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Proceedings of 10th International Conference ITELMS’2015
Spectral peculiarities of mono-crystalline silicon solar cells modified by twostage photo-anodizing process E. Shatkovskis, A. Jukna, V. Zagadskij, J. Stupakova, R. Mitkevičius Department of Physics, Faculty of Fundamental Sciences, Vilnius Gediminas Technical University, 11 Sauletekio av., Vilnius LT-10223, Lithuania,
[email protected] Abstract This article presents results of study of photoelectrical properties of mono-crystalline silicon solar cell, modified by means of building inside it a porous silicon (por-Si) structure, which was manufactured by means of two-stage photoelectro-chemical anodizing process. Solar cells made out of a 200-µm-thick Si(100) wafer with textured surface illuminated with visible and near-infrared light have been used in our experiments. A layer of por-Si buried in emitter’s volume causes noticeable change in short-circuit electric current, open-circuit electric voltage, filling factor of the current voltage dependence, and in maximum electric power generated by the modified solar cell illuminated by monochromatic light of wavelengths ranging from visible to near-infrared part of solar spectrum. Our measurements results let us to conclude that the 2nd-stage of two-stage photo-electro-chemical anodizing process forms the main portion of por-Si buried-layer. The results of measurements of photoelectric properties of chemically treated solar cells with buried por-Si layer in emitter’s volume are presented and discussed. KEY WORDS: silicon wafer, monocrystalline silicon solar cell, porous silicon, visible range of spectrum, conversion efficiency. 1. Introduction Solar cell research, development and industrial production experienced a breakthrough over the past 15 years. Manufacturing of solar cells, photoelectrical panels and construction of photoelectrical power plants has grown significantly. Solar cell manufactured of monocrystalline silicon (µ-Si) occupies a leading position among a wide range of products of the photovoltaic industry. Laboratory-derived conversion efficiency of silicon solar cells is ranging at 24.5 percent. However, the efficiency of µ-Si solar cells produced by modern industry does not exceed limit of 20 percent [1, 2]. Theoretical limit of µ-Si solar cell efficiency is considered to be at 31 percent [3]. Therefore, the main goal of research and development of µ-Si solar cells is to reach mentioned theoretical limit of efficiency in order to make photovoltaic based on silicon solar cells as more competitive with existing renewable energy sources. The most obvious and the most popular method, improving solar cell’s performance, is a reduction of optical reflection from the silicon wafer. For this purpose, a photo-sensing surface of the µ-Si solar cell is covered thin films of material, characterized refraction index lower than that of silicon. Choosing thicknesses of these films as quarter wavelength thick, one can select the maximized photo-sensitivity and simultaneously minimized impact of e.g. UV wavelengths from solar spectrum, which mostly affect rise in cell’s temperature and for that reason are not highly desired. One of the most significant improvements of efficiency was invented by Martin Green with co-authors, proposing to create solar cell with textured surfaces instead of mirror-like flat [1, 2]. Realizing advantages of this invention, most of manufactures start to produce µ-Si solar cells containing textured surfaces. Additional attempts to modify silicon surface by means of creating por-Si structures in it for efficiency enhancement was also very successful [4]. One of most effective way of application of por–Si structures is to fabricate por-Si anti-reflective coatings on top of solar cell [5-7]. Experiments confirm that manufacturing of approximately quarter-wavelength thick por-Si layer on the top of solar cell results in noticeable increase in efficiency of light energy conversion into electric energy if compare with efficiency of solar cells without por-Si coatings [8-10]. Experimentally shown that por-Si coating can affect decease in surface-recombination rate [10]. Essential reduction in optical reflection losses has been achieved in solar cells made of so-called black-silicon. The main point of black-silicon technology lies in nano-scale texturing of silicon surface using a local nano-scale metalcatalyzed wet chemical etching or other suitable technologies. It is shown that black-silicon nano-scale texturing of silicon surface results in an almost complete suppression of reflectivity in the broad spectral range, and in an considerable (to 36-42 percent) increase of the short-circuit electric current [11]. A black-silicon nano-scale wet texturing is applicable to all known crystal-forms of silicon solar cells such as µ-Si, poly-Si, or multi-Si, as well as to thin films of silicon. A quality of black-silicon nano-scale texturing process almost independent on surface orientation and doping. Por-Si structure tested also as a coating for epitaxial silicon solar cell [12]. Firstly, chirped Bragg reflector produced in a conventional p-type silicon substrate. The reflector itself was as a layer of por-Si produced by means of electrochemical etching. Secondly, epitaxial n-type Si emitter deposited on the top of the por-Si structure. Chirped broadband por-Si reflector, effectively redirecting light to p-n junction, increases the cell’s conversion efficiency.
220 Indeed, the Bragg reflector manufacture is relatively complex and may affect costs of technology and the price of final product. Recently we applied more cheap por-Si technology improving photo electrical characteristics of a regular, fully completed, µ-Si solar cell [13, 14]. Unique features of por-Si manufacturing technology enable producing of complex of por-Si structures built inside the silicon wafer. We choose a method of two-stage anodizing so, that top layer of the silicon wafer would be affected minimally. In contrast to known methods of formation of the functionally important por-Si structure on a top of photo-conducting surface of the solar cell, we create it buried in the emitter’s volume, in some extent analogously to production of the chirped Bragg reflector in silicon wafer [15]. However, chemically treated surface of solar cell was kept almost unchanged in this case. It was shown that constituted of por-Si structure buried in solar cell emitter’s volume markedly improve the solar cell’s characteristics such as open-circuit electric voltage, shortcircuit electric current and filling factor in region of 450-550 nm wavelengths of solar spectrum [13, 16]. It’s worth noting that similar improvement of cell’s performance in mentioned range of wavelengths was observed in the case of black-silicon nano-scale textured solar cells [17]. Our current work presents results of investigation of por-Si structure manufactured by two-stage anodizing method and its effect onto photoelectric characteristics of the µ-Si solar cells measured in visible and near-infrared range of wavelengths. 2. Experiment 200 µm thick silicon solar cells made from µ-Si(100) we used in our experiments. The n-type and almost 300 nm thick emitter has been produced by phosphorus diffusion procedure. The photosensitive surfaces of solar cells were textured in the form of regular inverse pyramids with facets and depth dimensions of about of 5 µm. Solar cells have the continuous lower and grid upper contacts deposited onto tens of nm-thick silicon nitride layer. Samples for our experiments we produced by cutting a ready-to-use solar cell into about of the 70-100 mm2 area pieces with further sticking of copper wires by means of silver paste to the photosensitive and bottom electrodes of the solar cell. All the metal electrodes were covered by a chemically resistant and electrically-tight sealing-wax to protect from chemicals. A procedure of por-Si layer formation in the µ-Si(100) solar cell took place in a electrochemical cell of polytetrafluoroethylene. We used wax-protected bottom-electrode as an anode and a platinum plate electrically contacting with electrolyte – as a cathode for two-stage photo-electro-chemical anodizing process. In order to remove silicon nitride as cell’s surface protective layer our samples were for short time immersed into fluoride acid. Quality of surface has been tested by means of measurement of surface electric resistance. A formation of por-Si structure has been carried out in the HF:ethanol = 1:2 volume ratio electrolyte with simultaneously illumination of samples by light from a 50 W halogen lamp. Manufacturing of por-Si structure has been carried out by means of two-stage photo-electro-chemical anodizing process performed at room temperature. For the 1st stage we succeeded in moving the etching front from cells textured surfaces towards theirs emitter’s volume due to cell’s chemical treatment in regime of comparatively weak electric current density. For the 2nd stage, we form por-Si buried layer in emitter’s volume by cell’s chemical treatment in regime of high density electric current. A computer controlled parameters of chemical etching procedures are presented in Tab. 1. Table 1 Parameters of a two-stage photo-electro-chemical anodizing procedure applied for manufacturing of por-Si structure in µ-Si(100) solar cells Sample Nr.
∆t1, s
j1, mA/cm2
∆t2, s
j2, mA/cm2
1 2 3 4
20 20 0 20
5 5 0 5
5 10 0 20
15 15 0 15
Our manufactured por-Si structure consists of two porous layers: low porosity P top layer (P about 20-40%) and main higher porosity lower layer with P ~ (50-70) %, most part of which buried in emitter’s volume. A quality of electrochemically formed por-Si depends on number of technological parameters, such as electrolyte density, concentration of charge carriers (mainly holes), temperature and others [4]. Some special efforts needs to apply in order to have a reiteration of properties of por-Si layer in all our samples. Samples reiteration becomes even more complicated because procedure of chemical treatment takes place under optical illumination. Current-voltage ( I - V ) characteristics in dark and under illumination from 5000 K Xenon lamp emitting spectrum of light similar to 5000 K black body radiation spectrum were measured at room temperature for chemically treated samples. A computerized setup, consisting of multimeter Tektronix CFG 253, Keithley 2000, Metex MXD 4660, oscilloscopes Tektronix TDS 3032B, have been used for our measurement. The optical illumination of samples in integral spectral mode was maintained at the level of 1.5·104 lx delivered from a diffraction-grating spectrometer and 50 W halogen lamp.
221 3. Results and discussion Plots of I - V characteristics measured under sample’s optical illumination emitted by the Xenon lamp depicted in Fig. 1. For the easier comparison, curves No. 1, 2, and 4 (open symbols) represent I - V dependences of por-Si buried layer-free samples and curves No. 1', 2', and 4' (full symbols) stand for same samples after their chemical treatment procedures, which form buried layer of por-Si in emitter’s volume. I - V dependence of a reference sample (i.e. chemically unmodified sample) designates curve No. 3. A photosensitive surface area varies with samples and this is the reason why do shapes of I - V dependence look-like different samples showing different values of short-circuit current and open-circuit voltage. The first fact is striking that the production of complex por-Si structure in emitter’s volume in all the samples gained a short-circuit current. Increase of about 12, 20 and 80 percent for the samples No. 1, 4 and 2, respectively.
Fig. 1. Current-voltage characteristics measured for four µ-Si solar cells containing por-Si layer buried in emitter volume by means of two-stage photo-electro-chemical anodizing process (full symbols) and for por-Si layerfree samples (open symbols) under optical illumination delivered from Xenon lamp. Numbers of curves correspond to samples chemically treated following regime presented by Tab. 1 The change in short-circuit current of chemically treated samples correlates with technological parameters presented in Table 1. It is worth noting that etching time and current density used for etching procedures kept constant in all cases of samples during the 1st stage of their two-stage photo-electro-chemical anodizing process. Consequently, the etching front in all samples moved almost to the same depth towards emitter’s volume. Thus, we assume that due to comparatively low porosity of top layer of por-Si formed onto solar cell’s surface, the chemical treatment of solar cell during the 2nd stage, which forms layer of por-Si buried in emitter’s volume, causes a formation of relatively denser top layer of por-Si structure formed during the 1st stage of two-stage photo-electro-chemical anodizing process. Etching current density during the 1st stage of treatment kept the same in all cases of our samples (Table 1), but varied for the 2nd stage of anodizing process in range from 5 to 20 s (see Tab. 1). Number of scientific works, systematically investigating the etching rate of por-Si are incomplete and sketchy, especially with respect to photo-electro-chemical etching. Moreover, available data are usually given for cases of planar surfaces etched along a direction of silicon surface normal. In our case, the surface of solar cell shaped in the form of regular inverse pyramids, which crystallographic orientation is different from that of crystallographic orientation of the surface of the silicon wafer itself. This is why it is complicated to determine exactly etching rate of por-Si from data of references and accurately calculate the thickness of the porous layer. However, typically the thickness of the porous layer increases linearly with etching time during an anodic etching of silicon [4]. Therefore, we have only the grub etching depth assessment for the purpose of the por-Si layer formed during the 2nd stage of our chemical treatment. This let us to assume that the por-Si layer did not extend to p-n junction area and did not damage it. Our results show that increase in time of etching process during the 2nd stage of photo-electro-chemical anodizing affects increase in thickness of por-Si layer. This also let us to conclude that a formation of por-Si buried layer, located deeper in the emitter’s volume, take place during the 2nd stage of anodizing of the solar cell. We assume that increase in etching time during the 2nd stage of anodizing also can be used as argument explaining behavior of I - V dependences (Fig. 1). Knowing that the 2nd stage of anodizing for sample No. 1 was the shortest in time scale, we can expect the finest thickness of the por-Si layer buried in sample No. 1 emitter’s volume. At a rough estimate, the thickness of the deeper por-Si layer (i.e. etched during the 2st stage of the anodizing process) in sample No.1 should be almost of the same scale as that one of por-Si layer located on the top of it (i.e. etched during the 1st stage of anodizing process). Assuming, that in both cases etching time is equal to 5 s, we can estimate thicknesses of por-Si layers to be in range of several tens of nm. This might be a reason why does short-circuit electric current change very little (curves No. 1 and 1' in Fig. 1). Relatively moderate improvement in performance observed in the sample No. 4. Taking the
222 longest etching time por-Si layer’s thickness in sample No. 4 should be at least hundred nm, but still the por-Si layer does not reach and destroy p-n junction of the solar cell. I - V dependences of the solar cell No. 4, which is illuminated by a monochromatic light of wavelengths ranging from 500 to 1150 nm are shown in Fig. 2.
Fig. 2
Current-voltage dependences of the sample No. 4 measured at various fixed wavelengths of monochromatic light incident onto the chemically treated solar cell. Different symbols correspond to different wavelength ranging between 500 and 1150 nm
Fig. 3. Filling factors of current-voltage dependences of solar cells, containing porSi layers (samples are same as presented by curves 1', 2', 3, 4' in Fig. 1) vs. wavelength of light incident onto the cell’s photo conducting surface
I - V dependences vs. wavelength of light incident onto sample reflect spectral sensitivity of the solar cell and simultaneously the spectral distribution of energy, which depends on number of photons emitted by the light source. Filling factor F of I - V dependence is one of the parameter often used to estimate efficiency η of the solar cell’s conversion. Factor F is defined as a ratio between area located under the experimentally measured I - V dependence curve with the product of short-circuit current Isc and open circuit voltage Uoc (e.m.f. of solar cell). Both parameters η and F are associated by nearly linear dependence. However, mostly used the simplified form defined as the maximal electric power Pmax generated by the solar cell divided over product of short-circuit electric current and the open-circuit electric voltage: F=
Vmax I max P = max I sc U oc I sc U oc
From the I - V dependences we can extract dependence of filling factor vs. wavelength of light λ incident onto chemically treated and untreated solar cells. Our results of calculations represents Fig. 3. F = f (λ) dependences look almost of the same shape in comparatively wide range of wavelengths ranging from 650 to 1150 nm for all our tested samples. A plot of calculated ratio (F4 – F3) / F3 vs. wavelength of light λ incident onto sample is shown in Fig. 4. Here F4 stands for the filling factor of the chemically modified sample No. 4, and F3 stands for the filling factor of unmodified (i.e. reference) sample No. 3. Course of the (F2 – Fo) / Fo = f (λ) dependence indicates that filling factor remains virtually unchanged in relatively large range of wavelengths of light incident on the chemically treated solar
Fig. 4. The current-voltage dependence filling factor relative ratio vs. wavelength of light incident onto solar cell. Here F4 stands for the filling factor of chemically modified sample No. 4 (for treatment regime see Tab. 1), and F3 stands for the filling factor of unmodified (i.e. reference) sample No. 3
223 cell containing por-Si layer buried in the emitter’s volume. Indirect evidence of behavior represented by Fig. 4, can be the fact that open-circuit electric voltage of the sample No. 4 in some extend decreased after it chemical treatment. The largest change of I - V dependence taking place after introduction of por-Si layer into µ-Si solar cell observed for the sample No. 2 (see Fig. 1). The open-circuit electric voltage of this sample does not noticeably change with introduction of por-Si layer. We expect that chosen by us regime for chemical treatment of the sample No. 2 is close to optimal, i.e. the etching front optimally distanced from the cells surface (i.e. chosen optimal regime for the 1st stage of anodizing) and the buried por-Si layer did not reach p-n junction of the solar cell and did not damage it. 4. Conclusion Our results let us to conclude that an introduction of por-Si layer into µ-Si solar cell by means of a two-stage photo-electro-chemical anodizing process might affect increase in filling factor of the cell’s current-voltage dependence and, if chemically treated at optimal conditions, it might affect a noticeable increase of short-circuit electric current keeping almost unchanged open-circuit electric voltage. The increase in filling factor of the chemically treated solar cell is positioned in the range of 450-1150 nm wavelengths, located in a part of characteristic solar A1.5 spectrum. Spectral I - V dependence of the solar cell measured under cell’s illumination with monochromatic light of various wavelengths can give a valuable information on how strongly the por-Si layer buried in the emitter’s volume affects photoelectric properties of the µ-Si solar cells. Acknowledgement This work was partly supported by the project VP1-3.1-ŠMM-08-K-01-009 of the National Programme “An improvement of the skills of researchers” launched by the Lithuanian Ministry of Education and Science. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
Zhao J., Wang A., Green M. A., Ferrazza F. Appl. Phys. Lett., 1998, V. 73, p. 14-17. Miles R. W., Zoppi G., Forbes I. Materials Today, 2007, 10 (11), p. 20-24. Schockley W., Queisser H. J. J. Appl. Phys. 1961, V. 32, p. 510-514. Lehmann V. Electrochemistry of silicon. Willey-VCH Weinheim, 2002, p. 277. Prasad A., Balakrishnan S., Jain S. K., Jain G. C. J. Electrochem. Soc., 1982, V. 129, p. 596- 601. Menna P., Di Francia G., La Ferrara V. Sol. Energy Mater. and Solar Cells, 1995, V. 37, p. 13-19. Pačebutas V., Grigoras K., Krotkus A. Physica Scripta., 1997, V. 69, p. 255 -250. Ramizy A., Hassan Z., Omar K., Al-Dauri Y, Mahdi M. A. Appl. Surface Sci., 2011, V. 257, p. 6112. Yerochov V. Y., Hezel R., Lipinski M., Ciach R., Nagel H., Mylyanych A., Panek P. Solar Energy Materials & Solar Cells, 2002, V. 72, p. 291-297. 10. Kopitkovas G., Mikulskas I., Grigoras K., Šimkienė I., Tamošiūnas R. Appl. Physics, 2001, V. A73, p. 495-501. 11. Koynov S., Brandt M. S., Stutzmann M. Applied Physics Letters, 2006, V. 88, p. 203107-203109. 12. Van Hoeymissen J., Depauv V., Kuzma-Filipek I., van Nieuwenhuysen K., Payo M. R., Qiu Yu, Gordon I., Poortmans A. Physica Status Solidi, 2011, V. A208 (6), p.1433-1439. 13. Shatkovskis E., Zagadskij V., Mitkevičius R., Stupakova J. Material Science, 2012, V. 18(3), p. 220-222. 14. Šatkovskis E., Zagadskij V., Mitkevičius R. LT Patent No. 5819E, Official Bulletin of State Patent. 15. Pavesi L. Revista del Nuovo Cimento, 1997, V. 20(10), p. 1-75. 16. Shatkovskis E., Mitkevičius R., Zagadskij V., Stupakova J. Tech. Phys. Lett., 2013, V. 39, p. 945-949. 17. Toor F., Page M. R., Branz H. M., Hao-Chin Yuan. Papers of the 37th IEEE Photovoltaic Specialists Conference (PVSC 37), June 19-24 Seattle, Washington, NREL/CP-5200-50707 July 2011, p. 1-5.
