All-polymer solar cells have shown great potential as flexible and portable power generators. These devices should offer good mechanical endurance with high power-conversion efficiency for viability in commercial applications. In this work, we develop highly efficient and mechanically robust all-polymer solar cells that are based on the PBDTTTPD polymer donor and the P(NDI2HD-T) polymer acceptor. These systems exhibit high power-conversion efficiency of 6.64%. Also, the proposed all-polymer solar cells have even better performance than the control polymer-fullerene devices with phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor (6.12%). More importantly, our all-polymer solar cells exhibit dramatically enhanced strength and flexibility compared with polymer/PCBM devices, with 60- and 470-fold improvements in elongation at break and toughness, respectively. The superior mechanical properties of all-polymer solar cells afford greater tolerance to severe deformations than conventional polymer-fullerene solar cells, making them much better candidates for applications in flexible and portable devices.

http://sciencemission.com/data/attachment.php?id=427&for_session=5f12dcbf83e038a697906b7b1b350568

Flexible, highly efficient all-polymer solar cells

Equations for polar and nonpolar interactions across the interface are developed by using energy additivity concept in a semi-continuum model. Interfacial and surface tensions of molten polymers are measured directly and used to test the resulting equations: The first expression may be called the harmonic-mean equation preferred for low energy systems such as organic liquids, water, polymers, and organic pigments. The second may be called the geometric-harmonic-mean equation preferred for high energy systems such as mercury, glass, metal oxides and graphite. The third may be called the geometric mean equation which is found unsatisfactory. The harmonic-mean equation is used to obtain the “optimum” wettability condition for adhesion. The importance of polar interactions and matching of the polarity are analyzed and emphasized.

Polar and Nonpolar Interactions in Adhesion

We present the results of a systematic study on how the processing conditions of spin casting affect the morphology of polymer thin films, and how the morphology affects polymer light-emitting diode (LED) performance. The absorption peaks of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) thin films, which reflects the conjugation of π electrons, are strongly correlated to the spin-casting conditions. At high spin speed, better conjugation is observed. In addition, the photoluminescence emission peak of MEH-PPV films at ∼630 nm has a strong correlation to polymer aggregation. By proper selection of organic solvents, polymer solution concentrations, and spin speeds, we are able to control the aggregation of the polymer chains. Subsequently, we are able to control the emission color and the quantum efficiency of the MEH-PPV LEDs by simply adjusting the spin-casting conditions. Although spin casting is the most commonly used technique for the preparation of polymer thin films, our fin...

/pdf/device-performance-and-polymer-morphology-in-polymer-light-4vogo5ehen.pdf

Device performance and polymer morphology in polymer light emitting diodes: The control of thin film morphology and device quantum efficiency

Purpose: The main purpose of this paper is the analysis of the most common methods for the calculation of the surface free energy (SFE) of solids, utilising the results of the contact angle measurements. The calculation deals also with the SFE at the interface, especially that at the surface of polymers and polymeric materials. The survey has been meant to ease the understanding of physical processes occurring at the solid-liquid interface and to help to find proper measuring methods with respect to various physical systems. Design/methodology/approach: The presented analysis has been based on the papers of the fundamental nature as well as on the specialised literature reports. The results of the experimental and theoretical studies of the author of this article are also considered. Findings: Different assumptions have been made in the individual methods for calculating the SFE of polymeric materials. Thus, the SFE values for a given material, obtained by various methods and with use of different measuring liquids, are not consistent. The method for the calculation of the SFE of porous or granulated materials, powders, and fibres, in which the Washburn equation is utilised, is very useful in practice. Currently, there is no appropriate alternative to this method. Research limitations/implications: The method for calculating the SFE with use of the equation of state requires further investigation, both experimental and theoretical. Further studies on the phenomena associated with penetration of the measuring liquids into the bulk of an examined material and on the relevant changes concerning this material, including the changes in its SFE, have also to be carried out. Practical implications: The presented results of the investigations may be applied in optimisation of the current and derivation of the new methods for calculating the SFE of solids and liquids, including mainly the SFE of polymers in the solid state. Originality/value: Because of differences in the assumptions made in most of the methods for the SFE calculation and of differences in the interactions between the individual measuring liquids and the examined material, the SFE values for various polymer materials may be compared with one another only when the contact angle measurements have been made using the same measuring liquids and when the SFE calculations have been performed by the same method.

Methods for the calculation of surface free energy of solids

http://www.zebraskimmers.com/pdf_files/metalworking_fluids-mechanisms%20_and_performance_small.pdf

Metalworking fluids—Mechanisms and performance

We propose an equation, based on “reciprocal” mean and force additivity, for calculating the interfacial tension between polymers or between a polymer and an ordinary liquid: 



where γ12 is the interfacial tension; γi the surface tension; γ and γ the dispersion and polar components of γi, respectively. This equation is shown to predict accurately the interfacial tension between polymers or between a polymer and an ordinary liquid. Fowkes' equation or Fowkes' equation with a geometric-mean polar term 2(γiPγ2p)1/2 is not applicable to polarlpolar systems. The interfacial tension arises mainly from disparity in the polarities of the two phases. The above equation can also be used to calculate the surface tension and polarity of polymers or organic solids from contact angle data.

Calculation of interfacial tension in polymer systems

A theory based on a simple buckling mechanism has been developed to describe the deformation and rupture mechanics of a polymer monolayer. It assumes that the closely packed monomer units are forced to buckle in successive pairs under compression. When the protruding angle of the rupture site reaches 90°, the monolayer collapses. This model is found to accommodate the essential features of compression isotherms of polymer mono-layers in the condensed region (below the closest packing area at zero surface pressure) up to and beyond the collapse point. The stress-bearing capacity of the interface is greatly influenced by the ratio of cohesion to adhesion. The compression isotherm may take either a positive or a negative slope beyond the collapse point, depending on whether adhesion is greater or smaller than cohesion.

Souheng Wu

Papers

Calculation of interfacial tension in polymer systems

Compression mechanics of polymer monolayers