CuIn1 − xGaxSe2 photovoltaic devices for tandem solar cell application

CuIn_1 − xGa_xSe₂ (CIGS) solar cells show a good spectral response in a wide range of the solar spectrum and the bandgap of CIGS can be adjusted from 1.0 eV to 1.7 eV by increasing the gallium-to-indium ratio of the absorber. While the bandgaps of Ga-rich CIGS or CGS devices make them suitable for top or intermediate cells, the In rich CIGS or CIS devices are well suited to be used as bottom cells in tandem solar cells. The photocurrent can be adapted to the desired value for current matching in tandem cells by changing the composition of CIGS which influences the absorption characteristics. Therefore, CIGS layers with different [Ga]/[In + Ga] ratios were grown on Mo and ZnO:Al coated glass substrates. The grain size, composition of the layers, and morphology strongly depend on the Ga content. Layers with Ga rich composition exhibit smaller grain size and poor photovoltaic performance. The current densities of CIGS solar cells on ZnO:Al/glass varied from 29 mA cm^− 2 to 13 mA cm^− 2 depending on the Ga content, and 13.5% efficient cells were achieved using a low temperature process (450 °C). However, Ga-rich solar cells exhibit lower transmission than dye sensitized solar cells (DSC). Prospects of tandem solar cells combining a DSC with CIGS are presented.     

Synthesis of Cu(In,Ga)Se2 absorber using one-step electrodeposition of Cu-In-Ga precursor

One-step Cu-In-Ga electrodeposition on Mo substrate is carried out by potentiostatic method in acidic aqueous media. The applied potential, the pH and the nature of the electrolyte are determined to obtain adequate precursor composition. The electrodeposit is found highly dendritic, due to Cu diffusion-controlled deposition. Selenization at temperatures ranging from 450 to 600 °C leads to Cu(In,Ga)Se₂ (CIGS) absorber. The influence of selenization temperature and duration on Ga distribution as well as on CIGS crystallinity is discussed. Although the precursor is dendritic, relatively compact absorbers can be obtained. The best solar cell, achieved on 0.1 cm², shows 9.3% efficiency (V_oc 456 mV; j_sc 33 mA cm⁻²; FF 62%).     

Comparative study of sputtered and electrodeposited CI(S,Se) and CIGSe thin films

Copper indium disulphide CuInS₂ (CIS) and diselenide CuInSe₂ (CISe) and their alloys with gallium CuIn_1 − xGa_xSe₂ (CIGSe) thin films have been prepared using both high- and non-vacuum processes. The well known two-stage process consisting in a sequential sputtering of Cu and In thin layers and a subsequent sulfurisation has led to the formation of good quality CuInS₂ ternary compound. The films exhibit the well known chalcopyrite structure with a preferential orientation in the (112) plane suitable for the production of the efficient solar cells. The absorption coefficient of the films is higher than 10⁴ cm^− 1 and the band gap value is about 1.43 eV. A non-vacuum technique was also used. It consists on a one step electrodeposition of Cu, In and Se and in a second time Cu, In, Se and Ga. From the morphological and structural point of view, the films obtained are similar to those prepared by the first technique. The band gap value increases up from 1 eV for the CIS films to 1.26 eV for the CuIn_1 − xGa_xSe₂ with 0 < x < 0.23. The resistivity at room temperature of the films was adjusted to 10 Ωcm after annealing. The films exhibit an absorption coefficient more than 10⁵ cm^− 1. The most important conclusion of this study is the interesting potential of electrodeposition as a promising option in low-cost CISe and CIGSe thin film based solar cells processing.     

Advantages of using amorphous indium zinc oxide films for window layer in Cu(In,Ga)Se2 solar cells

The advantages of using indium zinc oxide (IZO) films instead of conventional Ga-doped zinc oxide (ZnO:Ga) films for Cu(In,Ga)Se₂ (CIGS) solar cells are described. The electrical properties of IZO are independent of film thickness. IZO films have higher mobility (30-40 cm²/Vs) and lower resistivity (4-5 × 10^− 4 Ω cm) compared to ZnO:Ga films deposited without intentional heating, because the number of grain boundaries in amorphous IZO films is small. The properties of a CIGS solar cell using IZO at the window layer were better than those obtained using a conventional ZnO:Ga at the window layer; moreover, the properties tended to be independent of thickness. These results indicate that use of IZO as a transparent conducting oxide layer is expected to increase the efficiency of CIGS solar cells.     

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.     

Effects of Na on the properties of Cu(In,Ga)S2 solar cells

We have prepared Cu(In,Ga)S₂ solar cells with changing Na concentration using several preparation methods. We have investigated the hole density and solar cell properties. The hole density increases with increasing Na concentration up to 10¹⁷ cm^− 3, and stays almost constant for higher concentrations. It can be considered that Na might passivate a single type of donor up to 10¹⁷ cm^− 3. For all preparation methods, the efficiencies of solar cells prepared with the Cu(In,Ga)S₂ thin films increase with increasing Na concentration. We propose a Na diffusion model derived from depth profiles obtained for each of the preparation methods.     

Preparation of Cu2ZnSnS4 thin films by sulfurization of stacked metallic layers

Stacked precursors of Cu, Sn, and Zn were fabricated on glass/Mo substrates by electron beam evaporation. Six kinds of precursors with different stacking sequences were prepared by sequential evaporation of Cu, Sn, and Zn with substrate heating. The precursors were sulfurized at temperatures of 560 °C for 2 h in an atmosphere of N₂ + sulfur vapor to fabricate Cu₂ZnSnS₄ (CZTS) thin films for solar cells. The sulfurized films exhibited X-ray diffraction peaks attributable to CZTS. Solar cells using CZTS thin films prepared from six kinds of precursors were fabricated. As a result, the solar cell using a CZTS thin film produced by sulfurization of the Mo/Zn/Cu/Sn precursor exhibited an open-circuit voltage of 478 mV, a short-circuit current of 9.78 mA/cm², a fill factor of 0.38, and a conversion efficiency of 1.79%.     

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.     

Antimony sulfide thin films in chemically deposited thin film photovoltaic cells

Antimony sulfide thin films of thickness ≈ 500 nm have been deposited on glass slides from chemical baths constituted with SbCl₃ and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from − 3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb₂S_xSe_3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb₂S₃) to ≈ 1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb₂S₃-Ag₂Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)₂ with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO₂:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m² (tungsten halogen) are as follows: SnO₂:F-CdS-Sb₂S₃-Ag(paint) with open circuit voltage (V_oc) 470 mV and short circuit current density (J_sc) 0.02 mA/cm²; SnO₂:F-CdS-Sb₂S₃-CuS-Ag(paint), V_oc ≈ 460 mV and J_sc ≈ 0.4 mA/cm²; SnO₂:F-CdS-Sb₂S_xSe_3 − x-Ag(paint), V_oc ≈ 670 mV and J_sc ≈ 0.05 mA/cm²; SnO₂:F-CdS-Sb₂S₃-AgSb(S/Se)₂-Ag(paint), V_oc ≈ 450 mV and J_sc ≈ 1.4 mA/cm². We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’     

A new asymmetric supercapacitor based on λ-MnO2 and activated carbon electrodes

Lithium-ion intercalated compound λ-MnO₂ was used as positive electrode in asymmetric supercapacitor with activated carbon used as negative electrode in 1 mol L^− 1 Li₂SO₄ aqueous electrolyte solution. Phase composition, morphology and particle sizes of λ-MnO₂ were studied by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical capacitive performance of the asymmetric supercapacitor was tested by cyclic voltammetry and galvanostatic charge-discharge tests. The results show that the asymmetric supercapacitor has electrochemical capacitance performance within wide potential range of 0-2.2 V. The specific capacitance is 53 F g^− 1 at a constant current density of 10 mA cm^− 2. The energy density is 36 W h kg^− 1 with a power density of 314 W kg^− 1. It is obvious that λ-MnO₂ is a potential electrode material for asymmetric supercapacitor.     

High sensitivity and wide bandwidth image sensor using CuIn1 − xGaxSe2 thin films

We have fabricated a novel image sensor using Cu(In,Ga)Se₂ (CIGS). A combined process of dry etching using HBr and Ar gasses and wet etching using dilute HCl solution was developed as isolation process of CIGS photodiode deposited at 400 °C. Etchant residues of the dry etching, which consist of Cu complex, were almost completely cleaned using the wet etching process and favorable vertical side wall of CIGS films was obtained without mechanical damages. As a result, high performance image sensors with low leakage current of ~ 10^− 8 A/cm² and wide wavelength range up to ~ 1240 nm were achieved. The developed image sensor consisted of 352 × 288 pixels with 10 µm × 10 µm pixel sizes, was able to capture clear images of night scenes.     

Effect of electron irradiation on the photopleochroism of ZnO/CdS/Cu(In,Ga)Se<Subscript>2</Subscript> solar cells

The effect of irradiation with 1-MeV electrons to various doses on the photosensitivity of ZnO/CdS/Cu(In, Ga)Se₂ solar cells and related CdS/Cu(In,Ga)Se₂ and ZnO/Cu(In,Ga)Se₂ heterostructures has been studied. Both the photoconversion efficiency and the coefficient of induced photopleochroism of ZnO/CdS/CIGS solar cells remained practically unchanged upon irradiation up to a total dose of 10⁻¹⁷ cm⁻². It is suggested that the method of polarization photoelectric spectroscopy can be used for evaluating the effect of electron irradiation on the photosensitivity of semiconductor photoconverters.     

Cu2ZnSnS4 thin film solar cells from electroplated precursors: Novel low-cost perspective

Thin-film solar cells based on Cu₂ZnSnS₄ (CZTS) absorbers were fabricated successfully by solid-state reaction in H₂S atmosphere of electrodeposited Cu-Zn-Sn precursors. These ternary alloys were deposited in one step from a cyanide-free alkaline electrolyte containing Cu(II), Zn(II) and Sn(IV) metal salts on Mo-coated glass substrates. The solar cell was completed by a chemical bath-deposited CdS buffer layer and a sputtered i-ZnO/ZnO:Al bilayer. The best solar cell performance was obtained with Cu-poor samples. A total area (0.5 cm²) efficiency of 3.4% is achieved (V_oc = 563 mV, j_sc = 14.8 mA/cm², FF = 41%) with a maximum external quantum efficiency (EQE) of 80%. The estimated band-gap energy from the external quantum efficiency (EQE) measurements is about 1.54 eV. Electron backscatter-diffraction maps of cross-section samples revealed CZTS grain sizes of up to 10 µm. Elemental distribution maps of the CZTS absorber show Zn-rich precipitates, probably ZnS, and a Zn-poor region, presumably Cu₂SnS₃, close to the interface Mo/CZTS.     

Photoluminescence and Raman study of Cu2ZnSn(SexS1 − x)4 monograins for photovoltaic applications

The quaternary semiconductors Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ have attracted a lot of attention as possible absorber materials for solar cells due to their direct bandgap and high absorption coefficient (> 10⁴ cm⁻¹). In this study we investigate the optical properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograin powders that were synthesized from binary compounds in the liquid phase of potassium iodide (KI) flux materials in evacuated quartz ampoules. Radiative recombination processes in Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using low-temperature photoluminescence (PL) spectroscopy. A continuous shift from 1.3 eV to 0.95 eV of the PL emission peak position with increasing Se concentration was observed indicating the narrowing of the bandgap of the solid solutions. Recombination mechanisms responsible for the PL emission are discussed. Vibrational properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using micro-Raman spectroscopy. The frequencies of the optical modes in the given materials were detected and the bimodal behaviour of the A₁ Raman modes of Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ is established.     

Impact of the Se evaporation rate on the microstructure and texture of Cu(In,Ga)Se2 thin films for solar cells

Coevaporated Cu(In,Ga)Se₂ layers on Mo-coated soda-lime glass substrates were produced by a three-stage process using various Se overpressure conditions during the three stages. Cross-sections of these samples were analyzed by electron backscatter diffraction (EBSD) in a scanning electron microscope in order to reveal the microstructures in the Cu(In,Ga)Se₂ layers. In addition, the preferential orientations of these Cu(In,Ga)Se₂ layers were studied by plan-view EBSD measurements. It was found that Cu(In,Ga)Se₂ exhibits a texture in 110 orientation for Se/(Cu + In + Ga) atomic flux ratios R which are sufficiently large (≥ 4). In one Cu(In,Ga)Se₂ layer produced with approximately R = 4, a large density of (near) Σ3 (twin) boundaries were detected which are oriented preferentially perpendicular to the substrate. By comparison of the local textures of neighboring grains and the theoretically possible changes in orientation by twinning, it is possible to retrace how the twinning occurred.     

The effect of sodium doping to CuInSe2 monograin powder properties

The effect of sodium doping to the electrical and photoluminescence properties of CuInSe₂ monograin powders was studied. Sodium was added in controlled amounts from 5 × 10¹⁶ cm^− 3 to 1 × 10²⁰ cm^− 3. The photoluminescence spectra of Na-doped stoichiometric CuInSe₂ powders had two bands with peak positions at 0.97 and 0.99 eV. The photoluminescence bands showed the shift of peak positions depending on the Na doping level. Peak positions with maximum energy were observed if added sodium concentration was 1 × 10¹⁹ cm^− 3. This material had the highest carrier concentration 2 × 10¹⁷ cm^− 3. In the case of stoichiometric CuInSe₂ (Cu:In:Se = 25.7:25.3:49.0), Na doping at concentrations of 3 × 10¹⁷ cm^− 3 and higher avoided the precipitation of Cu-Se phase. Solar cells output parameters were dependent on the Na doping level. Sodium concentration 3 × 10¹⁸ cm^− 3 resulted in the best open-circuit voltage.     

Characterization of the CdS / Cu(In,Ga)Se2 interface by electron beam induced currents

The method of electron beam induced currents in junction configuration (JEBIC) has been employed to investigate carrier collection in Cu(In,Ga)Se₂ solar cells. A detailed analysis of JEBIC line-scans reveals unexpected carrier collection properties, which cannot be explained with common models. We ascribe this anomalous behavior to an electrostatic barrier effect at the Cu(In,Ga)Se₂ / CdS interface. We suggest the existence of a thin defect-layer on the surface of the Cu(In,Ga)Se₂ with high acceptor concentration and valence band edge that is energetically lower than that of the bulk. Using this model, we achieve a good agreement between experimental and simulated JEBIC line-scans. The influence of the barrier effect is considerably reduced by a metastable change of the interface properties induced by intensive electron irradiation of the interface. This effect is explained by a metastable decrease of the negative charge density in the defect-layer.     

The influence of gallium on phase transitions during the crystallisation of thin film absorber materials Cu(In,Ga)(S,Se)2 investigated by in-situ X-ray diffraction

Chalcopyrite based photovoltaic materials Cu(In_xGa_1 − x)(S_ySe_1 − y)₂ (CIGSSe) are substituted in the cation and anion lattice to adopt the semiconductor bandgap to the terrestrial solar spectrum. In-situ X-ray diffraction (XRD) investigations on the crystallisation of thin film absorber materials Cu(In,Ga)(S,Se)₂ while annealing stacked elemental layers (SEL) show phase transitions proceeding during the chalcopyrite synthesis.Thin layers of metals with elemental ratio Cu:In:Ga = 3:2:1 are deposited onto Mo-coated polyimide foil by DC-magnetron sputtering. The metal precursor is covered with S and subsequently Se by thermal evaporation of the elements in chalcogen excess (S + Se) / (Cu + In + Ga) = 2.3. Investigated chalcogen ratios reach from pure Se to pure S. Crystalline phases formed during the annealing of SEL are qualitatively determined. The results are compared to conclusions drawn from previous experiments on Ga-free CuIn(S,Se)₂ absorbers. The presence of Ga and S influences significantly the time-scale and the temperatures of phase transitions, i.e. the sulfoselenisation of precursor phases Cu₁₆(In,Ga)₉ and Cu₉(Ga,In)₄ proceeds faster with increasing S and is shifted to higher temperatures as compared to Ga-free Cu₁₁In₉/Cu₁₆In₉.     

Modification of the three-stage evaporation process for CuIn1−xGaxSe2 absorber deposition

CuIn_1−xGa_xSe₂ absorbers for highest efficiency state-of-the-art solar cells are generally deposited by a sequential three-stage coevaporation process from elemental sources. We investigated the influence of the maximum copper concentration used during processing in the second stage of the growth process. The impact on the Ga grading in the deposited layer was measured by SIMS. The position and slope of the Ga grading profiles were optimized for high efficiency solar cells. Effects on the phases found in the absorber layer were investigated by Raman spectroscopy. The recorded spectra show the formation of a group III rich phase in layers grown at high maximum Cu contents. Best PV parameters were achieved for solar cells developed with absorbers grown with [Cu]/[In + Ga] = 1.05 at the end of the 2nd stage.     

Preparation of wide gap Cu(In,Ga)S2 films on ZnO coated substrates

We have prepared Cu(In,Ga)S₂ films at growth temperatures from 300 °C to 580 °C with a homogeneous gallium depth distribution (estimated band gap 1.67 eV) onto soda lime glass (SLG) substrates with one of three different kinds of back contact: Mo(1000 nm), ZnO(500 nm), and Mo(30 nm)/ZnO(500 nm), respectively. We have also investigated the depth profiles of Zn and Na (diffused from SLG) in Cu(In,Ga)S₂ films by secondary ion mass spectroscopy (SIMS). The efficiency of solar cells on Mo increases with increasing growth temperature. It is higher on Mo/ZnO than on ZnO, and increases from 350 °C to 450 °C, then decreases above 450 °C. It was observed by SIMS that the amount of Zn in Cu(In,Ga)S₂ on Mo/ZnO is lower than it is on ZnO up to 450 °C, and a large amount of Zn diffuses into absorbers over 450 °C, which contributes to decreasing efficiency. The amount of Na in the back contact increases with growth temperature. The depth distribution of Na in Cu(In,Ga)S₂ films on Mo is almost constant in the order of 10¹⁷-10¹⁸ cm^− 3, on ZnO and Mo/ZnO the Na concentration increases towards the surface and is in the range of 10¹⁵-10¹⁷ cm^− 3.