Influence of Counter Ions Binary Doped Conducting Polymer on the Photovoltaic Properties of Dye Sensitized Solar Cells

Abstract

Dye-sensitized Solar Cells (DSSCs), different in principle from the conventional solar cells based on p–n junctions, are competitively cost-effective. For development of this kind of emerging solar cell technology, it is very significant to reduce cost and improve energy conversion efficiency of these cells to the maximum extent. The work presented in this thesis is focused on the synthesis of binary doped Polyaniline (PANI) salts and their use as counter electrodes (CEs) to replace the noble and expensive metal Pt in DSSCs. An effective material should exhibit high conductivity, high catalytic activity and favorable morphology to be used as a CE in DSSC. Therefore, a novel dopant couple comprising of an organic acid i.e Ammonium Lauryl Sulphate (ALS) and an inorganic acid i.e Sulfuric Acid (H2SO4) was employed with the aim to enhance the conductivity, favorable morphology and electrocatalytic activity of PANI. The synergy of the positive quality factors of these two acid dopants was realized by concurrent and optimized doping, resulting in a molecular composite material that renders good photovoltaic performance. This dissertation comprises of two parts: one is based on the synthesis and optimization of reaction parameters for obtaining binary doped PANI most suitable for utilization as CE in DSSCs whereas second part includes the study of the effect of counter ions of the binary dopant ions on the photovoltaic properties of the DSSC device by comparing its performance with DSSCs based on Pristine PANI, H2SO4 doped PANI, ALS doped PANI, and Pt CEs. Different morphological, optical and spectroscopic techniques were employed to characterize as synthesized PANI salts. The improvement in conductivity and decrease in band gap illustrated the positive effect of binary dopants on the binary doped PANI salts. These properties are known to be of great importance to the catalytic electrode material. Electrocatalytic properties of binary doped PANI CEs were investigated by Cyclic Voltammetry. ix The higher values of reduction current density (Jred) and diffusion coefficient (Dn) and lower values of peak to peak separation (Epp) also contributed to the desirable properties of the synthesized material for their better performance as a catalytic material in solar devices. These synthesized materials were tested for photovoltaic application. It was interesting to realize that these materials demonstrated good photovoltaic properties as compared to noble metal Pt. Furthermore, these fabricated devices were also checked through Electrochemical Impedance Spectroscopy EIS to understand the charge transfer processes and very encouraging results have been obtained with decrease in charge transfer resistance. In addition to these, the durability of DSSC based on binary doped PANI was also examined by start/stop switching. In the second part of dissertation, comparative study of optimized binary doped PANI with pristine PANI, H2SO4 doped PANI, ALS doped PANI and Pt is discussed. All the above mentioned techniques were used to study the physical, electrochemical properties and photovoltaic properties of the synthesized materials. Comparative analysis revealed that the counter ions of the dopant do have great impact on the properties of PANI based DSSCs. The binary doped PANI showed superior electrochemical properties, photovoltaic properties and facile charge transfer. At optimized fabrication conditions, the counter electrode shows significantly high photoelectric conversion efficiency of 4.54% compared to 4.02% for reference platinum counter electrode. Charge transfer resistance at the interface between electrolyte and counter-electrode is also decreased for the binary polyaniline based counter electrode. Furthermore, the devices also present the characteristics of multiple start/stop ability and fast activity. The simple preparation procedure, low cost and improved photovoltaic properties permit fabricated counter electrodes to be a reliable alternative for dye sensitized solar cells for their better performance as a catalytic material in solar devices. Keywords: Dye sensitized solar cells, counter ions, binary doped PANI, counter electrodes, electrocatalytic activities, start/stop switching