The photophysics of organic semiconducting nanospheres: a comprehensive study

Chemical Physics Letters 389 (2004) 7–13 www.elsevier.com/locate/cplett

The photophysics of organic semiconducting nanospheres: a comprehensive study T. Piok a, C. Gadermaier a, F.P. Wenzl b, S. Patil c, R. Montenegro d, K. Landfester d, G. Lanzani e, G. Cerullo e, U. Scherf c, E.J.W. List a,* a Christian Doppler Laboratory of Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria and Institute of Nanostructured Materials and Photonics, Franz Pichler Str. 30, A-8160 Weiz, Austria b Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria c Macromolecular Chemistry, Bergische Universit€at Wuppertal, Gauss-Str. 20, D-42097 Wuppertal, Germany d Organic Chemistry III/Macromolecular Chemistry, University of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany e National Laboratory of Ultrafast and Ultraintense Optical Science and INFM, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

Received 19 February 2004; in final form 4 March 2004 Published online:

Abstract The steady-state and non equilibrium photoexcitation dynamics of semiconducting polymer nanospheres is studied in detail and compared to a regular bulk film of the same polymer (methyl-substituted ladder-type poly(para)phenylene). It is found that the major optical properties of the polymer are left unaltered by this innovative material processing approach. However there is a clear influence of the film nanostructure on the generation, migration and recombination behavior of the involved photoexcited species. Ó 2004 Elsevier B.V. All rights reserved.

1. Introduction Over the last decade a lot of effort was put into the research on conjugated polymers for possible commercial applications [1]. Optoelectronic devices such as organic light emitting diodes (OLEDs) [2], solar cells [3], lasers [4], field-effect transistors [5], and all-polymer integrated electronic circuits [6] have successfully been realized. However, the quest for novel material properties and processing techniques stimulates research directed towards the controlled deposition of functional materials on the nano- to mesoscopic scale. Organizing the material at such a scale, enables to fabricate improved as well as novel devices based on organic semiconductors such as conjugated polymers in a cost-effective way. Structuring matter on the nanometer scale gives access to unprecedented combinations of material prop*

Corresponding author. Fax: +433168738478. E-mail address: [email protected] (E.J.W. List).

0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.03.018

erties and processability. One way to exploit these advantages for conjugated polymers are semiconducting polymer nanospheres (SPN) [7], which are accessible in a miniemulsion approach. A miniemulsion is a liquid heterophase system of high stability where small droplets imbedded in the continuous phase are created by using high shear forces [8]. For a typical oil-in-water miniemulsion, an oil, a hydrophobic agent, an emulsifier, and water are applied to high shear forces (ultrasonication) to obtain an emulsion with narrow distribution of the droplet sizes. After evaporation of the solvent a stable dispersion of SPN results (diameter of the particles from 30 to 500 nm). This novel approach enables the application of environmentally friendly water-based deposition techniques such as ink-jet printing and opens new ways for controlled multi- or heterolayer formation, simply by using one or more of the organic materials as an aqueous SPN dispersion. A first detailed study of the optoelectronic properties of SPNs fabricated from methyl-substituted ladder-type

T. Piok et al. / Chemical Physics Letters 389 (2004) 7–13

The preparation of the SPNs is described in detail in [11] while the synthesis of the utilized conjugated polymer m-LPPP is described in [12]. The molecular structure is shown in the inset of Fig. 2. Sodium dodecyl sulfate (SDS) was purchased by Sigma Aldrich (CAS Nr. 151-21-3). Commercially available glass substrates were cleaned by wiping with isopropyl alcohol and acetone and subsequently cleaned in an isopropyl alcohol ultrasonic bath. The SPN dispersion or toluene solution was spin-coated, and the films were dried in an oven for about 15 min at 75 °C. UV–Vis transmission spectra were measured using a Perkin–Elmer k9 spectrophotometer. Photoluminescence (PL) emission spectra were recorded using a Shimadzu RF5301 spectrofluorometer. For the PIA measurements the samples were mounted in a cryostat and cooled to a temperature of 80 K. As transmission source we used a standard halogen lamp operated at 160 W and as excitation source an Arþ laser operated in multiline UV mode (3.53, 3.41 eV) was used. As excitation source for the amplified spontaneous emission a frequency tripled Nd-YAG Laser (3.49 eV) with repetition rate of 10 Hz and pulse duration of
10 ns was used. The beam was focused to a strip like illumination zone via a cylindrical lens and the amplified spontaneous emission was recorded with an Oriel charge coupled device (CCD) spectrometer. A frequency-doubled Ti-sapphire laser system with a chirped pulse amplification stage provided the pump pulses with a duration of approx. 150 fs. The probe pulses, covering the spectral region of 1.2–3 eV, were

3. Results 3.1. Steady state absorption and emission properties In Fig. 1a the emission and absorption spectra of a spin-coated m-LPPP SPN film (particle size of the particle was 75 nm) are compared to those of a film spincoated from a common m-LPPP toluene solution. SPNbased and ‘common’ m-LPPP films, resp., display no significant differences in all recorded types of spectra. Note that also the emission and absorption spectra of SPN dispersions of different particle sizes did not reveal significant differences, i.e., the optical properties of the particles are not size-dependent, at least as tested for diameters from 160 nm down to 70 nm. The high energy tailing of the SPN thin film absorption spectra originates from scattering effects. All emission spectra are characterized by a steep onset at 2.7 eV accompanied by (a)

2.4

norm. absorbance (arb. units)

2. Experimental

obtained by super-continuum generation via self-phase modulation in a thin sapphire plate and focused via a band pass filter onto the photoexcited region of the sample. The pump pulse was focused onto a spot of 60 lm in diameter via a spherical lens. The signals are picked up by an Oriel MS125 spectrograph coupled to a CCD spectrometer Instaspec4 for recording spectra at fixed pump–probe delay, or via a photodiode preceded by an interference filter (bandwidth 10 nm) for measuring the temporal dynamics at selected wavelengths. A detailed description of the setup and measurement principles can be found in [13].

PL (arb. units)

poly(para)phenylene (m-LPPP) is given in [9] which is focused on the film formation and possible device applications. m-LPPP was chosen as model substance because it is one of the best characterized blue emitting conjugated polymers. For the application of SPNs in optoelectronic devices, it is essential to know whether the miniemulsification or the deposition/film formation of the SPNs alters the electronic and structural properties of the semiconducting polymers. This issue requires thorough investigations, since the preparation of the miniemulsion involves the application of ionic or non-ionic surfactants as well as strong ultrasonic shear. Thin films spin cast from SPN miniemulsion and regular toluene solution from the same batch of m-LPPP are characterized optically, via photoinduced absorption (PIA) and femto second pump–probe spectroscopy. The combination of these measurement methods leads to a comprehensive picture from the steady state optoelectronic properties down to the femto second dynamics. The used methods are very sensitive to possible defects [10] generated during the miniemulsifaction process.

norm. photoluminescence (arb. units)

8

2.6

Energy (eV)

(b)

1.5

2.0

2.5 3.0 energy (eV)

3.5

4.0

Fig. 1. (a) Normalized PL and absorbance spectra of a m-LPPP film (dashed lines), and normalized PL and absorbance spectra of a SPN film (solid lines). Inset: amplified spontaneous emission of a common m-LPPP film (dashed line) and a m-LPPP colloid film (solid line) on glass excited with frequency tripled Nd-Yag (3.49 eV). (b) Normalized PL and absorbance spectra of a m-LPPP film (dashed lines, open circles), and normalized PL and absorbance spectra of a m-LPPP/SDS blend film (dotted lines, triangles).

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well resolved vibronic side bands; they are a mirror image of the absorption spectrum as has been discussed in detail for common m-LPPP [14]. Moreover, a low energy tail due to an energy transfer to on chain emissive defects – as also known for thin films of the polymer [15] – can be observed in the PL spectra of the SPN film. Due to the preparation of the SPN, a surfactant (SDS) is present in the SPN films [16]. In order to compare the influence of the surfactant molecule SDS simple blend films of m-LPPP and SDS were produced. Comparing those films to pristine m-LPPP films neither a shift nor a change in the relative peak intensities both in absorbance and in PL was observed (Fig. 1b). From this we conclude that the electronic structure of m-LPPP is not influenced by SDS. Moreover, drop cast films of both materials show amplified spontaneous emission (ASE). In the inset of Fig. 1a the ASE measured at the same excitation densities is compared. The ASE onset of the common mLPPP film is 17 lJ, the SPN film showed an onset of 28 lJ observed at films with similar film thickness for equal spot sizes of
0.2 cm2 . The differences will be discussed below. 3.2. Steady state photoexcitation dynamics Long lived excited states such as triplet excitons or defect stabilized polarons can be revealed from PIA measurements [10,17]. The PIA spectra depicted in Fig. 2 show very similar shapes and relative intensities for bulk and SPN-based films. In both spectra the triplet absorption is observed at 1.3 eV [18], while a weak trace of the polaronic absorption is located near 1.9 eV [19]. This gives evidence that no additional electronic defects are generated upon processing of m-LPPP into SPNs. The frequency dependence of in-phase and out-of-phase components of the observed signal discloses the lifetime

9

of the electronic state assigned to a spectral feature. In Fig. 3 the excitation intensity dependent PIA at 1.3 eV and in the inset its out-of-phase component for both mLPPP and m-LPPP SPN films at different excitation intensity are shown. The intensity dependence of a PIA signal is linear (slope 1 in a double-logarithmic plot) for monomolecular decay while it has a square root dependence (slope 0.5) for a purely bimolecular recombination behavior. For both films at low excitation densities, the decay is essentially monomolecular. The lifetime can be calculated from the maximum in the frequency dependence of the out-of-phase component [18]. The lifetime for the monomolecular process for the m-LPPP film is smono ¼ 9 ms (excitation density 1.33E + 21 abs. phot./s cm3 ), for the colloid film smono ¼ 15 ms (excitation density 1.03E + 21 abs. phot./ s cm3 ) [20]. At high excitation energies, the sublinear slope indicates clearly a bimolecular recombination process. An equivalent bimolecular life time including the bimolecular annihilation parameter can be calculated from the maximum of the out-of-phase component. The bimolecular annihilation parameter for common m-LPPP film was found to c ¼ 6E ) 16 cm3 /s at an excitation density of 1.6E + 22 abs. phot./s cm3 . The c diffusion coefficient D ¼ 4pR [21] can be calculated QP with the relation between the diffusion coefficient D, annihilation parameter c and the radius of the triplet exciton, which has been assumed p toffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi be 0.35ffi nm [19]. The diffusion length l is given by l ¼ Z  D  s, where Z ¼ 6 for 3 dimension. In common m-LPPP the diffusion length of triplet excitons is found to be l ¼ 76 nm. In the SPN film an annihilation parameter c of 2E ) 16 cm3 /s at 1.2E + 22 abs. phot./s cm3 results with the corresponding diffusion length l of 54 nm. These results represent a first approximation based on a simple rate equation model which describes our data fairly well. A more complex modeling, accounting for a possible lifetime distribution, is beyond the scope of the present work [22].

C H

10 21

3.3. Non equilibrium photoexcitation dynamics CH C H 3

norm. ∆T/T (arb. units)

CH

6 13

6 13 *

*

n* *

CH

6 13

HC 3

CH

6 13

C H

10 21

1.2

1.4

1.6 energy (eV)

1.8

2.0

Fig. 2. Photoinduced absorption of a m-LPPP film (triplet exciton absorption at an energy of 1.3 eV, solid line) and of a m-LPPP SPN film (triplet exciton absorption at an energy of 1.3 eV, dashed line). The inset shows the molecular structure of m-LPPP.

In order to study the early electronic processes immediately following the optical excitation, we performed transient differential transmission measurements with a time resolution of 150 fs. The DT =T spectrum of an m-LPPP SPN and a ‘common’ m-LPPP film for various delays after excitation are shown in Fig. 4. Note that the optical density of both films is similar. Below 2.4 eV the signal is negative PIA with a broad peak at 1.5 eV (subsequently referred to as PA1 ). On the high energy tail of this feature are two peaks at 1.9 and 2.1 eV (PA2 ). In previous works PA1 has been assigned to an absorptive transition S1 –Sn from singlet excitons and PA2 to an overlap of PA1 with a transition D0 –Dn of charged polaron states (doublets)

T. Piok et al. / Chemical Physics Letters 389 (2004) 7–13

m-LPPP SPN film

bulk m-LPPP film

∆T/T

0.1

0.0001 100

50mW 400mW

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22

10

10

-3 -1

10

21

10 -1

Excitation Density (abs. photons cm s )

Excitation Density (abs. photons s cm

-3

22

)

Fig. 3. Intensity dependent photo induced absorption of an m-LPPP colloid film. The lines show the region of a monomolecular (solid, slope k ¼ 0:90) and bimolecular (dashed, slope k ¼ 0:51) excitation regions. Inset: Out of phase component of the frequency dependent photo induced absorption signal of an m-LPPP colloid film to determine the lifetime of the triplet excitons for the monomolecular (low laser intensity 50 mW, 1.0E + 21 abs. phot./s cm3 ) and bimolecular process (laser intensity 600 mW, 1.2E + 22 abs. phot./s cm3 ). The right figure shows the intensity dependent photo induced absorption of an m-LPPP film. The lines show the region of a monomolecular (solid, slope k ¼ 0:95) and bimolecular (dashed, slope 0.61) excitation regions. Inset: Out of phase component of the frequency dependent photo induced absorption signal of an m-LPPP colloid film to determine the lifetime of the triplet exciton for the monomolecular (low laser intensity 50 mW, 1.3E + 21 abs. phot./s cm3 ) and bimolecular process (laser intensity 600 mW, 1.6E + 22 abs. phot./s cm3 ).

m-LPPP SPN film

bulk m-LPPP film

200 ps (x10) 200 ps (x10)

∆T/T (arb. units)

∆T/T (arb. units)

40 ps (x10) 40 ps (x10)

4 ps (x2)

0 ps

0 ps

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 (a)

4 ps

Energy (eV)

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 (b)

Energy (eV)

Fig. 4. Transmission difference spectra of m-LPPP colloid film (a) and common m-LPPP film (b) at room temperature excited at 3.2 eV for various pump–probe delays.

[23]. The absorption edge of m-LPPP is 2.7 eV, hence the positive feature at 2.7 eV is assigned to photobleaching (PB). In common m-LPPP films stimulated emission (SE) from singlet states S1 has been reported for the spectral range from 2.2 to 2.6 eV, with two peaks at 2.3 and 2.5 eV. By comparing both spectra, all fea-

tures can be observed also for the SPN film at the same energy. However, comparing the spectra of the bulk and the SPN film, one finds slight differences, which grow with increasing pump–probe delay: PA2 becomes stronger in the SPN film, while SE becomes slightly weaker. Moreover, the vibronic replicas of PA2 are

T. Piok et al. / Chemical Physics Letters 389 (2004) 7–13 m-LPPP SPN film

bulk m-LPPP film 1.0

2.53eV 1.91eV 1.46eV

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0.0 -100

11

0.0 0

1

2

3

4

5

Time (ps)

-1

(b)

Time (ps)

Fig. 5. Temporal evolution of the differential transmission spectra of m-LPPP colloid film (a) and common m-LPPP film (b) at room temperature excited at 3.2 eV at different spectral positions.

much more pronounced for the SPN film, which supports the assumption of a higher polaron density [24]. Pump–probe studies as well as quantum chemical calculations on a ladder-type oligoparaphenyl, an oligomeric model compound of x bridged benzene rings with almost identical chemical structure to m-LPPP, showed that the absorption upon polarons can reach far into the SE region [25]. A similar effect can therefore be expected for m-LPPP, which explains the lower SE and the different shape at long delays, which is more pronounced in the SPNs. In Fig. 5 a comparison of the temporal evolution of PA1 , PA2 , and SE is shown, measured at 1.46, 1.91 and 2.53 eV, respectively. The time traces of PA1 and SE, which both originate from the singlet, show the same behavior with a lifetime of approx. 10 ps. No significant difference between the two samples can be discerned. The PA2 traces of both materials bear a considerably slower contribution, which originates from polarons (lifetime of ca. 100 ps). This contribution is slightly higher for the SPN film.

4. Discussion In general the basic optoelectronic properties are similar as it is shown in PL and absorption spectra. Also the spectral features of the polaron, singlet and triplet exciton show up at exactly the same energy. However, certain differences in the dynamics of the involved species’ can be found. The monomolecular decay of triplets is determined by its intersystem crossing rate, back transfer to the singlet manifold and by dissociation upon quenching sites [21]. When an exciton reaches an SPN boundary, it can be reflected, trapped or quenched. If a quenching at boundaries were efficient, the triplet lifetime would be shorter. Trapping and reflection on the other hand result

in a reduced mobility and hence in a longer lifetime. From our findings we conjecture that the shorter average diffusion length of the triplet exciton and the longer lifetime, respectively, can be attributed either to an enhanced trapping at the SPN boundaries or a reflection there. Although the SPN may come into close contact at certain surface regions, the SPN boundaries possess an average mutual distance which is much larger than the average distance between the polymer chains within the SPN [9]. This fact is of twofold origin, first the SPNs bare sufficient hardness to leave some interstice upon packing and second the SPN surfaces are at least partly covered with SDS molecules, which act as spacers. The SDS molecule adsorbs at the SPN with its hydrophobic alkyl chain, which is inert towards photoexcited states of conjugated polymers. Hence, the SPN surfaces act as barriers for exciton transfer. The annihilation parameter is significantly lower for the SPN film. Since annihilation involves migration of triplets, this finding can again be rationalized by a reduced migration and a lower diffusion length due to the film nanostructure. The equivalent bimolecular recombination, which is related to the triplet–triplet, triplet– polaron or triplet–singlet annihilation, is slower due to lower probability of an annihilation event. By comparing the absolute PIA signal referred to the excitation density, one can find significantly more triplets in the SPN film. By assuming a constant singlet generation rate the singlet–triplet transfer rate can be calculated. Via the triplet absorption cross section [26], the absolute triplet density can be calculated. By dividing it by the absolute number of absorbed photons the triplet generation rate gTE can be estimated under the condition that the triplet generation is much faster than its decay. Note that this calculation only gives a total value of all possible triplet generation processes: intersystem crossing, singlet fission and non-geminate polaron coalescence. The singlet–triplet generation ratio

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T. Piok et al. / Chemical Physics Letters 389 (2004) 7–13

gTE [27] is gTE ¼ 0:2% for the regular film and gTE ¼ 0:9% for the SPN film. In order to explain this difference we take a closer look at the early events after the photoexcitation. On the ultra fast timescale, the only perceivable difference between the two varieties of m-LPPP is a slightly higher density of polarons created either during the pump pulse or shortly afterwards from hot singlets. Since there is no indication concerning a difference in the electronic structure, we do not expect any differences in intrinsic polaron generation mechanisms such as sequential excitation of singlets [28]. Instead, the higher polaron yield can be ascribed either to a different alignment of polymer chains with respect to their neighbors, or to a higher number of extrinsic dissociation sites, either surface sites at the SPN boundaries or sites adjacent to SDS. From the transient differential transmission spectra a weak absorption overlapping the SE can be recognized. This causes a slightly lower gain in the SPN film already at very short pump–probe delays. Hence, the reason for the lowered gain is actually an overlap with an ultrafast forming absorption feature and not due to gradually formed species or quenching at polarons, which would both lead to a shorter singlet lifetime. This slightly reduced gain translates into a somewhat higher ASE threshold compared to the regular bulk film [29]. Moreover, the higher surface roughness inherent to the SPN film [9] may compromise to a certain extent the waveguiding necessary for ASE. The significantly higher polaron density observed in SPNs on the short time scale is a likely cause for the higher triplet generation rate in the steady state regime. The deactivation pathways for these polarons are geminate and non geminate recombination or coalescence. Recombination is direct deactivation to the ground state while coalescence results in a singlet exciton in the geminate case and in either a singlet or a triplet in the non geminate case. The triplet exciton formation in m-LPPP is a combination of two processes: intersystem crossing and nongeminate polaron coalescence. In the SPN film it is reasonable to assume that intersystem crossing has the same rate as in common m-LPPP. On the other hand the polaron contribution can be expected to be stronger in proportion to the higher polaron density.

5. Conclusion A detailed spectroscopic investigation of a common m-LPPP film and a film fabricated from m-LPPP semiconducting nanospheres (SPN) reveals that the processing of m-LPPP into SPNs does not significantly alter its basic spectroscopic properties. Nevertheless, some differences in the photo excitation kinetics have been found. The lifetime of the singlet excitons are

similar for both materials, the polaron density observed in SPN films is somewhat higher, the triplet lifetime is longer and a significantly higher triplet yield is found. The higher polaron density is attributed to a larger number of quenching sites (surface states). The longer triplet lifetime is the result of a hindered migration across the SPN boundaries. The higher triplet yield is ascribed to a consequence of a larger number of polarons, whose non-geminate coalescence can lead to the triplet formation. A higher ASE threshold has been found due to a slightly lower gain caused by the absorption upon polarons or/and a weaker waveguiding resulting from the rougher surface. However, although the SPN morphology brings about a slight disadvantage in this respect, it may one day be driven to such a degree of periodicity that it can be exploited for photonic purposes (photonic crystals), such as an intrinsic DFB structure.

Acknowledgements We thank G. Mauthner, A. Pogantsch, H. Wiesenhofer and E. Zojer for fruitful discussion and T. Virgili for experimental assistance. T.P. and C.G. gratefully acknowledge the financial support of European Community Access to Research Infrastructure action of the improving Human Potential Programme, Contract No. HPRI-CT-2001-00148 (Center for Ultrafast Science and Biomedical Optics, CUSBO). The CDL-AFM is an important part of the long term AT&S research strategies.

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