1. An optical technique for measuring surface recombination velocity
Sol. Energy Mater. Sol. Cells (2009)
An optical technique for measuring surface recombination velocity
R.K. Ahrenkiel, S.W. Johnston
National Renewable Energy Laboratory, Golden, CO, USA
Colorado School of Mines, Metallurgical and Materials Engineering, 1500 Illinois Street, Hill Hall, Room 309, Golden, CO 80401, USA
Speaker: Feng-Yu Chang
Advisor: Peichen Yu
Green Photonics Laboratory
Display Institute
National Chiao Tung University

2. Outline Introduction Theoretical analysis Calculation results
Experimental results
Data fitting of the modal function
Conclusions
Green Photonics Laboratory
2019/2/16

3. Introduction
The surface recombination velocity is a critical parameter in silicon device applications including solar cells.
To devolope an optical technique to provide quick, contactless measurement of the surface recombination velocity.
A curve fitting procedure provides a determination for both the bulk recombination lifetime and the surface recombination velocity.
Green Photonics Laboratory
2019/2/16

4. Theoretical analysis Excess carrier density as the Fourier modes
The total excess carrier concentration as a function of time
Green Photonics Laboratory
2019/2/16

5. Background Instantaneous lifetime (surface lifetime)
Steady-state lifetime
The excess carrier density is related to the conductivity by
Green Photonics Laboratory
2019/2/16

6. Calculation results
The calculated excess-carrier profiles at various times in the diffusion/decay process
The integrated excess-charge distribution as a function of time for a wide range of surface recombination velocities at the front and the back surfaces
The accelerated recombination rate is clearly
the result of the large excess-carrier density at the incident
surface. The charge distribution becomes symmetrical about the
midpoint of the wafer between 10 and 50 ms. The time required for
the profile to become symmetrical depends strongly on the
Diffusion coefficient, D.
We see from the figure that the
largest changes in the decay curve occur for S-values between 1
and 100 cm/s. For values of S larger than 1000 cm/s, the decay
curves overlap and are nearly independent of S. In practice, we can
only hope to accurately measure S-values in this range.
Green Photonics Laboratory
2019/2/16

7. Calculation results
The calculated photoconductive decay with the same transport parameters and S equals to 500 cm/s & 1 cm/s, and the excitation wavelengths are 750, 900, and 1000 nm.
When the surface is unpassivated. The decay curve is distinct in that a very fast,
initial decay occurs because of the high density of excess electrons at the front surface
Our calculations
show that the decay curves for excitation at 500, 600, and 700 nm
overlap. Excitation at 750 nm produces a slight shift in the decay
Curve from the latter wavelengths as shown in the figure We have found that this wavelength range is the most valuable for
the experimental determination of the surface recombination
velocity.
e. Similar behavior has been
observed in the laboratory for wafers immersed in iodine–metha-
nol solution. In this case, the character of the decay curve is
virtually independent of the wavelength. The slope of the decay in
this case is controlled by the bulk lifetime.
Green Photonics Laboratory
2019/2/16

8. Measurement technique
The techniques in our laboratory, used to measure the transient photoconductive decay, are resonant-coupled and microwave photoconductive decay (RCPCD and PCD)
The pulsed light is provided by an optical parametric oscillator (OPO) driven by a tripled YAG laser. The duration of the OPO pulse is about 5ns, and the repetition rate is 10Hz.
Green Photonics Laboratory
2019/2/16

9. Experimental results
The photoconductive decay data of an unpassivated, 1000
cm n-type float zone wafer as measured by the RCPCD
The same sample after an HF etch and during immersion in iodine–methanol solution
The decay curves are nearly identical in shape and are independent of the excitation wavelength.
We call this initial decay process as the surface lifetime
The surface lifetime effects are apparent
for wavelengths less than 1000 nm. These results are in quanti-
tative agreement with the simulation results of Fig. 3. The surface
lifetime increases with the increasing excitation wavelength as
predicted in Eq. (1). H
Our calculation assumes a constant bulk lifetime. Because of
the SRH filling effect, the model developed here is not completely
descriptive of the presented data
Green Photonics Laboratory
2019/2/16

10. Data fitting of the modal function
The RCPCD data measured at 900 nm and the fit of Eq. to the data
The data fit produced the S, , and D values that are shown in the figure
The data contained 1000 points and an infinite
series of Eq. (4), which was terminated after twenty terms(n ¼ 20).
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2019/2/16

11. Conclusions
The initial decay time was greatly reduced for wavelengths shorter than 1000 nm when the surface recombination velocity was larger than 10 cm/s.
Using a nonlinear least-squares technique, we have successfully fit the transient data using bulk lifetime, surface recombination velocity, and ambipolar diffusion coefficient as the parameters.
Green Photonics Laboratory
2019/2/16

12. Thanks for all your attention!