9 Parts-Per-Million-Level Doping Effects …
227
9.6 Junction Formation
9.6.1 Ohmic Junctions
9.6.1.1 Organic/Metal Ohmic Junction
It is very important to make the two organic/metal contacts in a photovoltaic cell
ohmic. When the region in the vicinity of a metal electrode is heavily doped, even if
there is a Schottky barrier, its width becomes extremely thin allowing charge-carrier
tunneling, and as a result, an ohmic contact is expected to be formed similar to
that on heavily p
+ - or n
+ -doped inorganic semiconductors [52–54]. Here, + means
heavily doped. Moreover, an ohmic contact can be formed irrespective of which electrode material is used, since tunneling is less dependent on the metal work function,
enabling the cell structure to be inverted. This technique would allow flexibility in
the design of the cell structure.
As a test case, two-layer cells consisting of C 60 and H 2 Pc (Fig. 9.5a, b) were
examined [55]. The electrode materials were ITO and Ag. Heavy doping of the order
of 10,000 ppm (1%) and 50,000 ppm (5%) was applied to thin 10 nm regions close
to the C 60 and H 2 Pc/metal interfaces. Figure 9.5c, d shows the current–voltage (J-V)
characteristics for the cells with heavily doped regions in Fig. 9.5a, b (red curves). For
the cell in Fig. 9.5a, the fill factor (FF) reaches a value of 0.59, and clear rectification
characteristics can be seen with heavily doped regions (red broken curve). Without
the heavily doped regions (blue curves), however, FF is only 0.29, and the forward
current is significantly suppressed. For the inverted cell (Fig. 9.5b), without heavily
doped regions (Fig. 9.5d, blue curves), photovoltaic and rectification behavior are
scarcely perceptible. However, with the heavily doped regions (red curves), the FF
recovers, reaching a value of 0.49, and rectification is clearly observed. Clearly, the
photovoltaic properties of the cells with thin heavily doped regions at the interfaces
are independent of the type of electrode material used. Thus, H 2 Pc/C 60 cells are
invertible using this interfacial heavy-doping technique.
Since electron extraction from C 60 to the ITO electrode is crucial for the operation
of the inverted cell (Fig. 9.5d (red curves)), we estimated the interfacial energy
band structure of ITO/10,000 ppm Cs 2 CO 3 -doped C 60 by Kelvin band-mapping
(Fig. 9.6) (Sect. 5.1.). There is a distinct barrier to electrons with a height of 0.34 eV
from the conduction band of C 60 to the ITO. However, since the band bends down
steeply within 5 nm of the interface, photogenerated electrons can tunnel through
this barrier. Heavily doped C 60 acts as an n
+ -type semiconductor and makes the n
+ -
C 60 /ITO junction ohmic. Organic/metal ohmic junctions can be fabricated by making
tunneling contacts with heavy interfacial doping.
227
9.6 Junction Formation
9.6.1 Ohmic Junctions
9.6.1.1 Organic/Metal Ohmic Junction
It is very important to make the two organic/metal contacts in a photovoltaic cell
ohmic. When the region in the vicinity of a metal electrode is heavily doped, even if
there is a Schottky barrier, its width becomes extremely thin allowing charge-carrier
tunneling, and as a result, an ohmic contact is expected to be formed similar to
that on heavily p
+ - or n
+ -doped inorganic semiconductors [52–54]. Here, + means
heavily doped. Moreover, an ohmic contact can be formed irrespective of which electrode material is used, since tunneling is less dependent on the metal work function,
enabling the cell structure to be inverted. This technique would allow flexibility in
the design of the cell structure.
As a test case, two-layer cells consisting of C 60 and H 2 Pc (Fig. 9.5a, b) were
examined [55]. The electrode materials were ITO and Ag. Heavy doping of the order
of 10,000 ppm (1%) and 50,000 ppm (5%) was applied to thin 10 nm regions close
to the C 60 and H 2 Pc/metal interfaces. Figure 9.5c, d shows the current–voltage (J-V)
characteristics for the cells with heavily doped regions in Fig. 9.5a, b (red curves). For
the cell in Fig. 9.5a, the fill factor (FF) reaches a value of 0.59, and clear rectification
characteristics can be seen with heavily doped regions (red broken curve). Without
the heavily doped regions (blue curves), however, FF is only 0.29, and the forward
current is significantly suppressed. For the inverted cell (Fig. 9.5b), without heavily
doped regions (Fig. 9.5d, blue curves), photovoltaic and rectification behavior are
scarcely perceptible. However, with the heavily doped regions (red curves), the FF
recovers, reaching a value of 0.49, and rectification is clearly observed. Clearly, the
photovoltaic properties of the cells with thin heavily doped regions at the interfaces
are independent of the type of electrode material used. Thus, H 2 Pc/C 60 cells are
invertible using this interfacial heavy-doping technique.
Since electron extraction from C 60 to the ITO electrode is crucial for the operation
of the inverted cell (Fig. 9.5d (red curves)), we estimated the interfacial energy
band structure of ITO/10,000 ppm Cs 2 CO 3 -doped C 60 by Kelvin band-mapping
(Fig. 9.6) (Sect. 5.1.). There is a distinct barrier to electrons with a height of 0.34 eV
from the conduction band of C 60 to the ITO. However, since the band bends down
steeply within 5 nm of the interface, photogenerated electrons can tunnel through
this barrier. Heavily doped C 60 acts as an n
+ -type semiconductor and makes the n
+ -
C 60 /ITO junction ohmic. Organic/metal ohmic junctions can be fabricated by making
tunneling contacts with heavy interfacial doping.