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There is an issue I'm facing that is very tricky.

A solar cell that is encapsulated in Sylgard 184 Silicone Elastomer with a front borofloat front glass. This cell is used to measure a light intensity.

The cell is enclosed in a chamber that can vary the temperature. The room ambient temperature is 25°C and humidity is 50%.

The cell has a temperature sensor applied to it that measures the cell's temperature.

The cell is shunted with a resistor (which has low temperature coefficient) in order to measure it's Isc value, and the whole is compensated in temperature, and is expected to behave linearly over temperature.

It does behave linearly when we raise the temperature, but when cooling, it goes off the chart and I can't really explain why.

One possibility is that as the cell cools, humidity diffuses into the silicone, which somehow changes its characteristics (refraction/transmission?) when the system cools down.

Perhaps, humidity seeps between the elastomer and the glass (a bonding agent is used) and when cooled, the dew point is reached causing the interface glass->elastomer to become glass->water->elastomer, changing the refractive index of the system.. ?

This graph shows the temperature VS the measured light intensity. 0-45 being the temperature in °C, irradiation being in W/m2 enter image description here

A clear change in behavior is noticed at around 25°C, which is the ambient temperature at which the cell rests when not performing tests.

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First, what are the coordinates of the graph? Temperature in °C and cell current in amperes? Light intensity in W/m2? In lux?? Edit the question to show the units. without that, the graph is meaningless.

However, my first assumption is that the bonding of sealant to cell is not effective. More likely than water seeping into the seal, which would happen slowly, would be a break in the silicone-photovoltaic device bond, which could occur rapidly, allowing air to seep in and forming extra interfaces, with concomitant Fresnel reflection at each boundary (air-silicone and air-solar cell).

Compare a test unit before temperature cycling with one after cycling. Is there a slight "silvery" reflective air layer in the cycled unit, or noticeable bubbling of the sealant? Try sliding the sealant atop the cell with a fingertip -- is it as tightly adhered?

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  • $\begingroup$ The coordinate are given above the graph "temperature VS the measured" 0-45 being the temperature in °C, irradiation being in W/m2 $\endgroup$
    – Damien
    Jul 14 at 3:23
  • $\begingroup$ Added, there is nothing visible, and it is repeatable. The cell is encapsulated on an aluminum body with a front glass so I cannot "peel" the sealant. $\endgroup$
    – Damien
    Jul 14 at 5:10

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