High-efficiency energy transfer in perovskite heterostructures

: Here, we report the energy transfer in (PEA) 2 PbI 4 /MAPbBr 3 perovskite heterostructures. Under two-photon excitation, the photoluminescence (PL) emission of the (PEA) 2 PbI 4 flake is nearly completely quenched, while that of the MAPbBr 3 microplate is greatly increased (6.5 folds higher) in the heterostructure. The opposite variation character of the PL emissions is attributed to the radiative energy transfer from the (PEA) 2 PbI 4 flake to the MAPbBr 3 microplate. The radiative energy transfer occurs on an ultrafast timescale with a high efficiency (~100%). In addition, a strongly thickness-and wavelength-dependent interlayer interaction is observed under one-photon excitation. This work advocates great promise for revealing the interlayer interaction of perovskite heterostructures and developing high-performance optoelectronic devices.


Introduction
Semiconductor heterostructures, composed by stacking of various semiconductor layers, has provided a versatile tool to engineer the band structure [1][2][3] and design novel optoelectronic devices [4][5][6].During the past years, interlayer charge transfer in semiconductor heterostructures has attracted lots of attentions due to the ultrafast dynamics and great tunability [7][8][9][10][11].However, the low charge transfer efficiency has seriously hindered the practical applications of semiconductor heterostructures.Energy transfer is also commonly observed in semiconductor heterostructures, particularly when the emission spectrum of one component (donor) is overlapped with the absorption spectrum of the other component (acceptor) [12,13].The energy of an excited donor can travel across the interface and interact with the acceptor, which shows great promise for the practical applications in fluorescent sensors, solar cells and light-emitting diodes (LEDs) [14][15][16][17].
Recently, organic-inorganic perovskites have been intensively studied due to their excellent optoelectronic properties, such as high-absorption coefficient, low-defect density and long-carrier diffusion [18][19][20].However, most of the previous reports mainly focused on the optoelectronic properties of individual perovskite components [21][22][23].This issue has seriously limited the study and development of integrated perovskite nanodevices.Moreover, organic-inorganic perovskites have a high quantum yield for carrier generation, which provides an ideal candidate for high-efficiency heterostructures [24,25].Therefore, revealing the interlayer interaction of perovskite heterostructures is essential.
In this work, we perform a systematical study on the energy transfer in organic-inorganic perovskite heterostructure.The perovskite heterostructure is prepared by stacking the flakes of two perovskites (PEA) 2 PbI 4 and MAPbBr 3 .Under two-photon excitation, the photoluminescence (PL) emission of the MAPbBr 3 microplate is largely increased, while that of the (PEA) 2 PbI 4 flake is completely quenched.Further study indicates that the opposite variation character of the PL emissions is originated from the radiative energy transfer from the (PEA) 2 PbI 4 flake to the MAPbBr 3 microplate.The radiative energy transfer occurs on an ultrafast timescale with a high efficiency (~100%).In addition, the case under one-photon excitation is investigated for comparison, and a thickness-and wavelength-dependent interlayer interaction is observed.

Experiments and results
(PEA) 2 PbI 4 monocrystalline films were synthesized by an anti-solvent vapor-assisted capping crystallization (AVCC) method [26].After that, (PEA) 2 PbI 4 flakes were obtained by mechanically exfoliating the monocrystalline films with a scotch tape.MAPbBr 3 microplates were prepared by a solution based self-assembly method [27].Figure 1(a) and Fig. 1(b) show the optical microscope images of a (PEA) 2 PbI 4 flake and a MAPbBr 3 microplate, respectively.Both of the two samples have a size over several ten micrometers.The uniform surfaces indicate a good crystallinity quality of the two perovskites.Figure 1(c) presents the XRD patterns of the two perovskite monocrystallines.The well-defined diffraction peaks of the XRD patterns correspond to the (00l) and (h00) series for (PEA) 2 PbI 4 and MAPbBr 3 respectively, which agree well with the reported results [28][29][30].To study the optical properties of the heterostructure composed of (PEA) 2 PbI 4 and MAPbBr 3 , a (PEA) 2 PbI 4 flake and a MAPbBr 3 microplate were stacked together by a dry-transfer method [31].Figure 1(d) shows the microscope image of a prepared perovskite heterostructure, in which the two perovskite components are well stacked, as labeled by the dashed lines.Figure 1(e) shows the normalized absorption spectrum of MAPbBr 3 (red curve) and PL spectrum of (PEA) 2 PbI 4 (green curve), respectively.Since these two spectra are well overlapped, an efficient energy transfer can occur in the perovskite heterostructure [32].For further investigation, an inverted-confocal-microscope configuration was used for optical measurements, as shown in Fig. 2(a).A mode-locked Ti/Sapphire oscillator (Vitara, 800 nm, ~8 fs, 80 MHz) was used as the pumping source.The laser beam was focused onto the samples by a 20 × objective (Olympus, NA = 0.4).The reflected signals were collected by the same objective, and then sent to a spectrometer (ANDOR 193i) for spectral measurement or to a CCD camera for imaging.A time-correlated single-photon counting (TCSPC) system (Picoquant, PicoHarp 300) was equipped behind the spectrometer for time-resolved PL (TRPL) measurement.Figure 2(b) presents the PL spectra for the bare (PEA) 2 PbI 4 flake (green curve), the bare MAPbBr 3 microplate (blue curve) and the heterostructure (red curve) under 800-nm excitation, respectively.The PL spectrum of the heterostructure was measured with the MAPbBr 3 microplate impinging the laser beam (forward excitation), as shown in the inset of Fig. 2(b).As the bandgaps of the two perovskites are larger than the one-photon energy (1.55 eV) but smaller than the two-photon energy (3.1 eV) of the pumping laser, the emissions are attributed to two-photon-absorption-induced PL (TPL).Figure 2(b) shows that the TPL intensity of the (PEA) 2 PbI 4 flake is much stronger than that of the MAPbBr 3 microplate under the same excitation.For the perovskite heterostructure, the TPL emission of the (PEA) 2 PbI 4 flake is nearly completely quenched, while that of the MAPbBr 3 microplate is greatly increased (6.5 folds higher) in the heterostructure area.In addition, the TPL emission from the heterostructure has a redshift relative to that of the bare MAPbBr 3 microplate.Figure 2(c) shows the integrated TPL mapping of the perovskite heterostructure.A similar result is obtained that the integrated TPL intensity from the heterostructure area is decreased relative to that of the (PEA) 2 PbI 4 flake, but it is largely increased relative to that of the MAPbBr 3 microplate.The opposite variation character of the TPL emission in the heterostructure indicates a strong interlayer interaction between the (PEA) 2 PbI 4 flake and MAPbBr 3 microplate.For comparison, the interlayer interaction is studied with the (PEA) 2 PbI 4 flake impinging the laser beam (backward excitation).Figure 2(d) shows the measured TPL spectra for the two perovskite components and heterostructure.In this case, the TPL spectrum from the heterostructure is similar to that of the bare (PEA) 2 PbI 4 flake, which indicates a weak interlayer interaction between the (PEA) 2 PbI 4 flake and MAPbBr 3 microplate.The difference in the TPL spectra of the perovskite heterostructure between forward and backward excitations implies that the interlayer interaction has a specific direction from the (PEA) 2 PbI 4 flake to the MAPbBr 3 microplate.
It has been well known that two typical processes exist in the semiconductor heterostructures: energy transfer and charge transfer.Since the emission spectrum of (PEA) 2 PbI 4 is strongly overlapped with the absorption spectrum of MAPbBr 3 , the emitted energy of the (PEA) 2 PbI 4 flake can travel across the interface and transfer to the MAPbBr 3 microplate, as illustrated in Fig. 3(a).As a result, the TPL emission of the (PEA) 2 PbI 4 flake will be largely decreased, and that of the MAPbBr 3 microplate will be greatly increased.
Besides, the energy bands of the two perovskites exhibit a type-II band alignment, indicating a possible charge transfer process in the perovskite heterostructure, as shown in Fig. 3(b).However, it is worth noting that the lattice constants are ~8.68Å and ~5.90 Å for (PEA) 2 PbI 4 and MAPbBr 3 respectively, which indicates a large lattice mismatch between the two perovskites [28,33].This mismatch makes it difficult for the two perovskite components to couple via interlayer charge transfer at the interface.For further demonstration, a heterostructure composed of MAPbBr 3 /SiO 2 /(PEA) 2 PbI 4 was prepared by depositing a SiO 2 dielectric layer (5 nm in thickness) on the MAPbBr 3 microplate.The SiO 2 layer will completely prevent the charge transfer between the two perovskite components.Figure 3(c) shows the TPL spectra for the two individual perovskite components and the heterostructure, respectively.Notably, the TPL emission of the (PEA) 2 PbI 4 flake is completely quenched, and that of the MAPbBr 3 microplate is largely increased.The result is similar to that of the MAPbBr 3 /(PEA) 2 PbI 4 heterostructure, implying that the possibility of charge transfer can be eliminated.In addition, the variation of the dielectric environment from the individual perovskite components to the heterostructure may also have influence on the PL emission [34].However, the contribution is quite limited because only a small dielectric mismatch exists between the (PEA) 2 PbI 4 and MAPbBr 3 [35,36].Therefore, we can conclude that the energy transfer is a dominant process in the perovskite heterostructure.The nonradiative energy transfer such as Förster resonance energy transfer (FRET) and Dexter transfer are originated from the dipole-dipole interaction in the donor and acceptor, which strongly depends on the distance between the two components [37].While the radiative energy transfer occurs in the form of photon radiation and absorption, which can still be efficient in a large distance [38].To further distinguish these two processes in the perovskite heterostructure, the TPL emission of more samples separated by a thicker (100 nm) dielectric layer were measured.Figure 3(d) shows that The TPL emission of the (PEA) 2 PbI 4 flake is also completely quenched, and that of the MAPbBr 3 microplate is increased.As the nonradiative energy transfer can hardly occur with such a thick dielectric layer, the variation of the TPL emission in the perovskite heterostructure can be only attributed to the radiative energy transfer, which agrees will with the overlap of the emission spectrum of (PEA) 2 PbI 4 and absorption spectrum of MAPbBr 3 .Moreover, Fig. 3(e) shows the result for a heterostructure sample separated by a 400-μm-thick dielectric layer.The emission spectrum of the heterostructure is composed of two components from both of the (PEA) 2 PbI 4 flake and MAPbBr 3 microplate.A part of the emission from the (PEA) 2 PbI 4 flake is quenched, and also leads to the increase of the emission from the MAPbBr 3 microplate, indicating that the radiative energy transfer in the micro/nanostructures can still occur efficiently at a very long distance.
To get further insights into the energy transfer dynamics of the perovskite heterostructure, TRPL were measured using a TCSPC system under 800 nm excitation.Figure 4(a) shows the decay traces of the (PEA) 2 PbI 4 flake, the MAPbBr 3 microplate and the perovskite heterostructure respectively.For the (PEA) 2 PbI 4 flake, the decay trace can be fitted to a biexponential decay function, with two lifetimes of 0.61 ns and 2.46 ns respectively.These two decay channels correspond to the radiative recombination and the trap-assisted recombination, respectively.For the MAPbBr 3 microplate, the decay trace can be fitted to a single-exponential decay function, and a long lifetime of 5.53 ns is obtained.For the perovskite heterostructure, the decay time of the donor in the presence (τ D ) and absence (τ 0 D ) of energy transfer obeys the following equation [32,39]: where k T is the transfer rate.Figure 4(b) shows that the decay trace of the heterostructure starts nearly simultaneously with that of the (PEA) 2 PbI 4 flake.The time contrast between these two processes (red and green curves) is beyond the resolution of the TCSPC system (~50 ps), indicating a ultrafast energy transfer rate (k T >>1/τ 0 D ) from the (PEA) 2 PbI 4 flake to the MAPbBr 3 microplate.Further, the energy transfer efficiency can be calculated to be [32]: The near-unity energy transfer efficiency is in good accordance with the TPL spectrum that the donor's emission is completely quenched in the perovskite heterostructure.For comparison, the energy transfer process under one-photon excitation was investigated.A 400 nm laser was obtained by frequency doubling the laser beam of the Ti/Sapphire oscillator and used for excitation.Figures 5(a) and 5(b) show the PL spectra and decay traces for the bare perovskite components and heterostructure respectively, under forward excitation.Notably, the PL spectrum and the decay trace of the heterostructure are the same as those of the MAPbBr 3 microplate, indicating a weak interlayer interaction.As MAPbBr 3 has a strong absorption (k = 0.39) at 400 nm [35], the penetration depth is determined to be approximately 163 nm, which is much smaller than the actual thickness of the MAPbBr 3 microplate (> 1 μm).Consequently, the 400-nm laser beam can hardly transmit the MAPbBr 3 microplate to excite the (PEA) 2 PbI 4 flake and no obvious energy transfer is observed.In contrast, with a thinner (215 nm) MAPbBr 3 microplate, the efficient energy transfer can also occur and lead to an increased PL emission of the acceptor, as shown in Fig. 5(c).For further confirmation, a 473 nm continuous-wave laser was used for excitation instead.The result is shown in Fig. 5(d).In this case, MAPbBr 3 has a smaller absorption of the laser beam (k = 0.22), and the energy transfer can be observed for the perovskite heterostructure with a thicker (420 nm) MAPbBr 3 microplate.Overall, the energy transfer process is strongly thickness-and wavelength-dependent under one-photon excitation, due to a much smaller penetration depth compared with that under two-photon excitation.

Conclusion
In conclusion, we study the energy transfer in perovskite heterostructure composed of (PEA) 2 PbI 4 flake and MAPbBr 3 microplate in this work.Under two-photon excitation, the PL intensity of the MAPbBr 3 microplate can be greatly increased by about 6.5 folds, while the PL of the (PEA) 2 PbI 4 flake was completely quenched in the heterostructure.The opposite variation of the PL intensity is attributed to the radiative energy transfer from the (PEA) 2 PbI 4 flake to the MAPbBr 3 microplate.Further investigation indicates that the energy transfer occurs on an ultrafast timescale with a high efficiency (~100%), and the radiative energy transfer can overcome the distance limitation of the nonradiative energy transfer.In addition, a strongly thickness-and wavelength-dependent energy transfer is observed under one-photon excitation.This work has performed a direct investigation on the interlayer interaction of perovskite heterostructures.As the radiative energy transfer is strongly dependent on the overlap of the absorption and emission spectra of the two components, our result has a broad scope for the similar heterostructures, which advocates great promise for understanding the interlayer interaction and improving the performance of the perovskite nanostructures.For practical applications like optoelectronic detection and energy harvesting, it is desirable to perform with a ultrafast and efficient response [40][41][42][43].The ultrafast and high-efficiency energy transfer in the perovskite heterostructures will have considerable potential for highperformance optoelectronic devices.

Fig. 5 .
Fig. 5. (a) PL spectra and (b) decay traces for the individual perovskite components and heterostructure (H) respectively, under forward excitation at 400 nm.The thickness of the MAPbBr 3 microplate (> 1 μm) is much larger than the penetration depth of the laser beam (163 nm).(c) PL spectra for the individual perovskite components and heterostructure (H) with a 215-nm-thick MAPbBr 3 microplate, under forward excitation at 400 nm.(d) PL spectra for the individual perovskite components and heterostructure (H) with a 420-nm-thick MAPbBr 3 microplate, under forward excitation at 473 nm.

Funding
973 Programs (2014CB921301); National Natural Science Foundation of China (11204097 and 11674117); Doctoral Fund of Ministry of Education of China (20130142110078).