Photovoltage enhancement of dye sensitised solar cells by using ZnO modified TiO2 electrode

Abstract

A ZnO modified TiO₂ (ZnO/TiO₂) film was prepared by immersing TiO₂ electrodes in Zn(Ac)₂ aqueous solution. The open circuit voltage of a dye sensitised solar cell (DSSC) with the ZnO/TiO2 film electrode has a dramatic enhancement, compared to the DSSC with the TiO₂ film electrode. However, the short circuit current density of the DSSC with the ZnO/TiO₂ film electrode is lower than that with TiO₂ electrode. The film electrodes were characterised by SEM, EDX and UV-vis, and the photoelectric performance of DSSCs were measured. The photovoltage enhancement is attributed to the formation of a flat-band potential energy barrier by ZnO at TiO₂/electrolyte interface. The decline of the photocurrent with ZnO/TiO₂ film electrode is due to poor dye absorption on larger particles of ZnO.     

Doubled layered ITO/SnO2 conducting glass for substrate of dye-sensitized solar cells

The effects of indium tin oxide (ITO) and ITO/SnO₂ conducting substrates on photovoltaic properties of dye-sensitized solar cells (DSCs) using nanocrystalline TiO₂ were studied. The decrease in fill factor of the DSCs was correlated to the increase in resistance of conducting substrate. The heat stability of ITO conducting glass was improved by depositing SnO₂ on ITO layer. The efficiency of the cells using double layered ITO/SnO₂ substrate remarkably increased comparing with that of the cells using ITO substrates. It is worth mentioning that increasing in sintering time, which enhanced the electronic contact between substrate and TiO₂, also modified the cell performance of MP-TiO₂ cells. Our experimental finding suggests that 3000 Å ITO substrate, which was covered by 1000 Å SnO₂ layer, exhibited the best properties for the DSCs.     

Controlled synthesis of ZnO and TiO2 nanotubes by chemical method and their application in dye-sensitized solar cells

In this paper, the center hollow ZnO and TiO₂ nanotubes arrays were synthesized by chemical etching ZnO nanorods and sol-gel process assisted by ZnO nanorods templates, respectively. And the process concerning the formation of nanotubes was analyzed. Furthermore, as an application of the ZnO and TiO₂ nanotubes, dye-sensitized solar cells (DSSCs) based on them were successfully fabricated and the cell performances were characterized. The efficiency of DSSCs based on ZnO and TiO₂ nanotubes was 1.2% and 2.1%, respectively.     

Improved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer

A comparative study of the cell performance of CIGS thin-film solar cells fabricated using ZnO:Al and ZnO:B window layers has been carried out. ZnO:B films were deposited by RF magnetron sputtering using an undoped ZnO target in a B₂H₆–Ar gas mixture. The short-circuit current (J_sc) was found to improve upon the replacement of the ZnO:Al layer with ZnO:B layers. This improvement in J_sc is attributed to an increase in quantum efficiency due to the higher optical transmission of the ZnO:B layer in the near-infrared region. The best cell fabricated with a MgF₂/ZnO:B/i-ZnO/CdS/CIGS/Mo structure yielded an active area efficiency of 18.0% with V_oc=0.645 V, J_sc=36.8 mA/cm², FF=0.76, and an active area of 0.2 cm² under AM 1.5 illumination.     

Light harvesting enhancement for dye-sensitized solar cells by novel anode containing cauliflower-like TiO2 spheres

Cauliflower-like TiO₂ rough spheres, which are about 200 nm large, have greatly enhanced light harvesting efficiency and energy conversion efficiency of dye-sensitized solar cells (DSC), due to their high light scattering effect and large BET surface area (80.7 m² g⁻¹) even after calcinations at 450 °C for 30 min. The large size TiO₂ rough and smooth spheres, produced at different initial temperatures by hydrolysis of Ti(OBu)₄ with P105 (EO₃₇PO₅₆EO₃₇) or F68 (EO₇₈PO₃₀EO₇₈) tri-block copolymer as structural agents, have nearly the same diameter of ∼275 nm and strong light scattering effects in the wavelength of 400–750 nm. However, rough spheres have even higher light scattering effect and larger BET surface area than smooth spheres for the roughness of the surface. By adding 25 wt.% large TiO₂ spheres into the over-layer of TiO₂ film composed of ∼20 nm TiO₂ particles as light scattering centers, the energy conversion efficiency of the film containing rough spheres reaches 7.36%, much larger than that of smooth spheres (6.25%). From another point of view, the TiO₂ rough spheres may have the satisfying ability in other fields of application such as photo-catalysis, drug carriers and so on.     

Tailor and functionalize TiO2 compact layer by acid treatment for high performance dye-sensitized solar cell and its enhancement mechanism

Surface tailoring and functionalization of an annealed TiO₂ compact layer by H₂SO₄ acid was performed to improve the dye-sensitized solar cell (DSSC) performance. Compared to untreated counterpart, the acid-treated compact layer possesses a rougher surface and more hydroxyl groups, which result in increased surface area and enhanced adherence of the compact layer with the mesoporous TiO₂ film by Ti–O–Ti bonds formed by a followed heating process. Impedance measurement was further used to investigate the enhancement mechanism, indicating the acid post treatment of the TiO₂ compact layer reduces the ohmic bulk resistivity while effectively suppressing charge recombination at FTO/electrolyte interface. In DSSCs with untreated TiO₂ compact layer, a significantly increased series resistivity is very likely to be the rate determining factor to limit the charge separation process. Thus, an optimal post acid treatment could reduce the resistivity for high charge transport, resulting in larger short-circuit current for further improvement of power conversion efficiency from 6.60% in DSSC with untreated compact layer to 7.21% in DSSC with acid-treated compact layer. This work also provides fundamental insight of the compact layer for DSSC performance improvement.     

TiO2/ZnO/TiO2 sandwich multi‐layer films as a hole‐blocking layer for efficient perovskite solar cells

A continuous and compact hole‐blocking layer is crucial to prevent photocurrent recombination at the photoanode/electrode interface of high‐performance mesostructure perovskite‐based solar cells. Novel TiO₂/ZnO/TiO₂ sandwich multi‐layer compact film prepared as hole‐blocking layer for perovskite solar cell. Herein, TiO₂, ZnO, and TiO₂ layers were successfully deposited by spin‐coating onto FTO glass substrate in sequence. The fill factor and power conversion efficiency of the perovskite solar cell are remarkably improved by the employment of a TiO₂/ZnO/TiO₂ sandwich compact layer. Perovskite solar cell based on TiO₂/ZnO/TiO₂ sandwich film has been observed to exhibit maximum incident‐photon‐to‐current conversion efficiency in the visible region (400–780 nm) and reach a power conversion efficiency of 12.8% under AM1.5G illumination. Copyright © 2016 John Wiley & Sons, Ltd.     

Efficiency enhancement by mixed cation effect in dye-sensitized solar cells with a PVdF based gel polymer electrolyte

This paper reports the effect of using a mixed iodide salt system with two dissimilar cations to enhance the efficiency of dye-sensitized solar cells made with polyvinylidenefluoride (PVdF) based gel electrolyte. Instead of a single iodide salt, a mixture of potassium iodide (KI) with a small K⁺ cation and tetrapropylammonium iodide (Pr₄NI) with a bulky Pr₄N⁺ cation were used to provide the required iodide ion conductivity. Solar cells of configuration FTO/TiO₂/Dye/electrolyte/Pt/FTO were fabricated using a mesoporous TiO₂ electrode sensitized with a Ruthenium dye (N719). With identical electrolyte compositions, the cells with KI and Pr₄NI alone gave efficiencies of 2.37% and 2.90% respectively. The cell with the mixed iodide system, KI:Pr₄NI = 16.6:83.4 (% weight ratio), however, showed an enhanced efficiency of 3.92% with a short circuit current density of 9.16 mA cm⁻², open circuit voltage of 674.4 mV and a fill factor of 63.4%.     

Preparation of transparent and conducting boron-doped ZnO electrode for its application in dye-sensitized solar cells

A simple and economic chemical spray pyrolysis method is used to prepare transparent and conducting boron-doped zinc oxide (B_nZnO) electrode having potential applications in dye-sensitized solar cells (DSSCs). The B_nZnO electrodes were critically characterized for their structural, morphological and electrical properties. The B_nZnO electrode with 2 at% boron doping showed average grain size of 20(±1) nm, surface roughness of 9 nm, ?95% transparency and resistivity of 4.5×10⁻³ Ω cm⁻¹. Furthermore, doping concentration of boron could also be easily controlled for achieving desired properties. Using this electrode as a substrate in DSSCs, the solar-to-electrical conversion efficiency with N3 dye as a sensitizer was noted to be 1.53%. This work suggests that the B_nZnO electrodes could be used as promising alternative to presently used indium- or fluorine-doped tin oxide as substrates.     

Improvement of the performance of dye-sensitized solar cells using Sn-doped ZnO nanoparticles

Sn-doped and undoped ZnO nanoparticles were synthesized by hydrothermal method and their performance as the photoanode of dye-sensitized solar cells (DSSCs) was investigated. Energy dispersive X-ray spectroscopy and X-ray diffraction showed that the Sn had been doped into the ZnO lattice. A red shift of photoluminescence spectra which was induced by Sn doping was observed. The photocurrent density-voltage curves of DSSCs indicated that the efficiency was increased by as high as 140% on bare-FTO substrate and 105% on ZnO compact layer/FTO substrate via Sn doping. Also the effect of the ZnO compact layer was discussed by both of Sn-doped or undoped DSSCs.     

ITO/ATO/TiO2 triple-layered transparent conducting substrates for dye-sensitized solar cells

A novel transparent conductive oxide film based on the triple-layered indium tin oxide (ITO)/antimony-doped tin oxide (ATO)/titanium oxide (TiO₂) has been developed for dye-sensitized solar cells by using radio frequency magnetron sputtering technique. Effects of the absence and presence of TiO₂ layer and the ITO layer thickness were investigated. Deposition of ATO layer was found to stabilize the thermal instability of ITO. Little change in sheet resistance and optical transmittance was observed by introduction of insulating thin TiO₂ layer on top of the ATO layer, whereas photovoltaic performance was significantly influenced. The conversion efficiency was improved from 4.57% without TiO₂ layer to 6.29% with TiO₂ layer. The enhanced photovoltaic performance with addition of TiO₂ layer was attributed mainly to the improved adhesion and partially to the reduced electron loss at the ITO/ATO conductive layer. Increase in the ITO layer thickness resulted in a slight decrease in photocurrent due to the reduced optical transmittance. When compared with the conventional fluorine-doped tin oxide (FTO), the ITO/ATO/TiO₂ conductive material exhibited similar photocurrent density but higher photovoltage and fill factor, resulting in better conversion efficiency.     

Improved performance of a dye-sensitized solar cell using a TiO2/ZnO/Eosin Y electrode

TiO₂/ZnO/Eosin Y structure films were prepared by a one-step cathodic electrodeposition method and used as a photoanode in a dye-sensitized solar cell (DSSC). Using this TiO₂/ZnO/Eosin Y electrode in DSSC, the degradation of the cell with time was reduced and I_SC, V_OC and fill factor values were increased. The use of a thin ZnO layer, permitted the formation of an energy barrier at the electrode/electrolyte interface, thus reducing recombination rate and improving cell performance. In addition, the adsorbed dye molecules prepared by one-step cathodic electrodeposition with ZnO were very stable compared with that prepared by conventional immersing method, as evidenced by UV/vis absorption spectroscopy measurements.     

Hydrogen treated TiO2 nanoparticles onto FTO glass as photoanode for dye-sensitized solar cells with remarkably enhanced performance

Hydrogen treatment is a facile and efficient approach for the enhancement in the functioning of TiO₂ nanoparticles for dye-sensitized solar cells (DSSC). In this work, TiO₂ nanoparticles have been synthesized in the hydrogen environment followed by the deposition onto FTO glass substrates with various film thickness as photoanodes for DSSC. The synthesized hydrogen treated TiO₂ nanoparticles based photoanodes have showed significantly improved photocurrent in the resulting fabricated devices. SEM and TEM analyses have confirmed the particle size and morphology of TiO₂ nanoparticles at various magnifications. The crystalline structure and phase identification were studied by XRD analysis and Raman spectroscopic measurements. The UV–Vis spectroscopy analysis was carried out to find the response of samples for ultraviolet and visible light. The current-voltage measurements have confirmed the improvement of photocurrent that is principally due to improved photo-activity of hydrogen treated TiO₂ nanoparticles. Moreover, hydrogen treated TiO₂ nanoparticles-based photoanode with the film thickness of 11.65 μm has remarkably enhanced power conversion efficiency of 6.05% in DSSCs. The ability of highly photoactive hydrogen treated TiO₂ nanoparticles will provide the new openings in different fields that include photo-electrochemical water splitting and in many other applications.     

Down-converting lanthanide doped TiO2 photoelectrodes for efficiency enhancement of dye-sensitized solar cells

Lanthanide (Ln³⁺) doped TiO₂ down-conversion photoelectrodes (Ln³⁺ = Eu³⁺ and Sm³⁺ ions) are used to enhance the photovoltaic efficiency of dye-sensitized solar cells (DSSC). We report on achieving fill factors of 0.67 and 0.69 and efficiencies of 5.81% and 5.16% for Sm³⁺ and Eu³⁺, respectively. This is compared to the 4.23% efficiency for the undoped-titania photoelectrodes. This enhancement is probably due to the improved UV radiation harvesting via a down-conversion luminescence process by the lanthanide ions. The structure, optical and photoluminescence properties of the down-converting photoelectrode are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDX) and room temperature photoluminescence excitation and emission spectrofluorimetric measurements.     

Influence of TiO2/electrode interface on electron transport properties in back contact dye-sensitized solar cells

The influence of the TiO₂/electrode interface was investigated on electron transport properties at the interface and in TiO₂ porous film in back contact dye-sensitized solar cells. Analysis of dye-sensitized solar cells (DSCs) with Ti and TCO indicated that electron transport properties at TiO₂/Ti and TiO₂/TCO interfaces are similar despite the former's lack of a ‘built-in potential’. The dependence of short circuit current density on TiO₂ thickness indicated that TiO₂ electron transport is not affected by ‘built-in potential’ or electrode structure. Electron transport thus appears similar in back contact dye-sensitized solar cells and DSCs. A back contact dye-sensitized solar cell fabricated with a Ti electrode and optimum TiO₂ porous film showed a conversion efficiency of 7.8% with a metal mask under an air mass of 1.5 sunlight.     

Enhancement of the photoelectric performance of dye-sensitized solar cells by using a CaCO3-coated TiO2 nanoparticle film as an electrode

Core-shell-type nanoparticles with TiO₂ cores and CaCO₃ shells were applied as the electrode of dye-sensitized solar cells. The performance of the cell was significantly improved (as high as 26.7%) compared to the case when un-coated TiO₂ particle film was used as electrode. The improved energy conversion efficiency has been ascribed to (i) enhanced dye adsorption due to the high isoelectric point of the overlayer, and (ii) the prevention of the back electron transfer by the insulating nature of the overlayer.     

A novel UV-mediated low-temperature sintering of TiO2 for dye-sensitized solar cells

Titania pastes were fired at 450 °C in oxygen to give white titania that was used to prepare dye-sensitized solar cells (DSSC). Titania fired at lower temperature and/or under inert atmosphere have brown stripes and cells made from these stripes had no measurable efficiency. When the titania paste was screen printed and then heated and simultaneously irradiated with UV light, white stripes were obtained. Improved efficiency was noted for PV cells made from pastes heated at lower temperature under irradiation vs. cells made from low-temperature heated paste but without irradiation. UV irradiation appears to facilitate clean oxidation of residual organic materials in the titania precursor pastes. The best cells in our study made with our titania paste treated at 450 °C in oxygen had the following characteristics: efficiency=3.45%; V_oc=630 mV; J_sc=8.5 mA/cm²; and a fill factor=0.64.     

Pore-size effect on photovoltaic performance of dye-sensitized solar cells composed of mesoporous anatase-titania

The effect of the pore size of mesoporous anatase-TiO₂ on the photovoltaic performance of dye-sensitized solar cells (DSSCs) is investigated. The mesoporous TiO₂ particles are synthesized by two different methods using a soft template of tri-block copolymer and a hard template of mesoporous ZnO/Zn(OH)₂-composite. These methods produce the same high surface area (S_BET ∼ 210 m² g⁻¹) but different pore sizes of 6.8 and 3.0 nm, respectively. With the mesoporous TiO₂ having larger pores, the photo-conversion efficiency (η) is increased significantly to 6.71%, compared with 5.62% that is typically achieved using P25 TiO₂ nanopowders. By comparison, only half the performance (3.05%) has been observed with mesoporous TiO₂ that has small pores. Mesoporous TiO₂ with suitable pore sizes (∼6.8 nm) makes the most of its high surface area and thereby allows a high uptake of dye to enhance the current density. In contrast, the low efficiency of mesoporous TiO₂ with small pores is attributed to the low uptake of dye due to the smaller pore size (∼3.0 nm), which blocks the diffusion and adsorption of dye molecules through the pores.     

Efficiency enhancement of solar cells by luminescent up-conversion of sunlight

Significant improvements in the efficiency of solar cells by combination with luminescent up- or down-converters have recently been predicted theoretically. Here, we extend the theoretical analysis of the limiting efficiency of the up-conversion (UC)-system to realistic Airmass spectra and analyse the spectral robustness of the UC-system. We also present initial experimental results from prototypes involving bifacial silicon solar cells with UC-phosphors attached to the rear surface, and discuss the possibility of realizing efficient UC with low-band-gap solar cells in combination with a light emitting diode.