The electrical responses of materials and devices subjected to thermal inputs, such as the Seebeck effect and pyroelectricity, are of great interest in thermal-electric energy conversion applications. Of particular interest are phenomena which exploit heterogeneities in the mechanics of heterostructured materials for novel and unexplored mechanisms in thermal-electric conversion. Here we introduce a new and universal mechanism for converting thermal stimuli into electricity via structural heterogeneities, which we term “pyro-paraelectricity.” Specifically, when a paraelectric material is grown on a substrate with a different lattice constant, the paraelectric layer experiences an inhomogeneous strain due to the lattice mismatch, establishing a strain gradient along the axis of the layer thickness. This induced strain gradient can be multiple orders of magnitude higher than strain gradients in bulk materials imparted by mechanical bending (0.1 m<sup>-1</sup>). Consequently, charge separation is induced in the paraelectric layer via flexoelectricity, leading to a polarization in proportion to the dielectric constant. The dielectric constant, and thus the polarization, changes with temperature. Therefore, when a strained metal-insulator-metal (MIM) heterostructure is subjected to a thermal input, changes in the permittivity generate an electrical response. We demonstrate this mechanism by employing a MIM heterostructure with a high permittivity sputtered barium strontium titanate (BST) film as the insulating layer in a platinum sandwich. The resulting strain gradient of more than 10<sup>4</sup> m<sup>-1</sup>, an enhancement of five orders of magnitude due to the structural heterogeneity, was verified by an X-ray diffraction scan. With an applied thermal input, the strained MIM heterostructure generated current which was highly correlated to the thermal input. A theoretical model was found to be consistent with the experimental data. These results demonstrate the existence of “pyro-paraelectricity,” a flexoelectricity-mediated mechanism for thermal-electrical conversion.
Key technical issues of flexible stainless steel foil substrates are addressed for OLED display backplane
applications. Surface roughness and corresponding planarization layer technology development will be the major factors
for the stainless steel foil substrates to be used for commercial applications. Promising candidates for the planarization
layer materials are reviewed and some of the properties are addressed. In addition, if the substrate is sustained to a
constant voltage for guaranteed circuit operation, capacitive coupling through the insulation and planarization dielectric
layer, from the conductive substrate to the electrode and circuit elements on it, is also carefully analyzed for panel
design and operation. Especially for large size high-resolution display applications, low k and thick planarization layer
should be used.
OLED display foils fitted conformally to goggles or cockpit canopies are of considerable interest. As films integrated onto pre-existing lenses or canopies they could provide visual information while adding little weight.
However, the conformal shaping of a displays to its mechanical support causes large deformation strain, in contrast to flexible displays whose bending to cylindrical shape can be managed with little strain. The deformation strain may easily exceed the critical strain of OLED materials, which then rupture and damage or destroy the OLED function. New fabrication techniques and OLED circuit architectures are required to prevent such rupture.
We report an experimental phosphorescent OLED array made on a dome shaped transparent plastic substrate. The pixellated array of OLEDs was fabricated and interconnected while flat. Late in the process sequence the array was shaped to a dome. The OLEDs are protected from rupture by their placement on ITO islands. These ITO islands are sized such that the shear strain developed along them does not reach the critical value. Most of the deformation strain is taken up by the plastic substrate that is exposed between the rigid islands. The metal interconnects do undergo this large deformation and must be designed to withstand it. The substrate was shaped to a dome of 6-cm diameter at its base, with a 10-cm radius of curvature. The radial strain at the apex of the dome is 1.5%.
The process produces bottom emitting phosphorescent OLEDs radiating into the hollow of the dome. OLED yields above 95% were achieved for up to 120-μm islands at area fill factors ranging from 4% to 44%.
Flexible displays fabricated using plastic substrates have a potential for being very thin, light weight, highly rugged with greatly minimized propensity for breakage, roll-to-roll manufacturing and lower cost. The emerging OLED display media offers the advantage of being a solid state and rugged structure for flexible displays in addition to the many potential advantages of an AM OLED over the currently dominant AM LCD. The current high level of interest in flexible displays is facilitating the development of the required enabling technologies which include development of plastic substrates, low temperature active matrix device and backplane fabrication, and display packaging. In the following we will first discuss our development efforts in the PEN based plastic substrates, active matrix backplane technology, low temperature (150°C) a-Si TFT devices and an AM OLED test chip used for evaluating various candidate designs. We will then describe the design, fabrication and successful evaluation and demonstration of a 64x64 pixel AM OLED test display using a-Si TFT backplane fabricated at 150°C on the flexible plastic substrate.
Dielectric elastomer actuators offer unprecedented opportunities for actuation in a wide range of applications. To make appealing large scale and efficient systems, new electronic devices combining high voltage and flexibility need to be designed. In this paper we report the first mechatronic system, made of an array of electro-active polymer based actuators integrated with thin film photoconductive high voltage switches fabricated on a plastic film substrate. The actuator is an acrylic elastomer diaphragm that expands under electrical stimulation. Each actuator is connected to the high voltage power supply through a photoconductive switch, which is addressed and closed by illumination. The amorphous silicon switches are made on flexible and transparent polyimide (Kapton E®) substrates. Individual switches were tested up to 8 kV and a nine-element array was successfully working at 5 kV.
Our motivation is to realize CMOS on plastic foil. We report the development of thin film transistors (TFTs) made of nanocrystalline silicon (nc-Si:H). nc-Si:H is compatible with present a-Si:H thin film technology. Because of the structural evolution of nc-Si:H with film thickness, it requires extensive experimentation with device geometry. For comparison we fabricate TFTs in (a) conventional coplanar top-gate, top-source/drain geometry and (b) staggered top-gate, bottom source/drain geometry. A seed layer is introduced in the latter case serves to develop the crystallinity of the intrinsic channel layer. While the coplanar geometry provides the shortest carrier path in the most crystalline channel region, the inverted staggered geometry ensures that the active channel is formed in the last-to-grow nc-Si:H layer, and also avoids exposure of the channel to reactive ion etching (RIE). The highest process temperature is 150°C. Both intrinsic and doped nc-Si:H layers are grown by plasma-enhanced chemical vapor deposition with an excitation frequency of 80MHz. Present p-channel TFTs reach a hole field-effect mobility of ~ 0.2 cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> in the staggered geometry, and an electron field-effect mobility of ~ 40 cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> in both geometries. These results suggest that directly deposited nc-Si:H is an attractive candidate material for CMOS capable electronics on plastic substrates.
This paper demonstrates the state of critical technologies for the integration of Thin Film Transistors (TFTs) onto plastic substrates for display applications. The transistor technologies examined include polyscrystalline silicon, amorphous silicon, organic semiconductor of TFTs. Fundamental work in new regimes of operation enabled by plastic substrates, such as the effect of rolling and 3-D deformation are also developed, leading to design guidelines. Finally, printing approaches for organic semiconductors are shown to demonstrate potential paths towards roll-to-roll display manufacturing. Altogether, the results point toward the possibility of printing transistors anywhere and bending them into nearly any shape.
In a collaboration between Pennsylvania State University and Princeton University, we have been laying the foundations for flexible display technology. Flexible substrates including plastic or steel foil, backplanes of organic or silicone transistors, and directly printed RGB organic light emitting diodes are issues central to this collaboration. We present an overview of key recent results. Silicon based thin film transistors have been processed at the ultralow temperatures required for processing on plastic substrates. Organic thin film transistors and circuits with record mobilities have been fabricated that are naturally matched to low temperature substrates. Organic light emitting diodes have been made by inkjet printing in an approach that solves the RGB patterning problem of OLED displays. The mechanics of flexible substrates have been defined and thin film silicon transistor performance is shown to be unaffected by bending. Substantial progress has been made toward the realization of rugged, lightweight, flexible and even conformal displays.
The emergence of wearable electronics is leading away form glass substrates for the display backplane, to plastic and metal. At the same time the substrate thickness is reduced to make displays lighter. These two trends cooperate toward the development of compliant substrates, which are designed to off load mechanical stress from the active circuit onto the substrate. Compliant substrates made the circuit particularly rugged against rolling and bending. Design principles for compliant substrates include: (a) Moving the circuit p;lane as close as possible to the neutral plane of the structure, and (b) Using substrate and encapsulation materials with low stiffness. Design principle (a) is demonstrated on thin-film transistors made on thin steel foil. Such transistors function well after the foils are rolled to small radii of curvature. Principle (b) of compliant substrates is demonstrated with bending experiments of a-Si:H TFTs made on thin substrates of polyimide foil. TFTs on 25-micrometers thick polyimide foil may be bent to radii of curvature as low as 0.5 mm without failing. The reduction in bending radius, from R-2 mm on same- thickness steel foil, agrees with the theoretical prediction that changing from a stiff to a compliant substrate reduces the bending strain in the device plane by a factor of up to 5.
Active matrix displays that are lightweight, rugged and bendable are a key DoD need for applications ranging from panoramic displays for aircraft cockpits to foldable maps. To achieve such displays compatible substrates, TFT backplanes, and light valve/light emissive materials systems must be developed. Advances toward this goal achieved in the joint Penn State/Princeton Display Program are discussed.
In an effort to raise the efficiency and speedup the rate of technology transfer from its university funded research programs, DARPA has ben encouraging the formation of industry/university teams to accelerate the development of backplane thin-film electronics for AMLCD displays. The effort among its university researchers has been carried forward through voluntary participation in a series of workshops cosponsored by DARPA and the Electric Power Research Institute. Evidence of the effectiveness of the teaming arrangement is shown by the many collaborations entered by the display industry participants.
A solution to the thin film silicon transistor gate metallization problem in active matrix liquid crystal displays is demonstrated in the form of a self-passivation process for copper. Bottom-level copper (Cu) lines are passivated by a self-aligned chromium oxide encapsulation formed by surface segregation of chromium (Cr) from dilute Cu<SUB>1-x</SUB>Cr<SUB>x</SUB> alloys at 400 degrees C. The encapsulation is an efficient barrier for Cu diffusion into the SiN<SUB>x</SUB> gate insulator during the plasma deposition and transistor processing, and solves the problems of oxidation and adhesion to the glass substrate without introducing additional mask steps into the manufacturing process. Gate line resistivities of 4.5 (mu) (Omega) cm are obtained. The performance of self-passivated Cu-gate thin film transistors is comparable to that of transistors with refractory metal gates.