Nanomanufacturing: ALD4PV CIGS solar cells: 11:05-12:30 Thursday 20 March 2014 De Zwarte Doos, Eindhoven University of Technology

PercIGS: Development of rear surface passivated Cu(In,Ga)Se2 (CIGS) solar cells

Si solar cell design = advanced: PERC/PERL See presentation of Dr. Joachim John (imec, Belgium)

– PERL= passivated emitter and rear locally diffused – World record efficiency of 25.0 % (UNSW, Australia) – Ever thinner wafers thanks to – Front: surface passivation and surface texturing – Rear/back: surface passivation and locally diffused point contacts

Cu

50 cm/s ≤ Sb ≤ 150 cm/s Rb ≥ 90%

100 µm ≤ tSi ≤ 160 µm

p-type Si Al

S = surface recombination velocity R = internal reflection t = thickness

Rear surface passivation layer with micron-sized point openings

CIGS solar cell design = simple: layer by layer growth See presentation of Dr. Tobias Törndahl (ÅSC, Sweden)

– Absorber, buffer and window layer between rear and front contact – World record efficiency of 20.8 % (ZSW, Germany) – New bottlenecks to increase efficiency further!

i-ZnO/ZnO:Al window CdS or Cd-free buffer p-CIGS absorber

Mo back contact Rigid or flexible substrate

Sb > 1x104 cm/s Rb < 60 %

2.5 µm ≤ tCIGS ≤ 3.0 µm

PercIGS: PERC meets CIGS Objective: to combine a highly reflective rear surface passivation layer with nano-sized local point contact openings in ultra-thin CIGS solar cells (i-)ZnO(:Al) n-CdS p-CIGS

Pass. layer

Nano-sized local point contact MgF2/Al2O3

Sb ≈ 100 cm/s Rb >> 60 %

tCIGS = 0.4 µm

Mo

Soda lime glass

PercIGS Outline

Approach – CIGS absorber layer – Nano-sized point openings – CIGS surface passivation layer

Their combination

Approach CIGS absorber layer – Co-evaporation: Tsubstrate,max = 540 ºC, Se in excess, and constant rates of Cu, In and Ga

[Ga]/([In]+[Ga])

• Uniform GGI and CGI throughout the full layer • Exclude any other rear surface passivation effects • Highly reproducible and long diffusion lengths

Uniform profile

Distance to SCR

30 %

Approach CIGS absorber layer – Standard Mo/CIGS: low Rb and high Sb – Loss in JSC and VOC for ever thinner CIGS absorber layers – Such solar cells with ultra-thin (< 500 nm) CIGS layers are great characterization devices to assess Rb and Sb!!! – Aim of PercIGS: increase JSC and VOC for ever thinner tCIGS

Approach Nano-sized point openings 1. Deposit (chemical bath deposition = CBD) a particle-rich CdS layer on Mo back contact 2. Deposit the surface passivation layer

3. Remove the CdS nano-particles

CdS

220 ± 25 nm

Mo rear contact Soda lime glass (SLG)

Surface passivation layer

Approach Nano-sized point openings – SEM picture of this particle-rich CdS layer • Particle diameter = 285 ± 30 nm

– These particles can be removed, leaving point openings • Point opening diameter = 220 ± 25 nm

Da 1 = 273 nm

Da 2 = 270 nm

Da 2 = 194 nm

Da 1 = 216 nm

Approach CIGS surface passivation layer See [1] J. Vac. Sci. Technol. A 2012; 30: 040802 (TU/e, The Netherlands), [2] Appl. Phys. Lett. 2012; 100: 023508 (NTU, Taiwan), [3] MSc thesis by S. Motahari (KTH, Sweden)

– Al2O3 • Excellent Si surface passivation [1] • ‘Strong’ material - as it ’survives’ CIGS deposition • (Integrated) photoluminescence (PL) intensities [2,3] – Of Al2O3 pass. CIGS >> unpass. CIGS

– ALD • Precise thickness control and very uniform and conformal deposition over large area surfaces

– DC- (and/or RF-)sputtering • Lower quality passivation layers • Directionality of sputtering processes is an advantage for the point opening approach at hand

Approach CIGS surface passivation layer See Appl. Phys. Lett. 2012; 100: 023508 (NTU, Taiwan)

– Chemical passivation (low Dit) and field effect passivation (Qf < 0) • First principle calculations – 35 % reduction as compared to unpass. CIGS surface

• CV measurements on MOS structures – Reduction of CIGS surface minority charge carrier concentration

Combination of surf. pass. layer & point opening approach – CdS particles embedded in too thick ALD Al2O3 films become irremovable ≈ 2.5 nm

≈ 5.0 nm

≈ 7.5 nm

≈ 10.0 nm

ALD Al2O3 layer thickness

– Solutions: • Stacking of thin ALD film on another layer: MgF2/ALD-Al2O3(5 nm) • Using DC- or RF-sputtering: Sputt. Al2O3(50 nm)

PercIGS Outline

Results & discussion – Open circuit voltage VOC • Back/rear surface recombination velocity Sb

– Short circuit current density JSC • Back/rear reflectance Rb

– Efficiency η

Results Open circuit voltage See also IEEE J. Photovoltaics 2014; 4: 486–492 (ÅSC, Sweden)

– Rear surf. pass. CIGS solar cells • Increase in VOC compared to unpass. thin (tCIGS = 0.4 µm) cells • Similar VOC as unpass. thick (tCIGS = 1.8 µm) cells (a)

(c) (b)

Rear surface passivation

tCIGS

None

1.8 µm

(a) ALD Al2O3(5 nm)

0.4 µm

(b) MgF2(60 nm)/ALD-Al2O3(5 nm)

0.4 µm

(c) Sputt. Al2O3(50 nm)

0.4 µm

None

0.4 µm

Discussion Open circuit voltage See Solar Energy Mater. Solar Cells 2013; 117: 505–511 (ÅSC, Sweden)

– Comparing SCAPS modeling to manufactured solar cell devices • ALD-Al2O3/CIGS interface – Decrease in Sb of a few orders compared to std. Mo/CIGS interface – Sb as low as 100 cm/s

Results Short circuit current density – Thin ALD-Al2O3 (5 nm) rear pass. cells • No increase in JSC

– Thick (MgF2/)Al2O3 (ttotal = 50-65 nm) rear pass. cells • Increase in JSC, close to unpass. thick CIGS cells

Discussion Short circuit current density – Thick (MgF2/)Al2O3 rear surf. pass. layers show improved EQE compared to unpass. ref. cells with equivalent CIGS thickness – This EQE enhancement can be explained by increased Rb

Discussion Short circuit current density – Loss in EQE for rear pass. thin cells compared to unpass. thick cells • EQE(pass. cells, tCIGS = 0.4 µm) ≈ EQE(unpass. cell, tCIGS = 1.0 µm) – Rb(MgF2/Al2O3) > Rb(Sputt. Al2O3) >> Rb(Mo/CIGS)

• Optical path lengthening

mostly caused by rear reflected photons escaping at the front (Resc) CdS/i-ZnO:Al

Resc CdS/i-ZnO:Al

tCIGS = 1.0 µm

tCIGS = 0.4 µm

Rb

=

Pass. layer Mo SLG

Rb low

Results & discussion Efficiency – Cell efficiency of all pass. cells is noticeably higher than the efficiency of unpass. ref. cells with equivalent CIGS thickness – Sb reduced at Al2O3/CIGS interface – Rb increased a.f.o. (MgF2/)Al2O3 thickness

– But still lower than the thick unpass. ref. cell efficiency – Further optimization: decrease Resc (and further increase Rb)

Front surface texturing (as in Si PV) ZnO:Al in diluted HCl

PercIGS Summary and outlook • As in Si solar cell design, highly reflective rear surface passivation layers with nano-sized point openings are implemented in ultra-thin CIGS solar cells (see TEM) – Increase in VOC, JSC and hence efficiency

• Ongoing – Optical confinement study • SE / ZnO:Al texturing – Well-ordered contacting development • E-beam lithography • Conductive Mo nano-particles (Rb ↑) – Ga grading in the absorber layer (≈ PERT) – Focus on CIGS surf. pass. understanding

(i-)ZnO(:Al) n-CdS

p-CIGS

Pass. layer

Nano-sized local point contact

MgF2/Al2O3

Mo

Soda lime glass

PercIGS Thank you ... … for your kind attention! … to all who contributed! Marika Edoff, Viktor Fjällström, Carl Hägglund, Jonathan Joel, Dorothea Ledinek, Jörgen Olsson, Jonas Pettersson, Fredrik Rostvall, Piotr Szaniawski, Jörn Timo Wätjen, Uwe Zimmermann (Ångström Solar Center, Sweden), Gao Xindong (KLA-Tencor, China), Adam Hultqvist (Stanford University, USA), Pedro Salomé (INL, Portugal), Angel Uruena (Imec, Belgium), Denis Flandre, Frederic Henry, Ratan Kotipalli (Université Catholique de Louvain, Belgium), Rickard Gunnarsson, Ulf Helmersson, Iris Pilch (Linköping University, Sweden)

… to our sponsors!

PercIGS References •

Vermang B, Fjällström V, Pettersson J, Salomé P, Edoff M. Development of rear surface passivated Cu(In,Ga)Se2 thin film solar cells with nano-sized local rear point contacts. Solar Energy Materials and Solar Cells 2013; 117: 505–511.

•

Vermang B, Fjällström V, Gao X, Edoff M. Improved rear surface passivation of Cu(In,Ga)Se2 solar cells: a combination of an Al2O3 rear surface passivation layer and nanosized local rear point contacts. IEEE Journal of Photovoltaics 2014; 4: 486–492.

•

Vermang B, Wätjen JT, Fjällström V, Rostvall V, Edoff M, Kotipalli R, Henry F, Flandre D. Employing Si solar cell technology to increase efficiency of ultra-thin Cu(In,Ga)Se2 solar cells. Progress in Photovoltaics: Research and Applications under review.