ORGANIC/INORGANIC SEMICONDUCTOR BASED OPTO-ELECTRONIC DEVICES: LIGHT DETECTORS AND DYE-SENSITIZED SOLAR CELLS.

Abstract

Emerging opto-electronic devices (e.g. light sensors and solar cells) based on thin-film devices have been emphasized, owing to their many interesting properties including low cost, reduced weight, high flexibility and solution process-ability. The present study aims to enhance the performance of opto-electronic devices by means of optimizing processes associated with their fabrication and investigating different device configurations. In the first phase, improvements in the electrical and photodetection properties of thin film devices were done. Sensitization with dyes is an emerging process to improve device optical properties. The devices sensitized using di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N719) dye showed large leakage currents (2.62 μA). To reduce this leakage current and improve rectification behavior, the N719 dye was co-sensitized with 5,12-bis([1,1':3',1''-terphenyl]-2'-yloxy)anthra[2,1,9-def:6,5,10-d'e'f']diisochromene-1,3,8,10-tetraone (AS-2). This has led to a significant reduction in the leakage current to around 0.07 μA and increased linearity and higher breakdown voltage. Nevertheless, properties like sensitivity and responsively resided within acceptable ranges. Promising photodetection results were also achieved when the titanium dioxide (TiO2) active layer was completely replaced with PCBM:P3HT, thereby creating a hybrid device BHJ based configuration. In the second phase, new type of copper nanowires (CuNWs), reduced graphene oxide (rGO) and poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS) based counter-electrode was developed to replace costly indium tin oxide/platinum (ITO/Pt) electrode. The fabricated hybrid electrode exhibited high transparency alongside low sheet resistance (20 Ω cm-2). Upon integration in the PV device, the CuNWs/rGO/PEDOT:PSS based counter-electrode displayed long-term stability and produced superior performance in terms of photodetection parameters such as response time, reset time and responsivity. Nonetheless, focus was paid on optimizing the photovoltaic properties of the TiO2 photo-electrodes, which were prepared by the electrophoretic deposition (EPD) technique. The post deposition-sintering process of these electrodes was also optimized. When integrated within DSSCs the photo-anode containing TiO2 semiconductor layer deposited at 5 VDC produced the highest efficiency of 4.2%. A further increase of 6.66% in efficiency was achieved due to the restrained nucleation and growth (i.e. altered crystallinity) of anatase nanoparticles, when single step sintering was replaced with two-step sintering process. These results are expected to have a significant effect on solar cell and photo-detection industry by fostering improvement of thin-film opto-electronic devices.