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High efficiency silicon heterojunction solar cells: From conventional concepts to a 3 rd generation bi-triplet exciton generating hybrid device M. Liebhaber E-mail: martin.liebhaber@helmholtz-berlin.de Helmholtz Center Berlin Institue for

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SLIDE 1

High efficiency silicon heterojunction solar cells: From conventional concepts to a 3rd generation bi-triplet exciton generating hybrid device

  • M. Liebhaber

E-mail: martin.liebhaber@helmholtz-berlin.de

Helmholtz Center Berlin Institue for Nanospectroscopy Energy Materials In-Situ Lab _______________________________ SPREE/UNSW Seminar 12/11/2015

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SLIDE 2

martin.liebhaber@helmholtz-berlin.de

Where I come from…

2

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SLIDE 3

martin.liebhaber@helmholtz-berlin.de

winter night life

Where I come from…

3

Helmholtz Center Berlin Berlin City

EMIL

Energy Materials In-Situ Lab sights sights

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SLIDE 4

martin.liebhaber@helmholtz-berlin.de

Part I – Conventional concept

4

Valence band offset and hole transport in c-Si/a-SiOx heterojunction solar cells

Lars Korte Mathias Mews Cooperation: Institut for Silicon Photovoltaics (HZB)

“SISSY” project

Grant No. 03SF0403 Grant No. 608498

Financial support:

…many more people involved

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SLIDE 5

martin.liebhaber@helmholtz-berlin.de

► excellently passivated contacts ► world record for Si based PV: η = 25.6%/ 22.5% (Panasonic, 2014/2015) ► reduce parasitic absorption

  • wide band gap materials
  • band offset modification

Introduction

5

(i)a-SiOx:H

SHJ solar cell

(Silicon Heterojunction)

passivation (i)a-Si:H (5 nm) TCO/metallization TCO emitter (p)a-Si:H (10 nm) base (n)c-Si (≈260 μm) (i/n+)a-Si:H

back contact p/n- junction front contact

amorphous crystalline heterojunction

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SLIDE 6

martin.liebhaber@helmholtz-berlin.de

Motivation

6

► heterojunction parameter

  • optical band gap Eg
  • band offsets ∆EV, ∆EC
  • surface passivation τeff

► FOCUS on ∆EV of

  • a-SiOx:H/c-Si heterointerface
  • hole transport mechanism
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SLIDE 7

martin.liebhaber@helmholtz-berlin.de

Sample preparation

7

► PECVD layer deposition

  • varying precursor gas mixtures
  • change of stoichiometry x in a-SiOx layers

(Plasma Enhanced Chemical Vapour Deposition)

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SLIDE 8

martin.liebhaber@helmholtz-berlin.de 8

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV) Intensity (10

3 cps)

108 106 104 102 100 98 0.0 0.5 1.0 1.5 Binding energy Ebind (eV)

CO2 flow

0 sccm 1 sccm 2 sccm 3 sccm 4 sccm 5 sccm 6 sccm 7 sccm 8 sccm

Intensity (10

3 cps)

CO2 flow

precursor gas mixture variation gas flow (sccm) 5 10

H2 SiH4 CO2 SiO2 (80% CO2) pure Si (0% CO2)

  • M. Liebhaber et al., APL 106, 031601 (2015)

a-Si:H SiO2

Stoichiometry (XPS)

► X-ray Photoelectron Spectroscopy ► Si 2p peak

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SLIDE 9

martin.liebhaber@helmholtz-berlin.de

SiO2 (Si4+)

9

  • M. Liebhaber et al., APL 106, 031601 (2015)

► chem. shift of core level peak depends on oxidation states ► peak intensity ratios ↔ oxygen concentration

pure Si (Si0+)

Si atom O atom

2p1/2 2p3/2

Si1+ Si3+ Si2+

Stoichiometry (XPS)

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SLIDE 10

martin.liebhaber@helmholtz-berlin.de 10

► cross-check with other peaks ► conversion: SiH4/CO2 ratio → oxygen conc. ► non linear dependency of stoichio- metry on gas phase composition

Stoichiometry (XPS)

  • M. Liebhaber et al., APL 106, 031601 (2015)

a-Si:H SiO2

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SLIDE 11

martin.liebhaber@helmholtz-berlin.de

Valence band (UPS)

11

► Ultraviolet Photoelectron Spectroscopy ► obtain valence band position relative to EF

  • He-UPS at 21.2 eV (standard)
  • CFSYS at 4.0-7.3 eV [1] (Constant Final State Yield Spectroscopy)

→ lower detection limit ↔ band tails & defect distribution

[1] L. Korte et al., JAP 109, 063714 (2011)

  • M. Liebhaber et al., APL 106, 031601 (2015)
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SLIDE 12

martin.liebhaber@helmholtz-berlin.de

increasing

  • xygen

content

∆EV (UPS & SPV)

  • UPS: valence band position (relative to EF)
  • SPV: band bending ≤ 150 meV

Valence band offset ∆EV (UPS & SPV)

12

(n)c-Si wafer a-Si EV EC EF EC EV

UPS

a-SiO2 EV

UPS E [eV]

  • 5.15
  • 1.15

𝑓𝜒

SPV

Ultraviolet Photoelectron Spectroscopy Surface PhotoVoltage

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SLIDE 13

martin.liebhaber@helmholtz-berlin.de

Valence band offset ∆EV

13

  • M. Liebhaber et al., APL 106, 031601 (2015)

► dependency of ∆EV on stoichiometry ► direct correlation of ∆EV on cell performance

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SLIDE 14

martin.liebhaber@helmholtz-berlin.de

passivation layers in SHJ solar cells

14

a-SiOx

  • M. Liebhaber et al., APL 106, 031601 (2015); M. Mews, M. Liebhaber et al., APL 107, 013902 (2015)
  • nly pass.

layer

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SLIDE 15

martin.liebhaber@helmholtz-berlin.de

passivation layers in SHJ solar cells

15

a-SiOx

► increasing Dit for rising oxygen concentration

  • but decrease drastically after emitter deposition
  • similar passivation quality as standard interface

► dangling bonds at the SHJ are saturated by hydrogen during additional plasma process (emitter deposition)

  • M. Liebhaber et al., APL 106, 031601 (2015); M. Mews, M. Liebhaber et al., APL 107, 013902 (2015)
  • nly pass.

layer additional emitter depo

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SLIDE 16

martin.liebhaber@helmholtz-berlin.de

[1] A. Kanevce et al., JAP 105, 094507 (2009)

  • M. Mews, M. Liebhaber et al., APL 107, 013902 (2015)

► hole transport mechanism

  • transport barrier reflected in FF
  • thermionic emission (for small ∆EV)
  • tunnel hopping through tail states

(additional for higher ∆EV [1]) ► solar cell parameter

  • surface passivation (→ Voc)
  • OK
  • widening of band gap (→ Isc)
  • thin layer (small effect expected)
  • band offsets (→ FF)
  • transport barrier

SHJ solar cells & hole transport

16

FF Voc Isc (n)c-Si wafer (i)a-SiOx:H EV EC EF

E

(p)a-Si:H ∆EV

emitter absorber layer pass.

(n)c-Si wafer (i)a-SiOx:H EV EC EF

E

(p)a-Si:H

+ + + + +

  • tunnel hopping/

thermionic emission

emitter absorber layer

∆EV

pass.

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SLIDE 17

martin.liebhaber@helmholtz-berlin.de

SHJ cell with stacked passivation layer

17

► “staircase” of valence band offsets

  • improved FF for HIT cell

with stacked passivation layers ► promising concept: combination of

  • moderate band gap passivation layer
  • high band gap hole contact layer
  • M. Mews, M. Liebhaber et al., APL 107, 013902 (2015)

single layer split into layer stack

∆EV,a-Si:H = 270 meV ∆EV,a-SiO0.3:H = 585 meV

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SLIDE 18

martin.liebhaber@helmholtz-berlin.de

Conclusion

18

► growth of (i)a-SiOx:H thin films (PECVD)

  • stoichiometry (XPS)
  • ∆EV (UPS/SPV)
  • sufficient passivation

► implemented into SHJ cells (low x-regime)

  • FF directly linked to ∆EV
  • discussion of hole transport mechanism

(thermionic emission vs. tunnel hopping) general challenge: transport limitation due to band offset induced by the high band gap of a-SiOx possible solution: band gap “staircase” also in combination with high band gap emitter

  • M. Liebhaber et al., APL 106, 031601 (2015); M. Mews, M. Liebhaber et al., APL 107, 013902 (2015)
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SLIDE 19

martin.liebhaber@helmholtz-berlin.de

Part II – 3rd generation concept

19

…many more people involved

June/July ´15 March ´15 November ´15

Tetracene/c-Si hybrid solar cell – multi-exciton generation via singlet fission –

Project-ID 57140921

Cooperation

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SLIDE 20

martin.liebhaber@helmholtz-berlin.de

Motivation

20

c-Si

adapted from: L. C. Hirst and N. J. Ekins-Daukes, PIP 19, 286 (2011)

a-Si:H/perovskite/OPV UC: TTA, Er-Yb,… MEG: Si nanocrystals, SF…

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SLIDE 21

martin.liebhaber@helmholtz-berlin.de

MEG via Singlet Fission (SF): 1 photon → 2 e--h+ pairs

21

► first observed in anthracene in 1965 by S. Singh et al. ► organic chromophore in excited singlet state ► shares its excitation energy with a neighboring ground-state chromophore ► both are converted into correlated triplet excited states molecular states of interest: ground state singlet state triplet state

ms=0 ms=0 ms=1

HOMO LUMO

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SLIDE 22

martin.liebhaber@helmholtz-berlin.de

MEG via Singlet Fission (SF): 1 photon → 2 e--h+ pairs

22

electron accepting material electron accepting material electron accepting material electron accepting material

S1

*

S0

+

T1

*

T1

*

h+ h+ e- e-

  • J. J. Burdett et al., J. Chem. Phys. 133, 144506 (2010)

time

excitation ≈100 fs fission ≈100 ps ionization ps – ms ?? triplet state ≈100 ns electronic transitions:

X X

→ direct triplet energy transfer ↔ challenge!

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SLIDE 23

martin.liebhaber@helmholtz-berlin.de

MEG device

23

absorption Exciton/ electron-hole-pair MEG event Exciton migration carrier migration

SF medium Solar cell

Prerequisite: ► triplet exciton diffusion length longer than thickness SF medium ► geminate triplet exciton pair dissociation at hybrid interface

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SLIDE 24

martin.liebhaber@helmholtz-berlin.de

How do we harvest triplets?

24

  • 1. Charge transfer state

direct electron transfer with the hole on the

  • rganic & the electron on the inorganic material
  • 2. Förster type transition
  • non-radiative transfer mechanism (long range)
  • dipole transition ↔ spin forbidden for triplets
  • 3. Dexter type transfer
  • charges/triplets move
  • wave function overlap (short range)

Challenges

  • momentum conservation & localization effect

→ direct vs. indirect semiconductors

  • loose spin correlation

→ easier in heavy atoms (spin-orbit coupling)

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SLIDE 25

martin.liebhaber@helmholtz-berlin.de

1)M. Tabachnyk et al., Nature mat. 13, 1033 (2014) 2)N. J. Thompson et al., Nature mat. 13, 1039 (2014)

Charge transfer mechanisms (nanocrystals)

25

triplet transfer (TT) “dexter type” followed by hole transfer (HT) back direct electron transfer (ET) via charge transfer state (CT) ► transient optical absorption measurements ↔ dynamics1) ► efficient transfer only at resonance ↔ size of NCs1) ► exponential dependency of transfer efficiency ↔ NC ligands2)

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SLIDE 26

martin.liebhaber@helmholtz-berlin.de

Successful harvesting of triplet excitons

26

EQE > 100% !!!

Singlet fission & triplet dissociation

cathode anode

exciton blocking

  • D. N. Congreve et al., Science 340, 334 (2013)

Side note:Tc/C60 proved by

  • T. C. Wu et al., APL 104, 193901 (2014)
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SLIDE 27

martin.liebhaber@helmholtz-berlin.de

Tetracene/c-Si hybrid cells

27

Tetracene

  • 2.7

PEDOT: PSS

  • 5.2

Tetracene

  • 5.4

i/n++a-Si:H BSF (n)c-Si

  • 4.1

(n)c-Si

  • 5.2

T1 Eb (eV)

200 µm 20 nm some Å ~30 nm ~80 nm

Interlayer aluminum Ag

1cm 0.5cm

photograph

  • f hybrid cells

► correct energy (band) alignment important motivation for c-silicon:

  • replace pentacene by higher band gap SF material
  • proof of charge separation of triplets at hybrid interface
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SLIDE 28

martin.liebhaber@helmholtz-berlin.de

PEDOT:PSS/c-Si hybrid front contact

28

► ≈14 % hybrid cell

  • optimized front contact
  • planar, w/o antireflection, “simple” back contact

►PEDOT:PSS forms a hybrid heterojunction with (n)c-Si → described by p+n-heterojunction (in lit. commonly assumed as Schottky junction)

  • S. Jäckle, M. Liebhaber et al., Sci. Rep. 5, 13008 (2015)

Sara Jäckle

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SLIDE 29

martin.liebhaber@helmholtz-berlin.de

PEDOT:PSS/c-Si hybrid front contact

29

► highly p-doped polymer

  • filled (valence) states at EF
  • valence band edge EV,P ≈ 80 meV above EF

► strong inversion of Si at the surface → intrinsic Fermi level EI,Si crosses EF (n-doped Si)

  • S. Jäckle, M. Liebhaber et al., Sci. Rep. 5, 13008 (2015)
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SLIDE 30

martin.liebhaber@helmholtz-berlin.de

Hybrid cell results

30

► absorption spectra reflected in IQE ► “filter effect” of Tc in IQE, no exciton dissociation

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SLIDE 31

martin.liebhaber@helmholtz-berlin.de

Hybrid cell results

31

► no field assisted exciton separation ► indirect band gap a problem? → phonon assistance? temperature, a-Si:H…

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SLIDE 32

martin.liebhaber@helmholtz-berlin.de

Tetracene film morphology – XPS

32

5 nm Pd-Porphyrin/c-Si ► no island growth 1 nm & 10 nm Tetracene/c-Si ► island growth Si 2p

10 nm 1 nm

Si 2p

5 nm

c-Si PEDOT:PSS PEDOT:PSS Tc Tc pinhole contact ultrathin layer

ZOOM ZOOM

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SLIDE 33

martin.liebhaber@helmholtz-berlin.de 33

20 40 60 80 100 120 140 160 180 200

ln (norm. Pl intensity) time (ns)

100nm Tc/glass 100nm Tc/c-Si 100nm Tc/glass 100nm Tc/c-Si

S1

*

T1

*

T1

*

long lived state (≈100 ns)

delayed PL ≈ 100 ns prompt PL ≈ 100 ps fission ≈100 ps

► first preliminary results: no differences in thickness & substrate variation ► triplets generated and long lived (≈100 ns) ► singlet fission in Tc layers (also on c-Si) but no injection ↔ in accordance to EQE

X X

Dynamics & charge transfer – ultrafast PL

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SLIDE 34

martin.liebhaber@helmholtz-berlin.de

Challenges & open questions

34

  • 1. Can correlated triplet excitons be transferred across a silicon interface?
  • 2. How long do the triplets generated upon singlet fission remain geminate?
  • 3. Which interlayers could help to dissociate triplet excitons?
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SLIDE 35

martin.liebhaber@helmholtz-berlin.de

Conclusion & Outlook

35

► overcome fundamental thermalization loss

  • MEG (“spectral downconversion”) via singlet fission

ultrafast PL measurements:

  • polycrystalline Tc thin layers show SF (glass & c-Si substrates)
  • no differences in triplet lifetime observed → no exciton dissociation

hybrid devices:

  • hybrid device works, but Tc layers act as filter
  • so far no hint for exciton dissociation & charge injection (EQE)

► search and implementation of organic intermediate layer

  • 1st step: charge separation at organic/organic interface works (shown in literature)
  • 2nd step: injection of separated charges into c-Si
  • correct band line-up!

► charge seperation at Pb-Nanocrystals works ↔ put on c-Si (Cambridge, MIT…)

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SLIDE 36

martin.liebhaber@helmholtz-berlin.de

Thank you…

36

“SISSY” project

  • M. Tayebjee, R. McQueen,
  • J. Niederhausen, J. Behrens,
  • K. Jäger, T. F. Schulze, T. W.

Schmidt, K. Schwarzberg, C. Bäcker, B. Rech, K. Lips

Project-ID 57140921 Grant No. 03SF0403

especially I would like to thank:

  • E. Conrad, T. Lußky, K. Jacob, M. Wittig, T. Hänel, K. Mack, F. Ruske,
  • C. Bäcker, L. Korte, M. Mews, F. Lang, S. Albrecht, M. Zellmeier

… and all colleagues at HZB: - Institute for Silicon Photovoltaics

  • Institute for Nanospectroscopy
  • PVcomB

collaboration: financial support:

Grant No. 608498

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SLIDE 37
  • M. Liebhaber

E-mail: martin.liebhaber@helmholtz-berlin.de

Helmholtz Center Berlin Institue for Nanospectroscopy Energy Materials In-Situ Lab

Thank you for your attention! Questions & discussion