Alyssa N. Brigeman

Random lasing in organo-lead halide perovskite microcrystal networks

We report optically pumped random lasing in planar methylammonium lead iodide perovskite microcrystal networks that form spontaneously from spin coating. Low thresholds ( 100 μm and spatially overlap with one another, resulting in chaotic pulse-to-pulse intensity fluctuations due to gain competition. These results demonstrate this class of hybrid organic-inorganic perovskite as a platform to study random lasing with well-defined, low-level disorder, and support the potential of these materials for use in semiconductor laser applications.

Diode-Pumped Organo-Lead Halide Perovskite Lasing in a Metal-Clad Distributed Feedback Resonator

Organic-inorganic lead halide perovskite semiconductors have recently reignited the prospect of a tunable, solution-processed diode laser, which has the potential to impact a wide range of optoelectronic applications. Here, we demonstrate a metal-clad, second-order distributed feedback methylammonium lead iodide perovskite laser that marks a significant step toward this goal. Optically pumping this device with an InGaN diode laser at low temperature, we achieve lasing above a threshold pump intensity of 5 kW/cm(2) for durations up to ∼25 ns at repetition rates exceeding 2 MHz. We show that the lasing duration is not limited by thermal runaway and propose instead that lasing ceases under continuous pumping due to a photoinduced structural change in the perovskite that reduces the gain on a submicrosecond time scale. Our results indicate that the architecture demonstrated here could provide the foundation for electrically pumped lasing with a threshold current density Jth < 5 kA/cm(2) under sub-20 ns pulsed drive.

Enhanced sub-bandgap efficiency of a solid-state organic intermediate band solar cell using triplet–triplet annihilation

Conventional solar cells absorb photons with energy above the bandgap of the active layer while sub-bandgap photons are unharvested. One way to overcome this loss is to capture the low energy light in the triplet state of a molecule capable of undergoing triplet–triplet annihilation (TTA), which pools the energy of two triplet states into one high energy singlet state that can then be utilized. This mechanism underlies the function of an organic intermediate band solar cell (IBSC). Here, we report a solid-state organic IBSC that shows enhanced photocurrent derived from TTA that converts sub-bandgap light into charge carriers. Femtosecond resolution transient absorption spectroscopy and delayed fluorescence spectroscopy provide evidence for the triplet sensitization and upconversion mechanisms, while external quantum efficiency measurements in the presence of a broadband background light demonstrate that sub-bandgap performance enhancements are achievable in this device. The solid-state architecture introduced in this work serves as an alternative to previously demonstrated solution-based IBSCs, and is a compelling model for future research efforts in this area.

Efficient Upper-Excited State Fluorescence in an Organic Hyperbolic Metamaterial

Upper-excited state emission is not usually observed from molecules owing to competition with much faster nonradiative relaxation pathways; however, it can be made more efficient by modifying the photonic density of states to enhance the radiative decay rate. Here, we show that embedding the small molecule zinc tetraphenylporphyrin (ZnTPP) in a hyperbolic metamaterial enables an ∼18-fold increase in fluorescence intensity from the second singlet excited state ( S2) relative to that from the lowest singlet excited state ( S1). By varying the number of periods in the HMM stack, we are able to systematically tune the ZnTPP fluorescence spectrum from red (dominated by emission from S1) to blue (dominated by emission from S2) with an instrument-limited decay lifetime <10 ps. Our results are consistent with a broadband Purcell enhancement in the radiative rate of both transitions predicted via transfer matrix modeling and point to a general opportunity to harness upper-excited states for spectrally tunable, ultrafast fluorescence via radiative decay engineering.

Nanotextured solar cells using aluminum as a catalyst and dopant

A black silicon solar cell fabricated using aluminum as both a catalyst and dopant is demonstrated. A nanowire/nanopyramid black silicon surface texture is grown via aluminum (Al)-catalyzed vapor-liquid-solid growth, and post-growth annealing diffuses the aluminum into the n-type substrate, forming a p-n junction. Devices with nanopyramid surface textures are found to have higher short-circuit currents and open-circuit voltages than nanowire surface textures grown at lower temperatures, and post-growth annealing times of 15-30 minutes are found to promote higher short-circuit current densities. External quantum efficiency measurements show that the highest photoconversion occurs in the red and IR regions for all devices, with low quantum efficiencies at shorter wavelengths even when the p-type silicon surface is passivated with alumina. The quantum efficiency spectra imply that the devices are limited by recombination on the illuminated side of the device. Based on these results and previous data on Al-catalyzed wires and pyramids, excess Al incorporation and Al cluster formation in the emitter are suggested as the primary factors currently limiting device performance.