SUPPLEMENTAL INFORMATION
Tuning of threshold voltage of organic field-effect transistors
by space charge polarization
Heisuke Sakai, Koudai Konno and Hideyuki Murata
UV-VIS reflection absorption spectra of the OFETs
Figure S1a shows the absorption spectra of MPA+ClO4- in benzonitrile (0.4 mM) and
pentacene thin film (50 nm) on quartz substrate. The absorption spectra of MPA+ClO4solution showed the peaks at around 370 nm and 420 nm. Figure S1b shows the
UV-VIS differential absorption spectra of the OFETs with and without buffer layer.
These spectra were obtained by subtracting the spectra at Vex = 0 V from those applied
at Vex = -60 V for Tbias = 10 min. Minakata et al. reported the absorption band at around
400 - 500 nm was increased due to the formation of pentacene cation and the absorption
band at around 500 - 700 nm was decreased due to the decrease of the amount of neutral
pentacene.1 The broad absorption at around 400 - 500 nm was increased in the OFETs
without buffer layer. At the same time, the absorption at 500 - 700 nm was significantly
decreased. Whereas, in the OFET with the buffer layer, the increase of the absorption
intensity around 400-500 nm and the bleaching at 500 – 700 nm were not observed.
These results clearly show that the electrochemical doping in the active layer was
effectively prevented by the buffer layer. The relatively sharp decrease of absorption
around 370 nm and 420 nm observed in the OFETs with the buffer layer would be
attributed to the stark effect or the changes of electron distribution in the MPA+
molecule caused by the dissociation of the MPA+ ClO4- ion pairs, because we observed
similar decreases of the spectra in the case of the absorption spectra of the MPA+ClO4ion dispersed PMMA film (glass/ITO/dielectrics/Al). The sharp decrease in the
absorption peak at 680 nm was observed in the OFETs with the buffer layer. This peak
would be also attributed to the stark effect of the pentacene absorption peak.
Fig. S1. (Color online) (a) UV-VIS absorption spectra of MPA+ClO4- in benzonitrile
(0.4 mM) and pentacene thin film (50 nm). (b) UV-VIS differential absorption spectra of
the OFETs with buffer layer (red) and without buffer layer (black) after applied Vex of
-60 V for 10 min.
References
1
T. Minakata, I. Nagoya, and M. Ozaki, J. Appl. Phys. 69, 7354 (1991).