1. Understanding the physics of
degradation of polymer solar cell.
Joydeep Bhattacharya
Final Oral –PhD
29th July , 2013
Dr Rana Biswas & Dr Vikram Dalal
The work is supported by
National Science Foundation & Iowa Power Fund

2. 2
 DISCUSSION ON PHOTONIC BACK REFLECTOR
 INTRODUCTION TO PLASTIC SOLAR CELLS
 DIFFERENT DEGRADATION & THEIR SIGNATURE
 INITIAL RESULTS
 MORE RESULTS
 PERFORMANCE RECOVERY
 PHOTO STABLE ZNO:AL BASED OPV
 CONCLUSION
OUTLINE

3. 3
Introduction
 Energy Requirement of Entire Earth- 1.99 TW only 0.02 % of the total solar
radiation that reaches Earth Surface ( 1.05E5 TW).
Market Share, Cost of the Module and status
Single Junction OPV can reach
7.2% while double junction
had achieved ~10.5% ( in lab)
International Energy Agency (IEA Report)2012

4. 4
Increase efficiency
Reduce costs
Improve stability
Some basic objective to reach-
Bandgap engineering, Introducing
Photonic –plasmonic architechture, New
architecture –super lattice,tandem
Addressed by roll to roll
fabricatioma/Use of Thinner
active layer
Better encapsulation from
environment

5. 5
Flexible
Easy to Fabricate-
Tunable properties
Why need organics?

6. 6
LOW EXCITON DIFFUSION
LENGTH
DEGRADATION
DIS-ADVANTAGES
TYPICAL EXCITON DIFFUSION LENGTH OF 10-15 nm
SEVERAL DEGRADATION FACTORS
INCLUDING OXYGEN, MOISTURE, LIGHT etc…

7. 7
FUNDAMENTAL DEGRADATION PATHWAYS

8. 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
Current(mA)
Voltage(Volt)
Pristine
After 56 Hrs
After 96 Hrs
8
SOLAR CELL STORRED IN AIR UNDER DARK CONDITION
The process is irreversible in nature
ROLE OF MOISTURE
Yue et al SOLMAT-2012/Kawano et al 2008-SOLMAT

9. 9
IN AMBIENT ATM IN N2 ATM
ROLE OF OXYGEN

10. 10
ROLE OF LIGHT
DEVICE PERFORMANCE CAN BE AFFECTED BY DIFFERENT
MECHENISM
1. Sub-band gap states creating recombination center due to photo oxidation.( This can be
initiated by even a trace amount of oxygen present in the active layer.
Craig H Peters et al. AFM 2012—McGeehe Group, Stanford
Absorbance(a.u)
Increase in sub band gap states shown by PDS

11. 11
ROLE OF LIGHT continued…
Ankit Kumar et al 2008 AFM, Yang Yang Group
Accumulation of space charge at D-A interface was held responsible

12. 12
ROLE OF LIGHT continued…
MO Reese et al 2010 AFM, NREL
Oxygen creates trap states in fullerene resulting in lower electron mobility

13. 13
RA Street et al 2013 APL & 2012-PRB, PARC
High energy photon can break C-H bond, and further theoretical studies show
that excess H can bind to other sites on the polymer creating localized
electronic states.
ROLE OF LIGHT continued…

14. 14
Oxygen held responsible for all photo degradation related
drop in OPV efficiency.
Under inert atmosphere- OPV are stable !!
MO Resse et al SOLMAT 2010/Krebs et al SOLMAT 2008
Summary

15. 15
What questions we wanted to address?

16. 16
Are polymer based cells stable under inert atmosphere and light?
If Unstable-What’s the reason of instability?
Which particular photon are harmful for OPV’s ?
What are the functional relationship between DOS and light intensity
Can we recover the loss of solar cell performances?
What could be the potential WAYOUT for photo-degradation?
Objectives..

17. 17
Environmental Chamber

18. 18
Can reach pressure to 1E-6 Torr with all optical & electrical systems inside
Degradation Chamber at Microelectronics Research Center
Such Low Pressure helps to eliminate the effect of moisture and oxygen
Except EQE all electrical measurement was done in-situ
ENVIRONMENTAL CHAMBER

19. 19
Initial Results

20. Voc
Isc
5%
12%
CHANGE IN LIGHT IV ( BIAS DEPENDENCE)

21. CHANGE IN BLEND ABSORBANCE UPON SOAKING

22. Before Exposure mt - 7.5×10-10 cm2/v
After Exposure mt - 1.5 ×10-10 cm2/v
Hetch Expression
5 times
CHANGE IN MOBILITY-LIFETIME

23. 0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
0 500 1000 1500
Mobility(cm2/V-s)
Time (min)
CHANGE IN MOBILITY

24. Io Before – 9.7E-12 Amps
Io After – 4.5E-11 Amps
No change observed in calculated
Eu( Urbach Energy)
5 times
CHANGE IN DARK CURRENT & SUBGAP EQE

25. De-noised DOS DataC-f Raw Data
Indicating the presence of mid-gap states away from
valence band of P3HT (Polymer)
Mid-gap
States
Contribution to
capacitance
CHANGE IN DOS SPECTRA

26. 26
Need extra electric field for separation and collection.
Indicating a loss mechanism at interface
e
BIAS DEPENDENT CURRENT(WHY?)

27. EFFECT OF DIFFERENT WAVELENGTH EXPOSURE
Blue photon is detrimental to active layer photo degradation

28. 28
More Results

29. 29
INTEGRATED DOS(in-situ) DIFFERENT INTENSITY
Study done on Regular PEDOT:PSS based cells

30. 30
0.3 0.4 0.5 0.6 0.7
1E16
1E17
1E18
CONTROL
EXP-0.5X
EXP-1X
EXP-2X
EXP-4X
DOS(cm
-3
ev
-1
)
E-Ehomo
(ev)
DEGRATION BY DIFFERENT INTENSITY

31. 31
DEGRATION BY DIFFERENT INTENSITY
0 2 4
1.0x10
16
4.0x10
16
7.0x10
16
INVERTED CELL
REGULAR CELL
INTEGRATEDDOS(cm-3ev-1)
INTENSITY OF EXPOSURE
CONTROL FOR INVERTED BATCH

32. 32
Street et al- APL 2013
DOS –Intensity relationship found in literature

33. 33
0.0 0.5
1E-12
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
Current(amps)
Volt(volt)
TYPICAL DARK CURRENT OF CONTROL DEVICE
due to series
resistance
𝐼𝑜1 exp −
𝑞𝑉
𝑛1 × 𝐾𝑇
+ 𝐼𝑜2exp(−
𝑞𝑉
𝑛2 × 𝐾𝑇
)

34. 34
0 2 4
10
-11
10
-10
10
-9
Io( reverse saturation current ( low voltage regime)
n( ideality factor at low voltage regime)
Exposure Intensity(X-sun)
Io1(amps)
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
n1(idealityfactor)
0 2 4
1E-13
1E-12
1E-11
Io2( Higher voltage regime)
n2 ( Higher Voltage regime)
Io(amps
1.0
1.2
1.4
1.6
1.8
2.0
EXPOSURE INTENSITY(X- SUN)
n(Idealityfactor)
Change in Io1 & n at Low voltage region

35. 35
Change in Io2 & n2 at High voltage region
0 2 4
1E-13
1E-12
1E-11
Io2( Higher voltage regime)
n2 ( Higher Voltage regime)
Io(amps
1.0
1.2
1.4
1.6
1.8
2.0
EXPOSURE INTENSITY(X- SUN)
n(Idealityfactor)
Band-band recombination dynamics remains roughly same

36. 36
Prolonged light soaking creates excess mid-gap
recombination center facilitating SRH
recombination
“ UV PHOTON breaks C-H bonds, and that excess H can bind to other sites
on the polymer creating localized Electronic states” –Street et al- APL 2013
What we learnt?

37. 37
Recovering solar cell performance by
post degradation –thermal anneal.
What we planned ?

38. 38
POST PRODUCTION ANNEALING
&
PERFORMANCE RECOVERY

39. 39
Cathode area 0.125 cm2
EFFECT OF PPA(ON PHOTO DEGRADED CELL)

40. 40
2x-96 Hours
CHANGE IN DOS UPON PPA

41. 41
EFFECT OF PPA ON SUCCESSIVE PPAAT DIFFERENT
TEMPERATURE

42. 42
CONSISTENT CHANGE IN DOS AND Io

43. PPA
POST PRODUCTION ANNEALING

44.  STANDARD
CELL
 INVERTED
CELL
PPA CONDITION-100 C -15 MIN INSIDE GB
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
Standard Cell
Cell With extended Annealing
PEDOT-ACTIVE LAYER-Ca(25 nm)/Al
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
VOLTAGE(VOLT)
CURRENT(ma)
PEDOT-ACTIVE LAYER-Ca(25 nm)/Al
PRISTINE
AFTER POST PROD ANNEALING
PPA
APPLIED
NO PPA,
EXTENDED
ANNEAL STEP
CHANGE IN Voc
TO SEE THE EXTRA EFFECT OF EXTENDED ANNEALING
( PPA)- EFFECT ON ACTIVE LAYER

45.  STANDARD
CELL
 INVERTED
CELL
PPA CONDITION-100 C -15 MIN INSIDE GB
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
PRISTINE
AFTER POST PROD ANNEALING
CURRENT(ma)
VOLTAGE(VOLT)
PEDOT-ACTIVE LAYER-Ca(25 nm)/Al
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
PRISTINE
AFTER POST PROD ANNEALING
CURRENT(ma)
VOLTAGE(VOLT)
PEDOT- ACTIVE LAYER -Al
NOTICED THIS?
( PPA)- EFFECT ON DIFFERENT CATHODE

46.  STANDARD
CELL
 INVERTED
CELL
PPA CONDITION-100 C -15 MIN INSIDE GB
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
VOLTAGE(VOLT)
CURRENT(ma)
Ca 10 nm
PRISTINE
AFTER POST PROD ANNEALING
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
CURRENT(ma)
VOLTAGE(VOLT)
PRISTINE
AFTER POST PROD ANNEALING
Ca 25 nm
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
VOLTAGE(VOLT)
CURRENT(ma)
PRISTINE
AFTER POST PROD ANNEALING
Ca 50 nm
ALMOST SAME DROP IN PERFORMANCES
( PPA)- EFFECT ON DIFFERENT ETL THICKNESS

47.  STANDARD
CELL
 INVERTED
CELL
PPA CONDITION-100 C -15 MIN INSIDE GB
-0.8 -0.4 0.0 0.4
0.0
0.5
1.0
1.5
Current(mA)
Voltage (volt)
Pristine
Post production annealing
10 nm MoOx
-0.8 -0.4 0.0 0.4
0.0
0.5
1.0
1.5
Voltage (volt)
Current(mA)
20 nm MoOx
Pristine
Post production annealing
-0.8 -0.4 0.0 0.4
0.0
0.5
1.0
1.5
Pristine
Post production annealing
Current(mA)
Voltage (volt)
30 nm MoOx
IDENTICAL DROP IN ALL THE THREE SAMPLES
( PPA)- EFFECT ON DIFFERENT HTL THICKNESS

48.  STANDARD
CELL
 INVERTED
CELL
PPA CONDITION-100 C -15 MIN INSIDE GB
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
1.6
Current(mA)
Voltage (volt)
Pristine
After PPA- 100C -15 min
Unannealed MoOx - 100 nm
-0.8 -0.4 0.0 0.4
0.0
0.4
0.8
1.2
1.6
Current(mA)
Voltage (volt)
Pristine
After PPA- 100C -15 min
Annealed MoOx - 100 nm -100C -10 min
( PPA)- EFFECT ON ANNEALED MoOx(HTL)

49. 49
-1.0 -0.5 0.0 0.5 1.0
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
CURRENT(mA)
Voltage(volt)
PRISTINE
POST EXP-2X-48 HRS
POST ANNEAL- 90C-1 HR
COMPLETE RECOVERY OF DEGRADED PERFORMANCE

50. 50
400 600 800
0.0
0.1
0.2
0.3
0.4
0.5
CONTROL
AFTER 2X EXP-48HRS
110C ANNEAL -1HR
90C ANNEAL-1HR
ABSEQE
WAVELENGTH (nm)
Jsc Recovery by THERMAL ANNEAL
Proof of Device active layer
being under annealed

51. 51
0.3 0.4 0.5 0.6 0.7
1E16
1E17
1E18
PRISTINE
POST EXP-2X -48HRS
POST 1HR-110C ANNEAL
POST 1HR-90C ANNEAL
DOS(cm
-3
ev
-1
)
E-Ehomo
(ev)
Proof of Device active laye
being under annealed
This region of defects are not entirely reversible.
CHANGE IN DOS AFTER THERMAL ANNEAL

52. 52
CHANGE IN DOS WITH DIFFERENT ANNEALING TEMP
LOW FREQUENCY CV(24Hz)INTEGRATED FROM CF DATA

53. 53
10
-9
10
-8
10
-7
10
-6
110C-1hr
90C-1hr
70C-1hr
2X-48hrs
I(-0.2V)-[AMPS]
reversebiasdarkcurrent
CONTROL
60C-1hr
CHANGE IN DOS AFTER THERMAL ANNEAL

54.  TCO
CHANGES
ARE MADE
IN TCO-
REST OF
THE
DEVICE
RECIPE
REMAINED
SAME
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
0.0
0.5
1.0
1.5
Current(mA)
Voltage(Volt)
ZnO:Al
Cs2
Co3
on ZnO:Al
Cs2Co3 on ITO
ONLY ZNO:AL AS ETL
( DIFFERENT TCO- ITO OR ZNO)

55.  TCO
Run No Time (Min) Rs ( Ω/□) Thickness(nm)
7964 20 6.44 1253
7965 15 10.05 933
7942 10.5 11.78 640
7967 5 43.67 312
MORE & MORE DROP IN BLUE PHOTON
TRANSMISSION AND DECREASE IN
SERIES RESISTANCE- WITH INCREASE IN
ZNO:AL THICKNESS
ZNO:AL FILM OPTIMIZATION

56.  TCO
-0.8 -0.4 0.0 0.4
0.0
0.5
1.0
1.5
Current(mA)
Voltage (volt)
20 min
15 min
5 min
10.5 min
400 500 600 700 800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
AbsEQE
Wavelength (nm)
20 min run -ZnO:Al
10 min run-ZnOLAl
5 min run-ZnO:Al
WITH INCREASE IN ZNO:AL THICKNESS
FF IMPROVES WITH DROP IN PHOTO CURRENT AT LOWER WAVELENGTH
CELL PERFORMANCES

57.  TCO
0 500 1000 1500 2000
0.90
0.92
0.94
0.96
0.98
1.00
THICK ZNO-20 MIN
THIN ZNO -10 MIN
CONTROL -ITO
NORMALIZEDDROPINVoc
TIME IN MIN
0 500 1000 1500 2000
0.75
0.80
0.85
0.90
0.95
1.00
THICK ZNO-20 MIN
THIN ZNO -10 MIN
CONTROL -ITO
NORMALIZEDDROPINVoc
TIME IN MIN
Isc
THICKER ZNO:AL BASED DEVICE MORE PHOTO
STABLE WITH HIGHER EFFICIENCY
2X- PHOTO DEGRADATION

58. 58
CONCLUSIONS..
1. Photo degradation is an active layer property, independent of oxygen and moisture.
2. The process is entirely reversible and related to change in recombination center DENSITY at
D-A interface
3. Follows a non linear DOS pattern with light intensity which is an excellent match with
previous work on both in-organic and organic solar cells.
Regarding Solutions...
Glass + ITO
Polymer cell
a-Si Cell
Efficient tandem structure – ( with >7% efficiency)

59. 59
Future work..
1. Injecting controlled oxygen in the degradation chamber and see the effect of oxygen in
addition to light.
2. Studying the effect of fullerene in the photo-degradation process.
3. Understanding the effect of cathode material on photo-degradation.
4. Photo-induced structural changes in the active layer with the help of Auger
microscopy, XPS, XRD technique.
5. Studying degradation of new polymer system with higher efficiency like PTB7 etc.
6. Studying photo-degradation on hybrid tandem junction solar cells.

60. 60
 Sincerely thanks to my advisor Dr. Vikram Dalal for giving me opportunity to
work under his guidance on the OPV project.
 To my team mates Bob Mayer, Mehran Samiee , Pranav Joshi & Balaji
Ganapathy
 Thanks to Max Noack and Bob Mayer for setting up the degradation chamber
and for many encouraging discussion.
 Thanks to my family and friends & all students with whom I worked at MRC.
Acknowledgement

61. 61
2010
R. Biswas, J.Bhattacharya , B.Lewis , N.Chakravarty , V.Dalal- Solar Energy Materials & Solar Cells 94
(2010)2337–2342-“ Enhanced nano crystalline silicon solar cell with a photonic crystal back-reflector”.
2011
J Bhattacharya, N Chakravarty, S Pattnaik, W. D Slafer, R Biswas and V L. Dalal- APPLIED PHYSICS
LETTERS 99, 131114 (2011)-“ A photonic-plasmonic structure for enhancing light absorption in thin film
solar cells”.
2012
J. Bhattacharya, N. Chakravarty , S. Pattnaik , W.D. Slafer , R. Biswas ,V. Dalal- In Press - Journal of Non-
Crystalline Solids-2012. “Comparison of optical properties of periodic photonic–plasmonic and randomly
textured back reflectors for nc-Si solar cells”.
2012
J. Bhattacharya, R. W. Mayer, M. Samiee, and V. L. Dalal –“Appl. Phys. Lett. 100, 193501 (2012)” –“Photo-
induced changes in fundamental properties of organic solar cells”.
List of Selected Publications

62. 62
Thanks for your attention