RESEARCH ARTICLE SUMMARY
◥SOLAR CELLS
Vapor-assisted deposition of highly efficient, stable
black-phase FAPbI 3 perovskite solar cells
Haizhou Lu, Yuhang Liu, Paramvir Ahlawat, Aditya Mishra, Wolfgang R. Tress, Felix T. Eickemeyer,
Yingguo Yang, Fan Fu, Zaiwei Wang, Claudia E. Avalos, Brian I. Carlsen, Anand Agarwalla, Xin Zhang,
Xiaoguo Li, Yiqiang Zhan, Shaik M. Zakeeruddin, Lyndon Emsley, Ursula Rothlisberger,
Lirong Zheng, Anders Hagfeldt, Michael Grätzel
INTRODUCTION:Metalhalideperovskitesolar
cells (PSCs) have reached a power-conversion
efficiency (PCE) of 25.2%, thus exceeding other
thin-film solar cells. FAPbI 3 (where FA is for-
mamidinium) has been shown to be an ideal
candidate for efficient, stable PSCs. Obtaining
highly crystalline, stable, and purea-phase
FAPbI 3 films has been of vital importance.
However, FAPbI 3 undergoes a phase transi-
tion from the blacka-phase to the photo-
inactived-phase below 150°C. Previous approaches
to overcoming this problem include mixing it
with MA, Cs or Br ions. Here, we report a de-
position method using methylammonium
thiocyanate (MASCN) vapor treatment to convert
d-FAPbI 3 to the desired purea-phase below
the thermodynamic phase-transition tempera-
ture. Molecular dynamics (MD) simulations show
that the SCN–anions promote the formation and
stabilization ofa-FAPbI 3. These vapor-treated
FAPbI 3 PSCs exhibit outstanding photovoltaic
and electroluminescent performance.RATIONALE:Although the phase transition from
d- toa-phase FAPbI 3 requires a high tempera-
ture, the treatment ofd-phase FAPbI 3 films with
MASCN vapor allows the conversion to occur at
temperatures below 150°C. MD simulations show
that SCN–ions preferentially adsorb on the sur-
face ofd-FAPbI 3 to replace iodide ions that are
bound to Pb2+. This process disintegrates the
top layer of face-sharing octahedra and induces
the transition to the corner-sharing architec-
ture ofa-FAPbI 3. Once the corner-sharinga-form
is formed on the top surface, this layer tem-
plates the progression of the phase transition
fromd- toa-FAPbI 3 toward the bulk. Once the
purea-FAPbI 3 is formed, its back conversion to
thed-phase is prevented by a high energy barrier.RESULTS:We show a complete conversion from
d- toa-FAPbI 3 at 100°C using the MASCN vapor
treatment method. This phase transition can
also be achieved using FASCN vapor. The vapor-
treated FAPbI 3 film remained in its pure black
phase even after 500 hours of annealing at 85°C,
whereas the reference FAPbI 3 film formed main-
ly PbI 2 during the heat exposure. X-ray diffraction
data showed an improved crystallinity and pre-
ferred orientation of the FAPbI 3 films after va-
por treatment. One- and two-dimensional NMR
experiments were used to probe changes in
symmetry and quantify the incorporation of MA
into the perovskite framework. Time-of-flight
secondary ion mass spectrometry measurements
confirmed that the MASCN content was mostly
located near the surface region of the FAPbI 3
films. We used these low-defect-densitya-FAPbI 3
films to make PSCs with >23% PCE, long-term
operational stability, low (330 mV) open-circuit
voltage (Voc) loss, and low (0.75 V) turn-on volt-
age of electroluminescence.CONCLUSION:SCN–anions play a key role in
promoting the formation and stabilization of
a-FAPbI 3. Vapor-treated FAPbI 3 films showed
long-term thermal stability. MD simulations
showed that the purea-FAPbI 3 remained ki-
netically stable. These findings are important
for developing stable and pure black-phase
FAPbI 3 -based PSCs. Our vapor-treated FAPbI 3
PSCs showed high efficiency and good long-
term stability under maximum power point
tracking conditions. Because of its highVocand
high external quantum efficiency electrolumi-
nescence yield, purea-FAPbI 3 will be useful
for other applications such as light-emitting
diodes and photodetectors.
▪RESEARCH
74 2 OCTOBER 2020•VOL 370 ISSUE 6512 sciencemag.org SCIENCE
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected] (Y.Z.);
[email protected] (L.Z.); [email protected] (A.H.);
[email protected] (M.G.)
Cite this article as H. Luet al.,Science 370 , eabb8985 (2020).
DOI: 10.1126/science.abb8985READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abb89850 100 200 300 400 5000%50%100%- FAPbl 3 MASCN or FASCN -FAPbl 3
SCN–FA+- FA P b l 3
- FAPbl 3
Energy barrier- phase percentage
VaporStable and phase pure MASCN vapor-treated FAPbI 3 films.Vapor-treated FAPbI 3 films were annealed at
85°C for 500 hours in an N 2 environment.