Photophysical and photoconductive aspects of donor-acceptor low band gap conjugated copolymer

This article presents the photophysical and photoconductive properties of a newly synthesized low band gap Donor-Acceptor conjugated poly (benzothiadiazole-triphenylamine) co-polymer labelled as P(BTZ-TPA) in which 2,1,3 benzothiadiazole (BTZ) act as the electron deficient moiety and triphenylamine (TPA) is used as donor moiety. Photophysical properties of this synthesized polymer were analyzed by recording steady state absorption and emission spectra of dilute solution and thin films respectively. In addition to this, solvatochromic experiments were carried out using a binary mixture consisting of toluene and acetonitrile to understand the solvent dependence on the ground state and excited states of this polymer. The optical characterization revealed high degree of intramolecular charge transfer through the polymer backbone. The P(BTZ-TPA) thin films exhibit good fluorescence emission with emission peak around 615 nm. Quantum yield of the fluorescence emission is measured in relative method. Also, fluorescence decay parameters were examined using time correlated single photon counting (TCSPC) technique. Carrier generation and transport of the generated carriers upon photo excitation is studied in pristine polymer films as well as in P(BTZ-TPA):PCBM blend films, by performing photoconductivity studies using broad spectral source and lasers of wavelengths 488 nm and 632 nm. P(BTZ-TPA) thin films exhibited good photoresponse over the entire visible region, with promising internal photo current efficiency. Quenching of fluorescence is observed in PCBM blend films. This drastic quenching of fluorescence emission is assigned to the charge transfer interaction between co-polymer and PCBM, and is confirmed by substantial increment in photoconductivity of the blend films. An internal photocurrent efficiency of 17.4% was obtained in the blend films for a biasing field of 10 V/mu m.