Solar Cells Based on Single PN Junctions - Nanophysics of Solar and Renewable Energy - page 136

light inside the silicon is easily internally re ected. In fact, since the critical angle y c is

16.6 , all light except that within 16.6 of the
surface normal is internally re ected. The corresponding angle at the Si/SiO 2
interface is 24.7 . Growing thermal oxide (quartz) on the back surface of the cell
as indicated in Figure 6.4 re ects light to increase the light path in the silicon, and
reduces the recombination rate of electrons and holes at the surface. Heating in a
hydrogen atmosphere (passivating) further reduces recombination at Si/SiO 2
interfaces, by lling dangling bonds. Any step to increase the lifetime of minority
carriers is bene cial to the operation of the solar cell, where the open-circuit voltage
(the separation of electron and hole quasi-Fermi levels as in Figure 6.2) arises as a
competition between photogeneration and recombination of carriers. (Figure 6.4 is
simpli ed, neither the PN junction nor the front antire ection layer is shown, but
these are clearly shown in Figure 6.5.) In Figure 6.4 an array of small contacts to the
rear is seen.
The most efficient single-crystalline silicon solar cell, evolving from Figures 3.17,
6.1 - 6.3, is shown in Figure 6.5 [67]. The faceting indicated in Figure 6.3 has been
implemented, showing the N þ P junction diffused into the faceted surface. The metal
conductive fingers easily form ohmic contacts with the N þ layer, and then the upper
surface is coatedwith aMgF 2 /ZnS antire

given by sin y c ¼ 1/ n Si is small, y c ¼

ection double layer applied above a thin and

Figure 6.5 Structure of the best single-crystal
Si solar cell, efficiency of 24.4%, developed by
M.A. Green and coworkers. Note that entrance
surface oxide is covered by an antireflection
coating and that the larger part of the rear
surface is oxidized to reduce recombination and
also to reflect light back into the silicon wafer.
The front top N-surface has locally diffused N þ
regions to facilitate formation of a low-
resistance ohmic contact to metal fingers and

also to collect current from the N-layer with
minimal voltage drop and reduced
recombination. On the top surface, the (thin-
oxide plus antireflection layer) is shown as dark,
while on the back surface thin thermal oxide
(quartz) is indicated white and the rear metal
contact is shown black. Locally diffused P þ
contacts connect the bulk P-region to the
metallic back contact [68].