Improvement of the efficiency of thiophene-bridged compounds for dye-sensitized solar cells

Theoretical, structural, and electronic analyses of pyridin-based dyes for dye-sensitized solar cells applications

CONTEXT

The new series of donor-π-acceptor dyes have been designed using pyridine derivatives as a donor group and thienothiophene as a π-spacer group, which were linked via 10 acceptor groups. The highest occupied molecular orbital energies range from - 6.177 to - 5.786 eV, whereas the lowest unoccupied molecular orbital energies range from - 2.181 to - 3.664 eV. A6 dye has smaller energy gap, lower hardness, higher electrophilicity index, and good photovoltaic performance than other sensitizers. The lowest dihedral angle is observed in A1, A2, A6, A7, and A8 which are appropriate for intramolecular charge transfer between the molecules. The A8 has higher light harvesting efficiency, which increases the photovoltaic efficiency of the designed dye. The A6, A7, and A8 dyes spend less time in the excited state, which means they emit photons more efficiently than other dyes. The interaction between donor to π-spacer (red line) parts of the dyes has the bonding interaction (positive), and π-spacer to acceptor (blue line) parts of the dyes have the bonding and antibonding (negative) behaviours. The dyes A5 and A9 have 305.79 and 357.71 times higher β0 values than urea (0.781 × 10-30 esu) molecules. The spectral properties of the A6 dye strongly affect the structural modification.

METHODS

The density functional theory (DFT) and time-dependent DFT (TD-DFT) approach B3LYP/6-311G (d,p) basic set were used to optimize the designed dyes. All the calculations are performed using Gauss view 6.0 and Gaussian 09 software. The density of state spectrum is plotted using Gauss sum 2.6.

Exploration of charge transfer analysis and photovoltaics properties of A-D-A type non-fullerene phenazine based molecules to enhance the organic solar cell properties

To intensify the photovoltaic properties of organic solar cells, density functional theory (DFT) based computational techniques were implemented on six non-fullerene A-D-A type small molecules (N1-N6) modified from reference molecule (R) which consists of phenazine fused with 1,4- Dimethyl-4H-3,7-dithia-4-aza- cyclopenta [α] pentalene on both sides with one of its phenyl rings acting as the central donor unit, further attached with 2-(5,6-Difluoro-2-methylene-3-oxo-indan-1-ylidene)-malononitrile acceptor groups at terminal sites. All proposed compounds have a phenazine base modified with a variety of substituents at the terminals. Transition density matrix, density of states, frontier molecular orbitals, intramolecular charge transfer abilities and optoelectronic properties of these compounds were investigated using B3LYP/6-31G (d, p) and B3LYP/6-31G++ (d,p) level of theory. All six designed compounds exhibited a bathochromic sift in their λmax as compared to the R molecule. All designed molecules also have reduced band gap and smaller excitation energy than R. Among all, N6 exhibited highest λmax and lowest bandgap as compared to reference molecule indicating its promising photovoltaic properties. Decreased hole and electron reorganization energy in several of the suggested compounds is indicative of greater charge mobility in them. PTB7-Th donor was employed to calculate open circuit voltage of all investigated molecules. N1-N5 molecules had improved optoelectronic properties, significant probable power conversion efficiency as evident from their absorption aspects, high values of Voc, and fill factor, compared to R molecule. Designed A-D-A type NF based molecules make OSCs ideal for use in wearable devices, building-integrated photovoltaics and smart fabrics.

The impact of aromatic π-spacers and internal acceptors in triphenylamine dyes for DSSCs: A DFT approach

In present work, the influence of internal acceptors and π-spacers on optoelectronic properties for dye-sensitized solar cells (DSSCs) is investigated. The dyes consist of various internal acceptors (A), a triphenylamine donor and π-spacers combined with cyanoacrylic acid acceptor. Density functional theory (DFT) was employed to inspect the dye geometries, charge transport characteristics and electronic excitations. The frontier molecular orbitals (FMOs), highest occupied molecular orbital, lowest unoccupied molecular orbital and HOMO-LUMO energy gap are assisted in the determination of suitable energy levels for electron transfer, electron injection and regeneration of dye. The required photovoltaic parameters like J_SC, ΔG_reg, ΔG_inj, LHE and other associated parameters are presented. The results demonstrate that altering the π-bridge and adding an internal acceptor to the D-π-A scaffold changes the photovoltaic properties and absorption energies. Therefore, the key objective of the current effort is to launch a theoretical groundwork for suitable operational alterations and scheme in creating successful DSSCs.

New insights into the alkoxy effects on auxiliary adsorption and inhibiting charge recombination in dye-sensitized solar cells with high open circuit voltage: a theoretical investigation

Although introducing an alkoxy group is one of the most popular methods to suppress the interfacial charge recombination process of dye-sensitized solar cells, understanding of its effects is still limited and a microscopic picture of the alkoxy effects is lacking. Two ullazine dyes with distinct alkoxy chains at the donor part are used to investigate the effects of the alkoxy group on the adsorption, dye aggregation and charge recombination process in our study. Different from the usual assumption, we find that alkoxy chains can not only play a shielding role, but can also assist dye adsorption and inhibit the charge recombination process more effectively by covering the TiO₂ surface. We also find that the existence of alkyl chains can well inhibit the aggregation of dyes and reduce intermolecular electron transfer. Furthermore, an important structural feature at the interface, the Ti-O interaction between the oxygen atom of the alkoxy group and the Ti atom of the surface is also found to contribute substantially to the interface stability. New insights into the effects of the alkoxy group on auxiliary adsorption and inhibiting charge recombination through reducing the recombination sites pave the way for rational design of sensitizers with high performance.

DFT Studies on a Metal Oxide@Graphene-Decorated D-π1-π2-A Novel Multi-Junction Light-Harvesting System for Efficient Dye-Sensitized Solar Cell Applications

Graphene nanocomposites have emerged as potential photoanode materials for increased performance of the dye-sensitized solar cells (DSSCs) via charge transfer. Various metal-oxide-decorated graphene nanocomposites have widespread applications in energy devices, such as solar cells, fuel cells, batteries, sensors, electrocatalysis, and photocatalysis. However, the possible role of these composites in DSSC applications has largely remained unexplored. Herein, we studied a Sb₂O₃-decorated graphene-D-π₁-π₂-A sensitized TiO₂ nanocomposite (dye-(TiO₂)₉/Sb₂O₃@GO) as a model multi-junction light-harvesting system and examined the impact of various π-bridges on the optical and photovoltaic properties of the push-pull dye system employed in this light-harvesting system. We have shown that by changing the spacer unit, the light sensitivity of nanocomposites can be varied from visible to near-infrared wavelengths. Furthermore, with the integration of metal-oxide-decorated graphene nanocomposites on D-π₁-π₂-A systems and D-π-A systems, composite photoelectrodes displayed better optical and photovoltaic characteristics with an enhanced absorption spectrum in the wavelength range of 800-1000 nm. The performance of the D-π₁-π₂-A system has been evaluated in terms of various photovoltaic parameters such as the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, excited-state oxidation potential (E _dye ^*), free energy of electron injection (G _inject), total reorganization energy (λ_total), and open-circuit voltage (V _oc). This work throws light on the current trends and the future opportunities in graphene-metal oxide nanocomposite-based DSSCs for better harvesting of the solar spectrum and better performance of solar devices.

Molecular engineering on D-π-A organic dyes with flavone-based different acceptors for highly efficient dye-sensitized solar cells using experimental and computational study

CONTEXT

The improvement of new organic flavone-based donor-spacer-acceptor (D-π-A) type dye molecules of the 3-(4-hydroxypiperidin-2-yloxy)-7-hydroxy-2-(3,4-dihydroxyphenyl)-4H-chromen-4-one (D1), 7-hydroxy-2-(3,4-dihydroxyphenyl)-3-(piperidin-4-yloxy)-4H-chromen-4-one (D2), and 3-((2-aminopyridin-4-yloxy)methoxy)-7-hydroxy-2-(3,4-dihydroxyphenyl)-4H-chromen-4-one (D3) were successfully designed and synthesized for dye-sensitized solar cells (DSSCs).

METHODS

Here, we discuss the synthesis of flavone compounds as well as their photophysical and electrochemical characterization. Using the Gaussian 09w software, the electronic structures and apsorption spectra have been calculated at the B3LYP, B3PW91, CAM-B3LYP, MPW1PW91, PBEPBE, and ωB97XD theory with the 6-311G(d,p) basis sets.

RESULTS

The computed values of the D2 molecule ground state optimized HOMOs-LUMOs energy is well positioned for advantageous charge transfer (CT) into the semiconducting material (TiO₂) as well as the electron injection process. With a high power conversion efficiency (PCE) of 3.46% (V_OC = 0.718 V, J_SC = 7.07 mA cm^-2, and FF = 0.68), the D2 compound also demonstrated good photovoltaic (PV) properties.

CONCLUSION

These findings unequivocally demonstrate that altering the D-π-A metal-free organic material electron-withdrawing capacity is a useful strategy for enhancing the optical and electrical characteristics of the organic PV system.

Advances in the Chemistry of 2,4,6-Tri(thiophen-2-yl)-1,3,5-triazine

Heterocyclic systems are now considered to be an integral part of material chemistry. Thiophene, selenophene, furan, pyrrole, carbazole, triazine and others are some such examples worth mentioning. 2,4,6-Tri(thiophen-2-yl)-1,3,5-triazine is a C3h -symmetric system with thiophene as the donor unit and s-triazine as the acceptor unit. This review gives an insight into the advances made in the thienyl-triazine chemistry over the past two to three decades. The synthetic pathways for arriving at this system and all its important derivatives are provided. The major focus is on the materials synthesized using the thienyl-triazine system, including star molecules, linear and hyperbranched polymers, porous materials and their diverse applications. This review will play a catalytic role for new dimensions to be explored in thienyl-triazine chemistry.

Efficient model photosensitizers based on metallocenyl complexes with thiophene-N = N-pyrimidine as π-conjugated bridge and cyanoacrylate as an anchoring group: a density functional theory study

Eight push-pull systems involving containing four transition metals (iron, ruthenium, cobalt, and nickel), metallocenes as donor groups, cyanoacrylate as electron attractor group, and thiophene-N = N- pyrimidine derivatives as π-conjugated bridges were designed and studied using DFT and TD-DFT methods involving B3LYP and CAM-B3LYP functionals combined with the cc-pVDZ/LANL2DZ basis sets. The main purpose of this work is to determine the effect of metallocene in improving the photosensitization property of such chromophores. This was done by calculating their light-harvesting efficiency LHE as well as other properties employed for DSSC application. The considered dyes were first studied in the gas phase, then in the presence of TiO₂ nanoparticles representing the semi-conductor, and finally in the presence of a specific implicit solvent. The presence of iron as metal involved in the metallocene group supplemented by extending the π-conjugated bridge by a cyanovinyl spacer was demonstrated so as to give the most optimal response taking into account the lower cost and toxicity as well as the friendliness to the environment of iron as metal.

A Theoretical Evaluation of the Efficiencies of Metal-Free 1,3,4-Oxadiazole Dye-Sensitized Solar Cells: Insights from Electron–Hole Separation Distance Analysis

Herein, some novel metal-free 1,3,4-oxadiazole compounds O1–O7 were evaluated for their photovoltaic properties using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to determine if they can serve as metal-free organic dyes in the use of dye-sensitized solar cells (DSSCs). To understand the trends in the relative efficiencies of the investigated compounds as dyes in DSSCs, their electron contributions, hole contributions, and electron–hole overlaps for each respective atom and fragment within the molecule were analyzed with a particular focus on the electron densities on the anchoring segments. As transition density matrices (TDM) provide details about the departure of each electron from its corresponding hole during excitations, which results in charge transfer (CT), the charge separation distance (Δr) between the electron and its corresponding hole was studied, in addition to the degree of electron–hole overlap (Λ). The latter, single-point excitation energy of each electron, the percentage electron contribution to the anchoring segments of each compound, the incident-photon-conversion-efficiency (IPCE), charge recombination, light harvesting efficiency (LHE), electron injection (Φinj), and charge collection efficiency (ncollect) were then compared to Δr to determine whether the expected relationships hold. Moreover, parameters such as diffusion constant (Dπ) and electron lifetime (t), amongst others, were also used to describe electron excitation processes. Since IPCE is the key parameter in determining the efficiency, O3 was found to be the best dye due to its highest value.

Role of the Backbone when Optimizing Functional Groups─A Theoretical Study Based on an Improved Inverse-Design Approach

We present an improved inverse-design approach for automatically identifying molecular (or other) systems with optimal values for prechosen properties. The new approach uses SMILES (simplified molecular input line entry system) to describe molecular structures efficiently, a genetic algorithm to optimize the molecules automatically, and the DFTB+ (self-consistent charge density functional tight-binding) method to calculate electronic properties. Thereby, almost every class of materials─even macromolecules or monomers─can be studied easily. Without crossover operators but with only mutation operators, the genetic algorithm is more adaptive to SMILES while keeping its efficiency. DFTB+ is more accurate than the DFTB method used in our previous inverse-design approach for the study of excited states and charge transfer processes. The improved approach is applied to optimize benzene, pyridine, pyridazine, pyrimidine, and pyrazine derivatives for seven electronic properties, which all are highly relevant and important for the performance of molecules in solar cells. We found that for some electronic properties, the precise composition and structure of the backbone have remarkable impacts on the value of the electronic properties and/or on the set of functional groups that leads to the best performance. On the contrary, for other properties, these effects are less pronounced. The reasonable optimal functional groups and/or substitution patterns are reported.

Tailoring benzo[α]phenoxazine moiety for efficient photosensitizers in dye sensitized solar cells via the DFT/TD-DFT method

Three benzo[α]phenoxazine-based dyes were designed by tailoring donor (D) and anchoring (A) moiety to benzo[α]phenoxazinetemplate via DFT and TD-DFT method for dye-sensitized solar cell (DSSC) applications.

Tuning the donating strength of dye sensitizers using molecular electrostatic potential analysis

The significance of electron releasing groups at the donors of D–π–A systems for improving the donating strength and power conversion efficiency of photosensitizers.

Aromaticity-Photovoltaic Property Relationship of Triphenylamine-Based D-π-A Dyes: Leads from DFT Calculations

D-π-A-based dyes find a wide range of applications in molecular electronics and photovoltaics in general and dye-sensitized solar cells (DSSC) in particular. We speculated whether there exists a relationship between the degree of aromaticity of the π-spacers used in the D-π-A type dyes and their structural, electronic, energetic, photophysical, and intramolecular charge transfer properties. Triphenylamine (TPA) and cyanoacrylic acid (CAA) have been chosen as the donor and acceptor, respectively. In order to carry out the investigation systematically the π-spacers have been logically chosen based on their experimental resonance energies, which follows the order, furan < pyrrole < thiophene < pyridine < benzene. All the properties have been discussed based on the degree of aromaticity of the π-spacers. Geometric properties such as dihedral angles and bond lengths have been discussed extensively. Energy levels of the frontier molecular orbitals, electrochemical properties, namely, ground and excited state oxidation potentials (GSOP/ESOP), and change in Gibbs free energy for electron injection and regeneration (ΔG_inj/ΔG_reg) have also been evaluated. Photophysical properties like wavelength of maximum absorption (λ_max), oscillator strength (f), light harvesting efficiency (LHE), and intramolecular charge transfer properties, viz., charge transfer distance (D_CT), fraction of charge transferred (q_CT), and change in dipole moment (μ_CT) have been assessed. The adsorption characteristics of dye with (TiO₂)₉ nanocluster have been studied along with their optical properties. Results reveal that the nature of the relationship between the aforementioned properties and the extent of aromaticity of the π-spacers is inherently multifaceted. It thus turns out that it is highly difficult to quantify the relationship. These properties of D-π₁-π₂-A molecules can be regarded to be arising from two groups, namely, π-spacers with lower and higher resonance energies. This results in a natural trade-off in selection of competing properties. The qualitative aromaticity photovoltaic property relationship thus obtained may serve as a guide to tailor-design various properties of D-π-A type dyes for application in the intramolecular charge transfer devices.

POM-based dyes featuring rigidified bithiophene π linkers: potential high-efficiency dyes for dye-sensitized solar cells

A series of POM-based dyes with a triphenylamine electron donor group, cyanoacrylic acid electron acceptor group and different π linkers of thiophene derivatives were systematically investigated to analyze the influence of a rigidified bithiophene with fastening atoms (C and N) on the performance of dye-sensitized solar cells (DSSCs) based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.

Computational Investigation of Tuning the Electron-Donating Ability in Metal-Free Organic Dyes Featuring an Azobenzene Spacer for Dye-Sensitized Solar Cells

A series of donor⁻π-conjugated spacer⁻acceptor (D⁻π⁻A) organic dyes featuring an azobenzene spacer were designed as chromic dyes and investigated computationally. The electron-donating strength was modified by introducing electron-donating units to the donor side. In particular, the trans⁻cis isomerization of the azobenzene-based dyes and its effect on the optical and electronic properties were further scrutinized. In both trans and cis conformers, a gradual increase in electron-donating strength promoted the natural charge separation between donor and acceptor moieties, thereby allowing the absorption of a longer wavelength of visible light. Importantly, the conformational change of the azobenzene bridge resulted in different absorption spectra and light-harvesting properties. The azobenzene-based dyes will open up a new research path for chromic dye-sensitized solar cells.

Design, Electron Transfer Process, and Opto-Electronic Property of Solar Cell Using Triphenylamine-Based D-π-A Architectures

A series of D-π-A type dyes were designed based on the experimentally synthesized A1 by introducing different functional groups on the donor and π-spacer, and the optical and electrical properties were calculated by using density functional theory (DFT) and time-dependent DFT (TD-DFT). P1⁻P6 present highest light harvesting efficiency (LHE), driving force of electron injection ( Δ G i n j e c t ), reorganization energy ( Δ G r e g ) and e V O C . These critical parameters have a close relationship with the short-circuit current density ( J S C ) and open-circuit photovoltage ( V O C ), and lead to P1⁻P6 will exhibit higher efficiency. D4 also exhibit superior properties in the driving force of electron injection ( Δ G i n j e c t ), reorganization energy ( Δ G r e g ), which will lead to a higher short-circuit current density ( J S C ). We hope that these results will be helpful for experiments to synthesize new and highly efficient dyes.

Theoretical investigation of auxiliary electronic acceptors in modifying D-D-π-A sensitizers for dye-sensitized solar cells

In light of the performance of the SD2 pigments in DSSC, in order to expand the absorption spectral scope, decrease the energy difference between the highest occupied and the lowest unoccupied molecular orbitals, with SD2 dye molecular electron donor and electron acceptor as the fundamental framework, the indole fragment and thiophene derivative in the prototype dye molecule were replaced by the two π-bridges (labeled PA, PB, respectively) and the four auxiliary electron acceptors (labeled A1, A2, A3, A4, respectively). For the sake of characterizing dye molecules as thoroughly as possible in DSSC, the frontier orbital energy levels, ultraviolet absorption spectra, natural bond orbital analysis, intramolecular charge transfer, charge and hole reorganization energies, parameters influencing the short-circuit current density and the open-circuit photovoltage for these eight individual dye molecules are carried out to try to fully characterize the properties of these dye molecules. According to these computational results of physical quantities and based on the performance of these dye molecules in the above aspects, in this paper, six free molecular models were picked out to combine with titanium dioxide cluster to calculate their geometrical structures, frontier orbital distributions, electron excitation energies, ultraviolet absorption spectra and the composition of the electronic transitions in chloroform solvent with polarizable continuum model. The results of these calculations show that the PA-A2 and PB-A4 dye molecule has better properties in electron transfer and spectral absorption range before and after the adsorption on the titanium dioxide.

Theoretical design of metal-phthalocyanine dye-sensitized solar cells with improved efficiency

The present work carried out a theoretical study of the electronic structures, absorption spectra, and photovoltaic performance of two series of transition metal-phthalocyanine derived from nonperipheral electron-donating substituents, either (2-phenyl) phenoxy(M-PC1) or quinoleinoxy(M-PC2). The DFT and TD-DFT were employed for this study. The effect of modifying the central metal atoms and the substitution on cell performance were investigated in terms of polarizability (α), hyper-polarizability (β), chemical potential (μ), chemical hardness (η), electrophilicity power (ω), FMOs, energy gaps, UV/vis absorption spectra and injected driving force (ΔG^inject), light harvesting efficiencies (LHE), total reorganization energy (λ_tot), open circuit photovoltage (V_oc), and life time of the excited state (τ). The results obtained by using these parameters showed that the replacement of (2-phenyl) phenoxy by a proposed substituent such as quinoleinoxy would increase the hyper-polarizability, light harvesting efficiency, and open circuit photovoltage, while on the other hand the reorganization energy and the injection driving force are decreased. Modifying central metal atoms, such as Zn, Cd, Pd, and Pt, exhibited good performance in terms of the driving force of electron injection, charge transfer characteristics, and dye reorganization as compared with the Cu reference dye. The findings provided a useful prediction and perspective for the promising future for high-efficiency dye-sensitized solar cells (DSSCs) with dyes based on phthalocyanine. Graphical abstract Photovoltaic performance of Metallo-phthalocyanine contening (2-phenyl) phenoxy and quinoleinoxy.

Physical Insight on Mechanism of Photoinduced Charge Transfer in Multipolar Photoactive Molecules

Two series of novel dyes were designed based on the multipolar structures of the red dye D35 and blue dye DB, by introducing the furan (F), benzene ring (B) and benzo[c]thiophene (BT) groups into the conjugated bridge of D35 in proper order and adjusting the position of diketopyrrolopyrrole(DPP) unit and the incorporation of fluorine in the conjugated bridge of DB, respectively. We performed the quantum chemistry calculation to investigate the ground state and excited properties in a direct correlation with the spectra properties and abilities of losing or accepting electron for the original and designed molecules. Furthermore, the absorption spectra characteristics in consideration of the aggregation of dyes on the TiO₂ layer and intermolecular charge transfer rate of the dimers were calculated. The obtained results indicate that the larger intermolecular charge transfer rate leads to the poor photoelectrical properties of the dyes, and the designed dyes D35-3 and DB-2 would exhibit the best photoelectrical properties among the investigated dyes due to their lower energy gaps, widened absorption spectra and prominent charge transfer properties.

Design of butterfly type organic dye sensitizers with double electron donors: The first principle study

In this work, we designed a series of butterfly type organic dyes, named ME07-ME13 by introducing such as triphenylamine, phenothiazine, coumarin groups etc. as electron donors and further investigated their absorption spectra using density functional theory (DFT) and time-dependent DFT (TDDFT). All designed dyes cover the entire visible absorption spectrum from 300 to 800nm. It's fascinating that ME13 molecule has two absorption peak and the molar coefficient of two absorption peaks are above 4.645×10⁴M^-1·cm^-1. The light absorption area of ME13 exhibits an increment of 16.5-19.1% compared to ME07-ME12. Furthermore, we performed a detailed analysis on their geometrical and electronic properties, including molecular structures, energy levels, light harvesting efficiency (LHE), driving force (ΔG_inject), regeneration (ΔG_regen),electron dipole moments (μ_normal), intermolecular electron transfer and dye/(TiO₂)₃₈ system electron transitions. The results of calculation reveal that double coumarin donors in ME13 are promising functional groups for butterfly type organic dye sensitizers. It is expected that the design of double donors can provide a new strategy and guidance for the investigation in high efficiency dye-sensitized devices.

Molecular design towards suppressing electron recombination and enhancing the light-absorbing ability of dyes for use in sensitized solar cells: a theoretical investigation

We report a molecular design strategy with a comprehensive consideration of both inhibiting electron recombination and enhancing the light-harvesting ability.

Tuning the physical properties of organic sensitizers by replacing triphenylamine with new donors for dye sensitized solar cells - a theoretical approach

New donor molecules with low ionization potential have been theoretically designed by replacing the benzene moieties in triphenylamine (TPA) with thiophene as well as furan. The designed new donors also exhibited planar structure, making an angle of 120° around the nitrogen atom that results in resonance effects through π-bonds of aryl rings. New sensitizers have been theoretically studied using DFT and TD-DFT by adopting these designed donors. UV-Vis absorption spectra, light harvesting ability (LHE) and electron injection ability (ΔG_inject) of the designed sensitizers have been calculated by taking L0 as reference. Orbital overlapping between donor and acceptor facilitates intra-molecular charge transfer, thereby increasing the interfacial electron injection from the sensitizer to the semiconductor nanoparticles. Our theoretical results demonstrate that sensitizers DTPA-AA and DFPA-AA have broader absorption and lower ΔG_inject respectively compare to L0, this opens a new way for designing sensitizers for dye sensitized solar cells (DSSCs). All the dyes designed were found to be good light harvesters.

Screening and design of high-performance indoline-based dyes for DSSCs

The photophysical properties of three D–A–π–A dyes (WS-2, WS-92 and WS-95) were calculated based on DFT and TDDFT.

Different Effect of the Additional Electron-Withdrawing Cyano Group in Different Conjugation Bridge: The Adjusted Molecular Energy Levels and Largely Improved Photovoltaic Performance

Four organic sensitizers (LI-68-LI-71) bearing various conjugated bridges were designed and synthesized, in which the only difference between LI-68 and LI-69 (or LI-70 and LI-71) was the absence/presence of the CN group as the auxiliary electron acceptor. Interestingly, compared to the reference dye of LI-68, LI-69 bearing the additional CN group exhibited the bad performance with the decreased Jsc and Voc values. However, once one thiophene moiety near the anchor group was replaced by pyrrole with the electron-rich property, the resultant LI-71 exhibited a photoelectric conversion efficiency increase by about 3 folds from 2.75% (LI-69) to 7.95% (LI-71), displaying the synergistic effect of the two moieties (CN and pyrrole). Computational analysis disclosed that pyrrole as the auxiliary electron donor (D') in the conjugated bridge can compensate for the lower negative charge in the electron acceptor, which was caused by the CN group as the electron trap, leading to the more efficient electron injection and better photovoltaic performance.

Does the position of the electron-donating nitrogen atom in the ring system influence the efficiency of a dye-sensitized solar cell? A computational study

We have reported a number of new metal-free organic dyes (2-6) that have cyclic asymmetric benzotripyrrole derivatives as donor groups with peripheral nitrogen atoms in the ring, fluorine and thiophene groups as π-spacers, and a cyanoacrylic acid acceptor group. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were employed to examine the influence of the position of the donor nitrogen atom and π-conjugation on solar cell performance. The calculated electron-injection driving force (ΔG inject), electron-regeneration driving force (ΔG regen), light-harvesting efficiency (LHE), dipole moment (μ normal), and number of electrons transferred (∆q) indicate that dyes 3, 4, and 6 have significantly higher efficiencies than reference dye 1, which exhibits high efficiency. We also extended our comparison to some other reported dyes, 7-9, which have a donor nitrogen atom in the middle of the ring system. The computed results suggest that dye 6 possesses a higher incident photon to current conversion efficiency (IPCE) than reported dyes 7-9. Thus, the use of donor groups with peripheral nitrogen atoms appears to lead to more efficient dyes than those in which the nitrogen atom is present in the middle of the donor ring system. Graphical Abstract The locations of the nitrogen atoms in the donor groups in the designed dye molecules have an important influence on DSSC efficiency.

Can fused-pyrrole rings act as better π-spacer units than fused-thiophene in dye-sensitized solar cells? A computational study

Fused-pyrrole rings can be potential π-spacers in dye-sensitized solar cells.

The electronic structure engineering of organic dye sensitizers for solar cells: The case of JK derivatives

The design and development of novel dye sensitizers are effective method to improve the performance of dye-sensitized solar cells (DSSCs) because dye sensitizers have significant influence on photo-to-current conversion efficiency. In the procedure of dye sensitizer design, it is very important to understand how to tune their electronic structures and related properties through the substitution of electronic donors, acceptors, and conjugated bridges in dye sensitizers. Here, the electronic structures and excited-state properties of organic JK dye sensitizers are calculated by using density functional theory (DFT) and time dependent DFT methods. Based upon the calculated results, we investigated the role of different electronic donors, acceptors, and π-conjugated bridges in the modification of electronic structures, absorption properties, as well as the free energy variations for electron injection and dye regeneration. In terms of the analysis of transition configurations and molecular orbitals, the effective chromophores which are favorable for electron injection in DSSCs are addressed. Meanwhile, considering the absorption spectra and free energy variation, the promising electronic donors, π-conjugated bridges, and acceptors are presented to design dye sensitizers.

Density functional theory study of new azo dyes with different π-spacers for dye-sensitized solar cells

Some of new azo-based metal-free dyes with different π-conjugation spacers, such as carbazole, fluorene, pyrrole, thiophene, furan and thiazole, have been investigated with density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Theoretical calculations allow us to quantify factors such as light harvesting efficiency (LHE), electron injection driving force (ΔG(inject)) and the weight of the LUMO orbital on the carboxylic group (QLUMO) related to the short-circuit photocurrent density (Jsc), and to evaluate both charge recombination between the semiconductor conduction band electrons and the oxidized dyes and/or electrolyte, and also the shift of the conduction band of the semiconductor as a result of the adsorption of the dyes onto the semiconductor surface, associated with the open-circuit photovoltage (Voc). According to the results, we could predict that how the π-conjugation spacers influence the Jsc as well as the Voc of DSSCs. Among these dyes, the carbazole and fluorene-based dyes (dyes 1 and 2) show the highest LHE, ΔG(inject), QLUMO, and the slowest recombination rate. Consequently, the obtained results show that the carbazole and fluorene-based dyes could have the better Jsc and Voc compared to the other dyes.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Can silicon substituted metal-free organic dyes achieve better efficiency compared to silicon free organic dyes? A computational study

DFT and TD-DFT calculations performed using metal free organic dyes containing silicon substituted silole units and/or donor systems exhibit significantly improved optical properties compared to their corresponding silicon free dyes.