1705998 (1 of 9)
2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Large-Grain Tin-Rich Perovskite Films for Efﬁcient Solar
Cells via Metal Alloying Technique
Mohammad Mahdi Tavakoli, Shaik Mohammed Zakeeruddin, Michael Grätzel,
and Zhiyong Fan*
Dr. M. M. Tavakoli, Prof. Z. Fan
Department of Electronics and Computer Engineering
The Hong Kong University of Science and Technology
Hong Kong, Clear Water Bay, Kowloon
Hong Kong SAR, China
Dr. M. M. Tavakoli, Dr. S. M. Zakeeruddin, Prof. M. Grätzel
Institut des Sciences et Ingénierie Chimiques
Ecole Polytechnique Fédérale de Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
The ORCID identiﬁcation number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201705998.
the past years to enhance the efﬁciency as
well as stability of solar cell devices.
Nevertheless, the lead toxicity is still a key
problem for commercialization of perov
skite solar cells. So far, many research
groups have explored replacement of Pb by
other metals such as Sn, In, Sr, Bi, Al, Ge,
Au, etc., into the perovskite structure.
However, to completely substitute Pb with
other metals still remains a big challenge.
Meanwhile, studies showed that adding
these elements at a low concentration to
the perovskite can improve its crystal
linity and as a result, the performance of
solar cell devices.
Up to now, Sn is the
only choice that can completely replace Pb
inside the perovskite lattice and Snbased
perovskite solar cell with efﬁciency around
6% has been reported. Thus, large efforts
are still needed to realize Snbased devices
whose performance is comparable with
Replacement of Pb by
Sn causes only a small perturbation in the
lattice due to their similar ionic radii. It
shifts the absorption of the perovskite to the nearinfrared range
decreasing its band gap down to below 1.3 eV.
the poor stability of Snbased perovskite is still a big challenge
for fabricating Pbfree solar cell devices due to fast oxidation of
and moisture attack.
In this regard, a binary
metal perovskite MAPb
(0 < x <1), i.e., partial replace
ment of Pb by Sn, is a viable way to fabricate more stable solar
cells with decent efﬁciency compared with pure Sn perovskites.
In the past few years, some research groups reported a binary
Pb–Sn perovskite solar cell with a maximum efﬁciency up to
Zhao et al.
fabricated binary Pb–Sn perovskite
solar cell devices using solution process with different con
centrations of Sn (0–100%) and they have reported an Snrich
(60% Sn) perovskite solar cell device with 10% efﬁciency which
is higher than other concentrations of Sn metal. In addition,
Li et al.
reported a 50% Snbased perovskite solar cell with
13.6% PCE using a twostep spin coating process, which is the
highest performance among MA(Sn
reported to date on single MA cationbased perovskite solar cell
devices. Recently, few research groups reported Pb–Sn pero
vskite solar cells with higher performance and stability based
on triple cation formulations, using a mixture of cesium, for
mamidinium, and methylammonium, which are good candi
dates for tandem solar cell application.
Herein, we propose
Fast research progress on lead halide perovskite solar cells has been achieved
in the past a few years. However, the presence of lead (Pb) in perovskite com-
position as a toxic element still remains a major issue for large-scale deploy-
ment. In this work, a novel and facile technique is presented to fabricate tin
(Sn)-rich perovskite ﬁlm using metal precursors and an alloying technique.
Herein, the perovskite ﬁlms are formed as a result of the reaction between
Sn/Pb binary alloy metal precursors and methylammonium iodide (MAI)
vapor in a chemical vapor deposition process carried out at 185 °C. It is found
that in this approach the Pb/Sn precursors are ﬁrst converted to (Pb/Sn)
and further reaction with MAI vapor leads to the formation of perovskite
ﬁlms. By using Pb–Sn eutectic alloy, perovskite ﬁlms with large grain sizes
up to 5 µm can be grown directly from liquid phase metal. Consequently,
using an alloying technique and this unique growth mechanism, a less-toxic
and efﬁcient perovskite solar cell with a power conversion efﬁciency (PCE) of
14.04% is demonstrated, while pure Sn and Pb perovskite solar cells prepared
in this manner yield PCEs of 4.62% and 14.21%, respectively. It is found that
this alloying technique can open up a new direction to further explore dif-
ferent alloy systems (binary or ternary alloys) with even lower melting point.
In the past few years, organic–inorganic perovskite solar cells
have been improved drastically in terms of solar to electric
power conversion efﬁciency (PCE) and stability.
high device performance, low fabrication cost and tempera
ture as well as the tunability of their composition and band
gap make this group of semiconductors highly promising for
Device architecture design,
material compositional engineering, and optimization of fab
rication process of perovskite materials have been explored in
Adv. Mater. 2018, 30, 1705998