Module-5_Unit-5_NSNT R&D on Photovoltaic or Solar cell Introduction A solar or photovoltaic cell (also known as ‘solar battery’) is electrical device used to convert energy from light into electrical energy. The working of solar cell is based on photovoltaic effect, which is a physicochemical process. It is a type of photoelectric cell, which can be described as the device whose electrical properties (e.g., resistance, current, etc.) change as light is exposed onto it. Large scale photovoltaic modules are termed as solar panels, and are made up of numerous solar cells. Figure 1 A traditional solar panel made up of silicon. Electrical contacts (silver-colored strips) are printed on the wafer. Regardless of the source, whether sunlight or artificial light, solar cells are termed as photovoltaic. They can be used as photodetectors to detect light or any electromagnetic wave in the visible range, examples include infrared detectors. Besides, solar cells can also be used to measure the intensity of the light incident on them. A photovoltaic cell operates on the following three attributes: • Excitons or electron-hole pairs are generated on absorption of light. • The charge carriers are separated as electrons and holes. • The separated charge carriers are collected at their respective electrodes, i.e., electrons at the positive electrode, and hole at the negative electrode. In addition to this, a solar thermal collector can be used for direct heating or indirect generation of electrical power from the heat generated. In these devices, heat is supplied when sunlight is absorbed. Further, a photoelectrolytic or photoelectrochemical cell is ether a type of photovoltaic cell (similar to dye-sensitized solar cells developed by Edmond Becquerel) or a device which directly splits water into its constituent hydrogen and oxygen by using solar irradiation. Theory The working of a solar cell can be summarised as (Figure 2): • Photons present in sunlight hitting the solar panel are absorbed by the material of the panel, e.g., Silicon. • Absorption of photons leads to the excitation of electrons from their atomic/molecular orbital. Upon excitation, the electron either dissipates energy as heat and return to its previous orbital, or travels through the cell to be collected at the electrode. A counter current flows through the material to cancel the potential developed by this extra electron. This current is captured as electricity. The nature of the chemical bonds of the material are crucial in this process. Most common used material is silicon, which is used in two layers: one layer is doped with boron, and other by phosphorous. The chemical electric charges in the two layers are different and subsequently both drive and direct the current of electrons. • An array of solar cells is used to convert solar energy into a usable amount of electricity (DC). • DC electricity produced can be converted to AC by using inverters. Figure 2 Schematics showing the working of a solar cell. Most common solar cell comprises a silicon based large area p-n junction. Other variants of solar cells include organic, dye-sensitised, perovskite, quantum dot solar cells, etc. To allow light into the active material and for collecting generated carriers, a transparent conductive film is usually coated on the illuminated side of the solar cell. Films deposited usually have high transmittance as well as high electronic conductivity, e.g., ITO (indium tin oxide), conducting polymers, conductive networks of nanowires, etc. Efficiency Figure 3 Shockley-Queisser limit for theoretical maximum efficiency of a solar cell. Semiconductors having bandgap from 1 to 1.15 eV (NIR light) have highest efficiencies among single-junction solar cells. Multi-junction cells may have higher efficiencies. The solar cell efficiency constitutes various efficiencies: (a) reflectance efficiency, (b) thermodynamic efficiency, (c) separation (of charge carriers) efficiency, and (d) conductive efficiency. The product of these components gives the overall efficiency of the cell. The solar cell efficiency depends upon voltage, temperature coefficients, and allowed shadow angles. However, the measurement of these factors is complex. To alleviate this problem, various other parameters are calculated and these parameters are used to determine the above mentioned factors. These parameters include: • Thermodynamic efficiency • Quantum efficiency (QE) • Integrated quantum efficiency • Open circuit voltage (VOC) ratio • Fill factor QE accounts for the reflectance and recombination losses,VOC ratio, as well as fill factor. Resistive losses affect the fill factor, QE, as well as VOC ratio. Fill factor, a crucial parameter in performance of the cell, can be obtained as the ratio of actual maximum power obtained to the product of open circuit voltage and closed circuit current. The efficiency of single junction (p-n junction) silicon cells has now reached ~33.16% (Shockley-Queisser limit). For infinite layers of single junction cells, the efficiency can theoretically reach ~86% by concentrated sunlight. Figure 3 Predicted timeline for solar cell efficiencies (National Renewable Energy Laboratory). Timeline of the Development of Material Usually, the solar cells are named after the semiconductor material used for their production. The material used for creating solar cell must be able to absorb sunlight. Solar cells can be configured to either capture sunlight reaching the surface of Earth, or work in space. Additionally both single layer (single-junction) or multiple layers (multi-junctions) of the material can be used to produce solar cells. Figure 4 Global market share of photovoltaic technology. There are three classifications of solar cells: a. First generation cells: These are the conventional or traditional wafer based cells which are produced form crystalline silicon. These are the dominant commercially available photovoltaic cells, and uses polysilicon, and single crystal silicon. b. Second generation cells: These include thin film solar cells made up of amorphous silicon, CdTe, etc. These are commercially used in photovoltaic power plants, small standalone power systems, etc. c. Third generation cells: These include various thin film technologies are are the future generation photovoltaics. Majority of them are at experimental stage and are not commercially available. These cells use both organic (e.g., organometallic) as well as inorganic materials. Although at this stage, they have poor absorption and low efficiencies, they are investigated for their potential of producing highly efficient and economic solar cells. We will now briefly discuss various materials used for solar cell production: Crystalline Silicon (c-Si) Crystalline silicon or c-Si is the most commonly used bulk material for producing solar cells. It is also termed as ‘solar grade silicon’. Depending upon the crystallinity, and crystal size, bulk silicon is separated in multiple categories. The cells use the concept of p-n junction. Typical wafers used in solar cell production are 160-240 mm thick. Monocrystalline Silicon (mono-Si) These are more efficient and more costly than other types of cells. The cell corners appear clipped, like an octagon, as the wafer material is cut from cylindrical ingots, which are usually grown via Czochralski method. Solar panels using mono-Si cells are characterised by a distinctive pattern of small white diamonds. Epitaxial Silicon Epitaxial wafers of crystalline silicon can be grown over monocrystalline silicon ‘seed’ wafer via CVD process. After growth, these wafers are detachable as self-standing wafers of controlled thickness. The efficiencies of solar cells produced form these wafers are comparable to that of wafer-cut cells, however, they are much cheaper when CVD is performed at atmospheric pressures. The light absorption can be enhanced by tailoring the surface of epitaxial wafers. Polycrystalline or Multicrystalline Silicon (multi-Si) These are made from cast square ingots which are large blocks of molten silicon, cautiously cooled and solidified. Due to the presence of multiple small crystals, they are characterised by metal flake effects. These are low cost and most common type of cells. However, they have lower efficiency than the monocrystalline type. Ribbon Silicon It is a type of polycrystalline silicon produced by drawing flat thin films from molten silicon. It is cheaper than polycrystalline silicon. Additionally, silicon waste is greatly reduced since sawing from ingots is not required. Their efficiency is usually poor. Mono-Like-Multi Silicon (MLM-Si) This is also known as cast-mono. Small mono material is placed as seeds in polycrystalline casting reactors. The grown crystal is similar to mono material, however, the edges are polycrystalline. After slicing, inner portions are high efficiency mono-like materials (however, these are square and not clipped), while the edges are polycrystalline. This processing produces mono-like cells at the cost of poly. Thin Films The amount of active material is greatly reduced in thin-film cells. Most common configurations involve sandwiching the active material between two glass panes. For comparison, silicon solar panels use single glass pane. This makes thin film cells around two times heavier than c-Si panels. However, the ecological impact (estimated by analysing life cycle) of thin film based cells is smaller. Cadmium Telluride (CdTe) The only thin film material comparable to silicon in terms of cost per watt is CdTe. Nonetheless, Cd is highly