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The authors thank P. Ambichl, J. Bertolotti, A. Haber, S. Hallegatte,
and J. Schwarz for fruitful discussions and C. Francois-Martin,
F. Pincet, and T. Narita for technical assistance with the dynamic
light scattering machine. This research was supported by the
European Research Council (project reference 278025). R.P. and
R.C. were supported by LABEX WIFI (Laboratory of Excellence
within the French Program “Investments for the Future”) under
references ANR-10-LABX-24 and ANR-10-IDEX-0001-02 PSL*.
S.R. was supported by the Austrian Science Fund (FWF) through
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Figs. S1 to S9
Tables S1 to S3
7 April 2017; resubmitted 23 June 2017
Accepted 3 October 2017
Perovskite solar cells with CuSCN
hole extraction layers yield stabilized
efficiencies greater than 20%
Neha Arora,1 M. Ibrahim Dar,1*† Alexander Hinderhofer,2 Norman Pellet,1
Frank Schreiber,2 Shaik Mohammed Zakeeruddin,1 Michael Grätzel1†
Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized only with
expensive organic hole-transporting materials. We demonstrate PSCs that achieve stabilized
efficiencies exceeding 20% with copper(I) thiocyanate (CuSCN) as the hole extraction
layer. A fast solvent removal method enabled the creation of compact, highly conformal
CuSCN layers that facilitate rapid carrier extraction and collection. The PSCs showed high
thermal stability under long-term heating, although their operational stability was poor.
This instability originated from potential-induced degradation of the CuSCN/Au contact.
The addition of a conductive reduced graphene oxide spacer layer between CuSCN and
gold allowed PSCs to retain >95% of their initial efficiency after aging at a maximum
power point for 1000 hours under full solar intensity at 60°C. Under both continuous
full-sun illumination and thermal stress, CuSCN-based devices surpassed the stability of
spiro-OMe TAD–based PSCs.
The tailoring of the formation and composi- tion of the absorber layer in organic-inorganic perovskite solar cells (PSCs) has resulted in certified power conversion efficiencies (PCEs) exceeding 20% (1, 2). These PCEs have been
obtained while retaining the electron-selective TiO2
layer and by using either spiro-OMe TAD [2,2′,7,7′-
bifluorene] or a polymer-based poly(triarylamine)
(PTAA) as the hole-transporting material (HTM)
(2, 3). However, the cost of these HTMs is pro-
hibitively high for large-scale applications, and
the archetype organic HTMs or their ingredients
apparently are a factor in the long-term opera-
tional and thermal instability of the PSCs that
use them (4). One strategy to combat these issues
of cost and instability could be the use of inex-
pensive inorganic hole extraction layers, similar
to the use of TiO2 as an electron-transporting
material (5). However, obtaining stable PCEs
of >20% with PSCs that use inorganic HTMs
(such as NiO, CuI, Cs2SnI6, and CuSCN) when
subjected to light soaking under realistic ope-
rational conditions (i.e., at maximum power point
and 60°C) has remained a challenge (6–9).
The realization of efficiencies of >20% from
PSCs using inorganic HTMs remains a key goal
that would foster the large-scale deployment of
PSCs. Among various inorganic HTMs, CuSCN
is an extremely cheap, abundant p-type semi-
conductor that exhibits high hole mobility, good
thermal stability, and a well-aligned work func-
tion (10). It is intrinsically p-doped and transmits
light across the entire visible and near-infrared
spectral region, so it is also attractive for tandem
cell applications where the PSC is placed on top
of a semiconductor with a lower band gap (11).
However, the stabilized PCE values reported with
CuSCN lag far behind devices based on the stan-
dard spiro-OMe TAD. CuSCN deposition methods
including doctor blading, electrodeposition, spin
coating, and spray coating have been tried (9, 12–16).
Of these, the solution-based bottom-up approaches
are more facile; however, a critical issue associated
with them is that most of the solvents in which
CuSCN shows high solubility degrade the perov-
skite layer (17). Because of the dearth of solvents
that readily dissolve CuSCN but not the perov-
skites, an inverted device architecture has been
used, albeit with moderate success (12).
To retain the mesoscopic TiO2-based normal
device architecture, we developed a simple dynamic deposition method. Typically, we deposited
a thin and uniform CuSCN layer on top of a
CsFAMAPbI3–xBrx [FA = CH(NH2)2+, MA =
CH3NH3+] perovskite layer. To do so without
compromising the quality of the perovskite layer,
we drop-cast a defined volume of CuSCN dissolved in diethyl sulfide (DES, 35 mg/ml) in 2
to 3 s while spinning the substrate at 5000 rpm
(18). The structural features of this CuSCN layer
were investigated by x-ray diffraction (XRD).
CuSCN crystallizes generally in two polymorphs,
a-CuSCN (19) and b-CuSCN (20, 21), where the
latter exhibits polytypism (i.e., variation in layer
stacking order). A comparison of the calculated
powder XRD spectra and grazing incidence XRD
data of CuSCN (Fig. 1A) shows that the dynamic
deposition method yielded b-CuSCN. A broad
reflection at q = 1.9 Å−1 established the presence
of different polytypes of b-CuSCN, predominantly 2H and 3R. Coherently scattering island
sizes of 17 and 18 nm were estimated from the
peak width of the (002) reflection of CuSCN
deposited, respectively, on the glass and the perovskite film. To determine the domain orientation, we acquired grazing incidence wide-angle
768 10 NOVEMBER 2017 • VOL 358 ISSUE 6364 sciencemag.org SCIENCE
1Laboratory of Photonics and Interfaces, Department of
Chemistry and Chemical Engineering, Ecole Polytechnique
Federale de Lausanne (EPFL), CH-1015 Lausanne,
Switzerland. 2Institut für Angewandte Physik, Universität
Tübingen, 72076 Tübingen, Germany.
*These authors contributed equally to this work.
†Corresponding author. Email: email@example.com (M.I.D.);