Long-distance charge carrier funneling in perovskite nanowires enabled by built-in halide gradient

Article abstract of DOI:10.1021/Jacs.6b10512

The excellent charge carrier transportation in organolead halide perovskites is one major contributor to the high performance of many perovskite-based devices. There still exists a possibility for further enhancement of carrier transportation through nanoscale engineering, owing to the versatile wet-chemistry synthesis and processing of perovskites. Here we report the successful synthesis of bromide-gradient CH₃NH₃PbBr_xI_3-x singlecrystalline nanowires (NWs) by a solid-to-solid ion exchange reaction starting from one end of pure CH₃NH₃PM₃ NWs, which was confirmed by local photoluminescence (PL) and energy dispersive X-ray spectroscopy (EDS) measurements. Due to the built-in halide gradient, the long-distance carrier transportation was driven by the energy funnel, rather than the spontaneous carrier diffusion. Indeed, local PL kinetics demonstrated effective charge carrier transportation only from the high-bandgap bromide-rich region to the lowbandgap iodine-rich region over a few micrometers. Therefore, these halide gradient NWs might find applications in various optoelectronic devices requiring long-distance and directional delivery of excitation energy.

Full text of DOI:10.1021/Jacs.6b10512

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Communication
Long-Distance Charge Carrier Funneling in Perovskite
Nanowires Enabled by Built-in Halide Gradient
Wenming Tian, Jing Leng, Chunyi Zhao, and Shengye Jin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10512 • Publication Date (Web): 30 Dec 2016
Downloaded from http://pubs.acs.org on December 30, 2016
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Long-Distance Charge Carrier Funneling in Perovskite Nanowires
Enabled by Built-in Halide Gradient
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†
†
Wenming Tian, Jing Leng, Chunyi Zhao and Shengye Jin*
State Key Laboratory of Molecular Reaction Dynamics and Collaborative Innovation Center of Chemistry for Energy Mateꢀ
rials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Rd., Dalian, China,
1
16023.
ABSTRACT: The excellent charge carrier transportation in
organolead halide perovskites is one major contributor to the
high performance of many perovskiteꢀbased devices. There
still exists a possibility for further enhancement of carrier
transportation through nanoscale engineering, owing to the
versatile wetꢀchemistry synthesis and processing of perovꢀ
skites. Here we report the successful synthesis of bromideꢀ
gradient CH NH PbBr I singleꢀcrystalline nanowires (NWs)
and achieved significantly improved lightꢀemitting perforꢀ
20
mance. These pioneer works opened a new avenue to manipꢀ
ulate carrier conveyance by tailoring the perovskite composiꢀ
tion. However, in comparison with the intrinsic carrier diffuꢀ
sion, the carrier transportation distance and efficiency to be
achieved by this compositional manipulation still remain unꢀ
known. It is therefore essential to develop a deep understandꢀ
ing of this energyꢀdriven carrier transportation in wellꢀdefined
perovskite structure to facilitate their application in practical
devices.
3
3
x 3ꢀx
by solidꢀtoꢀsolid ion exchange reaction starting from one end
of pure CH NH PbI NWs, which was confirmed by local
3
3
3
photoluminescence (PL) and energy dispersive xꢀray spectrosꢀ
copy (EDS) measurements. Due to the builtꢀin halide gradient,
the longꢀdistance carrier transportation was driven by the enꢀ
ergy funnel, rather than the spontaneous carrier diffusion. Inꢀ
deed, local PL kinetics demonstrated effective charge carrier
transportation only from the high bandgap bromide rich region
to low bandgap iodine rich region over a few micrometers.
Therefore, these halide gradient NWs might find applications
in various optoelectronic devices requiring longꢀdistance and
directional delivery of excitation energy.
Perovskite single crystals have been designated as standard
materials to explore the intrinsic properties of
15,17,21
perovskites.
In our previous work, we have successfully
demonstrated the application of a timeꢀresolved and PLꢀ
scanned imaging microscopy technique in studying carrier
diffusion dynamics in perovskite singleꢀcrystalline nanowires
16
(
NWs) and nanoplates (NPs). We therefore aimed to fabriꢀ
+
Organolead halide perovskites MAPbX (MA = CH NH ;
3
3
3
ꢀ
ꢀ
ꢀ
X = Cl , Br , I ) have emerged as attractive materials for phoꢀ
1
ꢀ6
7ꢀ12
tovoltaic and optoelectronic devices.
In perovskiteꢀbased
devices, a longꢀdistance carrier diffusion is essential to high
device performance. Diffusion lengths up to micrometers have
been reported in both polycrystalline and singleꢀcrystal perovꢀ
Figure 1. Halideꢀgradient MAPbBr I perovskite nanowires
x 3ꢀx
(NW) comprising a bromideꢀrich region and an iodideꢀrich reꢀ
gion. By changing the bandgap energy, the gradient distribution
of halide ion forms an energy funnel along the NW, which drives
carriers to transport from the highꢀbandgap bromideꢀrich region
to the lowꢀbandgap iodideꢀrich region.
1
3ꢀ17
skites.
In addition to utilizing the intrinsic transport properꢀ
ties of perovskites, the versatile wetꢀchemistry synthesis and
processing of perovskites allow further enhancement of carrier
transportation through nanoscale engineering. One of the stratꢀ
egies is recentlyꢀreported carrier transportation driven by
18ꢀ20
cate a halideꢀgradient MAPbBr I NW comprising bromideꢀ
bandgap energy gradient in alloyed perovskite materials.
x 3ꢀx
rich region (highꢀbandgap) at one side of the wire and iodideꢀ
rich region (lowꢀbandgap) at the other (Figure 1) such that the
energy difference between them can drive the unidirectional
transportation of charge carriers. It has been shown that perovꢀ
skite bulk films with halide gradient distribution can be preꢀ
pared by solidꢀtoꢀsolution or solidꢀtoꢀgas ion exchange reacꢀ
For example, Kamat et al. and Choi et al. have reported carrier
transportation in halideꢀgradient CsPbBr I and MAPbBr I
thin films fabricated by halide ion exchange reaction,
x 3ꢀx
x 3ꢀx
8,19
1
where the carriers were found to flow directionally from highꢀ
bandgap bromideꢀrich region to the lowꢀbandgap iodideꢀrich
region across the film. This carrierꢀfunneling was believed to
facilitate carrier extraction at perovskite/electrode interfaces
when applied in solar cells. Meanwhile, Sargent and coꢀ
workers also reported hybrid twoꢀdimensional (2D) perovskite
films comprising grains with different bandgap energy to funꢀ
nel the charge carriers to the lowestꢀbandgap emitting sites
18,19
tion.
However, using these methods to form a halide gradiꢀ
ent in individual perovskite NWs can be difficult.
Herein, we developed a “solidꢀtoꢀsolid” halide exchange reꢀ
action and successfully synthesized halideꢀgradient MAPꢀ
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bBr I perovskite NWs by directly contacting MAPbI NWs
bromideꢀrich and iodideꢀrich regions in the same NW. Beꢀ
cause replacing the iodide anion by bromide anion increases
the bandage energy, it is expected that a blueꢀshift in the emisꢀ
sion should be observed. We therefore compared the local
emission spectra extracted from two ends and the center in the
x 3ꢀx
3
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with bulk MAPbBr single crystals. The gradient distribution
3
of bromide ion in individual MAPbBr I NWs was confirmed
x 3ꢀx
by energy dispersive xꢀray spectroscopy (EDS) elemental
mapping and local emission spectra. Using timeꢀresolved PL
imaging and kinetics measurements, we observed severalꢀ
micrometers unidirectional charge carrier transportation from
bromideꢀrich region to the iodideꢀrich region. In comparison
with the intrinsic carrier diffusion, the carrier transportation
driven by bandgap energy gradient exhibited superior ability
in directional carrier transfer, making these NWs ideal candiꢀ
dates for applications in longꢀdistance photonꢀenergy delivery
and nanoscale optoelectronics.
same MAPbBr I NW (Figure 2b). A gradual blueꢀshift from
x 3ꢀx
the iodideꢀrich region (peak at 752 nm) to the bromideꢀrich
region (peak at ~711 nm) is observed, further confirming the
formation of the halide ion gradient within the single NW.
Based on the blueꢀshift in the emission spectra, the maximum
energy difference is calculated to be ꢁE = ~0.09 eV. This enꢀ
ergy difference is higher than room temperature thermal enerꢀ
gy (~0.025 meV) and hence is sufficient to create an effective
energy funnel within the NW. Among >100 examined NWs,
we observed ~50% of NWs showing clear gradient distribuꢀ
tion of halide ion. More examples of EDS and spectra measꢀ
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urements in other gradient MAPbBr I NWs are shown in
x 3ꢀx
Figure S4.
In order to understand the effect of the energy funnel on the
carrier transportation dynamics, we collected the PL images
and local PL kinetics of individual perovskite NWs using
timeꢀresolved and PLꢀscanned imaging microscopy (See Figꢀ
ure S5). The NWs were excited homogeneously (500 nm, 1
Figure 2. (a) SEM image and EDS mapping of lead (Pb), broꢀ
mine (Br) and iodine (I) elements in a typical gradient MAPꢀ
2
MHz, 67.8 nJ/cm /pulse) over the entire particle, and the PL
bBr I NW, showing a gradient distribution of halide ions along
x 3ꢀx
intensity was recorded by scanning the PL collection spot (difꢀ
fraction limited size) on the NW to construct the PL images.
Coupled with TCSPC module, the local PL kinetics at any
positions in the NW can be extracted. Under a homogeneous
excitation, net carrier diffusion is absent due to the lack of
the NW. (b) Local emission spectra collected (collection spot < 1
ꢂm in diameter) at the iodideꢀrich region (position 1), the central
area (position 2) and bromideꢀrich region (position 3) in the same
MAPbBr I NW as in panel (a). The inset is the optical image of
x 3ꢀx
the NW. The scale bars are 2 µm. The blueꢀshifted spectra are
due to the increase of bandgap energy induced by the mixture of
bromide ion in the perovskite.
carrier density gradient. The pure MAPbI NW thus exhibits a
3
uniform distribution in both PL intensity (Figure 3a) and local
PL dynamics (Figure 3b). In contrast, the MAPbBr I NW
x 3ꢀx
(ꢁE = ~0.07 eV as determined in Figure S6) shows a gradient
distribution in PL intensity, which decreases distinctly from
one end to the other (Figure 3c). The local emission spectra
We first grew MAPbI singleꢀcrystal NWs on a glass slide
3
by following the previously reported methods (see SI for deꢀ
(
Figure S6) identify the bright region in the iodideꢀrich side
11
tails). These NWs are 5 to 10 micrometers in length and a
and the dark region in the bromideꢀrich side in the NW. In
Figure 3d we also compared the local PL kinetics extracted
from different positions (spots 1 to 6 in Figure 3c) in the graꢀ
dient NW. These kinetics show fast decays in the bromideꢀrich
region and the decays become slower as the examining posiꢀ
tion move toward the iodideꢀrich region where the kinetics is
finally accompanied by the formation of a rising component
within 100 ns after excitation.
few hundreds of nanometers in the rectangular width (Figure
S1). In order to synthesize halideꢀgradient MAPbBr I NWs,
x 3ꢀx
preꢀprepared pure MAPbBr single crystals (2~ 3 mm in size,
3
see SI for details) were scattered on the top of asꢀgrown
MAPbI NW substrate (Figure S2). Since most MAPbI NWs
3
3
grew almost perpendicularly to the substrate (Figure S1), they
have only one end in contact with the large MAPbBr single
3
crystals, where the interfacial halide exchange reaction occurs.
1
2,22ꢀ25
Unlike the fast anion exchange in solution,
we found that
In our control experiments, we synthesized MAPbBr I
x 3ꢀx
this solidꢀtoꢀsolid halide exchange reaction took at least fifteen
days.
NWs with a uniform mixture of bromide ion. In such NWs,
the PL intensity and kinetics are also homogeneously distribꢀ
uted (Figure S7), and the PL lifetimes are found to be >200 ns.
Furthermore, we also performed excitation intensity dependent
transient PL measurements on an evenly Brꢀdoped MAPꢀ
After the anion exchange reaction, we carefully removed
the MAPbBr single crystals and transported the NWs to a
3
clean glass coverslip. Scanning electronic microscopy (SEM)
and energy dispersive xꢀray spectroscopy (EDS) were first
used to examine the morphology and compositional distribuꢀ
tion in the NWs. Figure 2a shows the SEM image and EDS
mapping of Pb, Br and I elements in a typical individual
bBr I NW (fabricated by ionꢀexchange reaction). The charge
x 3ꢀx
recombination constants are similar with those in the MAPbI3
NW (see Figure S8). These results suggest that the fast decays
in the bromideꢀrich region in the gradient MAPbBr I NW
x 3ꢀx
cannot be attributed to the fast carrier recombination by trapꢀ
ping states introduced in the ion exchange reaction.
MAPbBr I NW. Unlike the homogeneous distribution of
x 3ꢀx
lead over the entire NW, the iodide and bromide elements both
show gradient distributions with the concentration decreasing
in opposite directions along the wire. Such gradient distribuꢀ
We attribute the nonꢀuniform PL intensity and kinetics in
the gradient MAPbBr I NW to the carrier flow from broꢀ
x 3ꢀx
tion is not observed in pure MAPbI and MAPbBr NWs (Figꢀ
3
3
mideꢀrich region to the iodideꢀrich region driven by the energy
funnel in the NW. The fast PL decay in the bromideꢀrich reꢀ
gion is due to the flowꢀout of carriers to the iodideꢀrich region,
ure S3). These results confirm that the halide exchange reacꢀ
tion indeed started from one end of the NW, and formed both
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Figure 3. (a and c) Timeꢀresolved PL intensity images of a MAPꢀ
Figure 4. PL intensity images of (a) a gradient MAPbBr I NW
x 3ꢀx
bI NW and a gradient MAPbBr I NW under a homogeneous
and (b) a MAPbI NW excited with a focused laser beam (spot
3
x 3ꢀx
3
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2
excitation at 500 nm (1 MHz, 67.8 nJ/cm /pulse). (b and d) Comꢀ
diameter < 1 ꢂm, 500 nm, 1 MHz, 1.35 ꢂJ/cm /pulse) at one end
parisons of the local PL kinetics extracted from the indicated posiꢀ
of the wire. The scale bars are 2 µm. (c) Plots of relative carrier
density (in the percentage of total population) as a function of
distance to the excitation spot for the NWs shown in panel a and
b, indicating that carrier transportation driven by bandgap energy
tions (PL collection spot < 1 ꢂm in diameter) for the MAPbI NW
3
(
position 1ꢀ3) and the gradient MAPbBr I NW (position 1ꢀ6).
x 3ꢀx
The scale bars are 2 µm. The MAPbI NW exhibits a uniform
3
distribution in both PL intensity and kinetics; while the gradient
gradient in the gradient MAPbBr I NW delivers more carriers
x 3ꢀx
MAPbBr I NW shows much higher PL intensity in the iodideꢀ
to a distance > 2 um than the diffusion in the MAPbI NW. The
x 3ꢀx
3
rich region and dramatically changed local PL kinetics, which is
attributed to the carrier transportation from the bromideꢀrich reꢀ
gion to the iodideꢀrich region driven by the bandgap energy gradiꢀ
ent. The solid lines are the fits of the kinetics by the carrier transꢀ
portation model (Eq. S1 and S2), yielding the charge mobility and
diffusion coefficient of the NW.
solid lines are fits of the plots by Eq. S5 with the fitting parameꢀ
ters listed in Table S1.
positions other than the excitation spot (~1 ꢂm in diameter)
16
indicates the carrier transportation in the NW. In Figure 4a
and b, we compared the PL images of a gradient MAPbBr I
x 3ꢀx
NW and a pure MAPbI NW upon excitation at one end of the
3
where the flowꢀin of carriers generates a rising component in
PL kinetics. Consequently, this carrier flow accumulates the
carriers in the iodideꢀrich region, resulting in higher PL intenꢀ
sity. The electronic structure of perovskite has indicated that
wire. For the gradient MAPbBr I3ꢀx NW (ꢁE = ~0.06 eV as
x
determined in Figure S9), the excitation at the bromideꢀrich
region results in significantly higher PL intensity in the iodideꢀ
rich region that is away by a few micrometers; while for the
26
halide dominates in contribution to the valence band. Replacꢀ
ing iodide to bromide ion mainly raises the valence band to
higher energy levels. Within the bromide gradient in the
pure MAPbI
NW, PL intensity remains largely at the excitaꢀ
3
2
tion spot. Because PL intensity is proportional to n (n is the
27
carrier density), we calculated and plotted the relative local
carrier density as a function of distance to the excitation spot
along the NWs (Figure 4c). These plots and the local PL kinetꢀ
ics in the NWs (Figure S10) can also be described by the carꢀ
MAPbBr I NW, we anticipate that the energy funnel pushes
x 3ꢀx
the holes from the bromideꢀrich region to the iodideꢀrich reꢀ
gion. However, the observation of PL (due to recombination
of electrons and holes) on the gradient NW indicates that the
transportation of holes likely pulls the electrons to move toꢀ
gether by Coulombic attraction.
rier transportation model. In the gradient MAPbBr I3ꢀx NW,
x
carrier transportation driven by the energy funnel delivers
~60% of carriers over a distance > 2 ꢂm; in contrast, diffusion
driven by density gradient transports only 45% of carriers to
Figure 3d shows that the PL kinetics in the halideꢀgradient
NW can be well described by numerically solving a carrier
transportation model (Eq. S1 and S2), which includes both
carrier diffusion and funneling processes (see SI for the deꢀ
tails). The global fit of the kinetics yields a charge mobility (µ)
the same distance in the pure MAPbI NW. Indeed, no matter
3
where the gradient MAPbBrxI3ꢀx NW is excited, carriers alꢀ
ways accumulate at the iodideꢀrich region (Figure S11). These
results suggest that the energy gradient is the dominant driving
force for carrier transportation in the gradient MAPbBr I
x 3ꢀx
2
ꢀ1 ꢀ1
of ~41.5 cm v s and an ambipolar diffusion coefficient (D)
NW, which then ensures effective carrier transportation in
only oneꢀway direction. These results promise the potential
application of these NWs in light harvesting and longꢀdistance
energy delivery.
2
ꢀ1
of ~1.2 cm s (see Table S1). The measured µ should be close
to the hole mobility in the NW. These values are consistent
1
5ꢀ17
with those reported in literatures,
of the model.
confirming the validity
Finally, we investigated the stability of the compositional
gradient in the NWs. In mixed halide perovskite, it has been
shown that iodide and bromide ions segregate during phoꢀ
To compare the efficiency of carrier deliver (number of carꢀ
riers in total population that can transport over a specific disꢀ
tance) between the energyꢀdriven carrier transportation and the
spontaneous carrier diffusion, we also collected PL images of
NWs with a focused laser excitation (500 nm, 1 MHz, 1.35
2
8,29
toirradiation.
This phenomenon was not observed in the
3ꢀx NWs under our experimental condition.
gradient MAPbBr
I
x
The halideꢀgradient in the NWs exhibits nice stability at the
2
o
ꢂJ/cm /pulse). Under this condition, the PL emission from
examined temperatures from 20 to 100 C (Figure S12). Furꢀ
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thermore, we also frequently examined the local emission
spectra at the bromideꢀrich and iodideꢀrich regions during a
period of days. Although the NWs shows an overall blueꢀshift
in the spectra probably due to the intrinsic degradation of perꢀ
ovskite materials, the spectral shift between bromideꢀrich reꢀ
gion and iodideꢀrich region and the ensuing directional carrier
flow remain up to more than 50 days (See Figure S13). These
results suggest the possibility of these gradient NWs for longꢀ
term use in practical applications.
(4) McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.;
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In summary, we successfully fabricated halide compositionꢀ
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al gradient MAPbBr I perovskite NWs by a solidꢀtoꢀsolid
x 3ꢀx
halide exchange reaction. EDS mapping and PL spectra measꢀ
urements confirmed that the density of the mixed bromide
anion gradually decreases from one end to the other, forming a
bromideꢀrich region and iodideꢀrich region in single NWs. By
raising the bandgap energy, the halide gradient produced an
energy funnel in the NWs, which facilitated the transportation
of charge carriers in oneꢀway direction over a few micromeꢀ
ters and accumulated carriers in the lowꢀbandgap iodideꢀrich
region. The effective carrier transportation driven by the enerꢀ
gy funnel, along with other superior intrinsic photophysical
properties (e.g. long carrier lifetime and strong light harvestꢀ
ing), implies the applications of the halide gradient MAPꢀ
(
R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington,
D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Nat. Nanotech.
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These authors contributed equally.
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Notes
The authors declare no competing financial interest.
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