Polaron Generation Research Articles

Cutting-edge technologies and recent modernization on multifunctional perovskite materials for green energy conversion and spintronic applications

This featured letter summarizes the recent developments in green energy and spintronic technologies based on multifunctional i.e., multiferroic (MF) and magnetoelectric (ME) materials. These are non-toxic materials, find applications in spintronic, non-volatile memory devices, microelectromechanical systems (MEMS), photovoltaics and next generation IC miniaturization devices. Recent studies on MF and ME materials have probed and analyzed significant properties such as evidence of photo-response and significant power conversion efficiency (PCE) of multiferroic-based solar cells (SCs), evidence of electro-caloric effect (ECE) which find usage in non-toxic solid state refrigeration devices, photo- and electro-catalytic activities for green hydrogen generation, magnon-phonon coupling and existence of magnon polarons which can facilitate spintronic device technology and thermal Hall effect and, hybrid nano-generators utilized to empower portable electronic devices. All such applications are comprehensively analyzed in-depth in this letter.

Long-lived, pulse-induced transient absorption inLiNb1-x Ta x O3 (0≤x≤1) solid solutions: the case of three intrinsic defect sites for electron localization with strong coupling

Abstract Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied in LiNb1-x Ta x O3 (0≤x≤1, LNT) solid solutions with the aim of studying transient optical nonlinearities associated with the formation, transport and recombination of optically generated small bound electron polarons with strong coupling to the lattice. As a result, a pronounced, long-lived transient absorption is uncovered for LNT which exceed lifetimes and starting amplitudes of LiNbO3 (LN) and LiTaO3 (LT) by a factor of up to 100 and 10, respectively. The transients reveal a stretched-exponential decay behavior and a thermally activated process which provide strong evidence for an underlying hopping transport mechanism of small bound polarons. All findings are discussed in comparison to the model systems LN and LT within the framework of appropriate band models and optical generation of polarons via two-photon excitation. To explain the significant differences, the simultaneous presence of Nb5+ Li , Ta5+ Li antisites, and Ta5+ V interstitial defects, i.e. a mixture of the intrinsic defects widely established for LN and LT, is assumed for LNT.

Effect of temperature on polaronic transport in CeO2 thin-film.

The outstanding catalytic property of cerium oxide (CeO2) strongly depends on the polaron formation due to the oxygen vacancy (V̈O) defect and Ce4+ to Ce3+ transformation. Temperature plays an important role in the case of polaron generation in CeO2 and highly influences its electrical transport properties. Therefore, a much needed attention is required for detailed understanding of the effect of temperature on polaron formation and oxygen vacancy migration to get an idea about the improvement in the redox property of ceria. In this work, we have probed the generation of polarons in CeO2 thin-film deposited on a silicon (Si) substrate using the resonance photoemission spectroscopy (RPES) study. The RPES data show an increase in polaron density at the substrate-film interface of the thermally annealed film, indicating the formation of an interfacial Ce2O3 layer, which is, indeed, a phase change from the cubic to hexagonal structure. This leads to a modified electronic band structure, which has an impact on the capacitance-voltage (C-V) characteristics. This result nicely correlates the microscopic property of polarons and the macroscopic transport property of ceria.

Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation.

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices.

Recent advances of computational simulation on polarons in organic-inorganic hybrid perovskites

Organic-inorganic hybrid perovskites are prized for their exceptional properties and ease of use, finding applications in high-efficiency, cost-effective solar cells. Hybrid perovskite materials, belonging to the perovskite family, possess unique crystallographic and electronic structures that benefit from versatile phase transitions and component modifications. The crystal structure and electronic characteristics of perovskites facilitate the generation of polarons, which are quasiparticles that readily form in organic-inorganic hybrid perovskites due to the coupling of excess electrons or holes to ionic vibrations and play a pivotal role in shaping the electronic properties. This review delves into the polarons within organic-inorganic hybrid perovskites, in which the formation and interaction of polarons within the crystal lattice and their profound impact on material properties. Computational simulations offer insights into polaron characteristics, such as phonon density of states, carrier-phonon coupling, and polaron dynamics. Therefore, it is of vital importance to make clear the fundamental properties focusing on polarons in perovskite, since a comprehensive understanding of them would significantly facilitate the enhancement of the electronic and optoelectronic characteristics, and eventually contribute to the release of their full potential in real-world applications

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

Novel Organic Superbase Dopants for Ultraefficient N-Doping of Organic Semiconductors.

Doping is a powerful technique for engineering the electrical properties of organic semiconductors (OSCs), yet efficient n-doping of OSCs remains a central challenge. Herein, the discovery of two organic superbase dopants, namely P2-t-Bu and P4-t-Bu as ultra-efficient n-dopants for OSCs is reported. Typical n-type semiconductors such as N2200 and PC61 BM are shown to experience a significant increase of conductivity upon doping by the two dopants. In particular, the optimized electrical conductivity of P2-t-Bu-doped PC61 BM reaches a record-high value of 2.64Scm-1 . The polaron generation efficiency of P2-t-Bu-doped in PC61 BM is found to be over 35%, which is 2-3 times higher than that of benchmark n-dopant N-DMBI. In addition, a deprotonation-initiated, nucleophilic-attack-based n-doping mechanism is proposed for the organic superbases, which involves the deprotonation of OSC molecules, the nucleophilic attack of the resulting carbanions on the OSC's π-bonds, and the subsequent n-doping through single electron transfer process between the anionized and neutral OSCs. This work highlights organic superbases as promising n-dopants for OSCs and opens up opportunities to explore and develop highly efficient n-dopants.

Raman Snapshots of Side-Chain Dependent Polaron Dynamics in PolyThiophene Films.

Photogenerated polarons in π-conjugated polymers are the precursors to free charges at donor-acceptor interfaces. Unraveling the relationship between film morphology and polaron formation is conjectured to enable efficient charge generation in organic photovoltaic devices. However, it has been challenging to track the ultrafast dynamics of polarons selectively and thus evaluate the molecular coordinates that drive charge generation in films. Using a combination of broadband femtosecond transient absorption and resonance-selective femtosecond stimulated Raman spectroscopy, here, we investigate the polaron generation dynamics exclusively in traditional crystalline poly(3-hexylthiophene) (P3HT) and its amorphous side-chain variant poly(3-(2-ethylhexyl)thiophene-2,5-diyl) (P3EHT) films. The transient Raman data unequivocally provides evidence for an initial delocalization of the polaronic states via thiophene backbone planarization in ∼100 fs while capturing the subsequent morphology-dependent cooling dynamics in a few picoseconds. Our work highlights the structural significance of crystalline morphology in generating hot-charges and thereby emphasizes the importance of side-chain engineering in designing highly efficient conjugated polymer films for hot-carrier photovoltaic devices.

Singlet Fission, Polaron Generation and Intersystem Crossing in Hexaphenyl Film.

The ultrafast dynamics of triplet excitons and polarons in hexaphenyl film was investigated by time-resolved fluorescence and femtosecond transient absorption techniques under various excitation photon energies. Two distinct pathways of triplet formation were clearly observed. Long-lived triplet states are populated within 4.5 ps via singlet fission-intersystem crossing, while the short-lived triplet states (1.5 ns) are generated via singlet fission from vibrational electronic states. In the meantime, polarons were formed from hot excitons on a timescale of <30 fs and recombined in ultrafast lifetime (0.37 ps). In addition, the characterization of hexaphenyl film suggests the morphologies of crystal and aggregate to wide applications in organic electronic devices. The present study provides a universally applicable film fabrication in hexaphenyl system towards future singlet fission-based solar cells.

Intra- and interband excitations induced residue decay of the Bose polaron in a one-dimensional double-well

We investigate the polaronic properties of a single impurity immersed in a weakly interacting bosonic environment confined within a one-dimensional double-well potential using an exact diagonalization approach. We find that an increase of the impurity–bath coupling results in a vanishing residue, signifying the occurrence of the polaron orthogonality catastrophe. Asymptotic configurations of the systems’ ground state wave function in the strongly interacting regime are obtained by means of a Schmidt decomposition, which in turn accounts for the observed orthogonality catastrophe of the polaron. We exemplify that depending on the repulsion of the Bose gas, three distinct residue behaviors appear with respect to the impurity–bath coupling. These residue regimes are characterized by two critical values of the bosonic repulsion and originate from the interplay between the intra- and the interband excitations of the impurity. Moreover, they can be clearly distinguished in the corresponding species reduced density matrices with the latter revealing a phase separation on either the one- or the two-body level. The impact of the interspecies mass-imbalance on the impurity’s excitation processes is appreciated yielding an interaction shift of the residue regions. Our results explicate the interplay of intra- and interband excitation processes for the polaron generation in multiwell traps and for designing specific polaron entangled states motivating their exposure in current experiments.

High‐Performance n‐Type Organic Electrochemical Transistors Enabled by Aqueous Solution Processing of Amphiphilicity‐Driven Polymer Assembly

AbstractDespite the growing attention on organic electrochemical transistors (OECTs), most research has focused on the design of p‐type active materials, and the number of high‐performance n‐type materials is limited. Herein, a series of naphthalene diimide‐based polymers incorporated with asymmetrically branched oligo(ethylene glycol) (OEG) side chains are developed to enable green‐solvent‐processed, high‐performance n‐type OECTs. The branched OEG side chains afford sufficient solubility in eco‐friendly ethanol/water solvent mixtures. Importantly, taking advantage of the amphiphilic nature of OEG‐based polymers, ethanol/water solvents selectively solvate hydrophilic OEG side chains, while producing assembled π−π stacks of hydrophobic backbones. This enables highly ordered polymer packing with a preferential edge‐on orientation, and thus excellent lateral charge transport. In particular, the fine‐tuned OEG side chains of P(NDIMTEG‐T) provide compact backbone packing, effective polaron generation, and superior electrochemical stability with optimized swelling capability. The resultant n‐type OECT shows the best electrical/electrochemical performance in the family, represented by a high transconductance (gm) of 0.38 S cm−1 and a large figure‐of‐merit (µC*) of 0.56 F V−1 cm−1 s−1. This study demonstrates the use of aqueous processing in OECTs, for the first time, and suggests important guidelines for the design of n‐type organic mixed ionic‐electronic conductors with excellent OECT characteristics.

Superionic rubidium silver iodide gated low voltage synaptic transistor

Nonvolatile resistive switching based memristor and memtransistor devices have emerged as a leading platform in neuromorphic computing. In this work, we have fabricated a multifunctional synaptic transistor (ST) using a conjugated polymer P3HT channel and a superionic rubidium silver iodide (RbAg4I5) thin film coated over a polyethylene oxide (PEO) layer as the gate dielectric. Large hysteresis in the transfer curve represents the memristive behavior with at least 105 current On/Off ratio. Enormously large specific capacitance induced by the electrical double layers at the interfaces of PEO/RbAg4I5 dielectric induces polaron (P3HT+) generation in the channel through bound states formation by the electrons with Ag+ ions and consequent movement of iodine (I−) counter ions toward the P3HT channel under a negative gate bias stress. This is strongly supported by the blue shift of the Raman peak from 1444.2 to 1447.9 cm−1 and the appearance of a new peak at 1464.6 cm−1. Interestingly, the proposed ST device exhibits various synaptic actions, which include an excitatory postsynaptic current, paired-pulse facilitation, and short-term potentiation to long-term potentiation after repeated rehearsal on top of standard nonvolatile data storage capability. Our ST also depicts an enhanced retention to 103 s and more than 103 discrete On- and Off-states during potentiation and depression function modulation, respectively, just by consuming a very low energy of about 2.0 pJ per synaptic event. These results are very significant to make this organic synaptic transistor as a potential candidate in terms of the desired metrics for neuromorphic computation at low cost and improved accuracy in the future.

Rainbow polyaniline: chemical–electrochemical parallel polymerization for the production of rainbow color polyaniline as a morphological composite

ABSTRACT Polyaniline (PANI) was synthesized via a “chemical–electrochemical parallel polymerization (CEP)” wherein chemical and electrochemical polymerizations were simultaneously performed, which afforded the production of morphological composite in homopolymers. The chemical structure of the synthesized PANI was confirmed using infrared absorption spectroscopy. The generation of polarons (radical cations in the main chain) as charge carriers was confirmed through electron spin resonance in the doped state. The liquid surface on the polymerization solution (water) and PANI thin layer deposited on a glass substrate exhibited metallic color. Moreover, the surface reflection color was confirmed via measurements using angle-reflection-dependent spectroscopy. The structural color of the PANI was derived from a periodic structure prepared via CEP.

Influence of Intramolecular Charge-Transfer Characteristics of Excitons on Polaron Generation at the Donor/Acceptor Interface in Polymer Solar Cells

Influence of Intramolecular Charge-Transfer Characteristics of Excitons on Polaron Generation at the Donor/Acceptor Interface in Polymer Solar Cells

V OC variation with different molecular weight fractions in highly efficient organic photovoltaic bulk heterojunctions

Tuning molecular weight fractions is one of the most important strategies for optimizing the morphology of bulk heterojunctions (BHJs) to obtain high-performance organic solar cells. Here, we investigate the effects of molecular weight fractions on the performance of state-of-the-art PM6:Y6 BHJ solar cells. A remarkable blueshift in the photoluminescence spectra can be observed with the introduction of the PM6 component. The electroluminescent and morphological analyses indicate that this observation can be attributed to increased charge-transfer state energy and an intermolecular interaction between donor and acceptor, leading to open circuit voltage (V OC) variations in the photovoltaic device. The transient absorption spectra confirm that the optimized donor:acceptor (D:A) ratio can suppress the generation of polarons and achieve high efficiency exciton dissociation. The results of atomic force microscopy and neutron reflectometry reveal that higher acceptor contents promote the formation of a desirable interpenetrating network and enable high fill factors and short-circuit currents to be obtained.

Evidence of Polarons and Bipolarons in a Chemically Pressurized Nanoparticle Bi2Ba2Tb0.5Cu2O4+y

Previous researches on underdoped and overdoped Bi-2212 superconductor have shown that its mechanism follows the polarons-bipolarons theory of superconductivity. In this study, a chemically pressurized Bi-2212 compounds was synthesized and characterized. It was observed that phase analysis reveals the unique route of electron flow within the polycrystalline sample. The transmission electron microscopy (TEM) shows the presence of static and dynamic localizations that proves the possibly presence of Bose-Einstein condensation of inter-site bipolarons. This study on chemically pressurized Bi2Ba2Tb0.5Cu2O4+y compound has shown that the generation and dynamics of polarons and bipolarons are associated to the pentavalent post-transition nature of bismuth.

The Importance of Microstructure in Determining Polaron Generation Yield in Poly(9,9-dioctylfluorene)

Understanding the structure–property relationships that govern exciton dissociation into polarons in conjugated polymers is key in developing materials for optoelectronic applications such as light...

Effect of Post-Thermal Annealing on the Performance and Charge Photogeneration Dynamics of PffBT4T-2OD/PC71BM Solar Cells.

In this work, we studied influence of post thermal annealing on the performance and charge photogeneration processes of PffBT4T-2OD/PC71BM solar cells. As-prepared device exhibits a high-power conversion efficiency of 9.5%, much higher than that after thermal annealing. To understand this phenomenon, we studied charge photogeneration processes in these solar cells by means of time resolved spectroscopy. We associate the degradation of solar cell performance with the reduction of exciton dissociation efficiency and with increased bimolecular recombination of photogenerated charges as a result of annealing. We correlate the generation of localized PffBT4T-2OD polarons observed via spectro-electrochemical measurements with enhancement of the bimolecular charge recombination of annealed solar cells.

Role of Disorder in the Extent of Interchain Delocalization and Polaron Generation in Polythiophene Crystalline Domains.

To understand how disorder within conjugated polymer aggregates influences the polaron generation process, we investigated poly(3-hexylthiophene) (P3HT) and a congeneric random copolymer incorporating 33 mol % substituent-free thiophene units (RP33). Steady-state absorption and fluorescence spectra showed that increasing the intrachain torsional disorder in aggregates increases the energy and breadth of the density of states (DOS). By extracting polaron dynamics in the transient absorption spectra, we found that an activation energy barrier of 0.05 eV is imposed on the charge separation process in P3HT, whereas that in RP33 is essentially barrierless. We also found that a significant amount of excitons in P3HT are deactivated by traps, while no trapped excitons are generated in RP33. This efficient polaron generation in RP33 was attributed to the excess energy and enhanced interchain delocalization of precursor states provided by the intrachain torsional disorder and the close-packing structure in the absence of hexyl substituents.

Design of a Highly Crystalline Low-Band Gap Fused-Ring Electron Acceptor for High-Efficiency Solar Cells with Low Energy Loss

A fused-ring thiophene-thieno[3,2-b]thiophene-thiophene (4T)-based low-band gap electron acceptor, 4TIC, has been designed and synthesized for non-fullerene solar cells. The utilization of the 4T center core enhances the charge mobility of 4TIC and extends its absorption band edge to ∼900 nm, which facilitates its function as a very efficient low-band gap electron acceptor. The rigid, π-conjugated framework of 4T also offers a lower reorganization energy to facilitate lower VOC energy loss. Femtosecond transient spectroscopy showed a level of polaron generation in 4TIC results in the more efficient transfer of energetic carriers higher than that seen with the benchmarked molecule, ITIC. Film morphology analysis has also shown that 4TIC has structural order that is more prominent than that of ITIC with a multiscale phase separation in the blend with donor polymer PTB7-Th. As a result, solar cells based on PTB7-Th and 4TIC exhibit a high power conversion efficiency of 10.43% and a relatively low non-ideal p...