Effects of ZnO coating on the performance of TiO2 nanostructured thin films for dye-sensitized solar cells

ZnO-coated TiO₂ (ZTO) thin films were deposited on ITO substrates by a sol–gel method for application as the work electrode for dye-sensitized solar cells (DSSCs). The I–V curve and the incident photon-to-current conversion efficiency (IPCE) value of DSSCs for ZTO thin films were studied and compared with single TiO₂ films. The results show that the short-circuit photocurrent (J _sc) and open-circuit voltage (V _oc) values increased from 3.7 mA/cm² and 0.68 V for the DSSCs with a single TiO₂ film to 4.5 mA/cm² and 0.72 V, respectively, for the DSSCs with a ZTO thin film. It indicated that the DSSCs with a ZTO thin film contributed to provide an inherent energy barrier that suppressed charge recombination significantly. In addition, the higher IPCE value in the ZTO thin film is attributed to the better charge separation by a fast electron transfer process using two semiconductors with different conduction band edges and energy positions.     

Fabrication of a nanoparticle TiO<Subscript>2</Subscript> photoelectrochemical cell utilizing a solid polymeric electrolyte of PAN–PC–LiClO<Subscript>4</Subscript>

A nanoparticle TiO₂ solid-state photoelectrochemical cell utilizing as a solid electrolyte of poly(acrylonitrile)–propylene–carbonate–lithium perchlorate (PAN–PC–LiClO₄) has been fabricated. The performance of the device has been tested in the dark and under illumination of 100-mW cm⁻² light. A nanoparticle TiO₂ film was deposited onto indium tin oxide-covered glass substrate by controlled hydrolysis technique assisted with spin-coating technique. The average grain size for the TiO₂ film is 76 nm. LiClO₄ salt was used as a redox couple. The room temperature conductivity of the electrolyte is 4.2 × 10⁻⁴ S cm⁻¹. A graphite electrode was prepared onto a glass slide by electron beam evaporation technique. The device shows the rectification property in the dark and shows the photovoltaic effect under illumination. The best J _sc and V _oc of the device were 2.82 μA cm⁻² and V _oc of 0.58 V, respectively, obtained at the conductivity of 4.2 × 10⁻⁴ S cm⁻¹ and intensity of 100 mW cm⁻². The J _sc was improved by about three times by introducing nanoparticle TiO₂ and by using a solid electrolyte of PAN–PC–LiClO₄ replacing PVC–PC–LiClO₄ in the device. The current transport mechanism of the cell is also presented in this paper.     

Effects of preannealing temperature of ZnO thin films on the performance of dye-sensitized solar cells

The preferred (002) orientation zinc oxide (ZnO) nanocrystalline thin films have been deposited on FTO-coated glass substrates by sol–gel spin-coating technology and rapid thermal annealing for use in dye-sensitized solar cells (DSSC). The effects of preannealing temperature (100 and 300°C) on the microstructure, morphology and optical properties of ZnO thin films were studied. The ZnO thin films were characterized by X-ray diffraction (XRD), scanning electron microscopic (SEM) and Brunauer–Emmett–Teller (BET) analysis. The photoelectric performance of DSSC was studied by I–V curve and the incident photon-to-current conversion efficiency (IPCE), respectively. From the results, the intensities of (002) peaks of ZnO thin films increases with increasing preannealing temperature from 100°C to 300°C. The increase in pore size and surface area of ZnO films crystallized at the increased preannealing temperature contributed to the improvement on the absorption of N3 dye onto the films, the short-circuit photocurrent (J _sc) and open-circuit voltage (V _oc) of DSSC. The higher efficiency (η) of 2.5% with J _sc and V _oc of 8.2 mA/cm² and 0.64 V, respectively, was obtained by the ZnO film preannealed at 300°C.     

Fabrication and electrical characterization of p-Sb2S3/n-Si heterojunctions for solar cells application

Antimony trisulphide (Sb₂S₃) films were prepared by thermal evaporation technique on n-type single crystal Si substrates to fabricate p-Sb₂S₃/n-Si heterojunctions. The electrical transport properties of the p–Sb₂S₃/n-Si heterojunctions were investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The temperature-dependent I–V characteristics revealed that the forward conduction was determined by multi-step tunnelling current and the activation energy of saturation current was about 0.54 eV. The 1/C²–V plots indicated the junction was abrupt and the junction built-in potential was 0.6 V at room temperature and decreased with increasing temperature. The solar cell parameters have been calculated for the fabricated cell as V_oc = 0.50 V, J_sc = 14.53 mA cm⁻², FF = 0.32 and η = 4.65% under an illumination of 50 mW cm⁻².     

Light intensity and temperature dependence on performance of a photoelectrochemical cells of ITO/TiO<Subscript>2</Subscript>/PVC-LiClO<Subscript>4</Subscript>/graphite

The photovoltaic characteristics of a photoelectrochemical cell of ITO/TiO₂/PVC-LiClO₄/graphite are reported. This paper is concerned with the influence of light intensity and temperature on short-circuit current density, J_sc and open-circuit voltage, V_oc of the device. The photoelectrochemical cell material was a screen-printed layer of titanium dioxide onto an ITO-covered glass substrate, which was used as a working electrode of the cell. The solid electrolyte was polivinylchloride-lithium perchlorate. The graphite film serves as a counter electrode of the cell. The current density–voltage characteristics of the device under an illumination of 20, 40, 60, 80 and 100 mW cm⁻² light from a tungsten halogen lamp were recorded at 40 °C as well as under an illumination of 100 mW cm⁻² at 30, 35, 40, 45 and 50 °C, respectively. It was found that the short-circuit current density, J_sc of the device increases with both light intensity and temperature. The J_sc obtained at 100 mW cm⁻² was 1.0 μAcm⁻² and that at 50 °C was 0.7 μAcm⁻².     

Optimum design of InGaP/GaAs dual-junction solar cells with different tunnel diodes

We design the InGaP/GaAs dual-junction (DJ) solar cells by optimizing short-circuit current matching between top and bottom cells using the Silvaco ATLAS. The relatively thicker base layer of top cell exhibits a larger short-circuit current density (J _sc) while the thicker base layer of bottom cell allows for a smaller J _sc. The matched J _sc of 10.61±0.05 and 13.25±0.06 mA/cm ² under AM1.5G and AM0 illuminations, respectively, are obtained, leading to the increased conversion efficiency. The base thicknesses of top InGaP cells are optimized at 0.8 and 0.65 μm for AM1.5G and AM0 illuminations, respectively, and the base thicknesses of bottom GaAs cells are optimized at 2 μm. For the optimized solar cell structure, the maximum J _sc = 10.66 mA/cm ² (13.31 mA/cm ²), V _oc =  2.34 V (2.35 V), and fill factor =  87.84% (88.1%) are obtained under AM1.5G (AM0) illumination, exhibiting a maximum conversion efficiency of 25.78% (23.96%). The effect of tunnel diode structure, i.e, GaAs/GaAs, AlGaAs/AlGaAs, and InGaP/InGaP, on the characteristics of solar cells is investigated. The photogeneration rate in the DJ solar cell structure is also obtained by incident light of different wavelengths.     

Low resistance dye-sensitized solar cells based on all-titanium substrates using wires and sheets

Low resistance dye-sensitized solar cells (DSSCs) based on all-titanium substrates were proposed in this paper. To minimize the internal resistance of DSSCs, the titanium wires and titanium sheets were used as the substrates of the photoanode and the counter electrode, respectively. Compared with the FTO substrate, titanium wires could absorb much diffused light by back reflection since the reflectivity in the titanium sheet was highly increased up to 53.12%. Furthermore, the transmittance of the front cover was increased by 13.2% using the super white glass instead of FTO substrate. The thickness of TiO₂ thin film coated on titanium wire was optimized to achieve a high cell performance. The efficiency of 5.6% for the cell was obtained with a J_sc of 15.41 mA cm⁻², V_oc of 0.59 V, and FF of 0.62. The results showed that the titanium-based DSSCs had superiority for producing the large-scale DSSCs without metal grid line.     

Investigation of electrical and photovoltaic properties of Au/poly(propylene glycol)-b-polystyrene/n-Si diode at various illumination intensities

Electrical and photovoltaic properties of Au/poly(propylene glycol)-b-polystyrene/n-Si diode were investigated under various illumination intensities. Field emission scanning electron microscope micrographs of the interfacial polymer layer were provided for verification of the nano-fibre pattern. The current–voltage (I–V) measurements of the diode in the dark and under various illumination intensities (50–250?mW/cm²) were carried out and the main electrical parameters of the diode such as leakage current (I₀), ideality factor (n), zero-bias barrier height (Φ_B0), series and shunt resistance (R_s and R_sh), density of surface states (N_ss), open circuit voltage (V_oc) and short-circuit current (I_sc) were obtained in these conditions.     

Silicon heterojunction solar cell with amorphous silicon oxide buffer and microcrystalline silicon oxide contact layers

This Letter reports on the fabrication and characterization of silicon heterojunction solar cells with silicon oxide based buffer (intrinsic amorphous silicon oxide) and contact layers (doped microcrystalline silicon oxide) on flat p‐type wafers. The critical dependency of the cell performance on the front and rear buffer layer thickness reveals a trade‐off between the open circuit voltage V_oc and the fill factor FF. At the optimum, the highest efficiency of 18.5% (active area = 0.67 cm²) was achieved with V_oc = 664 mV, short circuit current J_sc = 35.7 mA/cm² and FF = 78.0%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of electrode film modifications on the open-circuit photovoltage in enhanced dye-sensitized solar cells

In this study, the open-circuit photovoltage (V _oc) decay technique was used to investigate the relationship between the electrode film morphology and the open-circuit photovoltage. Results indicate that dye-sensitized solar cells (DSCs) based on ordered arrays of TiO₂ nanostructures (100 nm external diameters and 20–50 nm internal diameters) generally show higher open-circuit photovoltage (V _oc) values than those based on sintered TiO₂ nanoparticles (20–40 nm diameters). In particular, cells based on thick nanotubules (wall thickness ≥ 45 nm in our research) and on nanorods (100 nm diameters) show particularly high V _oc values, indicating slow recombination kinetics under open-circuit conditions. It can be argued that the nanorods and the thick nanotubules act like singles crystals and therefore the injected electrons in the inner TiO₂ molecules are shielded from holes in the electrolyte under open-circuit conditions. The open-circuit recombination time constant of electrons accumulated in the TiO₂ conduction band is therefore prolonged and resulting in high V _oc values.     

Synthesis,characterization and photovoltaic properties of novel molecules based on triarylamine dyes

We report the synthesis, electrochemical and photovoltaic properties of triphenylamine based novel organic sensitizer, comprising donor, electron-conducting and anchoring group. Different triphenylamines were synthesized as donor unit and connected to cyano-acidic acid acceptor over vinyl group. In this work, we studied on two novel dyes (TPA2, TPA3) by attaching methyl to m-position and diphenylamine to p-position of the basic triphenylamine structure, respectively. All derivatives absorb at visible region of solar spectrum in the range of 350–632 nm. By introducing strong electron injection groups to triphenylamine ring, band gap becomes narrower while E_HOMO–E_LUMO levels are tuned. All current voltage (I–V) measurements were done under 100 mW/cm² light intensity and AM 1.5 conditions. TPA3 chromophore with the lowest band gap shows the best cell performance with an efficiency (η) of 4.12%, a short-circuit photocurrent density (I_sc) of 8.07 mA/cm², an open-circuit voltage (V_oc) of 714 mV and a fill factor (FF) of 0.72.     

AZO/Ag/AZO multilayer films prepared by DC magnetron sputtering for dye-sensitized solar cell application

Ag films were deposited on Al-doped ZnO (AZO) films and coated with AZO to fabricate AZO/Ag/AZO multilayer films by DC magnetron sputtering on glass substrates without heating of glass substrates. The best multilayer films have low sheet resistance of 19.8 Ω/Sq and average transmittance values of 61% in visible region. It was found that the highest figure of merit (F_TC) is 6.9 × 10⁻⁴ Ω⁻¹. For the dye-sensitized solar cell (DSSC) application, the multilayer films were used as transparent conductive electrode (multilayer films/ZnO + Eosin-Y/LiI + I₂/Pt/FTO). The best DSSC based on the multilayer films showed that open circuit voltage (V_oc) of 0.47 V, short circuit current density (J_sc) of 2.24 mA/cm², fill factor (FF) of 0.58 and incident photon-to-current conversion efficiency (η) of 0.61%. It was shown that the AZO/Ag/AZO multilayer films have potential for application in DSSC.     

Electrical and optical characterization of SiOxNy and SiO2 dielectric layers and rear surface passivation by using SiO2/SiOxNy stack layers with screen printed local Al-BSF for c-Si solar cells

In c-Si solar cells, surface recombination velocity increases as the wafer thickness decreases due to an increase in surface to volume ratio. For high efficiency, in addition to low surface recombination velocity at the rear side, a high internal reflection from the rear surface is also required. The SiO_xN_y film with low absorbance can act as rear surface reflector. In this study, industrially feasible SiO₂/SiO_xN_y stack for rear surface passivation and screen printed local aluminium back surface field were used in the cell structure. A 3 nm thick oxide layer has resulted in low fixed oxide charge density of 1.58 × 10¹¹ cm⁻² without parasitic shunting. The oxide layer capped with SiO_xN_y layer led to surface recombination velocity of 155 cm/s after firing. Using single layer (SiO₂) rear passivation, an efficiency of 18.13% has been obtained with V_oc of 625 mV, J_sc of 36.4 mA/cm² and fill factor of 78.7%. By using double layer (SiO₂/SiO_xN_y stack) passivation at the rear side, an efficiency of 18.59% has been achieved with V_oc of 632 mV, J_sc of 37.6 mA/cm², and fill factor of 78.3%. An improved cell performance was obtained with SiO₂/SiO_xN_y rear stack passivation and local BSF.     

Application of polyaniline nanowires electrodeposited on the FTO glass substrate as a counter electrode for low-cost dye-sensitized solar cells

Polyaniline nanowires (PANI NWs) were deposited onto fluorine-doped tin oxide (FTO) glass substrate using the cyclic voltammetric method with aniline monomer precursor in HCl aqueous solution. The secondary oxidation peak plays an important role in polymerization of aniline monomer and the optimization of catalytic activity of PANI-based counter electrodes was achieved by controlling the number of cycles. The photovoltaic performance of the dye-sensitized solar cells (DSSCs) with PANI NWs counter electrodes (CEs) was optimized at 4th cycles, and then following parameters were obtained: J_sc = 17.2 mA cm⁻², V_oc = 0.71 V, FF = 59.3%, and efficiency (η) = 7.24%. While, J_sc = 14.7 mA cm⁻², V_oc = 0.77 V, FF = 70.6%, and efficiency (η) = 7.98% in cells with Pt CEs. The PANI NWs were attractive as an alternative CEs for the low-cost DSSCs instead of Pt.     

Hydrothermal growth of large-scale macroporous TiO<Subscript>2</Subscript> nanowires and its application in 3D dye-sensitized solar cells

Large-scale macroporous TiO₂ nanowires (MTN) were directly grown on spiral-shaped titanium wires as photoanodes of dye-sensitized solar cells (DSSCs) via a facile hydrothermal reaction without any seeds, templates, and TiO₂ powder. The MTN thin film was characterized by SEM, XRD and TEM. The studies revealed that the MTN thin film had better mechanical properties and provided an efficient pathway for the diffusion of liquid electrolyte. The efficiency of 0.86% for the 3D DSSC was obtained with a J _sc of 2.30 mA/cm², V _oc of 616 mV, and FF of 0.61. This MNT-based mini 3D DSSC is a promising photovoltaic device for applications in the fields of high-integrated micro-electronic equipment.     

Poly(vinyl chloride)-<Emphasis Type="Italic">graft</Emphasis>-poly(<Emphasis Type="Italic">N</Emphasis>-vinyl caprolactam) graft copolymer: synthesis and use as template for porous TiO<Subscript>2</Subscript> thin films in dye-sensitized solar cells

Poly(N-vinyl caprolactam) (PNVCL) side chains were grafted to a poly(vinyl chloride) (PVC) backbone via atom transfer radical polymerization. The synthesized PVC-g-PNVCL graft copolymer was templated for the preparation of porous TiO₂ thin films, which involved a sol–gel reaction and calcination process. The interaction of the carbonyl groups in the PVC-g-PNVCL with the titania was revealed by FT-IR spectroscopy. X-ray diffraction and transmission electron microscopy analysis showed the formation of porous TiO₂ thin films with the anatase phase. A series of porous TiO₂ thin films with different pore sizes and porosities was prepared by varying the solution compositions and were used as photoelectrodes in dye-sensitized solar cells (DSSC) with a polymer electrolyte. The DSSC performed best when using the TiO₂ film with higher porosity, lower interfacial resistance, and longer electron life time. The highest energy conversion efficiency, photovoltage (V _oc), photocurrent density (J _sc), and fill factor (FF) were 1.2%, 0.68 V, 3.2 mA/cm², and 0.57 at 100 mW/cm², respectively, for the quasi-solid state DSSC with a 730-nm-thick TiO₂ film.     

Current transport mechanism and photovoltaic properties of photoelectrochemical cells of ITO/TiO2/PVC–LiClO4/graphite

This paper deals with the current transport mechanism of solid state photoelectrochemical cells of ITO/TiO₂/PVC–LiClO₄/graphite as well as the physical properties of a component of a device affecting its performance. The principle of operation and a schematic energy level diagram for the materials used in the photoelectrochemical cells are presented. The device makes use of ITO films, TiO₂ films, PVC–LiClO₄ and graphite films as photoanode, photovoltaic material, solid electrolyte and counter electrode, respectively. The device shows rectification. The J_sc and V_oc obtained at 100 mW cm⁻² were 0.95 μAcm⁻² and 180 mV, respectively.     

Ionic liquid type dye-sensitized solar cells: increases in photovoltaic performances by adding a small amount of water

Increases in photovoltaic performances for dye-sensitized solar cells having ionic liquid type electrolytes are reported. These results are explained by diffusion coefficient for I₃⁻, charge transfer resistances on counter electrodes, flat band potentials of TiO₂, redox potentials for I⁻/I₃⁻, electron diffusion constants, electron life time, and diffusion length in TiO₂ layers. Methylpropylimidazolium iodide is selected because of the lowest viscosity and the highest conductivity. Increases in the photovoltaic performance are observed when a small amount of water was added into the ionic liquid consisting of both LiI and t-butylpyridine as the additives. These improvements are brought about by enhancements of all of J_sc, ff and V_oc. The increases in J_sc and ff are associated with the decrease in charge transfer resistances on counter electrodes and increases in ionic conductivities. V_oc may be explained by an increase in the difference between redox potentials of I⁻/I₃⁻ and Fermi level.     

Synthesis of nanometric iron oxide films by RPLD and LCVD for thermo–photo sensors

Iron oxide films were deposited on <100> Si substrates by reactive pulsed laser deposition (RPLD) using a KrF laser (248 nm). These films were deposited too by laser (light) chemical vapor deposition (LCVD) using continuous ultraviolet photodiode radiation (360 nm). The deposited films demonstrated semiconducting properties. These films had large thermo-electromotive force (e.m.f.) coefficient (S) and high photosensitivity (F). For films deposited by RPLD the S coefficient varied in the range 0.8–1.65 mV/K at 205–322 K. This coefficient depended on the band gap (E _g ) of the semiconductor films, which varied in the range 0.43–0.93 eV. The largest F value found was 44 V_c/W for white light at power density I≅0.006 W/cm². Using LCVD, iron oxide films were deposited from iron carbonyl vapor. For these films, the S coefficient varied in the range −0.5 to 1.5 mV/K at 110–330 K. The S coefficient depended on E _g of the semiconductor films, which varied in the range 0.44–0.51 eV. The largest F value of these films was about 40 V_c/W at the same I≅0.006 W/cm². Our results showed that RPLD and LCVD can be used to synthesize iron oxide thin films with variable stoichiometry and, consequently, with different values of E _g . These films have large S coefficient and high photosensitivity F and therefore can be used as multi-parameter sensors: thermo–photo sensors.     

Effect of built-in electric field in photovoltaic InAs quantum dot embedded GaAs solar cell

In this paper, three p–i–n GaAs solar cells were grown and characterized, one with InAs quantum dot (QD) layers embedded in the depletion region (sample A), one with QD layers embedded in the n ⁻ base region (B), and the third without QDs (control sample C). QD-embedded solar cells (samples A and B) show broad photoluminescence spectra due to QD multi-level emissions but have lower open-circuit voltages V _oc and lower photovoltaic (PV) efficiencies than sample C. On the other hand, the short-circuit current density J _sc in sample A is increased while it is decreased in sample B. Theoretical analysis shows that in sample B where the built-in electric field in QDs is zero, electrons tend to occupy QDs and strong potential variations exist around QDs which deteriorate the electron mobility in the n ⁻ base region so that J _sc in sample B is decreased. Hole trapping and electron–hole recombination in QDs are also enhanced in sample B, resulting in a reduced V _oc and thus a worse PV effect. In sample A, a strong built-in field exists in QD layers, which facilitates photo-carrier extraction from QDs and thus J _sc is increased. However, QDs in the depletion region in sample A act also as recombination-generation centers so that the dark saturated current density is drastically increased, which reduces V _oc and the total PV effect. In conclusion, a nonzero built-in electric field around QDs is vital for using QDs to increase the PV effect in conventional p–i–n GaAs solar cells.