Development and characterization of flexible dye sensitized solar cells

Photovoltaic technology has been realized as a suitable renewable power source for the fulfillment of increasing world energy consumption with least impact on the environment. Dye sensitized solar cells (DSSCs) is one such photovoltaic technology which has gained a great deal of attention due to the low manufacturing cost compared to the conventional Si-based solar cell technologies. Fabrication of DSSCs on flexible substrates enables manufacturing solar cells using cost-effective and speedy roll-to-roll processing systems and also makes the device flexible and light-weight. However, polymer-based DSSCs set restrictions to their materials and fabrication processes. In this thesis, fabrication of DSSCs on flexible polymer substrates have been extensively studied concentrating on the factors related to the slurry preparation, deposition of films and processing of electrodes to improve the mechanical and photovoltaic properties of the device. Initially, mechanically-stable, well adhered TiO2 films based on nanocrystalline P-25 TiO2 slurries were fabricated on indium tin oxide (ITO) coated plastic substrates using ball milling as a part of the processing stage, without the need for binders or high temperature annealing. The strength of the TiO2 films was examined by a novel nanoscratch technique which was developed to assess inter-particle adhesion. Interfacial and photovoltaic properties of flexible dye sensitized solar cells with a ruthenium dye involving two tetra-butyl ammonium carboxylate groups (N-719) were studied. The maximum power conversion efficiency of 4.2% is obtained under illumination of 100mWcm-2, for the electrodes fabricated using 20 hour milled slurry and shorter or longer milling times were found to be less optimal. During the second stage of this work, binder-free titania pastes with high viscosity were developed for the preparation of improved quality electrodes for dye sensitized solar cells on plastic substrates. Rheological behavior of ethanol based titania pastes with the addition of ammonia, hydrochloric acid and water was investigated. The change in the viscosity is correlated with the measured zeta potential of the colloidal titania pastes. Improved inter-particle connectivity and hence better solar cell performance was found for the pastes containing acid or water. However, no such improvements were seen for the pastes containing ammonia. Maximum light to electrical energy conversion efficiencies of 4.9% and 5.0% were obtained for the plastic based dye-sensitized solar cells fabricated using water and acid-added slurries respectively. Thirdly, chemically-sintered, mesoporous ZnO electrodes with improved interparticle connectivity were prepared in the absence of any organic binders, using ammonia as the sole reagent to encourage interparticle connectivity. The reaction with ammonium hydroxide was found to increase the connections between ZnO grains by forming nanorods like a structure. The enhancement of adhesion among ZnO grains was evaluated from nano-scratch technique. Two different xanthene dyes were used to sensitize ZnO electrodes. The photo-voltage of 657 mV, fill-factor of 73 % and photo-current of 4.1 mAcm-2 with the maximum light to-electrical energy conversion efficiency of 2.0 % were obtained for plastic based ZnO|Mercurochrome|electrolyte solar cell under 100 mWcm-2 light intensity. One of the biggest challenges for DSSCs on plastic substrates is the difficulty in making good quality nano porous TiO2 films with both good mechanical stability and high electrical conductivity. Cold isostatic pressing (CIP) is a powder compaction technique that applies an isostatic pressure to a powder sample in all directions. It is particularly suitable for making thin films on plastic substrates and even on non-flat surfaces. During the final stage of this work, cold isostatically-pressed nanocrytalline TiO2 electrodes with excellent mechanical robustness are prepared on indium tin oxide (ITO) coated polyethylene naphthalate (PEN) substrates in the absence of organic binders, and without heat treatment. The morphology and the physical properties of the TiO2 films prepared by the CIP method were found to be very compatible with requirements for fabricating flexible DSSCs on plastics. This room-temperature processing technique has led to an important technical breakthrough in producing high efficiency flexible DSSCs. Devices fabricated on ITO/PEN films by this method using standard P-25 TiO2 films with a Ru-complex sensitizer yielded a maximum IPCE of 72% at the wavelength of 530 nm and showed high conversion efficiencies of 6.3% and 7.4% for incident light intensities of 100 and 15 mWcm-2, respectively, which are the highest power conversion efficiencies achieved so far for any DSSC on a polymer substrate using the widely-used, commercially-available P-25 TiO2 powder.