Spin-dependent recombination of the charge-transfer state in photovoltaic polymer/fullerene blends

ABSTRACT

Q-band electron spin echo (ESE) spectroscopy was applied for studying the spin-dependent recombination of charge transfer (CT) states in the benchmark organic photovoltaics (OPV) blend of poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT/PC₆₀BM). Selective microwave excitation and a special protocol for ESE data treatment allowed to suppress the ESE signal of thermalised polarons and weakly coupled CT states and to address CT states with a relatively short distance between positive and negative polarons (1.5 nm < r?<?2.5 nm). Inversion of the in-phase ESE signal with increase of the delay after laser flash was observed for the regioregular P3HT^?+/PC₆₀BM^?? CT state at a temperature of 40 K. This effect is very similar to the inversion of the time resolved (TR) EPR spectrum of the same system obtained previously. Both effects can be explained by spin-dependent recombination of the CT state, with the recombination via the triplet channel proceeding much slower than via the singlet channel. For the regiorandom P3HT^?+/PC₆₀BM^?? CT state no ESE sign inversion was observed in an analogous experiment. The result suggests the importance of CT state formation via a triplet exciton, a process which was not considered previously for the P3HT/PC₆₀BM blend.     

Single‐molecule surface‐enhanced Raman scattering of fullerene C60

We achieved single‐molecule surface‐enhanced Raman scattering (SM‐SERS) spectra from ultralow concentrations (10⁻¹⁵ M) of fullerene C₆₀ on uniformly assembled Au nanoparticles. It was found that resonant excitation at 785 nm is a powerful tool to probe SM‐SERS in this system. The appearance of additional bands and splitting of some vibrational modes were observed because of the symmetry reduction of the adsorbed molecule and a relaxation in the surface selection rules. Time‐evolved spectral fluctuation and ‘hot spot’ dependence in the SM‐SERS spectra were demonstrated to result from the single‐molecule Raman behavior of the spherical C₆₀ on Au nanoparticles. Copyright © 2010 John Wiley & Sons, Ltd.     

新型高效聚合物/富勒烯有机光伏电池研究进展

有机聚合物/富勒烯本体异质结光伏电池以其不断提高的能量转换效率受到了研究人员的广泛关注, 近年来成为光伏电池研究领域的热点之一. 本文主要通过对聚合物/富勒烯太阳能电池的内部机理,包括光吸收、激子扩散和解离以及自由载流子输运和提取等关键科学问题, 从器件材料和结构优化、形貌控制和界面修饰等不同侧面介绍了提高聚合物/富勒烯太阳能电池性能的方法, 讨论了各种器件的结构和能量转换效率, 对于进一步开展这方面的研究工作指明了方向, 最后对其未来的发展前景做出了展望.     

Raman spectroscopic analysis of sulphur-doped TiO2 by co-grinding with TiS2

Doping sulphur into titania has been tried using TiS₂ as a doper based on the mechanically induced solid-state reaction between TiO₂ and TiS₂. The prepared samples have been characterized by X-ray diffraction (XRD), Raman spectroscopy and UV-Vis reflectance spectroscopy. Raman analysis, particularly has been proved to be effective in assessing the sulphur doping by correlating the oxygen deficiency of the doped oxide with the change of active Eg mode of rutile phase.     

PVK/C_(60)组合体系薄膜的拉曼光谱和荧光光谱研究

我们采用物理喷束淀积技术制备了Ｃ６０、Ｃ７０及聚乙烯咔唑ＰＶＫ／Ｃ６０的混合和分层薄膜，拉曼光谱的测量表明，这种技术所制备的富勒烯薄膜中，富勒烯的笼型结构仍保持完整，而在ＰＶＫ／Ｃ６０组合薄膜中，拉曼光谱及荧光光谱测量表明：在ＰＶＫ和Ｃ６０分子之间存在激发传递过程，在混合膜中，这种激发传递过程要明显强于分层组合薄膜。     

High pressure Raman spectroscopic study of spinel MgCr2O4

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr₂O₄ spinel up to pressures of 76.4 GPa. Results indicate that MgCr₂O₄ spinel undergoes a phase transformation to the CaFe₂O₄ (or CaTi₂O₄) structure at 14.2 GPa, and this transition is complete at 30.1 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr₂O₄ at 5.7-18.8 GPa, predicted by the theoretical simulation. This high pressure MgCr₂O₄ polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.     

Direct characterization of phase behavior and compatibility in PET/HDPE polymer blends by confocal Raman mapping

Morphology, chemical distribution and domain size in poly(ethylene terephthalate)/high‐density poly(ethylene) (PET/HDPE) polymer blends of various ratios prepared with and without maleic anhydride have been analyzed with confocal Raman mapping and SEM. The ratioimage method introduced here allows us to obtain enhanced chemical images with higher contrast and reliability. Compatibility numbers (N_c) are calculated to evaluate the compatibility of the blends. The incompatible polymer blends show heterogeneous distribution with phase separation behavior, while the semicompatible blends prepared with maleic anhydride show much smaller subphase distributions with less distinct interphases. After the blending modification by maleic anhydride of only 0.5%, the viscosity status and dispersibility between PET and HDPE could be substantially improved, and the interactions that exist between the two phases have also been proved by ATR‐FT‐IR results. High‐spatial‐resolution confocal Raman mapping coupled with the ratioimage method provides a very attractive way to characterize the compatibility and phase behavior of the polymer blend through different blending methodologies. Copyright © 2006 John Wiley & Sons, Ltd.     

Photoinduced C70 radical anions in polymer:fullerene blends

Photoinduced polarons in solid films of polymer:fullerene blends were studied by photoluminescence (PL), photoinduced absorption (PIA) and electron spin resonance (ESR). The donor materials used were P3HT and MEH‐PPV. As acceptors we employed PC₆₀BM as reference and various soluble C₇₀‐derivates: PC₇₀BM, two different diphenylmethano‐[70]fullerene oligoether (C₇₀‐DPM‐OE) and two dimers, C₇₀–C₇₀ and C₆₀–C₇₀. Blend films containing C₇₀ revealed characteristic spectroscopic signatures not seen with C₆₀. Light‐induced ESR showed signals at g ≥ 2.005, assigned to an electron localized on the C₇₀ cage. The formation of C₇₀ radical anions also leads to a subgap PIA band at 0.92 eV, hidden in the spectra of C₇₀‐based P3HT and MEH‐PPV blends, which allows for more exact studies of charge separated states in conjugated polymer:C₇₀ blends. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Colour diversification in octocorals based on conjugated polyenes: A Raman spectroscopic view

Polyenic pigments in octocorals have been investigated by Raman spectroscopy using laser excitation at 532, 785 and 1064 nm. The spectral features suggest the structural nature of carotenoids from Phyllogorgia dilatata, Leptogorgia punicea, Muricea atlantica, Carijoa riisei and conjugated polyenals from L. punicea, L. setacea, Muricea flamma and Renilla muelleri. The observed vibrational bands at ca. 1540–1520 ν₁(C=C), 1159 ν₂(C–C) and 1005 cm⁻¹ ρ₃(C–CH₃) were assigned to carotenoids, whereas the identification of non‐methylated conjugated polyenals have been proposed due to two major Raman bands at ca. 1500 and 1120 cm^‐1, assigned to ν(C=C) and ν(C–C), respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman spectroscopic studies of ZnSe/GaAs interfaces

ZnSe/semi‐insulating GaAs interfaces were studied by observing photogenerated plasmon–LO (PPL) coupled modes by nonresonant micro‐Raman spectroscopy. The effect of the carriers generated by the focused laser beam was investigated for a series of different thicknesses of ZnSe epitaxial layers. The PPL mode in GaAs was observed in the micro‐Raman spectra for all samples, but with different magnitude. The plasma is believed to be an electron gas as a result of the negative nature of the interfacial region that contains predominantly hole traps. The free carrier concentration is estimated to be > 10¹⁸ cm⁻³ and their lifetime ∼0.1 ns. This relatively long lifetime suggests that the ZnSe/GaAs interface has to be of high structural quality leading to a low recombination velocity. ZnSe/GaAs heterostructures of less crystalline quality (as determined by resonant Raman measurements) shows the effect of photogenerated carriers only to lesser extent. Copyright © 2007 John Wiley & Sons, Ltd.     

Charge transport in low-concentration MEH-PPV conjugated polymer/fullerene composites

Charge transport was studied in composites of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) conjugate polymer and low-concentration fullerenes (C₆₀) below the percolation threshold. The electron mobility showed a linear increase with the fullerene concentration, to which the hole mobility was insensitive. Our results indicate that fullerene–polymer networks provide a conduction path to the electrons, whereas the holes are transported through the polymer-only paths. The microscopic environments of the two distinct conduction paths in the composites as revealed by the electric field dependence of the mobilities are also discussed.     

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm⁻¹, belonging to E₁ symmetry [SO(E₁)], is identified along with symmetry allowed Raman modes of E₂(low), E₂(high), and E₁(LO) at 87, 489, and 589 cm⁻¹, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E₁(LO) and SO(E₁) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E₁(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E₁) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman spectroscopic analysis and fast identification of several saturated monohydroxy alcohols

The Raman spectra of nine monohydroxy alcohols have been obtained by confocal Raman spectrometer at room temperature. Based on the Raman spectra, the density functional theory was used to analyze the characteristic Raman bands of monohydric alcohols. Through the discussion of the characteristic Raman bands and their corresponding assignment, four major Raman bands were selected to identify nine monohydric alcohols using principal component analysis and Euclidean distance. Finally, nine saturated monohydroxy alcohols can be distinguished exactly, and the recognition rate is 100%.     

Raman spectroscopic study of various types of tourmalines

Both polarized and unpolarized Raman scattering studies of seven tourmalines from the Lucyen mines in Vietnam are presented. These tourmalines, according to their chemical compositions, can be classified into four groups: G1, liddicoatite; G2, elbaite; G3, uvite; and G4, feruvite. The Raman spectra were recorded in two spectral ranges, i.e. 150–1600 cm⁻¹ and 3000–4000 cm⁻¹. In the lower spectral range, which covers the metal ion‐oxygen bond vibrations, all the observed A₁ and E modes are identified. In the higher spectral range, we investigated the OH stretching vibrations and showed that all the observed OH stretching modes have the A₁ character. In both spectral ranges, we found that the same group classification of tourmalines can be applied, and the grouping characterizations are consistent with the chemical composition results. Copyright © 2011 John Wiley & Sons, Ltd.     

银电极表面上C60薄膜的表面增强拉曼光谱研究

在银电极表面形成一层C60薄膜,分别在乙腈溶液和水溶液中进行表面增强拉曼光谱(SERS)研究并将两者进行比较,从而消除了溶液中的C60干扰表面吸附C60的SERS谱图的可能性.研究结果表明,C60分子对称性的降低导致SERS谱峰发生了分裂;表面电磁场的作用使得光谱选律在SERS效应中被拓宽,产生了新的拉曼谱峰.该结果与团簇吸附在粗糙银电极表面的C60分子的研究结果相似.与之不同的是在乙腈溶液和水溶液中的SERS谱图的低波数区内分别在348和311 cm-1左右出现一个新峰,经过分析可认为该峰与C60-金属基底的相互作用有关.     

Low‐temperature Raman spectroscopic studies in NaNbO3

Raman spectroscopic measurements were carried out in the temperature range 10–300 K to understand the low‐temperature antiferroelectric (AFE)–ferroelectric (FE) phase transition in NaNbO₃. Several modes in the low wavenumber range were found to disappear, while some new modes appeared across the transition. The temperature dependence of mode wavenumbers suggests that, during cooling, the AFE–FE phase transition begins to occur at 180 K, while the reverse transition starts at 260 K during heating. During cooling, the two phases were found to coexist in the temperature range of 220–160 K. Upon heating, the FE phase is retained up to 240 K and both FE and AFE phases coexist in the temperature range 240–300 K. In contrast to the earlier reports, the present results suggest a different coexistence region and the reverse transition temperature. The reported relaxor‐type FE behaviour over a broad temperature is consistent with the observed coexistence of phases during cooling and heating cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

Nanoparticles for dewetting suppression of thin polymer films used in chemical sensors

Addition of fullerenes (C₆₀ or buckyballs) to a linear polymer has been found to eliminate dewetting when a thin (∼50 nm) film is exposed to solvent vapor. Based on neutron reflectivity measurements, it is found that the fullerenes form a coherent layer approximately 2 nm thick at the substrate – polymer film interface during the spin-coating process. The thickness and relative fullerene concentration (∼29 vol%) is not altered during solvent vapor annealing and it is thought this layer forms a solid-like buffer shielding the adverse van der Waals forces promoted by the underlying substrate. Several polymer films produced by spin- or spray-coating were tested on both silicon wafers and live surface acoustic wave sensors demonstrating fullerenes stabilize many different polymer types, prepared by different procedures and on various surfaces. Further, the fullerenes drastically improve sensor performance since dewetted films produce a sensor that is effectively inoperable.     

Elastic strain at the interface in Ge clusters on Si substrate—a Raman spectroscopic measurement

Ge clusters are grown on Si substrate at room temperature (Ge-RT) and also at liquid nitrogen temperature (Ge-LNT) by cluster evaporation technique. These clusters show blue luminescence. Raman measurement demonstrates the increase in strain with annealing in diffused disordered Si at the interface between Ge-LNT clusters and Si substrate. This manifests in strain-relaxation in the clusters as observed by Photoluminescence (PL) measurements. The decrease in PL intensity for Ge-RT with annealing has been attributed to reduction in surface oxide species, which is supported by Raman spectroscopic measurements. The objective of the paper is to understand the effect of thermal annealing on both interfacial strain and interdiffusion of elemental Si at the interface, together with luminescence characteristics of the clusters.     

Identification of individual isotopes in a polymer blend using tip enhanced Raman spectroscopy

For the first time, tip enhanced Raman spectroscopy (TERS) blinking measurements are used to identify the individual isotopes of non‐Raman resonant polystyrene in a miscible, binary blend. This demonstrates the sensitivity and selectivity required for nanoscale chemical imaging and broadens the types of surface components potentially identifiable with TERS. Copyright © 2015 John Wiley & Sons, Ltd.     

Raman identification of H2CO in aqueous solutions

Aqueous formaldehyde solutions of mole fraction from 0.05 to 0.20 were studied by Raman spectroscopy at temperatures up to 180 °C. The previously unreported Raman spectrum of formaldehyde under its non‐hydrated form H₂CO has been identified, in addition to the already known bands from methanediol CH₂(OH)₂ and poly(oxymethylene)glycols HO(CH₂O)_nH formed by hydration and subsequent polymerization of the formaldehyde molecules. The experimental spectrum of formaldehyde in solutions agrees perfectly with the theoretical calculations. Copyright © 2010 John Wiley & Sons, Ltd.