3 Percolation Toward Lateral Junctions
59
3.4.1.2 NTCDA Single Crystal
We adopted single crystal of naphthalenetetracarboxylic dianhydride (NTCDA,
Fig. 3.12a) for the electron-transporting material for two reasons. (i) NTCDA is a
well-known electron-transporting organic semiconductor, and its use as an extremely
thick (2 μm) electron transport layer in organic solar cells has been reported [17].
(ii) A NTCDA single crystal is transparent in the visible region, allowing direct irradiation of the D/A interface. The charge separation energy relationship is shown in
Fig. 3.11c, and electrons are photogenerated only beneath the hole collection electrode. Hence, the starting point for electrons is aligned with the right-hand edge of
the hole collection electrode.
Plate-shaped transparent NTCDA single crystals with the sizes of around 2 mm ×
5 mm and thicknesses of around 50 μm were obtained (Fig. 3.11a) by physical vapor
transport [20] in N 2 (1 atm). The NTCDA molecules were stacked in a herringbone
structure in a single crystal with a monoclinic lattice [21].
3.4.1.3 Cell Fabrication
Figure 3.12b (right) shows the configuration of the NTCDA single-crystal cell
employing lateral electron transport and collection. Tetraphenyl dibenzoperiflanthene (DBP) acting as donor, MoO 3 and Ag were deposited at 10 -6 Pa in an oil-free
vacuum evaporator (EpiTech, VTS-350 M/ERH) built in a glove box purged with
N 2 gas. Metal masks with pairs of square apertures separated by distances of 20,
30, 50, and 100 μm were used to form the electrode pairs. A 30 nm-thick layer of
DBP and a MoO 3 (10 nm)/Ag (100 nm) hole collection electrode were deposited on
the NTCDA single crystal through one of the apertures. Then, an electron collection
electrode (Ag (100 nm)) was deposited through the other aperture. For reference, a
vertical cell (Fig. 3.12b (left)) was fabricated on an ITO substrate, which could be
regarded as a standard cell with L = 0.
3.4.1.4 Operation of Lateral Cell
Figure 3.13a shows the J-V characteristics of a lateral type solar cell with L = 30 μm
at solar light intensities of 1, 8, and 10 suns. The photocurrent is shown in the first
quadrant. Open-circuit voltage (V OC ) values of 0.42, 0.62, and 0.61 V were observed
for intensities of 1 (blue curve A), 8 (orange curve B), and 10 suns (green curve C).
For the vertical-type cell (Fig. 3.12b (left)), V OC values of 0.61, 0.66, and 0.67 V
were observed under the same light intensities. The magnitudes of V OC for the lateral
cell (L = 30 μm) were slightly smaller compared with the vertical cell (L = 0 μm).
This is a clear demonstration of the operation of the lateral cell with an extremely
long electron transport distance of 30 μm.
59
3.4.1.2 NTCDA Single Crystal
We adopted single crystal of naphthalenetetracarboxylic dianhydride (NTCDA,
Fig. 3.12a) for the electron-transporting material for two reasons. (i) NTCDA is a
well-known electron-transporting organic semiconductor, and its use as an extremely
thick (2 μm) electron transport layer in organic solar cells has been reported [17].
(ii) A NTCDA single crystal is transparent in the visible region, allowing direct irradiation of the D/A interface. The charge separation energy relationship is shown in
Fig. 3.11c, and electrons are photogenerated only beneath the hole collection electrode. Hence, the starting point for electrons is aligned with the right-hand edge of
the hole collection electrode.
Plate-shaped transparent NTCDA single crystals with the sizes of around 2 mm ×
5 mm and thicknesses of around 50 μm were obtained (Fig. 3.11a) by physical vapor
transport [20] in N 2 (1 atm). The NTCDA molecules were stacked in a herringbone
structure in a single crystal with a monoclinic lattice [21].
3.4.1.3 Cell Fabrication
Figure 3.12b (right) shows the configuration of the NTCDA single-crystal cell
employing lateral electron transport and collection. Tetraphenyl dibenzoperiflanthene (DBP) acting as donor, MoO 3 and Ag were deposited at 10 -6 Pa in an oil-free
vacuum evaporator (EpiTech, VTS-350 M/ERH) built in a glove box purged with
N 2 gas. Metal masks with pairs of square apertures separated by distances of 20,
30, 50, and 100 μm were used to form the electrode pairs. A 30 nm-thick layer of
DBP and a MoO 3 (10 nm)/Ag (100 nm) hole collection electrode were deposited on
the NTCDA single crystal through one of the apertures. Then, an electron collection
electrode (Ag (100 nm)) was deposited through the other aperture. For reference, a
vertical cell (Fig. 3.12b (left)) was fabricated on an ITO substrate, which could be
regarded as a standard cell with L = 0.
3.4.1.4 Operation of Lateral Cell
Figure 3.13a shows the J-V characteristics of a lateral type solar cell with L = 30 μm
at solar light intensities of 1, 8, and 10 suns. The photocurrent is shown in the first
quadrant. Open-circuit voltage (V OC ) values of 0.42, 0.62, and 0.61 V were observed
for intensities of 1 (blue curve A), 8 (orange curve B), and 10 suns (green curve C).
For the vertical-type cell (Fig. 3.12b (left)), V OC values of 0.61, 0.66, and 0.67 V
were observed under the same light intensities. The magnitudes of V OC for the lateral
cell (L = 30 μm) were slightly smaller compared with the vertical cell (L = 0 μm).
This is a clear demonstration of the operation of the lateral cell with an extremely
long electron transport distance of 30 μm.