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1. Earth‐Abundant Chalcogenide Photovoltaic Devices with over 5% Efficiency Based on a Cu 2 BaSn(S,Se) 4 Absorber

   https://doi.org/10.1002/adma.201606945  

   Shin, Donghyeop ; Zhu, Tong ; Huang, Xuan ; Gunawan, Oki ; Blum, Volker ; Mitzi, David B. ( April 2017 , Advanced Materials)

   In recent years, Cu₂ZnSn(S,Se)₄(CZTSSe) materials have enabled important progress in associated thin‐film photovoltaic (PV) technology, while avoiding scarce and/or toxic metals; however, cationic disorder and associated band tailing fundamentally limit device performance. Cu₂BaSnS₄(CBTS) has recently been proposed as a prospective alternative large bandgap (~2 eV), environmentally friendly PV material, with ~2% power conversion efficiency (PCE) already demonstrated in corresponding devices. In this study, a two‐step process (i.e., precursor sputter deposition followed by successive sulfurization/selenization) yields high‐quality nominally pinhole‐free films with large (>1 µm) grains of selenium‐incorporated (x= 3) Cu₂BaSnS_4−xSe_x(CBTSSe) for high‐efficiency PV devices. By incorporating Se in the sulfide film, absorber layers with 1.55 eV bandgap, ideal for single‐junction PV, have been achieved within the CBTSSe trigonal structural family. The abrupt transition in quantum efficiency data for wavelengths above the absorption edge, coupled with a strong sharp photoluminescence feature, confirms the relative absence of band tailing in CBTSSe compared to CZTSSe. For the first time, by combining bandgap tuning with an air‐annealing step, a CBTSSe‐based PV device with 5.2% PCE (total area 0.425 cm²) is reported, >2.5× better than the previous champion pure sulfide device. These results suggest substantial promise for the emerging Se‐rich Cu₂BaSnS_4–xSe_xfamily for high‐efficiency and earth‐abundant PV.

    

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2. Optical Properties of Magnesium-Zinc Oxide for Thin Film Photovoltaics

   https://doi.org/10.3390/ma14195649  

   Alaani, Mohammed A. ; Koirala, Prakash ; Phillips, Adam B. ; Liyanage, Geethika K. ; Awni, Rasha A. ; Sapkota, Dhurba R. ; Ramanujam, Balaji ; Heben, Michael J. ; O’Leary, Stephen K. ; Podraza, Nikolas J. ; et al ( October 2021 , Materials)

   null (Ed.)

   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. 

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3. Structural and Optical Properties of Two-Stage CuInSe 2 Thin Films Studied by Real Time Spectroscopic Ellipsometry

   https://doi.org/10.1109/PVSC40753.2019.8980671  

   Sapkota, Dhurba R. ; Collins, Robert W. ; Pradhan, Puja ; Koirala, Prakash ; Irving, Richard ; Phillips, Adam B. ; Ellingson, Randy J. ; Heben, Michael J. ; Marsillac, Sylvain ; Podraza, Nikolas J. ( June 2019 , 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC))

   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. 

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4. GaTlAs Quantum Well Solar Cells for Sub-band Gap Absorption

   https://doi.org/10.1557/adv.2019.334  

   Zayan, Ahmed ; Vandervelde, Thomas E. ( January 2019 , MRS Advances)

   ABSTRACT Despite the improvements seen in efficiency of GaAs cells over the years, there remains room for improvement for it to approach the theoretical single junction limit posited by Shockley and Quiesser decades ago. One of the more pursued options is the growth of quantum wells within the structure of GaAs to enhance its photon absorption below its bandgap. Multiple Quantum Wells (MQW) have been an ongoing topic of research and discussion for the scientific community with structures like InGaAs/GaAs and InGaP/GaAs quantum wells producing promising results that could potentially improve overall energy conversion. Here, we used WEIN2K, a commercial density functional theory package, to study the ternary compound Ga 1-x Tl x As and determine its electronic properties. Using these results combined with experimental confirmation we extend these properties to simulate its application to form a MQW GaAs/ Ga 1-x Tl x As solar cell. Ga 1-x Tl x As is a tunable ternary compound, with its bandgap being strongly dependent on the concentration of Tl present. Concentrations of Tl as low as 7% can reduce the bandgap of Ga 1-x Tl x As to roughly 1.30 eV from GaAs’s 1.45 eV at room temperature with as little as a 1.7% increase in lattice constant. The change in bandgap, accompanied by the relatively small change in lattice constant makes Ga 1-x Tl x As a strong candidate for a MQW cell with little to no strain balancing required within the structure to minimize unwanted defects that impede charge collection within the device. Our GaAs photodiode with TlGaAs MQWs shows an expanded absorption band and improved conversion efficiency over the standard GaAs photovoltaic cell with dilute concentrations of Tl incorporated into the compound. 

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5. Structural and electrical properties of thick κ -Ga 2 O 3 grown on GaN/sapphire templates

   https://doi.org/10.1063/5.0091653  

   Polyakov, A. Y. ; Nikolaev, V. I. ; Pechnikov, A. I. ; Stepanov, S. I. ; Yakimov, E. B. ; Scheglov, M. P. ; Shchemerov, I. V. ; Vasilev, A. A. ; Kochkova, A. A. ; Chernykh, A. V. ; et al ( June 2022 , APL Materials)

   Thick (23 µm) films of κ-Ga 2 O 3 were grown by Halide Vapor Phase Epitaxy (HVPE) on GaN/sapphire templates at 630 °C. X-ray analysis confirmed the formation of single-phase κ-Ga 2 O 3 with half-widths of the high-resolution x-ray diffraction (004), (006), and (008) symmetric reflections of 4.5 arc min and asymmetric (027) reflection of 14 arc min. Orthorhombic κ-Ga 2 O 3 polymorph formation was confirmed from analysis of the Kikuchi diffraction pattern in electron backscattering diffraction. Secondary electron imaging indicated a reasonably flat surface morphology with a few (area density ∼10 3  cm −2 ) approximately circular (diameter ∼50–100 µm) uncoalesced regions, containing κ-Ga 2 O 3 columns with in-plane dimensions and a height of about 10 µm. Micro-cathodoluminescence (MCL) spectra showed a wide 2–3.5 eV band that could be deconvoluted into narrower bands peaked at 2.59, 2.66, 2.86, and 3.12 eV. Ni Schottky diodes prepared on the films showed good rectification but a high series resistance. The films had a thin near-surface region dominated by E c − 0.7 eV deep centers and a deeper region (∼2 µm from the surface) dominated by shallow donors with concentrations of ≤10 16  cm −3 . Photocurrent and photocapacitance spectra showed the presence of deep compensating acceptors with optical ionization energies of ∼1.35 and 2.3 eV, the latter being close to the energy of one of the MCL bands. Deep level transient spectroscopy revealed deep traps with energies near 0.3, 0.6, 0.7, 0.8, and 1 eV from the conduction band edge. The results show the potential of HVPE to grow very thick κ-Ga 2 O 3 on GaN/sapphire templates. 

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