Organic Dye-sensitized Solar Cells Research Articles

Benzimidazole derivatives in the electrolyte of new-generation organic dye-sensitized solar cells with an iodine-free redox mediator

The effect of the addition of benzimidazole derivatives ( xBI) to new-generation organic dye-sensitized solar cells (DSSCs) was investigated by measuring the photovoltaic performance and by using a theoretical approach. An organic indoline dye (D149) and a stable organic radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), were employed as the sensitizer and redox mediator, respectively, for the fabrication of DSSCs. Three different xBI additives were added to the electrolyte with and without bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) in D149-DSSCs with a TEMPO/TEMPO + redox mediator. Competitive adsorption occurred significantly between D149 and xBI on TiO 2 particles in acetonitrile without the presence of LiTFSI in the electrolyte, implying a role of Li + in the electrolyte of D149-DSSCs. The addition of xBIs increased the open-circuit voltage ( V oc) by the band-edge shift upward when a negative surface charge was built up on the surface of the TiO 2 particles treated with Lewis base- xBIs. The addition of xBIs in the presence of LiTFSI/acetonitrile electrolytes would increased the photocurrent by more than 40% due to the suppression of recombination on the bare surface of the TiO 2 particles by the effective shielding of Li + on TiO 2. The value of V oc decreased due to the decreased redox potential of the redox mediators in the presence of xBI, which was confirmed by the theoretical and experimental methods.

Far-red absorbing squarylium dyes with terminally connected electron-accepting units for organic dye-sensitized solar cells

Novel squarylium dyes with either terminally placed tetracyanoquinodimethane or tetracyanoanthraquinodimethane moieties as strong electron acceptors were designed and synthesized for use as sensitizers for TiO 2-based dye-sensitized solar cells (DSSCs). The photovoltaic performance of DSSCs indicates that two chromophores, strong electron acceptors and squarylium components, make independent contributions to photosensitization of nanocrystalline TiO 2. DSSCs fabricated from squarylium-based diads as sensitizers and chenodeoxycholic acid as the coadsorbent exhibited a high open-circuit voltage (up to 0.66 V), suggesting that the electron-accepting components showed some effect on electron injection from the squarylium chromophore to the conduction band of TiO 2. Under the coadsorbent-free conditions, the short-circuit photocurrent and overall solar-to-electrical energy conversion efficiency of the photovoltaic cells was improved by the contribution of photosensitizing effect in the wavelength range derived from the electron-acceptor components rather than the squarylium component. This phenomenon implies the possibility that the electron-accepting group acts as an anchoring unit to the TiO 2 surface.

Redox couple related influences of π-conjugation extension in organic dye-sensitized mesoscopic solar cells

The iodide/triiodide redox shuttle is an unparalleled option in making efficient dye-sensitized solar cells (DSCs) but also presents some prominent constraints on the development of new photosensitizers, semiconductors and counter electrodes. In this paper, a cobalt electrolyte has been identified to offer an advantageous impact of π-conjugation extension in push–pull organic dyes, upon the interfacial charge recombination kinetics and thus the open-circuit photovoltage of DSCs, in sharp contrast with the conventional iodine congener. The usage of a solely visible-light-absorption dye has generated a DSC free of corrosive iodine, exhibiting an impressive overall power conversion efficiency of 8.0% at the 100 mW cm−2 AM1.5G conditions.

High-efficiency organic dye-sensitized mesoscopic solar cells with a copper redox shuttle

Based upon the bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) electron mediator, we present an iodine-free dye-sensitized solar cell exhibiting an impressive power conversion efficiency of 7.0% at 100 mW cm(-2) air mass global (AM1.5G) conditions, which rivals that of a control cell with a conventional iodine redox couple.

Efficient organic dye-sensitized thin-film solar cells based on the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle

We deploy the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle in conjunction with the high-absorption-coefficient organic photosensitizer C218 to fabricate an iodine-free dye-sensitized solar cell displaying an unprecedented 8.3% efficiency at an irradiation of AM1.5 full sunlight.

Geometrical effect of stilbene on the performance of organic dye-sensitized solar cells

New metal-free organic donor–bridge–acceptor dyes comprising a triphenylamine moiety as the electron donor (D), a cyanoacrylic acid moiety as the electron acceptor (A), and a conjugated chromophore between D and A as a spacer (S) were synthesized for the application of dye-sensitized solar cells (DSSCs). Two types of S were designed, one contains a normal stilbene backbone (dye 1) and the other contains a 2-phenylindene moiety (dye 2) with the central double bond locked in a transoid geometry. The performance of DSSCs made with these two types of dyes was compared, and it was found that the inhibition of trans/cis isomerization of the central double bond was advantageous to the efficiency of DSSCs. The phenomenon was investigated by a comparison of their photophysical properties, and rationalized by quantum mechanical calculations by using density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The electronic transitions were analyzed by time-dependent DFT, and the results were consistent with the experimental observations. The DSSCs were fabricated using both types of materials, and the performance was recorded under AM 1.5G irradiation (100 mW cm−2) condition. The overall conversion efficiencies of dye 2 (5.14–5.67%) were higher than those of dye 1 (4.52–4.98%). Both the short-circuit photocurrent density (Jsc) and the open-circuit voltage (Voc) were improved in 2 with respect to those of 1.

A thiolate/disulfide ionic liquid electrolyte for organic dye-sensitized solar cells based on Pt-free counter electrodes

The ionic liquid, 1-ethyl-3-methylimidazolium tetracyanoborate, was employed to prepare a thiolate/disulfide ionic liquid electrolyte with low viscosity for organic dye-sensitized solar cells (DSCs). CoS was introduced and showed better photovoltaic performance in DSCs than the ubiquitous platinized FTO CE.

Efficient organic DSSC sensitizers bearing an electron-deficient pyrimidine as an effective π-spacer

A series of new push–pull organic dyes incorporating electron-deficient pyrimidine as the π-spacer have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). In comparison with the model compound M-TP, which adopts phenylene as the π-spacer, the tailor-made dyes display enhanced spectral responses in the red portion of the solar spectrum. Through the introduction of two hexyloxy chains on the diphenylthienylamine donor, the DSSC employing dye OHexDPTP exhibited high power conversion efficiency up to 7.64% under AM1.5G irradiation.

Anion Effects in Organic Dye-Sensitized Mesoscopic Solar Cells with Ionic Liquid Electrolytes: Tetracyanoborate vs Dicyanamide

We compare the impacts of tetracyanoborate and dicyanamide anions in solvent-free ionic liquid electrolytes upon the optoelectronic features of dye-sensitized solar cells based on an organic push−pull chromophore. With respect to dicyanamide, tetracyanoborate confers a bathochromic and higher photocurrent response upon a dye-coated mesoporous titania film. Numerical simulations on electrical impedance spectroscopies disclose that tetracyanoborate anions evoke a downward displacement of the conduction-band edge of an electrolyte-sinking nanocrystalline film in comparison with dicyanamide, generating a more favorable energy-offset at the titania/dye interface and therefore a better exciton dissociation yield, as proved via the transient emission measurements. An effective recombination reaction rate constant U0k is used to depict the charge-transfer behavior at the titania/electrolyte interface in dye-sensitized solar cells. The negative impact of an about 4 times larger U0k of the dicyanamide cell is overwhelmed by the noticeably higher conduction band edge, leading to the experimentally observed anion-dependent open-circuit photovoltage.

Perylene imide dyes for solid-state dye-sensitized solar cells: Spectroscopy, energy levels and photovoltaic performance

We report the solid-state dye-sensitized solar cell performances of perylene imide using nanoporous TiO2 electrodes. Solid-state dye-sensitized solar cells were fabricated using the organic hole-transporting medium (HTM) 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD). The experimental ELUMO levels of perylene imide dyes are found to be 3.75 and 3.77eV, respectively. Therefore, perylene imide dyes can inject electrons to the conduction band of titanium dioxide in organic dye-sensitized solar cells. TiO2 thin films of about 2μm in thicknesses were prepared. Both preparation and thickness of the compact TiO2 layer were optimized using spray pyrolysis. The studies revealed that an optimum film thickness of 130–150nm of compact TiO2 yielded the best rectifying behavior and SDSC performance. In this work, our goal was to investigate the performance of perylene sensitizers in connection with spiro-OMeTAD hole transport material. Short-circuit current densities, open circuit voltages and overall conversion efficiencies of the solar cell with 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene(spiro-OMeTAD) as a hole conductor and perylene imide as sensitizer on mesoporous TiO2 were investigated.

SUCCESSIVE LARGE PERTURBATION METHOD FOR THE EXTRACTION OF MORE ACCURATE EQUIVALENT-CIRCUIT-PARAMETERS IN SOLAR CELLS

The numerical least-square fitting is a general method for the extraction of model parameter from experimental data. However, the method suffers from an initial value dependence problem when the method is used in I - V curve fitting. Recently we have proposed a successive large perturbation method (SLPM) to eliminate the initial value dependence problem. In this paper, we test the SLPM by comparing our results with previous published fitting results for various solar cells, such as traditional silicon solar cells and new type of organic solar cells and dye-sensitized solar cells. The method is available not only to the fitting for I - V curve, but also to the fitting for electrochemical impedance spectrum. Our test demonstrates that Newton-like method plus our SLPM is an accurate and robust method for the numerical extraction of equivalent circuit parameters in solar cells.

Electron transfer properties of organic dye-sensitized solar cells based on indolinesensitizers with ZnO nanoparticles

Two indoline dyes, coded D149 and D205, were used as the sensitizers ofZnO dye-sensitized solar cells (DSCs) with optimal energy conversionefficiencies of more than 5%, under AM 1.5 full sunlight illumination (100 mW cm − 2). Higher interfacial charge transfer rate and retardant fluorescence decay confirmed fromtransient fluorescence illustrated that D205-sensitized ZnO DSCs could possessbetter electron transport than D149-sensitized ZnO DSCs. The enhancement ofVoc andJsc for D205-sensitized ZnO DSCs was ascribed to the effective suppression of electronrecombination by extending the alkyl chain on the terminal rhodanine moiety from ethyl tooctyl. The evidence of enhanced electron diffusion coefficient was further shown byelectrochemical impedance spectroscopy (EIS).

Distance and Driving Force Dependencies of Electron Injection and Recombination Dynamics in Organic Dye-Sensitized Solar Cells

A series of dyes based on a triphenylamine donor and a rhodanine acetic acid anchor/acceptor for solar cell application has been studied with regards to electron injection and recombination kinetics using femtosecond transient absorption. The series contains three dyes, with estimated electron transfer distances ranging from 17.2 to 11.0 Å, and which have shown significant differences in energy conversion efficiencies. The injection and recombination kinetics were studied in the NIR region where electrons in the conduction band of the TiO(2) are suggested to absorb. For all dyes, the injection rate is larger than (200 fs)(-1) which implicates a quantitative injection efficiency. Surprisingly, the subsequent recombination reaction has a rate that increases with increasing linker length. On the other hand, this behavior is consistent with the concomitant decrease in driving force for this series of dyes. Moreover, the lifetimes show exponential distance dependence when corrected for driving force and reorganization energy, which indicates a superexchange interaction between the electrons in TiO(2) and the radical cations of the dyes. A dependence on probe wavelength of the attenuation factor was found, giving a β value of 0.38 Å(-1) at 940 nm and 0.49 Å(-1) at 1040 nm. The difference is suggested to be due to the difference in electronic coupling between fully separated dye cations and injected electrons versus geminate electron-hole pairs. Addition of tert-butylpyridine, which from previous work is known to give a substantial drop in the IPCE values for the studied dyes, was found to decrease the amount of long-lived electrons in the TiO(2) without affecting the injection rate.

Incorporating Hierarchical Nanostructured Carbon Counter Electrode into Metal-Free Organic Dye-Sensitized Solar Cell

Hierarchical nanostructured carbon with a hollow macroporous core of ca. 60 nm in diameter in combination with mesoporous shell of ca. 30 nm in thickness has been explored as counter electrode in metal-free organic dye-sensitized solar cell. Compared with other porous carbon counterparts such as activated carbon and ordered mesoporous carbon CMK-3 and Pt counter electrode, the superior structural characteristics including large specific surface area and mesoporous volume and particularly the unique hierarchical core/shell nanostructure along with 3D large interconnected interstitial volume guarantee fast mass transport in hollow macroporous core/mesoporous shell carbon (HCMSC), and enable HCMSC to have highly enhanced catalytic activity toward the reduction of I(3)(-), and accordingly considerably improved photovoltaic performance. HCMSC exhibits a V(oc) of 0.74 V, which is 20 mV higher than that (i.e., 0.72 V) of Pt. In addition, it also demonstrates a fill factor of 0.67 and an energy conversion efficiency of 7.56%, which are markedly higher than those of its carbon counterparts and comparable to that of Pt (i.e., fill factor of 0.70 and conversion efficiency of 7.79%). Furthermore, HCMSC possesses excellent chemical stability in the liquid electrolyte containing I(-)/I(3)(-) redox couples, namely, after 60 days of aging, ca. 87% of its initial efficiency is still achieved by the solar cell based on HCMSC counter electrode.

Synchronously Reduced Surface States, Charge Recombination, and Light Absorption Length for High-Performance Organic Dye-Sensitized Solar Cells

We employ the 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) segment as a conjugated spacer to construct an extremely high-molar-absorption-coefficient organic chromophore for dye-sensitized solar cells, exhibiting a high power conversion efficiency of 8.95% measured under irradiation of 100 mW cm(-2) AM1.5G sunlight. Our comparative experiments have proved the prominent merit of employing CPDT instead of the prevailing 2,2'-dithiophene (DT) as the building block for the further dye design. We also have demonstrated that a controllable coassembling of dye molecules and electrolyte components on semiconducting nanocrystals can reduce surface states and inhibit charge recombination synchronously.

Efficiency improvement of dye-sensitized tandem solar cell by increasing the photovoltage of the back sub-cell

Abstract In tandem-structured dye-sensitized solar cell (DSSC) composed of two parallel-connected sub-cells, the photovoltage (Voc) generated by the back sub-cell is usually rather low, resulting in low Voc and conversion efficiency (η) of the tandem cell. To solve this issue, two simple but very efficient strategies, namely, the filling of Li+-absent electrolyte and/or coating Al2O3 on TiO2 electrode surface in the back sub-cell, were explored to enhance the Voc of the back sub-cell and hence that of the tandem cell. The former strategy was expected to heighten the energy level of TiO2 conduction band, and the latter one to retard the charge recombination. The photovoltaic performance measurements reveal that in the both cases, although there was a slight decrease in the photocurrent (Jsc), an obvious rise in the Voc was achieved for the tandem cells, leading to significant improvements in η of the tandem cells. Compared to the individual organic dye-sensitized solar cell (the highest η is 7.58%), the tandem cell with two organic dyes having complementary absorption spectra demonstrates an improved efficiency of up to 8.33% by a combinational application of Li+-absent electrolyte and Al2O3 overcoat. The results presented in this study highlight that the efficiency of a parallel-connected tandem-structured DSSC can be improved significantly through enhancing the photovoltage of the back sub-cell, which is first time reported.

Influence of the electrolyte cation in organic dye-sensitized solar cells: lithium versus dimethylimidazolium

We investigate the influence of electrolyte cations such as lithium and dimethylimidazolium on the spectroscopic and electrical characteristics of dye-sensitized solar cells based on a metal-free chromophore C218. An evident bathochromic effect of lithium with respect to dimethylimidazolium is noticed for the C218 dye-coated nanocrystalline titania film via measuring electronic absorption and photocurrent action spectra. In comparison with dimethylimidazolium, the use of lithium as the electrolyte cation evokes a downward shift of the excited-state redox potential of the C218 sensitizer by 120 mV, and that of the conduction band edge of the nanocrystalline TiO2 film by 390 meV. The resultant remarkable variation in the energy alignment at the titania/dye interface brings on dissimilar electron injection yields as revealed by transient emission measurements. Modulating the titania surface states distribution with lithium relative to dimethylimidazolium induces a slightly declining electron diffusion coefficient in the mesoporous titania film. However, the adsorption of lithium cations on titania strongly retards the interfacial charge recombination compared to dimethylimidazolium, contributing to an over one order of magnitude of enhancement of electron diffusion length. The improvement of electron diffusion length has a noticeable effect on the charge collection yield, which can also be observed by measuring monochromatic incident photon-to-electron conversion efficiencies. Further interface engineering is needed to enhance both the photovoltage and photocurrent, making use of the full power of metal-free organic dyes.

Spirobifluorene-Bridged Donor/Acceptor Dye for Organic Dye-Sensitized Solar Cells

A new dye, SSD1, featuring two donor/acceptor chromophores aligned in a spiro configuration with two anchoring groups separated at a distance of 10.05 A (closely matching the distance between the adsorption sites of the anatase TiO(2) surface) undergoes efficient dye adherence on TiO(2) films. A dye-sensitized solar cell incorporating SSD1 exhibited a short-circuit current of 8.9 mA cm(-2), an open-circuit voltage of 0.63 V, a fill factor of 0.67, and a power conversion efficiency of 3.75%.

Enhanced Efficiency of Organic Dye-Sensitized Solar Cells: Triphenylamine Derivatives

A general TDDFT procedure has been set up that accurately evaluates the UV/vis absorption spectra of a series of new conjugated metal-free organic dyes based on the triphenylamine (TPA) moiety, which have recently been developed for dye-sensitized solar cells (DSSCs). It turns out that the BHandH functional, combined with the 6-311+G(2d,2p) basis set, gives reliable auxochromic shifts when the bulk solvation effects are included in the model. Indeed, the theoretical procedure provides λmax with a mean absolute deviation limited to ∼0.1 eV only. In addition, we give insights into the geometrical and electronic structures of the dyes, and we unravel the structural modifications allowing to optimize the properties of TPA-based DSSCs. This investigation aims at improving the electron-injection process, as well as the light-harvesting efficiency (LHE) of the dyes. To this purpose, we considered a set of about 20 new dyes, and starting from the TPC-1 structure, the following modifications help to get better ele...

Effect of Anchoring Group on Electron Injection and Recombination Dynamics in Organic Dye-Sensitized Solar Cells

In the field of dye-sensitized solar cells, the number of different sensitizing dyes is increasing rapidly. To produce low-cost dyes, much work is being directed toward synthesizing all-organic, ruthenium-free dyes with high extinction coefficients and broad absorption bands with large solar spectrum overlap. One of the best dyes, the polyene-diphenylaniline dye D5L2A1, has a rather blue absorption with an IPCE onset at around 650 nm, but it still has an energy conversion efficiency of almost 6%. To increase the overlap with the solar spectrum, the cyanoacrylic acid anchoring group was changed to rhodanine-3-acetic acid in complex D5L2A3. This gave an IPCE onset at around 750 nm, but unfortunately, it also decreased the overall efficiency to a modest 1.7%. By femtosecond transient absorption, we show that the electron injections into TiO(2) for the two dyes are ultrafast and indistinguishable with our time resolution (< 200 fs). However, charge recombination is also ultrafast, with different fractions of a ca. 500-fs component for the two dyes. Yet, the fraction of the faster decay component is larger for D5L2A3 than for D5L2A1. Here, we provide an interpretation of the implications of changing anchoring group. We believe that a lack of electron density oil the binding oxygens of the D5L2A3 LUMO, due to the rhodanine group, promotes a higher probability for electron injection to short-lived surface trap states compared to the situation for the fully conjugated D5L2A1.