Revealing the origin of magnetoresistance in unipolar amorphous organic field-effect transistors

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 0 ,7,7 0-tetrakis(diphenylamino)-9,9 0-spirobifluorene (Spiro-TAD). To scrutinize the origin of this effect two themes have been studied: (i) the influence of gate dielectric SiO 2 surface treatment and (ii) the importance of organic molecular p-dopant 1,3,4,5,7,8hexafluorotetracyanonaphthoquinodimethane (F6-TNAP) thin films sandwiched between SiO 2 and Spiro-TAD. A device fabricated on bare SiO 2 shows larger MR than one fabricated on hexamethyldisilazanetreated SiO 2 , 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 SiO 2 caused by dipolar disorder at the corresponding interface. Adding a thin layer of F6-TNAP is expected to increase the energetic disorder as well.

Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

“…This broader feature is attributed to Spiro-TAD + superimposed on F6-TCNNQ − absorption features (960 and 1140 nm). [13,21,22] The 480 nm feature can also be assigned to Spiro-TAD + , possibly superimposed on F6-TCNNQ and F6-TCNNQ − features. [13] Despite the overlapping absorption signatures of Spiro-TAD + and F6-TCNNQ − between 400 and 600 nm, these results still imply an intermolecular charge transfer between the Spiro-TAD HOMO and the F6-TCNNQ LUMO.…”

Magnetoresistance sign change in field-effect transistors via interface doping of Spiro-TAD with F6-TNAP

“…For a device with a channel length of 5 mm the hole mobility is 9 Â 10 À5 cm 2 /Vs, which is almost three times faster than that for a channel length of 20 mm [18]. The on/off ratio for a device with a channel length of 5 mm is 5 Â 10 4 , which is 50 times higher than that for a channel length of 20 mm [18]. Our results indicate a significant and observable sign change of MR between positive and negative values.…”

“…The magnetic field was varied between À100 mT and +100 mT and all experiments were performed in darkness and at room temperature. The data acquisition process and the calculation of MR values have been described in details elsewhere [16][17][18]. The hole mobility and the threshold voltage have been calculated at the saturation regime [26].…”

“…MR in this device has been observed in two different material systems and experimental conditions: (i) photoinduced MR in p-channel OFETs based on acene compounds [13][14][15] and (ii) MR in bipolar material systems such as in a binary mixture composed of electron donor and acceptor as well as in bipolar molecular glass [16,17]. Very recently, we reported MR effects in a unipolar p-channel OFET based on low molecular weight 2,2 0 ,7,7 0 -tetrakis-(diphenylamino)-9,9 0 -spirobifluorene (Spiro-TAD) and an interface doped system with organic molecular p-dopant 1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6-TNAP) [18]. In that paper we discussed the positive MR for devices with a channel length of 20 mm.…”

Sensitive Flexible Magnetic Sensors using Organic Transistors with Magnetic‐Functionalized Suspended Gate Electrodes

“…[ 2,3,6,8 ] The utilization of magnetosensitive organic materials as the active layer is considered an effective strategy for the fabrication of magnetic-fi eld-sensitive OFETs. [28][29][30][31][32][33] The construction of this kind of magnetic sensor with a typical OFET geometry, however, is limited by the development of organic semiconductors with both remarkable magnetosensitive property and prominent charge-transport …”