# Quasi-fermi levels in a solar cell?

I was wondering, if my fermi levels splits up due to n and p type doping into two fermi levels, one for the p type one for the n type, and now due to light radiation my fermi levels split up into 2 quasi fermi levels for the n and p carriers, do i have 4 quasi levels in total then? If i recall correctly the electrochemical potential is defined by the difference of quasi fermi levels, so which quasi fermi levels do i take the difference of, if there exist 4 levels?

sincerely

• Level not niveaus Sep 3, 2015 at 13:56

In the dark you have a single Fermi level which is spatially dependent on the net free charges $E_f(z)$.

When the semiconductor is perturbed by light the Fermi level splits into two quasi fermi levels for electrons $\mu_e(z)$ holes holes $\mu_h(z)$. The quasi fermi level separation (or electrochemical potential) is, $\Delta E_f(z) = \mu_e(z) - \mu_h(z)$.

• I thought that due to n and p type doping the fermi level already splits up in two, even though there is no light shining on our solar cell yet ?
– user85397
Sep 10, 2015 at 22:36
• No, because of the law of mass action $n_i(x)^2 = n(x) p(x)$. In the dark you always have one Fermi-level, because of the law of mass action that is all that is needed to get the correct electron and hole populations in the conduction and valence bands. When illuminated the Fermi-level can split and to get the correct number of electron and holes we need to use a quasi Fermi-level description. Sep 11, 2015 at 7:28