**4. Organic/organic composite films based on aromatic derivative inclusions for optoelectronic applications**

Lately, a special attention has been paid to composite materials based on different organic polymeric matrix and organic inclusions to obtain materials combining the properties of the both components [25; 34; 45; 62; 63]. This field of research has developed from fundamental investigations to the synthesis of new monomers to be introduced in polymeric matrix. The most important advantage of the polymeric matrix is the possibility to deposit thin films us‐ ing inexpensive methods, such as the deposition from solution by spin-coating. The limiting parameter is the quality of the layer and can be controlled by the control of the experimental conditions. A special attention is focused to the identification and development of π-conju‐ gated systems with functional groups that assure an improvement in the emission proper‐ ties and charge carrier mobility necessary for optoelectronic applications.

We have emphasised the effect of the polycarbonate of bisphenol A matrix on the properties of the synthesised amidic monomers with –CN and –NO2 substituent groups with the pur‐ pose to manipulate the local molecular environment of the monomer for changing the physi‐ cal properties of the films (transmission, luminescence, electrical transport) in correlation with the quality of the spin-coated layers.

The polycarbonate of bisphenol A, utilized as matrix, is characterised by a large domain of transparency, high transmission in visible, high refraction index, solubility in common sol‐ vents. As inclusions, to be embedded in the matrix, we have selected monomers character‐ ised by a maleamic acid structure with different functional groups:

where R=-NH, R1=-CN for (MM3); R=-NH, R1=-NO2; R2=-NO2 for (MM5) [64].

**Figure 39.** I-V characteristics of Si or ITO/organic layer(s)/Au or Cu heterostructures prepared by MAPLE: based on PTCDA (a) curves 2 and 3 under illumination; based on Alq3 (b) curve 3 under illumination; based on ZnPc (c) all in

In Figure 39 a, b, c are presented the I-V characteristics in dark and under illumination, the highest current (~10-4-10-3 A) being obtained in dark with the structure prepared with PTCDA, Alq3 or ZnPc on ITO substrate, at low applied voltage of 0.5 V [54]. This current is with three orders higher than the current for the same structure realized on Si. This behav‐ iour is correlated, in the first case, with the height of the energetic barriers at the interfaces that favour the injection of holes from ITO positively biased in organic. The I-V characteris‐ tics obtained under continuous illumination at an applied voltage of 1 V, indicate a higher current in the heterostructures realized with PTCDA and, Si and Cu electrodes, explained by the higher energetic barrier for electron injection at the contact Alq3/Cu (ΔE=1.5 eV) com‐ pared to PTCDA/Cu (ΔE=0.7 eV). The current is one order higher than the dark current con‐ firming the photo generation process [54]. In the heterostructures with double organic layer and, ITO and Cu electrodes, we have obtained a current of 2x10-3 A at 0.5 V, explained by the energetic barrier in ITO/ZnPc/Alq3/Cu heterostructure and by the presence of the inter‐ face dipoles reducing the energetic barrier and improving the conduction in ITO/ZnPc/

**4. Organic/organic composite films based on aromatic derivative**

ties and charge carrier mobility necessary for optoelectronic applications.

Lately, a special attention has been paid to composite materials based on different organic polymeric matrix and organic inclusions to obtain materials combining the properties of the both components [25; 34; 45; 62; 63]. This field of research has developed from fundamental investigations to the synthesis of new monomers to be introduced in polymeric matrix. The most important advantage of the polymeric matrix is the possibility to deposit thin films us‐ ing inexpensive methods, such as the deposition from solution by spin-coating. The limiting parameter is the quality of the layer and can be controlled by the control of the experimental conditions. A special attention is focused to the identification and development of π-conju‐ gated systems with functional groups that assure an improvement in the emission proper‐

dark [54].

PTCDA/Cu heterostructure.

326 Optoelectronics - Advanced Materials and Devices

**inclusions for optoelectronic applications**

After testing the process of layer formation in correlation with the surface energy by contact angle measurements using two different solvents, we have selected dimethylformamide (DMF) for the preparation of the "mother solution" that contain the both components, ma‐ trix and inclusion. We have varied the weight ratio between the components 1/3;1/2; 1/1, us‐ ing the pre-wetting of the surface and different duration and rotation speeds for the spreading stage (t1=3s; 6s; 9s; 12s; v1=0.5 krpm; 0.7 krpm; 0.9 krpm; 1.13 krpm) and homoge‐ nisation stages (t2= 10s; 20s; v2=1.6 krpm; 1.9 krpm; 2.2 krpm; 2.7 krpm; 3 krpm), with the purpose to identify the most adequate conditions for the deposition of layers [64].

UV-Vis transmission spectra presented in Figure 40 have evidenced differences in the be‐ haviour of the composite material prepared with (MM3) and (MM5), determined by differ‐ ences in the chemical structure of these components. The shape of the transmission curve is determined by the substituent to the aromatic nucleus and depends on the lone electron pairs of the oxygen atoms in the carbonyl and nitrous groups involving (n, π\*) state, which are splitted because of the interaction in the solid state between the polycarbonate matrix and (MM5) monomer. No significant difference has been emphasised, in the UV-VIS spec‐ tra, between the monomer deposited by vacuum evaporation and the same monomer em‐ bedded in a polymeric matrix and deposited by spin-coating. Although (MM3) shows also lone electron pairs the interaction between the cyan groups and the carbonyl groups is not so intense in the solid state to favour the splitting of the (n, π\*) energetic level.

The emission of the polymer/monomer composite material is determined by the interaction between the chromophoric groups in monomer with the polymeric matrix. This interaction

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The Figure 41 shows that the polymeric matrix significantly affects the emission spectra of the monomers characterised by a peak situated at 430 nm in (MM5) and 450 nm in (MM3). These shapes of the spectra can be correlated with the emission properties of the substituted benzene nucleus [65] and with the involvement of the (n, π\*) states lower than the usual sin‐ glet excited states. The strongest emission was obtained for monomer (MM5) in polycarbon‐ ate of bisphenol A matrix probably due to the strong absorption of the excitation radiation

In composite material based on (MM5) the emission spectra show a maximum around 510 nm and in composite material based on (MM3) a slightly weaker and broader maximum around 480 nm. Figure 41 has not evidenced a strong broadening effect of the matrix on the emission spectrum of monomers. The emission of polycarbonate:MM5 is not blue shifted and therefore we suppose that the monomer is not highly stressed in the polycarbonate ma‐

In Figure 42 are presented some results on the investigation of the effect of the polymeric matrix on the electrical transport properties of Si/monomer/Si and Si/polycarbonate:mono‐

**Figure 42.** I-V characteristics of Si/MM3/Si and Si/polycarbonate:MM3/Si heterostructures (a) and Si/MM5/Si and Si/

We have analysed the electrical properties of the heterostructure Si/monomer/Si and Si/ polycarbonate:monomer/Si at room temperature, testing the reproducibility of the measure‐

can generate the shift, broadening or strengthening of the emission peak [64].

(λ=335 nm) assuring a higher efficiency of the emission process.

trix.

mer/Si heterostructures.

polycarbonate/MM5/Si heterostructures (b) [64].

**Figure 40.** Comparative UV-VIS spectra of MM3 and MM5 monomers deposited by vacuum evaporation and polycar‐ bonate /MM3 and polycarbonate /MM5 deposited by spin coating, on glass substrate [64].

**Figure 41.** Photoluminescence spectra of monomers (MM3) and (MM5) deposited by vacuum evaporation and poly‐ carbonate/MM3 and polycarbonate/MM5 deposited by spin coating on glass substrates [64].

The emission of the polymer/monomer composite material is determined by the interaction between the chromophoric groups in monomer with the polymeric matrix. This interaction can generate the shift, broadening or strengthening of the emission peak [64].

The Figure 41 shows that the polymeric matrix significantly affects the emission spectra of the monomers characterised by a peak situated at 430 nm in (MM5) and 450 nm in (MM3). These shapes of the spectra can be correlated with the emission properties of the substituted benzene nucleus [65] and with the involvement of the (n, π\*) states lower than the usual sin‐ glet excited states. The strongest emission was obtained for monomer (MM5) in polycarbon‐ ate of bisphenol A matrix probably due to the strong absorption of the excitation radiation (λ=335 nm) assuring a higher efficiency of the emission process.

In composite material based on (MM5) the emission spectra show a maximum around 510 nm and in composite material based on (MM3) a slightly weaker and broader maximum around 480 nm. Figure 41 has not evidenced a strong broadening effect of the matrix on the emission spectrum of monomers. The emission of polycarbonate:MM5 is not blue shifted and therefore we suppose that the monomer is not highly stressed in the polycarbonate ma‐ trix.

In Figure 42 are presented some results on the investigation of the effect of the polymeric matrix on the electrical transport properties of Si/monomer/Si and Si/polycarbonate:mono‐ mer/Si heterostructures.

**Figure 40.** Comparative UV-VIS spectra of MM3 and MM5 monomers deposited by vacuum evaporation and polycar‐

**Figure 41.** Photoluminescence spectra of monomers (MM3) and (MM5) deposited by vacuum evaporation and poly‐

carbonate/MM3 and polycarbonate/MM5 deposited by spin coating on glass substrates [64].

bonate /MM3 and polycarbonate /MM5 deposited by spin coating, on glass substrate [64].

328 Optoelectronics - Advanced Materials and Devices

**Figure 42.** I-V characteristics of Si/MM3/Si and Si/polycarbonate:MM3/Si heterostructures (a) and Si/MM5/Si and Si/ polycarbonate/MM5/Si heterostructures (b) [64].

We have analysed the electrical properties of the heterostructure Si/monomer/Si and Si/ polycarbonate:monomer/Si at room temperature, testing the reproducibility of the measure‐ ments and considering only the typical behaviour. The heterostructures realized with com‐ posite materials show a current with 3 orders lower than the heterostructures realized only with monomers. The charge carrier transport is mainly affected by the insulating character of the polymeric matrix. The highest current (3x10-8 A) has been obtained in heterostructure Si/polycarbonate:MM5/Si for an applied voltage of 1 V and for voltages between 0.1 V and 1 V the characteristic is weakly superlinear [61]. For the heterostructures realized only with monomers the I-V characteristics are linear at low voltages and become strongly superlinear for voltages >0.2 V.

Important results have been bought in the field of molecular organic matrix based on aro‐ matic derivatives (benzil, m-DNB)/inorganic (iodine, silver, sodium) or/and organic (m-DNB, naphthalene, bulk) composite systems and in the field of composite films prepared from polymeric matrix and active monomeric inclusions based on π-conjugated systems containing functional groups with special properties, to improve the properties of film form‐ ing, emission properties of the matrix and the charge carrier mobility in the matrix, with the

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We have emphasised the effect of the polycarbonate of bisphenol A matrix on the properties of the synthesised amidic monomers with –CN and –NO2 substituent groups with the pur‐ pose to modify the local molecular environment of the monomer and change the optical and

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ces with improved stability. *Appl. Phys. Lett.*, 69(15), 2162-2160.

purpose to obtain materials for potential optoelectronic applications.

\*Address all correspondence to: fstanculescu@fpce1.fizica.unibuc.ro

2 National Institute of Materials Physics, Bucharest-Magurele,, Romania

1 University of Bucharest, Bucharest-Magurele,, Romania

tals. *Phys Rev. Lett.*, 14(7), 229-231.

Appl. Phys.Part 1 , 38(9A), 5274-5277.

Phys. Lett. , 51(12), 913-915.

electrical properties of the films.

Florin Stanculescu1\* and Anca Stanculescu2

**Author details**

**References**

152-154.

The films obtained from the polycarbonate containing the monomer with two nitrous sub‐ stituents (MM5) to the aromatic nucleus have shown good transparency, and photolumines‐ cence in the green region and promising electrical properties at voltages >0.6 V (I=10-8 A) with a close to linear characteristic at voltages between 1 V and 10 V [64].

Also this organic/organic composite material seems to be promising for optoelectronic appli‐ cations, the spin coated composite layers are characterised by a specific morphology and a high degree of disorder which affect the optical and electrical properties and make difficult their control.
