Tuning the Bandgap Character of Quantum‐Confined Si–Sn Alloyed Nanocrystals

Nanocrystals in the regime between molecules and bulk give rise to unique electronic properties. Here, a thorough study focusing on quantum‐confined nanocrystals (NCs) is provided. At the level of density functional theory an approximate (quasi) band structure which addresses both the molecular and bulk aspects of finite‐sized NCs is calculated. In particular, how band‐like features emerge with increasing particle diameter is shown. The quasiband structure is used to discuss technological‐relevant direct bandgap NCs. It is found that ultrasmall Sn NCs have a direct bandgap in their at‐nanoscale‐stable α‐phase and for high enough Sn concentration (≈41%) alloyed Si–Sn NCs transition from indirect to direct bandgap semiconductors. The calculations strongly support recent experiments suggesting a direct bandgap for these systems. For a quantitative comparison many‐body GW + Bethe–Salpeter equation (BSE) calculations are performed. The predicted optical gaps are close to the experimental data and the calculated absorbance spectra compare well with the corresponding measurements.     

Routes to Achieving High Quantum Yield Luminescence from Gas‐Phase‐Produced Silicon Nanocrystals

Plasma‐synthesized silicon nanocrystals with alkene ligands have shown the potential to exhibit high‐efficiency photoluminescence, but results reported in the literature have been inconsistent. Here, for the first time, the role of the immediate post‐synthesis “afterglow plasma” environment is explored. The significant impact of gas injection into the afterglow plasma on the photoluminescence efficiency of silicon nanocrystals is reprorted. Depending on the afterglow conditions, photoluminescence quantum yields of silicon nanocrystals synthesized under otherwise identical conditions can vary by a factor of almost five. It is demonstrated that achieving a fast quenching of the particle temperature and a high flux of atomic hydrogen to the nanocrystal surface are essential for a high photoluminescence quantum yield of the produced silicon nanocrystals.     

Silicon Nanocrystals in Liquid Media: Optical Properties and Surface Stabilization by Microplasma‐Induced Non‐Equilibrium Liquid Chemistry

Surface engineering of silicon nanocrystals directly in water or ethanol by atmospheric‐pressure dc microplasma is reported. In both liquids, microplasma processing stabilizes the optoelectronic properties of silicon nanocrystals. The microplasma treatment induces non‐equilibrium liquid chemistry that passivates the silicon nanocrystals surface with oxygen‐/organic‐based terminations. In particular, the microplasma treatment in ethanol drastically enhances the silicon nanocrystals photoluminescence intensity and causes a clear red‐shift (≈80 nm) of the photoluminescence maximum. The photoluminescence properties are stable after several days of storage in either ethanol or water. The surface chemistry induced by the microplasma treatment is analyzed and discussed.     

Interplay of Structural and Optoelectronic Properties in Formamidinium Mixed Tin–Lead Triiodide Perovskites

Mixed lead–tin triiodide perovskites are promising absorber materials for low bandgap bottom cells in all‐perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb_1−xSn_xI₃ perovskite are presented here as a function of lead:tin content across the alloy series. Temperature‐dependent photoluminescence and optical absorption measurements are used to identify changes in the bandgap and phase transition temperature. The large bandgap bowing parameter, a crucial element for the attainment of low bandgaps in this system, is shown to depend on the structural phase, reaching a value of 0.84 eV in the low‐temperature phase and 0.73 eV at room temperature. The parabolic nature of the bowing at all temperatures is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions. Charge‐carrier recombination dynamics are shown to fall into two regimes. Tin‐rich compositions exhibit fast, monoexponential recombination that is almost temperature‐independent, in accordance with high levels of electrical doping. Lead‐rich compositions show slower, stretched‐exponential charge‐carrier recombination that is strongly temperature‐dependent, in accordance with a multiphonon assisted process. These results highlight the importance of structure and composition for control of bandgap bowing and charge‐carrier recombination mechanisms in low bandgap absorbers for all‐perovskite tandem solar cells.     

Dramatic Enhancement of Photoluminescence Quantum Yields for Surface‐Engineered Si Nanocrystals within the Solar Spectrum

Substantial improvements of the absolute photoluminescence quantum yield (QY) for surfactant‐free silicon nanocrystals (Si‐ncs) by atmospheric pressure microplasma 3‐dimensional surface engineering are reported. The effect of surface characteristics on carrier multiplication mechanisms is explored using transient induced absorption and photoluminescence QY. Surface engineering of Si‐ncs is demonstrated to lead to more than 120 times increase in the absolute QY (from 0.1% up to 12%) within an important spectral range of the solar emission (2.3–3 eV). The Si‐ncs QY is shown to be stable when Si‐ncs are stored in ethanol at ambient conditions for three months.     

A Versatile Fluoro‐Containing Low‐Bandgap Polymer for Efficient Semitransparent and Tandem Polymer Solar Cells

The versatility of a fluoro‐containing low band‐gap polymer, poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b’]dithiophene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothia‐diazole)] (PCPDTFBT) in organic photovoltaics (OPVs) applications is demonstrated. High boiling point 1,3,5‐trichlorobenzene (TCB) is used as a solvent to manipulate PCPDTFBT:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) active layer morphology to obtain high‐performance single‐junction devices. It promotes the crystallization of PCPDTFBT polymer, thus improving the charge‐transport properties of the active layer. By combining the morphological manipulation with interfacial optimization and device engineering, the single‐junction device exhibits both good air stability and high power‐conversion efficiency (PCE, of 6.6%). This represents one of the highest PCE values for cyclopenta[2,1‐b;3,4‐b’]dithiophene (CPDT)‐based OPVs. This polymer is also utilized for constructing semitransparent solar cells and double‐junction tandem solar cells to demonstrate high PCEs of 5.0% and 8.2%, respectively.     

Efficient Directed Energy Transfer through Size‐Gradient Nanocrystal Layers into Silicon Substrates

Spectroscopic evidence of directed excitonic energy transfer (ET) is presented through size‐gradient CdSe/ZnS nanocrystal quantum dot (NQD) layers into an underlying Si substrate. NQD monolayers are chemically grafted on hydrogen‐terminated Si surfaces via a self‐assembled monolayer of amine modified carboxy‐alkyl chains. Subsequent NQD monolayers are linked with short alkyldiamines. The linking approach enables accurate positioning and enhanced passivation of the layers. Two different sizes of NQDs (energy donors emitting at 545 nm, and energy acceptors emitting at 585 nm) are used in comparing different monolayer and bilayer samples grafted on SiO₂ and oxide‐free Si surfaces via time‐resolved photoluminescence measurements. The overall efficiency of ET from the top‐layer donor NQDs into Si is estimated to approach ≈90% through a combination of different energy relaxation pathways. These include sequential ET through the intermediate acceptor layer realized mainly via the non‐radiative mechanism and direct ET into the Si substrate realized by means of the radiative coupling. The experimental observations are quantitatively rationalized by the theoretical modeling without introducing any extraneous energy scavenging processes. This indicates that the linker‐assisted fabrication enables the construction of defect‐free, bandgap‐gradient multilayer NQD/Si hybrid structures suitable for thin‐film photovoltaic applications.     

Solution‐Processed Metal Oxide Nanocrystals as Carrier Transport Layers in Organic and Perovskite Solar Cells

There has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.     

Dual Interfacial Modification Engineering with 2D MXene Quantum Dots and Copper Sulphide Nanocrystals Enabled High‐Performance Perovskite Solar Cells

The performance of perovskite solar cells (PSCs) strongly depends on the electron transport layer (ETL), perovskite absorber, hole transport layer (HTL), and their interfaces. Herein, the first approach to utilize ultrathin 2D titanium‐carbide MXenes (Ti₃C₂T_x quantum dots, TQD) by engineering the perovskite/TiO₂ ETL interface and perovskite absorber and introducing Cu_1.8S nanocrystals to perfect the Spiro‐OMeTAD HTL is represented. A significant hysteresis‐free power conversion efficiency improvement from 18.31% to 21.64% of PSCs is achieved after modifications with the enhanced short‐circuit current density, open‐circuit voltages, and fill factor. Various advanced characterizations, including femtosecond transient absorption spectroscopy, electrochemical impedance spectroscopy, and ultraviolet photoelectron spectroscopy, elucidate that the TQD/Cu_1.8S significantly contribute to the improved crystalline quality of the perovskite film with its large grain size and improved electron/holes extraction efficiencies at perovskite/ETL and perovskite/HTL interfaces. Furthermore, the long‐time ambient and light stability of PSCs are largely boosted through the TQD and/or Cu_1.8S nanocrystals doping, originating from the better crystallization of perovskite, suppressing the film aggregation and crystallization of HTL, and inhibiting the ultraviolet‐induced photocatalysis of the ETL. The findings highlight the TQD and Cu_1.8S can act as a superfast electrons and holes tunnel for the optoelectronic devices.     

Ligand‐Free Synthesis of Aluminum‐Doped Zinc Oxide Nanocrystals and their Use as Optical Spacers in Color‐Tuned Highly Efficient Organic Solar Cells

The color of polymer solar cells using an opaque electrode is given by the reflected light, which depends on the composition and thickness of each layer of the device. Metal‐oxide‐based optical spacers are intensively studied in polymer solar cells aiming to optimize the light absorption. However, the low conductivity of materials such as ZnO and TiO₂ limits the thickness of such optical spacers to tenths of nanometers. A novel synthesis route of cluster‐free Al‐doped ZnO (AZO) nanocrystals (NCs) is presented for solution processing of highly conductive layers without the need of temperature annealing, including thick optical spacers on top of polymer blends. The processing of 80 nm thick optical spacers based on AZO nanocrystal solutions on top of 200 nm thick polymer blend layer is demonstrated leading to improved photocurrent density of 17% compared to solar cells using standard active layers of 90 nm in combination with thin ZnO‐based optical spacers. These AZO NCs also open new opportunities for the processing of high‐efficiency color tuned solar cells. For the first time, it is shown that applying solution‐processed thick optical spacer with polymer blends of different thicknesses can process solar cells of similar efficiency over 7% but of different colors.     

Ultrasmall CuCo2S4 Nanocrystals: All‐in‐One Theragnosis Nanoplatform with Magnetic Resonance/Near‐Infrared Imaging for Efficiently Photothermal Therapy of Tumors

Copper‐based ternary bimetal chalcogenides have very promising potential as multifunctional theragnosis nanoplatform for photothermal treatment of tumors. However, the design and synthesis of such an effective platform remains challenging. In this study, hydrophilic CuCo₂S₄ nanocrystals (NCs) with a desirable size of ≈10 nm are synthesized by a simple one‐pot hydrothermal route. The as‐prepared ultrasmall CuCo₂S₄ NCs show: 1) intense near‐infrared absorption, which is attributed to 3d electronic transitions from the valence band to an intermediate band, as identified by density functional theory calculations; 2) high photothermal performance with a photothermal conversion efficiency up to 73.4%; and 3) capability for magnetic resonance (MR) imaging, as a result of the unpaired 3d electrons of cobalt. Finally, it is demonstrated that the CuCo₂S₄ NCs are a promising “all‐in‐one” photothermal theragnosis nanoplatform for photothermal cancer therapy under the irradiation of a 915 nm laser at a safe power density of 0.5 W cm^?2, guided by MR and infrared thermal imaging. This work further promotes the potential applications of ternary bimetal chalcogenides for photothermal theragnosis therapy.     

Highly Emissive and Color‐Tunable CuInS2‐Based Colloidal Semiconductor Nanocrystals: Off‐Stoichiometry Effects and Improved Electroluminescence Performance

The off‐stoichiometry effects and gram‐scale production of luminescent CuInS₂‐based semiconductor nanocrystals, as well as their application in electroluminescence devices are reported. The crystal structures and optical properties of CuInS₂ nanocrystals can be significantly influenced by controlling their [Cu]/[In] molar ratio. A simple model adapted from the bulk materials is proposed to explain their off‐stoichiometry effects. Highly emissive and color‐tunable CuInS₂‐based NCs are prepared by a combination of [Cu]/[In] molar ratio optimization, ZnS shell coating, and CuInS₂–ZnS alloying. The method is simple, hassle‐free, and easily scalable to fabricate tens of grams of nanocrystal powders with photoluminescence quantum yields up to around 65%. Furthermore, the performance of high‐quality CuInS₂‐based NCs in electroluminescence devices is examined. These devices have lower turn‐on voltages of around 5 V, brighter luminance up to approximately 2100 cd m^?2 and improved injection efficiency of around 0.3 lm W^?1 (at 100 cd m^?2) in comparison to recent reports.     

High‐Performance Hybrid Solar Cell Made from CdSe/CdTe Nanocrystals Supported on Reduced Graphene Oxide and PCDTBT

Core/shell tetrapods synthesized from CdSe and CdTe exhibit a type II band offset that induces separation of charge upon photoexcitation and localizes carriers to different regions of the tetrahedral geometry. CdSe/CdTe nanocrystals immobilized on oleylamine‐functionalized reduced graphene oxide (rGO) sheets can be homogeneously mixed with an organic dye (PCDTBT) to form donor–acceptor dispersed heterojunctions and exhibit a high power conversion efficiency of ～3.3% in solar cell devices. The near‐IR light absorbing type II nanocrystals complement the absorption spectrum of the visible light‐absorbing organics. The high efficiency is attributed to the amine‐functionalized rGO sheets, which allow intimate contact with the nanocrystals and efficient dispersal in the organic matrix, contributing to highly efficient charge separation and transfer at the nanocrystal, rGO, and polymer interfaces.     

Low‐Bandgap Mixed Tin‐Lead Perovskites and Their Applications in All‐Perovskite Tandem Solar Cells

Efficient organic–inorganic metal halide perovskite absorbers have gained tremendous research interest in the past decade due to their super optoelectronic properties and defect tolerance. Lead (Pb) halide perovskites enable highly efficient perovskite solar cells (PSCs) with a record power conversion efficiency (PCE) of over 23%. However, the energy bandgaps of Pb halide perovskites are larger than the optimal bandgap for single junction solar cells, governed by the Shockley–Queisser (SQ) radiative limit. Mixed tin (Sn)‐Pb halide perovskites have drawn significant attention, since their bandgap can be tuned to below 1.2 eV, which opens a door for fabricating all‐perovskite tandem solar cells that can break the SQ radiative limit. This review summarizes the development of low‐bandgap mixed Sn‐Pb PSCs and their applications in all‐perovskite tandem solar cells. Its aim is to facilitate the development of new approaches to achieve high efficiency low‐bandgap single‐junction mixed Sn‐Pb PSCs and all‐perovskite tandem solar cells.     

Wide Bandgap Copolymers Based on Quinoxalino[6,5‐f].quinoxaline for Highly Efficient Nonfullerene Polymer Solar Cells

Two novel wide bandgap copolymers based on quinoxalino[6,5‐f]quinoxaline (NQx) acceptor block, PBDT–NQx and PBDTS–NQx, are successfully synthesized for efficient nonfullerene polymer solar cells (PSCs). The attached conjugated side chains on both benzodithiophene (BDT) and NQx endow the resulting copolymers with low‐lying highest occupied molecular orbital (HOMO) levels. The sulfur atom insertion further reduces the HOMO level of PBDTS–NQx to ?5.31 eV, contributing to a high open‐circuit voltage, V _oc, of 0.91 V. Conjugated n ‐octylthienyl side chains attached on the NQx skeletons also significantly improve the π–π* transitions and optical absorptions of the copolymers in the region of short wavelengths, which induce a good complementary absorption when blending with the low bandgap small molecular acceptor of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene. The wide absorption range makes the active blends absorb more photons, giving rise to a high short‐circuit current density, J _sc, value of 15.62 mA cm^?2. The sulfur atom insertion also enhances the crystallinity of PBDTS–NQx and presents its blend film with a favorable nanophase separation, resulting in improved J _sc and fill factor (FF) values with a high power conversion efficiency of 11.47%. This work not only provides a new fused ring acceptor block (NQx) for constructing high‐performance wide bandgap copolymers but also provides the NQx‐based copolymers for achieving highly efficient nonfullerene PSCs.     

On the detection of shunts in silicon solar cells by photo‐ and electroluminescence imaging

Recently electroluminescence (EL) and photoluminescence (PL) imaging were reported to allow detection of strong ohmic shunts in silicon solar cells. Comparing lock‐in thermography (LIT) images with luminescence images of various shunted cells, measured under different conditions, the ability of luminescence techniques for shunt detection is investigated. Luminescence imaging allows identifying ohmic shunts only if they reach a certain strength. The detection limit for PL measurements of linear shunts was estimated to be in the order of 15 mA at 0·5 V bias for a point‐like shunt in multicrystalline (mc) cells. Pre‐breakdown sites can also be detected by electroluminescence under reverse bias. Copyright © 2007 John Wiley & Sons, Ltd.     

Thiolation and Cell‐Penetrating Peptide Surface Functionalization of Porous Silicon Nanoparticles for Oral Delivery of Insulin

During the last decades, advanced oral delivery systems to enhance the intestinal absorption of widely applicable proteins and peptides, particularly insulin, have been developed. Here, chitosan‐conjugated undecylenic acid‐modified thermally hydrocarbonized porous silicon nanoparticles (CSUn NPs) are used, which promote the mucoadhesion and cellular interactions, thus boosting the intestinal permeability of insulin. Then, to further potentiate the mucoadhesion and permeability enhancement of chitosan‐modified NPs, the surface of the NPs is further modified with either l ‐cysteine (CYS‐CSUn NPs) or a cell‐penetrating peptide (CPP‐CSUn NPs). CYS‐CSUn and CPP‐CSUn NPs show 17‐ and 12‐fold increase in the apparent permeability of insulin across cellular intestinal cells, respectively, with significant enhancement in the cellular interactions. The insulin uptake mechanism pathways in intestinal cells from the developed NPs are also unraveled, which demonstrates major involvement of active transport process and electrostatic interactions, along with adsorptive and clathrin‐mediated endocytic pathways. Moreover, after oral administration in diabetic rats, CYS‐CSUn NPs show 1.86‐ and 2.03‐fold increase in the relative bioavailability of insulin, as compared to empty NPs and oral insulin solution, respectively. In conclusion, this study presents l ‐cysteine modified CSUn NPs as a promising strategy with the ability to overcome the multiple barriers for oral delivery of insulin.     

Composition‐Tuned Wide Bandgap Perovskites: From Grain Engineering to Stability and Performance Improvement

Wide bandgap (WB) organic–inorganic hybrid perovskites (OIHPs) with a bandgap ranging between 1.7 and 2.0 eV have shown great potential to improve the efficiency of single‐junction silicon or thin‐film solar cells by forming a tandem structure with one of these cells or with a narrow bandgap perovskite cell. However, WB‐OIHPs suffer from a large open‐circuit voltage (V_oc) deficit in photovoltaic devices, which is associated with the phase segregation of the materials under light illumination. In this work the photoinstability is demonstrated and V_oc loss can be addressed by combining grain crystallization and grain boundary passivation, achieved simultaneously through tuning of perovskite precursor composition. Using FA_0.17Cs_0.83PbI_3–xBr_x (x = 0.8, 1.2 1.5, and 1.8), with a varied bandgap from 1.72 to 1.93 eV, as the model system it is illustrated how precursor additive Pb(SCN)₂ should be matched with a proper ratio of FAX (I and Br) to realize large grains with defect‐healed grain boundaries. The optimized WB‐OIHPs show good photostability at both room‐temperature and elevated temperature. Moreover, the corresponding solar cells exhibit excellent photovoltaic performances with the champion V_oc/stabilized power output efficiency reaching 1.244 V/18.60%, 1.284 V/16.51%, 1.296 V/15.01%, and 1.312 V/14.35% for WB‐OIHPs with x = 0.8, 1.2, 1.5, and 1.8, respectively.