Polyera Corporation

Introduction to Printed and Flexible Electronics

Printed and flexible electronics represent a nascent and fast-growing industry, enabled by new carbon-based materials. These new organic materials - semiconductors, dielectrics, etc. - function very similarly to traditional inorganic materials with a major difference being that they are solution-processable - they can be dissolved into a solution or an "ink".

The unique solution-processable property allows us to use organic materials differently from inorganic materials in two ways:

• Printability. These materials allows electronic devices to be printed using ink-jet printing, gravure printing, and other roll-to-roll printing processes. In contrast, traditional fabrication processes require facilities that are extremely costly (multi-billion dollar fabs), and have limited throughput using inorganic materials. In addition, printing processes are easily scalable, i.e., it is relatively straightforward to accomplish large-area manufacturing.

• Flexibility. These materials can be printed on flexible, light-weight substrates such as plastics, in contrast with traditional rigid substrates. This allows manufacturing of electronics with novel form factors such as roll-up displays and solar panels. In addition, flexibility provides robustness to impact since flexible things don't break easily when they get hit.

These 2 new abilities provide a host of unique and exciting benefits over traditional materials:

• Higher Throughput. Traditional manufacturing technologies such as photolithography have material throughputs around 0.1m²/s. In contrast, printing technologies such as gravure printing have throughputs close to 60m²/s, nearly 600 times faster.

• Lower Cost. Because of the lower cost of plastic substrates and the lower cost of manufacturing plants(a fab can cost $1-3 billion), printed and flexible electronic devices produced using organic materials are orders-of-magnitude cheaper than conventional electronics.

• Lighter Weight. Because these materials can be applied to light-weight, plastic substrates, the weight profile of the devices is significantly reduced.

• Robustness. In contrast with inorganic materials and substrates, both organic materials and plastic substrates they are deposited on are flexible, durable, and shock-resistant.

• Large-area Production. The ability of the materials to be printed allows for the manufacture of large-area devices, such as sheets of solar-cells and large-area displays.

• Rapid development cycles. Because circuits can be printed easily by a number of lab-scale devices, device designs can be prototyped, tested, and modified quickly, resulting in quicker commercialization.

There are also limitations with printed and flexible electronic devices. In particular, inorganic materials allow for higher-resolution fabrication (and thus smaller feature size), meaning that more circuitry can be packed into a given area. Furthermore, inorganic materials often have better performance characteristics such as higher mobility for transistors and higher efficiency for solar cells. The differences are summarized in the table below:

For this reason, it is not the aim of printed and flexible electronics to replace every conventional devices. While it is true that new devices are likely to displace conventional ones in some areas (e.g. displays), the most exciting opportunities come from their ability to open up new types of applications that are not practical with current materials.

Materials Basics

To make an electronic device, we need the materials listed below:

• Semiconductor. This is the basis for modern electronics. These materials can act either as a conductor or a dielectric, depending on conditions such as the presence of heat or light, an applied voltage, etc. These materials are designated as n-type if they transport electrons (negative charges), p-type if they transport holes (positive charges - the absence of electrons), or ambipolar if they transport both equally well. Learn more.

• Substrate. This is the foundation on which the device is built. Similar to the foundation of a house, substrate serves to hold the rest of the materials together. In printed and flexible electronics, this is usually plastic, but can be steel or even paper.

• Conductor. This is used to carry charge over long distances (i.e. millimeters), or anywhere that charge always needs to be able to move freely.

• Dielectric. This is a non-conductive (i.e. insulating) material used to block or inhibit charge migration.

Basic Devices

From these materials, one can construct a number of "building-block" devices, out of which most end-user applications can be developed:

• Organic Thin-Film Transistors (OTFTs). Often used in logic circuits, they act as "switches" that can turn voltage or current on and off based upon an applied voltage. They also function as amplifiers in analog circuits. Learn more.

• Organic Photovoltaics (OPVs). They generate electric currents when exposed to a light source. They are most often used in solar cells, and in senors and other applications. Learn more.

End-User Applications

With just these few building-block devices, the potential number of applications is significant:

• Display Backplanes. In most displays, the application is divided into two parts: the frontplane and the backplane. The frontplane creates the image you see, while the backplane regulates the flow of current to each individual pixel, turning it on or off as necessary. For eletrophoretic displays (EPVs) or OLED displays, the frontplanes are made from flexible materials. However, the backplanes are still made from non-flexible, traditional materials. The creation of flexible backplanes will allow, for the first time, the production of truly flexible displays. Learn more.

• Radio-frequency identification (RFID) Tags. An RFID tag is an object that can be applied to or incorporated into a product, animal, or person for the purpose of identification and tracking using radio waves. Some tags can be read from several meters away and beyond the line of sight of the reader. Currently limited in usefulness due to cost constraints, the much lower costs promised by printed electronics stands to unlock the power of this technology by making it ubiquitous on retail goods and novel applications. Learn more.

• Solar Cells. Organic photovoltaic devices, thanks to lower cost and faster manufacturing on a large scale, allows the possibility to make inexpensive solar cells to produce cheap, renewable energy all over the world. Lean more.

• Sensors. Printed and flexible circuits can be designed to detect a variety of stimuli, including temperature, pressure, radiation, and chemical identity. Learn more .

Manufacturing Techniques

Most of the benefit of these new materials derives from the higher-throughput, lower-cost manufacturing processes that can be used with them. Here we briefly cover the most common ones:

• Inkjet. Mechanically-controlled print heads move along the substrate while the flow of ink is selective deposited in the desired area.

• Gravure. Tiny cells are etched into a drum, which deposits onto the printing surface.

• Flexography. A regulator roll controls the amount of ink transferred from a pan onto a printing drum, which then makes contact with the printing surface.

• Screen Printing. This technique uses the creation of a "stencil", which defines the image to be printed.

• Spin Coating. A method that places a material on top of a substrate and then spins at high speeds to spread the material out evenly across the surface.

References