Chloride‐Substituted Perovskite Enables Thick Middle Cell for Perovskite/Perovskite/Silicon Triple‐Junction Solar Cells

Increasing the thickness of perovskite thin film absorbers allows for enhanced photo absorption, which increases the current and performance of the solar cells. In multijunction devices, thickness optimization is critical to achieve current matching between subcells, depending on the bandgap combination. However, maintaining film quality while balancing charge transport and recombination becomes more challenging as thickness increases. Herein, we investigate thickness limitations of wet‐chemically processed 1.56 eV mixed cation mixed halide perovskites as middle cell in monolithic perovskite/perovskite/silicon triple‐junction solar cells. It is found that Cs _0.05 (FA _0.90 MA _0.10 ) _0.95 Pb(I _0.95 Br _0.05 ) ₃ perovskite shows a decrease in performance for thickness >450 nm. Substituting bromide with chloride (Cl ⁻ ) in the composition allows to increase layer thickness up to ∼700 nm without sacrificing fill factor (FF) and open‐circuit voltage ( V _OC ) _. The Cl‐substituted absorbers show larger grain size, less nonradiative recombination loss, higher carrier lifetime, and mobility as evidenced by steady‐state and transient photoluminescence, and optical pump/terahertz probe spectroscopy measurements. Thick Cl‐substituted absorbers enhance performance up to 20% in 1 cm ² semitransparent single‐junction devices. By integrating the ∼700 nm perovskite as middle cell in a perovskite/perovskite/silicon triple‐junction device, an efficiency of 23.4% with a short circuit current density of 9.6 mA/cm ² , a FF of 80.5%, and a V _OC of 3026 mV is achieved.