31st European Photovoltaic Solar Energy Conference and Exhibition
21.40% EFFICIENT LARGE AREA SCREEN PRINTED INDUSTRIAL PERC SOLAR CELL Daming Chen, Weiwei Deng, Jianwen Dong, Feng Ye, Huijun Zhu, Hui Li, Yuling Jiang, Beibei Gao, Ming Zhong, Yanfeng Cui, Yifeng Chen, Yang Yang, Zhiqiang Feng, Pietro P. Altermatt, and Pierre J. Verlinden State Key Laboratory of PV Science and Technology, Trina Solar Co., Ltd. No.2 Trina Road, Trina PV Park, New District, Changzhou, Jiangsu, China, 213031.
[email protected] ABSTRACT: We report on a process and performance analysis of screen printed industrial PERC solar cells with champion efficiency of 21.40% (as confirmed by Fraunhofer CalLab). Advanced cell processing technologies such as 5 bus-bars, fine line printing, rear side passivation and local metallization were adopted. A detailed cell characterization and a loss analysis show that the fill factor is lowered by recombination saturation mechanisms. Overall, the front metal optical shading, bulk recombination and internal resistance loss are the three dominant power losses. Keywords: PERC, passivation, local metallization, loss analysis 1
INTRODUCTION
The first passivated emitter and rear cell (PERC) was introduced in 1989, with an energy conversion efficiency of 22.8% on 4 cm2 float zone (FZ) silicon substrates [1,2]. After the standard full-area Al-BSF solar cell has been the dominant commercial cell design for more than 30 years, the PERC concept has been commercialized by several PV manufacturers since 2012. Presently, module fabrication and installation contributes substantially to the total cost, there is thus a higher demand for reaching higher cell efficiencies, so the PERC technology is growing and expanding rapidly in mass production. By August 2014, PERC capacity was estimated to be 2.5 GW worldwide [3]. Going from the standard to the PERC design necessitates two major changes: firstly, the recombination losses in the emitter need to be greatly reduced so, secondly, the reduction of recombination losses at the rear side can contribute significantly to efficiency improvements. The latter can only do so if the emitter does not dominate the total recombination losses. The rear of industrial PERC cells is passivated with dielectric thin film stacks such as SiO2/SiNx or AlOx/SiNx, and is contacted only locally by line, segmented, or point contacts. For industrial applications, screen printing technology is favorable for PERC cells. The efficiency of such screen printed PERC solar cells has increased rapidly in recent years. According to the record efficiency map of such cells by Dullweber [4], the record efficiency increased from 19.2% in 2010 (from Centrotherm) to 21.0% in 2012 (Schott Solar), to 21.2% in July 2014 (ISFH), to 21.40% in October 2014 (Trina Solar), to now 21.7% (SolarWorld). In this paper, we describe and analyze the fabrication process and the performance of the PERC solar cells of Trina Solar. A detailed loss analysis is conducted. 2 HIGH EFFICIENCY FABRICATION
PERC
SOLAR
presently reach an average efficiency of 20.90% in the pilot line production. Fig. 2 shows the efficiency distribution histogram for 950 cells. As expected [5], this distribution is not symmetric but has a longer tail towards lower efficiencies. This distribution is rather broad mainly because, in this batch, we have targeted for maximum efficiency. For example, narrow rear contact fingers then cause a stronger fluctuation in Rs and rear recombination than if slightly wider fingers had been chosen for more stable fabrication. In this way, the champion cell reached an efficiency of 21.40% in October 2014, independently measured by Fraunhofer CalLab, as shown in in Table. 1.
Figure 1: Process flow of PERC solar cells at the Trina Solar pilot line.
CELL
PERC solar cells are fabricated on large area (156×156 mm2) Cz boron-doped silicon wafers, approximately 170 μm thick and with resistivity between 2 Ωcm and 3 Ωcm. The main process flow is illustrated in Fig. 1. We have so far focused mainly on the solar cell design optimization and on the process control, and
Figure 2: Histogram of conversion efficiency of 950 PERC solar cells fabricated at the Trina Solar pilot line. Table I: Electrical parameters of the champion PERC
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solar cell, measured by Fraunhofer CalLab in October 2014, and simulated with Sentaurus (in brackets).
3
Tot. area (cm2)
Voc (mV)
Isc (A)
η (%)
244.11 ±0.24
672.1 ±2.3 (672.4)
9.68 ±0.18
21.40 ±0.43 (21.4)
Jsc (mA/cm2) 39.65 ±0.75 (39.64)
Vmpp (V) 571.9
Impp (A) 9.13 is 37.4 (37.7)
FF (%) 80.31 ±0.52 (80.34)
(568)
CELL ANALYSIS
3.1 Measurements Some representative cells were thoroughly characterized. The 1-sun IV curve, measured in-house, is shifted by Jsc to the first quadrant, so it can be displayed in logarithmic scale, where details become visible, see Fig. 3. Plotted at the bottom of Fig. 3 is also the local ideality factor nloc, calculated from the Shockley equation as
Figure 4: Symbols: Total, lumped series resistivity, extracted from three IV curves [6]. Lines: a polynomial fit to the measured resistivity data, as well as the internal and total series resistivity simulated in Sec. 3.2. From the measured IV curves, the total, lumped series resistivity was extracted, using the triple light-level method [6] where three IV curves are measured with about 10% different illumination intensity. The result is shown in Fig. 4. Rs,mpp is 0.6 Ωcm2, which is in the expected range. By repeating the IV measurements, we found that the resulting Rs fluctuates the most at higher voltages and we had to apply data smoothing to obtain the shown results. Correcting the measured voltage of the measured IV curve by (2) Vcorr V IRs (V ) yields an IV curve freed from all resistive losses, which is depicted in Fig. 3 as green symbols. We used the polynomial fit shown in Fig. 4 for this procedure. The ideality factor of this Rs-free IV curve is not always unity and will be discussed in Sec. 3.3. In order to analyze and understand the resistive losses better, we characterize the front and rear metallization. The front metal finger geometry was measured using a Zeta-20 optical microscope and is exemplarily shown in Fig. 5. The average width of the 102 fingers is 57.3 μm, and the average finger height 10.7 μm. Considering the cross-sectional shape shown in Fig. 5, the average crosssectional area of the fingers is approximately 295 µm2. There are five bus-bars with an average width of 730.3 μm. The resulting metallization fraction at the front is 3.69% for the fingers and 2.35% for the bus-bars, in total 6.04%. The following resistivities were measured by the transfer length method (TLM): The contact resistivity and line resistivity of the front fingers are 1.4 mΩ.cm2 and 2.9 μΩ.cm, respectively, while the average contact resistivity of the rear local contacts is 2.2 mΩ.cm2.
(1) At low voltages, nloc is already high, which is commonly observed in PERC cells and will be interpreted in Sec. 3.3.
Figure 3: Top: Measured 1-sun IV curve (red symbols), Rs(V)-corrected by the fit to the Rs measurements shown in Fig. 4 (green symbols), and as simulated (red line). Bottom: smoothed local ideality factor of the measured IV curve (red line) and of the Rs(V)-corrected IV curve (green line).
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Figure 6: Emitter phosphorus dopant profile measured by ECV before and after etching.
Figure 5: Profile of screen-printed front metal finger, measured with an optical microscope. The emitter phosphorus dopant profile was measured by the electrochemical capacitance-voltage (ECV) technique using a CVP21 from Ingenieurbüro WEP, and following the measuring procedures described in Ref. [7]. The ECV technique monitors the active dopant concentration [8], regardless of the ionization fraction, and is depicted in Fig. 6 before and after etching. The saturation current-density J0 of these two profiles was extracted from lifetime measurements performed with a Sinton WCT-120 lifetime tester, applying the method of Kane and Swanson [9]. Wafers with a resistivity between 12 and 15 Ωcm were etched in KOH, diffused with POCl3, some of them etched, and some of them passivated with SiNx. For the extraction of J0 from lifetime measurements, the intrinsic carrier density of ni = 1010 cm-3 is used. The lifetime is monitored at the injection density Δn of 3-5×1015 cm-3 for the J0 extraction [10] so the resulting J0 values are not underestimated and are therefore relevant for 1-sun condition. J0 is 191 fA/cm2 for the passivated n++ part of the emitter, 713 fA/cm2 for the contacted (i.e. unpassivated) n++ part, and 65 fA/cm2 for the passivated n+ part, see Fig. 7. An IR filter was positioned in front of the flash lamp such that the mismatch between the collection efficiency of the n ++ emitter in the sample and of the emitter in the monitoring solar cell of the lifetime tester does not cause substantial overestimation of J0 [11]. Considering that the n++ part is 200 µm wide, and weighting by area, J0 of the total emitter is 127 fA/cm2.
Figure 7: Inverse lifetime over injection density for the J0 measurement of the passivated n+ part of the emitter. For measuring the J0 at the rear side of the cell, the same wafers as in the above lifetime measurements were polished, passivated, contact lines with four different pitches processed (at one side of the sample only), and the metal removed. The J0 values for the respective metallization fractions f are plotted in Fig. 8. There is: J 0,tot J 0,pass,front (1 f ) J 0,pass,rear fJ 0,cont,rear (3) ( J 0,cont,rear J 0,pass ) f 2 J 0,pass where in the second line it has been considered that the passivation is the same at the front as at the rear. Hence, the slope of J0 vs. f is equal to J0,cont – J0,pass and the intercept with the y-axis is 2J0,pass. In Fig. 8, the resulting J0,cont is 732 fA/cm2; the resulting J0,pass is 7.15 fA/cm2, which corresponds to an effective surface recombination velocity of Seff ≈ 5 cm/s, considering that the current required at the surface for supplying carriers for recombination is p2 J rec qpSeff J 0 2 , (4) ni where the hole density p can be replaced by the wafer dopant density. Considering that J0 measurements generally tend to underestimate J0 [10], an Seff ≈ 5 cm/s is a lower bound and may be higher for 2-3 Ωcm material used for cell fabrication.
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Figure 8: Measured total J0 vs. the metallization fraction of the lifetime samples for the extraction of the J0 at the rear side of the cells.
Figure 10: Measured internal quantum efficiency of the champion PERC solar cell. 3.2 Assessment of resistive losses Resistive losses can be characterized in three different, commonly pursued ways: as total (or regionresolved) lumped series resistivity [Ωcm2], as power loss [W], or as cell efficiency loss [% abs]. Each way has its strengths and weaknesses. Like e.g. reflectance values are an effective way of quantifying and judging optical material properties, lumped resistivity is effective for quantifying and judging the resistive properties. Overall, we know that if a cell with the size of 156×156 mm2 has an Rs,tot ≈ 1 Ωcm2, the resistive losses are rather large, while if Rs,tot < 0.5 Ωcm2, we are nearly as low as can be expected from screen-printing. The disadvantage of lumped series resistance is that we do not always directly know how much power is dissipated at MPP, since we cannot always simply multiply Rs,tot with Jmpp due to the spreading nature of resistance and the shift of the IV curve. On the other hand, resistive losses in terms of watts or absolute cell efficiency do not give us a direct measure for judging how good we already are. The series resistivity at MPP, Rs.mpp, is 0.6 Ωcm2, as was shown in Fig. 4 above. Rs(V) can be readily reproduced with the combination of numerical device and SPICE simulations to be discussed in Sec. 3.3. The device simulations also yield the internal resistivity, Rs.int(V), by simulating IV curves at three different light intensities and applying the triple light-level method [6], and is also shown in Fig. 4. The resistivity caused by the front metallization, Rs.met, is the difference between the internal and the total Rs(V) and is 0.2 Ωcm2. It is independent of voltage, as is expected from a lowresistivity metal grid, and confirms that also analytical expressions instead of SPICE modeling for calculating Rs.met can be applied. Removing Rs.met yields a cell efficiency of 21.63% instead of 21.40%, meaning that the potential for improving cell efficiency by improving the front metallization is 0.23% absolute. The efficiency potential for completely removing the internal resistive losses is 0.53% absolute. This value is not simulated, however, but results from a shift of the voltage according to Eq. (2). This is an approximation, because the resistive losses are strongly coupled to the recombination losses: removing resistive losses increases Vmpp and hence shifts the amount of recombination between the various cell regions, and this is not taken into account when removing Rs(V) from the IV curve. A simulation with practically zero resistivity would require that the carrier mobilities were very large, which would, in turn, change Jsc and e.g.
Figure 9 shows the injection-dependent effective excess carrier lifetime τeff of a p-type 2-3 Ωcm wafer, passivated on both surfaces with an AlOx/SiNx stack, and measured with the Sinton WCT-120 lifetime tester. The lifetime may be fitted at the lower injection range with the SRH parameters of the boron-oxygen (BO) complex [12-14], or in the whole injection range with an additional shallow defect situated 0.1 eV below the conduction band edge.
Figure 9: Measured effective lifetime of a CZ p-type 2-3 Ωcm wafer (red symbols). Top: Fit with the BO-complex; Bottom: fit with the BO-complex and a shallow defect level 0.1 eV below the conduction band edge. Finally, the quantum efficiency was measured with a “PV measurement” tester and is shown in Fig. 10.
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the influence of the rear surface recombination on cell efficiency, etc.
printed fingers from Ref. [18], and assuming a shading factor of 97% for the bus-bars, the total shading of the front metallization amounts to 4.83% (2.55% for the fingers and 2.28% for the bus-bars). This implies that removing the shading has a potential to increase cell efficiency by 1.09% absolute (evaluated by performing the SPICE simulation with zero shading factor but keeping the resistance unchanged). The internal quantum efficiency is composed of both, optical and electrical properties via IQE = Aηc, where A is the absorbance and ηc is the collection efficiency of charge carriers at the metal contacts. This gives on the one hand an overall impression of a cell, but on the other hand makes it difficult to extract sufficiently precise optical and electrical parameters. In a good cell, the diffusion length for minority carriers in the base region is large, so it is rather irrelevant where exactly in the device the carriers are photogenerated and, accordingly, it is difficult to discern between properties of the base region and of the rear surface. Also, it is well known that the IQE is not very sensitive to good emitters, making it rather impossible to extract for example the front surface recombination velocity. However, some optical properties of the cell can be extracted. An elegant method [19] is plotting 1/IQE vs. absorption length 1/α(λ), see Fig. 12, in the spectral range where silicon is weakly absorbing. In that range, the absorbance A in IQE = Aηc, can be expressed as 1 – e-α(λ)W, where the cell thickness W can be replaced by the effective path length Z(λ)W, so Z is a measure for the extended path length of light found in light-trapping structures [20]. Our extracted average Z value is 8.9 and compares well with experiments of other cells [14].
3.3 Assessment of recombination losses For the quantification of recombination losses, we use two-dimensional device modeling with Sentaurus, combined with a SPICE model for the metal grid and the optical shading. For the device modeling, the Si model parameters of Refs. [15-17] are taken.When taking all of the parameters measured in Sec. 3.1 as input, the simulations reproduce the measured IV curve shown in Fig. 3 without the need of adjusting parameters. The resulting IV parameters are shown in Table I as brackets. The recombination currents are shown in Fig. 11.
Figure 11: Recombination losses, simulated with Sentaurus, of the champion cell. At MPP, the recombination losses in the base dominate, closely followed by the recombination losses in the emitter. The recombination in the local BSFs also influences cell efficiency to some extent, but the recombination losses at the contacts and at the rear surface are so small that they influence cell efficiency only weakly. Note that the losses in the base region saturate towards higher injection levels (higher voltages). This occurs due to the improvement of the excess carrier lifetime, shown in Fig. 9, due to the asymmetric capture cross-section of the BO-complex [12-14]. Therefore, Voc is mainly influenced by the emitter, followed by the BSFs. This behavior is typical for PERC cells, where the recombination losses in the emitter and at the rear are reduced to an extent that the recombination losses in the base influence device behavior rather strongly. The main effect of the BO-complex is a reduction of the fill factor FF. Therefore, the ideality factor, calculated using Eq. (1), does not go to unity after removing the resistive losses, as is shown in Fig. 3 (bottom), except near Voc, where the emitter losses dominate the total losses. Hence, PERC cells tend to have a low FF, and in the optimization one has to carefully distinguish between the influence of resistive losses and of recombination losses that lower the FF. Also note that at short-circuit condition, the emitter dominates again. This influences the assessment of the optical losses in the Section below.
Figure 12: Inverse of IQE vs. absorption depth of silicon for the optical and electrical analysis.
3.4 Optical analysis As outlined in Sec. 2, the front metallization fraction is 6.04% (3.69% for the fingers and 2.35% for the busbars). Taking 69% as the shading factor for screen-
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REFERENCES [1] A. W. Blakers, A. Wang, A. M. Milne, J. Zhao, and M. A. Green, 22.8% efficient silicon solar cell, Applied Physics Letters 55, 1363-1365 (1989). [2] M. A. Green, The Passivated Emitter and Rear Cell (PERC): From conception to mass production, Solar Energy Materials & Solar Cells 143, 190–197 (2015). [3] NPD Solarbuzz, August 19, 2014, http://www.solarbuzz.com/resources/articles-andpresentations/perc-capacity-hits-25gw-and-offers-newtechnology-buy-cycle-option [4] T. Dullweber, H. Hannebauer, U. Baumann, T. Falcon, K. Bothe, S. Steckemetz, R. Brendel, Proceedings 29th European Photovoltaic Solar Energy Conference and Exhibition, Amsterdam, The Netherlands, (2014), pp. 22. [5] M. Müller, P.P. Altermatt, H. Wagner, G. Fischer, Sensitivity analysis of industrial multicrystalline PERC silicon solar cells by means of 3-D device simulation and metamodelling, IEEE Journal of Photovoltaics 4, 107 – 113 (2014). [6] K.C. Fong, K.R. Mcintosh, A.W. Blakers, Accurate series resistance measurement of solar cells, Prog. PV 21, 490 (2013). [7] R. Bock, P. P. Altermatt, and J. Schmidt, Accurate extraction of doping profiles from electrochemical capacitance voltage measurements, Proceedings 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain (2008), 1510. [8] M. Rauer, M. Rüdiger, C. Schmiga, H. Strutzberg, M. Bähr, M. Glatthaar, S.W. Glunz, Incomplete ionization of aluminum in silicon and its effect on accurate determination of doping profiles, J. Appl. Phys. 114, 203702 (2013). [9] D.E. Kane, R.M. Swanson, Measurement of the emitter saturation current by a contactless photoconductivity decay method, 18th IEEE Specialists Conference, Las Vegas (1985), 578. [10] B. Min, A. Dastgheib-Shirazi, P.P. Altermatt, H. Kurz, Accurate determination of the emitter saturation current density for industrial P-diffused emitters, Proceedings 29th European Photovoltaic Solar Energy Conference, Amsterdam (2014), 463. [11] T. Ohrdes, R. Peibst, N.P. Harder, P.P. Altermatt, R. Brendel, Characterization of the emitter collection efficiency by contactless photoconductance measurements, Proceedings 23rd Photovoltaic Science and Engineering Conference (PVSEC), Taipei, Taiwan (2013). [12] K. Bothe, R. Sinton, J. Schmidt, Fundamental boronoxygen-related carrier lifetime limit in mono- and multicrystalline silicon , Prog. PV 13 (2005) 287. [13] J. Schmidt, A. Cuevas, Electronic properties of lightinduced recombination centers in boron-doped Czochralski silicon, J. Appl. Phys. 86 (1999) 3175. [14] S. Rein, S.W. Glunz, Electronic properties of the metastable defect in boron-doped Czochralski silicon: Unambiguous determination by advanced lifetime spectroscopy, Appl. Phys. Lett. 82 (2003) 1054. [15] P. P. Altermatt, Models for numerical device simulations of crystalline silicon solar cells—a review, J. Computational Electronics 10 (2011) 314. [16] A. Richter, S.W. Glunz, F. Werner, J. Schmidt, A. Cuevas, Improved quantitative description of Auger
Figure 13: Since the optical losses are nearly the only losses that have an approimately linear behaviour in cell design, the optical losses can be represented in this additive column graph. When assessing optical losses, it is advantageous to quantify them in units mA/cm2 (which is the number of photons times unit charge), because this gives a direct comparison to the achieved Jsc. It is also common to consider the optical losses solely between λ = 300 nm and 1200 nm, because below 300 nm there exist hardly any photons in the am1.5g spectrum, and above 1200 nm Si is virtually transparent. In this spectral range, the am1.5g spectrum contains the equivalent of 46.3 mA/cm2 photons. In the Lambertian light trapping limit, 43.8 mA/cm2 photons can be maximally photogenerated in the 170 µm thick cell [21]. However, our PERC cells deliver a Jsc = 39.65 mA/cm2, so 4.15 mA/cm2 get lost. From our device simulation in Sec. 3.3 and Fig. 11 we know that 0.21 mA/cm2 is lost due to recombination at short-circuit condition. So the total optical losses amount to 43.8 – 39.65 – 0.21 = 3.94 mA/cm2. From our analysis [20] follows that 0.77 mA/cm2 is lost due to insufficient light trapping, 1.99 mA/cm2 by front metal shading, 0.46 mA/cm2 by front reflection, and 0.16 mA/cm2 by absorption in the front ARC (calculated with OPAL [22]). Hence, the remaining 0.56 mA/cm2 are lost due to freecarrier absorption in Si, parasitic absorption at the rear surface (mainly by Al), etc. 4 CONCLUSIONS A champion PERC solar cell with efficiency of 21.40%, confirmed by Fraunhofer CalLab, was fabricated employing the industrial screen printing technology. We characterized some cells from the same batch as the champion cell in detail, and performed a loss analysis by means of a combination of numerical device modeling and SPICE simulations, and an optical loss analysis. This analysis can be used to further improve the PERC cell fabrication and to understand fluctuations in fabrication better.
5 ACKNOWLEDGEMENTS The authors would like to thank Esther Lee for the detailed review. This work is supported by the National “863” Project, under the Project Number of 2015AA050302 and the Natural Science Foundation of Jiangsu Province under the Project Number of SBK2015022275.
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recombination in crystalline silicon, Physical Review B 86 (2012) 165202. [17] H. Steinkemper M. Rauer, P.P. Altermatt, F.D. Heinz, C. Schmiga, M. Hermle, Adapted parameterization of incomplete ionization in aluminumdoped silicon and impact on numerical device simulation, J. Appl. Phys.117 (2015) 074504. [18] R. Woehl, M. Hörteis, S. W. Glunz, „Determination of the effective optical width of screen-printed and aerosol-printed and plated fingers”, Proceedings 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain (2008), 1377. [19] P.A. Basore, Extended spectral analysis of internal quantum efficiency, Proceedings 23rd IEEE PV Specialists conference, Lousville (1993), 147. [20] J.A. Rand, P.A. Basore, Light-trapping silicon solar cells: experimental results and analysis, Proceedings 22nd IEEE Photovoltaic Specialist Conference, Las Vegas (1991), 192. [21] T. Tiedje, E. Yablonovitch, G. D. Cody, and B. G. Brooks, Limiting efficiency of silicon solar cells, IEEE Trans. Electron. Devices 31 (1984) 711. [22] OPAL2, www.pvlighthouse.com.au.
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