High performance organic-inorganic perovskite-optocoupler based on low-voltage and fast response perovskite compound photodetector

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Organic-inorganic hybrid photodetectors attract considerable attention because they can combine the advantages of both organic and inorganic systems. Here, a perovskite compound with a broad absorption spectrum and high power conversion efficiency is used as a photosensitive layer in an organic/inorganic hybrid heterojunction photodetector with a high and fast response. The high sensitivity exceeding 104 is obtained at bias of 0–4 V. Using a tandem organic light-emitting diode (OLED) as the light source, we fabricated an optocoupler device. The optocoupler achieved a maximum photoresponsivity of 1.0 A W−1 at 341.3 μWcm−2 at an input voltage of 6 V. The device also exhibits rapid response times of τrise ~ 20 μs and τfall ~ 17 μs; as well as a high current transfer ratio (CTR) of 28.2%. After applying an amplification circuit, the CTR of the optocoupler increases to 263.3%, which is comparable with that of commercial inorganic optocouplers. The developed hybrid optocoupler thus shows great promise for use in photonics.

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SUBJECT AREAS:

OPTOELECTRONIC

DEVICES AND
COMPONENTS

OPTICAL SENSORS

Received
29 September 2014

Accepted
18 December 2014

Published
20 January 2015

Correspondence and

requests for materials

should be addressed to

[G.D. (donggf@mail.](mailto:donggf@mail.tsinghua.edu.cn)

[tsinghua.edu.cn)](mailto:donggf@mail.tsinghua.edu.cn)

## High performance organic-inorganic perovskite-optocoupler based on low-voltage and fast response perovskite compound photodetector

Dong Li, Guifang Dong, Wenzhe Li & Liduo Wang

Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing 100084, China.

Organic-inorganic hybrid photodetectors attract considerable attention because they can combine the
advantages of both organic and inorganic systems. Here, a perovskite compound with a broad absorption
spectrum and high power conversion efficiency is used as a photosensitive layer in an organic/inorganic
hybrid heterojunction photodetector with a high and fast response. The high sensitivity exceeding 10 4 is
obtained at bias of 0–4 V. Using a tandem organic light-emitting diode (OLED) as the light source, we
fabricated an optocoupler device. The optocoupler achieved a maximum photoresponsivity of 1.0 A W 21 at
341.3 mWcm 22 at an input voltage of 6 V. The device also exhibits rapid response times of trise, 20 ms and
tfall, 17 ms; as well as a high current transfer ratio (CTR) of 28.2%. After applying an amplification circuit,
the CTR of the optocoupler increases to 263.3%, which is comparable with that of commercial inorganic
optocouplers. The developed hybrid optocoupler thus shows great promise for use in photonics.

ptocouplers are important photoelectric devices with fine anti-jamming ability for noise control in signal
communication and detection 1–3 . Structurally, an optocoupler is composed of a light emitter as input and
a photodetector (PD) as output. The input signal drives the light source and converts the electrical signal
# O
into an optical signal that is then reconverted into an electrical signal by the PD unit. Because the input and output
are not common ground, potential fluctuationit of the imput does not affect the output potential, and the load
fluctuation and noise of the output do not influence the input signal. Meanwhile, because an optocoupler transfers
electrical signals using light in a coupling medium to prevent electrical contact of its input and output, it is also an
excellent isolating device that can tolerate very large isolation voltages. For example, an optocoupler can control a
high-potential circuit of several thousand volts using a low-potential signal of less than 10 V. Therefore, the
optocoupler is an attractive prospect to realize both electrical isolation and signal conversion. However, as the
most important part in optocouplers, most PDs must apply a certain operating voltage (1–100 V), whick
currently maks the overall circuitry of optocouplers uneconomical. Therefore, the ‘self-powered’ or ‘low-voltage’
PDs, which can generate built-in potential for their own operation and do not need a high external applied
operating voltage, have attracted considerable attention 4 . Here, PDs with p-n or p-i-n junctions play an important
role because they have superiorities in photosensitivity and frequency response characteristics, and more importantly, they can operate in photovoltaic mode without a high external operating bias 5–7 .

Recently, with the development of organic materials, research on organic optoelectronic devices is burgeoning
because of merits including low cost, low process temperature, ease of large-area fabrication and flexibility 7,8 .
However, these device suffer from drawbacks such as relatively low carrier mobility and efficiency. The first polymer
organic optocoupler (OOC) was reported in 1994 by Yu and Heeger 9 . The current transfer ratio (CTR) of this device
was 0.2%. More recently, our group reported an OOC based on small molecules with a CTR of 0.17% 2 . At present, the
highest CTR of the OOC is 5.4%, which was achieved using a high-efficiency organic light-emitting diode (OLED)
and bulk heterojunction organic PD 10 . On the other hand, inorganic PDs 11,12 exhibit high carrier mobility and
absorption compared with their organic counterparts, but they have disadvantages such as high fabricating temperature and low flexibility. Therefore, organic-inorganic hybrid-PDs that combine the advantages of both are desired 4,13 .

Here, due to the broad absorption spectrum and high power-conversion efficiency 14–21, we use the perovskite
compound as the photosensitive layer in an organic/inorganic hybrid heterojunction PD with a high and fast

response. Inorganic/organic perovskite compounds such as methylammonium lead iodide (CH3NH3PbI3) have been widely used as
light harvesters in solar cells 14–19 because of their broad absorption
spectrum and high carrier mobility 20 . Due to their large short-circuit
current density (16.5 mA cm 22 ) and open-circuit voltage (0.997 V) 14,
hybrid PDs containing CH3NH3PbI3 have great potential to produce
optocouplers with large CTRs that can operate as ‘self-powered’ or at
‘low-voltage’. PDs with high responsivity and flexibility have been
reported 22,23, however, they exhibited very slow response times of
longer than 0.02 s, which limits their use in applications that require
a rapid frequency-response.

Here, we report a fast-response perovskite-optocoupler based on a
white tandem OLED light source and low-voltage organic/inorganic
hybrid-PD using CH3NH3PbI3 as a light harvester, p-type 2, 29, 7, 79tetrakis-(N,N-di-p-methoxyphenylamine)-9, 99-bifluorene(spiroMeOTAD) as a hole-transport material (HTM) and n-type mesoporous titania (mp-TiO2). The device shows ‘self-power’ property
operating at a zero-applied field, with a photoresponsivity of 284
mA W 21 at 0 V bias. The optocoupler device exhibits a maximum
photoresponsivity of 1.0 A W 21 at 341.3 mWcm 22 (VOLED 5 6 V)
and 3.8 A W 21 at 15.9 mWcm 22 (VOLED 5 5 V), which is higher than
that of many commercial inorganic PDs 24 . A maximum CTR of
28.2% is obtained, which is much higher than that of other OOCs
reported. By adding an amplification circuit, the optocoupler exhibits a maximum CTR of 263.3%, comparable with the commercial
inorganic ones (usually 50%–200%). The optocoupler also shows
rapid frequency-response with the response time on the order of
microseconds. The fast photoresponse and high photosensitivity
of this low-voltage hybrid-PD, make it, as well as the perovskiteoptocoupler, very promising candidates for practical use as photoelectric devices.

Results and Discussion
A CH3NH3PbI3 film was prepared by spin-coating PbI2 solution on
the mp-TiO2 substrate that was then dipped into CH3NH3I solution.
Figure 1a shows a cross-sectional micrograph of the CH3NH3PbI3
PD device taken by a scanning electron microscope (SEM). It is
apparent that the pores of mp-TiO2 film are infiltrated with
CH3NH3PbI3. Figure 1b presents a surface SEM image of the
CH3NH3PbI3 overlayers on top of the mp-TiO2 film. This image
shows that several nanometre-sized CH3NH3PbI3 islands form densely on top of mp-TiO2 film. The CH3NH3PbI3 domains do not
appear to be continuous, however, there should be large contact areas
for the interface between the CH3NH3PbI3 and HTM overlayers,
which should allow effective extraction of charge carriers to the
electrode. The CH3NH3PbI3 film coated on mp-TiO2 was characterized by X-ray diffraction (XRD). The diffraction pattern of
CH3NH3PbI3 (Figure S1) contains peaks at 14.21u, 28.57u and
31.91u, corresponding to the (110), (220) and (310) planes, respectively. It confirms the formation of a tetragonal perovskite structure
with lattice parameters of a 5 b 5 8.88 A ˚ and c 5 12.68 A ˚, which are
consistent with previous reports 15,16 .

Figure 1c depicts the schematic diagram of the device structure
based on the SEM analysis (Figure 1a), and Figure 1d shows the
corresponding energy level diagram. Under irradiation, when the
light energy is larger than the bandgap of CH3NH3PbI3, many excitons are photogenerated in the perovskite layer, and then dissociate
at both the TiO2/CH3NH3PbI3 and CH3NH3PbI3/HTM interfaces,
because CH3NH3PbI3 can act as both n- and p-type semiconductors 14 and form junctions with different types of neighboring
semiconductors. Besides, under an applied external electric field with
the same direction as that of the built-in electric field at the heterojunction interface, the photogenerated excitons could also be sepa

Figure 1 | Structure of the perovskite photodetector. (a) Cross-sectional SEM image of the CH3NH3PbI3 PD. (b) SEM image of the surface of
CH NH PbI overlayers on top of mp TiO film (c) Schematic diagram of the structureof the PD (d) Energy level diagram of the PD

rated in the CH3NH3PbI3 layer, and then the photogenerated carriers
can diffuse to the TiO2/CH3NH3PbI3 and CH3NH3PbI3/HTM interfaces. The lowest unoccupied molecular orbital (LUMO) level of
CH3NH3PbI3 (23.93 eV) is higher than the conduction band energy
level of TiO2 (24.0 eV) and the highest occupied molecular orbital
(HOMO) level of CH3NH3PbI3 (25.43 eV) is lower than that of
HTM (25.22 eV) (Figure 1d), so the electrons are injected into the
TiO2 conduction band and simultaneously the holes are transferred
to spiro-MeOTAD. It is clear that the favorable alignment of the
energy levels of these semiconductors make them suitable candidates
for PDs.

Further advantages of CH3NH3PbI3 are that it exhibits a long
electron lifetime and has a diffusion distance of nearly 500 nm (see
Supplementary Information and Figure S2), which is much higher
than that of most organic materials (usually less than 50 nm). The
carrier lifetime measured by intensity modulated photovoltage spectroscopy (IMVS) was in the order of 10 23 s, which is much longer
than that of many organic semiconductors (typically 10 27,
10 24 s) 25,26 . From the IMVS plot together with the intensity modulated photocurrent spectroscopy (IMPS) measurement, the electron
diffusion distance has an average value of 405 nm. As we know, thick
layers are generally used in devices to increase photon absorption,
but the relatively short diffusion distance of most semiconductors
can result in the recombination of many photogenerated carriers,
which seriously limits the increase of photocurrent. Here, the long
electron lifetime and diffusion distance of CH3NH3PbI3 mean that
most photogenerated carriers can be utilized to produce photocurrent. Therefore, we can effectively improve the light absorption
and exciton dissociation, thereby improving the performance of this
hybrid-PD.

We further investigated the spectral response and calculated the
wavelength-dependent responsivity of the device from the external
quantum efficiency (EQE) using the expression R(l) 5 EQE/hn,
where hn is the energy of the incident photon in electron volts
(eV). The absorption spectrum of CH3NH3PbI3 and incident
photon-to-electron conversion efficiency (IPCE) action spectrum
of the perovskite device are shown in Figure 2. The device exhibits
a broad photoresponse range from 400 to 750 nm, and maximum
responsivity of 284 mA W 21 at 490 nm and 0 V PD bias, which is
higher than some commercial PDs and previously reported self-powered
ones 4–6,12,13 . High photoresponse at 0 V bias is also obtained under
the illumination with a solar simulator (Supplementary Information
Figure S3). The different work functions of the electrodes (see
Figure 1d) mean that there could be parasitic injection of holes
and/or electrons from the electrodes 27,28, which may be why the
minimum dark current is observed at,20.5 V.

We coupled this perovskite hybrid-PD with an OLED to fabricate
an organic-inorganic perovskite optocoupler (Figure 3a). Here, the
OLED is the input component and the PD is the output component.
CTR, the ratio of output current density to input current density, is
the primary parameter to evaluate the efficiency of an optocoupler 1 .

In general, CTR can be described as:

CTR~ Jout

Jin

ð1Þ

Figure 2 | Spectral properties of the perovskite photodetector. (a) IPCE
spectrum and responsivity of the PD from 400 to 800 nm. (b) Absorption
spectrum of the CH3NH3PbI3 layer on top of mp-TiO2 film.

From Equation (1), for the input component, two important factors should be taken into consideration to achieve a high CTR. The
input component should have a high current efficiency (CDA), and
its electroluminescence (EL) spectrum should match the absorption
of the output component so that the output component obtains high
responsivity (R). The absorption bands of CH3NH3PbI3 are located
in the visible region from 400 to 750 nm (Figure 2b), so a tandem
white OLED with high current efficiency (44.0 cd A 21 ) and EL spectrum from 470 to 650 nm (Figure S4) was used as the input component. The active layers of the input component were thermally
evaporated sequentially and then encapsulated in N2 (Supplementary
Information).

We measured the current density (J)-voltage (V)characteristics of
the output component under different input voltages (Figure S5a). A
maximum photoresponse of 1.0 A W 21 at 341.3 mWcm 22 (Vinput 5
6 V) was observed as shown in Figure 3b, and a photoresponse of
3.8 A W 21 was demonstrated at 15.9 mWcm 22 (Vinput 5 5 V). The
highest detectivity value of 3.29 3 10 12 cm Hz 0.5 W 21 (Jones) is
obtained because of the high photocurrent and low dark current in
this device (Figure S5b).

The output-voltage-dependence of the CTR is presented in
Figure 3c. The maximum CTR is 28.2% at an output bias of 2–4 V
and input bias of 6 V, which is much higher than that of reported
OOC devices and comparable with that of inorganic ones 10,29 . The
transfer characteristics of the perovskite optocoupler are shown in
Figure 3d. The output current density is observed to be proportional
to the input current density over about three orders of magnitude

g OLED IQE

OLED IQE ðlÞ : goutðlÞ : ginðlÞ : g PD IQE

PD IQEð l Þdl

where g OLED IQE

OLED IQE ðlÞ and g PD IQE

where g OLED IQE ðlÞ and g PD IQEð l Þ are the single wavelength internal

quantum efficiency (IQE) of the OLED and PD at wavelength l,
respectively. gout(l) is the outcoupling efficiency of the OLED, and
g (l) is the incoupling efficiency of the PD

Figure 3 | Structure and properties of an organic/inorganic hybrid optocoupler. (a) Schematic diagram of the organic-inorganic hybrid optocoupler
device with a tandem OLED as the input light source and a perovskite PD as the output component. (b) Responsivity of the CH3NH3PbI3 perovskite PD
under the illumination of the tandem OLED with different input voltages (and thus different light intensities). (c) CTR of the optocoupler device under
different input OLED voltage and applied PD bias. (d) The output current density of the CH3NH3PbI3 perovskite PD under different OLED input current
densities. (The dots are a visual guide.)

suggesting that the perovskite optocoupler would be capable of
working in analog circuits. After applying an amplification circuit
in this optocoupler, a maximum CTR of 263.3% was obtained
according to Figure 4. CTR is magnified by about 10 times at an
applied bias of 3 V, while there is not any change in CTR at the
applied bias of 0 V after inclusion of an amplification circuit because
of the invalidation of the amplification circuit at zero bias.

Commercial inorganic optocouplers usually exhibit the CTR value
of 50, 200%, so this sort of perovskite optocoupler shows promise
as an alternative to commercial inorganic optocouplers in the applications such as image sensors and low-voltage-control circuit in an
intelligent robot 3 . These are the most important parts of a modern
electrical control system, because they avoid operation risk and allow
easy automation. Compared with commercial inorganic optocouplers, the perovskite optocoupler should have the superiorities in
lower cost, lower process temperature, easier large-area fabrication
and increased flexibility.

Response time is another significant parameter for high-speed
optoelectronic applications Figure 5 shows the frequency response

characteristics of the optocoupler corresponding to response time at
an input signal of 5 kHz. As designed, the phases of the output and
input signals are opposite, which proves that the optical signal is
successfully transported. The rise time (trise) and fall time (tfall) are
defined as the times taken by the signal to vary between 10% and 90%
of the saturation value. With a square-wave voltage applied as the
input signal and a constant voltage applied on output circuit, the
amplitude attenuation could be read out under different input frequencies. A clear saturation is observed in the transient signal with
trise, 20 ms and tfall, 17 ms for a device area of 16 mm 2 . The cutoff frequency is 40 kHz and the output signal is capable of following
the input signal at a modulation frequency of 200 kHz despite the
phase delay (Figure S6b).

Conclusions
In summary, we developed a PD composed of CH3NH3PbI3 with the
advantages of low-voltage operation, fast response and broad
absorption spectrum (from 400 to 750 nm). Combining this PD
exhibiting high photoresponsivity (1 0 A W 21 @ 341 3 mWcm 22 )

Figure 4 | Influence of an amplification circuit on the performance of the
optocoupler. CTR of the optocoupler device under different applied PD
bias with and without an amplification circuit. Inset: Circuit scheme of the
optocoupler applying the amplification circuit.

FTO substrate at 7000 rpm for 30 s, followed by heating at 500uC for 1 h. PbI2 in DMF
(Aldrich, 99.9%) was heated to 60uC for 2 h under magnetic stirring and then spin
coated on the as-prepared TiO2 film at 5000 rpm for 60 s in a glove box. The films
were dried at 70uC for 30 min. Subsequently, the dried films were dipped into a
solution of CH3NH3I in for 120 s and rinsed with 2-propanol. The films were heated to
70uC for 30 min. The dark red CH3NH3PbI3-coated TiO2 films were covered with
HTM solution by spin-coating method at 4000 rpm for 30 s. 60 nm-thick Au counter
electrode was deposited on the top of the HTM over layer by a thermal evaporation.
The active area was fixed at 4 3 4 mm 2 . The thickness of TiO2, CH3NH3PbI3 and
HTM films was 500, 200, and 80 nm, respectively.

Characterization. The UV-vis absorption spectra were used for testing the
absorption of perovskite sensitized TiO2 film with an absorption spectrometer
(Hitachi U-3010). XRD patterns were obtained with a diffractometer (Smart LAB
instruments) using Cu Ka beam (l 5 1.54 A ˚ ).The electrical characteristics were
measured using a semiconductor parameter analyzer (Keithley 4200 Semiconducting
System). And Thermo AM 1.5 Solar Simulator was used to measure the
photosensitivity of PD. IPCE spectrum was measured by ZAHNER CIMPS
electrochemical workstation, Germany. To test the frequency characteristic, an input
square-wave voltage applied on the OLED was generated by a GFG-8016Gfunction
generator and the output signal of the photodiode was enlarged by an amplification
circuit made by ourselves.The photodiode was inversely triggered at the falling edge of
input signals to eliminate the disturbance of induced overshoot noise between input
and output. The main body of the optocoupler was placed in a shielding box to isolate
environmental noises. Real photocurrent signal was recorded by Tektronix DPO 4104
oscilloscope.

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circuit in this optocoupler, we improve the CTR from 28.2% to
263.3%, which is comparable with that of commercial inorganic
optocouplers. The optocoupler exhibited a rapid photoresponse with
trise, 20 ms and tfall, 17 ms for an area of 16 mm 2 . The cut-off
frequency and maximum following frequency were 40 and 200 kHz,
respectively. Our research demonstrats that perovskite optocouplers
are attractive for use in high-efficiency, rapid-response photonics.

Methods
Materials preparation. Titanium (IV) isopropoxide (Ti[OCH(CH3)2]4, 1 mL) was
mixed with 2-methoxyethanol (CH3OCH2CH2OH, 5 mL) and ethanolamine
(H2NCH2CH2OH, Aldrich, 99.8%, 0.5 mL) and then heated to 80uC for 2 h under
magnetic stirring in a glove box. Nanocrystalline TiO2 paste (18NRT) was purchased
from Dyesol and diluted to w/w 33%. PbI2 was purchased from Aldrich and dissolved
in DMF to give a concentration of 462 mg mL 21 . The CH3NH3I was dissolved in 2propanol to give a concentration of 10 mg mL 21 . The HTM was prepared from a
solution of Spiro-MeOTAD (73 mg), Li-TFSI (99.95%, Aldrich) in acetonitrile
(17.5 uL, 520 mg mL 21 ) and TBP (96%, Aldrich, 37.5 uL) in chlorobenzene (99.8%,
Aldrich, 1 mL).

Device fabrication. The TiO2 precursor was spin coated on a cleaned fluorine-doped
tin oxide (FTO) substrate at 3000 rpm for 30 s, and then heated at 500uC for 60 min to
form a blocking layer. The nanocrystalline TiO2 paste was deposited on the pre-treated

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Acknowledgments
This work was financially supported by the National Natural Science Foundation of China
(No. 61177023, 61474069 and 51273104).

Author contributions

L.D.W. conceived and designed the experiments. D.L. and W.Z.L contributed equally to
perform the experiments. W.Z.L. helped with the sample preparation. D.L., W.Z.L and
G.F.D. contributed to data analysis. G.F.D., D.L. and L.D.W. wrote the manuscript. All
authors discussed the results and commented on the paper.

Additional information
[Supplementary information accompanies this paper at http://www.nature.com/](http://www.nature.com/scientificreports)
[scientificreports](http://www.nature.com/scientificreports)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Li, D., Dong, G., Li, W. & Wang, L. High performance
organic-inorganic perovskite-optocoupler based on low-voltage and fast response
perovskite compound photodetector. Sci. Rep. 5, 7902; DOI:10.1038/srep07902 (2015).

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