Exciton Diffusion Length in SolutionProcessed Small Molecules Organic Semiconductors Jason Lin Advisor: Thuc-Quyen Nguyen November 19, 2014 1
Why Study Organic Solar Cells? • Renewable energy source • Cleaner than fossil fuels • Lower fabrication cost than inorganic solar cells 2 sustainablecommunitydevelopmentgroup.org/wordpress/?page_id=99 earthtimes.org/pollution/record-global-fossil-fuel-emissions-2010/1685/
Device Operation of Organic Solar Cell Al 4
Acceptor
_ +
1 Donor (1) Charge generation (2) Exciton diffusion (3) Charge separation (4) Charge extraction and collection
3
_ +
2 4
ITO Glass 3
Main Projects in Thesis Systematic Study of Exciton Diffusion Length in Organic Semiconductors by Six Experimental Methods2
22 20
Exciton Diffusion Length [nm]
Effect of Structural Variation on Photovoltaic Characteristics of Phenyl Substituted Diketopyrrolopyrroles1
18 16 14 12 10 8 6 C6PT1C6
C6PT2C6
EHPT2C6
4
Temperature Dependence of Exciton Diffusion in a Small Molecule Organic Semiconductor Processed With and Without Additive3 8
Exciton Diffusion Length in Small Molecules Used in High Performance Organic Solar Cells4 C8H17
S N
C8H17
C8H17
O
S
S C8H17
S C8H17
C8H17
O
S NC
O
S
C8H17
S S
S
N
S
O
S
S
S O
C8H17
C8H17
O
CN H17C8 O
O S
S
S
C8H17
C8H17 S
S
S
S O
O
C8H17
C8H17
DR3TBDT
DCAO3TBDT
1D LD (nm)
6 S
N
N
S
S N
2
S
S
N
N
N
Si
S
N
N
S
DTS(PTTh2)2
4 S
N
Si
S N
S
S
S
S
S
S
S
S F
DTS(FBTTh2)2
S N
S N
N
N
Si
S S
N
Si
S
p-DTS(PTTh2)2
N
N
N
N
S
S
S F
S
F
S
S
S N
S F
S
S
p-DTS(FBTTh2)2 F
0
Si
S
N
S N
S S
0
100 200 Temperature (K)
300
N
N S
Si
S F
S
S
p-SIDT-(FBTTh2)2
1. Lin, J. D. A.; Liu, J.; Kim, C.; Tamayo, A. B.; Proctor, C. M.; Nguyen, T.-Q. T. RSC Adv. 2014, 4, 14101–14108. 2. Lin, J. D. A.; Mikhnenko, O. V.; Chen, J.; Masri, Z.; Ruseckas, A.; Mikhailovsky, A.; Raab, R. P.; Liu, J.; Blom, P. W. M.; Loi, M. A.; García-Cervera, C. J.; Samuel, I. D. W.; Nguyen, T.-Q. Mater. Horiz. 2014, 1, 280–285. 3. Lin, J. D. A.; Mikhnenko, O. V.; Van der Poll, T. S.; Bazan, G. C.; Nguyen, T.-Q. (Submitted) 4. Lin, J. D. A.; Li, Zhi; Mikhnenko, Oleksandr V.; Poll, Tom S. van der; Bazan, Guillermo C; Nguyen, Thuc‐Quyen. MRS Communications 2014, (Near submission).
4
Project 1
Systematic Study of Exciton Diffusion Length in Organic Semiconductors by Six Experimental Methods
5
Goal. Understand Relationship + _
5 nm
?
Exciton Diffusion Length
Chemical Modification
+ _
Chemical Structure
15 nm
6
Challenges with Current Literature • Compounds differ my a number of chemical modifications • Exciton diffusion length measured by different techniques Publication 1 Technique 1
Compound 1
5 nm
Publication 2 Technique 2
Compound 2
10 nm 7
Guiding Research Questions • How do different techniques compare with each other? Technique 1
Vs.
Technique 2 Vs. Technique 3
Vs. Technique 4
• How does chemical structure impact exciton diffusion length? Compound 1
Vs.
Compound 2
Vs.
Compound 3
8
Project Proposal • Compare six different techniques to measure the exciton diffusion length in a class of DPP small molecules. Technique 1 Fabrication
Measurement
Analysis
Compound 1
EDL?
Compound 2
EDL?
Compound 3
EDL?
9
Materials
Reduce Conjugation Length
B
Increase Molecular Bulkiness
A
C
10 1. Kim, C.; Liu, J.; Lin, J.; Tamayo, A. B.; Walker, B.; Wu, G.; Nguyen, T.-Q. Chem Mater 2012, 24, 1699–1709. 2. Lin, J. D. A.; Liu, J.; Kim, C.; Tamayo, A. B.; Proctor, C. M.; Nguyen, T.-Q. T. RSC Adv. 2014, 4, 14101–14108.
Techniques 1. Steady State Photoluminescence Surface Quenching (SS-SQ) 2. Time Resolved Photoluminescence Surface Quenching (TR-SQ) 3. Exciton-Exciton Annihilation (EEA) 4. Bulk Quenching with Monte Carlo Simulation (BQMC) 5. Bulk Quenching with Stern-Volmer Analysis (BQ-SV) 6. Förster resonance energy transfer (FRET) 11
Steady State Photoluminescence Surface Quenching (SS-SQ)
12
Fabrication Quencher
C60 Organic Semiconductor
Quencher
Quartz
Quartz
Evaporate C60
Spin Coat 13
Fabrication
Quencher
Organic Semiconductor
Quencher
Organic Semiconductor
Quencher Organic Semiconductor
Quartz
Thick Donor Film
Quartz
Quartz
Quencher Organic Semiconductor Quartz
Thin Donor Film
14
Photoluminescence and Quenching Under Room Light
Under UV Light
1
1
0.8
0.8
0.6 PL
0.6 PL
PL
PL
+ _
Organic Semiconductor Quartz
With PCBM Layer
Without PCBM Layer
C60
0.4
0.4
0.2
0.2
0
0
500 700 900 Wavelength (nm) Wavelength (nm)
500 700 (nm) 900 Wavelength Wavelength (nm) 15
+ _
Surface Quenching. Measurement 1 0.8
PL
Detector
0.6 PL 0.4
457 nm
0.2
0
Wavelength Wavelength (nm)
UV Filter
With Quencher
Without Quencher
1
1
0.8
0.8
0.6 PL
0.6 PL
PL
PL
Argon Laser
0.4
0.4
0.2
0.2
0
0
Wavelength Wavelength (nm)
Wavelength Wavelength (nm)
16
Quenching Efficiency 1
PL
0.8 0.6 PL 0.4
𝑄 =1−
0.2
0
Wavelength Wavelength (nm)
1
𝑄𝐸𝑥𝑝
PL
0.8 0.6 PL
𝑃𝐿𝑞𝑢𝑒𝑛𝑐ℎ𝑒𝑟 𝑑𝑥 𝑃𝐿𝑝𝑟𝑖𝑠𝑡𝑖𝑛𝑒 𝑑𝑥
𝐴𝑟𝑒𝑎 =1− 𝐴𝑟𝑒𝑎
0.4 0.2
0
Wavelength Wavelength (nm)
17
Analysis
n(x)
x
𝑰𝒇 𝑳𝑫 is Then 𝑸 𝒊𝒔 4 nm 0.3 5 nm 0.4 6 nm 0.5 7 nm 0.7
2 𝑛(𝑥) 𝑑 𝐿2𝐷 −𝑛 𝑥 +𝐺 𝑥 𝜏 =0 𝑑𝑥 2
18
Analysis Measurement
Model
1
𝐿2𝐷
PL
0.8 0.6
𝑑 2𝑛(𝑥) −𝑛 𝑥 +𝐺 𝑥 𝜏= 0 𝑑𝑥 2
0.4 0.2
n(x)
0 Wavelength (nm)
𝑸
=
−
= 0.5 Goal
x
𝑰𝒇 𝑳𝑫 is Then 𝑸 𝒊𝒔 4 nm 0.3 5 nm 0.4 6 nm 0.5 7 nm 0.7
19
Verdict on SS-SQ Technique • Fabrication: – Requires many films (10-20) – Difficult to find an efficient and robust quenching layer
• Measurement: – Demanding optical alignments – Multiple measurements needed
• Analysis: – Direct fitting of LD 20
Techniques 1. Steady State Photoluminescence Surface Quenching (SS-SQ) 2. Time Resolved Photoluminescence Surface Quenching (TR-SQ) 3. Exciton-Exciton Annihilation (EEA) 4. Bulk Quenching with Monte Carlo Simulation (BQMC) 5. Bulk Quenching with Stern-Volmer Analysis (BQ-SV) 6. Förster resonance energy transfer (FRET) 21
Bulk Quenching with Monte Carlo Simulation (BQ-MC)
22
Fabrication PCBM
Quartz
Quartz
Quartz
Quartz
23
PCBM = Phenyl-C61-butyric acid methyl ester
Measurement Titanium Sapphire Laser
Detector
PL
400 nm
Time
Monochromator
𝑡 −𝜏 𝑒
𝑃𝐿(𝑡) = 𝑃𝐿0
PL
PL Time
Time 24
Quenching Efficiency
𝑄 =1−
𝑃𝐿𝑞𝑢𝑒𝑛𝑐ℎ𝑒𝑟 𝑑𝑡 𝑃𝐿𝑝𝑟𝑖𝑠𝑡𝑖𝑛𝑒 𝑑𝑡
High PCBM Concentration
Quartz
Quartz
Quartz PL
PL
Low PCBM Concentration
PL
No PCBM
Time
Time
𝑄𝐿
𝑃
Time
𝐴𝑟𝑒𝑎 =1− 𝐴𝑟𝑒𝑎
𝑄
𝑸𝑳
𝑸
𝑃
𝒊
𝐴𝑟𝑒𝑎 = 1− 𝐴𝑟𝑒𝑎 25
Relative Quenching Efficiency Q
Quenching Efficiency 1 0.8 0.6
0.4 0.2 0 1.E-05
1.E-04 1.E-03 1.E-02 PCBM volume fraction, %
1.E-01
26
Analysis. Monte Carlo Simulation + _
Low D
Low Q Quartz
+ _
Medium D
Medium Q
Quartz
+ _
High D
High Q Quartz
27
Analysis Monte Carlo Modeling
Measurement QExp = 0.5 Goal
If D is
Then 𝑸 𝒊𝒔
10-5 10-4 10-3
0.2 0.5 0.6
𝐿𝐷 = 𝐷𝜏 =
10−4 𝜏 = 6 nm
28
Verdict on BQ-MC Technique • Fabrication: – Can be done with 8 films – Facile and robust
• Measurement: – Time-resolved photoluminescence is less sensitive to orientation than Steady-state Photoluminescence
• Analysis: – Can model films with multi-exponential decay 29
Techniques 1. Steady State Photoluminescence Surface Quenching (SS-SQ) 2. Time Resolved Photoluminescence Surface Quenching (TR-SQ) 3. Exciton-Exciton Annihilation (EEA) 4. Bulk Quenching with Monte Carlo Simulation (BQMC) 5. Bulk Quenching with Stern-Volmer Analysis (BQ-SV) 6. Förster resonance energy transfer (FRET) 30
Technique Comparison
22 22
18 16 14
12 10 8 6 4
𝐿𝐷 = 𝐷𝜏
Length[nm] DiffusionLength ExcitonDiffusion [nm] Exciton Exciton Diffusion Length [nm] Exciton Diffusion Length [nm]
Exciton Diffusion Length [nm]
20
A
20 22 2222 18 18 A AA 20 2020 16 Technique16 Symbol 2218 1818 14 SS-SQ 14 A 2016 1616 12 TR-SQ 12 EEA 1814 1414 10 BQ-MC 10 1612 1212 8 BQ-SV 8 1410 1010 6 FRET Theory 6 12 8 88 4 4 10 6 66
Exciton Diffusion Length [nm] Exciton Diffusion Length [nm]
22
20
A
Lin, J. D. A.; Mikhnenko, O. V.; Chen, J.; Masri, Z.; Ruseckas, A.; Mikhailovsky, 8A.;4Raab, 44 R. P.; Liu, J.; Blom, P. W. M.; Loi, M. A.; García-Cervera, C. J.; Samuel, I. D. W.; Nguyen, 6 T.-Q. Mater. Horiz. 2014, 1, 280–285.
B
B
31
Results • Comprehensive comparison of fabrication, measurement, and analysis with different techniques. • Exciton diffusion length should only be compared when measured by same technique. • Surface quenching techniques which require elaborate fabrication procedures, multiple measurements, and a number of assumptions in the analysis process. • Monte Carlo is the technique of choice due to facile fabrication and robust analysis. Ideal for systematic studies. • Exciton diffusion length correlates with relative degree of molecular ordering. 32
Future work • How does the analysis procedure between the works compare mathematically? • How does the photocurrent method compare with the photoluminescent techniques studied here? • Can exciton be measured in actual bulk heterojunction film rather than just bilayers or dilute PCBM films? 33
Project 2 Temperature Dependence of Exciton Diffusion in a Small Molecule Organic Semiconductor Processed With and Without Additive
34
Motivation DTS(FBTTh2)2
35 Van der Poll, T. S.; Love, J. A.; Nguyen, T.-Q.; Bazan, G. C. NonBasic High-Performance Molecules for Solution-Processed Organic Solar Cells. Adv. Mater. 2012, 24, 3646–3649.
Motivation 1 1 1 𝑐0 = − 4𝜋𝑟𝐷 𝜏𝑓 𝜏0
Legend: -Trap density (𝑐0 ) -Diffusion coefficient (D) -Pristine film lifetime (𝜏𝑓 ) -Solution lifetime (𝜏0 )
36 Mikhnenko, O. V.; Kuik, M.; Lin, J.; van der Kaap, N.; Nguyen, T.-Q.; Blom, P. W. M. Trap-Limited Exciton Diffusion in Organic Semiconductors. Adv. Mater. 2013, 1–6.
Goal. Understand Relationship
Processing Conditions
?
Exciton Diffusion Length
?
Trap Density
37
Guiding Research Questions • How does the mechanism of exciton diffusion in small molecules compare to polymers? • How does processing conditions impact exciton diffusion length? • What is the relationship between the exciton diffusion length and the exciton trap density?
38
Project Proposal • Used the BQ-MC and BQ-SV techniques to measure the temperature-dependent exciton diffusion length in a small molecule system which had previously shown significant enhancements in performance when processed with DIO
39
Fabrication
Spin-Drop Coated Film
Scratch off edge with sharp Teflon tweezers
Apply epoxy to edges of film
Add cover film
40
Fabrication Apply Scotch tape to section off measurement area
41
Measurement Cryostat
Flow Rate Controller
Cryopump
Liquid Helium Dewer
42
43
Temperature Dependence
8
5
As Cast DIO
6
3
1D LD (nm)
D × 10-4 (cm2/s)
4
2 1
4
2
0 0
-1 0
100 200 Temperature (K)
300
0
100 200 Temperature (K)
300
44
Lin, J. D. A.; Mikhnenko, O. V.; Van der Poll, T. S.; Bazan, G. C.; Nguyen, T.-Q. (Submitted)
Measuring Trap Density Trap Density = 𝒄𝟎 = 𝟒𝝅
𝑫 𝝉𝒇
D (cm2/s) Without DIO 4.6 With DIO 3.1 Condition
−𝝉
𝟎
LD (nm) 6.8 4.9
-Trap density (𝑐0 ) -Diffusion coefficient (D) -Pristine film lifetime (𝜏𝑓 ) -Solution lifetime (𝜏0 )
Trap Density (cm-3) 3.8 × 1017 15 × 1017
~ ×4
45
Results • Like polymers, exciton diffusion in small molecules consists of two parts: – (1) Downhill migration – (2) Thermally activated hopping
• Downhill migration is shorter in our small molecule system than a previously studied polymer system (MDMO-PPV). • Processing with DIO leads to a reduction in the exciton diffusion length • Processing with DIO leads to an increase in trap density 46
Future Work • What is the origin of these exciton trap states? • Are free charge carriers created at these trap sites? • How is an exciton trap related to charge trap?
47
Conclusions • Cleared the ambiguities between techniques used to measure exciton diffusion length. • Identified specific techniques which are idea for systematic studies of exciton diffusion length • Confirmed that the exciton diffusion length is directly limited by exciton trap states • Provided a clear roadmap for future studies 48
Acknowledgments •
•
•
• • •
UCSB: – Professor Thuc-Quyen Nguyen • Dr. Arnold Tamayo • Dr. Chunki Kim • Dr. Jianhua Liu • Dr. Oleksandr V. Mikhnenko • Reilly Raab • Dr. Mananya Tantiwiwat • Dr. Michelle Guide – Professor Carlos Garcia • Dr. Jingrun Chen – Dr. Alexander Mikhailovsky Zernike Institute for Advanced Materials, University of Groningen – Professor Maria Antonietta Loi – Professor Paul W. M. Blom School of Physics and Astronomy, University of St. Andrews – Professor Ifor Samuel • Zarifi Masri Nguyen & Bazan Group Family and friends Funding: National Science Foundation (NSF) Division of Materials Research and NSF-SOLAR
49
Thank you for your attention!
50
