Efficient iodine-free dye-sensitized solar cells employing truxene-based organic dyes

Two new truxene-based organic sensitizers (M15 and M16) featuring high extinction coefficients were synthesized for dye-sensitized solar cells employing cobalt electrolyte. The M16-sensitized device displays a 7.6% efficiency at an irradiation of AM1.5 full sunlight.     

5-Phenyl-iminostilbene based organic dyes for efficient dye-sensitized solar cells

Four new 5-phenyl-iminostilbene dyes (ISB-3–6) containing electron-withdrawing benzo-[c][1,2,5]thiadiazole have been designed and synthesized for use as DSSCs. Their absorption properties and electrochemical and photovoltaic performances have been investigated systematically. Among these dyes, DSSCs based on a dye containing benzo-[c][1,2,5]thiadiazole and benzene moieties (ISB-4) showed the best performance: a short-circuit photocurrent density (J_sc) of 13.69 mA cm⁻², an open-circuit photovoltage (V_oc) of 722 mV, and a fill factor (FF) of 0.71, which corresponds to a power conversion efficiency (PCE) of 6.71%, under optimized conditions. Additionally, long-term stability of the ISB-4 based DSSCs with ionic-liquid electrolytes was demonstrated under 1000 h of light soaking, the photovoltaic performance is up to 5.75%. The results suggest that 5-phenyl-iminostilbene containing dyes are promising candidates for application in DSSCs.     

Macrocyclic triphenylamine based organic dyes for efficient dye-sensitized solar cells

A novel class of organic D-π-A dyes employing macrocyclic triphenylamine dimer as electron donor was designed and synthesized for dye-sensitized solar cells. The prepared compounds showed high chemical and elelctrochemical stabilities as well as good long-wave absorption. Photovoltaic devices based on these dyes showed high open circuit voltage (higher than that of N3) and achieved a solar energy to electricity conversion efficiency of 6.31%. All the performances indicate the dyes containing macrocyclic triphenylamine dimer is a good candidate for dyes sensitized solar cells.     

Metal-free organic dyes for dye-sensitized solar cells: recent advances

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Molecular design of organic dyes based on vinylene hexylthiophene bridge for dye-sensitized solar cells

Three donor-(π-spacer)-acceptor (D-π-A) organic dyes, containing different groups (triphenylamine, di(p-tolyl)phenylamine, and 9-octylcarbazole moieties) as electron donors, were designed and synthesized. Nanocrystalline TiO₂ dye-sensitized solar cells were fabricated by using these dyes. It was found that the variation of electron donors in the D-π-A dyes played an important role in modifying and tuning photophysical properties of organic dyes. Under standard global AM 1.5 solar condition, the DSSC based on the dye D2 showed the best photovoltaic performance: a short-circuit photocurrent density (J _sc ) of 13.93 mA/cm², an open-circuit photovoltage (V _oc ) of 0.71 V, and a fill factor (FF) of 0.679, corresponding to solar-to-electric power conversion efficiency (η) of 6.72%. Supported by the Key Project of Hunan Province of China (Grant No. 2008FJ2004), Natural Science Foundation of Hunan Province of China (Grant Nos. 09JJ3020 & 09JJ4005), and Scientific Research Fund of Hunan Provincial Education Department (Grant No. 08C888).     

Y-shaped dyes based on triphenylamine for efficient dye-sensitized solar cells

Three new triphenylamine-based dyes with Y-shaped conformation bearing triphenylamino-vinyl, 10-octyl-10H-phenothiazine-vinyl and 9-octyl-9H-carbazole-vinyl as arms (TT, TP, and TC) have been synthesized. From electrochemical investigations it is found that they can be employed in DSSCs due to the balanced HOMO and LUMO energy levels. Notably, the photo-to-electrical conversion efficiency of the DSSCs sensitized with branched TT, TP, and TC reach 5.12%, 4.84%, and 3.63%, which are higher than that sensitized with T (2.79%), and the DSSC sensitized with TT shows higher IPCE response and better photovoltaic performances (J_sc=12.37 mA/cm², V_oc=0.72 V and ff=0.58) than others. These results reveal that the introduction of branched Y-shaped extended π-conjugated donors to D-π-A dyes cannot only enlarge the spectral response range, but also suppress the molecular aggregation on TiO₂ films to a certain extent, which would enhance the performance of DSSCs.     

High efficiency of dye-sensitized solar cells based on metal-free indoline dyes

We now report metal-free organic dyes having a new type of indoline structure, which exhibits high efficiencies in dye-sensitized solar cells. The solar energy to current conversion efficiencies with the new indoline dye was 6.51%. Under the same conditions, the N3 dye was 7.89% and the N719 dye was 8.26%. The new indoline dye was optimized for the amount of 4-tert-butyl pyridine in the electrolyte and cholic acid as a coadsorbent. Subsequently, the solar energy to current conversion efficiencies reached 8.00%. This value was the highest obtained efficiency for dye-sensitized solar cells based on metal-free organic dyes without an antireflection layer.     

Co-sensitization of organic dyes for efficient ionic liquid electrolyte-based dye-sensitized solar cells

The co-sensitization of two organic dyes (SQ1 and JK2), which are complementary in their spectral responses, shows enhanced photovoltaic performance compared with that of an individual organic dye-sensitized solar cell. The power conversion efficiency of the co-sensitized organic dye solar cell based on the newly developed binary ionic liquid (solvent-free) electrolyte gives 6.4% under AM 1.5 sunlight at 100 mW/cm2 irradiation, which is higher than that of individual dye-sensitized solar cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) of the co-sensitized solar cell shows typical absorption peaks at 530 and 650 nm corresponding to the two dyes and displays a broad spectral response over the entire visible spectrum with IPCE of >40% in the 400-700 nm wavelength domain.     

Efficient dye-sensitized solar cells based on surfactant-mediated TiO2 nanostructures

Journal of Solid State Electrochemistry - Dye-sensitized solar cells (DSSCs) based on TiO2 nanostructures have attracted much attention due to their high photoconversion efficiency. Here, we report...     

Organic dyes based on triphenylamine for dye-sensitized solar cells:Structure–property relationships

Three new organic dyes based on triphenylamine with a structure of A-D-A-D-A(D1),A-D-A(D2) and D-A(D3) were designed,theoretically calculated and synthesized for dye-sensitized solar cells.Dye D1 exhibits a broader absorption than D2 and D3,due to the intramolecular charge transfer between the donor triphenylamine and the acceptor benzothiadiazole.Dye D1 exhibits a lower HOMO and a lower LUMO than D2 and D3 due to the electron-withdrawing benzothiadiazole.The number of anchoring group cyanoacrylic acid has no obvious influence on absorption and energy levels of D2 and D3.The LUMO of D1 locates on benzothiadiazole rather than cyanoacrylic acid anchoring groups,while the LUMOs of D2 and D3 are localized on cyanoacrylic acid.D2 and D3 give higher short-circuit current density than D1.D3 with one anchoring group gives the highest open-circuit voltage.Consequently,the D3-based device gives the highest efficiency.     

Molecular engineering and theoretical investigation of organic sensitizers based on indoline dyes for quasi-solid state dye-sensitized solar cells

Novel indoline dyes, I-1-I-4, with structural modification of π-linker group in the D-π-A system have been synthesized and fully characterized. Molecular engineering through expanding the π-linker segment has been performed. The ground and excited state properties of the dyes have been studied by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). Larger π-conjugation linkers would lead to broader spectral response and higher molar extinction coefficient but would decrease dye-loaded amount on TiO(2) electrode and LUMO level. While applied in DSSCs, the variation trends in short-circuit current density (J(sc)) and open-circuit voltage (V(oc)) were observed to be opposite to each other. The internal reasons were studied by experimental data and theoretical calculations in detail. Notably, I-2 showed comparable photocurrent values with liquid and quasi-solid state electrolyte, which suggested through molecular engineering of organic sensitizers the dilemma between optical absorption and charge diffusion lengths can be balanced well. Through studies of photophysical, electrochemical, and theoretical calculation results, the internal relations between chemical structure and efficiency have been revealed, which serve to enhance our knowledge regarding design and optimization of new sensitizers for quasi-solid state DSSCs, providing a powerful strategy for prediction of photovoltaic performances.     

Molecular design of novel indacenodithiophene-based organic dyes for efficient dye-sensitized solar cells applications

Indacenodithiophene (IDT)-based high-efficiency photovoltaics have received increasing attention recently. This paper reports a density functional theory investigation of the electronic and optical properties of three IDT-based organic dyes together with the dye/(TiO₂)₄₆ interface. In order to enhance the photoelectric properties of IDT dyes, this paper considers two methods for the structure modification of the experimentally reported dye DPInDT (J. Org. Chem. 2011, 76, 8977): the extension of the conjugation length by dithienothiophene as well as the heteroatom substitution of the bridging atoms by electron-rich nitrogen atoms. Our calculations show that both methods obviously affect the distributions of the molecular orbitals and notably red shift the absorption peaks of around 20 nm, with the former method demonstrating enhanced light harvesting efficiency. The structure modifications proposed also enhance the emission spectrum properties for IDT-based organic dyes. The calculated ultrafast injection time of electrons from the excited state of IDT dyes to the (TiO₂)₄₆ belongs to the femtosecond order of magnitude, and is ideal for efficient photoelectric conversion process in dye-sensitized solar cells (DSSCs) applications. The IDT dyes designed in this paper have good electronic and spectroscopic properties. This study is expected to provide useful guidance for the development of novel IDT dyes for applications in DSSCs.     

Characterization of solid-state dye-sensitized solar cells utilizing high absorption coefficient metal-free organic dyes

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-MeOTAD), and three organic indoline-based sensitizer dyes with high molar extinction coefficients. The cells were characterized by several techniques, including spectral response measurements, photovoltage decay transients, intensity modulated photovoltage spectroscopy (IMVS), and charge extraction. The differences in apparent electron lifetime observed for cells fabricated using the three dyes are attributed in part to changes in the surface dipole potential at the TiO2/spiro-MeOTAD interface, which shift the TiO2 conduction band energy relative to the Fermi level of the HTM. These energy shifts influence both the open circuit voltage (as a result of changes in free electron density) and the short circuit current (as a consequence of changes in the overlap between the dye LUMO level and the conduction band). A self-consistent approach was used to derive the positions of the conduction band relative to the spiro-MeOTAD redox Fermi level for cells fabricated using the three dyes. The analysis also provided estimates of the free electron lifetime in spiro-MeOTAD cells. In order to evaluate the possible contribution of the adsorbed dyes to the observed changes in surface dipole potential, their dipole moments were estimated using ab initio density functional theory (DFT) calculations. Comparison of the calculated dipole contributions with the experimentally measured shifts in conduction band energy revealed that other factors such as proton adsorption may be predominant in determining the surface dipole potential.     

Efficient thiocyanate-free sensitizer: a viable alternative to N719 dye for dye-sensitized solar cells

We have designed and synthesized a new thiocyanate-free sensitizer coded as SPS-01 and used it as the sensitizer in a TiO(2) based nanocrystalline dye-sensitized solar cell (DSSC). SPS-01 exhibits strong visible absorption properties with maximum peak around at 532 nm. The overall power conversion efficiency (PCE) of a DSSC sensitized with SPS-01 (7.96%) is higher than that of N719 (7.30%) under identical experimental conditions. This high PCE is attributed mainly due to the improvement in the short circuit current.     

Nitro group as a new anchoring group for organic dyes in dye-sensitized solar cells

An organic dye JY1 bearing a nitro group was designed, synthesized and applied in DSCs. An unusual colour change was observed when the voltage applied to the device was reversed which was accompanied by a five-fold increase in the cell efficiency. We propose that applying a bias enabled the attachment of nitro groups to the TiO(2) surface.     

Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br-/Br3- electrolytes

A series of novel metal-free organic dyes TC301-TC310 with relatively high HOMO levels were synthesized and applied in dye-sensitized solar cells (DSCs) based on electrolytes that contain Br(-)/Br(3)(-) and I(-)/I(3)(-). The effects of additive Li(+) ions and the HOMO levels of the dyes have an important influence on properties of the dyes and performance of DSCs. The addition of Li(+) ions in electrolytes can broaden the absorption spectra of the dyes on TiO(2) films and shift both the LUMO levels of the dyes and the conduction band of TiO(2), thus leading to the increase of J(sc) and the decrease of V(oc). Upon using Br(-)/Br(3)(-) instead of I(-)/I(3)(-), a large increase of V(oc) is attributed to the enlarged energy difference between the redox potentials of electrolyte and the Fermi level of TiO(2), as well as the suppressed electron recombination. Incident photon to current efficiency (IPCE) action spectra, electrochemical impedance spectra, and nanosecond laser transient absorption reveal that both the electron collection yields and the dye regeneration yields (Φ(r)) depend on the potential difference (the driving forces) between the oxidized dyes and the Br(-)/Br(3)(-) redox couple. For the dyes for which the HOMO levels are more positive than the redox potential of Br(-)/Br(3)(-) sufficient driving forces lead to the longer effective electron-diffusion lengths and almost the same efficient dye regenerations, whereas for the dyes for which the HOMO levels are similar to the redox potential of Br(-)/Br(3)(-), insufficient driving forces lead to shorter effective electron-diffusion lengths and inefficient dye regenerations.     

Design of organic dyes and cobalt polypyridine redox mediators for high-efficiency dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) with cobalt-based mediators with efficiencies surpassing the record for DSCs with iodide-free electrolytes were developed by selecting a suitable combination of a cobalt polypyridine complex and an organic sensitizer. The effect of the steric properties of two triphenylamine-based organic sensitizers and a series of cobalt polypyridine redox mediators on the overall device performance in DSCs as well as on transport and recombination processes in these devices was compared. The recombination and mass-transport limitations that, previously, have been found to limit the performance of these mediators were avoided by matching the properties of the dye and the cobalt redox mediator. Organic dyes with higher extinction coefficients than the standard ruthenium sensitizers were employed in DSCs in combination with outer-sphere redox mediators, enabling thinner TiO(2) films to be used. Recombination was reduced further by introducing insulating butoxyl chains on the dye rather than on the cobalt redox mediator, enabling redox couples with higher diffusion coefficients and more suitable redox potential to be used, simultaneously improving the photocurrent and photovoltage of the device. Optimization of DSCs sensitized with a triphenylamine-based organic dye in combination with tris(2,2'-bipyridyl)cobalt(II/III) yielded solar cells with overall conversion efficiencies of 6.7% and open-circuit potentials of more than 0.9 V under 1000 W m(-2) AM1.5 G illumination. Excellent performance was also found under low light intensity indoor conditions.     

2,7-Diaminofluorene-based organic dyes for dye-sensitized solar cells: effect of auxiliary donor on optical and electrochemical properties

New organic dyes containing a diarylaminofluorene unit as an electron donor and cyanoacrylic acid as acceptor and anchoring group in a donor-π-donor-π-acceptor architecture have been synthesized and characterized as sensitizers for nanocrystalline TiO(2)-based dye-sensitized solar cells. They have shown three major electronic absorptions originating from the π-π* and charge-transfer transitions covering the broad visible range (250-550 nm) in solution. The charge-transfer transition of the dyes exhibited negative solvatochromism, suggesting a polarized ground state. They have also displayed acidochromism in solution owing to the presence of a protonation-deprotonation equilibrium. On comparison with the triphenylamine and carbazole-based parent dyes (E)-2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid and (E)-2-cyano-3-(9-ethyl-9H-carbazol-3-yl)acrylic acid they exhibited longer wavelength absorptions and facile oxidation, indicating the stronger electron-donating ability of the auxiliary chromophores. In addition, they exhibited nearly two times larger light-to-electron conversion efficiency under simulated AM 1.5 G irradiation (100 mW cm(-2)) with an aperture mask when compared to the parent dyes. Among the new dyes, the one containing the naphthylphenylamine segment showed better device characteristics attributable to the higher HOMO energy level which probably facilitates the regeneration of the dye and effective suppression of the back reaction of the injected electrons with the I(3)(-) in the electrolyte. The optical properties of the dyes were modeled using TDDFT simulations employing different theoretical models (B3LYP, CAM-B3LYP, and MPW1K), and the best correlations with the observed parameters have been found for CAM-B3LYP and MPW1K calculations. The electron lifetimes extracted from the electrochemical impedance measurements of the dye-sensitized solar cells were used to interpret the solar cell efficiency alternations.     

Improvement of dye-sensitized solar cells' performance via co-sensitization of new azo thiazole organic dyes with ruthenium (II) based N-719 dye

Three novel azo thiazole organic dyes, NA-1–3, have been synthesized and utilized as co-sensitizers in dye-sensitized solar cells (DSSCs). These co-sensitizers were designed with a thiazole ring π-bridge that mediates between the diazo (–N = N–) functional group and carboxylic acid anchoring unit. They possess a rod-like molecular structure and exhibit strong UV–vis absorption near 600 nm. Co-sensitization studies were also conducted with the ruthenium complex N719. The co-sensitized DSSCs showed enhanced short-circuit and open-circuit photocurrents (Jsc) and voltages (Voc), resulting in more efficient photovoltaic performance compared to N719 (PCE 7.25%). Electrochemical impedance spectroscopy (EIS) and incident photon to current efficiency (IPCE) were employed to investigate the underlying reasons for these improvements. It was found that co-sensitization effectively reduced electron recombination, resulting in a higher Voc without compromising photocurrent loss.     

Synthesis and photovoltaic properties of organic sensitizers containing electron-deficient and electron-rich fused thiophene for dye-sensitized solar cells

Diverse fused thiophenes with electron-rich and electron-deficient blocks have been synthesized and employed as the π-conjugated spacers of organic dyes for the dye-sensitized solar cells (DSSCs). The effects of these fused thiophenes were investigated by their absorption spectra, electrochemical and photovoltaic properties. For a typical device a maximum power conversion efficiency of 6.11% was obtained under simulated AM 1.5 irradiation (100 mW cm^?2): a short-circuit current (J_SC) of 14.47 mA cm^?2, an open-circuit voltage (V_OC) of 670 mV, and a fill factor (FF) of 0.63.