The dynamic surface evolution of halide perovskites induced by external energy stimulation.

Tracking the dynamic surface evolution of metal halide perovskite is crucial for understanding the corresponding fundamental principles of photoelectric properties and intrinsic instability. However, due to the volatility elements and soft lattice nature of perovskites, several important dynamic behaviors remain unclear. Here, an ultra-high vacuum (UHV) interconnection system integrated by surface-sensitive probing techniques has been developed to investigate the freshly cleaved surface of CH3NH3PbBr3  in situ under given energy stimulation. On this basis, the detailed three-step chemical decomposition pathway of perovskites has been clarified. Meanwhile, the evolution of crystal structure from cubic phase to tetragonal phase on the perovskite surface has been revealed under energy stimulation. Accompanied by chemical composition and crystal structure evolution, electronic structure changes including energy level position, hole effective mass, and Rashba splitting have also been accurately determined. These findings provide a clear perspective on the physical origin of optoelectronic properties and the decomposition mechanism of perovskites.

Pushing the Limit of Open-Circuit Voltage Deficit via Modifying Buried Interface in CsPbI3 Perovskite Solar Cells.

Although CsPbI3 perovskites have shown tremendous potential in the photovoltaic field owing to their excellent thermal stability, the device performance is seriously restricted by severe photovoltage loss. The buried titanium oxide/perovskite interface plays a critical role in interfacial charge transport and perovskite crystallization, which is closely related to open-circuit voltage deficit stemming from nonradiative recombination. Herein, target molecules named 3-sulphonatopropyl acrylate potassium salts are deliberately employed with special functional groups for modifying the buried interface, giving rise to favorable functions in terms of passivating interfacial defects, optimizing energetic alignment, and facilitating perovskite crystallization. Experimental characterizations and theoretical calculations reveal that the buried interface modification inhibits the electron transfer barrier and simultaneously improves perovskite crystal quality, thereby reducing trap-assisted charge recombination and interfacial energetic loss. Consequently, the omnibearing modification regarding the buried interface endows the devices with an impressive efficiency of 20.98%, achieving a record-low VOC deficit of 0.451 V. The as-proposed buried interface modification strategy renders with a universal prescription to push the limit of VOC deficit, showing a promising future in developing high-performance all-inorganic perovskite photovoltaics.

Two-dimensional wide-bandgap GeSe2 vertical ultraviolet photodetectors with high responsivity and ultrafast response speed.

Germanium selenide (GeSe2), as a typical member of 2D wide bandgap semiconductors (WBSs), shows great potential in ultraviolet (UV) optoelectronics due to its excellent flexibility, superior environmental stability, competitive UV absorption coefficient, and significant spectral selectivity. However, the GeSe2-based UV photodetector suffers from high operation voltages and low photocurrent, which prevents its practical imaging applications. In this work, we report an elevated photocurrent generation in a vertical stacking graphene/GeSe2/graphene heterostructure with low operation voltage and low power consumption. Efficient collection of photoexcited carriers in GeSe2 through graphene electrodes results in outstanding UV detection properties, including a pronounced responsivity of 37.1 A W-1, a specific detectivity of 8.83 × 1011 Jones, and an ultrahigh on/off ratio (∼105) at 355 nm. In addition, building a Schottky barrier between GeSe2 and graphene and reducing the channel length can increase the photoresponse speed to ∼300 µs. These accomplishments set the stage for future optoelectronic applications of vertical 2D WBS heterostructure UV photodetectors.

Asymmetric Ferroelectric-Gated Two-Dimensional Transistor Integrating Self-Rectifying Photoelectric Memory and Artificial Synapse.

Ferroelectric field-effect transistors (Fe-FET) are promising candidates for future information devices. However, they suffer from low endurance and short retention time, which retards the application of processing memory in the same physical processes. Here, inspired by the ferroelectric proximity effects, we design a reconfigurable two-dimensional (2D) MoS2 transistor featuring with asymmetric ferroelectric gate, exhibiting high memory and logic ability with a program/erase ratio of over 106 and a self-rectifying ratio of 103. Interestingly, the robust electric and optic cycling are obtained with a large switching ratio of 106 and nine distinct resistance states upon optical excitation with excellent nonvolatile characteristics. Meanwhile, the operation of memory mimics the synapse behavior in response to light spikes with different intensity and number. This design realizes an integration of robust processing memory in one single device, which demonstrates a considerable potential of an asymmetric ferroelectric gate in the development of Fe-FETs for logic processing and nonvolatile memory applications.

Interpretation of Rubidium-Based Perovskite Recipes toward Electronic Passivation and Ion-Diffusion Mitigation.

Rubidium cation (Rb+ ) addition is witnessed to play a pivotal role in boosting the comprehensive performance of organic-inorganic hybrid perovskite solar cells. However, the origin of such success derived from irreplaceable superiorities brought by Rb+ remains ambiguous. Herein, grain-boundary-including atomic models are adopted for the accurate theoretical analysis of practical Rb+ distribution in perovskite structures. The spatial distribution, covering both the grain interiors and boundaries, is thoroughly identified by virtue of synchrotron-based grazing-incidence X-ray diffraction. On this basis, the prominent elevation of the halogen vacancy formation energy, improved charge-carrier dynamics, and the electronic passivation mechanism in the grain interior are expounded. As evidenced by the increased energy barrier and suppressed microcurrent, the critical role of Rb+ addition in blocking the diffusion pathway along grain boundaries, inhibiting halide phase segregation, and eventually enhancing intrinsic stability is elucidated. Hence, the linkage avalanche effect of occupied location dominated by subtle changes in Rb+ concentration on electronic defects, ion migration, and phase stability is completely investigated in detail, shedding a new light on the advancement of high-efficiency cascade-incorporating strategies and perovskite compositional engineering.

Advent of alkali metal doping: a roadmap for the evolution of perovskite solar cells.

Metal-halide hybrid perovskites have prompted the prosperity of the sustainable energy field and simultaneously demonstrated their great potential in meeting both the growing consumption of energy and the increasing social development requirements. Their inimitable features such as strong absorption ability, direct photogeneration of free carriers, long carrier diffusion lengths, ease of fabrication, and low production cost triggered the development of perovskite solar cells (PSCs) at an incredible rate, which soon reached power conversion efficiencies up to the commercialized level. During their evolution process, it has been witnessed that alkali metal cations play a pivotal role in the crystal structure as well as intrinsic properties of hybrid perovskites, thus enabling the unique positioning of the correlated doping strategy in the development history of PSCs in the past decade. Herein, we summarize the growth and progress of the state-of-the-art alkali metal cation (Cs+, Rb+, K+, Na+, Li+) doping in the field of hybrid perovskite-based photovoltaics. To start with, the accurate identification of different alkali metal-occupied locations in the perovskite crystal lattice are discussed in detail with highlighted advanced characterization methods. Beyond that, the location-dependent functions induced by alkali metal doping are intensely focused upon and comprehensively assessed, indicating their versatile and special effects on perovskites in terms of bottleneck issues such as crystallinity modulation, crystal structure stabilization, defect passivation, and ion-migration inhibition. Thereafter, we are committed to analyze their responsible working mechanisms so as to unveil the relationship between occupied locations and crucial roles for each doped cation. The systematical overview and in-depth understanding of the superiorities of such strategies together with their future challenges and prospects would further boost the advancement of perovskite-related fields.

Graphdiyne: Bridging SnO2 and Perovskite in Planar Solar Cells.

The matching of charge transport layer and photoactive layer is critical in solar energy conversion devices, especially for planar perovskite solar cells based on the SnO2 electron-transfer layer (ETL) owing to its unmatched photogenerated electron and hole extraction rates. Graphdiyne (GDY) with multi-roles has been incorporated to maximize the matching between SnO2 and perovskite regarding electron extraction rate optimization and interface engineering towards both perovskite crystallization process and subsequent photovoltaic service duration. The GDY doped SnO2 layer has fourfold improved electron mobility due to freshly formed C-O σ bond and more facilitated band alignment. The enhanced hydrophobicity inhibits heterogeneous perovskite nucleation, contributing to a high-quality film with diminished grain boundaries and lower defect density. Also, the interfacial passivation of Pb-I anti-site defects has been demonstrated via GDY introduction.

A-Site Management for Highly Crystalline Perovskites.

An in-depth understanding and effective suppression of nonradiative recombination pathways in perovskites are crucial to their crystallization process, in which supersaturation discrepancies at different time scales between CH3 NH3 I (MAI, methylammonium iodide) and PbI2 remain a key issue. Here, an A-site management strategy via the introduction of an A-site placeholder cation, NH4 + , to offset the deficient MA+ precipitation by occupying the cavity of Pb-I framework, is proposed. The temporarily remaining NH4 + is substituted by subsequently precipitated MA+ . The temperature-dependent crystallization process with the generation and consumption of a transient phase is sufficiently demonstrated by the dynamic changes in crystal structure characteristic peaks through in situ grazing-incidence X-ray diffraction and the surface potential difference evolution through temperature-dependent Kelvin probe force microscopy. A highly crystalline perovskite is consequently acquired, indicated by the enlarged grain size, lowered nonradiative defect density, prolonged carrier lifetime, and fluorescence lifetime imaging. Most importantly, it is identified that the A-site IMA defect is responsible for such crystal quality optimization based on theoretical calculations, transient absorption, and deep-level transient spectroscopy. Furthermore, the universality of the proposed A-site management strategy is demonstrated with other mixed-cation perovskite systems, indicating that this methodology successfully provides guidance for synthesis route design of highly crystalline perovskites.

Facile one-pot synthesis of Mg-doped g-C3N4 for photocatalytic reduction of CO2.

Graphitic carbon nitride (g-C3N4) has attracted wide attention due to its potential in solving energy and environmental issues. However, rapid charge recombination and a narrow visible light absorption region limit its performance. In our study, Mg-doped g-C3N4 was synthesized through a facile one-pot strategy for CO2 reduction. After Mg doping, the light utilization efficiency and photo-induced electron-hole pair separation efficiency of the catalysts were improved, which could be due to the narrower band gap and introduced midgap states. The highest amounts of CO and CH4 were obtained on Mg-CN-4% under ultraviolet light illumination, which were about 5.1 and 3.8 times that of pristine g-C3N4, respectively; the yield of CO and CH4 reached 12.97 and 7.62 µmol g-1 under visible light irradiation. Our work may provide new insight for designing advanced photocatalysts in energy conversion applications.

Graphdiyne Nanowall for Enhanced Photoelectrochemical Performance of Si Heterojunction Photoanode.

Graphdiyne (GDY), a new member of 2D carbon material family, was introduced into a Si heterojunction (SiHJ)-based photoelectrochemical water splitting cell. With assistance of magnetron-sputtered NiOx, the plateau photocurrent density of SiHJ/GDY/NiO x-10 nm with optimized NiO x film thickness was twice higher than that of SiHJ/NiO x-10 nm, demonstrating the catalytic function of GDY itself as well as the synergistic effect between GDY and NiO x. The results verified that GDY is a promising photoelectrode material candidate to realize highly efficient PEC performance, and pave a novel pathway to further improve Si-based PEC system.

All-Inorganic Perovskite Quantum Dot-Monolayer MoS2 Mixed-Dimensional van der Waals Heterostructure for Ultrasensitive Photodetector.

2D transition metal dichalcogenide (2D-TMD) materials and their van der Waals heterostructures (vdWHs) have inspired worldwide efforts in the fields of electronics and optoelectronics. However, photodetectors based on 2D/2D vdWHs suffer from performance limitations due to the weak optical absorption of their atomically thin nature. In this work, taking advantage of an excellent light absorption coefficient, low-temperature solution-processability, and long charge carrier diffusion length, all-inorganic halides perovskite CsPbI3- x Br x quantum dots are integrated with monolayer MoS2 for high-performance and low-cost photodetectors. A favorable energy band alignment facilitating interfacial photocarrier separation and efficient carrier injection into the MoS2 layer inside the 0D-2D mixed-dimensional vdWHs are confirmed by a series of optical characterizations. Owing to the synergistic effect of the photogating mechanism and the modulation of Schottky barriers, the corresponding phototransistor exhibits a high photoresponsivity of 7.7 × 104 A W-1, a specific detectivity of ≈5.6 × 1011 Jones, and an external quantum efficiency exceeding 107%. The demonstration of such 0D-2D mixed-dimensional heterostructures proposed here would open up a wide realm of opportunities for designing low-cost, flexible transparent, and high-performance optoelectronics.