Semiconductor nanowires for solar cells
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Semiconductor nanowires for solar cells: Single junction and heterojunction photovoltaics Presented by Ted Kamins Consulting Professor Department of Electrical Engineering Stanford University Stanford CA 94305 Ge/Si core-shell nanowires and nanotubes N. J. Quitoriano, et al., Nano Letters 9, 1511-1516 (2009). Courtesy of Prof. N. J. Quitoriano, McGill University 20 nm For efficient solar cells 1) Increase absorption of solar spectrum Material selection Thickness of absorbing material Shape 2) Increase I collection ll ti off photogenerated h t t d carriers i Material quality Geometry How can semiconductor nanowires help increase efficiency? For efficient solar cells 1) Increase absorption of solar spectrum Material selection Thickness of absorbing material Shape 2) Increase I collection ll ti off photogenerated h t t d carriers i Material quality Geometry How can semiconductor nanowires help increase efficiency? What is a nanowire? Structure with one dimension (e.g., length) much greater than others “nano” implies other dimensions (e.g., diameter) < 100 nm Usuallyy single Usua s g e crystal c ysta for o good e electrical ect ca p properties ope t es Semiconductor nanowires for photovoltaics Limitations of planar solar cells Tradeoff: Photon absorption – carrier collection Limited heterostructures: different lattice parameters How nanowires can help overcome these limitations Nanowire growth F Focus on Vapor-Liquid-Solid V Li id S lid (VLS) growth th b by CVD Other techniques: MBE, solution growth, etching, anodization Catalyst Solar cells Single junction Axial: Bulk impurities and surface states can limit performance Core-shell structure Heterostructures Resonance effects Reusable substrates Focus on nanostructure growth and properties and their relation to solar cell efficiency Two-dimensional, planar solar cell Carriers generated within depletion regions are efficiently collected Carriers generated outside of depletion region diffuse (or drift) t depletion to d l ti region i tto b be collected ll t d Carrier collection Carrier recombination p+ p+ p+ n n+ n n+ n n+ Absorption and collection are coupled in planar solar cell Tradeoff for thicker absorbing g layer y More light absorption and carrier generation Less collection of photogenerated carriers More recombination Higher cost for thicker and higher higher-quality quality material 5 Light absorption in silicon 100 1 1 nm 1 µm 10-2 1 mm 10-4 10-6 1 cm 10-8 1m 10-100 200 Absorptio on Length Abssorption Co oefficient (µm-1) Absorption in Silicon 600 1000 Wavelength (nm) 1400 Reference: Wikipedia Reference: http://pvcdrom.pveducation.org/APPEND/OPTICAL.HTM Many carriers generated deep within silicon solar cell Not efficientlyy collected if diffusion length g limited by y material q quality y LD = 2(D) ( = minority-carrier recombination lifetime) Metal-catalyzed (VLS) nanowire growth “Bottom-up” nanowire fabrication SiH4 H2 Gas transport Si H SiH4 Si Metal catalyst particle (eg, TiSi2 ) y surface reaction Catalyzed Diffusion (bulk or surface) Precipitation Metal nanoparticle Si nanowire Si substrate Metal-catalyzed Si and Ge nanowires 2 µm 1 m 5µm Au NanoP+Bake + NW (363-3) 500 nm 1.5 µm Diameter: 5 nm to >1 µm Random or epitaxial alignment Aspect ratio for straight nanowires > 1000:1 Nanowire diameter depends on Catalyst nanoparticle size Nanowire growth conditions Nanowire properties can depend on catalyst material Temperature e (C) Material for catalytic nanoparticles Au: Liquid eutectic: ~360C 360 C for Si and Ge Growth temperature 400 – 1000C Nanoparticle in liquid state during growth Vapor-liquid-solid (VLS) growth Ti: Liquid eutectic: 1330C Growth temperature: ~600C Nanoparticle in solid state Vapor-solid-solid (VSS) growth Temp perature (C)) During nanowire growth catalyst nanoparticle can be in liquid state (VLS) or solid state (VSS) 1500 1414 Au-Si 1300 L 1100 1064 900 700 500 363C 300 1000 20 40 60 80 100 Au Atomic Percent Silicon Si 2200 2000 Ti-Si L 1800 1670 1600 1414 1400 1330 1330C 1200 1000 800 600 0 20 40 60 80 100 Ti Atomic Percent Silicon Si Catalyst Desired characteristics: Au Small nanoparticles; narrow size distribution Deposition temperature above or below liquid eutectic temperature (VLS vs. vapor-solid growth) 360 C ((Small size lowers eutectic temperature) p ) Low solid solubility in Si 1014-1016 Not a deep energy level in Si bandgap Et-Ei 0.01eV Ti 1330 C 1012 cm-3 0.34 eV Methods of forming catalyst on substrate: Random locations Deposit thin metal layer (~nm) and anneal Surface tension causes agglomeration into nanoparticles Disperse pre-formed catalyst nanoparticles Pre-determined locations – with low-cost techniques Imprint lithography Bl k co-polymer Block l Anodic aluminum oxide Nanoimprint lithography + electroless deposition Si(111) + NIL + electroless Au Si nanowires (~top view) OMIT: 60% 60% vertical vertical OMIT: Nanowires often grow in (111) directions Position catalyst using self-assembly Block co-polymers Use as mask for metal (electro)deposition or include metal in polymer Suitable for small small-diameter, diameter closely spaced nanowires Short-range order in array 100 nm Alternatively Porous anodic aluminum oxide Forms array with short-range order 200 nm Nanowire growth Materials Si nanowires: Precursors: Si2H6, SiH4, SiH2Cl2, SiCl4 Growth temperature 400 - 1000 C Ge nanowires: Precursor: Usually GeH4 Growth temperature: ~300 C Oth semiconductors: Other i d t e.g., GaP, G P GaAs, G A InP, I P InN I N Alignment Nanowires often grow in (111) directions U Si(111) substrate Use b t t ffor vertical ti l alignment li t Somewhat expensive Repositioning nanowires after growth C d Can detach t h nanowires i f from growth th substrate b t t Place on another substrate Benefits Decouple p high-temperature g p g growth from substrate Allows use of low-temperature substrates eg, Low-temperature (low cost) glasses Possible advantages of nanowire solar cells Decrease reflectance (light trapping) Increase junction area; i.e., increase surface area / unit volume Decrease distance between carrier generation and collection Wider range of materials and heterostructures Resonance effects Less material used Vertically aligned nanowire solar cell 1) Grow vertically aligned nanowires 2) Fill with transparent insulator 3) Chemical-mechanical polish 4) Dope top of nanowires (optional) 5) Add top transparent conductive electrode Nanowires often g grow in ((111)) directions Use Si(111) substrate for vertical alignment Decrease reflectance: “Light trapping” Multiple reflections reduce overall reflectance, increase absorption Sensitive to angle of incident light Increase light scattering between nanowires Light incident parallel to nanowires Low absorption Introduce scattering centers between nanowires M. D. Kelzenberg, et al, Nature Materials 9, 239-244 (March 2010) Random alignment of nanowires Less dependent on angle of incidence of light Can grow on non-single-crystal substrate Limited connection and packing density 1) Grow randomly aligned nanowires 2) Fill with transparent insulator 3) Chemical-mechanical polish 4) Dope top of nanowires (optional) 5) Add top transparent conductive electrode Deep energy levels within nanowire pn ni U Et-Ei 2 Et E i p n 2ni cosh kT 0 1 0 = LD = (D) vth Nt 0 55 0.55 Ti Au 0.34 0.01A 0.26D Concern: Catalyst metal incorporation into growing nanowire For Au, solid solubility (Nt) ~1014 cm-3 at 500C 0 ~ 100 ns; LD ~ 10 m Serious concern: High-temperature NW growth For Au, solid solubility (Nt) ~1016 cm-3 at 1000C 0 ~ 1 ns; LD ~ 1 m Also, worse if extra catalyst remains on NW surface Solutions: Use catalyst that does not introduce deep levels Build structure so photo-generated carriers are created within ~ LD of collecting region. Carrier recombination at surface states n+ n+ Surface states p+ p+ IIncrease carrier i absorption b i b by lilight h trapping, i b but …. Carriers recombine at surfaces before reaching collecting junction Carriers not collected reduced efficiency Improve by Passivating surface states For Si: With native oxide on surface: Ns = 2.3 1012 cm-2 After growing thermal oxide: Ns = 5 – 10 1011 cm-2 (Ns probably can be markedly reduced) Decreasing distance to collecting junction Ec Ei Ev Core-shell structure i p i n p n Decouple minority-carrier diffusion length from absorption depth Design so diameter <~ minority carrier diffusion length Maximize absorption length Minimize loss of photogenerated carriers by recombination Core-shell structure: VLS + uncatalyzed growth VLS growth p-type Uncatalyzed growth undoped (optional) Uncatalyzed growth n-type Grow p-type G t nanowire i by b catalyzed t l d VLS growth th Possibly remove catalyst NP Change growth conditions to favor uncatalyzed epitaxial growth Higher temperature temperature, more reactive precursor precursor, Grow undoped shell (optional) Grow n-type shell Core-shell nanowire solar cell p-n n p-i-n n p i p heterojunction Group IV, III-V, II-VI n i p Transparent electrode Transparent i insulator l t Insulator – eg, SiO2 Substrate - eg, Si(111) How to form core-shell structures 1) Grow nanowires (VLS), deposit epitaxial shell (non-catalyzed growth) Homojunction or heterojunction 2) Grow nanowires (VLS), (VLS) deposit amorphous Si Si, crystallize (optional) 3) Etch Si wafer to form nanowires, deposit amorphous Si, crystallize Grown Si nanowire + deposited amorphous Si shell Metal-foil substrate p-type p yp Si NW n-type amorphous Si (larger bandgap) Compared to planar structure Shorter collection distance Reduced optical reflectance L. Tsakalakos, et al, Appl. Phys. Lett. 91, 233117 (2007) Etched core + poly Si shell n-type single-crystal core l lli shell h ll p-type polycrystalline n-type Si wires formed by etching Si wafer p-type polysilicon shell by depositing amorphous Si + crystallization E. C. Garnett and P. Yang, J. Am. Chem. Soc. 130, 9224 (2008). Other materials: InP nanowire/polymer hybrid structure ITO/glass substrate Grow n-type InP nanowires Coat with high hole mobility conjugated polymer (poly(3 (poly(3-hyxylthiophene) hyxylthiophene) Low contact resistance to ITO Forward current increases by 106 compared to ITO alone Id lit ffactor Ideality t n ~1.33 1 33 Passivate InP NW surface states with sulfur Ammonium sulfide dip after NW growth Reduces surface recombination 500 nm C. J. Novotny, et al, Nano Lett. 8, 775-779 (2008) Resonance effects Tune wavelength of maximum absorption Absorption depends on diameter M lti l iinternal Multiple t l reflections fl ti Tapered nanowires Broader absorption? p D1 D2 Absorrption D2 Wavelength A Absorption n D1 Wavelength Resonance effects Single nanowires Increased absorption Weaker dependence on angle of incident light Multiple nanowires Control spacing p g between nanowires Increase absorption and Jsc L. Y. Cao, et al, Nano Lett. 10, 439-445 (2010) 28 28 Vertically stacked multi-junction solar cell Eg1 Eg2 Absorptio on Eg1 > Eg2 > Eg3 Eg3 Wavelength Different wavelengths absorbed in different materials in vertical stack of materials with different bandgaps g p Bandga ap Vertically stacked multi-junction solar cell: Lattice matched Lattice parameter Courtesy of C-Z Ning, Arizona State University Multi-junction solar cell Planar structure: Lattice mismatch limits materials that can be used Nanowires: Small diameter allows growth of lattice mismatched materials Heterojunction between nanowire and substrate Heterojunction within nanowire Nanowire growth on lattice mismatched substrate Grow high-quality direct bandgap materials on Si (Less expensive substrate) Small cross section Accommodates Lattice mismatch strain (8% for InP/Si) Thermal expansion mismatch Allows single domain Materials grown on Si substrate (partial list) GaP, GaAs, InP, InN S. S. Yi, et al, Appl. Phys. Lett. 89, 133121 (2006) 10s nm III-V V material Epitaxial growth of compound semiconductor nanowires on Si ~µm Si (111) substrate 6 µm Lattice mismatched nanowire growth Ge on Si: 4% lattice mismatch Radial: Ge Core – Si shell A i l Axial Ge HJ Si Courtesy of S. Sharma N JJ. Quitoriano, N. Quitoriano et al, al Nano Lett. Lett 9, 9 1511 (2009) Lateral composition variation along substrate Binary compound AxB1-x x= f(PA, PB, PT, exp(-Ea/kT), ….) Vary parameters over deposition zone varying composition Can extend to ternary and quaternary compounds CdS (~500nm) CdSe (~700nm) CdSxSe S 1-x Position along substrate General strategy: Combination of temperature gradient and source material t i l profiling fili Pan, Zhou, Sun, Leong, Chin, Liu, Zou, and Ning, Nano Lett. 9, 784 (2009) 34 Full-spectrum lateral multijunction solar cell Ning, Liu, and Pan, PVSC-34 Proceedings, June 7-12, 2009, Philadelphia, PA Possibly extend to composition variation along length of one nanowire Reuse substrate to reduce cost: Vertical nanowires Form oxide on Si(111) wafer Pattern oxide Electrodeposit catalyst (eg, Au or Cu) Grow VLS wires Fill with polymer (PDMS) ( S) Detach wires + polymer from substrate Etch residual Si nanowire from substrate (Anisotropic etch; slow Si(111) etch) Electrodeposit new catalyst Grow VLS wires J. M. Spurgeon, et al., Appl. Phys. Lett. 93, 032112 (2008) 40 µm Semiconductor nanowires for solar cells: Summary Limitations of planar solar cells Tradeoff: Photon absorption – carrier collection Li it d heterostructures: Limited h t t t diff different t llattice tti parameters t How nanowires can help overcome these limitations Nanowire growth Mainly Vapor-Liquid-Solid (VLS) growth by CVD Catalyst Solar cells Single g jjunction Axial: Bulk impurities of surface states can limit performance Core-shell structure Si, Ge and other semiconductor materials Heterostructures Resonance effects Reusable substrates F Focus on nanostructure t t growth th and d properties ti and d their th i relation l ti to solar cell efficiency Conclusion: Possible advantages of nanowire solar cell Decrease reflectance (light trapping) I Increase junction j ti area; i.e., i i increase surface f area / unit it volume l Decrease distance between carrier generation and collection Wider range of materials and heterostructures Resonance effects Less material used BACKUP Semiconductor nanowires for solar cells: Topics discussed Limitations of planar solar cells Tradeoff: Photon absorption – carrier collection Li it d heterostructures: Limited h t t t diff different t llattice tti parameters t Nanowire growth Focused on Vapor-Liquid-Solid (VLS) growth by CVD Other techniques: MBE, MBE solution growth growth, etching etching, anodization Catalyst Solar cells Decrease reflectance (light trapping) Single g jjunction Increase junction area; i.e., increase surface area / un Axial: Surface states, bulk impurities Decrease distance between carrier generation and co Core-shell structure Wider range of materials and heterostructures Si, Ge and other semiconductor materials R Resonance effects ff Heterostructures Less material used Resonance effects Reusable substrates F Focus on nanostructure t t growth th and d properties ti Surface / interface charge Surface (+) charges ( Ns [cm-2] ) Determine Ns from dependence of resistance on nanowire diameter Depleted (-) charges ( Na [cm-3] ) Dopant concentration Na = 1.9 1018 cm-3 r Φ ro Surface/interface charge density with native oxide on surface L Effective conducting area (Aeff) Ns = 2.3 1012 cm-2 Depleted region by surface charge -8 8 after ft thermal th l oxide id grown Ns = 5 – 10 1011 cm-2 Ns probably can be markedly reduced K.-II Seo, K Seo S. S Sharma, Sharma A. A Yasseri, Yasseri D D. Stewart Stewart, T T. II. Kamins Electrochemical and Solid-State Letters 9, G69 (2006) Conductance-L L ( S-cm) 3x10 -8 2x10 (b) with thermal SiO2 -8 1x10 with native SiOx 0 0 -10 2x10 -10 4x10 Aeff ( cm ) TiO2 nanotubes for dye-sensitized solar cells ZnO NWs 500 nm 5 µm TiO2 NTs 5 µm 5 µm Jsc, increase with ends of tubes open (more surface area) Hi h efficiency Higher ffi i compared d tto sintered i t d TiO2 nanoparticles: ti l Surface more accessible than disordered pores between nanoparticles Easier to fill with sensitizer (C use ZnO (Can Z O nanowires, i b t TiO2 more efficient) but ffi i t) C. K. Xu, et al., Chemistry of Materials, 22, 143 (2010) -10 6x10 2 Wire array detached from substrate SEM images of polymer-embedded Si wire array Al2O3 particles in polymer to scatter light M. D. Kelzenberg, g et al, Nature Materials 9, 239-244 ((March 2010)) Using nanowires Grow nanowires in location where they will be used (100) substrate (111) substrate Nanowires 1 500 m nm 1 m Si electrode Si electrode 337C (416-1) 500 nm 5 µm SiO2 Grow nanowires on separate substrate and then reposition them Manipulating nanowires after growth Horizontal arrangement Possible benefit: Efficiency only weak function of angle of incident light No tracking needed during day Possibly extend to multilayers Drawbacks Hard to make ohmic contacts Don’t know where ends of nanowires are located Unpatterned substrate Random Patterned substrate Semi random Semi-random One-dimensional fluidic alignment Possibly directed by channels or patterns on substrate Possibly use statistical assembly and/or post post-assembly assembly configuration Transfer printing Place nanowires on rubber (eg PDMS) stamp “P i t” onto “Print” t substrate b t t Roll (and slide) roller with nanowires on substrate (A. Javey, Berkeley) Manipulating nanowires after growth Horizontal arrangement Langmuir-Blodgett assembly Grow nanowires and detach from growth substrate Float on liquid phase C Compress llaterally t ll tto fform d dense array (“raft”) Transfer to substrate Possible advantages of nanowire solar cell Decrease reflectance (light trapping) I Increase junction j ti area; i.e., i i increase surface f area / unit it volume l Decrease distance between carrier generation and collection Wider range of materials and heterostructures Resonance effects Less material used Two-dimensional, planar solar cell Carriers generated within depletion regions are efficiently collected Carriers generated outside of depletion region diffuse (or drift) t depletion to d l ti region i tto b be collected ll t d p+ p+ i n n+ n+ p+ n n+ Two-dimensional, planar solar cell Carrier collection Carrier recombination p+ p+ n n n+ n+ Absorption and collection coupled in planar solar cell Tradeoff for thicker absorbing layer More light absorption and carrier generation Less collection of photogenerated carriers More recombination Higher cost when use expensive material Other materials: TiO2 nanotubes for dye-sensitized solar cells Catalyzed solution growth of ZnO nanowires Chemical conversion to TiO2 nanotubes: Simultaneous deposition of TiO2 and dissolution of ZnO Chemically open ends of TiO2 nanotubes Etch residual ZnO core nanocrystalline TiO2 nanotubes All wet processing; low cost, low temperature, scalable C. K. Xu, et al., Chemistry of Materials, 22, 143 (2010) TiO2 nanotubes for dye-sensitized solar cells TiO2 nanotubes formed by anodization of Ti Large surface area (1800X flat surface) Low recombination losses Charge transport along nanotube axis Increased optical absorption by light trapping K. Shankar, et al, Nano Lett. 8, 1654-1659 (2008) Resonance effects L. Y. Cao, et al,, Nano Lett. 10, 439-445 ((2010)) Resonance effects Single nanowires Take advantage of effects in individual nanowires Increased absorption Weaker dependence on angle of incident light Multiple nanowires Control spacing between nanowires Increase absorption and Jsc L. Y. Cao, et al, Nano Lett. 10, 439-445 (2010) Bandgap Multi-junction solar cell Lattice parameter Lattice mismatch limits materials that can be used Limits number of junctions in 2D structure Small diameter of nanowires allows growth of lattice mismatched materials Heterojunction between nanowire and substrate Heterojunction within nanowire Semiconductor nanowires for solar cells: Topics discussed Limitations of planar solar cells Tradeoff: Photon absorption – carrier collection Li it d heterostructures: Limited h t t t diff different t llattice tti parameters t How nanowires can help overcome these limitations Nanowire growth VLS growth by CVD Other techniques: MBE, solution growth, etching, anodization Catalyst Solar cells Initially focus on Si and Ge Nano to micro-scale diameters; aligned or random Surface states, bulk impurities Core-shell structure Resonance effects Other semiconductor materials H t Heterostructures t t Reusable substrates Focus on nanostructure growth and properties Can nanowires improve solar cell efficiency? What is a nanowire? Structure with one dimension much greater than others “nano” implies other dimensions (e.g., diameter) < 100 nm Usually single crystal for good electrical properties
