Off-diagonal disorder and activation energy of high-field hopping motion

Review of Radiation-Induced Effects in Polyimide

Polyimide (PI, Kapton-H®) films are widely utilized in the spacecraft industry for their insulating properties, mechanical durability, light weight, and chemical resistance to radiation. Still PI materials remain exposed to a combination of high-energy electrons, protons, and ultraviolet (UV) photons, particles primarily responsible for radiation-induced damage in geosynchronous Earth orbit (GEO), which drastically change PI’s properties. This work reviews the effect of electron, proton, and UV photon irradiation on the material properties (morphology, absorption, mechanical properties, and charge transport) of PI. The different damaging mechanisms and chemical consequences that drive changes in the material properties of PI caused by each individual kind of irradiation will be discussed in detail.

Monte Carlo Simulations of Charge Transport in 2D Organic Photovoltaics

The effect of morphology on charge transport in organic photovoltaics is assessed using Monte Carlo. In isotopic two-phase morphologies, increasing the domain size from 6.3 to 18.3 nm improves the fill factor by 11.6%, a result of decreased tortuosity and relaxation of Coulombic barriers. Additionally, when small aggregates of electron acceptors are interdispersed into the electron donor phase, charged defects form in the system, reducing fill factors by 23.3% on average, compared with systems without aggregates. In contrast, systems with idealized connectivity show a 3.31% decrease in fill factor when domain size was increased from 4 to 64 nm. We attribute this to a decreased rate of exciton separation at donor-acceptor interfaces. Finally, we notice that the presence of Coulomb interactions increases device performance as devices become smaller. The results suggest that for commonly found isotropic morphologies the Coulomb interactions between charge carriers dominates exciton separation effects.

Doping Highly Ordered Organic Semiconductors: Experimental Results and Fits to a Self-Consistent Model of Excitonic Processes, Doping, and Transport

An in-depth study of n-type doping in a crystalline perylene diimide organic semiconductor (PPEEB) reveals that electrostatic attractions between the dopant electron and its conjugate dopant cation cause the free carrier density to be much lower than the doping density. Measurements of the dark currents as a function of field, doping density, electrode spacing, and temperature are reported along with preliminary Hall-effect measurements. The activation energy of the current, E(aJ), decreases with increasing field and with increasing dopant density, n(d). It is the measured change in E(aJ) with n(d) that accounts primarily for the variations between PPEEB films; the two adjustable parameters employed to fit the current-voltage data proved to be almost constants, independent of n(d) and temperature. The free electron density and the electron mobility are nonlinearly coupled through their shared dependences on both field and temperature. The data are fit to a modified Poole-Frenkel-like model that is shown to be valid for three important electronic processes in organic (excitonic) semiconductors: excitonic effects, doping, and transport. At room temperature, the electron mobility in PPEEB films is estimated to be 0.3 cm(2)/Vs; the fitted value of the mobility for an ideal PPEEB crystal is 3.4 +/- 2.7 cm(2)/Vs. The modified Poole-Frenkel factor that describes the field dependence of the current is 2 +/- 1 x 10(-4) eV (cm/V)(1/2). The analytical model is surprisingly accurate for a system that would require a coupled set of nonlinear tensor equations to describe it precisely. Being based on general electrostatic considerations, our model can form the requisite foundation for treatments of more complex systems. Some analogies to adventitiously doped materials such as pi-conjugated polymers are proposed.