The effect of interfacial roughness on the thin film morphology and charge transport of high performance polythiophenes



            Roughness at the semiconductor-gate dielectric interface significantly influences the performance of organic field effect transistors (OFETs).  However, controlled studies of the effect of interfacial roughness on semiconducting polymers have not been reported.  In this work, we control and vary the interfacial roughness between a silicon oxide gate dielectric and a high performance polymer semiconductor, poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (pBTTT).  We investigate the effects of controlled interfacial roughness on the pBTTT thin film morphology and OFET performance.  The silicon oxide roughness was controlled by varying exposure to a reactive ion etch gas.  The roughness was then characterized with atomic force microscopy (AFM) for both root mean square (RMS) roughness and power spectral density (PSD) before and after functionalization with octyl trichlorosilane (OTS).  pBTTT forms large, highly oriented terraced domains with high hole charge carrier mobility when it is cast upon flat, low surface energy substrates and heated to a mesophase.  For surfaces with RMS roughness of greater than 0.5 nm, we find significant changes in the morphology of the pBTTT active layer and large reductions in charge carrier mobility.  The morphology of the pBTTT films on rough surfaces show significantly less order than that observed on smooth dielectric surfaces through characterization with AFM and x-ray diffraction (XRD) measurements.  These results show that even small RMS interfacial roughness can severely compromise the performance of OFETs.



Youngsuk Jung

Mentor: Eric K. Lin


Polymers Division

Materials science and Engineering Laboratory

National Institute of Standards and Technology

100 Bureau Drive, MS 8541

Gaithersburg , MD 20899-8541, USA

Building 224, Room B312

Tel: (301) 975-4769

Fax: (301) 975-3928



Category: Materials