Photoenergy Conversion Efficiency Research Articles

Influence of iodine concentration on the photoelectrochemical performance of dye-sensitized solar cells containing non-volatile electrolyte

The effect of iodine concentration in the electrolyte with non-volatile solvent of dye-sensitized solar cells (DSCs) on photovoltaic performance was studied. The electron transport and interfacial recombination kinetics were also systematically investigated by electron impedance spectroscopy (EIS). With the iodine concentration increased from 0.025 to 0.1 M, open-circuit voltage ( V oc) and photocurrent density ( J sc) decreased while fill factor ( ff) increased significantly. The decline of the V oc and J sc was mainly ascribed to increased electron recombination with tri-iodide ions (I 3 −). The increased fill factor was primarily brought by a decrease in the total resistance. From impedance spectra of the solar cells, it can be concluded that increasing the iodine concentration in electrolytes could decrease charge transfer resistance ( R ct) and the chemical capacitance ( C μ), increase the electron transport resistance ( R t), and hence decrease the electron lifetime ( τ) and the effective diffusion coefficient ( D n) of electrons in the TiO 2 semiconductor. With optimum iodine concentration, device showed a photocurrent density of 16.19 mA cm −2, an open-circuit voltage of 0.765 V, a fill factor of 0.66, and an overall photo-energy conversion efficiency of 8.15% at standard AM 1.5 simulated sunlight (100 mW cm −2).

Block-Copolymer-Nanowires with Nanosized Domain Segregation and High Charge Mobilities as Stacked p/n Heterojunction Arrays for Repeatable Photocurrent Switching

Development of materials for efficient photoenergy conversion is a subject of critical importance in current science and technology. Efficient performance requires well-controlled segregation of electron donor and acceptor moieties, which we have achieved using block copolymers of tetraphenylporphinatozinc(II) (donor) and C(60) fullerene (acceptor) using living ring-opening metathesis polymerization (ROMP). The resulting amphiphilic ROMP block copolymers undergo self-assembly into nanostructured phase-segregated 1-dimensional nanowires with an approximately 5.5 nm periodicity zebra-stripe-like morphology simply by drop-casting solutions of the polymers onto a substrate such as mica or highly oriented pyrolytic graphite (HOPG). Thin films of the self-assembled nanophase-segregated copolymers exhibit high charge carrier mobilities (approximately 0.26 cm(2) V(-1) s(-1)) and electrical conductivities (up to 6.4 x 10(-4) cm(2) V(-1) s(-1)) as well as highly repeatable photocurrent switching with rapid ON/OFF responses upon white light irradiation.

Electroluminescence and photovoltaic properties of light-emitting devices and solar cells comprising 2-pyran-4-ylidene-malononitrile conjugated polymers

Several main-chain type and triazine-branched, fluorene and thiophene copolymers containing a 2-pyran-4-ylidene-malononitrile derivative were synthesized using the Suzuki coupling reaction. A triazine-branched fluorene-based device displayed superior electroluminescence performance with a brightness ≤1031 cd m −2 and an efficiency of 1.97 cd A −1. A polymer solar cell, comprising an interpenetrating network of conjugated polymer as electron donor and a fullerene derivative, (6,6)-phenyl-C 61-butyric acid methyl ester, as the electron acceptor, exhibited photocurrent density and photo-energy conversion efficiency of 5.07 mA cm −2 and 0.50%, respectively.

Study of Dye‐Sensitized Solar Cells by Scanning Electron Micrograph Observation and Thickness Optimization of Porous TiO2 Electrodes

In order to improve the photoenergy conversion efficiency of dye‐sensitized solar cells (DSCs), it is important to optimize their porous TiO2 electrodes. This paper examines the surface and cross‐sectional views of the electrodes using scanning electron micrography. Two types of samples for cross‐sectional viewing were prepared by mechanically breaking the substrate and by using an Ar‐ion etching beam. The former displays the surface of the TiO2 particles and the latter shows the cross‐section of the TiO2 particles. We found interesting surface and cross‐sectional structures in the scattering layer containing the 400 nm diameter particles, which have an angular and horned shape. The influence of TiO2 particle size and the thickness of the nanocrystalline‐TiO2 electrode in DSCs using four kinds of sensitizing dyes (D149, K19, N719 and Z907) and two kinds of electrolytes (acetonitrile‐based and ionic‐liquid electrolytes) are discussed in regards to conversion efficiency, which this paper aims to optimize.

Functional Multilayered Transparent Conducting Oxide Thin Films for Photovoltaic Devices

In this study, we present a thermally stable multilayered transparent conducting oxide (TCO) functionalized for dye-sensitized solar cells (DSSCs). Nb-doped TiO2 (NTO) layers deposited on conventional Sn-doped In2O3 (ITO) substrates using pulsed laser deposition (PLD) enhanced the optical-to-electrical conversion efficiency of the DSSCs by as much as 17% compared to that of bare ITO-based DSSCs. The electrical properties and J−V characteristics of the multilayered NTO/ITO films showed that the improved cell performance was due to the facilitated charge injection from TiO2 to ITO that resulted from the formation of an ohmic contact with ITO, as well as the conserved high conductivity of ITO after the oxidizing annealing process. Moreover, the NTO/ITO-based DSSC exhibited higher efficiency than a F-doped SnO2(FTO)-based one, which demonstrates that optimization of multilayered NTO-based TCOs is a realistic approach for achieving highly efficient photoenergy conversion devices.

Bifacial dye-sensitized solar cells based on an ionic liquid electrolyte

Solar energy is a promising solution to global energy-related problems because it is clean, inexhaustible and readily available. However, the deployment of conventional photovoltaic cells based on silicon is still limited by cost, so alternative, more cost-effective approaches are sought. Here we report a bifacial dye-sensitized solar cell structure that provides high photo-energy conversion efficiency (∼6%) for incident light striking its front or rear surfaces. The design comprises a highly stable ruthenium dye (Z907Na) in combination with an ionic-liquid electrolyte and a porous TiO2 layer. The inclusion of a SiO2 layer between the electrodes to prevent generation of unwanted back current and optimization of the thickness of the TiO2 layer are responsible for the enhanced performance. Low-cost, efficient solar cells are sought as an alternative to silicon photovoltaics. Here a dye-based bifacial solar cell that is capable of efficient generation of electricity for light incident on either its front or rear face is demonstrated.

Ionic Liquid Based Electrolyte with Mesoporous Silica SBA‐15 as Framework for Quasi‐solid‐state Dye‐sensitized Solar Cells

AbstractQuasi‐solid‐state electrolytes were fabricated with mesoporous silica SBA‐15 as a framework material. Ionic conductivity measurements revealed that SBA‐15 can enhance the conductivity of the quasi‐solid‐state electrolyte. The diffusion coefficients of polyiodide ions such as I−3 and I−5 which were confirmed by Raman spectroscopic measurement, were about twice larger than that of I−. The optimized photoenergy conversion efficiency of dye‐sensitized solar cells (DSSC) with the quasi‐solid‐state electrolyte was 4.3% under AM 1.5 irradiation at 75 mW·cm−2 light intensity.

Improved quasi-solid dye-sensitized solar cells by composite ionic liquid electrolyte including layered α-zirconium phosphate

The authors reported a composite quasi-solid electrolyte by adding layered α-zirconium phosphate (α-ZrP) to the iodide/tri-iodide ionic liquid, 1-methyl-3-propylimidazolium dihydrophosphate, electrolyte including 4-tert-butylpyridine, which markedly improved photovoltaic properties of quasisolid dye-sensitized solar cells (DSSCs). When adding 6wt% α-ZrP, photoenergy conversion efficiency of the DSSC increases by a factor of more than 2 to 2.61%, compared to a DSSC without α-ZrP. This enhancement is primarily explained by studying dark reaction, diffusion coefficient of tri-iodide ions, exchange current density in the interface of electrolyte/Pt counterelectrode, and lifetime of electrons in mesoscopic TiO2 film.

Additives for Increased Photoenergy Conversion Efficiencies of Quasi-Solid, Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are solidified with gelators containing polyvinylpyridine and 1,2,4,5-tetra(bromomethyl)benzene. The photoconversion efficiencies are improved by new additives. LiI and -butylpyridine are commonly added in electrolytes for increasing short-circuit current (Jsc) and open-circuit voltage (Voc). These additives inhibit our gel electrolyte precursors from solidifying. We found that new additives, combinations of acetic acid, and methylpyrimidine or methylbenzimidazole, do not inhibit the solidification and are effective for increasing both Jsc and Voc. These mechanisms are discussed in terms of electron diffusion coefficients, ion diffusion coefficients, and charge-transfer resistances between counter electrodes and gel electrolytes.

The effect of particle size and conductivity of CuI layer on the performance of solid-state dye-sensitized photovoltaic cells

Abstract The conductivity of CuI film was greatly increased by the addition of the thiocyanate salts and the photoenergy conversion efficiency of DSSSCs made with this CuI film as a hole transporting material was greatly improved. The optimum amount of SCN salt for the cell performance was 4–5% to CuI, and an excessive addition declined cell performance in spite of conductivity increasing.

High performance dye-sensitized solar cells using ionic liquids as their electrolytes

Room temperature ionic liquids have been used as electrolytes to investigate the performance and the characteristics in dye-sensitized solar cells (DSSCs). The ionic liquids used are 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (EMImTFSI), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF 4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF 6), 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA), and 1-butylpyridinium bis(trifluoromethane sulfonyl)imide (BPTFSI), in which 1-ethyl-3-methylimidazolium iodide (EMImI) and I 2 are dissolved as a redox couple. The structure and the property greatly influence the DSSC performances. Especially, the photocurrents are affected by the ionic conductivity (viscosity) and charge transport by the exchange reaction between the iodide/tri-iodide redox couple. EMImDCA is specific in terms of enhancement of the open-circuit voltages. The photo-energy conversion efficiency of DSSCs with EMImDCA under 100 mW cm −2 can be optimized up to 5.5%, when [I −] + [I 3 −] = 2 M and [I −]:[I 2]=10:1 with the addition of 4- t-butylpyridine and LiI.

Enhancement of Light Harvesting and Photocurrent Generation by ITO Electrodes Modified with meso,meso-Linked Porphyrin Oligomers

ITO electrodes modified with a meso,meso-linked porphyrin oligomer have been prepared for the first time to achieve improved light-harvesting efficiency and a larger quantum yield for photocurrent generation as compared to the corresponding porphyrin monomer system. An increase in the number of the porphyrins in meso,meso-linked porphyrins results in an improvement of the photoenergy conversion efficiency per mol cm-2 in the visible light region as well as a larger quantum yield of photocurrent generation. The relative integrated values of the photocurrents for all of the frequencies in the action spectrum per mol cm-2 in the porphyrin dimer and tetramer systems are determined to be 2.5 and 6.5, respectively, as compared with the values of the porphyrin monomer system.

Quasi-Solid-State Dye-Sensitized TiO2 Solar Cells: Effective Charge Transport in Mesoporous Space Filled with Gel Electrolytes Containing Iodide and Iodine

Quasi-solid-state dye-sensitized solar cells were fabricated using low-molecular-weight gelators. They showed comparable photoenergy conversion efficiencies to the liquid cell at high illumination intensity up to AM 1.5 (1 sun). Conductivity measurements of the electrolyte phases revealed that the gelation does not affect the conductivity of the electrolyte and that the conductivity increased with an increase of iodine in both gel electrolytes and liquid electrolyte. The formation of polyiodide ions, such as I3- and I5-, caused by addition of iodine was confirmed by Raman spectroscopic measurement. The self-diffusion of iodide species in the gel electrolyte was found about a quarter of that of I- in acetonitrile. The formation of less-mobile polyiodide ions in electrolyte increased the conductivity in the mesoporous phase, which should be rationalized as due to the Grotthuss-type electron exchange mechanism caused by rather packed polyiodide species in the electrolytes. The optimized quasi-solid-state cell showed the values of 0.67 V for open-circuit voltage, 12.8 mA cm-2 for short-circuit photocurrent density, and 5.91% for photoenergy conversion efficiency under AM 1.5 irradiation with higher durability.

Mesoporous electrodes having tight agglomeration of single-phase anatase TiO 2 nanocrystallites: Application to dye-sensitized solar cells

Single-phase anatase nanocrystalline HyCOM-TiO 2 (Hydrothermal Crystallization in Organic Media) to label this method was synthesized by high-temperature hydrolysis of titanium tetrabutoxide in toluene. The resulting HyCOM-TiO 2 nanocrystallites were found to be covered by n-butoxide, yielding mesoporous, transparent anatase films with a narrow pore size distribution and good electron transport characteristic when sintered at 350–550°C on optically transparent conducting glass. Dye-sensitized solar cells made of the Ru-dye-adsorbed mesoporous HyCOM films as photoanodes achieved better photo-energy conversion efficiency as compared to those prepared using commercially available Degussa P25 films.

Novel sensitisers for photovoltaic cells. Structural variations of Ru(II) complexes containing 2,6-bis(1-methylbenzimidazol-2-yl)pyridine

A series of new ruthenium(II) complexes was synthesised for the application in photoelectrochemical solar cells based on dye-sensitised nanocrystalline titanium dioxide. Design strategies for the development of new dyes were tested via structural variations of the prototype complex K[Ru(II)(bmipy)(4,4′-dcbpy)(NCS)] (7) with bmipy = 2,6-bis(1-methyl-benzimidazol-2-yl)pyridine, and 4,4′-dcbpy = 2,2′-bipyridine-4,4′-dicarboxylate. A π ∗ level tuning was performed by replacing 4,4′-dcbpy with the low π ∗ level ligands dcbiq (2,2′-biquinoline-4,4′-dicarboxylate) and 5,5′-dcbpy. The resulting complexes showed decreased light-to-electric energy conversion efficiencies. K[Ru(II)(bmipy)(4-PO 3bpy)(NCS)] (4-PO 3H 2bpy=2,2′-bipyridine-4-phosphonic acid) also sensitised TiO 2 less efficiently than the model compound. The occurrence of two isomers was observed for the complexes containing 4-PO 3bpy. In Na[Ru(II)(bmipy)(4,4′-dcbpy)X] with X=substituted phenylcyanamide (pcyd −) anions, the influence of substitution on the phenyl group was investigated. MLCT absorption maxima of the phenylcyanamide complexes at around 510 nm were shifted to lower energies in comparison with the model complex, however photoenergy conversion efficiencies were reduced. When introduced into the complex, phenylcyanamide was coordinated via the nitrile or the amido nitrogen. With prolonged reaction time, the amido-bound isomer was partially transformed into the thermodynamically more stable nitrilo-bound isomer. Linkage isomerism of coordinated NCS − and 4-Clpcyd − was studied with multinuclear NMR ( 1H, 13C, 31P) spectroscopy and 13C-labelled ligands. Prospects for a substantial improvement of Ru(II) polypyridyl sensitisers for TiO 2 are discussed.

Importance of binding states between photosensitizing molecules and the TiO2 surface for efficiency in a dye-sensitized solar cell

The construction of dye-sensitized TiO2 solar cells is studied using a ruthenium complex, cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicar☐ylic acid)-ruthenium (II), as a photosensitizing molecule (dye) on TiO2 nanocrystallite films to determine the interfacial binding effect on photoenergy conversion efficiency. Photoconversion efficiency is improved by electrochemical treatment of TiO2 films, or reflux treatment for effective anchoring of the photosensitizing molecules on TiO2 nanocrystallite films. Characteristics of interfacial binding between dye molecules and the TiO2 surface is determined and compared using IR and UV-visible spectra. The ester-like linkage and the chelating car☐ylato linkage are formed between car☐ylic acid groups of ligands in dye molecules and TiO2 nanocrystallites. The ester-linkage is observed as major on the film which shows relatively high photoenergy conversion efficiency. The improvement of cell properties such as open circuit cell voltage (Voc) can be explained by the decrease in charge on bipyridine ligands of the dye molecule owing to the formation of the covalent-like linkage. The importance of binding states between photosensitizing molecules and TiO2 is discussed with a view to achieving high efficiency in dye-sensitized solar cell.