EXPERIMENTAL DETAILS
Device preparation
Reference device A (ITO-PEDOT:PSS-SQ:PCBM-Al). PEDOT:PSS (Clevios P VP AI 4083) was spin-coated from
aqueous solution at 2000 rpm onto glass-ITO substrates pre-treated with oxygen plasma. Following the
deposition, a thermal treatment at 100°C for 15 minutes under nitrogen was applied. The blend was
dissolved in chloroform (19.2 mg/ml) and spin-coated at 100 rpm in a glovebox for 1 minute (followed by 1
minute at 1000 rpm), giving a 220 nm thick film as deduced from absorbance measurements. Al was
evaporated at 10-6 mbar.
Device B (ITO-PEDOT:PSS-MEHPPV-SQ/PCBM-Al). For the cross-linked MEHPPV layer (Aldrich, average
Mn=150,000-250,000), a chloroform solution of MEHPPV together with photoinitiator (benzoin methyl
ether) and cross-linker (1,4-butanediol dimethacrylate) (weight ratio 1:0.06:0.6) was spin-coated onto ITOPEDOT:PSS substrates at 3000 rpm and treated in the glovebox with UV light (370 nm, 15 mW/cm2) for 15
minutes. Un-crosslinked materials were removed by spin-coating of pure chloroform. The crosslinked
MEHPPV film had a thickness of about 30 nm, obtained by Atomic Force Microscopy measurement
(Nanosurf Easyscan2) in non-contact mode on a reference film deposited onto glass slide. Photoinitiator
and cross-linker were purchased from Aldrich and used without further purification.
Device Characterization
All electrical measurements have been performed in vacuum (below 10-6 mbar).
Mobilities of pristine and crosslinked MEHPPV were extracted from transcharacteristic measurements in
saturation regime (gate voltage -30 V) on thin-film transistors in bottom contact, bottom gate
configuration, with SiO2 dielectric vapor primed with hexamethyldisilazane.
EQE measurement was performed through a set of LEDs (pulse length 500 s, incident power 12 mW/cm2).
The measurement of the detectors bandwidth was accomplished by means of Agilent Network Analyzer
5061B driving an LED emitting at 650 nm through a voltage/current converter.
Current-voltage measurements were performed both in dark and light condition. The latter was obtained
by illuminating the device with an optical power density of 0.63 mW/cm2 at 700 nm. While no significant
6
2
Photocurrent [nA/cm ]
10
5
10
4
10
3
10
2
10
1
10
0
10
Device A dark current
Device A illuminated
Device B dark current
Device B illuminated
-1
10
-2
10
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
V [V]
Figure 1. Current-Voltage curves of device A (red) and device B (black) in dark condition (squares) and
under 0.63 mW/cm2 optical power density (=700 nm) applied (triangles).
differences can be observed (Figure 1) between device A and device B with direct bias applied, a
remarkable discrepancy arises between the dark currents in reverse bias condition, stressing the key role
played by the MEHPPV-based blocking layer in suppressing electrons injection from the ITO/PEDOT:PSS
contact into the active material.
To investigate the working regime of the organic photodetectors, measurements of photocurrent
dependence on both the incident optical power (P) and the voltage bias were accomplished on devices with
ITO-PEDOT:PSS-SQ:PCBM-Al structure. Optical power output was varied by tuning the LEDs driving current
and was calibrated by means of a silicon photodetector with known responsivity. Photocurrent response at
different bias voltages was also recorded for detectors shined with continuous wave optical signal (P=1.6
mW/cm2, =700 nm).
From Figure 2(a) it can be observed that the photocurrent grows linearly with the incident light intensity
for values below 4 W/cm2, showing instead a P0.84 dependence at higher power densities. In Figure 2(b),
the dependence of the photocurrent on the effective bias voltage, calculated as the externally applied
voltage plus the built-in voltage, is shown: it can be noticed that the photocurrent grows with the square
root of voltage. Both these observations suggest that the photodetector operates in a space charge limited
current regime, accordingly to the demonstrations of Mihailetchi et al. (ref. 20) and Koster et al. (ref. 21).
20
Vapplied+Vbi=1.45 V
P = 1.6mW/cm
2
0
Iph [A]
Iph [A]
1x10
-1
1x10
10
-2
1x10
(b)
(a)
-3
1x10
0.1
1
10
100
2
Power density [W/cm ]
0.1
1
Vapplied + Vbi [V]
Figure 2. (a) Detector’s photocurrent as a function of the incident light intensity (Vbias=1 V, =700 nm).
The red line represents a linear increase of the current- (b) detector’s photocurrent as a function of bias
voltage (the built-in voltage is 0.45 V). The incident optical power is 1.6 mW/cm2 (=700 nm). The red
line represents the increase of the photocurrent with V0.55 on a logarithmic scale.