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^–3 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