Fill Factor Analysis of Organic Solar Cell

Fill Factor Analysis of Organic Solar Cell Rashmi Swami, Rajesh Awasthi, Sanjay Tiwari Abstract Solar cell is a device used to convert light into electricity. It can be made by organic and inorganic materials. Its most important parameters are open circuit voltage, short circuit current, fill factor and conversion efficiency. This paper is based on the analysis of factors that affect the fill factor of organic solar cell using MATLAB. Fill factor is calculated using conventional organic solar cell model without series and shunt resistances and constant light generated current for two different cases –first using Exponential dark characteristic and second using Polynomial dark characteristic. We get for exponential V-I relationship increase in ideality factor n, will reduce the fill factor and for polynomial V-I relationship increase in m will increase fill factor. A large dependence of light generated current Iph on increasing applied voltage would cause a significant drop in fill factor. Increase or decrease in an additional factor would accordingly change fill factor. Dark current can be varied in two ways, one by varying mobility and other by varying injection barrier heights. In both the cases fill factor increases proportionately with . Keywords – Organic solar cell, fill factor, ideality factor, open circuit voltage, HTL, ETL. Introduction Bilayer organic solar cell as shown in fig. 1(a) is a device in which thin layer of organic material (donor and acceptor) is used between electrodes to convert light into electricity. This work is completely based on bilayer structure of organic solar cell as shown in fig.1(a) in which poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4phenylenediamine) (PFB) is organic donor/HTL and poly(9,9-dioctylfluorene-co-benzothiadiazole) ( F8BT) is organic acceptor/ETL. Fig. 1(b) shows simplest conventional organic solar cell model without series and shunt resistances. Open circuit voltage, short circuit current, fill factor and efficiency are four important parameters of OSC. FF = Vmax Imax / VOC ISC When Vm= VOC and Im= ISC then (FF)max=1. For a good photo-voltaic device, all three factors FF, VOC, ISC should be large so that it can deliver large output power for the same incidental optical power. (b) Fig. 1 : (a) Bilayer organic solar cell structure. (b) Conventional organic solar cell model without series and shunt resistances. Simulation Model and Analysis of Fill Factor Two cases have been studied, one where dark characteristic is exponential like p-n junction and other where dark characteristics is polynomial like in space charge limited devices. 1.2.1Exponential Current Voltage Relationship – In this model, dark characteristic is assumed to follow exponential current voltage relationship and Iph is assumed to be constant. (1) where n is ideality factor and Vth is thermal voltage, Iph is light generated current, Id is dark current and I is net output current. Total output measured current can be written as a function of photo-generated current and dark current. (2) Output power of organic solar cell when it is operating at voltage V and giving current I- If maximum power is obtained at voltage Vm, , here assuming (3) Here y exp(y) is Lambert’s W function (4) and (5) At VOC net output current will be zero. At this condition eq. (2) will give (6) 1.2.2 Polynomial Current-Voltage Relationship In this case it is assumed that dark current depends on the applied voltage in the following manner- (7) Where K is constant and . (8) If photovoltaic is operated at voltage V and output current is I, output power will be- To calculate fill factor, one needs to find out the maximum power which photo-voltaic cell can supply. If maximum power is delivered at voltage Vm This will give, (9) and (10) At VOC net output current will be zero. At this condition eq. (8) will give (11) and (12) 1.2.3 Effect of Dark Current on Fill Factor – Simulation using 1D drift-diffusion electrical modeling of bilayer OSC in MATLAB is done. We obtained that the dependence of light generated current on the applied voltage means that fill factor would depend on it as well besides shape of dark characteristics. An estimate of variation of light current can be obtained by taking ratio of its value at short circuit and open circuit condition – At 0 volt, At VOC, i.e. The ratio is a measure of how drop in Iph with the voltage. This ratio can be written as – Thus shows an additional factor that would affect fill factor. As this factor increases or decreases, the fill factor should accordingly change too. Results and Conclusions Eq. (3) suggests that as ideality factor n is changed, keeping reverse saturation current I0 and photo-generated current Iph constant, Vm changes in such a manner that (Vm/n) remains constant. So Im will also be constant as it is a function of (Vm/n). From eq. (6) open circuit voltage is also changes with ideality factor n such that (VOC/n) remains constant. It follows from the above reasoning that (Im/ISC) and (Vm/VOC) will be unchanged if n will vary keeping the reverse saturation current constant. Hence as ideality factor n varies keeping the reverse saturation current I0 constant, fill factor of the device will remain unchanged. Though if open circuit voltage (VOC) assumed to be constant by varying reverse saturation current I0 as ideality factor n changes, fill factor will change accordingly. Assuming Iph to be 1 mA-cm-2, I0 to be mA-cm-2 and ideality factor n to be 1, open circuit voltage and fill factor come out to be 1.25 volts and 0.9 respectively. Taking Iph and VOC constant, the variation of fill factor with ideality factor n is shown in fig. 2. We get that increase in the value of ideality factor n, will reduce the value of fill factor Fig. 2 : Variation of fill factor with ideality constant n. open circuit voltage and light generated current are taken to be constant as 1.25 V and 1 mA-cm-2 respectively. Eq. (12) shows that fill factor is a function of m. Variation of fill factor with m is shown in fig. 3. For m = 1, FF = 0.25. As m increases fill factor also increases and approaches to 1. However, FF will become only 1 when m is infinity. In this case also, m is a measure of the sharpness of the characteristic curve. As m increases, I-V curve becomes increasingly sharper resulting in a high fill factor. For polynomial dark characteristic with constant light generated current we get that increase in m will increase fill factor which approaches to 1 Fig. 3 : Variation of fill factor with m. fill factor approaches to 1 as m becomes larger and larger. Simulation results revealed in fig. 4 show that light generated current Iph is a function of applied voltage, means FF would depend on it as well besides shape of dark characteristic. A large dependence of Iph on increasing applied voltage would cause a significant drop in FF. Increase or decrease in an additional factor would accordingly change fill factor. Dark current can be varied in two ways, one by varying mobility and other by varying injection barrier heights. In both the cases fill factor increases proportionately with as shown in fig. 5 and fig. 6. Fig. 4 : Dependence of light generated current on the applied voltage. and are the hole and electron mobilities respectively. and are the injection barriers at anode and cathode respectively. Fig. 5 : Variation of fill factor with for 0.1eV and 0.3eV injection barrier heights. Different points have been obtained by changing mobility. Fig. 6 : Variation of fill factor with for carrier mobilities and . Different points have been obtained by changing injection barrier height. References J. A. Barker, C. M. Ramsdale, and N. C. Greenham, â€Å"Modeling the current-voltage characteristics of bilayer polymer photovoltaic devices†, Physical Review B 67, (2003), 075205. D. P. Grubera, G. Meinhardtb and W. Papousekc, Modelling the light absorption in organic photovoltaic devices, Solar Energy Materials and Solar Cells, 87, (2005), 215-223. J. Wagner, T. Fritz, and H. Bottcher, Computer modelling of organic thin film solar cells exciton model of photocurrent generation, Physica Status Solidi A, 136, (1993), 423. Y. Roichman and N. Tessler, â€Å" Generalized Einstein relation for disordered semiconductors implications for device performance†, Appl. Phys. Lett., 80, (2002), 1948. J. C. Scott and G. G. Malliaras, â€Å"Charge injection and recombination at the metal-organic interface†, Chem. Phys. Lett., 299, (1999), 115. S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, and J. C. Hummelen,† 2.5 % efficient organic plastic solar cells†, Appl. Phys. Lett., 78, (2001), 841-843 B. K. Crone, P. S. Davids, I. H. Cambell and D. L. Smith, â€Å"Device model investigation of bilayer organic light emitting doide†, J. Appl. Phys., 84, (2000), 1974. P. W. M. Blom, M. J. M. de Jong and S. Breedijk, â€Å"Temperature dependent electron hole recombination in polymer light emitting diodes†, Appl. Phys. Lett., 71, (1997), 930. C. M. Ramsdale, J. A. Barker, A. C. Arias, J. D. MacKenzie, R. H. Friend and N. C. Greenham, â€Å"The origin of open circuit voltage in polyfluorene-based photovoltaic device†, J. Appl. Phys, 92, (2002),4266. B. Mazhari, â€Å"An improved solar cell circuit model for organic solar cells†, Solar Energy Materials Solar Cells, 90, (2002), 1021.

Digestion

The food that is ingested needs to go through several processes in the body before it is assimilated into the body. These processes occur in the alimentary tract which is a long structure beginning from the mouth to the anus. There are also other important chemicals, hormones and digestive juices which are involved in this whole process. These are found in specific portions of the tract and each food is digested or broken down in specific region. Therefore, the breakdown of food is either mechanical or chemically carried out.The end result is the division of the food into very small components or sizes that can pass through into the cells (Insel, Turner and Ross, 2004). Everyone has at one time embarked on a journey in their lifetime. In addition to this, you hope to have a smooth trip and reach your destination safely. Herbert the hamburger has prepared to go through a very long and tortuous journey of his life. Everything is in place and he only hopes to have a safe trip. The main purpose of this trip is to deliver very vital documents that are needed to a company known as Body Builders.If these documents do not reach their destination, the company dies. These documents includes; Fats, Carbohydrates, Proteins and Minerals. The journey begins in the mouth where Herbert goes through some rough machines known as the teeth. This breaks it down into small forms through mastication. The document known as carbohydrate starts to be processed in the mouth and it involves a good friend known as saliva who is the boss in this department. Saliva does this with the help of amylase. This breaks it down into simple forms. From the mouth, permission is anted to move to the next level.To do this, Herbert is rolled into a bolus by the tongue and thrown at the back of the mouth. The epiglottis prevents Herbert from going through the trachea which is a wrong route. Herbert moves down in a smooth wave known as peristalsis into the esophagus until it reaches the gate of another de partment known as the stomach. This gate is known as the cardiac sphincter (Insel, Turner and Ross, 2004). In the stomach, enzymes are also involved in the digestion process. Protein digesting enzymes are known as proteases. Amino acids are the building blocks of proteins.The bonds existing between them are broken down by hydrochloric acid from gastric wall. This is followed by breakdown of amino acids by the enzyme pepsin (Insel, Turner and Ross, 2004). The food enters the small intestines through the pyloric sphincter. In the small intestine, pancrease produces amylase which further breaks carbohydrates to lactose, sucrose and maltose. These are broken down by lactase, sucrase and maltase enzymes respectively. These three are converted to an important form known as glucose which is absorbed via the villi on the walls of the intestines and finally find its way into blood circulation.Glucose is necessary for cell metabolism and function. However, the amount of glucose is regulated b y the liver with the help of hormones. Too much glucose (hyperglycemia) is transformed into glycogen and stored in the liver through the help of insulin. In cases where glucose levels are low in the blood (hypoglycemia), the reverse happens i. e. conversion of glycogen to glucose with the help of the hormone glucagon. In cases where there is no glycogen, the hormone glucagons initiate formation of glucose from amino acids or fats.This process is known as gluconeogenesis (Insel, Turner and Ross, 2004). The digestion of fats is with the help of enzymes known as lipases. Fats are complex molecules and should be turned into small molecules. This happens with the help of lipase enzyme from the pancrease and the end result is glycerol and fatty acid molecules. In addition to this process, bile from liver enters through the bile duct and emulsifies the fat. This makes it easy for the enzyme (lipase) to break down the fats starting from the surface. Storage of the bile usually occurs in the gall bladder.The body absorbs fats through the villi that cover the small intestines. The structure of the villus is such that it has capillaries and lacteals (lymph vessels). Fatty acids and glycerol enter into the lacteals into lymphatic system and finally bloodstream. Fatty acids find its way into adipose cells for storage or as source of energy (Insel, Turner and Ross, 2004). Protein digestion also continues in the small intestines in duodenum. The pancrease produces protease enzyme namely trypsin. In addition, chymotrypsin is also secreted. It works in a similar manner as pepsin.The end result of breakdown of protein by trypsin is amino acids. This is done through hydrolysis with insertion of water molecule between the bonds of amino acids. This helps in separation of the bonds that hold the amino acids together. The amino acids can thus pass through the intestinal wall into bloodstream. Their importance is in the repair of structures of the body. The waste products and undige sted material move to the large intestines where water is re-absorbed. From there, it moves to the cecum and out through the anus (Insel, Turner and Ross, 2004).

“I Have a Dream” by Martin Luther King Essay

The effect of repetition of words and phrases on the main messages contained I have a dream is an inspirational speech delivered by Martin Luther king to emphasizeover the critical situation of negroes, who are still living in the dark age. Whereas he examinesthat how the blacks are suffering in the crucial situation, to convey the true feelings of AfricanAmericans. This is an argumentative and persuasive appeal to the readers in order to makethem aware about the duality that is among the blacks and whites; they are not treated equallyin America. The speech of the king become noticeable and effective to the readers because ofthe continuous use of repetition of words that is also the strategy of the king to give the nationa wakeup call to take a step towards Negroes for their welfare. The speech also leaves thetremendous effect on the minds of people. Although the speech was more effective when theking first delivered it in 1963. Hence, the whole speech contains several examples of repetition of words like onehundred year later, we must face the tragic fact that the Negro is still not free. One hundredyears later, the life of the negro is still sadly crippled by the manacles of segregation and thechains of discrimination. These lines describes the present unbearable condition of thenegroes that they are still grabbed in the terrible situation and they have a desire to get rid ofthis poverty and sadness which is not easy for them to tolerate. And they want to acquire theirown peaceful land where they get the equal rights and which should be free of all bounding andrestrictions. Coming up with the other repetition that is now is the time to in the last lines ofthe fourth paragraph he uses these words several times, where he strongly addresses that this isthe time to help yourself by making yourself independent like a rock and say goodbye to thedark side of life and by throwing away this separation and inequalit y because every humanbeing is equal in the eye of God. Furthermore, he also emphasize over the satisfaction of Negro by repeating the words wecan never be satisfied as long as on behalf of them. The Negroes are being the victims ofbrutality and at every step they are treated like animals that did not do anything without thepermission of their masters. Perhaps Blacks are not supposed to enter in the hotels and placeswhere the  whites live and their vote even do not matter in any elections. These are thedepressing things for them and they will never be satisfied until they get justice and a completefreedom. King sounds recommending the nation in the words go back to with these words healso took the names of those areas where black lives. He recommends looking forward theseblacks communities, then you come to know that the situation of blacks can be changed and itwill be changed. Instead, he is provoking those blacks that have lost their hope to live withtheir own identity and he also tells the blacks in a didactic way that they should leave thevalley of social outcast. Another repetition of word is I have a dream that begin from 10paragraph to 18 paragraph. Which indicates king emotional appeal towards the black that howimportant is freedom for them, their desire to be treated fairly not by ignorance and hatred,desire of holding the hands of whites to walk and live their lives together, the dream of havingjustice and the dream not to judge by their skin color but by their heart and character. Theydont want any separate nation for them but a nation, which is united and with equalopportunities as with the other white Americans. They want to destroy this inferiority amongthem, they want to low down all those hills and mountains where a person feel himself superiorto others and this is not only the dream of blacks but of America. Elaborating the other example with this faith we will be able to indicates that by this faithwe will definitely be able to come across the feelings of brotherhood and love for each other,and by this faith we will do every work of our routine life together. The word together willcome in our lives and with this togetherness we will understand the meanings of life that isgifted by God to us. By the help of the following words let freedom ring from the king tellsthat every state, valley and city will sing the song of freedom, love, prosperity, brotherhoodand justice. And we need to struggle for the day when we would say finally that we our free. In a nutshell the kings style in the whole essay is very effective and appealing. Therepetition of words and phrases used in this essay are very powerful which directly effect theheart of reader and make them realize that  this is the time to do something for the negroes andnot be late now.

Thomas Paine Common Sense Analysis - 772 Words

Paine’s Common Sense â€Å"My country is the world, and my religion is to do good† (Thomas Paine common sense). Thomas Paine was an English-American political activist and philosopher who changed the world through his words and writing. In Common Sense, distributed in January 1776, the pamphlets sold in the thousands and was extremely persuasive. It transformed a neighborhood uprising into a War of Independence and Thomas Paine was given the title Father of the American Revolution. He challenged great minds to advocate the idea of freedom of conscience. Thomas Paines history extraordinarily affected individuals during his own time and furthermore propelled numerous scholars. A few of Thomas Paine achievements were â€Å"The age of Reason† and†¦show more content†¦If Paine would have simply rewritten and altered this sentence to say, All plans and proposals are superseded. The reader perhaps would not think any more than â€Å"the orders were useless.† Thomas Paine uses rheto rical question to persuade and influence his audience. In these phrase, Paine uses rhetorical question to make his point clear and to make his audience think about his questions. â€Å"But admitting, that we were all of English descent, what does it amount to? Nothing.† (Common sense). Common sense was composed as a way to energize American independence. He is endeavoring to motivate his audience members and readers to answer these questions for themselves. According to law.gmu.edu an online PDF of common sense, In the beginning of page 91Thomas Paine asks questions rapidly, â€Å"I ask, hath your house been burnt? Hath your property been destroyed before your face? Are your wife and children destitute of a bed to lie on, or bread to live on?† (Thomas Paine). One question after another Paine tries to make his reader think for themselves and influences his audience to speak and repeal against Britain. In doing this the American Revolution was able to start. According t o an eBook found in Galileo (1776) â€Å"Thoughts on the Present State of American Affairs*IN the following pages I offer nothing more than simple facts, plain arguments, andShow MoreRelatedAnalysis Of Common Sense By Thomas Paine826 Words   |  4 PagesCommon Sense by Thomas Paine Analysis Initial Reaction The first sentence of the introduction, â€Å"a long habit of not thinking a thing wrong, gives it a superficial appearance of being right,† (Paine, 1776) is instantly captivating to me by the Paine’s acknowledgement that by not standing up to wrong and injustices when you see or experience it is the same as giving the injustice your seal of approval for the wrongs being done. 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