LEDs and Solar Cells Combine into Devices that Emit and Absorb Light

March 17, 2017 by Chantelle Dubois

Researchers have developed double-heterojunction nanorods which can be biased to both send and receive light—in other words, a device that is both an LED and solar cell, capable of emitting and absorbing light.

The solar industry is at a crossroads where consumer and research interest are increasing, but tax complications may its advancement. One such advancement is LEDs capable of emitting and absorbing light.

The Independent reports that the UK 2017 budget includes a proposed tax hike of up to 800% on roof-top solar microgenerators, costing users hundreds or even thousands of pounds.

The tax targets users who install and consume the energy of their own solar panels, which will severely disincentivize any further installations. With solar and wind energy becoming ever-cheaper, this move is a step backward, not only for those who wish to transition away from fossil fuel consumption but also for those who wish to pursue cheaper energy solutions. It's a confusing step for a number of reasons.


The Rise of Solar

This decision to punish future users of solar energy is a bit baffling considering consumer trends. 2016, in fact, was a record-breaking year for solar installation.


Image courtesy of SEIA.


In California, installation of solar panels has surged in the wake of the SA blackouts. The price of solar energy is also set to drop further in 2018 in India with capacity for solar energy increasing three-fold in the last three years.

Elon Musk made the headlines yet again by powering the entire island of T'au using exclusively solar energy in partnership with SolarCity. Musk is additionally investing in a massive solar farm and solar battery factory on the Hawaiian island Kauai in order to help the island transition from its current energy sources, which are 56% fossil fuel-based.

Combined with these business-backed deals is the development of Project Sunroof, a tool from Google created to calculate the amount of sunlight a given area in the US stands to receive. According to the tool, almost 80% of rooftops receive enough sunlight for solar panels.

Despite that the popularity of the solar industry is heavily dependent on net metering (which can be taken away without much ceremony), it's still drawing investors and, of course, researchers.


Advancements in Solar: LEDs that Emit and Absorb Light

Solar trends are not simply restricted to the consumer side—new innovations in double-heterojunction nanorods are poised to dramatically increase the capacity for relying upon solar energy. Although there has long been a body of work on double-heterojunction nanorods, recently researchers in South Korea and the USA have recently made great advances in real-world applications for them, including making solar energy a more practical option.


LEDs have become a familiar fixture, but they're getting a solar twist

Typical LEDs only permit current to flow in one direction when forward biased and only produce light (along with some heat). Alternatively, solar cells function by only collecting energy from light. Both use solid-state electronic principles, manipulating the flow of electrons through pn-junction materials.

The double-heterojunction nanorods are structured with materials that have properties which allow current flow in both directions, producing light, as well as absorbing light energy.

This is made possible via three different semiconductor materials: two type II bandgap materials with a smaller bandgap material in between (CdS, CdSe, and ZnSe), all suspended in a 5 nm colloidal nanorod. The type II bandgap materials dictate how electrons will flow through the intermediate material, which emits and absorbs light.

The nanorod does not emit and absorb light simultaneously. Rather, the current is switched back and forth between forward and reverse biasing at a rate quick enough to not be visibly detected.


Double-heterogeneous nanorods reacting to light. Image courtesy of the University of Illinois


The nanorods have been found to have a high luminescence capability (approximately 80,000 cd/m2) with a low turn-on voltage of 1.7 V, but the energy absorption is still relatively small (1000 cd/m2) with a 2.5 V bias. The team believes that it is possible to improve on this capability.

The nanorods so far have only been tested in a fairly basic 10x10 pixel array, and only with red LEDs. However, the team responsible is now working towards expanding to green and blue LEDs as well.

Applications for the Solar Industry

The nanorods can also be used for pixel-by-pixel screen brightness adjustment. This means that if the screen is partially shadowed or sunlight is reflecting off of a corner of the screen, then the pixels can adjust as necessary to maintain more consistent visibility. It also means that such displays may use less power overall since each pixel is only illuminating as much as it needs to for a specific application.

There are also possibilities with interactive displays which can detect objects (for example, a hand) and respond accordingly. A touchless display or a light-sensitive display could have different practical or artistic uses.



As dazzling as these uses are, there are far more impactful applications, particularly for green energy.

When coupled with a capacitor, the nanorods are able to absorb ambient light and store it, bearing great significance for solar energy applications. This feature not only reduces the energy consumption of screens,  making them self-sustaining, but also severely drops energy consumption. 

Additionally, the ability to store energy would potentially turn screens into functioning solar panels, providing significant opportunities for "greenifying" urban areas filled with billboards. 

This is simply one example of how innovations in one sector can be quickly and easily applied to filling the remaining gaps in green energy capacity.

Although there are challenges facing the solar industry, it's worthwhile to keep in mind that sustainable energy is on relatively stable footing. On its side are its broad market appeal, its eager investors, and the rapid development of game-changing technologies. With continued public support, solar may enjoy an even stronger 2017.