Photogeneration activation energies

Polypyrrole polymers have a maximum photosensitivity at 610 and 510 nm which increases to 1400 nm after thermal treatment for 24 h at 300 °C [317]. The established thermal activation energy of the photoconductivity excludes the one-step photogeneration process of the charge carriers. 

Another major application of Ti oxide species prepared within the zeolite cavities includes the photocatalytic decomposition of environmentally noxious substances into benign compounds. Examples include photodecomposition of chlor-ophenol [158] and of NO into N2 and O2 [159]. Ti02-loaded MCM-41 shows a much lower apparent activation energy for the photodegradation of acetophenone than that of Ti02-loaded zeolite-X, -Y and -A. The differences in reactivity with different frameworks was related to the efficiency of trapping of the photogenerated electrons and/or holes [160]. 

This cis-trans isomerization can be conveniently followed by monitoring the intense absorption at 320 nm due to n-n transition. The intensity of this absorbance decreases as more cis isomer is produced. The photogenerated cis form undergoes thermal transformation in the dark to trans form. This transformation was followed by UV spectroscopy. Similar results have been obtained with azoaromatic polyureas (16) and polyamides also. The increase in the peak intensity at 320 nm, in the dark, was monitored at various time intervals at six different temperatures (26, 29, 30, 33, 35 °C). It was found that the cis-trans isomerization follows first order kinetics. The rate of this isomerization was calculated from the slope of the plot of log(Aoo - At) vs. time, t, where Aoo and At are the absorbances at 360 nm before irradiation and at time t. It was possible to calculate the activation energies from the Arrhenius plots... 

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

---