2 research outputs found
Morphology Determines Conductivity and Seebeck Coefficient in Conjugated Polymer Blends
The impact of nanoscale
morphology on conductivity and Seebeck coefficient in p-type doped
all-polymer blend systems is investigated. For a strongly phase separated
system (P3HT:PTB7), we achieve a Seebeck coefficient that peaks at <i>S</i> ∼ 1100 μV/K with conductivity σ ∼
3 × 10<sup>–3</sup> S/cm for 90% PTB7. In marked contrast,
for well-mixed systems (P3HT:PTB7 with 5% diiodooctane (DIO), P3HT:PCPDTBT),
we find an almost constant <i>S</i> ∼ 140 μV/K
and σ ∼ 1 S/cm despite the energy levels being (virtually)
identical in both cases. The results are interpreted in terms of a
variable range hopping (VRH) model where a peak in <i>S</i> and a minimum in σ arise when the percolation pathway contains
both host and guest sites, in which the latter acts as energetic trap.
For well-mixed blends of the investigated compositions, VRH enables
percolation pathways that only involve isolated guest sites, whereas
the large distance between guest clusters in phase-separated blends
enforces (energetically unfavorable) hops via the host. The experimentally
observed trends are in good agreement with the results of atomistic
kinetic Monte Carlo simulations accounting for the differences in
nanoscale morphology
Molecular Doping and Trap Filling in Organic Semiconductor Host–Guest Systems
We
investigate conductivity and mobility of different hosts mixed
with different electron-withdrawing guests in concentrations ranging
from ultralow to high. The effect of the guest material on the mobility
and conductivity of the host material varies systematically with the
guests’ LUMO energy relative to the host HOMO, in quantitative
agreement with a recently developed model. For guests with a LUMO
within ∼0.5 eV of the host HOMO the dominant process governing
transport is the competition between the formation of a deep tail
in the host DOS and state filling. In other cases, the interaction
with the host is dominated by any polar side groups on the guest and
changes in the host morphology. For relatively amorphous hosts the
latter interaction can lead to a suppression of deep traps, causing
a surprising mobility increase by 1–2 orders of magnitude.
In order to analyze our data, we developed a simple method to diagnose
both the presence and the filling of traps