Modulating optical and electrical properties of TiO2 photoanode in dye-sensitized solar cells
Pham, Thi Thu Trang
Date of Issue2013
School of Materials Science and Engineering
Energy Research Institute @ NTU
Dye-sensitized solar cells (DSCs) are promising photovoltaics because of their low cost and facile fabrication process. The difference between DSC and other conventional solar cells is that it separates the photoelectron generation and charge transporting functions. The dye absorbs light, is excited and injects an electron into the semiconducting oxide which transports it away. The important reactions in DSC occur at the interfaces of these materials, hence the interactions between them are critical to the device performance. Light harvesting in DSCs can be improved by developing sensitizers with broad absorption spectra and high extinction coefficient or by designing semiconductor nanostructures that can enhance optical properties through phenomena such as plasmonics, photonic confinement or light scattering. This thesis focuses on a new TiO2 nanostructure for the DSC photoanode that can enhance light harvesting while increasing film porosity. The photoanode was modified by incorporating macropores within mesoporous TiO2 nanoparticles film to create a mixture of bimodal pore sizes. The macropores are filled with electrolyte and have different refractive index from the surrounding TiO2 nanoparticles, thus can scatter incident light, according to Mie theory. PS and ZnO were chosen as templating agents for the formation of macropores. The former created ellipsoidal pores while the latter, after removal, left behind more spherical pores. All films containing macropores had lower surface area compared to standard films. Film thickness and macropore concentration were investigated to optimize the photovoltaic performance with suitable dye/electrolyte combination. Optical properties of the mesoporous-macroporous films were investigated by measuring diffuse transmission and reflection in an integrating sphere, showing strong light scattering with increasing pore sizes and concentration. Finite-difference time-domain FDTD simulation was conducted to understand the influence of changing shape and dimensions of scatterers. The simulation revealed that ellipsoidal scatterers are as effective as spherical scatterers, if their concentration was sufficient. Better light harvesting from these photoanodes could be observed by investigating the IPCE (incident photon to current efficiency) of the corresponding devices. To study changes in charge recombination and electrolyte diffusion, several methods such as electrochemical impedance spectroscopy, intensity modulated voltage/photocurrent spectroscopy as well as photovoltage, photocurrent transient were employed to calculate the electron lifetime, recombination and diffusion resistance. Through these studies, it was revealed that the presence of macropores reduced the recombination between electrons in TiO2 and electrolyte as well as enhanced the diffusion of I3- or Co3+ ions in electrolyte to improve mass transport. These enhancements became more significant when the right combination of dye and electrolyte were used to fabricate DSCs from the mesoporous-macroporous films. By using high extinction coefficient organic dye and ionic liquid or cobalt electrolyte, the films containing macropores exhibited improved power conversion efficiency, mostly attributed to higher Voc, increasing FF and Jsc at high light intensity. The application of these novel photoanode structures can be extended to solid-state DSCs and other architectures which requires thinner photoabsorbers.