Promising pyridinium ylide based anchors towards high-efficiency dyes for dye-sensitized solar cells applications: Insights from theoretical investigations

Enhanced quantum efficiency of dicyano BODIPYs over difluoro BODIPYs: A DFT approach

The modified BODIPY (pyrromethene) dyes with cyano (-CN), substituting usual fluorine (-F), at the 4-position may give enhanced photochemical stability and quantum yield of fluorescence (QYF) when compared to the corresponding fluoro derivative. We have investigated and discussed comparatively the structural parameters of the ground (S0) and excited (S1) state geometries of a few widely used BODIPY dyes, both 4-cyano and 4-fluoro pyrromethene (PM), through DFT and TD-DFT studies and established the reasons for improved QYF of the cyano derivatives. The electrophilicity index indicates the stability of the excited geometry of the cyanated BODIPY and the calculated transition dipole moments reveal a longer lifetime of the excited state (S1) for the cyano BODIPY. The higher singlet and triplet gap in cyanated BODIPY accounts for higher QYF due to restricted transition from singlet to triplet state. This suggests that a population inversion, crucial for laser action, is easier to build up for cyano derivative than for the commonly used fluoro analogue while other gain/loss parameters of the PM dye lasers remain similar. Hence, 4-cyano derivatives of the PM dyes may be useful for more efficient lasing applications. The non-linear optical properties of the PM dyes are also investigated.

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.

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.

The role of the donor group and electron-accepting substitutions inserted in π-linkers in tuning the optoelectronic properties of D-π-A dye-sensitized solar cells: a DFT/TDDFT study

The design of low-cost and high-efficiency sensitizers is one of the most important factors in the expansion of dye-sensitized solar cells (DSSCs). To obtain effective sensitizer dyes for applications in dye-sensitized solar cells, a series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed by using computational methodologies based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We have designed JK-POZ(1–3) and JK-PTZ(1–3) D–π–A–A organic dyes by modifying the donor and π-linker units of the JK-201 reference dye. Computational calculations of the structural, photochemical properties and electrochemical properties, as well as the key parameters related to the short-circuit current density and open-circuit voltage, including light-harvesting efficiency (LHE), singlet excited state lifetime (τ), reorganization energies (λ_total), electronic injection-free energy (ΔG^inject) and regeneration driving forces (ΔG^reg) of dyes were calculated and analyzed. Moreover, charge transfer parameters, such as the amount of charge transfer (q^CT), the charge transfer distance (D^CT), and dipole moment changes (μ^CT), were investigated. The results show that ΔG^reg, λ_max, λ_total and τ of JK-POZ-3 and JK-PTZ-3 dyes are superior to those of JK-201, indicating that novel JK-POZ-3 and JK-PTZ-3 dyes could be promising candidates for improving the efficiency of the DSSCs devices.

A series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed theoretically.

Prediction of Power Conversion Efficiencies of Diphenylthienylamine-Based Dyes Adsorbed on the Titanium Dioxide Nanotube

The power conversion efficiency (η) is the most important key to determine the efficiency of dye-sensitized solar cell (DSSC) devices. However, the calculation of η theoretically is a challenging issue since it depends on a large number of experimental and theoretical parameters with extensive related data. In this work, η was successfully predicted using the improved normal model with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) for eight diphenylthienylamine-based (DP-based) dyes with various π-bridge adsorbed on titanium dioxide. The titanium dioxide is represented by a nanotube surface (TiO₂NT); this surface is rarely investigated in the literature. The π-linker consists of five (DP1)- or six (DP2)-membered rings and contains none to three nitrogen atoms (D0-D3). The reliability of the estimated values was confirmed by the excellent agreement with those available for the two experimentally tested ones (DP2-D0 and DP2-D2). The deviations between the experimental and estimated values were in the ranges of 0.03 to 0.06 mA cm^-2, 0.05 to 0.3 mV, and 0.37 to 0.18% for short-circuits current density (J _sc), open-circuit voltage (V _oc), power conversion efficiency (%η), respectively. More importantly, the results revealed that using pyridine (DP2-D1), pyrimidine (DP2-D2), and 1,2,4-triazine (DP2-D3) improves the power conversion efficiencies in the range of 6.03 to 6.90%. However, the cyclopenta-1,3-diene (DP1-D0) shows superior performance with a predicted η value that reaches 9.55%.

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.

Effect of graphene between photoanode and sensitizer on the intramolecular and intermolecular electron transfer process

The main goal of this work is to investigate the effects of a graphene layer between the photosensitive layer and semiconductor substrates on the electron transport performance of dye-sensitized solar cells from the perspective of intramolecular arrangement and interfaces. The benzothiadiazole sensitizer YKP-88 is used as the photosensitive layer and the influence of the graphene layer on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) is also discussed by exploring the frontier molecular orbitals, intramolecular charge transfer, weak interaction, interfacial electron dynamic propagations and other microscopic parameters after the anchoring of the graphene layer. The results demonstrate that the graphene layer can accelerate the electron injection of dye molecules into the semiconductor substrate, which not only has a qualitative reduction in injection time, but also has a qualitative change in the increase of the injection amount. In addition, it is also found that the graphene layer increases the stereoscopic effect, the absorption of long wavelength (>700 nm) photon flu and the amount of electron injection into the photoanode, which benefits the intramolecular charge transfer and increases the J_sc and V_oc of solar cells. The combination of intermolecular and interfacial perspectives indicates that the appropriate configuration of graphene layers can effectively improve the photoelectric conversion efficiency of dye-sensitized solar cells.

Structure and Electronic Properties of TiO₂ Nanoclusters and Dye⁻Nanocluster Systems Appropriate to Model Hybrid Photovoltaic or Photocatalytic Applications

We report the results of a computational study of TiO₂ nanoclusters of various sizes as well as of complex systems with various molecules adsorbed onto the clusters to set the ground for the modeling of charge transfer processes in hybrid organic⁻inorganic photovoltaics or photocatalytic degradation of pollutants. Despite the large number of existing computational studies of TiO₂ clusters and in spite of the higher computing power of the typical available hardware, allowing for calculations of larger systems, there are still studies that use cluster sizes that are too small and not appropriate to address particular problems or certain complex systems relevant in photovoltaic or photocatalytic applications. By means of density functional theory (DFT) calculations, we attempt to find acceptable minimal sizes of the TinO2n+2H₄ (n = 14, 24, 34, 44, 54) nanoclusters in correlation with the size of the adsorbed molecule and the rigidity of the backbone of the molecule to model systems and interface processes that occur in hybrid photovoltaics and photocatalysis. We illustrate various adsorption cases with a small rigid molecule based on coumarin, a larger rigid oligomethine cyanine dye with indol groups, and the penicillin V antibiotic having a flexible backbone. We find that the use of the n = 14 cluster to describe adsorption leads to significant distortions of both the cluster and the molecule and to unusual tridentate binding configurations not seen for larger clusters. Moreover, the significantly weaker bonding as well as the differences in the density of states and in the optical spectra suggest that the n = 14 cluster is a poor choice for simulating the materials used in the practical applications envisaged here. As the n = 24 cluster has provided mixed results, we argue that cluster sizes larger than or equal to n = 34 are necessary to provide the reliability required by photovoltaic and photocatalytic applications. Furthermore, the tendency to saturate the key quantities of interest when moving from n = 44 to n = 54 suggests that the largest cluster may bring little improvement at a significantly higher computational cost.

Performance Regulation of Thieno[3,2-b]benzothiophene π-Spacer-Based D-π-A Organic Dyes for Dye-Sensitized Solar Cell Applications: Insights From Computational Study

Dye-sensitized solar cells (DSSCs) have been widely investigated; however, the development of promising dye sensitizers is still appealing. In this work, we perform a detailed theoretical search for high-efficiency D-π-A organic dyes using density functional theory and time-dependent density functional theory calculations. Specifically, we perform geometric optimization, and electronic structure and absorption spectra calculations for isolated dyes for two thieno[3,2-b]benzothiophene π-spacer-based D-π-A organic dyes SGT129 and SGT130, which show significant efficiency difference, before and after binding to a TiO₂ semiconductor. The calculation results reveal that the coplanar configuration between the electron donor and the π-spacer can enhance electronic communication efficiently, thus facilitating intra-molecular charge transfer from the electron donor to the acceptor groups in SGT130. The absorption spectrum of SGT130 broadens and is red-shifted owing to the decreased bandgap. The higher light-harvesting efficiency, favorable intra-molecular charge transfer, larger shift of the conduction band edge in the TiO₂ semiconductor, and slower charge recombination between the injected electrons in the TiO₂ conduction band and the electrolyte explain the superior efficiency of SGT130 over that of SGT129. Using SGT130 as the reference dye, we further design four novel dyes 1-4 by modifying the π-spacer with electron-rich and electron-withdrawing moieties. Judging from the theoretical parameters influencing the short-circuit current and open-circuit voltage, we found that all dyes would perform better than SGT130 in terms of the favorable interfacial charge transfer (ICT) and light-harvesting efficiency, as well as the larger shift of the TiO₂ conduction band edge. Our theoretical research is expected to provide valuable insights into the molecular modification of TBT-based D-π-A organic dyes for DSSC applications.