Multi-stage evaporation of Cu2ZnSnS4 thin films

Multi-stage evaporation is a well-established method for the controlled growth of chalcopyrite thin films. To apply this technique to the deposition of Cu₂ZnSnS₄ thin films we investigated two different stage sequences: (A) using Cu₂SnS₃ as precursor to react with Zn-S and (B) using ZnS as precursor to react with Cu-Sn-S. Both Cu₂SnS₃ and ZnS are structurally related to Cu₂ZnSnS₄. In case (A) the formation of copper tin sulphide in the first stage was realized by depositing Mo/SnS_x/CuS (1 < x < 2) and subsequent annealing. In the second stage ZnS was evaporated in excess at different substrate temperatures. We assign a significant drop of ZnS incorporation at elevated temperatures to a decrease of ZnS surface adhesion, which indicates a self-limited process with solely reactive adsorption of ZnS at high temperatures. In case (B) firstly ZnS was deposited at a substrate temperature of 150 °C. In the second stage Cu, Sn and S were evaporated simultaneously at varying substrate temperatures. At temperatures above 400 °C we find a strong decrease of Sn-incorporation and also a Zn-loss in the layers. The re-evaporation of elemental Zn has to be assumed. XRD measurements after KCN-etch on the layers prepared at 380 °C show for both sample types clearly kesterite, though an additional share of ZnS and Cu₂SnS₃ can not be excluded. SEM micrographs reveal that films of sample type B are denser and have larger crystallites than for sample type A, where the porous morphology of the tin sulphide precursor is still observable. Solar cells of these absorbers reached conversion efficiencies of 1.1% and open circuit voltages of up to 500 mV.     

Study of optical and structural properties of Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ is a promising semiconductor to be used as absorber in thin film solar cells. In this work, we investigated optical and structural properties of Cu₂ZnSnS₄ thin films grown by sulphurization of metallic precursors deposited on soda lime glass substrates. The crystalline phases were studied by X-ray diffraction measurements showing the presence of only the Cu₂ZnSnS₄ phase. The studied films were copper poor and zinc rich as shown by inductively coupled plasma mass spectroscopy. Scanning electron microscopy revealed a good crystallinity and compactness. An absorption coefficient varying between 3 and 4 × 10⁴cm^− 1 was measured in the energy range between 1.75 and 3.5 eV. The band gap energy was estimated in 1.51 eV. Photoluminescence spectroscopy showed an asymmetric broad band emission. The dependence of this emission on the excitation power and temperature was investigated and compared to the predictions of the donor-acceptor-type transitions and radiative recombinations in the model of potential fluctuations. Experimental evidence was found to ascribe the observed emission to radiative transitions involving tail states created by potential fluctuations.     

Thin film preparation and characterization of wide band gap Cu3TaQ4 (Q = S or Se) p-type semiconductors

The structural, optical, and electronic properties of thin films of a family of wide band gap (E_g > 2.3 eV) p-type semiconductors Cu₃TaQ₄ (Q = S or Se) are presented. Thin films prepared by pulsed laser deposition of ceramic Cu₃TaQ₄ targets and ex-situ annealing of the as-deposited films in chalcogenide vapor exhibit mixed polycrystalline/[100]-directed growth on amorphous SiO₂ substrates and strong (100) preferential orientation on single-crystal yttria-stabilized zirconia substrates. Cu₃TaS₄ (E_g = 2.70 eV) thin films are transparent over the entire visible spectrum while Cu₃TaSe₄ (E_g = 2.35 eV) thin films show some absorption in the blue. Thin film solid solutions of Cu₃TaSe_4 − xS_x and Cu₃TaSe_4 − xTe_x can be prepared by annealing Cu₃TaSe₄ films in a mixed chalcogenide vapor. Powders and thin films of Cu₃TaS₄ exhibit visible photoluminescence when illuminated by UV light.     

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%.     

Structural and thermal characterization of La5Ca9Cu24O41 thin films grown by pulsed laser deposition on (1 1 0) SrTiO3 substrates

In the present study stoichiometric, b-axis oriented La₅Ca₉Cu₂₄O₄₁ thin films were grown by pulsed laser deposition on (1 1 0) SrTiO₃ substrates in the temperature range 600-750 °C. High resolution transmission electron microscopy was employed to investigate the growth mechanism and the epitaxial relationship between the SrTiO₃ substrates and the La₅Ca₉Cu₂₄O₄₁ films grown at 700 °C. The 3-ω method was used to measure the cross-plane thermal conductivity of La₅Ca₉Cu₂₄O₄₁ films in the temperature range 50-350 K. The observed glass-like behavior is attributed to atomic-scale defects, grain boundaries and an interfacial layer formed between film and substrate.     

Development of CZTS-based thin film solar cells

The low cost, environmental harmless Cu₂ZnSnS₄ (CZTS)-based thin film solar cells are fabricated by using abundant materials. The CZTS film possesses promising characteristic optical properties; band-gap energy of about 1.5 eV and large absorption coefficient in the order of 10⁴ cm^− 1. All constituents of this CZTS film, which are abundant in the crust of the earth, are non-toxic. Therefore, if we can use CZTS film practically as the absorber of solar cells, we will be free from both of the resource saving problem and the environmental pollution.In our CZTS project, CZTS absorber films were prepared by two independent techniques. One is three rf sources co-sputtering followed by annealing in sulfurized atmosphere. The latest conversion efficiency of over 6.7% was achieved by this technique. The other is co-evaporation technique. CZTS films were grown on Si (100) by vacuum co-evaporation using elemental Cu, Sn, S and binary ZnS as sources. XRD patterns indicated that the polycrystalline growth was suppressed and the orientational growth was relatively induced in a film grown at higher temperatures.In this presentation, the development of CZTS-based thin film solar cells will be surveyed.     

Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films

We fabricated Cu₂ZnSnS₄ (CZTS) thin films through sulfurization of stacked metallic films. Three types of Cu-Zn-Sn metallic films, i.e., Cu-rich, Cu-correct and Cu-poor precursor films were sputtered onto Mo-coated glass. The sulfurization of stacked Cu-Zn-Sn alloy films was performed at a relatively high temperature, 570 °C, with S-powder evaporation. CZTS films from Cu-rich and Cu-correct precursors showed a Cu_2 − xS phase on the film surface, while CZTS films from Cu-poor precursors didn't show the Cu_2 − xS phase. However, all films didn't exhibit any extra secondary phase and exhibited good crystalline textures even with Cu-ratio differences in metallic precursor films. Fabricated CZTS films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and Raman scattering measurements. SEM cross-section images of CZTS films showed that Cu-poor CZTS films were grown with more smooth film surface compared with other types of CZTS films.     

Microstructural study of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 heteroepitaxial trilayer films grown on (100) SrTiO3 substrates

Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa₂Cu₃O₇/SrTiO₃/YBa₂Cu₃O₇ trilayer thin films grown on (100)SrTiO₃ substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO₃ layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films.     

Radiative recombination in Cu2ZnSnSe4 monograins studied by photoluminescence spectroscopy

In this study we investigated the optical properties of Cu₂ZnSnSe₄ monograin powders that were synthesized from binary compounds in the liquid phase of flux material (KI) in evacuated quartz ampoules. The monograin powder had p-type conductivity. Radiative recombination processes in Cu₂ZnSnSe₄ monograins were studied using photoluminescence spectroscopy. The detected low-temperature (T = 10 K) photoluminescence band at 0.946 eV results from band-to-impurity recombination in Cu₂ZnSnSe₄. The ionization energy of the corresponding acceptor defect was found to be 69 ± 4 meV. Additional photoluminescence bands detected at 0.765 eV, 0.810 eV and 0.860 eV are proposed to result from Cu₂SnSe₃ phase whose presence in the as-grown monograins was detected by Raman spectroscopy and SEM analysis. Considering photoluminescence results, it is proposed that the optical bandgap energy of Cu₂ZnSnSe₄ is around 1.02 eV at 10 K.     

Epitaxial and polycrystalline CuInS2 thin films: A comparison of opto-electronic properties

Epitaxial and polycrystalline thin CuInS₂ (CIS) layers were grown by means of molecular beam epitaxy (MBE) on single crystalline silicon substrates of 4 inch diameter. Photoluminescence (PL) studies were performed to investigate the opto-electronic properties of these layers. For the epitaxial CIS, low-energy-hydrogen implantation leads to the passivation of deep defects and several donor-acceptor (DA) pair recombinations (from 1.034 eV to 1.439 eV) and two free-to-bound (FB) transitions (at 1.436 eV and 1.485 eV) become observable at low temperatures (5 to 100 K). Excitonic luminescence is completely absent for all investigated epitaxial CIS layers. This contrasts sharply with the PL of the polycrystalline films which is dominated by excitonic luminescence (1.527 eV). Also a donor-to-valence band transition at 1.465 eV (BF-1) and one donor-acceptor recombination at 1.435 eV (DA-1) were observed, while luminescence from deep levels is not present at all. Based on these data, a refined defect model for CuInS₂ with two donor and two acceptor states is presented. Under comparable growth conditions, the electronic quality of polycrystalline CIS is superior to epitaxially grown material.     

The crystallisation of Cu2ZnSnS4 thin film solar cell absorbers from co-electroplated Cu-Zn-Sn precursors

The best CZTS solar cell so far was produced by co-sputtering continued with vapour phase sulfurization method. Efficiencies of up to 5.74% were reached by Katagiri et al. The one step electrochemical deposition of copper, zinc, tin and subsequent sulfurization is an alternative fabrication technique for the production of Cu₂ZnSnS₄ based thin film solar cells. A kesterite based solar cell (size 0.5 cm²) with a conversion efficiency of 3.4% (AM1.5) was produced by vapour phase sulfurization of co-electroplated Cu-Zn-Sn films. We report on results of in-situ X-ray diffraction (XRD) experiments during crystallisation of kesterite thin films from electrochemically co-deposited metal films. The kesterite crystallisation is completed by the solid state reaction of Cu₂SnS₃ and ZnS. The measurements show two different reaction paths depending on the metal ratios in the as deposited films. In copper-rich metal films Cu₃Sn and CuZn were found after electrodeposition. In copper-poor or near stoichiometric precursors additional Cu₆Sn₅ and Sn phases were detected. The formation mechanism of Cu₂SnS₃ involves the binary sulphides Cu_2 − xS and SnS₂ in the absence of the binary precursor phase Cu₆Sn₅. The presence of Cu₆Sn₅ leads to a preferred formation of Cu₂SnS₃ via the reaction educts Cu_2 − xS and SnS₂ in the presence of a SnS₂(Cu₄SnS₆) melt. The melt phase may be advantageous in crystallising the kesterite, leading to enhanced grain growth in the presence of a liquid phase.     

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.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

Structure, morphology and optical properties of nanocrystalline yttrium oxide (Y2O3) thin films

Yttrium oxide (Y₂O₃) thin films were grown onto Si(1 0 0) substrates using reactive magnetron sputter-deposition at temperatures ranging from room temperature (RT) to 500 °C. The effect of growth temperature (T_s) on the growth behavior, microstructure and optical properties of Y₂O₃ films was investigated. The structural studies employing reflection high-energy electron diffraction RHEED indicate that the films grown at room temperature (RT) are amorphous while the films grown at T_s = 300-500 °C are nanocrystalline and crystallize in cubic structure. Grain-size (L) increases from ∼15 to 40 nm with increasing T_s. Spectroscopic ellipsometry measurements indicate that the size-effects and ultra-microstructure were significant on the optical constants and their dispersion profiles of Y₂O₃ films. A significant enhancement in the index of refraction (n) (from 2.03 to 2.25) is observed in well-defined Y₂O₃ nanocrystalline films compared to that of amorphous Y₂O₃. The observed changes in the optical constants were explained on the basis of increased packing density and crystallinity of the films with increasing T_s. The spectrophotometry analysis indicates the direct nature of the band gap (E_g) in Y₂O₃ films. E_g values vary in the range of 5.91-6.15 eV for Y₂O₃ films grown in the range of RT-500 °C, where the lower E_g values for films grown at lower temperature is attributed to incomplete oxidation and formation of chemical defects. A direct, linear relationship between microstructure and optical parameters found for Y₂O₃ films suggest that tuning optical properties for desired applications can be achieved by controlling the size and structure at the nanoscale dimensions.     

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.     

Film thickness dependence of critical current density for YBa2Cu3O7 films post-annealed at a low oxygen partial pressure

The variation of critical current density at 77 K as a function of film thickness was studied for YBa₂Cu₃O₇ films on (100) LaAlO₃ substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm^–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen.     

Effects of Mg doping on structural, optical, and electrical properties of CuScO2(0001) epitaxial films

CuScO₂ thin films with different Mg concentrations were grown on a-plane sapphire substrates by combining the two-step deposition and post-annealing techniques using Cu₂(Sc_1−xMg_x)₂O_y [X = 0.01, 0.05, 0.10] targets. The effects of the Mg doping in the Sc-site on the structural, optical, and electrical properties of the films were investigated. A Mg-doped CuScO₂[3R](0001) epitaxial film was obtained at a Mg concentration of 1 at%. The slight increase in the a-axis lattice constant and the slight decrease in the c-axis lattice constant of the film were confirmed using two-dimensional X-ray reciprocal space mapping. No significant increase in optical absorption was observed in the film, and the energy gap for direct allowed transition was estimated at 3.7 eV. The film showed an increase in the electrical conductivity and carrier concentration and a decrease in the Hall mobility compared with those of the non-doped epitaxial film. The decrease in the overlap of Cu 3d orbitals due to the increase in the a-axis lattice constant is one cause of the decrease in the Hall mobility of the film. The temperature dependence of the electrical transport properties of the film exhibited semiconducting characteristics, and the activation energy estimated from the temperature dependence of the carrier concentration was 0.55 eV.     

Thin film solar cells based on the ternary compound Cu2SnS3

Alongside with Cu₂ZnSnS₄ and SnS, the p-type semiconductor Cu₂SnS₃ also consists of only Earth abundant and low-cost elements and shows comparable opto-electronic properties, with respect to Cu₂ZnSnS₄ and SnS, making it a promising candidate for photovoltaic applications of the future. In this work, the ternary compound has been produced via the annealing of an electrodeposited precursor in a sulfur and tin sulfide environment. The obtained absorber layer has been structurally investigated by X-ray diffraction and results indicate the crystal structure to be monoclinic. Its optical properties have been measured via photoluminescence, where an asymmetric peak at 0.95 eV has been found. The evaluation of the photoluminescence spectrum indicates a band gap of 0.93 eV which agrees well with the results from the external quantum efficiency. Furthermore, this semiconductor layer has been processed into a photovoltaic device with a power conversion efficiency of 0.54%, a short circuit current of 17.1 mA/cm², an open circuit voltage of 104 mV hampered by a small shunt resistance, a fill factor of 30.4%, and a maximal external quantum efficiency of just less than 60%. In addition, the potential of this Cu₂SnS₃ absorber layer for photovoltaic applications is discussed.     

Optical and structural properties of CuSbS2 thin films grown by thermal evaporation method

Structural, optical and electrical properties of CuSbS₂ thin films grown by thermal evaporation have been studied relating the effects of substrate heating conditions of these properties. The CuSbS₂ thin films were carried out at substrate temperatures in the temperature range 100-200 °C. The structure and composition were characterized by XRD, SEM and EDX. X-ray diffraction revealed that the films are (111) oriented upon substrate temperature 170 °C and amorphous for the substrate temperatures below 170 °C. No secondary phases are observed for all the films. The optical absorption coefficients and band gaps of the films were estimated by optical transmission and reflection measurements at room temperature. Strong absorption coefficients in the range 10⁵-10⁶ cm^− 1 at 500 nm were found. The direct gaps Eg lie between 0.91-1.89 eV range. It is observed that there is a decrease in optical band gap Eg with increasing the substrate temperature. Resistivity of 0.03-0.96 Ω cm, in dependence on substrate temperature was characterized. The all unheated films exhibit p-type conductivity. The characteristics reported here also offer perspective for CuSbS₂ as an absorber material in solar cells applications.     

Influence of annealing treatments for ZnO-doped Zr0.8Sn0.2TiO4 thin films by RF magnetron sputtering

Zirconium tin titanium oxide doped 1 wt.% ZnO thin films on n-type Si substrate were deposited by rf magnetron sputtering at a fixed rf power of 300 W, a substrate temperature of 450 °C, a deposition pressure of 5 mTorr and an Ar/O₂ ratio of 100/0 with various annealing temperatures and annealing times. Electrical properties and microstructures of 1 wt.% ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by rf magnetron sputtering on n-type Si(100) substrates at different annealing temperatures (500 °C-700 °C) and annealing times (2 h-6 h) have been investigated. The structural and morphological characteristics analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM) were sensitive to the treatment conditions such as annealing temperature and annealing time. At an annealing temperature of 600 °C and an annealing time of 6 h, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films possess a dielectric constant of 46 (at f = 10 MHz), a dissipation factor of 0.059 (at f = 10 MHz), and a low leakage current density of 3.8 × 10^− 9 A/cm² at an electrical field of 1 kV/cm.