Synthesis and photovoltaic properties of new donor-acceptor benzodithiophene-containing copolymers

Four new alternating narrow band-gap copolymers containing benzodithiophene, 4,8-dithiophen-2-yl-benzo[1,2-c;4,5-c'-bis[1,2,5]thiadiazole, 4,9-bis(thiophen-2-yl)-6,7-di(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline, 5,8-dibromo-2,3-bis(5-octylthiophen-2-yl)quinoxaline, and 4,7-bis(5-bromothiophen-2-yl)benzo[1,2,5] thiadiazole units are synthesized under Stille reaction conditions. The structures, molecular masses, and physical properties of the copolymers are studied via H-1 NMR spectroscopy, GPC, cyclic voltammetry, and thermomechanical and thermogravimetric analyses. The polymers show solubility and a broad absorption region (with the band gap in the range from 0.81 to 1.53 eV). All of the polymers are photostable in air, and their levels of the highest occupied molecular orbital vary from -4.98 to -5.30 eV. Polymer solar cells based on these copolymers as donors and fullerene PC60BM as an acceptor show open-circuit voltages in the range 0.16-0.61 V, and the efficiencies of the devices are in the range 0.02-0.49%

Molecular Self-Doping Controls Luminescence of Pure Organic Single Crystals

Organic optoelectronics calls for materials combining bright luminescence and efficient charge transport. The former is readily achieved in isolated molecules, while the latter requires strong molecular aggregation, which usually quenches luminescence. This hurdle is generally resolved by doping the host material with highly luminescent molecules collecting the excitation energy from the host. Here, a novel concept of molecular self-doping is introduced in which a higher luminescent dopant emerges as a minute-amount byproduct during the host material synthesis. As a one-stage process, self-doping is more advantageous than widely used external doping. The concept is proved on thiophene-phenylene cooligomers (TPCO) consisting of four (host) and six (dopant) conjugated rings. It is shown tha

Charge Transfer Dynamics in Donor–Acceptor Complexes between a Conjugated Polymer and Fluorene Acceptors

We report on ground and excited state charge transfer in charge-transfer complexes in films formed between a semiconducting polymer, MEH-PPV (poly­[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]), and a series of fluorene electron acceptors. The latter were designed to vary systematically the electron affinity (EA) over ∼1.5 eV by attachment of various electron withdrawing groups to the fluorene core. The EAs of the acceptors are determined by cyclic voltammetry and compared with those from density functional theory calculations. The charge-transfer dynamics are studied using an ultrafast visible-pump–IR-probe photoinduced absorption technique. We demonstrate that the acceptor EA is the keybut not the onlyparameter that governs charge recombination rates that scale exponentially with the acceptor EA. From the time-resolved data we deduced a model that describes charge dynamics for acceptors with low and high EAs. The two opposite trendshigher acceptor EA increases the driving force for charge separation but also inevitably increases the rate of undesirable charge recombinationshould be carefully counterbalanced in designing novel polymer–fullerene bulk heterojunctions