These are the questions that one individual should ask himself if he/she does an research on these organic solar cells. The reason behind their entry is the scarcity of existing crystalline solar cell manufacturing base material, day by day hiking cost. As we all know that the polymers and simple molecules are abundant on earths surface and are cheaper in cost, and having desirable physical properties like light weight, flexibility etc. These are initial parameters those pushed the researchers to step in and take the motive of introducing organic solar cells into semiconductor device industry.
Initially silicon based solar cells were in many, but because of cost factor and their respective material properties they were moderately replaced by organic photo voltaic cells. As they are cheap, lightweight and are easy to fabricate they are in lead to crystalline organic photo voltaic cells. Devices made of these are supposed to have a theoretical efficiency of 10%. But, today, the efficiency we have for a small molecule organic photo voltaic cell is in the range of (0. 3-1. 3)%.
Hence, efforts for higher efficiencies are under research for many years, and these researchers have even a point to notice that the designing and synthesis of donor acceptor materials is vital for an efficient organic photo voltaic cell. Challenges so far we have are because of their low efficiency, low strength, low stability, and sensitiveness to environment. Factors like band gap, charge carrier density, absorption coefficient for incident solar spectrum in visible and near IR range are important to improve the efficiencies.
In this report we mainly focus on the organic solar cells based on small molecules. Having said that, we should know, why small molecules? The reason behind that is these small molecules has mostly hydrocarbons with carbon atoms alternatively linked to neighbor carbon atoms with alternate single-double bonding resulting into conjugated systems. In these systems the electrons in the p-z orbital’s idealizes to form a idealized bond with orbital and anti-bonding with orbital.
Where the orbital is considered as highest occupying molecular orbital (HOMO) and is least occupying molecular orbital(LOOM). Hence the gap between these two is considered to be the band-gap. Hence in small molecules the band-gap angels from 1-4 eve, which is moderately favorable for making a photo voltaic cell. When these systems absorb a photon, exited state is created in which e?ho pair is formed and is found to be bounded by electrostatic interaction. Hence, these conjugated systems are very vital to be maintained inside each small molecule.
Slightly introducing the type of materials required. We first of all concerned about the properties like band-gap, high temperature resistant, thickness, charge carrier density, ease of fabrication, and stability. For a material, to be a desired one, it should have moderate band-gap I. . In the semiconductor range with high charge carrier density, high strength and resistance at high temperatures, long life of functioning, moderate if possible low thickness, high absorption cross-section to incident solar spectrum and ease to process and fabricate.
As a part of our discussion in this term paper we will see what type of materials are necessary and how they function under applied light and generated electric field. For a small molecule based organic photo voltaic cell we generally choose indium oxide as electrode-I and metals with low work functions electrode-2. Generally we use Aluminum, Magnesium or Calcium. The difference between work functions between two electrodes develops electric field in organic layer. When light is incident on these elements sextons are produced as electrons are exited to LOOM leaving behind holes in HOMO.
Potential difference due to difference in work functions separates the electron-hole pair inside the organic layer and pulls them as electrons to positive electrode and holes to the negative electrode. Here, the positive electrode is an electrical conductor used to make contact with a non-metallic part of circuit. Most importantly, for high efficiencies we should have high charge carrier density inside organic layer and controlling the loss of energy as heat from the organic layer surface. Hence we adopt some efficiency improvement methods based on the properties and functioning nature of the materials.
Below are some of the methods we have reviewed to improve the efficiencies of organic photo voltaic cell based on small molecules. They are as follows: Methods to improve the efficiencies of organic solar cells based on small molecules: For a PVC (Organic Photo Voltaic Cell) based on copper pathetically-CO introducing beatification between buckminsterfullerene and aluminum cathodes gives the improvement of efficiency up to ;4%. Improving stability and lifetime of layered structure. Minimized charge recombination loss in Poops. Minimizing water and oxygen induced degradation of small molecules in Poops.
Improving the efficiency of solution processed small molecule Poops using thermal annealing. By combining two vacuum processed heterogeneous to optimize the absorption power of PVC and controlling the construction parameters thereby making them efficient. Using a unstructured sandwich processing. Controlling parameters like band gap and dissipation of energy in the form of heat. By making molecules face to face by maintaining optimum donor acceptor relative positions. By energy harvesting and improved storage methods. Solution processing of small molecules to improve efficiency by 6. %. We will now further proceed into each method of improving efficiency in-detail. As we have noticed from above methods, we have adopted, most of them are having their vital role during processing of an PVC rather than it is being a product. Hence, in an semiconductor industry the manufacturing processes of elements is given prior rarity. For a PVC (Organic Photo Voltaic Cell) based on copper pathetically-CO gives the improvement of efficiency up to ;4% Most efforts are being made from past few decades to improve the efficiency of Poops.
One among them is by inserting beatification(BCC) between fulfiller and aluminum cathode, organic thin film POP cells based on Chauncey-CO is observed to have an increased efficiency up to greater than 4%. Reason behind the insertion of this BCC layer is to minimize the exited loss during incident light and electric field. As the BCC inserted, obviously lays a very important roles in these cells by blocking the exited transport and hence named as Exited Blocking Layer.
Once the exited transport is blocked, increases the charge carrier density inside the organic layer and minimizes the losses related to them and finally increasing the efficiency grater than 4%. Before the insertion of this BCC layer, the efficiency is around 1-1. 5%. Hence it is considered to be a good move in increasing the efficiency. But, a new problem has arises because of BCC insertion. The Poops lifetime was decreased; now the lifetime is about only few hours. This ay be probably due to the instability of inserted BCC layer, as it is easily crystallized in moist environment.
BCC was already used for improving the electroluminescent efficiency in organic Leeds. However their lifetime is also found short because of instability of BCC layer. Hence for improving the stability we dope the BCC with PITCH (3,4,9,10-perpendicularity’s-licks-biomedical) Improving stability and lifetime of layered structure. In this section a small molecule PVC with the structure of TIT I Donor electoral I buffer layer I III is studied as a case where we can improve efficiency to some extent. In this experiment the usage of Alas s a buffer layer instead of BCC is seen.
Reason for using Alas instead of BCC is to use the effective blockage of waters lozenge to permeate through the acceptor layer. Another role of this new buffer layer is to block the diffusing cathode atoms into active layer during deposition and hence improve the efficiency. Describing the experiment, which is carried at room temperature without encapsulation, starting from deposition to the results in improved efficiency. Three types of buffer layers were used to study the I-V characteristics of the device in both light and dark.
The layered structure of the PVC is shown below. As we can see the buffer layer is separating the cathode from CO. The three layers are For device. A with BCC as buffer layer, for device. B with 50% BCC+50% Alas, for device. C with pure Alas as buffer layer. In this experiment the power conversion efficiencies for all the three are measured under the illumination Oriel Solar Simulator producing an AM 1. 56 spectrum. The I-V characteristics are recorded by Keith 2400. The calibrated I-V characteristic is shown below.
The experimental results from I-V characteristics ( Voce, Sis, IF) are tabulated below. Device Buffer Layer used Voce (V) Sis (ma/CM) Filling Factor(IF) Efficiency(%) A OHIOÂĽBCC. 42 5. 09 0. 49 1. 39 B 0. 49 5. 8 0. 464 1. 75 C 100% AlAlas. 506 6. 03 0. 52 2. 11 The irradiation intensity used is 75mawmCMf A. M 1. 5 simulated solar spectrum. Above values are same for the three devices under investigation. From the tabulated values it is clear that AlAlass most effective than BCBCCnd further experimental results have shown that the lifetime is increased up to 150 time the initial one.
Hence by inserting an buffer layer which acts as blocking layer without affecting the life time of evvices also one of the good options of improving the efficiencies. Minimized charge recombination loss in OPPoopsIn this section we discuss about minimizing the recombination losses of CuCupTPatchhotovoltaic cell by employing spin forbidden transition to reduce the recombination loss in OPPoopsWithout the mixing processes like spin-orbit coupling, spin-lattice interaction and hyperfine coupling he spin conversion process between singlet and triplet is not possible.
The molecular ground state have a singlet spin symmetry and hence the triplet CT states are forbidden to come and relax into the ground state. We an use this method to reduce the loss of recombination. The open circuit voltage and the fill factor can be improved by minimizing the recombination loss. Once VoVocend Fill factor are improved keeping IsSisonstant the efficiency improvement will take place. Minimizing water and oxygen induced degradation of small molecules in oppoopsThis section aims at complete knowledge of degradation of molecules because of water and oxygen during operation in small molecule based OPPoops
Once material degradation takes place, its consequences affect the physical, mechanical and electrical properties of materials, and hence the efficiency of device. The most significant degradation mechanisms for organic solar cells is diffusion of molecular oxygen and water into the device. The organic solar cell we discuss in this section is of the composition ITTITnd A1AYre the two electrodes where ITTITehaves like anode and A1AYs like cathode. In this experiment we have three sets of samples with same coconfigurationThree types of environments are used in dark and light modes.
Environments used are nitrogen, oxygen and water. The lifetime or end of lifetime of a solar cell is quoted as T88THr TSTTShich is the time taken by the device to reach 80% or 50% of its ininitialrreferenceWe use these values to record the lifetime of the above species in different environmental conditions. The results are tabulated below: Device name Intensity (W/m2mmTemperature (occoReReelHumidity(%) Atmosphere (1 atATMLife time( T88TH55THN2NOight 330 49 0 N2NO>l) 405/2700 N2NOark 0 26 0 N2NOlove box) – 1802 Light 330 49 0-17AAA3/74 1802 Dark 0 48 0-17AAA H2H2O80 Light 330 39 65 H2H2O80:N2NO. /11 H2H2O80 Dark 048 65 H2H2O80:N2NO As we can see from the tabulated results, the life time is decreasing for water case than in oxygen case. Hence degradation is seen largely in case of water than in xyoxygenFollowing table will provide the knowledge of how the efficiency changes with different types of injection. Water injection showing the effect on Short circuit current, open circuit voltage, fill factor, and efficiency. As we can see from above data that the efficiency after injection is gradually decreasing.
From the above data it is clear that the efficiency dropping Just after the oxygen injection is not as bad as water injection. Hence a moist free environment during preparing a layered structure, will play a vital role in improving the efficiencies of cells. Improving the efficiency of solution rorecessedmall molecule OPPoopssing thermal annealing. In this section we discuss about the insertion of (beFontainebleauBTBETunction into a ollighthousesackbones should lead to some interesting features for the photovoltaic applications.
In addition, to enhance the light absorption properties by thermal annealing of active layers, and increase the hole mobility by inducing the self-organization of conjugated polymers or small molecules thereby increasing the efficiency of OPPoopsAll experimental conditions were carried under argon atmosphere with the use of standard scschlepsechniques. After being made the cell is having an effective area of 4mmomThe thermal annealing is carried out by directly placing the completed devices directly on a digitally controlled hotplate at various temperatures in a argon filled glove-box.
The I-V characteristic curves of the photovoltaic cells are obtained by keKeith400 source measure unit. Results of the experiment with improved efficiencies is tabulated below: As we can see from the above three the normalized absorption region is high in particular range of wavelength. We can clearly see the LULUMPnd HOMO with their respective reduction and oxidation potential values. Following table gives the knowledge of effect of annealing on efficiency for different types of devices under different conditions.
Hence annealing a device at appropriate temperature and under appropriate conditions will change the PCPACEfEfficiencyup to a significant extent. By combining two vacuum processed hetero Junctions to optimize the absorption power of OPPVCnd controlling the construction parameters thereby making them efficient. In this part of discussion, we have the concept of combining two vacuum processed single hetero junction organic solar cell with complementary absorption are described. The red bsabsorbingetero Junction consists of C6COnd a fluorinated zinc ptpathologicallyF4IFZnZinc thThateads to 0. 1-0. V open circuit voltage than the commonly used ZnZincFrom the manufacturing point of view, tandem concepts offer an advantage, due to serial connection of sub-cells the efficiency is increased by adding up voltages rather than currents. To increase the efficiency, we have to increase the absorption cross-section of the OPPVCFor absorption at longer wavelengths the fluorinated ptpatheticallyerivative (F4IFnZincis used as donor instead of ZnZincAs F4IFnZincs efficient in lowering the ionominationotential from 5. 1 eVveto. 46 eVeveue to the electron withdrawing character of the fluorine atoms.
Higher fluorinated ptpathologicallytoms can be used as acceptors in organic solar cells. The absorption features of F4IFnZincn mixed layers with C6COre comparable to ZnZincexhibiting strong ababsorbencyn the red part of the spectrum peaking at 630 nmNMnd extending to above 800 nmNMThis is a good quality which is desired, as it increases the absorption coefficient of organic photovoltaic surface and thus increasing the efficiency. Heating the above mixed layer has the same effect that of common one but leads to an improvement in charge arairierensity. This can be seen through the mismatch of I-V characteristics.
Hence here above we found out two more concepts of increasing the efficiencies. Now we further look into the dependence of efficiency on factors like light intensity, angle of illumination and temperature. For a OPPVCith a given efficiency, thickness and incident wavelength the most important factors on which we should concern about are the performance of the device under different illumination intensities for varying illumination angles at typical operating temperatures. 343429000449580Fromig. : The nfinfluencef illumination intensity on the device performance from 0. 5mMomcmCMo 310 mWmmmCMThe illumination is provided by halogen lamp kept at constant temperature of 25ICCheHereJsSCs directly proportional to lint over the complete range of illumination intensities. VoVoces increasing continuously with lint but below logarithmic value. FFIFhows weak maximum at lint?70mawmCMfter which it decreases by approximately 4% up to 31mawmCMThis shows the maximum power point to be mostly sub-linear and the maximum efficiency approximately at 40mawcmCMFrom fig. : The influence of illumination intensity on the device performance roroom. 5mMomcmCMo 310 mWmmmCMThe illumination is provided by halogen lamp kept at constant temperature of 25ICCheHereJsSCs directly proportional to lint over the complete range of illumination intensities. VoVoces increasing continuously with lint but below logarithmic value. FFIFhows weak maximum at lint?70mawmCMfter which it decreases by approximately 4% up to 31mawmCMThis shows the maximum power cmCMGiven below is the calibrated data showing the dependence of illumination intensity on the device performance:leleptonsing a naunstructuredandwich processing.
In this section we will see the structural dependence of OPPVCevice layers on efficiency. Researchers found that the efficiency of a device is almost tripled when naunstructuredandwich metal and plastic are used. These type of structures can collects and traps the light more effectively than any other. The nanotechnology is used to overcome the challenges that cause solar cells to lose energy. Challenges like light reflecting from the layered surface and inability of the cell to capture the light fully that enters in.
But with the help of this NaAnnaaterial sandwich we can answer both the problems. The sandwich, which is generally called as sub-wavelength plplatonicavity. Have extraordinary characteristics of damping reflection and trapping light. Using this technique research studies have show us that only approximately 4% of the light is reflected and as much as 96% of the light is absorbed accounted for the increase in efficiency up to 52%. Above description is only for the direct sunlight(perpendicular light falling normal to the surface).
This efficiency is much more for a solar cell surface upon which the light incident is at large angles i.Ie.EonOnloudy days and when cells not facing the light directly/ erreproductivelyBy capturing these light incident at large angles one can boost up the efficiency up to extra 81%, combining a total of 175% increase in efficiency. Thought the physics behind the innovation is complex the device structure is simple. The window layer(top layer) is extremely fine metal mesh and of the size of 30NMhick with each mesh having diameter of 17Mannnd 25NMpart. This mesh layer can even replace the ITTITayer.
A picture of such naAnnaesh layer is shown below: The mesh window layer is placed very close to the bottom layer of the sandwich, the same metal film used in conventional solar cells. In between the two metal sheets is a thin strip of sesemiconductoraterial used in solar panels. It can be any type – silicon, plastic or gallium arsenide – although ChCoho’seam used an 85-nanometer- thick plastic. The solar cell’s features – the spacing of the mesh, the thickness of the sandwich, the diameter of the holes – are all smaller than the wavelength of the light being collected.
This is critical because light behaves in very unusual ways in sub- wavelength structures. ChCoho’seam discovered that using these sub-wavelength structures allowed them to create a trap in which light enters, with almost no effleetingand does not leave, and hence efficiency is drastically can be increased. Here we will see some of the common factors like band gap of semiconductor, electric charge recombination and energy dissipation in the form of heat, which are limiting factors for power conversion efficiency of a solar cell.
Light with less than the enenergyf band gap cannot be absorbed and cannot generate electricity inside the organic layer. Let say that the light with higher intensity and higher than the band gap is incident and generated the electricity inside the device and lets say that the charges aveaveeen generated. These generated charges if lost because of recombination; the electric current inside the device will decrease: and so the electric power of the cell will decrease.
The relation between the maximum conversion efficiency and optical absorption edge are given in the following pipickHere: The red line represents the conventional theoretical limit of single-junction- inorganic cell; blue line represents the theoretical limit of the single-junction-organic solar cell; the black line represents the theoretical limit of the multi-]unction-organic cell. The theoretically calculated photoelectric conversion efficiency for a single junction- organic photovoltaic cell is about 21%.
From the above picture we can see that for an appropriate range of optical absorption edge value only the maximum efficiency can be attained. Right side of pipicks the rough sketch of layered structure. Where, two active organic layers separate both the electrodes. By making molecules face to face by maintaining optimum donor acceptor relative positions. In this section we further precede our methods of improving the efficiency in organic solar cells aseasedn small molecules, and their dependence in structure at molecular levels.
Researchers have shown that to transfer maximum energy more effectively inside of complex three dimensional OPPoopss to align the donor molecules face-on, rather than relative to acceptor or edge-on to the acceptors. We know that exexitedn energy particle created when light is incident on the material and got absorbed. This efficiency of a cell depends on this exexitedThis exexiteds found at the donor acceptor interface inside the cell. At interface exexitedreates power by itself converting into haharearriers that travel to the electrodes.
Meanwhile inside at molecular level the atoms inside the donor acceptor levels itself mixes, cluster or both leading to the variances inside the domain leading to affect the conversion process and thereby affecting the efficiency. It is difficult for an individual to consider all changes at a time. 2525146001249680Buthen we consider the face-on orientation, which allows the favorable interactions for charge transfer and lessen the recombination loss can be thought of when efficiency is considered. The sample picture of face-on orientation eaeatingo favorable charge interactions is seen in the picture beside.
OBBouthen we consider the face-on orientation, which allows the favorable interactions for charge transfer and lessen the recombination loss can be thought of when efficiency is considered. The sample picture of face-on orientation leading to favorable charge interactions is seen in the picture beside. By energy harvesting and improved storage methods. Most significant problem now a day to an organic photovoltaic industry facing is loss of energy. Saving, storing the dissipated energy will improve the efficiency more than nyNYther methods we adopted so far.
Most of the dissipated energy is in the form of heat. Loss of energy is because of organic material’s lack in properties to hold them up. Energy harvesting is a process by which ambient neenergys captured and converted into electricity. These energy harvesting includes ththermostaticphotovoltaic, piezoelectric materials. More the harvesting and storage material is efficient, more the efficiency of the solar cell. Solution processing of small molecules to improve efficiency by 6. 7% In this section we see that the solution processed small molecule organic solar cells an perform up to efficiencies ranging 6-8%.
A combination of polymer design, morphology control, structural insight and device engineering lead us this much of efficiency. However solution processed small molecule BHBBCave their efficiencies below the polymeric ones. 33147005715 Normalized ababsorptionsvs.waWavelengthan be seen from adjacent fgfigures a result of solution processing. Normalized ababsorptionsvs.waWavelengthan be seen from adjacent figure as a result of solution processing. The component DIDIDs added to the solution to enhance the efficiency improving properties. We have a range of 0-1% v/v vixing.
