INVESTIGATION OF LASER SPECKLE PATTERNS OF SOLAR CELLS

H. El Ghandoor, G.M. Youssef, Mostafa A. El-Aasser and Alaa T. Ahmed. Department of Physics, Faculty of Science, Ain Shams University, Cairo, Egypt. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 02 June 2019 Final Accepted: 04 July 2019 Published: August 2019

Using an abrasive paper, the surface of commercial solar cells has been diffused with different micrometer features. Laser speckle patterns obtained from the different rough surface of solar cells were recorded using laser speckle photography technique. Different laser sources were used with a variable inclination with the solar cell optical axis. For each inclination angle, the contrast of the recorded speckle pattern was calculated. This speckle contrast gives a good indication about the solar cell roughness studied in this work. Furthermore, the speckle contrast changes significantly with the changing of the beam angle of incidence and its wavelength.

…………………………………………………………………………………………………….... Introduction:-
Solar cells use a virtually infinite source of energy to generate electrical current directly from sun white light in a one-step conversion process. No other renewable energy source comes close to such an application [1]. The need for clean, renewable energy technologies to reduce the environmental effect of our increasing energy consumption is by now well established [2]. One way to attain higher efficiency of the solar cell is to increase the fraction of light that is absorbed by the solar cell, a strategy now commonly known as light trapping [3]. Typical elements used to achieve light trapping enhancement include introducing light scattering at rough surfaces and increasing the path length are presented [4]. It was found out that the interface morphology can play an important role in light trapping [5]. Therefore, it is important to investigate the degree of roughness of the solar cell surface [6]. Formation of speckle is due to the incidence of coherent light on a rough surface of height variations greater than the wavelength λ of the light which is scattered in all directions [7]. These scattered wavelets interfere and form an interference pattern consisting of dark (distractive interference) and bright (constructive interference) spots named speckles which are randomly distributed in space [8]. Therefore, laser speckle is utilized for the analysis and study of the surface roughness of a solar wafer as well as for measuring the surface roughness of solar cells [9][10][11]. Also, the speckle pattern contrast gives a good implementation of the object surface roughness parameter [12][13][14][15]. The primary goal of this work is to study the relationship between the laser speckle contrast and the different time of ISSN: 2320-5407 Int. J. Adv. Res. 7(8), 61-70 62 mechanical roughness of the solar cell surface. Also, the behavior of the contrast at different angles of incidence and different wavelengths will be studied.

Theory
Surface texturing of the solar cell can enhance the light trapping by increasing the path length of the incident light as shown in Figure 1. This texturing can be realized mechanically or chemically [16][17][18]. Although the surface texturing increases the absorption of light, a part of the incident light will be scattered [19]. The intensity of the scattered wavelets carries important information about the surface texture. When illuminating the textured surface by coherent light such as laser, the scattered wavelets will interfere constructively and destructively to produce laser speckle patterns. The surface roughness can be extracted from the statistical properties "the contrast" of the speckle patterns [11]. The laser speckle contrast is based on the average intensity of the scattered light and it can be calculated from [10] C = (1) where I max is the average of the maximum intensity (bright spots) and I min is the average of the minimum intensity (dark spots).   He-Ne laser

Discussion:-
It is obvious from Figure 4 that the speckle contrast obtained using the blue laser diode increases with the increase of the time of roughening at all studied angles of incidence. This can be confirmed by Figure 3a whereas the speckle intensity distribution obtained using the blue laser diode has the highest I max value and the lowest I min value among the other laser sources used. This can give an impression of the blue laser beam "445 nm" scattering from the investigated solar cell. Such that, this wavelength "445nm" is the most scattered wavelength even with increasing the mechanical roughening time and the angle of incidence.
The speckle contrast obtained using the green laser diode is also increased with the mechanical time of roughening at the all studied angles of incidence except for =78 0 . This means that the green laser beam "536 nm" is strongly scattered by studied solar cell surface even after mechanical roughening. This is not the case when the green laser beam incident on the solar cell surface by =78 0 . The decrement in green speckle contrast with the mechanical roughening at =78 0 may refer to the decreasing of that wavelength "536 nm" scattering and increasing its absorption by the solar cell surface.
The contrast obtained using the red laser diode "670 nm" and the He-Ne laser "632.8 nm" decreased with the time of roughening at all angles of incidence except for =42 0 which has an increasing behavior with the time of the mechanical roughness. This may be resulted from increasing the solar cell absorption and decreasing scattering of the wavelengths 632.8, 670 nm which is not the same case at =42 0 .

Conclusion:-
The surface of a commercial thin film solar cell has been roughened for different times mechanically. The laser speckle patterns have been obtained using four different laser sources (different wavelengths). The angle of incidence varied from 42 to 78 angles. The contrast of the laser speckle patterns was calculated. It is found that the speckle contrast obtained using the blue laser diode increases with the time of roughening at all studied angles of incidence. The green laser diode speckle contrast increased with the mechanical time of roughening at the all studied angles of incidence except for =78 0 . The He-Ne and the red laser diode speckles contrast decreased with the time of roughening at all angles of incidence except for =42 0 . This can give an indication about the scattering and the absorption of this kind of solar cells upon increasing the surface roughness. As a result, this study has succeeded in monitoring the relationship between the speckle contrast and the time of mechanical roughening. Also, the effect of changing the wavelength used and the angle of its incidence on the relationship between speckle contrast and time of mechanical roughening is illustrated successfully in this study.