Film-based Dye-sensitized Solar Cells Research Articles

Effect of Dip-Coating Cycles on the Structural and Performance of ZnO Thin Film-based DSSC

Deposition of thin film with good thickness uniformity and quality for fabrication of thin film-based dye-sensitized solar cells is a critical factor that determines the reliability and consistency of its photovoltaic performance. In this work, dip-coating method was used for the deposition of ZnO thin films on fluorine-doped tin oxide glass substrates. The structural, electrical and optical properties of these ZnO thin films were characterized by XRD, FESEM, four-point probe, UV–Vis spectroscope and room temperature PL spectroscope. The study showed that the thickness of ZnO thin film could be adjusted by the number of dipping cycles. By increasing the dip-coating cycles, the thickness, crystal quality and absorbance of visible light of ZnO thin films increased whereas the sheet resistance of ZnO thin films decreased. As a consequence, the photovoltaic performance of DSSCs improved with maximum conversion efficiency of 0.68% at 3 cycles of dip coating. Nevertheless, formation of macro-defects such as pores and cracks in thick ZnO thin films became the dominant factor that deteriorated their conversion efficiency down to 0.19% (at 11 cycles).

Vertically Aligned Al-Doped ZnO Nanowire Arrays as Efficient Photoanode for Dye-Sensitized Solar Cells

In this communication, we report on the synthesis of vertically aligned aluminium (Al)-doped ZnO (ZnO:Al) nanowire (NW) thin films on FTO-coated glass substrates and their use as photoanode in dye-sensitized solar cells (DSSC). Very thin Al layers (∼3 nm, ∼6 nm and ∼10 nm) were deposited onto chemically synthesized ZnO nanowire film by electron-beam evaporation. The films were then subjected to rapid thermal annealing to incorporate different amounts of Al (∼0.98 at.%, 1.94 at.% and ∼2.89 at.%) into the ZnO nanowires. Optical, microstructural and compositional study of the films confirmed the growth of highly transparent and well-aligned ZnO:Al nanowires with a hexagonal crystal structure. The basic DSSC structure was fabricated using both undoped ZnO nanowire and ZnO:Al nanowire thin films as photoanode. In both cases, commercially available N3 dye was used as a photosensitizer, iodide/tri-iodide solution as electrolyte and FTO-coated glass as counter electrode. A significant increase in short-circuit current was observed, from 1.3 mA cm−2 for the pristine ZnO nanowire film-based DSSC to 4.4 mA cm−2 for the ZnO:Al (2.89 at.%) nanowire film-based DSSC. The overall power conversion efficiency (PCE) was also found to increase from 0.13% (for pristine ZnO nanowire thin film) to 0.49% for the ZnO:Al thin film-based DSSC.

A novel multilayer brookite TiO2 electrode for improved performance of pure brookite-based dye sensitized solar cells

A novel multilayer brookite TiO2 film-based dye-sensitized solar cell (DSSCs) is proposed and prepared by three kinds of brookite TiO2 nanoparticles. By optimizing the structure of multilayer brookite TiO2 electrode, the optimal photovoltaic conversion efficiency of multilayer brookite-based solar cell could reach to 8.16%, which is much higher than the current record (5.97%) of pure brookite-based one reported in the literature. The new structure with excellent light scattering effect can effectively extend the optical path length in the TiO2 film, which demonstrates a clear advance towards efficient improvement of the photovoltaic performance of pure brookite-based solar cells.

Sea urchin-like TiO2 microspheres as scattering layer of nanosized TiO2 film-based dye-sensitized solar cell with enhanced conversion efficiency

Sea urchin-like TiO2 microspheres are synthesized through a facile one-step solvothermal process by using titanium tetrabutoxide (TBOT) as titanium source. The obtained TiO2 microspheres with ∼3 μm diameter display anatase phase and hierarchical structures composed of TiO2 nanoribbons containing secondary nanoparticles with an average particle size of ∼15 nm. By using as photoanode material, the anatase TiO2 microspheres film-based dye-sensitized solar cells (DSSCs) show higher short-circuit current but slightly lower conversion efficiency than the TiO2 nanoparticles (P25) film-based one; while the bilayer film-based solar cell by using the TiO2 microspheres as overlayer of the P25 film exhibits significantly improved photovoltaic performance with an efficiency up to 7.14%, improved by 26.6% as compared to the P25 film-based one (5.64%). The anatase TiO2 microspheres as overlayer can not only enhance the light harvesting capability due to their scattering effect of the sea urchin-like hierarchical structures, but also contribute to the larger dye-loading amount, which lead to a decrease in the charge transfer resistance, charge recombination and dark current, and then resulting in the improved photovoltage and photovoltaic performance. The simple fabrication method of the bilayer solar cell, which contains a photoanode composed of P25 film as underlayer and sea urchin-like TiO2 hierarchical microspheres as overlayer, demonstrates a strategy for the development of the low cost and high efficiency solar cells through tuning the photoanode's component and structure.

Performance Enhancement in Dye-Sensitized Solar Cells with Composite Mixtures of TiO2 Nanoparticles and TiO2 Nanotubes

In the present investigation, a new composite nanostructured photoanodes were prepared using TiO2 nanotubes (TNTs) with TiO2 nanoparticles (TNPs). TNPs were synthesized by sol–gel method, and TNTs were prepared through alkali hydrothermal method. Dye-sensitized solar cells (DSSCs) were fabricated with different photoanodes comprising of various ratios of TNTs + TNPs, synthetic indigo dye as photosensitizer, PMII (1-propyl-3-methylimidazolium iodide) as ionic liquid electrolyte and cobalt sulfide as counter electrode. The structures and morphologies of TNPs and TNTs were analyzed through X-ray diffractometer, transmission electron microscope and scanning electron microscopes. The results of the investigation showed that the DSSC-4 made with composite photoanode structure (TNTs/TNPs) (90% of TNPs + 10% of TNTs) had improved photocurrent efficiency (2.11%) than pure TNPs (1.00%) and TNT film (0.78%). Electrochemical impedance spectra revealed that the composite TNTs/TNPs film-based DSSCs possessed the lowest charge-transfer resistances and longest electron lifetime. Hence, it could be concluded that the composite TNTs/TNPs photoanode facilitates the charge transport rate and enhances the efficiencies of DSSCs.

Highly Efficient SnO2/MgO Composite Film-Based Dye-Sensitized Solar Cells Sensitized with N719 and D358 Dyes

SnO 2/ MgO composite film-based dye-sensitized solar cells (DSCs), sensitized with both N719 dye and metal-free D358 dye, employing [Formula: see text] redox couple-based liquid electrolyte, show superior performance to those sensitized with only D358 dye, and N719 dye. A significant improvement in the power conversion efficiency was attained by co-sensitizing the N719-based DSCs with metal-free D358 dye when compared to those obtained for DSCs with individual dyes. As confirmed by UV-visible absorption spectra, N719 dye adsorption is more prominent than that of D358 dye when sensitizing the SnO 2/ MgO composite film with the two dyes, D358 and N719. However, N719 and D358 dyes, when used alone, are prone to form aggregates on the SnO 2/ MgO composite film, when N719 dye is used together with D358, the latter effectively suppresses the aggregation of N719 dye on the SnO 2/ MgO composite film, thereby enhancing the power conversion efficiency of the DSCs. Hence, the corresponding power conversion efficiency of the SnO 2/ MgO composite film-based DSCs can be significantly improved by sensitizing with both N719 and D358 dyes. The reported power conversion efficiencies for the SnO 2/ MgO composite film-based DSCs, sensitized with, (a) D358 dye, (b) N719 dye, and (c) both N719 dye and D358 dye, are 6.37%, 7.43% and 8.60% respectively, under AM 1.5 illumination.

Rice-like brookite titania as an efficient scattering layer for nanosized anatase titania film-based dye-sensitized solar cells

Rice-like brookite TiO2 particles with high phase purity and ∼600 nm particle diameter were synthesized through a hydrothermal method, and then bilayer TiO2 film-based dye-sensitized solar cells (DSSCs) were fabricated by using photoanode consisting of nanosized anatase TiO2 film as underlayer and the above brookite TiO2 submicrometer particle film as overlayer. The effects of the brookite overlayer on the light scattering and photoelectrochemical performances of the anatase TiO2 film-based DSSCs were investigated by using UV–vis diffused reflectance absorption spectra (DRS), open-circuit voltage decay (OCVD) curves and electrochemical impedance spectra (EIS) measurements. It is found that the bilayer film-based DSSCs have much better light harvesting capability, higher open-circuit voltage and longer electron lifetimes than the single anatase TiO2 film-based one. After optimizing the thicknesses of underlayer and overlayer, the corresponding bilayer TiO2 film-based DSSC gives conversion efficiency up to 8.83%, with a 29% improvement in the efficiency as compared to that (6.82%) of the single film-based one under AM 1.5G one sun irradiation. The present results represent a clear advance towards efficient light scattering materials for the nanosized TiO2 film-based solar cells with low-cost and high efficiency.

A dye-sensitized solar cell based on a boron-doped ZnO (BZO) film with double light-scattering-layers structured photoanode

A double light-scattering-layer boron-doped ZnO (DL-BZO) film based dye-sensitized solar cell (DSSC) is fabricated with a high energy conversion efficiency of 7.2%. The over layer is made of well dispersed sub-micrometer-sized boron-doped ZnO (BZO) sphere arrays (SSA-BZO) to facilitate the fastest movement of electrons towards the electrodes and also function as light scattering centers. The under-layer, consisting of a nanoporous BZO nanoparticulate (NP-BZO) film, increases the dye adsorption and also reduces the injected electrons recombination. The DL-BZO film boosted the cell efficiencies because of its improved light-harvesting ability, larger surface area, photon-to-electron mechanism and doping of the ZnO films. The efficiency of 7.2% achieved for DL-BZO is much larger than the 4.1% obtained for double light-scattering-layers for undoped ZnO (DL-ZnO) films and is also higher than the 2.1% achieved for mono-layer NP-BZO films. Addition of an SSA-BZO layer over the NP-BZO layer significantly improves the transport of electrons and their lifetime when compared to the NP-BZO film only. The DL-BZO film exhibit the slowest recombination rate of changes compared with the other films and is found to be a major factor for the improved cell efficiency. The cells exhibit high stability, in terms of cell performance, even after 35 days. The highest efficiency achieved in the present study is the highest ever reported for a double light-scattering-layer ZnO film-based DSSC.

Synthesis of size controllable and thermally stable rice-like brookite titania and its application as a scattering layer for nano-sized titania film-based dye-sensitized solar cells

Size controllable and thermally stable rice-like brookite TiO2 particles with high phase purity were synthesized through a hydrothermal process. By varying the reaction conditions, the average diameter (brachyaxis) of the rice-like brookite TiO2 particles can be tuned over a wide range from ca. 200 nm to 1200 nm. Moreover, the brookite phase can be maintained, even with calcination at a temperature up to 800 °C, and a brookite-to-anatase phase transition and then to rutile can be observed upon further enhancing the calcination temperature from 850 °C to 1000 °C. The obtained brookite TiO2 submicrometer particles were used as a light scattering overlayer on a nano-sized TiO2 (P25) film-based photoanode to fabricate bilayer TiO2 film-based dye-sensitized solar cells (DSSCs). It is found that the brookite TiO2 scattering layers can improve the performances of the P25 film-based solar cells to different extents by enhancing the light-harvesting capability, and the optimal diameter of the rice-like brookite TiO2 particles as a scattering layer material is determined to be ∼600 nm, its corresponding solar cell gives an overall conversion efficiency up to 7.57%, with a ∼33% improvement in the efficiency as compared to that (5.70%) of the individual P25 film-based one under standard AM 1.5G 1 sun irradiation. The above results on the brookite TiO2 particles represent a clear advance towards efficient light scattering materials for the nanosized TiO2 film-based solar cells.

A dual-functional double-layer film with indium-doped ZnO nanosheets/nanoparticles structured photoanodes for dye-sensitized solar cells

A double-layer film composed of an upper-layer of indium-doped ZnO (IZO) nanosheets and a lower-layer of IZO nanoparticles structure (designated as DL-NS,s/NP,s:IZO) has been successfully synthesized by a simple two step electrospray method and studied as the dye-sensitized solar-cell (DSSCs) electrodes with the aim of developing a cost-effective alternative fabrication method and improving the performance of ZnO-based DSSCs. The XRD patterns demonstrated that the films exhibited the hexagonal wurtzite structure with preferential orientation along the (0 0 2) crystal plane and all the peaks slightly shifted toward the larger angle side due to the indium (In) doping. The composition of the doped ZnO films was confirmed with the help of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and X-ray photospectroscopy (XPS). The upper-layer made of upright-standing IZO nanosheets (NS,s:IZO) acts as scattering centers to utilize more light as well as the direct pathway for the electrons to move towards the electrodes. The lower-layer composed of nonporous IZO nanoparticles increases the dye adsorption and also allowed for the suppression of the recombination of injected electrons. It was found that the DL-NS,s/NP,s:IZO film could significantly improve the efficiency (6.9%) of DSSCs owing to its enhanced light-scattering capability as well as intrinsic components of films, the photon-to-electron transfer process of the cells and relatively high surface area. This efficiency was significantly improved over the 4.8% obtained for double light-scattering-layer with DL-NS,s/NP,s:ZnO film and was also higher than the 3.2% achieved for single-layer of NP,s-IZO film. The fabricated cells showed highly durable cell performance, even after 40 days under atmospheric conditions. The efficiency achieved for the best DSSCs in this work is the highest ever reported value for a double light-scattering-layer ZnO film-based DSSCs.

Electrodeposited nanoporous ZnO films exhibiting enhanced performance in dye-sensitized solar cells

Electrodeposition of nanoporous ZnO films and their applications to dye-sensitized solar cells (DSSCs) were investigated in the aim of developing cost-effective alternative synthetic methods and improving the ZnO-based DSSCs performance. ZnO films were grown by cathodic electrodeposition from an aqueous zinc nitrate solution containing polyvinylpyrrolidone (PVP) surfactant. PVP concentration had strong effects on the grain sizes and surface morphologies of ZnO films. Nanoporous ZnO film with grain size of 20–40 nm was obtained in the electrolyte containing 4 g/L PVP. The X-ray diffraction pattern showed that nanoporous ZnO films had a hexagonal wurtzite structure. Optical properties of such films were studied and the results indicated that the films had a band gap of 3.3 eV. DSSCs were fabricated from nanoporous ZnO films and the cell performance could be greatly improved with the increase of ZnO film thickness. The highest solar-to-electric energy conversion efficiency of 5.08% was obtained by using the electrodeposited double-layer ZnO films (8 μm thick nanoporous ZnO films on a 200 nm thick compact nanocrystalline ZnO film). The performance of such cell surpassed levels attained in previous studies on ZnO film-based DSSCs and was among the highest for DSSCs containing electrodeposited film components.