Application Note

Light Emitting DiodeLED Photovoltaic Effect

Table of contents
1. Overview ............................................... 2
2. Photovoltaic Effect for LEDs ................ 2
3. Evaluations for the Photovoltage .......... 5
4. Summary ............................................... 8

NCSW170F, NCSA170F, NJSW170F, NCSW193F, NC5W193F, NFSW172C, and
NFSA172C refer to Nichia part numbers. These Nichia part numbers within thisdocument are merely Nichia's part numbers for those Nichia products and are not related
nor bear resemblance to any other company's product that might bear a trademark.

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Application Note
1. Overview
When a semiconductor is illuminated with light, an electromotive force may be generated. This iscalled the photovoltaic effect; the electromotive force generated by the photovoltaic effect is called
the photovoltage. The photovoltaic effect is used for photodiodes, solar cells, etc.
LEDs can have a photovoltaic effect since LEDs are a semiconductor. If a photovoltage isunintentionally generated in an LED, it may cause the operating circuit to malfunction and/or thecomponents used within the operating circuit to be damaged.
This application note provides general information on the photovoltaic effect and the results of theevaluations Nichia performed using Nichia’s LEDs.

Light Emitting Diode2. Photovoltaic Effect for LEDs
2.1 How an LED Emits Light and How a Photovoltage is Generated
Figures 1 and 2 are reference images for how an LED emits light and how a photovoltage is generated
in an LED; the LED referenced is a typical p-n junction LED.

How an LED emits lightLight emission

p-region

Depletion
layern-region
Travel directionof the electrons

Recombination+-

Light emission
Travel direction of
the electron holes

Figure 1. How an LED Emits Light
When electrical current flows through an LED, carriers (i.e. electrons and electron
holes) are provided to the LED. The electrons and the electron holes travel towards thep-region and the n-region respectively through the depletion layer1 to recombine witheach other. The energy that is lost during the recombination is released as light. The
wavelength of the released light (i.e. emitted color) depends on the energy band gapof the LED.

1

A region near the p-n junction interface where almost no carriers are present.

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Application Note
How a Photovoltage is Generated
p-region

Depletion
layern-region
Travel directionof the electrons

External light+-

External light

Recombination

Photoexcitation

Travel direction of
the electron holes

Diffusion

Light Emitting Diode

Figure 2. How a Photovoltage is Generated
When an LED is exposed to external light2 while current is not flowing through theLED, photoexcitation occurs and creates carriers. Carriers that are created within thedepletion layer do not recombine immediately and instead travel (i.e. diffuse) awayfrom the depletion layer due to the electric field in the depletion layer; the diffusioncauses a potential difference (i.e. an electromotive force is generated). If the LED is ina circuit, current may flow from the LED causing the circuit to malfunction and/or thecomponents used within the circuit to be damaged.

2.2 Photovoltaic Effect for LEDs
Figures 3 and 4 are reference images to show the structural difference between a typical rectifier diodeand an LED. For a rectifier diode, the chip is enclosed by a black resin package that shields the chipfrom external light; see Figure 3. Since the chip is not affected by external light, a photovoltage will
not be generated in a rectifier diode. An LED has a structure with which the light emitted from thechip and/or the phosphor can be released to the outside of the package; external light can penetrateinside the package with this structure. When the external light illuminates the surface of the LED, thelight can reach the chip through the phosphor to generate a photovoltage; see Figure 4.
External lightLight-shielding package
Chip

Figure 3. Example Image of the Structure of a Typical Diode
External light

Encapsulating resin
Chip

Figure 4. Example Image of the Structure of an LED
2

To generate a photovoltage, the LED needs to be illuminated with light that has an energy larger than the energy band gap of the LED.

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Application Note
2.3 Environment/Conditions Where the Photovoltage is Likely to be Generated
The brighter the light that illuminates an LED (i.e. high illuminance), the larger the photovoltage thatis generated in the LED. Figure 5 shows examples of illuminance values for the environments oftypical daily situations for reference. Since the illuminance of direct sunlight is very high outdoors, aphotovoltage is more likely to be generated for outdoor applications containing LEDs (e.g. traffic
lights, outdoor displays, automotive lighting systems, etc.).

Shop window

OfficeRoad at night

10lx

100lx

1,000lx

Outdoors(Sunny day)

10,000lx

100,000lx

Figure 5. Examples of Illuminance Measured for Daily Situations (for reference)
The illuminance varies depending on the distance between the light source and the illuminated object
even when the brightness of the light source is constant. For reference, Figure 6 shows the illuminancevalues for an automotive headlamp measured for different illumination distances. From this figure, itis easy to see that the rate of change in the illuminance increases as the illumination distance becomesshorter; it should be noted that the illuminance may be unexpectedly high when the illumination
distance is short.
Nichia performed evaluations for the relationship between the illuminance and photovoltage using
Nichia's LEDs. Refer to the next section for the evaluation results.
50,00040,000Illuminance (lx)Light Emitting Diode

Outdoors(Cloudy day)

Living room

30,00020,00010,000
0
0

1

2
3
4
Measured distance (m)56Figure 6. Illuminance for an Automotive Headlamp (for reference)

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Application Note
3. Evaluations for the Photovoltage
Nichia performed evaluations for the photovoltage (i.e. potential difference) that was generated when
Nichia's LEDs were illuminated on their emitting surfaces with an external light.
3.1 Evaluation Conditions
Evaluated LEDs:

Nichia selected LEDs with different structures. See Table 1.
Table 1. Evaluated LEDs

LED No.

1

2

3

4

5

6

7

Part No.

NCSW170F

NCSA170F

NJSW170F

NCSW193F

NC5W193F

NFSW172C

NFSA172C

Emittedcolor

Equivalent to
6000K

Amber

Equivalent to6000K

Equivalent to6000K

Equivalent to6000K

Equivalent to
6000K

Amber

ForwardVoltage3

3.25

3.25

3.25

3.25

16.25

3.0

3.0

Light Emitting Diode

Appearance

Test light source:

Nichia used a typical white LED to illuminate the evaluated LEDs.
See Figure 7 for its typical wavelength.

Figure 7. Wavelength of the Test Light Source
Evaluated illuminances:
Evaluation method:3500-100,000 lx

Nichia illuminated the evaluated LEDs at different levels of brightness (i.e.
illuminance) using the test light source and measured the photovoltages (i.e.
potential difference between the terminals of the LED) that were generated
with an oscilloscope. See Figures 8 and 9.

Typical forward voltage value when the LED is operated at its sorting current.

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Application Note
Test light source

Evaluated LED

V

Figure 8. Evaluation Circuits
Test light source

Power supply for the test lightsource: the illuminance was changed
by changing the input currents.

Evaluated LED

KA

Oscilloscope

Figure 9. Evaluation Set-up
3.2 Evaluation Results
Figures 10 and 11 show the evaluation results.

Photovoltage (V)1210

No.1: NCSW170F

8

No.2: NCSA170F

6

No.3: NJSW170F
No.4: NCSW193F

4

No.5: NC5W193F
2

No.6: NFSW172C

0
-2
100

No.7: NFSA172C

1,000

10,000100,000

Illuminance (lx)Figure 10. Evaluation Results
3

2.5
Photovoltage (V)Light Emitting Diode

Light-shielding box: external light
was shielded except for the lightfrom the test light source.

No.1: NCSW170F

2

No.2: NCSA170F

1.5

No.3: NJSW170F

1

No.4: NCSW193F

0.5

No.5: NC5W193F

0

No.6: NFSW172C-0.5

No.7: NFSA172C

-1
100

1,000

10,000100,000

Illuminance (lx)Figure 11. Evaluation Results (with smaller increments for the photovoltage)
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Application Note

Light Emitting DiodeThe evaluation results show the following:‧

The higher the illuminance was, the larger the photovoltage generated became. This result
indicates that a large photovoltage can be generated in outdoor applications or equivalentenvironments (i.e. equivalent to 10,000 lx) which may affect the components used within theoperating circuit.

‧

The amount of the photovoltage generated was different depending on the size of the LED chip,package structure, etc. The difference became smaller after the illuminance was high enough tohave the photovoltage saturate; see the measurement results for LEDs No. 1, 3, 4, and 6. The
maximum value for the photovoltage (i.e. saturation value) depends on the energy band gap of
the LED; the saturation values for those LEDs were almost the same. Nichia believes this isbecause the blue chips used within those LEDs had an equivalent energy band gap. Before
saturation, the amount of the generated photovoltage was different for each of those LEDs; Nichia
assumes it was because the amount of the external light that reached the chip was different due
to the difference in the package structure.

‧

LED No. 5 (NC5W193F) generated approximately a photovoltage five times larger than LED
No. 4 (NCSW193F); those LEDs had the same structure just with a different number of chips (i.e.
LED No. 4 had one chip and LED No. 5 had five chips connected in series). This result indicatesthat the amount of photovoltage generated in an LED is proportional to the number of chips thatare connected in series.

‧

LED No. 1 (NCSW170F) generated more photovoltage than LED No. 2 (NCSA170F); those
LEDs had the same structure just with different emission colors (i.e. white for LED No. 1 andamber for LED No. 2). The reason for this result is believed to be that the amount of the external
light that reached the chip was smaller for LED No. 2 than LED No. 1 because an amber LED
has a higher phosphor concentration than a white LED.

‧

For LED No. 7 (NFSA172C, amber), a photovoltage was generated in the reverse direction.
Nichia assumes this was due to the photovoltage generated in the protection device (i.e. Zener
diode) within the LED. As mentioned above, an amber LED has a higher phosphor concentration;
most of the external light is converted into yellow or red light by the phosphor before reaching
the chip. This converted light does not have a photovoltage generated in the chip if it is a blue
chip; this is because the energy for yellow or red light is smaller than that for blue light. The chipused within LED No. 7 was a blue chip; Nichia assumes a photovoltage was not generated in thechip due to this reason. However, a photovoltage can be generated in a Zener diode since theenergy band gap for a Zener diode is smaller than that for yellow or red light. As shown in Figure
12, a Zener diode is connected in parallel with reverse polarity with the LED to function as aprotection device; in the Zener diode, the photovoltage is generated in reverse polarity. LED No.
2 (NCSA170F) is also an amber LED with a Zener diode; however, that Zener diode has areflecting material covering it, that is why no photovoltage was generated in the reverse direction.

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Application Note
CathodeAnodeFigure 12. Example of the Internal Circuit for an LED with a Zener diodeLight Emitting Diode

4. Summary
An LED is a device in which a photovoltage is likely to be generated due to its structure. A large
photovoltage may be generated depending on the operating environments and/or conditions for the
LED causing the peripheral circuits in the chosen application to malfunction and/or the components
used within the circuits to be damaged. Examples for effective measures to prevent this fromoccurring are designing the chosen application to not allow an external light to enter the LED package,
incorporating a protection circuit that can prevent the generated photovoltage from affecting theperipheral circuits, etc.
In this application note, Nichia has provided the results for Nichia’s evaluations; however, the amountof the photovoltage generated varies depending on various factors. Ensure that the photovoltage
generated in the LED will not affect the peripheral circuits when the LED is operated in the conditions
under which the LED will actually be used.

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Application Note

Light Emitting Diode

Disclaimer
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within it and Nichia makes no guarantee that customers will see the same results for their chosen
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