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Solid State Solar Cells

Solid state electronics are the electronic equipment which use the semiconductor equipment similar to semiconductor diodes and integrated circuits (ICs). Solid state electronics are also used for devices which have semiconductor electronics with no moving parts replacing devices with moving parts.  

Solid state solar cells

In a normal solid state semiconductor, the  solar cell is composed of two doped crystals, one crystal is doped with n-type impurities – n-type semiconductor- which increases extra free conduction band electrons, and the other crystal is doped with p-type impurities – p-type semiconductor- which adds extra electron holes. When positioning the crystals in contact, some of the electrons from the n-type portion flow into the p-type to fill the free holes, also known as electron holes. Ultimately, enough electrons will flow across the boundary, equalizing the Fermi levels of the two materials. When placed under the sun, photons of the incident sunlight will excite electrons on the p type side of the semiconductor, which is known as photoexcitation process. In silicon cells, incident light can provide sufficient energy to push an electron out of the lower energy valence band to the higher energy conduction band. Sunlight creates an electric current from light as follows, when a load is connected to the cell, the electrons will flow from p-type side to the n-type side, losing energy while moving across the external circuit, and then flow back to the p-type material where the electrons can re-combine again with the valence band hole left behind.  

Function and efficiency of solid state solar cells

In semiconductors, the band gap defines the amount of energy that a photons must at least possess to contribute to producing an electrical current. For silicon, the majority of visible light range (red to violet) has enough energy to achieve this. Photons having higher energy ( in the spectrum’s blue and violet range), have excess to required energy for crossing the band gap; even though some of the excess energy is transferred to the electrons, most of the energy is wasted as heat. Another concern is that in order to have a reasonable opportunity for capturing a photon, the n-type layer has to be thick. This increases the chance of a freshly ejected electron meeting up with a hole already created in the material before reaching the p n junction. This leads to producing an upper limit on the silicon solar cells efficiency, currently ranging from 12 to 15% for common modules and can reach 25% for the optimum laboratory cells (as per Shockley–Queisser limit, the theoretical maximum efficiency for single band gap solar cell is 33.16%).  
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