layer yielded JSC = 23.24 mA cm−2, VOC = 1112 mV,
and FF = 78.2%, resulting in a PCE of 20.4% (Fig.
3B) (the role of rGO is discussed below). As
evident from the hysteresis index values, a hysteresis effect was discernable for spiro-OMe TAD
by comparing the forward and backward J-V
scan, but it was negligible for CuSCN (Fig. 3C)
(29). Figure 3, A and B, shows that the VOC yielded
by CuSCN-based devices was slightly lower than
that yielded by spiro-OMe TAD–based devices. To
understand the cause of the VOC deficit in CuSCN-based devices, we estimated the ideality factor
(n), which is an indicator of the dominant
recombination mechanism occurring within a
working device (30). By fitting the intensity dependence of the VOC curves (Fig. 3, A and B, insets)
[(18), equation S1], we estimated n = 1.46 and 1.50,
respectively, for the spiro-OMe TAD–based and
CuSCN-based devices, which indicates that the
difference in the VOC stemmed from marginally
higher monomolecular recombination occurring
within the CuSCN-based devices. JSC showed linear behavior with illumination intensity in both
PSCs (fig. S4).
Figure 3C summarizes the statistical analysis
of PV parameters extracted from the J-V curves
of 20 independent devices. The high PCEs were
reproducible for both spiro-OMe TAD–based and
CuSCN-based PSCs. For the CuSCN-based devices,
we observed an average JSC = 22.65 ± 0.60 mA cm−2,
VOC = 1090 ± 14 mV, and FF = 0.75 ± 0.02,
resulting in an average PCE of 19.22 ± 0.84%.
Similarly, for the spiro-OMeTAD–based devices,
we observed an average JSC = 22.6 ± 0.55 mA cm−2,
VOC = 1115 ± 15 mV, and FF = 0.75 ± 0.02, resulting in an average PCE of 19.6 ± 0.77%. To determine the stabilized (scan speed–independent)
PCEs, we probed the solar cells at their maximum power point under full-sun illumination
(Fig. 3D). We recorded a stabilized output power
corresponding to a PCE of 20.5% and 20.2%
for spiro-OMeTAD–based and CuSCN-based
devices, respectively, in close agreement with
the J-V measurements. The integrated photocurrent densities obtained from the external
quantum efficiency (EQE) spectra of spiro-OMe TAD–based and CuSCN-based devices agreed
closely with those obtained from the J-V curves
(Fig. 3E), indicating that any spectral mismatch
between our simulator and AM-1.5 standard
solar radiation was negligible.
The long-term thermal stability of devices at
high temperature has become a key problem, pri-
marily because the diffusion of metal through a
spiro-OMe TAD layer at higher temperatures leads
to the degradation of the devices (22). We exam-
ined the thermal stability of CuSCN-based devices
coated with a thin layer of poly(methyl meth-
acrylate) polymer (18) at 85°C in ambient condi-
tions in the dark. After 1000 hours, the CuSCN-based
devices retained >85% of their initial efficiency
(fig. S5). The formation of a uniform CuSCN film,
as evident from morphological analysis, blocked
the metal diffusion (22). Long-term operational
stability is a crucial requirement for future exploi-
tations of PSC-based technology (31). Under full-
sun illumination at their maximum power point,
(14), but atomic layer deposition of an insulat-
ing Al2O3 layer (~2 nm) between the perovskite
and CuSCN layers did not mitigate the initial
degradation (fig. S7). Instead, we introduced a
thin conductive rGO spacer layer (fig. S8) be-
tween the CuSCN and gold layers, leading to
excellent operational stability under full-sun
illumination at 60°C. The resulting PSCs re-
tained >95% of their initial efficiency after aging
for 1000 hours, apparently surpassing the stabil-
ity of spiro-OMeTAD devices recorded under
similar conditions (fig. S9).
We traced the photoeffect back to the positive
electrical polarization imposed on the gold when
the CuSCN device is illuminated at its maximum
power point or under open circuit conditions. We
confirmed the occurrence of potential-induced
degradation by applying a positive bias of 0.8 V
to the Au contact of a CuSCN device in the dark.
The results (fig. S10) illustrate the severe loss in
PV performance under these conditions. When
no electrical bias was applied to the cell during
aging in the dark, no appreciable degradation
was observed even after prolonged heating of
the CuSCN devices at 85°C (fig. S5). Thus, we
identify the cause of the degradation to be an
electrical potential–induced reaction of gold with
the thiocyanate anions forming an undesired
barrier, which is different from the degradation
occurring at the interfaces between perovskite
and selective contacts (32). Using x-ray photoelectron spectroscopy (XPS), we confirmed the
oxidation of gold (fig. S11) upon subjecting the
CuSCN devices to the light soaking test over extended time periods. We conclude that the instability of PSCs is not associated with the
degradation of CuSCN/perovskite interface, as
is generally believed, but rather originates mainly from the CuSCN/Au contact. The CuSCN film
did not require any additives to function as an
effective HTM, in contrast to PTAA and spiro-OMe TAD, which can reach their peak performance only in the presence of organic lithium
salt and 4-tert-butylpyridine and, for the latter,
also a Co(III) complex that acts as a p-dopant
(4); these additives readily cross into the photoactive PSC layer and adversely affect PV performance. Our results show that PSCs using
all-inorganic charge extraction layers (i.e., mesoporous TiO2 and CuSCN) display high PCE values
combined with remarkable operational and thermal stability, offering the potential for large-scale
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Author contributions: N.A., M.I.D., and M.G. conceived the
idea of the work; N.A. and M.I.D. designed the project
and fabricated and characterized devices; M.I.D. and N.A.
performed PL and SEM analysis; N.A., M.I.D., and A.H.
performed the XRD measurements; A.H. and F.S. analyzed and
discussed the XRD data; N.P. carried out hole mobility
experiments; N.P. and M.I.D. performed light soaking
measurements; M.I.D. wrote the manuscript; all the authors
contributed toward finalizing the draft; S.M.Z. coordinated the
project; and M.G. directed and supervised the research.
Supported by Greatcell Solar SA (N.A.) and by the European
Union’s Horizon 2020 program through a Future Emerging
Technologies (FET)–Open Research and Innovation action
under grant agreement 687008 and Graphene Flagship Core1
under grant agreement 696656 (M.I.D., S.M.Z., and M.G.). We
thank the European Synchrotron Radiation Facility for provision
of synchrotron radiation, A. Chumakov and F. Zontone for
assistance in using beamline ID10, J. Hagenlocher for
assistance with XRD analysis, M. Mayer for assistance with
ALD, F. Giordano and P. Yadav for helpful discussions,
and P. Mettraux (Molecular and Hybrid Materials
Characterization Center, EPFL) for carrying out XPS
measurements. All results are presented in the main
paper and supplement. We have applied for a patent on
inorganic hole conductor–based PSCs with high operational
Materials and Methods
Figs. S1 to S11
References (33, 34)
16 December 2016; resubmitted 4 August 2017
Accepted 14 September 2017
Published online 28 September 2017