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TitleStrong light-matter interactions and exciton-polaritons in carbon nanotubes
LanguageEnglish
File Size56.2 MB
Total Pages148
Table of Contents
                            List of Figures
Introduction
Background
	Single-walled carbon nanotubes (SWCNTs)
		Structural properties
		Electronic properties
		Excitons in SWCNTs
		Optical spectroscopy
		Synthesis and purification
	Devices for light generation
		Organic light-emitting diodes (OLED)
		Light-emitting field-effect transistor (LEFET)
		External quantum efficiency
	Cavity exciton-polaritons
		Planar cavities
		Strong light-matter coupling
		Exciton-polaritons
		Materials for strong coupling and polariton condensation
		Electrical pumping of polaritons
Experimental methods
	SWCNT selection
	Device fabrication
		OLEDs
		Metal-clad microcavities
		LEFET-cavities
	Characterization techniques
		Structural and optical properties
		Electrical properties
		Measures of efficiency
	Computational techniques
		Complex refractive index
		Transfer-matrix simulation
		Analysis of data from Fourier imaging
Selective dispersion of SWCNTs by shear-force mixing
	Introduction
	Dispersion by SFM
	Optical characterization of (6,5) SWCNTs from SFM
	Quality measures of selected SWCNTs
	Further characterization
	Summary & conclusions
Organic light-emitting diodes based on emissive SWCNTs
	Introduction
	SWCNTs in solid films & OLED-design
	Characterization of SWCNT-based OLEDs
	Trion emission
	Emitter orientation & efficiency calculations
	Summary & conclusions
Strong light-matter coupling in SWCNTs
	Introduction
	Strong light-matter coupling
	Coupled oscillator model & polariton emission
	Tuning exciton-polariton properties
	Further discussion
	Summary & conclusions
Electrical pumping & tuning of polaritons in SWCNTs
	Introduction
	Cavity-LEFETs based on SWCNT
	Electrically pumped exciton-polaritons
	Emission efficiency & wavelength tuning
	Polariton relaxation
	High current densities
	Polariton density & ground state occupation
		Toward polariton lasing & non-linear interactions
	Electrically tuned light-matter interaction
	Summary & conclusions
Conclusions & outlook
Bibliography
                        
Document Text Contents
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CHAPTER 5. ORGANIC LIGHT-EMITTING DIODES BASED ON EMISSIVE SWCNTS

1000 1200 1400
500

600

700

800

Emission wavelength (nm)

E
x
c

it
a

ti
o

n
w

a
v
e

le
n

g
th

(
n

m
)

(6,5) SWCNTs

1 µm

0

7

0

1

0.2 µm

ba

FIGURE 5.2. SWCNT-properties within OLEDs.
a, PLE map of the (6,5) SWCNT-layer within the completed OLED. No effect of
processing and interaction with adjacent layers was observed and spectral features
of the (6,5) SWCNTs were maintained. b, Topography scan of a thin (6,5) SWCNT-
layer. The layer had a RMS sur-face roughness of 0.97 nm.

and continuous network of nanotubes with strong planar orientation. While the overall surface

roughness was found to be similar to the nanotube diameter (0.97 nm), small aggregates and

SWCNT-bundles were found across the layer.

5.3 Characterization of SWCNT-based OLEDs

Three OLEDs with increasing SWCNT-layer thickness of 5, 16 and 39 nm are compared in the

following. The J-V-characteristics of all devices showed a clear diode-like behavior reaching a

current density of 180-320 mA cm−2 at 5 V (Figure 5.3a). These high currents are a result of the
use of doped charge transport layers enabling and supporting the injection and transport of these

currents. At applied biases larger than 2.5 V, all OLEDs showed emission in the nIR (Figure 5.3b).

For higher voltages, the generated nIR-irradiance varied significantly among the devices. Overall,

the thickest SWCNT layer (39 nm, red) gave the highest irradiance. In this device, the aerial

output power exceeded 900 µW cm−2 at 300 mA cm−2. Accordingly, the same OLED exhibited the
highest EQE as shown in Figure 5.3c. At low current density, a maximum EQE of 0.014% was

reached in the OLED with the 39 nm thick SWCNT-layer. The EQE for thinner SWCNT-layers

(5 nm thick film, green) decreased at these current densities to 0.006%. For all SWCNT-based

OLEDs, the EQE showed a moderate roll-off at high current densities. The reduced EQE at

high currents can be explained by exciton-exciton annihilation.[168] In general, the EQE of all

SWCNT-based OLEDs was limited by the PLQY (0.11%). A detailed discussion of additional loss

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5.3. CHARACTERIZATION OF SWCNT-BASED OLEDS

0 100 200 300
0.000

0.005

0.010

0.015

E
Q

E
(

%
)

Current density (mA cm–2)
0 2 4 6

10

100

1000

Ir
ra

d
ia

n
c

e
(

µ
W

c
m


2
)

Voltage (V)

0 2 4 6

100

200

300

C
u

rr
e

n
t

d
e

n
si

ty
(

m
A

c
m


2
)

Voltage (V)

39 nm

16 nm

5 nm

a b c

FIGURE 5.3. OLED characterization with nIR emission.
a, J-V-characteristics of SWCNT-based OLEDs fabricated with three different EML
thicknesses. Diode-like behavior is observed with a threshold-voltage of ~2.5 V. b,
Outcoupled nIR-irradiance of the OLEDs generated during operation in forward
direction. c, EQE versus cur-rent density of the OLEDs.

channels is presented and discussed in section 5.5.

The EL spectra of all OLEDs emitted at 62 mA cm−2 and in forward direction, i.e. at 0° with
respect to the normal of the substrate, are plotted in Figure 5.4. The emission was recorded with

an InGaAs detector (900-1300 nm) and a Si detector (400-900 nm) covering the entire visible

and nIR spectrum. OLEDs with as thick SWCNT-layer emitted purely in the nIR (>1000 nm). In

addition to the exciton peak at 1010 nm with a FWHM of 41 nm (cf. PL spectrum, Figure 5.1a), a

second intense peak was observed at 1177 nm with a FWHM of 58 nm. This peak is associated

with a charged exciton, called trion, that was observed in literature for (6,5) SWCNTs in LEFETs

and for electrochemical doping.[153, 186, 187] In case of thinner EMLs, the relative intensity of

the trion emission with respect to excitonic signal increased. The origin of the emission spectra

is more closely evaluated in the next section. Additionally, substantial EL in the visible was

detected for these devices with peaks at 430 nm and 570 nm. This emission is attributed to

exciplex formation at the interfaces to the adjacent layers as well as charge recombination within

these layers. Since BAlq has by far the lowest charge carrier mobility within the stack, this

effect is expected to take place predominantly at the electron EML-HBL interface and within the

HBL. From these observations, a SWCNT film thickness of 39 nm is recommended to electrically

generate pure nIR light.

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