Page 345 - Organic Electronics in Sensors and Biotechnology

322 Cha pte r Ei g h t

traps. Likewise, in the studies of photoresponse, an observation of
110
high responsivity R, which is defined as J /P, where J is the photo-
ph ph
current density (total drain current I upon illumination minus the
d
dark current per unit area) and P is the illumination power density,
101
can be affected by electron trapping at the interface near the gate
96
dielectric layer and the electrode work function. From the applica-
98
tion side, a distinct feature of the light-sensing properties of OFETs is
that R can be tuned by orders of magnitude by an applied V . The
g
largest measured R values are on the order of 1 to 100 A/W among all
organic devices prepared by Narayan and Kumar on single-layer poly
90
P3HT OFETs and by Noh et al. on single-layer 2,5-dibromothieno
101
111
[3,2-b]thiophene (BPTT). A photovoltaic effect and high R 112, 113 are
reported on ambipolar transistors based on a bulk heterojunction
concept. Such a highly photoresponsive OFET based on photoin-
duced charge transfer layer of C and ZnPc mixed layer as a photoac-
60
tive layer is shown in Fig. 8.17. In this experiment, using a mask to
illuminate only the active channel of the OFET with minimized arti-
1
facts, a large R of 10 to 10 A/W depending on the intensity of illu-
−3
mination and on applied V could be obtained. 114
g
Under illumination I in a transistor can be simply written as the sum
d
of dark current I and wavelength-dependent photocurrent I (λ):
dark ph
λ
I () = I + I () (8.1)
λ
d dark ph
I

I dark
ph
μ CW ⎡ 2 ⎤ ⎡N()λμ ⎤
e Wt
I () = e ins ( ⎢ V − V V − V D ⎥ + ⎢ e ⎥ V V ≤ V (8.2)
≤
λ
)
d L G T D 2 ⎣ L ⎦ D D G
⎣ ⎦

I
I
ph
dark
( ⎡
μ CW V − V ) ⎤ ⎡N()λμ ⎤
2
e Wt
λ
I () = e ins ⎢ G T ⎥ + ⎢ e ⎥ V V ≥ V (8.3)
d L 2 ⎣ L ⎦ D D G
⎣ ⎦
where C = capacitance per unit area
ins
W, L, and t = channel width, length, and the active layer
thickness, respectively
N = charge carrier density in the bulk
Charge carrier density N depends on the number of photogen-
erated charge carriers (PCCs), denoted by n .
ph
n =η Gτ (8.4)
ph