Challenges and Progress in Lead‐Free Halide Double Perovskite Solar Cells

Abstract

Lead-based metal halide perovskites have brought a revolution in photovoltaics in the last few years with a dramatic increase in the power conversion efficiency (PCE) from 3.8% in 2009 to 25.7% in 2022. [1,2] The incredible PCE improvement is mainly due to their remarkable optical and electrical properties, including suitable and tunable direct bandgap, high absorption coefficient, small exciton binding energy, long carrier diffusion length and lifetime, high carrier mobility, high defect tolerance, etc. [3][4][5][6][7] Besides, low-cost and printable devices have further prospered this fantastic technology. [8] Despite these exciting achievements, Pb-based perovskites suffer from two significant issues: toxicity and long-term instability. [9,10] The toxicity issue comes from the Pb which can induce detrimental environmental and health effects in the case of leakage. For long-term instability, Pd-based perovskites are susceptible to degradation after exposure to light, oxygen, moisture, or heat, ascribed to polymorphic transformation, hydration, or decomposition. Both of these issues hinder their further development and commercialization.A straightforward way to address the instability and Pb toxicity issues is to develop stable and lead-free perovskites. The first substitution candidate is Sn 2þ , as it is in the same group of the periodic table as Pb with similar lone pairs. As expected, these Sn 2þ -based materials maintain the 3D perovskite structure and exhibit relatively high PCE. [11] A critical issue for Sn-based perovskites is the easy oxidization of Sn 2þ to Sn 4þ . Moreover, Sn 2þ might be also harmful to the human body due to the strong acidification induced by the SnI 2 . [9] Instead of Sn 2þ , more stable elements such as Ag þ , Na þ , Bi 3þ , Sb 3þ , In 3þ , Fe 3þ , Ti 4þ , Pd 4þ , etc. were employed to form various novel lead-free perovskites or perovskite derivatives, including A 3 B 3þ 2 X 9 , [12] A 2 B 4þ X 6 , [13] B þ B 3þ 2 X 7 , [14] A 2 B þ B 2þ X 5 , [15] and halide double perovskites (HDPs) A 2 B þ B 3þ X 6 [16] (A = Cs þ , MA þ , FA þ ; B = metal ions; X = Cl À , Br À , I À ). Unfortunately, except for HDPs (A 2 B þ B 3þ X 6 ), the crystal dimensionalities of other materials are 2D or 0D, different from the 3D structure of connected BX 6 octahedra in Pb-based perovskites. Generally, the drawbacks of the low structural dimensionality (0D-2D) are poor carrier transport, high carrier effective masses, and exciton binding energies, limiting their applications in photovoltaics. Therefore, lead-free perovskite candidates with high structure dimensionality are more favorable for high-efficiency photovoltaics.Recently, HDPs (A 2 B þ B 3þ X 6 ) with 3D perovskite structure and exciting optoelectronic properties are becoming one of the most promising Pb-free perovskite candidates in photovoltaic applications. The benchmark HDP Cs 2 AgBiBr 6 for solar cells was first reported in 2017. [17] Afterward, this material has been explored for various photoelectric applications, including photodetectors...

Structural, electronic, optical, elastic, thermodynamic and thermal transport properties of Cs2AgInCl6 and Cs2AgSbCl6 double perovskite semiconductors using a first-principles study

“…The nontoxic double perovskites under consideration exhibit promising potential as photovoltaic absorbers, with the aim of replacing APbX 3 in halide perovskite solar cells. [43][44][45][46][47][48] Furthermore, these materials are being proposed as potential alternatives for various optoelectronic applications, including X-ray detectors, thermal neutral scintillators employed in nuclear monitoring applications, white emitting phosphors, and ultraviolet detectors. [49][50][51][52] HDP compounds have been successfully synthesized and observed in the historical world since the beginning of the 1970s.…”

Understanding the polaron behavior in Cs2CuSbCl6 halide double perovskite

“…; M = Ag + , K + , Li + ; M' = Sb 3+ , In 3+ or Bi 3+ and X = I À , Br À or Cl À ) double perovskites have become an object of significant interest due to their environmental and structural stability, low toxicity, defect tolerance, and exceptional optoelectronic properties. [1][2][3][4][5][6] These qualities have made them increasingly popular in applications such as solar cells, X-ray detectors, photodetectors, and photocatalysts. [7][8][9][10] Achieving high-performance for these applications often requires the use of high-quality perovskite single crystals (SCs).…”

Halide Double Perovskites Cs2PdBr6–xIx with Tunable Bandgaps for Solar Cells

“…Therefore, the exploration of inorganic lead-free perovskite materials is of great significance for the future of solar cell technology. Studies have shown that all-inorganic non-lead double perovskite structures A 2 B + B 3+ X 6 (double perovskite) and A 2 B 4+ X 6 (vacancy-ordered double perovskite) could be used to solve the above problems. , However, the double perovskite has the problem of too sizable a direct or indirect bandgap, showing lower photoelectric conversion efficiency. − Ion doping is an effective strategy to improve perovskite properties. , Anion mixing on the X-site has been shown to significantly improve both the photoelectric performance and the stability of perovskite. − The synthesized Ti-based halide perovskite, Cs 2 TiI x Br 6– x , has tunable bandgaps between 1.0 and 1.8 eV for PV applications . Zheng and colleagues successfully prepared a series of lead-free Rb 2 ZrCl 6– x Br x double perovskite nanocrystals with excellent crystallinity and explored the influence of mixed halogens on optical properties.…”