The effect of the extent of electron conjugation on the primary photophysics in semiconducting polymers is reported. A rapid depolarization of photoluminescence and transient absorption, which indicates a reorientation of the transition dipole moment by ϳ30°on a sub-100 fs time scale, is observed in the fully conjugated polymer poly͓2-͑2'-ethylhexyloxy͒-5-methoxy-1,4-phenylenevinylene͔ ͑MEH-PPV͒. In contrast, partially conjugated polymers exhibit a much slower depolarization. The results reveal rapid changes of exciton delocalization in the fully conjugated MEH-PPV driven by structural relaxation.

Articles you may be interested inCorrelation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes J. Chem. Phys. 139, 204306 (2013) In order to obtain a better understanding of the role of conformational disorder in the photophysics of conjugated polymers the ultrafast transient absorption anisotropy of partially deconjugated MEH-PPV has been measured. These data have been compared to the corresponding kinetics of Monte Carlo-simulated polymer chains, and estimates of the energy hopping time and energy migration distances for the polymers have been obtained. We find that the energy migration in the investigated MEH-PPV is approximately 3 times faster than in previously studied polythiophenes. We attribute this to a more disordered chain conformation in MEH-PPV.

The power conversion efficiency of organic and hybrid solar cells is commonly reduced by a low open‐circuit voltage (VOC). In these cases, the VOC is significantly less than the energy of the lowest energy absorbed photon, divided by the elementary charge q. The low photovoltage originates from characteristically large band offsets between the electron donor and acceptor species. Here a simple method is reported to systematically tune the band offset in a π‐conjugated polymer–metal oxide hybrid donor–acceptor system in order to maximize the VOC. It is demonstrated that substitution of magnesium into a zinc oxide acceptor (ZnMgO) reduces the band offset and results in a substantial increase in the VOC of poly(3‐hexylthiophene) (P3HT)–ZnMgO planar devices. The VOC is seen to increase from 500 mV at x = 0 up to values in excess of 900 mV for x = 0.35. A concomitant increase in overall device efficiency is seen as x is increased from 0 to 0.25, with a maximum power‐conversion efficiency of 0.5 % obtained at x = 0.25, beyond which the efficiency decreases because of increased series resistance in the device. This work provides a new tool for understanding the role of the donor–acceptor band offset in hybrid photovoltaics and for maximizing the photovoltage and power‐conversion efficiency in such devices.

Hydrogenation of carbon monoxide on the Ru(OO1) surface has been investigated using high-resolution electron energy loss spectroscopy and temperature-programmed desorption. Exposing gas-phase atomic hydrogen to a saturated carbon monoxide overlayer at 100 K results in reaction (via Eley-Rideal kinetics) under ultrahigh vacuum conditions. Both ql-and q2-formyl are clearly identified as initial reaction products at low atomic exposures. At higher exposures the production of q2-formaldehyde is observed. Annealing to 180 K decomposes some of the vl-formyl, leading to adsorbed CO and hydrogen desorption, with the remainder of the 7'-formyl converting to $-formyl. Upon annealing to 220 K, the $-formaldehyde decomposes to adsorbed CO and hydrogen which desorbs. Further annealing to 250 K leads to complete decomposition of the q2-formyl, resulting in hydrogen desorption and regeneration of the original CO overlayer. These identifications represent the first spectroscopic observation of a carbonyl insertion channel operating during carbon monoxide hydrogenation on a well-characterized transition metal surface.

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Pi-conjugated dendrimers are an important class of materials for optoelectronic devices, especially for light-harvesting systems. We report here a theoretical investigation of the optical response and of the excited-state properties of three-arm and four-arm phenyl-cored dendrimers for photovoltaic applications. A variety of theoretical methods are used and evaluated against each other to calculate vertical transition energies, absorption and excitation spectra with vibronic structure, charge transport, and excitonic behavior upon photoexcitation and photoemission processes. Photophysical phenomena in these dendrimers are, in general, better explained with ab initio methods rather than with semiempirical techniques. Calculated reorganization energies were found to correlate well with the device photocurrent data where available. The excitons formed during photoexcitation are calculated to be more delocalized than the ones formed after vibrational relaxation in the excited states for fluorescence emission. The localization of excitons in emission processes is a result of geometrical changes in the excited state coupled with vibronic modes. Correlated electron-hole pair diagrams illustrate breaking of pi-conjugation in three-arm dendrimers due to meta linkage of arms with the core, whereas four-arm dendrimers are not affected by such breaking due to presence of ortho and para branching. Yet, ortho branching causes large twist angles between the core and the arms that are detrimental to pi-electron system delocalization over the structure.

Bulk heterojunction organic photovoltaic devices have been fabricated by blending phenyl-cored thiophene dendrimers with a fullerene derivative. A power conversion efficiency of 1.3% under simulated AM1.5 illumination is obtained for a four-arm dendrimer, despite its large optical band gap of 2.1eV. The devices exhibit an increase in short-circuit current and power conversion efficiency as the length of the arm is increased. The fill factors of the devices studied are characteristically low, which is attributed to overly uniform mixing of the blend.

In this paper we describe the convergent synthesis of a new class of phenyl cored thiophene dendrimers, which are promising candidates for use in organic semiconductor devices. We have prepared dendrimers with three and four dendrons around the core as well as dendrimers with 1st and 2nd generation dendrons. All the dendrimers were soluble in common organic solvents such as chloroform, THF and toluene. The structures and size properties are confirmed by a number of techniques including NMR, GPC and MALDI-TOF-MS. Decomposition was studied by TGA with initial breakdown of the hexyl surface groups followed by the aromatic core. The spectroscopic properties were studied by UV-vis and PL spectrometry which demonstrated substantial differences between the dendrimers with three and four dendrons. Optical band gaps varied between 2.34 and 2.60 eV for thin films of the dendrimers and electronic band gaps were, on average, 0.3 eV greater than the optical band gaps. The smallest band gap was measured for the dendrimer with four 2nd generation dendrons around the phenyl core. Fluorescence lifetimes of the molecules in solution ranged from 200 to 560 ps. This range in values was attributed to differences in internal conversion rates associated with varying degrees of flexibility of the extended dendrons.