Dear Ashraful,

I am not too familiar with organic solar cells, but ideality factors greater than 2 are also often measured for other solar cell types.  In the case of thin film solar cells possible reasons for those high values might be:

• Tunneling enhanced interface recombination, see

http://scitation.aip.org/content/aip/journal/apl/74/1/10.1063/1.122967

• Superposition of 1-diode-IV curve by effects like rollover or IV-kinks, which change the IV-slope and therefore the determined ideality factor. In this case, the calculated ideality factor is systematically too high and cannot be used for further analysis.

Examples for those distortions can be found in

http://www.sciencedirect.com/science/article/pii/S0040609004016281

http://scitation.aip.org/content/aip/journal/jap/107/3/10.1063/1.3277043

http://www.sciencedirect.com/science/article/pii/S0040609006000472

• In the case of solar cells with large areas distributed series resistance effects lead to an apparent high ideality factor, which depends on the illumination density (the impact of the distributed series resistance decreases with decreasing illumination density) => Check, if n decreases with illumination density

See for example:

http://onlinelibrary.wiley.com/doi/10.1002/pip.784/abstract

• Voltage dependent carrier collection might also superimpose the IV curve slope, this will again lead to apparent illumination dependent ideality factors.

Example:

http://onlinelibrary.wiley.com/doi/10.1002/pip.518/abstract

I hope this might be helpful even if it is not directly connected to organic solar cells.

Stefan

http://scitation.aip.org/content/aip/journal/apl/74/1/10.1063/1.122967

http://www.sciencedirect.com/science/article/pii/S0040609004016281

http://scitation.aip.org/content/aip/journal/jap/107/3/10.1063/1.3277043

http://www.sciencedirect.com/science/article/pii/S0040609006000472

http://onlinelibrary.wiley.com/doi/10.1002/pip.784/abstract

http://onlinelibrary.wiley.com/doi/10.1002/pip.518/abstract

Dear Ashraful,

I am not too familiar with organic solar cells, but ideality factors greater than 2 are also often measured for other solar cell types.  In the case of thin film solar cells possible reasons for those high values might be:

• Tunneling enhanced interface recombination, see

http://scitation.aip.org/content/aip/journal/apl/74/1/10.1063/1.122967

• Superposition of 1-diode-IV curve by effects like rollover or IV-kinks, which change the IV-slope and therefore the determined ideality factor. In this case, the calculated ideality factor is systematically too high and cannot be used for further analysis.

Examples for those distortions can be found in

http://www.sciencedirect.com/science/article/pii/S0040609004016281

http://scitation.aip.org/content/aip/journal/jap/107/3/10.1063/1.3277043

http://www.sciencedirect.com/science/article/pii/S0040609006000472

• In the case of solar cells with large areas distributed series resistance effects lead to an apparent high ideality factor, which depends on the illumination density (the impact of the distributed series resistance decreases with decreasing illumination density) => Check, if n decreases with illumination density

See for example:

http://onlinelibrary.wiley.com/doi/10.1002/pip.784/abstract

• Voltage dependent carrier collection might also superimpose the IV curve slope, this will again lead to apparent illumination dependent ideality factors.

Example:

http://onlinelibrary.wiley.com/doi/10.1002/pip.518/abstract

I hope this might be helpful even if it is not directly connected to organic solar cells.

Stefan

http://scitation.aip.org/content/aip/journal/apl/74/1/10.1063/1.122967

http://www.sciencedirect.com/science/article/pii/S0040609004016281

http://scitation.aip.org/content/aip/journal/jap/107/3/10.1063/1.3277043

http://www.sciencedirect.com/science/article/pii/S0040609006000472

http://onlinelibrary.wiley.com/doi/10.1002/pip.784/abstract

http://onlinelibrary.wiley.com/doi/10.1002/pip.518/abstract

Thank you Mr. Stefan for your nice and detailed answer. Going through some research paper I have understood that Is depends on carrier recombination and n on the location of recombination.As recombination increases both Is and n increases.thus Rs increases and Rsh decreases.overall FF and PCE degrades. Hope I am thinking in the right way ( If wrong please clarify). I am searching for a paper that relates the PCE analysis with parameters like n,Is,Rsh,Rs etc.

Thanks in Advance

For well behaving solar cells (i.e. without rollover, kinks, too strong voltage dependent carrier collection, no distributed series resistance effects) n depends on the dominating recombination path, correct. A very good overview is given by Scheer and Schock in their book "Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices".

I0 increases with increasing recombination rate (hence often correlated to lower minorty carrier lifetimes, see e.g. http://scitation.aip.org/content/aip/journal/apl/73/9/10.1063/1.122134). But Voc and therefore I0 depends also on other influences as for instance net doping (e.g. http://www.sciencedirect.com/science/article/pii/S0927024813004376)

You cannot completely seperate between n and I0, they are connected by the open circuit voltage. For a fixed Voc lower n and lower I0 lead to higher fill factors (see file influence_I0.PNG).

Rs and Rsh are (except for distributed series resistance effects) considerd to be indepent of I0 and n. They influence both FF and might affect the determination of n and I0. But the "real" values of n and I0 have different physical origins compared to Rs and Rsh. For the influence of Rs and Rsh, see the file influence_Rs_Rp.PNG.

But at least for thin film solar cells a really detailed analysis of n and I0 make often only sense for solar cells with efficiency above 8-10%. Solar cells with lower efficiency behave often very differently from the ideal 1-diode model and parameter discussion might lead to wrong assumptions.

In Summary:

For high PCE, you need low Rs, low n, low I0 and high RSh, high IPh.

http://www.amazon.com/Chalcogenide-Photovoltaics-Physics-Technologies-Devices/dp/3527314598/ref=sr_1_1?ie=UTF8&qid=1415643438&sr=8-1&keywords=scheer+schock

http://scitation.aip.org/content/aip/journal/apl/73/9/10.1063/1.122134

http://www.sciencedirect.com/science/article/pii/S0927024813004376

Dear Ashraful Haiser,

I did not work with organic solar cells but from conceptual point of view they operate similarly but with different material parameters.The organic semiconductors  have low mobility because of their molecular nature. The solar cell is composed of a pn junction or a hetero junction and two collecting metallic electrodes on the p- and n- sides. The two collecting electrodes are metal semiconductor interfaces MS contact that must behave ohmic with low resistance.Therefore, a solar cell has in fact three junctions; one core junction and two MS contacts.

The core junction, assuming p-n one has an ideality factor between one and two as the current may be dominated by recombination in the neutral regions outside the space charge region in case of n=1 and n=2 for the recombination inside the space charge region. The two types can be competing leading to 1=2.

If the two MS contacts neighboring the core junction are low ohmic then one gets n between one and two. If the MS contacts behaves as nonlinear appreciable series resistance the the ideality factor will be greater than two and its specific value depends on the i-v characteristics of the MS contacts. In some cases when the i-v dominated by the M contacts the ideality factor may reach large values.

Conversely high ideality factor is a symptom of bad high resistive MS contacts and one has to get a solution for this problem by choosing the proper electrodes.

I spent many time investigating these phenomenon for metallic solar cells. The same can be mapped on the organic solar cells.

The solar cell model used to describe the I-V characteristics characteristic is the same.

This is for the ideality factor.

For PEC dependence on the solar cell parameters there will be separate post ISA.

wish you success.

Solar cells are in fact large area semiconductor diodes. Due to photovoltaic effect energy of light (energy of photons) converts into electrical current. At p-n junction, an electric field is built up which leads to the separation of the charge carriers (electrons and holes). At incidence of photon stream onto semiconductor material the electrons are released, if the energy of photons is sufficient. Contact to a solar cell is realised due to metal contacts. If the circuit is closed, meaning an electrical load is connected, then direct current flows. The energy of photons comes in "packages" which are called quants. The energy of each quantum depends on the wavelength of the visible light or electromagnetic waves. The electrons are released, however, the electric current flows only if the energy of each quantum is greater than WL - WV (boundaries of valence and conductive bands). 

Samples of solar cell I-V and power characteristics are presented on pictures below. Typical point on solar cell characteristics are open cirquit (when no load is connected), short cirquit and maximum power point. Presented characteristics were calculated for solar cell with following data: Voc = 0,595 mV, Isc = 4,6 A, IMPP = 4,25 A, VMPP = 0,51 V, and PMPP temperature coefficient γ = -0,005 %/K. Calculation algorithm presented in the book Photovoltaik Engineering (Wagner, see sources) was used.