Metal-free Organic Sensitizers Research Articles

Computational Study on D-π-A-Based Electron Donating and Withdrawing Effect of Metal-Free Organic Dye Sensitizers for Efficient Dye-Sensitized Solar Cells

A new generation of metal-free organic dyes with a range of donor (D1) and acceptors (A1-A3) were designed and examined for dye-sensitized solar cells (DSSCs) based on (3a) dye as a literature. Triphenylamine (TPA), thiophene ([Formula: see text] and 2-cyanoacrylic acid groups each perform the roles of an acceptor (A), donor (D) and spacer in order to produce a D-[Formula: see text]-A system. To investigate the intramolecular charge transfer (ICT), electronic distribution, ultra-violet visible (UV–Vis) absorption wavelengths, molecular electrostatic potential (MEP) and photovoltaic (PV) parameters of the D1 and A1–A3 molecules, density functional theory (DFT) and time-dependent DFT (TD-DFT) were used. The classification of the tunable donor D1 and A1–A3 determines the PV performance of the dye molecules. Results show that the A2 dye replacement group increases the performance of PV cells via red-shifting absorption spectra. Also, when compared to 3a, A2 dye have lower energy gap ([Formula: see text] and superior UV–Vis spectra that cover the full visible range. These results demonstrate the viability of molecular tailoring as an approach to improve D-[Formula: see text]-A sensitizer proposal for efficient DSSCs fabrication.

Synthesis and Photovoltaic Properties of Simple Fluorene-Based Triphenylamine Metal-Free Organic Sensitizers for Dye-Sensitized Solar Cells

Synthesis and Photovoltaic Properties of Simple Fluorene-Based Triphenylamine Metal-Free Organic Sensitizers for Dye-Sensitized Solar Cells

New photosensitizers that are based on carbazoles and have thiophene bridges with a low bandgap do 32% better than N719 metal complex dye

Synthesis of metal-free organic sensitizers is a promising route for obtaining low-cost, high-efficiency sensitizers for dye-sensitized solar cells (DSSCs). We present the results of a comprehensive photovoltaic analysis of four carbazole organic sensitizers with a D-A architecture in this study. MES 1-4 have been developed and used as sensitizers in DSSC applications. Using MES1-4 as sensitizers results in a broader light-harvesting ability (400–800 nm) and higher photovoltaic efficiency at a range of 6.06–9.55 %. Herein, the introduction of thiazolidine-4-one ring engineering into MES4 as a rich electron acceptor enhanced electron injection and inhibited electron recombination in thiazolidine-4-one sensitizers. The MES4 system has the highest light harvesting ability and reflectivity owing to the existence of distinct functional groups that enhanced the capacity to bind the dye with the semiconductor layer (TiO2 device's efficiency). In conclusion, the DSSC using the novel electron acceptor thiazolidine-4-one ring sensitizer achieves an unexpected PCE of 9.55 % and a 32 % increase over using the N719 metal complex dye. All sensitizers based on carbazole compounds had good photovoltaic performance.

Donor functionalized perylene and different π-spacer based sensitizers for dye-sensitized solar cell applications - a theoretical approach.

A series of perylene-based novel metal-free organic dye sensitizers are designed and optimized for dye-sensitized solar cell (DSSC) applications. The electronic and optical properties are analyzed through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approach. For perylene-based donors, the effects of additional donor units and different π-spacer positions were investigated. Cyanovinyl and thiophene are used as π-spacers, dimethylamine (DM) and N-N-dimethylaniline (DMA) are used as additional donors, and cyanoacrylic acid is used as mono acceptor unit for the designed sensitizers. Natural bonding orbitals (NBOs), frontier molecular orbitals (FMO), UV-Vis, and nonlinear orbital analysis were predicted to find the net electron transfer, energy gap, absorption spectra, and electronic charge distribution for perylene-based dye sensitizers, respectively. The electron injection and electron regeneration properties were also analyzed for perylene-based sensitizers.

Molecular engineering of the fused azacycle donors in the D-A-π-A metal-free organic dyes for efficient dye-sensitized solar cells

Four novel D-A-π-A metal-free organic sensitizers QL1–4 with the fused azacycle units as donors have been designed and synthesized to investigate the relationship between both the sizes and conjugations of azacycle and performances of dye-sensitized solar cells systematically. QL1 with the five-membered azacycle, i.e., with carbazole (Cz) as donor, exhibits the smallest molar extinction coefficients (ε). QL2 and QL3 with the seven-membered azacycle, i.e. with N-phenyl-iminostilbene (ISB) and N-phenyl-iminodibenzyl (IDB) as donors, respectively, show the red-shifted absorption spectra and higher ε compared to QL1. However, QL2 and QL3 with an only distinction in a single or double bond, display distinctive PCEs. QL4 with the six-membered azacycle, i.e., 9,9,10-triphenyl-9,10-dihydroacridine (TDD), exhibits a blue-shifted absorption spectrum due to a big dihedral angle, resulting in a lower photovoltaic performance. Especially, QL3 with the ISB donor is conducive to achieving a PCE of 6.22%, which is higher than QL2, due to its better planar structure and more conjugation. When co-adsorbed with CDCA, QL3 exhibits the highest PCE of 7.14% among all dyes, and up to 8.26% by co-sensitizing with dye WL5, which surpasses that of the reference dye N719 (7.75%). The results indicate that the size and conjugation of the fused azacycle donors may affect the photovoltaic performance significantly, and the ISB unit is demonstrated as a promising donor for efficient organic dyes.

Theoretical and photovoltaic investigations of 1,3,5-triazine-based photosensitizers achieving highly efficient DSSCs

By establishing a new push-pull relationship, three novel metal-free organic sensitizers based on A'–π–A framework were synthesized, in which A' and A represent the auxiliary acceptor (triazine) and the main acceptor (cyanoacrylic acid), respectively, coded ZS03, ZS04, and ZS05. In this paper, the molecular structure, photophysical property, electrochemical property, theoretical calculation, and photovoltaic property of these three sensitizers are described in detail. The results show that the additional acceptor and alkyl chains significantly affect the molecule’s energy level and spatial structure. In terms of photovoltaic characteristics, the ZS03-based device exhibits the best power conversion efficiency (PCE) of 7.11%, corresponding to a short-circuit photocurrent density (Jsc) of 14.31 mA cm−2, an open-circuit voltage (Voc) of 708 mV, and a fill factor (FF) of 0.723, which is the highest efficiency among the known A'–π–A type dyes. It can be determined that the improvement of PCE is mainly due to the combination of the following two factors: the π-spacer with appropriate length and the alkyl chain with a reasonable layout.

Starburst-Shaped D-π-A Chromophores Possessing a Hexaethynylbenzene Core for Dye-Sensitized Solar Cells.

Two starburst-shaped organic chromophores, incorporating a hexaethynylbenzene core modified by five donor branches (D-branches) of (p-dioctylaminostyryl)benzene and one acceptor/anchoring branch (A-branch) of either carboxylic acid-terminated phenylethynylbenzene (SB-07) or cyanoacrylic acid-terminated diketopyrrolopyrrole (DPP)-thiophene (SB-08), were synthesized and applied to dye-sensitized solar cells (DSSCs). In these chromophores, the common donor moiety, five (p-dioctylaminostyryl)phenyl groups, exhibits excellent optical absorption in the visible region (molar absorption coefficient ε > 105 M-1 cm-1 below 500 nm). The A-branch of SB-07 does not possess strong electron-accepting properties; however, the A-branch of SB-08, the DPP-thiophene moiety, serves as a strong electron acceptor site. Furthermore, the intramolecular charge-transfer (ICT) transition between the thiophene and DPP moieties extends the optical absorption range to the near-infrared region (∼800 nm). Optimized DSSC devices using SB-08 with coadsorption of chenodeoxycholic acid, in conjunction with iodide/triiodide-based electrolytes, exhibited incident photon-to-current conversion efficiency (IPCE) exceeding 70% in the 370-700 nm range and over 20% even at 800 nm, with a short-circuit photocurrent density (Jsc) of 19.3 mA cm-2 and a power conversion efficiency (PCE) of 6.4% under AM 1.5G illumination (100 mW cm-2). These results are considerably better than those of SB-07 (Jsc = 7.0 mA cm-2, PCE = 3.3%). The starburst-shaped architecture presented here can be used as a novel structural motif for metal-free organic sensitizers because it enables flexible modification of the multiple D-branches that enhance light-harvesting ability and the A-branch that serves as an excited electron transport pathway.

Exploring the screening of perylene based organic sensitizers with different lengths and functional groups of acceptors via computational spectroscopic analysis

Perylene based metal-free organic sensitizers in the dye sensitized solar cells are computed through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) for the ground state and excited state geometries using B3LYP/6–311G(d,p) basis set. In this present investigation, 2,5-dip-tolylperylene is chosen as electron donor, phenylthiophene as π-spacer, and eight acceptors, named as A1-A8 which have various blends of lengths and functional groups contiguous to the carboxyl acid group. Accordingly, A1-A8 sensitizers are systematically analyzed with the aid of various parameters, which includes light-harvesting ability, molecular orbital energy levels, electron injection and regeneration, ultraviolet-visible absorption spectrum, natural bond orbital analysis, and non-linear optical properties.

Donor group influence on dye-sensitized solar cell device performances: Balancing dye loading and donor size

Four high photocurrent near-infrared (NIR)-absorbing metal-free organic sensitizers (ND1, ND2, ND3, and AP25) with varying nitrogen-based donor groups are examined for use in dye-sensitized solar cell (DSC) devices. One of the highest photocurrent generating organic dyes based on a triarylamine donor reported in the literature (AP25 at 19 mA/cm2) is compared to dyes varying the donor group with constant intramolecular charge transfer π-bridge-acceptor systems. Specific popular donor groups include the bulky Hagfeldt donor (ND1), a carbazole donor (ND2), and an indoline donor (ND3) which varies a large range of sizes (maximum width varies between 8 Å and 26 Å). Computational analysis, dye energetics, absorption profiles (in solution and on TiO2), device incident photon-to-current conversion efficiencies (IPCEs), electrochemical impedance spectroscopy, small modulated photovoltage transient spectroscopy, and device current-voltage curves are used to probe dye behavior based on donor group selection. Balancing donor size and dye loadings was found to be critical for higher performances with the DSC dyes accessing low energy photons near 900 nm.

Photocatalytic H2 Production from Water by Metal-free Dye-sensitized TiO2 Semiconductors: The Role and Development Process of Organic Sensitizers.

The utilization of solar energy to produce hydrogen from water is showing increased importance and desirability in the field of artificial photosynthesis to produce clean and sustainable fuels. In a typical three-component dye-sensitized semiconductor system for photocatalysis, the dye sensitizer plays an essential role of energy antenna for harvesting visible light and promoting the reduction reaction to generate hydrogen. In recent decades, a lot of attention has focused on metal-free organic sensitizers, which have the advantages of low cost, high molar extinction coefficient, good modifiability and, most importantly, ability to avoid the use of noble metal ions. This Review enumerates the design strategies, specific properties and photocatalytic performances of metal-free sensitizers in the past 30 years and concludes their evolution process. The advantages of different types of metal-free sensitizers are highlighted and the instructively enlightening experiences are systematic summarized.

Dithienopyrrolobenzothiadiazole-based metal-free organic dyes with double anchors and thiophene spacers for efficient dye-sensitized solar cells

Five new dithienopyrrolobenzothiadiazole (DTPBT)-based organic sensitizers were prepared for dye-sensitized solar cells (DSSCs). In this work, DTPBT unit was used as a donor to construct mono- and di-anchored metal-free organic sensitizers for the first time. The DTPBT unit applied as a donor or a π-spacer in mono-anchored dyes (W1 and W2) greatly influences the absorption range, HOMO-LUMO energy level and the solar cell performance of the dyes. The results show that the DTPBT unit is more suitable for acting as a π-spacer in D-π-A dyes. Di-anchored dyes exhibit broader photo-spectra response and higher molar extinction coefficients than those of the mono-anchored dyes. Di-anchored dyes W4 and W5 with DTPBT as the donor, thiophene as the π-bridge achieve high power conversion efficiencies of 7.93%, 7.98% with 1 mM CDCA as the co-adsorbent, respectively. The results suggest that the DTPBT moiety is a promising donor candidate to develop efficient double anchored organic dyes for DSSCs.

Theoretical Study of the Effect of π-Bridge on Optical and Electronic Properties of Carbazole-Based Sensitizers for DSSCs.

Eight novel metal-free organic sensitizers were proposed for dye-sensitized solar cells (DSSCs), theoretically calculated and studied via density functional theory with D-π-A structure. These proposals were formed to study the effect of novel π-bridges, using carbazole as the donor group and cyanoacrylic acid as the anchorage group. Through the M06/6-31G(d) level of theory, ground state geometry optimization, vibrational frequencies, the highest occupied molecular orbital, the lowest unoccupied molecular orbital, and their energy levels were calculated. Further, chemical reactivity parameters were obtained and analyzed, such as chemical hardness (η), electrophilicity index (ω), electroaccepting power (ω+) and electrodonating power (ω-). Free energy of electron injection (ΔGinj) and light-harvesting efficiency (LHE) also were calculated and discussed. On the other hand, absorption wavelengths, oscillator strengths, and electron transitions were calculated through time-dependent density functional theory with the M06-2X/6-31G(d) level of theory. In conclusion, the inclusion of thiophene groups and the Si heteroatom in the π-bridge improved charge transfer, chemical stability, and other optoelectronic properties of carbazole-based dyes.

Bulky Phenanthroimidazole–Phenothiazine D−π–A Based Organic Sensitizers for Application in Efficient Dye-Sensitized Solar Cells

Metal-free organic sensitizers are important in dye-sensitized solar cells (DSSCs) to overcome the existing concerns such as synthetic feasibility and cost effectiveness. Herein, we developed four ...

Quantification of Dyes Generating Photocurrent and/or Photoluminescence in Dye-Sensitized Solar Cells Using Laser Scanning Microscopy

Photoconversion processes such as electron injection (photooxidation) and dye regeneration (reduction) in dye-sensitized solar cells (DSSCs) occur at considerably inhomogeneous semiconductor/dye/electrolyte interfaces, implying a very high heterogeneity of interfacial photoconversion kinetics. Herein, we present a temporally and spatially resolved investigation of DSSCs comprising a cover glass photoanode with a 100-nm thick TiO2 layer loaded with the metal-free organic dye sensitizer MK-2, which is performed by employing laser scanning microscopy (LSM) for the simultaneous measurement of the photocurrent (PC) and photoluminescence (PL) of DSSCs under short-circuit conditions. Analysis of PL decay curves and the excitation rate dependences of PC and PL obtained for local (or submicrometric) areas of the MK-2-DSSC allows disclosing and quantifying three types of dyes coexisting in the DSSCs: (i) a dye that only generates PC (“PC-dye,” 75% of total dye molecules in the DSSC), (ii) a dye that generates both PC and PL (“PCPL-dye,” 20%), and (iii) a dye that only generates PL (“PL-dye,” 5%). Considering recent theoretical reports on cyanoacrylic dyes, we propose that the PC-dye and the PCPL-dye are covalently bound on a TiO2 surface with different adsorption modes (presumably bidentate and tridentate bridging configurations), whereas the PL-dye is noncovalently trapped within a mesoporous TiO2 film.

Molecular engineering of metal-free organic sensitizers with polycyclic benzenoid hydrocarbon donor for DSSC applications: The effect of the conjugate mode

Optimizing the molecular structure to adjust the properties of photosensitizer is one of the crucial means to improve the power conversion efficiency of dye-sensitized solar cells (DSSCs). In this paper, four aromatic fused rings were selected as electron donor units to construct photosensitizers PBT–series. The effects of structural differences in the donor moiety on the photophysical and photochemical properties of the molecules were investigated in detail. The results show that the devices based on the dyes with triphenylene or fluoranthene as electron donor can yield higher short-circuit photocurrent densities (Jsc) and open-circuit voltages (Voc) than the devices based on the reference dyes with pyrene or phenanthrene as electron donor, and the power conversion efficiency (PCE) is increased by 37% and 23%, respectively. PBT–2 and PBT–4 sensitizers have better light absorption properties due to stronger intramolecular charge transfer (ICT) charge transitions, which helps to increase the photocurrent of devices, while they also exhibit significantly lower charge recombination rates. Eventually, PBT–2-based devices showed the best power conversion efficiency of 7.67% under the standard global AM 1.5 solar conditions.

Efficiency enhancement of ruthenium-based DSSCs employing A-π-D-π-A organic Co-sensitizers.

A new bipyridyl Ru(ii) sensitizer incorporating triphenylamine and the 3,4-ethylenedioxythiophene (EDOT) ancillary ligand IMA5 was synthesized for dye-sensitized solar cells (DSSCs). The performance of these DSSCs has been enhanced via di-anchoring metal-free organic sensitizers, denoted IMA1–4, with structural motif A–π–D–π–A and incorporating phenyl-dibenzothiophene-phenyl (Ph-DBT-Ph) as the main building block but with different anchoring groups (A). These new organic sensitizers were well-characterized and used as efficient co-sensitizers. Their photophysical, electrochemical and photovoltaic properties were studied. Furthermore, molecular modeling studies using DFT calculations were used to investigate their suitability as effective sensitizers/co-sensitizers. The molecular orbital isodensity showed distinguishable delocalization of the intramolecular charge in the DBT moiety. The photovoltaic characterization showed that IMA3 had the best DSSC performance (η = 2.41%). In addition, IMA1–4 was co-sensitized in conjunction with the newly synthesized IMA5 complex to enhance light harvesting across expanded spectral regions and thus improve efficiency. The solar cells co-sensitized with IMA2, IMA3 and IMA4 exhibited improved efficiency (η) of 6.25, 6.19 and 5.83%, respectively, which outperformed the device employing IMA5 alone (η = 5.54%) owing to the improvement in the loading of IMA2, IMA3 and IMA4 in the presence of IMA5 on the surface of the TiO2 nanoparticles, and charge recombination was suppressed.

9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole—A new donor building-block in the design of sensitizers for dye-sensitized solar cells

The novel donor building-block - 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole was designed and employed in the synthesis of dye-sensitized solar cell (DSSCs). An effective, high-yielding synthesis of 4,6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(p-tolyl)-1,2,3,4,4a,9a-hexahydrocarbazole from 1,2,3,4,4a,9a-hexahydrocarbazole was realized. Three new metal-free organic sensitizers, containing the new donor building block were prepared by a stepwise approach from 4,7-dibromobenzo[c][1,2,5]chacogenadiazoles. A 2,1,3-Benzothiadiazole dye containing hexahydrocarbazole donor, thiophene as π-spacer and cyanoacrylate as anchoring electron acceptor showed photovoltaic properties higher than the well-known WS-2 sensitizer with PCE = 5.86 %. Although benzoxa- and –selenadiazole dyes have a bathochromic shift (24−30 nm) in the UV–vis spectra, and smaller energy gap Eg (about 0.1 eV), they have lower photovoltaic parameters, including PCE of 1.5–2.3 %. Introducing a new donor 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole into the construction of the DSSCs has broadened possibilities for the optimization of their photovoltaic properties.

Novel metal-free organic dyes constructed with the D-D|A-π-A motif: Sensitization and co-sensitization study

A new structural motif, D-D|A-π-A, was suggested for the design of novel metal-free organic sensitizers, where “D|A” represents a fused donor-acceptor conjugated building block. Three organic dyes, coded FS07, FS08 and FS09, were synthesized accordingly with the incorporation of indolo [2,3-b] quinoxaline (IQ) as the main building block but different additional donors. The synthesized dyes were well-characterized and their photophysical and electrochemical properties were well-studied. Additionally, molecular modeling via DFT method was employed to probe their photovoltaic behaviors as sensitizers/co-sensitizers. The isodensity of the molecular orbitals displays the distinguished intramolecular charge delocalization in the IQ moiety, even though its donor and acceptor sections are coplanar-fused. In the photovoltaic characterization, FS08 exhibited the best DSSC performance. Moreover, FS07, FS08 and FS09 were co-sensitized along with a typical high efficient bipyridyl Ru(II) sensitizer, HD-2, in order to induce the light harvesting over expanded spectral region and hence improve the efficiency. The solar cell co-sensitized with FS08 displayed improved efficiency of 7.94% with JSC of 19.37 mA⋅cm−2, VOC of 0.654 V and FF of 62.7%, which outperformed the device employing HD-2 alone (efficiency of 7.46%). Owing to the appropriate dye loading of FS08 in the presence of HD-2 and CDCA mixed dye solution, the benign coordination for light harvesting was guaranteed and the photoexcitation was maximized, the co-sensitized JSC was improved; due to the well-modified TiO2 surface coverage with the tight attachment of FS08 and HD-2, the dye aggregation was diminished and the undesired charge recombination was suppressed, the co-sensitized VOC was improved.

Molecular Engineering of Simple Metal-Free Organic Dyes Derived from Triphenylamine for Dye-Sensitized Solar Cell Applications.

Two new metal-free organic sensitizers, L156 and L224, were designed, synthesized, and characterized for application in dye-sensitized solar cells (DSCs). The structures of the dyes contain a triphenylamine (TPA) segment and 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid as electron-rich and -deficient moieties, respectively. Two different π bridges, thiophene and 4,8-bis(4-hexylphenyl)benzo[1,2-b:4,5-b']dithiophene, were used for L156 and L224, respectively. The influence of iodide/triiodide, [Co(bpy)3 ]2+/3+ (bpy=2,2'-bipyridine), and [Cu(tmby)2 ]2+/+ (tmby=4,4',6,6'-tetramethyl-2,2'-bipyridine) complexes as redox electrolytes and 18 NR-T and 30 NR-D transparent TiO2 films on the DSC device performance was investigated. The L156-based DSC with [Cu(tmby)2 ]2+/+ complexes as the redox electrolyte resulted in the best performance of 9.26 % and a remarkably high open-circuit voltage value of 1.1 V (1.096 V), with a short-circuit current of 12.2 mA cm-2 and a fill factor of 0.692, by using 30 NR-D TiO2 films. An efficiency of up to 21.9 % was achieved under a 1000 lx indoor light source, which proved that dye L156 was also an excellent candidate for indoor applications. The maximal monochromatic incident-photon-to-current conversion efficiency of L156-30 NR-D reached up to 70 %.

Improving the efficiency of thin-film fiber-shaped dye-sensitized solar cells by using organic sensitizers

Fiber-shaped Dye-Sensitized Solar Cells (DSSFs) represent one of the most interesting technologies aimed at the light harvesting and the production of electricity for wearable applications. In order to boost DSSFs commercialization, their production costs and environmental impact must be reduced. To this end, a suitable strategy could be to build thin film-based devices endowed with metal-free organic sensitizers, exploiting their higher molar extinction coefficients compared to typical ruthenium-containing organometallic dyes. In this work, three thiazolo[5,4-d]thiazole-based organic dyes, TTZ3, TTZ5 and TTZ7, capable of strongly absorb visible light, were used for the first time to manufacture titanium wire-based DSSFs. DSSFs based on a thin TiO2 layer (5 μm) sensitized with the three organic dyes were prepared and tested and the obtained results show that power conversion efficiencies for the organic dyes (0.80%) are higher than that obtained with the reference N719 dye (0.45%). An efficiency of 0.99% with short circuit current density equal to 3 mA/cm2 was achieved when the TTZ7-based DSSFs were tested in diffuse illumination condition, highlighting the supremacy of these dyes compared to the metal-organic reference. The excellent photovoltaic performances of TTZ dyes were attributed to their better light harvesting properties, resulting in the production of higher photocurrent densities, which was confirmed by the electrochemical impedance spectroscopy (EIS) analysis. The superiority of organic dyes on DSSFs performances compared to N719, shown for the first time in this work, support the real possibility to apply these molecules for the preparation of efficient light-harvesting devices based on thin film photoanodes.