Advanced Energy Materials

Better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10%. In this work reported on p. 82, Michael Chabinyc, Craig Hawker, and co-workers use a model system, a bilayer of P3HT/PCBM, to develop a more complete understanding of miscibility and diffusion of PCBM within P3HT during thermal annealing. PCBM aggregates are miscible and mobile in disordered P3HT, but do not disrupt the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests favorable mixing in this system. Cover design by Peter Allen and Neil Treat.

The maximum energy density of the present state of the art in lithium ion batteries with topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal-air batteries, such as Li-air and Zn-air with conversion chemistry are considered as promising candidates. On p. 34, Kyu Tae Lee, Jaephil Cho, and co-workers review progress to date in this important field.

Three-dimensional (3D) solid-state microbatteries are promising for efficient and high energy density storage to power future generations of microelectronic devices. An overview of several 3D microbattery concepts proposed by various research groups is given. Several suitable electrode and electrolyte materials for these batteries are reviewed together with some key deposition methods to produce thin films of these materials.

Building a better battery: In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries. However, the maximum energy density of lithium ion batteries is not sufficient to the demands of new markets in such areas as electric vehicles. This review provides an overview of the fundamentals and recent progress in the area of Li-air and Zn-air batteries, with the aim of providing a better understanding of new electrochemical systems.

This image presents a scanning electron microscopy image of solid state dye-sensitized solar cell with a plasmonic back reflector, overlaid with simulated field intensity plots when monochromatic light is incident on the device. Plasmonic back reflectors, which consist of 2D arrays of silver nanodomes, can enhance absorption through excitation of plasmonic modes and increased light scattering, as reported by Michael D. McGehee, Yi Cui, and co-workers.

Plasmonic back reflectors, which consist of 2D arrays of silver nanodomes, are incorporated into solid-state dye-sensitized solar cells (ss-DSCs) by nanoimprint lithography. They enhance absorption through excitation of plasmonic modes and increased light scattering. SS-DSCs with plasmonic back reflectors show increased external quantum efficiency, particularly in the long wavelength region of the dye’s absorption band.

A binder-free LiNi_0.4Mn_0.4Co_0.2O₂ cathode fabricated with 5 wt.% single-walled carbon nanotubes (SWNTS) shows excellent cycling performance at rates of 10C (Charge/discharge in 6 minutes). In contrast, a LiNi_0.4Mn_0.4Co_0.2O₂ electrode prepared by conventional methods without SWNTs has a significantly lower capacity. Based on the morphology and structural analysis, this exceptional rate capability is due to highly intimate contact between the long crystalline SWNT ropes and the active cathode material, ensuring fast diffusion of ions and electrons during cycling and resulting in sustainable capacity at high rates for 500 cycles.

A mature approach: Allowing solutions of poly[(4,4-didodecyldithieno[3,2-b:2’,3’-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P1) to stand in a solvent of marginal quality leads to interchain aggregation. These supramolecular structures lead to thin films with higher charge carrier mobilities and internal order, as determined by the fabrication of thin film transistors and grazing-incidence wide angle X-ray scattering (GIWAXS) measurements, respectively. Aging of solutions is therefore a very straightforward method to modify the optoelectronic properties of solution-processable organic semiconductors.

The challenges of printing all layers in polymer solar cells from aqueous solution are met by design of inks for the electron-, hole-, active-, and metallic back electrode-layers. The conversion of each layer to an insoluble state after printing enables multilayer formation from the same solvent (water). The photograph here was taken just before screen printing of the aqueous silver ink.

The frontispiece shows an artist's impression of an electron transfer reaction from poly(3-n-hexylthiophene) to a new highly soluble electron-accepting small molecule for organic solar cell applications, as reported by Paul Meredith and co-workers. The background shows an AFM image of the nanoscale phase separation critical to the efficiency of devices using these materials. Efficiencies depend strongly on the ordering of the acceptor molecules, controllable by low temperature annealing.

A new highly soluble electron-accepting small molecule for organic solar cell applications is introduced. This new acceptor efficiently quenches the photoluminescence of the donor polymer (P3HT). The power conversion efficiencies of solution processed bulk heterojunction devices depend strongly on the ordering of the acceptor molecules, which can be controlled by low temperature annealing.

Polymer/fullerene blends: Bilayers of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric-acid-methyl-ester (PCBM) were fabricated to investigate the miscibility and mobility of the two components during thermal annealing. It is found that aggregates and/or molecular species of PCBM are miscible and mobile in disordered P3HT without disrupting the P3HT crystallite orientation or size.

The efficiency of ZnO:polythiophene hybrid solar cells depends critically on the intimacy of mixing of the ZnO and the polymer. An ester functionalized polythiophene copolymer is able to greatly enhance mixing of the materials, leading to superb exciton generation efficiency, but the fine phase separation hinders charge carrier transport to the electrodes, limiting device performance.

Direct conversion of heat to electricity can be achieved using the multiferroic alloy Ni₄₅Co₅Mn₄₀Sn₁₀. This alloy undergoes a phase transformation from a nonmagnetic martensite to a strongly ferromagnetic austenite phase upon heating. When biased by a permanent magnet heating through the transformation causes an increase of the magnetization that induces a current in the coil.

Polymer:fullerene blends were screened in a combinatorial approach using inkjet printing thin-film libraries for photovoltaic applications. Two polymers and two fullerene derivatives were studied in various blend ratios, concentrations, solvent ratios, and film thicknesses. Two promising candidates were tested for solar cell activity; a maximum power efficiency of the inkjet fabricated devices of 1.5% could be reached.

Partially graphitic mesoporous carbons can be synthesized by a direct triblock-copolymer-templating route, which possess both ordered mesostructure with high mesopore surface areas (∼1250 m²/g) and partially graphitic domains.

Understanding degradation in active layers: The importance of studying the effect of initial film morphology on the degradation rates of polymers is highlighted. For the P3HT: PCBM system, although similar efficiencies can be derived from four different processing solvents, the stability can be critically different. Transient absorption spectroscopy and charge extraction by linearly increasing voltage (CELIV) are used to elucidate the changes in transport properties and disorder before the widely acknowledged changes in polymer structure associated with photobleaching set in.