Organic solar cells. State of the art and outlooks. Gilles Horowitz LPICM, UMR7647 CNRS - Ecole Polytechnique

Transcription

1 Organic solar cells. State of the art and outlooks Gilles Horowitz LPICM, UMR7647 CNRS - Ecole Polytechnique

2 Solar energy Solar energy on earth: 75,000 tep/year 6000 times the world consumption in 2007 Solar energy conversion Thermal Biomass Photovoltaic 2

3 PV production 3

4 PV technologies Single crystal and multi-crystalline silicon 95% Thin film cells hydrogenated amorphous silicon (a-si:h) 4% Cu(InGa)Se2 (CIGS) and CdTe 1% micro and nano-crystalline Si organic 4

5 Outline Metals, insulators, semiconductors Organic semiconductors Solar cells Organic solar cells Operating mode Optimizing the performance New fabrication techniques Conclusions 5

6 Energy bands in solids 6

7 Filling the bands: Fermi distribution Fermi level 7

8 Zero temperature Fermi level 8

9 Zero temperature Conduction band Conduction band Fermi level Valence band Metal 8 Insulator

10 Zero temperature Conduction band Conduction band Fermi level gap Valence band Insulator Metal 8

11 Non zero temperature Conduction band Conduction band Conduction band Valence band Valence band Metal Insulator 9 Intrinsic semiconductor

12 Extrinsic semiconductor 1. n type Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si 10

13 Extrinsic semiconductor 1. n type Si Si Si Si Si Si Si Si Si Si P Si Si Si Si Si Si 10

14 Extrinsic semiconductor 1. n type Si Si Si Si Si Si Si Si P + Si Si Si Si Si Si Si Si 10

15 Extrinsic semiconductor 2. p type Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si 11

16 Extrinsic semiconductor 2. p type Si Si Si Si Si Si Si Si Si Si B Si Si Si Si Si Si 11

17 Extrinsic semiconductor 2. p type Si Si Si Si Si Si Si Si Si Si B - Si Si Si Si Si Si 11

18 Organic semiconductors? Organic compounds are mainly made of carbon and hydrogen The carbon atom is subject to hybridization of its electron orbitals 12

19 sp 3 hybridization divalent 2p 2s 1s 2 2s 2 2p 2 1s 13

20 sp 3 hybridization divalent tetravalent 2p 1s 2 2s 2 2p 2 2s sp 3 hybridiza6on 1s CH 4 13

21 sp 2 hybridization 2p 1s 2 2s 2 2p 2 2s sp 2 hybridiza6on p z 1s C 2 H 4 14

22 sp 2 carbon Ethylene 15

23 Benzene 1/2 16

24 Benzene 2/2 17

25 Poly(paraphenylenevinylene) (PPV) 18

26 Intrinsic parameters Si Pentacene Energy gap (ev) Dielectric constant Molecular density (cm - 3 ) n i (cm K µ (cm 2 /Vs) σ intr (S/cm)

27 Doping OSCs LUMO Si OSC HOMO LUMO Dopant density ppb to ppm 0.1 to 10 % HOMO Doping rate (%) 100 < 1% host (electron donor) dopant (acceptor) 20

28 Doping OSCs LUMO Si OSC HOMO charge transfer LUMO Dopant density ppb to ppm 0.1 to 10 % HOMO Doping rate (%) 100 < 1% host (electron donor) dopant (acceptor) 20

29 Doping OSCs LUMO Si OSC HOMO charge transfer LUMO Dopant density ppb to ppm 0.1 to 10 % HOMO Doping rate (%) 100 < 1% host (electron donor) dopant (acceptor) In practice, almost all organic semiconductors are undoped (non intentionally doped) 20

30 Where are charges coming from? 1. Charge injection metal LUMO electron barrier hole barrier HOMO OSC depending on the respective values of the barriers, the OSC is said to be n or p-type 2. Light illumination creates an electron-hole pair if the energy of the photon is higher than the energy gap 21

31 Polarons, excitons Organic semiconductors have much smaller dielectric constant than their inorganic counterpart organic: k ~ 3 5 Si: k = 12 22

32 Electronic polarization 23

33 Electronic polarization 24

34 Electronic polaron 25

35 Exciton Exciton: bound electron-hole pair (a) Frenkel s exciton high binding energy small size (b) Wannier s exciton low binding energy large size 26

36 Exciton binding energy OSC Transport gap Op6cal gap Binding energy Cu- Pc Zn- Pc PTCDA T pentacene Alq

37 Silicon solar cell n-type SCL p-type SCL:Space-charge layer built-in electric field separation of electron hole pairs Electron and holes generated outside the SCL reach the SCL through diffusion long diffusion length needed 28

38 Silicon solar cell n-type SCL p-type SCL:Space-charge layer built-in electric field separation of electron hole pairs Electron and holes generated outside the SCL reach the SCL through diffusion long diffusion length needed 28

39 Silicon solar cell light absorption SCL n-type p-type SCL:Space-charge layer built-in electric field separation of electron hole pairs Electron and holes generated outside the SCL reach the SCL through diffusion long diffusion length needed 28

40 Si solar cell drawbacks Low absorption coefficient thick layer required most of the electrons and holes are generated outside the SCL long diffusion length needed, requires high purity Raw material is abundant, but pure Si is costly and energy demanding the part of solar cells has overpassed that of microelectronic in the consumption of highly pure Si in

41 Si PV price 30

42 Organic solar cells Single layer cell (first report in 1958) ITO (anode) / OSC / low work function metal (cathode) Very low PCE ~ 0.01 % Advantages High light absorption; low cost Main drawback High exciton binding energy Solution:Two-layer structure 31

43 Exciton splitting Donor Acceptor C. W. Tang, Appl. Phys. Lett. 1986, 48, 183 N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 1992, 258,

44 Exciton splitting Donor Acceptor C. W. Tang, Appl. Phys. Lett. 1986, 48, 183 N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 1992, 258,

45 Exciton splitting Donor Acceptor Charge-Transfer exciton C. W. Tang, Appl. Phys. Lett. 1986, 48, 183 N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 1992, 258,

46 Charge carrier extraction E anode (TCO) ITO E cathode (low WF metal) Al, Mg, Ca 33

47 Double-layer cell Cathode Acceptor ~ 100 nm Donor Transparent anode light 34

48 Double-layer cell ~ 100 nm Cathode Acceptor Donor exciton diffusion length ~ 10 nm Transparent anode light 34

49 Bulk heterojunction (BHJ) cell Cathode Transparent anode light 35

50 Materials Acceptors Donors perylene derivative poly(para-phenylenevynilene) PCBM poly(3-hexylthiophene) P3HT 36

51 Power conversion efficiency (PCE) current dark Maximum power Pm = ImVm = IscVocFF Vm Voc voltage Isc: short-circuit current Voc: open-circuit voltage FF: fill factor Im PCE = Pm / Plight Isc 37

52 Optimizing Donor LUMO HOMO Acceptor LUMO Voc HOMO Voc = LUMOacceptor HOMOdonor Isc is a combination of light absorption high absorption coefficient best match with solar spectrum exciton collection large exciton diffusion length optimize morphology of the BHJ FF: minimize series resistance increase charge carrier mobility 38

53 Matching the solar spectrum P3HT Reducing the gap from 1.9 to 1.2 ev would increase the energy collection from 30 to 85 %! 39

54 Optimizing the donor M.C. Scharber Adv. Mater. 18, 789 (2006) 40

55 Rational design Blouin, N. et al. J. Am. Chem. Soc. 130, (2007) 41

56 42

57 Power conversion efficiency Research Industry 2009 Industry 2025 Single crystal Si a- Si:H CIGS CdTe Organic

58 Printing techniques Conventional printing with printing form Digital printing no printing form Heidelberger Intaglio printing - gravure printing Xenia Optomec Planographic printing - offset printing Relief printing - flexographic printing Print through - screen printing - Inkjet - Aerosol jet - Spray coating - Xerography - Thermography - Ionography - Magnetography - Laserphotography 44

59 Gravure printing Engraved elements on gravure cylinder Costly manufacturing of cylinder Mass-printing technique Web speed 100 m/min Resolution 355 dpi, 10µm Film thickness: µm 45

60 Flexography Raised elements on flexible printing plate Anilox roller made of chrome/ceramic Web speed 80 m/min Mass-printing technique Resolution 150 dpi, 20µm Film thickness: µm 46

61 Offset Indirect lithographic printing Planographic method Hydophobic properties of printing elements on plate Hydrophilic properties of nonprinting elements on plate Mass-printing technique Web speed 100 m/min Resolution 500 dpi, 10µm Film thickness: µm; zonal ink feeding 47

62 Screen printing Stencil is information carrier and screen Stencil elements are non-printing, screen is printing Small components and large areas, versatile process Many different substrates possible Printing on different geometrical objects possible Web speed 100 m/min Resolution 130 dpi, 50µm Film thickness: µm 48

63 Resolution / throughput 49

64 Example: Konarka 50

65 51