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Bandgap Narrowing in Bi-Doped CHNHPbCl Perovskite Single Crystals and Thin Films

Zhan Zhang, Lixia Ren, Hong Yan, Shujin Guo, Shuanhu Wang, Min Wang, and Kexin Jin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06248 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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The Journal of Physical Chemistry

Bandgap Narrowing in Bi-Doped CH3NH3PbCl3 Perovskite Single Crystals and Thin Films Zhan Zhang, Lixia Ren, Hong Yan, Shujin Guo, Shuanhu Wang, Min Wang and Kexin Jin* Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Science, Northwestern Polytechnical University, Xi’an 710072, China

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ABSTRACT: Single crystals of heterovalent Bi-doped CH3NH3PbCl3 perovskite have been successfully grown through the inverse temperature crystallization method. Bandgap narrowing of 300 meV (a 55 nm red-shifting absorption edge) is obtained for the nominal 20% Bi-doped crystal with the host structure and the energy at the top of valence band invariable. It is observed that the contact between the Au electrodes and single crystals transforms from Ohmic to Schottky and the conductivity increases with increasing the Bi doping. By contrast, the Bi-doped CH3NH3PbCl3 thin films are also prepared and the similar bandgap narrowing is found although the narrowing degree is less than that in single crystals. This work has a comprehensive understanding of Bi-doped single crystals and thin films, providing further optoelectronic applications for these promising solution-processed hybrid perovskite semiconductor materials.

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INTRODUCTION In the past several years, the organic-inorganic hybrid perovskite materials (especially, CH3NH3PbI3) have attracted much attention due to their excellent properties, such as high ultraviolet-visible (UV-vis) light absorption, tunable band gap, small exciton binding energy (20-50 meV) and long electron-hole diffusion length (> 175 µm). 1-4 Thus, the power conversion efficiency (PCE) has skyrocketed from 3.8% to 22.1% in a remarkably short time when using these materials as the light absorption layer in solar cells.

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Nevertheless, only CH3NH3PbI3 and CH3NH3PbBr3 (or Br/I mixed) materials

can be used in photovoltaic applications because CH3NH3PbCl3 (hereafter, MAPbCl3) perovskite, having a wide bandgap, shows high optical absorption coefficient only in the UV range. As a consequence, current researches on MAPbCl3 perovskite are mainly focused on visible-blind UV photodetectors. 7-11 Recently, it has also been used as a hole transport layer for highly efficient organic light-emitting diodes due to its transparency in the visible region.

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In addition, another feature of MAPbCl3 is its excellent stability. It

has been proved that no PbCl2 impurity phase is found in solution-processed MAPbCl3 thin films after stored in ambient conditions over more than 5 months.

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On the other

hand, doping in perovskites is an efficient way to improve the crystallization quality, increase the stability, and tune the bandgap and conduction type. For examples, Cl– doping at the I– site can significantly improve the crystallization and morphology of MAPbI3 films.14, 15 Inorganic Cs+ doping at the CH3NH3+ (MA) site can increase the stability.16 And CH(NH2)2+ and nontoxic Sn2+ doping at the MA and toxic Pb2+ sites, 3



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respectively, can narrow the bandgap and broaden the absorption range.17, 18 Additionally, Mn, Hg and Co doping in perovskites have also been proved an effective way to produce new physical properties or excellent device performance.19-23 Up to now, most researches are concentrated on the isovalent doping and the heterovalent doping seems to also have a great influence on perovskites. For instance, Al3+-doped MAPbI3-based solar cell shows an efficiency of 19.1% with negligible hysteresis.24 Moreover, Sb3+ doping improves the PCE of solar cells and tunes the energy states.25 In particular, Bi3+ ion has the same 6s26p0 electronic configuration of Pb2+ and thereby the Bi-doping has more important significance in changing the origin properties. Furthermore, the Bi-doping is a remarkable process for the bandgap narrowing, type engineering, conductivity increasing, charge carrier lifetime growth, carrier concentration and mobility improving, and photostriction enhancing. 26-31 On the other side, the physical properties of films prepared by different methods are divergent due to the influence of composition, crystal structure, and morphology. Thus, the phase-pure bulk perovskites, especially single crystals, should be considered to learn more about the intrinsic properties.32 Originally, the single crystals are grown by the cooling-induced crystallization (CIS) method.33 Recently, a lot of new approaches, including antisolvent vapor-assisted crystallization (AVC), top-seeded solution-growth, and inverse temperature crystallization (ITC) method, have been proposed to prepare single crystals with several millimeter-sizes.4,10, 34–36 Among these, it has been proved that the ITC method is a very quick process to prepare high-quality single crystals of all 4


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MAPbX3 (X = Cl, Br, and I) perovskites by using specific solvents for different halogens (DMSO or DMF/DMSO for Cl, DMF for Br, and GBL for I). Moreover, this method has been modified to develop a very simple laser direct-write synthesis of hybrid perovskites.37 Considering the researches of bandgap engineering and preparation process in Bi-doped perovskite single crystals based on those above, we first report the preparation of a series of undoped and Bi-doped MAPbCl3 single crystals through ITC method. And then the structural, optical, electrical properties and electronic structure of these single crystals are investigated. Moreover, it is observed that the bandgap narrowing in thin films is not obvious compared with that in single crystals, which is studied in Bi-doped thin films with high Bi doping level (20%) for the first time as far as we know. By doping Bi element into the MAPbCl3 single crystal, a red-shifting absorption edge of about 480 nm was obtained, making it possible to absorb violet and blue light. This seems that these Bi-doped MAPbCl3 perovskites, combining the stability and partial visible light absorption, provide a novel way to fabricate materials which can be used as the light absorption layer in solar cells for photovoltaic applications.

EXPERIMENTAL SECTION Single Crystal Preparation. Single crystals were grown by the ITC method as reported previously with a slight modification.10 1.25 M solution of MAPbCl3 was prepared by dissolving equimolar amounts of CH3NH3Cl (MACl, ≥99.5%, Xi’an Polymer Light 5



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Technology Corp.) and PbCl2 (99%, Aladdin Reagent Ltd.) in 2 ml DMSO-DMF (5:3 by volume) at room temperature. For Bi-doped solutions, x% (molar ratio) PbCl2 is replaced by BiCl3 (99%, Aladdin Reagent Ltd.) with 0.5%, 1%, 5%, 10%, and 20%, respectively. The solutions were then filtered using PTFE filters with a 0.22-µm pore size and the filtrate was transferred into 5-ml vials. The vials were placed in an oil bath, at 50 ℃. Then temperature was gradually increased to 90 ℃ at the rate of 3-5 ℃·h-1 and maintained at 90 ℃ for 12 h. Thin Film Preparation. Thin Films were prepared using one-step spin coating method in a nitrogen-filled glovebox. The precursor solutions were prepared as described above. The solutions were added onto pre-cleaned glass substrates and spin-coated at 5000 rpm for 40 s, and then heated on a hotplate at 100 °C for 10 min. Measurement and Characterization. Power X-ray diffraction (XRD) was performed using a X-ray diffractometer (D/max 2500, Rigaku) with Cu-Kα radiation source (λ = 1.5406 Å). Flame Atomic Absorption Spectroscopy (FAAS, ZEEnit 700P, Analytikjena) was used to quantify the Bi content in crystals. The binding energy and valence band spectra (uncertainty: ±0.05 eV) were analyzed by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos) using Al Kα radiation (hv = 1486.6 eV). The UV-vis absorption spectrum was measured using an UV-vis spectrophotometer (U-3010, Hitachi). The surface morphology of thin films was characterized using a scanning electron microscope (SEM, Phenom Pro, Phenom-World). The elemental composition of thin films was analyzed by Energy-dispersive X-ray spectroscopy (EDS). Current-voltage (I-V) 6


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measurements were performed by using a Keithley 6487 Picoammeter/Voltage Source.

RESULTS AND DISCUSSION Although we replace PbCl2 by equimolar BiCl3 in solutions, the chemical formula of Bi-doped perovskite single crystals should be written as MAPb1-xBi2x/3Cl3 (nominal x = 0%, 0.5%, 1%, 5%, 10%, and 20%) in order to maintain the electric neutrality because the different valence state between Bi3+ and Pb2+ would induce the formation of vacancy. These Bi-doped perovskite single crystals, grown by the ITC method, have a cuboid shape and a typical size of ~ 5 × 5 mm2 although the growth of some crystals is limited by the wall of vial as shown in Figure 1. The MAPbCl3 single crystal is transparent and colorless. For Bi-doped single crystals, the color gradually changes from colorless to yellow with increasing the molar ratio from 0% to 20% in the precursor solution, looking like Br/Cl mixed perovskite single crystals.11, 32 As shown in Table 1, the accurate Bi molar ratio of Bi-doped single crystals is obtained via FAAS. It is clear that the Bi content in crystal increases with increasing that in the feed solution. And the actual Bi content of doped crystal is ~ 0.3% for a maximum nominal 20% Bi doping, indicating a very low Bi doping level. This result is in agreement with that in Bi-doped MAPbBr3 and MAPbI3 single crystals prepared by the ITC method.28 In addition, the actual Bi content in crystals gradually decreases when substituting the halogen from I to Br, then to Cl (Table S1). This may originate from the larger difference of Goldschmidt tolerance factor due to the smaller ionic radius of Cl–, 7



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thus hindering the incorporation of Bi dopant into crystals (Table S2). As previously mentioned, three methods have been used for the growth of Bi-doped single crystals. The Bi molar ratios in crystals prepared by ITC, CIC, and AVC methods are less than 0.5%, 1.6%, and 10% for the nominal 10% doping, respectively.28-30 This means that low temperature contributes to the incorporation of Bi dopant due to the slow growth in near-equilibrium conditions. High-resolution XPS spectra are also performed to check the existence of Bi content in the nominal 20% Bi-doped single crystal (Figure S1). Despite of the weak signal strength due to low Bi content in crystal, Bi 4f signal still appears. And the Pb 4f doublet, Cl 2p and C 1s binding energies are observed at 137.6 eV (Pb 4f7/2) and 142.4 eV (Pb 4f5/2), 197.0 eV (Cl 2p3/2) and 198.6 eV (Cl 2p1/2), and 283.85 eV, respectively. Power XRD pattern (Figure 2) indicates that all Bi-doped single crystals exhibit the cubic perovskite structure without any other phase, e.g. BiCl3, compared with the pure MAPbCl3 single crystal. Namely, the Bi3+ ions substitute the Pb2+ ions in the crystal structures. Further analysis by calculating lattice parameters shows the same value of a = 5.67 Å between the undoped and doped crystals due to the close ionic radii of Bi3+ (1.03 Å) and Pb2+ (1.19 Å) and small amount of the Bi content in crystals. The normalized UV-vis absorption spectra of undoped and Bi-doped single crystals are shown in Figure 3a. The pure MAPbCl3 crystal shows a sharp absorption edge at about 425 nm. For Bi-doped crystals, a significant red-shift of absorption edge is observed with increasing Bi content. For the nominal 10% Bi-doped crystal, the absorption edge is about 480 nm. Continuing to increase Bi content to 20%, only a little 8


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shift emerges. Eventually, a maximum ~ 55 nm red-shifting absorption edge is obtained for Bi-doped MAPbCl3 crystals. In fact, we have tried to dope the Bi content with higher level (~50%) under the same experimental conditions and no single crystals are crystallized out from the solution. Bi element is introduced by replacing PbCl2 by BiCl3 and so the increase of Bi content means a decrease of Pb, which limits the formation of supersaturated solutions of MAPbCl3 and hinders the crystallization of single crystals. Furthermore, our work is mainly about the tune of bandgap (absorption edge) and the change of absorption edge is very small as increasing the Bi content from 10% to 20%, as shown in Figure 3a. Therefore, a maximum nominal 20% Bi doping level is used in this work. According to the absorbance, Tauc plots of (αhν)2-hν are used to determine the optical bandgap. With the increase of Bi content, a minimum bandgap of 2.62 eV is obtained, which is ~ 300 meV narrower than that of pure MAPbCl3 (2.92 eV). The bandgap values of undoped and Bi-doped single crystals are shown in Figure 3b. When comparing the red-shifting absorption edge, the variation (280 meV) of our nominal 10% Bi-doped MAPbCl3 is similar to that in Bi-doped MAPbBr3 (280meV) and MAPbI3 (230 meV).28 This indicates that Bi doping has almost same effect on the bandgap of MAPbX3 single crystals regardless of the halogen element. Meanwhile, Bi doping by ITC method might cause an explosive growth of defects (Pb vacancies), also resulting in the bandgap narrowing. 30 In addition, the absorption edge of Bi-doped crystals is not as sharp as that in the undoped crystal, which is similar to that of Bi-doped MAPbBr3 single crystals, Sb-incorporated MAPbI3 and Br/I mixed thin films.

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This might originate from the

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transition from a direct band gap to an indirect band gap or the impurity states near the band edge, that is, an increase of Urbach energy. 25 As we know, it has been verified that XPS valence spectra measurement is more reliable and reproducible than that of UPS for the MAPbX3 systems. 39 Thus, it is used here to check the valance band maximum (VBM) energy relative to the Fermi energy (EF–EVBM where EF = 0 eV) of single crystals from the onset energy value of spectra (Figure S2). As shown in Figure 3b, the VBM values (EF–EVBM) are nearly invariable considering the measurement uncertainty (±0.05 eV). The conduction band minimum (CBM) values are determined by adding the calculated optical bandgap to the VBM. It can be seen clearly that the CBM obviously shifts downward relative to the Fermi energy. For MAPbCl3, the upper edge of the valence band is dominated by the Cl 3p orbital (with minor anti-bonding contributions from Pb 6s), whereas only the Pb 6p orbitals contribute to the lower edge of the conduction band.40 By combining the theoretical calculation of Bi-doped and MASnI3, we speculate that the energy level of the empty 6p orbitals of Bi3+ is lower than that of the empty 6p orbitals of Pb2+.31 And so the CBM shifts downward with increasing Bi content and the energy of the valence band is unchanged. In other words, Bi doping results in the narrowing of band gap without changing the energy at the top of valence band. This variation in the conduction band level is critical for optimization of new device configurations. Next, we study the electronic properties of single crystals by sandwiching them between two Au electrodes (the thickness of Au is ~ 100 nm by plasma sputtering). The 10


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I-V characteristics are measured by forming a metal-semiconductor-metal configuration under dark at room temperature. As shown in Figure 4, the I-V curve of pure MAPbCl3 shows an Ohmic contact, and the conductivity (σ) is about 2.85 × 10–8 Ω–1 cm–1 estimated by Ohm's law. With an increase in Bi content, the I-V curves gradually show a nonlinear behavior, especially for single crystals with ≥ 5% Bi doping, indicating the formation of a Schottky contact. By an approximate linearization, we extract the σ to be 6.57 × 10–7 Ω–1 cm–1 and 2.19 × 10–6 Ω–1 cm–1 for 0.5% and 1% Bi-doped crystals, respectively, showing an increased conductivity with the Bi content in the crystals. The transformation of I-V relationship from Ohmic contact to Schottky contact might result from the potential barrier induced by the change of band alignment due to the bandgap narrowing mentioned above. Furthermore, this is evidenced by the slight asymmetrical I-V behavior even with the symmetry electrode structure since the potential barrier of Schottky contact is very sensitive to interface states.8 In our I-V measurement, we sandwich the single crystal sample (~ 2 mm) between two Au electrodes (~ 100 nm). Hence, the total resistance is composed of two contact resistances and the resistance of single crystal sample, which together form a series circuit. The measured conductivity derives from this whole system. In this case, the resistance of single crystal plays the leading role rather the contact resistances. Although a typical semiconductor with high conductivity is easier to form Ohmic contact with metals, it generally depends on the work function (Fermi level) of semiconductor and metals, and the conduction type of semiconductor. In our work, Bi doping increases conductivity and changes the band alignment of single crystals due to 11



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the bandgap narrowing. Therefore, the transformation from Ohmic to Schottky contact is observed at the interface between single crystal and Au electrodes. And the Schottky contact at the interface has no great influence on hindering the charge transport of whole system. Inspired by the noteworthy bandgap narrowing in Bi-doped MAPbCl3 single crystals, we also prepare the Bi-doped MAPbCl3 thin films using one-step spin coating method to check whether the bandgap narrowing still exists. The surface morphologies of undoped and Bi-doped MAPbCl3 thin films have been studied by SEM (Figure S3). All films exhibit rough, non-uniform surfaces with a large crystal domain size (~ 10 µm). And most of domains are flat-lying cuboids with some four-point stars when the Bi content is lower than 10%. For 20% Bi-doped thin film, it seems that only four-point star-shape domains can be observed. Besides that, the size of domains diminishes gradually and the coverage ratio of films increases with the increase of Bi content. Different from the low actual Bi content in single crystals, the EDS results (Table S3) show that the Bi molar ratio in thin films is close to that in precursor solutions, which is in agreement with the previously reported Br/I ratios in the spin-coating prepared MAPb(Br1–xIx)3.41 During the growth of Bi-doped perovskite single crystals, Pb element would grow into the crystal preferentially in order to maintain the original perovskite structure of acceptor system. Although the ionic radii of Bi3+ and Pb2+ are close, the difference between them is still large enough to hinder the incorporation of Bi element into the single crystals due to the change of Goldschmidt tolerance factor, which is an important parameter to form 12


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perovskite structure. Besides, the different valence state between Bi3+ and Pb2+ may also affect the incorporation of Bi element. Hence, a low Bi doping level is obtained in crystals compared with that in solutions. It is not clear why the doping level is so low, but our results are comparable to that in Bi-doped MAPbBr3 and MAPbI3 single crystals through the ITC method, indicating the reliability of our results.28 For Bi-doped thin films, they were prepared by spin-coating the solutions, following annealing to remove the residual solvent. Both these two stages have no obvious influence on the ratio of Bi/Pb, and thus the Bi doping level in thin films is almost the same with that in solutions. The XRD patterns (Figure 5a) of films also indicate that no BiCl3 impurity phases can be detected although the actual Bi content is high enough. The films show a preferential orientation along (l00) diffraction peak. Unexpectedly, the color of 20% Bi-doped thin film is only slight yellow compared with the colorless MAPbCl3 thin film. Considering the actual high Bi content in thin films, this color change is unapparent when compared with the Bi-doped MAPbCl3 single crystals. This weak color change may result from the thin thickness of films. Therefore, the UV-vis absorption spectra (normalized at 400 nm) are presented to acquire more accurate and quantitative information (Figure 5b). The pure MAPbCl3 thin film shows an obvious absorption peak at ~ 400 nm, which is the characteristic of MAPbCl3 and MAPbBr3 thin films.13 With the increase of Bi content, this absorption peak gradually disappears and the absorption edge slightly red-shifts, whereas the original position of absorption peak is almost unchanged, which is different from the Bi-doped MAPbCl3 single crystals. Eventually, an about 20 nm (90 meV) 13



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red-shift of bandgap is observed. This red-shift is less than that in Bi-doped single crystals (300 meV), which may originate from the different doping mechanisms between single crystals and thin films. Therefore, further study should be focused on finding out the different specific mechanisms between doped perovskite single crystals and thin films, and looking for more efficient ways to narrow the bandgap in Bi-doped thin films.

CONCLUSIONS In summary, we demonstrate herein that the bandgap narrowing exists in Bi-doped MAPbCl3 single crystals and thin films. A 300 meV bandgap narrowing is obtained without changing the host structure and the VBM in single crystals. These results indicate that obvious Bi doping-induced bandgap narrowing is suitable for all MAPbX3 (X = Cl, Br, and I) single crystals. In addition, this phenomenon also occurs in the Bi-doped MAPbCl3 thin films with high Bi doping level studied for the first time. Our results indicate that heterovalent Bi doping is a powerful means to tune the bandgap, electrical properties and electronic structure of MAPbX3 perovskites, which is promising for further improving the performance of perovskite-based optoelectronic devices.

ASSOCIATED CONTENT

Supporting Information The Supporting Information contains high resolution XPS spectra of nominal 20% Bi-doped single crystal, XPS valence band spectra, SEM images of Bi-doped thin films, 14


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and tables of FAAS results, Goldschmidt tolerance factor, and EDS results.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (51572222, 11604265), and sponsored by the Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University (Z2017195).

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Table 1 Bi and Pb concentrations in the feed solution and HNO3 solution determined by FAAS (0.01g/mL for all Bi-doped single crystals in HNO3) Bi molar ratio%

Bi concentration

Pb concentration

Bi molar ratio%

in feed solution

(µg/mL)

(µg/mL,400-time diluted)

in single crystal

0

–

–

0

0.5

0.8666 ± 0.0812

14.78 ± 0.0588

0.0147 ± 0.0014

1

1.436 ± 0.0066

15.45 ± 0.0823

0.0232 ± 0.0002

5

3.632 ± 0.1023

16.15 ± 0.1804

0.0562 ± 0.0022

10

7.562 ± 0.1429

15.05 ± 0.1429

0.1255 ± 0.0036

20

18.31 ± 0.1593

15.45 ± 0.0291

0.2954 ± 0.0031

22


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TOC Graphic

23



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Figure 1. Photographs of undoped and Bi-doped MAPbCl3 single crystals. 267x104mm (96 x 96 DPI)


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Figure 2. Powder XRD patterns of undoped and Bi-doped MAPbCl3 single crystals. 244x187mm (300 x 300 DPI)



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Figure 3. (a) Normalized absorption spectra of undoped and Bi-doped MAPbCl3 single crystals. Inset: corresponding Tauc plots. (b) Band alignment of undoped and Bi-doped MAPbCl3 single crystals. The CBM is determined by adding the optical bandgap to the VBM. 836x1173mm (96 x 96 DPI)


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Figure 4. Dark current–voltage (I–V) curves of undoped and Bi-doped MAPbCl3 single crystals. Inset: I–V curve of undoped MAPbCl3 crystal shows an Ohmic contact. 253x191mm (300 x 300 DPI)



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Figure 5. (a) XRD patterns of undoped and Bi-doped MAPbCl3 thin films. Inset: photographs of undoped and 20% Bi-doped thin films. (b) Normalized (at 400 nm) absorption spectra of undoped and Bi-doped MAPbCl3 thin films. Inset: corresponding Tauc plots. 258x384mm (300 x 300 DPI)


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Bandgap Narrowing in Bi-Doped CH3NH3PbCl3 ... - ACS Publications

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