Observation of internal photoinduced electron and hole separation in hybrid two-dimentional perovskite films

Article abstract of DOI:10.1021/jacs.6b12581

Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multilayered (BA)₂(MA)_n-1Pb_nI_3n+1 (BA = CH₃(CH₂)₃NH₃⁺, MA = CH₃NH₃⁺) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as n = 4 , are found to be mixture of multiple perovskite phases, with n = 2, 3, 4 and ≈ ∞, that naturally align in the order of n along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-n to large-n phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of ～358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices.

Full text of DOI:10.1021/jacs.6b12581

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Communication
Observation of Internal Photoinduced Electron and Hole
Separation in Hybrid 2-Dimentional Perovskite Films
Junxue Liu, Jing Leng, Kaifeng Wu, Jun Zhang, and Shengye Jin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12581 • Publication Date (Web): 17 Jan 2017
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Observation of Internal Photoinduced Electron and Hole Separation
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†
, ‡, §
†, ‡
‡
Jing Leng, Kaifeng Wu, Jun Zhang, Shengye Jin
*, §
*, ‡
Junxue Liu,
‡
State Key Laboratory of Molecular Reaction Dynamics and Collaborative Innovation Center of Chemistry for Energy Mate-
rials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Rd., Dalian, 116023,
China.
§
State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, 66
Changjiang West Rd., Huangdao District, Qingdao, 266580, China.
ABSTRACT: Two-dimensional (2D) organolead halide per-
ovskites are promising for various optoelectronic applications.
Here we report a unique spontaneous charge (electron/hole)
separation property in multi-layered (BA) (MA) Pb I (BA
The applications of 2D perovskites in various devices have
immediately triggered many fundamental studies focusing on
the carrier dynamics in single-phase 2D crystals and hybrid
2D films. Interestingly, recent reports have demonstrated
that the 2D multi-layered perovskite films actually comprised
24-27
2
n−1
n 3n+1
+
+
=
CH (CH ) NH , MA = CH NH ) 2D perovskite films by
3
3
2 3
3
3
studying the charge carrier dynamics using ultrafast transient
absorption and photoluminescence spectroscopy. Surprisingly,
the 2D perovskite films, although nominally prepared as “n =
multiple perovskite phases (with various n values from 1, 2, 3
6,7
and 4 to near ∞), even though the films were intended to be
prepared as a single-phase. This hybrid feature seems to be
ineluctable in fabricating 2D films. Because of the band
4
”, are found to be mixture of multiple perovskite phases, with
1
,7
alignment between perovskites with various n values, the
hybrid structure in 2D films should induce more complicated
internal charge carrier transportation than in 3D perovskites
where intrinsic carrier diffusion dominates.
n = 2, 3, 4 and ≈ ∞, that naturally align in the order of n along
the direction perpendicular to the substrate. Driven by the
band alignment between 2D perovskites phases, we observe
consecutive photoinduced electron transfer from small-n to
large-n phases and hole transfer in the opposite direction on
hundreds of picoseconds inside the 2D film of ~358 nm thick-
ness. This internal charge transfer efficiently separates elec-
trons and holes to the upper and bottom surfaces of the films,
which is a unique property beneficial for applications in pho-
tovoltaics and other optoelectronics devices.
Recently, Sargent et al. and Huang et al. have reported the
carrier funneling from small-n to large-n perovskite phases in
hybrid 2D films, presumably driven by exciton energy trans-
6,7
fer. However, two important questions remain yet-to-be-
answered: first, how the different perovskite phases align in
the hybrid films; second, whether the band alignment between
different phases induces energy funneling as reported in ref. 6
and 7 or instead charge separation. The latter is especially
important because it dictates the application of these hybrid
Two-dimensional (2D) organolead halide perovskites, in
addition to their three-dimensional (3D) analogues, have re-
cently emerged as an attractive material for applications in
2
D perovskite films: energy funneling is useful for light-
emitting applications, whereas charge separation would be
more beneficial for light conversion or detection.
1-16
photovoltaics and other optoelectronic devices.
The struc-
Herein, using ultrafast transient absorption (TA) and time-
resolved photoluminescence (PL) spectroscopy we studied the
tural formula of the 2D perovskite is generally given as
A) (CH NH ) M X3n+1, where A is a large aliphatic or aro-
(
2
3
3 n−1
n
charge carrier dynamics in (BA) (MA) Pb I 2D perov-
n 3n+1
2
n−1
matic alkylammonium cation working as an insulating layer,
M is the metal cation, and X is the halide anion. The integer n
is the number of inorganic perovskite layers between two or-
ganic cation spacers (A). Due to the geometric effect and die-
lectric contrast effect, the exciton binding energy in 2D perov-
+
skite films (BA = CH (CH ) NH , MA = CH NH ). We
+
3
2 3
3
3
3
found that indeed multiple perovskite phases with various n
values co-existed in the 2D perovskite films (although nomi-
nally prepared as “n = 4”), and more interestingly, these per-
ovskite phases were naturally aligned in the order of n along
the growth direction perpendicular to the substrate. Driven by
the built-in band alignment between different perovskite phas-
es, consecutive internal electron transfer (with a time of ~477
ps) from small-n to large-n perovskite phases and hole transfer
(with a time of ~987 ps) in the opposite direction were ob-
served in a film of ~358 nm thickness. This unique self-
charge-separation property of the 2D perovskite films can
facilitate their applications in photovoltaics and other optoe-
lectronics devices.
1
7-23
skites is much larger than that in 3D perovskites;
and con-
tinuous tunability in exciton absorption/emission energy can
be achieved by changing the number of n through quantum
confinement effect. These properties have led to successful
applications of 2D perovskites in light-harvesting and emitting
6-10
devices.
perovskites can also dramatically improve their moisture re-
Moreover, using the large organic cations in 2D
1-5
sistance, making this class of materials more attractive for
long-term use in real-life devices.
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The 2D perovskite films were prepared by spin-coating a
precursor solution of CH (CH ) NH I (BAI), CH NH I (MAI)
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and PbI2 with a molar ratio of 2:3:4 dissolved in N,N-
dimethylformamide (DMF) via a hot-casting process (see SI
2
for details and Figure S1 for SEM images). The precursor
mixtures crystalize to form perovskite phases when DMF sol-
vent evaporates. The molar ratio of precursor corresponds to a
nominal composition of (BA) (MA) Pb I (n = 4). Figure 1a
2
3
4 13
shows the UV-vis absorption spectrum of the 2D perovskite
film. In addition to the lowest bandgap absorption at ~740 nm
(
(
1.68 eV), three higher energy absorption peaks, at ~572 nm
2.17 eV), 608 nm (2.04 eV) and ~645 nm (1.93 eV), are also
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found in the spectrum. Comparing with the absorption of 2D-
perovskite single crystals (Figure S2), the high energy absorp-
tion peaks at ~572 nm, 608 nm and 645 nm can be assigned to
Figure 1. (a) UV-vis absorption spectrum of a typical
(BA) (MA)n−1Pb 3n+1 2D perovskite film (~358 nm thickness,
single-phase (BA) (MA) Pb I perovskites with n = 2, 3
n 3n+1
2
I
n
2
n−1
and 4, respectively. The lowest absorption peak at 740 nm,
which is close to the 3D perovskite, corresponds to a (or a
group of) perovskite phase(s) with much larger n values (n ≈
∞). This result suggests that the 2D perovskite films comprise
hybrid perovskite phases with different n values, even though
the molar ratio of used precursors is intended for a single
phase perovskite with n = 4. This hybrid feature of the 2D
perovskite film is consistent with recently reported films fabri-
prepared as n = 4). The perovskite film exhibited multiple ab-
sorption peaks which are identified as n = 2 (2.17 eV), n = 3
(
2.04 eV), n = 4 (1.93 eV) and n ≈ ∞ perovskite phases. (b)
Comparison of the emission spectra of the 2D perovskite film
illuminated from the front and back sides (as illustrated in the
inset) of the film. Under back-excitation, the spectrum show
emission peaks from n = 2, 3 and 4 phases in addition to the
dominant emission from n ≈ ∞ phase. (c) Comparative bandgap
6
,7
energy alignment of (BA) (MA)n−1Pb I3n+1 perovskites with
2 n
cated by similar methods.
different n values. Three possible carrier transfer mechanisms,
electron and energy transfers from small-n to large-n perovskites
and hole transfer from large-n to small-n perovskites, are all
energetically allowed.
In order to examine whether these different perovskite phas-
es distribute randomly or follow a specific order, the photolu-
minescence (PL) spectra of the 2D perovskite film were col-
lected under two different excitation configurations (Figure
1
b). The configuration with the laser beam (405 nm) imping-
ing the perovskite (or the glass substrate) is defined as the
front (or back) excitation (see the inset in Figure 1b). The
spectra with both excitations show a dominant emission peak
at ~750 nm (n ≈ ∞); however, under back-excitation three
emission peaks at higher energy were observed, which can be
attributed to the emission from the perovskite phases with n =
To elucidate the interaction mechanism between different-n
perovskite components, we investigated the charge carrier
dynamics in the hybrid 2D perovskite films using the femto-
second transient absorption (TA) and time-resolved PL spec-
troscopy. For TA measurements, a pump pulse was used to
excite the perovskite films and the induced absorption changes
(ꢀA), as functions of both wavelength and time, were record-
ed. Figure 2a shows the TA spectra at the indicated delay
times of the 2D perovskite film (358 nm thickness) under
2
, 3 and 4. The difference in the PL spectra between back- and
front- excitations implies that the small-n phases should ma-
jorly locate at the bottom surface and the large-n phase locates
at the upper surface of the film. Our TA measurements that
will be discussed below further confirm that the different per-
ovskite phases distribute in the order of their n values (from n
2
back-excitation (at 480 nm, 0.7 ꢁJ/cm /pulse). Consistent with
the static absorption spectrum, the TA spectra exhibit four
bleach peaks at n = 2, 3, 4 and n ≈ ∞ absorption bands due to
the charge carrier filling upon excitation. A small bleach peak
between the n = 4 and n ≈ ∞ bands is identified as the n = 5
perovskite phase (See Figure S2) whose absorption peak is
likely hidden in the broad absorption signal of n ≈ ∞ compo-
nent. For comparison, the TA spectra of the same film with the
front-excitation are shown in Figure 2b, where the TA spectra
are dominated by the bleach signals from the n ≈ ∞ phase.
This difference in TA spectra between back- and front- excita-
tions agrees with the observation in the PL spectra.
=
2 at the bottom surface to n ≈ ∞ at the upper surface). The
reason for forming this sequentially distributed perovskite
phases is unclear. We propose that in fabricating the films the
local molar ratio of BAI insulating cations to MAI and PbI2
precursors likely changes during the DMF solvent evaporation
process, leading to different local n values in perovskite crys-
tallization.
Because the perovskites with different n values exhibit a
certain band alignment, it is expected that the ordered distribu-
tion of perovskite phases should prompt directional carrier
transportation inside the film. In Figure 1c, we show the
bandgap diagram of the perovskite phases found in our films
using the reported valence band (VB) and conduction band
Under back-excitation, we observed that the bleach recovery
at n = 2, 3 and 4 is accompanied by the formation of the n ≈ ∞
species (Figure 2a). Such process is also observed under front-
excitation at very early delay times (Figure 2b), when the
small-n phases are excited by photons penetrating through the
n ≈ ∞ phase. This result suggests the charge carrier transpor-
tation from the small-n phases to n ≈ ∞ in the film. The TA
kinetics probed at these bands are shown in Figure 2c. The
bleach recovery kinetics (after subtracting the photo-induced
absorption (PIA) signal from the n ≈ ∞ phase) becomes slower
as the n value increases, and a rising kinetics appears for the n
≈ ∞ band, indicating carrier accumulation on this perovskite
(
CB) positions of 2D perovskites with n = 2 to 4 and 3D per-
1,7
ovskite. Based on the band alignment, photoinduced electron
transfer from small-n to large-n perovskites and hole transfer
in the opposite direction are both energetically allowed. Fur-
thermore, because of the spectral overlap between the emis-
sion of small-n perovskites and the absorption of large-n per-
ovskites, energy transfer from small-n to large-n perovskites is
also possible in the hybrid film.
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Figure 3. (a) TA spectra of the (BA) (MA) Pb I 2D per-
n 3n+1
2
n−1
ovskite film (prepared as n = 4, ~358 nm thickness) under front-
excitation at 740 nm. Bleach peaks at 608 and 645 nm are as-
signed to the n = 3 and 4 perovskite phases. (b) TA kinetics
(after subtracting PIA contribution) probed at n = 3 and 4 bands
showing the bleach formation processes due to the hole transfer
from the n ≈ ∞ phase. Solid lines are the fits of kinetics by
exponential function, yielding the hole transfer time of 192 ps to
n = 4 phase and 987 ps to n = 3 phase.
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Figure 2. TA spectra at different delay times of a typical
BA) (MA) Pb I 2D perovskite film (~358 nm thickness,
(
2
n−1
n 3n+1
prepared as n = 4) under (a) back- and (b) front- excitations.
Bleach peaks formed by carrier band edge filling at 572 nm, 608
nm, 645 nm and 740 nm are identified as n = 2, 3, 4 and n ≈ ∞
perovskite phases, respectively. Black arrows indicate the direc-
tion of the bleach peak evolution. (c) TA kinetics probed at n =
acceptor). With PCBM, the charge accumulation process at n
≈
∞ band disappears and the kinetics recovers much faster,
suggesting the efficient interfacial electron transfer from the n
∞ phase to PCBM. In contrast, the TA kinetics in the film
2
, 3, 4 and n ≈ ∞ bands under back-excitation. The slow rising
≈
kinetics at n ≈ ∞ band reflects the carrier (electron) populating
process to this phase. Solid lines are the fits of the kinetics by
exponential function, which yields the carrier populating time of
with the hole acceptor deposition remains unchanged relative
to that of the neat film. This result further confirms the internal
electron transfer rather than the energy transfer from small-n
to n ≈ ∞ perovskite phases in the 2D film.
4
77 ps. (d) Comparison of the time-resolved PL and TA kinetics
traces probed at the n ≈ ∞ band in the first two nanoseconds after
excitation, showing clear discrepancy in the rising kinetics. The
solid line is the fit of the PL kinetics with the rising kinetics
limited by the IRF.
Directionally opposite to the electron transfer, hole transfer
from large-n to small-n perovskite components is also energet-
ically allowed. To probe the hole transfer, we performed the
TA measurements on the film under front-excitation at 740 nm
2
(3.5 ꢁJ/cm /pulse). At this excitation wavelength only the n ≈
phase. The trend in these TA kinetics can be explained by the
consecutive carrier transfer through the perovskite phases,
which should be sequentially distributed in the order of n val-
ues in the film. Such sequential alignment of perovskite phases
and the corresponding consecutive carrier transportation are
further confirmed in a 2D perovskite film with a larger thick-
ness of ~523 nm (Figure S3 and Figure S4).
∞ perovskite phase should be excited. Figure 3a shows the TA
spectra of the film. In addition to the broad PIA signal from n
≈ ∞ band, two small bleach peaks growing larger at later delay
times are observed, and can be assigned to the n = 3 and 4
perovskite phases (Figure S6). The TA kinetics probed at these
bands (after subtracting the PIA contribution) (Figure 3b)
show the bleach formation processes with the times becoming
longer from n = 4 to n = 3 band. These TA results can be well
explained by the consecutive hole transfer from n ≈ ∞ to n = 4
and then to n = 3 phases. Furthermore, this hole transfer is also
confirmed from the fast PL decay (τ = 680 ps) probed at n ≈ ∞
band (Figure S7). The observation of hole transfer also sug-
gests the absence of energy transfer in the 2D perovskite films.
According to the diagram in Figure 1c, carrier transporta-
tion from small-n to n ≈ ∞ phases could result from electron or
energy transfers. In the latter case, the carrier (exciton) accu-
mulation at the n ≈ ∞ band observed in TA measurements
should be also seen in the time-resolved PL kinetics. To dif-
ferentiate these two mechanisms, we compared the TA kinet-
ics (probed at the n ≈ ∞ band) and the transient PL trace (col-
lected under back-excitation at 750 nm) (Figure 2d). It is
clearly that the rising kinetics in TA is slower than the rising
component in the PL trace (where the rising kinetics is deter-
mined by the instrument response function (IRF = ~300 ps)),
indicating that carrier populating process in these two meas-
urements are different. This discrepancy between TA and PL
kinetics was also observed in the thicker (523 nm) 2D film
Noticeably, the amplitudes of bleach peaks formed by hole
transfer are smaller than that by electron transfer. This is par-
tially due to the smaller contribution of hole-filling to the TA
28,29
bleach signal.
Another possible reason is the presence of
30
hole-trapping states in the perovskite film, which reduces the
number of holes during the transfer process; thus, the for-
mation of the bleach peak for n = 2 band that requests the
longest hole transfer distance is not observed.
(
Figure S4d). Based on these results, we confirm that the car-
By fitting the TA kinetics with simple multi-exponential
functions (fitting parameters in Table S1 and S2), we estimat-
ed the electron transfer time of ~ 477 ps (Figure 2c) and the
hole transfer time of ≥ 987 ps (Figure 3b) for transportation
across the entire film thickness of ~358 nm. The separated
electrons and holes exhibited a longer lifetime of >14 ns (Ta-
ble S1 and S2). By comparing the charge transfer times to the
intrinsic carrier lifetimes of single-phase 2D perovskites, we
estimated the theoretical charge separation efficiency of ≥92%
rier transfer from small-n to n ≈ ∞ perovskite phases should be
attributed to the electron transfer rather than the energy trans-
fer.. The observed PL from the n ≈ ∞ phase should be due to
the direct excitation by photons penetrating through small-n
phases under back-excitation.
In Figure S5, we also compared the TA kinetics (probed at n
≈ ∞) in the 2D perovskite films before and after the deposi-
tions of PCBM (electron acceptor) and Spiro-OMeTAD (hole
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by electron transfer (back-excitation) and ~70% by hole trans-
fer (front-excitation) in the 2D films of 358 nm thickness (see
Figure S8). In thicker films (e.g. ~500 nm) the charge separa-
tion efficiency is significantly reduced due to the slower
charge transfer process. We also observed that in a thin 2D
film of < 100 nm thickness the PL quantum yield (PLQY) is
enhanced by almost 4~7 times compared to the film of 358 nm
thickness (Figure S9b), even though the charge transfer time
increase to tens of ps (Figure S10). We proposed that in such a
thin film the perovskite phases with different n values are not
clearly discrete in space, and electrons and holes do not sepa-
rate in long distance to suppress their radiative recombination.
These results indicate that the thickness of 2D film is a key
factor for achieving the high internal charge separation effi-
ciency. Nevertheless, the PLQY of thin 2D perovskite film is
still higher than that of 3D film (Figure S9c), which may par-
tially explain the reported better performance of light-emitting
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AUTHOR INFORMATION
Corresponding Author
(
Milowska, K. Z.; Cortadella, R. G.; Nickel, B.; Cardenas-Daw, C.;
Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Nano Lett. 2015, 15, 6521-
6527.
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K.; Yamauchi, Y. J. Am. Chem. Soc. 2016, 138, 13874-13881.
(24) Milot, R. L.; Sutton, R. J.; Eperon, G. E.; Haghighirad, A. A.;
Martinez Hardigree, J.; Miranda, L.; Snaith, H. J.; Johnston, M. B.; Herz,
L. M. Nano Lett. 2016, 16, 7001-7007.
sjin@dicp.ac.cn
zhangj@upc.edu.cn
Author Contributions
†
These authors contributed equally.
(
1
25) Wu, X. X.; Trinh, M. T.; Zhu, X. Y. J. Phys. Chem. C. 2015, 119,
4714-14721.
Notes
The authors declare no competing financial interests.
(26) Straus, D. B.; Hurtado Parra, S.; Iotov, N.; Gebhardt, J.; Rappe, A.
M.; Subotnik, J. E.; Kikkawa, J. M.; Kagan, C. R. J. Am. Chem. Soc.
2016, 138, 13798-13801.
(27) Guo, Z.; Wu, X.; Zhu, T.; Zhu, X.; Huang, L. Acs Nano 2016, 10,
9992-9998.
ACKNOWLEDGMENT
S. J. acknowledges the financial support from the MOST
(2016YFA0200602) and NBRPC (2013CB834604). J. Z.
acknowledges the financial support from the NSFC (21471160)
(28) Wu, K. F.; Liang, G. J.; Shane, Q. Y.; Ren, Y. P.; Kong, D. G.; Lian,
T. Q. J. Am. Chem. Soc. 2015, 137, 12792-12795.
(
5
(
29) Leng, J.; Liu, J.; Zhang, J.; Jin, S. J. Phys. Chem. Lett. 2016, 7, 5056-
061.
30) Wu, X. X.; Trinh, M. T.; Niesner, D.; Zhu, H. M.; Norman, Z.;
Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X. Y. J. Am. Chem. Soc. 2015,
37, 2089-2096.
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