Minasadat Heydarian

Perovskite-based triple-junction solar cells offer a theoretical efficiency potential exceeding 47%. As an emerging photovoltaic technology, recent research has focused on optimizing the top, middle, and bottom subcells, as well as the recombination layers between the subcells.

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.

In recent years, the development of perovskite/perovskite/silicon triple junction solar cells has received considerable attention, aiming to exceed the efficiency of perovskite/silicon dual junction solar cells. Various challenges must be addressed to reach these efficiencies and justify the additional complexity. This article outlines a simulation‐based roadmap for improving the structure of these cells. It discusses critical issues and estimates potential improvements for perovskite/perovskite/silicon solar cells based on device architectures developed at Fraunhofer ISE and EPFL. The article describes potential challenges related to thickening the middle perovskite absorber to achieve current matching. It explains the observed losses in fill factor in the top cell with a high bandgap, which arise from an insufficient hole barrier caused by unfavorable valence‐band alignment between the perovskite and the electron transport layer. These aspects are examined through numerical device simulations.

Monolithic two‐terminal perovskite‐based multijunction solar cells are considered as the next generation technology in the photovoltaic market, thanks to their high power conversion efficiency at potentially low production costs. In these structures, an interlayer is required for subcell interconnection. Commonly used recombination layers are based on scarce and expensive materials such as gold or indium. Developing alternative candidates without compromising device performance is therefore crucial for the future of such multijunction technologies. Herein, low‐temperature sputtered zinc tin oxide (ZTO) is employed as an indium‐free recombination layer between two perovskite subcells. ZTO is deposited using scalable in‐line sputtering. Slight zinc doping of tin oxide is needed to enable the DC sputtering from a rotary target. Employing ZTO results in improved device performance compared to the reference devices with indium tin oxide recombination layer, deposited via a lab‐type sputtering tool. Perovskite/perovskite/silicon triple‐junction and all‐perovskite tandem solar cells show increased open‐circuit voltages by ≈60 and ≈30 mV, respectively. The work showcases that ZTO can be sputtered in a non‐destructive and scalable manner in different perovskite‐based multi‐junction technologies.

Following the impressive efficiencies achieved for two‐terminal perovskite/silicon dual–junction solar cells, perovskite/perovskite/silicon triple‐junction cells have now gained attention and are rapidly developing. In a two‐terminal triple‐junction cell, maximizing the open‐circuit voltage (V_OC) is not straightforward as it requires understanding and mitigating the dominant losses in such a complex structure. Herein, the high bandgap perovskite top cell is first identified as the main source of the V_OC loss in the triple‐junction cell. A multifaceted optimization approach is then implemented that improves the V_OC of the 1.83 eV perovskite. This approach consists of 1) replacing the reference triple‐cation/double‐halide with a triple‐cation/triple‐halide perovskite, which improves perovskite bulk quality and reduces transport losses, and 2) implementing a piperazinium iodide passivation between the perovskite and the electron transport layer, which reduces nonradiative recombination losses at this interface. Employing these optimizations in the top cell of the triple‐junction boost the V_OC by average 124 mV. A high V_OC of more than 3.00 V is achieved with a fill factor of 79.6%, a short‐circuit current density of 9.0 mA cm⁻², and an efficiency of 21.5%. Further study is conducted on the improvement of V_OC in the triple‐junction solar cell using subcell selective photoluminescence‐based implied V_OC imaging, which is applied for the first time to a perovskite‐based triple‐junction structure.

Monolithic perovskite-based triple junction solar cells have the potential to surpass the power conversion efficiency (PCE) limits of single and dual-junction solar cells.