Charge-carrier relaxation dynamics in highly ordered poly(p-phenylene vinylene): Effects of carrier bimolecular recombination and trapping

Abstract

We have studied the charge-carrier relaxation dynamics in highly ordered poly($p$-phenylene vinylene) over a broad time range using fast $(t>100\mathrm{ps})$ transient photoconductivity measurements. The carrier density was also monitored $(t>100\mathrm{fs})$ by means of photoinduced absorption probed at the infrared active vibrational modes. We find that promptly upon charge-carrier photogeneration, the initial polaron dynamics is governed by bimolecular recombination, while later in the subnanosecond time regime carrier trapping gives rise to an exponential decay of the photocurrent. The more sensitive transient photocurrent measurements indicate that in the low excitation regime, when the density of photocarriers is comparable to that of the trapping states $(\sim {10}^{16}{\mathrm{cm}}^{-3})$, carrier hopping between traps along with transport via extended states determines the carrier relaxation, a mechanism that is manifested by a long-lived photocurrent “tail.” This photocurrent tail is reduced by lowering the temperature and/or by increasing the excitation density. Based on these data, we develop a comprehensive kinetic model that takes into account the bipolar charge transport, the free-carrier bimolecular recombination, the carrier trapping, and the carrier recombination involving free and trapped carriers.