We report the use of solvent vapor swelling of ultrathin polymer films to determine Flory–Huggins solvent–polymer and polymer–polymer interaction parameters (χ i–j ) for poly(3-hexylthiophene) (P3HT) and polystyrene (PS) over a wide solvent composition range. From the calculated interaction parameters, we constructed a polymer/polymer/solvent phase diagram that was validated experimentally. χtetrahydrofuran–P3HT (1.04 ± 0.04) and χCHCl3–P3HT (0.99 ± 0.01) were determined through swelling of ultrathin P3HT films. Similar experiments using PS films gave χtetrahydrofuran–PS = 0.41 ± 0.02 and χCHCl3–PS = 0.39 ± 0.01, consistent with literature values. As expected, these χ i–j parameters indicated that P3HT is less compatible than PS with either solvent. From δPS (17.9 ± 0.2 MPa1/2) and δP3HT (14.8 ± 0.2 MPa1/2), determined through regular solution theory, we calculated χPS–P3HT = 0.48 ± 0.06 at 23 °C. The resulting phase diagram was validated by solution-based transmission measurements of PS/P3HT blends in o-xylene. Although we focused on PS/P3HT blends in this work, this approach is easily adaptable to other polymer/polymer combinations of interest.

Nanocrystal quantum dots are generally coated with an organic ligand layer. These layers are a necessary consequence of their chemical synthesis, and in addition they play a key role in controlling the optical and electronic properties of the system. Here we describe a method for quantitative measurement of the ligand layer in 3 nm diameter lead sulfide–oleic acid quantum dots. Complementary small-angle X-ray and neutron scattering (SAXS and SANS) studies give a complete and quantitative picture of the nanoparticle structure. We find greater-than-monolayer coverage of oleic acid and a significant proportion of ligand remaining in solution, and we demonstrate reversible thermal cycling of the oleic acid coverage. We outline the effectiveness of simple purification procedures with applications in preparing dots for efficient ligand exchange. Our method is transferrable to a wide range of colloidal nanocrystals and ligand chemistries, providing the quantitative means to enable the rational design of ligand-exchange procedures.

A series of novel block copolymers, processable from single organic solvents and subsequently rendered amphiphilic by thermolysis, have been synthesized using Grignard metathesis (GRIM) and reversible addition–fragmentation chain transfer (RAFT) polymerizations and azide–alkyne click chemistry. This chemistry is simple and allows the fabrication of well-defined block copolymers with controllable block lengths. The block copolymers, designed for use as interfacial adhesive layers in organic photovoltaics to enhance contact between the photoactive and hole transport layers, comprise printable poly­(3-hexylthiophene)-block-poly­(neopentyl p-styrenesulfonate), P3HT-b-PNSS. Subsequently, they are converted to P3HT-b-poly­(p-styrenesulfonate), P3HT-b-PSS, following deposition and thermal treatment at 150 °C. Grazing incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) revealed that thin films of the amphiphilic block copolymers comprise lamellar nanodomains of P3HT crystallites that can be pushed further apart by increasing the PSS block lengths. The approach of using a thermally modifiable block allows deposition of this copolymer from a single organic solvent and subsequent conversion to an amphiphilic layer by nonchemical means, particularly attractive to large scale roll-to-roll industrial printing processes.

Spin-coating offers a facile fabrication route for the production of high quality colloidal crystals, which have potential as photonic band-gap materials. This paper presents the results of direct observations of the self-assembly of latex colloids during spin-coating through the use of stroboscopic microscopy. We have been able to identify several mechanisms by which self-assembly occurs, depending upon the dispersion properties, such as particle weight fraction, solvent volatility and viscosity. Through the use of stroboscopic microscopy we have directly observed ordering occurring due to high concentrations of colloid particles (where volatility is relatively low), resulting in the formation of regular close packed ordered particle arrays. Conversely when the system in spun-cast from a much more volatile solvent, highly disordered non-equilibrium arrangements of particles form. When spin-coating a low concentration, low volatility dispersion, ordering is dominated by the occurrence of capillary drying fronts, which drag the particles into ordered arrangements. At a volatility intermediate to that of water and ethanol, ordering occurring predominantly via shear forces. Finally when the volatility is increased beyond the shear ordering regime, excessive shear leads to the occurrence of drying fronts within the system and so again, capillary forces induce a large degree of order within the final film.

No abstract

High quality, uniform thin polymer films are routinely produced by the technique of spin coating. Applications for such polymer films beyond photoresist layer fabrication include photovoltaics and light-emitting diodes, where device performance is dependent upon an appropriate phase separated morphology. Developing an understanding of the factors involved in the development of such morphologies is therefore an important goal. The spin coating of polymer blends is a high speed, nonequilibrium process and as such, it is difficult to monitor in situ, with most studies inferring structure development from the resultant final morphology. Over the past 20 years various in situ experimental techniques have been developed, providing insight into the details of the spin coating process itself and opening a route to understanding and controlling morphological development. The majority of studies have been based upon interferometry and light scattering, ''based in reciprocal space'' and have been able to verify theoretical models of spin coating and the associated phase separation of polymer blends, with direct real-space, in situ studies now a possibility.

understood. Triplet behavior in organic solar cells is one of the most understudied aspects. The reason of the relatively low number of studies is that triplet formation is typically considered a loss pathway, and thus associated with systems with low device efficiencies. Indeed, many previous studies indicated that triplets were formed only when charge separation did not occur or was very inefficient. [3][4][5][6] As such, triplets were considered irrelevant for the highest efficiency OPV devices. However, it has recently been demonstrated that OPV blends with non-fullerene acceptors (NFAs), the main driver of the current record OPV efficiencies, can show pronounced triplet formation and yet still provide high efficiencies. [7] In the benchmark PM6:Y6 blend, for example, NFA triplet exciton formation accounts for 90% of charge recombination at open circuit, leading to a 60 mV reduction of the open circuit voltage. As such, it is important to elucidate triplet pathways in organic solar cells, in particular how they affect charge photogeneration and recombination. Triplets can be generated via several different pathways in organic solar cells. [8,9] First, the photogenerated singlet exciton can undergo intersystem crossing (ISC). Second, triplets can form via charge transfer (CT) states at the donor/acceptor interface. These CT states can undergo rapid spin-mixing due to the low exchange energy between 3 CT and 1 CT states, often Organic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency. One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation. Herein, a small molecule donor, DRCN5T, is studied using a variety of morphology and spectroscopy techniques, and blended with both fullerene and non-fullerene acceptors. Specifically, grazing incidence wide-angle X-ray scattering and transient absorption, Raman, and electron paramagnetic resonance spectroscopies are focused on. It is shown that despite DRCN5T's ability to achieve OPV efficiencies of over 10%, it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state, and also undergo triplet-charge annihilation. As such, triplets have a dual role in DRCN5T device efficiency suppression: they both hinder free charge carrier formation and annihilate those free charges that do form. Using microsecond transient absorption spectroscopy under oxygen conditions, this triplet-charge annihilation (TCA) is directly observed as a general phenomenon in a variety of DRCN5T: fullerene and non-fullerene blends. Since TCA is usually inferred rather than directly observed, it is demonstrated that this technique is a reliable method to establish the presence of TCA.

Terpenes are ideal candidates for sustainable polymer feedstocks because of their natural abundance and availability from existing waste streams. Previously, we have shown that a range of terpene(meth)acrylate monomers can...