- Nano Express
- Open Access
Effect of Confinement on Photophysical Properties of P3HT Chains in PMMA Matrix
© The Author(s). 2017
- Received: 30 December 2016
- Accepted: 9 August 2017
- Published: 29 August 2017
The influence of arrangement of poly(3-hexylthiophene) (P3HT) chains embedded into poly(methyl methacrylate) (PMMA) matrix on photophysical properties, such as electronic absorption spectrum, band gap, and photoluminescence quantum yield, of the formed P3HT aggregates have been studied. It has been found that variation of P3HT fraction in PMMA matrix from 25 to 2 wt% is accompanied with the increasing quantum yield of photoluminescence, red shift of the band gap, and structural change of P3HT crystallites. The above changes are accompanied with disruption of the continuous network of P3HT fraction into smaller P3HT particles with size ranged from several microns to several tens of nanometers. The results are interpreted in terms of the changing intermolecular packing and reduced intramolecular torsional disorder. It is discussed that the most contribution to the above changes comes from P3HT molecules at the interface of P3HT cluster and PMMA environment.
- Poly(methyl methacrylate)
Photophysics of collapsed coils and nanoscale confined systems of conjugated polymers has attracted considerable interest during last decade [1–4]. Particularly, the processes of exciton generation, radiative recombination, and photogenerated charge transfer in poly(3-hexylthiophene) (P3HT) nanoscale aggregates and crystallites have a direct impact on performance of organic solar cells where this polymer is used as an active component. It was shown that the nature of emission in isolated P3HT molecules and P3HT aggregates is different. The molecular emission normally originates from a common intrachain exciton state corresponding to the relaxed chain with reduced torsional disorder . The emission spectrum of P3HT aggregates also originates from a common emitting state, but corresponding to the interchain singlet exciton that has fell down by single or multiple energy transfer steps to the domain with the lowest energy . Quantum yield (QY) of photoluminescence (PL) of ordered lamellae structure in P3HT films is strongly suppressed as compared to the free molecules in solutions due to interchain delocalization and dissipation of excitons in the condensed material . On the other hand, QY can be enhanced by control of temperature  or regioregularity of P3HT chains . It was shown, for example, that regioregular P3HT films has an order weaker optical transitions as compared to films of regiorandom P3HT due to a larger interchain contribution for the lowest exciton in lamellae compared to the intrachain character of exciton in regiorandom P3HT . Therefore, developing simple and effective strategies to manipulate the optical properties of conjugated macromolecules through changes in their intramolecular design and intermolecular assembly and ordering has significant potential for gaining further understanding of this interesting class of materials but also for their widespread application in organic electronics.
The goal of this work is to show how the changed arrangement of P3HT chains influences physical properties, such as electronic absorption spectrum, band gap, and emission QY of P3HT nanoscale particles. One promising strategy that enables one to tune photophysical properties of conjugated polymer films is blending with the other inert polymer. It is known that in the case of P3HT, its optical properties can be readily influenced by the presence of a suitable host medium. For example, Lee et al. showed that the optical transition energies in absorption and emission experiments of P3HT nanoparticles are affected by a hydrothermal (polar) treatment with deionized water at temperatures of up to 150 °C in an autoclave . Hellmann et al. showed that blending of P3HT with the polar poly(ethylene oxide) (PEO) leads to the increased 0-0 oscillator strength as well as to a considerable shift of the optical absorption spectrum by 0.1 eV . In addition, Kim et al. observed similar changes in the optical properties of electrospun P3HT nanofibers after blending the P3HT and PEO and spinning them from polar solvent mixtures . Other studies have demonstrated a minor redshift in the optical absorption spectrum of P3HT films by blending with poly(ethylene glycol) without the need for additional polar solvent additives . Thus, the above experiments have indicated that the photophysical properties of P3HT can be readily manipulated by processing means. Although above studies showed significant influence of the host environment on band gap of P3HT aggregates, the changes in emission QY have been paid less attention. For example, Kanemoto et al. have showed that PL of conjugated polymers can be enhanced in the solid state by dilution using moderate inert polymers such as polypropylene . However, this effect was achieved by conversion of aggregates to the molecular form of the conjugated polymer.
Here, we demonstrate that blending conjugated polymer P3HT with polar poly(methyl methacrylate) (PMMA), where P3HT particles of micro- and nanoscale are formed, induces systematic changes in physical characteristics of P3HT aggregates. We show that as the weight ratio of P3HT to PMMA decreases, the P3HT fraction demonstrates the redshifting band gap, the improvement in ordering, and the enhancement in QY of emission. We show that these changes very likely are due to planarization of the conjugated polymers’ backbone in the presence of PMMA under the action of hydrophobic forces from the host material.
Initial stock solution of regioregular P3HT (~ 93% RR, 99,995% trace metal basis, with number-average molecular weight (Mn) in the 15–45 kDa range, Sigma-Aldrich) was prepared with concentration of 1.0 wt% in chlorobenzene (CB). Binary mixtures of P3HT and PMMA were prepared by addition of a necessary amount of poly-(methyl methacrylate) (PMMA, average molecular weight (Mw) of 120 kDa, Sigma-Aldrich) to P3HT solution in CB followed by treatment in the ultrasound bath for 30 min. Films have been prepared by spin-coating onto glass substrates at 1500 rpm for 30 s.
For transmission electron microscopy (TEM) studies, the film was scraped away to the vessel with acetone, which then stayed several hours to ensure that all PMMA was completely dissolved releasing P3HT aggregates which are practically insoluble in acetone (solubility of P3HT in acetone is less than 0.1 mg/mL ). A small amount of the solution was drop-cast onto the TEM carbon grid followed by evaporation of acetone. PMMA solution in acetone was drop-cast on a separate grid in order to get images of the neat PMMA samples.
Absorption spectra were measured using a SPECORD M40 and an OLIS Cary 14 double beam spectrophotometers. Bare glass plate served as a reference. Fluorescence spectra were collected using a SPEX Fluorolog 1680 double spectrometer, with a Xe lamp as a light source. The excitation wavelength was selected at 468 nm. Absorption spectra are given below as normalized to their maximum in order to compare their spectral features, and the PL spectra are given corrected for the sensitivity of the registering system and normalized to the sample absorption at the excitation wavelength, i.e., the PL spectra are presented in terms of the relative QY of the sample emission.
The transient absorption (TA) pump–probe measurements were performed using a Ti:sapphire laser system. The excitation was set at a wavelength of 410 nm. The TA measurements were carried out with the pump (with repetition rate of 1 kHz and pulse duration of ~ 100 fs) and a white light continuum generated by a sapphire crystal as the probe. The pump beam was modulated mechanically at exactly half the repetition rate of the CPA system (500 Hz), and ΔT/T or ΔOD was detected with a phase sensitive technique using lock-in amplifiers. The polarization of the pump beam was at the magic angle (54.7°) relative to that of the probe beam. The measured fractional transmission signals, i.e., TA, are given by TA = −ΔT/T= (T on-T off)/T off, where T on denotes the probe transmission with pump on, and T off the probe transmission with pump off. The obtained spectra were rectified by wavelength calibration procedure.
Morphologies of the samples were studied both by optical microscopy and TEM. Optical micrographs of the samples were taken using optical microscope ULAB XY-B2 equipped with a photo-camera and a computer. TEM studies were performed using JEOL JEM-1400 instrument operating at 80 kV.
Decay time constants and amplitudes of TA spectra of P3HT and P3HT:PMMA films in the GSB and exciton regions; the dynamics is fitted by two-exponential decay OD(t) = A 1 exp(−t/t 1 ) + A 2 exp(−t/t 2 )
0-1 (560 nm)
0-0 (610 nm)
Exciton (1240 nm)
7.0 ps (60%)
295.1 ps (40%)
5.3 ps (63%)
424.7 ps (37%)
2.0 ps (51%)
40.2 ps (49%)
P3HT:PMMA (1:50 weight ratio)
1.8 ps (73%)
202.6 ps (27%)
1.9 ps (74%)
299.0 ps (26%)
0.6 ps (68%)
18.9 ps (32%)
Assignment of the Crystal Structure
The major finding of this work is that QY of emission of the P3HT condensed phase can be enhanced not due to disentanglement of tightly packed aggregates with substantial exciton quenching by neighboring molecules into molecular form of P3HT, but by simple reduction of the size of P3HT condensed phase to micro- and nanoparticles. Two main reasons can be considered which are responsible for the above phenomenon: First, the increase in the interface area of P3HT/PMMA, where the interfacial molecules increase their contribution to the emission properties due to increasing surface-to-volume ratio in the diminishing P3HT particles; Second, the changed arrangement of P3HT chains in the crystalline domains as a result of repulsive forces acting from PMMA, which affect more P3HT molecules as the ratio of P3HT to PMMA decreases.
The other important factor which can be particularly inferred from the spectral changes of P3HT-PMMA films is the change in mutual arrangement of polymer chains in P3HT domains being in PMMA matrix. It should be noted that P3HT crystals can adopt different forms, i.e., the form I which is being most commonly observed in thin films after annealing , or the form II which represents an energetically more stable situation . Form II can be obtained, for example, by synergetic action of hydrophilic polymer matrix and a poor solvent such as water on P3HT chains during film formation , and it displays a notable red shift in the absorption spectrum ; a similar tendency is observed in our results, showing red shift of the band gap from 1.92 to 1.89 eV (Fig. 1a). Interestingly, the π − π stacking distance reported for P3HT nanofibrillar crystals of form II is relatively large, being from 0.440 nm , as compared to the stacking distance found for the form I which is between 0.340 and 0.414 nm [48–50]. At the same time, there is a tighter alkyl side-chain inter-digitation in form II, with the interchain distance (in the direction the alkyl groups are pointing) of 1.20 to 1.31 nm  versus that varying from about 1.55 to 1.73 nm in crystals of form I [48, 50]; the tighter inter-digitation seems to better stabilize the intrachain ordering in crystals of form II.
The above discussion concerning different crystalline forms of P3HT is important for understanding of structural transformation of P3HT crystals formed in PMMA matrix at different weight ratios of P3HT to PMMA. First, it has been found that the maximum position related to the (0-0) band in spin-coated P3HT-PMMA films experiences slight red shift at small P3HT to PMMA ratio, i.e., from 602 to 608 nm (Fig. 1a). Second, microscopy studies allowed us to distinguish two types of crystals in the blend samples, which have the short interplanar distances in the stacking direction (along the b axis of P3HT crystal) to be 0.417 (that is characteristic of ball-like structures, see Fig. 5a and Fig. 7) and 0.445 nm (characteristic of lamella structure shown in Fig. 5b), respectively. The latter value agrees well with the crystalline form II as discussed above, whereas the former one better corresponds to an intermediate form I’ reported by Roehling et al. , which possesses the interplanar distance of 0.41–0.42 nm. They also showed that the form I’ demonstrates an increase in the coherent domain size in the π − π stacking direction by a factor of ~ 2 (from 6.3 to 12.4 nm), as compared to form I in samples prepared from p-xylene, which can be responsible for the enhancement of the (0-0) band relatively to the (0-1) band in P3HT samples .
An increasing QY of PL which has been found in P3HT particles embedded into PMMA matrix is an unusual phenomenon since it takes place when the polymer molecules are still aggregated and where a strong exciton quenching should be normally observed. The increasing QY is assigned due to the two factors. The minor factor is the changing dielectric constant which facilitates a modest increase in QY by about 14%. The major factor is due to rearrangement of the polymer chains themselves. Better chain ordering in P3HT domains embedded into the PMMA matrix has been unequivocally proved by spectroscopy methods and calculation of the exciton bandwidth as well. The reason of the structural changes is tentatively assigned to hydrophobic forces acting on P3HT chains being in polar environment, i.e., PMMA matrix, which forces P3HT aggregates to conform a specific arrangement inside the matrix. Such a process is most effective for smaller P3HT particles since the most influence is rendered onto the molecules being at the interface of P3HT-PMMA. Particularly, it can be concluded that the composite P3HT-PMMA samples contain P3HT crystals of two forms, i.e., I’ and II, in which the interchain stacking distance varies from 0.42 to 0.44 nm. In form I’, intramolecular torsional disorder is reduced and most probably it promotes the increasing coherent domain size in the π − π stacking direction of P3HT domains, respectively. This is accompanied by the increasing first absorption maximum in respect to sidebands in spectra of composite P3HT films and by narrowing free exciton bandwidth, respectively. It is interesting to note that the narrowing exciton bandwidth is a factor which is responsible for increasing QY of PL emission in semiconductor nanoparticles as compared to the bulk crystals possessing wide energetic bands . Narrow bands reduce smearing effect upon electronic transitions, thus facilitating more electrons falling to the same energy level. Thus, the observed enhanced QY of emission of P3HT particles can be interpreted in terms of the changing intermolecular packing and reduced intramolecular torsional disorder along with narrowing exciton bandwidth.
Part of this work has been performed in the frame of the Fulbright Scholar Program. The author is indebted to Prof. Blank’s group, especially Dr. B. Caplins, for technical assistance. The help of Dr. Sergey Voichuk in TEM studies is gratefully acknowledged.
The author declares that he has no competing interests.
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