Abstract
Bulk-heterojunction (BHJ) polymer solar cell (PSC) has attracted great attention because of its ease of fabrication, promising flexibility, and capability for large-scale and low-cost production. For common conjugated polymer Poly (3-hexylthiophene-2,5-diyl) (P3HT) : [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) system, the power conversion efficiency (PCE) has reached 5%. In order to achieve higher device performances, we replaced PC61BM with PbS quantum dot (QD) which possesses air stability, ease of synthesis, and low-cost fabrication. PbS QD has higher light absorption from the wavelength 200 nm to 400 nm than PC61BM, which is in complementary with P3HT that absorbs the light in long wavelength region 400 nm-600 nm. In this thesis, we dedicated to improve PCE of P3HT:PbS QD hybrid solar cell. We introduce the long-length (18 carbon length) oleic acid (designated as OA ligand) in the PbS QD synthesis procedure in order to stabilize the PbS QD. But the OA ligand is unfavorable for carrier transport, which results from long insulated alkyl, it is necessary to exchange long-length OA ligand to short-length ligand. There are two methods for exchanging the OA ligand of PbS QD. First, long ligand exchanges to short ligand after QD film formation, which is called post-treatment in this thesis and is a common method for ligand exchange. In this method, the carriers transport mobility are enhanced, which is resulted from the shorter distance between QD though some cracks in the film appear after the ligand exchange process. Second, in order to reduce the cracks in the post-treatment, we introduce short ligand into PbS QD solution for ligand exchange in advance, called pre-treatment in this thesis. This research then tried to find out the dispersion extents of PbS QD and P3HT by the pre-treatment. The long-length OA ligand of PbS QD exchange to short-length ligand 1-decanethiol (DT), 1-octanethiol (OT), 1-hexanethiol (HT) and 2-ethylhexanethiol (HET) in the pre-treatment. The OA ligand of PbS QD are designated as PbS-OA, and the PbS QD after the pre-treatment are designated as PbS-DT, PbS-OT, PbS-HT, and PbS-HET, respectively. Except for dispersing PbS QD by alkyl chain molecules, we also found that the short alkyl ligand PbS QD interacts well with the side-chain of P3HT. In the Atomic Force Microscope (AFM) topography and phase image, we found that the dispersion extent of P3HT and PbS QD increases when the ligand of PbS QD is exchanged in the pre-treatment, especially for the case exchanging by branched ligand (HET). Furthermore, we blend P3HT with both PbS-HT and PbS-HET together, the domain size of PbS QD is much smaller, the dispersion extent of P3HT and PbS QD is further improved, and a PCE of 3.95% is attained, which is the highest PCE in P3HT:PbS QD BHJ solar cell ever documented.