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JOURNAL OF DISPLAY TECHNOLOGY, VOL. 6, NO. 7, JULY 2010 247

Power Recycling of Large-Area OLEDsUsing Solar Cells

Jongwoon Park, Jongho Lee, and Dongchan Shin

AbstractWe demonstrate that power recycling is feasible bymerging a large-area (30 120 mm2 ) OLED panel and a solarcell into each other. The power recycling efficiency of 0.152% isachieved under the illumination of one side-emitting white OLEDat 2 450 cd/m2 when the conversion efficiency of a reference solarcell is 4% and the distance between the OLED and solar cell is1.5 mm. We have found that the power recycling efficiency isdecreased under high brightness due to a decrease in the powerefficiency of OLED and a loss of current induced by the resistanceof a transparent electrode. We have also shown that local heatgeneration of the large-area OLED panel would be an issue forpower recycling.

Index TermsOrganic light-emitting diodes (OLEDs), solarcells, power recycling, lightings.

I. INTRODUCTION

RECENTLY, extensive researches of flat panel lightings

using organic light-emitting diodes (OLEDs) have been

made due to their superior features such as low power con-

sumption, long lifetime, large surface-emitting area, low cost,

flexibility, etc. [1][12]. In particular, much attention has been

drawn to transparent flat panel OLED lightings, enabling a

variety of applications including light-emitting window glasses

for buildings, cars, and electronic goods. OSRAM Opto Semi-

conductors has developed transparent OLED lighting panels,exhibiting the transparency of 55% and power efficiency of 20

lm/W at a brightness of 1000 cd/m [13]. When transparent

OLED lightings are used for light-emitting windows, a light

emits not only from the front side but also from the rear side of

OLEDs. In this case, a light emitting from the front side would

be used for illuminations, but a light from the rear surface may

be wasted. Therefore, it would be desirable to make use of such

a wasted light for generating electrical power. A transparent

OLED lighting panel combined with a solar cell would be

such a case in point. A solar cell will absorb a light emitting

from the rear side of OLEDs and convert it to electrical power

that is fed to OLEDs for energy recycling. It is a matter ofcourse that the merged solar cell generates electrical power by

absorbing sunlight in the daytime. To this end, a transparent

solar cell like a dye sensitized solar cell (DSSC) or organic

Manuscript received September 18, 2009; revised March 17, 2010. Currentversion published May 12, 2010. This work was supported by the Ministry ofKnowledge Economy under 2008-10028527, Development of OLED auxiliarylighting system.

J. Park and J. Lee are with National Center for Nanoprocess and Equipment,Korea Institute of Industrial Technology, Gwangju 500-480, Korea (e-mail:[email protected]).

D. Shin is with the Department of Advanced Material Engineering, ChosunUniversity, Gwangju 501-759, Korea.

Digital Object Identifier 10.1109/JDT.2010.2046717

solar cell (OSC) need be integrated with a transparent OLED

lighting panel. There may exist several ways to implement such

configurations. One way is to deposit organic layers of OSC

and OLED in series on a common glass substrate [14]. This

approach was suggested mainly to reduce the ambient-light

reflection of OLED displays. With this scheme, the power recy-

cling efficiency (the ratio of the output power of solar cell to the

electrical input power of OLED) of 0.26% has been achieved

on a pixel scale. Another way is to deposit organic layers of

OSC on one side of the common glass substrate and organic

layers of OLED on the other side of the common substrate.

However, the simplest way would be to merge OSC and OLED

into each other after they are fabricated separately. With this

configuration, the power recycling efficiency can be enhanced

by: 1) reducing the operating voltage of OLED; 2) increasing

the power efficiency of OLED; 3) increasing the conversion

efficiency of solar cell; 4) reducing the distance between solar

cell and OLED; 5) reducing the panel size; and 6) matching

the absorption spectrum of solar cell with the emission one of

OLED. To our knowledge, there has not yet been any research

on a large-area OLED lighting panel merged with a solar cell

for power recycling. In the respect, a preliminary study of

the possibility of power recycling using a one side-emitting

large-area OLED panel is highly demanded. This paper iswritten with the following objectives. First, we clarify the

feasibility of power recycling from one side-emitting large-area

OLED lighting panels. Compared to one sun with the Air

Mass 1.5 (AM1.5) solar simulator, the luminous intensity of

large-area OLED lightings is relatively low, probably no power

recycling by which is achieved. Therefore, such a preliminary

study deserves to be made. Second, we investigate the effects

of the luminous intensity of OLED and the distance between

OLED and solar cell on the power recycling efficiency. Finally,

we demonstrate that large-area OLED lighting panels used for

obtaining high photo-current need heat radiation since the heat

distribution is highly inhomogeneous with increased injectioncurrent.

II. EXPERIMENT

All-fluorescent white OLED lighting panels we have fabri-

cated are composed of a 150-nm-thick indiumtinoxide (ITO)

pre-coated on a glass substrate, 35-nm-thick 4,4-bis[N-(1-

nathyl)-N-phenylamino]biphenyl (NPB) for a hole transport

layer (HTL), 30.5-nm-thick AGH-001 (SFC) for an emitting

layer (EML), 30-nm-thick ET-137 (SFC) for an electron

transport layer (ETL), 1-nm-thick lithium fluoride (LiF) for an

electron injection layer (EIL), and 100-nm-thick aluminum (Al)

1551-319X/$26.00 2010 IEEE

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248 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 6, NO. 7, JULY 2010

Fig. 1. (a) Real image of thelarge-areaOLED lighting panel (the emissionareais 302 120 mm and the panel size is 602 170 mm ). (b) Test setup; the OLEDpanel sheds white light on the reference solar cell.

for cathode. In the EML, 2 wt% NRD-129 (SFC) is doped in the

0.5-nm-thick EML layer (near the HTL/EML interface) for red

emission and 3 wt% BD-02 (SFC) is doped in the 30-nm-thick

EML layer for blue emission. To obtain homogeneous light

output, we have deposited chrome (Cr) on ITO, patterned Cr

in the form of a grid, and then covered the patterned Cr lines

with an insulating material. The Cr thickness is about 360 nm

and the thickness of insulator is 565 nm. The distance betweenthe patterned Cr lines is ten times greater than the width of

each Cr line. Namely, the ratio of the emitting area to the

non-emitting area is 10:1 [15]. Shown in Fig. 1(a) is the real

image of the large area (30 120 mm ) OLED lighting panel

and in Fig. 1(b) is the test setup where the OLED panel sheds

white light on a solar cell. For measurements, we have em-

ployed the Oriel mono-Si reference solar cell with the National

Renewable Energy Laboratory (NREL) KG5 filter. The active

area of the reference cell is 3.991 cm , a short-circuit current

density is 11.34 mA/cm , an open-circuit voltage

is 0.502 V, a filling factor (FF) is 70.46%, and the conversion

efficiency is 4.012% under AM1.5 with irradiation intensity of100 mW/cm .

Fig. 2. (a) Power efficiency and luminance of the white OLED as a function ofcurrent density. (b) CIE coordinates for different bias voltages (410 V).

III. RESULTS AND DISCUSSION

Since the conversion efficiency of a solar cell depends

strongly on the light intensity, white OLED showing high

luminance rather than high efficiency would be preferred for

the experiments. Fig. 2(a) shows the power efficiency and

luminance of the small-area (2 2 mm ) white OLED with

respect to the current density. It exhibits the power efficiency

of 10.7l m/W at 4 V (at 1 533 cd/m ) and the maximum lumi-

nance of 83 220 cd/m at 10 V. Though the power efficiency

of the white OLED is a little low, yet the luminance is high

enough for the experiments of power recycling efficiency. In

addition, white OLED showing good color stability versusdriving voltage is highly demanded. If the emission spectrum is

much swayed by driving voltage, the efficiency of a solar cell is

varied accordingly, thereby making an experiment less reliable.

We have measured the Commission Internationale dEclairage

(CIE) chromaticity coordinates for different driving voltages

and presented the results in Fig. 2(b). It is observed to be (0.32,

0.4) at 1 533 cd/m (at 4 V) and (0.32, 0.38) at 83 220 cd/m

(at 10 V), demonstrating very good color stability.

The conversion efficiency of a solar cell is known to be lim-

ited by the extent of an overlap between the emission spectrum

of a light source and the absorption one of the solar cell. Fig. 3

shows the emission spectrum of the white OLED and quantum

efficiency of the reference solar cell. It is obvious that the emis-sion spectrum of OLED stays well within the spectrum range of


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PARK et al.: POWER RECYCLING OF LARGE-AREA OLEDS USING SOLAR CELLS 249

TABLE ISUMMARY OF THE MEASURED VALUES

Fig. 3. Luminous spectrum of the white OLED measured at 4 V and quantumefficiency (QE) of the reference solar cell versus wavelength.

Fig. 4. I V characteristics of the reference solar cell under the illuminationof the large-area (30 2 120 mm ) OLED panel for different distances betweensolar cell and OLED and for different bias voltages of OLED.

the solar cell showing high quantum efficiency. However, it is

desirable to broaden the emission spectrum of the white OLED

further to make full use of the useful solar cell spectrum. In this

study, white-light generation is achieved with two complemen-

tary (sky blue orange red) colors using a fluorescent mate-

rial system. As such, broadening the emission spectrum further

would be doable with three colors; e.g., deep blue (430 nm)

green (520 nm) red (620 nm).

We have measured the characteristics of the ref-

erence solar cell under the illumination of the large-area

(30 120 mm ) white OLED panel and presented the results

in Fig. 4. To verify the effect of the distance between thesolar cell and OLED on the - characteristics of the solar

cell, we have carried out experiments at different distances of

1.5 and 9.5 mm. It is observed that a is increased from

0.477 to 0.621 mA/cm with decreasing distance, while a

is increased from 0.36 to 0.38 V and an FF from 54.52%

to 55.59%. Since the light intensity of the OLED panel that

reaches the solar cell is decreased when the distance between

them is large, they should be merged into each other as closely

as possible. With the distance fixed to be 1.5 mm, we have

then increased the bias voltage of the large-area OLED paneland observed the effect of the luminous intensity on the -

characteristics. The luminance of the large-area white OLED

is measured to be 2450 cd/m at 5 V, 5042 cd/m at 6 V, and

8953 cd/m at 7 V. Asthe luminance increases, , and FF

are shown to be raised to a great extent. A is increased up to

1.62 mA/cm , a up to 0.42 V, and a FF up to 63.14%. The

open-circuit voltage (0.42 V) measured under the illumination

of OLED at 7 V is still lower than that (0.502 V) observed under

AM1.5 with irradiation intensity of 100 mW/cm . Moreover,

the open-circuit voltage varies depending sensitively on the

short-circuit current density. The maximum voltage that can be

obtained from a solar cell is given as [16]

(1)

where is the short-circuit current and is the saturation

current in reverse bias. In reality, the dependence of on

is very small because is much larger than and thus no big

change in is observed. However, when is of the order of

, the dependence of on is large and hence is much

varied by the incident light intensity, which is also observed in

Fig. 4. It means that the luminous intensity (8 953 cd/m ) of the

white OLEDat 7 V is still too weak to reach a state in which

is independent of . We have summarized all the experimentresults in Table I.

Fig. 5 shows is the measured photo-current and power re-

cycling efficiency versus the luminance of the white OLED.

It is observed that the photo-current is increased, whereas the

power recycling efficiency is decreased with increasing lumi-

nance. The photo current is as high as 6.465 mA at 7 V and

the maximum power recycling efficiency is 0.152% at 5 V. As

also seen in Table I, the power recycling efficiency is decreased,

though the output power of the solar cell is raised with increased

input power of the white OLED. It is attributed to the fact that

the power efficiency of the white OLED is decreased as the cur-

rent density (input power) is increased [see Fig. 2(a)]. To en-

hance the power recycling efficiency, therefore, one needs to in-crease the power efficiency at high luminance or rather reduce


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250 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 6, NO. 7, JULY 2010

Fig. 5. Measured photo-current and power recycling efficiency versusluminance.

the driving voltage of white OLEDs. The low power recycling

efficiency also originates from the fact that the power consump-tion of OLED is getting high with increasing panel size due to

a loss of current induced by the resistance of a transparent elec-

trode [15]. As such, a smaller-area OLED lighting panel is pre-

ferred for high power recycling efficiency. In brief, the power

recycling efficiency of 0.152% can be boosted by reducing the

operating voltage of OLED, increasing the power efficiency of

OLED, reducing the panel size, reducing the distance between

solar cell and OLED, and matching the absorption spectrum of

solar cell with the emission one of OLED. It can be further in-

creased using a solar cell with higher conversion efficiency.

To get high output power of a solar cell, we need high bright-

ness white OLED lighting panels. Under high brightness, how-

ever, heat generation or inhomogeneity in the heat distributionconcerns us [12], which shortens the lifetime of OLEDs, shifts

the CIE coordinates, or damages an adjacent solar cell. This

problem is getting serious for transparent OLEDs since a light

is split and thus more current injection is required to get the

same radiation intensity. To estimate heat generation fromlarge-

area OLED panels under high brightness, we have measured the

heat distribution over the panel at different bias voltages with a

thermal imager (testo) and presented the results in Fig. 6 (also

in Table II). The ambient temperature was 25.1 C. One can see

that local heat generation is getting pronounced with increasing

injection current. Since the current flows mainly through the Cr

lines, it may be homogeneously distributed over the large-areaOLED panel. However, the vertical resistance of OLED may

be inhomogeneous due to the nonuniform thickness of organic

layers [15]. As such, the current crowds in local area, resulting

in highly inhomogeneous heat distribution. A heat sink may be

required for such a high-brightness large-area OLED panel.

IV. CONCLUSION

We have demonstrated that power recycling is feasible by

merging the one side-emitting large-area OLED panel and the

solar cell into each other. It is observed that the photo-current is

increased, whereas the power recycling efficiency is decreased

as the luminous intensity of OLED increases due to a decreasein the power efficiency of OLED and a loss of current induced

Fig. 6. Heat distribution at: (a) at 5 V; (b) 6 V; and (c) 7 V measured in fiveminutes.

TABLE IIMEASURED TEMPERATURES

by the resistance of a transparent electrode. We have also

shown that heat generation of the large-area OLED panel is

highly inhomogeneous under high brightness. This work pro-

vides useful information and design guidelines of a transparentOLED lighting panel integrated with a transparent solar cell.


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PARK et al.: POWER RECYCLING OF LARGE-AREA OLEDS USING SOLAR CELLS 251

ACKNOWLEDGMENT

The authors would like to thank Prof. Soohyoung Lee for his

help in measuring the - characteristics of the reference solar

cell.

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Jongwoon Park received the B.S. degree in radioengineering from Kyunghee University, Suwon,Korea, in 1999, the M.S. degree from the Universityof Victoria, BC, Canada, in 2001, and the Ph.D.degree in electrical engineering from McMasterUniversity, ON, Canada, in 2004.

He joined the Department of Electronic Scienceand Engineering at Kyoto University, Kyoto, Japan,as a Post-Doctoral fellow in 2005. Since 2007, hehas been working for Korea Institute of IndustrialTechnology (KITECH), Gwangju, Korea, as a senior

researcher. His research interests are organic light-emitting diodes (OLEDs),GaN-based light-emitting devices, fiber-optic communication systems, andnumerical modeling and simulation of semiconductor optoelectronic devices.

Jongho Lee, photograph and biography not available at time of publication.

Dongchan Shin received his B.S. degree in materialsscience and engineering at Korea University, Koreain 1991 and he received his M.S. and Ph.D. degreesin materials science and engineering at KoreaAdvanced Institute of Science and Technology(KAIST), Korea in 1993 and 1997, respectively. Hewas postdoctoral associate from 1997 until 2000at KAIST and Michigan Technological University,senior researcher at Samsung SDI from 2000 until2002. Since 2002, he has been a professor in Depart-ment of Advanced Materials Engineering at Chosun

University, Korea. His current research is nanophotonics area. Specificallyhis research interests are photonics crystal and its application to display,nanocrystalline solar cell device, nanopowder preparation and sintering. Hehas written 2 book chapters and more than 40 journal articles. He is memberof the Ceramic Society of Korea, the Korean Information Display Society, theMaterials Research Society of Korea, the Optical Society of Korea, and MRS.

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