**4. Organic heterostructures based on single and multilayer thin films: electrical properties for device applications**

The *I*‐*V* characteristics (**Figure 18**) for the heterostructures prepared by VTE were recorded under dark conditions, in the range of 0V–1V. By using an additional layer of PEDOT:PSS, the current value in the standard structures increased from 1.6 × 10−5 A (glass/ITO/ZnPc/C60/ NTCDA/Al) at 6 × 10−4A (glass/ITO/PEDOT:PSS/ZnPc/C60/NTCDA/Al). This supplementary layer favours the hole injection from the ITO electrode in the first organic film [52].

**Figure 18.** Current‐voltage characteristics of the organic heterostructures deposited by VTE: ITO/ZnPc/C60/NTCDA/Al (a), ITO/PEDOT:PSS/ZnPc/C60/NTCDA/Al (b) and glass/Al/NTCDA/C60/ZnPc/ITO (c) structures.

By the deposition of these materials in the inversed order from Al to ITO, an improvement in the current value was also obtained, from 1.6 × 10−5 (glass/ITO/ZnPc/C60/NTCDA/Al) A at 1 × 10−4 A (glass/Al/NTCDA/C60/ZnPc/ITO). Preparing the heterostructure in this way was avoided the interaction of the hot Al atoms with the organic layer which can determine the appearance of some recombination centres at the interface [87].

Thus, the high current values obtained for these heterostructures can be useful for the OPV applications. It was remarked that the current value can be increased either using a supple‐ mentary PEDOT:PSS layer or by preparing the heterostructure in the inverted way.

The resistivity for the AZO layers prepared by PLD was determined using a four‐point probe method, the values being between 2.7 × 10−4 and 3.2 × 10−4 Ω cm in the case of untreated layers and between 2.5 × 10−4 and 3.1 × 10−4 Ωcm in the case of the treated layers in oxygen plasma (**Table 1**).

For analysis of the NTCDA/ZnPc/AZO structures from electrical point of view, an injection con‐ tact behaviour was evidenced for both structures deposited on untreated AZO and treated AZO films (**Figure 19**). 5AZO and 10AZO films were characterised by a lower resistivity compared to that of the untreated AZO layer, and are chosen to facilitate the charge carrier injection. It was observed that the *I*‐*V* characteristics became asymmetric for the heterostructures prepared on

Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications http://dx.doi.org/10.5772/67702 107

**Figure 19.** Current‐voltage characteristics of the organic heterostructures deposited by MAPLE: AZO/ZnPc/NTCDA/Au structure on substrates untreated (curve 1), treated in oxygen plasma for 5 min (curve 2) and treated in oxygen plasma for 10 min (curve 3).

treated AZO substrates. At low voltages (under 0.4 V), the characteristics are linear and at higher voltages the effect of the space charge limited currents (SCLC) becomes dominant. At direct polarisation, at 1V, the current value increases from 3 × 10−3 A in the structure with AZO at 1.5 × 10−2 A in the structures with 5AZO and 10AZO (**Figure 19** Quadrant 1). An increase in the work function of the AZO electrode was induced by the oxygen plasma treatment [88], having as effect a decrease in the energetic barrier at the AZO/organic interface which improves the injection of the charge carriers from AZO in the organic layer.

Taking into consideration the properties of the AZO, this TCO can be integrated in organic heterostructure, instead of the ITO electrode. The heterostructures prepared on AZO are char‐ acterised by current values suitable for the photovoltaic applications. Moreover, treating in oxygen plasma the AZO substrate can be increased the current value in the heterostructures based on ZnPc and NTCDA.

By the deposition of these materials in the inversed order from Al to ITO, an improvement in the current value was also obtained, from 1.6 × 10−5 (glass/ITO/ZnPc/C60/NTCDA/Al) A at 1 × 10−4 A (glass/Al/NTCDA/C60/ZnPc/ITO). Preparing the heterostructure in this way was avoided the interaction of the hot Al atoms with the organic layer which can determine the

**Figure 18.** Current‐voltage characteristics of the organic heterostructures deposited by VTE: ITO/ZnPc/C60/NTCDA/Al

(a), ITO/PEDOT:PSS/ZnPc/C60/NTCDA/Al (b) and glass/Al/NTCDA/C60/ZnPc/ITO (c) structures.

Thus, the high current values obtained for these heterostructures can be useful for the OPV applications. It was remarked that the current value can be increased either using a supple‐

The resistivity for the AZO layers prepared by PLD was determined using a four‐point probe method, the values being between 2.7 × 10−4 and 3.2 × 10−4 Ω cm in the case of untreated layers and between 2.5 × 10−4 and 3.1 × 10−4 Ωcm in the case of the treated layers in oxygen plasma (**Table 1**). For analysis of the NTCDA/ZnPc/AZO structures from electrical point of view, an injection con‐ tact behaviour was evidenced for both structures deposited on untreated AZO and treated AZO films (**Figure 19**). 5AZO and 10AZO films were characterised by a lower resistivity compared to that of the untreated AZO layer, and are chosen to facilitate the charge carrier injection. It was observed that the *I*‐*V* characteristics became asymmetric for the heterostructures prepared on

mentary PEDOT:PSS layer or by preparing the heterostructure in the inverted way.

appearance of some recombination centres at the interface [87].

106 Phthalocyanines and Some Current Applications

The electrical properties of stacked and blend layers deposited by MAPLE on flexible sub‐ strate were also investigated. Regarding the stacked structures, they were energetically favourable, taking into account the ionisation potential (IP) and electron affinity (EA) levels in ZnPc (*E*IP;ZnPc = 5.28 eV and *E*EA;ZnPc = 3.28 eV [89]), MgPc (*E*IP;MgPc = 5.4eV and *E*EA;MgPc = 3.9 eV [90]) and TPyP (*E*IP;TPyP = 6.8 eV, *E*EA;TPyP = 4.1 eV [86]).

The *I*‐*V* characteristics recorded under dark and under illumination conditions, in 0V–1V domain, are near linear (**Figure 20**). In the dark, the higher value of the current (~10−6 A) was obtained for the Al/MgPc:TPyP/ITO structure, with ~ 3 orders of magnitude higher than the value presented by the Al/MgPc/TPyP/ITO structure. As was remarked in the AFM images (**Figure 10** g), MgPc:TPyP layer seems to be characterised by a larger roughness which can lead to the formations of some dipoles which reduce the energetic barrier at interfaces favouring the charge transport [83].

An increase in the current value and a photovoltaic effect was also evidenced in the Al/ ZnPc:TPyP/ITO structure after exposure to light (**Figure 21**). The solar cell parameters are:

**Figure 20.** Current‐voltage characteristics (in dark conditions—curves 1 and under illumination—curves 2) of the organic heterostructures deposited by MAPLE: PET/ITO/ZnPc/TPyP/Al (a), PET/ITO/MgPc/TPyP/Al (b), PET/ITO/ ZnPc:TPyP/Al (c) and PET/ITO/MgPc:TPyP/Al (d) structures.

**Figure 21.** Current‐voltage characteristic (under light, −1 V–1 V domain) of the PET/ITO/ZnPc:TPyP/Al heterostructure deposited by MAPLE.

*I* SC = 3.4 × 10−9 A; *U*OC = 0.77 V and FF = 0.28. Even if in the PL spectra of the MgPc:TPyP struc‐ ture (**Figure 17**) was remarked a quenching of the photoluminescence, in the *I*‐*V* characteristic recorded under illumination between −1 and 1V, the photovoltaic effect was not evidenced. This means that for this case the collection of the charge carrier at the electrodes has not occurred.

In analysis of the structures prepared with stacked layer, an increased current value was obtained in Al/TPyP/ZnPc/ITO structure comparing to that based on MgPc. This can be explained, considering the position of the HOMO and LUMO levels in these materials, a bet‐ ter hole injection from the ITO electrode in the ZnPc layer being ensured by the lower barrier at the ITO/ZnPc interface (WFITO = 4.6 eV [91] and ∆*E*ZnPc‐ITO = 0.68 eV).

Organic heterostructures based on ZnPc, MgPc and TPyP are suitable to be used in OPV due their electrical properties, especially in bulk forms instead of stacked layers.
