Solar batteries: the success story of a hybrid energy device

An image to illustrate hybrid energy

The ‘SolaBat’ research project has created a hybrid energy device using an organic solar cell and advanced
rechargeable battery.

The Internet of Things (IoT) is currently undergoing major technological developments. One critical challenge is supplying uninterrupted power to IoT-enabled devices. This is only one of many modern applications requiring a redundant, yet simple and reliable, power source. Since simplicity and redundancy are all too often antagonistic, a dual technological solution could be the answer. The topic of SolaBat, a fundamental research project funded by the Austrian Research Promotion Agency (FFG), is whether we can realise a hybrid between an organic solar cell and an advanced rechargeable battery.

What is a hybrid energy device?

From a principle point of view, a hybrid energy device consists of a solar cell connected to and integrated with a rechargeable battery. This simple concept leads to some practical
restrictions on the materials side. Firstly, the voltage generated by the solar cell must
slightly exceed the voltage of the battery so that charging of the battery is possible. This
translates into opposite requirements for the solar cell part and for the battery part.

Voltage requirements

On the one hand, a high voltage solar cell is required. With the majority of single solar cells
seldom exceeding 1 V, there is a necessity of designing relatively high voltage solar cells.
Thus, we are naturally oriented towards tandem organic solar cells (OPV) that are able to
reach higher voltages. On the other hand, a low voltage rechargeable battery would be
required even when used with a tandem solar cell. Indeed, Li-ion batteries (LIB) are typically charged to 3.6 – 4.2 V,while a tandem solar cell will only reach 1.8 – 2 V at open circuit; the voltage under load being lower. A simple estimation tells us that solar cell supplied voltage has to be at least 0.2 V above the nominal battery voltage. Therefore, we scouted for tandem solar cell configurations that reach 1.8 – 2 V and for battery materials that lead to a nominal voltage between 1.2 – 1.6 V.

New IoT technologies

Although such battery materials exist, they were never properly investigated and
characterised. Until very recently, such materials were merely perceived as scientific
curiosities, rather than seriously considered battery materials. This view is currently
changing under the impetus of new IoT technologies; batteries required for such devices are fully acceptable even when delivering low voltage at the terminals. Instead, good stability and long cycle-life would be the most critical selection criteria for such applications.

The success of the hybrid energy device

Once materials were selected, test batteries and test solar cells were first assembled
separately and then an important part was then dedicated to testing the chemical
compatibility of the components – in particular the liquid electrolyte used must not
interfere with the solar cell or lead to corrosion of any metal part that comes in contact withit. Then, the hybrid device was assembled, encapsulated and tested as a whole.
However, there were unexpected problems encountered and solved during this project
(such as electric contacts and encapsulation issues). The selected lithium storage materials
were found to possess lower than optimum stability – this was solved by changing the
battery cell chemistry to an alternative sodium-ion battery (SIB) that showed an almost
perfect stability and resilience to deep cycling. A detailed report on the successful
realisation of a hybrid solar cell – battery device is in preparation and will published within
this year.

To conclude, SolaBat conducted an exploratory work resulting in a successful and unique
hybrid energy device. I take this opportunity to thank all the members of the two research teams at Institute for Chemistry and Technology of Materials (ICTM) that essentially
contributed to this success. The dedicated support from the Austrian Centre for Electron
Microscopy and Nanoanalysis (ZFE) is equally acknowledged. I thank you all for a very
successful and valuable collaboration – the developed human resources and expertise is already paving the way towards further technological development.

Dr Ilie Hanzu
Graz University of Technology
+43 (0)316 873 32329

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