Research on the use of metal-free synthetic and commercial dyes as co-sensitizers in solar cells

The increasing population growth has led to a rise in the demand for energy. Due to the adverse effects of traditional energy sources such as natural gas, coal, and fossil fuels on the environment, there has been an increasing shift towards alternative energy sources around the world. Furthermore, the rapid depletion of fossil fuel reserves has underscored the importance of researching alternative and renewable energy sources that are environmentally friendly or associated with low pollution levels for many researchers worldwide. Renewable energy sources can be considered both as sustainable and as an infinite source of energy. In recent years, essential household demands in many countries, such as electricity, heating/cooling systems for air and water, and transportation, are increasingly being met by renewable energy sources. The most significant renewable energy sources include wind energy, hydro energy, tidal energy, geothermal energy, and biomass energy. According to the REN21 (Renewable Energy Policy Network for the 21st Century) report, 19.3% of the total energy consumed by the global population in the years 2015-2016 was derived from renewable energy sources. This overall contribution was divided into 8.9% from traditional biomass sources, 4.2% from geothermal and solar energy sources, 3.9% from hydroelectric power, and the remaining 2.2% from wind, solar, and geothermal sources. Investments and jobs associated with renewable energy continue to increase each year, and the efficiency of energy production is also on the rise, accompanied by a decrease in costs. The report emphasizes that solar energy, which is accessible, abundant, and cost-effective, plays a significant role among these renewable energy sources [1]. Therefore, as previously mentioned, solar energy provides an extraordinary capacity, abundant, and environmentally friendly resource to meet global energy consumption demands. In fact, the amount of solar energy reaching the Earth in one hour exceeds humanity's total annual energy needs. However, harnessing all this energy is challenging, and comprehensive research efforts are needed to obtain energy more efficiently. Despite being a significant renewable energy source due to its abundance and efficiency, one of the primary challenges in solar energy production is its fluctuating nature. Research in this field has been ongoing for many years, contributing to a better understanding of the principles underlying photovoltaic devices. The fundamental principle of photovoltaic systems involves coating a glass sheet with a metal oxide, such as TiO2, which provides electron conductivity, and then applying a sensitizing dye onto the TiO2 layer. These sensitizing coatings are used for photovoltaic measurements. In photovoltaic studies, the sensitizing dyes used are the most costly components, making it crucial to reduce this cost. The primaryfunction of sensitizing dyes is to absorb photons from sunlight, inducing electron excitation, and subsequently facilitating the transfer of excited electrons to the semiconductor's conduction band. Types of dyes used in Dye-Sensitized Solar Cells (DSSCs) can be categorized under fundamental headings such as organometallic dyes, metal-containing dyes, natural dyes, mordant dyes, quantum-dot dyes, and perovskite-based dyes. Metal-free organic dyes are typically designed with an electron donor (D), a π-conjugation bridge, and an electron acceptor (A). The D-π-A structure has a push-pull configuration. Photo-induced intramolecular electron transfer between the donor and acceptor is transmitted to the acceptor through the π-electron bridge, thereby achieving significant photovoltaic efficiency (PCE). The HOMO level is associated with the donor and π-conjugate bond, while the LUMO level is connected to the electron acceptor, and can be finely tuned for high cell efficiency. Electron-rich groups such as phenylamine, aminocoumarin, indoline, (di-fluoronil) TPA, and carbazole are chosen as donor groups. π-conjugated groups such as thiophenes, polyenes, and benzothiadiazole are selected for strong electron transmission. Electron acceptor groups such as cyanoacrylic acid, rhodamines, and pyridines are preferred. In this study, a metal-free dye molecule with a donor-π-acceptor (D-π-A) structure named BIM33 was synthesized, and photovoltaic examinations were conducted. The molecule contains tris(4-bromophenyl)amine as the donor, acridine as the π-bridge, and cyanoacrylic acid as the acceptor. The synthesis of a dye containing the acridine group in a D-π-A structure has not been conducted before this study. Characterization parameters were achieved for BIM33 through 1H NMR, 13C NMR, and High-Resolution Mass Spectrometry (HRMS) for structure characterization. The theoretical and electrochemical properties were characterized using UV-vis and cyclic voltammetry (CV) methods. Subsequently, the current density-voltage (J-V) measurements of the solar cells produced with this sensitizer and the efficiency characteristics based on the photon ratio obtained under sunlight (100 mW cm-2) were examined. Surface charge transfer processes were determined using electrochemical impedance spectroscopy (EIS). Optimization studies were conducted for the appropriate solvent, optimum duration, and maximum efficiency for the synthesized BIM33 dye. The best cell efficiency was determined for TiO2 types labeled T/SP, R/SP and D/SP. Current density-voltage curves were recorded for each study. The results indicated Voc 0.603 V, Jsc 8.81 mA.cm-2, FF 0.60, and PCE 3.18% for BIM33 dye. A co-adsorbent (CDCA) study was then conducted to prevent dye aggregation. The concentration of CDCA was increased from low to high to achieve the maximum cell efficiency. At a concentration of 3 mM, the current density-voltage curve values increased to Voc 0.622 V, Jsc 9.74 mA.cm-2, FF 0.70, and PCE 4.22%. Solid-UV (TiO2-UV) and solution-UV measurements were taken for the BIM33 dye. According to the obtained absorbance-wavelength graph, TiO2-UV exhibited maximum absorbance peaks at two different wavelengths, 409.5 nm and 523 nm. Solution-UV displayed peaks at 371 nm and 444 nm, with a shift towards the red spectrum compared to TiO2-UV. A cyclic voltammetry (CV) study was conducted with the BIM33 dye using ferrocene as a reference and dimethyl sulfoxide (DMSO) as the solvent. The results revealed that the HOMO (Highest Occupied Molecular Orbital) of the dye wasmeasured at -0.98 V, and the LUMO (Lowest Unoccupied Molecular Orbital) was measured at 1.48 V. In CV analysis, it is desirable for the dye to have a lower energy level than the conductivity band of TiO2. This ensures that the electron flow from the dye occurs easily. The applicability of this study was confirmed by observing that the HOMO value of the BIM33 dye is lower than the HOMO value of TiO2, which is -0.5 V. In order to enhance the current cell efficiency of the BIM33 dye, a co-sensitization study was conducted using three different commercially produced and sold reference dyes: D205, RK1, and N719. Two different co-sensitization approaches were employed: sequential co-sensitization and mixed co-sensitization. The BIM33 and RK1 dyes were subjected to sequential co-sensitization studies. In the study conducted with the RK1 dye, which had a current cell efficiency of Voc 0.686 V, Jsc 19.49 mA.cm-2, FF 0.62, and PCE 8.25%, the maximum cell efficiency was achieved with Voc 0.654 V, Jsc 19.13 mA.cm-2, FF 0.65, and PCE 8.09%. Subsequently, co-sensitization studies were conducted with both BIM33 and N719 dyes, in both sequential and mixed modes. In the mixed co-sensitization study with the N719 dye, which had a current cell efficiency of Voc 0.708 V, Jsc 15.23 mA.cm-2, FF 0.64, and PCE 6.90%, the maximum cell efficiency was obtained with Voc 0.645 V, Jsc 14.68 mA.cm-2, FF 0.60, and PCE 5.68%. The sequential co-sensitization study resulted in a maximum cell efficiency of Voc 0.606 V, Jsc 15.42 mA.cm-2, FF 0.58, and PCE 5.39%. Finally, sequential co-sensitization was conducted with BIM33 and D205 dyes. In the study conducted with the D205 dye, which had a current cell efficiency of Voc 0.665 V, Jsc 11.65 mA.cm-2, FF 0.68, and PCE 5.29%, the maximum cell efficiency was achieved with Voc 0.662 V, Jsc 17.02 mA.cm-2, FF 0.61, and PCE 6.87%. These results indicate that a higher cell efficiency was achieved in dye-sensitized solar cells using the newly synthesized BIM33 dye, surpassing the current efficiency of both BIM33 and D205 dyes.