Photo-generated Excitons Research Articles

Surface plasmon-mediated photoluminescence boost in graphene-covered CsPbBr3 quantum dots

The optical properties of graphene (Gr)-covered CsPbBr3 quantum dots (QDs) were investigated using micro-photoluminescence spectroscopy, revealing a remarkable three orders of magnitude enhancement in photoluminescence (PL) intensity compared to bare CsPbBr3 QDs. To elucidate the underlying mechanisms, we combined experimental techniques with density functional theory (DFT) calculations. DFT simulations showed that the graphene layer generates interfacial electrostatic potential barriers when in contact with the CsPbBr3 surface, impeding carrier leakage from perovskite to graphene and enhancing radiative recombination. Additionally, graphene passivates CsPbBr3 surface defect states, suppressing nonradiative recombination of photo-generated carriers. Our study also revealed that graphene becomes n-doped upon contact with CsPbBr3 QDs, activating its plasmon mode. This mode resonantly couples with photo-generated excitons in the perovskite. The momentum mismatch between graphene plasmons and free-space photons is resolved through plasmon scattering at Gr/CsPbBr3 interface corrugations, facilitating the observed super-bright emission. These findings highlight the critical role of graphene as a top contact in dramatically enhancing CsPbBr3 QDs’ PL. Our work advances the understanding of graphene-perovskite interfaces and opens new avenues for designing high-efficiency optoelectronic devices. The multifaceted enhancement mechanisms uncovered provide valuable insights for future research in nanophotonics and materials science, potentially leading to breakthroughs in light-emitting technologies.

Construction of a 2D/2D Vs-ZnIn2S4/Zn-TCPP heterojunction for effective charge transfer to enhance photocatalytic hydrogen evolution

The integration of inorganic and organic semiconductor components in composites has tremendous potential in photocatalysis, owing to the ultrafast photogenerated exciton dissociation and slow carrier recombination kinetics at their interfaces, which can result in enhanced photocatalytic performances. By exploiting the synergy among morphology modulation, sulfur vacancy (Vs) engineering, and close interface control, a novel inorganic–organic Vs-ZnIn2S4/ tetrakis (4-carboxyphenyl) zinc porphyrin (Zn-TCPP) composite for photocatalytic hydrogen evolution (PHE) was successfully synthesized via microwave-assisted solvothermal and self-assembly methods. The cocatalyst-free PHE rate of Vs-ZnIn2S4/Zn-TCPP (ZZT-20 %) reaches 8.997 mmol h−1 g−1 under visible light irradiation, which is 10.9, 2.2, and 67.1 times that of ZnIn2S4, Vs-ZnIn2S4, and Zn-TCPP, respectively. Moreover, the apparent quantum yield of ZZT-20 % under 420 nm monochromatic light (1.11 %) clearly exceeds that of Vs-ZnIn2S4. Testing experiments and DFT calculations revealed that the significantly enhanced PHE activity can be mainly attributed to the Z-scheme charge transfer mechanism, which greatly improves the visible light response range and redox potentials. Remarkably, Vs sites act as electron traps, inducing the vertical transport of electrons within the bulk phase and their accumulation at the surface Zn–S bonds. Simultaneously, van der Waals forces and weak hydrogen bonds between Vs-ZnIn2S4 and the Zn-TCPP interface cause the rapid migration of photogenerated electrons to Zn-TCPP. This work not only reveals new insights into the structure–property relationships of inorganic–organic Vs-ZnIn2S4 and Zn-TCPP composites, but also provides an appropriate strategy for cleaner water resourcelization and sustainable solar energy conversion.

P-Doped Carbon Nanotubes As Light Absorbers and Electron Donors in Photovoltaics

Doping is a ubiquitous and powerful tool used to alter a material’s electronic properties. Conventional Si solar cells would not exist without it, and it has been used to increase the efficiency of organic and inorganic photovoltaics alike. While doped carbon nanotubes (CNTs) have been the subject of numerous spectroscopic studies, few investigations have examined their performance in photovoltaic devices. Here, we incorporate chemically p-doped CNTs into photovoltaic heterojunctions with C60. In a conventional CNT–C60 heterojunction device, photocurrent can be produced when photogenerated excitons in the CNT layer diffuse to the CNT–C60 heterointerface. The offset in conduction band energy drives dissociation of these excitons via photoelectron transfer from CNT to C60, which yields separated charges that can be collected. Our objective here was to determine the following: 1) does p-doping increase or decrease the efficiency with which excitons dissociate into electrons and holes? 2) Do p-dopants trap excitons and inhibit their diffusion to the heterointerface? 3) How efficiently can a photogenerated trion (exciton coupled to an injected hole) be dissociated to produce a photocurrent?To answer these questions, we fabricated polymer wrapped (6,5) CNT–C60 heterojunctions over a range of CNT thickness with various levels of p-doping introduced by triethyloxonium hexachloroantimonate treatment. We measured photocurrent to quantify relative photoelectron transfer efficiency for both excitons and trions and compared this to photoluminescence and transient absorbance measurements on doped CNT films. We found that p-doping decreases photocurrent because injected holes bleach the CNTs’ S11 transition and decrease the amount of light absorbed. Meanwhile, the photogenerated exciton to collected electron transfer efficiency remains unchanged for doping densities up to 50μm-1. In contrast, photoluminescence quantum yield drops by over 90% at an equivalent doping density. We attribute this disparity to the fast electron transfer at the CNT–C60 interface, which occurs on the order of femtoseconds. We also found that moderate doping can promote photocurrent at the trion peak, with a photogenerated trion to collected electron transfer efficiency about 1/4 that of the photogenerated exciton to collected electron transfer efficiency. Photocurrent at the trion absorption peak is invariant with reverse bias up to 0.3 V, despite the charged nature of the trion, supporting evidence for the immobile nature of trions. These results indicate that p-CNTs are a viable material for use in photovoltaics, albeit with weaker absorption, and that photocurrent from trions is possible, although with lower quantum efficiency. Figure 1

Elucidating the Impact of the Dielectric Environment on Excitons and Trions in 2D Semiconductor Electrodes

Transition metal dichalcogenides (TMDs) (MX2; M = Mo, W; X = S, Se) are a promising class of materials for photoelectrochemical solar energy conversion applications due to their optimal bandgaps and natural abundance. In this configuration, TMDs are placed onto a conductive substrate and employed as a photoelectrode. A key step in the solar energy conversion process is to photo-excite the semiconductor and dissociate the photogenerated excitons to drive a reaction of interest (e.g., H2 evolution). Because these materials are so physically thin, the excitons are sensitive to the local environment. In this work, we characterize the exciton and trion formation behavior as a function of solvent dielectric, ionic strength, and chemistry. As the dielectric constant of the solvent decreases, trion formation increases. The lower dielectric solvent causes lower Coulombic screening, increasing the probability that an exciton will encounter another electron to generate a trion. I will discuss how these findings relate to a working photoelectrochemical cell.

Magnetic fields reveal signatures of triplet-pair multi-exciton photoluminescence in singlet fission

The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton, respectively. The role of the excimer state and the nature of the triplet-pair state in these processes have been a matter of contention. Here we analyse the room temperature time-resolved emission of a neat liquid singlet fission chromophore and show that it exhibits three spectral components: two that correspond to the bright singlet and excimer states and a third component that becomes more prominent during triplet fusion. This spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly coupled triplet pairs. It is thus attributed to a strongly coupled triplet pair state. These observations unite the view that there is an emissive intermediate in singlet fission and triplet fusion, distinct from the broad, unstructured excimer emission.

Size-Dependent Photocatalytic Reactivity of Conjugated Microporous Polymer Nanoparticles.

Particle size is a critical factor for improving photocatalytic reactivity of conjugated microporous polymers (CMPs) as mass transfer in the porous materials is often the rate-limiting step. However, due to the synthetic challenge of controlling the size of CMPs, the impact of particle size is yet to be investigated. To address this problem, a simple and versatile dispersion polymerization route that can synthesize dispersible CMP nanoparticles with controlled size from 15 to 180nm is proposed. Leveraging the precise control of the size, it is demonstrated that smaller CMP nanoparticles have dramatically higher photocatalytic reactivity in various organic transformations, achieving more than 1000% enhancement in the reaction rates by decreasing the size from 180 to 15nm. The size-dependent photocatalytic reactivity is further scrutinized using a kinetic model and transient absorption spectroscopy, revealing that only the initial 5nm-thick surface layer of CMP nanoparticles is involved in the photocatalytic reactions because of internal mass transfer limitations. This finding substantiates the potential of small CMP nanoparticles to efficiently use photo-generated excitons and improve energy-efficiency of numerous photocatalytic reactions.

Perovskite Quantum Heterostructure Constructed by Halide Mixing between a Single CsPbI3 Nanocrystal and an Individual CsPbBr3 Microplate.

Ion migration is significantly enhanced in lead-halide perovskites with a soft crystal lattice, which can promote the formation of a heterogeneous interface between two such materials with different halide-anion compositions. Here we have deposited a single CsPbI3 nanocrystal (NC) on top of an individual CsPbBr3 microplate to create a mixed-halide CsPbBrxI3-x (0 < x < 3) NC by means of the anion exchange process. The formation of a CsPbBrxI3-x/CsPbBr3 heterostructure is confirmed by the much-enlarged geometric volume of the CsPbBrxI3-x NC as compared to the original CsPbI3 one, as well as by its capability of receiving photogenerated excitons from the CsPbBr3 microplate with a larger bandgap energy. The quantum nature of this heterostructure is reflected from single-photon emission of the composing CsPbBrxI3-x NC, which can also be bulk-like during phase segregation to demonstrate a red shift in the photoluminescence peak that is opposite to the common trend observed in smaller-sized mixed-halide NCs.

Functionalized Linear Conjugated Polymer/TiO2 Heterojunctions for Significantly Enhancing Photocatalytic H2 Evolution.

Conjugated polymers (CPs) have attracted much attention in recent years due to their structural abundance and tunable energy bands. Compared with CP-based materials, the inorganic semiconductor TiO2 has the advantages of low cost, non-toxicity and high photocatalytic hydrogen production (PHP) performance. However, studies on polymeric-inorganic heterojunctions, composed of D-A type CPs and TiO2, for boosting the PHP efficiency are still rare. Herein, an elucidation that the photocatalytic hydrogen evolution activity can actually be improved by forming polymeric-inorganic heterojunctions TFl@TiO2, TS@TiO2 and TSO2@TiO2, facilely synthesized through efficient in situ direct C-H arylation polymerization, is given. The compatible energy levels between virgin TiO2 and polymeric semiconductors enable the resulting functionalized CP@TiO2 heterojunctions to exhibit a considerable photocatalytic hydrogen evolution rate (HER). Especially, the HER of TSO2@TiO2 heterojunction reaches up to 11,220 μmol g-1 h-1, approximately 5.47 and 1260 times higher than that of pristine TSO2 and TiO2 photocatalysts. The intrinsic merits of a donor-acceptor conjugated polymer and the interfacial interaction between CP and TiO2 account for the excellent PHP activity, facilitating the separation of photo-generated excitons. Considering the outstanding PHP behavior, our work discloses that the coupling of inorganic semiconductors and suitable D-A conjugated CPs would play significant roles in the photocatalysis community.

MSM UV photodetector with low dark current based on GaInO/SrTiO3 heterojunction

Doping and heterojunction manufacturing processes are widely employed techniques for modifying the energy band of wide band gap semiconductors. In this work, the sol-gel method was employed to introduce indium (In) into Ga2O3, leading to the formation of the GaInO (GIO) thin film, which was subsequently combined with SrTiO3 (STO) to form the heterojunction. The heterojunction was utilized for the fabrication of metal-semiconductor-metal (MSM) structured ultraviolet (UV) photodetectors (PDs). The existence of a built-in electric field (Ebi) generated by the heterojunction at the contact interface of GIO and STO can diminish the density of majority carriers in dark conditions, consequently leading to a decrease in the dark current. The photo-to-dark suppression ratio of GIO/STO exceeds three orders of magnitude (1.157 × 103), and the dark current of the composite GIO/STO MSM UVPDs (0.178 nA) is one order of magnitude lower than that of the individual GIO (4.028 nA) at 5V bias. Additionally, the Ebi may accelerate the exciton separation for photo-generated exciton, thereby accelerating the response speed. The GIO/STO device demonstrates a shorter rise time of 1.21 s and a decay time of 0.42 s. The utilization of both energy band modification methods offers promising prospects for the application of GIO/STO MSM UVPDs.

(Invited) Design of Perovskites for High-Performance Electronics and Optoelectronics

The success of lead halide perovskites as active materials in optoelectronic technologies is often associated with the unique properties of their optical excitations. Specifically, the photo-generated excitons in perovskites are responsible for the large absorption coefficients close to their band gap, reaching as high as 105 cm-1. Due to the advent of nanotechnology, perovskite materials can be readily fabricated into nanoscale configurations with different dimensionalities, and more importantly, the corresponding excitonic effects could be further enhanced. On the other hand, at length scales down to the nanoscale regime, surface features as well as quantum confinement effects, become dominant in regulating the advanced material properties of perovskite materials. [1, 2] In recent years, our group focused on the synthesis and characterization of metal halide perovskites with different forms, ranging from bulk film, microplate, and nanosheet, to nanowire. The corresponding physical properties and device applications were also systematically studied based on their widely tunable dimensionality, morphologies, and compositions.For perovskite bulk films, surface defects and bulk structural order significantly affect device performance. Through optimizing processing techniques, self-assemble quasi-2D perovskite films with graded phase distribution were successfully prepared by our group. [3] Gradient type-II band alignments along the out-of-plane direction of perovskites with spontaneous separation of photo-generated electrons and holes are obtained, which is further employed to construct self-powered vertical-structure photodetectors for the first time. Without any driving voltage, the device exhibited impressive performance with responsivity up to 444 mA/W and ultrashort response time down to 52 µs. In addition, to probe the intrinsic material properties of crystalline perovskites, freestanding MAPbI3 nanosheets, and lead-free Cs3Sb2I9 microplates were fabricated by two-step chemical vapor deposition method, in which excellent optoelectronic performance (e.g., responsively of MAPbI3 nanosheet is measured to be 40 A/W) together with ultra-fast response speed (down to 58 µs) and superior thermal stability were obtained. [4, 5]For nanostructured perovskites, understanding the dimensional features and their impact on the materials and devices is becoming increasingly important. For the first time, we reported the direct vapor-liquid-solid growth of single-crystalline all-inorganic lead halide perovskite (i.e., CsPbX3; X = Cl, Br, or I) NWs using Sn as catalyst seeds. [6] These NWs exhibited high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 1012 Jones toward the visible light regime. Field-effect transistors based on individual CsPbX3 NWs were also fabricated to show the impressive carrier mobility of 3.05 cm2/Vs, being higher than other all-inorganic perovskite devices. After that, the Au catalyst seeds were used to avoid the Sn-related impurity doping, by which lower dark current and higher detectivity were achieved in Au-seeded CsPbI3 NWs. Besides, the realization of high-mobility CsPbBr3 NW devices is reported via a simple surface charge transfer doping strategy. [7] After MoO3 decoration and device fabrication, the hole mobility of CsPbBr3/MoO3 core-shell NW device is significantly enhanced to 23.3 cm2/Vs. All these results provide important guidelines for the further improvement of these perovskite nanostructures for practical utilization.

(Allen J. Bard Award in Electrochemical Science) Toward Electrochemical Energy Conversion Via Metal-Organic Framework Materials

Consistent with the many energy-centric interests and pioneering contributions by Prof. Bard, this presentation will touch on catalytic electrochemical and photochemical energy conversion as enabled by metal-organic framework (MOF) materials. The presentation will focus on a couple of case studies that illustrate how nominally insulating MOF films can be persuaded to transport charge, deliver photo-generated molecular excitons, and catalytically drive energy-relevant, multi-electron chemical transformations.

Determination of Free Charge Carrier Luminescence and Quasi‐Fermi Level Separation in Organic Solar Cells via Transient Photoluminescence Measurements

AbstractThe detection of photoluminescence (PL) is extremely valuable for photovoltaic technologies as it provides direct information about the free charge carrier densities and therefore about the separation of the quasi‐Fermi levels (ΔEF). However, for organic solar cells the PL is strongly dominated by photogenerated excitons that in part decay radiatively before they form free charge carriers via dissociation at a donor/acceptor interface. For this reason, it is impossible to deduce the free charge carrier densities and ΔEF directly from the PL signal. To overcome this severe limitation, a new approach is developed that allows disentangling the luminescence of photogenerated excitons from the one of free charge carriers. Due to the large difference in respective lifetimes—for photogenerated excitons it is ≤ ns whereas for free charge carriers it is in the µs‐range—this is achieved by time‐resolved PL measurements. For highly efficient organic solar cells, it is found that ΔEF determined from these PL transients matches perfectly with the electrical voltage. Moreover, the new method also allows for the determination of ΔEF from mere absorber films without electrodes and thus paves the way for a better understanding of organic solar cells.

TiO2/WO3/graphene for photocatalytic H2 generation and benzene removal: Widely employed still an ambiguous system

Clean energy, as well as air and water pollution, have emerged as significant challenges in today's society. Photocatalysis offers a potential solution to address these issues. It is an advanced oxidation process that utilises light to activate a semiconductor. Among photocatalytically active materials, titanium dioxide (TiO2) semiconductors are widely recognised. However, the performance of TiO2 is hindered by its wide band gap (∼3.2 eV) and high recombination rate of photo-generated electron-hole (e−−h+) pairs. To overcome these limitations, TiO2 in heterojunction with tungsten trioxide (WO3) has gained substantial attention for various photocatalytic applications. However, the literature reports contradictory behaviours due to variations in synthesis techniques and photocatalytic applications.In this study, we extensively investigated the photocatalytic properties of the TiO2/WO3 system for the removal of gaseous benzene, and H2 generation. To enhance the transport and lifetime of photo-generated excitons, graphene nanoplatelets were incorporated into the TiO2/WO3 system. We examined several parameters that influenced the photocatalytic activity of the synthesised materials, including the WO3 to TiO2 ratio, the presence of graphene, and the specific photocatalytic application. Interestingly, the position of the conduction bands played a crucial role in hydrogen generation. The TiO2/WO3 system exhibited a type-II heterojunction. While the hybridisation of TiO2 with WO3 was found to be detrimental to light-induced benzene removal and H2 generation, the modification of TiO2/WO3 with graphene nanoplatelets significantly improved the photocatalytic hydrogen generation. Notably, the specimen with 15 mol% WO3 and 1 wt% graphene demonstrated a five-fold increase in yield compared to its counterpart without graphene. These findings provide valuable insights for data-driven catalysis research.

Organic near‐infrared photodetectors with photoconductivity‐enhanced performance

AbstractOrganic near‐infrared (NIR) photodetectors with essential applications in medical diagnostics, night vision, remote sensing, and optical communications have attracted intensive research interest. Compared with most conventional inorganic counterparts, organic semiconductors usually have higher absorption coefficients, and their thin active layer could be sufficient to absorb most incident light for effective photogeneration. However, due to the relatively poor charge mobility of organic materials, it remains challenging to inhibit the photogenerated exciton recombination and effectively extract carriers to their respective electrodes. Herein, this challenge was addressed by increasing matrix conductivities of a ternary active layer (D–A–D structure NIR absorber [2TT‐oC6B]:poly(N,N′‐bis‐4‐butylphenyl‐N,N′‐bisphenyl)benzidin [PolyTPD]:[6,6]‐phenyl‐C61‐butyric acid methyl ester [PCBM] = 1:1:1) upon in situ incident light illumination, significantly accelerating charge transport through percolated interpenetrating paths. The greatly enhanced photoconductivity under illumination is intrinsically related to the unique donor–acceptor molecular structures of PolyTPD and 2TT‐oC6B, whereas stable intermolecular interaction has been verified by systematic molecular dynamics simulation. In addition, an ultrafast charge transfer time of 0.56 ps from the NIR aggregation‐induced luminogens of 2TT‐oC6B absorber to PolyTPD and PCBM measured by femtosecond transient absorption spectroscopy is beneficial for effective exciton dissociation. The solution‐processed organic NIR photodetector exhibits a fast response time of 83 μs and a linear dynamic range value of 111 dB under illumination of 830 nm. Therefore, our work has opened up a pioneering window to enhance photoconductivity through in situ photoirradiation and benefit NIR photodetectors as well as other optoelectronic devices.

PbSe nanorod-quantum dot bulk nano-heterojunction solar cells generating multiple excitons with record photo conversion efficiencies

Multi-exciton-generation (MEG) process is critical due to its potential to push solar cell photoconversion efficiencies (PCEs) beyond theoretical limits. However, cells with this feature are rare, as the demonstration of this characteristic property in a working cell is challenging. Herein, a donor-acceptor type bulk nano-heterojunction layer, prepared by blending two nanostructures known to have high MEG yields, i.e., PbSe quantum dot-QDs and PbSe nanorods-NRs, is utilized as the light absorbing active layer. Transmission electron microscopy images show that NR-donors are surrounded by QD-acceptors, indicating the formation of optimal donor-acceptor interfaces that allow efficient dissociation of photo-generated excitons. Effective charge separation across the NR/QD interface is also demonstrated with valence and conduction band offsets measured in the range of 0.20–0.25 eV. It is found that the type of the hole transport layer has a profound effect in reducing the electron back injection and increasing the MEG efficiency/yield. Cells with 0.97 eV NR-donor band gap exhibit a PCE of 4.09% with a peak internal quantum efficiency greater than 100%. Cells, outperforming the previously reported MEG-based cells comprising PbSe QDs or NRs, demonstrate a staircase-like response in external quantum efficiency, which is considered as the most obvious indicator of an ideal MEG system.

Boron nitride quantum dots modified carbon-defects ultra-thin porous carbon nitride: double channels and quantum size effects facilitate photogenerated carrier migration and exciton dissociation

Graphite carbon nitride possesses great promise for visible photocatalysis, but the bulk carbon nitride prepared from nitrogen-rich precursors such as melamine has inherent drawbacks such as retarded photogenerated carrier migration and exciton effects, which limit its application. Herein, we constructed a novel Boron nitride quantum dots modified carbon-defects ultra-thin porous carbon nitride (BNQDs/Vc-UPCN). The double channels were constructed by carbon-defects structure and Boron nitride quantum effect to overcome its inherent drawbacks and applied to the photodegradation of common persistent organic pollutants (methylene blue). The structure, porosity, elemental composition, optical properties, photoelectrochemical properties, and photocatalytic properties of the prepared BNQDs/Vc-UPCN composites were investigated using various characterization methods. Meanwhile, the results of radical trapping experiments and electron spin resonance characterization demonstrated that BNQDs/Vc-UPCN promote molecular oxygen activation more than Vc-UPCN did. In terms of degradation effect, the best sample (BC-1) is 10 times more effective than the initial sample (BCN). This study proposes an effective mechanism for constructing novel visible-light-driven photocatalysts using carbon-defects ultra-thin structures and quantum dots, which can be used for the treatment of organic pollutants.

Broadening of the Response Spectrum and Gain in the Optoelectronic Performance of P3HT:PC61BM/ZnO NWs “Embedded” Heterojunction Photodetector

The combination of the inorganic semiconductor material zinc oxide (ZnO) and the organic material poly(3-hexylthiophene) (P3HT) with 6,6-phenyl C61-butyric acid methyl ester (PC61BM) blends (P3HT:PC61BM) has been widely studied for photovoltaic applications. In this paper, a P3HT:PC61BM/ZnO nanowires (NWs) photodetector (PD) with wide spectral responsivity was prepared. The photoresponse performance of PDs was compared and analyzed by tests and calculations, and it was demonstrated that the organic material could well broaden the absorption and response spectral range of the ZnO NWs PD, and the overall responsivity and external quantum efficiency were significantly improved. p-Type P3HT:PC61BM is “embedded” with n-type ZnO NWs in a large area, and the built-in electric field of the heterojunction can effectively separate photogenerated excitons and effectively promote electron transport. The ZnO NWs play the role of electron-transport “channels”, which lead to good improvement of the optoelectronic performance of the ZnO NWs PD.

Many-Body Correlations and Exciton Complexes in CsPbBr3 Quantum Dots.

All-inorganic lead-halide perovskite (LHP) (CsPbX3 , X=Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light-emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser), as well as for devices exploiting strong light-matter interaction at room temperature. Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, for example, by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity in quantum light sources. Here, by combining cryogenic single-QD photoluminescence spectroscopy with configuration-interaction (CI) calculations, the size-dependent trion and biexciton binding energies are addressed. Trion binding energies increase from 7to 17meV for QD sizes decreasing from 30to 9nm, while the biexciton binding energies increase from 15to 30meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates from the one of the single excitons, potentially as a result of coupling to the lattice in the multiexciton regime. The findings here provide a deep insight into the multiexciton properties in all-inorganic LHP QDs, essential for classical and quantum optoelectronic devices.

Spin Preserved Exciton Funneling through a Polaronic Intermediate State in Layered Perovskite Single Crystals

AbstractThe spintronic device is another application of perovskite beyond solar cells, light emitting diodes, and photodetectors. Spin relaxation with a lifetime of several ps has been observed in both 3D and 2D perovskite. In this study, Ruddlesden–Popper (RP) perovskite single crystals with mixed n and n + 1 phase are synthesized. It is found that the spin‐polarized exciton can funnel from n phase to n + 1 phase with spin information retained effectively. Interestingly, the spin lifetime of the funneled exciton in n + 1 phase is much longer than that in the pristine n phase. This is attributed to the fact that the funneling is realized through a polaronic intermediate state, in which the spin relaxation is much slower. The polaronic intermediate states originate from the interaction between photogenerated excitons and the lattice distortion. The formation of the polaronic states can reduce overlap of electron–hole wave function and modified lattice symmetry; these two changes contribute to retain the exciton spin information in such polaronic states. The findings deepen the understanding of spin‐polarized exciton transportation and relaxation dynamics and provide a potentially novel approach to enhance the spin relaxation lifetime of perovskite single crystals.

A Mini Review on the Development of Conjugated Polymers: Steps towards the Commercialization of Organic Solar Cells

This review article covers the synthesis and design of conjugated polymers for carefully adjusting energy levels and energy band gap (EBG) to achieve the desired photovoltaic performance. The formation of bonds and the delocalization of electrons over conjugated chains are both explained by the molecular orbital theory (MOT). The intrinsic characteristics that classify conjugated polymers as semiconducting materials come from the EBG of organic molecules. A quinoid mesomeric structure (D-A ↔ D+ = A-) forms across the major backbones of the polymer as a result of alternating donor-acceptor segments contributing to the pull-push driving force between neighboring units, resulting in a smaller optical EBG. Furthermore, one of the most crucial factors in achieving excellent performance of the polymer is improving the morphology of the active layer. In order to improve exciton diffusion, dissociation, and charge transport, the nanoscale morphology ensures nanometer phase separation between donor and acceptor components in the active layer. It was demonstrated that because of the exciton's short lifetime, only small diffusion distances (10-20 nm) are needed for all photo-generated excitons to reach the interfacial region where they can separate into free charge carriers. There is a comprehensive explanation of the architecture of organic solar cells using single layer, bilayer, and bulk heterojunction (BHJ) devices. The short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) all have a significant impact on the performance of organic solar cells (OSCs). Since the BHJ concept was first proposed, significant advancement and quick configuration development of these devices have been accomplished. Due to their ability to combine great optical and electronic properties with strong thermal and chemical stability, conjugated polymers are unique semiconducting materials that are used in a wide range of applications. According to the fundamental operating theories of OSCs, unlike inorganic semiconductors such as silicon solar cells, organic photovoltaic devices are unable to produce free carrier charges (holes and electrons). To overcome the Coulombic attraction and separate the excitons into free charges in the interfacial region, organic semiconductors require an additional thermodynamic driving force. From the molecular engineering of conjugated polymers, it was discovered that the most crucial obstacles to achieving the most desirable properties are the design and synthesis of conjugated polymers toward optimal p-type materials. Along with plastic solar cells (PSCs), these materials have extended to a number of different applications such as light-emitting diodes (LEDs) and field-effect transistors (FETs). Additionally, the topics of fluorene and carbazole as donor units in conjugated polymers are covered. The Stille, Suzuki, and Sonogashira coupling reactions widely used to synthesize alternating D-A copolymers are also presented. Moreover, conjugated polymers based on anthracene that can be used in solar cells are covered.