Preparation of Cu(In,Ga)(S,Se)2 thin films by sequential evaporation and annealing in sulfur atmosphere

Cu(In,Ga)(S,Se)₂ thin films with high Ga/III ratio (around 0.8) were prepared by sequential evaporation from CuGaSe₂, CuInSe₂, In₂Se₃ and Ga₂Se₃ compounds and then annealing in H₂S gas atmosphere. The annealing temperature was varied from 400 to 500 °C. These samples were characterized by means of XRF, EPMA, XRD and SEM. The S/(S+Se) mole ratio in the thin films increased with increase in the annealing temperature, keeping the Cu, In and Ga contents nearly constant. The open circuit voltage increased and the short circuit current density decreased with increase in the annealing temperature. The best solar cell using Cu(In,Ga)(S,Se)₂ thin film with Ga/(In+Ga)=0.79 and S/(S+Se)=0.11 annealed at 400 °C demonstrated V_oc=535 mV, I_sc=13.3 mA/cm², FF=0.61 and efficiency=4.34% without AR-coating.     

CuIn(Ga)Se2-based devices via a novel absorber formation process

A novel pathway for the formation of copper–indium (gallium) diselenide has been developed. This two-stage process consists of (a) the formation of Cu–In–(Ga)–Se precursors, and (b) subsequent thermal treatment to form CuIn(Ga)Se₂. The morphology, structure and growth mechanism for several different precursor structures prepared under various conditions were studied and correlated to the deposition parameters as well as the structure and morphology of the annealed films. Photovoltaic devices prepared from CuInSe₂ and CuIn_0.75Ga_0.25Se₂ resulted in efficiencies of 10% and 13%, respectively.     

Characterization of Cu(In,Ga)Se2 thin films prepared by thermal crystallization on Mo/glass substrate

Thin films of Cu(In,Ga)Se₂ were prepared by thermal crystallization on the sputtered Mo/substrate and characterized. MoSe₂ layer was formed at the interface between Cu(In,Ga)Se₂ and Mo layers after the thermal crystallization. The graded Ga concentration in crystallized Cu(In,Ga)Se₂ thin films was confirmed. Cu(In,Ga)Se₂ thin films prepared on the Mo/soda-lime glass had large and columnar grains rather than those on the Mo/quartz substrate.     

Compositional and optoelectronic properties of CIS and CIGS thin films formed by electrodeposition

CuInSe₂ (CIS) and Cu(In,Ga)Se₂ (CIGS) thin films were prepared by electrodeposition and processing. The influence of film deposition parameters such as bath composition, pH, deposition potential and material purity on film properties was studied. The structural, morphological, compositional and opto-electronic properties of electrodeposited and selenized CIS and CIGS thin films were characterized using various techniques. As-deposited as well as selenized films exhibited a compact or a granular morphology depending on the composition. The film stoichiometry was improved after selenization at 550°C in a tubular furnace. The films are formed with a mixed phase composition of CuInSe₂ and CuIn₂Se_3.5 ternary phases.     

Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique

The effects of sodium on off-stoichiometric Cu(In,Ga)Se₂ (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na₂Se on Mo-coated SiO_x/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10–13.5% range were obtained over an extremely wide Cu/(In + Ga) ratio range of 0.51–0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)₃Se₅ phase films with p-type conductvity. A new type of solar cell with p-type Cu(In,Ga)₃Se₅ phase absorber materials is also suggested.     

Structure, morphology and photoelectrochemical activity of CuInSe2 thin films as determined by the characteristics of evaporated metallic precursors

CuInSe₂ thin films have been obtained by the sequential evaporation of Cu and In layers, and subsequent reaction at 400°C with elemental selenium vapor. The individual metallic film thickness and the substrate temperature during evaporation have been varied in order to promote intermixing and alloy formation before the selenization. The structure, morphology and photoelectrochemical activity of the CuInSe₂ films have been determined by the characteristics of the evaporated metallic precursors. An improvement in the CuInSe₂ quantum efficiency, related mainly to the increased homogeneity and smoothing of the sample surface, can be gained by using as precursors multiple stacked Cu–In bilayers evaporated onto unheated substrates.     

CuGaxSey chalcopyrite-related thin films grown by chemical close-spaced vapor transport (CCSVT) for photovoltaic application: Surface- and bulk material properties, oxidation and surface Ge-doping

Device-grade ternary Cu-Ga-Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10×10 cm². A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe₂-related defect compound structures (DC) with stoichiometries of CuGa₃Se₅ and CuGa₅Se₈. In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe₂-related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa₃Se₅ formation in gallium-rich CCSVT-grown CuGa_xSe_y films. The chemical and electronic surface and interface structure of CuGaSe₂ thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced “cliff-like” conduction band offset at the respective interface.Furthermore, the CuGaSe₂ thin film degradation under ambient as well as under thermal conditions of CuGaSe₂ thin films has been studied by XPS. During thermal oxidation, the formation of predominantly Ga₂O₃ and some amount of SeO₂ were observed, but no copper oxides could be detected in the near-surface region of the thin films. The same oxides are found after native oxidation in air under ambient conditions. An additional sodium oxide compound formed at the thin film surface, Na_xO and Na₂CO₃ after thermal and native oxidation, respectively.Germanium ion implantation technique of the near-surface region of CuGaSe₂ thin films has been used in order to prove the feasibility of n-type doping. In photoluminescence (PL) studies, the occurrence of a new emission line is identified as Ge related and explained as a donor-acceptor-pair (DAP) recombination. The precise role the Ge is playing in this doping of CuGaSe₂ is revealed by X-ray absorption spectroscopy (XANES and EXAFS) and ab initio calculations based on the density functional theory. The studies indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe₂ originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted anti-bonding Ge-Se4sp³ defect band appearing well above the Fermi level for the Ge¹⁺_Ga point defect system can be directly linked to a Ge-dopant-related donor-acceptor-pair transition as observed in our photoluminescence spectra.     

The structural and material properties of CuInSe2 and Cu(In,Ga)Se2 prepared by selenization of stacks of metal and compound precursors by Se vapor for solar cell applications

CuInSe₂ films and related alloys were prepared by thermal evaporation of Cu, InSe and GaSe compounds instead of elemental sources. Band-gap tailoring in Cu(In,Ga)Se₂-based solar cells is an interesting path to improve their performance. In order to get comparable results, solar cells with Ga/(In+Ga) ratios x=0 and 0.3 were prepared, all with a simple two-step sequential evaporation process. The morphology of the resulting films grown at 550 °C was characterized by the presence of large facetted chalcopyrite grains, which are typical for device quality material. It is important to note that absorber films with elemental gallium resulted in a significant decrease in the average grain size of the film. The X-ray diffraction (XRD) diffraction pattern of single-phase Cu(In,Ga)Se₂ films depicts diffraction peaks shifting to higher 2θ values compared to that of pure CuInSe₂. The photoluminiscence (PL) spectrum of Cu(In,Ga)Se₂ thin films also depicts the presence of the peak at higher energy that is attributed to the incorporation of gallium into the chalcopyrite lattice. As the band gap of CIGS increases with gallium content, desirable effects of producing higher open-circuit voltage and low current density devices were achieved. A corresponding increase in device efficiency with gallium content caused by a higher fill factor was observed. The best results show passive area efficiencies of up to 10.2% and open-circuit voltage (V_oc) up to 519 mV at a minimum band gap of 1.18 eV.     

Cu(In,Ga)Se2 based photovoltaic structure by electrodeposition and processing

CuIn_1−xGa_xSe₂ (CIGS) thin films were formed from an electrodeposited CuInSe₂ (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 9.8% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF₂. The cell parameters such as J_sc, V_oc, FF and η were determined from I–V characterization of the cell.     

Increased homogeneity and open-circuit voltage of Cu(In,Ga)Se2 solar cells due to higher deposition temperature

Cu(In,Ga)Se₂-absorber deposition on commonly used soda lime glass is constrained in temperature by the softening of the substrate. To overcome this limitation, a high-temperature resistant glass was employed as a substrate for the growth of Cu(In,Ga)Se₂-absorbers by multi-stage coevaporation at standard (530 °C) and elevated (610 °C) temperatures. Absorbers were investigated using cathodoluminescence and X-ray diffraction and compared to the performance of solar cells fabricated from these absorbers. The higher deposition temperature is shown to lead to an increased homogeneity of the absorber layer both laterally and vertically and strongly enhanced open-circuit voltage. A best certified efficiency of 19.4% is reached.     

Large grain Cu(In,Ga)Se2 thin film growth using a Se-radical beam source

Cu(In,Ga)Se₂ (CIGS) thin films were grown by the three-stage process using a rf-plasma cracked Se-radical beam source. CuGaSe₂ (CGS) films grown at a maximum substrate temperature of 550 °C and CuInSe₂ (CIS) and CIGS films grown at the lower temperature of 400 °C exhibited highly dense surfaces and large grain size compared with films grown using a conventional Se-evaporative source. This result is attributed to the modification of the growth kinetics due to the presence of active Se-radical species and enhanced surface migration during growth. The effect on CIGS film properties and solar cell performance has been investigated. Enhancements in the cell efficiencies of 400 °C-grown CIS and CIGS solar cells have been demonstrated using a Se-radical source.     

Characterization of co-sputtered Cu-In alloy precursors for CuInSe2 thin films fabrication by close-spaced selenization

Sputtering technique for Cu–In precursor films fabrication using different Cu and In layer sequences have been widely investigated for CuInSe₂ production. But the CuInSe₂ films fabricated from these precursors using H₂Se or Se vapour selenization mostly exhibited poor microstructural properties. The co-sputtering technique for producing Cu–In alloy films and selenization within a close-spaced graphite box resulting in quality CuInSe₂ films was developed. All films were analysed using SEM, EDX, XRD and four-point probe measurements. Alloy films with a broad range of compositions were fabricated and XRD showed mainly In, CuIn₂ and Cu₁₁In₉ phases which were found to vary in intensities as the composition changes. Different morphological properties were displayed as the alloy composition changes. The selenized CuInSe₂ films exhibited different microstructural properties. Very In-rich films yielded the ODC compound with small crystal sizes whilst slightly In-rich or Cu-rich alloys yielded single phase CuInSe₂ films with dense crystals and sizes of about 5 μm. Film resistivities varied from 10⁻²–10⁸ Ω cm. The films had compositions with Cu/In of 0.40–2.3 and Se/(Cu+In) of 0.74–1.35. All CuInSe₂ films with the exception of very Cu-rich ones contained high amount of Se (>50%).     

Thin films of Cu(In,Ga)Se2 and ordered vacancy compound prepared by thermal crystallization and their photovoltaic applications

Thin films of Cu–In–Ga–Se alloy system with various composition were prepared by thermal crystallization from In/CuInGaSe/In precursor. Electron probe microanalysis and X-ray diffraction study revealed that these samples were assigned to chalcopyrite Cu(In,Ga)Se₂ or ordered vacancy compound Cu(In,Ga)₂Se_3.5. Solar cell with ZnO:Al/i–ZnO/CdS/Cu(In,Ga)Se₂/Mo/soda-lime glass substrate structure was fabricated by using thermal crystallization technique, and demonstrated a 9.58% efficiency without AR-coating.     

Role of incorporated sulfur into the surface of Cu(InGa)Se2 thin-film absorber

High-performance Cu(InGa)Se₂ (CIGS) thin-film absorbers with an intentionally graded band-gap structure have been fabricated by a simple two-stage method using In/Cu–Ga/Mo stacked precursors and H₂Se gas. Additional sulfurization step to form a thin Cu(InGa)(SeS)₂ (CIGSS) surface layer on the absorber is necesarry to improve the device performance. In order to understand the role of S incorporated into CIGS absorber, approaches with S are discussed. One approach is carried out by changing the condition of our absorber formation process. It is verified to be possible to incorporate more S into the CIGS absorber, but difficult to improve the device performance with higher S contained CIGS absorbers because of decrease in FF. The incorporated S is concluded to be effective to improve the pn heterojunction quality due to the passivation of surface and grain boundary of CIGS absorber through the formation of a thin CIGSS surface layer.     

Preparation and characterization of Cu(In,Ga)(Se,S)2 thin films from electrodeposited precursors for hydrogen production

Semiconducting Cu(In,Ga)(Se,S)₂ thin films were made from electrodeposited Cu(In,Ga)Se₂ precursors, followed by physical vapor deposition of In₂S₃, Ga, and Se. The bandgaps of these materials were found to be between 1.6 and 2.0 eV, which spans the optimal bandgap necessary for application for the top junction in photovoltaic multijunction devices and for unassisted water photolysis. These films were characterized by electron-probe microanalysis, scanning Auger spectroscopy, X-ray diffraction, and photocurrent spectroscopy.     

Selenization of Cu and In thin films for the preparation of selenide photo-absorber layers in solar cells using Se vapour source

Gas phase selenization of vacuum deposited Cu and In thin films employing an elemental Se vapour source is demonstrated as an essential first step in the search for optimized process parameters for the formation of single phase CuInSe₂ materials suitable for solar cell applications. The selenization was accomplished in Se vapour, derived from an elemental Se source, held at 240–260°C. This source was placed in a flow of nitrogen gas at 500 Torr to transport the Se vapour to the metal films. The selenization reaction readily occurs at Cu and In films kept at 340–400°C. Lower selenization temperatures invariably lead to the formation of Cu and In selenides with well defined crystalline microstructures. Hexagonal CuSe with an excess of Se in the matrix is the equilibrium growth phase, while the cubic Cu_2−xSe phase evolves under conditions of excess Se flux. Selenization of the In films consistently led to the formation of the β-form of hexagonal In₂Se₃. At high selenization temperatures (400°C), while the β-form still emerges as the major component, traces of the α-form of In₂Se₃ are also detected. Detailed X-ray diffraction, electron probe analysis and microstructure data are presented.     

The effects of the morphology on the CIGS thin films prepared by CuInGa single precursor

Cu(In_xGa_1−x)Se₂ (CIGS) thin films were prepared by selenization of CuInGa single-layer metallic precursors. At the first stage, CuInGa metallic precursors were deposited onto soda lime glass by direct current (DC) magnetron sputtering system using a CuInGa ternary alloy target with a composition ratio of Cu:In:Ga of 1:0.7:0.3. The precursor films were reacted with Se vapor in vacuum evaporation system. By means of X-ray diffraction (XRD), field emission scanning electron microscope (SEM) and electron probe microanalysis (EPMA), it was found that CIGS thin films exhibit large facetted grains and single chalcopyrite phase with preferred orientation along (1 1 2) plane. Meanwhile, the surface roughness of the CIGS films can be determined by the morphology of the precursor films.     

Preparation of CuIn1−xGaxSe2 thin films by sputtering and selenization process

CuIn_1−xGa_xSe₂ polycrystalline thin films were prepared by a two-step method. The metal precursors were deposited either sequentially or simultaneously using Cu–Ga (23 at%) alloy and In targets by DC magnetron sputtering. The Cu–In–Ga alloy precursor was deposited on glass or on Mo/glass substrates at either room temperature or 150°C. These metallic precursors were then selenized with Se pellets in a vacuum furnace. The CuIn_1−xGa_xSe₂ films had a smooth surface morphology and a single chalcopyrite phase.     

Effect of selenization pressure on CuInSe2 thin films selenized using co-sputtered Cu-In precursors

CuInSe₂ thin films were formed from the selenization of co-sputtered Cu–In alloy layers. These layers consisted of only two phases, CuIn₂ and Cu₁₁In₉, over broad Cu–In composition ratio. The concentration of Cu₁₁In₉ phase increased by varying the composition from In-rich to Cu-rich. The composition of co-sputtered Cu–In alloy layers was linearly dependent on the sputtering power of Cu and In targets. The metallic layers were selenized either at a low pressure of 10 mTorr or at 1 atm Ar. A small number of Cu–Se and In–Se compounds were observed during the early stage of selenization and single-phase CuInSe₂ was more easily formed in vacuum than at 1 atm Ar. Therefore, CuInSe₂ films selenized in vacuum showed smoother surface and denser microstructure than those selenized at 1 atm. The results showed that CuInSe₂ films selenized in vacuum had good properties suitable for a solar cell.     

Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.