1. INTRODUCTION
In the realm of organic electronics, the ability to manipulate material properties
to achieve desired electronic behavior is a pivotal aspect of research and development.
Among the various organic semiconductors, poly(3-hexylthiophene) (P3HT) has gained
significant attention thanks to its promising applications in field-effect transistors
(FETs) [1-4]. The performance of P3HT-based FETs is largely influenced by the microstructural
arrangement of polymer chains within the device, which dictate key characteristics
such as charge mobility and threshold voltage [4-6].
Recent studies have emphasized the role of external stimuli, such as mechanical or
ultrasonic agitation, and their influence on the physical properties of polymer semiconductors.
Spano et al. reported that conjugated polymer aggregates display two main electronic interactions:
intrachain (within chains) leading to J-aggregate behavior, and interchain (between
chains), resulting in H-aggregate behavior [5]. This dichotomy influences the photophysics of polymer films, as understood through
the concepts of J- and H-aggregation. The balance between these intrachain and interchain interactions shapes
the photophysical properties of common conjugated polymers like polythiophenes, phenylenevinylenes,
and polydiacetylenes, affecting their absorption and photoluminescence characteristics.
Alam et al. reported that blended films of poly(butyl acrylate)-grafted cellulose NCs and poly(3-hexylthiophene)
(P3HT) exhibited enhanced photoluminescence and longer emission lifetimes compared
to pure P3HT. These blends showed a shift from H-aggregate to J-aggregate behavior, indicated by increased molecular planarity and the reduced highest
occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy
gaps. Molecular dynamics and density functional theory calculations supported these
structural changes [7].
Baghgar et al. reported studies using photoluminescence imaging techniques on isolated P3HT nanofibers,
spanning molecular weights from 10 to 65 kDa, revealing a shift in exciton coupling
[8]. Nanofibers of lower molecular weight predominantly showed interchain (H-aggregate) coupling, whereas those with higher molecular weight demonstrated intrachain
(J-aggregate) coupling. This transition was attributed to the folding of polymer chains
within the lamellae, as indicated by transmission electron microscopy nanofiber width
measurements, leading to increased planarity of the chains and a decrease in torsional
disorder.
Sonication has become a key technique in studies exploring the relationship between
the structure and electrical properties of conducting polymers [9,10]. It's increasingly recognized for its ability to modify polymer microstructures,
thereby optimizing electrical properties for applications in optoelectronics. Aiyar
et al. reported the application lowintensity ultrasound to regioregular P3HT solutions
to enhance the crystallinity of spin-coated thin films, with fieldeffect mobilities
reaching 0.03 cm2V-1s-1 [11]. This technique induces π-stacking in P3HT, resulting in a multiphase morphology
and distinct charge transport behavior, where mobility initially increases sharply
with crystallinity and then plateaus. Kim et al. developed a method to assess charge transfer in an aluminum-doped ZnO/P3HT structure,
finding that sonication of P3HT enhances charge transfer efficiency at the interface,
critical for optoelectronic device performance [9]. This efficiency was linked to improved polymer chain arrangement, leading to reduced
contact resistance and better charge carrier mobility.
Although there has been significant research on modifying electrical properties through
sonication, the precise effects of sonication on the electrical behavior of P3HT FETs,
especially at the heterojunction interface under both dark and illuminated conditions,
have not been thoroughly investigated.
This study aims to bridge this gap by investigating the impact of ultrasound irradiation
(sonication) on the electrical gating caused by the aggregation behavior of polymer
chains in P3HT and its subsequent influence on the electronic properties of FETs.
Our research focuses on elucidating the mechanisms behind these changes, specifically
examining the interplay between sonication-induced aggregation, carrier concentration,
and interchain interactions. Furthermore, we explore how these alterations in microstructural
properties translate to changes in the electrical characteristics of P3HT FETs, with
a particular emphasis on electrical gating at the homo and heterojunction interface
in the dark and under illumination. The sonication process results in the formation
of dense clusters or structures within the polymer chains, which significantly alters
the electrical properties of the transistors.
A key observation from this study is the shift in the threshold voltage of the transistor
towards a higher, more positive value, suggesting that the concentration of charge
carriers in the transistor can be fine-tuned by managing the interchain interactions.
This phenomenon is comprehensively analyzed through structural and optical characterizations,
including grazing incidence X-ray diffraction (GIXD), photoluminescence (PL), and
optical absorption measurements. These techniques help in understanding how altering
the interchain interactions in P3HT can effectively control the activation behavior
of the transistor, as indicated by changes in the threshold voltage.
2. MATERIALS AND METHODS
2.1 Formation of ZnO NC film.
To synthesize ZnO nanocrystals (NCs), 1.097 grams of zinc acetate dihydrate was first
dissolved in 93.75 milliliters of ethanol, and this mixture was then stirred at a
temperature of 60°C for a duration of one hour. Simultaneously, in a separate setup,
0.9687 grams of tetramethylammonium hydroxide pentahydrate (TMAH) was dissolved in
4 milliliters of ethanol at room temperature, after ensuring the container was securely
sealed with para-film to prevent the evaporation of the solvent. Once an hour of stirring
was completed, the TMAH solution was gradually added to the zinc acetate solution
over a period of 15 minutes, maintaining a consistent temperature of 60°C. Following
the completion of this addition, the mixture was sustained at 60°C for an additional
30 minutes and then allowed to cool naturally to room temperature. For storage, the
resulting solution was refrigerated at approximately 5°C. Later, for the purposes
of device fabrication, the chilled solution was mixed with 30 mL of hexane and centrifuged
at 9000 rpm for 10 minutes. The entire procedure was executed in an environment with
standard air conditions.
2.2 FET Fabrication and Measurement.
To assess charge transport in the P3HT layers near the SiO2 interface and to study exciton dissociation at the P3HT/ZnO NC interface, a bottom-contact
FET configuration was utilized. This setup involved the fabrication of source and
drain electrodes, made of an 80 nm thick gold (Au) layer atop a 3 nm titanium (Ti)
layer, patterned photolithographically onto a SiO2 gate dielectric with a thickness of 200 nm. A heavily doped silicon substrate functioned
as the gate electrode.
To produce a film from sonicated P3HT, the polymer solution underwent a sonication
process at 1500 rpm for one minute, followed by an extended 30-minute sonication in
an ice bath. This procedure led to a visible transformation in the P3HT solutions,
altering their color from a transparent red to a deeper, dark wine shade, a change
attributed to the sonication-induced aggregation of the polymer chains. To safeguard
the integrity of both the pristine and sonicated P3HT films against oxidation, a protective
encapsulation was employed. This was accomplished by sealing the films beneath a glass
slide, utilizing a method that involves the careful application of adhesive tape to
ensure a robust and effective seal.
The P3HT solutions, prepared in concentrations of 10 mg/mL, were applied onto the
SiO2 gate dielectric using a spincoating technique. To facilitate molecular ordering
within the P3HT films, these solutions were sonicated in a chilled water bath for
a duration of 30 minutes. For experiments focusing on the photoinduced charge transfer
at the P3HT/ZnO interface, a thin film of ZnO NCs, roughly 7 nm thick, was deposited
before applying the P3HT layer.
The FET's characteristics were examined using an HP4145B semiconductor parameter analyzer.
A green laser diode, emitting at a wavelength of 530 nm and providing a power density
of 5 mWcm-2, was employed for illumination purposes during FET testing. This specific wavelength
was chosen to selectively excite the P3HT films while mitigating the UV-induced conductivity
enhancement often seen in the ZnO layer. All electrical measurements were meticulously
conducted in an argon-filled glove box to maintain a stable environment.
2.3 Determination of Threshold voltage
In the linear operating regime, the drain current (ID) of the FET exhibits a direct proportionality to the drain-source voltage (VD), as shown in Eq. The drain current can be mathematically formulated considering
several factors: the mobility of holes (μ), the capacitance per unit area at the semiconductor/gate insulator interface (Ci), and the dimensions of the device, specifically its width (Z) and length (L).
The threshold voltage (VT) is a crucial parameter in FETs, representing the minimum gate voltage required to
accumulate a sufficient number of mobile carriers within the channel to enable conduction.
Once this threshold is surpassed, the magnitude of the drain current follows a specific
relationship, as outlined in the Eq. In the linear operating regime of an FET, the
threshold voltage can be accurately determined by analyzing the transfer characteristic
curve of ID versus gate voltage (VG). Specifically, the threshold voltage is identified at the point where the extrapolated
line of the curve intersects the y-axis (drain current axis) at zero. This intersection
point provides a direct measure of the gate voltage needed to initiate significant
current flow in the device, marking the transition from the off-state to the active
conduction state of the FET.
3. RESULTS AND DISCUSSTION
In Fig 1(a), a comparison of absorption peaks between sonicated and pristine P3HT reveals a higher
A0-0/A0-1 ratio for the pristine P3HT. Upon sonication, there is a red shift in the absorption
peak, indicative of H-aggregate formation [8,12,13]. This phenomenon is more evident in the PL spectra shown in Fig 1(b), which highlights the impact of aggregation on luminescence [14,15]. The diminished PL intensity following sonication confirms the formation of interchain
species, specifically H-aggregates, which lead to non-emissive excitons.
The type of aggregates formed in P3HT significantly influences both the threshold
voltage and mobility. As depicted in the ID-VG curve in Fig 2, the charge transport properties of the sonicated and pristine P3HT differ markedly.
The field-effect transistor (FET) mobility (4.5×10-4 cm2V-1s-1) in sonicated P3HT was substantially higher than in pristine P3HT (2.6×10-5 cm2V-1s-1). Notably, the threshold voltage in the sonicated P3HT (2 V) was more positive than
that (-9 V) in the pristine P3HT, suggesting an increase in the presence of mobile
carriers even at zero gate voltage. This enhanced mobility can be attributed to differences
in crystallinity and stronger polymer chain interactions in the sonicated P3HT, leading
to an extended effective conjugation length, as corroborated by absorption spectroscopy,
which will be discussed later.
Sampling measurements (ID-time plots) under illuminated and dark conditions revealed distinct photocurrent behaviors
attributable to different aggregate types.
Under illumination, the drain current in pristine P3HT (J-aggregate) increased fivefold, as seen in Fig 3. In contrast, sonicated P3HT (H-aggregate) showed only a twofold increase in drain current, considerably less than
that observed in pristine P3HT. Intriguingly, for both samples, the drain current
continued to rise gradually after the light was switched on, and maintained this increase
until the light was turned off. This gradual increase in drain current upon illumination
in both pristine and sonicated P3HT can be ascribed to their photoconductive properties,
influenced by the respective aggregate types, which will be discussed later.
The electrical gating effects resulting from H- and J-type aggregates, differentiated by their PL spectra, as shown in Fig 4(a), are depicted at the heterojunction interface in Figs 4(b) and 4(c). In the pristine-P3HT/ZnO (J-type) system (Fig 4(b)), the drain current increased with an 8 V shift in the threshold voltage. Conversely,
in the sonicated P3HT (H-type) (Fig 4(c)), a high drain current in the dark remained relatively unchanged upon illumination,
maintaining the same threshold voltage, and the photocurrent (red) increase was negligible.
In the experiments illustrated in Fig 5, a marked difference in photocurrent response was observed between pristine P3HT
on ZnO and sonicated P3HT on ZnO when subjected to high-intensity green laser illumination.
Specifically, pristine P3HT on ZnO demonstrated a substantial increase in current
upon exposure to light, whereas the sonicated P3HT on ZnO exhibited only a minimal
current increase under the same lighting conditions.
Additionally, both types of samples showed a gradual increase in current when the
light was turned on and a corresponding gradual decrease when the light was turned
off. This behavior aligns with the observations made in Fig 3 for samples without ZnO, indicating a consistent photocurrent response pattern regardless
of the presence of the ZnO layer. This gradual change in current upon light activation
and deactivation reflects the photoconductive properties of the materials and their
response to changes in illumination.
Sonication led to the formation of H-aggregates in P3HT, as evidenced by the red shift in the absorption peak in Fig 1(a). This is contrasted with the higher A0-0/A0-1 ratio observed in pristine P3HT in Fig 1(b), indicating a dominance of J-aggregates. The PL intensity decreased after sonication,
suggesting the formation of non-emissive interchain species (H-aggregates) in Fig 1(b). This is in contrast to the behavior of pristine P3HT, where J-aggregates are more emissive. Upon illumination, pristine P3HT (J-aggregate) showed a larger increase in drain current compared to sonicated P3HT (H-aggregate). The slow increase in drain current upon illumination in both samples
in Fig 3 is attributed to the generation and recombination dynamics of charge carriers, as
well as trapping/de-trapping mechanisms [16,17].
The observed lack of immediate current recovery in P3HT samples post-illumination
can be attributed to persistent photoconductivity, where excited charge carriers take
time to recombine or become trapped within the material. Charge trapping, slow recombination
kinetics, and potential thermal effects from the illumination may also play a role.
Additionally, photoinduced changes in the material's properties and hysteresis effects
could cause sustained deviations from the original current levels even after the light
source is removed. These combined factors result in a current that does not immediately
revert to the pre-illumination level.
When light is shone on a semiconductor material like P3HT, it can excite electrons
from the valence band to the conduction band, creating electron-hole pairs. This process
increases the conductivity of the material, leading to an increase in the drain current.
P3HT can form different types of aggregates, notably J-aggregates and H-aggregates, as observed in Fig 1. These aggregates have distinct molecular packing and electronic structures. J-Aggregates (pristine P3HT) typically show strong intramolecular interactions, which
can lead to a more significant increase in photocurrent upon illumination. H-Aggregates, typically characterized by strong interchain interactions, can lead to
the "quenching" or neutralization of excitons by polarons, which are charged particles
within the material. This process effectively reduces the efficiency of exciton-based
charge transport. As a result of this quenching mechanism, there is a diminished increase
in photocurrent. The close packing of molecules in H-aggregates facilitates these interactions between excitons and polarons, thereby
impacting the material's photoconductive properties and leading to a less pronounced
photocurrent response when illuminated.
When the pristine P3HT is deposited on ZnO, as shown in Fig 4 and 5, the negatively charged ZnO surface can attract positive charges (hole polarons)
away from the P3HT [18,19]. This means that there are fewer polarons in the pristine P3HT to quench the excitons,
leading to higher efficiency (or higher photoluminescence quantum yields). In other
words, the P3HT chains in the pristine P3HT films don't pack too tightly together.
Conversely, in the case of sonicated-P3HT on ZnO, there is evidence of strong interchain
interactions, which is in line with the smaller d-spacing observed compared to pristine
P3HT.
This observation is clearly consistent with the change in the d-spacing, which represents the distance between molecular chains in the crystalline
structure of the polymer, from GIXD shown in Fig 5. The d-spacing decreased from 16.81 to 16.24 Å after sonication. In both scenarios, whether
with (Fig 5(b)) or without ZnO (Fig 5(a)), the d-spacing in sonicated P3HT was found to be smaller than that in pristine-P3HT, as
shown in Fig 5(a) and Fig 5(b). This reduced dspacing in the sonicated samples indicates closer packing of the polymer
chains by strong interchain interactions, which is characteristic of the formation
of H-aggregates. Such dense packing can facilitate stronger interchain interactions, influencing
the electronic and optical properties of the material, particularly in terms of charge
transport and photovoltaic efficiency.
The gradual increase in current, following an initial sharp rise due to band-to-band
photoconduction after the light is turned on, could be attributed to multiple factors.
Initially, the rate of generation of charge carriers (electrons and holes) may be
higher than their recombination rate, leading to a gradual increase in current. In
semiconducting polymers like P3HT, there can be traps for charges, which can temporarily
hold the charges before they contribute to the current. Over time, more charge carriers
might be released from these traps, contributing to the slow increase in current.
After light off, the current slowly decreases due to a dominant recombination process.
The increased current magnitude observed in sonicated P3HT/ZnO, as depicted in Fig 3, relative to pristine P3HT/ZnO, can be linked to improved molecular ordering in the
sonicated samples. This enhanced ordering is evidenced by the GIXD results presented
in Fig 5. In these GIXD patterns, the peaks corresponding to the sonicated P3HT are not only
sharper but also display greater intensity. The sharper and more intense peaks in
the GIXD pattern of the sonicated P3HT indicate a higher degree of crystallinity and
more ordered molecular arrangement. Such structural ordering can facilitate more efficient
charge transport pathways within the material. The closer packing and alignment of
the polymer chains in the sonicated P3HT likely contribute to improved electrical
conductivity, resulting in the observed increase in current when interfaced with ZnO.
This correlation between molecular ordering, as demonstrated by GIXD, and the electrical
performance of the material underscores the significant impact of sonication on the
microstructural properties of P3HT and, consequently, its electronic behavior in a
device context.
The observation of a more positive threshold voltage in sonicated P3HT FETs is attributed
to an increase in the presence of mobile carriers that fill the deep traps within
the material. This phenomenon is the basis for the slow increase and decrease in current
observed upon the switching on and off of light, respectively, as demonstrated in
the I-time measurements in Figs 3 and 5. Once these deep traps are filled with mobile carriers, particularly holes, the threshold
voltage shifts towards a more positive value. Consequently, the transistor becomes
operational at zero gate voltage.
In the pristine P3HT/ZnO FET, despite exhibiting a smaller drain current compared
to its sonicated counterpart, there is a distinct shift in the threshold voltage.
This shift is attributed to the trapping of excited electrons at the P3HT/ZnO interface,
which results in the bending of the P3HT energy levels upwards, thereby causing the
threshold voltage to shift to a more positive value. In other words, the LUMO and
HOMO energy levels of P3HT are elevated at the ZnO side due to photoinduced electron
transfer from P3HT to ZnO, leading to hole accumulation in P3HT adjacent to the ZnO.
This results in the shifting of the threshold voltage of the P3HT FET to a more positive
value, even without applying a gate voltage. Under illumination, the change in threshold
voltage (ΔVT) in the linear operating regime of the transistor is directly proportional to the
number of separated carriers at the interface (Δn), as described by the equation Δn = CiΔVT/q. This finding implies a higher efficiency of exciton dissociation at the interface
with pristine P3HT.
Conversely, in the sonicated P3HT/ZnO FET, excitons are predominantly quenched within
the sonicated P3HT layer, which is relatively distant from the P3HT/ZnO interface.
This spatial separation of exciton quenching sites from the interface prevents any
significant shift in the threshold voltage in the sonicated P3HT/ZnO FET. In other
words, the number of electrons are significantly reduced due to quenching, decreasing
the concentration of mobile holes in the P3HT, causing negligible change in the threshold
voltage of the sonicated P3HT/ZnO FET in Fig 4(c). Essentially, the interaction between the sonicated P3HT and the ZnO layer is different
from that in the pristine P3HT/ZnO system, leading to varied electronic behaviors
in these two configurations, particularly with respect to the threshold voltage and
the response to light illumination.
4. SUMMARY AND CONCLUSIONS
Our research delves into the profound effects of aggregation on the electronic behavior
of P3HT, both in its standalone configuration and when combined with ZnO to form a
planar-heterojunction structure. The study emphasizes the role that aggregation has
on electrical gating at the planarheterojunction interfaces of P3HT/SiO2 and P3HT/ZnO NC. We discovered that H- and J-type aggregation significantly alters molecular ordering and packing within P3HT,
leading to notable shifts in threshold voltage and changes in absorption and luminescence
properties.
Sonication, a key focus of our study, modifies the molecular alignment in P3HT, promoting
the formation of H-aggregates as opposed to the more commonly observed J-aggregates. We found that J-aggregates in pristine P3HT facilitate more efficient exciton movement and electricity
generation, resulting in enhanced photocurrents compared to sonicated P3HT, which
predominantly forms less efficient H-aggregates. In FETs incorporating sonicated P3HT, we observed a shift to a more positive
threshold voltage and an increase in mobility, indicating a higher presence of mobile
charge carriers even without the application of additional voltage. Upon integration
with ZnO, the pristine-P3HT demonstrated significant shifts in threshold voltage under
illumination, likely due to electron trapping. However, sonicated P3HT interfaced
with ZnO showed comparatively less trapping and a minimal shift in threshold voltage.
These observations underscore the vital role of aggregate type (J or H) in dictating the electronic properties and light-response behavior of P3HT, with
sonication yielding higher mobility but reduced photocurrent efficiency in H-aggregates compared to the J-aggregates present in pristine P3HT.
Our study suggests that electrical gating properties can be effectively controlled
through the manipulation of aggregate formation, driven by strong polymer chain interactions.
This understanding opens up new possibilities for optimizing the structural arrangement
in solution-state P3HT for specific applications. Such insights are particularly relevant
in fields like solar cell and sensor technology, where the efficiency of light-induced
current generation is a critical factor.