Laser-induced degradation studies of photoluminescence of PPV and CNPPV thin films

Abstract

Degradation of active polymer materials in presence of light, oxygen and moisture is known to be the most important reason limiting the lifetime of devices fabricated using light-emitting polymers. We focus on the monitoring of degradation of the two most popular light-emitting polymers PPV and CNPPV. We develop a procedure for accelerated laser-induced degradation and quantify degradation rate by monitoring loss of photoluminescence (PL) with irradiation time. We use this procedure to optimise thin film preparation conditions in terms of ambient and heat treatment schedule. Laser irradiation of PPV in air shows second-order exponential decay of PL intensity along with blue shift, while the decay is first order for CNPPV with no significant blue shift. Similar experiments with laser irradiation for vacuum-dried PPV films show PL enhancement under vacuum with slower subsequent degradation in air, demonstrating efficacy of vacuum drying in reducing oxygen content of processed films. No PL peak shift is observed with laser irradiation under vacuum, clearly indicating absence of contribution from chains of reduced conjugation length. In addition to PL enhancement, significant resistance to photodegradation is observed on laser irradiation in vacuum.