CuInSe2 Absorber Research Articles

Back Contact Materials for Superstrate CuInS2 Solar Cells

CuInS2 (CIS) solar cells were fabricated by spray pyrolysis method with different back contact electrodes of Al, Cu, Mo, Au, and C (carbon). The best conversion efficiencies of CuInS2 cells using Al, Cu, Mo, Au, and C back contacts were 1.96%, 2.17%, 2.08%, 1.87%, and 2.35%, respectively. The conversion efficiency of the cells using Al and Cu back contacts significantly deteriorated after one month keeping devices in air ambience. The device performance of the CIS cells using Au, Mo and Carbon is quite stable after one month. The effect of back contact on each photovoltaic parameter has been confirmed by impedance measurements. Carbon films were crack- free and showed adequate adhesion with the CuInS2 absorber layer. The annealing temperature for carbon films is in the range of 125- 240 °C. The cell parameters of the CIS cells using carbon back contact are Jsc = 10.36 mA/cm2, Voc = 0.50 V, fill factor = 0.44 and conversion efficiency = 2.35%. The resulting CIS solar cells, fabricated by spray-pyrolysis depositions for CIS layers and screen printings for carbon layers, are recognized as full non-vacuum fabricated solar cells.

Copper Indium Diselenide thin films using a hybrid method of chemical bath deposition and thermal evaporation

ABSTRACTCopper indium diselenide (CIS) based solar cells are one among the promising thin film solar cells. Most of the processes reported for the preparation of CIS directly or indirectly involve Se vapor or H2Se gases which are extremely toxic to health and environment. In this work, we report the preparation of CIS thin films by stacked layers of Glass/In/Se/Cu2Se and Glass/In/Se/Cu2Se/Se. For this, first indium (In) thin film was thermally evaporated on glass substrate on which selenium (Se) and copper selenide (Cu2Se) thin films were deposited sequentially by chemical bath deposition. Selenium thin films were grown from an aqueous solution containing Na2SeSO3 and CH3COOH at room temperature, triple deposition for 7, 7 and 10 min from consecutive baths. Copper selenide thin films were deposited at 35 °C for 1 hour from an aqueous bath containing CuSO4, Na2SeSO3 and NH4OH. Analysis of the X-ray diffraction patterns of the thin films formed at 400 °C from the precursor layer containing extra selenium layer showed the presence of chalcopyrite CuInSe2, without any secondary phase. Morphology of all the samples was analyzed using Scanning Electron Microscopy. Optical band gap was evaluated from the UV-Visible absorption spectra of these films and the values were 1.1 eV and 1 eV respectively for CIS thin films formed at 400 °C from the selenium deficient and selenium rich precursor layers. Electrical characterizations were done using photocurrent measurements. Thus preparation of a CuInSe2 absorber material by a non-toxic selenization process may open up a low cost technique for the fabrication of CIS based solar cells.

Synthesis of High Quality CuInSe2 Films with Oxygen Precursor by Non-vacuum Process

The CuInSe2 absorber was synthesized by non-vacuum process with a simple and low-cost method, which fabricated absorber layer of thin-film solar cell. The extra amount of Se was added into the ink to help reduction of the oxide and solid Se fountain was used to provide Se atmosphere during the selenization progress. The influence of same factors was investigated, such as the time of reduction in H2, the time of selenization and the Se vapor pressure. The selenizaion, processed at 550°C for 60 min with the Se vapor pressure at 1.90 kPa, resulted in high quality CuInSe2 layer with very good chemical composition.

Characterization of heterojunctions based on airless spray deposited CuInS2 thin films on different underlayers

Thin films of copper indium disulfide (CuInS2) are deposited on various substrates (In2S3/ZnO/SnO2/glass, ZnO/SnO2/glass), where In2S3 will be used as the optical window, ZnO as the optical window or wafer layer, CuInS2 as the absorber material in photovoltaic system and SnO2 as the ohmic contact. In2S3 is grown by chemical bath deposition; CuInS2, ZnO and SnO2 are grown by airless spray technique. The substrate temperature and the deposition duration of CuInS2 thin layer are fixed at 340 °C and 20 minutes respectively. The films were characterized by X-ray diffraction, Scanning electron microscope (SEM) and spectrophotometer. The X-ray diffraction revealed that the as-sprayed CuInS2 absorber films are of chalcopyrite crystalline phase with a highly (112) preferential orientation, the In2S3 deposited by CBD have a (400) preferential orientation, the ZnO sprayed thin films crystallizes in the hexagonal wurtzite form with a (002) principal orientation and the SnO2 films are well crystallized in the tetragonal structure and oriented preferentially in the (200) direction. The SEM showed that the thickness of the SnO2, ZnO, In2S3 and CuInS2 are 0.529; 0.684; 0.267 and 0.507 μm respectively. The optical band gap of the CuInS2 absorber material is 1,51 eV. The electrical properties of these structures are analysed from current-voltage (I-V) characteristics.

CuInSe2 (CIS) Thin Film Solar Cells by Direct Coating and Selenization of Solution Precursors

CuInSe2 (CIS) absorber layer was formed by a direct nonvacuum coating and a subsequent selenization of precursor solutions of Cu(NO3)2 and InCl3 dissolved in methanol. The viscosity of precursor solutions was adjusted by adding ethyl-cellulose (EC) to be suitable for the doctor-blade coating. During the coating and drying process Cu2+ ions in the starting solution were reduced to Cu+, resulting in precursor films consisting of CuCl crystals and amorphous In compound embedded in EC matrix. Selenization of the precursor films with Se vapor at elevated temperature generated double-layered films with an upper layer of chalcopyrite CIS and a carbon residue bottom layer. Significant In loss was observed during the selenization, which was attributed to the evaporation of the In2Se binary phase, confirmed by investigating the change in the Cu/In ratio of the selenized film as a function of Se flux and substrate temperature. As a proof-of-concept, thin film solar cells were fabricated with the double-layered absorber film and the devices exhibited reproducible conversion efficiency as high as about 2%.

The correlation between local defect absorbance and quasi-Fermi level splitting in CuInS2 from photoluminescence

Analogously with Cu(In,Ga)Se2, CuInS2 shows a high degree of spatial inhomogeneities in structural, optical and electronic properties on the length scale of grain sizes and above which is caused by the grainy structure and the inhomogeneous growth of absorber layers. To analyse these locally fluctuating magnitudes, spectrally resolved photoluminescence measurements with high lateral resolution (⩽1μm) have been performed in a confocal microscope setup. Based on these data sets and on Planck's generalized law determination of the spatial variation in the splitting of the quasi-Fermi levels and access to the local absorbance is possible. A detailed analysis of these properties, crucial for the solar light conversion efficiency of a final cell, is made for a CuInS2 absorber layer for data obtained from statistically representative scan areas. A cross-correlation between the splitting of the local quasi-Fermi levels and the local absorbance of an absorber leads to the conclusion that the splitting of quasi-Fermi levels is strongly governed by the excess-carrier recombination via deep defects.

Chemical Composition and Electronic Properties of CuInS2/Zn(S,O) Interfaces

AbstractWe report on the chemical deposition and electronic properties of CuInS2/Zn(S,O) interfaces. The Zn(S,O) buffer was grown by a new chemical bath deposition (CBD) process that allows the tailoring of the S/O ratio in the films. Resulting Zn(S,O) films exhibit transparencies above 80% (for λ>390 nm) and an optical energy band gap of 3.9 eV which decreases to 3.6 eV after annealing in air at 200°C. Production line CuInS2 (CIS) absorbers provided by Sulfurcell Solartechnik GmbH are used as substrates for the investigation of the CIS/Zn(S,O) interface and the chemical composition of Zn(S,O). A ZnS/(ZnS+ZnO) ratio of 0.5 is found by X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy (XPS and XAES). The valence band offset between the heterojunction partners (ΔEV = 1.8 ± 0.2 eV) has been determined by means of XPS and ultraviolet photoelectron spectroscopy (UPS). Considering the energy band gap of the CIS absorber and the measured band gap of Zn(S,O), the conduction band offset (ΔEC) is calculated as: resulting in a spike of 0.5±0.3 eV in the conduction band at the heterojunction before annealing. After the heat treatment, the valence band offset is reduced to 1.5±0.2 eV and the calculated conduction band offset remains at 0.5±0.3 eV.

Analysis of S-rich CuIn(S,Se)2 layers for photovoltaic applications: Influence of the sulfurization temperature on the crystalline properties of electrodeposited and sulfurized CuInSe2 precursors

This paper reports the microstructural analysis of S-rich CuIn(S,Se)2 layers produced by electrodeposition of CuInSe2 precursors and annealing under sulfurizing conditions as a function of the temperature of sulfurization. The characterization of the layers by Raman scattering, scanning electron microscopy, Auger electron spectroscopy, and XRD techniques has allowed observation of the strong dependence of the crystalline quality of these layers on the sulfurization temperature: Higher sulfurization temperatures lead to films with improved crystallinity, larger average grain size, and lower density of structural defects. However, it also favors the formation of a thicker MoS2 interphase layer between the CuInS2 absorber layer and the Mo back contact. Decreasing the temperature of sulfurization leads to a significant decrease in the thickness of this intermediate layer and is also accompanied by significant changes in the composition of the interface region between the absorber and the MoS2 layer, which becomes Cu rich. The characterization of devices fabricated with these absorbers corroborates the significant impact of all these features on device parameters as the open circuit voltage and fill factor that determine the efficiency of the solar cells.

Piezo-photovoltaic coupling in CdS-based thin-film photovoltaics

We report a summary of our recent study of piezo- and pyroelectricity in CdS-based photovoltaics including CdTe and CuIn(Ga)Se2 absorber devices. The strong pyro- and piezo-effects in CdS significantly affect the electric field distribution and the physics of device operations. We introduce the concept of pyro-photovoltaic coupling where the photovoltaic effect and the CdS polarization strongly depend on each other. Our quantitative modeling shows that the CdS polarization turns out to be beneficial for photovoltaic technology, making it more forgiving. Three experimental setups utilized in our work were aimed at exerting mechanical stresses corresponding to the device squeezing, bending, and flexing; we used the substrate, superstrate, and flexible substrate structures. Our understanding points at previously unexplored venues in thin-film photovoltaic technology.

Defect study on the indium-gallium alloy system of copper chalcopyrites performed on solar cell heterostructures

We report on defect spectroscopy on Cu(In,Ga)Se2 based solar cells with varying gallium content. Our investigations using deep level transient and admittance spectroscopy do not reveal a pronounced qualitative difference in the defect spectra for the case of absorbers containing indium and gallium. Therefore, we conclude that there exists no detrimental defect in the bulk material that generally prohibits one to achieve an increase in efficiency even for solar cells based on absorbers with a gallium to gallium plus indium ratio (GGI) larger than 0.3. The boundary compositions with CuInSe2 and CuGaSe2 absorbers (i.e., GGI=0 and GGI=1, respectively) show additional trap signals. Additionally, a peak width analysis was performed for two defect signals that occur for all absorber compositions. The relative peak width was found to be independent of GGI whereas the activation energies show some correlation with the gallium content of the absorber layer.

Pyroelectric coupling in thin film photovoltaics

AbstractWe propose a theory of thin film photovoltaics in which one of the polycrystalline films is made of a pyroelectric material grains such as CdS. That film is shown to generate strong polarization improving the device open circuit voltage. Implications and supporting facts for the major photovoltaic types based on CdTe and CuIn(Ga)Se2 absorber layers are discussed.magnified imageBand diagram of a pyroelectric (CdS) based PV junction. Arrows represent the charge carrier photo‐generation.(© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Junction formation of CuInSe 2 with CdS: A comparative study of “dry” and “wet” interfaces

The junction formation of polycrystalline CuInSe 2 absorbers (CIS) with thermally evaporated CdS was investigated by high-resolution synchrotron X-ray photoelectron spectroscopy. The chemistry and electronics of the interfaces of Cd partial electrolyte treated CIS (“wet” processed) and clean, decapped CIS (“dry” processed) were compared. A valence band offset of 0.96(10) eV was determined in both cases. The Cd(Se,OH) surface layer induced by the wet Cd partial electrolyte process does not significantly modify the band alignment at the CIS/CdS heterointerface from the “dry”, vacuum-processed CIS/CdS interface. During the stepwise interface formation the energy converting capability of the CIS/CdS heterojunction was assessed by in situ surface photovoltage measurements at room temperature. The evolution of the surface photovoltage significantly differs for the “wet” and the “dry” interfaces and is discussed in relation to the function in solar cell devices.

Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

AbstractRecent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (Eg = 3·6–3·8 eV) than the conventional buffer material CdS (Eg = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS2 (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm2 in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm2 (total area: 22·5 mA/cm2). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm2 substrates have been prepared and already reach an efficiency (active area: 17·2 cm2) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.

Thin Film Deposition Methods for CuInSe 2 Solar Cells

CuInSe2 and its alloys with Ga and/or S are among the most promising absorber materials for thin film solar cells. CuInSe2-based solar cells have shown long-term stability and the highest conversion efficiencies of all thin film solar cells, above 19%. Solar cells based on these materials are also very stable, thus allowing long operational lifetimes. The preparation of a thin film solar cell is a multistage process where every step affects the resulting cell performance and the production cost. CuInSe2 and other Cu chalcopyrites can be prepared by a variety of methods, ranging from physical vapor deposition methods such as evaporation and sputtering to low-temperature liquid phase methods such as electrodeposition. The present review discusses first the concept and operation principle of thin film solar cells, as well as the most important thin film solar cell materials. Next, the properties of CuInSe2 and related compounds, as well as features of solar cells made thereof are reviewed. The last part of the text deals with deposition methods used for the preparation of CuInSe2 and Cu(In,Ga)Se2 thin film absorbers and solar cells. Although the emphasis here is on absorber preparation methods, buffer and conducting oxide preparation are discussed as well.

Local segregation in Cu-In precursors and its effects on microstructures of selenized CuInSe2 thin films

Local segregation in Cu-In precursors and its effects on the element distribution and microstructures of selenized CuInSe2 thin films were investigated. Cu-In precursors with an ideal total mole ratio of Cu to In of 0.92 were prepared by middle frequency alternating current magnetron sputtering with Cu-In alloy target, then CuInSe2 absorbers for solar cells were formed by selenization process in selenium atmosphere. Scanning electron microscope and energy dispersive X-ray spectroscope were used respectively to observe the surface morphologies and determine the compositions of both Cu-In precursors and CuInSe2 thin films. Their microstructures were characterized by X-ray diffractometry and Raman spectroscope. The results show that Cu-In precursors are mainly composed of Cu11In9 phase with In-rich solid solution. Stoichiometric CuInSe2 thin films with a homogeneous element distribution and single chalcopyrite phase can be synthesized from a segregated Cu-In precursor film with an ideal total mole ratio of Cu to In of 0.92. CuInSe2 thin film shows P-type conductivity and its resistivity reaches 1.2×103 Θ·cm.

8% efficient CuInS2 solar cells by electrochemically removed Cu–S Phases

Investigations on how to replace the toxic KCN etching for the removal of Cu–S phases during the preparation of CuInS2 (CIS) absorber layers by electrochemical procedures are presented. Starting from a simple anodic treatment in V2+/V3+ electrolyte, a more complex photoelectrochemical technique is developed which consists of different consecutive etching steps for the dissolution of the predominant CuS and for the removal of remaining Cu2S and a small sacrificial layer of CuInS2. This new method also offers the possibility of in situ quality control of the CIS in a photoelectrochemical solar cell (PECS) setup. Further examination of the treated films is carried out using X-ray emission spectroscopy, X-ray photoelectron spectroscopy (XPS) and by further processing of the samples to create solid state solar cells. The specimen yield conversion efficiencies of 3.3% in PECS and 8% in solid state devices.

Glass/ITO/In(O,S)/CuIn(S,Se)2 solar cell with conductive polymer window layer

Hybrid solar cells based on the combination of conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) and inorganic semiconductor CuIn(S,Se)2 (CISSe) were investigated. The CuInSe2 (CISe) absorber layers were electrodeposited on ITO covered glasses from aqueous solutions with various ratios of elements. The ITO/In(O,S)/CISSe photovoltaic (PV) junctions were prepared by the sulfurization of ITO/CISe precursors at 450 °C in the H2S atmosphere. The PEDOT–PSS layer of p-type is considered an alternative to the traditional window top layer on the CISSe absorber layer in the cell structure. The polymer deposition was performed by help of the spin-casting technique. PV properties of the prepared ITO/In(O,S)/CISSe and ITO/In(O,S)/CISSe/PEDOT–PSS structures were studied, with emphasis on the role of conductive polymer layer in the cell structure.

Inx(OH)ySz as recombination barrier in TiO2/inorganic absorber heterojunctions

In extremely thin absorber (ETA) solar cells a transparent mesoporous semiconductor is sensitized for visible light by depositing an inorganic absorber into the pores. One of the factors limiting the efficiency in such a device is the fast recombination of electrons injected from the excited absorber into the semiconductor, with holes in the absorber. The approach of this paper is to introduce indium (hydroxy) sulfide Inx(OH)ySz (band gap 2.5 eV) as a buffer layer at the TiO2/absorber interface to suppress recombination. Photoelectrical measurements on solid-state cells with a stack of flat TiO2, a wet-chemically applied Inx(OH)ySz layer and a sprayed CuInS2 absorber layer on top of it reveal a characteristic diode curve with an energy conversion efficiency of 2.1%, in contrast to the short circuiting observed for cells with TiO2/CuInS2 alone. Since Inx(OH)ySz forms a thin surface layer on TiO2-nanoparticles it is interesting for application in ETA solar cells. A TiO2/ETA electrode with quantum-sized PbS particles has been compared to a stacked TiO2/Inx(OH)ySz/ETA-configuration. Surface photovoltage spectrosocopy (SPS) measurements have shown that the electron recombination rate reduces if compared to TiO2/PbS without intermediate Inx(OH)ySz layer. The detailed Inx(OH)ySz functioning is not yet clarified.

Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

An empirical diagnostic method for the evaluation of solar cell grade CuInS2 absorbers has been developed. The method involves the measurement of the contact angle between water and the CuInS2 absorber before fabrication of a solar cell. The contact angle is expected to depend upon local inhomogeneity, chemical composition and surface morphology of the CuInS2 absorber. The variation of these factors on the surface is supported with scanning electron micrographs, chemical analyses, laser scanning photocurrent mapping of various CuInS2 absorbers and measurements of the solar cell performance. The contact angle has been found to be different at different places on the CuInS2 surface. Empirically, it was found that for high conversion efficiency solar cells (>8–10.5%), the contact angle on CuInS2 absorbers ranges between 53° and 63°. For low conversion efficiency solar cells (<6%), it is between 48° and 50°. Therefore, it is seen that contact angle measurements on CuInS2 absorbers can be used to assess the quality of CuInS2 absorbers prior to solar cell fabrication.

CuInS2/PEDOT Photovoltaic Structure

AbstractStructures based on combination of electrically conductive polymers with inorganic semiconductors are currently intensively investigated with the aim to prepare low-cost, largearea and flexible photovoltaic devices. In this study, multilayer structures consisting of CuInS2 (CIS) and poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) thin films were prepared and investigated for photovoltaic applications. Polycrystalline CIS absorber layers were synthesized on top of a layered structure on Cu tape substrate using socalled non-vacuum CISCuT technique. Thin PEDOT buffer layers doped with PSS were deposited onto KCN etched and vacuum annealed CIS films. The deposition was performed using the spin-casting technique from an aqueous dispersion of PEDOT/PSS mixed with Nmethylpyrrolidone, isopropanol, glycerin and epoxysilane additives. Optimal deposition parameters for stable PEDOT films with a good adherence to the surface of CIS were selected experimentally. The morphology and thickness of prepared films and structures was determined using SEM technique. Average film thickness was about of 1.5 μm for CIS and 50 nm for PEDOT films. Current-voltage and impedance characteristics were measured. Significant photovoltage and photocurrent of the photovoltaic structures were observed under standard illumination intensity. The best structure showed an open-circuit voltage of 510 mV and a shortcircuit current density of 20.2 mA/cm2.