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HIGH PERFORMANCE MONOCRYSTALLINE SILICON SOLAR CELLS BY USING REAR SURFACE PASSIVATION OF Al2O3SiNx


23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain

HIGH PERFORMANCE MONOCRYSTALLINE SILICON SOLAR CELLS BY USING REAR SURFACE PASSIVATION OF Al

2O3/SiNx STACKS STRUCTURE W. L. Chang*, W, C. Sun, C. H. Lin and C. W. Lan Photovoltaics Technology Center Industrial Technology Research Institute, Hsinchu, 310 Taiwan, R. O. C. ABSTRACT: High performance p-type crystalline silicon(c-Si) solar cells rely on excellent surface passivation. In this study, we use PECVD for silicon nitride (SiNx) and atomic layer deposition for Al2O3 to produce two-layer rear surface passivation in low temperature. Based on cell efficiency measurement, this two-layer stacks structure (Al2O3/SiNx) with 20.1% is better than monolayer (SiNx) structure with 17.5%. Also, according to quantum efficiency measurement, the higher energy conversion efficiency, which is located in the IR range, explains better efficiency of this two-layer stacks structure as compared to non-two-layer one. Keywords: passivation, silicon nitride, atomic layer deposition, solar cells, photovoltacics 1 INTRODUCTION Figure 1 shows the layout of the fabricated solar cell structure used in this study to demonstrate the applicability of Al2O3 rear-side passivation for highefficiency solar cells and Table 1 shows the corresponding processing flow sequence.
Ag metal grid

The photovoltaic (PV) industry has been growing rapidly over the past several years because of the global warming and energy crisis. The current trend in c-Si based PV products towards thinner wafers and higher efficiencies coming from an effective reduction of surface recombination losses. Besides, adding a proper passivation layer would enhance the cell efficiency [1]. Some popular materials, like SiNx:H, SiO2 and SiCx, are adopted in passivation layers and have been applied on the silicon solar cell for a long time. For the p-type solar cell process, using the plasma enhanced chemical vapor deposition (PECVD) of hydrogenated silicon nitride (SiNx) has resulted in a poor passivation for n+ emitters [2-3]. Therefore, a better method to passivate n+ surfaces needs developing. Reported approaches to solve this issue are either by using an intermediate silicon dioxide (SiO2) layer between SiNx and the n+ diffused emitter, or by using PECVD silicon carbide (SiCx) as a passivation layer instead of SiNx [4-5]. However, the SiO2 layer is thermally grown at high temperature for a long time, which could result in reducing lifetime of Si wafers and increasing the production cost of the cells [6]. Recent results by Mihailetchi et al. demonstrated that an ultrathin SiO2 layer formed by nitric acid oxidation of Si and then combined with SiNx could result in good passivation of p+ emitters [7]. Hoex et al. also showed that Al2O3 films could have an excellent level of surface passivation on highly doped n+ or p+ emitters [8-9]. Still, a simple and effective method to passivate n+ emitters is on demand if p-type solar cells can be mass produced in a cost-effective way. In this investigation, we use quantum efficiency and solar simulation to evaluate the quantum yield and energy conversion efficiency of the solar cells, and demonstrate that aluminium oxide (Al2O3) thin film is an excellent alternative of surface passivation on n+ emitters to make a high-efficiency c-Si solar cell. Moreover, the SiO2 layer formed by standard RCA process and later combined with Al2O3/SiNx stacks structure to passivate n+emitters make it possible not only to have a high-efficiency solar cell but to provide a cost-down, mass-produced way. 2 EXPERIMENTAL

n+ Emitter

SiNx

p-type Si
p+ (Al-Si) BSF
Bare or SiNx or Al2O3/ SiNx stack

Al metal grid

Figure 1: Schematic layout of the fabricated solar cells used in this study to demonstrate the applicability of an Al2O3/ SiNx stack- rear surface passivation for highefficiency solar cells Initially, we used (100)-oriented boron-doped p-type Czochralski (Cz) Si wafers of 1.5?cm resistivity and 210μm thickness. The wafer was cleaned in a dilute HF solution prior to solar cell standard processing to remove the native oxide. Then, thermal oxide was grown using a wet oxidation furnace after texturization. Using a standard process for acidic etch recipe with texturization can remove saw damage in one single process step. Later, gaseous oxygen was passed through the heated DI water and led to the furnace. The temperature of furnace was below 850°C. The thickness of the thermal oxide was around 100nm. The oxide on the front side was removed to allow subsequent diffusion. The emitter was formed on the front surface by diffusion using POCl3 liquid as a doping source. The diffusion temperature was 840°C, and the flow rate of POCl3 was 600sccm. The sheet resistance of the n+ emitter was around 50 ? / square emitter. After the diffusion processes, all wafers received a standard cleaning followed by etching with a diluted hydrofluoric acid (HF) immediately before growing passivating layers. From here, the wafers were divided in three groups in which one group received directly bare (no passivating layers) on rear-sides. The other two 1349

23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain

groups were operated in RCA process after forming gas at 500°C for 30mins, resulting in a ~1.5 nm thick oxide layer [7, 10]. After rinsing with de-ionized water, one of the groups was coated by Al2O3 film. The ALD (Al2O3) film was then deposited using alternating pulses of Al(CH3)3 (trimethylaluminum (TMA), the Al precursor) and H2O (the oxygen precursor) in an N2 carrier gas flow on the SiO2 held at 250°C. The deposited thickness was 15nm. The ALD process was composed of a number of identical cycles, each involving the following sequence: TMA, 0.1s → N2 purge, 5s → H2O, 0.1s → N2 purge, 5s. Every ALD cycle deposited ~ 1 ? of Al2O3. The thickness of the deposited Al2O3 film was determined by the number of ALD cycles. Then, the SiNx:H was deposited on both sides of these two groups followed by plasma-enhanced chemical vapor deposition for antireflection purposes. The thickness of the antireflection layer was 90nm, and its reflective index was approximately 1.9. After the antireflection coating was deposited, the front and back electrodes (Ag paste: Du Pont PV145 and Al paste: Du Pont PV333) was printed and put in an IR furnace at the maximum zone temperature of 965°C. To compare the solar cell devices, all the groups were put in IR heated belt furnace under the same conditions (see Table 1). Table 1: The processing sequence of solar cell fabricated in this study
Solar cell processing sequence 1. Chemical isotexturing for simultaneous saw damage removing and surface texturing 2. Single-sided POCL3 diffusion followed by heated tube furnace for emitter diffusion during 25 minutes at 8400C to create a 50Ω/sq emitter 3. Phosphorous glass removal using HF dilute optionally followed by an additional RCA process after forming gas 4. Three different rear side passivations: (1)bare(0nm), (2) SiNx(100nm) and (3) Al2O3(15nm)/SiNx(100nm) 5. SiNHx:H deposition by a plasma-enhanced chemical vapor deposition for antireflection purposes 6. Screen printing of the Ag front side metallization and Al rear side metallization 7. Simultaneous firing of the front and rear side metallization and build-in Al BSF formation in heated tube furnace 8. Edge isolation

Table 2: Best efficiencies of the solar cells parameters measured under WACOM (AM1.5G, 100 mW/cm2, 25 °C).
Passivating layer(s) Bare SiNx Al2O3/ SiNx Jsc Voc F.F. (%) 76.7 73 80 η (%) 15.8 17.5 20.1 (mA/cm2) (mV) 33.78 39.5 40.73 610 611 618

Figure 2 shows the internal quantum efficiency (IQE) data of solar cells selected from each of the groups in Table 2. The IQE of these groups was taken at wavelengths of 300–1200nm. Two prominent features could be identified from the IQE results.

100 90 80 70 60 50 40 30

IQE(%)

Al2O3/SiNx SiNx bare

200

400

600 800 Wavelength(nm)

1000

1200

Figure 2: IQE data of p-type solar cells fabricated using different surface passivation layers at low temperature. First, IQE of the solar cells with SiO2/ SiNx (RCA process after forming gas) and with SiNx has strong absorption at wavelengths of 300–600nm near the front surface of the cell. Since the IQE at short wavelengths strongly reflects the recombination at the front surface of a cell and thus it reflects the passivation of n+ emitter in our solar cells. Also, the IQE of the solar cell using the SiO2 /SiNx stack shows a better improvement for wavelengths below 400nm. Second, the IQE at long wavelengths of 600nm– 1200nm is larger than that at short wavelengths of 400nm–600 nm in the Al2O3/SiNx stack, indicating a strong back side electric field exists at the rear-side of the silicon solar cell. According to recent results by Schmidt et al [11], depositing an Al2O3/SiO2 stack on a silicon substrate leads to net negative fixed charges. The electrical field generated by the negative fixed charges attracts holes near the silicon surface, resulting in accumulation on the p-Si surface and upward bending of the band near the surface. This accumulation significantly benefits the rearside passivation and hole extraction. In contrast, applying the Al2O3/SiNx stack to the front surface of the silicon solar cell only produced a limited effect. This structure is a disadvantage to electron extraction, owing to the depletion of the n-Si surface, which results in the upward bending of the band near the surface. The PC1D results demonstrated that the improvement of the cell efficiency is more significant in the rear-side passivation than in the front-side passivation. Therefore, an Al2O3/SiNx stack structure is a good candidate for rear-side passivation in achieving a high efficiency solar cell.

3. RESULTS AND DISCUSSION Table 2 shows the best results of the solar cell parameters using different passivation layers measured by a class-A solar simulators. The cell efficiency without rear passivation is 15.8%, representing a reasonable performance for monocrystalline silicon solar cells. As compared to bare one, a substantial enhancement of 2.6% in the conversion efficiency (η) is observed using SiO2 /Al2O3/SiNx stack layers. However, the SiO2 /Al2O3/SiNx stack layers has the highest efficiency up to 20.1%. Clearly, ultrathin (~ 1.5nm) SiO2 inserted between an Al2O3 /SiNx stack layers and the silicon substrate helps gain a better efficiency.

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23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain

Min., Ave. & Max. Jsc of Different Passivation
42 41 40 39
Jsc(mA/cm2)

efficiency of 20.1%. Figure 4 shows the measured IV curve of the best cell.

38 37
Current(A)

0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.0 0.1

36 35 34 33 bare SiNx
Passivation layer Jsc Min. Jsc Ave. Jsc Max.

η: 20.1% Voc: 618mV Jsc: 40.73mA/cm2 FF: 80%

Al2O3/SiNx

(a)
Min., Ave. & Max. Voc of Different Passivation
618 616 614 612
Voc(mV)

0.2

0.3

0.4

0.5

0.6

0.7

Voltage(V)

Figure 4: I-V curve of the best cell fabricated using a rear-side Al2O3/ SiNx stack layers. 4. CONCLUSION
Voc Min. Voc Ave. Voc Max.

610 608 606 604 602 600 bare SiNx
Passivation layer

Al2O3/SiNx

(b)
Min., Ave. & Max. FF of Different Passivation
80 79 78 77 76 75 74 73 72 71 70 69

A high-efficiency monocrystalline silicon solar cell is accomplished by an Al2O3/ SiNx stack layers as a passivation layer. This high-efficiency characteristic is shown from the enhancement of IQE in the range of 600nm– 1200nm, suggesting that an Al2O3/ SiNx stack is a good choice for the rear-side passivation. 5. REFERENCES [1] A. G. Aberle, Prog. Photovolt: Res. Appl, 8, 473 (2000). [2] H. F. W. Dekkers, S. De Wolf, G. Agostinelli, F. Duerinckx, G. Beaucarne, Sol. Energy Mater. Sol. Cells, 90, 3244 (2006) [3] S. Dauwe, L. Mittelstadt, A. Metz, and R. Hezel, Prog. Photovoltaics 10, 217 (2002). [4] R. Petres, J. Libal, R. Kopecek, M. Vetter, R. Ferre, I. Martín, D. Borchert, I. R?ver, K. Wambach, and P. Fath, Proceedings of the 15th International Photovoltaic Science and Engineering Conference, 2005, p.128. [5] I. Martín, M. Vetter, A. Orpella, J. Puigdollers, A. Cuevas, and R. Alcubilla, Appl. Phys. Lett. 79, 2199 (2001). [6] P. J. Cousins and J. E. Cotter, Sol. Energy Mater. Sol. Cells 90, 228(2006). [7] Valentin D. Mihailetchi, Yuji Komatsu, and L. J. Geerligs, Appl. Phys. Lett. 92, 063510 (2008). [8] B. Hoex, S.B.S. Heil, E. Langereis, M.C.M. Sanden, and W.M.M. Kessels, Appl. Phys. Lett. 89, 042112 (2006). [9] B. Hoex, J. Schmidt, R. Bock, P. P. Altermatt, M. C. M. van de Sanden, and W. M. M. Kessels, Appl. Phys. Lett. 91, 112107 (2007). [10] Hezel R, Metz A. Crystalline silicon solar cells with efficiencies above 20% suitable for mass production. Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, UK, 2000; 1091–1094. [11] J. Schmidt,y, A. Merkle, R. Brendel, B. Hoex, M. C. M. van de Sanden and W. M. M. Kessels, Prog. Photovolt: Res. Appl, 16, 461 (2008).

F.F.(%)

F.F. Min. F.F. Ave. F.F. Max.

bare

SiNx
Passivation layer

Al2O3/SiNx

(c)
Min., Ave. & Max. Eff. of Different Passivation
20 19 18
Eff.(%)

17 16 15 14 bare SiNx
Passivation layer Eff. Min. Eff. Ave. Eff. Max.

Al2O3/SiNx

(d)

Figure 3: Minimum, average and maximum value of Jsc, Voc, F.F. and efficiency for rear-side passivation cells. Figure 3 summarizes the results of three groups under one-sun parameters of the solar cells in this study. The cells were measured using an AM 1.5, 100mW / cm2 power WACOM solar simulator. Each average value was measured for five passivation cells and averaged out. The maximum and minimum values of each group were not obtained from the same cells. It shows that the efficiencies of solar cells strongly depend on the passivation layer. Al2O3/SiNx stack effectively improves the cell performance –especially Jsc and efficiency. The Vocs and field factor are also moderately affected by the passivation layer. This good rear-side passivation increases the average field factor up to 77.0, Jsc up to 40.73A/cm2, Voc up to 617mV, and efficiency up to 19.5%. The best result we can measure is a maximum

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