Complementary techniques to investigate degradation mechanisms in solar cells

Simon Züflea,b, Martin Neukoma, Beat Ruhstallera,b

10.09.2015

a: Fluxim AG, Winterthur, Switzerland b: Institute of Computational Physics, ZHAW, Winterthur, Switzerland

Who we are

2

Sim

ula

tion S

oft

ware

Me

asu

rem

ent

Ha

rdw

are

Research on

OLED and OPV

Stability of Solar Cells

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How to characterize and compare stability? Standardization! - IEC 61215 for c-Si - IEC 61646 for thin-film - ISOS Protocols for OPV But: focussed on steady-state, where valuable information on degradation is concealed.

Reese et al., Solar Energy Materials and

Solar Cells, (2011), 95, 1253

4

Impedance

Spectroscopy

Capacitance-Voltage

Dark Injection Transients

Transient Photocurrent

Photo-CELIV

Transient Photovoltage

IV-Curves

Techniques we propose:

t

Light

t

Current

• Cell is flashed at constant

voltage

• Qualitative investigation of

charge carrier mobility

• Qualitative investigation of

trapping dynamics

Transient Photocurrent TPC

Isc

DSC

Perovskite OPV

CdTe

P3HT:CdSe

Hybrid

Drift & Diffusion Ionic movement

Traps Other slow effect? Space-charge effects due to doping

µc-Si

Transient Photocurrent: Technology Comparison

Preconditioning -1V Preconditioning +1V

Repeated Transient Photocurrent on Perovskite SC

Stress-Test Module

Evidence for movement of ionic charges

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• Drift-Diffusion Simulation

• Simulate all experiments with one set of parameters

• Modeling helps to understand device physics

• Get further insight into device

Neukom et al., Org. El. 13,2910 (2012)

Modeling

IV

dark-CELIV

TPC

photo-CELIV

Measurement

and simulation

of an organic

solar cell

Global fit with

one set of

parameters!

Advanced Characterization Example

Neukom et al., Org. El. 13,2910 (2012)

Accelerated Ageing Study

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• Standard unencapsulated P3HT:PCBM organic solar cells with PEDOT:PSS as hole transport layer (unstable)

• In climate chamber at 45°C, 85%RH investigate influence of humidity

• Automated repetition of measurement routine

Highly systematic data!

P3HT:PCBM

PEDOT:PSS

Aluminum

ITO

Steady-State Measurements

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-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-10

-5

0

5

10

15

20

Voltage (V)

Curr

ent

density (

mA

/cm

2)

0 min

5 min

12 min

24 min

37 min

49 min

61 min

73 min

85 min

96 min

108 min

0 50 1000

0.2

0.4

0.6

Deg time (min)

Voc (

V)

Current-Voltage

0 20 40 60 80 100 1200

1

2

3

4

5

Degradation time (min)

Jsc (

mA

/cm

2)

Short-circuit current

No conclusions about degradation mechanism possible!

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-1 -0.5 0 0.5 1 1.50

50

100

150

Voltage (V)

Capacitance (

nF

/cm

2)

0 min

5 min

12 min

24 min

37 min

49 min

61 min

73 min

85 min

96 min

108 min

Hypothesis: Insulating Al2O3 interface layer grows, leading to MIS structure.

Cell B Cell B Cell A

Al2O3 Al2O3

Glass Substrate

PEDOT:PSS

P3HT:PCBM

Al

ITO

AC Measurements

-1 -0.5 0 0.5 1 1.5 20

20

40

60

80

100

120

140

160

180

200

Vdev (V)

capacitance (

nF

/cm

2)

Sim, x = 0, fresh

Sim, x = 0.42

Sim, x = 0.57

Sim, x = 0.91

Sim, x = 1, degraded

Capacitance-Voltage

Simulation of growing MIS

Surface A: (1-x)·Sini Surface B: (x)·Sini

0 10 20 30 40 50 60 70 80 90

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

Time (us)

Curr

ent

density (

mA

/cm

2)

0 min

5 min

12 min

24 min

37 min

49 min

61 min

73 min

85 min

96 min

108 min

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t

Lig

ht

Transient Measurements Transient Photocurrent

0 20 40 60 80 100

0

2

4

6

8

10

12

14

16

18

time (us)

J (

mA

/cm

2)

x = 0

x = 0.39

x = 0.55

x = 0.67

x = 0.77

x = 0.87

x = 0.95

«There is life after death!»

Charge extraction is hindered:

Blocking Al2O3 layer

Simulation

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-10

-5

0

5

10

15

20

Voltage (V)

Curr

ent

density (

mA

/cm

2)

Exp, t = 0 min

Exp, t = 5 min

Exp, t = 37 min

Exp, t = 102 min

Sim, x = 0, fresh

Sim, x = 0.42

Sim, x = 0.68

Sim, x = 0.94

Sim, x = 1, degraded

Comparison

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Model-based analysis helps to quantitatively validate the postulated degradation mechanism!

IV: Paios Measurements + Setfos Simulations

x describes effective area vs time, in agreement

with a 2D diffusion model

Electric field at short-circuit

Conclusions

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• Hypothesis: Al2O3 layer acts as insulating interface resulting in a local complete loss of current

Lateral instead of homogeneous degradation process!

This conclusion is possible without time-consuming, expensive and destructive methods!

Züfle et al., Adv. En. Mater., 2015, in press, 10.1002/aenm.201500835

Summary

• IV-curves alone are not enough to understand degradation processes

• Transient and impedance techniques reveal valuable information

• Systematic measurement data allows for combinatorial analysis

• Advanced Modeling helps to validate hypothesis and gives additional insight into the device

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Acknowledgement

Simon

Züfle

Beat Ruhstaller

Stephane

Altazin

Martin

Neukom

Adrian

Gentsch

Thank you for your attention!

Transient Photovoltage

DSC

Perovskite Organics

CdTe

P3HT:CdSe

Drift & Diffusion Traps

Recombination Ions?

Leakage Current

Capacitance-Voltage

Injection barriers Doping

Built-in voltage

DSC

Perovskite

P3HT

uc-Si

23

Perform all experiments at temperatures

from 150 to 350 K

Low Temperature Module

CELIV TPC

Transient and ac Measurements

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0 5 10 15

-30

-25

-20

-15

-10

-5

0

Time (us)

Curr

ent

density (

mA

/cm

2)

0 50 1000

2

4

6

8x 10

22

Deg time (min)Extr

acte

d c

harg

es (

1/m

3)Photo-CELIV

Dark-CELIV

0 10 20 30 40 50 60 70 80 90

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

Time (us)

Curr

ent

density (

mA

/cm

2)

0 min

5 min

12 min

24 min

37 min

49 min

61 min

73 min

85 min

96 min

108 min

-1 -0.5 0 0.5 1 1.50

50

100

150

Voltage (V)

Capacitance (

nF

/cm

2)

0 min

5 min

12 min

24 min

37 min

49 min

61 min

73 min

85 min

96 min

108 min

104

105

106

0

5

10

15

20

25

30

35

40

Frequency (Hz)

Capacitance (

nF

/cm

2)

0 min

5 min

12 min

24 min

37 min

49 min

61 min

73 min

85 min

96 min

108 min

C-V C-f

Systematic data of the same device during degradation!

Combinatorial Analysis

• Electrode workfunctions are constant

• Electrode resistance is constant

• Layer thickness is constant

• No doping is present

• Traps cannot be dominant mechanism

• Absorption loss cannot be dominant mechanism

• Decreased mobility cannot be dominant mechanism

• Space charge due to hampered extraction

There is life even in dead cells!

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