Solar Cells in Smartphone Screens

Researchers advance thin-film energy harvesters

3 min read
Solar Cells in Smartphone Screens

18 January 2012—A team at the University of Cambridge, led by IEEE Fellow Arokia Nathan, is working toward a simple goal: a mobile phone that requires charging less often. At the Materials Research Society's fall meeting in Boston, Arman Ahnood, a researcher on that team,  told scientists that eventually, we might see a phone that never needs to be plugged in.

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To extend the time between charges, Nathan's group built a prototype device that converts ambient light into electricity using an array of  solar cells made of thin-film hydrogenated amorphous silicon that's designed to sit within the phone's screen. The photovoltaic (PV) cell takes advantage of the smartphone display’s large footprint. In a typical organic light-emitting diode (OLED) display, only about 36 percent of the light generated is projected out of the front of the screen, says Ahnood. Much of it escapes at the edges of the OLED, where it is useless. So Nathan and his collaborators at his Canadian firm IGNIS Innovation set out to harness this wasted light by putting thin-film PV cells around the display’s edges as well.

Making the device work required sidestepping another problem: fluctuations in the voltage provided by the solar cell, which could have damaged the phone’s battery. The researchers, who were based at University College London until recently, designed a thin-film transistor circuit to smooth out voltage spikes and extract electricity more efficiently.

And instead of charging the battery directly, which would have involved adding complex circuitry, they worked with the energy group at Cambridge's Centre for Advanced Photonics and Electronics to integrate a thin-film supercapacitor for intermediate energy storage. This combination of photovoltaics, transistors, and supercapacitor yielded a system with an average efficiency of 11 percent and peak efficiency of 18 percent. If the PV array converts 5 percent of ambient light to electricity, the energy-harvesting system can generate as much as 165 microwatts per square centimeter under the right lighting conditions. For a typical 3.7-inch smartphone screen, that equates to a maximum power output of 5 milliwatts, "which is quite useful power," says Ahnood, though that’s only a fraction of a smartphone’s power needs.

There are existing CMOS-based switch mode voltage regulators that offer higher efficiency, says Ahnood, but they aren’t compatible with the thin-film technology used in smartphone displays. Furthermore, the team’s thin-film devices can be fabricated at temperatures below 150 °C on lightweight plastic, making them much more attractive for use in mobile phones, where every gram and every penny is a big deal.

The cellular handset prototype is just one example of such small-scale wireless energy harvesting. Another plug-free power source might be magnetic resonance coupling via an induction coil. Alternating current is run through a coil of conductive material, generating an oscillating magnetic field. That field, in turn, generates a current in a coil embedded in, say, a phone or an MP3 player.

Jun Yu, a doctoral student of Nathan's, reported preliminary work along those lines at the MRS meeting. He told scientists that the team had designed a flat thin-film coil that could be used as a receiver in a display. But the team doesn’t foresee the coil producing enough power to run an entire computer. It should, however, be possible to scale down the magnetic coupling scheme for use in mobile devices.

It will take quite a bit more research to get from prototype to consumer product. For example, the team is exploring different circuit designs and materials with the aim of increasing the energy harvesting system’s efficiency. Other energy scavenging schemes, such as MEMS-based kinetic energy harvesting, could contribute to further improvements, says Nathan.

This story was corrected on 19 January. The affiliations of the research team was in error.

About the Author

Neil Savage writes about strange semiconductors and amazing optoelectronics from Lowell, Mass. In the January 2012 issue he reported on the invention of conductive and semiconducting cotton fibers.

 

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
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A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay
Blue

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.

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