Elsevier

Thin Solid Films

Volume 638, 30 September 2017, Pages 277-281
Thin Solid Films

Dielectric relaxation of CH3NH3PbI3 thin film

https://doi.org/10.1016/j.tsf.2017.07.070Get rights and content

Highlights

  • A distribution of relaxation time is observed in CH3NH3PbI3 thin film.

  • The distribution of relaxation time is modeled by Cole-Cole method.

  • The frequency dependent conductivity spectra are found to follow the power law.

Abstract

A film of CH3NH3PbI3 of thickness 1200 nm is fabricated on fluorine doped tin oxide coated glass. The crystal structure of the material has been determined by X-ray diffraction method which shows that CH3NH3PbI3 is crystallized in the tetragonal phase with space group I4/mcm. The dielectric relaxation of the CH3NH3PbI3 thin film is investigated in the frequency range from 42 Hz to 4.8 MHz. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. Owing to the distribution of relaxation time, the relaxation mechanism in CH3NH3PbI3 is found to follow the Cole-Cole model. A comparison of the impedance with the electric modulus suggests the presence of localized relaxation process. The complex impedance plane plot is analyzed by a parallel electrical circuit consisting of a resistance and a capacitance. The frequency dependent conductivity spectra are found to follow the power law.

Introduction

CH3NH3PbI3, an organo-metallic perovskite material, is a potential candidate for photovoltaic research due to its high power conversion efficiency. At present, the power conversion efficiency of this material has just exceeded 20% [1]. Beyond the photovoltaic application, halide perovskite semiconductors are also expected to be used in other solution processed functional semiconductor devices such as photo detectors [2], [3], [4], [5], light emitting diodes [6], [7], lasers [8], [9], thin film transistors [10] and other optoelectronic or microelectronic devices. Due to their various technological applications, there is a growing research interest in these materials. The most studied hybrid perovskite CH3NH3PbI3 has been shown to possess suitable band gap to absorb sunlight [11], [12], large light absorption coefficient [13] and long charge carrier diffusion length [14], [15], [16], [17], [18] which are required for high photovoltaic performance. In addition to these interesting characteristics, CH3NH3PbI3 also exhibits some peculiar properties such as photo-induced giant dielectric constant [19], extremely slow photoconductivity response [20] and the existence of ferroelectric domains [21]. The ferroelectric domains produce internal electric field, which separates the photo-generated charge carriers by dielectric screening. This has been used to explain the slow charge carrier recombination [22], [23] and long charge carrier diffusion lengths [17], [18] in the halide perovskite. Thus the dielectric property of CH3NH3PbI3 plays an important role in its high photovoltaic performance. The dielectric relaxation (a conduction process) is a mechanism to understand the basic physical processes in organic-inorganic perovskite solar cells [24].

In the present work, we have investigated the dielectric relaxation mechanism of CH3NH3PbI3 thin film in the frequency range from 42 Hz to 4.8 MHz. It is to be mentioned here that the impedance spectroscopy is a very powerful tool for studying the dynamical properties (capacitance, conductance, permittivity and loss factor) of the materials.

Section snippets

Experimental details

For the preparation of CH3NH3PbI3 thin film, we first prepared 40 wt% precursor solution by dissolving PbI2 (Aldrich) and methyl ammonium iodide CH3NH3I (Aldrich) in N-N-di-methyl-formamide (Sigma-Aldrich). The fluorine doped tin oxide (FTO) coated glass plate (Aldrich, TEC 7) of dimension 3 cm × 2 cm was cleaned in an ultrasonic bath in a successive medium of soap water, millipore water, acetone and ethanol. The prepared precursor solution was spun on FTO coated glass plate at 2000 rpm for 35 s. The

Structural study

Fig. 1(a) represents the XRD pattern of CH3NH3PbI3 thin film. All the peaks are indexed according to the standard XRD pattern of tetragonal phase with space group I4/mcm [25], [26], [27]. Fig. 1(b) represents the surface morphology of CH3NH3PbI3 thin film. The thickness of the film has been measured from the cross-sectional SEM image and is found to be 1200 nm as shown in Fig. 1(c).

Dielectric relaxation

Generally, the dielectric relaxation of a material can be analyzed in terms of the dipolar and the conductivity

Conclusions

The CH3NH3PbI3 thin film is fabricated on FTO coated glass by spin coating method. The XRD data show that CH3NH3PbI3 is synthesized in the tetragonal phase with I4/mcm crystal symmetry. The dielectric relaxation of the CH3NH3PbI3 thin film is investigated in the frequency range from 42 Hz to 4.8 MHz. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The Nyquist plot (Z*) shows that the relaxation mechanism in CH3NH3PbI3 film is

Acknowledgement

Md S. Sheikh and A. P. Sakhya would like to acknowledge Department of Science and Technology(DST), Government of India for providing financial support in the form of DST Inspire Fellowship (IF150220 and IF110596, respectively).

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