Charge-injection energy barriers

In a typical polymer LED device, the anode is the electrode with a high work-function used for hole injection. Usually, the polymer thin film is spun on top of the anode. On the other hand, the cathode of the device is a metal electrode with a low workfunction used for electron injection. It is usually produced by the thermal evaporation of the metal on top of the polymer film. For the hole-only devices that will be discussed here, both electrodes consist of the same metal. Therefore, there is no logical cathode and anode. However, for the sake of consistency, we still 

According to this definition, the intrinsic energy barrier (pi represents the minimum energy required for the charge injection from the metal into the polymer molecule, which is a constant for a given polymer and metal pair. On the other hand, the contact-dependent component A p should depend on the quality of the metal/polymer interface, which is morphology dependent. 

The direct evidence for this morphological dependence of A p comes from the observation of the unsymmetrical I-V curves observed in a series of hole-only devices.25,37 For example, it is observed that a hole-only device consisting of Au(anode POM contact)/polymer/Au(cathode MOP contact) has different I-V curves under forward and reversed biases.37 This phenomenon is also observable with other high-workfunction metals such as Cu and Ag.25 In these devices, if the anode/polymer and the cathode/polymer interfaces have the same p values, it is expected that the I-V curves under forward and reverse biases should be exactly the same. In other words, the built-in potential of these devices should be equal to zero. However, it is observed that these devices have a built-in potential on the order of a few tenths of a volt.37 In contrast, for a device Cu(anode POM contact)/MEH-PPV/Al(cathode MOP contact), the I-V curves under forward and reversed biases are expected to be significantly different since the two metals have different workfunctions (4.5 eV for Cu and 4.3 eV for Al). However, it was found by Roman et al. that the I-V curves were almost identical under forward and reverse biases.36 

From the above discussion, one will expect that the Ar type of aggregation style (Fig. 6.4), which has the conducting polymer backbones exposed, should have a better electrical contact with the metal electrode (Fig. 6.12a) than the non-Ar type of aggregation (Fig. 6.5), where the metal electrode and the conducting n system 

Cu electrodes. Xylenes was the solvent used for spin-coatingS hoIe only device using 

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For typical polymer LED device parameters, currenl is space charge limited if the energy barrier to injection is less than about 0.3-0.4 eV and contact limited if it is laiger than that. Injection currents have a component due to thermionic emission and a component due to tunneling. Both thermionic emission and tunneling... 

For the high voltage regime it is reasonable to suppose that a semiconductor of thickness L is contacted with two electrodes which, by virtue of a low energy barrier at the interface, are able to support the transport of an infinite number of one type of mobile carrier. The current will then become limited by its own space charge, and this can in the extreme case reduce the electric field at the injecting contact to zero. This is realized when the number of carriers per unit area inside the sample approaches the capacitor charge of the diode, i.e., eeo/e. This number of carriers can be transported per unit transit time ttT = L/fj,. 

Defects are created by the recombination of photoexcited carriers, rather than by the optical absorption. The evidence for this conclusion is that defect creation also results from charge injection without illumination (see Section 6.5.2) and that defect creation by illumination is suppressed by a reverse bias across the sample which removes the excess carriers (Swartz 1984). The kinetics of defect creation are explained by the recombination model in Fig. 6.28, which assumes that the defect creation is initiated by the non-radiative band-to-band recombination of an electron and hole. The recombination releases about 1.5 eV of energy which breaks a weak bond and generates a defect. In terms of the configurational coordinate model of Fig. 6.1, the energy overcomes the barrier E. The defect creation rate is proportional to the recombination rate... 

The problem of detailed modeling can be divided into three parts (1) What is the shape of the energy bands within the device, particularly close to the electrodes (2) What is the mechanism for injection of charges through any barriers which are formed at the interface (3) To what extent are the characteristics of the device determined by transport and recombination within the bulk of the polymer, rather than by injection at the interfaces ... 

A), the metal/polymer interfaces, and the energy barriers for charge injection (Sec. 3.B), the device turn-on voltage (Sec. 3.C), the emission spectrum (Sec. 

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