Synthesis And Photovoltaic Properties Of Novel Fullerene Acceptors

Polymer solar cells (PSCs) have attracted great attention with the advantage of easy fabrication, low cost, light-weight, synthetic variety, mechanical flexibility and feasibility for large area fabrication. For the bulk heterojunction (BHJ) PSC devices, the photoactive layer is usually composed of a blend of a conjugated polymer donor and a soluble acceptor which is typically fullerene derivative. Fullerene has been commonly used as suitable acceptor material due to its strong electon accepting ability, high electron mobility and the ultrafast photoinduced charge transfer between conjugated polymer and fullerene. In this dissertation, we focused on the synthesis of novel fullerene derivatives and its application as acceptor materials in PSCs, and carried out the following works:(1) A dye unit,3-ethyl rhodanine, was attached onto (6,6)-phenyl-C61butyric acid methyl ester (PCBM) via a facile esterification reaction for the first time, affording a new fullerene acceptor PCBM-rhodanine (PCBRh) for PSCs with enhanced light absorption. UV-vis spectroscopic study indicated that PCBRh had stronger absorptions in the region of300-600nm than PCBM, and this was due to the high absorption coefficient of the dye unit of3-ethyl rhodanine. Cyclic voltammetric measurement revealed that LUMO level of PCBRh was0.10eV higher than that of PCBM, and this can be understood by considering the electron donating property of the rhodanine moiety. Using PCBRh as an acceptor blending with poly(3-hexylthiophene-2,5-diyl)(P3HT), the BHJ PSC device exhibited a power conversion efficiency (PCE) of1.46%under the optimized condition (blending ratio of P3HT:PCBM=1:1(w/w), annealing treatment at135℃for10min). The effect of annealing on the morphology of P3HT:PCBRh active layer and its correlation with the device performance were studied by atomic force microscopy (AFM), revealing that many separated spherical aggression existed in the unannealed P3HT:PCBRh blend film, but after annealing PCE improved because P3HT aggregated to long stripes with an average length of ca.20nm, which facilitates transport of charge carriers.(2) In order to raise the LUMO level of fullerene acceptor material, we synthesized two new fullerene derivatives, mono-and bis-adducts of di(5-methylphenyl)methano-C60, through a diazo addition reaction. UV-vis spectroscopic study and cyclic voltammetric measurements indicated that DMTCMA and DMTCBA were good acceptor materials with potential. LUMO levels of DMTCMA and DMTCBA were0.06eV and0.11eV higher than PCBM, respectively. As a result, the BHJ PSC device based on DMTCMA or DMTCBA as acceptor blending with P3HT showed remarkable increase of open circuit voltage (Voc), which was0.76V for DMTCBA-based device, whereas Voc of PCBM-based device is0.61V. The PCE of the PSC device based on DMTCMA or DMTCBA acceptor is1.45%and0.82%, respectively, lower than that of the reference device based on PCBM acceptor. AFM morphology analysis indicated that the inferior performance of DMTCBA-based device was probably because the amorphous structures of DMTCBA resulted in poor molecular stacking and the over-aggregation of P3HT hindering efficient exciton diffusion.