Fulleropyrrolidine Derivatives:synthesis, Characterizations And Photovoltaic Studies

Fullerenens C60 and C70 have been theoretically considered as ideal electron acceptors due to their high electron affinities and low reorganization energies. However, because of the poor solubilities and heavy aggregations of pristine fullerenes, fullerene derivatives such as PCBM and ICBA have been widely used as acceptor materials for practical applications. Among them, PCBM is the most widely used acceptor material for polymer solar cells(PSCs), and the PC71BM-based PSC gives rise to a record PCE of 11.3% very recently. In the past a few years, a large number of novel fullerene derivatives have been synthesized and reported, most of which have the advantages of simple preparations and good photovoltaic performances, and are expected to be alternatives to the PCBM for high performance PSCs.Fulleropyrrolidines are typical fullerene derivatives, which feature facile preparations and higher LUMO levels relative to that of PCBM. However, to date, only a few fulleropyrrolidines have been reported to be acceptors with comparable or superior photovaoltaic performance to that of PCBM. In this thesis, a series of fulleropyrrolidine derivatives(FPx) were prepared by 1,3-dipolar cycloaddition and characterized by means of MALDI-TOF mass, 1H NMR, UV-vis absorption spectroscopy, cyclic voltammogram, thermogravimetric analysis and DFT calculations. The results showed that fulleropyrrolidines with different substituents possess similar optical, electrochemical and thermal properties as well as comparable LUMO levels. Furthermore, the as-prepared fulleropyrrolidines were used as acceptors for PSCs. Device tests revealed that these fulleropyrrolidines showed very different photovoltaic properties. Particularly, the N-phenyl substituted FPs are much better acceptors than the N-methyl substituted FPs. The FP1-based device exhibited a PCE of 3.19%, almost comparable to that(3.31%) of PC61BM-based device. In order to shed light on these differences, the morphologies of the photoactive layers were examined by means of atomic force microscopy(AFM), transmission electron microscopy(TEM) and X-ray diffraction(XRD). Moreover, the electron mobilities of FP1 and FP5 were measured using space charge limited current(SCLC) method. The experimental results demonstrated that, comparing with the N-methyl substituted FPs, the N-phenyl substituted FPs exhibited higher electron mobilities and better miscibility with polymer donor. This might account for the superior photovoltaic performance of the latter. Therefore, we concluded that the photovoltaic properties of fulleropyrrolidines could be effectively improved by properly manipulating the N-substituents.In addition, we also synthesized FP-bisadducts FP11, FP12 and C70 adduct FP13, and applied them to PSCs. Specifically, the FP11 and F12-based devices gave Voc as high as 0.82～0.85 V, apparently higher than that(0.65 V) of PC61BM-based device. However, these devices showed poor Jsc and PCEs(<2%), which might be attributed to the presence of multiple isomers of FP11 and FP12. Accordingly, we may anticipate that the PSCs based on the isomerically pure bisadducts would exhibit not only high Voc, but also nice Jsc, FF and PCEs.