The homojunction barrier region
The electrostatic barrier region is the “charge separation engine” of homojunction cells. It is either of the p-n type seen in Figures 4.1a-e or of the p-i-n type seen in Figure 4.1f. These barriers break symmetry and make one direction different from the other, thereby causing charge separation and current flow. As may be noticed from the band diagrams of the p-n homojunction devices of Figures 4.1a-e, a characteristic feature of p-n cells is that they all have a barrier region that is constrained in its extent and flat band （no built-in electric field） regions on both sides of the electrostatic barrier in TE. Under illumination and current-flow conditions, our numerical analysis （Section 4.3） will show an electric field does develop in these flat band regions of a p-n cell but it usually tends to be small. When the electric field is small outside the barrier under light and non-existent in TE, the regions cannot have any significant charge density present under illumination. Because of this, they are often quasineutral regions. “Quasi” is Italian for “almost,” so we can have quasineutral regions and quasi-Fermi levels （defined in Appendices C and D）
in solar cell device physics. Since the electric field is small in these “almost” neutral regions of p-n homojunctions, we expect the minority-carrier collection to the barrier region to be dominated by diffusion when the cell is under illumination. Drift should be less important for these carriers in the quasi-neutral region because it would involve the product of a minority population and a very small electrostatic field. The p-i-n device of Figure 4.1f is the other extreme. It has a built-in electrostatic field, and therefore an electrostatic field barrier, that extends across the cell at TE and there are no flat band regions. Under operation, such cells have been designed to have collection of photogenerated carriers accomplished everywhere by drift. In many situations, analytical solutions to Poisson's equation （Eq. 2.45 in Section 2.3.4） yielding EC（x）， EV（x）， and EVL（x） and the electric field ￡，（x） can be obtained for p-n and p-i-n barrier regions as a function of voltage. Discussions of this analytical approach can be found in standard device physics books.17 Of course, we can find EC（x）， EV（x）， and EVL（x） and ￡，（x） for both the quasi-neutral and barrier regions in all situations using numerical solution techniques.