Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA11 / P23
Optimizing the Distance between Organic Light Emitting Diode (OLED) and Organic
Photodetector (OPD) for the Reflectance Pulse Meter Sensor
Muhamad Affiq Bin Misran1, Fahed Elsamnah1,Chang-Hoon Shim3 Anubha Bilgaiyan3, Ryutaro Sugawara4, Reiji Hattori2
1
Department of Applied Science for Electronics and Materials, Interdisciplinary Graduate School of EngineeringSciences, Kyushu University, Kasuga-city, Fukuoka 816-8580, Japan2
Global Innovation Center (GIC), Kyushu University, Kasuga-city, Fukuoka 816-8580, Japan
3
COI STREAM, Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuokacity, Fukuoka 819-0395, Japan
4Konica Minolta, Ishikawa-cho, Hachioji 192-8505, Japan
Email: muhamad.affiq90@gmail.com
Abstract: Pulse oximetry device has been used to observed the pulse rate and oxygen saturation in human blood at priorhealth examination before further diagnose can be performed. The reflection based pulse oximetry has been proposed bycomparing reflection and transmission type. The comparisons were performed by using the Monte Carlo simulation
method for human fingertip structure to find the optimum distance between light source and detector which shows closer
distance has higher signals compared with others. Actual devices were also fabricated to observe the differences between
closer and larger distance between the light source and detector. Higher signal-to-noise ratio (SNR) was observed for thecloser distance at low power consumption which brings to the possibility of lower power consumption pulse meter device
in the future.
Keywords: PPG sensor; pulse meter; Monte Carlo simulation
1. INTRODUCTION
Pulse oximeter is a device which using a noninvasive
technique to monitor oxygen saturation in the blood andalso the pulse rate as an initial procedure before further
diagnosing procedure can be perform onto a patient [1].
The signal and data obtained from the pulse oximeter
device will be used to appraised the numerous medical
conditions that related to the behavior of the heart andlungs [2]. In principle, there are two types of pulse
oximeter available in the market which are transmission
and reflection type as shown in Fig. 1. During pulsatile
condition, the volume of oxygenated blood (HbO2) willchanged repeatedly while the deoxygenated blood (Hb)and other components will remain the same. The
variations of the light absorption at different wavelengthfor each of the components also will determine theoxygen saturation and pulse rate. HbO2 component has
higher absorption coefficient at red wavelength (600-660nm) compared with the Hb component which has higher
absorption at green wavelength (550-550 nm) [3].

Fig. 1. (a) Principle of pulse oximeter. (b) Lightabsorption components.
In this study, the simulation process to determine theoptimal distance between light source and detector wasperformed for the fingertip structure. The comparisons
between reflection and transmission was performed inorder to observed the detection signal between these twotypes.

Previous study by Akl et al [4] demonstrated anexperiment to determine the optimal spacing between
transmitting and receiving optical fibers in reflectancepulse oximetry. In subsequent study, Akl et al [5] focusedon the optimal distance between the source and detector
by using point source Monte Carlo simulation. In contrast,
Huang et al [6] demonstrated a comparisons between
simulation and actual device for a ring-type pulse
oximeter with multi detector in order to increase theefficiency of the device. However, the comparisons
between the reflective and transmission type at finger
structure and also the comparisons between location ofthe pulse oximeter to be used are very important todetermine the efficiency and effectiveness of the device.

2. METHODOLOGY
2.1 Fingertip Structure Model
As for the fingertip structure used in this model, it
consists of skin, bone, arteries and veins. Fig. 3 shows thestructure and dimension of the fingertip structure.
2.2 Optical Properties
For the simulation process, several parameters need to betaken into consideration and all the parameters are shownin Table 1 [6].

- 27 -(2)2.5 Device Fabrication
The actual device was fabricated in order to verify theresults obtained from the simulation. Fig. 6 shows thedimensions for the Organic Photodetector (OPD) and red
light emission Organic Light Emitting Diode (OLED)
used in this device system. The OLED was located at thecenter and circulated by the OPDs at both sides. The
differences between these two devices are the distances
between the OLED and OPD and also the size of theOLED as shown in Fig. 6.

Fig. 3. Fingertip structure model.
2.3 Simulation Process
In order to determine the optimum distance between
source and detector, multiple simulations wereperformed by changing the distance between them at theinitial thickness of HbO2 and after increasing thethickness to imitate the pulsatile condition. Fig. 4 andFig. 5 show the location of source and detector for bothstructures and also at two different conditions which aretransmission and reflection.

Fig. 6. The OLED and OPD dimension in (A) Device-1and (B) Device-2 [7].
Fig. 5. Position of source and detector for fingertip
structure for transmission and reflection type.

The OPD and OLED were fabricated using the vacuum
process and the structure for both devices are illustrated
in Fig. 7.

2.4 Monte Carlo Simulation Method
This method has been widely use to study the lightscattering and distribution within the biological tissues
due to its flexibility and freedom. For this simulation,
Lighttools® software was used and it is based on theHenyey-Greenstein phase function where it describes thescattering behavior and given by [6]:,(1)Where PHG is the intensity in a specific degree. w and θrepresent the normalization parameter and the directionof scattering, respectively. The scattering angle from 0°
to 180° where the power distributes is represented by thisphase function. From here, the scattering anisotropy
parameter (g) [6]:-Fig. 7. The layer by layer structure for the OPD andOLED for this system [8].

Table 1. Optical properties for fingertip structure.
Refractive Index(η)

Anisotropy
parameter (g)Absorption
Coefficient (µa)
in mm-1

Scattering
Coefficient (µs)
in mm-1

MFP in mm

Epidermis

1.433

0.79

0.27

10.7

0.093458

Dermis and pulp

1.380

0.91

0.05

8.6

0.115600

Bone

1.400

0.92

0.05

33.1

0.030100

HbO2

1.363

0.98

0.20

77.50.012900

Hb

1.362

0.98

0.72

77.5

0.012800

Material- 28 -3. RESULTS AND DISCUSSION
3.1 Simulation Results
In the simulation, the light source given was 656 nm at
1 mW and the number of photons was 108. As in Fig. 8,
it shows the signal detected at the detector for reflection
and transmission type for fingertip structure. At 0 mm
distance, the signal obtained was 3.1x10-11 A/mm2 which
was higher than other distances and also with thetransmission type where the signal was 6.8x10-13 A/mm2.
However, the signal from the transmission type washigher than the reflection type when the distance between
the source and detector for the reflection type was at
4 mm and above. It shows that at some point, thetransmission has higher detection signal compared withthe reflection. Nevertheless, it still depends on thethickness of the fingertip since the thickness of fingertips
are varies upon each person.

currents were varied at 2.4 µA and 93.6 µA for bothdevices. Devicec-1 has the OLED size of 5.99 mm2 anddistance between OLED and OPD at 1.62 mm. Whereas,
Device-2 has the OLED size of 2.99 mm2 and thedistance of 2 mm between OLED and OPD.

Fig. 8. Signal detected at the detector from the reflection
and transmission mode at the fingertip structure.

Fig. 9. (a) The OLED’s electroluminescence spectrumwith respect to the wavelength (nm). (b) The OPD’s
external quantum efficiency (EQE) with respect to thewavelength.

The results showed that reflection type has higher
detection compared with the transmission type. The
reflection has more advantages from the transmission
type since it can be attached to any part of the body.
3.2 Device Characteristics
The actual device has been fabricated in order to observe
the comparisons between the simulation and actual data.
As has been showed previously in Fig. 6 and Fig. 7, around shape of red OLED at the center and the two OPDssurrounding the OLED so that the absorption of thesignal can be optimize. Fig. 9 shows the characteristics
and performances of both OPD and OLED devices. The
optimum intensity of light was at 625 nm based on theemission spectra in Fig. 9(a). The maximum External
Quantum Efficiency (EQE) for the OPD was about 37%
at 625 nm for zero-bias condition which sufficiently
overlapped with the peak of electroluminescence (EL) ofthe red OLED wavelength as shown in Fig. 9(b).

Fig. 10. The PPG signal obtained from (a) Device-1 and(b) Device-2.

3.3 PPG Signal
The photoplethysmogram (PPG) was measured by using
both fabricated devices. Based on Fig. 10, the signals
obtained were stable and clear for both devices. InFig. 10(a), the PPG amplitude was about 100 mV for theDevice-1 and less than 45 mV for the Device-2 inFig. 10(b) with a constant voltage of 5 V of the OLED
for both devices.

Table 2. Signal quality for both devices.
Source (µA)
93.6
2.4

Since both devices had different size of OLED anddistances between OLED and OPDs, a unify power wasused by supplying a constant current where the driving- 29 -Device

SNR (dB)

Device-1
Device-2
Device-1
Device-2

4546
188

Table 2 shows the signal quality from both devices atdifferent current source. The signal-to-noise (SNR) for
both devices at higher current show a very small
difference at 93.6 µA. It shows that the shorter distancewill has a possibility to increase the DC noise of the OPD
which then will reduce the SNR value at higher current
source. However, the SNR from Table 2 at lower current
source shows a significant difference between bothdevices. At 2.4 µA, Device-1 which has shorter distance
between OLED and OPD gives an acceptable SNR of thePPG signal of 18 dB and only 8 dB for the Device-2. It
shows that the shorter distance between OLED and OPD
gives better SNR value when considering with the power
or current source for the device.

plays an important role to further enhance the signal
detection for pulse detecting device in the future.
5. REFERENCES[1]
H. Lee, H. Ko, and J. Lee, “ScienceDirect
Reflectance pulse oximetry: Practical issues andlimitations,” ICT Express, vol. 2, pp. 195–198,
2016.
[2]
E. D. Chan, M. M. Chan, and M. M. Chan,“Pulse oximetry: Understanding its basic
principles facilitates appreciation of its
limitations,” Respir. Med., vol. 107, pp. 789–799, 2013.
[3]
C. M. Lochner, Y. Khan, A. Pierre, and A. C.
Arias, “ARTICLE All-organic optoelectronic
sensor for pulse oximetry,” Nat. Commun., vol.
5, 2014.
[4]
T. J. Akl, M. Hickey, and P. A. Kyriacou,“Optimal spacing between transmitting andreceiving optical fibres in reflectance pulse
oximetry Optimizing probe design for animplantable perfusion and oxygenation sensor
Optimal spacing between transmitting andreceiving optical fibres in reflectance pu,” J.
Phys. Conf. Ser. J. Phys. Conf. Ser, vol. 85.
[5]
T. J. Akl et al., “Optimizing source detector
separation for an implantable perfusion andoxygenation sensor,” 2011, p. 790605.
[6]
C. Y. Huang, M. C. Chan, C. Y. Chen, and B. S.
Lin, “Novel wearable and wireless ring-type
pulse oximeter with multi-detectors,” Sensors(Switzerland), vol. 14, no. 9, pp. 17586–17599,
2014.
[7]
F. Elsamnah, A. Bilgaiyan, M. Affiq, C.-H.
Shim, H. Ishidai, and R. Hattori, “ReflectanceBased Organic Pulse Meter Sensor for Wireless
Monitoring of Photoplethysmogram Signal,”
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F. Elsamnah, A. Bilgaiyan, M. Affiq, C. H.
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Even tough closer distance between OLED and OPD
gave better results at lower current source, the SNR wasstill at lower value compared with the higher current
source. Higher current source produced more lightintensity and irradiance which then probably gives better
SNR value. In this case, the Device-2 pattern will mostprobably give better SNR value compared with Device-1
based on the results from Table 2. Although thedifference was very small but there is still a possibility
that the SNR for Device-2 will increase greater thanDevice-1. This assumption was made due to thepossibility of the unwanted reflected light from the othercomponents of the skin will saturated the OPD andblocking the actual signal reflected from the blood
especially for the Device-1. In this case, the closer thedistance between OLED and OPD also will increase thepossibility of direct illumination from the OLED itself to
the OPD. In order to improve this device, new structureneed to be considered so that the closer distance between
OLED and OPD can be realize such as by adding ablocking layer or component between them. By doing
this, the direct illumination factor can also be eliminatedand further increase the signal quality of Device-1.
4. CONCLUSION
The reflection type was proposed at the beginning of thestudy due to the fact that it can be applied on any part of
the human body compared with the transmission typewhich is usually can be use at limited number of locationsuch as fingertip, earlobe and toe. The data obtained fromthe simulations showed that the reflection type has higher
signal detection compared with the transmission type.
The signal detected for the finger model structure for thereflection was 3.1x10-11 A/mm2 and higher comparedwith the transmission type which was 6.8x10-13 A/mm2.
By increasing the distance between the source anddetector will decrease the signal detection.
As for the experimental results, Device-1 shows anacceptable signal to produce the PPG signal comparedwith Device-2 which has larger distance between OLEDand OPD at lower current source which then brings to theprobability for fabricating a power saving device for thewearable pulse meter device in the future. In order toincrease the signal detection, several factor also need tobe considered for further study such as adding theblocking layer between OLED and OPD to eliminate thedirect illumination from the source to the detector. Other
factors such as size of OLED and OPD areas also will

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