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Cu(In,Ga)Se2 (CIGS) solar cells are sensitive to heat-light soaking (HLS) treatments performed at elevated temperatures. Such treatments often result in improved open-circuit voltage at the expense of short-circuit current and/or fill factor, and may be reversible. In this contribution, we analyze the changes brought in the devices by heat and light treatments to CIGS, to deliver hints regarding the fundamental reasons behind the observed changes and to ensure the long-term stability of the performance improvement. We apply HLS treatments to improve the performance of low-temperature grown CIGS solar cells on flexible polyimide and on glass substrates. We first investigate the treatment conditions favorable for device performances. A gain in open-circuit voltage is obtained, with no clear statistical impact on short-circuit current and fill factor. The changes are stable over periods of several months. Then we combine material analysis techniques to optoelectronic and electrical characterization on samples subject to a unique HLS treatment at the optimal conditions. The voltage gain arises from a large increase in the doping density in the absorber layer, correlated with increased concentrations of alkali elements Na and Rb. This improvement is partly offset by a reduction in minority charge carrier lifetime. The microscopic origin for the observed changes is discussed. At low temperatures, we also measure device enhancement due to the front interface behavior, however with unclear impact on standard operating conditions. The HLS treatment yielded a CIGS solar cell with certified record 21.4% power conversion efficiency on a flexible substrate.

The open-circuit voltage (VOC) is the main limitation to higher efficiencies of Cu(In,Ga)Se2 solar cells. One of the most critical parameters directly affecting VOC is the charge carrier lifetime. Therefore, it is essential to evaluate the extent to which inhomogeneities in material properties limit the carrier lifetime and how postdeposition treatments (PDTs) and growth conditions affect material properties. Time-resolved photoluminescence (TRPL) microscopy is employed at conditions similar to one sun to study carrier lifetime fluctuations in Cu(In,Ga)Se2 with light (Na) and heavy (Rb) alkalis, different substrates, and grown at different temperatures. PDT lowers the amplitude of minority carrier lifetime fluctuations, especially for Rb-treated samples. Upon PDT, the grains’ carrier lifetime increases, and the analysis suggests a reduction in grain boundary recombination. Furthermore, lifetime fluctuations have a small impact on device performance, whereas VOC calculated from TRPL (and continuous-wave PL) agrees with device values within the limits of investigated PDT samples. Finally, up to about half a per cent external radiative efficiencies are experimentally determined from TRPL metrics, and internal radiative efficiencies are approximated. The findings demonstrate that the highest absorber material quality investigated is still limited by nonradiative recombination (grain or grain boundary) and is comparable to state-of-the-art absorbers. This work received financial support in part from the Swiss State Secretary for Education, Research and Innovation (SERI) under Contract No. 17.00105 (EMPIR project HyMet) and from the Swiss Federal Office of Energy (SFOE) (SI/501614-01 ‘‘ImproCIS''). The EMPIR programme was cofinanced by the Participating States and by the European Union's Horizon 2020 research and innovation programme.

Thin film Cu(In,Ga)Se2 (CIGS) solar modules can be grown on flexible and lightweight substrates allowing their direct integration into building elements. Such multifunctional building elements would significantly reduce the installation cost of photovoltaic systems provided that CIGS solar cells with high conversion efficiency can be obtained. Also, there is a need for low cost substrate foil. Mild steel is a promising low cost substrate material due to its excellent mechanical stability and already wide acceptance as component in building envelopes and in numerous other applications. During the growth of the CIGS absorber layer certain elements, e.g. iron, can diffuse from the metallic substrate into the semiconductor deteriorating the device performance. Here we present an effective diffusion barrier and device architecture for processing of highly efficient CIGS solar cells on mild steel substrates. The CIGS absorber layers were grown on mild steel foils by a multistage co-evaporation process at different substrate temperatures. The mild steel substrates were plated with an industrially scalable electrodeposited Ni/Cr bi-layer. The diffusion barrier layer properties of this Ni/Cr coating were investigated directly by measuring the metallic impurities within the absorber by secondary ion mass spectrometry and indirectly by admittance spectroscopy. The Ni/Cr bi-layer was found to be effective up to a nominal CIGS growth temperature of 500 °C. A certified cell efficiency of 18.0% was achieved on a Ni/Cr coated mild steel substrate using a low temperature CIGS deposition process and a NaF and RbF post deposition treatment method.

Flexible perovskite solar cells (PSCs) hold great promise for the low-cost roll-to-roll production of lightweight single- and multijunction photovoltaic devices. Among the different deposition methods used for the perovskite absorber, the two-step hybrid vacuum-solution approach enables precise control over the thickness and morphology of PbI2. However, efficient conversion to perovskite is limited by diffusion of the organic cations in the compact lead halide layer. Herein, a multistage absorber deposition is developed by thermal evaporation of PbI2 and spin coating of CH3NH3I (MAI). The process relies on the different types of growth of vacuum-deposited PbI2 onto amorphous and crystalline surfaces. This approach represents a way to effectively increase the absorber thickness while tackling the limited MAI diffusion in the compact PbI2 film via a two-step deposition method. The efficiency of flexible PSCs is improved from 14.2 to 15.8% with multistage deposition. Furthermore, the use of an amorphous transparent conductive oxide (TCO), InZnO, enhances the mechanical resistance against bending with respect to conventional crystalline TCO-based flexible devices. Near-infrared transparent flexible PSCs are developed with an efficiency of 14.0% and average transmittance of ~74% between 800 and 1000 nm. Flexible perovskite/CIGS thin-film tandem devices are demonstrated with an efficiency of 19.6% measured in the four-terminal configuration. The efficiency of printable solar cells can be increased using a production method developed by researchers in Switzerland. Hybrid perovskites, a material that combines organic and inorganic components, are emerging as a competitive alternative to silicon for producing solar cells. One of their major advantages is that perovskite devices can be created on flexible substrates, making them compatible with a technology that prints devices on a roll of plastic. This means they can be cheaply mass produced, however their conversion efficiency needs to be improved. Stefano Pisoni and colleagues from the Swiss Federal Laboratories for Materials Science and Technology in Duebendorf constructed a perovskite solar cell using a two-step method that combined thermally evaporated lead iodide and a coating of methylammonium iodide. This design enabled better diffusion of the organic cations, which improved the device efficiency. Flexible perovskite solar cell with an efficiency of 15.8 % via tailoring of vacuum-deposited PbI2 growth morphology has been achieved. We demonstrated superior mechanical bending stability using amorphous TCO (retaining 80 % of the initial efficiency after 1000 bending cycles at 4 mm bending radius). Flexible NIR-transparent perovskite solar cell with an efficiency of 14.0 % and average transmittance of ~74 % between 800 and 1000 nm has been developed. Eventually, we proved a flexible perovskite/CIGS tandem solar cell with an efficiency of 19.6 % measured in four-terminal configuration.

We explore the feasibility of Ag fiber meshes as electron transport layer for high-efficiency flexible Cu(In,Ga)Se2 (CIGS) solar cells. Woven meshes of Ag fibers after UV illumination and millisecond flash-lamp treatment results in a sheet resistance of 17 Ω/sq and a visible transmittance above 85%. Conductive Ag meshes are integrated into flexible CIGS cells as transparent conductive electrode (TCE) alone or together with layers of Al-doped ZnO (AZO) with various thickness of 0…900 nm. The Ag mesh alone is not able to function as a current collector. If used together with a thin AZO layer (50 nm), the Ag mesh markedly improves the fill factor and cell efficiency, in spite of the adverse mesh shadowing. When Ag mesh is combined with thicker (200 nm or 900 nm) AZO layers, no improvements in photovoltaic parameters are obtained. When comparing a hybrid TCE consisting of 50 nm AZO and Ag fiber mesh with a thick 900 nm reference AZO device, an improved charge carrier collection in the near-infrared range is observed. Regardless of the AZO thickness, the presence of Ag mesh slows down cell degradation upon mechanical tensile stress, which could be interesting for implementation into flexible thin film CIGS modules., Graphical Abstract

The application of TiO2 as part of the buffer layer stack in thin film Cu(In,Ga)Se2 (CIGS) solar cells is investigated for the improvement of the photovoltaic device performance. In a standard device configuration a CdS/ZnO/Al:ZnO layer stack is applied onto the CIGS absorber layer. By decreasing the CdS buffer layer thickness a higher photocurrent is expected from a reduced parasitic absorption. When the CdS layer is not fully covering the CIGS surface, losses in VOC and FF are observed in I-V measurements due to the arising unfavorable CIGS/ZnO band alignment and sputter damage on the CIGS surface. Here we present thin TiO2 layers deposited by atomic layer deposition at low temperature as alternative to the unintentionally doped ZnO. With this approach, the photocurrent can be increased without adversely affecting VOC. Comparable device efficiency is achieved for the investigated structure and the reference process with the gain in current density being compensated by increased series resistance. Temperature dependent I-V measurements coupled with 1D-SCAPS simulations suggest a positive conduction band offset at the CdS/TiO2 interface limiting the FF. ALD-TiO2 is suggested as a more suitable intermediate buffer layer than sputtered ZnO when thin CdS buffer layers are applied.

Electronic transport in a semiconductor is key for the development of more efficient devices. In particular, the electronic transport parameters carrier lifetime and mobility are of paramount importance for the modeling, characterization, and development of new designs for solar cells and optoelectronic devices. Herein, time-resolved photoluminescence mapping under low injection and wide-field illumination conditions is used to measure the carrier lifetime and analyze the lateral charge carrier transport in Cu(In,Ga)Se2 absorbers grown at different temperatures, on different substrates, and subject to different postdeposition treatments (PDT) with light or heavy alkalis. To estimate the carrier mobility, numerical simulations of carrier diffusion transport to areas of increased recombination (defects) are used, similarly as observed experimentally. Mobilities are found in the range of 10–50 cm2 V−1 s−1, and effective minority carrier lifetime between 100 and 800 ns resulting in carrier diffusion lengths of 2–9 μm depending on the sample. Finally, the factors limiting carrier mobility and the implications of carrier diffusion on the measured carrier lifetimes are discussed. This work received financial support partially from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet) and from the Swiss Federal Office of Energy (SFOE) (SI/501614-01 ‘‘ImproCIS’’). The EMPIR program was cofinanced by the Participating States and by the European Union's Horizon 2020 research and innovation program.

In-situ Se flux control system is applied for the growth of Cu(In,Ga)Se 2 (CIGS) absorber layers by a multi stage thermal co-deposition of elements at low substrate temperature below 500 °C. It was revealed that the composition depth profile of the [Ga]/([In]+[Ga]) (GGI) ratio is affected by the [Se]/[Metal] flux ratio. It was observed for the solar cell properties that the change in the depth profiles of GGI induced by the change of [Se]/[Metal] resulted in an increase in the J SC and a decrease in the FF, and hence little influence on the efficiency. Under control of [Se]/[Metal] ratio during the deposition process, the effect of CIGS layer thickness on the solar cell properties was investigated. With increase in the thickness up to 2.8 μm, the short circuit current density was increased to over 35 mA/cm 2 w/o AR coating, resulting in conversion efficiency above 18% using low temperature deposition method.

The possibility of growing perovskite solar cells on flexible substrates can be seen as an exciting opportunity, allowing high throughput roll-to-roll manufacturing with a low embodied energy, and creating new applications in buildings, vehicles, portable electronics and internet-of-things based devices. Flexible perovskite solar cells have previously been developed on polymer substrates such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) which are vulnerable to the ingress of moisture. Here we report the development of flexible perovskite solar cells grown on a typical transparent front sheet which is generally used to encapsulate flexible Cu(In,Ga)Se2 (CIGS) solar cells. This type of substrate displays an ultra-low water vapor transmission rate and good UV blocking properties. Perovskite solar cells, grown on such flexible front sheets coated with a highly transparent conducting ZnO:Al (AZO) electrode and vacuum processed ZnO/C60 electron transport multilayer, yield 13.2% and 10.9% stabilized efficiencies for areas of 0.15 cm2 and 1.03 cm2, respectively. The substitution of an opaque rear contact with the transparent electrode enables the realization of flexible NIR-transparent perovskite solar cells with efficiencies above 12%. These devices display an average transmittance of 78% between 800 and 1000 nm and enable the development of 4-terminal polycrystalline all-thin-film flexible perovskite/CIGS tandem devices. In a first proof of concept 18.2% efficiency is obtained.

Highly efficient thin film solar cells based on co-evaporated Cu(In,Ga)Se2 (CIGS) absorbers are typically grown with a [Ga]/([Ga] + [In]) (GGI) gradient across the thickness and a Cu-poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased absorption in the near-infrared (NIR), diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the NIR response. In this contribution [Cu]/([In] + [Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometric point. In order to obtain comparable Ga contents at the interface for proper band alignment, the front GGI gradings were actively modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. By characterizing the modified absorber properties by reflection-transmission spectroscopy, we attribute the observed behavior to changes in the optical properties rather than to improved carrier collection. Cu-dependent modifications of the NIR-absorption coefficients are likely to be responsible for the variations in the optical properties, which is supported by device simulations. Adequate re-adjustments of the co-evaporation process and of the alkali-fluorides post-deposition treatments allow maintaining Voc and FF values, yielding an overall increase of efficiency as compared to a reference baseline. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.

Compositional grading has been widely exploited in highly efficient Cu(In,Ga)Se2, CdTe, GaAs, quantum dot solar cells, and this strategy has the potential to improve the performance of emerging perovskite solar cells. However, realizing and maintaining compositionally graded perovskite absorber from solution processing is challenging. Moreover, the operational stability of graded perovskite solar cells under long‐term heat/light soaking has not been demonstrated. In this study, a facile partial ion‐exchange approach is reported to achieve compositionally graded perovskite absorber layers. Incorporating compositional grading improves charge collection and suppresses interface recombination, enabling to fabricate near‐infrared‐transparent perovskite solar cells with power conversion efficiency of 16.8% in substrate configuration, and demonstrate 22.7% tandem efficiency with 3.3% absolute gain when mechanically stacked on a Cu(In,Ga)Se2 bottom cell. Non‐encapsulated graded perovskite device retains over 93% of its initial efficiency after 1000 h operation at maximum power point at 60 °C under equivalent 1 sun illumination. The results open an avenue in exploring partial ion‐exchange to design graded perovskite solar cells with improved efficiency and stability.

Co-evaporation of NaF during the 3rd stage of the low temperature Cu(In,Ga)Se2 multi-stage process is compared to post-deposition treatment (PDT) with NaF in view of their influence on the electronic and structural properties. In case of NaF co-evaporation, quantum efficiency losses in the near infrared region and thus lower short circuit current density cause a reduced efficiency compared to solar cells prepared with NaF PDT. The formation of a deep defect with activation energy of ~ 250 meV is measured by capacitance spectroscopy and can explain the deteriorated performance in such devices. In addition, NaF co-evaporation during the 3rd stage causes reduced grain size in the top part of Cu(In,Ga)Se2 and altered In, Ga, and Cu distribution.

In thin-film Cu(In,Ga)Se2 (CIGS) solar cells a large number of intrinsic defect types are possible in the chalcopyrite structure and can influence the efficiency of the solar cell. Defect characterization is therefore essential. In this study Deep Level Transient Spectroscopy (DLTS) is used as an electrical defect characterization technique. The detection of defect related signals might be hindered by signals originating from barriers caused by the multi-layer structure and by a possible type inversion layer at the interface. To investigate to which extent the DLTS signals effectively arise from the CIGS absorber, solar cells are simplified to a metal/semiconductor/metal (M/S/M) structure. This was done by etching away the buffer and window layer and subsequent metal evaporation. In this way structures closer to those normally measured in DLTS are obtained. Additional etch processes targeted at thinning the absorber layer and/or removing oxidation layers are also performed. In general very similar DLTS signals are recorded for complete cells and M/S/M structures before and after additional etches.

Thin-film solar cells based on the chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber material show high potential for further cost reduction in photovoltaics. Compared with polycrystalline silicon (p-Si) wafer technology, thin-film technology has inherent advantages due to lower energy and material consumption during production but has typically shown lower conversion efficiency. However, in the past two years, new scientific insights have enabled the processing of CIGS solar cells with efficiencies up to 21%, surpassing the p-Si wafer value of 20.4% efficiency for the first time. Now several research groups report record cell efficiency values above 20% using different deposition processes and buffer layers. The presence of potassium was observed in many CIGS devices over the years, but it is only very recently that differences with Na have started being taken into full consideration for device processing and that K was added intentionally to the absorber. In this study, previous reports showing the presence of potassium are reviewed and discussed in more detail. Furthermore, on a scale-up perspective, additional progress has also taken place with CIGS minimodules achieving efficiency up to almost 19% and where further increase can be expected in the near future with the improvements induced by the use of potassium. This shows that the CIGS technology is continuously progressing not only on scientific level but on technological level as well.

Monolithically integrated Cu(In,Ga)Se2 mini-modules were fabricated in order to reduce the width of patterning related dead area. The Cu(In,Ga)Se2 layers were prepared on soda-lime glasses using the multistage process at low substrate temperature below 500 °C. A picosecond laser with a wavelength of 532 nm was used for all of the structuring processes (P1, P2, and P3) for the monolithic integration. A “lift-off” type structuring was applied for P1 and P3, and an “ablation” type was for P2. The laser structuring was optimized to be minimizing the dead area width, and the width of about 70 µm was successfully achieved. A mini-module, in which the optimized structuring processes were applied for the integration, demonstrated a certified efficiency of 16.6%. Copyright © 2015 John Wiley & Sons, Ltd.

The introduction of a KF postdeposition treatment (KF PDT) of Cu(InGa)Se2 (CIGS) thin films has led to the achievement of several consecutive new world record efficiencies up to 21.7 for the CIGS solar cell technology. The beneficial effect of the KF PDT on the photovoltaic parameters was observed by several groups in spite of differing growth methods of the CIGS layer. For CIGS evaporated at lower temperature on alkali free flexible plastic substrates a postdeposition treatment to add Na was already successfully applied. However with the introduction of additional KF under comparable conditions distinctly different influences on the final absorber alkali content as well as surface properties are observed. In this work we discuss in more detail the intrinsically different role of both alkali treatments by combining several microstructural and compositional analysis methods. The ion exchange of Na by K in the bulk of the absorber is carefully analyzed and further evidence for the formation of a K containing layer on the CIGS surface with increased surface reactivity is given. These results shall serve as a basis for the further understanding of the effects of alkali PDT on CIGS and help identify research needs to achieve even higher efficiencies. (Figure Presented). © 2015 American Chemical Society.

In this paper, we investigate the laser processing of the CIGS thin-film solar cells in the case of the high-speed regime. The modern ultra-short pulsed laser was used exhibiting the pulse repetition rate of 1 MHz. Two main P3 scribing approaches were investigated – ablation of the full layer stack to expose the molybdenum back-contact, and removal of the front-contact only. The scribe quality was evaluated by SEM together with EDS spectrometer followed by electrical measurements. We also modelled the electrical behavior of a device at the mini-module scale taking into account the laser-induced damage. We demonstrated, that high-speed process at high laser pulse repetition rate induced thermal damage to the cell. However, the top-contact layer lift-off processing enabled us to reach 1.7 m/s scribing speed with a minimal device degradation. Also, we demonstrated the P3 processing in the ultra-high speed regime, where the scribing speed of 50 m/s was obtained. Finally, selected laser processes were tested in the case of mini-module scribing. Overall, we conclude, that the top-contact layer lift-off processing is the only reliable solution for high-speed P3 laser scribing, which can be implemented in the future terawatt-scale photovoltaic production facilities.

Recently recorded efficiencies of Cu(In,Ga)Se2 based solar cells were mainly achieved by surface treatment of the absorber that modifies the buffer–absorber interface region. However, only little is known about the electronic properties within this region. In this manuscript voltage dependent admittance spectroscopy is applied to low temperature grown Cu(In,Ga)Se2 based solar cells to detect near interface defect states in the absorber. Under non-equilibrium conditions even defect states close to the interface may cross the Fermi level and hence are detectable using capacitance based measurement methods, in contrast to the case of zero bias conditions. Such defects are of potential importance for understanding device limitations and hence, adequate characterization is necessary. A SCAPS model is developed including a near interface deep acceptor state, which explains the frequency and voltage dependence of the capacitance. Using the same model, also the experimental apparent doping density is explained.

The reliable and homogenous addition of Na to the Cu(In,Ga)Se 2 (CIGS) solar cell absorber represents one major challenge for the industrial implementation of the CIGS technology faces. In this study, an alternative Na source from a Na-doped Mo (MoNa) layer is compared to the conventionally used soda-lime glass (SLG) substrate. In particular, the Na out-diffusion mechanism from different MoNa back contacts into the CIGS layer is investigated for different multilayer designs and magnetron sputtering conditions. From the obtained experimental results a Na diffusion model for the MoNa back contact system is developed, which proposes that Na is mainly collected in the MoNa grain boundary region and then diffuses from these grain boundaries towards the CIGS layer. It is demonstrated that with increasing porosity of the MoNa layer the Na diffusion into the CIGS layer is enhanced. In addition, the low activation energy for Na diffusion found for the MoNa back contact proposes that this Na doping technology is suitable for both low and high substrate temperature CIGS processes. For CIGS solar cells with MoNa back contacts, best cell efficiency of 15% was achieved without anti-reflection coating, which exceeded the performance of the reference sample on SLG with standard Mo back contact.

Deposition of Cu(In,Ga)Se2 (CIGS) thin film solar cells on metallic substrate is an attractive approach for development of low cost solar modules. However, in such devices, special care has to be taken to avoid diffusion of impurities, such as Fe, Ni, and Cr, from the substrate into the active layers. In this work, the influence of Ni and Cr impurities on the electronic properties of CIGS thin film solar cells is investigated in detail. Impurities were introduced into the CIGS layer by diffusion during the CIGS deposition process from a Ni or Cr precursor layer below the Mo electrical back contact. A high temperature and a low temperature CIGS deposition process were applied in order to correlate the changes in the photovoltaic parameters with the amount of impurities diffused into the absorber layer. Solar cells with Ni and Cr impurities show a reduction in the device performance, whereas the effect was most pronounced in Ni containing devices. The presence of deep defect levels in the absorber layer was identified with admittance spectroscopy and can be related to Ni and Cr impurities, which diffused into the CIGS layer according to secondary ion mass spectroscopy depth profiles and inductively coupled plasma mass spectrometry. Copyright © 2014 John Wiley & Sons, Ltd.

The presence of Na in Cu(In,Ga)Se 2 (CIGS) is well known to improve the solar cell performance. To incorporate Na into the CIGS absorber, Na-doped Mo (MoNa) back contact layers were grown on stainless steel foils. Three different back contact designs deposited from MoNa sputtering targets with Na concentrations of 3 at.%, 5 at.% and 10 at.% were investigated. A multistage CIGS evaporation process at low (~ 450 °C) substrate temperature was used to deposit the absorbers. Measurements from time-of-flight secondary ion mass spectroscopy depth profiles indicate that Na is preferentially collected at the internal interfaces and out-diffuses from the grain boundaries of the multilayer MoNa back contact into the CIGS absorber. From the [Ga]/([In] + [Ga]) grading profiles, a more pronounced Ga dip was found with increasing Na concentration. Moreover, at high Na concentrations (10 at.% MoNa target), a change in the CIGS texture was observed by X-ray diffraction. Best solar cell efficiency of 14.4% was achieved for the 5 at.% MoNa sample without antireflective coating, which is a significant improvement compared to the 9.8% efficiency measured for the Na-free reference.

A multilayer Mo stack is conventionally used as the back contact for flexible Cu(In,Ga)Se 2 (CIGS) solar cells on polyimide foils, where record efficiencies of 18.7% were reported on lab-scale. The aim of this work was to develop alternative back contact designs on polyimide foils with the objective to reduce material costs, and to increase production throughput and yield. Two different back contact concepts are discussed, which are either based on Cu or Ti. The Cu-based back contact is a cost-effective design providing low sheet resistance of 0.2 Ω/square at ~ 200 nm stack thickness. To prevent Cu diffusion into the CIGS absorber, a TiN diffusion barrier was integrated into the back contact design. In addition, Cr was evaluated as a candidate of an alternative Cu-diffusion barrier material. Best cell efficiencies of 16.3% were achieved without antireflection coating on a Cu-based back contact design with a TiN diffusion barrier. For the Ti-based design, which is well adapted to the physical properties of the polyimide foil, efficiencies up to 18.1% were measured with antireflective coating.

State-of-the-art Cu(In,Ga)Se2 thin-film technology allows the industrial production of highly efficient solar modules. A significant growth of CIGS-based solar cell production volume can be expected for the coming years. One of the critical manufacturing steps in module production is thin-film patterning which allows the monolithic integration of cell-to-cell interconnects. Today, solar module manufacturers seek to replace sub-optimal needle scribing by suitable laser processes. It has been demonstrated, that ultra-short pulse laser scribing can reduce the overall width per interconnect and increase scribe quality. A promising tool for picosecond laser-scribing is the fiber laser. Here we present the successful implementation of all fiber laser patterning of CIGS modules.

Quantum efficiency measurements of state of the art Cu(In,Ga)Se2 (CIGS) thin film solar cells reveal current losses in the near infrared spectral region. These losses can be ascribed to inadequate optical absorption or poor collection of photogenerated charge carriers. Insight on the limiting mechanism is crucial for the development of more efficient devices. The electron beam induced current measurement technique applied on device cross-sections promises an experimental access to depth resolved information about the charge carrier collection probability. Here, this technique is used to show that charge carrier collection in CIGS deposited by multistage co-evaporation at low temperature is efficient over the optically active region and collection losses are minor as compared to the optical ones. Implications on the favorable absorber design are discussed. Furthermore, it is observed that the measurement is strongly affected by cross-section surface recombination and an accurate determination of the collection efficiency is not possible. Therefore it is proposed and shown that the use of an Al2 O3 layer deposited onto the cleaved cross-section significantly improves the accuracy of the measurement by reducing the surface recombination. A model for the passivation mechanism is presented and the passivation concept is extended to other solar cell technologies such as CdTe and Cu2 (Zn,Sn)(S,Se)4 .

Laser scribing is an indispensable step in the industrial production of Cu(In,Ga)Se 2 thin film solar modules. While cell separation (P1 and P3) is usually achieved using high velocity, low overlap lift-off processes, removal of the absorber layer for generating an electrical back-to-front interconnect (P2) is typically a slow process. In the present study we present an approach for scaling the classical P2 process velocity to an industrially exploitable level. We demonstrated successful P2 scribing at up to 1.7 m/ s in a single beam, single pass configuratio n using a linear focal spot. The presented process is robust against variations in the scribing velocity and focal position, a key point for successful machine integration. Keywords: laser scribing, CIGS, solar cell patterning, selective ablation, thin-film structuring, high throughput, process scaling, APPOLO 1. INTRODUCTION Laser scribing is replacing traditional mechanical scribing in the production of Cu(In,Ga)Se 2

In this paper, Cu(In,Ga)Se2 (CIGS) solar cells on stainless steel foils were developed using a three-stage evaporation processes at low and high substrate temperatures. Different CIGS back-contact designs were used: a thin Ti adhesion layer together with a Mo single layer, Mo bilayer, and Mo bilayer in combination with either an Si3N4 or TiN impurity diffusion barrier layer. Solar cells on contacts with an Si3N4 and a TiN barrier coating showed no improvement in performance compared with the cells on Ti/Mo/Mo triple-layer contact, regardless of the CIGS deposition temperature. The variation of the Mo back-contact designs with no impurity diffusion barrier showed a significant decrease in solar cell performance when thin Mo contacts were used. Furthermore, capacitance to voltage measurements showed a decrease in the carrier concentration in CIGS grown on thin Mo contacts compared with the layers for solar cells on Ti/Mo/Mo triple layer structures. Best cell efficiencies of 17.3% were obtained using a Mo back-contact with a thin Ti adhesion layer, in combination with a low-temperature CIGS deposition process with no additional oxide or nitride impurity diffusion barrier layer on stainless steel foils.

Co-evaporation of Cu, In, Ga, and Se to form CuIn 1− x Ga x Se 2 is known to yield high efficiency solar cells and modules. Highest efficiencies are achieved when using a multi-stage co-evaporation process, where the Cu–In–Ga–Se film enters a Cu rich composition during an intermediate stage of the growth. Furthermore, incorporation of sodium is crucial for high performance devices. We investigated the influence of varying the evaporation profile, especially the Cu excess during growth, on the microstructure of the final CIGS layer, on the distribution of sodium through the layer, and on the photovoltaic performance. Experiments were performed on CIGS layers grown at substrate temperatures of 450 and 600 ∘ C . With increasing maximum [Cu]/[In+Ga] ratio during the deposition, an increase in grain size of the CuIn 1− x Ga x Se 2 layer is observed. For high temperature grown samples, best efficiencies are achieved using minimal Cu excess during the second stage of the growth process. Results show a change in the sodium distribution across the absorber thickness for layers grown at high temperature when the duration of the Cu rich growth regime is changed. We observed that for layers exposed to a Cu rich regime for a short timeframe, Na accumulates at the surface of the layer while for longer exposure times it is more evenly distributed in the top region of the CIGS layer. Evidence for the suppression of the growth of a group III rich phase (ordered vacancy compound) by Na is found.

Development of a Cu(In,Ga)Se 2 thin film solar cell on a polyimide film with a conversion efficiency of 17.1 %, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process.

Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.

Concepts of localized contacts and junctions through surface passivation layers are already advantageously applied in Si wafer-based photovoltaic technologies. For Cu(In,Ga)Se2 thin film solar cells, such concepts are generally not applied, especially at the heterojunction, because of the lack of a simple method yielding features with the required size and distribution. Here, we show a novel, innovative surface nanopatterning approach to form homogeneously distributed nanostructures (

In the present work, we report the results of photoluminescence (PL) studies on Cu(In,Ga)Se2 thin films with varying Ga contents. The samples are epitaxially grown on a GaAs substrate by means of metal-organic vapour phase epitaxy (MOVPE) at slight Cu excess ([Cu]/[III]≈1,1). The polycrystalline samples are prepared by coevaporation on glass. To determine the nature of the observed emissions, PL experiments are performed with varying excitation intensities and temperatures. For the first time, excitonic luminescence is observed in Cu(In,Ga)Se2. It is seen for [Ga]/([Ga]+[In]) ratios (GGI) ≤0.25 and ≥0.75 that exciton binding energies and band gaps are found to rise with increasing Ga content and to be in good accordance with the hydrogen model. Emissions at lower energies are shown to be due to a donor–acceptor pair (DAP) transition equivalent to the DA1 transition in CuInSe2 and CuGaSe2, as well as their phonon replicas. The defect depth was found to increase with increasing Ga content, as expected from the hydrogen model. The study is supplemented by cathodoluminescence (CL) investigations of epitaxial layers and by PL measurements of polycrystalline thin films.

Cu-rich Cu–Ga–Se films were prepared on a Mo coated soda-lime glass substrate by thermal codeposition and the microstructures were characterized. An increase in the grain size of CuGaSe2 (CGS) as the Cu-rich composition approached stoichiometry was observed. In the Cu-rich film, precipitated Cu–Se grains were located between the CGS grains and were connected to the film surface, which suppressed the lateral growth of the CGS grains. On the film surface, a quasi-epitaxial growth of Cu2−xSe grains was found. In order to investigate the development of the microstructure together with the phase change from Cu–Se to CGS, a small amount of Ga and Se were deposited onto Cu-rich Cu–Ga–Se films. By transmission electron microscopy, Cu1.8Se and Cu1.5Se grains were identified in the Cu-rich film. A coherent lattice structure from the CGS into the Cu1.5Se grains was found. The microstructural development of the Cu-rich Cu–Ga–Se film is discussed and models of the growth mechanism are proposed.

We present an optimisation of our recipe for the CdS chemical bath deposition process as applied to solar cells based on polycrystalline CuGaSe 2 (CGSe) absorber layers prepared in two stages by physical vapour deposition. We investigate the influence of the ammonia (NH 3 ) and the thiourea (H 2 NCSNH 2 ) concentration, both being constituents of the chemical bath deposition (CBD) solution, at a deposition temperature of 80 °C on the microstructural and optical properties of CdS layers and on ZnO/CdS/CuGaSe 2 /Mo device parameters. The composition of the CdS layers and their thickness were determined using X-ray Fluorescence Analysis. Transmission and reflection measurements performed at 300 K were used for the calculation of absorption and optical band gap energy ( E g ). The E g values of the films varied from 2.41 to 2.46 eV depending on deposition conditions. Cubic phase of the as-grown layers was identified by X-ray diffraction analysis. An improvement in the investigated solar cells efficiency was achieved when the ammonia concentration was increased and the thiourea concentration was reduced, compared to the previously used standard HMI recipe. The influence of the CBD CdS preparation recipe on the ZnO/CdS/CuGaSe 2 /Mo electrical and photoelectrical properties is discussed.

Cu(In,Ga)Se2 (CIGS) surface was modified with Zn doping using vacuum evaporation. Substrate temperatures andexposure times d the Zn evaporation were changedto control a distribution of Zn in the CIGS films. Diffusion of Zn in the CIGS films was observed at the substrate temperature of over 2001C. The diffusion depth of Zn increases with increasing the exposure time at the substrate temperature of 3001C. Solar cells were fabricated using the Zn doped CIGS films. A distribution of the efficiencies decreases with increasing the exposure time of Zn vapor. The doping of Zn at the film surface improved reproducibility of a high fill factor andefficiency. A solar cell fabricatedusing the CIGS film mod ifiedwith Zn doping showed an efficiency of 14.8%. r 2002 Elsevier Science B.V. All rights reserved.

Kelvin probe force microscopy allows to determine not only the surface topography as does atomic force microscopy, but in addition also delivers images of the surface work function on a nanometer scale. Operation in ultrahigh vacuum improves the lateral and energy resolution and allows to obtain absolute work function values. In this paper we will introduce the method and give examples for the application to solar cell materials and devices. We review examples where the surface of an oriented CuGaSe2 film showed distinct work function values for differently oriented facets of single grains, with differences as high as 250 meV, possibly affecting the power conversion efficiency of a solar cell. A cross-sectional study of a complete solar cell device based on the CuGaSe2 absorber material revealed the formation of an additional MoSex layer between the Mo back contact and the absorber. We will present results of measurements at individual grain boundaries of the absorber material. Furthermore, band bending effects at these grain boundaries are discussed and compared to results from transport studies.

CuGaSe2 (CGS) thin films were prepared on tin-doped indium oxide (ITO) coated soda-lime glass substrates by thermal co-evaporation to fabricate transparent solar cells. The films consisted of columnar grains with a diameter of approximately 1 μm. Some deterioration of the transparency of the ITO was observed after deposition of the CGS film. The CGS solar cells were electrically connected in series with Cu(In,Ga)Se2 (CIGS) solar cells and mechanically stacked on the CIGS cells to construct tandem cells. The tandem solar cell with the CGS cell as the top cell showed an efficiency of 7.4% and an open-circuit voltage of 1.18 V (AM 1.5, total area). Copyright © 2003 John Wiley & Sons, Ltd.

The thin-film solar cell market has seen a period of consolidation during the last years and many involved companies were forced to stop production due to increasing price pressure from competing cell technologies. Today, thin-film solar industry is gaining momentum again. Especially Cu(In,Ga)Se2 technology evolves at high pace fired by recently achieved record efficiencies of 20.4 percent on flexible polyimide substrate [1] and 20.8 percent on glass substrate [2]. Fresh companies are preparing market entry with matured products and manufacturing technology suitable for high-volume and high-throughput production. Among these key-enabling technologies is laser patterning for cell-to-cell interconnects. Several research groups worked on efficient and reliable laser processes that are now ready for the industrial assessment. Here we present a set of work-horse processes for P1, P2 and P3 scribing of CIGS cells on glass substrate. Optimized parameters are presented for 532 nm and 1064 nm using 50 ps pulses from an all-in-fiber laser system. We further demonstrate the successful realization of functional 8-cell modules with a reduced “dead-zone” width of 70±5 μm and high efficiencies. The certified efficiency of 16.6 percent for our low-dead-zone champion module confirms the observation that shrinking of interconnects has no adverse effects on their electrical quality.

Sb doping of Cu(In,Ga)Se2 (CIGS) solar cells has been reported to exhibit a positive effect on the morphology of the absorber layer, offering a possibility to lower manufacturing cost by lowering the annealing temperatures during the CIGS deposition. In this work electron microscopy, energy-dispersive x-ray spectroscopy and photoluminescence experiments have been performed on cells deposited on soda-lime glass substrates, adding a thin Sb layer onto the Mo back contact prior to the CIGS absorber deposition. The defect structure of CIGS solar cells doped with Sb in this way has been investigated and is compared with that of undoped reference cells. The influence of substrate temperature during absorber growth has also been evaluated. For all samples the photoluminescence results can be explained by considering three donor–acceptor pair recombination processes involving the same defect pairs.