Dear Warnatha,

Welcome!

The ideality factor n is obtained from the dark IV characteristics. Its definition stems from the equation you wrote for dark current in the diode:

Id = I0( exp(q(V-I*Rs)/n*KB*T)-1,

Taking the part of the IV curve where the I RS drop is neglected and then taking the logarithm, it follows that :n= dV/ Vt (ln I2/ I1), where dV is the voltage difference between I2. So n is proportional to the inverse of the ln I versus V curve of the dark characteristics.This is the formal definition.

The detailed theory of the pn junction diode gives an ideality factor of one for if the current is limited by the recombination of minority carriers in the neutral region and n=2 fro the recombination current in the space charge region.

If the two effects are competing then one finds that n lies between one and 2 as hinted by Ahmed. 

However some types of solar cells shows an ideality factor greater than 2 and even may be as high as five. 

There are several reasons for such behavior.

- In hetero junction solar cells, the recombination at the interface of the heterojunction may have such high n value. I studied this effect and explained it in the paper given in the link:https://www.researchgate.net/publication/232710742_M._Abdelnaby_A._Zekry_F._Elakkad_and_H._F._Ragaie__Dependence_of_dark_current_on_zine_concentration_in_Znx_Cd1-x_SZn_Te_hetero__junctions_Solar_Energy_Materials_and_Solar_Cells_29_%281993%29

- the other major effect is the metal semiconductor contacts which may be nonohmic and has a relatively high resistance. I studied also this effect in the paper at the link:

https://www.researchgate.net/post/The_effect_of_MS_contacts_on_solar_cell_performance

In fact the model of the solar cell may be better represented by two diodes in parallel which are connected to an metal semiconductor diode representing the metal contacts. In this case n for a single diode model will be merely a fitting parameter.

Best wishes.

Article M. Abdelnaby, A. Zekry, F. Elakkad and H. F. Ragaie, “ Depen...

 Dear Prof. Tayyan  and Prof. Zekry ,

I thank you both for your detailed responses and the literature. 

I have only 2 JV curves (dark and light) taken at room temperature. I'm using these to extract the five parameters n, Rs, Rsh, J0, Jph.  I was wondering whether I could get the local ideality factor information from these alone. The curve fitting approach assign a constant for n, so I was wondering what approach is used for getting variable local ideality factor from JV data.

Dear Waranatha,

If you display your IV curves, one would have better insight on their shape.

Irrespective of that, my experience is that the ideality factor can be cosidered constant to large extent and RS is the parameter that changes with the current density in the cell. In order to experimentally verify that you need only to measure The short circuit current as function of the open circuit voltage at different illumination level as i did in the paper of the MS contact to get a physical insight in the real value of the ideality factor. 

However if you want to make curve fitting by using variable n, i would propose that you

divide your iv curve into sectors and fitting very sector with a different n keeping the other parameters fixed as you desire.

I am ready to discuss any matter with you.

Best wishes. 

Prof. Tayyan,

Thank you for the link. Actually going through the material there gave me the idea to see whether it is possible to extract the local ideality factor from my data.

Prof. Zekry,

Thank you for your detailed response. I have fabricated several devices sometime back and have only light characteristics for some and have both dark and light characteristics for the others. (only one illumination intensity). I tried to fit these data to the single diode model, and in some cases the fit is not perfect. so I was checking the alternative models. I wanted to check out the two options, the double diode model and the single diode model with variable ideality factor and compare the data fit. I have attached two of my typical JV curves. curve1 is where I get a very good fit, and curve2 is a typical curve where single diode model fit is not very good.

My main limitation is I don't have the option to do more experiments and  collect more data now. So my task is getting the maximum out of the current data set.

I would definitely try to extract the local ideality factor by fitting data by sectors. Thank you for your suggestion.

Dear Waranatha,

The upper iv curve points out to the presence of a relatively small shunting resistance because of the high leakage current in the reverse and zero bias. Also, the series resistance may be also, relatively large which is clear from the inverse of the slope at the the open circuit point. So the range of the dark diode current of the equivalent circuit is relatively small. For such smaller diode current range, one may achieve satisfactory fitting with single diode.

In the lower sample which is much better than the upper one since its iv curve approaches the typical illuminated solar cell curve, the diode current range is greater that the upper and hence it is more probable that there are more than one current controlling mechanism. This is also apparent from the less good fit with one diode model. So, the use of the two diode model is justified.

May be helpful to look at the link:https://www.researchgate.net/post/Can_I_use_the_three_diode_model_to_simulate_the_physical_behavior_of_any_pv_module

wish you success

http://link.springer.com/article/10.1007/s11082-016-0541-y

Dear all,

Are above equations and models also applicable for ferroelectric photovoltaic devices , such as BiFeO3 based switchable photo-diode ??

I am attaching one reference article for you. Please help me in this regard. thanks.

With the help of efficient ACT technique and its software application, which can be freely downloaded from the paper below, you can determine the accurate value of ideality factor and other parameters. n defines the type and process of recombination within the active layer of the devices.

Article Simple and efficient estimation of photovoltaic cells and mo...

Below you can also find detailed discussion about n and other parameters

Article Employment of single-diode model to elucidate the variations...

Dear Waranatha L Abeygunasekara I appreciate for your Exponential Relation for

Si - Solar cells . There are so many simulative approach to vary n of all researchers are given here . But being a non specialized person will you please

explain me what will be the general broad spectrum application to vary this parameter n .

It is basically interface defect associated. If the defects are more the ideality factor must be greater that can usually found in organic-inorganic heterojunction cells. However, inorganic cell it is a bit lower.

you can make use of this paper

What is the final equation of solar cell with two diode?

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwij5IuVv532AhVvzTgGHap5DSkQFnoECDwQAQ&url=https%3A%2F%2Fwww.researchgate.net%2Fpost%2FIdeality_factor_for_diodes&usg=AOvVaw392oURIdyQL8rNblogHlmj