacts and junctions through surface passivation layers are already advantageously applied in Si wafer-based photovoltaic technologies. For Cu­(In,Ga)­Se₂ thin film solar cells, such concepts are generally not applied, especially at the heterojunction, because of the lack of a simple method yielding features with the required size and distribution. Here, we show a novel, innovative surface nanopatterning approach to form homogeneously distributed nanostructures (<30 nm) on the faceted, rough surface of polycrystalline chalcogenide thin films. The method, based on selective dissolution of self-assembled and well-defined alkali condensates in water, opens up new research opportunities toward development of thin film solar cells with enhanced efficiency

Sodium Assisted Sintering of Chalcogenides and Its Application to Solution Processed Cu₂ZnSn(S,Se)₄ Thin Film Solar Cells

Cu₂ZnSn­(S,Se)₄ thin layers processed from solution-deposited earth-abundant precursors emerge as absorber materials for low-cost thin film solar cells. A frequently observed drawback of the chemical solution processingpoor crystallinity of the chalcogenide absorbercan be overcome by employing a sodium-containing reactive agent. We demonstrate a massive improvement in grain growth in the presence of sodium. It enhances the surface chemisorption of selenium molecules and can promote the formation of liquid Na₂Se_<i>x</i> phases during reactive annealing of the precursor. The sodium is also incorporated into the semiconductor absorber and significantly modifies its electronic properties. By adjusting the sodium precursor quantity, it is possible to tune doping levels and gradients to maximize the collection of photogenerated carriers in thin film Cu₂ZnSn­(S,Se)₄ solar cells. The presented approach can be extended to other solution-processed metal chalcogenides to enhance their structural and electronic properties, which are critical for applications such as thin film solar cells and transistors

Refractive indices of layers and optical simulations of Cu(In,Ga)Se₂ solar cells

<p>Cu(In,Ga)Se₂ -based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se₂ solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se₂ does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances.</p

Formation of a KInSe Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se₂ Thin-Film Solar Cell Absorbers

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu­(In,Ga)­Se₂ (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a KInSe species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber