Population and decay of keto states in conjugated polymers

Hintschich, Susanne I.; Rothe, Carsten; Sinha, Subrata; Monkman, Andrew P.; Freitas, P. Scandiucci de; Scherf, Ullrich

doi:10.1063/1.1624825

Cited by 67 publications

(56 citation statements)

References 24 publications

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“…The consequences of even small amount of traps in the material can have crucial effect on the photophysical properties and OLED device performance. The increase in the emission intensity from trap sites looks similar to the effects of keto defects in polyfluorene, 24 where one sees large increases in a LE, broad peak in solid state in comparison to solution and where longer wavelength peaks act as luminescence quencher sites via energy transfer mechanisms. 25 The quenching is so effective that very large changes in emission color of the material may be observed as in CBP.…”

Section: -5mentioning

confidence: 60%

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

Jankus

Winscom

Monkman

2009

The Journal of Chemical Physics

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In this paper we report the results of optical characterization of 4,4-N,N′-dicarbazolyl-1,1′-biphenyl (CBP), known as a host material for phosphorescent light emitting devices. Using absorption, steady state, and time-resolved spectroscopy, we explore the singlet and triplet states in solid and solution samples of CBP. In solutions we observe two distinct short-lived states with well-resolved emission originating from individual molecule singlet states (at 365 and 380 nm) and “quenching” low energy (LE) states (at 404 and 424 nm). The latter are seen only in saturated solutions and solid samples. Both of those species have different lifetimes. After UV exposure of very concentrated degassed solution the intensities of the LE bands starts to decrease. The longer the solution is exposed to UV, the less emission is seen at 404 and 424 nm, until it is totally gone. The spectrum of the highly concentrated solution is then the same as the spectrum of dilute solution, i.e., only emission at 365 and 380 nm is present. An increase in intensities of the singlet emission peaks correlates with an increase in UV exposure time. Similar behavior is observed in evaporated CBP film. We propose that this behavior is due to chemical instability of the weak N–C bonding of carbazolyl moiety—this creates new degradational species over time which dissociate after exposure to UV. We believe this to be the reason for variation in CBP fluorescence and delayed fluorescence spectra recorded by various research groups. Further, we detected two types of very long-lived states. One of these states (higher energy) is ascribed to molecular phosphorescence emission, the other to emission from low energy triplet trap states which we relate to degradational species. We propose that triplets are more easily caught by these latter sites when their hopping rate increases, and they emit inefficiently from these lower energy sites.

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Section: -5mentioning

confidence: 60%

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

Jankus

Winscom

Monkman

2009

The Journal of Chemical Physics

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“…20 Direct excitation of the red-shifted absorption band (450 nm), only gives the nonresolved emission band. 27,28 The exact nature of this CTS defect was identified by quantum-chemical calculations in model systems containing fluorene groups and one fluorenone unit in the center (FL) n -FLO-(FL) n . The energy of the CT state, which becomes the lowest energy state upon excitation, decreases by 0.32 eV when going from FLO to the three-unit system FL-FLO-FL and then remains constant upon further increasing the chain length, 29 showing that the system (FL-FLO-FL) is the appropriate model compound for our "CTS defect".…”

Section: Resultsmentioning

confidence: 99%

Fast and Slow Time Regimes of Fluorescence Quenching in Conjugated Polyfluorene−Fluorenone Random Copolymers: The Role of Exciton Hopping and Dexter Transfer along the Polymer Backbone

et al. 2006

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The luminescence decay kinetics of polyfluorene copolymers containing fluorenone units randomly distributed along the polymer chain have been studied by steady-state and time-resolved fluorescence techniques in toluene solution. The typical green emission from polyfluorenes containing 9-fluorenone moieties is only observed if the 9-fluorenone group is covalently attached to the polymer. Small-angle neutron scattering (SANS) measurements indicate that, independent of the 9-fluorenone fraction, all the studied copolymers adopt an open wormlike conformation. This prevalent 1-dimensional arrangement confirms that the green emission observed with polyfluorenes is not the result of excimer formation in the typical sandwich-like conformation. Analysis of time-resolved fluorescence decays by the maximum entropy method (MEM) collected at the polyfluorene emission (415 nm) and by global analysis of decays collected at 415 and 580 nm (the 9-fluorenone defect emission wavelength) clearly indicates two different time regimes in the population of the fluorenone defect: one occurring in the time interval of 10 to 30 ps and a second one occurring in the time range from 70 to 200 ps. While the slower process shows a linear dependence with the 9-fluorenone fraction, compatible with a hopping migration process along the polymer chain, the faster process does not show such a dependence and instead suggests a short-range Dexter mechanism. These findings are in agreement with our previous work where the presence of a faster component was suggested.

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“…Also this condition accounts for all exciton quenching mechanism as long as they apply in the same way for optically and electrically excited excitons. For example, quenching at the anode or impurity sites including the well-known (photooxidative) keto defect [94,95]. It is known that keto defects act as charge traps for electrical excitation, but this is still not a problem since it reduces the singlet and triplet density proportionally similar to a dark current.…”

Section: Experimental Observationsmentioning

confidence: 99%

“…Spirofluorene derivatives are chemically inert against backbone oxidation, which otherwise causes the formation of keto defects [94,95]. State-of-the-art diodes were fabricated at Philips Laboratories, Eindhoven using ITO and Ba/Al as electrode materials.…”

Section: Experimental Observationsmentioning

confidence: 99%

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

Monkman

2013

ISRN Materials Science

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A potential major drawback with organic light-emitting devices, (OLEDs) is the limit of 25% singlet exciton production through spin-dependent charge recombination. Recent device results, however, show that this limit does not hold and far higher efficiencies can be achieved in purely fluorescent-based systems (Wohlgenannt et al. (2012)). Here, the various routes by which triplet excitons can generate singlet states are discussed and their relative contributions to the overall electroluminescence yield are given. The materials requirements to obtain maximum singlet production from triplet states are discussed. These triplet contributions can give very high device yields for fluorescent emitters, which in the case of blue devices can be highly advantageous. Further, new devices architectures open up which are simple and have intrinsically low turn on voltages, ideal for large-area OLED lighting applications.

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Population and decay of keto states in conjugated polymers

Cited by 67 publications

References 24 publications

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

Fast and Slow Time Regimes of Fluorescence Quenching in Conjugated Polyfluorene−Fluorenone Random Copolymers: The Role of Exciton Hopping and Dexter Transfer along the Polymer Backbone

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

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