Formamidinium Incorporation into Compact Lead Iodide for Low Band Gap Perovskite Solar Cells with Open-Circuit Voltage Approaching the Radiative Limit

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of largecations to obtain a perovskite with larger lattice parameter together with a large crystalsize to minimize interface nonradiative recombination. When applying the two-stepmethod for a better crystal control, it is rather challenging to fabricate perovskites withFA+cations, given the small penetration depth of such large ions into a compact PbI2film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl-anions achieving,via a solvent annealing-controlled dissolution-recrystallization, larger than 1µmperovskite crystals in a solar cell. This solar cell, with a largely increasedfluorescencequantum yield, exhibited an open-circuit voltage equivalent to 93% of thecorresponding radiative limit one. This, together with the low band gap achieved(1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.Peer ReviewedPostprint (author's final draft

Natural Random Nanotexturing of the Au Interface for Light Backscattering Enhanced Performance in Perovskite Solar Cells

As the efficiency of a solar cell approaches its limits, photonic considerations to further enhance its performance overtake electronic ones. It has been theoretically shown for GaAs solar cells that with the combined effects of a surface random texturing and a perfectly reflecting rear mirror, efficiencies close to the Shockley–Queisser limit can be reached, even when the absorber layer is very thin. In here, we demonstrate a method for taking advantage of surface random texturing to enhance the efficiency of planar perovskite solar cells. By naturally transferring the perovskite random nanotexturing to the back semiconductor/metal interface, where the contrast in the imaginary part of the refractive index is very large, backscattering reduces light escape from the solar cell structure. This leads to a close to optimal light absorption that allows bringing the cell efficiency from 19.3% to 19.8%. Such a path we opened toward an ergodic behavior for maximum light absorption in perovskite cells may lead to the most efficient perovskite cells ever

Rare Earth-Ion/Nanosilicon Ultrathin Layer: A Versatile Nanohybrid Light-Emitting Building Block for Active Optical Metamaterials

We fabricate an Er³⁺/nano-Si ultrathin (≈ 4 nm) layer and explore its optical response from the near-UV to the near-IR, in the linear and nonlinear regimes. This nanohybrid layer combines the tunable broad-band light harvesting properties of nano-Si with the robust and sharp Er³⁺ light emission. Its unique nanostructure enables efficient nanometer-range transfer of the harvested energy to the Er³⁺ ions. Therefore, clear 1.54 μm Er³⁺ photoluminescence (PL) is observed under excitation at any photon energy (<i>E</i>_exc) from the visible to the near-UV, despite the small amount of Er³⁺ ions in the layer (<2.5% of atomic monolayer). In the linear regime, the Er³⁺ PL intensity can be tuned to a maximum by setting the amount of nano-Si (<i>Q</i>_Si) in the layer at a suitable value, independent of <i>E</i>_exc. In the nonlinear regime, adjustment of <i>Q</i>_Si allows the dependence of the Er³⁺ PL intensity on <i>E</i>_exc to be tuned and achievement of nonconventional saturation properties not reported so far in Er³⁺:nano-Si systems. Based on this characteristic tunability, at sufficiently low <i>Q</i>_Si the nanohybrid layer is an ideal candidate for efficient near-IR emission under intense near UV–visible broad-band excitation. Furthermore, the nanohybrid layers with high enough <i>Q</i>_Si show an interesting potential for the optical modulation of the PL intensity by using UV light in a pump–probe configuration. Therefore, this nanohybrid layer is an outstanding candidate as a pure-color light-emitting building block for the development of advanced multiscale active optical metamaterials