Preparation and properties of CdS/CdTe thin film solar cell produced by periodic pulse electrodeposition technique

CdS/CdTe solar cells have been prepared by periodic pulse electrodepositionmethod. 10.8% efficient cell was made with open circuit voltage (V_oc)≈753mV, short-circuit current (J_sc)≈23.6 mA/cm² and fill factor (FF)≈0.61. Current-voltage-temperature measurments showed the variation of ideality factor (A) from 1.88 at 344 K to 4.49 at 202 K whereas voltage factor (α) was almost constant above 276 K. The junction transport is possibly dominated by a tunneling mechanism. Capacitance measurements gave the value of diffusion potential as 1.2 eV, ionized charged density as 5.9 × 10¹⁵ cm⁻³ and number of interface states (N_IS) as 2.8 × 10¹¹ cm⁻² eV⁻¹ at zero volt bias. Measurements of open circuit voltage (V_oc) with temperature gave the value of barrier height as 1.42 eV.     

Bulk p-CuInSe2 photo-electrochemical solar cells

Dense CuInSe₂ of high quality, prepared by the fusion technique in evacuated quartz ampoule from stoichiometric melt, crystallizes in the chalcopyrite structure. Compositional analysis carried out by secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy (EDS) indicates a uniform distribution of elements through the depth and a composition close to the stoichiometry. The diffuse reflectance spectrum gives a band gap at 0.94 eV. The electrical conductivity follows an Arrhenius-type law with activation energy of 23 meV in conformity with polarons hopping. Above 320 °C, CuInSe₂ undergoes an irreversible oxidation. The thermal variation of the thermopower indicates p-type behavior attributed to copper deficiency and a hole mobility μ_300 K of 0.133 cm² V⁻¹ s⁻¹, thermally activated. In KCl media, the compound exhibits an excellent chemical stability with a corrosion rate of 8 μmol cm⁻² month⁻¹. The photo-electrochemical properties, investigated for the first time on the ingots, confirm the p-type conductivity. From the capacitance measurements, the flat band potential (V_fb=−0.62V_SCE) and the holes density (N_A=4×10¹⁷ cm⁻³) were determined. The valence band, located at 4.43 eV below vacuum, is made up of mainly Se orbital with little admixture of Cu character. The change of the electrolyte causes a variation in the potential V_fb (dV_fb/dpH=−0.058 V pH⁻¹) indicating strong OH⁻ adsorption. The fill factor in S²⁻ media was found to be 0.54; such result was corroborated by semi-logarithmic plots.     

Synthesis and assessment of La0.8Sr0.2ScyMn1−yO3−δ as cathodes for solid-oxide fuel cells on scandium-stabilized zirconia electrolyte

Perovskite-type La_0.8Sr_0.2Sc_yMn_1−yO_3−δ oxides (LSSMy, y = 0.0–0.2) were synthesized and investigated as cathodes for solid-oxide fuel cells (SOFCs) containing a stabilized zirconia electrolyte. The introduction of Sc³⁺ into the B-site of La_0.8Sr_0.2MnO_3−δ (LSM) led to a decrease in the oxides’ thermal expansion coefficients and electrical conductivities. Among the various LSSMy oxides tested, LSSM0.05 possessed the smallest area-specific cathodic polarization resistance, as a result of the suppressive effect of Sc³⁺ on surface SrO segregation and the optimization of the concentration of surface oxygen vacancies. At 850 °C, it was only 0.094 Ω cm² after a current passage of 400 mA cm⁻² for 30 min, significantly lower than that of LSM (0.25 Ω cm²). An anode-supported cell with a LSSM0.05 cathode demonstrated a peak power density of 1300 mW cm⁻² at 850 °C. The corresponding value for the cell with LSM cathode was 450 mW cm⁻² under the same conditions. The LSSM0.05 oxide may potentially be a good cathode material for IT-SOFCs containing doped zirconia electrolytes.     

Properties of CuIn1−xGaxSe2

Polycrystalline bulk samples of CuIn_1−xGa_xSe₂ weregrown with nominal x = 0.15, 0.25 and 0.5. Mobility, conductivity and band gap were measured at room and low temperatures. Mobilities for x = 0.21 were several hundred cm² V⁻¹s⁻¹ at room temperature and for x = 0.15 were 10³ cm² V⁻¹ s⁻¹, all n type. The band gaps were estimated from the spectra of photoelectrochemical cells at room temperature (with 8.5 K photoluminescence estimates shown in brackets), as 1.10 eV (1.14) for x = 0.21, and 1.07 eV (1.093) for x = 0.15. Crystal mechanical properties as regards cracks were not as good as for CuInSe₂, using similar growth techniques.     

Composition, surface topography, structure, Raman, and electrochemical/photoelectrochemical characterisation of CdxHg1−xTe films

Cd-rich Cd_xHg_{1 − x}Te films have been electrodeposited under potentiostatic conditions on conducting glass and Ti substrates from an acidic solution containing the respective ions as Cd²⁺:Hg²⁺:HTeO₂⁺ = 100:1:2. Six films one after another have been prepared from a single electrochemical cell. EDAX analysis of the air annealed films show decreasing Hg content in the deposit as the number of film preparation increases. SEM analysis indicate undulatory surface with Hg-rich clusters at the top surface. XRD analysis indicate the presence of Cd_xHg_{1 − x}Te along with . The Cd_xHg_{1 − x}Te alloy formation have been confirmed from Raman shift measurements which change with composition, x. The as-deposited films are n-type but converts to p-type after air annealing. Spectral response measurements gave band gap values that change with Hg content in the deposit. Band gap values ranging from 1.1 eV to 1.45 eV have been estimated. Photoelectrochemical solar cells using polysulphide electrolyte have been fabricated which gave an open-circuit photovoltage and short-circuit photocurrent, respectively, as 325 mV and 5.5 mA/cm² under 60 mW/cm² intensity of illumination.     

Enhanced performance of solid oxide fuel cells with Ni/CeO2 modified La0.75Sr0.25Cr0.5Mn0.5O3−δ anodes

The optimization of electrodes for solid oxide fuel cells (SOFCs) has been achieved via a wet impregnation method. Pure La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM) anodes are modified using Ni(NO₃)₂ and/or Ce(NO₃)₃/(Sm,Ce)(NO₃)_x solution. Several yttria-stabilized zirconia (YSZ) electrolyte-supported fuel cells are tested to clarify the contribution of Ni and/or CeO₂ to the cell performance. For the cell using pure-LSCrM anodes, the maximum power density (P_max) at 850 °C is 198 mW cm⁻² when dry H₂ and air are used as the fuel and oxidant, respectively. When H₂ is changed to CH₄, the value of P_max is 32 mW cm⁻². After 8.9 wt.% Ni and 5.8 wt.% CeO₂ are introduced into the LSCrM anode, the cell exhibits increased values of P_max 432, 681, 948 and 1135 mW cm⁻² at 700, 750, 800 and 850 °C, respectively, with dry H₂ as fuel and air as oxidant. When O₂ at 50 mL min⁻¹ is used as the oxidant, the value of P_max increases to 1450 mW cm⁻² at 850 °C. When dry CH₄ is used as fuel and air as oxidant, the values of P_max reach 95, 197, 421 and 645 mW cm⁻² at 750, 800, 850 and 900 °C, respectively. The introduction of Ni greatly improves the performance of the LSCrM anode but does not cause any carbon deposit.     

Electron irradiation and thermal annealing effect on GaAs solar cells

This paper describes the effect of electron irradiation and thermal annealing on LPE AlGaAs/GaAs heterojunction solar cells with various p/n junction depths. The electron irradiation experiments were performed with energy of 3 MeV, fluences ranging from 1×10¹⁴ to 5×10¹⁵ e/cm². The results obtained demonstrate that the irradiation-induced degradation of performances of the cells is mainly in the short circuit current and could be mostly recovered by annealing at 260°C for 30 min. Four electron traps, E_c−0.24 eV, E_c−0.41 eV, E_c−0.51 eV, E_c−0.59 eV, were found by DLTS analysis, only two shallow levels of which could be removed by the annealing.     

The obstructed diffusion of the I3 ion in mesoscopic TiO2 membranes

The diffusional permeability of I⁻₃ ion in acetonitrile in free standing TiO₂ membrane with a porosity of 55% was examined. The apparent diffusion coefficient, D_app at 25°C of the ion was found to be 3.4×10⁻⁶ cm^{2 −1}, an order of magnitude smaller than the free diffusion at the same temperature. The temperature dependency of D_app was measured in the range 0–30°C and analysed in terms of the Walden product. The diffusional activation energy was found to be 13.5 kJ/mol. The parameters of interest for the efficiency of mesoscopic wet solar cells are discussed. A back of an envelope calculation shows that although the obstructed diffusion coefficient of the I⁻₃ ion was an order of magnitude smaller than the free diffusion the diffusional flux is still sufficient to meet a current density of 50 mA cm⁻². At incident photon flux of 1 kW m⁻² and at a photopotential of 0.6 V this would correspond to a solar energy efficiency of approximately 30%.     

Optical and electrical studies on molybdenum sulphoselenide [Mo(S1−xSex)2] thin films prepared by arrested precipitation technique (APT)

Nanocrystalline stoichiometric [Mo(S_1−xSe_x)₂] thin films were deposited by using arrested precipitation technique (APT) developed in our laboratory. The precursors used for this are namely, molybdenum triethanolamine complex, thioacetamide and sodium selenosulphite; and various preparative conditions are finalised at the initial stages of deposition. Formation of [Mo(S_1−xSe_x)₂] semiconducting thin films are confirmed by studying growth mechanism, optical and electrical properties. X-ray diffraction analysis showed that the composites are nanocrystalline being mixed ternary chalcogenides of the general formula [Mo(S_1−xSe_x)₂]. The optical studies revealed that the films are highly absorptive (α×10⁴ cm⁻¹) with a band-to-band direct type of transitions and the energy gap decreased typically from 1.86 eV for pure MoS₂ down to 1.42 eV for MoSe₂. The thermoelectrical power measurement shows negative polarity for the generated voltage across the two ends of semiconductor thin films. This indicate that the [Mo(S_1−xSe_x)₂] thin film samples show n-type conduction.     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.     

Electrochromic properties of NiOxHy thin films

NiO_xH_y films were prepared by DC magnetron sputtering in H₂/O₂ atmosphere. NiO_xH_y coatings with transparency and high electrochromic efficiency were obtained by changing H₂ content. A 60 nm thick NiO_xH_y film with transmittance of 0.57 (as-deposited state), 0.78 (bleached state) and 0.24 (coloured state) at wavelength of 550 nm was deposited in an atmosphere of H₂(60%)+O₂(40%). Analysis of infrared spectra (60002400 cm⁻¹) showed that the absorption peaks for bleached and colored states are associated with free ‘OH’ and OH stretching vibrations, respectively. XPS Ni2p core level spectra of colored NiO_xH_y film exhibited a peak at 856.2±0.2 eV which is attributed to Ni³⁺. Ni2p core level spectra of the bleached and as-deposited films exhibited two peaks at 856.4±0.2 and 854.6±0.2 eV which are attributed to Ni³⁺ and Ni²⁺.     

Photovoltaic characteristics of CuInS2/CdS solar cell by electron beam evaporation

When a CuInS₂/CdS solar cell was fabricated by depositing CdS thin film with dopant In of 1.0 at% on ternary compound CuInS₂ thin film with the lowest resistivity of 5.59 × 10⁻² Ωcm, its best result was as follows: V_oc = 461 mV, I_sc = 26.9 mA, FF = 0.685, η = 5.66% under the illumination of 100 mW/cm². And its series resistance and lattice mismatch was 5.1 Ω and 3.2%, respectively.Besides, a 4 layer structure solar cell of -CuInS₂/high -CuInS₂/high -CdS/low - CdS has been fabricated. When thickness of high - CuInS₂ was 0.2 μm, its best result was as follows: V_oc = 580 mV, I_sc = 30.6 mA, FF = 0.697, η = 8.25%. An its series resistance and lattice mismatch were 4.3 Ω and 2.8%, respectively.     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

New trends to grow the n-CdZn (S1−xSex)2/p-CuIn(S1−xSex)2 heterojunction thin films for solar cell applications

The n-CdZn(S_1−xSe_x) and p-CuIn(S_1−xSe_x)₂ thin films have been grown by the solution growth technique (SGT) on glass substrates. Also the heterojunction (p–n) based on n-CdZn (S_1−xSe_x)₂ and p-CuIn (S_1−xSe_x)₂ thin films fabricated by same technique. The n-CdZn(S_1−xSe_x)₂ thin film has been used as a window material which reduced the lattice mismatch problem at the junction with CuIn (S_1−xSe_x)₂ thin film as an absorber layer for stable solar cell preparation. Elemental analysis of the n-CdZn (S_1−xSe_x)₂ and p-CuIn(S_1−xSe_x)₂ thin films was confirmed by energy-dispersive analysis of X-ray (EDAX). The structural and optical properties were changed with respect to composition ‘x’ values. The best results of these parameters were obtained at x=0.5 composition. The uniform morphology of each film as well as the continuous smooth thickness deposition onto the glass substrates was confirmed by SEM study. The optical band gaps were determined from transmittance spectra in the range of 350–1000 nm. These values are 1.22 and 2.39 eV for CuIn(S_0.5Se_0.5)₂ and CdZn(S_0.5Se_0.5)₂ thin films, respectively. J–V characteristic was measured for the n-CdZn(S_1−xSe_x)₂/p-CuIn(S_1−xSe_x)₂ heterojunction thin films under light illumination. The device parameters V_oc=474.4 mV, J_sc=13.21 mA/cm², FF=47.8% and η=3.5% under an illumination of 85 mW/cm² on a cell active area of 1 cm² have been calculated for solar cell fabrication. The J–V characteristic of the device under dark condition was also studied and the ideality factor was calculated which is equal to 1.9 for n-CdZn(S_0.5Se_0.5)₂/p-CuIn(S_0.5Se_0.5)₂ heterojunction thin films.     

Donor–acceptor interaction between non-aqueous solvents and I2 to generate I3, and its implication in dye sensitized solar cells

The spectrophotometric properties of I⁻, I₂ and the I⁻/I₂ mixture were studied in 1,2-dichloroethane (DCE), acetone (AC), acetonitrile (ACN), ethanol (EtOH), methanol (MeOH), tertiary-butanol (t-BuOH), dimethylformamide (DMF), propylenecarbonate (PC), 3-methoxypropionitrile (MePN), dimethylsulfoxide (DMSO), dioxane (DIO) and pyridine (PY) solutions. From the investigation it has been realized that in DCE, I⁻, I₂ and I⁻/I₂ mixture have the same absorption peak at 500 nm. I⁻ gives rise to the absorption spectra at about 220, 290 and 360 nm in t-BuOH and in PY solutions. However, in all other solvents the I⁻ generates peaks only around 220 nm. Similarly I₂ and the I⁻/I₂ mixture in all solvents except DCE have indicated similar absorption peaks around 220, 290 and 360 nm. On the other hand, except in PC and DMF, I₂ shows the additional peaks in the range of 380–500 nm which are assigned to the formation of a I₂–solvent complex. The peaks around 290 and 360 nm indicate the presence of I⁻₃ and around 220 nm is the peak of I⁻. The spectral shift of the I₂ solutions in the visible region is interesting and is the core of this report. It points to the importance of donor–acceptor interaction between solvents and iodine. The data obtained in these solvents were well correlated to the donor number (DN) of the solvents. From this correlation the DN of MePN was estimated to 14.6. The absorption peak of I₂ in DCE(DN=0.0) is 500 nm and in PY(DN=33.1) is 378 nm. This peak shift due to solvent effects corresponds to an energy difference close to 0.8 eV. The absorption peak shift due to addition of the 0.0080 vol%. PY(1 mM) in 1 mM I₂-ACN solutions corresponds to ca. 0.6 eV. The blue shift of I₂ absorption in basic solvents indicates the tendency to form a complex. The increase of the efficiency of the dye-sensitized solar cell by addition of PY to I⁻/I⁻₃ ACN solution is suggested to be due to the formation of the dipyridine complex, PY₂I⁺. Such complex formation decreases the amount of I₂ which is expected to be an electron scavenger. We also propose that the more bulky complex, PY₂I⁺ has a slower kinetics with the conduction band electrons, and thus decrease the losses of photocurrent and photopotentials in the solar cell.     

Structural and electrical properties of CuGaS2 thin films by electron beam evaporation

Single phase CuGaS₂ thin film with a highest diffraction peak of (1 1 2) at a diffraction angle (2θ) of 28.8° was made at a substrate temperature of 70°C, an annealing temperature of 350°C and an annealing time of 60 min. Second highest (2 0 4) peak was shown at diffraction angle of (2θ) 49.1°. Lattice constant of a and c of that CuGaS₂ thin film was 5.37 and 10.54 Å, respectively. The greatest grain size of the thin film was about 1 μm. The (1 1 2) peak of single phase of CuGaS₂ thin film at an annealing temperature of 350°C with excess S supply appeared at a little higher about 10% than that of no excess S supply. The resistivity, mobility and hole density at room temperature of p-type CuGaS₂ thin film was 1.4 Ω cm, 15 cm²/V s and 2.9×10¹⁷ cm⁻³, respectively. It was known that carrier concentration had considerable effect than mobility on a variety of resistivity of the fabricated CuGaS₂ thin film, and the polycrystalline CuGaS₂ thin films were made at these conditions were all p-type.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.     

Electronic and photovoltaic properties of Al/p-Si/copper phthalocyanine photodiode junction barrier

Al/p-Si/copper phthalocyanine photovoltaic device has been fabricated and characterised by current–voltage and capacitance–voltage measurements. Electrical properties of the device were determined by current–voltage characterizations under dark and illumination conditions. The density distribution of the interface states of the photodiode was found to vary from 8.88×10¹² eV⁻¹ cm⁻² in E_ss-0.54 eV to 4.51×10¹² eV⁻¹ cm⁻² in E_ss-0.61 eV. The device shows a photovoltaic behaviour with a maximum open circuit voltage Voc of 0.16 V and short-circuits current I_sc of 0.45 μA under 3500 lux light intensity.     

Photosensitization of nanocrystalline TiO2 films by a polymer with two carboxylic groups, poly (3-thiophenemalonic acid)

Photovoltaic devices were assembled using a conducting polymer; poly (3-thiophenemalonic acid) sensitized TiO₂ electrodes and an electrolyte containing I₃⁻/I⁻ redox couple. This cell exhibited a short-circuit photocurrent (J_sc) of 6.65 mA cm⁻², an open circuit voltage (V_oc) of 355 mV and an efficiency of 1.5% under the illumination of 100 mW cm⁻² (AM 1.5). Addition of an ionic liquid, 1-methyl 3-n-hexylimidazolium iodide, into the electrolyte led to an improvement in the cell performances, achieving an overall efficiency of 1.8% under the same illumination. The average cell characteristics of the later devices are , with a fill factor of 0.65.     

Optical and electrical properties of semiconducting WS2 thin films: From macroscopic to local probe measurements

Photosensitive WS₂ thin films are obtained by annealing in presence of a crystallization promoter like Ni or Co. Conventional optical and electrical measurements (conductivity, Hall effect, photoconductivity) are completed by various local probe investigations like scanning tunneling microscopy (STM) and conductive atomic force microscopy (AFM). This thorough study clarifies the respective role of the crystallites and the grain boundaries in the macroscopic measurements and gives information on the properties and on the photovoltaic prospect of the films. The optical properties of the thin films are comparable to those of WS₂ single crystals, with absorption excitonic peaks of same intensity at 1.94 and 2.36 eV. The films show a p-type behavior with a carrier concentration of p10²³ m⁻³ and a Hall mobility of μ_H10×10⁻⁴ m² V⁻¹ s⁻¹ at room temperature. The Hall mobility is thermally activated with an activation energy of 60–90 meV. The photoconductivity spectra show the first indirect transition at 1.35 eV and a decrease of the quantum efficiency at the excitonic-transitions energies. The transport in the film plane is mainly governed by the potential barriers at the grain boundaries. Using a conducting AFM, the crystallite edges are shown to be degenerate semiconductors, while STM current–voltage (I–V) spectroscopy indicates that the flat WS₂ crystallites have a low density of surface states on the basal planes. Submicron solid-state junctions are fabricated on the film by depositing gold electrodes on single WS₂ crystallites (with an electrode surface of 0.2 μm²). Under illumination the p-WS₂/Au micro-junctions show open circuit-voltages of up to 520 mV. The collection of photo-generated carriers is limited by recombination at the grain boundaries.