3 Percolation Toward Lateral Junctions
57
routes formed by percolation, which depends on accidental processes, are significantly inferior to the intentionally designed nanostructures with respect to the value
of [compare Fig. 3.11a and b]. This is a far from ideal structure.
The results of this study show that eliminating any dead areas, which do
not generate photocurrents, as well as ensuring the formation of a continuous
route to transport the spatially separated electrons and holes is critically important to obtaining high quantum efficiency in photoelectric conversion using organic
semiconductors.
The areas of the present vertical junctions utilizing the cross section of multilayers
are very small. From the standpoint of the practical fabrication of large area cells,
the required aspect ratio reaches 100 because the required distance between adjacent
D/A interfaces is less than 10 nm owing to the short exciton diffusion length, and
the film thickness required to absorb all incident solar light is 1,000 nm. Therefore,
the designated fabrication of vertical alternating multilayered structures when using
current lithography techniques remains very difficult.
3.4 Lateral Junctions
If carrier transport over a long distance parallel to the substrate is possible, the
formation of a vertical route is unnecessary. Based on this idea, we proposed and
developed lateral junctions. First, we investigated the possibility of lateral electron
transport in organic single crystals (Sect. 3.4.1) and then applied it to the lateral
junctions (Sect. 3.4.2).
3.4.1 Lateral Electron Transport in Organic Single Crystals
In this section, the operation of single-crystal organic solar cells using lateral electron
transport is demonstrated [19]. The spacing between the collection electrodes, which
is determined by the lateral range of the electrons, was estimated to be 30 μm, and
the possibility of millimeter-order electron ranges was indicated.
3.4.1.1 Concept
The concept of the lateral type cell is shown in Fig. 3.12b (right). Electrons move
laterally through the single-crystal substrate. A pair of electrodes separated by a
distance L is deposited in parallel on the crystal surface. A layer of the organic semiconductor acting as a donor is inserted between the organic single-crystal substrate,
which acts as an acceptor and the electrode for collecting holes. At the D/A interface,
the excitons dissociate into electrons and holes under light irradiation. Because the
photogenerated holes move in the vertical direction, the distance to the electrode is
57
routes formed by percolation, which depends on accidental processes, are significantly inferior to the intentionally designed nanostructures with respect to the value
of [compare Fig. 3.11a and b]. This is a far from ideal structure.
The results of this study show that eliminating any dead areas, which do
not generate photocurrents, as well as ensuring the formation of a continuous
route to transport the spatially separated electrons and holes is critically important to obtaining high quantum efficiency in photoelectric conversion using organic
semiconductors.
The areas of the present vertical junctions utilizing the cross section of multilayers
are very small. From the standpoint of the practical fabrication of large area cells,
the required aspect ratio reaches 100 because the required distance between adjacent
D/A interfaces is less than 10 nm owing to the short exciton diffusion length, and
the film thickness required to absorb all incident solar light is 1,000 nm. Therefore,
the designated fabrication of vertical alternating multilayered structures when using
current lithography techniques remains very difficult.
3.4 Lateral Junctions
If carrier transport over a long distance parallel to the substrate is possible, the
formation of a vertical route is unnecessary. Based on this idea, we proposed and
developed lateral junctions. First, we investigated the possibility of lateral electron
transport in organic single crystals (Sect. 3.4.1) and then applied it to the lateral
junctions (Sect. 3.4.2).
3.4.1 Lateral Electron Transport in Organic Single Crystals
In this section, the operation of single-crystal organic solar cells using lateral electron
transport is demonstrated [19]. The spacing between the collection electrodes, which
is determined by the lateral range of the electrons, was estimated to be 30 μm, and
the possibility of millimeter-order electron ranges was indicated.
3.4.1.1 Concept
The concept of the lateral type cell is shown in Fig. 3.12b (right). Electrons move
laterally through the single-crystal substrate. A pair of electrodes separated by a
distance L is deposited in parallel on the crystal surface. A layer of the organic semiconductor acting as a donor is inserted between the organic single-crystal substrate,
which acts as an acceptor and the electrode for collecting holes. At the D/A interface,
the excitons dissociate into electrons and holes under light irradiation. Because the
photogenerated holes move in the vertical direction, the distance to the electrode is