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.     

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%.     

World‐record Cu(In,Ga)Se2‐based thin‐film sub‐module with 17.4% efficiency

We report a new certified world‐record efficiency for thin‐film Cu(In,Ga)Se₂‐based photovoltaic sub‐modules of 17.4% (aperture area). The record efficiency of the 16 cm², monolithically integrated, sub‐module has been independently confirmed by Fraunhofer ISE. The record device is the result of extensive co‐optimization of all processing steps. During the optimization process, strong focus has been put on the scalability of processes to cost‐effective mass production, as reflected, for example, in Cu(In,Ga)Se₂ deposition time and substrate temperature. Device manufacturing as well as results of electrical and material characterization is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Inkjet printed metallizations for Cu(In1−x Ga x )Se2 photovoltaic cells

This study reports the inkjet printing of Ag front contacts on Aluminum doped Zinc Oxide (AZO)/intrinsic Zinc Oxide (i‐ZnO)/CdS/Cu(In_1−xGa_x)Se₂ (CIGS)/Mo thin film photovoltaic cells. The printed Ag contacts are being developed to replace the currently employed evaporated Ni/Al bi‐layer contacts. Inkjet deposition conditions were optimized to reduce line resistivity and reduce contact resistance to the Al:ZnO layer. Ag lines printed at a substrate temperature of 200°C showed a line resistivity of 2.06 µΩ · cm and a contact resistance to Al:ZnO of 8.2 ± 0.2 mΩ · cm² compared to 6.93 ± 0.3 mΩ · cm² for thermally evaporated contacts. These deposition conditions were used to deposit front contacts onto high quality CIGS thin film photovoltaic cells. The heating required to print the Ag contacts caused the performance to degrade compared to similar devices with evaporated Ni/Al contacts that were not heated. Devices with inkjet printed contacts showed 11.4% conversion efficiency compared to 14.8% with evaporated contacts. Strategies to minimize heating, which is detrimental for efficiency, during inkjet printing are proposed. Copyright © 2011 John Wiley & Sons, Ltd.     

Device characteristics of a 10.1% hydrazine‐processed Cu2ZnSn(Se,S)4 solar cell

A power conversion efficiency record of 10.1% was achieved for kesterite absorbers, using a Cu₂ZnSn(Se,S)₄ thin‐film solar cell made by hydrazine‐based solution processing. Key device characteristics were compiled, including light/dark J–V, quantum efficiency, temperature dependence of V_oc and series resistance, photoluminescence, and capacitance spectroscopy, providing important insight into how the devices compare with high‐performance Cu(In,Ga)Se₂. The record kesterite device was shown to be primarily limited by interface recombination, minority carrier lifetime, and series resistance. The new level of device performance points to the significant promise of the kesterites as an emerging and commercially interesting thin‐film technology. Copyright © 2011 John Wiley & Sons, Ltd.     

Atomic layer deposition of Zn1−xMgxO buffer layers for Cu(In,Ga)Se2 solar cells

Fabrication of Zn_1−xMg_xO films by atomic layer deposition (ALD) has been studied for use as buffer layers in Cu(In,Ga)Se₂ (CIGS)‐based solar cell devices. The Zn_1−xMg_xO films were grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in the temperature range from 105 to 180°C. Single‐phase ZnO‐like films were obtained for x < 0·2, followed by a two phase region of ZnO‐ and MgO‐like structures for higher Mg concentrations. Increasing optical band gaps of up to above 3·8 eV were obtained for Zn_1−xMg_xO with increasing x. It was found that the composition of the Zn_1−xMg_xO films varied as an effect of deposition temperature as well as by increasing the relative amount of magnesium precursor pulses during film growth. Completely Cd‐free CIGS‐based solar cells devices with ALD‐Zn_1−xMg_xO buffer layers were fabricated and showed efficiencies of up to 14·1%, which was higher than that of the CdS references. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

Improvement of the cell performance in the ZnS/Cu(In,Ga)Se2 solar cells by the sputter deposition of a bilayer ZnO : Al film

ZnS is a candidate to replace CdS as the buffer layer in Cu(In,Ga)Se₂ (CIGS) solar cells for Cd‐free commercial product. However, the resistance of ZnS is too large, and the photoconductivity is too small. Therefore, the thickness of the ZnS should be as thin as possible. However, a CIGS solar cell with a very thin ZnS buffer layer is vulnerable to the sputtering power of the ZnO : Al window layer deposition because of plasma damage. To improve the efficiency of CIGS solar cells with a chemical‐bath‐deposited ZnS buffer layer, the effect of the plasma damage by the sputter deposition of the ZnO : Al window layer should be understood. We have found that the efficiency of a CIGS solar cell consistently decreases with an increase in the sputtering power for the ZnO : Al window layer deposition onto the ZnS buffer layer because of plasma damage. To protect the ZnS/CIGS interface, a bilayer ZnO : Al film was developed. It consists of a 50‐nm‐thick ZnO : Al plasma protection layer deposited at a sputtering power of 50 W and a 100‐nm‐thick ZnO : Al conducting layer deposited at a sputtering power of 200 W. The introduction of a 50‐nm‐thick ZnO : Al layer deposited at 50 W prevented plasma damage by sputtering, resulting in a high open‐circuit voltage, a large fill factor, and shunt resistance. The ZnS/CIGS solar cell with the bilayer ZnO : Al film yielded a cell efficiency of 14.68%. Therefore, the application of bilayer ZnO : Al film to the window layer is suitable for CIGS solar cells with a ZnS buffer layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Electronic structure of the Zn(O,S)/Cu(In,Ga)Se2 thin‐film solar cell interface

The electronic band alignment of the Zn(O,S)/Cu(In,Ga)Se₂ interface in high‐efficiency thin‐film solar cells was derived using X‐ray photoelectron spectroscopy, ultra‐violet photoelectron spectroscopy, and inverse photoemission spectroscopy. Similar to the CdS/Cu(In,Ga)Se₂ system, we find an essentially flat (small‐spike) conduction band alignment (here: a conduction band offset of (0.09 ± 0.20) eV), allowing for largely unimpeded electron transfer and forming a likely basis for the success of high‐efficiency Zn(O,S)‐based chalcopyrite devices. Furthermore, we find evidence for multiple bonding environments of Zn and O in the Zn(O,S) film, including ZnO, ZnS, Zn(OH)₂, and possibly ZnSe. Copyright © 2016 John Wiley & Sons, Ltd.     

High quality baseline for high efficiency,Cu(In1−x,Gax)Se2 solar cells

We report on the progress that we have made in the quality of our baseline process for the production of high efficiency soda lime glass/Mo/Cu(In,Ga)Se₂ (CIGS)/CdS/i‐ZnO/ZnO:Al/MgF₂ solar cells. The enhancement of the average performance level has enabled us to reach conversion efficiencies of up to 19·3% (internal measurement). The new quality initiative uses process control, optical and electrical modelling, and the critical revision of all process steps as tools for the attainment of the 19% efficiency level. Our experiments show that the compositional process window for CIGS solar cells that have an efficiency of η ≈ 19% is very wide. Accordingly, we suggest that an efficiency of 19·0–19·5% is achievable in the following compositional process windows: 0·69 ≤ Cu/(Ga + In) ≤ 0·98 and 0·21 ≤ Ga/(Ga + In) ≤ 0·38. In addition, our results show that large CIGS grains are not a necessary requirement for high efficiencies up to 19%. These findings and the partly lacking ability to correlate certain aspects of our progress with experimental parameters lead us to the conclusion that there are still some important process variables undiscovered. From this conclusion and from the evaluation of the available data we infer that there is a potential for the enhancement of CIGS solar cell efficiencies beyond 20%. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigation of Cu(In,Ga)Se2 thin‐film formation during the multi‐stage co‐evaporation process

In order to transfer the potential for the high efficiencies seen for Cu(In,Ga)Se₂ (CIGSe) thin films from co‐evaporation processes to cheaper large‐scale deposition techniques, a more intricate understanding of the CIGSe growth process for high‐quality material is required. Hence, the growth mechanism for chalcopyrite‐type thin films when varying the Cu content during a multi‐stage deposition process is studied. Break‐off experiments help to understand the intermediate growth stages of the thin‐film formation. The film structure and morphology are studied by X‐ray diffraction and scanning electron microscopy. The different phases at the film surface are identified by Raman spectroscopy. Depth‐resolved compositional analysis is carried out via glow discharge optical emission spectrometry. The experimental results imply an affinity of Na for material phases with a Cu‐poor composition, affirming a possible interaction of sodium with Cu vacancies mainly via In(Ga)_Cu antisite defects. An efficiency of 12.7% for vacancy compound‐based devices is obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Reactive Sputtering Process for CuIn1‐xGaxSe2 Thin Film Solar Cells

CuIn_1‐xGa_xSe₂ (CIGS) thin films are grown on Mo/soda lime glass using a reactive sputtering process in which a Se cracker is used to deliver reactive Se molecules. The Cu and (In_0.7Ga_0.3)₂Se₃ targets are simultaneously sputtered under the delivery of reactive Se. The effects of Se flux on film composition are investigated. The Cu/(In+Ga) composition ratio increases as the Se flux increases at a plasma power of less than 30 W for the Cu target. The (112) crystal orientation becomes dominant, and crystal grain size is larger with Se flux. The power conversion efficiency of a solar cell fabricated using an 800‐nm CIGS film is 8.5%.     

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.     

Growth of Cu(In,Ga)Se2 thin films by a novel single‐stage route based on pulsed electron deposition

We report a novel route for growing Cu(In,Ga)Se₂ (CIGS) thin films, based upon the Pulsed Electron Deposition (PED) technique. Unlike other well‐known deposition techniques, PED process allows the stoichiometric deposition of CIGS layers in a single stage, without requiring any further treatments for Cu/(In + Ga) ratio adjustment nor selenization. The structural properties of polycrystalline CIGS films strongly depend on the growth temperature, whereas post‐deposition annealing enhances the grain size and the <112> out‐of‐plane preferred orientation of the chalcopyrite structure, without affecting the film composition. Preliminary measurements of the performances of solar cells based on these films confirm the great potentiality of PED‐grown CIGS as absorber layers. Copyright © 2011 John Wiley & Sons, Ltd.     

Cu(In,Ga)Se2 solar cell grown on flexible polymer substrate with efficiency exceeding 17%

Development of a Cu(In,Ga)Se₂ 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. Copyright © 2011 John Wiley & Sons, Ltd.     

High efficiency low temperature grown Cu(In,Ga)Se2 thin film solar cells on flexible substrates using NaF precursor layers

We report a total‐area efficiency of 15.9% for flexible Cu(In,Ga)Se₂ thin film solar cells on polyimide foil (cell area 0.95 cm²). The absorber layer was grown by a multi‐stage deposition process at a maximum nominal process temperature of 420°C. The Na was added via evaporation of a NaF layer prior to the absorber deposition leading to an enhanced V_oc and FF. Growth conditions and device characterization are described. Copyright © 2011 John Wiley & Sons, Ltd.     

Properties of ZnO/CdS/CuInSe2 solar cells with improved performance

We report the growth and characterization of low‐bandgap record‐efficiency ZnO/CdS/CuInSe₂ thin‐film solar cells. The total area conversion efficiency for this cell is 14·5%. This result has been measured and confirmed at the National Renewable Energy Laboratory under standard reporting conditions (1000 W/m², 25°C, AM1·5 Global). The improved performance of the CuInSe₂ solar cell is primarily due to a high current density. Material and device characterization data are presented.. Published in 2004 by John Wiley & Sons, Ltd.     

ILGAR‐ZnO Window Extension Layer: an adequate substitution of the conventional CBD‐CdS buffer in Cu(In,Ga) (S,Se)2‐based solar cells with superior device performance

A novel Window Extension Layer (WEL) concept for chalcopyrite‐based thin‐film solar cell devices has been developed. The optimization of its deposition is presented. WEL means the replacement of the conventional buffer layer by a layer consisting of the same material as the window, i.e., a part of the window is directly deposited onto the absorber by a soft process, such as ILGAR (Ion Layer Gas Reaction). This sequential cyclic technique has been applied to Cu(In,Ga)(S,Se)₂ absorber substrates. The ILGAR procedure was optimized with respect to the efficiency of the resulting Mo/Cu(In,Ga)(S,Se)₂/WEL/ZnO solar cells. The devices were characterized by J–V (under AM 1.5 and without illumination) as well as by quantum efficiency measurements. Devices with ZnO WEL yield total area (0.5 cm²) efficiencies of 14.6% (best cell) without any anti‐reflecting coating. The efficiencies are superior to those of the corresponding devices with CBD (Chemical Bath Deposition)‐CdS buffer (14.1%, best cell). Thus, in contrast to other ZnO buffers, ILGAR‐ZnO achieves record efficiencies exceeding those of CBD‐CdS buffered reference cells. Copyright © 2001 John Wiley & Sons, Ltd.     

High rate (~7 nm/s), atmospheric pressure deposition of ZnO front electrode for Cu(In,Ga)Se2 thin‐film solar cells with efficiency beyond 15%

Undoped zinc oxide (ZnO) films have been grown on a moving glass substrate by plasma‐enhanced chemical vapor deposition at atmospheric pressure. High deposition rates of ~7 nm/s are achieved at low temperature (200 °C) for a substrate speed from 20 to 60 mm/min. ZnO films are highly transparent in the visible range (90%). By a short (~minute) post‐deposition exposure to near‐ultraviolet light, a very low resistivity value of 1.6·10⁻³ Ω cm for undoped ZnO is achieved, which is independent on the film thickness in the range from 180 to 1200 nm. The photo‐enhanced conductivity is stable in time at room temperature when ZnO is coated by an Al₂O₃ barrier film, deposited by the industrially scalable spatial atomic layer deposition technique. ZnO and Al₂O₃ films have been used as front electrode and barrier, respectively, in Cu(In,Ga)Se2 (CIGS) solar cells. An average efficiency of 15.4 ± 0.2% (15 cells) is obtained that is similar to the efficiency of CIGS reference cells in which sputtered ZnO:Al is used as electrode. Copyright © 2013 John Wiley & Sons, Ltd.