the development of flexible modules and perovskite-based monolithic tandem devices. To overcome
this limitation, researchers have pursued planar PSCs that exploit low-temperature (typically
<150°C) solution-processed charge-selective layers.
Metal oxide materials such as TiO2, ZnO, SnO2,
and Zn2SnO4 colloidal nanoparticles synthesized
at low temperatures have commonly been used
as the ESL (8–16). Very recently, high-efficiency (
certified 19.9%), small-area PSCs have been achieved
using low-temperature–processed SnO2 (17, 18).
Unfortunately, long-term device operational
stability has remained far inferior to that of counterpart devices made using high-temperature–
processed ESLs (19–22). Furthermore, the high
efficiency and large area required for industrialization have yet to be demonstrated in low-temperature planar PSCs (table S5) (23–25).
We reasoned that performance and stability loss in low-temperature planar PSCs could
arise from imperfect interfaces and charge recombination between the selective contact at the
illumination side and the perovskite film grown
on top (4, 26, 27), as the perovskite active layers
themselves have excellent long-term photostability
upon the addition of formamidinium (FA), Cs,
and Br ions (19–22, 28). Once the impressively
long photocarrier diffusion lengths in perovskite films are achieved, attention must shift to
perfecting the interface (29–32). We reasoned
that deep trap states present at the perovskite/
ESL interface could potentially be addressed
by passivating the interface between the charge-selective contact and the perovskite absorber.
We devised a simple and effective interface-passivation method that leads to efficient and
stable low-temperature–processed planar PSCs.
Chlorine-capped TiO2 colloidal nanocrystal (NC)
films processed at <150°C were used as the ESL.
The chloride additive in perovskite precursor
solutions enhances grain-boundary passiva-
tion in MAPbI3–xClx (MA, methylammonium
cation, CH3NH3+) PSCs (33–36). We found that
the interfacial Cl atoms on the TiO2 NCs sup-
press deep trap states at the perovskite inter-
face and thus considerably reduce interface
recombination at the TiO2/perovskite contact.
The interfacial Cl atoms also lead to strong
electronic coupling and chemical binding at
the TiO2/perovskite planar junction, as projected
in previous theoretical studies (37). As a result, we fabricated hysteresis-free planar PSCs
with independently certified PCEs of 20.1% for
small-area devices (0.049 cm2) and 19.5% for
large-area devices (1.1 cm2). The low-temperature
planar PSCs with high initial PCE >20% exhibit excellent operational stability and retain
90% (97% after dark recovery) of their initial
performance after 500 hours operating at their
maximum power point (MPP) under constant
1-sun illumination (where 1 sun is defined as the
standard illumination at AM1.5, or 1 k W/m2).
We first used density functional theory (DFT)
to examine defect passivation and interface bind-
ing by interfacial chlorine at the TiO2/perovskite
interface (Fig. 1, fig. S1, and table S1) (38). We
found that Cl at the interface results in stronger
binding at the TiO2/perovskite interface for both
the cases of MAX- and PbX2-terminated (X = Cl,
I) perovskite surfaces. Perovskite films with the
PbX2-terminated interface are energetically fa-
vored to contact the TiO2. Previous studies (39–41)
have shown that, in bulk perovskites, the most
detrimental defects (deep-level defects) are anti-
sites but that their formation energy is relatively
high, which explains the low trap-state density in
MAPbI3 perovskite films and single crystals. Va-
cancies and interstitials, although much more
abundant, are shallow defects. We thus explored
the effect of Cl at the interface on both antisite
and vacancy defects. Without Cl, a Pb-I antisite
defect leads to localized states near the valence
band edge (Fig. 1A). These states can capture holes
and become nonradiative recombination centers.
In contrast, the formation energy of the Pb-Cl
antisite at the PbCl2-terminated interface is higher
(i.e., less favorable to form), which indicates that
antisite defects are suppressed in the presence
of interfacial Cl atoms. The Pb-Cl antisite defect
becomes much shallower and more delocalized
(Fig. 1B) and has little effect on interface recom-
bination. Overall, the incorporation of Cl atoms
at the TiO2/perovskite interface resulted in a low-
er density of interfacial trap states (fig. S1, B and
D), as well as stronger binding between TiO2 and
perovskite (table S1).
We devised a synthetic approach to obtain
Cl-capped TiO2 NCs as the ESL in solar cells.
We first synthesized ~5-nm-diameter anatase
TiO2 NCs (fig. S2) (38) via a nonhydrolytic method
through the reaction of TiCl4 and benzyl alco-
hol at 85°C under ambient atmosphere (42, 43).
This process resulted in Cl-capped TiO2 NCs
(TiO2-Cl) with 12 ± 2 atomic of Cl relative to
Ti atoms, as determined by x-ray photoelectron
spectroscopy (XPS) (Fig. 1C). A mixture of meth-
anol and chloroform was used to disperse the
NCs while preserving surface Cl ligands. XPS
confirmed that surface Cl ligands were well re-
tained after we formed films from a methanol-
chloroform cosolvent (Fig. 1C). In contrast, the
surface Cl ligands were detached from TiO2 sur-
faces when the washed NCs were redispersed
in ethanol with a stabilizer such as titanium
diisopropoxide bis(acetylacetonate) (TiAcAc). Such
TiO2 NCs that lack Cl ligands—the ESL materials
used in previous reports (8, 9)—were taken as
controls in the present study. Henceforth, we refer
to the TiO2 ESL with Cl ligands as TiO2-Cl and
the TiO2 ESL lacking the Cl ligands as TiO2. The
Cl atoms were strongly bound to TiO2, and the Cl
ligands of TiO2 thin film were retained on the
surface after annealing up to 250°C (Fig. 1D).
We fabricated planar PSCs with TiO2 as the
ESL with the device architecture of Fig. 2A.
The TiO2-Cl film on indium tin oxide (ITO)–
coated glass obtained by spin-coating was smooth
and pinhole-free (Fig. 2B and fig. S3A). The
film also exhibited negligible parasitic absorp-
tion loss over the entire visible–to–near-infrared
spectral range (fig. S3B). Post-annealing treat-
ment at moderate temperatures was applied
to improve the quality of the spin-cast TiO2-Cl
film. The best PV performance was achieved
with a TiO2-Cl annealing temperature of 150°C
(table S2 and fig. S4). The mixed cation-halide
perovskite layer FA0.85MA0.15PbI2.55Br0.45, with
a thickness of ~600 nm, was deposited on
Fig. 1. The effect of Cl on interface quality between perovskite and TiO2, and stabilization of Cl-capped TiO2 (TiO2-Cl) colloidal nanocrystals.
(A) Trap-like localized antisite defects form near the valence band edge for the PbI2-terminated TiO2/perovskite interface. (B) Shallow and delocalized
Pb-Cl antisite defects are seen for the PbCl2-terminated interface. (C and D) XPS spectra of Cl 2p peaks of (C) TiO2 NC films [as-synthesized,
redispersed in the cosolvent of methanol and chloroform (MeOH + CF), and redispersed in ethanol with titanium diisopropoxide bis(acetylacetonate)
as stabilizer (EtOH + TiAcAc)] and (D) TiO2-Cl NC films with various post-annealing temperatures [room temperature (RT) and 100°, 150°, and 250°C].
a.u., arbitrary units.