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.     

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.     

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.     

Band gap modified Al-doped Zn1?xMgxO and Zn1?yCdyO transparent conducting thin films

Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films were prepared on glass substrates by sol–gel method. The codoping thin films showed preferential c-axis orientation, and the lattice constant c evaluated from the shift of the position of (002) peak displayed an increasing evolution from x = 8 at.% to y = 8 at.%, indicating a roughly statistical substitution of Mg²⁺ and Cd²⁺ for Zn²⁺ in their solid solution. The effects of narrowing and widening band gap (E _g) on conductivity of (Cd, Al) and (Mg, Al) codoped ZnO thin films were simultaneously investigated using transmission spectra and electrical measurements. The transmittances of these films are obviously decreased by vacuum annealing to 50–60%. However, the carrier concentration and Hall mobility both increase, and resistivity decreases with narrowing band gap in 1 at.% Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films from x = 8 at.% to y = 8 at.%. It is revealed that the conductivity of Al-doped ZnO thin films could be enhanced by this simple band gap modification.     

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.     

Enhancement in photovoltaic performance of CZTS Thin-film solar cells through varying stacking order and sulfurization time

The CZTS samples were produced by a two-stage method, which includes deposition of Cu, Sn, Zn, and ZnS layers using magnetron sputtering to obtain CuSn/Zn/Cu and CuSn/ZnS/Cu stacks. The latter stage involves the sulfurization process of stacked films at 550 °C for varied sulfurization time (60, 90, 120, and 150 s) employing Rapid Thermal Processing (RTP) method to attain CZTS structure. The prepared CZTS thin films were analyzed utilizing several characterization methods. The energy-dispersive X-ray spectroscopy (EDX) measurements revealed that all sulfurized samples had Cu-poor and Zn-rich chemical composition. All samples showed that diffraction peaks belonged to pure kesterite CZTS phase subject to their XRD patterns. Besides, it was observed that the sulfurization time had a crucial effect on the crystal size of the samples. The Raman spectra of the samples verified the constitution of kesterite CZTS phase and it provides detection of some CTS-based secondary phases. The scanning electron microscopy (SEM) image of the films revealed that polycrystalline surface structures were observable in all the samples. However, plate-like surface features were observed in some samples that may refer to CTS-based secondary phases depending on chemical composition. From 1.40 to 1.48 eV optical band gap values were obtained from (αhν)² vs. photon energy (hν) plots. The Van der Pauw measurements exhibited that the CZTS samples produced employing CuSn/ZnS/Cu stack had lower resistivity (~?10^–3 Ω cm), higher carrier concentration values (~?10²¹ cm^?3), and higher charge mobility. The solar cells prepared using the most promising CZTS samples employing CuSn/Zn/Cu and CuSn/ZnS/Cu precursor films revealed 1.95% and 3.10% conversion efficiencies, respectively.

     

Effect of gas type,pressure and temperature on the electrical characteristics of Al-doped SnO2 thin films deposited by RGTO method for gas sensor application

In this paper, we present the experimental results of Al-doped SnO₂ thin films obtained by Rheotaxial Growth and Thermal Oxidation (RGTO) method. The effect of gas type (synthetic air, CO, NO₂ and H₂), pressure (10⁻⁴, 1 and 100 mbar) and temperature (in the range 300–650 K), on the electrical properties of Al-doped SnO₂ thin films were considered. The change of the work function of Al-doped SnO₂ thin films as a function of exposure time to synthetic air, CO (150 ppm), H₂ (1000 ppm) and NO₂ (80 ppm) was discussed under different pressures of 10⁻⁴, 1 and 100 mbar. The effect of ambient temperature at 303,373 and 433 K on the work function difference was investigated. The results reveal that the sensitivity of reaction to the gases was improved to high ambient temperature. The time and temperature dependent of electrical properties of the Al-doped SnO₂ films were studied using four probe method. The Al-doped SnO₂ films demonstrate negative temperature coefficient (NTC) characteristics of resistance in the higher temperature range as well as positive temperature coefficient (PTC) characteristics of resistance in the lower temperature range. The best sensitivity and the optimum work temperature were also considered.     

Effect of vacuum annealing on the phase composition of In/SnO/Si, In/SnO2/Si, and Sn/In2O3/Si heterostructures

The chemical interaction between indium and thin SnO and SnO₂ films and between tin and thin In₂O₃ films during vacuum annealing was studied. The metallic films were deposited onto single-crystal silicon substrates by magnetron sputtering, the SnO and SnO₂ films were produced by heat-treating the Sn film in flowing oxygen at 673 and 873 K, respectively, and the In₂O₃ film was produced by heat-treating the In film at 573 K. The results indicate that annealing of the In/SnO/Si and In/SnO₂/Si heterostructures in vacuum (residual pressure of 0.33 × 10^?2 Pa) at 773 K gives rise to the reduction of Sn and oxidation of In, whereas annealing of Sn/In₂O₃/Si causes partial tin substitution for indium in the cubic indium oxide lattice.     

Determination of optical constants of CdS x Se y electrodeposits

The optical constantsn(λ) and α(λ) have been determined for CdS_xSe _y films electrodeposited on to glass doped with SnO₂. From a (αhv)² versus photon energy plot, a value for the direct band gap of the electrodeposited polycrystalline semiconductor ofE _g=1.83 eV was obtained, which indicates a low proportion of S²⁻ in the thin films formed.     

Effects of deposition temperature on structure and properties of mo-doped indium oxide by radio frequency magnetron sputtering

Transparent conductive oxide (TCO) thin films of Mo-doped In₂O₃ (IMO) were prepared on glass substrates by radio frequency magnetron sputtering from the 2 wt% Mo-doped In₂O₃ ceramic target. The depositions were carried out under an oxygen-argon atmosphere by varying the deposition temperature from 200 °C to 350 °C. The crystal structure and thickness of IMO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of deposition temperature on the electrical and optical transmittance properties of IMO thin films were investigated by four-point probe Hall system and UV-VIS-NIR spectrophotometer separately. The optimum deposited IMO thin films were obtained with resistivity of 6.9 × 10⁻⁴ Ω cm and carrier mobility 45 cm²v⁻¹s⁻¹ at 350 °C. The average optical transmittance of IMO films on glass substrates are over 80% in the near-infrared region.     

Piezoresistive response of ITO films deposited at room temperature by magnetron sputtering

Indium tin oxide (ITO) thin films have been deposited on (100) Si substrates by RF magnetron sputtering from a compact target (90% In₂O₃–10% SnO₂ in weight) with 6 in. in diameter. In order to perform electromechanical characterizations of these films, strain gauges were fabricated. An experimental set-up based on bending beam theory was developed to determine the longitudinal piezoresistive coefficient (π_l) of the strain gauges fabricated. It has been confirmed that electrical resistance of the strain gauges decreases with load increases which results a negative gauge factor. A model based on the activation energy was used to explain the origin of this negative signal. The influence of the temperature on piezoresistive properties of ITO films was also evaluated.     

Preparation and properties of reactively co-sputtered transparent conducting films

The method of reactive co-sputtering was used to determine the optimum dopant concentration for low resistivity In₂O₃/SnO₂ and SnO₂/Sb₂O₅ films. The optimum concentration of SnO₂ in In₂O₃ was approximately 10 mol. % and of Sb₂O₅ in SnO₂ about 7 mol. %. The resistivity increased sharply at lower dopant concentrations but changed only slightly at higher dopant concentrations. The lowest resistivity for reactively sputtered highly transparent In₂O₃/SnO₂ films was 1.5 × 10^-3 Ω cm and for SnO₂/Sb₂O₅ films 3 × 10^-3 Ω cm. Reactively sputtered In₂O₃/SnO₂ films show a strong (111) texture and have an extremely smooth surface.     

Characterization of the chemical bath deposited In(OH)xSy films: Effect of the growth conditions

The In(OH)_xS_y thin films were deposited by chemical bath deposition (CBD) using three different deposition procedures: ‘hot’: starting the deposition at 70 °C, ‘cold’: starting the deposition at room temperature and pre-treatment with In³⁺ ions prior the ‘hot’ deposition. The analysis of the deposited In(OH)_xS_y layers on glass revealed that modifications in the chemical bath deposition procedure provoked significant changes in the nucleation process, the growth rate, the layer elemental composition and the layer morphology. With an additional In³⁺ pre-treatment or starting from a cold solution, the formation of a dense bottom layer has been observed, resulting in In(OH)_xS_y films with more compact structure with refractive index values of 2.6. The comparison of the measured In/S ratio with a thicker layer suggests, that the In(OH)_xS_y deposition starts with an OH-rich layer. Assuming the indirect allowed band gap transition type, an E_g of 2.2 eV was found independent of the procedure type or deposition time.     

Properties of indium oxide/tin oxide multilayered films prepared by ion-beam sputtering

The preparation and characterization of indium oxide (InO _x )/tin oxide (SnO _y ) multilayered films deposited by ion-beam sputtering are described and compared with indium tin oxide (ITO) films. The structure and the optoelectrical properties of the films are studied in relation to the layered structures and the post-deposition annealing. Low-angle X-ray diffraction analysis showed that most films retained the regular layered structures even after annealing at 500° C for 16 h. As an example, we obtained a resistivity of 6×10^–4 cm and a transparency of about 85% in the visible range at a thickness of 110 nm in a multilayered film of InO _x (2.0 nm)/SnO _y (0.2 nm)×50 pairs when annealed at 300° C for 0.5 h in air. Hall coefficient measurements showed that this film had a mobility of 17 cm² V^–1 sec^–1 and a carrier concentration (electron density) of 5×10²⁰ cm^–3.     

Influence of SnO2 films with high resistance on the performance of CdTe solar cells

In this paper, we report the SnO₂ thin films with high resistance prepared by magnetic reactive sputtering on the transparent conductive glass covered with SnO₂:F layers, and study the structural, optical, and electrical properties of the films before and after annealing, as well as the influence of the annealing on the performance of CdTe solar cells. The results have shown that SnO₂ thin films almost have the same structure as SnO₂:F, and SnO₂ high resistance transparent (HRT) insertion layer between transparent conductive oxide (TCO) and CdS affects little on light transmittance. After introducing the HRT layer, the tunneling leakage caused by the pinholes can be avoided, which effectively protects the p–n junction (Wu et al. Proceedings of NCPV program review meeting AIP, New York, 22–26, 2001). Meanwhile, higher parallel resistivity, fill factor (FF), short-wave response, carrier concentration, and lower dark saturation current density have been achieved. As a result, the conversion efficiency is improved by 14.5%, from 10.3% to 11.8%.     

Selenization of electrodeposited copper–indium alloy thin films for solar cell applications

Copper–Indium (Cu–In) alloys with sulfur and selenium have technological importance in the development of thin film solar cell technology. We have used potentiostatic electrochemical technique with three-electrode geometry for the deposition of Cu–In alloy thin films in an aqueous electrolyte. Cathodic voltammetry (CV) was thoroughly studied to optimize the electrodeposition parameters. The deposition potential for Cu–In alloy was found to be in the range ?0.70 to ?0.85 V versus Ag/AgCl reference electrode. Polycrystalline Cu_xIn_y thin films were electrodeposited from aqueous bath at room temperature and 45 °C. Effect of concentration of citric acid was extensively studied by CV measurements. The as-deposited Cu–In films were characterized with a range of characterization techniques to study the structural, morphological, compositional and electrical properties. Thin layers of Cu–In were selenized in a homemade tubular furnace at 400 ^°C, which reveals the formation of polycrystalline CuInSe₂ (CISe) thin films with tetragonal structure. The band gap of CISe thin film was estimated ~1.05 eV by optical absorption spectroscopy. Nearly stoichiometric CISe thin film, Cu = 25.25 %, In = 26.48 % and Se = 48.27 % was obtained after selenization. The linear behavior of current density–voltage (J–V) was observed for Cu–In alloy thin films whereas, the selenized Cu–In alloy films (CISe) possess rectifying properties.     

Effects of dopants on magnetic properties of Cu-doped ZnO thin films

We report the observation of room temperature ferromagnetism (FM) in Cu-doped ZnO (ZnO: Cu) thin films synthesized by sol–gel technique. While donor Al³⁺ cations are introduced into the ZnO: Cu films, the saturation magnetization decreases rapidly. Cu 2p core-level X-ray photoelectronic spectra demonstrate that the FM is strongly correlated with Cu²⁺ cations (3d ⁹ configuration). To further study the relationship between the FM and acceptors, Na⁺ cations are also introduced into the ZnO: Cu films to increase the saturation magnetization. The enhanced FM in the (Cu, Na)-codoped ZnO is suggested to be due to the hybridization between delocalized holes and spin-split Cu 3d states.     

Nanocrystalline sol–gel TiO2–SnO2 coatings: Preparation,characterization and photo-catalytic performance

In this study, preparation of SnO₂ (0–30 mol% SnO₂)–TiO₂ dip-coated thin films on glazed porcelain substrates via sol–gel process has been investigated. The effects of SnO₂ on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Surface topography and surface chemical state of thin films were examined by atomic force microscopy and X-ray photoelectron spectroscopy. XRD patterns showed an increase in peak intensities of the rutile crystalline phase by increasing the SnO₂ content. The prepared Sn doped TiO₂ photo-catalyst films showed optical absorption in the visible light area exhibited excellent photo-catalytic ability for the degradation of methylene blue under visible light irradiation. Best photo-catalytic activity of Sn doped TiO₂ thin films was measured in the TiO₂–15 mol% SnO₂ sample by the Sn⁴⁺ dopants presented substitution Ti⁴⁺ into the lattice of TiO₂ increasing the surface oxygen vacancies and the surface hydroxyl groups.     

Structural and optoelectronic properties of indium doped SnO2 thin films deposited by sol gel technique

Indium doped tin oxide (SnO₂:In) thin films were deposited on glass substrates by sol–gel dip coating technique. X-ray diffraction pattern of SnO₂:In thin films annealed at 500 °C showed tetragonal phase with preferred orientation in T (110) plane. The grain size of tin oxide (SnO₂) in SnO₂:In thin films are found to be 6 nm which makes them suitable for gas sensing applications. AFM studies showed an inhibition of grain growth with increase in indium concentration. The rms roughness value of SnO₂:In thin films are found to 1 % of film thickness which makes them suitable for optoelectronic applications. The film surface revealed a kurtosis values below 3 indicating relatively flat surface which make them favorable for the production of high-quality transparent conducting electrodes for organic light-emitting diodes and flexible displays. X-ray photoelectron spectroscopy gives Sn 3d, In 3d and O 1s spectra on SnO₂:In thin film which revealed the presence of oxygen vacancies in the SnO₂:In thin film. These SnO₂:In films acquire n-type conductivity for 0–3 mol% indium doping concentration and p type for 5 and 7 mol% indium doping concentration in SnO₂ films. An average transmittance of >80 % (in ultra-violet–Vis region) was observed for all the SnO₂:In films he In doped SnO₂ thin films demonstrated the tailoring of band gap values. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in indium doping concentration which may be due structural defects or luminescent centers, such as nanocrystals and defects in the SnO₂.     

Preparation and properties of thin ZnS:Cu films phosphors

Thin films of ZnS and ZnS:Cu were prepared by an original metalorganic chemical vapour deposition (MOCVD) method under atmospheric pressure onto a glass substrate heated up to 230–250 °C. The film thickness varied from 0.6 to 1 μm. The thin films were doped with Cu and Cl by the thermal treatment during 1 h at 600 °C at atmospheric pressure in the blend composed of a ZnS powder with Cu and Cl compounds. These films were used for fabrication of the thin film electroluminescent (TFEL) devices with a conventional double insulating structure. The structural properties were investigated by use of X-ray diffraction (XRD) techniques and atomic force microscopy (AFM). Electroluminescent (EL) spectra, electrical and EL characteristics were investigated. The EL spectra and characteristics as well as structural parameters depend on the growth conditions and significantly modified after the annealing. Blue color emission with brightness of 10 cd m^− 2 under a sine wave excitation at 60 V and 5 kHz was obtained. The degradation behavior of the TFEL devices with ZnS:[Cu, Cl] films fabricated using an original non-vacuum methods of deposition and annealing is the same as that of commercial thin film phosphor.