The Photovoltaic Behavior of Polymer Solar Cells Using Fulleropyrrolidines with Conjugated and Alkoxy Substituent as Electron Acceptor

This thesis mainly studies the photovoltaic behaviors of polymer solar cells in which new C60 derivatives that bear various functional groups were applied as electron acceptors. It essentially consists of two parts. Firstly, to overcome the drawback of low light˗absorptivity of C60 in the visible region, four different conjugated groups were
chemically bounded onto the N-methyl fulleropyrrolidine as a substituent. These molecules display distinct absorption bands and have a much higher molar absortivity than that of PCBM in the visible range. The photoluminescence (PL)experiments indicate a smooth transfer of photoexcited electrons from the conjugated substituent to the C60 cage, thereby creating free electron/hole pairs. Solar devices fabricated from the blends of poly(3-hexylthiophene) (P3HT) and these fullerene derivatives exhibit the best power conversion efficiency of 2.54%. More importantly, the comparison of the IPCE spectra of the P3HT/PCBM and P3HT/2,5-bis(4-hexylthiophen-2-yl)thiophenefulleropyrrolidine
(BTTC) devices reveals the IPCE of the latter cell is higher than that of the former cell by about 7% in the wavelength range of 350~400 nm, which corresponds to the absorption band of terthiophene. This finding clearly verifies the
excitons generated in the conjugated substituent can make contribution to the photocurrent. In addition, the thermal stability of the cells based on the blend of P3HT and these four C60 derivatives was examined by aging the film of photoavtive blend at 110 °C for various intervals before the evaporation of the metal cathode. Both OM and TEM images of the thermally aged blend films show the amorphous nature of BTTC,BTBTC and BTBSeC effectively suppresses the thermal-driven aggregation of C60 adducts during aging process, leading to a extremely stable blend morphology.
Consequently, these devices almost retain their initial PCE even after storing at 110 °C for 300 minutes.
In the second part, the influence of introducing an electron-donating, alkoxy, group into the C60 derivative and the anchoring position of such substituent on the
photovoltaics of polymer solar cells, was investigated. As expected, the LUMO of these alkoxy-bearing fulleropyrrolidines is higher than that of PCBM by 30~50 mV. Although the binding position of the alkoxy moiety on N-methyl-2-phenyl fulleropyrrolidine has no obvious effect on the optical properties of the molecule, the para-substituted
compound (4EHOBC) has a good solubility, which is about twice higher than that of the ortho-substituted compound (2EHOBC). This difference subsequently affects the carrier
mobilities and the film morphology of their blends with P3HT. As a result, the P3HT/4EHOBC and P3HT/2EHOBC devices exhibit an optimal power conversion efficiency of 3.37% and 3.01%, respectively.