Photoluminesence studies of CdTe/SnO2 and CdTe/CdS heterojunctions: The influence of oxygen and the CdCl2 heat treatment

The influence of oxygen and annealing in the presence of CdCl₂ on the photoluminescence (PL) spectra of CdTe, component of SnO₂/CdTe heterojunction (HJ), has been studied in a temperature range of 17-100 K. The changes in the photoluminescence spectra were studied as a function of excitation intensity. Analysis of the PL spectra was carried out with considerations of spectra obtained from CdS/CdTe heterojunctions. CdTe side PL (SnO₂/CdTe HJ) consisted of 1.450 eV-DA defect band and 1.243 eV band (17 K). Annealing resulted in the disappearance of 1.243 eV band in oxygen containing samples. Interface PL for the unannealed samples consisted of mainly the 1.264 eV and a trace of the defect band. The CdCl₂ treatment is responsible for an almost symmetrical 1.416 eV band.     

Structural, optical, and electrical properties of tin sulfide thin films grown by spray pyrolysis

Tin sulfide (SnS) thin films have been prepared by spray pyrolysis (SP) technique using tin chloride and N, N-dimethylthiourea as precursor compounds. Thin films prepared at different temperatures have been characterized using several techniques. X-ray diffraction studies have shown that substrate temperature (T_s) affects the crystalline structure of the deposited material as well as the optoelectronic properties. The calculated optical band gap (E_g) value for films deposited at T_s = 320-396 °C was 1.70 eV (SnS). Additional phases of SnS₂ at 455 °C and SnO₂ at 488 °C were formed. The measured electrical resistivity value for SnS films was ∼ 1 × 10⁴ Ω-cm.     

Photovoltaic structures using chemically deposited tin sulfide thin films

Chemically deposited thin films of tin sulfide forms in two crystalline structures depending on the bath compositions used: orthorhombic, SnS(OR), and zinc-blende, SnS(ZB). These films posses p-type electrical conductivity and have band gaps of 1.2 and 1.7 eV, respectively. The photovoltaic structure: SnO₂:F/CdS/SnS(ZB)/SnS(OR) with evaporated Ag-electrode reported here shows an open circuit voltage (V_OC) of 370 mV, a short circuit current density (J_SC) of 1.23 mA/cm², fill factor of 0.44 and conversion efficiency of 0.2% under 1 kW/m² illumination intensity. We present an evaluation for improvement in the light generated current density when the two types of SnS absorber films are used. Different evaporated electrode materials were tested, from which Ag-electrode was chosen for this work. The results given above were obtained with SnS(ZB) film of 0.1 µm and SnS(OR) film of 0.5 µm in thickness.     

Synthesis and characterization of tin disulfide hexagonal nanoflakes via solvothermal decomposition

Tin disulfide (SnS₂) hexagonal flakes with diameters in the range of 50−150 nm are synthesized by using SnCl₂.2H₂O and sodium diethyldithiocabamate as source materials via a solvothermal decomposition route. As-prepared SnS₂ hexagonal nanoflakes are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and ultraviolet-visible (UV-vis) spectroscopy. The band gap energy of the SnS₂ nanoflakes is measured to be 2.17 eV, and the conduction band (CB) and valence band (VB) levels of the SnS₂ nanoflakes are calculated to be − 4.34 eV and − 6.55 eV respectively, showing them to be suitable for optional and electronic applications.     

Optimization of parameters of chemical spray pyrolysis technique to get n and p-type layers of SnS

SnS thin films were prepared using automated chemical spray pyrolysis (CSP) technique. Single-phase, p-type, stoichiometric, SnS films with direct band gap of 1.33 eV and having very high absorption coefficient (> 10⁵/cm) were deposited at substrate temperature of 375 °C. The role of substrate temperature in determining the optoelectronic and structural properties of SnS films was established and concentration ratios of anionic and cationic precursor solutions were optimized. n-type SnS samples were also prepared using CSP technique at the same substrate temperature of 375 °C, which facilitates sequential deposition of SnS homojunction. A comprehensive analysis of both types of films was done using x-ray diffraction, energy dispersive x-ray analysis, scanning electron microscopy, atomic force microscopy, optical absorption and electrical measurements. Deposition temperatures required for growth of other binary sulfide phases of tin such as SnS₂, Sn₂S₃ were also determined.     

Sub-2 nm SnO2 nanocrystals: A reduction/oxidation chemical reaction synthesis and optical properties

A simple reduction/oxidation chemical solution approach at room temperature has been developed to synthesize ultrafine SnO₂ nanocrystals, in which NaBH₄ is used as a reducing agent instead of mineralizers such as sodium hydroxide, ammonia, and alcohol. The morphology, structure, and optical property of the ultrafine SnO₂ nanocrystals have been characterized by high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), differential scanning calorimetry and thermogravimetric analysis (DSC-TGA), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectroscopy. It is indicated that the uniform tetragonal ultrafine SnO₂ nanocrystals with the size below 2 nm have been fabricated at room temperature. The band gap of the ultrafine SnO₂ nanocrystals is about 4.1 eV, exhibiting 0.5 eV blue shift from that of the bulk SnO₂ (3.6 eV). Furthermore, the mechanism for the reduction/oxidation chemical reaction synthesis of the ultrafine SnO₂ nanocrystals has been preliminary presented.     

Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al co-doped SnO2 semiconductor thin films prepared by spray pyrolysis

In this paper, the effect of S and Al concentrations on the structural, electrical, optical, thermoelectric and photoconductive properties of the films was studied. The [Al]/[Sn] and [S]/[Sn] atomic ratios in the spray solutions were varied from 10 at.% to 40 at.% and 0 to 50 at.%, respectively. X-ray diffraction analysis showed the formation of SnO₂ cassiterite phase as a main phase and the numerous sulfur phases including S, SnS, SnS₂ and Sn₂S₃ in SnO₂:Al films. Scanning electron microscopy studies showed that in the absence of S, increasing the Al content results in a smaller grain size and with the addition of S, the films appear to contain small cracks and nodules. The minimum resistance of 0.175 (kΩ/□) was obtained for S-doped SnO₂:Al (40 at.%) film with 20 at.% S-doping. From the Hall effect measurements, the majority carrier concentration was obtained in order of 10¹⁷-10¹⁸ cm^− 3. The thermoelectric measurements showed that majority carriers change from electrons to holes for S-doping in SnO₂:Al (40 at.%) thin films. The maximum Seebeck coefficient of + 774 μV/K (at T = 370 K) was obtained for S-doped SnO₂:Al (10 at.%) film with 50 at.% S-doping. The band gap values were obtained in the range of 3.8-4.2 eV. The S-doped SnO₂:Al (40 at.%) films have shown considerably photoconductivity more than S-doped SnO₂:Al (10 at.%) with increasing S-doping. The best photoconductive property was obtained for co-doped SnO₂ thin film with 40 at.% Al and 5 at.% S concentration in solution.     

Performance of CuIn1 − xGaxSe2/(PVD)In2S3 solar cells versus gallium content

The present work studies the influence of the Ga content (x = Ga / (Ga + In)) in the absorber on the solar cell performance for devices using (PVD)In₂S₃-based buffers. Input to the hypothesis of the relative conduction band positions can be found in the evolution of the device parameters with x. For experiments with x between 0 and 0.5 devices using (PVD)In₂S₃-based buffers are compared to reference devices using (CBD)CdS. Both buffers give similar cell characteristics for narrow band gap absorbers, typically E_gCIGSe < 1.1 eV. However, the parameters of the cells buffered with (PVD)In₂S₃ are degraded when the absorber gap is widened whereas (CBD)CdS reference devices are only slightly affected. Consequently, the solar cell efficiency is similar for both buffer layers at the lower x values and increases with x only in the case of (CBD)CdS. These evolutions are coherent with the existence of a conduction band cliff at the CIGSe/(PVD)In₂S₃ interface.     

Ag2СdSnS4 single crystals as promising materials for optoelectronic

Single crystals of the quaternary single crystals Ag₂CdSnS₄ were grown for the first time using the horizontal gradient freeze technique. Optical spectral and photoelectric properties of obtained crystals were investigated. The band gap energy at 77 K according to the photoconductivity spectra is 1.94 eV. The energy levels of the major donor centers in the band gap were determined. The role of intrinsic defects in the observed dependences is analyzed. The energy levels of the major donor centers in the band gap were determined. A small photoconductivity maximum at low temperature is observed at wavelength λ_m = 640 nm (hν ∼ 1.94 eV); situated in the fundamental absorption band, which unambiguously corresponds to the intrinsic origin of photoconductivity. The increase of the extrinsic photoconductivity with the maximum at λ_m ∼ 800 nm with temperature leads to its domination above 240 K. The observed peculiarity can be explained by the photoexcitation of electrons from the valence band to the donor centers which are empty at high temperatures and with further thermal excitation to the conduction band.     

The structure and properties of SnS thin films prepared by pulse electro-deposition

SnS films were prepared onto the ITO-coated glass substrates by pulse-form electro-deposition. The potential applied to the substrates was of pulse-form and its “on” potential, V_on was − 0.75 V (vs. SCE )and “off ” potential, V_off was varied in the range of − 0.1-0.5 V. The SnS films deposited at different V_off values were characterized by XRD, EDX, SEM and optical measurements. It shows that all the films are polycrystalline orthorhombic SnS with grain sizes of 21.54-26.93 nm and lattice dimensions of a = 0.4426-0.4431 nm, b = 1.1124-1.1134 nm and c = 0.3970-0.3973 nm, though the V_off has some influence on the surface morphology of the films and Sn/S ratio. When V_off = 0.1-0.3 V, the SnS films have the best uniformity, density and adhesion, and the Sn/S ratio is close to 1/1. The direct band gap of the films was estimated to be between 1.23 and 1.33 eV with standard deviation within ± 0.03 eV, which is close to the theoretical value. The SnS films exhibit p-type or n-type conductivity and their resistivity was measured to be 16.8-43.1 Ω cm.     

Energetic alignment in nontoxic SnS quantum dot-sensitized solar cell employing spiro-OMeTAD as the solid-state electrolyte

An environmentally friendly solid-state quantum dot sensitized solar cell (ss-QDSSC) was prepared by combining colloidal SnS QDs as the sensitizer and organic hole scavenger spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene) as the solid-state electrolyte, and the energy alignment of SnS and TiO₂ was investigated. The bandgap of colloidal SnS QDs increased with decreasing particle size from 14 to 4 nm due to an upshift of the conduction band and a downshift of the valence band. In TiO₂/SnS heterojunctions, the conduction band minimum (CBM) difference between TiO₂ and SnS was as large as ～0.8 eV; this difference decreased with decreasing particle size, but was sufficient for electron injection from SnS nanoparticles of any size into TiO₂. Meanwhile, the sensitizer regeneration driving force, that is, the difference between the valence band maximum (VBM) of SnS and the work function of the electrolyte, showed an opposite behaviour with the SnS size due to a downward shift of the SnS VB. Consequently, smaller SnS QDs should result in a more efficient charge transfer in heterojunctions, revealing the advantages of QDs vs larger particles as sensitizers. This prediction was confirmed by the improved photovoltaic performance of ss-QDSSCs modified with SnS nanoparticles, which peaked for 5–6 nm sized SnS nanoparticles due to the balance between electron injection and sunlight absorption.     

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si₃N₄ (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si₃N₄ layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si₃N₄/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of β-Si₃N₄ and a type-II heterojunction. The conduction band offset was deduced to be ~ 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/β-Si₃N₄ interface formation.     

Effect of oxygen on tuning the TiNx metal gate work function on LaLuO3

This paper presents experimental evidence on effective work function tuning due to the presence of oxygen at the TiNx/LaLuO₃ interface. Two complementary techniques, internal photoemission and X-ray photoelectron spectroscopy, show good agreement on the position of the metal gate Fermi level to conduction (2.79 ± 0.25 eV) and valence (2.65 ± 0.08 eV) band edge for TiNx/bulk LaLuO₃ gate stacks. The chemical shifts of Ti2p and N1s core levels and different degree in ionicity of TiNx metal gates correlate with the observed valence band offset shifts. The results have significance for setting the band edge work function and resulting low threshold voltage for ultimately scaled LaLuO₃-based p-metal oxide semiconductor field effect transistor devices.     

Physical properties of very thin SnS films deposited by thermal evaporation

SnS films with thicknesses of 20-65 nm have been deposited on glass substrates by thermal evaporation. The physical properties of the films were investigated using X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and ultraviolet-visible-near infrared spectroscopy at room temperature. The results from XRD, XPS and Raman spectroscopy analyses indicate that the deposited films mainly exhibit SnS phase, but they may contain a tiny amount of Sn₂S₃. The deposited SnS films are pinhole free, smooth and strongly adherent to the surfaces of the substrates. The color of the SnS films changes from pale yellow to brown with the increase of the film thickness from 20 nm to 65 nm. The very smooth surfaces of the thin films result in their high reflectance. The direct bandgap of the films is between 2.15 eV and 2.28 eV which is much larger than 1.3 eV of bulk SnS, this is deserving to be investigated further.     

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.     

Band offset of the In2S3/indium tin oxide interface measured by X-ray photoelectron spectroscopy

In₂S₃ thin films have been grown on Indium Tin Oxide (ITO) by Chemical Spray Pyrolysis. The structural and physical-chemical properties of the films have been investigated by means of X-ray Diffraction and X-ray Photoelectron spectroscopy (XPS). The valence band discontinuity at the In₂S₃/ITO interface has been determined by XPS resulting in a value of 1.9 ± 0.2 eV. Consequently, the conduction band offset has been estimated to be 1.0 ± 0.4 eV.     

Antimony sulfide thin films in chemically deposited thin film photovoltaic cells

Antimony sulfide thin films of thickness ≈ 500 nm have been deposited on glass slides from chemical baths constituted with SbCl₃ and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from − 3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb₂S_xSe_3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb₂S₃) to ≈ 1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb₂S₃-Ag₂Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)₂ with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO₂:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m² (tungsten halogen) are as follows: SnO₂:F-CdS-Sb₂S₃-Ag(paint) with open circuit voltage (V_oc) 470 mV and short circuit current density (J_sc) 0.02 mA/cm²; SnO₂:F-CdS-Sb₂S₃-CuS-Ag(paint), V_oc ≈ 460 mV and J_sc ≈ 0.4 mA/cm²; SnO₂:F-CdS-Sb₂S_xSe_3 − x-Ag(paint), V_oc ≈ 670 mV and J_sc ≈ 0.05 mA/cm²; SnO₂:F-CdS-Sb₂S₃-AgSb(S/Se)₂-Ag(paint), V_oc ≈ 450 mV and J_sc ≈ 1.4 mA/cm². We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’     

Structural and chemical transformations in SnS thin films used in chemically deposited photovoltaic cells

Chemically deposited SnS thin films possess p-type electrical conductivity. We report a photovoltaic structure: SnO₂:F-CdS-SnS-(CuS)-silver print, with V_oc > 300 mV and J_sc up to 5 mA/cm² under 850 W/m² tungsten halogen illumination. Here, SnO₂:F is a commercial spray-CVD (Pilkington TEC-8) coating, and the rest deposited from different chemical baths: CdS (80 nm) at 333 K, SnS (450 nm) and CuS (80 nm) at 293-303 K. The structure may be heated in nitrogen at 573 K, before applying the silver print. The photovoltaic behavior of the structure varies with heating: V_oc ≈ 400 mV and J_sc < 1 mA/cm², when heated at 423 K in air, but V_oc decreases and J_sc increases when heated at higher temperatures. These photovoltaic structures have been found to be stable over a period extending over one year by now. The overall cost of materials, simplicity of the deposition process, and possibility of easily varying the parameters to improve the cell characteristics inspire further work. Here we report two different baths for the deposition of SnS thin films of about 500 nm by chemical deposition. There is a considerable difference in the nature of growth, crystalline structure and chemical stability of these films under air-heating at 623-823 K or while heating SnS-CuS layers, evidenced in XRF and grazing incidence angle XRD studies. Heating of SnS-CuS films results in the formation of SnS-Cu_xSnS_y. ‘All-chemically deposited photovoltaic structures’ involving these materials are presented.     

Optical and electrical characterization of CdS thin films

Thin CdS films have been grown by chemical bath (CdCl₂, thiourea, ammonia) deposition (CBD) on SnO₂ (TO)-coated glass substrate for use as window materials in CdS/CdTe solar cells. High-resolution transmission electron microscopy revealed grains with an average size of 10 nm. The structure was predominantly hexagonal with a high density of stacking faults. The film crystallinity improved with annealing in air. Annealing in a CdCl₂ flux increased the grain size considerably and reduced the density of stacking faults. The optical transmission of the as-deposited films indicated a band gap energy of 2.41 eV. Annealing in air reduced the band gap by 0.1 eV. Annealing in CdCl₂ led to a sharper optical absorption edge that remained at 2.41 eV. Similar band gap values were obtained by photocurrent spectroscopy and electroabsorption spectroscopy (EEA) using an electrolyte contact. EEA spectra were broad for the as-deposited and air-annealed samples, but narrower for the CdCl₂-annealed films, reflecting the reduction in stacking fault density. Donor densities of ca. 10¹⁷ cm ^–3 were derived from the film/electrolyte junction capacitance.     

Preparation of AgInS2 chalcopyrite thin films by chemical spray pyrolysis

AgInS₂ thin films were prepared by the spray pyrolysis technique using a water/ethanol solution containing silver acetate, indium chloride and thiourea. We reported our results on the characterization of tetragonal AgInS₂ (chalcopyrite type) films, which were grown from indium deficient spraying solution. The films displayed a n-type conductivity with room temperature resistivities in the range between 10³ and 10⁴ Ω cm. The absorption spectra of sprayed films revealed two direct band-gaps with characteristic energies around 1.87 and 2.01 eV, which are in good agreement with the reported energy values for interband transitions from the split p-like valence band to the s-like conduction band in tetragonal AgInS₂ single crystals.