4 Organic Solar Cell Materials and Their Characteristics Explained

Organic solar cell materials are; high work-function metal, low work-function metal, photosynthetic dye, and organic semiconductor. These materials are also the components of an organic solar cell, and are arranged in parallel, adjoined layers.

This study discusses organic solar cell materials, as follows;

1). High Work-Function Metal (as one of the Organic Solar Cell Materials)

Work function is a measure of the minimum amount of energy that is needed to liberate an electron from the surface of a metallic material [7].

This means that high work-function metals require large amounts of energy to liberate electrons from their surface. They can alternatively be described as having a high ionization potential [2].

High work-function metals generally have a low photoelectric effect due to their high energy threshold for electron release. However, they are very useful as organic solar cell materials.

The role of these metals in an organic solar cell is that of the anode. Due to their low-mobility interaction with electricity, they enable electrons to accumulate as the organic solar cell converts solar energy and generates electricity.

These accumulated charges are then conducted by the anode; which is usually a low work-function metal.

Examples of high work-function metals are gold, platinum, nickel, indium, titanium, and beryllium.

However, for practical purposes, the high work-function anode often occurs in alloy form. A common example of these metals in organic solar cells is indium tin oxide (ITO), which is an alloy of indium.

This alloy is used mainly for its transparency and high optical transmission, both of which help improve the performance of organic solar cells [4]. It also has a low electrical resistance compared to other high work-function metals.

2). Low Work-Function Metal

Low work-function metals generally serve as the cathode in an organic solar cell.

These metals require a relatively low amount of energy to liberate electrons from their surface, meaning that they have good photoelectric characteristics.

As an organic solar cell component, the low work-function metal occurs as a thin layer of about 1 nm width, which works alongside the high work-function metal (anode) to sandwich the organic semiconductor layer between them [5].

Also, in order to be effective for electric current transmission, the low work-function metal must have high conductivity.

Examples of low work-function metals are; calcium, aluminum, magnesium, and cesium.

Polymeric materials can also be used as the cathode in organic solar cells. These materials are required to have low work-function and high conductivity, but they may not always be metallic.

An example of polymeric low work-function cathodes is; poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) [9].

3). Photosynthetic Dye (as one of the Organic Solar Cell Materials)

Organic solar cell materials may include organic dyes with photosynthetic characteristics. Such solar cells are generally referred to as; ‘dye-sensitized solar cells (DSSCs) [1].

The function of dye in organic solar cell is similar to the natural function of chlorophyll pigment in photosynthesis which is carried out by plants.

Dye-sensitized solar cells can come in any of various designs. The most common of these is the thin-film design. Also, the dye is often held by a thin layer of nanoparticle materials, usually composed of metallic oxides, like titanium dioxide.

Examples of dyes used in organic solar cells are; bromophenol blue, methyl orange, eosin Y, fast green, aniline blue, carbol fuchsin, and crystal violet [3].

4). Organic Semiconductor

Organic semiconductors may either replace or work alongside the photosynthetic dye in an OSC.

Like dyes, their function is to convert solar energy which strikes the solar cell, into electricity.

Organic semiconductors may be polymeric or composed of a simple molecular structure. They are also often carbon-based.

The working principle of the organic semiconductor in an OSC is similar to that of an inorganic semiconductor in a conventional silicon solar cell. This principle is based on photoelectric conversion and charge flow.

However, organic semiconductors generally have a lower dielectric constant than silicon semiconductors. As a result, they do not work by instantaneous generation and flow of free charges, but rather by the accumulation and subsequent transmission of charges (which accumulate in pairs with holes).

The implication of this difference in electricity generation mechanism is that organic solar cell efficiency is generally lower than that of silicon solar cells, due to the need for charge-buildup before transmission.

There are some advantages of organic semiconductors, one of which is their mechanical characteristics. These materials are generally resilient, and are light, flexible, scalable and printable [8].

Organic semiconductor materials are also relatively cheap to utilize, and are less capable of causing environmental degradation due to their disposal or recycling, since they are organic and more susceptible to biodegradation than their inorganic counterparts.

Due to their tendency to decompose like biomass, humid air exposure is not always favorable for organic semiconductors, and can cause them to biodegrade [6].

Examples of organic photovoltaic semiconductors are; PET – polyethylene terephthalate, Poly(ortho phenylenediamine), and poly(3-nethyl-thiophene).

Conclusion

Organic solar cell components are the materials used to produce these solar cells.

There are three main components of OSCs; the anode, the cathode, and the photovoltaic unit.

Basically, the structural arrangement of organic solar cells requires the photovoltaic unit to be sandwiched between the two electrodes. This configuration allows the electricity generated by the unit to be transmitted by the electrode to electric wires which may further transmit them to the grid.

Since organic solar panels are not yet highly efficient, they are best as support systems for other energy facilities and power plants. They can serve this purpose effectively if integrated into a smart grid network, as a minor power supplier.

References

1). Adedokun, O.; Titilope, K.; Awodugba, A. (2016). “Review on Natural Dye-Sensitized Solar Cells (DSSCs).” International Journal of Engineering Technologies IJET 2(2):34. Available at: https://doi.org/10.19072/ijet.96456. (Accessed 14 September 2022).

2). Davydov, S. (2002). “On the relationship between the ionization potential and the work function: Metals.” Technical Physics 47(1):92-96. Available at: https://doi.org/10.1134/1.1435896. (Accessed 14 September 2022).

3). El-Agez, T. M.; Taya, S.; Elrefi, K. S.; Abdel-Latif, M. (2014). “Dye-sensitized solar cells using some organic dyes as photosensitizers.” Optica Applicata 44(2):345-351. Available at: https://doi.org/10.5277/oa140215. (Accessed 14 September 2022).

4). Jahng, W. S.; Francis, A. H.; Moon, H.; Nanos, J. I.; Curtis, M. D. (2006). “Is indium tin oxide a suitable electrode in organic solar cells? Photovoltaic properties of interfaces in organic p/n junction photodiodes.” Applied Physics Letters 88(9):093504-093504-3. Available at: https://doi.org/10.1063/1.2180881. (Accessed 13 September 2022).

5). Jiang, C. Y.; Sun, X. W.; Zhao, D.; Kyaw, A.; Li, Y. (2010). “Low work function metal modified ITO as cathode for inverted polymer solar cells.” Solar Energy Materials and Solar Cells 94(10):1618-1621. Available at: https://doi.org/10.1016/j.solmat.2010.04.082

6). Kawano, K.; Pacios, R.; Poplavskvy, D.; Al-Hashimi, M.; Bradley, D. D.; Durrant, J. R. (2006). “Degradation of organic solar cells due to air exposure.” Solar Energy Materials and Solar Cells 90(20):3520-3530. Available at: https://doi.org/10.1016/j.solmat.2006.06.041. (Accessed 14 September 2022).

7). Luo, Y.; Tang, Y.; Chung, T.; Tai, C.; Chen, C.; Yang, J.; Li, D. Y. (2021). “Electron work function: an indicative parameter towards a novel material design methodology.” Scientific Reports 11(1):11565. Available at: https://doi.org/10.1038/s41598-021-90715-4. (Accessed 14 September 2022).

8). Sulaiman, K.; Ahmad, Z.; Fakir, M. S.; Wahab, F. A.; Abdullah, S. M.; Rahman, Z. A. (2013). “Organic Semiconductors: Applications in Solar Photovoltaic and Sensor Devices.” Materials Science Forum 737:126..Available at: https://doi.org/10.4028/www.scientific.net/MSF.737.126. (Accessed 14 September 2022).

9). Zhang, Y.; Chen, L.; Hu, X.; Zhang, L.; Chen, Y. (2015). “Low Work-function Poly(3,4-ethylenedioxylenethiophene): Poly(styrene sulfonate) as Electron-transport Layer for High-efficient and Stable Polymer Solar Cells.” Scientific Reports 5(1):12839. Available at: https://doi.org/10.1038/srep12839. (Accessed 14 September 2022).