Doping thin films used for photovoltaic absorbers is both critical to maximize device voltage and challenging due to complex interactions between point defects in these materials. Such interactions can result in compensation of the intended dopant species, meaning that the active charge-carrier concentration is lower than the concentration incorporated dopants. Charge-carrier compensation is directly related to the open-circuit voltage (VOC) deficit, or magnitude of VOC relative to the theoretical limit. Understanding how the carrier concentration varies within thin-films is necessary to design material processing schedules to minimize this VOC deficit and produce more efficient devices. Unfortunately, measurements of the free carrier concentration are generally relevant at the device level and cannot resolve local differences. Resolving local doping differences in thin-films such as Cd(Se,Te), CZTS, and CIGSe requires techniques with micron or sub-micron spatial resolution due to the polycrystalline structure as well as intended and unintended composition variations in these materials. In this contribution, we show how simultaneous measurement of cathodoluminescence (CL) and electron-beam-induced current (EBIC) can be used to expose doping variations in Cd(Se,Te) thin-films. Simultaneous collection of these signals reveals unexpected differences in the electric field strength through the device thickness due to spatial variation in the carrier concentration.
KEYWORDS: Copper indium gallium selenide, Photovoltaics, Gallium, Transmission electron microscopy, Gallium arsenide, Solar energy, Scanning electron microscopy, Optoelectronics, Thin films
Over the last several decades, champion photovoltaic (PV) devices using CuInGaSe2 (CIGS) as the absorber material have been achieved using polycrystalline films exclusively. This has led to the assumption that polycrystalline CIGS generally outperform single-crystal CIGS in PV devices. However, recently, very high-quality epitaxial CIGS has been grown on GaAs substrates producing PV device efficiencies of 20.0%. These results have revived the debate on what effects grain boundaries have on PV device efficiencies. In this contribution, we compare the optoelectronic properties of polycrystalline CIGS films with those of high-efficiency epitaxial CIGS films. This comparison reveals that grain boundaries are associated with properties that negatively impact PV device efficiency. Additionally, we find that the grain interiors in polycrystalline films exhibit properties that are similar to the high-performance epitaxial films. Our results suggest that it may be possible to achieve higher device efficiencies with epitaxial CIGS than with polycrystalline films.
KEYWORDS: 3D modeling, Thin film solar cells, Monte Carlo methods, Electrons, Diffusion, Numerical simulations, MATLAB, Renewable energy, Electron holes, Thin films
Cathodoluminescence (CL) measurements can be applied to assess grain-boundary (GB) and grain-interior (GI) recombination in thin-film solar cell materials and made quantitative if we can develop CL models that account for material and measurement complexities. Recently, we developed a three-dimensional numerical CL model, based in MATLAB, that simulates the GI CL intensity as a function of four parameters: grain size, GI lifetime, and GB and surface recombination velocities. The model assumes that GB electrostatic potentials are screened by the high excesscarrier densities used in the CL measurement such that transport is governed by ambipolar diffusion. Here, we develop models to address directly GB potentials and their effects on these measurements. First, we transfer the MATLAB-based model to COMSOL software, and then introduce shallow donors to the GBs to produce potentials. We also develop a two-dimensional model in COMSOL to simulate CL GB contrast with GB potentials. Simulations indicate that GB potentials can increase or decrease CL intensities relative to the zero-potential case. However, the high electron-beam currents typically applied in CL measurements minimize the impact of GB potentials.
Alkali post-deposition treatments significantly improve the performance of CuInGaSe2 (CIGS) devices, but there is still room for improvement. Here, we investigate the effects of potassium fluoride alkali post-deposition treatment on the defect chemistry and recombination at grain boundaries and grain interiors using temperature- and injection-dependent cathodoluminescence (CL) spectrum imaging. We study CIGS thin films grown on alkali-free sapphire substrates to isolate the effects of alkali treatment from alkali metals that can diffuse from standard soda-lime glass substrates. We find that alkali treatment affects the energy and temperature dependence of the luminescence peaks, as well as the defect activation energies. CL spectrum images reveal that the luminescence transitions at grain boundaries have a distinct power dependence after alkali treatment and substantially different defect chemistry. This work shows that temperatureand injection-dependence CL spectrum images can provide unique insight into the defect chemistry and recombination behavior of CIGS thin films.
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