Organic Optocoupler with Simple Construction as an Effective Linear Current Transceiver

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 light-emitting 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: DC and AC properties; organic optocoupler; short circuited organic photodetector.

Figure 1

Schematic diagram of an optocoupler with dc $i_{in,dc}$ and ac $i_{in}\left(t\right)$ input current sources and a current source $i_{re,dc}$ designed to eliminate the DC component from the current at the output of the optocoupler.

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].

Figure 3

(a) Current–voltage characteristics $j_{OLED}\left(u_{OLED}\right)$ (circles) and the dependence of the total radiant emittance on the voltage $m\left(u_{OLED}\right)$ (stars) for OLED based on SY; the symbol $u_{ON}$ denotes the threshold voltage; the inset shows the dependence of the current efficiency $\eta$ on the current density $j_{OLED}$; (b) spectral emittance $m_{\lambda }$ for different voltage values $u_{OLED}$.

Figure 4

Comparison of relative overlap integrals ξ of spectral emittance ${\mathrm{m}}_{\mathsf{\lambda }}$ of OLED based on SY transmitter and responsivity $\mathcal{R}\left(\lambda \right)$ of OLSD photodetectors based on different donor–acceptor mixtures.

Figure 5

Dependence of photocurrent density at photodetector output $j_{ph}$ on current density at transmitter input $j_{in}$ for the OLED/OLSD optocoupler. The inset shows the dependence of current ratio CTR on $j_{in}$ in the linear range of the optocoupler operation.

Figure 6

Comparison of the variations in direct ${CTR}_{dc}$ and alternating ${CTR}_0$ current transfer ratios as a function of the density of direct current component $j_{in,dc}$ for (a) the experimentally determined DC and AC current transfer ratios for the OLED/OLSD optocoupler at reverse voltage $u_{rev}$= 0 and their equivalents estimated based on the experimental results obtained for the OLED transmitter and the OLSD photodetector; (b) direct ${CTR}_{dc}$, and (c) alternating ${CTR}_0$ current transfer ratios determined for the photodetector biased with the reverse voltage $u_{rev}$ of two different values.

Figure 7

(a) The highest possible value of the amplitude of the variable component of current density $j_{in0,m}$ as a function of $j_{in,dc}$ for the assumed range of linear operation of OLED/OLSD for a photodetector unpolarized by reverse voltage; (b) dependence of coefficient ${\delta }_0$ on $j_{in,0}$ for different values of the constant component of current $j_{in,dc}$ in the range 12–451 mAcm⁻²; (c,d) coefficient ${\delta }_0$ as a function of the constant component of current density $j_{in,dc}$ supplied to the OPC input for different values of the amplitude of variable component $j_{in,0}$, for two cases: (c) a non-polarized photodetector and (d) a photodetector polarized with a reverse voltage $u_{rev}$= −2 V.

Figure 8

Variable components of current density at the output of the OLED/OLSD optocoupler $j_{ph\sqcap }$ and $j_{ph~}$ as a function of time t (red) after applying a voltage wave (blue) to the transmitter input. (a,c) Rectangular $u_{in\sqcap }$ and (b,d) sinusoidal $u_{in~}$ of two frequencies of 100 Hz and 158 kHz; (c) the symbols ${\tau }_{rise}$ and ${\tau }_{fall}$ 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) ${\tau }_{del}$ is the delay time of the receiver photocurrent after applying a voltage harmonic component to the input.

Figure 9

Comparison of frequency spectra for relative amplitude of radiant emittance $m_{rel}$ of the OLED (blue circles), relative amplitude of photocurrent density $j_{ph,rel}$ of the OLSD photodiode (red circles), and relative transadmittance ${TA}_{rel}$ of the OLED/OLSD optocoupler (green circles). Dashed curve—spectrum resulting from multiplication of $m_{rel}$ and $j_{ph,rel}$ spectra. Vertical dotted lines indicate three-decibel cutoff frequencies of OLED $f_{-3\mathrm{d}\mathrm{B},SY}$, OLSD $f_{-3\mathrm{d}\mathrm{B},IPT}$ and OPC $f_{-3\mathrm{d}\mathrm{B},OPT}$.