Engineering interfacial charge transfer in CsPbBr3 perovskite nanocrystals by heterovalent doping

Article abstract of DOI:10.1021/jacs.6b09575

Since compelling device efficiencies of perovskite solar cells have been achieved, investigative efforts have turned to understand other key challenges in these systems, such as engineering interfacial energy-level alignment and charge transfer (CT). However, these types of studies on perovskite thin-film devices are impeded by the morphological and compositional heterogeneity of the films and their ill-defined surfaces. Here, we use well-defined ligand-protected perovskite nanocrystals (NCs) as model systems to elucidate the role of heterovalent doping on charge-carrier dynamics and energy level alignment at the interface of perovskite NCs with molecular acceptors. More specifically, we develop an in situ doping approach for colloidal CsPbBri perovskite NCs with heterovalent Bi3+ ions bv hot injection to precisely tune their band structure and excited-state dynamics. This synthetic method allowed us to map the impact of doping on CT from the NCs to different molecular acceptors. Using time-resolved spectroscopy with broadband capability, we clearly demonstrate that CT at the interface of NCs can be tuned and promoted by metal ion doping. We found that doping increases the energy difference between states of the molecular acceptor and the donor moieties, subsequently facilitating the interfacial CT process. This work highlights the key variable components not only for promoting interfacial CT in perovskites, but also for establishing a higher degree of precision and control over the surface and the interface of perovskite molecular acceptors.

Full text of DOI:10.1021/jacs.6b09575

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Article
Engineering-Interfacial Charge Transfer in CsPbBr3
Perovskite Nanocrystals by Heterovalent Doping
Raihana Begum, Manas R Parida, Ahmed L. Abdelhady, Banavoth Murali, Noktan Alyami,
Ghada H. Ahmed, Mohamed Nejib Hedhili, Osman M. Bakr, and Omar F. Mohammed
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Dec 2016
Downloaded from http://pubs.acs.org on December 16, 2016
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Engineering Interfacial Charge Transfer in CsPbBr Perovskite Nano-
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crystals by Heterovalent Doping
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†,§
†,§
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†
Raihana Begum , Manas R. Parida , Ahmed L. Abdelhady , Banavoth Murali , Noktan M. Al-
†
†
‡
†
yami , Ghada H. Ahmed , Mohamed Nejib Hedhili , Osman M. Bakr * and Omar F. Moham-
†
med *
†
King Abdullah University of Science and Technology (KAUST), KAUST Solar Center, Division of Physical Sciences
and Engineering, Thuwal 23955-6900, Kingdom of Saudi Arabia
‡
King Abdullah University of Science and Technology (KAUST), Imaging and Characterization Laboratory, Thuwal
23955-6900, Kingdom of Saudi Arabia
ABSTRACT: Since compelling device efficiencies of perovskite solar cells have been achieved, investigative efforts have
turned to understand other key challenges in these systems, such as engineering interfacial energy-level alignment and
charge transfer (CT). However, these types of studies on perovskite thin-film devices are impeded by the morphological
and compositional heterogeneity of the films and their ill-defined surfaces. Here, we use well-defined ligand-protected
perovskite nanocrystals (NCs) as model systems to elucidate the role of heterovalent doping on charge-carrier dynamics
and energy level alignment at the interface of perovskite NCs with molecular acceptors. More specifically, we develop an
3
+
in-situ doping approach for colloidal CsPbBr perovskite NCs with heterovalent Bi ions by hot injection to precisely tune
3
their band structure and excited-state dynamics. This synthetic method allowed us to map the impact of doping on CT
from the NCs to different molecular acceptors. Using time-resolved spectroscopy with broadband capability, we clearly
demonstrate that CT at the interface of NCs can be tuned and promoted by metal doping. We found that doping increases
the energy difference between states of the molecular acceptor and the donor moieties, subsequently facilitating the in-
terfacial CT process. This work highlights the key variable components not only for promoting interfacial CT in perov-
skites, but also for establishing a higher degree of precision and control over the surface and the interface of perovskite
molecular acceptors.
INTRODUCTION
1
1,15-18
Perovskite nanocrystals (NCs) are promising materials
for display devices, visible-light communication systems,
skites.
To date, the potential role of heterovalent-
metal doping on the charge transfer and energy-level
alignment at the interfaces of metal-halide perovskite
NCs remains unknown. Thus far, only isovalent doping in
NCs has been reported. Here, we were thus motivated to
design a synthetic method for the in situ heterovalent-
1
-4
lasers, and LEDs. Their optical and electronic properties
are easily tunable by adjusting their size, composition,
5
-10
19
and dimensionality. Moreover, their defined structure,
composition, and ligand-protected surfaces make them
unique model systems for investigating some of the most
perplexing challenges hampering the advancement of
bulk perovskite-based devices. Since achieving milestones
in device efficiencies, efforts to improve bulk perovskite-
based optoelectronics, such as solar cells and photodetec-
tors, have shifted to other central challenges, such as es-
tablishing stability and engineering charge transfer (CT)
metal doping of CsPbBr perovskite NCs. We developed a
3
3
+
hot-injection approach to dope perovskite NCs with Bi ,
while preserving the crystal structure of the host. This
method allowed us to reveal heterovalent-metal doping as
a tool for modulating interfacial CT and energy-level
alignment in NCs. Our time-resolved photoluminescence
(PL) and transient-absorption (TA) studies demonstrate
that the dopants not only introduce significant changes in
the band gaps of the perovskite NCs, but also facilitate CT
at the interface of these NCs with different molecular ac-
ceptors, including tetracyanoethylene (TCNE) and phe-
nyl-C -butyric acid methyl ester (PC BM).
1
1
at interfaces (e.g., through doping). The latter is difficult
to investigate in perovskite-based thin film devices be-
cause they suffer from ill-defined surfaces and morpholo-
gies, and because they have broad variations in their
1
2
compositions (grain-to-grain).
6
1
61
It is well documented that doping generally leads to
EXPERIMENTAL SECTION
1
3,14
new optical, magnetic, and electrical properties.
Het-
Synthesis of undoped and doped NCs
erovalent-metal doping causes the type of conductivity
significantly change, as in the case of chalcogenide and
pnictogenide binary NCs and bulk metal-halide perov-
Both Bi- doped and undoped CsPbBr NCs were syn-
thesized following the hot-injection method reported by
Protesescu et al. with some modifications. Cesium oleate
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(
(
Cs-oleate) was prepared by reacting cesium carbonate
0.203 g) with oleic acid (650 µL) in octadecene (10 ml)
Trace-metal analysis was carried out using induc-
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tively coupled plasma optical emission spectrometry
(ICP-OES) on a Varian 720-ES ICP-optical emission spec-
trometer. Dried powdered samples of NCs were first acid
digested in a microwave and then diluted prior to meas-
urements. ICP-OES measurements with a detection limit
ranging from parts per million (ppm) to parts per billion
(ppb) were performed twice on separately synthesized NC
samples (synthesized under identical reaction conditions)
and standard deviations were calculated accordingly.
solvent. The reaction was maintained under vacuum con-
ditions with continuous stirring at 120 C for 1 h followed
by purging of N gas until all solute had completely dis-
solved. Next, PbBr (0.069 g) and octadecene (5 ml) were
loaded in a two-neck flask and maintained under vacuum
conditions for about 30 min at 120 C. This was followed
by addition of oleic acid (1 ml) and oleylamine (550 µL).
After complete dissolution of the precursor, the vacuum
was stopped and the temperature was raised to 165 C
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Kelvin Probe (from KP Technology, KP software 9.1 )
measurement was done in a glove box. NCs samples were
drop cast on ITO glass substrates. A gold test sample was
used as a reference. The Kelvin probe measured the ener-
gy difference between the work function of the sample
and that of the reference metal plate.
while purging N gas. The Cs-oleate (400 µL) prepared in
the step above was then injected into the contents of the
two-necked flask, which was immediately transferred to
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an ice-water bath. The CsPbBr NCs that formed in the
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octadecene were centrifuged at 5000 rpm for 5 min and
the supernatant was discarded. The pellet was dispersed
in toluene, washed twice with acetonitrile, and then cen-
trifuged again at 7000 rpm for 5 min. The resultant NCs
were redispersed in toluene for a second time and centri-
fuged again at higher speed to discard the bigger parti-
cles. The NCs were doped by adding three concentrations
of BiBr and PbBr . Cs-oleate was injected at comparative-
X-ray photoelectron spectroscopy (XPS) measure-
ments were carried out in a Kratos Axis Ultra DLD spec-
trometer equipped with a monochromatic Al Kα X-ray
source (hν = 1486.6 eV) operating at 150 W, a multi-
-9
channel plate, and a delay line detector under 1.0 x10
Torr vacuum. The survey and the high-resolution spectra
were collected at fixed analyzer pass energies of 160 and
0 eV, respectively. Measurements were performed on
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0
ly lower temperature (165 C) to avoid surface adsorption
of the dopants.
2
NCs samples dispersed in toluene drop cast on a glass
substrate. Note that charge neutralization was required
for these measurements. Binding energies were refer-
Characterization
UV−vis absorption and PL spectra of freshly pre-
3
pared CsPbBr NCs doped and undoped redispersed in
3
enced to the C 1s peak (set at 284.8 eV) of the sp hybrid-
toluene were obtained using Cary-5000 spectrophotome-
ter from Agilent Technologies and FluoroMax-4 spectro-
fluorometer from Horiba Scientific.
ized (C-C) carbon (from oleylamine and oleic acid present
in the NCs). The data were analyzed with commercially
available software, CasaXPS. The individual peaks were
fitted by a Gaussian (70%)–Lorentzian (30%) (GL30) func-
tion after linear or Shirley-type background subtraction.
Powder X-ray diffraction (XRD) patterns were rec-
orded in thin film mode on a Bruker AXS D8 diffractome-
ter using CuKα radiation (λ = 1.54178 Å).
Femto- and nanosecond transient absorption
spectra were collected using an ultrafast Systems Helios
and EOS spectrometers with broadband capability and
time resolutions of 120 fs and 800 ps, respectively. The
probe pulse was generated in a 2-mm-thick calcium flo-
Transmission electron microscopy (TEM) data
were obtained using a Titan TEM (FEI Company) operat-
ing at a beam energy of 300 keV and equipped with a
Tridiem postcolumn energy filter (Gatan, IQD). The sam-
ples were imaged in EFTEM mode with a 20-eV energy slit
inserted around the zero-energy loss of electrons to ac-
quire high-resolution TEM (HRTEM) micrographs.
ride (CaF ) plate in the Helios spectrometer using a few µJ
2
of pulse energy from the fundamental output of a
Ti:sapphire femtosecond regenerative amplifier operating
at 800 nm with 35 fs pulses and a repetition rate of 1 kHz.
In EOS experiments, the white-light continuum probe
pulse was generated by a super continuum source instead
of CaF2 crystal used in Helios. A two-channel probe
Photoemission spectroscopy in air (PESA) meas-
urements were carried out using a Riken photoelectron
spectrometer (Model AC-2) with an ultraviolet (UV) lamp
at an intensity of 50 nW. PESA was used to measure the
yield of photo-emitted electrons (cube root of Yield) be-
ing ejected from the sample as a function of photon ener-
gy.
(
probe-reference) method was used for both spectrome-
ters. This method splits the probe beam into two before
passing through the sample. While one arm travels
through the sample, the other is sent directly to the refer-
ence spectrometer that monitors the fluctuations in the
probe beam intensity. The main advantage of this tech-
nique is that it allows the user to achieve the specified
signal-to-noise ratio with a lower number of averaged
laser pulses. The pump pulses at 370 nm were created
from spectrally tunable (240-2600 nm) femtosecond puls-
es generated in the optical parametric amplifier (Newport
Spectra-Physics). Finally, pump and probe beams were
Samples of doped and undoped NCs redispersed in
toluene were drop cast on glass for XRD and indium tin
oxide (ITO) glass substrates for PESA measurements.
Samples were dried at room temperature and measure-
ments were performed under ambient conditions. Sam-
ples were prepared similarly for TEM measurements with
the exception of drop casting on carbon-coated copper
grids.
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tions, the actual concentration in the NCs (after the sec-
focused on the sample solution, and the transmitted
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probe light from the samples was collected and focused
on the broadband UV-visible detector to record the time-
resolved transient absorption spectra.
ond wash) was estimated to be 0.25%, 0.8%, and 2.1%,
respectively. We also calculated the average amount of Bi
per NC (listed in Table S1 and S2) and plotted the Bi at-
oms incorporated in each NC, as determined by ICP-OES
in relation to the Bi/NC ratio in the feed solution (Figure
S6). The highest doping level achieved in our experiments
can be considered to be likened to the heavy doping de-
RESULTS AND DISCUSSION
Oleic acid capped CsPbBr perovskite NCs were syn-
3
20
1
8,26
thesized following a well-established method with mi-
nor modifications as represented in Scheme S1 (see sup-
porting information, SI). Three concentrations of bismuth
bromide – 2%, 5% and 10% – with respect to lead bromide
were used during synthesis. An optimal concentration of
oleic acid and oleylamine was used to facilitate the solu-
scribed in the literature.
The in situ hot-injection
method yielded 19% doping (calculated from the slope of
the curve in Figure S6) in the NCs that were used for
spectroscopic measurements.
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bilization of the Pb and Bi precursors as reported per-
21
viously. The NCs were size-selectively precipitated by
centrifugation at different speeds to ensure a narrow size
distribution. Finally, they were re-dispersed in toluene for
detailed optical characterizations.
We calculated the average size of the cubic-shaped
NCs from HRTEM images to be ~ 8.7 + 0.2 nm (Figure 1A-
B and Figure S1-S2, SI). The results indicated that the Bi-
doped and undoped NCs had the same particle size and
shape. We also used these images to calculate the inter-
planar distance (d= 0.58 nm) of doped and undoped NCs.
The Bragg reflections in powder XRD patterns (Figure S3)
22
were consistent with results published in the literature.
As shown in Figure S3, the peak positions of the XRD pat-
terns did not shift upon doping.
We performed several experiments confirming the
3+
Figure. 1 TEM images of CsPbBr NCs (A) undoped, B) 2.1%
successful incorporation of Bi into the NCs. Figure S4
3
3
+
Bi -doped, with HRTEM images shown in the insets, indicat-
ing no change in average size, shape, or crystal structure
illustrates XPS results revealing that both Bi-doped and
undoped NCs comprised Cs, Pb, Br, C, N, and O, con-
23
upon doping. (C) Absorption and (D) PL spectra (λ =365
ex
sistent with the literature. High-resolution spectra cor-
nm) of undoped and Bi-doped (0.25%, 0.8%, and 2.1%) CsP-
responding to the core levels of Cs 3d, Cs 4P, Pb 4f, and Br
bBr NCs showing the spectral shift and change in the band
3
3
d were recorded. Although the peak positions for both
gap energy, E (calculated from a Tauc plot).
g
doped and undoped NCs were the same, we observed an
additional peak from Bi-doped NCs at 164.4 eV, which we
attributed to the Bi 4f5/2 component of Bi . Another
Next, we compared the steady state absorption and PL
3+ 24
spectra of Bi-doped and undoped CsPbBr NCs as shown
3
component of the Bi 4f core level, Bi 4f , was difficult to
in Figures 1(C) and (D). As can be seen, the first excitonic
peak shifted to a lower energy when doped with lower
concentration (Bi/Pb=0.25%) and then it shifted to a
higher energy at high doping levels, providing another
7/2
distinguish because it overlapped with Cs (4P ) at 158.9
3
/2
eV, as shown in Figure S4(b). It also coincided with the
corresponding doublet of Cs (4P ) located at 170.3 eV.
1
/2
3+
We substantiated these XPS findings with results from
ICP-OES, which clearly indicated that the ratio of Bi/Pb
increased monotonically as the concentration of bismuth
bromide was increased in the feed solution (Figure S5).
Note that the amount of Bi decreased after the NCs were
washed a second time with acetonitrile, suggesting that
initially some loosely bonded or surface-adsorbed Bi at-
oms were on the doped NCs. A third wash of the NCs,
however, did not significantly change the dopant content,
indicating that the dopant was present in the core of the
NCs. It should be noted that we were unable to identify
the exact location of the dopant by X-ray absorption spec-
troscopy (XAS) due to the similar atomic emission fea-
piece of evidence for the successful incorporation of Bi
into the core of the NCs. An absence of Urbach tailing in
our doped NCs suggested that there was no lattice distor-
tion upon doping unlike that occurs in Bi-doped halide
11
perovskite single crystals. It is worth highlighting that
the absence of any tailing in the absorption spectrum up-
on doping is beneficial as a sharp absorption edge which
27
is a merit of halide perovskites. Optical band gaps were
2
quantified from the Tauc plot of (αhν) versus hν, which
illustrates the band gaps of undoped and Bi-doped NCs
(Figure S7). Undoped CsPbBr NCs showed a band gap of
3
28
2.39 eV, consistent with that previously reported. How-
ever, the plot of the energy shift (ΔE) of the first exciton
peak in relation to the Bi/NC ratio (estimated by ICP),
25
tures of Bi and Pb. In the 2%, 5%, and 10% Bi concentra-
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shown in Figure S8, further confirms the incorporation of
bBr host lattice, causing strong perturbation to the densi-
ty of states.
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Bi in the crystal lattice. A similar trend due to n-type
doping has previously been reported in Cu-doped InAs
To explore the impact of doping on the carrier lifetime
and the deactivation of the excited carriers, we performed
time-resolved PL and nanosecond transient absorption
spectroscopy, respectively. The experimental setups of
26
NC. Finally, we used Kelvin-probe measurements for un-
doped and Bi-doped NCs to determine the position of the
Fermi level (See Figure S9). The Fermi level (E ) of un-
F
doped CsPbBr NCs was close to the valence band edge.
3
our PL and TA spectroscopies have previously been de-
31-33
Meanwhile, as Bi content increased, the Fermi level gradu-
ally shifted toward the conduction band edge, further con-
firmed the presence of Bi doping in the crystal lattice.
scribed.
Optical excitation with 370 nm laser pulses
caused ground-state bleaching (GSB) at 506 and 499 nm
for undoped and doped NCs, respectively (as shown in
Figure 3(A) and (B)).
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Figure. 2 Schematic representation showing changes in the
band alignments of CsPbBr NCs upon doping with 0.25 or
3
2
.1% Bi. We measured the EV of NCs by PESA (Figure S14)
and E is estimated by subtracting the band-gap energy (cal-
C
culated from the Tauc Plot) from the respective E values; (E
V
v
and E are the valence and conduction band energies, respec-
c
tively).
Figure. 3 ns-TA spectra at indicated delay times after 370 nm
As can be seen in the PL spectra of the NCs samples in
Figure 1(D), a nominal red shift is followed by a blue shift
from 517 nm to 512 nm at higher Bi/Pb ratios. It is widely
accepted that as dopant content increases, impurity levels
interact with each other to form a band-like structure
optical excitation of (A) CsPbBr NCs (GSB ~ 506 nm) and (B)
3
0
.8% Bi- doped CsPbBr NCs (GSB ~ 499 nm) in toluene. (C)
3
Transient kinetic traces as a function of time-delay and (D)
time-resolved PL decays for excitation at 370 nm at three
doping levels (0%, 0.8%, and 2.1%).
1
8
within the band gap of the host (Figure 2). We attribute
the observed spectral shift to filling of the conduction
band with extra electrons donated by the dopants, also
The spectral changes in the TA of Bi-doped CsPbBr NCs
3
could be attributed to a strong perturbation in the elec-
tronic structure that occurs when Bi-dopant atoms are
29
known as the Burstein-Moss effect. At low doping con-
centrations, a red shift arises with fewer impurity levels or
3
4
introduced into the CsPbBr NC structure. We followed
3
the GSB recovery to study the charge carrier recombina-
tion dynamics. The GSB centered at 506 nm in undoped
trap states ‘below’ the first electronic state, 1S , of the
e
conduction band. At high-doping concentrations, a blue
shift is observed due to the evolution of the trap states
CsPbBr NCs revealed full recovery with a time constant
3
of 7 ns, while in 0.8% doped NCs, the GSB showed full
recovery with a time constant of 36 ns (Figure 3C). Fitting
‘above’ the 1S state (Figure 2). A similar result has been
e
26
reported for Cu-doped InAs NCs. Note that the full
width at half maximum of the emission peak in all sam-
ples was between 18-21 nm, which is indicative of a narrow
particle size distribution (see Figure 1). Moreover, we ob-
served that the emission intensity monotonically de-
creased with dopant concentration as was previously re-
of the kinetic profiles of 0.8% Bi-doped CsPbBr NCs
3
showed two lifetime components, a fast component of 3
ns and a long component of 36 ns, while undoped CsP-
bBr NCs showed just a single dominant lifetime. As can
3
be seen in Figure 3C, a greater increase in the long com-
ponent of charge recombination time was observed in
24
ported for Bi-doped PbS quantum dots. The quantum
2
.1% Bi- doped CsPbBr NCs. This implies the involvement
3
yield for undoped CsPbBr NCs was found to be 78% and
3
of change in the electronic band structure which delays
it decreased to 8% at high-doping concentration
3
5
carrier recombination in Bi-doped NCs. Time-resolved
PL decay measurements complement these results, as
shown in Figure 3(D), whereby undoped NCs showed ex-
(
Bi/Pb=2.1%). This decrease in emission intensity suggests
that Bi atoms may have been incorporated into the CsP-
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These rates suggest that the quenching mechanism static
ponential decay with a lifetime of 7 ns, results in agree-
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ment with a recent report. Noted that perovskite NCs
are known to have much shorter PL lifetimes than bulk
single crystals due to the stronger overlap between elec-
in nature (also, supported by fs-TA measurements).
Figure 4 shows the fs-TA spectra and corresponding
transient kinetic traces of undoped and 0.8 %-doped
samples with and without TCNE molecular acceptors. In
the presence of TCNE, GSB is slightly red-shifted, similar
to what we observed in the steady-state absorption spec-
tra (Figure S10). GSB occurs due to filling of 1S levels of
the conduction band by excited electrons and recovery
3
6
tron-hole wave functions in a spatially confined space.
Meanwhile, the doped NCs exhibited bi-exponential de-
cay with two lifetime components as presented in Figure
3
(D). The longer lifetime decreased to 5.2 ns and 4.5 ns
for 0.8% and 2.1% doped NCs, respectively. The emission
quenching accompanied by a decrease in the PL decay
lifetime in Bi-doped CsPbBr NCs further substantiated
the presence of trap states in the band gap of the host
CsPbBr NCs upon doping with Bi. Such trap states pro-
vide alternative ways for electronic relaxation and conse-
quently reduce radiative recombination.
occurs when the electrons are removed or the charge car-
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3
riers recombine.
This shift in the position of GSB in
presence of a molecular acceptor may be explained in
terms of change in the electron density upon the for-
mation of a ground-state complex. In addition, the recov-
ery of the GSB is ultrafast when coupled with TCNE, re-
gardless of whether NCs were doped or undoped. These
results suggest that TCNE provides an additional deacti-
vation channel for the transfer of photo-generated elec-
trons, accelerating recovery (see Figure 4(C) and (D)). We
also observed that GSB recovery was faster for Bi-doped
than for undoped NCs in presence of TCNE (Figure 4 (B)
and (D)). To obtain a quantitative picture of the carrier
dynamics in these NCs, we compared the fs-TA kinetics of
undoped and Bi-doped NCs with and without TCNE, as
shown in the insets of Figure 4. The kinetics data are fit-
ted with a bi-exponential function and the acquired time
components are presented in Table S3. Both the undoped
and 0.8% Bi-doped NCs have one fast component with
time constants of about 82 ps and 128 ps, respectively, and
one slow component with a few nanosecond time con-
stant. The fast component may arise from charge-carrier
3
To understand the impact of doping on the CT at the
interface of these NCs with molecular acceptors, so we
can take the work an important step further, we per-
formed femtosecond transient absorption (fs)-TA spec-
troscopy with broadband capability on undoped and
doped NCs in the presence of TCNE, a known electron
acceptor.
As TCNE was successively added (0.1 mM to 0.8 mM)
to doped and undoped CsPbBr NCs, we observed a grad-
3
ual decrease in emission intensity, as shown in Figure S10.
We observed maximum quenching of about 90% for 0.8%
doped NCs and 78% for undoped NCs under the same
experimental conditions. Although this degree of exciton
quenching could also be a consequence of energy transfer,
we excluded this possibility due to the lack of spectral
overlap between the absorption of TCNE and the emis-
sion of NCs, as shown in Figure S11. TEM images show
that treatment with TCNE (inset, Figure S10) did not
change the size or shape of the NCs. With the addition of
TCNE, we observed a red shift of 5-8 nm in the absorption
spectra of both undoped and doped NCs and a concomi-
tant red shift in the position of the PL emission maxi-
mum. These changes in the absorption and PL spectra of
the NCs upon addition of TCNE indicate the formation of
a ground-state complex between the donor and acceptor.
Such a strong electronic coupling between TCNE and the
excited state of the NCs might facilitate the photo-
trapping, whereas the slow component can be attributed
37,41
to excitonic recombination in the NCs.
We continued
to observe both components after adding TCNE, however,
the short component became much faster and the ratio
between the fast and slow components increased signifi-
cantly, reconfirming that TCNE provides an additional
channel for fast electron transfer from these NCs. The
faster rate of GSB recovery for doped than for undoped
NCs indicates that doping increases the rate of CT. We
also studied the excited-state dynamics of the undoped
and doped NCs in the presence of PC BM using fs-TA.
The steady-state and time-resolved results are shown in
Figure S13 & Figure 4(E) and (F). As the concentration of
61
37
induced electron transfer from NCs to TCNE. To under-
stand the mechanism of the photo-induced electron
transfer, we analyzed the PL data using the Stern-Volmer
equation. From the plot shown in Figure S12, we
estimated quenching rates at 9.1 × 10 M S and 7.1 × 10
PC BM was increased from 0.25 mM to 3.5 mM, the emis-
61
38
sion intensity of the NCs decreased (see Figure S13). Here,
we used two-photon excitations at 800 nm to selectively
excite the NCs.
12
-1 -1
12
-1 -1
M S for 0.8% doped and undoped NCs, respectively.
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Figure 4. fs- TA spectra at indicated delay times after 370-nm optical excitation of (A) CsPbBr NCs (B) 0.8%-doped CsPbBr NCs;
3
3
(C) and (D) are the same samples in (A) and (B), respectively, treated with 0.8-mM TCNE. Inset figures between (A) and (C), and
between (B) and (D) comparison fs-transient kinetics traces at the respective GSB maxima of the NC samples in the presence
and absence of TCNE. (E-F) fs- TA spectra of the undoped and 0.8%-doped NCs treated with 3.5 mM PC BM, Inset figures (right
61
panel) show comparison of fs-transient kinetics traces at the respective GSB maxima of the NC samples in the presence and ab-
sence of PC BM. The solid red lines in the insets represent the best fit for the kinetic traces.
61
The fs-TA spectra of undoped and Bi-doped samples
with PC BM is shown in Figure 4 (E and F), at two pho-
Marcus theory, as it correlates with Gibbs free energy (-
∆G), which is the energy difference between the acceptor
and donor moieties with the rate of electron transfer, the
rate of electron transfer increases as the difference in en-
61
ton laser excitation (at 800 nm). The insets illustrate the
fs-TA kinetics traces at the respective GSB positions (right
panel of Figure 4). Upon addition of PC BM, we observed
3
7,48,49
ergy increases.
We observed a clear difference in
61
faster GSB kinetics of NCs. The faster bleach recovery
observed upon coupling with a molecular acceptor was
due to electron injection from the conduction band of
NCs to the lowest-unoccupied molecular orbital (LUMO)
of the acceptor. This offset of the favorable band energy
facilitates the electron transfer. The electron transfer rate
electron affinities between doped and undoped NCs and
both the acceptors. The energy offset between the con-
duction band of the NCs and both of the molecular accep-
tors was higher for the Bi-doped NCs than for undoped
NCs, thus, accelerating the electron injection process.
was found to be relatively higher for 0.8% doped CsPbBr3
NCs than for undoped CsPbBr NCs, similar to what we
3
observed in the presence and absence of TCNE.
To gain a new physical insight into the mechanism of
the electron injection from Bi-doped and undoped NCs to
the two acceptors (TCNE and PC BM), we deciphered the
61
band alignment between the NCs and the molecular ac-
4
2,43
ceptor.
First, we measured the valence band edge (E )
V
44
of the NCs by PESA. We found that E = -5.5 eV and E =-
V
V
5.45 eV for undoped and 0.8% Bi- doped NCs, respectively
(Figure S14). Similar to that reported in the literature,
LUMO values were -3.77 eV and -3.9 eV for TCNE and
4
5-47
PC BM, respectively).
The band-edge alignment is
61
Scheme 1: Bandgap offset of undoped and Bi-doped CsP-
schematically demonstrated in Scheme 1. According to
bBr NCs with respect to the LUMO of TCNE.
3
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SUMMARY
In summary, we report the doping of heterovalent Bi
Journal of the American Chemical Society
Abdelhady, A. L.; Ng, T. K.; Mohammed, O. F.; Ooi, B. S.; Bakr,
O. M. ACS Photonics 2016, 3, 1150-1156.
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Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk,
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injection method to tune their electronic structure. We
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structure and carrier dynamics of CsPbBr NCs, but also
3
on the interfacial CT with two well-known molecular ac-
ceptors. The fs-TA results clearly show that interfacial CT
in this system can be tuned and facilitated by metal dop-
ing. This is due to the increase in the driving-force Gibbs
free energy (-∆G) between the molecular acceptor and
donor moieties upon metal doping, which leads to more
efficient CT from the NCs to molecular acceptors. This
novel finding provides a tool to precisely control the CT
process at the interface of this promising class of NCs and
paves the way for engineering interfacial charge transfer
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Abdelhady, A. L.; Saidaminov, M. I.; Murali, B.; Adinol-
ASSOCIATED CONTENT
Supporting figures S1-S14, table, TEM image, XRD, XPS,
PESA, Absorption and emission spectra.
fi, V.; Voznyy, O.; Katsiev, K.; Alarousu, E.; Comin, R.; Dursun, I.;
Sinatra, L.; Sargent, E. H.; Mohammed, O. F.; Bakr, O. M. J. Phys.
Chem. Lett. 2016, 7, 295-301.
(
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Li, J.-J.; Ma, J.-Y.; Ge, Q.-Q.; Hu, J.-S.; Wang, D.; Wan,
AUTHOR INFORMATION
Corresponding Author
L.-J. ACS Appl. Mater. & Interfaces 2015, 7, 28518-28523.
(
13)
Begum, R.; Chattopadhyay, A. J. Phys. Chem. Lett. 2014,
5
, 126-130.
(14)
*Email: omar.abdelsaboor@kaust.edu.sa
Lee, J.-W.; Son, D.-Y.; Ahn, T. K.; Shin, H.-W.; Kim, I.
Y.; Hwang, S.-J.; Ko, M. J.; Sul, S.; Han, H.; Park, N.-G. Sci. Rep.
*
Email: osman.bakr@kaust.edu.sa
Author Contributions
R.B and M.R. P contributed equally to the work.
2
013, 3, 1050.
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, 10790-10800.
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Banin, U.; Frenkel, A. I. J. Phys. Chem. C 2013, 117, 13688-13696.
17) Sahu, A.; Kang, M. S.; Kompch, A.; Notthoff, C.; Wills,
(
9
§
Present Addresses
(
A. W.; Deng, D.; Winterer, M.; Frisbie, C. D.; Norris, D. J. Nano
Lett. 2012, 12, 2587-2594.
King Abdullah University of Science and Technology
(
KAUST), KAUST Solar Center, Division of Physical Sci-
(18)
E.; Banin, U. Science 2011, 332, 77-81.
19) Navas, J.; Sanchez-Coronilla, A.; Gallardo, J. J.; Cruz
Mocatta, D.; Cohen, G.; Schattner, J.; Millo, O.; Rabani,
ences and Engineering, Thuwal 23955-6900, Kingdom of
Saudi Arabia.
(
Hernandez, N.; Pinero, J. C.; Alcantara, R.; Fernandez-Lorenzo,
C.; De los Santos, D. M.; Aguilar, T.; Martin-Calleja, J. Nanoscale
ACKNOWLEDGMENT
2015, 7, 6216-6229.
This work was supported by King Abdullah University of
Science and Technology (KAUST). Also, part of this work was
supported by a Saudi Arabia Basic Industries Corporation
(
20) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg,
F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko,
M. V. Nano Lett. 2015, 15, 3692-3696.
(
SABIC) grant RGC/3/2470-01.
(21)
De Roo, J.; Ibanez, M.; Geiregat, P.; Nedelcu, G.;
Walravens, W.; Maes, J.; Martins, J. C.; Van Driessche, I.; Ko-
valenko, M. V.; Hens, Z. ACS Nano 2016, 10, 2071-2081.
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