Page 56 - Applied Photovoltaics

Chapter

3

THE BEHAVIOUR

OF SOLAR CELLS

3.1 EFFECT OF LIGHT
A silicon solar cell is a diode formed by joining p-type (typically boron doped) and n-
type (typically phosphorous doped) silicon. Light shining on such a cell can behave in
a number of ways, as illustrated in Fig. 3.1. To maximise the power rating of a solar
cell, it must be designed so as to maximise desired absorption (3) and absorption after
reflection (5).
The electric field Ê at the p-n junction sweeps electrons to the n side and holes to the
p side. The ideal flow at short circuit is shown in Fig. 3.2. However, some electron-
hole (e-h) pairs get lost before collection, as shown in Fig. 3.3.
In general, the closer the point of e-h generation to the p-n junction, the better the
chance of ‘collection’. ‘Collected carriers’ are those that generate a finite current
when V = 0. Chances of collection are particularly good if the e-h pairs are generated
within a diffusion length of the junction, as discussed in Chapter 2.
The characteristic curves generated by plotting I against V for a diode (I-V curves)
were shown in Fig. 2.11 for I 0 , with no light falling on the cell. Illumination of a cell
merely adds to the normal ‘dark’ currents in the diode so that the diode law becomes