Special CPM Seminar

Direct observation of ultrafast long-range charge separation at polymer-fullerene heterojunctions

Carlos Silva

Université de Montréal

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. In this work, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on ≲ 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. Furthermore, we present a non-linear coherent spectroscopy, photocurrent-detected two-dimensional spectroscopy (2DPC), which is an ultrafast optical thechnique belonging to a family of 2D Fourier- domain spectroscopies that allows measurement of correlations between optical transitions induced by short optical pulses. In our implementation, spectral correlations are detected via the time-integrated photocurrent produced in a photovoltaic diode. We address current debates on the photocarrier generation mechanism at molecular heterojunctions.