Research

Ever since the successful demonstration of blue light-emitting diodes (LEDs) by Shuji Nakamura, Hiroshi Amano, and Isamu Akasaki (which, by the way, led to the 2014 Nobel Prize in Physics), our world has never been the same. Our room lighting, streetlights, computers, TVs, phones, and smartwatches benefit immensely from this discovery. Many commercial LEDs are made from III-V materials (i.e., materials from groups III and V of the periodic table) which can be both expensive and difficult to process. I believe there is an opportunity for next-generation light emission based on new classes of semiconducting optoelectronic nanomaterials. My research deals primarily with engineering alternative emissive metal halide perovskite materials and devices for next-generation lighting, display, public health, and 3D printing applications. Given the many advantages (e.g., inexpensive processing, sharp color purity, and bandgap tunability) of emissive perovskite semiconductors across both the visible and ultraviolet spectrums, this semiconductor class shows strong potential for next-generation LEDs!

Transition Metal Doped Perovskite Light-Emitting Diodes

Metal halide perovskites (MHPs) have inherent properties that show strong potential for light emission-based optoelectronic applications. MHPs typically employ an ABX3 crystal structure where A is an organic, inorganic, or hybrid cation, B is typically lead or tin, and X is a halide anion.

By introducing dopants into the B-site, one can improve the MHPs' optoelectronic properties, which can result into higher performance devices.

I have investigated the introduction of transition metal dopants (e.g., Mn2+, Ni2+, Zn2+, etc.) into the MHPs' B-site and their effects on perovskite LED performance. For instance, in our JACS paper we looked at the effects of transition metal dopants on MHP nanocrystals at device relevant conditions. We showed both theoretically and experimentally how the lattice periodicity breaking effect primarily enhances the radiative rates in doped MHP nanocrystals.

Zooming in on the effects of Mn2+ on perovskite LEDs, I engineered Mn2+ doped perovskite LEDs and studied their performance with respect to external quantum efficiency and operational stability. In our Device paper, by introducing a molecular additive known as tris(4-fluorophenyl)phosphine oxide (TFPPO), we were able to simultaneously achieve incredibly bright and efficient transition metal doped LEDs. However, we also found that the operational stability of these LEDs is degraded with higher concentrations of TFPPO. We studied this efficiency-stability trade-off with optoelectronic and photophysical characterization on the Mn2+ doped perovskite LEDs and found that while TFPPO treatment does initially improve LED performance, this enhancement is not resilient under identical device operating conditions. This implies that before phosphine oxide additives are universally adopted in perovskite LEDs, their stability degradation mechanisms must be further investigated.

Vacuum-Evaporated Perovskite Light-Emitting Diodes

Today, most perovskite LEDs are fabricated using spin-coating techniques for the solution-based deposition of the perovskite active layer. While these techniques have led to rapid progress in achieving high external quantum efficiencies(EQEs) in perovskite LEDs (within the last decade, >20% EQEs have been achieved for red and green perovskite LEDs), spin-coating based fabrication of perovskite LEDs limits their commercial viability. This is largely due to the large sample-sample variability and substrate area limitations incurred by spin-coating. In contrast, other LEDs such as organic LEDs (OLEDs) are fabricated using vacuum thermal evaporation techniques.

Vacuum thermal evaporation (VTE) holds the potential to reduce perovskite LED variability, increase pixel resolution by using precise metal masks, and enable cleaner production since no harmful solvents are needed. VTE involves heating perovskite precursor materials to form gaseous molecules that then interact with each other on the LED substrate surface to form a perovskite active layer. This process involves careful optimization of the nucleus formation, stoichiometric ratio of precursors, and regulation of evaporation rates to result in efficient vacuum-evaporated perovskite LEDs.

I have fabricated lead-free vacuum-evaporated rare-earth halide perovskite LEDs with ultraviolet photoluminescent light emission as short as 367 nm, electroluminescent light emission as short as 390 nm, and external quantum efficiencies as high as 0.33%. Currently, I am working to further optimize the performance of this LED by pushing deeper into the ultraviolet spectrum (i.e., < 390 nm) and further improve device efficiency.

Lead-Free Perovskite Nanoplatelets

While most metal halide perovskites (MHPs) will employ a 3D ABX3 structure, we can employ a ligand (or ligand-like) species, L, to limit the growth of the MHP in one dimension and result in a colloidally stable perovskite nanoplatelet structure. Thus, the formula of perovskite nanoplatelets is denoted by L2[ABX3]n-1BX4 where L is the ligand species and n-1 represents the thickness of the nanoplatelet in terms of the bulk perovskite.

Previously, the use of lead within colloidal perovskite nanoplatelets has been well studied and has demonstrated tunable emission across the visible spectrum. Additionally, the use of tin instead of lead has also been studied within perovskite nanoplatelets but generally lacks the stability and tunability offered by lead-based perovskite nanoplatelets.

In order to enable lead-free perovskite-based light emission, we must consider other candidates besides tin. In particular, there is substantial interest to enable lead-free light emission in the deep blue, violet, and near-ultraviolet ranges given the limited demonstrations.

By employing rare-earth elements, I have synthesized and characterized both solution- and solid-state 2D lead-free europium halide perovskite nanoplatelets that emit in the deep blue and near-ultraviolet ranges (between 400 and 450 nm) - demonstrating a new route towards non-toxic high energy visible light emission. For more details, please see our pre-print!

Solution-Processed Wide Bandgap Perovskite Light-Emitting Diodes

While the vast majority of perovskite LEDs have focused on light emission within the visible spectrum, there is significant interest in engineering perovskite LEDs that can emit at higher energies (i.e., the ultraviolet (UV) spectrum). UV LEDs have direct applications in human health applications (e.g., sterilization and water purification), 3D printing, sensing, and nanofabrication.

In order to design perovskites with wider bandgaps which can then emit higher energy light, we must consider additional tunability techniques in addition to halide engineering. One potential pathway is the integration of quantum confined 2D lead halide perovskites.

In our Matter paper, we introduced a trace amount of water into the perovskite precursor solution to control the crystallization of the 2D PEA2PbBr4 thin film. As a result, we were able to demonstrate a 408 nm violet-emitting perovskite LED with a maximum external quantum efficiency of 0.41%, a five-fold increase over control devices.

In our Device paper, we showed that by replacing 25% of the bromide within the 2D PEA2PbBr4 perovskite with chloride as well as employing a dual electron transport layer device architecture, we can achieve a 399 nm UV-emitting perovskite LED based on a PEA2PbCl1Br3 perovskite. This perovskite LED demonstrated a maximum external quantum efficiency of 0.16% and is, to our best knowledge, the first demonstration of UV light emission from a lead halide perovskite.