The main cause for the power conversion efficiency limitations in Cu(In,Ga)(S,Se)2 (CIGS) solar cells is still heavily debated in literature. Possible culprits for the limitation of the open circuit voltage of CIGS devices are conduction barriers, recombination in the bulk of the absorber, at grain boundaries, at the back contact or at the interface between the p-type absorber and the n-type buffer layer. In the present work we perform a large amount of bias-dependent admittance spectroscopy measurements on CIGS solar cells. We represent the data using CVf loss maps, comparing the measurement results to simulations, allowing us to draw conclusions about the recombination processes observed in the devices. Analyzing a range of devices consisting of state-of-the-art absorber layers with varying buffer layers and power conversion efficiencies, we could draw conclusions on the presence of an interface defect at the absorber-buffer interface. In fact, all devices, independent of power conversion efficiency, showed the presence of an admittance trace that could be related to a defect at the CIGS-buffer interface. A correlation could be found between the bias voltage position of the admittance trace with the open circuit voltage of the devices, indicating that the defect is limiting the photocurrent and open circuit voltage. A digital twin model involving only an interface defect at the CIGS-buffer interface was able to reproduce current voltage and admittance measurements of the best performing cell, proving the viability of the findings. We conclude that future improvements to the power conversion efficiency of these CIGS solar cells must come from interface engineering at the CIGS-buffer interface. Variations in doping of the absorber and buffer layer, the nature of the interface and buffer layer as well as the number of fixed charges at the interface all have the potential to drastically influence the significance and bias range of the interface recombination.

Two terminal (2T) perovskite /copper-indium-gallium-selenide (CIGS) tandem solar cells combine high conversion efficiency with lightweight flexible substrates which can decrease manufacturing and installation costs. In order to improve the power conversion efficiency of these tandem solar cells, the use of advanced simulation tools is crucial to estimate the loss mechanisms. In this regard, most of the available simulation works on tandem solar cells are oriented to minimize optical losses and assuming simplifications for the electrical simulations in particular in the top and bottom cell interconnection at the so-called tunnel recombination junction (TRJ) neglecting the inner physics of the complete tandem device. Therefore, the effect of charge exchange mechanism between top and bottom soler cells on the external parameters of a tandem devices is not fully understood yet. In this work, we present an experimentally validated opto-electrical model based on the fundamental semiconductor equations for the study of loss mechanisms of a reference perovskite/CIGS solar cell. Different from other numerical works, because our simulation platform includes the fundamental working mechanisms of the layers comprising the TRJ, we can properly calculate the losses related to it. We firstly present the calibration and validation of our opto-electrical model with respect to three fabricated reference solar cells: top cell only, bottom cell only and tandem device. Then, we use the calibrated model to evaluate main loss mechanisms affecting the baseline tandem device. Finally, we use the model to propose a roadmap for the optimization of monolithic perovskite/CIGS tandem solar cells.

In this work we employ the transfer matrix method for the analysis of optical materials properties to simulate and optimize monolithic tandem solar cell devices based on CuIn 1− x Ga x Se 2 , CI(G)S, and perovskite (PVK) absorbers. By finding models that fit well the experimental data of the CI(G)S solar cell, the semitransparent perovskite solar cell (PSC) and the PVK/CI(G)S monolithic tandem solar cell, we were able to perform a detailed optical loss analysis that allowed us to determine sources of parasitic absorption. We found better substitute materials for the transport layers to increase the power conversion efficiency and, in case of semitransparent PSCs, sub-bandgap transmittance. Our results set guidelines for the monolithic PVK/CI(G)S tandem solar cells development, predicting an achievable efficiency of 30%.

To increase the bankability of photovoltaics (PV), a more accurate prediction of the lifetime and degradation-rate of PV modules is required. In past studies, average degradation rates for different technologies have been derived from field data. However, in Cu(In,Ga)Se2 (CIGS) based PV, localized shunts are commonly found in modules coming out of the factory. Although the impact of such localized defects on initial module performance has been studied in the past, their evolution and impact on module performance upon aging remain mostly unknown. In this paper, it is demonstrated that exposure of CIGS devices to elevated temperatures and humidity can result in the partial or complete recovery of localized ohmic shunts. This was observed for three types of shunting defects, each on a different CIGS device characterized before and after damp heat exposure. In each device, the reduction in shunting after damp heat could be observed in illuminated lock-in thermography, photoluminescence and current-voltage measurements. For two of the devices, the positive effect of shunt reduction was counteracted by other degradation mechanisms, such as increase in series resistance of TCO or back contact, while a 4% absolute efficiency increase was observed in the third device. Our data suggests that the shunting is mainly reduced due to oxidation of the materials initially forming the shunting path. The combined effects of moisture degradation and shunt reduction may limit detectability of moisture ingress in electrical performance data. These results illustrate the added value of regular visual inspection and luminescence in quality control.

Copper indium gallium selenide (CIGS) thin film photovoltaic devices are in the early stages of large-scale commercialization. Their high performance, uniformity, reliability, and a low carbon footprint make them an attractive alternative to standard silicon solar cells. Due to the complex processing required and the associated manufacturing costs, reliable in-line quality control technology is needed. By identifying defective cells early in production, faulty batches can be excluded from further processing, saving resources and costs. We show that micro-Raman spectroscopy (RS) and hyper-spectral imaging (HSI) are powerful tools for quality control and process improvement. Distinctive features in the Raman spectra allow the estimation of the copper to gallium plus indium (CGI) ratio, which is an important criterion for the cell’s efficiency. With HSI in the visible and near infrared range (VNIR) and the near-infrared spectral range (NIR) in combination with machine learning techniques, the layer thickness and CGI ratio are accurately predicted.

Renewable energy sources such as photovoltaic (PV) technologies are considered to be key drivers towards climate neutrality. Thin-film PVs, and particularly copper indium gallium selenide (CIGS) technologies, will play a crucial role in the turnaround in energy policy due to their high efficiencies, high product flexibility, light weight, easy installation, lower labour-intensiveness, and lower carbon footprint when compared to silicon solar cells. Nonetheless, challenges regarding the CIGS fabrication process such as moderate reproducibility and process tolerance are still hindering a broad market penetration. Therefore, cost-efficient and easily implementable in-line process control methods are demanded that allow for identification and elimination of non-conformal cells at an early production step. As part of this work, a practical approach towards industrial in-line photoluminescence (PL) imaging as a contact-free quality inspection tool is presented. Performance parameters of 10 CIGS samples with 32 individually contacted cells each were correlated with results from PL imaging using green and red excitation light sources. The data analysis was fully automated using Python-based image processing, object detection, and non-linear regression modelling. Using the red excitation light source, the presented PL imaging and data processing approach allows for a quantitative assessment of the cell performance.

Monolithic two-terminal (2T) perovskite/CuInSe2 (CIS) tandem solar cells (TSCs) combine the promise of an efficient tandem photovoltaic (PV) technology with the simplicity of an all-thin-film device architecture that is compatible with flexible and lightweight PV. In this work, we present the first-ever 2T perovskite/CIS TSC with a power conversion efficiency (PCE) approaching 25% (23.5% certified, area 0.5 cm2). The relatively planar surface profile and narrow band gap (∼1.03 eV) of our CIS bottom cell allow us to exploit the optoelectronic properties and photostability of a low-Br-containing perovskite top cell as revealed by advanced characterization techniques. Current matching was attained by proper tuning of the thickness and bandgap of the perovskite, along with the optimization of an antireflective coating for improved light in-coupling. Our study sets the baseline for fabricating efficient perovskite/CIS TSCs, paving the way for future developments that might push the efficiencies to over 30%.

Novel thin-film solar cells based on Copper Indium Gallium Selenide (CIGS) are an alternative to standard crystalline silicon cells. This work tests whether two proposed optical methods: Micro-Raman spectroscopy (RS) and photoluminescence (PL) imaging, can measure quality parameters of CIGS PV plates during their manufacture. The investigation followed three steps. Step 1: semi-finished CIGS cells were deposited on a soda-lime glass carrier and measured with Raman and PL. The test cells consisted of a Molybdenum (Mo) back contact, a CIGS layer (varied in the absorber thickness), and a CdS layer. The measurements were used to train models for predictive quality monitoring. Step 2: the plates were finished by adding an iZnO buffer layer, ZnO:Al (AZO) front electrode and divided into 32 cells by scribing down to the Mo layer and electrically tested. I-V parameters such as the open circuit voltage VOC, shunt resistance Rsh, and EQE were measured. Step 3: the finished cells were again measured using the two proposed methods to estimate the composition, efficiency, and VOC of the thin-film cells. Our results show that the proposed methods can non-destructively predict the absorber composition and cell electrical parameters and can therefore be used to exclude samples with poor cell performance at an early production stage.

Copper Indium Gallium Selenide (CIGS) thin-film solar cells are a promising technology, but inline quality inspection systems are required for efficient high-volume production. Tests with two candidate methods: Raman spectroscopy and photo-luminescence imaging, are reported in this paper. The methods were used to estimate material compositions of CIGS samples that were varied in absorber thickness and the composition of the CIGS absorber layer. Our results indicate that both methods can be valuable for contact-free inline inspection during the manufacture of CIGS solar cells, both individually and in combination.

In modern Cu(In,Ga)Se2 (CIGS) solar cell production, alkali-poor substrates are becoming increasingly common. Thus, understanding the effect of alkali atoms on the growth of the CIGS absorber has become crucial. We studied this impact by comparing the bias dependent admittance spectroscopy response of CIGS solar cells grown with varying amounts of sodium using two different growth processes. The absence of sodium led to an intense response which is progressively replaced by other response domains as the sodium supply increases. We linked these new domains to Na accumulation and a resulting increase in band bending at the CIGS/CdS interface.

In order to control the bandgap profile of CIGSe absorbers, several key parameters of the absorber formation process have been investigated using semi-industrial 30×30 cm 2 chalcogenization equipment. It was found that the degree of gallium-indium interdiffusion depends on the thermal budget of the process and on the selenium vapor pressure at the utilized high process temperature. Additionally, it depends on the copper and sodium content of the precursor. As a result, the gallium depth profile could accurately be controlled and the minimum bandgap could be varied between 1.02 eV and 1.13 eV.

An interdigitated back-contacted (IBC) configuration is proposed for submicron copper indium gallium (di)selenide (CIGS). In a modelling platform, the structure was opto-electrically optimized for maximum efficiency. The results are compared with a reference front/back-contacted (FBC) solar cell with similar absorber thickness and exhibiting 11.9% efficiency. The electrical passivation at the front side is accomplished by an Al(2)O(3)layer, which is endowed with negative fixed charges. The results indicate that with an optimal geometry and engineered bandgap grading, the efficiency of the new IBC structure can reach 17%. Additionally, with a reasonably low defect density in the absorber layer, efficiencies as high as 19.7% and open-circuit voltage comparable with that of the record solar cell are possible with the IBC structure.

A simplified Cu(In, Ga)Se-2 (CIGS) solar cell structure based on a 500 nm thin CIGS layer is presented. The absorber layers are grown with a single-stage coevaporation process, and various KF post-deposition treatments (KF-PDT) are performed. The KF-PDT leads to an efficiency increase from 7% to 12%. For all cells an increase in open circuit voltage (V-oc) and fill factor is measured, which is attributed to an improved pn junction. By changing the annealing conditions, an additional V-oc increase is measured. This increase is attributed to the reduction of light-induced defects at the CIGS/CdS interface in addition to the improved pn junction. A reduction of defects is confirmed by reduced sub band gap emission in the photoluminescence spectra, an increased decay time, and increased quasi Fermi level splitting. With SCAPS the results are simulated, and it is concluded that after KF-PDT the V-oc is limited to 640 mV due to recombination at the back contact. A higher V-oc can then only be achieved by applying a passivation layer at the back. There are no indications that the single-stage process is limiting the efficiency, revealing the potential of the proposed simplified CIGS structure and the importance of interfaces for ultrathin CIGS solar cells.

In this work, we report results from the EU-funded project ARCIGS-M. The project started in 2016 and aims to reduce the use of indium and gallium by enabling the use of very thin Cu(In,Ga)Se2 (CIGS ...

KF and NaF treatments were done for single-stage co-evaporated CuIn0.7Ga0.3Se2. The absorber layers were grown on a substrate with an alkali barrier layer and NaF was either added before or after absorber layer growth. No differences were found on the device performance amongst the procedures to add Na. This is expected if the single-stage process does not have a copper rich stage or a Ga gradient, which is likely since there was no change of the elemental fluxes during absorber layer growth and no Ga profile was measured. KF was added by post-deposition only. Current-voltage characteristics were measured and net doping concentrations were determined from capacitance-voltage measurements (CV). We see an improvement of the open-circuit voltage (Voc) with increasing KF amount, and a marginal increase of the fill factor. CV measurements showed increasing net acceptor concentration with increasing KF amount. Time resolved photoluminescence (PL) showed an increased decay time for KF treated cells and the PL peak shape changed. Without KF treatment the PL peak is symmetric, after KF treatment a further peak appears at higher energy in the PL spectrum. This higher energy peak increases in intensity with increasing KF concentration. The same effects were seen in a sample without Na, but here the Voc was limited due to large tailing. Hence both Na and K are required for good cell efficiencies.

Due to ongoing improvements in polymer-based materials, they are being used in an increasingly wider range of applications. In addition to special bulk physical properties, specific surface properties are often required in particular applications. However, common polymers very often do not possess the desired surface properties.Several techniques have been developed to alter both the chemical and morphological properties of polymer surfaces in a desired way A versatile technique that is used more and more often in surface and interface engineering is low-temperature plasma processing. Among the various plasma sources, the surface dielectric barrier discharges, namely the Surface Barrier Discharge and the Coplanar Barrier Discharge, offer significant benefits for surface modification in many applications.This paper gives an overview of results achieved with both surface dielectric barrier discharges. The advantage of the Coplanar Barrier Discharge is generally a longer expected lifetime of the electrode system. However, with respect to efficiency and quality of the polymer surface modifications, the two types of discharges are considered to be equal.