Trap Generation Dynamics in Photo-Oxidised DEH Doped Polymers

Abstract

A series of polyester films doped with a hole transport molecule, p-diethylaminobenzaldehyde-1,1'-diphenylhydrazone (DEH), have been systematically exposed to ultraviolet radiation with a peak wavelength of about 375 nm. The electronic performance of the films, evaluated using time-of-flight and space-charge current injection methods, is observed to continuously degrade with increasing ultraviolet exposure. The degradation is attributed to photo cyclic oxidation of DEH that results in the creation of indazole (IND) molecules which function as bulk hole traps. A proposed model for the generation dynamics of the IND traps is capable of describing both the reduction in current injection and the associated time-of-flight hole mobility provided around 1% of the DEH population produce highly reactive photo-excited states which are completely converted to indazole during the UV exposure period. The rapid reaction of these states is incompatible with bulk oxygen diffusion-reaction kinetics within the films and is attributed to the creation of excited states within the reaction radius of soluble oxygen. It is suggested that encapsulation strategies to preserve the electronic integrity of the films should accordingly focus upon limiting the critical supply of oxygen for photo cyclic reaction.

Cellular Automata Modelling of Photo-Induced Oxidation Processes in Molecularly Doped Polymers

“…Optimised fitting could then be achieved as demonstrated by the symbols in Figure 2 provided   1 and it is consequently apparent that the photo-conversion dynamics may generally exhibit a stretched-exponential characteristic as opposed to exact ( = 1) exponential dynamics inferred from the previous rate-equation study [3]. The emergence of this subtly different dynamical behaviour is to some extent anticipated since the CA model is now capable of handling local cellular j,i effects which translate to an effective range of characteristic time constants throughout the film thickness.…”

“…These MDP samples comprised c M = 50% by weight doping of p-diethylaminobenzaldehyde-1,1 -diphenylhydrazone (DEH) in a commercial polyester binder and were subject to ultraviolet (UV) irradiation at 375 nm in order to induce a photo-oxidation conversion of the DEH to an indazole (IND) photo-product [8][9][10]. As demonstrated in an earlier paper [3] it is possible to deduce the concentration of IND molecules in such films using a combination of time-of-flight (TOF) and space-charge-limited current (SCLC) measurements. A restricted amount of photo-product dynamical information may thus be experimentally established for this MDP material using a finite number of samples which are exposed across a wide range of UV exposure times (10 2 s → 10 5 s).…”

“…An earlier study of photo-induced oxidation processes in MDPs [3] considered a situation in which the incident ultraviolet (UV) radiation was capable of inducing photo-excitation of the molecular dopant, but was below the absorption threshold of the binder polymer. Complications from possible degradation of the polymer itself are thus avoided and any measurable degradation in electronic performance may be attributed to photo-chemical conversion of the molecular dopant.…”

“…Complications from possible degradation of the polymer itself are thus avoided and any measurable degradation in electronic performance may be attributed to photo-chemical conversion of the molecular dopant. By setting up appropriate rate equations that describe the molecular photo-excitation rate (G), the molecular relaxation rate (β) and the fraction of photo-excited molecules that react with soluble oxygen (φ) it was found that under certain approximations the conversion dynamics of molecular dopant (M) to photo-product (P) could be analytically described as a function of photo-exposure time (t) by [3]:…”

An Evaluation of Optical Absorbance Kinetics for the Detection of Micro-Porosity in Molecularly Doped Polymer Thin-Films

“…Mitigation of photo-degradation processes in MDP thin-films consequently demands that a comprehensive understanding of the underlying processes and parameters that are dominant in driving these internal processes are fully identified and numerated. Previous work in this area has focused upon a specific photo-cyclic degradation process that is known to occur for a small hydrazone molecule p-diethylaminobenzaldehyde -1,1diphenylhydrazone (DEH) in MDP thin films [5][6][7][8]. The photo-cyclic conversion of DEH to the electronically inactive indazole (IND) product requires that the DEH molecule be first excited into a (reactive) state by appropriate absorption of UV radiation, and that excited DEH molecules have a supply of absorbed oxygen to then complete the reactive transition to IND.…”