9 Parts-Per-Million-Level Doping Effects …
221
lower right). The energy positions of their HOMO and LUMO are shifted in parallel
with each other. The photoexcitation of the donor molecule (Fig. 9.1b) and that of
the acceptor molecule (not shown) separate the positive and negative charges on
the adjacent donor and acceptor molecules and create a CT exciton (Fig. 9.1d) via
photoinduced ET. Thus, the CT exciton can dissociate thermally into a free electron
and a hole at RT. By utilizing D/A sensitization, OSCs can dissociate excitons and
generate photocurrent.
A pair of positive and negative charges separated to adjacent molecules (Fig. 9.1d)
can be considered a positively ionized donor dopant molecule and a negatively
charged neighboring semiconductor molecule (Fig. 9.1d, left) or as a negatively
ionized acceptor dopant molecule and a positively charged neighboring semiconductor molecule (Fig. 9.1d, right). Because the attractive force between negative and
positive charges in an organic solid is similar, a hole comparatively weakly bound to
a negatively ionized dopant molecule and vice versa can thermally dissociate into a
free hole at RT and become the majority carrier. In contrast with a CT exciton, the
charge of an ionized dopant molecule is spatially fixed in the molecular solid and
cannot move.
Since the distance between the orbital of the bound charge and the dopant ion
(Fig. 9.1d) is shorter than that in Si (Fig. 9.1c), the interaction between the dopant
molecule and semiconductor molecule may significantly influence the ionization
process. By forming the ground-state charge transfer complex by hybridization
(Fig. 9.1e), the ionization rate is expected to be lower than that of the simple ion
pair formation (Fig. 9.1a) [33, 34].
9.3 ppm-Level Doping Method
The keys to ppm-level doping are rigorous purification and an extremely low
deposition speed of 10
–9 nm s
−1 .
9.3.1 Purification
To clarify the effects of an extremely small doping amount reaching 1 ppm, the
organic semiconductor samples must be thoroughly purified to be free from residual
impurities, and external impurities from air must be avoided. Therefore, the air exposure of organic films has been completely avoided by using an evaporation chamber
built in a glove-box filled with circulating nitrogen gas (O 2 < 0.2 ppm, H 2 O < 0.5 ppm)
throughout depositions and measurements. The observed values of the Fermi level
(E F ) of organic films were changed irreversibly upon air exposure, and reproducible
and accurate values could not be obtained.
The organic semiconductors were purified through the formation of single crystals
by using train sublimation under nitrogen convection conditions at a pressure between
221
lower right). The energy positions of their HOMO and LUMO are shifted in parallel
with each other. The photoexcitation of the donor molecule (Fig. 9.1b) and that of
the acceptor molecule (not shown) separate the positive and negative charges on
the adjacent donor and acceptor molecules and create a CT exciton (Fig. 9.1d) via
photoinduced ET. Thus, the CT exciton can dissociate thermally into a free electron
and a hole at RT. By utilizing D/A sensitization, OSCs can dissociate excitons and
generate photocurrent.
A pair of positive and negative charges separated to adjacent molecules (Fig. 9.1d)
can be considered a positively ionized donor dopant molecule and a negatively
charged neighboring semiconductor molecule (Fig. 9.1d, left) or as a negatively
ionized acceptor dopant molecule and a positively charged neighboring semiconductor molecule (Fig. 9.1d, right). Because the attractive force between negative and
positive charges in an organic solid is similar, a hole comparatively weakly bound to
a negatively ionized dopant molecule and vice versa can thermally dissociate into a
free hole at RT and become the majority carrier. In contrast with a CT exciton, the
charge of an ionized dopant molecule is spatially fixed in the molecular solid and
cannot move.
Since the distance between the orbital of the bound charge and the dopant ion
(Fig. 9.1d) is shorter than that in Si (Fig. 9.1c), the interaction between the dopant
molecule and semiconductor molecule may significantly influence the ionization
process. By forming the ground-state charge transfer complex by hybridization
(Fig. 9.1e), the ionization rate is expected to be lower than that of the simple ion
pair formation (Fig. 9.1a) [33, 34].
9.3 ppm-Level Doping Method
The keys to ppm-level doping are rigorous purification and an extremely low
deposition speed of 10
–9 nm s
−1 .
9.3.1 Purification
To clarify the effects of an extremely small doping amount reaching 1 ppm, the
organic semiconductor samples must be thoroughly purified to be free from residual
impurities, and external impurities from air must be avoided. Therefore, the air exposure of organic films has been completely avoided by using an evaporation chamber
built in a glove-box filled with circulating nitrogen gas (O 2 < 0.2 ppm, H 2 O < 0.5 ppm)
throughout depositions and measurements. The observed values of the Fermi level
(E F ) of organic films were changed irreversibly upon air exposure, and reproducible
and accurate values could not be obtained.
The organic semiconductors were purified through the formation of single crystals
by using train sublimation under nitrogen convection conditions at a pressure between