A broadband photodetector based on Rhodamine B-sensitized ZnO nanowires film
A broadband photodetector based on Rhodamine B-sensitized ZnO nanowires film
Zheng Qi Bai
Ze Wen Liu
OPEN A broadband photodetector has been developed on the basis of ZnO nanowires (NWs)/Rhodamine B (RhB) hybrid system. The device is fabricated by spraying NWs on to gold interdigital electrodes followed by modifying the NWs via an RhB solution-casting process. Measurements show that the asfabricated device demonstrates photoresponsivity ranging from 300 nm to 700 nm with a bandwidth as large as 400 nm. The role of the dye sensitizer adsorbed on the surface of NWs is modeled to alter the transportation path of photo-generated carriers. The calculations based on the measurements reveal that the device exhibits a prominent responsivity in the interested band with maximum responsivity of 5.5A/W for ultraviolet (UV) light and 3A/W for visible (VIS) light under 8V bias, respectively. The sensitization not only widens the response spectrum with external quantum efficiency leaping up to 771% at VIS but also improves UV responsivity with maximum 51% enhancement. From the timedependent photo-current measurement, it is found that the response time (rise and decay times in total) of the device largely reduced from 17.5 s to 3.3 s after sensitization. A comparison of the obtained photodetector with other ZnO-based photodetectors is summarized from the view point of responsivity and bandwidth.
Continuous research interest has been attracted to zinc oxide (ZnO) nanostructures over twenty years for its
widespread potential applications in ultraviolet (UV) photodetectors1, 2, field effect transistors (FET)3, 4, laser
diodes (LDs)5, 6, and light emitting diode (LED)7 owing to its unique properties such as wide-bandgap (~3.37 eV),
larger specific surface area, carrier confinement in two or three dimensions, high exciton binding energy
(approximately 60 meV), and chemical stability8?12. Among various nanostructures, wurtzite (hexagonal) structured
ZnO nanowires (NWs) have been regarded as one kind of extremely promising building blocks for fabricating
high-gain optoelectronic nano-devices13?16 due to its excellent properties, relatively simplistic preparation
process, and the possibility of surface modification.
To date, the broadband photodetectors covering the visible band (VIS, wavelength ranges from 390 nm to
780 nm) have gradually become an important research domain due to the great demands of environmental
monitoring17, composition analysis18, photometric measurement19, communication20, and integrated optical system
such as micro-spectrometer21. To broaden the response bandwidth and overcome the ?visible-blind?
characteristics of low absorbance and weak responsivity of ZnO NWs for VIS light has become an interesting research focus
A variety of studies on strategies for widening the photosensitive bandwidth of ZnO have been reported,
including ZnO NWs doping with trivalent elements like Al, Ga and In24, 25 and co-growth with narrow
bandgap (2.0 eV~2.4 eV) materials CdS or Cu2O nanostructure26, 27. All these methods need complicated facilities
or demand carefully chemical synthesis process control, and still to be developed for practical application. Dye
sensitization provides an alternative method to widen the material response spectrum. It was firstly proposed by
O?regan B28 in 1991, and then reportedly used in solar cells to enhance its efficiency29?31. The attractions to this
method are its cost effectiveness and easy process handling, regardless of longtime material stability issues. The
possible breakthrough of this method is using novel low toxicity organic dye material and function material
combination for special potential application such as photodetectors.
In this work, we report a novel broadband photodetector using ZnO NWs surface-modified by an organic dye
called Rhodamine B (RhB) as function materials. We begin by introducing the fabrication process and
measurement methods. The active area of the broadband photodetector consists of pre-fabricated interdigital electrodes
covered by dispersive ZnO NWs, which are sensitized by dye molecule. And then, we present and discuss our
experimental results. Optic-electronic measurements show that photodetectors made of RhB-sensitized ZnO
NWs have a wide response spectrum from 300 nm to 700 nm with a high responsivity and bandwidth as large
as 400 nm. A model was established to explain the mechanism of broadening response spectrum of ZnO/RhB
hybrid system, in which carriers transport path and photoelectric property are greatly affected by the surface
modification of dye and the surface defect states of ZnO NWs. The optoelectronic and time-dependent photo
response properties of the photodetector were measured at certain UV and VIS wavelength, respectively. Based
on the measurement and the model, further calculations of responsivity and the external quantum efficiency
(EQE) were performed. An increase of approximate 2 to 26 times in the responsivity for VIS light is obtained after
sensitization, which corresponds to a 771% increment of the EQE at the maximum point in VIS band. Meanwhile,
the EQE in UV band is also enhanced with a maximum value of 51%. From the time?dependent photo-current
measurement, it shows that the response time (rise and decay times in total) of the device is significantly reduced
from 17.5 s to 3.3 s for UV and and 16 s to 4.3 s for VIS light by comparison with the devices made of only pure
ZnO NWs, respectively. Finally, we conclude that the demonstration of the as-fabricated device with a broad
bandwidth and enhanced responsivity envisions a feasible and cost-effective method to employ organic sensitizer
for the broadband photodetector.
Results and Discussion
The fabrication process of the device is depicted schematically in Fig.?1. The device can be fabricated by spraying
NWs on to gold interdigital electrodes followed by modifying NWs via an RhB solution-casting process. The
specific details are described in the method section.
The morphology analysis and material characteristics of ZnO NWs are shown in Fig.?2. Figure?2(a) shows the
SEM image of ZnO NWs before sensitization. The measured mean diameter of ZnO NWs is about 100nm and
the morphology of porous is beneficial to dye penetration. The ZnO NWs can interpenetrate each other densely
and form stacked network structures to bridge the gap between adjacent interdigital electrodes. The SEM image of
ZnO NWs after sensitization is shown in Fig.?2(b). Dye combined with NWs well without agglomeration and all
of the morphology, uniformity, and distribution have not been changed. As shown in the picture, the
photosensitive film is composed of NWs infiltrated by the crystallization of sensitized-dye. Figure?2(c) shows an enlarged
TEM image, which reveals the structure of single RhB-coated ZnO NWs clearly. Compared with the pure ZnO
NWs with a diameter of 107 nm displayed in the inset, the faint bright cladding circled the single-NWs may be
crystalline RhB. It is obvious that both ZnO NWs and ZnO NWs/RhB hybrid photosensitive film are uniform,
compact and densely packed. The XRD pattern of ZnO NWs and the ZnO NWs/RhB hybrid system were
characterized by XRD (Cu K?, ? = 0.15406 nm), which is shown in Fig.?2(d). All the sharp diffraction peaks can be well
indexed to hexagonal ZnO NWs with wurtzite structure. By comparing the intensity of these diffraction peaks,
the introduction of RhB slightly decreased the crystallinity of ZnO NWs and two relatively weak diffraction
signals of RhB appeared.
The absorption spectra (from 200 to 700nm) and photoluminescence (PL) spectra are plotted in Fig.?3(a). The
result indicates that RhB in the hybrid system has a significant role in broadening the absorption spectrum. An
obvious cut off phenomenon of ZnO NWs film at nearly 400nm appears while the effective absorption peak of
RhB concentrates ranging from 450 nm to 600 nm approximately.
The low-temperature (78K) photoluminescence of the as-prepared ZnO NWs and ZnO NWs/RhB hybrid
films were measured with an excitation power of 0.32 W/cm2, as shown in Fig.?3(b). Non-radiative
recombination can be suppressed significantly under low temperature, while a slight red shift of the emission peaks may be
inevitable cause of the contraction band phenomenon. There are three emission peaks: the ultraviolet emission
at 373 nm should correspond to the intrinsic transition between the band gap of ZnO, a weak shoulder peak at
382 nm is the first-order phonon line. The blue-green strong wide VIS emission with the center wavelength of
approximately 495 nm and full width at half maximum of 95 nm is possibly associated with deep-level defects and
the singly ionized oxygen vacancy in ZnO, resulting from the recombination of photo-generated hole with the
single ionized charge state of this defect32. As the result of sample infiltrated by RhB shows, a new wide emission
with the peak wavelength of 602 nm appears.
The quantitative measurements of the response current under different specific wavelengths were conducted
by the semiconductor characterization system (Keithley 1500-SCS) under dark condition, illuminated by UV light
(peak wavelength 350 nm, with power 2.4 mW) and VIS light (peak wavelength 650 nm with power 2.95 mW).
The current-voltage (I-V) characteristics of the photodetectors with or without dye sensitization are shown in
Fig.?3(c). The measurement results prove that the obvious role of dye sensitization in increasing responsivity to
VIS light band is effective. The time-dependent measurements of photo-response were employed to research the
rise and decay time with excitation (power: 2.4 mW @350 nm, 2.8 mW@550 nm, 2.95 mW@650 nm) switched on
and off for the same time intervals at 8 V bias. As shown in Fig.?3(d), the photocurrent could be reversibly
modulated by irradiation, increase rapidly and remain stable under the steady illumination at different wavelengths.
Drift in the dark current regarding time was observed, which can be explained by persistent photoconductivity
The mechanism to explain the photosensitive characteristic of ZnO NWs and the amazing effect of dye
molecules in broadening light responsivity spectrum are shown in Fig.?4, revealing the illustrative schematic of pure
ZnO NWs and dye-sensitized ZnO NWs. As shown in Fig.?4(a), Oxygen molecules are naturally adsorbed onto
the surface of ZnO NWs due to the existence of surface dangling bonds and free electrons from NWs are then
captured in dark conditions, leading to band bending, which can be expressed in equation (
). Therefore, a
depletion layer with low conductance and a high barrier comes into form near the surface of ZnO NWs, resulting in a
low dark current. Figure?4(b) describes the condition in illumination with UV light. Driven by the internal
potential, photogenerated holes are migrated to the surface of NWs and captured by the surface hole-traps, leading to
a reduction of the depletion barrier thickness and the height of the barrier. The reaction process of desorption
from oxygen adsorbents is shown in equation (
). In the meantime, the unpaired photo-generated electrons are
pumped to conducting band from valence band, which increases the conductivity of ZnO NWs and generates a
h+ + O2?(ad) ? O2(g)
This hole-trapping mechanism through the adsorption and desorption of oxygen molecule in ZnO NWs
accounts for the enhancement of the photo-response for UV light, but the responsivity decreases with the
increment of wavelength especially above 500 nm. In our research, dye RhB was adopted as the sensitizer in order to
alter the transfer pathway of the photo-generated carriers. The dye-sensitized mechanism is displayed in Fig.?4(c),
electrons are activated from the highest occupied molecular orbital (HOMO ~ ?5.3 eV) to the lowest unoccupied
molecular orbital (LUMO ~ ?3.2 eV) after the assimilation of low-frequency photons corresponding to a lower
energy band of VIS light, which subsequently injected into the bottom of the conducting band (Ec ~ ?4.2 eV) of
ZnO. At the same time, holes migrate from the top of the valence band (Ev ~ ?7.6 eV) of ZnO to HOMO of RhB.
Unavoidably, the carriers transfer between the HOMO of dye and the Ec of ZnO may slightly increase the dark
current after sensitization.
The responsivity as well as EQE among a wide bandwidth of the photodetector were further studied and
analyzed. The responsivity and its improvement of the broadband photodetector as a function of wavelength at
8 V bias are shown in Fig.?5(a). The responsivity is used to describe the photoelectric conversion capability of
the photodetector, which is expressed as R = I/P, where R is responsivity, I is photocurrent and P is incident light
power. The measurement results indicate that the responsivity rises firstly and then decreases with the increase
of wavelength. Compared with the device with only ZnO NWs, an extreme increase on responsivity (maximum
3 A/W at 550 nm) at VIS band has been achieved for our broadband photodetector modified and sensitized by
the dye. Specifically, the responsivity of the device with ZnO NWs decreases sharply as the increase of wavelength
and shows incredibly low response characteristic above 450 nm. As displayed, an improvement of approximate 2
to 26 times in responsivity to VIS and a maximum 51% growth to UV were obtained.
EQE is equal to responsivity multiplied by photon energy described as EQE = R ? hc/e?, where R is
responsivity calculated above, h is Planck constant, c is the velocity of light, e is the charge of an electron and ? is the
wavelength of incident light. Figure?5(b) manifests that an obvious improvement on EQE (maximum up to 771% for
VIS light) can be observed for this broadband photodetector based on dye-sensitized ZnO NWs film, illustrating
the excellent response characteristics of our novel device both in UV and VIS bands.
Besides responsivity and EQE, the specific detectivity is another one crucial figure-of-merits for the
photodetector, which is usually used to describe the minimal detectable signal33.
s ? ?f
where s is the effective area of the detector with the unit of cm2, ?f is bandwidth, NEP is the noise equivalent
power, in2 is the measured dark current and R? is the responsivity. In particular, at the operating bias of 8 V, the
D* of the photodetector was 2.34 ? 1011 cmHz1/2W?1. The results imply that our photodetector has a potential for
wide spectrum detection with simpler process and high performance.
Furthermore, the use of RhB decreases the response time by reducing the recombination of photo-generated
carriers prominently. According to Figs?3(d), 6(a) and (b) show the details on rising and decay time curve of the
broadband photodetector at two kinds of representative wavelength with similar light power density at 8 V bias.
The rise time (tr) is defined as the range that photocurrent rises from 10% to 90% of its maximum. The decay
time (td) is defined similarly. As shown in Fig.?6, the response time (the sum of the rise time and decay time) of
the device with dye sensitization significantly reduced from 17.5 s to 3.3 s for UV light and 16 s to 4.3 s for VIS
light, respectively, by comparison with devices with pure ZnO NWs only. More in details, the UV photocurrent
rise quickly and reach saturation with the tr of 1.5 s, while the photocurrent gradually decreased and recovered
to the initial state with the td of 1.8 s. Rise and decay time of the same order were demonstrated when using VIS
incident wavelength, showing a tr of 2 s and td of 2.3 s respectively. But without ignorance, an exciting reset time
(less than 800 ms) of ZnO NWs UV photodetector has been achieved by utilizing Schottky contact and surface
functionalization with polymers34.
Figure?7 exhibits the performance comparison of different ZnO based photodetectors by plotting the peak
responsivity as a function of detection bandwidth. A result of a 400 nm response bandwidth at least and 5.5 A/W
peak responsivity was realized by our broadband photodetector based on dye-sensitized ZnO NWs film with a
simple fabrication process. As displayed in above diagram, although a ZnO-based photodetector with high (about
460 A/W) responsivity has been achieved before, its response bandwidth is about 200 nm. Other ZnO-based
photodetectors have their own advantages in terms of bandwidth and responsivity severally. More works need
continued to further promote the responsivity of our device in the future.
In conclusion, a novel broadband photodetector based on dye-sensitized ZnO NWs film has been proposed,
fabricated and discussed. The RhB sensitization mechanism has been investigated. Through the sensitization
mechanism, it can be beneficial to increase the responsivity and EQE for VIS light, improves UV responsivity,
and also optimize the time resolved characteristics. The research results show that the broadband photodetectors
sensitized by the organic dye RhB molecules exhibit a prominent photo-response for a broadband light (300
to700 nm) with the approximate responsivity of 5.5 A/W to UV light and maximum 3 A/W to VIS light at 8 V
bias, respectively. By comparison with other ZnO-based photodetectors which have been reported, the response
bandwidth of our device reaches up to 400 nm. According to the results of this work, the demonstrated method
may be a promising, simple, cost-effective, and feasible technological means to realize broadband photo
detection. More theoretical and experimental studies are necessary to improve the performances of our device further.
Device fabrication. The ZnO/RhB hybrid broadband photodetector was fabricated via the following
processes in an ultra-clean environment. An N-type (100) oriented silicon wafer was used as the substrate, and a
300 nm thick silicon dioxide (SiO2) dielectric layer was thermally grown on the substrate for insulation
protection. The Au interdigital electrodes with same finger width and gap width of 50 ?m were fabricated by standard
photolithography, Titanium/Gold (Ti/Au, 20 nm/100 nm) RF sputtering, and the following lift-off process. Then
the ZnO NWs suspension was sprayed on interdigital electrodes to form a uniform photosensitive layer. The
dispersion solution was prepared by dispersing ZnO with an approximate average diameter of 100 nm in
ethanol with the ratio of 1 g (NW): 400 ml (E) achieving a concentration of 2.5 g/L and then ultra-sonicated for 2 h
before using. Then the interdigital electrodes coated with ZnO NWs was heated at 60?C for 30 s and annealed in
N2 atmosphere at 350 ?C for 8 min to achieve a better ohmic contact between metal electrodes and ZnO NWs.
Finally, the RhB solution with a concentration of 0.1 mg/ml was dropped onto the ZnO NWs under the
background of heating at 80 ?C for 30 s. The illuminated active area of the device is 5? 3 mm2, and the thickness of
ZnO NWs film is about 1 ?m.
Measurement details. The morphology of photosensitive materials is characterized by scanning electron
microscope (SEM) and transmission electron microscope (TEM). The crystalline structure of ZnO NWs and
ZnO NWs/RhB hybrid film were examined by X-ray diffraction (XRD) with scanning degree ranging from 25?to
85?. The absorption spectra of ZnO NWs and ZnO NWs/RhB hybrid films were recorded by the photometer. The
low-temperature photoluminescence (PL) measurement was performed at 78 K using 325-nm He-Cd laser as the
Current?voltage (I-V) characteristic curve was recorded by sweeping the bias voltage from 0 V to 10 V across
the electrodes using the Keithley 1500SCS semiconductor characterization system. All devices were kept in the
darkroom for more than 24 h to stabilize before measuring dark current. The photocurrent was tested by
illuminating the active area with discrete wavelengths through a monochromator (Triax190) from a Xenon Light
Source at 8 V bias. All measurements were performed in the atmosphere at room temperature except for the
This work was supported by Natural Science Foundation of China (Grant Nos 61273061).
Zheng Qi Bai completed all of the measurements, wrote the main manuscript text and prepared all the figures
independently. Ze Wen Liu is the corresponding author for this paper. All authors reviewed the manuscript.
Competing Interests: The authors declare that they have no competing interests.
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