3.3 Solvents, Deposition Conditions, and Post-deposition Treatment . . . . . . . . . . . . . .. . . . . 116
3.4 Dielectric Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
3.5 Alignment and Grain Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
3.6 Blends and Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
3.7 Doping and On/Off Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
3.8 Electrochemical Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4 New Polythiophene Derivatives for FETs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
1 Introduction
Long before the use of poly(3-hexylthiophene) (P3HT) in bulk-heterojunction solar
cells was proposed, this conjugated polymer was one of the first and strongest
contenders as a high mobility, solution-processable semiconductor for organic
field-effect transistors (FETs). The first P3HT-FETs had already been reported in
the late 1980s by Assadi et al. [1] and since the 1990s a large number of groups have
worked on P3HT transistors, steadily improving their performance and deepening
the understanding of the underlying physics [2–8]. Many of the fundamental charge
transport properties investigated for P3HT-FETs have informed subsequent studies
of P3HT-based solar cells and the development of other high-mobility
polythiophene-based polymers. Here we will give a brief overview of P3HTFETs, including the general working principles of polymer FETs, the various
factors influencing device performance, and ways to reach the theoretical limits
of charge transport in P3HT. Finally, we will discuss the new polythiophene
derivatives that have been developed over the last decade and may replace P3HT
as the most popular polymer semiconductor.
2 Transistor Structures and Working Principle
The basic function of a transistor is that of a switch and an amplifier. The amount of
current flowing between two terminals is determined by a voltage applied to a third
terminal, thus turning it on or off. This basic function makes transistors the
fundamental building block of all logic circuits necessary for electronics. Thin
film FETs generally consist of a semiconducting layer, which is separated from a
gate electrode by a thin insulating gate dielectric. A source and a drain electrode of
width W (channel width) separated by a distance L (channel length) are in direct
contact with the semiconducting layer. The most commonly found geometries
(in relation to the substrate) are the bottom contact/top gate (BC/TG, staggered),
top contact/bottom gate (TC/BG), and bottom contact/bottom gate (BC/BG, coplanar) geometries (shown in Fig. 1a–c).
108
J. Zaumseil
3.4 Dielectric Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
3.5 Alignment and Grain Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
3.6 Blends and Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
3.7 Doping and On/Off Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
3.8 Electrochemical Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4 New Polythiophene Derivatives for FETs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
1 Introduction
Long before the use of poly(3-hexylthiophene) (P3HT) in bulk-heterojunction solar
cells was proposed, this conjugated polymer was one of the first and strongest
contenders as a high mobility, solution-processable semiconductor for organic
field-effect transistors (FETs). The first P3HT-FETs had already been reported in
the late 1980s by Assadi et al. [1] and since the 1990s a large number of groups have
worked on P3HT transistors, steadily improving their performance and deepening
the understanding of the underlying physics [2–8]. Many of the fundamental charge
transport properties investigated for P3HT-FETs have informed subsequent studies
of P3HT-based solar cells and the development of other high-mobility
polythiophene-based polymers. Here we will give a brief overview of P3HTFETs, including the general working principles of polymer FETs, the various
factors influencing device performance, and ways to reach the theoretical limits
of charge transport in P3HT. Finally, we will discuss the new polythiophene
derivatives that have been developed over the last decade and may replace P3HT
as the most popular polymer semiconductor.
2 Transistor Structures and Working Principle
The basic function of a transistor is that of a switch and an amplifier. The amount of
current flowing between two terminals is determined by a voltage applied to a third
terminal, thus turning it on or off. This basic function makes transistors the
fundamental building block of all logic circuits necessary for electronics. Thin
film FETs generally consist of a semiconducting layer, which is separated from a
gate electrode by a thin insulating gate dielectric. A source and a drain electrode of
width W (channel width) separated by a distance L (channel length) are in direct
contact with the semiconducting layer. The most commonly found geometries
(in relation to the substrate) are the bottom contact/top gate (BC/TG, staggered),
top contact/bottom gate (TC/BG), and bottom contact/bottom gate (BC/BG, coplanar) geometries (shown in Fig. 1a–c).
108
J. Zaumseil