ALD as an enabling technology for new crystalline silicon solar cell concepts

Transcription

1 ALD as an enabling technology for new crystalline silicon solar cell concepts ALD4PV workshop Sjoerd Smit Bart Macco

2 In this presentation ALD 4 silicon heterojunction solar (SHJ) cells Reduction of deposition damage Precise and graded doping of s ALD 4 novel solar cell concepts Charge inversion cells Outlook: clever use of band structure

3 ALD 4 SHJ solar cells Solar cell basics The selectivity is usually achieved by doping Ag contacts Hole selective contact asi:h (p ) asi:h (i) csi (n) asi:h (i) asi:h (n ) Ag contact Electron selective contact Silicon heterojunction (SHJ) solar cell a.k.a. HIT solar cell

4 ALD 4 SHJ solar cells Two spots where ALD can improve SHJ solar cells: Ag contacts 1: Top layer Lateral conductivity Anti reflective coating asi:h (p ) asi:h (i) csi (n) 2: Backside reflector (BSR) Reflect light into silicon again Prevent light absorption in Ag back contact asi:h (i) asi:h (n ) Ag contact

5 ALD 4 SHJ solar cells: The frontside Top layer Important aspects: High transparency Low sheet resistance Light incoupling into silicon (ARC) Can ALD provide this? Industrial potential / Effect on passivation of the cell Ag contacts asi:h (p ) asi:h (i) csi (n) asi:h (i) asi:h (n ) Ag contact

6 ALD 4 SHJ solar cells: The frontside Top layer Important aspects: High transparency Low sheet resistance Light incoupling into silicon (ARC) Can ALD provide this? Industrial potential / Effect on passivation of the cell Ag contacts asi:h (p ) asi:h (i) csi (n) asi:h (i) asi:h (n ) Ag contact

7 ALD 4 SHJ solar cells: The frontside Top layer Important aspects: High transparency Low sheet resistance Light incoupling into silicon (ARC) Can ALD provide this? Industrial potential / Effect on passivation of the cell Front and rear can be deposited at the same time (even with different doping levels) Vacuum not strictly necessary Large batches possible Ag contacts asi:h (p ) asi:h (i) csi (n) asi:h (i) asi:h (n ) Ag contact

8 ALD 4 SHJ solar cells: The frontside Top layer Important aspects: High transparency Low sheet resistance Light incoupling into silicon (ARC) Can ALD provide this? Industrial potential / Effect on passivation of the cell Soft deposition Tunable properties (e.g. doping) Ag contacts asi:h (p ) asi:h (i) csi (n) asi:h (i) asi:h (n ) Ag contact

9 ALD 4 SHJ solar cells: The frontside : Effect on passivation E.g.: B. Demaurex (APL 2012): DOI: / Figure: B. Macco (APL, submitted for publication) Front has pronounced influence on the carrier lifetime of the solar cell. Causes: Sputtering damage. Can be prevented by using ALD Sputtered ITO ALD ZnO:Al asi:h csi 75 nm Lifetime decrease after sputtering (%) Lifetime decrease Sheet resistance ITO thickness (nm) ZnO:Al thickness (nm) Sheet resistance ( /sq)

10 ALD 4 SHJ solar cells: The frontside : Effect on passivation E.g.: B. Demaurex (APL 2012): DOI: / Figure: B. Macco (APL, submitted for publication) Front has pronounced influence on the lifetime of the solar cell. Causes: Sputtering damage. Can be prevented by using ALD Influence of field effect. Depends on doping. ALD ZnO:Al asi:h csi Minority carrier lifetime (ms) 10 1 High chemical passivation asi:h Recovery after HCL etch asi:h ZnO:Al (HD) Reduction due to field effect asi:h ZnO:Al (HD) etch of ZnO:Al by HCl Minority carrier density (cm3)

11 ALD 4 SHJ solar cells: The frontside : Effect on passivation Front has pronounced influence on the lifetime of the solar cell. Causes: Sputtering damage. Can be prevented by using ALD Influence of field effect. Depends on doping. ZnO Intrinsic ZnO Highly doped ZnO csi E C E Fn E Fp E V ALD ZnO:Al asi:h Field effect csi asih E.g.: B. Demaurex (APL 2012): DOI: / Figure: B. Macco (APL, submitted for publication)

12 ALD 4 SHJ solar cells: The frontside : Effect on passivation E.g.: B. Demaurex (APL 2012): DOI: / Figure: B. Macco (APL, submitted for publication) Front has pronounced influence on the lifetime of the solar cell. Causes: Sputtering damage. Can be prevented by using ALD Influence field effect. Doping can be accurately tuned by using ALD ALD ZnO:Al asi:h csi Minority carrier lifetime (ms) 10 1 Increasing ZnO doping V MPP asi:h Minority carrier density (cm 3 ) V OC asi:h ZnO (UD) asi:h ZnO:Al (LD) asi:h ZnO:Al (MD) asi:h ZnO:Al (HD)

13 ALD 4 SHJ solar cells: The frontside : Controlling the doping Example: Al doped ZnO Al content can be varied by tuning cycle ratio n Doping efficiency also depends on doping precursor (E.g. TMA vs DMAI) Other dopants also possible (e.g ZnO:B) More in Dr. Knoops presentation Y. Wu ( JAP 2013): DOI / Y. Wu (Chem. of Mater. 2013): DOI /cm402974j

14 ALD 4 SHJ solar cells: Decoupling of the functions Potential application of ALD: Graded doping and protection of the asi:h interface Top layer Important aspects: High transparency Low sheet resistance Light incoupling into silicon (ARC) Industrial potential Effect on passivation of the cell Decoupling of functions Ag contacts laterally conductive layer (any deposition method) interface (ALD) asi:h (p ) asi:h (i) csi (n) With ALD, the graded can, in principle, still be processed in one single deposition

15 In the rest of this presentation Key advantages ALD ALD 4 silicon heterojunction solar (SHJ) cells Reduction of deposition damage Graded doping of s ALD 4 novel solar cell concepts Al 2 O 3 based junctions TiO 2 based junctions

16 ALD 4 new cell concepts The selectivity is usually achieved by doping Selective hole membrane Absorber material Selective electron membrane However, other methods exist: 1. Inducing mirror charges in the absorber 2. By using band offsets 3. (exercise for the audience)

17 ALD 4 new cell concepts: Mirror charges by Al 2 O 3 The Al 2 O 3 /csi interface naturally contains a high density of negative charges (~5*10 12 e/cm 2 ) To maintain macroscopic charge neutrality, positive charge is induced in the csi, acting similarly to ptype doping Al 2 O 3 csi D. GarciaAlonso (SST 2013): DOI / /28/8/ S. Smit (SOLMAT 2014): DOI /j.solmat

18 ALD 4 new cell concepts: Mirror charges by Al 2 O 3 Al 2 O 3 csi D. GarciaAlonso (SST 2013): DOI / /28/8/ S. Smit (SOLMAT 2014): DOI /j.solmat

19 ALD 4 new cell concepts: Mirror charges by Al 2 O 3 Al 2 O 3 csi Y ALD 4 Al 2 O 3 and? Charge is collected by tunneling through the Al 2 O 3, so thickness control is crucial High fixed charge density possible (plasma ALD) Prevent mirror charges in (e.g. doping control) D. GarciaAlonso (SST 2013): DOI / /28/8/ S. Smit (SOLMAT 2014): DOI /j.solmat

20 ALD 4 new cell concepts: Measuring current through Al 2 O 3 /ZnO stacks 100 nm ZnO:Al 1 nm Al 2 O 3 Current high enough for csi solar cell applications ZnO doping control important for optimization

21 ALD 4 new cell concepts Outlook Selective hole membrane Absorber material Selective electron membrane However, other methods exist: 1. Inducing mirror charges in the absorber 2. By using band offsets 3. (exercise for the audience)

22 ALD 4 new cell concepts: The TiO 2 heterojunction with csi E Vacuum TiO 2 and csi conductions bands align, thus TiO 2 is electron selective Χ TiO2 = 4.0 ev Χ csi = 4.05 ev E C Unfortunately, TiO 2 does not passivate, so some sort of stack needs to be designed TiO 2 csi E V S. Avasthi (APL 2013) DOI: / (Not ALD, btw)

23 Conclusions/Summary The unique advantages of ALD 4 PV SHJ solar cells can benefit from the ability of ALD to grade the layer and the softness of the deposition ALD opens up new possibilities for fabricating carrierselective layers in csi solar cells ALD has unique industrial advantages, such as capability for simultaneous double sided depositions

24 Closing Thank you for your attention! Questions? Fire away!