User:Fraserhof/BiohybridSolarCells



Biohybrid solar cells (also called photovoltaic cells) are being investigated as green alternatives to non-renewable energy sources. These photovoltaic devices pair the natural process of photosynthesis with nanotechnology in order to harness solar energy through the direct conversion of light into electricity. For decades, photosynthesis has been an attractive source of alternative energy due to its limitless source, low cost and ecological impact, and high energy conversion efficiency.



Generally, the nanostructure of biohybrid cells are made up of three components; a photoactive organic pigment, a linker, and an electrode. It is the primary photosynthetic process, in which solar energy is converted to chemical energy within the bonds of the pigment (sensitizers) molecules, that have a very high quantum yield. Through a series of redox reactions, a charge separation is created and electrons are transferred to the electrode which is held close by way of the linker molecule. Challenges arise in the second step of the process when the electrons are transferred from the reaction centers and stored, usually with efficiencies under 27%. Currently, titanium dioxide (TiO2) nanostructures are being extensively researched as an electrode substrate for biohybrid solar cells. TiO2 is of interest for its high biocompatibility, the large specific surface area of its nanostructures, tunable electronic characteristics, and its conductivity.

=Introduction=

Two technologies dominate this area of research: i) Photobioelectric Cells (PBCEs), and ii) dye-sensitized solar cell (DSSCs). Although the general concept for these technologies is the same, they differ in where the charge separation step occurs. In PBCEs, the charge separation occurs within the photosynthetic complex. In DSSCs, the charge separation occurs at the interface between the dye and the electrode (usually no linker is necessary here). However, as more photosystems are being integrated into the design of DSSCs, the line is becoming blurred between these two technologies.

The photosynthetic components of biohybrid cells are complexes of pigment-proteins and cofactors, usually chlorophyll (Chl a, b, c, d, f) or bacteriochlorophyll. In the late 1990s, thylakoid membranes isolated from plant leaves were successfully immobilized onto platinum electrodes, generating a photocurrent. Since then, several studies have shown that solar cells can be created using the native thylakoids under various conditions. It is thought that keeping the transmembrane photo-complexes in their native states may confer greater stability than isolated photosystems. Other studies have focused on using photosystems I or II (PSI or PSII) as sensitizers. These are the isolated photo-catalytic subunits which make up the thylakoid membrane. Although they tend to have lower stability, the benefits of using these isolated systems include: i) less influence of other redox systems on the electron transport chain; and ii) reaction centres can be fixed closer to the electrode, facilitating electron transfer to the circuit. More recently, live cyanobacteria cells were used as sensitizers to build a photosynthetic microbial fuel cell (PMFC). Although these have yet to show the ability to produce as much power as the previously described sensitizers, they are promising as a better stress-resistant alternative.

=Principle=

General biohybrid solar cells operate based on Electron Transport Chain (ETC) as principal components. From one pigment-protein complex to another or to an enzyme in plasma in the thylakoid membrane. Electron transfer is activated when the electron donor with high negative potential passes the electron to a positive carrier. Light energy gives charge separation in the reaction center, causing electron transport in the photosynthetic membrane. Electron transfer is possible when an electron donor with high negative potential passes an electron to a positive carrier acceptor. This separation has a very large quantum yield due to the precisely ordered system of pigments within the reaction center and adjacent complex. By isolating the membrane proteins involved in conversion of photons to electrons and redirect those electrons into an electrical circuit, the flow of electrons can be collected. Furthermore, a system to optimize an electron mediator that will be functional with the reaction center. This technology is called Photo-bioelectrochemical cells (PBECs), use plant or bacterial photosynthetic complexes as biocatalysts. Biohybrid systems in these cells were built by immobilizing these catalysts immediately onto the electrode surface or via linker molecules.

The photoactive protein complex located within the thylakoid membrane is called Photosystem I. The uses a system of chlorophylls within protein scaffolding to collect energy from photons and transfer it to chlorophylls that make up the reaction center. This energy achieves an excited state denoted and oxidized to an electron deficient state, electron released down to intra-protein energy cascade. The terminal electron acceptor of this chain is an iron–sulfur cluster located on the stromal side of the protein complex and is reduced to when the electron arrives. In natural photosynthesis, electrons are shuttled away from by chloroplast ferredoxin, and resupplied to by another protein called plastocyanin.

=Structure=

By combining organic matter (photosystem I) and inorganic matter in solar cells, biohybrid solar cells are a hybrid recreate the natural process of photosynthesis to obtain high efficiency in solar energy conversion. Layers of photosystem I are stacked to collect photonic energy, convert it into chemical energy and create a current through the cell. The cell is made of the same non-organic materials that are found in other solar cells with few exception complexes. Thylakoid membranes were isolated and then went into a purification process to separate the photosystem I from the thylakoid membrane by using spinach as a source.

Photosystem I (PSI) complexes were then separated from the thylakoid membranes by additional centrifugation followed by purification using a chromatographic column packed with hydroxylapatite. The product was characterized via UV–VIS absorbance spectroscopy.

A schematic of the photoelectrochemical cell is a stack of layers. The base of the cell serves as the cathode and consists of a gold layer immobilized onto a silicon. The anode is made of indium tin oxide (ITO) and was modified with a small strip of copper tape on the surface of the ITO to allow lateral conduction of electrons across the cell. A layer of polydimethylsiloxane (PDMS) separates the two electrodes and effectively forms a channel in the middle of the two conductive surfaces. An aqueous electrolyte solution was injected into the reservoir until it was approximately half-filled. PSI complexes in solution assemble at the gold surface over a period of several days, after which the dense multilayer is a green film on the electrode surface.

=Types/Application=

Solar cells can be summarized into four generations organized by developing order. First-generation solar cells are silicon-based. With the longest developing history, the silicon wafer solar cell is a well-established technology. The second-generation solar cells are thin-film solar cells. The 2G solar cell is made by depositing the layer of compound semiconductor materials on a substrate like glass or plastic. The most advantage of 2G solar cells is the low cost. Due to the cheap price, it has been extensively applied in commercials. Based on thin-film technology, the third generation solar cells involved organic materials and nanostructures as layers. The 3G solar cell is aiming at higher efficiency, lower cost, and environment friendly. The fourth-generation solar cell combined 2G and 3G technology. “inorganic-in-organic” structure solved stability problems.

Wafer-Based Solar Cells
The first-generation solar cells can be divided into three specific types: Monocrystalline (single crystal) Silicon, Polycrystalline(muti-crystal) Silicon, and Amorphous (non-crystalline) Silicon.

Monocrystalline silicon solar cell
Monocrystalline silicon solar cell has a solar conversion efficiency of 25%. The advantage of Monocrystalline Silicon solar cell is the high solar conversion efficiency. It is widely used in rooftop solar panels. However, the high cost, high pollution, and sophisticated technological steps push the development of new kinds of solar cells.

Polycrystalline Silicon
Polycrystalline Silicon solar cell has a solar conversion efficiency of 20.4%. Even though it has a little lower efficiency than Monocrystalline Silicon solar cells, the price is more affordable. Therefore, polysilicon solar cells are popular in residential facilities.

Amorphous Silicon solar cell
Amorphous Silicon solar cell has the solar conversion efficiency of 10.1%. It is the most common kind of solar cell in our life, widely used in pocket calculators and power building. By depositing a thin layer of silicon on substrates, the solar cell moves to the second generation.

Thin-Film Solar Cells
The second-generation thin-film solar cells can be classified by compound semiconductor materials. The advantages of 2G solar cells are lower cost, better flexibility, and portability based on the quality of the substrate. Different semiconductor materials can make solar cells to meet different social needs.

CdTe solar cells
The solar energy conversion efficiency of CdTe solar cells is 19.6%. Due to the low cost, CdTe solar cell is used in power stations.

GaAs solar cells
GaAs is a type of high cost and high-efficiency solar cell. The solar conversion efficiency of 28.8% and good performance in high-temperature environments make GaAs solar cell was chosen in space missions.

Ternary/Tetranary compound（CIS/CIGS）
CIGS solar cells are manufactured by depositing a thin layer of copper, indium, gallium, and selenium on substrates. Depending on substrates, the CIGS solar cells can be thin, flexible, and lightweight. With 19.8% solar conversion efficiency CIGS solar cell has become the mainstream thin-film PV technologies as well as CdTe and amorphous solar cells.

Third-Generation Solar Cells
Based on the thin-film technology, the third generation solar cells involved organic materials and nanostructures as layers.

Dye-sensitized solar cells(DSSC)
DSSC works as nature’s absorption of light energy. It was called artificial photosynthesis. Due to chemical stability issues, commercial applications have been hampered. However, the advantages of low cost and environmental friendliness make DSSC solar cells still under development.

The Quantum dot solar cell(QDSC)
QDSC is using quantum dots as the absorbing photovoltaic material to replace the traditional bulk materials. It has an efficiency of 7.0%. The Quantum dot solar cells have the advantage of harvesting the sun's broad-spectrum, because of the size tunability of CQD's bandgap.

Biohybrid organic solar cells
Biohybrid organic solar cells combine photosensitive organic pigments (photosystem I) with inorganic materials. Even though the solar conversion efficiency is 4.53%, it provides an important idea for the future energy solution. Trying organic materials and nanostructures has led to the development of the 4G solar cell.

Fourth-generation solar cells
4G solar cells combine the polymer thin films with novel inorganic nanostructures. 4G solar cells based on carbon nanostructures, mNPs, metal oxides, and nanohybrids. Solved the 3G solar cell’s stability issues, efficiency increased to more than 9%.

=Challenges=

One of the main disadvantages to solar energy is intermittency due to weather. Solar energy in general is very weather dependent, therefore it cannot be acquired if there is no sunlight or secondary input as its power source. Decentralized energy from the cells cannot supply baseline power, or the constant required energy, without sunlight. While this is not the biggest issue for solar cells it remains a limitation until more efficient storage methods can be developed to conserve energy for when it is most needed.

More specific to solar cells, a Cadmium Telluride (CdTe) heterostructure will have low efficiency due to a short carrier lifetime, which is caused by the presence of background impurities in the CdTe layer. If the concentration of the impurities is decreased, it would translate in an increase of carrier lifetime. In a standard TiO2 cell, it is still far from its potential maximum value due to fundamental challenges with relation between the materials and protein complexes. Protein complex are needed for immobilization and multilayer structure and require regular modification. Biomolecules are fixed on the gold substrate due to the action of the self-assembled monolayer (SAM). While gold is useful for the electrodes of the cell, is has an inherent hydrophobic nature and does not interact favorable with these proteins. Furthermore, protein complexes are very susceptible to degradation in ambient environments. After extraction, they lack their repair mechanisms. Strangely, they can be stored in frozen areas up to -80 ֯c, yet when in elevated temperature (in solution), they will denature gradually and irreversibly.

Solar cells present certain issues in terms of fabrication. Electrodes in the voltaic system do not always provide clear mechanism due to random orientation, making it harder to control protein orientation at the electrode surface. There is potential for the protein portion of the cell to be exposed to the bacterial periplasm. Within the fabrication of the vacuum dried layer, there is some detachment of individual proteins allowing them to roam free in solution; thus affecting performance. Another issues found in biohybrid electrodes is that when base is not TiO2 the current generated is low, never exceeding over one milliampere.

Stability failure is also an issue that arises, it can happen in the form of extrinsic or intrinsic stability. Extrinsic instability occurs when sealant materials used lose function due to pressure build up or temperature increase. Intrinsic instability occurs when the biomaterials degrade due to high temperatures and light soaking. If the materials within the cell cannot function under proper conditions, then the cell losses energy potential through aging. An example is when the cell losses energy during regeneration due to loss in stable energy by the oxidized dies and the electrodes.

One challenge that relates back to the efficiency factor is the cost factor. Often when constructing a cell, it is a difficult to have a cost effective with maximum output. For the standard cell, using titanium is good for price but lacks efficiency and replacement metals are efficient but costly.

Environment impacts must also be accounted for. Solar energy presents burdens such as routine/accidental release of chemicals. Toxic and flammable materials are being used to construct these cells and can compromise the safety of the manufacturers and the environment.

=References=