A Study on Organic Solar Cells, Its Comparison with Inorganic Solar Cells, Its Disadvantages, and Alternatives

Introduction

The discovery of solar cells provided clean energy that is used today. The solar cells involve the conversion of light energy to electrical power. Over the years, solar energy technology has been gaining popularity. This paper covers organic solar cells and compares them with inorganic cells. From the comparison, it will be seen that organic solar cells are far much better than the inorganic ones especially in terms of cost and light absorption. The only disadvantages of organic solar cells are that they suffer photo-degradation and require to be encapsulated [1]. The paper presents alternatives that may be adopted to address challenges of organic solar cells.

Organic Solar Cells

Organic solar cells are photovoltaics that use organic electronics. These type of cells are made up of the first and the second electrodes, electron transport, photoactive and hole transport layers. The general principle behind the working of solar cells is the absorption of light, which leads to the holes and electrons to be created. The created holes and electrons are then separated from each other, and this results in the production of electrical power at the contacts of the solar cells. The difference between organic and inorganic solar cells is based on the working principles. While in inorganic solar cells, free charge carries are directly generated, this is not the case in organic solar cells. In the later type of cells, there is light absorption which leads to the creation of excitons with binding energy typically ranging between 0.3 to 0.5 eV [2]. 

Organic solar cell applications are deposited by spin coating and vapor-phase deposition (VPD). This kind of solar cells makes solar cells cheaper. The whole process idea is that planar waveguides with a thin-film organic coating on the face and organic solar cell attached to the edges. VPD provides a better way of controlling the structure and the morphology of the film. Solar cells fabricated using VPD have higher short-circuit current density than cells fabricated using other methods such as VTE (Vacuum thermal evaporation) [3].

Advantages of Organic Solar Cells

The benefits of organic solar cells are that they are cheap and are very thin. Comparing organic cells to inorganic cells, the organic solar cells are cheaper. This is because, during making of organic solars, less energy is used. In spite of organic materials being disordered, the materials can be put on a surface without taking into consideration their atomic order [4]. Let’s consider a typical example of an inorganic solar cell; silicon solar cell. More energy is needed for the creation of thin films of precisely crystalline silicon. The expenditure for making of organic and inorganic solar cells can be quantified by calculating the energy payback time. Energy payback time is the amount of time a solar cell takes to give the similar amount of energy that was invested during the making of that solar cell. The payback time for inorganic solar cells ranges between one to two years while organic cells take 1-2 days! Additionally, organic solar cells applications are deposited by spin coating and VPD, and this makes them cheaper [2].

The layers of an organic solar cell are very thin with a thickness within the range of 100 nanometers. This thickness is very small and facilitates light absorption. The aspect of being thin makes these solar cells have higher absorption coefficient than inorganic solar cells [1].

Disadvantages of Organic Solar Cells

The main challenges associated with organic solar cells is that they require to be encapsulated and experience significant photo degradation. These cells are typically encapsulated using Ethylene Vinyl Acetate (EVA). The reason for using EVA is because it is hard wearing, transparent, does not corrode and is inflammable [2]. The challenge of encapsulating organic solar cells arises from the vacuum lamination process that is involved when the EVA is sandwiched between two sheets of glass. Apart from increasing the overall weight of the solar panel, a gap is left between the glass sheets at the edges. This gap gives room for moisture and gas to enter the photoactive layers thus making the cells degrade. Encapsulating the solar cells requires more energy and material increasing the overall production of the solar panels. Additionally, so far, no technology has been developed to automate solar cell encapsulation, and this makes the whole process time consuming [4]. 

Photo degradation is one of the problems affecting organic solar cells. Organic solar cells are inherently photo-unstable, and this results in significant losses solar efficiency with irradiation. X-ray photoelectron spectroscopy measurements on organic solar cells show that by being exposed to light, the organic solar cell undergoes chemical changes which degrade their structure over time [1]. The modern organic solar cells are incorporated into interfacial layers between the layers as a way of reducing photo degradation of electrode interface although this has been proven to be ineffective. Despite interfacial layers having high photo-stability, the majority of the layers at the active layer/cathode interface experience thermal degradation because of increases in temperature when the solar cells are exposed to light [1]. Already organic solar cells have proven to outperform inorganic cells based on light absorption. They are also cheaper than inorganic type. Addressing their challenges could be a great solution to replace traditional inorganic solar cells that are used today.

Addressing Challenges of Organic Solar Cells

 Comparing the thickness of organic solar cells and the inorganic type, organic solar cells are thinner; the structure that makes them suffer photo degradation the most. Normally a thin film (interfacial film) is stack between several layers to absorb light and convert it to electrical power. If these interfacial films are not processed immediately, their ability to convert light energy to electrical energy declines [4].

When a laser is shown in a solar cell and the light released by the laser is measured, it could be seen that degradation occurs within first few minutes [1]. Degradation is reversible and can be prevented by simply adding another layer on top of the solar cell. This also helps to make the solar cell stable [2]. Thus designing solar cells with an additional layer on top of the cells may help to address the problem of photo degradation in organic solar cells.

Instead of using EVA that most organic solar cells use, this encapsulant may be replaced with optically clear silicone encapsulant. Using silicone encapsulant in organic solar cells may help to prevent challenges presented by EVA and ultimately help problems associated with encapsulation of organic solar cells. The following are advantages of silicone encapsulant [4];

  • Can operate over a wide range of temperature
  • Transmits light excellently
  • Can be easily repaired
  • Has high photo-stability and weather resistance
  • Is flexible
  • Easy to use

References

[1]

W. C. H. Choy, Organic solar cells: materials and device physics, London; New York: Springer, 2013.

[2]

J. Huang, Organic and hybrid solar cells, Cham: Springer, 2014.

[3]

C. Rolin, Vapor phase deposition of organic semiconductors, Saarbrucken, Deutschland: LAP LAMBERT Academic Publishing, 2011.

[4]

W. Tress, Organic solar cells, Cham: Springer, 2014.