Preface

Traditionally, printing is a process of reproducing text and images with ink on paper. More generally, it is a process of adhering one or more material layers on a surface. In the context of electrical and electronic applications it is understood as the process of adhering electronically functional materials to various flexible or rigid substrates. A significant distinctive feature of printed films as opposed to films deposited in the usual microelectronic technologies is that printed films are obtained via a (semi) direct-write approach, i.e. by means of an additive process, which minimizes material waste: no part of the coating has to be removed after the deposition. Also, generally speaking, the printed films are deposited in air: no vacuum chamber or inert atmosphere is required and most printed films are also stabilized in their functional structure by a post-deposition process, again in standard ambient.

According to this broad definition, the subject of printed films spans a wide range of materials and technologies. Very popular printed films in the electronic community are ‘thick-films’, also known as ‘screen-printed-and-fired films’. These films are prepared by means of a technology that mimics the ancient art of Greeks and Egyptians to produce scripts and drawings, for example on ceramic or glassy surfaces. The basic concept is in fact to force a paste through apertures of a stencil screen, or a mask, in order to deposit a pattern on the substrate. Although interest in this technology and its applications are far from faded away, they are currently joined by a wide arsenal of different printing techniques for functional films, either well assessed or under development. They are intended to add (or to extend) the benefits of this mature technology, in terms of its capacity for very efficient mass production of cheap functional films, environmentally compatible (and sometimes simply disposable) passive elements or even active devices. The perspective is to print not only dielectrics, conductors and resistive materials for integrated circuits, microelectro mechanical (MEMS) and labels for broad-band wireless communications but also light emitters and semiconductors for solar energy conversion, sensors, power, memory, logic, lighting and much else besides.

With these prospects, we are witnessing the birth of new companies, the substantial involvement of many large consortia, wide research programs on materials and ideas for innovative applications of printable materials and methods. Some commercial products of this ‘printed electronics’ have already reached the market, and many new announcements appear every day.

In this framework the novice may believe himself to be working in an almost virgin background and miss precious knowledge gained in the past. We hope that this book will bridge the gap between the mature field of screen-printing technology and the most promising newer printing technologies.

Hence we start with a description of screen-printing technology, with its intrinsic capabilities, limitations and variants, and then proceed to examine alternative processes such as ink-jet printing, laser micro-fabrication, writing through dispersing tools and direct gravure offset printing. The focus is on the interrelations of materials’ processing properties rather than on more technical aspects of design, manufacture and testing that have been extensively covered elsewhere.

Next, the applications of printed films in devices such as solar cells, sensors, actuators, heaters, fuel cells and microsystems are presented with a twofold view. One perspective is to look back at successful achievements, to review important learnt lessons and possibly to identify further advances in traditional fields that might increase possible future applications. The other view is to look forward, to new evolving technologies that may take advantage of recently developed or improved equipment and materials, as well as incorporate emerging methods for further future applications.

We have compelled ourselves and other contributors to present materials, processes and applications with constant care to underline the principles and methods of materials science which, according to our current level of knowledge, justify the choice of materials, procedures and systems appropriate to printed films. This has appeared a useful exercise because experience teaches that rapid development, along with an enthusiastic approach and/or prospects of new and profitable results tend to distract people from the task of taking into account the knowledge gained in the past and the responsibility of progressing rapidly and securely with a rigorous scientific understanding of the work.

We have been lucky enough to gain the cooperation of real specialists in well-assessed applications, and of scientists. Our unique regret is to have been able to cover only marginally the topics related to organic printed films, because of the late withdrawal of some of the experts in this scientific field. Our hope is, however, to have contributed an updated survey in the field of printed films, to offer food for thought to novices, and to provide suggestions to both scientists and practitioners in this fascinating area of research and development.