CZTSe Films Research Articles

Surfactant-free CZTS nanoparticles as building blocks for low-cost solar cell absorbers

A process route for the fabrication of solvent-redispersible, surfactant-free Cu2ZnSnS4 (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of ((Cu2+)a(Zn2+)b(Sn4+)c(Tu)d(OH−)e)t+, Tu = thiourea) oligomers, leading after temperature polycondensation and S2− exchange to highly concentrated (c > 100 g l−1), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells.

CZTSe thin film growth via a co-evaporation process using a ZnSe effusion source

The compositional distribution of elements is known to be significant for the high conversion efficiency of CZTSe solar cells. As detailed understanding of the Cu/(Zn+Sn) ratio in the light absorption layer is important, Cu2ZnSnSe4 (CZTSe) films grown via the co-evaporation process under different copper fluxes were characterized. It is difficult to evaluate the Cu content effect on the properties of CZTSe films grown using a co-evaporation process with Cu, Zn, Sn, and Se elemental effusion sources because the Cu flux variation during the process also induces other element ratio changes. Furthermore, the Zn/Sn ratio shows significant correlation to the Cu/(Zn+Sn) ratio variation in CZTSe thin films. Replacing the zinc metal effusion source with the ZnSe compound source resulted in less fluctuation in the Zn/Sn variation according to Cu flux change during the CZTSe co-evaporation. This can be useful in evaluating the effect of the different Cu ratios on the CZTSe solar cell characteristics.

Preparation and Analysis of Sputtered Cu<sub>2</sub>ZnSnSe<sub>4</sub> Thin Films

Cu2ZnSnSe4 (CZTSe) thin films with the advantages of low cost, abundance in resources, and the suitable band-gap of 0.9~1.1 eV have been the potential materials for solar cells, though the Cu(In,Ga)Se2 thin films have received most of the attentions. In this study, CZTSe thin films were prepared by direct-current (D.C.) sputtering using three self-made CZTSe targets in different compositions. The sputtered films displayed a preferred orientation in (112) by the X-ray diffraction analysis. The films were also characterized by field-emission scanning electron microscopy and energy dispersion spectroscopy. The films had the band-gap of 0.8~1.08 eV analyzed by absorption spectroscopy. CZTSe films were p-type and had a low electrical conductivity of 10-3 ohm-cm and a high carrier concentration of 1020~1021 cm-2.

The Preparation of Cu<sub>2</sub>ZnSnSe<sub>4</sub> Films by Selenization of the Oxide Precursors

A low-cost non-vacuum process for fabrication of Cu2ZnSnSe4 (CZTSe) films by solvent-free mechanochemical method and spin-coating process is described. First, highly monodisperse Cu, Zn, Sn oxides nanoparticles are synthesized via a facile, solvent-free route. Second, the oxide particulate precursors are deposited in a thin layer by spin-coating technique. Finally, the dry layers are sintered into CZTSe thin films selenization. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), it is found that near stoichiometric CZTSe films with a micron-sized grains are obtained in our work.

Characterization of Single Bath Electrodeposited Cu2ZnSnSe4 Thin Films

Cu2ZnSnSe4 (CZTSe), a chalcopyrite-like semiconductor, which has high optical absorption and p-type electrical conductivity, provide a low-cost mass production solution with abundant and cheap materials, Zn and Sn. In previous researches, CZTSe was produced from co-evaporated or sputtered precursors. In this study, electrodeposition, a non-vacuum and low-cost process, was used to grow CZTSe films. The films were grown by one-step cathodic electrodeposition in single electrolyte bath, which contained CuCl2, ZnCl2, SnCl2, and were deposited onto Mo coated glass substrates at room temperature. The CZTSe thin films have been obtained after annealing the as-deposited thin CZT films with powder of Se. The reduction potentials of each element in the electrolyte were determined by linear scan voltammetry (LSV) method. The film properties were characterized by field-emission scanning electron microscopy (FE-SEM) amd EDS.

Effect of Substrate Temperature on the Preparation of Cu<sub>2</sub>ZnSnSe<sub>4 </sub> Thin Films

The Cu2ZnSnSe4 (CZTSe) thin films were prepared by co-electroplating Cu-Zn-Sn precursors followed by selenization at different substrate temperatures. The effect of substrate temperatures on the morphologies and structures of CZTSe films were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and Raman scattering spectrum respectively. The results revealed that the impurity phases in CZTSe thin films such as CuSe and SnSe disappeared when the substrate temperatures were increased. The surface morphologies of CZTSe thin films were also strongly dependent on the substrate temperature treatment in the selenization process though the selenium temperature was kept at 340°C.

Growth of Cu2ZnSnSe4 thin films by selenization of sputtered single-layered Cu-Zn-Sn metallic precursors from a Cu-Zn-Sn alloy target

Cu2ZnSnSe4 (CZTSe) thin films were prepared by the simple process of selenization of single-layered metallic Cu-Zn-Sn precursors. These metallic precursors were deposited by radio frequency magnetron sputtering of a ternary Cu-Zn-Sn alloy target. Successive selenization was performed at various temperatures between 250°C and 500°C for 30 min. X-ray diffraction and Raman analysis showed that a single phase of the CZTSe compound can be obtained by selenization at 400°C, while increasing the selenization temperature to 500°C improves the grain size and crystal quality. The direct optical band gap of CZTSe films was calculated to be 1.06 eV to 1.09 eV with a high absorption coefficient on the order of 104 cm−1 for samples selenized at 400°C to 500°C. The obtained films are p-type semiconductors with bulk carrier concentrations of 2.41 to 7.96 × 1018 cm3, mobilities of 1.30 cm2 V−1 s−1 to 9.27 cm2 V−1 s−1, and resistivities of 0.20 Ωcm to 1.95 Ωcm.

Preparation and Characterization of Cu<sub>2</sub>ZnSnSe<sub>4</sub> Thin Films by Selenization of Electrodeposited Metal Precursors

Cu2ZnSnSe4thin films were prepared by selenization of electrodeposited Cu-Zn-Sn precursors. The Cu-Zn-Sn precursors were electrodeposited on the Mo-coated glass substrates from an electrolyte containing copper sulfate, zinc sulfate, tin (II) sulfate at a fixed potential between -1.2V and -1.25V vs. saturated calomel electrode, then the Cu2ZnSnSe4thin films were obtained by selenizing Cu-Zn-Sn precursors in elemental selenium atmosphere at different temperatures. The structure, composition and optical properties of the films were investigated by X-ray diffraction, Energy dispersive spectrometry and UV-VIS absorption spectroscopy. The CZTSe films have a stannite structure and an optical band-gap about 1.6 eV which is suitable for fabricating solar cells.

Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: On the discrepancies of reported band gap values

We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (Eg) of Cu2ZnSnSe4 (CZTSe) thin films, i.e., 1.0 or 1.5 eV. Eg of the coevaporated CZTSe film synthesized at substrate temperature (Tsub) of 370 °C, which was apparently phase pure CZTSe confirmed by x-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, Eg of the coevaporated films increases with Tsub due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the overestimation of overall Eg.

Effect of Cu/(Zn+Sn) ratio on the properties of co-evaporated Cu 2ZnSnSe 4 thin films

The effect of Cu/(Zn+Sn) ratio on the properties of Cu 2ZnSnSe 4 (CZTSe) thin films is investigated. CZTSe thin films with Cu/(Zn+Sn) ratio in the range 0.85–1.15 are deposited using 4-source co-evaporation technique onto glass substrates held at a substrate temperature T s=623 K and post-deposition annealed at T pa=723 K for 1 h in the selenium atmosphere. Powder X-ray diffraction (XRD) patterns reveal that CZTSe films deposited with Cu/(Zn+Sn) ratio in the range 0.90–1.10 are single phase and polycrystalline. CZTSe films, deposited with Cu/(Zn+Sn) ratio of 0.85 contain ZnSe as secondary phase and films with ratio of 1.15 contain Cu 2−XSe as the secondary phase. The films are found to exhibit kesterite structure. Band gap of the films is found to increase with decrease in Cu/(Zn+Sn) ratio. Electrical resistivity of the films is found to lie in the range 0.02–23-Ω-cm depending on Cu/(Zn+Sn) ratio.

Effect of post-deposition annealing on the growth of Cu2ZnSnSe4 thin films for a solar cell absorber layer

The effect of substrate temperature and post-deposition annealing on the growth and properties of Cu2ZnSnSe4 thin films, a potential candidate for a solar cell absorber layer, is investigated. The substrate temperature (Ts) is chosen to be in the range 523–673 K and the annealing temperature (Tpa) is kept at 723 K. Powder x-ray diffraction (XRD) patterns of as-deposited films revealed that the films deposited at Ts = 523 K and 573 K contain Cu2−xSe as a secondary phase. Single phase, polycrystalline Cu2ZnSnSe4 films are obtained at Ts = 623 K and films deposited at Ts = 673 K have ZnSe as a secondary phase along with Cu2ZnSnSe4. Direct band gap of as-deposited CZTSe films is found to lie between 1.40 eV and 1.65 eV depending on Ts. XRD patterns of post-deposition annealed films revealed that the films deposited at Ts = 523–623 K are single phase CZTSe and films deposited at Ts = 673 K still contain ZnSe secondary phase. CZTSe films are found to exhibit kesterite structure with the lattice parameters a = 0.568 nm and c = 1.136 nm. Optical absorption studies of post-deposition annealed films show that there is a slight increase in the band gap on annealing, due to decrease in the Cu content. Electrical resistivity of the films is found to lie in the range 0.02–2.6 Ω cm depending on Ts.

Growth and Properties of Stannite-Quaternary Cu<sub>2</sub>ZnSnSe<sub>4</sub> Thin Films Prepared by Selenization of Sputtered Binary Compound Precursors

Cu2ZnSnSe4 (CZTSe) thin films were grown on Corning glass 1737 by sequential methods of sputtering deposition and selenization process. As-grown films showed that elemental Cu, Zn, and Sn were in the nearly CZTSe stoichiometric ratio with Se-deficiency as detected by Energy Dispersive X-Ray spectrometry (EDX). In order to attain film stoichiometry, as-deposited films were subjected to selenization process in tube furnace under Ar ambient at different selenization temperatures for 10-60 min. It was found that compositions of binary compound in the sputtering target as well as selenization are critical for the growth of the CZTSe films. The structural characteristics of the selenized CZTSe films revealed a highly oriented stannite CZTSe phase with (112), (220/204) and (312/116) growth orientations and a CuSe secondary phase. By using 0.5% KCN solution, CuSe secondary phase could be totally etched from the CZTSe film surface.

Pulsed laser deposition of quaternary Cu2ZnSnSe4 thin films

AbstractWe report the successful growth of quaternary Cu2ZnSnSe4 (CZTSe) thin films by pulsed laser deposition (PLD) using Nd:YAG laser. It was found that CZTSe films atomic ratios were close to target atomic ratios with a slight metal excess and selenium deficiency. Quaternary CZTSe films grew and crystallized as a stannite‐type structure even at room temperature. All CZTSe films showed a p‐type electrical conductivity with a high absorption coefficient of 104–105 cm–1, a bandgap of 1.5 eV and a carrier concentration of the order of 1017–1018 cm–3. These results show that pulsed laser deposition could be employed as a particularly effective deposition method for the preparation of quaternary compounds thin films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single step preparation of quaternary [formula omitted] thin films by RF magnetron sputtering from binary chalcogenide targets

Cu 2 ZnSnSe 4 (CZTSe) thin films were grown in a single step procedure by RF magnetron sputtering from a compacted powder consisting of blended chalcogenides. Targets with various chalcogenide mole ratios were designed for the purpose of preparing stoichiometric as-grown films. The material concentrations of the films grown at room temperature were found to depend on the mole ratio of the chalcogenides in the targets. It was found that a significant deviation of material concentration of the films from ideal stoichiometry led to the formation of CuSe, ZnSe and SnSe secondary phases. CZTSe films with a stannite phase could be grown even at room temperature from the sputtering target containing Cu 2 Se with corresponding growth orientations of (101), (112), (220/204), (312/116) and (332/316). The p-type CZTSe film grown at a substrate temperature of 150 ∘ C showed a high absorption coefficient of 10 4 cm - 1 with an optical band gap of 1.56 eV, resistivity as low as 1.482 Ω cm and carrier concentration of 1 × 10 19 cm - 3 . These results suggested that the control of the target compositions was crucial to grow single phase and stoichiometric quaternary CZTSe films.