Hole Exciton Research Articles

Ultra-broadband Photodetectors Based on Formamidinium Lead Iodide Quantum Dots

Near-infrared photodetectors were fabricated by incorporating formamidinium lead iodide (FAPbI3) quantum dots (QDs) as the light-harvesting layer. Through systematic optimization of the device architecture, high device performance was achieved by utilizing PCBM as the electron transport material and a 50 nm-thick TAPC film as the hole transport layer. The energy level alignment between PCBM and the FAPbI3 QDs enabled efficient exciton dissociation and hole blocking, while the optimized TAPC thickness decrease current leakage pathways. The resulting photodetectors exhibited an impressive external quantum efficiency of 59.56 % at 750 nm, along with a high specific detectivity of 2.63 x 1011 Jones. A broadband photoresponse from 300–900 nm was observed, as well as a fast temporal response with 30.58/31.26 μs rise/fall times. A substantial linear dynamic range of 61.5 dB was achieved under 780 nm illumination. Furthermore, the low dark current densities facilitated by the judiciously selected materials and thicknesses contributed to the excellent overall device performance. The device performance of the PCBM/FAPbI3 QDs/TAPC system demonstrate its promising potential for near-infrared optoelectronic applications requiring high sensitivity, speed, and broad spectral response, opening up opportunities for further advances in photodetection as well as other relevant device technologies.

R‐NG●‐Regulated Electron‐Phonon Coupling to Enhance Hot‐Hole Injection for Inverted Perovskite Solar Cells

AbstractExtracting hot‐carriers before they relax back to the band edge will reduce thermal dissipation above the bandgap, which is one of the primary sources of efficiency loss in perovskite solar cells (PSCs). It requires slow cooling of hot‐carriers together with the efficient extraction of hot‐carriers. Here, a novel interface engineering is demonstrated by embedding radical‐containing nanographene (r‐NG●) between perovskite and poly[bis(4‐phenyl)(2,4‐dimethoxyphenyl)‐amine] (PTOAA) to harvest this excess energy. This strategy accelerates the extraction rate of the hot‐hole (3 times that of control at 400 K) by augmenting the relaxation channel, which is related to the “quasi‐SOMO” energy state of r‐NG●. Nonadiabatic molecular dynamics calculation further revealed that radical character promotes the rotation of the cation in perovskite, which contributes to generating strong nonadiabatic couplings and multiple phonon modes. In addition, the band bending effect of buried perovskite and improved conductivity of PTOAA also facilitate exciton dissociation and hole collection efficiency (97.5%). Consequently, the resultant r‐NG● decorated PSCs delivered an impressive efficiency of 24.20%, along with better thermal and humidity stability. Moreover, it can maintain 93% of its initial efficiency after 300 hours of illumination.

Eco-friendly solvent-processed layer-by-layer ternary all-polymer solar cells exhibiting over 18.5% efficiency

In this work, a series of eco-friendly solvent-processed all-polymer solar cells (APSCs) were fabricated by employing the sequentially spin-coating method. Two polymer donor materials PBQx-TCl and PM6 were deliberately selected due to their similar chemical structure, preferring to form the alloyed state for better exciton dissociation and hole transport in the layer-by-layer (LbL) APSCs. The better exciton dissociation between PBQx-TCl and PM6 can be confirmed from PL spectra of neat and blend films, as well as the short-circuit current densities (JSC) of special cells without acceptor layer. The optimized ternary LbL APSCs with 30 wt% PM6 in donor layer exhibit the power conversion efficiency (PCE) of 18.55 %, originating from the simultaneously improved JSC of 25.16 mA cm−2, open-circuit voltage (VOC) of 0.966 V, and fill factor of 76.33 %. The increased VOC of ternary LbL APSCs is mainly attributed to the decreased energy loss due to the good compatibility of the two polymer donors. The PCE of 18.55 % should be among the highest value for LbL APSCs processed by eco-friendly solvent.

Exciton Localization Modulated by Ultradeep Moiré Potential in Twisted Bilayer γ-Graphdiyne.

Twisted moiré superlattice is featured with its moiré potential energy, the depth of which renders an effective approach to strengthening the exciton-exciton interaction and exciton localization toward high-performance quantum photonic devices. However, it remains as a long-standing challenge to further push the limit of moiré potential depth. Herein, owing to the pz orbital induced band edge states enabled by the unique sp-C in bilayer γ-graphdiyne (GDY), an ultradeep moiré potential of ∼289 meV is yielded. After being twisted into the hole-to-hole layer stacking configuration, the interlayer coupling is substantially intensified to augment the lattice potential of bilayer GDY up to 475%. The presence of lateral constrained moiré potential shifts the spatial distribution of electrons and holes in excitons from the regular alternating mode to their respective separated and localized mode. According to the well-established wave function distribution of electrons contained in excitons, the AA-stacked site is identified to serve for exciton localization. This work extends the materials systems available for moiré superlattice design further to serial carbon allotropes featured with benzene ring-alkyne chain coupling, unlocking tremendous potential for twistronic-based quantum device applications.

Controllable Exciton Diffusion Length and Ultrafast Charge Generation in Ternary Organic Solar Cells

Comprehensive SummaryCharge generation, a critical process in the operation of organic solar cell (OSC), requires thorough investigation in an ultrafast perspective. This work demonstrates that the utilization of alloy model for the non‐fullerene acceptor (NFA) component can regulate the crystallization properties of active layer films, which in turn affects exciton diffusion and hole transfer (HT), ultimately influencing the charge generation process. By incorporating BTP‐eC7 as a third component, without expanding absorption range or changing molecular energy levels but regulating the ultrafast exciton diffusion and HT processes, the power conversion efficiency (PCE) of the optimized PM6:BTP‐eC9:BTP‐eC7 based ternary OSC is improved from 17.30% to 17.83%, primarily due to the enhancement of short‐circuit current density (JSC). Additionally, the introduction of BTP‐eC7 also reduces the trap state density in the photoactive layer which helps to reduce the loss of JSC. This study introduces a novel approach for employing ternary alloy models by incorporating dual acceptors with similar structures, and elucidates the underlying mechanism of charge generation and JSC in ternary OSCs.

Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure.

In two-dimensional semiconductors, cooperative and correlated interactions determine the material's excitonic properties and can even lead to the creation of correlated states of matter. Here, we study the fundamental two-particle correlated exciton state formed by the Coulomb interaction between single-particle holes and electrons. We find that the ultrafast transfer of an exciton's hole across a type II band-aligned semiconductor heterostructure leads to an unexpected sub-200-femtosecond upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this upshift is a clear fingerprint of the correlated interaction of the electron and hole parts of the exciton. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access electron-hole correlations and cooperative behavior in quantum materials. Our work highlights this capability and motivates the future study of optically inaccessible correlated excitonic and electronic states of matter.

Luminescence properties of CsPbBr3 single crystals and CsPbBr3 crystalline phases dispersed in a KBr matrix

Spectra of steady-state X-ray excited luminescence (XEL) and thermally stimulated luminescence (TSL) of CsPbBr3 single crystals in the temperature range of 80–294 K were studied. Each of the measured XEL spectra of CsPbBr3 single crystal contain only one elementary band peaking at 535 nm, which can be attributed to the radiative decay of the matrix exciton. The activation energy of exponential luminescence thermal quenching in this range, as determined by Mott's formula, is equal to 75 meV, close to the hole trap depth of 110 meV obtained for the main TSL peak of this crystal at 125 K. This value of the XEL quenching activation energy is associated with the thermal deactivation of the exciton hole component in the CsPbBr3 matrix.Doping of KBr crystals with CsPbBr3 perovskite impurities (0.05–1 mol. %) leads to the formation of CsPbBr3 micro- and nanocrystals in a KBr matrix with characteristic spectra of exciton photo- and XEL luminescence. A five-fold increase in the intensity of XEL was registered in KBr: CsPbBr3 (0.05 mol. %) samples at 294 K compared to that of CsPbBr3 single crystal.The temperature dependence of the XEL intensity of KBr: CsPbBr3 crystals and the analysis of their TSL integral curves allow us to make a conclusion about the hole mechanism of high-energy excitation energy transfer from the matrix to the activator resulting in the appearance of typical exciton luminescence in CsPbBr3 nanoparticles.

Ternary photocatalysts with electron modulation for efficient photocatalytic hydrogen evolution reactions: CdS-induced ring electrons transfer effect

To better utilize clean energy for the purpose of carbon neutrality, we designed and prepared the three-phase connected ternary tandem heterojunctions for the photocatalytic hydrogen evolution reaction. Notably, the variety bonding rings demonstrated by density functional theory raised the possibility of a special electrons transferred mechanism. Further, a unique ring electrons transfer mechanism extended the lifetime of the excitons and promoted their separation and transfer. In-situ XPS (X-ray photoelectron spectroscopy) and other characterizations were used to thoroughly characterize the ring electron transfer. The addition of the mid-component in three phase-connected heterojunctions changed the pathway of electrons transfer and increased photocatalytic hydrogen evolution reaction efficiency. As a consequence, this heterojunction exhibited an enhanced exciton lifetime, low exciton–hole recombination rate, and good photoelectric response, thus outperforming a traditional tandem heterojunction by 14.6%. This work provides novel insights into the directionally design, construction, and optimization of photocatalyst structures and exciton transfer mechanisms.

Enhanced Photodynamic of Carriers and Suppressed Charge Recombination Enable Approaching 18% Efficiency in Nonfullerene Organic Solar Cells.

Regulation of the exciton generation, diffusion, and carrier transport, as well as optimization of the non-radiative energy loss could further overcome the power conversion efficiency limitation of organic solar cells. However, the relationship between exciton properties and non-radiative energy loss has seldom been investigated. Herein, taking D18-series devices as the research model, the exciton diffusion length (LD) and hole transfer dynamics can be remarkably improved by the variation of electron-withdrawing halogen and the non-radiative energy loss simultaneously can be suppressed. By combining the analysis results of hole transfer, exciton diffusion, charge separation, and recombination, this work demonstrates that the photo-induced exciton in the chlorinated polymer donor can diffuse to a longer distance within the effective exciton lifetime, suppress the exciton recombination, and enhance device performance. The results define the relationship between the exciton behaviors and non-radiative energy loss and further reveal the significance of controlling the bulk heterojunction with superior photo-physical properties.

Single-atomic Co-N4-O site boosting exciton dissociation and hole extraction for improved photocatalytic hydrogen evolution in crystalline carbon nitride

Single atom co-catalyst loading has been demonstrated to be an effective strategy for achieving efficient photocatalytic water splitting. Unfortunately, the origins of the high activity of the single atom sites remain unrevealed owing to the lack of deep insight on their coordination environment. Herein, single-atom Co was loaded on crystalline g-C3N4 (CCN) nanorod in the form of five-coordination (Co-N4-O) at the heptazine cavities. Both experimental and theoretical evidences revealed that single-atomic Co-N4-O sites in CCN-Co played a key role in exciton dissociation and photogenerated charge carrier separation as well as subsequent hole extraction and transfer. Under visible light irradiation, the photogenerated holes in CCN directionally transferred to Co-N4-O sites and were rapidly extracted by hole sacrificial agent. As a result, the obtained CCN-Co sample with 0.32 wt. % Co and 1.0 wt. % Pt exhibited significantly improved photocatalytic hydrogen production rate of 32.1 mmol g−1 h−1, nearly 4 times and 38 times higher than that of CCN and bulk g-C3N4, respectively. The apparent quantum yield as high as 49.5 % was achieved at 420 nm. This work opens new insights for understanding the effect of single-atom active sites in promoting photocatalytic hydrogen production.

Contributions of exciton fine structure and hole trapping on the hole state filling effect in the transient absorption spectra of CdSe quantum dots.

The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the conduction band electrons to adsorbed methyl viologen molecules. We observe that in CdSe/CdS core/shell QDs with a high photoluminescence quantum yield of 81%, the valence band hole contributes to 22% ± 1% of the exciton bleach, while a negligible hole state filling signal is observed in CdSe core only QDs with a photoluminescence quantum yield of 17%. This hole state filling effect can be explained by a simplified valence band edge hole model that contains two sets of twofold degenerate hole levels that are responsible for the higher energy bright exciton and lower energy dark exciton states, respectively. Our result clarifies the TA spectral features of the valence band holes and provides insights into the nature of single hole states in CdSe-based QDs.

Hydrothermal Synthesis of Zinc-Doped Silica Nanospheres Simultaneously Featuring Stable Fluorescence and Long-Lived Room-Temperature Phosphorescence.

Fluorescence and phosphorescence are known as two kinds of fundamental optical signals, which have been used for myriad applications. To date, simultaneous activation of stable fluorescence and long-lived room-temperature phosphorescence (RTP) emission in the aqueous phase remains a big challenge. We prepare zinc-doped silica nanospheres (Zn@SiNSs) with fluorescence and RTP properties using a facile hydrothermal synthetic strategy. For the as-prepared Zn@SiNSs, the recombination of electrons and holes in defects and defect-stabilized excitons derived from oxygen vacancy/C=N bonds lead to the production of stable fluorescence and long-lived RTP (emission lasting for ≈9 s, quantum yield (QY): ≈33.6 %, RTP lifetime: ≈236 ms). The internal Si-O bonded networks and hydrophilic surface in Zn@SiNSs can reduce nonradiative decay to form self-protective RTP, and also provide high water solubility, excellent pH- and photostability.

Designing benzothiadiazole based non-fullerene acceptors with high open circuit voltage and higher LUMO level to increase the efficiency of organic solar cells

Non-fullerene acceptors have been widely used to fabricate the organic solar cells (OSCs) with enhanced photovoltaic efficiency owing to their superior and valuable properties over fullerene acceptors. In this regard, an effort has been made to design four new (A2-π-A1-π-A2) type acceptor molecules (MOL1, MOL2, MOL3, and MOL4) to study their optoelectronic properties and their application in improving the efficiency of OSCs. These designed molecules contain benzothiadiazole (BT) core unit which has been bridged up through (E)-2-(prop-1-enyl)thiophene with four different end capped acceptors namely 2-((5-vinylthiophen-2-yl)methylene)malononitrile (MOL1), methyl 2-cyanoacrylate (MOL2), 3-methyl-5-methylenethiazolidine-2,4-dione (MOL3), 4-vinyl- [1,2,5]thiadiazolo[3,4-c]pyridine (MOL4). Using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) optoelectronic properties have been analyzed and compared with renowned reference 4,7-bis(4-(N-hexyl-phthalimide)vinyl)benzo- [c]1,2,5-thiadiazole (PI-BT as RCAM). Appropriate frontier molecular orbitals (FMO) diagrams for each molecule have been drawn to study charge transfer. Amongst all four molecules, MOL1 with narrow bandgap is the best red-shifted compared to RCAM (545.02 nm). Furthermore, exciton binding energy, electron and hole mobility is theoretically calculated. Results showed the MOL4 with the highest VOC (3.35 V) and lowest Eb for best device efficiency. Electron and hole mobility calculations predicted these designed molecules to be better hole transporters compared to RCAM. Owing to their remarkable results these designed acceptor molecules are counted in to enhance the performance and efficiency of devices.

High Efficiency Polymer Solar Cells with Efficient Hole Transfer at Zero Highest Occupied Molecular Orbital Offset between Methylated Polymer Donor and Brominated Acceptor.

Achieving efficient charge transfer at small frontier molecular orbital offsets between donor and acceptor is crucial for high performance polymer solar cells (PSCs). Here we synthesize a new wide band gap polymer donor, PTQ11, and a new low band gap acceptor, TPT10, and report a high power conversion efficiency (PCE) PSC (PCE = 16.32%) based on PTQ11-TPT10 with zero HOMO (the highest occupied molecular orbital) offset (ΔEHOMO(D-A)). TPT10 is a derivative of Y6 with monobromine instead of bifluorine substitution, and possesses upshifted lowest unoccupied molecular orbital energy level (ELUMO) of -3.99 eV and EHOMO of -5.52 eV than Y6. PTQ11 is a derivative of low cost polymer donor PTQ10 with methyl substituent on its quinoxaline unit and shows upshifted EHOMO of -5.52 eV, stronger molecular crystallization, and better hole transport capability in comparison with PTQ10. The PSC based on PTQ11-TPT10 shows highly efficient exciton dissociation and hole transfer, so that it demonstrates a high PCE of 16.32% with a higher Voc of 0.88 V, a large Jsc of 24.79 mA cm-2, and a high FF of 74.8%, despite the zero ΔEHOMO(D-A) value between donor PTQ11 and acceptor TPT10. The PCE of 16.32% is one of the highest efficiencies in the PSCs. The results prove the feasibility of efficient hole transfer and high efficiency for the PSCs with zero ΔEHOMO(D-A), which is highly valuable for understanding the charge transfer process and achieving high PCE of PSCs.

Tailoring a Molecule’s Optical Absorbance Using Surface Plasmonics

The design of novel functional materials in silico is severely hampered by the lack of robust and computationally efficient methods for describing both molecular absorbance and screening on substrates. Here we employ our hybrid $G_0[W_0+\Delta W]$-BSE implementation, which incorporates the substrate via its screening $\Delta W$ at both the quasiparticle $G_0W_0$ level and when solving the Bethe-Salpeter equation (BSE). We show this method can be used to both efficiently and accurately describe the absorption spectra of physisorbed molecules on metal substrates and thereby tailor the molecule's absorbance by altering the surface plasmon's energy. Specifically, we investigate how the optical absorption spectra of three prototypical $\pi$-conjugated molecules: benzene (C$_6$H$_6$), terrylene (C$_{30}$H$_{16}$) and fullerene (C$_{60}$), depends on the Wigner-Seitz radius $r_s$ of the metallic substrate. To gain further understanding of the light--molecule/substrate interaction, we also study the bright exciton's electron and hole densities and their interactions with infrared active vibrational modes. Our results show that (1) benzene's bright $E_{1u}^1$ exciton at 7.0 eV, whose energy is insensitive to changes in $r_s$, could be relevant for photocatalytic dehydrogenation and polymerization reactions, (2) terrylene's bright $B_{3u}$ exciton at 2.3 eV hybridizes with the surface plasmon, allowing the tailoring of the excitonic energy and optical activation of a surface plasmon-like exciton, and (3) fullerene's $\pi-\pi^*$ bright and dark excitons at 6.4 and 6.8 eV hybridize with the surface plasmon, resulting in the tailoring of their excitonic energy and the activation of both a surface plasmon-like exciton and a dark quadrupolar mode via symmetry breaking by the substrate.

Understanding the enhancement of responsitivity in perovskite/organic semiconductor bilayer-structured photodetectors

Abstract Responsitivity is an important figure of merit for photodetectors. Here we demonstrated a simple yet efficient way to enhance the responsitivity of methylammonium lead iodide (MAPbI3)-based photodetectors by utilizing organic semiconductors (OSCs) to form MAPbI3/OSC bilayer structures. These bilayer photodetectors are fabricated with a facile and low-cost spin-coating method, yet the responsitivity can be more than one order of magnitude higher than that of pristine MAPbI3 photodetectors. More importantly, the physics behind the enhancement of responsitivity was analyzed and revealed by inspecting the physical processes in terms of light absorption, exciton dissociation and charge transport. The results indicate that the responsitivity enhancement is mainly attributable to the more efficient exciton dissociation and hole transport processes in the bilayer photodetectors.

Predicting Operational Stability for Organic Light-Emitting Diodes with Exciplex Cohosts.

Organic light‐emitting diodes (OLEDs) employing exciplex cohosts have gained attractive interest due to the promising high efficiency, low driving voltage, and potential low cost in future solid‐state lighting sources and full‐color displays. However, their device lifetime is still the most challenging weakness and rarely studied, which is regarded as a time consuming and complicated work. Therefore, a simplified but effective and comprehensive approach is demonstrated to give prediction for the exciplex cohosts operating lifespan and analyze their possible degradation mechanisms by considering molecular dissociated activation energy with internal exciton dynamics correlations. As a consequence, strong chemical bond stability for the hole transport moieties and rapid reactive exciton relaxation have the intrinsic talent to access potentially long‐lived exciplex cohosts, achieving an extended lifetime of 10169 h for the predicted long‐lived exciplex cohost OLEDs. Degradation behaviors further confirm that the deteriorated source is attributed to the formation of exciton quenchers and hole traps from excited states and charged‐excited states, respectively. The current findings establish a universal technique to screen the stable exciplex cohost candidates with economic time consumption and expenses.

One-Step Solvothermal Formation of Pt Nanoparticles Decorated Pt2+-Doped α-Fe2O3 Nanoplates with Enhanced Photocatalytic O2 Evolution

The photooxidation of water into O2 has been identified as the barrier of water-splitting, and light-driven water oxidation catalysts have been intensively explored to develop highly active water splitting materials. Despite the fascinating advantages for photocatalytic water oxidation, such as abundance in nature, inexpensiveness, low toxicity, thermo/photostability, and suitable electronic band structures, hematite α-Fe2O3 is a poor photocatalyst for water oxidation due to its short exciton lifetime and hole diffusion length, weak carrier mobility, and shallow sunlight penetration depth. In this work, we have synthesized Pt nanoparticles decorated Pt2+-doped α-Fe2O3 nanoplates (Pt/Pt-Fe2O3 NPs) by a one-step solvothermal route which exhibit the enhanced photoactivity and photostability for water oxidation. The introduction of the Pt into the α-Fe2O3 by the means of elemental doping and nanoparticle decoration accounts for the enhanced performance. The doping of Pt2+ into α-Fe2O3 improves the isolation e...

Electroabsorption Studies of Multicolored Lead Halide Perovskite Nanocrystalline Solid Films

Electroabsorption (E-A) spectra of lead halide perovskites, that is, CH3NH3PbI3–xBrx (x = 0–3), which show large spectral shift depending on the ratio between iodine and bromide, have been measured. By analyzing E-A spectra with the integral method, spectral shape of the absorption spectra for the first exciton band and binding energy of exciton have been determined. Magnitudes of the change in electric dipole moment and molecular polarizability following excitation into the exciton state have been also evaluated in these perovskites. The binding energy of electron and hole in exciton of these materials as well as the magnitude of change in electric dipole moment following exciton absorption is roughly the same, suggesting that the difference of the photoenergy conversion in photovoltaic cells using these materials comes from the difference in light harvesting effect and difference in carrier mobility, not from the difference in carrier generation efficiency. The frequency-dependent third-order nonlinear ...

Bayesian Spectroscopy of Admixed Photoluminescence Spectra with Exciton, Biexciton and Electron Hole Droplet States in a GaAs/AlAs Type-II Superlattice

We have analyzed a photoluminescence (PL) spectrum in a GaAs/AlAs type-II superlattice and have statistically confirmed stable existence of an electron-hole droplet (EHD) state. Although the PL spectrum consists of spectral components of the EHD, exciton and biexciton states, we have succeeded in decomposing the admixed spectrum into the respective components. In addition, posterior probability distributions of spectral parameters have been obtained in all spectral components from 30,000 times samplings through a replica exchange Monte Carlo method. By using mean values of their posterior probabilities, the PL spectrum can be completely reproduced by the sum of these spectral components. The stability energy of the EHD state has a sharp distribution in its posterior probability and has been confirmed to be enough larger than the ambient thermal energy. This fact is a strong evidence for the stable formation of the EHD state in the GaAs/AlAs type-II superlattice.