**3. Organic heterostructures with single and multilayer thin films: influence of the deposition technique type on their structural, morphological and optical properties**

Different organic heterostructures were obtained either by VTE or MAPLE on a solid glass substrate (covered with ITO or AZO) and on a flexible substrate (covered with ITO). The prepared layers and heterostructures were investigated by various techniques: X‐ray diffrac‐ tion (XRD), atomic force microscopy (AFM), ultraviolet–visible (UV–VIS) spectroscopy, pho‐ toluminescence spectroscopy (PL) and infrared Fourier transform spectroscopy (FTIR). The used organic materials were metal phthalocyanines (ZnPc or MgPc), porphyrins (15,10,15,20‐ tetra(4‐pyridyl)‐21H,23H‐porphine ‐TPyP) or other small molecule compounds (1,4,5,8‐naph‐ thalenetetracarboxylic dianhydride—NTCDA, fullerene‐C60), and their chemical structure is presented in **Figure 2**.

#### **3.1. Heterostructures based on ZnPc and NTCDA thin films obtained by VTE and MAPLE**

Phthalocyanines are materials that are often used in OPV due to their large absorption domain in the visible part of the spectrum. These compounds are characterised by a high chemical sta‐ bility having the property to form uniform layer on different solid substrates [52]. Thus, they can be easily deposited by the VTE method.

**Figure 2.** Chemical structure of the ZnPc, MgPc, TPyP, NTCDA and C60.

ZnPc, C60 and NTCDA layers were deposited by the VTE technique [53] on a ITO/glass, poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate)‐PEDOT:PSS/ITO/glass, silicon and glass using the following experimental conditions: 8 × 10−6 mbar pressure in the chamber, at ~218°C for ZnPc, ~255°C for C60 and ~166°C for NTCDA. A PEDOT:PSS layer (20 nm) was prepared by spin‐coating on ITO (15 Ω/sq) at a rotation speed of 3000 rot/min for 30 s. After that the obtained layers were supposed to a thermal treatment at 120°C for 5 min [53].

**3. Organic heterostructures with single and multilayer thin films: influence of the deposition technique type on their structural,** 

Different organic heterostructures were obtained either by VTE or MAPLE on a solid glass substrate (covered with ITO or AZO) and on a flexible substrate (covered with ITO). The prepared layers and heterostructures were investigated by various techniques: X‐ray diffrac‐ tion (XRD), atomic force microscopy (AFM), ultraviolet–visible (UV–VIS) spectroscopy, pho‐ toluminescence spectroscopy (PL) and infrared Fourier transform spectroscopy (FTIR). The used organic materials were metal phthalocyanines (ZnPc or MgPc), porphyrins (15,10,15,20‐ tetra(4‐pyridyl)‐21H,23H‐porphine ‐TPyP) or other small molecule compounds (1,4,5,8‐naph‐ thalenetetracarboxylic dianhydride—NTCDA, fullerene‐C60), and their chemical structure is

**3.1. Heterostructures based on ZnPc and NTCDA thin films obtained by VTE and MAPLE**

Phthalocyanines are materials that are often used in OPV due to their large absorption domain in the visible part of the spectrum. These compounds are characterised by a high chemical sta‐ bility having the property to form uniform layer on different solid substrates [52]. Thus, they

**morphological and optical properties**

90 Phthalocyanines and Some Current Applications

can be easily deposited by the VTE method.

**Figure 2.** Chemical structure of the ZnPc, MgPc, TPyP, NTCDA and C60.

presented in **Figure 2**.

The following heterostructures were fabricated: 1‐[(ITO/ZnPc(50 nm)/C60(30 nm)/NTCDA(110 nm)/Al)], 2‐[(ITO/PEDOT:PSS(20 nm)/ZnPc(50 nm)/C60(20 nm)/NTCDA(120 nm)/Al)] and 3‐ [glass/Al/NTCDA(90)/C60(20)/ZnPc(50)/ITO]. For the second type of heterostructure (starting from glass/Al), the ITO electrode (the last material deposited) was prepared by PLD with an excimer laser source (*λ* = 248 nm and *τ*FWHM ~25 ns) using the following experimental conditions: room temperature, 5 Hz repetition rate, 30,000 number of laser pulses, in oxygen atmosphere at 1.5 Pa pressure [53]. The resistivity of ITO was 3.9 × 10−4 Ωcm. Aluminium was used as a top metallic electrode (80 nm) being also obtained by VTE at 10−4 Pa pressure in the deposition cham‐ ber. The schematic representations of the prepared organic heterostructures are given in **Figure 3**.

The XRD diffractograms (**Figure 4**) indicated that the organic films obtained by VTE are not completely amorphous. The ZnPc layer presents lower diffraction peaks at 6.9, 9.6 and

**Figure 3.** Schematic representation of the organic heterostructures deposited by VTE: standard structure (a) and inverted structure (b).

**Figure 4.** XRD patterns of the ZnPc film (curve 1), C60 film (curve 2), NTCDA film (curve 3), standard structure (curve 4) and inverted structure (curve 5) deposited by VTE.

29.3° [29], attributed to the powder raw material [54]. For the C60 layer, remarked lines were obtained at 11.8 and 23.7° specific to the (111) and (311) diffraction plane of this material [55]. The XRD diagram of the NTCDA film presents three peaks, including an intense one at 11.9° obtained also in the diffractogram of the powder [53]. Depending on the method used for deposition of the organic layers, in the multilayer structures are remarked only the diffrac‐ tions lines originating from NTCDA, which are more intense when NTCDA is deposited on top (**Figure 4**, curve 4).

For the ZnPc and C60 films, the AFM images (**Figure 5**) show topography characteris‐ tic to these materials deposited by thermal evaporation [56, 57]. Thus, it can be observed a low roughness for the ZnPc film (root mean square, RMS = 5.1 nm) in comparison with C60 (RMS = 14.7 nm) and NTCDA (RMS = 20.9 nm). The RMS higher value of the NTCDA can be attributed to the layer thickness, this layer being thicker than ZnPc and C60 films.

For the heterostructures containing three organic layers, a reduced roughness was obtained in the inverted structure (RMS = 6.8 nm) compared to the normal structure (RMS = 8.3 nm) as it is expected, taking into account that in standard structure the top organic layer is NTCDA which is characterised by the highest roughness.

The vibrational properties of the raw materials were identified in the FTIR spectra (**Figure 6**) of ZnPc and NTCDA layers deposited by VTE, indicating that no chemical decomposition took place during the VTE transfer. The C60 film was too thin to remark some FTIR peaks on it. In the ZnPc layer, the peak from 727 cm−1 is specific to C‐H out of plane deformation, the peaks situated at 748, 1095, 1118 and 1288 cm−1 appear due to the in‐plane C‐H bending, the peak at 1333 cm−1 evidenced the C‐C stretching in isoindole and the peaks from 1481 to 1608 cm−1 are attributed to the C‐C stretching in benzene [56, 58].

**Figure 5.** AFM images of the ZnPc film (a), C60 film (b), NTCDA film (b), standard structure (d) and inverted structure (e) deposited by VTE.

**Figure 6.** FTIR spectra of the ZnPc (curve 1) and NTCDA (curve 2) single layers deposited by VTE.

29.3° [29], attributed to the powder raw material [54]. For the C60 layer, remarked lines were obtained at 11.8 and 23.7° specific to the (111) and (311) diffraction plane of this material [55]. The XRD diagram of the NTCDA film presents three peaks, including an intense one at 11.9° obtained also in the diffractogram of the powder [53]. Depending on the method used for deposition of the organic layers, in the multilayer structures are remarked only the diffrac‐ tions lines originating from NTCDA, which are more intense when NTCDA is deposited on

For the ZnPc and C60 films, the AFM images (**Figure 5**) show topography characteris‐ tic to these materials deposited by thermal evaporation [56, 57]. Thus, it can be observed a low roughness for the ZnPc film (root mean square, RMS = 5.1 nm) in comparison with C60 (RMS = 14.7 nm) and NTCDA (RMS = 20.9 nm). The RMS higher value of the NTCDA can be

For the heterostructures containing three organic layers, a reduced roughness was obtained in the inverted structure (RMS = 6.8 nm) compared to the normal structure (RMS = 8.3 nm) as it is expected, taking into account that in standard structure the top organic layer is NTCDA

The vibrational properties of the raw materials were identified in the FTIR spectra (**Figure 6**) of ZnPc and NTCDA layers deposited by VTE, indicating that no chemical decomposition took place during the VTE transfer. The C60 film was too thin to remark some FTIR peaks on it. In the ZnPc layer, the peak from 727 cm−1 is specific to C‐H out of plane deformation, the peaks situated at 748, 1095, 1118 and 1288 cm−1 appear due to the in‐plane C‐H bending, the peak at 1333 cm−1 evidenced the C‐C stretching in isoindole and the peaks from 1481 to

**Figure 5.** AFM images of the ZnPc film (a), C60 film (b), NTCDA film (b), standard structure (d) and inverted structure

attributed to the layer thickness, this layer being thicker than ZnPc and C60 films.

top (**Figure 4**, curve 4).

92 Phthalocyanines and Some Current Applications

(e) deposited by VTE.

which is characterised by the highest roughness.

1608 cm−1 are attributed to the C‐C stretching in benzene [56, 58].

For the NTCDA layer, the peaks at 1780 cm−1 (characteristic to the dianhidrydecarbonylic group [59]), at 543, 753 and 882 cm−1 (specific to the C‐H out‐of‐plane bending vibrations [60, 61]), at 698 and 754cm−1 (attributed to C‐H bending vibration [36]), at 1044, 1120, 1161, 1234 and 1293 cm−1 (characteristic to the stretching vibration of C‐O in the anhydride groups and the C‐H in‐plane bending vibration) and at 1442 and 1582 cm−1 due to the C‐C bending [61]) were evidenced.

The UV–VIS spectra of the VTE prepared organic thin films are given in **Figure 7**. For the ZnPc layer, a high transparency (~90% at 500 nm) was emphasised, covering a broad part of the VIS region and presenting the band B (so‐called Soret band) and band Q [62, 63]. Several absorption maxima are remarked for the C60 layer, at 340, 400 and 440 nm which are char‐ acteristic to this material prepared by the VTE technique [64, 65]. The NTCDA layer used as buffer in our structure reveals absorption maxima in UV (at 370 and 390 nm) attributed to the π‐π\* transition [66]. The structure comprising all the organic layers (**Figure 7**, curve 4) is characterised by a high transmittance, showing the absorption maxima of all components.

ZnPc, C60 and NTCDA thin films deposited by the VTE method are polycrystalline and have morphologies specific to raw materials (ZnPc, C60 and NTCDA), being characterised by different roughness values (RMS ranged between 5.1 and 20.9 nm). The materials present adequate absorption bands in the visible region. The peaks disclosed by the FTIR spectra are assigned to each organic material, evidencing that no chemical decomposition appears in the thin‐film deposition.

**Figure 7.** Transmission spectra of the ZnPc film (curve 1), C60 film (curve 2), NTCDA film (curve 3) and standard structure (curve 4) deposited by VTE.

Along the time, ITO was the most used transparent electrode, due to its high optical transmit‐ tance and reduced electrical resistivity. Because the required indium is rare and expensive, many attempts were made in order to replace the ITO in various applications, including the OPV field.

As transparent electrode we choose a large band gap semiconductor, ZnO doped with Al (AZO) because it presents adequate electrical resistivity (~10−4 Ωcm), a high optical trans‐ mission in the visible‐NIR domain, and a higher chemical stability in comparison with ITO [67–69].

AZO (ZnO doped with 2% Al) thin films were prepared at room temperature on a glass sub‐ strate by PLD using the KrF\* excimer laser in the following experimental conditions: 10 Hz repetition rate 5 cm, substrate‐target distance, 3 J/cm2 laser fluence, 32,000 laser pulses, in oxygen atmosphere at 10−2 mbar pressure [70]. Subsequently, the obtained AZO layers were treated in oxygen plasma at 0.6 mbar and *P*max = 130 W (for 5 and 10 min) in order to observe how this treatment affects the properties of the formed layers. The samples were labelled as fol‐ lows: AZO (untreated film), 5AZO (film treated for 5 min) and 10AZO (film treated for 10 min).

The MAPLE technique was used to process organic films from ZnPc and NTCDA on the AZO substrate. The same laser source was used to prepare thin films from a frozen target containing ZnPc or NTCDA and dimethyl sulphoxide (DMSO) as a solvent compatible with the laser wavelength. Two different laser fluences were used for the deposition of the ZnPc layer: 0.4 J/cm2 (1ZnPc) and 0.3 J/cm2 (2ZnPc). Organic heterostructures with two stacked layers were formed by the deposition of the NTCDA layer over ZnPc films. For the NTCDA, the deposition parameters were 0.3 J/cm2 laser fluence, 90,000 and 100,000 laser pulses [70]. **Table 1** presents the experimental conditions for the deposition of the organic layers.


Along the time, ITO was the most used transparent electrode, due to its high optical transmit‐ tance and reduced electrical resistivity. Because the required indium is rare and expensive, many attempts were made in order to replace the ITO in various applications, including the

**Figure 7.** Transmission spectra of the ZnPc film (curve 1), C60 film (curve 2), NTCDA film (curve 3) and standard

As transparent electrode we choose a large band gap semiconductor, ZnO doped with Al (AZO) because it presents adequate electrical resistivity (~10−4 Ωcm), a high optical trans‐ mission in the visible‐NIR domain, and a higher chemical stability in comparison with ITO

AZO (ZnO doped with 2% Al) thin films were prepared at room temperature on a glass sub‐ strate by PLD using the KrF\* excimer laser in the following experimental conditions: 10 Hz

oxygen atmosphere at 10−2 mbar pressure [70]. Subsequently, the obtained AZO layers were treated in oxygen plasma at 0.6 mbar and *P*max = 130 W (for 5 and 10 min) in order to observe how this treatment affects the properties of the formed layers. The samples were labelled as fol‐ lows: AZO (untreated film), 5AZO (film treated for 5 min) and 10AZO (film treated for 10 min). The MAPLE technique was used to process organic films from ZnPc and NTCDA on the AZO substrate. The same laser source was used to prepare thin films from a frozen target containing ZnPc or NTCDA and dimethyl sulphoxide (DMSO) as a solvent compatible with the laser wavelength. Two different laser fluences were used for the deposition of the ZnPc

layers were formed by the deposition of the NTCDA layer over ZnPc films. For the NTCDA,

[70]. **Table 1** presents the experimental conditions for the deposition of the organic layers.

laser fluence, 32,000 laser pulses, in

(2ZnPc). Organic heterostructures with two stacked

laser fluence, 90,000 and 100,000 laser pulses

repetition rate 5 cm, substrate‐target distance, 3 J/cm2

(1ZnPc) and 0.3 J/cm2

the deposition parameters were 0.3 J/cm2

OPV field.

structure (curve 4) deposited by VTE.

94 Phthalocyanines and Some Current Applications

[67–69].

layer: 0.4 J/cm2

**Table 1.** Resistivity of the AZO layers before and after treatment, used laser fluences in the MAPLE deposition, the thickness (d) of the layers and the roughness value obtained from AFM. The heterostructures (**Figure 8**) were carried out by the gold (Au) electrode of ~100 nm thick‐ ness deposited also by VTE.

The morphological investigations of the AZO substrates and of the ZnPc/NTCDA structures are represented in **Figure 9**. Only the AFM images collected for the structures with the ZnPc layer deposited at 0.4 J/cm2 laser fluence are presented, but the roughness (RMS) values both for structures with ZnPc deposited at 0.4 and 0.3 J/cm2 laser fluences are presented in **Table 1**.

Oxygen plasma treatment leads to a decrease in the RMS value of the AZO substrate, from 9.3 nm for the untreated film to 3.3 nm for the treated film for 10 min (**Table 1**). A similar behaviour was remarked by others authors [71]. The AFM images exhibit a topography char‐ acterised by small grains for the organic layers obtained on treated substrate compared to that formed on the untreated substrate. The RMS value increases from the single to bilayer structures prepared on the untreated AZO substrate. The RMS recorded for the ZnPc layer shows an increase when the AZO substrate is treated (**Table 1**). Probably, the ZnPc deposi‐ tion is affected by the surface energy of AZO layer modified during the oxygen plasma treat‐ ment. The higher roughness of the ZnPc layer obtained on the AZO‐treated substrate leads to a better arrangement of the NTCDA molecules having an effect on lowering the RMS value recorded for the bilayer heterostructures.

From the UV–VIS spectra, a transparency between 75 and 87% in the range 400–800 nm was obtained for the AZO layers (**Figure 10**, curve 1). The thickness of the AZO films was evalu‐ ated using the formula from [72] which takes into consideration successive interference max‐ ima and minima. The obtained values (between 940 and 1310 nm) are given in **Table 1**.

The UV–VIS spectra of the AZO layers revealed a slight improvement in the transparency with the increase in duration of the applied plasma treatment (**Figure 10**, curves 1' and 1''). This can be attributed either to a reduction of the defects number inside the AZO layer (these can act as scattering centres), due to decrease in the AZO layer thickness (**Table 1**) or to the reduction in scattering at the surface in wavelength domain (>750 nm).

The thickness of the organic films was also estimated from the UV–VIS spectra, using the absorption coefficients at λ = 355 nm reported in the literature, *α*ZnPc = 3.5 × 10<sup>4</sup> cm−1 [73] and *α*NTCDA = 2.1 × 105 cm−1 [74]. The thickness varied between 360 and 550 nm for the ZnPc layer and between 90 and 150 nm for NTCDA (**Table 2**).

**Figure 8.** Schematic representation of the organic heterostructure deposited by MAPLE on AZO substrate.

Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications http://dx.doi.org/10.5772/67702 97

The heterostructures (**Figure 8**) were carried out by the gold (Au) electrode of ~100 nm thick‐

The morphological investigations of the AZO substrates and of the ZnPc/NTCDA structures are represented in **Figure 9**. Only the AFM images collected for the structures with the ZnPc

Oxygen plasma treatment leads to a decrease in the RMS value of the AZO substrate, from 9.3 nm for the untreated film to 3.3 nm for the treated film for 10 min (**Table 1**). A similar behaviour was remarked by others authors [71]. The AFM images exhibit a topography char‐ acterised by small grains for the organic layers obtained on treated substrate compared to that formed on the untreated substrate. The RMS value increases from the single to bilayer structures prepared on the untreated AZO substrate. The RMS recorded for the ZnPc layer shows an increase when the AZO substrate is treated (**Table 1**). Probably, the ZnPc deposi‐ tion is affected by the surface energy of AZO layer modified during the oxygen plasma treat‐ ment. The higher roughness of the ZnPc layer obtained on the AZO‐treated substrate leads to a better arrangement of the NTCDA molecules having an effect on lowering the RMS value

From the UV–VIS spectra, a transparency between 75 and 87% in the range 400–800 nm was obtained for the AZO layers (**Figure 10**, curve 1). The thickness of the AZO films was evalu‐ ated using the formula from [72] which takes into consideration successive interference max‐ ima and minima. The obtained values (between 940 and 1310 nm) are given in **Table 1**.

The UV–VIS spectra of the AZO layers revealed a slight improvement in the transparency with the increase in duration of the applied plasma treatment (**Figure 10**, curves 1' and 1''). This can be attributed either to a reduction of the defects number inside the AZO layer (these can act as scattering centres), due to decrease in the AZO layer thickness (**Table 1**) or to the

The thickness of the organic films was also estimated from the UV–VIS spectra, using the

cm−1 [74]. The thickness varied between 360 and 550 nm for the ZnPc layer

reduction in scattering at the surface in wavelength domain (>750 nm).

and between 90 and 150 nm for NTCDA (**Table 2**).

absorption coefficients at λ = 355 nm reported in the literature, *α*ZnPc = 3.5 × 10<sup>4</sup>

**Figure 8.** Schematic representation of the organic heterostructure deposited by MAPLE on AZO substrate.

laser fluence are presented, but the roughness (RMS) values both

laser fluences are presented in **Table 1**.

cm−1 [73] and

ness deposited also by VTE.

96 Phthalocyanines and Some Current Applications

layer deposited at 0.4 J/cm2

for structures with ZnPc deposited at 0.4 and 0.3 J/cm2

recorded for the bilayer heterostructures.

*α*NTCDA = 2.1 × 105

**Figure 9.** AFM images of the glass/AZO substrates (a, a' and a'') and ZnPc/NTCDA layers deposited by MAPLE—0.4 J/cm2 fluence for ZnPc and 0.3 J/cm2 fluence for NTCDA (b, b' and b''): untreated (a, b), treated in oxygen plasma for 5 minutes (a', b') and treated in oxygen plasma for 10 minutes (a'', b'').

The ZnPc layers (**Figure 10**, curves 2 and 3) present a structured absorption in the range of 550–750 nm, this large absorption domain being useful in generation of the charge carriers. As mentioned above, the oxygen plasma treatment can modify the surface energy of the AZO layer, can change the way in which the organic molecules are arranged on the substrate and as consequence the optical properties of these organic layers. Comparison of the 1ZnPc and 2ZnPc samples was found that for the second film the absorption is smaller. Additionally, the NTCDA layer does not affect the shape of the transmission spectrum (**Figure 10**, curve 3).

The emission properties of the samples under excitation with *λ*exc = 335 nm were also inves‐ tigated (**Figure 11**). The AZO layer is characterised by an intense emission band with maxi‐ mum at ~430 nm and a shoulder at ~480 nm, linked to point defects as Zn2+ interstitial [75, 76].

**Figure 10.** Transmission spectra of the organic films deposited by MAPLE on a glass/AZO substrate untreated (1–5), treated in oxygen plasma for 5 min (1'‐5') and treated in oxygen plasma for 10 min (1"‐5"): glass/AZO substrate (curves 1, 1' and 1"), 1ZnPc film (curve 2, 2' and 2''), 2ZnPc film (curve 3, 3' and 3''), 1ZnPc/NTCDA layers (curve 4, 4' and 4'') and 2ZnPc/NTCDA layers (curve 5, 5' and 5'').


**Table 2.** MAPLE conditions used for the deposition of organic films and structures on ITO/PET, layer thickness and RMS values interpolated from AFM.

The oxygen radicals from the plasma can lower the number of the Zn2+ interstitials due to the reduction of the defects which appear near to the film surface [77]. The thickness of the samples has a decisive role in the intensity of the emission. In the AZO and 5AZO thicker layers, the

Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications http://dx.doi.org/10.5772/67702 99

**Figure 11.** Photoluminescence spectra of the organic films deposited by MAPLE on a glass/AZO substrate untreated (1–3), treated in oxygen plasma for 5 min (1'‐3') and treated in oxygen plasma for 10 min (1"‐3"): glass/AZO substrate (curves 1, 1' and 1"), 1ZnPc film (curve 2, 2' and 2'') and 1ZnPc/NTCDA layers (curve 3, 3' and 3'').

emission band attributed to the deep level point defects (2.6 eV) is lower while in the thinner 10AZO layer the emission increases (**Figure 11**, curves 1).

The emission band situated at 430 nm in the AZO spectrum can be remarked also in the structures prepared with ZnPc and ZnPc/NTCDA (**Figure 11**, curves 2 and 3). The ZnPc layer discloses also a peak in the range of 400–450 nm [78]. The shoulder situated at 480 nm from AZO became a well‐structured band in the structures containing ZnPc. No supplementary maxima were observed by adding NTCDA, probably because the emission bands specific to this material, one situated at ~430 nm and other situated in 475–575 nm range are masked by

The oxygen radicals from the plasma can lower the number of the Zn2+ interstitials due to the reduction of the defects which appear near to the film surface [77]. The thickness of the samples has a decisive role in the intensity of the emission. In the AZO and 5AZO thicker layers, the

**Table 2.** MAPLE conditions used for the deposition of organic films and structures on ITO/PET, layer thickness and RMS

**Figure 10.** Transmission spectra of the organic films deposited by MAPLE on a glass/AZO substrate untreated (1–5), treated in oxygen plasma for 5 min (1'‐5') and treated in oxygen plasma for 10 min (1"‐5"): glass/AZO substrate (curves 1, 1' and 1"), 1ZnPc film (curve 2, 2' and 2''), 2ZnPc film (curve 3, 3' and 3''), 1ZnPc/NTCDA layers (curve 4, 4' and 4'')

**Sample Laser pulses Thickness (nm) RMS (nm)**

ZnPc/ITO 100k 570 36 MgPc/ITO 85k 470 35 TPyP/ITO 74k 440 34 TPyP/ZnPc/ITO 30k/30k 430 25 TPyP:ZnPc/ITO 60k 380 32 TPyP/MgPc/ITO 30k/30k 530 49 MgPc:TPyP/ITO 60k 440 57

and 2ZnPc/NTCDA layers (curve 5, 5' and 5'').

98 Phthalocyanines and Some Current Applications

values interpolated from AFM.

the emission of the AZO and ZnPc layers, respectively [79]. The intensity of the emission band with the maximum at 480 nm from the AZO substrate decreases in the structures prepared with one or two organic layers.

AZO layers were successfully transferred by PLD, in order to be further used to prepare organic heterostructure by MAPLE. The oxygen plasma treatment influences the roughness of the AZO layers. A decrease of the films roughness is obtained with the increase duration of the applied treatment. For this TCO, a high transmittance and emission with maxima at about 430 and 480 nm were evidenced. The organic heterostructures formed on the AZO substrate present also a high transmittance in the visible domain. The surface topography of the organic heterostructures is characterised by grains, smaller grains being remarked for the AZO/1ZnPc/NTCDA structure made on the treated AZO substrate.

#### **3.2. Heterostructures based on metal phthalocyanines (ZnPc or MgPc) and TPyP thin films prepared by MAPLE**

Another type of organic heterostructure has bulk active layer. The bulk heterojunction concept [80] was introduced to overpass the mismatch between the energy bands of the constituent organic materials used to form an organic cell with different layers. A bulk het‐ erojunction can be obtained using a wet method for the deposition of the organic materi‐ als, these being mixed in a solution with an adequate solvent from which are subsequently deposited films. The organic p‐n materials form an interpenetrating network. In this way, the interface between them is enlarged, having effect on the exciton dissociation and the charge transport [81].

The MAPLE method described above was used for obtaining structures with metallic phthalo‐ cyanines (ZnPc or Mg Pc) and a non‐metallic porphyrin, 5,10,15,20‐tetra(4‐pyrydil)21H,23H‐ porphyne (TPyP) as a bulk active layer or as a stacked layer, to investigate the effect of the cell architecture on the properties. In these structures, the phthalocyanines are the p‐type material and the TPyP is the n‐type material.

A flexible ITO/PET substrate (14 Ω/sq resistivity) was used as a TCO electrode. For the MAPLE deposition, the same above presented laser was involved, keeping the constant experimental conditions: 2.5% concentration of the organic material in DMSO, 300 mJ/cm2 laser fluence, 5 Hz laser frequency and 5 cm target‐substrate distance. In order to obtain layer with appro‐ priate thickness, the number of the laser pulses was varied (**Table 2**). Besides ITO/PET, sub‐ strates as a glass and silicon were used. In the structures containing blends, the materials were used in the weight ratio of 1:1 and in those having two stacked layers: the first deposited layer was the metallic phthalocyanine [50]. A schematic representation of the transferred MAPLE layers is presented in **Figure 12**.

The layers prepared were analysed from structural point of view, the diffractograms of ZnPc, MgPc, TPyP and their structures are presented in **Figure 13**. The single layers and the heterostructure based on MgPc are amorphous. In the case of the diffractograms of het‐ erostructures based on ZnPc, some lines characteristic to this material are observed (6.8, 9.1 and 13.8°) [54, 63], meaning that ZnPc presents some degree of crystallinity. The amorphous

Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications http://dx.doi.org/10.5772/67702 101

the emission of the AZO and ZnPc layers, respectively [79]. The intensity of the emission band with the maximum at 480 nm from the AZO substrate decreases in the structures prepared

AZO layers were successfully transferred by PLD, in order to be further used to prepare organic heterostructure by MAPLE. The oxygen plasma treatment influences the roughness of the AZO layers. A decrease of the films roughness is obtained with the increase duration of the applied treatment. For this TCO, a high transmittance and emission with maxima at about 430 and 480 nm were evidenced. The organic heterostructures formed on the AZO substrate present also a high transmittance in the visible domain. The surface topography of the organic heterostructures is characterised by grains, smaller grains being remarked for the

**3.2. Heterostructures based on metal phthalocyanines (ZnPc or MgPc) and TPyP thin films** 

Another type of organic heterostructure has bulk active layer. The bulk heterojunction concept [80] was introduced to overpass the mismatch between the energy bands of the constituent organic materials used to form an organic cell with different layers. A bulk het‐ erojunction can be obtained using a wet method for the deposition of the organic materi‐ als, these being mixed in a solution with an adequate solvent from which are subsequently deposited films. The organic p‐n materials form an interpenetrating network. In this way, the interface between them is enlarged, having effect on the exciton dissociation and the

The MAPLE method described above was used for obtaining structures with metallic phthalo‐ cyanines (ZnPc or Mg Pc) and a non‐metallic porphyrin, 5,10,15,20‐tetra(4‐pyrydil)21H,23H‐ porphyne (TPyP) as a bulk active layer or as a stacked layer, to investigate the effect of the cell architecture on the properties. In these structures, the phthalocyanines are the p‐type material

A flexible ITO/PET substrate (14 Ω/sq resistivity) was used as a TCO electrode. For the MAPLE deposition, the same above presented laser was involved, keeping the constant experimental

5 Hz laser frequency and 5 cm target‐substrate distance. In order to obtain layer with appro‐ priate thickness, the number of the laser pulses was varied (**Table 2**). Besides ITO/PET, sub‐ strates as a glass and silicon were used. In the structures containing blends, the materials were used in the weight ratio of 1:1 and in those having two stacked layers: the first deposited layer was the metallic phthalocyanine [50]. A schematic representation of the transferred MAPLE

The layers prepared were analysed from structural point of view, the diffractograms of ZnPc, MgPc, TPyP and their structures are presented in **Figure 13**. The single layers and the heterostructure based on MgPc are amorphous. In the case of the diffractograms of het‐ erostructures based on ZnPc, some lines characteristic to this material are observed (6.8, 9.1 and 13.8°) [54, 63], meaning that ZnPc presents some degree of crystallinity. The amorphous

laser fluence,

conditions: 2.5% concentration of the organic material in DMSO, 300 mJ/cm2

AZO/1ZnPc/NTCDA structure made on the treated AZO substrate.

with one or two organic layers.

100 Phthalocyanines and Some Current Applications

**prepared by MAPLE**

charge transport [81].

and the TPyP is the n‐type material.

layers is presented in **Figure 12**.

**Figure 12.** Schematic representation of the organic heterostructures deposited by MAPLE with stacked films (a, b) and mixed layers (c, d).

**Figure 13.** XRD patterns of the organic layers deposited by MAPLE: single layers—ZnPc (curve 1), MgPc (curve 2), TPyP (curve 3), heterostructures containing stacked films—ZnPc/TPyP (curves 4), MgPc/TPyP (curve 5) and heterostructures with mixed layers—ZnPc:TPyP (curves 6), MgPc:TPyP (curve 7).

behaviour of the phthalocyanines was also reported for films prepared by VTE [82]. So, this behaviour is independent of a deposition technique.

The AFM images (**Figure 14**) were recorded on thin films and on the structures. The granu‐ lar morphology showed by the deposited films was also reported in other papers, this mor‐ phology being characteristic to the MAPLE prepared films but also to the phthalocyanines [56, 62, 83]. Small and large grains were disclosed by the AFM images performed on mixed layers (ZnPc:TPyP and MgPc:TPyP). The RMS values extracted from AFM are between 25.0 and 56.8 nm (**Table 2**).

A small RMS value is presented by the TPyP/ZnPc/ITO structure, meaning that in the stacked structure appears a better accommodation of the TPyP molecules on the rough ZnPc film (35.7 nm). The highest RMS value was obtained for the MgPc:TPyP/ITO structure. Probably, the MgPc:TPyP blend is less homogenous in DMSO, the obtained films being characterised by bigger grains comparable with ZnPc:TPyP blend.

The optical properties of the phthalocyanines and porphyrins films were also analysed. The FTIR spectra (**Figure 15**) were recorded in order to observe if some changes appear in the structure of the materials deposited by MAPLE. Thus, the FTIR spectra of the films are shown in comparison with those of the raw powders, the IR bands from the powders appearing also in the thin films, with lower intensity (due to the film thickness). In the phthalocyanines, films

**Figure 14.** AFM images of the organic layers deposited by MAPLE: single layers—ZnPc (a), MgPc (b), TPyP (c), heterostructures containing stacked films—ZnPc/TPyP (d), MgPc/TPyP (e) and heterostructures with mixed layers— ZnPc:TPyP (f), MgPc:TPyP (g).

Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications http://dx.doi.org/10.5772/67702 103

behaviour of the phthalocyanines was also reported for films prepared by VTE [82]. So, this

The AFM images (**Figure 14**) were recorded on thin films and on the structures. The granu‐ lar morphology showed by the deposited films was also reported in other papers, this mor‐ phology being characteristic to the MAPLE prepared films but also to the phthalocyanines [56, 62, 83]. Small and large grains were disclosed by the AFM images performed on mixed layers (ZnPc:TPyP and MgPc:TPyP). The RMS values extracted from AFM are between 25.0 and

A small RMS value is presented by the TPyP/ZnPc/ITO structure, meaning that in the stacked structure appears a better accommodation of the TPyP molecules on the rough ZnPc film (35.7 nm). The highest RMS value was obtained for the MgPc:TPyP/ITO structure. Probably, the MgPc:TPyP blend is less homogenous in DMSO, the obtained films being characterised by

The optical properties of the phthalocyanines and porphyrins films were also analysed. The FTIR spectra (**Figure 15**) were recorded in order to observe if some changes appear in the structure of the materials deposited by MAPLE. Thus, the FTIR spectra of the films are shown in comparison with those of the raw powders, the IR bands from the powders appearing also in the thin films, with lower intensity (due to the film thickness). In the phthalocyanines, films

**Figure 14.** AFM images of the organic layers deposited by MAPLE: single layers—ZnPc (a), MgPc (b), TPyP (c), heterostructures containing stacked films—ZnPc/TPyP (d), MgPc/TPyP (e) and heterostructures with mixed layers—

behaviour is independent of a deposition technique.

102 Phthalocyanines and Some Current Applications

bigger grains comparable with ZnPc:TPyP blend.

56.8 nm (**Table 2**).

ZnPc:TPyP (f), MgPc:TPyP (g).

**Figure 15.** FTIR spectra of the ZnPc (a), MgPc (b) and TPyP (c) as powders (curves 1) and single layers deposited by MAPLE (curves 2).

were identified the vibrations attributed to the C‐H out of plane deformation at 725 cm−1, in‐plane C‐H bend at 754, 1088, 1114 and 1285 cm−1, the C‐C stretching in isoindole at 1333 cm−1, the C‐C stretching in benzene at 1482 and 1606 cm−1, the C‐H bending in aryl at 1490 cm−1 [56, 58]. For the TPyP, the following vibrations were attributed: 798 cm−1 to the C‐H bond in pyrrole, 1500 and 1590 cm−1 to the C‐C stretching in the pyridyl aromatic ring, 970and 3306 cm−1 to porphyrin free‐base signature [15].

Based on these results, it can be concluded that no modification appears at the MAPLE trans‐ fer of the organic materials.

The UV–VIS spectra (**Figure 16**) of the organic thin films deposited on flexible substrates have identified the absorption maxima typical to the used compounds (**Figure 16**). It can be evi‐ denced the presence of the B and Q bands (between 550 and 750 nm) characteristic to ZnPc and MgPc [62, 63]. Two submaxima are remarked in the Q band due to the π‐π\* transition, this band being localised on the phthalocyanine ring [84, 85]. The π‐π\* absorption is emphasised

**Figure 16.** Transmission spectra of the organic layers deposited by MAPLE: single layers—ZnPc (curve 1), MgPc (curve 2), TPyP (curve 3), heterostructures containing stacked films—ZnPc/TPyP (curves 4), MgPc/TPyP (curve 5) and heterostructures with mixed layers—ZnPc:TPyP (curves 6), MgPc:TPyP (curve 7).

also in the TPyP film, being specific to the free‐base ethio‐type porphyrin, with the B (428 nm) and Q (with maxima at 520, 590 and 660 nm) bands [86].

Investigating the emission properties (at 435 nm excitation wavelength) of the samples based on phthalocyanines and porphyrins was noted that those containing ZnPc present a large emission band (**Figure 17**) with a maximum at 690 nm and those with MgPc show a broader band having the maximum at ~800 nm, associated with the Davidov coupling in the phthalo‐ cyanine solid films [84].

An emission band with two maxima at 660 and 713 nm was observed in the TPyP film, these peaks being characteristic to TPyP free base [86]. In the heterostructures prepared with stacked layers, the TPyP emission bands were also evidenced (**Figure 17** curves 4 and 5). For the heterostructures made with blends, a decrease in the emission intensity attributed to TPyP (leading even to its quenching) was observed (**Figure 17**, curves 6 and 7).

Layers based on ZnPc, MgPc and TPyP were successfully transferred by MAPLE on ITO flexible substrates. Only the ZnPc presents a certain crystallinity degree when is deposited both in stacked and blend forms with TPyP, all the others organic films being amorphous.

Heterostructures Based on Porphyrin/Phthalocyanine Thin Films for Organic Device Applications http://dx.doi.org/10.5772/67702 105

**Figure 17.** Photoluminescence spectra of the organic layers deposited by MAPLE: single layers—ZnPc (curve 1), MgPc (curve 2), TPyP (curve 3), heterostructures containing stacked films—ZnPc/TPyP (curves 4), MgPc/TPyP (curve 5) and heterostructures with mixed layers—ZnPc:TPyP (curves 6), MgPc:TPyP (curve 7).

also in the TPyP film, being specific to the free‐base ethio‐type porphyrin, with the B (428 nm)

**Figure 16.** Transmission spectra of the organic layers deposited by MAPLE: single layers—ZnPc (curve 1), MgPc (curve 2), TPyP (curve 3), heterostructures containing stacked films—ZnPc/TPyP (curves 4), MgPc/TPyP (curve 5) and

Investigating the emission properties (at 435 nm excitation wavelength) of the samples based on phthalocyanines and porphyrins was noted that those containing ZnPc present a large emission band (**Figure 17**) with a maximum at 690 nm and those with MgPc show a broader band having the maximum at ~800 nm, associated with the Davidov coupling in the phthalo‐

An emission band with two maxima at 660 and 713 nm was observed in the TPyP film, these peaks being characteristic to TPyP free base [86]. In the heterostructures prepared with stacked layers, the TPyP emission bands were also evidenced (**Figure 17** curves 4 and 5). For the heterostructures made with blends, a decrease in the emission intensity attributed to TPyP

Layers based on ZnPc, MgPc and TPyP were successfully transferred by MAPLE on ITO flexible substrates. Only the ZnPc presents a certain crystallinity degree when is deposited both in stacked and blend forms with TPyP, all the others organic films being amorphous.

(leading even to its quenching) was observed (**Figure 17**, curves 6 and 7).

and Q (with maxima at 520, 590 and 660 nm) bands [86].

heterostructures with mixed layers—ZnPc:TPyP (curves 6), MgPc:TPyP (curve 7).

cyanine solid films [84].

104 Phthalocyanines and Some Current Applications

The morphology with grains can be attributed also to the phthalocyanines but also to the MAPLE deposition method. The FTIR spectra confirm that the film deposited by MAPLE preserves the vibrational properties of the raw materials. The optical properties of the films evidenced a large absorption domain in the visible range. A quenching of the photolumines‐ cence in the bulk heterostructures was observed.
