Control of charge trapping in a photorefractive polymer

Malliaras, George G.; Krasnikov, V. V.; Bolink, Henk J.; Hadziioannou, Georges

doi:10.1063/1.114230

Cited by 55 publications

(28 citation statements)

References 8 publications

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“…The recovery of τ −1 for the samples with higher DEH concentration is due to the establishment of a new transport channel-in this regime, the DEH molecules are close enough so that direct hopping of charge from one DEH molecule to another is possible. The variation of the phase shift of the refractive index grating [20] and of the hole drift mobility [21] corroborate this picture. From the above it is clear that there is an optimum value for the concentration of the trapping species, in order to achieve the maximum trap density.…”

Section: Control Of Charge Trappingsupporting

confidence: 74%

Photorefractivity in poly(N-vinylcarbazole)-based polymer composites

et al. 1996

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Section: Control Of Charge Trappingsupporting

confidence: 74%

Photorefractivity in poly(N-vinylcarbazole)-based polymer composites

et al. 1996

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“…Surprisingly, the dd is faster in material CT than in C, whereas the recording process is much slower ͑about a factor of 5-6͒ in CT. The latter is in general agreement with earlier results reported by Malliaras et al 16 This finding may indicate, that even though the TPD content is small ͑about 10 21 cm Ϫ1 , one extrinsic trap per 100 transporting sites͒ it may contribute to charge transport in the dark. By contrast, the photoconductivity proceeds through the cabazole manifold and is hindered by the trapping in TPD, and therefore the recording in CT is slower than in C.…”

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confidence: 83%

Dark decay of holograms in photorefractive polymers

Bittner¹,

Meerholz²,

Steckman

2002

Applied Physics Letters

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The decay of holograms stored in photorefractive polymer composites based on poly͑N-vinyl-carbazole͒ with and without extrinsic deep traps is investigated. The photorefractive phase shift is identified as one of the key parameters determining the dark decay dynamics. This has important implications for all kinds of photorefractive imaging applications including holographic data storage. A trade off will be required between accepting a certain degree of hologram distortion due to two-beam coupling on the one hand and achieving high hologram stability during idle periods in the dark with the external field applied on the other. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1492848͔The photorefractive ͑PR͒ effect is one of the most promising reversible holographic storage mechanisms. 1 Under nonuniform illumination, the refractive index of the photosensitive material is modulated due to the generation of mobile charge carriers in the bright regions, their subsequent redistribution, and eventual trapping in the dark areas. This gives rise to a space-charge field E SC , which modulates the refractive index of the material through the linear electrooptic effect and orientational effects. 2,3 Photorefractivity in amorphous polymers has been intensively investigated, 4,5 and these systems have been widely recognized as potential active media in rewritable holographic optical memories for security applications, 6 in associative memories, 7 or in adaptive ultrasound sensors. 8 Due to the rather low dielectric constants of polymers ( Ͻ10), oppositely charged carriers show a rather strong tendency to recombine. As a result, only rather short storage times are anticipated. However, so far, the dark decay ͑referred to as ''dd'' hereafter͒ of the holograms in periods when the system is idle, i.e., held in the dark with the external field still applied, has been mostly neglected in literature on organic PR materials, even though it is important for the aforementioned applications. In this letter, we present systematic investigations of the dd of PR gratings in PR polymers. Our results will give evidence that the phase shift between the interference pattern and the recorded index grating, the commonly accepted fingerprint of photorefractivity, is one of the key parameters, yielding slower dd for a larger phase shift.The investigated materials contained the photoconductor poly͑N͒vinylcarbazole ͑PVK, 39 wt %͒, the plasticizer N-ethylcarbazole ͑10 wt %͒, the eutectic mixture of two EO chromophores 2,5-dimethyl-4͑p-nitrophenylazo͒-anisole ͑25 wt %͒, and 3-methoxy-4͑p-nitrophenylazo͒-anisole ͑25 wt %͒, and finally the sensitizer 2,4,7-trinitro-fluorenone ͑TNF, 1 wt %͒. We also prepared a similar material doped with 0.82 wt % ͑replacing PVK͒ of the commonly used hole conductor N,NЈ-bis͑3-tolyl͒-N,NЈ-diphenyl-benzidine ͑TPD͒, whose highest occupied molecular orbital levels are situated about 0.5 eV below those of PVK. Thus, TPD moieties constitute deep traps within the carbazole transport manifold, and therefore a longer storag...

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“…In a previous work 9 we have seen that addition of 1 DEH molecule every 1000 carbazole units in PVK:TNF:EPNA causes an increase in the trap density, as inferred from the decrease of the phase shift of the photorefractive grating. The presence of extra trapping centers should reduce the drift mobility of holes 12 and thus affect the rise time of the transient photorefractive grating.…”

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confidence: 99%

“…For this reason we have conducted transient photorefractivity experiments in the polymer composite poly͑N-vinyl carbazole͒ ͑PVK͒, 2,4,7-trinitro-9-fluorenone ͑TNF͒ and N,N-diethyl-para-nitroaniline ͑EPNA͒. 8 In a previous work 9 we have seen that the addition of various amounts of 4-͑diethylamino͒benzaldehyde diphenylhydrazone ͑DEH͒ in this polymer composite results in a modification of the trap density. With transient photorefractivity experiments the effect of DEH on hole mobility can now be seen.…”

mentioning

confidence: 99%