26
M. Hiramoto
Fig. 2.3 Schematic
illustrations of interfacial
doping. a H 2 Pc molecules
are doped at the Me-PTC/Au
interface. b Me-PTC
molecules are doped at the
H 2 Pc/ITO interface
positive photovoltage with respect to the indium tin oxide (ITO) electrode (Fig. 2.4,
curve a), implying that the Me-PTC film acts as an n-type semiconductor, and a
small value of 0.2 mAcm
−2 was obtained for J sc . By the interfacial doping of a
monolayer-like H 2 Pc layer with a thickness of 1.5 nm between Au and Me-PTC,
J sc was suddenly doubled to 0.4 mAcm
−2 , increased steeply by applying the small
reverse voltage, and reached close to 2 mAcm
−2 at −2 V (Fig. 2.4, curve b). At a
reverse bias of only −0.4 V, the photocurrent quantum efficiency for 480-nm light
reached a significantly high value of 40%. The author had a strong impression that
only molecular H 2 Pc doped at Me-PTC/Au interface has the dramatic ability for the
photocurrent enhancement into the milliampere level. Figure 2.5 shows a photograph
of a page of the author’s laboratory notebook from that time, recording the result of
interfacial doping. On the left middle side of the page, the author wrote a Japanese
sentence meaning “Increase of carrier generation efficiency!”.
Conversely, the interfacial doping of Me-PTC at H 2 Pc/ITO interface also caused
the drastic increase of carrier generation (Fig. 2.3b). It is clear that the molecular
contact between the Me-PTC and H 2 Pc molecules is a key for the photocarrier
generation enhancement.
To quantitatively check this hypothesis, we measured the action spectra of the
interfacial doped cells (Fig. 2.6b). Here, an observation window in the spectral region
around 500 nm is present. Namely, in this range only Me-PTC absorbs light strongly
while H 2 Pc shows little light absorption (Fig. 2.7a). At approximately 500 nm, a
drastic photocurrent enhancement was observed. The dramatic increase in J sc at
480 nm reveals that a molecular amount of H 2 Pc on the Me-PTC film apparently
enhances the carrier generation efficiency of Me-PTC (Fig. 2.7b) until the thickness of 20 nm at which point the entire surface of Me-PTC is covered by H 2 Pc
M. Hiramoto
Fig. 2.3 Schematic
illustrations of interfacial
doping. a H 2 Pc molecules
are doped at the Me-PTC/Au
interface. b Me-PTC
molecules are doped at the
H 2 Pc/ITO interface
positive photovoltage with respect to the indium tin oxide (ITO) electrode (Fig. 2.4,
curve a), implying that the Me-PTC film acts as an n-type semiconductor, and a
small value of 0.2 mAcm
−2 was obtained for J sc . By the interfacial doping of a
monolayer-like H 2 Pc layer with a thickness of 1.5 nm between Au and Me-PTC,
J sc was suddenly doubled to 0.4 mAcm
−2 , increased steeply by applying the small
reverse voltage, and reached close to 2 mAcm
−2 at −2 V (Fig. 2.4, curve b). At a
reverse bias of only −0.4 V, the photocurrent quantum efficiency for 480-nm light
reached a significantly high value of 40%. The author had a strong impression that
only molecular H 2 Pc doped at Me-PTC/Au interface has the dramatic ability for the
photocurrent enhancement into the milliampere level. Figure 2.5 shows a photograph
of a page of the author’s laboratory notebook from that time, recording the result of
interfacial doping. On the left middle side of the page, the author wrote a Japanese
sentence meaning “Increase of carrier generation efficiency!”.
Conversely, the interfacial doping of Me-PTC at H 2 Pc/ITO interface also caused
the drastic increase of carrier generation (Fig. 2.3b). It is clear that the molecular
contact between the Me-PTC and H 2 Pc molecules is a key for the photocarrier
generation enhancement.
To quantitatively check this hypothesis, we measured the action spectra of the
interfacial doped cells (Fig. 2.6b). Here, an observation window in the spectral region
around 500 nm is present. Namely, in this range only Me-PTC absorbs light strongly
while H 2 Pc shows little light absorption (Fig. 2.7a). At approximately 500 nm, a
drastic photocurrent enhancement was observed. The dramatic increase in J sc at
480 nm reveals that a molecular amount of H 2 Pc on the Me-PTC film apparently
enhances the carrier generation efficiency of Me-PTC (Fig. 2.7b) until the thickness of 20 nm at which point the entire surface of Me-PTC is covered by H 2 Pc