Polaron Recombination Research Articles

Hole Localization in Bulk and 2D Lead-Halide Perovskites Studied by Time-Resolved Infrared Spectroscopy.

Scattering and localization dynamics of charge carriers in the soft lattice of lead-halide perovskites impact polaron formation and recombination, which are key mechanisms of material function in optoelectronic devices. In this study, we probe the photoinduced lattice and carrier dynamics in perovskite thin films (CsFAPbX3, X = I, Br) using time-resolved infrared spectroscopy. We examine the CN stretching mode of formamidinium (FA) cations located within the lead-halide octahedra of the perovskite structure. Our investigation reveals the formation of an infrared mode due to spatial symmetry breaking within a hundred picoseconds in 3D perovskites. Experiments at cryogenic temperatures show much-reduced carrier localization, in agreement with a localization mechanism that is driven by the dynamic disorder. We extend our analysis to 2D perovskites, where the precise nature of charge carriers is uncertain. Remarkably, the signatures of charge localization we found in bulk perovskites are not observed for 2D Ruddlesden-Popper perovskites ((HexA)2FAPb2I7). This observation implies that the previously reported stabilization of free charge carriers in these materials follows different mechanisms than polaron formation in bulk perovskites. Through the exploration of heterostructures with electron/hole excess, we provide evidence that holes drive the formation of the emerging infrared mode.

Time-Domain View of Polaron Dynamics in Metal Oxide Photocatalysts.

The polaron is a fundamental physical phenomenon in transition metal oxides (TMOs), and it has been studied extensively for decades. However, the implication of a polaron on photochemistry is still ambiguous. As such, understanding the fundamental properties and controlling the dynamics of polarons at the atomistic level is desired. In this Perspective, we seek to highlight the recent advances in studying small polarons in TMOs, with a particular focus on nonadiabatic molecular dynamics at the ab initio level, and discuss the implications for photocatalysis from the aspects of the structure, intrinsic physical properties, formation, migration, and recombination of small polarons. Finally, various methods were proposed to advance our understanding of manipulating the small-polaron dynamics, and strategies to design high-performance TMO-based photoelectrodes were discussed.

Extracting Polaron Recombination from Electroluminescence in Organic Light-Emitting Diodes by Artificial Intelligence.

Direct exploring the electroluminescence (EL) of organic light-emitting diodes (OLEDs) is a challenge due to the complicated processes of polarons, excitons, and their interactions. This study demonstrated the extraction of the polaron dynamics from transient EL by predicting the recombination coefficient via artificial intelligence, overcoming multivariable kinetics problems. The performance of a machine learning (ML) model trained by various EL decay curves is significantly improved using a novel featurization method and input node optimization, achieving an R2 value of 0.947. The optimized ML model successfully predicts the recombination coefficients of actual OLEDs based on an exciplex-forming cohost, enabling the quantitative understanding of the overall polaron behavior under various electrical excitation conditions.

Conjugated Polymer Nanoparticles: Photothermal and Photodynamic Capabilities According to Molecular Ordering in Their Assembly Structures

We report that conjugated polymer nanoparticles (CPNs) coated with polyethylene glycols (PEGs) exhibit photothermal and photodynamic capabilities according to molecular ordering in their assembly structures. CPN-PEGs were made using three different methods: a dispersion process of phase-separated film assemblies of a conjugated polymer and a phospholipid-conjugated PEG (CPN-I), a dispersion process of a conjugated polymer and a phospholipid followed by surface conjugation with PEGs (CPN-II), and a miniemulsification of the conjugated polymer and the phospholipid-conjugated PEG. Our findings revealed that the ordered molecular assembly structures in CPN-I and CPN-II increased intermolecular interactions and decreased the optical band gap, promoting nonradiative exciton relaxation via the energy-gap law’s internal conversion mechanism and rationalizing CPN-I’s shorter singlet exciton lifetime (13 ps). Meanwhile, CPN-III with a disordered structure generated more singlet oxygen than CPN-I and CPN-II, indicating increased triplet exciton generation upon the polaron recombination. Our findings present that the photothermal and photodynamic properties of CPNs are obviously dependent on the assembly structure order and that CPNs with an ordered assembly of conjugated backbones have a stronger photothermal effect, whereas those with a disordered structure have a better photodynamic effect.

Management of Polaron Dynamics: A Key Factor for the Long Operational Stability of Exciplex‐Forming Cohost‐Based Organic Light‐Emitting Diodes

AbstractThe device performance of phosphorescent organic light‐emitting diodes (PHOLEDs) is improved by using the exciplex‐forming cohost with the thermally activated delayed fluorescence (TADF) property. This work comprehensively investigates the excitons and polarons management of exciplex host and dopant to understand the device degradation of PHOLEDs with various host materials. The recombination and accumulation of polarons is newly integrated in the numerical model for understanding transient electroluminescence in order to theoretically characterize a diversity of the polaron dynamics. Based on the unique excitons kinetics, polarons kinetics, and recombination coefficients coming from the exciplex host materials, the device degradation is examined. As a result, it is revealed that the longer device lifetime is attributed to the suppression of the exciton–polaron interaction by minimizing the steady‐state polarons via fast polaron recombination. The combination of the exciton–polaron coupled kinetics model and the device degradation model show that the recombination coefficient should be considered a key parameter of the host materials to design long‐lived PHOLEDs.

(Invited) Light-Induced Structural Deformations in BiVO4 Photoanodes

Monoclinic scheelite bismuth vandate (BiVO4) stands as one of the best performing thin film metal oxide photoanodes investigated to date. However, these desirable photoelectrochemical performance characteristics are surprising given the driving force for free charge carriers to self-trap as polarons. For the case of electrons, localization at vanadium sites leads to formation of small electron polarons with a large, thermally activated hopping barrier of several hundred meV. For the case of holes, several experimental and theoretical studies have indicated polaron formation. However, despite the importance of photogenerated hole extraction for photoanode function, significant uncertainties remain regarding the degree of hole localization, the associated structural configurations, and the transport barriers. In this work, we apply time resolved optical pump/X-ray diffraction probe experiments to investigate light excitation-induced structural transformations of BiVO4. At low pump fluences, we observe a distinct, non-thermal response that indicates increased symmetry and a partial relaxation of the monoclinic distortion within the BiO8 dodecahedra. We rationalize this finding in terms of the underlying electronic structure of BiVO4, photo-induced depopulation of valence band states, and the nature of the monoclinic-to-tetragonal phase transformation. The resulting structural change is expected to increase hole localization and play a key role in the formation and structure of photo-induced hole polarons within the material. Complimentary electronic transport and optical pump-probe spectroscopies provide additional support for this interpretation and shed light on the underlying mechanisms of polaron formation, transport, and recombination in this promising photoanode material.

Polaron mobility modulation by bandgap engineering in black phase α-FAPbI3

Lead halide hybrid perovskites (LHP) have emerged as one of the most promising photovoltaic materials for their remarkable solar energy conversion ability. The transportation of the photoinduced carriers in LHP could screen the defect recombination with the help of the large polaron formation. However, the physical insight of the relationship between the superior optical-electronic performance of perovskite and its polaron dynamics related to the electron-lattice strong coupling induced by the substitution engineering is still lack of investigation. Here, the bandgap modulated thin films of α-FAPbI3 with different element substitution is investigated by the time resolved Terahertz spectroscopy. We find the polaron recombination dynamics could be prolonged in LHP with a relatively smaller bandgap, even though the formation of polaron will not be affected apparently. Intuitively, the large polaron mobility in (FAPbI3)0.95(MAPbI3)0.05 thin film is ∼30% larger than that in (FAPbI3)0.85(MAPbBr3)0.15. The larger mobility in (FAPbI3)0.95(MAPbI3)0.05 could be assigned to the slowing down of the carrier scattering time. Therefore, the physical origin of the higher carrier mobility in the (FAPbI3)0.95(MAPbI3)0.05 should be related with the lattice distortion and enhanced electron–phonon coupling induced by the substitution. In addition, (FAPbI3)0.95(MAPbI3)0.05 will lose fewer active carriers during the polaron cooling process than that in (FAPbI3)0.85(MAPbBr3)0.15, indicating lower thermal dissipation in (FAPbI3)0.95(MAPbI3)0.05. Our results suggest that besides the smaller bandgap, the higher polaron mobility improved by the substitution engineering in α-FAPbI3 can also be an important factor for the high PCE of the black phase α-FAPbI3 based solar cell devices.

Photoinduced Small Hole Polarons Formation and Recombination in All-Inorganic Perovskite from Quantum Dynamics Simulation.

We conducted ab initio molecular dynamics (AIMD) and nonadiabatic MD to simulate polaron formation and recombination in all-inorganic Cs3Bi2Br9 perovskite. The meticulously designed AIMD simulations show that two types of small hole polaron, including localized and semidelocalized small hole polaron on either an intralayer or an interlayer Br dimer, are adiabatically formed within 1.71 ps. The localized small hole polaron reduces nonadiabatic coupling and decoherence time and, thus, delays charge recombination to 213 ns. In contrast, the dominant semidelocalized polaron increases nonadiabatic coupling by enhancing electron-hole overlap and restores the energy gap and decoherence time to the pristine system, accelerating recombination to 4.7 ns compared to a 10 ns charge carrier lifetime in the pristine system. All the obtained time scales agree well with experiments. The study offers a fundamental understanding of the excited-state dynamics of small hole polaron in Cs3Bi2Br9 and helps to design high-performance perovskite optoelectronics and photovoltaics.

Bound photoinduced giant spin polaron in EuTe

Previously, we showed that at low temperatures and very low excitation powers, EuTe displays band-edge photoluminescence with a well-resolved phonon structure, the so-called MX0 band, which is associated with the recombination of giant spin polarons (SPs) of magnetic moments of several hundred Bohr magnetons. Here, we investigate the MX0 band both experimentally and by Monte Carlo simulations. Results indicate that the SPs are bound to defects, with a localization energy of 0.29 eV. The density of defects harboring the SPs is estimated to be 4.3×1015 cm−3.

Liquid-like dielectric response is an origin of long polaron lifetime exceeding 10 μs in lead bromide perovskites.

Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic-inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1-10 THz region and liquid-like rotational relaxation in the 1-100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr3 and organic-inorganic hybrid (CH3NH3)PbBr3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1-100 GHz range. We confirmed the absence of the dielectric response in the range of 106-1010 Hz in CsPbBr3, which was clearly present in the hybrid (CH3NH3)PbBr3. This suggests that the response is almost purely due to the rotational motions of the organic dipoles in the hybrid perovskites. We evaluated the lifetimes of the polarons using surface-free transient photoluminescence. The lifetime in CsPbBr3 was up to 1.6 µs, while the lifetime in (CH3NH3)PbBr3 was 18 µs. The lifetime in the hybrid (CH3NH3)PbBr3 was significantly longer than in CsPbBr3, also confirmed by transient infrared spectroscopy. We concluded that the liquid-like dielectric response inhibits polaron recombination due to the efficient separation of opposite charges by the additional dynamic disorder.

Discerning Bulk and Interfacial Polarons in a Dual Electron Donor/Acceptor Polymer.

The active layer of organic solar cells typically possesses a complex morphology, with amorphous donor/acceptor mixed domains present in addition to purer, more crystalline domains. These crystalline domains may represent an energy sink for free charges that aids charge separation and suppresses bimolecular recombination. The first step in exploiting this behavior is the identification and characterization of charges located in these different domains. Herein, the generation and recombination of both bulk and interfacial polarons are demonstrated in the dual electron donor/acceptor polymer XIND using transient absorption spectroscopy. The absorption spectra of XIND bulk polarons, present in pristine polymer domains, are clearly distinguishable from those of polarons present at the donor/acceptor interface. Furthermore, it is shown that photogenerated polarons are transferred from the interface to the bulk. These findings support the energy sink hypothesis and offer a way to maximize morphology relationships to enhance charge generation and suppress recombination.

Radiative recombination of large polarons in halide perovskites

In hybrid organic/inorganic perovskites, the majority of charge carriers are large polarons. Slow recombination of the large polarons underlies long carrier lifetime and diffusion length, crucial to optoelectronic applications of the perovskites. However, microscopic mechanisms by which the large polarons recombine remain largely unknown. Here we propose a theoretical model to elucidate radiative recombination of large polarons. Six annihilation pathways are identified which involve the formation of mobile and immobile ‘dying pairs’, both responsible for charge recombination. The formation probability of the ‘dying pair’ and the corresponding annihilation rate can be estimated. The product of the formation probability and the annihilate rate gives rise to recombination rate along each pathway and weighted sum of the recombination rates yields an overall rate of radiative recombination in perovskites. The theory sheds light on the physical mechanism by which large polarons recombine via the formation of the dying pairs, assisted by thermal activation and quantum tunneling. The predictions from the theory in general agree well to corresponding experimental measurements on monomolecular and bimolecular recombination rates as well as peak frequency of photoluminescence spectrum.

Magnetic field effect on polaron recombination in conjugated polymers

By using the one-dimensional tight-binding model, we theoretically investigate the effect of magnetic field on the collision of the oppositely charged polarons with their spin antiparallel in cis-polyacetylene. We adopt the non-adiabatic approach to simulate the dynamical evolution of polaron spin. Under the effect of electric field with a moderate strength, the polarons initially localized at the chain-end of polyacetylene move towards each other. During the collision process, an obvious spin precession is obtained, and the precession period is found to be inversely proportional to the strength of magnetic field. While after including electron-electron interaction in form of Hubbard model, the period is no longer constant. With the decrease of distance between the oppositely charged polarons, the precession of their spin slows down. We also find that there is a critical value of electron-electron interaction strength, over which the spin precession of polaron disappears. In addition, we demonstrate that under the effect of magnetic field, the polarons could recombine or pass through each other to form the singlet exciton, rather than be dissociated after collision between them with the strong electron-electron interaction, and then the electroluminescence efficiency of organic optoelectronic devices is improved.

Polaron-Mediated Luminescence in Lithium Niobate and Lithium Tantalate and Its Domain Contrast

In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as on bipolarons. We discuss the absorption measurements on reduced as well as on doped LNO that made the characterization of the formed polaron states possible by their absorption bands. Next, we proceed by reporting on the two polaron-mediated photoluminescence bands that have been observed in LNO: (1) A near-infrared luminescence band in the range of 1.5 eV shows a mono-exponential decay and a strong dependence on iron doping. This luminescence is emitted by bound polarons returning from an excited state to the ground state. (2) A luminescence band at visible wavelengths with a maximum at 2.6 eV shows a stretched-exponential decay and is strongly enhanced by optical damage resistant doping around the doping threshold. This luminescence stems from the recombination of free electron and hole polarons. The next major topic of this review are domain contrasts of the visible photoluminescence that have been observed after electrical poling of the substrate, as singly inverted domains show a slightly reduced and faster decaying luminescence. Subsequent annealing results in an exponential decrease of that domain contrast. We show that this contrast decay is strongly related to the mobility of lithium ions, thus confirming the role of polar defect complexes, including lithium vacancies, for these domain contrasts. Finally we discuss the extension of our investigations to lithium tantalate (LTO) samples. While the results on the domain contrast and its decay are similar to LNO, there are remarkable differences in their luminescence spectra.

Optimization of the Electrochemically Generated Luminescence of Polyfluorene Films

Electrochemically generated chemiluminescence (ECL) from solid-state poly(9,9′-octylfluorene) films was obtained by the successive electrochemical injection of polarons with opposite sign (annihilation ECL) in double potential steps. The processes involved in the recombination of polarons, the quenching of the electrochemically generated excitons, and the emission stability have been also analyzed. A systematic study at various electrochemical potentials has been carried out to determine optimal conditions to obtain a maximum ECL intensity, particularly with potentials of 0.4 V beyond the doping onsets (for both p- and n-doping). Results show that while the ECL emission is slightly affected by the potential of the emission, it is strongly dependent on the charging potential. Under large charging potential, the electrochemically generated excitons formed in the emission step get quenched by an excess of polarons and bipolarons that are detrimental to ECL. The maximum emission is obtained when the applied p...

Effect of electron-electron interaction on scattering process of oppositely charged polarons in conjugated polymers

By using one-dimensional tight-binding Su-Schrieffer-Heeger model modified to include electron-electric field interaction and electron-electron interaction, we simulate the scattering process of oppositely charged polarons with spin parallel and antiparallel, respectively. It is found that at weak field, the electron-electron interaction is dominant. Under the effect of strong Coulomb attraction, the polarons with spin antiparallel could not be separated after collision, and ultimately recombine into the singlet exciton. While in case of spin parallel, the strong Coulomb repulsion makes no collision occur unless strengthening the electric field. At moderate field, the electron-electron interaction could facilitate the recombination of polarons into triplet exciton rather than singlet. It is obtained that there exists competition between electron-electric field interaction and electron-electron interaction, a suitable strength of which could promote the formation of exciton in conjugated polymers.

Temperature effect on polaron recombination in conjugated polymers

The microcosmic mechanism of electroluminescence in polymer light emitting diodes (PLEDs) is the recombination of the oppositely charged polarons. In previous studies, it has been demonstrated that the temperature-induced irregular lattice vibration may have non-negligible influence on polaron dynamics. Nevertheless, there are few reports about thermal effect on recombination process between polaron pair, although it is very important for the performance of PLEDs. In this paper, we adopt the modified one-dimensional tight-binding model, including to which the thermal random force, and explore the temperature effect on polaron collision driven by electric field with different strengths. The dynamical simulation is performed by using the non-adiabatic evolution method. The results show that under the influence of electric field, the oppositely charged polarons could recombine into either an exciton with one lattice distortion, or the mixed state of polaron pair and exciton with two lattice distortions. It depends on both field strength and temperature. Anyway, after including temperature effect, a significant improvement of exciton yield is obtained. In addition, the new-formed exciton could perform a random walk along the polymer chain driven by the thermal random force when its strength is large enough. If we further increase the temperature, the stability of exciton would become worse.

Electron–phonon coupling effects on intrachain polaron recombination in conjugated polymers

Based on a one-dimensional Su-Schrieffer-Heeger (SSH) model with electron correlation considered within the extended Hubbard model (EHM), we investigate the role played by electron-phonon coupling constant on intrachain polaron recombination in conjugated polymers. Our results suggest that a competition between external electric field and electron-phonon coupling on defining the behavior of the charge distribution of the system takes place. Whereas increasing electric field plays the role of destabilizing the charge carriers, an increase of the electron-phonon coupling has the opposite effect. Therefore, a suitable balance of these properties can give rise to the correct description of charge carrier dynamics in conducting polymers.

Derivation and solution of effective medium equations for bulk heterojunction organic solar cells

A drift-diffusion model for charge transport in an organic bulk heterojunction solar cell, formed by conjoined acceptor and donor materials sandwiched between two electrodes, is formulated. The model accounts for (i) bulk photogeneration of excitons, (ii) exciton drift and recombination, (iii) exciton dissociation (into polarons) on the acceptor–donor interface, (iv) polaron recombination, (v) polaron dissociation into a free electron (in the acceptor) and a hole (in the donor), (vi) electron/hole transport and (vii) electron–hole recombination on the acceptor–donor interface. A finite element method is employed to solve the model in a cell with a highly convoluted acceptor/donor interface. The solutions show that, with physically realistic parameters, and in the power generating regime, the solution varies little on the scale of the micro-structure. This motivates us to homogenise over the micro-structure; a process that yields a far simpler one-dimensional effective medium model on the cell scale. The comparison between the solution of the full model and the effective medium (homogenised) model is very favourable for applied voltages less than the built-in voltage (the power generating regime) but breaks down as the applied voltages increases above it. Furthermore, it is noted that the homogenisation technique provides a systematic way to relate effective medium modelling of bulk heterojunctions [19, 25, 36, 37, 42, 59] to a more fundamental approach that explicitly models the full micro-structure [8, 38, 39, 58] and that it allows the parameters in the effective medium model to be derived in terms of the geometry of the micro-structure. Finally, the effective medium model is used to investigate the effects of modifying the micro-structure geometry, of a device with an interdigitated acceptor/donor interface, on its current–voltage curve.

Spin-dependent photophysics in polymers lightly doped with fullerene derivatives: Photoluminescence and electrically detected magnetic resonance

Lightly fullerene-doped polymers are suitable composite systems to study spin-dependent bimolecular interactions among charge excitations due to their long lifetimes in these systems. These interactions can affect the photocurrent as well as the open-circuit voltage in an organic solar cell. Combining photoluminescence detected magnetic resonance (PLDMR) and electrically detected magnetic resonance (EDMR) spectroscopies we study films and devices of poly(p-phenylene vinylene) polymers poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and superyellow PPV (SY-PPV) lightly doped with various fullerene derivatives [6,6]-phenyl C61 butyric acid methyl ester (PCBM), bis[60]PCBM (bis-PCBM), indene-C60 bisadduct (ICBA), and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM). (i) We demonstrate strong fullerene triplet exciton (TE) production in SY-PPV:fullerene blends, whereas this is absent in MDMO-PPV:PCBM and only very weak in MDMO-PPV:ICBA. The low TE production in blends with MDMO-PPV is attributed to a weaker singlet-singlet energy-transfer coupling and an unfavorable triplet level alignment between the blend components. (ii) The fullerene TE spectra are analyzed on the basis of a single type of triplet excitation in PCBM, bis-PCBM, and ICBA, and two triplet species in PC70BM which are attributed to the α- and β-type isomers of the latter molecule. (iii) The sign change with increasing temperature of the g∼2 sharp central line in photo-EDMR, which is observed both in pristine SY-PPV and in blends with fullerene, is correlated to a transition from dominant TE-polaron annihilation to nongeminate polaron recombination processes.