2Pbbr4 and (C6H5CH2NH3)2Pbi4

applied sciences

Article Preparation and Two-Photon Photoluminescence Properties of Organic Inorganic Hybrid Perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4

Shuai Liu, Fang Li *, Xiaobo Han, Litu Xu, Fuqiang Yao and Yahui Liu Laboratory of Optical Information and Technology, School of Optoelectronics and Energy, Wuhan Institute of Technology, Wuhan 430073, China; [email protected] (S.L.); [email protected] (X.H.); [email protected] (L.X.); [email protected] (F.Y.); [email protected] (Y.L.) * Correspondence: [email protected] or [email protected]; Tel.: +86-027-8799-2024

Received: 13 October 2018; Accepted: 14 November 2018; Published: 19 November 2018

Abstract: Organic inorganic hybrid perovskites have potential applications in solar cells, electroluminescent devices and radiation detection because of their unique optoelectronic properties. In this paper, the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 were synthesized by solvent evaporation. The crystal structure, morphology, absorption spectrum, laser power dependence of the photoluminescence (PL) intensity and lifetime were studied. The results showed that the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 display a layered stacking structure of organic and inorganic components. The absorption peaks are located at 392 nm (3.16 eV) and 516 nm (2.40 eV), respectively. It was observed that the PL intensity and photoluminescence quantum yield (PLQY) increases with increasing laser power, and that the PL lifetime decreases with increasing laser power, which is mainly due to the non-geminate recombination.

Keywords: perovskites; two-photon excitation; photoluminescence spectrum; photoluminescence lifetime

1. Introduction In recent years, organic inorganic hybrid perovskites have received wide attention and investigation as the most competitive candidates in photovoltaic field development. Organic inorganic hybrid perovskites are a new subclass of material self-assembled at the molecular scale from organic and inorganic components [1–3]. The crystal structure of a perovskite can be described with the chemical formula ABX3, in which A-sites are the organic ammonium cations, B-sites are the inorganic metal cations, and X-sites are the halide anions. The B-site cation is six-coordinated by the X-site anion to form [BX6] octahedrons, which are corner-sharing to constitute three-dimensional frameworks [4,5]. These compounds have multiple quantum well structures with alternating organic and inorganic layers [6]. In these compounds, the excitons possess larger binding energy due to quantum confinement effects, which displays excellent optical properties such as second harmonic generation, exciton absorption and emission [7–11]. These properties make organic inorganic hybrid perovskites great candidates for applications in solar cells [12], electroluminescent devices [13–15] and radiation detection [16]. The perovskite solar cells with all solid state structure can prevent the problems from the liquid electrolyte and exhibit high power conversion efficiency (PCE), displaying great potential in photovoltaic devices. In 2009, Miyasaka et al. [17] firstly studied the solar cells with the perovskite CH3NH3PbI3, yielding a PCE of 3.8% due to the lower open voltage. In 2011, Zheng et al. [18] synthesized a series of the perovskites (C6H13NH3)2(CH3NH3)n−1PbnI3n+1, and investigated the effects

Appl. Sci. 2018, 8, 2286; doi:10.3390/app8112286 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2286 2 of 11 of the inorganic-sheet number on the crystal structure, bandgap energy, exciton binding energy and photoluminescent emission. In 2013, Stoumpos et al. [19] synthesized several perovskites CH3NH3SnI3, HC(NH2)2SnI3, CH3NH3PbI3, HC(NH2)2PbI3 and CH3NH3Sn1−xPbxI3, and studied the structure, morphology, phase transition and photoluminescence. In 2014, Kawano et al. [6] prepared three perovskites (C4H9NH3)2PbBr4, (C6H5CH2NH3)2PbBr4 and (C6H5C2H4NH3)2PbBr4, investigating the effects of organic moieties on luminescence properties of perovskite compounds. In 2015, Qin et al. [13] studied the performance of light-emitting diodes based on perovskite CH3NH3PbBr3, and a high luminance up to 1500 cd/m2 was achieved. In 2018, Luo et al. [20] studied solar cells with the inverted planar heterojunction perovskite, yielding a PCE of 21% due to the elimination of nonradiative charge-carrier recombination. Droseros et al. [21] studied the photoluminescence of the perovskite CH3NH3PbBr3, discovering the photoluminescence quantum yield (PLQY) increased with decreasing crystal size and explained the phenomenon. Organic inorganic hybrid perovskites combine distinct properties of organic and inorganic components within a single molecular composite. The inorganic component forms the octahedral framework by ionic bonds, to provide good conductivity and carrier mobility. The organic component facilitates the self-assembly, enabling the materials to be deposited using a simple way. These properties can be easily controlled by changing the organic ammonium, inorganic metal or halide [22]. Research on perovskites is now focusing on solar cells and light-emitting diodes, however reports on the photoluminescent properties of the perovskites with the two-photon excitation are rarely seen. In this paper, the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are synthesized by solvent evaporation. The laser power effect on the photoluminescence (PL) intensity and lifetime was investigated with femtosecond laser via the two-photon excitation.

2. Experimental Section

2.1. Synthesis of Organic Ammonium Salt

HBr and HI were added in the C6H5CH2NH2 solution respectively. The mixtures were heated at ◦ 50 C and cooled at room temperature. C6H5CH2NH3Br and C6H5CH2NH3I were obtained and then heat treated.

2.2. Synthesis of Perovskite Crystal

C6H5CH2NH3Br and PbBr2,C6H5CH2NH3I and PbI2 were then dissolved in N,N-dimethylformamide (DMF) in a molar ratio of 2:1 respectively. The mixtures were heated at ◦ 50 C and stirred for 1 h, then the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 were prepared by solvent evaporation. The products were washed with ethanol followed by heat treatment.

2.3. Synthesis of Perovskite Nanosheet The perovskite crystals were ground into powders, which were used for XRD measurement. The perovskite powders were put in aqueous solution, ultrasonically processed, and then centrifuged. After a period of time, large quantities of perovskite nanosheets were obtained for TEM measurement. The chemical reaction equations involved in the experiment are as follows:

C6H5CH2NH2 + HBr ⇒ C6H5CH2NH3Br (1)

2C6H5CH2NH3Br + PbBr2 ⇒ (C6H5CH2NH3)2PbBr4 (2)

C6H5CH2NH2 + HI ⇒ C6H5CH2NH3I (3)

2C6H5CH2NH3I + PbI2 ⇒ (C 6H5CH2NH3)2PbI4 (4) Appl. Sci. 2018, 8, 2286 3 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 11

2.4. Characterization The structure structure was was analyzed analyzed by by X-ray X-ray diffraction diffraction (Germany (Germany Bruker Bruker AXS, AXS, D8 Advance). D8 Advance). The morphologyThe morphology was wasobserved observed by bytransmission transmission elec electrontron microscopy microscopy (Japan (Japan JEOL, JEOL, JEM-2000) JEM-2000) and scanning electron electron microscopy microscopy (Holland (Holland FEI, FEI, SEM SEM 450). 450). The Theabsorption absorption spectra spectra were weremeasured measured with withUV-visible UV-visible spectrometer spectrometer (America (America PerkinElmer, PerkinElmer, Lambda Lambda 35). 35). The The optical optical measurements measurements were performed using a commercial optical microscopemicroscope systemsystem (Japan,(Japan, OlympusOlympus IX73).IX73). A Ti:sapphire oscillator (China Atop Electronic Technology Co. Ltd., Ltd., Vitara) Vitara) was was used used for two-photon excitation. excitation. The laser beam was focused by an objectiveobjective (Olympus,(Olympus, NA == 0.65) onto the samples. The reﬂectedreflected signal was collected by the same objective and then directed into a spectrometer (Andor 193i) for spectral measurement, or onto a CCD camera for imaging. The time correlated single photon count (TCSPC) systemsystem consisting consisting of of a PicoHarpa PicoHarp 300 300 controller, controller, a PDL a 800-BPDL 800-B reference reference and a SPDA-15 and a SPDA-15 detector wasdetector used forwas the used lifetime for measurements.the lifetime measurements Figure1 shows. theFigure optical 1 pathshows of thethe photoluminescence optical path of testthe photoluminescencesystem used in the experiment. test system used in the experiment.

Figure 1. The experiment optical path of photoluminescence test system. PZT, piezoelectric ceramic transducer; O, O, objective; objective; A, A, half-wave half-wave plate; plate; B, pola B, polarizer;rizer; C, low C, pass low filter; pass ﬁlter;M, reflector; M, reﬂector; CCD, charge CCD, couplecharge coupledevice; device;S, spectrometer; S, spectrometer; PMT, photomultiplier PMT, photomultiplier tube. tube.

The experimental results can be ﬁttedfitted to a biexponential decay function:function:

I=+ A exp(-t/τ ) A exp(-t/τ ) (5) I = A1 1122exp(−t/τ 1) +A2 exp(−t/τ 2) (5) where τ1 and τ2 are the radiative and nonradiative decay lifetime, respectively. where τ1 and τ2 are the radiative and nonradiative decay lifetime, respectively.

3. Results and Discussion

3.1. Structure Characterization

Figure 2a,b2a,b showsshows the the crystal crystal structure structure of of the the perovskites perovskites (C (C6H6H5CH5CH2NH2NH33))22PbBr4 and (C6HH55CHCH22NHNH3)32)PbI2PbI4. 4The. The Pb Pb atoms atoms are are six-coordinated six-coordinated by by the the halogen halogen to to form [PbX6]] octahedrons, octahedrons, which are corner-sharing to constituteconstitute three-dimensionalthree-dimensional frameworks. It It can can be observed that the perovskites (C(C66H5CH22NHNH33)2)PbBr2PbBr4 4andand (C (C66HH5CH5CH2NH2NH3)23PbI)2PbI4 exhibit4 exhibit the the structure structure with with alternating organic and inorganic layers. The The growth growth directions directions of of the the layers layers are are along along the the a a axis axis and and b b axis, axis, respectively. TheThe latticelattice parameters parameters of of the the perovskite perovskite (C (C6H65HCH5CH2NH2NH3)32)PbBr2PbBr4 4and and (C (C6H6H5CH5CH22NHNH33))22PbI44 are a a = = 3.3394 3.3394 nm, nm, b b= =0.8153 0.8153 nm, nm, c = c0.8131 = 0.8131 nm nmand anda = 0.8689 a = 0.8689 nm, b nm, = 2.878 b = nm, 2.878 c nm,= 0.9162 c = 0.9162nm, which nm, whichare consistent are consistent with reports with reports in the literature in the literature [23]. [23]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 11 Appl. Sci. 2018, 8, 2286x FOR PEER REVIEW 44 of of 11

Figure 2. The crystal structure of the perovskites (a) (C6H5CH2NH3)2PbBr4; (b) (C6H5CH2NH3)2PbI4. Figure 2. The crystal structure of the perovskites (a) (C6H5CH2NH3)2PbBr4;(b) (C6H5CH2NH3)2PbI4. FigureThe hydrogen 2. The crystalatoms bondedstructure to ofthe the ca rbonperovskites atoms are(a) omitted(C6H5CH for2NH clarity.3)2PbBr 4; (b) (C6H5CH2NH3)2PbI4. The hydrogen atoms bonded to thethe carboncarbon atomsatoms areare omittedomitted forfor clarity.clarity.

Figure 3a,b shows the XRD patterns of the perovskite powders (C6H5CH2NH3)2PbBr4 and Figure3a,b shows the XRD patterns of the perovskite powders (C 6H5CH2NH3)2PbBr4 and Figure 3a,b shows the XRD patterns of the perovskite powders (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4. The diffraction peaks of the perovskite (C6H5CH2NH3)2PbBr4 are located at 6.41°, (C6H5CH2NH3)2PbI4. The diffraction peaks of the perovskite (C6H5CH2NH3)2PbBr4 are located at (C6H5CH2NH3)2PbI4. The diffraction peaks of the perovskite (C6H5CH2NH3)2PbBr4 are located at 6.41°, 6.4110.63°,◦, 10.63 15.96°,◦, 15.96 21.32°,◦, 21.32 26.72°,◦, 26.72 32.18°◦, 32.18and ◦37.73°,and 37.73 which◦, which correspond correspond to the to lattice the lattice plane plane (X00, (X00, X = 2, X 4, = 6, 2, 10.63°, 15.96°, 21.32°, 26.72°, 32.18° and 37.73°, which correspond to the lattice plane (X00, X = 2, 4, 6, 8, …). The diffraction peaks of the perovskite (C6H5CH2NH3)2PbI4 are located at 6.14°, 12.29°,◦ 18.48°,◦ 4, 6, 8, ... ). The diffraction peaks of the perovskite (C6H5CH2NH3)2PbI4 are located at 6.14 , 12.29 , 8, …). The diffraction peaks of the perovskite (C6H5CH2NH3)2PbI4 are located at 6.14°, 12.29°, 18.48°, 18.4824.73°,◦, 24.7331.05°◦ ,and 31.05 37.47°,◦ and 37.47which◦, whichare correspond are correspond to the tolattice the lattice plane plane(0X0, (0X0,X = 2, X 4, = 2,6, 4,8, 6,…). 8, ...The). 24.73°, 31.05° and 37.47°, which are correspond to the lattice plane (0X0, X = 2, 4, 6, 8, …). The calculated interlayer spacing of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are The calculated interlayer spacing of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 calculated interlayer spacing of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are are1.44 1.44nm nmand and1.68 1.68nm, nm,respectively, respectively, using using the Bragg the Bragg formula: formula: 2dsin 2dsinθ = nθλ=. The nλ. experimental The experimental XRD 1.44 nm and 1.68 nm, respectively, using the Bragg formula: 2dsinθ = nλ. The experimental XRD patterns of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are consistent with XRD patterns of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are consistent with patterns of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are consistent with calculated values, confirming there is no tri-halide perovskite or PbX2 phase [23]. All the patterns calculated values, conﬁrming there is no tri-halide perovskite or PbX2 phase [23]. All the patterns calculated values, confirming there is no tri-halide perovskite or PbX2 phase [23]. All the patterns confirmed that the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are well crystallized conﬁrmed that the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are well crystallized confirmed that the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are well crystallized and oriented. and oriented.

Figure 3. TheThe XRD XRD patterns patterns of ofthethe perovskite perovskite powders powders (a) (a(C)6H (C56CHH52CHNH23NH)2PbBr3)2PbBr4; (b4); Figure 3. The XRD patterns of the perovskite powders (a) (C6H5CH2NH3)2PbBr4; (b) ((Cb)6H (C5CH6H52CHNH23NH)2PbI3)42. PbI4. (C6H5CH2NH3)2PbI4. 3.2. Morphology Characterization 3.2. Morphology Characterization 3.2. Morphology Characterization Figure4a,b shows the SEM images of the perovskites (C 6H5CH2NH3)2PbBr4 and Figure 4a,b shows the SEM images of the perovskites (C6H5CH2NH3)2PbBr4 and (C6HFigure5CH2NH 4a,b3)2PbI shows4. The perovskitethe SEM (Cim6Hages5CH 2ofNH 3the)2PbBr perovskites4 exhibits a(C layered6H5CH stacking2NH3)2PbBr structure4 and (C6H5CH2NH3)2PbI4. The perovskite (C6H5CH2NH3)2PbBr4 exhibits a layered stacking structure with with(C6H5 grainCH2NH size3)2 ofPbI 1–54. The um, perovskite and the perovskite (C6H5CH2 (CNH6H3)25PbBrCH2NH4 exhibits3)2PbI a4 alsolayered exhibits stacking a layered structure stacking with grain size of 1–5 um, and the perovskite (C6H5CH2NH3)2PbI4 also exhibits a layered stacking structure structuregrain size withof 1–5 grain um, and size the of perovskite 2–8 um. SEM(C6H5 imagesCH2NH of3)2PbI the4 perovskitesalso exhibits (Ca layered6H5CH 2stackingNH3)2PbBr structure4 and with grain size of 2–8 um. SEM images of the perovskites (C6H5CH2NH3)2PbBr4 and (Cwith6H 5CHgrain2NH size3)2PbI of4 are2–8 consistent um. SEM with thoseimages observed of the in theperovskites bright ﬁeld (C TEM6H5CH images.2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 are consistent with those observed in the bright field TEM images. (C6H5CH2NH3)2PbI4 are consistent with those observed in the bright field TEM images. Appl. Sci. 2018, 8, 2286 5 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 11

FigureFigure 4. 4.SEM SEM images images of of the the perovskites perovskites (a )(a (C) (C6H6H5CH5CH2NH2NH3)32)2PbBrPbBr44;(; (b)) (C(C66H5CHCH22NHNH3)32)PbI2PbI4. 4.

Figure5 5aa shows shows the the bright bright ﬁeld field TEM TEM images images of of the the perovskite perovskite (C (C66HH55CHCH22NH33)2PbBrPbBr44 nanosheets.nanosheets. The perovskite (C (C66HH55CHCH2NH2NH3)32PbBr)2PbBr4 displays4 displays a layered a layered stacking stacking stru structurecture with with crystal crystal geometry geometry size sizeof 600 of nm. 600 Figure nm. Figure5b shows5b showsthe HR the-TEM HR-TEM images of images the perovskite of the perovskite (C6H5CH2 (CNH63H)25PbBrCH24NH nanosheets.3)2PbBr4 nanosheets.It shows that Itthe shows lattice that plane the has lattice the planespacing has dist theance spacing of 0.294 distance nm, with of 0.294the growth nm, with direction the growth along direction[100]. The alongselected [100]. area Theelectron selected diffracti areaon electron(SAED) pattern diffraction of the (SAED) perovskite pattern (C6H of5CH the2NH perovskite3)2PbBr4 (Cnanosheets6H5CH2NH is 3shown)2PbBr 4in nanosheetsFigure 5c, indicating is shown the in Figureorthorhombic5c, indicating phase, space the orthorhombic group Cmca, phase,with a spacegrowth group direction Cmca, along with [100]. a growth Figure direction 5d shows along the [100]. bright Figure field 5TEMd shows images the brightof the ﬁeldperovskite TEM images(C6H5CH of2NH the3) perovskite2PbI4 nanosheets. (C6H5CH The2NH perovskite3)2PbI4 nanosheets. (C6H5CH2NH The3)2PbI perovskite4 displays (C 6aH layered5CH2NH stacking3)2PbI4 displaysstructure a with layered crystal stacking geometry structure size withof 1um. crystal Figu geometryre 5e shows size the of 1um.HR-TEM Figure images5e shows of the the perovskite HR-TEM images(C6H5CH of2NH the3) perovskite2PbI4 nanosheets. (C6H5 CHIt shows2NH3 )that2PbI the4 nanosheets. lattice plane It has shows the spacing that the distance lattice plane of 0.357 has nm, the spacingwith the distance growth of direction 0.357 nm, along with the [010]. growth The direction SAED pattern along [010]. of the The perovskite SAED pattern (C6H of5CH the2 perovskiteNH3)2PbI4 (Cnanosheets6H5CH2NH is 3shown)2PbI4 nanosheets in Figure is5f, shown indicating in Figure the 5orthorhombicf, indicating the phase, orthorhombic space group phase, Pbca, space with group a Pbca,growth with direction a growth along direction [010]. along [010].

Figure 5.5.( a()a) The The bright bright ﬁeld field TEM TEM images, images, (b) HR-TEM (b) HR-TEM images images and (c) Selectedand (c) areaSelected electron area diffraction electron

(SAED)diffraction pattern (SAED) of the pattern perovskite of the (C perovskite6H5CH2NH (C3)62HPbBr5CH42NHnanosheets;3)2PbBr4 nanosheets; (d) the bright (d ﬁeld) the TEM bright images, field (TEMe) HR-TEM images, images (e) HR-TEM and (f) SAED images pattern and of(f) theSAED perovskite pattern (C of6H 5theCH 2perovskiteNH3)2PbI 4(Cnanosheets.6H5CH2NH3)2PbI4 nanosheets.

Appl. Sci. 2018, 8, 2286 6 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11

3.3.3.3. Absorption Spectrum

Figure 6 shows the absorption spectrum of the perovskites (C6H5CH2NH3)2PbBr4 (red curve) and Figure6 shows the absorption spectrum of the perovskites (C 6H5CH2NH3)2PbBr4 (red curve) and (C6H5CH2NH3)2PbI4 (blue curve). The absorption peaks are located at 392 nm (3.16 eV) and 516 nm (C6H5CH2NH3)2PbI4 (blue curve). The absorption peaks are located at 392 nm (3.16 eV) and 516 nm (2.40 eV), which belong to the typical exciton absorption peaks. Organic inorganic hybrid perovskites (2.40 eV), which belong to the typical exciton absorption peaks. Organic inorganic hybrid perovskites have multiple quantum well structures with alternating organic and inorganic layers, therefore the have multiple quantum well structures with alternating organic and inorganic layers, therefore the excitons possess larger binding energy, and it is easy to generate exciton absorption due to quantum excitons possess larger binding energy, and it is easy to generate exciton absorption due to quantum confinement effects. conﬁnement effects.

FigureFigure 6.6. TheThe absorption spectra ofof thethe perovskitesperovskites (C(C6H6H5CH5CH22NHNH33)2PbBrPbBr44 (red(red curve) andand

(C(C66H5CH22NHNH33)2)PbI2PbI4 4(blue(blue curve). curve). 3.4. The Laser Power Effect on the PL Intensity 3.4. The Laser Power Effect on the PL Intensity Figure7a,b shows the bright ﬁeld and dark ﬁeld optical microscope images of the perovskites Figure 7a,b shows the bright field and dark field optical microscope images of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4. It is possible to observe the regular and transparent (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4. It is possible to observe the regular and transparent crystal geometry of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4, which emit blue crystal geometry of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4, which emit blue and green photoluminescence under the 800 nm femtosecond laser. Figure7c,d shows the two-photon and green photoluminescence under the 800 nm femtosecond laser. Figure 7c,d shows the two- PL spectrum of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4. PL peaks are photon PL spectrum of the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4. PL peaks are located at 415 nm (298 eV) and 540 nm (2.30 eV) respectively. PL intensity of the perovskite located at 415 nm (298 eV) and 540 nm (2.30 eV) respectively. PL intensity of the perovskite (C6H5CH2NH3)2PbBr4 increases from 117 photon counts to 2195 photon counts when laser power (C6H5CH2NH3)2PbBr4 increases from 117 photon counts to 2195 photon counts when laser power increases from 10 mW to 100 mW. In contrast, PL intensity of the perovskite (C6H5CH2NH3)2PbI4 increases from 10 mW to 100 mW. In contrast, PL intensity of the perovskite (C6H5CH2NH3)2PbI4 increases from 158 photon counts to 2771 photon counts when laser power increases from 10 mW to increases from 158 photon counts to 2771 photon counts when laser power increases from 10 mW to 100 mW. 100 mW. Figure8 shows the laser power dependence of the PL intensity of the perovskites Figure 8 shows the laser power dependence of the PL intensity of the perovskites (C6H5CH2NH3)2PbBr4 (red curve) and (C6H5CH2NH3)2PbI4 (blue curve). It can be observed the PL (C6H5CH2NH3)2PbBr4 (red curve) and (C6H5CH2NH3)2PbI4 (blue curve). It can be observed the PL intensity increases with increasing laser power. PL intensity of the perovskite (C6H5CH2NH3)2PbBr4 intensity increases with increasing laser power. PL intensity of the perovskite (C6H5CH2NH3)2PbBr4 is less than that of the perovskite (C6H5CH2NH3)2PbI4 at the same laser power. The radius of a Br is less than that of the perovskite (C6H5CH2NH3)2PbI4 at the same laser power. The radius of a Br atom atom is less than that of an I atom, and the lattice volume of (C6H5CH2NH3)2PbBr4 is less than that is less than that of an I atom, and the lattice volume of (C6H5CH2NH3)2PbBr4 is less than that of of (C6H5CH2NH3)2PbI4. Therefore the perovskite (C6H5CH2NH3)2PbI4 has a larger Bohr radius and (C6H5CH2NH3)2PbI4. Therefore the perovskite (C6H5CH2NH3)2PbI4 has a larger Bohr radius and smaller exciton binding energy, so the excitons of the perovskite (C6H5CH2NH3)2PbI4 will more easily smaller exciton binding energy, so the excitons of the perovskite (C6H5CH2NH3)2PbI4 will more easily separate and produce radiative recombination. separate and produce radiative recombination.

Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 11

Appl. Sci. 2018, 8, 2286 7 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 11

Figure 7. The bright field and dark field optical microscope images of the perovskites (a) Figure 7. The bright ﬁeld and dark ﬁeld optical microscope images of the perovskites (a) (C6H5FigureCH2NH 7.3 )2ThePbBr bright4; (b) (Cfield6H 5CHand2 NHdark3) 2PbIfield4; theoptical two-photon microscope PL images spectrum of theof theperovskites perovskites (a) (c) (C6H5CH2NH3)2PbBr4;(b) (C6H5CH2NH3)2PbI4; the two-photon PL spectrum of the perovskites (C6H5(CCH6H2NH5CH32NH)2PbBr3)2PbBr4; (d4;) (C(b)6 H(C56CHH5CH2NH2NH3)23PbI)2PbI4. 4; the two-photon PL spectrum of the perovskites (c) (c) (C H CH NH ) PbBr ;(d) (C H CH NH ) PbI . 6(C6H5 5CH22NH33)22PbBr4;4 (d) (C6H65CH5 2NH23)2PbI34.2 4

Figure 8. Laser power dependence of the photoluminescence (PL) intensity of the perovskites Figure 8. Laser power dependence of the photoluminescence (PL) intensity of the perovskites Figure(C 68.H 5CHLaser2NH power3)2PbBr 4 dependence(red curve) and of (C the6H5 CHphotolumin2NH3)2PbI4 escence(blue curve). (PL) intensity of the perovskites (C H CH NH ) PbBr (red curve) and (C H CH NH ) PbI (blue curve). (C6H5CH22NH3)32PbBr2 4 (red4 curve) and (C6H65CH5 2NH2 3)2PbI3 2 4 (blue4 curve). 3.5. The Laser Power Effect on the PL Lifetime 3.5. The Laser Power Effect on the PL Lifetime 3.5. The Laser Power Effect on the PL Lifetime Figure 9a,b shows the PL lifetime of the perovskites (C6H5CH2NH3)2PbBr4 and Figure9a,b shows the PL lifetime of the perovskites (C 6H5CH2NH3)2PbBr4 and Figure(C6H5CH 29a,bNH3 )2PbIshows4. PL lifetimethe PLof the lifetime perovskite of (C 6Hthe5CH 2NHperovskites3)2PbBr4 decreases (C6H5 fromCH2NH 0.753 )ns2PbBr to 0.504 and (C6H5nsCH when2NH laser3)2PbI power4. increases PL lifetime from of10 mW the to perovskite 100 mW. In (C contrast,6H5CH 2PLNH lifetime3)2PbBr of4 thedecreases perovskite from (C6H5CH2NH3)2PbI4. PL lifetime of the perovskite (C6H5CH2NH3)2PbBr4 decreases from 0.75 ns to 0.50 0.75 ns(C to6H5 0.50CH2NH ns when3)2PbI4 decreases laser power from increases 0.67 ns tofrom 0.46 ns 10 when mW laser to 100 power mW. increases In contrast, fromPL 10 mW lifetime to 100 of the ns when laser power increases from 10 mW to 100 mW. In contrast, PL lifetime of the perovskite perovskitemW. Figure (C6H5 CH9c 2showsNH3) 2PbIthe 4laserdecreases power from dependence 0.67 ns toof 0.46the nsPL when intensity laser of power the perovskites increases from (C6H5CH2NH3)2PbI4 decreases from 0.67 ns to 0.46 ns when laser power increases from 10 mW to 100 10 mW(C to6H5 100CH2 mW.NH3)2 FigurePbBr4 (red9c shows curve) theandlaser (C6H5 powerCH2NH dependence3)2PbI4 (blue curve). of the It PL can intensity be observed of the that perovskites the PL mW. Figure 9c shows the laser power dependence of the PL intensity of the perovskites lifetime decreases with increasing laser power. PL lifetime of the perovskite (C6H5CH2NH3)2PbBr4 is (C6H5CH2NH3)2PbBr4 (red curve) and (C6H5CH2NH3)2PbI4 (blue curve). It can be observed that the (C6H5largerCH2NH than3)2 PbBrthat of4 (redthe perovskite curve) and (C 6(CH56CHH5CH2NH2NH3)2PbI3)42 PbIat the4 (blue same curve). laser powe It canr. The be observedinternal quantum that the PL PL lifetime decreases with increasing laser power. PL lifetime of the perovskite (C6H5CH2NH3)2PbBr4 lifetimeefficiency decreases [6] is with calculated increasing as: laser power. PL lifetime of the perovskite (C6H5CH2NH3)2PbBr4 is is larger than that of the perovskite (C6H5CH2NH3)2PbI4 at the same laser power. The internal larger than that of the perovskite (C6H5CH2NH3)2PbI4 at the same laser power. The internal quantum quantum efﬁciency [6] is calculated as: efficiency [6] is calculated as:

Q = τnr/(τ r+τnr) (6) Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 11

Appl. Sci. 2018, 8, 2286 = 8 of 11 Q τnr/(τ r+τ nr ) (6) where τr and τnr are the radiative and nonradiative decay lifetime respectively. According to Equation where τr and τnr are the radiative and nonradiative decay lifetime respectively. According to (6), the internal quantum efficiency of the perovskite (C6H5CH2NH3)2PbBr4 varies from 81.48% to Equation (6), the internal quantum efﬁciency of the perovskite (C6H5CH2NH3)2PbBr4 varies from 90.91%, and the internal quantum efficiency of the perovskite (C6H5CH2NH3)2PbI4 varies from 83.55% 81.48% to 90.91%, and the internal quantum efﬁciency of the perovskite (C6H5CH2NH3)2PbI4 varies tofrom 91.28%. 83.55% to 91.28%.

Figure 9. The PL lifetime of the perovskites (a) (C6H5CH2NH3)2PbBr4;(b) (C6H5CH2NH3)2PbI4; Figure 9. The PL lifetime of the perovskites (a) (C6H5CH2NH3)2PbBr4; (b) (C6H5CH2NH3)2PbI4; (c) laser (c) laser power dependence of the PL lifetime of the perovskites (C6H5CH2NH3)2PbBr4 (red curve) power dependence of the PL lifetime of the perovskites (C6H5CH2NH3)2PbBr4 (red curve) and and (C6H5CH2NH3)2PbI4 (blue curve). (C6H5CH2NH3)2PbI4 (blue curve). The ﬂuence-dependence of PL intensity and lifetime with laser power can be explained with the energy level diagram [24–26], which is shown in Figure 10. The perovskite molecules will absorb two Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 11

Appl. Sci. 2018, 8, 2286 9 of 11 The fluence-dependence of PL intensity and lifetime with laser power can be explained with the energy level diagram [24–26], which is shown in Figure 10. The perovskite molecules will absorb two photonsphotons after after the the 800 800 nm nm laser laser excitation, excitation, promoting promoting the electron the electron transfer transfer from VB from to CB. VB Some to CB. electrons Some inelectrons CB will returnin CB will to VB return and produceto VB and exciton produce emission. exciton Meanwhile, emission. Meanwhile, some electrons some in electrons CB will move in CB towill ESS move by relaxation. to ESS by Therelaxation. relaxation The torelaxation ESS, or ESS to ESS, recombination, or ESS recombination, can possess can certain possess non-geminate certain non- recombinationgeminate recombination characteristics. characteristics. This emission This isemis a kindsion ofis non-geminatea kind of non-geminate recombination, recombination, which is thewhich intensity-dependent is the intensity-dependent two-photon two-photon absorption absorption cross-section cross-section [27]. When [27]. laser When power laser increases, power moreincreases, electrons more transfer electrons from transfer VB to from CB, andVB to move CB, toand ESS move by relaxationto ESS by relaxation after absorbing after absorbing two photons. two Asphotons. a consequence, As a consequence, the carrier the density carrier increases density andincreases the emission and the isemission dominated is dominated by non-geminate by non- recombination,geminate recombination, which results which in the results increase in the of increase PL intensity of PL and intensity decrease and of decrease lifetime. of lifetime.

FigureFigure 10. 10.Energy Energy level level diagram diagram of of the the perovskite perovskite molecule. molecule. VB, VB, valence valence band; band; CB, CB, conduction conduction band; band; ESS,ESS, electron electron surface surface states. states.

4.4. Conclusions Conclusions In this paper, we prepared the perovskites (C H CH NH ) PbBr and (C H CH NH ) PbI In this paper, we prepared the perovskites (C6 6H55CH22NH3)22PbBr44 and (C6H6 5CH5 2NH2 3)23PbI2 4 via4 via solvent evaporation. The perovskites (C H CH NH ) PbBr and (C H CH NH ) PbI were solvent evaporation. The perovskites (C6H65CH5 2NH2 3)2PbBr3 2 4 and4 (C6H56CH52NH23)2PbI3 42 were4 well wellcrystallized crystallized and oriented, and oriented, and exhibited and exhibited the layered the layered structures structures with alternating with alternating organic and organic inorganic and inorganiccomponents. components. The absorption The absorption peaks were peaks located were located at 392 at 392nm nm(3.16 (3.16 eV) eV) and and 516 516 nm nm (2.40(2.40 eV)eV) respectively. PL intensity of the perovskite (C H CH NH ) PbBr increased from 117 photon counts respectively. PL intensity of the perovskite (C6 6H5 5CH2 2NH3 32)2PbBr44 increased from 117 photon counts toto 2195 2195 photon photon counts, counts, PLQY PLQY varied varied from from 81.48% 81.48% to to 90.91%, 90.91%, and and lifetime lifetime decreased decreased from from 0.75 0.75 ns ns to to 0.500.50 ns ns when when laser laser power power increased increased from from 10 10 mW mW to to 100 100 mW. mW. In In contrast, contrast, PL PL intensity intensity of of the the perovskite perovskite (C H CH NH ) PbI increased from 158 photon counts to 2771 photon counts, PLQY varied from (C66H55CH2NH33)2PbI2 4 increased4 from 158 photon counts to 2771 photon counts, PLQY varied from 83.55% 83.55%to 91.28%, to 91.28%, and lifetime and lifetime decreased decreased from 0.67 from ns to 0.67 0.46 ns ns to when 0.46 nslaser when power laser increased power increased from 10 mW from to 10100 mW mW. to 100The mW. fluence-dependence The fluence-dependence of PL intensity of PL intensity and lifetime and lifetime with laser with power laser power is mainly is mainly due to due the tonon-geminate the non-geminate recombination. recombination. The Thephotoluminesce photoluminescencence properties properties of of organic organic inorganicinorganic hybridhybrid perovskitesperovskites can can be be tuned tuned to ato wide a wide range, range, which which has importanthas important applications applications in the in optoelectronic the optoelectronic field. Authorfield. Contributions: S.L. carried out the experiment and wrote the manuscript; F.L. helped in revising the manuscript; X.H., L.X., F.Y. and Y.L. helped in doing the experiment and analyzing the data. Author Contributions: S.L. carried out the experiment and wrote the manuscript; F.L. helped in revising the Funding: This research was supported by the National Natural Science Foundation of China (Grant No. 11204222), themanuscript; Natural Science X.H., L.X., Foundation F.Y. and ofY.L. Hubei helped Province, in doing China the experiment (Grant No. and 2013CFB316, analyzing the Grant data. No. 2014CFB793), theAcknowledgments: Postgraduate Education S.L. would Innovation like to Fund, thank Wuhan for the Institute support of Technologyfrom Laboratory (No. CX2016106). of Optical Information and Acknowledgments:Technology, WuhanS.L. Institute would of like Technology; to thank for Wuhan the support National from Laboratory Laboratory for of Op Opticaltoelectronics, Information Huazhong and Technology,University of Wuhan Science Institute and Technology. of Technology; Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology. Funding: This research was supported by the National Natural Science Foundation of China (Grant No. Conflicts of Interest: The authors declare no conflict of interest. 11204222), the Natural Science Foundation of Hubei Province, China (Grant No. 2013CFB316, Grant No. 2014CFB793), the Postgraduate Education Innovation Fund, Wuhan Institute of Technology (No. CX2016106).

Conflicts of Interest: The authors declare no conflicts of interest. Appl. Sci. 2018, 8, 2286 10 of 11

References

1. Egger, D.A.; Rappe, A.M.; Kronik, L. Hybrid Organic-Inorganic Perovskites on the Move. Acc. Chem. Res. 2016, 49, 573–581. [CrossRef][PubMed] 2. Li, W.; Wang, Z.M.; Deschler, F.; Gao, S.; Friend, R.H.; Cheetham, A.K. Chemically diverse and multifunctional hybrid, organic–inorganic perovskites. Nat. Rev. Mater. 2017, 2, 16099. [CrossRef] 3. Kim, T.W.; Uchida, S.; Matsushita, T.; Cojocaru, L.; Jono, R.; Kimura, K.; Matsubara, D.; Shirai, M.; Ito, K.; Matsumoto, H.; et al. Self-Organized Superlattice and Phase Coexistence inside Thin Film Organometal Halide Perovskite. Adv. Mater. 2018, 30, 1705230. [CrossRef][PubMed] 4. Gui, D.; Ji, L.J.; Muhammad, A.; Li, W.; Cai, W.Z.; Li, Y.C.; Li, X.D.; Wu, X.; Lu, P.X. Jahn-Teller Effect on Framework Flexibility of Hybrid Organic-inorganic Perovskites. Phys. Chem. Lett. 2018, 9, 751–7554. [CrossRef][PubMed] 5. Dong, L.Y.; Sun, S.J.; Deng, Z.Y.; Li, W.; Wei, F.X.; Qi, Y.J.; Li, Y.C.; Li, X.D.; Lu, P.X.; Ramamurty, U. Elastic

properties and thermal expansion of lead-free halide double perovskite Cs2AgBiBr6. Comput. Mater. Sci. 2018, 141, 49–58. [CrossRef] 6. Kawano, N.; Koshimizu, M.; Sun, Y.; Yahaha, N.; Fujimoto, Y.; Yanagida, T.; Asai, K. Effects of Organic Moieties on Luminescence Properties of Organic–Inorganic Layered Perovskite-Type Compounds. J. Phys. Chem. C 2014, 118, 9101–9106. [CrossRef] 7. Tanaka, K.; Kondo, T. Bandgap and exciton binding energies in lead-iodide-based natural quantum-well crystals. Sci. Technol. Adv. Mater. 2003, 4, 599–604. [CrossRef] 8. Zhang, S.J.; Audebert, P.; Wei, Y.; Choueiry, A.A.; Lanty, G.; Bréhier, A.; Galmiche, L.; Clavier, G.; Boissière, C.; Lauret, J.S.; Deleporte, E. Preparations and Characterizations of Luminescent Two Dimensional Organic-inorganic Perovskite Semiconductors. Materials 2010, 3, 3385–3406. [CrossRef] 9. Khadka, D.B.; Shirai, Y.; Yanagida, M.; Masuda, T.; Miyano, K. Enhancement in efﬁciency and optoelectronic quality of perovskite thin ﬁlms annealed in MACl vapor. Sustain. Energy Fuels 2017, 1, 755–766. [CrossRef]

10. Xu, L.T.; Li, F.; Wei, L.; Zhou, J.X.; Liu, S. Design of Surface Plasmon Nanolaser Based on MoS2. Appl. Sci. 2018, 8, 2110. [CrossRef] 11. Xu, L.T.; Li, F.; Liu, S.; Yao, F.Q.; Liu, Y.H. Low Threshold Plasmonic Nanolaser Based on Graphene. Appl. Sci. 2018, 8, 2186. [CrossRef] 12. Fang, H.H.; Raissa, R.; Aguye, M.A.; Adjokatse, S.; Blake, G.R.; Even, J.; Loi, M.A. Photophysics of Organic–Inorganic Hybrid Lead Iodide Perovskite Single Crystals. Adv. Funct. Mater. 2015, 25, 2346. [CrossRef] 13. Qin, X.; Dong, H.L.; Hu, W.P. Green light-emitting diode from bromine based organic inorganic halide perovskite. Sci. China Mater. 2015, 58, 186–191. [CrossRef] 14. Khadka, D.B.; Shirai, Y.; Yanagida, M.; Ryan, J.W.; Miyano, K. Exploring the effects of interfacial carrier transport layers on device performance and optoelectronic properties of planar perovskite solar cells. J. Mater. Chem. C 2017, 5, 8819–8827. [CrossRef] 15. Bandiello, E.; Avila, J.; Gil-Escrig, L.; Tekelenburg, E.; Sessolo, M.; Bolink, H.J. Inﬂuence of mobile ions on the electroluminescence characteristics of methylammonium lead iodide perovskite diodes. J. Mater. Chem. A. 2016, 4, 18614–18620. [CrossRef] 16. Stoumpos, C.C.; Malliakas, C.D.; Peters, J.A.; Liu, Z.F.; Sebastian, M.; Im, J.; Chasapis, T.C.; Wibowo, A.C.;

Chung, D.Y.; Freeman, A.J.; et al. Crystal Growth of the Perovskite Semiconductor CsPbBr3: A New Material for High-Energy Radiation Detection. Cryst. Growth Des. 2013, 13, 2722–2727. [CrossRef] 17. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-lightsensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050–6051. [CrossRef][PubMed] 18. Zheng, Y.Y.; Deng, H.T.; Wan, J.; Li, C.R. Bandgap energy tuning and photoelectrical properties of self-assembly quantum well structure in organic-inorganic hybrid perovskites. Acta Phys. Sin. 2011, 60, 646–652. 19. Stoumpos, C.C.; Malliakas, C.D.; Kanatzidis, M.G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019–9038. [CrossRef][PubMed] 20. Luo, D.; Yang, W.; Wang, Z.; Sadhanala, A.; Hu, Q.; Su, R. Enhanced photovoltage for inverted planar heterojunction perovskite solar cells. Science 2018, 360, 1442–1446. [CrossRef][PubMed] Appl. Sci. 2018, 8, 2286 11 of 11

21. Droseros, N.; Longo, G.; Brauer, J.C.; Sessolo, M.; Bolink, H.J.; Banerji, N. Origin of the Enhanced

Photoluminescence Quantum Yield in MAPbBr3 Perovskite with Reduced Crystal Size. ACS Energy Lett. 2018, 3, 1458–1466. [CrossRef] 22. Dou, S.Y.; Yan, L.T.; Liu, Y.C.; Du, G.; Zhou, P. Preparation and performance of organic–inorganic halide perovskites. J. Mater. Sci. 2013, 24, 4862–4867. [CrossRef] 23. Liao, W.Q.; Zhang, Y.; Hu, C.L.; Mao, J.G.; Ye, H.Y.; Li, P.F.; Huang, S.D.; Xiong, R.G. A lead-halide perovskite molecular ferroelectric semiconductor. Nat. Commun. 2015, 6, 7338. [CrossRef][PubMed] 24. Li, F.; He, Z.C.; Li, M.Y.; Zhang, J.P.; Han, J.B.; Lu, P.X. Photoinduced energy transfer in a CdTe quantum dot–copper phthalocyanine system via two-photon excitation. Mater. Lett. 2014, 132, 263–266. [CrossRef] 25. Li, M.Y.; Li, F.; He, Z.C.; Zhang, J.P.; Han, J.B.; Lu, P.X. Two-Photon-Excited Fluorescence Resonance Energy Transfer in an Aqueous System of CdTe Quantum Dots and Rhodamine B. J. Appl. Phys. 2014, 116, 233106. [CrossRef] 26. Li, F.; He, Z.C.; Li, M.Y.; Lu, P.X. Three-Photon Absorption of Copper Phthalocyanine Solution by Femtosecond Z-scan Technique. Mater. Lett. 2013, 111, 81–84. [CrossRef] 27. Liu, W.W.; Li, X.H.; Song, Y.L.; Zhang, C.; Han, X.B.; Long, H.; Wang, B.; Wang, K.; Lu, P.X. Cooperative Enhancement of Two-Photon-Absorption-Induced Photoluminescence from a 2D Perovskite-Microsphere Hybrid Dielectric Structure. Adv. Funct. Mater. 2018, 28, 1707550. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).