Exciton Binding Energy Research Articles

Room temperature growth of CsPbBr3 single crystal for asymmetric MSM structure photodetector

UV–visible broadband light harvesting ability is critical for photodetectors, and all inorganic perovskite CsPbBr3 is regarded as a promising candidate as the response active material. Herein, a saturated-solution crystallization method is employed to synthesize CsPbBr3 single crystal. Temperature-dependent photoluminescence spectra with one-photon and two-photon excitation are systematically investigated, determining a large exciton binding energy of 39.8 meV. This allows the stable excitonic emission of CsPbBr3 single crystal at room temperature. Subsequently, an asymmetric structure CsPbBr3 photodetector is fabricated by using InGa alloy as the Ohmic electrode and Au as the Schottky electrode. At -8 V, the device exhibits a prominent response to the irradiation from UV to green band with a maximum responsivity of 2.56 A/W, an external quantum efficiency of 580 %, and a detectivity of 1.24 × 1013 Hz1/2 W−1. In addition, the CsPbBr3 photodetector also presents rapid response speeds at both reverse and forward bias voltages and self-powered characteristics owing to the Schottky depletion layer underneath the Au electrode. The demonstration of asymmetric metal-semiconductor-metal (MSM) structure photodetector presents a promising pathway toward next-generation CsPbBr3 optoelectronic devices.

Trap‐Assisted Exciton Dissociation Through Continuous Trapping/Detrapping in Lithium Ions Inserted Crystalline Carbon Nitride

AbstractHere, lithium ions inserted crystalline carbon nitrides (LCCN) with different defect concentrations are prepared by elaborately regulating the mass transfer process in thermal polymerization. It is revealed that there is no conventional exciton–polarons‐induced exciton dissociation mechanism in LCCN due to the weak coupling between excitons and phonons. On the contrary, excitons continue to trap/detrap through defect‐induced traps, which weakens the Coulomb force between electron–hole pairs, thereby accelerating exciton dissociation. It is worth emphasizing that this is the first systematic research on the exciton dissociation characteristics and photocatalytic activity of LCCN of poly(heptazine imide) structure. A strategy of assisting exciton dissociation through defect‐induced traps to address the inherently high exciton binding energy and low exciton dissociation efficiency issues in polymer photocatalysts is proposed.

Theoretical discovery of the phase stability, mechanical, optoelectronic, and photovoltaic properties of AZn3N3 (A = Sc, Y, La)

In this study, the detailed first-principles calculations are employed to evaluate the structural stability, mechanical, optoelectronic, and photovoltaic properties of AZn3N3 (A = Sc, Y, La). Our calculations show that the thermodynamic, dynamical, and mechanical stability are firstly verified for the hexagonal phase with space group P63/mmc of AZn3N3. The direct band gap feature with the value of 1.7–1.8 eV is revealed for these novel compounds, and the energy gap variation is less than 100 meV. They have relatively high effective masses and exciton binding energies. Their similar absorption spectra are observed in the visible light region. Based on the spectroscopic limited maximum efficiency (SLME) model, their maximum efficiencies are about 18–20 % in the thickness of 0.5 μm. Given their optoelectronic and photovoltaic properties, ternary zinc nitrides AZn3N3 (A = Sc, Y, La) may be potential candidates for tandem solar cells.

Construction of Thiadiazole-Bridged sp2-Carbon-Conjugated Covalent Organic Frameworks with Diminished Excitation Binding Energy Toward Superior Photocatalysis.

Sp2-carbon-conjugated covalent organic frameworks (sp2c-COFs) have emerged as promising platforms for phototo-chemical energy conversion due to their tailorable optoelectronic properties, in-plane π-conjugations, and robust structures. However, the development of sp2c-COFs in photocatalysis is still highly hindered by their limited linkage chemistry. Herein, we report a novel thiadiazole-bridged sp2c-COF (sp2c-COF-ST) synthesized by thiadiazole-mediated aldol-type polycondensation. The resultant sp2c-COF-ST demonstrates high chemical stability under strong acids and bases (12 M HCl or 12 M NaOH). The electro-deficient thiadiazole together with fully conjugated and planar skeleton endows sp2c-COF-ST with superior photoelectrochemical performance and charge-carrier separation and migration ability. As a result, when employed as a photocathode, sp2c-COF-ST exhibits a significant photocurrent up to ∼14.5 μA cm-2 at 0.3 V vs reversible hydrogen electrode (RHE) under visible-light irradiation (>420 nm), which is much higher than those analogous COFs with partial imine linkages (mix-COF-SNT ∼ 9.5 μA cm-2) and full imine linkages (imi-COF-SNNT ∼ 4.9 μA cm-2), emphasizing the importance of the structure-property relationships. Further temperature-dependent photoluminescence spectra and density functional theory calculations demonstrate that the sp2c-COF-ST has smaller exciton binding energy as well as effective mass in comparison to mix-COF-SNT and imi-COF-SNNT, which suggests that the sp2c-conjugated skeleton enhances the exciton dissociation and carrier migration under light irradiation. This work highlights the design and preparation of thiadiazole-bridged sp2c-COFs with promising photocatalytic performance.

Computational design of new polymers having low exciton binding energy for organic solar cells fabrication: Chemical generation and visualization

Conventional organic solar cell (OSC) fabrication, based on the hit and trial methodology, is quite laborious and lethargic, requiring extensive human workforce and instrumental resources. Recent advancements in machine learning (ML) approaches have revolutionized selection of the suitable active polymeric layer for OSC devices. Among numerous aspects impacting efficacy of the OSC fabrication, the lower exciton binding energy (Eb) was found one of the most crucial factors. The dataset containing Eb values was prepared from already documented literature on polymeric active layers of OSC devices. Various ML models were applied and tested to predict Eb values accurately, however, the random forest regressor model was proved to be the most efficient one. The ML analysis was further utilized to develop a new and novel chemical database that could be utilized as an active layer for the fabrication of OSCs. The synthetic accessibility (SA) score was also investigated to explore the relative ease by which a proposed polymeric material could be synthesized in practical lab. Interestingly, analysis of the top thirty generated polymeric materials exhibited extremely decent SA score, and remarkably lower Eb values; both properties are highly desirable for the fabrication of efficient OSCs. Further, we also applied deep learning approach to generate and screen new monomers with lower Eb and SA score. This effort reveals significance of the computational data-driven approaches for real-life applications. Our current contribution of the novel polymeric materials database having extremely low Eb values could be valuable for the fabrication of efficient OSC devices.

High Open‐Circuit Voltage (1.197 V) in Large‐Area (1 cm2) Inverted Perovskite Solar Cell via Interface Planarization and Highly Polar Self‐Assembled Monolayer

AbstractThe efficiency loss caused by area scaling is one of the key factors hindering the industrial development of perovskite solar cells. The energy loss and contact issues in the buried interface are the main reasons. Here, a new self‐assembled monolayer (SAM), Ph‐4PACz, with a large dipole moment (2.32 D) is obtained . It is found that Ph‐4PACz with high polarity can improve the band alignment and minimize the energy loss , resulting in an open‐circuit voltage (Voc) as high as 1.2 V for 1.55 eV perovskite. However, when applied to large‐area devices, the fill factor (FF) still suffered from significant attenuation. Therefore, alumina nanoparticles (Al2O3‐NPs) are introduced to the interface between Ph‐4PACz and rough FTO substrate to further improve the flatness , resulting in a conformal perovskite film with almost no voids in the buried interface, thus promoting low exciton binding energy, fast hot‐carrier extraction and low non‐radiative recombination. The final devices achieved a small‐area power conversion efficiency (PCE) of 25.60% and a large‐area (1 cm2) PCE of 24.61% (certified at 24.48%), which represents one of the highest PCE for single device ≥ 1 cm2 area. Additionally, mini‐modules and stability testing are also carried out to demonstrate the feasibility of commercialization.

Inside Cover Picture

Organic photovoltaic (OPV) devices show promising as the next‐generation renewable energy source. Currently, the non‐radiative charge recombination induced energy loss is considered to be a major reason for the low device efficiency. This can be further attributed to the use of donor:acceptor (D:A) heterojunction structures, which resolves the high exciton binding energy induced low charge generation issue, but introduces low emissive charge transfer states. In this work, we assess the necessity of D:A heterojunction, and demonstrate that the high‐performance D:A heterojunction‐free OPV device is possible when both photo‐induced spontaneous polaron generation and good ambipolar charge transport are realized. Moreover, the D:A heterojunction‐free structure exhibits much less non‐radiative charge recombination and more stable morphology for high performance, and probably becomes the next game‐changer in OPVs. More details are discussed in the article by Zuo et al. on page 252—258.image

The effect of particle size on the optical and electronic properties of hydrogenated silicon nanoparticles.

We use a combination of many-body perturbation theory and time-dependent density functional theory to study the optical and electronic properties of hydrogen terminated silicon nanoparticles. We predict that the lowest excited states of these silicon nanoparticles are excitonic in character and that the corresponding excitons are completely delocalised over the volume of the particle. The size of the excitons is predicted to increase proportionally with the particle size. Conversely, we predict that the fundamental gap, the optical gap, and the exciton binding energy increase with decreasing particle size. The exciton binding energy is predicted to counter-act the variation in the fundamental gap and hence to reduce the variation of the optical gap with particle size. The variation in the exciton binding energy itself is probably caused by a reduction in the dielectric screening with decreasing particle size. The intensity of the excited state corresponding to the optical gap and other low energy excitations are predicted to increase with decreasing particle size. We explain this increase in terms of the 'band structure' becoming smeared out in reciprocal space with decreasing particle size, increasing the 'overlap' between the occupied and unoccupied quasiparticle states and thus, the oscillator strength. Fourier transforms of the lowest excitons show that they inherit the periodicity of the frontier quasiparticle states. This, combined with the delocalisation of the exciton and the large exciton binding energy, means that the excitons in silicon nanoparticles combine aspects of Wannier-Mott, delocalisation and effect of periodicity of the underlying structure, and Frenkel, large exciton binding energy, excitons.

Observation of spin-splitting energies on sp-d exchange interactions tailored in colloidal CdSe/CdMnS core/shell nanoplatelets: an atomistic tight-binding model.

Using the atomistic tight-binding model plus sp-d exchange term, the embedding of magnetic ions into CdSe/CdMnS core/shell nanoplatelets (NPLs) at different effective temperatures resulted in sp-d exchange interactions, which in turn cause modifications in electronic and magnetic characteristics. The influence of CdMnS monolayers on single-particle spectra, optical band gaps, wave function overlaps and exciton binding energies is more pronounced than that of the effective temperature. Due to the electron, hole and Zeeman splitting energies, with the growth of CdMnS shell monolayers, electron g-factor values are unchanged, but hole and exciton g-factor values are enhanced. Additionally, all g values decrease with increasing temperature, thus representing decreased magnetization of the paramagnetic system. By changing nanoplatelet architectures and temperatures, manipulation of s-d and p-d exchange interactions is accomplished. Overall, studied materials combine the merits of NPLs and magnetic ions, hence leading to alternate possibilities for active applications in spin-based devices.

Quantum-confined bismuth iodide perovskite nanocrystals in mesoporous matrices.

Bismuth iodide perovskite nanocrystals are considered a viable alternative to the Pb halide ones due to their reduced toxicity and increased stability. However, it is still challenging to fabricate nanocrystals with a small and controlled size, and their electronic properties are not well understood. Here, we propose the growth of Bi iodide perovskite nanocrystals using different mesoporous silica with ordered pores of controlled diameter as templates. We obtain a series of confined Cs3Bi2I9 and MA3Bi2I9 perovskites with diameters of 2.3, 3.7, 7.4, and 9.2 nm, and precise size control. The complex absorption spectra of the encapsulated perovskites cannot be properly fitted using classical Tauc or Elliott formalisms. By fitting the spectra with a modified Elliott formula, the bandgap values and exciton binding energies (70-400 meV) could be extracted. The calculated bandgaps scale with the pore sizes. Using a combined experimental and theoretical approach, we demonstrate for the first time quantum confinement in 0D Bi-iodide perovskite nanocrystals.

Strain-induced charge delocalization achieves ultralow exciton binding energy toward efficient photocatalysis.

The exciton effect is commonly observed in photocatalysts, where substantial exciton binding energy (E b) significantly hampers the efficient generation of photo-excited electron-hole pairs, thereby severely constraining photocatalysis. Herein, we propose a strategy to reduce E b through strain-induced charge delocalization. Taking Ta2O5 as a prototype, tensile strain was introduced by engineering a crystalline/amorphous interface, weakening the interaction between Ta 5d and O 2p orbitals, thus endowing a delocalized charge transport and significantly lowering E b. Consequently, the E b of strained Ta2O5 nanorods (s-Ta2O5 NRs) was reduced to 24.26 meV, below the ambient thermal energy (26 meV). The ultralow E b significantly enhanced the yield of free charges, resulting in a two-fold increase in carrier lifetime and surface potential. Remarkably, the hydrogen evolution rate of s-Ta2O5 NRs increased 51.5 times compared to that of commercial Ta2O5. This strategy of strain-induced charge delocalization to significantly reduce E b offers a promising avenue for developing advanced semiconductor photoconversion systems.

Exciton binding energies and polaron interplay in the optically excited state of organic–inorganic lead halide perovskites

Excitons and polarons are formed in organic–inorganic lead halide perovskites upon photoexcitation, accounting for most of their photovoltaic properties.

Study the optical properties of Cs3CeI6: First-principles calculations

The band structure, density of states, and optical properties of a novel material, Cs3CeI6 are calculated for the first time using the density functional theory method in first-principles calculations. It is found that Cs3CeI6 possesses a direct bandgap with an energy value of 3.05 eV. Examination of the density of states indicates that the conduction band minimum is primarily composed of Ce-5d and Ce-4f orbitals, while the valence band maximum is mainly contributed by Ce-4f orbitals. Photoluminescence (P.L.) spectroscopy reveals distinctive bimodal emission peaks at 432 and 468 nm, which serve as characteristic signatures of Ce3+ ions. This bimodal emission arises from spontaneous radiative transitions between excited 5d orbitals and the 2F7/2 and 2F5/2 states within the 4f orbital, as confirmed by crystal field calculations. The difference between these two emission peaks corresponds to variations in energy levels associated with Ce3+ ions due to crystal field disturbances. Moreover, Cs3CeI6 exhibits an exciton binding energy of 225 meV due to strong localization effects in Ce-4f orbitals and binding properties inherent in its zero-dimensional structure, promoting exciton formation. Such a substantial exciton binding energy offers significant advantages for potential electroluminescence applications. Based on these findings, we anticipate promising prospects for the use of Cs3CeI6 in electroluminescent devices.

Facet-specific ligand dynamics: scaling the descriptors of Cs2AgBiBr6 nanocrystals.

Facet control by primary amines can bolster the optoelectronic parameters of A2BIB'IIIX6 perovskite nanocrystals (NCs) with large indirect bandgaps. The 18-C amine competitively attaches to the (222) facet of Cs2AgBiBr6 (CABB) NCs, 16-C and 14-C bind to (400) and (440), and 12-C binds to (400). The NCs with only the (400) facet decrease the bandgap and exciton binding energy by 0.26 eV and 15 meV, respectively.

A first principles study on the stability and electronic and optical properties of 2D SbXY (X = Se/Te and Y = I/Br) Janus layers.

Motivated by the exceptional optoelectronic properties of 2D Janus layers (JLs), we explore the properties of group Va antimony-based JLs SbXY (X = Se/Te and Y = I/Br). Using Bader charges, the electric dipole moment in the out-of-plane direction of all the JLs is studied and the largest dipole moment is found to be in the SbSeI JL. Our results on the formation energy, phonon spectra, elastic constants, and ab initio molecular dynamics (AIMD) simulation provide insights into the energetic, vibrational, mechanical, and thermal stability of JLs. After confirming the stability, the three-dimensional phase diagram is investigated to propose the experimental conditions required to fabricate the predicted JLs. Then, the electronic band structure is calculated using different levels of theory, namely, the generalized gradient approximation (GGA), GGA + spin-orbit coupling (GGA + SOC), hybrid Heyd-Scuseria-Ernzerhof (HSE) functional, and many-body perturbation theory-based Green's function method (GW). According to the HSE results, JLs show band gaps between 1.653 and 1.852 eV. The GGA + SOC calculations reveal Rashba spin splitting in these JLs. The calculated carrier mobility using deformation potential theory shows that the electrons have exceptionally high mobility compared to holes, which assists the spatial separation of both charge carriers. The optical spectra are determined using GGA, HSE, and GW methods. With respect to GGA results, HSE and GW optical spectra show a blue shift. More accurate calculations using the GW-Bethe Salpeter equation (BSE) yield optical absorption spectra that are dominated by strong excitonic effects with the excitonic binding energy (BEex) in the range of 550-800 meV. Compared to the GW-BSE method, the Mott-Wannier (MW) model predicts a lower BEex. A strong e-h coupling is observed for dispersions along K-M in the Brillouin zone from the fat band analysis. Our study suggests that the SbSeI JL is a potential candidate for photocatalytic and photovoltaic applications due to its largest dipole moment and low excitonic binding energy.

Modulating photophysical properties in a flexible porous host by regulating guest-assisted charge transfer interactions.

The 1D array of electron donor-acceptor chromophoric organic molecules is of paramount importance for photovoltaic, catalytic and optoelectronic applications. Herein, we report coordination driven 1D arrays of an electron-donor guest (fluorene, carbazole, dibenzofuran, and dibenzothiophene) and o-phen chelator as an acceptor in a Zn-based porous coordination polymer, {[Zn(o-phen)(ndc)]·DMF} (PCP-1). All the guest-encapsulated PCPs were characterized by performing single-crystal structure determinations and showed emission driven by charge transfer. Exciton binding energies were calculated and correlated with electron-donating capabiliities of the guest molecules.

Composition-dependent band structure parameters and band-gap bowing effect in a caesium lead mixed halide system: a cyclic voltammetry investigation.

Cyclic voltammetry techniques have been employed to study the effect of halide substitution on the band edge parameters and band gap bowing effect in the case of CsPbX3 [X = I, Br, Cl] perovskite nanocrystals (PNCs). A series of compositions, viz. CsPbI3, CsPb(I-Br)3, CsPbBr3, CsPb(Br-Cl)3 and CsPbCl3, have been prepared by a hot injection method. From powder XRD and HR-TEM analysis, the formation of a highly crystalline, cubic phase of the perovskite having size in the range from 7-20 nm has been confirmed. Sharp peaks in the photoluminescence spectra suggest the formation of quantum dots with narrow-size distribution. The composition-dependent optical band gap (εopgap) for CsPbX3 displays a systematic shift towards shorter wavelengths from I to Br to Cl substitutions. The cyclic voltammetry investigation on the dispersion of PNCs in nonaqueous solvents yielded prominent cathodic and anodic peaks. These are correlated to conduction (e1) and valence band edge (h1) positions, respectively. The h1 has been decreased substantially with I to Br to Cl in CsPbX3. Meanwhile, e1 shows a marginal increase. The values derived from CV data demonstrated an excellent match with UVPS results, reported for a similar system. From these results, the quasi-particle gap (εqpgap) and exciton binding energy have been estimated for all the compositions. The negative band gap bowing effect noted in these PNCs is attributed to the size quantization effect. The band-edge parameters reported in this work will be valuable in matching these heterojunctions with suitable electron/hole transport materials for optimum device-performance.

Engineering highly efficient porphyrin sensitizers through metal, ligand and bridge modification: a DFT study.

This work presents a systematic investigation of porphyrin sensitizers for application in dye-sensitized solar cells (DSSCs). Density functional theory calculations, including both static and time-dependent methods, were employed to evaluate a series of candidate dyes for their potential to achieve high power conversion efficiency. The well-established SM315 dye, known for its record-breaking PCE of 13%, was adopted as a reference point. A range of metal atoms including alkaline-earth and 3d transition metals were screened, Ca was identified as the most promising metal for light capture and conversion. Ca-porphyrin-based sensitizer was further modified by introducing different axial ligands and four distinct bridging units. The designed dyes exhibit red-shifted absorption spectra and optimal frontier orbital alignment with the semiconductor's conduction band, promoting efficient light capture and charge transfer. In addition to these core parameters, a comprehensive analysis of light harvesting efficiency (LHE), reorganization energy (λ), short-circuit current density (JSC), exciton binding energy (EBE), open-circuit voltage (VOC), electron transfer rate (k), polarization (α) and hyperpolarization (βtot) collectively paint a clear picture of superior light capture, efficient charge transport dynamics, and minimized energy losses within the designed dyes. This ultimately translates to the remarkable power conversion efficiency (PCE) exceeding 27% achieved by the specifically designed dye with the Ca as metal atom, 4,4'-bipyridine as axial ligands and cyclopenta-1,3-diene as bridging unit, surpassing the performance of SM315 dye (13% PCE). This systematic study combines the design of high-performance porphyrin sensitizers through molecular engineering with a comprehensive investigation of their impact on DSSC function using advanced computational methods.

Boosting exciton dissociation in anion and cation co-doped polymeric semiconductor for selective oxidation reaction.

The inherently low dielectric properties and weak shielding effect of polymeric semiconductors cause excitons to dominate their photoexcitation process, which greatly restricts the photocatalytic performances mediated by charge carriers. Here, an anion and cation co-doping strategy was proposed to weaken the binding energy of excitons by forming distinct positive and negative charge regions, where the charge asymmetry produced an external potential to drive exciton dissociation. Using polymeric carbon nitride as a typical model framework, we show that the incorporation of anions (Cl-, Br-, I-) and cations (Na+, K+) could create a significant spatial separation of electrons and holes, thereby promoting exciton dissociation. Specifically, K+ and Cl- co-doped polymeric carbon nitride could effectively promote the dissociation of excitons into hot carriers, contributing to the outstanding efficiency in hot-electron-involved photocatalytic processes, such as the generation of superoxide radicals (O2˙-) and the oxidation of phenylboric acid under visible light. This work presents a practical approach for promoting excitons dissociation through the introduction of charge asymmetry.

FAPbBr3@GA2PbBr4 quantum dots: one step fabrication with improved stability for light-emitting applications

We proposed a new type of core–shell FAPbBr3@GA2PbBr4 QDs, which were demonstrated with an enhanced exciton binding energy and improved stability, making them promising candidates in perovskite based light-emitting applications.