In Part I [Appl.
Opt. 58, 6067
(2019)APOPAI003-693510.1364/AO.58.006067], we used a coupled
optoelectronic model to optimize a thin-film
C
u
I
n
1
−
ξ
G
a
ξ
S
e
2
(CIGS) solar cell with a
graded-bandgap photon-absorbing layer and a periodically corrugated
backreflector. The increase in efficiency due to the periodic
corrugation was found to be tiny and that, too, only for very thin
CIGS layers. Also, it was predicted that linear bandgap-grading
enhances the efficiency of the CIGS solar cells. However, a
significant improvement in solar cell efficiency was found using a
nonlinearly (sinusoidally) graded-bandgap CIGS photon-absorbing layer.
The optoelectronic model comprised two submodels: optical and
electrical. The electrical submodel applied the hybridizable
discontinuous Galerkin (HDG) scheme directly to equations for the
drift and diffusion of charge carriers. As our HDG scheme sometimes
fails due to negative carrier densities arising during the solution
process, we devised a new, to the best of our knowledge, computational
scheme using the finite-difference method, which also reduces the
overall computational cost of optimization. An unfortunate
normalization error in the electrical submodel in Part I came to
light. This normalization error did not change the overall conclusions
reported in Part I; however, some specifics did change. The new
algorithm for the electrical submodel is reported here along with
updated numerical results. We re-optimized the solar cells containing
a CIGS photon-absorbing layer with either (i) a homogeneous
bandgap, (ii) a linearly graded bandgap, or (iii) a
nonlinearly graded bandgap. Considering the meager increase in
efficiency with the periodic corrugation and additional complexity in
the fabrication process, we opted for a flat backreflector. The new
algorithm is significantly faster than the previous algorithm. Our new
results confirm efficiency enhancement of 84% (resp. 63%) when the
thickness of the CIGS layer is 600 nm (resp. 2200 nm),
similarly to Part I. A hundredfold concentration of sunlight can
increase the efficiency by an additional 27%. Finally, the currently
used 110-nm-thick layer of
M
g
F
2
performs almost as well as optimal
single- and double-layer antireflection coatings.