Local Charge Trapping in Conjugated Polymers Resolved by Scanning Kelvin Probe Microscopy

The microstructure of conjugated polymers is heterogeneous on the length scale of individual polymer chains, but little is known about how this affects their electronic properties. Here we use scanning Kelvin probe microscopy with resolution-enhancing carbon nanotube tips to study charge transport on a 100 nm scale in a chain-extended, semicrystalline conjugated polymer. We show that the disordered grain boundaries between crystalline domains constitute preferential charge trapping sites and lead to variations on a 100 nm scale of the carrier concentration under accumulation conditions.

Figure 1

(a) Normalized drain current of ribbon phase pBTTT FET during and after 1 h continuous gate bias stress at gate voltage $V_g=-80\mathrm{V}$, drain voltage $V_d=-5\mathrm{V}$. The device is turned off at time $t=0\mathrm{h}$, after which the current recovery is monitored periodically by pulsed current measurement at the same voltage conditions. The molecular structure of pBTTT and the device architecture with the source (S), drain (D), and gate (G) electrodes are shown as insets. (b) Average SKPM surface potential as a function of time after the stress. The dotted line indicates the surface potential measured before the stress. The red or gray points show the amplitude of the surface potential variations associated with the ribbon phase as determined from fast-Fourier-transform of potential line scans. The inset shows a schematic diagram of the pBTTT ribbon phase. The polymer chains are drawn as red or gray lines, and the transition regions between the chain-extended crystalline ribbons and the grain boundaries are indicated by dashed blue (or dashed dark gray) lines.

Figure 2

(a) AFM topograph of an 80 nm thick “ribbon phase” pBTTT film. (b) Corresponding surface potential image after 1 h stress at $V_g=-80\mathrm{V}$; (c),(d) Topography and surface potential of an unstressed pBTTT device. (e) Topography (black) and potential (red or gray) line scans of a stressed device; (f) SEM image of a metal-coated AFM cantilever with a single-walled CNT attached.

Figure 3

Topography (a) and corresponding surface potential images associated taken 1 h (b) and 3 h (c) after a 1 h stress at $V_g=-80\mathrm{V}$. Images are not corrected for lateral drift. (d) Histograms of the surface potential distribution taken from SKPM images as a function of recovery time.

Figure 4

Surface topography line scan (top) and surface potential profiles along the FET channel during operation at $V_g=-30\mathrm{V}$ with different values of $V_d$.