Thiri Htuna,
Amr Elattarbc,
Hytham Elbohyd,
Kosei Tsutsumia,
Kazumasa Horiganee,
Chiyu Nakanof,
Xiaoyu Gug,
Hiroo Suzukia,
Takeshi Nishikawaa,
Aung Ko Ko Kyaw*g and
Yasuhiko Hayashi*a
aGraduate School of Natural Science and Technology, Okayama University, Japan. E-mail: hayashi.yasuhiko@ec.okayama-u.ac.jp
bDepartment of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt
cIndustrial & Manufacturing Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer St, Tallahassee, Florida 32310, USA
dPhysics Department, Faculty of Science, Damietta University, Egypt
eResearch Institute for Interdisciplinary Science, Okayama University, Japan
fAdvanced Science Research Center, Okayama University, Okayama, Japan
gGuangdong University Key Laboratory for Advanced Quantum Dot Displays and Lighting and Department of Electronic & Electrical Engineering, Southern University of Science and Technology, P. R. China. E-mail: aung@sustech.edu.cn
First published on 23rd July 2024
Perovskite based on cesium bismuth bromide offers a compelling, non-toxic alternative to lead-containing counterparts in optoelectronic applications. However, its widespread usage is hindered by its wide bandgap. This study investigates a significant bandgap tunability achieved by introducing Fe doping into the inorganic, lead-free, non-toxic, and stable Cs3Bi2Br9 perovskite at varying concentrations. The materials were synthesized using a facile method, with the aim of tuning the optoelectronic properties of the perovskite materials. Characterization through techniques such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, energy dispersive spectroscopy (EDS), and UV-vis spectroscopy was conducted to elucidate the transformation mechanism of the doping materials. The substitution process results in a significant change in the bandgap energy, transforming from the pristine Cs3Bi2Br9 with a bandgap of 2.54 eV to 1.78 eV upon 70% Fe doping. The addition of 50% Fe in Cs3Bi2Br9 leads to the formation of the orthorhombic structure in Cs2(Bi,Fe)Br5 perovskite, while complete Fe alloying at 100% results in the phase formation of CsFeBr4 perovskite. Our findings on regulation of bandgap energy and crystal structure through B site substitution hold significant promise for applications in optoelectronics.
Within the A3B2X9 perovskite structure, diverse crystal structures exist, classified as 0D, 1D, and 2D depending on the B–X sublattice. The zero-dimensional (0D) structure have a hexagonal structure, in which A3X9 units are ordered with B cations in octahedral interstitial sites, resulting in isolated [B2X9]3− dimers. In a one-dimensional structure (1D), double chains form based on corner-sharing [BX6]3− octahedra. The two-dimensional (2D) structure, where the layers are corrugated like corner-connected [BX6]3− octahedra, is the most similar to the cubic perovskite that appeared by removing the third B site cation from cubic ABX3. The 2D structure has a narrow bandgap, superior defect tolerance, higher electron and hole mobility, and excellent stability compared to the 0D and 1D structures. Therefore, it plays a crucial role in controlling the bandgap and enhancing optical absorption in lead-free perovskite materials.22,23
Bismuth-based perovskites (A3Bi2X9) emerge as attractive alternatives due to their non-toxicity, desirable moisture and air stability,24 and excelle nt water stability. Bi3+ shares a similar electronic configuration and ionic radius with Pb2+, making it a viable substitute.14,25–30 All inorganic Bi3+-based perovskites have obtained increased attention in various optoelectronic applications owing to their superior stability and low toxicity.31–33 Various Bi3+ based perovskites, including Cs3BiBr6, CsBi3I10, Cs3Bi2Br9, Cs2AgBiBr6 and Cs3Bi2I9, have been successfully synthesized.34 Among these, all-inorganic Bi-based perovskites using Cs as A cation exhibit enhanced crystallinity and superior optoelectronic properties compared to organic–inorganic counterparts.2 The choice of Cs as the cation contributes to improve structure stability and overall performance of perovskite materials.35–37 Doping strategies can further be employed on pristine perovskite material to fine-tune their optical and electronic properties. Siqi Dai et al. investigated the effect of Sb doping on pristine Cs3Bi2Br9 perovskite, finding that it reduced the bandgap from 2.59 eV to 2.22 eV. The mixed alloy occupied a smaller bandgap than both the Bi-based and Sb-based perovskites.38 Amr Elattar et al. demonstrated that doping of Cu in Cs3Bi2Br9 perovskite significantly reduces the bandgap from 2.56 eV for pristine Cs3Bi2Br9 to 1.77 eV at 100% Cu.23 Mrinmoy Roy et al. studied the effect of Pb substitution in Cs3Bi2Br9 layered perovskites, finding that the bandgap is reduced from 2.62 eV to 2.23 eV due to the emergence of defect states between the bands.39 Fuxiang et al. investigated the impact of Fe doping in the double perovskite Cs2AgInxFe1−xCl6, observing a notable reduction in bandgap from 2.8 eV in the pristine material to 1.6 eV in the Fe-doped variants. This highlights the effectiveness of Fe in substantially reducing the bandgap of perovskite materials.40 Moreover, iron, being the most abundant element on Earth's surface, offers a non-toxic, environmentally friendly, and cost-effective option.41–44
Herein, we incorporated Fe ions into the bismuth-based (Cs3Bi2Br9) perovskite, effectively modulating the bandgap of Cs3Bi2−xFexBr9 over a wide range, from 2.54 eV to 1.78 eV. Furthermore, we investigated the impact of Fe doping on the pristine Cs3Bi2Br9 perovskite. The substitution of Fe3+ for Bi3+ in the matrix, resulted in the formation of a secondary phase (Cs2(Bi,Fe)Br5) and a ternary phase (CsFeBr4) with varying proportions of alloying elements. The incorporation of Fe3+ enables the fine-tuning of the optical properties of the doped materials, paving the way for potential applications in optoelectronic devices.
CsBr (mmol) | BiBr3 (mmol) | FeBr3 (mmol) | Alloying composition (%) |
---|---|---|---|
3 | 2 | w/0 | 0% |
3 | 1.9 | 0.1 | 5% |
3 | 1.6 | 0.4 | 20% |
3 | 1 | 1 | 50% |
3 | 0.6 | 1.4 | 70% |
3 | w/0 | 2 | 100% |
As shown in Fig. 1(b), the color of the doped material darkens with increasing Fe concentration, suggesting a variation in bandgap energy. This visual correlation is further supported by the UV-vis absorption spectroscopy presented in Fig. 2(a). The adsorption edge of the undoped material, observed around 430 nm, aligns with previous reports.22 As the concentration of Fe increases, the optical absorbance band edge of the doping materials also increases, reaching approximately 490 nm. The UV-vis spectra exhibit three distinct types of curves corresponding to different doping concentrations, each characterized by a unique crystal structure. The optical bandgap of the materials is estimated from UV spectra using Tauc plots of (αhν)n versus hν. The Cs3Bi2Br9 perovskite possesses both direct and indirect bandgaps.39,45 The indirect bandgap values of the pristine and doped Cs3Bi2Br9 are 2.54 eV, 2.53 eV, 2.39 eV, 1.80 eV, 1.78 eV, and 1.88 eV, and the corresponding direct bandgap values are 2.69 eV, 2.67 eV, 2.61 eV, 1.99 eV, 1.97 eV, and 2.17 eV for 0%, 5%, 20%, 50%, 70%, and 100% Fe doping, respectively as shown in Fig. 2(b) and (c). The bandgap value of the pristine Cs3Bi2Br9 perovskite material is in good agreement with previous reports.40,46 These tunable optical bandgap energies align with the observed color changes in different Fe doping concentrations. Both the direct and indirect bandgaps shift towards lower values with increasing Fe content compared to the pristine Cs3Bi2Br9 perovskite. The indirect bandgap values are consistently smaller than the direct bandgap values, although the trend is the same for both methods of calculation. The PL peak does not appear when we measure the Raman spectroscopy as illustrated in ESI Fig. S1.† Therefore, we assume that both the pristine and doped Cs3Bi2Br9 perovskite have an indirect bandgap. The indirect bandgap energies exhibit a gradual decrease, ranging from 2.54 eV to 2.39 eV for the 0%, 5%, and 20% compositions of the perovskite material characterized by the Cs3Bi2Br9 perovskite structure. For the Cs2(Bi,Fe)Br5 perovskite material, the bandgap energies shift from 1.80 eV to 1.78 eV in the case of the 50% and 70% Fe doping. Notably, 70% Fe doping stands out with the lowest bandgap energy of 1.78 eV, as shown in Fig. 2(d). The bandgap energy for the CsFeBr4 structure with 100% Fe occupancy is measured at 1.88 eV. This tunability in bandgap energy holds promise for applications that require tailored optical properties, emphasizing the influence on optoelectronic devices.
Fig. 2 (a) UV-Vis absorption spectra, Tauc plots of (b) indirect bandgap and (c) direct bandgap, and (d) indirect bandgap energies of different Fe doping materials. |
There is a possibility that variation in bandgap from 2.69 eV to 1.97 eV could be associated with crystal structure change due to Fe doping. Therefore, we further examined the crystal structure of the perovskites at various Fe doping using XRD and Rietveld analysis. Fig. 3 shows the XRD pattern of Fe-doped Cs3Bi2Br9 perovskite for concentrations ranging from 0% to 100% Fe. The powder XRD measurements of the pristine, 5% Fe-, and 20% Fe-doped perovskites are in agreement with the Cs3Bi2Br9 trigonal crystal structure illustrated in Fig. 4(a), maintaining the space group 3m1 as previously reported in the literature.23 The structural parameters were refined using Rietveld analysis of XRD data, based on the structural model as shown in in ESI Fig. S2.† The XRD patterns of the pristine, 5%, and 20% Fe-doped samples were consistent with the Cs3Bi2Br9 structure as depicted in ESI Fig. S2(a)–(c).† Notably, both the a and c axes increased with higher Fe doping levels, suggesting the substitution of Fe atoms into the Bi site. Additionally, a structural transformation becomes evident at 50% Fe doping, where the diffraction patterns show an orthorhombic structure characteristic of Cs2(Bi,Fe)Br5 perovskite with a space group Pnma, as illustrated in Fig. 4(b). This crystal structure has two metal-occupied sites, namely octahedral and tetrahedral sites. Consequently, we conducted Rietveld analysis on two models: one with Bi occupying an octahedral site and Fe occupying a tetrahedral site, and the other with Bi occupying a tetrahedral site and Fe occupying an octahedral site. When the Bi atoms occupied octahedral sites, the resulting reliability factor was Rwp = 6.33% as illustrated in ESI Fig. S2(d).† On the other hand, Bi atoms occupied at tetrahedral sites, the Rwp becomes worse from 6.33% to 12.85%. This outcome indicates that Bi atoms occupied octahedral sites and Fe atoms occupy tetrahedral sites. The diffraction pattern of the 70% Fe alloy sample could be indexed by the Cs2(Bi,Fe)Br5 structure, with lattice constants increasing with Fe doping. This tendency is similar to that of Fe doping in Cs3Bi2Br9, suggesting a substitution of Fe in the pristine material via the orthorhombic structure.23 For the 100% Fe alloying material, all diffraction peaks correspond to the orthorhombic structure of CsFeBr4 perovskite, characterized by the space group Pnma, presented in Fig. 4(c). The resulting reliability factor from the Rietveld refinement was Rwp = 4.04%, as shown in ESI Fig. S2(f).† Interestingly, in CsFeBr4, Fe ions exhibit tetrahedral coordination, suggesting that the structural change induced by Fe doping is associated with the preferential tetrahedral coordination of Fe. This leads to the presence of a tertiary phase (CsFeBr4) in the Cs3Bi2Br9 perovskite at 100% Fe alloying. From the XPD analysis, we postulate that the transformation of crystal structure from Cs3Bi2Br9 to Cs2(Bi,Fe)Br5 and CsFeBr4 at higher Fe doping may also contribute to significant bandgap change in the perovskites.
The elemental composition of the perovskite crystals was scrutinized using energy dispersive spectroscopy (EDS), and the alloying composition of all elements (Cs, Bi, Fe, and Br) is detailed in Table 2. Analysis reveals a correlation between the precursor and product compositions, indicating a preference for the incorporation of Fe into the pristine perovskite material. The EDS results further enable an estimation of the crystal structures. The A3B2X9 perovskite structure is observed for 0%, 5%, and 20% Fe doping, while the A2BX5 perovskite structure is identified for 50% and 70% Fe doping. Interestingly, the 100% Fe doping results in the ABX4 perovskite structure. These findings align with the XRD measurements, where the Cs3Bi2Br9 structure is retained for 0%, 5%, and 20% Fe doping, Cs2(Bi,Fe)Br5 structure emerges for 50% and 70% Fe doping, and CsFeBr4 structure is observed for 100% Fe doping. The comprehensive characterization sheds light on the structural transformations induced by varying Fe concentrations in the perovskite crystals. Energy dispersive spectroscopy (EDS) elemental mapping, shown in ESI Fig. S3,† demonstrates the same magnitude for all compositions. Despite some mappings being less clear, the overall tendency indicates a homogeneous decrease of Bi and an increase of Fe with increasing Fe content.
Cs (at%) | Fe (at%) | Bi (at%) | Br (at%) | Estimated structure | Alloying composition (%) |
---|---|---|---|---|---|
20.97 | w/0 | 12.85 | 66.17 | A3B2X9 | 0% |
24.85 | 1.61 | 11.91 | 61.63 | A3B2X9 | 5% |
23.32 | 4.88 | 7.08 | 64.71 | A3B2X9 | 20% |
25.41 | 6.45 | 6.38 | 61.76 | A2BX5 | 50% |
24.03 | 6.81 | 4.37 | 64.79 | A2BX5 | 70% |
15.16 | 16.99 | w/0 | 67.85 | ABX4 | 100% |
To verify the structural origin and phase purity of the pristine and alloyed perovskites, Raman spectra were obtained, as shown in Fig. 5(a). The peaks of undoped materials at 160 cm−1 and 187 cm−1 are attributed to the stretching vibrations of the Bi–Br bond with Eg and A1g symmetry in BiBr6 octahedra, respectively, which are vibrations with participation of Bi atoms. These results correspond well with prior reports.47,48 The absence of shifts in peak positions and intensity changes upon 20% or lower Fe doping suggests that Fe incorporation does not affect the crystal structure of the pristine material. New vibration mode at around 200 cm−1 appears from 50% to 100% alloyed materials, attributed to the stretching modes of Fe–Br bonds,49 absent in the parent Cs3Bi2Br9 material. The Raman peak at around 200 cm−1 broadens and shifts toward higher wavenumbers with 70% Fe and greater, as shown in Fig. 5(b), due to different ionic radius of the doping metal, Fe3+ (63 pm) and parent metal Bi3+(103 pm). This also indicates the substitution of Bi3+ with Fe3+ in the crystal structure.40
Fig. 5 (a) Raman spectra of the pristine Cs3Bi2Br9 and Fe doped perovskite crystals. (b) Expansion of Raman spectra. |
To investigate the incorporation of Fe into the pristine material, X-ray photoelectron spectroscopy (XPS) was utilized, revealing core levels of Cs 3d, Bi 4f, Br 3d, and an additional element, Fe, as depicted in Fig. 6. The Cs 3d5/2 and Cs 3d3/2 binding energies at 724 eV and 738 eV, respectively, align with previous reports.39 The Br 3d5/2 peak is located at 68 eV. Fig. 6(b) shows Bi exhibits doublet peaks corresponding to 4f7/2 at 159.3 eV and 4f5/2 at 164.6 eV, which matches with the previous results.41 At 100% Fe doping, the disappearance of these doublets is a result of the complete occupation by Fe. The binding energy of Bi 4f at 160 eV observed at 100% Fe doping is the spectral line from the 4p3/2 peak of Cs. The Fe 2p3/2 peak at 711 eV at 100% Fe doping confirms the successful doping of Fe into the pristine perovskite material as presented in Fig. 6(c). The consistency of this outcome aligns with other characterization results, including XRD and EDS analyses. Consequently, we have successfully doped Fe into the pristine Cs3Bi2Br9 perovskite material, leading to a potential reduction in its bandgap for optoelectronic applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra04062g |
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