Light-harvesting Devices Research Articles

Tuning Optical Absorption and Emission Using Strongly Coupled Dimers in Programmable DNA Scaffolds.

Molecular materials for light harvesting, computing, and fluorescence imaging require nanoscale integration of electronically active subunits. Variation in the optical absorption and emission properties of the subunits has primarily been achieved through modifications to the chemical structure, which is often synthetically challenging. Here, we introduce a facile method for varying optical absorption and emission properties by changing the geometry of a strongly coupled Cy3 dimer on a double-crossover (DX) DNA tile. Leveraging the versatility and programmability of DNA, we tune the length of the complementary strand so that it "pushes" or "pulls" the dimer, inducing dramatic changes in the photophysics including lifetime differences observable at the ensemble and single-molecule level. The separable lifetimes, along with environmental sensitivity also observed in the photophysics, suggest that the Cy3-DX tile constructs could serve as fluorescence probes for multiplexed imaging. More generally, these constructs establish a framework for easily controllable photophysics via geometric changes to coupled chromophores, which could be applied in light-harvesting devices and molecular electronics.

Subphthalocyanine-triangulene dyads: Property tuning for light-harvesting device applications.

Organic photovoltaics relies on the development of stable chromophores and redox‐active organic molecules with tailor‐made HOMO/LUMO energies. Here, we present the synthesis and properties of novel dyads composed of boron subphthalocyanine (SubPc) and triangulene units, connected either at the peripheral position of the subphthalocyanine or at the axial boron. The connectivity has strong implications for the absorption and fluorescence properties of the dyads, as well as their redox properties. While the SubPc unit has a bowl shape, triangulene is a planar structural unit that allows dyads to dimerize in the solid state on account of π‐stacking interactions as shown by X‐ray crystallography of one of the dyads. The electronic properties were also studied computationally by density functional theory methods. Excellent agreement between experimental and computed data were obtained, showing that our computational method is a strong tool in the rational design of optimum molecules to ultimately obtain finely tuned molecules for device applications.

An Investigation on the Effect of Extended Conjugation on the Photophysical Properties of Graphene Quantum Dot‐Porphyrin Dyads

AbstractNoncovalent dyads embodied with porphyrin (PPy) and graphene quantum dots (GQDs) were subjected to photophysical studies with the aim of a deep understanding on the electron transfer dynamics in such systems. Peculiarities of a single fluorophore seems to be inadequate in light harvesting materials for various applications which can be rectified by the method of co‐sensitization. In this work Tetraphenylporphyrin (TPP) and Tetranaphthylporphyrin (TNaP) have been utilized for the preparation of the nanohybrid systems to realize the effect of conjugation on the photophysical properties of these materials. The photophysical studies of these nanohybrids such as UV‐Visible spectra, Fluorescence emission spectra and Time‐correlated single photon counting (TCSPC) were carried out. Both the ground and excited state interaction studies indicate an effective association between the two components in these nanohybrids. The photoexcited porphyrin molecules behave as electron donors and GQDs as acceptors and the interaction studies show that the quenching of the porphyrin derivatives occurs through a combined static and dynamic quenching mechanism in the PPy‐GQD nanocomposites. Moreover the extended conjugation in naphthyl system exalted the binding with quantum dots through π‐π interaction and results in augmented electron transfer than that in phenyl nanocomposite. The higher non‐radiative rate constant from TCSPC authenticates the above results. Understanding the photodynamics of electrons in such systems is significant for their effective utilization and practical applications of these materials in light harvesting devices and photocatalysis.

Integration of topological insulator nanogap with atomic single layer for boosting photoluminescence

As a new class of nanomaterials, topological insulators with the Dirac surface conducting state and insulating bulk state offer a promising platform for plasmonics with breaking through the frequency and integration limitation of traditional materials. The combination of topological insulators with atomic-layer materials will be significant for the realization of integrated plasmonic functional devices. Herein, we experimentally report the plasmon-enhanced photoluminescence (PL) emission behavior of molybdenum disulfide (MoS2) single layer integrated with an antimony telluride (Sb2Te3) topological insulator nanogap. It is found that the PL response of MoS2 on the nanogap is dependent on the polarization of incident light. The MoS2 PL intensity can be distinctly improved with plasmonic excitation on Sb2Te3 nanogap. These results will contribute to exploring plasmonic behaviors of topological insulators and their applications in enhanced light-matter interaction for nanoscale light emission and harvesting devices.

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s−1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons

Luminescence and Stability Enhancement of Inorganic Perovskite Nanocrystals via Selective Surface Ligand Binding.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as one of the most promising light-emitting materials for optoelectronic devices with outstanding performance. However, the facile detachment of surface capping organic ligands from these NCs leads to very poor colloidal stability and durability. This is mainly due to the weak interfacial interactions between the inorganic perovskite core and ligands, high density of surface defect states, and aggregation of NCs. Here, using a combination of time-resolved laser spectroscopy and density functional theory (DFT) calculations, we explored the major impact of surface orientations and terminations for both CsPbBr3 and Cs4PbBr6 NCs not only on the interfacial binding affinities with organic ligands but also on surface defect formation and NC aggregation. By rationalizing that surface trap states are responsible for the decrease in photoluminescence (PL) upon fabrication and purification, we propose a powerful ligand-engineering strategy for eliminating these trap states and preventing the aggregation of CsPbBr3 and Cs4PbBr6 NCs. Interestingly, we find that the surface orientation and dimensionality determine the degree of interfacial interactions between the inorganic perovskite core and ligands and subsequently control the overall PL intensity and NC stability. Our results demonstrate that a treatment of as-synthesized CsPbBr3 NCs consisting of the addition of extra oleylammonium bromide (OAmBr) as a capping ligand, allows the CsPbBr3 NCs to retain their green emission with increased PL intensity and quantum yields and improves colloidal durability. On the other hand, the ultraviolet emissions of Cs4PbBr6 NCs are effectively increased upon addition of extra cesium oleate (CsOL) as the trap states induced by surface cesium ions are largely reduced by the formation of Cs-O bonds. Our work provides a robust and adequate ligand engineering approach to significantly enhance the optical behavior of perovskite NCs with different dimensionalities and various compositions and to achieve more efficient and stable light-harvesting devices.

Light-Modulated Cationic and Anionic Transport across Protein Biopolymers*.

Light is a convenient source of energy and the heart of light-harvesting natural systems and devices. Here, we show light-modulation of both the chemical nature and ionic charge carrier concentration within a protein-based biopolymer that was covalently functionalized with photoacids or photobases. We explore the capability of the biopolymer-tethered photoacids and photobases to undergo excited-state proton transfer and capture, respectively. Electrical measurements show that both the photoacid- and photobase-functionalized biopolymers exhibit an impressive light-modulated increase in ionic conductivity. Whereas cationic protons are the charge carriers for the photoacid-functionalized biopolymer, water-derived anionic hydroxides are the suggested charge carriers for the photobase-functionalized biopolymer. Our work introduces a versatile toolbox to photomodulate both protons and hydroxides as charge carriers in polymers, which can be of interest for a variety of applications.

Broadband and Highly Directional Visible Light Scattering by Laser-Splashed Lossless TiO2 Nanoparticles.

All-dielectric nanoparticles, as the counterpart of metallic nanostructures have recently attracted significant interest in manipulating light-matter interaction at a nanoscale. Directional scattering, as an important property of nanoparticles, has been investigated in traditional high refractive index materials, such as silicon, germanium and gallium arsenide in a narrow band range. Here in this paper, we demonstrate that a broadband forward scattering across the entire visible range can be achieved by the low loss TiO2 nanoparticles with moderate refractive index. This mainly stems from the optical interferences between the broadband electric dipole and the magnetic dipole modes. The forward/backward scattering ratio reaches maximum value at the wavelengths satisfying the first Kerker’s condition. Experimentally, the femtosecond pulsed laser was employed to splash different-sized nanoparticles from a thin TiO2 film deposited on the glass substrate. Single particle scattering measurement in both the forward and backward direction was performed by a homemade confocal microscopic system, demonstrating the broadband forward scattering feature. Our research holds great promise for many applications such as light harvesting, photodetection and on-chip photonic devices and so on.

Revealing the interrelation between C- and A-exciton dynamics in monolayer WS2 via transient absorption spectroscopy

Two-dimensional transition metal dichalcogenides (TMDs) are emerging as a promising complement for traditional semiconductor materials in ultrathin optoelectronic device fields. Developing a better understanding of high-energy C-exciton dynamics is essential for efficiently extracting hot carriers and building high-performance TMD-based light-harnessing devices; however, insight into the C-exciton dynamics remains scarce. To further understand the C-exciton dynamics, here, we have unraveled the interrelation between C-exciton and band edge A-exciton dynamics in monolayer WS2 by transient absorption spectroscopy. It is found that the band edge A-excitons could effectively generate high-energy C-excitons via the many-body process, and, in turn, the hot carriers relaxing from C-excitons to band edge states could compensate and slow the decay of the A-excitons. The comprehensive understanding of the interrelation between C-exciton and A-exciton dynamics in monolayer TMDs may trigger the potential applications for future TMD-based light-harvesting devices.

Nanotube Template-Directed Formation of Strongly Coupled Dye Aggregates with Tunable Exciton Fluorescence Controlled by Switching between J- and H-Type Electronic Coupling.

Strongly coupled dye aggregates with tailored exciton properties may find their use in developing artificial light-harvesting and optoelectronic devices. Here, we report the control of tubular pseudoisocyanine (PIC) dye J- and H-aggregate formation with tunable exciton fluorescence using lithocholic acid (LCA) as a template. The LCA-templated PIC J-aggregate nanotubes formed at a higher LCA/PIC molar ratio (≥5:1) exhibit a sharp, red-shifted absorption band (at 555 nm), intense fluorescence (at 565 nm), and shorter lifetime (200 ps), all indicating their strong superradiance properties. In contrast, the H-aggregate nanotubes formed at a lower LCA/PIC molar ratio (2:1) exhibit a significantly blue-shifted absorption band (at 420 nm) and highly red-shifted fluorescence emission (at 600 nm) with enhanced lifetime (4.40 ns). The controlled switching of the optical properties of the PIC dye aggregates achieved by controlling the LCA/PIC molar ratio could serve as an important guideline for the design of organic photo-functional materials.

Environmentally Improved Coherent Light Harvesting.

Coherence-enhanced light harvesting has not been directly observed experimentally, despite theoretical evidence that coherence can significantly enhance light-harvesting performance. The main experimental obstacle has been the difficulty in isolating the effect of coherence in the presence of confounding variables. Recent proposals for externally controlling coherence by manipulating the light's degree of polarization showed that coherent efficiency enhancements would be possible, but they were restricted to light-harvesting systems weakly coupled to their environment. Here, we show that increases in system-bath coupling strength can amplify coherent efficiency enhancements, rather than suppress them. This result dramatically broadens the range of systems that could be used to conclusively demonstrate coherence-enhanced light harvesting or to engineer coherent effects into artificial light-harvesting devices.

Graphene quantum dots with a Stone-Wales defect as a topologically tunable platform for visible-light harvesting

In this work, we report for the first time the crucial role of topological anomalies like Stone-Wales (SW) type bond rotations in tuning the optical properties of graphene quantum dots (GQDs). By means of first-principles calculations, we first show that the structural stability of GQDs strongly depends on position of SW defects. Optical absorption spectra is then computed using electron-correlated methodology to demonstrate that SW type reconstruction is responsible for the appearance of new defect-induced peaks below the optical gap and dramatically modifies the optical absorption profile. In addition, our investigations signify that electron correlation effects become more dominant for SW-defected GQDs. We finally establish that the introduction of SW defects at specific locations strongly enhances light absorption in visible range, which is of prime importance for designing light harvesting, photocatalytic and optoelectronic devices.

Transparent and Low-Loss Luminescent Solar Concentrators Based on Self-Trapped Exciton Emission in Lead-Free Double Perovskite Nanocrystals

Luminescent solar concentrators (LSCs) are light-harvesting devices that redirect solar light to an edge-attached photovoltaic cell, and thus, they have high potential to be incorporated directly into buildings’ windows to allow for generating electricity. Perovskite nanocrystals (PNCs) are promising materials for LSCs because their enticing optical properties can be engineered to provide a high photoluminescence (PL) quantum yield (QY) and low overlap between absorption and emission spectra. Replacement of toxic, lead-containing perovskites in LSCs by lead-free PNCs, while retaining high optical efficiency of the device, remains the key challenge, which needs to be overcome to build environmentally friendly solar-harvesting platforms. In this work, we use nanocrystals of Bi-doped Cs2Ag0.4Na0.6InCl6 double perovskites with a self-trapped exciton emission to realize for the first time a transparent, low-reabsorption, lead-free perovskite-based LSC. Fabricated 100 cm2 LSCs show an internal optical quantum efficiency of 21.2% with the corresponding internal concentration factor of 2.7. Monte Carlo (MC) ray-tracing simulations identified the loss caused by nonunity PL QY to be the most significant contribution to the overall efficiency loss. The MC simulations also allowed us to estimate the efficiency of 39.4% for 2,500 cm2 LSCs with hypothetical unity PL. These results demonstrate a significant promise held by Bi-doped lead-free PNCs for LSCs.

Using the Electrode Potential to Orient Single Molecules in Nanoscale Electrical Junctions and Determine Charge Transport Mechanism

Charge transport through and between molecules is central to important processes in nature. Studying the conductivity of single molecules can contribute to a better understanding of charge transport, and also help to develop building blocks of molecular electronics, light harvesting devices, etc. We measure the conductivity of molecules using the Scanning Tunneling Microscope break-junction (STM-BJ) method that utilizes repeatedly formed circuits where one or a few molecules are trapped between two electrodes, at least one of which has nanoscale dimensions. The statistical analysis of thousands of measurements yields the conductance of single molecules.One particular interest is the role of the molecule-electrode contact and orientation in charge transport. In the simplest analysis this contact can present a substantial barrier to charge injection, which can have important consequences in devices such as dye sensitized semiconductor nanoparticle solar cells. Our most recent developments include controlling the orientation of the molecule in the junction using the electrode potential so that we can measure conductance along different molecular axes, accessing elements of the anisotropy of the molecular conductance tensor. Specifically, when the molecule is oriented flat, with the molecular plane parallel to the electrode surface, charge transport is measured perpendicular to the molecular plane. Furthermore, the electrode dependence of the molecular conductivity provides insight into the transport mechanism. This approach can be applied to molecules that do not have suitable molecular anchoring groups for traditional STM-BJ measurements, and allows us to determine the importance of anchoring groups in charge transport in molecular junctions.

Photoelectrochemical Properties of Porphyrin-Functionalized Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDCs) show promising electrochemical properties for the ultrathin and flexible light-harvesting devices due to their absorption in the visible range and high conversion efficiencies. In this work, we demonstrate a strong enhancement in photoelectrochemical conversion using porphyrin-functionalization on TMDCs and elucidate related light-harvesting mechanisms. We prepared large-area samples from Zn-centered protoporphyrins (ZnPPs) functionalized on films of liquid-exfoliated MoS2, WS2, MoSe2, and WSe2 flakes. WS2 and MoS2 demonstrate an order of magnitude greater anodic photocurrents with ZnPP functionalization, where ZnPP is working as photoactive layers. In contrast, MoSe2 and WSe2 serve as charge transfer media, demonstrating strong cathodic photocurrents with the porphyrins. Two distinct mechanisms were identified for observed photocurrent behaviors, which largely depend on the differences in the energy levels of TMDCs against the HOMO/LUMO of ZnPP. Our findings should be useful for designing high-performance TMDC-based photoelectrochemical devices.

Passivation agent with dipole moment for surface modification towards efficient and stable perovskite solar cells

Recently, there has been renewed interest in interface engineering as a means to further push the performance of perovskite solar cells closer to the Schockly-Queisser limit. Herein, for the first time we employ a multi-functional 4-chlorobenzoic acid to produce a self-assembled monolayer on a perovskite surface. With this interlayer we observe passivation of perovskite surface defects and a significant suppression of non-radiative charge recombination. Furthermore, at the surface of the interlayer we observe, charge dipoles which tune the energy level alignment, enabling a larger energetic driving force for hole extraction. The perovskite surface becomes more hydrophilic due to the presence of the interlayer. Consequently, we observe an improvement in open-circuit voltage from 1.08 to 1.16 V, a power conversion efficiency improvement from 18% to 21% and an improved stability under ambient conditions. Our work highlights the potential of SAMs to engineer the photo-electronic properties and stability of perovskite interfaces to achieve high-performance light harvesting devices.

Pressing challenges in halide perovskite photovoltaics—from the atomic to module level

Carlo A.R. Perini is a post-doctoral researcher at Georgia Institute of Technology, Energy Materials Lab. His studies focus on the development and understanding of interface passivation of 3D metal halide perovskites using vapor deposition routes. He has a PhD in Physics at Politecnico di Milano, in collaboration with the Italian Institute of Technology (IIT), focused on the study of energy efficient, scalable, and environmentally friendly deposition routes for lead halide perovskites. Tiarnan A. S. Doherty is a PhD student in the Cavendish laboratory at the University of Cambridge working on nanoscale structure property relationships in metal-halide perovskites. He received the gold graduate student award at the Fall Materials Research Society meeting. Samuel D. Stranks is a University Lecturer in Energy and Royal Society University Research Fellow at the University of Cambridge. His group's research focuses on the optical and electronic properties of emerging semiconductors for low-cost, transformative electronics applications including light-harvesting (e.g., photovoltaic) and light-emission (e.g., LED) devices. He received the 2018 Henry Moseley Award and Medal from the Institute of Physics, the 2019 Marlow Award from the Royal Society of Chemistry, and is a co-founder of Swift Solar, a startup developing lightweight perovskite PV panels. Juan-Pablo Correa-Baena is an Assistant Professor and Goizueta Junior Faculty Chair in the School of Materials Science and Engineering at Georgia Institute of Technology. His group focuses on the understanding and control of electronic dynamics at the nanoscale for low-cost semiconductors, such as halide perovskites. He completed his PhD at the University of Connecticut, followed by a postdoctoral fellowship at EPFL, Switzerland, then a US Department of Energy postdoctoral fellowship at MIT, USA. Robert L. Z. Hoye is a Lecturer (Assistant Professor) at Imperial College London. He leads the Energy Materials and Devices group, which is working on developing defect-tolerant semiconductors and their application in optoelectronic devices. This includes tandem photovoltaics, indoor solar cells, and light-emitting diodes. Robert has received the Young Engineer of the Year Award from the Royal Academy of Engineering, the Sir Henry Royce Medal from the Institution of Engineering and Technology, and the Rising Star Award from the Waterloo Institute for Nanotechnology.

A Visible-Light-Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur-Carbon Cross-Coupling Dual Catalyst.

Covalent Organic Frameworks (COFs) have recently emerged as light-harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine-based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible-light-driven catalytic sulfur-carbon cross-coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two-component synergistic catalyst in organic transformation is demonstrated for the first time.

First-principles study of hybrid nanostructures formed by deposited phthalocyanine/porphyrin metal complexes on phosphorene

Porphyrin and phthalocyanine metal complexes are organic semiconductor molecules, also known as MN4 macrocyclic compounds, widely implemented as photosensitizers and electrocatalysts in solution. In this regard, phosphorene, an emerging layered material entirely composed of phosphorous atoms, shows excellent potential features to act as a building block of novel hybrid nanostructures with organic molecules. Here, we perform a first-principles study of hybrid nanostructures formed by phosphorene nanosheets and deposition of MN4 compounds (where M: Fe, Co, Ni, Zn). It is found that the MN4-phosphorene hybrids have remarkable stability in the gas phase and aqueous solution, comparable to that achieved with carbonaceous and metallic-based substrates. In terms of structure, the hybrid structures display non-degenerated adsorption patterns at room temperature, denoting the formation of stable ordered flat MN4 layers (self-assembly). To understand the origin of the binding stability, we conducted further analysis of binding and energy decomposition (ALMO-EDA). It is revealed that the stability is driven by the interplay between permanent electrostatic and dispersion physical effects, which stand for up to 90% of the stabilizing interaction energy. In terms of electronic properties, on the one hand, spin-polarized complexes do not show either switching in their spin state or substrate magnetization. On the other hand, all MN4 complexes act as mild n-dopants, slightly decreasing the phosphorene bandgap but retaining the topology of frontier orbitals concerning the free complexes for orbital-controlled reactions, e.g., electrochemical reactions in solution. In terms of optical properties, phosphorene induces shifting in the Q-band of phthalocyanines to longer wavelengths, which turns (metal)phthalocyanine-phosphorene hybrid materials into excellent candidates for the design of light-harvesting devices.

Tabletop X-ray spectroscopy probes electron dynamics on attosecond timescales

Ultrahigh time resolution tabletop X-ray spectroscopy is poised to further understanding of quantum materials, light harvesting devices, and ultrafast electronics.