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Ag alloying of Cu(In,Ga)Se2 (CIGSe) absorbers in thin‐film solar cells leads to improved crystallization of these absorber layers at lower substrate temperatures than for Ag‐free CIGSe thin films as well as to enhanced cation interdiffusion, resulting in reduced Ga/In gradients. However, the role of Ag in the microscopic structure–property relationships in the (Ag,Cu)(In,Ga)Se2 thin‐film solar cells as well as a correlation between the various microscopic properties of the polycrystalline ACIGSe absorber and open‐circuit voltage of the corresponding solar cell device has not been reported earlier. In the present work, we study the effect of Ag addition by analyzing the differences in the various bulk, grain‐boundary, optoelectronic, emission, and absorption‐edge properties of ACIGSe absorbers with that of a reference CIGSe absorber. By comparing thin‐film solar cells with similar band‐gap energies ranging from about 1.1 to about 1.2 eV, we were able to correlate the differences in their absorber material properties with the differences in the device performance of the corresponding solar cells. Various microscopic origins of open‐circuit voltage losses were identified, such as strong Ga/In gradients and local compositional variations within individual grains of ACIGSe layers, which are linked to absorption‐edge broadening, lateral fluctuations in luminescence‐energy distribution, and band tailing, thus contributing to radiative VOC losses. A correlation established between the effective electron lifetime, average grain size, and lifetime at the grain boundaries indicates that enhanced nonradiative recombination at grain boundaries is a major contributor to the overall VOC deficit in ACIGSe solar cells. Although the alloying with Ag has been effective in increasing the grain size and the effective electron lifetime, still, the Ga/In gradients and the grain‐boundary recombination in the ACIGSe absorbers must be reduced further to improve the solar‐cell performance. [ABSTRACT FROM AUTHOR]

Heavy alkali‐metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low‐cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO3) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin‐coating CsCO3 solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open‐circuit voltage. X‐ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High‐resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low‐cost route for improving the performance of CIGS solar cells by surface modification using a solution process. [ABSTRACT FROM AUTHOR]

Wide‐gap, high‐Ga (Ag,Cu)(In,Ga)Se2 thin‐film solar cells with a wide range of Ag contents are fabricated and characterized before and after dark storage, dark annealing at 85 °C, and light soaking. A 1:4 ratio of Ag to Cu enhances initial device performance significantly, with excess Ag enhancing carrier collection at the expense of open‐circuit voltage and fill factor for close‐stoichiometric devices. For off‐stoichiometric devices, increased open‐circuit voltages are offset by losses in carrier collection. Efficiency degradation after treatments is typically increased with additional Ag alloying. A second observation is a behavior threshold identified slightly below an Ag to Cu ratio of 1:1. For compositions below the threshold, the doping response to light soaking and dark annealing is similar to that exhibited by low‐Ga Cu(In,Ga)Se2. Above the threshold, light soaking reduces net doping and that dark annealing can even increase net doping. Furthermore, devices above the threshold exhibit a far greater doping responsivity than those below and display a strong dependence of initial performance and stability on group‐I/group‐III stoichiometry. A third observation is that all devices lose ≈1–2% (absolute) in efficiency after a 3 h light soak, indicating that this loss originates from the high‐Ga content (1:3 In:Ga), rather than the Ag alloying. [ABSTRACT FROM AUTHOR]

In response to climate and resource challenges, the transition to a renewable and decentralized energy system is imperative. Ultrathin Cu(In,Ga)Se2 (CIGS)‐based solar cells are compatible with such transition due to their low material usage and improved production throughput. Despite the benchmark efficiency of CIGS technology, ultrathin configurations face efficiency drops arising from increased rear interface recombination and incomplete light absorption. Dielectric passivation schemes address rear interface recombination, but achieving simultaneous electrical and optical gains is crucial for thinning down the absorber. Plasmonic nanoparticles emerge as a solution, enhancing light interaction through resonant scattering. In the proposed architecture, the nanoparticles are encapsulated within a dielectric rear passivation layer, combining effective passivation and light trapping. A controlled deposition and encapsulation of individualized nanoparticles is achieved by an optimized process flow using microfluidic devices and nanoimprint lithography. With the developed plasmonic and passivated architecture, a 3.7 mA cm−2 short‐circuit current density and a 23 mV open‐circuit voltage improvements are obtained, leading to an almost 2% increase in light‐to‐power conversion efficiency compared to a reference device. This work showcases the developed architecture potential to tackle the electrical and optical downfalls arising from the absorber thickness reduction, contributing to the dissemination of ultrathin technology. [ABSTRACT FROM AUTHOR]

Alkali treatments are crucial for low bandgap (Ag,Cu)InSe2 (ACIS) and Cu(In,Ga)Se2‐based solar cell performance. Traditionally, Ag‐alloying of CIS (ACIS) is grown on soda‐lime glass (SLG) at temperatures exceeding 500 °C, resulting in uncontrolled alkali diffusion from the substrate and variable photovoltaic properties. A substrate‐independent low‐bandgap ACIS growth process is introduced and the impact of controlled supplies of NaF and RbF alkali fluorides before and after absorber growth through precursor layers and post‐deposition treatments (PDT) are investigated. NaF and RbF precursor layers enhance carrier lifetimes and doping density, outperforming the previous SLG‐dependent strategy. Even small quantities of RbF significantly enhance device performance, while specific NaF amount during deposition are necessary to limit grain growth and achieve high doping densities and lifetimes. A certain density of grain boundaries appears crucial for high doping levels. Although subsequent NaF post‐deposition treatment (PDT) does not provide additional benefits with sufficient Na during growth, RbF‐PDT remains crucial. The best performance is achieved with a combination of NaF and RbF precursor layers along with RbF‐PDT, resulting in over 19% efficiency, 605 mV open‐circuit voltage (VOC), 73% fill factor (FF), a carrier density of 3 × 1016 cm−3, and a 700 ns lifetime. This approach supports high‐efficiency ACIS solar cell advancement, particularly for thin‐film tandem photovoltaic devices. [ABSTRACT FROM AUTHOR]

In this study, a feasible strategy is proposed for directly depositing high-quality Cu(In,Ga)Se2 (CIGS) films using Na-doped targets in a selenium-free atmosphere to boost the power conversion efficiency (PCE) of CIGS solar cells. Introducing a small amount of sodium dopant effectively promoted the textured growth of CIGS crystals in the prepared films, resulting in larger grain sizes and a smoother interface. The higher MoSe2 content at the CIGS/Mo interface increased the carrier lifetime in the films. In addition, sodium doping increased the proportion of Se atoms on the film surface and reduced the concentration of defects caused by the direct sputtering of the films in the selenium-free atmosphere. Therefore, the separation and transportation of photo-generated carriers in the devices were effectively enhanced. Using the optimized parameters, a record-high PCE of 17.26% was achieved for the 7.5% Na-doped devices, which represents an improvement of nearly 63%. [ABSTRACT FROM AUTHOR]

This study investigates the use of an alkaline earth metal precursor (MgF2) to enhance the performance of chalcogenide-based Cu(In,Ga)Se2 (CIGS) solar cells with a chemically bath deposited-Zn(O,S) (CBD-Zn(O,S)) buffer layer via post-deposited treatment (PDT). The optimal substrate temperature and layer thickness are 570 °C and 5 nm, and the light soaking (LS) treatment does not be required in this condition. The morphological properties and chemical reaction at the p-n junction of CIGS/CBD-Zn(O,S) are examined as a function of MgF2 PDT layer thickness. As the MgF2 PDT layer thickness increases, the CIGS surface becomes rough with vigorously agglomerated Cu clusters owing to the substantially high substrate temperature, which increases the incorporation of In-Se bonds and the oxygenation rate of MgF2. Density functional theory (DFT) clarifies the improved cell efficiency without the need for LS treatment (MgF2 PDT, 5 nm) by calculating the defect-related electronic behavior. The MgF2 phase effectively passivates metastable defect Cu-Se vacancy defects (VCu-Se), related to the LS effect without the additional formation of deep-level defect states into the CIGS bandgap. Moreover, VCu-Se states exert the most influence on the LS effect, and the control of defect states in the CIGS layer (not the buffer layer) is crucial for cell efficiency.

The stability of thin‐film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se2 material system is evaluated over time, after dry‐heat annealing and after light soaking, and the role of Ag and Ga content is explored. Ag‐free CuInSe2 is relatively stable to annealing and storage, while Cu(In,Ga)Se2 suffers a degradation of fill factor and carrier collection. High‐Ga (Ag,Cu)(In,Ga)Se2 suffers degradation of carrier collection after prolonged annealing, reducing the short‐circuit current by ≈12%. Ga‐free (Ag,Cu)InSe2 loses up to a third of open‐circuit voltage and a quarter of fill factor after all treatments are applied. All samples suffer voltage losses after light soaking, with the Ga‐free devices losing up to 50 mV and those containing Ga losing up to 90 mV. Ag incorporation leads to a significant reduction in doping, and a significant increase in the response of doping to treatments, with the depletion width of (Ag,Cu)(In,Ga)Se2 samples expanding from ≈0.1 μm as‐grown to beyond 1.0 μm after all treatments, compared to the Cu(In,Ga)Se2 sample variation of ≈0.1–0.3 μm. Connections between Ag content, doping instability, and performance degradation are discussed. [ABSTRACT FROM AUTHOR]

During the last few decades, major advances have been made in photovoltaic systems based on Cu(In,Ga)Se2 chalcopyrite. However, the most efficient photovoltaic cells are processed under high-energy-demanding vacuum conditions. To lower the costs and facilitate high-throughput production, printing/coating processes are proving to be effective solutions. This work combined printing, coating, and chemical bath deposition processes of photoabsorber, buffer, and transparent conductive layers for the development of solution-processed photovoltaic systems. Using a sustainable approach, all inks were formulated using water and ethanol as solvents. Screen printing of the photoabsorber on fluorine-doped tin-oxide-coated glass followed by selenization, chemical bath deposition of the cadmium sulfide buffer, and final sputtering of the intrinsic zinc oxide and aluminum-doped zinc oxide top conductive layers delivered a 6.6% maximum efficiency solar cell, a record for screen-printed Cu(In,Ga)Se2 solar cells. On the other hand, the all-non-vacuum-processed device with spray-coated intrinsic zinc-oxide- and tin-doped indium oxide top conductive layers delivered a 2.2% efficiency. The given approaches represent relevant steps towards the fabrication of sustainable and efficient Cu(In,Ga)Se2 solar cells. [ABSTRACT FROM AUTHOR]

The detrimental effect of tail states on the radiative and non‐radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage (VOC) in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single‐crystal films. The results demonstrate that sodium and potassium decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of sodium or potassium incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high‐efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC. [ABSTRACT FROM AUTHOR]

The thin‐film photovoltaic technology based on Cu(In,Ga)Se2 (CIGS) has reached high conversion efficiencies exceeding 23%. The gallium gradient in the bulk and the amount of alkali metals in and between CIGS grains strongly affect the cell performance and therefore need to be accurately measured and quantified. ToF‐SIMS is well suited for this task, but the spatial resolution is strongly influenced by the measurement parameters, especially at the required micrometer and nanometer scales. Such highly resolved measurements on CIGS surfaces are challenging. For instance, small copper aggregations occur in SEM images recorded after a focused ion beam (FIB) cut and change the energy‐dispersive spectroscopy (EDS) results. SIMS systems that use an argon ion source generate Cu lamellas, which also influence the measurement. Both effects result from preferential sputtering. Cooling with liquid nitrogen can minimize this negative effect for both techniques. Recent developments in SIMS systems provide more options to avoid these effects. New sources, like the Bi liquid metal gun and cluster guns, are available. However, a systematic investigation on the effect of the ion sources on the sputter craters and the resulting measurement of CIGS absorber is necessary to choose an adequate setup. Thus, we performed a comparison of four different ion sources (argon ion gun, oxygen gun, cesium metal gun, and oxygen cluster gun) and measured CIGS depth profiles at three temperatures [−120°C, −50°C, and room temperature (RT)]. We investigate the influence of these conditions not only on the resulting SIMS depth profile and quantification but also on the morphology of the sputter crater. [ABSTRACT FROM AUTHOR]

Voltage losses reduce the photovoltaic conversion efficiency of thin‐film solar cells and are a primary efficiency limitation in Cu(In,Ga)Se2. Herein, voltage loss analysis of Cu(In,Ga)Se2 solar cells fabricated at three institutions with variation in process, bandgap, absorber structure, postdeposition treatment (PDT), and efficiency is presented. Nonradiative voltage losses due to Shockley–Read–Hall charge carrier recombination dominate and constitute >75% of the total compared to <25% from radiative voltage losses. The radiative voltage loss results from nonideal absorption and carriers in band tails that stem from local composition‐driven potential fluctuations. It is shown that significant bulk lifetime improvements are achieved for all alkali PDT processed absorbers, chiefly associated with reductions in nonradiative recombination. Primary voltage loss contributions (radiative and nonradiative) change little across fabrication processes, but variation in submechanisms (bulk lifetime, net acceptor concentration, and interface recombination) differentiate nonradiative loss pathways in this series of solar cells. [ABSTRACT FROM AUTHOR]

Photovoltaic conversion efficiency of CuInSe2 solar cells is limited by low fill factor (FF) and open‐circuit‐voltage (VOC) values compared to Si or perovskite solar cells. Herein, small quantities of Ag to alloy CuInSe2 to improve its properties are used, such as enhanced grain growth, higher crystal quality, and less detrimental defects, to overcome the device limitations of low‐bandgap CuInSe2 absorbers. The impact of Ag on the electronic properties of the bulk material and the buffer–absorber interface is examined at different stoichiometric compositions. Ag alloying improves the morphology with larger grain sizes, extends carrier lifetimes, and elevates net doping densities. Ag alloying reduces Cu‐depleted ordered‐vacancy compounds at the buffer–absorber interface and causes a chalcopyrite phase of high‐crystal‐quality independent of the I/III ratio. It is suggested Ag affects the formation of the alkali‐rich surface layer (Rb–In–Se). The best solar cell with an Ag‐alloyed CuInSe2 absorber achieves a VOC over 600 mV and FF values of about 74%, which result in a power conversion efficiency of 18.7% for low‐bandgap energy of 1.0 eV with short‐circuit current above 42 mA cm−2. The advantage of Ag alloying on the device performance of CuInSe2 solar cells will impact future applications in tandem devices. [ABSTRACT FROM AUTHOR]

This study used AMPS-1D to peform numerical simulations and model the behavior of back-wall superstrate solar cells based on Cu(In,Ga)Se2 (CIGS) thin films to investigate optimal conditions and obtain maximum efficiency. The effects of absorber thickness and density of interface defects were examined along with the work function of the transparent conductive oxide (WTCO) to investigate their influence on the output parameters. Measurements of device performance (J-V) and Quantum Efficiency (QE) showed that the performance of the cell improved as the thickness of the CIGS layer decreased because photons were absorbed near the junction. The device achieved an efficiency of 16.4% using an optimal thickness for the CIGS layer on the order of 0.3µm, defect densities in the range of 1013-1015cm-3, doping concentration of the n-TCO back contact on the order of 1019cm-3, and WTCO in the range of 4.5-5.2eV. These results show that the generated electron-hole pairs had a high probability of separation and demonstrate the potential of this device structure. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se2 (CIGSe) solar cells are among the most efficient thin‐film solar cells on lab scale. However, this thin‐film technology has relatively large upscaling losses for commercial technology. To tackle this, paradigm shifts are proposed that allow for simpler, cost‐effective, and efficient CIGSe solar cells. Front passivation using dielectric layers is one of the options being investigated as this is widely used in Si technology. Research on front passivation for CIGSe is in an early stage and no improvements are made yet. A close comparison with silicon technology is made to understand why it seems to be more difficult for CIGSe solar cells. In general, chemical passivation is less effective, resulting in higher interface defect densities than seen for Si. Also, field‐effect passivation requires positive charges, which have not been implemented yet on the CIGSe front surface. Finally, for Si passivation, often a high‐temperature annealing step is applied, which is not possible for CIGSe. It is proposed to apply a dielectric tunneling layer with positive fixed charges in combination with an electron transport layer to move forward. A list of potential dielectric layers that could be suitable for CIGSe is provided.

(Ag,Cu)(In,Ga)Se2 solar cells with bandgaps of ≈1.45 eV with a large spread in absorber stoichiometry are characterized with the intention of assessing the effect of composition on the stability of the devices. This material is observed to have a poor diffusion length, leading to very strong dependence upon the depletion region width for charge carrier collection. The depletion width is observed to depend strongly upon the stoichiometry value and shrinks significantly after an initial period of dark storage. It is also seen that the depletion width can be varied strongly through light‐soaking and dry‐heat treatments, with prolonged annealing leading to detrimental contraction and light soaking leading to expansion which increases current collection. The extent of depletion width variation in response to the treatments is also clearly linked to absorber stoichiometry. Consequently, the device performance, particularly the current output, exhibits a stoichiometry dependence and is considerably affected after each round of treatment. Possible causes of this behavior are discussed. [ABSTRACT FROM AUTHOR]

Heterojunction Cu(In,Ga)Se2 (CIGS) solar cells comprise a substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al. Here, Al-doped zinc oxide (AZO) films were deposited by magnetron sputtering, and the substrate temperature was optimized for CIGS solar cells with two types of CIGS light absorbers with different material properties fabricated by three-stage co-evaporation and two-step metallization followed by sulfurization after selenization (SAS). The microstructure and optoelectronic properties of the AZO thin films fabricated at different substrate temperatures (150–550 °C) were analyzed along with their effects on the CIGS solar cell performance. X-ray diffraction results confirmed that all the deposited AZO films have a hexagonal wurtzite crystal structure regardless of substrate temperature. The optical and electrical properties of the AZO films improved significantly with increasing substrate temperature. Photovoltaic performances of the two types of CIGS solar cells were influenced by changes in the AZO substrate temperature. For the three-stage co-evaporated CIGS cell, as the sputter-deposition temperature of the AZO layer was raised from 150 °C to 550 °C, the efficiencies of CIGS devices decreased monotonically, which suggests the optimum AZO deposition temperature is 150 °C. In contrast, the cell efficiency of CIGS devices fabricated using the two-step SAS-processed CIGS absorbers improved with increasing the AZO deposition temperature from 150 to 350 °C. However, the rise in AZO deposition temperature to 550 °C decreased the cell efficiency, indicating that the optimum AZO deposition temperature was 350 °C. The findings of this study provide insights for the efficient fabrication of CIGS solar cells considering the correlation between CIGS absorber characteristics and AZO layer deposition temperature. [ABSTRACT FROM AUTHOR]

During the last few decades, major advances have been made in photovoltaic systems based on Cu(In,Ga)Se2 chalcopyrite. However, the most efficient photovoltaic cells are processed under high-energy-demanding vacuum conditions. To lower the costs and facilitate high-throughput production, printing/coating processes are proving to be effective solutions. This work combined printing, coating, and chemical bath deposition processes of photoabsorber, buffer, and transparent conductive layers for the development of solution-processed photovoltaic systems. Using a sustainable approach, all inks were formulated using water and ethanol as solvents. Screen printing of the photoabsorber on fluorine-doped tin-oxide-coated glass followed by selenization, chemical bath deposition of the cadmium sulfide buffer, and final sputtering of the intrinsic zinc oxide and aluminum-doped zinc oxide top conductive layers delivered a 6.6% maximum efficiency solar cell, a record for screen-printed Cu(In,Ga)Se2 solar cells. On the other hand, the all-non-vacuum-processed device with spray-coated intrinsic zinc-oxide- and tin-doped indium oxide top conductive layers delivered a 2.2% efficiency. The given approaches represent relevant steps towards the fabrication of sustainable and efficient Cu(In,Ga)Se2 solar cells.

Alkali post‐deposition treatment (PDT) of Cu(In,Ga)Se2 (CIGS) film is an effective approach to obtain high‐efficiency CIGS solar cells. Herein, a low‐cost and scalable surface passivation process is reported when the potassium iodide (KI) alkali source is employed instead of the conventional potassium fluoride (KF). CIGS surface is modified by spin‐coating of KI solution and subsequent annealing at 350 °C in a Se atmosphere. With the KI PDT, the cell efficiency is enhanced by almost 20% due to the increase of shunt resistance and the lowering of reverse saturation current. To clarify the origin of the efficiency enhancement, the chemical state and electronic structure at the CIGS surface are investigated. The KI PDT yields a valence band lowering by 0.18 eV at the CIGS/CdS interface, leading to the suppression of the interface recombination. This is possible by Cu depletion and Cu cation reduction at the CIGS surface after KI treatment. The developed process may be considered an upcoming alternative alkali PDT for CIGS absorbers. The results provide the knowledge base for further optimization of the interface properties to form high‐quality heterojunction in the CIGS solar cells. [ABSTRACT FROM AUTHOR]

Up to now, what we know about the impact of alkali post‐deposition treatment (PDT) on Cu(In,Ga)Se2 (CIGSe) absorber thin films is largely based on treatments performed in selenium atmosphere and only few studies have addressed the critical role of the chalcogen atmosphere during the PDT. The present study deals with an innovative process of alkali fluoride PDT under elemental sulfur atmosphere on co‐evaporated Cu(In,Ga)Se2 absorbers. With the aim to understand the effects of different the incorporated alkali element incorporated during the PDT, we investigate four different PDTs: CsF, NaF/RbF, RbF, and In + RbF—all under sulfur atmosphere. The treated absorbers are characterized by scanning electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. Our results show that for CIGSe compositions close to stoichiometry, forming a slightly Cu‐poor CIGSe at the surface during the PDT is beneficial. Cu(In,Ga)Se2/RbF(S) and Cu(In,Ga)Se2/In + RbF(S) exhibit the higher photoluminescence response probably due to decreased surface recombination. The quasi‐Fermi‐level splitting is in good agreement with the observed Voc difference between the treated and reference samples. The electronic properties of the Cu(In,Ga)Se2/In + RbF(S)‐based solar cells show a significantly improved performance with high Voc and FF. [ABSTRACT FROM AUTHOR]

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.

The Knudsen effusion cell is often used to grow high-quality Cu(In,Ga)Se2 (CIGS) thin film in co-evaporation processes. However, the traditional single-heating Knudsen effusion cell cannot deliver complete metal selenides during the whole deposition process, particularly for a low-temperature deposition process, which is probably due to the condensation and droplet ejection at the nozzle of the crucible. In this study, thermodynamics analysis is conducted to decipher the reason for this phenomenon. Furthermore, a new single-heating Knudsen effusion is proposed to solve this difficult problem, which leads to an improvement in the quality of CIGS film and a relative increase in conversion efficiency of 29% at a growth rate of about 230 nm·min−1, compared with the traditional efficiency in a low-temperature rapid-deposition process.

Open-circuit voltage (V OC) losses due to various recombination mechanisms are well-recognized limitations towards high-performance solar cells. Thin film Cu(In,Ga)Se 2 (CIGSe)-based solar cells have been well-developed and commercially used in the photovoltaic community due to their high power conversion efficiency (PCE) with high chemical stability and cost-effectiveness. Recently, numerous works on modification of the CIGSe/buffer band alignment and alkali post-deposition treatment on CIGSe have shown obvious enhancement of the solar cell efficiency. In this work, we explore the effects of subgap absorption driven by quasi-Fermi level splitting (QFLS) and Urbach tail (E U) states in CIGSe on V OC losses. In particular, we take all subgap absorption contributions into account to precisely fit the full photoluminescence (PL) spectra and obtain the QFLS and E U energies using an integrated model. We found that for CIGSe films with different Ga contents, the model can fit the PL spectra perfectly, which allows the extraction of reliable values of QFLS and E U. Using these values, we further calculated the V OC in both the SQ and radiative limit, as well as the non-radiative loss due to subgap absorption. We also showed that sample with a larger QFLS and a lower E U exhibits a smaller V OC loss and thus a higher PCE. Although the effects of QFLS on V OC is much smaller than Urbach tail, a more accurate estimation of V OC loss due to non-radiative recombination can be achieved when QFLS is considered. These results shed light on the origin of V OC losses in solar cells, which is potentially useful for the advancement of PV technology. • Cu(In,Ga)Se 2 solar cells were fabricated by 3-stage co-evaporation. • Urbach tail energy and QFLS energy are extracted by fitting the full PL spectrum. • Low Urbach tail energy and large QFLS energy minimize the V OC loss. • Accurate non-radiative V OC loss can be calculated when QFLS is considered. [ABSTRACT FROM AUTHOR]

Nowadays it is widely acknowledged that solar photovoltaic energy is one of the preferred options for sustainable management of the future energy needs of the world. For this, new technological processes, known as second and third generations, based on the use of thin films and nanomaterials, have recently been developed in order to reduce the cost of solar cells. Over the past few years, the yield of second-generation Cu(In, Ga)Se2 thin-film cells has exceeded 22 %. It was found that as nanostructured materials such as nanowire arrays often have a higher light absorption rate than thin films, they can therefore be used. This article aims to design and model nanostructured CIGS thin film solar cells based on indium tin oxide (ITO) nanowires. Modelling provides information on the operation of CIGS solar cells, as well as on the mechanisms of absorption and electric charge transport. The purpose of this work is to evaluate the electrical and optical characteristics (ISC, VOC, FF, η) of a ZnO/CdS/CIGS heterojunction thin film structure. Thus, an optimum efficiency of 17.57 % and a form factor of 76.56 % were achieved. Afterwards, the Mo film rear contact was replaced with ITO nanowires which were introduced into the CIGS-based solar cell. The results indicated that the solar cells under study exhibited very good photovoltaic performance, with an efficiency of 21.26 %. It is worth noting that this performance is higher than that of the corresponding CIGS thin film cells. In addition, the large active surface area of the ITO nanowire electrode and the short distance that the charge must travel helped to improve charge collection in the nanostructure. This would certainly increase the short circuit current ISC, and consequently the electrical efficiency. The simulation was based on the low-field mobility model, and on Shockley-Read-Hall (SRH) and Auger carrier transport and recombination models which may be activated in ATLAS-SILVACO (2D). [ABSTRACT FROM AUTHOR]

The pretreatment of the Cu(In,Ga)Se 2 (CIGS) absorption layer using an alkali element can effectively improve the photoelectric conversion efficiency (PCE) of CIGS solar cells. Here, we propose using NaF layer pretreatment below the CIGS absorption layer deposited by a three-stage process. Sodium ions in NaF can effectively suppress the diffusion of Ga elements and form a steep gradient backscatter layer on the back of the CIGS absorption layer, thereby passivating solar cell defects, inhibiting carrier recombination, promoting carrier transmission and collection, improving open circuit voltage ( V OC ), short circuit current ( J sc ), and filling factor (FF), and further improving the PCE.

[Display omitted] Given the societal concerns about the use of toxic chemicals and costly fabrication of functional materials and devices for photovoltaic applications, it is important to develop alternative sustainable methodologies. Previous studies have shown that cost-effective printing fabrication of Cu(In,Ga)Se 2 thin film photovoltaics represents an interesting alternative to energy-demanding vacuum-based deposition methods, commonly used to produce Cu(In,Ga)Se 2 photovoltaics. To enrich the field of printed Cu(In,Ga)Se 2 photoabsorber thin films and to develop associated eco-friendly solutions, two novel inks, consisting of non-toxic reagents and readily available oxide materials, are reported. Screen printing of the inks over fluorine-doped tin oxide conductive substrates followed by swift selenization of the resultant patterns provides a straightforward route to phase-pure, uniform, and compact Cu(In,Ga)Se 2 films with thickness and band gap energies ranging from 2.5 µm to 3.5 µm and from 0.97 eV to 1.08 eV, respectively. The present approach represents an important step forward in the sustainable fabrication of Cu(In,Ga)Se 2 photovoltaics, where the physical properties of the photoabsorber can be easily adjusted by tuning the conditions of the screen printing process and the metal ratios in the inks. [ABSTRACT FROM AUTHOR]

Crystalline gallium oxide is a promising wide‐bandgap semiconductor material, especially for applications in high‐frequency and high‐power devices. With an optical bandgap energy well above 4 eV, which implies no visible light absorption, it is also a candidate for one of the front‐side layers in thin‐film solar cells. X‐ray amorphous gallium oxide (a‐Ga2O3) deposited by RF magnetron sputtering is applied as an n‐type buffer layer in substrate‐type configuration solar cells based on industry‐relevant inline coevaporated Cu(In,Ga)Se2 (CIGS) absorbers, which include an i‐ZnO/ZnO:Al bilayer as the front electrode. The cells exhibit an efficiency peak at Ga2O3 deposition temperatures in the range of 140–180 °C and show mostly a gain in short‐circuit current density compared with the CdS‐buffered reference cells, as a result of the high optical bandgap of a‐Ga2O3 in the range of 4.6–4.8 eV. The CIGS solar cells with sputtered a‐Ga2O3 buffers reach efficiencies of almost 14%, lacking in open‐circuit voltage and fill factor, whereas the CdS‐buffered cells are on a 16–17% level. Light soaking of the cells leads to a slight improvement of the fill factor, but the gap in open‐circuit voltage of 80–100 mV in contrast to the CdS‐buffered reference cells remains unaffected. [ABSTRACT FROM AUTHOR]

The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells with bandgaps (Eg) > 1.40 eV is studied on a large sample set. It is confirmed that moving away in composition from ternary AgGaSe2 by simultaneous reduction in Ga and Ag content widens the chalcopyrite single‐phase region and thereby reduces the amount of ordered vacancy compounds (OVCs). As a consequence, a distortion in current−voltage characteristics, ascribed to OVCs at the back contact, can be successfully avoided. A clear anticorrelation between open‐circuit voltage (VOC) and short‐circuit current density (JSC) is detected with varying absorber stoichiometry, showing decreasing VOC and increasing JSC values for [I]/[III] > 0.9. Capacitance profiling reveals that the absorber doping gradually decreases toward stoichiometric composition, eventually leading to complete depletion. It is observed that only such fully depleted samples exhibit perfect carrier collection, evidencing a very low diffusion length in wide‐gap ACIGS films. The results indicate that OVCs at the surface play a minor or passive role for device performance. Finally, a solar cell with VOC = 0.916 V at Eg = 1.46 eV is measured, which is, to the best of our knowledge, the highest value reported for this bandgap to date. [ABSTRACT FROM AUTHOR]

Passivation of the Cu(In,Ga)Se2 (CIGS)/Mo back contact using AlOx is studied to reduce the recombination at this interface. Herein, RbF postdeposition treatment (RbF‐PDT), a well‐established method to improve absorber and front interface properties is used on back‐passivated solar cells. It is found that this combination deteriorates the performance due to formation of an injection barrier at the front and reduced acceptor concentration. Photoluminescence yield and decay times show no indication of increased defect recombination, as both are improved. With time‐of‐flight secondary ion mass spectroscopy, in‐depth and lateral alkali profiles are measured. It is shown that the Na concentration is higher at the AlOx/Mo back contact and that Rb accumulates at the CdS/CIGS interface. It is hypothesized that Na at the back is released during the RbF‐PDT and inhibits Rb diffusion into the CIGS layer. Rb remains at the front and acceptor concentration is reduced. Modeling of dark and light current–voltage characteristics shows that the injection barrier and low doping are responsible for the reduced Voc and fill factor (FF). It is suggested that the commonly observed FF losses upon heavier alkali PDT can be eliminated by adapting the initial Na amount. [ABSTRACT FROM AUTHOR]

This study investigated the effect of MoSe2 on the contact resistance (RC) of the transparent conducting oxide (TCO) and Mo contact in the P2 region of the CIGS photovoltaic module. MoSe2 formed in the process of making the Cu(In,Ga)Se2 (CIGS) absorber layer imparts ohmic contact properties to the CIGS/Mo contact. In the process of connecting cells in series to fabricate a CIGS photovoltaic module, TCO/MoSe2/Mo contact was formed, and it was confirmed that MoSe2 increased the RC of this contact using the transmission line method. It is estimated that the reason MoSe2 increases the RC is due to conduction band offset (CBO). When ZnO used as TCO forms a contact with MoSe2, 0.6 eV CBO is formed due to the difference in electron affinity. This CBO can act as a resistor that impedes the flow of current. Therefore, in order to reduce the contact resistance of the CIGS solar module and increase the power conversion efficiency, it is necessary to make the MoSe2 thin enough to facilitate carrier tunneling. [ABSTRACT FROM AUTHOR]

Heterojunction Cu(In,Ga)Se2 (CIGS) solar cells comprise a substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al. Here, Al-doped zinc oxide (AZO) films were deposited by magnetron sputtering, and the substrate temperature was optimized for CIGS solar cells with two types of CIGS light absorbers with different material properties fabricated by three-stage co-evaporation and two-step metallization followed by sulfurization after selenization (SAS). The microstructure and optoelectronic properties of the AZO thin films fabricated at different substrate temperatures (150–550 °C) were analyzed along with their effects on the CIGS solar cell performance. X-ray diffraction results confirmed that all the deposited AZO films have a hexagonal wurtzite crystal structure regardless of substrate temperature. The optical and electrical properties of the AZO films improved significantly with increasing substrate temperature. Photovoltaic performances of the two types of CIGS solar cells were influenced by changes in the AZO substrate temperature. For the three-stage co-evaporated CIGS cell, as the sputter-deposition temperature of the AZO layer was raised from 150 °C to 550 °C, the efficiencies of CIGS devices decreased monotonically, which suggests the optimum AZO deposition temperature is 150 °C. In contrast, the cell efficiency of CIGS devices fabricated using the two-step SAS-processed CIGS absorbers improved with increasing the AZO deposition temperature from 150 to 350 °C. However, the rise in AZO deposition temperature to 550 °C decreased the cell efficiency, indicating that the optimum AZO deposition temperature was 350 °C. The findings of this study provide insights for the efficient fabrication of CIGS solar cells considering the correlation between CIGS absorber characteristics and AZO layer deposition temperature.

Abstract While the major function of molybdenum (Mo) back contact on soda‐lime glass (SLG) substrate in Cu(In,Ga)Se2 (CIGS) solar cells is to provide good adhesion and good conductivity, its role as the transportation channel for alkali elements from SLG substrate to CIGS layer has often been overlooked. In this work, we have intentionally fabricated tri‐layered structure in contrast to the conventional bi‐layered structure for Mo back contact with the microstructure of the thin topmost Mo layer manipulated by the Ar pressure and water vapor during sputtering deposition. It has been discovered that the CIGS solar cell conversion efficiency can be greatly affected by this thin topmost Mo layer of the back contact. Investigations on the elemental depth profiles in the CIGS devices and the chemical states of Mo on the back contact surfaces have revealed that the surface layer of Mo back contact has a strong effect on the diffusion of alkali elements from SLG into CIGS absorber layer, which in turn influences the formation of Ga gradient in the CIGS layer and thus the solar cell performance. It was revealed that Mo(OH)6 and MoO3 formed on the surface of Mo back contact play a key role in determining the K atom distribution and Ga gradient. A reaction mechanism was also proposed.

Cu(In,Ga)Se2 -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)Se2 solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se2 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.

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. [ABSTRACT FROM AUTHOR]

• Device design is theoretically performed for bifacial Cu(In,Ga)Se 2 solar cells. • Bandgap grading in Cu(In,Ga)Se 2 layer improves performance under rear illumination. • Rear surface layer with widegap reduces carrier recombination at the back contact. • Increment of optical path length by forming the surface texture is useful. • These findings indicate a guideline for the high-performance bifacial solar cells. For bifacial solar cells, a gain of 10–50% of the electric power generation by harvesting albedo radiation is expected compared with mono-facial solar cells. Cu(In,Ga)Se 2 (CIGSe) is a promising photovoltaic material owing to its unique physical properties, whereas the knowledge of the bifacial CIGSe solar cells is not fully understood. In this article, high efficiencies under front and rear illuminations for the bifacial CIGSe solar cells are theoretically demonstrated. An adequate bandgap grading by Ga/(Ga + In) ratio in the CIGSe layer leads to a shift of the depth in carrier-generation toward pn -junction and driving force of electron transport by electric field, resulting in improvement of device performance. On the other hand, it is also disclosed that the effect of bandgap grading alone is not enough to obtain the high short-circuit current density (J SC) under rear illumination. It is found that improvement of a CIGSe quality and an introduction of a rear surface layer contribute to reduction of the carrier recombination at the bulk and back contact, respectively, leading to enhanced J SC in rear illumination. Furthermore, the effect of optical path length by the surface texture is discussed. These findings indicate a guideline for the promising high-performance bifacial CIGSe solar cells. [ABSTRACT FROM AUTHOR]

The power conversion efficiency boost of Cu(In,Ga)Se2 in the past years has been possible due to the incorporation of heavy alkali atoms. Their addition through postdeposition treatments results in an improvement of the open‐circuit voltage, the origin of which has been associated with grain boundaries. Herein, the effect of potassium fluoride postdeposition treatments on the optoelectronic properties of a series of sodium‐free Cu(In,Ga)Se2 single crystals with varying Cu and Ga content is discussed. Results suggest that improvement of the quasi‐Fermi level splitting can be achieved in the absence of grain boundaries, being greater in low‐gallium Cu‐poor absorbers. Secondary ion mass spectrometry reveals the presence of potassium inside the bulk of the films, suggesting that transport of potassium can occur through grain interiors. In addition, a type inversion from n to p in potassium fluoride‐treated low‐gallium Cu(In,Ga)Se2 is observed, which along with study of the carrier lifetime demonstrates that potassium can act as a dopant. The fact that potassium on its own can alter the optoelectronic properties of Cu(In,Ga)Se2 single crystals demonstrates that the effect of postdeposition treatments goes beyond grain boundary passivation. [ABSTRACT FROM AUTHOR]

Band gap tuning of electrodeposited Cu(In1-x,Gax)Se2 thin films through varying In3+ and Ga3+ concentrations in the electrolytic bath is demonstrated. Cyclic voltammetry was used to determine best Cu(In1-x,Gax)Se2 deposition potentials at the different bath conditions. Band gap tuning of Cu(In1-x,Gax)Se2 was achieved, through incorporation of varying In3+ and Ga3+ levels during film growth, in the range of 1 to 1.4 eV, corresponding to Ga content 0 ≤ Ga/(In + Ga) ≤ 0.64. Deposited films were characterized by EDS/SEM, XRD, and Raman spectroscopy to determine chemical composition, morphology, and crystal structure. Results show that as-deposited Cu(In1-x,Gax)Se2 thin films are of low crystallinity, with Se and Cu-Se compounds present as a secondary phase. Selenization of electrodeposited Cu(In1-x,Gax)Se2 films was performed using a rapid thermal processing system at 550 °C in an overpressure reactive atmosphere of N2/H2 (96%:4%) and elemental Se vapor. Selenization treatment promoted recrystallization and elimination of secondary phases, resulting in an increase in grain size while maintaining film composition. Processed films with Ga/(In + Ga) = 0.35 were processed into devices, achieving 2.6% efficiency. [ABSTRACT FROM AUTHOR]

Summary: In this study, a flexible polydimethylsiloxane (PDMS) ARL with embedded ZnO‐NRs was fabricated on the surfaces of Cu(In,Ga)Se2 (CIGS) solar cells using the chemical solution method. Under the conditions of an AM1.5G solar spectrum, the application of the PDMS ARL of embedded ZnO‐NRs on the surface of a CIGS solar cell can effectively increase the conversion efficiency of the solar cell from 7.23% to 7.79%. In addition, broadband and omnidirectional light harvesting are important as the sun moves over time; therefore, omnidirectional anti‐reflection applications of various ARLs on CIGS photovoltaic cells were also studied. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se2 absorbers were immerged in deionized water for different times, and specific chemical evolutions were monitored thanks to X‐ray photoemission spectroscopy. Cu(In,Ga)Se2 related dissolution products were studied in water through induced coupled plasma optical emission spectroscopy. From those analyses, specific surface network disorganization was observed, with Cu migration towards the surface, leading to different kinetics of oxidation and dissolution for each element that could be quantified. [ABSTRACT FROM AUTHOR]

This contribution evaluates the effect of absorber off‐stoichiometry in wide‐gap (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells. It is found that ACIGS films show an increased tendency to form ordered vacancy compounds (OVCs) with increasing Ga and Ag contents. Very little tolerance to off‐stoichiometry is detected for absorber compositions giving the desired properties of 1) an optimum bandgap (EG) for a top cell in tandem devices (EG = 1.6–1.7 eV) and at the same time 2) a favorable band alignment with a CdS buffer layer. Herein, massive formation of either In‐ or Ga‐enriched OVC patches is found for group I‐poor ACIGS. As a consequence, carrier transport and charge collection are significantly impeded in corresponding solar cells. The transport barrier appears to be increasing with storage time, questioning the long‐term stability of wide‐gap ACIGS solar cells. Furthermore, the efficiency of samples with very high Ga and Ag contents depends on the voltage sweep direction. It is proposed that the hysteresis behavior is caused by a redistribution of mobile Na ions in the 1:1:2 absorber lattice upon voltage bias. Finally, a broader perspective on OVC formation in the ACIGS system is provided and fundamental limitations for wide‐gap ACIGS solar cells are discussed. [ABSTRACT FROM AUTHOR]

• Beneficial effects of heavy alkali-metals on CuGaSe 2 films and devices are small. • LiF postdeposition treatment leads to few CuGaSe 2 film surface modifications. • Li may be more beneficial than Cs for CuGaSe 2 devices, unlike quaternary Cu(In,Ga)Se 2. Enhancement of photovoltaic performance due to alkali-metal doping in various thin-film compound solar cells such as chalcogenide Cu(In,Ga)Se 2 (CIGS), Cu 2 ZnSnS 4 (CZTS), and organic–inorganic hybrid perovskite, is significant in the development of photovoltaic technologies. In this work, the effects of light Li- and heavy Cs-doping in ternary CuGaSe 2 (CGS) and quaternary CIGS thin-films and devices are compared. The new findings are: (i) heavy alkali-metal halide CsF postdeposition treatment (PDT) modifies the CGS film surface morphology and increases the carrier density; however, the beneficial effects of Cs on the CGS photovoltaic performance are low when compared to the results observed from quaternary CIGS devices; (ii) LiF-PDT leads to no significant CGS and CIGS film surface (hetero p-n junction interface) modifications; thus, the use of a thin (approximately 30 nm) CdS buffer layer is not effective in enhancing photovoltaic performance; and (iii) the beneficial effects of either light Li or heavy Cs on ternary CGS devices are modest; nonetheless, Li may be more effective than Cs, unlike quaternary CIGS devices. [ABSTRACT FROM AUTHOR]

K-doped Cu(In,Ga)Se2 absorber thin films were prepared by a two-step process (sputtering/selenization) using a metal precursor of a KF-premixed In/CuGa/Mo/glass structure and Se vapor. KF was deposited on top of In, between In and CuGa, and below CuGa by thermal evaporation. The properties were compared with those of a precursor without KF inserted. As an effect of KF insertion, a large electrical change was observed, i.e., the disappearance of deep-level defects, and the grain size and Ga composition distribution of the absorber layer thin film were changed slightly. Nevertheless, there was no benefit in terms of the solar cell performance compared to a precursor without KF inserted. The K inserted in the precursor occupied a position competitively with Na diffused from the substrate, resulting in an undesirable carrier density profile in the surface region of the absorber layer thin film. [ABSTRACT FROM AUTHOR]

Cu (In,Ga)Se2 solar cells exhibit higher efficiencies if an appropriate Ga gradient is introduced and if the absorber is doped with sodium (Na) plus a heavier alkali atom such as potassium. However, a Gallium gradient in the presence of Na is challenging because sodium impedes the interdiffusion of elements and influences the gradient. In this contribution, we show that the presence of sodium during growth with the combination of high Ga concentration creates a pronounced gradient that is detrimental for carrier collection. One solution is to avoid Na abundance during absorber growth but to add it to the grown absorber layer via a postdeposition treatment. We investigate the effect of different Na incorporation methods simultaneously with KF‐PDT on wide band gap CIGSe absorbers. By preparation on alkali‐free substrates and application of alkalis (NaF and KF) onto a grown CIGSe layer, we show by smoothening of the Gallium gradient a large improvement in the solar cell performance from 4% to 8%. Another way to optimize the gradient in the presence of Na is the modification of the three‐stage method. This yields the best efficiency of 10% in our laboratory at an integral GGI of ~0.8. By means of temperature‐dependent JV measurements, we show that the additional postdeposition of KF induces a barrier for the diode current. We conclude that KF‐PDT induces a new thin layer at the CIGSe surface that has a lower valence band edge relative to the CIGSe bulk and is responsible for the double‐diode behavior. This barrier can also explain the Voc(T) saturation at low temperature. [ABSTRACT FROM AUTHOR]

CIGS-based thin film solar cell (TFSC) technology is emerging as a promising contributor to the solar photovoltaic industry next to the presently leading Si-based technology. Although the theoretical limit of power conversion efficiency (PCE) is as high as 33.5%, the highest experimental PCE so far just exceeded 20% in the past several years. Therefore, significant efforts are still continuing for further performance enhancement of these cells. Considering that the buffer layer has been identified as one of the key factors, the efforts to replace state-of-the-art but toxic CdS buffer layer have yielded promising results. Several studies showed that the alternative buffer layers grown with environmentally benign materials could even produce a better performance than the CdS-based TFSCs. In this regard, atomic layer deposition (ALD) has been proved as one of the best techniques for depositing the alternative buffer layers. Several Zn-based ternary and few other binary (e.g. In 2 S 3) compounds have been investigated to realize an optimum ALD-grown buffer layer. In the recent year, a record PCE of 23.35% was achieved using ALD-grown ZnMgO buffer layer along with chemical bath deposited Zn(O,S,OH) for CIGSSe TFSC. However, in general the ALD-grown buffer layers only could provide PCE s well below 20%. The article presents a comprehensive survey on rapid increase in PCE for several ALD-grown buffer layers during the early period followed by a trend of saturation. Finally, the article discusses the current challenges and future scopes/possibilities for the ALD-grown buffer layers as potential alternatives of CdS toward practical applications of CIGS TFSC. [ABSTRACT FROM AUTHOR]

This contribution evaluates a sequential post‐deposition treatment of Cu(In,Ga)Se2 (CIGS) films, consisting of 1) a post‐sulfurization in elemental S‐atmosphere and 2) a subsequent treatment by heavy alkali fluorides (Alk‐PDT). First, the effect of the sulfurization step on the corresponding solar cell performance is investigated and optimum process parameters, leading to an efficiency improvement, are identified. Losses in carrier collection observed after S‐incorporation are attributed to an increased grain boundary (GB) recombination. It is found that the corresponding reduction in short‐circuit current density can be mitigated by a RbF‐ or KF‐PDT, supposedly by depleting GBs in Cu. However, in strong contrast to non‐sulfurized CIGS, the Alk‐PDT results in a lower open‐circuit voltage and distortions in the current–voltage (I–V) characteristics for sulfurized absorbers. Possible explanations are the absence of a wide‐gap surface phase and/or air exposure between the post‐treatment steps. It is further proposed that a back contact barrier may be responsible for the distortions in I–V. [ABSTRACT FROM AUTHOR]