Doctor of Philosophy
Department of Chemistry
Panda, Dillip Kumar, Nanostructured organic solar cells, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3415
A wide range of state-of-the art solid-state dye-sensitized solar cells (ssDSCs) have been fabricated employing in situ photo-electrochemical polymerization (PEP). The ssDSCs were fabricated using nano-crystalline TiO2 as an electron acceptor, high molar extinction coefficient ruthenium-complex dyes with wide UV-Vis absorption range as photo-sensitizer, poly(3,4-ethylenedioxythiophene) (PEDOT) as hole transporting material (HTM), and both metallic and non-metallic counter electrodes. Potential problems with PEP are un-optimised filling and poor distribution of the conjugated polymer inside the pores of the nano-structured electrode. Better pore filling and hence better photovoltaic performance has been achieved here by optimising the polymerization conditions such as illumination orientation, light intensity, dopant used, electrolyte, solvent, PEDOT growth period and TiO2/PEDOT film thickness. This increased the power conversion efficiency of the ssDSC to 3 %, using the ruthenium dye Z907 (cis-Bis (isothiocyanato) (2, 2’-bipyridyl-4, 4’- dicarboxylato) (4, 4’-di-nonyl-2’-bipyridyl) ruthenium (II) and novel Goretex®-Au- PEDOT counter electrode, achieving the world’s most efficient ssDSC to date employing PEP fabrication methods for this dye.
PEP optimization was also combined with non-ruthenium dye and conjugated polymers enabling efficient charge transfer. Non-ruthenium dyes have also been studied, including zinc-porphyrin (GD2 and P-10), metal-free organic indoline dye (D-149), and small organic dyes (P-257), along with differing HTMs and counterelectrodes. Liquid DSCs (Grätzel cells) were also fabricated to compare efficiencies and to determine the achievable upper limit for the ssDSCs.
Current-voltage (JV) curves were recorded and Incident Photon to Current Conversion Efficiency (IPCE) determined at various polymer pore filling and distribution levels. Imaging and electrochemical mapping techniques including Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and SEMEnergy Dispersive X-ray Analyser (SEM-EDXA), have been employed to establish the morphology of the PEDOT/dyed TiO2 working electrode. HTM morphology and pore filling were characterised and related to the photovoltaic performance of the ssDSC. Raman spectroscopy, UV-Visible-near Infrared (UV-Vis-NIR) spectroscopy, matrix assisted laser desorption ionisation (MALDI) and mass spectroscopy were used to characterise the conductivity, doping level and the number of bis-EDOT units present in the photo-anodes.
The ruthenium dye regeneration kinetics in PEDOT-based ssDSCs were determined by Transient Absorption Spectroscopy (TAS) and compared with the common and conventional redox couple I-/I3 -. The stability and reproducibility of these solar cells were also demonstrated. The thesis concludes by pointing out the next steps to be taken for the fabrication of flexible solid-state dye-sensitized solar cells.