Defect Engineering of Metal Halide Perovskite Nanocrystals via Spontaneous Diffusion of Ag Nanocrystals.

Perovskite nanocrystals (NCs) have emerged as a promising building block for the fabrication of optic-/optoelectronic-/electronic devices owing to their superior characteristics, such as high absorption coefficient, rapid ion mobilities, and tunable energy levels. However, their low structural stability and poor surface passivation have restricted their application to next-generation devices. Herein, a drug delivery system (DDS)-inspired post-treatment strategy is reported for improving their structural stability by doping of Ag into CsPbBr3 (CPB) perovskite NCs; delivery to damaged sites can promote their structural recovery slowly and uniformly, averting the permanent loss of their intrinsic characteristics. Ag NCs are designed through surface-chemistry tuning and structural engineering to enable their circulation in CPB NC dispersions, followed by their delivery to the CPB NC surface, defect-site recovery, and defect prevention. The perovskite-structure healing process through the DDS-type process (with Ag NCs as the drug) is analyzed by a combination of theoretical calculations (with density functional theory) and experimental analyses. The proposed DDS-inspired healing strategy significantly enhances the optical properties and stability of perovskite NCs, enabling the fabrication of white light-emitting diodes.

Large-Scale Assembly of Peptide-Based Hierarchical Nanostructures and Their Antiferroelectric Properties.

An effective strategy is developed to create peptide-based hierarchical nanostructures through the meniscus-driven self-assembly in a large area and fabricate antiferroelectric devices based on these nanostructures for the first time. The diphenylalanine hierarchical nanostructures (FF-HNs) are self-assembled by vertically pulling a substrate from a diphenylalanine (FF) solution dissolved in a miscible solvent under precisely controlled conditions. Owing to the unique structural properties of FF nanostructures, including high crystallinity and α-helix structures, FF-HNs possess a net electrical dipole moment, which can be switched in an external electric field. The mass production of antiferroelectric devices based on FF-HNs can be successfully achieved by means of this biomimetic assembly technique. The devices show an evident antiferroelectric to ferroelectric transition under dark conditions, while the ferroelectricity is found to be tunable by light. Notably, it is discovered that the modulation of antiferroelectric behaviors of FF-HNs under glutaraldehyde exposure is due to the FF molecules that are transformed into cyclophenylalanine by glutaraldehyde. This work provides a stepping stone toward the mass production of self-assembled hierarchical nanostructures based on biomolecules as well as the mass fabrication of electronic devices based on biomolecular nanostructures for practical applications.

Enhanced Charge Transport in 2D Perovskites via Fluorination of Organic Cation.

Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky spacer cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluorophenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with better aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained a power conversion efficiency of >13% for (F-PEA)2MA4Pb5I16-based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.

Ultimate Charge Extraction of Monolayer PbS Quantum Dot for Observation of Multiple Exciton Generation.

Multiple exciton generation (MEG) has great potential to improve the Shockley-Queisser (S-Q) efficiency limitation for colloidal quantum dot (CQD) solar cells. However, MEG has rarely been observed in CQD solar cells because of the loss of carriers through the transport mechanism between adjacent QDs. Herein, we demonstrate that excess charge carriers produced via MEG can be efficiently extracted using monolayer PbS QDs. The monolayer PbS QDs solar cells exhibit α=1 in the light intensity dependence of the short-circuit current density Jsc (Jsc ∝Iα ) and an internal quantum efficiency (IQE) value of 100 % at 2.95 eV because of their very short charge extraction path. In addition, the measured MEG threshold is 2.23 times the bandgap energy (Eg ), which is the lowest value in PbS QD solar cells. We believe that this approach can provide a simple method to find suitable CQD materials and design interface engineering for MEG.

Do grain boundaries dominate non-radiative recombination in CH3NH3PbI3 perovskite thin films?

Here, we examine grain boundaries (GBs) with respect to non-GB regions (grain surfaces (GSs) and grain interiors (GIs)) in high-quality micrometer-sized perovskite CH3NH3PbI3 (or MAPbI3) thin films using high-resolution confocal fluorescence-lifetime imaging microscopy in conjunction with kinetic modeling of charge-transport and recombination processes. We show that, contrary to previous studies, GBs in our perovskite MAPbI3 thin films do not lead to increased recombination but that recombination in these films happens primarily in the non-GB regions (i.e., GSs or GIs). We also find that GBs in these films are not transparent to photogenerated carriers, which is likely associated with a potential barrier at GBs. Even though GBs generally display lower luminescence intensities than GSs/GIs, the lifetimes at GBs are no worse than those at GSs/GIs, further suggesting that GBs do not dominate non-radiative recombination in MAPbI3 thin films.

Anionic ligand assisted synthesis of 3-D hollow TiO2 architecture with enhanced photoelectrochemical performance.

Hollow structured materials have shown great advantages for use in photoelectrochemical devices. However, their poor charge transport limits overall device performance. Here, we report a unique 3-D hollow architecture of TiO2 that greatly improves charge transport properties. We found that citric acid (CA) plays crucial roles in the formation of the 3-D hollow architecture. First, CA controls the hydrolysis rate of Ti ions and facilitates surface hydrolysis on templates during hydrothermal synthesis. Second, CA suppresses the growth of the carbon template at the initial reaction stage, resulting in the formation of comparatively small hollow fibers. More importantly, a prolonged hydrothermal reaction with CA enables a hollow sphere to grow into entangled hollow fibers via biomimetic swallowing growth. To demonstrate advantages of the 3-D hollow architecture for photoelectrochemical devices, we evaluated its photoelectrochemical performance, specifically the electrolyte diffusion and electron dynamics, by employing dye-sensitized solar cells as a model device. A systemic analysis reveals that the 3-D hollow architecture greatly improves both the electrolyte diffusion and electron transport compared to those of the nanoparticle and hollow sphere due to the elongated porous hollow morphology as well as the densely interconnected nanoparticles at the wall layer.

Progress and outlook of Sn-Pb mixed perovskite solar cells.

Organic-inorganic hybrid perovskites have revolutionized solar cell research owing to their excellent material properties. Most previous research has been done on Pb-based perovskites. Recently, efforts to discover a Pb-free or Pb-less perovskite material with an ideal bandgap ranging 1.1-1.3 eV have led researchers to investigate Sn-Pb mixed perovskites. Sn-Pb mixed perovskites have a bandgap of ~ 1.25 eV, which is suitable for high-efficiency single-junction and perovskite/perovskite tandem solar cells. Moreover, the Pb content of Sn-Pb mixed perovskites is 50-60% lower than that of Pb-based perovskites, partially mitigating the Pb toxicity issue. However, incorporating Sn2+ into the crystal structure also causes various drawbacks, such as inhomogeneous thin film morphologies, easy oxidation of Sn2+, and more vulnerable surface properties. Researchers have made substantial progress in addressing these challenges through improvements in compositional design, structural optimization, precursor design, and surface treatments. In this review, we provide a comprehensive overview of the progress in Sn-Pb mixed perovskite solar cells. Furthermore, we analyze the key variables and trends as well as provide an outlook for future directions in the research on Sn-Pb mixed perovskites.

Influence of niobium doping in hierarchically organized titania nanostructure on performance of dye-sensitized solar cells.

Niobium doped hierarchically organized TiO2 nanostructures composed of 20 nm size anatase nanocrystals were synthesized using pulsed laser deposition (PLD). The Nb doping concentration could be facilely controlled by adjusting the concentration of Nb in target materials. We could investigate the influence of Nb doping in the TiO2 photoelectrode on the cell performance of dye-sensitized solar cells (DSSCs) by the exclusion of morphological effects using the prepared Nb-doped TiO2 anostructures. We found no significant change in short circuit current density (Jsc) as a function of Nb doping concentration. However, open circuit voltage (Voc) and fill factor (FF) monotonously decrease with increasing Nb concentration. Dark current characteristics of the DSSCs reveal that the decrease in Voc and FF is attributed to the decrease in shunt resistance due to the increase in conductivity TiO2 by Nb doping. However, electrochemical impedance spectra (EIS) analysis at open circuit condition under illumination showed that the resistance at the TiO2/dye/electrolyte interface increases with Nb concentration, revealing that Nb doping suppress the charge recombination at the interface. In addition, electron life time obtained using characteristic frequency in Bode plot increases from 14 msec to 56 msec with increasing Nb concentration from 0 to 1.2 at%. This implies that the improved light harvesting can be achieved by increasing diffusion length through Nb-doping in the conventional TiO2 photoelectrode.

Surface modified TiO2 nanostructure with 3D urchin-like morphology for dye-sensitized solar cell application.

Three-dimensional (3D) urchin-like rutile TiO2 powders were synthesized by a mild hydrothermal method without any templates. An individual urchin-like TiO2 powder consists of self-assembled nanorods with a length of about 150 nm and width of about 10 nm. Additionally, the urchin-like TiO2 nanopowders were coated with an ultra-thin ZnO layer in order to modify the surface properties of the nanopowders, and the ZnO layer was confirmed by high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analysis. The ZnO-modified TiO2 was used as a photoelectrode of a dye-sensitized solar cell (DSSC) and the solar cell performances were investigated. In comparison with bare TiO2, ZnO-modified TiO2 improved the photovoltaic performances, i.e., energy conversion efficiency, open circuit voltage, and short circuit current were increased. The higher DSSC performance of ZnO-modified TiO2 was attributed to its higher dye loading and lower charge recombination rate.

Perovskite microcells fabricated using swelling-induced crack propagation for colored solar windows.

Perovskite microcells have a great potential to be applied to diverse types of optoelectronic devices including light-emitting diodes, photodetectors, and solar cells. Although several perovskite fabrication methods have been researched, perovskite microcells without a significant efficiency drop during the patterning and fabrication process could not be developed yet. We herein report the fabrication of high-efficiency perovskite microcells using swelling-induced crack propagation and the application of the microcells to colored solar windows. The key procedure is a swelling-induced lift-off process that leads to patterned perovskite films with high-quality interfaces. Thus, a power conversion efficiency (PCE) of 20.1 % could be achieved with the perovskite microcell, which is nearly same as the PCE of our unpatterned perovskite photovoltaic device (PV). The semi-transparent PV based on microcells exhibited a light utilization efficiency of 4.67 and a color rendering index of 97.5 %. The metal-insulator-metal structure deposited on the semi-transparent PV enabled to fabricate solar windows with vivid colors and high color purity.

Synthesis and characteristics of tb-doped Y2SiO5 nanophosphors and luminescent layer for enhanced photovoltaic cell performance.

Nanophosphors based on green emitting terbium doped yttrium silicates with the general formula Y2SiO5:Tb3+ with a size of 30-60 nm were synthesized by the hydrothermal method. The prepared nanophosphors were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy and fluorescence spectroscopy. It was found that the nanophosphors crystallize in an X1-type monoclinic structure (P2(1)/c) and absorb UV light from 220 to 300 nm which they then down-convert into visible-light (strong green emission at around 545 nm (5D4-->7F(J. As TiO2-based dye-sensitized solar cells exhibit their maximum incident photon to current efficiency at around 500-600 nm, the wavelength-modulation characteristics of the nanophosphors can be efficient for dye-sensitized solar cell systems. Therefore, the Y2SiO5:Tb3+ nanophosphors were introduced into a TiO2-based dye-sensitized solar cell and their effects on the performance of the solar cell were investigated.

Wide-Bandgap Metal Halide Perovskites for Tandem Solar Cells.

Metal halide perovskite solar cells (PSCs) have become the most promising new-generation solar cell technology. To date, perovskites also represent the only polycrystalline thin-film absorber technology that has enabled >20% efficiency for wide-bandgap solar cells, making wide-bandgap PSCs uniquely positioned to enable high-efficiency and low-cost tandem solar cell technologies by coupling wide-bandgap perovskites with low-bandgap absorbers. In this Focus Review, we highlight recent research progress on developing wide-bandgap PSCs, including the key mechanisms associated with efficiency loss and instability as well as strategies for overcoming these challenges. We also discuss recent accomplishments and research trends on using wide-bandgap PSCs in perovskite-based tandem configurations, including perovskite/perovskite, perovskite/Si, perovskite/CIGS, and other emerging tandem technologies.

Intermediate Phase-Free Process for Methylammonium Lead Iodide Thin Film for High-Efficiency Perovskite Solar Cells.

Solvent engineering by Lewis-base solvent and anti-solvent is well known for forming uniform and stable perovskite thin films. The perovskite phase crystallizes from an intermediate Lewis-adduct upon annealing-induced crystallization. Herein, it is explored the effects of trimethyl phosphate (TMP), as a novel aprotic Lewis-base solvent with a low donor number for the perovskite film formation and photovoltaic characteristics of perovskite solar cells (PSCs). As compared to dimethylsulfoxide (DMSO) or dimethylformamide (DMF), the usage of TMP directly crystallizes the perovskite phase, i.e., reduces the intermediate phase to a negligible degree, right after the spin-coating, owing to the high miscibility of TMP with the anti-solvent and weak bonding in the Lewis adduct. Interestingly, the PSCs based on methylammonium lead iodide (MAPbI3 ) derived from TMP/DMF-mixed solvent exhibit a higher average power conversion efficiency of 19.68% (the best: 20.02%) with a smaller hysteresis in the current-voltage curve, compared to the PSCs that are fabricated using DMSO/DMF-mixed (19.14%) or DMF-only (18.55%) solvents. The superior photovoltaic properties are attributed to the lower defect density of the TMP/DMF-derived perovskite film. The results indicate that a high-performance PSC can be achieved by combining a weak Lewis base with a well-established solvent engineering process.

Synthesis and adsorption properties of gelatin-conjugated hematite (α-Fe2O3) nanoparticles for lead removal from wastewater.

Gelatin-conjugated hematite nanoparticles (HT NPs) are prepared through the solid-state phase transformation in the presence of phosphate. Their adsorption capacity and kinetics are investigated for Pb removal in wastewater. The gelatin-conjugated HT NPs with a size of 4-6 nm exhibit an excellent Pb removal performance, with a high adsorption capacity of 169.49 mg g-1 and a fast equilibrium adsorption kinetics, attributed to the large number of active sites and highly negative charge on the surface of HT NPs. Moreover, the magnetic property of HT NPs enables to selectively collect NPs in the wastewater by using a permanent magnet, leading to its high reusability.

Tailored 2D/3D Halide Perovskite Heterointerface for Substantially Enhanced Endurance in Conducting Bridge Resistive Switching Memory.

Hybrid organic-inorganic halide perovskites (HPs) have garnered significant attention for use in resistive switching (RS) memory devices due to their low cost, low operation voltage, high on/off ratio, and excellent mechanical properties. However, the HP-based RS memory devices continue to face several challenges owing to the short endurance and stability of the HP film. Herein, two-dimensional/three-dimensional (2D/3D) perovskite heterojunction films were prepared via a low-temperature all-solution process and their RS behavior was investigated for the first time. The 2D/3D perovskite RS devices exhibited excellent performance with an endurance of 2700 cycles, a high on/off ratio of 106, and an operation speed of 640 µs. The calculated thermally assisted ion hopping activation energy and the results of the time-of-flight secondary ion mass spectroscopy demonstrated that the 2D perovskite layer could efficiently prevent the Ag ion migration into the 3D perovskite film. Moreover, we found that owing to its high thermal conductivity, the 2D perovskite can control the rupture of the Ag conductive filament. Thus, the 2D perovskite layer enhances endurance by controlling both Ag migration and filament rupture. Hence, this study provides an alternate strategy for improving endurance of HP-based RS memory devices.

Efficient, stable silicon tandem cells enabled by anion-engineered wide-bandgap perovskites.

Maximizing the power conversion efficiency (PCE) of perovskite/silicon tandem solar cells that can exceed the Shockley-Queisser single-cell limit requires a high-performing, stable perovskite top cell with a wide bandgap. We developed a stable perovskite solar cell with a bandgap of ~1.7 electron volts that retained more than 80% of its initial PCE of 20.7% after 1000 hours of continuous illumination. Anion engineering of phenethylammonium-based two-dimensional (2D) additives was critical for controlling the structural and electrical properties of the 2D passivation layers based on a lead iodide framework. The high PCE of 26.7% of a monolithic two-terminal wide-bandgap perovskite/silicon tandem solar cell was made possible by the ideal combination of spectral responses of the top and bottom cells.

Room-Temperature-Processed Amorphous Sn-In-O Electron Transport Layer for Perovskite Solar Cells.

We report amorphous tin-indium-oxide (TIO, Sn fraction: >50 atomic percentage (at%)) thin films as a new electron transport layer (ETL) of perovskite solar cells (PSCs). TIO thin films with Sn fraction of 52, 77, 83, 92, and 100 at% were grown on crystalline indium-tin-oxide (ITO, Sn fraction: ~10 at%) thin films, a common transparent conducting oxide, by co-sputtering In2O3 and SnO2 at room temperature. The energy band structures of the amorphous TIO thin films were determined from the optical absorbance and the ultraviolet photoelectron spectra. All the examined compositions are characterized by a conduction band edge lying between that of ITO and that of perovskite (here, methylammonium lead triiodide), indicating that TIO is a potentially viable ETL of PSCs. The photovoltaic characteristics of the TIO-based PSCs were evaluated. Owing mainly to the highest fill factor and open circuit voltage, the optimal power conversion efficiency was obtained for the 77 at%-Sn TIO ETL with TiCl4 treatment. The fill factor and the open circuit voltage changes with varying the Sn fraction, despite similar conduction band edges. We attribute these differences to the considerable changes in the electrical resistivity of the TIO ETL. This would have a significant effect on the shunt and/or the series resistances. The TIO ETL can be continuously grown on an ITO TCO in a chamber, as ITO and TIO are composed of identical elements, which would help to reduce production time and costs.

Carrier lifetimes of >1 µs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells.

All-perovskite-based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite-based tandem devices has been limited by the lack of high-efficiency, low-band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs). We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low-band gap (~1.25 electron volt) perovskite films. The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers. These improved properties enable our demonstration of >20% efficient low-band gap PSCs. When combined with wider-band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite-based polycrystalline thin-film tandem solar cells.

Simultaneous Ligand Exchange Fabrication of Flexible Perovskite Solar Cells using Newly Synthesized Uniform Tin Oxide Quantum Dots.

Halide perovskite solar cells (HPSCs) have a significant potential for future photovoltaic systems because of a high power conversion efficiency (PCE) exceeding 23% using solution processing methods. A low-temperature processed oxide layer is a challenging issue for large-scale manufacture of flexible and low-cost HPSCs. Here, we propose a simple reverse micelle-water injection method for highly dispersed ligand-capped ultrafine SnO2 quantum dots (QD). Interestingly, we observed that the ligands, which help in the formation of a uniform SnO2 QD thin film, spontaneously exchange for halide through a perovskite solution, and finally we form a suitable SnO2 QD-halide junction for high-performance HPSCs. The flexible HPSC with the SnO2 QD-halide junction formed via the ligand exchange exhibits a high PCE of 17.7% using a flexible substrate. It also shows an excellent flexibility, where the initial PCE is maintained within 92% after 1000 bending cycles with a bending radius of 18 mm.

Highly Efficient and Uniform 1†cm2 Perovskite Solar Cells with an Electrochemically Deposited NiOx Hole-Extraction Layer.

Given that the highest certified conversion efficiency of the organic-inorganic perovskite solar cell (PSC) already exceeds 22 %, which is even higher than that of the polycrystalline silicon solar cell, the significance of new scalable processes that can be utilized for preparing large-area devices and their commercialization is rapidly increasing. From this perspective, the electrodeposition method is one of the most suitable processes for preparing large-area devices because it is an already commercialized process with proven controllability and scalability. Here, a highly uniform NiOx layer prepared by electrochemical deposition is reported as an efficient hole-extraction layer of a p-i-n-type planar PSC with a large active area of >1 cm2 . It is demonstrated that the increased surface roughness of the NiOx layer, achieved by controlling the deposition current density, facilitates the hole extraction at the interface between perovskite and NiOx , and thus increases the fill factor and the conversion efficiency. The electrochemically deposited NiOx layer also exhibits extremely uniform thickness and morphology, leading to highly efficient and uniform large-area PSCs. As a result, the p-i-n-type planar PSC with an area of 1.084 cm2 exhibits a stable conversion efficiency of 17.0 % (19.2 % for 0.1 cm2 ) without showing hysteresis effects.