12.8% Efficiency Cu(In,Ga)Se2 solar cell on a flexible polymer sheet

A lift-off process has been developed to obtain Cu(In,Ga)Se₂ solar cells on flexible polymer sheets. The absorber layer is grown by a co-evaporation method on a polyimide layer, which is spin coated on a NaCl covered glass substrate. The NaCl intermediate layer can provide Na to the Cu(In,Ga)Se₂ layer during deposition. After the complete processing of the cells, the NaCl buffer layer is dissolved to separate the glass substrate from the ZnO/CdS/Cu(In,Ga)Se₂/Mo/polyimide stack. A record conversion efficiency of 12.8% (total area) under AM1·5 illumination was independently measured at FhG/ISE, Freiburg, Germany. Such high efficiency solar cells on light weight and flexible substrates are needed for novel terrestrial and space applications. Copyright © 1999 John Wiley & Sons, Ltd.     

Quantitative analysis of optical and recombination losses in Cu(In,Ga)Se<Subscript>2</Subscript> thin-film solar cells

Optical and recombination losses in a Cu(In,Ga)Se₂ thin-film solar cell with a band gap of 1.36–1.38 eV are theoretically analyzed. The optical transmittance of the ZnO and CdS layers through which the radiation penetrates into the absorbing layer is determined. Using optical constants, the optical loss caused by reflection at the interfaces (7.5%) and absorption in the ZnO and CdS layers (10.2%) are found. To calculate the recombination loss, the spectral distribution of the quantum efficiency of CdS/CuIn_1–xGa_xSe₂ is investigated. It is demonstrated that, taking the drift and diffusion components of recombination at the front and rear surfaces of the absorber into account, the quantum efficiency spectra of the investigated solar cell can be analytically described in detail. The real parameters of the solar cell are determined by comparing the calculated results and experimental data. In addition, the losses caused by the recombination of photogenerated carriers at the front and rear surfaces of the absorbing layer (1.8% and <0.1%, respectively), at its neutral part (7.6%), and in the space-charge region of the p–n heterojunction (1.0%) are determined. A correction to the parameters of Cu(In,Ga)Se₂ is proposed, which enhances the charge-accumulation efficiency.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Diode characteristics in state‐of‐the‐art ZnO/CdS/Cu(In1−xGax)Se2 solar cells

We report a new state of the art in thin‐film polycrystalline Cu(In,Ga)Se₂‐based solar cells with the attainment of energy conversion efficiencies of 19·5%. An analysis of the performance of Cu(In,Ga)Se₂ solar cells in terms of some absorber properties and other derived diode parameters is presented. The analysis reveals that the highest‐performance cells can be associated with absorber bandgap values of ∼1·14 eV, resulting in devices with the lowest values of diode saturation current density (∼3×10⁻⁸ mA/cm²) and diode quality factors in the range 1·30 < A < 1·35. The data presented also support arguments of a reduced space charge region recombination as the reason for the improvement in the performance of such devices. In addition, a discussion is presented regarding the dependence of performance on energy bandgap, with an emphasis on wide‐bandgap Cu(In,Ga)Se₂ materials and views toward improving efficiency to > 1;20% in thin‐film polycrystalline Cu(In,Ga)Se₂ solar cells. Published in 2005 John Wiley & Sons, Ltd.     

Polarization photosensitivity of ZnO/CdS/Cu(In,Ga)Se2 solar cells

Results of the application of polarization spectroscopy of the photosensitivity of ZnO/CdS/Cu(In,Ga)Se₂ thin-film solar cells with different thicknesses of the CdS (50 and 100 nm) and ZnO (500 and 1000 nm) layers are considered. It is established that the induced photopleochroism coefficient is lowered while the quantum efficiency of photoconversion of the solar cells is raised by increasing the thickness of the front layer. The experimental conditions and spectral dependence of the induced photopleochroism are linked with the antireflection properties of the ZnO front layers. It is concluded that photosensitivity polarization spectroscopy can be used for rapid diagnostics of finished solar cells and to optimize their fabrication technology. Fiz. Tekh. Poluprovodn. 33, 484–487 (April 1999)     

13·7%-Efficient Zn(Se,OH)x/Cu(In,Ga)(SSe)2 thin-film solar cell

Cu(In,Ga)Se₂ (CIGS) and related semiconducting compounds have demonstrated their high potential for high-efficiency thin-film solar cells. The highest efficiency for CIGS-based thin-film solar cells has been achieved with CdS buffer layers prepared by a solution growth method known as chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based thin-film solar cells, Zn(Se,OH)_x buffer layers were deposited by CBD on polycrystalline Cu(In,Ga)(S,Se)₂ (CIGSS). A total-area conversion efficiency of 13·7% was certified by the Frauenhofer Institute for Solar Energy Systems. The CIGSS absorber was fabricated by Siemens Solar Industries (California). For device optimization, the thickness and good surface coverage were controlled by XPS–UPS photoemission spectroscopy. A Zn(Se,OH)_x thickness below 7 nm has been found to be optimum for achieving a homogeneous and compact buffer film on CIGSS, with open-circuit photovoltage V_oc=535 mV, fill factor FF=70·76% and a high short-circuit photocurrent density J_sc=36·1 mA cm⁻². Copyright © 1998 John Wiley & Sons, Ltd.     

Accelerated publication 16.4% total-area conversion efficiency thin-film polycrystalline MgF2/ZnO/CdS/Cu(In,Ga)Se2/Mo solar cell

This communication reports an MgF₂/ZnO/CdS/Cu(In,Ga)Se₂/Mo/glass polycrystalline solar cell with a confirmed total-area conversion efficiency of 16.4%. the thin-film Cu(In,Ga)Se₂ absorber was fabricated by computer-controlled physical vapor deposition (PVD) from the elemental sources. the resulting absorber has a Gal/In compositional grading that we refer to as a notch. Capacitance-voltage (C-V) measurements also reveal a graded doping profile in the region near the electronic p-n junction. the enhanced device performance is characterized by an open-circuit voltage (V_oc) of 660 mV and a particularly high fill factor (FF) of 78.7%.     

Cadmium-free thin-film Cu(In,Ga)Se<Subscript>2</Subscript>/(In<Subscript>2</Subscript>S<Subscript>3</Subscript>) heterophotoelements: Fabrication and properties

The method of heat treatment of metallic Cu-In-Ga layers in the N₂ inert atmosphere in the presence of selenium and sulfur vapors was used to grow homogeneous films of Cu(In,Ga)(S,Se)₂ alloys onto which the CdS or In₂S₃ films were deposited and, on the basis of these structures, the thin-film glass/Mo/p-Cu(In,Ga) (S,Se)₂/n-(In₂S₃,CdS)/n-ZnO/Ni-Al photoelements were fabricated. The mechanisms of charge transport and the processes of photosensitivity in the obtained structures subjected to irradiation with natural and linearly polarized light are discussed. The broadband photosensitivity of thin-film heterophotoelements and the induced photopleochroism were detected; these findings indicate that there is an interference-related blooming of the structures obtained. It is concluded that it is possible to use ecologically safe cadmium-free thin-film heterostructures as high-efficiency photoconverters of solar radiation.     

All Solution‐Processed Chalcogenide Solar Cells – from Single Functional Layers Towards a 13.8% Efficient CIGS Device

Solution processing of inorganic thin films has become an important thrust in material research community because it offers low‐cost and high‐throughput deposition of various functional coatings and devices. Especially inorganic thin film solar cells – macroelectronic devices that rely on consecutive deposition of layers on large‐area rigid and flexible substrates – could benefit from solution approaches in order to realize their low‐cost nature. This article critically reviews existing deposition approaches of functional layers for chalcogenide solar cells with an extension to other thin film technologies. Only true solutions of readily available metal salts in appropriate solvents are considered without the need of pre‐fabricated nanoparticles. By combining three promising approaches, an air‐stable Cu(In,Ga)Se₂ thin film solar cell with efficiency of 13.8% is demonstrated where all constituent layers (except the metal back contact) are processed from solutions. Notably, water is employed as the solvent in all steps, highlighting the potential for safe manufacturing with high utilization rates.     

Grain Boundaries in Cu(In,Ga)Se2: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Cu(In,Ga)Se₂ thin-film solar cells have attracted significant research interest in recent decades due to their high efficiency in converting solar energy into electricity for enabling a sustainable future. Although the Cu(In,Ga)Se₂ absorber can be grown as a single crystal, its polycrystalline form is dominating the market not only due to its lower costs, but also due to its unexpectedly higher cell efficiency. However, this absorber contains a high fraction of grain boundaries. These are structural defects where deep-trap states can be localized leading to an increase in recombination activity. This controversy is mirrored in the existing literature studies where two main contradictory believes exist: 1) to be crucial grain boundaries in Cu(In,Ga)Se₂ absorber are anomalous, being benign in terms of cell performance, and 2) grain boundaries are regions characterized by an increased recombination activity leading to deteriorated cell performance. Therefore, the present review tackles this issue from a novel perspective unraveling correlations between chemical composition of grain boundaries and their corresponding electronic properties. It is shown that features such as Cu depletion/In enrichment, segregation of 1-2at.% of alkali dopants, and passivation by a wide-bandgap or type inversion at grain boundaries are crucial ingredients for low open-circuit voltage loss and, hence, for superior cell performance.     

The charge-transport mechanisms and photosensitivity of <Emphasis Type="Italic">n</Emphasis>-ZnO:Al/Cu<Emphasis Type="Italic">Pc/p</Emphasis>-Cu(In,Ga)Se<Subscript>2</Subscript> structures

The first photosensitive n-ZnO:Al/CuPc/p-Cu(In,Ga)Se₂ structures are produced by a vacuum sublimation of copper phthalocyanine onto the surface of thin p-Cu(In,Ga)Se₂ films and a subsequent magnetron deposition of n-ZnO:Al films. The steady-state current-voltage characteristics of the resulting structures are studied. The charge-transport and photosensitivity mechanisms of the thin-film structures are discussed. The structures appear promising for the fabrication of wide-range (1.2–3.3 eV) thin-film photoelectric converters.     

Photosensitivity of thin-film ZnO/CdS/Cu(In, Ga)Se2 solar cells

The photoelectric properties of thin-film ZnO/CdS/Cu(In,Ga)Se₂ solar cells were studied by polarization photoactive absorption spectroscopy. It was shown that the thin-film solar cells have a high efficiency relative to the intensity of unpolarized radiation in the photon energy range from 1.2 to 2.5 eV. The induced photopleochroism coefficient P _I increases with the angle of incidence of the incident radiation as P _I ∼θ ² and at 70° it reaches 17–20% with photon energy 1.3 eV. Oscillations of the photopleochroism were also observed. These results are discussed taking into account the antireflection effect. The results obtained by us make it possible to use such solar cells as wide-band photosensors for linearly polarized radiation and for monitoring the production of high-efficiency, thin-film solar cells based on ternary semiconductors. Fiz. Tekh. Poluprovodn. 31, 806–810 (July 1997)     

Dynamic radiative properties of the Cu(In,Ga)Se2 layer during the co‐evaporation process

A study of the wavelength‐integrated emissivity has been performed on the optical stack Cu_xSe/Cu(In,Ga)Se₂/Mo. The wavelength interval used in the study was 2–20 µm, which covers 95% of the radiated heat from a black body heated to 500°C. Substrate temperatures around 500°C are commonly used in production of Cu(In,Ga)Se₂ thin films for solar cells. The integrated emissivity was obtained from directional reflectivity measurements of experimental samples with different thicknesses of the Cu_xSe layers. It was subsequently compared to the emissivity from numerical simulations based on newly obtained values of the refractive index values for Cu(In,Ga)Se₂ and Cu_xSe at these wavelengths. Good agreement was found between the measured and simulated values. At a Cu(In,Ga)Se₂ thickness of 1.8 µm and a Mo thickness of 400 nm, a maximum in the integrated emissivity was found for a Cu_xSe thickness of 30 nm. The results are valuable input into understanding the dynamics of the change in emissivity between Cu‐rich Cu(In,Ga)Se₂ with segregated Cu_xSe and Cu‐poor single phase Cu(In,Ga)Se₂ at temperatures around 500°C. In co‐evaporation of Cu(In,Ga)Se₂, this emissivity change is often monitored and used as a process control (end‐point detection). Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication and photoelectric properties of the ZnO-Cu(In,Ga)Se<Subscript>2</Subscript> heterojunctions

Thin-film n-ZnO:Al/p-Cu(In,Ga)Se₂ heterojunctions are fabricated by magnetron sputtering of an ZnO target, leading to a deposition of Cu(In,Ga)Se₂ films on the surface. The photoelectric properties of the fabricated heterojunctions are studied under exposure to natural and linearly polarized light. It is concluded that the resulting cadmium-free environmentally safe heterostructures can be used as high-efficiency broad-band photoconverters of natural and linearly polarized light.     

Defects in Cu(In,Ga) Se2 semiconductors and their role in the device performance of thin-film solar cells

This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chalcopyrite semiconductors and their relation to the device characteristics of thin-film solar cells, held on 3–5 June 1996 in Oberstdorf, Germany. Results of different characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations confirmed the presence of energetic distributions of defects throughout the bandgap of chalcopyrite thin films. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se₂ regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric field, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current–voltage characteristics of Cu(In, Ga)Se₂-based thin-film cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves.© 1997 John Wiley & Sons, Ltd.     

Flexible Cu(In,Ga)Se2 solar cells with reduced absorber thickness

Reduction of the absorber thickness combined with deposition on a flexible substrate is a technically viable strategy to allow lower cost manufacturing of Cu(In,Ga)Se₂ solar modules. Flexible plastic substrates, however, require a low‐temperature deposition process and appropriate control of the band gap grading for achieving high efficiencies. In this work, we developed solar cells on polyimide films using evaporated Cu(In,Ga)Se₂ absorbers with thickness of 0.8–1.3 µm. The double Ga‐grading profile across the absorber thickness was modified by varying the maximum Cu excess at the end of the second stage or by adapting the In and Ga evaporation flux profiles during the growth process. By minimizing the Cu excess during the intermediate stage of the growth process, no loss in open circuit voltage and fill factor is observed compared with a device having a thicker absorber. Efficiency of 16.3% was achieved for cells with an absorber thickness of 1.25 µm. Insufficient absorption of photons in the long wavelength region is mainly responsible for current loss. By changing the In and Ga evaporation profiles, the shape and position of the Ga notch were effectively modified, but it did not lead to a higher device performance. Modifications of the Ga compositional profile could not help to significantly reduce absorption losses or increase charge carrier collection in absorbers with thickness below 1 µm. Changes of open circuit voltage and fill factor are mostly related to differences in the net acceptor density or the reverse saturation current rather than changes of the double Ga grading. Copyright © 2013 John Wiley & Sons, Ltd.     

Properties of CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals and In/CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> structures

Using the method of planar crystallization from the melt with deviations from the stoichiometric composition, p-CuIn₃Se₅ single crystals are grown. The electrical properties of the homogeneous crystals are studied. It is found that the resistivity of the p-CuIn₃Se₅ crystals depends on the excess Se content in the melt. It is established that the voltaic photosensitivity of the In/CuIn₃Se₅ structures is enhanced with an increasing excess of Se content in the melt. The energy spectrum and the character of interband transitions in the CuIn₃Se₅ crystals are discussed. It is concluded that the CuIn₃Se₅ ternary compound can be used in high efficiency photoelectric converters of solar radiation.     

A stacked chalcopyrite thin‐film tandem solar cell with 1.2 V open‐circuit voltage

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.     

Structural and electrical properties of co-evaporated In,Garich CIGS thin films

The CIGS thin trims are prepared by co-evaporation of elemental In, Ga and Se on the substrates of Mo-coated glasses at 400℃ followed by co-evaporation of elemental Cu and Se at 550℃. We study the structural and electrical properties using XRD, XRF and Hall effect measurements. In general, Cu（In,Ga）sSes phase exists when Cu/（In＋Ga） ratio is from 0.17 to 0.27, Cu（In,Ga）3Se5 phase exists for Cu/（In＋Ga） ratio between 0.27 and 0.41, Cu2（in,Ga）4Se7 and Cu（In,Ga）2Se3.5 phases exist for Cu/0n＋Ga） ratio between 0.41 and 0.61, and OVC（or ODC） and CuIn0.7Ga0.3Se2 phases exist when Cu/（In＋Ga） ratio is from 0.61 to 0.88. With the increase of Cu/（In＋Ga） ratio, the carrier concentrations of the films gradually increase, but the electrical resistivity gradually decreases.     

Accelerated publication 17.1% efficient Cu(In,Ga)Se2-based thin-film solar cell

We report a world-record, total-area efficiency of 17.1% for a polycrystalline thin-film Cu(In,Ga)Se₂-based photovoltaic solar cell. the incorporation of Ga to raise the absorber bandgap has been accomplished successfully and in such a manner that an open-circuit voltage of 654 mV and a fill factor of greater than 77% have been achieved. We describe briefly the deposition process, the device structure, and the device performance characteristics.     

A comparison between thin film solar cells made from co‐evaporated CuIn1‐xGaxSe2 using a one‐stage process versus a three‐stage process

Until this day, the most efficient Cu(In,Ga)Se₂ thin film solar cells have been prepared using a rather complex growth process often referred to as three‐stage or multistage. This family of processes is mainly characterized by a first step deposited with only In, Ga and Se flux to form a first layer. Cu is added in a second step until the film becomes slightly Cu‐rich, where‐after the film is converted to its final Cu‐poor composition by a third stage, again with no or very little addition of Cu. In this paper, a comparison between solar cells prepared with the three‐stage process and a one‐stage/in‐line process with the same composition, thickness, and solar cell stack is made. The one‐stage process is easier to be used in an industrial scale and do not have Cu‐rich transitions. The samples were analyzed using glow discharge optical emission spectroscopy, scanning electron microscopy, X‐ray diffraction, current–voltage‐temperature, capacitance‐voltage, external quantum efficiency, transmission/reflection, and photoluminescence. It was concluded that in spite of differences in the texturing, morphology and Ga gradient, the electrical performance of the two types of samples is quite similar as demonstrated by the similar J–V behavior, quantum spectral response, and the estimated recombination losses. Copyright © 2014 John Wiley & Sons, Ltd.