Invalidity of Band-Gap Engineering Concept for Bi3+ Heterovalent

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Invalidity of Band Gap Engineering Concept for Bi3+ Heterovalent Doping in CsPbBr3 Halide Perovskite. Olga A. Lozhkina, Anna A. Murashkina, Vladimir V. Shilovskikh, Yury Kapitonov, Vladimir K Ryabchuk, Alexei V Emeline, and Tsutomu Miyasaka J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02178 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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The Journal of Physical Chemistry Letters

Invalidity of Band Gap Engineering Concept for Bi3+ Heterovalent Doping in CsPbBr3 Halide Perovskite. Olga A. Lozhkina1, Anna A. Murashkina1, Vladimir V. Shilovskikh1, Yury V. Kapitonov1, Vladimir K. Ryabchuk1, Alexei V. Emeline1*, Tsutomu Miyasaka 2 1

Saint-Petersburg State University, ul. Ulyanovskaya 1, Saint-Petersburg, 198504, Russia.

2

Toin University of Yokohama, and Peccell Technologies, Inc., 1614 Kurogane-cho, Aoba,

Yokohama, Kanagawa 225-8502, Japan.

Corresponding Author * [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Heterovalent CsPbBr3 doping with Bi results in a significant red shift of the optical absorption of both single crystal and powdered samples. The results of low-temperature (3,6 K) photoluminescence studies of perovskite single crystals indicate the position of the excitonic luminescence peak remains unaffected by Bi doping that in turn, infers that the band gap of Bi doped perovskite is not changed as well. The position and state density distribution of the valence band and Fermi level of single crystal perovskites were determined by another direct method of ultraviolet photoelectron spectroscopy. The obtained results show that Bi3+ doping causes no changes in the valence band structure but a raise in the Fermi level by 0.6 eV. The summary of the obtained results directly demonstrate that the concept of the band gap engineering in Bi3+ doped CsPbBr3 halide perovskite is not valid.

TOC GRAPHICS


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Lead-halide perovskites are of great current interest as high-performance semiconductors for cheap and efficient photonic and optoelectronic devices1-5 due to a combination of remarkable optical and charge transport properties and soft chemical synthesis6-7. A number of recent studies reports on the tuning of the optical and electronic characteristics of the halide perovskites by means of their doping with certain heterovalent cations8-13. In this respect, the cations In+, Tl+, Sb3+, and Bi3+ are particularly interesting since their valent electronic shell is isoelectronic to lead and ns2 lone-pair is essential for defect tolerance14. These ions being in a highly symmetric chemical environment form narrow-band compounds15 which is also important for the halide perovskite optical applications. Bi3+ draws particular interest as a dopant since it is stable and its ionic radius is close to the one of lead (1.03 and 1.19 Å, respectively16). One of the key issues addressed by doping halide perovskites with bismuth is a possibility to shift the absorption threshold toward longer wavelengths,13,17-21 that was assumed as a result of the optical bandgap narrowing13,

17-18

. However, such an interpretation of absorption shift has been recently

challenged20-21. In particular, the combination of spectroscopic ellipsometry and transmission spectroscopy used in the optical studies on bismuth doped MAPbBr3 single crystals21 suggested that no change of the band gap was caused by bismuth doping. The red absorption shift was associated with increase of the defect concentration and as a consequence, an increased density of states in the band gap, induced by dopant incorporation. Recent photoluminescence study suggested a similar conclusion21 regarding the insensibility of the perovskite band gap to Bi3+ incorporation. However, the photoluminescence studies were performed at room temperature and therefore, the luminescence bands were rather broad that resulted in poor resolution of the intrinsic excitonic luminescence and luminescence associated


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with the recombination on shallow traps. In addition, the theoretical modeling22 also showed that bismuth doping does not change band gap energy, but rises the Fermi level position. Thus, two very different concepts have been proposed to explain the observed red shift in absorption spectra of Bi doped perovskites comparing to undoped material, namely alteration of the intrinsic absorption vs. appearance of strong extrinsic absorption. In other words, the keyquestion is whether Bi doping of perovskites results in band gap engineering or not? Obviously, the establishing of which concept is a true one, has a crucial importance for various applications of perovskites in the fields of photovoltaics, optoelectronics and photonics. In this paper we demonstrate that bismuth doping does not affect either band gap energy or valence band structure of CsPbBr3 by two direct experimental methods: low temperature photoluminescence (PL) and angle resolved photoelectron spectroscopy (ARPES). Both CsPbBr3 single crystals and powders doped with bismuth have been studied. The detailed description of synthesis, characterization and sample preparation is presented in Supplementary materials (see synthesis and characterization section, Figures S1 and S2). The concentrations of bismuth incorporated in the crystal structure were determined by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy for single-crystal samples and by atomic emission spectroscopy for powders (Figure S3). The color of the samples varies from yellowish-orange to dark red with increase of the bismuth concentration as shown in TOC. The change in color corresponds to a significant red shift of the absorption (see Figure 1), in accordance to earlier reports13.


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Figure 1. Spectra of absorbance of pristine and Вi3+ doped powdered samples derived from diffuse reflectance spectra measured at ambient temperature.

The apparent band gap energies of pristine and doped powdered samples estimated from modified Tauc plot demonstrate red shift up to 140 meV (see Supplementary materials, estimation of apparent bang gap energy section, Figure S4). However, it is wise to note that application of the Tauc plot is based on several assumptions which are typically ignored. The detailed analysis of the Tauc plot transformation given in Supplementary materials demonstrates that the estimated band gap energies are rather false and the observed red shift in absorbance can originate from higher concentration of the defect states compensating the excess of charge brought by Bi3+ dopant states. Figure S5 in Supplementary materials demonstrates that the decrease of the temperature from 300 K to 100 K does not have a significant effect on the diffuse reflectance spectra of doped and undoped perovskites. At the same time, the low temperature (3.6 K) PL spectra demonstrate that the exciton peak positions of doped samples have only a slight red shift, which can be caused by inelastic scattering of excitons by crystal lattice (Figure 2). The half width at half maximum (HWHM) of PL peaks at low temperatures is close to the non-radiative relaxation rate value. HWHM value


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increases sequentially with the bismuth concentration and can be partly attributed to microstresses caused by doping21, while PL intensity decreases significantly (Figure 2 inset) primarily due to non-radiative recombination at traps. Thus, the low temperature PL of CsPbBr3 single crystals directly indicate that band gap remains unchanged with bismuth doping.

Figure 2. Normalized low temperature (3,6 K) PL spectra of CsPbBr3 single crystals doped with bismuth. The dependence of the exciton peak intensity on Bi concentration is plotted in the inset.

The results of ultraviolet photoelectron spectroscopy (UPS) measurements presented in Figure 3, undoubtedly indicate that the bismuth doping does not change the position and energy distribution of the valence band states in perovskite. At the same time, Bi3+ doping causes a raise in the Fermi level by 0.6 eV in accordance with theoretical modeling22. Note that results reported earlier17 also show the invariance of the VBM position of MAPbCl3 upon bismuth doping. The observed shift of the Fermi level toward the higher energy indicates the formation of the defect states located closer to the bottom of the perovskite conduction band and correlates well with the increase of conductivity and alteration of the conductivity type from p-type to n-type observed elsewhere.13


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Figure 3. UPS spectra and positions of the Fermi level (EF) for pristine and Bi3+doped CsPbBr3 single crystals.

The effect of bismuth doping is more pronounced in low-temperature PL of powder samples, since in this case the probability of exciton recombination on defects is much higher due to the exciton mean free path restriction. As a result PL spectra of perovskite powders demonstrate the resolved peak red shifted by 25 meV, which can be assign to recombination on the defects (see Figure S6). The PL results for powdered perovskite indicate that Вi3+ doping completely quenches the excitonic luminescence and luminescence associated with intrinsic defects decays with increase of Bi concentration. No new spectral features in PL spectra are induced by Bi3+ doping. This observation is in accordance with theoretical simulation indicating that Bi3+ incorporation stabilizes negatively charged cationic vacancies13,24. In conclusion, the UPS data clearly indicate that Bi3+ doping of perovskite does not induce any changes in the structure and the energy position of the valence band. At the same time, the shift of the Fermi level induced by Bi3+ inclusion in perovskite structure toward higher energy suggests the formation of the electron type defect states localized closer to the bottom of the conduction band that particularly, can explain the alteration of conductivity type from p-type to n-type observed elswhere.13 Invariability of the exciton luminescence peak position infers that


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the optical band gap of the perovskite is not changed by Bi3+ doping. Therefore, the observed significant red shift of optical absorption of Bi3+ doped CsPbBr3 perovskite is caused by an increase in the density of localized states associated with compensating defects and it is not assosiated with band gap narrowing. Bismuth doping leads to a broadening and weak red shift of the exciton luminescence peak apparently originated of the increasing micro-stresses and a decay of the PL intensity is caused by a nonradiative recombination through the defect states. In other words, the results of the present studies directly demonstrate that the concept of the band gap engineering in Bi3+ doped CsPbBr3 halide perovskite is not valid.

ACKNOWLEDGMENT The present study was performed within the Project “Establishment of the Laboratory “Photoactive Nanocomposite Materials” No. 14.Z50.31.0016 supported by a Mega-grant of the Government of the Russian Federation. A.A. Murashkina, A.V. Emeline and Yu.V. Kapitonov are grateful to RFBR 17-53-50083 JF_a grant which supported the studies concerning the synthesis and PL measurements. We are thankful very much to Dr. V.N. Kuznetsov for performing low temperature measurements of diffuse reflectance spectra. The work was carried out using equipment of the resource centers “Nanophotonics”, “Geomodel”, “PMSI”, “CAM” and “XRDS” of SPbSU.

Supporting Information Available: Description of the material included sample preparation and characterization of the obtained samples (including SEM, WDS, EBSD, XPS, UPS, XRD and ICP-AES), experimental setup description, diffuse reflection spectroscopy data analysis and low temperature photoluminescence data of powder samples.


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Invalidity of Band-Gap Engineering Concept for Bi3+ Heterovalent

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