**2. Bulk aromatic derivatives for optoelectronic applications**

The research was focalised on different topics such as: effect of doping of the organic semi‐ conductor to increase the "transparency" of the energetic barrier to the injection of electron from the contact [6], influence of the trapped and interfacial charges generated in multilayer organic heterostructures on the properties of the device [7], charge tunnelling in multilayer stack and at the interface between organic and anode [8], influence of the thickness and dop‐ ing of the emission layer on the properties of OLEDs [9], injection of the charge carriers from the electrodes and their migration in correlation with different types of cathodes [10-13], transport phenomena in organics [14; 15], stacked geometry for efficient double-sided emit‐ ting OLED [16], graded mixed layer as active layer to replace heterojunction in OLEDs [17]. The awarding in 2000 of the Nobel prize for researches in the field of conducting polymers has stimulated the development of OLEDs based on polymeric materials, application em‐ phasised years before [18], opening the way of reducing the applied voltages (< 10 V) and

The development of the technologies in the field of the structures for optoelectronic applica‐ tions based on organic compounds is dependent of the development of fundamental and ap‐ plied knowledge of all the optical and electrical processes involved, because many particularities of the organic solid state are not yet well known and understand. This is a re‐ al challenge because the number of the organic luminescent compounds is much larger than the number of inorganic compounds and it is continuously increasing. To increase the quan‐ tum efficiency, lifetime and thermal stability of these devices requires the separate optimisa‐ tion of the generation, injection and transport of the charge carriers and their controlled recombination in different layers. The electrical properties of the OLED are controlled by the mobility of the charge carriers and the heights of the barriers [19], whereas the optical prop‐ erties by the refractive index mismatches at the glass/air and organic/ITO interfaces that generate the trapping of a large fraction of the light by the mechanism of total internal re‐

Therefore the organic materials must be designed and selected in such a way to show spe‐ cial properties to satisfy these requirements. Organic luminescent materials can be divided considering their molecular structure and the macro scale organization. From the first point of view there are low-molecular compounds characterised by the possibility of high purifi‐ cation, easy vacuum deposition, high quantum yield fluorescence and large variety and high-molecular compounds (oligomers and polymers) characterised by mechanical strength, flexibility and luminescence over various spectral regions from near UV to near IR but, by small quantum yield of fluorescence. From the second point of view, macro scale organiza‐ tion, there are bulk crystalline organic and organic thin films and heterostructures to be

A special attention will be paid in this chapter to investigate the properties of bulk and thin films organic compounds showing both good optical, including luminescent, and transport

increasing the brightness and lifetime.

292 Optoelectronics - Advanced Materials and Devices

flection into glass and ITO [20].

used in devices' fabrication.

properties for potential optoelectronic applications.

Organic luminescent solids are attracting increasing interest in various field of application from optoelectronics to photonics. The interest in studying organic crystals is justified by the perspective to use these materials as a crystalline host matrix both for organic and inorganic guests (dopants) for developing new classes of materials combining the advantageous prop‐ erties of both components host and guest. The organic matrix can assure an efficient fluores‐ cence mechanism, can assure simple methods for processing and can contribute to electrical transport. On the other hand, the dopants could increase the charge carrier mobility and im‐ prove the emission properties and thermal stability of the organic.

Organic molecules containing electrons occupying nonlocalised molecular orbitals and strongly conjugated systems such as aromatic compounds, dyes, show important lumines‐ cence in solid state. This radiative emission involves transitions inside very well shielded systems of π-electrons. By light absorption, an electron is transferred to an antibonding π orbital on the lowest singlet excited state with a lifetime of 10-6-10-9 s, from which it decays by fluorescence emission.

The perspective to tailor the specific physical properties of a molecular solid by guest parti‐ cles (dopant) embedded in the crystalline organic matrix is very attractive, but not so acces‐ sible because some complications can appear both from the crystalline structure and/or dopant sites.

Special research has been devoted to the growth of organic crystals doped with rare earth metallic ions to prepare materials for luminescent and laser applications and benzil doped with Cd2+. The properties of the host/guest systems based on organic crystals depend on the crystalline perfection and chemical defects.

Growth of large and structural good organic crystals at good ratio cost/properties is very im‐ portant for theoretical understanding of the phenomena taking place in organic solid state and development of new organic-organic, organic-inorganic materials for a target applica‐ tion. The main limitations in large-scale using of aromatic derivatives as crystalline matrix are correlated with the requirements for crystals growth, which involves identification of particularised solutions to overpass the low melting point, supercooling and low thermal conductivity of organic compounds.

Substituted aromatic molecules are a class of organic materials containing weakly coupled, strongly polarisable delocalised π electrons. Concerning the bulk organic crystals, our inter‐ est was focalised on aromatic derivatives that contain one and two aromatic rings and sub‐ stituent groups which disturb the symmetry of the π-electrons cloud, such as metadinitrobenzene (m-DNB)/ C6H4N2O4 and benzil/ C6H5-CO-CO-C6H5, characterised by large transparency domain and good fluorescence emission.

**Figure 1.** Benzil (a) and m-DNB (b) molecular structure [21].

Benzil with the molecular structure presented in Figure 1a is an uniaxial crystal that belongs to the space-group *D*<sup>3</sup> 4 or *D*<sup>3</sup> 6 and it is known as "organic quartz" being isomorphic with α quartz. By similarity of the microstructures developed in quartz through the diffusion of metal atoms could be of great interest to study benzil as matrix for composite materials and the effect of dopant atoms on the matrix properties.

Meta-dinitrobenzene with the molecular structure presented in Figure 1b is a negative biax‐ ial crystal that belongs to the point group symmetry mm2 and space group Pbn21.

We have developed some investigations on the effect of dopant on the emission properties (shape of the spectra, position of the peaks) of the solid-state aromatic compounds by compar‐ ison with the emission properties of the pure organic matrix. We have also evidenced the differences between the influence of the inorganic dopant (iodine, sodium, silver) and/or organic dopant (m-DNB, naphthalene) on the luminescence of bulk m-DNB and benzil samples.

#### **2.1. Aromatic derivatives crystal growth**

The source materials used in crystal growth must be of high purity and the purification of organic compounds is a very long process. m-DNB was purified by three methods: chemical purification, vacuum distillation and two steps directional freezing in a horizontal configu‐ ration: length of the melted zone=2-3 cm, average travelling speed=2.5 cm/h [22; 23].

**Figure 2.** Experimental set-up for the growth of m-DNB crystals and the corresponding thermal profile [23].

cm and average moving speed of the ampoule in the furnace of 1-1.5 mm/h; 1 mm/h are very important for the crystal growth process because are determining the shape of the solid-liquid interface and position of the growth interface with effect on the properties of

A similar configuration, presented in Figure 3 has been used for the growth of the benzil crystals. Some differences result from the fact that benzil is characterised by a weaker adhe‐ sion to the quartz wall than m-DNB (the use of a Teflon crucible being not necessary in this case) and from the necessity to assure the control of the nucleation and solidification proc‐ esses in a configuration without crucible by the use of a conical shape of the ampoule tip

C/cm; 8.5-9 °

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C/

The two parameters, thermal gradient at the melt-crystal interface of 4.5-5 °

the crystals.

with a narrower zone [24].

Some factors have contributed in the selection of Bridgman-Stockbarger method in vertical configuration to grow m-DNB crystals: low melting point, low vapour pressure and no decom‐ position at the melting temperature. The supercooling tendency of the organic compound was counteracted in a special design system with two zones (a hot zone: 110-115 °C generated in a furnace that assures the melting of the charge and a cold zone: 50 °C generated by a thermo‐ stat) characterised by a steeper thermal gradient at the growth interface created by an oil bath. It is very important to correctly positioning the growth interface compared to the interface air/ oil. To allow the dissipation of the high melting heat in the organic material characterised by low thermal conductivity, the ampoule containing the crucible sealed under vacuum is moved slowly in the thermal field. The Teflon dismantle or undismantle crucible containing the organic compound powder has a special bottom configuration (a capillary tube with a diameter of 1 mm) to generate the nucleation and to favour the selection of the growth direction. Details about the experimental configuration are given in Figure 2.

**Figure 1.** Benzil (a) and m-DNB (b) molecular structure [21].

294 Optoelectronics - Advanced Materials and Devices

4 or *D*<sup>3</sup> 6

**2.1. Aromatic derivatives crystal growth**

about the experimental configuration are given in Figure 2.

the effect of dopant atoms on the matrix properties.

to the space-group *D*<sup>3</sup>

Benzil with the molecular structure presented in Figure 1a is an uniaxial crystal that belongs

quartz. By similarity of the microstructures developed in quartz through the diffusion of metal atoms could be of great interest to study benzil as matrix for composite materials and

Meta-dinitrobenzene with the molecular structure presented in Figure 1b is a negative biax‐

We have developed some investigations on the effect of dopant on the emission properties (shape of the spectra, position of the peaks) of the solid-state aromatic compounds by compar‐ ison with the emission properties of the pure organic matrix. We have also evidenced the differences between the influence of the inorganic dopant (iodine, sodium, silver) and/or organic dopant (m-DNB, naphthalene) on the luminescence of bulk m-DNB and benzil samples.

The source materials used in crystal growth must be of high purity and the purification of organic compounds is a very long process. m-DNB was purified by three methods: chemical purification, vacuum distillation and two steps directional freezing in a horizontal configu‐

Some factors have contributed in the selection of Bridgman-Stockbarger method in vertical configuration to grow m-DNB crystals: low melting point, low vapour pressure and no decom‐ position at the melting temperature. The supercooling tendency of the organic compound was counteracted in a special design system with two zones (a hot zone: 110-115 °C generated in a furnace that assures the melting of the charge and a cold zone: 50 °C generated by a thermo‐ stat) characterised by a steeper thermal gradient at the growth interface created by an oil bath. It is very important to correctly positioning the growth interface compared to the interface air/ oil. To allow the dissipation of the high melting heat in the organic material characterised by low thermal conductivity, the ampoule containing the crucible sealed under vacuum is moved slowly in the thermal field. The Teflon dismantle or undismantle crucible containing the organic compound powder has a special bottom configuration (a capillary tube with a diameter of 1 mm) to generate the nucleation and to favour the selection of the growth direction. Details

ration: length of the melted zone=2-3 cm, average travelling speed=2.5 cm/h [22; 23].

ial crystal that belongs to the point group symmetry mm2 and space group Pbn21.

and it is known as "organic quartz" being isomorphic with α

**Figure 2.** Experimental set-up for the growth of m-DNB crystals and the corresponding thermal profile [23].

The two parameters, thermal gradient at the melt-crystal interface of 4.5-5 ° C/cm; 8.5-9 ° C/ cm and average moving speed of the ampoule in the furnace of 1-1.5 mm/h; 1 mm/h are very important for the crystal growth process because are determining the shape of the solid-liquid interface and position of the growth interface with effect on the properties of the crystals.

A similar configuration, presented in Figure 3 has been used for the growth of the benzil crystals. Some differences result from the fact that benzil is characterised by a weaker adhe‐ sion to the quartz wall than m-DNB (the use of a Teflon crucible being not necessary in this case) and from the necessity to assure the control of the nucleation and solidification proc‐ esses in a configuration without crucible by the use of a conical shape of the ampoule tip with a narrower zone [24].

where ΔC=concentration gradient at interface; m=slope of the liquidus curve. *ΔCm*>0by con‐ vention, and for molecules rejected at the interface, that decrease the melting temperature,

The problem of the growth interface stability is very important because the growth interface has effect on the quality of the obtained crystals. Our benzil/dopant system was analysed us‐ ing the Mullin-Sekerka criterion [26-29], that fixes the limits of the stable growth and the conditions necessary to initiate instabilities in the growth system, and is defined by the fol‐

where V=ampoule moving speed; ρn=melted benzil density; km=melted benzil thermal con‐

gradient; ΔC=concentration gradient at the growth interface; m=slope of the liquidus curve.

In benzil/dopant system for the given experimental conditions, the stable and unstable growth zones were delimited by the curves *ΔC* ⋅*m*= *f* (|*ΔT* |) when V=constant or by the curves *ΔC* ⋅*m*= *f* (*V* ) for |*ΔT* |=constant. In the first case, as can be seen in Figure 4, at high concentration gradients (ΔC) the system moves through the unstable growth zone situated above the curve given by equation (3). For a given thermal gradient at the growth interface small variations in the interface moving speed have no significant influence on the area of the stable growth zone. The main consequences refer to an increase in the morphological in‐ stabilities and in crystal's homogeneity. In the second case presented in Figure 5, the area of the stable growth zone increases with the increase of the thermal gradient for a given mov‐ ing speed of the growth interface, the system remaining in the stable growth zone even for

For benzil crystals km>ks and as consequence the interface is more stable because the term

in equation (2) assures a large range of values situated in the stable

³

D × £ × - ×D *Cm V T* 10.0727 0.4382 (3)

+ × D +D × (2)

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H=solidification enthalpy; ΔT=thermal

( ( ) ) ( )( ) <sup>1</sup> *mf m s*

*V Hkk T k k T Cm* × ×D - - -D

*m s*

For the growth of pure benzil crystals the stability condition became:

r

ductivity; ks=thermal conductivity of benzil crystal; Δf

with ΔT<0 (Tfinal<Tinitial in the solidification process).

high concentration gradients at the interface.

(*km* + *ks*)

(*km* −*ks*) ⋅*ΔT*

growth zone.

m<0 [26].

lowing relation:

**Figure 3.** Experimental set-up for the growth of benzil crystals and the corresponding thermal profile [24].

A very important parameter in the process of crystals growth is the temperature, which has two counteracting actions:


In general, the organic compounds are characterised by a low thermal conductivity in solid phase and high values of the solidification enthalpies that must be liberated during the crys‐ tallization process. In the system matrix/solvent can be developed many flow cells leading to non-uniform distribution of the dopant in the matrix. Constitutional supercooling charac‐ terises the doped organic melt because the freezing front rejects the particles of dopant, which can accumulate in front of the moving solid-liquid interface, the equilibrium freezing temperature of the adjacent liquid is above the actual temperature and the gradient of the equilibrium temperature is:

$$
\Delta T\_e = \Delta C \cdot m \tag{1}
$$

where ΔC=concentration gradient at interface; m=slope of the liquidus curve. *ΔCm*>0by con‐ vention, and for molecules rejected at the interface, that decrease the melting temperature, m<0 [26].

The problem of the growth interface stability is very important because the growth interface has effect on the quality of the obtained crystals. Our benzil/dopant system was analysed us‐ ing the Mullin-Sekerka criterion [26-29], that fixes the limits of the stable growth and the conditions necessary to initiate instabilities in the growth system, and is defined by the fol‐ lowing relation:

$$\frac{\left(V \cdot \rho\_m \cdot \Delta\_f H - \left(k\_m - k\_s\right) - \Delta T\right)}{\left(k\_m + k\_s\right) \cdot \left(\Delta T + \Delta C \cdot m\right)} \ge 1\tag{2}$$

where V=ampoule moving speed; ρn=melted benzil density; km=melted benzil thermal con‐ ductivity; ks=thermal conductivity of benzil crystal; Δf H=solidification enthalpy; ΔT=thermal gradient; ΔC=concentration gradient at the growth interface; m=slope of the liquidus curve. For the growth of pure benzil crystals the stability condition became:

$$
\Delta C \cdot m \le 10.0727 \cdot V - 0.4382 \cdot \Delta T \tag{3}
$$

with ΔT<0 (Tfinal<Tinitial in the solidification process).

**Figure 3.** Experimental set-up for the growth of benzil crystals and the corresponding thermal profile [24].

two counteracting actions:

296 Optoelectronics - Advanced Materials and Devices

equilibrium temperature is:

cracks;

A very important parameter in the process of crystals growth is the temperature, which has

**1.** low thermal gradients at the growth interface are necessary to prevent the generation of mechanical defects, favoured by the accumulation of tensions inside the crystal, like

**2.** steep gradients are necessary at the same growth interface to counteract the supercool‐

In general, the organic compounds are characterised by a low thermal conductivity in solid phase and high values of the solidification enthalpies that must be liberated during the crys‐ tallization process. In the system matrix/solvent can be developed many flow cells leading to non-uniform distribution of the dopant in the matrix. Constitutional supercooling charac‐ terises the doped organic melt because the freezing front rejects the particles of dopant, which can accumulate in front of the moving solid-liquid interface, the equilibrium freezing temperature of the adjacent liquid is above the actual temperature and the gradient of the

D =D × *T Cm <sup>e</sup>* (1)

ing effect and the tendency to a facetted growth morphology [24; 25].

In benzil/dopant system for the given experimental conditions, the stable and unstable growth zones were delimited by the curves *ΔC* ⋅*m*= *f* (|*ΔT* |) when V=constant or by the curves *ΔC* ⋅*m*= *f* (*V* ) for |*ΔT* |=constant. In the first case, as can be seen in Figure 4, at high concentration gradients (ΔC) the system moves through the unstable growth zone situated above the curve given by equation (3). For a given thermal gradient at the growth interface small variations in the interface moving speed have no significant influence on the area of the stable growth zone. The main consequences refer to an increase in the morphological in‐ stabilities and in crystal's homogeneity. In the second case presented in Figure 5, the area of the stable growth zone increases with the increase of the thermal gradient for a given mov‐ ing speed of the growth interface, the system remaining in the stable growth zone even for high concentration gradients at the interface.

For benzil crystals km>ks and as consequence the interface is more stable because the term (*km* −*ks*) ⋅*ΔT* (*km* + *ks*) in equation (2) assures a large range of values situated in the stable growth zone.

The same parameters, thermal field and the interface moving speed are important in the en‐ gulfment or rejection of the dopant particles in the crystallisation front. Compositional varia‐ tions and growth micrononhomogeneities (named striations) appear because of the layer situated in front of the interface, which is enriched in foreign particles by rejection. The fac‐ tor which influences the incorporation of the dopant atoms/molecules in the matrix are: the shape, volume and intermolecular bonds of the dopants' molecules. A free space around 2.9 Ǻ has been evaluated considering the molecular structure and the geometry for both benzil and m-DNB [24]. The diameter of the dopant atoms favours the incorporation in interstitial positions and the incorporation is facilitated by the local deformation of the organic lattice

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Sodium atoms could be incorporated interstitially with difficulty because the atomic diame‐ ter is greater than the diameter of the free space in benzil solid state, while silver and iodine with a diameter smaller than the free space could be easily interstitially incorporated. In the case of metals characterised by a high first ionisation enthalpy is sustained the generation of

For sodium atoms the situation is a little bit more complicated because of the high reactivity of sodium [25]. The interaction between the organic molecules and the alkali and alkaliearth metal atoms is determined by a chemical reaction leading to an organometallic com‐ plex or a charge transfer generating anion-cation pairs. The first situation is characterised by a very low probability because the hydrogen in benzil has not a very strong acid character to be directly substituted by alkali metal atoms and the crystal growth in closed sealed systems under vacuum reduces the possibility of sodium oxides formation. The charge transfer in so‐ dium doped benzil crystals is caused by a nonbonding Van der Waals force present in all the organics and a dative bonding force corresponding to the situation in which two Na donor atoms could give each the outer 3s electron to two oxygen atoms from the carbonyl acceptor group in benzil. Because the ionisation energy for most alkali metal atoms is low (around 4-6 eV) the electron transfer from Na-guest to benzil-host is allowed without the formation

In the case of an organic dopant the situation is completely different and depends on the matrix. Because the organic molecules are big and can accommodate in the host lat‐ tice only substitutionally and not interstitially and must respect the condition for solubil‐ ity in solid phase and the criterion for the geometrical similarity between the molecule of the dopant and matrix [30; 31]. If there are geometrical differences, the substitution is less probable and the microinclusions of dopant can generate distortion of the lattice

The geometrical similarity is measured by the overlapping factor represented by the ra‐ tio between the unoverlapped and overlapped volume of the matrix and dopant [32]. This introduces a limitation of the doping level which can be allowed by each host/guest sys‐ tem. The volume of m-DNB, benzil and naphthalene molecules have been estimated sup‐ posing a spherical shape of the molecule and taking into account the length of the chemical bounds. Because the calculated unoccupied volume is much greater than the occupied volume, the possibility for m-DNB to replace benzil molecule and be included substitution‐

clusters, which can create difficulties in atoms incorporation (like iodine or silver).

characterised by weak Van der Waals forces [25; 24].

of a covalent bound [25].

and cracks.

**Figure 4.** Stable (bellow the curves) and unstable (above the curves) growth zones for the system benzil/dopant in Bridgman-Stockbarger configuration delimited by the curves:Δ*C* ⋅*m*= *f* ( |Δ*T* | ), V=constant [24].

These considerations are very important in choosing the parameters for a stable growth gen‐ erating homogeneous crystals. All the studied systems based on pure and doped benzil and m-DNB matrices are similar from the point of view of the solidus-liquidus interface stability criterion because the ratios |*ΔT* | / *V* are comparable [26; 24].

**Figure 5.** Stable (bellow the curves) and unstable (above the curves) growth zones for the system benzil/dopant in Bridgman-Stockbarger configuration delimited by the curves:Δ*C* ⋅*m*= *f* (*V* ), |Δ*T* | =*cons*tan*t*[24].

The same parameters, thermal field and the interface moving speed are important in the en‐ gulfment or rejection of the dopant particles in the crystallisation front. Compositional varia‐ tions and growth micrononhomogeneities (named striations) appear because of the layer situated in front of the interface, which is enriched in foreign particles by rejection. The fac‐ tor which influences the incorporation of the dopant atoms/molecules in the matrix are: the shape, volume and intermolecular bonds of the dopants' molecules. A free space around 2.9 Ǻ has been evaluated considering the molecular structure and the geometry for both benzil and m-DNB [24]. The diameter of the dopant atoms favours the incorporation in interstitial positions and the incorporation is facilitated by the local deformation of the organic lattice characterised by weak Van der Waals forces [25; 24].

Sodium atoms could be incorporated interstitially with difficulty because the atomic diame‐ ter is greater than the diameter of the free space in benzil solid state, while silver and iodine with a diameter smaller than the free space could be easily interstitially incorporated. In the case of metals characterised by a high first ionisation enthalpy is sustained the generation of clusters, which can create difficulties in atoms incorporation (like iodine or silver).

**Figure 4.** Stable (bellow the curves) and unstable (above the curves) growth zones for the system benzil/dopant in

These considerations are very important in choosing the parameters for a stable growth gen‐ erating homogeneous crystals. All the studied systems based on pure and doped benzil and m-DNB matrices are similar from the point of view of the solidus-liquidus interface stability

**Figure 5.** Stable (bellow the curves) and unstable (above the curves) growth zones for the system benzil/dopant in

Bridgman-Stockbarger configuration delimited by the curves:Δ*C* ⋅*m*= *f* (*V* ), |Δ*T* | =*cons*tan*t*[24].

Bridgman-Stockbarger configuration delimited by the curves:Δ*C* ⋅*m*= *f* ( |Δ*T* | ), V=constant [24].

criterion because the ratios |*ΔT* | / *V* are comparable [26; 24].

298 Optoelectronics - Advanced Materials and Devices

For sodium atoms the situation is a little bit more complicated because of the high reactivity of sodium [25]. The interaction between the organic molecules and the alkali and alkaliearth metal atoms is determined by a chemical reaction leading to an organometallic com‐ plex or a charge transfer generating anion-cation pairs. The first situation is characterised by a very low probability because the hydrogen in benzil has not a very strong acid character to be directly substituted by alkali metal atoms and the crystal growth in closed sealed systems under vacuum reduces the possibility of sodium oxides formation. The charge transfer in so‐ dium doped benzil crystals is caused by a nonbonding Van der Waals force present in all the organics and a dative bonding force corresponding to the situation in which two Na donor atoms could give each the outer 3s electron to two oxygen atoms from the carbonyl acceptor group in benzil. Because the ionisation energy for most alkali metal atoms is low (around 4-6 eV) the electron transfer from Na-guest to benzil-host is allowed without the formation of a covalent bound [25].

In the case of an organic dopant the situation is completely different and depends on the matrix. Because the organic molecules are big and can accommodate in the host lat‐ tice only substitutionally and not interstitially and must respect the condition for solubil‐ ity in solid phase and the criterion for the geometrical similarity between the molecule of the dopant and matrix [30; 31]. If there are geometrical differences, the substitution is less probable and the microinclusions of dopant can generate distortion of the lattice and cracks.

The geometrical similarity is measured by the overlapping factor represented by the ra‐ tio between the unoverlapped and overlapped volume of the matrix and dopant [32]. This introduces a limitation of the doping level which can be allowed by each host/guest sys‐ tem. The volume of m-DNB, benzil and naphthalene molecules have been estimated sup‐ posing a spherical shape of the molecule and taking into account the length of the chemical bounds. Because the calculated unoccupied volume is much greater than the occupied volume, the possibility for m-DNB to replace benzil molecule and be included substitution‐ ally in the lattice is very small. The m-DNB molecules, which are not completely dis‐ solved in benzil, segregate and generate microinclusions that favour the light scattering. The smaller geometrical differences between benzil and naphthalene generate weaker seg‐ regation effect.

#### **2.2. Optical properties of bulk aromatic derivatives**

The segregation effect of the dopant was investigated experimentally by UV-VIS measure‐ ments. The transmission of benzil doped with m-DNB sample is lower than the transmission of benzil doped with naphthalene sample (for the same thickness ~2 mm) as can be seen in Figure 6, suggesting a stronger segregation of m-DNB than naphthalene in benzil matrix. The segregation of iodine in benzil matrix with effect on the homogeneity, reflected in UV-VIS spectra, is stronger in the presence of another dopant (naphthalene or m-DNB) and less significant in the absence of any other organic dopant as presented in Figure 7.

**Figure 7.** The effect of dopant on the UV-VIS transmission spectra of m-DNB matrix [24].

and is attributed to the absorption on the dicarbonyl groups [33; 21].

The UV-VIS spectrum of pure benzil, presented in Figure 8, preserves the pattern by doping with Ag, which is not interacting with benzil molecules. As result, the fundamental absorp‐ tion edge is not affected and preserves the narrow peaks structure. The peak situated around 380 nm is correlated with some particularities of the benzil molecular configuration

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**Figure 8.** Comparison between the absorption spectra in bulk samples of pure benzil and benzil doped with Ag [25].

**Figure 6.** The effect of dopant on the UV-VIS transmission spectra of benzil matrix [24].

UV-VIS absorption spectra of pure benzil and benzil doped with Ag or Na, presented in Figure 8, Figure 9 and Figure 10, have specific shapes characterised by a narrow peaks struc‐ ture at wavelength < 450 nm.

**Figure 7.** The effect of dopant on the UV-VIS transmission spectra of m-DNB matrix [24].

ally in the lattice is very small. The m-DNB molecules, which are not completely dis‐ solved in benzil, segregate and generate microinclusions that favour the light scattering. The smaller geometrical differences between benzil and naphthalene generate weaker seg‐

The segregation effect of the dopant was investigated experimentally by UV-VIS measure‐ ments. The transmission of benzil doped with m-DNB sample is lower than the transmission of benzil doped with naphthalene sample (for the same thickness ~2 mm) as can be seen in Figure 6, suggesting a stronger segregation of m-DNB than naphthalene in benzil matrix. The segregation of iodine in benzil matrix with effect on the homogeneity, reflected in UV-VIS spectra, is stronger in the presence of another dopant (naphthalene or m-DNB) and less

significant in the absence of any other organic dopant as presented in Figure 7.

**Figure 6.** The effect of dopant on the UV-VIS transmission spectra of benzil matrix [24].

ture at wavelength < 450 nm.

UV-VIS absorption spectra of pure benzil and benzil doped with Ag or Na, presented in Figure 8, Figure 9 and Figure 10, have specific shapes characterised by a narrow peaks struc‐

regation effect.

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**2.2. Optical properties of bulk aromatic derivatives**

The UV-VIS spectrum of pure benzil, presented in Figure 8, preserves the pattern by doping with Ag, which is not interacting with benzil molecules. As result, the fundamental absorp‐ tion edge is not affected and preserves the narrow peaks structure. The peak situated around 380 nm is correlated with some particularities of the benzil molecular configuration and is attributed to the absorption on the dicarbonyl groups [33; 21].

**Figure 8.** Comparison between the absorption spectra in bulk samples of pure benzil and benzil doped with Ag [25].

From the transmission data near the fundamental absorption edge processed using a linearpower model characterised by a formula obtained by superimposing a linear function and a

> ( ) *d*

( ) *d g*

where: α=absorption coefficient; c=band gap energy, Eg, or edge of the fundamental absorp‐ tion, λg, respectively, d=coefficient that depends on the light absorption mechanism and (a +mE) or (a+mλ) respectively, define all the other parasitical processes, including scattering of light on the nonhomogeneities of the sample and affecting the band to band absorption

The light absorption process is characterised by the optical band gap, which in benzil has been evaluated at Eg=2.65 eV, emphasising the wide band gap semiconductor character of crystalline benzil. The narrowing of the optical band gap by introducing energetic levels in the band gap in pure crystal can be a consequence of physical defects or controlled doping

**Figure 11.** Fitting of the experimental data for bulk samples of pure and Na doped benzil [25].

ll

=+ - + *a b E c mE* (4)

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=+ - + *a bc m* (5)

*g*

a

a

that can generate chemical or structural defects.

power function [25]:

or

mechanism.

**Figure 9.** Comparison between the absorption spectra in bulk samples of pure benzil and benzil doped with Na [25].

At the contrary, doping with Na has introduced important changes in the shape of the funda‐ mental absorption edge in benzil, as can be observed in Figure 9, a large structured band replacing the narrow peaks. This can be explained by the light scattering on the nonhomoge‐ neities of the doped benzil or by changes in the forces acting between the host and guest [25].

**Figure 10.** Absorption spectra of bulk pure and Na doped benzil matrix. Detail between 375 nm and 435 nm [25].

From the transmission data near the fundamental absorption edge processed using a linearpower model characterised by a formula obtained by superimposing a linear function and a power function [25]:

$$\alpha = a + b\left(E\_g - c\right)^d + mE \tag{4}$$

or

**Figure 9.** Comparison between the absorption spectra in bulk samples of pure benzil and benzil doped with Na [25].

302 Optoelectronics - Advanced Materials and Devices

At the contrary, doping with Na has introduced important changes in the shape of the funda‐ mental absorption edge in benzil, as can be observed in Figure 9, a large structured band replacing the narrow peaks. This can be explained by the light scattering on the nonhomoge‐ neities of the doped benzil or by changes in the forces acting between the host and guest [25].

**Figure 10.** Absorption spectra of bulk pure and Na doped benzil matrix. Detail between 375 nm and 435 nm [25].

$$a = a + b\left(c - \lambda\_g\right)^d + m\lambda\tag{5}$$

where: α=absorption coefficient; c=band gap energy, Eg, or edge of the fundamental absorp‐ tion, λg, respectively, d=coefficient that depends on the light absorption mechanism and (a +mE) or (a+mλ) respectively, define all the other parasitical processes, including scattering of light on the nonhomogeneities of the sample and affecting the band to band absorption mechanism.

The light absorption process is characterised by the optical band gap, which in benzil has been evaluated at Eg=2.65 eV, emphasising the wide band gap semiconductor character of crystalline benzil. The narrowing of the optical band gap by introducing energetic levels in the band gap in pure crystal can be a consequence of physical defects or controlled doping that can generate chemical or structural defects.

**Figure 11.** Fitting of the experimental data for bulk samples of pure and Na doped benzil [25].

As can be seen in Figure 11, the narrowing in the optical band gap is correlated with the presence of physical defects because the large radius metal atoms disturb the organic lattice and the effect of the dopant is hidden by the structural imperfections [25]. The impurities migrate and concentrate at these defects, such as grain boundaries, twins, dislocations.

the vibrational interactions. The triplet state can be reached by a radiationless "intersystem

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**Figure 13.** Luminescence spectra of m-DNB crystals: (1) pure m-DNB; (2) m-DNB doped with 1 wt % iodine; (3) m-DNB

The m-DNB molecules contain oxygen atoms with lone electrons pairs which can be pro‐ moted to an unoccupied π orbital and give rise to a (n, π\*) singlet excite state and a triplet excited state, with an energy lower than the energy of usual (π, π\*) state. The (n,n\*)state in m-DNB is the lowest excited singlet state that favours the radiationless processes as "inter‐ system crossing" or "internal conversion" to a lower excited energetic level. The (n, π\*) tran‐ sition in m-DNB is confirmed by the small value of the "singlet-triplet" splitting evaluated

from the correlation of absorption and emission data [34].

crossing" process from the excited singlet state.

doped with 2 wt % iodine [34].

For benzil doped with Ag presented in Figure 12, the situation is different and the optical absorption involves energetic levels from the band gap associated with the generation of cluster, as a consequence of the high first ionisation energy and weak reactivity of Ag.

**Figure 12.** Fitting of the experimental data for bulk samples of pure and Ag doped benzil [25].

At excitation with λ=335 nm, the bulk sample of m-DNB shows a high, broad emission peak presented in Figure 13 [34], which correlated is with the radiative decays from the first excit‐ ed energetic level, peak situated at 2.85 eV with a shoulder at 2.95 eV generated probably by the radiative decay from another vibrational level of the same excited energetic level.

A modification of the spectrum could be evidenced when m-DNB is doped with iodine, the position of the emission slightly moving through shorter wavelengths, the difference between the peak and shoulder being attenuated by the increase of the iodine concentra‐ tion from 1 wt % to 2 wt %. The blue shift of the emission peak can be associated with the migration and trapping of the exciton on the defect zones characterised by slightly high‐ er energy compared to m-DNB without defects. Despite the strong interaction between the molecules, the energy of the level associated to the defect still remains under the energy of the exciton level.

The peak situated at 2.8 eV evidenced both in pure and doped m-DNB can be obtained by a radiative decay from the lowest excited triplet state to the ground state. This forbidden tran‐ sition became possible by the relaxation of the selection rule in m-DNB under the effect of the vibrational interactions. The triplet state can be reached by a radiationless "intersystem crossing" process from the excited singlet state.

As can be seen in Figure 11, the narrowing in the optical band gap is correlated with the presence of physical defects because the large radius metal atoms disturb the organic lattice and the effect of the dopant is hidden by the structural imperfections [25]. The impurities migrate and concentrate at these defects, such as grain boundaries, twins, dislocations.

304 Optoelectronics - Advanced Materials and Devices

For benzil doped with Ag presented in Figure 12, the situation is different and the optical absorption involves energetic levels from the band gap associated with the generation of cluster, as a consequence of the high first ionisation energy and weak reactivity of Ag.

**Figure 12.** Fitting of the experimental data for bulk samples of pure and Ag doped benzil [25].

of the exciton level.

At excitation with λ=335 nm, the bulk sample of m-DNB shows a high, broad emission peak presented in Figure 13 [34], which correlated is with the radiative decays from the first excit‐ ed energetic level, peak situated at 2.85 eV with a shoulder at 2.95 eV generated probably by

A modification of the spectrum could be evidenced when m-DNB is doped with iodine, the position of the emission slightly moving through shorter wavelengths, the difference between the peak and shoulder being attenuated by the increase of the iodine concentra‐ tion from 1 wt % to 2 wt %. The blue shift of the emission peak can be associated with the migration and trapping of the exciton on the defect zones characterised by slightly high‐ er energy compared to m-DNB without defects. Despite the strong interaction between the molecules, the energy of the level associated to the defect still remains under the energy

The peak situated at 2.8 eV evidenced both in pure and doped m-DNB can be obtained by a radiative decay from the lowest excited triplet state to the ground state. This forbidden tran‐ sition became possible by the relaxation of the selection rule in m-DNB under the effect of

the radiative decay from another vibrational level of the same excited energetic level.

**Figure 13.** Luminescence spectra of m-DNB crystals: (1) pure m-DNB; (2) m-DNB doped with 1 wt % iodine; (3) m-DNB doped with 2 wt % iodine [34].

The m-DNB molecules contain oxygen atoms with lone electrons pairs which can be pro‐ moted to an unoccupied π orbital and give rise to a (n, π\*) singlet excite state and a triplet excited state, with an energy lower than the energy of usual (π, π\*) state. The (n,n\*)state in m-DNB is the lowest excited singlet state that favours the radiationless processes as "inter‐ system crossing" or "internal conversion" to a lower excited energetic level. The (n, π\*) tran‐ sition in m-DNB is confirmed by the small value of the "singlet-triplet" splitting evaluated from the correlation of absorption and emission data [34].

The doping with other metallic impurities such as silver (2.4 wt%) or Na ( 1 wt %) has not modified the sharp peak situated at 2.37 eV, which is present in pure benzil, as it is emphas‐ ised in Figure 15, and this peak could not be correlated with an exciton trapping mechanism [34]. The peak at 2.37 eV could be generated by the radiative decay from the excited triplet state (T1) to the ground state (S0), which is a transition forbidden for separated molecules be‐ coming allowed through the vibrational interactions when the molecules are coupled in the

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In Figure 16 is presented the spectrum of benzil doped with m-DNB. The benzene deriva‐ tive, m-DNB, is active itself and has a direct action on the benzil matrix. The peak assigned to m-DNB is situated at 2.97 eV in the high-energy range of the emission spectrum. The pref‐ erential excitation of m-DNB molecule can be explained by the lower position of the excited singlet state (2.9 eV) in m-DNB compared to benzil (3.25 eV). At the contrary, naphthalene has not any significant influence on the emission spectrum of benzil presented in Figure 17, because the first singlet excited state in naphthalene is situated at ~ 3.84 eV and the triplet state at ~ 2.64 eV higher than the corresponding energetic levels in benzil (3.25 eV and 2.3 eV respectively). Therefore the peak observed in the emission spectrum of naphthalene doped

solid state, in a crystalline lattice [23; 35; 36].

benzil is obtained by the radiative deexcitation of benzil matrix.

**Figure 16.** Luminescence spectra of benzil crystals: (1) pure benzil; (2) m-DNB doped benzil (3 wt %) [34].

no significant effect on shape of the emission spectrum of benzil.

As can be seen from Figure 17, the simultaneous doping with naphthalene and iodine has

**Figure 14.** Luminescence spectra of benzil crystals: (1) pure benzil; (2) benzil doped with 1 wt % iodine; (3) benzil doped with 2 wt % iodine [34].

The luminescence spectra of pure benzil presented in Figure 14, shows an emission peak sit‐ uated at 2.37 eV generated by the lone electron pairs of oxygen atoms in carbonyl groups emitting only from planar configuration, on which are localized the emission transition in‐ volving (n, π\*) states.

**Figure 15.** Luminescence spectra of benzil crystals: (1) pure benzil; (2) benzil doped with Ag (2.4 wt %); (3) benzil dop‐ ed with Na (1 wt %) [34].

The doping with other metallic impurities such as silver (2.4 wt%) or Na ( 1 wt %) has not modified the sharp peak situated at 2.37 eV, which is present in pure benzil, as it is emphas‐ ised in Figure 15, and this peak could not be correlated with an exciton trapping mechanism [34]. The peak at 2.37 eV could be generated by the radiative decay from the excited triplet state (T1) to the ground state (S0), which is a transition forbidden for separated molecules be‐ coming allowed through the vibrational interactions when the molecules are coupled in the solid state, in a crystalline lattice [23; 35; 36].

In Figure 16 is presented the spectrum of benzil doped with m-DNB. The benzene deriva‐ tive, m-DNB, is active itself and has a direct action on the benzil matrix. The peak assigned to m-DNB is situated at 2.97 eV in the high-energy range of the emission spectrum. The pref‐ erential excitation of m-DNB molecule can be explained by the lower position of the excited singlet state (2.9 eV) in m-DNB compared to benzil (3.25 eV). At the contrary, naphthalene has not any significant influence on the emission spectrum of benzil presented in Figure 17, because the first singlet excited state in naphthalene is situated at ~ 3.84 eV and the triplet state at ~ 2.64 eV higher than the corresponding energetic levels in benzil (3.25 eV and 2.3 eV respectively). Therefore the peak observed in the emission spectrum of naphthalene doped benzil is obtained by the radiative deexcitation of benzil matrix.

**Figure 14.** Luminescence spectra of benzil crystals: (1) pure benzil; (2) benzil doped with 1 wt % iodine; (3) benzil

The luminescence spectra of pure benzil presented in Figure 14, shows an emission peak sit‐ uated at 2.37 eV generated by the lone electron pairs of oxygen atoms in carbonyl groups emitting only from planar configuration, on which are localized the emission transition in‐

**Figure 15.** Luminescence spectra of benzil crystals: (1) pure benzil; (2) benzil doped with Ag (2.4 wt %); (3) benzil dop‐

doped with 2 wt % iodine [34].

306 Optoelectronics - Advanced Materials and Devices

volving (n, π\*) states.

ed with Na (1 wt %) [34].

**Figure 16.** Luminescence spectra of benzil crystals: (1) pure benzil; (2) m-DNB doped benzil (3 wt %) [34].

As can be seen from Figure 17, the simultaneous doping with naphthalene and iodine has no significant effect on shape of the emission spectrum of benzil.

**a.** 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA);

**d.** 5, 10, 15,20-yetra(4-pyrydil)21H, 23H-porphyne (TPyP).

type conduction. They molecular structure is given in Figure 18.

mine a quasi-one-dimensional molecular crystal structure [41].

**Figure 18.** PTCDA (a), ZnPc (b), Alq3 (c) and TPyP (d) molecular structure.

range for light absorption [44].

PTCDA is known as having p type conduction while Alq3 and TPyP are characterized by n

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ZnPc is an electron donor forming highly ordered layer, with a broad transmission window

In PTCDA, an electron acceptor, the interaction between the π-electrons systems is favored by the planar molecule and the perpendicular stacks of molecular planes [40], which deter‐

Alq3 shows different stereoisomers (median, facial) determined by the mutual orientation of the ligands of hydroxyquinoline, which show different symmetries and as consequence dif‐

TPyP is a non-metallic porphyrin with an increased electron affinity obtained by the substi‐ tution of phenyl group by pyridyl group determining the n type conduction. The basic structure of porphyrin consists in four pyrrolic entities linked by four unsaturated methane bridges with a skeleton showing an extended π-electrons system assuring a large spectral

**c.** tris(8-hydroxyquinoline) aluminium (Alq3) and

**b.** Zinc phthalocyanine (ZnPc);

in visible region of the spectrum [39].

ferent properties [42; 43].

**Figure 17.** Luminescence spectra of benzil crystals: (1) pure benzil; (2) benzil doped with naphthalene (1.5 wt %); (3) benzil doped with naphthalene (1 wt %) and iodine (1 wt %) [34].
