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Cite this:Chem. Commun., 2014,

50, 11727

Composition-dependent photoluminescenceintensity and prolonged recombination lifetimeof perovskite CH3NH3PbBr3�xClx films†

Meng Zhang, Hua Yu, Miaoqiang Lyu, Qiong Wang, Jung-Ho Yun andLianzhou Wang*

Mixed halide perovskites CH3NH3PbBr3�xClx (x = 0.6–1.2) with different

compositions of halogens exhibit drastically changed optical properties.

In particular, the thin films prepared with these perovskites demon-

strate extraordinary photoluminescence emission intensities and

prolonged recombination lifetimes up to 446 ns, which are desirable

for light emitting and photovoltaic applications.

Organolead halide perovskite materials as light absorbers haveopened a new era for developing high efficiency solid state solarcells in the past few years.1–4 This group of perovskites hasa generic structure of APbX3 (A = cationic organic molecules;X = halogens), which can be made from abundant and low-coststarting compounds. Their large absorption coefficient and superiorcharge carrier mobility make them a cost-effective light absorbingmaterial for converting solar energy.5,6 To date, most of thesystematic studies have focused on the light absorbing proper-ties for solar cell application while less attention has been paidto some other optical properties of these perovskites for applica-tion in light emitting devices and lasing.7,8

Among the family of organolead halide perovskites, MAPbBr3�xClxhas a larger band gap than iodine based perovskites such asMAPbI3�xClx and MAPbI3�xBrx (MA abbreviates for CH3NH3).It has been applied as a light absorber especially for high energyphotons in photovoltaic applications.9,10 The correspondingdevices have achieved remarkably high open circuit voltages,suggesting that MAPbBr3�xClx have great potential in competingwith high cost absorber materials in tandem cells or other deviceswith spectral splitting.9,10 However, the optical properties and therecombination behaviour of these perovskite materials exhibit alarge variation based on different compositions of halogens,which can significantly influence the performance of the devicesassembled with these perovskites.11,12 A systematic study ofthese materials is highly demanded to better understand their

properties, and thereby optimize their chemical composition tomeet the requirement of different applications. In this work,the photoluminescence (PL) properties and the recombinationbehaviour of the solution processed MAPbBr3�xClx films withdifferent ratios of Br to Cl were characterized. Surprisingly, itwas found that the photoluminescence emission intensities ofMAPbBr3�xClx perovskites were drastically changed by simplyadjusting the Cl/Br ratio. In particular, MAPbBr2.25Cl0.75 exhibitsa remarkably long recombination lifetime with relatively goodstability, which could be of high potential for optoelectronicapplications, such as light emitting and photovoltaic devices.

The thin films of perovskite MAPbBr3�xClx were prepared bythe spin-coating method. The coating solution of MAPbBr3�xClx

was prepared by dissolving various amounts of MABr (0.75 M to2 M) and PbCl2 (0.25 M) in N,N-dimethylformamide (DMF).In this way, all the perovskites are comparable since they sharethe same concentration of Pb, and the ratio of Br to Cl is variedfrom 3 : 2 to 4 : 1. In order to obtain a smooth perovskite film,a B1.5 mm thick mesoporous scaffold layer stacked by Al2O3

nanoparticles (o50 nm) was pre-coated onto a glass substratefor the subsequent perovskite coating and penetration. Afterspin coating at a certain speed and a short period of subsequentheating at 100 1C, a smooth and uniform layer of MAPbBr3�xClx

was formed on the glass substrate (see more details in the ESI†).Note that most of the perovskite materials diffused into theporous scaffold Al2O3 layer are not easy to visualize due to thestrong emissions in microscopy observations, which will bediscussed in the following section.

XRD patterns of the as-prepared films (Fig. 1) revealed thecharacteristics of the perovskite cubic phase in addition to abroad halo that arose from the glass substrate. The peaks of Al2O3

are barely identified possibly due to the small thickness of theAl2O3 layer on substrates (Fig. S1, ESI†). Without Cl substitution,diffraction peaks of MAPbBr3 at 15.041, 21.281, 30.241, 33.881,37.261, 43.251 and 45.981 can be indexed to the (100), (110), (200),(210), (112), (220) and (300) planes, respectively, corresponding toa cubic phase of perovskite. The corresponding lattice constant is5.91 Å, which is consistent with the previously reported value.1,13

Nanomaterials Centre, School of Chemical Engineering and AIBN, The University of

Queensland, Qld 4072, Australia. E-mail: l.wang@uq.edu.au; Fax: +61 7 3365218

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc04973j

Received 30th June 2014,Accepted 13th August 2014

DOI: 10.1039/c4cc04973j

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11728 | Chem. Commun., 2014, 50, 11727--11730 This journal is©The Royal Society of Chemistry 2014

Upon increasing the ratio of Cl in the mixed halides, the latticeconstant was gradually decreased to 5.79 Å for the Cl-richperovskite sample (MAPbBr1.8Cl1.2). As a result, all diffractionpeaks slightly shift towards a larger angle along with theincrease in the Cl substitution ratio.

The normalized light absorption spectra of the MAPbBr3�xClx

perovskites are shown in Fig. 2a (original data are given inFig. S2, ESI†). The absorption onset is blue-shifted after substitutingBr with Cl, which is associated with band gap widening upon Cldoping.7,14 Fig. 2b shows the steady PL emission spectra peaks of theMAPbBr3�xClx perovskites, the emission peaks of each sampleexhibit a small shift to the longer wavelength side of the absorptionpeak (Fig. 2a), which is known as the Stokes-shift.15 Consequently,the films show different emission colours from green to blue whenilluminated with UV light as shown in Fig. 2c.

Apart from the tuneable emission peak wavelength, therelative intensity is more interesting. Note that all the perovs-kite precursor solutions share the same concentration of Pband all the samples were prepared and tested under the sameconditions. The result revealed that MAPbBr3 exhibited a muchstronger PL emission intensity compared to MAPbI3, MAPbI2Brand MAPbIBr2 (Fig. S3, ESI†). However, a small amount of

substituted Cl can lead to a distinct difference in PL properties.When compared with MAPbBr3�xClx the PL emission peaks ofMAPbBr3 can be almost negligible (Fig. 2b). We can evenvisually observe the difference in a dark room. As shown inFig. 2c, the MAPbBr3�xClx films exhibit bright colours while theMAPbBr3 film is almost dark. This implies that MAPbBr3�xClx

is the most efficient PL emitter among methylammonium leadhalide perovskites. Particularly, MAPbBr2.4Cl0.6 exhibits thestrongest PL emission among MAPbBr3�xClx perovskites. Theextraordinary PL emission intensity of MAPbBr3�xClx may stemfrom their high semiconducting quality and low bulk defectdensity. It is surprising that such a slight variation in thecomposition can lead to such a remarkable difference in theirPL emission intensities. The exact reason is still not clear, whilewe speculate that it might be associated with the orientation andvibration restraint of the MA cation in the perovskite lattice asthe Cl substitution rate varied.16–18 The relevant study includingsimulation studies on the exact mechanism is underway. Basedon the above superior light emitting properties, MAPbBr3�xClx,especially with x = 0.6 to 0.75 is considered to be a promisingcandidate in optical device applications.

The recombination lifetime of MAPbBr3�xClx was determinedby measuring PL decay at the emission peak wavelengths (lpeak).The time-resolved PL decay curves of MAPbBr3�xClx with differentCl substitution ratios are shown in Fig. 3. The curves are fittedwith a triexponential function of time (t):19

F(t) =P

aie�t/ti, i = 1, 2, 3 (1)

where ai is a prefactor and ti is the time constant. The averagerecombination lifetime (tave) is estimated with the ai and ti

values from the fitted curve data according to the followingequation:19

tave =P

aiti2/P

aiti, i = 1, 2, 3 (2)

The results are shown in Table 1. MAPbBr3�xClx exhibitslonger tave than MAPbBr3, especially for MAPbBr2.4Cl0.6, the tave

can be extended up to 446 ns. It indicates an extraordinary lowbulk recombination rate compared to other methylammoniumlead halide perovskites. On the other hand, the corresponding

Fig. 1 XRD patterns of the MAPbBr3�xClx films deposited on Al2O3 coatedglass substrates with different Br to Cl ratios.

Fig. 2 (a) Normalized light absorption spectra of MAPbBr3�xClx; (b) Steady-state PL emission spectra of MAPbBr3�xClx with excitation wavelength at400 nm, inset: an enlarged region between 440 and 580 nm; (c) MAPbBr3�xClxfilms illuminated with UV light in a darkroom. The MAPbBr3 sample as marked isalmost dark under the regular contrast compared to other samples.

Fig. 3 Time-resolved PL decay detected at the peak wavelength ofemission for various MAPbX3 (X = Cl, Br, I) perovskites.

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tave of MAPbBr3�xClx perovskite decreases with a larger Clsubstitution ratio. It was previously proved that only a littlesubstitution of Cl can dramatically improve the charge transportproperties within the MAPbI3�xClx perovskite layer,11,20 and theresult here suggests that the Cl substitution also improvesthe charge transport in MAPbBr3�xClx. As it is well known, therecombination lifetime is a crucial factor which contributes tothe charge diffusion length of the light absorbing materials andhence influences their photovoltaic performance.11,12 Forinstance, in the case of MAPbI3 and MAPbI3�xClx, both perovs-kites have a similar light absorption range, while MAPbI3�xClx

has a longer recombination lifetime than MAPbI3, followed by alonger electron diffusion length. In this regard, MAPbI3�xClx

does not require a mesoporous electron transport layer(e.g. TiO2) unlike MAPbI3.11,21 The long electron diffusion lengthenables MAPbI3�xClx to work perfectly on insulating scaffoldsand even in bulk films.22,23 According to our results, the tave ofthe MAPbBr3�xClx can be much longer than that of MAPbI3�xClx,suggesting that the charge diffusion properties of MAPbBr3�xClx

are potentially even better than those of MAPbI3�xClx. Becausethese light absorbers do not rely on electron transporting layers,the optimised thickness of the light absorbing layer is alsodetermined by the electron diffusion length. A longer electrondiffusion length facilitates a larger optimised thickness of thelight absorbing layer, where charge carriers can be timelyextracted before the recombination.11 And a thicker light absorb-ing layer will eventually contribute to a more effective lightutilization by employing more absorbers. Therefore, consideringthe absorption range, the MAPbBr3�xClx perovskites (in particularx = 0.6–0.75) can be employed as a thick light absorbing layerfor high energy photons.

Another critical issue of the organolead halide perovskitematerials that may cause problems in practical applications isthe stability. They suffered from a moisture-related decomposi-tion because of the hygroscopic amine salts.13,16 In the stabilitytest, however, the MAPbBr3�xClx behaves less moisture-sensitive.The as prepared MAPbBr3�xClx films were left in the dark atroom temperature with exposure to ambient air for 30 days.As shown in Fig. 4a, no apparent XRD pattern change is observedafter exposure to air for 30 days, indicating a good stabilityof MAPbBr3�xClx. As we know, MAPbI3 will be decomposed inone or two days when exposed to ambient air (Fig. S4, ESI†), butwith a small fraction of Br or Cl substitution the stability canbe significantly improved.3,13 Hence, the MAPbBr3�xClx is alsoreasonable to be more stable among the family of the organoleadhalide perovskites.

Although no evidence of decomposition was found in terms ofthe evolution of XRD patterns, some changes can still be observed bytracking their PL emission peak intensities. The emission peaks ofMAPbBr3�xClx films become weaker with time, and after 30 days theMAPbBr2.25Cl0.75 retains B20% of its initial intensity. It is possiblydue to some surface deterioration that may promote the recombina-tion. Nevertheless, MAPbBr3�xClx still can be considered as a stableclass of perovskites. The mixture of halogens may play an importantrole in protecting the methylammonium group from being affectedby moisture. The relevant mechanism study regarding the stabilityissue of the mixed halide perovskite is underway.

In summary, methylammonium lead halide perovskitesMAPbBr3�xClx exhibiting strong PL emission intensity wereprepared. The PL emission intensity and average recombinationlifetime of MAPbBr3�xClx can be drastically modified by changingthe Br to Cl ratio. MAPbBr2.4Cl0.6 exhibits the longest recombina-tion lifetime of 446 ns, while MAPbBr2.25Cl0.75 exhibits the strongestPL emission intensity and is also more stable than other perovskitecounterparts. The findings in this study may lead to promisingapplications in light emitting and photovoltaic sectors.

Financial support from CRC-Polymers programs and ARC DPsand FT programs is acknowledged. This work was performed inpart at the Queensland node of the Australian National FabricationFacility. M.Z. acknowledges the support from the ChineseScholarship Council (CSC).

Notes and references1 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc.,

2009, 131, 6050–6051.2 H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro,

S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Gratzel andN. G. Park, Sci. Rep., 2012, 2, 591.

3 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami andH. J. Snaith, Science, 2012, 338, 643–647.

4 B. Cai, Y. D. Xing, Z. Yang, W. H. Zhang and J. S. Qiu, Energy Environ.Sci., 2013, 6, 1480–1485.

5 N. G. Park, J. Phys. Chem. Lett., 2013, 4, 2423–2429.6 H. J. Snaith, J. Phys. Chem. Lett., 2013, 4, 3623–3630.7 G. Xing, N. Mathews, S. S. Lim, N. Yantara, X. Liu, D. Sabba, M. Gratzel,

S. Mhaisalkar and T. C. Sum, Nat. Mater., 2014, 13, 476–480.8 F. Deschler, M. Price, S. Pathak, L. E. Klintberg, D.-D. Jarausch, R. Higler,

S. Huttner, T. Leijtens, S. D. Stranks, H. J. Snaith, M. Atature, R. T. Phillipsand R. H. Friend, J. Phys. Chem. Lett., 2014, 5, 1421–1426.

9 E. Edri, S. Kirmayer, D. Cahen and G. Hodes, J. Phys. Chem. Lett.,2013, 4, 897–902.

10 E. Edri, S. Kirmayer, M. Kulbak, G. Hodes and D. Cahen, J. Phys.Chem. Lett., 2014, 5, 429–433.

Table 1 Calculated average recombination lifetime of various MAPbX3

(X = Cl, Br, I) perovskites at the emission wavelength

Perovskite lpeak (nm) tave (ns)

MAPbBr3 530 100MAPbBr2.4Cl0.6 525 446MAPbBr2.25Cl0.75 516 349MAPbBr2Cl1 498 172MAPbBr1.8Cl1.2 484 116MAPbI3�xClx 778 44

Fig. 4 Stability of MAPbBr3�xClx films in 30 days: (a) The XRD patternof the fresh samples and the samples after exposure to ambient air;(b) The corresponding PL emission intensity decay at the peak wavelengthduring 30 days.

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11 S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. Alcocer,T. Leijtens, L. M. Herz, A. Petrozza and H. J. Snaith, Science, 2013,342, 341–344.

12 G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gratzel,S. Mhaisalkar and T. C. Sum, Science, 2013, 342, 344–347.

13 J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal and S. I. Seok, Nano Lett.,2013, 13, 1764–1769.

14 E. Mosconi, A. Amat, M. K. Nazeeruddin, M. Gratzel and F. De Angelis,J. Phys. Chem. C, 2013, 117, 13902–13913.

15 J. R. Albani, Principles and Applications of Fluorescence Spectroscopy,Springer London, Limited, 2007.

16 J. M. Frost, K. T. Butler, F. Brivio, C. H. Hendon, M. van Schilfgaardeand A. Walsh, Nano Lett., 2014, 14, 2584–2590.

17 P. Li, Q. Li, Y. Ma and R. Fang, J. Appl. Phys., 1996, 80, 490–493.18 D. Li, J. Zhang and M. Anpo, J. Lumin., 2006, 116, 73–78.19 D. R. James, Y.-S. Liu, P. De Mayo and W. R. Ware, Chem. Phys. Lett.,

1985, 120, 460–465.20 S. Colella, E. Mosconi, P. Fedeli, A. Listorti, F. Gazza, F. Orlandi,

P. Ferro, T. Besagni, A. Rizzo, G. Calestani, G. Gigli, F. De Angelisand R. Mosca, Chem. Mater., 2013, 25, 4613–4618.

21 E. Edri, S. Kirmayer, A. Henning, S. Mukhopadhyay, K. Gartsman,Y. Rosenwaks, G. Hodes and D. Cahen, Nano Lett., 2014, 14,1000–1004.

22 J. M. Ball, M. M. Lee, A. Hey and H. J. Snaith, Energy Environ. Sci.,2013, 6, 1739–1743.

23 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395–398.

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