Conjugated polymers belong to a class of organic semiconductors that are used in a broad range of optoelectronic applications such as organic solar cells and organic light-emitting diodes. Thin films of two or more conjugated polymers or small molecules are coated from a solution that undergoes phase separation during drying and forms multiscale structures. In state-of-the-art bulk heterojunction organic solar cells, electron-donating and electron-accepting molecules form a network of donor-rich and acceptor-rich phases, whose domain sizes, composition, and interconnectivity play an important role in their power conversion efficiency. While these mesoscale structures formed in bulk heterojunction blend films under some circumstances can be observed by conventional scanning probe microscopy techniques, the task of mapping the film morphology becomes increasingly difficult when the donor and acceptor molecules are more chemically similar. Here we use AFM-IR, a combination of AFM (atomic force microscopy) and IR (infrared) spectroscopy, to image, with nanometer resolution, the morphology of a blend film of a donor polymer, TQ1, and an acceptor polymer, N2200, by using their distinct chemical composition contrast. These composition maps expose an interpenetrating network of the polymers that could not be distinguished by topography or phase imaging. Moreover, the dependence of the film structures, visualized by AFM-IR, on the molecular weight of the N2200 acceptor and the donor:acceptor blend ratio could be rationalized using Hansen solubility parameters (HSP).

We develop a mesoscopic lattice model to study the morphology formation in inter-acting ternary mixtures with the evaporation of one component. As concrete potentialapplication of our model, we wish to capture morphologies as they are typically arisingduring the fabrication of organic solar cells. In this context, we consider an evaporatingsolvent into which two other components are dissolved, as a model for a 2-componentcoating solution that is drying on a substrate. We propose a 3-spins dynamics to describethe evolution of the three interacting species. As main tool, we use a Monte CarloMetropolis-based algorithm, with the possibility of varying the system’s temperature,mixture composition, interaction strengths, and evaporation kinetics. The main novelty isthe structure of the mesoscopic model – a bi-dimensional lattice with periodic boundaryconditions, divided into square cells to encode a mesoscopic range interaction amongthe units. We investigate the effect of the model parameters on the structure of theresulting morphologies. Finally, we compare the results obtained with the mesoscopicmodel with corresponding ones based on an analogous lattice model with a short rangeinteraction among the units, i.e. when the mesoscopic length scale coincides with themicroscopic length scale of the lattice.

In all-polymer solar cells, high performance is attributed to the fine-grained morphology of the film in the active layer. However, the mechanism by which this fine-grained morphology is achieved remains unknown. Polymeric non-fullerene acceptors have the potential to restrict the self-aggregation, typical of non-fullerene small molecule acceptors. Here we employed a blend of the polymeric acceptor PF5-Y5 and the donor polymer PBDB-T to investigate the balance between molecular interactions in solution. Temperature-dependent absorption spectra show evidence of temperature-induced disaggregation of both donor and acceptor polymers, where the donor polymer disaggregation depends on the solvent polarity. Concentration-dependent fluorescence spectra of blend solutions display blue-shifted acceptor emission upon dilution, similar to that observed in acceptor solutions, and a decreased tendency for charge transfer from donor to acceptor upon dilution. Excitation spectra of dilute blend solutions contain an increased contribution to the long-wavelength acceptor emission, as compared to pure acceptor solutions, from a chromophore that absorbs in a region where the donor does not absorb. These observations can be explained by donor-acceptor complexation in dilute blend solutions, that is stabilized in more polar solvents. Moreover, the near IR-region of the absorption spectrum could be matched with the calculated electronic excitations of donor-acceptor complexes of PBDB-T and PF5-Y5 oligomers. The results corroborate that the interaction between segments of the donor and acceptor polymer chains favours the formation of donor-acceptor charge transfer complexes, stabilized by hybridization of the molecular orbitals, which reduces the electronic energy. The proposed donor-acceptor complex formation competes with the donor and acceptor self-aggregation and is influenced by the solvent environment. These pre-formed donor-acceptor complexes in low-concentration solutions can be expected to have important consequences on the film morphology of all-polymer blends. The results from this joint experimental-theoretical spectroscopy study provide insights that can guide the design of compatible donor and acceptor polymers for future high-performance organic solar cells.

All-polymer solar cells (all-PSCs) have achieved impressive progress by employing acceptors polymerized from well performing small-molecule non-fullerene acceptors. Herein, the device performance and morphology evolution in blade-coated all-PSCs based on PBDBT:PF5–Y5 blends prepared from two different solvents, chlorobenzene (CB), and ortho-xylene (o-XY) are studied. The absorption spectra in CB solution indicate more ordered conformation for PF5–Y5. The drying process of PBDBT:PF5–Y5 blends is monitored by in situ multifunctional spectroscopy and the final film morphology is characterized with ex situ techniques. Finer-mixed donor/acceptor nanostructures are obtained in CB-cast film than that in o-XY-cast ones, corresponding to more efficient charge generation in the solar cells. More importantly, the conformation of polymers in solution determines the overall film morphology and the device performance. The relatively more ordered structure in CB-cast films is beneficial for charge transport and reduced non-radiative energy loss. Therefore, to achieve high-performance all-PSCs with small energy loss, it is crucial to gain favorable aggregation in the initial stage in solution.

1-Chloronaphthalene (CN) has been a common solvent additive in both fullerene- A nd nonfullerene-based organic solar cells. In spite of this, its working mechanism is seldom investigated, in particular, during the drying process of bulk heterojunctions composed of a donor:acceptor mixture. In this work, the role of CN in all-polymer solar cells is investigated by in situ spectroscopies and ex situ characterization of blade-coated PBDB-T:PF5-Y5 blends. Our results suggest that the added CN promotes self-aggregation of polymer donor PBDB-T during the drying process of the blend film, resulting in enhanced crystallinity and hole mobility, which contribute to the increased fill factor and improved performance of PBDB-T:PF5-Y5 solar cells. Besides, the nonradiative energy loss of the corresponding device is also reduced by the addition of CN, corresponding to a slightly increased open-circuit voltage. Overall, our observations deepen our understanding of the drying dynamics, which may guide further development of all-polymer solar cells.

We perform a quantitative analysis of Monte Carlo simulation results of phase separation in ternary blends upon evaporation of one component. Specifically, we calculate the average domain size and plot it as a function of simulation time to compute the exponent of the obtained power law. We compare and discuss results obtained by two different methods, for three different models: two-dimensional (2D) binary-state model (Ising model), 2D ternary-state model with and without evaporation. For the ternary-state models, we study additionally the dependence of the domain growth on concentration, temperature and initial composition. We reproduce the expected 1/3 exponent for the Ising model, while for the ternary-state model without evaporation and for the one with evaporation we obtain lower values of the exponent. It turns out that phase separation patterns that can form in this type of systems are complex. The obtained quantitative results give valuable insights towards devising computable theoretical estimations of size effects on morphologies as they occur in the context of organic solar cells. 

This study explores the quenching of the dianionic fluorescent whitening agent, NFW, by various substances, including methyl viologen (MV), in water and in the presence of beta-cyclodextrin (beta-CD). Results of a fluorescence spectro-scopic examination of the beta-CD-NFW system are presented. It was found that NFW forms a 1:1 inclusion complex with b-CD with an association constant of 2540 +/- 380 M-1. The included NFW fluorescent state is protected by the beta-CD cavity from a range of water-based quenchers (neutral, anionic, and cationic). Quenching proceeds near the diffusion-controlled limit in water for the quenchers tested with the exception of the dicationic MV. Methyl viologen is an extremely efficient quencher of NFW fluorescence with a nominal K-SV similar to 5.0 x 10(3) M-1 in water alone, corresponding to a nominal k(q) of similar to 4 x 10(12) M-1 s(-1), which exceeds the diffusion-controlled limit in this solvent. The quenching efficiency of MV is strongly suppressed in the presence of 10 mM beta-CD (K-SV = 105 +/- 12 M-1) and in the presence of NaCl (K-SV = 106 +/- 9 M-1 at 0.5 M salt). In the absence of CD or salt, there is a strong contribution from static quenching in the MV system; the presence of these additives suppresses the static quenching. Various results suggest the static quenching is due to formation of a ground-state complex between the dianion NFW and the dication MV.

We report the design and testing of a custom-built experimental setup for dip-coating from volatile solutions under microgravity conditionsonboard an aircraft. Function and safety considerations for the equipment are described. The equipment proved to work well, both concerningthe safety and the preparation of thin films. No leakage of the solvents, nor the solvent vapors, was detected, not even in a situation with afluctuating gravitational field due to bad weather conditions. We have shown that the equipment can be used to prepare thin films of polymerblends, relevant for organic solar cells, from solution in a feasible procedure under microgravity conditions. The prepared films are similar tothe corresponding films prepared under 1 g conditions, but with differences that can be related to the absence of a gravitational field duringdrying of the applied liquid coating. We report on some introductory results from the characterization of the thin films that show differencesin film morphology and structure sizes.