Stability and Efficiency of Organic Solar cells - Help

Vikram Dalal

Iowa State University

* Supported in part by a NSF Grant

Stability and Efficiency of Organic

Solar cells

Acknowledgments

To M.Samiee, R. Shinar, J. Shinar, J. Bhattacharya, M. Noack,

T. Xiao, P. Joshi, M. Noack, S. Pattnaik

Outline

Degradation – Light induced and atmospheric

Changes in fundamental material properties under

degradation

Influence of changes in contacting layers

Hybrid organic/inorganic tandem cells

Potential efficiency

Problem

Organic cells degrade quite a bit under both light and

atmosphere

25-30% degradation in performance common; lifetime

prediction uncertain

Contrast with inorganic cells: Si cells last>30 years with

<10% degradation over that period

Can we do accelerated degradation?

What is the influence of the spectrum?

Can it be reversed?

How can we reduce degradation?

What measurements?

Light I-V Before and after degradation

Dark I-V Before and After degradation

Quantum efficiency-Before and after

Capacitance-frequency Before and after

Mobility measurements

Degradation in light, and in air

Experiments

Standard P3HT/PCBM cells

Glass

ITO

PEDOT/PSS or MoO3

P3HT/PCBM

Ca/Al

Light

Exposure system-high vacuum

• Unique system

• Full spectrum AM1.5 lamp

• Intensity can be changed to ~10X

• High vacuum followed by N2/Ar

• Measurements in-situ

• Can study influence of

various gases (eg: moisture)

Changes in performance, 2X sun intensity

Light Intensities adjusted for both cases – Current in organic cell

identical for the two cases

ORG 1023 Filter vs. No Filter

94.0%

95.0%

96.0%

97.0%

98.0%

99.0%

100.0%

101.0%

0 500 1000 1500

Time (Minutes)

% o

f In

itia

l V

alu

e

Voc Filter

Isc Filter

Voc No fi lter

Isc No fi lter

Full spectrum Xenon AM1.5

Blue a-(Si,C) filter

For accurate measurements, MUST

Use full spectrum light source!

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-0.5 -0.3 -0.1 0.1 0.3 0.5

Before Exposure

After Exposure

Light I-V Slope indicates decrease in

Collection length after degradation-

More recombination

Ref. Bhattacharya et al, APL, May 2012

Tail states and midgap states in P3HT-

measure subgap QE

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.8 1 1.2 1.4 1.6 1.8 2 2.2

Qu

antu

m e

ffic

ien

cy

Energy(eV)

BeforeDegradation

AfterDegradationTail states

Midgap

Midgap states Change, Tail states do not

Next, interface between P3HT and

PCBM

3 measurements

C-f – gives defect density vs. energy

Dark I-V-two regions?

Dark I-V vs temperature-relationship of Jo1 vs temperature

C-f data

Initial

With uv-blue filter

Full spectrum

DOS of defects in another device- 5X increase

at~0.5-0.6 eV

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

0 0.2 0.4 0.6 0.8VOLTAGE(VOLT)

After 2xIllumination for96 Hrs

Change in dark I-V-light exposure only-note the change in

earlier I-classic increasing midgap state problem:

5X increase in J01

Dark I-V vs. Temperature-Control sample

Pre exposure – dark I-V controlled by interface-

and interface changes upon exposure

-30

-29

-28

-27

-26

-25

-24

-23

-22

-21

-20

30 32 34 36 38 40

1/kT(kT in eV)

Ln

(Io

/sq

rt(T

))

slope=0.62 eV

The slope ~ ½ of LUMO-HOMO At interface

1.2 eV

P3HT

PCBM

LUMO level

HOMO level

P3HT/PCBM heterojunction

Bob Street’s Data

[Street et al,AFM,2012]

Influence of post-degradation thermal

anneal, 100 C

Changes in defect densities upon

thermal annealing after degradation

Next, influence of contacts

Anneal MoO3 or PEDOT/PSS at various temperatures

BEFORE depositing solar cells

Then study degradation in absence of oxygen/moisture

Changes in Isc and efficiency vs. pre-

annealing of contacts

McGehee group data on contacts

[Peters et al,AM,2012]

Bob Street Group’s Data

Conclusion on stability

UV and blue photons do much more damage than equivalent

dose of red photons

Contacts make a difference

Interface seems to be damaged

Midgap states in P3HT are also being generated by light

Thermal annealing seems to help partially

Q: Can we use a device design to not waste blue photons and

yet improve stability?

Basic device structure: Hybrid

inorganic/organic tandem

Light

The bandgap of inorganic can

Be changed at will to match

the organic cell

Intermediate layer-Critical:

Sputtered ITO

The intermediate layer MUST provide a good ohmic contact to the first cell,

and a good ohmic contact to the second. ITO good contact for n+ a-(Si,C)

(tunnel contact) and a good hole injector for P3HT (valence band match)

I-V curve of the P3HT:PCBM cell by

itself.

0.0 0.2 0.4 0.60

3

6

9

12

Cu

rre

nt

De

ns

ity

(m

A/c

m2)

Voltage (V)

Organic only

FF=64%

I-V curve of tandem

Note the voltage, ~1.5 V =

sum of amorphous + organic

Note the Fill factor, 77%- shows

No problem at interface

Between two cells

Ref: Pattnaik et al, IEEE J. Photovolt.(to be published)

QE of 1st and 2nd solar cells – Nicely

matched

400 500 600 700

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Jorg

sc = 4.97mA/cm

2

No

rmalized

EQ

E

Wavelength (nm)

Ja-(Si,C)

sc = 5.1mA/cm

2

(c)

What potential efficiency

With the right organic cells with LOWER gap, 12 mA/cm2

seems entirely feasible with ~0.8 V

A-Si will also give 12 mA

Combine, get 12 mA, 1.7V, 0.8 FF

Eff. ~16.3%!

And better stability – two reasons

blue photons absent, and less intensity on organic

In a triple, ~10 mA, 2.7V, 0.8 FF, Eff. Of = 21.6% possible

Influence of photonics and plasmonics

Must model it right-3D model needed – can increase current

by ~40% in P3HT

9

10

11

12

13

14

15

16

0 10 20 30 40 50 60 70 80

Variation with texture height

Jsc mA/cm2

Jsc

mA

/cm

2

height h nm

Open questions on stability What are the kinetics of degradation telling us-how do we model

them?

What is the relationship between stability and intensity of light –model needed

How about stability and temperature effects?

Can we anneal out defects?

Is there synergy between light induced and moisture induced degradation?

How do we reduce the influence of contacts on degradation ? What new contacting layers can be used?

How does structure change in degradation? What is the influence between structural changes and electronic property changes?

How do we minimize interfacial changes?

Open questions on multiple junction

cells

How do we optimize light collection using photonic and

plasmonic effects? Modeling and experiments needed for the

entire structure

What are the best intermediate connecting layers?

How do these layers affect stability of multi-junction cells?