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In a high efficiency CIGS solar cell, we observed a MoSe2 layer at the interface by SIMS, TEM and X-ray diffraction. MoSe2 had a layer structure and the layers were oriented perpendicular to the Mo layer. The MoSe2 contributes to the improvement of adhesion at the CIGS/Mo interface. The effects of the MoSe2 layer on the electrical and photovoltaic properties of CIGS solar cells were investigated. The CIGS/Mo heterocontact, including the MoSe2 layer, is not Schottky-type, but a favorable ohmic-type by the evaluation of dark I–V measurement at low temperature. A characteristic peak at 870 nm is observed in the differential quantum efficiency of a solar cell with a CIGS thickness of 500 nm. This peak relates to the absorption of the MoSe2 layer. The band gap of MoSe2 is calculated to be 1.41 eV from the absorption peak.

CIGS films were treated in In–S aqueous solution for high-efficiency CIGS solar cells. The In–S aqueous solution contained InCl 3 and CH 3 CSNH 2 (thioacetamide). The In–S treatment modified the CIGS surface favorably for high-efficiency CIGS solar cells as evidenced by the increase in V oc , J sc and FF. The In–S treatment formed thin CuInS 2 layer on the CIGS surface which contributes to the high efficiency and stable performance of the CIGS solar cell. The best cell showed an efficiency of 17.6% ( V oc =0.649 V, J sc =36.1 mA/cm 2 and FF=75.1%) without any annealing and light soaking before I – V measurement.

Improved preparation process of a device quality Cu(In,Ga)Se 2 (CIGS) thin film was proposed for production of CIGS solar cells. In-Ga-Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu-Se/In-Ga-Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF2/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V oc = 0.634 V, J sc = 36.4 mA/cm 2 , FF = 0.756).

We have proposed a composition-monitoring method using heat radiation for an in-line evaporation process. This method utilizes the di!erence of heat radiation from Cu- and (In,Ga)-rich "lms. With this method, we can successfully control chemical compositions easily and prepare device-quality Cu(In,Ga)Se 2 thin "lms. ( 2001 Elsevier Science B.V. All rights reserved.

An efficiency of over 18% have been achieved in Cu(In,Ga)Se 2 (CIGS) thin-film solar cells. Solar cell parameters were estimated for the cells with efficiencies of more and less than 18%. A diode quality factor n and forward current (saturated current) J 0 of the cell with over 18% efficiency are lower than those with below 18% efficiency. This would be attributed to sufficient coverage of the CdS film with excellent uniformity as a buffer and/or window layer over the CIGS film because the process of CdS film formation was improved.

We investigated the electrical properties of the Cu(In,Ga)Se 2 /MoSe 2 /Mo structure. CIGS/Mo heterocontact including the MoSe 2 layer is not Schottky-type but a favorable ohmic-type contact by the evaluation of dark I – V measurement at low temperature. A characteristic peak at 870 nm is observed in differential quantum efficiency of a solar cell with a CIGS thickness of 0.5 μm. This peak is considered with relating to the absorption of the MoSe 2 layer. The band gap of MoSe 2 is calculated to be 1.41 eV from the absorption peak. The band diagram is discussed on the basis of the electrical point of view.

An in-line process for deposition of Cu(In,Ga)Se 2 (CIGS) films on 10×10 cm 2 substrates has been developed in order to fabricate high-efficiency CIGS modules. At the first step, CIGS solar cells with a small area were fabricated by the in-line evaporation. The cells on different positions in 10×10 cm 2 area showed almost the same efficiency of over 14%. Twelve cells on a substrate of 5×10 cm 2 size showed an average efficiency of about 13%. Device-quality CIGS films can be prepared by the in-line evaporation. Furthermore, interconnected CIGS submodules were fabricated on 10×10 cm 2 size substrates. The CIGS submodule has achieved an efficiency of 10.5%. The segmental cells in the submodule showed an average open-circuit voltage higher than 0.6 V, comparable to that of a high-efficiency cell with a laboratory size.

Cu(In,Ga)Se2(CIGS) thin films were prepared at substrate temperatures of 350 to 500 °C. The (In,Ga)2Se2 precursor layers were deposited on Mo coated soda-lime glass and then exposed to Cu and Se fluxes to form CIGS films. The surface composition was probed by a real-time composition monitoring method. The CIGS films were characterized by x-ray diffraction, energy dispersive x-ray spectroscopy, secondary ion mass spectroscopy, and atomic force microscopy. The transient formation of a Cu–Se phase with a high thermal emissivity was observed during the deposition of Cu and Se at a substrate temperature of 350 °C. Faster diffusion of In than Ga from the (In,Ga)2Se3 precursor to the newly formed CIGS layer was observed. A growth model for CIGS films during the deposition of Cu and Se onto (In,Ga)2Se3 precursor is proposed. A solar cell using a CIGS film prepared at about 350 °C showed an efficiency of 12.4%.

II–VI widegap semiconductors such as ZnO are widely utilized in various electronic and optical devices. To widen the band gap of ZnO, we conducted a systematic investigation of solid solution thin films of Zn 1− x Mg x O, a group of ternary compounds of the ZnMgO system. We prepared the thin films by radio frequency (RF) magnetron co-sputtering on fused silica substrates at room temperature. The thin films were a single phase of Zn 1− x Mg x O having the basic structure of ZnO at x ≤0.46 and the basic structure of MgO at x ≥0.62, with segregation of the ZnO and MgO phases at x =0.58. The band gap of Zn 1− x Mg x O having the basic structure of ZnO increased from 3.24 eV at x =0 (ZnO) to 4.20 eV at x =0.46. Transmittances of Zn 1− x Mg x O thin films were nearly equivalent to those of ZnO. Zn 1− x Mg x O has a wider band gap than ZnO and can be expected to provide a useful window layer of solar cells that improves the overall efficiency by decreasing the absorption loss.

Growth of Cu(In,Ga)Se2 (CIGS) films from In–Ga–Se precursors was characterized by scanning Auger electron spectroscopy (SAES), secondary ion mass spectroscopy (SIMS), x-ray diffraction, scanning electron microscopy, and transmission electron microscopy (TEM). In–Ga–Se precursor layers were deposited on Mo-coated soda-lime glass, and then the layers were exposed to Cu and Se fluxes to form CIGS films. The SIMS and SAES analyses showed a homogeneous distribution of Cu throughout the CIGS films during the deposition of Cu and Se. The phase changes observed in the CIGS films during the deposition of Cu and Se on the In–Ga–Se precursor films were as follows: (In,Ga)2Se3 →[Cu(In,Ga)5Se8] →Cu(In,Ga)3Se5 →Cu(In,Ga)Se2. The grain size increased from the submicron grains of the (In,Ga)2Se3 precursor film to several micrometers in the stoichiometric Cu(In,Ga)Se2 film. A growth model of CIGS crystals is introduced on the basis of the results of TEM observations. CIGS crystals are mainly grown under (In,Ga)-rich conditions in the preparation from In–Ga–Se precursor films.

A solution deposition approach for high-performance aluminum-doped zinc oxide (AZO) thin films (visible transparency > 90% and sheet resistance down to 25 Ω/sq) with process temperatures not exceeding 85 °C is presented. This allows the non-vacuum deposition of AZO on temperature sensitive substrates such as polymer films for flexible and transparent electronics, or inorganic and organic thin film photovoltaics.

Thin film photovoltaic devices based on chalcopyrite Cu(InGa)Se 2 (CIGS) absorber layers show excellent light to power conversion efficiencies exceeding 20 (refs). This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals and so far mainly Na was believed to be the most favourable element whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes well known in other research areas are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction. © 2013 Macmillan Publishers Limited. All rights reserved.

New developments in the thin film solar market continue the trend towards solar modules with higher energy conversion while at the same time, reducing significantly manufacturing costs. Especially thin film technologies based on Cadmiumtellurid (CdTe) or Cu(In,Ga)(S,Se) 2 (CIGS) seem to be suited to improve the energy conversion and hence, take over larger market shares. With this work, we present our latest achievements towards a CIGS all laser scribing process with the emphasis on structuring the absorber layer and its implications to the production. While P1 laser scribing through the substrate is already implemented in production today a variety of different approaches, like lift-off, ablation, or remelting are possible for the P2 process where commonly a mechanical process is state of the art. One challenge which the P2 and P3 processes face is the layer side processing. Therefore a thorough investigation has been conducted including different laser wavelengths (355 nm to 1550 nm), pulse durations (10 ps to 100 ns), and beam shaping to find the best possible solution for each scribing process. Optimization took place utilizing not only resistance measurement and optical microscopy but also LSM, REM, EDX, EL, and Lock-In Thermography. Combining the best results of each scribing process and using a high speed, high accuracy motion system a functional lab size module has been produced with a reduced dead zone of below 200 m. In an outlook, a way is presented on how to take the lab results into a productive system and place it in a manufacturing environment.

Sr0.2Ba0.8Nb2O6 ceramics with 1mol% V2O5 addition were fabricated by liquid phase sintering. A compact sintered at 1300°C for 100h had the spontaneous polarization of 6.1×10-2C/m2 at 50°C. The sharp permittivity peak was observed at 293°C, corresponding to the Curie temperature. The crystal structure had the orthorhombic symmetry with a0=1.7654, b0=1.7662 and c0=0.7961nm at room temperature. The orthorhombic-tetragonal phase transition was found at about 120°C.

The development of a novel chemistry for the chemical bath deposition of Zn(O,S) buffer layers for Cu(In,Ga)Se2 (CIGS) solar cells is desired for a higher growth rate, hence reduced deposition time, while reducing simultaneously the required concentration of reactants. State-of-the-art recipes are based on thiourea as sulphide precursor requiring a high molarity of reactants and relatively long deposition times due to the slow decomposition rate of thiourea. In this contribution thioamide based sulphide precursors were investigated for their decomposition and growth behaviour. A co-solvent approach in an ethanolic/aqueous ammonia medium was evaluated omitting the need for additional complexants. By replacing thiourea with the investigated thioamides, homogeneous dense layers of around 30 nm were grown with a greatly decreased deposition time of 8 min compared to 25 min for thiourea. Likewise, the concentration of the sulphide precursor was 40-fold reduced. The photovoltaic performance as characterized by external quantum efficiency and current–voltage measurements, showed conversion efficiencies of 15% comparable to the thiourea based process.

Sr0.3Ba0.7Nb2O6(SBN30) compacts with 1mol% V2O5 addition were fabricated at relatively lower temperatures by liquid phase sintering. A considerable elongation and coarsening of grains were observed after prolonged sintering at 1200°C. The ferroelectric properties were improved by the addition of V2O5: sharpening of the dielectric peak and shift in Curie temperature from 175 to 225°C. The modified ferroelectric properties might be attributed to decrease in the internal stress and compositional fluctuation with coarsening of grains. The pyroelectric coefficient of V2O5-added compacts (1200°C, 14h) at room temperature was 8.0×10-5C/m2·K.

Recent achievements in Cu(In,Ga)Se2 (CIGS) thin film technology allow the industrial production ofhighly efficient solar modules. A large growth of the CIGS-based solar cell production volume can beexpected for the coming years thanks to some favorable properties inherent to this absorber type. A majordrawback of CIGS is the inefficient patterning process. Since CIGS is a particularly difficult material forlaser ablation there is still no industrial all-laser scribing solution available. Manufacturers fall back onmechanical needle scribing for the P2 and P3 scribing process and have to accept substantial broadeningof the electrical interconnects due to unpredictable chipping at the scribe borders. In the present study weexplored a large variety of possible processes for the P1-P3 scribing at different wavelengths and indifferent pulse duration regimes. Beside the direct ablation of CIGS with ultrashort pulses we alsoinvestigated more exotic processes like layer side lift-off variants. The resulting scribes were analyzedusing electron microscopy (EM), laser scanning microscopy (LSM), energy dispersive X-ray spectroscopy(EDX) and electrical conductivity measurements. The most promising processes were selected forproducing functional mini-modules. Multiple optimization cycles allowed us to select the processes withthe best performance in the mini-module.

Solar cells based on chalcopyrite Cu(In, Ga)Se 2 (CIGS) absorber layers show the highest potential for low-cost solar electricity by yielding comparable efficiencies to polycrystalline Si wafer-based cells, while also offering inherent advantages of thin-film technology for cost reduction.Highest efficiency of 20.3% was recently achieved on rigid glass substrate. Deposition of CIGS films onto flexible substrates opens new fields of applications and could significantly decrease production costs by employing roll-to-roll manufacturing and monolithic integration of solar cells to develop modules. Whereas, some years back, it seemed difficult to reach performance levels on flexible substrates similar to that obtained on glass, recent results on flexible polyimide prove that the efficiency gap can be significantly reduced. Different materials, i.e., mostly metals or plastics, have been used as flexible substrates, with highest cell efficiency of 18.7% demonstrated on a polyimide film. Improvements in efficiencies of flexible solar cells and modules achieved over the past few decades are discussed in this paper, addressing the main characteristics of substrate materials. The technology transfer from laboratory research to large-scale industrial production of CIGS modules leads to new manufacturing challenges, mainly for CIGS deposition, interconnections of cells, and long-term performance stability.

Recent developments in Cu(In,Ga)Se2 (CIGS) thin film photovoltaics enabled the manufacturers to produce highly efficient solar modules. Nevertheless, the production process still lacks a competitive process for module patterning. Today, the industry standard for the serial interconnection of cells is still based on mechanical scribing for the P2 and P3 process. A reduction of the non-productive "dead zone" between the P1 and P3 scribes is crucial for further increasing module efficiency. Compact and affordable picosecond pulsed laser sources are promising tools towards all-laser scribing of CIGS solar modules. We conducted an extensive parameter study comprising picosecond laser sources from 355 to 1064 nm wavelength and 10 to 50 ps pulse duration. Scribing results were analyzed by laser scanning microscope, scanning electron microscope and energy dispersive X-ray spectroscopy. We developed stable and reliable processes for the P1, P2 and P3 scribe. The best parameter sets were then used for the production of functional mini-modules. For comparison, the same was done for a selection of nanosecond pulsed lasers. Standardized analysis of the modules has shown superior electrical performance of the interconnections and confirmed the feasibility of a dead zone width of less than 200 im on an entire mini module.

The development of Cu(In, Ga)Se 2 (CIGS) thin film solar cells on flexible substrates shows high potential for reducing production costs by roll-to-roll manufacturing. However on steel foils, impurity diffusion from the steel into the CIGS absorber must be prevented to ensure high conversion efficiencies. We processed CIGS solar cells on stainless steel foils using different three-stage processes at substrate temperatures of ∼450°C, ∼500°C, and ∼600°C. Different CIGS back contact designs were used: Mo single layers, Mo bilayers, and Mo bilayers in combination with a Si 3 N 4 or TiN impurity diffusion barrier. The back contacts were deposited by an in-line magnetron sputtering process. Two different thicknesses of the Mo single layers were tested for different sputtering conditions. Solar cell experiments with Si 3 N 4 and TiN based contacts showed no improvements in performance compared to the Mo bilayer contact regardless of the CIGS deposition temperature. The variation of the Mo back contact designs without an impurity diffusion barrier showed a significant change in solar cell performance when thin contacts at certain sputtering conditions were used. Corresponding secondary ion mass spectroscopy measurements showed an increase in Fe diffusion from the steel into the absorber, whereas no influence on the Cr diffusion was found. The Fe diffusion intensity was in good correlation with the solar cell performance, which was measured by current density to voltage technique. Best cell efficiency of 17.7% (certified by Fraunhofer ISE) was obtained using a Mo bilayer back contact with appropriate sputtering conditions in combination with a low temperature CIGS process without any additional impurity diffusion barrier layer on stainless steel foils.

For the development of high efficiency Cu(In,Ga)Se 2 (CIGS) flexible solar cells on polymer films with efficient and robust deposition process the so-called three-stage evaporation processes were modified. CIGS layers were grown by co-evaporation of constituent elements at low substrate temperature compatible with the stability of the polyimide film. The CIGS layers grown with our standard process are compositionally graded, especially the Ga has a strong grading profile across the layer thickness. Different variations of the standard evaporation process were investigated, especially in view of changing the energy band gap profiles in the absorber layer for achieving further improvements in high efficiency solar cells. Further on, the incorporation of sodium during the CIGS deposition was optimized. Structural and chemical composition properties of CIGS layers were characterized with SEM, XRF, SIMS and the photovoltaic properties were characterized with I–V and quantum efficiency measurements. A flexible CIGS solar cell on polyimide film with a record efficiency of 17.6% has been developed.

Solar cells based on polycrystalline Cu(In,Ga)Se 2 (CIGS) absorber layers have shown high potential for low cost photovoltaic energy conversion. Our group has achieved 18.1% efficient cells on glass substrates and 14.1% on flexible polyimide foils [1] without antireflection (AR) coating. These results were achieved by applying a three-stage evaporation process with common growth rates of about 35 nm/min. Aim of this study was to enhance the growth rates of layers deposited at low substrate temperature (450°C) suitable for polyimide foils and to investigate structural properties of CIGS layers and photovoltaic properties of solar cells. It was found that very high deposition rates of 500 nm/min during the 2nd and 3rd stage of the evaporation process are feasible, maintaining high absorber quality and performance of solar cells.

Hydrogenated indium oxide (IOH) is implemented as transparent front contact in Cu(In,Ga)Se2 (CIGS) solar cells, leading to an open circuit voltage VOC enhanced by ∼20 mV as compared to reference devices with ZnO:Al (AZO) electrodes. This effect is reproducible in a wide range of contact sheet resistances corresponding to various IOH thicknesses. We present the detailed electrical characterization of glass/Mo/CIGS/CdS/intrinsic ZnO (i-ZnO)/transparent conductive oxide (TCO) with different IOH/AZO ratios in the front TCO contact in order to identify possible reasons for the enhanced VOC. Temperature and illumination intensity-dependent current-voltage measurements indicate that the dominant recombination path does not change when AZO is replaced by IOH, and it is mainly limited to recombination in the space charge region and at the junction interface of the solar cell. The main finding is that the introduction of even a 5 nm-thin IOH layer at the i-ZnO/TCO interface already results in a step-like increase i...

High mobility hydrogenated indium oxide is investigated as a transparent contact for thin film Cu(In,Ga)Se2 (CIGS) solar cells. Hydrogen doping of In2O3 thin films is achieved by injection of H2O water vapor or H2 gas during the sputter process. As-deposited amorphous In2O3:H films exhibit a high electron mobility of ∼50 cm2/Vs at room temperature. A bulk hydrogen concentration of ∼4 at. % was measured for both optimized H2O and H2-processed films, although the H2O-derived film exhibits a doping gradient as detected by elastic recoil detection analysis. Amorphous IOH films are implemented as front contacts in CIGS based solar cells, and their performance is compared with the reference ZnO:Al electrodes. The most significant feature of IOH containing devices is an enhanced open circuit voltage (VOC) of ∼20 mV regardless of the doping approach, whereas the short circuit current and fill factor remain the same for the H2O case or slightly decrease for H2. The overall power conversion efficiency is improved f...

A way to lower the manufacturing cost of Cu(In,Ga)Se-2 (CIGS) thin-film solar cells is to use flexible polymer substrates instead of rigid glass. Because such substrates require lower temperature during absorber deposition, the grain growth of the absorber layer can be hindered which leads to a lower cell performance. Partial compensation of this efficiency loss might be accomplished by growing the absorber in the presence of Sb, which is reported to promote grain growth. In this work CIGS solar cells, deposited on glass substrates, at a reduced substrate temperature with a thin Sb layer (7, 12 nm) on top of the Mo contact are investigated. The diffusion profile of Sb is measured with plasma profiling time of flight mass spectrometry. The beneficial effect of Sb on efficiency and grain size is shown in quantum efficiency measurements and with scanning electron microscopy, respectively. Electric spectroscopy is used to explore the possible effects on the defect structure, more in particular on the dominant shallow acceptor. Admittance spectra exhibit a capacitance step to the geometric capacitance plateau at low temperature (5-60 K). Analyzing this capacitance step, we obtained a good estimate of the activation energy of the intrinsic defects that provide the p-type conductivity of the CIGS absorber. The measurements did not show a change in the nature of the dominant acceptor upon Sb treatment.

APJ is a G-protein-coupled receptor with seven transmembrane domains, and its endogenous ligand, apelin, was identified recently. They are highly expressed in the cardiovascular system, suggesting that APJ is important in the regulation of blood pressure. To investigate the physiological functions of APJ, we have generated mice lacking the gene encoding APJ. The base-line blood pressure of APJ-deficient mice is equivalent to that of wild-type mice in the steady state. The administration of apelin transiently decreased the blood pressure of wild-type mice and a hypertensive model animal, a spontaneously hypertensive rat. On the other hand, this hypotensive response to apelin was abolished in APJ-deficient mice. This apelin-induced response was inhibited by pretreatment with a nitric-oxide synthase inhibitor, and apelin-induced phosphorylation of endothelial nitric-oxide synthase in lung endothelial cells from APJ-deficient mice disappeared. In addition, APJ-deficient mice showed an increased vasopressor response to the most potent vasoconstrictor angiotensin II, and the base-line blood pressure of double mutant mice homozygous for both APJ and angiotensin-type 1a receptor was significantly elevated compared with that of angiotensin-type 1a receptor-deficient mice. These results demonstrate that APJ exerts the hypotensive effect in vivo and plays a counterregulatory role against the pressor action of angiotensin II.

Temperature-dependent Hall measurements have been performed on thin films of the ternary chalcopyrite ${\mathrm{CuGaSe}}_{2}.$ Unintentionally doped samples and Na-containing samples are compared, as well as epitaxial and polycrystalline ones. Acceptor activation energies and acceptor and donor densities are extracted. Activation energies as well as defect densities vary over a wide range. We demonstrate that all samples are dominated by the same defect with an activation energy of 150 meV in the infinite-dilution limit. It is shown that the degree of compensation increases with increasing acceptor density. Thus direct evidence of self-compensation by intrinsic defects is given. ${\mathrm{CuGaSe}}_{2}$ containing Na shows the same defects as ${\mathrm{CuGaSe}}_{2}$ without Na: thus, it can be excluded that the dominant effect of Na is the introduction of a new acceptor. In addition, reduced compensation due to Na is not found; the net doping increases in spite of an increased compensation.

The mechanism of charge carrier transport in stoichiometric polycrystalline CuGaSe/sub 2/ has been studied in the temperature range of 80-350 K using Hall effect and conductivity measurements. The layers were grown on soda-lime glass substrates by physical vapour deposition. At 300 K we found mobility values in the range of 10-20 cm/sup 2//Vs. A thermally activated behavior of the mobilities in these films was observed in the temperature range of 150-300 K. Intergrain potential barriers limiting the charge carrier transport were determined to vary between 60 and 130 meV. Using the grain boundary barrier trapping model developed for polycrystalline Si the density of charged states at the grain boundaries was calculated to be about 1.2e12 cm/sup -2/. This value does not depend on the net doping concentration of the samples significantly, suggesting that the position of the relevant defect states at the grain boundaries is above the Fermi level. Above 300 K the carrier transport is limited not only by potential barriers but also the in-grain mobility.

Cu-Ga-Se films with an orderd vacancy compound (OVC) structure were prepared at substrate temperature about 500 °C by thermal co-deposition. With a preparation under extremely Se excess condition, films of the OVC were synthesized within the compositional ratio of 0.73 ≤ [Ga]/([Cu]+[Ga]) ≤ 0.86 along Cu2Se-Ga2Se3 pseudo binary system. The growth on soda-lime glass substrates improves the crystallinity compared to that on alkali-free glass. An increase in the optical bandgaps of OVC films from 1.85 eV to 1.94 eV was observed with an increase in the Ga content of the films. The deposition of Cu and Se onto Ga2Se3 films resulted in a vertically inhomogeneous film: the bottom layer with the OVC structure and the top layer with the chalcopyrite structure. A solar cell using the CuGa5.0Se8.1 film within a ZnO/CdS/CuGa-Se/Mo/soda-lime glass substrate structure showed an open circuit voltage of 947 mV, an efficiency of 2.2 %, a short circuit current density of 4.5 mA/cm2, and a fill factor of 0.52 (Air Mass 1.5, 0.5 cm2, total area).

CuxGaySe2MOCVD and PVD grown films were structurally and optically characterized by Raman, Micro-Raman and photoluminescence spectroscopy. Defect related photoluminescence excitation with wavelengths varying across the material band gap reveals: a) in Cu-rich CuGaSe2films, three band edge splitting due to the spin-orbit interaction and the crystal field, and donor-acceptor pair recombination between a shallow donor and two different acceptor levels, and b) in Ga-rich CuGaSe2films, donor-acceptor pair transitions between quasi-continua of donor and acceptor levels related to potential fluctuations. Raman spectra of CuxGaySe2films, excited by laser light near and below the material band gap, show intense modes at 197cm-1, 187cm-1, and 277cm-1, which can be used as indicators of crystallinity and Ga-content of the films. Polarization- and angular- dependent micro-Raman spectra of MOCVD CuGaSe2indicate that CuxSey-crystallites, dispersed on the surface of Cu-rich films, are grown oriented with their c-axis perpendicular to the film surface.

Thin film solar cells with a Cu(In,Ga)Se2 (CIGS) absorber layer achieved efficiencies above 20%. In order to achieve such high performance the absorber layer of the device has to be doped with alkaline material. One possibility to incorporate alkaline material is a post deposition treatment (PDT), where a thin layer of NaF and/or KF is deposited onto the completely grown CIGS layer. In this paper we discuss the effects of PDT with different alkaline elements (Na and K) on the electronic properties of CIGS solar cells. We demonstrate that whereas Na is more effective in increasing the hole concentration in CIGS, K significantly improves the pn-junction quality. The beneficial role of K in improving the PV performance is attributed to reduced recombination at the CdS/CIGS interface, as revealed by temperature dependent J-V measurements, due to a stronger electronically inverted CIGS surface region. Computer simulations with the software SCAPS are used to verify this model. Furthermore, we show that PDT with either KF or NaF has also a distinct influence on other electronic properties of the device such as the position of the N1 signal in admittance spectroscopy and the roll-over of the J-V curve at low temperature. In view of the presented results we conclude that a model based on a secondary diode at the CIGS/Mo interface can best explain these features.

For solar cells based on CuGaSe2(CGS) absorber layers physical vapour deposition (PVD) was employed to deposit polycrystalline CGS thin films. Efficiencies up to 7.9% were achieved by applying a two-stage deposition process for absorber preparation. The high structural quality of the absorbers is shown. For highest cell efficiencies our standard recipe for the CdS buffer deposition had to be re-adapted. Voltage dependent spectral response measurements of the device in conjunction with absorption measurements of the absorber layers allow calculation of the minority carrier diffusion length, space charge region (SCR) width, absorber carrier concentration and the built-in voltage. The defect state density at the buffer/absorber interface is estimated. It can be shown that the benefical effect of the modified buffer deposition is mainly due to a reduced acceptor concentration at the absorber/buffer interface.

The interface between a Cu(In,Ga)Se2 (CIGS) and an underlying Mo layer was studied by X-ray diffraction and high resolution transmission electron microscopy. The CIGS layer was deposited onto Mo coated soda-lime glass using the “3-stage” process. A MoSe2 layer found to form at the CIGS/Mo interface during the 2nd stage of the “3-stage” process. The thickness of the MoSe2 layer depended on the substrate temperature used for CIGS film deposition as well as the Na content of the CIGS and/or Mo layers. For higher substrate temperatures, thicker MoSe2 layers were observed. The Na in the CIGS and/or Mo layer is felt to assist in the formation of MoSe2. Current-Voltage measurements of the heterojunction formed by the CIGS/Mo interface were ohmic even at low temperature. The role of the MoSe2 layer in high efficiency CIGS solar cells is discussed.

Doping the Cu(In,Ga)Se2 (CIGS) absorber layer with alkaline metals is necessary to process high efficiency solar cells. When growth of CIGS solar cells is performed on soda-lime glass (SLG), the alkaline elements naturally diffuse from the substrate into the absorber layer. On the other hand, when CIGS is grown on alkaline free substrates, the alkaline metals have to be added from another source. In the past, Na was believed to be the most important dopant of the alkaline elements, even though K was also observed to diffuse into CIGS from the SLG. Recently, the beneficial effect of a post deposition treatment with KF was pointed out and enabled the production of a 20.4% CIGS solar cell grown at low substrate temperature (

The importance of Cu for CdTe solar cell absorber doping has been increasingly recognized in recent years. Currently different models are being discussed how to understand the case of CuCd substitutional doping in polycrystalline CdTe solar cells. In this work an understanding is developed which is based on a low concentration deep acceptor doped CdTe layer (N a ~ 5 × 1014 cm 3 Ea ~ 300 meV above the valence band). Despite their non shallow nature Cu Cd acceptors are fully or at least heavily (gt;30) ionized. The low hole concentration in CdTe (~1 × 1014 cm 3) originates directly from low Cu solubility in CdTe bulk material and is not caused by partial ionization or compensation as proposed by earlier models. The three to four orders of magnitude difference between bulk acceptor concentration and average Cu concentration in polycrystalline CdTe is attributed to grain boundary segregation of Cu. Our model is derived from substrate and superstrate CdTe solar cell measurements controlled CdTe doping and quenching Hall Effect measurements of CdTe films numerical and analytical calculations and a broad literature survey. Based on these results routes to improve the conversion efficiency of CdTe solar cells are discussed. © 2013 AIP Publishing LLC.

Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.

Cu(In,Ga)Se2 (CIGS) surface was modified with Zn doping using vacuum evaporation. The substrate temperature of CIGS films and exposed time during the deposition was changed to control the distribution of Zn in the CIGS films. The diffusion of Zn in CIGS film was observed at substrate temperature of over 200°C. The solar cell fabricated using the CIGS film modified with Zn showed an efficiency of 13.9 %, an open circuit voltage of 0.576 V, a short circuit current density of 34.9 mA/cm2, and a fill factor of 0.692. The effects of the amount and distribution of Zn on the performance of CIGS solar cells are discussed.

Growth of Cu(In,Ga)Se2 (CIGS) thin films during the physical vapor deposition (PVD) were characterized by scanning Auger electron spectroscopy (SAES), secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The deposition processes are comparable with the 2nd and 3rd stages of the “3-stage” process. The phase changes observed in CIGS films during the 2nd stage of the “3-stage” process were as follows: (In,Ga)2Se3 → [Cu(In,Ga)5Se8] → Cu(In,Ga)3Se5 → Cu(In,Ga)Se2. The grain size increased from the sub-micron grains of the (In,Ga)2Se3 precursor film to several micrometers in the stoichiometric Cu(In,Ga)Se2 film. Growth mechanism of CIGS crystal are discussed on the basis of the phase diagram of the Cu2Se-In2Se3 pseudo-binary system, and crystal structures and stacking sequences of Se atomic layers of compounds in the In2Se3–Cu2Se system.

The microstructure of Cu(In,Ga)Se2 (CIGS) films deposited under low Se flux was studied using scanning electron microscopy, scanning Auger electron spectroscopy and high resolution and analytical electron microscopy. CIGS films were deposited on Mo coated soda-lime glass substrate using the “3-stage” process in which the Se flux used during the third stage was restricted to a forth of standard value. In the as-grown CIGS films, voids were observed along the grain boundaries and a Cu2Se phase was identified at the surface and the grain boundaries. The voids and Cu2Se layer were produced by vaporization of an In–Se compound from the films during the third stage of deposition. A reaction model on the CIGS grain surface is proposed based on the microstructure observations.

MoSe2 layers formed at the interface between Cu-In-Ga-Se and Mo layers were studied by X-ray diffraction and high resolution transmission electron microscopy. Various composition Cu-In-Ga-Se films such as Se, Cu-Se, In-Ga-Se and Cu-rich Cu-In-Ga-Se were deposited on Mo coated glass substrates by physical vapor deposition. For the case of the Se/Mo interface, a MoSe2 layer of about 100 Å thickness was observed. The c-axis of the MoSe2 grains were found to be oriented normal to the surface of Mo layer. For the Cu-Se/Mo and Cu-rich Cu-In-Ga-Se/Mo structures, the thickness of the MoSe2 layers found at the interface was thin. For the case of the In-Ga-Se/Mo structure, a 0.1 µm thick interfacial MoSe2 layer was observed whose c-axis was oriented parallel to the Mo surface. The microstructure of the In-Ga-Se/Mo film was similar to that of the device quality CIGS/Mo structure deposited by the “3-stage” process. A formation mechanism for the MoSe2 layers occurring during the “3-stage” process is proposed.

Sr x Ba1-x Nb2O6 (0.2 ≤x ≤0.4: SBN) ceramics were fabricated by pressureless sintering with the addition of V2O5 (1 mol%) which considerably accelerated grain growth. X-ray diffraction analysis of elongated grains indicated that there is a morphotropic phase boundary between x=0.25 (tetragonal) and x=0.225 (orthorhombic) in the SBN system. The sharpening of a ferroelectric phase transition peak was maximum at x=0.2. This corresponded to a cation distribution in which the A2-sites in the tungsten bronze structure were fully occupied by Ba2+ ions. A phase transition temperature diagram was developed for the present composition region.

Sr0.3Ba0.7Nb2O6 (SBN30) ceramics with ∼97% theoretical density were fabricated by pressureless sintering at 1400° C for 4 h. The sintered sample with homogeneous and small-grained (∼2 µ m) microstructure shows a broadened permittivity (ε r)-temperature (T) curve with ε max =1600 at T C≈175° C. The microstructure and ferroelectric properties of the SBN30 ceramics can be substantially modified by Nb2O5 or V2O5 additive (0.5 mol%). The Nb2O5-added sample with isotropic grains of ∼4 µ m has ε max =2400 at T C≈165° C. Considerable coarsening and elongation of grains occur in the V2O5-added sample which has a markedly sharpened ε r-T character with ε max =3500 at T C≈220° C and improved spontaneous polarization of 6.4 µ C/cm2.

Knowledge of the absorber bandgap is often needed for assessing the junction quality of a thin film solar cell, for example when computing the open-circuit voltage deficit. The bandgap is typically estimated from the routine measurement of the external quantum efficiency (EQE) of finished devices. However the extraction becomes ambiguous in the case of very thin absorbers, or in presence of bandgap gradients or collection issues of charge carriers. This work reviews several methods for the determination of the bandgap from EQE measurements and discusses their validity conditions. The numerical results are compared based on experimental EQE of several different thin film solar cells such as CuInGaSe2, CuInSe2, CdTe, CuZnSnSe and perovskite, and systematic trends are identified. Numerical simulations are also performed to illustrate the behavior of the different bandgap extraction methods in presence of a bandgap gradient and different magnitudes of the exponential tail states.

We report on the application of ZnxTiyO deposited by atomic layer deposition (ALD) as buffer layer in thin film Cu(In,Ga)Se2 (CIGS) solar cells to improve the photovoltaic device performance. State-of-the-art CIGS devices employ a CdS/ZnO layer stack sandwiched between the absorber layer and the front contact. Replacing the sputter deposited ZnO with ALD-ZnxTiyO allowed a reduction of the CdS layer thickness without adversely affecting open-circuit voltage (VOC). This leads to an increased photocurrent density with a device efficiency of up to 20.8% by minimizing the parasitic absorption losses commonly observed for CdS. ALD was chosen as method to deposit homogeneous layers of ZnxTiyO with varying Ti content with a [Ti]/([Ti] + [Zn]) atomic fraction up to ∼0.35 at a relatively low temperature of 373 K. The Ti content influenced the absorption behavior of the ZnxTiyO layer by increasing the optical bandgap >3.5 eV in the investigated range. Temperature-dependent current–voltage (I–V) measurements of solar cells were performed to investigate the photocurrent blocking behavior observed for high Ti content. Possible conduction band discontinuities introduced by ZnxTiyO are discussed based on X-ray photoelectron spectroscopy (XPS) measurements.