Linear correlation between DSSC efficiency, intramolecular charge transfer characteristics, and NLO properties – DFT approach

Computational method on highly efficient D-π-A-π-D-based different molecular acceptors for organic solar cells applications and non-linear optical behaviour

Eight molecular structures (BT-A1 to BT-A8) with high-performance non-fullerene acceptor (NFA) were selected for organic solar cells (OSCs) and non-linear optical (NLO) applications. Their electronic, photovoltaic (PV) and optoelectronic properties were tuned by adding powerful electron-withdrawing groups to the acceptor (A) of the D-π-A-π-D structure. Using time-dependent density functional theory (TD-DFT) techniques, based on the laws of quantum chemical calculations, the absorption spectra, stability of the highest and lowest-energy molecular orbitals (HOMO/LUMOs), electron density, intramolecular charge transfer (ICT), transition density matrix (TDM), were examined. The binding energy (Eb) and density of states (DOS) were probed to realize the optoelectronic analysis of the structures BT-A1 to BT-A8. Noncovalent interactions (NCIs) based on a reduced density gradient (RDG) were used to describe the nature and strength of D-A interactions in the molecules BT-A1 to BT-A8. The new refined molecules BT-A1 to BT-A8 exhibited strong absorbance bands between 408-721 nm and high electron transfer contribution (ETC) ranges between 87-96 %, along with the smallest excitation energies (Ex) between 1.71-3.55 eV in the solvent dichloromethane. Dipolar moment strengths ranging from 0.38 to 4.72 Debye in both the excited and ground states have determined with good solubility properties of BT-A1 to BT-A8 in polar solvent. Highly effective charge mobilities and prevention of charge recombination have been demonstrated by the electron (0.18-0.41 eV) and hole RE values (0.13-0.89 eV) for the new compounds. Power conversion efficiencies (PCE) of BT-A1 to BT-A8 were nearly the same because of better outcomes compared to the molecules in the BT. Compared to poly[4.8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b: 4,5-b']dithiophene-2,6- diyl-alt-(4-2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th), the open circuit voltages (Voc) of compounds BT-A1 to BT-A8 were ranged from 1.52 to 2.13 eV. The polarizability (α) and hyperpolarizability (β) of the molecules BT-A1 to BT-A8 were used to determine the non-linear optical (NLO) properties. The results showed that BT-A2, BT-A6 and BT-A7 have good NLO activity. This computational analysis demonstrates the superiority of the molecules with NFA. Hence the compounds are advised for the use in production of high-performance OSCs and NLO activity.

Exploration of 4-substituted thiophene-based azo dyes for dye-sensitized solar cells and non-linear optical materials: synthesis and an in silico approach

This work examines the effects of changing the secondary donors' donating strengths at the thiophene ring's fourth position in a range of dyes designated SR1 to SR9. The DFT results indicate that the molecular planarity is greatly affected by the placement of the secondary donor at position four, which changes the charge transfer (CT) characteristics in the thiophene-azo-salicylic acid backbone. These results are corroborated by TD-DFT analysis, which indicates that as the secondary donor's donating strength increases, so does the vertical absorption maximum. Based on the computed photovoltaic characteristics, these dyes perform better from SR1 to SR5. Research on dye@TiO2 clusters suggests a potential for binding with TiO2, which might cause the dye@TiO2 clusters' absorbance to shift red. Additionally, computed linear and non-linear optical (NLO) properties exhibit similar trends to those observed for DSSC performance. The experimental results, which include HOMO and LUMO energies as well as initial absorption in dimethylformamide (DMF), match very well with the patterns seen in DFT and TD-DFT calculations. The high thermal stability of SR1 to SR4 is indicated by thermogravimetric analysis (TGA), indicating their practical applicability in non-linear optics (NLO) and DSSC applications. The efficiencies of the produced DSSCs vary; SR4 has the highest efficiency (4.50 ± 0.1), and SR1 has the lowest (0.37 ± 0.1). The combined effects of theoretical and experimental DSSC results demonstrate the vital role of secondary donors in influencing molecular characteristics and NLO and DSSC performance.

Study of the microscopic mechanism of stepwise charge injection in co-sensitive DSSCs in the framework of a D-π-A dye and chlorophyll

The newly synthesized dye molecules TY6 and CXC22 were selected to explain the influence of anthracene and acetylene groups on the power conversion efficiency (PCE) of the molecules at the microscopic level. Theoretical simulation was carried out to understand the properties of the two molecules, including frontier molecular orbitals, absorption spectra, light absorption efficiency, intramolecular charge transfer (ICT), dye regeneration, I-V prediction, etc. The results suggest that for CXC22, adding an anthracene and acetylene group in the conjugate bridge greatly enhances the molecule's absorption wavelength and molar extinction coefficient; CXC22 also has significant advantages in the intramolecular charge transfer and comparatively better dye regeneration and electron injection. These parameters cause CXC22 to have a higher PCE. Subsequently, CXC22 and the chlorophyll molecule (CHL7) were selected for co-sensitization to regulate performance. The stable structure in the co-sensitization configuration was screened, and the absorption spectrum characteristics and charge transfer mechanisms were revealed for the co-sensitization system. The designed evaluation model predicted that the PCE of CO1 (the cosensitive system of CXC22 and TY6 in H-H configuration is referred to as CO1) could reach 16.78%. This work provides an idea for developing an efficient dye-sensitized solar cell system.

In silico strategy to design an efficient organic photoswitch based on excited-state cation transfer

A new class of photoswitches and the corresponding elementary photoinduced reaction, the so-called Excited-State Cation Transfer (ESCT), are investigated. This reaction relies on an intramolecular photo-release/photo-complexation of cation: after irradiation, the cation is translocated from a complexation site 1 to a site 2 during the excited state lifetime. Our purpose is thus to develop a computational strategy based on Density Functional theory (DFT) and its time-dependent counterpart (TD-DFT) to improve the different properties of the ESCT photoswitches, namely (i) the ground state complexation constant K, (ii) the excited state complexation constant K*, (iii) the photoejection properties and (iv) the population of the triplet states from a singlet state via intersystem crossing to increase the lifetime of the excited state. In this work, we are interested in optimizing the ESCT properties of a betaine pyridinium chromophore substituted by a 15-aza-5-crown, that was previously shown to efficiently photoeject a Ca2+ cation from the site 1 but no photo-recapture was observed in the site 2 [Aloïse et al., Phys. Chem. Chem. Phys., 2016, 22, 15384]. To this purpose, we have investigated the impact of the modification of the site 1 on the ESCT properties by introducing different substituents (EDG groups, halogen atoms) at different positions. So far, promising systems have been identified but a simultaneous improvement of all the ESCT photoswitches properties has yet not been achieved.

Theoretical investigation of the effect of changing the auxiliary acceptor on the performance of organic D-A'-A dyes used as sensitizers in DSSCs

CONTEXT

Dye-sensitized solar cells (DSSCs) have displayed huge potential in inexpensive, efficient, and clean solar energy technology. In this work, seven new dyes with the structure D-A'-A were designed in which the thiophene in the reference dye was replaced by auxiliary acceptors (A'). These dyes consist mainly of a pyranylidene-based electron donor D and the cyanoacrylic acid moiety as acceptor A. A computational investigation was carried out on the effect of various auxiliary acceptors A' on the efficiency of D-A'-A dyes in isolation and after binding to the semiconductor TiO2. Optimized structures, geometrical, optoelectronic, and photovoltaic parameters were calculated to predict promising dyes for potential use as solar cell sensitizers, including band gap (Egap), natural bond orbital (NBO) analysis, nonlinear optical properties (NLO), UV-Vis absorption spectra, maximum absorption wavelength (λmax), reorganization energy (λtotal), light-harvesting efficiency (LHE), electron injection driving force (ΔGinject) and open-circuit photovoltage (VOC). The results of this study revealed that all designed dyes, compared to the reference dye, are characterized by small Egap and λtotal values as well as large λmax, in addition to significant NLO properties and large adsorption energy (Eads). Therefore, all studied dyes can be used as sensitizers in DSSC.

METHODS

Using Density Functional Theory (DFT) approaches with the B3LYP functional and the 6-31G(d,p) basis set, all ground state geometries of the isolated dyes were fully optimized. Time-Dependent Density Functional Theory (TD-DFT) method using the CAM-B3LYP/6-31G(d,p)/IEF-PCM level was applied to simulate the UV-visible absorption properties. All isolated dye calculations were performed using the Gaussian 09 software package. DFT calculations have been carried out with the DMol3 package included in Materials Studio for simulating the adsorption of the investigated dyestuff on the TiO2 surface of anatase (101), using the generalized gradient corrected approximation (GGA) approach of the Perdew-Burke-Ernzerhof (PBE) functional with the basic set of digital double polarisation (DNP). To study the optical performance of dye@TiO2 the PBE/DNP method present in DMol3 was applied.

Computational insights into bis-N,N-dimethylaniline based D-π-A photosensitizers bearing divergent-type of π-linkers for DSSCs

In this study, theoretically designed D-π-A derivatives containing different π-subunits as linkers were investigated to enlighten their potential applicability in photovoltaic applications. For this aim, we first focused on clarifying the effect of tailored π-linker scaffolds on the frontier orbital energies of the investigated photosensitizers. In the concomitant step, global descriptors, TiO₂ adsorption, maximum absorbance wavelength, light-harvesting efficiency (LHE), energy conversion efficiency (η), short circuit current density (J_SC), open circuit photovoltage (V_OC), fill factor (FF), and reorganization energy (λ_e, λ_h, λ_T) values, electron density differentiation maps (EDDM), transition density matrices (TDM), fragmental contributions on electron-hole overlap were investigated in detail. Based on the trend of the calculated properties, 2,3-dimethylthieno [3,4-b]pyrazine (D-Ɛ₃-δ_n-A; n = 1-3) and 5-isobutyl-10,11-dimethyl-10,11-dihydro-5H-pyrrolo [3,4-e]thieno [2',3':4,5]pyrrolo [3,2-g]thieno [3,2-b]indole (D-Ɛ₆-δ_n-A; n = 1-3) bearing molecules were identified as the best-suited and improved dye candidates for DSSC applications. Following the prediction of photovoltaic properties for the pristine dye molecules, our consecutive efforts have contributed to a similar calculation protocol comprising DFT and subsequent TD-DFT computations for the D-Ɛ_n-δ_n-A@Ti₅O₁₀ clusters to elucidate the interaction of the investigated photosensitizers with the semiconductor layer (TiO₂).

Designing and theoretical study of benzocarbazole-based D-π-D type small molecules donor for organic solar cells

Seven new molecules (S1-S7) of D-π-D type have been designed for organic photovoltaic applications. The DFT and TD-DFT methods were used to investigate the effect of different central bridge groups on the geometric, optoelectronic, and charge transport properties of the constructed molecules. Among them, S4 and S6 have the lowest energy band gap and a red shift in the absorption spectra, revealing the perfect relationship between the central bridge and the strong electron withdrawal character through extended conjugation. Similarly, S6 explored the lowest reorganization energy (RE) value for electron and hole revealing its enhanced charge transition, also shows better ICT characteristics with its highest NLO properties. Compound S4 showed the smallest value of ΔE_L-L and E_b, and the highest V_oc due to its low HOMO, which improves the photocurrent density of the devices. Thus, the results suggest that bridge modification is a practical strategy to improve the efficiency of OSCs.

Role of anchoring groups on the light harvesting and optoelectronic properties of triphenylamine derivatives: insights from theory

BACKGROUND

In the present work, DFT and time-dependent DFT calculations were performed to investigate the role of anchoring groups on the photophysical properties and reveal structure-property correlations of triphenylamine (TPA) derivatives. The selected anchoring groups are tetrazole, acrylamide, hydantoin, and rhodanine.

RESULTS

Our results show that the different anchoring groups employed alter the planarity, intramolecular charge transfer properties, and HOMO-LUMO gap and hence influence the optoelectronic properties of the dyes. Although all molecules fulfill the basic requirements with suitable energy levels, band gap, absorption, and charge transfer properties, the dye with rhodanine acceptor (TPA4) was the most promising candidate due to its lowest HOMO-LUMO gap, red-shifted highest λ_max absorption value, better ICT pattern, low total reorganization energy, and good electron injection properties. Overall, it is anticipated that the results of this investigation will point to new avenues for the experimental fabrication of remarkably effective metal-free organic dyes for solar cell applications.

New organic dye-sensitized solar cells based on the D-A-π-A structure for efficient DSSCs: DFT/TD-DFT investigations

Global energy consumption has increased due to population growth and economic development. Solar energy is one of the most important renewable energy sources for human consumption. In this research, four novel organic dyes (D2-D5) of the D-A-π-A structure based on triphenylamine (TPA) were studied theoretically using DFT and TD-DFT techniques for future usage as dye-sensitized solar cells (DSSCs). The effects of modifying the π-spacer of the reference molecule D1 on the structural, electronic, photovoltaic, and optical characteristics of the D2-D5 dyes were studied in detail. D2-D5 exhibited band gaps (E gap) in the range from 1.89 to 2.10 eV with λ abs in the range of 508 to 563 nm. The results obtained show that modifying the π-spacer of the dye D1 increased its hole injection and reinforced the intramolecular charge-transfer (ICT) impact, which resulted in a red-shifted ICT absorption with a greater molar extinction coefficient. The theoretically calculated open-circuit voltage (V oc) values ranged from 0.69 to 1.06 eV, while the light-harvesting efficiency (LHE) values varied from 0.95 to 0.99. Indeed, this theoretical research could guide chemists to synthesize effective dyes for DSSCs.

Environmental effects on the singlet fission phenomenon: a model Hamiltonian-based study

In the screening of compounds for singlet fission, the relative energies of the constitutive units are decisive to fulfil the thermodynamic rules. From a model Hamiltonian constructed on the local spin states of an active chromophore and its environment, it is suggested that embedding greatly influences the energy differences of the active monomer spin states. Even in the absence of charge transfer, the field generated by a singlet environment produces an increase of the [E(S₁) - E(S₀)]/[E(T₁) - E(S₀)] critical ratio by up to 6% as compared to the one of a free chromophore. Besides, variations are observed when the intimate electronic structure of the singlet environment is modified. This propensity towards singlet fission is even more pronounced (10%) when the environment is switched to the triplet state. Finally, the embedding is likely to reverse the spin state ordering in the limit of vanishing atomic orbital overlaps. Despite its simplicity, the model stresses the importance of the environment spin nature in the quest for singlet fission candidates, and more generally in spectroscopy analysis.

Novel gamma-irradiated chitosan-doped reduced graphene-CuInS2 composites as counter electrodes for dye-sensitized solar cells

To address the issues associated with traditional counter electrodes, a novel gamma-irradiated chitosan-doped reduced graphene-CuInS₂ composite (Chi@RGO-CIS) was used as the counter electrode (CE). The system was fabricated following a simple hydrothermal method. The prepared Chi@RGO-CIS was characterized by various spectroscopic and microscopic techniques. The synergistic effect between chitosan, CuInS₂, and reduced graphene oxide can help in producing a large surface area. It can also help in the generation of catalytic sites toward I-/I₃-redox electrolytes. We used a composite (based on electrical considerations) to study the effect of the amount of graphene on the characteristics and photovoltaic efficiency of the Chi@RGO-CIS composites. The solar cell assembled with 1.5% Chi@RGO-CIS exhibited an efficiency of 12.21%. The efficiency was higher than that of a Pt-based device (9.96%) fabricated under the same conditions. Hence, Chi@RGO-CIS can be potentially used as the CE of dye-sensitized solar cells (DSSCs). It can be used as a substitute for Pt in DSSCs.

To address the issues associated with traditional counter electrodes, a novel gamma-irradiated chitosan-doped reduced graphene-CuInS₂ composite (Chi@RGO-CIS) was used as the counter electrode (CE).

Synthesis, spectral characterizations, molecular geometries and electronic properties of phenothiazine based organic dyes for dye-sensitized solar cells

A number of organic dye compounds is developed and used as dye-sensitized solar cells in order to produce cost-effective devices and enhance cell performance. In this aspect, phenothiazine based organic dye compounds such as (E)-3-(7-bromo-10-phenyl-10H-phenothiazine-3-yl) acrylic acid and (E)-3-(7-bromo-10-phenyl-10H-phenothiazine-3-yl)-2-cyanoacrylic acid have been synthesized. The synthesized dye compounds have been characterized through Fourier-transform infrared, Fourier-transform Raman and nuclear magnetic resonance spectroscopic method. The Ultraviolet–Visible spectra were recorded and electronic features were discussed with the theoretically calculated bands using time-dependent density functional theory. Frontier molecular orbital, natural bond orbital and non-linear optical properties have been calculated for these compounds using density functional theory. The photosensitization properties such as light harvesting efficiency and electron injection driving force (∆G inject) have also been discussed.

A rational design of excellent light-absorbing dyes with different N-substituents at the phenothiazine for high efficiency solar cells

Dye-sensitized solar cells (DSSCs) have attracted great interest due to their simple fabrication process and low cost. However, most organic dyes with D-π-A configuration usually exhibit narrow absorption band, leading to poor light harvesting ability and great loss on photon conversion efficiency. In this research, a series of excellent light-absorbing dyes (CC202-I - CC202-III) with different N-substituents at phenothiazine entities based on the champion dye CC202 were designed and investigated by density functional theory (DFT) and time-dependent DFT (TD- DFT). According to the analysis of absorption property, the results demonstrated that different N-substituents (12-crown-4-substituted phenyl, 4-hexoxyphenyl, and bare phenyl) at phenothiazine entities lead to stronger and broader absorption band as well as red-shifted spectra; moreover, larger electronic injection driving force (ΔG^inject), regeneration driving force (ΔG^reg), capability of light harvested (η_LHE(λ_strong)/η-_LHEλ) and maximal photon generated current (J_ph) in CC202-I - CC202-III are observed compared to that of CC202, which further increase J_SC. Additionally, a larger V_OC can be obtained in CC202-I - CC202-III due to larger dipole moment (u_normal) and slow electron recombination rate. Considering the all calculated characteristics related to J_SC and V_OC, dyes with 12-crown-4-substituted phenyl, 4-hexoxyphenyl, and bare phenyl substituent on phenothiazine can effectively enhance the photoelectric conversion efficiency of DSSCs.

DFT characterization and design of anthracene-based molecules for improving spectra and charge transfer

Four anthracene-based dyes (AN-3, AN-11, AN-12, AN-14) are investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) for dye-sensitized solar cells (DSSCs), involving the stable molecular geometries, the electronic structures, the absorption and fluorescence spectra, and the performance of photoelectric properties. For the simulation of the realistic environment, some important parameters, including energy levels, energy gaps, electron density, projected density of states (PDOS), absorption, vertical dipole moment, first hyperpolarizability, light-harvesting efficiency (LHE), evaluation on electron injection, are calculated for each dye molecule. The relevant electron transfer (ET) and dynamic processes were studied by using the charge different density (CDD) and Newns-Anderson model. The relationship between structure and performance are established. Furthermore, six dyes are designed and examined on the basis of AN-11 to improve optical response and electron injection. It is expected that this study will give theoretical guidance and ideas for finding potential solar cell materials.