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Nanoscale phase separation of organic blends as key for high-performance OFETs

The nanoscale investigation of OFETs by means of a combination of FFM and KPFM allows unveiling the nanoscale phase separation in organic blend that play a major role in the high-performance of solution-processed OFETs and identifying optimization routes

As core component of integrated circuits, the realization of high-performance organic field effect transistors (OFETs) from solution processing requires uniform and crystalline films with optimal electronic properties. One of the advances in the solution manufacturing process is raised from the idea of blending small conjugated semiconductor molecules with an amorphous insulating polymer to combine the advantageous properties of the individual components. This strategy has led to an overall rise in the charge carrier mobility and an improvement of device processability, reproducibility, and stability [1]. The key of superior performance of OFETs fabricated with blended films seems to be the spontaneous vertical phase separation of the two material components. In this article we elucidate the structural details at nanoscale level of blends based on a relevant OSC, 2,7-dioctyl[1]-benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and polystyrene (PS) by employing friction force microscopy (FFM) to identify the two different blend constituents [2]. The results reveal a three-layer stratification: a C8-BTBT crystalline layer (for efficient carrier transport) sandwiched between two PS layers, one at the bottom acting as a passivating dielectric layer and a PS-rich skin layer on the top (∼1 nm) conferring stability to the devices. The investigation of the electronic properties at the nanoscale by Kelvin Probe Force Microscopy (KPFM) demonstrates that contact resistance is the critical factor limiting the device performance, which is significantly improved by doping the contacts with a molecular dopant. Our findings, obtained via a combination of FFM and KPFM, allows identifying optimization routes for high-throughput solution-processed OFETs.

Authors:
Ana Pérez-Rodríguez,1 Inés Temiño,1 Carmen Ocal,1 Marta Mas-Torrent,1,2 Esther Barrena1

Affiliations:
1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
2CIBER-BBN, Spain

Publication:
Decoding the Vertical Phase Separation and Its Impact on C8-BTBT/PS Transistor Properties
ACS Applied Materials & Interfaces 10, 8, 7296-7303 (2018)
DOI: 10.1021/acsami.7b19279

Figure:
Schematic view of the three-layer stratification in the blend. Topography lateral force image by FFM (blue image) showing the frictional contrast caused by the two materials in the blend

References:
[1] Temiño, I.; Del Pozo, F. G.; Ajayakumar, M. R.; Galindo, S.; Puigdollers, J.; Mas-Torrent, M. A. Adv. Mater. Technol. 2016, 1, 1600090
[2] Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C. B.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z.  Nat. Commun. 2014, 5, 3005

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