Organic Solar Cells - Report Example

Organic Solar Cells By Lecturer’s and 1 Introduction There has been increased search of renewable sources of energy due to the current increase in demand for energy supply such as oil, coal and uranium. According to research, statistic oil prices are anticipated to increase as compared to the past prices so as to meet increased demand for energy. It is worth noting that, non-renewable sources of energy have high impacts to the environment as they cause a lot of pollution leading to depletion of the ozone layer and to curb such effects requires quite a lot of time and process (WYNANDS, 2010). The major current sources of energy are fossil fuels, which are the major contributors to global warming leading to increased temperatures and raise in sea levels. Below is a graph showing global energy consumption (PÉREZ LEÓN, 2006). Fig 1: Historic and projected global energy demand for period 1980-2030 Today, there is need for a better energy policy. This involves coming up with a cheaper source of energy, and more so, renewable sources. This is due to the fact that the government and different companies are not in the position to renew fossil fuels. Due to this, there is a lot of concern on the creation of a more cost friendly sources of energy, and more so, a renewal one. Despite the fact that the sun seems to be much promising, less costly, non pollutant or cleaner and a renewable source of energy, its intensity is not often enough as it is 1 kW/m squared and this amount vary from one place to another depending on the time of the day. From researches from different scholars, it is evident that the amount of the sun rays that hit the ground in one hour is equivalent to the amount that the whole world utilizes in one year (WYNANDS, 2010). One of the ways through which the sun’s energy can be renewed into electricity is through photovoltaic devices. These devices were discovered in the year 1839 by Edmund Bequeral. He came up with this idea after a thorough study on the production of light from a silver covered platinum electrode dipped in an electrolyte. In the year 1883, Charles Fritts developed the first photovoltaic gadget, which was a continuation of the work of Edmund development of the photovoltaic effect. He came up with a cell whose efficiency was rated at 1%. He created it by the use of semi-conductor selenium, which was covered with a slight coating of gold forming a junction. Thereafter, this photovoltaic cell was demonstrated by the use the theory of metal-semiconductor obstacle coating (PAGLIARO, PALMISANO & CIRIMINNA, 2008). Later on in 1954, Chapin developed a photovoltaic cell with a power change efficiency of 6%. This was the first Silicon solar cell. This was further enhanced to an efficiency of 37%, which is held even today (WYNANDS, 2010). This is very reliable and performs some work in more time as compared to the past ones. Despite its increased efficiency, this requires a lot of resources for its development, and therefore, costly and more so, needs more energy. More research has been conducted in the last few years in an effort to improve this cell to achieve one that is less costly and with higher efficiency. Different resources have been gathered by different individual to meet these goals. One of these less costly and high efficiency materials, among different researchers, is an Organic Semiconductor material (PÉREZ LEÓN, 2006). Such materials are less costly due to the fact that they can be developed using low temperatures, and that their absorption coefficients is very high allowing low films being used lowering the use of resources which in turn lowers material costs. The production these kinds of molecules are very elastic and therefore give a room for electronic constraint and solubility to be changed (WYNANDS, 2010). This has further been enhanced by the introduction of plastic substrates as compared to the conservative silicon solar cells making the structure more elastic. Through chemical alteration, the properties of different polymers and small molecule materials can be changed, making it possible to alter the electronic and chemical properties of organic materials. This is one of the major advantages of the use of organic materials (PÉREZ LEÓN, 2006). Additionally, through the use of different morphology contained in the thin film on the organic materials gives a room for control through the use of chemical alterations. A good example is an energy band gap in resources used in OLEDs, which can be altered into a given wavelength of light. Through different research, different organic materials, classified in terms of class, have been made and they are really promising in their application in photovoltaic cells such as conjugated polymers and dendrimers (RIEDE, 2007). The uses of organic semiconductor materials have proved to be very successful as light emitting diodes. This has been highly advocated for due to their low manufacturing cost and high efficiency. Despite this invention, there is still a room for improvement through the use of technology to come up with major sources of power generation systems (RIEDE, 2007). 1.2 History of Solar Cells The history of photovoltaic effect is dated back in the year 1839 with the first invention by Edmund Becquerel. However, the very first cell to be developed was created by Charles Fritts. Through his creativity, he covered selenium with a slight coating of gold, which in turn formed a Scotty obstacle like gadget with the power of change efficiency (PCE) of about 1%. The current semiconductor junction was made solar cell was developed in the year 1946 by RusselOhl. This made further discovery of p-n junction with the use of crystalline silicon. Through this discovery the first experiment was carried out by the Bell Laboratories coming up with the first cell with a diffused silicon p-n junction. (KREBS, 2012). The fact that the cost of producing the cell was very high; there was low production and use of such cells. The only suitable use at such moment was the use in satellites. This was due to the fact that the use of such cells was not a barrier and the advantage of PV was suitable for the use. The efficiency of single junction mono-crystalline silicon is rated has developed to about 24.4% today. However, such cells are very costly due to its manufacturing costs. In the manufacture of a flaw free crystal cell, high temperatures are required and more so such processing should be conducted in a vacuum. This makes large scale production of such cells much expensive (CHOY, 2013). To reduce such costs, amorphous silicon was discovered. However, the efficiency of the cell is highly affected, leading to reducing it by a very significant amount of approximately 10%. Such silicon cells are also referred to as first generation cells (RIEDE, 2007). Through technological advancement, the cell development was enhanced giving rise to second generation solar cells. This involved the use of diverse inorganic materials in slight films so as to cut costs of materials in the cell manufacture in comparison with the first generation cells. The use of different mixture such as gallium arsenide (GaAs) or cadmium telluride (CdTe) gave a room for the development of slender inorganic films which their band gap is manageable. This allows tailoring to different parts of the solar cell. It is through this that the first multi-junction gadgets were developed referred to as the third generation solar cells in which different junction absorbed different spectrums (CHOY, 2013). This was a huge step in that since this reduced the loss of energy through thermalization of hot-carriers. With the use of solar concentrators, these cells have a very high power conversion efficiency of about 41%. Despite its increased efficiency the solar cell is still very expensive due to the increased cost of production and more so limited materials for its development (LIN, (2012).Additional limitation to these kinds of cells is emission of environmental hazard gases during its manufacture. According to the National Renewable Energy Laboratory referred to as the Emergent Photovoltaics are the most recent generation cells, which involve advancement and a shift in its material choice. This has been made possible by the development of organic semiconductors a good example being organic light emitting diodes (OLEDs). Such kinds of cell put into practice the organic semiconductor junctions or organic /inorganic fusion junction in the generation of photovoltaic effect or the organic-photovoltaics (OPV). These cells utilize dye or chromophore to collect sunlight. Fused cells utilize some high absorbers of organic semiconductors to give a room for slight and cheap absorbing layers while on the other hand utilizing an inorganic acceptor due to their high strength and conductivity (KREBS, 2012). Organic compounds are made up of elements such as carbon, nitrogen, oxygen and hydrogen. The major advantage of these elements is that they all exist in abundance and therefore can be altered easily at very low temperatures, hence cheaper than the inorganic materials (CHOY, 2013). However, their efficiencies are much lower as compared to the use inorganic materials. The figure below shows the advancement of technologies related to cell development and their comparison between them. Fig 1.2: Research Cells Efficiencies from 1975 to 2015 1.3 The Photovoltaic Effect When any material absorbs light the electrons in it are excited since such electrons gain energy equivalent to the energy absorbed by the photon. A photoelectric effect is created when enough energy is absorbed. This is created when the photon absorbs large amounts of energy, causing the electron to be displaced from its location. The first proof that light exist in a discrete amount was first demonstrated by Einstein in the year 1905 (WYNANDS, 2010). Often when the amount of light is not enough to displace the electron, it does make it just excited and later relaxes in its original position by an internal conversion. On the other hand, in photovoltaic gadgets, electrons gains a kinetic energy changing its state into a stable charge carrying, and its development in asymmetry makes the election in the state of motion all through the device. This in turn produces a current which can be extracted using an outside loading circuit to perform some work (PAGLIARO, PALMISANO & CIRIMINNA, 2008). 1.4 Organic Solar Cells Development These types of cells are divided into three, namely dye-sensitized solar cells (DSSCs), small molecule cells and polymer cell (PAGLIARO, PALMISANO & CIRIMINNA, 2008). On the other hand, a hybrid cell is a cell that combines both organic and inorganic materials in its development whereby the organic materials are the donors and the inorganic ones are the acceptors (LIN, 2012). DSSCs came into being in the 1991 after O’Regan and Gratzel developed it. This had an efficiency of about 7.4%. Through technological advancement its efficiency had increased to about 10% in the year 1994 and in 2001 there was a dramatic increase to about 10, 4% while in 2005 it was about 11.1%. Cell development is related to the redox reaction in chemistry by utilization of a dye in electrolyte and a solid electron acceptor such as titanium dioxide (TiO2) (CHOY, 2013). The dye takes in light leading to electron gaining kinetic energy which in turn transfers the charge to the titanium dioxide. By use of the electrolyte the dye neutralizes itself through oxidation. The oxidized electrolyte diffuses to the other electrode where it is reduced, thereby completing the circuit. On the other hand the polymer and small cell molecule cells are developed on almost the same ideology. It is worth noting that polymer layers are normally solution processed while the small molecules are whirl-coated, spray painted or deposited in a vacuum using an organic molecular beam deposition taking into consideration their features (LIN, (2012). Before technological advancement such as before 1986, organic cell development was made using slight films of one coating of minute molecules such as porphyrins or phthalocyanines (KREBS, 2012). Such devices are deemed to be very inefficient due to the fact that it is defect guided as compared to inorganic semiconductors. The large amount of impurities and defect guided dissociation lead to interference of the electrical features important in the solar cells (RIEDE, 2007). Due to the need to improve the excision dissociation different researchers developed a double layer donor/acceptor heterojunction design. In the heterojunction between materials excision can easily dissociate into free charges easily and more efficiently as compared to a Schottky cell (WYNANDS, 2010). The earliest cell made use of Copper Phthalocyanine (CuPc) and a perylenetetracarboxylic to generate a power with an efficiency of about 1%. Through the advancement of technology, there has been the development of better materials which have led to dramatic improvement of the power conversion efficiency. This has been contributed but the use of C60 as the acceptor being a very good organic conductor with ability to absorb shorter and UV wavelength and more so possess enough energy levels for excision dissociation with large number of organic donor resources (CHOY, 2013). In the year 1995, there was further development of the heterojunction cell with the creation of bulk heterojunction. This was achieved by whirl covering of a polymer, poly (2-methoxy-5-(2’-ethyly-hexyloxy)-1, 4phenylene vinylene) with dissolvable fullerene. In this the coating is all done at the same time rather than creating junction between layers. In this regard the polymer and the fullerene dissociated into two domains such that from one molecule to the next molecules is a very small distance. This has highly helped to increase the current cells to an efficiency of about 7.4% (RIEDE, 2007). However, to formation of different domains, there is a limitation to access of the paths hence reducing its efficiencies in charge transport. There has been an effort to improve the process of charge transportation in the bulk heterojunction (LIN, (2012). To deal with this problem, researchers have tried the use of polystyrene sphere to come up with a better arrangement of these charges by coming up with a 3D interpretation of the donor and the acceptor materials (CHOY, 2013). In these sphere all round vacuum and void spaces if filled with donors and acceptors there will be continuous transportation of charge all round. Tandem cell can be created by stacking more layers to create another cell at the top of the first cell. This increases the absorption levels while at the same time allowing the cell to be thin giving a room for the electron excitement and its movement. This involves the use of different materials, both at the front, back and tandem cell so to increase its absorption level, thereby increasing the efficiency of the cell (WYNANDS, 2010). The first tandem cell was first created by Hiramoto et al in the 1990. It was made of two metal-free phthalocyanine, perylenetetracarboxylic derive sub cells divided by gold. This was very vital in that it prevented the concentration of charges between the sub cells as this made available combination centre of all charges from both sides (KREBS, 2012). There has been a lot of effort to develop organic solar cell with the best power conversion efficiencies by combining different materials while minimizing their disadvantages of combining them (LIN, 2012). 1.5 Thesis Outline There have been continuous efforts to develop an organic solar cell, BHJ solar cell and more so improving its power conversion efficiencies to 6-8% today. Additionally, there has been an attempt of gathering knowledge on the important role of non-scale morphology and the derivation of open circuit voltage (Voc). Despite the increased efficiencies, fullerene acceptors have so many disadvantages in OPVs such as they have negligible light absorption the close visible IR region, second is that they have low photochemical and chemical ability, thirds is that they have higher rates of molecular diffusion and crystallization abilities, next is that it has limited accessible photo-voltage, next is that it has limited fullerene synthesis derivative and lastly the recycling process of fullerene is expensive (CHOY, 2013). Therefore, to improve the efficiency there is need to develop a new organic semiconductor to replace fullerene. In my discussion, three types of solar cell tackled the organic bilayer solar cell (OHJ), organic bulk heterjunction cell (OBHJ) and organic tandem solar cell. Development of these cells has been made by use of different materials such as Poly (3-hexylthiophene-2-5-diyl) (P3HT) utilized as donors among others. In addition, introduction of solar cell and its important elements in its functioning is also discussed. Both organic and inorganic cells are discussed in detail, giving its pros and cons of its use (WYNANDS, 2010). Reference List CHOY, W. C. H. (2013). Organic solar cells materials and device physics. London, Springer. http://dx.doi.org/10.1007/978-1-4471-4823-4. KREBS, F. C. (2012). Stability and degradation of organic and polymer solar cells. Chichester, West Sussex, U.K., Wiley. http://public.eblib.com/EBLPublic/PublicView.do?ptiID=887260. LIN, C.-F. (2012). Organic, inorganic, and hybrid solar cells: principles and practice. Hoboken, NJ, Wiley. PAGLIARO, M., PALMISANO, G., & CIRIMINNA, R. (2008). Flexible solar cells. Weinheim [Germany], Wiley-VCH. PÉREZ LEÓN, C. (2006). Vibrational spectroscopy of photosensitizer dyes for organic solar cells. Göttingen, Cuvillier. http://opus.ub.uni-bayreuth.de/volltexte/2006/251/index.html. RIEDE, M. K. (2007). Identification and analysis of key parameters in organic solar cells. Konstanz, University, Diss., 2006. http://deposit.d-nb.de/cgi-bin/dokserv?idn=982924534. WYNANDS, D. (2010). Strategies for Optimizing Organic Solar Cells Correlation between Morphology and Performance in DCV6T - C60 Heterojunctions. Strategies for Optimizing Organic Solar Cells. S.l, s.n.]. Read More