Photovoltaics and Photodetectors - part II
• Organic Heterojunction Photovoltaic Cell
• Organic Multilayer Photodetector
Data on Solar Cells and Photodetectors taken from
Peumans, Bulovic, and Forrest.,
Appl. Phys. Lett. 76, 2650 (2000) – solar cell
Appl. Phys. Lett. 76, 3855 (2000) - photodetector
@ MIT
March 11, 2003 – Organic Optoelectronics - Lecture 10
1
Organic Heterojunction PVs
VB
V
LD
CB
LD
Incident
Light
2
Optical power ⇒ electrical power
Photovoltaics
FIGURES OF MERRIT:
Power conversion efficiency
Full solar intensities
Reliability
Consume power to detect a signal
Photodetectors
FIGURES OF MERRIT:
High external quantum efficiency
High bandwidth
Low noise, low power consumption
3
Solid state organic solar cells
•high absorbtion in the visible spectrum
•have relaxed deposition requirements
•can be manufactured in a low cost process
(roll-to-roll, web-processing, etc.)
•can be grown on thin, flexible substrates → light weight
•can add value to existing products (window coatings, etc.)
CHALLENGE!
Current power conversion efficiencies are
too low for commercial implementation
(especially at full solar intensities)
4
Solar Cell Power Efficiency
20
10
[%]
15
5
0
Or
ga
nic
Am
o
rph
ou
s-S
i
Production
Po
lySi
Cr
ys
ta
llin
eSi
Laboratory
5
Device Preparation and Growth
substrate
holder
thickness
monitor
• Glass substrates precoated with ITO
- 94% transparent
- 15 Ω/square
• Precleaning
Tergitol, TCE
Acetone, 2-Propanol
• Growth
- 5 x 10-7 Torr
- Room T
substrate
shutter
TURBO
PUMP
COLD
TRAP
ROUGHING
PUMP
source
boats
VACUUM
CHAMBER
- 20 to 2000 Å
layer thickness
POWER
SUPPLIES
GND
6
History and Progress
‘86: Heterojunction Solar Cell
C.W. Tang
Tang, Appl Phys Lett. 48, 183 (1986).
‘90s: Polymer Networks
3
Current [mA/cm2]
• first heterojunction for efficient
charge generation
• ~0.95% conversion efficiency
• nearly ideal IVs (FF~0.65)
• under full solar illumination
(1 sun)
2
1
Ag
PTCBI (500Å)
CuPc (300Å)
ITO
Glass
VOC = 450mV
0
-1
η = 0.95%
ISC = 2.3mA/cm 2
ff = 0.65
-2
-3
-0.4
G. Yu et al., Science 270, 1789 (1995).
• series resistance problem (low FF)
• calculated power conversion efficiency of ~1.5%
• not well matched to solar spectrum
Shaheen et al. , Appl. Phys. Lett. 78, 841 (2001).
• power conversion efficiency ~2.5%
• not well matched to solar spectrum
• long term stability ?
-0.2
0.0
0.2
0.4
Voltage [V]
7
Photoinduced Charge-Transfer
Processes occuring at a Donor-Acceptor heterojunction
LUMO
1
2
3
4
1 Exciton generation by
absorption of light
2 Exciton diffusion over ~LD
4
3 Exciton dissociation by
HOMO
rapid and efficient
charge transfer
D: CuPc
4 Charge extraction by the
A: PTCBI
internal electric field
8
Exciton Diffusion: Experiment and Theory
•Photoluminescence (PL) probes the exciton lifetime
•Exciton lifetime depends on proximity of donoracceptor interface
540nm
0.8
CuPc
LD=(30±3)Å
PTCBI
PL1
0.6
(
(
⎡1 − exp −2 ⋅
PL
L ⎢
η = 1 = 1− D ⋅ ⎣
PL2
t ⎡1 + exp −2 ⋅
⎢⎣
0.4
0.2
⎤
)
L ⎥⎦
⎤
t
L ) ⎥⎦
t
D
D
PL2
PTCBI
PL efficiency ratio η
1.0
experiment
theory
0.0
0
40
80
120
160
LPTCBI [Angstrom]
9
Double Heterojunction
Problem
Solution
Ag
Ag
EBL
ITO
~200Å
ITO
•cathode metal diffusion
Introduce ‘Exciton Blocking
•deposition damage
Layer’ (EBL) to:
•exciton-plasmon interaction •confine excitons to active region
•vanishing optical field
•act as a damage-absorber
•electrical shorts
10
Exciton Blocking Layer (EBL)
BCP
(2, 9-dimethyl, 4, 7-diphenyl,
1, 10-phenanthroline)
(aka bathocuproine)
CuPc PTCBI BCP
EG=1.7 E =1.7 EG=3.5
G
ITO
• conducts electrons
• transparent
• effectively blocks excitons
• absorbs damage
• separates active layers from metal
Ag
LUMO
Ag
0.7
0.9
0.85 eV HOMO
BCP
PTCBI
CuPc
ITO
glass substrate
Energy Band Diagram
courtesy of I. Hill & A. Kahn
11
Quantum Efficiency, η [%]
Exciton Blocking Layer (EBL)
Improves Thin Cell Efficiency
>2.5-fold improvement
in efficiency
30
25
η INTERNAL
20
15
Previous best
devices (Tang, ‘86)
w/ EBL
10
η EXTERNAL
5
no EBL
0
100
200
400
800
CuPc and PTCBI Layer Thickness [Å]
12
Practical Realization:
MicroMolded Winston Collectors
CROSS SECTION
Solar
Illumination
3-D VIEW
molded substrate
with collector profiles
and coated with Ag
organics + ITO (anode)
Ag mirror / cathode
13
Quantum Efficiency, η [%]
90Å CuPc/ 90Å PTCBI
Solar Spectrum
AM1.5
4
30
η INTERNAL
3
20
light trapping
η EXTERNAL
2
10
1
0
400
0
600
800
λ [nm]
1000
Photon Flux [1018 s-1 m-2nm-1]
Broad Spectral Response
Matches Solar Spectrum
14
I-V Response under Solar Illumination
•Highest illumination levels to date
•Record-high short circuit currents
90Å CuPc/ 90Å PTCBI
Current [mA/cm2]
20.0
0.0
VMAX
2
19
64
200
IMAX
-20.0
620
-40.0
VOC
ISC
960
-60.0
1300 mW/cm2 = 17 suns (AM1.5)
-80.0
-0.4
-0.2
0.0
0.2
Voltage [V]
0.4
0.6
15
ISC [mA/cm2]
I-V Response under Varying Solar Illumination Intensity
Optical power [# suns] (AM1.5 spectrum)
0.1
1
10
100
10
linear
1
ηP [%]
FF
VOC [V]
0.6
0.5
monotonic increase
0.4
0.6
0.5
0.4
0.3
1.2
1.0
0.8
0.6
no drastic decrease
broad plateau
10
100
1000
Optical power [mW/cm2] (AM1.5 spectrum)
16
Light Trapping Improves PV Efficiency ~2.5 Fold
60Å CuPc/ 60Å PTCBI
Power Conversion Efficiency [%]
light trapping configuration
Ag aperture
Glass Substrate
ITO / Organic Layers
Ag cathode
3.0
2.5
~10 suns
(2.4 + 0.3)%
2.0
w/ light trapping
no light trapping
1.5
(1.0 + 0.1)%
1.0
no light trapping
0.5
0.0
1
10
100
Optical power [mW/cm2]
1000
17
Practical Realization:
MicroMolded Winston Collectors
•Raytracing calculations
•Microcavity effects ignored
•1D geometry
•Mirror/cathode reflectivity: 95%
•Cell absorption: 30%
•ηEXT = 90% of ηINT
•ηP = 2.7% (ideal = 3.0%)
•Local intensity never exceeds 3 suns
COLLECTOR
MIRROR
REFLECTIVE
UNDERCOAT
GLASS SUBSTRATE
THIN PV
REFLECTING CATHODE
Peumans et al, US Patents
#6440, 769, Aug 27, 2002
18
Collector Fabrication
PRESSURE
MOLD
POLYMER
SUPPORT
100-200µm
HEAT
SHAPED
POLYMERIC FILM
100-200µm
Peumans et al, US Patents #6440, 769, Aug 27, 2002
19
Collector Fabrication
METAL COATING
200-400µm
ANTI-REFLECTION COATING
TRANSPARENT POLYMERIC FILLER
ITO ANODE
ORGANICS: CuPc/PTCBI/BCP
DEVICE DEPOSITION
Ag CATHODE
ENCAPSULATING FILM
20
Internal Quantum Efficiency, η [%]
Outlook – towards future PVs
improved material
growth
100
60
20Å
40Å
thinner
30
cells
LD=100Å
ηP=2.4%
10
6
3
Single
Heterojunction
ηP~1.0%
Peumans et al., APL 76 (19), p. 2650 (2000).
1
10
30
60
100
300
600 1000
CuPc and PTCBI Layer Thickness [Å]
21
Multilayer Organic PV
V-
V+
Incident
Light
THIN sub-PVs connected in PARALLEL
• increased likelihood of exciton dissociation
• decreased R
• ISC of all sub-PVs add while VOC ~constant
Bulović and Forrest, patents: US 6,198,091 (2001); US 6,198,092 (2001); US 6,278,055 (2001) .
22
Short circuit current density (closed squares, left
axis) and open circles, right axis) for dual cells
having Ag interlayers of different average
thickness. The measurements were performed
under AM 1.5, 100mW/cm2 (1 sun) illumination.
Also shown is the proposed energy level diagram
of the dual-HJ device.
Adapted from A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 80, 1667 (2002).
23
dark
18 mW/cm2
58 mW/cm2
116 mW/cm2
372 mW/cm2
Open circuit voltage dependence on the incident light
intensity for single (squares), dual (circles), triple
(triangles), and fivefold (diamonds) HJ photovoltaic
cells. The inset shows the maximum open circuit
voltage achievable vs. the number of heterojunctions
in the stacked devices.
Current density-voltage characteristics of a triple-HJ
cell at different incident light intensities with an AM
1.5 solar spectrum. Here ~100 mW/cm2 corresponds
to 1 sun intensity.
Power efficiencies (ηP) and open circuit voltages (VOC) under AM 1.5 illumination:
Single HJ cell
ηP = 1.1 ± 0.1%
VOC = 0.43 V
Double HJ cell
ηP = 2.5 ± 0.1%
VOC = 0.93 V
Triple HJ cell
ηP = 2.3 ± 0.1%
VOC = 1.2 V
A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 80, 1667 (2002).
24
Applications
Photodetectors - Motivation
Molecular Organic Photonic Integrated Circuits (MOPICs) Organic LEDs, transistors, photovoltaic cells demonstrated. There is a need
for efficient, high-bandwidth photodetectors.
Large Area Photodetection - Solid-state scanner, solid-state X-Ray
plate (in conjunction with scinitillator downconverter), very large area
imaging arrays, etc.
Photodetectors on any Substrate - Organic photodetectors can be
Physics
deposited on a variety of substrates, including low-cost, flexible foil.
Exciton Dynamics in Organic (Ultra)Thin Films - Organic donoracceptor multilayers as probes for exciton dynamics.
Carrier Dynamics in Organic Heterostructures - Information about
carrier transport across heterojunction barriers.
25
Organic PDs: Status
Yu et al.,
Synth. Metal 102 (1-3) 904 (1999), Science 270 1789 (1995)
•operates over visible + near UV
•ηEXT=45% @ -10V, IDARK=1×10-8A/cm2 @ -5V
•functional 1D array
•response time?
Halls et al.,
•Polymeric
•µ~10-6-10-3cm2/Vs
•τTR>50ns
Nature 395 6699 (1998)
•operates over visible + near UV
•ηEXT=58%, IDARK=? (RR=103) @ -2V
•response time?
So et al.,
IEEE TED 36 (1) 66 (1989)
•PTCDA, CuPc, ... on Si
•500MHz possible for t<500Å
•Crystalline layers
•µ~1cm2/Vs
•τTR~1ns
26
VOPc/PTCDA Multilayers
CB
% INCIDENT LIGHT QUANTUM EFFICIENCY
VB
7
6
# of interfaces
Au/[VOPc(10nm)\PTCDA(10nm)]8
5
15
4
3
Au/[VOPc(10nm)\PTCDA(20nm)]4
7
2
Au/[VOPc(10nm)\PTCDA(78nm)]
1
1
0
350
450
550
650
750
850
WAVELENGTH (nm)
Adapted from: Arbour, et al., Mol. Cryst. Liq. Cryst. 183, 307 (1990).
27
Donor-Acceptor Multilayer Structure
CuPc
(EG=1.7)
BCP
(EG=3.5)
ITO
•Growth:
LUMO
Ag
0.7
0.9
PTCBI
(EG=1.7)
0.85 eV
t=5Å to 160Å
HOMO
•Anode=ITO (ρsheet~40Ω/ )
•Growth: UHV (pbase~1×10-10Torr)
•320Å thick multilayer (2×160Å to
64×5Å) or codeposited (mixed)
layer.
•~300Å BCP cap
•Cathode=Ag (pbase~1×10-6Torr)
CuPc
PTCBI
BCP
copper phthalocyanine
3,4,9,10-perylenetetracarboxylic
bis-benzimidazole
bathocuproine
28
Dark Current
Dark current shows Frenkel-Poole dependence.
1x10
-6
Frenkel-Poole Emission
Current Density [A/cm2]
t=160Å
1x10
⎛ q
J = κ E exp ⎜
⎝ rkT
80Å
-7
20Å
1x10
E = (V − Vbi ) d
10Å
-8
mixed
1x10
qE ⎞
⎟
πε i ⎠
d (V ) = ε i A C (V )
-9
κ = (2.1 ± 1.5)× 1018 A / Vcm
1x10
-10
10
-11
10
-12
-10
r = (1.8 ± 0.1)
-5
0
5
Voltage [V]
29
Electrical Characteristics
A possible origin for the
compensating traps are O2- centers
which act as deep acceptor levels in CuPc.
depleted
compensated
p
i
n
= p-n junction diode
compensated
p
i
n
= p-i-n junction diode
30
Spectral + Voltage Dependence of the EQE
•Sensitive to visible + NIR wavelengths
•Strong dependence on bias: EQE~75% @ -10V
External quantum efficiency
1.00
t=5Å (64 layers)
-11-10
-9
-8
-7
-6
0.75
-5
-4
-3
0.50
-2
0.25
-1
0V
0.00
500
600
700
800
λ [nm]
31
Response Time
Thinner individual layers makes faster devices due to a reduced exciton lifetime
100µm diameter, -9V, 1.4ps excitation @ 670nm under
an average optical power of (1.0±0.3)W/cm2. Estimated
4
carrier velocities: v = d τ = (1.1 ± 0.1) × 10 cm s
f3dB=(430±40)MHz
Normalized Response
Normalized Response
PTCBI lifetime=(1.8±0.1)ns
t=160Å
20Å
10Å
5Å
1
0.1
0.01
10
100
1000
10000
Frequency [Hz]
FWHM=(720±50)ps
0
1
2
3
Time [ns]
4
5
6
32
Model
Predicts trend in τFWHM
3
2
τCT=(300±200)fs, LD=30Å, τ=2ns
10
100
PL lifetime of
pristine PTCBI
τFWHM [ns]
1
0.9
0.8
0.7
0.6
t [Å]
exciton lifetime << transit time
response time ~ transit time
Response
t=160Å
exciton lifetime > transit time
response time ~ exciton lifetime
t=10Å
-5
0
5
10
time [ns]
15
20
33