Electrical transport properties characterization of PVK (poly N-vinylcarbazole) for electroluminescent devices applications

D’Angelo, Pasquale; Barra, Mario; Cassinese, A.; Maglione, Maria Grazia; Vacca, Paolo; Minarini, Carla; Rubino, Alfredo

doi:10.1016/j.sse.2006.11.008

Cited by 74 publications

(58 citation statements)

References 16 publications

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“…There are two items to note: the first is that the cell responds differently to the positive and the negative parts of the cycle. This has already been reported in the literature: the mobility of electrons and holes in PVK:C 60 is different [18][19][20], and C 60 has different hole/electron acceptor levels [21]. The second observation is that the largest beam coupling is at intermediate voltages.…”

Section: Time-dependent Beam Couplingsupporting

confidence: 65%

“…There are already in the literature detailed studies [22,26,27] of the role that the cell parameters, e.g., the cell tilt angle, the LC pretilt, and the anchoring energy, play on beam coupling. There are also detailed measurements of the electrical properties of PVK with or without C 60 doping [18][19][20][21]28]. These could be coupled with existing models of ion currents in LCs [29][30][31] and the role of Debye layers [32] at the LCpolymer interface.…”

Section: Modeling Analysis Of Beam Couplingmentioning

confidence: 99%

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Light-activated modulation and coupling in integrated polymer–liquid crystal systems

et al. 2014

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We explore the transfer of an incident light pattern onto the liquid crystal (LC) bulk in a photorefractive cell through an integrated photoconducting layer that modulates the electric field applied to the device. The electrical properties and the strength of modulation are investigated as a function of the incident light intensity as well as the frequency and amplitude of the applied voltage, for two LCs with very different electrical conductivity. A simplified electrical model of the cell is proposed, demonstrating that the LC conductivity is a key factor determining the beam-coupling strength.

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Section: Time-dependent Beam Couplingsupporting

confidence: 65%

Section: Modeling Analysis Of Beam Couplingmentioning

confidence: 99%

Light-activated modulation and coupling in integrated polymer–liquid crystal systems

et al. 2014

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“…If we assume that the conductance of E7 is 10 −8 Sm −1 [17], that the thickness of the PVK layer is 50 nm, that its relative dielectric permittivity is approximately 3 [18] and neglecting all other layers, we obtain τ VTF ∼ 4 s, a value compatible, in view of the large uncertainty on the data, with that obtained using our model. We have not found precise values of the conductance of TL205 in the literature, but other groups have reported that the conductivity of TL205 is much lower [19] than for E7; this is consistent with the higher value of τ VTF for TL205 measured here, which suggests that it is at least two orders of magnitude smaller than that of E7.…”

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confidence: 66%

Voltage transfer function as an optical method to characterize electrical properties of liquid crystal devices

et al. 2014

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The voltage transfer function is a rapid and visually effective method to determine the electrical response of liquid crystal (LC) systems using optical measurements. This method relies on crosspolarized intensity measurements as a function of the frequency and amplitude of the voltage applied to the device. Coupled with a mathematical model of the device it can be used to determine the device time constants and electrical properties. We validate the method using photorefractive LC cells and determine the main time constants and the voltage dropped across the layers using a simple nonlinear filter model. Typically, such devices rely on having a LC layer aligned between two substrates that can be reoriented with an applied electric field. The alignment is usually controlled through thin layers of polymers or surfactants on the substrates that, in most cases, play a passive role when considering the optical or electrical response of LC devices. However, this assumption is not always valid as even nominally passive polymers, and their interface with LCs, can influence the electric field profile extending into the LC bulk and can introduce ionic effects [2]. Therefore, one of the most important parameters that has to be established in systems with LCs integrated with other layers is the actual value of the voltage dropped across the LC bulk. This question is particularly relevant if the alignment layers themselves respond to an electrical or optical field. Such "active" layers can be used to enhance the functionality of LC devices. Suitable examples include the use of electrically commanded surfaces of ferroelectric LC polymer combined with nematic LCs [3] and those that include photoconducting polymers such as polyvinyl carbazole (PVK) [4,5].To optimize the performance of different polymer-LC systems, a reliable and universal technique is needed to determine a cross-section of the voltage profile across each of the layers and visualize their frequency response. Such a technique can provide an informative snapshot of the suitability of the aligning layer and LC materials for effective and stable light modulation [6].We have developed an optical technique that we have termed the voltage transfer function (VTF) to visualize the electrical response of an LC device and, in particular, to measure the ratio of the voltage dropped across the LC layer and the voltage applied to the whole device. This method is based on crosspolarized intensity measurements at different frequencies and amplitudes of the voltage applied to the device. As the electro-optic response of the LC is independent from the other layers, the VTF method can be applied to a wide variety of LC cells and can be used to calculate important parameters, such as birefringence and elastic constants. The VTF function can be shown in a visually instructive way and data can be extracted from the cells by comparison with suitable LC models that may include ion motion [7] and the effect of Debye layers [8]. Here we use, for illustration purposes, a simple, but intui...

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“…It is worth to note that up to now the excimeric phosphorescence in the PVK films was related to the emission in the range 480-530 nm; however lower energetic tail down above 650 nm was observed in both phosphorescence and EL spectra [6,8]. This emission bands about 590 nm and above 600 nm were attributed to several different mechanisms, such as aggregation, [7,9] electroplex [3,9] or phosphorescence [13,14].…”

Section: Introductionmentioning

confidence: 99%

The nature of trapping sites and recombination centres in PVK and PVK–PBD electroluminescent matrices seen by spectrally resolved thermoluminescence

Glowacki¹,

Szamel²

2010

J. Phys. D: Appl. Phys.

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To cite this version:Ireneusz Glowacki, Zbigniew Szamel. The nature of trapping sites and recombination centres in PVK and PVKPBD electroluminescent matrices seen by spectrally resolved thermoluminescence. with the electroluminescence (EL) spectra has allowed identifying the localized (trapping) sites and the radiative recombination centres present in the investigated matrices. In the neat PVK films deep traps with a depth about 200 meV, related to triplet excimers dominate, while in the PVK-PBD (40 wt %) blend films the traps that are related to triplet exciplexes formed by the carbazole groups and the PBD molecules dominate. Depth of the traps in the PVK-PBD blend is somewhat lower than that in the neat PVK. An analysis of the EL spectra shows that in the PVK and in the PVK-PBD blend the dominant radiative centres are singlet excimers and singlet exciplexes, respectively. However in the neat PVK some contributions of the triplet monomer and the triplet excimer states in the electroluminescence were also detected.

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Electrical transport properties characterization of PVK (poly N-vinylcarbazole) for electroluminescent devices applications

Cited by 74 publications

References 16 publications

Light-activated modulation and coupling in integrated polymer–liquid crystal systems

Light-activated modulation and coupling in integrated polymer–liquid crystal systems

Voltage transfer function as an optical method to characterize electrical properties of liquid crystal devices

The nature of trapping sites and recombination centres in PVK and PVK–PBD electroluminescent matrices seen by spectrally resolved thermoluminescence

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