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Hybrid inorganic/organic semiconductor structures for opto-electronics.

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Plenary lecture of the XVIII B-MRS Meeting given by Prof. Norbert Koch (HU and HZB, Germany) on September 23, 2019 at Balneário Camboriú (Brazil).

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Hybrid inorganic/organic semiconductor structures for opto-electronics.

  1. 1. Hybrid inorganic/organic semiconductor structures for opto-electronics Norbert Koch Institut für Physik & IRIS Adlershof Humboldt-Universität zu Berlin Helmholtz Zentrum Berlin für Materialien und Energie GmbH
  2. 2. Collaborations & Acknowledgements Financial Support: SFB 951 "HIOS" (DFG) EC (GENIUS, iSwitch, UHMob) @ HZB Jie Ma Ahmed Mansour Thorsten Schultz @ HU Patrick Amsalem Dominique Lungwitz Andreas Opitz Soohyung Park Maryline Ralaiarisoa Qiang Wang Qiankun Wang Rongbin Wang Xiaomin Xu Fengshuo Zu NUS Goki Eda Ziyu Qin KAUST Lain-Jong Li Arrej Aljarb IMS / UVSOR Satoshi Kera HU Berlin Emil List-Kratochvil Sylke Blumstengel Stefan Hecht Georgia Tech Seth Marder Stephen Barlow Princeton U Antoine Kahn U Strasbourg Paolo Samorí
  3. 3. Electronic and optoelectronic devices: Functional elements for information processing and display switch Source Drain Gate Gate insulator Organic channel VDS VG light emission energy conversion memory sensor
  4. 4. Societal relevance of opto-electronics these depend on electronic & optoelectronic devices  transistor  diode  LED  PV  capacitor  sensor biotechnology – services – energy health & food – information & communication mobility – nanotechnology – photonics production – security – material science (Stories from the future 2030)
  5. 5. biotechnology – services – energy health & food – information & communication mobility – nanotechnology – photonics production – security – material science  faster  multifunctional  energy efficient  low cost  large area & flexible  more & everywhere these depend on electronic & optoelectronic devices  transistor  diode  LED  PV  capacitor  sensor (Stories from the future 2030) Societal relevance of opto-electronics
  6. 6. Electronics & Optoelectronics material components: dielectrics – semiconductors – conductors devices are multicomponent structures: omnipresent interfaces modern transparent OLED stack Yanko Design defined & controlled by bulk & interface charge density distribution "The interface is the device", Herbert Kroemer, Nobel Lecture 2000 "The interface is still the device", Editorial, Nature Materials 2012 "charge density @ interfaces", every pertinent major conference 2019+ few nm
  7. 7. The mission: Understand interface phenomena & develop methods for energy level management for all relevant present, emerging, future electronic materials & applications inorganic & organic semiconductors, oxides, carbon allotropes, 2D semiconductors, perovskites, …
  8. 8. cathode anode organic material EF hn EF Evac U-     VB (HOMO) CB (LUMO) SE U - (1 - 2) Electrode semiconductor contacts: Charge injection / extraction efficiency hn        Tk ATj B barrierinjectioncharge exp2 Injection-limited current: electron injection barrier (EIB) hole injection barrier (HIB)  minimize ohmic losses CB VB
  9. 9. Heterojunction energy levels found in literature Evac CB/LUMO VB/HOMO vacuum level alignment no charge carriers Evac interface dipole maybe charge carriers Evac band bending certainly charge carriers ? ?
  10. 10. Direct & inverse photoelectron spectroscopy Challenge: adapt IPES, UPS, XPS to the special requirements of emerging semiconductors chemical environment stoichiometry electron affinity band structure Fermi level ionization energy work function
  11. 11. Today's menu Organic semiconductors Fermi level pinning operando energy level switching bulk doping Organic/inorganic hybrids light emission photovoltaic cell Perovskites surface states & impact on level alignment 2D semiconductors exciton binding energy doping with molecular acceptors
  12. 12. Interfacial Fermi level pinning
  13. 13. Ideal Schottky contact (no interface states) and band bending EF electrostat. potential = 0 (Evac) EF - - - - ++ ++ EA IE m eVbi s width of depletion region W W 𝑾 = 𝟐𝜺 𝒓 𝜺 𝟎 𝑽 𝒃𝒊 𝒆𝑵 intrinsic carrier density for Eg = 2 eV:  3×104 cm-3  W > cm low "doping" concentration  1014-16 cm-3  W  µm
  14. 14. Clean/undoped organic semiconductors: device thickness << W EF - - - - ++ ++ eVbi W organic layer thickness in devices  100 nm << µm depletion layer width W - seemingly flat band conditions - EF not representative of bulk semiconductor Fermi level - electrical contact dictates position of EF in semiconductor - charges induced by contact dominate (e.g., Fermi level pinning) EF W Ishii, Hayashi, Ito, Washizu, Sugi, Kimura, Niwano, Ouchi, Seki, phys. stat. sol. (a) 201, 1075 (2004)
  15. 15. "Intrinsic" Fermi level pinning: organic semiconductors at electrodes d dE S gap F  Weak electronic coupling: S  1 (Schottky-Mott limit) Braun, Salaneck, Fahlman, Adv. Mater. (2009) Strong electronic coupling: 0 < S < 1 Vazquez, Flores, Kahn, Org. Electron. (2007) • crit for Fermi-level pinning: sample dependent  molecular orientation (IE, EA) Duhm, et al., Nat. Mater. (2008)  gap states & DOS Bussolotti, et al., Phys. Rev. Lett. (2013) Oehzelt, Koch, Heimel, Nat. Commun. (2014) •  above/below crit beneficial for ohmic contact formation
  16. 16. Electrode work function tuning with molecular agents: donor/acceptor interlayers  changes due to molecule-metal charge transfer induced dipoles µ Koch, Duhm, Rabe, Vollmer, Johnson, Phys. Rev. Lett. 95 (2005) 237601    0 eN Helmholtz equation: µ N N N N F F FF Bröker, et al., Appl. Phys. Lett. 93, 243303 (2008) µ N N donors acceptors
  17. 17. -tuning of electrode / semiconductor surfaces with molecular agents work function of metals, oxides, semiconductors: 2.2 eV ̶ 6.5 eV  beyond crit for most semiconductors Koch, et al., Phys. Rev. Lett. 95, 237601 (2005) Heimel, et al., Nat. Chem. 5, 187 (2013) Schlesinger, et al., Nat. Commun. 6, 6754 (2015) Schultz, et al., Phys. Rev. B 93, 125309 (2016) Akaike, et al., Adv. Funct. Mater. 26, 2493 (2016) N N pristine materials w/ molecular acceptors w/ molecular donors
  18. 18. Photochromic molecules: towards multi- functionality & operando energy level switching photochromic diarylethene (DAE) derivatives • open & closed forms have different energy levels • must match energy levels of other components • must switch in the solid with high yield
  19. 19. LUMO level switching in/out of resonance with polymer electron transport levels Mosciatti, del Rosso, Herder, Frisch, Koch, Hecht, Orgiu, Samorì, Adv. Mater. 28 (2016) 6606 Wang, Frisch, Herder, Hecht, Koch, ChemPhysChem 18 (2017) 717 Wang, et al., Adv. Funct. Mater 28 (2018) 1800716
  20. 20. Optically switchable n-type OFET Mosciatti, del Rosso, Herder, Frisch, Koch, Hecht, Orgiu, Samorì, Adv. Mater. 28 (2016) 6606 photochromic diarylethenes (DAE) as source/drain - polymer interlayer OFET can be addressed: - electrically - optically  multifunctional
  21. 21. Molecular acceptors/donors for bulk doping of organic semiconductors • two mechanisms of doping – Ion Pair (IPA) formation – Charge Transfer Complex (CPX) formation • n-type doping “beyond thermodynamic limit”  conductivity increases by several orders of magnitude  increased carrier mobility Pfeiffer, Fritz, Blochwitz, Nollau, Plönnigs, Beyer, Leo Advances in Solid State Physics, 39, 77 (1999) Gao, Kahn, Appl. Phys. Lett. 79, 4040 (2001) Olthof, Mehraeen, Mohapatra, Barlow, Coropceanu, Bredas, Marder, Kahn, Phys. Rev. Lett. 109, 176601 (2012) Lu, Blakesley, Himmelberger, Pingel, Frisch, Lieberwirth, Salzmann, Oehzelt, Di Pietro, Salleo, Koch, Neher, Nat. Commun., 4, 1588 (2013)
  22. 22. Ion pair (IPA) formation: diagnostic spectral features CN CN F F FF NC NC S n electronic transitions: F4TCNQ anion P3HT cation (polaron) vibrations: C≡N stretch indicative of charge transfer amount (d = 1) Salzmann, Oehzelt, Heimel, Koch, Acc. Chem. Res. 49 (2016) 370
  23. 23. Charge transfer complex (CPX) formation: diagnostic spectral features electronic transitions: no ion absorptions CPX absorption vibrations: C≡N stretch indicative of charge transfer d < 1 CN CN F F FF NC NC Salzmann, Oehzelt, Heimel, Koch, Acc. Chem. Res. 49 (2016) 370
  24. 24. Doping efficiency: IPA vs. CPX IPA (strong dopant) CPXIPA (weak dopant) hole density: IPA (strong) > IPA (weak) > CPX  avoid CPX formation hole density Salzmann, Oehzelt, Heimel, Koch, Acc. Chem. Res. 49 (2016) 370 140 15 0.6 Méndez, et al., Nat. Commun. 6 (2015) 8560 Méndez, et al., Angew. Chem. 125 (2013) 7905
  25. 25. Challenge: n-type doping of organic semiconductors
  26. 26. n-type doping of organic electron transport materials Challenge: • good donor = extremely low ionization energy (< 2.2 eV for many materials) • most donors (dopants) not air-stable monomer oxidation (not available/air stable) dimer oxidation oxidation/reduction potentials: just mixing dopant dimer and POPy2 will not result in doping
  27. 27. Doping by photo-activation: POPy2 and [RuCp*Mes]2 thin films Lin, Wegner, Lee, Fusella, Zhang, Moudgil, Rand, Barlow, Marder, Koch, Kahn, Nature Mater. 16 (2017) 1209  orders of magnitude conductivity (s) increase only upon light irradiation  saturation s depends on light energy  stable s for > year 1 year
  28. 28. Light energy dependence of doping photoactivation Lin, Wegner, Lee, Fusella, Zhang, Moudgil, Rand, Barlow, Marder, Koch, Kahn, Nature Mater. 16 (2017) 1209 absorption charge transfer absorption / ET ET back-reaction thermodynamically prevented
  29. 29. inorganic semiconductors  highest purity levels  high excitation density  high carrier mobility Hybrid materials: synergy rationale • combine & take advantage of individual material strengths • compensate weaknesses • new opto-electronic properties via hybridization conjugated organic materials  tunable energy range  strong light-matter coupling  high frequency response
  30. 30. Inorganic/organic semiconductor heterojunctions energy level alignment determines function  molecular interlayers for energy level tuning 1. molecular layer with built-in dipole 2. donor/acceptor molecules
  31. 31. One targeted function of a hybrid: energy transfer & radiative emission requirements: •spectral matching for energy transfer •type-I energy level alignment
  32. 32. Inorganic/organic semiconductor level tuning "intrinsic" type-II alignment "perfect" type-I alignment ad donor interlayer Schlesinger, Bianchi, Blumstengel, Christodoulou, Ovsyannikov, Kobin, Moudgil, Barlow, Hecht, Marder, Henneberger, Koch, Nat. Commun. 6 (2015) 6754
  33. 33. Functionality of hybrid structure with tuned energy levels PL emission from L4P-sp3 in hybrid overall PL yield from 5% to 35% • type I: charge transfer • type II: energy transfer Schlesinger, Bianchi, Blumstengel, Christodoulou, Ovsyannikov, Kobin, Moudgil, Barlow, Hecht, Marder, Henneberger, Koch, Nat. Commun. 6 (2015) 6754 2.8 2.9 3.0 3.1 3.2 3.3 PLIntensity Energy (eV)
  34. 34. Polymer-induced inversion layer in n-Si • high  polymer (PEDOT:PSS) induces band bending in n-Si • band bending magnitude depends on polymer formulation  inversion layer in n-Si, “spontaneous pn-junction” SO3H SO3 H SO3 H SO3HSO3 -SO3 - O OO OO O S S S S S S O O O O O O ( ) m ( ) n 2 +
  35. 35. PEDOT:PSS/n-Si solar cells 4.5% 3.0% 2.3% 10.2% Wang, Wang, Wu, Zhai, Yang, Sun, Duhm, Koch, submitted highest PCE: - good polymer wetting - strong induced inversion - high polymer conductivity - Si passivation
  36. 36. metal halide perovskites for photovoltaic cells & …
  37. 37. Hybrid inorganic/organic perovskite solar cells PCE > 25%
  38. 38. Hybrid inorganic/organic perovskite solar cells Kojima, et al., JACS 131 (2009) 6050 Im, et al., Nanoscale 3 (2011) 4088 Lee, et al., Science 338 (2012) 643 ABX3 A = cation(s), B = Pb, X = Cl, I orthorhombic tetragonal cubic Korshunova, et al., Phys. Status Solidi B 253 (2016) 1907 ?Methylammonium (MA) EF VBM CBM
  39. 39. Surface photovoltage of perovskite thin films in dark: • surface states partially empty • EF pinned to DoSS at surface • surface band bending (downwards) surface appears n-type while bulk is  intrinsic • significant density of surface states (DoSS) close to the conduction band edge • bulk Fermi level  midgap under illumination: • photo-generated charges • electrons fill empty DoSS • bands flatten at surface flat band conditions give bulk EF Zu, Amsalem, Salzmann, Wang, Ralaiarisoa, Kowarik, Duhm, Koch, Adv. Opt. Mater. 5 (2017) 1700139 DoSS DoSS
  40. 40. Surface photovoltage induced by UV in UPS Caution: UV light used to photo-emit electrons in UPS causes SPV! Zu, Wolff, Ralaiarisoa, Amsalem, Neher, Koch, ACS Appl. Mater. Interfaces 11, 21578 (2019)
  41. 41. Controlling DoSS (Pb0 defects) by light before illumination: • only Pb2+ • "clean" gap after illumination: • substantial Pb0 • clear gap states up to EF white light induces metallic Pb precipitates in perovskites Zu, Amsalem, Salzmann, Wang, Ralaiarisoa, Kowarik, Duhm, Koch, Adv. Opt. Mater. 5 (2017) 1700139
  42. 42. DoSS determines level alignment with electron transport materials Zu, Amsalem, Ralaiarisoa, Schultz, Schlesinger, Koch, ACS Appl. Mater. Interfaces 9 (2017) 41546  electron trapping in perovskite  good for electron transport  flat bands
  43. 43. 2D semiconductors …
  44. 44. 2D semiconductors for inorganic/organic hybrids transition metal dichalcogenides (TMDCs): defined structure & robust monolayer: direct semiconductor  strong light-matter coupling from: Ramasubramaniam, Phys. Rev. B 86, 115409 (2012)
  45. 45. Exciton binding energy • TMDC monolayers: excitonic semiconductors • reduced dielectric screening in 2D • screening depends on dielectric environment • optical excitations below band gap Eb,exc Exciton binding energy Eb,exc = band gap – exciton energy images from: Wang, Chernikov, Glazov, Heinz, Rev. Mod. Phys. 90, 021001 (2018)
  46. 46. Exciton binding energy determination 𝑬 𝒃,𝒆𝒙𝒄 (𝒏) = 𝝁𝒆 𝟒 𝟐ℏ 𝟐 𝜺 𝒆𝒇𝒇 (𝒏) 𝒏 − 𝟏 𝟐 𝟐 indirect from fitting exciton energies to modified Rydberg series: challenges: • observation of excitons n > 1 • only from fitting 𝜺 𝒆𝒇𝒇 (𝒏) Chernikov, et al., Phys. Rev. Lett. 113, 076802 (2014) direct: single-particle band gap from direct and inverse photoelectron spectroscopy (PES, IPES) exciton (n = 1) energy (absorption/reflectance) Eb,exc = band gap – exciton energy Eb,exc
  47. 47. Challenges of PES/IPES on TMDC monolayers *Xu, Schultz, Qin, Severin, Haas, Shen, Kirchhof, Opitz, Koch, Bolotin, Rabe, Eda, Koch, Adv. Mater. 30, 1803748 (2018) • insulating substrates single crystal grain coalesced but azimuthally disordered grains* (CVD-grown 2D powders) direct gap at K • angle-resolved (I)PES of 2D powders !
  48. 48. ARPES on WSe2 monolayer 2D powder on HOPG single crystal Brillouin zone (BZ) azimuthally averaged 2D powder “BZ” ARPES data calculated band structure along Γ-K-M (path 1) Γ-M-Γ (path 2) • ARPES seemingly shows band dispersion along high-symmetry directions • why? seen before for HOPG (assigned to Van Hove singularities in density of states): Zhou, Gweon, Spataru, Graf, Lee, Louie, Lanzara, Phys. Rev. B 71, 161403 (2005)
  49. 49. TMDCs: selective photoemission intensity in BZ I E, kr ~ N E, kr = δE Δφ ∙ kr φ=0 2𝜋 1 |𝛻φE kr, φ | 𝛻φE(kr, φ) → 0 photoemission intensity for N grains with j randomized orientation calculated valence band energy map 1 |𝛻φE kr, φ | high photoemission intensity when azimuthal integration: only Γ-K and Γ-M have high intensity at each kr Park, Schultz, Han, Aljarb, Xu, Beyer, Ovsyannikov, Li, Meissner, Yamaguchi, Kera, Amsalem, Koch, Commun. Phys. 2, 68 (2019)
  50. 50. ARPES from TMDC monolayer 2D powders • band dispersion determination in most relevant directions • on insulating substrates (SiO2, sapphire, …) Park, Schultz, Han, Aljarb, Xu, Beyer, Ovsyannikov, Li, Meissner, Yamaguchi, Kera, Amsalem, Koch, Commun. Phys. 2, 68 (2019)
  51. 51. Exciton binding energy determination direct: single-particle band gap from direct and inverse photoelectron spectroscopy (PES, IPES) exciton (n = 1) energy (absorption/reflectance) Eb,exc = band gap – exciton energy Eb,exc
  52. 52. Angle-resolved PES & IPES to derive band gap Eg MoS2 WSe2 on sapphire (r = 11.5) on Au (r = ) • both TMDC monolayers direct semiconductors at K-point • substantial Eg reduction on Au vs. sapphire
  53. 53. Optical gap (exciton energy) Eexc from reflectivity • small changes of Eexc depending on substrate r Park, Mutz, Schultz, Blumstengel, Han, Aljarb, Li, List-Kratochvil, Amsalem, Koch, 2D Mater. 5 (2018) 025003
  54. 54. Exciton binding energy Eb,exc dependence on dielectric environment • Eg and Eb,exc depend strongly on substrate r (single charges) • Eexc depends weakly on r (coupled e-h pair) Park, Mutz, Schultz, Blumstengel, Han, Aljarb, Li, List-Kratochvil, Amsalem, Koch, 2D Mater. 5 (2018) 025003 Eb,exc 240 meV  90 meV 240 meV  140 meV
  55. 55. Controlling charge density in TMDC monolayer: doping with molecular electron acceptors following concept for work function tuning of electrodes & 3D semiconductors*:  strong molecular electron acceptor captures an electron  hole remains in TMDC monolayer  p-type doping (EF shifts towards valence band) * Schlesinger, et al., Nat. Commun. 6, 6754 (2015); Koch, et al., Phys. Rev. Lett. 95, 237601 (2005) F6TCNNQ 
  56. 56. F6TCNNQ / MoS2 / Au  work function increase  no valence band shift towards EF  no MoS2 core level shift  no substrate core level shift electron transfer to F6TCNNQ but no doping?
  57. 57. F6TCNNQ / MoS2 / HOPG  work function increase  valence band shifts towards EF  MoS2 core level shifts like VB  no substrate core level shift electron transfer & doping ?
  58. 58. F6TCNNQ / MoS2 / sapphire  work function increase  2x larger valence band shift towards EF  MoS2 core level shift similar VB  substrate core level shift! stronger electron transfer & doping? sapphire involved ? BUT: IE of MoS2 (> 6.2 eV) much higher than EA of F6TCNNQ (5.6 eV)  charge transfer uphill?
  59. 59. Evidence for MoS2 gap states & F6TCNNQ anions bare MoS2: gap states just below EF (only on sapphire); source of electrons with acceptors: F6TCNNQ anion features for all substrates Park, Schultz, Xu, Wegner, Aljarb, Han, Li, Tung, Amsalem, Koch, Commun. Phys. 2, 109 (2019)
  60. 60. 3 substrate-dependent charge transfer mechanisms Park, Schultz, Xu, Wegner, Aljarb, Han, Li, Tung, Amsalem, Koch, Commun. Phys. 2, 109 (2019) Bruix, et al., Phys. Rev. B 93, 165422 (2016)
  61. 61. Organic semiconductors & hybrids  electrode-induced Fermi level pinning enables ohmic contacts to (almost) any semiconductor  multifunctional devices with photochromic molecules  n-doping beyond thermodynamic limit Perovskites  surface states impact level alignment, and must be controlled 2D semiconductors  exciton binding energy & doping mechanisms depend on dielectric environment  enhanced or novel functionality Conclusions

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