Transparent oxides for selective contacts and passivation in heterojunction silicon solar cells

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1 Transparent oxides for selective contacts and passivation in heterojunction silicon solar cells Francesca Menchini Photovoltaic Technologies Laboratory, ENEA Casaccia LIMS maggio 2018

2 Outline Introduction Photovoltaic solar cells Solar cell losses Laboratory activities Selectivity and passivation a-sio x :H and MoO x Antireflection coatings and contacting ITO Laboratory projects 2

3 Photovoltaics 3

4 Solar radiation Out of atmosphere 1361 W/m 2 Sea 46 lat 1000 W/m 2 4

5 Solar radiation Sunlight is sufficient to supply the power needed by humanity 25 TW SUN Actual consumption TW needed AM1.5G W/m 2 Module Efficiency % PV Surface needed km 2 Side of the PV square km 1.66x

6 Cell structure Direct energy conversion I max V max FF 6

7 Cell structure 7

8 Cell structure 7

9 Record efficiencies of Silicon solar cells UNSW (2008) homojunction PERL Voc = 706 mv Jsc = 42.7 ma/cm 2 FF = 82.8% Eff = 25.0% Area = 4 cm 2 Sanyo/Panasonic (2014) heterojunction HIT Voc = 740 mv Jsc = 41.8 ma/cm 2 FF = 82.7% Eff = 25.6% Area = cm 2 Kaneka (2017) heterojunction HJ-IBC Voc = 744 mv Jsc = 42.3 ma/cm 2 FF = 83.8% Eff = 26.35% Area = cm 2 9

10 Resistance losses (FF) Solar cell losses Optical losses (J sc ) metal contacts Reflection Absorption Shading TCO p-a-si a-si n-si Si + - absorber 80 nm 10 nm 5 nm 200 m n-a-si TCO Recombination losses (V oc ) metal contacts 10

11 Some solutions Key factors for high cell performances: Transparency Reduced recombination (passivation) Selective contacts Technological solutions AND intrinsic properties of the materials Si absor 11

12 Chemical passivation Passivation: reduction of recombination D it = superficial density of states Q f = fixed charge within the film Ga 2 O 3 (ALD) Al 2 O 3 (ALD) HfO 2 (ALD) SiOx (PECVD) D it Charge-assisted carrier density control SiN x (PECVD) a-si:h SiO 2 (PECVD) Q f 1 eff passivation c-si passivation 1 bulk 2 S eff W eff = effective lifetime S eff = surface recombination velocity bulk = bulk lifetime W = wafer thickness Chemical compatibility is the key for c-si surface passivation 12

13 Passivation: a-sio x :H vs a-si:h a-si:h a-sio x :H High absorption coefficient in UV Eg = 1.7eV Low thermal stability to thermal treatment (T<200 C) Damages during ITO deposition -> recovery low temperature processes. Lower absorption coefficient in UV Eg = eV Higher thermal stability to thermal treatment (T>250 C) ITO deposition followed by annealing improves surface passivation Same industrial deposition technique: PECVD 13

14 Passivation: a-sio x :H vs a-si:h 3 2 Implied Voc=751 mv a-siox:h enhances passivation Lower absorption ensured by wider gap eff (ms) 1 Higher IQE with higher passivation and transparency 0 SiOx a-si:h Implied Voc=730 mv x x x10 16 Minority Carrier Density (cm -3 ) Absorption coefficient (cm -1 ) a-si:h a-sio x :H ITO n a-siox:h i a-siox:h n-si i a-si:h p a-si:h ITO IQE n a-sio x :H / i a-sio x :H n a-sio x :H / i a-si:h n a-si:h / i a-sio x :H p a-si:h / i a-si:h Wavelength (nm) Wavelength (nm) 14

15 Selectivity Solar cell efficiency means selective contacts without recombination electron extraction Conduction band Electron collector E fn E fp hole extraction Valence band Hole collector Light absorber holes / electrons that stray into the wrong contact will recombine through defects at the edges Perfect surface passivation is needed to achieve faultless selectivity to reduce J 0 15

16 Selectivity 0 2 Vacuum level 4 Energy (ev) Si a-si:h SiO 2 Al 2 O 3 HfO 2 WO 3 MoO 3 V 2 O 5 Ga 2 O 3 NiO Ta 2 O ZrO 2 5 TiO 2 ZnO In 2 O passivation layer h-extracting layer e-extracting layer TCO 16

17 Hole-selective layer: MoO x UPS Transmittance (%) nm 12 nm 6 nm Wavelength (nm) Counts/s (norm) as dep ann 180 C 5.2 ev 5.5 ev Kinetic Energy (ev) MoO x films have high transparency and high work function MoO x films are very sensitive to air exposure, temperature and plasma treatments, which affect solar cell performances. 17

18 Hole-selective layer: MoO x ITO MoO x SiO x n-c-si SiO x n-a-si:h 1 Energy (ev) E f Thickness ( m) Hole selectivity ensured by high difference in Electron Affinity values Eg (ev) (ev) d (nm) ITO MoO x a-sio x :H n-c-si m 18

19 Hole-selective layer: MoO x ITO MoOx SiOx n-c-si SiOx n-a-si:h Energy (ev) E fn E fp Multi-step tunneling thickness ( m) Hole selectivity ensured by high difference in Work Function values Eg (ev) (ev) d (nm) ITO MoO x a-sio x :H n-c-si m 19

20 Hole-selective layer: MoO x n-c-si Silver grid ITO MoO x a-sio x :H a-sio x :H n-a-si:h ITO Silver contact 80nm 12nm 5nm 5nm 10nm 80nm Optimal thickness for a-sio x :H and MoO x : compromise between barrier effects and passivation Suitable ITO deposition: low temperature (<150 C) and low DC power (1.3 W/cm 2 ) Suitable process to avoid parasitic surface oxidations on amorphous surfaces Internal Quantum Efficiency as dep ann 130 C ann 180 C Wavelength (nm) J (ma/cm 2 ) as dep ann 130 C ann 180 C V (V) V oc = 675 mv J sc = 31.7 ma/cm 2 FF = 77.4% Eff = 16.6% Thermal annealing C N 2 V oc = 680 mv J sc = 31.7 ma/cm 2 FF = 77.4% Eff = 16.7% 20

21 Hole-selective layer: MoO x n-c-si Silver grid ITO MoO x a-sio x :H a-sio x :H n-a-si:h ITO Silver contact 80nm 12nm 5nm 5nm 10nm 80nm Optimal thickness for a-sio x :H and MoO x : compromise between barrier effects and passivation Suitable ITO deposition: low temperature (<150 C) and low DC power (1.3 W/cm 2 ) Suitable process to avoid parasitic surface oxidations on amorphous surfaces Normalized IQE p-a-si:h / a-si:h MoOx / a-siox:h Wavelength (nm) J (ma/cm 2 ) as dep ann 130 C ann 180 C V (V) V oc = 675 mv J sc = 31.7 ma/cm 2 FF = 77.4% Eff = 16.6% Thermal annealing C N 2 V oc = 680 mv J sc = 31.7 ma/cm 2 FF = 77.4% Eff = 16.7% 21

22 Antireflection coating and contacting: ITO Innovative developments for next generation production lines N (cm -3 ) 3x x x10 21 (cm 2 /Vs) (10-3 cm) DC bias RT DC bias 180 C RF RT RF 180 C DC bias RT DC bias 180 C RF RT RF 180 C T ann ( C) DC bias 250RT 300 DC bias 180 C T ann ( C) RF RT RF 180 C Free carrier absorption is an issue Target: Higher and lower N (cm -1 ) K 633 nm 1000 nm DC bias RT DC bias 180 C RF RT RF 180 C INES reference RF RT as grown RF RT C RF 180 C DC bias 180 C DC bias RT DC bias RT C Wavelength (nm) Annealing temperature ( C) 22

23 Conclusions Amorphous hydrogenated silicon oxide (a-sio x :H) with good transparency and high passivation capabilities can succesfully substitute amorphous hydrogenated silicon (a-si:h) as passivation layer in heterojunction solar cells Substoichiometric Molybdenum Oxide (MoO x ) can be beneficially used in silicon heterojuynction solar cells, also in conjunction with a-siox:h ITO films produced by RF and DC sputtering show different characteristics and behaviors when varying depostionand annealing temperature, which could be be exploited when designing the cells manufacturing process sequence 23

24 Progetto Ampere 24

25 Thanks to Mario Tucci Alberto Mittiga Massimo Izzi Luca Serenelli Luca Martini Glauco Stracci Enrico Salza Pietro Mangiapane 25