complementary techniques to investigate degradation ... · complementary techniques to investigate...

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

11.09.2015 3

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

11.09.2015 8

• 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

11.09.2015 10

• 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

11.09.2015 11

-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!

11.09.2015 12

-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

11.09.2015 13

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

11.09.2015 14

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

11.09.2015 15

• 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

11.09.2015 16

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

11.09.2015 25

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!

11.09.2015 26