N-PERT BACK JUNCTION SOLAR CELLS: AN OPTION FOR THE NEXT INDUSTRIAL TECHNOLOGY GENERATION?

N-PERT BACK JUNCTION SOLAR CELLS: AN OPTION FOR THE NEXT INDUSTRIAL TECHNOLOGY GENERATION? Bianca Lim *, Till Brendemühl, Miriam Berger, Anja Christ, Thorsten Dullweber Institute for Solar Energy Research Hamelin (ISFH) Am Ohrberg 1, D-31860 Emmerthal, Germany * telephone: +49 5151 999 313, fax: +49 5151 999 400, email address: b.lim@isfh.de ABSTRACT: In this work, we present back-junction (BJ) n-pert (Passivated Emitter, Rear Totally Diffused) solar cells with a processing sequence based on an industrial-type p-perc (Passivated Emitter and Rear Cell) process, with the addition of a boron diffusion. Through this, we achieve efficiencies up to 20.5% on n-pert BJ solar cells, which do not degrade under subsequent illumination. For comparison, reference p-perc solar cells fabricated on 1 and 3 cm boron-doped Cz-Si achieve efficiencies up to 20.6% before light-induced degradation (LID) and 20.1% (3 cm) and 19.7% (1 cm), respectively, after LID. We find that the width of the laser contact opening (LCO) on the rear strongly influences the n-pert BJ solar cell performance. Wider LCOs significantly increase the shortcircuit current, open-circuit voltage, and pseudo fill factor, resulting in efficiency increase of up to 1.2% absolute. We attribute this to an increased thickness and homogeneity of the Al-p + regions beneath the contacts, which effectively reduces recombination at the contacts. By varying the metallization fraction on the rear side, we determine the specific contact resistance of the Al contact to be c = (8 ± 2) m cm 2 and the saturation current density to be J 0.met of (320 ± 50) fa/cm 2. Keywords: n-type, Silicon Solar Cell, Recombination 1 INTRODUCTION The vast majority of crystalline silicon solar cells are based on boron-doped (B) silicon and feature a full-area aluminum (Al) contact on the rear side, which results in strong rear surface recombination. Currently, a number of solar cell manufacturers are introducing a new solar cell design: the Passivated Emitter and Rear Cell (PERC), which features a full-area dielectric rear passivation and only local Al rear contacts (see Fig. 1(a)). Through this, recombination losses are strongly reduced and higher open-circuit voltages can be obtained. In addition, the presence of the dielectric increases the rear side reflectance, thus increasing the generated current in the solar cell [1]. Using B-doped Czochralski-grown silicon (Cz-Si), PERC solar cell efficiencies between 20.0% and 20.9% have been obtained by several solar cell manufacturers [2-5]. Recently, a new record efficiency of 21.2% was reported for industrial p-perc solar cells [6], using a processing sequence very similar to the one used in this study. However, after light-induced degradation (LID) the efficiency of PERC solar cells decreases between 0.5% abs. and 1.0% abs. depending on the wafer resistivity [7,8]. A possibility to circumvent the detrimental effect of LID is the use of n-type Si wafers and the concept of the Passivated Emitter, Rear Totally Diffused (PERT) solar cell, which typically features a boron-doped emitter at the front and a phosphorus-doped BSF at the rear and applies screen-printed contacts grids on both the front and the rear side. Such bifacial PERT solar cells have so far achieved energy conversion efficiencies up to 20.5% [9]. At the same time, n-pert back-junction (BJ) solar cells, which have the boron emitter at the rear as shown in Fig. 1(b) and are very similar to p-perc solar cells in terms of cell architecture and processing sequence, have already achieved efficiencies up to 20.7% [10]. In this work, we process p-perc and n-pert BJ solar cells in parallel and analyze the performance of both. We determine the saturation current densities of the passivated diffusions as well as of the contacted areas. In addition, we determine the specific contact resistance of the Al contact on the rear side by varying the rear side metallization fraction from 3% to 30%. 2 SOLAR CELL PROCESS Figure 1: Schematic drawing of (a) p-type PERC solar cells and (b) n-type PERT back-junction solar cells. For the p-perc reference solar cells, we use 239 cm 2 B-doped Cz-Si wafers with resistivities between 1 and 3 cm. The n-pert BJ solar cells are fabricated on 6 cm P-doped Cz-Si (also 239 cm 2 ). After damage etching, the n-pert BJ solar cells undergo a BBr 3 quartz furnace diffusion. Subsequently, the rear side of all solar cells (p- PERC and n-pert BJ) is coated with a protection layer, which acts as etching and diffusion barrier in the following alkaline texturing and phosphorus diffusion. After a POCl 3 quartz furnace diffusion with a sheet resistance of 80 /sq., the protection layer and the phosphorus glass are removed by wet chemistry and the

Table I: Processing steps for p-perc and n-pert BJ solar cells. Blue processing steps are identical for both solar cells concepts. p-perc n-pert BJ Wafer cleaning Rear protection layer Texturing P-diffusion PSG + dielectric etch Passivation Laser contact opening Screen-printing Co-firing Wafer cleaning B-diffusion Rear protection layer Texturing P-diffusion PSG + dielectric etch Passivation Laser contact opening Screen-printing Co-firing rear side is passivated using a stack of atomic layer deposited Al 2 O 3 and plasma-enhanced chemical vapor deposited (PECVD) SiN x. Then, the front side is passivated with PECVD SiN x. Laser contact openings (LCO) are formed on the rear side using a picosecond laser with 532 nm wavelength. For the silver front side metallization we use print-on-print as a fine line printing technique. The rear is full-area printed with a commercially available Al paste which has been specifically designed for p-perc cell applications. Both the front and the rear contacts are fired in a single step. An overview of the processing steps is given in Tab. I. As can be seen, the only difference in the processing sequence of p-perc and n-pert BJ cells is the additional boron diffusion. 3 IMPACT OF REAR CONTACT WIDTH Despite the strong resemblance of p-perc and n- PERT BJ solar cells, there are some notable differences, especially in mode of operation. In n-pert BJ solar cells, the Al-p + region underneath the Al rear contacts acts as part of the rear-side emitter, in particular since the B-diffusion profile in that region is completely removed through the Al-Si alloying process during the firing step. As a consequence, a continuous Al-p + region of good quality is crucial for n-pert BJ solar cells in order to avoid shunts or enhanced space charge region recombination. In contrast, in p-perc solar cells the Alp + acts as a back surface field (BSF), which repels minority charge carriers from the rear contacts. The Al- BSF is not critical for shunts and enhanced rear contact recombination would not impact the diode quality factor. Also, the boron-diffused emitter has a much higher conductivity than an average p-type wafer, allowing for wider contact spacing on the rear side for n-pert BJ cells in comparison to p-perc cells. As a consequence, we investigated the impact of different LCO widths on the performance of n-pert BJ solar cells. We kept the total metallization fraction of the rear constant at 10%, adjusting the contact spacing accordingly. After firing, the width of the rear contacts increases by 20 µm to 30 µm due to the alloying process, as has been reported before [11]. Figure 2(a) depicts the dependence of the efficiency on the relative LCO width, where 1 corresponds to the LCO width used for the p-perc baseline process at Figure 2: (a) Efficiency, (b) open-circuit voltage V oc, (c) short-circuit current density J sc, and (d) pseudo fill factor pff of n-pert BJ (red symbols) and p-perc solar cells (blue symbols) as a function of relative laser contact opening (LCO) width. ISFH. The red symbols correspond to n-pert BJ solar cells, whereas the blue symbols refer to p-perc solar cells. Each data point represents one solar cell, however, the same trend is observed for the average value of 5

solar cells. In Fig. 2(b) the dependence of the open-circuit voltage V oc is displayed, Fig. 2(c) shows the short-circuit current density J sc and Fig. 2(d) the pseudo fill factor pff. For the n-pert BJ solar cells, all parameters show a strong increase with increasing LCO width: V oc increases from 650 mv to 667 mv, while J sc increases from 38.4 ma/cm 2 to 38.9 ma/cm 2. At the same time, the pseudo fill factor pff increases from 81.0% to 83.0%. In total, these gains result in an efficiency increase from 19.4% to 20.6%. However, beyond a relative LCO width of 2.5 the efficiency decreases due to an increased series resistance caused by the wide rear contact spacing. In p-perc solar cells, on the other hand, J sc and pff are unaffected by the LCO width while V oc changes by 2 mv. While this is close to the measurement uncertainty, it is also comparable to the change observed for n-pert BJ solar cells. For further characterization, we measured J sc V oc curves and fitted them using the two-diode-model, thus obtaining J 01 and J 02. Figure 3 depicts the J sc V oc curve of a p-perc (black circles) and n-pert BJ (red diamonds) solar cell. In addition, the slope of a diode with an ideality factor of 1 (dashed blue line) and 2 (dashed green line) is shown. The data of the p-perc solar cell closely follows a diode with an ideality of 1, resulting in a low J 02 value of 1 na/cm 2. In contrast, the n-pert BJ solar cell converges to a diode with ideality factor of 2 at voltages lower than 550 mv, which corresponds to larger J 02 values of 5 na/cm 2 at optimal LCO width and up to 20 na/cm 2 at narrowest LCO width. The increase of J 02 increases the ideality factor at maximum power point and thus decreases the pff. At the same time, we observe that J 01 increases from 200 fa/cm 2 at optimal LCO width to 300 fa/cm 2 at the narrowest investigated LCO width, which corresponds to the decrease of V oc with decreasing LCO width. Further investigations using scanning electron microscopy (SEM) indicate that narrower LCOs more often exhibit thinner or even missing Al-p + regions, in particular in combination with voids (i.e. missing Al-Si eutectic beneath the Al). Wider lines, on the other hand, exhibit continuous and homogeneous Al-p + regions, even in the presence of voids, which is similar to the results reported in Ref. [12]. The increased J 01 term could thus be a result of increased recombination at the Al contact, caused by an increased minority-carrier gradient in the emitter due to the thinner Al-p + region. The increased J 02 term, on the other hand, indicates recombination in the space charge region between n-type wafer and p + -type Al-emitter, and thus indicates the presence of defects in the Al-p + region. Note that such defects would not induce increased J 02 terms in p-perc solar cells, since the Al-p + acts as a BSF. However, even at optimal LCO width, J 02 of the n- PERT BJ solar cells is about 3 times higher than for the p-perc solar cells, resulting in 0.6% (absolute) lower pff. Further improvement of the quality of the Al-p + region will thus be important to fully realize the potential of n-pert BJ solar cells. The results of the best solar cells are summarized in Tab. II. Using 1 cm p-si as well as 3 cm p-si, the p- PERC solar cells achieve energy conversion efficiencies up to 20.6% (before light-induced degradation), whereas the n-pert BJ solar cells yield up to 20.5% (independently measured at ISE CalLab). While the Figure 3: J sc V oc curves of a p-perc (black circles) and n-pert BJ (red diamonds) solar cell. The dashed blue line indicates the slope of a diode with an ideality factor of 1, whereas the green dashed line indicates the slope of a diode with an ideality factor of 2. The p-perc solar cell follows the n = 1 curve closely, whereas the n-pert BJ solar cell starts to converge to n = 2 around 0.55 V. As a consequence, the pseudo fill factor pff of the n- PERT BJ solar cell is lower than for the p-perc solar cell. short-circuit current density J sc is very similar for both solar cell types, the open-circuit voltage V oc and fill factor FF differ slightly. The p-perc solar cells feature a lower V oc but a higher FF. As already discussed above, the higher FF is not exclusively due to a lower series resistance but is also a result of a higher pseudo fill factor pff. After complete LID (16 h of illumination at room temperature for 1 cm p-si and 32 h for 3 cm p-si), the p-perc solar cells fabricated on 1 cm Cz-Si achieve efficiencies up to 19.7% ( = 0.9%) while the 3 cm solar cells yield 20.1% ( = 0.5%). The n-pert BJ solar cells, on the other hand, are stable under illumination (efficiencies ±0.1% were measured after 16h of illumination, which is within the measurement uncertainty) and thus yield 0.4% higher efficiencies than the degraded p-perc solar cells. Table II: Solar cell parameters of the best p-type PERC solar cells (before and after LID) and the best n-pert BJ solar cell obtained from IV measurements performed at standard testing conditions (25 C, AM1.5G spectrum). Best cell η [%] J sc [ma/cm 2 ] V oc [mv] FF [%] p-perc, 1 cm 20.6* 38.8* 658* 80.5* p-perc, 1 cm (degraded) 19.7 38.1 645 80.1 p-perc, 3 cm 20.6 38.9 661 80.1 p-perc, 3 cm (degraded) 20.1 38.8 657 79.0 n-pert BJ, 6 cm 20.5* 38.7* 665* 79.8* *independently measured at ISE CalLab

4 CHARACTERIZATION OF LOCAL AL CONTACTS After finding an optimal LCO width of 2.5 times the standard p-perc LCO width, we further characterized the local Al contacts and the local Al-p + emitter on the n- PERT BJ solar cells by varying the rear contact pitch and consequently the metallization fraction f of the rear. Changing f from 3% to 30%, we observe a strong dependence of both the open-circuit voltage V oc and the series resistance R s. The V oc decreases from 662 mv to 655 mv with increasing metallization fraction f due to increased recombination. The overall recombination is the sum of recombination in the base, the FSF, the emitter, and at the contacts. By varying the rear side metallization fraction, we are able to extract the saturation current density at the rear contacts, assuming all other recombination currents remain the same. This is true for all contributions except the passivated emitter, since its fraction decreases with increasing emitter contact area. However, the change is in the range of 3 fa/cm 2 and can thus be neglected. Instead of converting the open-circuit voltage of the solar cells into an overall J 0, which relies on the assumption that the ideality of the solar cells equals 1, we analyze the J 01 term of the 2-diode-model fit of the J sc V oc curve. We plot J 01 as a function of f in Fig. 4. The linear fit to the experimental data (red line) yields (320 ± 50) fa/cm 2 for the saturation current density at the rear contacts. Note that a similar study using the standard p-perc LCO width yielded a higher J 0.met value of 650 fa/cm 2. The latter value is in good agreement with experimental data of the surface recombination velocity at local Al contacts published in Ref. [12], where S met = 400 cm/s was reported, corresponding to a saturation current density of approximately 750 fa/cm 2. In addition, we have measured the saturation current densities J 0 of the wafer, the passivated emitter, and the passivated FSF using non-metallized lifetime samples and assuming an intrinsic carrier concentration n i = 9.65 10 9 cm 3. Using published data for the contacted FSF [13] as well as for the rear surface recombination velocity Figure 4: Total saturation current density J 01 of n-pert BJ solar cells determined from the 2-diode fit as a function of rear side metallization fraction f. The red line corresponds to a linear fit of the data. The gradient yields the J 0 contribution of the Al contact J 0.met = (320 ± 50) fa/cm 2. Table III: Contributions to the overall saturation current density J 0 of the different solar cell regions. n-pert BJ J 0 Area fraction Weighted Contact P-diff (FSF) 500 0.061 31 Pass. P-diff (FSF) 120 0.939 113 Wafer 10 1 10 Pass. B-diff (emitter) 25 0.90 22 Contact B-diff (emitter) 320 0.10 32 p-perc, 3 cm Corresponding V oc J 0 Total 208 Area fraction 667 mv Weighted Contact P-diff (emitter) 500 0.061 31 Pass. P-diff (emitter) 120 0.939 113 Wafer (before LID) 40 1 40 Pass. base 20 0.89 18 Contact base 750 0.11 83 Corresponding V oc Total 285 659 mv of p-perc solar cells [12] and area weighting all contributions, we obtain J 01.total = 285 fa/cm 2 for the p- PERC solar cells (before LID) and J 01.total = 208 fa/cm 2 for the n-pert BJ solar cells (see Tab. III). Assuming a short-circuit current density J sc of 39.0 ma/cm 2, these J 01.total values imply open-circuit voltages V oc of 667 mv (n-pert BJ) and 659 mv (3 cm p-si), respectively, which is in good agreement with the open-circuitvoltages obtained on the finished solar cells (see Tab. II). The decrease of the series resistance R s from 2.25 cm 2 to 0.54 cm 2 with increasing metallization fraction is mainly due to a reduced series resistance contribution of the emitter. However, the larger contact area and thus a lower overall contact resistance also reduces the total series resistance. Taking into account the resistance contribution of the emitter R em through two-dimensional simulations using the conductive boundary (CoBo) model [14] and Quokka software [15] and assuming that all other resistance contributions remain unchanged, the contact resistance of the local Al contact can be extracted by plotting the difference of total series resistance R s and emitter resistance contribution R em as a function of inverse metallization fraction 1/f, as shown in Fig. 5. The linear fit (red line) yields R s R em = 1/f 8 m cm 2 + 0.49 cm 2 (1) i.e. c = (8 ± 2) m cm 2, which is close to what Urrejola et al. found (8 to 18 m cm 2 ) [16] but about a factor 5 smaller than what has been published by Gatz et al. (40 to 55 m cm 2 ) [11]. Interestingly, Müller and Lottspeich very recently

PERT BJ solar cell, these defects would lie within the pnjunction, inducing non-ideal recombination, whereas in p-perc solar cells, the Al-p + acts as BSF and defects at the pp + -junction would contribute to J 01. For an optimal LCO width, we also determined the saturation current density of the Al contact J 0.met of (320 ± 50) fa/cm 2 and a specific contact resistance of (8 ± 2) m cm 2. Using the former in an analysis of the overall recombination, we find that for our current n-pert BJ solar cells more than 50% of the total recombination is caused by the passivated phosphorus-diffusion, i.e. the front surface field. In order to further improve the n- PERT BJ solar cells, improvement of the FSF is thus mandatory. Figure 5: Series resistance R s of n-pert BJ solar cells, determined using the double-light-level method, plotted as a function of inverse rear side metallization fraction 1/f. The experimental data is fitted by a linear function (red line): R s R em = 1/f 8 m cm 2 + 0.49 cm 2. We thus determine a contact resistance of the Al to the Al-p + emitter of (8 ± 2) m cm 2. observed that previous publications might have overestimated c [17]. Their own findings yield an absolute contact resistance per line of 0.46 cm. Assuming the contact width of 60 µm used in their device simulations, this corresponds to a specific contact resistance of 3 m cm 2, which is even lower than what we found. 5 CONCLUSIONS We have fabricated fully screen-printed n-pert BJ solar cells with efficiencies up to 20.5% based on a well established processing sequence for p-perc solar cells. p-perc reference solar cells made on 1 cm and 3 cm B-doped Cz-Si, respectively, achieved efficiencies up to 20.6%. However, during illumination at room temperature the 1 cm p-perc solar cell degraded by 0.9% to 19.7% and the 3 cm p-perc solar cell degraded by 0.5% to 20.1%, whereas the n-pert BJ solar was stable under illumination, thus yielding 0.4% higher efficiency. Varying the width of the laser contact openings on the rear side, we observed a strong increase of opencircuit voltage, short-circuit current density, pseudo fill factor, and consequently of solar cell efficiency, for the n-pert BJ solar cells with increasing LCO width. Comparing the solar cell characteristics at narrowest investigated and optimal LCO width, we find that J sc increases by 0.4 ma/cm 2, V oc increases by about 15 mv, pff increases by 2.0%, and increases by more than 1% absolute. The improved solar cell performance with wider LCO contacts may be due to thicker and more homogeneous Al-p + regions underneath the Al contacts, as indicated by SEM investigations. The increased thickness of the Al-p + region reduces the minority charge carrier gradient in the p + region, thereby reducing the contact recombination. In addition, the reduced pff of n- PERT solar cells in comparison to p-perc solar cells suggests the presence of defects in the Al-p + region. In n- References [1] T. Dullweber et al., Prog. Photovolt 20, p. 630 (2012) [2] P. Engelhart et al., in: Proc. 26th EUPVSEC, Hamburg, Germany, pp. 821 826 (2011) [3] D. Chen et al., in: Proc. 28th EUPVSEC, Paris, France, pp. 770 774 (2013) [4] B. Tjahjono et al., in: Proc. 28th EUPVSEC, Paris, France, pp. 775 779 (2013) [5] G. Fischer et al., in: Proc. 4th SiliconPV, s Hertogenbosch, Netherlands, in press (2014) [6] H. Hannebauer et al., Phys. Status Solidi, 8 (8), pp. 675-679 (2014) [7] Y. Gassenbauer et al., IEEE J. Photovolt. 3, p. 125 (2013) [8] T. Dullweber et al., Proc. 39th IEEE PVSC, Tampa, USA, in press (2013) [9] T. S. Böschke et al, IEEE J. Photovolt. 4 (1), p. 48 (2014) [10] V. Mertens et al., Proc. 28th EUPVSEC, Paris, France (2013) [11] S. Gatz, T. Dullweber, and R. Brendel, IEEE J. Photovolt. 1 (1), p. 37 (2011) [12] S. Gatz et al., Energy Procedia 27, pp. 95-102 (2012) [13] C. Kranz et al., Proc. 27th EUPVSEC, Frankfurt, Germany (2012) [14] R. Brendel, Prog. Photovolt: Res. Appl. 20, pp. 31 43 (2012) [15] A. Fell, IEEE Trans. Electron Devices 60 (2), pp. 733-738 (2012) [16] E. Urrejola et al., J. Appl. Phys. 107, p. 124516 (2010) [17] M. Müller and F. Lottspeich, J. Appl. Phys. 115, p. 084505 (2014)