Tuning the bandgap of double perovskites

Yu Zou, Wenjin Yu, Lixiu Zhang, Cuncun Wu, Lixin Xiao,†, and Liming Ding,†

1State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, China

2School of Materials Science and Engineering, State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology, Tianjin 300130, China

3Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China

With great achievements in efficiency, stability, and large-scale preparation of perovskite solar cells (PSCs), the commercialization of PSC is ongoing, but there is still an issue on lead toxicity. Although lead content in the device is low, the water solubility of lead salts leads to potential environmental pollution. At present, the non-lead perovskites studied include: divalent metal perovskite (e.g., Sn2+, Ge2+, Cu2+),trivalent metal perovskite (e.g., Bi3+, Sb3+, In3+), tetravalent metal double perovskite (e.g., Sn4+, Pd4+, Ti4+, Pt4+) and monotrivalent mixed double perovskite (e.g., Ag+and Bi3+, Ag+and In3+, Ag+and Sb3+). Their properties are summarized in Table 1. Since the first report on non-lead double perovskite Cs2AgBiBr6in 2016[1], this material has caused extensive research in optoelectronic devices because of its long carrier lifetime[1]and good stability[2]. However, owing to its wide and indirect bandgap (Eg) (～2.0 eV), its light absorption range is narrow, which limits its application in photovoltaics. At present, the power conversion efficiency (PCE) of Cs2AgBiBr6device is ～3%[3]. To enhance PCE, theEgneeds to be narrowed. The efforts in this area focus on adjusting chemical composition and physical structure (Fig. 1) to tuneEg.

A common attempt to tune theEgof double perovskite is to adjust the chemical composition of compound A2B(I)B(III)X6.

A-site. Doping alkali metal ions like Rb+, K+, and Na+made little effect onEg[4]. Chenet al.[5]introduced Rb+into Cs2AgBiBr6to form (Cs1-xRbx)2AgBiBr6and effectively passivated the defects of double perovskite, thus increasing PCE to 1.5%. But this doping could not significantly change theEg.When doping A-site to change the dimension of perovskite from 3D to 2D, theEgand electronic structure can be tuned.In 2019, Mitziet al.[6]reported that a stable 2D double perovskite [AE2T]2AgBiI8was formed by using multifunctional organic molecules with a directEgof 2.01 eV.

X-site. For lead-based perovskites, theEgdecreases with the increasing of halogen atom radius, and similar results are also observed in double perovskites. The introduction of Cl-can greatly increaseEg, while I-doping can significantly reduceEg, which is 2.77 eV (X = Cl), 1.95 eV (X = Br), and 1.75 eV (X = I) for Cs2AgBiX6[7]. When X site is I, the stability for double perovskite decreases. It is difficult to prepare Cs2AgBiI6experimentally, because the formation energy of Cs3Bi2I9is lower. As reported by Maet al.[8], theEgof Cs2NaBiI6was relatively narrow (1.66 eV), but it can easily decompose into Cs3Bi2I9, yielding a 0.42% PCE.

B-site. The structure of double perovskite consists of B(I)X6and B(III)X6with alternating octahedrons connected by vertices. Substituting B-site elements can tuneEg. Karunadasaet al.[9]found that doping low content of toxic Tl (Tl+,Tl3+) into Cs2AgBiBr6to form Cs2(Ag1-aBi1-b)TlxBr6(0.003

When using Cu+/Cu2+to replace part of Ag+in Cs2Ag-BiBr6, there is no obvious effect onEg[14]. Cu+(ionic radius 77 pm,hereafter omit) and Cu2+(73 pm) are much smaller than Ag+(115 pm). They can cause lattice defects, and the absorption tail extends from 610 to 860 nm. This was due to the absorption of the defect intermediate state. Similarly, Fe3+(65 pm)was used to replace part of Bi3+(103 pm)[15]. Since the size of Fe3+is quite different from Bi3+, it can easily cause lattice distortion and defects. The lattice constant was reduced from 11.27 to 11.25 Å, and Cs2AgBi0.886Fe0.114Br6was obtained as a black crystal. ThoughEgdid not change obviously, the light absorption was greatly increased by defects. Recently, Gaoet al.[16]demonstrated that Fe3+could replace diamagnetic In3+to yield Cs2AgIn1-xFexCl6and form [FeCl6]3-·[AgCl6]5-domains,and the connection of larger [FeCl6]3-·[AgCl6]5-domains leads to segregated Fe3+-rich phases. TheEgof Cs2AgInCl6was tuned from 2.8 to 1.6 eVviaFe3+-alloying.

The second approach to tuneEgof double perovskite is to adjust the physical structure. First-principles calculations indicated that changing the order of Ag+and Bi3+in space can significantly reduceEg. When the arrangement of Ag+andeffective for reducingEg(Table 2). We need further studyEgnarrowing mechanism of double perovskites and find effective methods to realize it.Bi3+was completely disordered, theEgcould be reduced to 0.44 eV[17]. Gaoet al.[18]reported that through thermallyinduced defects, as the temperature increased, the color of Cs2AgBiBr6single crystal and film can change from red to black, theEgdecreased, but the original color of the film recovered after cooling. The finite-temperature molecular dynamics simulations indicated the synergistic effect of the incongruous bond lengths (RAg-BrandRBi-Br) fluctuations and the related electron-phonon coupling, as well as the special spinorbit coupling effect, were the cause for the thermochromism. Gaoet al.[19]increased the disorder degree of [Ag-Br6] and [BiBr6] octahedral by adjusting the crystallization speed of Cs2AgBiBr6. As the temperature increases, the disorder degree gradually increases, the lattice shrinks, initially leading to the formation of isolated defect states in the forbidden band, and finally a series of defect states, and reducedEg(60 °C, 1.98 eV; 150 °C, 1.72 eV). And this reducedEgcan keep stable at room temperature. Zouet al.[20]found that high pressure can also significantly reduceEgof Cs2AgBiBr6single crystal. Under 15 GPa pressure, theEgwas reduced from 2.2 to 1.7 eV. When the pressure was released, theEgwas relatively lower than that at atmospheric pressure.

Table 1. Properties of lead-free perovskites.

Table 2. Tuning Eg of double perovskites.

Fig. 1. (Color online) Tuning Eg of double perovskites.

In summary, for double perovskite Cs2AgBiBr6, the effect of A-site doping onEgis not obvious, the stability of X-site doping is intractable, and only B-site doping can significantly tuneEg. Changing the order of Ag+and Bi3+in space is much

Acknowledgments

This work was supported by the National Natural Science Foundation of China (61775004, 61935016). L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045, 21772030, 51922032,21961160720) for financial support.