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M. Hiramoto
9.1.2 History
Organic semiconductors were previously thought to be insensitive to doping. Now,
researchers recognize that external impurities due to imperfect refinement and
contaminants from the ambient atmosphere have hidden the true properties of OSCs
for a long time. A representative contaminant is oxygen. Because oxygen acts as an
acceptor and is doped in various kinds of OSCs, such as phthalocyanines, these OSCs
always behave as p-type semiconductors. Completely eliminating oxygen under
ultra-high vacuum throughout film fabrication and measurements has shown that
phthalocyanines are essentially n-type [1, 2]. A few kinds of OSCs, such as perylene
pigments, are not influenced by oxygen and behave as n-type semiconductors [3].
Though impurity doping in OSCs has a long history, very few dopants were
previously available. Halogen vapors such as I 2 and Br 2 have been typical acceptor
dopants [4, 5]. A perylene pigment was converted from n- to p-type by Br 2 doping,
and pn-homojunction behaviors were observed by forming a junction between nand p-type regions [6, 7]. Moreover, available donor dopants were limited to alkaline
metals such as Na and Ca, which are easily deteriorated by oxidation in the ambient
air.
In the last ten years, however, because of the research outcomes on organic electroluminescence, many novel dopants have been developed. Organic acceptor dopants
such as F 4 -TCNQ [8, 9], F6-TCNNQ [10, 11], HAT-CN [12], and F 36 C 60 [13] and
inorganic acceptor dopants such as MoO 3 , V 2 O 5 , and Fe 2 Cl 6 [14, 15] have been
found. Ru- [16–18], Co- [19], and W-complexes [10, 20] and Cs 2 CO 3 [21, 22] have
been discovered to be relatively stable donors in air.
Simultaneously, important physics were reported, for example, the realization of
a pin-homojunction fabricated by connecting p- and n-doped zinc phthalocyanines
(ZnPc) [16] and p- and n-type pentacenes [18] and the formation of an ohmic junction
for carrier injection [15].
Recently, even an extremely small amount of dopant reaching the ppm level
was revealed to affect the electrical characteristics of OSC films [10, 23, 24] and
organic single crystals [25]. Some papers reported significantly higher doping efficiency [10, 25, 26]. Such recent progress on doping strongly suggests that the field
of organic electronics is now shifting from conventional intrinsic organic semiconductor electronics to doped organic semiconductor electronics [27–30]. Today,
organic semiconductors are very sensitive to doping.
9.2 Principles
In this section, the processes behind doping, including charge transfer and ionization
to create carriers, are described.
M. Hiramoto
9.1.2 History
Organic semiconductors were previously thought to be insensitive to doping. Now,
researchers recognize that external impurities due to imperfect refinement and
contaminants from the ambient atmosphere have hidden the true properties of OSCs
for a long time. A representative contaminant is oxygen. Because oxygen acts as an
acceptor and is doped in various kinds of OSCs, such as phthalocyanines, these OSCs
always behave as p-type semiconductors. Completely eliminating oxygen under
ultra-high vacuum throughout film fabrication and measurements has shown that
phthalocyanines are essentially n-type [1, 2]. A few kinds of OSCs, such as perylene
pigments, are not influenced by oxygen and behave as n-type semiconductors [3].
Though impurity doping in OSCs has a long history, very few dopants were
previously available. Halogen vapors such as I 2 and Br 2 have been typical acceptor
dopants [4, 5]. A perylene pigment was converted from n- to p-type by Br 2 doping,
and pn-homojunction behaviors were observed by forming a junction between nand p-type regions [6, 7]. Moreover, available donor dopants were limited to alkaline
metals such as Na and Ca, which are easily deteriorated by oxidation in the ambient
air.
In the last ten years, however, because of the research outcomes on organic electroluminescence, many novel dopants have been developed. Organic acceptor dopants
such as F 4 -TCNQ [8, 9], F6-TCNNQ [10, 11], HAT-CN [12], and F 36 C 60 [13] and
inorganic acceptor dopants such as MoO 3 , V 2 O 5 , and Fe 2 Cl 6 [14, 15] have been
found. Ru- [16–18], Co- [19], and W-complexes [10, 20] and Cs 2 CO 3 [21, 22] have
been discovered to be relatively stable donors in air.
Simultaneously, important physics were reported, for example, the realization of
a pin-homojunction fabricated by connecting p- and n-doped zinc phthalocyanines
(ZnPc) [16] and p- and n-type pentacenes [18] and the formation of an ohmic junction
for carrier injection [15].
Recently, even an extremely small amount of dopant reaching the ppm level
was revealed to affect the electrical characteristics of OSC films [10, 23, 24] and
organic single crystals [25]. Some papers reported significantly higher doping efficiency [10, 25, 26]. Such recent progress on doping strongly suggests that the field
of organic electronics is now shifting from conventional intrinsic organic semiconductor electronics to doped organic semiconductor electronics [27–30]. Today,
organic semiconductors are very sensitive to doping.
9.2 Principles
In this section, the processes behind doping, including charge transfer and ionization
to create carriers, are described.