Preparation And Optical Properties Of Functionalized Carbon Microspheres And Graphene

With increasing global fossil energy depletion and environmental pollution, solar energy as a new environmentally friendly renewable energy and inexhaustible energy has drawn enormous research concerns. Thus, solar cells, which convert solar energy directly into electrical energy, have come into being and become a hot topic. Polymer solar cells (PSCs) based on electron-donating conjugated polymers and electron-accepting carbon materials have attracted general attention because of their low cost, easy availability, light weight, flexibility, simple device fabrication, and many other advantages. The most successful acceptor material applied in PSCs is fullerene derivative (PCBM), but the structure of PCBM is difficult to be further optimized for solution processed PSCs. Moreover, the power conversion efficiency (PCE) of PSCs is still lower than that of inorganic solar cells. Carbon microspheres (CMSs) and graphene, because of their unique nature of structures, excellent optical, electrical, thermal and mechanical characteristics, have became potential electron acceptor materials for polymer solar cell.Based on preparation of carbon microspheres and graphene, this thesis prepared the porous carbon microspheres (PCMSs) by hydrothermal and annealing methods, which were further modified by silver nanoparticles (Ag NPs) at their surface; The reduced graphene oxide was prepared by hydrothermal method and then functionalized. Functionalized PCMSs and graphene were prepared, characterized, and their feasibility as acceptor material of PSCs was evaluated.The main results are as follows:1. Preparation and optical properties of porous carbon microspheres and poly (3-hexylthiophene) (P3HT) composite films. PCMSs were prepared by hydrothermal method and annealing treatment; they were of rough surface, uniform particle size, with a lot of oxygen-containing functional groups.(1) As the concentration of the glucose increased the particle size of PCMSs increased, with a diameter of 100 nm and a porous structure; PCMSs matched well with P3HT in level and had compatibility in organic solvents, promising for use as acceptor material of PSCs.(2) The ratio of P3HT to PCMSs of 2:1, spinning rate of 2000 rpm and suspension concentration of P3HT of 20 mg/mL (suspension concentration of PCMSs of 10 mg/mL) are appropriate for fabricating P3HT:PCMSs composite films for high performance polymer solar cells based on PCMSs. The optimal annealing temperature and time for the composite films of P3HT:PCMSs were 120℃ and 10 minutes, respectively.2. Preparation and optical properties of Ag/PCMSs and P3HT composite films. Two kinds of PCMSs were prepared by hydrothermal method with glucose as carbon source and annealed at 600℃ and 800℃ separately, and then the composite materials of Ag/PCMSs were prepared, in which Ag NPs were loaded on the surfaces of PCMSs through impregnation reduction method.(1) Annealing temperature did not significantly affect the structure of PCMSs, and the crystal strueture of PCMSs was not much changed; PCMSs have uniform particle size, without agglomeration. Ag NPs with an average particle size of 16 nm were decorated on the surface of two kinds of PCMSs in uniform and regular stacks by means of physical attachment. The stability of Ag/PCMSs was higher with the increased annealing temperature.(2) Ag/PCMSs was prepared from PCMSs annealed at 600℃, which exhibit good dispersibility in chlorobenzene. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy gaps of Ag/PCMSs and P3HT were greater than 0.3 eV, as calculated by cyclic voltammetry measurements. Therefore, Ag/PCMSs meets the requirements of energy levels for acceptor material of PSCs.(3) The composite materials 1.0-Ag/PCMSs were prepared in which Ag NPs (added in an amount of 1 mL at a concentration of lmol/L) were loaded on the surfaces of PCMSs (50 mg) annealed at 600℃. The 1.0-Ag/PCMSs and P3HT composite film exhibits strong light absorption and obvious fluorescence quenching.3. Preparation and optical properties of Ag@C and P3HT composite films. The core-shell Ag@C composites were prepared hydrothermally at 200℃, for 4 h with glucose as carbon source and silver nitrate as silver source.(1) As the concentration of glucose increased the particle size of Ag@C increased.0.3-Ag@C has the smallest particle size, the narrowest particle size distribution and the largest silver content. The energy gap between donor and acceptor is greater than 0.3 eV by cyclic voltammetry measurements. Therefore, 0.3-Ag@C can be used as acceptor material of PSCs.(2) The Ag@C and P3HT composite film exhibits significantly lower PL intensity than pure P3HT, and obvious fluorescence quenching, indicating that the photogenerated excitons are effectively separated at the interface of the composite film. The composite film with the weight ratio of P3HT to PCMSs of 2:1 shows a broadened spectral absorption range and obvious fluorescence quenching.4. Preparation and optical properties of the functionalized reduced grapheme oxide and P3HT composite films. The graphene oxide (GO) was prepared by Hummers method. Two kinds of reduced graphene oxide (5-RGO and 10-RGO) were prepared by hydrothermal method after reducing GO for 5 h and 10 h, respectively. Then the GO and RGO were reacted separately with phenyl isocyanate to obtain three kinds of solution processable functionalized graphenes (SPFGO,5-SPFRGO and 10-SPFRGO). The composite films were prepared with the functionalized graphene materials as acceptor and P3HT as donor.(1) The reduced graphene oxide (RGO) was prepared by hydrothermally reducing GO; the crystal structure of the products was destroyed to some extent and the degree of disorder increased, The oxygen content decreased, while oxygen-containing functional groups were partly removed.(2) Three kinds of functionalized graphenes(SPFGO,5-SPFRGO and 10-SPFRGO) containing functional groups -CONH, CN, NH, etc. were prepared after phenyl isocyanate functionalization treatment; three functionalized graphene materials have good dispersion in o-dichlorobenzene (o-DCB); their LUMO energy gap and HOMO energy difference with respect to P3HT are greater than 0.3 eV, as analyzed from CV measurements; These functional graphene materials were proved to meet the requirements for acceptor material of PSCs.(3) The composite films of P3HT:SPFGO, P3HT:5-SPFRGO and P3HT:10-SPFRGO were analyzed by atomic force microscopy (AFM), ultraviolet-visible spectrophotometry and fluorescent spectrometry. The results indicate that the composite films of P3HT:5-SPFRGO are dense and tactic. With GO reduced for 5 h, the light absorption efficiency of the composite film was improved, the intensity of PL spectra was reduced, and the electrical performance was improved. Moreover, the carrier mobility was increased, which could effectively reduce the exciton complexes. Therefore,5-SPFRGO is the optimal functionalized graphene material for acceptor material of PSCs.