Page 60 - Applied Photovoltaics

nkT § V mp ·
V mp V oc ln ¨ ¨ ¸ 1 ¸ (3.4)
© q q nkT ¹
For example, if n = 1.3 and V oc = 600 mV, as for a typical silicon cell, V mp is about
93 mV smaller than V oc .
2
The power output at the maximum power point under strong sunlight (1 kW/m ) is
known as the ‘peak power’ of the cell. Hence photovoltaic panels are usually rated in
terms of their ‘peak’ watts (W p ).
The fill factor (FF), is a measure of the junction quality and series resistance of a cell.
It is defined as
V mp I mp
FF (3.5)
V oc I sc

Hence

P V I FF (3.6)
mp oc sc
Obviously, the nearer the fill factor is to unity, the higher the quality of the cell.
Ideally, it is a function only of the open circuit voltage and can be calculated using
the approximate empirical expression (Green, 1982)

ln v oc . 0 72 v
oc
FF (3.7)
v 1
oc
where v oc is defined as a ‘normalised V oc ’; that is
V
v oc (3.8)
oc q nkt

The above expression applies to ideal cases only, with no parasitic resistance losses,
and is accurate to about one digit in the fourth decimal place for these cases.

3.2 SPECTRAL RESPONSE
Solar cells respond to individual photons of incident light by absorbing them to
produce an electron-hole pair, provided the photon energy (E ph ) is greater than the
bandgap energy (E g ). Photon energy in excess of E g is quickly dissipated as heat, as
shown in Fig. 3.6.

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