# I Voltage output of a photodiode (solar cell)

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1. Jun 16, 2017

### Smooth

Hi,
I'm trying to figure out how the angle of incidence will affect the voltage output of a photodiode. The obvious answer that comes to my mind is that the voltage output should be proportional to the cos(α), where α is the angle of incidence of light. However, I couldn't find any formula/article that states this.

Also, an important thing is that I'm interested in the case when the photodiode is in space – so I can assume that there is no reflected light interfering with the readings. Cause I found some people with experimental data that sort of goes along with my assumption:
http://www.diva-portal.org/smash/get/diva2:858954/FULLTEXT02
https://www.researchgate.net/public...rientation_on_Photovoltaic_Module_Performance

2. Jun 16, 2017

### Nidum

A quick search on Google returned 391000 results . The subject is well dealt with just in articles which came up on the first page .

What key words have you been using for your searches ?

3. Jun 16, 2017

### Smooth

solar cell voltage angle of incidence
photodiode voltage angle

different variations of these.
I couldn't find anything that would give the relationship between the voltage output and the angle of incidence.

4. Jun 16, 2017

### Nidum

photodiode angle of incidence

5. Jun 16, 2017

Most of the time, it is best to use the photodiode at near normal incidence for consistent results. The reason is that there is often a window on them, and the reflectivity/transmission of the window will vary with incident angle, and this is in addition to the $cos(\alpha)$ factor. The reflective losses can be quite significant at steep angles of incidence and will also be polarization dependent. The same even applies for reflections off the photodiode itself. $\\$ I believe the Halliday-Resnick physics text shows $R_{parallel}$ and $R_{perpendicular}$ as a function of incident angle $\theta$ for a typical glass. Both curves have low (and equal) reflectivity values at normal incidence and go to $R=1.0$ at 90 degree incidence, with a dip for the $R_{parallel}$ case with a Brewster angle value of $R_{parallel}=0$ for some angle $\theta_B$ before it rises to $R=1.0$ at 90 degrees.

Last edited: Jun 16, 2017
6. Jun 16, 2017

### marcusl

Clarification: A solar cell is called a photovoltaic device. A photodiode is reverse biased detector that generates a current when light falls on it. It does not generate voltage on its own.

Solar cells are flat and have the cosine projected area dependence that you mention. Photodiodes are often mounted behind a lens, so behavior could differ.

7. Jun 16, 2017

Additional item is that a photodiode is often used with a current-to-voltage amplifier, where the photodiode is operated at zero voltage (they can also be reverse-biased for faster response), but ultimately a voltage is what is observed at the output. A photodiode would generate a voltage in the open circuit state (note: in the current mode with the current-to-voltage amplifier it is essentially short -circuited), but the voltage it generates would be less than linear with light intensity. In the current mode, a photodiode is meanwhile quite linear in its response.

8. Jun 16, 2017

### Smooth

My photodiodes are flat though, so I suppose that they too are proportional to the angle of incidence, right?

And I just wanted to thank everybody for helping me! Thank you!

9. Jun 18, 2017

### sophiecentaur

You can't be sure of that. You can get the spec of all these devices on line. I suggest you search for it (if it's very important to you). Put in the device number and "PV"; that should give you the info.