Search

Polarized spectroscopic photodetection enables numerous applications in diverse areas such as sensing, industrial quality control, and visible light communications. Although organic photodetectors (OPDs) can offer a cost-effective alternative to silicon-based technology—particularly when flexibility and large-area arrays are desired—polarized OPDs are only beginning to receive due research interest. Instead of resorting to external polarization optics, this report presents polarized OPDs based on directionally oriented blends of poly(3-hexylthiophene) (P3HT) and benchmark polymer or nonfullerene acceptors fabricated using a versatile solution-based method. Furthermore, a novel postprocessing scheme based on backfilling and plasma etching is advanced to ameliorate high dark-currents that are otherwise inherent to fibrillar active layers. The resulting polarized P3HT:N2200 OPDs exhibit a broad enhancement across all principal figures of merit compared to reference isotropic devices, including peak responsivities of 70 mA W$^{-1}$ and up to a threefold increase in 3 dB bandwidth to 0.75 MHz under parallel-polarized illumination. Polarization ratios of up to 3.5 are obtained across a spectral range that is determined by the specific donor–acceptor combinations. Finally, as a proof-of-concept demonstration, polarized OPDs are used for photoelasticity analysis of rubber films under tensile deformation, highlighting their potential for existing and emerging applications in advanced optical sensing.

The competitive performance of optoelectronic devices based on advanced organic semiconductors increasingly calls for suitably scalable processing schemes to capitalise on their application potential. With performance benchmarks typically established by spin-coating fabrication, doctor-blade deposition represents a widely available roll-to-roll-compatible means for the preparation of large-area samples and establishing the device upscaling potential. However, the inherently slower film formation kinetics often result in unfavourable active layer microstructures, requiring empirical and material-inefficient optimisation of solutions to reach the performance of spin-coated devices. Here we present a versatile approach to achieving performance parity for spin- and blade-coated devices using in situ gas-assisted drying enabled by a modular 3D-printed attachment. This is illustrated for organic photodetectors (OPDs) featuring bulk heterojunction active layers comprising blends of P3HT and PM6 polymer donors with the nonfullerene acceptor ITIC. Compared to conventionally blade-coated devices, mild drying gas pressures of 0.5–2 bar yield up to a 10-fold enhancement of specific detectivity by maximising external quantum efficiency and suppressing dark-current. Furthermore, controlling gas flux distribution enables one-step fabrication of 1D chain conformation and 2D chain orientation patterns in, respectively, PFO and P3HT:N2200 blend films, opening the possibility for high-throughput fabrication of devices with complex structured active layers.

The correct determination of the exciton diffusion length (LD) in novel organic photovoltaics (OPV) materials is an important, albeit challenging, task required to understand these systems. Herein, a high-throughput approach to probe LD in nonfullerene acceptors (NFAs) is reported, that builds upon the conventional photoluminescence (PL) surface quenching method using NFA layers with a graded thickness variation in combination with spectroscopic PL mapping. The method is explored for two archetypal NFAs, namely, ITIC and IT-4F, using PEDOT:PSS and the donor polymer PM6 as two distinct and practically relevant quencher materials. Interestingly, conventional analysis of quenching efficiency as a function of acceptor layer thickness results in a threefold difference in LD values depending on the specific quencher. This discrepancy can be reconciled by accounting for the differences in light in- and outcoupling efficiency for different multilayer architectures. In particular, it is shown that the analysis of glass/acceptor/PM6 structures results in a major overestimation of LD, whereas glass/acceptor/PEDOT:PSS structures give no significant contribution to outcoupling, yielding LD values of 6−12 and 8−18 nm for ITIC and IT-4F, respectively. Hence, practical guidelines for quencher choice, sample geometries, and analysis approach for the accurate assessment of LD are provided., V.B., A.P., and J.G. contributed equally to this work. The authors acknowledge that this research was financially supported by the European Research Council (ERC) under grant agreement no. 648901. The authors also acknowledge financial support from the Spanish Ministry of Science and Innovation through the Severo Ochoa Program for Centers of Excellence in R&D (CEX2019-000917-S) and project PGC2018-095411-B-I00. This publication was based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award no: OSR-2018-CARF/CCF-3079 and award no. OSR-CRG2018-3746. The authors thank Anastasia Ragulskaya (The University of Tübingen) for contributing to the development of the computational model.

We present a simple technique for measuring haze and transmittance of materials that uses inexpensive and virtually ubiquitous hardware, and is based on photographic imaging of at least partially transparent samples backlit through a “knife-edge” array mask. Its performance is tested for a series of injection-molded polyethylene plaques containing a varied fraction of a clarifying agent, as well as commercially available ASTM D1003 haze standards. The results show excellent agreement with those obtained using a conventional haze meter adhering to the ASTM D1003 standard and, moreover, are specifically indicative of the overall contact haze—a desirable feature for characterization of, for instance, commodity plastics for packaging applications. Notably, the quantification of haze is based explicitly on the reduction of perceived image quality of objects viewed through a material sample and, hence, quantifies haze in a more practically useful way compared with the, arguably, more arbitrary ASTM definition. The demonstrated technique can be further used to extract more sophisticated, local information on the microstructure and optical properties, which cannot be obtained with a conventional haze meter. Its potential for being standardized, as well as additional modifications that can enable improved performance, are also discussed. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers

Here we show that molecular doping of polymer thermoelectrics increases the electrical conductivity while reducing the thermal conductivity. A high-throughput methodology based on annealing and doping gradients within individual films is employed to self-consistently analyze and correlate electrical and thermal characteristics for the equivalent of >100 samples. We focus on the benchmark material system poly(2,5- bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) doped with molecular acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The thermal conductivity of neat PBTTT films is dominated by the degree of crystallinity, with thermal percolation observed for annealing temperatures >170 °C. Upon doping the samples with a relatively low amount of F4TCNQ (anion content, We acknowledge financial support from the Spanish Ministry of Science and Innovation through projects PGC2018-095411- B-I00 and MAT2017-90024-P (TANGENTS)-EI/FEDER, UE projects; the Generalitat de Catalunya through grants 2017SGR488 and AGAUR 2018 PROD 00191; and from the European Research Council (ERC) under grant agreement no. 648901. O.Z.-A. acknowledges CONACYT-SENER for his Ph.D. scholarship (no. 472571). J.M. thanks MCIU for the Ramon y Cajal contract and grant PGC2018-094620-A-I00. ́ We thank Dr. Agusti ́ n Mihi for the access to and support with the FTIR equipment. We acknowledge the technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). We thank Andreś Gomez Rodri ́ ́ guez from the Scanning Probe Microscopy Laboratory (ICMAB-CSIC) for a set of AFM measurements. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

The surprising optical properties of the non--conjugated polymer poly(phenylene methylene) (PPM) and its derivatives—that is, absorption in the 350–450 nm and photoluminescence (PL) in the 400–600 nm spectral regions—have been attributed to chromophores formed by homoconjugation along the polymer chain. The enabling role of homoconjugation, however, was hitherto ascertained primarily by excluding alternative origins of luminescence. The present study offers direct evidence for homoconjugation by employing optical and vibrational spectroscopy to investigate the interplay between microstructure and solid-state optical properties of PPM and its derivative poly(2,4,6-trimethylphenylene methylene) (PTMPM). In particular, polarized Raman and PL spectroscopy of melt-drawn fibers reveal a preferentially perpendicular orientation of the phenylene rings relative to the fiber axis, while, simultaneously, a preferentially parallel orientation of the transition dipole moment. PL spectroscopy under applied hydrostatic pressure yields a nearly 4-fold increase in PL intensity at 8 GPa, together with a surprising absence of excimer emission. These characteristics, being highly atypical of conventional -conjugated polymers, highlight the different origin of the optical properties of poly(phenylene methylene)s as well as unique opportunities for applications., The authors thank Drs Nathan Cheetham (Imperial College London) and Johannes Gierschner (IMDEA Nanociencia) for stimulating discussions. A.P. is indebted to the Great European Lockdown (GEL) network for enabling Prof. Paul Smith (ETH Zürich) to comprehensively review this manuscript on one of his lesser favored topics, for which he is also thanked sincerely. The Spanish Ministerio de Ciencia, Innovación y Universidades is gratefully acknowledged for its support through grant no. SEV-2015-0496 in the framework of the Spanish Severo Ochoa Centre of Excellence Program and through grant PGC2018-095411-B-100 (RAINBOW). We also thank the Catalan agency AGAUR for grant 2017-SGR-00488. AFL acknowledges a FPI fellowship (BES-2016-076913) from the Spanish Ministerio cofinanced by the European Social Fund and the Ph.D. program in Materials Science from Universitat Autònoma de Barcelona in which he is enrolled. Financial support is also acknowledged from the European Research Council through project ERC CoG648901.

Two doping mechanisms are known for the well-studied materials poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), namely, integer charge transfer (ICT) and charge transfer complex (CTC) formation. Yet, there is poor understanding of the effect of doping mechanism on thermal stability and the thermoelectric properties. In this work, we present a method to finely adjust the ICT to CTC ratio. Using it, we characterize electrical and thermal conductivities as well as the Seebeck coefficient and the long-term stability under thermal stress of P3HT and PBTTT of different ICT/CTC ratios. We establish that doping through the CTC results in more stable, yet lower conductivity samples compared to ICT doped films. Importantly, moderate CTC fractions of ∼33% are found to improve the long-term stability without a significant sacrifice in electrical conductivity. Through visible and IR spectroscopies, polarized optical microscopy, and grazing-incidence wide-angle X-ray scattering, we find that the CTC dopant molecule access sites within the polymer network are less prone to dedoping upon thermal exposure., We thank Dr. Martijn Kemerink for fruitful discussions at the early stages of this project. The authors acknowledge financial support from the Spanish Ministry of Economy, Industry, and Competitiveness through the “Severo Ochoa” Program for Centers of Excellence in R&D (SEV-2015-0496), MAT2015-70850-P, PGC2018-095411-B-I00, and No. MAT2017-90024-P (TANGENTS)-EI/FEDER, UE projects; from the Generalitat de Catalunya through grants 2017SGR488 and AGAUR 2018 PROD 00191; from CSIC through project 201560I032; and from the European Research Council (ERC) under grant agreement no. 648901. O.Z.-A. acknowledges CONACYT-SENER for his Ph.D. scholarship (No. 472571). J.M. furthermore thanks MINECO for the Ramón y Cajal contract and the PGC2018-094620-A-I00 grant. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

ISSN:1438-7492 ISSN:1439-2054

A versatile imaging‐based method is presented for quantifying the transparency of materials based on “illumination diffusion” (ID), representing scattering‐ and refraction‐induced change in the spatial distribution of transmitted light intensity. Samples are backlit through a graticule mask, with analysis performed by comparative evaluation of graticule images recorded as‐is and viewed through a sample, mimicking visual perception. ID‐haze is quantified as the reduction of contrast, while ID‐sharpness is derived from imaged knife‐edge acuity. Measurements are performed for diverse materials, including clarified polyolefins, silica‐filled amorphous polymers, semicrystalline films, and etched polymer sheets. Comparisons with the respective haze and clarity values obtained using a common ASTM D1003 haze‐meter are made in terms of their quantitative correlation and suitability for applications. In particular, unlike conventional instruments, ID‐based analysis captures the variation of transparency with sample‐to‐object “airgap” distance. Gratifyingly, ID‐haze generally features a one‐to‐one correlation with standard ASTM haze, when determined at a specific distance. The presented method also enables sensitive detection of local defects—differentiating them from large‐area characteristics—and accurately extracts the contribution of luminescence to loss of transparency. ID‐based method therewith offers unique opportunities for application‐ and airgap‐specific transparency analysis, and advanced options for optical process‐ and quality control.

Synthesis, processing, and characterization are reported for a series of tetracyanoplatinate Magnus' salt (TCN-MS) derivatives-soluble derivatives of the generally intractable Magnus' green salt-that feature the general structure [Pt(NH2R)(4)] [Pt(CN)(4)] where R is a branched alkyl group or a w-phenylalkyl group. In solutions, these coordination compounds generally dissolve on the level of individual ion pairs as shown by X-ray diffraction analysis. To enable the formation of quasi-one-dimensional linear stacks of Pt(II) atoms in thin films, the matrix-assisted assembly is employed, whereby the compounds are codissolved with poly(ethylene oxide) (PEO), followed by film casting, thermally activated assembly, and eventual removal of PEO. Remarkably, assembled TCN-MS inorganic polymers exhibit bright blue-green photoluminescence. A detailed investigation of the assembly process and simultaneously modified solid-state optical properties is performed using a range of microscopy, optical and vibrational spectroscopy, and thermal analysis techniques. Given their unusual combination of optical properties, namely, transparency in the visible region, high photoluminescence quantum efficiencies (up to 13% in first-demonstration samples), and large Stokes shifts (up to 1 eV), TCN-MS derivatives are proposed as a promising class of light-emitting materials for emerging applications in molecular optoelectronics, the potential and challenges of which are discussed., The work in Barcelona was financially supported by the Ministerio de Economía y Competitividad of Spain through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496) and project MAT2015-70850-P and the European Research Council (ERC) under grant agreement no. 648901. The work in Madrid and Valencia was supported by the Spanish Ministerio de Economía y Competitividad (MINECO-FEDER project CTQ2017-87054); the work in Madrid was further supported by the Severo Ochoa Programme for Centers of Excellence in R&D program of the MINECO (SEV-2016-0686) and by the Campus of International Excellence (CEI) UAM + CSIC.

Phase-change memory materials refer to a class of materials that can exist in amorphous and crystalline phases with distinctly different electrical or optical properties, as well as exhibit outstanding crystallization kinetics and optimal phase transition temperatures. This paper focuses on the potential of colloids as phase-change memory materials. We report a novel synthesis for amorphous GeTe nanoparticles based on an amide-promoted approach that enables accurate size control of GeTe nanoparticles between 4 and 9 nm, narrow size distributions down to 9-10%, and synthesis upscaling to reach multigram chemical yields per batch. We then quantify the crystallization phase transition for GeTe nanoparticles, employing high-temperature X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. We show that GeTe nanoparticles crystallize at higher temperatures than the bulk GeTe material and that crystallization temperature increases with decreasing size. We can explain this size-dependence using the entropy of crystallization model and classical nucleation theory. The size-dependences quantified here highlight possible benefits of nanoparticles for phase-change memory applications.

Polymer-solvent compound formation, occurring via co-crystallization of polymer chains and selected small-molecular species, is demonstrated for the conjugated polymer poly(9,9-dioctylfluorene) (PFO) and a range of organic solvents. The resulting crystallization and gelation processes in PFO solutions are studied by differential scanning calorimetry, with X-ray diffraction providing additional information on the resulting microstructure. It is shown that PFO-solvent compounds comprise an ultra-regular molecular-level arrangement of the semiconducting polymer host and small-molecular solvent guest. Crystals form following adoption of the planar-zigzag β-phase chain conformation, which, due to its geometry, creates periodic cavities that accommodate the ordered inclusion of solvent molecules of matching volume. The findings are formalized in terms of nonequilibrium temperature–composition phase diagrams. The potential applications of these compounds and the new functionalities that they might enable are also discussed., Journal of Polymer Science. Part B, Polymer Physics, 53 (21), ISSN:0887-6266, ISSN:0098-1273, ISSN:1099-0488

Solution-crystallization is studied for two polyfluorene polymers possessing different side-chain structures. Thermal analysis and temperature-dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X-ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar-zigzag chain conformation termed the β-phase, which is observed for certain linear-side-chain polyfluorenes, is necessary for the formation of so-called polymer-solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side-chains prevents formation of the β-phase conformation and results in non-solvated, i.e. melt-crystallization-type, polymer crystals. Unlike non-solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the β-phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter-chain interactions. The presented results clarify the fundamental differences between the β-phase and other conformational/crystalline forms of polyfluorenes., Journal of Polymer Science. Part B, Polymer Physics, 53 (21), ISSN:0887-6266, ISSN:0098-1273, ISSN:1099-0488

Poly(phenylene methylene) (PPM) exhibits pronounced blue fluorescence in solutions as well as in the solid state despite its non-π-conjugated nature. Optical spectroscopy was used to explore the characteristics and the physical origin of its unexpected optical properties, namely absorption in the 350–450 nm and photoluminescence in the 400–600 nm spectral regions. It is shown that PPM possesses two discrete optically active species, and a relatively long photoluminescence lifetime (>8 ns) in the solid-state. Given the evidence reported herein, π-stacking and aggregation/crystallization, as well as the formation of anthracene-related impurities, are excluded as the probable origins of the optical properties. Instead there is sufficient evidence that PPM supports homoconjugation, that is: π-orbital overlap across adjacent repeat units enabled by particular chain conformation(s), which is confirmed by DFT calculations. Furthermore, poly(2-methylphenylene methylene) and poly(2,4,6-trimethylphenylene methylene) – two derivatives of PPM – were synthesized and found to exhibit comparable spectroscopic properties, confirming the generality of the findings reported for PPM. Cyclic voltammetry measurements revealed the HOMO–LUMO gap to be 3.2–3.3 eV for all three polymers. This study illustrates a new approach to the design of light-emitting polymers possessing hitherto unknown optical properties. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 707–720

Poly(9,9‐dioctylfluorene) (PFO) is a widely studied blue‐emitting conjugated polymer, the optoelectronic properties of which are strongly affected by the presence of a well‐defined chain‐extended “β‐phase” conformational isomer. In this study, optical and Raman spectroscopy are used to systematically investigate the properties of PFO thin films featuring a varied fraction of β‐phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the β‐phase fraction and the method by which it was induced. Notably, a PLQE of ∼69% is measured for PFO films possessing a ∼6% β‐phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of ∼55% recorded for other β‐phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the β‐phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the β‐phase fraction. As a specific example, spatial Raman mapping is used to image a mm‐scale β‐phase stripe patterned in a glassy PFO film, with the extracted β‐phase fraction showing excellent agreement with the results of optical spectroscopy. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1995–2006

Coupled-cavity passive harmonic mode-locking of a quantum well based vertical-external-cavity surface-emitting laser has been demonstrated, yielding an output pulse train of 1.5 ps pulses at a repetition rate of 80 GHz and with an average power of 80 mW. Harmonic mode-locking results from coupling between the main laser cavity and a cavity formed within the substrate of the saturable absorber structure. Mode-locking on the second harmonic of the substrate cavity allows a train of 1.1 ps pulses to be generated at a repetition rate of 147 GHz with 40 mW average power.