C. Rothe

Population and decay of keto states in conjugated polymers

Using time-resolved and steady-state photoluminescence techniques, fluorene/fluorenone copolymers have been studied to investigate the role of keto defects in degraded polyfluorene. Keto sites can be populated via migration from polyfluorene singlets, thereby quenching the polyfluorene fluorescence, and via direct photon absorption. In the former case, the migration process dominates all thermal and interchain variability in the efficiency of quenching. No annihilation process of fluorenone triplets and no interchain processes such as excimer formation participate in the defect emission itself.

Trap influenced properties of the delayed luminescence in thin solid films of the conjugated polymer Poly (9,9-di(ethylhexyl)fluorene)

Spectra, kinetics, temperature, and excitation-dose dependencies of phosphorescence and delayed fluorescence intensities in thin films of the conjugated polymer poly(9,9-di(ethylhexyl)fluorene) [PF2/6] have been investigated via time-resolved spectroscopy. The results reflect a limited number of available energy sites (traps) for triplet-excited states in the long time region after laser excitation. Triplets captured in these traps decay mono-exponential with a time constant of several 100 ms. In the first few milliseconds after laser excitation, pronounced delayed fluorescence is observed due to triplet–triplet annihilation. The decay kinetics of the delayed fluorescence obeys a power law at all times in accordance with the framework in which triplets find each other after performing a random walk (hopping) in an energetic disordered media, which is represented by variations of the conjugation length in the polymer.

Triplet build in and decay of isolated polyspirobifluorene chains in dilute solution

The triplet kinetics of a conjugated polymer, polyspirobifluorene, have been studied using time resolved photoinduced absorption spectroscopy and gated emission delayed fluorescence. Working on isolated polymer chains in dilute solution, we pay particular attention to the buildup and decay of the triplet states following intersystem crossing from the excited singlet state. Confirmation of intersystem crossing as a monomolecular cold process has been made. At high excitation powers an initial fast decay of the triplet has been observed; this is attributed to intrachain triplet-triplet annihilation. From this observation we estimate the lower bound of the intersystem crossing yield as 1.2%. We also calculate the intrachain annihilation constant to be (2.9+/-0.1)x 10(8) cm(3) s(-1).

The Complex Excited-state Behavior of a Polyspirobifluorene Derivative: The Role of Spiroconjugation and Mixed Charge Transfer Character on Excited-state Stabilization and Radiative Lifetime

In this study, we report on the unusual fluorescence decay of an alkoxy-substituted polyspirobifluorene. Excited state behavior has been probed as a function of time, using femtosecond photobleaching, single photon counting, and streak camera techniques. Unusually complex decay kinetics are observed, which strongly depend on solvent viscosity and polarity, featuring decay components in both the tens of picoseconds and in the nanosecond time domain. These findings are explained by the consequences of spiroconjugation in combination with excited-state conformational relaxation. We propose that exciton wave function delocalization into the spiro units effectively traps the exciton, allowing it to relax further into a highly emissive state with a very long lifetime as compared to non-spiroconjugated polymer analogues. Frontier molecular orbitals and exciton orbitals have been calculated using a first principles density functional theory (DFT) approach. These results confirm the importance of spiroconjugation as both the highest occupied molecular orbital (HOMO) and the (lowest) exciton level are not localized on the polymer backbone but strongly extend into the side fluorene groups of the spirobifluorene units. The results of our calculations are very sensitive to the substitution pattern on the spirobifluorene units, in particular when oxygen is included. This finding may lead to new materials of this kind with optimized charge carrier transport properties and high luminescence quantum yields.

Regarding the origin of the delayed fluorescence of conjugated polymers

In the first part of this work we revisit and reevaluate the experimental data that lead to the assignment of the origin of the delayed fluorescence (DF) to triplet-triplet annihilation for polyfluorene and to geminate pair recombination in the case of the ladder-type polyparaphenylene (MeLPPP); the ambiguity of this classification is unveiled. Next, new data about the DF of MeLPPP under applied electric field are presented. Here, the DF intensity completely recovers once the field is turned off, which rules out geminate pairs as the origin of the DF and in turn provides clear evidence of the triplet-triplet annihilation picture. Finally, we show and discuss how recombination of space charge layers may also give rise to electric field induced delayed fluorescence, whereby the formation of these space charge layers strongly depends on device configuration and purity of the materials.

Femtosecond ground state recovery: Measuring the intersystem crossing yield of polyspirobifluorene

Measurement of the quantum yield of triplet formation has been made for the prototypical conjugated polymer polyspirobifluorene in solution and solid state. An updated method has been described based on femtosecond time resolved ground state recovery following photoexcitation of the polymer. The two components to the recovery of the ground state due to the decay of the singlet and triplet excited states are clearly visible and from these it is possible to calculate Phi(T)=0.05+/-0.01 in solution, this gives k(isc)=5.4 x 10(7) s(-1) which compares favorably with other conjugated polymers. In polymer films an increased triplet yield of Phi(T)=0.12+/-0.02 is found to be independent of temperature, the increased yield is attributed to triplet recombination from charged states.

Singlet and triplet energy transfer in a benzil-doped, light emitting, solid-state conjugated polymer

Abstract The luminescence emitted from pure and benzil-doped thin films of the conjugated polymer polyfluorene [PF2/6] are compared. The prompt fluorescence from the first singlet-excited state of the polymer is quenched by 90% in the presence of 10% per weight benzil. In addition to the prompt fluorescence, time-resolved spectroscopy at low temperature also allows the detection of phosphorescence and delayed fluorescence from the host polymer. Again the delayed fluorescence is strongly quenched but the phosphorescence is enhanced in doped samples. An explanation of the results is given in terms of singlet energy transfer from the host to benzil and triplet energy transfer from the dopant back to PF2/6. We have applied this to enable better understanding of the photophysics in PF2/6 doped with a platinum porphyrin complex.

Singlet Generation Yields in Organic Light-Emitting Diodes

A major potential difference between polymer and small molecule organic light-emitting devices is the possibility of spin-dependent charge recombination in the latter. In this case, the singlet exciton formation yield, one of the key parameters that determines device efficiencies, deviates from the simple quantum mechanical spin statistics prediction of 25%. For polymers, the departure from this limit has been demonstrated by several indirect measurements yielding a wide spread values from 25% up to 95% and by ourselves using a direct measurement on working devices at 44%. These results clearly indicate that polymer-based devices hold a unique advantage over small molecule-based devices. Comment will be made to these various measurement techniques along with an insight into current theoretical ideas that can explain the departure from the quantum spin statistical limit.OLEDs.

Spectroscopic investigation of the different long-lived photoexcitations in a polythiophene

Using time-resolved photoinduced transient absorption and gated emission techniques long-lived excitations of the solid-state conjugated polymer poly(3-methyl-4-octyl-thiophene) (PMOT) have been detected and analyzed at different temperatures. At 15 K phosphorescence and delayed fluorescence resulting from triplet and geminate pair decay, respectively, are observed. A redshifted emission (with respect to the prompt fluorescence) detected at ambient temperatures is assigned to excimer fluorescence. We also observed excitations with extremely long lifetimes of 300 ms at 289 K that we allocate to triplet excimer formation.

Temperature dependence of the space-charge distribution in injection limited conjugated polymer structures

Temperature dependent pyroelectric measurements have been carried out on high barrier devices of conjugated polymers. At room temperature we find significant buildup of space charge at the metal/polymer interface as well as in the bulk despite the very low device current. At lower temperature (180 K) the charge profile is “frozen in” and shows nearly no bias dependence, while above room temperature the space charge seems to become more mobile and leads to broadening of the charge profile. By applying an injection model that accounts for charge backflow via interface recombination we demonstrate that the high amount of space charge detected in our films indicates the existence of deep traps in the surface region that can effectively collect charge from the electrode. In the bulk of the film we find space charge of different sign, which is assumed to be due to separation in the field of charge from intrinsic defect states. These bulk charges are also immobile at low temperatures, indicating that they corres...