Investigation of photovoltaic properties of solar cells sensitized by acridine-based dyes

Energy is the driving force for economic development, progress, and modernization. Energy demand and usage are increasing globally, and researchers are making significant efforts to meet future energy needs. Currently, 80% of the world's energy consumption comes from natural gas, fossil fuels, and coal. However, directly obtaining energy from fossil fuels is not the most viable and sustainable approach due to significant limitations such as the depletion of natural resources, being a major cause of CO2 emissions and global warming, non-renewability, and ecological pollution. Therefore, fossil fuels need to be replaced with renewable energy sources. Renewable energy technologies generate power, mechanical energy, or thermal energy by utilizing and converting these resources, and transform them into electromotive force or electricity. The use of renewable energy offers various advantages that contribute to environmental sustainability, such as reducing the release of harmful air pollutants and reducing greenhouse gas emissions. Due to its advantages such as being environmentally friendly, easy to use, inexhaustible, renewable, producing no CO2 emissions, being a natural resource, suitability for various applications, and not generating waste products, solar energy is superior to other renewable energies. Therefore, photovoltaic (PV) cells are designed to convert sunlight directly into electricity. The energy flow from the sun to the Earth is 3 × 1024 joules per year, which is about ten thousand times more than what humanity currently consumes. This means that even if only 0.1% of the Earth's surface were covered with photovoltaic cells with an efficiency of 10%, it would be sufficient to meet our current needs. Third-generation solar cells—organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs)—exhibit better stability than earlier generations and have reported efficiencies above 20%. Among the third-generation PV technologies, dye-sensitized solar cells (DSSCs) have garnered significant attention since their introduction by Grätzel and O'Regan in 1991. This is due to their environmental friendliness, good stability, simple production, low cost, and relatively high efficiency under both outdoor and indoor lighting conditions. DSSCs provide very high efficiency of up to 28.9% under indoor lighting conditions compared to other photovoltaic technologies, which can deliver sufficient power for the autonomous operation of small electronic devices. DSSCs have proven particularly beneficial for applications such as wireless sensor networks, sports and medical devices, cameras, security sensors, and wearable electronics. Researchers have been drawn to Dye Sensitized Solar Cells (DSSCs) due to their eco-friendliness, good stabilities, simple production processes, low cost, and relatively high efficiencies under both outdoor and indoor lighting conditions. The dyes used in DSSCs are categorized into three types: metal complexes, natural, and organic dyes. Among all these types, metal-free organic dyes are more advantageous due to their versatility, low toxicity, and relatively simple syntheses compared to others. These metal-free dyes possess a donor (D)-π-acceptor (A) skeleton, and the π-bridge facilitates intermolecular charge transfer (ICT) by connecting the electron donor (D) to the electron acceptor (A). Alternatively, efficient and stable organic sensitizers have been developed by adding electron-withdrawing groups, namely auxiliary acceptors, between the π and D regions The auxiliary acceptor extends the absorption wavelength, thereby significantly increasing cell efficiency. Furthermore, dyes with D-A-π-A skeletons improve photostability and photovoltaic performance by preventing aggregation and electron recombination. Polycyclic acridine derivatives are compounds with a conjugated planar structure that are useful in many applications. Apart from their utilization as pigments and dyes, they are utilized in various applications due to their intense fluorescence, superior electron transfer properties, strong absorptivity, and high photo, thermal, and electrochemical stability. In the scope of thesis study, a new organic dye (BIM37) with a donor–acceptor–π–acceptor (D–A–π–A) structure based on acridine π-bridge has been synthesized and used in DSSCs. Unlike the previously synthesized acridine-based D–π–A–π–A dyes (BIM25 and BIM26) in the literature, an additional acceptor has been inserted between the donor and the π-bridge in BIM37 dye, and this modification has led to a significant redshift in the absorption spectrum compared to other dyes. Therefore, BIM37-based DSSCs achieved a power conversion efficiency (PCE) of 5.03%, which is higher than the efficiencies of other acridine-based BIM25 (3.76%) or BIM26 (4.56%) dyes reported in the literature. In DSSCs, co-adsorbents enhance electron injection by preventing aggregation of the dye and decrease charge recombination, thus improving cell efficiency. Additionally, when used with dyes, they form a denser semiconductor surface layer. Among these, chenodeoxycholic acid (CDCA) is commonly used. The fundamental principle of co-sensitization method is to enhance cell efficiency by achieving a broader absorption spectrum through panchromatic light harvesting. In this method, the light harvesting capabilities of DSSCs are increased by adsorbing two or more dyes on the semiconductor surface that complement each other spectrally. There are two approaches for sensitization: cocktail method and sequential method. The cocktail method, which involves immersing working electrodes containing metal oxide semiconductors into a mixture solution containing dyes, has a simpler production process than the sequential method and is therefore more advantageous. Additionally, this method helps create a denser molecular layer by enabling the dye molecules to naturally settle on the TiO2 surface, which is a metal oxide semiconductor, in the most suitable adsorption arrangement. To further enhance photovoltaic performance, one of the co-sensitization methods, the cocktail strategy, has been implemented. When chenodeoxycholic acid (CDCA) is used as a co-adsorbent, the BIM37+BIM26-based co-sensitized DSSCs exhibited a high efficiency of 6.30%. This efficiency is higher than that of both CDCA-containing and non-CDCA-containing solely sensitized DSSCs. The synthesized BIM37 dye was optically characterized in the study, revealing an absorption peak at 485 nm. Electrochemical characterization, including cyclic voltammetry (CV) measurements, resulted in a HOMO energy level of 0.91 eV and a LUMO energy level of -1.32 eV. These findings indicate that BIM37 dye is well-suited for light harvesting due to the absorbed region and capable of electron injection and dye regeneration within the cell. The quantity of dye adsorbed onto the TiO2 surface, which is the metal oxide, also impacts the cell's efficiency. Therefore solvent optimizations were conducted for the BIM37 dye using methanol, acetonitrile:tert-butanol, and tetrahydrofuran (THF). Subsequently, a duration test was performed in the appropriate solvent selected, and the maximum efficiency was determined by finding the duration yielding the highest power conversion efficiency (%η). Throughout all optimization steps, efficiencies were calculated based on the power conversion efficiency (PCE). After determining the appropriate duration, TiO2 anodes with different particle sizes were prepared for anode selection. One type of anode consisted of transparent TiO2 (TSP/TSP) paste having particle sizes ranging from 18-20 nm for both layers. The other type of anode was prepared with the first layer using TSP paste and the second layer using TiO2 paste with particle sizes exceeding 100 nm and reflective properties (TSP/RSP). Subsequently, both types of anodes were immersed in BIM37 dye with THF as the solvent for their maximum durations. According to the results, the best type of anode observed was TSP/TSP. The PCE for BIM37 was found to be 5.03% with a 10-hour adsorption time. To enhance this efficiency, the cocktail method, one of the co-sensitization strategies with previously synthesized BIM26 dye, was employed. For this purpose, dye mixtures were prepared at different molar ratios, and the highest efficiency (5.41%) was achieved when BIM37+BIM26 (1:3), where the voluminous molecule BIM37 was used in a lesser amount. To further increase the efficiency obtained through co-sensitization, chenodeoxycholic acid (CDCA), acting as a co-adsorbent, was introduced into the cocktail mixture to facilitate a more uniform arrangement of molecules without π stacking. This resulted in an efficiency of 6.30% at the maximum duration again. Finally, the photovoltaic performance of the cells based on BIM37 dye and all co-sensitized cells was measured using electrochemical impedance spectroscopy (EIS) and incident photon-to-current efficiency (IPCE) techniques. According to the results obtained, molecular engineering is an effective approach for enhancing the performance of DSSCs, particularly through co-sensitization.