Sigurd Wagner

Stretchable gold conductors on elastomeric substrates

Stripes of thin gold films are made on an elastomeric substrate with built-in compressive stress to form surface waves. Because these waves can be stretched flat they function as elastic electrical conductors. Surprisingly, we observe electrical continuity not only up to an external strain of ∼2% reached by stretching the films first flat (∼0.4%) and then to the fracture strain of free-standing gold films (∼1%), but up to ∼22%. Such large strains will permit making stretchable electric conductors that will be essential to three-dimensional electronic circuits.

MECHANICS OF ROLLABLE AND FOLDABLE FILM-ON-FOIL ELECTRONICS

The mechanics of film-on-foil devices is presented in the context of thin-film transistors on steel and plastic foils. Provided the substrates are thin, such transistors function well after the foils are rolled to small radii of curvature. When a substrate with a lower elastic modulus is used, smaller radii of curvature can be achieved. Furthermore, when the transistors are placed in the neutral surface by sandwiching between a substrate and an encapsulation layer, even smaller radii of curvature can be attained. Transistor failure clearly shows when externally forced and thermally induced strains add to, or subtract from, each other.

Stretchable Interconnects for Elastic Electronic Surfaces

Elastic electronic surfaces will integrate stiff thin film devices onto compliant polymer substrates. These surfaces may be stretched once or many times, by up to tens of percent strain. One way to make such an elastic electronic surface is to distribute rigid subcircuit islands over the polymer surface, and then fabricate active devices on the islands. These islands need to be interconnected with stretchable metallization. We describe stretchable interconnects made of stripes of thin gold film patterned on elastomeric membranes. These membranes can be stretched by up to twice their initial length and maintain electrical conduction. We review the fabrication of these conductors, present their electrical and mechanical properties, and summarize our model for their extreme stretchability. Using such stretchable interconnects, we made the first elastic circuit, an inverter of thin film transistors. The circuit remains functional when stretched and relaxed by 12% strain.

CuInSe2/CdS heterojunction photovoltaic detectors

We report CuInSe2/CdS p‐n heterojunction photovoltaic detectors which display uniform quantum efficiencies of up to ∼70% between 0.55 and 1.25 μ. Response times as short as 5 nsec have been observed. A weak electroluminescence (0.01% external quantum efficiency) peaking near 1.4 μ has also been observed at room temperature.

Mechanisms of reversible stretchability of thin metal films on elastomeric substrates

Gold films on an elastomeric substrate can be stretched and relaxed reversibly by tens of percent. The films initially form in two different structures, one continuous and the other containing tribranched microcracks. We have identified the mechanism of elastic stretchability in the films with microcracks. The metal, which is much stiffer than the elastomer, forms a percolating network. To accommodate the large elongation of the elastomeric substrate, the metal network twists and deflects out of the plane but remains bonded to the soft substrate. Consequently, the metal film experiences only small strains and deforms elastically without suffering fatigue.

Design and performance of thin metal film interconnects for skin-like electronic circuits

We prepare stretchable electrical conductors of 25-nm-thick gold films on elastomeric substrates prestretched by 15%. When the substrates relax from the prestretch, the gold stripes form surface waves with /spl sim/8.4-/spl mu/m wavelength and /spl sim/1.2-/spl mu/m amplitude. When the strain is cycled between 0 and 15%, both the wave pattern and the electrical resistance of the gold stripes change in reproducible cycles. Such repeatedly stretchable metallization can serve as interconnects for skin-like, conformal, and electroactive polymer circuits.

Stretchability of thin metal films on elastomer substrates

Many flexible electronic surfaces comprise inorganic films on organic substrates. Mechanical failure of such integrated structures of stiff and compliant materials poses a significant challenge. This letter studies the stretchability of metal films on elastomer substrates. Our experiment shows that, when stretched, elastomer-supported metal films rupture at strains larger than those reported for freestanding films. We use a finite element code to simulate the rupture process of metal films. A freestanding metal film ruptures by forming a single neck. By contrast, a metal film on an elastomer substrate may develop an array of necks before rupture. While the pre-rupture necks do not change the electrical conductance appreciably, they elongate the metal film, leading to a large overall rupture strain.

Efficient CuInSe2/CdS solar cells

We report the preparation of a CuInSe2/CdS heterojunction solar cell having a solar power conversion efficiency of 12% measured on a clear day in New Jersey (∼92‐mW/cm2 solar intensity).

Heterojunction band discontinuities

The discontinuity ΔEc=0.56 eV in the conduction band edge at n‐CdS/p‐InP junctions is reported. This discontinuity and others are compared with photoemission data and with Van Vechten’s extension of these data to many tetrahedrally coordinated semiconductors. Agreement between measured discontinuities and theoretical predictions is very good. Predictions are made for band parameters pertinent to interfaces involving AIIBIVCV2 compounds with zinc blende, chalcopyrite, or wurtzite crystal structures.

Failure resistance of amorphous silicon transistors under extreme in-plane strain

We have applied strain on thin-film transistors (TFTs) made of hydrogenated amorphous silicon on polyimide foil. In tension, the amorphous layers of the TFT fail by periodic cracks at a strain of ∼0.5%. In compression, the TFTs do not fail when strained by up to 2%, which is the highest value we can set controllably. The amorphous transistor materials can support such large strains because they lack a mechanism for dislocation motion. While the tensile driving force is sufficient to overcome the resistance to crack formation, the compressive failure mechanism of delamination is not activated because of the large delamination length required between transistor layers and polymer substrate.