Introduction:
Discovery of electricity brought a revolution to human society. Since then the demand of electricity is escalating day by day. In 2004, the total world energy consumption was found roughly 14.5 terawatts (TW) per-day. This huge consumption of energy was first highlighted by Professor Richard E. Smalley as terawatts challenge (December 2004, Materials Research Society Fall Meeting in Boston). The Energy Information Administration (EIA) of United States regularly published survey reports on total world energy consumptions. According to EIA reports, the consumption was 104,426 TWh in 2012, increased to 113,009 TWh in 2017 (wikipedia.org/wiki/World energy consumption). The energy consumption is typically measured per year basis and it includes all possible energy harnessed from all possible energy resources; such as fossil fuels, hydrothermal power, nuclear power, and others (solar, wind, geothermal, biomass, known as renewable energy sources). To meet-up this ever-increasing energy needs, we need to develop and deploy technology to use renewable resources available to us alongwith the improvement of electrical grid and energy storage devices.
Electricity Generation:
Electricity generated from different energy resources is given in the figure here (provided by International Energy Agency, 2019). We can see even today the electricity generation mostly (~70 %) depend on fossil fuels. Nearly 20 % is coming from hydrothermal power-plant and only 7 % is coming from solar energy and wind power. Hydropower is an excellent renewable energy source but it requires reserved water in reservoirs at high altitudes, therefore not suitable for every place in earth. Another choice is off-course geothermal energy. It can provide almost what we need at present but generation and extraction of electricity from geothermal-source into usable from is very difficult. Solar energy and wind power are very promising renewable energy resources. Future of electricity generation and supply will possibly depend on these two. I highlight the importance of solar energy as a renewable source and development of solar cells in the following sections.
Why do we need solar energy?
The main concern is that we need huge amount of energy per-day (roughly 15 TW). At the moment most of the energy is coming from fossil fuels (coal, petroleum, natural-gases), non-renewable resources. These fossil fuels take millions of years to reform and if we consumed at the present rate then the available amount of reserved oil, natural gases and coal will end up soon. So, we need to reduce our dependence on fossil fuels and look for alternatives. In addition, burning of fossil fuels generates CO2/CO gases causing environmental pollution. We are looking for some alternatives that are more environment friendly.
The Sun is a massive source of energy. Nuclear fusion reactions always go on inside the Sun, in which hydrogen atoms fused into helium atoms, realising energy into space in the form of radiation. Our Earth receives energy from the Sun in two forms : heat and light. Direct radiation from the Sun reaches almost every corners of earth. Total energy received by earth from Sun is roughly 1.75×1017 Watts. Here we need to consider loss of radiation due to atmospheric scattering and absorption of radiation in the upper atmosphere. Almost 30% of the incoming radiation is reflected back into space and rest portion is absorbed by clouds, oceans and land-masses (wikipedia.org/wiki/solar-energy). We consider the energy absorbed by the land-mass only, termed as technical potential of direct radiation; it is Ptechnical = 26,000 TW. We see that Ptechnical is several times larger than total energy consumption per day. It is really remarkable and makes solar energy a captivating source for generation of electricity. What we need is to develop an efficient device (solar cells) to produce electricity directly from sunlight. Solar technology it is quite expensive initially but in the last 5 to 10 year development in photovoltaics is promising and prices are coming down very fast. The following sections I will focus more about solar-cells and the three generations of solar-cells.
History of Solar cells:
The photovoltaic effect was discovered in 1839, by Alexandre Edmond Becquerel and with this the journey of solar cells began. In 1983, selenium solar cell with efficiency roughly 1% (fraction of incident power converted to electricity) was developed. However next significant breakthrough in this field came in 1954, with the discovery of modern silicon solar cells in Bell Laboratory, efficiency of 6%. After that in 1959, Hoffman Electronics manufactured commercial solar cells of 10 % efficiency. Initially solar devices were quite expensive and were used in space-applications mainly. As it is not possible to carry huge batteries into space, solar cells proved to be a good alternative although it is expensive. The silicon solar module was first installed in satellite Vanguard-1, in 1958. Thereafter solar cells have been regularly used as main power source for satellites.
Figure 1: Use of solar cells in space applications; Vanguard-1, Explorer 6 and Telstar (source: wikipedia.org/wiki/Vanguard 1, Explorer 6, Telstar)
Since 1990, solar technology has been shifted slowly from silicon to gallium arsenide (GaAs) based technology. Subsequently thin film solar-cells, polymer solar-cells, multi-junction devices are developed. In this long journey, since 1839 to 2020 several milestones have been reached. Different Companies in collaboration with International Labs have participated in the race of developing solar technology; in this way efficiency and stability of the solar devices are improving day by day.
Three Generations of Solar cells:
First generation: Silicon solar-cells belong to this group, are really dominating in the market at present. Either mono-crystalline or polycrystalline silicon are used to fabricate solar cells. Generally silicon cells are thick so materials cost is huge, they are efficient and stable with reasonably high energy payback time; it means more energy/cost has been spent in manufacturing the solar-cell than electricity gained from it.
Second generation: Thin films solar cells belong to this group. Main 3 candidates are copper-indium-gallium-selenide (CIGS), cadmium-telluride (CdTe) and amorphous silicon solar cells. Materials used here are direct band-gap semiconductors and thickness of the absorbing layer varies from few nano-meters to micro-meters. The main advantages are; much thinner than silicon cells so material cost is minimized, can be prepared on flexible substrates and have much lower energy payback time. The main drawback is they are less efficient than conventional solar cells.
Figure 2: 2nd generation Photovoltaic solar-cells (wikipedia.org/wiki/Thin-film solar cell)
Third generation: Actually various different types belong to this third generation. It includes polymers solar cells, dye-sensitized solar cells, multi-junction solar cells, quantum-dot solar cells, and perovskite solar cells (wikipedia.org/wiki/Third-generation photovoltaic cell).
- Polymers cells come under organic solar-cells. Polymers cells can prepared directly from solutions, they can be printed or coated over a flexible substrate. The production time is less hence they have really low energy payback time.
- Multi-junction cells consist of multiple p-n They are GaAs based solar-cells, are really very efficient and expansive, mainly used for space applications.
- Quantum dot cells are basically nano-crystals of semiconducting materials like; cadmium sulphide (CdS), cadmium selenide (CdSe), lead sulphide (PbS). Main advantage here is one can tune the band-gap of these semiconductors by changing the size of nano-crystals.
- Perovskites solar cells are very new and fastest developing technology. In 2009 the recorded efficiency was 3.8% and it increases to 22% in 2016. They can be made like polymer solar-cells but stability is the main challenge that is to be improved.
Conclusion:
Solar cells are capable of generating electricity from Sun light. They are environment friendly as they do not emit any greenhouse gases like fossil fuels or toxic wastes like nuclear energy. The main drawback is their efficiency and high cost. Lots of research is going on in this direction and we believe that in future many efficient and cost-effective photovoltaic devices will be available for us and would resolve our energy need.
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