I am working on perovskite solar cells and I am interested in finding the traps using Capacitance-Voltage and Capacitance-Frequency. But I don't know how to choose the frequency for CV measurements. In most of the papers people just say that they have used 10KHz frequency and a small input voltage of 30mV. But, they never explained in the papers why they are using that values. So, is it trivial? or Can anyone explain/suggest me some papers where this has been well discussed.
Sathy,
welcome,
We measured the c-v and the g-v properties of a heterojunction solar cell at different frequencies ranging from 10 kHz to one MHz in steps. This range was sufficient to characterize the electronic processes in the junction.
The concept is that the structure at reverse bias do not behave an ideal capacitor. The junction capacitance has a series resistance and a leakage shunting resistance. In very low frequencies the shunting resistance will be dominant, at very high frequencies the series Resistance will be dominant ,
At the intermediate frequencies, the junction capacitance nay be dominant.
Our study is comprehensive and will lead you to achieve your target.
Please for the paper follow the link: Article Capacitance and conductance of ZnxCd1-xS/ZnTe heterojunctions
Best wishes
As Prof Zekry well states, you are examining an electronic entity that does NOT show conventional properties. It will have constant phase elements and Warburg impedances as well. Refer to Dr Bisquert et al at the University Jaimes Salinas for his work on such circuit elements. You will need to scan your circuit from very low frequency to high, as Prof Zekry states. I suggest you might have to go a lot lower than 10 kHz - but I am not an expert in perovskite cells. In DSSC you need to go well below 1 Hz.
Dear zekry, I really appreciate for your time and valuable answer. I've read the paper which you have sent. From that what I got is that you have to choose a frequency which doesnot depend on the junction capacitance. In your paper, you are modelling and discussing about the junction capacitance of only one junction but, in my case I have multiple junctions. So, Iam wondering how does that effect?
In some other paper, they are measuring the phase versus frequency of the device at various bias voltages and said that at 10KHz the angle is 12 degrees and any measurement below 10 degrees shows error in measurement. Hence, you have to consider the phase of slightly higher than 10 degrees. Is that correct? But this is in contrast with what Tony Maine has mentioned.
In yet another paper they are measuring the impedance spectroscopy and fitting with semi circle. They are considering the frequency where it deviates with the semicircle. Shown in the attached figure. So, I am really confused on to which method should I choose.
Sathy, Your answer will depend on your measurement method and the frequency chosen. So - ask yourself carefully what you are really trying to discover.These cells are complex structures and you will not be able to define a method that yields a consistent answer for cells of different internal chemistries. So - WHY do you need to do WHICH measurement and WHAT practically useful result do you want from it?
Dear Tony Maine, thanks for your answer. I interested in finding the built in potential using the CV. Could you please tell me what frequency should I use for this measurement.
Dear Sathy,
I would like to thank the respected colleague Dr. Tony for referring to my answer. You have to assume some junction model to your perovskite solar cells. Then you have to work out the theory to extract the built in potential.
Conceptually the built in potential can be obtained from the extrapolation of 1/c^2 versus the reverse voltage. It is assumed that only that one junction is active and its depletion region widens with applied reverse bias voltage. Normally one constructs the solar cell such that metal contacts to the solar ell behave ohmic, otherwise the the structure will not operate properly.
In order to determine a suitable operating frequency, you can bias the solar cell at zero voltage, and then measure the equivalent capacitance and resistance as a function frequency. Then you can choose any frequency in the range where the capacitance is independent of frequency.
Best wihses
I worked on C-V measurement and I used V amplitude ~50 mV with frequency 1KHz in OPV devices. In regards, C-f the frequency range 10 Hz-1 MHz.
Regards
Dear zekry, when you said equivalent capacitance and resistance. Do you mean to say that I should do an impedance measurement to determine the equivalent capacitance and resistance? But the equivalent capacitance and resistance you get from the impedance spectrum fitting is just a discrete value and doesnot depend on the frequencies right? or is there any other way to find the equivalent capacitance and resistance as a function of frequency.
Yes you do an impedance measurement as a function of frequency while keeping the bias of the solar cell at zero voltage for example. Then you calculate c from c= 1/x w, where x is the reactance of the impedance and w is the angular frequency. When you plot c versus frequency you will get the range where c is independent of frequency. Choose one frequency well inside this range and measure the c-V curve.
You can also share your c versus f curve with us and we help you to choose a suitable frequency for measuring c-V curve.
Best wishes
In order to determine the contact difference of potential, you have to measure your C-V at low frequencies in reverse biasing not forward biasing as you did in your displayed curve.
Best wishes
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