Aerosol-Printed Silicon Solar Cell Exceeding 20% Efficieny

Transcription

1 Aerosol-Printed Silicon Solar Cell Exceeding 20% Efficieny Crystal-Clear-Workshop M. Hörteis Fraunhofer-Institut für Solare Energiesysteme ISE Utrecht,

2 Motivation, Screen print contact - new -contact Reduced contact area reduced shading losses reduced recombination Aspect ratio higher conductivity reduced series resistance Good electrical contacts on lowly doped emitters improved blue response reduced recombination 15 µm 100 µm

3 Content Seed layer concepts Aerosol print technique Ink preparation at Fraunhofer ISE Contact structure and formation Application: High efficiency solar cell

4 Two layer concept: Seed-layer and conductive-layer Seed-layer Evaporation of contact metals trough a mask Direct laser scribing Electroless plating of contact metals Fine line screen print Aerosol-jet print / Ink-jet print Conductive-layer Plating (light induced or electroless) Multiple printing of highly conductive materials

5 Light-induced plating Crystal-Clear-Workshop, Metallization 2008 Backside connected via external power supply to Silver anode Illumination induces negative potential on front side Positively charged Ag + -ions are attracted by the illuminated front side and deposited along the seed layer

6 Light-induced-plating industrial realization Inline LIP at Fraunhofer ISE Two machines are available, for Ag and Cu

7 Schematic of an aerosol printer Crystal-Clear-Workshop, Metallization 2008 Atomization gas Atomizer Virtual impactor Sheath gas Print head Nozzle Substrate XY-Table

8 Aerosol print head Metal Aerosol Sheath gas d=18 µm Ø=100 µm (nozzle opening) Nozzle Aerosol Jet XY-Table

9 Printing systems Optomec

10 Simulation: optimized contact width Influence of optical losses Influence of electrical losses Dependent on contact resistivity Total loss p [% rel ] el. losses ρ c = 0.01 mω cm 2 ρ c > 10 mω cm 2 10 minimum R sh = 55 Ω/sq Contact width w c [μm] opt.losses Mette A., New Concepts for Front Side Metallization of Industrial Silicon Solar Cells, Universität Freiburg

11 Functional ink materials Crystal-Clear-Workshop, Metallization 2008 Metal powder - Silver Conductivity and contact Glass frit (Metal Oxides) Contact formation and adhesion Organic vehicle system (Solvents, binder, dispersant agents, rheological additives ) Adjusting the ink on the used printing system (Screen print, Aerosol print, Inkjet print )

12 Ink - Preparation Tools: Crushing Mortar Ball mill Mixing Agitatiors spatula and beaker Dispersing Ultra sonic finger/bath 3 mill chair

13 Aerosol contact after printing Crystal-Clear-Workshop, Metallization 2008 contact height [µm] 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 after printing contact width [µm] 38 µm Single printed, dense line Line width <40 µm using a 200 µm nozzle Line height about 1-2 µm Height:width < 1/20

14 Aerosol contact after firing Crystal-Clear-Workshop, Metallization 2008 contact height [µm] 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 after printing after firing contact width [µm] Evaporation of solvent and binder Sintering of silver particles Reduced line conductivity Light induced plating 35 µm

15 Aerosol contact after LIP Crystal-Clear-Workshop, Metallization 2008 contact height [µm] after printing after firing after plating contact width [µm] Aspect ratio increases 1:4 improved conductivity ρ f ~ Ωm Higher reflection (optical width about 70% of real contact width) 35 µm 75 µm 20 μm height 40 µm seed layer 80 µm contact after light induced plating

16 Reverse contact formation Crystal-Clear-Workshop, Metallization 2008 plated silver LIP-Silber n-emitter p-base

17 Reverse contact formation Crystal-Clear-Workshop, Metallization 2008 Seed layer n-emitter n-emitter p-base LIP silver is removed by nitric acid p-base

18 Reverse contact formation Crystal-Clear-Workshop, Metallization 2008 Glass layer n-emitter p-base LIP silver is removed by nitric acid Printed silver is removed n-emitter

19 Reverse contact formation Crystal-Clear-Workshop, Metallization 2008 Silver crystallites n-emitter p-base LIP silver is removed by nitric acid Printed silver is removed HF dip to remove the glass layer

20 Reverse contact formation Crystal-Clear-Workshop, Metallization 2008 Imprints of silver crystallites n-emitter p-base LIP silver is removed by nitric acid Printed silver is removed HF dip to remove the n-emitter glass layer Nitric acid to remove the silver crystallites

21 Current model of the contact formation Glass melts at about 500 C and wets the interface inksolar cell PbO, dissolved in the glass reacts with the SiNx-layer and opens the ARC T<500 C 700<T<830 C 700<T<830 C Both, silver and oxidized silicon is dissolved in the glass melt at about 800 C Dissolved silver crystallizes in form of small silvercrystallites during cooling T<830 C RT Schubert, G., Thick film metallisation of crystalline silicon solar cells 2006, Universität Konstanz

22 Mögliche Mechanismen des Stromflusses ohne LIP plated silver n-si 5 6 Precipitate Glass SiNx layer plated silver screen-printed contact n-emitter p-base SiNx layer plated silver crystallite glass precipitate screen-printed contact glass Mette A., New Concepts for Front Side Metallization of Industrial Silicon Solar Cells, Universität Freiburg

23 Solar cells on Fz-Wafers Crystal-Clear-Workshop, Metallization 2008 Two different inks Diluted screen print paste (ink A) Designed at ISE (ink B) On three different emitters 50 Ω/sq. 70 Ω/sq. 110 Ω/sq.

24 Process flow Front and rear passivated solar cell

25 Process flow Aerosol seed layer print

26 Process flow Contact firing

27 Process flow Evaporation of Al on the rear

28 Process flow Laser fired contacts (LFC)

29 Process flow Light induced plating (LIP)

30 Process flow Forming gas annealing (FGA)

31 Cell structure LIP-Silver Aerosol-printed seed layer Antireflexion coating and emitter LFC point contacts Thermal oxide Evaporated aluminum

32 Current and voltage 39, , ,0 660 j sc [ma] 37,5 V oc [mv] , ,5 55 (ink A) 70 (ink A) 110 (ink A) R sh [Ω/sq] 110 (ISE ink) (ink A) 70 (ink A) 110 (ink A) R sh [Ω/sq] 110 (ink B)

33 IQE of the best cells of each emitter Influence of the emitter sheet resistance is visible in the short wave length region Comparable quantum efficiencies for all cells in the long wave length region Good reflection, due to a passivated rear and a aluminum mirror R, IQE Ω/sq ink A 70 Ω/sq ink A 110 Ω/sq ink A 110 Ω/sq ink B Sim. IQE 110 Ω/sq ink B λ (nm)

34 Fill-Factor and efficiency Crystal-Clear-Workshop, Metallization FF 0.75 η [%] (ink A) 70 (ink A) 110 (ink A) 110 (ink B) R sh (Ω/sq.) (ink A) 70 (ink A) 110 (ink A) R sh (Ω/sq.) 110 (ink B)

35 Efficiency over one wafer Crystal-Clear-Workshop, Metallization ,3% 19,8% 20,2% 20,3% 20,6% (20.3%*) 20,4% 20,6% (20.3%*) *)independently confirmed by Callab ISE

36 Contact resistance vs. emitter sheet resistance Contact resistance increases with increasing emitter sheet resistance 1,6 1,4 The contact resistance R c W for ink B (designed at ISE) is below 0,5 Ωcm for all emitter sheet resistances R c *W [Ω*cm] 1,2 1,0 0,8 0,6 Ink B Ink A 0,4 0, R sh [Ω/sq.]

37 Crystal-Clear-Workshop, Metallization 2008 SEM image of Ag-crystallites on a 110 Ω/sq. Emitter

38 Conclusion Fine line printing is possible Low ohmic contacts can be formed on lowly doped emitters using ISE ink Processed solar cells achieving 20% efficiency

39 Thank you for your attention 25 µm More information: M. Hörteis, S. W. Glunz PIP-850 (online available) 55 μm 55 μm 25 μm