No abstract

We reveal substantial luminescence yield heterogeneity among individual subdiffraction grains of high-performing methylammonium lead halide perovskite films by using highresolution cathodoluminescence microscopy. Using considerably lower accelerating voltages than is conventional in scanning electron microscopy, we image the electron beam-induced luminescence of the films and statistically characterize the depthdependent role of defects that promote nonradiative recombination losses. The highest variability in the luminescence intensity is observed at the exposed grain surfaces, which we attribute to surface defects. By probing deeper into the film, it appears that bulk defects are more homogeneously distributed. By identifying the origin and variability of a surface-specific loss mechanism that deleteriously impacts device efficiency, we suggest that producing films homogeneously composed of the highest-luminescence grains found in this study could result in a dramatic improvement of overall device efficiency. We also show that although cathodoluminescence microscopy is generally used only to image inorganic materials it can be a powerful tool to investigate radiative and nonradiative charge carrier recombination on the nanoscale in organic−inorganic hybrid materials.

We study the interaction of mobile ions and electronic charges to form nonradiative defects during electric biasing of methylammonium lead triiodide (MAPbI 3 ) and formamidinium lead triiodide (FAPbI 3 ) thin films. Using multimodal microscopy that combines in situ photoluminescence and scanning Kelvin probe microscopy in a lateral electrode geometry, we correlate temporal changes in radiative recombination with the spatial movement of ionic and electronic charge carriers. Importantly, we compare trap formation with both charge injecting and blocking contacts. Even though ion migration takes place in both cases, we observe the formation of new nonradiative defects in MAPbI 3 only in the presence of injected electrons, suggesting that redox processes play a key role. On the basis of density functional theory (DFT) simulations, we propose that reduction of Pb 2+ to Pb 0 is responsible for the new defects formed in our films. These results underscore that defect properties in metal halide perovskites are not only determined by the migration of mobile ions but are also highly sensitive to their interaction with injected electronic charge.

We study light-induced dynamics in thin films comprising Ruddlesden–Popper phases of the layered 2D perovskite (C4H9NH3)2PbI4. We probe ionic and electronic carrier dynamics using two complementary scanning probe methods, time-resolved G-mode Kelvin probe force microscopy and fast free time-resolved electrostatic force microscopy, as a function of position, time, and illumination. We show that the average surface photovoltage sign is dominated by the band bending at the buried perovskite–substrate interface. However, the film exhibits substantial variations in the spatial and temporal response of the photovoltage. Under illumination, the photovoltage equilibrates over hundreds of microseconds, a time scale associated with ionic motion and trapped electronic carriers. Surprisingly, we observe that the surface photovoltage of the 2D grain centers evolves more rapidly in time than at the grain boundaries. We propose that the slower evolution at grain boundaries is due to a combination of ion migration occurring between PbI4 planes, as well as electronic carriers traversing grain boundary traps, thereby changing the time-dependent band unbending at grain boundaries. These results provide a model for the photoinduced dynamics in 2D perovskites and are a useful basis for interpreting photovoltage dynamics on hybrid 2D/3D structures.

Ion migration is seen as a primary stability concern of halide perovskite-based photovoltaic and optoelectronic devices. Here, we provide experimental studies of long-distance, reversible ion migration in methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3) films. We use time-resolved scanning Kelvin probe microscopy on insulator-coated lateral electrodes to probe the dynamic redistribution of charged Frenkel defects over micrometer distances after application of an electric field. We combine these dynamic measurements with drift–diffusion simulations that yield self-consistent pictures of the sign, distribution, mobility, and activation energy of the associated, mobile Frenkel defects. This comprehensive approach is applied to study the impact of an organic cation on ionic mobility in metal halide perovskites, which we find to be significantly reduced in the case of FAPbI3 films compared to MAPbI3 films.

We conduct correlated laser scanning confocal photoluminescence (PL) microscopy, scanning kelvin probe microscopy, and conductive atomic force microscopy (c-AFM) to understand the origins and effects of local heterogeneity in films of the hybrid organic–inorganic perovskite semiconductor methylammonium lead tribromide (MAPbBr3). We compare PL between perovskite films deposited on glass and on hole-transporting contacts. In both systems, we observe heterogeneous PL, but this heterogeneity is due to different mechanisms. On glass substrates, we observe that the PL maps are dominated by lateral carrier diffusion, and on hole-transporting contacts, we observe an anticorrelation between PL and local hole injected current as measured by c-AFM. We conclude that the local variations are due to heterogeneous electronic coupling at the perovskite–electrode interface. We also show that correlated PL and AFM studies are expected to play a key role in studying the electronic heterogeneities in the perovskite itself, which are currently screened by the perovskite–contact interfaces. Our results suggest the need for new selective contacts to improve the charge transfer at the perovskite–contact interfaces.

The effect of carrier–carrier and carrier–phonon interactions is presented in n = 1 (2D) (BA)2PbI4 Ruddlesden–Popper thin films, and their effect is compared to that of conventional MAPbI3. While temperature-dependent photoluminescence shows the well-studied structural phase transitions and evidence of longitudinal optical (LO) phonon scattering in MPbI3, the 2D (BA)2PbI4 films produce subtler properties. At low temperatures, evidence of two competing intrinsic exciton transitions is observed (P1 and P3), in addition to several extrinsic transitions attributed to the impurities in the (BA)2PbI4 film. At higher temperatures, (BA)2PbI4 is dominated by the two intrinsic excitons that are attributed to varying degrees of localization caused by deformations of the PbI4 framework mediated by structural relaxation. Although both exciton complexes are well separated from the continuum (490 meV or greater), their interaction with phonons is quite different. In the case of the more strongly bound complex (P3), the strong excitonic nature and short-range interaction are mediated by the emission of LO phonon replicas. In the case of the exciton with the smaller binding energy (P1), the more extended wavefunction manifests itself in strong carrier–phonon scattering and evidence of large, stable polarons.

We demonstrate a new nanoimaging platform in which optical excitations generated by a low-energy electron beam in an ultrathin scintillator are used as a noninvasive, near-field optical scanning probe of an underlying sample. We obtain optical images of Al nanostructures with 46 nm resolution and validate the noninvasiveness of this approach by imaging a conjugated polymer film otherwise incompatible with electron microscopy due to electron-induced damage. The high resolution, speed, and noninvasiveness of this "cathodoluminescence-activated" platform also show promise for super-resolution bioimaging.