Two-dimensional (2D) perovskites have attracted considerable interest for their promising applications for solar cells and other optoelectronics, such as light-emitting diodes, spintronics, and photodetectors. Here, we review the recent achievements of 2D perovskites for various optoelectronic applications. First, we discuss the basic structure and optoelectronic properties of 2D perovskites, including band structure, optical properties, and charge transport. We then highlight recent achievements using 2D perovskites in solar cells and beyond solar cells, including progress on various synthesis strategies and their impact on structural and optoelectronic properties. Finally, we discuss current challenges and future opportunities to further develop 2D perovskites for various applications.

Advances in two-dimensional organic–inorganic hybrid perovskites

Hybrid organic–inorganic perovskites (HOIPs) offer long carrier-diffusion lengths, high absorption coefficients, tunable band gaps and long spin lifetimes. The flexible crystal structure and ionic nature of HOIPs make it possible to allow tuning of their material properties through rational design, including the incorporation of chiral organic ligands. Recently, chiral HOIPs have emerged as promising materials for chiroptoelectronics, spintronics and ferroelectrics. They exhibit high photoluminescence polarization (17% without an external magnetic field), good device performance (a circularly polarized photodetector had 100 times higher responsivity than one based on a chiral metasurface) and high saturated polarization (~2 times higher than that of barium titanate). Here, we review the latest advances in chiral HOIPs and investigate the specific benefits of combining chiral organic and inorganic components in perovskites. We discuss demonstrations of chiroptical and ferroelectric applications, and conclude with our perspective on the future opportunities for chiral HOIPs. Chiral hybrid organic–inorganic perovskites combine the remarkable optical, electrical and spintronic properties of perovskites with chirality. This Review systematically introduces the latest advances in chiral perovskites, surveys their structure–property relationships and details their chiroptical and ferroelectric applications.

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Chiral perovskite optoelectronics

Photonics and optoelectronics using nano-structured hybrid perovskite media and their optical cavities

Hybrid organic–inorganic perovskites exhibit strong spin–orbit coupling1, spin-dependent optical selection rules2,3 and large Rashba splitting4–8. These characteristics make them promising candidates for spintronic devices9 with photonic interfaces. Here we report that spin polarization in perovskites can be controlled through chemical design as well as by a magnetic field. We obtain both spin-polarized photon absorption and spin-polarized photoluminescence in reduced-dimensional chiral perovskites through combined strategies of chirality transfer and energy funnelling. A 3% spin-polarized photoluminescence is observed even in the absence of an applied external magnetic field owing to the different emission rates of σ+ and σ− polarized photoluminescence. Three-dimensional perovskites achieve a comparable degree of photoluminescence polarization only under an external magnetic field of 5 T. Our findings pave the way for chiral perovskites as powerful spintronic materials.

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Spin control in reduced-dimensional chiral perovskites

Hybrid lead halide perovskite materials are used in solar cells and show efficiencies greater than 23%. Furthermore, they are applied in light-emitting diodes, X-ray detectors, thin-film transistors, thermoelectrics, and memory devices. Lead trihalide hybrid materials contain methylammonium (MA) or formamidinium (FA) (or a mixture), or long alkylammonium halides, as alternative organic cations. However, the intrinsic stability of hybrid lead halide perovskites is not very high, and they are chemically unstable when exposed to moisture, light, or heat because of their organic contents and low formation energies. Therefore, although improvements in the chemical stability are crucial, changing the material composition is challenging because it is directly related to the desired application requirements. Fortunately, hybrid lead halide perovskites have a very high tolerance toward changes in physical properties arising from doping or addition of different cations and anions, in many cases showing improved properties. Here, the intrinsic instability of hybrid lead halide perovskites is reviewed in relation to the crystal phase and chemical stability. It is suggested that FA should be used for lead halide perovskites for chemical stability instead of MA. Furthermore, additives that stabilize the crystal phase with α-FAPbI3 should eschew MA.

Intrinsic Instability of Inorganic-Organic Hybrid Halide Perovskite Materials.

Hybrid organic–inorganic perovskites have emerged as a new class of semiconductors that exhibit excellent performance as active layers in photovoltaic solar cells. These compounds are also highly promising materials for the field of spintronics due to their large and tunable spin–orbit coupling, spin-dependent optical selection rules, and their predicted electrically tunable Rashba spin splitting. Here we demonstrate the optical orientation of excitons and optical detection of spin-polarized exciton quantum beating in polycrystalline films of the hybrid perovskite CH3NH3PbClxI3−x. Time-resolved Faraday rotation measurement in zero magnetic field reveals unexpectedly long spin lifetimes exceeding 1 ns at 4 K, despite the large spin–orbit couplings of the heavy lead and iodine atoms. The quantum beating of exciton states in transverse magnetic fields shows two distinct frequencies, corresponding to two g-factors of 2.63 and −0.33, which we assign to electrons and holes, respectively. These results provide a basic picture of the exciton states in hybrid perovskites, and suggest they hold potential for spintronic applications. Hybrid perovskites are known to have excellent optoelectronic properties, but the observation of exciton states with long spin lifetimes suggests that they may also have potential spintronics applications.

Ruizhi Wang

Papers

Spin-polarized exciton quantum beating in hybrid organic-inorganic perovskites