84
T. Kaji
when comparing the J sc at the same thicknesses in Figs. 4.1 and 4.4, here, J sc of
20.04 mA/cm
2 and photoelectric conversion efficiency of 4.76%. The V oc was 0.45 V,
and FF was 52.6%.
For this better performance, the J sc of the devices was improved by an additional
layer of 40-nm rubrene:C 60 (ratio = 1.5:1, deposited with PDMS co-evaporant and
substrate heating at 70 °C) inserted between the active layer and the upper side
interfacial layers in Fig. 4.1a. Although this insertion increased short-circuit current
density up to around 2 mA/cm
2 for the devices with the active layer thickness of
over 400 nm, there was dispersion of the increase, and this could not be used for the
thickness dependence comparison.
Judging from the incident photon to current conversion efficiency (IPCE) spectra
in Figs. 4.5b and 4.6b, these current increases are mainly attributed to the absorption
of crystalline C 60 around 450 nm of wave length [7]; it is considered that the charge
carrier recombination at the upper interface has been suppressed by hole blocking
because of the deeper highest occupied molecular orbital (HOMO) of rubrene: 5.4 eV
[34], than it of ZnPc: 5.2 eV [11]. We note that almost the same phenomena were
also observed when rubrene was replaced by the other donor materials, such as CBP:
6.0 eV [35], DTDCTB: 5.3 eV [36], and α-6T: 5.3 eV [37], but were not observed
by the acceptor material of Alq 3 : 5.7 eV [38]; even it has similar HOMO level to the
donors above.
4.3.2 Effect of Antireflection Film
Finally, we confirmed the effect of the antireflection film to further improve J sc in this
device (Fig. 4.6a). Pasting an antireflection film onto the surface of a glass substrate
has been known as one of the methods for improving J sc and efficiency [39, 40].
This film reduces the loss due to the reflection of light on the outermost surface of
the glass and increases the effective incident light to the OPV cell. Introducing of
antireflection film reduced from 0.69% to 4.06% of the reflectance depending on the
wavelength in the entire visible light range (Fig. 4.6b). As a result, J sc could reach
up to 21.16 mA/cm
2 , even with ZnPc:C 60 active layer, which is the most standard
combination of organic pigments for OPVs made by vacuum evaporation. By using
the effects of the antireflection film, crystallization, and optimization of the interface
layer, the photocurrent of the standard materials was maximized, and FF was kept
at 52.1%, but because the V oc was restricted by the materials themselves to 0.46 V,
the resultant photoelectric conversion efficiency was up to 5.03%.
4.4 Summary and Outlook
The authors have discovered that if an organic pigment is properly crystallized, it
can be used as a photoelectric conversion layer of an OPV even if it is thickened
T. Kaji
when comparing the J sc at the same thicknesses in Figs. 4.1 and 4.4, here, J sc of
20.04 mA/cm
2 and photoelectric conversion efficiency of 4.76%. The V oc was 0.45 V,
and FF was 52.6%.
For this better performance, the J sc of the devices was improved by an additional
layer of 40-nm rubrene:C 60 (ratio = 1.5:1, deposited with PDMS co-evaporant and
substrate heating at 70 °C) inserted between the active layer and the upper side
interfacial layers in Fig. 4.1a. Although this insertion increased short-circuit current
density up to around 2 mA/cm
2 for the devices with the active layer thickness of
over 400 nm, there was dispersion of the increase, and this could not be used for the
thickness dependence comparison.
Judging from the incident photon to current conversion efficiency (IPCE) spectra
in Figs. 4.5b and 4.6b, these current increases are mainly attributed to the absorption
of crystalline C 60 around 450 nm of wave length [7]; it is considered that the charge
carrier recombination at the upper interface has been suppressed by hole blocking
because of the deeper highest occupied molecular orbital (HOMO) of rubrene: 5.4 eV
[34], than it of ZnPc: 5.2 eV [11]. We note that almost the same phenomena were
also observed when rubrene was replaced by the other donor materials, such as CBP:
6.0 eV [35], DTDCTB: 5.3 eV [36], and α-6T: 5.3 eV [37], but were not observed
by the acceptor material of Alq 3 : 5.7 eV [38]; even it has similar HOMO level to the
donors above.
4.3.2 Effect of Antireflection Film
Finally, we confirmed the effect of the antireflection film to further improve J sc in this
device (Fig. 4.6a). Pasting an antireflection film onto the surface of a glass substrate
has been known as one of the methods for improving J sc and efficiency [39, 40].
This film reduces the loss due to the reflection of light on the outermost surface of
the glass and increases the effective incident light to the OPV cell. Introducing of
antireflection film reduced from 0.69% to 4.06% of the reflectance depending on the
wavelength in the entire visible light range (Fig. 4.6b). As a result, J sc could reach
up to 21.16 mA/cm
2 , even with ZnPc:C 60 active layer, which is the most standard
combination of organic pigments for OPVs made by vacuum evaporation. By using
the effects of the antireflection film, crystallization, and optimization of the interface
layer, the photocurrent of the standard materials was maximized, and FF was kept
at 52.1%, but because the V oc was restricted by the materials themselves to 0.46 V,
the resultant photoelectric conversion efficiency was up to 5.03%.
4.4 Summary and Outlook
The authors have discovered that if an organic pigment is properly crystallized, it
can be used as a photoelectric conversion layer of an OPV even if it is thickened