3. Results and discussion
Fig. 3 shows the output characteristics of the doped and the
undoped OFETs. The drain–source voltage VDS is swept from 0 to –
60 V, while the gate–source voltage VGS is changed stepwise from
0 to –60 V in 15 V steps. It is evident from Fig. 3 that the devices
exhibit the I–V characteristics of unipolar OFETs with saturation
behavior and therefore the OFETs operate in the hole accumulation
mode. There are two main differences between the doped
and the undoped devices: the light emission is only detected only
from the doped devices and the drain–source current IDS of the
doped devices (Fig. 3a) is almost four orders of magnitude higher
those of than the undoped devices (Fig. 3b) under the same
experimental conditions.The optical output characteristics at different gate voltages of
the doped devices are shown in Fig. 4. It can be seen that the
optical output does not directly follow the current. At a fixed VGS,light emission intensity increases with the increase of VDS. Fig. 5
shows the light intensity as a function of the drain-source current
IDS which is recorded by sweeping the drain–source voltage at
various constant gate–source voltages.
But interestingly, the light emission is not observed in the first
sweeps in the doped devices. After several repetitions, the light
emission is observed in the saturation regime. On further sweeping
cycles, the light emission was observed even from linear regions.
Moreover in each next sweeping turn, the doped devices are
operating at lower and lower voltages indicating that the injection
barrier is going down via consecutive doping near the electrodes.
All these show that during operation of the electrochemical OLEFET
devices there is a continued ‘‘in situ’’ doping and increase in the
conductivity occurring. When a negative gate voltage is applied,
positive charges are induced at the interface between the active
layer and the dielectric layer. The holes are injected from the source
electrode and transferred through the channel to the drain electrode.
At sufficient drain voltages, electrons are also injected into
the active layer from the drain electrode, which results in carrier
recombination and light emission. Because of the presence of the
electrolyte in the active layer of the doped device, MDMO–PPV gets
electrochemically doped at the opposite electrodes and the light
emission can be observed at lower voltages even using symmetric
Au source and drain electrodes.
Fig. 6a shows the transfer characteristics of a doped OFET
along with simultaneously measured light emission intensity
data. Fig. 6b shows the transfer characteristics of an undoped
OFET as comparison (no light emission is detected while sweeping
the gate–source voltage). IDS is measured keeping the drain–
source voltage constant at 40 V, while sweeping the gate–
source voltage from 0 to –60 V in 1 V steps. Note the close
correspondence of both the channel current and the emitted light
intensity vs gate voltage. Fig. 6a indicates that the gate bias
controls not only the current flow but also the light intensity. The
light intensity increases with the gate voltage. On the other hand,
the doped device exhibits high field effect hole mobility of about
3 cm2 V1 s1 calculated in the linear regime using the standard
transistor equation [69] whereas hole mobility of the undoped
device (standard OFET) is calculated to be 103 cm2 V1 s1
which is three orders of magnitude lower than that of the doped
device.
Fig. 7 shows the light intensity as a function of IDS which is
recorded by sweeping the gate voltage at a constant drain voltage
as in Fig. 6a. The light output is directly proportional to the drain
current, too.
Fig.peak appeared at a longer wavelength of ca. 750 nm, possibly due
to polaronic effects of doping (polaron-exciton).
The doped OLEFET emits orange–yellow light which could be
seen with the naked eye. Light emission is observed adjacent to
the negative biased electrode, which we assign as the drain (D)
electrode. This region of emission indicates the charge carrier
recombination zone close to the cathode. It is conceivable since
the electron mobility is much smaller than the hole mobility in
such devices, bringing the statistical recombination zone near to
the electron injecting electrode (drain). A series of pictures were
taken with constant source–drain bias (VDS) and increasing gate
bias (Fig. 9). One realizes that the emission is inhomogeneous and
occurs close to the drain electrode.