The simplification of perovskite solar cells (PSCs), by replacing the mesoporous electron selective layer (ESL) with a planar one, is advantageous for large-scale manufacturing. PSCs with a planar TiO2 ESL have been demonstrated, but these exhibit unstabilized power conversion efficiencies (PCEs). Herein we show that planar PSCs using TiO2 are inherently limited due to conduction band misalignment and demonstrate, with a variety of characterization techniques, for the first time that SnO2 achieves a barrier-free energetic configuration, obtaining almost hysteresis-free PCEs of over 18% with record high voltages of up to 1.19 V

Only a selected group of two-dimensional (2D) lead-halide perovskites shows a peculiar broad-band photoluminescence. Here we show that the structural distortions of the perovskite lattice can determine the defectivity of the material by modulating the defect formation energies. By selecting and comparing two archetype systems, namely, (NBT)PbI and (EDBE)PbI perovskites (NBT = n-butylammonium and EDBE = 2,2-(ethylenedioxy)bis(ethylammonium)), we find that only the latter, subject to larger deformation of the Pb-X bond length and X-Pb-X bond angles, sees the formation of V color centers whose radiative decay ultimately leads to broadened PL. These findings highlight the importance of structural engineering to control the optoelectronic properties of this class of soft materials.

We present here a planar perovskite solar cell with a stabilized power conversion efficiency (PCE) of 17.6% at the maximum power point and a PCE of 17% extracted from quasi-static J–V with an open-circuit voltage of 1.11 V. Such excellent figures of merit can be achieved by engineering a solution-processed electron buffer layer that does not require high temperature steps. A compact thin film of perovskite absorber is grown onto a PCBM-based electron extraction layer by implementing a novel two-step procedure which preserves the soluble organic interlayer during the deposition of successive layers. We demonstrate that efficient charge extraction is the key for high steady state efficiency in perovskite solar cells with a highly integrable architecture

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. Here, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic counter-cation. We present a phenomenological, yet quantitative framework to simulate excitonic absorption lineshapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by 35 ± 5 meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the lineshape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Our work highlights the complexity of excitonic properties in this class of nanostructured materials. * carlos.silva@gatech.edu † srinivasa.srimath@iit.it 1 arXiv:1803.02455v3 [cond-mat.mtrl-sci] 30 Apr 2018 I. INTRODUCTION Organic-inorganic hybrid perovskites (HOIPs) consist of metal-halide octahedral motifs that form multi-dimensional lattice planes structurally separated by coordinating organic counter cations [1]. While the frontier orbitals that give rise to the semiconductor electronic structure are contributed by the metal-halide network, the organic cation plays a key role in the structural configuration as well as the stability of the lattice [2]. When the organic moieties are long enough to isolate the lattice planes electronically, the latter form quantum-well-like structures with strong two-dimensional (2D) electronic confinement within the metal-halide layer [3]. A consequence of such confinement is the creation of strongly bound excitons, which have been reported as early as 1989 by Ishihara et al. [4], with binding energies of 200-300 meV. In a general context, a variational approach of electron-hole correlations predicts that excitons in strongly confined quantum wells experience a four-fold enhancement in binding energy with respect to the bulk semiconductor [5], assuming a smooth dielectric environment around the well. This enhancement is generally observed in systems such as GaAs, which is characterized by an exciton binding energy of 4 meV in the bulk and 16 meV in quantum wells [6]. Intriguingly, there is more than a ten-fold increase in the binding energies going from 3D lead-halide perovskites (10-20 meV [7]) to their 2D counterparts [4].Beyond quantum confinement, dielectric confinement arising from the intercalating organic layers increases the Coulomb correlations substantially, resulting in such a strong increase in the exciton binding energy [3,4,8,9].There is now an increasing consensus ...

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors -chemical, electronic and structural -that govern strong multi-exciton correlations.Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA) 2 PbI 4 (PEA = phenylethylammonium). We determine the binding energy of biexcitons -correlated two-electron, two-hole quasiparticles -to be 44 ± 5 meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalchogenides. Importantly, we show that this binding energy increases by ∼ 25% upon cooling to 5 K. Our work highlights the importance of multi-exciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder. * FT and SN are to be considered first co-authors of this manuscript. †

groups have recently and independently demonstrated that, by applying an electric fi eld across a pristine fi lm of 3D hybrid perovskites of different chemical composition, a self-sustained fi eld is induced in the semiconductor as a consequence of ion migration toward the electrode regions. [10][11][12][13] The formation of a self-sustained internal fi eld upon device polarization is also in good agreement with the observations reported by Tan et al. when testing perovskite-based light emitting diodes. [ 14 ] This concept has also been the base of the explanation proposed by Tress et al. for the rate-dependent hysteresis seen in current-voltage scans of solar cells. [ 15 ] So far reports suggest that transient electrical characteristics are due to a polarization response of the perovskite active layer that results in changes in the photocurrent extraction effi ciency of the device. [ 11,15 ] However, it must be noted that a variety of dynamics have been reported, which differ in magnitude and time scale, depending both on the specifi c device architecture and, in particular, on the adopted charge extraction layer. [ 7,9 ] This indicates that contact interfaces have a considerable effect on transients in perovskite based devices.In this Communication we investigate the role played by charge extracting layers on the slow transient behavior of CH 3 NH 3 PbI 3 perovskite based solar cells. Such transients, which typically affect both short-circuit currents and open circuit voltage of hysteretic devices, are found to notably modify the open circuit voltage also in the very fi rst J -V scans of so-called "hysteresis-free" devices integrating a phenyl-C61-butyric acid methyl ester (PCBM) charge extraction layer. Here a preconditioning of the device, i.e., a repetition of J -V scans, is needed to achieve completely stable J -V characteristics under illumination. In particular, we fi nd that under device operation, iodide ions migrate to the electron extracting layer. We fi rst show that the use of an organic extraction layer such as PCBM, albeit not hampering ions motion, evidently improves charge extraction with respect to interfaces involving compact TiO 2 , in agreement with what is suggested in other seminal investigations, [ 16,17 ] and makes the short-circuit current density virtually insensitive to the transient phenomena related to ions migration. Moreover, while self-doping of the perovskite fi lm close to the contact has been generally put forward in the study of transient phenomena, [ 12,13 ] here we show that ions can specifi cally interact with the organic electron extracting layer, inducing electronic doping and that such I − /PCBM interaction is at the origin of the preconditioning requirement for stabilizing the device and for improving its open circuit voltage with respect to the fi rst scan.Solution-processable hybrid perovskite semiconductors have risen to the forefront of photovoltaics research, offering the potential to combine low-cost fabrication with high-powerconversion effi ciency. Originally u...

Metal-halide perovskites are promising lasing materials for the realization of monolithically integrated laser sources, the key components of silicon photonic integrated circuits (PICs). Perovskites can be deposited from solution and require only low-temperature processing, leading to significant cost reduction and enabling new PIC architectures compared to state-of-the-art lasers realized through the costly and inefficient hybrid integration of III−V semiconductors. Until now, however, due to the chemical sensitivity of perovskites, no microfabrication process based on optical lithography (and, therefore, on existing semiconductor manufacturing infrastructure) has been established. Here, the first methylammonium lead iodide perovskite microdisc lasers monolithically integrated into silicon nitride PICs by such a top-down process are presented. The lasers show a record low lasing threshold of 4.7 μJcm–2 at room temperature for monolithically integrated lasers, which are complementary metal–oxide–semiconductor compatible and can be integrated in the back-end-of-line processes.

The simple solution processability at room temperature exposes lead halide perovskite semiconductors to a non-negligible level of unintentional structural and chemical defects. Ascertained that their primary optoelectronic properties meet the requirement for high efficiency optoelectronic technologies, a lack of knowledge about the nature of defects and their role in the device operation currently is a major challenge for their market-scale application due to the issues with stability and reliability. Here, we use excitation correlation photoluminescence (ECPL) spectroscopy to investigate the recombination dynamics of the photogenerated carriers in lead bromide perovskites and quantitatively describe the carrier trapping dynamics within a generalization of the Shockley-Read-Hall formalism. The superior sensitivity of our spectroscopic tool to the many-body interactions enables us to identify the energetics of the defects. In fact, in the case of polycrystalline films, depending on the synthetic route, we demonstrate the presence of both deep and shallow carrier traps. The shallow defects, which are situated at about 20 meV below the conduction band, dope the semiconductor, leading to a substantial enhancement of the photoluminescence quantum yield despite carrier trapping. At excitation densities relevant for lasing, we observe breakdown of the rate-equation model, indicating a buildup of a highly correlated regime of the photocarrier population that suppresses the nonradiative Auger recombination. Furthermore, we demonstrate that colloidal nanocrystals represent virtually defect-free systems, suffering from nonradiative quenching only due to subpicosecond Auger-like interactions at high excitation density. By correlating the fabrication conditions to the nonradiative loss channels, this work provides guidelines for material engineering towards superior optoelectronic devices.