230
M. Hiramoto
9.6.1.2 Organic/Organic Ohmic Junction
By making an n
+ p
+ heavily doped double layer, organic/organic ohmic junctions can
be fabricated [47, 56]. Figure 9.7d shows an n
+ p
+ -homojunction device fabricated
in a C 60 :6 T co-deposited film. MoO 3 and Cs 2 CO 3 were heavily doped (50,000 ppm
(5%)) for the p
+ and n
+ -regions, respectively. Obviously, the n
+ p
+ -homojunction
showed good ohmic properties (Fig. 9.7e). We think that the origin of the ohmic
behavior of the n
+ p
+ -homojunction can be attributed to carrier recombination or
tunneling. Similarly, organic/organic ohmic junctions can be fabricated also for a
C 60 single film (Fig. 9.6).
9.6.2 Homojunctions
9.6.2.1 pn-Homojunction in Single C 60 Films
Since both p- and n-type C 60 were formed, we tried to fabricate pn-homojunctions in
single C 60 films [56, 57]. Figure 9.6b shows the structure of a pn-homojunction. MoO 3
and Cs 2 CO 3 dopants in volume concentrations of 3,000 and 500 ppm, respectively,
were used. Ohmic contacts to the ITO and Ag electrodes were formed by heavily
doping 10 nm thick layers adjacent to the electrodes with MoO 3 (50,000 ppm) for
the p
+ contact and Cs 2 CO 3 (10,000 ppm) for the n
+ contact, respectively. Figure 9.6a
shows the structure of a tandem cell consisting of two pn-C 60 unit cells (Fig. 9.6b).
To make the n
+ p
+ -homojunction, both doping concentrations were increased to
50,000 ppm.
Curves A and B in Fig. 9.6c show the current–voltage (J-V ) characteristics for a
tandem cell (Fig. 9.6a) and a unit cell (Fig. 9.6b), respectively. For the tandem cell,
the V oc value reaches 1.90 V. For the unit cell, the V oc value is 1.03 V. V oc has almost
doubled (84% increase) by connecting two unit cells. When the n
+ p
+ -homojunction
between the two unit cells of the tandem cell (Fig. 9.6a) was removed, V oc remained
the same (1.03 V), but the photocurrent density decreased significantly (curve C)
compared to the unit cell (curve B). Thus, an n
+ p
+ -interconnecting homojunction is
indispensable for doubling the V oc value. A heavily doped n
+ p
+ -homojunction acted
as an ohmic interlayer between the two pn-homojunction cells.
Figure 9.6d shows the energy structure of tandem cell in which n
+ p
+ -homojunction
(Fig. 9.6d, red-shaded region) connecting two pn-homojunctions (Fig. 9.6d, blueshaded regions). The width of the depletion layers of the pn-homojunctions (blueshaded) and the n
+ p
+ -homojunction (red-shaded) is 130 and 20 nm, respectively.
The former and the latter act as the active regions for photocurrent generation and
the ohmic interlayer, respectively. Thus, the depletion layer width, which is closely
related to the behavior of the homojunction, was intentionally controlled by the
doping concentration.
M. Hiramoto
9.6.1.2 Organic/Organic Ohmic Junction
By making an n
+ p
+ heavily doped double layer, organic/organic ohmic junctions can
be fabricated [47, 56]. Figure 9.7d shows an n
+ p
+ -homojunction device fabricated
in a C 60 :6 T co-deposited film. MoO 3 and Cs 2 CO 3 were heavily doped (50,000 ppm
(5%)) for the p
+ and n
+ -regions, respectively. Obviously, the n
+ p
+ -homojunction
showed good ohmic properties (Fig. 9.7e). We think that the origin of the ohmic
behavior of the n
+ p
+ -homojunction can be attributed to carrier recombination or
tunneling. Similarly, organic/organic ohmic junctions can be fabricated also for a
C 60 single film (Fig. 9.6).
9.6.2 Homojunctions
9.6.2.1 pn-Homojunction in Single C 60 Films
Since both p- and n-type C 60 were formed, we tried to fabricate pn-homojunctions in
single C 60 films [56, 57]. Figure 9.6b shows the structure of a pn-homojunction. MoO 3
and Cs 2 CO 3 dopants in volume concentrations of 3,000 and 500 ppm, respectively,
were used. Ohmic contacts to the ITO and Ag electrodes were formed by heavily
doping 10 nm thick layers adjacent to the electrodes with MoO 3 (50,000 ppm) for
the p
+ contact and Cs 2 CO 3 (10,000 ppm) for the n
+ contact, respectively. Figure 9.6a
shows the structure of a tandem cell consisting of two pn-C 60 unit cells (Fig. 9.6b).
To make the n
+ p
+ -homojunction, both doping concentrations were increased to
50,000 ppm.
Curves A and B in Fig. 9.6c show the current–voltage (J-V ) characteristics for a
tandem cell (Fig. 9.6a) and a unit cell (Fig. 9.6b), respectively. For the tandem cell,
the V oc value reaches 1.90 V. For the unit cell, the V oc value is 1.03 V. V oc has almost
doubled (84% increase) by connecting two unit cells. When the n
+ p
+ -homojunction
between the two unit cells of the tandem cell (Fig. 9.6a) was removed, V oc remained
the same (1.03 V), but the photocurrent density decreased significantly (curve C)
compared to the unit cell (curve B). Thus, an n
+ p
+ -interconnecting homojunction is
indispensable for doubling the V oc value. A heavily doped n
+ p
+ -homojunction acted
as an ohmic interlayer between the two pn-homojunction cells.
Figure 9.6d shows the energy structure of tandem cell in which n
+ p
+ -homojunction
(Fig. 9.6d, red-shaded region) connecting two pn-homojunctions (Fig. 9.6d, blueshaded regions). The width of the depletion layers of the pn-homojunctions (blueshaded) and the n
+ p
+ -homojunction (red-shaded) is 130 and 20 nm, respectively.
The former and the latter act as the active regions for photocurrent generation and
the ohmic interlayer, respectively. Thus, the depletion layer width, which is closely
related to the behavior of the homojunction, was intentionally controlled by the
doping concentration.