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

The influence of the position and quantity of thiophene or acetylene groups on the photoelectric properties of dye-sensitized solar cells

To investigate the influence of the position and quantity of thiophene or acetylene groups on the photoelectric properties of dye-sensitized solar cells (DSSCs), density functional theory (DFT) were employed to simulate five zinc porphyrin dye molecules (T-3, T-3-D, T-3-A, T-3-AD, and T-3-ace). The optimized geometry indicated that T-3-ace possessed superior planar properties, attributed to incorporating the acetylene groups, facilitating the charge transfer process. The lower lowest unoccupied molecular orbital (LUMO) energy levels of T-3-ace and T-3-D suggested that introducing thiophene or acetylene groups on the donor side enhanced the electron absorption capability of the dyes. The analysis of optical properties revealed that the incorporation of thiophene or acetylene groups on the donor side (T-3-D or T-3-ace) exhibited a more prominent red shift and a broader absorption range, which was beneficial for promoting electron excitation and optical properties. The low reorganization energy suggested these two molecules have better structural stability during photoexcitation. The prediction of photoelectric conversion efficiency (PCE) showed that introducing thiophene was beneficial for improving the PCE, with the most significant effect observed when introducing thiophene groups on the donor side (T-3-D). The T-3-ace demonstrated the highest maximum short circuit current density Jmax; however, the lower electron injection efficiency (φinject) led to a lower short circuit current density JSC and consequently a lower PCE. This work is conducive to providing valuable insights into the molecular structure of zinc porphyrin dyes and their photoelectric properties.

Photovoltaic performance and power conversion efficiency prediction of double fence porphyrins

To explore high efficiency dye-sensitized solar cells (DSSCs), two experimentally derived (single fence and double fence porphyrins) and two theoretically designed zinc porphyrin molecules with D-D-π-A-A configurations were studied. Density functional theory and time-dependent density functional theory were employed to simulate these two experimental dyes and dye@TiO₂ systems to understand why the double fence porphyrin molecule exhibits better photovoltaic performance than the single fence porphyrin molecule. For the short-circuit current (J_SC), the various parameters that affected the experimental magnitude of J_SC were analyzed from different aspects of absorption, charge transfer and chemical parameters as well as an electron injection process. The almost equal open-circuit voltages (V_OC) in the experiment were predicted by theoretical V_OC calculations. Our model predicted power conversion efficiencies (PCEs) of 1.993% and 10.866% for the single and double fence molecules, respectively, which are in accordance with the experimental values of 3.48% and 10.69%, respectively. In addition, one designed two new molecules based on the double fence porphyrin molecule with a 2-methyl-2H-benzo[d][1,2,3]triazole (BTA) unit bearing one fluorine and two fluorine atoms as the guest acceptor, respectively. Compared to the original molecules, the engineered molecules significantly improved the photovoltaic parameters, J_SC and V_OC, thereby causing excellent PCEs. The most outstanding designed molecule reached a PCE of 12.155%, and is considered a candidate dye for high-efficiency DSSC. This study provides insights into the photoelectric properties of single and double fence porphyrins. It also demonstrated that the strong electron-withdrawing ability of fluorine atoms would enhance the photovoltaic performance and provide a guideline for the further design of double fence porphyrins.

Enhancing the Photoelectric Properties of Zinc Porphyrin Dyes by Introducing Five-Membered Heterocyclic Rings into the Electron Donor: A Density Functional Theory and Time-Dependent Density Functional Theory Study

To fabricate highly efficient dye sensitizers for dye-sensitized solar cells, new zinc porphyrin dye sensitizers were designed based on one of the most efficient dyes, YD2-o-C8, by introducing electron-rich heterocyclic rings into the electron donor. Five potentially efficient dyes, Dye1-5, were obtained by replacing the phenyl group of the donor in YD2-o-C8 with pyrrolyl, furyl, and thienyl groups. The electronic structures, absorption spectra, intramolecular charge-transfer characteristics, and excited-state lifetimes of the designed dyes were investigated using the density functional theory and time-dependent density functional theory methods. All the designed dyes exhibit better photoelectric properties than those of YD2-o-C8. Compared with YD2-o-C8, the designed new dyes have smaller frontier molecular orbital energy gaps and obvious red-shifting absorption spectra in the Q band. The analyses of charge density difference plots and intramolecular charge-transfer characteristics indicated that the designed dyes can better promote intramolecular charge transfer and electron-hole separation. Among the five designed dyes, Dye1 with a pyrrolyl group exhibits the best performance. Dye3 and Dye5 with methyl-furyl and methyl-thienyl groups, respectively, exhibit the next best performance. Dye2 and Dye4 with furyl and thienyl groups, respectively, are the worst performers. The introduced methyl group can further improve the electron-donating ability of heterocyclic rings and promote the red shift of the Q bands and intramolecular charge transfer of dyes. The excited-state lifetimes of the new dyes were in the following order: YD2-o-C8 < Dye4 < Dye2 < Dye5 < Dye3 < Dye1, which shows their stronger abilities to inject electrons into semiconductor films.

Systematic theoretical investigation of two novel molecules BtyC-1 and BtyC-2 based on ESIPT mechanism

Inexperiment, Song et al. have successfully synthesizedtwo novel molecules BtyC-1 and BtyC-2 and observedasingle and dual fluorescence peaks in these two molecules respectively. (Song et al. Tetrahedron Lett. 2019, 60, 1696-1701) However, they still lack a detailed and reasonable theoretical explanation. Then we wonder why these two similar structures behave so much differently? In this work, we focus on explaining the photochemical and photophysical properties of BtyC-1 and BtyC-2 by studying the excited state intramolecular proton transfer (ESIPT) mechanisms. Based on the optimized geometric configurations, the calculated infrared spectra indicate the intramolecular hydrogen bonding interactions are heightened in their excited states. The frontier molecular orbitals reflect the charge redistribution in photoinduced process, which explains that the driving force of ESIPT process is provided by enhanced hydrogen bonding interactions. In the meantime, the calculations of potential energy curves vividly explain the principle of the experimental dual fluorescence phenomenon. The analysis of Mulliken charges deepens the discussion of molecular structures on the potential energy barriers. Calculated absorption spectra via using density functional theory and emission spectra via using time-dependent density functional theory are consistent with the experimental data, which confirms the correctness of our calculation methods. The reduced density gradient isosurfaces help us distinguish the complex non-covalent bonds. Base on the above analyses, we conclude that there is no stable structure for BtyC-1 in excited state, which make it occur the ESIPT reaction spontaneously. BtyC-2 exists a stable normal structure in excited state. Its dual fluorescence signals are emitted by its normal and isomer structures, respectively.

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%.

Enhancement of one- and two-photon absorption and visualization of intramolecular charge transfer of pyrenyl-contained derivatives

To further improve the pyrenyl-contained derivatives two-photon absorption (TPA) and third-order nonlinear optical (NLO) properties, three steps of optimization are employed based on experimental molecule PCVS-B: heteroatomic substitution, exchanging the position of double bonds and adding a branch. The contributions of π electrons to localized orbital locators and Mayer bond orders (LOL-π and I_AB^π) show that the second step can enhance the chemical interaction between pyrenyl and the branched-chain. Two visual methods of charge density difference (CDD) and transition density matrix (TDM) are combined to intuitively analyze the intramolecular charge transfer (ICT) process of one (two) photon absorption; results show that both following two steps can increase the degree of ICT on the conjugated plane of the pyrenyl. The sum over state (SOS) model was used to find out the dominant two-photon transition process. The difference between the dipole moments obtained by the McRae equation is applied to the three-state model, revealing the inherent law of the second static hyperpolarizability.

Theoretical Investigation on Photophysical Properties of Triphenylamine and Coumarin Dyes

Organic molecules with donor and acceptor configures are widely used in optoelectronic materials. Triphenylamine dyes (TPCTh and TPCRh) are investigated via density functional theory (DFT) and time-dependent DFT. Some microscopic parameters related to light absorption and photoelectric formation are calculated to interpret the experimental performance in dye-sensitized solar cells (DSSCS). Considering that coumarin derivatives (Dye 10 and Dye 11) have good donor and acceptor structures, they also have a COOH group used as an anchoring group to connect with semiconductors. Thus, the two dyes' photophysical and photoelectric properties are analyzed to estimate the performance and application in DSSCs.

Probing the charge-transfer of Ag/PEDOT:PSS/4-MBA by surface-enhanced raman scattering

A metal-organic semiconductor-molecule model was developed with Ag nanoparticles (NPs), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and 4-mercaptobenzoic acid (4-MBA) via the layer-by-layer self-assembly method. In the SERS spectrum of the Ag/PEDOT:PSS/4-MBA system, structural changes in the PEDOT chain were discovered, which provides a deeper understanding of the charge transfer (CT) mechanism in SERS and helps in the development of a method to construct metal-organic semiconductor SERS substrates. A quantitative calculation of the degree of charge transfer (ρCT(κ)) determines the CT contribution of PEDOT:PSS to the SERS intensity of the Ag/PEDOT:PSS/4-MBA system. On this basis, we propose the formation of a resonance complex between Ag NPs and PEDOT:PSS to explore the CT mechanism, which is beneficial for studying interface CT and for understanding the CT mechanism in SERS. The introduction of organic semiconductors in this study not only broadens the research scope of SERS substrates but also contributes to the exploration of SERS mechanisms.

Photoinduced Charge Transfer in Push/Pull Systems of Two-Photon Absorption

A series of stilbene derivatives have been constructed by modifying the stilbene systems with different H, CN, NH₂, NMe₂, and NO₂ groups. In a vacuum, it was found that a redshift in the ultraviolet-visible spectrum occurred because of the enhancement of the donor/acceptor capabilities of this group, with the order of redshift being NO₂ > NMe₂ > CN > NH₂ > H. For stilbene molecular systems, the peak of two-photon absorption (TPA) observed in the simulated spectra should be attributed to two transitions that are contributed by two excited states with similar energy. In the case of derivatives, such a transition is separated by energy, and two TPA peaks can be clearly observed (derivatives containing NO₂ and NMe₂ groups have two TPA peaks), where the magnitude of the separation is directly related to the intensity of the peripheral group. In addition, the S₁ state is the intermediate state in the TPA transitions to both of the final excited states.

2-Thiohydantoin Moiety as a Novel Acceptor/Anchoring Group of Photosensitizers for Dye-Sensitized Solar Cells

Very recently, we have reported the synthesis and evaluation of biological properties of new merocyanine dyes composed of triphenylamine moiety, π-aromatic spacer, and rhodanine/2-thiohydantoin-based moiety. Interestingly, 2-thiohydantoin has never been studied before as an electron-accepting/anchoring group for the dye-sensitized solar cells (DSSCs). In the presented study, we examined the applicability of 2-thiohydantoin, an analog of rhodanine, in DSSC technology. The research included theoretical calculations, electrochemical measurements, optical characterization, and tests of the solar cells. As a result, we proved that 2-thiohydantoin might be considered as an acceptor/anchoring group since all the compounds examined in this study were active. The most efficient device showed power conversion efficiency of 2.59%, which is a promising value for molecules of such a simple structure. It was found that the cells' performances were mainly attributed to the dye loading and the ICT molecular absorption coefficients, both affected by the differences in the chemical structure of the dyes. Moreover, the effect of the aromatic spacer size and the introduction of carboxymethyl co-anchoring group on photovoltaic properties was observed and discussed.

Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(diimine)copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?

The syntheses of 4,4'-bis(4-dimethylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (1), 4,4'-bis(4-dimethylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (2), 4,4'-bis(4-diphenylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (3), and 4,4'-bis(4-diphenylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (4) are reported along with the preparations and characterisations of their homoleptic copper(I) complexes [CuL2][PF6] (L = 1-4). The solution absorption spectra of the complexes exhibit ligand-centred absorptions in addition to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu(5)(Lancillary)]+ dyes in which 5 is the anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) and Lancillary = 1-4 have been assembled on fluorine-doped tin oxide (FTO)-TiO2 electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu(5)(1)]+ and [Cu(5)(3)]+. The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit current density (JSC), confirmed by lower values of EQEmax. Addition of a co-absorbent (n-decylphosphonic acid) to [Cu(5)(1)]+ lead to no significant enhancement of performance for DSCs sensitized with [Cu(5)(1)]+. Electrochemical impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the analysis shows that more favourable electron injection into TiO2 is observed for sensitizers without the alkynyl spacer and confirms higher JSC values for [Cu(5)(1)]+.