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INTRODUCTION Hybrid metal halide perovskites are highly favorable materials for efficient photovoltaic and optoelectronic applications. The mechanisms behind their impressive performance have yet to be fully understood, but they likely depend on atomic-level properties that may be unique to these perovskites. Atomic-resolution transmission electron microscopy is well suited to provide new insights but is challenging because of the highly beam-sensitive nature of hybrid perovskites. RATIONALE We used low-dose scanning transmission electron microscopy (STEM) imaging to determine the microstructure of thin hybrid perovskite films. Thermally evaporated thin films of formamidinium and methylammonium lead triiodide (FAPbI3 and MAPbI3, respectively) were examined on ultrathin carbon-coated copper TEM grids to reveal the nature of boundaries, defects, and decomposition pathways. RESULTS Using low-dose low-angle annular dark field (LAADF) STEM imaging, we obtained atomic-resolution micrographs of FAPbI3 films in the cubic phase. We found that prolonged electron irradiation leads to a loss of FA+ ions, which initially causes the perovskite structure to change to a partially FA+-depleted but ordered perovskite lattice, apparent as light-and-dark checkered patterns in STEM images. Further electron beam exposure leads to the expected deterioration to PbI2 as the final decomposition product. We propose that the observed intermediate checkered pattern is triggered by an initially random, beam-induced loss of FA+, followed by subsequent reordering of FA+ ions. The discovery of this intermediate structure explains why the perovskite structure can sustain significant deviations from stoichiometry and recovers remarkably well from damage. We further revealed the atomic arrangement at interfaces within the hybrid perovskite films. We found that PbI2 precursor remnants commonly encountered in hybrid perovskite films readily and seamlessly intergrow with the FAPbI3 and MAPbI3 lattice and can distort from their bulk hexagonal structure to form a surprisingly coherent transition boundary, exhibiting low lattice misfit and strain. We observed PbI2 domains that nearly perfectly follow the surrounding perovskite structure and orientation, which suggests that PbI2 may seed perovskite growth. These observations help to explain why the presence of excess PbI2 tends not to impede solar cell performance. Images of FAPbI3 grain boundaries further revealed that the long-range perovskite structure is preserved up to the grain boundaries, where sharp interfaces are generally present, without any obvious preferred orientation. Near-120° triple boundaries are most commonly observed at the intersection of three grains, which we generally found to be crystallographically continuous and associated with minimal lattice distortion. Finally, we identified the nature of defects, dislocations, and stacking faults in the FAPbI3 lattice. We discovered dislocations that are dissociated in a direction perpendicular to their glide plane (climb-dissociated), aligned point defects in the form of vacancies on the Pb-I sublattice, and stacking faults corresponding to a shift of half a unit cell, connecting Pb-I columns with I– columns rather than with FA+ columns. CONCLUSION Our findings provide an atomic-level understanding of the technologically important class of hybrid lead halide perovskites, revealing several mechanisms that underpin their remarkable performance. The highly adaptive nature of the perovskite structure upon organic cation loss yields exceptional regenerative properties of partly degraded material. The observation of coherent perovskite interfaces with PbI2 explains the barely diminished optoelectronic performance upon such precursor inclusions, while sharp interfaces between perovskite grains grant a benign nature. Such atomically localized information enables the targeted design of methods to eliminate defects and optimize interfaces in these materials.

The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THz-TDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 102Ω− 1cm− 1. We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 104Ω− 1cm− 1 and provides a very high precision for calculating photoconductivity up to 104Ω− 1cm− 1, and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility by over 50%.

Nanowire-based THz detection Terahertz (THz) radiation is an interesting region of the electromagnetic spectrum lying between microwaves and infrared. Non-ionizing and transparent to most fabrics, it is finding application in security screening and imaging but is also being developed for communication and chemical sensing. To date, most THz detectors have focused just on signal intensity, an effort that discards half the signal in terms of the full optical state, including polarization. Peng et al. developed a THz detector based on crossed nanowires (arranged in a hash structure) that is capable of resolving the full state of the THz light. The approach provides a nanophotonic platform for the further development of THz-based technologies. Science , this issue p. 510

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr

Mixed-halide perovskites are essential for use in all-perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed-halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed-halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge-carriers in the perovskite, a charge-neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA+ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge-carriers can be extracted from the iodide-rich low-bandgap regions of the phase-segregated perovskite formed under illumination, suggesting the existence of charge-carrier percolation pathways through grain boundaries where phase-segregation may occur.

The tunable bandgap of metal-halide perovskites has opened up the possibility of tandem solar cells with over 30% efficiency. Iodide-Bromide (I-Br) mixed-halide perovskites are crucial to achieve the optimum bandgap for such tandems. However, when the Br content is increased to widen the bandgap, cells fail to deliver the expected increase in open-circuit voltage (VOC). This loss in VOC has been attributed to photo-induced halide segregation. Here, we combine Fourier Transform Photocurrent Spectroscopy (FTPS) with detailed balance calculations to quantify the voltage loss expected from the halide segregation, providing a means to quantify the VOC losses arising from the formation of low bandgap iodide-rich phases during halide segregation. Our results indicate that, contrary to popular belief, halide segregation is not the dominant VOC loss mechanism in Br-rich wide bandgap cells. Rather, the loss is dominated by the relatively low initial radiative efficiency of the cells, which arises from both imperfections within the absorber layer, and at the perovskite/charge extraction layer heterojunctions. We thus identify that focussing on maximising the initial radiative efficiency of the mixed-halide films and devices is more important than attempting to suppress halide segeregation. Our results suggest that a VOC of up to 1.33 V is within reach for a 1.77 eV bandgap perovskite, even if halide segregation cannot be supressed

Understanding the electronic energy landscape in metal halide perovskites is essential for further improvements in their promising performance in thin-film photovoltaics. Here, we uncover the presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films. We attribute these discrete features to intrinsically occurring quantum confinement effects, for which the related energies change with temperature according to the inverse square of the intrinsic lattice parameter, and with peak index in a quadratic manner. By determining the threshold film thickness at which the amplitude of the peaks is appreciably decreased, and through ab initio simulations of the absorption features, we estimate the length scale of confinement to be 10–20 nm. Such absorption peaks present a new and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps. Oscillatory features in the absorption spectra of formamidinium lead triiodide perovskite thin films reveal the occurrence of intrinsic quantum confinement effects with confinement on the scale of tens of nanometres.

Stable perovskites with ionic salts Ionic liquids have been shown to stabilize organic-inorganic perovskite solar cells with metal oxide carrier-transport layers, but they are incompatible with more readily processible organic analogs. Lin et al. found that an ionic solid, a piperidinium salt, enhanced the efficiency of positive-intrinsic-negative layered perovskite solar cells with organic electron and hole extraction layers. Aggressive aging testing showed that this additive retarded segregation into impurity phases and pinhole formation in the perovskite layer. Science , this issue p. 96

One-dimensional nanostructures commonly refer to nanomaterials with a large length-to-diameter ratio, such as nanowires, nanotubes, nanorods, and nanopillars. The nanoscale lateral dimensions and high aspect ratios of these (quasi) one-dimensional nanostructures result in fascinating optical and electrical properties, including strongly anisotropic optical absorption, controlled directionality of light emission, confined charge-carrier transport and/or ballistic transport, which make one-dimensional nanostructures ideal building blocks for applications in highly integrated photonic, electronic, and optoelectronic systems. In this article, we review recent developments of very high (terahertz) frequency devices based on these one-dimensional nanostructures, particularly focusing on carbon nanotubes and semiconductor nanowires. We discuss state-of-the-art nanomaterials synthesis, device-fabrication techniques, device-operating mechanisms, and device performance. The combination of nanotechnology and terahertz science is a nascent research field which has created advanced THz sources, detectors, and modulators, leading to terahertz systems with extended functionalities. The goal of this article is to present the up-to-date worldwide status of this field and to highlight the current challenges and future opportunities.

Perovskite thin-film solar cells are one of the most promising emerging renewable energy technologies because of their potential for low-cost, large-area fabrication combined with high energy conversion efficiencies. Recently, formamidinium lead triiodide (FAPbI3) and other formamidinium (CH(NH2)2) based perovskites have been explored as interesting alternatives to methylammonium lead triiodide (MAPbI3) because they exhibit better thermal stability. However, at present a major challenge is the scale-up of perovskite solar cells from small test-cells to full solar modules. We show that coevaporation is a scalable method for the deposition of homogeneous FAPbI3 thin films over large areas. The method allows precise control over film thickness and results in highly uniform, pinhole-free layers. Our films exhibited a high charge-carrier mobility of 26 cm2 V–1s–1, excellent optical properties, and a bimolecular recombination constant of 7 × 10–11 cm3 s–1. Solar cells fabricated using these vapor-deposited layers within a regular device architecture produced stabilized power conversion efficiencies of up to 14.2%. Thus, we demonstrate that efficient FAPbI3 solar cells can be vapor-deposited, which opens up a pathway toward large-area stable perovskite photovoltaics.

Progress in ultrafast terahertz (THz) communications has been limited due to the lack of picosecond switchable modulators with sufficient modulation depth. Gallium arsenide nanowires are ideal candidates for THz modulators as they absorb THz radiation, only when photoexcited – giving the potential for picosecend speed switching and high modulation depth. By embedding the nanowires in a polymer matrix and laminating together several nanowire–polymer films, we increase the areal density of nanowires, resulting in greater modulation of THz radiation. In this paper, we compare PDMS and Parylene C polymers for nanowire encapsulation and show that a high modulation depth is possible using Parylene C due to its thinness and its ability to be laminated. We characterize the modulator behavior and switching speed using optical pump–THz probe spectroscopy, and demonstrate a parylene–nanowire THz modulator with 13.5% modulation depth and 1ps switching speed.

The production of highly efficient single‐ and multijunction metal halide perovskite (MHP) solar cells requires careful optimization of the optical and electrical properties of these devices. Here, precise control of CH3NH3PbI3 perovskite layers is demonstrated in solar cell devices through the use of dual source coevaporation. Light absorption and device performance are tracked for incorporated MHP films ranging from ≈67 nm to ≈1.4 µm thickness and transfer‐matrix optical modeling is utilized to quantify optical losses that arise from interference effects. Based on these results, a device with 19.2% steady‐state power conversion efficiency is achieved through incorporation of a perovskite film with near‐optimum predicted thickness (≈709 nm). Significantly, a clear signature of photon reabsorption is observed in perovskite films that have the same thickness (≈709 nm) as in the optimized device. Despite the positive effect of photon recycling associated with photon reabsorption, devices with thicker (>750 nm) MHP layers exhibit poor performance owing to competing nonradiative charge recombination in a “dead‐volume” of MHP. Overall, these findings demonstrate the need for fine control over MHP thickness to achieve the highest efficiency cells, and accurate consideration of photon reabsorption, optical interference, and charge transport properties.

In this work, we will demonstrate a novel ultrafast THz modulator based on GaAs semiconductor nanowires at very high THz bandwidth. The modulator devices were fabricated using a highly flexible and cheap substrate – parylene-c. Through the use of multiple layers of this substrate, we show that for higher layers, a higher modulation depth is achieved without affecting the bandwidth over which the devices can operate. A terahertz air-plasma setup was built which allowed us to assess the devices over a broad range bandwidth of 0.1 THz – 40 THz.

In this study the photoconductivity and charge-carrier dynamics of the metal-halide perovskite MAPbI3 is investigated via optical-pump-THz-probe spectroscopy (OPTPS). We perform OPTPS on polycrystalline MAPbI3 thin films in both transmission and reflection modes, and demonstrate the consistency of extracted mobility and recombination parameters between the two geometries. Furthermore, we performed OPTP measurement on MAPbI3 single crystal in reflection and compare the mobility and charge recombination dynamics between the polycrystalline thin films and perovskite single crystals. Finally, the consequence of our results for future metal halide optoelectronic devices are discussed.

Due to their exceptional optoelectronic properties, hybrid metal halide perovskites thin films have shown extraordinary success as active layers in solar cells. One potential drawback of hybrid perovskites, however, is that the highest performing devices are currently based around toxic, lead-containing materials. Although tin is a promising replacement for lead, p-doping in tin-based materials has prevented them from achieving the efficiencies of their leadbased analogues. Using optical-pump/THz-probe spectroscopy and THz time-domain spectroscopy, we compare the intrinsic and extrinsic optoelectronic properties formamidinium tin triiodide thin films and comment on the fundamental limits to charge transport.

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI3 films with average thicknesses from 2-320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI3 films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices.

Terahertz (THz) photonic devices are now exploiting emerging materials systems, while novel device designs utilise plasmonic effects, nanophotinics, and metamaterials. The scope of this special topic highlights and reviews the recent cutting-edge THz photonic devices which have been revolutionised from the advances in the above research areas.

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI3 films with average thicknesses from 2 – 320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI3 films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices.

The addition of large hydrophobic cations to lead halide perovskites has significantly enhanced the environmental stability of photovoltaic cells based on these materials. However, the associated formation of two-dimensional structures inside the material can lead to dielectric confinement, higher exciton binding energies, wider bandgaps and limited charge-carrier mobilities. Here we show that such effects are not detrimental to the charge transport for carefully processed films comprising a self-assembled thin layer of quasi-two-dimensional (2D) perovskite interfaced with a 3D MAPbI3perovskite layer. We apply a combination of time-resolved photoluminescence and photoconductivity spectroscopy to reveal the charge-carrier recombination and transport through the film profile, when either the quasi-2D or the 3D layers are selectively excited. Through modeling of the recorded dynamics, we demonstrate that while the charge-carrier mobility is lower within the quasi-2D region, charge-carrier diffusion to the 3D phase leads to a rapid recovery in photoconductivity even when the quasi-2D region is initially photoexcited. In addition, the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective “heterogeneous photon recycling”. We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.

Summary Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH3CN)/methylamine(CH3NH2) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA0.83Cs0.17Pb(Br0.7I0.3)3 (1.94 eV) and MAPbI3 (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb0.75Sn0.25I3 (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells.

Many optoelectronic properties have been reported for lead halide perovskite polycrystalline films. However, ambiguities in the evaluation of these properties remain, especially for long-range lateral charge transport, where ionic conduction can complicate interpretation of data. Here we demonstrate a new technique to measure the long-range charge carrier mobility in such materials. We combine quasi-steady-state photo-conductivity measurements (electrical probe) with photo-induced transmission and reflection measurements (optical probe) to simultaneously evaluate the conductivity and charge carrier density. With this knowledge we determine the lateral mobility to be ∼2 cm2 V−1 s−1 for CH3NH3PbI3 (MAPbI3) polycrystalline perovskite films prepared from the acetonitrile/methylamine solvent system. Furthermore, we present significant differences in long-range charge carrier mobilities, from 2.2 to 0.2 cm2 V−1 s−1, between films of contemporary perovskite compositions prepared via different fabrication processes, including solution and vapour phase deposition techniques. Arguably, our work provides the first accurate evaluation of the long-range lateral charge carrier mobility in lead halide perovskite films, with charge carrier density in the range typically achieved under photovoltaic operation.

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH2PO3 and MAH2PO2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

We describe how optimized growth processes and contact-free electrical characterization techniques are accelerating the development of III–V nanowire-based optoelectronic devices with new and enhanced performance. © 2019 The Author(s).

We demonstrate four-and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2-electron volt band-gap perovskite, FA(0.75)Cs(0.25)Sn(0.5)Pb(0.5)I(3), that can deliver 14.8% efficiency. By combining this material with a wider-band gap FA(0.83)Cs(0.17)Pb(I0.5Br0.5)(3) material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with > 1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable "all-perovskite" thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs. We thank M. T. Horantner for performing the Shockley-Queisser calculation. The research leading to these results has received funding from the Graphene Flagship (Horizon 2020 grant no. 696656 - GrapheneCore1), the Leverhulme Trust (grant RL-2012-001), the UK Engineering and Physical Sciences Research Council (grant EP/J009857/1 and EP/M020517/1), and the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement nos. 239578 (ALIGN) and 604032 (MESO). T.L. is funded by a Marie Sklodowska Curie International Fellowship under grant agreement H2O2IF-GA-2015-659225. A.B. is financed by IMEC (Leuven) in the framework of a joint Ph.D. program with Hasselt University. B.C. is a postdoctoral research fellow of the Research Fund Flanders (FWO). We also acknowledge the U.S. Office of Naval Research for support. We acknowledge the use of the University of Oxford Advanced Research Computing (ARC) facility (http://dx.doi.org/10.5281/zenodo.22558) and the ARCHER UK National Super-computing Service under the "AMSEC" Leadership project. We thank the Global Climate and Energy Project (GCEP) at Stanford University. All data pertaining to the conclusions of this work can be found in the main paper and the supplementary materials.

Trap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA0.95Cs0.05PbI3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

Metal‐halide perovskites (MHP) are highly promising semiconductors for light‐emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NCs) is an effective approach for obtaining nearly defect‐free MHP that can be processed into inks for low‐cost, high‐performance device fabrication. However, disentangling the effects of surface ligands, morphology, and boundaries on charge‐carrier transport in thin films fabricated with these high‐quality NCs is inherently difficult. To overcome this fundamental challenge, terahertz (THz) spectroscopy is employed to optically probe the photoconductivity of CsPbBr3 NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge‐carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue‐shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.

III–V semiconductor nanowires combine the properties of III–V materials with the unique advantages of the nanowire geometry, allowing efficient room temperature photodetection across a wide range of photon energies, from a few eV down to meV. For example, due to their nanoscale size, these show great promise as sub-wavelength terahertz (THz) detectors for near-field imaging or detecting elements within a highly integrated on-chip THz spectrometer. We discuss recent advances in engineering a number of sensitive photonic devices based on III–V nanowires, including InAs nanowires with tunable photoresponse, THz polarisers and THz detectors.

Characterisation of materials often requires the use of a substrate to support the sample being investigated. For optical characterisation at terahertz frequencies, quartz is commonly used owing to its high transmission and low absorption at these frequencies. Knowledge of the complex refractive index of quartz is required for analysis of time-domain terahertz spectroscopy and optical pump terahertz probe spectroscopy for samples on a quartz substrate. Here, we present the refractive index and extinction coefficient for α-quartz between 0.5 THz and 5.5 THz (17–183 cm− 1) taken at 10, 40, 80, 120, 160, 200 and 300 K. Quartz shows excellent transmission and is thus an ideal optical substrate over the THz band, apart from the region 3.9 ± 0.1 THz owing to a spectral feature originating from the lowest energy optical phonon modes. We also present the experimentally measured polariton dispersion of α-quartz over this frequency range.

To date, the most efficient hybrid metal halide peroskite solar cells employ TiO2 as electron-transporting material (ETM), making these devices unstable under UV light exposure. Replacing TiO2 with fullerene derivatives has been shown to result in improved electronic contact and increased device lifetime, making it of interest to assess whether similar improvements can be achieved by using other organic semiconductors as ETMs. In this work, we investigate perylene-3,4:9,10-tetracarboxylic bis(benzimidazole) as a vacuum-processable ETM, and we minimize electron-collection losses at the electron-selective contact by depositing pentamethylcyclopentadienyl cyclopentadienyl rhodium dimer, (RhCp*Cp)2, on fluorinated tin oxide. With (RhCp*Cp)2 as an interlayer, ohmic contacts can be formed, there is interfacial doping of the ETM, and stabilized power conversion efficiencies of up to 14.2% are obtained.

We investigate the Raman spectrum of the low-temperature orthorhombic phase of the organic-inorganic halide perovskite CH3NH3PbI3, by combining first-principles calculations with high-resolution low-temperature Raman measurements. We find good agreement between theory and experiment, and successfully assign each of the Raman peaks to the underlying vibrational modes. In the low-frequency spectral range (below 60 cm􀀀1) we assign the prominent Raman signals at 26, 32, 42 and 49 cm􀀀1 to the Pb-I-Pb bending modes with either Ag or B2g symmetry, and the signal at 58 cm􀀀1 to the librational mode of the organic cation. Owing to their significant intensity, we propose that these peaks can serve as clear markers of the vibrations of the [PbI3]􀀀 network and of the CH3NH+ 3 cations in this perovskite, respectively. In particular, the ratios of the intensities of these peaks might be used to monitor possible deviations from the ideal stoichiometry of CH3NH3PbI3.

In this work, we demonstrate novel THz detectors and modulators based on semiconductor nanowires. We show that single nanowire PC THz receivers exhibit excellent sensitivity, high SNR of 40, and a broad detection bandwidth up to 2 THz, comparable to the bulk InP PC receivers. We also demonstrate ultrafast THz modulation by GaAs nanowire polarisers with a less than 5ps switching time and a modulation depth of −8dB, We show an extinction of over 13% and dynamic range of −9dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators.

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI3 (thin film) and find remarkably different behavior across the β–γ phase transition compared with MAPbI3. While MAPbI3 has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI3 vary smoothly over a range of 100–160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI3 exhibits a clear T–0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron–phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T–1.5 dependence previously observed for MAPbI3. Finally, we directly observe intraexcitonic transitions in FAPbI3 at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.

Traditionally, III–V multi-junction cells have been used in concentrator photovoltaic (CPV) applications, which deliver extremely high efficiencies but have failed to compete with ‘flat-plate’ silicon technologies owing to cost. Here, we assess the feasibility of using metal halide perovskites for CPVs, and we evaluate their device performance and stability under concentrated light. Under simulated sunlight, we achieve a peak efficiency of 23.6% under 14 Suns (that is, 14 times the standard solar irradiance), as compared to 21.1% under 1 Sun, and measure 1.26 V open-circuit voltage under 53 Suns, for a material with a bandgap of 1.63 eV. Importantly, our encapsulated devices maintain over 90% of their original efficiency after 150 h aging under 10 Suns at maximum power point. Our work reveals the potential of perovskite CPVs, and may lead to new PV deployment strategies combining perovskites with low-concentration factor and lower-accuracy solar tracking systems. Metal halide perovskites offer the potential for high-efficiency, low-fabrication-cost solar cells. This study now explores their prospects if deployed in concentrator photovoltaics and finds they perform well up to a concentration of 53 Suns and retain good stability under 10 Suns for over 150 h.

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI

GaAs nanowires are promising candidates for advanced optoelectronic devices, despite their high surface recombination velocity and large surface-area-to-volume ratio, which renders them problematic for applications that require efficient charge collection and long charge-carrier lifetimes. Overcoating a bare GaAs nanowire core with an optimized larger-bandgap AlGaAs shell, followed by a capping layer of GaAs to prevent oxidation, has proven an effective way to passivate the nanowire surface and thereby improve electrical properties for enhanced device performance. However, it is difficult to quantify and distinguish the contributions between the nanowire core and cap layer when measuring the optoelectronic properties of a nanowire device. Here, we investigated the photoconductive terahertz (THz) response characteristics of single GaAs/AlGaAs/GaAs core–shell–cap nanowire detectors designed for THz time-domain spectroscopy. We present a detailed study of the contributions of the GaAs cap layer and GaAs core on the ultrafast optoelectronic performance of the detector. We show that both the GaAs cap and core contribute to the photoconductive signal in proportion to their relative volume in the nanowire. By increasing the cap volume ratio to above 90% of the total GaAs volume, a quasi-direct-sampling type photoconductive nanowire detector can be achieved that is highly desirable for low-noise and fast data acquisition detection.

The rise of metal-halide perovskite solar cells has captivated the research community, promising to disrupt the current energy landscape. While a sizable percentage of the research done on this class of materials has been focused on the neat and iodide-rich perovskites, bromide-based perovskites can deliver substantially higher voltages because of their relatively wide band gaps of over 2 eV. The potential for efficient, high-voltage devices makes materials such as these incredibly attractive for multijunction photovoltaic applications. Here, we use the acetonitrile/methylamine solvent system to deposit smooth, highly crystalline films of CH3NH3PbBr3. By using choline chloride as a passivating agent for these films, we achieve photoluminescence quantum efficiencies of up to 5.5% and demonstrate charge-carrier mobilities of 17.8 cm2/(V s). Incorporating these films into photovoltaic devices, we achieve scanned power conversion efficiencies of up to 8.9%, with stabilized efficiencies of 7.6%, providing a simple route to realizing efficient, high-voltage CH3NH3PbBr3 planar-heterojunction devices.

Already exhibiting solar to electrical power conversion efficiencies of over 17%, organic-inorganic lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicology issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While organic-inorganic tin halide perovskites have shown good semiconducting behaviour, the instability of tin in its 2+ oxidation state has thus far proved to be an overwhelming challenge. Here, we report the first completely lead-free, CH3NH 3SnI3 perovskite solar cell processed on a mesoporous TiO2 scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap. This journal is © the Partner Organisations 2014.

Downloading of the abstract is permitted for personal use only. Nanowires show unique promise as nanoscale building blocks for a multitude of optoelectronic devices, ranging from solar cells to terahertz photonic devices. We will discuss the epitaxial growth of these nanowires in novel geometries and crystallographic phases, and the use of terahertz conductivity spectroscopy to guide the development of nanowire-based devices. As an example, we will focus on the development of nanowire-based polarization modulators for terahertz communications systems.

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs0.65Sb0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs0.65Sb0.35 nanowires to date, exceeding 16,000 cm2 V–1 s–1 at 10 K.

Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that can boost efficiencies beyond the Shockley–Queisser limit stipulated for a single‐junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has not been sufficiently explored so far for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. Here, the prospects of hybrid perovskites are assessed for use in concentrator solar cells. Solar cell performance parameters are theoretically predicted as a function of solar concentration levels, based on representative assumptions of charge‐carrier recombination and extraction rates in the device. It is demonstrated that perovskite solar cells can fundamentally exhibit appreciably higher energy‐conversion efficiencies under solar concentration, where they are able to exceed the Shockley–Queisser limit and exhibit strongly elevated open‐circuit voltages. It is therefore concluded that sufficient material and device stability under increased illumination levels will be the only significant challenge to perovskite concentrator solar cell applications.

Organic–inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (~7 µm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

The performance of perovskite solar cells has been progressing over the past few years and efficiency is likely to continue to increase. However, a negative aspect for the integration of perovskite solar cells in the built environment is that the color gamut available in these materials is very limited and does not cover the green-to-blue region of the visible spectrum, which has been a big selling point for organic photovoltaics. Here, we integrate a porous photonic crystal (PC) scaffold within the photoactive layer of an opaque perovskite solar cell following a bottom-up approach employing inexpensive and scalable liquid processing techniques. The photovoltaic devices presented herein show high efficiency with tunable color across the visible spectrum. This now imbues the perovskite solar cells with highly desirable properties for cladding in the built environment and encourages design of sustainable colorful buildings and iridescent electric vehicles as future power generation sources.

Solar cells based on metal halide perovskite thin films show great promise for energy generation in a range of environments from terrestrial installations to space applications. Here we assess the device characteristics of the prototypical perovskite solar cells based on methylammonium lead triiodide (CH3NH3PbI3) over a broad temperature range from 15 to 350 K (−258 to 77 °C). For these devices, we observe a peak in the short-circuit current density and open-circuit voltage at 200 K (−73 °C) with decent operation maintained up to 350 K. We identify the clear signature of crystalline PbI2 contributing directly to the low-temperature photocurrent spectra, showing that PbI2 plays an active role (beyond passivation) in CH3NH3PbI3 solar cells. Finally we observe a blue-shift in the photocurrent spectrum with respect to the absorption spectrum at low temperature (15 K), allowing us to extract a lower limit on the exciton binding energy of 9.1 meV for CH3NH3PbI3.

Nanowires show unique promise for a multitude of optoelectronic devices, ranging from solar cells to terahertz (THz) photonic devices. Here, we discuss how THz spectroscopy is guiding the development of such nanowire-based devices. As an example, we focus on developing nanowire-based THz polarization modulators.

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH