suggesting that PAA played a key role in form-
ing the uniform and contiguous layer of paa-
QD-SnO 2 ETL, as shown in fig. S3.
Element mapping with energy-disperse x-ray
spectroscopy of Ti (Fig. 1, C and D) and Sn
(Fig. 1, E and F) revealed a coverage of TiO 2
and SnO 2 over the FTO surface for both QD-
SnO 2 @c-TiO 2 and paa-QD-SnO 2 @c-TiO 2 bi-
layers. The selected-area electron diffraction
generated by TEM for the paa-QD-SnO 2 @c-
TiO 2 bilayer (fig. S4) showed that both QD-
SnO 2 and c-TiO 2 were polycrystalline. The paa-
QD-SnO 2 had a particle size of ~4 nm (fig. S4C),
which is also confirmed by the TEM images
(fig. S5, A to C) and dynamic light scattering
analysis (fig. S5, D and E).
The interactions between PAA and QD-SnO 2
were studied by the x-ray photoelectron spec-
troscopy (XPS) (fig. S6) and Fourier transform
infrared spectroscopy (FTIR) measurements(fig. S7). It is clear from the XPS measurements
that both QD-SnO 2 @c-TiO 2 and paa-QD-
SnO 2 @c-TiO 2 bilayers showed characteris-
tic peaks attributed to Sn (fig. S6A); however,
the Sn 3d peaks of paa-QD-SnO 2 shifted to
high binding energy by ~0.2 eV compared
with that of QD-SnO 2 , indicating that PAA was
bonded to the QD-SnO 2. No obvious difference
was observed for the O 1s characteristic peaks
(fig. S6, B to D). FTIR measurements showedSCIENCEscience.org 21 JANUARY 2022¥VOL 375 ISSUE 6578 303
Fig. 1. Microstructures of the ETLs.
(AandB) The cross-sectional TEM images
of QD-SnO 2 @c-TiO 2 (A) and paa-QD-SnO 2 @
c-TiO 2 (B) over the FTO substrates. Scale
bars, 0.1mm. (CandD) EDS elemental
analysis of Ti for both QD-SnO 2 @c-TiO 2 (C)
and paa-QD-SnO 2 @c-TiO 2 (D) over the FTO
surface. Scale bars, 100 nm. (EandF) EDS
elemental analysis of Sn for both QD-SnO 2 @
c-TiO 2 (E) and paa-QD-SnO 2 @c-TiO 2 (F) over
the FTO surface. Scale bars, 100 nm.
QD-SnO 2 @c-TiO 2paa-QD-SnO 2 @c-TiO 2Ti SnTi SnABCD FE20 21 22 23 24 25 26
0510150.1 1 10 100051015200.0 0.2 0.4 0.6 0.8 1.0 1.20510152025700 750 800 850 9000.00.51.01.52.02.53.0300 400 500 600 700 800 900
020406080100051015202530Current density (mA/cm^2 )A B CDE FVoltage (V)PCE (%)c-TiO 2
m-TiO 2 @c-TiO 2
paa-QD-SnO 2 @c-TiO 2EQEEL(%)Wavelength (nm)EQE (%)Efficiency (%)CountsLight intensity (mW/cm^2 )Voc(V)Target cellPhoton flux
(1/[s m2
nm]) x 1018
Target cellWavelength (nm)12.5%2.5%QD-SnO 2 @c-TiO 2IntegratedJ
sc(mA/cm2 )m-TiO 2 @c-TiO 2
paa-QD-SnO 2 @c-TiO 2QD-SnO 2 @c-TiO 210 1001.01.11.2QD-SnO 2 @c-TiO 2m-TiO 2 @c-TiO 2Target cellm-TiO 2 @c-TiO 2
QD-SnO 2 @c-TiO 2m-TiO 2 @c-TiO 2
QD-SnO 2 @c-TiO 28.3%QD-SnO 2 @c-TiO 2m-TiO 2 @c-TiO 2paa-QD-SnO 2 @c-TiO 2Fig. 2. Characterization of the PSCs.(A) TheJ-Vcurves of the PSCs with different ETLs measured under the QSS-IVmethod. (B) A statistical distribution of the
PCE for PSCs with different ETLs. (CtoF) The EQE and integratedJsc(C), steady-state PL spectral photon flux (D), EQEEL(E), and the light-dependentVoc(F) for the
PSCs with different ETLs.
RESEARCH | REPORTS