78
T. Kaji
PDMS was not induced and when the ZnPc:C 60 layer became thicker than 40 nm,
the short-circuit current density J sc of the OPV cell drastically decreased to less than
1 mA/cm
2 , and the photoelectric conversion efficiency also drastically decreased.
This is as reported by many past research groups [10, 11]. The cause of the decrease
is often thought to be an increase in electron–hole recombination in the ZnPc:C 60
mixed layer. This ZnPc:C 60 mixed layer was also vapor-deposited at a substrate
temperature of 70 °C, but crystallization of the active layer could not be promoted
only by heating the substrate. When crystallization was induced by co-evaporation
of PDMS, J sc increased dramatically with increasing active layer thickness and was
maintained at about 18 mA/cm
2 between 400 and 1000 nm.
There was almost no effect of crystallization on the open-circuit voltage V oc , and
it varied between 0.40 and 0.45 V over the entire film thickness range. The fill factor
(FF) was highly dependent on crystallization. Without crystallization, FF gradually
decreased as the thickness of the mixed film increased, and leveled off at 25% above
400 nm, whereas with optimal crystallization, FF was maintained at more than 60%
and in a thin-film thickness range and more than 55% in the 400–1000 nm range. As
a result, the photoelectric conversion efficiency (PCE) of the crystallized device was
around 4.0% even when the mixed layer thickness was 1,000 nm.
Here, the question arises as to how much thickness this characteristic can be maintained, and when we fabricated devices with successively increasing film thickness,
we confirmed that the characteristic was almost maintained up to 10 μm, as shown
in Fig. 4.1b. When the thickness reached 10 μm, the deposition time of the active
layer reached three days and we were very tired to continue; then, this experiment
was completed. The J sc was about 16 mA/cm
2 at 2–10 μm, and the maximum value
at 0.5–1.0 μm was 18 mA/cm
2 . The FF varied at 2–10 μm and gradually decreased
from 60 to 50%. In these devices, the Ag electrode surface had lost its metallic luster,
so the surface became rougher as the active layer became thicker, and this is thought
to cause problems with the contact of the electrodes. In particular, in order to measure
the characteristics of devices with a very thick active layer (>4 μm), it was necessary
to deposit a very thick Ag electrode (>1,500 nm). As a result of a gradual change
in V oc between 0.40 and 0.48 V, the photoelectric conversion efficiency (PCE) was
able to maintain a value of about 4.0% in devices of all thicknesses up to 10 μm. It
was a great surprise.
4.2.3 Film Morphology and Crystallinity of 10-µm
Crystallized OPV
Using ultraviolet–visible absorption spectroscopy and X-ray diffraction (XRD), we
confirmed the difference in the deposited film depending on the presence or absence of
co-evaporant molecules in the 600-nm-thick ZnPc:C 60 film. Comparing the absorption spectrum (Fig. 4.2a) with the XRD pattern (Fig. 4.2b), it can be seen that the
absorption peak (about 445 nm) and the XRD peak (10.3° and 10.8°) due to the C 60
T. Kaji
PDMS was not induced and when the ZnPc:C 60 layer became thicker than 40 nm,
the short-circuit current density J sc of the OPV cell drastically decreased to less than
1 mA/cm
2 , and the photoelectric conversion efficiency also drastically decreased.
This is as reported by many past research groups [10, 11]. The cause of the decrease
is often thought to be an increase in electron–hole recombination in the ZnPc:C 60
mixed layer. This ZnPc:C 60 mixed layer was also vapor-deposited at a substrate
temperature of 70 °C, but crystallization of the active layer could not be promoted
only by heating the substrate. When crystallization was induced by co-evaporation
of PDMS, J sc increased dramatically with increasing active layer thickness and was
maintained at about 18 mA/cm
2 between 400 and 1000 nm.
There was almost no effect of crystallization on the open-circuit voltage V oc , and
it varied between 0.40 and 0.45 V over the entire film thickness range. The fill factor
(FF) was highly dependent on crystallization. Without crystallization, FF gradually
decreased as the thickness of the mixed film increased, and leveled off at 25% above
400 nm, whereas with optimal crystallization, FF was maintained at more than 60%
and in a thin-film thickness range and more than 55% in the 400–1000 nm range. As
a result, the photoelectric conversion efficiency (PCE) of the crystallized device was
around 4.0% even when the mixed layer thickness was 1,000 nm.
Here, the question arises as to how much thickness this characteristic can be maintained, and when we fabricated devices with successively increasing film thickness,
we confirmed that the characteristic was almost maintained up to 10 μm, as shown
in Fig. 4.1b. When the thickness reached 10 μm, the deposition time of the active
layer reached three days and we were very tired to continue; then, this experiment
was completed. The J sc was about 16 mA/cm
2 at 2–10 μm, and the maximum value
at 0.5–1.0 μm was 18 mA/cm
2 . The FF varied at 2–10 μm and gradually decreased
from 60 to 50%. In these devices, the Ag electrode surface had lost its metallic luster,
so the surface became rougher as the active layer became thicker, and this is thought
to cause problems with the contact of the electrodes. In particular, in order to measure
the characteristics of devices with a very thick active layer (>4 μm), it was necessary
to deposit a very thick Ag electrode (>1,500 nm). As a result of a gradual change
in V oc between 0.40 and 0.48 V, the photoelectric conversion efficiency (PCE) was
able to maintain a value of about 4.0% in devices of all thicknesses up to 10 μm. It
was a great surprise.
4.2.3 Film Morphology and Crystallinity of 10-µm
Crystallized OPV
Using ultraviolet–visible absorption spectroscopy and X-ray diffraction (XRD), we
confirmed the difference in the deposited film depending on the presence or absence of
co-evaporant molecules in the 600-nm-thick ZnPc:C 60 film. Comparing the absorption spectrum (Fig. 4.2a) with the XRD pattern (Fig. 4.2b), it can be seen that the
absorption peak (about 445 nm) and the XRD peak (10.3° and 10.8°) due to the C 60