Exciton Binding Energy Research Articles

Carrier modulation of one-dimensional MAPbxSr1-x(IyCl1-y)3 core-shell perovskite nanowires.

Low dimensional hybrid organic-inorganic perovskites (HOIPs) have become one of the most promising materials in solar cells, photodetectors, and lasers due to their low exciton binding energy, high bipolar carrier mobility, and long carrier lifetime. The effective separation and collection of photo-generated electrons and holes have always been crucial for a perovskite as a working medium for optoelectronic devices. However, the surface state of pristine perovskite nanowires causes recombination of electrons and holes at the edge of the energy band, leading to deactivation of charge carriers. In this work, we constructed MAPbxSr1-xI3 core-shell nanowires, which adjust the band gap and control the spatial distribution of charge carriers by changing the Pb composition ratio, achieving spontaneous separation of electron-hole pairs to avoid surface recombination. In addition, MAPbxSr1-x(IyCl1-y)3 core-shell nanowires with different component ratios (x, y) can be constructed by further doping the Cl element, and the spatial distribution of charge carriers in the nanowires can be flexibly manipulated to achieve exchange between type-I and type-II band alignment orders. This study provides a feasible method for enhancing the carrier separation of organic-inorganic hybrid perovskites and improving the performance of optoelectronic devices.

Enhanced exciton-phonon coupling in pseudohalide 2D perovskite for X-ray to visible light detection.

Two-dimensional Cs2Pb(SCN)2I2 perovskite has an ultra-small interlayer spacing, inducing a series of interesting semiconductor properties, such as efficient interlayer carrier transport as well as lower exciton binding energy compared to perovskites with long alkylammonium cations. The prepared Cs2Pb(SCN)2I2 photodetector achieves a high on/off ratio of 2.1 × 104 with a specific detectivity of 6.2 × 1011 jones to 450 nm visible light, and a high sensitivity of 2747 μC Gyair-1 cm-2 to X-rays.

Organic cations promote exciton dissociation in Ruddlesden-Popper lead iodide perovskites: a theoretical study.

Two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) are a class of quantum well (QW) materials showing large exciton binding energy owing to quantum confinement. The existence of localized edge states was proposed to accelerate exciton dissociation into long-lived charge carriers in 2D RPPs, but recent experimental reports suggested that highly efficient internal exciton dissociation is achievable in 2D RPPs despite the absence of edge states. Herein, we adopt first-principles calculations to unveil the physical origin of the high internal quantum efficiency in the bulk region of widely familiar (BA)2(MA)n-1PbnI3n+1 (BA = butylammonium; MA = methylammonium) materials. We discover that the dipolar nature of MA cations provides the driving force for the separation of photoexcited electron-hole pairs inside QWs as the inorganic layer thickens from n = 1 to n = 3. Concurrently, electronic coupling between organic spacer layers and QWs is enhanced in the energetically favorable configurations where MA cations orient with their CH3 groups towards the exterior PbI2 layers of QWs in the n = 3 structure. Consequently, hole delocalization is promoted along the out-of-plane direction of QWs, which in turn facilitates exciton dissociation into free charge carriers despite large exciton binding energy. Our simulations reveal that the hydrogen bonding between organic species (including both MA and BA cations) and iodine atoms, which is subtly interconnected, engineers the response of morphology in QWs and electronic interactions at organic-inorganic interfaces, providing novel insights for the exciton-free carrier behavior in the bulk area of 2D RPPs.

Determination of energetic positions of electronic states and the exciton dynamics in a π-expanded N-heterotriangulene derivative adsorbed on Au(111).

Bridged triarylamines, so-called N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in optoelectronic devices. Thereby the electronic structure at organic/metal interfaces and within thin films as well as the electronically excited states dynamics after optical excitation is essential for the performance of organic-molecule-based devices. Here, we investigated the energy level alignment and the excited state dynamics of a N-HTA derivative adsorbed on Au(111) by means of energy- and time-resolved two-photon photoemission spectroscopy. We quantitatively determined the energetic positions of several occupied and unoccupied molecular (transport levels) and excitonic states (optical gap) in detail. A transport gap of 3.20 eV and an optical gap of 2.58 eV is determined, resulting in an exciton binding energy of 0.62 eV. With the first time-resolved investigation on a N-HTA compound we gained insights into the exciton dynamics and resolved processes on the femtosecond to picosecond timescale.

First-principles investigation of photovoltaic material based on lanthanide metals

Searching for photovoltaic materials with high efficiency conforms to the goal for accessing low carbon and sustainable energy. This work explores the properties of RA3X3 compounds (R = La, Sm, Gd, Tb, Dy, Ho, Er; A = Cd, Zn; X = As, P) through a screening process, focusing on the huge potential of RCd3P3 series for further investigation in photovoltaic applications. Results show that RCd3P3 series with a direct or quasi-direct bandgap have an ultra-low exciton binding energy, which is favorable for separating electron and hole pairs. Moreover, these candidates possess high optical absorption in the visible light regions, and the power conversion efficiency limit of RCd3P3 series can be as high as 24.5 % at the thickness of 4 μm. These results could enrich the database of potential solar cells based on lanthanide materials.

Exploration of isoelectronic substitution in graphene dioxide for photocatalytic and photovoltaic applications - an ab-initio study.

Herein, we propose graphene dioxide (GDO) derivatives as promising materials for green hydrogen production by photocatalytic water splitting. The optoelectronic and photocatalytic properties of GDO, an insulator with a wide band gap, are tuned by designing new compositions through isovalent substitution of S/Se at the O site, Si and (B,N) at the C site. The newly predicted GDO derivatives were studied using hybrid functional calculations and our results show that several of these materials exhibit semiconducting behavior with a direct band gap value higher than 1.23 eV, hence appropriate for visible light-driven photocatalytic water splitting. The structural stability of these materials was analyzed by total energy and lattice dynamical calculations. The photo generated charge carriers possess lower effective mass and hence higher carrier mobility resulting in suppressed recombination rate and hence improving the water splitting efficiency. Apart from low excitonic binding energy, the electronic structure analysis shows that in several of these compounds the electrons and holes reside in two different atomic sites ensuring further reduction in recombination rate. The relatively higher absorption coefficient of GDO derivatives in the visible part of the solar spectrum indicates enhanced photoconversion efficiency suitable for solar cell applications also and it was further determined by photovoltaic performance parameter analysis. The band edge potential of GDO derivatives is well straddled by the water redox potential at different pHs, suggesting their potential for water splitting along with the possibility of CO2 reduction. Our findings indicate that the newly predicted compositions hold significant promise for photocatalytic as well as photovoltaic applications.

Exclusive generation of a superoxide radical by a porous aromatic framework for fast photocatalytic decontamination of mustard gas simulant in room air.

Mustard gas and other chemical warfare agents (CWAs) are a global threat to public security, arising from unpredictable emergencies and chemical spill accidents. So far, photocatalysts such as metal clusters, polyoxometalates and porous solids have been exploited for oxidative degradation of mustard gas, commonly with 1O2 as reactive species. However, the production of 1O2 is oxygen-dependent and requires a high oxygen concentration to sustain the detoxication process. For safety and operation process considerations, it is always preferable to rapidly detoxify dangerous chemicals in the atmosphere of room air. In this work, a porous aromatic framework, PAF-68, was synthesized as a metal-free photocatalyst. In the presence of PAF-68, fast detoxication occurred in typical room air atmosphere. The half-life (t 1/2) for the complete conversion of mustard gas simulant to nontoxic product in room air was only 1.7 min, which is comparable to the performance in pure oxygen, surpassing that of any other porous photocatalysts. It was found that ˙O2 - rather than 1O2 is the predominant reactive species initiated by PAF-68 for mustard gas detoxication. Unlike the formation of 1O2 which prefers the environment of pure oxygen, generation of the ˙O2 - is an oxygen-independent process. It is suggested that amorphous PAFs possess low exciton binding energy and long decay lifetime, which facilitate the generation of ˙O2 -, and this offers a general design strategy to detoxifying chemical warfare agents under real-world conditions.

Temperature-driven journey of dark excitons to efficient photocatalytic water splitting in β-AsP.

Limited availability of photogenerated charge carriers in two-dimensional (2D) materials, due to high exciton binding energies, is a major bottleneck in achieving efficient photocatalytic water splitting (PWS). Strong excitonic effects in 2D materials demand precise attention to electron-electron correlation, electron-hole interaction and electron-phonon coupling simultaneously. In this work, we explore the temperature-dependent electronic and optical responses of an efficient photocatalyst, blue-AsP (β-AsP), by integrating electron-phonon coupling into state-of-the-art GW + BSE calculations. Interestingly, strong electron-lattice interaction at high temperature promotes photocatalytic water splitting with an increasing supply of long-lived dark excitons. This work presents an atypical observation contrary to the general assumption that only bright excitons enhance the PWS due to prominent absorption. Dark excitons, due to the low recombination rate, exhibit long-lived photogenerated electron-hole pairs with high exciton lifetime increasing with temperature up to ∼0.25 μs.

Ionization-triggered low exciton binding energy in covalent organic frameworks for efficient photocatalytic synthesis of benzimidazole

Ionization of covalent organic frameworks (COFs) can lower the exciton dissociation energy, thus contributing to the high photocatalytic performance for benzimidazole synthesis.

Superalkali halide perovskites with suitable direct band gaps for photovoltaic applications.

The construction of superalkali halide perovskites has attracted attention for the development of new photovoltaic materials, but stable superalkalis have not been found until now. Herein, to construct new three-dimensional superalkali halide perovskites with a MI3 frame (M = Sn and Pb), a new Li(H2O)3+ superalkali cation is designed and selected based on low vertical ionization potential, suitable tolerance factor, small ionic radius and large dissociation energy. High-throughput first-principles calculations show that superalkalis with lower vertical ionization potentials exhibit stronger interactions with the MI3 frame. The normal and cubic Li(H2O)3MI3 perovskites and cubic Li(H2O)4PbI3 perovskites have direct band gaps, s-p and p-p electron transitions, effective carrier masses of less than 0.45me and exciton binding energies of less than 291 meV. Moreover, the cubic Li(H2O)3PbI3 perovskite with a direct band gap of 1.40 eV can in theory show a power conversion efficiency of 33.49%. These results strongly suggest that superalkali cations with large dissociation energy can be used to develop stable superalkali perovskites for photovoltaic applications.

Twist angle and electric gating controllable electronic structure of the two-dimensional stacked BP homo-structure.

The boron phosphide (BP) van der Waals (vdW) homostructure is designed to construct high-performance nano-optoelectronic devices due to its distinctive photoelectric properties. Using density functional theory, the electronic properties of twisted and untwisted BP bilayer structures are systematically calculated. We found that the 0° structure is a direct band gap semiconductor with a type II band alignment, the carrier mobility of which is increased to 104, and its photoelectric conversion efficiency is 17.3%. By analyzing the band structure and exciton binding energy calculated at 0° under an electric field, it is further found that 0° is a superior photoelectric material. As for the twist BP bilayer, the band gap changes with torsional structures under the applied electric field, which generates the Stark effect. The twist angles of bilayer BP, specifically 13.17°, 21.79°, 38.21°, and 46.83°, always maintain a direct band gap under the influence of an electric field. While 60° is an indirect band gap, the structure exhibits high resistance to the electric field. Our results reveal that bilayer BP is a potential application prospect in photovoltaic and optoelectronic fields and can provide more insights into optoelectronic devices.

Exploring the photovoltaic properties of naphthalene-1,5-diamine-based functionalized materials in aprotic polar medium: a combined experimental and DFT approach.

In this study, a series of naphthalene-1,5-diamine-based donor chromophores (ND1-ND9) with A-D-A architecture was synthesized through a condensation reaction between amines and substituted aldehydes. Various spectroscopic techniques i.e., FTIR, UV-Vis, 1HNMR and 13CNMR were performed for structural elucidation of naphthalene-1,5-diamine-based chromophores. Accompanying the synthesis, quantum chemical calculations were also accomplished at MPW1PW91/6-311G (d,p) functional of DFT/TD-DFT approaches to explore the photovoltaic properties of ND1-ND9 compounds. A comparative study between experimental and DFT results of vibrational and UV-Vis analyses showed a good harmony. All compounds showed band gaps in the range of 3.804-3.900 eV with absorption spectra in the UV region (397.169-408.822 nm). Frontier molecular orbital (FMO) findings revealed an efficient intramolecular charge transfer (ICT) from the central naphthalene-1,5-diamine-based donor core towards terminal acceptors. This significant charge transfer was also supported by the density of states (DOS) and transition density matrix (TDM) maps. All synthesized chromophores showed lower exciton binding energy values (E b = 0.670-0.785 eV), illustrating higher exciton dissociation rates with greater charge transfer in the studied chromophores. A reasonable harmony was obtained by comparative investigations of a standard hole transport material (HTM), Spiro-OMe TAD, with ND1-ND9 compounds, which illustrated that these synthesized chromophores might be considered as good HTMs. Therefore, all analyses indicated that the naphthalene-1,5-diamine-based chromophores might be utilized as efficient photovoltaic materials.

Synthesis, Characterization, and Applications of ZnO, Ag2O, and ZnO/Ag2O Nanocomposites: A Review

Nanoscale dimensional materials represent a captivating area of research in modern technology, owing to their unique properties and vast potential applications. The distinct advantage of nanosized materials lies in their large surface area to volume ratio, which endows them with enhanced stability and significant functional characteristics. Among these, metal oxide nanomaterials have garnered significant attention due to their substantial energy bandgaps and high exciton binding energies, making them suitable for a wide range of applications, including antimicrobial activity and optoelectronics. This review delves into the synthesis methods of various metal oxides, emphasizing the advantages and drawbacks of physical, chemical, and biological approaches. Specifically, it focuses on the synthesis, characterization, and applications of single metal oxide nanoparticles (NPs), such as zinc oxide (ZnO) and silver oxide (Ag2O), as well as binary or composite metal oxides like ZnO/Ag2O nanocomposites. Through this comprehensive literature review, readers will gain insights into the fabrication techniques and explore the intrinsic properties of both single and composite metal oxide NPs, thereby broadening their understanding of these innovative materials and their potential technological impacts.

Promising TMDC-like optical and excitonic properties of the TiBr2 2H monolayer.

The presented simulation protocol provides a solid foundation for exploring two-dimensional materials. Taking the TiBr2 2H monolayer as an example, this material displays promising TMDC-like optical and excitonic properties, making it an excellent candidate for optoelectronic and valleytronic applications. The direct band gap semiconductor (1.19 eV) is both structurally and thermodynamically stable, with spin-orbit coupling effects revealing a broken mirror symmetry in the K and K' valleys of the band structure, as confirmed by opposite values of the Berry curvature. A direct and bright exciton ground state was found, with an exciton binding energy of 0.56 eV. The study also revealed an optical helicity selection rule, suggesting selectivity in the valley excitation by specific circular light polarizations.

Two-dimensional Be2P4 as a promising thermoelectric material and anode for Na/K-ion batteries.

Incredibly effective and flexible energy conversion and storage systems hold great promise for portable self-powered electronic devices. Owing to their large surface area, exceptional atomic structures, superior electrical conductivity and good mechanical flexibility, two-dimensional (2D) materials are recognized as an attractive option for energy conversion and storage application. In this work, we examined the stability, electronic, thermoelectric and electrochemical aspects of a novel 2D Be2P4 monolayer by adopting density functional theory (DFT). The Be2P4 monolayer exhibits a direct semiconductor gap of 0.9 eV (HSE06), large Young's modulus (∼198 GPa), high carrier mobility (∼104 cm2 V-1 s-1) and a low excitonic binding energy of 0.11 eV. Our calculated findings suggest that Be2P4 shows a lattice thermal conductivity of 1.02 W m K-1 at 700 K, resulting in moderate thermoelectric performance (ZT ∼ 0.7), encouraging its use in thermoelectric materials. In addition, a higher adsorption energy of -2.28 eV (-2.52 eV) and less diffusion barrier of 0.22 eV (0.17 eV) for Na(K)-ion batteries promote fast ion transport in the Be2P4 monolayer. This material also shows a high specific capacity and superior energy density of 8460 W h kg-1 (8883 W h kg-1) for Na(K)-ion batteries. Thus, our results offer insightful information for investigating potential thermoelectric and flexible anode materials based on the Be2P4 monolayer.

Ultra-broadband bright light emission from a one-dimensional inorganic van der Waals material

One-dimensional (1D) van der Waals materials have emerged as an intriguing playground to explore novel electronic and optical effects. We report on inorganic one-dimensional SbPS4 nanotube bundles obtained via mechanical exfoliation from bulk crystals. The ability to mechanically exfoliate SbPS4 nanobundles offers the possibility of applying modern 2D material fabrication techniques to create mixed-dimensional van der Waals heterostructures. We find that SbPS4 can readily be exfoliated to yield long (>10 μm) nanobundles with thicknesses that range from 1.3 to 200 nm. We investigated the optical response of semiconducting SbPS4 nanobundles and discovered that upon excitation with blue light, they emit bright and ultra-broadband red light with a quantum yield similar to that of hBN-encapsulated MoSe2. We discovered that the ultra-broadband red light emission is a result of a large ∼1 eV exciton binding energy and a ∼200 meV exciton self-trapping energy, unprecedented in previous material studies. Due to the bright and ultra-broadband light emission, we believe that this class of inorganic 1D van der Waals semiconductors has numerous potential applications, including on-chip tunable nanolasers, and applications that require ultraviolet to visible light conversion, such as lighting and sensing. Overall, our findings open avenues for harnessing the unique characteristics of these nanomaterials, advancing both fundamental research and practical optoelectronic applications.

Exploring Acceptor Modification in Helicene‐Phenylamine‐Based Small Molecules for Organic and Perovskite Solar Cells

The current quantum mechanical study is focused on the fabrication of eight helicene‐phenylamine‐based molecules (MPAM1–MPAM8) to investigate their photovoltaic (PV) and optoelectronic properties for photovoltaic cells (PVCs). The outcomes revealed that fabricated chromophores showed deeper highest occupied molecular orbital levels, higher solubility, low exciton binding energy, greater hole mobility with low band gap energy than the model compound (MPAR). Moreover, the reorganization energy values interpret that newly fabricated compounds possess high charge mobility compared to the representative molecule (MPAR) which encourages their usage as HTM in PSCs. All the freshly designed molecules revealed greater estimated open circuit voltage values contrasted to the reference molecule (MPAR) which ensures their prominent working efficacy. The results signify the competence of this strategic approach, paving a new path for the fabrication of hole transport material (HTM) for perovskite solar cells (PSCs) and donor molecules for organic solar cells (OSCs).

A computational investigation of the effects of changes of donor–acceptor linker length and acceptor on electronic spectra of small-molecule single-component organic solar cells

Small-molecule single-component organic solar cells (SM-SCOSCs) have great potential for commercial application. However, the effects of changes of donor–acceptor linker length and acceptor unit of small molecules of active layers on their photoelectric properties are still not well understood. In this work, four state-of-the-art dyads 1–4 were selected to investigate these effects since dyad 4 created the highest power conversion efficiency (PCE) of 5.34% of SM-SCOSC; and their electronic structures were investigated. The results indicate that increasing the length of the donor–acceptor connecting bridge leads to a decrease in exciton binding energy, and so does replacing the acceptor component PC61BM with PC71BM. Furthermore, the simulated electronic absorption spectrum of dyad 4 is enhanced with respect to others. We also find that the introduction of PC71BM creates more channels for transitions from local excited states to charge transfer states. This work gains a deep insight into the relationship between molecular structure and performance of SM-SCOSC.

Mechanistic Insights into the Photoluminescence Enhancement in Surface Ligand Modified CsPbBr3 Perovskite Nanocrystals.

We report a mechanistic study of the photoluminescence (PL) enhancement in CsPbBr3 perovskite nanocrystals (PNCs) induced by organic/inorganic hybrid ligand engineering. Compared to the as-synthesized oleic acid-oleylamine modified PNCs, the tributylphosphine oxide-CaBr2 modified PNCs can achieve a better passivation effect due to strong P═O-Pb coordination and Br-vacancy remedy, resulting in enhanced PL efficiency. We employ steady-state/time-resolved/temperature-dependent PL and fluence/polarization-dependent ultrafast transient absorption spectroscopy to obtain a mechanistic understanding of such an enhancement effect from both nonradiative and radiative perspectives. As for the dominating nonradiative recombination suppression, we quantitatively evaluate the contributions from channels of exciton dissociation and exciton trapping, which are connected to exciton binding energy and activation energy of exciton trapping to surface defect-induced trap states, respectively. We also look into the radiative recombination enhancement, which is likely due to the increase in electron-hole overlap of photogenerated excitons induced by slight Ca-doping. These mechanistic insights would be of guiding value for perovskite-based light-emitting applications.

Vertical Graphene/Pentacene Single Crystal/Graphene Transistors for Self‐Powered Weak Light Detection and High‐Speed Imaging

AbstractOrganic semiconductors are promising candidates for next‐generation photodetectors owing to their unique properties including large absorption coefficient and intrinsic mechanical flexibility. However, limited by high defect density, large exciton binding energy, and short exciton diffusion length, many challenges remain for fabricating high‐performance organic photodetectors, especially for scenarios where weak light sensing and high‐speed imaging are required. Herein, by coupling single‐crystal pentacene with monolayer graphene, a vertical organic phototransistor with excellent zero‐bias photon detecting capacity is demonstrated. Under self‐powered weak light condition, the responsivity and specific detectivity are 2.86 AW−1 and 3.3 × 1011 Jones (based on experiment spectral noise density), respectively. It is demonstrated that weak light down to 25 nWcm−2 can be detected. When operating with an external bias, the responsivity of the device increases dramatically (up to 105 AW−1), and a better 3 dB bandwidth (≈74 kHz) is obtained. Finally, imaging functionality is demonstrated by employing this vertical transistor as a single‐sensing pixel. This work suggests organic single crystals exhibit enormous potential in advanced optoelectronic systems, and may provide a viable route for weak light detecting and high‐speed imaging.