Abstract
We report on the magnetoresistance (MR) effect in a unipolar p-channel field-effect transistor based on amorphous thin film of low molecular weight 2,2′,7,7′-tetrakis(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD). To scrutinize the origin of this effect two themes have been studied: (i) the influence of gate dielectric SiO2 surface treatment and (ii) the importance of organic molecular p-dopant 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6-TNAP) thin films sandwiched between SiO2 and Spiro-TAD. A device fabricated on bare SiO2 shows larger MR than one fabricated on hexamethyldisilazane-treated SiO2, suggesting the bipolaron species as the origin of this effect. This could be understood through two aspects. (i) The stabilization of bipolaron, i.e. trapped charge-stabilized bipolaron. Due to large energy cost for bipolaron formation, the stabilization of bipolaron by a trap with opposite sign is favored. Additionally, interface doping of Spiro-TAD with F6-TNAP reveals a significant enhancement in MR caused by an increase in trap density and charge carriers, boosting the effectiveness of bipolaron stabilization. (ii) Larger energetic disorder that might compensate the energy cost for bipolaron formation is expected to be obtained for a device fabricated on bare SiO2 caused by dipolar disorder at the corresponding interface. Adding a thin layer of F6-TNAP is expected to increase the energetic disorder as well.
Highlights
Organic semiconductors have attracted much interest in the scienti c community since the rst report on high conductivity in the conjugated polymer cis-polyacetylene doped with AsF5.1 This research eld has been extensively studied over more than three decades, leading to a new research area called “organic electronics”
We report on the magnetoresistance (MR) effect in a unipolar p-channel field-effect transistor based on amorphous thin film of low molecular weight 2,20,7,70-tetrakis(diphenylamino)-9,90-spirobifluorene (Spiro-TAD)
Interface doping of Spiro-TAD with F6-TNAP reveals a significant enhancement in MR caused by an increase in trap density and charge carriers, boosting the effectiveness of bipolaron stabilization. (ii) Larger energetic disorder that might compensate the energy cost for bipolaron formation is expected to be obtained for a device fabricated on bare SiO2 caused by dipolar disorder at the corresponding interface
Summary
Organic semiconductors have attracted much interest in the scienti c community since the rst report on high conductivity in the conjugated polymer cis-polyacetylene doped with AsF5.1 This research eld has been extensively studied over more than three decades, leading to a new research area called “organic electronics”. The applications of organic p-conjugated molecules or polymers can already be found in light-emitting devices,[2] solar cells,[3] and eld-effect transistors.[4]. Research in spintronics has been carried out for many material systems such as metals, ferromagnets and semiconductors.[5] one may think to utilize thin lms of organic p-conjugated molecules and polymers as spacers or tunnel barriers in spintronic devices due to the abundance of physical aspects and potential practical applications.
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