The thickness of solar cells should be less than the diffusion length of the induced carriers. To avoid the unnecessary resistance and to reduce the production cost thickness should be equal or less than the width of the depletion region. Much smaller thickness produces weak static electric fields in the depletion region.
Hello!
The problem in organic solar cells is mainly related to the poor charge carrier mobilities. Moreover, some systems show Langevin-type bimolecular charge recombination and some show non-Langevin recombination. For the latter case we can give an example of P3HT/PCBM solar cells where betta is typically equal to 0.001. This means that recombination is not influenced significantly by the active layer thickness. As a result, films with a thickness of 60 nm, 100 nm and 1000 nm can give equally good performance.
In other case (in this is vast majority of the systems) the recombination follows Langevin law. In this case we observe extremelly strong dependence of the photovoltaic parameters on the film thickness. The current and FF start to decrease typically for the films thicker than 150 nm and for 1000 nm thick films we have just an extremely poor performance. Therefore, if you work with solution processible materials, you need to vary the spin-coating frequency to obtain a set of films of different thickness. You can also change concentrations of your materials (=viscosity of the solution) to achieve the same effect. Note that optimal film thickness depends strongly on the donor-acceptor ratio and the morphology of the blend. Therefore, while optimizing the thickness one needs to play with other parameters as well.
Regarding the theoretical aspects mentioned by colleagues above, optical interference effects play a significant role in these systems. If you are inetersted in this aspect, you can look up the publications of H. Hoppe who did optical modelling for a range of systems.
Good luck!
The thickness requirement for solar cell depends on absorption coefficient of semiconducting material. The absorbance increases exponentially with thickness.if absorption coefficient is high, less thickness is required to absorb light. If absorption coefficient is low, we have to use thicker sample to absorb the light, otherwise part of it will be transmitted. Absorption coefficient of direct band semiconductor is higher than indirect band gap semiconductors. One can calculate the requirement of thickness from absorption coefficient of the semiconductor for maximum efficiency.
In terms of efficiency, you want thick absorbers to maximise absorption, and thin absorbers to maximise current collection (thinner than diffusion length as you said, or drift length on p-i-n cells). For most materials there will thus be an optimum thickness than depends on the balance between absorptivity and diffusion or drift length. This is the thickness that gives optimum efficiency.
In addition to this, there is the cost factor - thinner absorbers are cheaper, so you may get the best cost efficiency with absorbers that are thinner than the optimum thickness for maximum efficiency.
In addition to that said by the colleagues above, The absorption thickness of the material is approximately equals 1/alpha, with alpha the smallest absorption coefficient. However the the thickness can be reduced to half this value by metallic reflecting back contact which is practically the case where the back of he cell is covered with metal electrode. To collect the photo generated electron hole pair in such thickens,it must be smaller equal the diffusion length.
For more information please see the link:
https://www.researchgate.net/publication/236002879_A_course_on_photo-voltaic_array_systems
Book A course on photo-voltaic array systems
Thank you very much to the colleagues above for answering my question. However my question was on the optimization of the thickness for the particular material/s without considering the absorption. Some researchers1-2) show almost all spectrum of solar radiation is been absorbed as the surface plasmon polarization within the thickness of nm range by simply adding a metallic nano pattern. Is this ultra thin thickness enough to make strong static electric field in the depletion region of a solar cell? To make strong static electric field a large amount of carriers should be transport from one side to other side.
1) Vivian E. Ferry, Albert Polman, and Harry A. Atwater, ACSNANO, VOL. 5, NO. 12 (2011) 10055–10064.
2) Vivian E. Ferry, Marc A. Verschuuren, Hongbo B. T. Li, Ewold Verhagen, Robert J. Walters, Ruud E. I. Schropp, Harry A. Atwater, and Albert Polman, OPTICS EXPRESS, Vol. 18, No. 102 (2010) A237-A245.
Dear Md,
In order to have a photovoltaic effect you have to create a field region to separate the electron hole pairs generated by the absorbed light. If the eh pairs are generated within a diffusion length Ln and Lp from the edges of he field region hey can be successfully separated. So, principley the active thickness must be= Ln +Lp + WSR, So, if you can make Ln+Lp very small tending to Zero Then the active region will be only the space charge region. If the absorption coefficient of the material is high such that 1/ alpha is smaller or equal WSR then the EQE will be high and the photocurrent will be also high.
Now let us come to the other parameter, the OPEN CIRCUIT VOLTAGE Voc it is proportional to the contact difference of potential or the potential barrier on the SCR
This potential barrier Vb= Ei times WSR, where Ei i the average internal electric field.
So for high Voc , Vb must be made high which means that you to make WCR smaller we have to increase Ei. But there is a limit to Ei , the breakdown field of the material.
It must be said in this context, that Vb i defined by the contact difference of potential of materials creating the junction.
Further to the above, the electrical field is simply related to the thickness E=V/d, so less thickness is required for high electric field. you should take into account that the lower thickness than the optimum could increase the device resistance as well ( become highly resistive).
Hello!
The problem in organic solar cells is mainly related to the poor charge carrier mobilities. Moreover, some systems show Langevin-type bimolecular charge recombination and some show non-Langevin recombination. For the latter case we can give an example of P3HT/PCBM solar cells where betta is typically equal to 0.001. This means that recombination is not influenced significantly by the active layer thickness. As a result, films with a thickness of 60 nm, 100 nm and 1000 nm can give equally good performance.
In other case (in this is vast majority of the systems) the recombination follows Langevin law. In this case we observe extremelly strong dependence of the photovoltaic parameters on the film thickness. The current and FF start to decrease typically for the films thicker than 150 nm and for 1000 nm thick films we have just an extremely poor performance. Therefore, if you work with solution processible materials, you need to vary the spin-coating frequency to obtain a set of films of different thickness. You can also change concentrations of your materials (=viscosity of the solution) to achieve the same effect. Note that optimal film thickness depends strongly on the donor-acceptor ratio and the morphology of the blend. Therefore, while optimizing the thickness one needs to play with other parameters as well.
Regarding the theoretical aspects mentioned by colleagues above, optical interference effects play a significant role in these systems. If you are inetersted in this aspect, you can look up the publications of H. Hoppe who did optical modelling for a range of systems.
Good luck!
In addition: my answer is related to polymer solar cells or bulk heterojuction solar cells in general. If you use flat heterojunctions, the situation is somewhat different. Indeed, you need to think about optical properties or the materials (see ref above) and exciton diffusion lengths.
@ Dr. Pavel A Troshin,
Your comments regarding the recombination mechanism and thickness independent performance of P3HT:PCBM devices is very interesting. Can you please direct me to some publications which elaborate on these comments.
Thank you.
Dear Waranatha,
please, look up publications of Atilla Mozer and his colleagues. People from Konarka (C. Brabec as coathor) also did some work in that direction.
And also please follow the citattions for early reports in the respective publications of A. Mozer.
Dear Pavel,
You can use solar cell simulation softwares such as SCAPS to optimize the thickness of layers for solar cells.
Best regards,
Morteza
I believe you can define the generated current (or the absorptance spectra) as a function of the thickness (t) of any given layer (say active region) of your device, and try to optimise the efficiency as a function of t using optimisation tools; e. g. genetic algorithm. This will also give you an idea how to design an optimised solar cell device by optimising thickness or composition of the layers.
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