E. Schabowska-Osiowska

Z. Porada

Active and Passive Electronic Components

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Thin Film PC-EL Type Electric and Optical Signal Delaying Systems

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Active and Passive Elec. Comp., 1994, Voi. 17, 91-97
Reprints available directly from the publisher
Photocopying permitted by license only) 1994 Gordon and Breach Science Publishers S.A.
Printed in Malaysia
THIN FILM PC-EL TYPE ELECTRIC AND
OPTICAL SIGNAL DELAYING SYSTEMS
Z. PORADA
Institute of Engineering and Electronics, Technical University, Warszawska 24, 31-1.5.5
Krak6w (Poland)
E. SCHABOWSKA-OSIOWSKA
Institute of Electronics, Academy of Mining and Metallurgy, Al.Mickiewicza 30, 30-1.59
Krak6w (Poland)
(Received February 24, 1994; in final form April 13, 1994)
1. INTRODUCTION
The rapid development of optoelectronics and more widespread utilization in tech-
nology of various types of optoelectronic systems has resulted in an increase in
interest in thin film PC-EL type systems. They are increasingly being used as logic
circuits, as amplifiers of weak light signals, and can be used to delay electric and
optical signals. The electric and optical signal delay lines find application in various
types of industrial telecommunication systems.
In the presented work, the dependence of the delay times of electric pulse signals
and optical signals in the form of rectangular light pulses on the parameters of PC-
EL system was investigated. Characteristics obtained were compared with the
curves calculated from a proposed mathematical model.
2. THEORETICAL CONSIDERATION
A diagram of electric signal delaying system is shown in Fig. 1. In such a system,
the input signal is the voltage U,p supplying the element ELt, and the output signal
is the current lout through the element PC.
The switching on of the voltage U.o will cause the emission of light from the
element ELI of luminance B, which can be described by the relation:
B,(t) B0t exp(-yt t) exp[bt/([U,pl)’21 (1)
where Bo, ’)/i and b are the parameters, constant for given electroluminescent cell
EL.
91
92 Z. PORADA AND E. SCHABOWSKA-OSIOWSKA
FIGURE
PC2 EL2 PC1
PC-EL type electric signal delaying system.
The light with luminance Bl will then illuminate the element PC,., causing an
increase of its conductance Gpc2, which can be described by following equation:
d Gpc2 G02
d- Gpc2 a2/3al (2)
where G02 is the "dark" conductance of non-illuminated element PC2, ’rE is the
photoconductivity rise time for this element, a2 is a parameter, constant for given
element PC2, and/31 is the unidirectional optical coupling coefficient of the element
ELI with the element PC2.
The increase of the conductance Gpc2 will cause an increase of the voltage on
the element EL2, so that it will emit the light of luminance BE, which can be
described with a formula similar as in the case of luminance Bl"
B2(t) B,,2 exp(- 720 exp[- b2/(IUEL2I) 1’2] (3)
where the voltage UEL on the element EL2 is given by the formula:
d
CEL2
=
UEL2 + UEL2(GEL2 + Gpc2) GpcEU2 (4)
where Bo2, Y2, and b2 are parameters of electroluminescent cell EL, CEL is its
capacity, GEL2 is its leakage conductance, and U2 is the voltage supplying the PCE-
EL2 system.
The light emitted by the element EL2 illuminates the photoconducting element
PC, causing an increase of its conductance Gacl and thereby an increase in the
output current Io,t. The course of this current vs. time Io,t(t) can be expressed by
the formula:
lout(t) Opcl(t)Ui (5)
where the conductance Gpcl(t) is described by the eqution:
d Gpcl Go
d Gpc, a,/i2B2 (6)"rl
THIN FILM PC-EL DELAYING SYSTEMS 93
FIGURE 2 Diagram of an optical signal delaying PC-EL system.
where Got is the "dark" conductance of non-illuminated element PC, r is the
photoconductivity rise time for this element, at is a parameter, constant for given
element PC, /32 is the unidirectional optical coupling coefficient of the element
EL2 with the element PC, and U is the voltage supplying the circuit with the
element PCI.
The solution of the set of equations (1)-(6) allows one to calculate the delay
time z2 of the output signal lout in relation to the input signal U,p.
In Fig. 2, the diagram of an optical signal delaying PC-EL system is shown. In
the circuit under consideration, the input signal is a light pulse illuminating PC
element, the illumination of which is L, and the output signal is the light emitted
by the element EL2, the luminance of which is B2.
The illumination of the element PC causes an increase in the conductance Gec
of this element, the result of which the voltage on the element EL will increase
so that the electroluminescent cell EL will emit the light of luminance B. This
luminance can be described by the expression (1), where U,p UEL. The voltage
UEL is given by the formula:
dCEL
-
UEL + UELI(GEL "+" Gp,) Op,U, (7)
where CELt are, respectively, the capacity and the leakage conductance of the
element EL, U is the voltage supplying the PC-EL circuit, and the conductance
Gvc is described by the formula:
d Gpc G0
d- Gpct aLi (8)TI
where a is a parameter, constant for the element PCt, zt is the photoconductivity
rise time for this element, and Got is the "dark" conductance of non-illuminated
element PC.
As can be seen on Fig. 3, the element EL is optically coupled with the element
PC2 and, thereby, the illumination of the element PC2 will be L2 /3B, where/3
is the unidirectional optical coupling coefficient. Thus, the output signal in the
94 Z. PORADA AND E. SCHABOWSKA-OSIOWSKA
cas(c ,cO CaS( CO
z o3(s,O
FIGURE 3 Thin film PC-EL type optical signal delaying circuit.
form of luminance BE of the light emitted by the element EL2 can be described
using the dependence"
B2(t) B,,2 exp( y2t) exp[- bE/(IUEL2I) ’/2] (9)
where the voltage UEL is described by the formula:
dCEL UEL -k UEL2(GEL2 -t- Gp) GpcU (10)
and the conductance Gpc2 Of the element PC2 is given by the equation:
d Gpc2 GII2
d Gpc2 a2/3B (11)’2
where B02, ’)’2, and b2 are parameters, constant for given element EL2, CEL2 is the
electric capacity of this element, GEL is its leakage conductance, a2 is a parameter,
constant for the element PC2, G02 is the "dark" conductance of non-illuminated
element PC2, and U2 is the voltage supplying the PCE-EL2 circuit.
After solving the set of equations (7)-(11) the delay time % of output signal
BE in relation to input signal L can be determined.
3. RESULTS OF EXPERIMENTAL INVESTIGATION
The circuit PC-EL delaying the optical signal (Fig. 3) was composed of two pho-
toconducting elements, PC and PC2, and two electroluminescent cells, EL and
EL2.
In the elements PC, the photoconducting layer was made of copper- and chlorine-
doped cadmium sulphide, the transparent electrode was a tin-doped indium oxide
layer, and the second electrode an indium layer3.
THIN FILM PC-EL DELAYING SYSTEMS 95
15
10
5
FIGURE 4 The dependence of luminance B2 and illumination L on time for the delaying circuit PC-
EL supplied with equal sinusoidal voltages U U2 420 V of the frequency 70 Hz.
In the elements EL, the electroluminescent layer was copper-, chlorine- and
manganese-doped zinc sulphide, the transparent electrode was a tin-doped indium
oxide layer, and the second electrode was an aluminium layer4.
In the circuit under consideration, the dependence of luminance B2 on time, as
well as the dependence of the illumination L on time, for the delaying circuit PC-
EL supplied with equal sinusoidal voltages U1 and U2 (fl f2 f) of the frequency
f 70 Hz (Fig. 4) were investigated.
o
FIGURE 5 The dependence of the delay time ’op on the frequency of the voltage supplying the PC-
EL circuit.
96 Z. PORADA AND E. SCHABOWSKA-OSIOWSKA
Z S(C ,Ct,M )
ISOx/I ,gto.s
cds(c ,cO
FIGURE 6 Thin film PC-EL type electric signal delaying circuit.
In Fig. 5, the dependence of the delay time Zop on the frequency of the voltage
supplying the PC-EL circuit (fl f2 f) is presented. The electric signal delaying
circuit is shown in Fig. 6. In the considered circuit, the dependence of the voltage
U,p and the current Io,t on time was investigated (Fig. 7). In Fig. 8, the dependence
of delay time z0 on the frequency of voltage supplying the PCE-EL2 circuit is pre-
sented.
4. CONCLUSIONS
Results from the analysis of proposed theoretical model indicate that the delay
time of the optical signal Zop depends, in the first place, on the photoconductivity
FIGURE 7 Time dependences of the input signal Uin and of the output signal Io.t; f 140 Hz.
THIN FILM PC-EL DELAYING SYSTEMS 97
o (.z)
FIGURE 8 Dependence of delay time ,, on the frequency of supplying voltage U2.
rise times r and rE, as well as on the frequency of supplying voltages U and
the value top can be increased, in the circuit under consideration, by using a greater
number of PCI-EL2 circuits.
The delay time of electric signal r0 depends also on the values of photoconduc-
tivity rise times rl and rE, as well as on the frequency f of voltage U2 supplying the
PCE-EL2 circuit.
The PC-EL type electric signals delaying systems, as well as optical delaying
system, have an additional advantage; namely, the connection of input circuits with
output circuits in these systems is not galvanic, i.e., from the point of view of
electric circuits they perform the function of a separator.
REFERENCES
1. Z. Porada and E. Sehabowska, J. Luminescence, 21, 129 (1980).
2. P.S. Kireev, Semiconductor Physics, Mir, Moscow, 1978 (English translation).
3. Z. Porada and E. Schabowska-Osiowska, Active and Passive Elec. Comp., 13, 151 (1988).
4. Z. Porada and E. Schabowska, Vacuum, 33, 179 (1983).
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Thin Film PC-EL Type Electric and Optical Signal Delaying Systems

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