Electron-hole capture

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. 

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... 

In Fig. 2.21, micrographs of the PL and EL emission from the same region of a TFB F8BT blend device are shown. The polymer film was spun from xylene solution, so that distinct phases rich in either of the polymers of several im in size are seen. Comparison of the PL and the EL images reveals that EL emission is predominantly produced at the interfaces between the TFB-rich and F8BT-rich phases. This is consistent with electron-hole capture occurring at the interface. 

Whereas under photoexcitation the exciplex is excited indirectly via energy transfer from the excitons, it is the primary neutral excitation in electroluminescence. This is shown in Fig. 2.24, parts (a) and (b), where the EL emission for both TFB and PFB blends is dominated by the exciplexes. This becomes particularly clear when comparing the EL spectra with the delayed emission spectra in Fig. 2.23, parts (c) and (d). In contrast, the time-integrated PL from similarly prepared blend films (also plotted in Fig. 2.24) is primarily due to bulk excitons. We note that exciplex EL emission has been observed previously, which suggests that these exciplexes may also be formed by the mechanism of direct electron-hole capture at the interface [37, 41, 42]. 

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,... 

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

The appearance of both the semicircle and the capacitive peak were accounted for by Vanmaekelbergh and coworkers, by considering recombination of photogenerated holes with GB electrons at the semiconductor surface [51, 55-57]. The recombination mechanism assumed by these authors consists of the successive capture of an electron in an empty surface state and of a hole in an occupied surface state. Taking the rates of the electron (hole) capture steps to be first order in the GB electron density s (valence band hole density p ) and the density of empty (filled) surface states, an electrochemical impedance corresponding to the equivalent circuit shown in Fig. 9 was calculated for this recombination mechanism. The... 

The process of electron/hole capture in these devices is not well studied. However, to get efficient capture in these very thin structures (typically 100 nm total thickness of polymer layers), it is necessary that one or the other charge carrier be of very low mobility so that the local charge density is sufficiently high to ensure that the other charge carrier will pass within a collision capture... 

Besides differing efficiencies of hole and electron injection at the electrode/CP interface, hole and electron mobilities within the CP also differ substantially, with electron mobilities being impeded through trapping by impurities such as oxygen. Additionally, if electron-hole recombination occurs near one of the electrodes, quenching is more likely to occur. Thus, it is preferred to somehow be able to keep electron-hole capture, i.e. exciton formation, away from the electrodes and in the interior of the device. 

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