RESUMEN
We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.
RESUMEN
We investigate the loss mechanisms in hybrid photovoltaics based on blends of poly(3-hexylthiophene) with CdSe nanocrystals of various sizes. By combining the spectroscopic and electrical measurements on working devices as well as films, we identify that high trap-mediated recombination is responsible for the loss of photogenerated charge carriers in devices with small nanocrystals. In addition, we demonstrate that the reduced open-circuit voltage for devices with small nanocrystals is also caused by the traps.