Silicon cell concepts - trends in research

Transcription

1 PV Module Technology & Applications Forum 2018 TÜV Rheinland, Köln, Silicon cell concepts - trends in research R. Brendel 1,2 & B. Lim 1 1 Institut für Solarenergieforschung Hameln ISFH in Hameln/Emmerthal 2 Institut für Festkörperphysik Leibniz Universität Hannover Contact: 1

2 Quality of monocrystalline Si wafer \\ Improving carrier lifetimes Deactivation of BO defect Lifetime Dn=0.1 p 0 [ms] Resistivity r [Wcm] [ p-type Cz-Si n=0.1 p 0 [µs] BO activated (2005) Intrinsic limit (2012) BO deactivated (2016) BO deactivated (2013) Improvements due to rapid thermal processes Approaching intrinsic lifetime Concentrate on surface and contacts here Doping concentration p 0 [cm -3 ] Figure from: D. C. Walter, B. Lim, and J. Schmidt, Prog. Photovolt. Res. Appl. 24, (2016) 2

3 Overview Al-BSF baseline cell (20%) Selective contacts PERC technology (22%) Improved selectivity for holes POLO junctions (25%) Improved selectivity for electrons and holes Screen-printed PERC+ cell New materials and designs (27%) Screening options 3

4 Al-BSF cell Working principle Carrier selective contacts (CSC) 4

5 Al-BSF cell \\ Carrier selectivity S 10 PECVD SiN sun P-diff. Al V th J c r c Al- BSF For illustration only, not a POCl diffusion 2 Jc 550 fa/cm Definition of selectivity of a contact V S. th 10 log10 log J c rc rc 5 mwcm 2 Introduction of selectivity: R. Brendel and R. Peibst, IEEE Journal of Photovoltaics 6, (2016). 5

6 Synergistic efficiency gain analysis (SEGA) of an Al-BSF solar cell Largest losses/ gain options: 1.0% reflection at Al-BSF 1.7% rec. at Al-BSF Full-area Al-BSF is limiting Introduction SEGA: R. Brendel, T. Dullweber, R. Peibst, C. Kranz, A. Merkle, and D. Walter, Prog. Photovolt. Res. Appl. 24, (2016) Conductive Boundary model: R. Brendel, Prog. Photovolt. Res. Appl 20, 31 (2012). 6

7 Efficiency limit due to Al-BSF contact in an otherwise ideal cell Al-BSF Perfect contacts: Maximum efficiency h max = 29% Limiting efficiency for Si:. Richter, M. Hermle and S. W. Glunz, IEEE Journal of Photovoltaics 3, (2013). With full area Al-BSF: Maximum efficiency h max = 23.5% Can we do better? 7

8 PERC cell Reduced recombination at the hole contact by reduced contact area 8

9 PERC cell \\ Reducing rear contact area Al-BSF f 1 c PERC f 01. c PERC concept: A. W. Blakers, A. Wang, A. M. Milne, J. Zhao and M. A. Green, Applied Physics Letters 55, (1989). Partial contacts Laser contact opening Introduction of LCO: Preu, S. W. Glunz, S. Schäfer, R. Lüdemann, W. Wettling, and W. Pfleging, in Proc. 16th EUPCSEC, Glasgow, United Kingdom, 2000, pp Firing stable rear passivation Al 2 O 3 /SiN x Firing stable AlO x -passivation: J. Schmidt, B. Veith, and R. Brendel, Phys. Status Solidi RRL 3, (2009). Dielectric interlayer reduces reflection loss Dielectric interlayer: P. Campbell, S. R. Wenham and M. A. Green, Solar Energy Mat. and Solar Cells 31, 133 (1993). 9

10 Efficiency limit of Al-BSF increases when reducing contact area Al-BSF 100 Changing contact area fraction f c keeps selectivity S 10 constant Vth S10 log rc fc Jc f c 100 Optimum area fraction f c,max 1% Reduce f c to below current technological limits! 10

11 13.0 Maximum efficiency depends on selectivity 27.2% Maximum efficiency h max at optimum f c,max increases linearly with selectivity S 10 until running into intrinsic limitation J sc = 43.6 ma/cm 2 S 10 > 15 means perfect contact Selectivity-efficiency relation: R. Brendel and R. Peibst, IEEE Journal of Photovoltaics 6, (2016). 11

12 PERC+ solar cell \\ Reduced contact area & bifacial screen-printed Ag PECVD SiN P-diff P-diff. Al- BSF For illustration only, not a POCl diffusion screen-printed Al 50 µm PERC+ concept: T. Dullweber, C. Kranz, R. Peibst, U. Baumann, H Hannebauer, A. Fülle, S. Steckemetz, T. Weber, M. Kutzer, M. Müller, G. Fischer, P. Palinginis, and H. Neuhaus, Prog. Photovolt. Res. Appl. 24, 1487 (2016) 12

13 22.1% PERC+ solar cell at ISFH *independently confirmed by ISE CalLab Missing BB shadowing increases h front by 0.4% abs. Front Al paste reduced by 90% Deeper BSFs Rear Bifaciality: 5 to 10% gain (w/o tracking) 22% PERC+: T. Dullweber et al., Photovoltaics International 38 (2017), in press. 13

14 Analyzing a PERC+ solar cell \\ Enhanced rear reflectance Open rear enhances rear reflection 14

15 Synergistic efficiency gain analysis (SEGA) of a PERC+ solar cell Rear recombination: From 1.7% (Al-BSF) to 0.14% Rear reflection: From 1.0% (Al-BSF) to 0.4% Largest losses / gain options: 0.7% finger shading 1.0% emitter rec. Electron contact is limiting Improve emitter or 15

16 Poly-Si on Oxide (POLO) junctions as an example for many new approaches for forming carrier selective contacts 16

17 Poly-Si on Oxide (POLO) junctions \\ It s a Nano-PERC! PERC POLO 100 nm 2 nm 50 µm Growth of 1 to 3 nm thick oxide Thermal, wet chemical, 1 mm Al-doping 5x10 18 cm -3 P-doping cm -3 5 nm 1 µm Deposition of doped a-si LPCVD, PECVD, In-situ, implant, diffusion, High temperature annealing Oxide break up Higher doping higher selectivity S 10 Pin-hole concept: R. Peibst, U. Römer, K. R. Hofmann, B. Lim, T. F. Wietler, J. Krügener, N. P. Harder and R. Brendel, IEEE Journal of Photovoltaics 4, 841- (2014). 17

18 POLO junctions \\ Observation of pinholes Annealed at 1020 C 1035 C 1050 C Scanning electron microscope J c = 1.4 fa/cm 2 J c = 1.5 fa/cm 2 J c = 55 fa/cm 2 Transmission electron microscope TEM observation: D. Tetzlaff, J. Krügener, Y. Larionova, S. Reiter, M. Turcu, R. Peibst, U. Höhne, J. D. Kähler and T. Wietler, 43rd IEEE Photovoltaic Specialists Conf. (PVSC), 2016, pp SEM oberservation: T. F. Wietler, D. Tetzlaff, J. Krügener, M. Rienäcker, F. Haase, Y. Larionova, R. Brendel and R. Peibst, Appl. Phys. Lett. 110, (2017). Electron-microscopic observation of pinholes in oxide Increasing pinhole density for higher annealing temperatures More pin-holes causes larger J c and smaller r c 18

19 Selectivity of POLO junctions \\ Contacts invisible to the cell Al-BSF S 10 _ f c,max n-type POLO close to invisible (S 10 = 15.9) Small contact f c = 5% required Rest needs to be passivated too! 19

20 25.0% * efficient POLO cell at ISFH *independently confirmed by ISFH CalTeC DL ARC Al 2 O 3 evaporated Al p + -POLO n + -POLO 25%-efficient POLO cell: F. Haase, F. Kiefer, S. Schäfer, C. Kruse, J. Krügener, R. Brendel, and R. Peibst, Japanese Journal of Applied Physics 56, 08MB15 (2017) 20

21 Synergistic eff. gain analysis (SEGA) of a POLO solar cell Electron contact recombination From to 1.2% (PERC) to 0.1% Largest losses / gain options: 0.5% front surface reflection 0.6% front surface recomb. Front surface is limiting SEGA of POLO cell: F. Haase, F. Kiefer, S. Schäfer, C. Kruse, J. Krügener, R. Brendel, and R. Peibst, Japanese Journal of Applied Physics 56, 08MB15 (2017) 21

22 Screening combinations of different materials for carrier selective contacts (CSC) 22

23 Overview carrier selective contacts Al-BSF PERC Many material options for electrons and holes POLO contacts achieve highest selectivity S 10 HIT Other combinations? Figure from: J. Schmidt, R. Peibst, and R. Brendel, Solar Energy Mat. Solar Cells, submitted Jan

24 Screening of material combinations (disregarding optics, ) High Selectivity S 10 > 13.0 High efficiency h max > 27.2% Full area: f f c, e, max c, e, max High efficiency & Full area Model used here: R. Brendel, M. Rienaecker and R. Peibst, in Proc. 32 nd EUPVSEC (WIP-Renewable Energies, Munich, 2016), pp

25 Attractive combinations of CSC (disregarding optics, ) TCO a-si(i)/a-si(p) a-si(i)/a-si(n) a-si(n) & a-si(p) (3T-HIT) h max = 27.5% HIT: 26.7% M. A. Green, et al., Prog Photov. Res Appl. 25, 668 (2017). a-si(i)/a-si(p) POLO(n) a-si:h and POLO (ASAP) h max = 27.9% > h max,hit Al-BSF AlO/SiN POLO(n) POLO and Al-BSF (PAL) due to h max = 27.1% 26.9% AlO/SiN 25

26 Summary PERC is successor of Al-BSF cell PERC+ is bifacial (22%, further potential when using selective emitters) High contact selectivities S 10 required for >25% (e.g. a-si, POLO, ) Highly asymmetric carrier concentrations is key Doping, band bending! POLO is Nano-PERC. S 10,POLO = 15.9 Contact invisible to cell Various new designs (e.g. 3T-HIT, ASAP, PAL) have a potential of 27% Thanks to T. Dullweber, R. Peibst, all ISFH team, all partners, BMWi, and you for your attention! Contact: More information: 26