Amplified Spontaneous Emission Optimization in ... - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Amplified Spontaneous Emission Optimization in Regio RegularPoly(3-Hexylthiophene) (rrP3HT):poly(9,9-Dioctylfluorene-CoBenzothiadiazole) (F8BT) Thin Films Through Control of the Morphology Marco Anni, and Sandro Lattante J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05008 • Publication Date (Web): 31 Aug 2015 Downloaded from http://pubs.acs.org on September 1, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Amplified Spontaneous Emission Optimization in Regio Regular-poly(3-hexylthiophene) (rrP3HT):poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT) Thin Films Through Control of the Morphology M. Annia∗ and S. Lattante Dipartimento di Matematica e Fisica ”Ennio De Giorgi”,Università del Salento, Via per Arnesano, 73100 Lecce, Italy E-mail: [email protected] a

Corresponding author’s e-mail: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

Abstract The maximization of the optical gain of organic active materials is a crucial step for the development of organic lasers. In this paper we demonstrate that the Amplified Spontaneous Emission (ASE) of regio regular-poly(3-hexylthiophene) (rrP3HT):poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT) thin films is strongly affected by the film morphology, controlled by changing solvent used for the spin coating deposition. The solvent variation results in ASE threshold changes up to 8 times, due to the variation of the uniformity of the two polymers mixing, determining the optical gain, and the morphology uniformity, affecting the losses. Our results demonstrate that the morphology optimization is a very important step of the development of organic material with high optical gain.

Introduction Since the first demonstration of light amplification 1 and optically pumped lasing, 2 organic conjugated molecules received large attention as potential interesting systems for laser applications. Organic materials show broadly tunable optical gain in the visible 3,4 and near infrared range, 5 they can be deposited with simple techniques from solutions using a wide variety of solvents, on large area and eventually flexible substrates. To date Amplified Spontaneous Emission (ASE) and optically pumped lasing have been demonstrated in many classes of active molecules and by exploiting a large variety of laser resonators. 6 Moreover in the last few years the first demonstrations of practical application of organic lasers have been reported. 7,8 Despite this the research of novel active systems for organic laser application is still running, aiming to develop materials with high optical gain, 9 possibly in strategic spectral ranges. 10 The measurement of the eventual presence of ASE in thin film waveguides under strong optical pumping is a fast and easy approach to check the presence of optical gain in a 2 ACS Paragon Plus Environment

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


novel active system, while the dependence of the emission spectrum on the pumped area geometry allows the quantitative measurement of the optical gain value. For these reasons the measurement of the ASE properties is the typical initial step for the characterization of a new material for laser applications and the value of the ASE threshold is often used as an indicator of the potentiality of the material as active material in lasers. Thus materials showing low ASE threshold are considered good laser materials and viceversa. A particularly effective approach to minimize the ASE threshold is the realization of active host-guest blends, interacting by Forster Resonant Energy Transfer (FRET), in which the pump laser is mainly absorbed by the host matrix, and the excitation is then transfered by FRET to a guest with high optical gain, emitting at higher wavelength. These systems thus combine a high absorption, an effective pumping of the active molecule, and low losses due to self absorption and aggregation of the guest molecule. It has been demonstrated that the gain of a polymer waveguide depends on several factors, beyond the gain cross section of the active molecule, like the pumping efficiency of the active molecule, 10,11 the aggregation of the active molecule 10 and the active material thickness. 12 A particularly critical aspect in the determination of the properties of conjugated molecule films is the control of the film morphology. In particular it has been demonstrated that the film morphology, depending on the deposition conditions and on the eventual post deposition sample processing, has important effects on the photoluminescence efficiency, 13 on the electroluminescence efficiency, 14,15 and on the photovoltaic efficiency 15,16 of organic systems. On the contrary, rather surprisingly, the morphology dependence of the optical gain of organic waveguides has received almost no attention, despite the evidence that ASE depends on the solvent use for the spin coating already in the first paper demonstrating ASE from MeHPPV neat films. 1 The only exceptions are few recent studies on the morphology dependence of the ASE of MeHPPV, with the investigations of the thermal annealing effects, 17,18 indicating that despite the morphology variation the thermal annealing does not significantly affect the ASE properties up to about 80◦ C , while it drastically reduces the ASE



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

at 120◦ C. Moreover the role of the solvent has been recently investigated, suggesting that the solvent dependent MeHPPV aggregation has marginal effects on the ASE properties. 19 In this paper we investigate the role of the local emission properties and morphology on the ASE properties of an organic layer with fixed composition and thickness, deposited by using different solvents for the active material solution. Blends of F8BT and rrP3HT are taken as prototype material of polymeric host:guest system, after the recent demonstration of high optical gain in F8BT:rrP3HT blends close to 650 nm, which is a strategic wavelength for polymethylmethacrylate (PMMA) based polymer optical fibre (POF) datacommunications. 10 We demonstrate that the choice of the solvent is critical in determining the ASE threshold, and thus the net gain, with variations up to 8 times between the worst solvent (xylene) and the best one (toluene), due to a strong solvent dependence both of the active molecule gain and the waveguide propagation losses. We show, by confocal laser spectroscopy, that the final ASE properties of the investigated films are related to the solvent dependence of the mixing uniformity of the two polymers and to rrP3HT clusters and islands formation, affecting the gain and the losses of the film. Our results evidence that the choice of the solvent can be a very important element in determining the ASE performances of organic waveguides, thus providing a simple but powerful degree of freedom for the gain maximization of organic waveguides. Considering that the emission properties dependence on the film preparation condition is a common property of most of the organic molecules our results are expected to be valid for many other organic systems showing optical gain.

Experimental F8BT and rrP3HT were provided by ADS dyes and Sigma Aldrich, respectively, and used as received. The samples were realized by spin coating at 2500 rpm of a F8BT:rrP3HT blend with a relative concentration of 80:20 in weight, in four different solvents, namely chloroform (CF in the following), toluene (TOL), chlorobenzene (CB) and xylene (XYL). The used


Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


concentration is the one of minimum ASE threshold in samples deposited from toluene according to Xia et al. 10 In order to avoid rrP3HT aggregation the solutions were heated at about 60◦ for few minutes, and the spin coating was done using the hot solutions. All the samples were realized with the same thickness of 130 nm, within about 10%, by acting on the total solution concentration in order to compensate the differences in the solvents vapour pressure 20 (34.0 kPa for chloroform,4.97kPa for toluene, 3.60 kPa for chlorobenzene and 1.33 kPa for xylene). A concentration of 10 mg/ml was used for the CF sample, of 20mg/ml for TOL and CB, and of 30 mg/ml for XYL. For the low excitation density photoluminescence (PL) measurements the samples were pumped by a 405 nm laser diode, with a incident power of about 1 mW. For the ASE and gain measurements the samples were pumped by a Nitrogen laser (337 nm) delivering 3 ns pulses with a repetition rate of 10 Hz and a maximum pulse energy of 155μJ, focused in a 6.8 mm × 100 μm rectangular stripe by a cylindrical lens. The excitation density on the sample was changed with a variable neutral density filter. The PL was collected from the sample edge and detected by a Spectral Product SM442 CCD spectrometer, with a spectral resolution of 0.6 nm. In order to avoid photodegradation all the measurements were performed in vacuum, at room temperature. The net gain and the losses were measured by varying the pumped stripe length 21 and position. 22 The local PL properties were investigated by confocal laser spectroscopy using a Nikon Eclipse C1 Confocal Laser Scanning inverted microscope (20X DIC Plan Apochromat objective, 0.50 numerical aperture) exciting the samples with the 488 nm line of an argon laser. The PL was collected in backscattering configuration and detected by a couple of photomultipliers (PMTs). The PL map were measured in the range 530 ± 20 nm, close to the F8BT PL peak wavelength, 23 and 605 ± 30 nm, close to the rrP3HT PL peak wavelength, 10 using two band-pass filters, The incident excitation power was P = 1.4μW , measured by a power meter placed on the objective focal plane.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

Results and discussion PL and ASE properties The absorbance spectra of all the samples (see inset of Fig. 1) show a shoulder at about 550 nm, ascribed to rrP3HT, and evident peaks at the absorption characteristic wavelengths of F8BT. 10 No evident variations are present in the spectra, evidencing comparable composition of the four samples and comparable excited state energies. The PL spectra of all the samples (see Fig. 1) show a main peak at about 610 nm, with a high wavelength shoulder at about 660 nm, which is typical of amorphous rrP3HT. 24 A weaker peak is observed at about 530 nm, that is ascribed to a residual F8BT PL. 3 The presence of PL dominated by the rrP3HT, which is the minority component of the active blend, is a clear evidence of Forster Resonant Energy Transfer (FRET) from F8BT to rrP3HT. Moreover the similar rrP3HT/F8BT PL intensity ratio suggest that similar FRET efficiency is present in all the samples, thus that a similar excitation regime of rrP3HT characterizes all the investigated samples (further discussions on the solvent independence of the FRET efficiency are reported in the Supplementary Informations). The excitation density dependence of the PL spectra, under Nitrogen laser pumping, allows to observe the sample spontaneous emission spectrum at low excitation density. As the excitation density increases a narrow band at about 670 nm appears in the spectra of all the investigated samples (see Fig. 2 and Fig. 3 for the spectra of the TOL and XYL sample, respectively), with a strongly increasing intensity, and a progressively decreasing Full Width at Half Maximum (FWHM) down to about 7 nm. These results are typical of Amplified Spontaneous Emission (ASE) and are qualitatively consistent with the results reported by Xia et al. 10 Despite the evidence of ASE in all the investigated samples, by looking at the spectra it is very evident that the minimum excitation density necessary to observe ASE is strongly dependent on the solvent used, evidencing differences in the ASE threshold excitation density.


1.00

TOL CB CF

0.75

XYL

Absorbance (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PL Intensity (arb. units)

Page 7 of 27

TOL

1.2

CB

1.0

CF

0.8

XYL

0.6 0.4 0.2

0.50

300

400

500

600

Wavelength

0.25 500 550 600 650 700 750 800 850 Wavelength(nm)

Figure 1: PL spectra of all the investigated samples. Inset: absorbance spectra of the investigated samples, the spectra are vertically translated for clarity. The ASE threshold was estimated from the excitation density dependence of the PL spectra FWHM (see inset of Fig. 4), considering the same criterion of Xia et al., 10 i. e. the ASE threshold is the excitation density at which the linewidth decreases down to one half of the low excitation density value. The minimum ASE threshold, of about 45 μJcm−2 , is observed in TOL sample, followed by CB (54 μJcm−2 ), CF (116 μJcm−2 ) and XYL (340 μJcm−2 ). The remarkable difference between the highest and the lowest ASE threshold (about 8 times), despite the identical composition and thickness of the samples, clearly evidences that the solvent choice is critical in determining the ASE properties. In order to understand the origin of this huge ASE threshold variation we measured the propagation losses and the net gain in all the samples, at a common absorbed excitation 7 ACS Paragon Plus Environment

Page 8 of 27

9000 8000 0.25 mJcm-2 8000 6000 0.17 mJcm-2 4000 0.13 mJcm-2 7000 0.10 mJcm-2 2000 6000 -2 0.067 mJcm 0 0 50 100 150 200 250 -2 5000 0.050 mJcm Excitation density ( Jcm-2) 4000 3000 2000 1000 0 575 600 625 650 675 700 725 750 775 800 Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Wavelength (nm)

Figure 2: Excitation density dependence of the PL spectra of the sample spin coated from toluene. Inset:excitation density dependence of the intensity at the ASE band peak wavelength, evidencing the superlinear intensity increase above threshold (the line is a guide for the eyes). density of 700 μJcm−2 (see Fig.5). We remember that ASE is observed when the waveguide gain g becomes larger than the propagation losses α or, equivalently, when the waveguide net gain g = g − α becomes positive. A maximum net gain g = 21.4cm−1 is observed for the TOL sample, with a progressive decrease to 17.4 cm−1 in CB, 13.4 cm−1 in CF, and 7.6cm−1 in XYL, which is consistent with the observed progressive ASE threshold increase. Concerning the origin of the solvent dependence of the net gain values we observe that the g decrease from TOL to CB is mainly due to an increase of the propagation losses (from 1.7 to 7.7 cm−1 ) that overcomes the gain increase from 23.1 to 25.1 cm−1 . The further g decrease in CF and XYL samples is instead due to the decrease of the gain, to 18.7 cm−1 and 10 cm−1 , respectively, despite


14000 PL Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12000 10000 8000 6000

1.06 mJcm-2 0.93 mJcm-2 0.80 mJcm-2 0.66 mJcm-2 0.53 mJcm-2 0.40 mJcm-2 0.27 mJcm-2 0.21 mJcm-2

Intensity (arb. units)

Page 9 of 27

8000 6000 4000 2000 0

0.2

0.4

0.6

0.8

1.0

Excitation density (mJcm-2)

4000 2000 0 575 600 625 650 675 700 725 750 775 800

Wavelength (nm)

Figure 3: Excitation density dependence of the PL spectra of the sample spin coated from xylene.Inset:excitation density dependence of the intensity at the ASE band peak wavelength, evidencing the superlinear intensity increase above threshold (the line is a guide for the eyes). the progressive reduction of the propagation losses to 5.3 cm−1 and 2.4 cm−1 , respectively.

Sample morphology and confocal PL mapping In order to investigate the origin of the strong observed solvent dependence of the ASE properties we started from a simple investigation of the samples roughness (that can affect the waveguide propagation losses) by profilometry, determined from the average roughness of three independent linescans of 200 μm each on the sample surface, in positions significant of the observed local film morphology. A roughness of 2.5 nm is obtained both for TOL and XYL, while CB sample show a larger roughness of 4.5 nm. In CF sample wavelike structures



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

Figure 4: ASE threshold of all the investigated samples. Inset: Excitation density dependence of the Full Width at Half Maximum (FWHM) of the PL spectra of the four samples. The continuous line indicate a linewidth of 50 % of the low excitation density FWHM value. are observed, with a peak to peak typical distance of about 100 μm and a peak to valley hight of about 150 nm. The obtained results suggest that the observed ASE variations are not significantly due to differences in the sample roughness. Actually XYL sample, showing the highest ASE threshold, has the same roughness of the TOL sample (with the lowest ASE threshold), and a roughness of about one half of the CB sample, also showing low ASE threshold. Moreover CF film shows the clearly least uniform thickness, but shows propagation losses 30% lower than CB sample with a uniform thickness (with local variation lower than 10 nm across the sample). As a further step we investigated the local PL properties of the sample by measuring the 10 ACS Paragon Plus Environment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Gain and losses (cm -1)

Page 11 of 27

28 26 24 22 20 18 16 14 12 10 8 6 -2 -4 -6 -8 -10

net gain losses gain

L

TO

F

B

C

C

L

XY

Figure 5: Gain, loss and net gain as a function of the solvent. F8BT and the rrP3HT PL maps (see Fig.6) by Confocal Laser Spectroscopy. 25 The F8BT and rrP3HT PL maps of the TOL film show uniform PL intensity, evidencing the absence of relevant morphology and optical properties irregularities. Few bright spots are observed in the rrP3HT map, ascribed to rrP3HT clusters, with an average size of about 9 μm and a surface density of about 65 clusters/mm2 . Similar features are observed in the CB sample PL maps, with uniform F8BT PL intensity across the sample, and with the presence of bright rrP3HT clusters with an average size of about 7 μm and a clearly higher surface density of about 330 clusters/mm2 (see Table 1). A clearly non uniform emission is instead observed in the CF sample, showing an evident stripe like structure in both the F8BT and rrP3HT PL maps, similar to the one observed



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

Figure 6: 300 μm ×300 μm confocal PL maps at 530 nm (left) and 605 nm (right) for the different solvents. during profilometry measurements. These stripes are mainly oriented in radial direction, suggesting their formation during the spin coating, due to the fast chloroform evaporation. A careful observation of the position of high PL intensity allows to observe that the regions of high F8BT PL coincide with low rrP3HT PL intensity regions indicating that the fast CF evaporation during the spin coating results not only in thickness fluctuations, but also in a non uniform local relative composition of the blend, leading to non uniform FRET from F8BT to rrP3HT and then anticorrelated local PL intensity of the two materials. Some bright cluster of rrP3HT with a typical size of about 8 μm are also observed, with a surface 12 ACS Paragon Plus Environment

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


density of about 80 clusters/mm2 . Finally in XYL sample a uniform F8BT PL map is observed, while dark rounded islands are observed in the rrP3HT map, with an average diameter of about 15 μm and a surface density of about 900 islands/mm2 . Considering that rrP3HT is known to have a PL lineshape depending on its aggregation state 24 we investigated the origin of these islands by measuring also a PL map at λ > 700 nm (see inset of Fig. 6). All the dark islands at 605 nm are instead bright above 700 nm, clearly evidencing that in these island rrP3HT is mainly in an aggregated phase. Table 1: rrP3HT cluster average size and density in the investigated samples. Sample TOL CB CF XYL

Cluster average size (μm) 9 7 8 15

Cluster density (mm−2 ) 65 330 80 900

Discussion In order to correlate the measured optical gain and losses with the local emission and morphology properties of the samples we start by observing that the TOL and the CB samples, showing the highest and comparable optical gain, are the only two samples showing almost uniform rrP3HT PL maps, indicating a uniform distribution and excitation of the active molecule across the sample. On the other side the CB sample show a rrP3HT cluster density about 5 times larger than the TOL sample, which is in remarkable agreement with the increase of the waveguide losses (about 4.5 times). This result suggests that the optical losses in these two samples are mainly due to scattering of the waveguided light from the rrP3HT clusters. Concerning the CF sample the profilometer measurement evidences the presence of thickness fluctuation across the sample, resulting in a non uniform PL intensity both of F8BT and 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

rrP3HT, indicating a non uniform local blend composition and rrP3HT excitation across the sample. As the average composition of the blend is the one of maximum optical gain, 10 the lack of composition uniformity results in regions with rrP3HT below the optimal one and in regions with rrP3HT concentration above the optimal one, thus overall in a sub-optimal optical gain, in agreement with the decrease of gain in the CF sample with respect to the TOL and CB ones. In order to determine the origin of the losses we observe that the cluster density of the CF sample is comparable to the TOL one, thus allowing to estimate a similar contribution to the losses of about 1.7 cm−1 . As the total losses of the CF sample are 5.3 cm−1 , the contribution of thickness irregularities should be of about 3.6 cm−1 , thus indicating that the waveguide losses in the CF sample are mainly due to thickness non uniformity. Finally the XYL sample show the worst ASE threshold, mainly determined by the low gain value of 10 cm−1 , which is 2.5 times lower than the gain of the CB sample. This results can be ascribed to the presence of a high density of islands of aggregated rrP3HT across the film, as it has been demonstrated that rrP3HT aggregation suppresses the optical gain in F8BT:rrP3HT blends. 10 Moreover the change of the aggregation of the rrP3HT in the islands are expected to lead to local variations of the refractive index, 26 and thus to light scattering from the islands edges. Considering that the F8BT PL in the islands is comparable to the intensity in the rest of the film, the blend composition is expected to be uniform across the film. As the rrP3HT is the minority component in the blend, the variation of the rrP3HT aggregation is expected to result in a low refractive index contrast, and thus in a low islands scattering cross section, in qualitative agreement with a loss increase of about 40% with respect to TOL sample, despite a defect density about 14 times larger. We then ascribe the losses in the XYL sample to scattering from the aggregated rrP3HT emitting islands.


Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Conclusions In conclusions we investigated the ASE properties of four nominally identical F8BT:rrP3HT blends by changing only the solvent using for the solutions. We demonstrate that the ASE threshold is strongly different from sample to sample, with maximum variations up to 8 times between the best and the worst sample. The ASE threshold solvent dependence is due to a solvent dependence of both the gain and the losses in the film. Confocal microscopy evidences that the solvent variation affect both the mixing uniformity of the two polymers in the blend, related to the gain, and the morphology uniformity, related to the losses. Our results evidence that the solvent optimization is a powerful tool to improve the gain of a given active molecule. As the morphology dependence on the preparation conditions is typical of most of the conjugated molecules our results are expected to be of general validity, suggesting that a systematic investigation of the solvent dependence of the ASE can lead to an ASE improvement in many other conjugated systems showing optical gain.

Acknowledgments The authors acknowledge Andrea Perulli and Concetta Martucci for the profilometry measurements. This work has been supported by Regione Puglia through the project Sens and MicroLAB and it has been partly funded by the Project TASMA-Tecnologie Abilitanti per Sistemi di Monitoraggio Aeroportuale cod. PON01 02876.

References (1) Hide, F.; Diaz-Garcia, M.; Schwartz, B.; Andersson, M.; Pei, Q.; Heeger, A. Semiconducting Polymers: A New Class of Solid-State Laser Materials. Science 1996, 273, 1833–1836.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

(2) Tessler, N.; Denton, G. J.; Friend, R. H. Lasing from conjugated-polymer microcavities. Nature 1996, 382, 695–697. (3) Xia, R.; Heliotis, G.; Bradley, D. D. C. Fluorene-based polymer gain media for solidstate laser emission across the full visible spectrum. Appl. Phys. Lett. 2003, 89, 3599– 3601. (4) Lattante, S.; Giorgi, M. D.; Barbarella, G.; Favaretto, L.; Gigli, G.; Cingolani, R.; Anni, M. Interplay between stimulated emission and singlet-singlet annihilation in oligothiophene dioxide thin films. J. Appl. Phys. 2006, 100, 023530. (5) Thompson, J.; Anni, M.; Lattante, S.; Pisignano, D.; Blyth, R. I. R.; Gigli, G.; Cingolani, R. Amplified spontaneous emission in the near infrared from a dye-doped polymer thin film. Synthetic Metals 2004, 143, 305–307. (6) Samuel, I. D. W.; Turnbull, G. A. Organic Semiconductor Lasers. Chemical Review 2007, 107, 12721295. (7) Liu, X.; Stefanou, P.; Wang, B.; Woggon, T.; Mappes, T.; Lemmer, U. Organic semiconductor distributed feedback (DFB) laser as excitation source in Raman spectroscopy. Optics Express 2013, 21, 28941–28947. (8) Wang, Y.; Morawska, P. O.; Kanibolotsky, A. L.; Skabara, P. J.; Turnbull, G. A.; Samuel, I. D. W. LED pumped polymer laser sensor for explosives. Laser and Photonics Reviews 2013, 7, L71–L76. (9) Cerdan, L.; Costela, A.; Duran-Sampedro, G.; Garcia-Moreno, I.; Calle, M.; Juan-YSeva, M.; Abajo, J. D.; Turnbull, G. A. New perylene-doped polymeric thin films for efficient and long-lasting lasers. Journal of Materials Chemistry 2012, 22, 8938–8947. (10) Xia, R.; Stavrinou, P. N.; Bradley, D. D. C.; Kim, Y. Efficient optical gain media


Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


comprising binary blends of poly(3-hexylthiophene) and poly(9,9-dioctylfluorene-cobenzothiadiazole). Journal of Applied Physics 2012, 111, 123107–1,123107–8. (11) Anni, M. The role of the β-phase content on the stimulated emission of poly(9,9dioctylfluorene) thin films. Applied Physics Letters 2008, 93, 023308. (12) Anni, M.; Perulli, A.; Monti, G. Thickness dependence of the Amplified Spontaneous Emission threshold and operational stability in poly(9,9-dioctylfluorene) active waveguides. J. Appl. Phys. 2012, 111, 093109–1, 093109–5. (13) Ariu, M.; Lidzey, D. G.; Sims, M.; Cadby, A. J.; Lane, P. A.; Bradley, D. D. C. The effect of morphology on the temperature-dependent photoluminescence quantum efficiency of the conjugated polymer poly(9, 9-dioctylfluorene). J . Phys.: Condens. Matter 2002, 14, 9975–9986. (14) Moons, E. Conjugated polymer blends: linking film morphology to performance of light emitting diodes and photodiodes. J. Phys.: Condens. Matter 2002, 14, 12235–12260. (15) Sekine, C.; Tsubata, Y.; Yamada, T.; Kitano, M.; Doi, S. Recent progress of high performance polymer OLED and OPV materials for organic printed electronics. Sci. Technol. Adv. Mater. 2014, 15, 034203–1, 034203–15. (16) Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins, B. A.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H. et al. The Importance of Fullerene Percolation in the Mixed Regions of PolymerFullerene Bulk Heterojunction Solar Cells. Advanced Energy Materials 2013, 3, 364–374. (17) Lampert, Z. E.; Lappi, S. E.; Papanikolas, J. M.; Jr., C. L. R.; Aboelfotoh, M. O. Morphology and chain aggregation dependence of optical gain in thermally annealed films of the conjugated polymer poly[2-methoxy-5-(2’- ethylhexyloxy)-p-phenylene vinylene]. Journal of Applied Physics 2015, 113, 233509–1, 233509–9.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

(18) Wang, Y.; Yang, X.; Wang, R.; Li, L.; Li, H. Morphology dependent amplified spontaneous emission in p-conjugated polymer. Optics and Laser Technology 2015, 71, 73–77. (19) Lampert, Z. E.; Jr., C. L. R.; Papanikolas, J. M.; Aboelfotoh, M. O. Controlling Morphology and Chain Aggregation in Semiconducting Conjugated Polymers: The Role of Solvent on Optical Gain in MEH-PPV. J. Phys .Chem. B 2015, 116, 12835–12841. (20) sconosciuto, A. CRC Handbook of Chemistry and Physics, 89th Edition; CRC press, 2008. (21) Shaklee, K.; Nahaori, R.; Leheny, L. Optical gain in semiconductors. Journal of Luminescence 1973, 7, 284–309. (22) Valenta, J.; Pelant, I.; Linnros, J. Waveguiding effects in the measurement of optical gain in a layer of Si nanocrystals. Applied Physics Letters 2002, 81, 1396–1398. (23) Chappell, J.; Lidzey, D. G.; Jukes, P. C.; Higgins, A. M.; Thompson, R. L.; O’Connor, S.; Grizzi, I.; Fletcher, R.; O’Brien, J.; Geoghegan, M. et al. Correlating structure with fluorescence emission in phase-separated conjugated-polymer blends. Nature Materials 2003, 2, 616–621. (24) Samuel, I. D. W.; Magnani, L.; Rumbles, G.; Murray, K.; Stone, B. M.; Moratti, S. C.; Holmes, A. B. Photoluminescence in Poly(3-hexylthiophene). Proc. SPIE Int. Soc. Opt. Eng. 1997, 3145, 163–170. (25) No evidences of micrometric phase separation of the two materials were obtained in measurements with higher magnification, thus allowing us to exclude the presence of relevant local variation of the active material properties on a size scale smaller than the discussed one. (26) Morfa, A. J.; Barnes, T. M.; Ferguson, A. J.; Levi, D. H.; Rumbles, G.; Rowlen, K. L.;


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


van de Lagemaat, J. Optical characterization of pristine poly(3-hexyl thiophene) films. Journal of Polymer Science Part B: Polymer Physics 2015, 49, 186–194.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Figure Captions Figure 1: PL spectra of all the investigated samples. Inset: absorbance spectra of the investigated samples, the spectra are vertically translated for clarity. Figure 2: Excitation density dependence of the PL spectra of the sample spin coated from toluene. Figure 3: Excitation density dependence of the PL spectra of the sample spin coated from xylene. Figure 4: ASE threshold of all the investigated samples. Inset: Excitation density dependence of the Full Width at Half Maximum (FWHM) of the PL spectra of the four samples. The continuous line indicate a linewidth of 50 % of the low excitation density FWHM value.. Figure 5: Gain, loss and net gain as a function of the solvent. Figure 6: 300 μm ×300 μm confocal PL maps at 530 nm (left) and 605 nm (right) for the different solvents.


Page 21 of 27

1.00

TOL CB CF

0.75

XYL

Absorbance (arb. units)

Figures


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOL

1.2

CB

1.0

CF

0.8

XYL

0.6 0.4 0.2

0.50

300

400

500

600

Wavelength

0.25 500 550 600 650 700 750 800 850 Wavelength(nm)

M. Anni et al Figure 1


9000 8000 0.25 mJcm-2 8000 6000 0.17 mJcm-2 -2 4000 0.13 mJcm 7000 -2 0.10 mJcm 2000 6000 -2 0.067 mJcm 0 0 50 100 150 200 250 -2 5000 0.050 mJcm Excitation density ( Jcm-2) 4000 3000 2000 1000 0 575 600 625 650 675 700 725 750 775 800 Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Wavelength (nm)



Page 22 of 27

14000 PL Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12000 10000 8000 6000

1.06 mJcm-2 0.93 mJcm-2 0.80 mJcm-2 0.66 mJcm-2 0.53 mJcm-2 0.40 mJcm-2 0.27 mJcm-2 0.21 mJcm-2

Intensity (arb. units)

Page 23 of 27

8000 6000 4000 2000 0

0.2

0.4

0.6

0.8

1.0

Excitation density (mJcm-2)

4000 2000 0 575 600 625 650 675 700 725 750 775 800

Wavelength (nm)




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 24 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 25 of 27

28 26 24 22 20 18 16 14 12 10 8 6 -2 -4 -6 -8 -10


L

TO

F

B

C

C



L

XY


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 26 of 27

Page 27 of 27

TOC entry The role of the morphology in the Amplified Spontaneous Emission properties of organic active layer has been poorly investigated to date. We show that the ASE properties of regio regular-poly(3-hexylthiophene) (rrP3HT):poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) thin films is strongly affected by the film morphology, thus showing that the morphology optimization is a very important step of the development of organic material with high optical gain.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28 26 24 22 20 18 16 14 12 10 8 6 -2 -4 -6 -8 -10


L

TO

F

B

C

C


L

XY

Amplified Spontaneous Emission Optimization in ... - ACS Publications

Recommend Documents