The use of non-metallic, nano-element arrays for light trapping in substrate a-Si:H single junction solar cell
structures is found to dramatically increase power conversion efficiency. This enhancement is consistent with significant
light diffraction from the array into plasmon-polariton modes, photonic modes, or both. However, modeling shows that
photonic effects dominate in substrate structures and that the light and carrier collection advantages of such arrays can
result in short circuit current densities of 17.3 mA/cm2 for nominally 200nm a-Si:H substrate cells, giving a 56%
increase in efficiency over planar 200nm controls while keeping photocarriers within 224nm from a collecting electrode.
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.
We have developed a process for fabricating reproducible nanostructured silicon materials at low temperatures (<100C) using high density plasma chemical vapor deposition. These films have a column/void network morphology and they can be deposited on glass, on plastics, on metal foils, or even on substrates with previously existing, completed structures or circuits. The films have absorption properties that qualify them for the description " molecular VelcroTM In addition their optical properties can be tailored and they can have very low reflectance with high absorption in the UV. These films can easily be chemically modified and functionalized. In this report we discuss the deposition and morphology of these films. We also outline several bio-medical applications: substrates for cell growth, substrates for mass analysis for proteomics, and sacrificial layer applications for nano-and micro-channel and reaction chamber formation.
A new measurement technique for process monitoring that consists of observing current or charge transients after each voltage step during cyclic voltage sweeps is presented. This technique provides a quick turn-around alternative to monitoring with full-flow CMOS devices since it uses simple ultrathin oxide MOS structures. Analysis of the resulting cyclic I-V and Q-V data provides important physical information about the status of the ultrathin oxide, the interface and the near surface Si region.
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.
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.
The integration of thin film electronics and opto-electronics with MEMs is discussed. Thin film electronics and opto-electronics based on amorphous silicon or polycrystalline silicon are considered for integration with MEMs devices based on piezoelectric thin films or on amorphous or poly-Si thin films.
Silicon micromachining is, at the present time, the tool of choice for the fabrication of microelectromechanical systems (MEMS) and, in general, miniature devices. Silicon micromachining techniques include some of the basic processing steps of IC technology especially plasma etching for the fabrication of submicron structures. However, inherent to plasma etching are damage effects to Si that can negatively influence the MEMS device performance. In this study we have explored the effects of conventional reactive ion etching (RIE) and magnetically enhanced RIE (MERIE) on the properties of Si exposed to fluorocarbon based oxide etch chemistries. By using combinations of spectroscopic ellipsometry (SE), secondary ion mass spectrometry (SIMS), and Schottky diode current-voltage-temperature (IVT) measurements, we show that damage in the form of thin layers of residual polymer and heavily damaged silicon can be induced by plasma exposure on the Si surface. SE and IVT measurements have shown that the thickness of the heavy damage layer decreases with magnetic field. This layer is proposed to comprise amorphous Si and voids and the density of the latter increases with the magnetic field. The polymer residue layer, detected by both SE and SIMS on the plasma-exposed Si surface, is observed to be thinner the higher the magnetic field.
Although amorphous silicon (a-Si:H) is very photosensitive and can be doped n and p type, it does not give effective phototransistors because of the extremely poor diffusion lengths. Hence enhanced photodetection in two-terminal a-Si:H devices is of considerable interest. Using the Analysis of Microelectronic and Photonic Structures (AMPS) computer model, we explore enhanced photodetection possibilities in two-terminal a-Si:H structures and show situations where it can occur. These situations, which we then experimentally verify, are of two types: one can yield quantum efficiencies greater than unity and the other can yield gains of 103. Both of these enhanced photodetection situations occur because of what we term photogating.
Photodetection mechanisms can be quite complex in amorphous semiconductors due to the extensive trapping and electric field redistribution. When properly understood and exploited, this rich complexity can lead to enhanced photodetection. Using the AMPS computer model, we explore two such experimentally verified situations: one is an example of a primary photoconductivity type of effect which can yield quantum efficiencies greater than unity and the other is an example of a secondary photoconductivity type of effect which can yield gains of 103.
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