Characterisation of ultrasonically sprayed InxSy buffer layers for Cu(In,Ga)Se2 solar cells

In order to replace chemical bath deposited (CBD) CdS buffer layers in Cu(In,Ga)Se₂ (CIGS) solar cells by an alternative material, In_xS_y thin-film buffer layers were prepared by ultrasonic spray pyrolysis at various substrate temperatures. X-ray Diffraction measurements confirmed that the films contained primarily the tetragonal In₂S₃ phase. X-ray Photoelectron Spectroscopy measurements revealed a small concentration of chlorine impurity throughout the In_xS_y layer. By depositing the indium sulphide layer as buffer layer in the CIGS solar cell configuration, a maximum solar cell efficiency of 8.9% was achieved, whilst the reference cell with CdS/CIGS on a similar absorber exhibited 12.7% efficiency. Additionally, light soaking enhanced the efficiency of In_xS_y/CIGS cells primarily by improvements in fill factor and open circuit voltage.     

Thermal stability of sputtered Mo/polyimide films and formation of MoSe2 and MoS2 layers for application in flexible Cu(In,Ga)(Se,S)2 based solar cells

Molybdenum (Mo) films with a thickness of about 800 nm were room temperature sputtered onto flexible polymeric substrates. Upilex® films were chosen as substrates on the basis of their high thermal endurance and reduced coefficient of thermal expansion. Thermal stability of Mo films has been proved by heat treatment of the Mo/Upilex® structures at a temperature comparable to that used in the preparation of the Cu(In,Ga)(Se,S)₂ absorber layer. A combination of high optical reflectance (maximum values of 75-80%), low electrical resistivity (about 30 μΩ cm) and a smooth surface free of cracks for heated films highlights their good thermal stability. The formation of MoSe₂ and MoS₂ layers, after selenization/sulfurization of the Mo/Upilex® structures, has been further investigated in view of their application as back contact layers in flexible CIGS based solar cells.     

Influence of sodium compounds at the Cu(In,Ga)Se2/(PVD)In2S3 interface on solar cell properties

The present contribution deals with indium sulfide buffer layers grown by thermal co-evaporation of elemental indium and sulfur. It has been found necessary to deposit these buffer layers at low substrate temperatures in order to reach V_oc values similar to those with (CBD)CdS. However, such deposition conditions lead to the formation of a highly recombinative Cu(In,Ga)Se₂/indium sulfide interface. This behaviour may be associated to the presence of sodium carbonates/oxides at the interface even though the Cu(In,Ga)Se₂ surface was cleaned in NH₃ (1 M, room temperature) prior to the indium sulfide deposition. An explanation is that, despite the chemical etch, sodium carbonates/oxides remain in the air exposed Cu(In,Ga)Se₂ grain boundaries and can migrate towards the surface when the Cu(In,Ga)Se₂ is heated under vacuum. These polluted interface areas act as recombination zones and thus inferior devices. A possibility to improve the device performance (i.e. improve the interface quality) is to sulfurize the remaining sodium carbonates/oxides. The resulting Na₂S can then leave the interface by formation of a solid solution with the indium sulfide. By adapting the buffer layer deposition process, 13.3% efficiency devices with co-evaporated indium sulfide are realized, performance which is close to that reached with (CBD)CdS.     

Alternative sodium sources for Cu(In,Ga)Se2 thin-film solar cells on flexible substrates

Sodium (Na) is an important doping element for Cu(In,Ga)Se₂ (CIGS) solar cells. However, when using Na-free flexible substrates like steel foil or polyimide film, it is necessary to ensure an efficient supply of sodium to achieve high cell efficiencies. The common incorporation methods for Na on these Na-free substrates are either to deposit a Na-containing precursor layer (e.g. NaF) onto the molybdenum (Mo) back contact prior to CIGS growth or to coevaporate a Na compound during CIGS growth. Another way is to incorporate sodium after CIGS growth by a post-deposition treatment with NaF. In this work, we tested two alternative Na doping methods which are well suited for a production line due to their easy controllability. One approach is to dope the molybdenum target with Na. With Na-doped Mo layers (Mo:Na) as the back contact, we could achieve efficiencies of 13.1% both on titanium (Ti) and stainless Cr steel foil using a single-stage inline CIGS process. With a low-temperature single-stage CIGS process on polyimide (PI) we reached an efficiency of 11.2% using a Mo:Na back contact. Another doping method involves sol-gel-deposited silicon oxide layers which contain Na (SiO_x:Na). We have successfully deposited these sol-gel layers onto stainless steel foil by a roll-to-roll (R2R) method with short annealing times as needed in production. With these SiO_x:Na layers we could achieve efficiencies of 13.7% on stainless steel foil and 11.5% on mild steel sheet using a single-stage inline CIGS process.     

Optimization of ALD-(Zn,Mg)O buffer layers and (Zn,Mg)O/Cu(In,Ga)Se2 interfaces for thin film solar cells

(Zn,Mg)O films, fabricated by atomic layer deposition, ALD, are investigated as buffer layers in Cu(In,Ga)Se₂-based thin film solar cells. Optimization of the buffer layer is performed in terms of thickness, deposition temperature and composition. High efficiency devices are obtained for deposition at 105-135 °C, whereas losses in open circuit voltage are observed at higher deposition temperatures. The optimal compositional region for (Zn,Mg)O buffer layers in this study is for Mg/(Zn + Mg) contents of about 0.1-0.2, giving band gap values of 3.5-3.7 eV. These devices appear insensitive to thickness variations between 80 and 600 nm. Efficiencies of up to 16.2% are obtained for completely Cd- and S-free devices with (Zn,Mg)O buffer layers deposited with 1000 cycles at 120 °C and having a band gap of 3.6 eV.     

Comparative study of Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS heterojunction based solar cells by admittance spectroscopy, current-voltage and spectral response measurements

Co-evaporated Cu(In,Ga)Se₂ (CIGSe) based solar cells with Physical Vapour Deposited (PVD) Indium Sulphide (In₂S₃) as buffer layer have been studied by admittance spectroscopy and current-voltage characteristics measurements. The results have been compared to those obtained with a reference CBD-CdS/CIGSe device. In darkness, the PVD-In₂S₃ buffer layer devices exhibit higher densities of trapping defects and low values of shunt resistance. However, under illumination we have observed an important improvement of the In₂S₃/CIGSe electronic transport properties. This behavior seems to be linked to the presence of a metastable defect with activation energy of 0.3 eV.     

Effect of copper-deficiency on multi-stage co-evaporated Cu(In,Ga)S2 absorber layers and solar cells

Solar cell absorber films of Cu(In,Ga)S₂ have been fabricated by multi-stage co-evaporation resulting in compositional ratios [Cu]/([In] + [Ga]) = 0.93-0.99 and [Ga]/([In] + [Ga]) = 0.15. Intentional doping is provided by sodium supplied from NaF precursor layers of different thicknesses. Phases, structure and morphology of the resulting films are investigated by X-ray diffraction (XRD) and scanning electron microscopy. The XRD patterns show CuIn₅S₈ thiospinel formation predominantly at the surface in order to accommodate decreasing Cu content. Correlated with the CuIn₅S₈ formation, a Ga-enrichment of the chalcopyrite phase is seen at the surface. Since no CuS layer is present on the as-deposited films, functioning solar cells with CdS buffer and ZnO window layers were fabricated without KCN etch. The open-circuit voltage of solar cells correlates with the copper content and with the amount of sodium supplied. The highest efficiency cell (open-circuit voltage 738 mV, short-circuit current 19.3 mA/cm², fill factor 65%, efficiency 9.3%) is based on the absorber with the least Cu deficiency, [Cu]/([In] + [Ga]) = 0.99. The activation energy of the diode saturation current density of such a cell is extracted from temperature- and illumination-dependent current-voltage measurements. A value of 1.04 eV, less than the band gap, suggests the heterojunction interface as the dominant recombination zone, just as in cells based on Cu-rich grown Cu(In,Ga)S₂.     

Structural and chemical analyses of sputtered InxSy buffer layers in Cu(In,Ga)Se2 thin-film solar cells

Sputtered In_xS_y layers deposited on borosilicate glass and Si at substrate temperatures ranging from about 60 °C to 340 °C were analyzed by means of X-ray diffraction, energy-dispersive X-ray spectrometry, and optical transmission and reflection measurements. With increasing substrate temperature, the In_xS_y layers exhibit increasing sulfur concentration and also increasing absorption-edge energies. In_xS_y layers on Cu(In,Ga)Se₂(CIGS)/Mo/glass stacks were additionally studied by scanning and transmission electron microscopy. With increasing substrate temperature, Cu, Ga, and In interdiffusion between CIGS and In_xS_y becomes more enhanced. At 340 °C, CuIn₅S₈ forms instead of In_xS_y. The CuIn₅S₈ formation at elevated temperatures may be the reason for the very low efficiency of solar cells with indium sulfide buffers deposited at temperatures above about 250 °C by various techniques.     

Optical properties of Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4

The optical properties of CuInSe₂, CuGaSe₂, Cu₂ZnSnS₄, and Cu₂ZnSnSe₄ are investigated using three different first-principles methods, namely the generalized gradient approximation by Perdew, Burke, and Ernzerhof (PBE), the hybrid Hartree-Fock-like functional by Heyd, Scuseria, and Ernzerhof (HSE), and a Green's function approach (GW). The density-of-states, the complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω), and the optical absorption coefficient α(ω) are determined, providing fundamental understanding of these materials. We find that even though the PBE method generates fairly accurate effective crystal potentials, the HSE and GW methods improve considerably the band-gap energies E_g and also the localization of the semicore states, thereby describing the optical properties much better. Furthermore, we also present optimized convergence parameters for the self-consistent HSE calculation in order to reduce the computational time of this orbital-dependent method.     

Influence of absorber copper concentration on the Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS based solar cells performance

The present contribution deals with the influence of the copper concentration in Cu(In,Ga)Se₂ (CIGSe) on the solar cells based on CIGSe/(PVD)In₂S₃ and CIGSe/(CBD)CdS. We find that, depending on the buffer layer, the optimum open circuit voltage (Voc) is not reached for the same copper concentration. The values of Voc for the CIGSe/(CBD)CdS solar cells are higher when the copper content is very close to stoichiometry (25%), whereas, the Voc values for CIGSe/(PVD)In₂S₃ solar cells attain their maximum for lower copper contents. On the other hand, contrary to the case of the (CBD)CdS buffer, the Jsc is strongly hindered for the (PVD)In₂S₃ buffered cells when the copper content is lowered. The study has been made for different absorber gallium contents and the evolution is coherent with the presence of a cliff at the CIGSe/(PVD)In₂S₃ interface.     

Investigation of Cu(In,Ga)Se2/In2S3 diffuse interface by Raman scattering

The CIGSe/In₂S₃ interface is known to be highly diffuse because of the migration of Cu from the CIGSe into the In₂S₃. Most of the analytical techniques allowing the determination of composition profiles throughout this interface involve ion etching either during the samples preparation or during data acquisitions. In the present work, we have explored the potential of the Raman scattering for the characterization of such interfaces. This technique is non destructive and provides information on both the composition and the structure of the materials that are probed. Three CIGSe/In₂S₃ structures have been investigated; the parameter varying being the substrate temperature during the In₂S₃ deposition. For the first time we could demonstrate that at high temperature, the CuInS₂ Cu-Au phase is formed at the CIGSe/In₂S₃ interface. Furthermore, the thickness of the ordered defect compound at the CIGSe surface increases with the deposition temperature. All of the new knowledge collected during this work shows the relevance of using the Raman scattering technique for the characterization of the CIGSe/In₂S₃ interface.     

Fabrication and characterisation of Cu(In,Ga)Se2 solar cells on polyimide

Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se₂/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se₂ (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E_g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed.     

Time-resolved photoluminescence in Cu(In,Ga)Se2 thin films and solar cells

Room temperature time-resolved photoluminescence (TR-PL) measurements have been performed on Cu(In,Ga)Se₂ (CIGS) thin films and solar cells to clarify the recombination process of the photo-generated minority carrier. Both films and solar cells exhibited PL decay curves composed of the dominant fast (0.7-2 ns) and weak slow (3-10 ns) exponential decay curves. PL lifetime of the cell is longer than that of the thin films, indicating the longer minority carrier lifetime for the hetero-structures than in thin films. The increase of PL lifetime is consistent with the enhancement of the PL intensity and the elimination of defect-related PL as a result of the solar cell formation. These results are discussed in terms of the recombination process of carriers in films and hetero-structures. The relationship between the PL lifetime of the CIGS solar cells and the cell conversion efficiency is described.     

Analysis of electrical properties of CIGSSe and Cd-free buffer CIGSSe solar cells

We investigated the influence of different buffer layers to the electrical parameters (J_sc, V_oc, QE and efficiency η) of solar cells. The cells with an In₂S₃ and a ZnMgO buffer layer were compared with a reference cell with a CdS buffer layer. We performed temperature and light dependent current-voltage measurements, temperature dependent capacitance measurements and quantum efficiency measurements.The cells with In₂S₃ and ZnMgO buffers differ not too much in J_sc, but they do differ in V_oc and their electrical properties — fill factor FF, diode saturation current J₀ and efficiency η. They also do differ in their spectral response, both at short and long wavelengths, and in their ideality factor. This indicates a different current transport mechanism. The device simulation program SCAPS is used for further interpretation of the measurements. After exploring the parameters we found an acceptable agreement between simulated and measured J-V and QE(λ) curves. The simulated QE curves fit well over the whole spectrum, except for the CdS buffer cell, where there is an overestimation for the intermediate wavelengths. Because of this the simulated J_sc is higher than the measured one. The simulated V_oc agrees well for all cells. For the ZnMgO buffer cell it was necessary to include a buried homo-junction.     

Cu(In,Ga)Se2 solar cells with double layered buffers grown by chemical bath deposition

In based mixture In_x(OH,S)_y buffer layers deposited by chemical bath deposition technique are a viable alternative to the traditional cadmium sulfide buffer layer in thin film solar cells. We report on the results of manipulating the absorber/buffer interface between the chalcopyrite Cu(In,Ga)Se₂ (CIGS) absorber and CdS or ZnS buffer by addition of a thin In based mixture layer. It is shown that the presence of thin In_x(OH,S)_y at the CIGS absorber/CdS or ZnS buffer interfaces greatly improve the solar cell performances. The performances of CIGS cells using dual buffer layers composed of In_x(OH,S)_y/CdS or In_x(OH,S)_y/ZnS increased by 22.4% and 51.6%, as compared to the single and standard CdS or ZnS buffered cells, respectively.     

The performance of CuInSe2 monograin layer solar cells with variable indium content

The recent results on the characterization of CuInSe₂ monograin layer solar cells are presented. The influence of different Cu/In ratio on solar cell characteristics was studied. It was determined that device-quality CuInSe₂ monograin powder could be grown from CuIn precursor alloys with a composition between 0.9 < Cu/In < 1. For absorber material, these powders were post-treated in sulphur vapour. The cells based on these absorbers showed efficiencies up to 9.5%. However, the quantum efficiency measurements revealed a significant loss in the long-wavelength range of photons λ > 800 nm. The derivative of QE with respect to wavelength showed two peaks at the energy values of ∼ 1.4 eV and ∼ 1.03 eV, which proves that these sulphurised absorber materials consist of two different phases.     

Interfacial chemistry control in thin film solar cells based on electrodeposited CuIn(S,Se)2

In this work, we present a study on CuIn(S,Se)₂ absorbers prepared by electrodeposition followed by rapid thermal annealing promising to lower manufacturing cost. However the annealed material contains copper sulpho-selenide of Cu(S_y,Se_1 − y) type which is harmful for the electrical properties of photovoltaic devices. These phases are removed by a cyanide etching. Because of an intrinsic variability of absorber fabrication process, the presented survey is based on statistic approach. We highlighted the influence of a cyanide treatment on surface and bulk compositions. The surface composition follows a distribution according to a Cu(S,Se)-CuIn(S,Se)₂ system and the bulk composition agrees with Cu(S,Se)₂-CuIn₃(S,Se)₅ system. Moreover, surface composition can be modified by adjusting the cyanide concentrations of etching solution without any changes in the bulk one. It ensues that Cu(S,Se) is not only present on the surface but also in the bulk of samples.     

Structural properties of Cu(In,Ga)Se2 thin films prepared from chemically processed precursor layers

We have developed a chemical process for incorporating copper into indium gallium selenide layers with the goal of creating a precursor structure for the formation of copper indium gallium diselenide (CIGS) photovoltaic absorbers. Stylus profilometry, EDX, Raman spectroscopy, XRD and SIMS measurements show that when indium gallium selenide layers are immersed in a hot copper chloride solution, copper is incorporated as copper selenide with no increase in the thickness of the layers. Further measurements show that annealing this precursor structure in the presence of selenium results in the formation of CIGS and that the supply of selenium during the annealing process has a strong effect on the morphology and preferred orientation of these layers. When the supply of Se during annealing begins only once the substrate temperature reaches ≈ 400 °C, the resulting CIGS layers are smoother and have more pronounced preferred orientation than when Se is supplied throughout the entire annealing process.     

Characterisation of Cu(In,Ga)Se2-based thin film solar cells on polyimide

Thin films of Cu(In,Ga)Se₂ (CIGS) were deposited at temperatures below 450 °C on polyimide (PI) substrates coated with Mo in a roll-to-roll set up by a combination of co-evaporation and ion-beam techniques. Flexible solar cells ITO/i-ZnO/CdS/CIGS/Mo/PI with and without Na incorporation were then fabricated. The films and solar cells were examined by: X-ray fluorescence spectroscopy (XRF) and Auger electron spectroscopy (AES), to determine the elemental composition, as well as by X-ray diffraction for structure- and scanning electron microscopy (SEM) for morphology-analysis. Photoluminescence (PL) and PL-excitation (PLE) at temperatures from 4.2 to 78 K were also used to estimate the band-gap energy of CIGS, examine the electronic properties and defect nature. The aim of this study was to correlate the incorporation of Na with optical and structural parameters of the CIGS layers as well as with the solar cell performance.     

Electrodeposition based synthesis of S-rich CuIn(S,Se)2 layers for photovoltaic applications: Raman scattering analysis of electrodeposited CuInSe2 precursors

Single step electrodeposition (ED) of Se-rich CuInSe₂ precursors, followed by RTP annealing under sulphurising conditions leading to S-rich CuIn(S,Se)₂ films, constitutes a promising technology for low cost high efficiency solar cells. In this work, a Raman scattering (RS) analysis of Se rich precursors grown under ED conditions leading to different chemical compositions is reported. RS has allowed identification of the main secondary phases in these layers with elemental Se, Cu-Se binary and ordered vacancy compound (OVC) phases. The experimental data show a strong dependence of the spectral contributions related to Se and Cu-Se with the layer molecularity, and the formation of these phases is mainly determined by the content of excess Se in the layers. The correlation of these data with the characteristics of the solar cells fabricated with these precursors, shows the strong impact of the presence of the Cu-Se phase on the performance of the final devices. These results point out the key role played by this binary phase on the formation of secondary phases after the sulphurising step.