Effects of Confinement on Microstructure and Charge Transport in High Performance Semicrystalline Polymer Semiconductors

Abstract

The film thickness of one of the most crystalline and highest performing polymer semiconductors, poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT), is varied in order to determine the effects of interfaces and confinement on the microstructure and performance in organic field effect transistors (OFETs). Crystalline texture and overall film crystallinity are found to depend strongly on film thickness and thermal processing. The angular distribution of crystallites narrows upon both a decrease in film thickness and thermal annealing. These changes in the film microstructure are paired with thin‐film transistor characterization and shown to be directly correlated with variations in charge carrier mobility. Charge transport is shown to be governed by film crystallinity in films below 20 nm and by crystalline orientation for thicker films. An optimal thickness is found for PBTTT at which the mobility is maximized in unannealed films and where mobility reaches a plateau at its highest value for annealed films.

Vertical confinement and interface effects on the microstructure and charge transport of P3HT thin films

“…A number of studies have documented the trend of better crystalline texture resulting in improved electronic performance. 16,23,[46][47][48] Our data confirms and expands that structural details of semicrystalline polymers have an important role in controlling charge transport.…”

“…We have previously performed similar measurements in PBTTT, and found that crystallites exhibit comparable trends in texture as a function of thickness. 16 Similar to P3HT, mobility also drops sharply in extremely thin films due a suppression of overall film crystallinity, however PBTTT mobility also falls in thicker films. We have attributed this to the higher inherent crystallinity of PBTTT which causes the connections between crystalline grains, and hence film texture, to have a greater impact on charge carrier mobility.…”

“…a method to measure XRD pole figures for fiber textured thin films using three separate measurements, taking advantage of the benefits of an area detector. 15,16,27 By appropriately combining data from grazing incidence diffraction patterns with data from local-specular diffraction patterns (where the proper choice of incidence angle prevents distortion near the Bragg reflection), 27 we obtain diffracted intensity that represents the true intensity of a Bragg reflection across the entire span of polar angles (À90 to 90 ). The polar angle v is defined such that 0 corresponds to the substrate normal and 90 corresponds to the substrate plane.…”

“…14 Although there exist some important examples of using quantitative XRD to study semicrystalline, semiconducting polymer microstructures, 11,[15][16][17][18][19] Xray analysis in this field is often limited to qualitative observations. Moreover, only a few studies are able to make direct observations about interface structure using XRD.…”

Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors

“…Remarkably, x-ray diffraction (Fig. 4b) reveals that the width of the (020) diffraction peak related to the p-p stacking direction of P3HT chains is much narrower 43 in the P3HT/PS (5% P3HT) blend than in neat P3HT, implying a better order of P3HT chain in the PS matrix. Here the coherence length is B12 nm, as compared with B7 nm of neat P3HT.…”