Application Challenges Optoelectronics

Although high-performance TFTs are needed for several electronic applications, the potential for printed, inorganic electronics encompasses other devices and applications. A major opportunity is in optoelectronic applications, which impose different requirements, challenges, and opportunities (see Chapters 6, 7, 9, and 11 for discussion of solution-processed solar cells and other printed optical devices). 

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The conductivity of ZnO, ITO, and Sn02 can be controlled across an extremely wide range such that they can behave as insulators, semiconductors, or metal-like materials. However, these materials are all n-type electrical conductors in nature. Their applications for optoelectronics are rather restricted. The lack of p-type conducting TCOs prevent fabrication of p-n junction composed from transparent oxide semiconductors [2], The fabrication of highly conducting p-type TCOs is, indeed, still a challenge. 

Although this discussion has focused on challenges for solution-based processing for PV, the fundamentals apply to other optoelectronic applications to be discussed in the following sections. First, the material characteristics... 

During the fabrication process the surface of the semiconductor is etched and metal contacts are deposited. These features can represent a topographical challenge to subsequent metal wiring levels. For this reason it is important that the dielectric film used tends to smooth out such discontinuities as metal and etched edges (150,217). Additional applications for spin-on dielectrics include forming integrated microlenses for optoelectronics (218). 

This movement is a key challenge for the entire field of advanced materials, but it is a particularly exciting challenge for silicon-based polymers. From the point of view of materials, silicon-based polymers span the three traditional domains plastics, ceramics, and metals. Potential applications are equally diverse. Silicon-based polymers range from structural materials, to optoelectronic devices, and to speciality materials for biomedical applications. We are in a unique position to capture the benefits of this merger of materials and polymer science. 

Some other interesting applications include the CMP of high dielectric constant materials (e.g., BaTiOj) that could be used for increasing capacitance, high T -superconductors used for zero-resistance interconnections, and optoelectronic materials, especially waveguides, where surfaces will play an important role. There is considerable interest in these areas and there are quite a few challenges to encounter in each case. Unfortunately, very little is disclosed in the published literature. We shall review and discuss each of these areas in the following, with more focus on areas of immediate interest to microelectronic industry, the areas (a), (b), (c), (d), and (h). 

From the heterostructures that make possible the use of exotic electronic states in optoelectronic devices to the application of shape memory alloys as filters for blood clots, the inception of novel materials is a central part of modern invention. While in the nineteenth century, invention was acknowledged through the celebrity of inventors like Nikola Tesla, it has become such a constant part of everyday life that inventors have been thrust into anonymity and we are faced daily with the temptation to forget to what incredible levels of advancement man s use of materials has been taken. Part of the challenge that attends these novel and sophisticated uses of materials is that of constructing reliable insights into the origins of the properties that make them attractive. The aim of the present chapter is to examine the intellectual constructs that have been put forth to characterize material response, and to take a first look at the types of models that have been advanced to explain this response. 

Despite these successes, several problems remain, including the need for expensive reactants and the large number of parameters that must be controlled precisely to obtain the necessary uniformity and reproducibility for microelectronic and optoelectronic applications. Additional challenges to overcome include the use of hazardous materials and the generation of a substantial volume vapor phase waste stream. 

Despite the challenge of controlling the morphology of polymer composite the excellent bipolar performance of these blends may hold promise for easy, cheap, and solution-processed bipolar optoelectronic applications. 

Ohmic and rectifying contacts to porous silicon are important and challenging for the commercial applications of PS-based electronic, optoelectronic, and sensor devices. The choice of contact materials and the nature of PS surface (including porosity) are the prime factors for the successful achievement in the contact formation with the desired specific contact resistance. Surface modification of porous silicon by Pd improves both ohmic and rectifying contacts along with the stability as it was verified by intermittent I-V studies. Verification of specific contact resistance at regular intervals can be an alternative method to study the junction stability. 

Although many of the problems that are of concern for optoelectronic applications of organic materials have been overcome, there are still many challenges ahead in the field. Many of the polymers that have been made recently have not yet been investigated in detail, and more work is needed to optimize these materials for specific apphcations. Fimdamental questions also remain, such as the exact nature of the charge carriers in PPV films, and fiirther studies on the photophysical behavior of these polymers are required. 

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