Unipolar injection

A similar method can also be used to inject positive charge carriers (Reinis and Cressman, 1979). The anode is a semiconductor film in contact with the liquid. Illumination leads to the formation of electrons and holes in the film. The holes are drawn toward the liquid where they transfer their charge to suitable molecules in the liquid. 

Thin-film Me/Ins/Me diodes can be used to generate hot electrons with mean energies of 1 eV (Tsuchida and Ishii, 1990). An aluminum cathode is covered by thin 

Optically excited states of solute molecules in the liquid can form positive ions upon contact with the anode. The electron from the excited state tunnels into the metal and a positive charge carrier is injected into the liquid (Romanets et al, 1970). In Table 1 the injection methods are summarized. The primary spatial extension can be modified by the application of an electric field. 

Method / process Electrons Positive Ions extension 

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FIGURE 5.19. Distribution of the density of positive and negative Q-charges (a) when there are Debye layers near the electrodes [105], (b) imder conditions of unipolar injection [102], and (c) under conditions of bipolar injection [104]. 

Tobazeon, R., Electrohydrodynamic instabilities and electroconvection in the transient and a.c. regimes of unipolar injection in insulating liquids a review,/. Electrostatics, 15,359, 1984. 

Lacroix, J.C., Atten, P., and Hopfinger, E.J., 1975, Electro-convection in a dielectric liquid layer subjected to unipolar injection, Phys. Fluid Mech., 69 539. 

Currem field characteristics measured wiih conjugated polymers sandwiched between an indium-tin oxide (ITO) anode and an aluminum cathode are usually hole dominated and are, consequently, appropriate for testing injection/lransport models for the case of unipolar current How. Data shown in Figure 12-1 refer to injection-limited currents recorded on typically 100 nm thick spin-coated films of derivatives of poly(y d/"fi-phenylenevinylene) (PPV) and a planarized poly(/ /" -pheny-leue) employing a Keilhley source measure unit. The polymers were ... 

It has been shown that the EL of polysilane-based LEDs is emitted near the interface between the polysilane and the electron injecting electrode, because of the strong unipolar (hole conductive) nature of polysilanes. Defect levels existing at the interface are considered to play an essential role in the emission of EL in the visible region,93 and have both positive and negative effects on the LED characteristics. The positive space charges generated by... 

By exchanging one of the electrodes, such a diode can be altered from a unipolar hole device into an ambipolar device. Figure 5.10 shows the I/V characteristics of an ITO/PEDOT/MDMO-PPV/LiF-Al device. Here, the LiF-Al electrode should guarantee electron injection under forward bias. 

Figure 64 Current-voltage characteristics for single-layer (SL) EL cells based on anthracene (a) and tetracene (b) single crystals with unipolar and double injection contacts. Na/K-Na/K mononegative carrier injection, Au-H20 mono-positive carrier injection, Au-Na/K double injection. The crystal thicknesses are 98 and 108 pm for anthracene and tetracene, respectively. The slopes of the straight-line segments for tetracene characteristics are given nearby the curves. After Ref. 51.

In this chapter, we showed that OLED performance is clearly defined based on the simplified working mechanism of carrier injection and successive SCLC. The balance of holes and electrons injected within an emitter layer is a major factor contributing to overall EL quantum efficiency. The unipolar-charged-transport layers, HTL and ETL, contribute to the increase of quantum efficiency. 

Here I, represents the drain current and ju, jUp the respective electron and hole mobility. C defines the area capacitance of the insulator. The channel geometry is defined by the channel width W and length L. The ambipolar range, described by Eq. (3), is only valid as long as both electrons and holes can be injected and further transported in the active layer of the transistor. However, in most cases the injection and/or the transport in the transistor channel are suppressed for one charge carrier type. In that case, the FET operates only in the unipolar and saturation range as described by Eqs. (1) and (2). 

By eonsidering the inset of Figure 24.14a, the origin of the increase in unipolar hole eurrent injected from the drain electrode can be understood. In this graph the square root of the ambipolar drain current (Eq. (3)) [3],... 

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