Spin-Dependent Photoinduced Absorption

Photoinduced absorption (PA) measurements provide useful information about relaxation processes of photocarriers and tail and gap states in a-Si H [Tauc, (1982) see also Chapter 9 by Tauc in Volume 21B]. Photoinduced absorption also depends on the recombination processes of those trapped electrons and holes that are created under optical excitation. Thus it is expected that PA will be spin-dependent, as is luminescence. Hirabayashi and Morigaki (1983a,b) have observed, for the first time, spin-dependent PA at 2°K in a-Si H, monitoring the PA intensity while irradiating the sample with microwaves at 9.6 GHz. This is called photoinduced absorption-detected ESR (PADESR), which provides complementary information to the conventional ODMR (luminescence-detected ESR). However, since the PADESR technique can be applied to nonluminescent materials, it may provide a more general means for highly sensitive detection of ESR than conventional ODMR. 

From stimulated emission measurements on picosecond timescales, Rothberg and co-workers found that 90% of photons absorbed in their PPV samples produced nonemissive species, and they proposed that these nonemissive species were po-laron pairs.22 These polaron pairs are believed to be responsible for an induced absorption which overlaps with the PL.23 For PPV prepared in Cambridge with special care taken to avoid photo-oxidation, we have found that a much smaller fraction of absorbed photons produce polaron pairs, with a consequent decrease in the picosecond photoinduced absorption.24 For the soluble PPV derivatives, the formation of interchain excited states is strongly dependent on the film morphology, which can be controlled through the choice of solvent for spin-coating.25... 

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