Wing Chung Tsoi

The nature of in-plane skeleton Raman modes of P3HT and their correlation to the degree of molecular order in P3HT:PCBM blend thin films.

The nature of main in-plane skeleton Raman modes (C=C and C-C stretch) of poly(3-hexylthiophene) (P3HT) in pristine and its blend thin films with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) is studied by resonant and nonresonant Raman spectroscopy and Raman simulations. Under resonant conditions, the ordered phase of P3HT with respect to its disordered phase is identified by (a) a large shift in the C=C mode peak position to lower wavenumber (~21 cm(-1) shift), (b) a narrower fwhm of the C=C mode (~9 cm(-1) narrower), (c) a larger intensity of the C-C mode relative to the C=C mode (~56% larger), and (d) a very small Raman dispersion (~5 cm(-1)) of the C=C mode. The behavior of the C=C and C-C modes of the ordered and disordered phases of P3HT can be explained in terms of different molecular conformations. The C=C mode of P3HT in P3HT:PCBM blend films can be reproduced by simple superposition of the two peaks observed in different phases of P3HT (ordered and disordered). We quantify the molecular order of P3HT after blending with PCBM and the subsequent thermal annealing to be 42 ± 5% and 94 ± 5% in terms of the fraction of ordered P3HT phase, respectively. The increased molecular order of P3HT in blends upon annealing correlates well with enhanced device performance (J(SC), -4.79 to -8.72 mA/cm(2) and PCE, 1.07% to 3.39%). We demonstrate that Raman spectroscopy (particularly under resonant conditions) is a simple and powerful technique to study molecular order of conjugated polymers and their blend films.

Performance enhancement of fullerene-based solar cells by light processing.

A key challenge to the commercialization of organic bulk heterojunction solar cells is the achievement of morphological stability, particularly under thermal stress conditions. Here we show that a low-level light exposure processing step during fabrication of blend polymer:PC60BM solar cells can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerization of PC60BM that effectively hinders their diffusion and crystallization in the blend. We thus suggest that light processing may be a promising, general and cost-effective strategy to optimize fullerene-based solar cell performance. The low level of light exposure required suggests not only that this may be an easily implementable strategy to enhance performance, but also that light-induced PC60BM oligomerization may have inadvertently influenced previous studies of organic solar cell device behaviour.

Photochemical stability of high efficiency PTB7:PC70BM solar cell blends

Thieno[3,4 b]thiophene-alt-benzodithiophene (PTB7) is a promising donor–acceptor copolymer that has achieved high efficiencies (7–9%) in organic solar cells but suffers from poor stability and degrades when exposed to light and oxygen. Using resonant Raman spectroscopy to examine the nature of this photo-oxidation, three main changes to the vibrations of the conjugated backbone are observed: (1) shift of the benzodithiophene (BDT) CC stretch peak at ∼1489 cm−1 up to ∼1499 cm−1; (2) increase in the relative intensity of coupled fused thiophene and benzene CC stretch peaks at ∼1535 and ∼1575 cm−1; (3) appearance of a new peak at ∼1650 cm−1; which suggest oxidation takes place on the BDT unit without loss of conjugation. In situ accelerated photo-degradation reveals that the observed oxidation is the initial step of degradation, which is followed by reductions in absorption and Raman scattering intensities that indicate the loss of chromophores by a second, more extensive oxidation step. Blending PTB7 with PC70BM is found to accelerate the polymers degradation, and further shift the BDT peak to ∼1509 cm−1. Using density functional theory to simulate Raman spectra for several possible oxidised products, the initial oxidation is best described by hydroxylation of 3rd and 7th positions on the BDT donor unit.

Blue polymer optical fiber amplifiers based on conjugated fluorene oligomers

We fabricated polymer optical fiber (POF) amplifiers operating between 440 and 480 nm, using POFs doped with a series of fluorene oligomers, including tri-, penta-(9,9-dioctylfluorene) and hepta-(9,9-dihexylfluorene). The gain properties of pure oligofluorene films demonstrate gain coefficients on the order of 250 dB/cm and amplified spontaneous emission thresholds between 1 and 8 μJ c m-2 , significantly lower than other fluorene gain media. The optical and morphological characteristics of PMMA thin films doped with the oligomers demonstrate that the oligomers are largely isolated within the PMMA. The optical and gain properties of POFs produced using an adapted preform-drawing technique and doped with the oligofluorenes provide gain values on the order of 0.07 dB for 2 mm of doped POF. The oligofluorenes are largely isolated within the POFs, paving the way for all optical gain-switching.

Is organic photovoltaics promising for indoor applications

This work utilizes organic photovoltaics (OPV) for indoor applications, such as powering small electronic devices or wireless connected Internet of Things. Three representative polymer-based OPV systems, namely, poly(3-hexylthiophene-2,5-diyl), poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]], were selected as the donor materials in blend with fullerene derivatives for comparison under low light level condition using fluorescent lamps. PCDTBT based devices are found to be the best performing system, generating 13.9 μW/cm2 corresponding to 16.6% power conversion efficiency at 300 lx, although PTB7 based devices show the highest efficiency under one sun conditions. This high performance suggests that OPV is competitive to the other PV technologies under low light condition despite much lower performance under one sun conditi...

Surface and subsurface morphology of operating nanowire:fullerene solar cells revealed by photoconductive-AFM

The 3D nanometer scale phase separated morphology of organic solar cells crucially affects performance. We demonstrate that photoconductive atomic force microscopy can provide both surface and subsurface information in operating organic solar cells providing direct correlation between 3D film morphology, local nanoscale optoelectronic properties and device characteristics. P3HT nanowire:PCBM bulk-heterojunction working devices were investigated. The macroscopic solar cell performance improvements upon thermal annealing, such as an increase in the short circuit current, the open circuit voltage and the fill factor, are consistent with observed enrichment of PCBM at the air interface and increased nanowire crystallinity. PC-AFM is able to directly resolve the associated changes in charge transport and collection at the local scale, with an estimated depth resolution of at least 20 nm inside the film.

Understanding the relationship between molecular order and charge transport properties in conjugated polymer based organic blend photovoltaic devices

We report a detailed characterization of the thin film morphology of all-polymer blend devices by applying a combined analysis of physical, chemical, optical, and charge transport properties. This is exemplified by considering a model system comprising poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). We show that the interactions between the two conjugated polymer components can be controlled by pre-forming the P3HT into highly ordered nanowire structures prior to blending with F8BT, and by varying the molecular weight of the F8BT. As a result, it is possible to produce films containing highly ordered P3HT with hole mobilities enhanced by three orders of magnitude over the pristine blends. Raman spectroscopy under resonant excitation conditions is used to probe the molecular order of both P3HT and F8BT phases within the blend films and these morphological studies are complemented by measurements of photocurrent generation. The resultant increase in photocurrent is associated with the enhanced charge carrier mobilities. The complementary analytical method demonstrated here is applicable to a wide range of polymer blend systems for all applications where the relationships between morphology and device performance are of interest.

Directly probing the molecular order of conjugated polymer in OPV blends induced by different film thicknesses, substrates and additives

In organic bulk heterojunction photovoltaic (OPV) devices, formation of a phase-separated morphology of the blend thin film with a high degree of molecular order is required for efficient device performance. Using resonant Raman spectroscopy we monitor in situ the P3HT molecular order in P3HT:PCBM blend films influenced by the substrate, film thickness and additives. We report that molecular order depends on substrate for as-cast films, consistent with vertical phase separation driven by a surface energy gradient, but is standardised to a highly ordered state by thermal annealing. In situ Raman spectroscopy reveals this phase transition to a more ordered state begins at 40–60 °C for ∼120 nm thick blend films, which corresponds to the glass transition temperature (Tg). Ultra-thin (<10 nm thick) blend films had greater P3HT order than the bulk and reorganised at lower temperatures, which we propose is due to a P3HT-rich interfacial layer at the film/air interface, and that extra disordered component retained despite annealing is due to P3HT trapped in a disordered state within the corresponding PCBM-rich substrate interface. Finally we probe how the 1,8-octanedithiol (ODT) additive improves P3HT molecular order in blends by increasing phase separation during deposition, finding that 3% ODT by volume presents a saturation point for improving molecular order, and the improvement is comparable to that by thermal annealing. Through in situ experiments and varied fabrication conditions, we have built an understanding of how processing conditions determine conjugated polymer molecular order in blends, with the aim of controlling morphology for higher OPV efficiencies.

A characterization of the Raman modes in a J-aggregate-forming dye: a comparison between theory and experiment.

J-Aggregates are a class of organic molecules that possess several interesting characteristics that make them attractive for a range of organic-based optoelectronic devices. We present experimental and computer-simulation studies of the Raman-active vibrational modes in the J-aggregate-forming dye 5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)benzimidazol-2-ylidene]propenyl]-1-ethyl-3-(4-sulfobutyl)benzimidazolium hydroxide, sodium salt, inner salt. The molecular monomer and dimer are analyzed computationally and the Raman mode energies extracted. There is a good agreement between the energies of the theoretical and experimental Raman modes. Experimentally, an enhancement is seen in the intensity of two low frequency modes upon aggregation of the dye. This is attributed to aggregation-enhanced Raman scattering. An enhancement is also observed in certain modes of the calculated spectra upon changing from a monomer to dimeric arrangement. A link is suggested between the Raman-active vibrational modes of the molecule, and a time-dependent electronic coupling present over several molecules.

Raman spectroscopy of fluorene oligomers in the α-, β- and γ-phases

We present Raman spectroscopy measurements on a series of fluorene oligomers, and compare them with those of the polymer poly(9,9-dioctylfluorene) [PF8]. We show that such measurements can be used as evidence for the formation of the so-called β-phase in an oligofluorene, and confirm the picture in which the β-phase is stabilized by the adoption of an anti-gauche–gauche side chain conformation. We also demonstrate that Raman spectroscopy can be used to identify the formation of the so-called γ-phase (crystalline phase) in oligofluorene thin films. Our measurements suggest that the rich phase morphology observed in PF8 can often be replicated in thin films of its model oligomers.