Organic Electronics: Foundations to Applications - Optical Detectors 1 · 2021. 5. 5. · Organic...
Transcript of Organic Electronics: Foundations to Applications - Optical Detectors 1 · 2021. 5. 5. · Organic...
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Organic ElectronicsStephen R. Forrest
Week 2-6
Optical Detectors 1
Photodetection BasicsOrganic photoconductors and photodiodes
Chapter 7.1-7.2
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Organic ElectronicsStephen R. Forrest
Objectives• Understand the physics of photodetection in organic
photoconductors and photodiodes• Understand OPD performance characteristics
– Dark current– Efficiency and responsivity– Bandwidth– Noise
• Learn about OPD applications• Solar cells: what makes OPVs a compelling story?• Learn how to characterize solar cell performance• Solar cell architectures
– Thermodynamic efficiency limits to single junction cells– Multijunction cells and other architectures– The role of morphology– Some materials
• What lies beyond the horizon?
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Organic ElectronicsStephen R. Forrest
Photodetectors• Transducers that convert light to another energy form
(in our case, electricity)• Types
– Photoconductors– Photodiodes
• These are operated in the reverse-biased (photodetection) or photovoltaic mode
• Properties– Sensitivity & Efficiency– Spectral range– Bandwidth– Dynamic range
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Organic ElectronicsStephen R. Forrest
Photoconductors• Earliest organic electronic devices• Simplest (no HJs needed)
When illuminated, conductivity changes
σ = q µnn + µ p p( )
n = nph + n0
p = pph + p0
Without background doping: n0 = p0 = ni
nph = pph
L
hn
Semiconductor
ContactContact
II
jpjnd
W
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Organic ElectronicsStephen R. Forrest
energy
Exciton
HOMO
LUMO
krecnp
kdissN
kDN
jX/d
GroundState
FreeCharge
kDN0
n,p
jT /qd
Photocharge generation• Generation does not occur through an intermediate CT state
as it does at OPD heterojunctions:
Gph = kDnph =
ηext Pincλ hc( )dWL
tD = 1/kD = lifetime of chargehext = external quantum efficiency (electrons out/photons in)
jph =σ F = qnph µn + µ p( )Va
L= q
ηext Pincλ hc( )kD
µn + µ p( ) Va
dWL2
⇒ Photocurrent:
Generation rate:
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Organic ElectronicsStephen R. Forrest
Quantum Efficiency and Responsivity
External quantum efficiency = No. electrons generated No. of photons incident = 𝜂!"# =
ℎ𝑐𝑗$%𝑞𝜆𝑃&'(
Responsivity = [A/W]Current generatedPower incident = ℛ = !!"
"#$%= #$
%&𝜂'()
Internal quantum efficiency = No. electrons generated No. of photons absorbed = 𝜂&'# =
ℎ𝑐𝑗$%𝑞𝜆𝑛$%𝑃&'(
where: 𝑛$% =)*+, ∫-
, 𝑒𝑥𝑝 −𝛼 𝜆 𝑥 𝑑𝑥
for a total reflection coeff’t, R, from the surface, and an absorption coeff’t of a in an active region of thickness, d.Note: the total thickness must account for internal reflections and other cavity effects
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Organic ElectronicsStephen R. Forrest
Gain and bandwidth
• Bandwidth: Df = 1/2πtD
• Leading to a gain-bandwidth product: gDf= 1/2πttr
jph =σ F = qnph µn + µ p( )Va
L= q
ηext Pincλ hc( )kD
µn + µ p( ) Va
dWL2
gηext =jph A
q Pincλ hc( )Quantum efficiency cannot be separated from gain
⇒ A photoconductor has gain: g =
jph
j0
= τ D µn + µ p( )Va
L2
j0 = qηEQE Pincλ hc( ) dWWhere:
That is: gain = tD/ ttr, where the carrier transit time is ttr = L/v = L/µF = L2/µV
ext
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Organic ElectronicsStephen R. Forrest
Calculating the Noise Current
• Determines the sensitivity of a photodetector to low intensity signals
• Signal-to-noise ratio: 𝑖!"# = mean square photocurrent𝑖$# = mean square noise current
minimum level of detectability
(after Rose, 1963. Concepts in Photoconductivity and Allied Problems)
Consider a “general” photodetector. It has randomly generated particles, each carrying charge zq in time interval, t, between electrodes, resulting in current, j.
Then, the noise current is:
AB =
C!"#
C$#> 1
in2 1/2
=n1/2
τζq
where 𝑛 1/2 is the rms number of particles collected in t.
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Organic ElectronicsStephen R. Forrest
Calculating Noise Current, con’tThus, in terms of the total mean current, iT, the mean square noise current is:
in2 =
nτ 2
ζq( )2 = qiTζτSince the bandwidth is Df=1/2t, and accounting for both generation and recombination, we get a shot noise current of:
is2 = 4qgiTΔf
If diffusion is dominant, then the charge delivered per particle is reduced by thefraction of charge diffusing to the contacts for a slab of length, L: z = LD/L. Using 𝐿% = 𝐷𝜏 and the Einstein relation for mobility, we obtain the thermal, or Johnson noise:
ith2 =
4kBTΔfRPC
RPC is the resistance of the conductor
Finally, there is flicker, or 1/f noise: i f2 = κΔf
f αk, a are empirical constants
The total noise current is then the sum of the squares of the various contributions (they are uncorrelated): 𝑖$# = 𝑖&# + 𝑖'"# + 𝑖(# +...
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Organic ElectronicsStephen R. Forrest
Graphically, Noise Spectra Look Like...
if2 = κΔf
f α
ist2 = 4qgtiTΔft
i 2th =4kBTΔfRPC
a > 1
(2 for OPD)
i f2 = κΔf
f α
ith2 =
4kBTΔfRPC
𝑖&#
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Organic ElectronicsStephen R. Forrest
Photodiodes and solar cells• Many of the same considerations as photoconductors except
there is a junction for efficient charge separation.
23
1 Exciton generation by absorption of light (abs length~1/α
4
Exciton diffusion over ~LD
Exciton dissociation by rapid and efficient charge transferCharge extraction by the internal electric field
Typically: LD<<1/α
ηint = ηAηEDηCTηCCext
11
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Organic ElectronicsStephen R. Forrest
Basic OPD/OPV structure
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Organic ElectronicsStephen R. Forrest
• Recall (Ch. 4) that the j-V characteristics are given by:
j0 = qa0krecNS2 1−ηPPd( )exp −ΔEHL kBT( )
j = j0 exp q Va − jARser( ) nSkBT( ) − kPPdkPPd ,eq
⎡
⎣⎢⎢
⎤
⎦⎥⎥+Va − jARserRshunt
− jph
free carriers(nI, pI )
�
kPPrζ�
kPPdζ
�
krecnI pI ener
gy
�
kPPrζeq
�
JX a0
�
J qa0excitons
reaching to HJ
polaron pairs at HJ
ground13
Saturation current
Equivalent circuit
Current generation
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Organic ElectronicsStephen R. Forrest
Current-Voltage Characteristics
Rshunt =1AdVadj
Va=0Curr
ent d
ensit
yVoltage
jSCjph VOC
Rshunt Rser
Photodetector mode Photovoltaic mode
• In the photovoltaic mode, the power is P = jV < 0; i.e. the device delivers power to the external circuit.
• In the photodetector mode, P > 0 and the detector dissipates power.
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Organic ElectronicsStephen R. Forrest
Photodetector Equivalent Circuit & Frequency Response
Δf = 1
2π1ttr
+ 1τ ED
+ 1τ RC
⎛⎝⎜
⎞⎠⎟
: RC time constant
tED : exciton diffusion time
𝑡') = ⁄𝑤# µ𝑉 : transit time through depleted regions of the device (w)
Gain-Bandwidth product =gDf = Df since in a PD, g = 1.
𝜏*+ = (𝑅&,) +𝑅-||𝑅.$)(𝐶/ + 𝐶0) (𝑅/→ ∞)
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Organic ElectronicsStephen R. Forrest
Pentacene/C60 OPD Frequency Response
TIA: Transimpedance amplifier through which the diode is biased
High frequency response due to high pentacene mobility
Tsai et al. Appl. Phys. Lett., 95, 213308 (2009)
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Organic ElectronicsStephen R. Forrest
Heterojunction MorphologiesBreaking the tradeoff between LD and a with BHJs
Bulk HJ Mixed HJ Annealed BHJ Controlled BHJ 17
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Organic ElectronicsStephen R. Forrest
Polymer Bulk HJ
18Yu et al. Science, 270, (1995), 1789Halls et al., (1995) Nature, 376, 498.
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Organic ElectronicsStephen R. Forrest
Small Molecule Planar-Mixed HJSmall molecule blends:
ηCC =LCxM
1− exp − xM LC( )( )
hED = 1
J. Xue, Adv. Mater., vol. 17, p. 66, 2005. 19
Charge carrier collection length, LC, replaces diffusion length since excitons dissociate at point of generation without diffusion to HJ
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Organic ElectronicsStephen R. Forrest
Comparison of OPDs and OPCs
Table 7.2: Comparison of operating parameters of photoconductors and photodiodes
Parameter Photoconductor Photodiode Operating
voltage Near equilibrium (!! → 0) Reverse bias
Photocurrent gain (g)
! !!"⁄ (1-106) 1
ηint
ηext
Responsivity
Bandwidth (Δf) 1 2!!!⁄ 1 2!!!"⁄ Gain-
bandwidth product (gΔf)
1 2!!!"⁄
1 2!!!"⁄
Specific detectivity (D*)
PPr
20
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Organic ElectronicsStephen R. Forrest
The first bilayer OPD/OPV
Tang, Applied Phys. Lett., (1986) 48, 183. 21
-0.4 -0.2 0.0 0.2 0.4
-3
-2
-1
0
1
2
3
Voltage [V]
Curr
ent [
mA/
cm2 ]
ISC = 2.3mA/cm 2
VOC = 450mVCuPc (300Å)
Glass
Ag
ITO
PTCBI (500Å)
hP = 0.95%FF = 0.65
acceptor donor
hP = power conversion efficiencyFF = fill factor
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Organic ElectronicsStephen R. Forrest
Photodetector Materials• Good materials absorb in the region of interest• Morphology promotes exciton diffusion and charge conduction (high mobility)
Ultra-violet
Selected donors
Generally, donors employ fullerene acceptors in OPDs
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Organic ElectronicsStephen R. Forrest
RT
RR
High Bandwidth Multilayer Photodetectors
Place all D/A junctionswithin LD of absorption site
Stack layers until total thickness d ~ 1/α
Apply voltage to sweep chargeout of potential wells
Bandwidth due to transit timeacross d.
d
23
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Organic ElectronicsStephen R. Forrest
Spectral + Voltage Dependence of the EQE•Sensitive to visible + NIR wavelengths•Strong dependence on bias: EQE~75% @ -10V
500 600 700 8000.00
0.25
0.50
0.75
1.00-11-10
-9-8-7-6
-5-4-3
-2
-1
0V
Exte
rnal
qua
ntum
effi
cien
cy
λ [nm]
t=5Å (64 layers)
Peumans, et al. Appl. Phys. Lett., 76, 3855 (2000). 24
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Organic ElectronicsStephen R. Forrest
Response TimeThinner individual layers makes faster devices due to a reduced exciton lifetime
t=160Å20Å
5Å
0 1 2 3 4 5 6
Time [ns]
Nor
mal
ized
Res
pons
e
f3dB=(430±40)MHz
FWHM=(720±50)ps
10Å
10 100 1000 100000.01
0.1
1
Nor
mal
ized
Res
pons
e
Frequency [MHz]
PTCBI lifetime=(1.8±0.1)ns
100 µm diameter, -9V, 1.4ps excitation @ 670nm at (1.0±0.3)W/cm2.
Estimated carrier velocities: ( ) 41.1 0.1 10v d cm st= = ± ´
25Peumans, et al. Appl. Phys. Lett., 76, 3855 (2000).
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Organic ElectronicsStephen R. Forrest
26
Long wavelength DetectorsCarbon Nanotubes Can Stretch Detection to NIR
Chirality determines if CNTis metallic, semiconducting or insulating
Ch = na1 + ma2
n = m: Metallicn-m = 3i (i integer), n≠m, nm≠0: semimetalotherwise: semiconductor
Organic/CNT Detector
CNT:MDMO-PPV composite Mat of bare CNT
500 nm 500 nm
Arnold, et al., Nano Letters, 9, 3354, 2009.
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Organic ElectronicsStephen R. Forrest
Long wavelength DetectorsSingle Walled Nanotubes Wrapped in Polymer
Responsivity and Specific Detectivity:
27
D* = AΔfNEP
=R AΔfin2 [cm-Hz1/2/W]
M. S. Arnold, et al., Nano Letters, 9, 3354, 2009.
=jph A
Pinc= qgηext
λhc
⎛⎝⎜
⎞⎠⎟
R [A/W]D* = AΔfNEP
=R AΔfin2
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Organic ElectronicsStephen R. Forrest
Position Sensitive Detectors
• Mechanism of operation– Extended junction transports charge vertically (no
current spreading)– Current divided by linear resistance of ITO strip
R1 R2I1 I2A A
V
ID/2 ID/2Iph
PEDOT:PSS
ITO
CuPcPTCBI
BCPAg
Glass Substrate
Rand, et al. IEEE Photon. Technol. Lett., 15, 1279 (2003).
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Organic ElectronicsStephen R. Forrest
-15 -10 -5 0 5 10 15
-15
-10
-5
0
5
10
15
M
easu
red
Beam
Pos
ition
, Dx
(mm
)
Actual Beam Position (mm)
0 V 0.7%-0.5 V 0.1%-1 V 0.1%
-1.5 V 0.8%-2 V 1.3%
120 μW 0.8%
510 μW 0.1%
1.3 mW 0.1%
Position Detection Characteristics
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Organic ElectronicsStephen R. Forrest
Applications of PSDs
• Machine vision– Part location and positioning– Robot servo feedback– 2D possible
• Lab bench positioning• Free space communication
(1 to 1 correlation betweenobject location and position of image)
PSD