Short Circuit current, Series Resistance, Shunt resistance and Fill factor are important figures of merit of organic solar cell. But what exactly they depend upon ?
Dear Rajdeep,
Adding to the answer of Yonghai, the short circuit of the organic solar cells depends on the following physical parameters:
- The absorption efficient of the active material. The absorption of the active material must be very effective on the most wavelengths of the incident solar radiation. The higher the absorption coefficient, the higher the photo current will be.
- The second important parameter is the thickness of the absorber. It must be thick enough to absorb the highest cut off wavelength of the incident solar radiation.
This means that the absorber must be effective and generate excitons for the incident solar cradiation. The efficacy of this process is the called the internal quantum efficiency.
The other part is the collection efficiency of the generated excitons:
It depends on the following parameters:
- The dissociation rate of excitos in which they will be transferred to free electrons and free holes. The dissociation of excitons is accomplished by the high built in field in the absorber and by donor acceptor where the electron acceptor molecules are made to contact each other as the excitons have a very short lifetime and consequently a very short diffusion length. So, the distance between the donors which are the active molecules and the acceptors which are the elctron extracting molecules must be smaller equal to the exciton diffusion length. This is practically accomplished by mixing the donor and acceptor molecules to form the socalled bulk heterogeneous organic solar cell interface. The mixed phase absorbing thickness must be made sufficiently thick to absorb the incident solar radiation.
- After dissociation the the electrons will be collected by the electron transport layer and holes will be collected by the hole transport layer. It happens that at the interface of the etl with acceptor material that the dissociated electrons can recombine with holes and get lost.
Accordingly There are two loss mechanisms:
The recombination of the excitons if they left travelling more than their diffusion length before dissociation
Recombinations of electrons and holes at the interface between the acceptor and the hole transport layer.
To collect as much as possible from the photogenerated electrons and holes one has to control the mixing process of the D-A blend, control the thickness of this blend and choose the proper materials from the start to form the bulk heterogeneous phase.
In the next post i will discuss the parameters affecting the open circuit violatge and the fill factor
Best wishes
But fill factor also has relationship with series resistance. Doesn't it ?
Dear Rajdeep,
Adding to the answer of Yonghai, the short circuit of the organic solar cells depends on the following physical parameters:
- The absorption efficient of the active material. The absorption of the active material must be very effective on the most wavelengths of the incident solar radiation. The higher the absorption coefficient, the higher the photo current will be.
- The second important parameter is the thickness of the absorber. It must be thick enough to absorb the highest cut off wavelength of the incident solar radiation.
This means that the absorber must be effective and generate excitons for the incident solar cradiation. The efficacy of this process is the called the internal quantum efficiency.
The other part is the collection efficiency of the generated excitons:
It depends on the following parameters:
- The dissociation rate of excitos in which they will be transferred to free electrons and free holes. The dissociation of excitons is accomplished by the high built in field in the absorber and by donor acceptor where the electron acceptor molecules are made to contact each other as the excitons have a very short lifetime and consequently a very short diffusion length. So, the distance between the donors which are the active molecules and the acceptors which are the elctron extracting molecules must be smaller equal to the exciton diffusion length. This is practically accomplished by mixing the donor and acceptor molecules to form the socalled bulk heterogeneous organic solar cell interface. The mixed phase absorbing thickness must be made sufficiently thick to absorb the incident solar radiation.
- After dissociation the the electrons will be collected by the electron transport layer and holes will be collected by the hole transport layer. It happens that at the interface of the etl with acceptor material that the dissociated electrons can recombine with holes and get lost.
Accordingly There are two loss mechanisms:
The recombination of the excitons if they left travelling more than their diffusion length before dissociation
Recombinations of electrons and holes at the interface between the acceptor and the hole transport layer.
To collect as much as possible from the photogenerated electrons and holes one has to control the mixing process of the D-A blend, control the thickness of this blend and choose the proper materials from the start to form the bulk heterogeneous phase.
In the next post i will discuss the parameters affecting the open circuit violatge and the fill factor
Best wishes
Welcome again,
Now i will post about the important factors governing the open circuit voltage,
The first factor affecting the open circuit voltage is contact difference of potential between the hole transport layer material htl and the electron transport layer etl provided that the these layers make ohmic contacts with the transport layer. If they are nonohmic they will affect much the performance of the solar cells where they reduce the contact difference pf potential. The contact difference of potential must approach the energy difference between the lumolevel of the acceptor and the homolevel of the donor. To separate the electrons and holes in the exciton the lumolevel of the electron in the acceptor layer must lie under that of the donor absorber by an energy difference greater equal to the dissociation energy of the excitons. So, as a rule of thumb, when all above assumptions are valid the open circuit voltage will be = Ealumo - Edhomo - d E aetl- dEdhtl - dEexciton dissociation,
where the energies in order,
The acceptor lumo level, The donor lumolevel, The enrgy mismatch between the acceprotr layer and the etl, The energy mismatch between the donor absorber and the htl and the exciton dissociation energy.
So one has to maximize the gain and feminize the losses by choosing the suitable matched st of materials.
Extra loss on the open circuit voltage is due to recombination of the either the photo generated carriers and the injected charges in case of the dark currents.
In fact the open circuit is achieved when the photo current is balanced by the dark current.
So in addition to the proper choice of the materials concerning the energy levels, one has to minimize the recombination by high quality layers.
In the next post i will speak isa about the fill factor.
Best wishes
Welcome Rajdeep,
Now, we come to the fill factor. FF,
By definition the fill factor is defined by Vp Ip/ Voc Isc which is the peak power at the maximum power point, Voc and Isc have their usual meaning.
Graphically it is a measure for the squatness of the I-V curve in the solar cell mode.
The factors governing the fill factor are:
-The reverse saturation current of the dark current. This current decreases as the minority lifetime increases. Therefore one has to reduce the recombination leading to obtain higher minority carrier lifetime. This factor is as before for higher open circuit voltage and short circuit current,
- Lower ideality factor for the dark current. For pin diode model of organic solar cells n=2. but if the recombination at the interface between the donor material and the acceptance material is large, the ideality factor may be greater than two which results in less squatness of the i-v curve leading to lower FF.
- Then comes the series resistance of the device. Every layer contribute to the series resistance. That is the resistance of the acceptor donor blend, the Resistance of the transport layers, and the resistance of the metal semiconductor contacts. Some of these resistances may be linear and other may be nonlinear such as the resistance of the M-S contacts.
- The shunt resistance as the shunt resistance decreases the FF decreases
- Finally, In organic solar cells the collected short circuit current may not remain constant with the cells voltage. Specifically it decreases with the forward cell voltage. This also, will lead to smaller fill factor because it reduces the squareness of the i-v curve in the solar cell mode.
In the next comment i will speak isa on the deficiencies in real solar cells.
Best wishes
Welcome,
This comment is dedicated to outline the observed anomaly in the solar cell characteristics and their possible causes:
-The first anomaly is the small short circuit resistance Rsh and high leakage current. This may be due to the following deficiencies:
The formation of pinholes in the active layer and the reach through of the top metal to the bottom metal. This can be considered as partial shorts across the junction.
One has to examine the structure layer by layer during the fabrication processes and develop the deposition process to avoid the formation of voids and pin holes.
- No shorts but there is high ideality factor. For BH OSC, the ideality factor is equal to 2 for nominal pin diode model. However, if there is a large interface recombination and the current is dominated by such interface recombination between the donor and acceptor, the ideality factor may be appreciably larger than 2 causing a decrease in the fillfactor
- Too large reverse saturation current is a consequence of excessive recombination at the interface of the donor acceptor blend.
All these deviations from the nominal behavior will lead to smaller open circuit voltage and smaller fill factor.
- The higher recombination and inside the layers will decrease the the collected short circuit current and will lead to the decrease of the collected photo current with the forward voltage.
We see that the recombination at the D-A hetero interface is very serious and must be minimized to improve the solar cell performance.
In the next post i will discuss the effect of layer and interface resistances on the solar cell performance.
Abdelhalim Zekry
sir can we look at the condition in which Jsc, Voc do not change much but FF decreases significantly ?
Dear Shailendra,
welcome,
If Jsc and Voc are constants and the fill factor decreases significantly, this means that the series resistance may be increased significantly.
Best wishes
Abdelhalim Zekry
First of all, thank you for your extended responses.
I have an issue related to the topic of the question. Sometimes, I find that Jsc have increased (32%) after a period of 4 months exposure in a sunny region. However, the Voc still the same and FF have decreased of about 2,5%. Are the variation of the parameters you cited above are able to generate all this impact?
Thanks in advance
Dear Abdelouahed,
welcome,
It may be that the temperature increased in addition to an increase in the insolation leading to an increase in the short circuit current while the temperature decrease of the open circuit voltage is compensated by its increase due to the shortcircuit current. The slight decreases in the FF is due to the increase of the series resistance due to increases in the temperature.
Please check and report!
Thank you
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