Organic Solar Cells History, Principles and Efficiency

Organic Solar Cells History, conventions and clevernessSolar CellsSolar cellphones atomic number 18 cells or devices use for conver provideg sunlight into voltaic current (electricity) or voltage. They ar also called imagevoltaic cells (PV) or devices and the process of genera plateg electricity from sunlight is called photoelectric effect. Solar efficiency conversion through with(p) photovoltaic effect lavatory be touchd with many materials at polar lifetimes. Over the geezerhood many research and development shake off been conducted in the argona of solar capability ( keen film applications)1-3. But most of these developments gull been in in total solar cells with conventional atomic number 14 give solar cells dominating in the output signal of solar vim in the commercialised-grade market 4-5. Silicon base cells for repress film application have enormous advantages like bang-up absorption rate of sunlight, suitable band crack for photovoltaic applications, l onger lifetimes and improving efficiency. But the process of silicon base cells generation of voltage is tedious and above all very expensive for the commercial market. Research for alternatives to silicon has been ongoing for many time now with or so other in constitutional materials like Copper Indium Gallium Selenium (Cu-In-Ga-Se)6, certificate of deposit Sulfide (CdS)7, Lead Cadmium Sulfide (PbCdS)8, etc. But some have similar production problems like the silicon and as good expensive. Others also are of dangerous elements which are non environmentally companionable (CdS, PbCdS, etc). Another alternative to silicon base cells in terms of thin film (solar cells) research for photovoltaic application could be constituent(a) solar cells (also cognize as plastic solar cells)9. With this, photocurrents are generated from fundamental materials. In this review, picture history of organic solar cells is discussed, the basic principle of operation is outlined and some proceed ing in terms of the materials ingress rate, efficiency, constancy and abjection and comparison between organic solar cells and inorganic solar cells (silicon) are also discussed.Chapter 2Organic Solar cells (Plastic Solar cells)The infancy of organic solar cells began in the late 1950s 10. At this time, photoconductivity in some organic semiconductor cells (anthracene, chlorophyll) were measured with voltage of 1 V by some research groups1112.They proposed that if a single degree PV cell is illuminated consisting of an organic layer, sandwich cell with gloomy work lead metal (aluminum, Al) and a conducting glass of high work function (indium tin oxide, ITO), photoconductivity will be observed. With this interesting result and less bell hard-hitting of these organic semiconductor cells and also a possibility of doping these materials to achieve more encouraging results caught up with many researchers in this line of business. The work done since has been unprecedented as sho wn in figure 2.1 on the next page.In the 1960s, semiconducting properties were observed in dyes particularly in methylene blue 13. faculty of 105 % in sunlight conversion was reported in the early 1970s to an improvement of 1 % in the early eighties 14. This was achieved through an interesting phenomenon known as heterojunction15. This phenomenon is a surface between semiconducting materials of dissimilar layers. Photovoltaic devices were applied with heterojunction where donor-acceptor organic cells were tailored together. In youthful years, photoconductivity has been measured in dyes and the dye solar cells have progressively been improved for laboratory cells16. Currently male monarch conversion efficiency of organic photovoltaics in single-junction devices is over 9 %17 and that of multi-junction cell is over 12 %18. most materials of organic solar cells are dyes and some polymers like origomers19, dendrimers20, liquid lechatelierite materials21 and self-assembled monolayer s 22. All these need to be prepared carefully to obtain optimal efficiency and stability23 judge 2.1 Number of publications is plotted against the year of publications. This shows the inception of organic solar cells and how much interest the field has generated among scientists and the commercial entities over the years. eld below 1990 saw less publication (1960 to 1970 -10 and 1980 to 1990 29) compared to the years in the figure.Principle of Operations.In recent time, organic solar cells are of different trading operations due to their usage. Similar to inorganic solar cells, organic solar cells tolerate be used to convert sunlight into electricity with the aid of a semiconductor. The basic principle behind this operation is outline below most(prenominal) organic solar cells have very thin material layer either single or multi-layer where there is a strong absorption of light sandwich between two electrodes, an anode (A) and a cathode (C). The anode (usually indium tin oxide ITO) is transparent and has a high work function. The cathode (aluminum) is opaque and has a low work function. The material layer is usually a photosensitive organic semiconductor. When light of appropriate zero (sunlight) is incident on it, an negatron is aflame from the highest occupied molecular orbital (HOMO) to a lower uncopied area called terminal uncopied molecular orbital (LUMO) leaving a clutter in the HOMO. This leads to exciton formation. That is, there is a creation of an electron-hole pair which is strongly bounded together. As the electron stays at the LUMO, there is a loss in cleverness by the electron through thermal liberalisation as the electron penetrates the skill band violate. The electron-hole pair diffuses independent of the electric field and are separated (exciton dissociation) at the interface between the donor state (HOMO) and the accepter state (LUMO). The electron is collected at one end of the electrode (cathode) and the hole at the other end of the electrode (anode) thereby generation photocurrent in the process. If the electron and the hole after separation do not reach the interface, their wrapped energies are dissipated out and no photocurrent is generated. Step by smell principle is illustrated in pictorial form belowFigure 3.2 a) gay is incident on an electron (red). (b) Electron is excited from the HOMO to the LUMO creating a hole (black) at the HOMO. (c) Exciton formation of electronhole pair. (d) Diffusion of exciton independent of electric field. (e) Exciton dissociation. (f) accretion of charges.Chapter 4Performance4.1 Absorption of light.In organic solar cells, the thin organic semiconducting layer is responsible for light absorption. This layer has a valence band which is densed with electrons and a conduction band. These bands are separated by an naught rift. When the layer absorbs light, an excited state is created. This state is characterized by an efficacy gap. The faculty gap is the brawniness difference between the higher energy state (LUMO) and the lower energy state (HOMO). It is usually of the range of (1.0 -4.0) eV24 and it is located asEg = ELUMO EHOMO . (4.1)Where Eg is the energy gap in electron volts (eV), ELUMO is the energy at LUMO (higher energy state) and EHOMO is the energy at HOMO (lower energy state).The energy gap usually serves as an activation energy barrier. This activation energy barrier inevitably to be overcome before an electron is excited from the lower energy state to the higher energy state. The excited electron has energy greater than or equal to this activation energy barrier. This energy is determined ash.cEphoton = Eg . (4.2)photonWhere Ephoton is the energy of the incident photon (light), h is Plancks constant (6.626 1034 Js), c is speed of light (2.997 108 ms1) and photon is wavelength of the photon ( (400 -700) nm).As the excited electron mud at the LUMO, a hole is created in the HOMO. The electron undergoes thermal relaxation as it body at the LUMO and this result in loss of energy by the electron. This energy loss is compensated for asEl = Eelectron Eg . (4.3)Where El is thermal energy loss of the electron, Eelectron is the energy of the electron at the LUMO and Eg is the energy gap.Figure 4.1 (a) Thin organic semiconductor layer (with both LUMO and HOMO) with energy gap (Eg). (b) Incident light of greater energy than the energy gap excites electron (red) from HOMO to LUMO. This creates a hole (black) at the HOMO (c) Energy lost by the electron through thermal relaxation.4.2 Stability and abasementIn solar cell application, long operational lifetime performance is required. To achieve this, stability and degradation are few of the key of import issues to look at in real-time application. Over the years, stability of organic solar cells has improved very much in terms of their power conversions25. This is clearly shown in the figure belowIdeally the advantages of organic solar cells with their low equal m aterials, recyclable, easy production and production in king-size quantities, exibility and durability (low weight), stability should be optimum. These advantages somehow also affect the stability of the organic cells. The active layer (thin organic semiconducting layer) component which is a core component of the cells is sometimes prone to degradations. These degradations occur during their production (printing in bulk quantities and rolling them together thereby introducing some mechanical properties which then affect the morphology of the active layer) and also reactions from weathering (UV light, oxygen, water). Extensive work on photo stability of some organic solar cells (large number of polymers) has been investigated by Manceau et al27.Figure 4.2 Organic Photovoltaic (OPV) production with progression in years shown. The years below 2010 had lower production of OPVs ( 0.5 MW) 26. Chapter 5Comparism between organic solar cells and inorganic solar cells (Silicon base solar ce lls).Organic and inorganic solar cells serve similar applications but they interesting differences in terms of how they are made. Organic solar cells are cheap in terms of materials, production and are recyclable, they have very thin solar cells with little energy in making them, they are exible, durable and have low weight, they are colourful and they have easy production and tooshie be produced in large areas. But they have low efficiency and lifetime compared to silicon base solar cells. Inorganic solar cells are cost effective in terms of materials, production and are not recyclable, much energy is need to have thin layer cells, they are rigid and not durable, they are of dark grey materials with dark blue to black show uping, they have complicated production and are difficult to produce in large areas. But they have good light absorption rate, reform efficiency and longer lifetime.Chapter 6ConclusionOrganic solar cells can be alternative to silicon base solar cells with its interesting applications. They can be fabricated into our day to day usage materials and equipment with low cost technology in serving their purpose. Efficiency and stability still remains areas that should be addressed in the future to optimally have good power conversions.