Engineering Chemistry S Datta

SOLID STATE CHEMISTRY 501
Ring
Light
Transparent film
Barrier layer
Selenium
Iron Base
Electron
flow
A
Fig. 22.27 Photovoltaic cell.
The most common photovoltaic cells are of the barrier-layer type, like iron-selenium
cells. In the iron-selenium cell, selenium layer is placed on an iron disc and then extremely
thin transparent film of gold or silver is formed on the selenium to act as front electrode. The
barrier layer is formed by cathode sputtering the semi-transparent film on the selenium. A
contact ring on the silver layer acts as one electrode and the iron base as the other.
Working
When radiant flux falls on semiconductor i.e. selenium, it ejects electrons which travel
from selenium to the front silver electrode through the barrier layer (Fig. 22.27). The flow in
the opposite direction is not permitted by the barrier layer because it acts as a rectifier. The
e.m.f. generated internally between silver electrode and selenium is directly proportional to
the incident flux.
Application
l The main advantage of a photovoltaic cell is that it requires no external battery for its
operation, i.e. it is self-generating.
l The internal e.m.f. and hence current generated by it are large enough to be measured
by a pointer galvanometer.
l Such cells are used
(a) in devices like portable exposure meters,
(b) in direct reading illumination meters and with low resistance relays for on and off
operations and
(c) in other monitoring operations in industries.
PROBLEMS
Problem 1. Calculate the Miller indices of crystal planes whose Weiss indices are :
(a) a/2, 2b/3, ∞ c; (b) 2a/3, 2b, c/3.
Solution:
(a)
(b)
Weiss indices 1/2, 2/3, ∞ 2/3, 2, 1/3
Reciprocal of
2, 3 2 , 1 ∞
Weiss indices
3/2, 1 2 , 3
Clear fractions 4, 3, 0 3, 1, 6
Miller indices (430) (316)