Introduction
Symmetry is captivating, but asymmetry is also intriguing. In life, we witness some objects exhibit right-handedness while others showcase left-handedness. This exciting concept known as chirality refers to objects that cannot be superimposed on their mirror images. From seashells to glucose and amino acids, chirality permeates various aspects of nature. In everyday life chirality plays an important role in liquid crystal displays, pharmaceutical manufacturing, cosmetics, agrochemicals, and the fragrance industry. The study of chirality in functional materials has yielded remarkable applications, and the Center for Hybrid Organic-Inorganic Semiconductors for Energy (CHOISE) is at the forefront of this research.
CHOISE
Established in 2018, CHOISE is an Energy Frontier Research Center (EFRC) led by the National Renewable Energy Laboratory (NREL). Comprised of experts from multiple academic and research institutions, the Center focuses on unraveling how light and electrons interact with emerging semiconductor compositions. With a primary goal of understanding and advancing hybrid organic-inorganic semiconductors, CHOISE strives to revolutionize technologies like photovoltaics and create new technologies exploiting the unique nature of organic-inorganic coupling in materials at the leading edge of science.
Continuous exploration and development of novel material systems drive technological progress. Hybrid organic–inorganic semiconductors, which combine organic and inorganic components within a single structure, offer unparalleled properties stemming from synergistic effects. One notable example is hybrid halide perovskites, which have garnered immense attention in the past decade for their potential applications in electronics, most notably as light absorbers in photovoltaic panels [1,2]. Hybrid perovskites are defined by the general formula ABX3, where A is an organic cation, B is a metal cation, and X is halide anion. These perovskites exhibit diverse functions, tunability, and solution processing capabilities, complementing traditional semiconductors like Silicon or Gallium Arsenide. Moreover, they possess fascinating spintronic properties and introduce structural chirality into perovskite structures, leading to the emergence of chiral perovskites with implications beyond our current fundamental understanding [3]. Spintronics, derived from spin electronics, explores the utilization of both the electron's intrinsic spin and its accompanying magnetic moment, alongside its basic electronic charge, within solid-state devices. This field aims to go beyond traditional electronics, which focus solely on charge-based information processing.
CHOISE's Contributions
CHOISE has played a pivotal role in the development of chiral perovskites. Most optoelectronic devices involve interactions between light and electric current. Photovoltaic (PV) panels are a perfect example, and perovskites are regularly making headlines in PV research, promising to bring nearly 50% more power to silicon panels. However, electrons also have a “spin” character that can be utilized to enable new technologies that we haven’t imagined yet. The Center achieved a groundbreaking feat by creating a solution-processed perovskite spin polarized light-emitting diode (SpinLED) that emits polarized light at room temperature without the need for a magnetic field, enabling a simpler and cost-effective technology [4]. The SpinLED employs chiral perovskites, which act as electronic spin filters, enabling the emission of polarized light. In general, manipulating the spin properties in a semiconductor requires the use of ferromagnetic contacts under an applied magnetic field. However, chiral perovskites uniquely achieve spin control by polarizing the carrier spins as they pass through them. This opens a range of possibilities for futuristic technologies like 3D displays, holography, optical communication, and computing. Such spin-controlled semiconductors could offer high-fidelity computation, reducing electricity waste. Another exciting application of chiral perovskites lies in optical detectors for sensing polarized light. By integrating chiral perovskites into such detectors, the need for external polarizers can be eliminated, streamlining the sensing process and making it more efficient [5].
Chiral properties closely relate to the symmetry of the material, and to control the spin, one must break the structural symmetry. Chiral organic molecules in halide perovskites enable this symmetry breaking by distorting the crystal structure [6]. Controlling the spin transport behavior of halide perovskites holds promise for designing energy-efficient and advanced devices. Recent research supported by CHOISE demonstrated the generation of spin current in chiral 2D halide perovskites through the chiral phonon-activated spin Seebeck effect (CPASS) [7]. This effect occurs due the temperature difference, which causes certain vibrations called “phonons” in the chiral material to carry the spin information, resulting in the creation of a spin current. This groundbreaking combination of spintronics and thermoelectrics simplifies device architecture by eliminating the need for a ferromagnetic contact. CPASS can be observed in chiral semiconductors and chiral insulators at room temperature, paving the way for spin control and applications of halide perovskites in spintronic devices.
Future Outlook
Although significant progress has been made in synthesizing various chiral perovskites and studying their chiroptical properties, understanding the intricacies of spin transport in these semiconductors is still in the early stages of exploration. Looking ahead, CHOISE aims to propel future advancements in this field through several exciting directions. These include compositional tuning for higher asymmetry, establishing universal guidelines for chirality transfer, and developing emissive chiral layers and crystals. Another ambitious goal is to integrate chiral perovskites with existing semiconductor technologies to further enhance their efficacy. As CHOISE continues its research journey, the future holds immense possibilities for chiral perovskites to drive technological advancements and shape a more sustainable and efficient future.
