Se Ambient Research Articles

Effects of Growth Temperature on the Morphological, Structural, and Electrical Properties of CIGS Thin Film for Use in Solar Cell Applications

Cu-In-Ga-Se nanoparticles (NPs) were synthesized using a colloidal route process. The effects of growth temperature (GT) on the properties of CuInGaSe2 (CIGS) thin films made from these nanoparticles were investigated using TEM, PL, XRD, and SEM techniques. The Cu-In-Ga-Se NPs were synthesized at growth temperatures ranging from 90 °C to 105 °C and then annealed at 550 °C for 7 min under a Se ambient. The resulting CIGS thin film, formed from Cu-In-Ga-Se NPs synthesized at a GT of 90 °C (referred to as GT90-CIGS), showed a tetragonal structure, large grain size, and high sunlight absorption. It had a band gap energy (Eg) of approximately 0.94 eV. Non-vacuum GT90-CIGS-based solar cells were investigated and fabricated using varying thicknesses of a CdS buffer layer. The maximum power conversion efficiency achieved was approximately 8.3% with an optimized device structure of Al/ITO/ZnO/CdS/CIGS/Mo.

Modification of back electrode with WO3 layer and its effect on Cu2ZnSn(S,Se)4-based solar cells

In the present work, we designed and prepared Cu2ZnSn(S,Se)4 (CZTSSe)-based solar cells with a new structure of Al/ITO/ZnO/CdS/CZTSSe/WO3/Mo/SLG (S1-5) by depositing about 5-nm-thick WO3 layer with monoclinic structure on the back electrode Mo/SLG of solar cells with the convention structure of Al/ITO/ZnO/CdS/CZTSSe/Mo/SLG (S2), with the aim of improving the power conversion efficiency (PCE) of CZTSSe-based solar cells. It is found that the average open circuit voltage (Voc) increases from 346.7 mV of the S2 cells to 400.9 mV of the S1-5 cells, the average short circuit current density (Jsc) from 26.4 mA/cm2 to 32.1 mA/cm2 and the filling factor (FF) from 33.8 to 40.0 by addition of the WO3 layer, which results in that the average PCE increases from 3.10% of the S2 cells to 5.14% of the S1-5 cells. The average increasing percent of the PCE is 65.8%. The increase in Voc, Jsc and FF of the S1-5 cells compared to the S2 cells is attributed to that the WO3 layer prevent the Se coming from Se ambient and CZTSSe to react with the Mo to form MoSe2 and other second phases, which makes the shunt resistance (Rsh) of the S1-5 increase and the series resistance (Rs) and reverse saturation current density (J0) decrease compared to the S2 cells. The decreased J0 is main factor of improvement of the PCE. A mechanism of influence of the Rsh, Rs and J0 on the PCE is also revealed. Our result demonstrates that addition of the WO3 layer with a reasonable thickness can be a promising technical route of improving the PCE of the CZTSSe-based solar cell.

Chemistry of K in Cu(In,Ga)Se2 photovoltaic absorbers: Effects of temperature on Cu-K-In-Se films

Incorporation of K has led to world record Cu(In,Ga)Se2 photovoltaic power conversion efficiencies, but there is poor consensus about the role of phase impurities in these advances. This work lays a foundation for identifying and controlling these phase impurities. Films of Cu-K-In-Se were co-evaporated at varied K/(K + Cu) compositions and substrate temperatures (with constant (K + Cu)/In ∼ 0.85). Increased Na composition on the substrate's surface and decreased growth temperature were both found to favor Cu1-xKxInSe2 alloy formation, relative to two-phase CuInSe2+KInSe2 formation. Structures from X-ray diffraction (XRD), band gaps, resistivities, minority carrier lifetimes and carrier concentrations from time-resolved photoluminescence were in agreement with previous reports, where low K/(K + Cu) composition films exhibited properties promising for photovoltaic absorbers. Films grown at 400–500 °C were then annealed to 600 °C in a controlled Se ambient, which caused K loss by evaporation in proportion to the initial K/(K + Cu) composition. Similar to growth temperature, annealing drove Cu1-xKxInSe2 alloy consumption and CuInSe2+KInSe2 production, as evidenced by high temperature XRD. Annealing also decomposed KInSe2 and formed K2In12Se19. At high temperature, the KInSe2 crystal lattice gradually contracted as temperature and time increased, as well as just time. Evaporative loss of K during annealing could accompany the generation of vacancies on K lattice sites, and may explain the KInSe2 lattice contraction. This knowledge of Cu-K-In-Se material chemistry may be used to predict and control minor phase impurities in Cu(In,Ga)(Se,S)2 photovoltaic absorbers—where impurities below typical detection limits may have played a role in recent world record photovoltaic efficiencies that utilized KF post-deposition treatments.

Fabrication, characterization and application of Cu2ZnSn(S,Se)4 absorber layer via a hybrid ink containing ball milled powders

Cu2ZnSnS4 (CZTS) powder with kesterite structure was prepared by ball milling of mixture of Cu2S, ZnS and SnS2 powders for more than 15h. By dispersing the milled CZTS powder in a Cu-, Zn- and Sn-chalcogenide precursor solution, a hybrid ink was fabricated. With the hybrid ink, a precursor CZTS film was deposited on Mo coated soda-lime glass by spin-coating. In order to obtain Cu2ZnSn(S,Se)4 (CZTSSe) absorber film with kesterite structure, the CZTS film was annealed at 560°C for 15min in Se ambient. It is demonstrated that the annealed film is dominated by a thick layer of kesterite CZTSSe with larger grain size and Cu8Fe3Sn2(S,Se)12 impurity phase with the exception of a very thin layer of kesterite CZTS with smaller grain size at interface between the CZTSSe and Mo layers. Solar cell device was fabricated by using the annealed CZTSSe film as absorber layer, and its conversion efficiency reached 4.2%. Mechanism of formation of the kesterite CZTS powder and CZTSSe film as well as effect of impurity phases on conversion efficiency are discussed in the present paper. The present results suggest that the hybrid ink approach combining with ball milling is a simple, low cost and promising method for preparation of kesterite CZTSSe absorber film and CZTSSe-based solar cell.

Control of an interfacial MoSe2 layer in Cu2ZnSnSe4 thin film solar cells: 8.9% power conversion efficiency with a TiN diffusion barrier

We have examined Cu2ZnSnSe4 (CZTSe) solar cells prepared by thermal co-evaporation on Mo-coated glass substrates followed by post-deposition annealing under Se ambient. We show that the control of an interfacial MoSe2 layer thickness and the introduction of an adequate Se partial pressure (PSe) during annealing are essential to achieve high efficiency CZTSe solar cells—a reverse correlation between device performance and MoSe2 thickness is observed, and insufficient PSe leads to the formation of defects within the bandgap as revealed by photoluminescence measurements. Using a TiN diffusion barrier, we demonstrate 8.9% efficiency CZTSe devices with a long lifetime of photo-generated carriers.

Effects of different sulfurization conditions on the characterization of CuIn(SxSe1−x)2 thin films

Abstract

Effect of substrate temperature on the optical, structural and morphological properties of In2Se3 thin films grown by a two-step process

Polycrystalline γ – In2Se3 thin films with adequate properties to use them as buffer layer in solar cells, were grown on corning glass substrates using a novel procedure which includes the formation of the α- In2Se3 phase in a first step followed by thermal annealing in Se ambient to activate the formation of the γ- In2Se3 phase. X-ray diffraction (XRD) measurements revealed that the substrate temperature strongly affects the phase in which the indium selenide films grow; at substrate temperatures of around 300°C the indium selenide grow in the α-In2Se3 phase, whereas the samples deposited at temperatures between 300 and 550°C grow with a mixture of the α-In2Se3 and γ-In2Se3 phases. The α-In2Se3 samples change into the γ-In2Se3 phase when subjected to heat treatment around 550°C in Se ambient. Spectrophotometric measurements also revealed that the phase in which the indium selenide films grow, significantly affects the optical gap Eg. Eg values of 1.47 eV and 2.11 eV were determined for the α-In2Se3 and γ-In2Se3 films respectively, indicating that this γ-In2Se3 compound has better properties to perform as buffer layer in thin film solar cells. The effect of substrate temperature on the structural, optical and morphological properties was investigated using XRD, spectral transmittance and atomic force microscope (AFM) measurements. Theoretical simulation of the XRD pattern carried out with the help of the PowderCell package, allowed us to identify the phases associated to the X-Ray reflections, with a good degree of confidence.

Behavior of Point Defects in CuInSe2 Layers Post-Annealed in a Cu, In or Se Ambient

Behavior of Point Defects in CuInSe2 Layers Post-Annealed in a Cu, In or Se Ambient

Fabrication of Cu(In,Ga)Se2 thin films by a combination of mechanochemical and screen‐printing/sintering processes

AbstractWe prepared fine Cu(In,Ga)Se2 (CIGS) powder suitable for screen printing using a mechanochemical process. Particulate precursors were deposited in a thin layer by a screen‐printing technique, the remaining organic solvent was removed from the screen‐printed CIGS film and finally the porous precursor layer was sintered into a dense polycrystalline film by atmospheric‐pressure firing. The crystal structure of the film was analyzed by X‐ray diffraction and the microstructure was observed in a SEM. The thickness of the film was 5–10 μm with a grain size of about 2 μm. The films were also observed in a TEM. The grain size of the as‐prepared powder was less than 1 μm; however, it enlarged to 2–3 μm after firing at 575 °C under a Se ambient. Preliminary CIGS solar cells with our standard Al grid/B‐doped ZnO/i‐ZnO/ CdS/CIGS/Mo/soda‐lime glass structure were fabricated. An efficiency of 2.7%, a V oc of 0.325 V, a J sc of 28.3 mA/cm2 and a FF of 0.295 was obtained. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transmission Photoacoustic Spectroscopy Analysis of CuIn<SUB>0.75</SUB>Ga<SUB>0.25</SUB>Se<SUB>2</SUB> Thin Films

Photoacoustic spectroscopy (PAS) has proved to be an effective technique for the evaluation of the inherent defect population in a wide range of materials for various applications. This paper demonstrates the use of this technique in transmission mode, and hence, evaluates the optical properties of flash-evaporated CuIn0.75Ga0.25Se2 (CIGS) thin films. Both the photoacoustic and transmission spectra were recorded at room temperature using high-resolution near-IR of the gas-microphone-type PAS, which revealed a very broad transmission region (about 300 meV) near the fundamental band edge in the as-grown CIGS thin films due to the presence of several shallow defect levels. The postdeposition heat treatment of the samples under Se ambient, followed by annealing under inert and forming gas ambient, showed significant changes in the behavior of the PAS spectra, particularly near the fundamental band edge. The absorption coefficient has been derived from these spectra to determine the energy band gap values and the activation energies for several defect related energy levels. Using PAS, the energy band gap values were in the range of 1.197 to 1.202 eV. The optical transmission spectra were also recorded from the routinely used spectrophotometer. The transmission data was used to determine the energy band-gap values, which were calculated to be in the range from 1.159 to 1.194 eV. These values were found to be in good agreement with each other, as well as with values reported in the literature.

Preparation and structure of annealed CuInSe 2 electrodeposited films

Thin CuInSe 2 films were prepared by one-step electrodeposition process. The deposition was done in potentiostatic regime from an original electrolyte containing Cu +, In 3+, Se 4+ ions and thiocyanate as a complexing agent. The influence of deposition parameters (electrolyte composition and concentration, temperature of the electrolyte and deposition potential) on film composition was studied. Technological parameters for preparation of films with a desired composition were found. The as-deposited films were heat treated in Ar and in Se ambient. Morphology and structure of as-deposited and of annealed films were investigated. It was established that Se treatment was more effective than the annealing in Ar in addition to crystallite size. Phase formation after Se treatment was elucidated. It was found that composition of as-deposited films mainly conditioned phase composition after the selenization.

Synthesis and characterization of CuInSe2 thin films from Cu, In and Se stacked layers using a closed graphite box

Various techniques have been used to produce CuInSe 2 but the problem of producing films with the desired properties for efficient device fabrication over large areas has always persisted. The Stacked Elemental Layer (SEL) technique has been demonstrated as a method for producing films over a large area, but the films normally annealed in vacuum or in Se ambient, mostly exhibited poor morphology with small grain sizes which result in poor devices. A method of synthesizing CuInSe 2 films by annealing or selenization of the Cu, In and Se elemental layers using a closed graphite box was developed. SEM, EDX, XRD, spectrophotometric and Hall measurements were used to characterize all annealed films. Results have shown single phase chalcopyrite films with improved crystal sizes of about 4 μm The film composition varied from Cu-rich to In-rich with electrical resistivities of 10 −3 to 10 4 Ωcm, cattier concentrations of 5 × 10 15 to 10 17 cm −3 and mobilities of 0.6 to 7.8 cm 2 V −1 s −1 An energy band gap of 0.99 eV and 1.02 eV was obtained for a Cu-rich and near stoichiometric In-rich films respectively. Heterojunction devices using the structure ZnO/CdS/CuInSe 2 were fabricated with electrical conversion efficiencies of 6.5%.