Computational studies on optoelectronic and charge transfer properties of some perylene‐based donor‐π‐acceptor systems for dye sensitized solar cell applications

We report DFT studies on some perylene‐based dyes for their electron transfer properties in solar cell applications. The study involves modeling of different donor‐π‐acceptor type sensitizers, with perylene as the donor, furan/pyrrole/thiophene as the π‐bridge and cyanoacrylic group as the acceptor. The effect of different π‐bridges and various substituents on the perylene donor was evaluated in terms of opto‐electronic and photovoltaic parameters such as HOMO‐LUMO energy gap, λ_max, light harvesting efficiency(LHE), electron injection efficiency (Ø_inject), excited state dye potential (E^dye*), reorganization energy(λ), and free energy of dye regeneration (). The effect of various substituents on the dye–I₂ interaction and hence recombination process was also evaluated. We found that the furan‐based dimethylamine derivative exhibits a better balance of the various optical and photovoltaic properties. Finally, we evaluated the overall opto‐electronic and transport parameters of the TiO₂‐dye assembly after anchoring the dyes on the model TiO₂ cluster assembly.     

Exploring the relevance of thiophene rings as bridge unit in acceptor‐bridge‐donor dyes on self‐aggregation and performance in DSSCs

The possibility of dye charge recombination in DSSCs remains a challenge for the field. This consists of: (a) back‐transfer from the TiO₂ to the oxidized dye and (b) intermolecular electron transfer between dyes. The latter is attributed to dye aggregation due to dimeric conformations. This leads to poor electron injection which decreases the photocurrent conversion efficiency. Most organic sensitizers are characterized by an Acceptor‐Bridge‐Donor ( A ‐Bridge‐ D ) arrangement that is commonly employed to provide charge separation and, therefore, lowering the unwanted back‐transfer. Here, we address the intermolecular electron transfer by studying the dimerization and photovoltaic performance of a group of A ‐Bridge‐ D structured dyes. Specifically, eight famous sulfur containing π‐bridges were analyzed ( A and D remained fixed). Through quantum mechanical and molecular dynamics approaches, it was found that the formation of weakly stabilized dimers is allowed. The dyes with covalently bonded and fused thiophene rings as Bridges, 6d and 7d as well as 8d with a fluorene, would present high aggregation and, therefore, high probability of recombination processes. Conversely, using TiO₂ cluster and surface models, delineated the shortest bridges to improve the adsorption energy and the stability of the system. Finally, the elongation of the bridge up to 2 and 3 units and their photovoltaic parameters were studied. These results showed that all the sensitizers are able to provide similar photocurrent outcomes, regardless of whether the bridge is elongated. © 2017 Wiley Periodicals, Inc.     

Fine Tuning the Performance of DSSCs by Variation of the π‐Spacers in Organic Dyes that Contain a 2,7‐Diaminofluorene Donor

Organic dyes that contain a 2,7‐diaminofluorene‐based donor, a cyanoacrylic‐acid acceptor, and various aromatic conjugation segments, which are composed of benzene, fluorene, carbazole, and thiophene units, as a π‐bridge have been synthesized and characterized by optical, electrochemical, and theoretical investigations. The trends in the absorption and electrochemical properties of these dyes are in accordance with the electron‐donating ability of the conjugating segment. Consequently, the dyes that contained a 2,7‐carbazole unit in the π‐spacer exhibited red‐shifted absorption and lower oxidation potentials than their corresponding fluorene‐ and phenylene‐bridged dyes. However, the enhanced power‐conversion efficiency that was exhibited by the fluorene‐bridged dyes in the DSSCs was attributed to their broader and intense absorption. Despite the longer‐wavelength absorption and reasonable optical density, carbazole‐bridged dyes exhibited lower power‐conversion efficiencies, which were ascribed to the poor alignment of the LUMO level in these dyes, thereby leading to the inhibition of electron injection into the TiO₂ conduction band.     

Theoretical rationalization for reduced charge recombination in bulky carbazole‐based sensitizers in solar cells

The search for greater efficiency in organic dye‐sensitized solar cells (DSCs) and in their perovskite cousins is greatly aided by a more complete understanding of the spectral and morphological properties of the photoactive layer. This investigation resolves a discrepancy in the observed photoconversion efficiency (PCE) of two closely related DSCs based on carbazole‐containing D–π–A organic sensitizers. Detailed theoretical characterization of the absorption spectra, dye adsorption on TiO₂, and electronic couplings for charge separation and recombination permit a systematic determination of the origin of the difference in PCE. Although the two dyes produce similar spectral features, ground‐ and excited‐state density functional theory (DFT) simulations reveal that the dye with the bulkier donor group adsorbs more strongly to TiO₂, experiences limited π–π aggregation, and is more resistant to loss of excitation energy via charge recombination on the dye. The effects of conformational flexibility on absorption spectra and on the electronic coupling between the bright exciton and charge‐transfer states are revealed to be substantial and are characterized through density‐functional tight‐binding (DFTB) molecular dynamics sampling. These simulations offer a mechanistic explanation for the superior open‐circuit voltage and short‐circuit current of the bulky‐donor dye sensitizer and provide theoretical justification of an important design feature for the pursuit of greater photocurrent efficiency in DSCs. © 2017 Wiley Periodicals, Inc.     

Organic Dyes Containing Pyrenylamine‐Based Cascade Donor Systems with Different Aromatic π Linkers for Dye‐Sensitized Solar Cells: Optical,Electrochemical, and Device Characteristics

New organic dyes containing pyrenylamine donors in a cascade arrangement and cyanoacrylic acid acceptors have been synthesized and characterized by optical, electrochemical, and theoretical studies. The dyes inherit a D ‐π¹‐D ‐π²‐A (D=donor, A=acceptor) molecular architecture where the π linkers π¹ are changed from phenyl to biphenyl and fluorene, whereas the π linker π² that connects the donor fragment with the acceptor is a phenyl unit. The conjugation pathway linking the two donor segments has been found to play a major role in the optical and electrochemical properties. Shorter π linkers such as phenyl groups facilitate the donor–acceptor interaction while the nonplanar biphenyl spacer decreases the electronic communication between the donors and enhances the oxidation propensity of the corresponding dye. All the dyes display an intense longer wavelength electronic transition,which is attributable to the amine‐to‐cyanoacrylic acid charge transfer. The extinction coefficient of this peak grows dramatically on increasing the conjugation pathway length between the two donor segments. The dyes were used as sensitizers in nanocrystalline TiO₂‐based dye‐sensitized solar cells (DSSCs) and the cascade donor system contributed to the enhancement in the device efficiency due to favorable absorption and redox properties.     

Donor‐π‐Acceptors Containing the 10‐(1,3‐Dithiol‐2‐ylidene)anthracene Unit for Dye‐Sensitized Solar Cells

Two donor–acceptor molecular tweezers incorporating the 10‐(1,3‐dithiol‐2‐ylidene)anthracene unit as donor group and two cyanoacrylic units as accepting/anchoring groups are reported as metal‐free sensitizers for dye‐sensitized solar cells. By changing the phenyl spacer with 3,4‐ethylenedioxythiophene (EDOT) units, the absorption spectrum of the sensitizer is red‐shifted with a corresponding increase in the molar absorptivity. Density functional calculations confirmed the intramolecular charge‐transfer nature of the lowest‐energy absorption bands. The new dyes are highly distorted from planarity and are bound to the TiO₂ surface through the two anchoring groups in a unidentate binding form. A power‐conversion efficiency of 3.7 % was obtained with a volatile CH₃CN‐based electrolyte, under air mass 1.5 global sunlight. Photovoltage decay transients and ATR‐FTIR measurements allowed us to understand the photovoltaic performance, as well as the surface binding, of these new sensitizers.     

Elementary Photoelectronic Processes at a Porphyrin Dye/Single‐Walled TiO2 Nanotube Hetero‐interface in Dye‐Sensitized Solar Cells: A First‐Principles Study

A unique one‐dimensional (1D) sandwich single‐walled TiO₂ nanotube (STNT) is proposed as a photoanode nanomaterial with perfect morphology and large specific surface area. We have thoroughly examined the elementary photoelectronic processes occurring at the porphyrin dye/STNT hetero‐interface in dye‐sensitized solar cells (DSSCs) by theoretical simulation. It is desirable to investigate the interfacial photoelectronic processes to elucidate the electron transfer and transport mechanism in 1D STNT‐based DSSCs. We have found that the photoexcitation and interfacial charge separation mechanism can be described as follows. A ground‐state electron of the dye molecule (localized around the electron donor) is first promoted to the excited state (distributed electron donor), and then undergoes ultrafast injection into the conduction band of the STNT, leaving a hole around the oxidized dye. Significantly, the injected electron in the conduction band is transported along the STNT by means of Ti 3d orbitals, offering a unidirectional electron pathway toward the electrode for massive collection without the observation of trap states. Our study not only provides theoretical guidelines for the modification of TiO₂ nanotubes as a photoanode material, but also opens a new perspective for the development of a novel class of TiO₂ nanotubes with high power‐generation efficiency.     

Bi‐anchoring Organic Dyes that Contain Benzimidazole Branches for Dye‐Sensitized Solar Cells: Effects of π Spacer and Peripheral Donor Groups

Benzimidazole‐branched bi‐anchoring organic dyes that contained triphenylamine/phenothiazine donors, 2‐cyanoacrylic acid acceptors, and various π linkers were synthesized and examined as sensitizers for dye‐sensitized solar cells. The structure–activity relationships in these dyes were systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The wavelength of the absorption peak was more‐heavily influenced by the nature of the π linker than by the nature of the donor. For a given donor, the absorption maximum (λ_max) was red‐shifted on changing the π linker from phenyl to 2,2′‐bithiophene, whilst the dyes that contained triphenylamine units displayed higher molar extinction coefficients (?) than their analogous phenothiazine‐based triphenylamine dyes, which led to good light‐harvesting properties in the triphenylamine‐based dyes. Electrochemical data for the dyes indicated that the triphenylamine‐based dyes possessed relatively low‐lying HOMOs, which could be beneficial for suppressing back electron transfer from the conduction band of TiO₂ to the oxidized dyes, owing to facile regeneration of the oxidized dye by the electrolyte. The best performance in the DSSCs was observed for a dye that possessed a triphenylamine donor and 2,2′‐bithiophene π linkers. Electron impedance spectroscopy (EIS) studies revealed that the use of triphenylamine as the donor and phenyl or 2,2′‐bithiophene as the π linkers was beneficial for disrupting the dark current and charge‐recombination kinetics, which led to a long electron lifetime of the injected electrons in the conduction band of TiO₂.     

Tris‐Heteroleptic Ruthenium–Dipyrrinate Chromophores in a Dye‐Sensitized Solar Cell

Two novel tris‐heteroleptic Ru–dipyrrinates were prepared and tested as sensitizers in the dye‐sensitized solar cell (DSSC). Under AM 1.5 sunlight, DSSCs employing these dyes achieved power conversion efficiencies (PCEs) of 3.4 and 2.2 %, substantially exceeding the value achieved previously with a bis‐heteroleptic dye (0.75 %). As shown by electrochemical measurements and DFT calculations, the improved PCEs stem from the synthetically tuned electronic structure, which affords more negative excited state redox potentials and favorable electron injection into the TiO₂ conduction band. Electron injection was quantified by nanosecond transient absorption spectroscopy, which revealed that the highest injection yield is achieved with the dye that acts as the strongest photoreductant.     

Enhanced Charge Separation Efficiency in Pyridine‐Anchored Phthalocyanine‐Sensitized Solar Cells by Linker Elongation

A series of zinc phthalocyanine sensitizers ( PcS22 – 24 ) having a pyridine anchoring group are designed and synthesized to investigate the structural dependence on performance in dye‐sensitized solar cells. The pyridine‐anchor zinc phthalocyanine sensitizer PcS23 shows 79 % incident‐photon to current‐conversion efficiency (IPCE) and 6.1 % energy conversion efficiency, which are comparable with similar phthalocyanine dyes having a carboxylic acid anchoring group. Based on DFT calculations, the high IPCE is attributed with the mixture of an excited‐state molecular orbital of the sensitizer and the orbitals of TiO₂. Between pyridine and carboxylic acid anchor dyes, opposite trends are observed in the linker‐length dependence of the IPCE. The red‐absorbing PcS23 is applied for co‐sensitization with a carboxyl‐anchor organic dye D131 that has a complementary spectral response. The site‐selective adsorption of PcS23 and D131 on the TiO₂ surface results in a panchromatic photocurrent response for the whole visible‐light region of sun light.     

Molecular design of corrole‐based D‐π‐A sensitizers for dye‐sensitized solar cell applications

First principles calculations based on density functional theory (DFT) have been performed to design a new set of donor‐corrole‐bridge‐acceptor type systems based on the gallium corroles for dye‐sensitized solar cell applications. The design strategy for these systems is based on the benchmark studies done on the experimentally tested aluminum, gallium, and tin metallocorroles. Unfortunately, corrole analogues display poor light to current conversion efficiencies in spite of their desirable photophysical properties. Thus, improving the efficiency of corrole analogues has become a major challenge and ways to identify solutions to this is of outstanding fundamental importance. This study shows the lack of charge directionality toward anchoring group as plausible reason for the poor efficiencies of reported corrole systems, which enabled us to fine‐tune the electronic and optical properties of new D‐π‐A type systems, COR1‐COR4. The molecular geometries, electronic structure, and binding orientation of these systems on TiO₂ surface were investigated using DFT, TD‐DFT, and PBC methods. When compared with the reported corroles, COR1‐COR4 have a smaller band gaps, red‐shifted absorption spectra with higher extinction coefficients (10⁵ M^?1 cm^?1) and improved nonlinear optical properties. Importantly, results revealed that these dyes bind with two‐arm mode to TiO₂ surface and the density of states of the dye@TiO₂ elucidate strong coupling between the dyes and TiO₂ surface. We anticipate that the unique photophysical properties of these sensitizers will trigger the experimental efforts to yield a new generation of sensitizers based on corrole macrocyle. © 2015 Wiley Periodicals, Inc.     

Tailoring Excited State Properties and Energy Levels Arrangement via Subtle Structural Design on D‐π‐A Materials

The donor‐π‐conjugated‐acceptor (D‐π‐A) structure is an important design for the luminescent materials because of its diversity in the selections of donor, π‐bridge and acceptor groups. Herein, we demonstrate two examples of D‐π‐A structures capable to finely modulate the excited state properties and arrangement of energy levels, TPA‐AN‐BP and CZP‐AN‐BP , which possess the same acceptor and π‐bridge but different donor. The investigation of their photophysical properties and DFT calculation revealed that the D‐π‐A structure with proper donor, π‐bridge and acceptor can result in separation of frontier molecular orbitals on the corresponding donor and acceptor with an obvious overlap on the π‐bridge, resulting in a hybridized local and charge‐transfer (HLCT ) excited state with high photoluminescent (PL ) efficiencies. Meanwhile, their singlet and triplet states are arranged on corresponding moieties with large energy gap between T₂ and T₁ , and a small energy gap between S₁ and T₂ , which favor the reverse intersystem crossing (RISC ) from high‐lying triplet levels to singlet levels. As a result, the sky‐blue emission non‐doped OLED based on the TPA‐AN‐BP reached maximum external quantum efficiency (EQE ) of 4.39% and a high exciton utilization efficiency (EUE ) of 77%. This study demonstrates a new strategy to construct highly efficient OLED materials.     

High‐Performance Dipolar Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety in the π‐Conjugation Framework for Dye‐Sensitized Solar Cells

We report here the synthesis and electrochemical and photophysical properties of a series of easily prepared dipolar organic dyes and their application in dye‐sensitized solar cells (DSSCs). For the six organic dyes, the molecular structures comprised a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron‐deficient diphenylquinoxaline moiety integrated in the π‐conjugated spacer between the electron donor and acceptor moieties. The incorporation of the electron‐deficient diphenylquinoxaline moiety effectively reduces the energy gap of the dyes and broadly extends the spectral coverage. DSSCs based on dye 6 produced the best overall cell performance of 7.35 %, which translates to approximately 79 % of the intrinsic efficiency of the DSSCs based on the standard N719 dye under identical experimental conditions. The high performance of DSSCs based on dye 6 among the six dyes explored is attributed to the combined effects of high dye loading on a TiO₂ surface, rapid dye regeneration, and effective retardation of charge recombination.     

Molecular Design of D‐π‐A Type II Organic Sensitizers for Dye Sensitized Solar Cells

Four new type II organic dyes with D‐π‐A structure (donor‐π‐conjugated‐acceptor) and two typical type II sensitizers based on catechol as reference dyes are synthesized and applied in dye sensitized solar cells (DSCs). The four dyes can be adsorbed on TiO₂ through hydroxyl group directly. Electron injection can occur not only through the anchoring group (hydroxyl group) but also through the electron‐withdrawing group (? CN) located close to the semiconductor surface. Experimental results show that the type II sensitizers with a D‐π‐A system obviously outperform the typical type II sensitizers providing much higher conversion efficiency due to the strong electronic push‐pull effect. Among these dyes, LS223 gives the best solar energy conversion efficiency of 3.6%, with J_sc=7.3 mA·cm^?2, V_oc=0.69 V, FF=0.71, the maximum IPCE value reaches 74.9%.     

Dipolar Compounds Containing Fluorene and a Heteroaromatic Ring as the Conjugating Bridge for High‐Performance Dye‐Sensitized Solar Cells

A novel series of dipolar organic dyes containing diarylamine as the electron donor, 2‐cyanoacrylic acid as the electron acceptor, and fluorene and a heteroaromatic ring as the conjugating bridge have been developed and characterized. These metal‐free dyes exhibited very high molar extinction coefficients in the electronic absorption spectra and have been successfully fabricated as efficient nanocrystalline TiO₂ dye‐sensitized solar cells (DSSCs). The solar‐energy‐to‐electricity conversion efficiencies of DSSCs ranged from 4.92 to 6.88 %, which reached 68–96 % of a standard device of N719 fabricated and measured under the same conditions. With a TiO₂ film thickness of 6 μm, DSSCs based on these dyes had photocurrents surpassing that of the N719‐based device. DFT computation results on these dyes also provide detailed structural information in connection with their high cell performance.     

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2′‐hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor–acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b , which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π‐stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene‐containing dye 4 , for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone‐based compounds 1 b , 2 b , and 3 b showed no evidence of degradation as solutions in CH₂Cl₂ that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ_f=45 000 M ^?1 cm^?1) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid‐state NIR fluorophores for applications in materials science.     

Dye‐Sensitized Solar Cells Based on (Donor‐π‐Acceptor)2 Dyes With Dithiafulvalene as the Donor

Dipolar metal‐free sensitizers (D‐π‐A; D=donor, π=conjugated bridge, A=acceptor) consisting of a dithiafulvalene (DTF) unit as the electron donor, a benzene, thiophene, or fluorene moiety as the conjugated spacer, and 2‐cyanoacrylic acid as the electron acceptor have been synthesized. Dimeric congeners of these dyes, (D‐π‐A)₂, were also synthesized through iodine‐induced dimerization of an appropriate DTF‐containing segment. Dye‐sensitized solar cells (DSSCs) with the new dyes as the sensitizers have cell efficiencies that range from 2.11 to 5.24 %. In addition to better light harvesting, more effective suppression of the dark current than the D‐π‐A dyes is possible with the (D‐π‐A)₂ dyes.     

Modulating the Ground‐ and Excited‐State Oxidation Potentials of Diaminonaphthalene by Sequential N‐Methylation

A series of 1,5‐diaminonaphthalene derivatives were synthesized and characterized to provide ground‐ and excited‐state electron donors of similar structure but varying potential. Electrochemical and spectroscopic properties of the series are reported and together illustrate two opposing consequences of alkyl substitution on the aryl amines. Inductive effects of methylation are evident from the decrease in ground‐state oxidation potential for derivatives containing monomethylamino substituents. In contrast, steric effects seem to dominate the increase in the ground‐state oxidation potential of derivatives containing dimethylamino substituents since the conformational constraints created by dimethylation suppress delocalization of the nonbonding electrons. Absorption and emission properties also respond to increasing levels of N‐methylation, and the excited‐state oxidation potentials of the parent 1,5‐diaminonaphthalene and its monomethylamine derivatives (ca. ?3.2 V) are approximately 200 mV lower than the corresponding dimethylamino derivatives (?3.0 V).     

Theoretical studies on triaryamine‐based p‐type D‐D‐π‐A sensitizer

Development of triaryamine‐based nonmetallic dye sensitizers is a hot topic in the solar cell research. A series of triaryamine‐based dyes WS1 – WS7 were designed with W1 as the prototype. Density functional theory (DFT) and time‐dependent‐DFT calculations were used to investigate the effects of the attached donor D on the absorption spectra and electronic properties of the dyes. The light‐harvesting efficiency (LHE), hole injection force (ΔG_inj), dye regeneration force (ΔG_reg), and charge recombination force (ΔG_CR) for all the dyes were predicted. The insertion of D not only results in a red shift in the absorption spectra for all dyes but also achieves a broader absorption for visible light. Compared with that of the prototype, the absorption peak of the dye WS7 has a red shift of 95 nm and an oscillator strength increase of 29%. The absorption peak of WS7 is wider and stronger, and the absorption range extends to 900 nm. The LHE and ΔG_reg values of WS7 are 0.991 and ?1.49 eV, respectively. On overall evaluation, WS7 is a promising candidate of a p‐type dye sensitizer with good light absorption and dye regeneration efficiency.     

Discovery of Novel D‐A and D‐π‐A Structured Carbazole‐Based Formazan Dyes: Synthesis,Characterization, and Spectroscopic Studies

In the current study, new carbazole‐based formazan dyes, D‐A and D‐π‐A, were synthesized, and their spectroscopic properties were studied for the first time. For this aim, carbazole aldehyde compounds were modified by the derivatization of carbazole, a natural electron‐donor compound, from 3‐ and 9‐position. Then, hydrazone derivatives were synthesized from these aldehyde derivatives. Finally, D‐A ( 5A–C ) and D‐π‐A ( 6A–C ) carbazole–formazan dyes were obtained by the interaction of the hydrazone compounds with p‐substituted aniline. After characterization of the structures of these compounds, photophysical properties of the carbazole–formazans were studied in the different polarity media (i.e., acetonitrile, toluene, and chloroform) in order to detect the solvent effects. Because of the π‐conjugated bridge and the electron acceptor nitro group at the para position, D‐π‐A structured carbazole–formazan dye 6C showed the highest Stokes shift of 155 nm.