Functionalization Of Silicon Nanoparticles And Their Application In Polymer Solar Cells

Organic photovoltaics (OPVs) take advantages of wide source, light weight and easy fabrication, despite the disadvantages of low power conversion efficiency(EQE), short lifetime and low charge mobility. Silicon nanoparticles (SiNPs) gain extensive attraction as solar energy harvesting materials, because they are nontoxic and have the inherent superiority of semiconductor nanocrystals, such as quantum confinement effects and band gap tunability. Fabricating organic/SiNPs composites is believed to be a possible route to enhance the performance of solar cells. In order to overcome the disavantages of organic semiconductor, we plan to graft organic semiconductor molecular onto the surface of SiNPs by covalent bond. Exploring the usage of SiNPs modified with organic semiconductor in OPVs, which will be a new way to functionalize the SiNPs, can obtain important scientific significance and potential applications.In Chapter1, the whole world energy problem, the working principle of OPVs, device structures and progresses on organic solar cells are reviewed. The preparation and modification of SiNPs and their application in solar cells are concluded,In Chapter2, we studied the surface modification of SiNPs by3,3"’-didodecylquaterthiophene (QT). H1-NMR shows that SiNPs were grafted with functional groups via strong and stable covalent Si-C bond. The red shifts observed in the UV-visible and fluorescence emission spectra prove the chemical linking between SiNPs and QT group. Charge transfer from SiNPs to QT is observed and surface functionalized SiNPs are photochemically stable and soluble in nonpolar organic solvents. Characterized by surface photovoltage spectroscopy (SPS), SiNPs-QT is turned out to be a P-type semiconductor material, which can be used as donor in hybrid solar cells. Device of SiNPs-QT/PCBM shows much better performance than that based on QT/PCBM system, which proves that organic semiconductor combined with inorganic nanoparticles gain greater performance. In Chapter3, stable aqueous amino-grafted silicon nanoparticles (SiNPs-NH2) were prepared via one-pot solution method. By grafting amino groups on the particle surface, the SiNPs can disperse uniformly in water to form clear solution. By incorporating SiNPs-NH2into the hole transport layer of the poly (3,4-ethylenedioxythiophene)/polystyrene sulfonic acid (PEDOT:PSS), the phase separation and charge transport can be improved. What’s more, the roughness of PEDOT:PSS films increases by addition of SiNPs-NH2, which help form better connection between PEDOT:PSS and active layer. Thus, compared with the devices fabricated by original PEDOT:PSS, OPVs with PEDOT:PSS containing1wt%of SiNPs-NH2exhibits a9.8%PCE enhancement.In Chapter4, we have investigated the effect of silicon nanocrystals (SiNCs) as a third component on performance of OPVs composed of poly[2-methoxy,5-(2’-ethylhexyloxy)-1,4-phenylene vinylene](MEH-PPV):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film. By adding suitable amounts of SiNCs into MEH-PPV:PCBM blend, the device performance such as external quantum efficiency (EQE), short circuit current density (Jsc), and PCE improved. Incorporation of2.5%SiNCs in the blend led to13.6%improvement of Jsc, which in turn resulted in18%improvement of PCE up to2.28%. The improved performance was mainly due to the improvements both in the charge generation from the interface of MEH-PPV/SiNCs and the charge collection at the cathode.In Chapter5, chromatographic column was used in the purification of butyl-coated SiNPs. After purification, pure SiNPs-butyl with low size distribution was gained, showing a very high fluorescence quantum yield (QY). With the excitation wavelength of310nm, the QY was as high as64.1%, which is the highest QY of SiNPs prepared from solution method. SiNPs also show the potential application in light emitting diode (LED).