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Motivated by their utility in CdTe-based thin film photovoltaics (PV) devices, an investigation of thin films of the magnesium-zinc oxide (MgxZn1−xO or MZO) alloy system was undertaken applying spectroscopic ellipsometry (SE). Dominant wurtzite phase MZO thin films with Mg contents in the range 0 ≤ x ≤ 0.42 were deposited on room temperature soda lime glass (SLG) substrates by magnetron co-sputtering of MgO and ZnO targets followed by annealing. The complex dielectric functions ε of these films were determined and parameterized over the photon energy range from 0.73 to 6.5 eV using an analytical model consisting of two critical point (CP) oscillators. The CP parameters in this model are expressed as polynomial functions of the best fitting lowest CP energy or bandgap E0 = Eg, which in turn is a quadratic function of x. As functions of x, both the lowest energy CP broadening and the Urbach parameter show minima for x ~ 0.3, which corresponds to a bandgap of 3.65 eV. As a result, it is concluded that for this composition and bandgap, the MZO exhibits either a minimum concentration of defects in the bulk of the crystallites or a maximum in the grain size, an observation consistent with measured X-ray diffraction line broadenings. The parametric expression for ε developed here is expected to be useful in future mapping and through-the-glass SE analyses of partial and complete PV device structures incorporating MZO. 

CuInSe 2 (CIS) thin films ~ 500-650 Å in thickness have been deposited on c-Si substrates by two-stage thermal co-evaporation starting either from In 2 Se 3 [according to In 2 Se 3 + (2Cu+Se) → 2(CuInSe 2 )] or from Cu 2-x Se [according to Cu 2 Se + (2In+3Se) → 2(CuInSe 2 )]. The design of such processes is facilitated by accurate calibrations of Cu and In 2 Se 3 growth rates on substrate/film surfaces obtained by real time spectroscopic ellipsometry (RTSE). The two-stage deposited CIS films were also studied by RTSE to deduce (i) the evolution of film structure upon conversion of the starting In 2 Se 3 or Cu 2-x Se films to CIS via Cu+Se or In+Se co-evaporation, respectively, and (ii) the complex dielectric functions of the starting films as well as the resulting CIS. The goal is to fabricate CIS that develops large grains as early as possible during growth for high quality materials in tandem solar cell applications. Results indicate that by depositing Cu 2-x Se in the first stage and exposing the film to In+Se flux in the second stage [as in the third stage of a three-stage CIS process] well-defined bandgap critical points with no detectable subgap absorption are noted in films as thin as 650 Å. 

Optical and microstructural properties of as-deposited CdTe films deposited on soda lime glass by magnetron sputtering at various source flux angles have been investigated using GIXRD, SEM, unpolarized transmittance / reflectance, and spectroscopic ellipsometry. Influence of deposition angle on resultant crystalline grain size and orientation are tracked for these films. All CdTe films studied are found to have cubic crystal structure and (111) preferential grain orientation. Films deposited at 0° and 45° are almost entirely (111) oriented, whereas films deposited at higher angles exhibit a wider variety of competing grain orientations, suggesting that deposition angle can be used as an effective parameter towards controlling grain orientation. With increasing numbers of grain orientations, grain size is found to decrease. Ex-situ spectroscopic ellipsometry is used to obtain the structural and optical properties. Stress induced in the film is calculated based on shifts of critical point energies. 

Spectroscopic ellipsometry (SE) was performed on CuIn Se 2 (CIS) thin films and solar cells with a goal toward optimizing this low bandgap absorber for tandem applications. The CIS thin films and the absorbers in devices were deposited by one-stage thermal co-evaporation on silicon and on Mo-coated soda-lime glass substrates in a deposition system that has yielded CuIn 1-x Ga x Se 2 (CIGS) cells with > 17% efficiency using standard thickness (2.0 μm)x = 0.3 absorbers and > 13% using 0.7 μm low-Ga absorbers. In this study, a mapping capability for CIS Cu stoichiometry y = [Cu]/[In] over the film area was established based on a y-dependent parametric dielectric function (ε 1 , ε 2 ) with bandgap critical point E g decreasing linearly from 1.030 eV for y = 0.7 to 1.016 eV for y = 1.1. In addition, a full set of (ε 1 , ε 2 ) spectra measured for the CIS cell components enables analysis of SE data in terms of an accurate structural model for the device. With this model, spectra in the external quantum efficiency can be predicted, and deviations from this prediction can be attributed to incomplete collection of photogenerated electrons and holes as simulated with a carrier collection profile. 

Abstract: Monolithic integrated thin film tandem solar cells consisting of a high bandgap perovskite top cell and a low bandgap thin film bottom cell are expected to reach higher power conversion efficiencies (PCEs) with lower manufacturing cost and environmental impacts than the market-dominant crystalline silicon photovoltaics. There have been several demonstrations of 4-terminal and 2-terminal perovskite tandem devices with CuInGaSe 2 (CIGS) or CuInSe 2 (CIS) and, similar to the other tandem structures, the optimization of this device relies on optimal choice for the perovskite bandgap and thickness. Therefore, further advancement will be enabled by tuning the perovskite absorber to maximize the photocurrent limited by the current match condition. Here, we systematically study the optical absorption and transmission of perovskite thin films with varying absorber band gap. Based on these results, we model the photocurrent generations in both perovskite and CIS subcells and estimate the performances of projected tandem devices by considering the ideally functioning perovskite and CIS device. Our results show that for perovskite layers with 500 nm thickness the optimal bandgap is around 1.6 eV. With these configurations, PCEs above 20% could be achieved by monolithically integrated perovskite/CIS tandem solar cells. Also by modelling the absorption at every layer we calculate the quantum efficiency at each subcell in addition to tracking optical losses.