Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

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Organolead trihalide perovskites are shown to exhibit the best of both worlds: charge‐carrier mobilities around 10 cm2 V−1 s−1 and low bi‐molecular charge‐recombination constants. The ratio of the two is found to defy the Langevin limit of kinetic charge capture by over four orders of magnitude. This mechanism causes long (micrometer) charge‐pair diffusion lengths crucial for flat‐heterojunction photovoltaics.

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).

Perovskite solar cells based on abundant low cost materials promise to compete on performance with mainstream PV. Here we demonstrate lead-free perovskite solar cells, removing a potential barrier to widespread deployment.