In this paper, wide-gap Cu (In,Ga)(S,Se)2 thin-film solar cells are studied in view of their performance, limitations, and opportunities for further optimization. To this end, a wide variety of properties is investigated. This includes the role of gallium gradients, grain size effects, electronic properties, doping metastabilities, and minority carrier lifetime. Particular emphasis is placed on the impact of alkali atoms. A comparison of surface, interface, and grain boundary chemistry shows systematic atomic accumulation and depletion effects. This leads to electronic modifications in the grain boundary regions of the absorber. Heavy alkali treatments also influence the device properties, giving a clear boost of open-circuit voltage. By the combination of different experimental results, this positive open-circuit voltage effect has been explained in terms of reduction of interface recombination. The latter effects are discussed in view of a possible alkali-indium-selenium bond formation at the interface between the absorber and the buffer layer. The properties of a 14.2%-efficient Cu (In,Ga)Se2-based device with [Ga]/([Ga] + [In]) = 0.8 and a wide optical band gap of 1.48 eV are investigated, also in view of further opportunities for improvement.

Based on the high power conversion efficiencies and compatibility toward large-area deposition techniques, Cu(In,Ga)(S,Se) _2 (CIGSSe) phototvoltaic absorbers are currently at the forefront of chalcopyrite thin-film solar cell technology. The performance of these solar cells is critically dependent on the properties of the interface between the p-type chalcopyrite absorber and the n-type buffer and window layers. Due to the complex defect physics of the chalcopyrites in general, the defect-electronic properties of the absorber surface is of particular concern. In this regard, the CIGSSe surfaces are considerably less understood compared with their S-free counterparts (e.g. Cu(In,Ga)Se _2 ). In the present work, by applying high-resolution scanning probe techniques such as atomic force microscopy and scanning tunneling spectroscopy (STS), combined with electron backscatter diffraction, the morphology, the crystallographic orientation, and the defect electronic properties of CIGSSe thin-film surfaces were investigated. Our work highlights distinct differences as well as similarities between S-containing and S-free chalcopyrite thin films. Three types of features were found on the CIGSSe surface, which were found to be exclusively made of polar facets. This is different from S-free absorbers that are known to facet in both, polar and non-polar planes with distinct electronic properties. Defect density mapping using STS revealed a highly defective surface with significant lateral inhomogeneities. Furthermore, grain boundary band bending detected in S-free absorber surfaces was absent. However, similar to S-free absorbers, annealing under ultra-high vacuum conditions was found to electronically passivate the CIGSSe surface. Our results shed light on the fundamental properties of these S-containing chalcopyrite-type surfaces and demonstrate a valuable platform for further optimization of this promising solar cell technology.

We report on crossing the 20% efficiency line for thin-film solar modules. The efficiency of our cadmium-free Cu(In,Ga)(S,Se) 2 (CIGSSe) mid-sized modules (30 × 30 cm 2 ) based on the cost-efficient AVANCIS stacked elemental layer – rapid thermal processing absorber process has evolved in the last two years reaching 19.6%, 19.8% and recently we have achieved an efficiency level of 20.3% as independently measured by NREL. The recent improvements were made possible by thorough variations in absorber composition and elemental distribution. The optimization of the absorber thickness, and of the band gap profile through the engineering of sulfur content and gradient at the absorber surface induces an improved absorber quality leading to a distinct increase in the product of short circuit current density and open circuit voltage (J SC × V OC ). Moreover, improving the absorber homogeneity and adjusting the absorber-buffer interface play an important role in enhancing the fill factor.

Alloying small quantities of silver into Cu(In,Ga)Se-2 is shown to improve the efficiency for wide and low bandgap solar cells. Low bandgap industrial Cu(In,Ga)(S,Se)(2) absorbers are studied, substituting less than 10% of the copper with silver, using absolute photoluminescence and cathodoluminescence spectroscopy. Silver improves the grain size and promotes the interdiffusion of Ga and In across the depth of the absorber, resulting in a smoother bandgap gradient. However, a certain lateral inhomogeneity is observed near the front and back sides. The nonradiative losses in the bare absorbers are reduced by up to 30 meV.

Copper indium gallium selenide (CIGS)-based solar cells are a type of thin-film photovoltaic technology used to convert sunlight into electricity. They are one of the most promising thin-film technologies with high efficiency and low-cost potential. CIGS is a direct band gap material with a high absorption coefficient, around 2 mu m thickness can absorb most of the light which can reduce the usage of material. CIGS solar cells have a better temperature coefficient, meaning their efficiency decreases less in high-temperature environments compared to other solar technologies. Furthermore, CIGS solar cells also have excellent low-light performance due to their broad absorption spectrum. This allows them to generate electricity even in partially shaded or cloudy conditions, which can be common in urban environments with tall buildings or trees casting shadows. Thus, CIGS solar cells are also a good option to be used for building-integrated PV (BIPV) systems. The latest cell efficiency record was reached in 2023 with 23.6%. Ongoing research aims to increase efficiency, durability, and cost-effectiveness, making the CIGS thin-film technology a mainstream option for solar energy generation.

The chemical and electronic structure of the front contact i-ZnO/InxSy:Na interface for Cu(In,Ga)(S,Se)(2)-based thin-film solar cells is investigated using a combination of x-ray and electron spectroscopies. Upon i-ZnO sputter deposition on the InxSy:Na buffer layer, we find an intermixed heterojunction and the formation of InOx and Na2SO4. The window layer is shown to consist of a mixture of Zn(OH)(2) and ZnO, with decreasing relative Zn(OH)(2) content for thicker window layers. Moreover, we observe diffusion of sodium to the surface of the window layer. We derive electronic surface band gaps of the i-ZnO and InxSy:Na layers of 3.86 +/- 0.18 eV and 2.60 +/- 0.18 eV, respectively, and find a largely flat conduction band alignment at the i-ZnO/InxSy:Na interface.

Record efficiency in chalcopyrite based solar cells Cu(In,Ga)(S,Se)2 (CIGSSe) is achieved using the standard gallium gradient structure to increase the band gap of the absorber towards the back side. Although this structure has reduced the recombination at the back side, we demonstrate that in sequentially processed absorbers with intentional gallium gradient, the back side is a source of non-radiative recombinations. Photoluminescence measurements performed on both the front and back sides reveal two main radiative recombination paths, the dominant one corresponding to band-to-band recombination at the band gap minimum and the second one attributed to a secondary phase, mainly located towards the back. A linear relation between the non-radiative voltage losses and the contribution of the secondary PL peak is observed and suggests that reducing the amount of secondary phase could decrease these losses by up to 180mV. Additionally, we show that as the back side of the absorber is etched away until complete removal of the gallium gradient, the photoluminescence intensity increases by 1 order of magnitude, which translates into an increase of 60meV in quasi Fermi level splitting or voltage.

A 1D single-cell simulation model for a laminated Ga-rich 30x30 cm(2) n-ZnO/ZnOS/Cu(In,Ga)(S,Se)(2)/Mo(Se,S)(2)-based solar module with a power conversion efficiency of close to 19% is developed and calibrated on its current-voltage (IV), capacitance-voltage (CV), external quantum efficiency(EQE) and V-oc(G) 2 crystal and can be solved by a modification of the sulfur GDOES profile in accordance with grain sizes, (b) the saturation current density and the diode factor can be calibrated by electron and hole capture cross-sections of the absorber to match the measured IV data, and (c) a modification of the illumination spectrum corresponding to reflection losses can account for missing real part values of the complex refractive index. The resulting interplay of recombination mechanisms in the simulation model is then verified by a generation-dependent V-oc measurement.

The chemical and electronic structure of the interface between a sputter‐deposited Zn(O,S) buffer layer and an industrial Cu(In,Ga)(S,Se) 2 (CIGSSe) absorber for thin‐film solar cells is investigated with X‐ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy. We find a CIGSSe absorber surface band gap of 1.61 (±0.14) eV, which is significantly increased as compared to the minimal value derived with bulk‐sensitive methods (≈1.1 eV). We find no indication for diffusion of absorber elements into the buffer layer. Surface‐ and bulk‐sensitive measurements of the buffer layer suggest the presence of S‐Zn and S‐O bonds in the Zn(O,S) layer. We find that the naturally existing downward band bending toward the CIGSSe absorber surface is increased by the formation of the interface, likely enhancing carrier separation under illumination. We also derive a flat conduction band alignment, in line with the reported high conversion efficiencies of corresponding large‐area solar cells.

With our mid-sized Cd-free R&D modules based on the cost-efficient AVANCIS SEL-RTP absorber process we have recently breached the 20% efficiency level with an efficiency of 20.3% as independently certified by NREL. We could transfer this best value to an average value of 19.6% with standard cell width in our pilotline by introduction of a screen-printed grid process. For the scale-up and industrialization of the new champion stack we developed and manufactured a new PVD tool for the key Na-PDT process required together with the sputterd ZnOS buffer process. For the fullsize module we, thereby, achieved a mean module power of 164W with fullsize champion module power at 176W. The combined process and equipment development for CAPEX and OPEX optimization will path the way for further expansion of Cd-free CIS manufacturing capacity.

An intriguing area in the field of alkali fluoride post-deposition treatment (PDT) on Cu(In,Ga)(S,Se)2 (CIGSSe) absorbers is the effect of the chalcogen atmosphere on the absorber's surface during the PDT process. In this work, we focus on RbF-PDTs and we study (i) the effect of Se and S atmosphere and (ii) the impact of In and RbF co-evaporation under S vapor on the vibrational and chemical properties of the absorber's surface. Using micro -Raman and X-ray photoelectron spectroscopy, we examine three different PDT processes: RbF under S atmo-sphere (RbF(S)-PDT), In and RbF co-evaporation under S atmosphere (In + RbF(S)-PDT) and In and RbF co -evaporation under Se atmosphere (In + RbF(Se)-PDT). Our results show that under Se atmosphere the forma-tion of a Cu-poor chalcopyrite phase is enhanced, while under S atmosphere Cu-poor chalcopyrite phases are hindered. In addition, the In + RbF(S)-PDT leads to the formation of a Rb:InxSy compound at the surface of the absorber. This almost Cu -free surface compound seems to impedes Cu inclusion during the In + RbF(S)-PDT in the indium sulfide spinelle lattice thanks to the occupation of its cationic vacancies by Rb.

Record efficiency in chalcopyrite-based solar cells Cu(In,Ga)(S,Se)2 is achieved using a gallium gradient to increase the band gap of the absorber towards the back side. Although this structure has successfully reduced recombination at the back contact, we demonstrate that in industrial absorbers grown in the pilot line of Avancis, the back part is a source of non-radiative recombination. Depth-resolved photoluminescence (PL) measurements reveal two main radiative recombination paths at 1.04 eV and 1.5-1.6 eV, attributed to two phases of low and high band gap material, respectively. Instead of a continuous change in the band gap throughout the thickness of the absorber, we propose a model where discrete band gap phases interlace, creating an apparent gradient. Cathodoluminescence and Raman scattering spectroscopy confirm this result. Additionally, deep defects associated to the high gap phase reduce the absorber performance. Etching away the back part of the absorber leads to an increase of one order of magnitude in the PL intensity, i.e., 60 meV in quasi Fermi level splitting. Non-radiative voltage losses correlate linearly with the relative contribution of the high energy PL peak, suggesting that reducing the high gap phase could increase the open circuit voltage by up to 180 mV.

Electronic defect levels in semiconductor devices are often investigated by thermal admittance spectroscopy, a method in which the device capacity is measured as a function of frequency and temperature. When applied to Cu(In,Ga)(S,Se)2-based thin film solar cells, these commonly show a characteristic signature which has an activation energy of roughly 0.1 eV and has been termed the "N1" signature. However, even though this feature has been observed for at least two decades, the origin of the N1 signature is still under quite some debate in the community. Explanations include a defect level in the absorber bulk or near the heterointerface, a transport barrier at the back contact or at the buffer/i-layer interface, as well as mobility freeze-out. In order to contribute to an answer on this issue, in this study we fabricated several Cu(In,Ga)(S,Se)2 devices in which the individual layers of the cell stack were systematically varied, so that between two variations only one layer was different. This approach allowed us to test different hypotheses on the origin of the N1 signature by measuring admittance spectroscopy on the variations and checking whether a change in a specific layer led to a change in the N1 signature. Additionally, we investigated the role of the N1 signature in non-radiative recombination losses. The results suggest that the N1 signature is caused by a transport barrier in the buffer/absorber region in the investigated devices.

In this work, the impact of different Cu(In 1‐x Ga x )(Se 1‐y S y ) 2 solar cell structures on the shift of the dominant recombination region at varying light intensities was investigated. A new parameter was proposed to account for the dominant recombination region relative to the minimum band gap location in a graded absorber. Additionally, the influence of shunt resistances on the dominant recombination location for different CIGSSe solar cell structures was modeled. Within the investigated illumination range, correlations between the dominant recombination location and solar cell parameters as well as their temperature dependence were discussed.

One of the advantages of Cu(In,Ga)(S,Se)2 (CIGSSe) thin film solar cells is the lower temperature sensitivity of the solar cell parameters as compared to conventional silicon-based technologies. To further improve and to tailor CIGSSe temperature behavior for different environments, it is important to gain a detailed understanding of the microscopic mechanisms involved. The goal of the current study is to elucidate the impact of the individual functional layers within the CIGSSe solar cell stack on the temperature dependence of the overall cell performance. For this purpose, temperaturedependent current-voltage measurements were performed on a selected set of CIGSSe cells, containing a systematic variation of functional layers. The experimental results demonstrate that the absorber layer exhibits the largest influence on the temperature dependence of the power efficiency. An analysis of the contributing parameters revealed that the temperaturedependence of the power efficiency is mainly governed by a change in the open-circuit voltage. The temperature coefficient of the open-circuit voltage is analyzed in terms of the cell?s physical parameters, including their minimum band gap and the temperature-dependence of the dark saturation current.

The metastable behavior of the fill factor temperature coefficient (βFF,rel) in Cu(In,Ga)(S,Se)2 solar cells is investigated for absorbers with a different Ga content in the front surface, and for different buffer materials. The buffer/absorber interface region is suggested to be the location which causes the metastability in βFF,rel. Temperature dependent current–voltage curves, light‐biased external quantum efficiency, and low temperature capacitance–voltage measurements are performed to characterize the electrical properties and metastable defects in the investigated solar cell devices. The presence of Ga in the absorber front surface, together with the buffer layer material plays a significant role in the metastable behavior. Additionally, energy dispersive X‐ray spectroscopy indicates that the InxSy buffer layers allow for elemental interdiffusion from absorbers with a Ga‐richer front surface, whereas a Zn(O,S) buffer shows to be rather resilient to atomic diffusion. The elemental interdiffusion between the buffer and absorber layers is proposed to be the cause of the defects causing the observed βFF,rel metastability.

Dry buffer layer deposition techniques for chalcopyrite (CIGSSe)-based thin-film solar cells lack the surface-cleaning characteristics of the commonly used CdS or Zn(O,S) wet-chemical bath deposition. A UV-induced ozone and/or a low-energy Ar+-ion treatment could provide dry CIGSSe surface cleaning steps. To study the impact of these treatments, the chemical surface structure of a CIGSSe absorber is investigated. For this purpose, a set of surface-sensitive spectroscopic methods, i.e., laboratory-based x-ray photoelectron spectroscopy and x-ray-excited Auger electron spectroscopy, is combined with synchrotron-based soft x-ray emission spectroscopy. After treatment times as short as 15s, the UV-induced ozone treatment decreases the amount of carbon adsorbates at the CIGSSe surface significantly, while the oxygen content increases. This is accompanied by the oxidation of all absorber surface elements, i.e., indium, selenium, sulfur, and copper. Short (60s) low-energy Ar+-ion treatments, in contrast, primarily remove oxygen from the surface. Longer treatment times also lead to a removal of carbon, while extremely long treatment times can also lead to additional (likely metallic) Cu phases at the absorber surface as well.

Doping an indium sulfide buffer layer with sodium is a promising route to replace the “state‐of‐the‐art” CdS buffer layer in chalcopyrite‐based thin‐film solar cells, as it achieves efficiencies as high as 17.9% for large‐area devices (30 cm × 30 cm). We report on the chemical and electronic structure of the InxSy:Na/CuIn(S,Se)2 (CISSe) interface for thin‐film solar cells by means of photoelectron, soft x‐ray emission, and inverse photoemission spectroscopy. For as‐deposited InxSy:Na buffer layers, we find a sulfur‐poor surface and, in comparison to undoped InxSy and the standard CdS buffer, derive a large electronic surface band gap of 2.60 ± 0.11 eV. The conduction band offset at the buffer/absorber interface is a spike of 0.32 ± 0.10 eV. After annealing at 200°C to simulate the thermal load of subsequent cell manufacturing processes, an additional diffusion of copper and selenium from the absorber towards the buffer layer surface is observed, leading to a distinct electronic surface band gap decrease of the InxSy:Na buffer layer (to 2.11 ± 0.11 eV). We speculate that the diffusion of absorber elements causes a band gap widening at the former absorber surface and that both effects lead to a reduction of the conduction band spike for the buried InxSy:Na/CISSe interface after annealing.

We present a one-dimensional simulation model for high efficiency Cu(In,Ga)(Se,S)2 solar cells with a novel band alignment at the hetero-junction. The simulation study is based on new findings about the doping concentration of the InxSy:Na buffer and i-ZnO layers as well as comprehensive solar cell characterization by means of capacitance, current voltage, and external quantum efficiency measurements. The simulation results show good agreement with the experimental data over a broad temperature range, suggesting the simulation model with an interface-near region (INR) of approximately 100 nm around the buffer/absorber interface that is of great importance for the solar cell performance. The INR exhibits an inhomogeneous doping and defect density profile as well as interface traps at the i-layer/buffer and buffer/absorber interfaces. These crucial parameters could be accessed via their opposing behavior on the simulative reconstruction of different measurement characteristics. In this work, we emphasize the necessity to reconstruct the results of a set of experimental methods by means of simulation to find the most appropriate model for the solar cell. Lowly doped buffer and intrinsic window layers in combination with a high space charge at the front of the absorber lead to a novel band alignment in the simulated band structure of the solar cell. The presented insights may guide the strategy of further solar cell optimization including (alkali-) post deposition treatments.

The Munich-based AVANCIS' R&D Pilot line drives the efficiency development of medium-sized 30x30cm 2 CIGSSe modules using the AVANCIS' SEL-RTP process (stacked elemental layer – rapid thermal processing) in combination with a dry Cd-free PVD buffer process. Applied buffer processes comprise the thermal evaporated InxSy:Na buffer as well as the sputtered Zn(O,S) buffer. The band gap of the absorber layer, especially the band gap at the absorber surface, can be varied in the SEL-RTP process by a) the Gallium (Ga) content in the precursor layer, i.e. the sputter target and/or b) the Sulphur (S) content incorporated during the RTP process. In this presentation we will illustrate how the band gap tuning for the InxSy:Na buffer system is achieved by varying the Na content in the buffer layer during the thermal evaporation process, and how the tuning is achieved in case of the Zn(O,S) buffer system by varying the S/S+O content in the purchased sputter targets. The presented efficiency improvement results from a systematic increase of the absorber band gap in combination with a buffer layer with well-aligned band gap towards the absorber surface. Our current 30x30cm 2 champion module surpasses the 18% efficiency level with an in-house measured value of 18.2% (see Figure 1) for an InxSy:Na-buffered device. We will show how the appropriate matching of the buffer band gap towards the absorber surface band gap for the two buffer materials InxSy:Na and Zn(O,S) affects the diode parameters as extracted from the respective I-V curves based on the 1-diode-model. The band alignment of the wider band gap absorber with the wide band gap buffer layers leads to an improvement of the open-circuit voltage Voc as wells as the fill factor FF at the expense of the short-circuit current density Jsc (see Figure 1).