Bioinspired fractal designs for laser scribed graphene energy storage integrated with silicon solar cells

In the 21st century, technology has undertaken a fast growth with the evolution of flexible, wearable, and on-chip portable applications. The developments in the areas of nano-micro fabrication, low-dimensional materials, and advanced characterisation techniques have contributed to this growth. Rapid expansion in the technological field shortens the distribution of total electricity generated across the world. An efficient way to overcome this issue is to create energy storages with large storage capacities and longer lifespans.

In addition, the effective utilisation of the world’s sustainable energy resources, like solar energy, is a priority importance for the research community, due to the fast depletion of the current major energy resources, like oil and coal. A bottle neck issue associated with the harnessing of solar energy for the production of electricity is the intermittent nature of solar power, which limits its use to certain climatic conditions and geographical regions. The currently available solution to the issue is to provide battery supplies with the solar module, and these comprise almost 30% of the total cost.

Batteries are the most commonly used energy storage units in the commercial market, due to their higher energy storage capacities. But these energy storage units are known for their non-ecofriendliness, short life spans, and long charging hours. These drawbacks create demand for other types of durable energy storage that can provide green, on-chip platforms for modern technologies at a low cost.

A recent addition to the energy storage family are micro-supercapacitors based on laser scribing methods, and are useful for the on-chip energy storage integration with buildings, vehicles, wearable and portable devices. These energy storage units differ from batteries due to the electrostatic adsorption and desorption phenomena that occur between the electrodes and electrolyte ions, while an electrochemical interaction occurs in batteries. In general, electrodes in micro-supercapacitors are made of carbon materials, due to their excellent physical properties. The common electrolytes used in micro-supercapacitors mainly belong to the aqueous, organic electrolyte, and ionic liquid families. For all solid state energy storage devices, ionic gels are preferred as the electrolyte.

Micro-supercapacitors are known for their higher charge transfer rates and green nature. But they lack high energy storage capacities,which make them a less substantial to substitute for batteries. The limitation for energy storage capacity is due to the use of an interdigital design for electrodes, which limits the accessible surface area and, in turn creates lower energy storage densities. However, in recent times, new materials like oxides and conducting polymers are used with carbon materials to improve their performance. The use of additional materials can result in the increase of cost of energy storage, however, less preferable.

To overcome the limitation of accessible surface area of micro-supercapacitors, we adopt fractal, self-repeating structures inspired by the Sword fern or American fern leaf for the electrode designs of the micro-supercapacitors. In the fern leaf, the enhancement of the water transport is contributed from its vein density, which directly contributes to photosynthesis process improvement. We use those principles in our micro-supercapacitor design and perform a theoretical investigation of different fractal electrode designs using the Stern-Gouy model.

We demonstrate that the Hilbert fractals have the highest surface area, and are more similar to a fern leaf than other designs within the same family, like Sierpinski and Peano’s fractals. Inspired by these findings, we fabricate the electrode patterns on graphene oxides with optimum photoreduction conditions. We use the direct laser writing technique and electrode dimensions based on electrical conductivity measurements using direct laser writing in an area that is 4×4 cm2 .

We use reduced graphene oxides as the electrode material for fractal micro-supercapacitors due to the enhancement of its physical properties, like electrical conductivity and refractive index, after the reduction using thermal, chemical, or photoreduction methods. The reduction results in the removal of oxygen groups and leads to the formation of carbon clusters. The photoreduction using the direct laser writing technique is the most efficient reduction method for developing patterned applications such as microcircuits in flexible electronics. It induces changes in the pre-defined regions of the graphene oxide film, which afterward are known as laser scribed graphene or laser-induced graphene regions.

We study the photoreduction of graphene oxides under different laser beam irradiation conditions. The study is performed using a continuous wave 1064 nm carbon dioxide laser and an 800 nm ultrafast femtosecond laser beam with a pulse width of 100 fs. We study the influence of different repetition rates during the femtosecond laser beam-induced photoreduction of graphene oxides. The influence of parameters like scanning speed, laser fluences, and various objectives are envisaged for the photoreduction process.

The obtaining bioinspired fractal electrode patterns are filled with the electrolytes of two types: an ionic liquid and ionic gel. The ionic gel is used due to the requirement of securing an all solid-state device. The characterisation of the energy storage units is performed using the electrochemical measurements. The resultant micro-supercapacitor increased the energy storage density up to 30 times more than the existing other laser scribed graphene micro-supercapacitors, and is closer to lithium-ion batteries. We further demonstrate a flexible version of bioinspired frac-tal electrode-based micro-supercapacitors that retain their capacitance up to 90% in bent and twisted conditions up to 60◦ .

In addition, direct laser writing technique with ultrafast laser beams is utilised for the realisation of two-photon induced-graphene, three-dimensional micro-supercapacitors with high spatial resolution features in confined footprints with potential for large area fabrication. The energy storages are fabricated by optimising the repetition rates of the femtosecond ultrafast laser beam for the photoreduction of graphene oxides. The non-thermal laser processing offered by the femtosecond laser beam results in electrode patterns closer to 1 µm and even down to 600 nm using a 100x high NA oil objective, where our fabricated two-dimensional micro-supercapacitors are limited to an 80 µm resolution by the commercial carbon dioxide laser printing system.

We extend the optimised Hilbert fractal design pattern to become three-dimensional for the fabrication of miniaturised energy storage units using a layer-by-layer approach in an area of 1 mm2 of thickness, 23 µm. The resultant energy storage density exceeds the lithium-ion batteries with a 2-fold increase in rate transfer capability (power density) using an ionic gel electrolyte. In addition, we study the stretchability of the energy storage up to 150%. Our demonstration firmly confirms that this additive nanofabrication method is highly desirable for the development of self-sustainable stretchable energy storage.

The new stretchable supercapacitors enhanced by three times in the stretchability and 100 times in the the volumetric capacitance compared to the existing microsupercapacitors could open a new branch of technologies for flexible and wearable electronics, light-weight energy storages for space shuttles, displays and biomedical applications. Thus, our result is of interest to the general energy storages. These fractal energy storages with a high energy density in two-dimensional and three-dimensional opens the new opportunity for on-chip sensing, imaging, and monitoring.

Further, we use the two-dimensional, laser scribed, graphene micro-supercapacitors to obtain an on-chip solar-powered solution for the energy storage required by integrated, self-reliant buildings and portable applications. Initially, we fabricate a solar energy storage unit using interdigital laser scribed graphene electrodes on the reverse side of crystalline silicon solar cells. Even though we could demonstrate the concept of on-chip solar energy storage, the performance was limited to 62% due to the limited electrode surface area. Next, we denote high-performance bioinspired fractal electrodes for integration with the thin-film amorphous silicon solar cells by adopting the same technology used for energy storage integration with crystalline silicon solar cells. The efficiency attained was around 95% without the loss of solar cells, and the energy storage performance which can be useful for the future green self-powered platforms.

The research investigations and methodologies described in this thesis present an approach to developing high-performance, on-chip micro-supercapacitors in two and three-dimensional fractal designs in theoretical and experimental aspects. A successful demonstration of on-chip solar energy storages without damaging the performances of solar cells and energy storage is presented.