An oligothiophene dye with triphenylamine as side chains for efficient dye-sensitized solar cells

A novel oligothiophene-cyanoacrylic acid photosensitizer with two triphenylamine side chains (7T-2TPA) is designed and synthesized for dye-sensitized solar cells. 7T-2TPA exhibits broad (250-600 nm) and strong absorption (ε = 5.0 × 10⁴ L mol⁻¹ cm⁻¹ at 496 nm). The optical band gap (E_g) is estimated from the onset absorption edge to be 2.07 eV. The oxidation potential E_ox and reduction potential E_red vs NHE of the dye is 0.93 and −1.14 V, respectively. Dye-sensitized solar cell (DSSC) based on 7T-2TPA exhibits an open-circuit voltage (V_oc) of 724 mV, a short-circuit current density (J_sc) of 16.28 mA cm⁻², a fill factor (FF) of 0.684 and a power conversion efficiency of 8.06%. The efficiency of 8.06% is similar to that for widely used N719-based cell fabricated and measured under the same conditions.     

Enhanced performance of dye-sensitized solar cells with novel 2,6-diphenyl-4H-pyranylidene dyes

Novel 2,6-diphenyl-4H-pyranylidene derivatives were designed and synthesized as dyes for dye-sensitized solar cells (DSSC). Dyes 2a, b with a phenyl substituent showed high DSSC energy conversion efficiencies of 5.3% (J_sc = 10.3 mA/cm², V_oc = 0.72 V, FF = 0.72) and 4.7% (J_sc = 8.9 mA/cm², V_oc = 0.73 V, FF = 0.72) at 100 mW/cm² under simulated AM 1.5 G solar light conditions. These values are twice better than that of dye 1 without the phenyl substituent under the same conditions. Both the photocurrent density (J_sc) and open circuit voltage (V_oc) of DSSCs based on dyes 2a, b are increased compared with 1. It can be attributed to their twisted structures, absorption abilities and proper energy levels. This result shows that the tetraphenylpyranylidene is a promising electron-donor unit for high-efficiency DSSCs.     

Efficient co-sensitization of dye-sensitized solar cells by novel porphyrin/triazine dye and tertiary aryl-amine organic dye

A novel porphyrin dye ZnP-triazine-(gly)₂, consisting of a zinc-metallated porphyrin unit covalently linked through its peripheral aryl-amino group with a 1,3,5-triazine group which is functionalized by two carboxylic acid groups of glycine moieties, has been synthesized. Photophysical and electrochemical measurements, as well as theoretical DFT calculations, suggest that the compound exhibits appropriate light absorption characteristics and frontier molecular orbital levels for use as sensitizer in dye-sensitized solar cells (DSSCs). The ZnP-triazine-(gly)₂ based solar cell was found to exhibit a power conversion efficiency (PCE) value of 4.72%. A significant improvement of the overall photovoltaic efficiency of the solar cell was achieved up to 7.34% upon co-sensitization with a tertiary aryl-amine D with two ethynyl-pyridine substituents and cyano-acetic acid anchoring group, which exhibits complementary light absorption characteristics with the porphyrin dye. The higher PCE value of the co-sensitized solar cell is attributed to its enhanced short circuit current (J_sc), which is due to improved light harvesting efficiency, reduced porphyrin aggregation, and faster electron injection and charge collection, as well as its enhanced open circuit voltage (V_oc), which is due to increased electron density in the TiO₂ conduction band of the photoanode. These results are in accordance with electrochemical impedance spectra (EIS) of the solar cells, which revealed higher charge recombination resistance (R_rec), longer electron lifetime (τ_e), and shorter electron transport time (τ_d) for the co-sensitized solar cell.     

Effects of different electron donating groups on dye regeneration and aggregation in phenothiazine-based dye-sensitized solar cells

A series of phenothiazine-based dyes containing different auxiliary chromophores (TP, TTP, EP, and EEP) bring about unusual power conversion efficiency (PCE) for the corresponding dye-sensitized solar cells (DSSCs): EEP with the best electron-donating capability provides the lowest PCE of 2.24%, while TP with the weakest electron-donating capability leads to the highest PCE of 8.07%. The underlying influencing factors have been investigated by considering the electronic structures and aggregation properties based on density functional theory and Marcus theory. We found that the energy-mismatch between electron-donating units and the PTZ moiety results in poor EEP dye regeneration. Additionally, molecular dynamics simulations illustrate that the increased intermolecular interaction energy induced by preferable electron-donating groups aggravates the intermolecular aggregation. Especially, the calculated average values of the time-dependent intermolecular lateral charge transfer rate k for (EEP)₂ are nearly one order of magnitude higher than those of (TP)₂, revealing a more robust π-π stacking interaction induced by the donor unit of EEP. Importantly, the dye-TiO₂ interactions have been taken into account, which are absent in many previous theoretical work but crucial for accurate describing the aggregations. These deeper insights into the regeneration process and the aggregation mechanism induced by different donor units encourage researchers to balance various properties in designing novel components for photovoltaic devices.     

Novel organic dye employing dithiafulvenyl-substituted arylamine hybrid donor unit for dye-sensitized solar cells

In order to increase electron-donating ability of the donor part of the organic dye, two dithiafulvenyl (DTF) units were introduced into a triphenylamine unit to form dithiafulvenyl-substituted triphenylamine hybrid donor for dye-sensitized solar cells (DSSCs) for the first time. Novel donor–acceptor organic dye WD-10 containing this hybrid donor and 2-cyanoacetic acid acceptor has been designed, synthesized and applied in DSSCs. The influence of the substituent unit DTF in the dye on the device performance has been investigated. It was found that the dye with dithiafulvenyl-substituted triphenylamine hybrid donor gave higher photocurrent, open-circuit voltage, and efficiency value. The DSSC based on organic dye WD-10 displayed a short-circuit current (J_sc) of 9.58 mA cm^?2, an open-circuit voltage (V_oc) of 648 mV, and a fill factor (ff) of 0.71, corresponding to an overall conversion efficiency of 4.41%. An increase in η of about 79% was obtained from simple triphenylamine dye L0 to WD-10. The different photovoltaic behaviors of the solar cells based on the organic dyes were further elucidated by the electrochemical impedance spectroscopy. This work identifies that the introduction of DTF unit into the simple triphenylamine dye could significant improve the photovoltaic performance.     

Phenoxazine-based organic dyes with different chromophores for dye-sensitized solar cells

A series of organic dyes (POZ-2, POZ-3, POZ-4 and POZ-5) involving phenoxazine were synthesized as sensitizers for application in dye-sensitized solar cells (DSSCs). For comparison, three different electron donors namely 10-phenyl-10H-phe-nothiazine, 10-phenyl-10H-phenoxazine and triphenylamine were separately appended onto the 7-position of the model dye (POZ-2). The obtained four dyes exhibit considerably high values of conversion efficiencies of 6.6%, 7.8%, 7.1% and 6.4%, respectively, under the simulated AM1.5G conditions. The geometries of the dyes were optimized to gain insight into the molecular structure and electron distribution, and then the charge extraction and transient photovoltage decay measurements were further performed to understand the influence of electron donors on the photovoltaic behaviors.     

N-phenylindole-diketopyrrolopyrrole-containing narrow band-gap materials for dopant-free hole transporting layer of perovskite solar cell

Novel conjugated materials, DPIO and DPIE, having same molecular configuration of both an electron donating N-phenylindole and an electron accepting diketopyrrolopyrrole derivative, exhibited different aggregation behavior because of the applied side chains. When DPIO and DPIE were applied to as hole transporting materials in perovskite solar cell, DPIO showed better device performance than ones with DPIE, mostly due to the aggregation-assisted enhanced electrical property. DPIO effectively extracted hole from the perovskite layer, providing over 10% PCE of cell efficiency without any chemical doping. Incident-photon-to-electron conversion efficiency (IPCE) measurement confirmed that DPIO’s strong absorption in the longer wavelength region partly contributed to the light harvesting of the solar cell device. In addition, time-resolved photoluminescence (TRPL) and transient photovoltage (TPV) studies proved that the DPIO-based device, compared to the conventional Spiro-MeOTAD-based device, has better charge extraction ability and reduced charge recombination.     

Uses of new natural dye photosensitizers in fabrication of high potential dye-sensitized solar cells (DSSCs)

In the present study, dye-sensitized solar cells (DSSCs) were fabricated by sensitizing TiO₂ P25-based electrodes using a series of natural dye extracted from plant sources of Reseda luteola, Berberis integerrima, Panica granatum Pleniflora, Consolida orientalis, Reseda gredensis, Clemetis orientalis, Adonis flammea, Salvia sclarea, and Consolida ajacis. The optical properties of the natural dye extracts were investigated by UV–vis spectroscopy. The crystalline structure and morphological characteristic of TiO₂ electrode were analyzed by XRD and SEM, respectively. Our findings showed that due to Delphinidin is the main pigment of C. ajacis and interaction between the hydroxyl groups of the Delphinidin and the TiO₂ surface is very efficient, this sensitizer owns the best photovoltaic performance among the all natural dyes. Photoelectrochemical performance of the natural dyes based-DSSCs illustrated short-circuit photocurrent (Jsc) and open-circuit voltages (Voc) ranging from 0.004 to 0.68 mA and 0.37 to 0.64, respectively.     

Panchromatic dyes having diketopyrrolopyrrole and ethylenedioxythiophene applied to dye-sensitized solar cells

Novel panchromatic donor-acceptor-π-acceptor dyes (DPP1–4) containing diketopyrrolopyrrole (DPP) and 3,4-ethylenedioxythiophene (EDOT) have been designed and synthesized. Introduction of both DPP and EDOT units into a dye framework induced a remarkable red-shift of absorption bands and thus photoactive spectral regions of the dye-sensitized solar cells (DSSCs) based on the panchromatic DPP1–4 dyes were extended far above 800 nm. Influences of alkyl substituents in DPP1–4 on the performances of the DSSCs were discussed, and also those of co-adsorbate on TiO₂ and additive in the electrolyte used in the DSSC preparation were studied to optimize photovoltaic performances of the DSSCs.     

Carbazole-based hole transporting material for solid state dye-sensitized solar cells: Influence of the purification methods

The synthesis and properties of a glass-forming carbazole compound 9-(ethyl)-3,6-bis(4,4′-dimethoxydiphenylaminyl)-carbazole are reported. The thermal, optical and electrochemical properties of the hole-transporting molecule were studied by differential scanning calorimetry, thermogravimetric analysis, UV/Vis spectroscopy and cyclic voltammetry. The molecular glass exhibits a thermal stability as high as 370 °C and a glass transition temperature of 68 °C. The synthesized coumpound absorbs in the 250–400 nm range and possesses an optical band gap of 2.76 eV, avoiding any screening effect with the dye. The solid state ionization potential (IPss) of the molecule, estimated by cyclic voltammetry is around 4.77 eV, higher than the standard spiro-OMeTAD hole-transporting material. The compound was finally assessed as hole-transporting material in the solid state dye-sensitized solar cells with (5-(1,2,3,3 a,4,8b-hexahydro-4-[4-(2,2-diphenylvinyl)phenyl]-cyclopenta[b]indole-7-ylmethylene)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid (D102) as sensitizer. The effect of the purity of the glassy molecule on photovoltaic performances is discussed and showed a two-fold increase of the power conversion efficiency after purification by sublimation, going from 0.82% to 1.62% under standard AM 1.5 G solar irradiation (100 mW/cm²).     

Efficient inverted polymer solar cells incorporating doped organic electron transporting layer

An efficient inverted polymer solar cell is enabled by incorporating an n-type doped wide-gap organic electron transporting layer (ETL) between the indium tin oxide cathode and the photoactive layer for electron extraction. The ETL is formed by a thermal-deposited cesium carbonate-doped 4,7-diphenyl-1,10-phenanthroline (Cs₂CO₃:BPhen) layer. The cell response parameters critically depended on the doping concentration and film thickness of the Cs₂CO₃:BPhen ETL. Inverted polymer solar cell with an optimized Cs₂CO₃:BPhen ETL exhibits a power conversion efficiency of 4.12% as compared to 1.34% for the device with a pristine BPhen ETL. The enhanced performance in the inverted device is associated with the favorable energy level alignment between Cs₂CO₃:BPhen and the electron-acceptor material, as well as increased conductivity in the doped organic ETL for electron extraction. The method reported here provides a facile approach to optimize the performance of inverted polymer solar cells in terms of easy control of film morphology, chemical composition, conductivity at low processing temperature, as well as compatibility with fabrication on flexible substrates.     

Efficient dye-sensitized solar cells based on CNTs and Zr-doped TiO2 nanoparticles

The light scattering, harvesting and adsorption effects in dye-sensitized solar cells (DSSCs) are studied by preparation of coated carbon nanotubes (CNTs) with TiO₂ and Zr-doped TiO₂ nanoparticles in the forms of mono- and double-layer cells. X-ray diffraction (XRD) analysis reveals that the phase composition of Zr-doped TiO₂ electrode is a mixture of anatase and rutile phases with major rutile content, whereas it is the same mixture with major anatase content for coated CNTs with TiO₂. Furthermore, the average crystallite size of Zr-doped TiO₂ electrode is slightly decreased with Zr introduction. Field emission scanning electron microscope (FE-SEM) images show that the porosity of Zr-doped TiO₂ electrodes is higher than that of undoped electrode, enhancing dye adsorption. UV–visible spectroscopy analysis reveals that the absorption onset of Zr-doped TiO₂ electrodes is slightly shifted to longer wavelength (the red-shift) in comparison with that of undoped TiO₂ electrode. Moreover, the band gap energy of TiO₂ nanoparticles is decreased by Zr introduction, enhancing light absorption. It is found that electron injection of monolayer TiO₂ electrode is improved by introduction of 0.025 mol% Zr, resulted in enhancement of its power conversion efficiency (PCE) up to 6.81% compared with 6.17% for pure TiO₂ electrode. Moreover, electron transport and light scattering are enhanced by incorporation of 0.025 wt% coated CNTs with TiO₂ in the over-layer of double layer electrode. Therefore, double layer solar cell composed of 0.025 mol% Zr-doped TiO₂ nanoparticles as the under-layer and mixtures of these nanoparticles and 0.025 wt% coated CNTs with TiO₂ as the over-layer shows the highest PCE of 8.19%.     

Effect of imidazole derivatives in triphenylamine-based organic dyes for dye-sensitized solar cells

In order to study the influence of imidazole derivatives in triphenylamine-based organic dyes, two different imidazole derivatives are introduced into the phenyl ring of the triphenylamine core, coded as TPA-B5 and TPA-B6, respectively. The photophysical and electrochemical properties of the dyes are investigated by UV–vis spectroscopy and cyclic voltammetry. TPA-B5 increases the molar extinction coefficients and λ_max because of the extension of the π-conjugation structure of the dye and non-planar structure of imidazole derivative. However, TPA-B6 does not increase the molar extinction coefficients and λ_max compared with a simple triphenylamine derived dye (TPA-1), which may be due to the planar structure of imidazole derivative and benzene ring. The structure of TPA-B6 is in favor of the formation of dye aggregates on the semiconductor surface and the recombination of conduction band electrons with triiodide in the electrolyte. Overall conversion efficiencies of 3.13% and 1.21% under full sunlight (AM 1.5G, 100 mW cm^?2) irradiation are obtained for DSSCs based on the two new dyes, under the same conditions, the dye TPA-1 and di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) (N719) give overall conversion efficiencies of 2.23% and 5.38%, respectively. Although the overall conversion efficiencies of these dyes are not very high, the results will still afford significant value for future development of efficient D–π–A sensitizers.     

Enhanced short circuit current density of dye-sensitized solar cells aided by Sr,V co-doped TiO2 particles

This study comes up with a straightforward method for preparing uniform electrodes containing Sr,V co-doped TiO₂ particles for dye-sensitized solar cells (DSCs) applications. The spherical particles with the average diameter around 2.5 µm are assembled from small nanoparticles with the average grain size of 60 nm. X-ray diffraction (XRD) reveals that the introduction of dopants not only inhibits the growth of rutile phase, but also results in smaller primary crystallites, improving the surface area and dye adsorption ability of the electrodes. X-ray photoelectron spectroscopy (XPS) showed that Sr²⁺ and V⁵⁺ ions are well incorporated into the titania crystal lattice without forming specific Strontium and Vanadium compositions. UV–visible spectra show that the co-doped TiO₂ films have lower band gap energy than that of undoped-TiO₂, extending the absorption of TiO₂ into visible region. Isolated energy levels in band structure of TiO₂ as well as local lattice distortions due to dopants introduction are the parameters enhanced the short circuit current density of the cells. The TiO₂ DSC co-doped with 0.075 at% Sr and 1.5 at% V (i.e., S7V15 cell) had the highest circuit current density and power conversion efficiency of 18.57 and 7.76%, respectively, as a result of less recombination, which is demonstrated by electrochemical impedance spectroscopy (EIS).     

Significant improvement of phenothiazine organic dye-sensitized solar cell performance using dithiafulvenyl unit as additional donor

The simple D-π-A phenothiazine organic dye (C₆PTZ) was modified by introducing excellent electron donor dithiafulvenyl unit with alkyl chains as additional donor to form novel D-D-π-A organic dyes WD14 and WD15 for the first time. These organic dyes were successfully applied in dye-sensitized solar cells and the photovoltaic properties were investigated. Compared with the reference dye C₆PTZ, the power conversion efficiencies increased significantly from 4.16% to 5.87% (WD14) and 6.63% (WD15). The performance improvement is due to the following advantages of the introduction of dithiafulvenyl unit with alkyl chains. Firstly, the light-harvesting capability is improved by increasing electron-donating ability. Secondly, it leads to more efficient inhibition of aggregation between dye molecules. Thirdly, the charge recombination between photoelectrons injected into the conduction band of TiO₂ and the oxidized form $\left({{\text{I}}_3}^{-}\right)$ of the redox couple is restricted due to the double blocking effect originating from the long hexyl chains of the dithiafulvenyl unit and hexyl side chain attached to phenothiazine unit. This work indicates that the incorporation of dithiafulvenyl unit into the simple phenothiazine organic dye greatly improves the solar cell performance, and it will be an effective approach to develop high-performance metal-free organic dyes.     

Porphyrins modified with a low-band-gap chromophore for dye-sensitized solar cells

Two novel porphyrin dyes (PMBTZ and PHBTZ) modified with alkyl-thiophene and 2,1,3-benzothiadiazole (BTZ) moieties were designed and synthesized. The optical and electrochemical properties were characterized by UV-visible, fluorescence spectroscopy and cyclic voltammetry. With the introduction of the low-band-gap chromophore onto the porphyrins, the absorption spectra of the two porphyrin dyes in the range of 450-600 nm were broadened and the maximum wavelength was red-shifted compared with P_Zn as expected. The first oxidation potentials (E_ox1) were altered to the negative, which lowered from 1.27 to 1.11 and 1.15 eV, respectively. For a typical solar cell device based on dye PMBTZ, the maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach to 65%, with a broad respondent region of 350-800 nm. Under standard global AM 1.5 solar condition, the dye-sensitized solar cell (DSSC) based on the dye PMBTZ showed the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 14.11 mA/cm², an open-circuit photo voltage (V_oc) of 0.59 V, and a fill factor (ff) of 0.66, corresponding to solar-to-electric power conversion efficiency (η) of 5.46%.     

CuAlO2 powder dispersed in composite gel electrolyte for application in quasi-solid state dye-sensitized solar cells

The use of delafossite CuAlO₂ (CAO) powder as an additive in composite gel electrolyte (CGE) of the quasi-solid state dye-sensitized solar cell (DSSC) is first reported. In order to achieve an improvement of power conversion and long-term performance of the quasi-solid state DSSC, different contents of CAO powder containing in CGE, a mixture of polyethylene glycol (PEG), iodide/tri-iodide (I⁻/I₃⁻) liquid electrolyte (LE) and 4-tertbutylpyridine (4-tBP), were used in the present study. The photocurrent density–voltage characteristic (J–V curve) and photovoltaic performance parameters of the cells, such as the short-circuit current density (J_sc), open-circuit voltage (V_oc), power conversion efficiency (η) and fill factor (FF) were investigated. The CGE containing the dispersed CAO powder exhibited high ionic conductivity due to the charge diffusion through free channels. The power conversion efficiency of the quasi-solid state DSSC was significantly improved by adding CAO powder to the CGE. The optimum CAO powder content in the CGE was 0.05 wt%. In this research, the power conversion efficiency was 1.71 times of the LE and 2.85 times of the CGE with no CAO powder adding. The quasi-solid state DSSC based on the addition of CAO powder to CGE had long-term stability better than the normal DSSC based on the LE.     

Fabrication of a counter electrode using glucose as carbon material for dye sensitized solar cells

Carbon material was produced from the graphitization of glucose at high temperature in flowing argon. The produced carbon material was characterized using Scanning electron microscopy, Transmission electron microscopy, Raman spectroscopy and XRD. Carbon slurry of the produced carbon was made in ethanol by using polyvinylpyrrolidone (PVP) as surfactant. Carbon slurry was coated homogeneously on fluorine doped tin oxide (FTO) glass by a doctor blade technique and applied as counter electrode for dye synthesized solar cell. The current density (J) and open circuit voltage (V_OC) of fabricated cell was 8.30 mA cm⁻² and 0.77 V respectively. The efficiency of the cell was 3.63%, which is comparable to 5.82% of cell with platinum counter electrode under the same experimental conditions.     

Low glass transition temperature hole transport material in enhanced-performance solid-state dye-sensitized solar cell

A new triarylamine-based hole transport materials (HTM) with branched side chain giving low glass transition temperature (T_g) is synthesized and incorporated into a solid state dye sensitized solar cell. This designing of molecular structure of HTM for lowering the T_g along with viscosity and surface tension of the casting solution effectively increases the pore-filling fraction (PFF) as the cell is heated during the fabrication, leading to an 8 fold increasing in cell efficiency over cells without heat treatment. We relate the cell performance improvement not only due to the PFF of TiO₂ by the (HTM), but also because of morphological and thickness changes in the hole transport material (HTM) capping layer.     

High performance platinum nanofibers with interconnecting structure using in dye-sensitized solar cells

Platinum nanofibers (PtNFs) with interconnecting structure are prepared by a simple electrospun method and novel film forming technique, which are used as the counter electrode (CE) materials for the dye-sensitized solar cells (DSSCs). Electron microscope images demonstrate that PtNFs are constituted by Pt nanoparticles. Cyclic voltammetry measurements indicate that PtNF CE has an excellent catalytic activity for the reduction of triiodide to iodide. DSSC based on PtNF CE achieves an enhanced photovoltaic conversion efficiency of 9.31% compared to that of DSSC based on Pt CE (7.32%) prepared by the thermal decomposition, owing to the three-dimensional interconnecting nanostructure has abundant catalytic surfaces, large contact area with the electrolyte, and lots of electronic transmission channels. DSSC efficiency based on PtNF CE remains 90.08% after continuous scanning 1000 s under an open system without any sealing in ambient atmosphere, and 83.19% of the efficiency is remained after 1000 h under a strictly packaging for the device application.