Altermatt Lecture:   The Solar Spectrum

 
 

6.2:  Calculation of the solar spectrum with SMARTS

1) Calculate the AM1.5g and AM1.5d standard spectra

SMARTS uses an extraterrestrial spectrum and calculates the influence of the earth atmosphere. By default, the starting spectrum is the standard AM0 spectrum.

To calculate the AM1.5g standard spectrum, go to the the page "The global standard spectrum (AM1.5g)". and type in the values given on that page into the SMARTS panels.

Press the button 12 (Output), where you can choose the output. The "global tilted irradiance" is the output for the AM1.5g spectrum, the "Experimental direct circumsolar" is the AM1.5d spectrum.

Finally, press the button "Run Model". Keep in mind that each calculation overwrites the output file *.out.txt. If you keep this file opened in another application, you many not be able to run the model because *.out.txt may be locked by that application from overwriting, and SMARTS gives you a "permission denied" error message.

2) Calculate the influence from water on the AM1.5g standard spectrum

Proceed as in exercise 1 but additionally press button 4 (water vapor) and type in 7 cm, which is observed in the tropics.

In buttom 12 (Output), choose "Water vapor transmittance", and you obtain the dashed blue curve on the page "Variability of the terrestrial spectrum".

3) Calculate the electrical current of a Si solar cell under standard conditions

Proceed as in exercise 1, but as output, choose in button 12 the option "Global tilted photon flux". This gives you the photon flux in units of 1/(cm2⋅s⋅nm) and also takes into account that the solar cell is fixed at a tilted angle given in button 10b.

Also, choose the wavelength range from 300 to 1200 nm, because this is the wavelength range where Si solar cells absorb sunlight.

In the next lecture on photogeneration, we will learn that, usually, every absorbed photon creates an electron–hole pair. However, not all photons can be absorbed: some reflect at the front surface, some leave the solar cell again due to weak absorption. And in a later lesson, you will learn that not all electrons and holes reach the metal contacts to contribute to the output power. These optical and transport losses are quantified by the factor called "quantum efficiency" (QE). Download the quantum efficiency as an Excel file or as an OpenOffice file measured on a typical industrial cell.

Because the quantum efficiency is given in intervals of 10 nm, choose in button 12 the interval of the spectral range as 10 nm.

Now, run SMARTS. Load the output to Excel. To calculate the electrical current of a solar cell, you need to multiply the SMARTS output values with the QE of that cell at every wavelength, and to sum over all wavelengths. Because the wavelength interval is 10nm, but the output of SMARTS is in units of 1/(cm2⋅s⋅nm), you need to multiply by 10 nm to obtain 1/(cm2⋅s). And because the unit should be mA/cm2, this means electrical current instead of a photon rate, multiply by the unit charge, 1.602 x 10–19.

If you do everything correct, you obtain about 36.73 mA/cm2.

 

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