Ultrafast transient absorption spectroscopy of doped P3HT films: distinguishing free and trapped polarons

Abstract

It is generally presumed that the vast majority of carriers created by chemical doping of semiconducting polymer films are coulombically trapped by the counteranion, with only a small fraction that are free and responsible for the increased conductivity essential for organic electronic applications.

Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers

“…In most chemically doped conjugated polymers, including PTB7 as shown in the Supporting Information, the new low‐energy optical absorption that results from polarons, labeled P1 in Figure 1a, appears near 0.5 eV (≈2480 nm). [ 30 ] We [ 33,34 ] and others [ 35,36 ] have argued that when the dopant counterion resides close to the polaron on the polymer backbone the polaron can be trapped by the attractive Coulomb interaction, lowering its mobility and blueshifting the absorption of the P1 transition. As shown in Figure 1b, bipolarons are also expected to have an optical transition (BP1) that is higher in energy than that of single polarons.…”

“…In previous work, we showed that we could distinguish free and Coulombically trapped polarons using ultrafast transient absorption spectroscopy. [ 33 ] The idea behind the experiment is shown in Figure 1a. By exciting the polaron P1 transition, an electron from the valence band is moved to the half‐filled state in the bandgap (left energy level diagram); in essence, the excitation is a photoinduced charge transfer taking an electron from a neutral region of the polymer and filling the hole, thus moving the hole to a new location on the polymer backbone (center diagram).…”

“…The optical signatures of this process are a bleach (loss) of the bandgap transition, a bleach of the P1 transition, and an increase in absorption of the P2 transition, all of which uniformly and smoothly recover on a time scale of a few ps. [ 33 ] If the photoexcited carriers are Coulombically trapped polarons, then the photoinduced P2 transition is blueshifted, and the recovery time lengthens to tens of ps because the relocalized carrier needs time to diffuse back to the place where it was Coulombically trapped. Thus, time‐resolved spectroscopy is capable of separating the presence of free and trapped polaron species.…”

“…Thus, time‐resolved spectroscopy is capable of separating the presence of free and trapped polaron species. [ 33 ] We note that although many other groups have explored the time‐resolved spectroscopy of charge carriers on conjugated polymers, [ 37–41 ] none that we are aware of used photoexcitation directly into the P1 band, explaining why it previously had been difficult to interpret the results. [ 37 ]…”

“…Perhaps more importantly, the shape and dynamics of the transient absorption signals from doped PTB7 (see the Supporting Information) all are very much like that of doped P3HT. [ 33 ] Figure S10 in the Supporting Information shows the results of ultrafast transient absorption experiments on 0.025 m FeCl 3 ‐doped PTB7 films excited at ≈0.69 eV (1800 nm). The results show a bleach of the P1 band, seen at the red edge of the detection window, an increase of the P2 peak, and the bleach of a small P3 peak seen at the blue side of the spectrum, all of which decay in a few ps, exactly as predicted from the simple diagram in Figure 1a.…”