Nanowire Solar Cells

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1 Nanowire olar Cells Peidong Yang Department of Chemistry University of California, Berkeley Materials cience Division Lawrence Berkeley National Laboratory Materials ciences Division

2 Emerging PV Low cost Intermediate efficiency Environmental benign Possible solar paint Materials ciences Division MR Bulletin, Jan 2005

3 Emerging PV Grätzel, M. Nature 414, Alivisatos et al. cience 2002, 295, Why nanowires are important? Materials ciences Division

4 PV Performance Metrics FF = J MV J V C M OC Efficiency Pout = = P in FF Voc J P Materials ciences Division in sc

5 η A = (1 e αd ) η = d ED e / L D IQE(λ) = η A (λ) η ED η CT η CC P out η PCE = = FF x V OC q F(λ)IQE(λ)dλ P in P in Materials ciences Division. Forrest. MR Bulletin, Jan 2005

6 Emerging PV Use of solar at terawatt levels requires drop in $/W p 3N: New materials, New designs, New tricks dirty semiconductors organics oxides absorbers biological subunits dye-sensitized cells bulk heterojunction cells (polymer, organic-inorganic) quantum effects carrier multiplication frequency shifting Interface engineering Materials ciences Division

7 Dye-sensitized Photoelectrochemical Cell electron diffusivity: 10-4 cm 2 /s Poor charge collectors? Absorbanc e MLCT Grätzel, M. Nature 414, DC characteristics surface area of cm 2 per cm 2 η of 5-10% with TiO 2 nanoparticles electron transport via trap-mediated diffusion Wavelength(nm) Materials ciences Division Low efficiency at long wavelength

8 The three ways to improve DC efficiency 1) Find dyes that function efficiently across the visible and near-ir 2) Raise open-circuit voltage closer to its theoretical maximum 3) Increase the electron diffusion length in the oxide anode, L d = (D e τ) 1/2 speed up electron transport slow recombination adopt a nanowire geometry engineer the active interface Nanoparticle DC Nanowire DC random, polycrystalline network oriented single-crystalline channels slow diffusive transport fast band conduction (field-assisted) efficient for films ~10 μm thick in principle, efficient for much thicker cells high internal surface area smaller internal surface area Materials ciences Division

9 Nanowire DC: Design Principle high nanowire density long, thin nanowires electrode length (μm) diameter (nm) density (x10 10 cm -2 ) A nanoparticle n/a ideal nanowire achieved NW ~200 Materials ciences Division

10 Large-cale Nanowire Array ynthesis 1 st : dip-coat to get ZnO quantum dots 2 nd : grow nanowires from QD seeds zinc salt hydrolysis, HMTA C Nanowire densities of 1-40 billion cm -2 ingle-crystalline wires in direct contact with the substrate Inexpensive and environmentally benign Compatible with arbitrary substrates of any size Materials ciences Division L. Greene et al. Angew Chem. Int. Ed. 42, 3031, 2003.

11 Control of Nanowire Aspect Ratio Poly-ethylenimine (PEI): H 2 N-(CH 2 CH 2 N) x -(CH 2 CH 2 NH) y -]- CH 2 CH 2 NH 2 diameter (nm) Hybrid length (microns) without PEI with PEI DC 500 nm 5 μm Hybrid: aspect ratio = 10 DC: aspect ratio > 150 Materials ciences Division

12 Alignment Control 500 nm 200 nm Greene, L.E., Law, M. et al. Nano Letters 5, 1231 (2005). Materials ciences Division

13 High Optical Quality A / a.u. E / a.u. I/a.u. 5 K 25 K 45 K 65 K 85 K 105 K 125 K 145 K 165 K 185 K 215 K 245 K 275 K 300 K I/a.u λ/nm λ / nm λ /nm TEM shows that the nanowires are single crystals Wire surfaces are clean (Raman, EEL) after 400 C treatment Materials ciences Division

14 Characterization of Nanowire Arrays Electrical: Ohmic wire-substrate contacts FETs: Wires have high e - mobility I D (μa) I D (na) 125 V 100 D = 100 mv V G (V) 40 V 20 V 0 V -20 V -40 V Individual wires are electrically conductive ρ = Ω cm mobility: 1-5 cm 2 V -1 s -1 electron diffusitivity: D n = cm 2 s -1 [D = k B Tμ/e ] Ensure larger electron diffusion length, avoiding possible interfacial recombination Materials ciences Division V D (V) Law, M., Greene, L. et al. Nature Mater. 4, 455 (2005).

15 Nanowire based DC Black = 12 nm TiO 2 NP Blue = 30 nm ZnO NP Green = 200 nm ZnO NP Red = ZnO nanowires η PCE = 1.5% under AM 1.5 G conditions Materials ciences Division NW cells are competitive with thin TiO 2 nanoparticle cells (η cc ~ 100%) NW cells outperform ZnO nanoparticle cells Law, M., Greene, L. et al. Nature Mater. 4, 455 (2005).

16 Nanowire DC Faster electron injection in NW cell Bi-exponential (<250fs, 3ps) vs. Tri-exponential (<250fs, 20ps, 200ps) Materials ciences Division

17 Time cale for Electron Injection and Transport Grätzel, M, MR Bulletin, Jan 2005 Materials ciences Division

18 Engineer active interface to reduce recombination Core-sheath Nanowire Cells Overcoat the nanostructured electrode with an insulating or semiconducting oxide Reduce recombination Physically separate electrons and holes ubstrate Form a tunneling barrier Passivate recombination centers on oxide surface Dye Redox hift band edge to increase V oc Use an oxide with a higher band edge energy Form dipole layer that bends band upwards Materials ciences Division Gregg, B. NREL.

19 Atomic Layer Deposition (ALD) Oxides: Al 2 O 3, TiO 2, Ta 2 O 5, Nb 2 O 5, ZrO 2, HfO 2, no 2, ZnO, La 2 O 3, Y 2 O 3, CeO 2, c 2 O 3, Er 2 O 3, V 2 O 5, io 2, In 2 O 3,... Perovskites: rtio 3, BaTiO 3, LiNbO 3, LaMnO 3 Nitrides: AlN, TaN x, NbN, TiN, MoN, ZrN, HfN, GaN,... Fluorides: CaF 2, rf 2, ZnF 2,... Metals: Pt, Ru, Ir, Pd, Cu, Fe, Co, Ni,... Carbides: TiC, NbC, TaC,... Mixed structures: AlTiN x, AlTiO x, AlHfO x, io2:al, HfiO x,... ulfides: Zn, r, Ca, Pb,... Nanolaminates: HfO 2 /Ta 2 O 5, TiO2/Ta 2 O 5, TiO 2 /Al 2 O 3, Zn/Al 2 O 3, ATO (AlTiO)... Doping: ZnO:Al, Zn:Mn, r:ce, Al 2 O 3 :Er, ZrO 2 :Y,... rare earth metals (Ce3+, Tb3+ etc.) also co-doping Example: Zn Planar ystems, Inc. Materials ciences Division

20 Core-sheath Nanowire Dye-sensitized olar Cells Materials ciences Division

21 Nanowire-polymer Hybrid Cell Poly(3-hexylthiophene) C 6 H 13 0 vacuum ] ]- 2L D ~ 20 nm -3.5 ~2.1 ev -4.5 ITO ZnO P3HT Ag )- CH 3 CH 3 2L D Target nanowire array CH 3 CH 3 )- a 1) ultrahigh nanowire density 2) short, thin nanowires 3) nanowires normal to substrate CH 3 CH 3 c Materials ciences Division

22 Nanowire-polymer Composite Film Aligned wires Inter-wire spacing is nm 2L D for P3HT ~ 20 nm Thickness nm 200 nm Materials ciences Division

23 The Ideal Nanowire Cell Low cost Intermediate efficiency Environmental benign Possible solar paint Fully interdigitated donor-acceptor interface Acceptor wire array: high density, smaller band gap Donor: polymer/nanoparticles, maximize absorption Interface engineering: reduce recombination. Applicable to DC, hybrid, and conventional semiconductor cells. Materials ciences Division N. Lewis

24 Acknowledgement Dr. Matt Law Lori Geene Dwaud Tan Funding DOE ITRI Materials ciences Division