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.     

Performance of CuIn1 − xGaxSe2/(PVD)In2S3 solar cells versus gallium content

The present work studies the influence of the Ga content (x = Ga / (Ga + In)) in the absorber on the solar cell performance for devices using (PVD)In₂S₃-based buffers. Input to the hypothesis of the relative conduction band positions can be found in the evolution of the device parameters with x. For experiments with x between 0 and 0.5 devices using (PVD)In₂S₃-based buffers are compared to reference devices using (CBD)CdS. Both buffers give similar cell characteristics for narrow band gap absorbers, typically E_gCIGSe < 1.1 eV. However, the parameters of the cells buffered with (PVD)In₂S₃ are degraded when the absorber gap is widened whereas (CBD)CdS reference devices are only slightly affected. Consequently, the solar cell efficiency is similar for both buffer layers at the lower x values and increases with x only in the case of (CBD)CdS. These evolutions are coherent with the existence of a conduction band cliff at the CIGSe/(PVD)In₂S₃ interface.     

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.     

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.     

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.     

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.     

Ultrasonically sprayed indium sulfide buffer layers for Cu(In,Ga)(S,Se)2 thin-film solar cells

Indium sulfide layers were grown by an ultrasonic spray pyrolysis method for application in Cu(In,Ga)(S,Se)₂ solar cells. X-ray diffraction measurements of layers on soda lime glass showed polycrystalline In₂S₃ with preferential orientation along the [103] direction and X-ray photoelectron spectroscopy revealed presence or absence of oxygen and chlorine impurities depending on the composition of the spray solution. For more quantitative chemical composition measurements In₂S₃ layers were sprayed on silicon substrates and analyzed with Rutherford backscattering spectrometry. The structural and chemical information on the In₂S₃ layer sprayed with different sulfur concentrations in the chemical precursor solution are correlated to the photovoltaic performance of solar cells. Best cell efficiency of 12.4% was achieved with an ultrasonically sprayed In₂S₃ buffer layer on a Cu(In,Ga)(S,Se)₂ absorber.     

Fast Cu(In,Ga)Se2 precursor growth: Impact on solar cell

In order to reduce the co-evaporation time of Cu(In,Ga)Se₂ (CIGSe) thin film absorber, a sequential approach has been investigated. CIGSe layers have been grown using the three-step based CUPRO (Cu-Poor/Rich/Off) process at substrate temperature of 600 and 500 °C. The first step consists in the growth of Cu-poor ([Cu]/[In + Ga] = 0.9) precursor layers. This paper aims at investigating the impact of this layer deposition duration on the CIGSe and respective solar cell properties. It is observed that for the two substrate temperatures investigated, the morphological and structural properties of the CIGSe layers do not change with increasing precursor deposition speed, even when it is increased by ten. Furthermore, the respective device performance also appears not affected by this reduction of the precursor growth time; all cells demonstrate 15% efficiency. From this work, the duration of our standard deposition process could be decreased from 23 to 14 min without performance loss independently of the substrate temperature.     

Flexible Cu(In,Ga)Se2 on Al foils and the effects of Al during chemical bath deposition

Cu(In,Ga)Se₂ (CIGS) solar cells on aluminum foils offer the advantage to be flexible, lightweight and, because of the low cost substrate, can be used for several applications, especially in buildings, where aluminum is already commonly used. There are reports of a-Si solar cells on Al foil, but to our knowledge development of CIGS solar cells on Al foils has not been reported. We have developed CIGS solar cells on coated Al-foil samples. When using Al as substrate, CIGS layers of suitable structural and opto-electronic properties should be grown at low (< 450 °C) deposition temperatures, because of the difference in the thermo-physical properties of layers and substrates. We have grown CIGS layers by evaporation of elemental Cu, In, Ga, and Se at different substrate temperatures and investigated the properties of these CIGS layers by different methods (SEM, SIMS, and EDX). The photovoltaic properties of small area solar cells were characterized with I-V and quantum efficiency measurements. An efficiency of 6.6% has been achieved. We have also observed that some Al from the foil dissolves during chemical bath deposition (CBD) of CdS. The presence of Al in the bath seems, in some cases, to be beneficial for the electrical properties of the CIGS solar cells. Thinner and more homogenous CdS layers are obtained. Elastic Recoil Detection Analysis (ERDA) and SIMS measurements have shown incorporation of Al in the CdS.     

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.     

Study of CuInS2/buffer/ZnO solar cells, with chemically deposited ZnS-In2S3buffer layers

Thin film solar cells based on CuInS₂/buffer/ZnO have been prepared with different buffer layers of mixed ZnS-In₂S₃ composition. The buffer films are grown by chemical bath deposition (CBD) from acidic solutions of InCl₃, ZnSO₄ and thioacetamide (TA). A kinetics study of the growth of the buffer layers is carried out with the quartz crystal microbalance. The influence of bath conditions (solution, temperature and composition) on the growth rate is studied. Films are obtained with different physical, chemical and morphological properties. The absorption coefficient spectra of the films show a variation depending on the CBD conditions, with absorption edges between 2.6 and 3.35 eV. Surface morphology of the films, observed with scanning electron microscopy, reveals that the presence of Zn produces characteristic structures with microtubular form, compared with pure In₂S₃ buffer films. Solar cell results show a significant increase of Voc and FF upon introducing Zn²⁺ ion in the buffer layer.     

Evaluation of electron beam induced current profiles of Cu(In,Ga)Se2 solar cells with different Ga-contents

The measurement of electron beam induced current profiles in junction configuration (JEBIC) is a settled method for several semiconductor devices. We discuss the JEBIC method in the light of the special conditions present in the case of thin film Cu(In,Ga)Se₂ solar cells.Our previously published results indicate that the charge state of defects close to or at the Cu(In,Ga)Se₂/CdS interface depends on the minority carrier distribution, which changes strongly during a scan of the cross section with an electron beam. The charge distribution influences the electrostatic potential and therewith the collection of minority carriers.Here, we present an evaluation method of JEBIC profiles that accounts for this effect. Monte Carlo simulations of the carrier generation help us to consider in detail the influence of surface recombination. We determine the diffusion length, space charge width, surface- and back contact recombination velocity of Cu(In_(1-r),Ga_r)Se₂ devices with different Ga-contents r from JEBIC line scans.     

Near-interface doping by ion implantation in Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se₂ absorber layers were implanted with 20 keV Cd ions in order to investigate the influence of changes in the near-interface doping profile. Modifications in this region are shown by AMPS-1D simulations to have substantial impact on solar cell properties. Ion implantation and subsequent thermal annealing steps were monitored by SIMS measurements to control the thermal diffusion of the dopant. Solar cells both with and without CdS buffer layer were made from the implanted absorbers and characterized by j-V and EQE measurements. These experimental results in conjunction with simulations of the quantum efficiency show that a well-defined type-inversion of the implanted layer can be achieved by low-energy ion implantation.     

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.     

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.     

Cu2ZnSnS4 solar cell prepared entirely by non-vacuum processes

Cu₂ZnSnS₄ (CZTS) solar cell with superstrate structure of fluorine-doped tin oxide glass/TiO₂/In₂S₃/CZTS/Carbon was prepared entirely by non-vacuum processes. The compact TiO₂ window and In₂S₃ buffer layers, CZTS absorber layer and Carbon electrode layer were prepared by spray pyrolysis method, ball milling and screen printing combination processes and screen printing process, respectively. The short-circuit current density, open-circuit voltage, fill factor and conversion efficiency of the best fabricated solar cell are 8.76 mA/cm², 250 mV, 0.27 and 0.6%, respectively. The fabrication process for the CZTS solar cell did not employ any vacuum conditions or high-toxic materials (such as CdS, H₂Se, H₂S or Se).     

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.     

Optical and electrical properties of electron-irradiated Cu(In,Ga)Se2 solar cells

The optical and electrical properties of electron-irradiated Cu(In,Ga)Se₂ (CIGS) solar cells and the thin films that composed the CIGS solar cell structure were investigated. The transmittance of indium tin oxide (ITO), ZnO:Al, ZnO:Ga, undoped ZnO, and CdS thin films did not change for a fluence of up to 1.5 × 10¹⁸ cm^− 2. However, the resistivity of ZnO:Al and ZnO:Ga, which are generally used as window layers for CIGS solar cells, increased with increasing irradiation fluence. For CIGS thin films, the photoluminescence peak intensity due to Cu-related point defects, which do not significantly affect solar cell performance, increased with increasing electron irradiation. In CIGS solar cells, decreasing J_SC and increasing R_s reflected the influence of irradiated ZnO:Al, and decreasing V_OC and increasing R_sh mainly tended to reflect the pn-interface properties. These results may indicate that the surface ZnO:Al thin film and several heterojunctions tend to degrade easily by electron irradiation as compared with the bulk of semiconductor-composed solar cells.     

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.     

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.