Efficiency from hole-selective contacts: Perovskite/silicon tandem solar cells must stabilize a perovskite material with a wide bandgap and also maintain efficient charge carrier transport. Al-Ashouri et al. stabilized a perovskite with a 1.68–electron volt bandgap with a self-assembled monolayer that acted as an efficient hole-selective contact that minimizes nonradiative carrier recombination. In air without encapsulation, a tandem silicon cell retained 95% of its initial power conversion efficiency of 29% after 300 hours of operation. Science , this issue p. 1300

Stable perovskites with ionic salts: Ionic liquids have been shown to stabilize organic-inorganic perovskite solar cells with metal oxide carrier-transport layers, but they are incompatible with more readily processible organic analogs. Lin et al. found that an ionic solid, a piperidinium salt, enhanced the efficiency of positive-intrinsic-negative layered perovskite solar cells with organic electron and hole extraction layers. Aggressive aging testing showed that this additive retarded segregation into impurity phases and pinhole formation in the perovskite layer. Science , this issue p. 96

25.5% efficiency is demonstrated for monolithic perovskite/silicon tandem solar cell using textured foil and the impact of texture position on performance and energy yield is simulated.

Metal halide perovskites show great promise to enable highly efficient and low cost tandem solar cells when being combined with silicon. Here, we combine rear junction silicon heterojunction bottom cells with p–i–n perovskite top cells into highly efficient monolithic tandem solar cells with a certified power conversion efficiency (PCE) of 25.0%. Further improvements are reached by reducing the current mismatch of the certified device. The top contact and perovskite thickness optimization allowed increasing the J_SC above 19.5 mA cm⁻², enabling a remarkable tandem PCE of 26.0%, however with a slightly limited fill factor (FF). To test the dependency of the FF on the current mismatch between the sub-cells, the tandems' J–V curves are measured under various illumination spectra. Interestingly, the reduced J_SC in unmatched conditions is partially compensated by an enhancement of the FF. This finding is confirmed by electrical simulations based on input parameters from reference single junction devices. The simulations reveal that especially the FF in the experiment is below the expected value and show that with improved design we could reach 29% PCE for our monolithic perovskite/silicon tandem device and 31% PCE if record perovskite and silicon cell single junctions could be combined in tandem solar cells.

Monolithic perovskite/silicon tandem solar cells recently surpass the efficiency of silicon single‐junction solar cells. Most tandem cells utilize >250 μm thick, planarized float‐zone (FZ) silicon, which is not compatible with commercial production using <200 μm thick Czochralski (CZ) silicon. The perovskite/silicon tandem cells based on industrially relevant 100 μm thick CZ‐silicon without mechanical planarization are demonstrated. The best power conversion efficiency (PCE) of 27.9% is only marginally below the 28.2% reference value obtained on the commonly used front‐side polished FZ‐Si, which are about three times thicker. With both wafer types showing the same median PCE of 27.8%, the thin CZ‐Si‐based devices are preferred for economic reasons. To investigate perspectives for improved current matching and, therefore, further efficiency improvement, optical simulations with planar and textured silicon have been conducted: the perovskite's bandgap needs to be increased by ≈0.02 eV when reducing the silicon thickness from 280 to 100 μm. The need for bandgap enlargement has a strong impact on future tandem developments ensuring photostable compositions with lossless interfaces at bandgaps around or above 1.7 eV.

Silicon heterojunction (SHJ) solar cells have been increasingly attracting attention to the photovoltaic community in the last years due to their high efficiency potential and the lean production process. We report on the development of a stable baseline process for SHJ cells with focus on the optical improvement of the solar cells’ front side. An amorphous silicon oxide layer (a-SiO $_{2}$ was used as an antireflective coating (AR) on the front side the finished SHJ devices. Both optical simulations and experimental results demonstrate a short-circuit current density (J $_{sc}$ improvement of 0.4 mA/cm2 when applying the a-SiO $_{2}$ AR, yielding maximum conversion efficiencies of 23.0%. Full-size cells with 244 cm2 total area have been produced using three front contact stacks: indium tin oxide (ITO) as reference, ZnO:Al, and ZnO:Al/SiO $_{2}$ showing the J $_{sc}$ improvement with the double AR configuration. Damp-heat tests on those samples demonstrate an enhanced stability of cells with ZnO:Al front TCO when capped with SiO $_{2}$ .