Porous silicon used as an oxide diffusion mask to produce a periodic micro doped n++/n regions
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Phys. Status Solidi C 8, No. 6, 1774–1778 (2011) / DOI 10.1002/pssc.201000185
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Wissem Dimassi*, Hayet Jafel, Mohamed Lajnef, M. Ali Kanzari, Mongi Bouaïcha, Brahim Bessaïs, and Hatem Ezzaouia Laboratoire de Photovoltaïque, Centre de Recherche et des Technologies de l’Energie, PB : 95, Hammam Lif 2050, Tunisia Received 29 April 2010, revised 24 May 2010, accepted 29 May 2010 Published online 1 December 2010 Keywords porous silicon, micro grooving, solar cell and selective emitter * Corresponding author: e-mail
[email protected], Phone: +216 97 828 000
The realization of screen-printed contacts on silicon solar cells requires highly doped regions under the fingers and lowly doped and thin ones between them. In this work, we present a low-cost approach to fabricate selective emitter (n++/n doped silicon regions), using oxidized porous silicon (ox-PS) as a mask. Micro-periodic fingers were opened on the porous silicon layer using a micro groove machining process. Optimized phosphorous diffusion through the micro grooved ox-PS let us obtain
n++ doped regions in opened zones and n doped large regions underneath the ox-PS layer. The dark I-V characteristics of the obtained device and Fourier transform infrared (FTIR) spectroscopy investigations of the PS layer show the possibility to use PS as a dielectric layer. The Light Beam Induced Current (LBIC) mapping of the realized device, confirm the presence of a micro periodic n++/n type structure.
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1 Introduction One of the possible ways to reduce cost production of silicon solar cells, is the use of porous silicon (PS) in a specific step of the whole process [1,2]. It was demonstrated that a simple formation of PS on the front surface of a silicon solar cell could simultaneously replace in a single step texturization, passivation, antireflection coating (ARC) deposition and the removal of the dead layer [1-3]. To reduce the high surface recombination velocity, Suresh Kumar Dhungel et al. reduce the back contact area by forming a local back surface field (LBSF) using porous silicon as a dielectric layer [4]. The Formation of a selective emitter is an important step in industrial screen-printed silicon solar cells [5]. The fabrication of screen-printed contacts for high efficiency solar cells requires high doping and deep diffusion under the fingers (metal contacts) and a lowly doped and thin zone between them. Such an emitter structure can be realized using two diffusion steps and photolithography, but this does not meet the industrial requirements [6]. Indeed, this technology is time consuming and expensive because of the use of photo-resist mask patterning and
metal e-beam evaporation. This has led to the investigation, development, and use of other simpler metallization techniques. To improve energy conversion efficiency, laser microgrooved passivated emitter solar cells (PESC) were developed. The approach is characterised by the use of a thin thermally grown oxide that reduce the electric activity of the top surface. The photolithography technique was often used to pattern the top contact fingers and to align them with the oxide openings. In the case of the rear surface, the oxide serves as a diffusion mask to confine the heavy diffusion to the laser grooved areas. In this work we present the possibility of realizing a low cost selective emitter, where micro highly doped regions are separated with large lowly doped regions using the PS-based mechanical micro-grooving. This method can reduce the two-steps phosphorus diffusion in one step. We show that we can replace phtolithography by achieving n++ doping under the contacts.
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2.1 The insulator characteristic of PS In order to investigate the insulator characteristic of PS, Al/PS-Si/Al (where the PS layer is formed for 2 min, 4 min, 6 min and 13 min) and Al/Si/Al (taken as a reference) devices were realized and characterized by the dark current-voltage (I-V) measurements. The I–V characteristics (Fig. 1) measured on the above mentioned Schottky diodes allow a quantitative determination of the series resistance associated to the bulk material, PS and contacts [8–10]. To extract the series resistance Rs different methods have been proposed [11, 12]. We applied the method developed by Cheung and Cheung [13]. In Fig. 2 we report the Rs values as a function of anodization time. In view of these results, one notices an important increase of the Rs value after the formation of the porous silicon layer. The increase of the Rs value indicates the insulator characteristic of the PS layer.
oxidized polished monocrystalline silicon substrate (ox-Si) together with the oxidized PS layers (ox-PS) after thermal oxidation.
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2 Experiment and results The bare substrates are solar grade (100) oriented p-type monocrystalline silicon wafers (Czochralski), with a resistivity ranging from 1 to 3 Ωcm and a thickness of 450 µm. The wafers were dipped in an acid mixture (CP4) (HF: 16%, HNO3:64%, CH3COOH:20%) for a few seconds in order to obtain a cleaned surface, then rinsed in deionized water. A thin PS layer was formed on the front surface of the cell by electrochemical etching the wafers in a HF: C2H5OH (1:1) solution. The anodization time was varied from 2 min to 13 min.
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The band centred at 1080 cm-1 corresponding to the Si–O stretching vibration mode widens. The shoulder in the 1100–1250 cm-1 region, corresponds to the out-of-phase Si–O stretching vibration mode [14-17]. Compared with the (ox-Si) spectrum, the Si-Ox related vibration bands are largely strengthened in the (ox-PS) spectra. We can conclude that thermal oxidation of PS at 750 °C for 5min under O2/N2, is sufficient to obtain a good and thick oxide layer [18, 19].
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Figure 1 Dark I–V characteristics measured on the Al/Si/Al and on three Al/PS-Si/Al Schottky diodes.
2.2 Rapid thermal oxidation of porous silicon The PS layers realized at different anodization (2 min, 4 min, 6 min and 13 min) time and the CP4 polished substrates (reference samples) were subjected to a rapid thermal oxidation in an IR furnace at 750 °C for 5 min under O2/N2 atmosphere. The possibility of using PS as an oxide diffusion mask was investigated by FTIR spectroscopy in the 500 cm−1 - 3500 cm−1 wavenumber spectral range. Figure 3 depicts FTIR spectra of the www.pss-c.com
Figure 3 FTIR spectra of monocrystalline silicon (ox-Si) and porous silicon after thermal oxidation.
The dark I-V characteristics and the FTIR spectroscopy investigations show the possibility to use PS as an oxide layer. Therefore (ox-PS) can be used as a low cost mask layer to realize a micro periodic n++/n doped silicon regions. 2.3 Porous silicon as an oxide diffusion mask Figure 4 describes the process that we use to determine the optimal thickness of the (ox-PS) layer to obtain an n-doped layer under PS. Using a phosphorous-based paste, the © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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samples were subjected to heavy phosphorous diffusion in an infrared furnace at 930 °C for 30 min. Using these conditions, n++ highly doped layer was achieved on CP4 polished silicon. For PS-treated samples, phosphorosilica glass and (ox-PS) were removed using a HF dilute solution followed by a NaOH etching. Finally, an Al/Ag paste was screen-printed on the back surface, and a very effective Al back surface field was performed via a 2 min drive-in at 860 °C in a belt furnace; Ag front contacts was screen printed and fired at 600 °C. The realized devices were subjected to dark I-V measurements to determine the optimal anodisation time (PS formation) that gives an ndoped layer after diffusion and PS removing.
Micro periodic linear windows were performed on the (oxPS) layer using a micro-motorized diamond tip. Two sets of samples were prepared.
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Figure 5 Dark I–V characteristics after heavy phosphorous diffusion were PS (3 min, 4 min, 6 min, 8 min and 13 min) is used as oxide mask
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The first set was used for LBIC characterization, wherein 50 µm and 100 µm grooving steps were employed. The second set with a 2000 µm step grooved areas is used for solar cell application. P-doped monocrystallin silicon Surface cleaning with CP4
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Step 5: Back and front screen-printing contacts Figure 4 Process descriptions enabling optimization of the phosphorus diffusion through PS.
Figure 5 depicts the dark I–V characteristics of the samples prepared using the process presented in Fig. 4. The 8 min anodisation sample presents the better I-V characteristic. The (ox- PS) obtained from 8 min anodisation time can be used as a mask to achieve n-doped layer. In a single step, we can obtain n++-doped layer on silicon and n-doped layer under PS when a heavy phosphorous diffusion is performed in an infrared furnace at 930 °C for 30 min. In the following, we use the 8 min anodisation based (ox-PS). 2.4 n++/n selective emitter using PS We describe the process adopted to achieve the micro n++ doped zones under the Ag front contact, and the active junction of the solar cell. In Fig. 6 we illustrate the fabrication sequences: the samples were subjected to CP4 etching, then PS formation using the 8 min optimal anodisation time and oxidation in an IR furnace at 750°C for 5min under an O2/N2 atmosphere.
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Micro-grooving of PS
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(a) Structure for LBIC scan
(b) Back and front screen-printing contacts Figure 6 Steps performed to achieve solar cells with selective n++/n-doped structures for LBIC investigation (a), solar cells application (b).
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For LBIC investigations, the grid front contacts were deposited away from the grooved areas, to enable LBIC laser scans. For the other samples the grid is deposited on the grooved area to realize selective emitter solar cells. Figure 7 shows Scanning Electron Microscopy (SEM) image of micro periodic linear windows (steps of 50 µm and 100 µm) opened throughout the oxidized PS layer using the micro-motorized diamond tip.
We notice a decrease of the LBIC signal when the laser beam is onto the grooved regions, outside these areas, the LBIC signal increases (the laser beam is on the active region). Figure 9 depicts the LBIC mapping of the grooved regions, showing the uniformity and the periodicity of the realized structure. In Fig. 10 we give the I-V characteristics of n++/n selective emitter based solar cells under AM1.5 illumination, and standard solar cell realized using the standard industrial process (reference). 50
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Figure 7 Scanning electron microscopy image showing PS and a 50 µm step mechanically micro-gooved areas.
The width of the micro grooved zone is 10 µm wide. When the distance between grooved fingers is less than 50 µm, we cannot obtain periodic grooves and the PS layer is scratched. After heavy phosphorous diffusion, n++-doped zones and n-doped regions between them were obtained. In Fig. 8 we present LBIC profiles corresponding to 50 µm and 100 µm grooving steps.
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Figure 9 LBIC mapping of the n++ micro-grid.
One may observe (Fig. 10) an enhancement of the solar cell performances. The short-circuit current density (Jsc) enhanced from 27.9 mA/cm2 to 29 mA/cm2 and the open circuit voltage (Voc) from 549.9 mV to 555.6 mV. The Jsc and Voc enhancements are essentially ascribed to the decrease of the contact resistances produced by the n++micro doped regions. Another important effect could be the phosphorus-porous silicon co-gettering [7, 20]. The external P-gettering that uses a sacrificial PS layer was reported to substantially enhance the majority carrier mobility in monocrystalline silicon. The heat treatment diffusion steps (throughout the PS layer) can getter eventual metal impurities towards the phosphorus-doped PS layer. The PS-phosphorus cogettering induces a decrease of the resistivity close to the surface in the active region and enhances the minority carrier diffusion lengths [21].
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Figure 8 Scanning Electron Micrograph of micro periodic linear windows (50 µm and 100 µm step).
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3 Conclusion Dark I-V characteristics and FTIR spectroscopy show that we can oxidize PS and use it as a low cost dielectric layer. The thicknesses of this layer can vary from a few nanometers to a few tens of micrometers. Due to its fragile structure, the PS layer can be easily grooved and employed to achieve micro periodic n++/n-doped silicon regions. Therefore oxidized PS can be utilized as a diffusion mask to realize a low cost selective emitter, particularly in the case of industrial screen-printed-based silicon solar cells. In some extent, and at this scale, this technology can replace the photolithography and the laser grooving techniques.
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