Photoluminescence analysis of Post Deposition
Treated CIGSe absorber
Ultrafast Nanoscale Dynamics (UND), Institute of Physics, University of Oldenburg,
D-26111 Oldenburg, Germany
Author: Ojoma Abamu
Supervisors: Dr. Stephan Heise and
M.Sc. Ashwin Hariharan
Introduction
Crystalline Silicon-based material has dominated the PV industry for almost the last 5 decades. But due to the poor absorption coefficient, the absorber has to be made significantly thicker to gain
more photo-current. CIGS-based solar cell offers a significant advantage over c-Si in the fact that the absorber can be made much thinner and consequently the production time and cost are lower.
Researchers are constantly trying to push the efficiency limits of CIGS solar cells through various processing methods. One such method is the alkali based PDT. In order to understand the effects of PDT
on the device, it is critical to understand its effect on the absorber material.
Thus we use room temperature steady-state PL spectroscopy as a tool to probe the effects of PDT on CdS/CIGSe system, and the spectrum is analysed with the use of the
generalized planck‘s equation. At room temperature the quasi fermi level splitting and absorption coefficient of the material would be determined. We would see if there is any
significant improvement in the device when the Rb based PDT is made.
Experimental Procedure
The PL spectra is measured at 300K at an excitation wavelength of 620nm. The laser
beam is focused to a spot of 400µm in diameter. The beam intensity is modulated using
a mechanical chopper, in which the reference frequency is fed into a lock-in amplifier.
The beam hits the absorber sample and the emitted light is passed through a lens into
a monochromator through an entrance slit. The luminescence is scanned over the
wavelength range of 950nm to 1400nm by a grating and dispersed signal is fed into the
detector through an exit slit and the corresponding signal is recorded in the wavelength
bins.
Figures
Fig. 1: Room temperature PL spectrum of CIGS devices
Fig. 2: Determination of QFLS from caliberated room
temperature PL
Fig. 3: Absorptivity determined from the calibrated PL
measurements
Fig. 4: Absorption coefficient determined from the absorptivity
Method
The luminescence signal is recorded in bins of wavelength. For a quantitative analysis
of the PL spectra, noting the wavelength vs intensity is insufficient. The wavelength bin
of the signal needs to be converted into the energy bin which proceeds via the familiar
𝒉𝒄
𝑬=
(1)
𝝀
Due to the inverse relationship between energy and wavelength, the intervals d𝛌 in the
wavelength spectrum is not evenly sized across the energy spectrum. If the recorded
signal is considered some function f(𝛌), then from conservation of energy 𝒇 𝑬 𝒅𝑬 =
𝒇 𝝀 𝒅, combining both equations gives the correct scaling for the energy value. [1]
𝒇 𝝀 =𝒇 𝑬
𝒅𝑬
𝒅𝝀
= 𝒇 𝑬
𝒅
𝒅𝝀
𝒉𝒄
𝝀
= 𝒇 𝑬
𝒉𝒄
𝝀𝟐
(2)
If the magnitude of the luminescence photon flux is available, the QFLS (µ), of the
material can be derived from the spectral shape of the PL signal. The equation is known
as the generalized Planck‘s law:
𝒀𝒇𝒍𝒖𝒙 (E)
𝟏
= 𝟒𝝅𝟐ħ𝟑𝒄𝟐
𝑨 𝑬 𝑬𝟐
(3)
𝑬−∆𝝁
𝒆𝒙𝒑 𝒌 𝑻 −𝟏
𝑩
Where Δµ represents the quasi fermi level splitting (QFLS). A(E) is the absorptivity
which is given by 𝐴 𝐸 = 1 − 𝑅𝑓 (1 − 𝑒𝑥 𝑝( − 𝛼 𝐸 𝑑
with Rf being the surface
reflectance and d is the thickness of the layer.
If you take the natural logarithm of both sides, Eq. 3 can be rewritten as
𝒍𝒏
𝒀
𝑪.𝑬𝟐
= 𝒍𝒏 𝑨 𝑬
−
𝑬 −𝛥µ
𝒌𝑻
(4)
Where 𝑪 =
𝟏
𝟒𝝅𝟐 ħ𝟑 𝒄𝟐
The absolute QFLS is estimated by plotting the quantity ln(Y/C/𝐸 𝟐 ) vs photon energy
using Eq. 4. From the PL spectrum, the absorptivity in the region above the band gap
(E>Eg) is approximated to be constant A(E)≈1, assuming an absorber layer with
homogeneous phase composition throughout the material. This value is used to extract
µ from the linear fit of the high energy wing of the curve (Fig. 2). Once µ is determined,
the value is put back into Eq. 3 and the spectral absorptivity for the lower energy wing
(E<Eg) can be extracted. If the temperature isn’t known, one can simply extract it from
the slope, m, of the linear fit [T= (1/kB/m)]. [2][3]
The energy dependent absorptivity, A(E) can be determined by plugging in the derived
values of temperature and µ back into Eq. 3, and the absorption coefficient α E can be
derived from the absorptivity.
𝑨 𝑬 = 𝟏 − 𝒆𝒙 𝒑 −𝜶 𝑬 𝒅
𝜶 𝑬 =
𝟏
−
𝒅
∙ 𝒍𝒐 𝒈( 𝟏 − 𝑨 𝑬
(5)
(6)
Fig. 5: Tauc plot of sample with No PDT
Fig 6: Tauc plot of sample with PDT
Results
From the experiments, it was observed that the addition of Rb based PDT on CIGS
absorbers lower the PL peak by about ~10 meV compared to the sample with no PDT
(Fig.1). It was expected that the QFLS would follow in a similar pattern. But, to our
surprise, the QFLS of the PDT sample was higher than that with no PDT (Fig. 2).
There could be two possible reason for this: a) From CV measurements, the doping
value of non-PDT samples found to be around 2e16 cm-3. The corresponding
increase in doping to compensate for the bandgap decrease is around 5.5e16 cm-3.
But our CV measurement only measured around 3e16 for samples with PDT. Thus
conclusive effects of QFLS increase due to PDT not fully visible. b) The other reason
could simply be that our error bar in the fitting is also around 10 meV this making the
difference resolution very difficult. A detailed error analysis of the PL setup was
beyond the scope of this project.
Absorbance curve and Tauc plots further indicate a marginal decrease in bandgap of
PDT samples.
Conclusion
• Steady-state PL measurements were carried on CIGS absorbers to study the effects
of PDT. The results were analyzed using the generalized Planck’s law.
• The experiment showed a decrease in optical bandgap for PDT based sample by
about ~10 meV. QFLS was shown to increase by ~10 meV.
• Possible reasons for this contradicting behavior were provided based on
complimentary measurements and data analysis point of view.
• Thus a fully conclusive possible positive effect of Rb-PDT on CIGS absorber was not
able to be provided. But further measurements with additional emphasis on data
analysis could provide a more conclusive result.
References
[1] The Journal of Physical Chemistry Letters 2013, 4, 3316-3318
[2] Charakterisierung opto-elektronischer Eingeschaften mit lateraler Sub-Mikrometer-Auflösung: Dr. Levent Gütay
[3] Advanced Characterization of Thin Film Solar Cells: Dr. Daniel Abou‐Ras, Dr. Thomas Kirchartz, Prof. Dr. Uwe Rau (P 151-170)
[4] Optoelectronic Characterization of Thin-Film Solar Cells by Electroreflectance and Luminescence Spectroscopy: Dipl.-Phys. Christoph Daniel Krämmer
[5] IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 8, NO. 5, SEPTEMBER 2018 pages 1320