Due to the growing global population and rapid depletion of fossil fuels, people have started to focus on renewable energy sources, and solar energy has become the first choice for researchers to carry out research due to its ease of use [[1], [2], [3], [4], [5], [6], [7]]. Solar cells are an important way to use solar energy efficiently, among which dye-sensitized solar cells (DSSCs) have attracted more and more researchers due to their low manufacturing cost, less restrictive use environment and high power conversion efficiency (PCE) [8].DSSCs are mainly composed of a five-part structure, consisting of semiconductors (+ transparent conductive glass), counter electrodes, electrolyte, and dye sensitizers, which can be improved by improving the above five parts. The energy conversion efficiency can be improved by improving the materials of the above five parts, which are the sensitizer and porous oxide semiconductor substrate are used to perform the light absorption process and electron harvesting process, respectively [9]. The principle of its operation is shown in Scheme 1, and the specific process is as follows: (i) the incident light irradiated to the electrode surface makes the electrons in the dye molecules transition from their ground state to the excited state; (ii) the dye molecules in the excited state will rapidly inject the electrons into the conduction band (CB) of the semiconductor substrate. These electrons will rapidly collect on the transparent conductive glass and flow through the connected external circuit to the counter electrode, and the semiconductor substrate of the battery is usually a TiO2 nanocrystalline porous film, and can also be used in ZnO and SnO2, etc.; (iii) the electron donor in the electrolyte provide electrons and make dye molecules in the oxidation state are restored to the reduction state, which allows the dyes to be regenerated. The redox pair in the electrolyte is generally I−/I3−, most of the counter electrodes are made of platinum (Pt), which is used to accelerate the conversion of redox pairs in the electrolyte; (iv) (I−) in the electrolyte is changed to (I3−) upon provision of electrons and subsequently diffuses to the counter electrode where it is reduced due to the acquisition of electrons at the electrode surface. There are also other processes electron transfer, such as the electrons injected into the CB will complex with the dye molecules in the oxidation state as well as the I3− in the electrolyte [10,11]. Dye molecules are dye sensitizers, sensitizers can be classified into metal complex dyes and metal-free organic dyes, the highest performing metal-free organic dyes are D-π-A type dye sensitizers, where (D) is the electron donor, (A) is the electron acceptor, and π is the conjugate bridge for transferring electrons. This type of dye molecule causes charge transfer of D-A through the π conjugate bridge and possesses the characteristics of high absorption intensity, low production cost and easy regulation of electron complexes [12].

According to literature reports, the electrochemical and photophysical properties of dye molecules can be improved by replacing different units in the D-π-A moiety [[13], [14], [15], [16], [17], [18], [19]]. Based on extensive literature study, common electron donors include: triphenylamine [13], phenothiazine, indoline and its derivatives, etc. [14], and typical electron acceptors include: cyanoacrylic acid, benzoic acid, and hydroxypyridine [15]. Due to the extensive application of organic dyes in DSSCs, various researchers have been trying to find more efficient dyes for DSSCs. The triphenylamine (TPA) group, as an electronextractive dye group, has superior photovoltaic performance, with an energy conversion efficiency (PCE) that has long exceeded 10 %, and at the same time effectively avoids the problem of low absorption of TPA dyes in the near-infrared (NIR) spectra as described by Tayebeh et al. [16]; Anbarasan and colleagues firstly carried out a preliminary evaluation of common hybridisation floodgates, and chose the one that was closest to the experimental data The light-harvesting efficiencies (LHE) of a series of phenothiazine derivatives based on phenothiazine studied exceeded 75 %, with the largest LHE value having reached 92 % [17]; Simon Mathew's team designed a group of macrocyclic porphyrin-based compounds, which resulted in better intermolecular charge transfer performance of the surface electron-D and electron-A when connected in parallel to the ligand macrocycle than the adjacent ligand-ligand substituted isomers, with the largest PCE value of 13 % [18]; Said et al. reported nine compounds containing phenylhydrazone functionalities and N,N chelation sites, and these coordination compounds with open circuit voltage values between 0.61–1.06 eV, which is sufficient for effective electron injection [19]; Kar's team reported DSSCs based on several aryl amines have even approximately reached 20 % PCE, while discussing the significance of tetrahydroquinoline dyes as valuable for photovoltaic applications [12]. Asif Mahmood et al. optimized an organic dye of structure (AA)-π-D- π-(AA) with a PCE value of 7.54 % in laboratory test [20]; Ailing Tang's group took advantage of the blue-shifted absorption of the thiazole bridge and the deeper energy levels to design a molecule with a thiazole ring as a conjugated bridge, which has a PCE value of about 5.8 % [21]; Muhammad Haroon's team designed five fullerene-modified acceptor molecules and showed that all of these molecules are perfect materials for high-performance indoor and outdoor organic solar cells [22]; and almost simultaneously, another set of S-type (twisted) fullerene-free acceptor molecules were reported, which likewise proved to be promising in organic solar cells [23]; Muhammad Ramzan Saeed Ashraf Janjua et al. reported six improved A-D-A type molecules, all of them with large absorption redshifts and small bandgaps, which dramatically reduce the maximum charge transfer rate and thus enhance their own photovoltaic properties [24].

In recent years our lab has been long engaging in designing and optimizing dye molecular structures to study their intramolecular charge transport efficiency and excited state dynamics [[25], [26], [27], [28], [29], [30]]. In this work, a series of novel organic dyes SH-1, SH-2, SH-3, SH-4 and ZD-1, ZD2, ZD-3, ZD-4 were designed whose molecular structures are shown in Fig. 1. The eight compounds are typical D-π-A structures, which have the same π-conjugated bridge and electron acceptor. Meanwhile, the π-bridge is a thiophene group and a carbon-carbon double bond, and the acceptor group is cyanoacrylic acid. The electron-D of the SH series has a core of tritylene as well as two identical branched chains, and the tritylene molecule, by combining with the electron-withdrawing group, can act as pushing and pulling of electrons within the molecule.

The triphenylamine group of SH-1 does not have any branched chain, the branched chain of SH-2 is the alkyl chain of C4H9, the branched chain of SH-3 replaces the original hydrogen atoms with two perfluoroalkyl chains (C4F9) with greater electron-egativity to form a new group with greater electron absorption capacity, fluorine is the most electronegative of all the elements, and when the fluorine atom replaces the hydrogen atom, it will usually cause the electronic properties of the molecule to undergo a large change. SH-4 donor replaces the hydrogen atom with two methoxy groups at the end of triphenylamine, which behaves as an electron-D group on the benzene ring due to the greater charge density of the methoxy groups. The molecules of the ZD series, on the other hand, replace all the electron-D triphenylamine with 9-phenylcarbazole, with three branched groups identical to those of the SH2-4 series, and the two strong donors are able to promote the generation of short circuit photocurrents in order to facilitate the compensation of the loss of photocurrents due to the loss of I−/I3− in the redox electrolyte [31]. By introducing additional electron-D groups, the degree of conjugation of the molecular system can be altered to increase the light absorption intensity of the dye, which is an effective strategy to modulate the electron complex and electrochemically relevant properties. By introducing π-conjugated bridges will result in increasing the mobility of the electron cloud between the two groups, which in turn helps to polarize the electron cloud as well as increase the intermolecular charge transfer (ICT) efficiency and luminescence properties can be regulated by the type and length of the electron-D, among others [32]. In addition, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods were used to study these molecules theoretically, to observe intramolecular charge excitation and transfer, and to conduct spectroscopic studies, with the aim of enhancing the degree of ICT, decreasing the intermolecular energy bandgaps, and improving the linear and nonlinear optical properties while preserving the planarity of the molecules in order to designing DSSCs dyes with higher performances in the later stages. The photovoltaic and quantum chemical parameters of several dyes were also calculated to qualitatively compare the theoretical PCE values of each molecule by comparing each parameter, and the global reactivity characters of each sample were also discussed to determine their chemical properties.