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A post-mortem analysis is conducted after potential-induced degradation (PID) of a commercial copper-indium-gallium-selenide (CIGS) photovoltaic module. After PID, the conversion efficiency of the total module decreased by 62%. Electroluminescence images of the module show that the edges of the modules were much more affected by the PID than the middle part of the module. Coring samples were prepared of the different areas and chemical compositional information of the various areas was combined with electrical characterisation, cell modelling and luminescence data to obtain an overall perspective on the root cause of degradation in these modules during high voltage stress. Consistent with earlier studies on cell level, the module analysis shows the occurrence of alkali migration. From current–voltage modelling, it was concluded that the degradation of the most affected areas is due to an increase in bulk and CdS/CIGS interface defects, likely induced by ion migration. Further degradation on the same samples occurred when they are taken out of the argon-filled glovebox and stored under ambient conditions. Remarkably, the PID-degraded areas show stronger degradation when left in ambient atmosphere, as well as a stronger Na redistribution. These new results show that ion migration not only causes the immediate degradation but also strongly affects the longer-term stability of the cells in ambient atmosphere. This indicates that PID degradation makes CIGS devices more vulnerable to hermeticity problems, which are most prominent at the module edges.

In this work, recombination mechanisms are investigated in Cu(In, Ga)Se 2 solar cells based on numerical modelling and verified by regression analysis of the dark and light current density–voltage (J–V) curves. Loss mechanisms such as a back contact barrier, deep level defects in the absorber layer are determined by fitting the simulated cell performance with the measurements using global optimisation algorithm differential evolution. The cell performance in the fitting process includes J–V curves recorded at different temperatures and open-circuit voltage(V o c ) under different illumination intensities at 300K. The results show that for CIGSe solar cells with different preparation methods and absorber thickness, the main loss mechanisms are different. Based on the proposed numerical model and the observed loss mechanisms, suggestions are given for further improving the solar cell efficiency in each cell.

Tandem solar cells present a promising solution to overcome the Schottky–Queisser efficiency limit of single-junction solar cells. In this article, an all–thin-film tandem solar cell based on perovskite (PK) top cell and chalcopyrite Cu (In, Ga) Se2 (CIGS) bottom cell is researched. Device optical simulations are validated on the top and bottom cells and employed for the analysis of PK/CIGS tandem cells. In particular, the optical effects of introduced laser reduced graphene oxide (rGO) layers at two positions in the tandem cell: 1) at the position between the top PK and bottom CIGS cell and 2) underneath the front transparent electrode. The purpose of introducing rGO layers is to improve the optoelectrical properties of the device, based on the tunable electronic and optical characteristics of rGO layers. Optical simulation results show that the parasitic absorption in rGO layers may noticeably affect the optical performance of the tandem cell if the layers are not optimized. The use of a thin and a few nanometer-thick rGO is suggested from the analysis if its parasitic absorption is not reduced. Directions for further optimization of optical rGO, including the reduction of parasitic absorption and tuning of the real part of the refractive index, are performed.

The implementation of anoxide layerwith contact openings to the Cu(In,Ga)Se2(CIGS) interfaces has become especially popular to reduce the recombination losses, resulting in a higher open-circuit voltage. While implementing this approach on the back surface is straightforward, it is more complicated for the front surface due to the roughness of the absorber as well as material selection constraints imposed by further processing steps. In this contribution, an AlOx/HfOxmulti-stack oxide layer with contact openings is applied between the CIGS and CdS layers. Two different approaches to create contact openings in multi-stack oxide layer are presented, and their advantages and disadvantages are investigated. In the bottom-up approach (BU), a NaCl salt pattern is created on the CIGS absorber surface, while in the top-down approach (TD), the pattern is created on the AlOxlayer surface. Time-resolved photoluminescence (TR-PL) shows that the multi-stack design improves the PL decay time regardless of the approach, but when the CdS layer is deposited, the PL decay time decreases. A more severe decrease is observed in case of the BU approach. Capacitance-Voltage measurements show that there is no impact on doping level when the NaCl pattern is created on an AlOxsurface. This work shows that any oxide material, even if not chemically resistant to the chemical bath deposition, can be applied at the CIGS/buffer layer interface, and additionally, any template can be used to create contact openings while preventing interaction between the material used for patterning and the CIGS, using the TD approach.

In the present contribution, we have measured and simulated room temperature bias- and frequency-dependent capacitances of thin-film solar cell devices. The results of both the simulations and experimental measurements are represented as 2-D contour plots showing the derivative of the capacitance with respect to the frequency multiplied by the frequency. These plots are called “loss maps,” because responses in these contour plots correspond to responses of different nonidealities in the devices. Using a 1-D drift-diffusion solver (SCAPS), we have simulated the responses of different nonidealities of the solar cell devices, such as series resistance, bulk defects, interface defects, back contact barrier, and absorber-buffer barrier. We have shown that some nonidealities have a quite recognizable trace in the loss map. Other nonidealities on the other hand show responses that look quite similar in the bias voltage and frequency space, making exact conclusions on the nature and position of the defect responses in thin-film solar cells most of the times difficult. We have compared the simulated results with experimental measurements of one of our Cu(In,Ga)Se2 (CIGS) solar cell devices and came to the conclusion that there is likely a bulk defect or a spike-like barrier at the CIGS-CdS interface present in our particular device. The loss map can, in some cases, be useful in order to analyze admittance spectroscopy data in a graphical and relatively intuitive way.

Thin (640 mV were achieved after the KF treatment. From SCAPS simulations and temperature dependent current voltage-measurements, it is concluded that the V-oc of these cells are limited by backcontact surface recombination and, thus, further improvements require passivation of the back contact. This work was supported by the European Union’s H2020 Research and Innovation Program under Grant 715027.

Herein, using the spin-coating process as a sodium pre-deposition technique for Cu(In,Ga)Se2, ultrathin film solar cells is discussed. Throughout this study, several challenges are detected to overcome, such as unevenly deposited salt layers and unplanned clusters (even islands) of sodium salts due to accumulation. The solar cell performances with the optimal and nonoptimal amounts of sodium are shared to assess the effect of varying sodium amounts on performance. The hyperspectral photoluminescence (PL) mapping and imaging and reflection measurements are also used to emphasize the importance of homogeneity of the sodium layer.

Cu(In,Ga)Se-2 (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. Dr. J.D.W., Dr. G.B., and Professor B.V. received funding from the European Union’s H2020 research and innovation programme under grant agreement no. 715027 for this work. G.B acknowledges FWO postdoctoral fellowship 1219423N and R.S FWO Ph.D. fellowship fundamental research 1178022N.

This report highlights the combination of the MicroTime 100 upright confocal fluorescence lifetime microscope with a Single Quantum Eos Superconducting Nanowire Single-Photon Detector (SNSPD) system as a powerful tool for photophysical research and applications. We focus on an application in materials science, photoluminescence imaging, and lifetime characterization of Cu(InGa)Se-2 (CIGS) devices intended for solar cells. We demonstrate improved sensitivity, signal-to-noise ratio, and time-resolution in combination with confocal spatial resolution in the near-infrared (NIR) range, specifically in the 1000-1300 nm range. The MicroTime 100-Single Quantum Eos system shows two orders of magnitude higher signal-to-noise ratio for CIGS devices' photoluminescence imaging compared to a standard NIR-photomultiplier tube (NIR-PMT) and a three-fold improvement in time resolution, which is now limited by the laser pulse width. Our results demonstrate the advantages in terms of image quality and time resolution of SNSPDs technology for imaging in materials science.(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). PicoQuant and Single Quantum thank the European Union’s Horizon 2020 research and innovation program [ATTRACT Phase 2 (Grant Agreement No. 101004462), project: MicroQuaD] for the financial support of this work.

Interface recombination is one of the factors limiting the performance of Cu(In,Ga)Se2(CIGS). Especially in the absence of band grading at the front and rear surface, interface passivation approaches become important to improve device performance. The integration of an oxide layer as passivation layer at the front surface of the CIGS requires meticulous considerations in order not to impact the further steps of the solar cell production. In this article, a novel approach is reported to try to tackle the problem of interface recombination at the front surface of CIGS without affecting further solar cell production steps. In this approach, an Al2O3/HfO2multi-stack layer with contact openings is applied. NaCl template patterning with preliminarily selected parameters was used to create a homogeneous pattern of contact opening on the CIGS surface and allow the current flow in the device. After the removal of the NaCl islands, the holes in the multi-stack (openings) were visualized by scanning electron microscopy. In addition, energy-dispersive X-ray spectroscopy (EDS) was performed before and after chemical bath deposition of the buffer layer. The EDS result confirmed that the undesired etching of the Al2O3layer during buffer layer deposition was prevented by using a thin HfO2layer. Solar cells were produced by using preliminarily selected parameters for the multi-stack design. As a result, without having a significant negative impact on the solar cell parameters, a device design was achieved which is almost comparable with the reference device. In addition, options for future improvement and development are discussed.

Ultrathin Cu(In,Ga)Se-2 (CIGS) absorber layers of 550 nm were grown on Ag/AlOx stacks. The addition of the stack resulted in solar cells with improved fill factor, open circuit voltage and short circuit current density. The efficiency was increased from 7% to almost 12%. Photoluminescence (PL) and time resolved PL were improved, which was attributed to the passivating properties of AlOx. A current increase of almost 2 mA/cm(2) was measured, due to increased light scattering and surface roughness. With time of flight-secondary ion mass spectroscopy, the elemental profiles were measured. It was found that the Ag is incorporated through the whole CIGS layer. Secondary electron microscopic images of the Mo back revealed residuals of the Ag/AlOx stack, which was confirmed by energy dispersive X-ray spectroscopy measurements. It is assumed to induce the increased surface roughness and scattering properties. At the front, large stains are visible for the cells with the Ag/AlOx back contact. An ammonia sulfide etching step was therefore applied on the bare absorber improving the efficiency further to 11.7%. It shows the potential of utilizing an Ag/AlOx stack at the back to improve both electrical and optical properties of ultrathin CIGS solar cells. This research was funded by European Union H2020 research and innovation program grant number 715027.

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. J. de Wild, D.G. Buldu, T. Kohl, G. Birant, and B. Vermang received funding from the European Union’s H2020 research and innovation program under grant agreement no. 715027.

A new multi-stack approach with contact openings was investigated for front surface of Cu(In,Ga)Se 2 thin film solar cells. In this multi-stack design, a thin HfO x layer was used to protect a thicker AlO x layer from the presence of ammonia in the chemical bath deposition. The contact openings were created by using alkali solution. The lifetime decay was improved by implementing multi-stack passivation layer between CIGS/buffer layer interface. It was shown that open circuit voltage was increased by up to 30mV.

In this contribution, a cost- and time- effective contacting approach is presented for dielectric-based rear surface passivation layers up to 30 nm. Nano-size and homogenously distributed openings are realized in AlOx passivation layer by selenization of LiF salt on top, and monitored via SEM. Further, this layer was implemented into sub-micron thick CIGS solar cells. Consequently, contact openings facilitate the current collection through the dielectric layer up to 30 nm thick. However, solar cell parameters remain the same, even worse than the reference. Thus, this approach is still open for optimizations.

Several optoelectronic issues, such as poor optical absorption and recombination, limit the power conversion efficiency of ultrathin Cu(In,Ga)Se

In Cu(In,Ga)Se2(CIGS) thin-film solar cells, interface recombination is one of the most important limiting factors with respect to device performance. Herein, metal–insulator–semiconductor samples are used to investigate and compare the passivation effects of Al2O3and HfO2at the interface with CIGS. Capacitance–voltage–frequency measurements allow to qualitatively and quantitatively assess the existence of high negative charge density (Qf ≈ −1012 cm−2) and low interface-trap density (Dit ≈ 1011 cm−2 eV−1). At the rear interface of CIGS solar cells, these, respectively, induce field-effect and chemical passivation. A trade-off is highlighted between stronger field-effect for HfO2and lower interface-trap density for Al2O3. This motivates the usage of Al2O3to induce chemical passivation at the front interface of CIGS solar cells but raises the issue of its processing compatibility with the buffer layer. Therefore, an innovative Al2O3/HfO2multistack design is proposed and investigated for the first time. Effective chemical passivation is similarly demonstrated for this novel design, suggesting potential decrease in recombination rate at the front interface in CIGS solar cells and increased efficiency. 300 °C annealing in N2environment enable to enhance passivation effectiveness by reducingDitwhile surface cleaning may reveal useful for alternative CIGS processing methods.

This work presents a novel method of local contact openings formation in an aluminum oxide (Al2O3) rear surface passivation layer by the selenization of the lithium fluoride (LiF) salt on top of the Al2O3for ultra-thin copper indium gallium (di)selenide (CIGS) solar cells (SCs). This study introduces the potentially cost-effective, fast, industrially viable, and environmentally friendly way to create the nano-sized contact openings with the homogeneous distribution in the thick, i.e. up to 30 nm, Al2O3passivation layer. The passivation layer is deposited by atomic layer deposition, while the LiF layer is spin-coated. Selenization is done in the H2Se atmosphere and the optimal process parameters are deduced to obtain nano-sized and uniformly allocated openings as confirmed by scanning electron microscopy images. The contact openings were produced in the different thicknesses of the alumina layer from 6 nm to 30 nm. Furthermore, the Al2O3rear surface passivation layer with the contact openings was implemented into ultra-thin CIGS SC design, and one trial set was produced. We demonstrated that the created openings facilitate the effective current collection through the dielectric Al2O3layer up to 30 nm thick. However, the upper limit of Al2O3thickness in which the contact openings can be created by the described method is not established yet. The produced passivated CIGS SCs show increased external quantum efficiency response due to the optical enhancement of the passivated cells. However, the production of SCs on the Al2O3passivation layer with the openings created by selenization of LiF is not optimized yet.

Several optoelectronic issues, such as poor optical absorption and recombination, limit the power conversion efficiency of ultrathin Cu(In,Ga)Se-2 (CIGS) solar cells. To mitigate recombination losses, two combined strategies were implemented: a potassium fluoride (KF) post-deposition treatment (PDT) and a rear interface passivation strategy based on an aluminum oxide (Al2O3) point contact structure. The simultaneous implementation of both strategies is reported for the first time on ultrathin CIGS devices. Electrical measurements and 1D simulations demonstrate that in specific conditions, devices with only KF-PDT may outperform rear interface passivation based devices. By combining KF-PDT and rear interface passivation, an enhancement in an open-circuit voltage of 178 mV is reached over devices that have a rear passivation only, and of 85 mV over devices with only a KF-PDT process. Time-Resolved Photoluminescence measurements showed the beneficial effects of combining KF-PDT and the rear interface passivation at decreasing recombination losses in the studied devices, enhancing charge carrier lifetime. X-ray photoelectron spectroscopy measurements indicate the presence of an In and Se-rich layer that we linked to be a KInSe2 layer. Our results suggest that when bulk and front interface recombination values are very high, they dominate, and individual passivation strategies work poorly. Hence, this work shows that for ultrathin devices, passivation mitigation strategies need to be implemented in tandem. Funding Fundaca̧ o para a Cie ̃ ncia e Tecnologia through grants IF/ ̂00133/2015, PD/BD/142780/2018, and SFRH/BD/146776/2019 and projects PDTC/CTM-CTM/28075/2017, PTDC/FISMAC/29696/2017, and UIDB/04730/2020 is acknowledged. European Union’s Horizon 2020 through grant nos. 720887 and 715027 and the Special Research Fund (BOF) of Hasselt University are also acknowledged. ACKNOWLEDGMENTS Fundaca̧ o para a Cie ̃ ncia e a Tecnologia (FCT) is acknowledged through the project IF/00133/2015, PD/BD/142780/2018, and SFRH/BD/146776/2019. The authors want to acknowledge the European Union’s Horizon 2020 research and innovation program under the agreement nos. 720887 (ARCIGS-M project) and 715027. The Special Research Fund (BOF) of Hasselt University, the FCT through the project NovaCell (PDTC/CTM-CTM/28075/2017), and InovSolarCells (PTDC/FISMAC/29696/2017) co-funded by FCT and the ERD through COMPETE2020 is also acknowledged. P. A. Fernandes would like to acknowledge FCT for the support of the project FCT UIDB/04730/2020.

The incorporation of nanostructures in optoelectronic devices for enhancing their optical performance is widely studied. However, several problems related to the processing complexity and the low performance of the nanostructures have hindered such actions in real-life devices. Herein, a novel way of introducing gold nanoparticles in a solar cell structure is proposed in which the nanostructures are encapsulated with a dielectric layer, shielding them from high temperatures and harsh growth processing conditions of the remaining device. Through optical simulations, an enhancement of the effective optical path length of approximately four times the nominal thickness of the absorber layer is verified with the new architecture. Furthermore, the proposed concept in a Cu(In,Ga)Se2 solar cell device is demonstrated, where the short-circuit current density is increased by 17.4%. The novel structure presented in this work is achieved by combining a bottom-up chemical approach of depositing the nanostructures with a top-down photolithographic process, which allows for an electrical contact., This work was funded in part by the Fundação para a Ciência e a Tecnologia (FCT) under Grants IF/00133/2015, PD/BD/142780/2018 and SFRH/BD/ 146776/2019. The authors also want to acknowledge the European Union’s Horizon 2020 Research and Innovation Programme through the ARCIGS-M project under Grant 720887, the Special Research Fund (BOF) of Hasselt University, the FCT through the project NovaCell (PTDC/CTM-CTM/28075/ 2017), and InovSolarCells (PTDC/FISMAC/29696/2017) co-funded by FCT and the ERDF through COMPETE2020. The authors also want to acknowledge Sandra Maya for the production of images used in this work.

In ultrathin Cu(In,Ga)Se2(CIGS) film solar cells, the CdS/CIGS interface may become one of the limiting factors for efficiency. The first step toward reducing the impact of this problem could be a surface treatment process to improve the quality of the front interface. The purpose of this study is to have a better understanding of the effect of wet chemical surface treatment, using ammonium sulfide ((NH4)2S), on CIGS thin film layers with different Cu/(Ga + In) (CGI) ratios. Herein, photoluminescence (PL) and time-resolved PL (TRPL) studies are conducted on bare CIGS, ammonium sulfide–treated CIGS thin films, and samples with CdS. In bare CIGS, CGI ratio–dependent changes in PL are observed on both a low-energy (defect related transition) and a high-energy peak (band-to-band transition). After the surface treatment, the PL maximum increases by factors ranging from 4 to 11 depending on the CGI ratio, accompanied by a slower decay. Trends with similar improvement as in the PL study are observed in the performance of the solar cells. It is shown that the impact of the surface treatment is beneficial independently of the CGI ratio of the absorber layers. In all cases, the treatment is shown to improve the efficiency.

In this work, hafnium oxide layer is investigated as rear surface passivation layer for ultra-thin (550 nm) CIGS solar cells. Point contact openings in the passivation layer are realized by spin-coating potassium fluoride prior to absorber layer growth. Contacts are formed during absorber layer growth and visualized with scanning electron microscopy (SEM). To assess the passivating qualities, HfOx was applied in a metal-insulator-semiconductor (MIS) structure, and it demonstrates a low interface trap density in combination with a negative density of charges. Since we used ultra-thin devices that are ideal to probe improvements at the rear, solar cell results indicated improvements in all cell parameters by the addition of 2 nm thick HfOx passivation layer with contact openings.

Interface quality plays a key role in solar cell applications. Interface recombination at the front and rear surfaces, which determine this quality, have significant effects on open circuit voltage and fill factor values. In this work, several surface treatments were applied on Cu(In,Ga)Se2 (CIGS) surfaces to improve the interface quality. Besides, the passivation layer implementation was investigated to reduce interface recombination between the buffer and absorber layers.

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order to establish relationships between group IV composition and structural, vibrational and opto-electronic properties. Seven single phase Cu2Sn1-xGexS3 films are prepared from x = 0 to 1, showing a uniform distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation. The shift of the main Raman peak from x = 0 to x = 1 can be explained as being half due to the change in atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing Ge. The net acceptor density of all films is around 1018 cm−3. These analyses have established that the alloy forms a solid solution over the entire composition range meaning that intentional band gap grading should be possible for future absorber layers. The linear variation of the unit cell parameters and the band gap with group IV content allows composition determination by scattering or optical measurements. Further research is required to reduce the doping density by two orders of magnitude in order to improve the current collection within a solar cell device structure.

The p-type semiconductor Cu2SnS3 is alloyed with Ag to investigate its effect on absorber layer and solar cell properties. Ag replaces the Cu in (Cu1‐ x Ag x )2SnS3 (ACTS) up to x ≤ 6% at 550 °C. Above this percentage, Ag forms secondary phases. We find a significant increase in grain size, from hundreds of nanometers to several microns, and increased photoluminescence yield with increasing Ag concentration. Low-temperature photoluminescence measurements show that compensation is increased for the ACTS absorber layers, which could be beneficial for CTS, but also that the electrostatic band gap fluctuations are increased. The external quantum efficiency of the solar cells made from ACTS shows an increased carrier collection length from 320 nm for CTS to 700 nm and a thicker buffer layer. We attribute the increase in collection length to both increased depletion width (increased compensation) and diffusion length (larger grains). Overall the ACTS solar cells have a lower power conversion efficiency due to lower shunt resistance and open-circuit voltage, which are attributed to increase in pinholes, electrostatic fluctuation, and changes at the CdS/ACTS interface.

This research reveals the missing or often undisclosed experimental data on processing conditions of Cu(In,Ga)(S,Se)2 absorbers by the two-step sequential technique. Accordingly, by employing a one-variable-at-a-time approach- where each factor is exclusively changed with respect to a defined center point, we identified a handful of significant parameters which play the most significant role in an absorber's properties and consequently, prospective high-performing solar cells. This was done by analyzing their impact on a few most important metrics. Hence, the vast processing window with numerous factors and levels for development of Cu(In,Ga)(S,Se)2 by this method becomes constrained and feasible to optimize in future endeavours.

Cu2(Sn,Ge)(S,Se)3 alloys are new semiconductor absorber layers of interest in thin film photovoltaics. This paper presents a new synthesis of Cu2GeS3 and Cu2Sn(S,Se)3 layers from a Cu thin film reacting with two gas phase species, MX (SnS(e), GeS) and X (S, Se). Equilibrium reactions of Cu2MX3 ternaries with their gaseous binaries MX are examined for various annealing temperatures and background pressures. Incorporation of Sn and Ge only occurs when Cu2S(e) is formed and there is a sufficient partial pressure of MX. Observation of a lateral composition gradient across the sample for the SnS and SnSe containing absorbers suggest a limiting factor during the annealing. A theoretical model shows that the generation rate of these species is responsible for this gradient. This result highlights the importance of controlling the vapour pressure uniformity of MX during absorber annealing for the synthesis of homogeneous compound semiconductor layers that contain volatile group IV chalcogenide species, such as kesterite.

Air annealing of chalcopyrite solar cells at either 200 °C or higher is often known to increase their power conversion efficiency. In this paper, we investigate the nature of this effect for Cu2SnS3 (CTS) solar cells by modeling the experimental external quantum efficiency. We find that the cell efficiency increase stems from increased diffusion length and depletion width and decreased interface recombination at the p-n junction. The increased diffusion length is also reproduced when only the absorber layer is air annealed. When solar cells are annealed in N2, no increase in diffusion length is measured. Hence, we attribute the increase in diffusion length to passivation of the grain boundaries in the bulk by oxygen. The larger depletion width on air and N2 annealing in the devices is independent of the CdS buffer layer thickness and occurs in its absence. We ascribe it to copper diffusion from the absorber layer to the n-type buffer and window layers. Interface recombination positively correlates with increasing buffer layer thickness. Based on our modeling, we conclude that the CTS absorber layer is still too highly doped to obtain large depletion widths and is highly recombination active at the p–n interface.

Thin ( 640 mV were achieved after the KF treatment. From SCAPS simulations and temperature dependent current voltage measurements, it is concluded that the Voc of these cells are limited by back contact surface recombination and, thus, further improvements require passivation of the back contact.

A simplified Cu(In, Ga)Se2(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 (Voc) and fill factor is measured, which is attributed to an improved pn junction. By changing the annealing conditions, an additionalVocincrease 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 theVocis limited to 640 mV due to recombination at the back contact. A higherVoccan 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.

Room temperature photoluminescence measurement has been carried out on 1-stage co-evaporated Cu(In,Ga)Se 2 thin film in order to investigate the effect of ammonium sulfide (NH 4 ) 2 S surface treatment. The defect related PL peak in the as-deposited CIGS thin film changes with CGI ratio. The surface treatment leads to an increase in the main PL intensity by a factor of ~10. The lifetime of minority carriers is slightly increased after surface treatment, and it is observed that a low-Cu region is more affected than a high-Cu region.

This review summarizes all studies which used dielectric-based materials as a passivation layer at the rear surface of copper indium gallium (di)selenide, Cu(In,Ga)Se2, (CIGS)-based thin film solar cells, up to 2019. The results regarding the kind of dielectric materials, the deposition techniques, contacting approaches, the existence of additional treatments, and current–voltage characteristics (J–V) of passivated devices are emphasized by a detailed table. The techniques used to implement the passivation layer, the contacting approach for the realization of the current flow between rear contact and absorber layer, additional light management techniques if applicable, the solar simulator results, and further characterization techniques, i.e., external quantum efficiency (EQE) and photoluminescence (PL), are shared and discussed. Three graphs show the difference between the reference and passivated devices in terms of open-circuit voltage (Voc), short-circuit current (Jsc), and efficiency (η), with respect to the thicknesses of the absorber layer. The effects of the passivation layer at the rear surface are discussed based on these three graphs. Furthermore, an additional section is dedicated to the theoretical aspects of the passivation mechanism.

This review summarizes all studies which used dielectric-based materials as a passivation layer at the rear surface of copper indium gallium (di)selenide, Cu(In,Ga)Se2, (CIGS)-based thin film solar cells, up to 2019. The results regarding the kind of dielectric materials, the deposition techniques, contacting approaches, the existence of additional treatments, and current–voltage characteristics (J–V) of passivated devices are emphasized by a detailed table. The techniques used to implement the passivation layer, the contacting approach for the realization of the current flow between rear contact and absorber layer, additional light management techniques if applicable, the solar simulator results, and further characterization techniques, i.e., external quantum efficiency (EQE) and photoluminescence (PL), are shared and discussed. Three graphs show the difference between the reference and passivated devices in terms of open-circuit voltage (Voc), short-circuit current (Jsc), and efficiency (η), with respect to the thicknesses of the absorber layer. The effects of the passivation layer at the rear surface are discussed based on these three graphs. Furthermore, an additional section is dedicated to the theoretical aspects of the passivation mechanism. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027

Room temperature photoluminescence (PL) and intensity dependent PL measurements were used as a tool to investigate alkali treatment (KF and RbF) of thin (< 500 nm) single-stage absorber layers and compared with similar 3-stage samples. Single-stage absorber layers have a clear distinguishable PL peak below the band gap, which is absent in 3-stage absorber layers. We attribute this peak to a defect band arising due to the planar defects and the small grains. We find that this low energy peak can be largely reduced with alkali treatment, leading to similar decay times and PL yield as thin 3 stage samples. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027

Cu2SnS3 is investigated as a potential solar absorber material, which is known to exist in several polymorphs. In the present work, the transition to the most efficient monoclinic polymorph is investigated and an explanation is given why this one seems to be the best for solar cells. Cubic, mixed polymorphs and monoclinic polymorphs are synthesised with increasing temperature. Photoluminescence spectra of the mixed and cubic polymorphs show mainly defect emissions below the band-gap energy, while the monoclinic modification shows only one sharp peak, which is attributed to the conduction to valence band transition. The monoclinic polymorph also grows in combination with a secondary phase containing sodium, NaxCuSnS3, with x between 0.5 and 1. It exhibits a cubic (F43m) crystal structure like CuSn3.75S8 and a band-gap of around 1.6 eV. Devices with absorber layers of monoclinic Cu2SnS3, in which NaxCuSnS3 is either present or removed, gave similar power conversion efficiencies of above 1%.

In ultrathin Cu(In,Ga)Se-2(CIGS) film solar cells, the CdS/CIGS interface may become one of the limiting factors for efficiency. The first step toward reducing the impact of this problem could be a surface treatment process to improve the quality of the front interface. The purpose of this study is to have a better understanding of the effect of wet chemical surface treatment, using ammonium sulfide ((NH4)(2)S), on CIGS thin film layers with different Cu/(Ga + In) (CGI) ratios. Herein, photoluminescence (PL) and time-resolved PL (TRPL) studies are conducted on bare CIGS, ammonium sulfide-treated CIGS thin films, and samples with CdS. In bare CIGS, CGI ratio-dependent changes in PL are observed on both a low-energy (defect related transition) and a high-energy peak (band-to-band transition). After the surface treatment, the PL maximum increases by factors ranging from 4 to 11 depending on the CGI ratio, accompanied by a slower decay. Trends with similar improvement as in the PL study are observed in the performance of the solar cells. It is shown that the impact of the surface treatment is beneficial independently of the CGI ratio of the absorber layers. In all cases, the treatment is shown to improve the efficiency. This work received funding from the European Union's H2020 research and innovation program under grant agreement No. 715027. Buldu, DG (corresponding author), Hasselt Univ Partner Solliance, Inst Mat Res IMO, Agoralaan Gebouw H, B-3590 Diepenbeek, Belgium ; Imec Div IMOMEC Partner Solliance, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; EnergyVille, Thorpk,Poort Genk 8310 & 8320, B-3600 Genk, Belgium. Dilara.Gokcen.Buldu@imec.be

Ultrathin Cu(In,Ga)Se2solar cells are a promising way to reduce costs and to increase the electrical performance of thin film solar cells. An optical lithography process that can produce sub‐micrometer contacts in a SiO2passivation layer at the CIGS rear contact is developed in this work. Furthermore, an optimization of the patterning dimensions reveals constrains over the features sizes. High passivation areas of the rear contact are needed to passivate the CIGS interface so that high performing solar cells can be obtained. However, these dimensions should not be achieved by using long distances between the contacts as they lead to poor electrical performance due to poor carrier extraction. This study expands the choice of passivation materials already known for ultrathin solar cells and its fabrication techniques.

Interface quality plays a key role in solar cell applications. Interface recombination at the front and rear surfaces, which determine this quality, have significant effects on open circuit voltage and fill factor values. In this work, several surface treatments were applied on Cu(In,Ga)Se2 (CIGS) surfaces to improve the interface quality. Besides, the passivation layer implementation was investigated to reduce interface recombination between the buffer and absorber layers.

The replacement dynamics of Se by S is investigated in kesterite Cu 2 ZnSn(S,Se) 4 and Cu 2 ZnGe(S,Se) 4 . Two different post-sulphurization techniques yield the same results. In the initial stages of the replacement, the original selenium-rich phase coexists with a second mixed phase. This phase has an increased sulphur content of 30 %. In later stages of the reaction, the initial phase disappears in favor of the second phase. Thus, sulphur inclusions of less than 30% seem impossible while above 30% the sulphur amount is adjustable.

Ultrathin Cu(In,Ga)Se2 solar cells are a promising way to reduce costs and to increase the electrical performance of thin film solar cells. In this work, we develop an optical lithography process that can produce sub-micrometer contacts in a SiO2 passivation layer at the CIGS rear contact. Furthermore, an optimization of the patterning dimensions reveals constrains over the features sizes. High passivation areas of the rear contact are needed to passivate the CIGS interface so that high performing solar cells can be obtained. However, these dimensions should not be achieved by using long distances between the contacts as they lead to poor electrical performance due to poor carrier extraction. This study expands the choice of passivation materials already known for ultrathin solar cells and its fabrication techniques. published

In this work, we obtain the highest reported power conversion efficiency (13.3%) for sulfur-free CIGSe solar cells by solution processing. Intentional sodium incorporation is achieved directly by dissolving sodium salts in the ink. The effect on solar cell properties of three sodium sources is investigated: NaCl, NaHCO2, and NaSCN. A comparison is made with absorbers grown on soda-lime glass substrates and in the absence of sodium sources. The incorporation via sodium salts yields significantly better results, which is attributed to enhanced-sodium availability. A comparison with identically annealed sputtered metal precursor layers capable of delivering 15.1% module efficiency suggests that the cell results are limited by the selenization procedure. Copyright © 2015 John Wiley & Sons, Ltd.

Monoclinic Cu2SnS3 was made by solution based processing of the precursor metals after which the samples are annealed in a sulphur environment. XRD and Raman spectra shows that the monoclinic phase was synthesised. One sample was further etched in KCN and HCl to remove possible secondary phases. Transmission spectra show that the material has two optical transitions and in conjunction with reflection data absorption spectra were calculated. The two optical transitions are determined to be 0.91 and 0.98 for the unetched sample and 0.90 and 0.95 eV for the etched sample. The values of the optical transitions are within the error the same and thus etching does not affect the values of these optical transitions. Photoluminescence spectra map show only one luminescence peak with a maximum at 0.95 eV, which is consistent with the values found by absorption spectra. This in combination with the Raman spectra and XRD indicates that the sample contains only one polymorph of Cu2SnS3, which is monoclinic. Therefore the two optical transitions are intrinsic to monoclinic Cu2SnS3.