Demixing In Blends Research Articles

Degradation through Directional Self‐Doping and Homogeneous Density of Recombination Centers Hindered by 1,8‐Diiodooctane Additive in Non‐Fullerene Organic Solar Cells

Non‐fullerene‐based organic solar cells (OSCs) have recently proven to perform with efficiencies above 18%. This is an important milestone for one of the most promising technologies in the fields of flexible and transparent/semitransparent photovoltaics. However, the stability of OSCs is still a challenging issue to meet the industry requirements. Herein, several devices with IT‐4F:PM6 as the active layer with and without 1,8‐Diiodooctane (DIO) additive are characterized before and after a 1400 h degradation test under 1 sun white light‐emitting diode (LED) illumination intensity. The optoelectronic study via impedance spectroscopy under illumination at quasi‐open‐circuit correlates the use of DIO as an additive with a retarded degradation behavior and an overall improved device performance. In dark conditions, the Mott–Schottky analysis suggests that samples without DIO develop self‐doping during degradation, changing the p‐i‐n doping profile into a p–n type, most likely related to the evolution of the blend demixing. These mechanisms are further confirmed by drift‐diffusion simulations. Space‐oriented redistribution of shallow trap levels (self‐doping) and homogeneous increase in deep‐trap levels (nonradiative recombination) are shown to be hindered by the use of the DIO additive.

Effects of Aging and Annealing on the Density of Trap States in Organic Photovoltaic Materials

The concentration of charge carriers in bulk heterojunction solar cells is limited by recombination processes whose rates may be affected by trapping in deep localized states within the band gap. Trapping reduces carriers mobility and causes dispersive charge transport with negative impact on device performance. Aging and thermal annealing may cause variations of the trap state distribution in the photoactive materials. Variable-temperature light-induced electron spin resonance (LESR) measurements are one method for revealing the distribution of deep trap states (DOS). The LESR signal intensity is in fact proportional to the concentration of trapped charges that survive recombination long enough for ESR detection at the chosen experimental temperature. We used LESR to study the effect of aging and thermal annealing on drop-casted blend films composed of regioregular poly(3-dodecylthiophene-2,5-diyl) (rrP3DDT) as charge donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor materials. The trapped carriers concentration reduced slightly after aging and strongly after annealing; lower recombination rates after aging tended to be restored after annealing. We ascribe the variations in charge concentration to two effects: (1) a decrease in charge generation efficiency due to blend demixing, causing less exciton splitting and charge separation; and (2) the presence of oxygen contaminant acting as additional trap states. The experimental DOS can be modeled by Gaussian curves whose parameters (average and standard deviation) are influenced by aging and annealing. Aging shifts the average binding energy to higher values and increases Gaussian width (i.e., energetic disorder). Annealing tends to restore the average energy to the original value, while curve width is invariant or increases. We conclude that the effects of aging and annealing on the DOS are due to the presence of reversible oxygen contamination and to the changes in the blend film microscopic morphology. Both are favorably modified by thermal annealing, although some (apparently irreversible) features of the energetic disorder deserve further investigation.

Insoluble Patterns of Cross-Linkable Conjugated Polymers from Blend Demixing in Spin Cast Films

Insoluble Patterns of Cross-Linkable Conjugated Polymers from Blend Demixing in Spin Cast Films

Insoluble Patterns of Cross-Linkable Conjugated Polymers from Blend Demixing in Spin Cast Films

Insoluble patterns of cross-linkable conjugated polymers, CPs, are obtained from spin coating their blends with polystyrene, PS, following an approach first demonstrated for poly [2-methoxy-5-(2′-ethyl)hexyloxy-2,4-phenylenevinylene] (MEH-PPV) by Castro et al. (Chem. Mater. 2006, 18, 5504). Taking advantage of the CP and PS tendency to phase separate, we make use of CP functionality to be cross-linked, so PS can be removed, leaving behind a solvent-resistant pattern. Columnar, spike, and porous structures can be obtained. Furthermore, we show that these patterns morphologies and dimensions can be controlled by adjusting the experimental conditions and the PS molecular weight. We present strategies to tune cross-linkable CP’s films morphologies aiming for applications in organic solar cells and light-emitting devices.

PC-SAFT Equation: A Predictive Tool to Determine Experimental Conditions for Polymer Blend Demixing

The perturbed-chain statistical associating fluid theory equation of state (PC-SAFT) was applied to predict the phase behavior of polymer solutions in order to determine the pressure – temperature region for the high molecular weight polymer blend separation by using n-alkanes at high pressure and high temperature. The polymer blends selected were physical blends of polyethylene (PE)/polystyrene (PS), and polypropylene (PP)/PS. The miscibility and immiscibility region of each polymer in different alkanes (n-pentane, n-hexane, and n-heptane) was studied and from this analysis, the experimental conditions of the polymer blend demixing were predetermined. The results obtained were validated with experimental data indicating that the PC-SAFT equation is a good tool to predict experimental conditions for the processing windows of polymer blend separation.

The influence of nanoparticle fillers on the morphology of a spin-cast thin film polymer blend

Polymer/nanoparticle composite films are receiving growing attention thanks to their potential for application in ultra-thin electronic and optical devices. Polymer blend demixing has been shown to be a suitable technique for the structuring of polymer thin films and the patterning of nanoparticles (NP) within them. In this work we show that the morphology of thin polymer films made by spin-casting a polymer blend solution containing NP fillers on a surface depends strongly on the concentration of NP fillers. More specifically, polystyrene/polymethylmethacrylate (PS/PMMA) films formed from a toluene solution, and which demix following a nucleation and growth mechanism, were studied. It was found that both the height and the surface density of PMMA domains increased as the concentration of CoPt:Cu NPs in the film was increased. We find that similar effects are induced in a NP-free PS/PMMA demixed film upon increasing the molecular weight of the PS molecules. This suggests that under certain conditions the NPs and the polymer molecules in the blend do not behave as separate species but form aggregates.

Thermoplastic blend demixing by a high‐pressure, high‐temperature process

AbstractThe analysis of a thermoplastic polymer blend requires a precise separation of the blend components, which is usually performed by selective solvent extraction. However, when the components are high‐molecular‐weight polymers, a complete separation is very difficult. The use of fluids in near critical and supercritical conditions becomes a promising alternative to reach a much more precise separation. In this work, a method to separate reactive and physical blends from high‐molecular‐weight commercial polymers is proposed. Polyethylene (PE)/polystyrene (PS) blends were separated into their components with n‐propane, n‐pentane, and n‐heptane at near critical and supercritical conditions. The selectivity of each solvent was experimentally studied over a wide range of temperatures for assessing the processing windows for the separation of pure components. The entire PE phase was solubilized by n‐pentane and n‐heptane at similar temperatures, whereas propane at supercritical conditions could not dissolve the fraction of high‐molecular‐weight PE. The influence of the blend morphology and composition on the efficiency of the polymer separation was studied. In reactive blends, the in situ copolymer formed was solubilized with the PE phase by chemical affinity. The method proposed for blend separation is easy, rapid, and selective and seems to be a promising tool for blend separation, particularly for reactive blends, for which the isolation of the copolymer is essential for characterization © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2361–2369, 2005

The use of thermal analysis in the study of liquid—liquid phase separation in blends of some crystallizable homopolymers with their branched copolymers

It is difficult to determine the degree of mixing or demixing in blends in which the two components have closely similar physical properties, such as homopolymer—lightly branched copolymer blends. Direct methods of observation of liquid—liquid phase separation are not possible, but in the course of our studies of blends of linear with branched polyethylenes we have developed several indirect techniques. We examine samples after rapid quenching from the melt in order to determine whether the melts were mixed or demixed prior to quenching. One of the most useful techniques is DSC; however, there are limitations to its applicability and it is usually necessary to use a second technique. This paper describes the techniques in detail and pays particular attention to the strengths and weaknesses of DSC in determining phase separation.