THE EFFECT OF INTERFACE MODIFICATION BY PEDOT: PSS ON THE HOLE MOBILITY OF THE LEC DEVICE

International Journal on Organic Electronics (IJOE) Vol.11, No.1, January 2022
DOI: 10.5121/ijoe.2022.11101 1
THE EFFECT OF INTERFACE MODIFICATION
BY PEDOT: PSS ON THE HOLE MOBILITY
OF THE LEC DEVICE
Ali Kemal Havare
Electrical and Electronics Engineering, Photoelectronics Lab (PEL),
Toros University, Mersin, Turkey
ABSTRACT
The purpose of the work is to understand how to effect the interface PEDOT: PSS on the hole mobility of
the LEC device by Space Charge Limited Current (SCLC) approaches technique. PEDOT: PSS plays a
significant role in organic electronics device as interface modification, particularly on Light-emitting
electrochemical cells (LEC) due to fundamental structure of hole only device. This study analyses the hole
mobility of the device based on current-voltage characteristic approach at room temperature. It has been
observed that the PEDOT: PSS interface increases the hole mobility of the LEC device by a factor of 108
.
KEYWORDS
Hole mobility, LEC, PEDOT: PSS, OLED, organometallic Iridium (III) Complex
1. INTRODUCTION
Light-emitting electrochemical cells (LECs) containing ionic compounds in the light-emitting
layer have attracted interest in organic electronics in recent years due to their simple structure and
easy flexible fabrication techniques [1, 2]. This simple structure of LEC devices compared to
organic light-emitting diodes (OLEDs), LECs achieve the multi-tasking electron-hole
transmission and their assembly in the light-emitting layer at the same time with a single active
layer [3, 4]. Also, LECs are coated easily on transparent conducting films and then electrodes can
be formed on this layer with Al, Ag or Au. Since holes are injected in a balance with the electric
field applied to the electrodes, high brightness and efficiency can be achieved at low operating
voltage for single-layer LEC devices. All these features are the reasons why LECs are studied a
lot in the literature as an alternative light emitting diode. Metal complexes are mostly used as
emissive materials in LEC devices. Ionic transition metal complexes (iTMCs) have been widely
applied as a single layer in LEC devices [5-11]. The most prominent feature of iTMC materials is
that they catalyse charge transfer reactions with their tunable oxidation and reduction properties.
Ir(III) complex-based iTMC-LEC devices allow to create high electric fields in the device, thanks
to their ability to work with air-resistant electrodes [12-14]. Hole injection at the
oxide/organometallic interfaces, which form the basic structure of the LEC device and allow the
light to be formed and emitted here, is a crucial parameter that affects the performance of the
device [15-21]. The transparent conductive oxide, indium tin oxide (ITO), which forms the anode
electrode of this interface in LEC, is a factor that limits the performance of the device because it
is both rough and unsuitable for hole injection [22-24]. To overcome such problems, PEDOT:
PSS is formed at the interface of the organometallic light-emitting layer with the ITO. Thus, the
PEDOT: PSS interface provides an increase in hole injection in the LEC device, a decrease in the
operating voltage, and an improvement of the light performance due to the transparency of

2
PEDOT: PSS to visible light. [25-31]. Moreover, the high energy of the occupied electronic
levels of the PEDOT moiety in the molecular chain of PEDOT: PSS compared to ITO results in a
lower value of the barrier potential energy at the ITO/organometallic interface. [32-37].
In this work, the theoretical calculations of the hole mobility 𝜇 bases on the current-voltage
values of the LEC device. The active layer of the LEC is commercial material Iridium(III)
Complex (Ir[dF(CF3)ppy]2(dtbpy))PF6). This paper just focus on how to effect the PEDOT:PSS
interface on calculation and comparing of the hole mobility of the LEC devices. The optical
characteristic does not issue of this study.
2. MATERIAL AND METHODS
In order to fabricate a LEC device an ITO glass as anode and the bottom-up technique is used.
The ITO transparent conductive films are cut as 1.5x1.5 cm. After cleaning procedure [37, 38] of
ITO substrates, the PEDOT-PSS is coated with 3000 rpm for 45 seconds by Spin coater (model
SPIN150). The PEDOT: PSS thin layer was annealed in a vacuum oven at 110 °C for 25 minutes.
The Iridium (III) Complex is purchased from Sigma Aldrich to modify the LEC devices consist
of just one active an emitter layer. The solutions of Ir(III) complexes (20 mg/mL in acetonitrile)
were layered by spin coating and dried at 110 °C for 25 minutes in a vacuum oven to obtained a
smooth film. Thermal evaporation system (NANOVAK Corporation) is used to evaporate the
aluminum under 10-6
Torr vacuum (100nm). After all stages the LEC devices (ITO/Iridium
(III)Complex/Al and ITO/PEDOT:PSS/Iridium(III)Complex/Al) are ready to characterize using
a Keithley 2401 source. The Current-Voltage (I-V) data of the LEC diode is obtained by Keithley
2401 source under room temperature (Fig. 1). The hole mobility characterization of LEC diode
can be calculated using the current-voltage curves. The hole mobility calculations [39] of LEC
were made with the Space Charge Limited Current (SCLC) [40] approach considering the high
potential differences. The SCLC curve trend can be given by the following equations; . 𝐽 =
9
8
𝜀𝜀0𝜇
𝐸2
𝐿
(1) and 𝜇(𝐸) = 𝜇0𝑒𝑥𝑝(𝛽√𝐸) (Poole-Frenkel equation) (2). The equation (1) become,
𝐽 =
9
8
𝜀𝜀0
𝐸2
𝐿
𝜇0𝑒𝑥𝑝(𝛽√𝐸) (3). In eq. (1): 𝐽, current density; 𝜀 and 𝜀0, the dielectric constant and
the permeability of empty space; E, the electric field; L, the organic layer thickness. In eq. (2): 𝜇,
the mobility; 𝜇0, the mobility in the zero electric field; β, the Poole-Frenkel coefficient. The
concept of the mobility is the ability of charges to move. The J-V characteristics allow
investigating that how the PEDOT: PSS interface effect the hole mobility of the diode by using
the specified equations.
3. RESULT AND DISCUSSION
3.1. The hole mobility calculation
The forward bias current-voltage characteristics of the LEC structure at 300 K is showed in Fig.
1.

3
Figure 1. The forward bias current-voltage characteristics of the LEC structure and LEC devices are
fabricated at Photoelectronics Lab (PEL), Toros University, LEC devices under measurement in glove box
at 300 K°.
Figure 2. Plot of log J versus E1/2 of the ITO/PEDOT:PSS/Iridium(III)Complex/Al and
ITO/Iridium(III)Complex/Al diode.

4
Figure 3. Plot of mobility values 𝜇 versus E1/2
of the ITO /PEDOT:PSS/Iridium (III) Complex/Al and
ITO/Iridium (III) Complex/Al diodes.
By Fig. (2), Fig. (3) and equation (3), β, Poole-Frenkel coefficient and μ0 are calculated. In
addition, ε = 3 and ε0 = 8.85×10-12
Farad/meter were taken for all calculations. By Equation (3),
mobility values μ versus E1/2
of the ITO/PEDOT: PSS/Iridium (III) Complex/Al and ITO/Iridium
(III) Complex/Al diodes are plotted in order to get an idea how the mobility changes in diodes up
to electric field. The mobility values of ITO /PEDOT:PSS/Iridium (III) Complex/Al diode are
found as 21x10-5
cm2/Vs as the highest mobility value. The mobility of the
ITO/PEDOT:PSS/Iridium(III)Complex/Al and ITO/Iridium(III)Complex/Al diode are 21×10-5
cm2/Vs and 15×10-13
cm2
/Vs respectively under 495 (V/cm2
)1/2
electric field. The LEC devices
fabricated in this work showed an ideal I–V behavior (Fig. 1). The hole mobility of the LEC
device is calculated by SCLC approach technique. According to SCLC approach the mobility 𝜇
of ITO/PEDOT: PSS/Iridium(III)Complex-PF6/Al diode is found by factor ~108
comparing the
ITO/PEDOT:PSS/Iridium (III) Complex/Al diode and it is given the Table 1. The main reason for
this is thought to be the high energy of the occupied electronic levels of PEDOT in PEDOT: PSS
compared to ITO leads to a reduction of the barrier potential energy at the ITO/organometallic
interface. The I-V characteristics of the LEC structures show that more holes tunnel from ITO to
PEDOT molecular orbitals at the same time a decrease in the barrier height of the diode. In
addition, considering the 100 nm and 200 nm film thicknesses of the LEC devices, it was
observed that there was an inverse correlation between the hole mobility and the increasing
distance, thanks to the extra energy levels created by PEDOT: PSS, despite the increase in the
distance for tunneling the holes at the ITO/organometallic interface. This is one of the important
parameters to be considered when manufacturing LEC devices.

5
4. CONCLUSIONS
In summary, the I–V characteristics of ITO/PEDOT:PSS/Iridium(III) Complex-PF6/Al junctions
are studied. In this study, the SCLC method is used for calculating the hole mobility of the LEC
device in order to understand how effect the PEDOT: PSS layer as interface on the hole mobility
of the cell. The results show that PEDOT: PSS is enhance the hole mobility. According to the
SCLC analysis results, the diode modified with PEDOT:PSS material possesses the best
performance. The used PEDOT: PSS can be attributed to the change in barrier height values and
the increase in hole injection.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this paper.
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AUTHOR
Ali Kemal Havare has been a research scientist at Photoelectronics Lab in the
department of Electrical-Electronic Engineering at Toros University since 2012. He
holds the MSc and PhD in Mathematical Physics from Mersin University and in
Energy from the Solar Energy Institute at Ege University, respectively. He studied at
University Illinois at Chicago in the Microphysics Lab. on X-Ray Photoelectron
Spectroscopy during the PhD. His research interest includes applications and the
electrical characterizations of organic semiconductor on electronic devices.

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