Understanding the complex interactions associated with charge, spin, lattice and orbital degrees of freedom is fundamental for emerging applications of quantum materials. In this context, ultrafast optical spectroscopy systems are promising tools to study the origin of complex orders. Here, an intense optical pulse brings the system out-of-equilibrium, providing an excellent opportunity to distinguish the dynamics of each subsystem. Using ultrafast techniques, we investigated charge density wave (CDW) behavior in transition-metal dichalcogenides (TMDs) after photo-excitation and during the relaxation time. To unravel the mechanisms underlying the correlations in CDW systems, we combined time resolved re ectivity (TRR) and time and angle resolved photoemission spectroscopy (TARPES). Our approach provides clear evidence of the phononic contribution to CDW phenomena in 1T-TiSe2.
We demonstrate a simple and novel technique to improve the power conversion efficiency of polymer:fullerene solar cells based on the low bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and the soluble fullerene derivative PCBM. We dope the blend via cosolution with the electron accepting molecule tetrafluoro-tetracyanoquinodimethane
(F4-TCNQ). Doping concentrations of F4-CNQ from 0 - 0.3 %, with respect to the weight of the polymer, were
investigated. The dark current-voltage (IV) characteristics, the illuminated IV characteristics, the external quantum efficiency, and the hole mobility in dependence of the doping concentration were studied. We observe
an increase in the photocurrent resulting in improved solar cell efficiencies, which corresponds to an increase in the hole mobility in the polymer. We explain this enhancement in the solar cell performance in terms of decreased carrier recombination.