Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH₃NH₃PbI₃ Perovskite Nanocrystals

Photoinduced degradation of individual methylammonium lead triiodide (MAPbI₃) perovskite nanocrystals was studied using super-resolution luminescence microspectroscopy under intense light excitation. The photoluminescence (PL) intensity decrease and blue-shift of the PL spectrum up to 60 nm together with spatial shifts in the emission localization position up to a few hundred nanometers were visualized in real time. PL blinking was found to temporarily suspend the degradation process, indicating that the degradation needs a high concentration of mobile photogenerated charges to occur. We propose that the mechanistic process of degradation occurs as the three-dimensional MAPbI₃ crystal structure smoothly collapses to the two-dimensional layered PbI₂ structure. The degradation starts locally and then spreads over the whole crystal. The structural collapse is primarily due to migration of methylammonium ions (MA⁺), which distorts the lattice structure causing alterations to the Pb–I–Pb bond angle and in turn changes the effective band gap

Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH₃NH₃PbI₃ Perovskite Nanocrystals

Photoinduced degradation of individual methylammonium lead triiodide (MAPbI₃) perovskite nanocrystals was studied using super-resolution luminescence microspectroscopy under intense light excitation. The photoluminescence (PL) intensity decrease and blue-shift of the PL spectrum up to 60 nm together with spatial shifts in the emission localization position up to a few hundred nanometers were visualized in real time. PL blinking was found to temporarily suspend the degradation process, indicating that the degradation needs a high concentration of mobile photogenerated charges to occur. We propose that the mechanistic process of degradation occurs as the three-dimensional MAPbI₃ crystal structure smoothly collapses to the two-dimensional layered PbI₂ structure. The degradation starts locally and then spreads over the whole crystal. The structural collapse is primarily due to migration of methylammonium ions (MA⁺), which distorts the lattice structure causing alterations to the Pb–I–Pb bond angle and in turn changes the effective band gap

Tunable Percolation in Semiconducting Binary Polymer Nanoparticle Glasses

Binary polymer nanoparticle glasses provide opportunities to realize the facile assembly of disparate components, with control over nanoscale and mesoscale domains, for the development of functional materials. This work demonstrates that tunable electrical percolation can be achieved through semiconducting/insulating polymer nanoparticle glasses by varying the relative percentages of equal-sized nanoparticle constituents of the binary assembly. Using time-of-flight charge carrier mobility measurements and conducting atomic force microscopy, we show that these systems exhibit power law scaling percolation behavior with percolation thresholds of ∼24–30%. We develop a simple resistor network model, which can reproduce the experimental data, and can be used to predict percolation trends in binary polymer nanoparticle glasses. Finally, we analyze the cluster statistics of simulated binary nanoparticle glasses, and characterize them according to their predominant local motifs as (<i>p</i>_<i>i</i>, <i>p</i>_1‑<i>i</i>)-connected networks that can be used as a supramolecular toolbox for rational material design based on polymer nanoparticles

Multiscale Active Layer Morphologies for Organic Photovoltaics Through Self-Assembly of Nanospheres

We address here the need for a general strategy to control molecular assembly over multiple length scales. Efficient organic photovoltaics require an active layer comprised of a mesoscale interconnected networks of nanoscale aggregates of semiconductors. We demonstrate a method, using principles of molecular self-assembly and geometric packing, for controlled assembly of semiconductors at the nanoscale and mesoscale. Nanoparticles of poly­(3-hexylthiophene) (P3HT) or [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) were fabricated with targeted sizes. Nanoparticles containing a blend of both P3HT and PCBM were also fabricated. The active layer morphology was tuned by the changing particle composition, particle radii, and the ratios of P3HT:PCBM particles. Photovoltaic devices were fabricated from these aqueous nanoparticle dispersions with comparable device performance to typical bulk-heterojunction devices. Our strategy opens a revolutionary pathway to study and tune the active layer morphology systematically while exercising control of the component assembly at multiple length scales

Interplay between Ion Transport, Applied Bias, and Degradation under Illumination in Hybrid Perovskite p‑i‑n Devices

We studied ion transport in hybrid organic–inorganic perovskite p-i-n devices as a function of applied bias under device operating conditions. Using electrochemical impedance spectroscopy (EIS) and equivalent circuit modeling, we elucidated various resistive and capacitive elements in the device. We show that ion migration is predictably influenced by a low applied forward bias, characterized by an increased capacitance at the hole-transporting (HTM) and electron-transporting material (ETM) interfaces, as well as in bulk. However, unlike observations in n-i-p devices, we found that there is a capacitive discharge leading to ion redistribution in the bulk at high forward biases. Furthermore, we show that a chemical double-layer capacitance buildup as a result of ion accumulation impacts the electronic properties of the device, likely by inducing either charge pinning or charge screening, depending on the direction of the ion-induced field. Lastly, we extrapolate ion diffusion coefficients (∼10^–7 cm² s^–1) and ionic conductivities (∼10^–7 S cm^–1) from the Warburg mass (ion) diffusion response and show that, as the device degrades, there is an overall depletion of capacitive effects coupled with increased ion mobility

High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer

Perovskite-containing tandem solar cells are attracting attention for their potential to achieve high efficiencies. We demonstrate a series connection of a ∼90 nm thick perovskite front subcell and a ∼100 nm thick polymer:fullerene blend back subcell that benefits from an efficient graded recombination layer containing a zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO₃). This methodology eliminates the adverse effects of thermal annealing or chemical treatment that occurs during perovskite fabrication on polymer-based front subcells. The record tandem perovskite/polymer solar cell efficiency of 16.0%, with low hysteresis, is 75% greater than that of the corresponding ∼90 nm thick perovskite single-junction device and 65% greater than that of the polymer single-junction device. The high efficiency of this hybrid tandem device, achieved using only a ∼90 nm thick perovskite layer, provides an opportunity to substantially reduce the lead content in the device, while maintaining the high performance derived from perovskites

Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability

Solar cells fabricated using alkyl ammonium metal halides as light absorbers have the right combination of high power conversion efficiency and ease of fabrication to realize inexpensive but efficient thin film solar cells. However, they degrade under prolonged exposure to sunlight. Herein, we show that this degradation is quasi-reversible, and that it can be greatly lessened by simple modifications of the solar cell operating conditions. We studied perovskite devices using electrochemical impedance spectroscopy (EIS) with methylammonium (MA)-, formamidinium (FA)-, and MA_<i>x</i>FA_1–<i>x</i> lead triiodide as active layers. From variable temperature EIS studies, we found that the diffusion coefficient using MA ions was greater than when using FA ions. Structural studies using powder X-ray diffraction (PXRD) show that for MAPbI₃ a structural change and lattice expansion occurs at device operating temperatures. On the basis of EIS and PXRD studies, we postulate that in MAPbI₃ the predominant mechanism of accelerated device degradation under sunlight involves thermally activated fast ion transport coupled with a lattice-expanding phase transition, both of which are facilitated by absorption of the infrared component of the solar spectrum. Using these findings, we show that the devices show greatly improved operation lifetimes and stability under white-light emitting diodes, or under a solar simulator with an infrared cutoff filter or with cooling