Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ~ 20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV), can be achieved by varying halide composition but to date, such materials have had poor photostability and thermal stability. Here, we present a highly crystalline and compositionally photostable material, [HC(NH 2 One concept for improving the efficiency of photovoltaics (PVs) is to create a "tandem junction," for example, by placing a wide band gap "top cell" above a silicon "bottom cell." This approach could realistically increase the efficiency of the Si cell from 25.6% to beyond 30% (1, 2). Given the crystalline silicon (c-Si) band gap of 1.1 eV, the top cell material requires a band gap of ~1.75 eV, in order to current-match both junctions (3). However, suitable wide-band-gap top-cell materials for Si or thin film technologies that offer stability, high performance, and low cost have been lacking. In recent years, metal halide perovskite-based PVs have gained attention because of their high power conversion efficiencies (PCE) and low processing cost (4-11). An attractive feature of this material is the ability to tune its band gap from 1.48 to 2.3 eV (12, 13), implying that we could potentially fabricate an ideal material for tandem cell applications.Perovskite-based PVs are generally fabricated with organic-inorganic trihalide perovskites with the formulation ABX 3 , where A is the methylammonium (CH 3 NH 3 ) (MA) or formamidinium (HC(NH 2 ) 2 ) (FA) cation, B is commonly lead (Pb), while X is a halide (Cl, Br, and I). Although these perovskite structures offer high power conversion efficiencies (PCE), reaching > 20% PCE with band gaps of around 1.5 eV (14), fundamental issues have been discovered when attempting to tune their band gaps to the optimum 1.7 to 1.8 eV range. In the case of MAPb(I(1-x)Brx) 3 , Hoke et al. reported that light-soaking induces a halide segregation within the perovskite (15), The formation of iodiderich domains with lower band gap result in an increase in sub-gap absorption and a red-shift of photoluminescence (PL). The lower band gap regions limit the voltage attainable with such a material, so this band gap "photoinstability" limits the use of MAPb(I(1-x)Brx) 3 in tandem devices (15). In addition, when considering real-world applications, MAPbI3 is inherently thermally unstable at 85°C, even in an inert atmosphere (international regulations require a commercial PV product to withstand this temperature) (16).

Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.

We introduce new hole-selective contacts for next-generation perovskite photovoltaics and point to design paths for molecular engineering of perfect interfaces.

We present a monolithic perovskite/silicon tandem cell enabling a high Voc and a stabilized power output of 18%.

Solar cells based on metal-halide perovskite semiconductors inspire high hopes for efficient low-cost solar energy conversion technology. We here reflect on recent progress and potential limitations of the band gap tunability of these compound semiconductors.