Environmental Engineering Reference
In-Depth Information
Chapter 7
Multi-Layer Graded Bandgap Solar Cells
7.1 Introduction
In order to effectively harvest photons from all spectral regions
(ultraviolet [UV], visible, and infrared [IR]) and combine the impact
ionisation and impurity photovoltaic (PV) effects, a multi-layer
graded bandgap solar cell structure was introduced in chapter
6. This structure is based on an n-type window material layer.
However, improved results can be obtained from device structures
starting with p-type window materials. As indicated in section 1.6
of chapter 1, these two structures are shown in Fig. 1.16. The
structure with p-type window material has additional advantages
over that with n-type window material. The potential barrier height
(
φ
b
) for electron flow through the device structure is determined
by the large bandgap of the window material. This is, therefore,
capable of producing high potential barriers and, hence, large
V
oc
values. In addition, the electrons produced by high-energy photons
can accelerate and create more charge carriers due to impact
ionisation,becauseofthesmallerbandgappresentattherearofthe
solar cell. Because of these additional advantages, the experimental
testing was carried out only on this device structure with p-
type window material. This chapter presents the experimental
