Nanomaterials for Hybrid Solar Cells Silvija Gradečak Department of Materials Science and Engineering, MIT November 14 th, 2012 1
Solar energy Hybrid organic-inorganic photovoltaics Most of the world's energy resources are from the Sun's rays hitting the Earth The total annual downward solar energy at the Earth surface is approx. 8000 times more than the world annual energy consumption 2
Solar cells Multijunction GaAs Silicon Thin film Organic 3
Hybrid organic/inorganic photovoltaics Organic Bulk Heterojunction Organic/Inorganic Hybrid? Benefits of organic PV devices Flexible substrates Roll-to-roll processing Reduced installation costs Challenges Poor donor-acceptor ordering Poor carrier transport 4
Nanomaterials for PV 0-D: Nanoparticles 1-D: Nanowires 2-D: Nanowalls 500 nm 500 nm 500 nm Image taken by Joshua Leu 5
Outline Hybrid nanowire-polymer photovoltaics Addressing ordering Hybrid nanowire-quantum dot photovoltaics Ordered nanowire arrays and polymer infiltration Graphene electrodes for flexible photovoltaics Looking ahead: multijunction devices 6
Hybrid organic/inorganic photovoltaics Electron donor (hole transport) Electron acceptor (electron transport) 1. Photon absorption exciton generation Complimentary absorption 2. Exciton diffusion Ordered structures Donor-acceptor interface in the order of diffusion length (10 nm) 3. Exciton dissociation Band edge offsets of around exciton binding energy Recombination minimization 4. Carrier transport Continuous charge transport Improved mobilities 5. Carrier collection 7
Hybrid nanowire-polymer photovoltaics source: wikipedia.org 8
Hybrid nanowire-polymer photovoltaics Arrays of nanowires: enhanced charge transfer and enhanced light absorption GaAs (electron acceptor) and P3HT (electron donor) GaAs NWs (diameter = 25 nm) are blended with P3HT in a single solvent Drying mediated self-assembly leads to vertically inclined NWs (top-rich) Charge transport in the vertical direction, vertical separation reduces leakage current S. Ren, N. Zhao, S. C. Crawford, M. J. Tambe, V. Bulovic, and S. Gradečak, Nano Lett. 11, 408 (2011) 9
Hybrid nanowire-polymer photovoltaics Ordering? In-Plane GIXS GaAs NWs extend the photoabsorption to NIR range P3HT molecular ordering is enhanced as NWs concentration increases A threshold NW concentration exists Vertically inclined NWs stimulate the P3HT chain alignment along NW surface S. Ren, N. Zhao, S. C. Crawford, M. J. Tambe, V. Bulovic, and S. Gradečak, Nano Lett. 11, 408 (2011) 10
Hybrid nanowire-polymer photovoltaics Heterojunction photovoltaics using GaAs nanowires and conjugated polymers 2.3% power conversion efficiency devices realized using GaAs nanowires S. Ren, N. Zhao, S. C. Crawford, M. J. Tambe, V. Bulovic, and S. Gradečak, Nano Lett. 11, 408 (2011) 11
Hybrid nanowire-polymer photovoltaics S = 900 660 EQE(λ) I(λ)dλ 900 350 EQE(λ) I(λ)dλ Ordering plays a critical role in the enhanced device performance. S. Ren, N. Zhao, S. C. Crawford, M. J. Tambe, V. Bulovic, and S. Gradečak, Nano Lett. 11, 408 (2011) 12
Enhancing ordering Crystalline P3HT nanowires synthesized using solvent self-assembly Photoabsorption red-shifted and molecular ordering: mobility P3HT nanowire diameter ~ exciton diffusion length: exciton dissociation S. Ren, L.-Y. Chang, S. K. Lim, J. Zhao, M. Smith, N. Zhao, V. Bulovic, M. Bawendi, S. Gradečak, Nano Lett. 11, 3998 (2011) S. Ren, M. Bernardi, R. R. Lunt, V. Bulovic, J. C. Grossman, S. Gradečak, Nano Lett. 11, 5316 (2011) 13
Bridging quantum dots to conjugated polymer nanowires 4 nm CdS quantum dots capped by oleic acid ligands 14
Chemical grafting XPS S 2p peaks Non-grafting (same solvent) and chemical grafting (mixed solvent) Solvent-assisted chemical grafting decorates CdS QDS onto P3HT NW surface Grafting improves the efficiency of exciton dissociation Chemical grafting is confirmed using XPS and supported by time-resolved PL measurement 15
Device structure and performance Controlled morphology, interpenetrating and percolating P3HT/CdS BHJ network gives rise to efficient charge separation and charge transport Power conversion efficiencies of 4.1% S. Ren, L.-Y. Chang, S. K. Lim, J. Zhao, M. Smith, N. Zhao, V. Bulovic, M. Bawendi, S. Gradečak, Nano Lett. 11, 3998 (2011) S. Ren, M. Bernardi, R. R. Lunt, V. Bulovic, J. C. Grossman, S. Gradečak, Nano Lett. 11, 5316 (2011) 16
Enhancing ordering Electron beam lithography and galvanic process: selective deposition of small-diameter gold nanoparticles Galvanic process locally consumes the substrate etch pits are formed preventing Au diffusion during high-t nanowire growth Small-diameter (20 nm) nanowires grown with controlled position and density C.-H. Tseng, M. J. Tambe, S. K. Lim, M. J. Smith, S. Gradečak, Nanotechnology 21, 165605 (2010) 17
Hydrothermal (all-solution) synthesis of ZnO nanowires 18
Flexible substrates: beyond ITO Spin-coat PEDOT:PE(PC) or RG1200 zinc acetate dihydrate in 2- methoxyethanol with ethanolamine for ZnO seed layer Hydrothermal growth, 90 ºC Zinc nitrate hexahydrate and Hexamethylenetetramine (HMTA) solution Graphene ZnO seed layer ZnO NWs Quartz/Graphene Quartz/graphene/ZnO Quartz/graphene/RG-1200/ZnO Growth of high-quality ZnO nanowires on graphene via hydrothermal method enabled by novel non-destructive interfacial modification of graphene Good wetting property of zinc acetate dihydrate in 2-methoxyethanol on graphene observed when the surface is modified with interfacial conductive polymer layers H. Park, S. Chang, J. Jean, J. Cheng, M. Bawendi, V. Bulović, J. Kong, S. Gradečak, submitted 19
Flexible substrates: graphene electrode ITO Graphene/PEDOT:PEG(PC) ZnO nanowire arrays on ITO as well as on the modified graphene surface were grown under the same conditions The alignment and quality of nanowires on the modified graphene substrates is comparable to the results obtained on ITO Graphene/RG-1200 200 nm H. Park, S. Chang, J. Jean, J. Cheng, M. Bawendi, V. Bulović, J. Kong, S. Gradečak, submitted 20
Graphene-nanowire photovoltaics ZnO nanowire-p3ht Au First inverted organic/inorganic PV device using graphene as a cathode MoO3 P3HT Anode Interface layer ITO -Graphene PEDOT:PEG(PC) ZnO nanowires Graphene RG-1200 Graphene HTL JSC (ma/cm2) VOC (V) FF PCE (%) P3HT P3HT P3HT 1.67 1.89 2.44 0.59 0.49 0.57 0.37 0.34 0.33 0.36 0.31 0.46 200 nm H. Park, S. Chang, J. Jean, J. Cheng, M. Bawendi, V. Bulović, J. Kong, S. Gradečak, submitted 21
Graphene-nanowire photovoltaics ZnO nanowire-pbs QD Au MoO3 PbS QD Anode Interface layer ITO -Graphene PEDOT:PEG(PC) ZnO nanowires RG-1200 Graphene 200 nm HTL JSC (ma/cm2) VOC (V) FF PCE (%) PbS QD PbS QD PbS QD 24.57 24.39 20.78 0.54 0.50 0.54 0.39 0.35 0.35 5.14 4.18 3.91 Graphene H. Park, S. Chang, J. Jean, J. Cheng, M. Bawendi, V. Bulović, J. Kong, S. Gradečak, submitted 22
Looking ahead: multijunction devices Al x Ga 1-x As grown at 480 C and V/III=80 on GaAs (100)B substrate Zincblende structure and <111> growth direction High temperatures and high V/III ratios: highly tapered structure and compositional gradient EDS: higher Al composition at the nanowire top than at the base Top Middle S. K. Lim, M. Tambe, M. Brewster, S. Gradečak, Nano. Lett. 8, 1386 (2008) 23
MOCVD growth of III-V ternary nanowires base decomposition diffusion length Two competing growth mechanisms Relevant parameters: diffusion length of adatoms, concentration and decomposition rates of MO precursors Temperature, V/III ratio E MMA > E MMG MG E Al-As > E Ga-As S. K. Lim, M. Tambe, M. Brewster, S. Gradečak, Nano. Lett. 8, 1386 (2008) 24
MOCVD growth of III-V ternary nanowires Uniform composition Non-tapered structure 420ºC V/III=20 S. K. Lim, M. Tambe, M. Brewster, S. Gradečak, Nano. Lett. 8, 1386 (2008) 25
n-type doping of GaAs nanowires 2-step process consisting of a shell deposition after the nanowire growth n-type doping in GaAs nanowires realized Au seed particle is removed before the shell deposition M. Tambe, S. Ren, S. Gradečak, Nano Lett. 10, 4584 (2010). 26
Looking ahead: axial heterostructures 100 nm m-directional InN/InGaN axial heterostructure (non-polar direction) S. K. Lim, S. Crawford, G. Haberfehner, S. Gradečak, Nano Letters ASAP (2012) 27
Looking ahead: axial heterostructures Electron tomography: In collaboration with CEA-LETI S. K. Lim, S. Crawford, G. Haberfehner, S. Gradečak, Nano Letters ASAP (2012) 28
Looking ahead: axial heterostructures S. K. Lim, S. Crawford, G. Haberfehner, S. Gradečak, Nano Letters ASAP (2012) 29
Conclusions Hybrid nanowire-polymer photovoltaics Addressing ordering Hybrid nanowire-quantum dot photovoltaics Ordered nanowire arrays and polymer infiltration Graphene electrodes for flexible photovoltaics Looking ahead: multijunction devices 1. S. Ren, N. Zhao, S. C. Crawford, M. J. Tambe, V. Bulovic, and S. Gradečak, Nano Letters 11, 408 (2011) 2. S. Ren, L.-Y. Chang, S. K. Lim, J. Zhao, M. Smith, N. Zhao, V. Bulovic, M. Bawendi, S. Gradečak, Nano Letters 11, 3998 (2011) 3. S. Ren, M. Bernardi, R. R. Lunt, V. Bulovic, J. C. Grossman, S. Gradečak, Nano Letters 11, 5316 (2011) 4. S. K. Lim, S. Crawford, G. Haberfehner, S. Gradečak, Nano Letters ASAP (2012) 5. C.-H. Tseng, M. J. Tambe, S. K. Lim, M. J. Smith, S. Gradečak, Nanotechnology 21, 165605 (2010) 6. H. Park, S. Chang, J. Cheng, J. Kong, S. Gradečak, in preparation 30
Group members Graduate students: Megan Brewster Jian Wei Jayce Cheng Jordan Chesin Samuel Crawford Sema Emrez John Hanson Eric Jones Sung Keun Lim Paul Rekemeyer Matthew Smith Michael Tambe Xiang Zhou Postdocs: Kamal Baloch Seehon Chang Ming-Yen Lu Hyesung Park Shenqian Ren Chun-Hao Tseng Mingsheng Wang Undergraduate students: Ellen McIsaac Christopher Francis Sandra Abago http://web.mit.edu/gradecakgroup/ gradecak@mit.edu 31
Acknowledgements DOE- Excitonics EFRC Eni-MIT Alliance Solar Frontiers Program CMSE NSF-MRSEC (DMR-0213282) NSF CAREER award (DMR-0745555) 3M Innovation Award 32