9 Parts-Per-Million-Level Doping Effects …
243
FF increase at 10 ppm can be attributed to the emergence of majority carriers
after all traps in the p-region are filled (0–1 ppm).
(iii) Built-in potential formation (10–100 ppm): J sc increase continues, while
FF becomes constant. The J sc increase is clearly synchronized with the
built-in potential (V bi ) increase (Fig. 9.12c, left and center). Thus, the J sc
increase at 100 ppm can be attributed to the creation of a built-in field, i.e.,
n
+ p-homojunction formation.
(iv) Mobility decrease (100–1,000 ppm): Decreases in FF and J sc . Both decreases
are clearly synchronized with the hole mobility decrease and the drastic increase
in cell resistance from 2 to 50 cm
2 . Thus, the FF and J sc decreases can be
attributed to the hindered carrier transport due to the negatively ionized and
neutral accepter dopant molecules. In addition, the decrease in the depletion
layer thickness (W dep ) causes J sc to decrease (Fig. 9.12c, center and right).
High doping concentrations reaching 1,000 ppm have harmful side effects, such
as a mobility decrease caused by adding dopant molecules. Harada et al. [68, 69]
reported decreased mobility in C 60 films at high concentrations of doping. These
doping-induced side effects must be clarified to develop effective doping technology
for organic semiconductors. From this point of view, doping has been applied to
organic single crystals with strictly defined spatial and energetic structures.
9.9 Bulk-Doped Organic Single Crystals
In the research on organic electronics, doping bulk organic single crystals and
measuring their Hall effect have not been attempted. In addition, for vacuumdeposited films, it is difficult to exclude the possibility that the dopant molecules
exist at the grain boundaries and that the essential nature of doping is hidden by
the defects. In this section, ppm-level doping effects on bulk-doped rubrene single
crystals measured by the Hall effect are described. Rubrene single crystals, which
have inherent band conduction, showed a very high doping efficiency of 24%.
9.9.1 Doped Homoepitaxy
The Hall voltage of undoped rubrene single crystals was measured with the
help of trap healing [70] and charge accumulation by a field-effect transistor (FET)
[71, 72]. The bulk-doped rubrene single crystal was obtained by homoepitaxy, i.e.,
a homoepitaxial layer of rubrene was grown on a single-crystal rubrene substrate
(Fig. 9.13a) [73]. When evaporated at a conventional speed of 0.2 nm s
−1 , rubrene
formed an amorphous film even on the single-crystal rubrene substrate, but homoepitaxial growth could be achieved at a low speed of 3.3 × 10
–3 nm s
−1 (Fig. 9.13b). An
extremely low speed of 10
–9 nm s
−1 is needed for 1 ppm Fe 2 Cl 6 doping due to the low
243
FF increase at 10 ppm can be attributed to the emergence of majority carriers
after all traps in the p-region are filled (0–1 ppm).
(iii) Built-in potential formation (10–100 ppm): J sc increase continues, while
FF becomes constant. The J sc increase is clearly synchronized with the
built-in potential (V bi ) increase (Fig. 9.12c, left and center). Thus, the J sc
increase at 100 ppm can be attributed to the creation of a built-in field, i.e.,
n
+ p-homojunction formation.
(iv) Mobility decrease (100–1,000 ppm): Decreases in FF and J sc . Both decreases
are clearly synchronized with the hole mobility decrease and the drastic increase
in cell resistance from 2 to 50 cm
2 . Thus, the FF and J sc decreases can be
attributed to the hindered carrier transport due to the negatively ionized and
neutral accepter dopant molecules. In addition, the decrease in the depletion
layer thickness (W dep ) causes J sc to decrease (Fig. 9.12c, center and right).
High doping concentrations reaching 1,000 ppm have harmful side effects, such
as a mobility decrease caused by adding dopant molecules. Harada et al. [68, 69]
reported decreased mobility in C 60 films at high concentrations of doping. These
doping-induced side effects must be clarified to develop effective doping technology
for organic semiconductors. From this point of view, doping has been applied to
organic single crystals with strictly defined spatial and energetic structures.
9.9 Bulk-Doped Organic Single Crystals
In the research on organic electronics, doping bulk organic single crystals and
measuring their Hall effect have not been attempted. In addition, for vacuumdeposited films, it is difficult to exclude the possibility that the dopant molecules
exist at the grain boundaries and that the essential nature of doping is hidden by
the defects. In this section, ppm-level doping effects on bulk-doped rubrene single
crystals measured by the Hall effect are described. Rubrene single crystals, which
have inherent band conduction, showed a very high doping efficiency of 24%.
9.9.1 Doped Homoepitaxy
The Hall voltage of undoped rubrene single crystals was measured with the
help of trap healing [70] and charge accumulation by a field-effect transistor (FET)
[71, 72]. The bulk-doped rubrene single crystal was obtained by homoepitaxy, i.e.,
a homoepitaxial layer of rubrene was grown on a single-crystal rubrene substrate
(Fig. 9.13a) [73]. When evaporated at a conventional speed of 0.2 nm s
−1 , rubrene
formed an amorphous film even on the single-crystal rubrene substrate, but homoepitaxial growth could be achieved at a low speed of 3.3 × 10
–3 nm s
−1 (Fig. 9.13b). An
extremely low speed of 10
–9 nm s
−1 is needed for 1 ppm Fe 2 Cl 6 doping due to the low