Effect of Thermal Annealing on Aggregations in MEH-PPV Films

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Effect of Thermal Annealing on Aggregations in MEH-PPV Films Ruizhi Wang, Xiao Yang, Shu Hu, Yang Zhang, Xiaoliang Yan, Yuchen Wang, Chuang Zhang, and Chuanxiang Sheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11991 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry

Effect of Thermal Annealing on Aggregations in MEH-PPV Films Ruizhi Wang†,‡, Xiao Yang*,† , Shu Hu‡, Yang Zhang‡, Xiaoliang Yan‡, Yuchen Wang‡, Chuang Zhang*,§, ChuanXiang Sheng*, ‡ †Jiangsu

Laboratory of Lake Environment Remote Sensing Technologies, Huaiyin Institute of

Technology, Huai’an, 223003, China ‡School

of Electronic and Optical Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences,

§

Beijing 100190, China.

Corresponding Author *E-mail: [email protected] (C.X.S.) *E-mail: [email protected] (X.Y.) *E-mail: [email protected] (C.Z.)

ACS Paragon Plus Environment

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ABSTRACT Absorption and photoluminescence spectra at various temperatures below 200 K had been used to study the optical properties of poly [2-methoxy-5-(2-ethylhexyloxy) -1,4-phenylenevinylene] (MEH-PPV) films. In the films without annealing, the PL emission was dominated by intrachain excitons with J-aggregate-like properties, where 0-0/0-1 PL ratio decreases with increasing temperatures; although it was obvious that interchain coupling of E was stronger in film from chlorobenzene solution (E ~7.5 meV) than that from chloroform solution (E ~2 meV). For MEH-PPV films annealed at 420 K and 520 K, the E was enhanced dramatically to 76 meV and 89 meV, respectively. Thus, the interchain interaction (H-aggregate-like behavior) dominated PL properties in annealed films, resulting in the enhancement of 0-0/0-1 PL ratio with increasing temperatures. Our work experimentally proved that HJ-aggregate model may be proper to describe the photophysical properties of -conjugated polymer films using adjustable E.


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INTRODUCTION -conjugated polymers (PCPs) are promising for potential applications in luminescent,1,2 biomolecular sensing3,4 and photovoltaics.5,6 The films of PCPs are often considered being amorphous; however, intermolecular and intramolecular interactions cause the formation of phases with some degree of order in many cases. These ordered polymer molecular chains were usually referred as “aggregates”, which have strong impact on the photophysical processes in films and devices of PCPs. For examples, in 2003, P. J. Brown et al. showed that both interchain and intrachain states were exhibited in the emission of P3HT;7 and Z. Zhu et al. discovered the correlation between morphology of P3HT nanoparticle and emission state.8 The molecular aggregates, which are normally formed in -stacked arrangements, are described as H-, J- or recently introduced HJ-types, respectively.9-12 H-aggregates are easily found in materials with a face to face packing motif, such as in thin films of regioregular poly (3-hexythiophene) (P3HT).13 On the other hand, J-aggregates (head-to-tail packing arrangements) were usually observed in a variety of small-molecule such as porphyrins14 and in single polymer chain of polydiacetylenes.15 Recently, the HJ model, which includes the interplay between interchain (H-aggregates) and intrachain (J-aggregates) couplings, had been introduced theoretically.16 The photophysical properties of polymers with HJ-aggregates are determined by the competition between intrachain interaction inducing J-like behavior, and interchain interaction inducing H-like behavior.17 In this work, using one of the most studied PCPs, poly [2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV), we found such competition can be tuned by film morphology which can be manipulated by procedures of sample preparation. Here, we measured and analyzed absorption and photoluminescence spectra of MEH-PPV films,


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in particular, at temperatures lower than 200 K. PL 0-0/0-1 ratio decreases with increasing temperatures in as-cast films; this is J-aggregate-like behavior, even if the interchain coupling of E is roughly ~2 meV and ~7.5 meV in films from chloroform and chlorobenzene solution, respectively. For films annealed at 420 K and 520 K (above melting point of 450 K18), the interchain coupling strength of E was enhanced to be 76 meV and 89 meV, respectively. Correspondingly, PL 0-0/0-1 ratio decreases with decreasing temperatures; this is H-aggregatelike behavior. Our work experimentally proved that the aggregate of MEH-PPV films can be characterized by HJ-type, in which the photophysical properties resulting from the competition between intrachain interaction (J-aggregate-like) and interchain interaction (H-aggregation like) can be manipulated by solvent, and more effectively, by thermal annealing. EXPERIMENTAL SECTION Sample Preparation. MEH-PPV was purchased from Sigma-Aldrich with average molecular weight 106 g/mol. 10 mg of MEH-PPV was dissolved in 10 mL solvent (chloroform or chlorobenzene) and stirred whole night. Glass or CaF2 substrates were cleaned by ultrasonication for 30 minutes in acetone and ethanol, followed by UV ozone treatment for another 30 minutes. The films were then drop-casted onto prepared substrates from the solution and fully dried at room temperature. These films were so-called as-cast films. In order to obtain annealed films, some ascast films were heated at 420 K or 450 K for 30 min, and then naturally cooled to room temperature. In order to obtain fast cooling samples, some as-cast films were annealed at 420 K (520 K) for 20 min, then were dipped into liquid nitrogen immediately to achieve fast cooling. After one minute in liquid nitrogen, the films were taken out and warmed up to room temperature naturally. All samples were prepared in a glove box filled with nitrogen atmosphere.


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Optical Measurements. Temperature dependent PL measurements were performed in liquid nitrogen cooled cryostat in which temperature could vary from 80 K to 350 K. Absorption spectra were collected with halogen tungsten lamp and monochromator (BOCI, WDG30-Z). Optical excitation was carried out by 447 nm diode laser from Changchun New Industries Optoelectronics Tech. Co., Ltd (MDL-III-447L). Mid-IR photoinduced absorption spectra were measured using IR emitter (New Port, 6363IR), matching Zolix monochromator (Omni-λ150) connected with Mercury Cadmium Telluride detectors (Teledyne Judson Technologies, J15D12-M204-S01M-60). Photoluminescence was collected using Idea Optics PG-4000 spectrometer. RESULTS AND DISCUSSION The absorption spectra near band edge of as-cast and annealed MEH-PPV films measured at 80 K (LT) and 300 K (RT) are shown in Figure 1a,b, respectively. While the films were annealed, the polymer chain could relax to reduce the conformational defects such as twists and torsions. As a result, the length of the conjugation segments increases.19-21 At the same time, the chain-chain interaction could be enhanced due to the formation of aggregate in films. Thus, red shift of band edge for film annealed at 420 K can be ascribed to either increases of average conjugation length of the polymer chains or enhancement of chain-chain interaction, which both lower the energy of band edge. On the contrary, the absorption edge blue-shifts in film annealed at 520 K, which is above the melting point (~450 K) of the MEH-PPV.18 The high temperature may damage the polymer chain to reduce the conjugation length, i.e., to increase the energy of the π-π* transition.22,23 Moreover, the most interesting spectral feature in Figure 1a,b is the enhancement of absorbance below 2 eV in annealed films compared to that in as-cast film at 80 K, but such enhancement is NOT observable in same films at room temperature. This phenomenon could be easily ignored in thin films. From literatures, we noticed obvious new absorption peak emerges at


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low temperatures in pentacene derivative functionalized with the triisopropylsilylethynyl (TIPS) side group24 and in highly ordered poly (p-phenylene vinylene),25 indicating the formation of Haggregates in highly ordered molecule and polymer. In MEH-PPV, the observation shown in Figure 1a,b may also indicate the formation of H-aggregates, however, with only small fraction in films after annealing. In Figure 1c, we present the X-ray diffraction (XRD) pattern of same films in Figure 1a. The diffraction peaks around 2θ peaked at 23° (from 19° to 27°), corresponding to the regular stack of the main chains of PPV with d-spacing of 0.39 nm.26 It is obvious that stronger diffraction peak is observed in film annealed at 420 K, indicating higher crystallinity. In film annealed at 520 K, the less aggregates could be due to the damage of the polymer chains, being consistent with the blue shift of the absorption edge in Fig. 1a and 1b. To further study the photophysics of polymer films, we applied photoinduced absorption (PIA) spectroscopy in mid-IR for as-cast and two annealed films, as shown in Figure 1d. The PIA band due to photogenerated polarons27 and photoinduced infrared-active vibrations (IRAVs) in mid infrared spectral range were not detectable.28 For comparison, we also present PIA spectra of a blended film of MEH-PPV/PCBM, in which the IRAVs due to photogenerated polarons are prominent.28 Therefore, the PIA spectra proved that the high temperature annealing did not generate large amount of defects like UV illumination did.29 The photoexcitations, no matter intrachain or interchain, are still neutral in both annealed films. Figure 2a shows the PL spectra at various temperatures ranging from 80 K to 300 K for as-cast MEH-PPV film from chloroform solution. Other than peak positions, the PL of as-cast film shows the similar spectrum as that of MEH-PPV solution:30 the 0-0 electronic transition band and its first (0-1) and second (0-2) vibrational bands can be discerned clearly at low temperatures. The PL spectra of 0-0, 0-1 and 0-2 blue-shift and their total intensities decrease with increasing


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temperatures, where the PL were concluded to be radiative recombination of intrachain excitons.31 In PL spectra of annealed films shown in Figure 2b,c, there are three different features compared to Figure 2a. Firstly, both annealed films show obvious less-featured and relatively red-shifted PL spectra than that of as-cast one, where the 0-0 transition for PL of 420 K (520 K) annealed film is at ~626 nm (~633 nm) and as-cast film at ~613 nm. This was regarded as one of evidences for aggregates formed in polymer films,31-34 i.e., in annealed MEH-PPV films here. After annealing, red-shift of PL can be explained as delocalization of the excited state wave function. It is very possible that the interchain interaction is enhanced after annealing 31-34 , this is also consistent with the analysis in absorption spectra of Figure 1a,b. Figure 2d shows the integrated intensity of PL decreases a lot for the as-cast film while temperature increases, but not for annealed films. It is known that binding energy of exciton for MEH-PPV is in the order of 500 meV.35 Therefore, decreasing PL intensity with increasing temperatures cannot be attributed to the dissociation of excitons. This could be due to higher probability of electron scattering at higher temperature by conformational defects such as twists and torsions.31 For annealed films, the PL intensities changes much slower with increasing temperatures. Which may be partially due to the smaller amount of defects after annealing;36 meanwhile, from the analysis of absorption spectra of Figure 1a,b, the existence of H-aggregation phase in annealed film may also be one of the reasons for relation between PL-intensity and temperature.37 Secondly, over 200 K, additional spectral shoulder appears at ~575 nm in PL spectra of both annealed and as-cast films. This can be ascribed to the existence of disordered coils in MEH-PPV film,30,38 which contains shorter effective conjugation length with higher energy levels. At higher


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temperature, the thermal activation could make the occupation of such states, resulting in the emission at higher energy.30,39 Thirdly and the most striking difference in PL shown in Figure 2a-c is the intensity ratio between PL 0-0 and 0-1 peaks. For as-cast film, the relative intensity of 0-1 peak compared to 0-0 one increases with increasing temperatures obviously. However, in annealed film, the PL 0-1 peak, which is relatively stronger than 0-0 peak at low temperature, almost vanishes at room temperature, presenting very different temperature dependence after annealing. Those opposite trends in the behavior of ratio between PL 0-0 and 0-1 transitions are labeled for discrepancy between emission from J-aggregates and H-aggregates.40 Moreover, cooling rate after annealing is known to be very important for crystallization processes.41-43 To test the relation between PL 0-0 and 0-1 peaks is NOT accidental after annealing. In glove box filled with nitrogen we prepared two fasting cooling MEH-PPV films by annealing them separately at 420 K and 520 K for twenty minutes, then quenching the film into liquid nitrogen immediately; after one minute in liquid nitrogen, the films were taken out and warmed up to room temperature naturally. Figure 2e,f show the temperature dependent PL measurements on fast cooling films, respectively; it is obvious that the intensity ratio of PL 0-0 and 0-1 increases with increasing temperatures in both fast cooling films. This is same with naturally cooling films shown in Figure 2b,c, although spectra and the relation between PL intensity and temperature show different features. In ideal H-aggregate (no disorder), the emission of 0-0 transition is forbidden at 0 K.44 At T > 0 K, some 0-0 emission may occur due to thermal activation of the exciton to the top of the band.16 Therefore, the ratio of I0-0 to I0-1 increases with increasing temperatures, this is one important character of H-aggregates.17 On the other hand, J-aggregate formation has been extensively studied in a variety of small molecules, from which the delocalized nature of the excited electronic states


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was derived.40 For J-aggregation, the scaling of PL 0-0/0-1ratio is characterized with inverse square root of temperature.11,17 Recently, so called HJ-aggregate model was introduced to describe interplay between intrachain and interchain interaction in semiconducting polymers, in which, PL 0-0/0-1 ratio can be presented theoretically as Equation 1 for disorder-free dimers,16

00 0 1 I PL / I PL 

1+e

e E / kBT E / k BT



k BT

(1)

where kB is Boltzmann constant, and T is temperature., E is proportional to the strength of interchain coupling, which leads to the energy splitting of intrachain excitons. When E = 0, Equation 1 reduces to the single chain result.16,40 The temperature dependence of I0-0/I0-1 embodied here consists of H-like thermally activated term and J-like diminishing term as T-1/2. Therefore, to achieve the ratio, we fit the PL spectrum with three Gaussian functions, which are roughly equal spaced in energy of ~0.17 eV. Here we note that although the Gaussian function was traditionally used to analyze PL spectra of MEH-PPV films,31 Franck-Condon analysis could be properly used to extract the superposition of different vibronic progressions, as Reference 31 did. On the other hand, according to the analysis in Reference 16, two close phonon modes of 155 meV and 192 meV contribute mostly in PL (The 192 meV mode reflects a carbon-carbon stretching vibration, the 155 meV mode is associated with a coupled carbon-carbon stretching/carbonhydrogen bending motion); this cause the well-separated single peak 0-0 and 0-1 transitions, in particular at low temperatures. The fitting stops at 200 K, to avoid the influence of additional emission around 575 nm due to disordered coils in MEH-PPV film at higher temperatures.30,38 Figure 3a-c present the PL spectra along with fitting using three Gaussian functions for PL at 80 K, for films of as-cast, annealed at 420 K and 520 K, respectively. The fitting results of ratio of


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I0-0 and I0-1 are shown in Figure 3d. We note that the ratio of I0-0/I0-1 in our work is smaller than reported value of MEH-PPV,16 this could be due to the spectral response of the system. Using Equation 1, the annealed film was perfectly fitted, achieving E = 89.2 meV and 75.6 meV for 520 K and 420 K annealed films, respectively. However, we found it is impossible to fit the curve of as-cast film very well, the best fitting result for as-cast film is included in Figure 3d too, with E = 2  1 meV. The inset of Fig. 3d shows the PL ratio scaling with the inverse square root of temperature above 150 K. At low temperatures (below 140 K), the ratio of PL 0-0/0-1 is practically temperature independent, deviating the prediction of Equation 1. This results from the disorders in film, which cause finite size effects to confine wave function in the polymer chain, as described in References 16, 45, 46. Note disorders for temperature independent PL 0-0/0-1 ratio are the disorders in aggregates,16 not the disorder coils responsible for the additional PL emission at 575 nm above 200 K.30,38 The transition temperature from T-1/2 to steadfast is determined by interchain interaction: stronger interchain interaction, higher the transition temperature, according to the theoretical simulation.40 However, the direct relation of transition temperature with E between experimental results and theoretical models is not well understood yet, partially because of unavoidable defect/impurity in polymer experimentally as well as disorder-free model theoretically. The inverse square root dependence of the PL ratio on temperature was considered as the defining signature of linear J-aggregates.45 Therefore, we conclude that the intrachain J-aggregate occurs in as-cast MEH-PPV films from chloroform soluiton, with the existence of weak interchain interaction. In other word, the intrachain excitons dominate photoexcitations in MEH-PPV film from chloroform solution, being consistent with the conclusion from ultrafast transient spectroscopy.47 On the other hand, after annealing, the interchain H-aggregates dominate


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photoexcitations with adjustable interchain coupling strength of E. Here we may point out that different types of interchain photoexcitation species could be existing in PCPs.31,41 For example, the spatially indirect exciton (polaron pair), in which the electron resides on one chain and the hole on an adjacent chain48,49 or excimer: the excited state has no significant degree of charge separation and is shared equally between multiple chains,50-52 or exciplex: a neutral excited state shared between unequal chains (or unequally between similar chains).53,54 Because we do not know more details on photoexcitations, such as the extent of charge transfer and wave function sharing between the chains, so the exactly distinction between varieties of interchain species could be impossible in current work. Furthermore, it is known that the morphology of MEH-PPV films can also be influenced by solvent for solution.55 In Figure 4a, we show temperature dependent absorption edge of MEH-PPV film for as-cast and annealed films prepared from chlorobenzene solution. Compared to the film from chloroform solution (Figure 2a), the most noticed feature of those spectra is the enhancement of absorption below 2 eV for as-cast film at 80 K, suggesting the formation of aggregates in ascast film from chlorobenzene solution. This is confirmed by XRD shown in Figure 4b. However, for as-cast film, PL of MEH-PPV film from chlorobenzene solution did not present big difference compared to that of chloroform one. We conclude that the intrachain excitons are still responsible for PL whereas the peak position moves from 602 nm to 615 nm, as an evidence for more delocalization.56 After carefully fitting of PL spectra up to 200 K, shown in Figure 4d, we found the ratio of PL0-0 and PL0-1 decreases with increasing temperatures slowly, meeting the signature of J-aggregate-like emission loosely. This is also consistent with conclusion of Figure 4a, i.e., the intrachain excitons are responsible for PL in as-cast film. However, comparing to the ratio with that of as-cast film from chloroform solution (dashed line in Figure 4d), the transition temperature


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from steadfast to inverse square root dependence of the PL ratio does not present, which supplies an evidence of larger interchain interaction in films from chlorobenzene solution than that from chloroform one. Again, we found it is also impossible to fit the PL 0-0/0-1 ratio of as-cast film from chlorobenzene solution precisely using equation (1); the best result is presented in Figure 4d, with E = 7.5  2.5 meV. Here we note that as the temperature increases from 80 K to 300 K, the PL intensity of as-cast chlorobenzene film decreases just about 20%. Furthermore, we present the PL of 420 K annealed film in Figure 4c. It is obvious the ratio of PL 0-0 and 0-1 have the opposite trend compared to as-cast film. CONCLUSIONS In summary, we analyzed absorption and photoluminescence spectra to study the optical properties of MEH-PPV films, in which we found both intrachain (J-aggregates) and interchain (H-aggregates) played roles, in particular, at low temperature lower than 200 K. While the temperature is above 200 K, the random coil of polymer chain could be thermally activated for emission. As summarized by schematic diagram shown in Figure 5, in the films without annealing, the PL spectra was dominated by intrachain excitons with J-aggregate-like properties, where the PL 0-0/0-1 ratio decreases with increasing temperatures, although the interchain coupling of E was roughly 2 meV and 7.5 meV in films from chloroform and chlorobenzene solution, respectively. In particular, the scaling of PL 0-0/0-1 ratio with the inverse square root of temperatures, which is the characteristic of linear disorder-free J-aggregates, was observed in the MEH-PPV film from chloroform solution above 150 K. For films annealed at 420 K and 520 K, interchain coupling of E was enhanced to 76 meV and 89 meV, respectively. Therefore, the interchain interaction (H-aggregate-like behavior) dominates the PL properties, resulting in the


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enhancement of PL 0-0/0-1 ratio with increasing temperatures. Our work experimentally proved that the aggregate in MEH-PPV films can be characterized as HJ type. The photophysical properties from a competition between intrachain interaction (J-aggregate-like) and interchain interaction (H-aggregation like) can be tuned by solvent and thermal annealing effectively. ACKNOWLEDGMENTS The work was supported by NSF China (Nos. 61574078, 61874056, 61704063), and the Project of Jiangsu Laboratory of Lake Environment Remote Sensing Technologies (JSLERS-2017-001).


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FIGURES AND CAPTIONS

Figure 1. Absorption spectra of as cast (red square line), 420 K annealed (green circle line) and 520 K annealed (blue triangle line) MEH-PPV films in (a) 80 K (LT) and (b) 300 K (RT). (c) XRD patterns of as cast, 420 K and 520 K annealed MEH-PPV films. Dash line provides substrate background. (d) Mid-IR Photoinduced absorption spectra of MEH-PPV films (color solid lines around zero) and MEH-PPV/PCBM films (purple circle line). All sample films are generated from chloroform dispersions.


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Figure 2. Temperature dependent photoluminescence spectra of (a) as cast, (b) 420 K annealed and (c) 520 K annealed MEH-PPV films. (d) Integrated photoluminescence intensity of as cast (red square line), 420 K (green circle line) and 520 K annealed (blue triangle line) MEH-PPV films in different temperatures. (e) and (f) are temperature dependent photoluminescence spectrum of MEH-PPV 420 K and 520 K fast cooling samples, respectively. All sample films are prepared from chloroform dispersions.


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Figure 3. Photoluminescence spectra with perfect fitting using three Gaussian functions at 80 K for (a) as-cast, (b) 420 K and (c) 520 K annealed MEH-PPV films, generated from chloroform dispersions. (d) Ratio of I0-0 and I0-1 with temperature dependence in all sample films. Inset is the relationship between the ratio of I0-0 and I0-1 with the inverse of square root in temperatures for as cast films.


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Figure 4. Samples for (a), (b) and (c) are prepared from chlorobenzene dispersions. (a) Absorption spectra of as cast (red square line), 420 K annealed (green circle line) and 520 K annealed (blue triangle line) MEH-PPV films in 80 K as well as XRD patterns in 300 K for inset. Temperature dependent photoluminescence spectra of as cast (b) and 420 K annealed (c) MEH-PPV films. (d) Comparison between MEH-PPV films generating from chloroform and chlorobenzene dispersions in term of relation between temperature inverse square root and ratio of I0-0/I0-1.


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Figure 5. Schematic description of aggregates, as well as annealing effects on aggregates in MEHPPV films. IC: internal conversion.


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Effect of Thermal Annealing on Aggregations in MEH-PPV Films

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