| Summary: | This doctoral dissertation presents the study of the development and characterization of solar energy conversion devices based on nanocomposite semiconductors. The PhD research is focused on third generation solar cells, namely in dye−sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). The innovation of this study is detected on the use of novel materials by well−known methods in order to intervene in the structural and morphological properties of the photoelectrode, the investigation of newly synthesized sensitizers and hybrid organic/inorganic materials employed in quasi−solid state electrolytes for their jellification, alternative counter electrodes to replace platinum and composite anodes for efficient electron transport to enhance the electrical characteristics of the final devices. Additionally the upscaling of transparent strip−shaped DSSC of various lengths (1 cm to 45 cm) was carried out by inkjet printing the electrodes to ensure the reproducibility and the accuracy of the research findings and the obtained results are evaluated.
The study regarding DSSCs is limited on TiO2 solar cells and is divided into individual sections each one confining in one of the components of a DSSC. Firstly, the research is concentrated on the photoanode and specifically on different approaches to modify the films to optimize the performance of the corresponding devices. The methods that were tested were the incorporation of a small amount of carbonaceous materials (carbon black powder (CBP) or multi−walled carbon nanotubes (MWCNTs)), the surface treatment of the TiO2 photoanode films by soaking them in a TiO2 solution and the testing of different TiO2 precursor materials or surfactants.
The use of template free P25−TiO2 solutions modified with CBP or MWCNTs showed that the incorporation of MWCNTs improved the overall performance of the DSSCs, in both sintering temperatures examined (100oC and 500oC) with the highest efficiency recorded for the devices employing P25−TiO2 electrodes with 0.1 wt% of MWCNTs. However, the CBP modified P25−TiO2 films resulted in poorer performing devices for all cases tested. The DSSCs with the post−treated P25−TiO2 films, where the anodes were immersed in two TiO2 solutions used for the first time (Titanium(IV) (triethanolaminato)isopropoxide and Titanium(IV) bis(ammonium lactate)dihydroxide solution) followed by annealing at 300oC and 500oC, demonstrated improved short−circuit current density promoting their overall efficiency up to 26−30% compared with solar cells with untreated electrodes. Among the titanium precursor materials used to fabricate transparent TiO2 films for DSSC, the solar cells with the Titanium(IV) butoxide photoanodes displayed the best electrical parameters and an 11.2% higher power conversion efficiency compared with the solar cells with the Titanium(IV) isopropoxide anode. In contrast, the last modification method tested using the surfactant Dioctyl sulfosuccinate sodium salt (AOT) in the TiO2 solution which could be easily removed by rinsing without damaging the film, didn’t reach to the desired results and the attempt to simplify the fabrication of TiO2 films through this procedure was given up.
Regarding the dye complexes tested as sensitizers in quasi−solid state DSSCs, initially six newly synthesized ruthenium dye complexes with different pyridine and bipyridine side groups were studied. The DSSCs that were sensitized with the dyes having two bipyridines (bpy-bpy) in their structure demonstrated almost the same performance as the solar cells that were sensitized with the commercially available dye D907 which has a similar structure as the new complexes. In particular, the solar cell sensitized with the dye CS28 displayed a slightly better power conversion efficiency than the one corresponding to the DSSC sensitized with D907 (3.28% and 3.26% respectively). Subsequently, two new triphenylamine based organic dyes with or without the additional electron donating hexyloxy groups, having a benzimidazole derivative as π−bridge were studied. The DSSCs sensitized with the dye without the additional hexyloxy groups (MZ−341) were more efficient compared with the ones sensitized with the dye with the hexyloxy groups (MZ−235) because of the higher absorbance of the former on the TiO2 photoelectrode. Finally, a water based natural dye solution was prepared by extracting the phycoerythrin pigment from red algae and was used as a sensitizer for DSSCs. The best results were obtained for an acidic dye solution (pH=3) and for a temperature of 35oC during the sensitization of the anode.
In the electrolyte solution only a minor alteration was examined regarding the hybrid material used for the gradual solidification of the electrolyte. All five hybrid organic/inorganic materials that were synthesized with polypropylene or polyethylene of different oligomer chain length as organic sub−phase demonstrated good thermal stability up to 150oC with ED600−ICS slightly standing out. Nevertheless no obvious differences were detected in the electrical parameters of the DSSCs that employed these hybrid materials in their electrolyte solution. As far as the alternative counter electrodes tested to replace platinum is concerned, DSSCs with the nickel doped CoS2 and the polypyrrole counter electrode displayed better electrical characteristics compared with the solar cells with platinum. In fact in both cases an enhanced short−circuit current density was recorded improving the solar cells’ overall efficiency.
Proceeding to the manufacture of large area transparent DSSCs with electrodes fabricated by inkjet printing, the main variations observed at these strip−shaped DSSC of various lengths (1 cm−45 cm) were at the JSC and the FF values which decreased as the length increased. Even so, the construction of strip−shaped DSSCs of intermediate length (20 cm−25 cm) connected in order to form a solar module in an almost square form could be a viable solution providing satisfactory results.
Finally, the PSCs with the TiO2−In2O3 binary electron transport layer which was used as a scaffold in order for the mixed halide perovskite (CH3NH3PbI3-xClx) to infiltrate exhibited a maximum power conversion efficiency of 12.86% which improved the efficiency of the PSCs with the pristine TiO2 over 28%. This is attributed to the better interfacial connection between TiO2−In2O3/CH3NH3PbI3-xClx, the efficient electron transport due to optimized alignment of the energy levels of the mixed oxides and the perovskite, as well as the reduction of the recombination processes. Baring in mind that all manufacturing processes for the perovskite solar cells were carried out in ambient conditions renders these results quite impressive.
Overall, it is presumed that the obtained research results, their interpretation and the final conclusions derived from this work will give an insight for future research prospects on third generation solar cells.
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