Nanocrystalline CsPbBr3 thin films: a grain boundary opto-electronic study

phys. stat. sol. (c) 2, No. 1, 306–309 (2005) / DOI 10.1002/pssc.200460171

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nanocrystalline CsPbBr3 thin films: a grain boundary opto-

electronic study

G. Conte*1

, F. Somma1, and M. Nikl

3

1 INFM and Electronic Engineering Dept., Univ. “Roma Tre”, Via Vasca Navale, 84 – 00146 Rome, Italy 2 INFM and Dept. of Physics, Univ. “Roma Tre”, Via Vasca Navale, 84 – 00146 Rome, Italy 3 Institute of Physics, AS CR, Cukrovarnicka 10, 16253 Prague, Czech Republic

Received 11 July 2004, revised 7 September 2004, accepted 17 September 2004 Published online 20 January 2005

PACS 72.20.Ee, 72.40.+w, 73.61.Le, 73.63.Bd

CsPbBr3 thin films with nanocrystalline morphology were studied by using optoelectronic techniques to infer the grain boundary region in respect of the crystallite’s interior performance. Co-evaporation of puri-fied powders or crushed Bridgman single crystals were used to deposit materials and compare recombina-tion mechanism and dielectric relaxation processes within them. Nanosecond photoconduction decay was observed on both materials as well as activated hopping transport. An asymmetric Debye-like peak was evaluated from impedance spectroscopy with a FWHM value, which remains constant for 1.25±0.02 deca-des, addressing the presence of a tight conductivity relaxation times distribution. The evaluated acti-vation energy, equal to 0.72±0.05 eV, similar to that estimated by DC measurements, is well smaller then that expected for an intrinsic material with exciton absorption at 2.36 eV. A simple model based on Voigt’s elements was used to model the electronic characteristics of these nanostructured materials, to discuss observed results and define the role played by grain boundaries.


1 Introduction Very interesting luminescence properties have been reported for ternary com-pounds of

Cs:Pb:Br [1–3]. Two stable compositions, CsPbBr3 and Cs4PbBr6 can be deposited in the thin film form

with the relative percentage as a function of deposition conditions. Both com-pounds have demonstrated

to organize in aggregates with nanometer sized dimension, showing exciton luminescence, as evidenced

by optical spectroscopy and X-Ray diffraction [1–4]. In such systems, the grain boundary (GB) and sur-

face related defects play an important role in the charge carrier trapping and release processes since a

majority of atoms reside within few atomic layers from the boundary. The reason for this is the fact that

surface states are usually involved in non-radiative processes and small particles have a large surface-to-

volume ratio. Furthermore, small binding energy of the exciton state in CsPbX3 semiconducting com-

pounds [5] leads to its early thermal disintegration and electron-hole pairs are dominating at room tem-

perature. In a previous study on nanocrystalline CsPbX3 (X=Cl, Br and I) [6] it has been evidenced a

metal-like transport at very low T, probably associated with a very small excess of Pb within the material.

This paper is aimed to characterize nanoaggregates of CsPbBr3 from an optoelectronic point of view and

to investigate the role played by GBs in determining the material performance.

2 Experimental Samples used in this study have been realized starting from purified CsBr and PbBr2 powders by using two differently driven molybdenum crucibles and hereafter indicated as “co-evaporated” samples, whereas those deposited from crushed Bridgman single crystals have been used as

* Corresponding author: e-mail: [email protected]; Phone: +39 06 5517 7268; Fax: +39 06 5579 078

phys. stat. sol. (c) 2, No. 1 (2005) / www.pss-c.com 307


comparison specimens and will be referred as “bulk-evaporated” films. Opti-cal glass heated at 100 °C has been used as a substrate. Deposited films were 0.3–1.5 µm thick, typically. AFM and XRD evi-denced a material constituted by nanometer sized crystallites with a needle-like structure elongated in the growing direction. Photoelectrical measurements were made in a planar geometry by using silver elec-trodes. Obtained curves were ohmic and sym-metric while the Arrhenius plot leaded to an average acti-vation energy equal to 0.85±0.05 eV. It is worth to note that this value is lower then the expected 1.18 eV of an intrinsic material. Im-pedance spectroscopy measurements were carried out by using a So-lartron SI-1250 equipped with a SI-1296 dielectric interface. All measurements were made under vac-uum in the range 300–490 K with an accuracy of ±1K. A Neweks PSX100 ArF laser (4.5 mJ per pulse) was used to shine light on the material and to test the performance in respect of the beam intensity varia-tions between 1 and 100 µJ/cm2. The photocurrent signal was recorded as the voltage across the 50 Ω input impedance of a Le Croy Wavepro 960 oscilloscope.

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10-3

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CsPbBr3

Co-evaporate d

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,T)

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Angular Frequency, ω (rad/s) Fig. 1 Normalized transient photoconductivity response of a co-evaporated film (square) compared with the intrinsic line shape (line) as measured by a Hamamatsu detector. Inset: photoconductivity peak values versus the density of energy of a 193 nm UV ArF laser beam. The slope is equal to 0.5. Fig. 2 AC conductivity real component of a co-evaporated thin film at different temperatures.

3 Results and discussion Spectral photoconductivity experiments have reported a very steep Urbach tail, with a characteristic energy equal to 50 meV on both materials: by co-evaporation and by crushed CsPbBr3 Bridgman crystals. This result is in good agreement with observation of the exciton peak ab-sorption also at room temperature and with luminescence emission. From this point of view, the nanocrystal’s interior appears to be perfect and the luminescence and steady state photoconductive per-formance only determined by thermal equilibrium between the exciton and electron-hole states con-fined within the nanocrystalline domain. On the other hand, the electronic behaviour of the grain bound-ary phase is not evidenced by these observa-tions. The transient photoconductive response of a co-evaporated film to an UV beam is report-ed in Fig. 1. As it is apparent, the material responds with a rise time less then 0.5 ns as demon-strated by the comparison with the Hamamatsu vacuum tube used to monitor the excimer laser intrinsic pulse shape. The asymmetry of CsPbBr3 and reference detector re-sponse is typical of the experiment and associated to the speed of response of the laser gas to the pump-ing lamp. It is noteworthy the fact that the recombination time of photogenerated carriers is around 2.5 ns, practically equal to the response of the vacuum tube. Moreover, if we look at the inset, showing the peak voltage versus the beam intensity, we observe a power law response with exponent equal to 0.5. This result is in good agreement with Rose’s theory [7] indicating that bi-molecular recombination

308 G. Conte et al.: Nanocrystalline CsPbBr3 thin films


mechanisms (band-to-band) are active. These observations fairly agree with previously reported lumi-nescence decay time measurements [1,2] and address the quality of the grain interior and the control of optoelectronic performance.

These results, in any case, risen a number of questions, the most important among others is: al-though the material appears to be a perfect intrinsic semiconductor, why the activation energy of conduc-tivity is smaller then the expected Eg/2 value?

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Fig. 3 Imaginary component of Impedance, Z”, and Electric Modulus (M”=ωC0Z’) vs the frequency at different temperatures evaluated by data in figure 2. C0 is the empty cell capacitance. Fig. 4 Arrhenius plot of resistance and capacitance values of the grain (G) and grain boundaries (GB) as evaluated by using a two Voigt’s element circuit model.

To answer this question we analysed AC electrical data obtained from impedance and modulus spectros-

copy as a function of T. To shine light on the problem we used various formalisms: σ∗(ω), ε∗(ω) and

Μ∗(ω) related each to the other by well known expression [8], being ε0 the free space permittivity and

where σ, ε and M are the complex conductivity, the material permittivity and the electric modulus

(M*=1/ε*), respectively. It is clear that these quantities are alternative and interchangeable representations

of the same dielectric relaxation data, and their use is mat-ter of convenience to put in evidence different

characteristics of the material. The real compo-nent of the AC conductivity of a co-evaporated sample is

reported in Fig. 2 as a function of temperature. The observed trend at each T resemble the response of a

leaking insulator. Data exhibit a frequency independence at low frequencies, a plateau, identified with

electronic con-duction along percolating paths at the grain boundaries, because of the correspondence

with the activation energy estimated by DC analysis. At high frequency, another term adds varying ap-

proximately in a power law fashion where the exponent is a function of temperature and was found to be

decreased with increase in T. This trend is common to many other systems and typical of polycrystalline

materials. It is worth to note that data in Fig. 2 can be reduced to a master curve as addressed by hopping

models [9]. Figure 3 reports the imaginary components of impedance and electric modulus evaluated from data in the previous figure. At low T, a single Z” peak is observed at low frequency whereas, in-creasing T, a second one becomes evident; both peaks are temperature dependent. A different behaviour is observed for the Debye-like peak in the imaginary modulus representation. A single asymmetric peak is found, and the peak position shifts toward higher frequencies with increasing temperatures. Moreover, the peak frequency of M” maximum is at higher values then the corresponding peak in Z”.

Aimed at understanding this different behaviour, we model the material by two different phases: the former insulating and the latter resistive, and appointing to each of them a Voigt’s RC element. De-tailed consideration of the equation for Z” and M” shows that for a circuit con-sisting of a single parallel

phys. stat. sol. (c) 2, No. 1 (2005) / www.pss-c.com 309


RC element, Z” and M” have the same functional form, giving rise to a Debye peak when plotted against frequency, but have different weighting factors in that the Z” peak is scaled according to R whereas the M” peak is scaled according to C-1. Hence, Z” and M” spectra highlight different aspects of the same data. Since Z” plots are dominated by the grain resistance Rg, if Rg >> Rgb, they provide useful informa-tion about the bulk but little or no information about the grain boundaries response. Therefore, also M” plots are dominated by the bulk response since Cb << Cgb is expected and provide little or no information about the grain boundary. On these basis, we can now discuss data reported in figure 3.

The discussion stems on the assumption that GBs are more conductive then CsPbBr3 grains. At low T, the Z” maximum - corresponding to one-half the grain resistance - reduces in value and shifts in frequency with the temperature increasing. The Arrhenius plot of calculated values (see Fig. 4) leads to an activation energy equal to 0.72±0.05 eV, similar to that found by DC analysis. This activation is asso-ciated to the average energy barrier that charge carriers must overcome to be collected to the external electrodes. Conversely, the grain capacitance stays constant at all T and equal to 2 pF. This result address the possibility to have completely de-pleted nanocrystalline grains. The pre-exponential factor of the conductivity relaxation time (τσ=RgCg) Arrhenius plot leads to a phonon frequency, νph, equal to 4x1014 Hz in good agree-ment with values reported in literature for similar materials. The GB impedance peak, located at smaller frequencies, starts to become evident at high T, only. Evaluated resistance increases with T increasing confirming the suspected metal-like nature of this contribution. The estimated GB conductivity relaxation time, stays quite constant around 1.5 ms. M” Debye-like peaks show a FWHM that stays constant around 1.22-1.28 decades. These values must be compared with the intrinsic line width of a single relaxation time Debye’s peak (i.e. 1.14) and evidence a very tight distribution, confirm-ing that most part of the charge carriers generation and transport takes place in regions depleted of de-fects.

In conclusion, we can affirm that the conductive GB does not influence the material opto-electronic behaviour but only plays the role of a distributed metal contact. The fast photo-transient re-sponse is indicative of collection distances determined by the nanometer sized mate-rial. The GB’s ca-pacitance is thought to be associated to the defective shell inducing a band-bending around 0.6 eV. Measured activation energies correspond to one-half the average dis-tance from the conduction band. This indication is sustained by the evidence of mastering AC conductivity data as proposed by jump relaxation and hopping models.

References

[1] M. Nikl, K. Nitsch, E. Mihokova, K. Polak, P. Fabeni, G. P. Pazzi, M. Gurioli, S. Santucci, R. Phani, A. Scac-co, and F. Somma, Physica E 4, 323 (1999).

[2] M. Nikl, K. Nitsch, E. Mihokova, K. Polak, P. Fabeni, G.P. Pazzi, M. Gurioli, R. Phani, S. Santucci, A. Scacco, and F. Somma, Radiat. Eff. Defects Solids 150, 341 (1999).

[3] M. Nikl, E. Mihokova, K. Nitsch, F. Somma, C. Giampaolo, G. P. Pazzi, P. Fabeni, and S. Zazubovich, Chem. Phys. Lett. 306, 280 (1999).

[4] F. Somma, P. Aloe, S. Lo Mastro, C. Giampaolo, M. Nikl, K. Nitsch, P. Fabeni, and G.P. Pazzi, Radiat. Eff.

Defects Solids 156, 103 (2001).

[5] I.P. Pashuk, N.S. Pydzirailo, and M.G. Macko, Sov. Phys. Solid State 23, 1263 (1981).

[6] G. Conte, F. Somma, and M. Nikl, Mater. Sci. Eng. C 19, 63 (2002).

[7] R. H. Bube, Photoelectronic Properties of Semiconductors, Cambridge University (1992).

[8] J. Ross Macdonald, Impedance Spectroscopy (John Wiley, New York, 1987). [9] T. B. Schroeder and J. C. Dyre, Phys. Rev. Lett. 84, 310 (2000).

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Nanocrystalline CsPbBr3 thin films: a grain boundary opto-electronic study