How to Measure Dev Eff

8/3/2019 How to Measure Dev Eff

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Accurate measurement of the

AM1.5G solar cell efficiency

Not really byJoey Luther because I stole all the content from the internet



Certified solar cell efficiencies

This shows the maximum certified efficiency (under simulated AM1.5G conditions) for each type of device structure.

This talk will explain how that is measured.



Simple measurement, right?η = VOC × JSC × FF

Measuring the efficiency

But measuring the device should be as easy as shining light on a device and measuring the max power point



The energy flow within the sun results in a surface temperature of

around 5800 K, so the spectrum of the radiation from the sun is similar

to that of a 5800 K blackbody with fine structure due to absorption in

the cool peripheral solar gas (Fraunhofer lines).

By the time the radiation reaches our power infrastructure,

atmospheric inference reduces the spectum to the solid line.

Solar radiation reaching the earth's surface varies significantly with location,

atmospheric conditions including cloud cover, aerosol content, and ozonelayer condition, and time of day, earth/sun distance, solar rotation and activity.

Since the solar spectra depend on so many variables, standard spectra have

been developed to provide a basis for theoretical evaluation of the effects of

solar radiation and as a basis for simulator design.

Basic solar radiation

http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/catalog.aspx



AM1.5G spectrum

http://rredc.nrel.gov/solar/spectra/am1.5/

Download the .xls file here:



Many different light bulb options

http://www.newport.com/file_store/Supporting_Documents/Tech_Ref_Solar_Simulation2.pdf

Light bulbs don’t look like the

AM1.5G spectrum. Here are some

examples of common light bulb

spectra.



Choosing Light bulb spectra

No light bulb perfectly mimics the sun, look at the spectrum for various bulb types.

Consider which part of the spectrum is most important for the type of cell you plan

to measure.



Various other factors to consider

Additional tradeoffs in available light bulbs



Solar simulators

Even custom designed solar simulators poorly mimic the true spectrum.



What you need to measure

the efficiency of a solar cell• Solar cell (need to measure device area, quantum

efficiency spectrum)

• Light bulb (need to know spectrum)

• IV source measurement unit

• A calibrated reference cell or pyrometer (need EQE spectrum and it must must encompass

full range of test device)

What you need to accurately measure the device. What you need to know about them is in parentheses.

The talk is based on this paper.



NREL’s procedure• 1st, Acquire the QE spectrum of the device

Then choose an appropriate reference cell based on the

spectrum of your cell.

Example reference cells

Obvious choice for a Si cell is

a Si ref cell

KG5 filtered Si cell serves as

a good ref cell for

P3HT:PCBM cells because

they have similar spectal

response.

Pick an appropriate reference cell. The reference cell mustbe active over the same area as your solar cell. If your

device has a narrower spectrum, you can use a filter to

block the light that isn’t of interest. For example, if you are

measuring a P3HT:PCBM cell active between 350 and 700

nm, and a Si diode as your reference detector, a spectral

spike or dip at 900 nm could cause your reference cell

current to be incorrect.



IV scan

2nd. Tune the lamp current such that reference cell

reads the 1-sun AM1.5G current, as provided by the

calibration

NREL shines the lamp and reads the VOC.

Then scans from -0.1V to 10% higher than the VOC

The QE spectrum is acquired and compared to the



Spectral Mismatch

E REF = solar spectrum

E S = lamp spectrum

S R = ref cell spectral responseS T = test cell spectral response

Term 1 = current of ref cell in AM1.5G

Term 2 = current of device under lamp

Term 3 = current of device under AM1.5G

Term 4 = current of ref cell under lamp

VOC

(V) ISC

(mA) FF(%)

A(cm2) MηAM1.5G =

3rd, spectral mismatch is calculated (not as difficult as it looks)

Again, see the Yang

Paper referenced earlier



Device Area

Overlap of ITO and Metal contact

Active area

Certified efficiency

Valid if A ≥ 1 cm2

Carefully measure the device area using a calibrated microscope

With many of these device having a small area, errors in device area are easily the cause for overly stated efficiencies.

Thermal evaporation of a top contact of Calcium followed by aluminum, from 2 different sources can cause a shadowing.

It’s important to accurately measure the device area. NREL won’t give a certified value for devices with area < 1 cm^2. You

can get a certified efficiency value, but it cannot be considered a record efficiency and won’t make the chart shown at thebeginning of this talk.



Typical report given to someone who submitted adevice to NREL for accurate efficiency value.



Other considerations

• Light spectrum is generally dependent on bulb age,

optical settings, and current thru the bulb.

• Devices sometimes exhibit intensity dependent QE

• Response time of current collection



ConclusionsTo accurately measure a device, it’s

important to:

Have a calibrated reference detector with response over the

same range as the test cell.

Know the lamp spectrum.

Calculate the spectral mismatch and divide by that value

accurately measure the device area

The external quantum efficiency can be used as a check to seeif the Jsc value is correct by integrating the QE vs the lampspectrum.

How to Measure Dev Eff

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