Push-Pull N-Annulated Perylene-Based Sensitizers for Dye-Sensitized Solar Cells: Theoretical Property Tuning by DFT/TDDFT

Visualizing and characterizing excited states from time-dependent density functional theory

Time-dependent density functional theory (TD-DFT) is the most widely-used electronic structure method for excited states, due to a favorable combination of low cost and semi-quantitative accuracy in many contexts, even if there are well recognized limitations. This Perspective describes various ways in which excited states from TD-DFT calculations can be visualized and analyzed, both qualitatively and quantitatively. This includes not just orbitals and densities but also well-defined statistical measures of electron-hole separation and of Frenkel-type exciton delocalization. Emphasis is placed on mathematical connections between methods that have often been discussed separately. Particular attention is paid to charge-transfer diagnostics, which provide indicators of when TD-DFT may not be trustworthy due to its categorical failure to describe long-range electron transfer. Measures of exciton size and charge separation that are directly connected to the underlying transition density are recommended over more ad hoc metrics for quantifying charge-transfer character.

Synthesis and Optical Properties of a Series of Push-Pull Dyes Based on Pyrene as the Electron Donor

Fifteen push-pull dyes comprising the tetracyclic polyaromatic pyrene have been designed and synthesized. The optical properties of the fifteen dyes have been examined in twenty-two solvents of different polarities. Surprisingly, contrarily to what is classically observed for push-pull dyes of D-π-A structures, a negative solvatochromism could be found for numerous dyes. The photoluminescence and thermal properties of the dyes were also examined. Theoretical calculations were carried out to support the experimental results.

Schiff base as n-type semiconductor: synthesis, characterization, and diode features

A pyrene-based Schiff base, PyFA, with D-π-A structure was synthesized and the chemical structure was established using IR, NMR, and mass spectrometry. The photoluminescence studies revealed intramolecular charge transfer in PyFA with green fluorescence emission. PyFA with pyrene and fluorene as molecular units was thermally stable up to 300 °C and exhibited an electrochemical band gap of 3.5 eV. The thin films of PyFA exhibited higher carrier concentration, n-type semiconducting property, and a dielectric constant of 6.69. The thin films of PyFA displayed uniform and homogenous surface morphology, which is desirable for device application. A p-n type diode was constructed using PyFA and the diode parameters were extracted from the DC and AC measurements.

Graphical abstract

Fusing Thienyl with N-Annulated Perylene Dyes and Photovoltaic Parameters for Dye-Sensitized Solar Cells

Due to the role of dyes in dye-sensitized solar cells (DSSCs), designing novel dye sensitizers is an effective strategy to improve the power conversion efficiency. To this end, the fundamental issue is understanding the sensitizer's trilateral relationship among its molecular structure, optoelectronic properties, and photovoltaic performance. Considering the good performance of N-annulated perlyene dye sensitizers, the geometries, electronic structures, and excitations of the selected representative organic dye sensitizers C276, C277, and C278 as well as dyes adsorbed on TiO₂ clusters were calculated in order to investigate the relationship between molecular structures and properties. It was found that fusing thienyl to N-annulated perlyene can elevate the highest occupied molecular orbital (HOMO) energy, reduce the orbital energy gap, increase the density of states, expand the HOMO to the benzothiadiazole moiety, enhance the charge transfer excitation, elongate the fluorescence lifetime, amplify the light harvesting efficiency, and induce a red-shift of the absorption spectra. The transition configurations and molecular orbitals of the dye-adsorbed systems support that the electron injection in DSSCs based on these dyes is a fast mode. Based on extensive analysis of the electronic structures and excitation properties of these dye sensitizers and the dye-adsorbed systems, we present new quantities as open-circuit voltage and short-circuit current density descriptors that celebrate the quantitative bridge between the photovoltaic parameters and the electronic structure-related properties in order to expose the relationship between properties and performance. The results of this work are critical for the design of novel dye sensitizers for solar cells.

Solvent-sensitive circularly polarized luminescent compounds bearing a 9,9′-spirobi[fluorene] skeleton

Chiral 9,9′-spirobi[fluorene] derivatives with a donor–π–acceptor system were prepared and found to exhibit solvent-sensitive circularly polarized luminescence.

Molecular engineering mechanism of organic photoactive layer by alkyl chains, 4-butoxyphenyl and cyanogroup

The three organic dye molecules (JY31, JY32 and JY33) were applied to the photoactive layer in solar cells. Photophysical and photochemical characteristic have been investigated with natural bond orbital (NBO), frontier molecular orbital, ionization potentials, electron affinities, absorption properties, reorganization energies, static first hyperpolarizability, emission characteristics, IR spectra, charge density difference; the influence of alkyl chains and 4-butoxyphenyl on properties were revealed; Subsequently, three new molecules JY33-1, JY33-2 and JY33-3 were designed by inserting the electron withdrawing group -CN into the acceptor part of JY33 in order to understand molecular engineering mechanism. The results show that the three original molecules have relatively high molar extinction coefficients, and the molecule of JY33 with a 4-butoxyphenyl group enables a bathochromic shift in absorption spectrum and is beneficial to improve the hole transport, injection capacity and ICT properties as well as better energy levels matching. The current study provides an effective channel for manipulating performance in materials design of solar cells.