Thermal admittance spectroscopy is a technique for the measurement of deep trap levels within pn junctions. By measuring the small-signal ac admittance of the junction under different conditions, e.g., with small-signal frequency and sample temperature as parameters, one can extract the density of states, activation energy, and capture cross sections of the traps. According to the applied dc voltage, zeroand nonzero-bias admittance spectroscopy are distinguished.

Source : Admittance spectroscopy of efficient thin film solar cells Joachim Kneisel, Kai Siemer, Ilka Luck, and Dieter Bräunig

An introduction[1], about the basics, could be found in some of the related companies, gamry[2-4], bio-logic[5], etc., as well as some materialized views and cases for the technique's implementations in journals[6-9].

1. Electrochemical Impedance Spectroscopy and its Applications http://pirg.ch.pw.edu.pl/instrukcje/eis_extensive_introduction.pdf

2. DSSC: Dye Sensitized Solar Cells https://www.gamry.com/application-notes/physechem/dssc-dye-sensitized-solar-cells/

3. Dye Solar Cells – Part 2: Impedance Measurements https://www.gamry.com/application-notes/physechem/dye-solar-cells-impedance-measurements/

4. Dye Solar Cells – Part 3: IMPS and IMVS Measurements https://www.gamry.com/application-notes/physechem/dye-solar-cells-imps-imvs/

5. Photovoltaic Characterizations https://www.bio-logic.net/wp-content/uploads/20101210-Application-note-24.pdf ; [https://www.bio-logic.net/references/solarphotovoltaics-cells/]

6. Electrodeposited tin selenide thin films for photovoltaic applications https://www.sciencedirect.com/science/article/pii/S0038092X11002155?via%3Dihub

7. Low temperature sintering of aqueous TiO2 colloids for flexible, co-sensitized dye-sensitized solar cells https://www.sciencedirect.com/science/article/pii/S0167577X18316975?via%3Dihub

8. see Figure 9 in: New D–D′–A Configured Dye for Efficient Dye-Sensitized Solar Cells https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.8b05477

9. Using EIS for diagnosis of dye-sensitized solar cells performance Article Using EIS for diagnosis of dye-sensitized solar cells performance

Dear Muhammad Awais,

Complementary to defect spectroscopy by means of photoluminescence, the admittance spectroscopy is a convenient electrical characterization method to investigate the complete and finished solar cell.

In homogeneously distributed shallow doping, the doping concentration defines the width of the depletion region of the p/n-junction. In this situation the thin film solar cell is equivalent to a plate capacitor of the area A (cell area), with a plate distance of W (extension depletion region) and causes a capacitance value C = εε A/W whereas the dielectric constant ε is a property of the investigated semiconductor used in the solar cell. By measuring the capacitance value in dependence of an applied dc bias sweep, a spatial resolved defect distribution can be extracted and the depletion width can be estimated.

The energetic position of defect levels in the band gap, their capture cross-section and defect concentration can be derived by measuring the admittance of the solar cell for a wide temperature range (40K – 320K) and also in dependence of a varied applied frequency of the alternating current.

Application of Admittance spectroscopy:-

Admittance spectroscopy is used to characterize majority- carrier trapping defects. Defects are undoubtedly among the most critical physical issues to address for the photovoltaic (PV) industry.

Admittance spectroscopy, among other capacitance based techniques such as capacitance-voltage and deep level transient spectroscopy, is used to characterize majority-carrier trapping defects in PV materials and devices.

Admittance spectroscopy inspects the current response of a device to small AC bias voltage modulation and its dependence on frequency and temperature, presumably due to the capture and emission of the electrically active defects. In a simple single-junction device, it is possible to extract defect parameters such as activation energy, capture cross-section, and density of states.

Application of admittance spectroscopy to deep levels in the absorber of a typical PV device usually assumes that several conditions are satisfied:

01. Absorber is conductive enough and the dielectric relaxation frequency of bulk absorber material is out of the range of interest;

02. Only one junction is present and both contacts are ohmic;

03. Only majority-carrier traps are observed; and

04. The junction is in reverse bias. With these simplifications, it is safe to consider that admittance spectroscopy reflects the behavior of majority-carrier trapping defects. However, the complex nature of most practical PV devices means that one or more of these conditions may often be violated in the admittance measurement.

Under most circumstances, admittance spectroscopy measures contributions from the response of majority carriers. Admittance spectroscopy is usually carried out with the device in reverse bias to avoid interference from the diffusion capacitance. If the diffusion capacitance cannot be neglected, then the basic physical assumptions of capacitance-voltage technique and admittance spectroscopy technique are violated.

For your reference and better understanding few Research Articles are mentioned below:

01. Article Admittance spectroscopy of thin film solar cells

02. Article Admittance spectroscopy of efficient CuInS2 thin film solar cells

03. Article Buffer Layers, Defects, and the Capacitance Step in the Admi...

04. Article Admittance spectroscopy of CdTe-based solar cells

05. Article Admittance spectroscopy characterize graphite paste for back...

I hope I have answered your question.

With Best Wishes,

Samir G. Pandya

There is the impedance spectroscopy and the admittance spectroscopy where one bias the solar cell at specific DC operating condition (I,V) and then measure the small signal impedance of the solar cell diode or its admittance as a function of frequency. Then one plots the complex impedance in the impedance diagram or the admittance in the admittance dungaree, where one plots imaginary part versus the real part. Normally one gets half circles for every interface.

The method is based on the small signal equivalent circuit of a junction diode where it is equivalent to a conductance in parallel with a capacitance. Normally they are independent of frequency. In addition to a bulk series resistance in series.

By drawing the x versus r one gets a semicircle.

One uses this plot to characterize the junction.

For more information please follow the original paper:Article A distributed SPICE-model of a solar cell

Best wishes