Design and Synthesis ofNaphthobisthiadiazole-based Semiconducting Polymers
for Non-Fullerene Organic Photovoltaics（非フラーレン型有機太陽電池に向けた
ナフトビスチアジアゾール系ポリマーの開発）

March 2022

周 俐慧
LI-HUI CHOU

Department of Applied Chemistry, Graduate School of Engineering
Hiroshima University

ABSTRACT
π-Conjugated (semiconductive) polymers have attracted considerable attention because of
their solution-processability and excellent electronic properties. These interesting features
enable their application as semiconductive materials for organic electronic devices. A setof devices that strongly rely on the properties of semiconductive polymers is organic
photovoltaics, in which said polymers are typically used as a p-type (donor) material in
combination with fullerene derivatives or non-fullerene π-conjugated compounds as ntype (acceptor) materials. We have been focusing on a series of semiconductive polymers
based on naphthobisthiadiazole (NTz); these polymers exhibited power conversion
efficiencies (PCEs) of over 10% in combination with fullerene acceptors, which is among
the highest values obtained for fullerene-based cells. Based on this, we envisioned that theuse of NTz-based semiconductive polymers in photovoltaic cells combined with emerging
non-fullerene acceptors can provide even higher PCEs than those previously obtained withfullerene acceptors.
In this dissertation, we report the design and synthesis of two series of NTz-based
semiconductive polymers through different polymer designs, namely NTz-BDT polymers
and NTz-TPTz random copolymers. First, the NTz-BDT polymers exhibited betterisolubility because of their suppressed polymer crystallinity and weaker sidechaininterdigitation, compared with that of PNTz4T, which may afford more well-mixed
donor/acceptor blend films that are effective for increasing the photocurrent. In addition,
the NTz-BDT polymers exhibited deeper highest occupied molecular orbital (HOMO)
energy levels than PNTz4T, which was beneficial for improving the photovoltage. Second,
the NTz-TPTz random copolymers exhibited significantly higher solubility than the NTzBDT polymers even with the addition of only a small amount of the V-shaped TPTz unit
(2.5%–10%) into the PNTz4T backbone. Interestingly, NTz-TPTz random copolymers
were observed to exhibit a face-on backbone orientation that was favorable forphotovoltaic cells, whereas PNTz4T is known for an edge-on orientation. Consequently,
we obtained PCEs of approximately 13% and 11% for the NTz-BDT polymers and NTzTPTz random copolymers, respectively, which were higher than those for PNTz4T. Herein,
we discuss the structure–property–photovoltaic performance relationships of the polymers,in comparison with PNTz4T.

ii

TABLE of CONTENTS
ABSTRACT ····························································································· i
TABLE of CONTENTS ·············································································· iii
LIST of FIGURES ···················································································· vi
LIST of TABLES ······················································································ x
CHAPTER 1

INTRODUCTION ································································ 1

1-1

General ······················································································ 1

1-2

Evolution of Photovoltaic/Solar Cell Technology ····································· 3

1-3

Characteristics of Solar Cell ······························································ 7
1-3-1

Basic Introduction ································································ 7

1-3-2

Characteristics Curves ···························································· 8

CHAPTER 2

Introduction of Organic Solar Cells ···········································122-1Working Principle of Organic Solar Cells ·············································122-2Requirements for Semiconducting Polymers ·········································12

2-3

Semiconducting Polymers ·······························································15

2-4Evolution of Semiconducting Materials for Organic Solar Cells ···················16

2-5Naphthobisthiadiazole-based Semiconducting Polymers ···························18

CHAPTER 3
3-1

EXPERIMENTAL SECTION ··················································21

Materials ···················································································21
3-1-1

NTz Monomer ····································································21

3-1-2

NTz-BDT Polymers ·····························································22

3-1-3

NTz-TPTz Random Copolymers ··············································27

iii

3-2

3-3

Solar Cell Fabrication ····································································33
3-2-1

Solar Cell Fabrication of NTz-BDT Polymers ·······························33

3-2-2

Solar Cell Fabrication of NTz-TPTz Random Copolymers ················33

Instrumentation ············································································34
3-3-1

Cyclic Voltammetry Measurements ···········································35

3-3-2

Hole/Electron-Only Device Fabrication & Measurements ·················36

CHAPTER 4

PNTz4T in Non-Fullerene Solar Cells ········································37

CHAPTER 5

NTz-BDT Polymers ·····························································41

5-1

Physical Property Evaluation ····························································44

5-2

OPV Properties of NTz-BDT Polymers in Non-Fullerene System ················48

5-3

Reason for the Differences in OPV Performances ···································50

5-4

5-3-1

Charge Transport Ability ························································50

5-3-2Charge Recombination Behavior ··············································51

5-3-3

Thin-Film Structure ······························································53

Summary ···················································································60

CHAPTER 6
6-1

6-2

6-3

NTz-TPTz Random Copolymers ··············································61

Physical Property Evaluation ····························································64
6-1-1

Different Alkyl Sidechain on TPTz Unit······································65

6-1-2

Different TPTz Ratio ····························································68

OPV Properties of NTz-TPTz Polymers ···············································71
6-2-1

Different Alkyl Sidechain on TPTz Unit······································71

6-2-2

Different TPTz Ratio ····························································73

Charge Recombination Behavior ·······················································75
iv

6-4

6-5

6-3-1

Different Alkyl Sidechain on TPTz Unit······································75

6-3-2

Different TPTz Ratio ····························································77

Thin Film Structure ·······································································78
6-4-1

Different Alkyl Sidechain on TPTz Unit······································78

6-4-2

Different TPTz Ratio ····························································82

Summary ···················································································85

CHAPTER 7

CONCLUSION ··································································86

ACKNOWLEGEMENTS ···········································································87
REFERENCE··························································································88v

LIST of FIGURES
Figure 1 Annual renewable energy output and non-renewable energy storage, and global
energy consumption. 1 ................................................................................................................ 2
Figure 2 Generation of photovoltaic technology 2 .................................................................... 5
Figure 3 Best research-cell efficiency chart reported by NREL in November 2021, and themagnification for emerging PVs including OPVs...................................................................... 6
Figure 4 Basic structure of the solar cell. .................................................................................. 7
Figure 5 Characteristics current- voltage (I-V) curve under dark and light illumination. ......... 8
Figure 6 (a) Important parameters in the I–V curve and (b) a quantum efficiency spectrum. .. 9
Figure 7 Schematic diagram of band structure of organic photovoltaic. ................................ 13
Figure 8 Representation of backbone orientation for semiconducting polymers with respect
to the substrate.29 ...................................................................................................................... 14
Figure 9 (a) Chemical structures of representative semiconductive polymers. (b) Images oforganic electronics such as organic light-emitting diodes (OLEDs), organic field-effecttransistors (OFETs), and organic photovoltaics (OPVs).34-36 ................................................... 15
Figure 10 Chemical structure of semiconducting polymers, and non-fullerene acceptor
materials. .................................................................................................................................. 17
Figure 11 Chemical structure of acceptor unit: NTz. .............................................................. 18
Figure 12 Chemical structure of PNTz4T and a series of PNTz4T family: PNTz4T2F,
PNTz4T4F, PFN4T and PFN4T2F.17, 48, 61 ............................................................................ 19
Figure 13 Chemical structure of semiconducting polymer, PNTz4T; and non-fullerene
acceptor Y6............................................................................................................................... 37vi

Figure 14 UV-vis absorption spectra of the PNTz4T thin film. The absorption of Y6 is also
shown in the thin film spectra. ................................................................................................. 38
Figure 15 J−V curves of the optimized organic solar cells using the PNTz4T combined with
Y6. ............................................................................................................................................ 39
Figure 16 Chemical structure of PNTz4T and NTz-BDT polymers, i.e., PNTzBDT,PNTzBDT-F and PNTzBDT-Cl. ............................................................................................. 42
Figure 17 DSC thermograms of PNTz4T and NTz-BDT polymers. ...................................... 44
Figure 18 (a,b) UV-vis absorption spectra of the NTz-BDT polymers in the solution (a) andthin film (b). The absorption of Y6 is also shown in the spectra. (c) CV of the thin films for
the NTz-BDT polymers vs Ag/Ag+ as the reference electrode. ............................................... 46
Figure 19 Optimized geometry of the polymer backbone, and ELUMOs and EHOMOs for thecompounds calculated by the density functional theory (DFT) method at the B3LYP/6-31G(d)level. ......................................................................................................................................... 46
Figure 20 (a) J−V curves and (b) EQE spectra of the optimized organic solar cells using theNTz-BDT polymers combined with Y6. .................................................................................. 49
Figure 21 J–V curves of the (a) hole-only devices; and (b) electron-only devices using thepolymer/Y6 blend films based on the space-charge limited current (SCLC) model. .............. 51
Figure 22 Light-intensity-dependence of (a) JSC and (b) VOC for the polymer/Y6 devices. ... 52
Figure 23 GIWAXS patterns of (a-d) polymer neat films and, (e-h) polymer/Y6 blend films.
Corresponding intensity profiles along the out-of-plane (solid line) and in-plane (dotted lines)
direction of (i) polymer neat films and, (j) polymer/Y6 blend films. ...................................... 55
Figure 24 GIWAXS patterns of Y6 non-fullerene acceptor neat film. .................................... 55
Figure 25 TEM and AFM images of the Y6-blend films for (a, e) PNTzBDT/Y6, (b, f)vii

PNTzBDT-F/Y6, (c, g) PNTzBDT-Cl/Y6, and (d, h) PNTz4T/Y6. ...................................... 57
Figure 26 Contact angle of water and glycerol on PNTzBDT, PNTzBDT-F, PNTzBDT-Cl,
PNTz4T and Y6 films. ............................................................................................................. 58
Figure 27 Chemical structure of TPTz unit unit and the illustration of V-shape structure. .... 61
Figure 28 Chemical structure of PNTz4T and NTz-TPTz random copolymers. ................... 63
Figure 29 DSC thermograms of PNTz4T and NTz-TPTz random copolymers. .................... 64
Figure 30 (a,b) UV-vis absorption spectra of the NTz-TPTz random copolymers (different
alkyl sidechain on TPTz unit depend) in the solution (a) and thin film (b). (c) Cyclic
voltammograms of the thin films for the NTz-TPTz random copolymers vs Ag/Ag+ as thereference electrode. .................................................................................................................. 66
Figure 31 (a,b) UV-vis absorption spectra of the NTz-TPTz random copolymers in thesolution (a) and thin film (b). (c) Cyclic voltammograms of the thin films for the NTz-TPTz
polymers random copolymers vs Ag/Ag+ as the reference electrode....................................... 69
Figure 32 (a) J−V curves and (b) EQE spectra of the optimized organic solar cells using the
NTz-TPTz random copolymers combined with Y6. ................................................................ 72
Figure 33 (a) J−V curves and (b) EQE spectra of the optimized organic solar cells using the
NTz-TPTz random copolymers combined with Y6. ................................................................ 74
Figure 34 Light-intensity dependence of (a) JSC and (b) VOC for the polymer/Y6 devices. .... 76
Figure 35 Light-intensity-dependence of (a) JSC and (b) VOC for the polymer/Y6 devices. ... 77
Figure 36 GIWAXS patterns of (a-e) polymer neat films and, (f-j) polymer/Y6 blend films.
Corresponding intensity profiles along the out-of-plane (solid line) and in-plane (dotted lines)
direction of (k) polymer neat films and, (l) polymer/Y6 blend films. ..................................... 80
Figure 37 TEM & AFM images of polymer/Y6 blend films. (a) PNTz4T/Y6, (b) C8-5/Y6,
viii

(c) EH-5/Y6, (d) C12-5/Y6 and (e) BO-5/Y6. ........................................................................ 81
Figure 38 GIWAXS patterns of (a-d) polymer neat films and, (e-h) polymer/Y6 blend films.
Corresponding intensity profiles along the out-of-plane (solid line) and in-plane (dotted lines)
direction of (i) polymer neat films and, (j) polymer/Y6 blend films. ...................................... 83
Figure 39 TEM and AFM images of polymer/Y6 blend films. (a) C12-2.5/Y6, (b) C12-5/Y6,(c) C12-7.5/Y6 and (d) C12-10/Y6. ........................................................................................ 84
Figure 40 Chemical structure of (a) NTz-BDT polymers, and (b) NTz-TPTz random
copolymers. .............................................................................................................................. 86ix

LIST of TABLES
Table 1 Polymerization results of NTz-BDT polymers. .......................................................... 26
Table 2 Polymerization results of NTz-TPTz polymers. ......................................................... 32
Table 3 Optical and electrochemical properties of the polymer thin films.............................. 47
Table 4 Summary of photovoltaic parameters of the optimized polymer/Y6 devices. ........... 49
Table 5 Summary of contact angle of water and glycerol on each polymer, the surface
tensions of and the corresponding χ parameters of all blend films. ......................................... 59
Table 6 Solubility Estimation of PNTz4T and NTz-TPTz random copolymers..................... 63
Table 7 Optical and electrochemical properties of the polymer thin films.............................. 67
Table 8 Optical and electrochemical properties of the polymer thin films.............................. 70
Table 9 Summary of photovoltaic parameters of the optimized polymer/Y6 devices. ........... 72
Table 10 Summary of photovoltaic parameters of the optimized polymer/Y6 devices. ......... 74x

CHAPTER 1 INTRODUCTION
1-1 General
Over 200 years after the industrial evolution, people began to use petrochemicals as theirmain source of energy. In particular, the more technology flourished, the more energy
sources were required. Although petroleum is a powerful energy source and can be aconstant support for human beings, it can harm the environment even for the coming
generations. In addition, its sources are limited, similar to other natural fossil fuels. The
most serious problem for petrochemicals is that burning and extracting petroleum could
generate greenhouse gases that monumentally contribute to environmental pollution.
Additionally, certain hazardous substances extracted during petroleum production may
threaten human lives and the planet.
Furthermore, oil shocks affect the prices of petrochemical resources, petroleum,
natural fossil fuel, coal, and even staple merchandise. Consequently, other energy
resources are required, such as nuclear, wind, tidal, biomass, geothermal, solar, andhydropower energies. However, nuclear energy is nonrenewable, and there are issues
concerning nuclear waste. Therefore, countries have put a lot of effort into achieving thesustainable use of renewable energy.
Among these renewable resources, solar energy appears to be one of the most ideal
resources for the next generation because it is renewable, and no byproducts are obtained
during the energy-generating process. In addition, the annual energy emitted from the sunand received by the earth is approximately 23000 TW/year, whereas the global annual
consumption is only 16 TW/years1 (Figure 1). This indicates that the annual energy
1potential that the earth receives exposed to the sun would be sufficient for our lives. Hence,the effective use of solar energy is a global issue.

Figure 1 Annual renewable energy output and non-renewable energy storage, and global
energy consumption. 12

1-2 Evolution of Photovoltaic/Solar Cell Technology
Photovoltaic (PV)/solar cell technology can directly convert solar energy into
electricity. Dissimilar to other energy resources, solar energy can be environmentallyfriendly for sustainable use, which is ideal. PV technology can be divided into three
generations (Figure 2). First-generation PV technologies are mainly based on an elemental
semiconductor (silicon). In 1954, Bell Laboratories first presented silicon-based PVs that
exclusively relied on solar power. The New York Times wrote, “the silicon solar cell may
mark the beginning of a new era, leading eventually to the realization of one of mankind’s
most cherished dreams–the harnessing of the almost limitless energy of the sun for the useof civilization.” Silicon-based PVs, specifically crystalline silicon PVs, have beendeveloped and are one of the most historic technologies. Silicon-based PVs demonstratehigh power conversion efficiencies (PCEs) of approximately 23%–27%, depending on the
crystalline motifs.
Second-generation PV technologies include silicon-based PVs that use amorphoussilicon (thin film) and polycrystalline silicon. Amorphous silicon that exhibits a PCE of
up to 14% can be deposited as a thin film and can be processed on glass substrates or evenon flexible substrates. The second generations include PVs based on compound
semiconductors, such as copper indium gallium selenide (CIGS) and cadmium telluride(CdTe), which can also be made as thin films. CIGS and CdTe typically exhibit PCEs in
the range of 20%–23% that are slightly lower than that of crystalline silicon. The use ofsilicon wafers is avoided for second-generation PVs to lower the consumption of materials
and possibly reduce the production cost. However, second-generation cells require a
vacuum and high-temperature process, which largely consume energy. There is also the3pollution of heavy metals, particularly in compound semiconductor-based PVs, in whichthose scarce elements are a limiting factor for this generation.
The third-generation PVs are based on organic or organic–inorganic hybrid
semiconductive materials, such as dye-sensitized solar cells (DSSC), organic solar cells(organic photovoltaics (OPVs)), and perovskite solar cells. A striking feature of these PVs
is solution-processability, which should result in low production costs, compared withthose of other PVs. As the solution process is a low-temperature methodology, these PVs
can even be manufactured on plastic substrates. Recently, the PCE of perovskite solar cells
dramatically increased to 25%, which is comparable to that of crystalline silicon-based
PVs. However, perovskite solar cells use toxic lead-based semiconductors, which is aserious issue in practical application. OPVs, without the problem of materials, arepromising and flexible. Owing to the high light absorption of organic materials, thethickness of the photoactive layer of OPVs is in the order of hundreds of nanometers.
Therefore, OPVs can be semitransparent PVs, which greatly expands the scope of
application. A major issue for OPVs is their lower PCE, compared with that of inorganic
solar cells, such as silicon solar cells.

4Figure 2 Generation of photovoltaic technology 2

The National Renewable Energy Laboratory (NREL) reported a chart of PCEs fordifferent types of solar cells, called the “Best Research-Cell Efficiency Chart”, as shownin Figure 3. OPVs had been known to exhibit PCEs of only approximately 5% in the 2000s.
However, owing to the development of organic semiconductors, specifically p-typesemiconductive polymers, the PCE of OPVs has remarkably increased in the early 2010s
to 10%. Recently, owing to the development of n-type organic semiconductors, called“non-fullerene acceptors (NFAs),” the PCE has exceeded 18%. Although OPVs still face
several challenges including stability and low PCEs, their features such as low-cost andlow-energy consumption production, flexibility, semitransparency, and lightweight offer
great possibilities to become a popular PV technology.

5

Figure 3 Best research-cell efficiency chart reported by NREL in November 2021, andthe magnification for emerging PVs including OPVs.

6

1-3 Characteristics of Solar Cell
1-3-1 Basic Introduction
A solar cell is a semiconductor/PN junction diode. Normally, cells do not have an externalbias. Nevertheless, they can provide/convert sunlight into electricity when solar cells are
illuminated. (Figure 4)

Figure 4 Basic structure of the solar cell.

The typical characteristics of solar (photovoltaic) cells are shown as a current–voltage
curve (I–V). Figure 5 shows the I–V curve of a cell without light illumination as a green
line. In an ideal situation, without illumination, there is no current flow through the solar
cell unless an external bias is applied. With incident sunlight, the I–V curve shifts down,
generating an external current flow.
As the solar cell is illuminated by sunlight, the active (photoactive) layer creates
several electron-hole pairs (excitons) by the photoelectric effect. The incident light breaks
the thermal equilibrium condition of the junction. The free electrons in the depletion region
diffuse into the n-type side of the junction. Similarly, the holes diffuse to the p-type side.
When an external potential bias that is greater than the depletion barrier potential is applied,the carriers migrate throughout the solar cell, thereby generating electricity.
7Figure 5 Characteristics current- voltage (I-V) curve under dark and light illumination.

1-3-2 Characteristics Curves
Through the I–V measurement, we obtained various parameters that determine the qualityof a solar cell (Figure 6). One of such parameters is the short-circuit current (ISC),
commonly shown as the short-circuit current density (JSC) when considering the area ofthe solar cell. Other parameters include the open-circuit voltage (VOC), fill factor (FF),maximum current density (Jmax), maximum voltage (Vmax), and maximum output power(Pmax). Certain additional parameters, including series and shunt resistances (Rs and Rsh),
are also used to fully characterize the solar cell.
The quantum efficiency of a solar cell represents the current that the cell would
generate when irradiated by photons through wavelength. The current can be evaluated byintegrating the quantum efficiency over the spectrum. Two types of quantum efficiency ofthe solar cell are typically considered: External/internal quantum efficiency (EQE and
IQE). EQE is defined as the ratio of the number of electrons in an external circuit to thenumber of incident photons (Figure 6b); it can be directly obtained from the spectral
8response, which is also known as the incident photon to current conversion efficiency(IPCE). The EQE result can be modified to factor in the reflectance and transmittance toconsider the portion of the incident light in an active region. IQE is defined as the numberof absorbed photons in the solar cell to the number of collected carriers. This allows abetter understanding of the material properties of solar cells.

Figure 6 (a) Important parameters in the I–V curve and (b) a quantum efficiency
spectrum.

(1) Open-circuit Voltage, VOC
The open-circuit voltage is the maximum voltage available from a solar cell when thecurrent is zero. This voltage corresponds to the amount of forward bias on the solar
cell due to the bias of the solar cell junction with the light-generated current. The
open-circuit voltage is shown on the J–V curve above.

(2) Short-circuit Current, ISC (Short-circuit Current Density, JSC)
The short-circuit current is the current through the solar cell when the voltage acrossthe cell is zero. The ISC is due to the generation and collection of light-generated
9

carriers, which is the largest current that may be drawn from the solar cell. The ISC
from a solar cell is directly dependent on the illumination light intensity; it decreases
at a lower intensity of illumination light.

(3) Fill Factor, FF
The FF is used to define the ratio of cell to the ideal cell. The maximum power point(Pmax) is the multiplication of the maximum voltage output point (Vmax) and the
maximum current density output point (Jmax) (yellow area). The multiplication of VOC
and JSC can obtain the blue area in Figure 6. Owing to the structural design and process
conditions of solar cells, the cell cannot achieve the ideal output power, and there are
new parameters to define the cells. The yellow area represents the maximum ideal
power that the cell can output. The ratio of the blue to the yellow areas represents the
FF. Additionally, Equation (1-1) can be used to express it as follows:
The FF varies with the type of solar cell and the quality of the solar cell andgenerally falls between 0.5 and 0.85. The larger the fill factor, the closer the solar cellto the ideal diode.

FF = $!!" × $#"$%& × !$%&

(1-1)

(4) Power Conversion Efficiency, PCE
The photoelectric conversion efficiency is the ability to convert light energy into
electrical energy. Equation (1-2) shows the definition of PCE as the ratio of the
maximum power to the energy (E) of the incident sunlight.
The PCE would be different if the irradiated sunlight energy changed; therefore, it
10has been internationally defined as uniform standard solar energy. Solar radiation isapproximately 6000 K blackbody radiation, and the radiant energy reaching thesurface of the earth’s atmosphere is approximately 1353 kW/m2. The radiant energy
will be reduced because of atmospheric absorption, and the strength of the paththrough different atmospheres is also different. Here, the solar spectrum outside theatmosphere is defined as AM0, where the air mass (AM) is used to express thecharacteristics of the absorption of the atmosphere, which leads to a decrease in thesolar spectrum and radiant energy. Standard sunlight is defined when the air quality ofsolar radiation is AM1.5, and the temperature is 25 ℃; the incident sunlight intensity(Plight) at this time is 100 mW cm−2, as shown in Equation (1-3). Therefore, to improve
the PCE, it is necessary to improve the quality of the solar cell and increase the VOC,
JSC, and FF. These parameters reflect the quality of the material. In addition, the overallcomponent structure and manufacturing process of the solar cell is a controlling factor.

!

×#

%

𝑃𝐶𝐸 = !"#$ !"# × 100% = !"#× 100%$
𝑃𝐶𝐸 =!$% ×#&% ×&&%'()*+

× 100% =11!$% ×#&% ×&&
'((

!,
-!.

× 100%

(1-2)
(1-3)

CHAPTER 2 Introduction of Organic Solar Cells
2-1

Working Principle of Organic Solar Cells
The organic solar cell is a promising device for solar energy conversion. It has attracted
considerable interest among researchers and the industrial community because of its
potential advantages (lightweight, low cost, and mechanical flexibility).3-7 Typically,
semiconductive polymers have been used as an electron donor (p-type material), combinedwith a fullerene derivative 8-10 or non-fullerene material 11-14 as an electron acceptor (ntype material). In the past decade, extensive efforts were devoted to developing a wide
variety of p-type semiconductive polymers 15-17 to obtain high-performance organic solar
cells and n-type semiconductive polymers. 18-22
It is possible to promote charge carrier transport ability through molecular design.
Therefore, the design of the semiconductive polymer is important for a high-performancesolar cell. The chemistry of semiconductive polymers offers powerful methods for easy
control of their properties. Recently, the PCE of single-junction devices exceeded 19%,
owing to the fine-tuning of light utilization and the photophysical process of the active
layers 23.

2-2

Requirements for Semiconducting Polymers
For semiconductive polymers, the optical bandgap (Eg), deep highest occupied molecularorbital (EHOMO), and lowest unoccupied molecular orbital (ELUMO) energy levels areimportant parameters for organic solar cells. The energy difference between the EHOMO ofa p-type material and the ELUMO of an n-type material (∆EHL) is known to correlate with12

the VOC (Figure 7).24 Regarding the electronic properties, the requirements to improve theperformance include a narrow Eg with a deep EHOMO, which is crucial for optimizing the
VOC and JSC. Additionally, the energy difference (∆ELL) between the p-type and n-typematerials serves as a driving force to ensure an efficient photoinduced charge transfer, inwhich ∆ELL ≥ 0.3 eV is necessary.

Figure 7 Schematic diagram of band structure of organic photovoltaic.

Organic solar cells exhibit lower efficiency, compared with inorganic photovoltaic cells
based on Si, CIGS, or perovskite solar cells, owing to a serious loss in photoenergy.
Photoenergy loss in organic solar cells is defined as Eloss = Eg − eVOC, where e is theelementary charge.25-26 To further enhance the PCE, the Eloss must be reduced. The ELUMOof the p-type material should be as close as the n-type material to minimize the offset
energy loss and increase the VOC.
Charge carrier transport is an important function in semiconductive polymers mainly
governed by intramolecular transport along the polymer backbone and intermoleculartransport along the π–π overlaps that significantly influence the performance.27 Therefore,
the desired structural features for semiconductive polymers are as follows. First, the13polymer backbone must be coplanar, and the π–π stacking distance should be as close as
possible; this can be regarded as a highly crystalline structure. Second, the backbone
orientation should be ‘‘face-on’’ (Figure 8b) because the out-of-plane charge carriertransport is required for organic solar cells (‘‘edge-on’’ orientation is essential for in-planetransport in organic field-effect transistors (OFETs), 28 as shown in Figure 8a.

Figure 8 Representation of backbone orientation for semiconducting polymers withrespect to the substrate.29

14

2-3

Semiconducting Polymers
π-Conjugated (semiconductive) polymers have attracted considerable attention because of
their feature advantages of solution processability and excellent electronic properties.
Representative semiconductive polymers are shown in Figure 9a. These polymers are
significant functional materials that can be utilized for organic electronic devices. Studies
on organic electronics such as organic light-emitting diodes (OLEDs) 30-31, organic fieldeffect transistors (OFETs) 32-33, and organic solar cells (Figure 9b) have recently been
conducted worldwide.

Figure 9 (a) Chemical structures of representative semiconductive polymers. (b) Imagesof organic electronics such as organic light-emitting diodes (OLEDs), organic fieldeffect transistors (OFETs), and organic photovoltaics (OPVs).34-36

15

2-4

Evolution of Semiconducting Materials for Organic Solar Cells
In general, semiconductive polymers comprise electron-rich (D) and electron-deficient (A)heterocyclic π-building units alternately incorporated in the backbone, which is a state ofan art material system that greatly functions in organic solar cells, called donor–acceptor
semiconductive polymers. Polymer solar cells based on conjugated p-type polymers andfullerene derivatives have been intensively investigated in the last decade.8 Regioregularpoly(3-hexylthiophene) (P3HT, Figure 10) and [6,6]-phenyl C61vbutyric acid methyl ester(PC61BM) represent two of the most successful systems with PCEs in the range of 4–5%
(Figure 10).37-39 However, further improvement of the performance was limited to its
narrow absorption spectrum (300–650 nm) and high EHOMO (−4.9 eV). Therefore, great
effort has been devoted to developing new conjugated polymer donors and new fullerene
derivative acceptors 40-42 to solve these issues.43-44 An efficient way to broaden absorption
without sacrificing the EHOMO is to narrow the optical bandgaps of the polymers. The
combination of electron-rich and electron-deficient moieties (so-called donor–acceptor
systems) has been effectively demonstrated to obtain a small bandgap of alternating
conjugated polymers, which has greatly influenced solar cells. 45-48 Poly[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) (Figure 10) was developed andexhibited a PCE of 7.4%, which indicated great potential and a bright future for polymer
solar cells.45
Considerable efforts have been dedicated to the development of NFAs since 2012.49-51
A novel electron acceptor, ITIC (Figure 10), was designed, and a PTB7:ITIC-based solar
cell exhibited better performance than a fullerene-based solar cell. This result greatly
16promoted the development of alternative acceptors for organic solar cells.11 The Eloss of
the NFA-based organic solar cells was significantly suppressed, thereby improving the
PCE to 15%.52 Recently, an NFA (Y6, Figure 10) and its Y-derivative attracted hugeattention because of their remarkable photovoltaic performances. 12-14, 53-55 These advances
are extremely encouraging and initiate a critical question of how to further improve thePCEs by designing new materials.
C4H9

C2H5
O

S

C6H13
S

P3HT

S

O

PTB7

HOOC

P3HT:PC61BM PCE>4%Nat. Mater. 2005, 4, 864

n

S

F

n

S

C4H9

C4H9

PTB7:PC71BM PCE>7%
Adv. Mater. 2010, 22, E135

C6H13
C6H13
S

O
S
NC

CN

NC

N
CN

CN

O

S

O

C6H13

ITIC

F

PTB7:ITIC PCE>7%

F

S

S
C2H5

NC

C6H13

N

S

C11H23

S

S

N

N

C4H9

C4H9

C11H23
NC

S
C2H5

Y6
PM6:Y6 PCE>15%

CN
O

F

Joule. 2019, 4, 1140

Adv. Mater. 2015, 27, 1170

Figure 10 Chemical structure of semiconducting polymers, and non-fullerene acceptor
materials.

17

F

2-5

Naphthobisthiadiazole-based Semiconducting Polymers
Amongvariousbuilding

units

for

D–A

polymers,

naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz) (Figure 11) has been confirmed to be a high-potential A unit
because of its electron-deficient nature that offers a relatively deep EHOMO, small bandgaps,
and rigid and large planar structures that promote strong intermolecular interactions.56-57
N

S

N

N

S

N

NTz

Figure 11 Chemical structure of acceptor unit: NTz.

In previous studies, we demonstrated that a semiconductive polymer based on NTz
with a quarter-thiophene moiety and long branched alkyl chains, PNTz4T (Figure 12),
exhibited a highly crystalline structure with the face-on orientation in combination with
PC71BM and achieved a PCE of approximately 10%, which was extremely high for a
fullerene-based solar cell.48, 58 However, the energy level of PNTz4T limited the VOC andaffected the performance. The introduction of halogen substitution is an efficient strategy
to tune the energy level and ensure the coplanarity of the polymer backbone.44, 59-60
Recently, we developed a PNTz4T family (Figure 12), in which fluorine atoms wereintroduced into the polymer backbone.17, 61

18

N

S

N

S

S

S
N

C10H21

C12H25

C12H25

C10H21S

S

N

n

PNTz4T

VOC = 0.71 V, PCE = 9.6%
N

S

N

C10H21NF

S

C12H25

C12H25

C10H21

N

S

S

N

F

n

PNTz4T2F

C12H25

C10H21SN

C10H21

C10H21

F

N

S

F

N

S
F

S

N

S

N

S

n

C10H21

C12H25
F

S

S

N

F

PNTz4T4F

F

F
C12H25

C12H25SS

C12H25F
S

N

C10H21

VOC = 0.93 V, PCE = 6.5%

VOC = 0.81 V, PCE = 10.1%

N

N

S

S

S

S

S

S

S

F
n

PFN4T

C12H25

VOC = 0.73 V, PCE = 9.6%

C10H21

N

S

N

S
F

n

PFN4T2F

VOC = 0.84 V, PCE = 10.8%

Figure 12 Chemical structure of PNTz4T and a series of PNTz4T family: PNTz4T2F,
PNTz4T4F, PFN4T and PFN4T2F.17, 48, 61

All the fluorinated polymers had a deeper EHOMO than PNTz4T, which improved the VOC
to 0.90 V, thereby achieving a high PCE of 11%. However, the introduction of the fluorine
atom enhanced the backbone coplanarity, owing to the intramolecular noncovalentinteractions between the fluorine and sulfur atoms in the thiophene ring, thereby
significantly influencing the solubility. PNTz4T and these fluorinated polymers had
sufficiently high Mn values in the range of 40000–50000 kDa, which were soluble in hot
chlorinated solvents, such as chlorobenzene (CB) and o-dichlorobenzene (DCB).
Therefore, these studies were fundamentally based on a fullerene-based solar cell system19

(PC71BM). The solubility issues of these polymers can limit their solution-processability
and affect their miscibility with acceptor materials, resulting in poor photovoltaic
performances, particularly for NFA-based solar cells.
To obtain good solubility and high photovoltaic performance, we alternatively
designed NTz-based D–A polymers for a non-fullerene solar cell. First, we discuss theperformances of PNTz4T in the role of a non-fullerene solar cell. Afterward, the new πconjugated polymers (NTz-BDT polymers), which possess a benzodithiophene (BDT) unit
with alkylthienyl substituents, are discussed. The energetic properties, photovoltaic
performances, polymer packing, and film morphology were investigated. Consequently,
PNTzBDT-F obtained a PCE exceeding 13% in combination with Y6. Thereafter, the NTzTPTz random copolymers were investigated while collaborating with Prof. Tajima in
Riken. Triphenyleno[1,2-c:7,8-c’]bis([1,2,5]thiadiazole) (TPTz) was introduced in a small
amount (2.5%–10%) while preserving the intermolecular interaction of PNTz4T and
improving the solubility. The characteristics and photovoltaic performances of theserandom copolymers were carefully investigated.

20

CHAPTER 3 EXPERIMENTAL SECTION
3-1

Materials

3-1-1 NTz Monomer
The synthetic route of NTz monomer was listed in Scheme 1, and further synthesized thebrominated NTz2T with different alkyl sidechain (R = BO, DT). And 5,10-bis(5-bromo4-(2-butyloctyl)thiophen-2-yl)naphtho[1,2-c:5,6- c']bis([1,2,5]thiadiazole (NTz2T-Br2)
will be used for further polymerization through stille-coupling reaction in this research.48

COOH

NHCO2Et

NCO

CON3
DPPA

NHCO2Et

EtOH-N2

f-HNO3
AcOH

O2N

91%

EtO2CHN

DIPEA
DIoxane

HOOC

N3OC

quant

1

NH2
KOH aq.
EG/Dioxane

OCN

NH2

HCl
Zn

NO2

H2N

H2N

quant

NBS

S

Me3Sn

Br

Br

H2SO4/MeSO3HNSN

NTz-Br2

N

R

S

90%

S

N

R

Chloroform, r.t.

Br

NTz2T

Scheme 1. Synthetic route to the NTz monomer.

21

N

N

S

S

N

C4H9

N
S

S

80%

S

NTz

NBS

S

N

S
NH

N
H

N

R
S

N

HN
S

ClSSHNNS

Pd(PPh3)4/Toluene

H
N

H2N

40%

R

N

3

NH2

Pyridine

5

4

N

SOCl2

4HCl

H2N

2

NHSSClNH2

Dioxane

O2N

50%

EtO2CHN

NO2

N

S

N

NTz2T-Br2

Br

BO =

R

C12H25

DT =

C6H13

C10H21

3-1-2 NTz-BDT Polymers
All the reagents were purchased from commercial sources and used without further
purification.

2,6-bis(trimethytin)-4,8-bis(5-(2-ethylhexyl)-4-thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene

(1)

and

2,6-bis(trimethytin)-4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (2) were purchased from Ossila Ltd.
2,6-Bis(trimethytin)-4,8-bis(5-(2-ethylhexyl)-4-chlorothiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene (3)62, 5,10-bis(5-bromo-4-(2-butyloctyl)thiophen-2-yl)naphtho[1,2-c:5,6c′]bis([1,2,5]thiadiazole (4)48 and PNTz4T48 were synthesized according to publishedprocedure. Polymerization was carried out with a microwave reactor, Biotage Initiator.
Molecular weight of the polymers was evaluated by a high-temperature GPC (140 °C),TOSOH HLC-8121GPC/HT, using o-dichlorobenzene as the eluent and calibrated with
polystyrene standard. High-resolution mass spectrometry (HRMS) was performed with an
LTQ Orbitrap XL spectrometer.
Polymerization was carried out with a microwave reactor, Biotage Initiator. Molecularweight of the polymers was evaluated by a high-temperature GPC (140 °C), TOSOH HLC8121GPC/HT, using o-dichlorobenzene as the eluent and calibrated with polystyrene
standard. High-resolution mass spectrometry (HRMS) was performed with an LTQ
Orbitrap XL spectrometer.

22

PNTzBDT: 1 (45.2 mg, 0.05 mmol), 4 (45.2 mg, 0.05 mmol), Pd(PPh3)4 (1.16 mg, 2 mol%)and toluene (2 mL) were added to reaction tube, then heated to 200 °C for 2 hours inmicrowave reactor. After cooling to room temperature, the reaction solution was poured
into 50 mL of methanol containing 2 mL of hydrochloric acid and stirred for 1 hour. Thenthe precipitate was collected sequentially with methanol, hexane, and dichloromethane
through Soxhlet extraction to remove the impurities and low-molecular weight fractions.
The residue was then extracted with chloroform, and reprecipitated in 50 mL of methanol.
The precipitate was collected and dried in vacuum to yield the polymer as dark blue solid(56 mg, 85%). Anal. Calced for C78H100N4S8: C, 69.15; H, 7.18, N, 4.24. Found: C, 68.94;
H, 7.18, N, 4.12. Mn = 63 kDa, Mw = 106 kDa, Ð = 1.7.

1

H-NMR spectrum of PNTzBDT.
23

PNTzBDT-F: 2 (47.0 mg, 0.05 mmol), 4 (45.2 mg, 0.05 mmol), Pd(PPh3)4 (1.16 mg, 2mol%) and toluene (2 mL) were added to reaction tube, then heated to 200 °C for 2 hours
in microwave reactor. After cooling to room temperature, the reaction solution was poured
into 50 mL of methanol containing 2 mL of hydrochloric acid and stirred for 1 hour. Thenthe precipitate was collected sequentially with methanol, hexane, and dichloromethane
through Soxhlet extraction to remove the impurities and low-molecular weight fractions.
The residue was then extracted with chloroform, and reprecipitated in 50 mL of methanol.
The precipitate was collected and dried in vacuum to yield the polymer as dark blue solid(62 mg, 91%). Anal. Calced for C78H98F2N4S8: C, 67.31; H, 6.84, N, 4.13. Found: C, 67.02;
H, 6.75, N, 4.06. Mn = 41 kDa, Mw = 94 kDa, Ð = 2.3.

1

H-NMR spectrum of PNTzBDT-F.

24

PNTzBDT-Cl: 3 (48.7 mg, 0.05 mmol), 4 (45.2 mg, 0.05 mmol), Pd(PPh3)4 (1.16 mg, 2mol%) and toluene (2 mL) were added to reaction tube, then heated to 200 °C for 2 hours
in microwave reactor. After cooling to room temperature, the reaction solution was poured
into 50 mL of methanol containing 2 mL of hydrochloric acid and stirred for 1 hour. Thenthe precipitate was collected sequentially with methanol, hexane, and dichloromethane
through Soxhlet extraction to remove the impurities and low-molecular weight fractions.
The residue was then extracted with chloroform, and reprecipitated in 50 mL of methanol.
The precipitate was collected and dried in vacuum to yield the polymer as dark blue solid(54 mg, 78%). Anal. Calced for C78H98Cl2N4S8: C, 65.72; H, 6.68, N, 4.03. Found: C,
65.81; H, 6.64, N, 3.90. Mn = 63 kDa, Mw = 166 kDa, Ð = 2.7.

25

1

H-NMR spectrum of PNTzBDT-Cl.

Table 1 Polymerization results of NTz-BDT polymers.
Mna (g mol–1)

Mwa (g mol–1)

Ð

PNTz4T

31,000110,000

3.5

PNTzBDT

63,000106,000

1.7

PNTzBDT-F

41,00094,000

2.3

PNTzBDT-Cl

63,000166,000

2.7

Polymera

Determined by high-temperature GPC (o-dichlorobenzene, 140 °C) calibrated with polystyrene

standard.

26

3-1-3 NTz-TPTz Random Copolymers
All the reagents were purchased from commercial sources and used without further
purification.

5,10-bis(5-bromo-4-(2-butyloctyl)thiophen-2-yl)naphtho[1,2-c:5,6-c']bis([1,2,5]thiadiazole (NTz2T-Br2), distannylated bithiophene (2T-(SnMe3)2) and
PNTz4T were synthesized according to published procedure.48 4,7-Dibromo-12,13bis((2-R)oxy)triphenyleno[1,2-c:7,8- c']bis([1,2,5]thiadiazole) (TPTz-Br) with different
alkyl side chain (R= C8, CEH, C12 and CBO) were provided by Prof. Tajima andsynthesized according to published procedure.63
Polymerization was carried out with a microwave reactor, Biotage Initiator. Molecularweight of the polymers was evaluated by a high-temperature GPC (140 °C), TOSOH HLC8121GPC/HT, using o-dichlorobenzene as the eluent and calibrated with polystyrene
standard. High-resolution mass spectrometry (HRMS) was performed with an LTQ
Orbitrap XL spectrometer.

CR-5% (R=C8, C12, EH, BO): NTz2T-Br2 (0.0475 mmol), A3-CR (0.0025 mmol), 2T(SnMe3)2 (0.05 mmol), Pd(PPh3)4 (1.16 mg, 2 mol%) and toluene (2 mL) were added toreaction tube, then heated to 200 °C for 2 hours in microwave reactor. After cooling toroom temperature, the reaction solution was poured into 50 mL of methanol containing 2
mL of hydrochloric acid and stirred for 1 hour. Then the precipitate was collectedsequentially with methanol, hexane, and dichloromethane through Soxhlet extraction toremove the impurities and low-molecular weight fractions. The residue was then extracted27

with chloroform, and reprecipitated in 50 mL of methanol. The precipitate was collected
and dried in vacuum to yield the polymer as dark blue solid (85% for C8-5, 91% for C125, 95% for EH-5 and 85% for BO-5).

1

H-NMR spectrum of C8-5.

28

1

H-NMR spectrum of EH-5.

1

H-NMR spectrum of BO-5.

C12-x (x= 2.5, 5, 7.5 & 10%): NTz2T-Br2 (0.04875, 0.0475, 0.04625 or 0.045 mmol for
2.5%, 5%, 7.5% or 10%, respectively), A3-CR (0.00125, 0.0025, 0.00375 or 0.005 mmol
29

for 2.5%, 5%, 7.5% or 10%, respectively), 2T-(SnMe3)2 (0.05 mmol), Pd(PPh3)4 (1.16 mg,
2 mol%) and toluene (2 mL) were added to reaction tube, then heated to 200 °C for 2 hours
in microwave reactor. After cooling to room temperature, the reaction solution was poured
into 50 mL of methanol containing 2 mL of hydrochloric acid and stirred for 1 hour. Thenthe precipitate was collected sequentially with methanol, hexane, and dichloromethane
through Soxhlet extraction to remove the impurities and low-molecular weight fractions.
The residue was then extracted with chloroform, and reprecipitated in 50 mL of methanol.
The precipitate was collected and dried in vacuum to yield the polymer as dark blue solid(yield: 87% for C12-2.5, 91% for C12-5, 89% for C12-7.5 and 93% for C12-10).

1

H-NMR spectrum of C12-2.5.
30

1

1

H-NMR spectrum of C12-5.

H-NMR spectrum of C12-7.5.

31

1

H-NMR spectrum of C12-10.

Table 2 Polymerization results of NTz-TPTz polymers.
Mna (g mol–1)

Mwa (g mol–1)

Ð

PNTz4T

31,000110,000

3.5

C8-5

35,00080,5002.3

EH-533,00069,3002.1

BO-5

46,00096,6002.1

C12-2.5

35,00066,5001.9

C12-5

35,00094,5002.7

C12-7.5

36,00068,4001.9

C12-1048,00086,400

1.8

Polymera

Determined by high-temperature gel permeation chromatography (GPC) using DCB at

140 °C.
32

3-2

Solar Cell Fabrication

3-2-1 Solar Cell Fabrication of NTz-BDT Polymers
Indium tin-oxide (ITO) substrates were first pre-cleaned sequentially by sonicating in a
detergent bath, deionized water, acetone, and 2-propanol at room temperature, and inboiled 2-propanol for 10 min each, and then baked at 120 °C for 10 min in air. Following
the substrates were then subjected to a UV/ozone treatment at room temperature for 20
min. The PEDOT:PSS layer was prepared by spin-coating at 5000 rpm for 30 s thenannealed at 120 °C for 10 min. The photoactive layers were fabricated in glove box(KOREA KIYON, KK-011AS-EXTRA) by spin-coating with chloroform solution
containing 3.5 mg mL−1 of the polymer sample with Y6 (polymer: Y6 = 1:1.2 w/w). The
solution was kept at 50 °C for 30 min and the hot solution was spin-coated on the substrate
at 600 rpm for 20 s and thermal annealing at 90 °C for 5 min. The thickness of the optimal
device was around 100 nm. A PNDI-F3N-Br layer was prepared by spin-coating (1000rpm, 30 s) with methanol solution at concentration of 0.5 mg mL–1 (AcOH 0.5 vol%).
Finally, thin films were transferred to vacuum evaporator (ALS Technology, E-100J)
which it is connected to the glove box and Ag (220 nm) were deposited by thermal
evaporation under ~10–5 Pa, where the active area of the cells was 0.1256 cm2.

3-2-2 Solar Cell Fabrication of NTz-TPTz Random Copolymers
Indium tin-oxide (ITO) substrates were first pre-cleaned sequentially by sonicating in a
detergent bath, deionized water, acetone, and 2-propanol at room temperature, and inboiled 2-propanol for 10 min each, and then baked at 120 °C for 10 min in air. Following
the substrates were then subjected to a UV/ozone treatment at room temperature for 20
33

min. The PEDOT:PSS layer was prepared by spin-coating at 5000 rpm for 30 s thenannealed at 120 °C for 10 min. The photoactive layers were fabricated in glove box byspin-coating with chloroform solution containing 3.5 mg mL−1 of the polymer sample withY6 (polymer: Y6 = 1:1.2 w/w, without solvent additive). The solution was kept at 50 °C
for 30 min and the hot solution was spin-coated on the substrate at 600 rpm for 20 s andthermal annealing at 90 °C for 5 min. The thickness of the optimal device was around 90100 nm. Then, ZnO layer was prepared by spin-coating (6000 rpm, 20 s) from a diluted
solution of ZnO nanoparticles.
Finally, thin films were transferred to vacuum evaporator which it is connected to theglove box and Ag (220 nm) were deposited by thermal evaporation under ~10–5 Pa, where
the active area of the cells was 0.1256 cm2.

3-3

InstrumentationJ-V characteristics of the solar cells were measured using Keithley 2400 source in the
glove box under 100 mW cm–2 (AM 1.5G) condition using solar simulator (SAN-EIElectric, XES-40S1). The light intensity was calibrated with a reference PV cell (KONICAMINOLTA AK-100 certified at National Institute of Advanced Industrial Science andTechnology, Japan). EQE spectra was measured with a Spectral Response Measuring
System (Soma Optics, Ltd., S-9241). The thickness of the photoactive layers were
measured with an ET200 (Kosaka Laboratory, Ltd.).
UV-vis absorption spectra were measured using a Shimadzu UV-3600 spectrometer.
2D-grazing incidence X-ray diffraction (GIXD) measurements were performed on thebeamline BL46XU at the SPring-8. The sample was irradiated at a fixed incident angle on34

the order of 0.12° through a Huber diffractometer with an X-ray energy of 12.39 keV (λ =1 Å).
Two dimensional (2D) GIXD patterns were recorded with a 2D image detector (Pilatus
300K). The polymer neat films were coated on the PEDOT:PSS-coated ITO glass substrateunder same conditions that used in device fabrication while the blend thin films weredirectly used for measurement after device fabrication. Atomic force microscopy (AFM)
was carried out with a SPM-9700HT scanning probe microscope (Shimadzu Corp).

3-3-1 Cyclic Voltammetry Measurements
Cyclic voltammetry (CV) measurements were carried out with ALS Electrochemical
Analyzer

Model

610E,

in

acetonitrile

containing

tetrabuthylammonium

hexafluorophosphate (Bu4NPF6, 0.1 M) as the supporting electrolyte. The counter andworking electrodes were made of Pt while the reference electrode was Ag/Ag+. The
polymer thin films were obtained on the working electrode through directly dipping theelectrode into the polymer solution. All potentials were calibrated with the standard
ferrocene/ferrocenium redox couple (Fc/Fc+: E1/2 = + 0.13 V measured under the identicalconditions). HOMO energy levels (EH) and LUMO energy levels (EL) were calculated
through the following equations:
EH (eV) = −4.67 − Eox
EL (eV) = −4.67 + Ered
where Eox and Ered are the onset oxidation and reduction potential of CV, respectively, and−4.80 eV represent the EH of ferrocene against the vacuum level.

35

3-3-2 Hole/Electron-Only Device Fabrication & Measurements
For hole-only devices, the pre-cleaned ITO glass were coated with PEDOT:PSS by spincoating at 5000 rpm for 30 s. The polymer blend film (polymer/Y6=1:1.2) was thendeposited through spin-coating as described in solar cell fabrication. Then, they weretransferred into vacuum evaporator, and MoOx (7 nm) and Ag (100 nm) were deposited
sequentially through a shadow mask. For electron-only devices, a ZnO layer was preparedon the pre-cleaned ITO glass by spin-coating (1200 rpm) from a diluted solution of ZnO
nanoparticles. The polymer blend film was deposited as described. Then, the ZnO layer
was directly spin-coated on the photoactive layer. The thin films were transferred into
vacuum evaporator and Ag (100 nm) were deposited through a shadow mask under ~10-5
Pa.
The J–V characteristics were measured in the range of 0–7 V using a Keithley 2400
source–measure unit in the dark in the glovebox. The mobility (μSCLC) was calculated byfitting the J–V curves to a space charge limited current model described by the following
equation:
8
𝑉𝐽 = 𝜀) 𝜀( 𝜇*+,+ .
9𝐿where εr is the relative dielectric constant of the polymer, ε0 is the permittivity of free space,μSCLC is the mobility, V = Vappl − Vbi, where Vappl is the applied voltage to the device andVbi is the built-in voltage due to the difference in work function of the two electrodes(determined to be 0.1 V), and L is the polymer thickness. The dielectric constant εr isassumed to be 3, which is a typical value for semiconducting polymers.

36

CHAPTER 4 PNTz4T in Non-Fullerene Solar Cells
PNTz4T, as a novel semiconductive polymer, exhibited a crystalline structure with theface-on orientation in the PNTz4T:PC71BM blend film, resulting in a PCE of
approximately 10% in the fullerene solar cell.17, 48 The emergence of the so-called Y-seriesof acceptors is the latest impetus to further push OPVs.12, 64 In 2019, Zou et al. firstreported Y6 (Figure 13) as a new class of near-infrared (NIR)-absorbing NFA material
based on a multifused ring central unit with an electron-deficient benzothiadiazole unit.12
Solar cells based on Y6 and polymer, PM615 , exhibited low energy losses and achieved a
PCE of 15.7%. Y6 was also compatible with other polymer donor materials.16, 65-68
Therefore, to understand the role of PNTz4T in the non-fullerene solar cell, weinvestigated PNTz4T in combination with Y6.

Figure 13 Chemical structure of semiconducting polymer, PNTz4T; and non-fullerene
acceptor Y6.

The ultraviolet–visible (UV–vis) absorption spectra for PNTz4T in the polymer thin films
are shown in Figure 14. The absorption of Y6 is shown in the thin film spectra. The
PNTz4T thin film offered a spectrum with the main absorption band ≈ 500–800 nm,
37whereas Y6 offered a spectrum in a longer wavelength with the main absorption band ≈
600–900 nm. PNTz4T exhibited λmax = 714 nm, in which a 0–0 band was observed,
indicating good molecular ordering, probably due to the rigid π-extended structure. The
absorption onset in the thin film for PNTz4T was 776 nm, and Eg was calculated to be
1.60 eV. To effectively harvest the solar energy, the optical absorption of the active layer
in the polymer organic solar cells must optimally match the region of the maximum photon
flux. As shown in Figure 14, the main absorption band for PNTz4T and Y6 was rather
overlapped, which affected the light harvesting, thereby lowering the JSC.

Figure 14 UV-vis absorption spectra of the PNTz4T thin film. The absorption of Y6 isalso shown in the thin film spectra.

The EHOMO and ELUMO of PNTz4T and Y6 were estimated by conducting cyclic
voltammetry (CV) using the thin films. PNTz4T had an EHOMO and ELUMO of −5.20 and−3.46 eV, respectively, whereas Y6 had an EHOMO and ELUMO of −5.62 and −4.11 eV. The
VOC linearly correlated with the difference between the EHOMO of the donor and the ELUMO

38

of the acceptor.69 Narrowing the Eg of the polymer while maintaining a proper driving
force for charge separation would minimize the Eloss, resulting in the maximum value ofthe VOC.24 The mismatch of the energy level of PNTz4T and Y6 would lead to a poor VOC.
The photovoltaic performances of PNTz4T were investigated by fabricating thedevices with a conventional architecture (ITO/PEDOT:PSS/PNTz4T/Y6/PNDI-F3NBr/Ag). The optimal weight ratio of PNTz4T to Y6 was observed to be 1:1.2, and the
optimized thickness of the active layer was within a range of 90–100 nm. Figure 15 shows
the representative current density versus voltage (J−V). PNTz4T exhibited a VOC of 0.71
V, JSC of 23.4 mA cm−2, and FF of 67%, thereby achieving a PCE of 11.1%. As expectedfrom the energy level mismatch, the VOC was rather low in organic solar cells.

Figure 15 J−V curves of the optimized organic solar cells using the PNTz4T combinedwith Y6.

The solubility of PNTz4T is an issue because it was soluble in hot chloroform (CF), whichhampered the solution-processability, owing to its high crystallinity. In addition, it further
39affected the miscibility with the acceptor whether with Y6 or other acceptor materials.
Therefore, it is necessary to design a new series of polymers based on the NTz unit withthe appropriate energy level for a high VOC with a short wavelength absorption band andgood solubility.
Subsequently, we demonstrated a new series of NTz-based polymers, NTz-BDT
polymers (Chapter 5) and NTz-TPTz random copolymers (Chapter 6), which exhibited
better performances in non-fullerene solar cells. The optical and electrochemical
properties, photovoltaic performances, polymer packing, and film morphology were
investigated and are discussed herein.

40

CHAPTER 5 NTz-BDT Polymers
It is unsatisfactory that these low solubilities in π-conjugated polymers would limit thesolution-processability and miscibility with acceptor materials, in particular, nonfullerenes, resulting in poor photovoltaic performances. To develop highly soluble
polymers, benzo[1,2-b:4,5-b′]dithiophene (BDT) is important as a high-potential D unit.7071

BDT possesses a planar conjugated structure, thereby 70, 72-73 allowing the attachment ofvarious side chains at the 4 and 8 positions to relatively easily tune the solubility. Thus,
we expected that the introduction of a BDT unit in the polymer backbone would endow
the NTz-based polymers with high solubility, thereby ensuring high photovoltaic
performances in non-fullerene cells. 62, 74-76
Here, we synthesized a series of NTz-based polymers with BDT units that possessalkylthienyl substituents, named PNTzBDT and PNTzBDT-F and PNTzBDT-Cl
containing fluorine and chlorine groups in the alkylthienyl side chain, respectively (Figure16). The alkylthienyl side chains provided torsion with respect to the BDT moiety, owing
to the steric repulsion from the peripheral hydrogen atom, which reduced theintermolecular interaction between the backbones. As a result, these NTz-BDT polymers
exhibited significantly improved solubility, compared with PNTz4T. In addition, they had
slightly wider Eg and deeper HOMO energy levels because of the weaker electrondonating nature of BDT, compared with that of oligothiophenes; halogenation further
deepened the HOMO energy level. As a result, the photovoltaic parameters of the cells
based on the NTz-BDT polymers, where Y6 was blended, simultaneously improved
relative to that based on PNTz4T.
41

Figure 16 Chemical structure of PNTz4T and NTz-BDT polymers, i.e., PNTzBDT,PNTzBDT-F and PNTzBDT-Cl.

The synthetic route for the NTz-BDT polymers is shown in Scheme 2, where the BDTcontaining monomers (1–3) were copolymerized with the NTz-containing monomer (4)via the Stille coupling reaction to yield the corresponding copolymers, PNTzBDT,PNTzBDT-F, and PNTzBDT-Cl, at yields in the range of 80%–90%. The number-average
and weight-average molecular weights (Mn and Mw) of the polymers are summarized inTable 1. The polymers had Mn values within the range of 40000–60000 with
polydispersities (Ð) of approximately 2.0. In addition, they were soluble in CF even atroom temperature. PNTz4T was also synthesized according to our previous report.48
Notably, the molecular weight of PNTz4T was controlled by modifying the synthetic
conditions to be soluble in CF to fabricate the solar cells under conditions similar to thoseof the NTz-BDT polymers. PNTz4T had an Mn of 34200 (Ð = 2.5) and was soluble in hot
CF. Typically, PNTz4T is obtained with a high Mn of over 50000 and is soluble in hot CB(insoluble in CF). Thus, the NTz-BDT polymers had a significantly improved solubility
than PNTz4T.

42

C4H9

C2H5

C2H5
X

X
S

Me3Sn

N

S
S

SnMe3

+

Br

X
C2H5

N

C4H9

S

Pd(PPh3)4
S

C6H13

C6H13

C4H9

S

S

C4H9

S

N

S

Br

N

S

N

S

C4H9

S

C4H9

X
C2H5

S

S

N
C6H13

S

N

C4H9

PNTzBDT (X = H)
PNTzBDT-F (X = F)PNTzBDT-Cl (X = Cl)

1 (X = H)
2 (X = F)
3 (X = Cl)

Scheme 2. Synthetic route to the NTz-BDT polymers.

43

C6H13

N
S

Toluenemicrowave, 200 oC, 2hr

4

C4H9

n

5-1

Physical Property Evaluation
The chemical structure of PNTz4T and the NTz-BDT polymers are shown in Figure 16.
The use of the alkylthienyl chain on the BDT moiety provided the torsion of the peripheral
thienyl rings relative to BDT, which reduced the intramolecular interaction, thereby
increasing the solubility. The NTz-BDT polymers were soluble in CF solvent at room
temperature, whereas PNTz4T needed to be dissolved in hot CF, which might haveaffected the solution processability. The thermal properties of these polymers were
examined by differential scanning calorimetry (DSC) (Figure 17). The DSC curves
showed that PNTzBDT, PNTzBDT-F, and PNTzBDT-Cl had no melting peak and phasetransition peaks below 350 °C, whereas PNTz4T exhibited a melting peak at 317 °C. Thisimplied that the polymers were thermally stable in this temperature range.

Figure 17 DSC thermograms of PNTz4T and NTz-BDT polymers.

44

The UV–VIS absorption spectra of the solution and polymer thin films are presented in
Figure 18. The absorption maximum (λmax), absorption edge (λedge), and optical bandgap(Egopt) were calculated and are summarized in Table 3. The polymers exhibited a spectrumwith the main absorption band ≈ 500–800 nm. In the polymer solution (Figure 18a), theNTz-BDT polymers exhibited spectra with a broad single peak, where λmax wasapproximately 650 nm. However, PNTz4T exhibited a relatively sharp peak with a λmaxof 716 nm assignable to the 0–0 transition band along with a shoulder at approximately
650 nm assignable to the 0–1 band. The change in the absorption shape of the NTz-BDT
polymers, compared with that of PNTz4T, was probably due to the enhancement of thelinkage rotation along the backbone originating in a higher solubility and suppressedintermolecular interactions. PNTzBDT-Cl exhibited a blue-shifted spectrum, comparedwith those of PNTzBDT and PNTzBDT-F, which could be correlated with the shift of theenergy levels, as discussed below. In the thin film (Figure 18b), the NTz-BDT polymers
exhibited absorption spectra similar to those in the solution, whereas PNTz4T exhibited aspectrum that was more sharpened, compared to that in the solution. This suggested thatthe aggregation was significantly suppressed by the introduction of the BDT unit in the
backbone.
The HOMO and LUMO energy levels (EHOMO and ELUMO) of the polymers were
estimated by conducting CV using the polymer thin films (Figure 18c). PNTzBDT
exhibited an EHOMO and ELUMO of −5.26 and −3.46 eV, respectively, in which the EHOMO
slightly downshifted from that of PNTz4T (−5.20 eV). The ELUMO remained unchanged,
resulting in a larger HOMO–LUMO gap, compared with that of PNTz4T. This suggested
that the replacement of the bithiophene moiety with the BDT moiety had a greater45

influence on the EHOMO than on the ELUMO. The EHOMO and ELUMO of PNTzBDT-F (−5.36
and −3.56 eV) and PNTzBDT-Cl (−5.41 and −3.60 eV) were observed to be lower thanthose of PNTzBDT. This can be explained by the effect of the electron-withdrawing natureof the fluorine and chlorine groups on the alkylthienyl sidechains. These trends in theenergy levels correlated with the results obtained by the density functional theory (DFT)
calculation (B3LYP/6-31g(d)) using the model compounds (Figure 19).

Figure 18 (a,b) UV-vis absorption spectra of the NTz-BDT polymers in the solution (a)and thin film (b). The absorption of Y6 is also shown in the spectra. (c) CV of the thinfilms for the NTz-BDT polymers vs Ag/Ag+ as the reference electrode.

Figure 19 Optimized geometry of the polymer backbone, and ELUMOs and EHOMOs for thecompounds calculated by the density functional theory (DFT) method at the B3LYP/631G(d) level.
46

Table 3 Optical and electrochemical properties of the polymer thin films.
Polymerλmax (nm)a

λedge

Egopt

ELUMO

EHOMO

solution

film

(nm)b

(eV)c

(eV)d

(eV)d

PNTzBDT

664

666

762

1.63

−3.46

−5.26

PNTzBDT-F

657

643

786

1.58

−3.56

−5.36

PNTzBDT-Cl

643

628

776

1.60

−3.60

−5.41

PNTz4T

716

657, 714

776

1.60

−3.46

−5.20

a

Absorption maximum. bAbsorption onset. cOptical bandgap determined from theabsorption onset. dEHUMO and ELUMO determined by the onset oxidation and reduction
potential of the cyclic voltammograms, respectively,

47

5-2

OPV Properties of NTz-BDT Polymers in Non-Fullerene System

The photovoltaic performances of the NTz-BDT polymers and PNTz4T were investigatedbyfabricatingorganic

solar

cells

with

conventionalarchitecture(ITO/PEDOT:PSS/polymer/Y6/PNDI-F3N-Br/Ag). The optimal weight ratio of polymer/Y6
was 1:1.2, and the optimal active layer thickness was ≈ 90–100 nm. The representative J–V
characteristic and the EQE of the optimized devices are shown in Figures 20a and b,
respectively, and the corresponding parameters including the VOC, JSC, FF, and PCE aresummarized in Table 4. As expected from the deeper EHOMO of PNTzBDT, the device
exhibited a higher VOC of 0.77 V, compared to that of the PNTz4T device (0.71 V). Moreover,the EHOMO shifted deeper by the halogenation alkylthienyl side chain of the BDT moiety.
PNTzBDT-F and PNTzBDT-Cl exhibited significantly improved VOC values of 0.79 and0.83 V, respectively. PNTzBDT-F exhibited a JSC of 24.5 mA cm−2, which was slightly higher
than those of PNTzBDT (24.0 mA cm−2) and PNTz4T (23.4 mA cm−2). The slight difference
between PNTzBDT-F and PNTz4T for the JSC might be attributed to the blueshift of theabsorption that offered a better combination with Y6, and this was consistent with the resultsof the EQE investigation. Nevertheless, the PNTzBDT-F devices achieved a PCE of 13.3%,
which was higher than those of the PNTz4T (11.1%) and PNTzBDT devices (12.6%).
Although the PNTzBDT-Cl devices exhibited good performances for the VOC over 0.80 V, it
was discovered that the poor JSC and FF mainly restricted the device performances, resulting
in a low PCE of 10.3%.

48

Figure 20 (a) J−V curves and (b) EQE spectra of the optimized organic solar cells using
the NTz-BDT polymers combined with Y6.

Table 4 Summary of photovoltaic parameters of the optimized polymer/Y6 devices.
VOC

JSC [JSCEQE]a(V)(mA cm−2)

PNTzBDT

0.77

24.0 [24.2]

0.68

12.6 [11.9 ± 0.8]

PNTzBDT-F

0.79

24.5 [24.4]

0.69

13.3 [12.9 ± 0.4]

PNTzBDT-Cl

0.83

21.0 [18.1]

0.60

10.3 [10.0 ± 0.5]

PNTz4T

0.71

23.4 [21.6]

0.67

11.1 [10.4 ± 0.6]

PolymeraFF (–)

PCEmax [PCEave](%) b

JSC calculated from the EQE spectrum bMaximum PCE and the average PCE, obtained

from more than 10 cells, with standard deviation in the square bracket.

49

5-3

Reason for the Differences in OPV Performances

5-3-1 Charge Transport Ability
The charge carrier mobilities of the blend films were evaluated based on the space-charge
limited current (SCLC) model to correlate with the performance. Hole-only and electrononly devices with device structures of ITO/PEDOT:PSS/active layer/MoOx/Ag and
ITO/ZnO/ active layer /ZnO/Ag, respectively, were prepared. Typical J–V curves of thehole-only and electron-only devices are shown in Figure 21. According to the Mott–
Gurney equation, the hole mobility (µh) for the PNTzBDT/Y6 blend film was observed to
be 4.2 × 10−4 cm2 V−1 s−1, which was slightly higher than that of the PNTz4T/Y6 blend
film (3.3 × 10−4 cm2 V−1 s−1). Although the PNTzBDT-F/Y6 blend film exhibited a slightly
higher µh (5.3 × 10−4 cm2 V−1 s−1) than the PNTzBDT/Y6 blend film (4.2 × 10−4 cm2 V−1s−1), the PNTzBDT-Cl/Y6 exhibited a slightly lower µh (3.1 × 10−4 cm2 V−1 s−1). These
data were consistent with the trend in the performances, although the difference may beinsignificant. The electron mobilities (𝜇e) of PNTz4T/Y6, PNTzBDT/Y6, PNTzBDTF/Y6, and PNTzBDT-Cl/Y6 were 4.5 × 10−4, 6.2 × 10−4, 8.4 × 10−4 and 5.8 × 10−4 cm2V−1 s−1, respectively. The electron mobilities were similar for PNTz4T/Y6 and the NTzBDT polymer/Y6, which was reasonable because the major electron carrier in the device
was Y6.

50

Figure 21 J–V curves of the (a) hole-only devices; and (b) electron-only devices using thepolymer/Y6 blend films based on the space-charge limited current (SCLC) model.

5-3-2 Charge Recombination Behavior
Charge recombination is a crucial factor that affects photovoltaic performance. Thus, weinvestigated the devices by probing the dependence of JSC and VOC on the light intensity(Plight). Figure 22a shows the light-intensity dependence of JSC. According to the following
equation, JSC ∝ Plight 𝛼,77-78 an 𝛼 value closer to 1 indicates that the charge carriers can be
efficiently collected at the electrodes by avoiding bimolecular recombination, whereas an
𝛼 value varying from 1 indicates that bimolecular recombination is not negligible under
the short-circuit condition that the charge carrier can be efficiently collected at theelectrode. As shown in Figure 22a, the devices showed similar 𝛼 values in the range of
0.98–1.01, suggesting that the bimolecular recombination was not serious and that there
was no significant difference between these devices.
Figure 22b illustrates the light-intensity dependence of the VOC, which can be used toanalyze trap-assisted recombination. The VOC was fitted to the linear law, VOC ∝nkBT/qln(Plight),78-79 where kB, q, and T are the Boltzmann constant, elementary charge,
51

and Kelvin temperature, respectively. Factor n for the VOC–ln(Plight) plot being equal to 1
suggested that the trap-assisted recombination was negligible; however, n deviating from1 suggested that the trap-assisted recombination existed under the open-circuit condition.
As shown in Figure 22b, the PNTzBDT device exhibited an n of 1.41 that was slightly
closer to 1, compared with that for the PNTz4T device (1.48), indicating that the trapassisted recombination slightly reduced. This correlated with the fact that the PNTzBDT
device exhibited a slightly larger FF than that of the PNTz4T device. Furthermore,
although the PNTzBDT-F devices exhibited an n considerably closer to 1 (1.18), the
PNTzBDT-Cl device exhibited an n significantly farther from 1 (1.83), which correlatedwith the variation in the FF.

Figure 22 Light-intensity-dependence of (a) JSC and (b) VOC for the polymer/Y6 devices.

52

5-3-3 Thin-Film Structure
To understand the differences in the photovoltaic performances, we conducted grazingincidence wide-angle X-ray scattering (GIWAXS) measurements and studied thestructural order of the materials, such as the backbone orientation and packing. Figures
23a–d depict the two-dimensional (2D) diffraction patterns of the polymer neat films, andthe corresponding diffraction profiles along the qz and qxy axes extracted from the 2D
patterns are shown in Figure 23i. For the PNTz4T neat film (Figure 23a), diffractions
assignable to the lamellar (h 0 0) and π–π stacking (0 1 0) structures were observed alongthe ~ qz and qxy axes, respectively, as previously reported, suggesting that the polymer
formed the edge-on orientation. For the NTz-BDT polymers, contrarily, the lamellar and
π–π stacking diffractions were observed along the qxy and ~ qz axes, respectively,
indicating the face-on orientation. These results indicated that the use of alkylthienyl-BDT
as a donor unit made the polymer backbone oriented parallel to the substrate plane. This
probably originated from the weaker intermolecular interactions in the NTz-BDT
polymers, compared with that of PNTz4T, probably because the bulky alkylthienyl side
chains on the BDT unit was twisted from the backbone plane. This was evident because
the d-spacing for the π–π stacking (dπ) of the NTz-BDT polymers (3.7–3.9 Å) was
significantly wider than that for PNTz4T (3.54 Å). Furthermore, the NTz-BDT polymers
provided smaller coherence length (LC) for the π–π stacking (16–18 Å), calculated using
the Scherrer’s equation, compared with that for PNTz4T (32 Å). This result was
reasonable because the torsion of the alkylthienyl side chain with respect to the BDT unit
might have affected the intramolecular interaction, thereby decreasing the polymer
crystallinity. Notably, halogenated polymers, PNTzBDT-F and PNTzBDT-Cl, had dπ
53

values of 3.71 and 3.81 Å, respectively, which were shorter than that of PNTzBDT (3.91)
Å. This was probably because the halogenation induced the local dipole, owing to theirelectron-withdrawing nature, which enhanced the intermolecular interaction.
The 2D GIWAXS patterns of the polymer/Y6 blend films are shown in Figures 23e–h,
and the corresponding diffraction profiles are shown in Figure 23j. The blend thin films
exhibited a π–π stacking diffraction along the qz axis. Thus, it was assumed that theorientation of PNTz4T changed to the face-on orientation in the blend film, which was
also observed when PNTz4T was blended with PCBM. Here, the dπ observed for the blend
films of the NTz-BDT polymers were approximately 3.6 Å, which was significantly
shorter than that observed for the neat film (3.7–3.9 Å) and was similar to that observed
for the Y6 neat film (3.60 Å) (Figure 24). Therefore, the π–π stacking diffraction observed
for the blend films of the NTz-BDT polymers were assignable to that of Y6. This indicatedthat the NTz-BDT polymers were possibly amorphous in these blend films. For PNTz4T,
in contrast, the dπ observed for the neat and blend films were practically unchanged at
approximately 3.6 Å. Thus, we assumed that the π–π stacking diffraction of PNTz4T andY6 overlapped in the blend film. Therefore, it was difficult to compare the crystallinity ofthe blend films for PNTz4T and the NTz-BDT polymers. Although we can compare thecrystallinity of Y6 in the blend films of the NTz-BDT polymers, the LC values of the π–πstacking of Y6 were practically the same for all the blend films of PNTzBDT (19 Å),
PNTzBDT-F (20 Å), and PNTzBDT-Cl (18 Å).

54

Figure 23 GIWAXS patterns of (a-d) polymer neat films and, (e-h) polymer/Y6 blend
films. Corresponding intensity profiles along the out-of-plane (solid line) and in-plane(dotted lines) direction of (i) polymer neat films and, (j) polymer/Y6 blend films.

Figure 24 GIWAXS patterns of Y6 non-fullerene acceptor neat film.

55

Subsequently, we conducted transmission electron microscopy (TEM) to investigate themorphology of the blend films (Figures 25a–d). The PNTz4T/Y6 and PNTzBDT/Y6
blend films formed similar fibrillar structures with fine networks as typically observed inmany blend systems that provide high photovoltaic performances. This facilitated exciton
dissociation and charge transport, although PNTz4T and PNTzBDT exhibited a rather
distinct solubility. For the halogenated polymers, the PNTzBDT-F/Y6 blend film also
exhibited a similar morphology, and the PNTzBDT-Cl/Y6 blend film exhibited large
aggregations, which might have been detrimental to their photovoltaic performance.
Therefore, the difference in the photovoltaic performance between PNTz4T and the NTzBDT polymers cannot be well explained by TEM. The low performance for the
PNTzBDT-Cl can be ascribed to the large aggregations. We performed atomic force
microscopy (AFM) to further investigate the morphology of the blend films (Figures 25e–h). The blend films of the NTz-BDT polymers exhibited smaller domains with smoothersurfaces than those of the PNTz4T/Y6 blend film. These internal and surface
morphologies could partially explain the difference in the photovoltaic performances inthese polymers.

56

Figure 25 TEM and AFM images of the Y6-blend films for (a, e) PNTzBDT/Y6, (b, f)
PNTzBDT-F/Y6, (c, g) PNTzBDT-Cl/Y6, and (d, h) PNTz4T/Y6.

In addition, we measured the contact angles of the neat films of the polymers and Y6(Figure 26) and calculated their surface and interfacial energies, which are summarized inTable 5, to discuss their miscibility.80-81 The interfacial energy, i.e., the Flory–Huggins
interaction parameter, χ, is expressed by the following equation, χ = k(√γD –√γA)2, where
k is a constant, and γD and γA are the surface energies of the neat films of the polymers andY6, respectively. The (√γD –√γA)2 values are also summarized in Table 4. The large
difference in the surface energy corresponded to a large interfacial energy and thus large
repulsive interaction (low miscibility) between the two materials.82 The NTz-BDT
polymers exhibited higher γD values (28–31 mN m−1) than that of PNTz4T (24 mN m−1).
Among the NTz-BDT polymers, PNTzBDT exhibited lower surface energy than
PNTzBDT-F and PNTzBDT-Cl, which could have originated from the larger polarity of57

the halogen atoms.83 Although the NTz-BDT polymers exhibited significantly higher
solubility than PNTz4T, the (√γD –√γA)2 values for the blend films of the NTz-BDT
polymers (0.035–0.191) were larger than that for the PNTz4T (0.030). Notably, because
the surface roughness and crystallinity between the neat films of the NTz-BDT polymers
and PNTz4T differed, it may be inappropriate to discuss the miscibility between polymer
and Y6 using the (√γD –√γA)2 values. Nevertheless, the NTz-BDT polymers mostly
exhibited similar surface roughnesses and crystallinities, which enabled us to compare themiscibility using this method. PNTzBDT/Y6 and PNTzBDT-F/Y6 had (√γD –√γA)2 valuesof 0.035 and 0.082, respectively, which were smaller than that of PNTzBDT-Cl/Y6(0.191), suggesting that the first two exhibited better miscibility than the latter, correlatingwith the TEM results.

Figure 26 Contact angle of water and glycerol on PNTzBDT, PNTzBDT-F, PNTzBDTCl, PNTz4T and Y6 films.

58

Table 5 Summary of contact angle of water and glycerol on each polymer, the surface
tensions of and the corresponding χ parameters of all blend films.
polymer

θwaterθglycerolγD (mN m−1) a

γA (mN m−1) b

PNTzBDT

99

82

27.86

–

0.035

PNTzBDT-F

101

82

28.94

–

0.082

PNTzBDT-Cl

102

82

30.58

–

0.191

PNTz4T

104

89

24.21

–

0.030

Y6

91

81

–

25.94

-

a

(√γD –√γA)2

γD: surface energy of the polymers. γA: surface energy of Y6. bparameter for the interfacial

energy χ, which is defined as 𝜒 = 𝑘(√𝛾/ − √𝛾0 )-

59

5-4

Summary
Here, we synthesized a series of NTz-BDT semiconductive polymers, PNTzBDT,PNTzBDT-F, and PNTzBDT-Cl. On the one hand, the alkylthienyl side chains provided
torsion with respect to the BDT moiety, which suppressed the crystallinity of the polymers,compared with that of the crystalline NTz-based polymer, PNTz4T. On the other hand,these side chains probably improved the solubility of the polymer and in turn, the solution
processability and possibly better miscibility with Y6. The NTz-BDT polymers exhibitedwider optical bandgaps and deeper HOMO energy levels than PNTz4T because of the
weak electron-donating nature of BDT; the halogenation of the alkylthienyl substituents
further deepened the EHOMO. The photovoltaic performance of the NTz-BDT polymers wasstudied by fabricating photovoltaic cells with a conventional structure using Y6 as theacceptor material. All the cells based on the NTz-BDT polymers exhibited a higher VOC
than that of PNTz4T. Among them, the PNTzBDT-F cell exhibited the highest PCE of
13.3%, which was significantly higher than that of the PNTz4T cell (11.1%). To the bestof our knowledge, we reported for the first time organic photovoltaic cells using NTzbased polymers. The results indicated that semiconductive polymers based on NTz, by
rational molecular design, offer high potential for use in non-fullerene cells.

60CHAPTER 6 NTz-TPTz Random Copolymers
Triphenyleno[1,2-c:7,8-c’]bis([1,2,5]thiadiazole (TPTz, Figure 27) was designed andsynthesized by Dr. Tajima’s research group in Riken. The TPTz unit exhibited strong
electron-accepting capability and an expanded aromatic plane. The target was to use the
TPTz unit to construct a copolymer with a V-shape structure through the D–A system,
which offers a new way to design a new semiconductive polymer.63 Therefore, what is thereason behind using this kind of TPTz unit in this study?

Figure 27 Chemical structure of TPTz unit unit and the illustration of V-shape structure.

The natural structure of PNTz4T constituting a linear and rigid polymer structure
backbone enabled PNTz4T to densely pack together, thereby ensuring high polymer
crystallinity. In contrast, it indicated that PNTz4T had relatively low solubility. Therefore,
we considered that the polymer structure would be a critical point affecting the solubility.
Here, our strategy was to introduce a small amount of the TPTz unit into PNTz4T topartially destroy the linear structure and construct a foldable NTz-based random
copolymer. It was expected that the random copolymers could improve the solubility while
preserving the polymer crystallinity. Here, we investigated NTz-TPTz random copolymers
in which the TPTz unit was added in small amounts to construct a new semiconductive
random polymer. According to the results of a previous study, the side chain on the TPTz
61

unit had a major effect on performance and solubility. The effect of the different alkyl
sidechains on the TPTz unit and the adding ratio of TPTz on the photovoltaic performanceand molecular orientation are discussed herein.
The synthetic route for the NTz-TPTz random copolymers is shown in Scheme 3,
where NTz2T-Br2 and the corresponding TPTz unit were copolymerized via the Stillecoupling reaction in a yield of approximately 90%. NTz2T-Br2 and PNTz4T weresynthesized according to the previous report.48 Here, we synthesized NTz-TPTz random
copolymers with a TPTz unit that possesses different alkyl sidechain substituents,
including C8-5, EH-5, C12-5, and BO-5 (Figure 28). Furthermore, we synthesized NTzTPTz random copolymers in which the blending ratio of the TPTz units was investigated(blending ratio of 2.5%–10%), namely C12-2.5, C12-5, C12-7.5, and C12-10. The NTzTPTz random copolymers exhibited a significantly improved solubility, compared withthat of PNTz4T (Table 6). PNTz4T exhibited a solubility of 3 mg mL−1. When 2.5% of
TPTz was introduced into PNTz4T, the solubility significantly improved (27 mg mL−1).
However, as the ratio increased to 10%, the solubility (15.6 mg mL−1) decreased but
remained better than the initial value, which was consistent with a previous report63 thatthe D–A copolymer of the TPTz unit exhibited a low solubility (0.3 mg mL−1). The opticaland electrochemical properties, photovoltaic performances, and morphology were
carefully investigated. Consequently, we obtained a PCE of 10.8% in the non-fullerene
solar cell.

62

N
Br

S

N

S
S

C12H25

C10H21NS

RO

C12H25

C10H21

+

Br

S
N

S

N

3

C10H21

N

S

Toluenemicrowave 200 oC, 2 hr

C12H25

C12H25SS

Pd(PPh3)4

2

1

S

Me3Sn

Br

Br

SnMe3

S

N

N

S
N

N

OR

C10H21

N

S

S

N

S

TPTz co1-nreNTz-TPTz random copolymer(R= C8, CEH, C12, CBO)(n = 2.5%, 5%, 7.5%, 10%)

Scheme 3. Synthetic route to the NTz-TPTz random copolymers.

N

S

N

N

S

N

S

S

N

n

C10H21

C12H25N

C4 H 9
OC8H17

N

N

Br

Br

A3-C8 (n = 5%)

1-nTPTz coreS(x= 2.5%, 5%, 7.5%, 10%)

TPTz Unit

S
N

S

S

N

PNTz4T-A3CR-x%

PNTz4T

C8H17OSS

S

S

C10H21

C12H25

C10H21NSS

C12H25

C12H25

C10H21C2 H 5 C2 H 5
C4H9
O

S
N

S
N

C12H25OO

C4 H 9OC12H25

C6H13 C6H13
C4H9
O

N

N

Br

Br

S
N

S
N

N

Br

A3-CEH (n = 5%)

N

S
N

S
N

O

N

N

Br

Br

S
N

Br

A3-C12(n = 2.5, 5, 7.5, 10%)

A3-CBO (n = 5%)

Figure 28 Chemical structure of PNTz4T and NTz-TPTz random copolymers.

Table 6 Solubility Estimation of PNTz4T and NTz-TPTz random copolymers.
polymersolubility(mg ml–1)

polymersolubility(mg ml–1)

PNTz4T

3.0

C12-2.5

27.3

C8-5

23.1

C12-5

15.4

EH-5

26.2

C12-7.5

27.2

BO-5

27.9

C12-10

15.6

63

S

n

S

n

6-1

Physical Property Evaluation
The chemical structure of PNTz4T and the NTz-TPTz random copolymers are shown in
Figure 28. The NTz-TPTz random copolymers were soluble in CF solvent at room
temperature whereas PNTz4T needed to be dissolved in hot CF, which might have affected
its solution processability. The thermal properties of these polymers were examined by
DSC (Figure 29). C8-5, EH-5, C12-5, and BO-5 exhibited melting points at 313, 313, 312,
and 316 °C, respectively. Additionally, C12-2.5, C12-7.5, and C12-10 exhibited melting
points at 313, 308, and 308°C, respectively, whereas PNTz4T exhibited a melting peak at
317 °C. This implied that the polymers were thermally stable in this temperature range.

Figure 29 DSC thermograms of PNTz4T and NTz-TPTz random copolymers.

646-1-1 Different Alkyl Sidechain on TPTz Unit
The UV–VIS absorption spectra for these polymers in solution and the polymer thin films
are shown in Figure 30. The absorption maximum (λmax), absorption edge (λedge), andoptical bandgap (Egopt) were calculated and are summarized in Table 7. The polymer
solution and thin film exhibited a spectrum with the main absorption band ≈ 500–800 nm.
In the polymer solution (Figure 30a), PNTz4T exhibited λmax = 716 nm, in which a 0–0
band was observed, indicating strong aggregation in the solution. The NTz-TPTz random
copolymers exhibited λmax = 664, 666, 672, and 661 nm for C8-5, EH-5, C12-5, and BO5, respectively. The introduction of TPTz reduced the polymer coplanarity, producing asignificant blueshift, compared with PNTz4T, indicating the weaker intermolecularinteractions of the NTz-TPTz random copolymers. A platform-like absorption peak was
observed in the solution. In the thin film (Figure 30b), the NTz-TPTz random copolymers
significantly redshifted from that of PNTz4T (λmax = 714 nm), indicating a stronger
aggregation for the NTz-TPTz random copolymers in the thin film. Additionally, PNTz4T
and the NTz-TPTz random copolymers exhibited two peaks, revealing that the polymers
were highly ordered in the thin film, and the alkyl side chain on the TPTz unit hardly
influenced the molecular ordering.
The EHOMO and ELUMO of the polymers were estimated by conducting CV using thepolymer thin films (Figure 30c). First, PNTz4T had an EHOMO of −5.20 eV and ELUMO of−3.46 eV, whereas the NTz-TPTz random copolymers exhibited a lower EHOMO than thatof PNTz4T, resulting in a larger electrochemical HOMO–LUMO gap of > 1.80 eV for the
NTz-TPTz random copolymers, compared with that for PNTz4T (1.7 eV). This suggested
that the introduction of TPTz (with different alkyl sidechains) had a greater influence on65the EHOMO than the ELUMO. Notably, the NTz-TPTz random copolymers exhibited similarenergy levels, C8-5 (−5.36 and −3.60 eV), EH-5 (−5.37 and −3.57 eV), C12-5 (−5.36 and−3.56 eV), and BO-5 (−5.38 and −3.54 eV) because of the use of the electron-withdrawing
TPTz unit. However, the alkyl sidechain hardly influenced the energy level.

Figure 30 (a,b) UV-vis absorption spectra of the NTz-TPTz random copolymers (different
alkyl sidechain on TPTz unit depend) in the solution (a) and thin film (b). (c) Cyclic
voltammograms of the thin films for the NTz-TPTz random copolymers vs Ag/Ag+ as thereference electrode.

66

Table 7 Optical and electrochemical properties of the polymer thin films.
ELUMO

EHOMO

λmaxsol

λmaxfilm

λedge

Egopt

(eV) a(eV) a(nm)b(nm) c(nm) c(eV) b

PNTz4T

−3.46

−5.20

716

657, 714

776

1.60

C8-5

-3.60

−5.36

664

650, 722

805

1.54

C12-5

−3.56

−5.36

672

651, 721

803

1.55

EH-5

−3.57

−5.37

666

653, 720

808

1.53

BO-5

−3.54

−5.38

661

664, 717

803

1.55

Polymera

ELUMO and EHOMO determined by the onset oxidation and reduction potential of the cyclic

voltammograms; b Absorption maximum determined from solution absorption spectrum; cAbsorption maximum, absorption edge and optical bandgap determined from thin film absorption
spectrum.

67

6-1-2 Different TPTz Ratio
The UV–VIS absorption spectra for these polymers in solution and the polymer thin films
are shown in Figure 31. The absorption maximum (λmax), absorption edge (λedge), andoptical bandgap (Egopt) were calculated and are summarized in Table 8. The polymer
solution and thin film exhibited a spectrum with the main absorption band ≈ 500–800 nm.
In the polymer solution (Figure 31a), the NTz-TPTz random copolymers significantly
blue-shifted, compared with that of PNTz4T (λmax = 716), which afforded λmax = 678, 672,
669, and 669 nm for C12-2.5, C12-5, C12-7.5, and C12-10, respectively. This suggested
that the introduction of the TPTz unit affected the coplanarity, owing to the weakintermolecular interactions of the NTz-TPTz random copolymers. In the thin film (Figure31b), C12-2.5 (λmax = 720), C12-5 (λmax = 721), and C12-7.5 (λmax = 717) slightly redshiftedfrom that of PNTz4T (λmax = 714), implying that the small proportion of TPTz in PNTz4T
slightly enhanced the intermolecular interactions of the NTz-TPTz random copolymers.
Nevertheless, C12-10 blueshifted, compared with PNTz4T and the NTz-TPTz random
copolymers (2.5%–7.5%), owing to the strong electron-withdrawing effect of the TPTz
unit, in which the donor–acceptor interaction weakened because the TPTz unit exceededa certain amount. Moreover, PNTz4T and the NTz-TPTz random copolymers wereobserved to exhibit two peaks; the peak in the shorter wavelength region appeared as ashoulder. The peaks were assigned to the 0–0 (700–720 nm) and 0–1 (650–670 nm)
vibrational bands. The intensity ratio of the 0–0 to 0–1 band decreased as the ratio
increased, implying that C12-2.5 and C12-5 were relatively ordered, compared with C127.5 and C12-10.
The EHOMO and ELUMO of the polymers were estimated by conducting CV using the68polymer thin films (Figure 31c). The NTz-TPTz random copolymers exhibited a lower
EHOMO than those of PNTz4T (−5.20 eV), C12-2.5 (−5.38 eV), C12-5 (−5.36 eV), C127.5 (−5.37 eV), and C12-10 (−5.37 eV). However, the adding ratio of TPTz did not affectthe energy level, resulting in larger electrochemical HOMO–LUMO gaps of C12-2.5 (1.84eV), C12-5 (1.80 eV), C12-7.5 (1.83 eV) and C12-10 (1.80 eV), compared with that of
PNTz4T (1.75 eV).

Figure 31 (a,b) UV-vis absorption spectra of the NTz-TPTz random copolymers in thesolution (a) and thin film (b). (c) Cyclic voltammograms of the thin films for the NTzTPTz polymers random copolymers vs Ag/Ag+ as the reference electrode.

69

Table 8 Optical and electrochemical properties of the polymer thin films.
ELUMO

EHOMO

λmaxsol

λmaxfilm

λedge

Egopt

(eV) a(eV) a(nm) b(nm) c(nm) c(eV) b

PNTz4T

−3.46

−5.20

716

657, 714

776

1.60

C12-2.5

−3.54

−5.38

678

654, 720

800

1.55

C12-5

−3.56

−5.36

672

651, 721

803

1.55

C12-7.5

−3.54

−5.37

669

663, 717

795

1.56

C12-10

−3.57

−5.37

669

654, 702

795

1.56

Polymera

EHOMO and ELUMO determined by the onset oxidation and reduction potential of the cyclic

voltammograms; b Absorption maximum determined from solution absorption spectrum; cAbsorption maximum, absorption edge and optical bandgap determined from thin film absorption
spectrum.

70

6-2

OPV Properties of NTz-TPTz Polymers

6-2-1

Different Alkyl Sidechain on TPTz Unit

The photovoltaic performances of the polymers were investigated by fabricating devices
with a conventional architecture (ITO/PEDOT:PSS/active layer/ZnO/Ag). The optimal
polymer to Y6 weight ratio was observed to be 1:1.2, and the optimal active layer thickness
was 90–100 nm for all cases. The representative J−V and EQE characteristics of the
optimized cells are shown in Figures 32a and b, respectively. The corresponding
photovoltaic parameters including the VOC, JSC, FF, and PCE (maximum and average) aresummarized in Table 9.
Although C8-5, EH-5, C12-5, and BO-5 (−5.37 eV) exhibited slightly lower EHOMO
values than that of PNTz4T (−5.31 eV), the cells with NTz-TPTz random copolymers
exhibited a slightly higher VOC ≈ 0.72–0.74 V, compared with that of PNTz4T (0.72 V).
The devices exhibited a similar JSC of approximately 21 mA cm−2. Notably, the maindifference was the FF. The PNTz4T devices had an FF of 0.68. After the introduction of
the TPTz unit, the FF slightly decreased for the C8-5 and EH-5 devices (0.65 and 0.62,
respectively). As the alkyl sidechain on the TPTz unit lengthened, the C12-5 device
exhibited a higher FF (0.70), compared with that of PNTz4T (0.68), but the FF reduced
for the BO-5 device (0.64). This indicated that although the alkyl sidechain length was thesame, the linear or branch alkyl chain could still influence the polymer structure, resulting
in a slight difference in the performances. These JSC values were consistent with the EQE
spectra. Overall, the C12-5 device achieved a PCE of 10.8%, which was higher than thatof the PNTz4T device (10.1%), owing to the slightly higher VOC and FF. In Chapter 6-22, we discuss the NTz-TPTz random copolymers with different TPTz ratios, in which the71alkyl sidechain on the TPTz unit is fixed with C12.

Figure 32 (a) J−V curves and (b) EQE spectra of the optimized organic solar cells using
the NTz-TPTz random copolymers combined with Y6.

Table 9 Summary of photovoltaic parameters of the optimized polymer/Y6 devices.
Polymer

VOC (V)

JSC [JSCEQE]a(mA cm−2)

FF (–)

PCEmax [PCEave](%) b

PNTz4T

0.72

20.7 [20.7]

0.68

10.1 [9.7 ± 0.6]

C8-5

0.72

20.7 [20.5]

0.65

9.8 [9.5 ± 0.2]

C12-5

0.74

20.7 [20.8]

0.70

10.8 [10.4 ± 0.4]

EH-5

0.73

20.1 [18.0]

0.62

9.1 [8.8 ± 0.3]

BO-5

0.73

20.2 [19.5]

0.64

9.5 [9.3 ± 0.2]a

JSC calculated from the EQE spectrum bMaximum PCE and the average PCE, obtained

from more than 10 cells, with standard deviation in the square bracket.

72

6-2-2 Different TPTz Ratio
The photovoltaic performances of the polymers were investigated by fabricating devices
with a conventional architecture (ITO/PEDOT:PSS/polymer/Y6/ZnO/Ag). The optimal
polymer to Y6 weight ratio was observed to be 1:1.2, and the optimal active layer thickness
was 90–100 nm for all cases. The representative J–V and EQE characteristics of the
optimized organic solar cells are shown in Figures 33a and b, respectively. The
corresponding photovoltaic parameters including the VOC, JSC, FF, and PCE (maximum
and average) are summarized in Table 10.
The NTz-TPTz random copolymers with different TPTz ratios exhibited similar EHOMO
values but were lower than that of PNTz4T. The cells with NTz-TPTz random copolymers
exhibited VOC values in the range of 0.74−0.75 V, which were slightly higher than thoseof the cells with PNTz4T (0.72 V). C12-2.5 exhibited a JSC of 19.6 mA cm−2, which was
similar to that of PNTz4T (20.7 mA cm−2). When the TPTz ratio increased to 5%, C12-5
exhibited a JSC of 20.7 mA cm−2, which was slightly higher than that of C12-2.5. However,as the TPTz ratio exceeded 7.5%, the cells with C12-7.5 and C12-10 exhibited
significantly lower JSC values of 18.3 and 19.1 mA cm−2, respectively. These JSC values
were consistent with the EQE spectra. The C12-2.5, C12-7.5, and C12-10 devices
exhibited high VOC; their slightly low JSC and FF limited their photovoltaic performances.

73

Figure 33 (a) J−V curves and (b) EQE spectra of the optimized organic solar cells using
the NTz-TPTz random copolymers combined with Y6.

Table 10 Summary of photovoltaic parameters of the optimized polymer/Y6 devices.
Polymera

VOC (V)

JSC [JSCEQE]a(mA cm−2)

FF (–)

PCEmax [PCEave] (%) b

PNTz4T

0.72

20.7 [20.7]

0.68

10.1 [9.7 ± 0.6]

C12-2.5

0.74

19.6 [18.4]

0.66

9.6 [9.0 ± 0.6]

C12-5

0.74

20.7 [20.8]

0.70

10.8 [10.4 ± 0.4]

C12-7.5

0.75

18.3 [17.4]

0.64

8.8 [8.1 ± 0.9]

C12-100.74

19.1 [18.5]

0.65

9.1 [8.3 ± 0.6]

JSC calculated from the EQE spectrum bMaximum PCE and the average PCE, obtained

from more than 10 cells, with standard deviation in the square bracket.

74

6-3

Charge Recombination Behavior

6-3-1 Different Alkyl Sidechain on TPTz Unit
Charge recombination is a crucial factor that affects photovoltaic performance. Thus, weinvestigated the devices by probing the dependence of JSC and VOC on the light intensity(Plight). Figure 34a displays the light-intensity dependence of JSC. According to thefollowing equation, JSC ∝ Plight 𝛼,77-78 the 𝛼 value closer to 1 indicates that the charge
carriers can be efficiently collected at the electrodes by avoiding the bimolecular
recombination. The 𝛼 value varying from 1 indicates that bimolecular recombination is
not negligible under the short-circuit condition. As shown in Figure 34a, the devices
exhibited similar 𝛼 values in the range of 0.99–1.01, suggesting that the bimolecular
recombination was not serious, and there was no significant difference for these devices.
Figure 34b illustrates the light-intensity dependence of the VOC, which can be used toanalyze trap-assisted recombination. The VOC was fitted to the linear law VOC ∝nkBT/qln(Plight),78-79 where kB, q, and T are the Boltzmann constant, elementary charge,
and Kelvin temperature, respectively. Factor n for the VOC–ln(Plight) plot being equal to 1
suggested that the trap-assisted recombination was negligible; however, n deviating from1 suggested that the trap-assisted recombination existed under the open-circuit condition.
As shown in Figure 34b, the C12-5 device exhibited an n of 1.06 that was slightly closer
to 1, compared to that of the PNTz4T device (n = 1.18), indicating that the trap-assisted
recombination slightly reduced. This correlated with the results that the C12-5 device
exhibited a slightly larger FF than that of the PNTz4T device. Furthermore, the C8-5 (n =1.24), EH-5 (n = 1.41), and BO-5 (n = 1.33) devices exhibited n values higher than thatof C12-5, which correlated with the variation in FF.
75

Figure 34 Light-intensity dependence of (a) JSC and (b) VOC for the polymer/Y6 devices.

76

6-3-2 Different TPTz Ratio
As previously mentioned in Chapter 5-3-1, we can analyze the bimolecular and trapassisted recombination behavior of the devices by probing the dependence of the JSC and
VOC on the light intensity. As shown in Figure 35a, the devices exhibited similar 𝛼 values
in the range of 0.99–1.01, suggesting that the bimolecular recombination was not serious,and there was no significant difference for these devices. Figure 35b illustrates the lightintensity dependence of the VOC. The C12-7.5, C12-7.5, and C12-10 (n = 1.20, 1.40, and1.38, respectively) devices exhibited n values significantly higher than that of the C12-5
device (n = 1.06). This suggested that the trap-assisted recombination was serious, and theresults correlated with the variation in FF. As a result, the C12-5 device suppressed thecharge recombination, resulting in the highest FF among all the devices.

Figure 35 Light-intensity-dependence of (a) JSC and (b) VOC for the polymer/Y6 devices.

77

6-4

Thin Film Structure

6-4-1 Different Alkyl Sidechain on TPTz Unit
We performed GIWAXS measurements and studied the structural order of the materials,
such as the backbone orientation and packing to understand the differences in thephotovoltaic performances. Figures 36a–e show the 2D diffraction patterns of the polymer
neat films with different alkyl side chains on the TPTz unit, and the corresponding
diffraction profiles along the qz and qxy axes extracted from the 2D patterns are shown in
Figure 36k. For the PNTz4T neat film (Figure 36a), diffractions assignable to lamellar (h0 0) and π–π stacking (0 1 0) structures were observed along the ~qz and qxy axes,
respectively, indicating that the PNTz4T formed the edge-on orientation. In contrast, theNTz-TPTz random copolymers exhibited the lamellar and π–π stacking diffractions along
the qxy and ~qz axes, respectively, indicating the face-on orientation for these polymers.
The results indicated that the introduction of the TPTz unit into PNTz4T made the polymer
backbone oriented parallel to the substrate. The d-spacing of the π–π stacking (dπ) for the
NTz-TPTz random copolymers (~3.55 Å) was similar with that of PNTz4T (3.57 Å).
Although the introduction of TPTz did not significantly affect the intermolecularinteraction, the lamellar distance of these NTz-TPTz random copolymers became wider(~24 Å) than that of PNTz4T (22 Å), implying that the introduction of TPTz suppressed
the lamellar interaction, providing a better solubility than that of PNTz4T. Additionally,
the NTz-TPTz random copolymers exhibited slightly smaller coherence length (LC) fromthe π–π stacking (17–23 Å), compared to that of PNTz4T (32 Å), calculated using theScherrer’s equation. The results indicated that the introduction of TPTz suppressed thelamellar interaction.
78

The GIWAXS patterns of the polymer/Y6 blend films are shown in Figures 36f–j, andthe corresponding diffraction profiles are shown in Figure 36l. The blend thin films wereobserved to exhibit a π–π stacking diffraction along the qz axis. The dπ observed for the
blend films of the NTz-TPTz random copolymers was shorter than that observed for theneat film and far away from that observed for the Y6 neat film (3.60 Å). Therefore, the π–
π stacking diffraction in the blend films of the NTz-TPTz random copolymers was
assigned to the polymer. In contrast, the dπ for the PNTz4T neat and blend films werevirtually unchanged at approximately 3.6 Å, and the diffraction patterns were formed
differently. We assumed that the π–π stacking diffractions of PNTz4T and Y6 overlapped
for the PNTz4T blend film (Figure 36f). We calculated the variation in the crystallinity ofthe blend films of the NTz-TPTz random copolymers. The LC values for the π–π stacking
in the blend films of C8-5, EH-5, C12-5, and BO-5 were 25, 26, 26, and 20 Å, respectively.
As a result, the NTz-TPTz random copolymers with different alkyl side chains provided
better solubility than that of PNTz4T while preserving the crystallinity of the polymer thinfilms.

79

Figure 36 GIWAXS patterns of (a-e) polymer neat films and, (f-j) polymer/Y6 blend films.
Corresponding intensity profiles along the out-of-plane (solid line) and in-plane (dottedlines) direction of (k) polymer neat films and, (l) polymer/Y6 blend films.

We conducted TEM to investigate the morphology of the blend films (Figure 37). The
PNTz4T/Y6 blend film formed similar fibrillar structures with fine networks, as typicallyobserved in many blend systems that provide high photovoltaic performance, whichfacilitated exciton dissociation and charge transport. The NTz-TPTz/Y6 blend film formeda similar grid-like structure different from that of the PNTz4T/Y6 blend film. However,the difference in the photovoltaic performance between PNTz4T and the NTz-TPTz
random copolymers could not be explained by TEM. Therefore, we probed the relationship80

between the thin film morphology and surface roughness by AFM. The PNTz4T/Y6 blend
film (Figure 37a, down) formed a large polymer aggregate domain that led to a coarse
morphology (root mean square (RMS) = 2.0 nm). The NTz-TPTz/Y6 blend films exhibiteda smoother surface than that of the PNTz4T/Y6 blend film, particularly the random
copolymers with a linear alkyl sidechain on the TPTz unit, C8-5/Y6 and C12-5/Y6 (RMS
~ 0.8 nm). Therefore, the highest FF of the cells (0.70) with C12-5 contributed to a morefavorable surface morphology, compared with the PNTz4T/Y6 blend film. We observed
an increase in surface roughness to RMS ~1.5 and ~1.1 nm for EH-5/Y6 and BO-5/Y6,
respectively, which might have been detrimental to the FF; hence, their low photovoltaic
performances.

Figure 37 TEM & AFM images of polymer/Y6 blend films. (a) PNTz4T/Y6, (b) C8-5/Y6,(c) EH-5/Y6, (d) C12-5/Y6 and (e) BO-5/Y6.

81

6-4-2 Different TPTz Ratio
We performed GIWAXS measurements and studied the structural order of the materials tounderstand the differences in the photovoltaic performances. Figures 38a–e shows the 2D
diffraction patterns of the polymer neat films with different TPTz ratios, and thecorresponding diffraction profiles along the qz and qxy axes extracted from the 2D patterns
are shown in Figure 38k. First, C12-2.5 exhibited lamellar and π–π stacking diffractions
along the ~qz and qxy axes, respectively, indicating edge-on orientation. Interestingly, when
the ratio of the TPTz unit increased to 5%, C12-5 exhibited π–π stacking diffractions alongthe ~ qz and qxy axes, suggesting that both edge-on and face-on orientations existed for theC12-5 neat film. Furthermore, as the ratio exceeded a certain amount, C12-7.5 and C1210 exhibited lamellar and π–π stacking diffractions along the qxy and ~qz axes, respectively,
indicating face-on orientation. The results indicated that the introduction of the TPTz unit
into PNTz4T gradually turned the polymer backbone into a face-on orientation. The dπ ofthe neat films (C12-2.5, C12-5, C12-7.5 and C12-10) were 3.54, 3.55, 3.60, and 3.60 Å,
respectively. The NTz-TPTz random copolymers were observed to relatively maintaintheir intermolecular interaction. However, lamellar interaction was suppressed because of
the wider lamellar distance between these NTz-TPTz random copolymers (~24 Å),compared with that of PNTz4T (22 Å), which caused the better solubility of the NTzTPTz random copolymers. Additionally, the NTz-TPTz random copolymers exhibited
smaller coherence length (LC) from the π–π stacking (16–21 Å), compared with that of
PNTz4T (32 Å), suggesting that the introduction of TPTz slightly affected the crystallinity.
The GIWAXS patterns of the polymer/Y6 blend films are shown in Figures 38f–j, andthe corresponding diffraction profiles are shown in Figure 38l. The blend thin films were82

observed to exhibit a π–π stacking diffraction along the qz axis. First, the dπ observed forthe blend films were similar, approximately ~3.58 Å, which was extremely close to thatobserved for the Y6 neat film (3.60 Å). Therefore, the π–π stacking diffraction observed
for the polymer blend films was assigned to that of Y6. The LC values for the π–π stacking
in the blend films of C12-2.5, C12-5, C12-7.5, and C12-10 were 27, 26, 22, and 20 Å,
respectively. Although it was difficult to use the π–π stacking diffraction distance to clarify
the difference for these polymer blend films, the crystallinity gradually decreased as theratio increased, which correlated to the PCE variation in the solar cells.

Figure 38 GIWAXS patterns of (a-d) polymer neat films and, (e-h) polymer/Y6 blend
films. Corresponding intensity profiles along the out-of-plane (solid line) and in-plane(dotted lines) direction of (i) polymer neat films and, (j) polymer/Y6 blend films.

83

Next, we performed TEM and AFM to further understand the impact of the TPTz ratio on
the surface morphology (Figure 39). The NTz-TPTz/Y6 blend films exhibited similar gridlike structures, as shown in the TEM images; there was no huge difference. For the AFM
analysis, C12-2.5/Y6 and C12-5/Y6 (Figures 39a and b downward) demonstrated goodphase separation in their blend thin films that afforded lower surface roughness, comparedwith that of PNTz4T/Y6 (RMS = 2 nm, Figure 37a). However, the polymer aggregateddomain increased as the TPTz ratio increased, particularly the C12-10/Y6 blend film thatexhibited a coarse thin film (RMS = 2.4 nm). The results indicated that the surface
morphology greatly influenced the photovoltaic performance. A smooth surface wasbeneficial for interfacial charge transfer, and it improved the performance.

Figure 39 TEM and AFM images of polymer/Y6 blend films. (a) C12-2.5/Y6, (b) C125/Y6, (c) C12-7.5/Y6 and (d) C12-10/Y6.

84

6-5

Summary
We synthesized new semiconductive polymers, NTz-TPTz random copolymers, with a
TPTz unit possessing different lengths of alkyl substituents, including C8-5, EH-5, C125, and BO-5. Furthermore, the impact of the TPTz ratio on C12-2.5, C12-5, C12-7.5, andC12-10 are discussed. These NTz-TPTz random copolymers exhibited better solubility
than that of PNTz4T because of the suppressed lamellar interaction through the partiallydestroyed linear polymer structure of PNTz4T. Additionally, the increase in the TPTz ratio
caused the polymer packing to gradually change from edge-on (C12-2.5) and bimodal(C12-5) to face-on orientation as the ratio exceeded 7.5% (C12-7.5 and C12-10).
Moreover, the NTz-TPTz/Y6 polymer blend film exhibited decreased surface roughness,compared with that of PNTz4T/Y6, whose surface morphology was a critical point that
affected the photovoltaic performances. As a result, the C12-5 devices exhibited thehighest PCE of 10.8% in combination with Y6.

85

CHAPTER 7 CONCLUSION
We designed and synthesized two series of NTz-based polymers, BDT-NTz polymers and
NTz-TPTz random copolymers as shown below. These NTz-based polymers exhibited
better solubility than the crystalline polymer (PNTz4T) that our group previously
developed. In the BDT-NTz polymers, the BDT moiety with alkylthienyl substituentsafforded steric hindrance between the polymer backbones that suppressed the crystallinity,
which enhanced the solubility and electronic properties. Both of which are important forhigher efficiencies in non-fullerene-based cells. In contrast, the introduction of TPTz
partially destroyed the linear structure of PNTz4T, enhancing the solubility without
sacrificing the polymer crystallinity. The BDT-NTz polymers and NTz-TPTz random
copolymers exhibited face-on orientation, which was favorable for charge transport inphotovoltaic cells. Consequently, these new NTz-based polymers exhibited relatively highefficiencies of ~ 13.3% (BDT-NTz polymers) and 10.8% (NTz-TPTz random copolymers)
in non-fullerene-based photovoltaic cells, which were even higher than the efficiencies
obtained for PNTz4T/non-fullerene photovoltaic cells.

Figure 40 Chemical structure of (a) NTz-BDT polymers, and (b) NTz-TPTz random
copolymers.

86

ACKNOWLEGEMENTSFirst, I would like to express my sincere gratitude to my advisor, Prof. Itaru Osaka, for the
support for my PhD, patience, enthusiasm, and immense knowledge. His guidance helped
me to complete my PhD.
I would not have completed this study without the support of Dr. Masahiko Saito, Dr.
Tsubasa Mikie, and my lab members, including Yoshikazu Teshima, Naoya Nakao, Kodai
Yamanaka, Takumi Tanaka, and Yuka Iwasaki. In addition, I would like to thank Dr.
Keisuke Tajima, Dr. Kyohei Nakano and Dr. Feng-Kung Chen of Riken for providing the
TPTz unit, Dr. Tomoyuki Koganezawa of the Japan Synchrotron Radiation ResearchInstitute (JASRI) for the GIWAXS measurement, Dr. Ryo Tanaka for the GPC
measurement, and Dr. Makoto Meada for the TEM measurement.
Finally, I would like to sincerely appreciate my parents and brothers for their love,
prayers, care, and financial support. They kept me going, and this study would not havebeen possible without them.

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