Solar cells

Solar cells

Introduction of Solar Cells

Solar Cells are devices that are intended to capture sunlight and convert them into electricity via the photovoltaic effect. These solar cells are generally made of semiconductors which are individual atoms bonded together in a regular structure so that each atom is surrounded by 8 electrons. Most semiconductors are found in groups II, IV or V, in which silicon is one of the most common semiconductor that is highly used for solar cells. The electrons around the atoms are involved in covalent bonds between the atoms so that there are 8 electrons surrounding the atom (in the case of silicon, covalent bonds are made with 4 different silicon atoms around the arbitrary silicon atom so that there are 8 electrons around the atom) , hence are localized to the region surrounding the atoms. Since such bonded electrons are localized, they are unable to move or change their energy, therefore, are unable to act as charged carriers for electrical conduction. However, at higher temperatures, there is a chance that the electrons may escape from its bond, allowing it to move freely around the crystal lattice and participate in electrical conduction. As such, a semiconductor can be said to have conductive properties at higher temperatures yet also have insulating properties at lower temperatures.

Solar Cells - Band gap

Band gap is a concept whereby electrons in a semiconductor need a certain energy level to leave their bonds. In essence, band gap is the amount of energy needed for an electron in the semiconductor to leave its respective covalent bond, allowing the electron to participate in conduction. As such, the lower energy level of the semiconductor is known as the valence band, whereby electrons are rather static and are unable to participate in conduction while the higher energy level of the semiconductor is known as the conduction band whereby electrons in this band are mobile and can participate in conduction. When an electron gains enough energy to leave the valence band and enters the conduction band, it leaves behind a space in the valence band. An electron in a neighbouring atom can occupy this space, but the electron will also leaves behind another hole when it leaves. Hence, the movement of the space of an electron, called a “hole”, can be thought of as the movement of a positively-charged particle through the crystal structure in the valence band. This movement of electrons in the valence band results in the movement of this electron hole.

Light Reactions in Photosynthesis

When a photon of specific wavelength 680nm strikes a pigment molecule of the Photosystem II, one of the electrons of the molecule gets excited. As the electron returns to ground state, a photon of the same wavelength is released and hits an adjacent pigment molecule. This continues until the photon strikes the P680 dimer. Electrons of the P680 dimer will excite and transfer to a primary electron acceptor, forming P680+. An enzyme from the oxygen-evolving complex acts as a catalyst for the splitting of water into 2 hydrogen ions, 2e- and 2 O2 molecules. Since P680+ is the strongest biological oxidizing agent, its electron “hole” must be filled first, hence this facilitates the water-splitting reaction. The 2e- from the reaction will be supplied to the P680+ dimer pair. The electron transfer chain will bring down the energy level of the 2e- and the energy from lowering the energy level of the electrons is used to power the chemiosmotic synthesis of Adenosine Triphosphate (ATP). Similar to the Photosystem II, pigment molecules get excited by photons of wavelength 700nm and the energy is transferred in the same way as that of the photon in Photosystem II. The P700 dimer, after getting excited, will also donate one electron each to the primary acceptor. The 2e- that has just exited the electron transport chain will then be supplied to the P700+ ion pair. Meanwhile, the 2e- donated by the P700 to the primary acceptor will be brought down a shorter energy chain to the NADP+ reductase that will catalyze the reaction for NADP+ to become NADPH which is an energy storage carrier chemical used to power the secondary step of photosynthesis, the Calvin's Cycle. The equation is as follows:

NADP++H++4e-→NADPH

Original Proposed Idea

Originally, I intended to combine the solar cell as a photoreceptor to capture the photon, with a tank to extract the other enzymes and chlorophyll dimers to simulate the Z scheme and Calvin's Cycle occurring in the plants during photosynthesis. However, upon further evaluation, I realised that this original idea was impractical and hence, the idea was edited and changed to the current one stated below in the Experiment and Explanation section. Further comparisons and evaluations of the original idea with the proposed experimental design is in Discussions and Considerations section.

Experiment and Explanation

In essence, the solar cells can act as the chlorophyll dimers that capture light to power the photosynthesis process. However, As we do not have the technology to replicate the said receptors, the solar cells can capture the photons and convert it into electricity which can then be used to split the water molecules into hydrogen and oxygen molecules. The electrons meant to be removed from the hydrogen molecules in the plants to excite the P680 chlorophyll dimers, on the other hand, would not be removed. Instead, the hydrogen molecules will be harvested as a form of storage clean energy source, since hydrogen when combusted yield water. The byproduct of the reaction, oxygen is also useful to our earth since we need oxygen to respire and due to the burning of trees, the amount of oxygen molecules are quickly being replaced by carbon dioxide molecules.

Discussions and Considerations

Originally, the idea for this project was to follow almost exactly how plants photosynthesise. In this sense, it means that the solar cells will act as a replacement for the

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