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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.

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.

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.

Light management strategies are of utmost importance to allow Cu(In,Ga)Se2 (CIGS) technology market expansion, as it would enable a conversion efficiency boost as well as thinner absorber layers, increasing sustainability and reducing production costs. However, fabrication and architecture constraints hamper the direct transfer of light management architectures from other photovoltaic technologies. The demand for light management in thin and ultrathin CIGS cells is analyzed by a critical description of the optical loss mechanisms in these devices. Three main pathways to tackle the optical losses are identified: front light management architectures that assist for an omnidirectional low reflection; rear architectures that enable an enhanced optical path length; and unconventional spectral conversion strategies for full spectral harvesting. An outlook over the challenges and developments of light management architectures is performed, establishing a research roadmap for future works in light management for CIGS technology. Following the extensive review, it is expected that combining antireflection, light trapping, and conversion mechanisms, a 27% CIGS solar cell can be achieved. Advanced light management strategies are still not fully exploited in state‐of‐the‐art Cu(In,Ga)Se2 (CIGS) devices. Leaps and bounds still must be made to reach or even surpass its optical limits. Hence, a critical overview of the developed light management strategies in CIGS solar cells is provided, establishing a research roadmap for future works in light management for CIGS solar cells.

A graphene–cadmium sulfide (Gr–CdS) nanocomposite was prepared by a chemical solution method, and its material properties were characterized by several analysis techniques. The synthesized pure CdS nanoparticles (NPs) and Gr–CdS nanocomposites were confirmed to have a stoichiometric atomic ratio (Cd/S = 1:1). The Cd 3d and S 2p peaks of the Gr–CdS nanocomposite appeared at lower binding energies compared to those of the pure CdS NPs according to X-ray photoelectron spectroscopy analyses. The formation of the Gr–CdS nanocomposite was also evidenced by the structural analysis using Raman spectroscopy and X-ray diffraction. Transmission electron microscopy confirmed that CdS NPs were uniformly distributed on the graphene sheets. The absorption spectra of both the Gr–CdS nanocomposite and pure CdS NPs thin films showed an absorption edge at 550 nm related to the energy band gap of CdS (~2.42 eV). The Cu(In,Ga)Se2 thin film photovoltaic device with Gr–CdS nanocomposite buffer layer showed a higher electrical conversion efficiency than that with pure CdS NPs thin film buffer layer. In addition, the water splitting efficiency of the Gr–CdS nanocomposite was almost three times higher than that of pure CdS NPs.

Adding potassium to Cu(In,Ga)Se 2 absorbers has been shown to enhance photovoltaic power conversion efficiency. To illuminate possible mechanisms for this enhancement and limits to beneficial K incorporation, the properties of Cu 1− x K x InSe 2 (CKIS) thin-film alloys have been studied. Films with K/(K + Cu), or x , from 0 to 1 were grown by co-evaporation, and probed by XRF, EPMA, SEM, XRD, UV–Visible spectroscopy, current–voltage, and TRPL measurements. Composition from in situ quartz crystal and EIES monitoring was well correlated with final film composition. Crystal lattice parameters showed linear dependence on x , indicating complete K incorporation and coherent structural character at all compositions in the and lattice directions, despite the different symmetries of CuInSe 2 and KInSe 2 . The band gap energy showed pronounced bowing with x composition, in excellent agreement with experimental reports and semiconductor theory. Films of Mo/CKIS/Ni were non-ohmic, and increasing x from 0 to 0.58 decreased the apparent CKIS resistivity. Further evidence of decreased CKIS resistivity was observed with photoluminescence response, which increased by about half a decade for x  > 0, and indicates increased majority carrier concentration. Minority carrier lifetimes increased by about an order of magnitude for films grown at x  = 0.07 and 0.14, relative to CuInSe 2 and x  ≥ 0.30. This is the first report of a Cu-K-In-Se film with >1 at.% K, and the observed property changes at increased x (wider band gap; lower resistivity; increased lifetime) comprise valuable photovoltaic performance-enhancement strategies, suggesting that CKIS alloys have a role to play in future engineering advances.

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%.

Ultra‐thin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide back contact reduce the material consumption of rare indium and gallium and simultaneously exhibit great potential for semi‐transparent bifacial application. For highly efficient CIGSe solar cells, a steep back Ga grading and Na treatment are expected. However, Na will promote the formation of highly resistive GaOx at the rear interface owing to Ga accumulation. In this work, the three‐stage co‐evaporation process is renewed and the effect of the deposition sequence in the first stage on the Ga distribution as well as the cross‐correlated influence of Na is explored. In particular, the standard deposition sequence of Ga+In is altered to start with In. When a thin In layer is pre‐deposited on the back contact, the fill factor and efficiency increase. The deposition of In+Ga+In in the first stage of CIGSe growth leads to efficiencies 28% (on average) higher than for the standard deposition sequence of Ga+In. Additionally, 2.74% efficiency is reached under rear and 9.32% under simultaneous front and rear illumination. Therefore, adapting the deposition sequence in the first stage of CIGSe growth is identified as a key to improving the device performance on transparent back contact. The deposition sequence in the first stage of the three‐stage co‐evaporation process is adapted to address the challenge of GaOx formation at the CIGSe/ITO interface. The sequence In+Ga+In moves the Ga peak from the interface while keeping a steep back Ga‐gradient and a U‐shaped bandgap profile. As a result, the photovoltaic performance can be improved, especially the Voc and FF.

In this study, the origins of efficiency gains in Cu(In,Ga)Se2 (CIGS) solar cells are investigated by introducing an Al2O3 passivation layer in terms of the oxidation condition of Mo back contact, alkali‐metal diffusion, minority carrier lifetimes (τ), and charge conditions. The study reveals that introduction of an Al2O3 back‐contact passivation layer into solar cells yields multiple impacts. Al2O3 deposition enhances the oxidation of the Mo back contacts, increasing Na solubility in Mo and Na diffusion from Mo into the CIGS layer, thereby modifying the metastable properties of CIGS. The charge condition at the CIGS/Al2O3 interface is not fixed negative charge but variable, dependent on whether electrons or holes are supplied. During solar cell operation, the interfacial charge condition is expected to be neutral or positive for Al2O3 grown using plasma or thermal atomic layer deposition techniques, respectively. Moreover, the mechanical peeling off of CIGS from Mo back contact enhanced τ in a similar way as with the insertion of Al2O3. Based on this study, the enhancement of alkali metal supply and the removal of direct contact of CIGS to the metal contact (Mo) can play crucial roles in improving the performance of CIGS solar cell. Introduction of Al2O3 passivation layer into Cu(In,Ga)Se2 (CIGS) solar cell improves device performance by making multiple impacts. Oxidation of the Mo back contacts increase Na solubility in the Mo and accordingly enhanced Na diffusion from the Mo into CIGS layer, resulting in modified bulk property of CIGS. Also, the removal of direct contact to metal contact (Mo) reduces back‐contact recombination.