Analysis of Cu(In,Ga)(S,Se)2 thin-film solar cells by means of electron microscopy

The present work gives an overview of how electron microscopy and its related techniques are used to analyze individual layers and their interfaces in Cu(In,Ga)(S,Se)₂ thin-film solar cells. Imaging of samples can be performed at scales of down to the (sub)angstroms range. At similar spatial resolutions, information on composition can be gathered by means of energy-dispersive X-ray spectroscopy (EDX) and on spatial distributions of electrostatic Coulomb potentials in the specimen by applying electron holography. Microstructural and compositional properties as well as charge-carrier collection and radiative recombination behavior of the individual layers are accessible by use of electron backscatter diffraction, EDX, electron-beam-induced current (EBIC) and cathodoluminescence measurements, available in scanning electron microscopy. The present contribution gives an overview of the various scanning and transmission electron microscopy techniques applied on Cu(In,Ga)(S,Se)₂ thin-film solar cells, examples from case studies, and also demonstrates how these techniques may be combined in order to improve the analysis. Particularly, EBIC results show a reduced charge-carrier collection at Cu(In,Ga)Se₂ grain boundaries, while no indication was found for a charge accumulation at the grain boundaries by electron holography.     

Preparation of Cu(In,Ga)(S,Se)2 thin films by sequential evaporation and annealing in sulfur atmosphere

Cu(In,Ga)(S,Se)₂ thin films with high Ga/III ratio (around 0.8) were prepared by sequential evaporation from CuGaSe₂, CuInSe₂, In₂Se₃ and Ga₂Se₃ compounds and then annealing in H₂S gas atmosphere. The annealing temperature was varied from 400 to 500 °C. These samples were characterized by means of XRF, EPMA, XRD and SEM. The S/(S+Se) mole ratio in the thin films increased with increase in the annealing temperature, keeping the Cu, In and Ga contents nearly constant. The open circuit voltage increased and the short circuit current density decreased with increase in the annealing temperature. The best solar cell using Cu(In,Ga)(S,Se)₂ thin film with Ga/(In+Ga)=0.79 and S/(S+Se)=0.11 annealed at 400 °C demonstrated V_oc=535 mV, I_sc=13.3 mA/cm², FF=0.61 and efficiency=4.34% without AR-coating.     

Quantum efficiency and admittance spectroscopy on Cu(In,Ga)Se2 solar cells

We investigate the electronic transport properties of Cu(In,Ga)Se₂ solar cells by means of quantum efficiency and temperature dependent admittance spectroscopy. A simple evaluation scheme of quantum efficiency data is introduced which accounts for recombinatoric losses in the Us buffer layer and in the Cu(In,Ga)Se₂ absorber. By admittance spectroscopy, we find that the controlled incorporation of Na into the absorber material leads to a shallow acceptor state at about 75 meV above the valence band.     

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)₂/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se₂ (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H₂S---Ar mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)₂ absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with V_oc = 528 mV, J_sc = 39.9 mA/cm² and FF = 0.68 after the sulfurization process.     

In situ monitoring of the growth of Cu(In,Ga)Se2 thin films

Thin films of Cu(In,Ga)Se₂ were grown by co-evaporation process in a conventional vacuum coating system at the base pressure of 10⁻⁵ mbar. The controllable of the two-stage growth process was developed. During the growth process, substrate temperature, graphite heater temperature, heating output power and temperature of the CIGS surface were monitored simultaneously. In this setup, we use the change in the thermal behavior of the substrate due to the variations in emissivity of CIGS film during the transition of Cu-rich to Cu-poor in the second stage corresponding to the change of power fed into the substrate heater as the control signal. By observing the variation of control signals, the desired final composition of the film can be obtained. Using our device fabrication, Al/ZnO(Al)/CdS/CIGS/Mo/SLG solar cells with efficiency over 14% (without AR) were achieved.     

Lift-off process reducing crack formation and its Cu(In,Ga)Se2 thin film solar cell applications

To reduce cracks caused by the lift-off process in a Cu(In,Ga)Se₂ (CIGS) layer, we focused on increasing the transferred layer thickness. We investigated the relationship between crack formation and the transferred layer thickness which is controlled by a Mo back electrode thickness. We found that the cracks were reduced by increasing the back electrode thickness. We suggest that the dominant factor of the crack reduction is attributed to the increase of the film hardness by increasing the Mo back electrode thickness. Next, we applied this crack reduction method to the solar cell fabrication. From the comparison of the 0.2-μm-thick Au single and 0.2-μm-thick Au/1.6-μm-thick Mo stacked back electrode lift-off CIGS solar cells, we investigated advantages of our crack reduction method. The crack formation was reduced only for the stacked back electrode lift-off solar cell. From the spatial distribution evaluation of an external quantum efficiency (EQE), we found that the crack reduction leads to not only the increase of an average EQE but also the decrease of EQE dispersion. In the solar cell parameters, the stacked back electrode lift-off solar cell without cracks showed the short-circuit current density and fill factor higher than those of the single back electrode lift-off solar cell with cracks. As a result, the conversion efficiency improvement as high as approximately 1% (an absolute value) was obtained. Moreover, the stacked back electrode lift-off solar cell showed the diode parameters (the diode ideality factor, the saturation current density, and series resistance) better than those of the single back electrode lift-off solar cell in the dark current density-voltage characteristics. We concluded that this high fill factor was attributed to the better diode performance. We therefore found that the stacked back electrode structure was very effective for improving the solar cell performance using the lift-off process.     

XPS, TEM and NRA investigations of Zn(Se,OH)/Zn(OH)2 films on Cu(In,Ga)(S,Se)2 substrates for highly efficient solar cells

Structural and compositional properties of Zn(Se,OH)/Zn(OH)₂ buffer layers deposited by chemical bath deposition(CBD) on Cu(In,Ga)(S,Se)₂ (CIGSS) absorbers are investigated. Due to the aqueous nature of the CBD process, oxygen and hydrogen were incorporated into the ‘ZnSe’ buffer layer mainly in the form of Zn(OH)₂ as is shown by X-ray photoelectron spectroscopy and nuclear reaction analysis (NRA) measurements leading to the nomenclature ‘Zn(Se,OH)’. Prior to the deposition of Zn(Se,OH), a zinc treatment of the absorber was performed. During that treatment a layer mainly consisting of Zn(OH)₂ grew to a thickness of several nanometer. The whole buffer layer therefore consists of a Zn(Se,OH)/Zn(OH)₂ structure on CIGSS. Part of the Zn(OH)₂ in both layers (i.e. the Zn(Se,OH) and the Zn(OH)₂ layer) might be converted into ZnO during measurements or storage. Scanning electron microscopy pictures showed that a complete coverage of the absorber with the buffer layer was achieved. Transmission electron microscopy revealed the different regions of the buffer layer: An amorphous area (possibly Zn(OH)₂) and a partly nanocrystalline area, where lattice planes of ZnSe could be identified. Solar cell efficiencies of ZnO/Zn(Se,OH)/Zn(OH)₂/CIGSS devices exceed 14% (total area).     

Microdefects and point defects optically detected in Cu(In,Ga)Se2 thin film solar cells exposed to the damp and heating

Optically detected changes have been studied in Cu(In,Ga)Se₂ (CIGS) thin film solar cells, which were exposed to the damp and heat test by IEC 1215 international standard recommendations. High-resolution optical microscopic images at T=300 K and emission properties at T=20 K of ZnO/CdS/CIGS devices were characterized and compared to the tested non-incapsulated device. The near-gap photoluminescence peak at 1.191 eV for the baseline device drastically decreases after the test. Long wavelength emission bands at 1.13 and 1.07 eV, associated with optical transitions through defect levels in absorber, retain their intensity and spectral position. Microscopic surface morphology deteriorates after the test: appearance of micro-scale defects and reduction of optical reflectivity have been observed in blue-violet light and polarization with good contrast. A decrease of conversion efficiency of the exposed solar cell is caused by the degradation of upper wide-gap films and heteroboundary between CdS and CIGS.     

A superstrate solar cell based on In2(Se,S)3 and CuIn(Se,S)2 thin films fabricated by electrodeposition combined with annealing

Stacked thin films composed of In₂(Se,S)₃ and CuIn(Se,S)₂ layers were grown on a fluorine-doped tin oxide (FTO)-coated glass substrate using electrodeposition of the corresponding selenide (In₂Se₃ and CuInSe₂) precursors followed by annealing in H₂S flow (5% in Ar). Structural characterizations of both layers revealed that the resulting film quality strongly depended on annealing conditions of both CuIn(Se,S)₂ and In₂(Se,S)₃ layers: a compact and uniform film was obtained by annealing both layers at 400 °C. Performance of Au/CuIn(Se,S)₂/In₂(Se,S)₃/FTO superstrate-type solar cells also followed these structural characteristics, i.e., a preliminary conversion efficiency of 2.9% was obtained on the device based on 400 °C-annealed In₂(Se,S)₃ and CuIn(Se,S)₂ layers.     

效率为12.1%的Cu(In,Ga)Se2薄膜太阳电池

利用共蒸发的三步法制备了较高质量的四元化合物Cu(In，Ga)Se2(CIGS)薄膜，并采用Mo／CIGS／CdS／ZnO结构为基础做出转换效率超过10％的薄膜太阳电池，其最高转换效率达到12．1％(测试条件为：AM1．5，Global 1000W／m^2)。通过与国际最高水平的CIGS太阳电池各参数的比较，分析了我们所制备的CIGS太阳电池在工艺和物理方面存在的问题。     

Preparation and characterization of Cu(In,Ga)(Se,S)2 thin films from electrodeposited precursors for hydrogen production

Semiconducting Cu(In,Ga)(Se,S)₂ thin films were made from electrodeposited Cu(In,Ga)Se₂ precursors, followed by physical vapor deposition of In₂S₃, Ga, and Se. The bandgaps of these materials were found to be between 1.6 and 2.0 eV, which spans the optimal bandgap necessary for application for the top junction in photovoltaic multijunction devices and for unassisted water photolysis. These films were characterized by electron-probe microanalysis, scanning Auger spectroscopy, X-ray diffraction, and photocurrent spectroscopy.     

Pentanary chalcopyrite compounds without tetragonal deformation in the heptanary system Cu(Al,Ga,In)(S,Se,Te)2

The c/a ratios of the nine ternary Cu–III–VI₂ compounds in the system Cu(Al,Ga,In)(S,Se,Te)₂ range from 1.939 for CuAlSe₂ to 2.014 (CuInS₂) at room temperature. The validity of Vegard's law was presumed to determine the tetragonal deformation 1–c/(2a) for all pentanary mixed crystal compounds Cu–(III,III)–(VI–VI)₂. For six of these nine compounds it is possible to achieve zero tetragonal deformation by adjusting the correct cation and anion substitution. The calculations are performed for room temperature as well as for 500 °C, which is a typical temperature for the synthesis of thin films of these semiconductor compounds.     

The effect of NaF on Cu(In, Ga)Se2 thin film solar cells

This work investigates NaF, on Mo coated sodium barrier glass, as a sodium precursor for the growth of Cu(In, Ga)Se₂ for thin film solar cells. These precursor layers are investigated by X-ray photoelectron spectroscopy (XPS) before and after annealing, and after exposure to selenium. XPS is also performed on the Cu(In, Ga)Se₂ layer, deposited with NaF. The influence of the NaF on the absorber growth is studied by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). The electrical properties are investigated by cell fabrication and characterization. Cell results are comparable when NaF or out-diffusion of sodium from the soda lime glass are used.     

Fabrication of Cu(In,Ga)Se2 thin films solar cell by selenization process with Se vapor

CIGS films were prepared on Mo-coated glass by sputtering and selenization processes. The metallic precursors were selenized under higher pressure in selenium vapor instead of H₂Se. In order to improve the performance of CIGS thin film solar cells, the morphologies of CIGS thin films were studied carefully by various temperature profiles. The relationship between temperature decrease rate and fill factor (FF) of solar cells was investigated thoroughly. On the other hand the value of open circuit voltage (V_oc) was improved by increasing the gallium content near the surface of CIGS thin film. A glass/Mo/CIGS/CdS/ZnO cell was fabricated and the conversion efficiency of 9.4% was obtained without antireflective film.     

Formation of CuIn(S,Se)2 thin film by thermal diffusion of sulfur and selenium vapours into Cu–In alloy within a closed graphite container

The formation of CuIn(S,Se)₂ thin films by thermal diffusion of sulfur (S) and selenium (Se) vapours into co-sputtered Cu–In alloy within a closed-space graphite container is reported. All films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Four-point-probe and hot-probe measurements. Cu–In alloy films with composition varying from Cu-rich to In-rich were deposited. The synthesized In-rich films yielded CuIn₅(S,Se)₈ spinel compound which gradually transformed into a single phase CuIn(S,Se)₂ as the film composition approached the Cu-rich region. The morphology of the CuIn₅(S,Se)₈ was found to differ from the stoichiometric and Cu-rich CuIn(S,Se)₂ as observed from SEM. EDX composition analysis of the films showed a Cu/In ratio varying from 0.36 to 1.54 and a (S+Se)/(Cu+In) varying from 0.97 to 1.32. The amount of S incorporated in the films was found to differ with changes in the composition. The resistivity of the films ranged between 10⁻¹ and 10⁷ Ω cm and it strongly followed the change in the alloy film composition.     

New developments in Cu(In,Ga)(S, Se)2 thin film modules formed by rapid thermal processing of stacked elemental layers

A second-generation process for high-efficiency large-area Cu(In,Ga)(Se,S)₂ thin film (CIGSSe) solar modules has been developed applying controlled sodium doping and rapid thermal processing for absorber formation. The pilot line delivers aperture area efficiencies of 12.2%±0.5% (average) for 30×30 cm² modules and a certified champion efficiency of 13.1% for an unencapsulated 60×90 cm² demonstrator module. The stability of the frameless pilot line modules with a low-cost package against humidity is confirmed externally by passing the damp heat test sequence according to IEC 61646. Substitution of CBD-CdS by CBD-Zn(S, OH) buffer layers yields efficiencies up to 12% on 30×30 cm² circuits. First CIGSSe-cells on flexible substrates were also developed applying 30 μm thin titanium foils, 8×8 cm² in size. Average cell efficiencies on a substrate up to 12.4% were achieved.     

Near-band-edge photoluminescnce in Cu(In,Ga)Se2 solar cells

Photoluminescence (PL) and PL decay characteristics of the near-band-edge (NBE) PL at room temperature have been studied on the Cu(In,Ga)Se₂ (CIGS) solar cells. The carrier recombination process has been discussed with emphasis on the photovoltaic properties of the solar cell. It has been found that: (i) PL intensity of the CIGS solar cells is much stronger than that in the corresponding CIGS thin films, (ii) the PL decay time of the cell is longer than that of the CIGS film, and (iii) the PL decay time of the CIGS solar cell exhibits strong dependence on the PL excitation intensity. In the CIGS solar cell, intense PL is obtained under the open circuit condition (oc), in contrast to the very low PL yield under the short circuit (sc) condition. The PL decay time under the sc condition is much shorter than that under the oc condition. Excitation intensity dependence of PL intensity and the PL decay time have been studied, and they are discussed with relation to the photo-voltage due to the PL excitation light. PL and injection EL under the external DC bias have been studied. The mapping image of NBE-PL intensity has been compared with that of the laser beam induced current (LBIC), and the PL intensity image reflects the photovoltaic properties of the CIGS solar cells. We demonstrated that NBE-PL of the CIGS solar cell reflects the photovoltaic effect, and it can be utilized as a powerful characterization method.     

Fabrication of pentanary Cu(InGa)(SeS)2 absorbers by selenization and sulfurization

The objective of this study is to find the key factors to improve V_oc. In this study, pentanary Cu(InGa)(SeS)₂ absorbers were prepared by selenization and sulfurization or a sulfurization after selenization (SAS) method. It is found that the “sulfurization degree” defined as a function of temperature and holding time at the sulfurization step is a key factor to enhance the Ga diffusion and improve V_oc. It is also verified that increase in the temperature difference between selenization and sulfurization enhances the incorporation of S into the selenide absorber. Applying these findings related to Ga and S, V_oc of 642 mV/cell and efficiency of 14.3% are achieved on a 30 cm×30 cm-sized soda-lime glass substrate.     

Band-gap engineering in CuIn(Se,S)2 absorbers for solar cells

Thin films based on CuInSe₂ have become very successful as absorber layers for solar cells. It is only in the recent past that gallium (Ga) and sulfur (S) were incorporated into CuInSe₂ in order to increase the energy band gap of the film to an optimum value with the ultimate aim of producing more efficient devices. This paper focuses on the incorporation of S into partly selenized CuInSe₂ films in order to produce CuIn(Se,S)₂ films with varying S/Se+S ratios, resulting in different band-gap energies. This was achieved by varying the conditions when selenizing Cu/In alloys in H₂Se/Ar, and then exposing these various partly selenized films to H₂S/Ar under identical conditions.     

Cu(In,Ga)Se2 solar cells on stainless steel substrates covered with insulating layers

We have developed the flexible Cu(In,Ga)Se₂ (CIGS) solar cells on the stainless steel substrates with the insulating layer for the fabrication of the integrated module. The CIGS films have strong adhesion to the Mo films with insulating layers. An efficiency of 12.3% was achieved by the flexible CIGS solar cell with a structure of ITO/ZnO/CdS/CIGS/Mo/SiO₂/stainless steel. The insertion of the SiO₂ insulating layer did not have an influence on the formation of the CIGS film and solar cell performances.