Photo-electro-striction in halide perovskite semiconductors
Cite as: Appl. Phys. Lett.121, 041102 (2022);doi: 10.1063/5.0099954 Submitted: 19 May 2022
.
Accepted: 10 July 2022.
Published Online: 26 July 2022
ZeenZhao,1YechengDing,1XuefengZhao,1YaojinWang,1BenXu,2GuanghuaLiu,2GuoliangYuan,1,a) and Jun-MingLiu3
AFFILIATIONS
1School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China
2School of Materials Science and Engineering, Tsinghua University, Beijing 10084, China
3National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
a)Author to whom correspondence should be addressed:[email protected]
ABSTRACT
MAPbI3, MAPbBr3, and CsPbBr3are excellent halide perovskite semiconductors with super long carrier diffusion length, long minority carrier lifetime, and large light absorption coefficient. Compared with the small intrinsic electrostriction, photocarriers induce a large photostriction in the surface layer. Furthermore, an electric field can efficiently separate the light excited electron–hole pairs, enhance photocarriers diffusion, and finally increase the crystal expansion, i.e., photo-electro-striction. For each crystal under 30 V/mm and in light with 450 nm wavelength and 840 mW/cm2, the photo-electro-striction is over four times of the pure electrostriction and is larger than the sum of photostriction and electrostriction. Most importantly, MAPbI3single crystal shows a large photostriction of0.35% and the photo- electro-striction of0.64%. This work proves a very large photo-electro-striction as a result of the strong coupling among photocarriers, electric fields, and crystal lattices, which is important to develop semiconductor devices.
Published under an exclusive license by AIP Publishing.https://doi.org/10.1063/5.0099954
Halide perovskites (HP), such as MAPbX3and CsPbX3(X¼I, Br, or Cl), have attracted extensive interest for potential applications in next-generation solar cells, LEDs, photodetectors, and so on due to their demonstrated high power conversion efficiency (PCE) and low fabrication cost. For example, the updating solar cells based on doped MAPbI3thin films have achieved a certified PCE of 25%.1,2These materials display extraordinary optoelectronic properties, such as a large absorption coefficient, long carrier diffusion length (LD), high carrier mobility (l), and long carrier recombination lifetimes (s0), which benefit their outstanding performance. In fact, Shi et al.
reported largeLD>10lm in MAPbX3(X¼Br or I) single crystals, and Donget al.found that theLDof photocarriers are over 0.175 mm in MAPbI3single crystals under 1 sun illumination.3,4The super long LDmay come from the screened Coulomb potential created by large ferroelectric polarons, which can reduce the carrier scattering by charged defects, other charge carriers, and phonons.5–7 Once these high-density photocarriers diffuse from the surface to the bulk sample, some coupling and physic phenomenon being different from those of the conventional semiconductors may occur in these HP single crystals.
Photocarriers of HP single crystals can trigger a large photostric- tion and may enlarge electrostriction even in the bulk crystal. The inci- dent light can excite photocarriers and change some special chemical bonds. Recently, Tsai reported a strain of 6.35103 in MA0.25FA0.7Cs0.05PbI3films with 400 nm thickness.2Aside from that, these photocarriers with super longLD can diffuse in the mm thick MAPbI3single crystal. Zhouet al.reported a light-induced strain as large as 5105in a 1.0 mm thick MAPbI3single crystal.8Lv per- formed a systematic investigation of this effect in bulk halide perov- skites and observed a large photon-driven bulk strain in the 0.5 mm thick MAPbI3single crystals.9The photostriction is different from a typical photostriction of conventional ferroelectric oxides, which origi- nates from the inverse piezoelectric effect and the photocarrriers screening the ferroelectric polarization.9–15Here, there is a significant crystal expansion for the MAPbI3, MAPbBr3, and CsPbBr3 single crystals under an external electric field (Eex) in light and with both conditions, which correspond to electrostriction, photostriction, and photo-electro-striction, respectively. The production and diffusion of photocarriers introduce a large photostriction, and anEexcan enhance the efficiency of the production and diffusion and further enhance the
photo-electro-striction of MAPbI3, MAPbBr3, and CsPbBr3 single crystals. Most importantly, the MAPbI3 single crystal shows an electrostriction of 0.158% at anEexof 30 V/mm, a large photostriction of 0.35% for the crystal in light with 532 nm wavelength and 840 mW/cm2, and a very large photo-electro-striction of 0.637% with both conditions.
MAPbI3, MAPbBr3, and CsPbBr3single crystals were grown with the solution method at 60–100C (supplementary materialFig. S1).
The absorbance (a) of these crystals shows a clear band edge cutoff, and the optical band gaps (Eg) of 1.55, 2.21, and 2.25 eV are extracted from Tauc plots for MAPbI3, MAPbBr3, and CsPbBr3, respectively (supplementary materialFig. S2).3,16Most photons are absorbed in the light absorption layer with the depth of 1/a, which is 70–75 nm for the light with 405–550 nm wavelength. The MAPbI3single crystal in the dark shows a small electrostriction according to the XRD pattern anal- ysis. The semitransparent Au film with10 nm thickness is deposited on the pseudo-cubic (001)c crystal surface, and its transmittance is 53%–65% for the laser light with thekof 405–650 nm (supplementary materialFig. S3). After that, XRD patterns are characterized for the (001)ccrystal in the dark under a series of bias voltages. The diffraction angle of the (004)ccrystal plane slightly decreases with the bias voltage [Figs. 1(a)and1(b)]. The calculated crystal latticec increases from 0.627 83 nm at zero bias to 0.628 23 nm at 10 V/mm and 0.628 82 nm at 30 V/mm, corresponding to the electrostriction of 0.063% at 10 V/mm and 0.158% at 30 V/mm. This small positive electrostriction at a middle electric field is intrinsic. On the contrary, Chen reported a
giant extrinsic electrostriction of1% due to a mass of defects intro- duced by a super high electric field of 3.7 kV/mm in the MAPbI3single crystal.10
Furthermore, the 450 nm light excites photocarriers and introdu- ces the large photostriction and photo-electro-striction in the MAPbI3
single crystal according to the crystal lattice evolvement. With the intensity (p) of 450 nm light increasing to 840 mW/cm2, the diffraction angle of the (004)ccrystal plane decreases [Fig. 1(c)andsupplemen- tary materialFig. S4], and the derived crystal latticecincreases from 0.62783 to 0.63002 nm, which corresponds to the photostriction of 0.35%.17When an electric field of 30 V/mm (or30 V/cm) applies on the crystal in light withp¼840 mW/cm2, the photo-electro-striction is as high as 0.637% (or 0.605%) that is about four times of the pure electrostriction of 0.158% (or 0.127%) for the crystal in the dark [Figs.
1(d),1(e), andsupplementary materialFig. S5].
Similarly, the other two famous HP single crystals, i.e., MAPbBr3 and CsPbBr3, also show the obvious electrostrictions, the large photo- strictions, and the gaint photo-electro-strictions. The diffraction angles of these two single crystals decrease further when these crystals are under bias voltage, in light or both [Figs. 2(a)and2(c),supplementary material Figs. S6 and S7]. The crystal lattice c was calculated and shown inFigs. 2(b)and2(d). Thecvalue of MAPbBr3(CsPbBr3) is 0.5925 nm (0.5891 nm), 0.5927 nm (0.5894 nm), 0.5932 nm (0.5900 nm), and 0.5940 nm (0.5908 nm) for the crystal in the dark, under 30 V/mm, in light with 840 mW/cm2, and in light with 840 mW/cm2 and under 30 V/mm, respectively. So, thec values of the
FIG. 1.(a) Sketch of electrostriction, photostriction, and photo-electro-striction of HPSC. The (004) diffraction peak of the MAPbI3crystal in the dark (b) under bias voltages, (c) in light, and (d) in light under bias voltage. (e) Crystal lattice dependence on voltage bias for MAPbI3single crystals.
crystals MAPbBr3 (CsPbBr3) just increase about 0.03% (0.05%) under 30 V/mm,0.12% (0.15%) in light and0.25% (0.30%) under 30 V/mm and in light. These results prove that the photo-electro- striction is far larger than the pure electrostriction for these crystals in deed (supplementary materialFig. S8). The photostriction of MAPbI3
and MAPbBr3nearly linearly depends on the light intensity according to previous studies.9,18 Therefore, the large photo-electro-striction should come from the strong coupling among photocarriers, electric fields, and crystal lattices rather than the combination of inverse piezo- electric effects and the screen of ferroelectric polarization by photocar- riers.9–15In fact, the amplitude vs tip bias curves of these crystals were characterized by piezoelectric force microscopy (PFM) with a conduc- tive tip, where the positive amplitude means the increase in the lattice constant along the out-of-plane direction. The amplitude at63 V tip bias in light of a 450 nm laser withp¼840 mW/cm2is over nine times of the amplitude at63 V in the dark for all three crystals (supplemen- tary materialFig. S9). The amplitude nearly linearly increases with the absolute values of tip bias, which does not show the characteristics of conventional ferroelectricity and piezoelectricity.
Phase transition and thermal decomposition do not occur, and thermal expansion is much smaller than the observed photostriction and photo-electro-striction during our measurements. At first, each x- ray diffraction peak before and after the crystal in light for 5 s nearly
overlaps [Figs. 1(c),2(a), and2(c)], which excludes the crystals’ decom- position.19In fact, the highest temperature of these crystals in light with 840 mW/cm2for 5 s (<55C) (supplementary materialFig. S10) is lower than the temperature of the tetragonal-cubic phase transition of MAPbI3at58C and the decomposition temperature of MAPbI3and MAPbBr3(>120C).20–24So, these crystals do not have phase transi- tion or decompose during the measurements of XRD patterns. Aside from that, thermal expansion is much lower than the observed photo- striction. The 980 nm light withhv<Egcannot excite a mass of photo- carriers and the corresponding large photostriction. For the Au/crystal/
Ga structure, the 980 nm light was mainly absorbed by Au/Ga electrodes and the bulk crystal together, so the 450 nm light and 980 nm light with the same intensity introduce the similar temperature increase in each crystal (supplementary materialFig. S11). There is a very small thermal expansion of 0.01%–0.032% for the crystals in light with 980 nm wave- length and 830 mW/cm2for 5 s, so the temperature increase only intro- duces a small crystal expansion compared with the large photostriction and photo-electro-striction introduced by light with 450 nm wavelength.
Aside from that, the electric field of 30 V/mm introduces a small current and a temperature increase in<2C during the photo-electro-striction measurement of these crystals.
The large photostriction and photo-electro-striction should origi- nate from photocarriers, which are studied by the photocurrent and FIG. 2.Amplified diffraction peak and crystal latticecof (a) and (b) MAPbBr3and (c) and (d) CsPbBr3crystal in the dark, under bias voltages, in light with 450 nm wavelength and in light under bias voltage simultaneously, respectively.
photoimpedance. At first, the photocarriers are excited in the light absorption layer of HP by the incident light and then they diffuse in the crystal and induce a high photocurrent [Fig. 3(a)]. As a result, the photocurrent is over 103 higher than the dark current of the Au/
MAPbI3/Ga solar cell at 2 V bias.3,4Moreover, the photocarriers also introduce a low photoimpedance, and the equivalent circuit is achieved through simulating the Z00–Z0impedance curve [Figs. 3(b) and S12].25
The large photocapacitance proves the high surface polarization triggered by photocarriers in these HP single crystals. The photocapa- citance of a 0.5 mm thick MAPbI3single crystal increases12 times at 1 kHz and4% at 1 MHz compared with the corresponding dark capacitance [Fig. 3(c)], and the large photocapacitance fast recovers to the small dark capacitance after the light switches off [Fig. 3(d)]. In addition, there is a large photocapacitance in MAPbBr3and CsPbBr3
single crystals, especially at low frequencies (supplementary material Figs. S13 and S14).26,27In principle, the capacitance comes from (1) electronic polarization, (2) interfacial or space charge polarization, (3) ionic polarization, and (4) dipolar polarization.28Since the electronic polarization can respond with the AC signal within the ns scale, the large photocapacitance below 1 MHz should mainly come from the polarization increasing of (2)–(4).28–30In a word, the large photocapa- citance proves the polarization increasing in the photocarriers’ diffu- sion layer with the thicknessLDof hundreds oflm.9,18
The generation and diffusion of photocarriers largely increase pho- tostriction and photo-electro-striction. Although a medium Eex can introduce an intrinsic electrostriction at an electric field of30 V/mm mainly due to dipole rotation and ion movement [Figs. 4(a)and4(b)],
such electrostriction is much smaller than the photostriction of MAPbBr3, MAPbI3, and CsPbBr3 single crystals discussed above.
For these crystals in light, both the generation and the diffusion of pho- tocarriers introduce a large photostriction. At first, most photons with hv Egare absorbed and excited photocarriers in the surface light absorption layer with 100 nm thickness.9The photon-excited elec- trons transfer from hybridized Pb 6s–X (X¼I or Br) 5p orbitals to the Pb 6p orbitals, which reduce the coulomb interaction with amine, straighten the Pb–X–Pb bond, and introduce a large photostriction.2,8 Second, the photocarriers diffuse into the crystal interior and introduce a large photostriction in the photocarriers’ diffusion layer with the depth ofLD[Fig. 4(c)]. Since the electrons and holes diffuse into the crystal interior with differentlandLD, the separation of electrons and holes would induce a diffusion potential, increase interfacial space charge polarization, and the corresponding inner electric field (Ein).9,31The large photostriction and photo-electron-striction concern interfacial polarization, ionic polarization, and dipolar polarization. For example, the potential barriers of MAþ, I, and Br ions diffusing are below 0.4 eV in MAPbI3, MAPbBr3, and CsPbBr3, and these ions can diffuse and introduce additional ionic polarization under an electric field.10,32,33 BothEinandEexcan rotate these polarizations and introduce a large photostriction and photo-electro-striction in the diffusion layer.
Furthermore, once anEexincreases the separation efficiency of the photo excited electron–hole pairs, more photocarriers diffuse into the crystal interior, which abruptly increases the photo-electro-stric- tion in the diffusion layer [Fig. 4(d)].34,35Most importantly, the photo- electro-striction is much larger than the sum of electrostriction and photostriction (supplementary material Fig. S10). Aside from that, since both photostriction and photo-electro-striction effects are the surface effects in the photocarriers’ diffusion layer with the depth of LD, they do not seriously depend on the crystal orientation. In fact, these crystals with (001)c, (010)c, and (100)cplanes show the similar FIG. 3.(a) Current density vs voltage curves, (b) impedance vs frequency curves,
(c) capacitance vs frequency curves, and (d) capacitance vs time curves of the Au/
MAPbI3/Ga solar cell in the dark or in light, where the top Au electrode is semitrans- parent for light illumination and the MAPbI3single crystal is 0.5 mm thick.
FIG. 4.Sketches of (a) origin crystal and the crystals with (b) electrostriction, (c) photostriction, and (d) photo-electro-striction.
photostriction and photo-electro-striction. For the pure photostriction, Tsai reported the simultaneous expansions ofa,b, andccrystal lattices in MA0.25FA0.7Cs0.05PbI3 films in light.2 For the photo-electro- striction,a,b, andccrystal lattices should expand simultaneously in the photocarriers’ diffusion layer as the photostriction does, since photocarriers are the essential factor for both photostriction and photo-electro-striction. The crystal expansion of all axes is different from that of the photostriction in piezoelectric oxides, where only the crystal lattice along the photovoltaic electric field increases and the other two vertical directions often shrink.
The MAPbI3single crystal shows the largest photostriction and photo-electro-striction among the three crystals mainly due to three possible factors. At first, the electron transfer straightens the Pb–X–Pb bond when electron–hole pairs are excited by incident light. The Pb–I–Pb bond deflection of MAPbI3may be stronger than that of Pb–Br–Pb in MAPbBr3and CsPbBr3in the surface light absorption layer.9Therefore, the difference between Pb–I and Pb–Br bonds as well as the electronic structure might be also responsible for the differ- ence in photostriction and photo-electro-striction among three crys- tals. Second, the MAPbI3 semiconductor has a higher external quantum efficiency and a longerLD of photocarriers than those of MAPbBr3 and CsPbBr3; thus, more photocarriers can diffuse and introduce a higher interfacial, ionic and dipolar polarization and intro- duce a larger crystal lattice in the photocarriers’ diffusion layer. Third, dipoles may be an important factor for photostriction and photo- electro-striction. The dipoles of these crystals may rotate to increase the crystal lattice underEin or Eex.36–41 In fact, both intrinsic and dynamical dipoles are confirmed in MAPbI3 and MAPbBr3,42 and there is the local polar fluctuations due to the head-to head Cs motion coupling with Br face in CsPbBr3.43,44
In conclusion, a large photostriction and a giant photo- electro-striction are discovered, and their physics mechanisms are clarified for MAPbBr3, MAPbI3, and CsPbBr3single crystals. The large photostriction is introduced by the production of photocar- riers in the light absorption layer and the photocarriers’ movement in the diffusing layer with the thickness ofLD. Furthermore, anEex
increases the efficiencies of light excited electron–hole pairs’ separa- tion, enhances the photocarriers’ diffusion, and finally introduces a large photo-electro-striction, which is higher than the sum of pure photostriction and electrostriction. Most importantly, the MAPbI3 crystal shows a large photostriction of0.35% and a giant photo- electro-striction of0.64%. This study not only discovers a coupling among photocarriers, electric fields, and crystal lattices but also adds a freedom to design photoelectric devices.
See thesupplementary materialfor detailed experimental growth processes, optical images, and surface morphologies of MAPbI3, MAPbBr3, and CsPbBr3single crystals, absorbance and the penetra- tion depth, transmittance of the Au film, XRD patterns, PFM mea- surements, photostriction, electrostriction, and photo-electro-striction of MAPbI3, MAPbBr3, and CsPbBr3single crystals, dependence of the temperature, impedance spectroscopy, current density vs voltage curves, impedance vs frequency curves, capacitance vs frequency curves, and capacitance vs time curves.
This work was supported by the National Natural Science Foundation of China (Nos. 51790492, 92163210, 51790494,
11974167, and 61874055) and the Fundamental Research Funds for the Central Universities (No. 30921013108).
AUTHOR DECLARATIONS Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Zeen Zhao:Conceptualization (equal); Data curation (equal); Writing – original draft (equal). Yecheng Ding: Conceptualization (equal).
Xuefeng Zhao: Conceptualization (equal). Yaojin Wang:
Conceptualization (equal); Formal analysis (equal). Ben Xu:Formal analysis (equal); Writing – review and editing (equal).Guanghua Liu:
Conceptualization (equal); Formal analysis (equal).Guoliang Yuan:
Conceptualization (equal); Formal analysis (equal); Writing – review and editing (equal).Junming Liu:Formal analysis (equal); Writing – review and editing (equal).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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