Effect of condensed phase environment on luminescent properties of MEH-PPV thin films

We study sensitivity of photoluminescence emission properties to state of aggregation in MEH-PPV thin films, and its solution in chloroform and xylene (50:50) mixture from which these films are either spin-cast or drop-cast. The choice of composition of solvent is dictated by the ease of fabrication of devices. We demonstrate unambiguous correlation between relative peak heights of the two characteristic major peaks in steady state photoluminescence with processing parameters, such as spin speed, and show that their ratio can be used as a robust measure of degree of entangled packing both in films and solutions. In thin films, a small but discernible red shift of about 3-10 nm in PL peak positions, and increase in width of peaks is observed with changes in degree of packing. A comparison of PL decay dynamics at the 588 nm emission shows that the decay develops slower components with increase in entangled packing in the case of thin films as compared to the fast nearly exponential decay in the case of solution. The progressive slowing down of dynamics is attributed to shift in distribution of inter-chain species of excitons in thin films.