1 Basic Principles of Modern Organic Solar Cells
11
an i-interlayer sandwiched between n- and p-layers, similar to the case of amorphous
silicon incorporating a p-i-n junction.
1.2.1.5 Route Formation
Irrespective of the occurrence of exciton dissociation, the nanostructure control of the
co-deposited films, i.e., route formation for electrons and holes generated by exciton
dissociation, is crucial to extract a significant portion of the photogenerated charge
to the external circuit. After the two types of organic semiconductors acting as the
donor and acceptor layers are blended, the extraction of photogenerated holes and
electrons becomes an inevitable and difficult problem because they can only move
through the A and D molecules.
(i) Molecular mixture: When the two types of organic molecules are blended
using the co-deposition technique at room temperature, the co-deposited films
usually have an amorphous molecularly blended structure (Fig. 1.9a), and only
a minimal number of carriers can be extracted because there are few routes for
electron and hole transport.
(ii) Percolation: The routes for the electrons and holes are formed by percolation,
which is promoted by annealing, while maintaining D/A molecular contacts in
the bulk of the organic film (Fig. 1.10a) (see Chap. 3, Sect. 3.2) [22–24].
(iii) Vertical superlattice: An ideal nanostructure is the “vertical superlattice” structure (Fig. 1.10b) (see Chap. 3, Sect. 3.3). This structure, which can be made
artificially by cutting out the cross section from the multilayer [20], enables the
efficient dissociation of photogenerated excitons at the D/A interfaces within
the exciton diffusion length (5–10 nm) and the transport of electrons and holes
to the respective electrodes. However, it is a difficult task to construct such a
structure by artificial design over a large area.
(iv) Lateral superlattice: The photogenerated holes and electrons are laterally
transported and extracted to the respective electrodes. The lateral superlattice
structure (Fig. 1.10c) corresponds to the 90° rotated vertical superlattice structure (Fig. 1.10b). This structure eventually became possible by the emergence
of high-mobility organic semiconductors. The lateral alternating multilayered
junctions (Fig. 1.10c) can collect both excitons and carriers with an efficiency
of approximately 100%, as well as fabricate cells that have sufficient areas.
Therefore, the lateral junction is used as an alternative blended junction (see
Chap. 3, Sect. 3.4.2) [25].
Here, two examples of the successful fabrication of ideal vertical superlattices
(Fig. 1.10b) are introduced. When one succeeds in fabricating the pseudo-vertical
superlattice, the photocurrent increases with the thickness of vertical superlattice,
while the fill factor (FF) maintains a constant value. On the contrary, since the
photocurrent and FF of the co-deposited film that has a molecular mixture steeply
decrease for film thicknesses above several tens nm, thicker co-deposited films cannot
be used for the solar cell.
11
an i-interlayer sandwiched between n- and p-layers, similar to the case of amorphous
silicon incorporating a p-i-n junction.
1.2.1.5 Route Formation
Irrespective of the occurrence of exciton dissociation, the nanostructure control of the
co-deposited films, i.e., route formation for electrons and holes generated by exciton
dissociation, is crucial to extract a significant portion of the photogenerated charge
to the external circuit. After the two types of organic semiconductors acting as the
donor and acceptor layers are blended, the extraction of photogenerated holes and
electrons becomes an inevitable and difficult problem because they can only move
through the A and D molecules.
(i) Molecular mixture: When the two types of organic molecules are blended
using the co-deposition technique at room temperature, the co-deposited films
usually have an amorphous molecularly blended structure (Fig. 1.9a), and only
a minimal number of carriers can be extracted because there are few routes for
electron and hole transport.
(ii) Percolation: The routes for the electrons and holes are formed by percolation,
which is promoted by annealing, while maintaining D/A molecular contacts in
the bulk of the organic film (Fig. 1.10a) (see Chap. 3, Sect. 3.2) [22–24].
(iii) Vertical superlattice: An ideal nanostructure is the “vertical superlattice” structure (Fig. 1.10b) (see Chap. 3, Sect. 3.3). This structure, which can be made
artificially by cutting out the cross section from the multilayer [20], enables the
efficient dissociation of photogenerated excitons at the D/A interfaces within
the exciton diffusion length (5–10 nm) and the transport of electrons and holes
to the respective electrodes. However, it is a difficult task to construct such a
structure by artificial design over a large area.
(iv) Lateral superlattice: The photogenerated holes and electrons are laterally
transported and extracted to the respective electrodes. The lateral superlattice
structure (Fig. 1.10c) corresponds to the 90° rotated vertical superlattice structure (Fig. 1.10b). This structure eventually became possible by the emergence
of high-mobility organic semiconductors. The lateral alternating multilayered
junctions (Fig. 1.10c) can collect both excitons and carriers with an efficiency
of approximately 100%, as well as fabricate cells that have sufficient areas.
Therefore, the lateral junction is used as an alternative blended junction (see
Chap. 3, Sect. 3.4.2) [25].
Here, two examples of the successful fabrication of ideal vertical superlattices
(Fig. 1.10b) are introduced. When one succeeds in fabricating the pseudo-vertical
superlattice, the photocurrent increases with the thickness of vertical superlattice,
while the fill factor (FF) maintains a constant value. On the contrary, since the
photocurrent and FF of the co-deposited film that has a molecular mixture steeply
decrease for film thicknesses above several tens nm, thicker co-deposited films cannot
be used for the solar cell.