By Dr Franky So, Chief Technology Officer, NextGen Nano
In the days gone by, many ancient civilisations, from the Aztecs to ancient Egyptians, worshipped the Sun as a god or deity that could bestow prosperity and maintain the order of the immediate universe. In today’s slightly more enlightened times, we look to the Sun once more, albeit less as a deity but more to solve our energy crisis, sustainable as it may be.
The Sun bombards the Earth with 430 quintillion Joules of energy every hour: that’s a lot more power than the entire world consumes in a year. Solar power doesn’t affect our carbon footprint and doesn’t make global warming worse, yet, so far, it only accounts for about 1.7% of the global power generation.
Nevertheless, harnessing that energy is easier said than done: One of the biggest challenges is capturing this energy efficiently. Traditional silicon solar photovoltaic (PV) cells are brittle, expensive and inefficient, generating energy to 17% efficiency at the most. In addition, despite being heavily subsidised, the prices of silicon-based solar panels are still not competitive with other conventional combustion techniques – which affect the environment.
One way to lower PV panels’ manufacturing costs and improve their efficiency is to use organic materials that require less demanding conditions for processing. Realising this, the opportunity presented for solar technology all of a sudden looks quite tremendous.
The birth of solar technology
The photovoltaic effect was first observed by Edmond Becquerel in 1839, with PV panels introduced in 1954 by Bell Labs, becoming the primary means of harnessing solar energy. More recently, greater research has gone into this field, especially in making more efficient and cheaper PV panels.
There are three specific processes that need to take place in PV cells for them to function properly. First, the cells need to absorb photons in semiconducting materials. These photons then excite charge carriers in the semiconductor material, subsequently extracted to electrodes, generating current. To convert this current to electricity, the generated direct current (DC) goes through inverters which convert it to alternating current (AC) for consumption.
The first PV cell by Bell Labs was only 6% efficient in converting sunlight to electricity, meaning it only successfully turned a small fraction of the photons that hit it into electricity.
First-generation PV panels consisted of wafer-based cells, which were typically made using materials such as crystalline silicon. The overall design of these panels was based around that of a single p-n junction, where positive (p) and negative (n) type semiconductor materials form a “junction”. An n-doped material has a greater abundance of negative charges whereas the p-doped layer the opposite, or positive charges. The polar opposites of the two generates an electric field at the junction between the layers, pushing photo-generated charge carriers out of the p-n junction toward the electrodes.
The second generation
The second generation solar technology was born in the 1980s, promising to overcome some of the technical and physical limitations introduced by their 30-year-old predecessors. In 1999, a new solar cell was born, which offered a 33% efficiency in laboratory setting. Where the first generation was primarily silicon-based, this new generation cells used a more diverse range of materials − from amorphous silicon to cadmium telluride. Because of the strong light absorption compared with silicon, these are known as “thin-film” solar cells, as thin layers of materials are deposited onto substrates such as glass or plastic.
The key difference between the two generation PV panels is that the slimmer footprint of the latter cells allowed for new PV panel designs that could circumvent the theoretical Shockley-Queisser limit, which is the maximum theoretical efficiency of a solar cell using a single p-n junction to collect power from the cell where the only loss mechanism is radiative recombination. This is predominately achieved by introducing multiple photoactive materials to create multi-junction cells, and extend the spectral response of the final cell and increase the power conversion efficiency.
Of course, the second generation of solar cells had their own limitations. The substrate materials were still inflexible and, with efficiencies running only to a maximum of 30%, they required substantial surface areas. In addition, some of the materials used – such as cadmium – proved carcinogenic, as they produce toxic by-products during recycling or when exposed to certain environmental conditions.
Humanity has collectively accelerated the depletion of natural resources for use as energy sources, and the changes to the environment linked to the use of these resources, has increased the need for sustainable energy generation. Combining all these factors with requirements for being environmentally-friendly and non-toxic has spurred further research into solar energy harvesting – bringing about the third generation of PV cells.
Next-generation PV technology
The third generation of PV cells incorporates several types of solar technologies, most notably organic photovoltaic (OPV) cells that promise greater efficiencies whilst reducing environmental impacts. This is the technology the team of researchers at NextGen Nano has been exploring, with the goal of applying nanotechnology to develop a highly-efficient, flexible, organic solar cell that also supports the move to decentralised energy generation.
The result of this research is PolyPower, which blends earth-friendly organic materials to provide a lightweight, flexible and potentially inexpensive solutions to solar energy harvesting – lightweight organic polymers are used in place of silicon. This technology achieves all the beneficial characteristics whilst still being robust, mitigating the problems associated with brittle panels.
Because PolyPower is developed at a nano level, with transparency and flexibility, it can effectively be applied as a semi-transparent thin layer or solar panels to the building surfaces, windows and rooftops – including that of vehicles. Applications could use organic polymer solar cells as a design feature, charging with energy during the day, and independent of a centralised power supply. They are also far less expensive to manufacture than silicon-based solar cells, since organic materials like conjugated polymers are easy to make.
The technology holds great promise for future use in many wide-ranging applications, avoiding the limitations of current PV panels.
We no longer worship the Sun as a sacred deity, but there is now a greater understating of its importance in future applications and energy use. In essence, the Sun is one enormous, cosmic battery, boasting practically bottomless energy and with half of its (approximately) ten-billion-year run-time remaining. All that’s needed is the right technology to harness this energy, in an ethical, environmentally-friendly and adaptable way to applications and industries of today and the future.