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This manuscript presents a simple method for excess minority carriers’ lifetime measurement within the base region of p-n junction polycrystalline solar cell in transient mode. This work is an experimental transient 3-Dimensionnal study. The magnitude of the magnetic field B is varied from 0 mT to 0.045 mT. Indeed, the solar cell is illuminated by a stroboscopic flash with air mass 1.5 and under magnetic field in transient state. The experimental details are assumed in a figure. The procedure is outlined by the Open Circuit Voltage Decay analysis. Effective minority carrier life-time is calculated by fitting the linear zone of the transient voltage decay curve because linear decay is an ideal decay. The kaleidagraph software permits access to the slope of the curve which is inversely proportional to the lifetime. The external magnetic effects on minority carriers’ effective lifetime is then presented and analyzed. The analysis show s that the charge carrier ’ s effective lifetime decrease with the magnetic field increase.

The lifetime of minority carriers is an important parameter and its determination is essential to improve solar cells high efficiency. The dark and illuminated characteristics are affected by the charge carriers’ lifetime. This electronic parameter restricts the open-circuit voltage, short circuit courant and the net output power. Therefore, with the increasing interest in photovoltaic energy conversion, fresh attempts have been made in the recent past for accurate lifetime measurement. Suitability for the earlier methods for p-n junctions, e.g. open-circuit voltage decay [

Among all methods, we are chosen the open-circuit voltage decay method. This method is practice for our experimental conditions. However, two methods are used to induce the initial voltage: the conventional method [

In this study, firstly we present the experimental set-up. Secondly, the experimental conditions will be described and assumptions made will be presented. Thirdly, the method for charge carriers’ effective lifetime measurement will be explained and the results obtained will be presented and analyzed.

The experimental set-up is composed of: a mono-facial silicon solar cell manufactured by MOTCH INDUSTRY; a pulsed light source MINISTROB PHIWE, a digital oscilloscope TECKTRONIX model TDS 10013, a computer INTEL 586, a power supply 0 - 12V DC/6V-12V AC, a teslameter and Helmholtz Coil.

The experimental set-up is presented in

The experimental system principle operating is the same described by Sam et al. and R. Sam et al. [

The Kaleidagraph software which is a thoughtfully designed graphing and data analysis application for research scientists permit us to convert complex data obtained during experimentation into transient curves.

The transient response is obtained by disturbance of steady state. While exciting the solar cell, we carry it towards a state characterized by a balance between the phenomena of recombination and generation of pairs electron-hole [

Considering the identical optoelectronic properties of all the lot of grains, then the study of phenomena of generation, diffusion and recombination of the charge carriers in the solar cell during the transient state can be described a only one grain.

The calculation of transient voltage decay expression by using boundaries conditions and quasi-neutral base theory gives [

– For exponential zone

V ( t ) = V T F c ( k 1 , l ∗ 1 , μ 1 ) r exp ( − β ∗ ( t − T e ) ) (1)

This is a time dependent exponential decay function

– For linear zone

V ( t ) = V T ( − β ∗ ( t − T e ) + r ln ( 1 + F v ( k 1 , l ∗ 1 , μ 1 ) ) ) (2)

This is a linear function of time with a negative slope − V T β ∗

∂ δ ( x , y , z , t ) ∂ z | z = 0 = S f D ∗ δ ( x , y , z = 0 , t ) (3)

∂ δ ( x , y , z , t ) ∂ z | z = H = S b D ∗ δ ( x , y , z = H , t ) (4)

∂ δ ( x , y , z , t ) ∂ x | x = ± a = ± S g x D ∗ δ ( x = ± a , y , z , t ) (5)

∂ δ ( x , y , z , t ) ∂ y | y = ± b = ± S g y D ∗ δ ( x , y = ± b , z , t ) (6)

Equations (3)-(6) are boundaries conditions. S f , S b , S g are the recombination velocity of minority charge carriers respectively at surfaces z = 0, z = H and x = ±a (or y = ±b). a, b and H are the grain sizes as indicated in

D ∗ , L ∗ are also respectively the electrons diffusion coefficient and length diffusion of charge carriers

Our approach is based on the linear approximation of transient voltage decay because in low injection it permits to avoid impedances effects [

After identification of linear zone of the curve, we fit this zone and we get a linear regression line indicated in

In

| m | = V T τ e f f

where m is the slope of the transient voltage curve, V_{T} is the thermic voltage and τ e f f is the charge carrier effective lifetime.

After registration of transient voltage data on digital scope, we use kaleidagraph software to make simulations. Then, we get the curves of transient voltage decay for various magnetic field values under AM1.5 spectrum (Figures 6-13).

B (mT) | 0 | 0.01 | 0.015 | 0.02 | 0.03 | 0.035 | 0.04 | 0.045 |
---|---|---|---|---|---|---|---|---|

τ e f f L ( μs ) | 3.6 | 3.6 | 3.3 | 3.2 | 1 | 1 | 1 | 1 |

R_{E} | 0.9916 | 0.9927 | 0.9824 | 0.9625 | 0.9639 | 0.9727 | 0.9828 | 0.9404 |

R_{E} is correlation coefficient which show the quality of the fit.

By observations of the curves, we note two types of decay: linear decay and exponential decay. Also, we remark that for magnetic field values B ≥ 0.03 mT , the decay is very fast and the linear decay is major than exponential decay.

After the fitting of all the analyzed curves, we note that the exponential zone is minor than the linear zone. We got ideal types of open circuit transient voltage decay according to Dariwhal and Mahan conditions [

The analysis of the results show that the charge carriers’ effective lifetime decreases with the magnetic field. These results are in agreement with theoretical results obtained by Alain et al. [

In this manuscript, we have developed an experimental technic of measurement of minority charge carriers’ effective lifetime under various magnetic fields with air mass AM1.5. Our approach is based on the method of Open Circuit Transient Voltage Decay. From the experimental set-up described and presented, we got transient voltage decay data and used them to plot the curves. We have done a linear fit on the analyzed curves in their linear zone because this zone is major than exponential zone for all transient voltage curves and did not present impedance effects. These results are in agreement with those obtained and published by other authors in the literature. We validate our results because the theoretical study is in good agreement with the experimental study.

The authors declare no conflicts of interest regarding the publication of this paper.

Diasso, A., Sam, R., Zouma, B. and Zougmoré, F. (2020) Experimental Measurement of Minority Carriers Effective Lifetime in Silicon Solar Cell Using Open Circuit Voltage Decay under Magnetic Field in Transient Mode. Smart Grid and Renewable Energy, 11, 181-190. https://doi.org/10.4236/sgre.2020.1111011