Computational Study of the Influence of Polymer/Polymer Interface Formation on Bilayer-LED Functioning (original) (raw)

Mesoscopic modelling of 2-CN-PPV/PPV polymer LED

Journal of Materials Science: Materials in Electronics, 2007

Although optoelectronic devices made of polymers are very attractive ones (low cost, easy to make), problems related to charge transport, exciton quenching, among others, can be an obstacle for their performance. The use of heterojunctions made of two polymers can be a strategy for improving the efficiency of polymer light emitting diodes (PLEDs) at low bias. Here we present a theoretical study of the influence of bilayer structure in a PLED made of PPV and 2-CN-PPV, by adopting a mesoscopic approach. Our results show that the presence of the polymer/polymer interface improves charge injection and leads to a confinement of charges near it, which will increase the number recombination events in the middle of the device compared to the equivalent single-layer PLEDs.

Role of thin organic interlayers inserted at the electrode interfaces for efficient polymer LEDs

2017

This thesis discusses role of thin organic interlayers at the electrode interfaces of polymer light-emitting diodes (LEDs) in increasing their efficiencies. The effect of varying the anode-side IL material, its thickness, and its p-doping level on poly(dioctylfluorene-alt-benzothiadiazole) (F8BT) PLEDs is examined. Then the impact of varying the IL material is explored in Lumation Green 1300 PLEDs to determine whether a relation exists between the role of the IL and the light emitting layer's properties. It is found that excitons are formed in F8BT adjacent to the interface with the IL and are thus exceptionally sensitive to the energetics at that interface, with wide energy gaps helping to reduce luminance quenching significantly. The general effects of ionic cathode-side ILs on PLED efficiencies and response times are investigated. The novel materials used contain imidazolium cationic groups. The impact of varying the ionic IL main conjugated backbone on PLED characteristics i...

Effect of molecular properties on the performance of polymer light-emitting diodes

Applied Surface Science, 2004

The performance of a single layer polymer light-emitting diode depends on several interdependent factors, although recombination between electrons and holes within the polymer layer is believed to play an important role. Our aim is to carry out computer experiments in which bipolar charge carriers are injected in polymer networks made of poly(p-phenylene vinylene) chains randomly oriented. In these simulations, we follow the charge evolution in time from some initial state to the steady state. The intra-molecular properties of the polymer molecules obtained from self-consistent quantum molecular dynamics calculations are used in the mesoscopic model. The purpose of the present work is to clarify the effects of intra-molecular charge mobility and energy disorder on recombination efficiency. In particular, we find that charge mobility along the polymer chains has a serious influence on recombination within the polymer layer. Our results also show that energy disorder due to differences in ionization potential and electron affinity of neighbouring molecules affects mainly recombinations that occur near the electrodes at polymer chains parallel to them.

Polymer Photovoltaic Devices from Stratified Multilayers of Donor-Acceptor Blends

Advanced Materials, 2000

The possibility that fully plastic flexible photovoltaic devices made of conjugated polymers and fullerene derivatives may be manufactured has recently been explored by a number of groups [1±5] in Europe and United States. Methods such as solvent casting, and inclusion in polymer matrices were applied to make large-area flexible devices. Spin-coating from solution has been successfully applied to produce blend-based devices with the formation of so-called bulk heterojunctions in polymer/fullerene blends as well as in polymer/polymer blends, and lamination techniques have been used to obtain layered devices. The conversion efficiency of these devices was substantially improved due to the enhanced interface area. Photoluminescence quenching due to photoinduced charge transfer is close to complete, but highly efficient collection of the photoinduced charges requires the ratio of the fullerene derivatives to conjugated polymer as high as 3:1. The observation that photocurrent also originates by absorption in the acceptor phase in organic photodiodes and in polymer/fullerene bilayer diodes suggests that the requirements for the acceptor phase are not only determined by transport properties but also by optical absorption. We therefore need to utilize absorption from this phase. Another disadvantage of the homogeneous blends is low values of fill-factor due to bad rectification, which is presumably caused by the existence of continuous and separate pathways for the two types of charge carriers from anode to cathode. While we have not yet obtained experimental and microscopic proof of this assertion, it is a plausible interpretation of a wealth of observations.

Temperature-dependent recombination in polymer composite light-emitting diodes

Applied Physics Letters, 1998

We study the temperature dependence of the current-voltage and radiance-voltage curves in double-carrier injected polymer light-emitting devices comprised of poly͑2-methoxy,5-͑2Ј-ethyl-hexoxy͒p-phenylene vinylene) ͑MEH-PPV͒ and MEH-PPV/SiO 2 as the active layer. The quantum efficiency increases significantly as the temperature is decreased in agreement with an increase in the recombination efficiency with decreasing temperature. Moreover, the bimolecular recombination efficiency saturates at low temperatures and high currents to a very high value for both the composite and plain MEH-PPV devices with the nanoparticles serving as charge traps only at moderately low current densities. Finally, we find that the order of magnitude improvement in radiance observed in some polymer/nanoparticle composites is due to an increase in the effective electric field across the device.

Photoresponse and electroresponse studies of polymer light-emitting diodes

Synthetic Metals, 1997

We have developed and applied a non-destructive and non-invasive technique, the saturated photovoltage (SPV), to measure the internal built-in potential, Vbi, in conducting polymer based light-emitting diodes (LED). We found that Vbi scales with the difference in work function between the two capping metal--electrodes in a variety of polymer LEDs, independent of the constituent polymer. The SPV technique can be used as a diagnostic tool to study the polymer device quality and degradation. In addition, the photocurrent excitation spectra of polymer LEDs are shown to be sensitive to states in the gap and also to spectral features above the optical gap.

Charge transfer processes in polymer light-emitting diodes

Materials Science and Engineering: C, 2002

Optical characterisation of an organic soluble poly (paraphenylene vinylene) (PPV) derivative involving an alkyloxy pendant group on the phenylene ring has shown a shift of the absorption and emission towards longer wavelengths in comparison to PPV ones. The changes of the optical characteristics are ascribed to the redistribution of the k molecular orbital over the phenylene ring induced by the alkyloxy substituent. The comparison of the absorption and emission of the PPV derivative in solution or as a thin layer shows the role of the interchain interactions in condensed matter. Another example of the importance of charge transfer processes in PLED is provided by the indium tin oxide (ITO)/poly (vinyl carbazole) (PVK) hole transport layer interface. The weakening of the ITO absorption in the UV resulting from interband transitions has been interpreted by the depletion of the ITO free carriers at the contact of PVK. The shift towards longer wavelengths of the plasma frequency in the near infrared confirms the reduction of the free electron concentration in ITO, which has been estimated to 30% using the Drude free electron theory. The formation of a dipole layer at the interface can account for such charge transfers. D

Substrate/semiconductor interface effects on the emission efficiency of luminescent polymers

Journal of Applied Physics, 2011

The importance of interface effects for organic devices has long been recognized, but getting detailed knowledge of the extent of such effects remains a major challenge because of the difficulty in distinguishing from bulk effects. This paper addresses the interface effects on the emission efficiency of poly(p-phenylene vinylene) (PPV), by producing layer-by-layer (LBL) films of PPV alternated with dodecylbenzenesulfonate. Films with thickness varying from $15 to 225 nm had the structural defects controlled empirically by converting the films at two temperatures, 110 and 230 C, while the optical properties were characterized by using optical absorption, photoluminescence (PL), and photoluminescence excitation spectra. Blueshifts in the absorption and PL spectra for LBL films with less than 25 bilayers (<40-50 nm) pointed to a larger number of PPV segments with low conjugation degree, regardless of the conversion temperature. For these thin films, the mean free-path for diffusion of photoexcited carriers decreased, and energy transfer may have been hampered owing to the low mobility of the excited carriers. The emission efficiency was then found to depend on the concentration of structural defects, i.e., on the conversion temperature. For thick films with more than 25 bilayers, on the other hand, the PL signal did not depend on the PPV conversion temperature. We also checked that the interface effects were not caused by waveguiding properties of the excited light. Overall, the electronic states at the interface were more localized, and this applied to film thickness of up to 40-50 nm. Because this is a typical film thickness in devices, the implication from the findings here is that interface phenomena should be a primary concern for the design of any organic device.