Metal-free Organic Sensitizers Research Articles

Stable and charge recombination minimized π-extended thioalkyl substituted tetrathiafulvalene dye-sensitized solar cells

Two new metal-free organic sensitizers were designed based on tetrathiafulvalene scaffolds by adopting a donor–π–acceptor (D–π–A) approach.

Recent developments in tetrathiafulvalene and dithiafulvalene based metal-free organic sensitizers for dye-sensitized solar cells: a mini-review

Tetrathiafulvalene and dithiafulvalene are thermally stable, versatile and their HOMO–LUMO levels can be easily through molecular engineering to yield metal-free organic dyes for dye-sensitized solar cells, which are not restricted by the availability of natural resources.

Effect of spacers and anchoring groups of extended π-conjugated tetrathiafulvalene based sensitizers on the performance of dye sensitized solar cells

Four novel metal-free organic sensitizers based on thioalkyl substituted tetrathiafulvalene scaffolds using the donor–π–acceptor approach were tested in dye-sensitized solar cells and had efficiency in the range of 4.55–6.36%.

D-A-π-A Motif Quinoxaline-Based Sensitizers with High Molar Extinction Coefficient for Quasi-Solid-State Dye-Sensitized Solar Cells.

To meet the requirement of high molar extinction coefficient, broad absorption spectrum, and photo/thermal stability for sensitizers of quasi-solid-state dye-sensitized solar cells (Qs-DSSCs) with reduced film thickness, a novel D-A-π-A configuration organic sensitizer IQ22 was specifically designed, in which the conjugation bridge of cyclopentadithiophene (CPDT) unit was incorporated to widen the light response and enhance molar coefficients for increasing the short-circuit current density (JSC), and the octane chain on CPDT was targeted for suppressing the charge recombination and improving the open-circuit voltage (VOC). As a result, the Qs-DSSC based on IQ22 exhibits very promising conversion efficiency as high as 8.76%, with a JSC of 18.19 mA cm-2, a VOC of 715 mV, and a fill factor (FF) of 0.67 under AM 1.5 illumination (100 mW cm-2), standing out in the Qs-DSSCs utilizing metal-free organic sensitizers.

Design of high-performance dye-sensitized solar cells by variation of the dihedral angles of dyes

Two new metal-free organic sensitizers (i.e., IAH and IDH) based on a D−π−A configuration were designed to elucidate the effect of the alkyl chain location at the thiophene π-bridge on their photovoltaic performance. IAH, in which the alkyl chain is closer to the acceptor moiety, is more planar than IDH, in which the alkyl chain is closer to the donor moiety. Therefore, IAH induces more effective π-conjugation and electron injection from the dye excited state to the TiO2 conduction band, leading to a higher short-circuit photocurrent (Jsc). In contrast, IDH more effectively inhibits electron transfer from the conduction band of TiO2 to the dye excited state, resulting in a higher open-circuit photovoltage (Voc). Under standard AM 1.5 simulated sunlight, the IAH-based cells exhibited an overall conversion efficiency of 6.90%; this value is higher than that of the IDH-sensitized cells, which reached 86% of the conversion efficiency of N719 cells.

Aggregation of metal-free organic sensitizers on TiO2(1 0 1) surface for use in dye-sensitized solar cells: A computational investigation

We present a theoretical investigation of the aggregation effects of two 4H-Pyran-4-ylidene-based organic sensitizers on the TiO2(101) surface, which are used in dye-sensitized solar cells (DSSCs). Our results combining density functional theory (DFT), time-dependent DFT, and density functional tight binding (DFTB) methods show that 4H-Pyran-4-ylidene-based organic sensitizers are prone to aggregate on the TiO2 surface due to the strong intermolecular π…π interactions. The aggregation has obvious effects on electronic structures and absorption spectra of the organic sensitizers. The incorporation of an electron-deficient benzothiadiazole (BT) unit in the spacer part of the organic dye leads to a broader absorption band in the red portion of the UV–visible spectra, which is desirable for enhanced spectral responses. However, the BT moiety also leads to more aggregation on the TiO2 surface and stronger electron coupling within the dimers, which might retard the electron transfer at the dye-TiO2 interface and therefore limit the photovoltaic efficiency. This study sheds light on the aggregation effects of metal-free organic dyes and is expected to assist in the rational design of novel and efficient all-organic sensitizers for the further optimization of DSSCs.

Modified pyrene based organic sensitizers with thiophene-2-acetonitrile as π-spacer for dye sensitized solar cell applications

Designed, synthesized and characterized two new metal free organic sensitizers for dye-sensitized solar cells (DSSCs) applications comprising pyrene-1-carbaldehyde as donor, thiophene-2-acetonitrile as π-spacer and cyano acetic acid or rhodanine-3-acetic acid as electron acceptor moieties. From photophysical properties, J and H- type dye aggregations observed for the cyano acetic acid and rhodanine-3-acetic acid anchor group based dyes respectively. The electrochemical experiments and time-correlated single-photon counting (TCSPC) measurements indicate the ability of electron injection into the TiO2 conduction band from the excited sensitizer. Theoretical calculations were performed to rationalize the experimental data and both experimental and theoretical studies indicate that rhodanine-3-acetic acid anchor groups at the acceptor moiety inhibit the electron injection process due to H-type dye aggregation which in turn decreases the open circuit voltage (Voc). Furthermore, the electrochemical impedance spectroscopy (EIS) experiments suggest that the effective electron injection, low charge transfer resistance (Rct), longer lifetime of dye on TiO2 film assists the increase of Voc, achieved by the addition of co-adsorbent deoxycholic acid which in turn enhances the power conversion efficiency (η%).

A new D–D–π–A dye for efficient dye-sensitized solar cells

New metal-free organic dye sensitizers containing mono-triphenylamine or bis-triphenylamine as the electron donor, a thiophene as the π-conjugated system, and a cyanoacrylic acid moiety as the electron acceptor were synthesized. The optical and electrochemical properties of the dyes were investigated, and their performance as sensitizers in solar cells was evaluated. Dye-sensitized solar cells based on dye containing bis-triphenylamine as the electron donor produced a photon-to-current conversion efficiency of 6.06% (Jsc=14.21mA/cm2, Voc=0.62V, ff=0.69) under 100mW/cm2 simulated AM 1.5G solar irradiation (100mW/cm2).

Organic sensitizers for dye-sensitized solar cell (DSSC): Properties from computation, progress and future perspectives

The advent of the dye-sensitized solar cells (DSSCs) came at a time when the quest for alternative energy was high, replacing p-n junction photovoltaic devices. Its uniqueness arises from the fact that unlike the conventional systems where the semiconductor assumes the task of light absorption and charge transport, the two functions are separated in DSSC. Organic sensitizers have been used to harvest a large fraction of sunlight ranging from the UV region to the near infrared region of the spectrum leading to power conversion efficiencies of up to ∼ 10.65 % for metal-free organic sensitizers. Currently, experimental analysis of photo sensitizers utilized in DSSCs is often a trial and error process, often laborious and require extensive and expensive chemical synthesis. In most cases, disappointing results from late-stage of the dye synthesis indicate an urgent need to understand the properties of the dyes at a molecular level, before experiments take place. Fortunately, the use of quantum chemical calculations especially Density Functional Theory (DFT) to screen potential dyes has helped in developing efficient sensitizers and to reduce cost. In the present review article, we discuss the current state of the field, new concepts, design strategies, challenges facing the theoretical design and development of organic sensitizers for DSSCs and future perspectives.

Novel metal-free organic dyes possessing fused heterocyclic structural motifs for efficient molecular photovoltaics.

Reported herein are six novel metal free organic dyes such as PCA1-PCA3 and PCTA1-PCTA3 featuring fused heterocyclic structural motifs such as phenothiazine and alkylcarbazole. Photophysical/electrochemical properties and nanocrystalline TiO2 based dye-sensitized solar cell performance of the same have been investigated. Electronic distribution within the molecules has been determined through a computational approach. The overall power conversion efficiencies of the devices with the utilization of dyes PCA1-PCA3 and PCTA1-PCTA3 as sensitizers ranges between 4.67 and 8.08%. The novel dyes possessing cyanoacrylic acid as electron acceptor, PCA1-PCA3, exhibit higher power conversion efficiency, short-circuit current, open-circuit voltage, and electron lifetime those with thioxothiazolidinylacetic acid, PCTA1-PCTA3, as the same. Of the devices fabricated by employing the new metal-free organic sensitizers, that with the dye PCA2 exerted a power conversion efficiency (PCE, η) of 8.08% with a short circuit current density (JSC) of 16.45 mA cm-2, an open circuit voltage (VOC) of 735 mV and a fill factor (ff) of 0.68; this PCE is the highest amongst the devices fabricated.

V-shaped organic dyes with triphenylamine core for dye-sensitized solar cells: Simple synthesis with enhanced open-circuit voltage

A new series V-shaped organic dyes featured with triphenylamine as core/π-linker has been synthesized for the application of dye-sensitized solar cells. Four different polycyclic aromatic subunits have been introduced as donor group in this series of dyes (named DH-6 to DH-9). These dyes have been systematically characterized and employed in DSSC devices. Among the fabricated devices, DH-6-based cell exhibits the highest power conversion efficiency of 4.04% with a short-circuit current density of 8.02mAcm−2, an open-circuit photo voltage of 0.73V and a fill factor of 0.69 under AM 1.5 illumination without coadsorbent that reaches 83% of the reference dye N719-based cell. This result indicates that other than electron-donor, triphenylamine could also be a promising π-linker to construct high-performance metal-free organic sensitizer.

End-capped “thiophene-free” organic dye for dye-sensitized solar cell: Optimized donor, broadened spectra and enhanced open-circuit voltage

We report on a new series “thiophene-free” organic dyes featured with 2,7-disubstituted carbazole as a π-linker for dye-sensitized solar cells (DSSCs). Three different tert-butyl-capped N-arylamine has been introduced as donor group of this series of dyes (coded DH-13 to DH-15). These dyes have been systematically characterized and employed in DSSC devices, leading to the power conversion efficiencies of 5.06%, 4.38% and 1.75% for DH-13, DH-14 and DH-15, respectively. For a further modification of DH-13, 2,1,3-benzothiadiazole has been inserted to construct a D-A-π-A structured dye (DH-16) which achieves a power efficiency of 6.76% without coadsorbent that reached 89% of the reference dye N719-based cell. This result indicates that 2,7-disubstituted carbazole could be a promising building block to construct high-performance metal-free organic sensitizer even without most commonly used thiophene derivatives.

Nonplanar Organic Sensitizers Featuring a Tetraphenylethene Structure and Double Electron-Withdrawing Anchoring Groups.

Two metal-free organic sensitizers containing two N,N-diethylaniline (DEA) moieties and a twisted 1,1,2,2-tetraphenylethene (TPE) structure, dye SD with one anchoring group and dye DD with two anchoring groups, were synthesized and applied in dye-sensitized solar cells (DSSCs). The introduction of a nonplanar TPE structure was used to form a series of propeller-like structures and reduce the tendency of dyes to randomly aggregate on TiO2 surface, but without importing an aggregation-induced emission (AIE) property. The thermal stabilities, UV-vis absorption spectra, electrochemical properties, and photovoltaic parameters of DSSCs with these two dyes were systematically studied and compared with each other. The overall conversion efficiencies (η) of 4.56% for dye SD and 6.08% for dye DD were obtained under AM 1.5 G irradiation.

Enhancing the Spectral Response of Mesoporous ZnO Films of Dye–sensitized Solar Cells by Incorporating Metal-free Organic Sensitizer and N719 dye

To enhance the spectral responses of the dye-sensitized solar cells (DSSCs), utilizing two or more dyes with complementary absorption spectra is a feasible way to absorb light in various regions of the solar spectrum. The organic dye, J1 dye, is self-synthesized for the first time, which shows a high absorption coefficient of 38,500M−1cm−1 and an absorption spectrum ranged from 400 to 600nm. By co-sensitizing the J1 dye with the commercial N719 dye, the absorption region of the J1/N719 dyes can be extended to 700nm, which nearly covers all the visible region of the solar spectrum. A light-to-electricity conversion efficiency (η) of 4.67% is achieved for the co-sensitized DSSC, which is higher than those of the cells sensitized with individual J1 dye (3.26%) or N719 dye (3.27%), probably due to the enhanced light absorption and more complete coverage of the small organic J1 dye on the ZnO surfaces for the co-sensitized DSSC. The ultraviolet-visible (UV-vis) absorption spectra and the incident photon-to-current conversion efficiency (IPCE) measurement are also utilized to analyse the wavelength-dependent light-to-electricity efficiency of the DSSCs.

Improved photovoltaic performances of dye-sensitized solar cells with ZnO films co-sensitized by metal-free organic sensitizer and N719 dye

The co-sensitization is an effective way to enhance the spectral responses of the dye-sensitized solar cells (DSSCs) by using multiple dyes to absorb different parts of the solar spectrum. The organic sensitizers of the triphenylamine derivatives, W3 and W4 dyes, are synthesized with complementary light absorption spectra to that of the commercially available N719 dye. The main absorption peaks are at 400–500nm and 500–600nm for the triphenylamine derivatives and the N719 dye, respectively. Owing to the outstanding cell performance and the higher absorption coefficient, the W4 dye was applied to co-sensitize with the N719 dye. A wide absorption wavelength (400–700nm) for light harvesting is obtained for the co-sensitized ZnO film soaking in the N719 dye for 2h and subsequently in the W4 dye for 1.5h, and thereby an light-to-electricity conversion efficiency (η) value of 4.34% is achieved for the pertinent DSSC, which is higher than those of the cells sensitized with individual W4 dye (3.38%) and N719 dye (3.76%). The co-sensitized ZnO film cannot only possess wider light absorption range but also better sensitizer coverage since the small size of the organic dye (W4) can adsorb on the gaps between the three dimensional N719 dye on the ZnO surface. The electrochemical impedance spectroscopy (EIS) and the incident photo-to-current conversion efficiency (IPCE) are utilized to analyse the interfacial resistance and the wavelength-dependent light-to-electricity efficiency for the DSSCs, respectively.

All-Solid-State Dye-Sensitized Solar Cells Based on Push-Pull Zinc Porphyrin and Metal-Free Organic Sensitizers

Porphyrins have been designed and synthesized based on the molecular structure of YD2-oC8 for applications of dye-sensitized solar cell (DSSC) to reach PCE 13 % using cobalt-based liquid-type electrolyte. However, traditional liquid-type devices suffered from leaking of electrolyte and the best solution is to replace it with solid hole transporting materials like spiro-OMeTAD. In this lecture, I will present our recent results of all-solid-state DSSC based on a series of novel porphyrin sensitizers together with co-sensitization of porphyrins with organic dyes and/or porphyrin dimers. Extension of p-conjugation can be achieved by attaching various cyclic aromatic hydrocarbon substituents, such as anthracene, tetracene, pyrene, fluorene, and amino-substituted electron-donating groups opposite to the anchoring group of the target porphyrin. The reference porphyrin is the LD14 dye, which makes a traditional DSSC attaining PCE 9.2 % using iodine-based electrolyte. When the functional group has been inserted into the position between porphyrin core and the donor group, the efficiency of the device made of spiro-OMeTAD HTM attained PCE 4.1 %. The solid state device has the configuration FTO/TiO2 (1.8 mm)/porphyrin/spiro (180 nm)/Ag. Another new push-pull porphyrin was designed to insert the functional group Benzothiadiazole (BTD) in the position between the porphyrin core and the anchoring group. By co-sensitization of this porphyrin with a newly designed organic dye on a TiO2 film of 1.8 mm, the solid-state device showed a promising performance with PCE 5.5 %. Both time-resolved (TAS) and frequency-domain (EIS) techniques were applied to understand charge transport and recombination kinetics with respect to their photovoltaic performances.

Metal-free tetrathienoacene sensitizers for high-performance dye-sensitized solar cells.

A new series of metal-free organic chromophores (TPA-TTAR-A (1), TPA-T-TTAR-A (2), TPA-TTAR-T-A (3), and TPA-T-TTAR-T-A (4)) are synthesized for application in dye-sensitized solar cells (DSSC) based on a donor-π-bridge-acceptor (D-π-A) design. Here a simple triphenylamine (TPA) moiety serves as the electron donor, a cyanoacrylic acid as the electron acceptor and anchoring group, and a novel tetrathienoacene (TTA) as the π-bridge unit. Because of the extensively conjugated TTA π-bridge, these dyes exhibit high extinction coefficients (4.5-5.2 × 10(4) M(-1) cm(-1)). By strategically inserting a thiophene spacer on the donor or acceptor side of the molecules, the electronic structures of these TTA-based dyes can be readily tuned. Furthermore, addition of a thiophene spacer has a significant influence on the dye orientation and self-assembly modality on TiO2 surfaces. The insertion of a thiophene between the π-bridge and the cyanoacrylic acid anchoring group in TPA-TTAR-T-A (dye 3) promotes more vertical dye orientation and denser packing on TiO2 (molecular footprint = 79 Å(2)), thus enabling optimal dye loading. Using dye 3, a DSSC power conversion efficiency (PCE) of 10.1% with Voc = 0.833 V, Jsc = 16.5 mA/cm(2), and FF = 70.0% is achieved, among the highest reported to date for metal-free organic DSSC sensitizers using an I(-)/I3(-) redox shuttle. Photophysical measurements on dye-grafted TiO2 films reveal that the additional thiophene unit in dye 3 enhances the electron injection efficiency, in agreement with the high quantum efficiency.

Double D-π-A dye linked by 2,2'-bipyridine dicarboxylic acid: influence of para- and meta-substituted carboxyl anchoring group.

Starting from 2,2'-bipyridine dicarboxylic acid, two new (D-π-A)2 sensitizers, including m-DA with the carboxyl anchoring group substituted meta to the donor-bridge moiety and p-DA with a para-substituted anchoring group, were synthesized in order to evaluate the impact of the position of the anchoring group on the optical, electrochemical, and photovoltaic properties of dye-sensitized solar cells. p-DA exhibits red-shifted absorption behavior compared to m-DA, owing to the more efficiently extended π-conjugation with para substitution. Both m-DA and p-DA are adsorbed on the mesoporous TiO2 surface by using both of their carboxylic acid groups in a bianchoring mode, which is confirmed through attenuated total reflectance FTIR analysis. Red-shifted absorption of p-DA assists the achievement of a red-shifted incident photon-to-electron conversion efficiency and a higher short-circuit current density than m-DA. The photogenerated electron lifetime in TiO2 is also found to be higher for para substituted p-DA than the meta-substituted m-DA, which results in a higher open-circuit voltage. All of the results suggest that dicarboxyl-2,2'-bipyridine can be used as an acceptor for metal-free organic sensitizers. However, the anchoring segments should be adjusted to the favorable position of the corresponding donor-bridge moieties for better conjugation.

Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells

Solar fuel generation requires the efficient capture and conversion of visible light. In both natural and artificial systems, molecular sensitizers can be tuned to capture, convert, and transfer visible light energy. We demonstrate that a series of metal-free porphyrins can drive photoelectrochemical water splitting under broadband and red light (λ > 590 nm) illumination in a dye-sensitized TiO2 solar cell. We report the synthesis, spectral, and electrochemical properties of the sensitizers. Despite slow recombination of photoinjected electrons with oxidized porphyrins, photocurrents are low because of low injection yields and slow electron self-exchange between oxidized porphyrins. The free-base porphyrins are stable under conditions of water photoelectrolysis and in some cases photovoltages in excess of 1 V are observed.

N-Annulated perylene-based metal-free organic sensitizers for dye-sensitized solar cells.

A series of novel metal-free organic sensitizers based on N-annulated perylene derivatives have been designed and synthesized, and exhibit broad absorption spectra in the visible region. The dye-sensitized solar cells exhibit overall conversion efficiencies ranging from 4.90% to 8.28% under AM 1.5 solar conditions.