Photoconversion Efficiency Of Dye-sensitized Solar Cells Research Articles

Few-layer graphene supported polyaniline (PANI) film as a transparent counter electrode for dye-sensitized solar cells

Polyaniline (PANI) is an attractive alternative to platinum counter electrode for dye-sensitized solar cell (DSSC) due to its exceptionally low-cost. However, deposition of PANI film over FTO (Fluorine doped tin oxide) glass surface is very difficult. Here, we demonstrate a novel approach to grow PANI via in-situ polymerization on a porous few-layer graphene (FLG) support which has been prepared by simple solvent exfoliation method and deposited on FTO glass by tape casting. The existence of PANI on few-layer graphene is verified by scanning electron microscopy (SEM) images/energy-dispersive X-ray spectroscopy (EDX) mapping, Raman spectrum, X-ray photoelectron spectroscopy (XPS). While, the presence of PANI in the form of emeraldine salt is confirmed by Fourier transformation infrared spectroscopy (FTIR) and UV-Vis absorbance spectroscopy. DSSC based on Gr/PANI and Pt counter electrode (CE) exhibited photoconversion efficiency (PCE) of 3.58% and 3.97% respectively, under one sun at 1 cm2 active area. Electrochemical impedance spectroscopy (EIS) reveals that Gr/PANI based symmetrical cells exhibited higher double layer capacitance than Pt based symmetrical cells, believed to be attributed to higher catalytic active sites. However, Gr/PANI and Gr CEs displayed higher charge transfer resistance and Warburg impedance compared to Pt CE, resulting in superior PCE performance for Pt DSSC. Although the PCE of DSSC with Gr/PANI CE could not exceed the DSSC with Pt CE, but its optical properties and photovoltaic performance are comparatively good for consideration as the alternative.

Enhanced Efficiency of Dye-sensitized Solar Cells Using rGO@TiO2 Nanotube Hybrids

We established a novel strategy for the synthesis of reduced graphene oxide(rGO)@TiO2 nanotube hybrids using an 18 W UV-assisted photo-catalytic reduction method for utilization as photo-anode of dye-sensitized solar cells(DSSCs). The photo-conversion efficiency of DSSCs was significantly enhanced after the addition of rGO, and in addition, the photo-anode showed decreased internal resistance. Analysis of rGO@TiO2 hybrids by transmissions scanning electron microscopy(TEM), X-ray diffraction(XRD), Raman spectra, N2 adsorption and desorption, atomic force microscopy(AFM) and X-ray photoelectron spectroscopy(XPS) demonstrates that the rGO modified TiO2 nanotubes can increase the short-circuit current and the conversion efficiency of dye-sensitized solar cells. The efficiency is improved by almost two folds as much compared to those of the bare TiO2 nanotubes.

Band edge movement and structural modifications in transition metal doped TiO2 nanocrystals for the application of DSSC

Nanocrystalline undoped and transition metal ion doped (TM:Cu2+, Mn2+ and Fe3+) TiO2 nanoparticles, with 1 mol% were synthesized by a simple and cost effective modified co-precipitation method at room temperature and were successfully used as photoanode for dye sensitized solar cell (DSSC). The effect of transition metal ions into TiO2 nano crystalline powder has been systematically investigated using x-ray diffraction (XRD), UV–Vis spectroscope, scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive x-ray spectroscopy (EDX). The results of XRD confirm nanocrystalline anatase tetragonal structure of prepared undoped and TM doped TiO2 semiconductor. The influence of doping on band edge movement has been estimated using UV–visible spectroscopy. The SEM results indicate that microscopic effect of doping on morphology of the TiO2. The peaks of EDX signify incorporation of transition metal cations into TiO2 lattice. The effect of doping on flat band potential was estimated using interpolation on Mott-Schottky plot. The performances of DSSCs of undoped and doped TiO2 photoelectrodes were investigated under light illumination. In comparison with undoped and (Cu2+, Fe3+) doped TiO2 photoanodes we found that incorporation of Mn2+ into TiO2 exhibits improvement in photoconversion efficiency (η). There is increase in photoconversion efficiency of DSSCs with Mn2+ doped TiO2 by 6% as compared to that of undoped TiO2 photoanode.

Boosting the Photovoltage of Dye-Sensitized Solar Cells with Thiolated Gold Nanoclusters.

Glutathione-capped gold nanoclusters (Aux-GSH NCs) are anchored along with a sensitizing squaraine dye on a TiO2 surface to evaluate the cosensitizing role of Au(x)-GSH NCs in dye-sensitized solar cells (DSSCs). Photoelectrochemical measurements show an increase in the photoconversion efficiency of DSSCs when both sensitizers are present. The observed photoelectrochemical improvements in cosensitized DSSCs are more than additive effects as evident from the increase in photovoltage (ΔV as high as 0.24 V) when Au(x)-GSH NCs are present. Electron equilibration and accumulation within gold nanoclusters increase the quasi-Fermi level of TiO2 closer to the conduction band and thus decrease the photovoltage penalty. A similar beneficial role of gold nanoclusters toward boosting the V(oc) and enhancing the efficiency of Ru(II) polypyridyl complex-sensitized solar cells is also discussed.

Ultrafast Spectroscopy and Numerical Analysis of Inhomogeneous Electron Injection in Dye-Sensitized Solar Cells: Effect of Electrolyte

Ultrafast electron injection from a photo-excited dye molecule to semiconductor electrode is one of the most important reactions in the dye-sensitized solar cell (DSSC). The reaction occurs in the wide time region from femtoseconds to nanoseconds with a inhomogeneous manner.[1,2] Since the origin of such inhomogeneity has not been understood well, there still is many problems to improve the photo conversion efficiency of DSSC.[2]Here, we propose a quantitative reaction model of the electron injection dynamics to discuss the origin of the inhomogeneity. Our experimental results of transient absorption spectroscopy (TAS) were successfully analyzed by the model.Dye sensitized TiO2 films with Ru-complex (N719) were prepared. The fs-TAS and ns-TAS measurements were carried out for the films in acetonitrile-based electrolytes. The results were numerically analyzed as a sum of non-adiabatic electron transfer reactions, which supported our experimental finding that the inhomogeneous energy distribution of the energy gap between the donor and acceptor states (Δ) was present.[2] Fig.1 shows the results of population time profile of injected electron in DSSC. The profiles are measured by fs-TAS at <1 ns and sub-nsTAS >1 ns. The simulation result based on our model showed a good agreement with the experimental result. Furthermore, the effects of electrolyte with different potential controlling ion (Li+) concentrations and the electron concentration in semiconductor electrode were investigated experimentally and the obtained electron injection kinetics were well explained by the numerical simulation, indicating that the model is reasonable.In some cases, however, the model does not seem to be easily applicable. For example, when we measured electron injection kinetics from excited N719 to TiO2 in the presence of ionic liquid (IL) as surrounding solvent, less efficiency was observed for some ILs with higher viscosity (> 30 cP). This suggests that not only the driving force but also restriction of molecular reorientation of the sensitizer may affect the reaction.Some other examples of electron injection dynamics relating to the effect of electrolytes and solvents will be shown and the mechanism will be discussed [3, 4]. Acknowledgement: A part of this work is supported by NEDO Japan. Fig.1Population time profile of injected electron in DSSC. Solid line shows a simulation result.

Effects of ligand LUMO levels, anchoring groups and spacers in Ru(II)-based terpyridine and dipyrazinylpyridine complexes on adsorption and photoconversion efficiency in DSSCs

The photophysical and electrochemical properties of a new series of Ru complexes adsorbed onto P25 TiO2 films and their performance in dye-sensitized solar cells (DSSCs) are reported, in order to examine the effects of their various structural differences. The dyes are all homoleptic tridentate Ru(II) complexes differing in the nature of the tridentate chromophore, being either terpyridine (tpy) or 2,6-dipyrazinylpyridine (dpp), in the nature of the anchoring group, either carboxylic acid or ester groups, and in the presence or absence of a phenylene spacer between the chromophore and the anchoring group. The dpp dyes performed poorest, largely due to too-low excited state energies, and phenylene spacers were found to be deleterious to loading levels, while ester anchoring groups led to lower loading levels even after heating to promote transesterification. Further, tert-butylpyridine in the electrolyte inhibited their performance. The best dye in the series was bis(4′-carboxy-2,2′:6′,2″-terpyridine)ruthenium(II) bis(hexafluorophosphate) (H2) for which a total solar energy conversion efficiency of 1.56% was achieved under the best-measured conditions.

Preparation of composite light-scattering layer for dye sensitized solar cells

In this work, the dye sensitized solar cells (DSSCs) were fabricated with bilayered structure TiO2 based photoelectrodes, active and composite light scattering layers. The photoconversion efficiencies of these cells were significantly greater than those of the cells made by single active layered TiO2 films. The composite scattering layers consist of mixed commercial TiO2 particles (ST41) and anatase TiO2 nanoparticles (NPs). The effect of scattering layer composition (ST41:NP weight ratio) on the photovoltaic performance of DSSCs has also been investigated. The best photoconversion efficiency of DSSC was 9.02% (open circuit voltage (Voc)=0.76V, short-circuit photocurrent density (Jsc)=17.6mA/cm2, fill factor (FF)=0.68) for ST41+NP cell (40% ST41 and 60% NPs) with the TiO2 film thickness of 15.3μm, which was 58% enhancement compared with the TiO2 film without scattering-layered TiO2. The scattering layer effect was vindicated by the incident photon-to-current conversion efficiency measurement.

The effect of Li+ intercalation on different sized TiO2 nanoparticles and the performance of dye-sensitized solar cells

The effect of Li+ insertion into different sized TiO2 nanoparticles and their influences on the photoconversion efficiency of dye-sensitized solar cells (DSSC) were investigated. TiO2 nanoparticles with different particle sizes (22nm, 14nm and 6nm) doped with Li+ were employed to form thin film electrodes and their properties were characterized by X-ray diffraction (XRD) and electrochemical impedance spectroscopy analysis. XRD evidenced the presence of anatase as the main phase. From the XRD analysis, it was observed that the Li+ ions could be inserted into both the surface and bulk of the TiO2 nanoparticles. In the larger particle size, the Li+ ions are inserted into the bulk anatase where as Li+ ions bounded on the TiO2 surface for the smaller crystallite size. The photovoltaic properties were measured by a current–voltage meter under AM1.5 simulated light radiation. It exhibited that the overall photoconversion efficiency of DSSC was decreased in the larger particles while it was enhanced in the smaller nanoparticles when Li+ was doped into the TiO2 nanoparticles. A nearly 40% decrease in the efficiency (η) of DSSC was observed upon intercalation of Li+ ions into 22nm sized TiO2 nanoparticles (P25). The 14nm sized TiO2 nanoparticles (P90) showed slightly less efficiency (η) upon Li+ doping than that of the undoped sample. However, the smallest sized TiO2 nanoparticles (6nm) showed higher efficiency than that of the undoped one. This phenomenon is explained based on electron trapping and charge recombination due to lithium doping.

Surface modification of TiO 2 electrode by various over-layer coatings and O 2 plasma treatment for dye sensitized solar cells

The anti-recombination property and the surface area of TiO 2 influence the efficiency of dye-sensitized solar cells (DSSCs). The effects of various over-layers and O 2 plasma treatment on TiO 2 electrodes have been investigated. Over-layers were coated by dip coating in a solution of saturated Ba(NO 3) 2, Mg(NO 3) 2 and N 2O 6Sr, which reduced the recombination of electrons from the photo excited state of Ru dye. O 2 plasma treatment was applied to improve both the contact ability with the dye and the electron takeover capability by reducing oxygen vacancies on both the TiO 2 and the over-layer surface. The photo conversion efficiency of DSSC was improved by both over-layer coating and O 2 plasma treatment. In particular, dipping in saturated Ba(NO 3) 2 solution and O 2 plasma treatment (BP), the photo conversion efficiency was greatly improved from 5.27% to 6.76%. The reason was that the over-layers and O 2 plasma treatment decreased electron recombination. The energy barrier of the modified over-layers blocks the electron transfer from TiO 2 to the electrolyte, and consequently increases electron density by extending the electron lifetime. O 2 plasma treatment also increased the Ru dye absorption.