Page 242 - A Comprehensive Guide to Solar Energy Systems

Chapter 11 • Hybrid Organic–Inorganic Metal Halide Perovskite Solar Cells 245

these organic HTL materials have good transport properties, they unfortunately have poor
stability. Inorganic HTL materials, on the other hand, are cost effective and demonstrate
long-term stability although the device efficiency is lower than using organic HTLs. Exam-
ples of inorganic HTL materials are CuI [115], Cu:NiOx [116], NiO [117], CuSCN [118,119],
and iron pyrite [120].

11.5 Stability Issues and Challenges of Perovskite Solar Cells

11.5.1 Stability Issues

The present research on perovskite solar cells mainly focused on material design, novel
cell structures, and the underlying mechanisms. The issues of degradation of perovskite
and the stability of the devices are huge challenges to the PV communities. It is very ur-
gent to address these challenges to achieve good reproducibility and long lifetime for so-
lar cells. Organo-metallic halide perovskite undergoes series of chemical reactions even
under ambient atmospheric conditions and either decomposes into their components or
the film can directly degrade into other chemicals. Niu et al. [121] identified four factors
responsible for the degradation of perovskite films such as oxygen and moisture, UV light,
solution processing, and temperatures.
Since the material is quite sensitive to oxygen and moisture, most of the fabrication
processes are conducted in an inert atmosphere in a glove box. Significant degrada-
tion occurs in the solar cells during their testing under ambient conditions. The report
by Seok et al. [90] indicates that degradation of perovskite film starts at a humidity of
55% and higher, displaying a color change from dark brown to yellow. This degradation
prevents perovskite solar cells for outdoor applications. The degradation of perovskite
film caused by oxygen and moisture is irreversible [121]. Niu et al. [121] found that
absorption of TiO 2 /MAPbI 3 film in the spectral range of 530–800 nm is greatly reduced
after exposure to air with a humidity of 60% at 35°C for 18 h and the material’s X-ray
diffraction (XRd) peaks had completely disappeared. Leijtens et al. [61] demonstrated
that the cause of degradation of perovskite solar cells is due to the degradation of TiO 2
in UV light. Thermal stability was tested for semi-finished perovskite solar cells by an-
nealing them at 85°C for 24 h; they found only the PbI 2 remaining when analysed using
XRd [122].
Even with these challenges, efficient and stable solar cells have been demonstrated
in recent years with materials and interface engineering [8,105,123–125]. Kim et al. [18]
reported stable performances of solar cells over 500 h for devices stored in air at room
temperature and occasional exposed to air mass 1.5 global (AM1.5G) light illumination.
The stability of perovskite solar cells under high humidity and temperature conditions
has been improved by employing a moisture-resistant layer to prevent water entrance
[104,126,127]. Leijtens et al. [128] demonstrated encapsulation techniques using glass
sealing or laminate plastic films to improve device stability to over 125 days at 60°C under
simulated sunlight. Stability of perovskite solar cells could also be improved by composi-
tional engineering of the films [31,90,129].