Organic solar cells

Introduction

The organic solar cell (plastic solar cell) is a form of photovoltaic that makes the use of organic electronics. It is the branch of the electronics which deals with the conductive organic polymers or the organic molecules that are usually small in size meant for the absorption of light used for the production of the electricity from the sunlight through photovoltaic effects. The organic materials have the potential of developing a long-term technology which is viable economically for the generation of the large-scale power depending on the availability environment safe materials. The materials designed using organic semiconductor materials are less expensive in comparison to inorganic semiconductor materials. The organic solar cell aims to enhance the earth-abundant and low energy production at a lower cost compared with other methods of energy production. It is a technology that is used in some part of the world, and when it is embraced completely, it will lead to the great transformation in the energy sector. They are highly used in the industry since they are preferable to another form of the cell energy used. Their use is not quite complicated; it is only a little skill that is used in their operation. (Jeon & Lee, 2012)

Fabrication methods

The organic solar cell which is produced by the semiconductor has the potential to deliver effective solar energy conversation with quite a low-cost fabrication. The main challenge that the designers seek to overcome is improving the efficiency. It is quite important to come up with light absorbing materials that contain a high-efficiency charge separation as well as the charge transport properties which are fabricated into the active layers of the solar cell that are controlled by nanomorphology.

The light absorbing layer of the organic solar cell consist of the p-type (the electron donor D) and an n-type (an electron acceptor, A) material. The p-type organic semiconductor that is commonly used include the polymers that are designed from the thiophene building blocks the likes of PBTTT, P3HT, PCPDTBT. The most efficient n-type materials are the fullerene derivatives, for example, PC, BM, and PDI. The active layer composite is inserted between the transparent anode like the ITO and indium tin oxide and the metallic such as the aluminum cathode. Below is the energy diagram of the organic solar cell which demonstrates the process that is involved in the generation of the photocurrent

For the generation of a charge, the incident light excites an electron from the donor material from its ground or HOMO (Highest Occupied Molecular Orbital) to the excited state; it leaves a hole that is commonly known as the positive charge (first step). Afterward, the exciton travels to the D-A interface and experiences a charge transfer to the lowest unoccupied molecular orbital (LUMO) of the acceptor (second step). Electron transport from the electron and the recombination of the hole in along the external circuit gives a photocurrent (third step). One of the main challenges associated with the organic materials is their low dielectric constants that lead to a relatively short electrons diffusions length as compared to the inorganic semiconductor. For one to achieve an effective electronic transfer between the donor and the acceptor, the two materials are supposed to be within 10mn proximity. However, notwithstanding the high absorption in the coefficient of the organic dyes, the minimum thickness of 100nm is necessary for the light absorption to be maximized.

Mechanism of the organic solar cell

The organic solar cell experiences device degradation in different undesirable ambient conditions that limit their application as well as the efficiencies a lot. The S-shaped current density-voltage (J-V) characteristic that appears frequently reflects this effects and leads to its underlying mechanisms become obscure for the experimentalists. When a device model simulation is performed, the J-V curve of the organic bulk heterojunction solar cell is investigated for various interfacial charge injection conditions. The S-shaped kinks appear when both the hole injection and the electrons barriers become more than 0.3 eV, it is caused by the shortage of dark injection carriers. For close Ohmic contacts, the open circuit voltage is highly reduced. It rises from the accumulation of the majority carrier as well as the variation of the locally induced field at the vicinity of the contacts. The increase of the carrier mobility leads to the gradual elimination of the S-shaped kink. The fill factor decreased as enhanced by the light absorption that is caused by the drastic bimolecular recombination leads to significantly remarkable losses. (KOEPPE, BOSSART, CALZAFERRI, & SARICIFTCI, 2007)

In the case where the aluminum cathode is used, it experiences a high degradation from the air. Through an interface modification which combined with the chemical as well as the electrical characterization, there is a rapid degradation that originates from the formation of the charge blocking layer that is between the evaporated aluminum cathode and organic active layer. When a thin interfacial layer of thermally evaporated CrOx is inserted between the organic active layer and aluminum cathode, the stability of the device can be significantly improved. Under normal circumstance, the interfacial layer of the CrOx provides a protective layer through minimization or stopping the penetration of the thermally evaporated aluminum and turn into an active layer which forms a diffused organic aluminum interface. When it is exposed to the air, it results in a large oxidized interfacial area. Therefore, the organic solar cells do not have a complicated function mechanism. It is the reason they are highly available in the industry and are used to in performing most of the functions.

Advantage and disadvantage of the solar cells

Advantages

The organic solar cell has the lasted advances in the molecular engineering which have unfolded the series of benefits in the organic cell potential which will eventually lead to the outbalancing of benefits of the silicon-based solar cell. Despite the fact that conventional solar cells dominate the existing market currently, shortly the case will be different.

The manufacturing process and cost of the organic solar cell is relatively low compared to the silicon-based cells. Such low cost is caused by the molecular nature of the material used. The molecules are quite easier to work with and they can be utilized with thin film substrates which are almost 1000 times thinners compared to the silicon cells. It is this fact which reduces the cost of production significantly.

Another advantage of the organic material in the solar cell manufacturing is their ability to tailor the properties of the molecules so that they can fit the application. The molecular engineering is used to alter the molecular mass, ability to generate charges, band gap through modifying the length as well as the functional group of the polymers. Besides, there are new unique formulations that are developed from the combination of both the organic and inorganic molecules which makes it possible to print the organic cell in whichever desirable pattern.

Organic materials have a series of desirable properties, they are lighter and quite flexible compared to their rigid and heavy counterparts, and therefore they are less prone to failure and damage. Also, they can exist in different portable forms while their flexibility enhances their installation, storage, and transport. Other advantages of the organic materials include being environmental friendly and multiple uses and applications.

Disadvantages

Organic materials are affected by some challenges which make them undesirable to some extent. For instance, they have a low external quantum efficiency of up to 70% in comparison to the inorganic compounds photovoltaic devices.  It is caused by the insufficient absorption with the active layers that is in the order of nanometers. The instabilities against the reduction and oxidation, temperature variation as well as the crystallization also causes the device degradation as well as decreasing in the performance with time. It happens because of the various extents for devices which have the different compositions. Other undesirable factors of the organic materials include the exciton diffusion length, charge separation and collection that are affected by the impurities.

Future of the organic solar cell

As the technology advances, the organic solar cell seems to have a bright future ahead. Despite the fact that the amount of the energy that is generated by the organic solar cells from sunlight is lower compared to the conventional cells, experts assert that there is a big room for improvement. Many of those experts argue that more tools are being invented that will help in slashing the emission of the carbon dioxide which will make the organic materials to be more efficient.

The organic solar cells are made up through coating the polymer on different materials like the plastics hence their light weight. Besides, the cells can be crafted in a condition of fewer than 100 degrees of temperature which makes them be more environments friendly. They are flexible because the polymer based cells are coated on the plastic film which makes it possible for the solar cells to be stuck vertically or installed on the weak structure. Due to this pliability, there is a likelihood of the potential usage expanding in the future. They may be installed on the exteriors of the electric cars so that the car can be able to generate electricity while driving. (Fara & Yamaguchi, 2013)

Research have shown that the polymer-based solar cells have had an explosion in the late 2000s a period when the scientist have started uncovering the combinations of the organic materials which has led to the increase in efficiency of the power conversion. There are infinite ways to change the structures of the organic compounds which will improve their usage in the future. Such discovering changes offer a high power conversion and will boost organic photovoltaic usages in the future.  However, there are some shortcomings in the organic materials that need to be improved for it to be more effective in the future. The experts are still looking for ways to enhance it usage as well as the applications in the day to day life.

References

Fara, L. & Yamaguchi, M. (2013). Advanced solar cell materials, technology, modeling, and simulation (1st ed.). Hershey, Pa.: Engineering Science Reference.

Jeon, S. & Lee, J. (2012). Improved lifetime in organic solar cells using a bilayer cathode of organic interlayer/Al. Solar Energy Materials And Solar Cells, 101, 160-165.

KOEPPE, R., BOSSART, O., CALZAFERRI, G., & SARICIFTCI, N. (2007). Advanced photon-harvesting concepts for low-energy gap organic solar cells. Solar Energy Materials And Solar Cells, 91(11), 986-995.

Schünemann, C., Leo, K., Leo, K., & Stamm, M. (2013). Organic Small Molecules: Correlation between Molecular Structure, Thin Film Growth, and Solar Cell Performance (1st ed.). Dresden: Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden.