Fluorescence Quenching Studies Fluorescence emission spectra were recorded for polymers P1-P4 with various PCBM quencher concentrations (1 x 10 -5 M to 1 x 10 -3 M). Polymers P1, P2, P3 and P4 were excited at the wavelength of 460 nm, 400 nm, 448 nm, and 547 nm, respectively. The decrease in the fluorescence emission intensity upon addition of PCBM reveals the efficient quenching of excitons by PCBM. These quenching results were analyzed by the following Stern-Volmer relationship. M 0 /M = 1 +

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... sv [Q] Wherein, M 0 and M are the fluorescence intensity of polymers with and without quencher respectively, [Q] is the quencher concentration in moles per litre, K sv is the Stern-Volmer quenching constant. The plot between the ratio M 0 /M of polymers (P1-P4) and the Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics This journal is