Dear Idris Bouchama ,

welcome!

May be the deep level transient spectroscopy is one of the effective methods to measure the energy levels of the defects.

However to measure the effectiveness of these levels as a recombination centers I would advise you to measure the minority carrier lifetime Tau. It is really very sensitive to the density of the recombination centers and their capture cross section. Also, only the defect levels laying in and around the middle of the energy gap are effective recombination centers.

One of the staight forward method to assess the minority carrier lifetime is the open circuit voltage decay. The other method is the reverse recovery time when reverse biasing the cell after a forward bias.

I an astonishing how these elegant methods are neglected in characterizing the solar ells.

Also, the parameter which is sensitive to the minority carrier lifetime is the reverse saturation current. It is inversely proportional to the minority carrier lifetime.

I would like that you see the paper in the link:Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells

For the open circuit voltage decay method please refer to the paper:Article A. Zekry and G. El-Dllal, “ effect of MS contact on the elec...

Best wishes

Hi Idris, indeed defects and the levels they cause in the bandage energy of a pv absorber can be very detrimental to solar cell efficiency. Artur mentioned some methods but unfortunately these are not sensitive enough to detect concentrations in the order of 1e10-1e13/cm3, in term of defects, which can already lower performance substantially. The best known technique to do so is called DLTS. I think your best starting reference to learn about it is Semiconductor Material and Device Characterization (Wiley - IEEE) 3rd by Dieter K. Schroder . I just found it available in http://www.optima.ufam.edu.br/SemPhys/Downloads/Dieter.pdf

Check out Chapter 5 and 6. For example this plot for deciding what sort of impedance/admittance spectroscopy method to use.

Reference 20-38 in Ch5 are all pretty good. Specially for example, Anthony. R. Peaker is still actively working in the subject (ref 38, probably one of the initial papers on it).

After reading this, I think the next best reference is the book of Nicoliand and Brews: https://www.amazon.com/Metal-Oxide-Semiconductor-Physics-Technology/dp/047143079X

I hope this helps.

Sebastian

Dear Idris,

Please consider to use photoluminescence (PL) spectroscopy. Then, fit the PL spectra using Gaussian fit peaks, which could be related to transitions involving defect states.

Best regards,

SM

I tend to agree with the reply above, on the DLTS. I woud also add ADSPEC, for shallow levels.

One important thing to mention is that DLTS does not give you any information on the nature of the electrically active defects. For this, you may need to

1)perform irradiation/implantation studies

2)complement DLTS with PAS, EPR etc.

3)take a look at DFT studies.

Dear colleague Dr. Idris

Have a nice time and good day.

the following discussion may help you. if some symbols not clear please open my own attached files.

Good luck.

A. El-Denglawey Ph.D, Full Prof.

This can be partially explained using the density of states model in amorphous solids proposed by Mott and Davis [N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1979,p. 210, and M. Dongol, M. Abou Zied, G.A. Gamal, A. EI-Denglawey, Physica B 353 (2004) 169–175, A. EI-Denglawey, Journal of Luminescence 194 (2018) 381–386].

According to this model, the width of the localized states near the mobility edge depends on the degree of disorder and on the defects present in the amorphous structure. In particular, it is known that unsaturated bonds are responsible for the formation of some defects in amorphous solids. Such defects produce localized states in the band structure.

The presence of a high concentration of localized states is responsible for relatively

low values of the optical gap in the case of as-deposited amorphous films. In the process of heat treatment, the unsaturated defects are gradually annealed, producing a large number of structure bonds. The reduction of the number of unsaturated defects reduces the density of localized states in the band structure and consequently increases the optical gap.

Amorphous nature and localized states can distinguished using X-Ray Diffraction (XRD), Scanning electron microscope(SEM) and Transmission electron microscope (TEM) and optical gap and Urbach tail as well.

First: There are four techniques; three related to structure measurement and the

other for optical measurement.

Using x-ray diffraction (XRD), Scanning electron microscope(SEM) and

Transmission electron microscope (TEM).

1- XRD the produced curve of amorphous materials has a broad hump and no any sharp diffraction peaks, the presence of sharp diffraction peaks is an indication of crystalline material.

2- SEM of of amorphous material don't release any features and no particles can be distinguished otherwise crystalline material.

3- TEM of amorphous material contains a hallow (white) and no diffraction rings otherwise crystalline material. Through the SAED pattern by TEM if you have the lattice fringes then the sample has poly crystalline structure, the presence of spotlight is an indication of single crystals.

Second: Optically using Urbach tail and/or optical gap

The Urbach tail

The exponential tail occurring at the absorption of most amorphous semiconductors, could be interpreted as arising from transition between localized states, if the density of states in the deep tails varies exponentially with energy, such an edge is expected Davis et al[*] gave the following agreements as evidence against such an interpretation.

(1) The empirical relation for this tail can be represented as :

a = a0 exp {hν/Ec}

Ln (a) = Ln (a0) + hν/Ec

Y = C + X slope

The slope = 1/ Ec

The intercept = Ln (a0)

; a0 is a constant, and Ec band tail width

(2) Urbach edges occur in many semiconductors.

(3) The temperature dependence of the slopes frequently follows the above relation at least at high temperature.

The observed exponential edges suggest that the breading is dominated by the random internal electric fields due to either to the lack of long range order or the presence of defects.

[*] E. A. Davis and N. F. Mott, Phil. Mag. 22 (1970) 903.

Ec can be calculated from the plot of ln(alfa)-Y axes vs. (hv)-X-axes for the films represent a straight line, indicating that the absorption threshold characteristic an exponential absorption edge. The Urbach tail increases as the increasing of disorder and hence the localized states, which cause increasing of band tailing.

I wish this issue is useful.

yours

A. El-Denglawey Ph.D, Full Prof.

Thnak you Mr Ruy Sebastian Bonilla, Artur Braun , Shibghatullah Muhammady , Giovanni Alfieri and Mr A. El-Denglawey

One can easily depicts the defect states or oxygen vacancies using PL and XPS measurements.