Condensed Matter Physics
During 2014, the research activities of the Condensed Matter Physics project were focused on (a) synthesis and characterization of “polymer free” gel electrolytes and their use in dye sensitized solar cells and (b) Optimization of CdS films prepared by chemical bath deposition method for possible use in CdS/CdTe p-n junction solar cells. Under the first project, we have been successful in obtaining “polymer free” gel electrolytes using tetrapropyl ammonium iodide and potassium iodide dissolved in ethylene carbonate/propylene carbonate co-solvent and incorporating fume silica (EC:PC:Pr4NI:KI:I2:SiO2). This electrolyte composition has been optimized for the iodide salt concentration and fume silica content to obtain the gel electrolyte with highest ionic conductivity. Dye sensitized solar cells incorporating this electrolyte and nanoporous TiO2 anode exhibited a power conversion efficiency of 5.32% under the illumination of 100 mW cm-2 (AM 1.5).
The second project on optimization of cadmium sulfide (CdS) films for possible use in CdS/CdTe p-n junction solar cells was supported by the National Science Foundation. CdS thin films have been grown by chemical bath deposition (CBD) technique. Energy band gap, optical absorption and sheet resistance were studied as a function of the film thickness and annealing temperature and time. Indium doped CdS films and also CdS films grown on TiO2 buffer layer were studied in detail to determine their effect on energy band gap. According to our observations, the energy band gap values increase with In substitution as well as with TiO2 buffer layer. CdS/CdTe thin film solar cells will be fabricated using modified CdS films.
Solid State Chemistry
During 2014, Solid State Chemistry research focused on three projects: (a) Efficiency enhancement by mixed cation effect in dye-sensitized solar cells with liquid based electrolytes, (b) Solid state magnesium batteries with polyethylene oxide based gel polymer electrolyte and SnO2 cathode and (c) Efficiency enhancement of dye sensitized solar cells by TiO2 photo-anode modification with silver and gold nano particles.
For project (a) solution electrolyte was prepared by using ethylene carbonate (EC), propylene carbonate (PC), acetonitrile (ACN), potassium iodide (KI), tetrapropyle ammonium iodide (Pr4NI) and iodine (I2) The electrolyte was sandwiched between a TiO2 photoanode and platinum counter electrode. The current-voltage (I-V) characteristic of the cells was measured using simulated sunlight with 100 mW cm-2 (AM 1.5). The solar cell containing the electrolyte with composition KI 33.3% W/W shows 7.22 % efficiency, while the solar cells contain only KI and only Pr4NI showed efficiencies of 5.77 % and 4.02 %, respectively. Therefore it is clear that the mixed cation effect can be used to enhance the DSSC efficiencies in solution electrolyte based dye sensitized solar cells. Under project (b), SnO2 was used as the cathode material in rechargeable magnesium batteries fabricated with Mg++ ion conducting, quasi solid (gel) polymeric electrolyte based on polyethylene oxide (PEO) as the host matrix. Batteries fabricated with cell configuration Mg/PEO:EC:PC:Mg(CF3SO3)2/SnO2 exhibited a discharge capacity of 220 mAh/g with 1.90 V open circuit voltage.
Under project (c) we have studied the use of silver nanoparticles (Ag NPs) incorporated TiO2 photoanode, for the enhancement of the photocurrent of the DSSC by the surface Plasmon effect. It was found that the energy conversion efficiency of the DSSC was strongly dependent on the amount of Ag NPs incorporated into the TiO2 matrix. The conversion efficiency of the DSSC increased from 5.00 % to 6.12% due to the incorporation of Ag NPs. This efficiency enhancement has been attributed to the increase of the short circuit photo current density of DSSCs by the Plasmon effect of the metal nanoparticles.
Fabrication of highly efficient polythiophene sensitized metal oxide photovoltaic cells G.K.R. Senadeera, K. Nakamura, T. Kitamura, Y. Wada and S. Yanagida Applied Physics Letters (American Institute of Physics) 83, (2003), 5470.
Highly stable dye-sensitized solid-state solar cell with the semiconductor 4CuBr.3S(C4H9)2 as the hole collector. K. Tennakone, G.K.R. Senadeera, D.B.R.A. De Silva and I.R.M. Kottegoda Applied Physics Letters (American Institute of Physics) 77, 15, (2000) 2367.
Ph.D, M.Phil., M.Sc. students, undergraduate students trained during 2014 with their affiliations, completed graduated during 2014 Postgraduate students trained/supervised by Prof. M.A.K.L. Dissanayake and Dr. G.K.R. Senadeera during 2013/2014: