ABSTRACT Improved back contacts can benefit CdTe photovoltaics (PV). In this work, Cd(Se,Te) PV absorbers are cooled during Au evaporation to thermally quench a chemical reaction occurring between gold and CdTe and the generation of a reaction product that lowers device efficiency. Reducing substrate temperature enhances PV power conversion efficiency via open‐circuit voltage and fill factor increases. X‐ray photoelectron spectroscopy (XPS) reveals that lower temperature also reduces chemical perturbations of the CdTe, potentially linking back contact formation to a CdTe degradation product that hinders PV performance. Comparing reaction enthalpy and substrate heating energy shows that back contact formation by sputtering elemental metals onto ZnTe may exhibit a degradation pathway analogous to that of CdTe/Au reported here. Degradation‐diminishing contact formation processes are therefore of general interest for optoelectronic devices, and the reduced substrate temperature in this study is one example.

We report on the optimization of in situ passivation of ink-based CuIn(S,Se)(2) thin-film solar cells via controlled incorporation of Al2O3 in CuIn(S,Se)(2) films by the addition of Al(NO3)(3) to the molecular ink precursor. For this purpose, the Al/(Al + In) (AAI) metal ratio was varied from 0.05 to 0.30. We observe that the efficiency of the cells made of Al2O3-incorporated CuIn(S,Se)(2) is consistently higher than those without Al2O3, especially due to an improvement in open-circuit voltage (V-OC) and fill factor (FF), for all tested AAI ratios. With an AAI of 0.05, a maximum efficiency of 11.2% and an average efficiency of 8.5% (measured across 18 cells) was achieved, compared to 8.5% maximum efficiency and 6.5% average efficiency for Al-free CuIn(S,Se)(2). Furthermore, we find that cells made of Al2O3-incorporated CuIn(S,Se)(2) with an AAI of 0.2 show a narrow distribution in the photovoltaic performance, indicating higher reproducibility and higher FF. Energy-dispersive X-ray spectroscopy shows that, at AAI = 0.2, Al2O3 is distributed more homogeneously at the surface of the Al2O3-incorporated CISSe. Capacitance-voltage measurements reveal a reduced defect density by incorporation of Al2O3, which could be partly responsible for the higher V-OC. Furthermore, using detailed surface analysis with various X-ray and electron spectroscopy methods, we derive chemical and electronic structure information from the surface. With ultraviolet photoelectron (UPS) and inverse photoemission spectroscopies (IPES), the electronic band gap of the CuIn(S,Se)(2) thin-film surface is found to increase from 1.22 to 1.88 eV (+/- 0.12 eV) with Al2O3 incorporation. This is accompanied by a clear reduction of the conduction band spike at the CdS/CISSe interface due to Al2O3 addition, as derived by both UPS and IPES as well as temperature-dependent V-OC measurements.

Nitrogen-doped zinc selenide telluride (N:ZnSexTe1-x) is of interest because it is one of the widest-gap II-VI semiconductors that can still be doped p-type with reasonably high hole concentrations. We sputter deposit N:ZnSexTe1-x films, varying Se/(Se + Te), or x, from 0 to 0.7, N2 flow rate from 0.25 to 0.75 sccm, and substrate temperature from 250 to 370 degrees C. Increasing x from 0 to 0.39 at the optimal temperature of 370 degrees C and N2 flow rate of 0.5 sccm leads to 1.3 atomic % nitrogen incorporation and wurtzite phase stabilization. Such doping and alloying increases hole concentration from 3 & times; 1018 cm-3 to 3 & times; 1019 cm-3, although mobility drops from 0.4 to 0.02 cm2 V-1 s-1. Our sputtered N:ZnSe0.38Te0.62 has an absorption onset 0.1 eV greater than the ZnTe value of 1.87 eV. Increasing x from 0 to 0.51 enhances transmittance by moving absorption onset from 1.87 to 2.11 eV with diminished band gap bowing likely due to disorder, and increases the work function from 5.12 to 5.42 eV. This combination of tunable properties makes sputtered N:ZnSexTe1-x desirable for transparent p-type contacts in polycrystalline Cd(Se,Te) optoelectronic devices.

Ternary Zintl phosphides are promising light-absorbing semiconductors for thin-film optoelectronic applications, but strategies for controlling their microstructure and optoelectronic quality remain underexplored. Here, we report the synthesis of phase-pure SrZn2P2 thin films using radio-frequency co-sputtering in a PH3 + Ar atmosphere and investigate the impact of post-growth processing on their structural and optical properties. Grazing-incidence X-ray scattering and Raman spectroscopy confirm the formation of crystalline SrZn2P2 films over a finite compositional window. Optical measurements reveal strong absorption near the direct-band-gap energy ( 1.8 eV) and near-band-edge photoluminescence. Further, we have studied the effects of chemically compatible halide-assisted annealing. It is found that SrI2 treatments lead to pronounced grain growth and reduced diffraction peak broadening while preserving phase purity, in contrast to rapid thermal or forming-gas annealing. Notably, annealing with SrI2 at 450 °C significantly enhances both the intensity and spatial uniformity of the photoluminescence, thus connecting the observed microstructural consolidation with improved radiative recombination. Our study demonstrates that halide-assisted annealing provides an effective pathway for microstructural control in SrZn2P2 thin films and highlights a generalizable processing strategy for advancing Zintl phosphide semiconductors toward optoelectronic applications.

Ga x In1-x Se (GIS) alloys are two-dimensional (2D) layered materials with band gaps and lattice parameters of interest for many energy and electronic applications. They can be fabricated using van der Waals epitaxy, which is an emerging technique that offers unprecedented opportunities for 2D optoelectronic devices and epitaxy processes. This work has demonstrated van der Waals epitaxy of GIS alloys for the first time. Films with x compositions of 0, 0.062, 0.164, 0.680, 0.894, and 1 and tunable lattice constants were grown on Si(111) substrates, and characterized by X-ray diffraction pole figure and transmission electron microscope analysis. In spite of the lattice mismatches (InSe is 4.1% too large and GaSe is 2.8% too small), these alloys grow epitaxially, with Si(111) || GIS(001) and Si[1-10] || GIS[100] orientation. Photoluminescence was used to measure tunable band gaps in the absorber-relevant 1.3-2.0 eV range as a function of x composition and showed GIS did not degrade after capping with Se and prolonged storage. Therefore, GIS alloys exhibit a technologically advantageous combination of tunable band gap and photoluminescence with relaxed lattice parameter and rotational registry with the substrate.

Rare-earth nickelates exhibit valuable behavior for neuromorphic computing at low temperature: Building blocks for biologically inspired microelectronic neurons like electrically driven insulator-metal transitions (IMTs), negative differential resistance, and self-oscillations have been shown up to 230 K for SmNiO3 and NdNiO3. EuNiO3 raises the IMT far above room temperature (460 K) but high-quality thin films are challenging to synthesize. Here, we explore the epitaxial stabilization of EuNiO3 using pulsed laser deposition. X-ray diffraction reciprocal space maps, x-ray absorption spectroscopy, and transmission electron microscopy show that higher growth temperature (800 degrees C) reduces oxygen vacancy concentrations in EuNiO3. Pseudomorphic EuNiO3 is demonstrated on both SrLaAlO4 and NdGaO3 substrates, and LaNiO3 buffer layers are incorporated to facilitate future vertical device fabrication. In contrast to bulk thermodynamic predictions, the greater oxidation and crystallinity at higher temperature we observe indicates that epitaxial substrates can stabilize EuNiO3 at O-2 pressures less than 1 atm. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Tetragonal ZnGa 2 Te 4 thin films with direct bandgap ∼1.86 eV show −2 mA cm −2 photocurrent, highlighting their potential as efficient photocathodes for photoelectrochemical CO 2 reduction.

Utilizing sunlight for photoelectrochemical carbon dioxide reduction reaction (PEC CO2 RR) is a carbon-neutral path to valuable liquid fuels. Higher quality photoabsorbers are needed to improve the efficiency of the PEC CO2 RR process. We show how the optoelectronic properties of sputtered ZnTe absorbers can be improved for this purpose via chloride treatments. MnCl2 and MgCl2 heat treatments recrystallize ZnTe absorbers to enlarge grains and improve photoluminescence. These material improvements result in the highest PEC CO2 RR photocurrent density reported for planar ZnTe and >50% Faradaic efficiency to CO formation with diaryliodonium additive in the solution. These results pave the way to integration of polycrystalline thin-film photoabsorbers in PEC CO2 RR systems.

We demonstrate the new concept of using unit cell volume coefficient of variation to approximate the enthalpic penalty of high-entropy alloy (HEA) candidates, and use it along with configurational entropy to map promising HEA halide perovskites.

Despite the outstanding progress in performance of halide perovskite solar cell absorbers fabricated via vapor-based approaches, increasing deposition rates as well as enabling continuous deposition has been woefully neglected. In fact, recent reports show deposition times for the fabrication of high performing absorbers typically in the range of hours, being orders of magnitude away from industrially reasonable process times. In this work, continuous flash sublimation (CFS) of halide perovskite absorbers is introduced as a concept to overcome the fundamental rate and continuity limitations of current approaches, while at the same time maintaining the performance of previously reported vapor deposition approaches. Using CFS, we reduce the time required to deposit a fully absorbing layer to less than 5 minutes, demonstrating the applicability of vapor deposited halide perovskites for commercialization. Additionally, the approach enables continuous deposition, thus circumventing another major bottleneck of established vapor deposition approaches. Continuous Flash Sublimation (CFS) is a vapor deposition technique that shows commercially relevant deposition rates, is amenable to continuous operation, and exceeds previously reported efficiencies for the same material class.

A hierarchical transparent back contact leveraging an AlGaOx passivating layer, Ti3C2Tx MXene with a high work function, and a transparent cracked film lithography (CFL) templated nanogrid is demonstrated on copper-free cadmium telluride (CdTe) devices. AlGaOx improves device open-circuit voltage but reduces the fill factor when using a CFL-templated metal contact. Including a Ti3C2Tx interlayer improves the fill factor, lowers detrimental Schottky barriers, and enables metallization with CFL by providing transverse conduction into the nanogrid. The bifacial performance of an AlGaOx/Ti3C2Tx/CFL gold contact is evaluated, reaching 19.5% frontside efficiency and 2.8% backside efficiency under 1-sun illumination for a copper-free, group-V doped CdTe device. Under dual illumination, device power generation reached 200 W/m2 with 0.1 sun backside illumination.

Although foundational to multi-junction (MJ) photoelectrochemical (PEC) device synthesis, monolithic integration presents major limitations in process compatibility. Consequently, conductive adhesive-based interconnection scheme emerged as a suitable method to overcome limitations and to integrate incompatible material classes into MJ devices without compromising the integrity of the constituent layers. As the conductive adhesive generally employ a polymer matrix, the conductivity of this composite material depends on the conductive filler material. Among which, a core-shell type silver coated PMMA (Ag-PMMA) conductive microsphere filler has gained significant interest in the PV and PEC community due to its ability to exhibit reliable out-of-plane electrical conductivity (0.1 Ω-cm2) and good optical transparency (T > 90%), as we demonstrate for an epoxy-based transparent conductive composite (TCC) consisting of low particle loading (0.1 - 5 vol%). By implementation of the Ag-PMMA based TCC paired with a device exfoliation method, further referred to as semi-monolithic integration, independently processed substrate-grown single junction (SJ) devices were successively bonded and transferred via exfoliation onto a single host substrate to create a MJ device. As a proof-of-concept demonstration, we constructed the world’s first whole-chalcopyrite triple junction MJ device comprising 1.13 eV and 1.44 eV Cu(In,Ga)Se2 and 1.85 eV CuGa3Se5 sub-cells, with the TCC acting as the recombination layer between each sub-cell. The device exhibited an open circuit voltage of 1.85 V and is capable of splitting water with an STH efficiency of 3% in a PV-electrolysis configuration. Furthermore, a TCC-based bifacial MJ device comprising 1.7eV perovskite and 1.1eV Cu(In,Ga)Se2 sub-cells exhibited STH efficiency exceeding 10% in a PV-electrolysis configuration. Although the use of Ag-PMMA based TCC has shown to be an effective approach to prepare MJ devices, mechanical and electrical design and optimization is required to reliably appropriate the TCC as an optoelectronic device interconnect and to scale the semi-monolithic integration method. In the context of mechanical optimization, it has been shown that the degree of deformation of conductive microsphere filler is a key factor in establishing electrical connections. Up until now, the load-deformation characteristic of a single Ag-PMMA microsphere has not been measured. As such, we present in this communication a mechanical model based on measured load-deformation characteristics of a single Ag-PMMA microsphere to facilitate a spring model to better predict the deformation of multi-particle systems with respect to applied load. Furthermore, a finite element electrical model is implemented to determine the effects of electrical contacts across the sub-cell emitter layer and the resulting charge transport resistive losses. Hence, we demonstrate the critical factors influencing the quality of TCC as an anisotropic electrical interconnection layer, and provide a computational mechanical and electrical model to understand and mitigate parasitic resistive losses.

Bifacial CdTe solar cells with greater power density than the monofacial baselines are demonstrated by using a CuGaOx rear interface buffer that passivates while reducing sheet resistance and contact resistance. Inserting CuGaOx between the CdTe and Au increases mean power density from 18.0 ± 0.5 to 19.8 ± 0.4 mW cm-2 for one sun front illumination. However, coupling CuGaOx with a transparent conductive oxide leads to an electrical barrier. Instead, CuGaOx is integrated with cracked film lithography (CFL)-patterned metal grids. CFL grid wires are spaced narrowly enough (≈10 µm) to alleviate semiconductor resistance while retaining enough passivation and transmittance for a bifacial power gain: bifacial CuGaOx /CFL grids generate 19.1 ± 0.6 mW cm-2 for 1 sun front + 0.08 sun rear illumination and 20.0 ± 0.6 mW cm-2 at 1 sun front + 0.52 sun rear-the highest reported power density at field albedo conditions for a scaled polycrystalline absorber.

Nitride perovskites are an emerging class of materials that have been predicted to display a range of interesting physics and functional properties, but they are under-explored due to the difficulty of synthesizing oxygen-free nitrides. LaWN3, recently reported as the first oxygen-free nitride perovskite, exhibited polar symmetry and a large piezoelectric coefficient. However, the predicted ferroelectric switching was hindered by large leakage current, which motivates better understanding of its electronic structure and optical properties. Here, we study the structure and optoelectronic properties of thin film LaWN3 in greater detail, employing combinatorial techniques to correlate these properties with cation stoichiometry. We report a two-step synthesis that utilizes a more common RF substrate bias instead of a nitrogen plasma source, yielding nanocrystalline films that are crystallized by ex-situ annealing. We investigate the structure and composition of these films, finding polycrystalline La-rich and highly textured W-rich films. The optical absorption onset and temperature- and magnetic field-dependent resistivity are consistent with semiconducting behavior and are highly sensitive to cation stoichiometry, which may be related to amorphous impurities: metallic W or WNx in W-rich samples and insulating La2O3 in La-rich samples. The fractional magnetoresistance is linear and small, consistent with defect scattering, and a W-rich sample has n-type carriers with high densities and low mobilities. We demonstrate a photoresponse in LaWN3: the resistivity of a La-rich sample is enhanced by 28% at low temperature, likely due to a defect trapping mechanism. The physical properties of LaWN3 are highly sensitive to cation stoichiometry, like many oxide perovskites, which therefore calls for precise composition control to utilize the interesting properties observed in this nitride perovskite.

K, Rb, and Cs improve Cu(In,Ga)Se-2 (CIGS) solar cell performance, but the mechanism remains unclear. Here we use air-free transfer of multiple samples to study KF post-deposition treatments (PDTs) by X-ray photoelectron spectroscopy. The KF PDTs do not change the majority carrier concentration or Cd in diffusion, but they boost efficiency by 6.1% absolute, improve minority carrier lifetime, and shift the surface valence band further from the Fermi energy. Unlike former reports, the valence-band shift is not a result of lower Cu/(Ga+In) or higher K/(K+Cu) composition ratios. We propose that instead, KF PDTs alter the surface valence-band alignment through a reconstructive phase transformation from chalcopyrite CIGS to K2CuIn3Se6 or KInSe2 , which have layered structures. These compounds can leave Cu-free cation planes after K is rinsed away, unlike the structure of chalcopyrite CIGS or CuIn3Se5.

Anand et al. introduce an implicit equation for transparent conductive electrode (TCE) merit in photovoltaics (PV) by lumping series resistance in with the diode. However, an explicit equation has been previously derived by lumping series resistance external to the diode. For TCEs of practical interest for PV, the choice of where to lump series resistance does not affect merit, so external lumping is preferred. On the other hand, monolith deadspace, metal grid shadowing, and grid resistance shift TCE merit away from the figure of merit behavior published by Anand et al.

All-back-contact perovskite solar cells promise greater power conversion efficiency compared to conventional planar device architectures. However, the best-performing devices to date use photolithography to fabricate electrodes, which is expensive for deployment and a barrier for research facilities. Herein, we utilize cracked film lithography, a solution-processed micropatterning technique, to form an interconnected, defect-tolerant back-contact electrode network. We introduce a crack widening technique to control the optical transparency and sheet resistance while decoupling the relative areas of the electron and hole contacts in the back-contact network. Wider cracks increase the area of the hole-selective contact, which increases photocurrent and power conversion efficiency.

Despite the outstanding progress in performance of halide perovskite solar cell absorbers fabricated via vapor-based approaches, increasing deposition rate in these approaches has been neglected nearly completely. In fact, today’ deposition times for the fabrication of high performing absorbers are typically in the range of up to several hours, being orders of magnitude away from industrially reasonable deposition times. In this work, continuous flash sublimation of inorganic halide perovskite absorbers is introduced as a concept to overcome this fundamental limitation of state-of-the-art approaches. Solar cells employing flash-sublimed CsPbI2Br absorbers reach power conversion efficiencies as high as 12.3%, making them one of the most efficient and, with deposition times below five minutes, at the same time the fastest vapor-processed inorganic perovskite absorbers reported so far. Besides the promising performance and significantly higher deposition rates, the approach enables continuous deposition, thus, circumventing another major bottleneck of established vapor deposition approaches toward commercialization.

Yungchieh Lai, Nicholas B. Watkins, Christopher Muzzillo, Matthias Richter, Kevin Kan, Lan Zhou, Joel A. Haber, Andriy Zakutayev, Jonas C. Peters, Theodor Agapie, John M. Gregoire 1 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA 2 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA 3 Materials Science Center, National Renewable Energy Laboratory, Golden, CO, USA E-mail: gregoire@caltech.edu (J.M.G.), agapie@caltech.edu (T.A.), jpeters@caltech.edu (J.C.P.)

In this work we test field-relevant potential-induced degradation (PID) behavior by encapsulating laboratory Cu (In,Ga)Se-2 (CIGS) solar cells and applying +1000 V uniformly on the face of the front glass. In this configuration, we find that K-rich borosilicate glass reduces the extent of PID relative to Na-rich soda-lime glass. We also find that the standard testing protocol of stressing cells at short-circuit leads to faster PID than stressing cells at open circuit. We characterize two types of CIGS PID: The first, front shunting PID, is driven by front-glass stress and occurs when alkali metal cations accumulate in the i-ZnO buffer, where they increase shunt conductance to reduce fill factor. The second, p-n junction PID, results from back-glass stress as alkali metal cations pile up near the CIGS surface/CdS buffer, where they reduce charge carrier concentration, open-circuit voltage, and fill factor to degrade efficiency similar to 160 times faster than front shunting PID.