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

Abstract

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. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its timeresolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on t50-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. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

Probing polaron excitation spectra in organic semiconductors by photoinduced-absorption-detected two-dimensional coherent spectroscopy

“…At each laser shot, the experiment is repeated with a sequence of pulses phase-shifted with respect to the previous shot, thus creating two collinear trains of phase-modulated pulse pairs. Those interfering excitation pulses produce a population signal oscillating at Ω 21 and Ω 43 in the kHz range. It can be readily shown that [6], within this phase modulation scheme, the non-linear signals of interest, which in analogy to four wave mixing experiments are referred to as rephasing and non-rephasing signals, oscillate at the frequencies Ω 43 − Ω 21 and Ω 43 + Ω 21 respectively.…”

“…Those interfering excitation pulses produce a population signal oscillating at Ω 21 and Ω 43 in the kHz range. It can be readily shown that [6], within this phase modulation scheme, the non-linear signals of interest, which in analogy to four wave mixing experiments are referred to as rephasing and non-rephasing signals, oscillate at the frequencies Ω 43 − Ω 21 and Ω 43 + Ω 21 respectively. These frequency components are extracted simultaneously from the overall signal by dual lock-in detection.…”

“…This frequency downshift results in an improvement of the signal-to-noise ratio, which is inversely proportional to the frequency downshift itself, and which virtually removes the impact of the mechanical fluctuations occurring in the setup on the signals of interest [6]. In order to obtain the reference signals at the frequencies of the rephasing and non-rephasing signals, Ω 43 − Ω 21 and Ω 43 + Ω 21 , one of the two photodiode outputs (typically the one at higher frequency) undergoes amplitude modulation (AM) by the output of the other photodiode. The AM signal so obtained carries the two sideband frequencies of interest (Ω 43 −Ω 21 and Ω 43 +Ω 21 ) is then used for the lock-in demodulation.…”

“…In each arm of the two twin interferometers, an acousto-optic Bragg cell modulates each pulse at an unique frequency (Ω i , i = 1, 2, 3 and 4) close to 200 MHz. The exact Ω i values are chosen so that the difference between the modulation frequencies of the two pulses within a pair (Ω 21 and Ω 43 ) is of the order of ∼ 10 kHz.…”

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

“…For both material compositions, the effect of the push pulse builds-up fast (within ~200 fs time resolution) at time zero, which is a clear indication of the bound CT state formation happening at the ultrafast time scale as observed by others before. 61 After this, the PPP response decays on a time scale of ~500 ps, which we assign with the loss of bound CT states. Because this process occurs at the fast (sub-ns) timescale and does not depend on the pump we associate it with the geminate recombination of photogenerated species.…”