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1. Photovoltaic Response of Thin-Film CdTe Solar Cells under Accelerated Neutron Radiation in a TRIGA Reactor

   Powell, Kaden ; Lund, Matthew ; Wang, Meng-Jen ; Qian, Yang ; Magginetti, David ; Reese, Matthew ; Cazalas, Edward ; Sjoden, Glenn ; Yoon, Heayoung ( June 2020 , Electronic Materials Symposium)

   Cadmium telluride (CdTe) solar cells are a promising photovoltaic (PV) technology for producing power in space owing to their high-efficiency (> 22.1 %), potential for specific power, and cost-effective manufacturing processes. In contrast to traditional space PVs, the high-Z (atomic number) CdTe absorbers can be intrinsically robust under extreme space radiation, offering long-term stability. Despite these advantages, the performance assessment of CdTe solar cells under high-energy particle irradiation (e.g., photons, neutrons, charged particles) is limited in the literature, and their stability is not comprehensively studied. In this work, we present the PV response of n-CdS / p-CdTe PVs under acceleratedmore »neutron irradiation. We measure PV properties of the devices at different neutron/photon doses. The equivalent dose deposited in the CdTe samples is simulated with deterministic and Monte Carlo radiation transport methods. Thin-film CdTe solar cells were synthesized on a fluorine-doped tin oxide (FTO) coated glass substrate (≈ 4 cm × 4 cm). CdS:O (≈ 100 nm) was reactively RF sputtered in an oxygen/argon ambient followed by a close-spaced sublimation deposition of CdTe (≈ 3.5 μm) in an oxygen/helium ambient. The sample was exposed to a 10 min vapor CdCl2 in oxygen/helium ambient at 430˚C. The samples were exposed to a wet CuCl2 solution prior to anneal 200ºC. A gold back-contact was formed on CdTe via thermal evaporation. The final sample contains 16 CdTe devices. For neutron irradiation, we cleaved the CdTe substrate into four samples and exposed two samples to ≈ 90 kW reactor power neutron radiation for 5.5 hours and 8.2 hours, respectively, in our TRIGA (Training, Research, Isotopes, General Atomics) reactor. We observed a noticeable color change of the glass substrates to brown after the neutron/gamma reactor exposure. Presumably, the injected high-energy neutrons caused the breaking of chemical bonds and the displacement of atoms in the glass substrates, creating point defects and color centers. The I-V characteristics showed noticeable deterioration with over 8 hour radiations. Specifically, the saturation current of the control devices was ≈ 25 nA increasing to 1 μA and 10 μA for the 5.5-hour and 8.2-hour radiated samples, respectively. The turn-on voltage of the control devices (≈ 0.85 V) decreased with the irradiated sample (≈ 0.75 V for 5.5-hour and ≈ 0.5 V for 8.2-hour exposures), implying noticeable radiation damage occurred at the heterojunction. The higher values of the ideality factor for irradiated devices (n > 2.2) compared to that of the control devices (n ≈ 1.3) also support the deterioration of the p-n junction. We observed the notable decrease in shunt resistance (RSH) and the increase in series resistance (Rs) with the neutron dose. It is possible that Cu ions introduced during the CuCl2 treatment may migrate into CdTe grain boundaries (GBs). The presence of Cu ions at GBs can create additional leakage paths for photocarrier transport, deteriorating the overall PV performance. We estimated the radiation dose of CdTe in comparison to Si (conventional PV) using a UUTR model (e.g., MCNP6 2D UTR Reactor simulations). In this model, we simulated Si and CdTe at the center point of the triangular fuel lattice and used an “unperturbed flux” tally in the water. Our simulations yielded a dose rate of 6916 Gy/s of neutrons and 16 Gy/s of photons for CdTe, and 1 Gy/s of neutrons and 21 Gy/s of photons for Si (doses +/- <1%). The large dose rate of neutrons in CdTe is mainly attributed to the large thermal neutron absorption cross-section of 113Cd. Based on this estimation, we calculate that the exposure of our CdTe PVs is equivalent to several million years in LEO (Low-Earth Orbit), or about 10,000 years for Si in LEO. Currently, we are working on a low-dose neutron/photon radiation on CdTe PVs and their light I-Vs and microstructural characterizations to gain better understanding on the degradation of CdTe PVs.« less
2. Impact of cesium on the phase and device stability of triple cation Pb–Sn double halide perovskite films and solar cells

   https://doi.org/10.1039/c8ta06391e  

   Tosado, Gabriella A. ; Lin, Yi-Yu ; Zheng, Erjin ; Yu, Qiuming ( January 2018 , Journal of Materials Chemistry A)

   Triple cation Cs/methylammonium (MA)/formamidinium (FA) and double halide Br/I lead perovskites improved the stability and efficiency of perovskite solar cells (PVSCs). However, their effects on alloyed Pb–Sn perovskites are unexplored. In this work, perovskite thin films with the composition Cs x (MA 0.17 FA 0.83 ) 1−x Pb 1−y Sn y (I 0.83 Br 0.17 ) 3 are synthesized utilizing a one-step solution process plus an anti-solvent wash technique and deployed in PVSCs with an inverted architecture. All films show a cubic crystal structure, demonstrating that compositional tuning of both the tolerance factor and crystallization rate allows for dense, singlemore »phase formation. The band gaps, affected by both lattice constriction and octahedral tilting, show opposite trends in Pb-rich or Sn-rich perovskites with the increase of Cs for fixed Sn compositions. The Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb 0.25 Sn 0.75 (I 0.83 Br 0.17 ) 3 PVSCs achieve a power conversion efficiency (PCE) of 11.05%, a record for any PVSC containing 75% Sn perovskites, and the Cs 0.10 (MA 0.17 FA 0.83 ) 0.90 Pb 0.75 Sn 0.25 (I 0.83 Br 0.17 ) 3 PVSCs reach a record PCE of 15.78%. Moreover, the triple cation and double halide alloyed Pb–Sn perovskites exhibit improved device stability under inert and ambient conditions. This study, which illustrates the impact of cation and halide tuning on alloyed Pb–Sn perovskites, can be used to further eliminate Pb and improve device performance of high Sn PVSCs and other optoelectronic devices.« less
3. Detailed-balance analysis of Yb ³⁺ :CsPb(Cl _1−x Br _x ) ₃ quantum-cutting layers for high-efficiency photovoltaics under real-world conditions

   https://doi.org/10.1039/c9ee01493d  

   Crane, Matthew J. ; Kroupa, Daniel M. ; Gamelin, Daniel R. ( January 2019 , Energy & Environmental Science)

   Yb 3+ -Doped lead-halide perovskites (Yb 3+ :CsPb(Cl 1−x Br x ) 3 ) have emerged as unique materials combining strong, tunable broadband absorption with near-infrared photoluminescence quantum yields (PLQYs) approaching 200% at ambient temperature. These remarkable properties make Yb 3+ :CsPb(Cl 1−x Br x ) 3 an extremely promising candidate for spectral shaping in high-efficiency photovoltaic devices. Previous theoretical assessments of such “downconversion” devices have predicted single-junction efficiencies up to 40%, but have been highly idealized. Real materials like Yb 3+ :CsPb(Cl 1−x Br x ) 3 have practical limitations such as constrained band-gap and PL energies, non-directional emission,more »and an excitation-power-dependent PLQY. Hence, it is unclear whether Yb 3+ :CsPb(Cl 1−x Br x ) 3 , or any other non-ideal quantum-cutting material, can indeed boost the efficiencies of real high-performance PV. Here, we examine the thermodynamic, detailed-balance efficiency limit of Yb 3+ :CsPb(Cl 1−x Br x ) 3 on different existing PV under real-world conditions. Among these, we identify silicon heterojunction technology as very promising for achieving significant performance gains when paired with Yb 3+ :CsPb(Cl 1−x Br x ) 3 , and we predict power-conversion efficiencies of up to 32% for this combination. Surprisingly, PL saturation does not negate the improved device performance. Calculations accounting for actual hourly incident solar photon fluxes show that Yb 3+ :CsPb(Cl 1−x Br x ) 3 boosts power-conversion efficiencies at all times of day and year in two representative geographic locations. Predicted annual energy yields are comparable to those of tandem perovskite-on-silicon technologies, but without the need for current matching, tracking, or additional electrodes and inverters. In addition, we show that band-gap optimization in real quantum cutters is inherently a function of their PLQY and the ability to capture that PL. These results provide key design rules needed for development of high-efficiency quantum-cutting photovoltaic devices based on Yb 3+ :CsPb(Cl 1−x Br x ) 3 .« less
4. Study of disorder in pulsed laser deposited double perovskite oxides by first-principle structure prediction

   https://doi.org/10.1038/s41524-021-00561-1  

   Fertitta, Edoardo ; Das, Sujit ; Banerjee, Debalina ; Ebrahimi, Farbod ; Barraud, Clément ; Du, Kai ; Tian, He ; Pickard, Chris J. ; Weber, Cedric ; Ramesh, Ramamoorthy ; et al ( June 2021 , npj Computational Materials)

   Double perovskite oxides, with generalized formula A₂BB$$^{\prime}$$${}^{\prime }$O₆, attract wide interest due to their multiferroic and charge transfer properties. They offer a wide range of potential applications such as spintronics and electrically tunable devices. However, great practical limitations are encountered, since a spontaneous order of the B-site cations is notoriously hard to achieve. In this joint experimental-theoretical work, we focused on the characterization of double perovskites La₂TiFeO₆and La₂VCuO₆films grown by pulsed laser deposition and interpretation of the observed B-site disorder and partial charge transfer between the B-site ions. A random structure sampling method was used to show that several phasesmore »compete due to their corresponding configurational entropy. In order to capture a representative picture of the most relevant competing microstates in realistic experimental conditions, this search included the potential formation of non-stoichiometric phases as well, which could also be directly related to the observed partial charge transfer. We optimized the information encapsulated in the potential energy landscape, captured via structure sampling, by evaluating both enthalpic and entropic terms. These terms were employed as a metric for the competition of different phases. This approach, applied herein specifically to La₂TiFeO₆, highlights the presence of highly entropic phases above the ground state which can explain the disorder observed frequently in the broader class of double perovskite oxides.

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5. Two-dimensional halide perovskites featuring semiconducting organic building blocks

   https://doi.org/10.1039/d0qm00233j  

   Gao, Yao ; Wei, Zitang ; Hsu, Sheng-Ning ; Boudouris, Bryan W. ; Dou, Letian ( January 2020 , Materials Chemistry Frontiers)

   Two-dimensional (2D) organic–inorganic hybrid halide perovskites exhibit unique properties, such as long charge carrier lifetimes, high photoluminescence quantum efficiencies, and great tolerance to defects. Over the last several decades tremendous progress has occurred in the development of 2D layered halide perovskite semiconductor materials and devices. Chemical functionalization of 2D halide perovskites is an effective approach for tuning their electronic properties. A large amount of effort has been made in compositional engineering of the cations and anions in the perovskite lattice. However, few efforts have incorporated rationally designed semiconducting organic moieties into these systems to alter the overall chemical and optoelectronicmore »properties of 2D perovskites. In fact, incorporation of large conjugated organic groups in the spatially confined inorganic perovskite matrix was found to be challenging, and this synthetic challenge hinders a deeper understanding of the materials’ structure–property relationships. Recently, exciting progress has been made regarding the molecular design, optical characterization, and device fabrication of novel 2D halide perovskite materials that incorporate functional organic semiconducting building blocks. In this article, we provide a timely review regarding this recent progress. Moreover, we discuss successes and current challenges regarding the synthesis, characterization, and device applications of such hybrid materials and provide a perspective on the true future promise of these advanced nanomaterials.« less