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We explored how tellurium (Te) anion distribution as a function of doping concentration and substrate temperature effects on the photovoltaic (PV) performance of narrow-bandgap Cu(In, Ga)Se 2 thin film solar cells grown by a three-stage co-evaporation process. At low and moderate substrate temperatures (480 ℃ and 550 ℃), the inclusion of Te anions in the structure has negative impacts on photovoltaic parameters due to formation of disorder in the crystal structure, the deepening of the notch in the GGI gradient and the increase of defect-assisted recombination. At high substrate temperature (620 ℃), conversely, the presence of Te not only serves to improve the crystal structure and mitigate recombination through defect suppression but also facilitates the growth of large grains and attains a near-optimal GGI ratio (0.3) with a better gradient, which has a beneficial impact on the PV performance. These results highlight the role of Te anion distribution on PV performance of Cu(In, Ga)Se 2 thin film solar cells and its potential for highly efficient commercial thin-film solar cells in reasonable and effective ways as a function of substrate temperature and doping concentration.

Cu(In, Ga)Se2 (CIGSe) solar cells with a tunable bandgap stand out as a promising technology for tandem applications. Addressing the environmental concerns associated with Cd‐based buffers, this study investigates the suitability of zinc tin oxide (ZTO), deposited via chemical bath deposition (CBD), as a Cd‐free alternative for both low‐bandgap CIGSe and wide‐bandgap (Ag, Cu)(In, Ga)Se2 (ACIGSe) solar cells. Best ZTO‐buffered devices exhibit competitive power conversion efficiencies (PCE) of 14% and 7% for low‐bandgap and wide‐bandgap absorbers, respectively. The optimal tin concentration for ZTO buffer layers vary, with 10% [Sn]/([Sn] + [Zn]) ratio (TTZ) identified as optimal for wide‐gap ACIGSe and 20% TTZ for low‐gap CIGSe. A performance decline beyond optimal tin concentrations could be linked to losses in open‐circuit voltage. In summary, ZTO‐based devices showcase promising photovoltaic performance, emphasizing ZTO's potential as a practical and nontoxic alternative, deposited by CBD, to traditional CdS for diverse CIGSe solar cell applications. Considering the importance of Cu(In, Ga)Se2 (CIGS) solar cells for tandem applications, Cd‐free low and wide‐bandgap CIGSe solar cells are studied, based on the chemical bath deposition of Zn–Sn–O as an alternative buffer layer. The fabricated solar cells show a similar performance as CdS‐based reference, which proves the viability of this nontoxic buffer layer.

The partial replacement of Cu by Ag in Cu(In,Ga)Se2 thin-film solar cells is strategically interesting to achieve smooth devices with high conversion efficiencies. Yet, the industrial exploitation requires further understanding of the deposition process and control of the absorber layer properties. In this study, three-stage co-evaporation of (Ag,Cu)(Ga,In)Se2 films with [Ag]/([Ag] + [Cu]) contents up to 0.2 was investigated. Deep crevices and voids, sometimes extending down to the rear contact, were found. They mainly occur for high Ag contents and excessive group-I richness during the second stage of the deposition. The formation of cavities is attributed to the segregation of Ag-Se phases and slow Ag diffusion into the chalcopyrite during the deposition. Another identified challenge is the flattening of the desired bandgap grading which is correlated with the Ag content. Optimized process conditions allow fabrication of smooth (Ag,Cu)(Ga,In)Se2 films in a manufacturing-like inline deposition with cell efficiencies up to 20.5%.

In this contribution, voltage- and temperature-dependent admittance spectroscopy is used in concert with other electrical characterisation techniques to gain insight into composition-dependent variations in the meta-stable behaviour of the (Ag,Cu)(In,Ga)Se2 system. The extended thermal admittance spectroscopy techniques are powerful tools for the evaluation of meta-stabilities, providing multiple approaches for the determination of the likely origins of observed capacitance features. These capabilities, which enable cross-referencing and verification of capacitance feature labelling, enhance the confidence in conclusions deduced from the often ambiguous and complex admittance spectroscopy methods. Our results indicate that high levels of Ag alloying lead to charge transport barriers active at low temperatures (below ∼220 K) and, for the off-stoichiometric case, a shallow acceptor state ∼150meV above the valence band edge. Lightsoaking (LS)-induced increases in the activation energy and, consequently, decreases in the occupation of this acceptor state closely reflect the decrease in net doping concentration induced by LS in the off-stoichiometric high-Ag device. The presence of the acceptor state also correlates with the large difference in net doping concentration between the close- and off-stoichiometric high-Ag devices. For moderate Ag-alloying and Ag-free devices, it is shown that off-stoichiometry introduces secondary capacitance features, alongside a common feature observed in both close- and off-stoichiometric devices. These features are determined to originate from a combination of deep defects and charge transport barriers.

Photovoltaic (PV) energy conversion of sunlight into electricity is now a well-established technology and a strong further expansion of PV will be seen in the future to answer the increasing demand for clean and renewable energy. Concentrator PV (CPV) employs optical elements to concentrate sunlight onto small solar cells, offering the possibility of replacing expensive solar cells with more economic optical elements, and higher device power conversion efficiencies. While CPV has mainly been explored for highly efficient single-crystalline and multi-junction solar cells, the combination of thin-film solar cells with the concentration approach opens up new horizons in CPV. Typical fabrication of thin-film solar cells can be modified for efficient, high-throughput and parallel production of organized arrays of micro solar cells. Their combination with microlens arrays promises to deliver micro-concentrator solar modules with a similar form factor to present day flat-panel PV. Such thin-film micro-concentrator PV modules would use significantly less semiconductor solar cell material (reducing the use of critical raw materials) and lead to a higher energy production (by means of concentrated sunlight), with the potential to lead to a lower levelized cost of electricity. This review article gives an overview of the present state-of-the-art in the fabrication of thin-film micro solar cells based on Cu(In,Ga)Se2 absorber materials and introduces optical concentration systems that can be combined to build the future thin-film micro-concentrator PV technology.

In this paper, we report the development of Cd-free buffers using atomic layer deposition (ALD) for Cu(In,Ga)(S,Se)2-based solar cells. The ALD process gives good control of thickness and the S/S +O ratio content of the films. The influence of the growth per cycle (GPC) and the S/(S+O) ratio, and the glass temperature of the atomic layer deposited Zn(O,S) buffer layers on the efficiency of the Cu(In,Ga)(S,Se)2 solar cells were investigated. We present the first results from our work on cadmium-free CIGS solar cells on substrates with an aperture area of 0.4 cm2. These Zn(O,S) layers were deposited by atomic layer deposition at 120 °C with S/Zn ratios of 0.7, and layers of around 30 nm. The Zn(O,S) 20% (Pulse Ratio: H2S/H2O+H2S) process results in a S/Zn ratio of 0.7. We achieved independently certified aperture area efficiencies of 17.1% for 0.4 cm2 cells.

Cu(In,Ga)Se 2 -based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se 2 solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se 2 does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances.

This paper details the enhancement of the optoelectronic properties of Cu-(In, Ga)-Se2 (CIGSe) solar cells through a two-segment process in the ultraviolet (UV)–visible spectral range. These include fine-tuning the DC sputtering power of the absorber layer (ranging from 20 to 40 W at segment I) and thoroughly checking the trace micro-chemistry composition of the absorber layer (CdS, ZnO/CdS, ZnMgO/CdS, and ZnMgO at segment II). After segment I of treatment, the optimal 30 W CIGSe absorber layer (i.e., with a 0.95 CGI ratio) can be obtained, it can be seen that the Cu-rich film exhibits the ability to significantly promote grain growth and can effectively reduce its trap state density. After the segment II process aimed at replacing toxic CdS, the optimal metal alloy (Zn0.9Mg0.1O) composition (buffer layer) achieved the highest conversion efficiency (η) of 8.70%, also emphasizing its role in environmental protection. Especially within the tunable bandgap range (2.48–3.62 eV), the developed overall internal and external quantum efficiency (IQE/EQE) is significantly improved by 13.15% at shorter wavelengths. A photovoltaic (PV) module designed with nine optimal CIGSe cells demonstrated commendable stability. Variation remained within ±5% throughout the 60-day experiment. The PV modules in this study represent a breakthrough benchmark toward a significant advance in the scientific understanding of renewable energy. Furthermore, this research clearly promotes the practical application of PV modules, harmonizes with sustainable goals, and actively contributes to the creation of eco-friendly communities.

The chemical and electronic structure of the CdS/(Ag,Cu)(In,Ga)Se2 (CdS/ACIGSe) interface for thin‐film solar cells, involving an absorber with a bulk [Ag]/([Ag]+[Cu]) (AAC) ratio of 0.06, a state‐of‐the‐art RbF post‐deposition treatment (PDT), and a chemical‐bath deposited CdS buffer layer, is studied. To gain a detailed and depth‐resolved picture of the CdS/ACIGSe interface, synchrotron‐ and laboratory‐based hard X‐ray, soft X‐ray, and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy are combined. Compared to the bulk of the absorber, a Cu‐ and Ga‐poor ACIGSe surface is found, with a slightly increased AAC ratio. Strong evidence of a Rb–In–Se species (possibly with some Ag) at the absorber surface is compiled, with a corresponding band gap of 2.79 ± 0.12 eV. This finding is in clear contrast to comparable Ag‐free Cu(In,Ga)Se2 absorbers with RbF‐PDT. The Rb–In–Se surface species is not removed by the (wet‐chemical) CdS deposition process, while some Se diffuses into the CdS layer and segregates at its surface. The CdS buffer layer shows a band gap of 2.48 ± 0.12 eV, and a cliff (≈ −0.4 eV) is determined in the conduction band alignment at the interface between the Rb–In–Se species and the CdS buffer. The chemical and electronic structure of the CdS/(Ag,Cu)(In,Ga)(S,Se)2 interface with RbF treatment is investigated using a combination of synchrotron‐ and laboratory‐based soft and hard X‐ray spectroscopies. A Rb–In–Se species is found at the absorber surface, with a band gap of 2.8 eV, in contrast to comparable Ag‐free Cu(In,Ga)Se2 absorbers with RbF‐PDT.

Tandem solar cells can surpass the limitations of single‐junction devices, promising increased performance due to lower thermalization losses. Even though many research and industrial upscaling efforts are based on perovskite‐Si tandems, all‐thin‐film photovoltaic (PV) devices, for instance with chalcopyrite (CIGSe) and perovskite, can offer many advantages such as significant cost and material savings and access to niche markets like building integrated‐ and flexible PV. However, long‐term stability and outdoor performance of perovskite‐based tandem devices is to this day challenging. This work presents the first data analysis of year‐round outdoor measurements (mpp‐tracked) of a perovskite‐chalcopyrite tandem device with a starting efficiency of about 23.14% before encapsulation. The maximum outdoor performance of the tandem device changed during the period of observation, reaching the peak performance in April and then decreased due to the device degradation. At its maximum outdoor performance, the tandem could reach up to 68% higher instantaneous power output, relative to its single‐junction reference (CIGSe‐SJ). In addition, a quantitative time series performance analysis, exemplary qualitative imaging characterization of the tandem before and after outdoor exposure, is shown. Finally, the possibility of predicting the immediate performance of an all‐thin‐film tandem is verified by using a multiple linear regression model with accuracies generally exceeding 90%. This work presents the first data analysis of year‐round measurements under outdoor conditions of a perovskite‐CIGS tandem solar cell. In addition, the data acquired is used to verify the possibility of predicting the immediate performance of an all‐thin‐film tandem using a multiple linear regression model with accuracies generally exceeding 90%.