easily functionalized with self-assembled monolayers of alkyl silanes to minimize
the trap state density and change the surface energy, which influences the morphology of the polymer layer. Top contacts may be evaporated onto the semiconductor
through a shadow mask. Bottom contacts are usually photolithographically patterned before the semiconductor is deposited. Even when the same electrode
material (e.g., gold) is used, the injection properties and thus the contact resistance
of top and bottom contact devices are different, which can affect the calculated
mobilities. Also, although silicon substrates are convenient for basic research one
should keep in mind that flexible polymer substrates and complex circuits require
different processing techniques and materials, which can substantially affect device
performance.
A new type of thin-film transistor, the electrolyte-gated transistor (EGT), has
recently been introduced that allows low-voltage operation and high carrier densities [13, 14]. In these electrochemical transistors, the dielectric is replaced by an
electrolyte with highly mobile anions and cations. The electrolyte is ionically
conducting but electrically insulating. Typical examples of such electrolytes are
blends of polyethylene oxide (PEO) with LiClO 4 [15], or imidazolium-based ionic
liquids and iongels containing these ionic liquids [16, 17]. When a gate voltage is
applied to such an electrolyte, the ions move according to their charge polarity. For
example, in the case of a negative gate voltage, the cations move towards the gate
electrode and the anions start to penetrate the semiconducting polymer (e.g.,
P3HT). In order to compensate for the extra negative charge in the polymer layer,
holes are injected by the source electrode that can move through the channel to the
drain (see Fig. 1d). Similar to the electrostatic accumulation regime in FETs, the
source-drain current is modulated by the gate voltage, which determines the number
of anions in the film. The advantage of electrolyte gating is the large amount of
charge that can be accumulated in the channel at very low voltages. EGTs operate
within a voltage range of less than 2 V and at high carrier densities (up to
10
14 cm
À2 ) [18]. However, because the ions need to move in and out of the polymer
layer, the switching speed of EGTs is slower than for traditional FETs. Also, the
exact determination of carrier mobility in these devices is even less straightforward
than for FETs with a dielectric of constant capacitance. The number of injected
charges for these FETs must be determined by independent measurements of the
displacement current at different gate voltage sweep rates [19] in order to extract a
useful mobility number.
3 Improving and Understanding Device Performance
of P3HT-FETs
The device performance of P3HT transistors depends sensitively on many factors
that can be partially controlled through synthesis and processing. These factors
influence the field-effect mobility, the threshold voltage, on/off current ratio, and
112
J. Zaumseil
the trap state density and change the surface energy, which influences the morphology of the polymer layer. Top contacts may be evaporated onto the semiconductor
through a shadow mask. Bottom contacts are usually photolithographically patterned before the semiconductor is deposited. Even when the same electrode
material (e.g., gold) is used, the injection properties and thus the contact resistance
of top and bottom contact devices are different, which can affect the calculated
mobilities. Also, although silicon substrates are convenient for basic research one
should keep in mind that flexible polymer substrates and complex circuits require
different processing techniques and materials, which can substantially affect device
performance.
A new type of thin-film transistor, the electrolyte-gated transistor (EGT), has
recently been introduced that allows low-voltage operation and high carrier densities [13, 14]. In these electrochemical transistors, the dielectric is replaced by an
electrolyte with highly mobile anions and cations. The electrolyte is ionically
conducting but electrically insulating. Typical examples of such electrolytes are
blends of polyethylene oxide (PEO) with LiClO 4 [15], or imidazolium-based ionic
liquids and iongels containing these ionic liquids [16, 17]. When a gate voltage is
applied to such an electrolyte, the ions move according to their charge polarity. For
example, in the case of a negative gate voltage, the cations move towards the gate
electrode and the anions start to penetrate the semiconducting polymer (e.g.,
P3HT). In order to compensate for the extra negative charge in the polymer layer,
holes are injected by the source electrode that can move through the channel to the
drain (see Fig. 1d). Similar to the electrostatic accumulation regime in FETs, the
source-drain current is modulated by the gate voltage, which determines the number
of anions in the film. The advantage of electrolyte gating is the large amount of
charge that can be accumulated in the channel at very low voltages. EGTs operate
within a voltage range of less than 2 V and at high carrier densities (up to
10
14 cm
À2 ) [18]. However, because the ions need to move in and out of the polymer
layer, the switching speed of EGTs is slower than for traditional FETs. Also, the
exact determination of carrier mobility in these devices is even less straightforward
than for FETs with a dielectric of constant capacitance. The number of injected
charges for these FETs must be determined by independent measurements of the
displacement current at different gate voltage sweep rates [19] in order to extract a
useful mobility number.
3 Improving and Understanding Device Performance
of P3HT-FETs
The device performance of P3HT transistors depends sensitively on many factors
that can be partially controlled through synthesis and processing. These factors
influence the field-effect mobility, the threshold voltage, on/off current ratio, and
112
J. Zaumseil