224
M. Hiramoto
9.4.1 Single Films
Figure 9.3 shows the energy diagrams of OSC films. The Fermi levels (E F s) for the
undoped (black-dashed line), acceptor-doped (red-dashed line), and donor-doped
(blue-dashed line) films were measured using a Kelvin probe [37–39]. Generally,
single OSCs, such as fullerenes, phthalocyanines, rubrene, sexithiophene (6 T), and
pentacene, can be changed to n- and p-type only by doping.
Here, the case of fullerenes (C 60 ) (Fig. 9.3, left) is described. The E F of the MoO 3
acceptor has a remarkably positive value of 6.69 eV (Fig. 9.3), which is more positive
than the valence band of C 60 (6.2 eV) [40, 41]. Thus, an electron can transfer from C 60
to MoO 3 . When MoO 3 was doped at the 3,000 ppm level, the E F moved positively
to 5.88 eV and approached the valence band (6.2 eV). A positive charge on C 60
+
was released thermally from the negative charge on MoO 3
− at RT (Fig. 9.1d, right).
Thus, C 60 showed p-type behavior upon doping with MoO 3 [42]. Upon doping the
acceptor (3,000 ppm Cs 2 CO 3 ), the E F negatively moved to 4.40 eV and approached
the conduction band of C 60 (4.0 eV) [39]. The n-type behavior of C 60 upon doping
with Cs 2 CO 3 results from the opposite mechanism of MoO 3 doping (Fig. 9.1d, left).
9.4.2 Blended Films
pn-control by doping can be extended to blended films fabricated by co-deposition,
which consists of D:A (donor:acceptor) organic semiconductors, such as H 2 Pc:C 60
and 6 T:C 60 , having the energy relationship shown in Fig. 9.1b. Blended films are
indispensable in device applications, especially for organic photovoltaic cells. The
organic blended junction (Fig. 9.3, lower right) was proposed by Hiramoto in 1991
[43, 44]. By doping a H 2 Pc:C 60 co-deposited film with a donor (Cs 2 CO 3 ), the E F
moved negatively to 4.22 eV (blue-dashed line) and approached the conduction band
of C 60 (Fig. 9.3, right). In contrast, by acceptor (V 2 O 5 ) doping, the E F moved to
4.95 eV (red-dashed line) and approached the valence band of H 2 Pc. For pn-control
of the blended films [45, 46], the E F shift occurs in “the bandgap of the blended film,”
i.e., between the C 60 conduction band and H 2 Pc valence band.
9.5 Doping Efficiency
The doping efficiency of single organic semiconductor films is less than 10%. With
D/A sensitization, the doping efficiency of blended films can be increased to 97%.
M. Hiramoto
9.4.1 Single Films
Figure 9.3 shows the energy diagrams of OSC films. The Fermi levels (E F s) for the
undoped (black-dashed line), acceptor-doped (red-dashed line), and donor-doped
(blue-dashed line) films were measured using a Kelvin probe [37–39]. Generally,
single OSCs, such as fullerenes, phthalocyanines, rubrene, sexithiophene (6 T), and
pentacene, can be changed to n- and p-type only by doping.
Here, the case of fullerenes (C 60 ) (Fig. 9.3, left) is described. The E F of the MoO 3
acceptor has a remarkably positive value of 6.69 eV (Fig. 9.3), which is more positive
than the valence band of C 60 (6.2 eV) [40, 41]. Thus, an electron can transfer from C 60
to MoO 3 . When MoO 3 was doped at the 3,000 ppm level, the E F moved positively
to 5.88 eV and approached the valence band (6.2 eV). A positive charge on C 60
+
was released thermally from the negative charge on MoO 3
− at RT (Fig. 9.1d, right).
Thus, C 60 showed p-type behavior upon doping with MoO 3 [42]. Upon doping the
acceptor (3,000 ppm Cs 2 CO 3 ), the E F negatively moved to 4.40 eV and approached
the conduction band of C 60 (4.0 eV) [39]. The n-type behavior of C 60 upon doping
with Cs 2 CO 3 results from the opposite mechanism of MoO 3 doping (Fig. 9.1d, left).
9.4.2 Blended Films
pn-control by doping can be extended to blended films fabricated by co-deposition,
which consists of D:A (donor:acceptor) organic semiconductors, such as H 2 Pc:C 60
and 6 T:C 60 , having the energy relationship shown in Fig. 9.1b. Blended films are
indispensable in device applications, especially for organic photovoltaic cells. The
organic blended junction (Fig. 9.3, lower right) was proposed by Hiramoto in 1991
[43, 44]. By doping a H 2 Pc:C 60 co-deposited film with a donor (Cs 2 CO 3 ), the E F
moved negatively to 4.22 eV (blue-dashed line) and approached the conduction band
of C 60 (Fig. 9.3, right). In contrast, by acceptor (V 2 O 5 ) doping, the E F moved to
4.95 eV (red-dashed line) and approached the valence band of H 2 Pc. For pn-control
of the blended films [45, 46], the E F shift occurs in “the bandgap of the blended film,”
i.e., between the C 60 conduction band and H 2 Pc valence band.
9.5 Doping Efficiency
The doping efficiency of single organic semiconductor films is less than 10%. With
D/A sensitization, the doping efficiency of blended films can be increased to 97%.