3 Percolation Toward Lateral Junctions
65
3.4.3 Carrier Pathway Unit Cells
Hole and electron pathway unit cells are shown in Figs. 3.16a and d, respectively.
For the hole pathway unit cell (Fig. 3.16a), C8-BTBT, which shows a hole mobility
(μ h ) of up to 43 cm
2 ·V
−1 ·s
−1 [24, 28], was used as the hole transporter. A pair of
electrodes separated by distance L was deposited on the film surface, and cells with
an L of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mm were fabricated. Fullerene (C 60 ), the
acceptor (A), was inserted between the C8-BTBT, the donor (D), and the electroncollecting electrode. At the D/A interface, excitons dissociate into electrons and
holes under light irradiation. Because the photogenerated electrons move vertically,
the distance to the electron-collecting electrode is equal to the C 60 film thickness
(50 nm). Conversely, because the photogenerated holes move in the lateral direction,
the distance to the hole-collecting electrode is equal to the inter-electrode distance (L),
which is longer than 0.05 mm. Because the lateral distance is 1,000 times longer than
the vertical distance, the characteristics of the hole pathway unit cells are dominated
by the lateral hole transport. The left-hand edge of the electron-collecting electrode
can be regarded as the starting line for the holes (solid red line). The range of the
holes (L h ) is expressed by Eq. (3.2) [29–33],
L h = μ h τ h E,
(3.2)
where μ h , τ h , and E are the hole drift mobility, the lifetime of holes, and the electric
field, respectively. When L is shorter than the range L h (L h > L) (Fig. 3.16a, electrode A), the holes are collected by the hole-collecting electrode. However, when
L is longer than the range L h (L h < L) (Fig. 3.16a, electrode C), the holes are not
collected. Thus, the magnitude of the photocurrent decreases at approximately L =
L h (Fig. 3.16a, electrode B), and L h can be determined by observing the sudden drop
in the photocurrent. By using the electron pathway unit cells (Fig. 3.16d), the electron range (L e ) can be determined in a similar manner. PTCDI-C8 (Fig. 3.16d) which
has a high electron mobility (μ e = 1.7 cm
2 ·V
−1 ·s
−1 [22]) was used as the electron
transporter. Phthalocyanine (H 2 Pc), D, was inserted between the PTCDI-C8, A, and
the hole-collecting electrode.
3.4.4 Carrier Ranges
Figure 3.16c shows the current–voltage (J-V) characteristics of the hole-transporting
cells. At L = 0.05 mm, a clear photovoltaic curve was observed (red curve). Photovoltaic effects were observed for L values of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mm.
A sudden decrease in the photocurrent was observed between L = 0.4 and 0.5 mm,
and the photocurrent disappeared at L = 0.6 mm.
The dependence of the photocurrent on the electrode distance (L) for constant
electric field strengths (E) between the lateral electrodes of 0 (black), −10 (red), −
65
3.4.3 Carrier Pathway Unit Cells
Hole and electron pathway unit cells are shown in Figs. 3.16a and d, respectively.
For the hole pathway unit cell (Fig. 3.16a), C8-BTBT, which shows a hole mobility
(μ h ) of up to 43 cm
2 ·V
−1 ·s
−1 [24, 28], was used as the hole transporter. A pair of
electrodes separated by distance L was deposited on the film surface, and cells with
an L of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mm were fabricated. Fullerene (C 60 ), the
acceptor (A), was inserted between the C8-BTBT, the donor (D), and the electroncollecting electrode. At the D/A interface, excitons dissociate into electrons and
holes under light irradiation. Because the photogenerated electrons move vertically,
the distance to the electron-collecting electrode is equal to the C 60 film thickness
(50 nm). Conversely, because the photogenerated holes move in the lateral direction,
the distance to the hole-collecting electrode is equal to the inter-electrode distance (L),
which is longer than 0.05 mm. Because the lateral distance is 1,000 times longer than
the vertical distance, the characteristics of the hole pathway unit cells are dominated
by the lateral hole transport. The left-hand edge of the electron-collecting electrode
can be regarded as the starting line for the holes (solid red line). The range of the
holes (L h ) is expressed by Eq. (3.2) [29–33],
L h = μ h τ h E,
(3.2)
where μ h , τ h , and E are the hole drift mobility, the lifetime of holes, and the electric
field, respectively. When L is shorter than the range L h (L h > L) (Fig. 3.16a, electrode A), the holes are collected by the hole-collecting electrode. However, when
L is longer than the range L h (L h < L) (Fig. 3.16a, electrode C), the holes are not
collected. Thus, the magnitude of the photocurrent decreases at approximately L =
L h (Fig. 3.16a, electrode B), and L h can be determined by observing the sudden drop
in the photocurrent. By using the electron pathway unit cells (Fig. 3.16d), the electron range (L e ) can be determined in a similar manner. PTCDI-C8 (Fig. 3.16d) which
has a high electron mobility (μ e = 1.7 cm
2 ·V
−1 ·s
−1 [22]) was used as the electron
transporter. Phthalocyanine (H 2 Pc), D, was inserted between the PTCDI-C8, A, and
the hole-collecting electrode.
3.4.4 Carrier Ranges
Figure 3.16c shows the current–voltage (J-V) characteristics of the hole-transporting
cells. At L = 0.05 mm, a clear photovoltaic curve was observed (red curve). Photovoltaic effects were observed for L values of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mm.
A sudden decrease in the photocurrent was observed between L = 0.4 and 0.5 mm,
and the photocurrent disappeared at L = 0.6 mm.
The dependence of the photocurrent on the electrode distance (L) for constant
electric field strengths (E) between the lateral electrodes of 0 (black), −10 (red), −