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.

Solution processed metal halide perovskite materials have revealed outstanding optoelectronic features that make them uniquely suited for photovoltaic applications. Although a rapid progress has led to performances similar to inorganic thin film technologies, the fabrication method of some of the most widely used electron selective layers, based on either mesoporous architectures or high annealing temperatures, may limit yet a future large scale production. In that regard, planar perovskite solar cell configurations that can be processed at low temperatures are more desirable. Herein, we demonstrate that a few tens of nanometers thick bilayer, made of two types of inorganic oxide nanoparticles, can perform as a robust and low temperature processed electron selective contact for planar perovskite solar cells. Aside from boosting the average efficiency of planar opaque devices, the proposed method allowed us to preserve the main photovoltaic characteristics when thinner active layers, usually exhibiting a non-continuous morphology, were integrated for semi-transparent cells. By providing excellent electronic and coverage features against the bottom electrode, this novel configuration may hence offer an alternative route to approach future inexpensive printable methodologies for the fabrication of efficient low temperature perovskite solar cells.

In the subwavelength regime, several nanophotonic configurations have been proposed to overcome the conventional light trapping or light absorption enhancement limit in solar cells also known as the Yablonovitch limit. It has been recently suggested that establishing such limit should rely on computational inverse electromagnetic design instead of the traditional approach combining intuition and a priori known physical effect. In the present work, by applying an inverse full wave vector electromagnetic computational approach, a 1D nanostructured optical cavity with a new resonance configuration is designed that provides an ultrabroadband (≈450 nm) light absorption enhancement when applied to a 107 nm thick active layer organic solar cell based on a low‐bandgap (1.32 eV) nonfullerene acceptor. It is demonstrated computationally and experimentally that the absorption enhancement provided by such a cavity surpasses the conventional limit resulting from an ergodic optical geometry by a 7% average over a 450 nm band and by more than 20% in the NIR. In such a cavity configuration the solar cells exhibit a maximum power conversion efficiency above 14%, corresponding to the highest ever measured for devices based on the specific nonfullerene acceptor used.

Bringing the Voc of a perovskite solar cell toward its radiative value, corresponding to a 100% external fluorescence quantum yield (QY) of the cell, has been pursued to reach the highest performance photovoltaic devices. Therefore, much research has been focused on maximizing the QY of the active layer isolated from the rest of the cell layers. However, such quantity does not often correlate with the Voc following the ideal diode relation. Herein, the QYs of complete FA0.8MA0.2PbI3−yBry solar cells are reported, ranging from 0.1% to 3%, and compared with their Vocs, ranging from 1 to 1.13 V. By combining these measurements with electromagnetic simulations based on a full‐wavevector detailed balance and a fluorescence power‐loss model, it is demonstrated that a nonoptimal Voc in mixed‐cation lead halide perovskite cells is not only due to nonradiative photocarrier recombination at traps. In addition to the expected parasitic absorption of the emitted photons in the electrode layers, discrepancies appear between Voc and QY. These discrepancies are attributed to the rise of energy barriers, a side effect of trap removal. Indeed, although surface passivation may enhance the QY, its beneficial effect may be counterbalanced by the emergence of such barriers between active and charge‐transporting layers.

To bring hybrid lead halide perovskite solar cells toward the Shockley− Queisser limit requires lowering the band gap while simultaneously increasing the opencircuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface nonradiative recombination. When applying the two-step method for a better crystal control, it is rather challenging to fabricate perovskites with FA + cations, given the small penetration depth of such large ions into a compact PbI 2 film. In here, to successfully incorporate such large cations, we used a highconcentration solution of the organic precursor containing small Cl − anions achieving, via a solvent annealing-controlled dissolution−recrystallization, larger than 1 μm perovskite crystals in a solar cell. This solar cell, with a largely increased fluorescence quantum yield, exhibited an open-circuit voltage equivalent to 93% of the corresponding 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.