Polarons in Halide Perovskites: A Perspective.

Metal halide perovskites (MHPs) have rapidly emerged as leading contenders in photovoltaic technology and other optoelectronic applications owing to their outstanding optoelectronic properties. After a decade of intense research, an in-depth understanding of the charge carrier transport in MHPs is still an active topic of debate. In this Perspective, we discuss the current state of the field by summarizing the most extensively studied carrier transport mechanisms, such as electron-phonon scattering limited dynamics, ferroelectric effects, Rashba-type band splitting, and polaronic transport. We further extensively discuss the emerging experimental and computational evidence for dominant polaronic carrier dynamics in MHPs. Focusing on both small and large polarons, we explore the fundamental aspects of their motion through the lattice, protecting the photogenerated charge carriers from the recombination process. Finally, we outline different physical and chemical approaches considered recently to study and exploit the polaron transport in MHPs.

Low temperature atomic layer deposition of zirconium oxide for inkjet printed transistor applications.

We report the growth of zirconium oxide (ZrO2) as a high-k gate dielectric for an inkjet-printed transistor using a low-temperature atomic layer deposition (ALD) from tetrakis(dimethylamido)zirconium (TDMAZr) and water precursors. All the samples are deposited at low-temperature ranges of 150-250 °C. The films are very uniform with RMS roughness less than 4% with respect to their thickness. The atomic force microscopy (AFM) shows a significant change in surface morphology from tapered posts to undulating mountain-like structures with several hundreds of ALD cycles. The results from X-ray diffraction (XRD) analysis exhibit an amorphous to the crystalline structure with temperature variation, which is independent of the thickness of the films. All our samples are hydrophilic as contact angles are less than 90°. The capacitance-voltage (C-V) and conductance-voltage (G p/ω-V) characteristics of ZrO2 dielectrics for silicon metal-oxide-semiconductor (MOS) capacitors are studied for different temperatures. For the n-type substrate MOS capacitors, the dielectric constants are estimated to be 7.5-11. Due to the low deposition temperature, a hydrophilic surface, and high k value, the ALD-ZrO2 dielectric can be compatible for printed transistors. The processes of fabrication and characterization of inkjet-printed graphene transistors is demonstrated using the ZrO2 dielectric. The possible solvents, surfactant, and the dielectric induced modifications in graphene flakes are demonstrated by Raman spectra. The graphene flakes spread uniformly on the ZrO2 surface. The functional inkjet-printed graphene transistor characteristics are demonstrated to illustrate the field effect behavior with the ALD-ZrO2 dielectric.

Photoactivated Mixed In-Plane and Edge-Enriched p-Type MoS2 Flake-Based NO2 Sensor Working at Room Temperature.

Toxic gases are produced during the burning of fossil fuels. Room temperature (RT) fast detection of toxic gases is still challenging. Recently, MoS2 transition metal dichalcogenides have sparked great attention in the research community due to their performance in gas sensing applications. However, MoS2 based gas sensors still suffer from long response and recovery times, especially at RT. Considering this challenge, here, we report photoactivated highly reversible and fast detection of NO2 sensors at room temperature (RT) by using mixed in-plane and edge-enriched p-MoS2 flakes (mixed MoS2). The sensor showed fast response with good sensitivity of ∼10.36% for 10 ppm of NO2 at RT without complete recovery. However, complete recovery was obtained with better sensor performance under UV light illumination at RT. The UV assisted NO2 sensing showed improved performance in terms of fast response and recovery kinetics with enhanced sensitivity to 10 ppm NO2 concentration. The sensor performance is also investigated under thermal energy, and a better sensor performance with reduced sensitivity and high selectivity toward NO2 was observed. A detailed gas sensing mechanism based on the density functional theory (DFT) calculations for favorable NO2 adsorption sites on in-plane and edge-enriched MoS2 flakes is proposed. This study revealed the role of favorable adsorption sites in MoS2 flakes for the enhanced interaction of target gases and developed a highly sensitive, reversible, and fast gas sensor for next-generation toxic gases at room temperature.

Solvent Toolkit for Electrochemical Characterization of Hybrid Perovskite Films.

Organohalide lead (hybrid) perovskites have emerged as competitive semiconducting materials for photovoltaic devices due to their high performance and low cost. To further the understanding and optimization of these materials, solution-based methods for interrogating and modifying perovskite thin films are needed. In this work, we report a hydrofluoroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of organic-inorganic trihalide lead perovskite thin films. Organic perovskite films are soluble in most of the polar organic solvents, and thus until now, they were not considered suitable for electrochemical processing. We have enabled electrochemical characterization and demonstrated a processing toolset for these materials utilizing highly fluorinated electrolytes based on a HFE solvent. Our results show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties, and potential electrochemical doping of the films with in situ diagnostic capabilities.

Nanoimprinted Perovskite Nanograting Photodetector with Improved Efficiency.

Recently, organolead halide-based perovskites have emerged as promising materials for optoelectronic applications, particularly for photovoltaics, photodetectors, and lasing, with low cost and high performance. Meanwhile, nanoscale photodetectors have attracted tremendous attention toward realizing miniaturized optoelectronic systems, as they offer high sensitivity, ultrafast response, and the capability to detect beyond the diffraction limit. Here we report high-performance nanoscale-patterned perovskite photodetectors implemented by nanoimprint lithography (NIL). The spin-coated lead methylammonium triiodide perovskite shows improved crystallinity and optical properties after NIL. The nanoimprinted metal-semiconductor-metal photodetectors demonstrate significantly improved performance compared to the nonimprinted conventional thin-film devices. The effects of NIL pattern geometries on the optoelectronic characteristics were studied, and the nanograting pattern based photodetectors demonstrated the best performance, showing approximately 35 times improvement on responsivity and 7 times improvement on on/off ratio compared with the nonimprinted devices. The high performance of NIL-nanograting photodetectors likely results from high crystallinity and favored nanostructure morphology, which contribute to higher mobility, longer diffusion length, and better photon absorption. Our results have demonstrated that the NIL is a cost-effective method to fabricate high-performance perovskite nanoscale optoelectronic devices, which may be suitable for manufacturing of high-density perovskite nanophotodetector arrays and to provide integration with state-of-the-art electronic circuits.