Abstract
We report on solution processed, highly light sensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-
co
-bithiophene) (F8T2). Transistors without heat treatment showed the highest saturation mobility, while devices annealed at
280
°
C
showed the highest drain current. The latter annealed transistors were found to give highly stable and reproducible performance over many light cycles. Measurements were carried out using an inorganic light emitting diode (LED) light source with a peak wavelength of
465
nm
and
19
nm
bandwidth from
0
to
400
μ
W
∕
cm
2
light intensity on TFTs with an F8T2 film thickness of
30
nm
. The TFT OFF current was found to increase both with light intensity and gate bias. The bulk photogenerated carrier density was calculated to change from
5
×
10
11
to
1
×
10
13
cm
−
3
over the measured light intensity range. The TFT saturation mobility did not change with light intensity, remaining constant at
1.2
×
10
−
4
cm
2
∕
V
s
. The TFT ON current instead increased due to a shift in the turn-on voltage
V
T
. This changed from
−
27
to
−
20
V
over the measured light intensity range, initially changing rapidly but then saturating at higher intensity values. Contact resistance
R
C
measurements showed large values in the dark.
R
C
rapidly decreases with increasing light intensity, again saturating at higher values. From these results, we propose a phototransistor model in which illumination varies the device performance by effecting injection. By considering this shift in
R
C
as photoassisted barrier lowering which additionally varies the width of the region depleted of carriers between the injecting interface and the channel, it is possible to explain the observed shift in
V
T
as a change in the fraction of the gate bias dropped across the contact capacitance
C
C
. By operating the phototransistor at a value of
V
g
=
−
5
V
(below
V
T
), it was possible to achieve a highly linear response of the photocurrent with light intensity. Alternatively, by operating at a value of
V
g
=
−
40
V
(above
V
T
), it was possible to maximize the photoresponsivity within the measured range. A photoresponsivity of
18.5
A
∕
W
at
5
μ
W
∕
cm
2
light intensity was achieved.