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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.