1

PhotovoltaicsPhotovoltaics and Photodetectors and Photodetectors -- part IIpart II

@ MITMarch 11, 2003 – Organic Optoelectronics - Lecture 10

• 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

2

Organic Heterojunction PVs

CB

VBLD

LD

IncidentLight

V

3

PhotovoltaicsPhotovoltaics

PhotodetectorsPhotodetectors

Optical power ⇒ electrical power

FIGURES OF MERRIT:

Power conversion efficiencyFull solar intensitiesReliability

Consume power to detect a signal

FIGURES OF MERRIT:

High external quantum efficiencyHigh bandwidthLow noise, low power consumption

4

•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.)

Solid state organic solar cells

CHALLENGE!Current power conversion efficiencies are

too low for commercial implementation (especially at full solar intensities)

5

0

5

10

15

20

Laboratory

Production

Crystal

line-

Si

Poly-

Si

Amor

phou

s-Si

Orga

nic

[ % ]

Solar Cell Power Efficiency

6

VACUUMCHAMBER

TURBOPUMP

COLDTRAP

ROUGHINGPUMP

substrateholder

thicknessmonitor

shutter

GND

substrate

POWERSUPPLIES

sourceboats

Device Preparation and Growth

• Glass substrates precoated with ITO- 94% transparent- 15 Ω/square

• PrecleaningTergitol, TCEAcetone, 2-Propanol

• Growth- 5 x 10-7 Torr- Room T

- 20 to 2000 Ålayer thickness

7

History and Progress ‘86: Heterojunction Solar Cell

• first heterojunction for efficient charge generation

• ~0.95% conversion efficiency• nearly ideal IVs (FF~0.65)• under full solar illumination

(1 sun)

‘90s: Polymer NetworksG. Yu et al., Science 270, 1789 (1995).• series resistance problem (low FF)• calculated power conversion efficiency of ~1.5%• not well matched to solar spectrumShaheen et al. , Appl. Phys. Lett. 78, 841 (2001).• power conversion efficiency ~2.5%• not well matched to solar spectrum• long term stability ?

C.W. Tang

η = 0.95%ff = 0.65η = 0.95%ff = 0.65

-0.4 -0.2 0.0 0.2 0.4

-3

-2

-1

0

1

2

3

Voltage [V]

Cur

rent

[mA

/cm

2 ]

ISC = 2.3mA/cm 2

VOC = 450mV

Tang, Appl Phys Lett. 48, 183 (1986).

CuPc (300Å)PTCBI (500Å)

GlassITO

Ag

8

LUMO

HOMO

D: CuPc

A: PTCBI

2 3 4

4

Photoinduced Charge-Transfer

1

23

1 Exciton generation by absorption of light

4

Exciton diffusion over ~LD

Exciton dissociation by rapid and efficient charge transfer

Charge extraction by the internal electric field

Processes occuring at a Donor-Acceptor heterojunction

1 2 3 4

4

9

Exciton Diffusion: Experiment and Theory

0 40 80 120 1600.0

0.2

0.4

0.6

0.8

1.0

experimenttheory

PL

effic

ienc

y ra

tio η

LPTCBI [Angstrom]

CuP

cP

TCB

IP

TCB

I

540nm

PL1

PL2

( )( )

1

2

1 exp 21

1 exp 2

DD

D

tLPL L

PL t tL

η

⎡ ⎤− − ⋅⎢ ⎥⎣ ⎦= = − ⋅⎡ ⎤+ − ⋅⎢ ⎥⎣ ⎦

•Photoluminescence (PL) probes the exciton lifetime•Exciton lifetime depends on proximity of donor-acceptor interface

LD=(30±3)Å

10

Double Heterojunction

Ag

Problem Solution

•cathode metal diffusion•deposition damage•exciton-plasmon interaction•vanishing optical field•electrical shorts

Introduce ‘Exciton Blocking Layer’ (EBL) to:

•confine excitons to active region•act as a damage-absorber

Ag

ITO ITO

EBL~200Å

11

ITOCuPc

PTCBIBCP

Ag

glass substrate

Exciton Blocking Layer (EBL)

0.7

0.90.85 eV

CuPcEG=1.7

PTCBIEG=1.7

BCPEG=3.5

ITOAg

HOMO

LUMO

Energy Band Diagramcourtesy of I. Hill & A. Kahn

• conducts electrons• transparent• effectively blocks excitons• absorbs damage• separates active layers from metal

BCP(2, 9-dimethyl, 4, 7-diphenyl,

1, 10-phenanthroline)(aka bathocuproine)

12

0

5

10

15

20

25

30

CuPc and PTCBI Layer Thickness [Å]

Qua

ntum

Effi

cien

cy, η

[%]

100 400200 800

η INTERNAL

η EXTERNAL

w/ EBL

no EBL

Exciton Blocking Layer (EBL)Improves Thin Cell Efficiency

>2.5-fold improvementin efficiency

Previous best devices (Tang, ‘86)

13

Ag mirror / cathode

organics + ITO (anode)

molded substrate with collector profiles and coated with Ag

SolarIllumination

CROSS SECTION 3-D VIEW

Practical Realization:MicroMolded Winston Collectors

14

400 600 800 10000

10

20

30

λ [nm]

0

2

4

Pho

ton

Flux

[101

8s-

1m

-2nm

-1]

1

3

Qua

ntum

Effi

cien

cy, η

[%]

η INTERNAL

AM1.5

η EXTERNAL

Solar Spectrum

Broad Spectral ResponseMatches Solar Spectrum

light trapping

90Å CuPc/ 90Å PTCBI

15

ISC

VOC

960

620

20064

192

-0.4 -0.2 0.0 0.2 0.4 0.6-80.0

-60.0

-40.0

-20.0

0.0

20.0

Voltage [V]

Cur

rent

[mA

/cm

2 ]

1300 mW/cm2 = 17 suns (AM1.5)

VMAX

IMAX

I-V Response under Solar Illumination•Highest illumination levels to date•Record-high short circuit currents

90Å CuPc/ 90Å PTCBI

16

I SC

[mA

/cm

2 ]

110

100

0.4

0.5

0.6

VO

C[V

]

10 100 10000.60.81.01.2

Optical power [# suns] (AM1.5 spectrum)

Optical power [mW/cm2] (AM1.5 spectrum)

η P[%

]

0.30.40.50.6

FF0.1 1 10

I-V Response under Varying Solar Illumination Intensity

linear

monotonic increase

broad plateau

no drastic decrease

17

1 10 100 10000.0

0.5

1.0

1.5

2.0

2.5

3.0

no light trapping

w/ light trapping

Optical power [mW/cm2]

Pow

er C

onve

rsio

n E

ffici

ency

[%]

(2.4 + 0.3)%

Ag apertureGlass Substrate

ITO / Organic LayersAg cathode

~10 suns

Light Trapping Improves PV Efficiency ~2.5 Fold

no light trapping

light trapping configuration

(1.0 + 0.1)%

60Å CuPc/ 60Å PTCBI

18REFLECTING CATHODE

COLLECTORMIRROR

REFLECTIVEUNDERCOAT

THIN PV

GLASS SUBSTRATE

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

Peumans et al, US Patents #6440, 769, Aug 27, 2002

19

HEAT

PRESSURE

MOLD

SUPPORT

POLYMER

SHAPEDPOLYMERIC FILM

Collector Fabrication

100-

200µ

m

100-200µm

Peumans et al, US Patents #6440, 769, Aug 27, 2002

20

METAL COATING

ANTI-REFLECTION COATING

DEVICE DEPOSITION

TRANSPARENT POLYMERIC FILLERITO ANODEORGANICS: CuPc/PTCBI/BCPAg CATHODE

200-

400µ

m

ENCAPSULATING FILM

Collector Fabrication

21

10 100 10001

100

CuPc and PTCBI Layer Thickness [Å]

Inte

rnal

Qua

ntum

Effi

cien

cy, η

[%]

30 60 300 600

30

60 LD=100Å40Å

20Å

ηP=2.4%

SingleHeterojunction

ηP~1.0%

10

3

6

improved materialgrowth

thinnercells

Outlook – towards future PVs

Peumans et al., APL 76 (19), p. 2650 (2000).

22

Multilayer Organic PV

IncidentLight

V+ V-

Bulović and Forrest, patents: US 6,198,091 (2001); US 6,198,092 (2001); US 6,278,055 (2001) .

THIN sub-PVs connected in PARALLEL

• increased likelihood of exciton dissociation• decreased R

• ISC of all sub-PVs add while VOC ~constant

23Adapted from A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 80, 1667 (2002).

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.

24A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 80, 1667 (2002).

Power efficiencies (ηP) and open circuit voltages (VOC) under AM 1.5 illumination:Single HJ cell ηP = 1.1 ± 0.1% VOC = 0.43 VDouble HJ cell ηP = 2.5 ± 0.1% VOC = 0.93 VTriple HJ cell ηP = 2.3 ± 0.1% VOC = 1.2 V

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 heterojunctionsin the stacked devices.

Current density-voltage characteristics of a triple-HJcell at different incident light intensities with an AM1.5 solar spectrum. Here ~100 mW/cm2 correspondsto 1 sun intensity.

18 mW/cm2

58 mW/cm2

116 mW/cm2

372 mW/cm2

dark

25

Photodetectors - MotivationMolecular Organic Photonic Integrated Circuits (MOPICs) -Organic LEDs, transistors, photovoltaic cells demonstrated. There is a needfor 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 deposited on a variety of substrates, including low-cost, flexible foil.

App

licat

ions

Phys

ics Exciton Dynamics in Organic (Ultra)Thin Films - Organic donor-

acceptor multilayers as probes for exciton dynamics.

Carrier Dynamics in Organic Heterostructures - Information about carrier transport across heterojunction barriers.

26

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.,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

•Polymeric•µ~10-6-10-3cm2/Vs•τTR>50ns

•Crystalline layers•µ~1cm2/Vs•τTR~1ns

27

VOPc/PTCDA Multilayers

% IN

CID

EN

T LI

GH

T Q

UA

NTU

M E

FFIC

IEN

CY 7

6

5

4

3

2

1

0350 650450 550 750 850

WAVELENGTH (nm)

CB

VB

Adapted from: Arbour, et al., Mol. Cryst. Liq. Cryst. 183, 307 (1990).

# of interfaces

15

7

1

Au/[VOPc(10nm)\PTCDA(10nm)]8

Au/[VOPc(10nm)\PTCDA(20nm)]4

Au/[VOPc(10nm)\PTCDA(78nm)]

28

Donor-Acceptor Multilayer Structure

•Growth:•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)

0.7

0.9

0.85 eVPTCBI(EG=1.7)

BCP(EG=3.5)

ITO AgLUMO

HOMO

CuPc(EG=1.7)

CuPccopper phthalocyanine

PTCBI3,4,9,10-perylenetetracarboxylic

bis-benzimidazole

BCPbathocuproine

t=5Å to 160Å

29

-10 -5 0 510-12

10-11

1x10-10

1x10-9

1x10-8

1x10-7

1x10-6

mixed10Å20Å80Å

t=160Å

Voltage [V]

Cur

rent

Den

sity

[A/c

m2 ]

Dark Current

expi

q qEJ ErkT

κπε

⎛ ⎞= ⎜ ⎟

⎝ ⎠

( ) dVVE bi−=

( ) VcmA /105.11.2 18×±=κ

( )1.08.1 ±=r

Frenkel-Poole Emission

)()( VCAVd iε=

Dark current shows Frenkel-Poole dependence.

30

Electrical Characteristics

compensated

depleted

= p-n junction diode

compensated

p ni ip n

= p-i-n junction diode

A possible origin for thecompensating traps are O2

- centers which act as deep acceptor levels in CuPc.

31

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

Ext

erna

l qua

ntum

effi

cien

cy

λ [nm]

t=5Å (64 layers)

32

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 [Hz]

PTCBI lifetime=(1.8±0.1)ns

100µm diameter, -9V, 1.4ps excitation @ 670nm under an average optical power of (1.0±0.3)W/cm2. Estimated carrier velocities: ( ) 41.1 0.1 10v d cm sτ= = ± ×

33

10 1000.60.70.80.9

1

2

3

-5 0 5 10 15 20

ModelPredicts trend in τFWHM

t=10Å

t=160Å

t [Å]

time [ns]

τ FW

HM

[ns]

Res

pons

e

exciton lifetime << transit timeresponse time ~ transit time

exciton lifetime > transit time response time ~ exciton lifetime

PL lifetime of pristine PTCBI

τCT=(300±200)fs, LD=30Å, τ=2ns