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We describe highly efficient and stable bifacial perovskite solar cells incorporating carbon nanotube network films as a back contact enabling perovskite/CIS 4-terminal tandem solar cells to reach apower conversion efficiency of over 27%.

The scalability of highly efficient organic–inorganic perovskite solar cells (PSCs) is one of the major challenges of solar module manufacturing. Various scalable methods have been explored to strive for uniform perovskite films of high crystal quality on large-area substrates, but each of these methods has individual limitations on the potential of successful commercialization of perovskite photovoltaics. Here, we report a fully scalable hybrid process, which combines vapor- and solution-based techniques to deposit high quality uniform perovskite films on large-area substrates. This two-step process does not use toxic solvents, and it further allows easy implementation of passivation strategies and additives. We fabricate PSCs based on this process and use blade coating to deposit a SnO2 electron transporting layer and Spiro-OMeTAD hole transporting layer without halogenated solvents in ambient air. The fabricated PSCs have achieved open-circuit voltage up to 1.16 V and power conversion efficiency of 18.7% with good uniformity on 5 cm × 5 cm substrates., A novel and scalable PVD/blade coating method of the perovskite absorber is presented. In conjunction with charge transporting layers blade coated in ambient air using non-toxic solvents, these perovskite solar cells achieved 18.7% efficiency.

Lithium garnet Li7La3Zr2O12 (LLZO) is being investigated as a potential solid electrolyte for next-generation solid-state batteries owing to its high ionic conductivity and electrochemical stability against metallic lithium and high potential cathodes. While the LLZO/Li metal anode interface has been thoroughly investigated to achieve almost negligible interface resistances, the LLZO/cathode interface still suffers from high interfacial resistances mainly due to the high-temperature sintering required for proper ceramic bonding. In this work, the LLZO solid electrolyte/LiCoO2 (LCO) cathode interface is investigated in an all-thin-film model system. This architecture provides an easy access to the interface for in situ and ex situ characterization, allowing one to identify the degradation processes taking place under high-temperature cosintering and to test solutions such as interface modifications. Introducing an in situ-lithiated Nb2O5 diffusion barrier at the interface, we were able to lower the LLZO/LCO charge transfer resistance to about 50 Ω cm2, a 3-fold reduction with respect to previously reported values. The low interfacial resistance combined with the high conductance through the LLZO thin-film electrolyte allows one to investigate the charge transfer at high charge-discharge rates, unlike in bulk systems. At 1C, discharge capacities of about 140 mA h g-1 were measured, and at 10C, 60% of the theoretical capacity was retained with a cycle life over 100 cycles. Besides the role of this architecture in the interface investigation, this work also constitutes a milestone in the development of thin-film solid-state batteries with higher power densities.

y High specific power, high stowed packing efficiency, low processing cost, and high tolerance against environmental threats (high energy and charged particle radiation) make perovskite solar cell (PSC) a promising candidate for power generation in space. However, vacuum, as encountered in space, causes perovskite outgassing, raising concern for its long-term stability. In this work, we find that PSCs (ITO/SnO2/perovskite/Spiro-MeOTAD/Au) degrade ten times faster upon reducing the pressure from 9 x 10(4) to 5 x 10(3) Pa during operation, due to acceleration of the perovskite transformation and ion migration. Gas permeability of the layers atop perovskite and mobile ion-induced chemical reactions at charge transporting layers and related interfaces are two critical factors. We develop a PSC structure (ITO/PTAA/perovskite/PCBM/ZnO/AZO/[Ni/Al grid]) that effectively mitigates vacuum and illumination-induced degradation pathways, enabling PSCs to realize a low PCE loss rate of 0.007%/h over 1,037 h at the maximum power point under 100 mW cm(-2) illumination at 5 x 10(3) Pa.

Cu(In,Ga)Se2 photovoltaic technology has notably progressed over the past years. Power conversion efficiencies above 23% were reached in spite of the polycrystalline nature of the absorber. Although efficiencies are still far from the practical limits, the material quality is approaching that of III?V compounds that yield the most efficient solar cells. The high carrier lifetime, low open circuit voltage deficit and external radiative efficiency in the single-digit percentage range suggest that the next efficiency boost may arise from the implementation of alternative device architectures. In this perspective paper, we describe the current challenges and pathways to enhance the power conversion efficiency of Cu(In,Ga)Se2 solar cells. Specifically, we suggest the use of non-graded absorbers, integration of charge selective contacts and maximization of photon recycling. We examine these concepts by a semi-empirical device modelling approach, and show that these strategies can lead to efficiencies of 29% under the AM1.5 global spectrum. An analysis of whether or not current state-of-the-art Cu(In,Ga)Se2 solar cells already benefit from photon recycling is also presented. This work received financial support partially from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet) and from the Swiss Federal Office of Energy (SFOE) (SI/501614-01 “ImproCIS”). The EMPIR programme is co-financed by the Participating States and by the European Union's Horizon 2020 research and innovation programme.

The open-circuit voltage (VOC) is the main limitation to higher efficiencies of Cu(In,Ga)Se2 solar cells. One of the most critical parameters directly affecting VOC is the charge carrier lifetime. Therefore, it is essential to evaluate the extent to which inhomogeneities in material properties limit the carrier lifetime and how postdeposition treatments (PDTs) and growth conditions affect material properties. Time-resolved photoluminescence (TRPL) microscopy is employed at conditions similar to one sun to study carrier lifetime fluctuations in Cu(In,Ga)Se2 with light (Na) and heavy (Rb) alkalis, different substrates, and grown at different temperatures. PDT lowers the amplitude of minority carrier lifetime fluctuations, especially for Rb-treated samples. Upon PDT, the grains’ carrier lifetime increases, and the analysis suggests a reduction in grain boundary recombination. Furthermore, lifetime fluctuations have a small impact on device performance, whereas VOC calculated from TRPL (and continuous-wave PL) agrees with device values within the limits of investigated PDT samples. Finally, up to about half a per cent external radiative efficiencies are experimentally determined from TRPL metrics, and internal radiative efficiencies are approximated. The findings demonstrate that the highest absorber material quality investigated is still limited by nonradiative recombination (grain or grain boundary) and is comparable to state-of-the-art absorbers. This work received financial support in part from the Swiss State Secretary for Education, Research and Innovation (SERI) under Contract No. 17.00105 (EMPIR project HyMet) and from the Swiss Federal Office of Energy (SFOE) (SI/501614-01 ‘‘ImproCIS''). The EMPIR programme was cofinanced by the Participating States and by the European Union's Horizon 2020 research and innovation programme.

Among cathode materials, LiNi$_{0.8}$Mn$_{0.1}$Co$_{0.1}$O$_2$ (NMC811) is the most discussed for high performance Li-ion batteries, thanks to its capacity of ≈200 mAh g$^{-1}$ and low Co content. Here, it is demonstrated that NMC811 can reversibly accommodate more than one Li-ion per formula unit when coupled with a solid-state electrolyte, thus significantly increasing its capacity. Sputtered Li-rich NMC811 cathodes are tested with lithium–phosphorus–oxynitride as a solid-state electrolyte in a thin-film architecture, which is a simplified 2D model with direct access to the cathode-electrolyte interface. The solid-state electrolyte helps to stabilize the interface and prevents capacity fading, voltage decay, and interface resistance growth, thus allowing cycling at extended voltage ranges of 1.5–4.7 V. While the liquid electrolyte cells suffer from rapid capacity decay, the Li-rich NMC811 cells with the solid-state electrolyte can cycle at a fast rate and an initial capacity of 149 mAh g$^{-1}$ from 1.5 to 4.3 V for 1000 cycles. The all-solid-state thin-film cells with a lithium metal anode yield a discharge capacity of up to 350 mAh g$^{-1} $at C/10 because of multi-electron cycling with a coulombic efficiency of 90.1%. The results demonstrate how solid-state electrolytes that are stable against NMC811 cathodes can unlock the full potential of this Li-rich and Ni-rich cathode class., Advanced Energy Materials, 12 (40)

Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF2 additive. Herein, we revealed the spatial distribution of SnF2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF2 to a fluorinated oxy-phase on the Pb-Sn perovskite film surface, due to its rapid oxidation. In addition, at the buried perovskite interface, we detected and visualized the accumulation of F- ions. We found that the photoluminescence quantum yield of Pb-Sn perovskite reached the highest value with 10 mol% SnF2 in the precursor solution. When integrating the optimized absorber in flexible devices, we obtained the flexible Pb-Sn perovskite narrow bandgap (1.24 eV) solar cells with an efficiency of 18.5% and demonstrated 23.1%-efficient flexible 4-terminal all-perovskite tandem cells.

Among various types of perovskite based tandem solar cells TSCs , all perovskite TSCs are of particular attractiveness for building and vehicle integrated photovoltaics, or space energy areas as they can be fabricated on flexible and lightweight substrates with a very high power to weight ratio. However, the efficiency of flexible all perovskite tandems is lagging far behind their rigid counterparts primarily due to the challenges in developing efficient wide bandgap WBG perovskite solar cells on the flexible substrates as well as their low open circuit voltage VOC . Here, it is reported that the use of self assembled monolayers as hole selective contact effectively suppresses the interfacial recombination and allows the subsequent uniform growth of a 1.77 eV WBG perovskite with superior optoelectronic quality. In addition, a postdeposition treatment with 2 thiopheneethylammonium chloride is employed to further suppress the bulk and interfacial recombination, boosting the VOC of the WBG top cell to 1.29 V. Based on this, the first proof of concept four terminal all perovskite flexible TSC with a power conversion efficiency of 22.6 is presented. When integrating into two terminal flexible tandems, 23.8 flexible all perovskite TSCs with a superior VOC of 2.1 V is achieved, which is on par with the VOC reported on the 28 all perovskite tandems grown on the rigid substrate

Garnet Li7La3Zr2O12 (LLZO) is a promising solid-state electrolyte due to its wide electrochemical stability window and high Li-ion conductivity. This electrolyte has potential to be employed in the form of thin films for solid-state batteries, a promising approach in the quest for safer batteries with higher energy densities at lower fabrication costs. In this study, we use a scalable cosputtering process to fabricate LLZO thin films with subsequent postannealing at a temperature of 700 °C, significantly below the sintering temperatures employed in ceramic pellet processing. We investigate the roles that Li excess and incorporation of Al play in the film’s crystalline phase, microstructure, phase stability, and, ultimately, ionic conductivity. Our results reveal that improving the conductivity of LLZO thin films requires not only the stabilization of the cubic phase but especially the densification of the film and the minimization of the proton exchange degradation mechanism in the presence of moisture an...

Two key strategies for enhancing the efficiency of Cu(In,Ga)Se2 solar cells are the bandgap gradient across the absorber and the incorporation of alkali atoms. The combined incorporation of Na and Rb into the absorber has brought large efficiency gains compared to Na-containing or alkali-free layers. Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition-graded Cu(In,Ga)Se2 solar cells. Electron lifetimes are found to be highly dependent on the film composition and morphology, varying from tens of nanoseconds in the energy notch to only ≈100 ps in the Ga-rich region near the Mo-back contact. NaF PDT improves recombination lifetimes by a factor of 2–2.5 in all regions of the absorber, whereas the effectiveness of the RbF PDT is found to decrease for higher Ga-concentrations. Electron mobility measured in the absorber region with large grains is promoted by both alkali PDTs. The data suggest that NaF PDT passivates shallow defect states (Urbach tail) throughout the Cu(In,Ga)Se2 film (including the interior of large grains), whereas the additional RbF PDT is effective at grain boundary surfaces (predominantly in regions with medium to low Ga-concentrations). Y.-H.C. thanks the Ministry of Education of Taiwan for her Ph.D. scholarship, Dr. Michael Sachs, and Dr. Carlota Bozal-Ginesta from Imperial College London for the fruitful discussions and aid on TA data. J.R.D. would like to thank the UKRI Global Challenge Research Fund project SUNRISE (EP/P032591/1). L.S. acknowledges funding from the European Research Council (H2020-MSCA-IF-2016, Grant No. 749231). This work also received financial support partially from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is co-financed by the Participating States and by the European Union's Horizon 2020 research and innovation programme.

Passivation and interlayer engineering are important approaches to increase the efficiency and stability of perovskite solar cells. Thin insulating dielectric films at the interface between the perovskite and the charge carrier transport layers have been suggested to passivate surface defects. Here, we analyze the effect of depositing poly(methyl methacrylate) (PMMA) from a very low-concentration solution. Spatial- and time-resolved photoluminescence and atomic force microscopy analyses of samples with diverse morphologies demonstrate the preferential deposition of PMMA in topographic depressions of the perovskite layer, such as grain and domain boundaries. This treatment results in an increase in the fill factor of more than 4% and an absolute efficiency boost exceeding 1%, with a maximum efficiency of 20.4%. Based on these results, we propose a physical isolation mechanism rather than a chemical passivation of perovskite defects, which explains not only the data of this study but also most results found in earlier works. This work was partially funded by a Swiss Government Excellence Scholarship (2017.1080) and by the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie project PerSisTanCe with grant agreement No. 841005 (E.O.M.). It also received financial support from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is cofinanced by the Participating States and by the European Union’s Horizon 2020 research and innovation programme. E.O.M., R.D.O., P.F., and U.S. acknowledge financial support by the Adolphe Merkle Foundation. E.O.M.would like to thank Dr. Silver Hamill Turren-Cruz from Helmholtz-Zentrum Berlin for fruitful discussions and Dr. Jovana Milic from the University of Fribourg and Brian Carlsen from the Laboratory of Photomolecular Science (EPFL) for facilitating and carrying out stability measurements.

The performance of Cu(In,Ga)Se2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (VOC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (ΔGGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the ΔGGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both VOC deficit and time-resolved photoluminescence lifetime are observed with increasing ΔGGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a ΔGGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high-performance CIGS solar cells and devices. Bundesamt für Energie, Grant/Award Number: SI/501614-01; Horizon 2020 Framework Programme, Grant/Award Number: EMPIR project HyMet; Swiss State Secretary for Education, Research and Innovation (SERI), Grant/Award Number: 17.00105 (EMPIR project HyMet)

The major obstacle to commercialization of high-voltage Li-ion batteries is the lack of oxidatively stable and inexpensive current collectors that can operate at potentials of up to 5 V vs Li+/Li. In this work, we present titanium nitride as a compelling cathode current collector for high-voltage Li-ion batteries exhibiting higher oxidative stability in LiPF6 and lithium bis(fluorosulfonyl)imide electrolytes than aluminum or stainless steel current collectors. Its high oxidative stability has been assessed with a high-voltage LiMn1.5Ni0.5O4 cathode. TiN/LiMn1.5Ni0.5O4 half cells demonstrated a high Coulombic efficiency of 98.5% at a low C-rate of 0.2 C after 100 cycles.

Over the last years, the operational stability of perovskite solar cells has been significantly improved by compositional engineering, interface modification, and improved encapsulation techniques. However, irreversible degradation is still ubiquitously observed during the first 1000 hours of operation, particularly at elevated temperatures. In this work, we elucidate a major mechanism controlling this degradation. For that, formamidinium lead iodide-based perovskite solar cells were stressed under continuous 1 sun illumination at 80 °C in N2, before extensive characterization of their microstructure and composition. The cell active area and hence the photocurrent are found to decrease with time due to the growth of porous PbI2-rich regions. This degradation was observed to originate from a few seed points in the perovskite bulk rather than from the interfaces with the charge-selective layers or from the cell edges. I2 vapor, first released at these defective points and then further released from the decomposition of the perovskite exposed to this vapor, controls the degradation process. Furthermore, this autocatalytic degradation process is shown to locally rupture the top electrode due to vapor pressure build-up. In addition to highlighting the detrimental influence of residual PbI2, we show that such a degradation pathway can be alleviated by reducing the methylammonium and/or iodine content, providing a path to more stable perovskite solar cells.

The performance improvement of conventional CdTe solar cells is mainly limited by doping concentration and minority carrier life time. Alloying CdTe with an isovalent element changes its properties, for example its band gap and behaviour of dopants, which has a significant impact on its performance as a solar cell absorber. In this work, the structural, optical, and electronic properties of CdTe1-xSex films are examined for different Se concentrations. The band gap of this compound changes with composition with a minimum of 1.40 eV for x = 0.3. We show that with increasing x, the lattice constant of CdTe1-xSex decreases, which can influence the solubility of dopants. We find that alloying CdTe with Se changes the effect of Cu doping on the p-type conductivity in CdTe1-xSex, reducing the achievable charge carrier concentration with increasing x. Using a front surface CdTe1-xSex layer, compositional, structural and electronic grading is introduced to solar cells. The efficiency is increased, mostly due to an increase in the short-circuit current density caused by a combination of lower band gap and a better interface between the absorber and window layer, despite a loss in the open-circuit voltage caused by the lower band gap and reduced charge carrier concentration.

Structural defects such as voids and compositional inhomogeneities may affect the performance of Cu(In,Ga)Se2 (CIGS) solar cells. We analyzed the morphology and elemental distributions in co-evaporated CIGS thin films at the different stages of the CIGS growth by energy-dispersive x-ray spectroscopy in a transmission electron microscope. Accumulation of Cu-Se phases was found at crevices and at grain boundaries after the Cu-rich intermediate stage of the CIGS deposition sequence. It was found, that voids are caused by Cu out-diffusion from crevices and GBs during the final deposition stage. The Cu inhomogeneities lead to non-uniform diffusivities of In and Ga, resulting in lateral inhomogeneities of the In and Ga distribution. Two and three-dimensional simulations were used to investigate the impact of the inhomogeneities and voids on the solar cell performance. A significant impact of voids was found, indicating that the unpassivated voids reduce the open-circuit voltage and fill factor due to the introduction of free surfaces with high recombination velocities close to the CIGS/CdS junction. We thus suggest that voids, and possibly inhomogeneities, limit the efficiency of solar cells based on three-stage co-evaporated CIGS thin films. Passivation of the voids’ internal surface may reduce their detrimental effects., Graphical Abstract

Flexible perovskite solar cells (PSCs) hold great promise for the low-cost roll-to-roll production of lightweight single- and multijunction photovoltaic devices. Among the different deposition methods used for the perovskite absorber, the two-step hybrid vacuum-solution approach enables precise control over the thickness and morphology of PbI2. However, efficient conversion to perovskite is limited by diffusion of the organic cations in the compact lead halide layer. Herein, a multistage absorber deposition is developed by thermal evaporation of PbI2 and spin coating of CH3NH3I (MAI). The process relies on the different types of growth of vacuum-deposited PbI2 onto amorphous and crystalline surfaces. This approach represents a way to effectively increase the absorber thickness while tackling the limited MAI diffusion in the compact PbI2 film via a two-step deposition method. The efficiency of flexible PSCs is improved from 14.2 to 15.8% with multistage deposition. Furthermore, the use of an amorphous transparent conductive oxide (TCO), InZnO, enhances the mechanical resistance against bending with respect to conventional crystalline TCO-based flexible devices. Near-infrared transparent flexible PSCs are developed with an efficiency of 14.0% and average transmittance of ~74% between 800 and 1000 nm. Flexible perovskite/CIGS thin-film tandem devices are demonstrated with an efficiency of 19.6% measured in the four-terminal configuration. The efficiency of printable solar cells can be increased using a production method developed by researchers in Switzerland. Hybrid perovskites, a material that combines organic and inorganic components, are emerging as a competitive alternative to silicon for producing solar cells. One of their major advantages is that perovskite devices can be created on flexible substrates, making them compatible with a technology that prints devices on a roll of plastic. This means they can be cheaply mass produced, however their conversion efficiency needs to be improved. Stefano Pisoni and colleagues from the Swiss Federal Laboratories for Materials Science and Technology in Duebendorf constructed a perovskite solar cell using a two-step method that combined thermally evaporated lead iodide and a coating of methylammonium iodide. This design enabled better diffusion of the organic cations, which improved the device efficiency. Flexible perovskite solar cell with an efficiency of 15.8 % via tailoring of vacuum-deposited PbI2 growth morphology has been achieved. We demonstrated superior mechanical bending stability using amorphous TCO (retaining 80 % of the initial efficiency after 1000 bending cycles at 4 mm bending radius). Flexible NIR-transparent perovskite solar cell with an efficiency of 14.0 % and average transmittance of ~74 % between 800 and 1000 nm has been developed. Eventually, we proved a flexible perovskite/CIGS tandem solar cell with an efficiency of 19.6 % measured in four-terminal configuration.

We explore the feasibility of Ag fiber meshes as electron transport layer for high-efficiency flexible Cu(In,Ga)Se2 (CIGS) solar cells. Woven meshes of Ag fibers after UV illumination and millisecond flash-lamp treatment results in a sheet resistance of 17 Ω/sq and a visible transmittance above 85%. Conductive Ag meshes are integrated into flexible CIGS cells as transparent conductive electrode (TCE) alone or together with layers of Al-doped ZnO (AZO) with various thickness of 0…900 nm. The Ag mesh alone is not able to function as a current collector. If used together with a thin AZO layer (50 nm), the Ag mesh markedly improves the fill factor and cell efficiency, in spite of the adverse mesh shadowing. When Ag mesh is combined with thicker (200 nm or 900 nm) AZO layers, no improvements in photovoltaic parameters are obtained. When comparing a hybrid TCE consisting of 50 nm AZO and Ag fiber mesh with a thick 900 nm reference AZO device, an improved charge carrier collection in the near-infrared range is observed. Regardless of the AZO thickness, the presence of Ag mesh slows down cell degradation upon mechanical tensile stress, which could be interesting for implementation into flexible thin film CIGS modules., Graphical Abstract

Bandgap grading is often employed in thin film solar cell absorbers for creating the back surface field that can reduce interface recombination at the back contact. Here, we investigate different pathways to obtain back graded Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells based on a co-sputtered metal precursor and rapid thermal annealing route. The absorber bandgap can be precisely tuned for the whole compositional range of x = 0…1. While Ge does not accumulate towards the back in absorbers fabricated from uniform precursor, Ge-back graded absorbers can be obtained from stacked metal precursors. A linear back grading with a bandgap energy difference of up to 40 meV has been achieved. However, no significant improvement in open-circuit voltage and near-infrared response could be observed for the kesterite devices. This indicates that even steeper gradients are required to obtain an effective back surface field.

Sodium is typically used during the synthesis of kesterite thin films to enhance the performance of solar cells. As sodium tends to affect grain growth and morphology, it is difficult to analyse solely the electronic effects of sodium as dopant. To decouple the structural and electronic effects from each other, two processes were designed in this work to successfully incorporate sodium into a vacuum-processed Cu2ZnSnSe4absorber without changing the morphology. A thin layer of NaF is deposited before precursor deposition (Pre-NaF) or after absorber synthesis to undergo a post deposition treatment (NaF-PDT). While composition and distribution of matrix elements remain unchanged, the sodium concentration is increased upon sodium treatment up to 140 ppm as measured by inductively coupled plasma mass spectrometry. X-ray photoelectron spectroscopy showed that the surface composition was not altered. Within its detection limit, sodium was not present at the absorber surface. For a Pre-NaF sample measured with atom probe tomography a sodium concentration of 30 ppm was measured in a grain, suggesting that sodium might segregate at grain boundaries. The additional sodium content in the film leads to an increased acceptor concentration, which results in improved open-circuit voltage and fill factor., Financial support from the Swiss National Science Foundation (SNF) in the network of the Indo-Swiss Joint Research Programme (ISJRP) [IZLIZ2_157140/1] is gratefully acknowledged. T. Schwarz is grateful for the support of the German Research Foundation (DFG) [Contract GA 2450/1-1]. R. Caballero acknowledges financial support from Spanish MINECO within the Ramón y Cajal program [RYC-2011-08521], MINECO project WINCOST [ENE2016-80788-C5-2-R] and from Spanish Ministry of Education, Culture and Sport within the José Castillejo program [CAS 15/00070]

Thin film Cu(In,Ga)Se2 (CIGS) solar modules can be grown on flexible and lightweight substrates allowing their direct integration into building elements. Such multifunctional building elements would significantly reduce the installation cost of photovoltaic systems provided that CIGS solar cells with high conversion efficiency can be obtained. Also, there is a need for low cost substrate foil. Mild steel is a promising low cost substrate material due to its excellent mechanical stability and already wide acceptance as component in building envelopes and in numerous other applications. During the growth of the CIGS absorber layer certain elements, e.g. iron, can diffuse from the metallic substrate into the semiconductor deteriorating the device performance. Here we present an effective diffusion barrier and device architecture for processing of highly efficient CIGS solar cells on mild steel substrates. The CIGS absorber layers were grown on mild steel foils by a multistage co-evaporation process at different substrate temperatures. The mild steel substrates were plated with an industrially scalable electrodeposited Ni/Cr bi-layer. The diffusion barrier layer properties of this Ni/Cr coating were investigated directly by measuring the metallic impurities within the absorber by secondary ion mass spectrometry and indirectly by admittance spectroscopy. The Ni/Cr bi-layer was found to be effective up to a nominal CIGS growth temperature of 500 °C. A certified cell efficiency of 18.0% was achieved on a Ni/Cr coated mild steel substrate using a low temperature CIGS deposition process and a NaF and RbF post deposition treatment method.

The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses (Δ

Lithium metal dendrites have become a roadblock in the realization of next-generation solid-state batteries with lithium metal as high-capacity anode. The presence of surface and bulk inhomogeneities with non-negligible electronic conductivity in crystalline electrolytes such as the lithium garnet Li7La3Zr2O12 (LLZO) facilitates the growth of lithium filaments, posing a critical safety risk. Here we explore the amorphous phase of LLZO (aLLZO) as a lithium dendrite shield owing to its grain-boundary-free microstructure, stability against metallic lithium, and high electronic insulation. We demonstrate that by tuning the lithium stoichiometry in sputtered aLLZO films, the ionic conductivity can be increased up to 10-7 S cm-1 while retaining an ultralow electronic conductivity of 10-14 S cm-1. In Li/aLLZO/Li symmetric cells, plating-stripping results in no degradation of the films and current densities up to 3.2 mA cm-2 can be applied with no signs of lithium penetration. The defect-free and conformal nature of the films enables microbatteries with an electrolyte thickness as low as 70 nm, which withstand charge-discharge at 0.2 mA cm-2 for over 500 cycles. Finally, we demonstrate that the application of aLLZO as a coating on crystalline LLZO lowers the interface resistance and significantly impedes the formation of lithium dendrites, increasing the critical current density of a symmetric cell up to 1.3 mA cm-2 at room temperature and without external pressure. The effectiveness of the amorphous Li-La-Zr-O as lithium dendrite blocking layer can accelerate the development of more powerful and safer solid-state batteries.

Protective coatings applied to cathodes help to overcome interface stability issues and extend the cycle life of Li-ion batteries. However, it is difficult to isolate the effect of the coating because of the additives and non-ideal interfaces within 3D cathode composites. In this study we investigate niobium oxide (NbOx) as cathode coating in a thin-film model system, which allows assessing the cathode-coating-electrolyte interfaces. The conformal NbOx coating was applied by atomic layer deposition (ALD) onto thin-film LiCoO2 cathodes. The cathode/coating stacks were annealed to lithiate and ensure sufficient ionic conductivity. A range of different coating thicknesses were investigated to improve the electrochemical cycling as compared to the uncoated cathodes. At a NbOx thickness of 30 nm, the cells retained 80% of the initial capacity after 493 cycles at 10 C, more than doubling the cycle life of the uncoated cathode. At the same thickness, a residual initial capacitance of 47% remained even at very high charge-discharge rates of 100 C. Using impedance spectroscopy measurements, we find that the enhanced performance is due to suppressed interfacial resistance growth during cycling. Elemental analysis using TOF-SIMS and XPS further revealed a bulk and surface contribution of the NbOx coating. These results show that lithiated ALD NbOx can significantly improve the performance of layered oxide cathodes by inhibiting the cathode degradation, resulting in prolonged cycle life.

Achieving high power conversion efficiencies with Cu(In,Ga)Se2 (CIGS) solar cells grown at low temperature is challenging because of insufficient thermal energy for grain growth and defect annihilation, resulting in poor crystallinity, higher defect concentration, and degraded device performance. Herein, the possibilities for high-performing devices produced at very low temperatures (≤450 °C) are explored. By alloying CIGS with Ag by the precursor layer method, (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells grown at about 450 °C reach an efficiency of 20.1%. Only a small efficiency degradation (0.5% and 1.6% absolute) is observed for ACIGS absorbers deposited at 60 and 110 °C lower substrate temperature. CIGS devices exhibit a stronger efficiency degradation, driven by a decrease in the open-circuit voltage (Voc). The root cause of the Voc difference between ACIGS and CIGS devices is investigated by advanced characterization techniques, which show improved morphology, reduced tail states, and higher doping density in ACIGS absorbers. The proposed approach offers several benefits in view of depositions on temperature-sensitive substrates. Increased Cu diffusion promoted by Ag allows end-point detection in the three-stage process at the substrate temperatures below 300 °C. The modified process requires minimal modification of existing processes and equipment and shows the potential for the use of different flexible substrates and device architectures. This work received funding from the Swiss Federal Office of Energy (SFOE) under ImproCIS project (Contract no.: SI/501614-01) and from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is co-financed by the participating States and by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 850937. X.S. acknowledges funding from the ETH Zurich Postdoctoral Fellowship. M.K. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 850937. J.S. acknowledges funding from the Swiss National Science Foundation (grant number 200021_172764).

The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses amp; 916;Voc,nrad because of the reported decomposition reactions, an uncontrolled growth of MoSe2, or a nonoptimal electrical contact with high recombination. Several intermediate layers IL , such as MoO3, TiN, and ZnO, have been tested to mitigate these issues, and efficiency improvements have been reported. However, the introduction of IL also triggers other effects such as changes in alkali diffusion, altered morphology, and modifications in the absorber composition, all factors that can also influence amp; 916;Voc,nrad. In this study, the different effects are decoupled by designing a special sample that directly compares four rear structures SLG, SLG Mo, SLG Al2O3, and SLG Mo Al2O3 with a Na doped kesterite absorber optimized for a device efficiency gt;10 . The IL of choice is Al2O3 because of its reported beneficial effect to reduce the surface recombination velocity at the rear interface of solar cell absorbers. Identical annealing conditions and alkali distribution in the kesterite absorber are preserved, as measured by time of flight secondary ion mass spectrometry and energy dispersive X ray spectroscopy. The lowest amp; 916;Voc,nrad of 290 mV is measured for kesterite grown on Mo, whereas the kesterite absorber on Al2O3 exhibits higher nonradiative losses up to 350 mV. The anticipated field effect passivation from Al2O3 at the rear interface could not be observed for the kesterite absorbers prepared by the two step process, further confirmed by an additional experiment with air annealing. Our results suggest that Mo with an in situ formed MoSe2 remains a suitable back contact for high efficiency kesterite devices

Cu(In,Ga)Se2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post-deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga)Se2 films, the role of the Ga-gradient on carrier mobility has been less explored. Here, transient absorption spectroscopy (TAS) is utilized to investigate the impact of the Ga-gradient profile on charge carrier dynamics. Minority carriers excited in large Cu(In,Ga)Se2 grains with a [Ga]/([Ga]+[In]) ratio between 0.2–0.5 are found to drift-diffuse across ≈1/3 of the absorber layer to the engineered bandgap minimum within 2 ns, which corresponds to a mobility range of 8.7–58.9 cm2 V−1 s−1. In addition, the recombination times strongly depend on the Ga-content, ranging from 19.1 ns in the energy minimum to 85 ps in the high Ga-content region near the Mo-back contact. An analytical model, as well as drift-diffusion numerical simulations, fully decouple carrier transport and recombination behaviour in this complex composition-graded absorber structure, demonstrating the potential of TAS. Y.-H.C. Chang thanks the Ministry of Education of Taiwan for her Ph.D. scholarship, and Dr. Michael Sachs for fruitful discussions on TA data. J.R.D. would like to thank the UKRI Global Challenge Research Fund project SUNRISE (EP/P032591/1). L.S. acknowledges funding from the European Research Council (H2020-MSCA-IF-2016, Grant No. 749231). This work received financial support from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is co-financed by the Participating States and by the European Union’s Horizon 2020 research and innovation programme.

Amongst a number of deposition methods for perovskite layers, vapour based ones are promising for large area industrial production of solar cells. Different variants of such methods and high efficiencies have been reported recently, but there remains a lack of understanding on the formation process of perovskite layers with 2-step vapour deposition. Here, we present a new reactor design for a controlled investigation of the reaction kinetics for conversion of an evaporated metal halide precursor layer (such as a mixture of lead iodide and cesium bromide) into a perovskite layer by exposure to an organo-halide (such as formamidinium iodide) vapour under stable isobaric–isothermal conditions. With this new concept of gas flow reversal in a tubular reactor, we overcome an inherent problem of the lack of control over the precise start and end of the conversion process. We investigated the formation reaction of a mixed cation (Cs0.04FA0.96)PbI3perovskite in well-defined intermediate states to elucidate the influence of processing conditions on the kinetics of perovskite and other phase formations. A high conversion rate of up to 60 nm min−1is achieved with a well-controlled abrupt start and end of the vapor supply. Using our deposition method, a semitransparent solar cell with a power conversion efficiency (maximum power tracking) of 9.6% on a designated area of 0.27 cm2is achieved in the initial phase of development where the charge extracting layers and interfaces are yet to be optimised.

Four-terminal (4-T) tandem solar cells (e.g., perovskite/CuInSe2 (CIS)) rely on three transparent conductive oxide electrodes with high mobility and low free carrier absorption in the near-infrared (NIR) region. In this work, a reproducible In2O3:H (IO:H) film deposition process is developed by independently controlling H2 and O2 gas flows during magnetron sputtering, yielding a high mobility value up to 129 cm2 V–1 s–1 in highly crystallized IO:H films annealed at 230 °C. Optimization of H2 and O2 partial pressures further decreases the crystallization temperature to 130 °C. By using a highly crystallized IO:H film as the front electrode in NIR-transparent perovskite solar cell (PSC), a 17.3% steady-state power conversion efficiency and an 82% average transmittance between 820 and 1300 nm are achieved. In combination with an 18.1% CIS solar cell, a 24.6% perovskite/CIS tandem device in 4-T configuration is demonstrated. Optical analysis suggests that an amorphous IO:H film (without postannealing) and a partially crystallized IO:H film (postannealed at 150 °C), when used as a rear electrode in a NIR-transparent PSC and a front electrode in a CIS solar cell, respectively, can outperform the widely used indium-doped zinc oxide (IZO) electrodes, leading to a 1.38 mA/cm2 short-circuit current (Jsc) gain in the bottom CIS cell of 4-T tandems.

Direct nano-scale microanalysis is important for photovoltaic functional thin films to characterize their homogeneity and purity. This demands combining spatial resolution in the micro/nano-scale and sensitivity in the trace-level range, which is at the moment beyond state-of-the-art. As dictated by counting statistics, the reduction of the spot size degrades the detection limit. The utilization of a tabletop XUV laser at λ = 46.9 nm has shown to dramatically improve the ablation efficiency with respect to that of visible lasers, such that ablation spot of 1 μm limits. Li-doped Cu2ZnSn(S,Se)4 (so-called kesterite) thin films were irradiated across 3D ablation arrays for hyperspectral mapping by means of time-of-flight mass spectrometry. The nominal 3D data node lattices were the initialisation perceptron, filled with measured values, and for a detailed supervised learning postprocessing, the node-to-node links were analysed by means of a 2D-kernel covariance algorithm. The latter permitted to obtain robust 3D elemental distribution functions well below the measurement spacing, giving insights into the inhomogeneity and impurities.

Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post-deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se-2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open-circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single-phase Cu-alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries. This work was supported by the European Union's Horizon 2020 research and innovation program under grant agreement no. 641004 (Sharc25) and by the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number 15.0158. Siebentritt, S (reprint author), Univ Luxembourg, Lab Photovolta, Phys & Mat Sci Res Unit, 41 Rue Brill, L-4422 Belvaux, Luxembourg. susanne.siebentritt@uni.lu