Synthesis of some new acridine-based compounds and applications in dye-sensitive solar cell

Acridines are an important class of nitrogen-containing heterocyclic compounds. Acridine compounds with planar tricyclic aromatic structures possess pigment and dyeing properties, as well as good photochemical/physical and electrochemical characteristics. Due to these properties, acridines find various applications. They are used in creating new fluorescent probes due to their fluorescent nature. These compounds can emit light at longer wavelengths to prevent undesired absorption, autofluorescence, and scattering by nearby biological tissues. Additionally, they have been used as active environment for laser dyes. Iridium complexes of acridines have been utilized as light-emitting materials in OLED applications. In light of this information, certain new acridine derivatives have been synthesized and for the first time applied as sensitizers in dye-sensitized solar cells (DSSCs). In order to enhance the efficiency of dyes to be used in Dye-Sensitized Solar Cells (DSSCs), they need to possess a specific structural sequence. Fundamentally, the design of organic dyes is based on a D-π-A structure, where the first part (D) represents the donor group, and the second part (A) represents the acceptor group. The middle part (π-bridge) plays a crucial role as a conjugating linker between D and A within the D-π-A framework, facilitating intramolecular charge transfer (ICT) upon excitation and characterized by good synthetic accessibility and high-performance organic sensitizers. Recently, numerous studies have elucidated the role of materials with a D-π-A structure, highlighting the significance of these compounds in organic solar cells. In recent studies, acceptors and new π-bridges are being incorporated to assist the dyes with D-π-A structures, resulting in transformations to either D-A-π-A or D-π-A-π-A forms. Despite these advancements, due to strong molecular interactions, organic dyes tend to exhibit prolonged π-conjugation and π-π stacking, which can lead to aggregation. While this expansion of the absorption spectrum can benefit light harvesting, π-stacked aggregates often lead to inefficient electron injection, resulting in low Power Conversion Efficiency (PCE). To overcome this issue, modifying the donor portions of the dye molecule with branched alkoxy groups has proven to be an effective strategy, although bulky substituents may not entirely suppress dye aggregation.To address this concern, the use of a co-adsorbent is a practical approach. Co-adsorbents improve electron injection by suppressing dye aggregation and reduce charge recombination, thereby enhancing cell performance. Additionally, the combination of dye and co-adsorbent prevents the formation of gaps on the semiconductor surface, resulting in a more compact layer compared to the dye layer alone. Some effective co-adsorbents for enhancing the photovoltaic performance of DSSCs include phosphonic acids, organic bases, carboxylic acids, and acetylacetone. Naturally occurring saturated polycyclic molecules like chenodeoxycholic acid (CDCA), with a steroid structure containing a carboxylic acid binding group, have proven to be versatile and popular co-adsorbents. CDCA compounds contain one to three hydroxyl groups and a flexible carboxylic acid chain. As a result, they are amphiphilic and chiral compounds. Later, it was discovered that they could also attach to the surface of the semiconductor material in photovoltaic cells, helping to eliminate drawbacks that could occur between the dye and semiconductor material. As part of the thesis study, dyes were synthesized in the D-π-A-π-A structure, where tris(alkoxyphenyl)amine containing bulky alkoxy groups was utilized as the donor, acridine was employed as the π-bridge for the first time, and benzothiadiazole (BIM25) and benzotriazole (BIM26) compounds were introduced as co-acceptors. Additionally, a phenyl compound was incorporated as the π-bridge and cyano acrylic acid compound was synthesized as the acceptor, both for the first time. In Dye-Sensitized Solar Cells (DSSCs), the most common semiconductor material is TiO2, and the commonly used electrolyte contains the I-/I3- redox couple. For a dye to effectively function in DSSCs, the HOMO energy level of the dye needs to be more positive than the Nernst potential of the electrolyte (0,4 eV), and the LUMO energy level should be more negative than the conduction band of TiO2 (-0,5 eV). The synthesized dyes (BIM25 and BIM26) in the study were initially optically characterized, and it was determined that they exhibited absorption peaks at 402 nm and 422 nm, respectively. Subsequently, they were electrochemically characterized, revealing the HOMO energy levels for each dye (0,96 eV for BIM25, 0,99 eV for BIM26) and LUMO energy levels ( -1,47 eV for BIM25, -1,46 eV for BIM26). All these findings have demonstrated that the dyes will effectively engage in light harvesting and can readily facilitate sufficient electron injection and dye regeneration processes in the prepared Dye-Sensitized Solar Cells (DSSCs). The amount of dye adsorbed onto the TiO2 surface is one of the factors that affect the power conversion efficiency. Therefore, before examining the photovoltaic performance of BIM25 and BIM26 dyes, an appropriate solvent was investigated. To carry out this process, the electrodes were immersed in dye solutions prepared in tetrahydrofuran (THF) and dichloromethane (DCM) for specific durations, and the power conversion efficiency of Dye-Sensitized Solar Cells (DSSCs) was determined. The solvent with the highest efficiency was identified and used for further experimentation. For solvent selection, transparent bilayer (TSP/TSP) TiO2 electrodes were immersed in the solvent for half an hour. After determining the solvent, two different anode (substrate) types were prepared based on the structural characteristics of TiO2. The anodes were applied onto a transparent bilayer (TSP/TSP) TiO2 with particle sizes of 18-20 nm. The second type of anode was prepared by applying a layer of transparent bilayer (TSP/TSP) TiO2 as the first layer and a mixture of small and large particle-sized diffused TiO2 (TSP/DSP) as the second layer. Both types of anodes for each dye were immersed in the previously determined THF solution for a specific duration. The anode with the higher achieved power conversion efficiency, the TSP/TSP photoanode, was selected for further experimentation. The suitable solvent and appropriate TiO2 pastes were chosen, and the immersion durations of the selected Dye-Sensitized Solar Cells (DSSCs) in the solvent were determined. This duration corresponds to the time at which the maximum amount of dye is adsorbed onto the semiconductor surface, thus achieving the highest power conversion efficiency. THF was identified as the solvent for both BIM25 and BIM26 dyes, with TSP/TSP-type TiO2 pastes selected. The power conversion efficiencies were found to be 3,75% for BIM25 with a 24-hour adsorption duration and 4,56% for BIM26 with a 16-hour adsorption duration. To demonstrate the functionality of these compounds in Dye-Sensitized Solar Cells (DSSCs), their photovoltaic performance was assessed using various techniques, including % power conversion efficiency (η), % incident photon-to-current conversion efficiency (IPCE), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS).