4.2. OPD characteristics
OPDs were made by sequential deposition of the two different
blends of PTB3-PC61BM and P3HT-PC61BM. Fig. 5 shows the EQE
that wasmeasured for blends of PTB3 and P3HT composing the photodetectors.
Their absorption spectra are very broad and they have
an EQE of over 46% (at 680 nm) and 55% (at 480 nm), respectively.
The maximum value of EQE for the P3HT/PC61BM OPD is situated
between 450 nm and 600 nm and between 600 nm and 720 nm for
the PTB3/PC61BM OPD, with higher than 40% value. Depending on
the application, OPDs based on P3HT can be adapted to detect the
wavelengths in the range of 450–600 nm while OPDs based on PTB3
are more suitable to measure 600–720 nm wavelengths. However,
the broad absorption bands of OPDs overlap the emission spectra
of the OLEDs, requiring the use of an excitation filter to avoid the
detection of the signal coming from the photons directly emitted
by the OLED and not absorbed by the media.
Signal-to-noise ratio is a concern for OPDs performance for measuring
low fluorescence signals. The signal-to-noise ratio of the
PTB3/PC61BM OPD has been plotted in Fig. 6. This curve represents
the current intensity generated in the presence of light (at
100 mW/cm2), divided by the dark current of the OPD as a function
of voltage. From Fig. 6, we can conclude that the maximum signalto-noise
ratio of PTB3 OPD is at 0V, with a value larger than 106 for
a light intensity of 100 mW/cm2.
The bandwidth of a photodetector is directly related to its speed
of operation. Depending on the application, the response time of a
photodetector can be critical, especially when recording the fluorescence
kinetics of fluorophores is needed. The photodetector
bandwidth has been measured at 0V in order to have the best signal/noise
ratio (Fig. 6), which results in the best sensitivity. To do
so, we have measured the OPD response to laser diode pulses at
different frequencies. Fig. 7b shows an example of measurements
Fig. 6. A plot of signal-to-noise ratio of the PTB3/PC61BM OPD.
Table 1
Comparison of the OPD characteristics with literature.
Peumans et al.
[24]
Ramuz et al. [4] This article
Type Small molecule
layers
P3HT/PC61BM PTB3/PC61BM
EQEmax >75%
(550–670 nm)
at −9V
>70%
(450–620 nm)
at 0V
>40%
(550–720 nm)
at 0V
Jdark (mA/cm2) 12E−3 1E−7