Accepted: January 29, 2024 | Published Online: January 31, 2024

Citation: Benchaabane A, Lahmar A (2024) Optical Study of Hybrid Colloidal Solution Based on P3OT Polymer and Cdse Nanoparticles. Int J Opt Photonic Eng 9:057.

Copyright: © 2024 Benchaabane A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

The effect of oleic acid-capped CdSe nanoparticles incorporation into poly(8-octylthiophene) (P3OT) colloidal solutions on optical properties was investigated. The photoluminescence study was carried out on both P3OT and P3OT:wt%CdSe solutions. The results showed that the inclusion of oleic acid-capped CdSe strongly influenced the fluorescence spectra. This trend was described within the Förster formalism, which involves a non-radiative energy transfer from the donor P3OT to the acceptor CdSe NPs. Furthermore, the P3OT solution with a high oleic acid-capped CdSe concentration level was found to exhibit a clear splitting, assigned to the existence of different emitting chromophore species. A modified Stern-Volmer model was used to interpret the fluorescence quenching in this system.

Keywords

Colloidal solution, Poly(8-octylthiophene)(P3OT), Oleic acid-capped cadmium selenide nanoparticles (CdSe)

Introduction

Conjugated conducting polymers (CPs), such as polypyrrole (PPy), polyaniline (Pani), and polythiophene (PTh) emerge as an alternative matrix for replacing silicon based materials. This interest resides in their flexible manufacturing and low-cost fabrication, which attracted much attention in both fundamental research and technological applications, especially in electronics and optoelectronics domains [1-4]. Besides, polythiophenes gained particular attentiveness because their end-group functionalization could lead to an attractive prospect of having a molecular handle for charge transfer, surface modification, self assembly, growth of block copolymers, and biomarking [5-7]. In addition, polythiophenes are environmentally stable, easily processed in solution, and can show good synthetic versatility including their regiospecific synthesis [8,9]. It should be noted however that the superiority of polytiophenes as promising organic materials depends upon the possibilities of being combined with different inorganic nanoparticles (NPs) such as cadmium selenide (CdSe) [10], titanium dioxide (TiO ₂ ) [11], ZnO [12], cadmium sulfide (CdS) [13] or other inorganic nanoparticles [14] to create organic/inorganic nanocomposites. This would, therefore, offer the possibility of achieving an unlimited variety of hybrid organic/inorganic materials with diverse functional properties and hence would open new horizons in advanced materials research [15,16]. In this context, hybrid materials composed of polymer and inorganic nanoparticles have shown excellent optoelectronic properties promising for potential applications such as photovoltaic cells (PV), light emitting diodes (LEDs), photodetectors, sensors, and transistors [11,17-20]. Numerous investigations reported that these applications benefit from improved carrier generation and enhanced charge transport as well as efficient exciton dissociation in PTh-containing organic/inorganic nanocomposites [21-28]. However, a particular interest is reserved to poly(octyltiophene) P3OT polymer and oleic acid (OA) capped CdSe NPs system [29-31] as bulk heterojunction (BHJ) photoactive layers in hybrid solar cells. It should be pointed out that the morphologies and distribution of the NPs in such a polymer matrix strongly influence the optical and electrical properties, and in particular the photovoltaic performances of the composites [32].

It is agreed upon that bulk heterojunction BHJ photovoltaic cells can be processed from liquid solution where the chemical synthetic process and molecular design play an important role for their efficiency [33,34]. However, most of the investigations were carried out mainly on optical properties of BHJ thin films rather than on starting solutions, which play a key role for predicting the performance of final BHJ solar cell thin films.

In the present work, we focus more particularly on the study of the influence of oleic acid- capped CdSe NPs incorporation into P3OT matrix in solution state. We use photoluminescence analysis as a preferential tool to study the charge transfer occurring at the donor-acceptor interface. The effect of NPs concentration on photoluminescence properties is also discussed.

Experimental

For the colloidal solution preparation , the first step consists of the preparation of CdSe nanoparticules (NPs) from a mixture of equal volume ratio of cadmium acetate and oleic acid in diphenyl ether. The NPs are capped with oleic acid by precipitation with copious amounts of methanol and are collected by centrifugation and decantation as previously reported for ZnSe [35].

The precipitated nanoparticles were recovered by adding a small amount of chloroform and reprecipitated with methanol. This purification process was repeated three times with a dilution in chloroform (20 g/l) and the obtained solution was stirred for 4h at room temperature to increase the solubility of P3OT. The composite solution was then prepared by adding CdSe nanoparticles to P3OT solution. For our study, five different nanocomposites were prepared corresponding to the concentrations 0 wt% (pure polymer), 20, 40, 60 and 80 wt% of CdSe, respectively (Figure 1a). Figure 2b describes the obtained P3OT:CdSe hybrid materials.

X-rays diffraction analysis was carried out on the CdSe powder using a Bruker D4 Diffractometer (lambda Kα ₁ = 1.54056 Å, lambda Kα ₂ = 1.54439 Å) operating at 40 kV and 40 mA. The morphologies and size of CdSe- NPs were examined by a transmission electron microscope (FEI Tecnai G2 20). The photoluminescence (PL) spectra for all investigated solutions were obtained using a LS 55 Fluorescence Spectrometer (PerkinElmer) with a 400 nm excitation wavelength.

Results and Discussion

Figure 2a represents the XRD diffractogram of CdSe NPs sample powder, which shows three main broad diffraction peaks, of cubic zincblend CdSe located at 25.7°, 42.3° and 49.8° corresponding to the favorite orientations (111), (220) and (311), respectively. All Bragg’s reflections well matched the previously reported literature values [36] and prove thecrystallinity of CdSe NPs. The average crystallite size of CdSe was calculated from the highest peaks intensity by using Debye Scherrer’s equation [37]:

$$\text{D = }\frac{0.9\text{λ}}{\text{βcosθ}}$$

Where D is the nanoparticle size, l is the X-ray wavelength of 1.5406 Å, β is the corrected full width at half maximum (FWHM) of the coherent domain along a direction normal and θ is Bragg diffraction angle of the most intense peaks.

By applying the Scherrer equation for the three peaks, CdSe NPs size values of 6.5, 10.3 and 6.3 nm, respectively were obtained. An average value of (8 ± 2) nm was deduced, which agrees with those reported in previous works [38,39]. The size of NPs is also confirmed with transmission electron microscopy (TEM) image presented in Figure 2b, which clearly reveals a homogeneous dispersion of CdSe nanoparticles with an average diameter of (8 ± 5) nm.

Figure 3 represents comparative emission spectra of CdSeNPs and P3OT solutions. For cadmium selenide nanoparticles, the emission spectra under excitation of 400 nm show a maximum of emission at 2.18 eV. This maximum is significantly shifted compared to the results reported by Okamoto, et al. [40] for CdSe-based QDs (1.99 eV) in toluene solution. For instance, Ananthakumar, et al. [41] showed that the nature of the medium influences the absorption spectra of CdSe; the authors attributed this dependence to the medium dissociation constant and to the Gibbs free energy of the entire phase transfer process that could exist. For poly(8-octylthiophene), the emission band response is in the range of 1.8-2.2 eV, and no P3OT aggregates are detected for which the emission occurs in the range 1.3-1.8 eV [42]. The maximum of emission observed in our measurement is around 2.12 eV. In addition, the profile is not symmetric and a shoulder could be albeit detected around 2 eV. It should be noticed that the emission spectrum in solid state is generally expected to split because of the local single chain conformation and chain-chain interactions [43]. In our case, chloroform, which is a good solvent for the title polymer, might induce suppression of such interactions and only the solvent-solute interaction would be predominating.

To have a better understanding of the photoluminescence PL properties of pure P3OT polymer and P3OT:%CdSe hybrid material, the PL curves were analyzed using a multi-Gaussian functions, as shown in Figure 4. The profile of the photoluminescence band is typical of a multi-phonon process, i.e., a material in which the relaxation occurs by several paths, involving the participation of numerous states within the band gap of the material [44]. This behavior is probably related to the presence of additional electronic levels in the forbidden band gap of the material.

The P3OT spectrum displays four peaks of fluorescence centered at 2.12 eV (583 nm), 2.08 eV (595 nm), 2.01 eV (616 nm) and 1.92 eV (645 nm), respectively. We assume in the present work the existence of at least four bands characteristics for the invariable matrix ascribed to carbonyl-related chromophore groups that are inherent to P3OT. The analysis of the spectrum shows that the difference in the energy between consecutive peaks is in the range of 0.04-0.07 eV, which proves that the photoluminescence is governed by vibronic transitions [45]. Furthermore, the contribution centered at 583 nm (2.12 eV) can be attributed to the formation of the quinone structure, while that centered at 645 nm (1.92 eV) may be assigned to the pristine structure [46-48]. Moreover, the maxima situated at 2.08 eV and 2.01 eV are probably specific to vibronic transitions of the C=C stretching mode in the thiophene rings [49].

As it can be seen from Figure 5, the photoluminescence spectra of CdSe NPs show an intense emission peak at 567 nm with the presence of two other shoulders may be originate from the nom homogeneity of the NPs size. Kumari, et al. found a maximum of photoluminescence intensity at 550 nm for CdSe nanoparticles capped with oleic acid of size equal to 7 nm in toluene solution [50]. Sharma, et al. proved that oleic acid is a particular ligand that yields much more stable active metal nanoparticles than other saturated fatty acids. Moreover, they showed that OA passivates most of the vacancies and trap sites on the CdSe surface thus increasing the photostability of nanoparticles [51]. Gerou, et al. studied CdSe NPs emission spectrum with deconvolution method. The spectrum was decomposed into a main PL band, positioned at 543 nm and two weak bands shifted to the red, at 576 and 670 nm [52].

As shown in Figure 4a, the PL spectrum of pure P3OT polymer consists of four PL components. The first feature around 583 nm and the second at about 595 nm corresponding to a yellow component, the third one (≈ 616 nm) corresponds to orange component and the last peak (≈ 645 nm) corresponds to a red component.”

The same order of colors is observed for P3OT:20%CdSe and P3OT:40%CdSe. Noting that each color represents different types of electronic transition linked to a specific structural molecule arrangement [53]. On the other hand, a red shift is observed for samples containing a large number of nanoparticles (P3OT:60%CdSe and P3OT:80%CdSe samples). The last peak (» 716-727 nm) observed for P3OT:60%CdSe and P3OT:80%CdSe samples, is attributed to red component color and it represents different types of electronic transition linked to a specific structural nanocomposite arrangement [53]. All the obtained components with the related assignment are gathered in Table 1.

Furthermore, we observe a slight red shift of the third and the fourth features (Figure 4d and Figure 4e). Such a behavior could be assigned to a large number of NPs incorporated in polymer matrix. Similar case is observed in P3EEET-ZnO nanocomposite reported by Huan-Ming Xiong where the red shift was attributed to NPs aggregation causing a significant decrease in polymer photoluminescence [54].

The PL intensity was at the maximum for pure P3OT and decreases with increasing CdSe NPs concentration (Figure 6a). The observed quenching phenomenon is mainly due to the concentration effect. However, one of the possible mechanisms that may occur, is the direct charge transfer between CdSe NPs and P3OT, leading to the formation of isolated electron-hole pairs. Such a situation could induce a non-radiative energy transfer responsible for emission quenching [55].

It is well established that exciton energy transfer process of this P3OT:%CdSe composite is followed by Förster energy transfer (FRET). FRET arises from the oscillating dipole–dipole interaction that occurs when the donor emission (P3OT emission spectra) overlaps the acceptor absorption (CdSe absorption spectra) as well as to the small distance between the donor and the acceptor, which would be of few nanometers. Noting that both FRET and Förster energy transfers are important in understanding the optoelectronic properties of hybrid nanosystems [56-58]. The key condition for efficient energy transfer is that the absorbance spectrum of acceptor must overlap adequately with the emission spectrum of donor. Figure 7 illustrates this overlap. Figure 8 summarizes the different routes expected for exciton and charge transfer in P3OT:CdSe blend.

In addition, the introduction of CdSe NPs, acting as a modifying agent, may affect the effective conjugation length of P3OT that influences both the dynamics of excited species and recombination. To explain the fluorescence quenching, Ananthakumar, et al. reported that the presence of CdSe in P3HT matrix suppressed the recombination by splitting the charges at the interface, resulting in the production of holes in polymer and electron in CdSe NPs [41].

To shed more light into the role of CdSe NPs and for better comparison, we normalized the PL intensities in Figure 6b. As shown in this figure, no clear change could be observed for P3OT:20%CdSe NPs compared to the pristine P3OT. As soon as the concentration of NPs increases, a clear second band appeared at ca. 2 eV as a shoulder for 40wt% and became clearly visible in P3OT:60%CdSe sample. This feature may be attributed to the emission of a second type of chromophores with shorter conjugation length. However, the second resolved broad band observed for 60 wt% NPs concentration at ca.1.76 eV is a consequence of inter chain interaction. At high CdSe NPs concentration levels, two clear distinct bands are observed at 2.15 and 1.70 eV, respectively. A closer look at these energy values, we suggest that the first emission peak can be attributed to CdSe NPs (see Figure 3). In contrast, the second one, observed at lower energy, may correspond to a small amount of chains-chain aggregation of the P3OT matrix. Note that the existence of two emission centres (yellow/red) could be rationalized in terms of chain packing effects, resulting from the presence of high CdSe-NPs concentration that would create high and low energy chromophores, in agreement with the literature results [42]. A number of processes can result in a decrease of photoluminescence intensity, which is referred to as quenching. In the simplest case, the collisional quenching follows the Stern-Volmer equation [58]:

I ₀ /I = 1 + K _sv [Q]                                                                   (1)

Where I ₀ and I are the fluorescence intensities observed in the absence and presence of quenchers, respectively, [Q] is the quencher concentration and K _sv is the Stern-Volmer quenching constant. The K _sv is associated to the excited state lifetime in the absence of quencher τ ₀ and the bimolecular quenching rate constant K _q as follows [59]:

K _sv = K _q τ ₀                                                               (2)

Kumari, et al. found straight line plots of I ₀ /I versus the CdSe concentration [Q] of P3HT and MEH-PPV polymers, respectively by CdSe capped with oleic acid (OA) and CdSe capped with tri-n-octylphosphene-oxide (TOPO) and confirm the occurrence of dynamic quenching [50]. In our case, it is observed that I ₀ /I plot shows that experimental data result in positive deviation from a linear S-V relation [60-62]. This positive deviation is attributed to various processes such as formation of charge transfer complexes, intersystem crossing, static and dynamic quenching [63].

Recall that for the quenching phenomena in no composite solutions, the fractional intensity (I ₀ /I) is given by the product of both static and dynamic quenching [64]:

I ₀ /I = (1 + K _sv [Q]).(1 + K _s [Q])                                                           (3)

Where K _sv and K _s the dynamic and static quenching constants, respectively. Figure 9 shows the fraction I ₀ /I as function of the oleic acid-capped CdSe nanoparticle concentrations for the nanocomposite solutions. A positive deviation was observed in the Stern-Volmer (S-V) plot for both the quenchers. This indicates the presence of either static or transient quenching component in the overall dynamic quenching.

The phenomena being related to the quenching of the host P3OT matrix for which fluorescence decreases even upon the embedding of fraction of oleic acid capped CdSe nanoparticles, is described by Stern-Volmer (S-V) model as evidenced by the fitting of the experimental data according to Eq. (3). The non-radiative complex formation was corroborated by the FTIR data published in a previous work [65]. By applying the above equation, static and dynamic quenching constants are being calculated. The K _sv and K _s values are found at 4980 and 7590 mol.L ^-1 , respectively. The value found for K _sv is in the same order of that reported by Kumar, et al. [66]. It is worthy to mention that dynamic quenching of P3OT polymer nanoparticles by quencher CdSe nanoparticles occurred due to long range electronic energy transfer process, which mainly depends upon the overlap between the absorption spectra of CdSe nanoparticles and the emission spectra of P3OT polymer [67].

Conclusion

Cadmium selenide nanoparticles with diameter of about 8 nm were synthesized and incorporated into poly(3-octylthiophene) conducting polymer. Photoluminescence spectra of P3OT:%CdSe nanocomposite solution based on the embedding of CdSe NPs in P3OT matrix were shown to be mainly governed by P3OT and strongly affected by NPs concentration. It turns out that firstly the NPs act as an efficient quencher for the polymer fluorescence and secondly for high concentration the overall fluorescence spectrum is split out into two separate bands for higher concentrations. The higher energy band corresponds to the NPs photoluminescence and the lower energy one is attributed to the P3OT aggregate emission. This was further confirmed by the Stern-Volmer model which showed the most sound description of P3OT photoluminescence quenching phenomena in P3OT:%CdSe nanocomposite solution. The energy transfer accompanying this quenching process, which is of FRET type, results in an efficient fluorescence of CdSe NPs. However, the photoluminescence quenching mechanism would be restricted by the presence of residual ligand species and phase segregation effects in P3OT:CdSe nanocomposite.

Conflicts of Interest

There are no conflicts to declare.

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