Organic Light Emitting Devices Integrated with

Solar Cells

Manikandan Sampathkumar4 /16 / 2012U84037635

Outline1. Introduction

2. Process and impact

3. Organic light emitting devices integrated with solar cells for “high contrast and energy recycling “.

I. Conventional methodsII. Integrating with solar cellsIII. FabricationIV. Results and application

4. Power recycling of “transparent large area OLED panels “ using solar cells

V. Integrating with solar cells .VI. Power recycleVII. FabricationVIII. Advantages and resultsIX. Challenges

5. Conclusion

6. References

MotivationThe diversity in Organic display's

has amazed me from the moment I was introduced to them . But I had noticed that there was a lot of free energy wasted in their operation . I was motivated in making use of this useful energy for a better operation of the device, which made me choose the topic of integrating organic display’s with solar cells .

Brief Introduction• Organic light emitting diodes(OLED) are

devices which exhibit luminescence due to the recombination of electron – hole pair. This recombination releases a photon.

• Organic photovoltaic (OPV) devices are those which absorb a photon to release useful electrical energy .

• Here we are going to observe how the photon emitted by the OLED is converted to useful electrical energy by integrating it with an OPV device .

OLED OPV

Process and impact Let’s compare the impact of solar cells (OPV) when we integrate them with OLED’s to just the normal operation of OLED’s without any solar cells .

The two process am going to discuss their impact on are :

Organic light emitting devices integrated with solar cells for “ high contrast and energy recycling “.

Power recycling of “transparent large

area organic light emitting devices” using solar cells .

Organic light emitting devices integrated with solar cells for “high contrast and energy recycling”

OLEDs in general are composed of a reflective back electrode , organic layers and a semi-transparent electrode for light coupling . Due to the reflective electrode , there is a strong light reflection which would degrade the contrast of an OLED display under strong lighting environment . Light

Electron transport

Hole transport

Anode

Cathode

Organic layers

Optical Enhancer

Conventional MethodsA few approaches have been used to reduce the ambient-light reflection and the contrast ratio of OLED displays since even at the off state the device will still exhibit some brightness due to ambient-light reflection :

Laminating polarizer or filter films on the surface of the display panel .

Inserting layers of optical absorbers into the devices .

Light Polarizer

Integrating with solar cells

Electron transport

Hole transport

Donor

Acceptor

Anode of OLED

Anode of solar cell

Common cathode

Light Photons

Fabrication

The solar cell layers are first deposited by sputtering after which the layers required for OLED are deposited by thermal evaporation . We can see that solar cells layers are deposited vertically and the OLED layers are deposited horizontally .

Results

The comparison between the conventional OLED , OLED with polarizer and OLED with solar cell is made .We can see that the light reflection in OLED + solar cell is much lower than conventional OLED and the OLED with polarizer. Apart from lower light reflection we also get energy . The light reflection is reduced to as low as 1.4%.

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Application

Phot

ons

It is apt for portable electronics that are highly power aware . When a cell phone is made with an organic display , this combination of OLED and solar cells not only enhances the contrast but also powers the device even when it is in an off state . The application can also be extended to laptops.

Power recycling of “transparent large area OLED panels “ using solar cells

Lately much attention has been given to transparent OLED’s ,which enables a variety of applications such as light-emitting window glasses for buildings, cars, and advertising bill boards. 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 OLED. 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.

Integrating with solar cells

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 is a good example of making use of the wasted energy.

Also during the day time when the OLED’s are not in use , by integrating them with solar cells they can be used to absorb the photons from the sun and convert them into useful electrical energy .

“OLED + Solar” = Power recycle

Fabrication techniqueIn our previous fabrication technique we deposited organic layers of Organic solar cell and OLED in series on a common glass substrate . This approach was suggested mainly to reduce the ambient-light reflection of OLED displays. With this scheme, the power recycling 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.

However a more modern technique compared to the previous technique is followed , it is a simple but efficient technique . The technique is to merge the OLED and organic solar cells after they are fabricated separately.

AdvantagesBy using the modern fabrication the distance between the two devices is drastically reduced and the following advantages are achieved :

• Increasing the conversion efficiency of solar cell

• Increasing the power efficiency of OLED

• Cost efficient

• Reducing the panel size

• Matching the absorption spectrum of solar cell with the emission one of OLED.

* Work on progress

Results

Color WavelengthViolet 380–450 nmBlue 450–475 nmCyan 476–495 nmGreen 495–570 nmYellow 570–590 nmOrange 590–620 nmRed 620–750 nm

Luminous spectrum of white OLED measured at 4V and Quantum efficiency of reference solar cell vs Wavelength

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Challenges • The power efficiency of the solar cell decreases with increasing luminescence and current density .

• Under high brightness, how- ever, heat generation or heat distribution concerns us , it shortens the lifetime of OLED’s, or damages an adjacent solar cell. • Since we are talking about transparent OLED’s the integrated solar cell must also be transparent , which gives the challenge of making the solar cell not only efficient but also transparent .

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Futuristic ?

ConclusionBy integrating solar cells with OLED’s :

• We can have a low ambient light reflection which helps improves the contrast ratio .

• Absorb the reflected light within the OLED .

• Converts the rear light from a transparent OLED to useful energy and recycles it .

• Makes use of the incident light energy even when the device is off during daytime .

• Since organic solar cells are cheap and flexible the overall price is also not varied a lot compared to conventional OLED’s .

References1. Chih-Jen Yang, Ting-Yi Cho, Chun-Liang Lin,

and Chung-Chih Wu, Organic light-emitting devices integrated with solar cells: High contrast and energy recycling, 2007 American institute of physics .

2. Jongwoon Park, Jongho Lee, and Dongchan Shin, Power recycling of large area OLEDs using solar cells, Journal of display technology, Vol.6, No.7, July 2010.

3. www. Wikipedia .org /organic solar cells

4. www.princeton.edu/ research group/research/Mems