Page 166 - A Comprehensive Guide to Solar Energy Systems

Chapter 8 • Photovoltaics: The Basics 167

FIGURE 8.15 (A) In-series connection of PV cells. (B) Cells of identical I–V characteristics. (C) Cells with different I–V
characteristics.

FIGURE 8.16 (A) In-parallel connection of PV cells. (B) Cells of identical I–V characteristics. (C) Cells with different I–V
characteristics.

(e.g., local shading) can cause differences both in the photo-generated current and in the
voltage. Furthermore, nonuniform temperatures can cause differences in the cell output
voltage and negatively influence the characteristics of cells.

8.4 Principles of Solar Cell Construction

From eq. (8.18) it follows that for obtaining a high efficient solar cell, it is necessary to max-
imize the photo-generated current I PV and minimize losses due to parasitic resistances R p
and R s . At first, we concentrate on optimizing cell construction with the aim of obtaining a
maximum photo-generated current.
The structure of a solar cell can be approximated by the diagram, as shown in Fig. 8.17.
In the illuminated area excess carriers are generated, which diffuse toward the Pn junc-
tion. The current density J FV is the current density of carriers collected by the junction
space charge region and it can be expressed by

J () () () λ (), (8.23) JPV(λ)=JPVn(λ)+JPVP(λ)+JoPn(λ),
λ + J OPN
λ + J PVP
PV
λ = J PVN