All ink-jet printed polyfluorene photosensor for high illuminance

All ink-jet printed polyfluorene photosensor for high
illuminance detection
Leah Lucas Lavery*, Gregory L. Whiting and Ana Claudia Arias+
Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304, USA
ABSTRACT
An all-printed photosensor based on polyfluorene derivatives has been developed for high illuminance
range detection (>100 klux). The active layer is a blend of hole-accepting poly(9,9’-dioctylfluorene-cobis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine)
(PFB)
and
electron-accepting
poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT). A top-absorbing architecture was used with
poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as the transparent electrode.
The surface energy of the PEDOT:PSS ink was modified in order to allow printing directly onto the
hydrophobic surface of the PFB:F8BT blend layer, placing the transparent electrode at the top of the
light sensor structure. The all-printed photosensor shows a linear response over multiple measurements
within the illuminance range of (100 to 400) klux.
*[email protected]
+ Current address: Department of Electrical Engineering and Computer Sciences, University of
California, Berkeley, Berkeley, CA 94720-1770, USA.
KEYWORDS. INK-JET, PHOTOSENSOR, PFB:F8BT
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High intensity light sensors are needed in order to monitor working environmental conditions and
assist with the diagnosis of blast-related injuries1 which include traumatic brain injury (TBI).2 Lightemitted from blasts or explosions can be of very high illuminance3 (greater than 100 klux) and cause
photo-stress, temporary blindness and disorientation.4 We have been developing a monitoring system
that could be worn by soldiers to collect daily information that can be incorporated as part of their
medical record. This device is designed to be disposable, inexpensive and adhered directly to objects
(including non-planar surfaces). This requires the development of light sensors with transparent top
electrodes, high light intensity response and process compatibility with flexible substrates. In our
approach we use inkjet printing as a deposition and patterning technique and use solution processible
materials such as conjugated polymers and metal nanoparticles as active layers in the device. Ink-jet
printing has been previously used to print the active layers of photovoltaics,5 organic light emitting
diodes,6 field effect transistors7and display backplanes.8
Typically organic-based light sensors are fabricated on glass substrates with a sputtered layer of
indium tin oxide (ITO) as the transparent anode,9 with light being absorbed through the glass. As
described above, our application requires a top-absorbing device architecture with the transparent
conductor deposited directly on top of the semiconducting material, which can sometimes damage the
organic layer.
The surface of the organic photoactive layer is hydrophobic and presents a challenge
when fabricating top-absorbing devices from solution. We have been able to address this fabrication
challenge by employing a modified conductive polymer ink as the transparent electrode in order to
improve wetting and printing characteristics onto the photoactive layer.
A cross-sectional view of the all-printed light sensor is shown in Figure 1.
Poly(3,4-
ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS from H.C. Starck) was used as the
transparent anode as an alternative to conventional sputtered-ITO and ITO nanodispersions.10 As
illustrated in Figure 1(a), light absorption occurs directly through the top electrode PEDOT:PSS layer.
The sessile drop contact angle of conventional PEDOT:PSS solution on the polyfluorene blend
2
(photoactive layer) surface was θ = 104 ± 5° (Figure 1b). The surface tension of PEDOT:PSS ink was
modified by mixing Zonyl fluorosurfactant (0.1-0.2 wt%) to the original formulation. This new ink
composition reduced the contact angle to θ = 22 ± 4° (Figure 1c) allowing printing of PEDOT:PSS (film
thickness 25-30 nm) directly onto the semiconducting film surface. While making the PEDOT ink
compatible with other layers of devices, the addition of the surfactant did not alter the PEDOT:PSS
conductivity. The measured conductivity of films with and without the fluorosurfactant was 29±4 S/cm
and 31±6 S/cm, respectively.
The printed PEDOT:PSS electrode area defines the active area of the
light sensor, and it varied from 6 to 9 mm2 in the experiments described here.
The organic photoactive layer was composed of a blend of electron accepting poly(9,9’dioctylfluorene-co-benzothiadiazole) (F8BT) and the electron donor poly(9,9’-dioctylfluorene-co-bisN,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB). The molecular weights of PFB
and F8BT were 46k and 27k, respectively.
The PFB:F8BT blend layer was ink-jet printed, in air, from
a 1:1 weight ratio solution in dichlorobenzene atop a printed silver nanoparticle electrode8 using a
previously described custom-built system.7 Our printer system consists of a piezoelectric print head,
translation stages, substrate holder and alignment camera. The printed silver nanoparticle electrode
(Cabot Corporation) was annealed at 130°C for 10 min, typically ≈ 50 nm thick. The printed PFB:F8BT
layers were optically smooth thin films of ≈ 35 mm2 area and thicknesses of 0.8 - 1.0 µm. We have
adopted this thickness to ensure high yield and reproducibility in our fabrication process. As we are
printing all layers in the device and PEDOT:PSS directly onto the photoactive blend layer, we need to
take into account the roughness of the printed silver electrodes. This blended system has been used
previously in polymer based photovoltaics,11 and it has been shown that the degree of phase separation
of the polymer components influences the resulting morphology and device performance.11, 12 Methods
used to vary the thin film phase separation include altering: polymer component ratio,11 selection of
solvent based on boiling point,12 substrate temperature,12 molecular weight,13 and the formation of
polymer nanospheres from a miniemulsion process.14
These methods alter the domain size from
nanometers to microns when observed by atomic force microscopy (AFM) or fluorescence microscopy.
3
Compositional mapping via synchrotron-based scanning X-ray,15 scanning near-field optical,16 and
electrostatic force microscopies17 have shown that these micron-sized domains are not composed of a
single phase but contain proportions of both PFB and F8BT. Using synchrotron-based scanning X-ray
microscopy,15 McNeill, et al. have mapped the composition of the enclosed domains to be PFB-rich
(70 ± 3 wt%) and the surrounding majority phase to be F8BT-rich (95 wt%).
Ink-jet printing can also be used as a method to vary the thin film morphology that has lead to
improved device performance over other processing methods such as spin-coating.18 Xia et al. observed
increased external quantum efficiency (EQE) in ink-jet printed devices attributed to improved charge
transport compared to spin-coated devices.18 The external quantum efficiency of the printed PFB:F8BT
photosensor is shown in Figure 2. EQE was calculated by
EQE =
hcJ sc
eλP
(1)
where λ is the incident wavelength, JSC the short-circuit photocurrent density, e the elementary charge, h
is the Planck constant, c is the speed of light, and P is light the incident light power. Spectral response
was measured under zero bias using an Oriel Cornerstone Monochromator with Oriel Instruments
Quartz Tungsten Halogen lamp (100 W). Photoresponsivity was at wavelengths below 550 nm and
EQE reaches a maximum of 5.9% at 400 nm. The photocurrent increases at higher excitation energy,
which has been explained by the theoretical analysis of the nature of excited states in PPV19 and applied
to the PFB:F8BT system,11 namely that excitons at higher excitation energies have smaller binding
energies and greater number of delocalized wavefunctions, making disassociation more probable. For
comparison, the absorption spectra measured by UV-Vis spectroscopy of a printed blend layer
(~ 220 nm thickness) is also plotted in Figure 2.
The photoaction spectrum of the blend devices was
mostly symbatic with the absorption curves for excitation energies below ~3.75 eV. Examination of the
printed surface topology by AFM (Veeco Dimension 3100) revealed a fine phase separation of the
polyfluorene components as shown in the inset of Figure 2. The ink-jet printed features are on the
nanometer-scale measuring ≈ 60-150 nm in size. Xia and Friend observed device improvements and
4
finer phase separation in ink-jet printed devices compared to spin-coated devices.18 They observed little
change in the photoluminescence (PL) efficiency between devices with micron-sized domains to
nanometer-sized domains indicating almost no change due to charge dissociation and therefore
attributed the improvement in EQE of their ink-jetted devices to improved charge transport. Ink-jet
printed devices likely have finer phase separation because solvent drying rates are more rapid in ink-jet
printing due to the small volumes of liquid (≈ 100 pL) being dispensed, allowing for less time for the
polyfluorene blend components to phase-separate.
The printed high light intensity sensors were tested under illumination provided by a Newport 1600W
solar simulator (Xenon short arc lamp). The illuminance was varied from 100 to 400 klux which was
achieved by changing the vertical distance between the photosensor and the vertex of the light source
since illuminance = I/d2, where d is the distance from the sample to the illumination source and I is the
intensity of the illumination source.
Illuminance was measured by an Extech EA30 light meter.
Figure 3 shows current-voltage (I-V) characteristics for the all-printed sensor measured by a Keithley
6487 electrometer. The dark current at -1 V was 1 nA/cm2.
The photocurrent (measured at -1 V)
increases linearly with light intensity and no current saturation is observed (Figure 3 inset). For all
sensors, the open circuit voltage (Voc) was within the range of 0.1 - 0.2 V, however, for an individual
sensor, the Voc did not shift as illuminance increased. Previous researchers20 observed a logarithmic
increase in the Voc in PFB:F8BT bilayer devices from increasing Argon-ion laser intensity (based on
458 nm line), but for those measurements, the Voc saturated at radiometric intensities above
0.7 mW/cm2, which is significantly lower than the testing conditions used in the experiments described
here.
Further examination of the printed PFB:F8BT photosensors was completed under short illumination
pulses. Light exposure from a explosion or blast was simulated by exposing the PFB:F8BT photosensor
to light from the solar simulator for only 0.5 - 0.75 s, by toggling the mechanical shutter of the Newport
solar simulator open and closed. The change in voltage was recorded by a Tektronix TDS2014B digital
oscilloscope at different illuminance levels. The signal from the PFB:F8BT photosensor at zero bias
5
was amplified by a Keithley 427 current amplifier (gain 107 V/A) and the oscilloscope was triggered
using a calibrated silicon photodiode (Newport) from a secondary input channel. An example of the
oscilloscope voltage readout of the PFB:F8BT photosensor is shown in Figure 4a. Plotting the change
of voltage (∆V) from the initial background voltage (Vo ≈ 0) versus illuminance (Figure 4b) shows that
the photosensor voltage increased ≈ 0.3 V per 100 klux, and linear photoresponse as a function of
illuminance over multiple measurements.
Future experiments will further examine the transient
photocurrent response as well as device reliability, which is necessary to understand the dynamics of
charge transport and charge separation at these high illuminance values.
In summary, we have demonstrated an all ink-jet printed top-absorbing photosensor fabricated from
printed layers of a PFB:F8BT polymer blend, and Ag nanoparticles and PEDOT:PSS electrodes. Ink-jet
printing of the PEDOT:PSS top electrode directly onto the hydrophobic PFB:F8BT layer was made
possible by the addition of a fluorosurfactant to the PEDOT:PSS ink. The photosensors were examined
under short illumination pulses (0.5 - 0.75 s), and showed good linearity in photoresponse over multiple
measurements at high illuminance values of 100 - 400 klux.
ACKNOWLEDGMENTS. The authors thank T.N. Ng, B. Russo and B. Krusor for assistance with
experiments and helpful discussions. The authors thank A. Kalio for use of solar simulator and N.
Johnson and R.A. Street for use of monochromator equipment. This work is partially supported by
Sensor Tape Program of the Defense Advanced Research Projects Agency (DARPA) under contract
W81XWH-08-0065. Approved for Public Release Distribution Distribution Unlimited.
6
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Figure 1. a) Cross-sectional view of the all-printed polyfluorene PFB:F8BT photosensor. Illumination
is through the PEDOT:PSS top electrode layer. b) PEDOT:PSS droplet on PFB:F8BT semiconducting
film c) Modified PEDOT:PSS droplet with addition of Zonyl fluorosurfactant on a PFB:F8BT film
Figure 2. External quantum efficiency (EQE) and UV-Vis absorption spectra for printed PFB:F8BT
photosensor. Atomic force micrograph (inset) of ink-jet printed film. Z-height scale is 30 nm and scale
bar is 1.25 µm.
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Figure 3. Current-Voltage (I-V) characteristics for the printed polymer blend photosensor collected at
increasing illuminance values (inset legend, measured in klux, photocurrent measured at -1 V).
Figure 4. (a) Change in voltage (∆V) as the photosensor is exposed to ≈ 0.5 s of illumination from solar
simulator at varying levels of illuminance (see inset legend). (b) Change in voltage (∆V) from (a)
plotted as a function of illuminance for multiple measurements.
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