CHEMICAL INTERACTIONS ON COUNTER ELECTRODE SURFACE OF NANOSTRUCTURED POLYIANILINE IN DYE-SENSITIZED SOLAR CELLS (DSSC)

• Main Author: Reza, Muhammad
• Format: Dissertations
• Language: Indonesia
• Subjects: Kimia
• Online Access: https://digilib.itb.ac.id/gdl/view/57307
• Institution: Institut Teknologi Bandung

With the accelerating development of platinum-free Dye-Sensitized Solar Cells (Pt- Free DSSC), many researchers have studied ways to integrate carbonaceous materials as an efficient counter electrode (CE) in Pt-Free DSSC. Among conductive polymers, polyaniline (PANI) has received remarkable attention because it has controllable electrical conductivity, easy synthesis method, good environmental stability, and interesting redox properties associated with the chain heteroatoms. The problem occurs during the electrochemical reaction between PANI Emeraldine Salt (PANI ES) and triiodide (I3-) ion. The redox reaction between PANI ES and I3- ion leads to a negative standard potential cell at the CE surfaces. However, the potential cell value (Ecell) under nonstandard conditions follows the Nernst equation to facilitate the reaction. Therefore, a positive Ecell can still be obtained by increasing the adsorbed I3- ions on the PANI surface. Increasing the PANI surface area at the CE/electrolyte interface is another approach that can be carried out besides increasing the I3- ion concentration in the electrolyte solution. However, PANI ES is synthesized using a rapid mixing method that produces irregularly shaped particles with smooth surface morphology and low surface area. Meanwhile, the adsorption process is crucial in determining the sustainability of I3- reduction and charge circulation in DSSC. We report a reverse-micelle emulsion polymerization of nanostructured PANI (NPES) using a nonionic surfactant Polyglyceryl-2-Dipolyhydroxystearate (PGPH) at various concentrations from 2 to 6% (v/v). SEM images show that the obtained morphologies are irregular agglomerates at low PGPH concentration and relatively regular granules at high PGPH concentration. FTIR and Raman spectra show that NPES is in the form of Emeraldine salt with electrical conductivity around 10?3 S cm?1. Photovoltaic current-voltage (J-V) measurements show the highest power conversion efficiency (PCE) is achieved at 1.71% at 6% (v/v) of PGPH. We investigate the adsorption behavior of PANI towards I3- ion solution in acetonitrile. Two different types of PANI, namely PANI ES and NPES, were prepared by the rapid mixing and interfacial polymerization methods, respectively. NPES particles are interconnected granules with thorny durian-like surface morphology and better shape uniformity than PANI ES. The maximum adsorption capacity of NPES towards I3- ions is higher than that of PANI ES; however, both obey the dual-site Langmuir–Freundlich isotherm. The calculated thermodynamic parameters reveal the spontaneous and endothermic adsorption processes for both types of PANI. The positively charged group in PANI (C–N+) provides more favorable adsorption sites for I3- ions through electrostatic interactions, as confirmed by Raman spectroscopy. The photovoltaic and electrochemical performance data confirm the higher PCE (~30%) of the NPES CE compared to PANI ES. A density functional theory (DFT) study has been carried out to evaluate the adsorption ability of PANI for I3- ion. Two different structures, namely Bipolaron (Bip) and Polaron (Pol), and their corresponding complexes with I3- ion were studied. The spatial structure, energetics, vibrational spectra, and frontier molecular orbitals energy information was used to investigate the effect of different PANI structures on adsorption ability toward I3- ion. Structural and charge distribution changes were observed during the adsorption process for both PANI and I3- ion. Raman analysis revealed that the adsorbed I3- ion affects the PANI chain, mainly the C~N+ group, consistent with experimental data. The electronic properties of PANI are also affected by the presence of I3- ion. Interaction energy (?H) results suggest the first adsorption of I3- ion is slightly favorable on Pol while the second adsorption of I3- ion is more favorable on Bip. The further addition of I3- ion on PANI leads to a significant increase in ?H due to the steric hindrance. The reversible redox reaction between Bip and Pol having a positive value of ?H is also responsible for the observed ?H. We demonstrated surface modifications of NPES-based CE using microwave plasma. Two different types of plasma were used: oxygen and hydrogen plasma (O2- and H2-plasma). Optical emission spectra confirm the presence of reactive species in both plasmas, like O2+, O+, O*, and H. Microwave plasma treatment does not affect the surface morphology of NPES CE, which conserves its granule nanostructures, as revealed by SEM. Raman spectroscopy results show significant intrinsic oxidation state changes and the number of semiquinonoid species (SQ) of NPES after plasma treatment. Generally, O2-plasma increases the intrinsic oxidation state and SQ number, while H2-plasma usage causes both properties to decrease. The optimum condition of microwave plasma-assisted surface modification was O2-plasma usage for one minute, producing NPES with the highest intrinsic oxidation state and SQ number. Using this NPES as CE, we obtained a DSSC with a PCE of 3.17%.