Optimization of the Electrical Properties of PVDF Nanofibers with the Addition of P3HT and PCBM Processed by Electrospinning

Abstract

In this study, electrospun nanofibers of poly(vinylidene fluoride) (PVDF) were produced with the addition of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT), for application as a nanostructured active layer of bulk heterojunction polymer with fullerene and polythiophene derivatives in third-generation photovoltaic devices. The electrospinning technique was used to favor the formation of the β phase, exhibiting ferroelectric properties, improving the electrical performance of the device. The morphological, structural and electrical characteristics of the nanofibers were investigated using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and DC electrical measurements. The synergism between nanofiber processing and the addition of P3HT and PCBM improved electrical conductivity by up to three orders of magnitude, furthermore, the nanofibers incorporating P3HT exhibited responses to solar stimuli.

Keywords:  Organic materials; OPVs; Conductive polymer

1. Introduction

In recent years, there has been a significant increase in awareness regarding the importance of planetary preservation, achievable through sustainable socio-economic development. According to the International Energy Agency (IEA), global energy demand is rising by approximately 2% per year¹. Currently, around 65% of the world's electricity generation relies on fossil fuels and other non-renewable sources². This growing energy demand underscores the need for innovative solutions, such as sensors and actuators, that are better integrated into systems and improved in both economic and ecological production processes, leading to the development of smart and functional materials³-⁵.

One promising solution to address this issue is the implementation of solar cells, which harness the vast abundance of solar energy available on Earth⁶. Solar cells are categorized into three generations. The first two generations consist of solar cells based on inorganic compounds like silicon and cadmium telluride. Although these cells are efficient, their production costs are high due to the complexity of their processing⁷. The third generation includes photovoltaic cells made from organic materials, known as organic photovoltaic cells (OPVs)⁸. These cells offer several advantages over their inorganic counterparts, such as easier processing, lower production costs, flexibility, and potential optical transparency, making them a viable option for sustainable energy generation and diverse applications⁹–¹¹.

OPVs are composed of various organic materials organized into layers, with one crucial layer being the active layer. This layer contains the compounds responsible for the photovoltaic effect and consists of a donor material and an electron acceptor¹²,¹³. OPVs are essentially semiconductor diodes produced from these materials, capable of generating electrical current through the photovoltaic effect¹⁴. The selection of materials for the active layer's architecture is vital, as it directly influences the device's performance. Among the most popular organic materials used for this purpose are P3HT and PCBM¹⁵.

PCBM derivatives, belonging to the fullerene class, are widely used in photovoltaic devices due to their high electron affinity and mobility, making them excellent electron acceptors in OPVs¹⁶-¹⁹ Poly(3-hexylthiophene) (P3HT) is a regio-regular alkylated derivative of polythiophene and acts as a p-type semiconductor²⁰. P3HT was first synthesized in 2002²¹ and is favored for its low bandgap value and high hole mobility, showing good performance when combined with PCBM²²,²³.

OPVs can adopt different architectures depending on the arrangement of materials in the active layer. A common strategy to enhance the interaction between materials in this layer is the formation of a bulk heterojunction (BHJ) through the simultaneous mixing and processing of the donor and acceptor²⁴. One method to construct this layer is electrospinning, electrospinning involves the deposition of polymeric fibers onto a collector plate by applying electrostatic force, producing fibers in the range of 20 to 500 nm²⁵-²⁸.

Successful electrospinning requires the solution to have adequate viscosity, determined by the material concentration relative to the solvent²⁵,²⁹,³⁰. Achieving this concentration can be costly with materials like PCBM and P3HT alone, necessitating the use of a third polymer, PVDF, to attain the required viscosity⁷,²⁰,³¹.

Poly(vinylidene fluoride) (PVDF), is extensively studied for its unique electrical and optical properties, flexibility, processability, chemical stability, and mechanical resistance. PVDF can be used as a protective agent by coating the materials of the active layer. The phases of PVDF differ by chain conformations, with the β phase being highly polar and enhancing its piezoelectric, pyroelectric, and ferroelectric properties⁹,³²,³³. These properties are particularly useful for charge transport and storage, as the internal electric field of the β phase reduces electron-hole recombination, thereby improving device performance³⁴-³⁶.

In summary, the integration of PVDF in OPVs not only enhances the mechanical and chemical stability of the active layer but also contributes to improved photovoltaic performance through its ferroelectric properties¹⁸. This study aims to explore the incorporation of PVDF into organic photovoltaic cells, hypothesizing that the inclusion of PVDF will significantly improve the efficiency and durability of OPVs, thus providing a more viable solution for sustainable energy generation. Additionally, this work will investigate the effectiveness of the electrospinning process in forming bulk heterojunctions, with the goal of optimizing the interaction between donor and acceptor materials in the active layer³⁷,³⁸.

2. Experimental

2.1. Materials

Poly(vinylidene fluoride) (PVDF) - Arkema Kynar PVDF 761/Polyvinylidene fluoride resin (average molecular weight 20,000 ~ 45,000), acquired from Arkema. Phenyl-C61-butyric acid methyl ester (PCBM) (EPCP:3315) it was acquired from Solenne BV. The polymer Poly(3-hexylthiophene-2,5-diyl) (P3HT) regioregular (average molecular weight 20,000 ~ 45,000) was acquired from Sigma Aldrich. The solvents used were N,N-Dimethylformamide (DMF) and Toluene (Toluol), both from the Synth.

2.2. Preparation

2.2.1. Preparation of PVDF, PCBM:PVDF, P3HT:PVDF and P3HT:PCBM:PVDF

PVDF was diluted in DMF with a ratio of 1 g of PVDF to 6 mL of DMF, using a magnetic stirrer, and the solution was stirred at 70°C for 2 hours. The PCBM solution was diluted at a ratio of 10 mg of PCBM to 1 mL of Toluene, using ultrasonic bath for 30 minutes. Finally, the P3HT solution was diluted at a ratio of 1 mg of P3HT to 1 mL of Toluene, stirred on a magnetic stirrer for 10 minutes at 50°C, and then stirred for 110 minutes at 25 ^oC, totaling 2 hours. PCBM and P3HT were added to the PVDF solution at a mass ratio of 1%, resulting in four different solutions for the study: PVDF, PCBM:PVDF, P3HT:PVDF and P3HT:PCBM:PVDF.

2.2.2. Electrospinning parameters

We assembled a system for electrospinning using a syringe pump (New Era Pump System), a 3 mL syringe, a needle with diameter of 0.55 mm and a high voltage power supply capable of providing a continuous current of up to 10 kV, along with a rotating collector. To produce nanofibers, the syringe was placed in the infusion pump, maintaining a fixed flow rate of 0.5 mL/h, and the needle was connected to the positive output of the high voltage power supply, applying a voltage of 10 kV. Simultaneous, the negative output (grounding) was connected to the rotating collector. This setup generated the electromagnetic field necessary to propel the material from the syringe to the collector, forming the nanofibers. Electrospinning of the polymer solutions was performed under fixed parameters of 70% relative humidity, with the rotation collector set at 400 rpm and positioned 15 cm from the needle tip, as per the parameter optimizations previously investigated in prior works. The high relative humidity during the application of the technique is intended to induce fibers with a higher content of the electroactive β phase and to achieve a porous surface morphology for the fibers, which aids in the infiltration of sunlight through the nanofibrous network³⁹-⁴¹.

2.3. Substrate

The deposition of nanofibers was carried out on two substrates according to the desired characterization. For morphological analysis, a 10x10 cm aluminum foil was used. For electrical characterizations, gold interdigitated electrodes (IDEs - Interdigitated Electrodes), represented in Figure 1, were used at the Microfabrication and Thin Films Laboratory (LMF) of the National Nanotechnology Laboratory (LNNano) at the National Center for Research in Energy and Materials (CNPEM). The used of IDEs is essential for this work and for the characterization of the electrical properties of the materials. This is because knowledge of their geometric factors, such as the distance between the digits and their lengths, allows for the determination of the cell constant k og the electrode and the quantitative analysis of the data obtained to determine the electrical properties of the samples, such as their conductivity (σ) and their resistance (R), before being inserted as a layer in the final device.

Figure 1
Interdigitated gold electrode (IDE) with 100 digits produced by Microfabrication and Thin Films Laboratory (LMF) of the National Nanotechnology Laboratory (LNNano) at the National Center for Research in Energy and Materials (CNPEM).

3. Characterization

3.1. Scanning Electron Microscopy (SEM)

The morphology of the nanofibers was analyzed using Scanning Electron Microscopy (SEM) on the Carl Zeiss EVO LS15 model, equipped with secondary electron detector (SE) under high vacuum and constant temperature of 25°C. The average diameter of the fibers was calculated from the SEM images of the electrospun nanofibers using the ImageJ software.

3.2. Fourier transform infrared spectroscopy (FTIR)

FTIR analysis was performed on the Frontier FTIR spectrophotometer (Perkin Elmer, USA), equipped with a diamond attenuated total reflection (ATR) module, at the Laboratory of Organometallic Catalysis and Materials (LaCOM), UNESP-FCT, Presidente Prudente campus. Measurements were carried out with a resolution of 0.5 cm⁻¹, 128 scans, and a range from 400 to 4000 cm⁻¹.

3.3. Direct current (DC) measurements

DC measurements allowed the study of the electrical properties of the materials. A Keysight B290IA voltage source was used to perform the measurements, enabling the determination of the electrical current response according to the applied voltage (I x V), both under sunlight and in the dark. The Oriel VERASOL solar simulator was employed. For the I x V study, a voltage from -5 V to 5 V was applied in 0.5 V intervals, and the generated amperage was recorded for each voltage. Measurements were also conducted under sunlight. Additionally, a study of the device response over time was conducted by repeating the measurements on the first, fifth, tenth, fifteenth, twentieth, and thirtieth days. The I x t measurements were performed with the incidence of light pulses. The device was subjected to 0 V and 5 V voltages, kept in the dark for 5 minutes, and then light pulses were initiated in cycles of 1 minute, with and without light.

4. Results and Discussion

4.1. Scanning Electron Microscopy (SEM)

The SEM images, depicted in Figure 2, illustrate the morphology of the fibers for the samples of pure PVDF, PCBM:PVDF, P3HT:PVDF, and P3HT:PCBM:PVDF. The characterizations were conducted using Scanning Electron Microscopy (SEM) equipment, with magnifications of 1000x and 15000x. Additionally, the graphs related to the frequency distributions of the nanofiber diameters were obtained from these analyses.

Figure 2
SEM images of pure PVDF (a-b), PCBM:PVDF (c-d) electrospun nanofibers. P3HT:PVDF (e-f) and P3HT:PCBM:PVDF (g-h) at 1000x, 15000x and histograms with diameter frequencies.

All samples exhibited a significant quantity of high-quality fibers. The appropriate quantity of fibers is intrinsically related to the deposition time during their formation. For electrospinning, the infusion pump is programmed with a fixed flow rate; however, not all the ejected material is deposited on the collector⁴²-⁴⁵. To maximize fiber formation in the shortest possible time, we determined the appropriate parameters, which allowed for the sufficient quantity of fibers to cover the substrate to be deposited in 10 minutes, thereby optimizing the amount of material used.

The quality of the fibers is determined by various factors, such as morphology, including diameter, uniformity, and surface structure (smooth or porous, suitable for different functions). Additionally, the fibers may exhibit defects, such as beads. Although there are regions in the fibers with these defects, the reduction in surface area relative to volume due to the presence of beads is not as significant when compared to the defect-free area in the entire sample. The presence of beads is inevitable; however, mitigating these structures by adjusting optimal parameters contributes to a superior effective area of the device.

The images in Figure 2(a-b) depict the electrospun nanofibers of pure PVDF, highlighting an excellent fiber formation, with the most frequent diameter ranging between 300-350 nm and an average diameter of approximately 410 nm. Although the presence of beads along the nanofibers is observed, these are outnumbered by those without such deformation. Overall, the parameters employed in the electrospinning of pure PVDF nanofibers demonstrate to be ideal for the production of these structures.

In Figures 2(c-d), the SEM images of PCBM:PVDF nanofibers exhibit a morphology very similar to that of pure PVDF nanofibers, with a significant quantity of fibers. The PCBM:PVDF nanofibers also show a higher frequency of fibers with diameters between 300 and 350 nm, but with an average diameter of approximately 360 nm. This reduction is due to the addition of PCBM, semiconductor polymers enhance the interaction between the needle and the collector during electrospinning. Studies have shown that adding semiconductor polymers to the solution used in electrospinning is an excellent strategy for obtaining nanofibers with improved properties, as they interact more effectively with the electric field generated by the high-voltage source²⁴,⁴⁶-⁴⁸.

In Figures 2(e-f), the SEM images of P3HT:PVDF electrospun nanofibers reveal a morphology similar to that of pure PVDF nanofibers, with a noticeable formation of nanofibers. Among the different solutions, this one presents the highest quantity of beads, yet it has the smallest average diameter, approximately 250 nm, and the highest fiber formation (30%) occurs at diameters of 250 nm and 300 nm. Finally, in Figures 2(f-g), the electrospun nanofibers of P3HT:PCBM:PVDF also maintain the pattern of pure PVDF nanofibers. These exhibit 25% of the fibers formed with diameters between 200 and 250 nm, the smallest among the samples, but with an average diameter of approximately 350 nm. Thus, it was possible to electrospin the nanofibers of pure PVDF, PCBM:PVDF, P3HT:PVDF, and P3HT:PCBM:PVDF, maintaining their original characteristics, justified by the significant amount of PVDF in the samples. However, the addition of conductive polymers resulted in a decrease in the diameters of the electrospun nanofibers.

4.2. Fourier transform infrared spectroscopy (FTIR)

The Fourier transform infrared (FTIR) spectra of PVDF and PCBM:PVDF, P3HT:PVDF, P3HT:PCBM:PVDF hybrid nanofibers were measured and shown in Figure 3. Curve PVDV pure in Figure 3 represents the PVDF nanofibers sample. The peak at 1454 cm⁻¹ corresponds to angular deformations (scissors) and asymmetric deformations of CH and CH₃⁴⁹. PVDF exhibits distinctive peaks at 1402 and 876 cm⁻¹, while peaks at 1274, 1073, and 837 cm⁻¹ are indicative of the formation of the β phase, renowned for its piezoelectric properties⁵⁰,⁵¹.

Figure 3
FTIR spectra of pure PVDF, PCBM:PVDF, P3HT:PVDF, and P3HT:PCBM:PVDF nanofibers according to transmittance.

Curve PCBM:PVDF in Figure 3 for PCBM:PVDF nanofibers, a distinct peak at 1737 cm−1 ⁴⁹,⁵², associated with PCBM, is observed, is the characteristic peak of C=O group. This peak is characteristic of PCBM, although its relatively low intensity may be due to the reduced proportion of PCBM compared to PVDF. Additionally, the elongated peak at 1107 cm−1 ⁵³, typically indicative of C-N chain elongation, is a consequence of the electrospinning process. Notably, characteristic PCBM peaks at 1737 and 1188 cm⁻¹ were not observed, likely due to the predominance of PVDF in the material composition⁵².

Curve P3HT:PVDF in Figure 3 for P3HT:PVDF nanofibers, the band at 1260 cm⁻¹ is associated with the vector derived from the dipole perpendicular to the plane of the P3HT structure⁴⁹,⁵³,⁵⁴.

Curve P3HT:PCBM:PVDF in Figure 3 for the blend of P3HT:PCBM:PVDF nanofibers, the most prominent bands correspond to PVDF, the material present in greater quantity in the samples. The electrospinning process elongates the polymeric chains, promoting the formation of this phase. Alongside the β phase, PVDF also manifests the α and γ phases, discernible by bands observed at 762, 614, and 1232 cm−1 ⁵⁰,⁵⁵ In Table 1, the peaks attributed to the transmittances of electrospun nanofibers are presented.

Table 1
Assignments for transmittances in FTIR of nanofibers.

Peak (cm-1)	Assignment	Reference
1737	Characteristic PCBM	49,52
1454	CH and CH3 deformation	53
1402	PVDF characteristic	51
1274	PVDF β phase	50
1260	P3HT ring	49,52
1232	PVDF γ phase	55
1172	C-C bond	50
1107	C-N chain elongation	53
1073	fase β do PVDF	50
876	PVDF characteristic	51
837	PVDF β phase	50
762	α phase PVDF	55
614	α phase PVDF	55

Transmittance analysis reveals the material's behavior concerning specific wavenumber, especially in the bands related to PCBM and P3HT. A discrete peak at 1737 cm⁻¹, observed in the nanofibers containing PCBM, distinguishes this material, although its relatively low intensity may be attributed to the reduced proportion of PCBM in relation to PVDF. The elongated peak at 1107 cm⁻¹ commonly occurs in cases of elongation of C-N chains, a result of the electrospinning process of PVD F. On the other hand, the band at 1260 cm−¹ is associated with the vector derived from the dipole perpendicular to the plane of the P3HT structure⁴⁹,⁵³,⁵⁴. The FTIR technique allowed for the detection of the presence of PCBM and P3HT in the electrospun nanofibers of PVDF.

Some peaks observed in all curves are attributed to PVDF, due to the high concentration of this material in the samples. For example, the peak at 1454 cm⁻¹ is related to angular deformations (scissors) and asymmetric deformations of CH and CH₃⁴⁹. The distinctive peaks of PVDF, at 1402 and 876 cm⁻¹, are also identified. However, characteristic PCBM peaks, such as those found at 1737 and 1188 cm⁻¹, were not observed, possibly due to the predominance of PVDF⁵².

The most prominent bands correspond to PVDF, the material present in greater quantity in the samples. For instance, peaks at 1274, 1073, and 837 cm⁻¹ are associated with the formation of the β phase, notable for its piezoelectric properties⁵¹. The electrospinning process stretches the polymeric chains, favoring the formation of this phase. In addition to the β phase, PVDF has the α and γ phases, identified by the bands at 762, 614, and 1232 cm−1 ⁵⁰. Table 1 was prepared to assign the peaks to the transmittances in the FTIR of the nanofibers.

From absorbance measurements, the percentage of the β phase in the material can be calculated using Equation 1. This equation utilizes the absorbance peaks observed at 840 cm⁻¹, indicative of the β phase, and at 760 cm⁻¹, indicative of the α phase, as depicted in Figure 4. The results are summarized in Table 2, revealing a predominant formation of the β phase, particularly in comparison to the α phase.

Figure 4
FTIR spectra of pure PVDF, PCBM:PVDF, P3HT:PVDF, and P3HT:PCBM:PVDF nanofibers according to absorbance.

Table 2
Formation of β on Nanofibers.

Sample	β phase (%)
PVDF pure	80.03
PCBM:PVDF	73.41
P3HT:PVDF	79.28
P3HT:PCBM:PVDF	80.92

Equation 1 expresses the ratio of the β phase percentage relative to the α phase (F_α), where A_β and A_α represent the absorbance values at 840 cm⁻¹ and 760 cm⁻¹, respectively. The constants K_β and K_α are calibration factors. For PVDF, the ratio of these constants ($K_{\beta }/K_{\alpha }$) is determined to be 1.26 ⁵⁵,⁵⁶.

$F_{\beta }=\frac{A_{\beta }}{\left(1.26\right).A_{\alpha }+A_{\beta }}$ (1)

4.3. Direct current (DC) measurements

The direct current (DC) measurements were carried out using pure polyvinylidenefluoride (PVDF) nanofibers, as well as mixtures of PCBM, P3HT, and P3HT:PCBM.These materials were carefully deposited on the interdigitated electrodes (IDE), as depicted in Figure 5. By conducting these measurements, researchers were able to determine the conductivity and electrical resistance of the materials over a period of time. This, in turn, provided valuable insights into the charge transport mechanisms within the pure PVDF, PCBM, P3HT, and P3HT:PCBM devices, contributing to a deeper understanding of their electrical properties and potential applications.

Figure 5
Electrospun nanofibers on the IDE.

Figure 6 presents the graph of the I x V direct current measurements over time for PVDF pure nanofibers. This graph contains information about the material's electrical behavior and conductivity.

Figure 6
I x V measurements of direct current for PVDF pure nanofibers and Conductivity of pure PVDF nanofibers.

The curve depicted by the current (I) versus voltage (V) graph in Figure 6 shows a linear relationship between the electric current I and the potential difference V applied to the interdigitated electrode (IDE) terminals. However, it is notable that the I values do not intersect the origin of the Cartesian plane (0,0) when the voltage is zero. This deviation might be attributed to charge accumulation within the nanofibers. Upon the application of voltage to the device, the current response ranges from -2.0 nA to 2.5 nA, which is within the expected range for an insulating material. A similar work is the Forouzan et. al. optimized the electromechanical performance of piezoelectric nanofibers fabricated from electrospun polyvinylidene fluoride (PVDF) by using IDE and yarns through various electrical connection configurations and electrode designs and connection in core-sheath yarns resulted in large output voltage magnitude (~1.4 V)⁵⁷.

A plausible explanation for the accumulation of residual charges in the material is linked to the behavior of the PVDF polymer chain when exposed to an external electric field during DC measurements. This is particularly relevant because the PVDF nanofibers, after undergoing electrospinning, predominantly exhibit the β phase, as indicated in Table 2. Consequently, they display a resulting dipole moment due to the orientation of the hydrogen and fluorine side groups at opposite ends of the polymer chain, creating permanent dipoles because of the difference in electronegativity between these elements.

Supporting this hypothesis, measurements of the material under a 0 V voltage over time were conducted, as shown in Figure 7a). The resulting I versus time (t) graph reveals an initial non-zero electric current that gradually tends towards zero over time. This behavior indicates the dissipation of accumulated charge within the material, essentially grounding the sample.

Figure 7
a) Measurements obtained through grounding of pure PVDF and b) I x V measurements of pure PVDF starting from 0 V.

Complementarily, Figure 7b) illustrates the current (I) generated by the same sample, after charge dissipation, when subjected to a variable voltage (V) with an amplitude ranging from -5 V to 5 V. Under initial conditions, when the voltage is 0 V, the electric current is zero, contrary to the behavior analyzed in Figure 6. As the voltage gradually increases to its maximum of 5 V, the electric current also increases, reaching more positive values.

When the voltage polarity is inverted, thus altering the polarity of the electric field acting on the nanofibers, there is a gradual reduction in the current. This process eventually brings the system back to the initial conditions when the voltage was 0 V. However, at this point, the current does not return to the initial zero value, indicating an accumulation of charges that causes a difference in the I vs V trajectory. The same polarity reversal is seen in the work of Yousry et. al. at explore and understand the synergistic interaction between hydrated salt and electric field during electrospinning, aiming to enhance polarization and alignment for achieving high-performance piezoelectric nanofiber films⁵⁸. This phenomenon is observed again when the voltage reaches negative values and subsequently inverts its polarity once more.

These results suggest the presence of residual polarization of the dipoles in the PVDF polymer chain. During the process of inverting the electric field in the nanofibers, the dipoles partially orient themselves, resulting in a non-zero electric dipole moment and a corresponding non-zero electric current. This behavior describes a graph similar to the hysteresis observed in magnetic materials.

Understanding the reasons why the current values measured at 0 V are not 0 A, it is possible to calculate the conductivity of pure PVDF on each of the days. For this, some procedures are necessary, including adjusting the curves of the graph to first-degree equations (f(x) = ax + b) and using Equations 2 and 3 to determine the conductivity (σ).

$$V=r.i$$ (2)

$$\sigma =\frac{1}{r}\cdot \frac{l}{A}$$ (3)

In this case, the value of r^-1 can be determined by the slope of the lines in the I x V graphs (considering the appropriate approximations), where L is the length of the IDE digits and N is the number of digits, as shown in Figure 3. Finally, to determine $\frac{l}{A}$, it is necessary to calculate the cell constant k [cm⁻¹] of the device.

This constant for an electrolytic conductivity sensor is defined as the proportionality factor between the specific resistance of the electrolyte and the measured resistance. This constant is determined by the sensor's geometry. The equation used to calculate k can be seen in Equation 4.

$$k=\frac{1}{{(N-1)}^L}\cdot \frac{2K(k)}{K\left[{\left(1-k^2\right)}^{\frac{1}{2}}\right]}$$ (4)

$$K(k)={\int }_{t=0}^1\frac{dt}{{\left[\left(1-t^2\right)\left(1-k^2{t}^2\right)\right]}^{1/2}}$$ (5)

Thus, we obtain the elliptic integral $K\left(k\right)$, as observed in Equation 5. For this study, the value of the cell constant (k) is 5.1 m−1 ⁵⁶ determined solely by the architecture of the IDE. Therefore, to determine the material's conductivity, simply multiply the cell constant by the slope of the material's line; these calculations apply to all materials deposited on the IDEs. Table 3 and Figure 6 were prepared for pure PVDF nanofibers, representing the conductivity values over time.

Table 3
Conductivity of pure PVDF nanofibers.

Days	Conductivity σ (S/m)
1	$2.29*{10}^{-9}$
5	$3.53*{10}^{-9}$
10	$1.87*{10}^{-9}$
15	$0.19*{10}^{-9}$
20	$1.99*{10}^{-10}$
30	$1.98*{10}^{-10}$

In pure PVDF nanofibers, a considerable difference in measured conductivities was observed over time, with the conductivity on the fifth and tenth day being significantly higher than the others. Material degradation occurred after the fifteenth day, justifying the decrease in conductivity.

On the first day, it is possible that some of the solvent remained in the nanofiber membrane. As the solvent evaporated, a more accurate response was obtained. This evaporation period is prolonged due to the low evaporation rate of DMF (Dimethylformamide), which has a slow evaporation rate⁵⁹.

Moreover, considering the study of these materials for application in photovoltaic devices, it is crucial to account for the influence of light on the material. Therefore, observing the materials behavior under light exposure is necessary. Performing current versus voltage (I x V) measurements both in the dark and under light allows for investigating the influence of photons on current generation and material recovery in the absence of light.

PVDF is a material that lacks photoactive properties, and therefore, light does not seem to significantly influence the I x V measurements, as demonstrated in Figure 8. A very similar behavior between measurements conducted in the dark and under light, and upon reevaluating measurements in the dark, the material maintained the same response. Thus, the conductivity of pure PVDF nanofibers was not affected by light exposure. Despite not having light-sensitive properties, PVDF is widely used in photoactuator devices as a support material for different radiation-sensitive layers, as seen in the work of Sultana et. al. The authors highlight that Methylammonium lead iodide on PVDF porous films increasing the electroactive phase in PVDF⁶⁰.

Figure 8
a) Current-voltage I x V measurements of PVDF pure nanofibers in the dark and under light exposure. b) Respective conductivities.

Subsequently, PCBM was added to create PCBM nanofibers. The current versus voltage (I x V) measurements for these fibers are presented in Figure 9. Similar to the behavior observed with pure PVDF nanofibers, the PCBM fibers did not reach their highest conductivity on the day of electrospinning. Instead, their peak conductivity occurred on the tenth day, after which it decreased and stabilized at less than 0.15 µS/m from the fifteenth day onwards.

Figure 9
I x V measurements of direct current for PCBM:PVDF 1% nanofibers and Conductivity of PCBM:PVDF nanofibers.

It is observed that the I x V curve did not pass through the origin point, indicating that the material exhibits current even in the absence of applied voltage in the device. Similar to pure PVDF, there is an accumulation of charges in the nanofibers due to the initial use of negative voltages, so when the voltage reaches 0 V, there is an accumulation of charges resulting in the presence of current even without the application of voltage. This behavior is consistent, as we are dealing only with polymeric materials²⁸, as was the case with pure PVDF.

However, when repeating the measurement starting at 0 V, in the same manner as was done for pure PVDF, it was possible to generate the graph shown in Figure 10. The current values found for 0 V are approximately 0 A, confirming that the current values observed at 0 V in the previous cases are due to the presence of accumulated charges in the material.

Figure 10
I x V measurements of PCBM:PVDF starting from 0 V.

Regarding the material conductivity, by examining the curves depicted in Figure 9 , it was possible to compile the data into Table 4 and create the graph shown in Figure 9, which displays the material's conductivity values. When comparing the conductivities of pure PVDF nanofibers with those of PCBM:PVDF nanofibers, it is observed that the nanofibers containing PCBM exhibit significantly higher conductivity, up to three orders of magnitude higher than pure PVDF nanofibers. The joining of PVDF and PCBM was reported in the work of Zhang C. et. al. which shows to develop and enhance the energy storage performance of dielectric capacitors using all-organic composite dielectrics, addressing the increasing demand for clean energy solutions⁶⁰.

Table 4
Conductivity of PCBM:PVDF nanofibers.

Days	Conductivity σ (S/m)
1	$4.80*{10}^{-6}$
5	$6.19*{10}^{-6}$
10	$2.39*{10}^{-5}$
15	$6.56*{10}^{-6}$
20	$1.55*{10}^{-7}$
30	$2.56*{10}^{-8}$

Similar to the observations with pure PVDF nanofibers, the most notable conductivity responses for PCBM:PVDF nanofibers were not observed on the first day but rather increased over time. Only on the fifteenth day did a decrease in conductivity occur, followed by a considerable decrease on the twentieth and thirtieth days of up to three orders of magnitude. An important result to highlight is the significant improvement in the conductivity of the material when PCBM is added to PVDF nanofibers. This addition increased the conductivity by up to 3 orders of magnitude, primarily due to the charge carrier properties of PCBM¹⁹.

PCBM, like PVDF, is a material that lacks photoactive properties. Therefore, its addition to the nanofibers is not influenced by light in the I x V measurements, as shown in Figure 11. The graph shows a very similar behavior between measurements performed in the dark and under light, and when the measurements are repeated in the dark, the material maintains the same response. Consequently, the conductivity of PCBM:PVDF nanofibers was not affected by light exposure.

Figure 11
a) Direct current (I x V) measurements of PCBM:PVDF nanofibers in the dark and in the presence of light. b) Respective conductivities.

The next step involved the characterization of P3HT:PVDF nanofibers. Unlike PVDF and PCBM, P3HT exhibits photovoltaic properties. Therefore, when exposed to light, the material displays a different response compared to darkness. To assess this behavior, measurements were conducted using the solar simulator.

In the graph (Figure 12) of I x V measurements of the device taken in the dark and in the light, it is observed that the responses varied in the microampere scale up to the fifth day, after which the material started responding in nanoamperes. The conductivity values of the material on each day are presented in Table 5, with the respective conductivity graph in Figure 13 for measurements taken in the dark and in the light.

Figure 12
Direct current (I x V) measurements of 1% P3HT:PVDF nanofibers in the dark and in the light.

Table 5
Conductivity of P3HT:PVDF nanofibers.

Days	Conductivity in the dark σ (S/m)	Conductivity in the light σ (S/m)
1	$3.57*{10}^{-7}$	$1.23*{10}^{-7}$
5	$4.36*{10}^{-7}$	$2.27*{10}^{-7}$
10	$2.29*{10}^{-9}$	$4.18*{10}^{-10}$
15	$2.24*{10}^{-9}$	$1.76*{10}^{-9}$
20	$2.86*{10}^{-10}$	$1.86*{10}^{-10}$
30	$2.35*{10}^{-10}$	$1.70*{10}^{-10}$

Figure 13
Conductivity of P3HT:PVDF nanofibers in the dark and in the light.

The material degradation in P3HT was more aggressive compared to PCBM. On the tenth day, there was a significant drop in conductivity response, which was less than 1% of the highest conductivity peak observed on the fifth day. Although both P3HT and PCBM are classified as conducting polymers, P3HT was found to be less conductive than PCBM when comparing the conductivity of PVDF nanofibers with P3HT and those with PCBM.

Comparing the responses of measurements conducted in the dark and under light, a considerable decrease in material conductivity is observed when exposed to light. Although the responses remained proportional over the days, it is clear that when the material is exposed to light, the current response of the material is reduced, indicating the presence of P3HT in the nanofibers.

Figure 14 illustrates the I x V measurements of the P3HT:PCBM:PVDF nanofibers conducted over time, both in the dark and under light. The P3HT:PCBM:PVDF nanofibers showed combined results between P3HT:PVDF and PCBM:PVDF nanofibers. Although they exhibited conductivity similar to that of PCBM:PVDF nanofibers, they responded to stimuli caused by light incidence due to the presence of P3HT.

Figure 14
I x V measurements of direct current for 1% P3HT:PCBM:PVDF nanofibers in the dark and in the light.

In Table 6 and Figure 15, the conductivities of the nanofibers for the characterizations performed in the dark and under light conditions over time are presented. Material degradation was more pronounced from the tenth day onwards in both conditions. The conductivity for measurements conducted under light was significantly lower, and it was noted that for measurements in the dark, the highest conductivity occurred on the first day, while for measurements under light, it occurred on the fifth day.

Table 6
Conductivity of P3HT:PCBM:PVDF nanofibers.

Days	Conductivity in the dark σ (S/m)	Conductivity in the light σ (S/m)
1	$8.79*{10}^{-9}$	$1.44*{10}^{-9}$
5	$4.86*{10}^{-9}$	$1.60*{10}^{-9}$
10	$3.52*{10}^{-10}$	$1.96*{10}^{-10}$
15	$1.77*{10}^{-10}$	$1.52*{10}^{-10}$
20	$8.36*{10}^{-10}$	$3.83*{10}^{-10}$
30	$1.24*{10}^{-10}$	$3.98*{10}^{-10}$

Figure 15
Conductivity of P3HT:PCBM:PVDF nanofibers in the dark and in the light.

When using the blend of p-type and n-type polymers in the P3HT:PCBM:PVDF nanofibers, a considerable increase in device conductivity was not achieved compared to PCBM:PVDF nanofibers, which exhibited considerably higher conductivities. However, when both semiconductors were combined, the difference in current between measurements conducted in the dark and under light was more pronounced compared to P3HT:PVDF nanofibers, consolidating that the influence of light becomes more significant when combining the donor and acceptor materials.

5. Conclusions

The electrospinning technique was applied to produce nanofibers from three different polymers, PVDF, PCBM and P3HT, for use in organic photovoltaic devices. It was possible to form nanofibers with PVDF, PCBM and P3HT combined, improving the electrical conductivity of the combined materials P3HT:PVDF and PCBM:PVDF by 2-3 orders of magnitude, however with the three materials combined it was not possible to verify the improvement in conductivity electrical. The P3HT:PVDF and P3HT:PCBM:PVDF nanofibers showed changes in electrical conductivity when exposed to light. The samples produced showed a decrease in conductivity over the days, demonstrating the need to protect devices from the atmosphere to prevent oxidation.

6. Acknowledgments

The authors would like to thank CNPq (308846/2022-2 and 407863/2023-0), LabMEV–FCT/UNESP, INEO (14/50869-6), and FAPESP. CAPES-PRINT-UNESP 2022 - ORGFLEX. This research utilized facilities at the National Nanotechnology Laboratory (LNNano), part of the National Center for Research in Energy and Materials (CNPEM), a private nonprofit organization under the supervision of the Ministry of Science, Technology, and Innovations of Brazil (MCTI). The LNNano team is acknowledged for their assistance during the experiments (20230903). This work was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Financing Code 001.

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Publication Dates

• Publication in this collection
  09 Dec 2024
• Date of issue
  2024

History

• Received
  30 Apr 2024
• Reviewed
  18 Sept 2024
• Accepted
  22 Sept 2024