**3.1. Preparation of the aromatic derivatives thin films**

#### *3.1.1. Aromatic derivatives thin films preparation by directional solidification process*

It is not very easy to obtain organic thin films because of the same difficulties which affect the preparation of organic crystals and the quality of the organic layer is strongly influenced by the method, which have been selected to grow the film. For example there is a high con‐ centration of structural defects in the benzil thin films which have been grown by a rapid directional solidification process, characterised by a non rigourous control of the thermal re‐ gime compared to the crystal growth process and these defects have caused the red shift of the emission peak. It is very difficult to grow, by vacuum evaporation, thin films of organic compounds characterized by a melting point Tm<100 ° C (including benzil and m-DNB) be‐ cause the heating of the substrate during the evaporation process can favor a strong desorp‐ tion of the organic molecules from the substrate. By the directional solidification process could be prepared organic thin films between two substrates, like quartz or glass substrates, during a rapid thermal solidification characterized by a temperature gradient for solidifica‐ tion, ΔT>50 ° C, necessary to counteract the supercooling phenomenon [45]. Crystalline frag‐ ments from organic ingots of pure and doped m-DNB (Tm=89.9 o C) or benzil (Tm=95 o C), grown by Bridgman-Stockbarger method presented in paragraph 2.1, have been melted be‐ tween the substrates by the hot plate technique and after that rapidly frozen by the cold plate technique obtaining films with a columnar structure with large dendrites branches in the plane of the film determined by the low thermal conductivity and anisotropy of these organic compounds. The thickness of the films has been evaluated (using the density in sol‐ id state) from geometrical considerations presuming that the total volume of the substance didn't change during the melting-solidification cycles.

absorbing solvent, like dimethylsulphoxide (DMSO) or chloroform. Deposition was realized with a KrF\* laser, Coherent ComplexPro 205 characterised by λ=248 nm, τFWHM~25 ns, repeti‐ tion rate=10 Hz [47]. The incident laser energy absorbed by the solvent molecules is convert‐ ed into thermal energy determining the heating and simultaneous evaporation of the two components. The solvent molecules are pumped away by the vacuum pump that maintains a pressure of 10-2-10-1 mbar in the deposition chamber, while the less volatile molecules of the organic compound deposit on the substrate maintained at room temperature. The low

and 430 mJ/cm<sup>2</sup>

organic molecule and the number of pulses between 10000 and 120000, with effect on the

The absorption spectrum of m-DNB, presented in Figure 20, which is similar to a classical semiconductor, could be correlated with the strong interactions between the polar molecules and with the partial superposition of the π-electrons clouds from neighbour molecules gen‐ erating narrower valence and conduction bands. This spectrum is different from that of ben‐ zil presented in Figure 19, which is characterized by a two edges of the fundamental absorption, with a subband light absorption peak situated at 380 nm, attributed to absorp‐ tion by dicarbonyl groups, strongly interacting in the solid state and producing the split of

films' thickness (40 nm-150 nm), which has been evaluated by ellipsometry.

**3.2. Optical properties of aromatic derivatives thin films**

**Figure 19.** Absorption spectra of benzil film grown between two quartz plates [45].

to avoid the deterioration of the

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value fluence varied between 160 mJ/cm<sup>2</sup>

the energetic level (n, π\*).

#### *3.1.2. Aromatic derivatives thin films and heterostructures preparation by vacuum evaporation*

Thin films of PTCDA, Alq3 and TPyP have been prepared by vacuum evaporation and dep‐ osition on different substrates (glass/ITO, quartz, Si), which have been cleaned in acetone (glass/ITO, quartz) and with acetone, hydrofluoric acid and distillate water (Si).

Stable, homogeneous organic films, with good adhesion to the substrates have been pre‐ pared, by the evaporation of the organic powder contained in the quartz crucible heated by a self-sustaining kanthal winding, in an Alcatel system with turbo molecular pump [40; 46]. During the deposition with a duration between 10-15 min, the temperature was measured by a thermocouple situated at the bottom of the crucible and varied between 220-240 o C for PTCDA [40], 150-160 o C for Alq3 [40] and 175-185 o C for TPyP [46].

#### *3.1.3. Aromatic derivatives thin films an heterostructures preparation by MAPLE*

A special type of Pulsed laser Deposition (PLD) technique, Matrix Assisted Pulsed Laser Evaporation (MAPLE), has been used for the deposition of small molecule organic films (PTCDA, ZnPc, Alq3). This technique involves the ablation of a target formed by the frozen solution of the organic compound in a high molecular weight and strong laser wavelength absorbing solvent, like dimethylsulphoxide (DMSO) or chloroform. Deposition was realized with a KrF\* laser, Coherent ComplexPro 205 characterised by λ=248 nm, τFWHM~25 ns, repeti‐ tion rate=10 Hz [47]. The incident laser energy absorbed by the solvent molecules is convert‐ ed into thermal energy determining the heating and simultaneous evaporation of the two components. The solvent molecules are pumped away by the vacuum pump that maintains a pressure of 10-2-10-1 mbar in the deposition chamber, while the less volatile molecules of the organic compound deposit on the substrate maintained at room temperature. The low value fluence varied between 160 mJ/cm<sup>2</sup> and 430 mJ/cm<sup>2</sup> to avoid the deterioration of the organic molecule and the number of pulses between 10000 and 120000, with effect on the films' thickness (40 nm-150 nm), which has been evaluated by ellipsometry.

#### **3.2. Optical properties of aromatic derivatives thin films**

**3.1. Preparation of the aromatic derivatives thin films**

310 Optoelectronics - Advanced Materials and Devices

compounds characterized by a melting point Tm<100 °

didn't change during the melting-solidification cycles.

ments from organic ingots of pure and doped m-DNB (Tm=89.9 o

tion, ΔT>50 °

PTCDA [40], 150-160 o

*3.1.1. Aromatic derivatives thin films preparation by directional solidification process*

It is not very easy to obtain organic thin films because of the same difficulties which affect the preparation of organic crystals and the quality of the organic layer is strongly influenced by the method, which have been selected to grow the film. For example there is a high con‐ centration of structural defects in the benzil thin films which have been grown by a rapid directional solidification process, characterised by a non rigourous control of the thermal re‐ gime compared to the crystal growth process and these defects have caused the red shift of the emission peak. It is very difficult to grow, by vacuum evaporation, thin films of organic

cause the heating of the substrate during the evaporation process can favor a strong desorp‐ tion of the organic molecules from the substrate. By the directional solidification process could be prepared organic thin films between two substrates, like quartz or glass substrates, during a rapid thermal solidification characterized by a temperature gradient for solidifica‐

grown by Bridgman-Stockbarger method presented in paragraph 2.1, have been melted be‐ tween the substrates by the hot plate technique and after that rapidly frozen by the cold plate technique obtaining films with a columnar structure with large dendrites branches in the plane of the film determined by the low thermal conductivity and anisotropy of these organic compounds. The thickness of the films has been evaluated (using the density in sol‐ id state) from geometrical considerations presuming that the total volume of the substance

*3.1.2. Aromatic derivatives thin films and heterostructures preparation by vacuum evaporation*

(glass/ITO, quartz) and with acetone, hydrofluoric acid and distillate water (Si).

C for Alq3 [40] and 175-185 o

*3.1.3. Aromatic derivatives thin films an heterostructures preparation by MAPLE*

Thin films of PTCDA, Alq3 and TPyP have been prepared by vacuum evaporation and dep‐ osition on different substrates (glass/ITO, quartz, Si), which have been cleaned in acetone

Stable, homogeneous organic films, with good adhesion to the substrates have been pre‐ pared, by the evaporation of the organic powder contained in the quartz crucible heated by a self-sustaining kanthal winding, in an Alcatel system with turbo molecular pump [40; 46]. During the deposition with a duration between 10-15 min, the temperature was measured by a thermocouple situated at the bottom of the crucible and varied between 220-240 o

A special type of Pulsed laser Deposition (PLD) technique, Matrix Assisted Pulsed Laser Evaporation (MAPLE), has been used for the deposition of small molecule organic films (PTCDA, ZnPc, Alq3). This technique involves the ablation of a target formed by the frozen solution of the organic compound in a high molecular weight and strong laser wavelength

C for TPyP [46].

C, necessary to counteract the supercooling phenomenon [45]. Crystalline frag‐

C (including benzil and m-DNB) be‐

C) or benzil (Tm=95 o

C),

C for

The absorption spectrum of m-DNB, presented in Figure 20, which is similar to a classical semiconductor, could be correlated with the strong interactions between the polar molecules and with the partial superposition of the π-electrons clouds from neighbour molecules gen‐ erating narrower valence and conduction bands. This spectrum is different from that of ben‐ zil presented in Figure 19, which is characterized by a two edges of the fundamental absorption, with a subband light absorption peak situated at 380 nm, attributed to absorp‐ tion by dicarbonyl groups, strongly interacting in the solid state and producing the split of the energetic level (n, π\*).

**Figure 19.** Absorption spectra of benzil film grown between two quartz plates [45].

The effect of the impurities on the shape and position of the absorption peak in benzil situat‐ ed at 3.25 eV and assigned to dicarbonyl group absorption is not important and no other ab‐ sorption peaks have been evidenced in the longer wavelength range to sustain the trapping of the excitation energy by theses impurities. Therefore we can conclude that the energetic levels of these impurities are not significantly lower than the lowest energetic level which

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characterizes the crystalline assembly [21].

**Figure 22.** Fitting of the experimental data for pure m-DNB film [45].

**Figure 23.** Fitting of the experimental data for pure benzil film in the range 2.6 eV-3.1 eV [45].

**Figure 20.** Absorption spectra of m-DNB film grown between two quartz plates [45].

From Figure 21 it can be emphasized that the shape of the fundamental absorption edge is not affected by the presence of impurities. No important changes have been evidenced at the absorption edge characterized by a lower energetic threshold. But the absorption at the edge characterized by the higher energetic threshold is attenuated in benzil doped with m-DNB or sodium compare to pure benzil because of the light scattering process on the nonhomoge‐ neities of the films. This effect is stronger in benzil doped with Na because it is not com‐ pletely dissolved, segregates and generates microinclusions as a distinct phase.

**Figure 21.** Comparative absorption spectra of: pure benzil (a); benzil doped with m-DNB (3 wt %) (b); benzil doped with Na (6 wt %) (c): grown between two quartz plates [45].

The effect of the impurities on the shape and position of the absorption peak in benzil situat‐ ed at 3.25 eV and assigned to dicarbonyl group absorption is not important and no other ab‐ sorption peaks have been evidenced in the longer wavelength range to sustain the trapping of the excitation energy by theses impurities. Therefore we can conclude that the energetic levels of these impurities are not significantly lower than the lowest energetic level which characterizes the crystalline assembly [21].

**Figure 22.** Fitting of the experimental data for pure m-DNB film [45].

**Figure 20.** Absorption spectra of m-DNB film grown between two quartz plates [45].

312 Optoelectronics - Advanced Materials and Devices

From Figure 21 it can be emphasized that the shape of the fundamental absorption edge is not affected by the presence of impurities. No important changes have been evidenced at the absorption edge characterized by a lower energetic threshold. But the absorption at the edge characterized by the higher energetic threshold is attenuated in benzil doped with m-DNB or sodium compare to pure benzil because of the light scattering process on the nonhomoge‐ neities of the films. This effect is stronger in benzil doped with Na because it is not com‐

**Figure 21.** Comparative absorption spectra of: pure benzil (a); benzil doped with m-DNB (3 wt %) (b); benzil doped

with Na (6 wt %) (c): grown between two quartz plates [45].

pletely dissolved, segregates and generates microinclusions as a distinct phase.

**Figure 23.** Fitting of the experimental data for pure benzil film in the range 2.6 eV-3.1 eV [45].

benzil, using the formula (4) and we have obtained Eg=2.90 eV for m-DNB and Eg1=2.79 eV

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Supplementary information about the optical properties of theses aromatic derivatives in solid state have been obtained from the luminescence measurements. At excitation with λ=300 nm, the emission spectra of pure m-DNB present a peak situated at 2.92 eV with a shoulder at 2.63 eV, as it is shown in Figure 25. The apparent red shift of the emission peak situated at 2.92 eV to 2.78 eV in thicker film is due to a self-absorption process of the emitted radiation and not to a recombination on the energetic levels associated with physical defects

Luminescence spectra of benzil films presented in Figure 26 show a peak at 2.30 eV attribut‐ ed also to the radiative decay from the excited triplet state (T1) to the ground state (S0), tran‐ sition possible because of the vibrational interactions as was mentioned for bulk samples in

**Figure 26.** Emission spectra of pure and doped benzil films: (1) benzil doped with Na (1 wt %); (2) benzil doped with m-DNB (3 wt %); (3) benzil doped with Ag (2.4 wt %); (4) benzil pure; (5) benzil doped with Na (6 wt %) [21].

The excited molecules of benzil are more sensitive to a radiationless process and the con‐ version from the first singlet excited state to the lowest triplet excited state becomes pos‐ sible by "intersystem crossing", transition between states with different multiplicity, very efficient in systems containing carbonyl groups, like benzil. This radiationless process is followed by a radiative decay through phosphorescence from (T1) to (S0). The intersys‐ tem crossing appears in benzil as a transition between (n, π\*) states of singlet and trip‐ let levels rather than between (π, π\*) states. Both absorption and emission transitions involve states localized on carbonyl groups, which emit only from planar configuration.

and Eg2=3.54 eV for the two absorption edges in benzil.

or impurities.

paragraph 2.2.

**Figure 24.** Fitting of the experimental data for pure benzil film in the range 3.49 eV-3.65 eV [45].

**Figure 25.** Emission spectra of pure m-DNB films of different thickness for λexcitation=300 nm [21].

The band gap energy has been evaluated from the experimental data fitting, near the funda‐ mental absorption edge, shown in Figure 22 for m-DNB and, Figure 23 and Figure 24 for benzil, using the formula (4) and we have obtained Eg=2.90 eV for m-DNB and Eg1=2.79 eV and Eg2=3.54 eV for the two absorption edges in benzil.

Supplementary information about the optical properties of theses aromatic derivatives in solid state have been obtained from the luminescence measurements. At excitation with λ=300 nm, the emission spectra of pure m-DNB present a peak situated at 2.92 eV with a shoulder at 2.63 eV, as it is shown in Figure 25. The apparent red shift of the emission peak situated at 2.92 eV to 2.78 eV in thicker film is due to a self-absorption process of the emitted radiation and not to a recombination on the energetic levels associated with physical defects or impurities.

Luminescence spectra of benzil films presented in Figure 26 show a peak at 2.30 eV attribut‐ ed also to the radiative decay from the excited triplet state (T1) to the ground state (S0), tran‐ sition possible because of the vibrational interactions as was mentioned for bulk samples in paragraph 2.2.

**Figure 24.** Fitting of the experimental data for pure benzil film in the range 3.49 eV-3.65 eV [45].

314 Optoelectronics - Advanced Materials and Devices

**Figure 25.** Emission spectra of pure m-DNB films of different thickness for λexcitation=300 nm [21].

The band gap energy has been evaluated from the experimental data fitting, near the funda‐ mental absorption edge, shown in Figure 22 for m-DNB and, Figure 23 and Figure 24 for

**Figure 26.** Emission spectra of pure and doped benzil films: (1) benzil doped with Na (1 wt %); (2) benzil doped with m-DNB (3 wt %); (3) benzil doped with Ag (2.4 wt %); (4) benzil pure; (5) benzil doped with Na (6 wt %) [21].

The excited molecules of benzil are more sensitive to a radiationless process and the con‐ version from the first singlet excited state to the lowest triplet excited state becomes pos‐ sible by "intersystem crossing", transition between states with different multiplicity, very efficient in systems containing carbonyl groups, like benzil. This radiationless process is followed by a radiative decay through phosphorescence from (T1) to (S0). The intersys‐ tem crossing appears in benzil as a transition between (n, π\*) states of singlet and trip‐ let levels rather than between (π, π\*) states. Both absorption and emission transitions involve states localized on carbonyl groups, which emit only from planar configuration. The behaviour of benzil molecule depends on the flexible conformation [48] because the torsion angle around the central bond C-C can change and the geometry of the mole‐ cule can change after excitation with light. In the ground state the benzil molecule has a skew configuration and can twist around the carbonyl-carbonyl bound with little interac‐ tion between the two benzyl halves of the molecule. This interaction becomes strong in excited molecule, which rearranges in a new configuration with a trans-planar dicarbon‐ yl system characterized by a redistribution of the energy followed by the process of "in‐ tersystem crossing". By phosphorescent emission the system passes from a trans-planar configuration to the ground state with also a trans-planar configuration considering the Frank-Condon principle [49]. In the next step the molecule relax from the emissive transplanar configuration to a skew configuration and the differences in the emission spectra of benzil can be determined by changes in the molecular conformation of the ground state [50]. For most of the dopants we have not remarked any modification in the shape and position of the emission peak that sustains no modification in the molecular conforma‐ tion of the (n, π\*) state with effect on the angle between the carbonyl groups. The only changes, a slightly blue shift and significant attenuation in intensity, have been re‐ marked in Figure 26 for benzil highly doped with Na (6 wt %). A possible charge trans‐ fer from Na atoms to oxygen atoms in carbonyl groups can generate conformational changes, and the shift could be explained by the decrease in the dihedral angle between the two carbonyl groups [21]. The only impurity active in emission is sodium at high concentration because of the conformational changes in the emitting triplet state, while m-DNB, Ag, Na are not active in absorption.

In Figure 27 and Figure 28, we have emphasized the band structure of the absorption spec‐ tra in PTCDA and Alq3 with peaks situated at 358 nm, 374 nm, 475 nm, 552 nm and 232 nm, 261 nm, 380 nm respectively. This confirms the presence of median isomer in Alq3 film. The position of the two important peaks situated at 2.25 eV and 2.61 eV remain unchanged in the absorption spectrum of PTCDA deposited on glass covered by ITO while the absorption spectrum of the heterostructure with double organic layer, glass/ITO/PTCDA/Alq3, pre‐ serves the pattern of the absorption spectrum of PTCDA between 400 nm and 600 nm, which is very important in the stage of the charge carrier's generation.

**Figure 27.** Absorption spectra of PTCDA thin films deposited by vacuum evaporation [40].

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In Figure 29 have been evidenced the presence of π-π\* absorption bands characteristic for free-base ethio type porphyrin. These bands are associated with π-π\* transition between bonding and anti-bonding molecular orbitals. Other bands which have been also evidenced are: an intense Soret band (B) with a peak centred at 430 nm and 4 Q bands situated at 520 nm, 555 nm, 590 nm and 645 nm.

The behaviour of benzil molecule depends on the flexible conformation [48] because the torsion angle around the central bond C-C can change and the geometry of the mole‐ cule can change after excitation with light. In the ground state the benzil molecule has a skew configuration and can twist around the carbonyl-carbonyl bound with little interac‐ tion between the two benzyl halves of the molecule. This interaction becomes strong in excited molecule, which rearranges in a new configuration with a trans-planar dicarbon‐ yl system characterized by a redistribution of the energy followed by the process of "in‐ tersystem crossing". By phosphorescent emission the system passes from a trans-planar configuration to the ground state with also a trans-planar configuration considering the Frank-Condon principle [49]. In the next step the molecule relax from the emissive transplanar configuration to a skew configuration and the differences in the emission spectra of benzil can be determined by changes in the molecular conformation of the ground state [50]. For most of the dopants we have not remarked any modification in the shape and position of the emission peak that sustains no modification in the molecular conforma‐ tion of the (n, π\*) state with effect on the angle between the carbonyl groups. The only changes, a slightly blue shift and significant attenuation in intensity, have been re‐ marked in Figure 26 for benzil highly doped with Na (6 wt %). A possible charge trans‐ fer from Na atoms to oxygen atoms in carbonyl groups can generate conformational changes, and the shift could be explained by the decrease in the dihedral angle between the two carbonyl groups [21]. The only impurity active in emission is sodium at high concentration because of the conformational changes in the emitting triplet state, while

In Figure 27 and Figure 28, we have emphasized the band structure of the absorption spec‐ tra in PTCDA and Alq3 with peaks situated at 358 nm, 374 nm, 475 nm, 552 nm and 232 nm, 261 nm, 380 nm respectively. This confirms the presence of median isomer in Alq3 film. The position of the two important peaks situated at 2.25 eV and 2.61 eV remain unchanged in the absorption spectrum of PTCDA deposited on glass covered by ITO while the absorption spectrum of the heterostructure with double organic layer, glass/ITO/PTCDA/Alq3, pre‐ serves the pattern of the absorption spectrum of PTCDA between 400 nm and 600 nm,

In Figure 29 have been evidenced the presence of π-π\* absorption bands characteristic for free-base ethio type porphyrin. These bands are associated with π-π\* transition between bonding and anti-bonding molecular orbitals. Other bands which have been also evidenced are: an intense Soret band (B) with a peak centred at 430 nm and 4 Q bands situated at 520

which is very important in the stage of the charge carrier's generation.

m-DNB, Ag, Na are not active in absorption.

316 Optoelectronics - Advanced Materials and Devices

nm, 555 nm, 590 nm and 645 nm.

**Figure 27.** Absorption spectra of PTCDA thin films deposited by vacuum evaporation [40].

**Figure 29.** Absorption spectra of TPyP thin films deposited by vacuum evaporation [46].

of the pyridyl group and the pyrrole moieties [53]).

The shape of the absorption spectra of TPyP thin films deposited on different substrates is preserved at λ> 430 nm. The slight red shift of the Soret band can be determined by the or‐ der induced by the interaction between the molecules in solid state influenced by the inter‐ action with the substrate [51]. A possible bonding mechanism can be based on the pyridylsurface interaction mediating the deformation of the molecule after adsorption on the substrate's surface. Subsequent packing of the molecules can be determined by the non-co‐ valent interactions mediated by the terminal pyridyl groups and these interactions seem to prevail over the site-specific adsorption [52]. During these intermolecular interactions the porphyrin core can be deformed and the symmetry of the TPyP molecule modified because the conformation of this molecule is defined by several degree of freedom (dihedral angle in correlation with the rotation of the pyridyl group about C-C bond, inclination angle of the same bond and distortion angle determined by the steric repulsion between hydrogen atoms

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The spectra in Figure 30 have revealed a wide absorption band situated between 400 nm and 600 nm in PTCDA with a maximum at 480 nm and a shoulder at 550 nm, this shape being determined by the interactions of the π-electrons system of the neighbour planar mol‐ ecules very closed packed in solid state [21; 54]. The excited states can be the result of the superposition of the intramolecular Frenkel excitons and intermolecular charge transfer ex‐ citons existing near the excitation threshold [55]. The UV-VIS spectra of Alq3 confirm that the low temperature isomer (median) correlated with the presence of the weak absorption band situated at 380 nm, dominates in the films deposited at room temperature [56]. For ZnPc we have evidenced two absorption peaks situated at 690 nm, a strong band corre‐ sponding to Q band and a weak band situated at 330 nm corresponding to B band [57].

**Figure 28.** Absorption spectra of Alq3 thin films deposited by vacuum evaporation [40].

**Figure 29.** Absorption spectra of TPyP thin films deposited by vacuum evaporation [46].

**Figure 28.** Absorption spectra of Alq3 thin films deposited by vacuum evaporation [40].

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The shape of the absorption spectra of TPyP thin films deposited on different substrates is preserved at λ> 430 nm. The slight red shift of the Soret band can be determined by the or‐ der induced by the interaction between the molecules in solid state influenced by the inter‐ action with the substrate [51]. A possible bonding mechanism can be based on the pyridylsurface interaction mediating the deformation of the molecule after adsorption on the substrate's surface. Subsequent packing of the molecules can be determined by the non-co‐ valent interactions mediated by the terminal pyridyl groups and these interactions seem to prevail over the site-specific adsorption [52]. During these intermolecular interactions the porphyrin core can be deformed and the symmetry of the TPyP molecule modified because the conformation of this molecule is defined by several degree of freedom (dihedral angle in correlation with the rotation of the pyridyl group about C-C bond, inclination angle of the same bond and distortion angle determined by the steric repulsion between hydrogen atoms of the pyridyl group and the pyrrole moieties [53]).

The spectra in Figure 30 have revealed a wide absorption band situated between 400 nm and 600 nm in PTCDA with a maximum at 480 nm and a shoulder at 550 nm, this shape being determined by the interactions of the π-electrons system of the neighbour planar mol‐ ecules very closed packed in solid state [21; 54]. The excited states can be the result of the superposition of the intramolecular Frenkel excitons and intermolecular charge transfer ex‐ citons existing near the excitation threshold [55]. The UV-VIS spectra of Alq3 confirm that the low temperature isomer (median) correlated with the presence of the weak absorption band situated at 380 nm, dominates in the films deposited at room temperature [56]. For ZnPc we have evidenced two absorption peaks situated at 690 nm, a strong band corre‐ sponding to Q band and a weak band situated at 330 nm corresponding to B band [57]. Modifications in the deposition parameters (target concentration, fluence and number of pulses) are reflected in the thickness of the layer and not in the shape of the transmission.

**Figure 30.** UV-VIS spectra of organic thin films deposited by MAPLE: PTCDA on quartz (1 and 2); PTCDA on ITO (7); Alq3 on quartz (3 and 4); ZnPc on quartz (5 and 6) [54].

**Figure 32.** Emission spectra of Alq3 film deposited by vacuum evaporation on glass for two excitation wavelength

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The fluorescence emission spectrum of PTCDA film presented in Figure 31 shows a broad structureless band shifted significantly to the red compared to the PTCDA fluorescence spectrum in solution as a consequence of a strong interaction between the organic molecules in the solid state favoured by the close spacing and important overlap of the molecular planes. The emission takes place in PTCDA from the lowest excited singlet state (S1) by the relaxation of the electron transferred by the light absorption on an antibonding π orbital [40]. The preservation of the emission peak situated at λ=680 nm for two different excitation wavelengths sustains the compositional homogeneity of the film. The luminescence in Alq3, presented in Figure 32, is generated by excitations localized on individual molecule with op‐ tical properties independent of molecular environment [58]. The presence of two isomers with different spatial configurations is sustained by the generation of different emission peaks for different excitation wavelengths. The significant Stocks shift in the both spectra of PTCDA (ΔE=0.40 eV) and Alq3 (ΔE=0.9 eV) between the peaks of the lowest level absorption and highest fluorescence emission level, and large Frank-Condon shift (0.40-2.3 eV) meas‐ ured peak to peak between the absorption and emission spectra, can be correlated with ef‐ fects determined by the solid state structure and with important conformational differences

between the ground state and the excited state [40].

[40].

**Figure 31.** Emission spectra of PTCDA film deposited by vacuum evaporation on glass for two excitation wave‐ lengths [40].

Modifications in the deposition parameters (target concentration, fluence and number of

**Figure 30.** UV-VIS spectra of organic thin films deposited by MAPLE: PTCDA on quartz (1 and 2); PTCDA on ITO (7);

**Figure 31.** Emission spectra of PTCDA film deposited by vacuum evaporation on glass for two excitation wave‐

Alq3 on quartz (3 and 4); ZnPc on quartz (5 and 6) [54].

320 Optoelectronics - Advanced Materials and Devices

lengths [40].

pulses) are reflected in the thickness of the layer and not in the shape of the transmission.

**Figure 32.** Emission spectra of Alq3 film deposited by vacuum evaporation on glass for two excitation wavelength [40].

The fluorescence emission spectrum of PTCDA film presented in Figure 31 shows a broad structureless band shifted significantly to the red compared to the PTCDA fluorescence spectrum in solution as a consequence of a strong interaction between the organic molecules in the solid state favoured by the close spacing and important overlap of the molecular planes. The emission takes place in PTCDA from the lowest excited singlet state (S1) by the relaxation of the electron transferred by the light absorption on an antibonding π orbital [40]. The preservation of the emission peak situated at λ=680 nm for two different excitation wavelengths sustains the compositional homogeneity of the film. The luminescence in Alq3, presented in Figure 32, is generated by excitations localized on individual molecule with op‐ tical properties independent of molecular environment [58]. The presence of two isomers with different spatial configurations is sustained by the generation of different emission peaks for different excitation wavelengths. The significant Stocks shift in the both spectra of PTCDA (ΔE=0.40 eV) and Alq3 (ΔE=0.9 eV) between the peaks of the lowest level absorption and highest fluorescence emission level, and large Frank-Condon shift (0.40-2.3 eV) meas‐ ured peak to peak between the absorption and emission spectra, can be correlated with ef‐ fects determined by the solid state structure and with important conformational differences between the ground state and the excited state [40].

The photoluminescence investigations have evidenced the preservation of the chemical structure of the compounds (PTCDA, Alq3, ZnPc) during the deposition by MAPLE, be‐ cause we have identified the characteristic emission peaks corresponding to each com‐ pound, as can be seen in Figure 34. The emission peak situated at 500 nm, in PTCDA deposited on Si, is associated with monomer-like species and that situated at 650 nm can be associated with two excimeric states [54]. The emission peak in Alq3 film deposited on Si is situated at 520 nm being associated with the excitation of median isomer dominant at the deposition temperature. The emission peak in ZnPc film deposited on Si is situated between 650 nm and 750 nm and can be associated with the deexcitation from the first excited singlet state with an energy of 1.8 eV [54]. In the heterostructures with double organic layer the

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weak emission of ZnPc is masked by the stronger emission of PTCDA and Alq3.

A material could become interesting for optoelectronic application if it is adequate from the point of view of both optical and electrical properties (such as good contact inject and trans‐ port the charge carriers). A good injection of the charge carrier was evidenced at the ITO/m-DNB contact compared to ITO/benzil contact as can be shown in Figure 35. This can be explain by the difference in the energetic contact barrier, which is higher between ITO and benzil than ITO and m-DNB, as a consequence of the position of the highest occupied mo‐ lecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in these aromatic

**Figure 35.** I-V characteristics of the benzil and m-DNB based heterostructures with Si(p) anode and different cathode (Cu, ITO, Al): (a) Cu for benzil ; (b) ITO for benzil; (c) Cu for m-DNB; (d) ITO for m-DNB, (e) Al for m-DNB [61]. For con‐

**3.3. Electrical properties of aromatic derivatives thin films**

derivative compounds.

tacting we have used very high purity In.

**Figure 33.** Photoluminescence spectra of TPyP films deposited by vacuum evaporation on different types of Si sub‐ strates [46].

**Figure 34.** Photoluminescence spectra of organic thin films deposited by MAPLE for two excitation wavelengths: (a) PTCDA on Si (1 and 2), PTCDA on ITO/ZnPc (4); (b) Alq3 on Si (1 and 3), Alq3 on ITO/ZnPc (5); (c) ZnPc on Si (1) [54].

The photoluminescence spectra of TPyP films deposited on Si by vacuum evaporation have revealed in Figure 33 two emission bands situated at 660 nm and 700 nm associated to Q bands and corresponding to free-base ethio type porphyrin [46].

The photoluminescence investigations have evidenced the preservation of the chemical structure of the compounds (PTCDA, Alq3, ZnPc) during the deposition by MAPLE, be‐ cause we have identified the characteristic emission peaks corresponding to each com‐ pound, as can be seen in Figure 34. The emission peak situated at 500 nm, in PTCDA deposited on Si, is associated with monomer-like species and that situated at 650 nm can be associated with two excimeric states [54]. The emission peak in Alq3 film deposited on Si is situated at 520 nm being associated with the excitation of median isomer dominant at the deposition temperature. The emission peak in ZnPc film deposited on Si is situated between 650 nm and 750 nm and can be associated with the deexcitation from the first excited singlet state with an energy of 1.8 eV [54]. In the heterostructures with double organic layer the weak emission of ZnPc is masked by the stronger emission of PTCDA and Alq3.

#### **3.3. Electrical properties of aromatic derivatives thin films**

**Figure 33.** Photoluminescence spectra of TPyP films deposited by vacuum evaporation on different types of Si sub‐

**Figure 34.** Photoluminescence spectra of organic thin films deposited by MAPLE for two excitation wavelengths: (a) PTCDA on Si (1 and 2), PTCDA on ITO/ZnPc (4); (b) Alq3 on Si (1 and 3), Alq3 on ITO/ZnPc (5); (c) ZnPc on Si (1) [54].

The photoluminescence spectra of TPyP films deposited on Si by vacuum evaporation have revealed in Figure 33 two emission bands situated at 660 nm and 700 nm associated to Q

bands and corresponding to free-base ethio type porphyrin [46].

strates [46].

322 Optoelectronics - Advanced Materials and Devices

A material could become interesting for optoelectronic application if it is adequate from the point of view of both optical and electrical properties (such as good contact inject and trans‐ port the charge carriers). A good injection of the charge carrier was evidenced at the ITO/m-DNB contact compared to ITO/benzil contact as can be shown in Figure 35. This can be explain by the difference in the energetic contact barrier, which is higher between ITO and benzil than ITO and m-DNB, as a consequence of the position of the highest occupied mo‐ lecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in these aromatic derivative compounds.

**Figure 35.** I-V characteristics of the benzil and m-DNB based heterostructures with Si(p) anode and different cathode (Cu, ITO, Al): (a) Cu for benzil ; (b) ITO for benzil; (c) Cu for m-DNB; (d) ITO for m-DNB, (e) Al for m-DNB [61]. For con‐ tacting we have used very high purity In.

For the sample based on m-DNB two different regions were identified on the I-V character‐ istic: the ohmic behaviour region at low voltages and a region with a behaviour associated to the space charge limited effect at voltages > 5 V. Both in Figure 35 and Figure 36 can be seen that at voltages > 5 V the effect of the space charge limitation of the current becomes important in the heterostructure ITO/m-DNB/Si(p). The steep increase in the current at volt‐ age ~ 10 V for Al/m-DNB/Si(p) and ITO/m-DNB/Si(p) can be associated with an avalanche generation mechanism involving energetic states situated in the band gap, in the interface region, state generated by the easy diffusion of Al in organic layer favoured by the first ioni‐ zation potential of Al (5.98 eV).

**Figure 37.** I-V characteristics for ITO/TPyP/Si heterostructures [46].

er is too high and can't be surpass by the charge carriers [46].

**Figure 38.** I-V characteristic of the Au/TPyP/Si heterostructure [46].

The heterostructure Au/TPyP/Si, at an illumination through the metallic electrode and direct polarization, shows a rectifier behaviour presented in Figure 38, determined by the energetic barrier at the contact Au/TPyP, which can be lowered applying a voltage > 0.30 eV. The line‐ ar behaviour at low applied voltages became a power dependence with n>2 at voltages >0.1 V and corresponds to trap charge limited current. At reverse bias, the same heterostructure shows a blocking behaviour independent of the applied voltage, because the energetic barri‐

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**Figure 36.** I-V characteristics for ITO/m-DNB/Si(p) heterostructures for different type ITO contacts. For contacting we have used very high purity In [61].

The different ITO/m-DNB/Si heterostructures have shown significantly different shapes of the I-V characteristics as a consequence of the crystalline quality of the organic layer, in cor‐ relation with the preparation method (rapid thermal directional solidification).

A blocking diode behaviour, both at direct and reverse bias, has been emphasized in Figure 37 at low applied voltages in ITO/TPyP/Si heterostructures, independent of the type of con‐ duction of the Si electrode. For these heterostructures no photoelectric effect has been evi‐ denced. These I-V characteristics are quasi-linear at low voltages and at higher voltages the limitation of the current determined by the space charge and/or by trap-charge became very important as can be seen in Figure 37.

**Figure 37.** I-V characteristics for ITO/TPyP/Si heterostructures [46].

For the sample based on m-DNB two different regions were identified on the I-V character‐ istic: the ohmic behaviour region at low voltages and a region with a behaviour associated to the space charge limited effect at voltages > 5 V. Both in Figure 35 and Figure 36 can be seen that at voltages > 5 V the effect of the space charge limitation of the current becomes important in the heterostructure ITO/m-DNB/Si(p). The steep increase in the current at volt‐ age ~ 10 V for Al/m-DNB/Si(p) and ITO/m-DNB/Si(p) can be associated with an avalanche generation mechanism involving energetic states situated in the band gap, in the interface region, state generated by the easy diffusion of Al in organic layer favoured by the first ioni‐

**Figure 36.** I-V characteristics for ITO/m-DNB/Si(p) heterostructures for different type ITO contacts. For contacting we

The different ITO/m-DNB/Si heterostructures have shown significantly different shapes of the I-V characteristics as a consequence of the crystalline quality of the organic layer, in cor‐

A blocking diode behaviour, both at direct and reverse bias, has been emphasized in Figure 37 at low applied voltages in ITO/TPyP/Si heterostructures, independent of the type of con‐ duction of the Si electrode. For these heterostructures no photoelectric effect has been evi‐ denced. These I-V characteristics are quasi-linear at low voltages and at higher voltages the limitation of the current determined by the space charge and/or by trap-charge became very

relation with the preparation method (rapid thermal directional solidification).

zation potential of Al (5.98 eV).

324 Optoelectronics - Advanced Materials and Devices

have used very high purity In [61].

important as can be seen in Figure 37.

The heterostructure Au/TPyP/Si, at an illumination through the metallic electrode and direct polarization, shows a rectifier behaviour presented in Figure 38, determined by the energetic barrier at the contact Au/TPyP, which can be lowered applying a voltage > 0.30 eV. The line‐ ar behaviour at low applied voltages became a power dependence with n>2 at voltages >0.1 V and corresponds to trap charge limited current. At reverse bias, the same heterostructure shows a blocking behaviour independent of the applied voltage, because the energetic barri‐ er is too high and can't be surpass by the charge carriers [46].

**Figure 38.** I-V characteristic of the Au/TPyP/Si heterostructure [46].

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

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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].

purpose to identify the most adequate conditions for the deposition of layers [64].

so intense in the solid state to favour the splitting of the (n, π\*) energetic level.

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

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

with the quality of the spin-coated layers.

**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 dark [54].

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/ PTCDA/Cu heterostructure.
