Black single crystals of lead-free perovskite Cs₂Ag(Bi:Ru)Br₆ with an intermediate band

Abstract

The toxicity of lead halide perovskites hinders their application in optoelectronics. Lead-free Cs₂AgBiBr₆ is regarded as a promising candidate due to its excellent photoelectric properties. However, its excessively wide band gap limits its absorption in the visible region. Herein, we incorporated the transition metal Ru³⁺ into Cs₂AgBiBr₆ and obtained black single crystals by a hydrothermal method. The absorption edge was extended from ∼660 nm to the near-infrared region (∼1200 nm). When 1.8% of Ru was doped, the double perovskite structure was still maintained and the lattice shrank since some Bi³⁺ was replaced by smaller Ru³⁺. Theoretical calculation indicates that after Ru-doping, a new intermediate band was generated inside the pristine band gap of Cs₂AgBiBr₆ as experimentally confirmed by Valence Band X-ray Photoelectron Spectroscopy. The intermediate band lies below the Fermi surface and is mainly dominated by the Ru-d orbital. Moreover, with the fabrication of a near-infrared (NIR) photodetector, a NIR response from the Ru-doped Cs₂AgBiBr₆ was realized. This work provides an effective way of regulating the energy band structure of Cs₂AgBiBr₆ and extends the application of lead-free perovskites for optoelectronic devices, such as NIR detectors and intermediate band solar cells.

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1. Introduction

Lead halide perovskites have rapidly advanced in the field of optoelectronics due to their adjustable bandgaps,¹ long carrier diffusion length,² high defect tolerance³ and low-cost process.⁴ However, the toxicity of water-soluble Pb²⁺ hinders their broad application.⁵ The research of low-toxicity elements to replace Pb has become more attractive for the development of next generation perovskites. The Sn-based perovskite that uses Sn²⁺ to replace Pb²⁺ exhibits extremely good photoelectric properties, but Sn²⁺ is easily oxidized to Sn⁴⁺.^6,7 In addition, most of the perovskite derivatives including 2D A₃B₂³⁺X₉,⁸ 0D A₂B⁴⁺X₆⁹ and 0D A₃B₂³⁺X₉¹⁰ (A: Cs⁺, MA⁺, FA⁺; B: metal ions; X: Cl⁻, Br⁻, I⁻) have slightly worse optoelectronic properties, due to lower electronic dimensions, large effective mass and poor carrier transport capabilities.^11–14 Recently, 3D double perovskites (A₂B⁺B³⁺X₆), especially Cs₂AgBiBr₆, are considered to be promising candidates for photovoltaics^15,16 and detectors,^17,18 due to their intrinsic thermodynamic stability and small carrier effective masses.¹⁹ However, compared with lead halide perovskites, the excessively wide band gap of Cs₂AgBiBr₆^15,20 provides inferior light absorption, which severely limits its application in the photovoltaic field. Therefore, exploring effective strategies to expand its optical absorption is crucial to the development of highly efficient photovoltaics and detectors.

Element doping is an effective method to modulate the band structure of a semiconductor. According to previous reports,^21,22 however, it is not successful in achieving a narrower band gap through A-site or X-site doping. Therefore, more research groups focus on B-site doping. Karunadasa et al. reduced the band gap of Cs₂AgBiBr₆ through Tl-doping, and the indirect band gap reaches about 1.4 eV (∼890 nm).²³ However, Tl is highly toxic. In their subsequent work, non-toxic but unstable Sn²⁺ was selected as the doping element, and the band gap of the double perovskite was also significantly reduced, with the indirect band gap reaching 1.48 eV (∼840 nm).²⁴ In addition, there are some reports that Sb-doping can also reduce the band gap to a certain extent.^25,26 Gao et al. successfully broadened the absorption edge of Cs₂AgBiBr₆ to the near-infrared (NIR) region to ∼860 nm via Cu-doping.²⁷ However, according to their experimental results, Cu cannot enter the crystal lattice of Cs₂AgBiBr₆, and it is also difficult to change the band structure of the double perovskite. The shift of the absorption edge after doping is due to the introduction of sub-bandgap states. Their group also successfully incorporated Fe³⁺ into Cs₂AgInCl₆ to realize the red shift of the absorption edge to about 800 nm.²⁸ In addition, they also successfully incorporated Fe³⁺ into Cs₂AgBiBr₆ and developed new spintronic materials.²⁹ It is worth noting that based on their sample photos, the original red single crystal changed to black after Fe³⁺ doping and its absorption tailing reaches ∼800 nm.²⁹ Searching for a stable and non-toxic element to effectively extend the absorption of Cs₂AgBiBr₆ remains challenging.

In previous work,³⁰ we successfully incorporated Fe into Cs₂AgBiBr₆ to extend the absorption to the NIR region. As a congener of Fe, Ru has a wide application in dye-sensitized solar cells (DSSC), e.g., N719 dye is a Ru complex.³¹ Gu et al. reported on the incorporation of Ru³⁺ into Cs₃Bi₂I₉ to replace partial Bi³⁺ and experimentally confirmed the feasibility of Ru³⁺ replacing Bi³⁺ in perovskite crystals.³² Although Fe and Ru, from the same family, have similar characteristics, they differ in electron configuration (Fe: 3d⁶4s², Ru: 4d⁷5s¹), electronegativity (Ru: 2.2, Fe: 1.8) and ion size.³³ To investigate the doping thermodynamic process, we studied the Ru-doping effect on double perovskite. In this work, we successfully doped Ru³⁺ into Cs₂AgBiBr₆ to partially replace Bi³⁺ to form Cs₂Ag(Bi:Ru)Br₆ and obtained black single crystals. Benefiting from the similar properties, Cs₂Ag(Bi:Ru)Br₆ has similar characteristics to the Fe-doped one. The absorption edge of Cs₂Ag(Bi:Ru)Br₆ was extended to ∼1200 nm and the lattice shrunk. Theoretical calculation indicates that after the introduction of Ru³⁺, a new intermediate band, which was mainly dominated by the Ru-d orbital, was generated inside the pristine band gap. Due to the different electronegativity and ionic size of Fe and Ru, the bond lengths of Ru–Br and Fe–Br in metal bromide are different, which are 2.5 Å and 2.4 Å, respectively. Meanwhile, the bond length of Bi–Br is 2.9 Å. (Crystal data is referenced from mp-22892, mp-752602 and mp-23232, MaterialsProject). Compared with Fe–Br, the bond length of Ru–Br is closer to Bi–Br, so Ru can more easily replace Bi in the lattice than Fe. Therefore, doping can be achieved with Ru at a lower concentration in the precursor. Furthermore, the intermediate band lies below the Fermi surface. Finally, we observed experimentally the existence of the intermediate band by Valence Band X-ray Photoelectron Spectroscope (VB-XPS), revealing that the intermediate band is located at a position about 1 eV above the original valence band maximum (VBM). Cs₂Ag(Bi:Ru)Br₆ has the potential to be applied in intermediate band solar cells, which may break the Shockley–Queisser limit.³⁴ In addition, a NIR response was observed for Cs₂Ag(Bi:Ru)Br₆ single crystal, indicating its potential application in NIR detection and other optoelectronic fields.

2. Results and discussion

Ru-doped Cs₂AgBiBr₆ single crystals (shown in Fig. 1a) were synthesized by a hydrothermal method. The doping level of Ru was tuned by different amounts of RuBr₃ in the precursor solution. We named samples according to the molar ratio of Ru (Ru/(Ru + Bi)) in the precursor, e.g., Ru-5 means Ru/(Ru + Bi) × 100% = 5%. And Ru-0 is pristine Cs₂AgBiBr₆ without Ru-doping. It can be clearly seen that Ru doping causes the single crystals to gradually change their colour from red to black, i.e., Ru-0.5, Ru-0.7, Ru-1, Ru-3 and Ru-5, as shown in Fig. 1. The molar ratio of Ru in the precursor solution does not represent the specific ratio of Ru in the resultant single crystal. Therefore, we used inductively coupled plasma optical emission spectrometry (ICP-OES) to determine the specific molar ratio of Ru in the single crystal. The data is shown in Table S1 (ESI†). Since Ag and Br will form AgBr precipitates during the digestion process, the content of Ag and Br is not measured. This result indicates that Ru actually replaced about 0.30%, 1.07% and 1.85% of Bi in Ru-1, Ru-3 and Ru-5, respectively. Since the concentration of Ru-doping in Ru-0.7 is ultra-low, the actual content of Ru is hardly measured. According to the absorption spectrum (Fig. 1b), the absorption edge of Ru-0 is at about 630 nm, which is consistent with the previous reports.^15,20 After introducing Ru-dopants, the absorption spectra of the samples changed significantly. The absorption edges of Ru-3 and Ru-5 were very close, as shown in Fig. S1 (ESI†). For Ru-5, the new absorption edge was located close to 1200 nm.

Fig. 1 (a) Photographs and (b) the normalized absorption of the samples.

However, by characterizing the PL spectrum of the sample (Fig. S2, ESI†), it is found that the peak position was basically unchanged. However, after Ru doping, the PL peaks of the samples were obviously weakened, indicating that the doping brought defects and led to the enhancement of non-radiative recombination.

XRD measurement was carried out to study the effect of Ru doping on the crystal structure of the double perovskite. According to the XRD patterns (Fig. 2), the crystals of Ru-1, 3 and 5 have a double perovskite structure (Fig. 2a), and their diffraction peaks (220) shift towards a larger angle than that of Ru-0 (Fig. 2b). According to the Bragg equation, a larger diffraction angle means a smaller lattice constant. This lattice shrink phenomenon is caused by the partial replacement of Bi³⁺ (1.03 Å) by the smaller Ru³⁺ (0.68 Å).³³ As the doping concentration increases, the lattice shrinkage is enhanced. The specific lattice constants are shown in Table S2 (ESI†). Ru-5 is the sample with the largest doping concentration we can get, and thus the following discussion will focus on Ru-5.

Fig. 2 (a) XRD patterns of Cs₂Ag(Bi:Ru)Br₆. (b) An expansion of the diffraction peak of (220).

To further clarify the effect of Ru-doping on the band structure of the double perovskite and to find out the reason for the change in the absorption spectra, we carried out DFT calculations to study the energy band structure. The simulated crystal structure of Cs₂Ag(Bi:Ru)Br₆ is shown in Fig. S3 (ESI†). The calculated lattice constant of (11.292 Å) Cs₂AgBiBr₆ is consistent with the experimental one (11.280 Å), as listed in Table S2 (ESI†). And the experimental and calculated XRD spectra are accordant (Fig. S4, ESI†). The aforementioned results ensure the calculating precision generally. To reduce computational cost, we constructed some small models that were sufficient to describe the effects of Ru doping. For Cs₂AgBiBr₆, we observed bandgap underestimation (Fig. 3a) which was consistent with previous reports.²⁴Fig. 3b shows that the calculated band gap of Cs₂AgBiBr₆ is reduced after 25% Ru-doping, which stems from the huge effect on the band structure of Ru. A new energy band appeared after Ru doping, which we named the intermediate band. Obviously, the intermediate band below the Fermi level is dominated by Ru-d orbitals mainly (Fig. S5, ESI†), which causes additional transition channels from the intermediate band to the unoccupied bands. The intermediate band is quite flat, which would lead to high photo-carrier effective mass and small mobility, i.e., the transport property of Cs₂AgBiBr₆ weakens after Ru-doping. To further discuss the effect of Ru-doping concentration on the band structure of Cs₂Ag(Bi:Ru)Br₆, we calculate the band structures of Cs₂Ag(Bi₁Ru₀)Br₆, Cs₂Ag(Bi_0.75Ru_0.25)Br₆, Cs₂Ag(Bi_0.5Ru_0.5)Br₆ and Cs₂Ag(Bi_0.25Ru_0.75)Br₆, as exhibited in Fig. S6 (ESI†). The energy difference between the intermediate band and CBM decreased with the increase of the Ru doping concentration. The energy level of the intermediate band remains almost constant near the Fermi level for various Ru-doping concentrations. A similar phenomenon was also found in our previous report on Fe-doped Cs₂AgBiBr₆.³⁰ Therefore, we have abundant proof that there is also such an intermediate band in Cs₂Ag(Bi:Ru)Br₆, which lead to absorption in NIR region.

Fig. 3 The calculated band structures and partial density of states (PDOS) of (a) Cs₂AgBiBr₆ and (b) Cs₂Ag(Bi_0.75Ru_0.25)Br₆. The Fermi level is set at 0 eV.

VB-XPS was used to confirm experimentally the influence of Ru-doping on the band structure (Fig. 4). The bands with binding energy less than 8 eV show obvious changes (Fig. 4b). In the case of Cs₂AgBiBr₆, two signal peaks with binding energies of 3.91 eV and 5.64 eV were observed, respectively (Fig. 4c). There are three signal peaks in the spectrum of Ru-5, corresponding to three energy levels with binding energies of 2.74 eV, 3.72 eV and 5.54 eV, respectively (Fig. 4d). Therefore, the intermediate band after Ru-doping was experimentally confirmed at the position of ∼1 eV above the original VBM. It is the intermediate band that causes the absorption edge to redshift towards the NIR region. Materials with intermediate bands have been proposed to fabricate intermediate band solar cells, which may break the Shockley-Queisser efficiency limitation.³⁴

Fig. 4 (a) The VB-XPS spectrum of Ru-0 and Ru-5. (b) An expansion of the region with binding energy below 8 eV. The multi-peak fitting of the spectra of (c) Ru-0 and (d) Ru-5.

The transition process of Ru-doped samples is shown in Fig. 5a. Process (1) is the transition from the VBM to the CBM of Cs₂AgBiBr₆ constituting the visible part of the absorption spectrum, and process (2) is the transition from the intermediate band to the CBM constituting the near-infrared part of the absorption spectrum. The data of VBM and CBM are referenced from our previous work.¹⁵ Motivated by the absorption of Ru-doping samples in the NIR region, we fabricated the device by evaporating Au electrodes on a single crystal of Ru-5. For comparison, a device based on Ru-0 was also manufactured. The schematic diagram of the devices is shown in Fig. 5b. The Volt-Ampere characteristic curves of the devices are shown in Fig. 5c and d. It can be clearly observed that the dark current of the devices after Ru-doping is significantly reduced and there is a clear photoelectric response at 980 nm (the longest wavelength of light source available in our lab). However, the Cs₂AgBiBr₆ device has no photoelectric conversion capability for 980 nm, but it is suitable for UV and visible light detection as reported in our previous work.¹⁷ Thus, Cs₂Ag(Bi:Ru)Br₆ shows potential for a lead-free perovskite infrared detector.

Fig. 5 (a) Schematic illustration of the transition process of the Ru-doping samples. (b) Schematic diagram of the devices. (c) The Volt-Ampere characteristic curves of the devices under dark conditions. (d) On/off ratios of the devices under 980 nm.

3. Conclusion

Similar to Fe doping, we have successfully obtained black single crystals and extended the absorption edge of Cs₂AgBiBr₆ to the NIR region (∼1200 nm) with an intermediate band via incorporating transition metal Ru. The partial substitution of Bi by Ru (<2 mol.%) led to the crystal lattice shrinking. The intermediate band, mainly dominated by Ru-d orbitals, resulted in the absorption redshift. However, due to the difference of Ru and Fe, Ru is more easily doped into Cs₂AgBiBr₆, and the position of the intermediate band is below the Fermi surface. Moreover, we experimentally confirmed that the intermediate band was located at ∼1 eV above the original VBM of Cs₂AgBiBr₆ by VB-XPS. The confirmed NIR photoelectric response implied the potential application for intermediate band solar cells and NIR photodetectors. Our work provides an efficient method to reduce the band gap of lead-free perovskites, and extending the optoelectronic response to the NIR region.

4. Experimental Methods

The experimental section is provided in the ESI.†

Author contributions

Zehao Zhang, Ganghong Liu and Wenhui Guo contributed equally.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (12174013, 61935016, 52173153, 12074011, 62104059). The DFT calculations are supported by the High-performance Computing Platform of Peking University.

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Footnotes

† Electronic supplementary information (ESI) available: The experimental details, the data of ICP-OES, lattice constants and the additional figures of XRD patterns. Simulated lattice structure used in calculations and the partial density of states of Ru-s/p/d. See DOI: https://doi.org/10.1039/d2ma00264g

‡ These authors contributed equally.

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