Optimisation and characterisation of alternative hole transporting media of dye-sensitised solar cells and stability study of perovskite solar cells

Abstract

Dye-sensitised solar cells (DSSCs) are regarded as a possible alternative to silicon-based photovoltaics because of their potential for low-cost production. The processing of two alternative hole transport media, one for liquid-state DSSCs and the other for solid-state DSSCs is studied in this thesis. Also, research interest in methyl ammonium lead iodide perovskite solar cells has been increasing quickly. This thesis also reports some preliminary studies on the stability of TiO2/CH3NH3PbI3 perovskite solar cells. Water is not commonly used as a solvent in liquid electrolyte DSSCs, but there are many reasons to re-examine water, ranging from cost advantage to fundamental science. The first part of the thesis addresses the wetting and recombination issues of water-based DSSCs. DSSCs using only water as the solvent and guanidinium iodide/iodine as the redox couple have been fabricated and they operate at 4% energy efficiency under 1-sun illumination. The second part of this thesis studies melt-processing of hole transport materials. This technique overcomes the problem of poor pore filling which is commonly observed in solid-state dye-sensitised solar cells. It is found that the low efficiency of melt-processed DSSCs is due to the heat applied during the melting process which causes a decrease in recombination lifetime. Solid-state DSSCs made with melt-processed spiro-OMeTAD are shown, with a maximum efficiency of 0.45 %. Stability of TiO2/CH3NH3PbI3 perovskite solar cells is examined in the third part of the thesis. Most literature in the perovskite solar cells focuses on the efficiency of devices, with little attention being paid to stability. A TiO2/CH3NH3PbI3 solar cell has been exposed to 40 sun-equivalent constant illumination for 63 hours (which delivers over 2700 hours equivalent of 1 sun photo-excitations). The loss in the cell’s Jsc was only 7 %, however the loss in Voc was 190 mV (24 %) at 1 sun.