Ambipolar Charge Transport in Air‐Stable Polymer Blend Thin‐Film Transistors

Abstract

Ambipolar thin‐film transistors based on a series of air‐stable, solution‐processed blends of an n‐type polymer poly(benzobisimidazobenzophenanthroline) (BBL) and a p‐type small molecule, copper phthalocyanine (CuPc) are demonstrated, where all fabrication and measurements are performed under ambient conditions. The hole mobilities are in the range of 6.0 × 10–6 to 2.0 × 10–4 cm2 V–1 s–1 and electron mobilities are in the range of 2.0 × 10–6 to 3.0 × 10–5 cm2 V–1 s–1, depending on the blend composition. UV‐vis spectroscopy and electron diffraction show crystallization of CuPc in the metastable α‐crystal form within the semicrystalline BBL matrix. These CuPc domains develop into elongated ribbon‐like crystalline nanostructures when the blend films are processed in methanol, but not when they are processed in water. On methylene chloride vapor annealing of the blend films, a phase transformation of CuPc from the α‐form to the β‐form is observed, as shown by optical absorption spectroscopy and electron diffraction. Ambipolar charge transport is only observed in the blend films where CuPc crystallized in the elongated ribbon‐like nanostructures (α‐form). Ambipolar behavior is not observed with CuPc in the β‐polymorph. Unipolar hole mobilities as high as 2.0 × 10–3 cm2 V–1 s–1 are observed in these solution‐processed blend field‐effect transistors (FETs) on prolonged treatment in methanol, comparable to previously reported hole mobilities in thermally evaporated CuPc FETs. These results show that ambipolar charge transport and carrier mobilities in multicomponent organic semiconductors are intricately related to the phase‐separated nanoscale and crystalline morphology.

Morphology and Field-Effect Mobility of Charge Carriers in Binary Blends of Poly(3-hexylthiophene) with Poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylenevinylene] and Polystyrene

“…To improve the off state characteristics, thermal annealing at 100 °C for 5 min was done, and this resulted in increase of the I on / I off ratio by at least 2 orders of magnitude. These results are in close agreement with the reported mobility of PHT devices fabricated under similar conditions. 4a,13b The field-effect mobility of holes in MEH−PPV was measured to be 2 × 10 -6 cm 2 /(V s), which is in close agreement with an earlier reported hole mobility ((0.5−2.5) × 10 -6 cm 2 /(V s)) determined by the time-of-flight method 15c. However, a recent experimental and numerical analysis of MEH−PPV FETs gave a field-effect mobility of holes to be (5−8) × 10 -5 cm 2 /(V s) 15d…”

“…Figure shows the AFM topographic images of PHT/MEH−PPV blend thin films on Si/SiO 2 substrates as a function of composition. The AFM images of PHT homopolymer showed a homogeneous surface morphology with some nanorod-like crystalline domain characteristic of the ordered regioregular PHT thin films 13b. The AFM images of MEH−PPV homopolymer showed a smooth and featureless surface morphology.…”

Air-Stable Ambipolar Field-Effect Transistors and Complementary Logic Circuits from Solution-Processed n/p Polymer Heterojunctions

“…In P3HT-on-BBL devices, the mobilities are rather low and asymmetric compared to the other systems (Figure 3c). The electron mobility of 0.002 cm 2 /(V s) is similar to the previously reported values of BBL on SiO 2 (9,10). It is interesting that µ e in n-on-p devices is generally higher than in p-on-n, despite potential electron-injection barriers of n-on-p devices.…”

“…Both electron mobility (µ e ) and hole mobility (µ h ) of n-on-p devices are in the range of ∼1 × 10 -3 to 1 × 10 -2 cm 2 /(V s). The similarity of mobilities to those of singlelayer OFETs comes from the fact that charge transport occurs in thin conduction sheet defined by the channel (9,10). It is interesting that µ e in n-on-p devices is generally higher than in p-on-n, despite potential electron-injection barriers of n-on-p devices.…”

Ambipolar Transport in Phase-Separated Thin Films of p- and n-Type Vanadylporphyrazines with Two-Dimensional Percolation

“…Figure indicates that the transistor has well-balanced high ambipolar characteristics, in which the threshold voltages for the electron and hole injections are both lower than 1 V. The p- and n-type mobilities are 2.3 × 10 –3 and 1.0 × 10 –4 cm 2 V –1 s –1 , respectively, which are larger than those of the corresponding SiO 2 gate TFTs by 3 orders of magnitude. The p- and n-type mobilities are relatively high in the reported TFTs for the ambipolar transistors consisting of p- and n-type semiconductors. − …”