Organic Optocoupler with Simple Construction as an Effective Linear Current Transceiver

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_Article_
# Organic Optocoupler with Simple Construction as an Effective Linear Current Transceiver

Jaroslaw Jung 1, * , Arkadiusz Selerowicz 1 , Jacek Ulanski 1 , Ruslana Udovytska 1 , Beata Luszczynska 1 ,
Artur Zawadzki 2 and Andrzej Rybak 2

1 Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, 116 Zeromskiego Str.,
90-924 Lodz, Poland; arkadiusz.selerowicz@dokt.p.lodz.pl (A.S.); jacek.ulanski@p.lodz.pl (J.U.);
ruslana.udovytska@p.lodz.pl (R.U.); beata.luszczynska@p.lodz.pl (B.L.)
2 ABB Corporate Technology Center, 13A Starowislna Str., 31-038 Krakow, Poland;
artur.zawadzki@pl.abb.com (A.Z.); andrzej.rybak@pl.abb.com (A.R.)

* Correspondence: jaroslaw.jung@p.lodz.pl

Academic Editors: Madhav Baral and

Charles Lu

Received: 6 December 2024

Revised: 24 December 2024

Accepted: 30 December 2024

Published: 2 January 2025

Citation: Jung, J.; Selerowicz, A.;

Ulanski, J.; Udovytska, R.;

Luszczynska, B.; Zawadzki, A.; Rybak,

A. Organic Optocoupler with Simple

Construction as an Effective Linear

Current Transceiver. _Materials_ 2025,

_18_, 152. [https://doi.org/10.3390/](https://doi.org/10.3390/ma18010152)

[ma18010152](https://doi.org/10.3390/ma18010152)

Licensee MDPI, Basel, Switzerland.

This article is an open access article

distributed under the terms and

conditions of the Creative Commons

Attribution (CC BY) license

[(https://creativecommons.org/](https://creativecommons.org/licenses/by/4.0/)

[licenses/by/4.0/).](https://creativecommons.org/licenses/by/4.0/)

Abstract: In this study, it is shown that an efficient organic optocoupler (OPC) can be
fabricated using commercially available and solution-processable organic semiconductors.
The transmitter is a single-active-layer organic light-emitting diode (OLED) made from a
well-known polyparavinylene derivative, Super Yellow. The receiver is an organic lightemitting diode (OLSD) with a single active layer consisting of a mixture of the polymer
donor PTB7-Th and the low-molecular-weight acceptor ITIC; the receiver operates without
an applied reverse voltage. OLED and OLSD have the same geometry and simple structure
without any interlayers: glass/ITO/PEDOT:PSS/(active layer)/Ca/Al; the OPC is formed
by OLED and OLSD which adhere tightly to each other. Despite its simple structure,
the OPC showed a current transfer ratio of 0.13%, good linearity, and good dynamic
performance: a three-decibel cutoff frequency of 170 kHz and response times to a step
change in current at the OPC input of 2 µs. Compared to most organic OPC devices
with similar performance parameters, where the transmitter and receiver have complex
structures with additional interlayers between the active layers and electrodes and the need
to apply a reverse voltage to the receiver, the simple design of our OPC reduces the number
of fabrication steps and greatly simplifies the device fabrication process.

Keywords: organic optocoupler; short circuited organic photodetector; DC and AC properties

1. Introduction

An optocoupler is a device that can be used to transmit alternating signals between
galvanically separated electrical circuits [1,2]. In real organic optocouplers (OPCs), the
ability to transmit an AC signal without distortion is limited [3,4]. These limitations
result from external conditions (in the case of hermetically encapsulated optocouplers, it
is mainly the operating temperature) and the materials and manufacturing technology
used [5–10]. These limitations include the three-decibel cutoff frequency ( _f−_ 3 _dB_, _OPC_ ) for the
highest harmonic of the alternative current transmitted to the receiver without amplitude
attenuation and the current transfer coefficient (CTR), which indicates the efficiency with
which the optocoupler converts the current applied to the input into the current detected at
the output.

It should be noted that even with a transmitter and receiver with very good operating
parameters, it is not always possible to design an optocoupler that can transmit AC signals
with low nonlinear distortion. To obtain an effective device, transmitters and receivers

_Materials_ 2025, _18_, 152 2 of 18

should be selected so that their light emission and absorption spectra and linear operating
ranges overlap partially [11] or completely [12,13]. In addition, additional interlayers are
often used in photodiodes to prevent current leakage when reverse bias is applied [14–17].
However, it is possible to use simpler photodetector designs and achieve good results, as
we demonstrate in this paper.

Organic optocouplers are constructed using an organic light-emitting diode (OLED) as
the transmitter and an organic light-sensitive diode (OLSD) as the receiver. OLSD devices
can operate as photodetectors with short-circuited electrodes. The active layers in the
OLED and OLSD are made of organic semiconductors that can be processed by solution
methods, making them easy to manufacture. Encapsulated OPCs exhibit relatively good
long-term stability and can transmit current signals in a wide frequency bandwidth while
maintaining good linearity [18–21]. A diagram of an optocoupler with a photodetector
with short-circuited electrodes in the receiver circuit is shown in Figure 1.

Optocoupler

Transmitter Receiver

Transimpedance

Amplifier

_ire_ ( _t_ )

_iin_ ( _t_ ) _iin,dc_ η

_m_ λ = _e_ λ

( λ )

_Ire,dc_

Dc current
compensation

_uout_ ( _t_ )

Figure 1. Schematic diagram of an optocoupler with dc _iin_, _dc_ and ac _iin_ ( _t_ ) input current sources and
a current source _ire_, _dc_ designed to eliminate the DC component from the current at the output of
the optocoupler.

In order to obtain the least distorted signals at the output of the optocoupler, it is
necessary that in the frequency range corresponding to the frequency band of the transmitted signal, the current recorded at the output ( _ire_ ) is proportional to the current supplied
to the input ( _iin_ ). For this purpose, a direct current ( _iin_, _dc_ ), is supplied to the input of the
device, so that the device works in the linear range of the current transfer characteristic.
An alternating current ( _iin_ ( _t_ )), by means of which the signal is transmitted, is added to the
constant component of current _iin_, _dc_ . The transmitter (i.e., OLED) emits alternating light
with spectral emittance _mλ_ (where _λ_ is the light wavelength) proportional to the variable
component of current _iin_ ( _t_ ). The receiver (i.e., OLSD), exposed to radiation, converts the
light energy into current _iph_ . The output circuit contains a current source for eliminating
the DC component of the receiver current ( _ire_, _dc_ ). The transimpedance amplifier (serving as
an ideal ammeter with zero internal resistance) connected to the correction circuits converts
the AC component of the receiver current ( _ire_ ( _t_ )) into voltage ( _uout_ ( _t_ )).

This paper presents an organic optocoupler with a simple structure consisting of the
combination of an OLED as a transmitter and an OLSD as a receiver, wherein the active
layers in the OLED and OLSD are fabricated by solution methods from well-known and
widely studied organic semiconductors. The advantage of the device is the single-layer
structure of both the OLED and OLSD, without interlayers. It is also important that, unlike
most known organic optocouplers [9,22,23], the devices described in this paper do not
require a reverse voltage on the receiver side. As the active layer in the OLED, a poly(paraphenylenevinylene) copolymer with the trade name SuperYellow (SY) (Figure 2a) [24–28]
was used. SY is a well-known material that emits light in a yellow color and transports
both holes and electrons. This material allows for the construction of OLEDs with a
single active layer, which greatly simplifies the device fabrication process compared to

_Materials_ 2025, _18_, 152 3 of 18

most OLED devices with a complicated multi-layer structure. The active layer of the
OLSD was a mixture of a donor and acceptor forming a bulk heterojunction. The donor
was the polymer poly([2,6 _′_ -4,8-di(5-ethylhexylthienyl)benzo [1,2-b;3,3-b]dithiophene]{3fluoro-2[(2-ethylhexyl)carbonyl]thieno [3,4-b]thiophenediyl}) with the commercial name
PTB7-Th [29–31] (Figure 2c), and the acceptor was the low-molecular-weight, non-fullerene
compound 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4hexylphenyl)-dithieno [2,3-d:2 _′_,3 _′_ -d’]-s-indaceno [1,2-b:5,6-b’]dithiophene with the abbreviation ITIC [14,32–34] (Figure 2b). The OLED and OLSD had the same architecture,
ITO/PEDOT:PSS/(active layer)/Ca/Al, and the same electrode geometry and were separated only by thin glass plates, i.e., glass supports of OLED and OLSD. It was therefore
assumed that the spectral emittance of the emitter ( _mλ_ ) was equal to the spectral irradiance
( _eλ_ ) of the receiver.

Figure 2. Materials and devices. Organic semiconductors used to produce active layer of ( a ) OLED
(Super Yellow) and of OLSD (( b ) acceptor ITIC and ( c ) donor PTB7-Th); photos of ( d ) OLED, ( e ) OPC,
and ( f ) OLSD; ( g, h ) work functions for electrodes (Al, ITO), hole-transporting layer (PEDOT:PSS),
and electron-transporting layer (Cu) and LUMO and HOMO energies of semiconductors [35,36].

Optocouplers manufactured in this way have good performance parameters, such as
a high current transfer ratio (CTR) of 0.13%, a wide frequency band (three-decibel cutoff
frequency of 170 kHz), and good linearity.

2. Materials and Methods

_2.1._ _Sample Preparation and Characterization_

Optocouplers were fabricated in two ways. One was to produce OLED devices
(Figure 2d) and OLSD devices (Figure 2f) separately on glass substrates with six indium
tin oxide (ITO) electrodes (anodes) (purchased from Ossila, S101, Sheffield, UK) and then
bond the two devices closely together. The second method was to fabricate an optocoupler
with ITO electrodes pre-patterned on both sides of the glass substrate (made by Instytut Fotonowy, Kraków, Poland) (Figure 2e). The geometry and area of the electrodes
( _S_ = 4.5 _×_ 10 _−_ 6 m 2 ) for both one-sided and two-sided covered glass were identical. Substrates were ultrasonically washed twice in acetone and then twice in isopropanol for 5 min
each time. Then, they were dried in a stream of nitrogen and then exposed to oxygen

_Materials_ 2025, _18_, 152 4 of 18

plasma for 150 s using a plasma surface treatment machine with a power of 200 W (Diener,
Atto, Ebhausen, Germany).

The hole transport layers were prepared from a commercial PEDOT:PSS solution
(purchased from Ossila, Al 4083) by spin coating them on ITO plates at a speed of 3000 rpm
for 30 s. The substrates were then heated at 120 _◦_ C for 10 min in a nitrogen atmosphere.
The active layer of the luminescent diodes was made by spin-coating (2000 rpm for 30 s)
from a solution of Super Yellow (Sigma-Aldrich, St. Louis, MO, USA) in toluene (5 mg of
polymer in 1 mL of solvent). The active layer of the OLSD was deposited from a mixture
of ITIC and PTB7-Th in a mass ratio of 1:1. The total concentration of the solution was
20 mg of ITIC:PTB7-Th mixture per 1 mL of chloroform. Similar to OLED, the active layer
of the OLSD device was prepared by spin coating (2500 rpm, 60 s). A scheme of the layer
structure of the optocoupler is presented in Figure S1 in Supplementary Information.

The thicknesses of the prepared layers, measured using a Bruker DEKTAK XT needle
profilometer (Billerica, MA, USA), were as follows: PEDOT:PSS-30 nm, SY-70 nm, and
ITIC:PTB7-Th-150 nm. Finally, the metallic shared cathode was vacuum-evaporated on
all six devices. This electrode was composed of a 10 nm layer of calcium evaporated
directly on the surface of the active layer and a 100 nm thick protective layer of aluminum.
The samples were prepared in a glovebox system with a controlled nitrogen atmosphere
and then encapsulated to protect the organic layers from oxygen and moisture using an
adhesive, non-destructive matrix encapsulation epoxy (Ossila, E132). Curing of epoxy resin
was achieved by UV illumination at a wavelength of about 350 nm for 2 min. The resin in
conjunction with a glass coverslip provided a robust barrier, ensuring an extended service
life during the measurement and storage of samples.

_2.2._ _Optical and Electrical Properties of Devices_

DC Characterization
The DC current–voltage characteristics of OLSD and OLED were collected using a
Keithley 2400 source measurement unit. The photocurrent was measured under OLSD illumination with the use of a solar simulator (AM1.5, 1000 Wm _−_ 2, LOT Oriel, LS 0110-1000).
The density of radiant flux per unit area of OLED were controlled with homemade equipment consisting of an Ocean Optics spectrometer (365–900 nm), the Keithley 2400 sourcemeter with a computer, and appropriate software. The external quantum efficiency ( _EQE_ )
spectra for OPDs were determined using a photoelectric spectrometer (Instytut Fotonowy).
The OLEDs were powered by a Matrix MPS-3005L-3 DC power supply and the current was
measured with the use of a Fluke 8808A multimeter. The bias voltage applied to the OLSD
and the photocurrent control were performed using a Keithley 2400 source meter.

AC Characterization
The OLEDs were powered by the sum of the DC and AC voltage from the sinusoidal- or
square-wave generator (Matrix 8216A). The DC component of the bias voltage determined
the operating conditions of the OLED devices (the operating point on the current–voltage
characteristics). The AC analysis of the photocurrent was performed by illuminating the
OLSD with an inorganic white light-emitting diode. Two Rigol DS1052E l oscilloscopes,
electrically isolated from each other, were used to measure the dynamics of the signal
changes. One was used to measure the LED or OLED supply voltage and the other to
monitor the output voltage of the high-speed transimpedance amplifier (Femto, DHPCA-100) that converts the OLSD photocurrent into voltage. The output voltage of the
transimpedance amplifier was proportional to the measured current over the full range
of the frequency spectrum. A bias voltage applied to the photodiode was supplied by the
voltage source (Matrix MPS-3005L-3). A silicon photodiode with a dedicated amplifier
(ThorLabs, PDA36A-EC) was used to analyze the changes in the light emitted by the OLED

_Materials_ 2025, _18_, 152 5 of 18

over time. A scheme of the DC and AC control systems is presented in Figure S2 in the
Supplementary Information.

3. Results and Discussion

An example of the current–voltage characteristics of an OLED based on SY is shown
in Figure 3a. For a supply voltage ( _uOLED_ ) lower than the turn-on voltage _uON_ = 2.1 V, a
current flows in the diode, for which the density ( _jOLED_ ) is a linear function of the voltage.
When the turn-on voltage is exceeded, the current density suddenly increases. According to
the literature [36,37], the value of _uON_ corresponds to the voltage of the built-in field, which,
after multiplying by the elementary charge ( _q_ ), is equal to the energy difference between
the work functions of electrons exiting the anode and cathode (made of PEDOT:PSS and
calcium layers, respectively). For voltages above this, light emission occurs.

10 4

10 3

10 2

10 1

10 0

10 -1

10 -2

0.06

0.05

0.04

0.03

0.02

0.01

0.00
500 600 700

λ [nm]

10 -2

10 -3
0 400 800

_jOLED_ [mAcm -2 ]

10 3

10 2

10 1

10 0

10 -1

10 -2

10 -3

10 -4

10 -3
10 0 10 1

_uON_

_uOLED_ [V]

( a ) ( b )

Figure 3. ( a ) Current–voltage characteristics _jOLED_ ( _uOLED_ ) (circles) and the dependence of the total
radiant emittance on the voltage _m_ ( _uOLED_ ) (stars) for OLED based on SY; the symbol _uON_ denotes
the threshold voltage; the inset shows the dependence of the current efficiency _η_ on the current
density _jOLED_ ; ( b ) spectral emittance _mλ_ for different voltage values _uOLED_ .

_3.1._ _Characteristics of the OLED Emitter_

In optocouplers, the detector is an electronic device and not the human eye, so instead
of photometric quantities (such as luminance), spectroscopic quantities such as the spectral
density of radiation emittance _mλ_ and radiation emittance _m_ were used to describe the
operation of the OLED transmitter properties. The spectra _mλ_ measured for different values
of voltage _uOLED_ are shown in Figure 3b. The dependence of the total radiant emittance
on voltage _m_ ( _jOLED_ ) shown in Figure 3a was determined from the spectral emittance
_mλ_ ( _jOLED_ ) measured using Equation (1):

_m_ ( _jOLED_ ) =

- ∞

_mλ_ ( _jOLED_ ) _dλ_ (1)
0

Based on the _I-V_ characteristic and the _m_ ( _uOLED_ ) dependence, current efficiency was
determined as a function of current density ( _η_ ( _jOLED_ ) = _m_ ( _jOLED_ )/ _jOLED_ ). The graph
in the inset of Figure 3a shows that as current density increases from 0.1 to 800 mA/cm,
current efficiency slowly decreases from 85 to 60 A/W.

An important parameter of the transmitter is the speed at which the OLED emits light
after the voltage is applied. To test the effectiveness of the emission of alternating light,

_Materials_ 2025, _18_, 152 6 of 18

the OLED devices were supplied with a voltage containing a constant time component,
_uOLED_, _dc_, and a harmonic variable component: _uOLED∼_ ( _t_ ) = _u_ 0, _OLED_ sin(2 _π f t_ ). For the
value of the constant voltage component _uOLED_, _dc_ = 7 V and the amplitude of the variable
component _u_ 0, _OLED_ = 0.1 V, the current density varied from 190 to 210 mAcm _−_ 2 . The
operating point of the OLED was the current density _jOLED_, _dc_ = 200 mAcm _−_ 2 and the
corresponding radiation emittance _mdc_ = 16 Wm _−_ 2 .

Based on the series of measured oscillograms for different frequencies, the relative
amplitude of the radiation emittance, _m_ 0, _rel_ ( _f_ ), expressed in decibels, was calculated:

_m_ 0, _rel_ ( _f_ ) = 20log[ _m_ 0( _f_ )/ _m_ 0(10Hz)] (2)

where _m_ 0( _f_ ) is the amplitude of the alternating component of the radiation emittance.
Then, the amplitude spectrum of the radiation emittance was prepared and on its basis, the
three-decibel cutoff frequency, _f−_ 3dB, _OLED_, was determined to be _f−_ 3dB, _OLED_ _≈_ 640 kHz.

The speed of OLED devices was limited by contact resistance, _Rs_, _OLED_, differential
resistance of the OLED, _Rdi f f_, _OLED_ (equal to the inverse derivative of the I-V characteristic
for a fixed voltage value), and geometric capacitance _COLED_ = _Sεε_ 0/ _L_ (where _ε_, _ε_ 0, _S_, and
_L_ are the vacuum permittivity, the relative permittivity of SY, the OLED area, and the SY
layer thickness, respectively).

From the I-V characteristic presented in Figure 3a, it follows that for a voltage of 7 V,
resistance _Rdi f f_, _OLED_ was equal to 220 Ω, and based on the electrical equivalent model
of OLED (made on the basis of impedance tests, described in [36]), contact resistance
_Rs_, _OLED_ = 100 Ω was calculated. The three-decibel cutoff frequency is

_f−_ 3 _dB_, _OLED_ = 1/(2 _πτRC_, _OLED_ ) (3)

-
where _τRC_, _OLED_ = 2 _πCOLEDRdi f f_, _OLEDRs_, _OLED_ / _Rdi f f_, _OLED_ + _Rs_, _OLED_ is the time con
stant for the simplified OLED equivalent circuit shown in Figure S3 in the Supplementary
Information. Based on the above formula and the experimentally obtained results, the
value of the dielectric constant of SY, _ε_, was calculated from the following formula:

where _τRC_, _OLED_ = 2 _πCOLEDRdi f f_, _OLEDRs_, _OLED_ /

_Rdi f f_, _OLED_ + _Rs_, _OLED_

_L_

_Rdi f f_, _OLED_ + _Rs_, _OLED_

_ε_ =

(4)
2 _π f−_ 3 _dB_, _OLEDSε_ 0 _Rdi f f_, _OLEDRs_, _OLED_

The estimated _ε_ value was 6.3.

_3.2._ _Characteristics of the OLSD Photodetectors_

Five different OLSD photodetectors were tested as a receiver in optocouplers with the
OLED transmitter based on SY. It was assumed that the optocouplers should fulfill three
basic conditions: (a) they should transmit signals from the transmitter to the photodetector
with high efficiency, (b) they should be characterized by a high signal transmission rate
while maintaining good DC parameters, and (c) they should not lose their electro-optical
properties after encapsulation and operate stably under normal atmospheric conditions for
at least several weeks. The characteristics of the tested OLSD devices with active layers
made of mixtures of different semiconductors are given in the Supplementary Information
(Figure S4 and Table S1). Based on the measured spectra of emittance spectral density m_λ
for the OLED emitter (measured for _jOLED_ = 0.11 mAcm _−_ 2 ) and the OLSD response _R_ ( _λ_ ),
the overlap integrals were calculated. These were used to determine the ability of each
transmitter/receiver pair to effectively convert light energy into photocurrent. The highest

_Materials_ 2025, _18_, 152 7 of 18

values of the relative overlap integral _ξ_ (see Figure 4) were found for the OLED/IPT1:1chb
and OLED/IPT1:1chf systems.

_ξ_ =

�300800nmnm _mλR_ ( _λ_ ) _dλ_
(5)
�300800nmnm _mλRIPT_ 1:1 _chr_ ( _λ_ ) _dλ_

1.0

0.8

0.6

0.4

0.2

0.0

Figure 4. Comparison of relative overlap integrals _ξ_ of spectral emittance mλ of OLED based on SY
transmitter and responsivity _R_ ( _λ_ ) of OLSD photodetectors based on different donor–acceptor mixtures.

The semiconductors that make up the active layer of OLSD devices are unstable in air,
so OLSD devices were fabricated and characterized in glove boxes filled with nitrogen. The
IPT1:1chb system was found to have the best stability in air after encapsulation of the OLSD
devices tested. It was also found that this OLSD converted light signals to photocurrent at
a much higher rate than the other OLSD devices and showed good DC properties without
an applied reverse voltage. Therefore, this system was selected for use in the optocoupler
together with SY-based OLED.

_3.3._ _Characteristics of the Optocoupler_

Constant current measurements were performed on the constructed OLED/OLSD
devices and the dependence of the photocurrent density on input current _jph_, _dc_ ( _jin_, _dc_ )
was determined. The current generated by light in the receiver, _jre_, was measured using a transimpedance amplifier without reverse-biasing the OLSD photodetector. It was
assumed that the current density in the circuit at the receiver output was equal to the
photocurrent density _jph_ in OLSD; according to the optocoupler model shown in Figure 1,

_jre_ = _jphRsh_, _re_ /( _Rsh_, _re_ + _Rs_, _re_ ), which for _Rsh_, _re_ _≫_ _Rs_, _re_ gives _jre_ _∼_ = _jph_ . Figure 5 shows the
measured current characteristic for an example optocoupler. The characteristic is linear for
the current density at the OPC input in the range from 55 to 580 mAcm _−_ 2 .

The key parameter that characterizes an optocoupler is the current transfer ratio ( _CTR_ ),
which is defined as the ratio of the current detected at the receiver output to the current
supplied to the transmitter input:

_ire_

_iin_ _≈_ _iph_

_iin_

_Sph_
_Sem_

_CTR_ = _ire_

_iin_

_jph_

_jin_

(6)

where _Sem_ and _Sph_ are the surfaces of the emitter and photodetector, respectively (for the
tested OLED/OLSD, both surfaces were the same; _Sph_ / _Sem_ = 1).

_Materials_ 2025, _18_, 152 8 of 18

0.1

0.01

10 -2

10 -3

10 100

_Jin_ [mAcm -2 ]

Figure 5. Dependence of photocurrent density at photodetector output _jph_ on current density at
transmitter input _jin_ for the OLED/OLSD optocoupler. The inset shows the dependence of current
ratio _CTR_ on _jin_ in the linear range of the optocoupler operation.

An important requirement is that the range of variations in the current applied to
the device input, for which the value of _CRT_ coefficient remains constant, should be as
wide as possible. For a current density range _jin_ from 55 to 580 mAcm _−_ 2, where the
dependence _jph_, _dc_ ( _jin_, _dc_ ) could be considered linear, the _CRT_ value was about 0.13% and
was similar to those found in the literature [4,8,38]. However, in the tested systems, the
_CTR_ value was only approximately constant over a limited range of current density _jin_, _dc_ .
Moreover, the current transfer ratios for the DC and AC components, _CTRdc_ = _iph_, _dc_ / _iin_, _dc_
and _CTRac_ = _iph_, _rms_ / _iin_, _rms_, respectively (where the rms subscript denotes the root mean
square value), were not equal for all current density values.

For time-varying signals, for frequencies above the three-decibel optocoupler cutoff
frequency ( _f−_ 3dB, _OPC_ ), _CTRac_ decreased with increasing _f_ according to the formula

- _−_ 1/2
(7)

_CTRac_ ( _f_ ) = _CTR_ 0

- _f_ 2
1 +

_f_ 2

_−_ 3dB, _SY_

where _CTR_ 0 is the current transfer ratio for _f_ _≪_ _f−_ 3dB, _OPC_ .

In Figure 6a, it is visible that the experimentally determined _CTRdc_ and _CTR_ 0 values
decrease with current density _jin_ below a certain threshold value of current density _jin_, _t_ .
This threshold value _jin_, _t_ could be reduced by biasing the OLSD photodetector with a
reverse voltage ( _urev_ ). In order to check what effect the reverse voltage polarization of
the OLSD photodetector may have on the linearity of current transfer, the characteristics
_jph_, _dc_ ( _jin_, _dc_ ) were measured for _urev_ = _−_ 1 V and _urev_ = _−_ 2 V. The photocurrent density
_jph_, _dc_ was calculated as the difference between receiver current density ( _jre_, _dc_ ) for _jin_, _dc_ _>_ 0 A
and the leakage current ( _jre_, _l_ ) of the receiver at _jin_, _dc_ = 0 A ( _jph_, _dc_ = _jre_, _dc −_ _jre_, _l_ ).

To estimate the values of _CTRdc_ and _CTR_ 0, two appro ~~aches wer~~ e used. In the first one,
the coefficients were estimated based on independent tests performed separately for the
receiver and transmitter (these coefficients are marked with the upper index _e_ —estimated),
and the second way was to determine these coefficients according to the definition as the
ratio of the measured current at the OPC output and the current supplied to its input (these
coefficients are marked with the upper index _d_ —determined).

_Materials_ 2025, _18_, 152 9 of 18

10 0

10 -2

( a )

( b )

( c )

10 -4

10 -6

10 -8

10 -2

10 -4

10 -6

10 -8

10 -2

10 -4

10 -6

estimated
determined at _Urev_ = 0V
determined at _Urev_ = -1V
determined at _Urev_ = -2V

_CTRdc_ _CTR0_

estimated
determined at _Urev_ = 0V

estimated
determined at _Urev_ = 0V
determined at _Urev_ = -1V
determined at _Urev_ = -2V

10 -8
10 1 10 2 10 3

_jin,t_,(-2) _jin,t_,(-0)

_jin,dc_ [mA/cm 2 ]

Figure 6. Comparison of the variations in direct _CTRdc_ and alternating _CTR_ 0 current transfer ratios as
a function of the density of direct current component _jin_, _dc_ for ( a ) the experimentally determined DC
and AC current transfer ratios for the OLED/OLSD optocoupler at reverse voltage _urev_ = 0 and their
equivalents estimated based on the experimental results obtained for the OLED transmitter and the
OLSD photodetector; ( b ) direct _CTRdc_, and ( c ) alternating _CTR_ 0 current transfer ratios determined
for the photodetector biased with the reverse voltage _urev_ of two different values.

First, according to the definition of responsivity, the total photocurrent density was
determined according to the formula _jph_ = � _λ_ _λ_ 1 2 _mλR_ ( _λ_ ) _dλ_ . The dependence of the spec-

tral density of emittance of the transmitter radiation _mλ_ on the current density at the
transmitter input _jin_, _dc_ = _jOLED_, _dc_ for OLED (Figure 3) and the responsivity spectrum
( _R_ ( _λ_ )) of the receiver) for OLSD were determined experimentally (see Supplementary

Information Figure S4). Then, according to Equation (6), _CTR_ ( _e_ )

_dc_ was calculated. It was
assumed that for small signals of the variable component, the approximate equation holds

_jph_, _rms_ / _jpin_, _rms≈_ d _jph_, _rms_ /d _jpin_, _rms_ . Based on this, _CTR_ 0 ( _e_ ) was calculated as follows:

0 ( _e_ )( _jin_ , _dc_ ) = - _λ_ 2

_dmλ_ ( _jin_ , _dc_ )

_djin_, _dc_

_CTR_ 0 ( _e_ )

_R_ ( _λ_ ) _dλ_ (8)

_λ_ 1

The determined dependencies _CTR_ ( _dc_ _e_ )

0 ( _e_ )( _jin_ , _dc_ ) were treated as ref-

erence curves for coefficients _CTR_ ( _dc_ _d_ )

( _dc_ _d_ )( _jin_ , _dc_ ) = _jph_, _dc_ ( _jin_, _dc_ )/ _jin_, _dc_ and _CTR_ 0 ( _d_ )

( _dc_ _e_ )( _jin_ , _dc_ ) and _CTR_ 0 ( _e_ )

erence curves for coefficients _CTR_ ( _dc_ _d_ )( _jin_ , _dc_ ) = _jph_, _dc_ ( _jin_, _dc_ )/ _jin_, _dc_ and _CTR_ 0 ( _d_ )( _jin_ , _dc_ ) =

_djph_, _dc_ ( _jin_, _dc_ )/ _djin_, _dc_ determined for the optocoupler created by assembling (i.e., simply by
putting together) this transmitter–receiver pair.

Figure 6a compares the plots of the _CTRdc_ ( _jin_, _dc_ ) and _CTR_ 0( _jin_, _dc_ ) dependences determined by using the two methods described above for the case where the photodetec

_Materials_ 2025, _18_, 152 10 of 18

tor was not reverse-biased ( _urev_ = 0 V). For a current density higher than 20 mAcm _−_ 2,
all determined DC coefficients had very similar values in the range from 1.3 _×_ 10 _−_ 3 to
1.7 _×_ 10 _−_ 3 . However, as the current density decreased below 20 mAcm _−_ 2, the coefficients

_CTR_ ( _dc_ _d_ )

( _dc_ _d_ ) had lower values in relation to _CTR_ ( _dc_ _e_ )

_CTR_ ( _dc_ _d_ ) had lower values in relation to _CTR_ ( _dc_ _e_ ). A similar trend was observed for the AC

coefficients, with significant differences between the estimated and experimentally determined _CTR_ 0 values occurring only for the density of the direct current component _jin_, _dc_
below 6 mAcm _−_ 2 .

As shown in Figure 6b, the values of coefficients _CTR_ ( _dc_ _e_ ) and _CTR_ ( _dc_ _d_ ) have approxi-

mately similar values for a current density _jin_, _dc_ higher than 15 mAcm _−_ 2, independently of
the reverse voltage values applied to the OLSD photodetector. When the current density

As shown in Figure 6b, the values of coefficients _CTR_ ( _dc_ _e_ )

( _dc_ _e_ ) and _CTR_ ( _dc_ _d_ )

_jin_, _dc_ is lower than 15 mAcm _−_ 2, the values of coefficients _CTR_ ( _dc_ _e_ )

( _dc_ _e_ ) and _CTR_ ( _dc_ _d_ )

_jin_, _dc_ is lower than 15 mAcm _−_ 2, the values of coefficients _CTR_ ( _dc_ _e_ ) and _CTR_ ( _dc_ _d_ ) are different.

However, after applying the reverse voltage to the photodetector, these differences decrease;
the higher the absolute value of the _|urev|_, the smaller this difference is. A similar effect of
applying the reverse voltage to the photodetectors was observed for the AC coefficients
_CTR_ 0 (Figure 6c).

The discrepancy between the estimated and experimentally determined _CTRdc_ and
_CTR_ 0 coefficients could be due to the phenomenon of electron trapping in the acceptor phase
or/and holes in the donor phase in the bulk heterojunction in OLSD. Trapping reduces
the population of mobile charge carriers and therefore reduces the photocurrent intensity.
For a given trap concentration, the trapping effect will be particularly pronounced for low
photocurrents because a relatively large fraction of the photogenerated charge carriers can
get stuck in the traps. After applying the reverse voltage _urev_ an additional external electric
field is created in the heterojunction, which, according to the Poole–Frenkel mechanism,
reduces the energy barrier for carriers to leave the traps [39,40]. At a sufficiently high
voltage, the residence time of the carriers in the traps is significantly reduced, and the
trapping of the charge carriers has little effect on the current flow.

In the case where _f_ _<_ _f−_ 3dB,OPC and _jin_ _>_ _jin_, _t_, the DC and AC current transfer ratios
have similar values, _CTRdc_ _≈_ _CTR_ 0, which apparently do not depend on the current
intensity, as can be seen in Figure 6. However, the DC and AC _CTR_ values are not exactly
equal and constant in the OPC input current density ranges described above. The _CTR_
coefficients changed slightly with the change in current density _jin_, _dc_, resulting in the deterioration of linearity of the transmitting of high-amplitude AC currents ( _jin_ 0). In order to
estimate the degree of this nonlinearity, we assumed that the range of linear operation of
the example OLED/OLSD optocoupler for _urev_ = 0 V was within the current density limits
_jin_, _dc_ from the threshold value _jin_, _t_ = 6 mAcm _−_ 2 to the maximum value _jin_, _m_ = 500 mAcm _−_ 2

(Figure 6c). For the assumed range of the constant component of current density, the
maximum possible amplitude of the variable component of current density ( _jin_ 0, _m_ ) depended on the value of the constant component _jin_, _dc_ and was _jin_ 0, _m_ = ( _jin_, _dc −_ _jin_, _t_ )/2 for

_jin_, _dc_ _≤_ ( _jin_, _m −_ _jin_, _t_ )/2 or _jin_ 0, _m_ = ( _jin_, _m −_ _jin_, _dc_ )/2 for _jin_, _dc_ _≥_ ( _jin_, _m_ + _jin_, _t_ )/2, as shown in
Figure 7a.

For _jin_, _dc_ _∈_ ( _jin_, _t_, _jin_, _m_ ), the coefficients _CTR_ 0 changed slightly with the changing
current, and their deviation from the mean value ( _CTRo_ ) was expressed by coefficient _δ_ 0
defined as

� _CTRo_ ( _ξ_ ) _−_ _CTRo_ ( _jin_, _dc_, _jin_ 0)

_CTRo_ ( _jin_, _dc_, _jin_ 0)

(9)

�2 _dξ_

_δ_ 0( _jin_, _dc_, _jin_ 0) =

�� _jin_, _dc_ + _jin_ 0
_jin_, _dc−jin_ 0

where

_CTRo_ ( _jin_, _dc_, _jin_ 0) = 1

2 _j_ 0

- _jin_, _dc_ + _jin_ 0

_jin_, _dc−jin_ 0

( _CTRo_ ( _ξ_ ) _−_ _CTRo_ ( _jin_, _dc_ )) 2 _dξ_ (10)

_Materials_ 2025, _18_, 152 11 of 18

Figure 7. ( a ) The highest possible value of the amplitude of the variable component of current
density _jin_ 0, _m_ as a function of _jin_, _dc_ for the assumed range of linear operation of OLED/OLSD for a
photodetector unpolarized by reverse voltage; ( b ) dependence of coefficient _δ_ 0 on _jin_,0 for different
values of the constant component of current _jin_, _dc_ in the range 12–451 mAcm _−_ 2 ; ( c, d ) coefficient _δ_ 0 as
a function of the constant component of current density _jin_, _dc_ supplied to the OPC input for different
values of the amplitude of variable component _jin_,0, for two cases: ( c ) a non-polarized photodetector
and ( d ) a photodetector polarized with a reverse voltage _urev_ = _−_ 2 V.

The value of _δ_ 0 depended on the choice of the constant component of current density _jin_, _dc_ (OPC operating point) and the amplitude of the variable component of signal
_jin_ 0. Figure 7b shows the dependencies of the _δ_ 0 coefficient on the amplitude of the variable signal for different values of the constant component of current _jin_, _dc_ in the range
12–451 mAcm _−_ 2 . In each case, regardless of the value of the constant component of the current adopted, the nonlinearity increased as the amplitude of the variable component _jin_ 0, _m_
increased. This is intuitive, since one might expect that the smaller the signal amplitude
relative to the DC component of the input current, the better the linear function that can be
used to approximate the current characteristic of the optocoupler in the immediate vicinity
of the operating point (Figure 5).

Plots of the dependence of the _δ_ 0 coefficient on the DC component of the current
density for different values of the AC signal amplitude have been drawn for an optocoupler with an unpolarized photodetector ( _urev_ = 0 V—Figure 7c) and a polarized
one ( _urev_ = _−_ 2 V—Figure 7d). After exceeding the current density _jin_, _dc_ (e.g., 100 mAcm _−_ 2 ),

_Materials_ 2025, _18_, 152 12 of 18

which is characteristic for the amplitude _jin_,0 (e.g., 4.5 mAcm _−_ 2 ), the _δ_ 0 coefficient decreased,
which was caused by the slightly decreasing current efficiency _η_ with increasing current
characteristic of OLED devices (see the inset in Figure 3a). In the whole range of current
densities tested at the OPC input, _δ_ 0 had a higher value for _urev_ = _−_ 2 V, which indicates a
deterioration in the linearity of light conversion into photocurrent by the optocoupler after
polarizing the receiver with a voltage.

To operate in the third quadrant of the I-V characteristic and to broaden the frequency
band photodiodes working as the photodetectors in optocoupler require reverse polarization [7,41–45]. However, from the above analysis, it is clear that in order to minimalize
linearity degradation, the photodetector should be polarized with the lowest possible
reverse voltage or should not be polarized at all. In addition, the following rules must be
followed: (a) small values of direct current from the range in which _δ_ 0 reaches the lowest
value should be used for alternating signals with low amplitude, or (b) for higher-amplitude
alternating signals, the highest possible values of direct current should be used. On the
other hand, one should be aware that too small an amplitude of AC current and/or too
large a DC component can hinder transmission. The limitation of a small AC amplitude is
the noise occurring in the transmitter, receiver, and amplifier, as well as the fluctuations
of the power of the emitted light. It is often necessary to use a preamplifier to increase
the AC amplitude and improve the signal-to-noise ratio before feeding the signal to the
OPC input. However, this may preclude the use of variant a), as small values of direct
current are only suitable for small AC signal amplitudes (Figure 7c,d). In turn, the use of
large values of direct current (variant b) will contribute to an increase in shot noise in the
photodetector, which can also worsen the signal-to-noise ratio at the OPC output. In the
case of polarization of the OLSD photodetector with reverse voltage, the leakage current
can increase significantly with the increasing voltage _urev_ and then the detectivity [44,46,47]
of the receiver is drastically worsened.

Time-varying signals transmission tests
Taking into account the above analysis, in order to ensure low nonlinear distortions during the transmission of time-varying signals through the OLED(SY)/OLSD(IPT)
optocoupler, the operating point determined by the input voltage _uin_, _dc_ = 7 V (current density _jin_, _dc_ = 200 mAcm _−_ 2 ) was chosen. The corresponding constant component of the radiation emittance was _mdc_ = 16 Wm _−_ 2 . The variable component
consisted of sinusoidal voltage waves _uin∼_ ( _t_ ) = _uin_ 0sin(2 _π f t_ ) and rectangular
_uin⊓_ ( _t_ ) = _uin_ 0[1( _t_ ) _−_ 1( _t −_ 0.5/ _f_ ) + 1( _t −_ 1/ _f_ ) _−_ _t −_ 1.5/ _f_ + _· · ·_ .], where _t_ is time, _f_ is
frequency, and 1( _t_ ) is the Heaviside step function. The amplitude of the variable component _uin_ 0 was 0.1 V. It corresponded to the range of transmitter current changes from
190 to 210 mAcm _−_ 2 . For the adopted operating point and the range of transmitter current
variation, both the emitter radiative emittance values and the corresponding photodetector photocurrent were within the ranges of linear operation of the OPC (see the inset in
Figure 5).

Figure 8 shows sample waveforms of the variable components of the voltage applied to the device input _uin⊓_ and _uin∼_ and the corresponding current densities at the
photodetector output _jph⊓_ and _jph∼_ developed based on the measured oscillograms (see
Supplementary Information Figure S5). The results are presented for two selected frequency
values of 100 Hz and 158 kHz. At 100 Hz, there was no attenuation of the signal at the
output of the device. There was no phase shift in the output current relative to the input
voltage, and the input current increased immediately after the voltage jump (on a time
scale corresponding to the period of a rectangular voltage wave). However, at a frequency
of 158 kHz, the response to a rectangular wave did not resemble the shape of the input

_Materials_ 2025, _18_, 152 13 of 18

voltage. The rise and fall of the output current corresponded to the rise ( _τrise_ ) and relaxation
( _τf all_ ) times of 2 µs, respectively.

( a ) ( b )

0.2

0.1

0.0

-0.1

-0.2

0.2

0.1

0.0

-0.1

τ _del_

10 ~~0~~ Hz

0.04

0.02

0.00

-0.02

-0.04
0 5 10 15 20 5 10 15 20

( c ) _t_ [ms] ( d ) _t_ [ms]

5 10 15 20

( c ) _t_ [ms] ( d )

_t_ [ms]

0.04

τ _fall_

15 ~~8~~ kHz

0.02

0.00

-0.02

τ _rise_

-0.2
0 3 6 9 12 15

_~~t~~_ ~~[~~ ~~μ~~ ~~s]~~ -0.04

3 6 9 12 15

_t_ [ μ s]

Figure 8. Variable components of current density at the output of the OLED/OLSD optocoupler _jph⊓_
and _jph∼_ as a function of time t (red) after applying a voltage wave (blue) to the transmitter input.
( a, c ) Rectangular _uin⊓_ and ( b, d ) sinusoidal _uin∼_ of two frequencies of 100 Hz and 158 kHz; ( c ) the
symbols _τrise_ and _τf all_ denote the times of the rise and fall of the current at the output, respectively,
after applying a rectangular voltage wave to the input; ( d ) _τdel_ is the delay time of the receiver
photocurrent after applying a voltage harmonic component to the input.

After the deconvolution of the waveform of the variable component shown in the
oscillogram in Figure S5, we obtained two time constants _τ_ 1 = 249 ns and _τ_ 2 = 440 ns.
For the former, the three-decibel cutoff frequency (1/(2 _πτ_ 1) was 660 kHz, which roughly
corresponded to the transmitter cutoff frequency _f−_ 3dB,OLED. The second time constant
_τ_ 2 indicated that the three-decibel receiver cutoff frequency should be about 360 kHz,
which is higher than _f−_ 3dB,OLSD = 250 kHz for OLSD(IPT) (see Supplementary Information,
Table S1).

For a sinusoidal signal with a frequency of 158 kHz, the output current amplitude

_√_

decreased by 2 compared to the amplitude for lower frequencies (100 Hz). This would

indicate that 158 kHz is equal to the three-decibel cutoff frequency _f−_ 3dB,OPC, but the phase
shift between the input voltage and the output current was less than π/4 ( _τrise_ = 1/(8 _f_ )).
This in turn would mean that the three-decibel cutoff frequency was higher than 158 kHz.
In order to determine _f−_ 3dB,OPC, the frequency dependence of the relative transadmittance
_TArel_ ( _f_ ) was investigated, calculated as the quotient of the amplitudes of the variable
components of the current at the photodetector output, and the voltage applied to the
transmitter input for frequency _f_ was studied by referring to the same quotient, but for
the undamped frequency of 10 Hz: _TArel_ ( _f_ ) = [ _iin_ 0( _f_ ) _uin_ 0(10Hz)]/[ _uin_ 0( _f_ ) _iin_ 0(10Hz)].
Figure 9 shows a comparison of the frequency spectra for the relative amplitude of radiant

_Materials_ 2025, _18_, 152 14 of 18

emittance ( _mrel_ ( _f_ ) = _m_ 0( _f_ )/ _m_ 0(10Hz)) for OLED, relative amplitude of photocurrent
density _jph_, _rel_ ( _f_ ) = _jph_,0( _f_ )/ _jph_,0(10Hz) recorded by OLSD, product _mrel_ ( _f_ ) _jph_, _rel_ ( _f_ ), and
relative transadmittance _TArel_ ( _f_ ). As expected, the spectrum obtained by multiplying the
transmitter and receiver spectra ( _mrel_ ( _f_ ) _jph_, _rel_ ( _f_ )) completely overlapped with the relative
transadmittance spectrum of the optocoupler in a wide range of considered frequencies
(from 10 Hz to 640 kHz). The determined three-decibel cutoff frequency of the OLED/OLSD
optocoupler was 170 kHz.

-3

-6

-9

-12

-15

-18

-21

OLED
OLSD
OPT

_f-3dB_,OLED

_f-3dB_,OLSD

_f-3dB_,OPC

-24
10 -2 10 -1 10 0 10 1 10 2 10 3 10 4

_f_ [kHz]

Figure 9. Comparison of frequency spectra for relative amplitude of radiant emittance _mrel_ of the
OLED (blue circles), relative amplitude of photocurrent density _jph_, _rel_ of the OLSD photodiode (red
circles), and relative transadmittance _TArel_ of the OLED/OLSD optocoupler (green circles). Dashed
curve—spectrum resulting from multiplication of _mrel_ and _jph_, _rel_ spectra. Vertical dotted lines indicate
three-decibel cutoff frequencies of OLED _f−_ 3dB, _SY,_ OLSD _f−_ 3dB, _IPT_ and OPC _f−_ 3dB, _OPT_ .

The spectrum obtained by mathematically combining the frequency spectra of the
transmitter and photodetector matched the transadmittance spectrum measured for the
optocoupler almost perfectly. Considering the considerable t time interval (several weeks)
between the time of conducting a complete set of experiments, first performed separately
for the transmitter and photodetector, and then the production of the optocoupler and its
characterization, the overlap of the spectra indicated the stability of the fabricated devices.
It could be expected that applying a reverse voltage to the photodetector would increase
the cutoff frequency of the device. This broadening of the bandwidth also follows from the
analysis of the OPC equivalent circuit (see Figure S3 in Supplementary Information), where
capacitor _Cre_ symbolizes the photodiode capacitance, which is the resultant of the geometric
capacitance, the capacitance of the electrode–semiconductor junctions, and the capacitance
of the bulk heterojunction. The application of an external electric field causes a decrease
in junction capacitance [48–50] and this in turn causes an increase in limit frequency

_f−_ 3 _dB_, _OPC_ . As a result, the frequency band for signals that can be transmitted without
distortion is extended. However, for the OLED/OLSD optocouplers tested, no effect of
increasing the limit frequency _f−_ 3 _dB_, _OPC_ was observed after applying a reverse voltage
to the photodetector. This may be explained by the low leakage resistance and the small
number of junctions in the simple structure of the OLSD photodetector tested compared to
typical multilayer photodiodes described in the literature. In these photodiodes, the leakage
current reduction is achieved by introducing additional interlayers [14,41] to block the
injection of holes from the cathode and electrons from the anode. However, the additional
junctions created by the interlayers hinder the charge carrier transport for alternating
signals, because charges are accumulated near these junctions. In order to accelerate the

_Materials_ 2025, _18_, 152 15 of 18

operation of the device, these space-charges should be removed, which is achieved by
applying a reverse voltage to the photodiode. In the case of our single-layer OLSD, the
charges from the active layer and the areas near the junctions are quickly removed even
with shorted electrodes due to the low leakage resistance; in this case, the application of a
reverse voltage does not improve the photodiode dynamics.

It should be emphasized that it is difficult to produce an optocoupler that has both
a high _CTR_ and fast response time. In general, it can be seen that optocouplers with
high _CTR_ values tend to operate slower than optocouplers with low _CTR_ [8,41,51–53].
The improvement in receiver speed is usually achieved by biasing the OPDs with the
voltage _urev_ . The assumption is that an increase in the effective electric field leads to less
carrier trapping and higher effective mobility, resulting in faster carrier extraction from the
active layer. However, in our devices, we obtained a high value of _f−_ 3 _dB_, _OPC_ = 170 kHz,
comparable to other OPC devices fabricated using similar organic semiconductors, but
without the need to apply _urev_ . To the best of our knowledge, the three-decibel frequency
for the photodetectors with an ITIC:PTB7-Th mixture layer has not yet been determined.
The published data for OPD with different mixtures of the PTB7-Th/acceptor [29,30] or
with ITIC/donor [14,32] show that only photodetectors with the mixture PTB7-Th:CO1-4Cl
had cutoff frequencies _f−_ 3 _dB_, _OPC_ = 240 kHz comparable to our results [54,55].

The results presented in this article allow us to formulate several detailed summary
statements. An organic light-emitting diode with a single active layer made of the wellknown Super Yellow can be used as an efficient transmitter. An organic light-sensitive
diode with a single active layer made of a mixture of a polymer donor PTB7-Th and a
low-molecular-weight non-fullerene acceptor ITIC can be used as an efficient receiver.
An OPC can be easily formed by inducing a tight adhesion of the glass substrates of
OLED and OLSD, since the OLED and OLSD have the same geometry and structure:
glass/ITO/PEDOT:PSS/(active layer)/Ca/Al. Despite their simplified structure, our OPC
devices showed the ability to transmit current signals over a wide frequency band with
a good linearity of current transmission. The current transfer ratio (CTR) was 0.13%; the
three-decibel cutoff frequency was 170 kHz for harmonic signals, and the response time to
a step change in the current at the OPC input was 2 µs.

4. Conclusions

- We have shown that it is possible to produce an optocoupler that meets the application requirements using commercially available organic semiconductors that can be
processed using solution techniques.

- The organic light-sensitive diode can efficiently operate as a receiver with shortcircuited electrodes, i.e., without any reverse voltage applied. It is often stated in the
literature that it is necessary to apply a reverse voltage to the photodiode to make
it operate in the third quadrant of its I-V characteristic and to extend the frequency
bandwidth of the device. However, from our analysis, it is clear that in order to
minimize linearity degradation, the photodetector should not be biased or should be
biased with the lowest possible reverse voltage.

- Compared to most OPC devices with complex multi-layer transmitter and receiver
structures and the need to apply reverse voltage to the receiver, the simplest possible
design of our OPC reduces the number of manufacturing steps and greatly simplifies
the device manufacturing process.

- The simple structure of our OPC, if the vacuum-deposited Ca/Al electrodes are replaced with a conductive ink (e.g., zinc oxide and nanosilver suspensions), enables the
production of optocouplers by printing on flexible double-sided ITO-coated substrates.
This structure will also reduce light losses resulting from reflections at interfaces.

_Materials_ 2025, _18_, 152 16 of 18

Supplementary Materials: [The following supporting information can be downloaded at https://www.](https://www.mdpi.com/article/10.3390/ma18010152/s1)
[mdpi.com/article/10.3390/ma18010152/s1.](https://www.mdpi.com/article/10.3390/ma18010152/s1)

Author Contributions: Conceptualization, J.J., J.U. and B.L.; Methodology, A.S., A.Z. and A.R.;
Formal analysis, J.J., A.Z. and A.R.; Investigation, A.S. and R.U.; Resources, A.S. and R.U.; Writing—
original draft, J.J., J.U. and B.L.; Writing—review & editing, J.J., J.U., R.U. and B.L.; Supervision, J.J.
All authors have read and agreed to the published version of the manuscript.

Funding: This work was financially supported by the National Science Centre (Poland) under grant
2020/37/B/ST5/03929.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The original contributions presented in this study are included in the
article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments: The authors would like to thank Allison Proszowska for her support in proofreading the English language of the article.

Conflicts of Interest: Authors Artur Zawadzki and Andrzej Rybak were employed by ABB Corporate
Technology Center. The remaining authors declare that the research was conducted in the absence of
any commercial or financial relationships that could be construed as a potential conflict of interest.

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