Towards an all-polymer cathode for dye sensitized photovoltaic cells

Highly conducting conjugated polymer composite was deposited by vapour phase polymerization. Poly[3,4-ethylenedioxythiophene:para-toluenesulfonate] (PEDOT:PTS) itself was used as the counter electrode in a dye sensitized solar cell. The maximum photocurrent of 9.7 mA/cm², open circuit voltage of 759 mV, fill factor of 0.71 with a power conversion efficiency of 5.25% were observed for glass based wet type dye sensitized solar cell, under illumination of 100 mW/cm². It was observed that the resistance, during operation of the dye sensitized solar cells, due to the I₃⁻ conversion was less with PEDOT:PTS coated cathodes than with standard platinum coated fluorine doped tin oxide and was confirmed by steady state electrochemical measurements.     

Solar cells with Sb2S3 absorber films

CdS/Sb₂S₃/PbS structures were prepared by sequential chemical deposition of CdS, Sb₂S₃ and PbS thin films on TEC-8 (Pilkington) transparent electrically conductive SnO₂ (TCO) coatings. CdS thin films (100 nm) were deposited with hexagonal structure from Cd-citrate bath and of cubic structure from Cd-ammine/triethanolamine bath. Sb₂S₃ thin films were deposited at 40 °C from a solution mixture of potassium antimony tartrate, triethanolamine, ammonia and thioacetamide(TA) or at 1 to 10 °C from a mixture of antimony trichloride and thiosulfate (TS). These films were made photoconductive by heating at temperatures 250 to 300 °C. When heated in the presence of a chemically deposited Se thin film of 300 nm, a solid solution Sb₂S_1.8Se_1.2 resulted. PbS thin films of 100-200 nm thickness were deposited on the TCO/CdS/Sb₂S₃ or TCO/CdS/Sb₂S_1.8Se_1.2 structure. Graphite paint was applied on the PbS film prior to applying a silver epoxy paint. The cell structures were of area 0.4 cm². The best results reported here is for a cell: TCO/CdS(hex-100 nm)/Sb₂S₃(TS-100 nm)/PbS(200 nm) with open circuit voltage (V_oc) 640 mV, short circuit current density 3.73 mA/cm², fill factor 0.29, and conversion efficiency 0.7% under 1000 Wm^− 2 sunlight. Four series-connected cells of area 1 cm² each gave V_oc of 2 V and short circuit current of 1.15 mA.     

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.     

Amorphous In-Ga-Zn-O thin-film transistor with coplanar homojunction structure

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) with a coplanar homojunction structure are demonstrated. The coplanar source and drain regions made of a-IGZO were formed by depositing a hydrogenated silicon nitride (SiN_X:H) layer onto the a-IGZO layer. The a-IGZO regions on which the SiN_X:H layer was directly deposited showed the low resistivity of 4.7 × 10⁻³  Ω cm and degenerated conduction. The fabricated TFT showed excellent transfer and output characteristics with a field-effect mobility of 11 cm² V^− 1 s^− 1, a subthreshold swing of 0.17 V decade^− 1, and an on-to-off current ratio larger than 1 × 10⁹. The width-normalized source-to-drain resistance (R_sdW) calculated using a channel resistance method was 51 Ω cm. This TFT also showed good stability over environment change and under electrical stress.     

Transparent amorphous In-Ga-Zn-O thin film as function of various gas flows for TFT applications

The electrical and optical properties of amorphous indium gallium zinc oxide (a-IGZO) films, which can be used as a channel layer, deposited by radio frequency (rf) magnetron sputtering system at room temperature (RT), were investigated as function of various gas flows. The optical transmittance of films deposited under Ar, O₂ / Ar + O₂ and O₂ / Ar-4% H₂ + O₂ atmospheres in the visible wavelength was consistently above 90% at a wavelength of 550 nm at all gas flows, although the film deposited under Ar-4% H₂ atmosphere exhibited a transmittance of below 50%. The carrier concentration and mobility of the a-IGZO films fabricated under Ar and Ar-4% H₂ were observed slight decrease as a function of the flow, respectively. The thin film transistors (TFTs) with an a-IGZO channel deposited under Ar and Ar-4% H₂ atmosphere exhibited the following good characteristics: V_th of 0.34 V, µ_FE of 3.6 cm² V^− 1 s^− 1, on/off ratio of 10⁶, and S value of 0.04 V decade^− 1.     

Synthesis and optical characterization of n-ZnO and p-Cu2ZnSnS4 nanocrystalline thin films for low cost solar cells

High quality ZnO/Cu₂ZnSnS₄ thin films as a window/absorber layers were successfully synthesized via spin coating the sol-gel precursor of each composition without using any vacuum facilities. In this study, the impact of annealing temperature (400 °C, 3 h) on the ZnO window layer and different thickness (3 and 5 layers) of the Cu₂ZnSnS₄ (CZTS) absorber layer were investigated. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM) and UV–vis–NIR spectroscopy were used for the structural, compositional, morphological and optical absorption analysis of each layer. ZnO exhibits wurtzite hexagonal crystal structure with particle size equals to 8.60 and 28.59 nm for fresh and annealed films, respectively. Micro-strain and dislocations density decreased with the annealing temperature. X-ray diffraction patterns for CZTS films show small peak at (112) according to the kesterite structure with particle size in nano-scale for the two thicknesses. ZnO films demonstrated direct optical band gap of 3.23 and 3.21 eV for fresh and annealed films, respectively. CZTS films (3 and 5 layers) also have direct optical band with optimum value (1.51 eV) for thickness of 5 layers. The J-V characteristics of the CZTS-based thin film solar cells (CZTS/ZnO/ZnO:Ag) were measured under air mass AM 1.5 and 100 mW/cm² illumination. The values of the short circuit current (J_sc), open circuit voltage (V_oc) and fill factor (FF) also have been obtained.     

Metal-free indoline dye sensitized zinc oxide nanowires solar cell

Dye sensitized solar cell (DSSC) based on metal-free indoline dye D102 sensitized zinc oxide (ZnO) nanowires (NWs) derived from aqueous solution on seeded substrate was investigated. The morphology, composition and crystalline structure of the highly oriented ZnO NWs were characterized by field-emission scanning electron microscope, energy dispersive X-ray spectrum spectroscopy and X-ray diffraction, respectively. The chemical bond between D102 and ZnO NWs was confirmed by Fourier transfer infrared spectra. The photovoltaic property of DSSC was characterized at full sun intensity of 100 mW/cm² (AM 1.5) with short circuit current J_sc = 14.06 mA/cm² and energy conversion efficiency η = 2.6%.     

CuIn1 − xGaxSe2 photovoltaic devices for tandem solar cell application

CuIn_1 − xGa_xSe₂ (CIGS) solar cells show a good spectral response in a wide range of the solar spectrum and the bandgap of CIGS can be adjusted from 1.0 eV to 1.7 eV by increasing the gallium-to-indium ratio of the absorber. While the bandgaps of Ga-rich CIGS or CGS devices make them suitable for top or intermediate cells, the In rich CIGS or CIS devices are well suited to be used as bottom cells in tandem solar cells. The photocurrent can be adapted to the desired value for current matching in tandem cells by changing the composition of CIGS which influences the absorption characteristics. Therefore, CIGS layers with different [Ga]/[In + Ga] ratios were grown on Mo and ZnO:Al coated glass substrates. The grain size, composition of the layers, and morphology strongly depend on the Ga content. Layers with Ga rich composition exhibit smaller grain size and poor photovoltaic performance. The current densities of CIGS solar cells on ZnO:Al/glass varied from 29 mA cm^− 2 to 13 mA cm^− 2 depending on the Ga content, and 13.5% efficient cells were achieved using a low temperature process (450 °C). However, Ga-rich solar cells exhibit lower transmission than dye sensitized solar cells (DSC). Prospects of tandem solar cells combining a DSC with CIGS are presented.     

Synthesis,optical and thermal characterization of novel thiazolyl heterocyclic azo dye

Thiazolyl heterocyclic azo dye and its metal (Ni²⁺, Co²⁺)–azo complexes were synthesized. Their structures were confirmed by elemental analysis, UV–VIS absorption spectra, FT-IR, ¹H NMR and MALDI-MS. The thermal properties of metal complexes were studied by DSC–TGA. The optical constants (complex refractive index N = n + ik) and thickness of the complex thin films on polished single-crystal silicon substrates were investigated on a scanning ellipsometer. Results indicate that thiazolyl metal–azo complexes possess good optical and thermal properties. They would be a promising recording medium candidate for NVD with the Super-resolution near field structure (Super-RENS) technology.     

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.’     

Effect of the substrate temperature on the properties of Ga-doped ZnO films for photovoltaic cell applications deposited by a pulsed DC magnetron sputtering with a rotating cylindrical target

Effect of substrate temperature on the properties of Ga-doped ZnO (GZO) films was investigated by pulsed DC magnetron sputtering with a rotating cylindrical target with an aim to establish suitable process conditions for their photovoltaic (PV) cell applications. Without formation of undesirable secondary oxide phases such as Ga₂O₃ and ZnGa₂O₄, the GZO film having mixed orientation at lower deposition temperature evolved into the c-axis oriented one with increasing deposition temperature to 230 °C, which accompanied morphological evolution to vertically oriented dense columnar structure and improved doping efficiency. Correlated with this, crater-like surface texturing was possible only on the sample deposited at 230 °C. Electrical resistivity and diffuse surface reflectance over the spectral range of 200-1200 nm of this GZO film after surface texturing were 8.73 × 10⁻⁴ Ω cm and 3.32%, respectively, indicating that the film has application potential as anti-reflection coating and front electrode of PV cells. Morphological features, surface texturing behavior, electrical and optical properties of the GZO films in this study suggest that this novel technique would be applicable to the fabrication of anti-reflection coating and front electrode of PV cells only when substrate temperature is sufficiently high.     

Callindra haematocephata and Peltophorum pterocarpum flowers as natural sensitizers for TiO2 thin film based dye-sensitized solar cells

We have studied the performance of dye-sensitized solar cells employing natural dye extracted from the flowers Callindra haematocephata and Peltophorum pterocarpum as sensitizers for TiO₂ photoanode. The extracts have shown appreciable absorption in the visible region. FTIR studies indicated the presence of anthocyanins and β-carotene in the flowers of C. haematocephata and P. pterocarpum respectively. The extracts were anchored on TiO₂ film deposited on transparent conductive glass (FTO) which were used as photoanode. The dye coated TiO₂ film electrode, Pt counter electrode and electrolyte (I⁻₃) assembled into a cell module was illuminated by a light source with intensity 100 mW/cm² to measure the photoelectric conversion efficiency of the DSSCs. From the J-V characteristic curves of cells, the parameters related to the solar cell performance were determined. The conversion efficiency of the DSSC employing natural dye extract from the flower C. haematocephata and P. pterocarpumwere was found as 0.06% and 0.04%, with open-circuit voltage (V_OC) of 370 mV & 400 mV, short-circuit current density (J_SC) of 0.25 mA/cm² & 0.15 mA/cm², fill factor (FF) of 0.70 & 0.71 and P_max of 65 & 45 μW cm⁻² respectively. The extract of the flower C. haematocephata exhibited better photosensitization action compared to the flower of P. pterocarpum.     

Photovoltaic performance of dye-sensitized solar cells fabricated with polyvinylidene fluoride–polyacrylonitrile–silicondioxide hybrid composite membrane

Electrospun fibrous membranes of hybrid composites of polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN) and silicon dioxide (SiO₂) (PVdF–PAN–SiO₂) are prepared with different proportions of SiO₂ (3, 5 and 7% w/w). The field emission scanning electron microscopy (FE-SEM) reveals that these membranes have three-dimensional, fully interconnected network structures, which are combined with micropores of fine SiO₂ distribution. The surface roughness of the membranes increases with increasing the SiO₂ content. It is found that 7 wt% SiO₂/PVdF–PAN electrolyte membrane has the highest ionic conductivity (6.96 × 10⁻² S cm⁻¹) due to the large liquid electrolyte uptake (about 570%). As the concentration of SiO₂ nanoparticles increase, the contact angle value also increases, ranging from 135.70° to 140.60° which indicates that the membrane has higher hydrophobicity. The dye sensitized solar cells (DSSCs) are fabricated using the hybrid composite membrane with PVdF–PAN with 7 wt % SiO₂. Its photovoltaic performance exhibits an open circuit voltage (V_oc) of 0.79 V and a short circuit current 11.6 mA cm⁻² at an incident light intensity of 100 mW cm⁻², producing an efficiency of 5.61%. DSSC, using the hybrid composite electrospun membrane which shows more stable photovoltaic performance than other assembled DSSCs.     

Low energy O2 and N2O ion beam modification of polyimide for improving adhesive strength of Cu/PI in roll-to-roll process

To enhance the weak mechanical durability of directly deposited copper layers on polyimide (PI) film due to their poor adhesive strength, a continuous roll-to-roll process involving surface modification using a reactive ion beam irradiation and in-situ deposition process is studied. The polyimide film is modified by an ion source with a linear stationary plasma thruster (LSPT) in the vacuum roll-to-roll process. An O₂ ion beam, with beam energy of 214 eV and beam current density of 0.78 mA/cm², and N₂O ion beam, with 220 eV and 0.69 mA/cm², irradiate PI film in winding speed of 0.5 m/min. The surface energy increases from 38 mN/m for the pristine PI film to 80 mN/m after beam irradiation at an ion fluence of 3.5 × 10¹⁶ ions/s. After beam irradiation, a 10 nm thick tie layer and 200 nm thick copper layer are successively deposited by in-situ DC magnetron web coating. The peel strength of the copper layer on the PI film is enhanced from 0.4 kgf/cm without ion beam treatment to 0.71 kgf/cm after O₂ beam treatment and to 0.75 kgf/cm after N₂O beam treatment. This enhancement is closely related to the increase in the polar force originating from the formation of hydrophilic CO (carbonyl) groups on the modified PI surface.     

HFCVD grown graphene like carbon–nickel nanocomposite thin film as effective counter electrode for dye sensitized solar cells

The two step processes of hot filament chemical vapor deposition (HFCVD) and DC sputtering were used to grow graphene like carbon (GLC)–nickel (Ni) nanocomposite thin film on fluorine-doped tin oxide (FTO) glass and applied as counter electrode (CE) for dye sensitized solar cells (DSSCs). The morphological and absorption properties revealed uniform GLC–Ni thin film with reasonable transmittance. The GLC–Ni thin film showed enhanced electrical conductivity as compared to FTO. The good electrocatalytic activity towards iodide ions in redox electrolyte was showed by the prepared GLC–Ni/FTO thin film electrode. The fabricated DSSC with GLC–Ni/FTO counter electrode (CE) presented relatively moderate solar-to-electrical conversion efficiency of ∼3.1% with high short-circuit current density (J_SC) of ∼10.03 mA/cm², open circuit voltage (V_OC) of ∼0.663 V with fill factor (FF) of ∼0.45, which might attribute to enhanced electrical conductivity and the electrocatalytic activity of GLC–Ni/FTO CE.     

Supermagnetron plasma CVD of highly effective a-CNx:H electron-transport and hole-blocking films suited to Au/a-CNx:H/p-Si photovoltaic cells

Hydrogenated carbon nitride (a-CN_x:H) films (0-500 nm) were deposited on p-Si wafers to make Au/a-CN_x:H/p-Si photovoltaic cells using i-C₄H₁₀/N₂ supermagnetron plasma chemical vapor deposition. At a lower electrode RF power (LORF) of 50 W and an upper electrode RF power (UPRF) of 50-800 W, hard a-CN_x:H films with optical band gaps of 0.7-1.0 eV were formed. At a film thickness of 25 nm (UPRF of 500 W), the open circuit voltage and short circuit current density were 247 mV and 2.62 mA/cm², respectively. The highest energy conversion efficiency was 0.29%. The appearance of the photovoltaic phenomenon was found to be due to the electron-transport and hole-blocking effect of thin a-CN_x:H film.     

Optical study of ion-induced effects in Ge20Se80−xBix thin films

Incorporation of impurities into the semiconductors by high energy heavy ions is a non-equilibrium process, which can result in intriguing near surface or subsurface property changes depending upon the energy of the ion. Present work deals with the optical study of Ni ion irradiated Ge₂₀Se_80−xBi_x thin films samples. The thin film samples were deposited by the flash evaporation method at 10⁻⁵ Torr and were characterized by XRD, XRF, EPMA and DSC. Irradiation was done on samples with a Ni ion beam of 75 MeV energy. The optical spectra of the films were recorded and the optical band gap study was done both as a function of dose (5 × 10¹² to 10¹⁴ ions/cm²) as well as composition of the samples. The optical band gap was found to decrease with increasing Bi content as well as with increasing dose.     

Influence of substrate temperature on structural,optical, and electrical properties of flash evaporated CuIn0.81Al0.19Se2 thin films

Chalcopyrite copper indium aluminum diselenide (CuIn_0.81Al_0.19Se₂) compound is prepared by direct reaction of high purity elemental copper, indium, aluminum and selenium in their stoichiometric proportion. Structural and compositional characterizations of pulverized material confirm the formation of a single phase, polycrystalline nature. CuInAlSe₂ (CIAS) thin films are deposited on organically cleaned soda lime glass substrates using flash evaporation technique by varying the substrate temperatures in the range from 423 K to 573 K. Influence of substrate temperature observed by X-ray diffraction (XRD), scanning electron microscope (SEM), optical and electrical measurement. CIAS Films grown at different substrate temperatures are polycrystalline in nature, exhibiting a chalcopyrite structure with lattice parameters a = ∼0.576 nm and c = ∼1.151 nm. The crystallinity in the films increases with increasing substrate temperature up to 473 K, and tend to degrade at higher substrate temperatures. Optical band gap is in the range of 1.20 eV–1.38 eV and the absorption coefficient is close to 10⁵ cm⁻¹. Electrical characterization reveals p-type conductivity and the structural, morphological and optical properties indicate potential use of CIAS thin films as an absorber layer for thin film solar cell applications.     

Development of microcrystalline silicon carbide window layers by hot-wire CVD and their applications in microcrystalline silicon thin film solar cells

Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E₀₄ is about 3.0-3.2 eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5 μm, the short circuit current density (j_SC) increases from 23 to 26 mA/cm² with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j_SC = 29.6 mA/cm² and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.     

All-chemically deposited Bi2S3/PbS solar cells

We report on solar cells with a cross-sectional layout: TCO/window/Bi₂S₃/PbS, in which a commercial SnO₂ transparent conductive oxide (TCO-PPG Sungate 500); chemically deposited window layers of CdS, ZnS or their oxides; n-type Bi₂S₃ (100 nm) and p-type PbS (360-550 nm) absorber films constitute the cell structures. The crystalline structure, optical, and electrical properties of the constituent films are presented. The open circuit voltage (V_oc) and short-circuit current density (J_sc), for 1000 W/m² solar radiation, of these solar cells depend on the window layers, and vary in the range, 130-310 mV and 0.5-5 mA/cm², respectively. The typical fill factors (FF) of these cells are 0.25-0.42, and conversion efficiency, 0.1-0.4%.