Novel redox mediators for dye sensitized solar cells

2017-01-24T00:28:09Z (GMT) by Hetti Arachchige, Ishanie Rangeeka Perera
With the depletion of the earth’s oil reserves, the likely doubling of the world’s energy consumption over the next three decades and the startling climatic consequences caused by fossil fuel combustion, the development of new technologies that employ various sources of renewable energy is an urgent necessity. Among the sources of renewable energy, solar energy is considered the most promising since the energy reaching earth in one hour is greater than the annual consumption by human beings. Thus, various types of photovoltaic devices have been invented to harvest solar energy. However, the construction of these devices can be quite expensive and required advanced processing techniques. The dye sensitized solar cell (DSC) is one promising technology which can potentially be constructed at low cost with simple procedures and with lower environmental impact than most other available photovoltaics. DSCs typically consist of a working electrode with a mesoporous oxide layer composed of nano-size particles coated on a conducting glass. Attached to the surface of the nanocrystalline film is a monolayer of a photoactive dye. An electrolyte consisting of a redox couple is filled between the working electrode and the counter electrode, a conducting glass coated with a catalyst. The function of the electrolyte is to transport charge between two electrodes and to regenerate the photo-oxidized/reduced dye, following photo-induced injection into the semiconductor. This thesis focuses on the development of new DSC electrolytes based on transition metal complexes as redox couples with a view to improving the environmental friendliness and performance in both n-type and p-type devices. In n-type DSCs (n-DSCs), light illumination initiates the photoexcitation of the dye which then injects an electron in to the conduction band of the n-type semiconductor. The injected electron flows through the external circuit towards the counter electrode while the reduced redox mediator in the electrolyte regenerates the photo-oxidized dye. Then, the oxidized redox mediator undergoes reduction by electron injection at the counter electrode completing the charge transfer cycle. In contrast, the electron flow in p-type DSCs (p-DSCs) is directly opposite to that of n-DSCs. In these devices the photo-excited dye injects a hole into the valence band of the p-type semiconductor. In other words, an electron is transferred to the photo-excited dye from the valence band of the p–type semiconductor followed by photo-excitation. The reduced dye is then regenerated by the oxidized redox mediator in the electrolyte. The oxidized redox mediator is restored by injection of an electron from the reduced redox mediator into the counter electrode which then flows through the external circuit towards the working electrode to complete the cycle. In searching for environmental friendly redox mediators, a manganese based redox mediator, tris(acetylacetonato)manganese(III)/(IV), was utilized in n-DSCs. In addition, to the low toxicity of acetylacetonate complexes, there is the possibility to fine tuning of the redox potential by applying a wide variety of acetylacetone derivatives. Tris(acetylacetonato)manganese(III)/(IV) mediated devices demonstrated promising performances with both organic and metal based sensitizers leading to a vast selection of sensitizers. A good overall energy conversion of 4.4% was demonstrated for these devices in conjunction with both types of sensitizers. Due to the impressive properties of acetylacetonate complexes and suitable redox potentials, the tris(acetylacetonato)iron(II)/(III) complexes were used as redox mediators in p-DSCs. Development of redox shuttles with suitable redox potentials and fast dye regeneration kinetics is vital for further improvement in p-DSC efficiencies. Application of a nickel oxide (NiO) blocking layer on the working electrode suppressed unfavourable back reactions at the electrolyte/working electrode interface. Furthermore, fast dye regeneration kinetics were observed for the tris(acetylacetonato)iron(II)/(III) redox mediator which led to the best current density and the best overall energy conversion efficiency (2.5%) reported so far for p-DSCs. The efficiency of p-DSCs is limited mainly due to the highly coloured semiconductor material in use (NiO). None of the alternative materials introduced, such as; copper based semiconductor materials and boron-doped diamond, could surpass the performance of the devices based on NiO. Recently, surprising p-type conductivity was revealed on well-known n-type degenerate semiconductor, indium doped tin oxide (ITO). Yet, the reported performance for p-DSCs based on ITO was quite low compared to NiO based devices. To further exploit this property and develop devices with better efficiencies, mesoporous ITO was applied in p-DSCs with three different redox mediators iodide/triiodide, tris(ethylenediamine)cobalt(II)/(III) and tris(acetylacetonato)iron(II)/(III). For tris(acetylacetonato)iron(II)/(III) mediated devices an efficiency of 2% was reached, which rivalled our best result of 2.5% for devices based on NiO. The mechanism of operation of the ITO based devices was also analysed and ITO shows a significant local density of states arising below −4.8 eV, that initiates electron transfer from the ITO to the excited dye leading to the photocathodic current within the device. Comparative studies also indicated that the rate of recombination at the ITO/electrolyte interface was faster than in the case of NiO. Therefore, development of redox mediators with faster dye regeneration kinetics or designing sensitizers with longer charge separated state may facilitate construction of more efficient p-DSCs based on mesoporous ITO. Finally, p-DSCs were constructed with an aqueous electrolyte based on (diaminosarcophagine)cobalt(II)/(III). Properties, such as low light absorption in visible region and more negative redox potential (vs. NHE), encouraged their use as p-DSCs redox mediators. The highest reported voltages for p-DSCs were measured (>800 mV) for these devices and were attributed to more negative redox potential. Moreover, the dependence of the device performance on pH of the electrolyte was studied. The redox potential of the electrolyte shifted from −0.10 V to −0.34 V (vs. NHE) when pH was varied from 4 to 7 and then remained the same up to pH 11. The best device performance (1.33%) was recorded at pH 8. In summary, a series of transition metal complexes were introduced as redox mediators for either n-type or p-type DSCs that pave the way to enhance device performance. Furthermore, ITO was shown to be a suitable semiconductor material for p-DSCs that could overcome the major drawback in these devices.