Iodine Doping Implementation Effect on the Electrical Response in Metallophthalocyanines (M = Cu, Co, Zn), for Electronic and Photovoltaic Applications

Abstract

We report the structural organization and its effect on the current response of the conducting domains in MPcs (M = Cu, Co, Zn) films, deposited by vacuum thermal evaporation and doped by the presence of iodine vapors. Structural and surface features of the doped metallophthalocyanines (MPcs) were studied by using IR spectroscopy, X-ray diffraction, atomic force microscope (AFM) and scanning electron microscope (SEM). DFT calculations were carried to study the interaction between iodine and MPcs molecules and establish the influence of iodine on the electronic behavior of these species and the changes on the frontier molecular orbitals. This interaction is thermodynamically favored, and the mechanism of electronic transit involving the iodine atoms providing electrons to the transfer. The I-MPc films have a mainly amorphous structure, some crystallinity in the MPcs α and β forms. A roughness between 18.41 and 99.02 nm and particle size between 1.35 and 15 μm. By evaluating the electrical behavior of the flexible PET/ITO/I-MPc/Ag devices, it was found that J-V curves under illuminated conditions show an increase of curves values upon the I-MPc, indicating that the flexible films are photosensible. J_sc between 1.59 × 10⁻⁵ and 2.41 × 10⁻⁷ A/cm², conductivities between 6.17 × 10⁻⁸–2.54 × 10⁻⁷ Scm⁻¹ and photosensibility values of up to 133%.

1. Introduction

In recent years, electronics has had a good development in the organic semiconductive materials, especially in optoelectronics devices and biomedicine applications. Some examples are in the construction of electronic microdevices for monitoring the heart rate [1], or implants of biological materials in biosensors [1], in the generation of electric current as energy-carrying coupled to organic molecules in cells made from fuel [1]. Furthermore, the control of the orientation and spacing of functional groups in organic molecules, and the formation of organic monolayers on inorganic surfaces is, within molecular electronics, one of the topics of leading interest. This is due to its potential applications in the manufacture of biosensors or of other types of devices such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) and in photovoltaic devices [2]. Therefore, the development of organic semiconducting materials is an area that has found multiple applications for organic and heterocyclic conjugated molecules with high chemical stability and one important example of this is represented by phthalocyanines (Pcs) [3,4,5,6,7,8]. Pcs are a class of organic materials which is constituted by four isoindole units through nitrogen atoms. These macrocycles are made up of 42 π electrons, which span 32 carbon and 8 nitrogen atoms. [5,9]. Its electronic delocalization takes place on the inner ring system, so the phthalocyanine ligand is formally considered as an aromatic system formed by 16 atoms and 18 π electrons, condensed in four benzene rings that retain their electronic structure [5,9]. The most stable structure of the Pcs is the flat one with a C_4h molecular symmetry [6,7]. Pcs are stable up to temperatures above 500 °C, which is not typical for organic materials. This thermal stability is due to the conjugation of bonds throughout their supramolecular structure [10]. In addition, Pcs have high absorption in the near-IR UV-vis region of the electromagnetic spectrum, which is very characteristic due to the existence of two easily distinguishable absorption bands: The B band, which is the least intense and it is around 300–400 nm, and the Q band, with the highest intensity between 630–700 nm. Both arise as products of π-π transitions [11]. A vibrionic band is also observed around 600 nm, a result of higher vibration levels of the electronic state [11]. Pcs are used as colorants and pigments in the textile and paint industries and as photosensitizers in the photodynamic therapy of cancer [12]. Additionally, Pcs are compounds that possess high chemical versatility. The two hydrogens that Pc presents in the cavity of the molecule (H₂Pc) can be replaced by more than 70 types of metal atoms, which gives rise to metallophthalocyanines (MPcs). The MPcs have a symmetrical structure composed of eight rings that surround the metal nucleus and have different spacing, which is where these metal atoms can be attached [13]. MPcs are insoluble in water, which makes them attractive to the pigment industry. However, if the need arises to make them soluble in water, hydrophilic groups are introduced into the molecule as substituents [10]. MPcs are structures that can incorporate a great variety of substituents on the outside of the ring (peripheral and non-peripheral positions) or bind them to the metal in axial positions, modifying their physical and chemical properties [2]. In terms of properties, semiconductivity is a characteristic of MPcs, which can behave both as p-type semiconductors and n-type semiconductors. The above depends on the metal bond in the axial positions of the metal coordination sphere and the substituent present on the outside of its ring. The inclusion of substituents in the MPcs as dopants can cause a structural rearrangement in the MPc packaging, affecting its semiconductivity. In this sense, investigating the changes in the semiconductor behavior of the Pc in the presence of different metals and substituents is of fundamental importance to evaluate the suitability of the resulting system, in terms of applications [13,14]. One of the most studied MPcs is copper phthalocyanine (CuPc) due to its large ring binders closely related to porphyrins, and because constitutes an important class of tissue dyes [15]. Its strong absorption in the Q band (600–800 nm) and its excellent photoinduced charge generation efficiency ease its application in xerographic photoreceptors of laser printers [16]. CuPc is mainly used in thin films, which can be easily deposited by evaporation in a high vacuum [16]. The introduction of various substituents in the CuPc macrocycle can significantly alter the structure and morphology of thin films, which lead to a change in their electrical and detection properties [17]. ZnPc is another example of well-known MPcs, used as a paint pigment [6] and in the industry of catalysts, photoconductors and photodynamic therapy [18]. Thanks to its thermal stability [6], it can be obtained with high purity by successive sublimations; therefore, it is possible to use ZnPc as an electronic component. Although its conductivity is quite low (σ = 10⁻⁹ − 10⁻¹⁰ Scm⁻¹) for a thin film, it is not much lower than the usual conductivity of messy mineral electronic components such as SiH [6] and can be increased by the introduction of dopants. For example, doping with halide ions can alter the properties of MPcs for applications in optoelectronics [19,20,21].

In the present work, all these factors were considered, and the CuPc, ZnPc and CoPc were doped with iodine for the manufacture of hybrid semiconductors, with improvements in their electronic properties [20]. Initially, the interactions of phthalocyanine molecules with iodine were theoretically studied by Density Functional Theory (DFT) calculations and additionally were obtained the frontier molecular orbitals HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital). Subsequently, MPc (M = Cu, Co, Zn) films were deposited using the high vacuum evaporation technique and iodine doping was performed by exposing the deposited MPc film to iodine vapors (I-MPc). The structural and morphological properties of doped films are also studied, and finally, flexible devices PET/ITO/I-MPc/Ag were manufactured and the electronical properties of I-MPc are reported.

2. Theoretical Calculations

The optimization process of all species was carried out in two stages. In a first step, 10,000 stage Molecular Mechanic conformational search using Amber force field with the Amber parameter set [22] was used, in order to achieve the global minima. Additionally, the used version was included in the NWChem package [23]. Next, in a second step, a DFT method based on the combination of Becke’s gradient corrections [24] for exchange and Perdew-Wang’s for correlation [25] was applied. This is the scheme for the B3PW91 method included in the Gaussian16 package [26,27]. All the calculations were performed using the 6–31 g * basis set. Frequency calculations were carried out at the same level of theory in order to confirm that the optimized structures were at the minimum on the potential surfaces. Frequencies calculations were carried out at the same level of theory in order to confirm that the optimized structures are at the minimum on the potential surfaces. The thermodynamic values obtained in this step were used to evaluate the free energy of the isodesmic reactions.

3. Materials and Methods

CuPc (copper(II) phthalocyanine: C₃₂H₁₆CuN₈) with 99% purity, CoPc (cobalt(II) phthalocyanine: C₃₂H₁₆CoN₈) with 97% purity and ZnPc (zinc phthalocyanine: C₃₂H₁₆N₈Zn) with 97% purity were obtained from commercial suppliers (Sigma-Aldrich, Saint Louis, MO, USA) and required no further purification. MPcs films were deposited on polyethylene terephthalate film, indium tin oxide coated (PET-ITO), corning glass, silicon p-type and silicon n-type substrates. All substrates, excluding PET-ITO, were cleansed under an ultrasonic process and dried in vacuum. Each MPc was placed in a molybdenum crucible for evaporation and were heated at different temperatures to produce their phase change. The films were deposited upon contact with the substrates, which were at room temperature and at a vacuum pressure of 1 × 10⁻⁵ Torr. The data obtained from evaporation measurements for each compound were based on the deposition rate and its thickness: for CuPc, this was 0.3 Å/seg and 135 Å, for CoPc, this was 1.8 Å/seg and 263 Å and for ZnPc, this was 0.1 Å/seg and 73 Å. Each layer thickness was monitored thought a quartz-crystal microbalance monitor connected to a thickness sensor (Intercovamex, S.A. de C.V., Cuernavaca, Morelos, Mexico). After depositing the MPc by the high vacuum method, the MPc films were doped with iodine. Iodine doping of all the films was achieved by the sublimation of the vapors from iodine crystals contained in a sealed vessel at a temperature of 45 °C for 20 min. The amount of iodine used was 2.6 g for each MPc. To verify the main functional groups of the semiconductors, infrared (IR) spectroscopy analysis was performed on a Nicolet iS5-FT spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) on a wavelength range of 4000 to 500 cm⁻¹ using KBr pellets and thin films on n-silicon wafers. The X-ray diffraction (XRD) analysis was performed with the θ–2θ technique using a Bragg-Brentano Geometry with a Bruker (Bruker Nano GmbH, Berlin, Germany), D8 Advance diffractometer and working with CuK-α (λ = 0.15405 nm) radiation. The samples were measured at 0.6°/min, interval 5–70 and the grazing angle was 1.0°. Atomic Force Microscopy (AFM) measurements were performed for the films on silicon substrates under ambient conditions, in contact mode with a Nanosurf Naio AFM (Nanosurf, Liestal, Switzerland). Scanning electron microscopy (SEM) was performed on a ZEISS EVO LS 10 scanning electron microscope (Zeiss International Inc., Göttingen, Germany). The electrical behavior of the flexible devices PET/ITO/I-MPc/ag was evaluated by the four-point method using ITO as anode and Ag as cathode. Silver paint was used for the contacts. Electrical properties were measured using a sensing station with a Next Robotix (Comercializadora KMox, S.A. de C.V., Mexico City, Mexico) lighting and temperature controller circuit and a Keithley 4200-SCS-PK1 auto-ranging picoammeter (Tektronix Inc., Beaverton, OR, USA).

4. Results and Discussion

4.1. DFT Calculations

The complexes were built by placing the MPc structures of both complexes (M = Cu, Co and Zn) in front of a layer of iodine atoms. This layer was built as a frontal face of an orthorhombic crystal of pure iodine. Considering the centered shape of the crystal, the layer contains five iodine atoms, as shown in Figure 1.

This layer was optimized in the same conditions of all the other structures and the result was a regular trapezium. On the other hand, the structures of the MPcs (M = Cu, Co, Zn) were optimized, in both cases a deformation of the planar shape is present and the definitive geometries are shown in Figure 2.

There is a pyramidalization process where the metal atom leaves the plane to come closer to an iodine atom, while the other atoms move in a minor degree to join the remaining iodine atoms. These bonds can be considered as ionic attractions because the net Mulliken charge of the metal atom is 1.2 for Cu, 1.38 for Co and 0.95 for Zn. The average charge on the iodine atoms is 0.35, and the average length between iodine atoms and the surface of the ring in the optimized cases is 2.89 Å. The energy results derived from the thermochemistry study reveal a preference of the species to associate with the iodine layer; the results arise from the next isodesmic reaction:

Metal-complex + iodine layer → iodine complex

It is −69 kcal/mol for the copper, −51.6 kcal/mol the cobalt and −48.8 kcal/mol for the zinc complexes, respectively. Additionally, the process free energy was obtained taking advantage of the thermodynamic results from the frequencies calculations, and the results are −25.9, −21.6 and −19.8 kcal/mol, respectively. The calculated HOMO-LUMO gap values for all complexes without interactions is 1.36 for the Cu, 1.17 for the Co and 2.34 eV for the Zn complexes; all of them can be considered as good targets in semiconductor behavior. These values dramatically change when the iodine surface interacts with the naked complexes; the new values are 0.99, 0.84 and 0.52 eV again for Cu, Co and Zn iodine doped complexes, respectively. The transit of electrons also changes when iodine is included because it provides the electrons of the HOMO. Figure 3 shows the shape of HOMO and LUMO for all cases and in

Table 1. HOMO, LUMO and bandgap of the intrinsic and doped MPcs (M = Cu, Co, Zn).

Molecule	HOMO	LUMO	Bandgap (eV)
CuPc	−8.43	−7.07	1.36
CuPc iodine doped	−5.30	−4.31	0.99
CoPc	−5.47	−4.30	1.17
CoPc iodine doped	−8.38	−7.53	0.84
ZnPc	−5.28	−2.94	2.35
ZnPc iodine doped	−5.55	−4.38	0.52

the energy values for the orbitals are shown, as well as the band gap for the I-MPc system; their low band gap energy values allow their application in optoelectronic devices. As a conclusion, it is possible to suggest that the doping with iodine consists in the ionic interaction between the naked complexes and iodine surfaces of crystal iodine. This interaction is thermodynamically favored and the mechanism of electronic transit changes involving the new iodine atoms, which provide electrons to the transfer.

4.2. Deposit and Characterization of Thin Films

The results obtained with the DFT calculations indicate that it is possible to generate an interaction between iodine and MPcs molecules. In addition, the presence of iodine considerably decreases the HOMO-LUMO gap, which can result in an improvement in the semiconductor behavior of I-MPc films. Based on the above results, MPc (M = Cu, Co, Zn) films were deposited, subsequently doped with gaseous iodine and characterized with IR spectroscopy. The IR spectra shown in Figure 4a were obtained in order to verify the stability of the doped phthalocyanines. In the spectra are observed the bands responsible for C = N at 1471 ± 3 and 1333 ± 2 cm⁻¹, while the bands located at 1166 ± 3, 1122 ± 2 and 755 ± 4 cm⁻¹ are the result from the C-H interaction [28,29]. The bands in 1606 ± 6 and 1087 ± 2 cm⁻¹ result from a C = C stretch within the macrocyclic ring [30]. The presence of the characteristic signals of the MPcs are an indication of the stability of the films after the deposition process and in the presence of iodine. Additionally, the IR bands in the 700–800 cm⁻¹ region of the spectra are used to identify different polymorphs α and β in MPcs [28,31]. The phases are categorized according to the crystalline stacking angle between the molecular plane and b-axis, 65° and 45° for α and β, respectively [19]. The characteristic signals of the α and β forms in the IR spectra are found around 720 cm⁻¹ for the α-form and around 777 cm⁻¹ for the β-form [28,31,32]. In the case of the three films, the spectra present the signals in 725 ± 1 and 776 ± 1 cm⁻¹. From these results, it is clear that the α and β forms are present in the MPcs and to complement the above information XRD was performed. According to the XRD patterns shown in Figure 4b, the presence of the central metal ion in the phthalocyanine is responsible for the crystalline or amorphous structure in the film and the iodine doping responsible of disorder [19]. It is evident that the I-CuPc film has the highest crystallinity, followed by the I-CoPc film. Although the I-ZnPc film is the least crystalline, it also shows the diffraction peak at 2θ = 6.8° (d = 12.98 Å) that corresponds to the (100) plane of β-form in MPcs [19]. For I-CuPc the intensity of this peak has diminished indicating that the structural organization of the β-form is disrupted [19]. Additionally, the peak around 2θ = 27° (d = 3.21, 3.31 Å) for I-CoPc and I-CuPc films indicates the growth of small crystallites lying parallel to the substrate surface plane and their stacking axes being inclined to it [33,34]. This peak also corresponds to the β-form [35,36]. It is important to consider that, due to the low crystallinity of the films, it is not possible to distinguish the presence of the α-form. Moreover, a representative grazing incidence X-ray diffraction measurement was conducted for I-CoPc, which supports the XRD patterns of I-MPc films (Figure 4) and the previous observations. The obtained pattern can be found in the Supplementary Materials (Figure S1).

The morphological features of I-MPc films were studied with the help of AFM and SEM and the images are shown in Figure 5 and Figure 6, respectively. The topography shows marked differences between the I-MPc films. According to Madhuri et al. [19], doping causes a structural rearrangement in MPc packing, resulting in a structural disorder and a morphology change. For the silicon substrate, especially in the case of I-ZnPc, the film is formed by to elongated and flat features, and smaller individual nanocrystallites are found beneath the larger features. The elongated structures are oriented at an angle from the substrate surface [37]. The RMS (root mean square) roughness is also different in each film, 43.32 nm for I-CuPc, 99.02 nm for I-CoPc and 18.41 nm for I-ZnPc. It is the I-ZnPc film which presents a lower roughness and similar to that obtained by Madhuri et al. [19] for iodine-doped PbPc films. Apparently, this film would be the one that would present the best semiconductor behavior. Therefore, to complement this information, the particle size, morphology and size distribution were determined by SEM. With respect to the SEM studies shown in Figure 6, small differences are observed for the three I-MPc films. However, the size of the particles that integrate them changes. According to the graphs in Figure 6, the average diameter for the particles that form the I-CuPc film is between 1.35–3.35 μm, between 3.1–4.75 μm for the I-CoPc film and between 1.50–3.85 μm for the I-ZnPc film. The particles of the films with CuPc and ZnPc have quite similar average size, but it is the film with ZnPc which presents particles of a more uniform size. Additionally, it is observed that the CoPc presents clusters formed by small particles, where the maximum cluster size is of approximately 15 μm. ZnPc has the higher particle density among the films. Complimentary SEM images of the I-MPc films can be found in the Supplementary Materials (Figures S2–S4).

The iodine induces changes in the morphology, higher degree of disorder and increase in surface roughness in MPcs films [19,37,38,39]. However, the metal present in the phthalocyanine and the iodine used as a dopant are the main causes of the change; for example, Yao et al. [38] studied the presence of iodine in CoPc films deposited on Au and found that the changes in the morphology of the films are due to the fact that the iondine atoms become embedded among the CoPc molecules. Similar rearrangements and disorder were visualized in planar MnPc films for Madhuri et al. [19] and for Kobayashi et al. in NiPc [39] doped with iodine. In the present study, the crystallinity of the I-ZnPc film, added to its lower roughness and higher particle distribution, may cause better electronic properties in this film compared to I-CuPc and I-CoPc.

Electrical characterization was conducted for the flexible doped films PET/ITO/I-MPc/Ag and J-V curves were obtained shown in Figure 7a for dark and illuminated conditions. First, it can be observed that most of the J-V curves are not symmetrical. For the forward bias region, I-ZnPc presents the highest current values, followed by I-CoPc and I-CuPc, respectively. The curves shape resembles a Schottky diode curve and varies upon the MPc. The curves on the forward bias region present higher current density values than for the reverse bias region. The J-V curves under illuminated conditions presented in Figure 7a show an increase of the current density values for all the I-MPcs curves, but the degree of change due to the incident light changes upon the I-MPc, indicating that the flexible films are photosensible. This may be indicative of the photovoltaic effect essential for the suggested application. However, an apparent higher current change is observed for the I-CuPc. On the other hand, for the reverse bias region, I-ZnPc shows an opposite change consequence of the illumination conditions, which may be related to a change in the conduction mechanisms and the interaction with light, due to an increase of trapping sites. Figure 7b shows the semi-Log J-V curve for the flexible films, where the previous mentioned results can be observed. Additionally, a change in the J_sc (short circuit current density) with the I-MPc can be observed and changes for the illuminated condition.

Table 2. Electrical properties of doped MPcs (M = Co, Zn, Cu) for darkness and illuminated conditions.

Sample	Conductivity	Jsc	Photo-Current Density @ 0 V	Ideality Factor	Io
	S/cm	A/cm2	A/cm2	n	A
I-CoPc darkness	2.54 × 10−7	2.41 × 10−7	1.35 × 10−7	2.26	3.90 × 10−6
I-ZnPc darkness	2.02 × 10−7	1.59 × 10−5	2.33 × 10−6	2.37	1.28 × 10−5
I-CuPc darkness	6.17 × 10−8	1.26 × 10−6	5.09 × 10−6	2.20	1.79 × 10−6
I-CoPc illuminated	3.00 × 10−7	3.76 × 10−7		2.25	4.59 × 10−6
I-ZnPc illuminated	2.25 × 10−7	1.83 × 10−5		2.33	1.34 × 10−5
I-CuPc illuminated	1.44 × 10−7	6.36 × 10−6		2.14	4.01 × 10−6

shows the electrical properties of doped MPcs (M = Co, Zn, Cu) for dark and illuminated conditions, where the conductivity, J_sc, Photo-current density @ 0V, Ideality Factor and I_o resulted from the obtained J-V curves (Figure 7a). Comparing the I-MPcs on darkness conditions, the highest J_sc is observed for I-ZnPc (1.59 × 10⁻⁵ A/cm²) and the lowest for I-CoPc (2.41 × 10⁻⁷ A/cm²), which is two orders of magnitude larger for I-ZnPc. For all the films, an increase of the J_sc is observed for the illuminated conditions, where the photo-current density at 0V is more pronounced for the I-CuPc, followed by I-ZnPc and I-CoPc. However, the increased ratio of the current density is observed to be the largest for I-CuPc (504%), followed by I-CoPc (156%) and I-ZnPc (115%), which may be related to the films’ crystalline ratio and may increase the charge carrier generation and transport efficiency. The resulting conductivity shows an increase for the illuminated condition (1.44 × 10⁻⁷–3.00 × 10⁻⁷ Scm⁻¹) compared with the dark condition (6.17 × 10⁻⁸–2.54 × 10⁻⁷ Scm⁻¹), where the lowest conductivity is observed for I-CuPc, while the highest is observed for I-CoPc. However, the photosensibility is observed to be the largest for I-CuPc (133%), followed by I-CoPc (18%) and I-ZnPc (11%). The obtained conductivity values yield close to the literature [40,41,42]. Considering a diode curve and equation, the ideality factor was calculated and resulted in values larger than the ideal diode (1), where the obtained values for the films lay between 2.20 (CuPc) and 2.37 (ZnPc) for the dark condition, and 2.14 (CuPc) and 2.33 (ZnPc) for the illuminated condition. The previous indicates that the shape of the curve varies compared to the ideal diode curve and that the observed decrease shows that the illumination affect the shape of the curve tending to the ideal curve. Moreover, the I_o calculation resulted in values between 10⁻⁶ and 10⁻⁵ A and increases with the incident light. However, it is interesting to note that I-CuPc also presents the lowest I_o.

Figure 7c,d show the flexible I-MPc films voltage-dependent resistance and voltage-dependent normalized resistance. The resistance values for all the films were between 1 kΩ and 18 kΩ. It can be observed that I-CuPc presents the largest resistance for the voltage interval and a decreasing trend is observed with the voltage. Moreover, a change in the curve behavior is observed for the reverse bias region compared to the forward bias region for all the thin films. I-CoPc shows an increasing trend to a maximum at 0 V (5 kΩ) and a further decrease to the approximately the initial value. I-ZnPc presents the lowest resistance among the films, and an almost constant value of approximately 1 kΩ. To analyze the curves in detail, the curves where normalized to their maximum and plotted in Figure 7d. The I-ZnPc curve presents an increase on the resistance with the voltage to a maximum and a step decrease at 0 V, which is followed by a slight decrease on the resistance. Comparing the darkness and illuminated curves (Figure 7c), it can be observed that there is a large decrease in the resistance values for the I-CuPc, which may cause the previous observations in Table 2. Additionally, a decrease of the resistance, while in a smaller manner, is observed for I-CoPc. However, for I-ZnPc, two different behaviors are observed: for negative voltages, there is an increase on the resistance values with the light, while for positive voltages, a decrease is observed.

To further investigate the previous photovoltaic effect, J-V and semi-Log J-V curves for doped CoPc, ZnPc and CuPc were obtained for different incident light colors (green, yellow, orange, red, UV) and plotted in Figure 8. The dark J-V curve was included for comparison. Figure 8a shows the J-V curve for I-CoPc, where a similar curve is observed for the different incident light colors, but with small increment of the values than the dark conditions curve. Figure 8b shows the J-V curve for I-ZnPc, where a slight variation of the curves is observed for the different incident light colors, but with larger increment of the values than the dark case. Additionally, it can be observed that the incident light color effect on the J-V curve is more pronounced for positive voltages, and the greatest is for the yellow light. Figure 8c shows the J-V curve for I-CuPc, where an incident light color-dependent variation of the curves is observed. An increment of the current density values than the darkness curve is observed, where the incident wavelength apparently shows a marked variation compared to the other films. Additionally, it can be observed that the incident light color effect on the J-V curve is dependent of the polarization voltage, and the greatest change is for the yellow light.

Photo-current density against voltage was plotted in Figure 9 to analyze in more detail the light effect and the possible application of these flexible films in optoelectronic applications. Figure 9a–c show the resulting plots for I-CoPc, I-ZnPc and I-CuPc, respectively. The results show that the highest photocurrent density is obtained for I-ZnPc (10⁻⁴ A/cm²), followed by I-CuPc (10⁻⁵ A/cm²) and I-CoPc (10⁻⁷ A/cm²). Depending on the MPc, the incident light color affects in a different manner the photogenerated current, which might be related to a variation in an absorption properties and photo-generated charge carriers’ mobility. For instance, I-CoPc shows the largest photo-current with a green color, while the least occurs with the UV radiation. For I-ZnPc, the yellow is the largest and the UV the least, and for the I-CuPc case, the yellow is the largest and the red the least. Additionally, the change caused by the incident light in the photo-current is more pronounced for the I-CuPc, and less pronounced for I-CoPc. All of this indicates that these flexible thin films may be used for different optoelectronic applications, for example on photosensors and solar cells.

5. Conclusions

The effect of iodine doping exposure on the current response of the conducting domains in metallophthalocyanines (M = Cu, Co, Zn) films are revealed in the present study. The initial results of the DFT calculations showed that an interaction between iodine and MPc takes place, giving rise to ionic complexes in which sets of iodine atoms interact with the MPc molecules breaking the planarity and causing strong electronic changes. An isodesmic reaction simulating the formation of the iodine complex was carried out, taking advantage of the obtained thermodynamic values from the calculations. In the three cases, negative free energy values were obtained showing the natural preference to form the complex. In this way, the iodine atoms provide electrons to the transfer and the HOMO-LUMO gap decreases. With respect to the I-MPc films, there is a marked difference in their topography and morphology, which depends on the dopant, but more importantly, it depends on the central metal of the phthalocyanine. The devices present a photoresponse that is dependent of the central atom, resulting in photosensibility values of up to 133%, which may be used for photovoltaic devices.