# Pn junction in solar cell case

1. Nov 21, 2008

### Rossomingus

HI. I got some question about the derivation of the I-V characteristic in a solar cell.
The first step is to solve the minority carrier diffusion equation with appropriate boundary conditions: at the edges of the cell and at the edges of the depletion zone.
At the edges of the d.z. the conditions for the concentrations p_N and n_P can be found using quasi-Fermi levels....

1) Suppose to be in case of forward applied voltage V. In the quasi neutral regions, far from the d.z, and into the d.z., the quasi-Fermi levels are constant. This means that here currents J_p and J_n equal to 0. How is it possible to have currents just in teh little spaces out of the d.z. when a votage is applied?

2) Why qV= F_n - F_p ?

In thee next step of the derivation, they integrate the electron continuity equation over the d.z. ,
dJn/dx = q[ R(x) - G(x)]
with R recombinatio rate ad G generation.
For R they consider just midgap single level trap mechanism. They keep a point Xm inner the d.z. at wich p(xm)=n(xm) and then consider R(xm) for the whole d.z.

3) Does is exst a point xm where p=n?