University of Groningen
Structure-property and film formation mechanism in PEDOT:PSS based and perovskites systems
Dong, Jingjin
DOI:
10.33612/diss.166892884
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Publication date: 2021
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Dong, J. (2021). Structure-property and film formation mechanism in PEDOT:PSS based and perovskites systems. University of Groningen. https://doi.org/10.33612/diss.166892884
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Chapter 1 Introduction
1.1 Organic semiconductors (OSCs)
Plastics (organic polymers) were defined as insulators in the earlier versions of scientific textbook. The synthesis of highly conductive polyacetylene by A. J. Heeger, A. G. Macdiarmid, and H. Shirakawa changed people’s mind completely in 1977.[1] Their original
work revealed that properly doped conjugated organic polymers are capable of presenting great electrical properties. This discovery was so revolutionary that the three scientists were awarded the 2000 Nobel Prize in Chemistry.
At a molecular level, conductive polymers feature a long sequence of conjugated double bonds ( conjugation) as shown in Figure 1.1. The delocalized electrons along the backbone compose the base of polymer electrical conductivity. However, due to the small amount of overlap between two adjacent -bonds, it is difficult to form a continuous energy band for a single chain, which leads to poor conductivity. This makes inter-molecular - coupling fundamental, as now the electrons can delocalize among different molecules, leading to the formation of a less discrete energy band that in the end leads to great conductivity.[2]
Figure 1.1 Scheme of conjugated double bonds along a polyacetylene chain and the overlap
between two adjacent -bonds.
According to quantum mechanics and quantum chemistry, the electronic behavior of semiconductors derives from the energy level transitions between intra- and inter-molecules. Various theories have been developed to describe the electron energy level structure and to predict the properties, including molecular orbital (MO) theory, ligand field theory (LFT) and energy band theory. Among them, MO and LFT mainly focus on the electron interactions, together with the energy level distributions and the electron behavior within the molecule, while energy band theory also applies when large amount of molecules pack together to form
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a solid state. Although the energy band theory was first developed for inorganic materials, it also works well for OSCs. In Figure 1.2(a-c), examples for molecular orbital of two-atom systems, delocalized molecular orbital of an octahedral compound and the electron band structure of polyacetylene are shown. A diagram of the specific energy level variation from isolated molecule to crystalline solid is shown in Figure 1.3, where HOMO stands for the "highest occupied molecular orbital" and LUMO stands for and the "lowest unoccupied molecular orbital". Normally, the HOMO and LUMO are located at the top of the valence band (VB) and at the bottom of the conduction band (CB), respectively. The band gap Eg
stands lies in between the VB and the CB.
Figure 1.2 Scheme of molecular orbital of (a) a two identical atom system; (b) delocalized
molecular orbital of an octahedral metal-ligand compound; (c) the electron band structure of polyacetylene. Adapted with permission from Springer.[3]
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Figure 1.3 Diagram of the specific energy level variation from isolated molecule to bulk
semiconductor. Adapted with permission from Wiley.[4]
Of course, some differences exist between organic and inorganic semiconductors. As mentioned above, the intermolecular interactions in organic solids are mainly van der Waals or London dispersion force, which is much weaker than the covalent interactions in the inorganic semiconductors. Thus, OSCs typically have relatively narrow energy bands and HOMO and LUMO can be easily disrupted by structural or chemical defects, which means the electrons in LUMO (or holes in HOMO) are more localized and thus take the way of hopping transport instead of band transport. The difference between these two transport mechanisms is summarized in Figure 1.4. In a perfect crystal, depicted as a straight line, free carriers are delocalized. However, there are always lattice vibrations that disrupt the crystal symmetry and cause scattering of the carriers at these phonons, leading to a limit of carrier mobility. For hopping transport, which is the case of most amorphous organic semiconductor materials, the carriers are localized due to large amount of defects, disorder etc., so that the carriers are more localized and the lattice vibrations are essential for the mobility (indicating a different temperature dependence).[5] Even in the case of semi-crystalline organic solids,
the concepts of allowed energy band is of limited validity, and excitations and interactions localized on individual molecules play a predominant role.[6] This explains why OSCs usually
have much lower charge carrier mobility.
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material and the same material with a phonon scattering spot. (b) hopping transport.
Furthermore, because of the localization effect, when an electron moves into the LUMO (in the case of n-type doping) or moves out from HOMO (in the case of p-type doping), there will be an electron-polaron or an hole-polaron formation, resulting in a relatively big relaxation energy (hundreds meV).[7,8] This process is illustrated in Figure 1.5. What can be
inferred is that the doping state will affect the electronic structure of OSCs, which is the basis of some characterization method applied in the following chapters. The main differences between the OSCs and inorganic materials are summarized in Table 1.1.
Figure 1.5 From ground state (left), removal of an electron from HOMO leads to
hole-polaron (middle), while injection an electron into LUMO leads to electron-hole-polaron (right). The energy level has a variation because of the relaxation between electrons and the crystalline lattice.
Table 1.1 Difference between OSCs and inorganic semiconductors.
Organic semiconductors Inorganic semiconductors
Material used conjugated organic compounds inorganic materials like Si,
GaAs
Bonding
covalent within the molecules and van der Waal interactions between the molecules
Covalent
Transport hopping transport band transport
Charge carrier mobility low, 10-5~40 cm2 (V s)-1 high, 102~104 cm2 (V s)-1
Crystallinity polycrystaline or amorphous single crystals or amorphous
Stability in air poor good
Absorption and emission high low
Dielectric constant low, 3-5 high, 10-15
Exciton binding energy high, 0.5-0.7 eV low, around 0.025 eV
Production cost cheap complex and expensive
Although suffering from the relatively low carrier mobility and poor stability (due to the chemical instability of the double bonds in air), OSCs hold great potential in various fields
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because of its low fabrication cost, varieties in structures and chemistry, simplicity in designing the molecular structure and mechanical flexibility suitable for soft, flexible devices. Beside the most famous organic light-emitting diode (OLED) display which has been commercialized, OSCs are showing more and more promising developments in fields such as organic field effect transistor (OFET), organic solar cells, thermoelectric (TE) generators, piezoelectricity and memristors.[9–15] Moreover, these organic based devices represent the
next generation flexible electronics, which would make a huge impact on our daily life in the future.
1.2 Poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonate) (PEDOT:PSS) 1.2.1 General information
PEDOT:PSS is one of the most stable conductive polymers exhibiting great electric properties, making it the most popular organic electronic material of the past decade.[16–19]
The chemical structure is shown in Figure 1.6a. PEDOT:PSS is a p-type semiconductor, with PSS acting at the same time as a dopant and as the matrix. The sulfonate acid groups in PSS can efficiently dope the PEDOT by accepting the electrons from PEDOT to form the ionic complex PEDOT+:PSS-. As PEDOT is almost insoluble in any solvent, PSS is essential to
help dispersing PEDOT in water.[20] Due to the electrostatic force, PEDOT and PSS form a
zipper-like structure which reduces the surface energy of PEDOT in water.[21] Commercially
available PEDOT:PSS water dispersion is a dark blue liquid as shown in Figure 1.6b. For device fabrication, different methods including spin coating, drop casting, slot-die, etc. can be applied to deposit thin films. The typical thin film structure from pristine PEDOT:PSS is shown schematically in Figure 1.6c. PEDOT nanocrystals are distributed inside the PSS matrix. PEDOT nanocrystallite domains are usually described as a core-shell structure in literature.[22] The crystal packing of PEDOT is driven by - stacking and holes hop among
these crystals providing good electrical current. Beside thin films, nanogels in which PEDOT:PSS is considered as a mixed ionic and electronic conductor, have recently became very popular because of many interesting applications such as flexible electrodes and biosensors.[23]
Figure 1.6 (a) Chemical structure of PEDOT:PSS; (b) a photo of the commercially available
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thin film.
1.2.2 PEDOT:PSS as organic TE material
TE materials allow energy harvesting by collecting the waste heat generated during the energy production, transport and consumption processes and converting it into electricity. This is a very important field as it was reported that more than 50 % of the energy in fuels is wasted in the form of heat.[24,25] A significant portion of this heat can thus be recovered by
using efficiently enough TE devices and this would be of great help for the environment. By applying a temperature gradient to a TE material, the charge carriers (holes in p-type materials or electrons in n-type materials) will diffuse from the hot side to the cold side, thus inducing an electrostatic potential (ΔV) between the two ends. This effect was discovered by Thomas Johann Seebeck in 1821 and it is named as Seebeck effect in his memory. As shown in Figure 1.7, a simple TE generator composes of a couple of p-type and n-type materials by connecting the hot side of them. In order to get a powerful device with great energy output, a typical TE generator usually holds hundreds of TE couples.
Figure 1.7 Scheme of the working mechanism of (a) simple TE generator and (b) practical
cell composed of hundreds of TE couples. Reprint with permission from Applied Thermoelectric Solutions LLC© Copyright 2020.[26]
Besides the engineering aspects, the device performance mainly depends on the TE properties of the material. The efficiency of a TE system can be quantified by the following equation:
𝜂 =THT-TC H √1+ZT-1 √1+ZT+TC TH (1)
where TH and TC represent the temperatures at the hot and cold sides, respectively, and ZT is
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ZT = S2T (2)
where σ stands the electrical conductivity, S for Seebeck coefficient (which is defined as ΔV/ΔT), T for absolute temperature and κ for thermal conductivity. For organic TE materials,
κ is intrinsically much lower than for inorganic materials, so the power factor (PF = S2) is used instead. The operating temperature is usually below 250 °C due to the low melting and decomposition temperature of most of the conducting polymers.[27]
For PEDOT:PSS, the values of and S vary a lot depending on the chemistry and the applied processing conditions. For example, can be as high as 4000 S cm-1 in thin films post-treated
by sulfuric acid,[28] while for a pristine PEDOT:PSS film prepared from the commercial
product CleviosTM PH1000, is about 0.2~1 S cm-1. Moreover, researchers further exposed
the sulfuric acid treated film into base solutions and successfully got the Seebeck coefficient dramatically enhanced from 17 to 39.2 V K-1 ( slightly decreased). This significant
improvement shows a great potential in morphology modification and electronic structure engineering to PEDOT:PSS. A large variety of processing methods have been developed to achieve performance enhancement including organic solvent treatment, ionic liquids treatment, salt solution treatment, acid or alkali treatment and so on.[29] The principle of these
methods are summarized as following. Electrical conductivity can be expressed as:
𝜎 = 𝑛𝑒𝜇 (3) in which the carrier charge (e) is a constant, while the carrier concentration (n in n. of charges cm-3) and the carrier mobility (μ in cm2V-1 s-1 ) are the main parameters to play with. The
Seebeck coefficient describes the electronic transport property of the material and it depends on the entropy of a carrier with unit charge. In the case of a simple system with charge carriers avoiding strong interactions (which is the case for PEDOT:PSS based films), the entropy of a carrier is statistically expressed as:[29]
𝑞𝑆 = 𝑘𝐵𝑙𝑛 [1−𝜌
𝜌 ] (4)
where 𝑘𝐵 is the Boltzmann constant and ρ is the charge density for each state. In the framework of energy band theory and Boltzmann distribution, this equation can be further transformed into:
S=kB q
E-Ef
kB T (5)
where Ef is Fermi energy level and E is the energy level occupied by the carrier. An increase
of the carrier concentration (n) will lead to a change of Ef making it shifting into the CB
which will have a negative effect on S according to equation (5).[31] Consequently, in a TE
material, when the carrier mobility and carrier concentration both increase, the conductivity increases while the Seebeck coefficient decreases. Conversely, when the carrier mobility increases and the carrier concentration decreases, both the conductivity and the Seebeck coefficient will increase provided that the carrier mobility increase effect overwhelms the
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effect of carrier concentration decrease.[29]
Another strategy to improve the TE properties of PEDOT:PSS films is based on the so-called energy filtering effect.[32] As shown in Figure 1.8, carriers with low energy contribute
negatively to the Seebeck coefficient, indicating that S can be enhanced by filtering the low energy carriers.[33] Normally, energy barriers (or traps) are introduced by phase boundaries
(interfacial scattering), so that holes will get trapped during the temperature gradient driven diffusion.[34]
Figure 1.8 The calculated spectrally resolved normalized Seebeck coefficient. It is shown
that low-energy electrons negatively contribute to the total Seebeck coefficient due to the appearance of grain boundaries. Adapted with permission from Royal Society of Chemistry.[33]
A brief review of the most effective research efforts to improve the PF of PEDOT:PSS is presented in the following section of this chapter.
1.3 Strategies to improve the properties of PEDOT:PSS films 1.3.1 Treatment using polar organic solvents
Polar organic solvents (POSs) treatment can be applied in at least three different ways: addition, drop or immersion post-treatment and solvent vapor treatment. Dimethyl sulfoxide (DMSO) and ethylene glycol (EG) were first introduced as additives into the aqueous PEDOT:PSS dispersion. In 2002 Kim et al. reported the efficient use of DMSO and DMF for electrical conductivity enhancement with up to 80 S cm−1.[35]
Soon the method was extended to improve the TE properties as well.[36] The most
representative work was done by Pipe et al. who reported promising thermoelectric properties for PEDOT:PSS processed by EG dip treatment following pre-doping PEDOT:PSS nanofilms with DMSO or EG.[37] The DMSO and EG pre-doped films exhibited σ > 600 S
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cm−1. After dipping them into an EG bath, conductivities up to 1000 S cm−1 were obtained,
which were three orders of magnitude higher than pristine PEDOT:PSS. The Seebeck coefficient was about 33.4 V K-1 after the DMSO addition and raised to 60-70 V K-1 after
EG post-treatment. With a remained low thermal conductivity, a record ZT value as 0.42 was reported for PEDOT:PSS based TE materials. The mechanism can be summarized as: DMSO or EG addition helps refine the nanostructure of the film changing PEDOT crystal grains from coiled-like to fibrillar-like structures, which benefits a lot to the carrier transport (via formation of a percolated structure).[38] The subsequent EG post-treatment sufficiently
removes the excessive PSS and reduces the dopant concentration, thus increasing the Seebeck coefficient. The addition method is so simple and effective that now an optimized 5 v/v% concentration DMSO addition has nearly become a ‘standard’ pre-treatment when preparing PEDTO:PSS for TE studies.[30,39–41]
Solvent vapor was also used to tune the TE properties of PEDOT:PSS.[42,43] However, even
though the electrical conductivity can be significantly increased, solvent vapor exposure did not allow for a great enhancement of the Seebeck coefficient.
A scheme of the polar solvent effect on PEDOT:PSS films is shown in Figure 1.9.
Figure 1.9 Scheme of the morphology in (a) pristine and (b) polar solvent treated
PEDOT:PSS. Hole (h+) transport (dash line) gets significantly improved by transforming the
crystals into elongated structure as indicated by the thicker line. Proton (H+) transport (solid
line), which determines the ionic TE properties, gets slightly reduced due to the interactions between the ions and crystallites. Adapted with the permission from Nature Publishing Group.[44]
To note, polar solvents also have effect on the film thickness and surface roughness, which may be a critical factor to take into account for applications like photovoltaics.[45]
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1.3.2 Acids or bases treatment
A pristine PEDOT:PSS dispersion has a pH of about 2.3 and playing with pH has been proven effective to tune its TE properties.[46,47] Normally, the electrical conductivity will be promoted
by low pH, while the opposite is true for high pH. For instance, Kong et al. investigated the TE properties of PEDOT:PSS processed at different pH ranging from 0.2 to 12, by adding acids (HCl or oxalic acid) and alkalis (ethylenediamine, NaOH or NH4OH), respectively. A
maximum of the PF of about 1.35 μW m−1 K−2, together with a maximum ZT value of
3.68 × 10−3 was achieved with CleviosTM PH1000.
When employing acids and alkalis, the most effective way is to use the post-treatment processing. Xia et al. post-treated spin coated PEDOT:PSS thin films with 1M H2SO4 at
160 °C for three times and got an optimized conductivity of 3065 S cm-1, which is comparable
to many inorganic conductors.[48] In 2014, Kim et al. increased the value to 4380 S cm-1 by
rigorously controlling the conditions.[49] The highly concentrated H
2SO4 undergoes an
autoproteolysis as:
2H2SO4 ↔ H3SO4+ + HSO4−
The two ionic species can stabilize the segregated state of the positively charged PEDOT and negatively charged PSS, which improves the doping efficiency and charge carrier delocalization. The intensity increase observed by X-ray diffraction of PEDOT:PSS films post-treated at high pH suggests a significant crystalline enhancement. At the same time, a dramatic structural rearrangement from coiled-like to fibrillar-like morphology is observed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), indicating the formation of highly ordered and densely packed PEDOT:PSS nanofibrillar crystals. This morphological transition is closely related to an high carrier mobility (see Figure 1.10).
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Figure 1.10 (a) HAADF-STEM images of PEDOT:PSS films treated with pure H2SO4. (b)
XRD patterns of PEDOT:PSS films treated with various concentrations of H2SO4. (c) Scheme
of the structural rearrangement occurring in PEDOT:PSS upon high pH post-treatment. The amorphous PEDOT:PSS grains (left) reorganize into crystalline PEDOT:PSS nanofibrils (right) via a charge-separated transition mechanism (middle) upon concentrated H2SO4
treatment. Adapted with permission from Wiley.[49]
Inspired by these findings, the Ouyang’s group proposed a sequential post-treatment of acid and base.[28] Treatment with bases leads to sufficiently dedoping (removal of PSS) of the acid
treated PEDOT:PSS and causes Seebeck coefficient enhancement due to the reduction in the number of charge carriers. The retaining of the fibrillar-like structure caused by H2SO4 is
fundamental, which helps to maintain the relatively high electron conductivity. In such a way an extremely high PF around 334 µW (m−1 K−2) could be obtained.
In addition, post-treatment using a mixture of an organic acid and organic solvent has also been explored. For example, Mukherjee et al. achieved exciting results by combining the effect of p-toluenesulfonic acid and DMSO, with the conductivity of the PEDOT:PSS film significantly increased to about 3500 S cm-1 and transparency as high as over 94%.[50] This
is also a great result for photovoltaic applications.
1.3.3 Treatment using ionic liquids
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cations and anions are normally very large so that the Coulomb force required little energy to be overcome. ILs have some unique properties such as good chemical stability, low flammability and negligible vapor pressure. Moreover, they show great affinity with many conducting polymers and the ability of supramolecular ordering.[51] Dobbelin et al. were the
first to explore the possibility to employ ILs with the aim to increase the PEDOT:PSS conductivity.[52] They tested five different ionic liquids, including
1-butyl-3-methylimidazolium tetrafluoroborate ((BMIm)BF4), 1-butyl-3-methylimidazolium bromide
((BMIm)Br), 1-ethyl-3-methylimidazolium chloride ((EMIm)Cl), 1-benzyl-3-methylimidazolium chloride ((BzMIm)Cl), and 1-butyl-1-methylpyrrolidium chloride (BMPro)Cl), added into PEDO:PSS solution with different percentages. The highest electrical conductivity achieved was around 136 S cm-1 with the addition of (BMIm)BF
4,
while Badre et al. raised the value to 2084 S cm-1 with the addition of
1-ethyl-3-methylimidazolium tetracyanoborate ((EMIm)TCB).[53]
Besides the direct addition, ILs can be used to perform post-treatment processing as well. Luo et al. proposed a novel method to treat PEDOT:PSS films by using a mixture of an organic solvent and ILs.[53] However, the results were not as good as the ones obtained by
treatment with organic solvents only. Although treatment with the ILs allowed Seebeck coefficient enhancement, the electrical conductivity resulted too much compromised. This was attributed to the presence of globular PEDOT grains. Mazaheripour et al. further reported that ILs additives can tailor the thermoelectric performance of PEDOT:PSS films by decoupling morphological and electronic effects.[54] By adjusting ion stoichiometry, they
observed that if cations are added in excess and are free to interact with the PSS- groups, the
relative amount of polarons and bipolarons can be changed and thus the Seebeck coefficient can be enhanced. Ouyang et al. studied different ILs layers applied on top of the PEDOT:PSS films and probed the TE properties of the double layers.[55] They found that the Seebeck
coefficient can be improved to about 65 mV K-1 with a 1-ethyl-3-methylimida-zolium
dicyanamide (EMIM-DCA) coating on an acid-base treated PEDOT:PSS film, while the electrical conductivity was only slightly affected. They explained the mechanism by the energy filtering effect. In the top ILs layer, IL anions moves from the hot side to the cold side under a temperature gradient leaving the protons accumulated at the cold side. These protons can generate an electric field that scatters the low energy holes and thus increase the mean energy of the them at the cold side of the PEDOT:PSS films. The reduction of low energy holes successfully increases the Seebeck coefficient according to the mechanism discussed above (see Figure 1.8 and relative text).
1.3.4 Blending with inorganic nanoparticles
An alternative approach to modify the polymer properties is blending with inorganic nanomaterials. On one hand, organic nanomaterials offer the high density of interfaces, which is very important for scattering phonons that carry heat.[56] This helps decreasing the thermal
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polymeric TE properties, while keeping the flexibility and lightweight of the polymer matrix for daily usage. For example, Zhang et al. investigated the properties of PH1000 mixed with ball-milled Bi2Te3 particles.[57] However, their experimental results were far lower than the
theoretical values expected for the hybrid system due to the contact resistance from interfaces between PEDOT:PSS and Bi2Te3. Nevertheless, the achieved PF of 70 μW m-1 K-2 with 30
wt% Bi2Te3 loading is encouraging and calls for the use of tailored nanostructured materials
in order to acquire better contact with the polymer. By chemical synthesis, Toshima et al. prepared gold nanoparticles and successfully mixed them with PEDOT:PSS.[58] Different
effects between terthiophenethiol (TSH) and dodecanethiol (DT) coatings were found and an optimized PF of 11.68 μW m-1 K-2 was achieved with the addition of 10-5 w/w% DT coated
Au nanoparticles, while the TSH coated NPs exhibited no obvious positive effect on the TE properties of PEDOT:PSS films. The enhancement caused by the DT coated NPs was attributed to the superior dispersion property.
In contrast to nanoparticles, 1D nanotubes and 2D nanosheets are believed to be more effective due to the interfacial ordering effect.[36] In this case, the anisotropic nanoparticles
act as templates that induce nanostructuring of PEDOT:PSS. For example, See et al. synthesized in-situ water soluble Te nanorods which were successfully blended to PEDOT:PSS and, thanks to the great transport property, the hybrid material exhibited a PF of 70.9 μW m-1 K-2.[59] Ju et al. successfully blended chemically exfoliated SnSe nanosheets
with PEDOT:PSS films and got the PF increased to 386 μW m-1 K-2 (ZT as high as 0.32 at
300K).[60] By liquid exfoliation Jiang et al. also prepared nanosheets of MoS
2 and got the TE
properties of the hybrid system efficiently improved to PF ~ 45.6 μW m-1 K-2.[61] The process
of PEDOT:PSS:MoS2 nanocomposite formation is shown in Figure 1.11. To note, carbon
fillers like graphene and carbon nanotube act in a similar way of the traditional 1D/2D inorganic materials.[62,63]
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Adapted with permission from Royal Society of Chemistry.[61]
Beside the nanostructuring effect, graphene is also discovered to have great - stacking with PEDOT which facilitated the carrier transfer between the organic-inorganic interfaces.[64]
Kim et al. reported a PF of 11.09 μW m-1 K-2 with 2 % graphene added PEDOT:PSS, thanks
to good interface interactions and high electron mobility of graphene.
However, blending with inorganic nanoparticles is an underexplored strategy so far, and an easy and at the same time efficient method to incorporate PEDOT:PSS film with inorganic nanomaterials is still missing. This can represent an important step towards industrialized mass production. To tackle this issue, a promising inorganic-organic hybrid strategy for future industrialized fabrication of PEDOT:PSS based TE devices is presented in Chapter 4.
1.4 Critical role of in situ structural information: grazing incidence wide angle X-ray scattering as an efficient tool to probe thin film structural morphology
Following what reported above, an important conclusion independently from the strategy adopted to alter the PEDOT:PSS properties can be drawn: in order to improve the thermoelectric properties, the morphology of PEDOT:PSS films has to be tuned in order to obtain improved PEDOT intermolecular packing and inter crystal connectivity which greatly affects the carrier mobility.[65]
Many techniques can be efficiently applied for ex situ characterization of the thin film structure, such as atomic force microscopy (AFM), scanning electron microscope (SEM), transmission electron microscopy (TEM), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR), ultraviolet–visible spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS), etc. However, it is difficult to use most of these techniques to conduct in situ measurements during thin film processing because of the relatively long measurement time required or of the non-suitable sample environment (i.e. high vacuum). Compared to the lab X-ray devices, synchrotron radiation light holds great characteristics including high flux, broad energy spectrum, small beam size and polarization which make in
situ studies possible on time scale of seconds and sub-seconds.[66] Nowadays, there are more
than 50 synchrotron light sources worldwide, providing great opportunities for high quality science (Figure 1.12).[65]
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Figure 1.12 Map of the different synchrotron light sources worldwide. Reprinted with
permission from Elsevier.[65]
Grazing incidence wide angle X-ray scattering (GIWAXS) is a powerful technique allowing to detect the (semi)crystalline structure of organic thin films.[67–69] While its resolution could
not be as high as X-ray diffraction (XRD), GIWAXS makes use of 2D detectors, allowing collection of scattering or diffraction signals along the whole spectrum of azimuthal angles (from 0° for in-plane to 90° for out-of-plane direction). This characteristic enables GIWAXS to analyze not only the degree of crystallinity, interplanar spacing and crystallite size, but also the crystal orientation and phase composition inside a (thin) film. Scheme of the GIWAXS setup and representative crystal orientations of organic films and their expected GIWAXS patterns are shown in Figure 1.13. Moreover, when synchrotron light is used, in
situ GIWAXS measurements are well feasible during the film processing. For example,
Müller-Buschbaum et al. reported the in situ GIWAXS/GISAXS (grazing incidence small angle X-ray scattering) study on the P3HT (poly(3-hexylthiophene-2,5-diyl)) based active layer for organic photovoltaics (OPVs).[70,71] They observed clear structural changes during
the spin coating and annealing processes. In addition, they pointed out some specific technical issues that need to be considered, such as the high flux X-ray damage of soft materials. Ueda et al. studied the effect of various drying rates on P3HT:PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) films.[72] The different crystallinity tendency between P3HT
and PCBM suggests a fine control of the film preparation for OPVs applications. Besides, many in situ study have been done to optimize the morphology and performance of P3HT based films in the field of not only OPVs but also organic field effect transistors (OFETs).[73,74]
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Figure 1.13 (a) Diagram of GIWAXS technique and different film crystalline and
corresponding GIWAXS images in case of (b) vertical lamellar stacking, (c) mixture of vertical and horizontal oriented crystals, (d) oriented domains with minor disorder and (e) full rotational disorder of crystallites. The GISAXS signal is blocked by the beam stop (black box). Adapted with permission from Wiley.[70]
In 2015, the Müller-Buschbaum’s group reported the first real time study on the slot-die processed PEDOT:PSS films’ formation process.[75] Their findings helped understanding of
the film evolution during the printing process, allowing for tailored modifications of the solutions and printed films thereby gaining further enhanced organic electronic device performances. More recently, Dauzon et al. performed time-resolved GIWAXS studies during spin coating and thermal annealing process of PEDOT:PSS, with different additives, namely 5% DMSO, 1% Zonyl, and mixed 5% DMSO/1 % Zonyl.[76] By comparing the
acquired data for the different additive used, they revealed the mechanism of film formation and the different effects of DMSO and Zonyl. It was found that DMSO promoted the crystallization of PEDOT, creating a pathway for the carrier transport, while Zonyl additive involved twisted fibrils with thicker PEDOT core. Their synergetic effect improved the film morphology and led to great transport properties.
Achieving control over the film preparation process is critical not only for OPV application but most importantly for other inorganic and organic-inorganic hybrid materials such as perovskites. Thus, in situ GIWAXS has a primary role in elucidating the perovskites film formation mechanism.[77,78] Zhang et al. discovered a ‘surprisingly complex journey’ for
(MA)nPbnI3n+1 (n = 3) perovskites through disordered colloidal sol−gel precursors,
intermediate phases, and finally ACI (alternating cations in the interlayer space) perovskites by in situ GIWAXS measurement.[79] Intermediate phase control was found to be essential
for better PV performances and they successfully improved the optoelectronic properties by tuning several kinetically arrested phases. Barrows et al. applied in situ GIWAXS to study the film formation process (from precursor solution to the end of thermal annealing) of CH3NH3PbI3− xClx.[80] It was found that the parallel-to-substrate orientation of perovskite
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crystalline planes was preferred throughout the annealing process, as suggested by the signal along out-of-plane direction. They also applied in situ GISAXS measurements to study the evolution in length scales, which was correlated with an increase in film surface coverage. With highly oriented perovskite crystalline and great surface coverage, the device fabricated got an average efficiency of 12.2%.
The examples provided above sufficiently prove the power of in situ GIWAXS in the field of organic electronics and perovskite photovoltaics. The insights provided by using this technique will pave the way for achieving desirable final perovskite layers and increases in photovoltaic performance.
1.5 Motivation and outline 1.5.1 Motivation
Electronic devices have been playing an increasingly important role in the modern life since the invention of computers. From the clumsy computer which could be as big as a house to the portable mobile smart phones, the development of semiconductor materials constitutes the foundation of the electronic industry upgrade. At this point, the industry is about to have another breakthrough due to the development of soft, flexible electronic materials. Some commercial products with foldable screens have already appeared on the market, commercialized by the most admired companies like Huawei and Samsung.[81–83] In the near
future, electronic devices will be extremely light, portable and at the same time shock resistant.[84–86]
Beside the innovation in display, energy harvesting is another possible application of soft electronic materials. A lot of research has already been carried out to develop flexible and efficient solar cells produced by various methods also suitable for large scale fabrication, such as ink printing methods.[68,87,88] Moreover, new materials are even going to change the
way of power supply. Self-powered devices have been a research hit for years and may be widely applied in the future life.[89–91] Considering all kinds of promising results, the day
when people use their body temperature or body movement to generate electricity and to power their personal devices is not too far.
With soft electronic materials, scientists are also trying to make something completely different, for example, artificial skin for intelligent robots.[92,93] Different function units of a
real skin have been mimicked successfully, including biosensing, neural signal transportation and even stimuli responsive. These new materials offer a great potential to get extremely ‘smart’ robots that can help human beings in the future.
As an uprising field, there are so many things that still need to be improved and developed, including fundamental theory studies, enhancement of efficiency and development of cheap and powerful devices fabrication technologies. In order to get efficient devices, materials with superb electrical properties are highly in demand. Essential here is to understand the link between material composition and material properties. Thus, researchers in this field aim to build this ‘bridge’ called structure-property relationship (SPR) that can help designing
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materials of improved and tailored properties.
1.5.2 Scope of this thesis
With this thesis, we aim to further advance in the SPR of different materials for energy applications such as one of the most promising soft electronic materials, poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonate) (PEDOT:PSS) based thin films (Chapter 2-4) and lead-free hybrid perovskites for PV applications (Chapter 5).
Chapter 1 demonstrates the developing history of the organic semiconductor field and gives
a short introduction of PEDOT:PSS and some strategies to improve the its thermoelectric properties. The relevance of the in situ grazing incidence wide angle X-ray scattering (GIWAXS) technique to study the structure of the materials studied in this thesis is also discussed in this chapter.
In Chapter 2, the structure and formation mechanism of PEDOT:PSS films processed with solvents of different polarity with both addition and post-treatment methods are unveiled by combining results from in situ GIWAXS measurements and other extensive ex situ characterization methods. The relationship between electrical conductivity and film structure determined by the nature of different solvent processing is established.
Chapter 3 elucidates the structure and TE properties of PEDOT:PSS-5%DMSO (5 % volume
ratio DMSO added into the pristine PEDOT:PSS solution) thin films post-treated with different alkali base solutions. The results show great potential on further tuning the electrical properties of PEDOT:PSS based materials in a simple and green way and hold promises for designing future pH/ion responsive devices for different applications.
In Chapter 4, a new strategy to create efficient nanocomposites with improved thermoelectric (TE) properties is discussed. PEDOT:PSS thin films were blended with spark generated "naked" tin oxide nanoparticles, deposited with a low energy diffusive method. The significant increased power factor proves the efficiency of this strategy and develops the insights into the organic-inorganic interactions.
Finally, beside studying PEDOT:PSS based materials, the in situ X-ray capabilities developed during this thesis are further applied to different materials for energy applications such as mixed perovskites. In Chapter 5, the structure and formation mechanism during spin coating of environmental friendly lead-free Sn-based Ruddlesden-Popper (Sn-RDP) perovskites are unveiled.
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