Polaron Absorption Research Articles

(Invited) Influence of Y6 Aggregation on Energetics and Charge Generation in High Performance OPV Blends

We investigated the origin of the high ratio of open circuit voltage to charge transfer state energy (eVOC/ECT) of OPV polymer blends consisting of the electron donating polymer PM6 and the non-fullerene electron acceptor Y6. PM6-Y6 and related donor-acceptor OPV blends exhibit remarkable optoelectronic properties and record power conversion efficiencies in part because of their unusually high eVOC/ECT ratios, which is believed to be related to the small energetic offset of the HOMO levels of PM6 and Y6. In this study, we varied the amount of Y6 in PM6 blends as a means to control the aggregation of Y6 molecules. We examined the corresponding influence that Y6 aggregation has on the energetic offsets and the charge generation efficiency of the materials by probing the appearance of polaron absorption signals in the mid-infrared using time-resolved infrared spectroscopy. Furthermore, we examined the optical absorption and emission spectra of the PM6-Y6 blends over the range of compositions to correlate the efficiency of charge generation with the signatures of Y6 aggregation in the optical spectra. We observed an abrupt transition around 30 mass% Y6 content at which polarons were efficiently generated by charge separation from PM6 to Y6. Comparison to composition dependent GIWAXS studies of PM6-Y6 blends revealed the formation of Y6 aggregates around the same 30 mass% threshold. These observations demonstrate that aggregation of Y6 molecules is required for charge separation to occur from PM6 to Y6 as measured through the mid-infrared absorption of polarons in the TRIR spectra. Although charges were efficiently generated in blends with only 30 mass% Y6 content, OPV device studies revealed that 50-60 mass% Y6 was needed to reach optimized short-circuit current and OPV device efficiency because these measurements convolve charge generation with charge transport. These findings clarify the optoelectronic properties of the novel class of Y6 acceptors and emphasize the importance of molecular aggregation for tuning the energetics of high performance systems that minimize energy offsets for maximum OPV power conversion efficiencies.

Polaron absorption in aligned conjugated polymer films: breakdown of adiabatic treatments and going beyond the conventional mid-gap state model.

This study provides the first experimental polarized intermolecular and intramolecular optical absorption components of field-induced polarons in regioregular poly(3-hexylthiophene-2,5-diyl), rr-P3HT, a polymer semiconductor. Highly aligned rr-P3HT thin films were prepared by a high temperature shear-alignment process that orients polymer backbones along the shearing direction. rr-P3HT in-plane molecular orientation was measured by electron diffraction, and out-of-plane orientation was measured through series of synchrotron X-ray scattering techniques. Then, with molecular orientation quantified, polarized charge modulation spectroscopy was used to probe mid-IR polaron absorption in the ℏω = 0.075 - 0.75 eV range and unambiguously assign intermolecular and intramolecular optical absorption components of hole polarons in rr-P3HT. This data represents the first experimental quantification of these polarized components and allowed long-standing theoretical predictions to be compared to experimental results. The experimental data is discrepant with predictions of polaron absorption based on an adiabatic framework that works under the Born-Oppenheimer approximation, but the data is entirely consistent with a more recent nonadiabatic treatment of absorption based on a modified Holstein Hamiltonian. This nonadiabatic treatment was used to show that both intermolecular and intramolecular polaron coherence break down at length scales significantly smaller than estimated structural coherence in either direction. This strongly suggests that polaron delocalization is fundamentally limited by energetic disorder in rr-P3HT.

Photoluminescence/optical-absorption modulation of layered lanthanide molybdates for chemical sensing of reductive materials

Abstract This work has found K1−δHδEu(MoO4)2 as a novel material for detecting reducing agents. The red emission from Eu3+ was notably diminished in the presence of the reducing agent, followed by its restoration in the presence of the oxidizing agent, demonstrating a significant luminescence modulation. This effect is believed to be induced by polaron absorption between Mo5+ and Mo6+, deactivating the red luminescence of Eu3+. Notably, the facile proton deinsertion stemming from the layered structure contributed to its exceptional restoration capability.

Silver Nanoparticle-PEDOT:PSS Composites as Water-Processable Anodes: Correlation between the Synthetic Parameters and the Optical/Morphological Properties.

The morphological, spectroscopic and rheological properties of silver nanoparticles (AgNPs) synthesized in situ within commercial PEDOT:PSS formulations, labeled PP@NPs, were systematically investigated by varying different synthetic parameters (NaBH4/AgNO3 molar ratio, PEDOT:PSS formulation and silver and PEDOT:PSS concentration in the reaction medium), revealing that only the reagent ratio affected the properties of the resulting nanoparticles. Combining the results obtained from the field-emission scanning electron microscopy analysis and UV-Vis characterization, it could be assumed that PP@NPs' stabilization occurs by means of PSS chains, preferably outside of the PEDOT:PSS domains with low silver content. Conversely, with high silver content, the particles also formed in PEDOT-rich domains with the consequent perturbation of the polaron absorption features of the conjugated polymer. Atomic force microscopy was used to characterize the films deposited on glass from the particle-containing PEDOT:PSS suspensions. The film with an optimized morphology, obtained from the suspension sample characterized by the lowest silver and NaBH4 content, was used to fabricate a very initial prototype of a water-processable anode in a solar cell prepared with an active layer constituted by the benchmark blend poly(3-hexylthiophene) and [6,6]-Phenyl C61 butyric acid methyl ester (PC60BM) and a low-temperature, not-evaporated cathode (Field's metal).

Enhancing the NIR shielding ability and controlling the surface wettability of RF magnetron sputter deposited WO3-x thin films by tuning the oxygen vacancies

Enhancement in the near-infrared (NIR) shielding property of tungsten oxide (WO3-x, 0 < x < 1) thin films by the induction of further oxygen vacancies via vacuum annealing is discussed in this work. The property is comprehensively analysed by UV–Vis–NIR spectroscopy and variable angle spectroscopic ellipsometry techniques. From transmittance spectra, an average visible transmittance of ∼48% along with an average NIR transmittance of ∼15% (with Tvis/Tsol ratio of 1.49) is obtained for WO3-x thin films vacuum annealed at 500 °C for 1.5 h. The pronounced NIR absorption is attributed to the enhanced polaron absorption and surface plasmon resonance effects produced by the increased oxygen vacancies in the film. In order to realize their potential as energy saving window coatings, thermal shielding test is performed in which vacuum annealed WO3-x thin films could restraint the indoor temperature of the heat box efficiently. X-ray diffractometry, field emission scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy measurements were also performed on the films for elucidating structural, morphological, and compositional characteristics. In addition, contact angle measurements revealed that vacuum annealing could reduce the surface wettability of WO3-x thin films considerably. The present work is significant in the field of development of NIR shielding materials for energy conserving smart window applications, especially for buildings and automobiles.

Optical conductivity of an anharmonic large polaron gas at weak coupling

In a polar solid, electrons or other charge carriers can interact with the phonons of the ionic lattice, leading to the formation of polaron quasiparticles. The optical conductivity and optical absorption spectrum of a material are affected by this electron-phonon coupling, most notably leading to an absorption peak in the midinfrared region. Recently, a model Hamiltonian for anharmonic electron-phonon coupling was derived [M. Houtput and J. Tempere, Phys. Rev. B 103, 184306 (2021)] that includes both the conventional Fr\"ohlich interaction as well as an interaction in which an electron interacts with two phonons simultaneously. In this article, we calculate and investigate the optical conductivity of the anharmonic large polaron gas, and we show that an additional characteristic absorption peak appears due to this one-electron--two-phonon interaction. We calculate a semianalytical expression for the optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ at finite temperatures and weak coupling using the Kubo formula. The electronic and phononic contributions can be split and treated separately, such that the many-body effects of the electron gas may be taken into account through the well-known dynamical structure factor $S(\mathbf{k},\ensuremath{\omega})$. From the resulting optical conductivity, we calculate the polaron effective mass, an estimate for the electron-phonon scattering times, and the optical absorption spectrum of the anharmonic polaron gas. It is shown that the effects are negligible for four common III-V semiconductors (BN, AlN, BP, AlP) in the zinc-blende structure, which justifies the commonly used harmonic approximation in these materials. We show that alongside the well-known polaron absorption peak at the phonon energy $\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}$, the one-electron--two-phonon interaction leads to an additional absorption peak at $2\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}$. We propose this absorption peak as an experimentally measurable indicator for non-negligible one-electron--two-phonon interaction in a material, since the height of this peak is proportional to the strength of this anharmonic interaction.

Infrared and Near-Infrared Spectrometry of Anatase and Rutile Particles Bandgap Excited in Liquid.

Chemical conversion of materials is completed in milliseconds or seconds by assembling atoms over semiconductor photocatalysts. Bandgap-excited electrons and holes reactive on this time scale are key to efficient atom assembly to yield the desired products. In this study, attenuated total reflection of infrared and near-infrared light was applied to characterize and quantify the electronic absorption of TiO2 photocatalysts excited in liquid. Nanoparticles of rutile or anatase were placed on a diamond prism, covered with liquid, and irradiated by steady UV light through the prism. Electrons excited in rutile particles (JRC-TIO-6) formed small polarons characterized by a symmetric absorption band spread over 10000-700 cm-1 with a maximum at 6000 cm-1. Electrons in anatase particles (JRC-TIO-7) created large polarons and produced an asymmetric absorption band that gradually strengthened at wavenumbers below 5000 cm-1 and sharply weakened at 1000 cm-1. The absorption spectrum of large electron polarons in TIO-7 was compared with the absorption reported in a Sr-doped NaTaO3 photocatalyst, and it was suggested that excited electrons were accommodated as large polarons in NaTaO3 photocatalysts efficient for artificial photosynthesis. UV-light power dependence of the absorption bands was observed in N2-exposed decane liquid to deduce electron-hole recombination kinetics. With light power density P > 200 W m-2 (TIO-6) and 2000 W m-2 (TIO-7), the polaron absorptions were enhanced with absorbance being proportional to P1/2. The observed 1/2-order power law suggested recombination of multiple electrons and holes randomly moving in each particle. Upon excitation with smaller P, the power-law order increased to unity. The unity-order power law was interpreted with recombination of an electron and a hole that were excited by the same photon. In addition, an average lifetime of 1 ms was estimated with electron polarons in TIO-6 when weakly excited at P = 20 W m-2 to simulate solar-light irradiation.

Effect of substrate temperature on the near-infrared shielding properties of WO3-x thin films deposited by RF magnetron sputtering

Near-infrared (NIR) shielding materials emerge as potential candidates for energy conservation applications. In this work, tungsten oxide (WO3-x, 0 < x < 1) thin films were deposited on soda-lime glass substrate by RF magnetron sputtering at various substrate temperatures and NIR shielding properties of the films were studied. Structural, morphological and compositional properties of the films were also investigated. Optical constants, thickness and roughness of the films were extracted from spectroscopic ellipsometric (SE) measurements. The correlativity of SE with UV–Vis–NIR spectrophotometry, cross-sectional field emission scanning electron microscopy and atomic force microscopy was investigated. Of all the films, WO3-x thin films deposited at 500 °C showed the lowest NIR transmittance of about 23% with the highest visible transmittance of above 70% corresponding to a Tvis/Tsol ratio of 1.35 and NIR shielding value of 75%. WO3-x coated soda lime glass shows better NIR shielding performance compared to commercial soda lime glass in the thermal shielding test. The underlying reason for NIR absorption is the presence of oxygen vacancies which promote formation of W5+ ions and the resultant combined effects of polaron absorption and surface plasmon resonance. This work is significant in utilizing a facile technique for developing undoped WO3-x thin films for efficient NIR shielding applications.

Nonradiative Recombination via Charge‐Transfer‐Exciton to Polaron Energy Transfer Limits Photocurrent in Organic Solar Cells

AbstractA recombination and exciton loss mechanism is reported in organic solar cells involving energy transfer between charge transfer (CT) excitons and polarons, impacting photocurrent generation, particularly in the near‐infrared where polaronic transitions typically reside. This process sets a low‐energy cut‐off in the external quantum efficiency spectrum of an excitonic donor/acceptor interface, determined by the low‐energy polaron absorption peak and the CT state reorganization energy. Furthermore, this process explains the deviation from unity and bias dependence of the CT state's internal quantum efficiency at low photon energies. This process is demonstrated in a variety of systems and it is hypothesized that CT state to polaron energy transfer recombination may be responsible for a share of nonradiative recombination in all organic photovoltaics and can explain numerous experimentally observed device trends regarding photocurrent generation and energy losses. Overall, this work enhances the understanding of photophysical processes in organic materials and allows the design of systems that can avoid this recombination pathway.

Chemically dedoped PEDOT:PSS films with an amidine/polymer overcoating and their applications as a semiconducting channel in organic field-effect transistors and inverters

We investigate the properties of a conducting polymer composite of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) after chemical dedoping with an amidine base and demonstrate the applications of the dedoped films as a semiconducting channel in a transistor and an inverter. We observe that PEDOT:PSS can be chemically transformed from a conductor to a semiconductor through a post-treatment with 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), which can donate electrons to the oxidized PEDOT chains and screen negative charges of PSS by salt formation. Both appearance of visible absorption bands and disappearance of (bi)polaronic absorption bands are observed in the DBN-treated PEDOT:PSS film, suggesting the effective dedoping. Compositional depth profiling and morphological analyses show that two distinct layers evolve after chemical dedoping and that the majority of internal PEDOT chains are dedoped over the whole film. We also introduce an overcoating layer using polyvinylpyrrolidone (PVP) with DBN to get more stable PEDOT:PSS films in a dedoped state. Organic field-effect transistors (OFETs) with the dedoped PEDOT:PSS film as a semiconducting channel show the typical p-type enhancement-mode characteristics without any doping-induced shift of threshold voltage. The pinch-off, hard saturation, and distinguishable switching characteristics with on-to-off current ratio of 104 are observed without redoping. We also demonstrate a logic circuit from the OFET as a proof-of-concept of semiconducting nature of PEDOT:PSS.

Persistent photoconductivity in barium titanate

Annealed bulk crystals of barium titanate (BaTiO3) exhibit persistent photoconductivity (PPC) at room temperature. Samples were annealed in a flowing gas of humid argon and hydrogen, with a higher flow rate corresponding to larger PPC. When exposed to sub-bandgap light, a broad infrared (IR) absorption peak appears at 5000 cm−1 (2 μm), attributed to polaronic or free-carrier absorption from electrons in the conduction band. Along with the increased IR absorption, electrical resistance is reduced by a factor of approximately two. The threshold photon energy for PPC is 2.9 eV, similar to the case of SrTiO3. This similarity suggests that the mechanisms are similar: an electron in substitutional hydrogen (HO) is photoexcited into the conduction band, causing the proton to leave the oxygen vacancy and attach to a host oxygen atom. The barrier to recover to the ground state is large such that PPC persists at room temperature.

Organic-inorganic hybrid material: synthesis, characterization for solar cell application

We report a hybrid system based on conducting polymer-semiconductor from poly(otoluidine) (POT) doped with camphor sulfonic acid(CSA) and cadmium selenide capped with Tri sodium citrate(CdSe-TSC) by the physical mixing method. The influence of CdSe nanoparticles weight percent on the hybrid thin films have been investigated by means of XRD, FESEM, UV-VIS absorption and photoluminescence spectroscopy (PL) measurements. The main diffraction peaks of POT/CdSe hybrids are analogous to the neat CdSe nanoparticles. The absorption measurement reveals that the interaction between CdSe and POT in the hybrid materials led to the blue shift and broadening of the polaron absorption band. The energy gap has been decreased with an increase in the concentration of CdSe NPs. the PL intensity has been totaly quenched when the tow materials are bring to other. The hybrid nanocomposite material and provides useful evidence apropos the optimum use of CdSe nanoparticles in conductive polymer based optoelectronics.

Controlling morphology, adhesion, and electrochromic behavior of PEDOT films through molecular design and processing

AbstractPoly(3,4‐ethylenedioxythiophene) (PEDOT) is a well‐known semiconducting polymer with favorable properties which find it often applied as the active material in biological sensors and electrochromic devices. However, PEDOT has several drawbacks which can prohibit its effective or long‐term use, including weak adhesion to substrates such as ITO‐coated glass, poorly controlled surface morphology, and reduced electrochemical stability over time. While a diverse range of approaches have been explored to overcome these issues, most involve additives or substrate modification, while solutions based on direct covalent adaptation are relatively lacking. We present a novel polymer based on covalently modified EDOT (PEDOT‐Crown), featuring polar motifs and a 15‐crown‐5 moiety. Compared to PEDOT, PEDOT‐Crown demonstrates a wealth of advantageous properties including: superior adhesion to ITO under physical and electrochemical duress; a more uniform surface morphology; and electrochemical properties including a higher contrast ratio, red‐shifted polaron and bipolaron absorption features, bleaching of the neutral absorption band across a narrower voltage range, and more Faradaic rather than capacitive behavior. Additionally, we note that in the presence of Na+, PEDOT‐Crown appears to show modified behavior in long‐term electrochemical experiments. These features make PEDOT‐Crown a material with improved suitability for application in biological sensing and electrochromic devices, compared to PEDOT.

Thermodynamic Properties and Optical Absorption of Polaron in Monolayer Graphene Under Laser Field

In this work, we use the variational method to investigate thermal properties and optical absorption of polaron in monolayer graphene under laser field. We have shown that the energies and the optical absorption of the system strongly depend on laser parameters and graphene characteristics. We found that the simple model adopted to calculate the optical absorption is enough accurate and interesting to investigate the optical absorption coefficient of the polarons in graphene. We observe that the laser assists the polaron in the optical absoprtion phenomenon. We observe that temperature, the coupling between electron and quantum of lattice vibration, laser parameters and wave number affect the disorder in the system. Contrary to temperature, the laser increases the disorder in the system. At low temperature, the polaronic system becomes decoherent for low values of wave number and gains coherency for high values of wave number. In addition, the coherence of the system is not significantly affected by the laser field but there is a considerable change at higher temperatures. We also observed that laser increases the capacity of system to store energy.

Dirac polaron dynamics in the correlated semimetal of perovskite CaIrO3

We have investigated the charge dynamics of the correlated Dirac semimetallic perovskite ${\mathrm{CaIrO}}_{3}$ by infrared spectroscopy. The optical conductivity spectra show the $\ensuremath{\omega}$-linear-type interband transition and a tiny Drude response at low temperatures, suggesting that the Dirac node is pinned near the Fermi energy due to the Mott criticality. Moreover, a large polaron absorption appears at around 0.04 eV, when thermally excited carriers diminish and the plasma frequency decreases below the optical phonon energy. The electron correlation and resultant reduced plasma frequency are likely to enhance the electron-phonon interaction via the insufficient electronic screening, realizing the Dirac polaron.

Donors and polaronic absorption in rutile TiO2 single crystals

We have used a combination of optical absorption and electrical conductivity measurements to study the effect of the main donor on small polarons in rutile TiO2 single crystals rendered n-type conductive by hydrogenation or doping with Nb. The electrical conductivity measured at 295 K for hydrogenated samples shows a clear correlation with the interstitial hydrogen (Hi) concentration, which is consistent with reports that Hi is the main shallow donor in rutile TiO2. Conductive samples exhibit two distinct optical absorption bands in the IR spectral region, at ω1 = 6500 cm−1 (∼0.8 eV) and ω2 = 3100 cm−1 (∼0.4 eV), which are present in both hydrogen-rich and Nb-doped samples. The intensities of the absorption bands are proportional to the electrical conductivity, and they exhibit an Arrhenius-like temperature dependence for temperatures between 25–50 K and 50–100 K for H-doped and Nb-doped samples, respectively. The thermal activation energies (EAs) for the absorption bands depend strongly on the main donor: ω2 exhibits EA(H) and EA(Nb) of ∼4 and ∼10 meV, respectively, whereas ω1 shows EA(H) and EA(Nb) of ∼1 and ∼2 meV, respectively. The combination of temperature-dependent data for the optical absorption bands and interstitial deuterium (Di)-small polaron vibrational lines support a model where the thermal activation is associated with the reconfiguration of small polarons involving Ti sites far away from the donor. The thermal activation of the optical absorption bands gives us insight into the dynamics of donor-dependent small polaron reconfiguration in rutile TiO2.

The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated mo...

Resolution of polaron spin states by the infrared absorption in monolayer transition metal dichalcogenides

Abstract We theoretically investigate the infrared absorption of polaron with spin-orbit coupling in monolayer transition metal dichalcogenides, in which the polaron states are formed arising from the electron coupled with the intrinsic longitudinal optical phonon and the induced surface optical phonon modes. We find that the magnitude of optical absorption are different for the polaron with spin-up and spin-down states in the same valley. Moreover, the absorption peaks of spin-up and spin-down states show the opposite changing trends with increasing the strength of Rashba spin-orbit coupling. These results indicate that the polaron spin state could be used for an effective valley information carrier in two-dimensional valleytronic materials by the infrared optical absorption, which offers new opportunities for potential applications in infrared and valleytronic devices.

Optical Absorption and Tsallis Entropy of Polaron in Monolayer Graphene

We studied the effect of polar vibration modes on optical absorption of polaron and the rate of disorder in a graphene monolayer. Our investigation focused more particularly on the role played by phonons. We found that the amplitude of optical absorption decreases with increasing photon energy and with increasing magnetic field. We also found that the rate of local disorder in the system increases with increasing temperature and with increasing graphene-substrate distance. However, it decreases with the increase in magnetic field. These observations show that the absorption of a polaron by the photon creates a relative disorder in the graphene lattice. It is also shown that entropy and optical absorption of polaron strongly depend on the type of substrates.

The optical signatures of molecular-doping induced polarons in poly(3-hexylthiophene-2,5-diyl): individual polymer chainsversusaggregates

For molecularly doped poly(3-hexyl-thiophene) solvated individual chains can be unambiguously differentiated from aggregated ones by diagnostic polaron absorption.