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 mono-trivalent 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 Cs₂AgBiBr₆ in 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 (E_g) (~2.0 eV), its light absorption range is narrow, which limits its application in photovoltaics. At present, the power conversion efficiency (PCE) of Cs₂AgBiBr₆ device is ~3%^[3]. To enhance PCE, the E_g needs to be narrowed. The efforts in this area focus on adjusting chemical composition and physical structure (Fig. 1) to tune E_g.

A common attempt to tune the E_g of 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 on E_g^[4]. Chen et al.^[5] introduced Rb⁺ into Cs₂AgBiBr₆ to form (Cs_1–xRb_x)₂AgBiBr₆ and effectively passivated the defects of double perovskite, thus increasing PCE to 1.5%. But this doping could not significantly change the E_g. When doping A-site to change the dimension of perovskite from 3D to 2D, the E_g and electronic structure can be tuned. In 2019, Mitzi et al.^[6] reported that a stable 2D double perovskite [AE2T]₂AgBiI₈ was formed by using multifunctional organic molecules with a direct E_g of 2.01 eV.

X-site. For lead-based perovskites, the E_g decreases with the increasing of halogen atom radius, and similar results are also observed in double perovskites. The introduction of Cl^– can greatly increase E_g, while I^– doping can significantly reduce E_g, 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 Cs₂AgBiI₆ experimentally, because the formation energy of Cs₃Bi₂I₉ is lower. As reported by Ma et al.^[8], the E_g of Cs₂NaBiI₆ was 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)X₆ and B(III)X₆ with alternating octahedrons connected by vertices. Substituting B-site elements can tune E_g. Karunadasa et al.^[9] found that doping low content of toxic Tl (Tl⁺, Tl³⁺) into Cs₂AgBiBr₆ to form Cs₂(Ag_1–aBi_1–b)Tl_xBr₆ (0.003 < x = a + b < 0.075) can significantly reduce its E_g to 1.40 eV for Cs₂(Ag_1–aBi_1–b)Tl_0.075Br₆. Replacing Ag⁺ and Bi³⁺ by Tl⁺/Tl³⁺ yielded defects, leading to band-edge reconstruction. Using unstable Sn²⁺ (can be oxidized to Sn⁴⁺) to replace both Ag⁺ and Bi³⁺ can also reduce E_g to 1.71 eV (indirect) and 1.48 eV (direct)^[10]. These results indicate that the band-edge reconstruction caused by the lattice distortion due to B-site ion doping can tune the E_g of double perovskite. Mitzi et al.^[11] reported that the use of trivalent metal In³⁺ or Sb³⁺ can also significantly tune E_g, which could be increased to 2.4 eV by replacing 75% of Bi³⁺ with In³⁺. While replacing 37.5% Bi³⁺ with Sb³⁺ could reduce E_g to 1.86 eV. When using In⁺ to replace Ag⁺, it was predicted that the E_g of Cs₂InBiCl₆ (1.02 eV) and Cs₂InSbCl₆ (0.91 eV) would be very suitable for photovoltaics^[12]. However, In⁺ is extremely unstable and can be easily oxidized into In^3+[13].

When using Cu⁺/Cu²⁺ to replace part of Ag⁺ in Cs₂AgBiBr₆, there is no obvious effect on E_g^[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 Cs₂AgBi_0.886Fe_0.114Br₆ was obtained as a black crystal. Though E_g did not change obviously, the light absorption was greatly increased by defects. Recently, Gao et al.^[16] demonstrated that Fe³⁺ could replace diamagnetic In³⁺ to yield Cs₂AgIn_1–xFe_xCl₆ and form [FeCl₆]^3–·[AgCl₆]^5– domains, and the connection of larger [FeCl₆]^3–·[AgCl₆]^5– domains leads to segregated Fe³⁺-rich phases. The E_g of Cs₂AgInCl₆ was tuned from 2.8 to 1.6 eV via Fe³⁺-alloying.

The second approach to tune E_g of double perovskite is to adjust the physical structure. First-principles calculations indicated that changing the order of Ag⁺ and Bi³⁺ in space can significantly reduce E_g. When the arrangement of Ag⁺ and Bi³⁺ was completely disordered, the E_g could be reduced to 0.44 eV^[17]. Gao et al.^[18] reported that through thermally-induced defects, as the temperature increased, the color of Cs₂AgBiBr₆ single crystal and film can change from red to black, the E_g decreased, 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 (R_Ag–Br and R_Bi–Br) fluctuations and the related electron-phonon coupling, as well as the special spin-orbit coupling effect, were the cause for the thermochromism. Gao et al.^[19] increased the disorder degree of [AgBr₆] 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 reduced E_g (60 °C, 1.98 eV; 150 °C, 1.72 eV). And this reduced E_g can keep stable at room temperature. Zou et al.^[20] found that high pressure can also significantly reduce E_g of Cs₂AgBiBr₆ single crystal. Under 15 GPa pressure, the E_g was reduced from 2.2 to 1.7 eV. When the pressure was released, the E_g was relatively lower than that at atmospheric pressure.

In summary, for double perovskite Cs₂AgBiBr₆, the effect of A-site doping on E_g is not obvious, the stability of X-site doping is intractable, and only B-site doping can significantly tune E_g. Changing the order of Ag⁺ and Bi³⁺ in space is much effective for reducing E_g (Table 2). We need further study E_g-narrowing mechanism of double perovskites and find effective methods to realize it.

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.