Layered Dion-Jacobson-Type Chalcogenide Perovskite CsLaMX (M = Ta/Nb; X = S/Se) with a Narrow Band Gap of ∼1 eV as a Promising Rear Cell for All-Perovskite Tandem Solar Cells.

Perovskite-perovskite tandem solar cells have bright prospects to improve the power conversion efficiency (PCE) beyond the Shockley-Queisser (SQ) limit of single-junction solar cells. The star lead-based halide perovskites are well-recognized as suitable candidates for the front cell, thanks to their suitable band gap (∼1.8 eV), strong optical absorption, and high certified PCE. However, the toxicity of lead for the front cell and the lack of a narrow band gap (∼1.1 eV) for the rear cell seriously restrict the development of the two-junction tandem cell. To break through this bottleneck, a novel Dion-Jacobson (DJ)-type ( = 2) chalcogenide perovskite CsLaMX (M = Ta, Nb; X = S, Se) has been found based on the powerful first-principles and advanced many-body perturbation GW calculations. Their excellent electronic, transport, and optical properties can be summarized as follows. (1) They are stable and environmentally friendly lead-free materials. (2) The direct band gap of CsLaTaSe (0.96-1.10 eV) is much smaller than those of lead-based halide perovskites and very suitable for the rear cell in the two-junction tandem cell. (3) The carrier mobility in CsLaTaSe reaches 1.6 × 10 cm V s at room temperature. (4) The absorption coefficients (3-5 × 10 cm) are 1 order higher than that of Si (10 cm). (5) The estimated PCEs of the CsSbBr-CsLaTaSe tandem cell (33.3%) and the concentrator solar cell (35.8% in 100 suns) are higher than those of the best recorded GaAs-Si tandem cell (32.8%) and the perovskite-perovskite tandem solar cell (24.8%). These energetic results strongly demonstrate that the novel lead-free chalcogenide perovskites CsLaMX are good candidates for the rear cell of tandem cells.