effect of polymer molecular weight, binary processing additives, two-dimensional polymer on ...

145 Pages·2014·2.53 MB·English

EFFECT OF POLYMER MOLECULAR WEIGHT, BINARY PROCESSING
ADDITIVES, TWO-DIMENSIONAL POLYMER ON EFFICIENCY OF POLYMER
SOLAR CELLS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree
Master of Science

Chang Liu
May, 2014
EFFECT OF POLYMER MOLECULAR WEIGHT, BINARY PROCESSING
ADDITIVES, TWO-DIMENSIONAL POLYMER ON EFFICIENCY OF POLYMER
SOLAR CELLS
Chang Liu
Thesis

Approved: Accepted:

Advisor Department Chair
Dr. Xiong Gong Dr. Robert Weiss

Committee Member Dean of the College
Dr. Alamgir Karim Dr. Stephen. Z. D. Cheng

Committee Member Dean of the Graduate School
Dr. Zhu Yu Dr. George R. Newkome

Date

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ABSTRACT
In recent years, bulk heterojunction (BHJ) polymer solar cells (PSCs) have received
great attention from both academic and industrial sectors as they are flexible and have
large scale production potential and low-cost grid power generation. However, PSCs are
still inferior to their inorganic counterparts in efficiency and stability. In order to advance
large-scale commercialization and implementation, the power conversion efficiency
(PCE) and stability of PSCs must be improved.
There are four chapters in this thesis.
In Chapter 1, the development of PSCs with an inverted device structure was
overviewed.
In Chapter 2, we study the effect of polymer molecular weight on the efficiency of
PSCs. It was found that the PSCs performances are largely dependent on the molecular
weight of PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-
diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]). PCE significantly
enhanced from 5.41% to 6.27% and 8.50% along with the MW of PTB7 increased from
18 kg/mol to 40 kg/mol and 128 kg/mol, respectively. This enhancement is attributed to
the enhanced light absorption and increased charge carrier mobility of PTB7 with high
MW, and a proper phase separation in BHJ composite of PTB7:PC BM ([6,6]-phenyl-
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C butyric acid methyl ester) interpenetrating network. Then, by using binary solvent
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additives to tune the phase separation domains between PTB7
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and PC BM, we successfully boosted PCE from 5.60% (pristine device) to 8.55% (BHJ
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composites with binary additives).
In Chapter 3, we report the influence of binary additives: 1,8-octanedithiol (ODT)
and 1-chloronaphthalene (CN) on the performance of PSCs. It was found that
performance of PSCs based on PTB7:PC BM as the active layer can be largely
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improved with incorporation of binary additives ODT and CN. PCE increased from 5.60%
to 8.55%, which originates from ameliorated thin film morphology of active layer. With
binary additives, crystallinity of PTB7 is demonstrated to be elevated, which in turn
facilitates charge transport within the BHJ layer. Moreover, the phase separation is
testified to form a more homogenous film. Through the optimization of film morphology,
recombination loss mechanisms in PSCs were proved to be drastically lowered, with the
elimination of geminate recombination and decreasing the nongeminate recombination to
the lowest level.
In Chapter 4, we report single-junction PSCs with high short circuit current density
(J ) large open circuit voltage (V ) and high fill factor (FF) by combining
SC OC
complementary materials design, synthesis, and device engineering strategies. A two-

dimensional donor-acceptor conjugated copolymer, poly[[2,6’-4,8-di(5-

ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2[(2-

ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-DT) was developed and utilized

in BHJ PSCs. Introduction of 5-alkylthiophene-2-yl unit as a conjugated side chain

lowers the bandgap and facilities the π-π stacking, and improves the charge carrier

mobility of PTB7-DT. These results are consistent with the observation from first-

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principle calculations. Consequently, the resulting polymer exhibits considerably better
photovoltaic performance with PCE of 10.12 % from single-junction PSCs.

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PUBLICATIONS
1. Chang Liu, Kai Wang, Xiaowen Hu, Wei Zhang, Yali Yang, Steven Xiao, Xiong
Gong* and Yong Cao, “Molecular Weight Effect on the Efficiency of Polymer Solar
Cells”, ACS Appl. Mater. & Interfaces 2013, 5, 12163-12167.
2. Chang Liu, Chao Yi, Yali Yang, Ram S. Bhatta, Kai Wang, Mesfin Tsige, Steven Xiao
and Xiong Gong*. “A Novel Donor-Acceptor Conjugated Polymer for Single-
Junction Polymer Solar Cell with 10% Power Conversion Efficiency”, Adv. Energy
Mater. 2014, under revision.
3. Chang Liu, Chengmei Zhong, Xiaowen Hu, Mingjun Huang, Zhan Zhang, Kai Wang,
Xiong Gong*and Alan J. Heeger, “Influence of Binary Processing Additives on the
Recombination in Polymer Solar Cells”. ACS NANO, 2014, Submitted.
4. Chang Liu, Kai Wang, Tianyu Meng, Zhan Zhang and Xiong Gong*, “Inverted
Polymer Solar Cell”, Chem. Soc. Rev, 2014, Submitted.
5. Hangxing Wang, Xinfei Yu, Chao Yi, He Ren, Chang Liu, Yali Yang, Steven Xiao, Jie
Zheng, Alamgir Karim, Stephen Z. D. Cheng, Xiong Gong*, “Fine-Tuning of
Fluorinated Thieno[3,4-B]thiophene Copolymer for Efficient Polymer Solar Cells”,
Journal of Physical Chemistry C 2013, 117, 4358-4363.
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6. Kai Wang, He Ren, Chao Yi, Chang Liu, Hangxing Wang, Lin Huang, Haoli Zhang,
Alamgir Karim and Xiong Gong*, “Solution-Processed Fe O Magnetic Nanoparticle
3 4
Thin Film Aligned by an External Magnetostatic Field as a Hole Extraction Layer for
Polymer Solar Cells”, ACS Appl. Mater. & Interfaces2013,5, 10325–10330.

7. Praveen Pitliya, Yan Sun, Jose C Garza, Chang Liu, Xiong Gong, Alamgir Karim

and D. Raghavan*, “Synthesis and Characterization of Novel Fulleropyrrolidine in

P3HT blended Bulk Heterojunction Solar Cells”, Polymer 2014, 55, 1769-1781.

8. Xiaowen Hu, Kai Wang, Chang Liu, Tianyu Meng, Yang Dong, Shengjian Liu, Fei

Huang and Xiong Gong*, “High-Detectivity Inverted Near-Infrared Polymer

Photodetectors using Cross-Linkable Conjugated Polyfluorene as an Electron

Extraction Layer”, ACS Appl. Mater. & Interfaces, 2014, Submitted.

9. Xiong Gong, Kai Wang and Chang Liu, provisional patent application for UA 1119,

“Solution-Processed Perovskite Based Organic Inorganic Hybrid Photodetectors”.

This application was filed 3/12/2014, USPTO: 61/951,567.

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ACKNOWLEGEMENTS
I would like to express my appreciation to my advisor Dr. Xiong Gong for his
invaluable supervision, both in scientific research and in daily life, and the benefit from
his suggestion let me get confident in myself. Also I would like to thank my committee
members, Dr. A. Karim and Dr. Y, Zhu. I’m also very grateful for Dr. S. Cheng for wide-
angle x-ray diffraction measurement.
I would like to acknowledge Prof. Alan J. Heeger at the University of California
Santa Barbara (UCSB) for collaboration in the investigation of charge recombination
mechanism in polymer solar cells. I am grateful for Dr. Chengmei Zhong at UCSB for
transient absorption measurement. I would like to appreciate Dr. Yali Yang and Dr.
Steven Xiao from 1-Material Inc. for providing materials. I also would like to appreciate
Prof. Mesfin Tsige and Dr. Ram S. Bhatta for their collorborations.
In addition, I appreciate collaborations and suggestions from our group members.
Finally, I would like to express my gratitude towards my beloved parents for their
patience and financial support to make this thesis complete.
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TABLE OF CONTENTS
Page

LIST OF FIGURES ......................................................................................................... xiii

CHAPTER

I. INVERTED POLYMER SOLAR CELLS ...................................................................... 1

1.1 Energy Source ........................................................................................................... 1

1.2 Solar Energy & Solar Cells ....................................................................................... 2

1.3 Device Structures for Polymer Solar Cells ............................................................... 3

1.3.1 Conventional device structure ............................................................................ 3

1.3.2 The inverted device structure.............................................................................. 5

1.3.3 Top-illuminated device structure ........................................................................ 7

1.3.4 Bottom-illuminated device structure .................................................................. 9

1.4 Electrode Buffer Layer ............................................................................................ 10

1.4.1 Electron transfer/extraction layer ......................................................................11

1.4.1.1 Transition metal oxides ...............................(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:20)(cid:20)

1.4.1.2 Alkali-metal compounds ............................................................................ 20

1.4.1.3 Ultrathin layer ............................................................................................ 23

1.4.2 Hole transfer/extraction layer ........................................................................... 24

1.4.2.1 n-type transition metal oxides .................................................................... 24

1.5 Interfacial Modification Layer ................................................................................ 33

1.5.1 Non-cross-linkable self assembled monolayers ............................................... 33
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1.6 Processing Active Layer .......................................................................................... 54

1.6.1 Thermal annealing ............................................................................................ 55

1.6.2 External electric field ........................................................................................ 58

1.6.3 Solvent additives............................................................................................... 60

1.7 Top Anode Electrode ............................................................................................... 62

1.7.1 Silver nanowires ............................................................................................... 63

1.7.2 PEDOT:PSS ...................................................................................................... 66

1.7.3 Dielectric/metal/dielectric structure ................................................................. 68

1.8 Device Stability ....................................................................................................... 70

1.8.1 Shelf stability .................................................................................................... 71

1.8.2 Operation stability ............................................................................................ 72

1.8. Conclusion .............................................................................................................. 73

II. INFLUENCE OF POLYMER MOLECULAR WEIGHT ON THE EFFICIENCY OF
POLYMER SOLAR CELLS ............................................................................................ 74

2.1 Motivation for the research ..................................................................................... 74

2.2 Experimental section ............................................................................................... 75

2.2.1 Molecular weight measurement ........................................................................ 75

2.2.2 Cyclic voltammetry .......................................................................................... 75

2.2.3 UV-vis absorption spectra ................................................................................ 76

2.2.4 Thin film morphology ...................................................................................... 76

2.2.5 Charge carrier mobility ..................................................................................... 76

2.2.6 PSCs fabrication and characterization .............................................................. 77

2.3.1 Synthesis and characterization of PTB7 ........................................................... 78

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