Charge injection model

Due to the low mobility in organic semiconductors, diffusion transport is also very important for the charge injection process. Therefore, an analytical diffusion controlled charge injection model particularly suited for OLEDs has been developed [33]. This model is based on drift-diffusion and multiple trapping theory. The latter can also be used to describe hopping transport in organic semiconductors [34],... 

Li L, Meller G, Kosina H (2007) Diffusion-controlled charge injection model for organic light-... 

The hole current in this LED is space charge limited and the electron current is contact limited. There are many more holes than electrons in the device and all of the injected electrons recombine in the device. The measured external quantum efficiency of the device is about 0.5% al a current density of 0.1 A/cm. The recombination current calculated from the device model is in reasonable agreement with the observed quantum efficiency. The quantum efficiency of this device is limited by the asymmetric charge injection. Most of the injected holes traverse the structure without recombining because there are few electrons available to form excilons. 

It is worth repeating the above. T o date, the effects of the presence of interfacial barriers or interface regions have not been explicitly considered in models of charge injection at such interfaces. The... 

When the interfacial supramolecular triad is irradiated in the presence of I- under solar cells conditions, appreciable photocurrents are obtained. The profile of the photoaction spectrum shows clearly that photoinjection into TiC>2 takes place upon excitation of the ruthenium center. However, the IPCE values obtained are lower than those observed for the model compound, thus suggesting that injection is less efficient in the heterotriad. Of major interest is the mechanism for charge injection. Two different pathways can be envisaged. First, the charge injection may be a two-step process and takes place via the rhodium center as shown in the following equations ... 

S.M. Jeong, et al., Charge injection and transport model in organic ligt emitting diodes, Solid-State Electronics 49 (2005) 205-212. 

An important parameter is the ionization potential Ip. It is needed to model the energetics of p-doping, in which an electron is removed from the topmost valence levels (see Chapter 13), and also electrical contacts on CPs, in order to understand charge injection, and therefore Schottky barriers, field-effect transistors, and light-emitting diodes, which are studied in Section V. 

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... 

An important practical problem idiich requires for its solution the concepts developed in the preceeding section is that of contact charge exchange between insulators. Long viewed in terms of the traditional semiconductor model (, 32), charge injection and motion in high polymers have successfully eluded quantitative interpretation until the recognition in recent years ( 5, 18,... 

Table 3 gives a summary of the interfacial charge injection and recombination rate constants determined by direct spectroscopic techniques. The data are not directly comparable as different Ti02 preparations, solvents, electrolytes, time-scales and kinetic models were used by different experimentalists. Nevertheless, the table demonstrates the wide range of sensitized materials reported in the literature and provides a basis for further discussion. 

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