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1 Exciton dissociation in solar cells: Xiaoyang Zhu Department of Chemistry University of Minnesota, Minneapolis t (fs) 3h! E, k h! Pc Bi e - 1

2 Acknowledgement Organic semiconductors: Mutthias Muntwiler, Qingxin Yang Inorganic quantum dots: William Tisdale, Kenrick Williams Prof. Eray Aydil, Prof. David Norris Funding: NSF (DMR, MRSEC, NIRT) DOE (Solar Initiative) MN-IREE (Initiative for Renewable Energy & Environment) 2

3 Outline Introduction - Excitons & photovoltaics Charge transfer (CT) excitons on organic semiconductor surfaces: Direct time domain observation of 2D polarons Extracting hot or multiple carriers from inorganic quantum dots: 3

4 The conventional solar cell: p-n junction Fermi p e n Conduction band h Valence band Light absorption & charge separation occur in same location Charge separation by build-in electric field UMN solar car 4

5 Excitonic solar cells electrolyte I - /I 3 - transparent anode cathode M Gratzel, Nature (21), Leschkies et al., Nano Lett (27) e A D Tang, APL (1986) Acceptor h Donor 5

6 The Coulomb barrier problem Exciton: an electron-hole pair Molecular or Frenkel exciton in an organic semiconductor or inorganic quantum dot Charge transfer exciton at a D/A interface D A V = e 2-4!""o r Large binding energy (>> kt) due to the low dielectric constant! 6

7 What drives exciton dissociation? BE ex How does a Frenkel exciton dissociate? BE CT # LUMO > BE ex How does a CT exciton separate? #µ, but Frenkel or 7

8 Probing electron dynamics in the time domain: femtosecond two-photon photoemission! e -.2 Binding Energy (ev) n = 1 image state, 2 ML nonane/cu(111) Pump-Probe Delay (fs) 4 Lindstrom, Zhu Chem. Rev. (6) 8

9 How do charge carriers separate in an organic heterojunction solar cell? A D 9

10 Previous suggestion of long distance geminate e-h pair in exciton dissociation Morteani et al. PRL 92, (24) Adv. Mater. 15, 178 (23) 1

11 Previous evidence for the need of excess energetic driving force in exciton dissociation Coulombically bound radical pair (BRP) Nanocrystalline polythiophene/pcbm Amorphous polythiophene/pcbm Ohkita et al. JACS 13, 33 (28) 11

12 Charge transfer exciton: a toy model..2 1s 1p 2s 2s 1d D + r o r A E B (ev).4.6 1p e 1s.8 1.eV x (Å) z (Å) 12

13 Experimental realization of the toy model for CT excitons " Pc = 5.3," o = 1. # " =

14 Image potential state as the electron acceptor (Geminate pair) What is the binding energy of a CT exciton (geminate pair)? How to overcome this binding energy in charge separation? 14

15 The model system: crystalline pentacene thin films grown on Bi Sadowski et al, Appl Phys Lett 25 Thayer et al, Phys Rev Lett 25 15

16 2PPE spectra: IPS & CT exciton 3 1h! Pc coverage (ML) CT 1s h! 2 = 1.39 ev h! 1 = 4.16 ev Bi 2 1 IPS ev 1..5 E B (ev) ev -1.2 (Bi) 16

17 Spatial distribution of IPS & CT exciton " Nonane overlayer quenches both IPS and the CT exciton state IPS shows dispersion & CT exciton state is localized. m eff = 1.5 m e 13 fs 7 fs " 2 is on the surface 17

18 Origin of charge transfer excitons E r r o x z E (atomic units)!! = " # 1 " + 1 [jackson, classical electrodynamics] $ = 2 " + 1 y x 18

19 Simulation results: the delocalized image band 2p 2s 1p 1s?= 519

20 Simulation results: the localized CT excitons angular momentum l z 1s 1 Å 1p 1d 1f E = ev s 2p 2d 2f nodes s 3p 3d 3f

21 Simulation: CT exciton binding energies a. ECTE (ev).4 c.2 E,1s (ev) s 2p 3s 1d 2s 1p 2 4 x (Å) 6 6 _ 5 b energy scale E CTE 2 _ _ internal Coulomb 1s energy of 1 the exciton = bottom of IPS band (continuum)! 1s 1?= -1 1s.45 ev 1p.265 ev 1 2s.197 1p ev 1d.156 ev 2p.132 ev -1 3s.17 ev z (Å) y (Å) y (Å) (Å) 1 2s 21

22 Seeing hot excitons at higher h! a c ECTE (ev) ECTE,1s (ev) UV pump from HOMO.4 _ s 3s 2p 1d 2s 1p 2 4 x (Å) b z (Å) kcounts/s y (Å) 2 _ _ y (Å) y (Å) s 1s 1p 2s 2.8 1s UV only or long pump-probe delay 3. ev HOMO 1p forbidden 3.2 2s 3s 1 ML pentacene Pump: 4.38 ev Probe: 1.46 ev 3.4 IPS

23 CT excitons - the complete picture 3.6 IPS 6x1 5 CT 1s E - E Fermi (ev) s+ 2s 1s 2 4 Pump-probe delay (fs) Photoelectron intensity (c/s) CT 2s CT 3s +... IPS! t = fs 16 fs 1 ML pentacene fs Pump: 4.38 ev; Probe: 1.46 ev E - E Fermi (ev) 23

24 All CT excitons show no dispersion: photo-ionization destroys the quasi-particle! IPS E kin = ( hk)2 2m E - E Fermi (ev) ML pentacene s 1d? 2s 1s m= 1.5 m e Dispersion measurement carried out at h! 1 = 4.38 ev & h! 2 = 1.46 ev; time delay = 8 fs Detection angle 24

25 CT excitons on thick pentacene films >2 nm pentacene/si(111) 6x CT 1s E-E F (ev) Pump-probe delay (fs) x1 5 Photoelectron intensity (c/s) ! t / fs E - E Fermi (ev) CT 2s CT 3s +... IPS Pump: 4.59 ev; Probe: 1.53 ev 25

26 Why do CT excitons on pentacene decay so fast? ML > 1 ML 11 fs fs E-E F (ev) fs E-E F (ev) fs fs Pump-probe delay (fs) Pump-probe delay (fs) Short lifetimes: resonance with LUMO+x bands 26

27 Surface CT excitons are general: tetracene 4.2!2 nm tetracene/si(111)!tuning h! to reach each resonance IPS (standing up) E-E F (ev) h! 1 =4.28 ev CT 1s IPS (lying down) Time delay (fs) IPS (n=1) E-E F (ev) h! 1 =4.77 ev E-E F (ev) h! 1 =4.59 ev 3.4 CT 1s 3.4 CT 1s Time delay (fs) Time delay (fs) 27

28 The difference between surface CT excitons and the toy D/A model. CT exciton at D/A interface. CT exciton at surface.2 1s 1p 2s 1d.2 E B (ev).4.6 2s E B (ev).4.6 1d 2s 2s IPS band bottom 1p.8.8 1s 1.eV 2 4 z (Å) eV 2 4 z (Å) 6 8 1A 28

29 The hot mechanism for charge carrier separation in organic photovoltaic Conclusion: Charge separation at a D/A interface in OPV must involve hot CT excitons! n > 1 Less CT binding energy Longer lifetime More delocalization Better electronic coupling (energy denominator!) 29

30 Other interesting systems Donor Acceptor Donor Acceptor 3

31 Summary 1 Charge transfer excitons are seen on the crystalline pentacene and tetracene surfaces. Hot CT excitons are necessary in organic heterojunction photovoltaic. 31

32 Challenge to theory/simulation: breaking up the quasiparticle D k A A + $ e k = k A - k D k D "( k, $ ) What is the role of delocalization on interfacial charge separation? 32

33 Polarons & localization Electronic-nuclear coupling Defects Appl. Phys. Lett. 5 Energetics of self-trapping: E LUMO band "E ST = E pol + E loc + E bond BW! Polarization gain (negative) Localization penalty (positive) ".5# BW "p# "x $ h k 33

34 Self trapping not observed. Tetracene/Silicon(111), 15 K, thick film (> 2 nm) 4.2 n = s ev n = s Energy relaxation negligible: No lattice polaron formation. 34

35 2D polarons that are absent in 3D Cl - Na + e Na + Cl - Cl - e Na+ Na + Cl - Not an organic, but same physics Model: NaCl/Cu(111) 35

36 Polaron formation in time-energy domain NaCl conduction band (CB) # trap = 62 ± 8 fs, # decay = 12 ± 2 fs Interface $E = 6±1 mev polaron NaCl interface band (IB) # trap = 135 ± 47 fs, # decay = 23 image ± 2 fs $E = -85 ± 1 mev polaron Pump-probe delay (fs) 3ML NaCl/Cu(111) Muntwiler & Zhu, Phys. Rev. Lett

37 Where is the % 2? Effect of an overlayer bulk polaron NaCl/Cu Interface polaron image polaron 3ML NaCl/Cu(111) 1 ML nonane/3ml NaCl/Cu(111) 37

38 Polaron formation in the momentum domain Parallel dispersions 3 ML NaCl/Cu(111) Conduction band Interface band 38

39 Why does an electron polaron form in a 2D NaCl film but not in 3D NaCl? Cl - Na + e Na + Cl - Cl - Na Na + Cl - Conduction band width = 5. ev "E ST = E pol + E loc + E bond Bulk crystal -.5 ev 2.5 ev -1. ev "E ST #1. ev! Thin film (1 unit cell thick) -1. ev 1.7 ev -1. ev More deformable and narrower bandwidth in a thin film. Delocalization localization!! Does it apply to thin films in general? Quantum wells?! "E ST # $.3eV 39

40 Conclusion 2 TR-2PPE of NaCl/Cu(111) provides time-domain evidence of a 2D polaron. Polarons or self-trapped excitons must be important to exciton dissociation at organic D/A interfaces (not directly observed in the time domain yet..) 4

41 Challenge to theory/simulation: delocalization + localization D k A A + $ e k = k A - k D k D "( k, $ ) Now add localization from electronnuclear coupling & trapping! My head is spinning 41

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