# How does increasing temperature reduce the efficiency of solar cells?

I've seen several answers and trying to confirm.

1) Does temperature increase the ground state energy of electrons once they reach the positively doped semiconductor, thus reducing the voltage between excited electrons from negatively doped semiconductor and ground state they will reach on positive side.

-This was the most compelling and easy to understand answer for me.

-However I'm not sure if it makes sense that "ground states" would rise with increasing temp.

2) Does temperature increase vibrations in wires and semiconductors, such that excited electrons following voltage difference lose energy in collisions along their path from N-doped to P-doped?

Combination of the two? Am I way off?

I am a molecular biology student with some basics in photochemistry. Somewhat familiar with solar cells and looking to dive deeper on the subject so feel free to reference.

## 1 Answer

While perhaps better suited to Physics SE, I will quote from Sze's Physics of Semiconductor Devices:

As the temperature increases, the diffusion lengths in Si and GaAs will increase ... and the minority lifetime increases with temperature. The increase in minority-carrier diffusion length causes an increase in $$J_{L}$$. However, $$V_{oc}$$ will rapidly decrease because of the exponential dependence of the saturation current on temperature. The increase in the "softness" (roundness) in the knee of the $$I-V$$ curve as temperature increases will also degrade the fill factor. Therefore, the overall effect causes a reduction of efficiency as the temperature increases.

Here, $$J_{L}$$ is the current caused by light generation of electron-hole pairs. So, it goes up. But, the device is still a diode and must follow the standard diode equation, so the open circuit voltage ($$V_{oc}$$) drops. This then also impacts the power output.

In brief, neither of your proposed reasons is the basis for it. Instead, you have to probe deeper into the physics of semiconductor devices.

• Oh great a third approach XP. Hmm can you translate this information into plainer English (/ describe the concept) to help me get into the subject? – dlight Feb 14 at 8:29
• However,in semiconductors, mobility decreases when temperature increases. The second part of the Q suggest a correct answer. Though the final output of the cell is dictated by more factors as addressed in the answer. Additional note/curiosity: organic solar cells (not yet mature) generally show opposite behaviour as for in disordered material conduction is hopping or hopping-assisted, ie scattering by vibrations is now a positive factor. – Alchimista Feb 14 at 9:05
• @Alchimista - The increases in lifetime and diffusion lengths more than compensate for a decrease in mobility, since the light-induced current contribution increases. But, you are absolutely correct that it is the diode equation (through Shockley-Read-Hall) and saturation current that kills the efficiency. For organic cells, I think you get to through SRH out the window and start with a different theory of carrier dynamics. – Jon Custer Feb 14 at 14:01
• Ok SRH got to fame for several reasons but the core is kind of Huckel calculation. Not that I wanted to enter the realm of organic solar cells. The fact that semiconductor physics took over more basic consideration is making even a bigger mess. Those are still speaking of dumpling bonds in conjugated polymers, to give you an idea. Of course any IV curve can be modelled using concept at work in Si, but the facts behinds can be totally different. Other point: yes my comment was more on mobility alone rather than the specific of the question, ie a traditional solar cell. – Alchimista Feb 14 at 14:57