**3. Aromatic derivatives thin films for optoelectronic applications**

The major problems for large-scale application of crystalline matrices from these aromatic derivatives materials are associated with the difficulties to grow (low melting point, super‐ cooling, and low thermal conductivity), process (weak mechanical properties determined by weak bonding forces between molecules) and doping organic crystals to assure an homoge‐ neous distribution of the guest atoms.

Crystalline organic films are preferred in a variety of applications because of the complexity of the processes involved and long time necessary to grow bulk organic crystals. The use of thin films is more promising because represents an optimum between the cost of manufac‐ turing and properties of interest for special and oriented applications.

Investigation of the properties of organic thin films is a very important aim because these films are components of the organic heterostructures as fundamental elements of any organ‐ ic devices. It is also necessary to investigate the properties of heterostructures because the junction between two different semiconductors (organic/organic; organic/inorganic) or be‐ tween a metal and a semiconductor (metal/organic) is the key building block of any modern electronic, photovoltaic and optoelectronic devices. Heterojunction technology has known a continuous development from the first heterojunction transistor, realized by Bardeen in 1948 at Bell Laboratory [37], to p-n junction transistor, realized by Schockley in 1949 [38], and to nowadays devices based on multilayer heterostructures.

The organic compounds which have been investigated as components of organic compound based heterostructures are:


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

The major problems for large-scale application of crystalline matrices from these aromatic derivatives materials are associated with the difficulties to grow (low melting point, super‐ cooling, and low thermal conductivity), process (weak mechanical properties determined by weak bonding forces between molecules) and doping organic crystals to assure an homoge‐

Crystalline organic films are preferred in a variety of applications because of the complexity of the processes involved and long time necessary to grow bulk organic crystals. The use of thin films is more promising because represents an optimum between the cost of manufac‐

Investigation of the properties of organic thin films is a very important aim because these films are components of the organic heterostructures as fundamental elements of any organ‐ ic devices. It is also necessary to investigate the properties of heterostructures because the junction between two different semiconductors (organic/organic; organic/inorganic) or be‐ tween a metal and a semiconductor (metal/organic) is the key building block of any modern electronic, photovoltaic and optoelectronic devices. Heterojunction technology has known a continuous development from the first heterojunction transistor, realized by Bardeen in 1948 at Bell Laboratory [37], to p-n junction transistor, realized by Schockley in 1949 [38], and to

The organic compounds which have been investigated as components of organic compound

**3. Aromatic derivatives thin films for optoelectronic applications**

turing and properties of interest for special and oriented applications.

nowadays devices based on multilayer heterostructures.

benzil doped with naphthalene (1 wt %) and iodine (1 wt %) [34].

neous distribution of the guest atoms.

308 Optoelectronics - Advanced Materials and Devices

based heterostructures are:


PTCDA is known as having p type conduction while Alq3 and TPyP are characterized by n type conduction. They molecular structure is given in Figure 18.

ZnPc is an electron donor forming highly ordered layer, with a broad transmission window in visible region of the spectrum [39].

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

Alq3 shows different stereoisomers (median, facial) determined by the mutual orientation of the ligands of hydroxyquinoline, which show different symmetries and as consequence dif‐ ferent properties [42; 43].

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

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 range for light absorption [44].
