PHOTOCURRENT MEASUREMENTS ON AGGREGATES IN LADDER-TYPE POLY(P-PHENYLENE)

We report measurements of the photovoltaic effect on single layer devices made with a ladder-type poly-(paraphenylene) (LPPP). Besides the photocurrent in the blue spectral range associated with pi to pi* intra-chain absorption we observe an additional response in the yellow part of the spectrum centred at 2.25 eV. We also observe a weak absorption feature at the same energy, and we consider that both arise from absorption into pi electron states formed over several aggregated polymer chains. The photocurrent due to excitation of these states is enhanced by two orders of magnitude compared to blue excitation and vanishes under reverse bias (i.e. ITO negative). We find that the photocurrent response due to the aggregate states is more sensitive to oxygen exposure than the response due to intra-chain absorption.