Sulfurization Of Metallic Precursors Research Articles

Current transport mechanisms in anisotype heterojunctions p-Cu2ZnSnS4/n-Si

Anisotype heterojunctions p-Cu2ZnSnS4/n-Si were fabricated by sulfurization of metalprecursors which were deposited on a poly-Si substrate. Current -voltage characteristics are discussed and the dominating mechanisms of current transfer are determined: at a direct bias tunnel recombination processes prevail with participation of defect states on interface of heterojunction, when voltage increases Newman-s tunnel mechanism dominates. The reverse current through the heterojunctions under investigationwas analyzed within the tunnel mechanism.

CZTS thin film solar cells utilizing sulfurization of metallic precursors

During this decade, slow but steady progress in copper tin zinc sulfide (CZTS) photovoltaics can be seen as a deep research interest and an extensive development effort in the Earth-abundant and non-toxic thin-film solar cells have continued worldwide. Sulfurization of metallic precursors is one of the most promising preparation processes for this optical absorber because it is considered favorable from a productivity viewpoint. Two types of precursors were studied in this paper. One was a multiple stack of pure metal layers and the other was an alloy layer with or without an overlayer consisting of pure metal. By investigating how the stacking sequence of each metallic precursor layer and a heating rate for sulfurization had an influence on the formation of secondary phases on the CTZS surface and also in the bulk, we found that the sulfurization process of Zn-covered metallic precursors had a beneficial effect on the improvement of device performance.

Cu2ZnSnS4 electrodeposition By Sulfurization of Metallic Precursors

Cu2ZnSnS4 (CZTS) is a kesterite-type material with promising properties among the materials suitable to be used as absorbers in solar cells. It is highly absorbent (α ≥ 105 cm-1), with a direct bandgap energy value (E GAP) suitable for solar light absorption (E GAP close to 1.5 eV) [1]. The constituting elements are not toxic, expensive or scarce, as is the case in competing family compounds such as chalcopyrites (CuInGaSe2). CZTS films are thus excellent candidates for low-cost, highly efficient and environmentally friendly solar cells [2]. There are different electroplating routes to obtain CZTS. In a previous work [3] we explored electrodeposition from a single bath containing the four elements and found it difficult to stabilize the sulfur source in the precursor mixture. Moreover, sulfur was found to incorporate into the material mainly during the sulfurization stage. In this work, the synthesis started with the deposition of a Cu-Zn-Sn precursor (CZT), to be continued by the incorporation of S during a heat treatment in the presence of vaporized sulfur that transforms the precursor into CZTS. The purpose of this investigation was to adjust the pH of the precursor bath to 5.9, so that CZTS could be deposited on top of ZnO, in order to prepare solar cells in superstrate configuration. The effect of electrodeposition time and annealing temperature were also explored. Thin films of CZTS were prepared by sulfurization of metallic precursors (CZT) previously electrodeposited during 5 to 10 min at -1.15 VSCE using conducting glass (FTO) as primary substrate. The precursor solution consisted of 100 mmol/L ZnSO4, 30 mmol/L CuSO4, 10 mmol/L SnSO4 and 100 mmol/L of sodium citrate as complexing agent. Citrates and hydroxyls concentrations were adjusted so that pH = 5.9. Electrodeposited CZT precursors underwent a reactive sulfurization annealing stage in a three-temperature zone thermal reactor. First, the samples were placed in the cold zone of the tube (room temperature). Then the second zone was heated to 500 °C and in the third zone the sulfur powder (0.3 g) was allowed to reach 350 °C (melting starts at around 150 °C). The vaporized sulfur reached the samples assisted by argon flux. After 30 to 60 minutes, the samples were left to cool down naturally. CZTS films were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electronic microscopy (SEM), UV-Vis spectroscopy and photoelectrochemical techniques. SEM results showed good coverage and homogenous morphology. Also, cross-section images showed thickness values close to 1 micron and higher than expected, even for samples electrodeposited for just 10 min. Raman spectroscopy and XRD results confirmed the formation of crystalline CZTS after sulfurization, with no secondary phases detected in the film. Photocurrent analysis confirmed the p-type nature of the semiconductor. Direct energy gap values close to 1.5 eV were estimated for CZTS films using transmittance spectra. CZTS films obtained with short electrodeposition times are good-quality absorbers suitable to be used in kesterite thin films solar cells. Preliminary results from solar cells in superstrate configuration show a promising photovoltaic response.

Investigate the growth mechanism of Cu2FeSnS4 thin films by sulfurization of metallic precursor

Cu2FeSnS4 (CFTS) thin films have been prepared on glass substrates by a process of sputtering metallic precursor with post-sulfurization. By means of various holding times, the growth mechanism of CFTS has been determined by X-ray diffraction and Raman patterns. Three steps have been involved, the binary-phases CuSx, FeSx and SnSx are synthesized in the preliminary stage, and afterwards, CuSx reacts with FeSx to generate ternary-compound CuFeS2. At last, the target product CFTS is obtained with the increasing holding time of sulfurization. All the thin films have smooth and closely packed surface, and longer holding time is in favor of the grain growth. The thickness of these thin films is around 1µm.

Heating rate tuning in structure, morphology and electricity properties of Cu2FeSnS4 thin films prepared by sulfurization of metallic precursors

Cu2FeSnS4 (CFTS) thin films have been synthesized by sulfurization of the sputtered metallic layers. The disappeared coexistence phase (Cu2FeSn3S8) and reduced strain have been observed in CFTS thin films accompanying the ritardando heating rate of sulfurization, and no impurity phase is detected in the samples. It is found that the S content at the bottom of CFTS thin films turns out to be very sensitive to the heating rate. The CFTS thin film sulfurized under faster heating rate has S-poor state with a bilayer structure and lots of micro-grains occupy the bottom of the CFTS thin film, while it has been obviously improved by reducing the heating rate. The optical transmission spectrum reveals that valence band maximum and conduction band minimum of CFTS are away from one another gradually with the decreasing heating rate, corresponding to the enhanced band gap energy of CFTS from 1.42 to 1.47 eV. As for these four CFTS devices, the highest open circuit voltage is 129 mV with short circuit current of 3.25 mA/cm2, which increased by 17.3% and 30.0%, respectively, compared to the previous work.

Influence of sulfurization pressure on Cu2ZnSnS4 thin films and solar cells prepared by sulfurization of metallic precursors

Effects of sulfurization pressure on composition, morphology and microstructure of kesterite Cu2ZnSnS4 (CZTS) thin films obtained by sulfurization of the metallic layers have been investigated in detail. It is found that the S content in the CZTS thin films is strongly dependent on the sulfurization pressure. The CZTS thin films sulfurized under low sulfurization pressure have S-poor state with a bilayer structure, while it exhibits sufficient amounts of sulfur under high sulfurization pressure with grain growth throughout the entire absorber film. X-ray diffraction data indicate lower sulfurization pressure during the CZTS grain growth process can induce the formation of more structural defects in the CZTS lattice and the CZTS thin films sulfurized under high sulfurization pressure have more random orientation. Furthermore, ZnS and MoS2 phase exist in all samples determined by Fourier transform infrared reflectance spectroscopy as complementary to Raman spectroscopy. The solar cell fabricated with the CZTS thin film under 10 Torr sulfurization pressure shows the best conversion efficiency of 3.52% (VOC = 484 mV, JSC = 14.56 mA cm−2, FF = 50.1%).

Effects of Cu/In compositional ratio on the characteristics of CuInS2 absorber layers prepared by sulfurization of metallic precursors

This paper investigates the effects of the Cu/In compositional ratio on morphological, structural and optical properties of CuInS2 (CIS) absorber layers formed by sulfurization of In/Cu stacked precursors. In/Cu stacked precursors were prepared on Mo-coated soda-lime glass substrates by DC magnetron sputtering method. The Cu/In compositional ratio in the precursor thin film was varied from 0.55 to 1.44. The as-deposited stacked precursor thin films were sulfurized using a tubular furnace annealing system in a mixture of N2 (95%) + H2S (5%) atmosphere at 450°C for 1 hour. X-ray diffraction patterns and Raman spectra results showed that the sulfurized thin films contained both tetragonal CIS and a Cu-based secondary phase, except for the film with a Cu/In compositional ratio of 0.55. Field emission-scanning electron microscopy study showed that the microstructure of the sulfurized CIS thin films became denser with increasing Cu/In compositional ratio. Optical properties of the CIS thin films showed that all the CIS thin films had a good absorption coefficient over 104 cm−1 in the visible region. The direct band gap energy of the sulfurized CIS thin films decreased from 1.39 eV to 1.08 eV with increasing Cu/In compositional ratio. These results demonstrated the effect of the Cu/In compositional ratio on the properties of the CIS thin films and the consequent importance of precisely controlling the metal ratio in the precursor film in order to control the properties of absorber layers in thin film solar cells.

CuIn 1− xAl xS 2 thin films prepared by sulfurization of metallic precursors

CuIn 1− x Al x S 2 thin films ( x = 0, 0.09, 0.27, 0.46, 0.64, 0.82 and 1) with thicknesses of approximately 1 μm were formed by the sulfurization of DC sputtered Cu–In–Al precursors. All samples were sulfurized in a graphite container for 90 min at 650 °C in a 150 kPa Ar + S atmosphere. Final films were studied via X-ray diffraction (XRD), scanning electron microscopy (SEM) and micro-Raman spectroscopy. It was found that all samples were polycrystalline in nature and their lattice parameters varied slightly nonlinearly from { a = 5.49 Å, c = 11.02 Å} for CuInS 2 to { a = 5.30 Å, c = 10.36 Å} for CuAlS 2. No unwanted phases such as Cu 2− x S or others were observed. Raman were recorded at a room temperature and the most intensive and dominant A 1 phonon frequency varied nonlinearly from 294 cm −1 (CuInS 2) to 314 cm −1 (CuAlS 2).

Current Transport in Cu(In,Ga)S2 Based Solar Cells with High Open Circuit Voltage - Bulk vs. Interface

AbstractCu(In,Ga)S2 thin films prepared by rapid thermal sulfurization of metallic precursors yielded solar cells with efficiencies reaching 12.9% [1]. A good short circuit current density was observed together with open circuit voltages up to 850 mV. However, the fill factor was close to, but typically did not exceed 70%. In this contribution we report on the role of junction formation by chemical bath deposition on these parameters. Concentrations in the bath and deposition times were varied. A comparison is made between CdS and Zn(S,O) buffer layers. The influence of the incorporated gallium on surface properties was investigated by ultraviolet photoelectron spectroscopy (UPS) for the valence band edge and near edge X-ray absorption fine structure (NEXAFS) for the conduction band edge. Even in our best cell (13.1%) the activation energy of the saturation current is found to be still smaller than the band gap. High diode ideality factors and voltage dependent current collection prevent higher fill factors.

CuAlS2 Thin-Films Prepared by Sulfurization of Metallic Precursors and their Properties

CuAlS2 thin-films with the (112) preferred orientation have been prepared for the first time by sulfurization of metallic precursors in a vacuum-sealed quartz ampule at temperatures ranging from 530°C to 850°C. The film had resistivity as low as 62 Ω·cm, and exhibited p-type conduction. The structure of the film was characterized by X-ray diffraction, scanning electron microscopy and electron probe micro-analysis. The bandgap estimated by an optical transmission measurement was 3.50 eV, in fair agreement with that of the single crystal.

Quasi real-time Raman studies on the growth of Cu–In–S thin films

In this work annealing and growth of CuInS2 thin films is investigated with quasireal-time in situ Raman spectroscopy. During the annealing a shift of the Raman A1 mode towards lower wave numbers with increasing temperature is observed. A linear temperature dependence of the phonon branch of −2 cm−1/100 K is evaluated. The investigation of the growth process (sulfurization of metallic precursors) with high surface sensitivity reveals the occurrence of phases which are not detected with bulk sensitive methods. This allows a detailed insight in the formation of the CuInS2 phases. Independent from stoichiometry and doping of the starting precursors the CuAu ordering of CuInS2 initially forms as the dominating ordering. The transformation of the CuAu ordering into the chalcopyrite one is, in contrast, strongly dependent on the precursor composition and requires high temperatures.