Study On The Optoelectric Properties Of Efficient Polymer And Pbs Colloidal Quantum Dot Solar Cells

Harvesting energy directly from the sunlight by using photovoltaic (PV) technology isbeing widely recognized as one of the most promising ways to tackle today’s energy issues.Despite of the great progress in inorganic solar cells that plays a leading role in the field,nevertheless, high material and production costs limit its large-scale applications. In recentyears, as emerging alternative energy sources, polymer solar cells (PSCs) and colloid quantumdot solar cells have attracted increasing attention in both academic and industrial area. Thecharacteristics of polymer photovoltaic materials, such as bandgap and charge carrier mobilitycan be tuned readily through molecular design, thus can provide wide selection for PSCs.Similarly, the electrical and optical properties of quantum dots (QDs) can be tuned throughadjusting their size, shape, composition and aggregation morphology, allowing efficientutilization of major part of the visible and infrared of the solar spectrum. Due to ability ofsolution-processing, large-area and flexible devices can potentially be fabricated in a low-costway. Therefore, these two photovoltaic technologies have become hot research topics in thefield of optoelectronic materials and devices. Nowadays, the highest power conversationefficiencies (PCE) of the solution-processed PSCs and PbS colloid quantum dot solar cellshave exceeded10.6%and7.4%, respectively. Despite the significant progress, there is still along way to go toward the practical application of these two photovoltaic technologies. Themain aim of this thesis focuses on the optoelectric properties of efficient polymer and PbScolloidal quantum dot solar cells, and the influences of solvent annealing on the morphologyof the active layer and the concomitant effect on the device performances. The researchresults provide valuable references for the low-cost and solution-processed solar celltechnologies.In the first part, we demonstrate highly efficient flexible PSCs with a PCE of8.71%, ascertified by the National Center of Supervision&Inspection on Solar Photovoltaic ProductsQuality of China (CPVT), by using an inverted architecture. The reported efficiency is thehighest value for flexible PSCs reported to date. After a thin layer of alcohol conjugatedpolymer, poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7–fluorene)-alt-2,7-(9,9–dioctyl-fluorene)](PFN) as surface modification layer atop of PET/indium tin oxide (ITO), the work function and the RMS roughness of ITO were reduced simultaneously. The modified ITO canform ohmic contact with the photoactive layer which is beneficial for efficient electronextraction. The flexible PSCs retained92%of its initial efficiency when stored in ambient aircondition for120days, representing very good shelf storage stability. In the bending testconditions, no decay of device’s photovoltaic performance was observed. The fabricateddevice also has a very high specific power of400W kg-1, thus this kind of flexible PSCs canopen new opportunities for many novel terrestrial and space applications. It is worthy tomention that the processing method for the flexible PSCs is solution-based and all of the filmdeposition processes are completed at room temperature, thus this simplicity of preparingflexible PSCs can offer easy-processibility over large area size on an industrial scale at roomtemperature.Device performance of a solar cell is closely related with the morphology of the active layer.In the second chapter, solvent annealing was adopted to optimize morphology of the activelayer and improved performances of PSCs was achieved. We realized a PCE of4.53%basedon P3HT/PCBM active layer by using high boiling point solvent annealing (HBPSA) method.The conduction band of ZnO electron extraction layer was tuned by PFN modified interlayer,facilitating electron transport and extraction. When the thermally evaporated MoO3wasreplaced by the solution processed vanadium pentoxide, which was used as efficient holeextraction layer in P3HT/PCBM PSCs, and in conjunction with alcohol soluble conjugatedpolymer PFN as electron extraction layer, a high PCE of3.65%was achieved in P3HT/PCBMdevices. Furthermore, low boiling point solvent annealing (LBPSA) was employed toimprove the performance of D-A low bandgap polymer based PSCs. It was found that thistreatment also lead to enhancement in light absorption and favorable donor/acceptor phaseseparation. As a result, improved charge carrier mobilities and concomitant improved fillfactor and device performance was observed. This LBPSA method was also applicable tovarious donor-acceptor blend systems. For instance, upon the treatment, the PCE ofPDTBDTFTQ/PC71BM blending system increased from5.21%to7.25%, with the FFincreased from49.9%to74.2%. The PCE of SFTBT/PC71BM based-small molecular organicsolar cells and PCDTBT/PC71BM based-PSCs increased from1.88%to3.39%and5.16%to7.03%, respectively. Moreover, the performance of PSCs was found to be very sensitive to the treating time. Too long treating time led to an enlarged phase separation and significantrecombination loss of charge carriers, as indicated by reduced the short current density (Jsc)and PCE of the device.In the third chapter, PbS QDs with good dispersibility and uniform size were obtained byusing organic/inorganic hybrid ligands passivation scheme. We demonstrate highly efficientschottky PbS quantum dot solar cells with open-circuit voltage of0.54V and PCE of3.72%.The impact of alkyl chain length of the solvent for PbS colloidal quantum dot solarphotovoltaic performance was explored. Through the used of oleylamine, which has longeralkyl chain length, as the protection solvent in the synthesis process of PbS colloidal QDs, amaximal PCE of3.52%for these schottky-type PbS quantum dot solar cells was reached. It isworthy to note that the incorporation of alcohol-soluble conjugated polymer PFN interlayercan effectively depressed the leakage current, as indicated by a much decreased darksaturation current, thus responsible for the improvement in FF and PCE.