Major Processes in Organic Solar Cells - Revision history

Cmditradmin: /* Exciton Migration */

2010-07-13T18:17:23Z

Exciton Migration

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	<swf width="500" height="400" >http://depts.washington.edu/cmditr/~~media~~/Excitonmigration.swf</swf>		<swf width="500" height="400" >http://depts.washington.edu/cmditr/mediawiki/images/4/42/Excitonmigration.swf</swf>

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Cmditradmin: /* Exciton Migration */

2010-07-08T20:16:16Z

Exciton Migration

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	As you can see, the equation is an important parameter. The diffusion coefficient D represents how fast the exciton can diffuse within the material or how fast the exciton can hop from molecule to molecule, or from chain segment to chain segment. The larger the diffusion coefficient, the longer the lifetime, which means the further the exciton can go before it decays back to the ground state. It is better if the exciton has a long diffusion length L<sub>D</sub> because it allows the exciton to have a higher chance of reaching the interface with the acceptor within its lifetime. If this process does not occur, no charge will generate and the exciton will decay back to the ground state. This decay will either produce heat, vibrations, or release the photons it once absorbed-- also called photoluminescence. But photoluminescence is not what you want in solar cell.		As you can see, the equation is an important parameter. The diffusion coefficient D represents how fast the exciton can diffuse within the material or how fast the exciton can hop from molecule to molecule, or from chain segment to chain segment. The larger the diffusion coefficient, the longer the lifetime, which means the further the exciton can go before it decays back to the ground state. It is better if the exciton has a long diffusion length L<sub>D</sub> because it allows the exciton to have a higher chance of reaching the interface with the acceptor within its lifetime. If this process does not occur, no charge will generate and the exciton will decay back to the ground state. This decay will either produce heat, vibrations, or release the photons it once absorbed-- also called photoluminescence. But photoluminescence is not what you want in solar cell.

	~~:<math>\~~alpha L~~\,\!</math>~~ is the figure of merit. It is the product of the diffusion length and the absorption coefficient or the absorbance of the material. The most efficient organic layer would be very absorbing, have a large alpha, and the excitons would have a large diffusion length so that the exciton will have a large probability of finding the interface during its lifetime.		αL is the figure of merit. It is the product of the diffusion length and the absorption coefficient or the absorbance of the material. The most efficient organic layer would be very absorbing, have a large alpha, and the excitons would have a large diffusion length so that the exciton will have a large probability of finding the interface during its lifetime.

	[[Image:opvdesignimprove.jpg\|thumb\|400px\|Light absorption and exciton diffusion. The red line shows the intensity of light as a function of thickness of donor layer. It shows an exponential drop indicating that a thicker layer could absorb all the light. The dashed yellow line indicates the exciton migration before it dies. A thicker layer absorbs more light but the excitons do not make it to the interface to generate a charge. As consequence organic dye layers must be really thin. ]]		[[Image:opvdesignimprove.jpg\|thumb\|400px\|Light absorption and exciton diffusion. The red line shows the intensity of light as a function of thickness of donor layer. It shows an exponential drop indicating that a thicker layer could absorb all the light. The dashed yellow line indicates the exciton migration before it dies. A thicker layer absorbs more light but the excitons do not make it to the interface to generate a charge. As consequence organic dye layers must be really thin. ]]

Cmditradmin: /* Exciton Migration */

2010-07-08T20:15:12Z

Exciton Migration

← Older revision		Revision as of 13:15, 8 July 2010
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	:<math>\alpha L\,\!</math> is the figure of merit. It is the product of the diffusion length and the absorption coefficient or the absorbance of the material. The most efficient organic layer would be very absorbing, have a large alpha, and the excitons would have a large diffusion length so that the exciton will have a large probability of finding the interface during its lifetime.		:<math>\alpha L\,\!</math> is the figure of merit. It is the product of the diffusion length and the absorption coefficient or the absorbance of the material. The most efficient organic layer would be very absorbing, have a large alpha, and the excitons would have a large diffusion length so that the exciton will have a large probability of finding the interface during its lifetime.

			[[Image:opvdesignimprove.jpg\|thumb\|400px\|Light absorption and exciton diffusion. The red line shows the intensity of light as a function of thickness of donor layer. It shows an exponential drop indicating that a thicker layer could absorb all the light. The dashed yellow line indicates the exciton migration before it dies. A thicker layer absorbs more light but the excitons do not make it to the interface to generate a charge. As consequence organic dye layers must be really thin. ]]
			<div id="Flash">Exciton Diffusion Animation</div>
	In the following Flash simulation the path of an exciton is visualized as you control the thickness of the electon donor layer. You can also experiment with the use of projecting "fingers", a dendritic surface morphology that is being attempted. Note that exciton migration is random diffusion process that does not have a directionality as shown here for instructive purposes. We are only showing the excitons that migrate in a productive direction.		In the following Flash simulation the path of an exciton is visualized as you control the thickness of the electon donor layer. You can also experiment with the use of projecting "fingers", a dendritic surface morphology that is being attempted. Note that exciton migration is random diffusion process that does not have a directionality as shown here for instructive purposes. We are only showing the excitons that migrate in a productive direction.

	[[Image:opvdesignimprove.jpg\|thumb\|400px\|Light absorption and exciton diffusion. The red line shows the intensity of light as a function of thickness of donor layer. It shows an exponential drop indicating that a thicker layer could absorb all the light. The dashed yellow line indicates the exciton migration before it dies. A thicker layer absorbs more light but the excitons do not make it to the interface to generate a charge. As consequence organic dye layers must be really thin. ]]

	~~<div id="Flash">Exciton Diffusion Animation</div>~~

	<swf width="500" height="400" >http://depts.washington.edu/cmditr/media/Excitonmigration.swf</swf>		<swf width="500" height="400" >http://depts.washington.edu/cmditr/media/Excitonmigration.swf</swf>

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	=== Charge Separation (exciton dissociation)===		=== Charge Separation (exciton dissociation)===

Cmditradmin: /* Steps in the Organic Photoelectric Process */

2010-07-08T20:11:33Z

Steps in the Organic Photoelectric Process

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	There are five different steps in excitonic solar cells. These 5 steps should also be valid for the organic light emitting diodes-- the fifth step would be the out coupling. The slide shows the energy scheme.		There are five different steps in excitonic solar cells. These 5 steps should also be valid for the organic light emitting diodes-- the fifth step would be the out coupling. The slide shows the energy scheme.

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	===Light Absorption (exciton formation)===		===Light Absorption (exciton formation)===
	[[Image:Opv15-photonabsorption.JPG\|thumb\|300px\|OPV Step 1 Photon absorption]]		[[Image:Opv15-photonabsorption.JPG\|thumb\|300px\|OPV Step 1 Photon absorption]]

Cmditradmin at 20:11, 8 July 2010

2010-07-08T20:11:13Z

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 == Steps in the Organic Photoelectric Process ==
 ===Light Absorption (exciton formation)===
 [[Image:Opv15-photonabsorption.JPG|thumb|300px|OPV Step 1 Photon absorption]]
 Suppose you have crystalline silicon that absorbs a photon from the solar spectrum, which leads to an excited state. The exciton binding energy is so small that the electron and hole can separate and thus, a current can be produced. However, in an organic material, the absorption of a photon creates a strongly bound exciton. Remember that an exciton is a neutral species. When the electron and the hole are next to one another, the charge is 0 since 1+ + 1- = 0. A current cannot be generated from an exciton. This is why crystalline silicon can perfectly lead to an absorption to the generation of free carriers also known as the band-to-band process. Suppose an exciton is formed on a polythiophene chain. To generate current the exciton must disassociate, meaning the electron and the hole in the exciton must separate, move away from one another, and generate current.
 ===Exciton Migration===
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 Excitons travel a certain distance during their lifetime. This distance is called the exciton diffusion length, and is a critical component in device efficiency. To produce current, an exciton must be able to diffuse and reach the interface between the donor and acceptor components during its lifetime. If the interface is 20 nanometers from where the exciton is generated, but during its lifetime the exciton only diffuses 5 nanometers, no current will be produced. If a system has large diffusion lengths but the migration of the exciton takes too long, then the distance between where the exciton is generated and the interface must be reduced, and will result in thinner films. However, thinner films absorb fewer photons. Organic cell engineers must factor in exciton diffusion lengths, exciton lifetimes, and film thickness to design efficient cells. The film must be thick enough to absorb a reasonable amount of photons, but also be thin enough so that the generated excitons can reach the interface efficiently.
 === Charge Separation (exciton dissociation)===

Cmditradmin: /* Charge Separation (exciton dissociation) */

2010-07-08T20:07:43Z

Charge Separation (exciton dissociation)

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	=== Charge Separation (exciton dissociation)===		=== Charge Separation (exciton dissociation)===
	~~[[Image:opv17_chargeseparation.JPG\|thumb\|300px\|OPV Step 3 charge separation (exciton dissociation)]]~~
	~~This diagram shows the events that occur when an exciton reaches the interface between the donor component (D) and the acceptor component (A).~~

			[[Image:opv17_chargeseparation.JPG\|thumb\|300px\|OPV Step 3 charge separation (exciton dissociation)This diagram shows the events that occur when an exciton reaches the interface between the donor component (D) and the acceptor component (A). ]]

			Disassociation, or charge separation, occurs at the interface. Once the electron and the hole have separated, the built in potential cause the electron to drift toward the cathode and the hole to drift toward the anode. If there was no built in potential, the electron and the hole will simply diffuse without any directionality to their motion.
	At the interface there are donor molecules or donor polymer chains next to the acceptor molecules or acceptor polymer chains. The lowest energy state is the ground state of the two components.		At the interface there are donor molecules or donor polymer chains next to the acceptor molecules or acceptor polymer chains. The lowest energy state is the ground state of the two components.

			[[Image:ET_P3HT_to_PCBM.png‎ \|thumb\|300px\|Charge is passed from P3HT to PCBM]]

	Suppose that the exciton is formed within the donor. This exciton will be labeled D*, an excited state in the donor. The exciton reaches the interface. There is a state and energy there that corresponds to the exciton energy above the ground state. The exciton transfers its electron to the acceptor, forming a hole on the donor component, and creating the charge transfer state (D<sup>+</sup> A<sup>-</sup>). The + and – charges move away from one another, forming a charge separated state.		Suppose that the exciton is formed within the donor. This exciton will be labeled D*, an excited state in the donor. The exciton reaches the interface. There is a state and energy there that corresponds to the exciton energy above the ground state. The exciton transfers its electron to the acceptor, forming a hole on the donor component, and creating the charge transfer state (D<sup>+</sup> A<sup>-</sup>). The + and – charges move away from one another, forming a charge separated state.

Cmditradmin: /* Charge Migration */

2010-07-08T20:05:41Z

Charge Migration

← Older revision		Revision as of 13:05, 8 July 2010
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	Efficient cells will maximize the rate of charge transfer (k<sub>ct</sub>) and charge separation (k<sub>cs</sub>) and minimize the rate of charge recombination (k<sub>cr</sub>). With those rates, you can also evaluate the Marcus Theory expressions.		Efficient cells will maximize the rate of charge transfer (k<sub>ct</sub>) and charge separation (k<sub>cs</sub>) and minimize the rate of charge recombination (k<sub>cr</sub>). With those rates, you can also evaluate the Marcus Theory expressions.

	===Charge Migration===		===Charge Migration and Collection===
	[[Image:Opv19migration.JPG\|thumb\|300px\|OPV Step 4 Charge Migration]]		[[Image:Opv19migration.JPG\|thumb\|300px\|OPV Step 4 Charge Migration]]
	During migration the charge move away from the heterojunction interface toward the electrodes.		During migration the charge move away from the heterojunction interface toward the electrodes. The electron and the hole will be collected efficiently at their respective electrodes, generate a current, and enter the external circuit

	Inorganic solar cells can be very efficient because a single inorganic material can be used with an exciton binding energy that is negligible at room temperature. The physics of organic solar cells is very different because the exciton is strongly bound. You need to have an acceptor and donor component with as much interface as possible between them in order that the formed excitons can quickly reach the interface to disassociate.		Inorganic solar cells can be very efficient because a single inorganic material can be used with an exciton binding energy that is negligible at room temperature. The physics of organic solar cells is very different because the exciton is strongly bound. You need to have an acceptor and donor component with as much interface as possible between them in order that the formed excitons can quickly reach the interface to disassociate.

Cmditradmin: /* Charge Migration */

2010-07-08T20:04:45Z

Charge Migration

← Older revision		Revision as of 13:04, 8 July 2010
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	===Charge Migration===		===Charge Migration===
	[[Image:Opv19migration.JPG\|thumb\|300px\|OPV Step 4 Charge Migration]]		[[Image:Opv19migration.JPG\|thumb\|300px\|OPV Step 4 Charge Migration]]
			During migration the charge move away from the heterojunction interface toward the electrodes.

	Inorganic solar cells can be very efficient because a single inorganic material can be used with an exciton binding energy that is negligible at room temperature. The physics of organic solar cells is very different because the exciton is strongly bound. You need to have an acceptor and donor component with as much interface as possible between them in order that the formed excitons can quickly reach the interface to disassociate.		Inorganic solar cells can be very efficient because a single inorganic material can be used with an exciton binding energy that is negligible at room temperature. The physics of organic solar cells is very different because the exciton is strongly bound. You need to have an acceptor and donor component with as much interface as possible between them in order that the formed excitons can quickly reach the interface to disassociate.

Cmditradmin: /* Electronic Energy Levels */

2010-07-08T19:55:31Z

Electronic Energy Levels

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	~~==Electronic Energy Levels==~~
	~~[[Image:opv18_energylevel.JPG\|thumb\|300px\|Electron State diagram for OPV cycle]]~~
	The literature refers to the exciton states and the charge transfer state which is when the donor + and the acceptor – are right next to each other. But the electron and the hole still feel each other rather strongly. This charge transfer state CT<sub>1</sub> is the lowest in energy precisely because the electron and the hole are strongly bound together. Remember, the stronger the attraction between the + and – charges, the lower the energy state. So the lowest energy charge transfer state has the strongest coulomb attraction between the hole and electron. To separate the charges, energy must be applied and a higher energy state must be acquired. In the literature there are many models that demonstrate how this can be achieved. But it would be much simpler to go directly from S<sub>1</sub> to higher lying charge states CT<sub>n</sub> and then to CS=E<sub>final</sub> rather then going all the way down to CT<sub>1</sub> and then to the final state.

	~~With atomic energy levels, the higher the energy level of a state, the farther away the electron is from the nucleus.~~
	Compare this with CT: the higher the energy of a charge transfer state, the lesser the attraction is between the electron and the hole, and the further away the two species are from one another. So if a higher lying charge transfer state is achieved, that state will be more diffused and the wave functions will be more delocalized. Alternatively, if instead of the exciton occurring at the interface and the charges being right next to each other, if the plus is further from the minus because the wave functions are more delocalized, it will be easier for the plus and minus to move away from one another.

	[[category:organic solar cell]]		[[category:organic solar cell]]

Cmditradmin: /* Charge Migration */

2010-07-08T19:54:50Z

Charge Migration

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	Once the excitons have disassociated into two separated species, the electron and the hole, they must move efficiently toward the electrodes. Remember that the more you can order your material, in general, the better the mobility will be. The faster the electrons and the holes can move away from each other, the more efficient their separation will be.		Once the excitons have disassociated into two separated species, the electron and the hole, they must move efficiently toward the electrodes. Remember that the more you can order your material, in general, the better the mobility will be. The faster the electrons and the holes can move away from each other, the more efficient their separation will be.

			<embed_document width="100%" height="500">http://depts.washington.edu/cmditr/media/type2-heterojunctionOPV.pdf</embed_document>
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