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Although the power conversion efficiency of perovskite solar cells (PSCs) has reached 25.7%, there is still great potential for improvement in their performance and stability. In the past few years, ionic liquids (ILs) have been extensively investigated and demonstrated to enhance the efficiency and stability of devices substantially. Herein, the role of ILs in PSCs as additives, solvents, interface engineering, and charge transport layer is reviewed. Also, this review will guide the researchers in understanding bulk doping and interface engineering for efficient and stable PSCs.

Hybrid lead halide perovskites have reached comparable efficiencies to state-of-the-art silicon solar cell technologies. However, a remaining key challenge toward commercialization is the resolution of the perovskite device instability. In this work, we identify for the first time the mobile nature of bis(trifluoromethanesulfonyl)imide (TFSI-), a typical anion extensively employed in p-type dopants for 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'spirofluorene (spiro-OMeTAD). We demonstrate that TFSI- can migrate through the perovskite layer via the grain boundaries and accumulate at the perovskite/electron-transporting layer (ETL) interface. Our findings reveal that the migration of TFSI- enhances the device performance and stability, resulting in highly stable p-i-n cells that retain 90% of their initial performance after 1600 h of continuous testing. Our systematic study, which targeted the effect of the nature of the dopant and its concentration, also shows that TFSI- acts as a dynamic defect-healing agent, which self-passivates the perovskite crystal defects during the migration process and thereby decreases nonradiative recombination pathways.

Non-fullerene ETMs are emerging as a hot topic in the field of PSCs due to the inherent shortcomings of fullerene derivatives. Herein, the potential of SubPcs as ETMs in PSCs has been evaluated. To this end, ETMs based on two SubPcs (with chlorine or fluorine at the axial position) have been incorporated in PSCs, where the perovskite is deposited by either solution processing (CsFAMAPbIBr) or thermal evaporation (MAPI). The device performance and morphology of these devices are analyzed in depth using several techniques, and the interfacial effects induced by the SubPcs are studied using PL and TR-PL. In all cases, the SubPc with axial chlorine led to higher efficiencies. Thus, a maximum PCE of 10.8% was achieved with the champion device using a SubPc/C60 double-layer ETM.

Carrier multiplication (CM) generates multiple electron-hole pairs in a semiconductor from a single absorbed photon with energy exceeding twice the band gap. Thus, CM provides a promising way to circumvent the Shockley-Queisser limit of solar cells. The ideal material for CM should have significant overlap with the solar spectrum and should be able to fully utilize the excess energy above the band gap for additional charge carrier generation. We report efficient CM in mixed Sn/Pb halide perovskites (band gap of 1.28 eV) with onset just above twice the band gap. The CM rate outcompetes the carrier cooling process leading to efficient CM with a quantum yield of 2 for photoexcitation at 2.8 times the band gap. Such efficient CM characteristics add to the many advantageous properties of mixed Sn/Pb metal halide perovskites for photovoltaic applications.

Assembled perovskite nanocrystals (NCs), known as supercrystals (SCs), can have many exotic optical and electronic properties different from the individual NCs due to energy transfer and electronic coupling in the dense superstructures. We investigate the optical properties and ultrafast carrier dynamics of highly ordered SCs and the dispersed NCs by absorption, photoluminescence (PL), and femtosecond transient absorption (TA) spectroscopy to determine the influence of the assembly on the excitonic properties. Next to a red shift of absorption and PL peak with respect to the individual NCs, we identify signatures of the collective band-like states in the SCs. A smaller Stokes shift, decreased biexciton binding energy, and increased carrier cooling rates support the formation of delocalized states as a result of the coupling between the individual NC states. These results open perspectives for assembled perovskite NCs for application in optoelectronic devices, with design opportunities exceeding the level of NCs and bulk materials.

The development of a scalable chemical bath deposition (CBD) process facilitates the realization of electron-transporting layers (ETLs) for large-area perovskite solar modules (PSMs). Herein, a method to prepare a uniform and scalable thick Zn

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo0.6W0.4S2 compared to pure MoS2 and WS2 grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. The effects of temperature on A and B exciton peak energies and line widths in optical transmission spectra were compared between the alloy and pure MoS2 and WS2. On increasing the temperature from 25 to 293 K, the energy of the A and B exciton peaks decreases, while their line width increases due to exciton-phonon interactions. The exciton-phonon interactions in the alloy are closer to those for MoS2 than those for WS2. This suggests that exciton wave functions in the alloy have a larger amplitude on Mo atoms than that on W atoms. The experimental absorption spectra could be reproduced by TDDFT calculations. Interestingly, for the alloy, the Mo and W atoms had to be distributed over all layers. Conversely, we could not reproduce the experimental alloy spectrum by calculations on a structure with alternating layers, in which every other layer contains only Mo atoms and the layers in between also contain W atoms. For the latter atomic arrangement, the TDDFT calculations yielded an additional optical absorption peak that could be due to excitons with some charge transfer character. From these results, we conclude that ALD yields an alloy in which Mo and W atoms are distributed uniformly among all layers.

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo

Photoelectrochemical (PEC) reactors based on polymer electrolyte membrane (PEM) electrolyzers are an attractive alternative to improve scalability compared to conventional monolithic devices. To introduce narrow band gap photoabsorbers such as BiVO4in PEM–PEC system requires cost-effective and scalable deposition techniques beyond those previously demonstrated on monolithic FTO-coated glass substrates, followed by the preparation of membrane electrode assemblies. Herein, we address the significant challenges in coating narrow band gap metal-oxides on porous substrates as suitable photoelectrodes for the PEM–PEC configuration. In particular, we demonstrate the deposition and integration of W-doped BiVO4on porous conductive substrates by a simple, cost-effective, and scalable deposition based on the SILAR (successive ionic layer adsorption and reaction) technique. The resultant W-doped BiVO4photoanode exhibits a photocurrent density of 2.1 mA·cm–2, @ 1.23 V vs RHE, the highest reported so far for the BiVO4on any porous substrates. Furthermore, we integrated the BiVO4on the PEM–PEC reactor to demonstrate the solar hydrogen production from ambient air with humidity as the only water source, retaining 1.55 mA·cm–2, @ 1.23 V vs RHE. The concept provides insights into the features necessary for the successful development of materials suitable for the PEM–PEC tandem configuration reactors and the gas-phase operation of the reactor, which is a promising approach for low-cost, large-scale solar hydrogen production.

Light-emitting diodes (LEDs) based on excitonic material systems, in which tightly bound photoexcited electron–hole pairs migrate together rather than as individual charge carriers, offer an attractive route to developing solution-processed, high-performance light emitters. Here, we demonstrate bright, efficient, excitonic infrared LEDs through the incorporation of quantum dots (QDs)1 into a low-dimensional perovskite matrix. We program the surface of the QDs to trigger fast perovskite nucleation to achieve homogeneous incorporation of QDs into the matrix without detrimental QD aggregation, as verified by in situ grazing incidence wide-angle X-ray spectroscopy. We tailor the distribution of the perovskites to drive balanced ultrafast excitonic energy transfer to the QDs. The resulting LEDs operate in the short-wavelength infrared region, an important regime for imaging and sensing applications, and exhibit a high external quantum efficiency of 8.1% at 980 nm at a radiance of up to 7.4 W Sr−1 m−2. Embedding perovskite quantum dots in perovskite leads to bright, efficient 980 nm LEDs with applications in imaging and sensing.

Exfoliated transition metal dichalcogenide (TMD) nanomaterials possess remarkable and tunable semiconducting properties which make them competitive for use in ultrathin, flexible devices such as ph...

Colloidal quantum dots (QDs) allow great flexibility in the design of optoelectronic devices, thanks to their size-dependent optical and electronic properties and the possibility to fabricate thin films with solution-based processing. In particular, in QD-based heterojunctions, the band gap of both components can be controlled by varying the size of the QDs. However, control over the band alignment between the two materials is required to tune the dynamics of carrier transfer across a heterostructure. We demonstrate that ligand exchange strategies can be used to control the band alignment of PbSe and CdSe QDs in a mixed QD solid, shifting it from a type-I to a type-II alignment. The change in alignment is observed in both spectroelectrochemical and transient absorption measurements, leading to a change in the energy of the conduction band edges in the two materials and in the direction of electron transfer upon photoexcitation. Our work demonstrates the possibility to tune the band offset of QD heterostructures via control of the chemical species passivating the QD surface, allowing full control over the energetics of the heterostructure without requiring changes in the QD composition.

Colloidal quantum dots (CQDs) have recently gained attention as materials for manufacturing optoelectronic devices in view of their tunable light absorption and emission properties and compatibility with low-temperature thin-film manufacture. The realization of CQD inkjet-printed infrared photodetectors has thus far been hindered by incompatibility between the chemical processes that produce state-of-the-art CQD solution-exchanged inks and the requirements of ink formulations for inkjet materials processing. To achieve inkjet-printed CQD solids with a high degree of reproducibility, as well as with the needed morphological and optoelectronic characteristics, we sought to overcome the mismatch among these processing conditions. In this study, we design CQD inks by simultaneous evaluation of requirements regarding ink colloidal stability, jetting conditions, and film morphology for different dots and solvents. The new inks remain colloidally stable, achieved through a design that suppresses the reductant properties of amines on the dots' surface. After drop ejection from the nozzle, the quantum dot material is immobilized on the substrate surface due to the rapid evaporation of the low boiling point amine-based compound. Concurrently, the high boiling point solvent allows for the formation of a thin film of high uniformity, as is required for the fabrication of high-performance IR photodetectors. We fabricate inkjet-printed photodetectors exhibiting the highest specific detectivities reported to date (above 10

Photon recycling, the iterative process of re-absorption and re-emission of photons in an absorbing medium, can play an important role in the power-conversion efficiency of photovoltaic cells. To date, several studies have proposed that this process may occur in bulk or thin films of inorganic lead-halide perovskites, but conclusive proof of the occurrence and magnitude of this effect is missing. Here, we provide clear evidence and quantitative estimation of photon recycling in CsPbBr3 nanocrystal suspensions by combining measurements of steady-state and time-resolved photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. The steady-state PL shows clear spectral modifications including red shifts and quantum yield decrease, while the time-resolved measurements show prolonged PL decay and rise times. These effects grow as the nanocrystal concentration and distance traveled through the suspension increase. Monte Carlo simulations of photons diffusing through the medium and exhibiting absorption and re-emission account quantitatively for the observed trends and show that up to five re-emission cycles are involved. We thus identify 4 quantifiable measures, PL red shift, PL QY, PL decay time, and PL rise time that together all point toward repeated, energy-directed radiative transfer between nanocrystals. These results highlight the importance of photon recycling for both optical properties and photovoltaic applications of inorganic perovskite nanocrystals.

This paper reports on the development of a transportable high data rate quantum absolute gravimeter for gravity map matching navigation. Its target sensitivity is 10−7 g/ ✓ Hz at 100 Hz data rate with a stability of 10−9 g over a month. The design issues, sensitivity analysis and an evaluation of noises are discussed.

Few-layered transition-metal dichalcogenides (TMDs) are increasingly popular materials for optoelectronics and catalysis. Among the various types of TMDs available today, rhenium chalcogenides (ReX2) stand out due to their remarkable electronic structure, such as the occurrence of anisotropic excitons and potential direct band gap behavior throughout multilayered stacks. In this paper, we have analyzed the nature and dynamics of charge carriers in highly crystalline liquid-phase exfoliated ReS2, using a unique combination of optical pump-THz probe and broad-band transient absorption spectroscopy. Two distinct time regimes are identified, both of which are dominated by unbound charge carriers despite the high exciton binding energy. In the first time regime, the unbound charge carriers cause an increase and a broadening of the exciton absorption band. In the second time regime, a peculiar narrowing of the excitonic absorption profile is observed, which we assign to the presence of built-in fields and/or charged defects. Our results pave the way to analyze spectrally complex transient absorption measurements on layered TMD materials and indicate the potential for ReS2 to produce mobile free charge carriers, a feat relevant for photovoltaic applications.

Photon recycling, the iterative process of re-absorption and re-emission of photons in an absorbing medium, can play an important role in the power-conversion efficiency of photovoltaic cells. To date, several studies have proposed that this process may occur in bulk or thin films of inorganic lead-halide perovskites, but conclusive proof of the occurrence and magnitude of this effect is missing. Here, we provide clear evidence and quantitative estimation of photon recycling in CsPbBr3 nanocrystal suspensions by combining measurements of steady-state and time-resolved photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. The steady-state PL shows clear spectral modifications including red shifts and quantum yield decrease, while the time-resolved measurements show prolonged PL decay and rise times. These effects grow as the nanocrystal concentration and distance traveled through the suspension increase. Monte Carlo simulations of photons diffusing through the medium and exhibiting absorption and re-emission account quantitatively for the observed trends and show that up to five re-emission cycles are involved. We thus identify 4 quantifiable measures, PL red shift, PL QY, PL decay time, and PL rise time that together all point toward repeated, energy-directed radiative transfer between nanocrystals. These results highlight the importance of photon recycling for both optical properties and photovoltaic applications of inorganic perovskite nanocrystals., This research received funding from The Netherlands Organisation for Scientific Research (NWO) in the framework of the Materials for Sustainability Programme (MAT4SUS Project No. 739.017.011).

In conventional solar cell semiconductor materials (predominantly Si) photons with energy higher than the band gap initially generate hot electrons and holes, which subsequently cool down to the band edge by phonon emission. Due to the latter process, the energy of the charge carriers in excess of the band gap is lost as heat and does not contribute to the conversion of solar to electrical power. If the excess energy is more than the band gap it can in principle be utilized through a process known as carrier multiplication (CM) in which a single absorbed photon generates two (or more) pairs of electrons and holes. Thus, through CM the photon energy above twice the band gap enhances the photocurrent of a solar cell. In this review, we discuss recent progress in CM research in terms of fundamental understanding, emergence of new materials for efficient CM, and CM based solar cell applications. Based on our current understanding, the CM threshold can get close to the minimal value of twice the band gap in materials where a photon induces an asymmetric electronic transition from a deeper valence band or to a higher conduction band. In addition, the material must have a low exciton binding energy and high charge carrier mobility, so that photoexcitation leads directly to the formation of free charges that can readily be extracted at external electrodes of a photovoltaic device. Percolative networks of coupled PbSe quantum dots, Sn/Pb based halide perovskites, and transition metal dichalcogenides such as MoTe2 fulfill these requirements to a large extent. These findings point towards promising prospects for further development of new materials for highly efficient photovoltaics.

InP and InZnP colloidal quantum dots (QDs) are promising materials for application in light-emitting devices, transistors, photovoltaics, and photocatalytic cells. In addition to possessing an appropriate bandgap, high absorption coefficient, and high bulk carrier mobilities, the intrinsic toxicity of InP and InZnP is much lower than for competing QDs that contain Cd or Pb–providing a potentially safer commercial product. However, compared to other colloidal QDs, InP QDs remain sparsely used in devices and their electronic transport properties are largely unexplored. Here, we use time-resolved microwave conductivity measurements to study charge transport in films of InP and InZnP colloidal quantum dots capped with a variety of short ligands. We find that transport in InP QDs is dominated by trapping effects, which are mitigated in InZnP QDs. We improve charge carrier mobilities with a range of ligand-exchange treatments and for the best treatments reach mobilities and lifetimes on par with those of PbS QD films used in efficient solar cells. To demonstrate the device-grade quality of these films, we construct solar cells based on InP & InZnP QDs with power conversion efficiencies of 0.65 and 1.2%, respectively. This represents a large step forward in developing Cd- and Pb-free next-generation optoelectronic devices.

Mg2Si is apart from its conductivity properties expected to be a promising candidate for thermoelectric applications due to its low toxicity, low costs, and the high abundance of its precursor chemicals. Through the addition of a homogeneous distribution of nanoparticles (e.g. MgO) and by reducing the size of Mg2Si to the nanometer regime, it is possible to decrease the thermal conductivity by increasing phonon-interface scattering and, as a result, improve the thermoelectric properties. However, classical approaches do not allow for the synthesis of nanocomposites from Mg2Si and MgO. In this work, a straightforward route is presented towards homogeneously mixed Mg2Si/MgO via a two-step magnesiothermic reduction process starting from sol–gel derived hierarchically organized porous silica. Monolithic materials composed of Mg2Si and MgO in variable molar ratios are built up from a macroporous network of Mg2Si with homogeneously distributed MgO particles exhibiting a crystallite size in the range of 24–37 nm.

Perovskite solar modules have been attracting increasing attention due to their market potential, yet publications concerned with theintrinsic scale-up potential of different perovskite compositions remain relatively scarce. On the other hand, while great success is being made towards improving the power conversion efficiency (PCE) of perovskite solar cells (PSCs) by cesium cation (Cs+) doping of the perovskite, more attention is being paid to the perovskite phase stabilization effect of Cs+ doping, and less to other properties that are critical to understand and futher improve the PSC’s. In this work, moderately-Cs-doped MAPbI3 was employed as a model perovskite material in order to exclude the phase stabilization effect. Our systematic study revealed the influence of Cs+ in organic-inorganic hybrid perovskites on the crystal structure, crystallization process, trap state density, band structure and charge (i.e., ions or photo-carriers) transport. Markedly, it has been observed that Cs+ doping can greatly increase the carrier diffusion length in the perovskite films, thus improving the potential to scale-up PSC’s.The PCE of small area devices (0.09 cm2) was increased to 21.72% from 19.73%, with decreased hysteresis behavior and increased operational stability (T85 = 1000 h) after Cs+ doped, where T85 refers to the retention of 85% of the initial PCE. Moreover, a PCE of 21.08% was obtained for a Cs+-containing perovskite module with an active area >30 cm2, which demonstrates a better “reproducibility” than the reference sample (MAPbI3-based perovskite modules, PCE = 18.26%).

We functionalize PbS nanocrystals with the organic semiconductor Zn β-tetraaminophthalocyanine to design a nanostructured solid-state material with frequent organic-inorganic interfaces. By transient absorption and fluorescence spectroscopy, we investigate the optoelectronic response of this hybrid material under near-infrared excitation to find efficient charge transfer from the nanocrystals to the molecules. We demonstrate that the material undergoes cooperative sensitization of two nanocrystals followed by photon upconversion and singlet emission of the organic semiconductor. The upconversion efficiency resembles that of comparable systems in solution, which we attribute to the large amount of interfaces present in this solid-state film. We anticipate that this synthetic strategy has great prospects for increasing the open-circuit voltage in PbS nanocrystal-based solar cells.

In this work, we investigate the occurrence of localized surface plasmon resonances (LSPRs) in different nickel sulfide nanostructures. Therefore, spherical and anisotropic nickel sulfide nanoparticles (NPs) are synthesized and analyzed regarding their optical properties by UV/vis/NIR and transient absorption spectroscopy. Furthermore, new pathways for the synthesis of spherical Ni3S2 nanodots with an extremely narrow size distribution, as well as Au–Ni3S2 core–shell NPs with controllable shell thickness, are presented. Our results show that NPs of different metallic nickel sulfide phases like Ni3S2 and Ni3S4 exhibit LSPR bands in the visible regime of the electromagnetic spectrum, which possibly makes them a comparably cheaper alternative to NPs consisting of noble metals like Au and Ag. In case of the presented plasmonic core–shell particles, the resonance frequency of the plasmon can be tuned between those of pure gold and pure Ni3S2 NPs by varying the Ni3S2 shell thickness.

Singlet fission in tetracene generates two triplet excitons per absorbed photon. If these triplet excitons can be effectively transferred into silicon (Si) then additional photocurrent can be generated from photons above the bandgap of Si. This could alleviate the thermalization loss and increase the efficiency of conventional Si solar cells. Here we show that a change in the polymorphism of tetracene deposited on Si due to air exposure, facilitates triplet transfer from tetracene into Si. Magnetic field-dependent photocurrent measurements confirm that triplet excitons contribute to the photocurrent. The decay of tetracene delayed photoluminescence was used to determine a triplet transfer time of 215 ns and a maximum yield of triplet transfer into Si of ~50 %. Our study suggests that control over the morphology of tetracene during deposition will be of great importance to boost the triplet transfer yield further.

International audience; In this study, we report the synthesis and characterization of nonprecious bimetallic iron–molybdenum sulfide bio-inspired electrocatalysts for the hydrogen evolution reaction (HER). Iron–molybdenum sulfide materials were obtained through three distinct scalable synthetic approaches using microwave irradiation or heat treatment in furnace under inert and reductive atmospheres. These electrocatalysts were combined with carbon nanotubes (CNTs), and their activity for the hydrogen evolution reaction (HER) was studied in acidic environment. The most efficient composite material of the series was obtained by microwave synthesis and displays a current density per geometric area of 10 mA·cm–2 at an overpotential of 140 mV with unity faradic efficiency for hydrogen evolution. This composite cathode catalyst was implemented into a proton exchange membrane (PEM) electrolyzer single cell with iridium black as the anode catalyst and could be operated at 0.5 A·cm–2, requiring less than 300 mV additional voltage compared to Pt and with remarkable stability during accelerated stress testing.

Surface passivation of the perovskite photo absorber is a key factor to improve the photovoltaic performance. So far robust passivation strategies have not yet been revealed. Here, we demonstrate a successful passivation strategy which controls the Fermi-level of the perovskite surface by improving the surface states. Such Fermi-level control caused band-bending between the surface and bulk of the perovskite, which enhanced the hole-extraction from the absorber bulk to the HTM side. As an added benefit, the inorganic passivation layer improved the device light stability. By depositing a thick protection layer on the complete device, a remarkable waterproofing effect was obtained. As a result, an enhancement ofVOCand the conversion efficiency from 20.5% to 22.1% was achieved. We revealed these passivation mechanisms and used perhydropoly(silazane) (PHPS) derived silica to control the perovskite surface states.

Summary Organic-inorganic metal-halide perovskite solar cells (PSCs) have achieved a certified power-conversion efficiency (PCE) of 25.5%. However, long-term stability and air stability of the PSCs are still major concerns and hamper commercialization. Herein, we employ a multi-functional ionic liquid (IL), 1,3-bis(cyanomethyl)imidazolium bis(trifluoromethylsulfonyl)imide ([Bcim][TFSI]), which contains an imidazolium cation functionalized with nitrile (CN) groups and the TFSI anion and is, additionally, able to form an extensive network of low-barrier H bonds, as an additive in perovskite-precursor solutions. Because of a combination of the Lewis-basic CN groups and the hydrophobic TFSI anion in the IL, highly crystalline perovskite films with large grain sizes are obtained. The IL-modified perovskite films afford PSCs with long-term stability and PCEs > 21%. The stability of unencapsulated devices retain >95% of their original efficiency after 1,000 h of aging. This study demonstrates the considerable potential of TFSI-based ILs to improve the performance of PSCs.

Triarylamine-substituted bithiophene (BT-4D), terthiophene (TT-4D), and quarterthiophene (QT-4D) small molecules are synthesized and used as low-cost hole-transporting materials (HTMs) for perovskite solar cells (PSCs). The optoelectronic, electrochemical, and thermal properties of the compounds are investigated systematically. The BT-4D, TT-4D, and QT-4D compounds exhibit thermal decomposition temperature over 400 °C. The n-i-p configured perovskite solar cells (PSCs) fabricated with BT-4D as HTM show the maximum power conversion efficiency (PCE) of 19.34% owing to its better hole-extracting properties and film formation compared to TT-4D and QT-4D, which exhibit PCE of 17% and 16%, respectively. Importantly, PSCs using BT-4D demonstrate exceptional stability by retaining 98% of its initial PCE after 1186 h of continuous 1 sun illumination. The remarkable long-term stability and facile synthetic procedure of BT-4D show a great promise for efficient, stable, and low-cost HTMs for PSCs for commercial applications.

Quantum-dot-in-perovskite solids are excellent candidates for infrared light-emitting applications. The first generation of dot-in-perovskite light-emitting diodes (LEDs) has shown bright infrared electroluminescence with tunable emission wavelength; however, their performance has been limited by degradation of the active layer at practical operating voltages. This arises from the instability of the three-dimensional (3D) organolead halide perovskite matrix. Herein we report the first dot-in-perovskite solids that employ two-dimensional (2D) perovskites as the matrix. 2D perovskite passivation is achieved via an in situ alkylammonium/alkylamine substitution carried out during the quantum dot (QD) ligand exchange process. This single-step film preparation process enables deposition of the QD/perovskite active layers with thicknesses of 40 nm, over seven times thinner than the first-generation dot-in-perovskite thin films that relied on a multistep synthesis. The dot-in-perovskite film roughness improved fr...

We simultaneously surface-functionalize PbS nanocrystals with Cu 4,4′,4′′,4′′′-tetraaminophthalocyanine and assemble this hybrid material into macroscopic monolayers. Electron microscopy and X-ray scattering reveal a granular mesocrystalline structure with strong coherence between the atomic lattice and the superlattice of nanocrystals within each domain. Terahertz spectroscopy and field-effect transistor measurements indicate efficient coupling of holes throughout the hybrid thin film, in conjunction with a pronounced photoresponse. We demonstrate the potential of this material for optoelectronic applications by fabricating a light-effect transistor.

Two-dimensional (2D) semiconductors hold high potential for the implementation of efficient ultrathin electronics (e.g. field-effect transistors, light emitting diodes and solar cell devices). In recent years, colloidal methods to synthesize ultrathin 2D materials have been developed that offer alternatives (like the production of non-layered 2D materials and upscaling) to mechanical exfoliation methods. By focusing on optoelectronic applications, it is important to characterize the nature and dynamics of photoexcited states in these materials. In this paper, we use ultrafast transient absorption (TA) and terahertz (THz) spectroscopy as optimal tools for such a characterization. We choose recently synthesized ultrathin colloidal 2D InSe nanosheets (inorganic layer thickness 0.8–1.7 nm; ≤5 nm including ligands) for discussing TA and THz spectroscopic studies and elucidate their charge carrier dynamics under photoexcitation with TA. THz spectroscopy is then used to extract contactless AC mobilities as high as 20±2 cm2/Vs in single InSe layers. The obtained results underpin the general applicability of TA and THz spectroscopy for characterizing photoexcited states in 2D semiconductors.

The implementation of next generation ultrathin electronics by applying highly promising dimensionality-dependent physical properties of two-dimensional (2D) semiconductors is ever increasing. In this context, the van der Waals layered semiconductor InSe has proven its potential as photodetecting material with high charge carrier mobility. We have determined the photogeneration charge carrier quantum yield and mobility in atomically thin colloidal InSe nanosheets (inorganic layer thickness 0.8-1.7 nm, mono/double-layers, ≤ 5 nm including ligands) by ultrafast transient terahertz (THz) spectroscopy. A near unity quantum yield of free charge carriers is determined for low photoexcitation density. The charge carrier quantum yield decreases at higher excitation density due to recombination of electrons and holes, leading to the formation of neutral excitons. In the THz frequency domain, we probe a charge mobility as high as 20 ± 2 cm2/(V s). The THz mobility is similar to field-effect transistor mobilities extracted from unmodified exfoliated thin InSe devices. The current work provides the first results on charge carrier dynamics in ultrathin colloidal InSe nanosheets.

ZnS–CuInS2 (ZCIS) alloy nanostructures are becoming increasingly important materials because of their photoluminescence properties. Here we explore the emission properties of ZCIS quantum dots (QDs) capped with dodecanethiol, which exhibit Zn:Cu-dependent emission properties. Absorption and photoluminescence excitation spectra indicate a single, composition-independent light absorbing state. The emission spectra point out the existence of two emissive states with lifetimes of ∼10 ns and ∼100 ns. The photoluminescence and time-resolved emission analysis provide insight into the synergy between the two intraband states and the possibility of modulating the emission through variation in the Zn/Cu ratio. Better understanding of light absorbing and emission mechanisms in alloyed nanostructures is essential for future development of photoelectric and display devices.

Correction for ‘Cation optimization for burn-in loss-free perovskite solar devices’ by Sanghyun Paek et al., J. Mater. Chem. A, 2021, 9, 5374–5380, DOI: 10.1039/D1TA00472G.

The exceptional semiconducting properties of exfoliated transition metal dichalcogenide (TMDs) nanosheets have long inspired their use in photoelectrochemical applications; in particular, photoelectrodes for solar-to-chemical energy conversion. Indeed, past demonstrations using non-exfoliated, bulk material gave solar-to-hydrogen efficiencies greater than 7%.1 In taking advantage of the exfoliable nature of these materials, nano-TMDs not only provide a new range of tunable optoelectronic properties, but also create a direct path toward ultrathin, flexible devices. However, a major challenge in commercializing TMD-based devices is their large-scale production.2 Our group has recently addressed this issue by developing methods for solution-based fabrication of nanoflake TMD thin films for large area solar energy conversion applications.3 In this presentation we highlight our results employing WSe2 for solar water splitting and report the further advancement of our preparation methods toward a continuous roll-to-roll deposition of TMD-based thin films from nanoflake dispersions using a liquid-liquid interfacial self-assembly technique.3,4 We demonstrate reproducible printing of 100mm wide TMD flake films on plastic substrates at 10 mm s-1 with nanoflake loadings of 35 mg m-2.4 Importantly we show that these large-scale films of semiconducting TMDs are robust and capable of solar-to-chemical energy conversion comparable to that of their batch-scale counterparts.4 In addition, we address the effects of processing conditions on TMD flake quality and optoelectronic device performance. We find that the type of exfoliation and related parameters have a significant impact on the defect density within the flakes, which effects the harvesting of photogenerated charge carriers. We observe this for MoS2 and WSe2-based devices, suggesting that this is a fundamental limitation of liquid exfoliated TMDs. To overcome this limitation we present distinct defect mitigation strategies that can drastically improve the solar energy conversion ability of TMD nanoflake films.5 These techniques lead to state-of -the-art performance of solution-processed WSe2 nanoflake films for photoelectrochemical H2 production with internal quantum efficiency of over 50% and 1 sun photocurrent density over 4 mA cm-2.5 References: 1. J. Am. Chem. Soc., 1, 223-231 (2013). 2. ACS Energy Lett., 1, 315–322 (2016). 3. Nat. Commun., 6, 7596 (2015). 4. ACS Appl. Nano Mater., 2, 7705-7712 (2019). 5. Nano Lett., 18, 215–222 (2018). Figure 1

Multi-layer graphene flakes with superior properties can be produced by chemical vapor deposition (CVD) on Ni substrates over large areas. We report a detailed investigation of lateral structures corresponding to correlated variations in Ni roughness and multilayer graphene thickness uniformity that may form under certain conditions over the wafer with large (mm to cm) feature size. Investigating these structures is important for understanding the interplay between carbon diffusion, segregation, metal grain formation and its role on graphene growth. In this study, we investigated such structures formed after CVD graphene growth on 200 nm Ni thin films on SiO2 (300 nm)/Si substrates by optical microscopy, atomic force microscopy, scanning electron microscopy and Raman microscopy. Our findings showed large-scale Voronoi-like structures featuring roughness variations in the Ni film correlated with the thickness distribution of the graphene multilayers. Ni recrystallization during the CVD process plays a pivotal role in forming these structures. Similar roughness-related structures do not form without a source of carbon (methane) present during the process, which shows that carbon diffusion/segregation and Ni recrystallization are mutually dependent.

Solution-processable two-dimensional (2D) semiconductors with chemically tunable thickness and associated tunable band gaps are highly promising materials for ultrathin optoelectronics. Here, the properties of free charge carriers and excitons in 2D PbS nanosheets of different thickness are investigated by means of optical pump-terahertz probe spectroscopy. By analyzing the frequency-dependent THz response, a large quantum yield of excitons is found. The scattering time of free charge carriers increases with nanosheet thickness, which is ascribed to reduced effects of surface defects and ligands in thicker nanosheets. The data discussed provide values for the DC mobility in the range 550-1000 cm2 V-1 s-1 for PbS nanosheets with thicknesses ranging from 4 to 16 nm. Results underpin the suitability of colloidal 2D PbS nanosheets for optoelectronic applications.