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).

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Interfacial Fermi level pinning

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. 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. "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. 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. -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. 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. 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. 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. 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. 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. 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. 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. Challenge: n-type doping of organic semiconductors

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. 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. 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

30. 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

31. 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

32. One targeted function of a hybrid:
energy transfer & radiative emission
requirements:
•spectral matching for energy transfer
•type-I energy level alignment

33. 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

34. 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)

35. 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 +

36. 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

37. metal halide perovskites for photovoltaic cells & …

38. Hybrid inorganic/organic perovskite solar cells
PCE > 25%

39. 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

40. 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

41. 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)

42. 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

43. 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

44. 2D semiconductors …

45. 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)

46. 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)

47. 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

48. 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 !

49. 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)

50. 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)

51. 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)

52. 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

53. 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

54. 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

55. 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

56. 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


57. 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?

58. 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 ?

59. 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?

60. 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)

61. 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)

62. 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