Page 240 - A Comprehensive Guide to Solar Energy Systems

Chapter 11 • Hybrid Organic–Inorganic Metal Halide Perovskite Solar Cells 243

The post-deposition annealing process serves to remove residual solvent from the
solution-process, assist in perovskite formation from its precursors, and enhance crys-
tallization and grain growth. A mixed halide perovskite like MAPbI 3−x Cl x has to be an-
nealed longer, to accomplish full conversion, compared to the single halide system. These
perovskite films are more stable at room temperature. The surface coverage of perovskite
films decreases when the annealing takes a long time (e.g., more than 30 min at 110°C)
at higher temperature due to the decomposition of the perovskite phase [51]. Annealing
at higher temperatures (higher than 80°C) results in MAI loss and increases the relative
PbI 2 content resulting in a decrease in device performance [43,82]. In general, annealing is
done either in dry air or in an inert atmosphere like a nitrogen environment. Annealing of
the perovskite films can also be conducted in pyridine or MAI vapor with the achievement
of enhanced luminescence and carrier lifetime [83,84]. Similarly, Xiao et al. [85] conducted
solvent annealing with dMF achieving improve crystallinity of perovskite films and en-
hancement of performance.
The annealing of the perovskite films discussed above is known as hotplate annealing
under different atmospheres. Optical annealing approaches have also been reported for
perovskite films using either a halogen lamp (near-infrared radiation) or a xenon lamp
(photonic flash and intense pulse light) sources [86–88]. Optical annealing process is con-
sidered effective because heat light/radiation is absorbed by absorber layer as well as FTO
layers. However, it was found that the power conversion efficiencies of perovskite solar
cells are not as good as hot plate annealing processes. This might be because the tempera-
ture cannot be measured accurately in optical annealing method. Saliba et al. [89] intro-
duced a concept of “flash” annealing, which was particularly effective for planar architec-
ture cells, where the sample was annealed at high to a low temperature range at different
time intervals.

11.4.2 Band Gap Optimization

It has been shown that chemical modification of the anions in organic–inorganic metal
halide perovskite can tune the band gap over a wide range of the solar spectrum. For ex-
ample, the band gap of the perovskite can be tuned to cover almost the entire visible range
from 1.5 eV for MAPbI 3 to 2.3 eV for MAPb(Br x I 1−x ) 3 when x = 1, with a significant modula-
tion in photo-conversion efficiency (PCE) of perovskite solar cells [31,90]. The introduc-
tion of Br not only changes the band gap of the material but also makes the material more
stable in humid condition. The band gap of perovskite can also be changed by the modifi-
cation of the cation such as the replacement of methylammonium with a formamidinium
ion (FA) to form formamidinium lead halide [(HC(NH 2 ) 2 PbI 3 ] perovskite structure having
a band gap of 1.48 eV [31,91,92]. The reduction in band gap in FAPbI 3 allows for absorption
of photons over a wide range of the spectrum providing for a greater current collection.
Instead of complete replacement, only a partial replacement of cation can also be done to
change the band gap of the perovskite. Pellet et al. worked on an alloy of MA x FA 1−x PbI 3 to
extend the absorption to a longer spectral range and to enhance the thermal stability [93].