NANO SILICON DOTS EMBEDDED SIO 2 /SIO 2 MULTILAYERS FOR PV HIGH EFFICIENCY APPLICATION

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1 NANO SILICON DOTS EMBEDDED SIO 2 /SIO 2 MULTILAYERS FOR PV HIGH EFFICIENCY APPLICATION Olivier Palais, Damien Barakel, David Maestre, Fabrice Gourbilleau and Marcel Pasquinelli 1

2 Outline Photovoltaic today short view ( cells 3G cells (case of tandem solar All Si tandem solar cells:different possibilties for nc-si fabrication ( each (problem and interest of nc-si in multilayers SRSO A technique to grow Si nanostructures Control of the size and the density Evidence of a photoconductance effect 2

3 Photovoltaic today : Solar Cells technology η Multijunction Concentrators Three-junction (2-terminal, monolithic) ( monolithic Two-junction (2-terminal, Crystalline Si Cells Single crystal Multicrystalline Thin Si Thin Film Technologies Cu(In,Ga)Se 2 CdTe ( stabilized ) Amorphous Si:H Emerging PV Dye cells Organic cells ( technologies (various Claudio Pelosi and Matteo Bosi Solar & Alternative Energy, SPIE Newsroom

4 3G photovoltaic cells: tandem cells Power loss mechanisms in single band gap Solar Cells Typical III-V Tandem stack cell Shorter wavelength Longer wavelength ( ev 56% of solar spectrum lost with c-si (1.12 High efficiency(>39%) but very expensive ( MOCVD ) This work : ANR DUOSIL Use of nano-crystalline silicon for gap engineering: tandem solar cell 4

5 All Si tandem solar cells nanostructured silicon layer(s) hν higher gap Eg 1 hν>eg 1 ( ev ) Eg 2 Eg 1 >hν>eg 2 lower gap Eg 2 Silicon tandem cell ( ev ) Eg 1 Quantum efficiency of tandem silicon cell versus band gaps Choice for all Si : 1.7 ev on 1.12 ev. 5

6 du maximum Energie PL (ev) de d'onde du maximum Longueur PL (nm) de All Si tandem solar cells Silicon quantum dots: gap versus diameter 2,0 1, ,6 1,4 1,2 1,0 Bande interdite du c-si Schuppler et al., 1995 Fujii et al., 1998 Takagi et al., 1990 Takeoka et al., 2000 Ledoux et al., 2000 Ehbrecht et al., 1997 Garrido et al., 2002 Ledoux et al., Diamètre des grains (nm) (synthesis of experimental results fom Céline Ternon Thesis, Université de Caen, Décembre 2002). Our goal is to obtain 3nm diameter quantum dots 6

7 All Si tandem solar cells Various method for nc-si elaboration + - Silicon nano-wire (CVD VLS) - Stability of diameter (use of ( colloïd - Need of matrix (?) - Conductivity Nano-silicon particles SiNx matrix (PECVD) - Low temperature - Stable matrix - Stability of diameter - Modest NP density Si NP in SiNx matrix SiO 2 matrix (RMS) - Stability of diameter, easy to select - High NP density - Time consuming 7

8 All Si tandem solar cells GROWTH : Reactive Magnetron Sputtering Key parameters : Hydrogen rate r H = P H2 /(P H2 +P ar ) Substrate temperature RF power Use of an argon-hydrogen mixture to allow the incorporation of a Si excess in the growing film 8

9 All Si tandem solar cells Reactive Magnetron Sputtering C O M P O S I T E e T=60 C r H =80% Effect of hydrogen and temperature M U L T I - L A Y E R S T=500 C r H =80% e T=500 C r H =50% e T=500 C r H =25% e 9

10 Reactive Magnetron Sputtering Elaboration sequence for N multi-layers Multilayers of 3nm-thick SRSO and 3nm thick SiO 2 (60 SRSO/SiO 2 pattern) plan view 3 nm thick Ar + H 2 Plasma substrate cross section SRSO Si nanostructure for PV 10

11 Reactive Magnetron Sputtering Elaboration sequence for N multi-layers plan view Multilayers of 3nm-thick SRSO and 3nm thick SiO 2 (60 SRSO/SiO 2 pattern) cross section 3 nm thick 3 nm thick Shutter + H 2 cut off Ar + H 2 Plasma substrate SRSO substrate SiO 2 Ar Plasma 11

12 Reactive Magnetron Sputtering Elaboration sequence for N multi-layers Multilayers of 3nm-thick SRSO and 3nm thick SiO 2 (60 SRSO/SiO 2 pattern) 3 nm thick Ar + H 2 Plasma substrate plan view cross section 3 nm thick Shutter + H 2 cut off SRSO substrate SiO 2 Ar Plasma Shutter + H 2 entry 12

13 Caractérisations techniques TEM and Energy Filtered TEM Multilayers of 3nm-thick SRSO and 3nm thick SiO 2 (60 SRSO/SiO 2 pattern) plan view cross section Filtered images at 16 ev (nc-si) Presence of Si nanoparticles as evidenced by EFTEM experiments. Good homogeneity with Si size Distance Si-np Si-np 1.5 nm 13

14 Caractérisations techniques TEM and Energy plan Filtered view TEM Multilayers of 3nm-thick SRSO and 3nm thick SiO 2 (60 SRSO/SiO 2 pattern) cross section HRTEM plan view 5 nm 5 nm Filtered images at 16 ev (nc-si) Presence of Si nanoparticles as evidenced by EF-TEM experiments. Good homogeneity with Si size Si-np density /cm-3 Distance Si-np Si-np 1.5 nm 14

15 Caractérisations techniques effect of the post annealing temperature PL intensity (a.u.) Energy (ev) Energy (ev) Energy (ev) Best annealing 600 C treatment 700 C for the R 2 = 0, C R 2 = 0, R 2 = 0,993 optical properties : 1100 C during 40 one hour 1200 C R 2 = 0, C R 2 = 0,999 Photoluminescence C R 2 = 0, C R 2 = 0, Wavelength (nm) Wavelength (nm) Wavelength (nm) 15

16 Caractérisations techniques Optical measurements Absorption, α grain (cm -1 ) Energy (ev) Si nc diameter 8 nm 6 nm 3 nm 1.5 nm Intensity (a.u.) 5.0x x x x x x x x x x Si-nc diameter 8 nm 6 nm 3 nm 1.5 nm Raman Shift (cm -1 ) Crystalline phase for Si-nc having a diameter ranging from 3 to 8 nm For φ Si-nc = 1.5 nm, the system is amorphous 16

17 Caractérisations techniques Optical measurements Absorption, α grain (cm -1 ) Energy (ev) Si nc diameter 8 nm 6 nm 3 nm 1.5 nm Intensity (a.u.) 5.0x x x x x x x x x x Si-nc diameter 8 nm 6 nm 3 nm 1.5 nm Raman Shift (cm -1 ) Crystalline phase for Si-nc having a diameter ranging from 3 to 8 nm For φ Si = 1.5 nm, the system is amorphous Similar absorption for Si nanocrystals ( 3nm to 8 nm) Increase of the absorption coefficient with the nature of the Si S phase ( amorphous ) 17

18 Caractérisations techniques Electrical measurements Sheet resistance : Van Der Pauw Technique I V V I R sheet R sheet around 250 kω Resistivity of ML around 100 Ω.cm

19 Caractérisations techniques Electrical measurements I-V; Photoconductance 9.0x10-5 Darkness Illumination Green et al. 6.0x10-5 I (A) 3.0x kωkω kωkω - Al Contacts 500 ºC (15 h) N 2 - I/V curves illumination and darkness V (V) Improvement of the conductivity (~20%) under illumination 19

20 Caractérisations techniques Electrical measurements 9.0x x10-5 I (A) 3.0x10-5 I-V; Photoconductance / effect of wavelength White source Darkness Illumination 37 kω KΩ KΩ 48 kω Monochromatic source 9.0x x10-5 I (A) 3.0x10-5 Darkness UV-Blue (λ = 375 nm) IR-Red (λ = 940 nm) kω kω V (V) Improvement of the conductivity (~20%) under white illumination V (V) Dependence of the conductivity with λ. No effect of the substrate Clear evidence of the contribution of the nanostructured multilayers 20

21 Caractérisations techniques optical measurements Transverse probe optical lifetime (Free Carrier Absorption) Parameter measured : Lifetime of free carriers (τ eff ) Detector Nd:YAG Laser λ = 1064 nm Lased Diode λ = 1.55 µm Absorption peak A 481 A.Irace (Dpt.E.T.E. Univ. Naples) - Probe: optical (λ=1.55µm) - Pump : optical (λ=1,064 µm) - Values of τ eff ( 30 ns) are rather low, even for µw-pcd (τ eff 100 ns) - Probably due to induced defect or contamination during thermal treatment 21

22 Conclusion for the PV approach A reactive process allowing the growth of Si nanoclusters in multilayer structures Control of the Si size through the deposition parameters High Si-nc density (~ cm -3 ) Multilayers is a promising structure for the conduction (control of the SiO 2 sublayer thickness) Evidence of a photocurrent on 360 nm-thick SRSO/SiO 2 multilayers. Next step under progress : Effect of the Si-nc size on the photoconductance measurements. p-n junction must be processed 22

23 Thank you for your attention

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