Barrier-free capture

TFB(50 nm)/anode device assuming charge capture via charge injection (a) and barrier-free capture into an exciplex (b). When assuming barrier-free capture, much lower electric fields and charge densities are required at the heterojunction. Graph copied directly from [53]. 

Barrier-free Capture in Polymer Bilayer LEDs at Low Temperatures... 

This means that in these blends the spectral shape of the electroluminescence would be independent of whether process (a) or (b) in Fig. 2.1 occurs because ex-citons could quickly transfer towards a nearby interface site. In well-mixed blends it is therefore impossible to show that the exciplex is the primary product of charge capture and hence to prove that barrier-free capture occurs. We address these concerns here by presenting electroluminescence measurements from bilayer light-emitting diodes, where leakage currents are prevented, while a low density of interface sites (in fact the lowest possible) is maintained. 

By investigating bilayer and blend light-emitting diodes at different temperatures we give conclusive evidence that barrier-free capture is the only capture mechanism occurring at the hetero junction between PFB and F8BT (at moderate voltages). 

The High-voltage Limit of the Barrier-free Capture Regime... 

High Interface Densities Lower the Barrier-free Capture and Exciton-Regeneration Efficiencies... 

A.C. Morteani, A.S. Dhoot, J.S. Kim, C. Silva, N.C. Greenham, C. Murphy, E. Moons, and R.H. Friend, Barrier-free electron-hole capture in polymer blend heterojunction light-emitting diodes, Adv. Mater., 15 1708-1712, 2003. 

Morteani A. C., Dhoot A. S., Kim J.-S., Silva C., Greenham N. C., Murphy C., Moons E., Cina S., Burronghes J. H. and Friend R. H. (2003), Barrier-free electron-hole capture in polymer blend heterojunction light-emitting diodes , Adv. Mat. 15, 1708-1712. 

The study of glycoside formation and breaking in the gas phase is not as esoteric as it might appear. The very short lifetimes of glycosyl oxocarbenium ions in solution require that their study be via indirect competitive kinetic experiments. Knowledge of the barrier for capture of these species, whether as free oxocarbenium ions or as ion molecule pairs, is difficult to obtain by direct methods. On the other... 

Fig. 2.1 Illustration of the two mechanisms for electron-hole capture discussed in the text. Electrons and holes are transported through their respective transport materials and accumulate at the heterojunction, a) Injection of one of the charges into the opposite polymer makes possible charge capture within the polymer bulk and formation of intramolecular excitons. b) Barrier-free electron-hole capture...

Barrier-free Electron-Hole Capture in Polymer Blend LEDs... 

We have shown an extreme enhancement of the exciplex emission in bilayer EL as compared to PL and the appearance of weak exciton EL only through thermal activation from the exciplex at higher temperatures. At low temperatures, the exciton contribution is frozen out completely and only exciplex electroluminescence is seen. This demonstrates unambiguously that the only source of bulk excitons during electrical excitation is endothermic energy transfer from exciplex states that are generated via barrier-free electron-hole capture and confirms the work presented in Section 2.2.1 that was based on room-temperature emission from polymer blend LEDs and time-resolved PL. 

As explained in Section 2.2.1, the barrier-free electron-hole capture mechanism (depicted in Fig. 2.1(b)) relies on the fact that charge transport is blocked by the barriers that result from the offsets of the HOMO and LUMO levels of the two polymers. Since the carriers cannot progress across the heterojunction,... 

Fig. 2.46 III ustration of exciton retrapping at PFB F8BT heterojunctions. An exciplex state that was formed at the heterojunction via barrier-free charge capture (Section 2.2) or relaxation of an interfacial geminate pair (Section 2.3) can either decay radiatively and emit (1) or transfer endothermically to the bulk...

Electron-Hole Capture can Occur Barrier-free via an Exciplex Intermediate... 

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