Polyera Corporation

An Introduction to Printable & Flexible Electronics

Printable and flexible electronics represent a nascent and rapidly-growing industry enabled by revolutionary materials technology. Most commonly made from organic, carbon-based compounds, these materials - primarily consisting of semiconductors and dielectrics (insulators) - function very similarly to their traditional cousins, with the primary difference being that they are solution-processable - that is, dissolvable into a solution or ink.

This makes these materials differ from traditional electronics materials in three substantial ways:

• Printability. Because these can essentially be converted into an 'ink', devices using these materials can be printed using techniques like ink-jet printed, gravuere printing, and other roll-to-roll processes. This is in contrast to historical manufacturing processes, which required large, costly, and difficult-to-maintain fabrication facilities.

• Flexibility. In addition to traditional substrates used in the electronics industry, these materials can also placed upon flexible and robust substrates such as plastic.

These properties convey 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. Printing technologies such as gravure printing, by contrast, have throughputs close to 60m²/s, or nearly 600 times faster.

• Lower cost. Because of the lower cost of the substrates (plastic is relatively inexpensive) and the substantially lower cost of the manufacturing processes (a single Silicon fabrication plant can cost $1-3 billion), the cost of producing devices using printed and flexible electronic materials is orders-of-magnitude cheaper than conventional electronics.

• Lower weight. Because these materials can be applied to lightweight substrates such as plastic, the overall weight profile of the application can be significantly reduced.

• Better robustness. Unlike traditional materials used such as crystalline silicon, both the semiconductor materials themselves and the substrates they are deposited on are flexible, durable, and shock-resistant, allowing for devices to be placed in areas where traditional electronics would not be suitable.

• 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, application designs can be prototyped, tested, and modified quickly, resulting in quicker commercialization.

There are limitations, however. In particular, conventional electronics materials allow for higher-resolution fabrication (and thus smaller feature size), meaning that you can pack more circuitry into a given area. Furthermore, the materials used in conventional electronics often display 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 flexible and printed electronics to fully replace conventional ones. While is true that these technologies are likely to displace conventional ones in some areas (displays, for example), the most exciting opportunities come from their ability to open up new types of applications that were either impossible or impractical with conventional materials and methods.

Materials Basics

To make any type of electronic device or application, some basic materials types are needed:

Semiconductor. The basis for most modern electronics. These materials can act either as a conductor or a dielectric, depending other conditions (such as the presence of heat or light, an applied voltage, or the like). These materials are designated as n-type if they favorably transport electrons (negative charges), p-type if favorably transport holes (positive charges - really the absence of an electron - more on this in our section on p-types), or ambipolar if they transport both equally well. Learn more.

Substrate. This is the foundation on which the device is built. Like the foundation of a house, this does not usually serve any purpose other than to hold the rest of the structure. In printed and flexible electronics, this is usually plastic, but can be steel or even paper.

Conductor. 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 circuitry, these devices at as 'switches' that can turn voltage or current on and off based upon an applied voltage. They can also act as amplifiers for use in analog circuits. They usually use either an n- or p-type semiconductor along with a dielectric. Learn more.

Organic Photovoltaics (OPVs). These materials generate an electric current when exposed to a light source, they are most often used in solar cells, but also in senors and other applications. The most promising OPVs contain both n- and p-type materials along with interlayers which facilitates proper device functioning. Learn more.

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. Even in the case of eletrophoretic displays (EPVs) or OLED displays, where the frontplanes are made of flexible material, historically the backplanes have been made from non-flexible, traditional electronics. 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. Traditionally limited in usefulness due to cost constraints, the significantly-lower costs promised by printable-electronics technology stands to unlock the power of this application by making it ubiquitous on all types of retail goods. Learn more.

Solar cells. Using organic photovoltaic devices, and capitalizing on the ability to cheaply and quickly manufacture these devices on a large scale, the possibility exists to make inexpensive solar cells ubiquitous, both in the developed and developing worlds. 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

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

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 using the creation of a 'stencil' that 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