br Acknowledgments br Data The

Acknowledgments
Data The dataset of this paper provides additional information to Ref. [1]. The rear surface passivation quality in relation to the ALD Al2O3 processing parameters is reported in Figs. 1–5, the PERC cell properties are reported in Figs. 6–8 and Table 1, and the metal contact design pattern and parameters are reported in Table 2.
Experimental design, materials and methods The procedure prior to the minority carrier lifetime measurements on the ALD Al2O3 rear surface passivation is described in detail in Fig. 1 in Ref. [1] and the most relevant aspects are reported here. All samples underwent saw damage removal and pre-ALD clean, followed by ALD Al2O3 deposition with varying parameters. Post-ALD anneal was performed on most of the samples, and then followed by Embelin enhanced chemical vapor deposition (PECVD) of the SiNx capping layer. Afterwards, since lifetime cannot be measured after metallization, the samples were divided into two groups; one group directly underwent rapid thermal processing (RTP) firing, whereas a second group first underwent screen-printing of Al paste, then RTP firing followed by Al paste removal. All the samples were then measured under similar parameters. The interface defect density (Dit), the negative fixed charges (Qf) and open circuit voltage (Voc) as a function of the pre-ALD clean parameters are reported in Fig. 1. SC1 and SC2 pre-clean can achieve the best passivation quality (both regarding Dit and Voc). The pre-ALD clean does not affect the amount of Qf. Note that in the industrial PERC cell process, edge isolation and rear side polishing prior to the Al2O3 process normally terminate with HCl–HF clean sub-step. The effect of the ALD process temperature for Al2O3 passivation on effective minority carrier lifetime and PERC cell performance (as Voc and cell efficiency (ηcell)) is reported in Fig. 2, while the effect on growth rate and refractive index (nk) of the dielectric layer is shown in Fig. 3. Our data show that the optimal temperature for passivation (in relation to both the effective lifetime and ηcell) is in the range of 200–240°C, whereas the passivation level drops sharply at 275°C. Note also that the process window between 160 and 240°C is broad. In addition, the growth rate begins to decrease for temperatures above 200°C. The refractive index is relatively low below 200°C (nk=1.57), and it is stable (nk=1.63–1.65) between 200 and 400°C. The optimal temperature for growth rate and refractive index is also around 200°C. Fig. 4 shows the passivation quality as effective minority carrier lifetime, PERC cell performance (Voc and ηcell), Dit and Qf as a function of the Al2O3 film thickness. Effective minority lifetime and cell performance gradually increase with thickness increasing from 3.5 to 16nm, where the optimal thickness is ~10nm. When the film thickness increases from 3.5 to 16nm, cell performances and Dit gradually improve, while Qf is at the same level. Note that even a thin Al2O3 film of about 3.5nm can work well on PERC cell (Voc≈652mV and ηcell≈20.3%). SEM/energy dispersive X-ray spectroscopy (EDS) analysis on the effect of voids on the local back surface field (LBSF) rear local contact was performed and is shown in Fig. 5. Note that the spectra in the figure are from the Al-Si eutectic, the Al LBSF and the Si substrate, respectively. The detailed interpretation of the differences among the spectra is given in Ref. [1]. The thickness of the Al LBSF layer is ~10μm, thus revealing that the junction depth is deeper than 10μm. Representative examples of industrial PERC cell performances are reported in Figs. 6,7. The certified current-voltage (I-V) curve and cell performance of one SiO2-PERC cell was measured at Fraunhofer ISE in July 2013 under standard test conditions (see Fig. 6). Note that the key process in this structure is rear surface passivation with thermal oxidation and dielectric opening with Merck׳s chemical paste. Fig. 7 shows a typical example of the cell efficiency of an average industrial batch of China Sunergy׳s Al2O3 PERC cell, tested in house under standard conditions (i.e., the same I-V test criteria used as Fraunhofer ISE).