D. W. Zhang

Graphene nanosheet counter-electrodes for dye-sensitized solar cells

Graphene nanosheets (GNs) have been investigated as a counter electrode for dye-sensitized solar cells (DSCs). Mesoporous TiO2 films are prepared from the commercial TiO2 nano-powders by screen-printing technique on fluorine-doped tin oxide (FTO) slides. GNs are applied to substitute for platinum as counter-electrode materials. GN films are screen printed on FTO glass using a paste based on GNs dispersed in a mixture of terpineol and ethylcellulose. GN counter-electrodes were prepared by annealing the GN films at different temperatures. A metal-free organic dye (indoline dye D102) is used as a sensitizer. Morphological and electrochemical properties of the formed counter-electrodes are investigated by scanning electronic microscopy and electrochemical impedance spectroscopy (EIS), respectively. The electronic and ionic processes in platinum and GNs based DSCs are analysized and discussed. A conversion efficiency of 2.94 % has been obtained for GNs based DSCs. It is found that the quality of the GN counter-electrode and the photovoltaic performance are strongly affected by the annealing temperature of GN materials.

Fabrication and Evaluation of Low-cost Cu 2 ZnSn(S,Se) 4 Counter Electrodes for Dye- sensitized Solar Cells

We explore a simple and eco-friendly approach for preparing CZTS powders and a screen-printing process for Cu2ZnSn(S,Se)4 (CZTSSe) counter electrodes (CEs) in dye-sensitized solar cells (DSCs). Cu2ZnSnS4 (CZTS) nanoparticles have been synthesized via a hydrazine-free solvothermal approach without the assistance of organic ligands. CZTS has been prepared by directly drop-casting the CZTS ink on the cleaned FTO glass, while CZTSSe CEs have been fabricated by screen-printing CZTS pastes, followed by post selenization using Se vapor obtained from elemental Se pellets. The crystal structure, composition and morphology of the as-deposited CZTS nanoparticles and CZTSSe electrodes are characterized by X-ray diffractometer, energy dispersive spectrometer, field emission scanning electron microscopy and transmission electron microscopy. The electrochemical properties of CZTS, CZTSSe and Pt CE based DSCs are examined and analyzed by electrochemical impedance spectroscopy. The prepared CZTS and CZTSSe CEs exhibit a cellular structure with high porosity. DSCs fabricated with CZTSSe CEs achieve a power conversion efficiency of 5.75% under AM 1.5 G illumination with an intensity of 100 mW/cm2, which is higher than that (3.22%) of the cell using the CZTS CE. The results demonstrate that the CZTSSe CE possesses good electrocatalytic activity for the reduction of charge carriers in electrolyte. The comprehensive CZTSSe CE process is cheap and scalable. It can make large-scale electro-catalytic film fabrication cost competitive for both energy harvesting and storage applications.

Dye-sensitized Solar Cells with Higher Jsc by Using Polyvinylidene Fluoride Membrane Counter Electrodes

A flexible counter electrode (CE) for dye-sensitized solar cells (DSCs) has been fabricated using a micro-porous polyvinylidene fluoride membrane as support media and sputtered Pt as the catalytic material. Non-conventional structure DSCs have been developed by the fabricated CEs. The Pt metal was sputtered onto one surface of the membrane as the catalytic material. DSCs were assembled by attaching the TiO2 electrode to the membrane surface without Pt coating. The membrane was with cylindrical pore geometry. It served not only as a substrate for the CE but also as a spacer for the DSC. The fabricated DSC with the flexible membrane CE showed higher photocurrent density than the conventional sandwich devices based on chemically deposited Pt/FTO glass, achieving a photovoltaic conversion efficiency of 4.43%. The results provides useful information in investigation and development of stable, low-cost, simple-design, flexible and lightweight DSCs.

Stability of transparent conducting oxide films deposited by sputtering for solar cells applications

The stability of transparent conducting oxide (TCO) films with bi-layered structures has been tested in air at temperatures up to 550 °C. Aluminum-doped ZnO and Sn-doped In2O3 (AZO/ITO) transparent conducting oxide (TCO) films were deposited on glass substrates by a home made radio-frequency (RF) magnetron sputtering system at room temperature in pure argon ambient. A typical commercial ITO and AZO films by mid-frequency (MF) magnetron sputtering at a substrate temperature of 350 °C were also investigated for comparison. A strong decrease of electrical conductivity was observed after testing at temperature above 350°C for RF deposited bi-layered TCO as well as for the commercial ITO, while the AZO films deposited by MF magnetron sputtering showed a quite stable electrical property at temperature not greater than 500 °C. The optical absorption edge was found to shift to the longer wavelength with an increase in testing temperature for all the TCO. MF sputtered AZO films were used as transparent front contacts for the fabrication of copper indium gallium selenide (CIGS) superstrate thin film solar cells.

One-step growth of structured ZnO thin films by chemical bath deposition in aqueous ammonia solution

Structured ZnO films have been fabricated on soda-lime glass slides at a low temperature (80–85 °C) by a chemical bath deposition method in one step without seed layers. Mixed aqueous solutions of zinc sulfate, ammonia and thiourea were used at alkaline conditions. The influence of the ammonia concentration in the initial solution on the property of the deposited film was investigated systematically. The morphology, structural and optical properties of the deposited films were examined and characterized by x-ray diffraction (XRD), energy-dispersive spectroscopy x-ray diffraction (EDX), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) spectroscopy. Structural analyses with XRD, EDX and SEM revealed that the formed films exhibit a wurtzite hexagonal phase. The deposited film was more preferentially oriented in the (0 0 2) direction with an increase in the ammonia concentration from 0.75 to 2 mol l−1. The optical-phonon E2 mode at 437 cm−1 in the Raman spectrum, together with the XRD and EDX analyses, showed that flower-like and columnar crystalline ZnO films were formed in two ammonia concentration ranges, 0.75–1.4 mol l−1 and 1.6–2.0 mol l−1, respectively. Furthermore, PL spectra showed strong and high intensity peaks of UV emission with suppressed green emission for these deposited ZnO films. ZnS films were formed with a high ammonia concentration of 3.0 M. The formation mechanisms of ZnO, Zn(OH)2 and ZnS phases were discussed.

Open-circuit voltage improvement by using TiO 2 nanotubes as a working electrode of dye-sensitized solar cell

The titanium dioxide nanotubes (TiNTs) were directly fabricated from commercial P25 TiO2 via alkali hydrothermal transformation. The optimized synthesis, thermal and hydrothermal stability, and consequent optical properties of the titanate nanotubes were systematically studied. The TiO2 nanotubes were characterized by transmission electron microscopy. Dye-sensitized solar cells (DSSCs) were constructed with films made of grown TiO2 nanotubes as working electrodes. The nanocrystalline TiO2 pastes were prepared with PEG (Mw 20000) and as made TiO2 nanotubes. The titania thin films were grown by screen printing method in order to efficiently control the DSSC fabrication process. The microstructures of nanoporous films in solar cells were characterized by scanning electron microscope (SEM) and Brunauer Emmett Teller (BET) analysis. A metal-free organic dye (referred to as D102 dye) was used as a sensitizer. A high conversion efficiency of light-to-electricity of around 6% under illumination of simulated AM1.5 sunlight (65mW/cm2) was achieved with the TiO2 nanotube cell. Compared with the case of DSSCs with TiO2 nanoparticles, the open-circuit voltage and fill factor of DSSCs with TiO2 nanotubes increased significantly. The related mechanisms are discussed.

Electrochemical study on the TiO2 porous electrodes for metal-free dye-sensitized solar cells

Nanocrystalline TiO2 porous electrodes were prepared by screen-printing method in order to efficiently control the fabrication process. TiO2 viscous pastes were prepared from commercial TiO2 nano powder using ethyl cellulose as a porosity controlling agent. A metal-free organic dye (indoline dye D102) was used as a sensitizer. TiO2 porous electrodes with different thicknesses were investigated. The optical and physical properties of the TiO2 films, dye adsorption behavior and performance of dye-sensitized solar cells (DSCs) were investigated systemically. The electronic and ionic processes in DSCs were analysized and discussed by electrochemical impedance spectroscopy (EIS). High conversion efficiencies over 8.00 % under illumination of simulated AM1.5 sunlight (60mW/cm2) were achieved.

Cadmium sulfide quantum dots grown by chemical bath deposition for sensitized solar cell applications

Cadmium sulfide (CdS)-sensitized solar cells have been investigated. Chemical bath deposition (CBD) based on an aqueous medium involving cadmium acetate, thiourea, ammonium acetate and ammonia was utilized to assemble CdS quantum dots (QDs) onto mesoporous TiO2 films for dye-sensitized solar cell (DSC) applications. Physical and chemical properties of the formed TiO2/CdS bilayers were analyzed using field emission scanning electron microscope (FE-SEM), Raman spectroscope, ultraviolet-visible light (UV-VIS) spectrometer and X-ray diffraction (XRD). An efficiency as high as 1.50% for the CdS Q dots-sensitized DSC was achieved using the present method. The results have demonstrated the potential applications of CBD CdS Q dots for sensitized solar cells.

Quasi-solid-state dye-sensitized solar cells prepared with a D 102 sensitizer and a polymer electrolyte

Dye-sensitized solar cells (DSSCs) have been constructed by using quasi-solid polymer electrolytes containing polyethylene oxide (PEO). TiO2 viscous pastes were prepared by grinding TiO2 powder (P25) with terpineol, ethylcellulose and acetylacetone. Nanostructured mesoporous TiO2 photoanodes were prepared by the screen-printing method on FTO substrates. TiO2 porous electrodes were sensitized using a metal-free organic dye named D102. The quasi-solid-state electrolytes were grown by incorporating LiI-I2 solutions into a polyethylene oxide (PEO) matrix supported by TiO2 filler, and polyvinylidene fluoride (PVDF) was added to enhance the conductivity of the slurry. The morphologies of dried electrolytes were characterized by scanning electron microscope (SEM). The dependence of the conductivity of the system on the weight ratio of PEO to PVDF in the composite electrolyte was investigated. The polymer gel electrolytes were incorporated in dye sensitized solar cells and the measured energy conversion efficiencies were successfully correlated with their morphological and conducting properties.