Mobile Charge-Induced Fluorescence Intermittency in

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Mobile Charge Induced Fluorescence Intermittency in Methylammonium Lead Bromide Perovskite Xiaoming Wen, Anita Ho-Baillie, Shujuan Huang, Rui Sheng, Sheng Chen, HsienChen Ko, and Martin A. Green Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b01405 • Publication Date (Web): 18 Jun 2015 Downloaded from http://pubs.acs.org on June 20, 2015

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Mobile Charge Induced Fluorescence Intermittency in Methylammonium Lead Bromide Perovskite Xiaoming Wen*1, Anita Ho-Baillie,1 Shujuan Huang,1 Rui Sheng,1 Sheng Chen,1 Hsien-chen Ko,2 and Martin A. Green1 1: Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia 2: Institute of Physics, Academia Sinica, Nankang, Taipei, 105, Taiwan

Abstract Organic-inorganic halide perovskite has emerged as a very promising material for solar cells due to its excellent photovoltaic enabling properties resulting in rapid increase in device efficiency over the last 3 years. Extensive knowledge and in-depth physical understanding in the excited state carrier dynamics are urgently required. Here we investigate the fluorescence intermittency (also known as blinking) in vapour-assisted fabricated CH3NH3PbBr3 perovskite. The evident fluorescence blinking is observed in a dense CH3NH3PbBr3 perovskite film which is composed of nano-particles in close contact with each other. In the case of an isolated nanoparticle no fluorescence blinking is observed. The ON probability of fluorescence is dependent on the excitation intensity and exhibits a similar power rule to semiconductor quantum dots at higher excitation intensity. As the vapour-assisted fabricated CH3NH3PbBr3 perovskite film is a cluster of nanoparticles forming a dense film, it facilitates mobile charge migration between the nanoparticles and charge accumulation at the surface or at the boundary of the nanoparticles. This leads to enhanced Auger-like non-radiative recombination contributing to the fluorescence intermittency observed. This finding provides unique insight into the charge accumulation and migration, and thus is of crucial importance for device design and improvement.

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Keyword: fluorescence intermittency, perovskite; nanoparticle, mobile charge; methylammonium lead bromide; blinking


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The emergence of organic-inorganic halide perovskite solar cells has attracted great attention over the past few years.1-3 Significant progress has been achieved such as the most recent demonstration of independently certified efficiency of 20.1%.4 During the last few years, the electronic and optical properties of organic-inorganic halide perovskites have been extensively studied by various techniques.5-9 The high performance of perovskite solar cells was usually attributed to the high and broad-spectra absorption, slow carrier recombination, long diffusion length of electron and holes.5 In addition, the superior performance of perovskite was partly ascribed to their exceptional defects which do not create a detrimental deep defect state that acts as traps and recombination centres for carriers in other solar cells.5,

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In particular, the photoexcited carrier dynamics has been the

research focus because the understanding of the photoexcited carrier dynamics is of critical importance for improving perovskite based solar cells.6,

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In addition to the application in

photovoltaics, organometal halide perovskite has also been shown to be promising as luminescent and lasing materials due to their high luminescence quantum efficiency and excellent carrier transport.20, 21 Although many investigations have been carried out,22, 23 the photo-physical properties of the excited carriers are still far from fully being understood. Fluorescence intermittency, also referred as to blinking, randomly switching between states of high (ON) and low (OFF) emissions, is a universal property of molecular emitters found in dyes, polymers, biological molecules as well as artificial nanostructures, such as nanocrystal, quantum dots, carbon nanotubes and nanowires.24-26 Fluorescence blinking has been extensively investigated in semiconductor nanoparticles and organic molecules, providing unique insight into their photoexcited carrier dynamics.24,

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Based on confocal microscopy and super-resolution

techniques, single molecule spectroscopy provides a powerful tool to investigate the fluorescence behaviour and carrier dynamics in a single nanoparticle. Here we investigate the fluorescence intermittency in a CH3NH3PbBr3 perovskite film and isolated nanoparticles. We found that blinking is present in the CH3NH3PbBr3 perovskite film whereby nanoparticles are in close contact with one another. On the other hand, blinking is not present in isolated CH3NH3PbBr3 nanoparticle. We are able to describe the probability density of the fluorescence intermittency in the CH3NH3PbBr3 perovskite film by truncated power-law dependence. The exponent mon of the probability density found is consistent to those of other semiconductor nanoparticles where blinking has been observed. The time correlated single photon counting (TCSPC) carried out under various excitation densities indicate Auger recombination from charge accumulation exemplified by the mobile charge migration in particular under high excitation is responsible for the blinking.


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The samples of CH3NH3PbBr3 films and isolated nanoparticles used in this study were fabricated by vapour-assisted deposition33 on a glass substrate in the absence of any inter-layer or quencher. The isolated nanoparticles were fabricated by the same procedures except using diluted PbBr2 solution. The size of nanoparticles can be estimated to be 150-250 nm by scanning electron microscopy (SEM). The details of the fabrication are described in the supplementary information (SI). The x-ray diffraction (XRD) pattern shows that the CH3NH3PbBr3 perovskite film is well crystallized, see Figure S1. The SEM and optical images for the CH3NH3PbBr3 perovskite film and isolated nanoparticles are shown in Figure S2 and S3, respectively. In particular, a nano-granular structure can be clearly seen in optical image in Figure S2 (d). A strong fluorescence PL peak can be observed at 536 nm, see Figure S4, consistent to other observations.34-36 The laser illumination induced degradation can be excluded because the PL measurement can be performed repeatedly. At the highest excitation intensity, the PL spectra keep identically before and after the illumination; the PL intensity can be repeated after keeping the sample in the dark for a few minutes. The fluorescence was observed as function of time in both CH3NH3PbBr3 film and isolated nanoparticles in Microtime-200 confocal microscopy system under an excitation of 470 nm and detected through a bandpass filter at 536 nm. Figure 1 shows the time traces of the fluorescence intensity of an isolated nano-particle and a single point of the CH3NH3PbBr3 film respectively in Figure 1(a) and (b); and their corresponding fluorescence microscopy images respectively in Figure 1 (c) and (d). As evident in the time trace using the same microscopy system under a continuous excitation of 470 nm, blinking can be observed. It is interesting to note that the fluorescence blinking was only observed in the CH3NH3PbBr3 film. In contrast, no fluorescence blinking is observed in isolated nanoparticles.

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Figure 1. Time traces of the fluorescence intensity of (a) an isolated nanoparticle at different excitation intensities; and (b) a single point of the CH3NH3PbBr3 film. Fluorescence microscopy images of (c) isolated nanoparticles and (d) the CH3NH3PbBr3 film.

The fluorescence occurrence of ON and OFF periods in nanoparticles has been usually attributed to the presence of an additional charge, which results in fluorescence quenching by nonradiative Auger recombination.27, 28 The charge induced blinking, ON and OFF event probability density, can be described by a truncated power-law dependence:29, 30

P(t ) ∝ t m exp(−t / τ )

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where m is exponent and τ is truncation time (or saturation time). The exponent m of some semiconductor nanoparticles studied represents characteristics of blinking. The blinking in semiconductor single nanoparticle has been intensively studied and exponent m has been shown different from the ideal -1.5 due to dispersive diffusion correlation times, and related to temperature, intensity and the size and shape of nanoparticles.37-41 In this work, a 470nm laser is used for the excitation source with a NA1.4 oil objective. Figure 2 shows the PL time traces of the CH3NH3PbBr3 perovskite film, under low (80 mW/cm2) and high (2400 mW/cm2) excitation intensities in Figure 2 (a) and (b) respectively. It should be noted that the occurrence of fluorescence ON event (whereby the fluorescence intensity is above 8 counts) is higher under low excitation. Using equation 1, its probability density is obtained, see Figure 2 (c) and the exponent mon is found to be dependent on excitation intensity, see Figure 2 (d). At lower excitation intensity, more ON events can be confirmed; therefore the ON probability is relatively larger with a


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small magnitude of the exponent mon. At very low excitation density (< 60 mW/cm2), the PL intensity is very weak that no blinking is observed. With increasing excitation intensity, the ON event decreases and an increased mon is obtained, from ~ -0.4 to ~ -1.5. Further increasing the excitation intensity, the mon does not change evidently, staying at ~ -1.5. 40

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Note the magnitude of the exponent mon ~-1.5 at higher excitation for the perovskite film is similar to those reported for some semiconductor nanoparticles; such as those by Shimizu et al. where by

mon = -1.5 for CdSe; mon = -1.6 for CdTe and mon = -1.4 for CdSe on gold substrate. reported mon = -1.6 for CdSe and mon = -2.0 for InP.

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Effort has been made to understand the blinking mechanism in other material systems. The fluorescence intermittency has been associated with Auger recombination assisted quenching,27, 28, 37, 42

although the detailed mechanism is still under debate.46,

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intermittency in nanoparticles can be due to (i) significant difference between electron and hole mobilities, or (ii) the presence of an additional charge.27, 28, 41 In our case it is more likely to be the latter due to the similar electron and hole mobilities found for the organic-inorganic perovskites and thus their similar probabilities to be trapped by defect and/or surface states.5,

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usually observed in isolated nanoparticles in conventional semiconductors. In these semiconductors, the minimum of conduction band is mostly contributed by s orbitals of cations and anions; the maximum of the valance band is mostly contributed by p orbital of anions. High energy level s orbital are more delocalized than low energy level p orbital and thus the lowest conduction band is more dispersive than the highest valance band. Consequently, the effective mass of electrons is much smaller than that of holes; and the mobility of electrons is much larger than that of holes. Therefore, electrons have much higher possibility to be trapped, which results in the charged nanoparticles. Dramatically differently, the electronic structure of CH3NH3PbBr3 is inverted due to existence of lone-pair Pb s electrons compared to conventional p-s semiconductors.48, 49 The lower conduction band of the perovskite is more dispersive than the upper valance band in p-s semiconductors. Due to strong s-p coupling around the maximum of valance band, the upper valance band of perovskite is dispersive. The effective mass of electrons and holes is balanced, which results in ambipolar conductivity and balanced mobilities of electrons and holes.48-52 When formation of the condensed CH3NH3PbBr3 film, a large number of nanoparticles are accumulated together, which results in free electrons and/or holes that can easily drift among the nanoparticles, referred as to mobile charges.53, 54 These charges can accumulate on the surface of nanoparticles, also as grain boundaries, and results in enhanced Auger nonradiative recombination, that is OFF state. In this case, the ON and OFF states will be relevant to the density of mobile charges and mobility among the nanoparticles. It has been shown that the electron and hole of CH3NH3PbBr3 perovskite exhibit similar mobilities and the diffusion length is as long as micron,33 which facilitates the charge drift between the nanoparticles. The presence of mobile charges and their migration result in fluorescence quenching by nonradiative Auger recombination, 27, 28 contributing to the OFF event in blinking. To ascertain this, we investigate the excitation density dependent carrier dynamics which can be generally described by the three terms

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in

dn = −C1n − C2 n 2 − C3n 3 dt

depending on the dominant recombination mechanism. The


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three terms correspond to the Shockley-Read-Hall (SRH) recombination via sub-gap trap states, free electron-hole recombination (bimolecular) and Auger recombination, respectively. The TCSPC is done with ten consecutive measurements (each with an integration time of 60 seconds), see Figures 3 (a) and (b). To minimise the effect from inhomogeneous surface from the sample, an air objective of NA0.4 was used. Under very low excitation density (< 60 mW/cm2), the dominant mechanism of carrier relaxation is defect trapping. A bi-exponential fitting y = A1 exp(-t / τ1) + A2 exp(-t / τ 2 ) determines the trapping and the recombination times to be 1.9 and 62 ns at 30 mW/cm2, respectively, see Figure S5 and Table 1.The latter being ascribed to electron-hole recombination.55,

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In addition, there is no evident

variation in PL intensity at each consecutive measurement (illumination), Figure 3 (a). In contrast, under high excitation intensity, the PL exhibits significantly faster decay, and the lifetimes vary at each consecutive measurement (excitation), see Figure 3 (b). This suggests increased contributions from higher order recombination such as bimolecular free electron-hole and/or Auger recombination.

Table 1 Lifetimes of CH3NH3PbBr3 perovskite film under various excitation intensities from dualexponential fitting of results in Figures 3 (a) and 3 (c). Excitation intensity (mW/cm2)

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With increasing excitation intensity, the traps are filled and gradually saturate,57 increasing the bimolecular carrier recombination component in the PL trace. With further increase in the excitation intensity, the PL decay becomes faster due to the increased contribution of Auger recombination,20 see Figures 3 (b) and (c), resulting in shorter lifetimes, see in Table 1, extracted from bi-exponential fitting. It should be emphasized that the PL traces at high excitation intensity are evidently different from those at low excitation, due to the different decay mechanism.

Manser et al. attributed the faster decay excitation density dependent carrier dynamics to the predominantly enhanced free electron-hole (bimolecular) recombination; evidenced by the strong linearity of the maximum change in inverse absorbance (ΔA-1) with time.9 Figure 3 (c) and (d) show


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the PL decays and their reciprocal under various excitation intensities. Evidently, the decay curves deviate significantly from linearity with increasing excitation (carrier) intensity. In other words, the dominant mechanism resulting in the fast decay in our case is Auger nonradiative recombination. Under consecutive laser illuminations, the PL exhibits further faster decay, due to charge accumulation at the surface of each nanoparticle that is in close contact with each other when within a continuous dense film. The charge migration and accumulation therefore induces Auger-like nonradiative behaviour which increases with illumination contributing to more frequent OFF event during blinking as observed in Figure 2 (c). Under low excitation, the density of the photogenerated mobile charges is relatively lower and thus the probability of the ON event is higher as observed in Figure 2. Fluorescence blinking is not present in isolated nanoparticle as fluorescence is continuous in these isolated nanoparticles under continuous illumination. This confirms that the CH3NH3PbBr3 has balanced mobilities between electrons and holes that would otherwise cause blinking in isolated nanoparticle. The blinking observed in this work is in the millisecond timescale, which is of a much slower dynamic process. This is very similar to the slow transient processes observed in other works in similar timescales which are attributed to the presence of mobile ions.58-61 The density and lifetime of the mobile charges would also increase upon increasing excitation intensity as their migration and accumulation at the surfaces and grain boundaries are enhanced further enhancing Auger-like behaviour. It is also worth noting that the isolated CH3NH3PbBr3 nanoparticles are different from a single photon source or semiconductor quantum dot62 exhibiting enhanced quantum confinement. Rather, the perovskite nanoparticles in this study are in the range of hundreds of nanometres. Photon antibunching measurements were carried out on both isolated CH3NH3PbBr3 nanoparticle and nanoparticle in CH3NH3PbBr3 film, as shown in Figures 4 for nanoparticle in film. The correlations observed demonstrate that neither the isolated nanoparticle nor the film (although obvious) shows single photon source property.


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Figure 4 Photon anti-bunching measurements of CH3NH3PbBr3 nanoparticle in film.

In summary, we have observed fluorescence intermittency present in CH3NH3PbBr3 perovskite film. The CH3NH3PbBr3 perovskite film is composed of a closely packed nanoparticles facilitating


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photoexcited charge migration between nanoparticles and accumulation at the surface of the nanoparticles which results in enhanced Auger-like nonradiative behaviour contributing to the fluorescence quenching (OFF state). The ON probability of fluorescence is dependent on the excitation intensity and exhibits a similar power rule to semiconductor quantum dots at higher excitation intensity. In contrast, fluorescence intermittency does not present in isolated nanoparticles due to similar effective masses of electrons and holes in CH3NH3PbBr3 perovskite and the absence of mobile charges. This finding provides unique insight into the charge accumulation and migration, and thus is of crucial importance for device design and improvement. Associated Content Support information Experimental section, SEM and optical images of CH3NH3PbBr3 perovskite film and isolated nanoparticles, PL and absorption spectra, video of blinking in CH3NH3PbBr3 perovskite film. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgement The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-based activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA).

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32. Stein, I. H.; Capone, S.; Smit, J. H.; Baumann, F.; Cordes, T.; Tinnefeld, P. Chemphyschem 2012, 13, 931-937. 33. Sheng, R.; Ho-Baillie, A.; Huang, S.; Chen, S.; Wen, X.; Hao, X.; Green, M. A. J Phys. Chem. C 2015, 119, 3545-3549. 34. Schmidt, L. C.; Pertegas, A.; Gonzalez-Carrero, S.; Malinkiewicz, O.; Agouram, S.; Minguez Espallargas, G.; Bolink, H. J.; Galian, R. E.; Perez-Prieto, J. J. Am. Chem. Soc. 2014, 136, 850853. 35. Zhang, M.; Yu, H.; Lyu, M.; Wang, Q.; Yun, J.-H.; Wang, L. Chem. Commun. 2014, 50, 1172711730. 36. Lewis, D. J.; O'Brien, P. Chem. Commun. 2014, 50, 6319-6321. 37. Yuan, C.; Yu, P.; Ko, H.; Huang, J.; Tang, J. ACS nano 2009, 3, 3051-3056. 38. Stefani, F.; Knoll, W.; Kreiter, M.; Zhong, X.; Han, M. Phys. Rev. B 2005, 72, 125304. 39. Wang, S.; Querner, C.; Emmons, T.; Drndic, M.; Crouch, C. H. J. Phys. Chem. B 2006, 110, 23221-23227. 40. Wang, S.; Querner, C.; Fischbein, M. D.; Willis, L.; Novikov, D. S.; Crouch, C. H.; Drndic, M. Nano Lett. 2008, 8, 4020-4026. 41. Ko, H.; Yuan, C.; Lin, S.; Tang, J. J Phys. Chem. C 2011, 115, 13977-13984. 42. Shimizu, K.; Neuhauser, R.; Leatherdale, C.; Empedocles, S.; Woo, W.; Bawendi, M. Phys. Rev. B 2001, 63, 205316. 43. Shimizu, K.; Woo, W.; Fisher, B.; Eisler, H.; Bawendi, M. Phys. Rev. Lett. 2002, 89, 117401. 44. Kuno, M.; Fromm, D.; Hamann, H.; Gallagher, A.; Nesbitt, D. J. Chem. Phys. 2001, 115, 10281040. 45. Kuno, M.; Fromm, D.; Johnson, S.; Gallagher, A.; Nesbitt, D. Phys. Rev. B 2003, 67, 125304. 46. Zhao, J.; Nair, G.; Fisher, B. R.; Bawendi, M. G. Phys. Rev. Lett. 2010, 104, 157403. 47. Rosen, S.; Schwartz, O.; Oron, D. Phys. Rev. Lett. 2010, 104, 157404. 48. Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Nano Lett. 2014, 14, 2584-2590. 49. Giorgi, G.; Fujisawa, J.-I.; Segawa, H.; Yamashita, K. J. Phys. Chem. C 2014, 118, 1217612183. 50. Giorgi, G.; Fujisawa, J.-I.; Segawa, H.; Yamashita, K. J. Phys. Chem. Lett. 2013, 4, 4213-4216. 51. Brivio, F.; Butler, K. T.; Walsh, A.; van Schilfgaarde, M. Phys. Rev. B 2014, 89, 155204. 52. Brivio, F.; Walker, A. B.; Walsh, A. Apl Materials 2013, 1, 042111. 53. Kim, H.-S.; Mora-Sero, I.; Gonzalez-Pedro, V.; Fabregat-Santiago, F.; Juarez-Perez, E. J.; Park, N.-G.; Bisquert, J. Nat. Comm. 2013, 4, 2242:4. 54. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Science 2013, 342, 341-344. 55. Wu, X.; Trinh, M. T.; Niesner, D.; Zhu, H.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X. J. Am. Chem. Soc. 2015, 137, 2089–2096. 56. Wetzelaer, G. J. A.; Scheepers, M.; Sempere, A. M.; Momblona, C.; Ávila, J.; Bolink, H. J. Adv. Mater. 2015, 27, 1837-1841. 57. Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. J. Phys. Chem. Lett. 2014, 5, 1300-1306. 58. Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S.; Nazeeruddin, M. K.; Grätzel, M. Eng. Environ. Sci. 2015, 8, 995-1004. 59. Unger, E.; Hoke, E.; Bailie, C.; Nguyen, W.; Bowring, A.; Heumüller, T.; Christoforo, M.; McGehee, M. Eng. Environ. Sci. 2014, 7, 3690-3698. 60. Zhao, Y.; Liang, C.; Zhang, H.; Li, D.; Tian, D.; Li, G.; Jing, X.; Zhang, W.; Xiao, W.; Liu, Q. Eng. Environ. Sci. 2015, 8, 1256-1260. 61. Zhang, Y.; Liu, M.; Eperon, G. E.; Leijtens, T. C.; McMeekin, D.; Saliba, M.; Zhang, W.; De Bastiani, M.; Petrozza, A.; Herz, L. M. Materials Horizons 2015, 2, 315-322.


Nano Letters

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62. Cui, J.; Beyler, A. P.; Bischof, T. S.; Wilson, M. W.; Bawendi, M. G. Chem. Soc. Rev. 2014, 43, 1287-1310.


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