Abstract
The properties and performance of polycrystalline materials depend critically on the properties of their grain boundaries. Polycrystalline photovoltaic materials – e.g. hybrid halide ...perovskites, copper indium gallium diselenide (CIGSe) and cadmium telluride – have already demonstrated high efficiencies and promise cost-effective electricity supply. For CIGSe-based solar cells, an efficiency above 23% has recently been achieved using an alkali-fluoride post-deposition treatment; however, its full impact and functional principle are not yet fully understood. Here, we show direct evidence for the passivation of grain boundaries in CIGSe treated with three different alkali-fluorides through a detailed study of the nanoscale optoelectronic properties. We determine a correlation of the surface potential change at grain boundaries with the open-circuit voltage, which is supported by numerical simulations. Our results suggest that heavier alkali elements might lead to better passivation by reducing the density of charged defects and increasing the formation of secondary phases at grain boundaries.
Thin-film solar cells based on polycrystalline absorbers have reached very high conversion efficiencies of up to 23-25%. In order to elucidate the limiting factors that need to be overcome for even ...higher efficiency levels, it is essential to investigate microscopic origins of loss mechanisms in these devices. In the present work, a high efficiency (21% without anti-reflection coating) copper indium gallium diselenide (CIGSe) solar cell is characterized by means of a correlative microscopy approach and corroborated by means of photoluminescence spectroscopy. The values obtained by the experimental characterization are used as input parameters for two-dimensional device simulations, for which a real microstructure was used. It can be shown that electrostatic potential and lifetime fluctuations exhibit no substantial impact on the device performance. In contrast, nonradiative recombination at random grain boundaries can be identified as a significant loss mechanism for CIGSe solar cells, even for devices at a very high performance level.Achieving higher efficiencies for thin-film solar cells always requires identification of the limiting factors. Here Krause et al. show that inhomogeneously distributed net doping or lifetime have little impact while recombination at grain boundaries is one of the main loss mechanisms for high performance Cu(In,Ga)Se2 solar cells.
We report on the analytical description of high-efficiency Cu(In,Ga)Se2-based solar cells produced with a static coevaporation process. We discuss classic quality markers such as grain morphology, ...composition, vertical compositional gradings, and grain orientation in these cells. We then describe the successful transfer of such results to industrially relevant inline processes in our module production line. Finally, we explicate one of the many optimisation routes for the further improvement of these Cu(In,Ga)Se2-based solar cells: Zn(O,S) buffer layers.
•Quality markers of high-efficiency Cu(In,Ga)Se2 (CIGS) cells re-examined.•Efficiency development of CIGS modules.•Zn(O,S) alternative buffer optimization.•Technology transfer from lab to industry
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A new method for efficiently converting electron backscatter diffraction data obtained using serial sectioning by focused ion beam of a polycrystalline thin film into a computational, ...three-dimensional (3D) structure is presented. The reported data processing method results in a more accurate representation of the grain surfaces, reduced computer memory usage, and improved processing speed compared to traditional voxel methods. The grain structure of a polycrystalline absorption layer from a high-efficiency Cu(In,Ga)Se2 solar cell (19.5%) is reconstructed in 3D and the grain size and surface distribution is investigated. The grain size distribution is found to be best fitted by a log-normal distribution. We further find that the grain size is determined by the Ga/(Ga + In) ratio in vertical direction, which was measured by glow discharge optical emission spectroscopy. Finally, the 3D model derived from the structural information is applied in optoelectronic simulations, revealing insights into the effects of grain boundary recombination on the open-circuit voltage of the solar cell. An accurate 3D structure like the one obtained with our method is a prerequisite for a detailed understanding of mechanical properties and for advanced optical and electronic simulations of polycrystalline thin films.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
In this article, we discuss the leading thin-film photovoltaic (PV) technology based on the Cu(In,Ga)Se2 (CIGS) compound semiconductor. This contribution includes a general comparison with the ...conventional Si-wafer-based PV technology and discusses the basics of the CIGS technology as well as advances in world-record-level conversion efficiency, production, applications, stability, and future developments with respect to a flexible product. Once in large-scale mass production, the CIGS technology has the highest potential of all PV technologies for cost-efficient clean energy generation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In Cu(In,Ga)Se2 (CIGS) thin‐film solar cells, laterally inhomogeneous distributions of point defects may induce electrostatic potential fluctuations and thus reduce the open‐circuit voltage (Voc). In ...the present work, we investigate possible origins of fluctuating potentials and estimate the amplitude of fluctuations and Voc losses in solar cells with various Ga in the CIGS absorber, with different buffer layers and with different durations of an RbF postdeposition treatment (PDT). Electron‐beam‐induced current measurements were employed to study the local difference in the width of the space‐charge region (wSCR). It is shown that the amplitude of fluctuations in the wSCR depends significantly on the choice of buffer system and on the duration of the RbF PDT. In addition, energy‐dispersive X‐ray spectroscopy and cathodoluminescence measurements reveal that band‐gap fluctuations do not have substantial impact on the device performance. Finally, some of the investigated cells were exposed to light soaking, which was found to be a means to reduce the detected electrostatic potential fluctuations and also to increase the effective electron diffusion length in the CIGS absorber for a part of the investigated cells.
It is shown that the dominant effect of electrostatic potential fluctuations in Cu(In,Ga)Se2 solar cells is linked to the local variations in the doping densities ND and the interface‐charge density NIF introduced via the buffer layer deposition or duration of RbF postdeposition treatment. Furthermore, light soaking was found to reduce electrostatic fluctuations in cells with Zn(O,S) buffer. To explain this, we proposed a model based on changes in charge states of defects at the CIGS/buffer interface.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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