List of Figures

• 1.1. Two examples of use of OLEDs in commercial products. The image on the left shows a new Philips shave introduced to the market in 2002. The image on the right shows the OLED TV produced by Sony recently. Images were taken from [6].
• 1.2. Charge transport mechanism in solids. The left image describes the band transport. In a perfect crystal, depicted as a straight line, free carriers are delocalized. There are always lattice vibrations that disrupt the crystal symmetry. Carriers are scattered at these phonons, which limit the carriers mobility. The image on the right describes hopping transport. If a carrier is localized due to defects, disorder or selflocalization, the lattice vibrations are essential for a carrier to move from one site to another. The figure is from [8].
• 1.3. Left: device layout of a typical organic light-emitting diode (OLED). It consists of a glass substrate with an indium-tin-oxide (ITO) coating functioning as anode, a spin-coated layer of an organic semiconductor as the active layer, and an evaporated metal cathode. Right: working principle of an OLED. Four important processes are shown: (1) Charge injection (2) Transport (3) Exciton formation (4) Photon emission. The last two steps form the recombination process.
• 1.4. Left: A schematic view of a bottom contact OFET. The source electrode is grounded, while the drain and the gate are biased negatively. In this mode, holes are injected from the source and collected at the drain. Right: a top contact OFET with the electrodes patterned on top of the organic semiconductor.
• 2.1. Comparison between the analytical model (2.9) and empirical model $\mu \approx \textrm { exp } \left (-\left (C\sigma /{k_BT}\right )^2\right )$ for different temperature.
• 2.2. The mobility as a function of $\left (T_\sigma /T\right )^{1/3}$ for different $\alpha$.
• 2.3. Fermi-energy as a function of the carrier occupation probability. The symbols represent Fermi-Dirac and the solid lines Boltzmann represent statistics. Panel (a) shows the case of carrier occupation between $10^{-40}$ and 1. Panel (b) shows the case of carrier occupation bigger than $10^{-10}$.
• 2.4. The calculated mobility versus carrier occupation at different temperature.
• 2.5. Comparison between calculation and typical experimental results [41].
• 2.6. Plot of $\textrm { log }\sigma$ versus $T^{-1/4}$ at the electric field $100 V/cm$.
• 2.7. Conductivity and mobility versus temperature for ZnPc as obtained from the model (2.13) and (2.14) in comparison with experimental data (symbols).
• 2.8. Logarithm of the mobility versus $T^{-1}$. The electric field is $10^5 V/$cm, $\sigma _0=1.1\times 10^9 S/cm$, $T_0=340$K, $\alpha ^{-1}=0.5$ $\AA$
• 2.9. The same data as in Fig 2.8 plotted versus $T^{-2}$.
• 2.10. Plot of $\textrm { log }\sigma$ versus $F^{1/2}$ at $T=204$ K.
• 2.11. Electric field dependence of the mobility at 290K. Symbols represent Monte Carlo results [49], the line represents our work with parameter $T_0$=$852$K.
• 2.12. Field dependence of the conductivity at different temperatures.
• 2.13. Temperature dependence of the conductivity at different electric fields.
• 2.14. The calculated mobility (symbols) as a function of $\sigma /{kT}$.
• 2.15. The calculated mobility (symbols) as a function of $\left (\sigma /{kT}\right )^2$.
• 2.16. The conductivity as a function of the scaled electric field, $\beta =F/F_0$.
• 2.17. Comparison between our mobility model and analytical expression (2.30) with $p=1/3$ and $C=0.7$.
• 2.18. Temperature dependences of parameters $C$ and $p$ extracted from the analytical model.
• 2.19. Electric field dependence of parameter $C$ extracted from the analytical model.
• 2.20. Electric field dependence of parameter $p$ extracted from the analytical model.
• 2.21. Effect of $\alpha$ on the values of parameters $p$ and $C$ extracted from the analytical model.
• 3.1. Comparison between the model (3.8) and Baranovskii's model for the temperature characteristics of $E_{tr}$.
• 3.2. The transport energy versus the chemical potential for different standard deviations $a$ of the DOS.
• 3.3. The transport energy versus the relative carrier concentration for different standard deviations $a$ of the DOS.
• 3.4. Dependence of the relaxation time on the chemical potential for different standard deviations $a$ of the DOS .
• 3.5. Temperature dependence of the carrier mobility in organic semiconductors. In (a) the data are plotted versus $T^{-1}$, in (b) the same data are plotted versus $T^{-2}$.
• 3.6. Carrier concentration dependence of the mobility in organic semiconductors.
• 4.1. Temperature dependence of the conductivity in a disordered hopping system at different doping concentrations.
• 4.2. Temperature dependence of the conductivity in an organic semiconductor plotted as $\textrm { log }\sigma$ versus $T^{-2}$. The dashed line is to guide the eye.
• 4.3. Conductivity of doped ZnPc at various doping ratios as a function of temperature. The lines represent the analytical model, experiments (symbols) are from [63].
• 4.4. Conductivity as a function of the dopant concentration with temperature as a parameter.
• 4.5. Conductivity of PPEEB films versus the dopant concentration. The line represents the analytical model. Experiments (symbols) are from [139].
• 4.6. Conductivity as a function of the doping ratio with temperature as a parameter.
• 4.7. Conductivity at T=200K as a function of the doping ratio. The dashed line is to guide the eye.
• 4.8. Activation energy ($E_A$) as a function of the doping ratio.
• 4.9. Conductivity of an organic semiconductor versus $T^{-1}$ for different trap concentrations.
• 4.10. Conductivity of an organic semiconductor versus $T^{-2}$ for different trap concentrations.
• 4.11. Temperature dependence of the zero-field mobility for TTA doped with DAT. Symbols represent experimental data from [99].
• 4.12. Conductivity of an organic semiconductor versus the width of the trap distribution, $T_1$.
• 4.13. The dependence of the conductivity on the trap concentration.
• 4.14. The dependence of the conductivity on the Coulombic trap energy.
• 5.1. Dependence of the injection current on the barrier height.
• 5.2. Temperature dependencies of the injection current.
• 5.3. Comparison between the model and experimental data.
• 5.4. Comparison between injection currents for field dependent mobility and constant mobility.
• 5.5. Field dependence of the net, injection, and backflow currents.
• 5.6. Relation between injection current and $F^{1/2}$.
• 5.7. Barrier height dependence of the injection current.
• 5.8. Comparison between calculation and experimental data at $T=123K$.
• 6.1. Gaussian density of states with zero mean energy. The vertical axis corresponds to energy, the horizontal axis reflects the site density. The center of the Gaussian DOS is at zero energy.
• 6.2. Spatial distribution of the quasi Fermi energy for different current densities.
• 6.3. Spatial distribution of the carrier concentration near the contact for $j=0.4$A/cm$^2$.
• 6.4. Spatial distribution of the electric field at different current densities.
• 6.5. Current-voltage characteristics of a sample with Gaussian DOS distribution parametric in temperature.
• 6.6. The effect of the field dependent mobility on the space charge limited current.
• 6.7. The relation between organic layer thickness and space charge limited current.
• 7.1. Schematic structure of an organic thin film transistor.
• 7.2. Geometric definition.
• 7.3. The electrostatic surface potential as a function of gate voltage obtained by the implicit relation (7.13) and the approximation (7.14) (solid line).
• 7.4. Sheet conductance from numerical calculation (symbols) and the approximation.
• 7.5. Measured (symbols) and calculated transfer characteristics of a pentacence OTFT at room temperature.
• 7.6. Measured (symbols) and calculated transfer characteristics of a pentacence OTFT at different temperatures at $V_{D}=-2V$.
• 7.7. Measured (symbols) and calculated transfer characteristics of a PTV OTFT at room temperature at $V_{D}=-2V$.
• 7.8. Modeled output characteristics of a pentacene OTFT.
• 7.9. Barrier height dependence of current/voltage characteristics for unbipolar OLED.
• 7.10. Comparison between the model and experimental data for unbipolar OLED.