Passivation of interfacial defects serves as an effective means to realize highly efficient and stable perovskite solar cells (PSCs). However, most molecular modulators currently used to mitigate such defects form poorly conductive aggregates at the perovskite interface with the charge collection layer, impeding the extraction of photogenerated charge carriers. Here, a judiciously engineered passivator, 4‐tert‐butyl‐benzylammonium iodide (tBBAI), is introduced, whose bulky tert‐butyl groups prevent the unwanted aggregation by steric repulsion. It is found that simple surface treatment with tBBAI significantly accelerates the charge extraction from the perovskite into the spiro‐OMeTAD hole‐transporter, while retarding the nonradiative charge carrier recombination. This boosts the power conversion efficiency (PCE) of the PSC from ≈20% to 23.5% reducing the hysteresis to barely detectable levels. Importantly, the tBBAI treatment raises the fill factor from 0.75 to the very high value of 0.82, which concurs with a decrease in the ideality factor from 1.72 to 1.34, confirming the suppression of radiation‐less carrier recombination. The tert‐butyl group also provides a hydrophobic umbrella protecting the perovskite film from attack by ambient moisture. As a result, the PSCs show excellent operational stability retaining over 95% of their initial PCE after 500 h full‐sun illumination under maximum‐power‐point tracking under continuous simulated solar irradiation.

We report the observation of a very large negative magnetoresistance effect in a van der Waals tunnel junction incorporating a thin magnetic semiconductor, CrI, as the active layer. At constant voltage bias, current increases by nearly one million percent upon application of a 2 T field. The effect arises from a change between antiparallel to parallel alignment of spins across the different CrI layers. Our results elucidate the nature of the magnetic state in ultrathin CrI and present new opportunities for spintronics based on two-dimensional materials.

We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-and bi-layer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, as well as magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk.

We demonstrate clear weak anti-localization (WAL) effect arising from induced Rashba spin-orbit coupling (SOC) in WS2-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single-layer graphene flake, WAL in WS2-covered graphene occurs over a wide range of carrier densities on both electron and hole sides. At high carrier densities, we estimate the Rashba SOC relaxation rate to be ∼ 0.2ps −1 and show that it can be tuned by transverse electric fields. In addition to the Rashba SOC, we also predict the existence of a'valley-Zeeman' SOC from first-principles calculations. The interplay between these two SOC's can open a non-topological but interesting gap in graphene; in particular, zigzag boundaries host four sub-gap edge states protected by time-reversal and crystalline symmetries. The graphene/WS2 system provides a possible platform for these novel edge states.

Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials. So far, a comprehensive understanding of the magnetic excitations in such 2D magnets remains missing. Here we report polarized micro-Raman spectroscopy studies on a 2D honeycomb ferromagnet CrI3. We show the definitive evidence of two sets of zero-momentum spin waves at frequencies of 2.28 terahertz (THz) and 3.75 THz, respectively, that are three orders of magnitude higher than those of conventional ferromagnets. By tracking the thickness dependence of both spin waves, we reveal that both are surface spin waves with lifetimes an order of magnitude longer than their temporal periods. Our results of two branches of high-frequency, long-lived surface spin waves in 2D CrI3 demonstrate intriguing spin dynamics and intricate interplay with fluctuations in the 2D limit, thus opening up opportunities for ultrafast spintronics incorporating 2D magnets.

Carbon-based electrodes have been widely applied in perovskite solar cells (PSCs) because of their chemical inertness and compatibility with up-scalable techniques, signifying their solid potential for mass-production. The material scarcity...

Record power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have been obtained with the organic hole transporter 2,2′,7,7′-tetrakis( N , N -di- p -methoxyphenyl-amine)9,9′-spirobifluorene (spiro-OMeTAD). Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4- tert -butylpyridine is a time-consuming process and also leads to poor device stability. We developed a new doping strategy for spiro-OMeTAD that avoids post-oxidation by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated radical doping). We achieved PCEs of >25% and much-improved device stability under harsh conditions. The radicals provide hole polarons that instantly increase the conductivity and work function (WF), and ionic salts further modulate the WF by affecting the energetics of the hole polarons. This organic semiconductor doping strategy, which decouples conductivity and WF tunability, could inspire further optimization in other optoelectronic devices.

Despite its extremely weak intrinsic spin-orbit coupling (SOC), graphene has been shown to acquire considerable SOC by proximity coupling with exfoliated transition metal dichalcogenides (TMDs). Here we demonstrate strong induced Rashba SOC in graphene that is proximity coupled to a monolayer TMD film, MoS 2 or WSe 2 , grown by chemical vapor deposition with drastically different Fermi level positions. Graphene/TMD heterostructures are fabricated with a pickup-transfer technique utilizing hexagonal boron nitride, which serves as a flat template to promote intimate contact and therefore a strong interfacial interaction between TMD and graphene as evidenced by quenching of the TMD photoluminescence. We observe strong induced graphene SOC that manifests itself in a pronounced weak anti-localization (WAL) effect in the graphene magnetoconductance. The spin relaxation rate extracted from the WAL analysis varies linearly with the momentum scattering time and is independent of the carrier type. This indicates a dominantly Dyakonov-Perel spin relaxation mechanism caused by the induced Rashba SOC. Our analysis yields a Rashba SOC energy of ~1.5 meV in graphene/WSe 2 and ~0.9 meV in graphene/MoS 2 , respectively. The nearly electron-hole symmetric nature of the induced Rashba SOC provides a clue to possible underlying SOC mechanisms.