(Byoung-Nam Park)
1*
Copyright © 2024 The Korean Institute of Metals and Materials
Key words(Korean)
P3HT colloids, mini-emulsion, aggregates, surfactant, conjugation
1. INTRODUCTION
The dynamic field of organic electronics, marked by rapid advances and growing applications,
is undergoing a paradigm shift with the emergence of polymer-based nanomaterials[1-3]. Among these, poly(3-hexylthiophene) (P3HT), a semiconducting polymer, has been
the subject of extensive research due to its promising applications in organic solar
cells, field-effect transistors, and organic light-emitting diodes[4-7]. The allure of P3HT stems from its favorable electronic properties, ease of processing,
and the potential for tunable optoelectronic characteristics.
In this study the mini-emulsion technique, noted for its ability to control particle
size and distribution, crucial determinants of the material's electronic properties,
was employed to synthesize P3HT nanoparticles (NPs).[8-10] This method, characterized by its use of surfactants to stabilize the emulsion,
ensures the formation of NPs with defined dimensions and morphologies, directly influencing
the conjugation length and intermolecular interactions within the polymer.
In this realm, the choice of surfactant is not merely a procedural necessity but a
strategic tool that can be used to manipulate the NP properties. Here, we specifically
focus on sodium dodecyl sulfate (SDS) due to its compatibility with P3HT, its cost-effectiveness,
and the ability to form stable emulsions. The concentration of SDS was hypothesized
to play a pivotal role in determining the size of the P3HT NPs. This size factor,
in turn, has profound implications on the optoelectronic properties of the polymer,
as it influences the extent of pi-pi stacking and conjugation length within the polymer
chains.
The electronic structure of conjugated polymers like P3HT is inherently linked to
their physical dimensions and morphological characteristics. The highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels,
key parameters defining the electronic behavior of these materials, are sensitive
to changes in molecular ordering and interchain interactions[11-14]. Consequently, controlling the NP size through surfactant concentration provides
a way of fine-tuning these energy levels, thereby tailoring the material's absorption,
emission, and charge transport properties.
Beyond size control, this study also investigated the structural properties of the
P3HT NPs, particularly focusing on the conjugation length and pi-pi stacking. These
features are integral to the optoelectronic performance of the polymer. The conjugation
length affects the effective bandgap, influencing light absorption and emission characteristics,
while pi-pi stacking is crucial for charge mobility, a key factor in device efficiency.
This paper aims to elucidate the relationship between surfactant composition, specifically
SDS concentration, and the resultant size and structural properties of P3HT NPs synthesized
via the mini-emulsion technique. Through a systematic study combining NP characterization,
including their electrochemical and optoelectronic properties, with electronic property
analysis, we seek to provide deeper insights into how surfactant-induced changes at
the nanoscale can be leveraged to optimize the optoelectronic behavior of P3HT. Here,
we investigated P3HT colloid size dependent optoelectronic properties, including the
HOMO and the LUMO levels, using photoelectron spectroscopy in air, UVVis absorption
spectroscopy and cyclic voltammetry measurements. This research not only contributes
to the fundamental understanding of polymer NP synthesis but also has significant
implications for the design and development of next-generation organic electronic
devices.
2. MATERIALS AND METHODS
2.1. Mini-Emulsion Technique for P3HT Colloid Formation
High-regioregularity poly(3-hexylthiophene) (P3HT), characterized by a regioregularity
exceeding 96%, was acquired from Sigma-Aldrich. Complementary materials, including
sodium dodecylbenzene sulfonate (SDS) and anhydrous chloroform, were also sourced
from Sigma-Aldrich. We utilized the mini-emulsion technique for the synthesis of P3HT
colloids. The process began by dissolving 10.0 mg of P3HT in 1 mL of chloroform. This
mixture was stirred continuously for 12 hours at a stable temperature of 45°C, resulting
in a 10 mg/mL P3HT solution with an estimated thickness of 60 nm. Subsequently, this
P3HT solution was mixed with pre-prepared SDS aqueous solutions of varying concentrations
(1.0, 1.7, 3.4, and 10 mM). Following sonication for 5 minutes, the mixture was heated
at 70°C for one hour to ensure complete chloroform evaporation. This process ultimately
yielded an aqueous suspension of P3HT colloids. A comprehensive schematic of the mini-emulsion
technique, delineating the synthesis stages, is illustrated in Figure 1.
2.2. Structural and Optoelectronic/Electrochemical Characterizations of P3HT Colloids
For morphological analysis, we first coated silicon substrates with a thin layer of
P3HT colloids, followed by air-drying at 70°C. The structural features of these colloids
were then examined using a scanning electron microscope (SEM), as illustrated in Figure 2(a).
SEM analysis was carried out post-sample preparation, which involved drop-casting
the P3HT colloids onto silicon substrates and subsequently drying them at 70°C. To
explore the impact of SDS concentration on P3HT nanoparticle size, we performed SEM
image analysis. This analysis revealed size distributions that varied with SDS concentration,
as shown in Figure 2(b).
To study size dependent structural properties, optical absorption spectroscopic methods
were used with a photo-spectrometer after coating P3HT colloids on glass substrates.
The size dependent ionization energy of P3HT NPs was measured using photoelectron
spectroscopy in air (PESA). CV measurements were conducted to determine the LUMOs
and HOMOs of P3HT NPs.
The redox potentials were measured in a solution of 0.1 M tetrabutylammonium perchlorate
(TBAP) in acetonitrile (ACN), using platinum electrodes for both the working and counter
electrodes at a scan rate of 10 mV/s. The measurements were referenced against an
Ag/AgCl electrode. The ferrocenium/ferrocene (Fc+/Fc) couple exhibited an oxidation potential of +0.51 V and a reduction potential
of +0.43 V, both values being relative to the Ag/AgCl reference electrode. The HOMO
levels were calculated by HOMO (eV)=e(-4.8 V-(Eonset=E1/2(Fc+/Fc))[15].
2.3. Electrical Characterization Using Field Effect Transistor Devices
We fabricated a bottom-contact field-effect transistor (FET) to investigate charge
transport in the P3HT colloid films near the SiO2 interface. The source and drain electrodes, composed of Au (80 nm) over Ti (3 nm),
were patterned on a 200 nm SiO2 gate dielectric via photolithography. A highly-doped silicon substrate served as
the gate electrode. The P3HT NP colloids were then drop-casted onto this pre-patterned
substrate and subsequently heated at 80°C to evaporate water. For the FET measurements,
we covered the P3HT channel region with glass. Electrical characterizations of the
FET were conducted in air using a semiconductor parameter analyzer (HP4145B).
3. RESULTS AND DISCUSSION
The relationship between the size of the synthesized P3HT NPs and the concentration
of SDS is illustrated in Figure 2. Figure 2(a) shows SEM images of the P3HT NP colloids synthesized with different concentrations
of SDS. Figure 2(b) offers a detailed analysis of the size distribution of these nanoparticles, showcasing
how changes in SDS concentration affected NP size. Notably, the size distribution
of P3HT NPs varied with different concentrations of SDS. A clear trend was observed,
where an increase in SDS concentration resulted in a corresponding decrease in the
size of the NPs.
The observed dependence of P3HT NP size on the concentration of SDS in the mini-emulsion
synthesis process, specifically the trend of decreasing NP size with increasing SDS
concentration, can be explained by several key factors in colloidal chemistry and
NP formation. SDS, being a surfactant, forms micelles in aqueous solutions above its
critical micelle concentration. As the concentration of SDS increases, the number
of micelles in the solution also increases. These micelles act as templates or nano-reactors
for the formation of nanoparticles. With more micelles present at higher SDS concentrations,
the P3HT is distributed among a greater number of micelles, leading to the formation
of smaller NPs. At higher concentrations, more SDS molecules are available to stabilize
the NPs, effectively preventing them from aggregating or growing too large. This stabilization
is critical to the final size of the NPs. The process of NP formation involves nucleation
followed by growth. Higher SDS concentrations can lead to a higher nucleation rate
due to the increased number of micelles. This results in a larger number of smaller
nuclei, which eventually grow into smaller NPs. Conversely, at lower SDS concentrations,
fewer nuclei form, which can grow larger due to the availability of more P3HT per
nucleus.
The energy band gap of the P3HT NPs was determined using the Tauc plot derived from
optical absorption data presented in Figure 3. We observed that as the size of the P3HT NP decreased from 227 nm to 127 nm, the
energy band gap correspondingly increased, moving from 1.85 eV to 1.95 eV, respectively.
Using the PESA analysis, the ionization energy of P3HT NPs with different sizes was
measured, as shown in Figure 4. PESA is a technique used to measure the ionization potential of materials, and is
particularly useful for the analysis of thin films and surfaces[16]. Unlike vacuum-based photoelectron spectroscopy methods, PESA allows measurements
to be conducted in an ambient air environment, which can be advantageous for studying
materials sensitive to vacuum conditions, or those that require a rapid analysis.
In PESA, the sample is irradiated with photons of varying energies from 4.2 to 5.3
eV. As the photon energy increases, electrons are emitted from the sample surface.
The number of emitted electrons (emission yield) is measured and plotted against the
photon energy. We measured the ionization potential by identifying the threshold energy
at which electrons start to be emitted from the sample. This threshold corresponds
to the minimum photon energy required to overcome the binding energy of the outermost
electrons in the material. Initially, at lower photon energies, no electrons are emitted,
and the emission yield is zero. As the photon energy reaches the ionization threshold,
electrons begin to be emitted, and the emission yield increases sharply. The plot
is analyzed by looking for the point where the emission yield graph intersects the
photon energy axis. This intersection point is indicative of the threshold energy.
Utilizing data from PESA and optical absorption studies, we determined the LUMO and
HOMO levels of P3HT NPs of varying sizes. An increase in NP size was found to correspond
with a rise in ionization potential, as depicted in Figure 4(e). Smaller NPs have a larger surface-to-volume ratio, which can influence their electronic
properties due to surface states. These surface states can alter the energy levels
of the electronic states of the NPs. As the NP size increases, the influence of surface
states diminishes, potentially leading to a higher ionization potential.
CV measurements were conducted to verify the size-dependent energy levels of P3HT
NP colloids in Figure 5. The HOMO and LUMO levels of a material can be estimated from CV measurements by
analyzing the onset of oxidation and reduction processes. The onset of the oxidation
peak corresponds to the removal of an electron from the HOMO level. This is where
the material starts to oxidize. The potential at this onset can be related to the
HOMO energy level relative to the reference electrode.
Similarly, the onset of the reduction peak corresponds to the addition of an electron
to the LUMO level. This is where the material starts to reduce. The potential at this
onset gives information about the LUMO energy level. The potentials obtained for the
HOMO and LUMO levels are referenced to the Fermi level of the standard electrode.
Table 1 summarizes the general trend of band gap variation with the size of P3HT NPs. Notably,
there's an observed decrease in the band gap as the NP size increases from 127 nm
to 229 nm. However, the 199 nm P3HT NPs deviate from this trend. This anomaly could
potentially be explained by polydispersity within the NP samples. If the 199 nm batch
exhibits a broader size distribution, it would influence the averaged energy levels
obtained from CV measurements, possibly accounting for the unexpected increase in
the band gap.
Size dependent aggregate structural information was extracted from the absorption
spectra. The peaks around 550 nm and 600 nm in Figure 6(a) were assigned as A0–1 and A0–0, respectively. The A0-0/A0-1 ratio in the absorption spectrum is a crucial indicator for distinguishing between
H-aggregates and J-aggregates in conjugated polymers like P3HT[17,18]. In J-aggregates, the chromophores are aligned in such a way that their transition
dipoles are coherently additive. This leads to a sharp, intense 0-0 (first electronic
transition) peak and a weaker 0-1 peak. As a result, J-aggregates typically exhibit
a high A0-0/A0-1 ratio. J-aggregates are indicative of strong intrachain interactions and extended
conjugation.
For H-aggregates, the chromophores are aligned in a manner that causes their transition
dipoles to more or less cancel each other out. This alignment typically results in
a less intense or even absent 0-0 peak and a more pronounced 0-1 peak. Therefore,
H-aggregates show a low A0-0/A0-1 ratio. H-aggregates are characteristic of strong interchain interactions and possibly
shorter conjugation lengths. So, if the A0-0/A0-1 ratio decreases, it suggests a transition from J-aggregate dominance to H-aggregate
dominance. This shift indicates an increase in interchain interactions relative to
intrachain interactions, which can significantly affect the material's optical and
electronic properties.
Figure 6(b) displays the A0-0/A0-1 ratios for all P3HT NP samples, which were found to be less than 1. This suggests
a predominance of H-type aggregation, indicative of robust interchain interactions[19]. As the size of the P3HT NPs diminished, there was a corresponding decrease in the
A0-0/A0-1 ratio, underscoring stronger H-aggregate formation. The exception was observed in
the 156 nm sample, where the A0-0/A0-1 ratio did not follow the general trend, potentially due to a broad size distribution
within the sample. The decrease in the A0-0/A0-1 ratio in the absorption spectrum of P3HT NPs with decreasing size, especially when
synthesized via the mini-emulsion technique, can be attributed to changes in the molecular
and aggregate structure within the NPs.
As the size of P3HT NPs decreases, there's a likelihood of increased pi-pi stacking
between polymer chains (interchain coupling). This interchain coupling is characteristic
of H-aggregates, which typically show a lower A0-0/A0-1 ratio. The smaller the NPs, the greater the surface area relative to volume, which
could enhance interactions between polymer chains, leading to more pronounced H-aggregate
formation. Further, in smaller NPs, the confined space may force polymer chains into
closer proximity, favoring interchain interactions over intrachain interactions. J-aggregates,
which are indicative of strong intrachain interactions and show a higher A0-0/A0-1 ratio, might be less favored in these confined environments.
The electrical characteristics of P3HT NPs were investigated using field-effect transistors
(FETs). Transfer characteristic curves indicated that P3HT NPs constitute p-channel FETs, with the gate voltage magnitude inversely related to the drain voltage.
However, output transistor curves did not exhibit clear current saturation. This absence
of saturation could stem from several factors. Notably, high contact resistance at
the source/drain interfaces might obscure the pinch-off phenomenon due to dense packing
of P3HT NPs near these contacts[20]. This, in turn, can cause a significant voltage drop at the contacts, which may
lead to misinterpretation of the channel's actual voltage. Furthermore, undetectable
current at low drain voltage could imply insufficient drive voltage for carrier injection
or high resistance pathways. Additionally, traps and defects within the P3HT semiconductor
can hinder carrier mobility and alter recombination dynamics, potentially resulting
in atypical I-V characteristics that deviate from the expected pinch-off behavior[21,22].
4. SUMMARY AND CONCLUSIONS
In this study, we investigated the correlation between the size of P3HT NPs and their
aggregate formation and electronic properties. Our findings demonstrate a clear relationship
between the nanoparticle size and the type of chromophore aggregation, as well as
the energy band gap of the material.
As the size of P3HT NPs decreases, there is a notable increase in H-aggregate formation.
This increase can be attributed to enhanced interchain interactions between the polymer
chains. In smaller NPs, the surface area-to-volume ratio is higher, leading to more
significant interactions between adjacent polymer chains. These interactions promote
the formation of H-aggregates, characterized by their interchain pi-pi stacking, which
differs from the intrachain coupling observed in J-aggregates.
Concurrently, a decrease in P3HT NP size results in an increase in the energy band
gap. This phenomenon is likely linked to the enhanced formation of H-aggregates and
the associated changes in electronic structure. As the NPs become smaller, the altered
molecular packing and increased interchain interactions lead to a reduction in the
conjugation length and an increase in the band gap. This shift in the band gap is
indicative of changes in the electronic and optical properties of the P3HT material,
which are crucial for applications in organic electronics and photovoltaics.
In conclusion, the size of the P3HT NPs plays a critical role in determining their
aggregate structure and electronic properties. The observed increase in H-aggregate
formation and the corresponding rise in the energy band gap with decreasing nanoparticle
size provide valuable insights for tailoring the material properties of P3HT for specific
applications in nanotechnology and materials science. Understanding these relationships
is essential for the development and optimization of organic electronic devices, where
the control of material properties at the nanoscale can significantly impact performance
and efficiency.