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).

We establish compositional effects on stability, crystallinity, charge-carrier life times and mobilities in mixed-cation lead iodide-bromide perovskites as band gap tunable materials for multi-junction photovoltaic cells.

by at least four orders of magnitude. [ 17,18 ] MAPbI 3 also appears to exhibit only shallow trap-levels and although the grain boundaries have recently been shown to induce nonradiative decay, [ 19 ] regions only a few tens of nm away from the grain boundaries appear to be unaffected. [ 20,21 ] Furthermore, low Urbach energies, which are extracted from near band edge optical absorption measurements and serve as a benchmark for crystalline phase disorder, indicate low disorder and sharp band edges for lead tri-halide perovskites (15-23 meV). [ 22,23 ] All of these properties contribute to high open-circuit voltages, long charge-carrier lifetimes, and micrometer diffusion lengths, which are crucial for planar-hetero junction photovoltaics. [ 1,24 ] Nonetheless, these parameters are also expected to depend on the infl uence of crystallization condition on perovskite morphology in fabricated fi lms. [ 25 ] A distinct benefi t of organic-inorganic perovskite materials (e.g., over silicon) is that their bandgap can be tuned relatively easily with chemical composition, allowing attractive coloration and multijunction or tandem cell designs. For example, changing the metal cation at the M site from Pb 2+ to the less toxic Sn 2+ to form CH 3 NH 3 SnI 3 shifts the optical bandgap from 1.55 to 1.3 eV into the range of the "ideal" single-junction solar cell bandgap between 1.1 and 1.4 eV. [ 26,27 ] However, stability issues arising from the oxidation of tin have so far prevented widespread use. Alternatively, tuning the size of the A site cation has been proven to change optical and electronic properties of the perovskite and to signifi cantly infl uence solar cell performance. [ 28 ] Replacing MA in MAPbI 3 by the larger cation formamidinium HC(NH 2 ) 2 + (FA) was found to decrease the bandgap from 1.57 to 1.48 eV, [29][30][31] yield long photoluminescence (PL) lifetimes, high PCEs, [ 28 ] and lower recombination and device hysteresis. [ 32 ] Highest PCEs of 20.1% have been reported [12][13][14] to date for solar cells based on FAPbI 3 making this an attractive system to explore. In addition, the gradual replacement of the MA cation by FA through the fi lm was shown to create a mixed cation-lead-iodide PSC allowing for energetic gradients. [ 33 ] However, mixing of the halide component in the perovskite offers the fi nest tuning of the optical properties of the perovskite fi lm. Here, the mixed organic lead iodide/bromide system has recently gained strong interest for application in PSCs. [ 11,28 ] By changing the ratio between bromide and iodide (at the X site anion), the bandgap can be tailored between 1.55 eV (MAPbI 3 ) and 2.3 eV (MAPbBr 3 ), which results in the coverage of much of the visible spectrum and paves the way for the development of tandem solar cells. [ 11 ] In addition to MAPb(Br y I 1-y ) 3 , its formamidinium relative FAPb(Br y I 1-y ) 3 has been explored. [ 28 ] Most fractional mixtures of FAPb(Br y I 1-y ) 3 were found to be crystalline, with the exception of the region between y = 0.3 and Recent year...