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 Nanospectroscopy Energy Materials In-Situ Lab _______________________________ SPREE/UNSW Seminar 12/11/2015
Where I come from … martin.liebhaber@helmholtz-berlin.de 2
Where I come from … Berlin City Helmholtz Center Berlin sights night life EMIL sights winter E nergy M aterials I n-Situ L ab martin.liebhaber@helmholtz-berlin.de 3
Part I – Conventional concept Valence band offset and hole transport in c-Si/a-SiO x heterojunction solar cells Cooperation: Financial support: Institut for Silicon Photovoltaics (HZB) “SISSY” project Grant No. 03SF0403 Lars Korte Mathias Mews … many more people involved Grant No. 608498 martin.liebhaber@helmholtz-berlin.de 4
Introduction SHJ solar cell ( S ilicon H etero j unction) front amorphous crystalline heterojunction TCO contact emitter (p)a-Si:H (10 nm) p/n- passivation (i)a-Si:H (5 nm) (i)a-SiO x :H junction base (n)c-Si (≈260 μ m) ► excellently passivated contacts ► world record for Si based PV: η = 25.6%/ 22.5% (Panasonic, 2014/2015) (i/n + )a-Si:H back contact ► reduce parasitic absorption TCO/metallization ◦ wide band gap materials ◦ band offset modification martin.liebhaber@helmholtz-berlin.de 5
Motivation ► heterojunction parameter ◦ optical band gap E g ◦ band offsets ∆ E V , ∆ E C ◦ surface passivation τ eff ► FOCUS on ∆E V of ◦ a-SiO x :H/c-Si heterointerface ◦ hole transport mechanism martin.liebhaber@helmholtz-berlin.de 6
Sample preparation ► PECVD layer deposition ( P lasma E nhanced C hemical V apour D eposition) ◦ varying precursor gas mixtures ◦ change of stoichiometry x in a-SiO x layers martin.liebhaber@helmholtz-berlin.de 7
Stoichiometry (XPS) gas flow (sccm) ► X -ray P hotoelectron S pectroscopy a-Si:H SiO 2 SiH 4 CO 2 ► Si 2p peak 10 5 H 2 pure Si precursor gas mixture variation (0% CO 2 ) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 SiO 2 CO 2 flow 3 cps) 3 cps) 3 cps) 3 cps) 3 cps) 3 cps) 3 cps) 3 cps) 3 cps) 3 cps) (80% CO 2 ) 0 sccm 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1 sccm CO 2 flow Intensity (10 Intensity (10 Intensity (10 Intensity (10 Intensity (10 Intensity (10 Intensity (10 Intensity (10 Intensity (10 Intensity (10 2 sccm 3 sccm 4 sccm 5 sccm 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 6 sccm 7 sccm 8 sccm 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 98 98 98 98 98 98 98 98 98 98 100 100 100 100 100 100 100 100 100 100 102 102 102 102 102 102 102 102 102 102 104 104 104 104 104 104 104 104 104 104 106 106 106 106 106 106 106 106 106 106 108 108 108 108 108 108 108 108 108 108 Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) Binding energy E bind (eV) M. Liebhaber et al ., APL 106 , 031601 (2015) martin.liebhaber@helmholtz-berlin.de 8
Stoichiometry (XPS) ► chem. shift of core level peak depends on oxidation states ► peak intensity ratios ↔ oxygen concentration Si atom O atom pure Si SiO 2 (Si 0+ ) (Si 4+ ) Si 1+ Si 2+ Si 3+ 2p 1/2 2p 3/2 M. Liebhaber et al ., APL 106 , 031601 (2015) martin.liebhaber@helmholtz-berlin.de 9
Stoichiometry (XPS) SiO 2 ► cross-check with other peaks ► conversion: SiH 4 /CO 2 ratio → oxygen conc. ► non linear dependency of stoichio- metry on gas phase composition a-Si:H M. Liebhaber et al ., APL 106 , 031601 (2015) martin.liebhaber@helmholtz-berlin.de 10
Valence band (UPS) ► U ltraviolet P hotoelectron S pectroscopy ► obtain valence band position relative to E F • He-UPS at 21.2 eV (standard) • CFSYS at 4.0-7.3 eV [1] ( C onstant F inal S tate Y ield S pectroscopy) → lower detection limit ↔ band tails & defect distribution [1] L. Korte et al ., JAP 109 , 063714 (2011) M. Liebhaber et al. , APL 106 , 031601 (2015) martin.liebhaber@helmholtz-berlin.de 11
Valence band offset ∆E V (UPS & SPV) • UPS: valence band position (relative to E F ) U ltraviolet P hotoelectron S pectroscopy • SPV: band bending ≤ 150 meV S urface P hoto V oltage E [eV] E C E C 0 E F 𝑓𝜒 SPV E V UPS E V -1.15 (n)c-Si a-Si wafer increasing ∆E V (UPS & SPV) oxygen content UPS E V -5.15 a-SiO 2 martin.liebhaber@helmholtz-berlin.de 12
Valence band offset ∆ E V ► dependency of ∆E V on stoichiometry ► direct correlation of ∆E V on cell performance M. Liebhaber et al ., APL 106 , 031601 (2015) martin.liebhaber@helmholtz-berlin.de 13
passivation layers in SHJ solar cells a-SiO x only pass. layer M. Liebhaber et al ., APL 106 , 031601 (2015); M. Mews, M. Liebhaber et al. , APL 107 , 013902 (2015) martin.liebhaber@helmholtz-berlin.de 14
passivation layers in SHJ solar cells a-SiO x ► increasing D it 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) only pass. layer additional emitter depo M. Liebhaber et al ., APL 106 , 031601 (2015); M. Mews, M. Liebhaber et al. , APL 107 , 013902 (2015) martin.liebhaber@helmholtz-berlin.de 15
SHJ solar cells & hole transport pass. pass. emitter emitter layer layer absorber absorber E E - E C E C 0 0 E F E F V oc FF + + + E V E V + ∆E V ∆E V + tunnel hopping/ (i)a-SiO x :H (i)a-SiO x :H thermionic emission I sc (p)a-Si:H (p)a-Si:H (n)c-Si wafer (n)c-Si wafer ► solar cell parameter ► hole transport mechanism • surface passivation (→ V oc ) ◦ OK • transport barrier reflected in FF ◦ thermionic emission (for small ∆E V ) • widening of band gap (→ I sc ) ◦ tunnel hopping through tail states ◦ thin layer (small effect expected) (additional for higher ∆E V [1]) • band offsets (→ FF) ◦ transport barrier [1] A. Kanevce et al ., JAP 105 , 094507 (2009) M. Mews, M. Liebhaber et al. , APL 107 , 013902 (2015) martin.liebhaber@helmholtz-berlin.de 16
SHJ cell with stacked passivation layer ► “staircase” of valence band offsets single layer • improved FF for HIT cell split into with stacked passivation layers layer stack ► promising concept: combination of • moderate band gap passivation layer ∆ E V,a-Si:H = 270 meV • high band gap hole contact layer ∆E V,a-SiO0.3:H = 585 meV M. Mews, M. Liebhaber et al. , APL 107 , 013902 (2015) martin.liebhaber@helmholtz-berlin.de 17
Conclusion ► growth of (i)a-SiO x :H thin films (PECVD) • stoichiometry (XPS) • ∆E V (UPS/SPV) • sufficient passivation ► implemented into SHJ cells (low x -regime) • FF directly linked to ∆ E V • 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-SiO x 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) martin.liebhaber@helmholtz-berlin.de 18
Part II – 3 rd generation concept Tetracene/c-Si hybrid solar cell – multi-exciton generation via singlet fission – March ´ 15 Project-ID 57140921 June/July ´ 15 Cooperation November ´ 15 … many more people involved martin.liebhaber@helmholtz-berlin.de 19
Motivation c-Si a-Si:H/perovskite/OPV UC: TTA, Er-Yb ,… MEG: Si nanocrystals, SF … adapted from: L. C. Hirst and N. J. Ekins-Daukes, PIP 19 , 286 (2011) martin.liebhaber@helmholtz-berlin.de 20
MEG via Singlet Fission (SF): 1 photon → 2 e - -h + pairs ► 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: LUMO m s =0 m s =0 m s =1 HOMO ground state singlet state triplet state martin.liebhaber@helmholtz-berlin.de 21
MEG via Singlet Fission (SF): 1 photon → 2 e - -h + pairs time electronic transitions: excitation fission triplet state ionization ≈100 ps ≈100 ns ≈100 fs ps – ms ?? → direct triplet energy transfer ↔ challenge! X X h + h + + * * * S 0 S 1 T 1 T 1 e - electron electron electron electron e - accepting accepting accepting accepting material material material material J. J. Burdett et al. , J. Chem. Phys. 133 , 144506 (2010) martin.liebhaber@helmholtz-berlin.de 22
MEG device Prerequisite: SF medium Solar cell ► triplet exciton diffusion length longer than thickness SF medium ► geminate triplet exciton pair dissociation at hybrid interface Exciton/ Exciton migration electron-hole-pair MEG event carrier migration absorption martin.liebhaber@helmholtz-berlin.de 23
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