Simple Processing Additive-Driven 20% Efficiency for Inverted Planar Heterojunction Perovskite Solar Cells.

Compositional engineering has been a strong tool to improve the quality of the perovskite materials and, in turn, the reproducibility of the solar cells. However, the control over the active layer uniformity, one of the most important requirements for the obtainment of efficient devices, is still a weak point of perovskite solar cells (PSCs) manufacturing. Here, we develop an approach to grow a uniform mixed cation perovskite layer, foreseeing its implementation in inverted solar cells endowing organic transporting layers, through the addition of a stoiochiometric amount of tropolone as chelating agent for the lead. Thanks to low melting and boiling temperatures, tropolone is present in the system only during the colloidal liquid phase, leaving the film during its formation; this unique characteristic promotes the obtainment of ideal perovskite surface morphologies and an increased short circuit current of photovoltaic devices. A maximum power conversion efficiency of 20% was obtained, with a 25% increase with respect to the reference.

Novel Physical Vapor Deposition Approach to Hybrid Perovskites: Growth of MAPbI3 Thin Films by RF-Magnetron Sputtering.

Solution-based methods represent the most widespread approach used to deposit hybrid organic-inorganic perovskite films for low-cost but efficient solar cells. However, solution-process techniques offer limited control over film morphology and crystallinity, and most importantly do not allow sequential film deposition to produce perovskite-perovskite heterostructures. Here the successful deposition of CH3NH3PbI3 (MAPI) thin films by RF-magnetron sputtering is reported, an industry-tested method to grow large area devices with precisely controlled stoichiometry. MAPI films are grown starting from a single-target made of CH3NH3I (MAI) and PbI2. Films are single-phase, with a barely detectable content of unreacted PbI2, full surface coverage and thickness ranging from less than 200 nm to more than 3 µm. Light absorption and emission properties of the deposited films are comparable to as-grown solution-processed MAPI films. The development of vapor-phase deposition methods is of interest to advance perovskite photovoltaic devices with the possibility of fabricating perovskite multijunction solar cells or multicolor bright light-emitting devices in the whole visible spectrum.

Ultra-Bright Near-Infrared Perovskite Light-Emitting Diodes with Reduced Efficiency Roll-off.

Herein, an insulating biopolymer is exploited to guide the controlled formation of micro/nano-structure and physical confinement of α-δ mixed phase crystalline grains of formamidinium lead iodide (FAPbI3) perovskite, functioning as charge carrier concentrators and ensuring improved radiative recombination and photoluminescence quantum yield (PLQY). This composite material is used to build highly efficient near-infrared (NIR) FAPbI3 Perovskite light-emitting diodes (PeLEDs) that exhibit a high radiance of 206.7 W/sr*m2, among the highest reported for NIR-PeLEDs, obtained at a very high current density of 1000 mA/cm2, while importantly avoiding the efficiency roll-off effect. In depth photophysical characterization allows to identify the possible role of the biopolymer in i) enhancing the radiative recombination coefficient, improving light extraction by reducing the refractive index, or ii) enhancing the effective optical absorption because of dielectric scattering at the polymer-perovskite interfaces. Our study reveals how the use of insulating matrixes for the growth of perovskites represents a step towards high power applications of PeLEDs.

Doping porous silicon with erbium: pores filling as a method to limit the Er-clustering effects and increasing its light emission.

Er clustering plays a major role in hindering sufficient optical gain in Er-doped Si materials. For porous Si, the long-standing failure to govern the clustering has been attributed to insufficient knowledge of the several, concomitant and complex processes occurring during the electrochemical Er-doping. We propose here an alternative road to solve the issue: instead of looking for an equilibrium between Er content and light emission using 1-2% Er, we propose to significantly increase the electrochemical doping level to reach the filling the porous silicon pores with luminescent Er-rich material. To better understand the intricate and superposing phenomena of this process, we exploit an original approach based on needle electron tomography, EXAFS and photoluminescence. Needle electron tomography surprisingly shows a heterogeneous distribution of Er content in the silicon thin pores that until now couldn't be revealed by the sole use of scanning electron microscopy compositional mapping. Besides, while showing that pore filling leads to enhanced photoluminescence emission, we demonstrate that the latter is originated from both erbium oxide and silicate. These results give a much deeper understanding of the photoluminescence origin down to nanoscale and could lead to novel approaches focused on noteworthy enhancement of Er-related photoluminescence in porous silicon.

Optical determination of Shockley-Read-Hall and interface recombination currents in hybrid perovskites.

Metal-halide perovskite solar cells rival the best inorganic solar cells in power conversion efficiency, providing the outlook for efficient, cheap devices. In order for the technology to mature and approach the ideal Shockley-Queissier efficiency, experimental tools are needed to diagnose what processes limit performances, beyond simply measuring electrical characteristics often affected by parasitic effects and difficult to interpret. Here we study the microscopic origin of recombination currents causing photoconversion losses with an all-optical technique, measuring the electron-hole free energy as a function of the exciting light intensity. Our method allows assessing the ideality factor and breaks down the electron-hole recombination current into bulk defect and interface contributions, providing an estimate of the limit photoconversion efficiency, without any real charge current flowing through the device. We identify Shockley-Read-Hall recombination as the main decay process in insulated perovskite layers and quantify the additional performance degradation due to interface recombination in heterojunctions.

Absorption F-sum rule for the exciton binding energy in methylammonium lead halide perovskites.

Advances of optoelectronic devices based on methylammonium lead halide perovskites depend on understanding the role of excitons, whether it is marginal as in inorganic semiconductors, or crucial, like in organics. However, a consensus on the exciton binding energy and its temperature dependence is still lacking, even for widely studied methylammonium lead iodide and bromide materials (MAPbI3, MAPbBr3). Here we determine the exciton binding energy based on an f-sum rule for integrated UV-vis absorption spectra, circumventing the pitfalls of least-squares fitting procedures. In the temperature range 80-300 K, we find that the exciton binding energy in MAPbBr3 is EB = (60 ± 3) meV, independent of temperature; for MAPbI3, in the orthorhombic phase (below 140 K) EB = (34 ± 3) meV, while in the tetragonal phase the binding energy softens to 29 meV at 170 K and stays constant up to 300 K. Implications of binding energy values on solar cell and LED workings are discussed.

Correlated electron-hole plasma in organometal perovskites.

Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance.