Photocurrent measurements for solar cells characterization E. P´erez, M. Maestro, H. Garc´ıa, H. Cast´an, S. Due˜nas and L. Bail´on Dpto. de Electricidad y Electr´onica E.T.S.I. de Telecomunicaci´on, Universidad de Valladolid, Paseo de Bel´en 15, 47011 Valladolid, Spain e-mail:
[email protected] Abstract—In order to find the regions in solar cells where the efficiency drops an experimental setup is tuned up. Through this equipment a set of samples are characterized checking that its response is the expected. The photocurrent maps obtained allow us to determine the regions with higher defects concentration. These regions will be characterized using electrical techniques which will give us additional information of the nature of these defects. Keywords—Solar cell; defects; photocurrent
I. I NTRODUCTION Quantum efficiency is a key parameter of solar cells. The best efficiency results obtained in polycrystalline (pc-Si) and monocrystalline (c-Si) silicon solar cells have been 20.4% and 27.6%, respectively, until now. The theoretical efficiency for silicon solar cells is over 30%, so indicating that several unsolved questions still remain preventing the theoretical efficiency to be reached. To obtain better solar cells is of great importance to identify the causes affecting the efficiency. One of the most important is the existence of recombination centers which trap photogenerated carriers before they reach the solar cell terminals. On the other hand, the efficiency losses are not uniform on all the solar cell area. Therefore, the different nature and/or concentration of these traps on different locations in the solar cells could be the responsible for the efficiency variability throughout the solar cell surface. Nowadays, the photoconductance measurement techniques are usually used to obtain different results close related with the solar cells efficiency [1]–[3]. In this work, we have tuned up an experimental setup which allows us to obtain efficiency maps in solar cells and to determine the defects characteristics which are the responsible of these efficiency losses, when correlating these measurements with other characterization techniques. II. M EASURING
Figure 1.
Experimental setup
A. Laser Provide the monochromatic light which lead in conduction in the solar cell. Three different lasers are used, with different wavelengths: 405nm, 543nm and 633nm. The use of different wavelengths allows studying the efficiency losses in specific zones of the solar spectrum. B. Beam spliter Separates the laser beam into two beams: one of them of about the 90% of power and the other one the remaining 10%. The first beam is used to illuminate the solar cell and the second one is lead towards photodiode to monitoring it. The Beam splitter is composed by a BK7 glass type with A grade. It has a diameter of 25.4mm and a width of 6.10mm. C. Mirror Allows the laser beams to perpendicularly impinge over the solar cell. It consists on a 25.4mm diameter Pyrex mirror with a mean reflectivity in the visible range greater than 93%. D. Lens
EQUIPMENT
The experimental setup is based on a commercial kit [4] of Newport Corporation, which we have customized for our requirements obtaining the configuration shown in Fig. 1. In the following lines we enumerate the different elements of the setup.
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With a 50.8mm focal distance the lens is used to achieve a lightly better spatial resolution since laser beam, at the output, presents an elliptical shape. This lens lets to focus the laser beam over the solar cell with a spot. It is actually a composition of two plane-convex lenses.
E. Photodiode Measures the laser power in real time allowing us to monitor it during the photocurrent measurement. The silicon sensor of the photodiode has an active diameter of 1.13cm (that is, an active area of 1cm2 ). The operation range is from 400nm to 1100nm. The sensor presents a perfect linear response among pW and dozens of mW. F. Motion controlers Two motion controllers are used to scan the solar cell area during the current measurements. One of them carry out the movement along X axis and the other along Y axis, up to 45mm each. Therefore, photocurrent maps of 202.5mm2 are obtained in one scanning. Each motion controller incorporates a specific module to be controlled by the computer through a USB port. By means of this module, velocity of the movement, step between movements and other motion controller parameters can be modified as long as it was necessary.
Figure 2.
Current density vs. voltage curves in the dark
G. Power meter The Newport 2936-C Series is the instrument which turns the analog signal coming from the photodiode into a digital signal that is visualized in a display and can be sent to the computer through a USB port. H. Electrometer To measure the photocurrent induced by the light beam we use an Keithley 195A electrometer. The measured values are sent to the control computer by a GPIB bus connection. I. Control computer Runs the LabVIEW program which controls the operation of the measuring equipment. Through its connections to the electrometer and the power meter devices make a file which stores the information necessary to do the photocurrent map.
Figure 3.
Current density vs. voltage curves with ilumination
III. E XPERIMENTAL MEASUREMENTS As we have said previously, our purpose is to use the above described setup to find the regions in solar cells where the efficiency fall down and try to discover why. We have used it over a set of solar cell samples to test its operation and to give the first conclusions from the photocurrent maps of these samples. The samples set consists in two polycrystalline solar cells with an efficiency of 16% and 16.4%, labeled Poly160 and Poly164 respectively, and a monocrystalline solar cell called Mono. All three cut in a way that allow us put them in a 45mm∗45mm area. A. Electrical measurements Before doing the optical measurements we obtained some electrical features of the samples used. Particularly the J-V characteristics can give us interesting information to contrast it with the photocurrent measurements. In Fig. 2 we show the J-V curves of the three samples in the dark. Moreover, in Fig. 3 we show the same curves but with ambient lighting. The first conclusion we can get from this
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curves is that Poly164 give much current (due to the impinging light) than Poly160 which is expected due to the efficiencies of each sample. Another conclusion is that Mono sample give photocurrent closer to Poly164 than to Poly160. B. Optical measurements These were the first measurements done with commercial samples using the measuring equipment. So, the results give us an idea about if its behavior is the expected or not. Besides, they will allow get the first conclusions related with efficiency over these samples. The measurements consist in 45mm∗45mm scans with the three laser wavelengths. To examine the differences in the results due to a different wavelength used we will focus in the Poly160 sample. The first measurement was done with the laser of 405nm (violet). The light power measured was 206μW and the photocurrent map is shown in Fig. 4. The second one was done with the laser of 543nm (green). The light power measured
was 10.7μW and the photocurrent map is shown in Fig. 5. And the last one was done with the laser of 633nm (red). In this case the light power measured was 25μW and we can see the photocurrent map in Fig. 6. If we consider the current generated in the solar cell related with the impinging light power we can conclude that it increases with the wavelength increase. The reason could be that as higher is the wavelength more photons compose
the impinging beam so, more number of carriers will be generated and therefore the current will be bigger. Another reason, which does not exclude the previous one, is that the response (reflexivity, absorption...) of the solar cell to different wavelengths is not the same. Also, we can see in all three wavelengths regions with a lower photocurrent response. For example, for 633nm in the left bottom corner appears this kind of region. So, the detailed study of this region using other characterization techniques could give us the reason why this fall in efficiency. Now we will examine the differences among the photocurrent maps of each sample at one wavelength: 543nm. The first idea we can extract is that at 543nm Poly160 (Fig. 5) and Poly164 (Fig. 7) have a very similar behavoir. But if we focus in left region of the photocurrent map of Poly164 we can see a region with higher photocurrent response. This peak in the photocurrent response could explain the 0.4% difference in efficiency between Poly160 and Poly164. The second idea is that Mono (Fig. 8) have a current response bigger than the polycrystallines. This result match with what we could expect, in general monocrystalline substrates obtain higher efficiencies than polycrystalline ones.
Figure 4.
Photocurrent map at 405 nm
Figure 5.
Photocurrent map at 543 nm
Figure 6.
Photocurrent map at 633 nm
Figure 7.
Figure 8.
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Photocurrent map for Poly164 at 543 nm
Photocurrent map for Mono at 543 nm
IV. C ONCLUSIONS In this work we show an experimental setup which is able to give us the surface distribution of solar cells quantum efficiency. Measurements carried out on commercial samples indicate that the system may be used to find the locations where the efficiency drops. Once these defective regions are identified, additional information about the defects present in the solar cells can be obtained by contrasting the results with other electrical techniques available in our lab. ACKNOWLEDGMENT This work was partially supported by VA128A11-2 funded by the Junta de Castilla y Le´on. Research of E. P´erez was supported by a University of Valladolid FPI grant. R EFERENCES [1] P. Rosenits, T. Roth, W. Warta, and S.W. Glunz, ”Influence of different excitation spectra on the measured carrier lifetimes in quasi-steady-state photoconductance measurements,” Sol. Energy Mater. Sol. Cells, vol. 94, pp. 767-773, May 2010 [2] P. Campbell, M. Keevers, and B. Vogl, ”Characterization of light trapping in silicon films by spectral photoconductance measurements,” Sol. Energy Mater. Sol. Cells, vol. 66, pp. 187-193, February 2001 [3] A. Cuevas, R.A. Sinton, M. Kerr, D. Macdonald, and H. M¨ackel, ”A contactless photoconductance technique to evaluate the quantum efficiency of solar cell emiters,” Sol. Energy Mater. Sol. Cells, vol. 71, pp. 295-312, February 2002 [4] Newport, ”Photoresponse mapping of photovoltaic cells. App note 40,” Newport, 2009
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