Both CdTe and Cu(In,Ga)Se2 have produced highly efficient thin film solar cells, exceeding 22% in champion devices. Both are also manufactured in large scales and show promise as future energy technologies. However, understanding the current collection mechanisms and mechanisms of instability in the devices remain a concern. To address these questions, we have used scanning probe and photoemission spectroscopies to study the response of chalcogenide materials to light and how charge is collected. Results of scanning microwave impedance microscopy and conductive atomic force microscopy show dramatic differences in the behavior of CdTe and Cu(In,Ga)Se2 (CIGS). The results include characterization of the effect of CdCl2 treatment on the properties CdTe grains and grain boundaries. This treatment dramatically increases the current collection in the grain boundaries. Thus we show that CdTe solar cells operate apparently by generation of electron hole pairs in the CdTe grains and collection of electrons to the grain boundaries. By contrast, CIGS grains show little or no contrast between the grains and grain boundaries and no obvious conduction pathway through the grain boundaries appears to exist. Our surface analysis results are supplemented with other measurements of both surface and bulk microchemistry and microstructure.
KEYWORDS: Interfaces, Copper indium gallium selenide, Crystals, Thin films, Process control, Thin film solar cells, Manufacturing, Thin film manufacturing, Thin film devices, Control systems
Optimizing the buffer layer in manufactured thin-film PV is essential to maximize device efficiency. Here, we describe a combined synthesis, characterization, and theory effort to design optimal buffers based on the (Cd,Zn)(O,S) alloy system for CIGS devices. Optimization of buffer composition and absorber/buffer interface properties in light of several competing requirements for maximum device efficiency were performed, along with process variations to control the film and interface quality. The most relevant buffer properties controlling performance include band gap, conduction band offset with absorber, dopability, interface quality, and film crystallinity. Control of an all-PVD deposition process enabled variation of buffer composition, crystallinity, doping, and quality of the absorber/buffer interface. Analytical electron microscopy was used to characterize the film composition and morphology, while hybrid density functional theory was used to predict optimal compositions and growth parameters based on computed material properties.
Process variations were developed to produce layers with controlled crystallinity, varying from amorphous to fully epitaxial, depending primarily on oxygen content. Elemental intermixing between buffer and absorber, particularly involving Cd and Cu, also is controlled and significantly affects device performance. Secondary phase formation at the interface is observed for some conditions and may be detrimental depending on the morphology. Theoretical calculations suggest optimal composition ranges for the buffer based on a suite of computed properties and drive process optimizations connected with observed film properties.
Prepared by LLNL under Contract DE-AC52-07NA27344.
Advances in thin-film photovoltaics have largely focused on modifying the absorber layer(s), while the choices for other layers in the solar cell stack have remained somewhat limited. In particular, cadmium sulfide (CdS) is widely used as the buffer layer in typical record devices utilizing absorbers like Cu(In,Ga)Se2 (CIGSe) or Cu2ZnSnS4 (CZTS) despite leading to a loss of solar photocurrent due to its band gap of 2.4 eV. While different buffers such as Zn(S,O,OH) are beginning to become competitive with CdS, the identification of additional wider-band gap alternatives with electrical properties comparable to or better than CdS is highly desirable.
Here we use hybrid density functional calculations to characterize CdxZn1-xOyS1-y candidate buffer layers in the quaternary phase space composed by Cd, Zn, O, and S. We focus on the band gaps and band offsets of the alloys to assess strategies for improving absorption losses from conventional CdS buffers while maintaining similar conduction band offsets known to facilitate good device performance. We also consider additional criteria such as lattice matching to identify regions in the composition space that may provide improved epitaxy to CIGSe and CZTS absorbers. Lastly, we incorporate our calculated alloy properties into device model simulations of typical CIGSe devices to identify the CdxZn1-xOyS1-y buffer compositions that lead to the best performance.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Department of Energy office of Energy Efficiency and Renewable Energy (EERE) through the SunShot Bridging Research Interactions through collaborative Development Grants in Energy (BRIDGE) program.
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