the risks associated with a new PV technology.
Thus, it is important to accurately predict the
time dependence of the PCE. The market reference
is crystalline silicon solar cells with an average
degradation rate of 0.5% per year, which is often
ensured for 25 years under operational conditions.
To compete within the PV market, PSCs must
reach similar levels of stability (e.g., 0.25 to 0.5%
losses per year).
Extrinsic degradation factors
Before investigating the stability of perovskite
materials, it is important to understand external
factors that may enhance degradation in PSCs,
which tends to be irreversible. One of the main
degradation pathways has been linked to high-
temperature device testing (above 60°C) or de-
vices heated up by constant illumination. Until
recently, it was believed that the MA cation is
thermally unstable in the perovskite and is solely
responsible for degradation. Several studies have
now shown that spiro-OMe TAD [2,2′,7,7′-tetrakis
crystallizes under thermal stress, which then creates
pathways that allow for an interaction of the metal
electrode and the perovskite (Fig. 4A) (21, 68–70).
The use of carbon electrodes has helped to alleviate
this issue because these materials are thermally
stable and do not seem to interact with the perovskite layer (71–73). Layers of a polymeric hole
conductor such as polytriarylamine (PTAA) are
robust to temperatures up to 85°C, and devices
have been shown to maintain 95% of their efficiency after 500 hours at continuous maximum
power point tracking (MPPT) and light soaking
Other extrinsic degradation factors have been
ascribed to hole conductor dopant migration, such
as lithium salts migrating through the perovskite
layer affecting the efficiency of devices (74). Ultraviolet (UV) light, present in the full solar spectrum, has been reported to be detrimental to the
long-term stability of perovskites as it is absorbed
by the electron-selective contact, TiO2, initiating
a chemical degradation (75). To partially circumvent this issue, electron-selective contacts with
wide band gaps have been developed, offering
superior UV stability relative to TiO2 (Fig. 4C)
(23, 75, 76). Alternatively, when using TiO2, UV
filters can be easily applied, offsetting this effect.
For instance, with a coating of UV fluorophores,
UV photons are downconverted to visible photons, boosting the photocurrent and filtering
out UV photons (77); therefore, UV stability
seems not to be the main concern in the quest
Analogous to organic PVs, the stability of perovskite solar cells is severely influenced by moisture. Proper encapsulation with the elastomeric
polymer ethylene-vinyl acetate has been demonstrated to protect the PSCs against moisture and
heat in the commonly used damp heat testing
(26). Promising long-term stability results have
been reported for unencapsulated Si-perovskite
tandems (Fig. 4D) (26) and single junctions (70),
topped by an ITO electrode that acts as a barrier
to moisture. Other simple encapsulation schemes
involving polymers have been successfully used.
Polymer-coated devices tested on a roof withstood rain and variable temperature conditions
for more than 90 days, showing no sign of degradation (77).
Fig. 4. Long-term stability of perovskite solar cells. (A) Gold migration
through spiro-OMe TAD under light, MPPT, nitrogen flow at 75°C. [Adapted
from (69) with permission] (B) The use of multiple cations and a PTAA hole
contact shows losses of ~5% in 500 hours of MPPT in nitrogen at 85°C
(21). (C) Photostability test under AM 1.5G illumination, including UV
radiation for encapsulated devices based on different metal oxides (23).
(D) Unencapsulated Si-perovskite tandems show remarkable MPPTstability,
with a slight increase in performance and an almost unchanged performance
at the end of the >1000-hour test (26). (E) MPPTof devices showing reversible
losses before going through permanent degradation (78). (F and G) Room-
temperature test with light soaking of planar p-i-n for 150 hours (24) (F)
and light soaking of n-i-p for 500 hours (22) (G); the devices exhibit small
losses under MPPT. Configurations: M, mesoporous; N, planar n-i-p; P,
planar p-i-n; Si-T, Si-perovskite tandem. The layer compositions of the device
stacks are summarized at the top of each graph. Each graph includes a
summary of the aging conditions used.