76
T. Kaji
is often placed about 200 nm from the metal electrode that acts as a mirror for the
benefit from the second standing wave [1, 14, 16]. The thicker active layer is often
used in OPV devices made of polymer, and the antinode of the third standing wave
is often located at about 300 nm. However, beyond this thickness, the efficiency
of polymer devices also tends to decrease, or peak, in both J sc and fill factor, FF
[16–19].
However, in some exceptional cases, it has been reported that OPV devices show
high J sc even in a very thick active layer of 1 μm [17, 20, 21]. This anomalous
behavior has been reported both in the vacuum-deposited pigment devices [20] and
in the solution spin-coated polymer devices [17, 21], although in these cases the
relationship between the optical effects of standing waves has not been clarified as
well, as a result of actual device operations.
4.2 Change in Relation Between Film Thickness of Active
Layer and Solar Cell Characteristics of OPV
by Crystallization
This section summarizes performance difference of thickness-dependent characteristics of crystallized and non-crystallized ZnPc:C 60 OPVs. The thickness ranges from
40 nm to 10 μm [22].
4.2.1 Fabrication of Pigmented OPV Device with Crystallized
Active Layer
In this study, in order to show the thickness dependence of solar cell characteristics of OPV devices, a typical combination of donor:acceptor mixed film, zinc
phthalocyanine, and fullerene (ZnPc:C 60 ) was chosen [11–14, 23, 24]. The thickness range of the active layer is 40–10,000 nm, and the order, composition, and
thickness of the layers of the fabricated OPV cell are as follows: [ITO /F 4 TCNQ
(0.6 nm)/CuI (3 nm)/ZnPc (2 nm)/C 60 (2.5 nm)/ZnPc (1 nm)/ZnPc: C 60 (volume
ratio 1.5:1, 40–10000 nm)/C 60 (5 nm)/Alq 3 : C 60 (1: 1, 25 nm)/LiF (2 nm)/Ag
(100/600/1500 nm)]. ITO is a transparent electrode of indium-doped tin oxide,
F 4 TCNQ is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane, and Alq 3 is an
abbreviation for tris(8-quinolinolato)aluminum. In order to cover the thick active
layer uniformly, it was necessary to change the thickness of the Ag electrode.
For the vacuum evaporation of ZnPc, C 60 , and ZnPc:C 60 mixed layers, the “coevaporant induced crystallization” method [7, 22, 23, 25] was used, which has been
devised by the authors and is still under research and development. In this method,
“co-evaporant” enables crystallization and growth control of organic pigment film
during vacuum evaporation. “Co-evaporant” liquid molecules are simultaneously
vaporized during the deposition of the organic pigment thin film. The role of this
T. Kaji
is often placed about 200 nm from the metal electrode that acts as a mirror for the
benefit from the second standing wave [1, 14, 16]. The thicker active layer is often
used in OPV devices made of polymer, and the antinode of the third standing wave
is often located at about 300 nm. However, beyond this thickness, the efficiency
of polymer devices also tends to decrease, or peak, in both J sc and fill factor, FF
[16–19].
However, in some exceptional cases, it has been reported that OPV devices show
high J sc even in a very thick active layer of 1 μm [17, 20, 21]. This anomalous
behavior has been reported both in the vacuum-deposited pigment devices [20] and
in the solution spin-coated polymer devices [17, 21], although in these cases the
relationship between the optical effects of standing waves has not been clarified as
well, as a result of actual device operations.
4.2 Change in Relation Between Film Thickness of Active
Layer and Solar Cell Characteristics of OPV
by Crystallization
This section summarizes performance difference of thickness-dependent characteristics of crystallized and non-crystallized ZnPc:C 60 OPVs. The thickness ranges from
40 nm to 10 μm [22].
4.2.1 Fabrication of Pigmented OPV Device with Crystallized
Active Layer
In this study, in order to show the thickness dependence of solar cell characteristics of OPV devices, a typical combination of donor:acceptor mixed film, zinc
phthalocyanine, and fullerene (ZnPc:C 60 ) was chosen [11–14, 23, 24]. The thickness range of the active layer is 40–10,000 nm, and the order, composition, and
thickness of the layers of the fabricated OPV cell are as follows: [ITO /F 4 TCNQ
(0.6 nm)/CuI (3 nm)/ZnPc (2 nm)/C 60 (2.5 nm)/ZnPc (1 nm)/ZnPc: C 60 (volume
ratio 1.5:1, 40–10000 nm)/C 60 (5 nm)/Alq 3 : C 60 (1: 1, 25 nm)/LiF (2 nm)/Ag
(100/600/1500 nm)]. ITO is a transparent electrode of indium-doped tin oxide,
F 4 TCNQ is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane, and Alq 3 is an
abbreviation for tris(8-quinolinolato)aluminum. In order to cover the thick active
layer uniformly, it was necessary to change the thickness of the Ag electrode.
For the vacuum evaporation of ZnPc, C 60 , and ZnPc:C 60 mixed layers, the “coevaporant induced crystallization” method [7, 22, 23, 25] was used, which has been
devised by the authors and is still under research and development. In this method,
“co-evaporant” enables crystallization and growth control of organic pigment film
during vacuum evaporation. “Co-evaporant” liquid molecules are simultaneously
vaporized during the deposition of the organic pigment thin film. The role of this