Control of defect processes in photovoltaic materials
is essential
for
realising high
-
efficiency
solar cells and related optoelectronic devices.
The concentrations of n
ative defects and
e
xtrinsic dopants tune the Fermi level and
enable
semiconducting p
-
n junctions; however,
fundamental limits to doping exist in many compounds. Optical transitions involving defect
states can enhance photocurrent generation through sub
-
bandgap absorption; ho
wever,
such states are often responsible for
carrier trapping and
non
-
radiative recombination
events that limit open
-
circuit voltage. Many classes of materials
–
including metal oxides,
chalcogenides
,
and halides
–
are being examined for next
-
generation so
lar
energy
applications, and each technology faces distinct challenges
that could benefit from point
defect engineering
. We review the evolution in point defect behaviour from Si
-
based
photovoltaics to thin
-
film CdTe and Cu(In,Ga)Se
2
technologies,
through
to the latest
generation halide perovskite (CH
3
NH
3
PbI
3
) and kesterite (Cu
2
ZnSnS
4
) devices. We focus on
the chemical bonding that underpins the defect chemistry, and the atomistic processes
associated with the photophysics of charge carrier generation, trap
ping
,
and recombination
in solar cells. Finally, we
outline
general
principles
to enable defect
control
in
complex
semico
nducting materials.