![]() ![]() For inorganic semiconductors such as silicon, other materials are often added to the semiconductor (a process known as doping) to create regions of high (n-type) and low (p-type) electron density. However, the origin of the built-in field depends on the type of semiconductor being used. The built-in electric field of a device arises from the relative energy levels of the materials that make up the cell. Once dissociated, the free charges diffuse to the electrodes of the cell (where they are collected) - this is assisted by built-in and applied electric fields. d) Finally, the charges are transported to the electrodes. Here, c) the electron (or hole) transfers to material 2, and the exciton is split. b) The exciton diffuses to an interface with material 2 which has offset energy levels. a) A photon is absorbed by material 1, generating an exciton. ![]() Solar cell operation for a material with a low dielectric constant. This enables the electron (or hole) to transfer to the other material, and dissociate the exciton. A common method is to get the exciton to an interface between materials with energy levels that have an offset greater than the exciton’s binding energy. Excitons in materials with low ε r have high binding energies, preventing thermal dissociation - thus requiring a different method of dissociation. In materials with high ε r, excitons have low binding energies - enabling dissociation to occur thermally at ambient temperatures. c) The electron and hole are transported to electrodes to be collected. a) A photon is absorbed by the semiconductor, b) an electron is promoted from the valence band to the conduction band, leaving a hole in the valence band. This describes the level of screening between charges in a semiconducting material and affects the binding energy of the exciton. The energy required to do this is dependent on the dielectric constant (ε r) of the material. This exciton must be split (also known as ‘dissociation’) before the charge carriers can be collected and used. Due to their opposite charge, the excited electron and hole are coulombically bound in a state known as an ‘exciton’. This is known as a ‘hole’, and behaves like a particle analogous to an electron in the conduction band (albeit with positive charge). With the electron now in the conduction band, an unoccupied state is left in the valence band. If a photon incident on the semiconductor has energy (E γ) greater than the band gap, it will be absorbed - enabling an electron to transfer from the valence band into the conduction band. This small band gap is what enables some semiconductors to generate electricity using light. Energy bands for metals, insulators, and semiconductors. Conversely, the band gap in semiconductors is relatively small, enabling some electrons to move to the conduction band by injecting small amounts of energy. Insulators have very large band gaps which require copious amounts of energy to cross - and as such, inhibits the movement of electrons from the valence band to the conduction band. In a conductor, there is no band gap as the valence band is not filled completely - thus allowing the free movement of electrons through the material. ![]() The energy difference between the top of the valence band and bottom of the conduction band is known as the ‘band gap’ (E g). The valence band contains the highest occupied electron energy levels, whilst the conduction band contains the lowest unoccupied electron energy levels. Electron energy levels are generally categorised into two bands: the ‘valence band’ and the ‘conduction band’. Semiconductors can carry out this conversion due to the structure of their electron energy levels. The main component of a solar cell is the semiconductor, as this is the part that converts light into electricity. Therefore, there is arguably a much greater potential for solar to fulfil our energy requirements than other renewable sources. Clearly, the Sun provides more than enough energy to satisfy global energy needs. Meanwhile, the total annual solar energy that falls upon the Earth’s landmasses is estimated to be 1,575 - 49,837 EJ. In 2012, the total global energy usage was approximately 559 EJ (exajoules, x10 18). At the most basic level, the semiconductor absorbs a photon, exciting an electron which can then be extracted into an electrical circuit by built-in and applied electric fields.ĭue to the increased desire for more renewable sources of energy in recent years, solar power has seen increasing popularity. The photovoltaic effect is a process that occurs in some semiconducting materials, such as silicon. They are also commonly called ‘photovoltaic cells’ after this phenomenon, and also to differentiate them from solar thermal devices. Solar Cells: A Guide to Theory and MeasurementĪ solar cell is a device that converts light into electricity via the ‘photovoltaic effect’. ![]()
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