Effect of surface modification of TiO2 on the photovoltaic performance of the quasi solid state dye sensitized solar cells using a benzothiadiazole-based dye

We report the preparation of nanoporous TiO₂ electrode modified with an insulating material—BaCO₃ and used as electrode for quasi solid state dye sensitized solar cells (DSSCs), with a benzothiadiazole-based dye (BTDR2) as sensitizer and PEDOT:PSS coated FTO as counter electrode. We found that the BaCO₃ modification significantly increases the dye adsorption, resulting from the fact that the surface of BaCO₃ is more basic than TiO₂. The performance of the DSSCs with and without BaCO₃ modified TiO₂ electrodes were analyzed by cyclic voltammograms, optical absorption spectra, current-voltage characteristics in dark and under illumination and electrochemical impedance spectra. The photovoltaic performance has been significantly improved for the BaCO₃ modified electrode as compared to bare TiO₂ electrode having the same other components in the DSSCs. The value of the overall power conversion efficiency (η) improves from 2.42% to 4.38% under illumination intensity, when BaCO₃ modified electrode is used instead of bare TiO₂ electrode. The improvement in η has been attributed to the increased dye adsorption, reduction in recombination rate and enhancement in the electron lifetime when the modified TiO₂ electrode is used.     

Dye-sensitized solar cells based on anatase TiO2 hollow spheres/carbon nanotube composite films

Dye-sensitized solar cells (DSSCs) based on anatase TiO₂ hollow spheres (TiO2HS)/multi-walled carbon nanotubes (CNT) nanocomposite films are prepared by a directly mechanical mixing and doctor blade method. The prepared samples are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy and N₂ adsorption-desorption isotherms. The photoelectric conversion performances of the DSSCs based on TiO2HS/CNT composite film electrodes are also compared with commercial-grade Degussa P25 TiO₂ nanoparticles (P25)/CNT composite solar cells at the same film thickness. The results indicate that the photoelectric conversion efficiencies (η) of the TiO2HS/CNT composite DSSCs are dependent on CNT loading in the electrodes. A small amount of CNT clearly enhances DSSC efficiency, while excessive CNT loading significantly lowers their performance. The former is because CNT enhance the transport of electrons from the films to FTO substrates. The latter is due to high CNT loading shielding the visible light from being adsorbed by dyes.     

Fabrication of multilayer TiO2 thin films for dye-sensitized solar cells with high conversion efficiency by electrophoresis deposition

This research coats a commercial TiO₂ nanoparticle Degussa P25 with good roundness and size uniformity on an indium tin oxide (ITO) glass substrate and to be photoelectrical electrode by electrophoresis deposition. It combined with dye N719, electrolyte I^-/ and counter-electrode of Pt layer to produce dye-sensitized solar cells (DSSCs). Through the electrophoretic technique, a multilayer film of an appropriate thickness is deposited in the suspension containing TiO₂ nanoparticles and isopropanol. In this process, electric current, voltage, and the number of deposition cycles are well controlled to obtain a single TiO₂ film of around 3.3 μm thick. Stacking is then performed to obtain a multilayer-typed TiO₂ film of around 12 μm thick. As the sintering temperature reaches 400 °C, the prepared multilayer TiO₂ film with a good compactness can increase the dye adsorption capability of the thin film and enhance its adsorption percentage. In addition, the heat treatment will transfer a portion of the rutile crystalline into the anatase crystalline, resulting in better material properties for DSSCs application. DSSCs produced are exposed to metal halide lamp and their energy conversion efficiency is measured. The I-V curve of the produced DSSCs shows that it has an excellent energy conversion efficiency of 6.9%.     

The influence of surface morphology of TiO2 coating on the performance of dye-sensitized solar cells

The optimization of solar energy conversion efficiency of dye-sensitized solar cells (DSSCs) was investigated by the tuning of TiO₂ photoelectrode's surface morphology. Double-layered TiO₂ photoelectrodes with four different structures were designed by the coating of TiO₂ suspension, incorporated with low and high molecular weight poly(ethylene glycol) as a binder. Among these four systems, P2P1, where P1 and P2 correspond to the molecular weight of 20,000 and 200,000, respectively, showed the highest efficiency under the conditions of identical film thickness and constant irradiation. This can be explained by the larger pore size and higher surface area of P2P1 TiO₂ electrode than the other materials as revealed by scanning electron microscopic (SEM) and Brunauer–Emmett–Teller (BET) analyses. Electrochemical Impedance Spectroscopy (EIS) analysis shows that P2P1 formulation displayed a smaller resistance than the others at the TiO₂/electrolyte interface. The best efficiency (η) of 9.04% with the short-circuit photocurrent density (J_sc) and open-circuit voltage (V_oc) of 18.9 mA/cm² and 0.74 V, respectively, was obtained for a solar cell by introducing the light-scattering particles to the TiO₂ nanoparticles matrix coated on FTO electrode having the sheet resistivity of 8 Ω/sq.     

TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact

In traditional solar cells, metal-semiconductor contacts used to extract photogenerated carriers are very important. In dye-sensitized solar cells (DSSC) not much attention has been given to contact between the TiO₂ and the transparent conducting glass (TCO), which is used instead of a metal contact to extract electrons. TiO₂ layers obtained by microwave-activated chemical-bath deposition (MW-CBD) are proposed to improve TiO₂ contact to conducting glass. Spectra of incident photon to current conversion efficiency (IPCE) are obtained for two-photoelectrode TiO₂ photoelectrochemical cells. IPCE spectra show higher values when TiO₂ double layer photoelectrodes are used. In these, the first layer or contacting layer is made by MW-CBD. Best results are obtained for double layer photoelectrodes on FTO (SnO₂:F) as conducting oxide substrate. Modeling of IPCE spectra reveals the importance of electrical contact and electron extraction rate at the TiO₂/TCO interface.     

Preparation of TiO2 nanocrystalline electrode for dye-sensitized solar cells by 28 GHz microwave irradiation

Microwave preparation of TiO₂ nanocrystalline electrode for use in dye-sensitized solar cells is examined. A multi-mode microwave heating system operating at a frequency of 28 GHz is used to produce rapid processing. Well-sintered TiO₂ nanocrystalline thin film is successfully fabricated on transparent conductive FTO glass electrode. Photoelectron energy conversion efficiency of 5.51% is achieved in an electrode prepared by 28 GHz microwave irradiation at 0.7 kW for 5 min.     

Influences of different TiO2 morphologies and solvents on the photovoltaic performance of dye-sensitized solar cells

The effects of TiO₂ photoelectrode's surface morphology and different solvents on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were studied. By successive coating of TiO₂ suspension, composed of low and high molecular weight poly(ethylene)glycol (PEG) as a binder, double layered TiO₂ photoelectrodes with four different structures were obtained. Among the DSSCs with different TiO₂ electrodes, DSSC with P2P1 electrode (P2 and P1 correspond to PEG molecular weights of 20,000 and 200,000, respectively) showed higher performance under identical film thickness at a constant irradiation of 100 mW cm⁻², which may be correlated with large pore size and high surface area of the corresponding TiO₂ electrode. This was confirmed by electrochemical impedance spectroscopy (EIS) analysis of the DSSC and the transient photovoltage measurement of electrons in the TiO₂ electrode. Among the different solvents investigated here, the DSSC containing acetonitrile showed high conversion efficiency and the order of performance of the DSSCs with different solvents were AN > MPN > PC > GBL > DMA > DMF > DMSO. Better correlation was observed between the donor number of solvents and photoelectrochemical parameters of the DSSCs containing different solvents rather than the measured viscosity and dielectric constant of solvents. The reasons for the low performance of the DSSCs containing DMA, DMSO and DMF, respectively, were due to the negative shift of TiO₂ conduction band and the desorption of dye molecules from the TiO₂ photoelectrode by those solvents.     

Improvement in dye-sensitized solar cells employing TiO2 electrodes coated with Al2O3 by reactive direct current magnetron sputtering

A novel surface modification method was carried out by reactive dc magnetron sputtering to fabricate TiO₂ electrodes coated with Al₂O₃ for improving the performance of dye-sensitized solar cells (DSSCs). The Al₂O₃-coated TiO₂ electrodes had been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–vis spectrophotometer, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The study results revealed that the modification to TiO₂ increases dye absorption amount, reduces trap sites on TiO₂, and suppresses interfacial recombination. The impact of sputtering time on photoelectric performance of DSSCs was investigated. Sputtering Al₂O₃ for 4 min on 5-μm thick TiO₂ greatly improves all cell parameters, resulting in enhancing the conversion efficiency from 3.93% to 5.91%. Further increasing sputtering time decreases conversion efficiency.     

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.     

The effects of hydrothermal temperature and thickness of TiO2 film on the performance of a dye-sensitized solar cell

The effects of hydrothermal temperature on the preparation of TiO₂ colloids, and their film thickness on fluorine-doped tin oxide (FTO) glass, toward the performance of a dye-sensitized solar cell (DSSC) were investigated. Pore diameter and surface area of the TiO₂ are of paramount importance in determining the cell efficiency. With the increase of hydrothermal temperature, the pore diameter increases linearly; however, the surface area shows the reverse effect. It is found that the DSSC assembled with the TiO₂ films prepared under the hydrothermal temperature of 240 °C, and the film thickness larger than 10 μm gives optimal performance. The effect of film thickness of TiO₂ on the performance of the DSSC can be explained by the relative size of reactive species diffusing into the thin film and the lifetime of injected electrons. Electrochemical impedance spectroscopy (EIS) was also used to analyze the resistance of the cell, developed as a result of the change in the thickness of the TiO₂ thin film. The at-rest stability for over 200 days was monitored and the results show that the solar energy conversion efficiency was found to decrease from 5.0% of initial value to 3.0% at the end.     

Effect of TiO2 photo-electrode growth condition on dye-sensitised solar cells

Titanium dioxide- (TiO₂) based photo-electrode for dye-sensitised solar cells (DSSCs) use is fabricated with the electro-phoretic deposition (EPD) technique on indium-tin-oxide (ITO). TiO₂ films were synthesised at different EPD biases ranging from 70 to 110 V. We correlate the morphological and optical properties of formed films to the electrical characteristics of fabricated DSSCs. In addition, by neglecting the tunnelling and the tunnelling-assisted thermo-ionic currents with respect to the pure thermo-ionic current at the ITO/TiO₂ interface, we evaluate the high potential barrier, e?_B. Investigation of the electrical properties of the formed DSSCs shows a best result when the TiO₂ film is elaborated at 100 V. Furthermore, by taking into account the high band energy of 0.6 eV at an aluminium-based counter electrode/electrolyte interface, we deduce that aluminium reduces drastically the short-circuit current of the DSSC.     

Al2O3-coated nanoporous TiO2 electrode for solid-state dye-sensitized solar cell

This paper reports the preparation of a core-shell nanoporous electrode consisting of an inner TiO₂ porous matrix and a thin overlayer of Al₂O₃, and its application for solid-state dye-sensitized solar cell using p-CuI as hole conductor. Al₂O₃ overlayer was coated onto TiO₂ porous film by the surface sol–gel process. The role of Al₂O₃ layer thickness on the cell performance was investigated. The solar cells fabricated from Al₂O₃-coated electrodes showed superior performance to the bare TiO₂ electrode. Under illumination of AM 1.5 simulated sunlight (89 mW/cm²), a ca. 0.19 nm Al₂O₃ overlayer increased the photo-to-electric conversion efficiency from 1.94% to 2.59%.     

Triphenylamine- and benzothiadiazole-based dyes with multiple acceptors for application in dye-sensitized solar cells

Two novel dyes TPAR3 and BTDR2 based on triphenylamine and benzothiadiazole, respectively, with multiple electron acceptors were synthesized and characterized by FT-IR, ¹H NMR, TGA and thermomechanical analysis (TMA). They carried terminal cyanoacrylic acid electron acceptors/anchoring moieties, which were connected with the central unit through a thiophene ring. The absorption bands of the dyes were extended up to ∼570 nm with long-wave absorption maximum at 425-455 nm and optical band gap of 2.10-2.17 eV. The dyes emitted yellow-orange light with photoluminescence maximum at 547-615 nm. We have investigated the photovoltaic properties of quasi solid state dye sensitized solar cells (DSSCs) based on these metal free organic dyes. It has been found that the power conversion efficiency of the DSSCs based on composite zinc titanium oxide (ZTO) nanocrystalline photoelectrode is higher than that for TiO₂ based DSSCs. This has been attributed to the longer electron lifetime and more negative conduction band edge of ZTO. The overall power conversion efficiency of the DSSCs based on TPAR3 and BTDR2 employing ZTO photoelectrode is 6.3% and 3.6%, respectively. These results indicate that both the acceptor moiety of metal free organic dyes and ZTO photoelectrode have an effect on the photovoltaic performance of DSSCs.     

Low-temperature oxygen plasma treatment of TiO2 film for enhanced performance of dye-sensitized solar cells

The effects of low-temperature O₂ plasma treatment of a TiO₂ film are studied with the objective of improving the performance of dye-sensitized solar cells (DSSCs). X-ray photoelectron spectra (XPS) reveal that the ratio of titanium dioxide to titanium sub-oxides is increased in the O₂ plasma-treated TiO₂ film, compared with that of the untreated TiO₂ film. This increase suggests that the oxygen vacancies in the film are effectively reduced. The near-edge X-ray absorption fine structure (NEXAFS) spectra results agree with the XPS result. It is proposed that there is a correlation between the shifts of the peaks in the NEXAFS spectra and the adsorption of N719 dye on the TiO₂ particles. A DSSC having an O₂ plasma-treated, 4 μm thick TiO₂ film electrode renders a short-circuit photocurrent of 7.59 mA cm⁻², compared with 6.53 mA cm⁻² for a reference cell with an untreated TiO₂ electrode of the same thickness. As a result of these changes, the solar-to-electricity conversion efficiency of the O₂ plasma-treated cell is found to be 4.0% as compared with 3.5% for the untreated cell. This improvement in the performance is rationalized on the basis of increased N719 dye adsorption on to the TiO₂, due to the reduction in the number of oxygen vacancies caused by the oxygen plasma treatment.     

A high-performance counter electrode based on poly(3,4-alkylenedioxythiophene) for dye-sensitized solar cells

A poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et₂) counter electrode prepared by electrochemical polymerization on a fluorine-doped tin oxide (FTO) glass substrate was incorporated in a platinum-free dye-sensitized solar cell (DSSC). The surface roughness and I⁻/I₃⁻ redox reaction behaviors based on PProDOT-Et₂, poly(3,4-propylenedioxythiophene) (PProDOT), poly(3,4-ethylenedioxythiophene) (PEDOT), and sputtered-Pt electrodes were characterized, and their performances as counter electrodes in DSSCs were compared. Cells fabricated with a PProDOT-Et₂ counter electrode showed a higher conversion efficiency of 7.88% compared to cells fabricated with PEDOT (3.93%), PProDOT (7.08%), and sputtered-Pt (7.77%) electrodes. This enhancement was attributed to increases in the effective surface area and good catalytic properties for I₃⁻ reduction. In terms of the film thickness effect, the fill factor was strongly dependent on the deposition charge capacity of the PProDOT-Et₂ layer, but the aggregation of PProDOT-Et₂ in thicker layers (>80 mC cm⁻²) resulted in decreases in J_SC and the cell conversion efficiency. The charge transfer resistances (R_ct1) of the PProDOT-Et₂ counter electrodes had the lowest value of ∼18 Ω at a deposition charge capacity of 40 mC cm⁻². These results indicate that films with high conductivity, high active surface area, and good catalytic properties for I₃⁻ reduction can potentially be used as the counter electrode in a high-performance DSSC.     

Plasmon-enhanced photocurrent in dye-sensitized solar cells

Plasmonic structures of FTO/TiO₂/NPs-Ag and FTO/NPs-Ag/TiO₂ electrodes were fabricated by sputter technology and the sol–gel & spin coating procedure. These electrodes with similar optical absorptions in the visible region enhanced by the surface plasmon resonance of silver nanoparticles have different photovoltaic properties, revealing that the significant design can be used to identify the favorably enhanced direction of plasmonic structure resulting from plasmonic scattering to trap light which confines light within the active TiO₂ layer to promote dye absorption in dye-sensitized solar cells (DSSCs). In the FTO/TiO₂/NPs-Ag, a 60% enhancement in photocurrent and an improvement in photovoltage were observed and the increased incident photon-to-photocurrent efficiency (IPCE) was consistent with the enhanced absorption spectrum. However, the photovoltaic properties of the FTO/NPs-Ag/TiO₂ were similar to those of the standard electrode. This concept is potentially applicable to new kinds of solar cells.     

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cells

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cell (DSSC) has been investigated to optimize the device geometry for reliable energy conversion efficiency assessment. Mesoporous TiO₂ layers with an identical active area (0.40 cm²) and different dimension are prepared on FTO glass substrate by the screen printing method and used as photoanodes for DSSCs. Under 1 sun illumination (AM 1.5G, 100 mW cm⁻²), both the open-circuit voltage and the short-circuit current density are independent of electrode geometry whereas the fill factor and hence energy conversion efficiency show strong dependency. Electrochemical impedance spectroscopy analysis indicates that the distance between active layer and ohmic contact directly contributes to internal series resistance and influence photovoltaic performance.     

Efficiency enhancement in dye-sensitized solar cells by interfacial modification of conducting glass/mesoporous TiO2 using a novel ZnO compact blocking film

A novel and thin ZnO compact blocking film is employed at the interface of fluorine-doped tin oxide (FTO) substrate and mesoporous TiO₂, and its influence on dye-sensitized solar cells (DSSCs) is investigated. The ZnO film prepared by spin-coating method on FTO is characterized by energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and UV–vis spectrophotometer. The ZnO film is firstly employed as an energy barrier between FTO and mesoporous TiO₂ film in DSSCs, which improves open-circuit photovoltage (V_oc) and fill factor (FF) with compensation of J_sc decrease, finally increasing energy conversion efficiency from 5.85% to 6.70%. Electrochemical impedance spectra (EIS) analysis and open-circuit voltage decay (OCVD) technique reveal that the existence of the energy barrier not only resulted in the effect of suppressing back electrons transfer from FTO to electrolyte but also blocking the electrons injection from the conductive band of TiO₂ to FTO. The former effect effectively reduces the recombination which occurs in the region of FTO substrate, and the latter leads to remarkable increment of electron density in the TiO₂, thus resulting in enhanced V_oc and FF. These results suggest that the methodology of introducing the semiconductor with a more negative conduction band edge than TiO₂ as the compact blocking film in DSSCs may be feasible.     

A comparison of fluorine tin oxide and indium tin oxide as the transparent electrode for P3OT/TiO2 solar cells

Open circuit voltage (V_oc) and other photovoltaic parameters from fluorine tin oxide (FTO) P3OT/TiO₂ composite solar cells have been investigated in comparison with those from the indium tin oxide (ITO) devices with the same device structure and fabrication process. From the experimental results, the performance of FTO-based devices is better than that of ITO devices in terms of V_oc, short circuit current density (J_sc), and power conversion efficiency. The origin of V_oc and the higher V_oc of FTO can be explained and estimated by metal–insulator–metal model with a non-ohmic cathode contact.     

The effect of a blocking layer on the photovoltaic performance in CdS quantum-dot-sensitized solar cells

In order to reduce the surface recombination at the interface between the fluorine-doped tin oxide (FTO) substrate and the polysulfide electrolyte in CdS quantum-dot-sensitized solar cells (QDSCs), compact TiO₂ is deposited on the FTO electrode by sputtering. The TiO₂-coated CdS-sensitized solar cell exhibits enhanced power-conversion efficiency (0.52%) compared with a bare CdS-sensitized solar cell (0.23%). Charge-transfer kinetics are analyzed by impedance spectroscopy, open-circuit decay, and cyclic voltammetry. The TiO₂ layer deposited on the FTO substrate acts as a blocking layer, which plays a significant role in reducing the electron back transfer from the FTO to the polysulfide electrolyte. Interestingly, with respect to the incident photon-to-current conversion efficiency (IPCE) data, asymmetric enhancement is observed from the sample with a thicker blocking layer. This is because CdS quantum dots absorb ultraviolet light completely with the TiO₂ layer because of the high extinction coefficient of the CdS quantum dots compared with dye molecules.