All-polymer organic solar cells with nano-to-micron hierarchical morphology and large light receiving angle.

Distributed photovoltaics in living environment harvest the sunlight in different incident angles throughout the day. The development of planer solar cells with large light-receiving angle can reduce the requirements in installation form factor and is therefore urgently required. Here, thin film organic photovoltaics with nano-sized phase separation integrated in micro-sized surface topology is demonstrated as an ideal solution to proposed applications.

1,10-Phenanthroline as an Efficient Bifunctional Passivating Agent for MAPbI Perovskite Solar Cells.

Passivation is one of the most promising concepts to heal defects created at the surface and grain boundaries of polycrystalline perovskite thin films, which significantly deteriorate the photovoltaic performance and stability of corresponding devices. Here, 1,10-phenanthroline, known as a bidentate chelating ligand, is implemented between the methylammonium lead iodide (MAPbI) film and the hole-transport layer for both passivating the lead-based surface defects (undercoordinated lead ions) and converting the excess/unreacted lead iodide (PbI) buried at interfaces, which is problematic for the long-term stability, into "neutralized" and beneficial species (PbI(1,10-phen), = 1, 2) for efficient hole transfer at the modified interface. The defect healing ability of 1,10-phenanthroline is verified with a set of complementary techniques including photoluminescence (steady-state and time-resolved), space-charge-limited current (SCLC) measurements, light intensity dependent measurements, and Fourier-transform photocurrent spectroscopy (FTPS).

On-surface photopolymerization of two-dimensional polymers ordered on the mesoscale.

The use of solid supports and ultra-high vacuum conditions for the synthesis of two-dimensional polymers is attractive, as it can enable thorough characterization, often with submolecular resolution, and prevent contamination. However, most on-surface polymerizations are thermally activated, which often leads to high defect densities and relatively small domain sizes. Here, we have obtained a porous two-dimensional polymer that is ordered on the mesoscale by the two-staged topochemical photopolymerization of fluorinated anthracene triptycene (fantrip) monomers on alkane-passivated graphite surfaces under ultra-high vacuum.

High-Efficiency Organic Photovoltaics using Eutectic Acceptor Fibrils to Achieve Current Amplification.

The intrinsic electronic properties of donor (D) and acceptor (A) materials in coupling with morphological features dictate the output in organic solar cells (OSCs). New physical properties of intimate eutectic mixing are used in nonfullerene-acceptor-based D-A -A ternary blends to fine-tune the bulk heterojunction thin film morphology as well as their electronic properties. With enhanced thin film crystallinity and improved carrier transport, a significant J amplification is achieved due to the formation of eutectic fibrillar lamellae and reduced defects state density.

Probing Trions at Chemically Tailored Trapping Defects.

Trions, charged excitons that are reminiscent of hydrogen and positronium ions, have been intensively studied for energy harvesting, light-emitting diodes, lasing, and quantum computing applications because of their inherent connection with electron spin and dark excitons. However, these quasi-particles are typically present as a minority species at room temperature making it difficult for quantitative experimental measurements. Here, we show that by chemically engineering the well depth of sp quantum defects through a series of alkyl functional groups covalently attached to semiconducting carbon nanotube hosts, trions can be efficiently generated and localized at the trapping chemical defects.

Uniaxial transition dipole moments in semiconductor quantum rings caused by broken rotational symmetry.

Semiconductor quantum rings are topological structures that support fascinating phenomena such as the Aharonov-Bohm effect and persistent current, which are of high relevance in the research of quantum information devices. The annular shape of quantum rings distinguishes them from other low-dimensional materials, and enables topologically induced properties such as geometry-dependent spin manipulation and emission. While optical transition dipole moments (TDMs) in zero to two-dimensional optical emitters have been well investigated, those in quantum rings remain obscure despite their utmost relevance to the quantum photonic applications of quantum rings.

Light-Emitting Metasurfaces: Simultaneous Control of Spontaneous Emission and Far-Field Radiation.

Light-emitting sources and devices permeate every aspect of our lives and are used in lighting, communications, transportation, computing, and medicine. Advances in multifunctional and "smart lighting" would require revolutionary concepts in the control of emission spectra and directionality. Such control might be possible with new schemes and regimes of light-matter interaction paired with developments in light-emitting materials.

Mapping Structure-Property Relationships of Organic Color Centers.

Organic color centers are an emergent class of quantum emitters that hold vast potential for applications in bioimaging, chemical sensing, and quantum information processing. Here, we show that these synthetic color centers follow interesting structure-property relationships through comparative spectral studies of 14 purified single-walled carbon nanotube chiralities and 30 different functional groups that vary in electron-withdrawing capability and bonding configurations. The defect emission is tunable by as much as 400 meV in the near-infrared as a function of host structure and the chemical nature of the color centers.

Solvent- and Wavelength-Dependent Photoluminescence Relaxation Dynamics of Carbon Nanotube sp Defect States.

Photoluminescent sp defect states introduced to single wall carbon nanotubes (SWCNTs) through low-level covalent functionalization create new photophysical behaviors and functionality as a result of defect sites acting as exciton traps. Evaluation of relaxation dynamics in varying dielectric environments can aid in advancing a more complete description of defect-state relaxation pathways and electronic structure. Here, we exploit helical wrapping polymers as a route to suspending (6,5) SWCNTs covalently functionalized with 4-methoxybenzene in solvent systems including HO, DO, methanol, dimethylformamide, tetrahydrofuran, and toluene, spanning a range of dielectric constants from 80 to 3.

Suppression of exciton dephasing in sidewall-functionalized carbon nanotubes embedded into metallo-dielectric antennas.

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) is a promising route to enhance the quantum yield of exciton emission and can lead to single-photon emission at room temperature. However, the spectral linewidth of the defect-related E11* emission remains rather broad. Here, we systematically investigate the low-temperature exciton emission of individual SWCNTs that have been dispersed with sodium-deoxycholate (DOC) and polyfluorene (PFO-BPy), are grown by laser vaporization (LV) or by CoMoCat techniques and are functionalized with oxygen as well as 3,5-dichlorobenzene groups.

Ultrashort Carbon Nanotubes That Fluoresce Brightly in the Near-Infrared.

The intrinsic near-infrared photoluminescence observed in long single-walled carbon nanotubes is known to be quenched in ultrashort nanotubes due to their tiny size as compared to the exciton diffusion length in these materials (>100 nm). Here, we show that intense photoluminescence can be created in ultrashort nanotubes (∼40 nm length) upon incorporation of exciton-trapping sp defect sites. Using super-resolution photoluminescence imaging at <25 nm resolution, we directly show the preferential localization of excitons at the nanotube ends, which separate by less than 40 nm and behave as independent emitters.

Remote excitation and detection of surface-enhanced Raman scattering from graphene.

We demonstrate the remote excitation and detection of surface-enhanced Raman scattering (SERS) from graphene using a silver nanowire as a plasmonic waveguide. By investigating a nanowire touching a graphene sheet at only one terminal, we first show the remote excitation of SERS from graphene by propagating surface plasmon polaritons (SPPs) launched by a focused laser over distances on the order of 10 μm. Remote detection of SERS is then demonstrated for the same nanowire by detecting light emission at the distal end of the nanowire that was launched by graphene Raman scattering and carried to the end of the nanowire by SPPs.

Enhanced Single-Photon Emission from Carbon-Nanotube Dopant States Coupled to Silicon Microcavities.

Single-walled carbon nanotubes are a promising material as quantum light sources at room temperature and as nanoscale light sources for integrated photonic circuits on silicon. Here, we show that the integration of dopant states in carbon nanotubes and silicon microcavities can provide bright and high-purity single-photon emitters on a silicon photonics platform at room temperature. We perform photoluminescence spectroscopy and observe the enhancement of emission from the dopant states by a factor of ∼50, and cavity-enhanced radiative decay is confirmed using time-resolved measurements, in which a ∼30% decrease of emission lifetime is observed.

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films.

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects.

Multi-exciton emission from solitary dopant states of carbon nanotubes.

By separating the photons from slow and fast decays of single and multi-exciton states in a time gated 2 order photon correlation experiment, we show that solitary oxygen dopant states of single-walled carbon nanotubes (SWCNTs) allow emission of photon pairs with efficiencies as high as 44% of single exciton emission. Our pump dependent time resolved photoluminescence (PL) studies further reveal diffusion-limited exciton-exciton annihilation as the key process that limits the emission of multi-excitons at high pump fluences. We further postulate that creation of additional permanent exciton quenching sites occurring under intense laser irradiation leads to permanent PL quenching.

Charge Transport Limitations in Perovskite Solar Cells: The Effect of Charge Extraction Layers.

Understanding the charge transport characteristics and their limiting factors in organolead halide perovskites is of great importance for the development of competitive and economically advantageous photovoltaic systems derived from these materials. In the present work, we examine the charge carrier mobilities in CHNHPbI (MAPI) thin films obtained from a one-step synthesis procedure and in planar n-i-p devices based on these films. By performing time-of-flight measurements, we find mobilities around 6 cm/V s for electrons and holes in MAPI thin films, whereas in working solar cells, the respective effective mobility values are reduced by 3 orders of magnitude.

Low-Temperature Single Carbon Nanotube Spectroscopy of sp Quantum Defects.

Aiming to unravel the relationship between chemical configuration and electronic structure of sp defects of aryl-functionalized (6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature single nanotube photoluminescence (PL) spectroscopy studies and correlate our observations with quantum chemistry simulations. We observe sharp emission peaks from individual defect sites that are spread over an extremely broad, 1000-1350 nm, spectral range. Our simulations allow us to attribute this spectral diversity to the occurrence of six chemically and energetically distinct defect states resulting from topological variation in the chemical binding configuration of the monovalent aryl groups.

Giant PbSe/CdSe/CdSe Quantum Dots: Crystal-Structure-Defined Ultrastable Near-Infrared Photoluminescence from Single Nanocrystals.

Toward a truly photostable PbSe quantum dot (QD), we apply the thick-shell or "giant" QD structural motif to this notoriously environmentally sensitive nanocrystal system. Namely, using a sequential application of two shell-growth techniques-partial-cation exchange and successive ionic layer adsorption and reaction (SILAR)-we are able to overcoat the PbSe QDs with sufficiently thick CdSe shells to impart new single-QD-level photostability, as evidenced by suppression of both photobleaching and blinking behavior. We further reveal that the crystal structure of the CdSe shell (cubic zinc-blende or hexagonal wurtzite) plays a key role in determining the photoluminescence properties of these giant QDs, with only cubic nanocrystals sufficiently bright and stable to be observed as single emitters.

Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces.

All-dielectric metasurfaces made from arrays of high index nanoresonators supporting strong magnetic dipole modes have emerged as a low-loss alternative to plasmonic metasurfaces. Here we use oxygen-doped single-walled carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon metasurfaces to study effects of the magnetic dipole modes of the constituent nanoresonators on the photoluminescence (PL) of individual SWCNTs. We find that when in resonance, the magnetic mode of the silicon nanoresonators can lead to a moderate average PL enhancement of 0.

Photoluminescence Imaging of Polyfluorene Surface Structures on Semiconducting Carbon Nanotubes: Implications for Thin Film Exciton Transport.

Single-walled carbon nanotubes (SWCNTs) have potential to act as light-harvesting elements in thin film photovoltaic devices, but performance is in part limited by the efficiency of exciton diffusion processes within the films. Factors contributing to exciton transport can include film morphology encompassing nanotube orientation, connectivity, and interaction geometry. Such factors are often defined by nanotube surface structures that are not yet well understood.

Photoluminescence Dynamics of Aryl sp(3) Defect States in Single-Walled Carbon Nanotubes.

Photoluminescent defect states introduced by sp(3) functionalization of semiconducting carbon nanotubes are rapidly emerging as important routes for boosting emission quantum yields and introducing new functionality. Knowledge of the relaxation dynamics of these states is required for understanding how functionalizing agents (molecular dopants) may be designed to access specific behaviors. We measure photoluminescence (PL) decay dynamics of sp(3) defect states introduced by aryl functionalization of the carbon nanotube surface.

Photoluminescence imaging of solitary dopant sites in covalently doped single-wall carbon nanotubes.

Covalent dopants in semiconducting single wall carbon nanotubes (SWCNTs) are becoming important as routes for introducing new photoluminescent emitting states with potential for enhanced quantum yields, new functionality, and as species capable of near-IR room-temperature single photon emission. The origin and behavior of the dopant-induced emission is thus important to understand as a key requirement for successful room-T photonics and optoelectronics applications. Here, we use direct correlated two-color photoluminescence imaging to probe how the interplay between the SWCNT bright E(11) exciton and solitary dopant sites yields the dopant-induced emission for three different dopant species: oxygen, 4-methoxybenzene, and 4-bromobenzene.

Room-temperature single-photon generation from solitary dopants of carbon nanotubes.

On-demand single-photon sources capable of operating at room temperature and the telecom wavelength range of 1,300-1,500 nm hold the key to the realization of novel technologies that span from sub-diffraction imaging to quantum key distribution and photonic quantum information processing. Here, we show that incorporation of undoped (6,5) single-walled carbon nanotubes into a SiO2 matrix can lead to the creation of solitary oxygen dopant states capable of fluctuation-free, room-temperature single-photon emission in the 1,100-1,300 nm wavelength range. We investigated the effects of temperature on photoluminescence emission efficiencies, fluctuations and decay dynamics of the dopant states and determined the conditions most suitable for the observation of single-photon emission.

Antenna-enhanced optoelectronic probing of carbon nanotubes.

We report on the first antenna-enhanced optoelectronic microscopy studies on nanoscale devices. By coupling the emission and excitation to a scanning optical antenna, we are able to locally enhance the electroluminescence and photocurrent along a carbon nanotube device. We show that the emission source of the electroluminescence can be pointlike with a spatial extension below 20 nm.

Radiation channels close to a plasmonic nanowire visualized by back focal plane imaging.

We investigated the angular radiation patterns, a key characteristic of an emitting system, from individual silver nanowires decorated with rare earth ion-doped nanocrystals. Back focal plane radiation patterns of the nanocrystal photoluminescence after local two-photon excitation can be described by two emission channels: excitation of propagating surface plasmons in the nanowire followed by leakage radiation and direct dipolar emission observed also in the absence of the nanowire. Theoretical modeling reproduces the observed radiation patterns which strongly depend on the position of excitation along the nanowire.