Thermal-stable mixed-cation lead halide perovskite
solar cells
Shuai Gu (顾 帅), Pengchen Zhu (朱鹏臣), Renxing Lin (林仁兴), Mingyao Tang (唐明瑶),
Shining Zhu (祝世宁), and Jia Zhu (朱 嘉)*
National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and
School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University,
Nanjing 210093, China
*Corresponding author: jiazhu@nju.edu.cn
Received April 27, 2017; accepted May 22, 2017; posted online June 16, 2017
As perovskite solar cells show tremendous potential for widespread applications, we find that adding inorganic
thermal-stable cesium ions into MAPbI
3
results in significantly improves thermal stability. For un-encapsulated
perovskite devices, the energy conversion efficiency maintains about 75% of its original value (over 15%) in the
MA
0.85
Cs
0.05
PbI
3
device under 80 min of heating at 140°C in a dry atmosphere (RH ≤ 30% ). With significantly
improved thermal stability achieved by a convenient process, it is expected that this type of mixed-cation
perovskites can further facilitate large scale applications.
OCIS codes: 350.6050, 230.0250, 310.6845, 310.6860.
doi: 10.3788/COL201715.093501.
In the past few years, organolead halide perovskites have
captured intensive attention due to their remarkable
optoelectronic properties
[1–4]
. With a high absorption
coefficient, long diffusion length, and feasible fabrication
processes
[3,5,6]
, perovskites have been widely employed
in various optoelectronic applications, such as solar
cells
[7–12]
, photodetectors
[13,14]
, light-emitting diodes
(LEDs)
[2,15,16]
, and lasers
[17–20]
. Particularly in the field of
photovoltaics (PV)
[21–23]
, the past years have witnessed
the rapid development of efficiency from 3.8%
[24]
to
22.1%
[25]
. However, due to intrinsic ionic characters and
low crystallization energy
[26]
, the stability issue of perov-
skites (the most used form is MAPbI
3
) has become one
of the major obstacles toward large scale applications.
Compared to significant efforts devoted to the efficiency
improvement, much less attention has been paid to en-
hance the stability
[26–31]
.
As inorganic cesium lead halide perovskites (CsPbX
3
,
X ¼ Cl, Br, I) have demonstrated advantageous thermal
stability
[32–34]
, there have been efforts to develop mixed-
cation perovskites with high efficiency and good thermal
stability at the same time
[26,27,29–31]
. Several groups have
reported on Cs/FA mixtures, with enhanced stability of
photo, moisture, and heat compared to the pure one
(FAPbI
3
). They attribute the stability improvement to
lattice shrinkage and entropic stabilization. Saliba et al .
have fabricated solar cells using triple-cation perovskites
(MA/FA/Cs), also revealing improved r eproducibility
and stability. The investigation of MA/Cs mixtures
and their thermal stabilities are rather limited
[27]
. Here,
by partially substituting CH
3
NH
þ
3
(MA
þ
) ions with
cesium (Cs
þ
) to form a mixed-cation perovskite
(MA
1-x
Cs
x
PbI
3
), it is found that the mixed-cation perov-
skite films exhibit improved thermal durability when
exposed at a high temperature (150°C) compared to the
pristine perovskite (MAPbI
3
) and confirmed by the ab-
sorption spectra and X-ray diffraction (XRD) results.
In addition, MA
0.85
Cs
0.15
PbI
3
-based solar cells exhibit
more steady performance than MAPbI
3
-based ones under
heat stress tests. To be specific, the power conversion ef-
ficiency (PCE) degrades to about 75% of its original value
in the MA
0.85
Cs
0.15
PbI
3
-based devices after 80 min of
heating at 140°C in the dry atmosphere (RH ≤ 30%).
By contrast, the PCE degrades quickly to only about
15% in the MAPbI
3
devices under the same condition.
In this work, we investigated mixed-cation perovskites
with the general formula MA
1-x
Cs
x
PbI
3
and systemati-
cally adjusted x from 0 to 0.30 (x represents the molar
ratio of cesium added initially) to examine their PV per-
formances and thermal stabilities. In Fig.
1(a), we show
the XRD data of perovskites with different cesium con-
tents. With a small cesium concentration (x ≤ 0.15), only
MAPbI
3
peaks are observed, and no new phase corre-
sponding to cesium compounds is detected. This demon-
strates the complete integration of Cs
þ
into the perovskite
lattice. When the cesium concentration increases to 20%
Fig. 1. (Color online) (a) XRD patterns of MA
1-x
Cs
x
PbI
3
films
(x ¼ 0–0.30). (b) Absorption spectra of MA
1-x
Cs
x
PbI
3
films
(x ¼ 0–0.30).
COL 15(9), 093501(2017) CHINESE OPTICS LETTERS September 10, 2017
1671-7694/2017/093501(5) 093501-1 © 2017 Chinese Optics Letters