Towards metal free counter electrodes for dye-sensitized solar cells

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Syed Ghufran Hashmi Name of the doctoral dissertation Towards metal free counter electrodes for dye-sensitized solar cells Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 41/2014 Field of research Photovoltaics, Micro and nanotechnology, Renewable energy Manuscript submitted 17 January 2014 Date of the defence 16 May 2014 Permission to publish granted (date) 11 March 2014 Language English Monograph Article dissertation (summary + original articles) Abstract This thesis provides a technical overview of results from addressing the critical steps in preparing counter electrodes for dye-sensitized solar cells (DSC) on flexible substrates (mainly plastics) to pave the way towards low-cost DSCs. The aim was to develop low-temperature counter electrode inks and pastes that could be suitable for high volume manufacturing processes. A special focus was given on the mechanical stability (flexibility and adhesion) of the materials that were deposited through a low-temperature route.This thesis provides a technical overview of results from addressing the critical steps in preparing counter electrodes for dye-sensitized solar cells (DSC) on flexible substrates (mainly plastics) to pave the way towards low-cost DSCs. The aim was to develop low-temperature counter electrode inks and pastes that could be suitable for high volume manufacturing processes. A special focus was given on the mechanical stability (flexibility and adhesion) of the materials that were deposited through a low-temperature route. Different low temperature inks and pastes based on carbon composites were tested as a replacement to expensive platinum nanoparticles based catalyst layer and showed good mechanical stability, comparable photovoltaic performance and low charge transfer resistance in complete dye-sensitized solar cells. Indium doped tin oxide (ITO) layer that is normally used as a conductor on plastic substrates was successfully replaced with a single walled carbon nanotube (SWCNT) film deposited as a low-temperature curable ink on polyvinyl chloride (PVC) substrate. The SWCNT films exhibited remarkably good mechanical stability when subjected to bending and tape adhesion tests. Coating the film with a thin layer of conducting polymer poly (3, 4ethylenedioxythiophene (PEDOT) increased its catalytic performance comparable to a reference platinum counter electrode. Relevant to scaling up DSC preparation, a two way electrolyte filling method was studied as a means to suppress performance losses that occur due to non-homogenous distribution of electrolyte components. The method enhanced the photovoltaic performance of a segmented measurement cell by up to 42% compared to traditional one way filling of the electrolyte. The aforementioned results provide pathways for the future development of robust flexible counter electrodes that can be beneficial especially from the perspective of high throughput roll-to-roll processing of flexible DSCs on polymer substrates.