A team of researchers have successfully developed a new kind of solar battery that can store energy directly from sunlight.

The team, a collaboration between the University of Córdoba and the Max Planck Institute for Solid State Research, reports that their new battery uses an abundant, non-toxic, easy-to-use material called 2D carbon nitride. But, the battery is something of an accidental discovery.

Absorbs and stores light

“In the group led by Professor Bettina V Lotsch, from the Max Planck Institute, they had achieved synthesising a material capable of absorbing light and store that energy to use it later on demand,” explained team member Alberto Jiménez-Solano “and it occurred to us to use it to create a solar battery". 

To make the battery, the team first had to deposit a thin layer of 2D potassium carbon nitride, poly(heptazine imide), K-PHI for short, to create a stable structure for the battery. Usually, the researchers point out this material is found as a powder or in an aqueous solution, so this was quite a challenge. To this, the team added a material sandwich to form a photovoltaic cell. 

This sandwich consists of “a highly transparent glass which presents a transparent conductive coating (to allow charge transport), and a series of layers of semitransparent materials (with different functions), and another conductive glass that closes the circuit”, explained Jiménez-Solano. 

The sandwich enables the creation of photogenerated electrons that become trapped within the structure. These photoelectrons are then able to persist for some time.

"The remaining photo-electrons persist as long-lived anion radicals akin to small polarons and are macroscopically evident by a colour change from the yellow material to a blue hue in aqueous conditions.

Provided that the surrounding is free of electron acceptors such as dissolved oxygen gas, the trapped charges can persist for days (albeit with progressively decreasing concentration) and may later be applied to initiate reduction reactions, such as reduction of protons to H2as a chemical fuel," explained Bettina V Lotsch et al in a paper on the subject.

This setup of varying layers with thicknesses optimised for the task could, the researchers believed, enable the material to absorb light from either side (since it is semi-transparent). It turned out, during experimentation, it could. This, the researchers point out, means the device has great versatility as it can provide both a large current quickly (like for a photography flash) and a sustained smaller current over time (like for a mobile phone).

Batteries from urea

This study showcases the effectiveness of the device, which is made from urea – an abundant, environmentally sustainable, and easy-to-synthesise material. Further research is needed to explore its performance in different real-world scenarios and to adapt it for manufacturing purposes. 

It could also prove pivotal for the solar storage industry, as low-cost, environmentally friendly energy capture and storage devices will prove critically important. Especially if they can be produced at scale and relatively cheaply.