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Silicon Films on Metals for Energy Applications

Final Report Summary - ENERGOSIL (Silicon Films on Metals for Energy Applications)

Silicon solar photovoltaics (PV) are the most widely employed solar technology as, generally, PV cells have low maintenance costs, no moving parts, operate at near ambient temperature, and enable generation at any scale. As an example, a 10-square-metre (m2) PV array is, in theory, no less efficient per unit area than a 10-square kilometre (km2) array which contrasts with other renewable energy sources, such as hydroelectric or wind turbines, which lose efficiency at reduced scales meaning only large-scale installations are economically feasible. During operation, the front surface of the PV module is illuminated by light. Solar photons are transmitted into each cell, and those photons with sufficiently high energy are absorbed. An absorbed photon transfers its energy to an electron and its positively charged counterpart (a hole) creating a “pool” of free charge carriers within the material. An internal electric field drives electrons toward one electrode and holes toward the other, creating a flow of current.

For technical and historical reasons the vast majority of commercial PV module production has been, and remains, silicon based. Currently (2015) over 90% of worldwide PV energy production is based upon silicon solar technologies. Silicon can be manufactured into non-toxic, efficient, and extremely robust solar cells, making use of the cumulative knowledge of more than 60 years of semiconductor processing and manufacturing technologies. Crystalline silicon (c-Si) solar cells are divided into two categories: single- crystalline (sc-Si) and multicrystalline (mc-Si). sc-Si affords higher crystal quality and so improves charge generation and power conversion efficiencies, but requires more expensive wafers. Efforts are also underway to improve the energy efficiency of thin film silicon solar cells, thin film panels are not as efficient as crystalline cells and therefore more thin film panels are required to generate the same amount of electricity. A thin film installation can take up to 35 % more space (i.e. land) to achieve the same total power output as a premium crystalline installation. Despite small advances in new PV materials, crystalline silicon utterly dominates today’s PV landscape, and will continue to be the leading deployed solar PV technology over the next two decades.
The price of solar power continues to fall, and is now on a par or cheaper than grid electricity in many areas of the world. Unsubsidized rooftop solar electricity costs between
Eu0.08-Eu0.13/kWh which is around 30-40% below the retail price of electricity in many global markets. It is widely predicted that solar systems will be at grid parity in up to 80 % of the global market within two years. Despite the massive growth in solar capacity worldwide (40 GW capacity was added in 2014 and 57 GW is expected to be added in 2015, largely in China the cost of silicon solar cells PV, makes up less than 1 % of the electricity market today but could be the world’s cheapest energy source by 2030, and the world’s biggest single source by 2050.

Photovoltaic (PV) electricity generation is a rapidly expanding industry with the European scenarios to reach 15% solar energy contribution to the total EU electricity market by 2020. There is an increasing public awareness in the PV technology as a provider of clean, sustainable and secure energy from the most abundant source, which is free. Cost reduction of
photo-electricity, therefore, is high on agenda of PV engineers and materials scientists.
The performed project, in a broad sense, was aimed to contribute to more wide application of the renewable energies. Vast majority of the solar cells are produced from silicon, where the material cost makes up to 70 % in total balance. Our goal was to find new methods of silicon surface treatments in order to increase its efficiency in solar energy harvesting.
Silicon surface nano-micro architectures were created, which absorb about 99% of the incident light.

Solar cell efficiencies can be improved based on the following aspects: (i) absorbing more incident photons to create more photocarriers and (ii) efficiently collecting more photocarriers to generate higher flow of current. The employment of surface nanostructures provides economic potential for capturing more light owing to their unique architectures, large surface-area-to-volume ratio, and quantum confinement effects, which substantially differ from those of bulk materials. From an optical viewpoint, nanostructures show significant photon capturing and photon confinement abilities to enhance light absorption. Light management is crucial to solar cell design as it increases the path length of light in the absorber layer, thereby enhancing the probability of electron-hole pair generation. By engineering the reflective and refractive properties of the solar cell surfaces, light can be trapped within the active region more efficiently. Better photon trapping allows for physically thin, but optically thick active layers in the solar cells to not only reduce the processing costs and amount of material used, but also to decrease electrical losses during the photocarrier transport.
The experiments have been performed by means of electrochemistry in molten salts and ionic liquids. The developed new methods are promising also for production of chemical sensors and hydrogen photo-generation.

The method used was based on the FFC-Cambridge process, where a silicon disk was exposed to a cathodic potential which caused the oxygen in the oxide to ionise and dissolve in the salt leaving a matrix of silicon needles creating a black surface which could absorb up to 95% 0f the incident light over a wide range of wavelengths. Furthermore, distinctive surface architectures of stainless steel were created, which could be applied for production of efficient heat concentrating solar energy devices. The results have been published and patented. A company, BlackSilicon Ltd, has been formed and financed which will be incorporated in the UK.

The work carried out addressed well the Energy Roadmap 2050 for the decarbonisation of electricity production and achievement reduction in greenhouse gas emissions (Communication from the Commission to the European Parliament, the council, the European economic and social committee and the committee of the regions, ‘A Roadmap for moving to a competitive low carbon economy in 2050’, 2011). The obtained results, first of all, are relevant to PV industry. A black silicon company was established to commercialize the innovations. Venture capital was also attracted and invested to seed this early-stage company with a novel silicon technology.

Further details can be obtained from Prof. D. J Fray – or Prof. E. Juzeliunas