Inline-high-rate thermal evaporation of aluminum for novel industrial solar cell metallization

Transcription

1 2nd Workshop on Metallization Konstanz, April 14 th -15 th, 2010 Inline-high-rate thermal evaporation of aluminum for novel industrial solar cell metallization Frank Heinemeyer 1

2 Motivation Development of high-efficiency solar cell concepts at ISFH Searching for a contact-free preparation as alternative to screen printing Back contact metallisation with evaporated aluminum due to better contact resistance and line resistance compared to screen printing Required aluminum thickness: from 2 µm to 20 µm Industrial aluminum evaporation systems for solar cell production with high throughput have recently become available 2

3 Challenges High throughput High deposition rates High temperatures (thermal damage of passivation layers, spiking, wafer bow) Adaption of evaporation systems to requirements of PV industry Adaption of solar cell structures to evaporation processes (passivation, local openings, LFCs) 3

4 Outline Inline high-rate thermal evaporation system at ISFH Results Homogeneity Temperature measurements Temperature simulation Contact resistance Surface recombination velocity Solar cell results Summary 4

5 Thermal evaporation 5

6 Evaporation chamber Depositon width of 500mm 10 evaporation boats Separatly adjustable evaporation boats and wire feed Up to 720 wafers per hour (2 µm Al) in this laboratory system Live-view camerasystem 6

7 Live view image during evaporation 7

8 Scheme of the ATON system Loading Entry Buffer Transfer Process Transfer Buffer Exit Unloading Turbopump Turbopump Turbopump Turbopump Turbopump Pumpunit evaporator Pumpunit Pumpunit Continuous substrate flow Oscillating substrate flow 8

9 ATON500 system at ISFH 9

10 Homogeneity FZ-Si Wafer after deposition 10 µm aluminum 6 oscillations 10

11 Homogeneity of a 10 µm Al layer Aluminum layer thickness [µm] Average thickness: 9.97 µm Variation: ±3.4 % Measurement line on carrier 11

12 In-situ temperature measurements Oven tracker system mounted on carrier Up to 6 thermocouples Connected to the front side of the wafer Data collection during deposition Evaluation of data after deposition

13 Measured temperatures One Oscillation Three Oscillations

14 Finite element modelling Si-Wafer Evaporator

15 Comparison of simulation with experiment Simulation according to model (no fit!) Temperature increase is well described by model Maximum temperature differs only by 20 C

16 Influence of wafer thickness on temperature Simulated values ddr = 5 µm*m/min d Al = 2µm 156x156mm² Temperature T [ C] d Si = 120µm 50 0 d Si = 150µm d Si = 190µm Time t [s]

17 Influence of wafer thickness on temperature Simulated values Measured values ddr = 5 µm*m/min d Al = 2µm 156x156mm² ddr = 5 µm*m/min d Al = 2µm 156x156mm² Temperature T [ C] d Si = 120µm Temperature T [ C] d Si = 120µm 50 d Si = 150µm d Si = 190µm 50 d Si = 150µm d Si = 190µm Time t [s] Time t [s]

18 Influence of dynamic deposition rates on maximum temperature Maximum wafer temperature [ C] µm x m/min 2.9 µm x m/min 0.97 µm x m/min Aluminum layer thickness [µm] Same layer thickness with different dynamic deposition rates by variation of carrier speed Significant temperature differences for thinner aluminum layers Temperatures up to 370 C are suited for solar cell production

19 Contact resistance measurement Specific contact resistance [mωcm²] ,1 BAK EVO liner BAK EVO crucible BAK EVO thermal BAK 550 BAK 600 ATON Aluminum contact to boron doped p + - emitter (30 Ω/sq) Comparison of low deposition rate static systems to a high deposition rate inline evaporation system Static systems need subsequent annealing to reach a minimum contact resistance of 1.0 mωcm 2 ATON reveals specific contact resistance of 1.7 mωcm 2 without subsequent annealing Annealing temperature [ C] 19

20 Surface recombination velocity Lokal opening of passivation layer with laser Variation of pitch p, constant radius r Aluminum deposition in ATON system with different ddrs Aluminum Passivation Layers SVR measurement dependent on pitch 20

21 Surface recombination velocity Effective rear SRV S eff [cm/s] f =4% f =2% S pass =2.2 cm/s S met =2.2x10 4 cm/s f =0.25% f =0.15% f =0.07% f =0.02% SRV range from: 2.2 cm/s for contact opening of 0.02% up to 450 cm/s for contact opening of 4% No significant differences for Aluminum layers, deposited with tree different ddrs Values well suited for solar cell production Period length p [µm] 21

22 Solar cell results 50 Current density [ma/cm 2 ] A = 3.97 cm 2 V oc = mv J sc = 41.3 ma/cm² FF = 77.0 % η = 20.6 % Buried-emitter rear-contact solar cell without photolithography Voltage [mv] Harder, N.-P. et.al., physica status solidi (RRL) 2 (4), , (2008) 22

23 Summary All relevant aluminum layer thicknesses from 2 µm to more than 20 µm, depending on different solar cell concepts, can be fabricated Deposition temperatures 200 C up to 370 C (depending on layer thickness) Up to 720 wafers per hour Specific contact resistance of 1.7 mωcm 2 without subsequent annealing Solar cell with 20.6 % conversion efficiency We demonstrate the applicability of the novel inline evaporation system to the production of industrial next-generation high-efficiency solar cells 23

24 Thanks to: Christoph Mader and Daniel Münster all colleagues at ISFH the Federal Ministry for the Environment for granting this projekt under contract number and 24

25 Thank you for your attention! 25