232
M. Hiramoto
Tandem photovoltaic cells consisting of two pn-homojunctions were fabricated in
single fullerene films only by impurity doping. pn-homojunctions were also reported
in C 60 [57], H 2 Pc [62], ZnPc [16], and pentacene [18] single films.
9.6.2.2 pn-Homojunction in Co-Deposited Films
Blended film consisting of donor and acceptor molecules is indispensable in the
modern organic solar cells. Therefore, we think that the doping technology for the
blended films should be developed. Based on this consideration, we have designed
and fabricated a variety of organic homojunctions in the bulk of co-deposited organic
films by doping only. As an example, a p
+ in
+ p
+ in
+ -homojunction was fabricated
only by doping. The homojunction consisted of a couple of p
+ in
+ -homojunctions in
a uniform 6 T:C 60 blended film (Fig. 9.7a) [47]. Here, “+” means heavily doped. By
comparing the photovoltaic characteristics of p
+ in
+ - and p
+ in
+ p
+ in
+ -homojunctions
(Fig. 9.7a), the open-circuit photovoltage (V oc ) was found to double from 0.85 V
(blue curve) to 1.69 V (red curve), representing a tandem organic photovoltaic
cell [58, 59] made by doping alone (Fig. 9.7b). The precise potential profile of
the n
+ p
+ -homojunction can be illustrated based on the conventional junction theory
(Fig. 9.6c) [60, 61]. An organic/organic tunneling ohmic junction is formed at the
n
+ p
+ -homojunction, which is doped heavily to connect two p
+ in
+ -homojunctions.
Heavily doped p
+ /ITO and n
+ /metal junctions form metal/organic ohmic junctions
via tunneling [55].
9.6.3 Band-Mapping of Organic pn-Homojunctions
In this section, precise band-mapping in organic pn-homojunctions with various
balances between the doping concentrations of the p- and n-type layers, i.e., (i) a
p
+ n
+ -homojunction in 6 T:C 60 (p
+ layer doped with MoO 3 at 50,000 ppm and n
+
layer doped with Cs 2 CO 3 at 50,000 ppm), (ii) a pn-homojunction in 6 T:C 60 (p-layer
doped with MoO 3 at 3,000 ppm and n
+ layer doped with Cs 2 CO 3 at 500 ppm), and
(iii) a pn
+ -homojunction in H 2 Pc:C 60 (p-layer doped with V 2 O 5 at 1,000 ppm and
n
+ layer doped with Cs 2 CO 3 at 10,000 ppm) was performed [61]. Here, “ + ” means
heavily doped.
In order to obtain the potential profiles of the pn-homojunctions, the film thickness
dependence of the work function both in p-type layers on n-type layers and vice
versa were measured using a Kelvin probe. The work function shift for p-side can
be observed by repeat of deposition and work function measurement of p-type layer
on 100 nm-thick n-type layer. Similarly, the work function shift for n-side can be
observed by repeat of deposition and work function measurement of n-type layer on
100 nm-thick p-type layer.
For the heavily doped p
+ n
+ -homojunction (i), the bulk values of E F in 100 nmthick p
+ and n
+ layers were determined to be 5.58 eV and 4.38 eV, respectively. The
M. Hiramoto
Tandem photovoltaic cells consisting of two pn-homojunctions were fabricated in
single fullerene films only by impurity doping. pn-homojunctions were also reported
in C 60 [57], H 2 Pc [62], ZnPc [16], and pentacene [18] single films.
9.6.2.2 pn-Homojunction in Co-Deposited Films
Blended film consisting of donor and acceptor molecules is indispensable in the
modern organic solar cells. Therefore, we think that the doping technology for the
blended films should be developed. Based on this consideration, we have designed
and fabricated a variety of organic homojunctions in the bulk of co-deposited organic
films by doping only. As an example, a p
+ in
+ p
+ in
+ -homojunction was fabricated
only by doping. The homojunction consisted of a couple of p
+ in
+ -homojunctions in
a uniform 6 T:C 60 blended film (Fig. 9.7a) [47]. Here, “+” means heavily doped. By
comparing the photovoltaic characteristics of p
+ in
+ - and p
+ in
+ p
+ in
+ -homojunctions
(Fig. 9.7a), the open-circuit photovoltage (V oc ) was found to double from 0.85 V
(blue curve) to 1.69 V (red curve), representing a tandem organic photovoltaic
cell [58, 59] made by doping alone (Fig. 9.7b). The precise potential profile of
the n
+ p
+ -homojunction can be illustrated based on the conventional junction theory
(Fig. 9.6c) [60, 61]. An organic/organic tunneling ohmic junction is formed at the
n
+ p
+ -homojunction, which is doped heavily to connect two p
+ in
+ -homojunctions.
Heavily doped p
+ /ITO and n
+ /metal junctions form metal/organic ohmic junctions
via tunneling [55].
9.6.3 Band-Mapping of Organic pn-Homojunctions
In this section, precise band-mapping in organic pn-homojunctions with various
balances between the doping concentrations of the p- and n-type layers, i.e., (i) a
p
+ n
+ -homojunction in 6 T:C 60 (p
+ layer doped with MoO 3 at 50,000 ppm and n
+
layer doped with Cs 2 CO 3 at 50,000 ppm), (ii) a pn-homojunction in 6 T:C 60 (p-layer
doped with MoO 3 at 3,000 ppm and n
+ layer doped with Cs 2 CO 3 at 500 ppm), and
(iii) a pn
+ -homojunction in H 2 Pc:C 60 (p-layer doped with V 2 O 5 at 1,000 ppm and
n
+ layer doped with Cs 2 CO 3 at 10,000 ppm) was performed [61]. Here, “ + ” means
heavily doped.
In order to obtain the potential profiles of the pn-homojunctions, the film thickness
dependence of the work function both in p-type layers on n-type layers and vice
versa were measured using a Kelvin probe. The work function shift for p-side can
be observed by repeat of deposition and work function measurement of p-type layer
on 100 nm-thick n-type layer. Similarly, the work function shift for n-side can be
observed by repeat of deposition and work function measurement of n-type layer on
100 nm-thick p-type layer.
For the heavily doped p
+ n
+ -homojunction (i), the bulk values of E F in 100 nmthick p
+ and n
+ layers were determined to be 5.58 eV and 4.38 eV, respectively. The