The first demonstration of entirely roll-to-roll fabricated perovskite solar cell modules under ambient room conditions.

The rapid development of organic-inorganic hybrid perovskite solar cells has resulted in laboratory-scale devices having power conversion efficiencies that are competitive with commercialised technologies. However, hybrid perovskite solar cells are yet to make an impact beyond the research community, with translation to large-area devices fabricated by industry-relevant manufacturing methods remaining a critical challenge. Here we report the first demonstration of hybrid perovskite solar cell modules, comprising serially-interconnected cells, produced entirely using industrial roll-to-roll printing tools under ambient room conditions. As part of this development, costly vacuum-deposited metal electrodes are replaced with printed carbon electrodes. A high-throughput experiment involving the analysis of batches of 1600 cells produced using 20 parameter combinations enabled rapid optimisation over a large parameter space. The optimised roll-to-roll fabricated hybrid perovskite solar cells show power conversion efficiencies of up to 15.5% for individual small-area cells and 11.0% for serially-interconnected cells in large-area modules. Based on the devices produced in this work, a cost of ~0.7 USD W-1 is predicted for a production rate of 1,000,000 m² per year in Australia, with potential for further significant cost reductions.

Direct synthesis of CsPbX3 perovskite nanocrystal assemblies.

Inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) possess many advantageous optoelectronic properties, making them an attractive candidate for light emitting diodes, lasers, or photodetector applications. Such perovskite NCs can form extended assemblies that further modify their bandgap and emission wavelength. In this article, a facile direct synthesis of CsPbX3 NC assemblies that are 1 µm in size and are composed of 10 nm-sized NC building blocks is reported. The direct synthesis of these assemblies with a conventional hot-injection method of the NCs is achieved through the judicious selection of the solvent, ligands, and reaction stoichiometry. Only under selective reaction conditions where the surface ligand environment is tuned to enhance the hydrophobic interactions between ligand chains of neighbouring NCs is self-assembly achieved. These assemblies possess narrow and red-shifted photoluminescence compared to their isolated NC counterparts, which further expands the colour gamut that can be rendered from inorganic perovskites. This is demonstrated through simple down-converting light emitters.

Printable Perovskite Diodes for Broad-Spectrum Multienergy X-Ray Detection.

Multienergy X-ray detection is critical to effectively differentiate materials in a variety of diagnostic radiology and nondestructive testing applications. Silicon and selenium X-ray detectors are the most common for multienergy detection; however, these present poor energy discrimination across the broad X-ray spectrum and exhibit limited spatial resolution due to the high thicknesses required for radiation attenuation. Here, an X-ray detector based on solution-processed thin-film metal halide perovskite that overcomes these challenges is introduced. By harnessing an optimized n-i-p diode configuration, operation is achieved across a broad range of soft and hard X-ray energies stemming from 0.1 to 10's of keV. Through detailed experimental and simulation work, it is shown that optimized Cs0.1 FA0.9 PbI3 perovskites effectively attenuate soft and hard X-rays, while also possessing excellent electrical properties to result in X-ray detectors with high sensitivity factors that exceed 5 × 103 µ C G y Vac - 1 cm - 2 $\mu {\rm{C}}\;{{\bf Gy}}_{{\rm{Vac}}}^{ - 1}\;{\rm{c}}{{\rm{m}}^{ - 2}}$ and 6 × 104 µC Gy-1 cm-2 within soft and hard X-ray regimes, respectively. Harnessing the solution-processable nature of the perovskites, roll-to-roll printable X-ray detectors on flexible substrates are also demonstrated.

Observation of Phonon Cascades in Cu-Doped Colloidal Quantum Wells.

Electronic doping has endowed colloidal quantum wells (CQWs) with unique optical and electronic properties, holding great potential for future optoelectronic device concepts. Unfortunately, how photogenerated hot carriers interact with phonons in these doped CQWs still remains an open question. Here, through investigating the emission properties, we have observed an efficient phonon cascade process (i.e., up to 27 longitudinal optical phonon replicas are revealed in the broad Cu emission band at room temperature) and identified a giant Huang-Rhys factor (S ≈ 12.4, more than 1 order of magnitude larger than reported values of other inorganic semiconductor nanomaterials) in Cu-doped CQWs. We argue that such an ultrastrong electron-phonon coupling in Cu-doped CQWs is due to the dopant-induced lattice distortion and the dopant-enhanced density of states. These findings break the widely accepted consensus that electron-phonon coupling is typically weak in quantum-confined systems, which are crucial for optoelectronic applications of doped electronic nanomaterials.

Macroporous perovskite nanocrystal composites for ultrasensitive copper ion detection.

Accumulation of heavy metal ions, including copper ions (Cu2+), presents a serious threat to human health and to the environment. A substantial amount of research has focused on detecting such species in aqueous solutions. However, progress towards ultrasensitive and easy-to-use sensors for non-aqueous solutions is still limited. Here, we focus on the detection of copper species in hexane, realising ultra-sensitive detection through a fluorescence-based approach. To achieve this, a novel macroporous composite material has been developed featuring luminescent CsPbBr3 nanocrystals (NCs) chemically adhered to a polymerized high internal phase emulsion (polyHIPE) substrate through surface thiol groups. Due to this thiol functionality, sub-monolayer NC formation is realised, which also renders outstanding stability of the composite in the ambient environment. Copper detection is achieved through a direct solution based immersion of the CsPbBr3-(SH)polyHIPE composite, which results in concentration-dependent quenching of the NC photoluminescence. This newly developed sensor has a limit of detection (LOD) for copper as low as 1 × 10-16 M, and a wide operating window spanning 10-2 to 10-16 M. Moreover, the composite exhibits excellent selectivity among different transition metals.

High-Performance and Stable Semi-Transparent Perovskite Solar Cells through Composition Engineering.

Semi-transparent perovskite solar cells (ST-PeSCs) have tremendous potential as solar windows owing to their higher efficiency and visible transmittance. However, studies toward this application are still nascent, particularly in unraveling the interplay between how the perovskite composition impacts the achievable device performance and stability. Here, the role of A- and X-site modification in APbX3 perovskites is studied to understand their influence on these factors. Through detailed experimental and simulation work, it is found that a perovskite composition consisting of cesium (Cs) and formamidinium (FA) at the A-site delivers the best device performance over a range of band gaps, which are tuned by changes to the X-site anion. Using this optimized perovskite composition, power conversion efficiencies of 15.5% and 4.1% are achieved for ST-PeSCs with average visible transmittance values between 20.7% and 52.4%, respectively. Furthermore, the CsFA-based ST-PeSCs show excellent long-term stability under continuous illumination and heating. The stability of the precursor solutions across each of the studied compositions has also been considered, showing dramatic differences in the structural properties of the perovskites and their device performance for all mixed A-site compositions possessing the archetypal methyl ammonium species, while also confirming the superior stability of the CsFA precursor solutions.

Intrinsic Green Fluorescent Cross-Linked Poly(ester amide)s by Spontaneous Zwitterionic Copolymerization.

The spontaneous zwitterionic copolymerization (SZWIP) of 2-oxazolines and acrylic acid affords biocompatible but low molecular weight linear N-acylated poly(amino ester)s (NPAEs). Here, we present a facile one-step approach to prepare functional higher molar mass cross-linked NPAEs using 2,2'-bis(2-oxazoline)s (BOx). In the absence of solvent, insoluble free-standing gels were formed from BOx with different length n-alkyl bridging units, which when butylene-bridged BOx was used possessed an inherent green fluorescence, a behavior not previously observed for 2-oxazoline-based polymeric materials. We propose that this surprising polymerization-induced emission can be classified as nontraditional intrinsic luminescence. Solution phase and oil-in-oil emulsion approaches were investigated as means to prepare solution processable fluorescent NPAEs, with both resulting in water dispersible network polymers. The emulsion-derived system was investigated further, revealing pH-responsive intensity of emission and excellent photostability. Residual vinyl groups were shown to be available for modifications without affecting the intrinsic fluorescence. Finally, these systems were shown to be cytocompatible and to function as fluorescent bioimaging agents for in vitro imaging.

Prospects of Z-Scheme Photocatalytic Systems Based on Metal Halide Perovskites.

Considering the attractive optoelectronic properties of metal halide perovskites (MHPs), their introduction to the field of photocatalysis was only a matter of time. Thus far, MHPs have been explored for the photocatalytic generation of hydrogen, carbon dioxide reduction, organic synthesis, and pollutant degradation applications. Of growing research interest and possible applied significance are the currently emerging developments of MHP-based Z-scheme heterostructures, which can potentially enable efficient photocatalysis of highly energy-demanding redox processes. In this Perspective, we discuss the advantages and limitations of MHPs compared to traditional semiconductor materials for applications as photocatalysts and describe emerging examples in the construction of MHP-based Z-scheme systems. We discuss the principles and material properties that are required for the development of such Z-scheme heterostructure photocatalysts and consider the ongoing challenges and opportunities in this emerging field.

Detection of Halomethanes Using Cesium Lead Halide Perovskite Nanocrystals.

The extensive use of halomethanes (CH3X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH3I) as a model system, it is shown that cesium lead bromide (CsPbBr3) nanocrystals can undergo rapid (<5 s) halide exchange, but only after exposure to oleylamine to induce nucleophilic substitution of the CH3I and release the iodide species. The extent of the halide exchange is directly dependent on the CH3I concentration, with the photoluminescence emission of the CsPbBr3 nanocrystals exhibiting a redshift of more than 150 nm upon the addition of 10 ppmv of CH3I. This represents the widest detection range and the highest sensitivity to the detection of halomethanes using a low-cost and portable approach reported to date. Furthermore, inherent selectivity for halomethanes compared to other organohalide analogues is achieved through the dramatic differences in their alkylation reactivity.

Scalable synthesis of colloidal CsPbBr3 perovskite nanocrystals with high reaction yields through solvent and ligand engineering.

The ligand assisted reprecipitation (LARP) technique is an accessible and facile method that can synthesize metal halide perovskite nanocrystals (PNCs) under ambient conditions. However, low product yields of less than 30% for LARP and its contemporary methods are indicative of highly inefficient reactions. In this work we apply the principles of green chemistry to the LARP technique for synthesizing CsPbBr3 PNCs and help address this issue. Through these efforts, high product yields of ∼70% are achieved using stochiometric Cs : Pb precursor ratios. This is realized by (i) substituting the conventional toluene (TOL) anti-solvent with ethyl acetate (EA) and (ii) replacing the conventionally used unsaturated oleylamine ligand with the shorter saturated octylamine ligand. These changes also result in a 60% molar reduction in total ligand concentration and a 62.5% reduction in solvent waste during purification. The synthesized PNCs are comparable to the TOL-LARP reference in crystal quality, morphology and phase, with their photoluminescence quantum yields being readily enhanced to over 80% through additions of RNH3Br ligands. The spectral versatility of these materials is demonstrated through post-synthetic chloride and iodide halide anion exchange, which readily yields tunable CsPbX3 derivatives across the visible spectrum. Our EA-LARP protocol is further shown to be readily upscaled to ∼0.5 L, while maintaining good nanocrystal properties and a product yield of 60%.

Facile purification of CsPbX3 (X = Cl-, Br-, I-) perovskite nanocrystals.

CsPbI3 perovskite nanocrystals are a promising optoelectronic material when stabilized in their cubic phase. While ongoing efforts have addressed this structural challenge through a variety of meta-stabilization approaches, the postsynthesis purification of these nanocrystal dispersions has remained a challenge. In this article, we undertake a detailed investigation into the chemical, optical, and structural changes that arise during purification of CsPbI3 nanocrystals. It is found that nanocrystal degradation can only be avoided through the judicious control of additives within each purification cycle. Under optimized additive-to-nanocrystal ratios, multiple purification cycles can be readily achieved, while retaining the quality and phase stability of the CsPbI3. This facile purification protocol ensures the preparation of high purity and high quality CsPbI3 nanocrystal inks that are suitable for better characterization or integration in optoelectronic devices. The approach has been generalized for CsPbX3 (X = Cl-, Br-, and I-).

Synthetic Evolution of Colloidal Metal Halide Perovskite Nanocrystals.

Metal halide perovskite semiconductor nanocrystals have emerged as a lucrative class of materials for many optoelectronic applications. By leveraging the synthetic toolboxes developed from decades of research into more traditional semiconductor nanocrystals, remarkable progress has been made across these materials in terms of their structural, compositional, and optoelectronic control. Here, we review this progress in terms of their underlying formation stages, synthetic approaches, and postsynthetic treatment steps. This assessment highlights the rapidly maturing nature of the perovskite nanocrystal field, particularly with regard to their lead-based derivatives. It further demonstrates that significant challenges remain around precisely controlling their nucleation and growth processes. In going forward, a deeper understanding of the role of precursors and ligands will significantly bolster the versatility in the size, shape, composition, and functional properties of these exciting materials.

Flexible photodetectors based on reticulated SWNT/perovskite quantum dot heterostructures with ultrahigh durability.

Recently, single-walled carbon nanotube (SWNT) films have been regarded as a promising channel material for flexible photodetectors due to their high intrinsic carrier mobility, conductivity, and mechanical flexibility. However, the application of SWNTs in photonic devices is limited due to their weak light absorption and the absence of a gain mechanism. Here, we demonstrate a high-performance flexible photodetector that consists of a reticulated SWNT film covered with a thin film of CsPbI3 perovskite colloidal quantum dots. The unique hierarchical reticulated structure of the SWNTs provides such films with extremely high tensile strength and great extensibility, which can ensure the appropriate toughness for achieving flexible photodetectors. Meanwhile, the perovskite quantum dots enhance light absorption, thereby sensitizing the creation of free electrical carriers within the SWNTs. This hybrid photodetector exhibits an extended photonic response and gain compared with the original pure SWNT devices. In addition, the device exhibits good robustness against repetitive bending and stretching, suggesting its applicability as a large-area wearable flexible photodetector.

Aqueous Synthesis of High-Quality Cu2ZnSnS4 Nanocrystals and Their Thermal Annealing Characteristics.

Copper zinc tin sulfide (CZTS) nanocrystal inks are promising candidates for the development of cheap, efficient, scalable, and nontoxic photovoltaic (PV) devices. However, optimization of the synthetic chemistry to achieve these goals remains a key challenge. Herein we describe a single-step, aqueous-based synthesis that yields high-quality CZTS nanocrystal inks while also minimizing residual organic impurities. By exploiting simultaneous redox and crystal formation reactions, square-platelet-like CZTS nanocrystals stabilized by Sn2S64- and thiourea are produced. The CZTS synthesis is optimized by using a combination of inductively coupled plasma analysis, Raman spectroscopy, Fourier transform infrared spectroscopy, and synchrotron powder X-ray diffraction to assess the versatility of the synthesis and identify suitable composition ranges for achieving phase-pure CZTS. It is found that mild heat treatment between 185 and 220 °C is most suitable for achieving this because this temperature range is sufficiently high to thermalize existing ligands and ink additives while minimizing tin loss, which is problematic at higher temperatures. The low temperatures required to process these nanocrystal inks to give CZTS thin films are readily amenable to production-scale processes.

Binding and Packing in Two-Component Colloidal Quantum Dot Ligand Shells: Linear versus Branched Carboxylates.

In this work, we present a combined experimental and theoretical analysis of two-component ligand shells passivating CdSe quantum dots. Using nuclear magnetic resonance spectroscopy, we first show that exposing oleate-capped quantum dots to primary carboxylic acids results in a one-for-one exchange that preserves the overall ligand surface concentration. Exposure to straight-chain acids leads to a binary ligand shell that behaves as an ideal mixture and that has a composition matching the overall acid composition of the dispersion. In the case of branched-chain acids, the exchange is restricted to about 25% of the original ligands. Based on molecular dynamics simulations, we argue that this behavior reflects the more favorable packing of oleates compared to branched carboxylates on the (100) facets of CdSe quantum dots.

Stabilizing the cubic perovskite phase of CsPbI3 nanocrystals by using an alkyl phosphinic acid.

CsPbI3 nanocrystals suffer from a facile cubic perovskite to orthorhombic phase transformation, which deteriorates their appealing optoelectronic properties. Here, we report a new colloidal synthesis that replaces the conventionally used oleic acid with an alkyl phosphinic acid to grow high-quality, phase-stable cubic perovskite CsPbI3 nanocrystals.

Engineering of Semiconductor Nanocrystals for Light Emitting Applications.

Semiconductor nanocrystals are rapidly spreading into the display and lighting markets. Compared with liquid crystal and organic LED displays, nanocrystalline quantum dots (QDs) provide highly saturated colors, wide color gamut, resolution, rapid response time, optical efficiency, durability and low cost. This remarkable progress has been made possible by the rapid advances in the synthesis of colloidal QDs and by the progress in understanding the intriguing new physics exhibited by these nanoparticles. In this review, we provide support to the idea that suitably engineered core/graded-shell QDs exhibit exceptionally favorable optical properties, photoluminescence and optical gain, while keeping the synthesis facile and producing QDs well suited for light emitting applications. Solid-state laser emitters can greatly profit from QDs as efficient gain materials. Progress towards fabricating low threshold, solution processed DFB lasers that are optically pumped using one- and two-photon absorption is reviewed. In the field of display technologies, the exploitation of the exceptional photoluminescence properties of QDs for LCD backlighting has already advanced to commercial levels. The next big challenge is to develop the electroluminescence properties of QD to a similar state. We present an overview of QLED devices and of the great perspectives for next generation display and lighting technologies.

Interfacial Characteristics of Efficient Bulk Heterojunction Solar Cells Fabricated on MoOx Anode Interlayers.

The role of the interface between an MoOx anode interlayer and a polymer:fullerene bulk heterojunction is investigated. Processing differences in the MoOx induce large variations in the vertical stratification of the bulk heterojunction films. These variations are found to be inconsistent in predicting device performance, with a much better gauge being the quantity of polymer chemisorbed to the anode interlayer.

Photonic Sintering of Copper through the Controlled Reduction of Printed CuO Nanocrystals.

The ability to control chemical reactions using ultrafast light exposure has the potential to dramatically advance materials and their processing toward device integration. In this study, we show how intense pulsed light (IPL) can be used to trigger and modulate the chemical transformations of printed copper oxide features into metallic copper. By varying the energy of the IPL, CuO films deposited from nanocrystal inks can be reduced to metallic Cu via a Cu2O intermediate using single light flashes of 2 ms duration. Moreover, the morphological transformation from isolated Cu nanoparticles to fully sintered Cu films can also be controlled by selecting the appropriate light intensity. The control over such transformations enables for the fabrication of sintered Cu electrodes that show excellent electrical and mechanical properties, good environmental stability, and applications in a variety of flexible devices.