Optical and electrical evaluation of a nylon 11-ZnO film incorporation in disodium phthalocyanine heterostructures

Abstract

Disodium phthalocyanine (Na₂Pc) has been considered as promising organic semiconductor for optoelectronic applications, however, Na₂Pc-based devices are not being fully studied. To study the Na₂Pc as active layer for optoelectronic devices, and also the effect of the incorporation of a nylon 11-ZnO film within the device heterostructure, in this work two heterojunction cells of Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al were fabricated by vacuum thermal evaporation technique. The topography of the heterostructures was analyzed by atomic force microscopy. Later, the films that form the heterostructure were optically characterized by Ultraviolet-visible spectroscopy and the optical band gap and the Urbach energy were obtained. The values of the optical gap for the films are between 2.08 and 2.93 eV. The value of Urbach energy is lower for the pristine triphenylamine film, and higher for the film composed of nylon and ZnO. Moreover, the two heterostructures present a strong photoluminescence in the visible region. On other hand, by introducing the nylon 11-ZnO film in the heterostructure shows an important increase of the short circuit current density (Jsc), conductivity and photocurrent values, indicative of a better mobility and more carriers generation. For this heterostructure a mean 79.7% external quantum efficiency and 0.48 mA/cm² Jsc were obtained. These results indicate that these heterostructures can be used toward organic solar cell applications and remark on the large effect of the nylon 11-ZnO dielectric film addition on their performance.

Introduction

The cofacial stacking of the metallic phthalocyanines (MPcs) allows the electrical current conduction in one-direction, along their stacking axis. This conduction is due to the generation of conducting bands by overlapping and unfolding of the π orbitals, when MPc molecules overlap with other molecules. Their organization in condensed phases has been carried out by various methods, such as columnar mesophases [1–4], skewered polymer [5, 6], aclipped columnar aggregates mediated by alkali metals [7, 8] or in thin films [9–11]. Diverse organizations have eased the application of MPcs in optoelectronic devices, where the conductivity may be increased by oxidant or reductor doping [10, 12, 13], fulfilling the application requirements. Additionally, properties such as its electrochromism have demonstrated their potential use in electrochromic devices [14, 15]. Also, water-soluble MPcs have been studied as photosensitizers, demonstrating their effectiveness in cancer photodynamic therapy [16, 17]. Moreover, it has been studied the MPcs capacity as optical limiters [18–20], thus the possibility to apply these organic semiconductors in such devices that show an intensity dependent light transmission. Finally, it must be highlighted the applications of the MPcs in what refers to photovoltaic cells [21–23]. Although the materials most commonly used in photovoltaic devices are inorganic such as silicon, over the last few decades, significant efforts have been conducted in the development of organic solar cells. In relation to their optical properties, the MPcs present the most important absorption around 700 nm with a high extinction coefficient. This wavelength roughly corresponds to the highest emission flux intensity of the solar spectrum, making the MPcs excellent photosensitizers or light-harvesting molecules (antennas). Besides, when they are incorporated in donor-acceptor systems, in a covalent way or in homogeneous mixtures with other photo and electroactive molecules, they exhibit photoinduced electron transfer properties. This phenomenon together with photoconductivity, is fundamental in the manufacture of solar cells and solar energy conversion devices [24, 25]. There are two types of organic photovoltaic devices where phthalocyanines have been incorporated: hybrid solar cells sensitized by dye and organic solar cells. The first type corresponds to electrochemical solar cells, also known as Grätzel type cells [26, 27]. These cells consist of the covalent attachment of a dye, like the MPc, to the surface of a binary mesoporous semiconductor, like TiO₂ and In₂O₃ or ternary material like Sr₃TiO₃ and Zn₂SnO₄. The most relevant examples have been those that use zinc carboxyphthalocyanines anchored to the TiO₂ surface[28]. External quantum efficiencies of 80% at the maximum absorption wavelength of the phthalocyanine (686 nm), with global efficiencies of around 4% have been reached with these kind of devices [29]. When these photovoltaic cells are cosensitivized with other dyes, enlarging the device absorption spectrum range of solar flow emission, global efficiencies around 8% have been reached [30]. The second type of solar cells with MPcs are those where the active layer is located between the two electrodes and it perform the task of capturing light and allowing the charge carriers movement to the electrodes, after charge separation process is generated. Although their efficiency is inferior to the hybrid Grátzel type devices, they have the advantage of being cheaper, easy to process and easier to deposit on different types of substrates. These MPcs cell types have stood out with interesting results, however their power conversion efficiencies are still very low [10, 30, 31]. The above makes it necessary to broaden their study with (i) different metallic atoms in the macrocycle center, (ii) changes in the substituents of the MPcs external structure, or (iii) different solar cells architecture type. Respect to the architecture, tandem type photovoltaic cells that present a multilayer architecture consisting of the overlapping of at least two heterojunctions of polymer organic semiconductors [32], molecular semiconductors [33, 34] or both [35], have been developed. Where one of them is an electronic donor and the other an electronic acceptor type. Although these solar cells are more efficient, they have not been optimized yet in such a way that allows them to achieve the Grátzel type device efficiency. Nevertheless, for phthalocyanine base devices, a 10⁻⁴–10⁻⁶ s carrier lifetime depending on their architecture and composition was reported, directly affecting the organic solar cells performance [36, 37].

Because of the above, in the current study it is proposed the development of two organic devices where the active layer was formed by the Na₂Pc, in a new way as a disperse heterojunction with TCNQ. Although MPcs have been widely studied as components in optoelectronic cells and devices, the inclusion of two metal atoms in the Pc macrocycle has been little studied. Some authors of this work have been pioneers in the study of the electrical behavior of M₂Pcs such as Na₂Pc or Li₂Pc with dopants like tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) [38, 39]. Mainly in the comparative study of the charge transport mechanism between Na₂Pc and PbPc with functionalized 1,4 dihydropyridine [40], and in the study of Na₂Pc film as part of the electronic devices deposited on poly(ethylene terephthalate) [39]. Additionally, Alosabi et al. [41] have studied the electrical and dielectrical properties of Na₂Pc, resulting a good candidate for optoelectronic applications. However, the Na₂Pc application in optoelectronic devices like solar cells has been very little explored. Also, to explore possible improvements to the proposed device architecture, the present work deals with the effect of inserting a dielectric layer of nylon 11-ZnO on the electrical properties of Na₂Pc + TCNQ/triphenylamine/p-Si cells and comparative characterization of these devices as a new type of organic solar cell configuration. The addition of this dielectric layer in contact with the electrode allows the increase of light absorption by trapping the light within the device structure, but this should be transparent to the visible light and with relatively high refractive index [42]. On the other hand, by using a nylon 11-ZnO layer the carrier diffusion length of nylon 11 may be increased by adding the ZnO particles, compared to other efficient dielectric materials [43, 44]. Therefore, in this work the devices with the architecture Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al were manufactured, and were compared and evaluated in their optical, and electrical behavior toward their photovoltaic application as light absorbers.

Materials and methods

All reagents and solvents were obtained from commercial suppliers and used without further purification. The active layer of the organic cell is made up of a disperse heterojunction: disodium phthalocyanine (Na₂Pc:C₃₂H₁₆N₈Na₂) + 7,7,8,8-tetracyanoquinodimethane (TCNQ:C₁₂H₄N₄). This active layer, of Na₂Pc with TCNQ, was mechanically doped using an agate mortar. Subsequently, triphenylamine ((C₆H₅)₃ N) and Na₂Pc + TCNQ, were sequentially deposed on p-Si by vacuum thermal evaporation technique to fabricate the organic device Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al (see Fig. 1). A p-Si substrate was used due to its ability to form hetero-junctions with organic compounds films [21, 45, 46] and its features are: silicon wafer single side polished, backside etched, Root Mean Square (RMS) = 2.37 nm ± 0.29 nm, < 100 > ± 0.9°, contains boron as dopant, thickness 381 μm ± 25 μm and resistance 1–100 Ωcm. This silicon substrate was first washed with a p-solution (10 ml HF, 15 ml HNO₃, and 300 ml H₂O), in order to clean and remove surface oxide. All the films were deposited sequentially according to the scheme in Fig. 1a, and through a vacuum thermal evaporation system, HVTE using tantalum crucibles. The aluminum sublimated at vacuum pressure of 10⁻⁶ Torr and the deposition rate was 8 Å/s, the triphenylamine sublimated at a vacuum pressure of 10⁻⁶ Torr and the deposition rate was of 50 Å/s and for the Na₂Pc + TCNQ film the deposition rate was 10 Å/s at 10⁻⁵ Torr. The thickness was evaluated using rate/thickness monitor Inficon-SQM-160 of a high-resolution quartz microbalance, connected to a thickness sensor. The thicknesses obtained are: 2400 Å for the Al film, 700 Å for the triphenylamine film and 188 Å for the Na₂Pc + TCNQ film. The next stage for the fabrication of the cell: Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al (Fig. 1b) includes all manufacturing steps of the previous cell and additionally, the deposit of the dielectric film, composed of zinc oxide (ZnO) particles embedded in a nylon 11 (polyundecanolactam [–NH(CH₂)₁₀CO–]_n) matrix via a vacuum thermal evaporation deposition technique at a constant pressure of 1 × 10⁻⁶ Torr and a constant deposition rate of 28 Å/s and the thickness for this compound film was 2199 Å. After deposition, in order to penetrate ZnO particles into nylon 11 matrix a thermal relaxation technique was made at a temperature of 120 °C for 12 min, and followed by the electrode contacts deposition. The thermal relaxation technique relies on (i) the consecutive evaporation of a nylon 11 pellets and ZnO particles, and then (ii) the heat treatment. The heat treatment subsequent to film deposition leads to dispersion of ZnO particles owing to the structural relaxation of polymer matrix [43, 44]. After fabricating the heterostructures, their topographical characteristics were investigated with an atomic force microscope (AFM) Nanosurf using a Ntegra platform. The optical parameters of the films were obtained in the 200–1100 nm wavelength range, on an UV–Vis 300 Unicam spectrophotometer. Photoluminescence (PL) was measured at room temperature using a He–Cd laser with an excitation wavelength of 325 nm, constant power, and integration time of 100 ms. The emission spectra were recorded over a range of 350–1050 nm with a 10 nm step. The electrical characterization was carried out on the 5.97 cm² surface devices using the two-probe method by an auto-ranging Keithley 4200-SCS-PK1 on a − 1.5–1.5 V range, where silver contacts were painted using a silver paint and with no specific configuration. The latter due to firstly study the device behavior under darkness and light conditions, but for future work a contact design to optimize the output should be conducted. The devices were illuminated using a lighting and temperature controller circuit from Next Robotix, using commercial LEDs. For these LEDs, typical operating voltages varied between 1.8 and 2.8 V, with an operating current of 18 mA. The external quantum efficiency (EQE) of the devices was obtained using a QUESA-1200 system with a LED light source from TFSC Instrument Inc. under a white light irradiation of 100 mW/cm² illumination from an AM1.5 solar simulator.

Fig. 1 [Images not available. See PDF.]

Diagram of a Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and b Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al manufactured devices

Results and discussion

Deposit and characterization of heterostructures

After the films that make up the heterostructures were deposited, their topography was analyzed, since to carry out efficient charge, homogeneity and low roughness are required. Efficient charge transport requires that inside the Na₂Pc + TCNQ and triphenylamine films, the charges can move from one molecule to another quickly and not get trapped or scattered. The heterogeneity within the films can affect molecular packing and it can also affect the interactions between different molecules such as Na₂Pc with TCNQ. With respect to roughness, if it is high, it can affect the interaction between the surface of the films that make up the heterostructure and can also affect the film interface with the Ag that acts as a cathode. The AFM is one of the best methods for topography measuring and analysis, which provides information about grain scale and roughness of the prepared heterostructures [47]. Figure 2 shows a 2D and 3D AFM images for the two heterostructures and for both structures holes are not observed in the images. The topography of the heterostructure is made up of ridges and valleys in both cases, the films are made of round and elliptical clusters on the surface that can reach sizes greater than 1 μm. According to AFM results obtained for films of other phthalocyanines such as ZnPc [23], the heterostructures surface presents the formation of localized clusters. These clusters were formed by the generation of preferential growth directions of the films during their deposition. The obtained Root Mean Square (RMS) values were 29.704 and 69.225 nm for Na₂Pc + TCNQ/triphenylamine/p-Si and nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si, respectively. As expected, the highest RMS is presented in the heterostructure with the nylon 11-ZnO layer however, their roughness values are relatively low. For example, the roughness reported by El-Zaidia [47] for films with non-planar trivalent MPcs and chloride substituent such as MnClPc is between 46 and 150 nm. Although the roughness obtained for these heterostructures is higher than for planar divalent MPcs such as ZnPc (around 15 nm) [23], they are expected to present adequate optoelectronic properties.

Fig. 2 [Images not available. See PDF.]

2D and 3D AFM images of a 5 × 5 μm on a Na₂Pc + TCNQ/triphenylamine/p-Si and b nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si

The formation of semiconductors films and their structure is a process that can be influenced by factors such as deposition parameters and material properties. Differences in packing structure determine solid state properties such as absorbance [10]. According to Darwish et al. [48] the absorption of light by organic semiconductors gives us a great insight into the electrons transfer between levels. Based on this, the absorbance spectra for Na₂Pc + TCNQ/triphenylamine and nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine heterostructures in quartz are presented in Fig. 3a. The UV–Vis spectra are related to the π-molecular orbitals of the Na₂Pc and present two absorption peaks around 320 and 600 nm. The absorbance peaks between 290 and 365 nm correspond to the B- or Soret-band, which is related to the electronic transition between the HOMO (Highest Occupied Molecular Orbital) to the LUMO (Lowest Unoccupied Molecular Orbital) orbitals [49, 50]. The absorbance peaks between 560 and 710 nm correspond to the Q-band, which represents the transition between the HOMO to the LUMO orbitals [50]. The Q-band is integrated by the Qx peak at 660–707 nm, as well as a shoulder peak Qy at 550–650 nm, the two peaks originate from the π–π* transitions and the inter-molecule interactions, which might be accounted for the Davidoff splitting or charge-transfer excitation [50, 51]. It is interesting to note that the spectrum for the heterostructure with the nylon 11-ZnO film presents a blue shift and, even with the greater roughness and thickness, its absorbance is greater for wavelengths greater than 400 nm (Fig. 3a). In other hand, Fig. 3b presents the transmittance for Na₂Pc + TCNQ/triphenylamine and nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine structures. The transmittance maximums of the heterostructure systems are 91.58% at a wavelength of 844 nm for the Na₂Pc + TCNQ/triphenylamine and 65.77% at a wavelength of 980 nm for the nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine. In Fig. 3c the low transmittance of the nylon 11-ZnO film is evident, hence this film considerably decreases the transmittance in the heterostructure. On the other hand, it is interesting how this film, being part of the heterostructure, does not affect the absorbance of nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine (see Fig. 3a). Although the nylon 11-ZnO film also presents low absorbance (Fig. 3c), its effect on the heterostructure is more significant in terms of transmittance. Even the maximum absorbance of this film is greater than the absorbance of nylon 11 with Ag nanoparticles, reported by Akamatsu et al. [43]. These results are due to the presence of ZnO in nylon, which is a well-known n-type semiconductor widely used in optoelectronics [52].

Fig. 3 [Images not available. See PDF.]

a UV–Vis absorbance and b UV−Vis transmittance spectra for heterostructures. c UV–Vis absorbance and transmittance for nylon 11-ZnO film

In order to complement the information on optical parameters, it is necessary to obtain the energy bandgap in the components of the heterostructures. The Tauc relationship was used to obtain the optical bandgap energy, according to the type of optically induced electronic transitions in crystalline and amorphous semiconductors films [41, 53, 54]:

1

$$(\alpha hv{)}^B=A(hv-E_g)$$where E_g is the energy bandgap, A is constant, α is the absorption coefficient obtained from Eq. (2):

2

$$\alpha =ln\left(T,/,d\right)$$where T is the transmittance and d is the film thickness. In the Eq. (1) hν is the photon energy, h is Planck’s constant, and ν is the frequency that can be obtained from Eq. (3):

3

$$\nu =\frac{c}{\lambda }$$where λ is the wavelength obtained from the UV–Vis spectrum, and c is the speed of light. For Eq. (1), the constant B depends on the electronic transition type and takes 2 or ½ for direct and indirect forbidden transitions respectively. For these films, indirect electronic transitions are considered, due to their amorphous character, typical of films deposited by vacuum thermal evaporation technique [41, 47, 53, 54]. During the evaporation of the material, the molecules lose any order that they had in the solid state. When the molecules come in contact with the substrates that are at room temperature, the thermal gradient favors the phase change, and with it, the random deposition of the material on the substrate and the formation of amorphous films. About Eq. (1), the onset gap (E_g^onset) and optical gap (E_g^opt) values could be calculated by extrapolating a tangent line to the hν axis in the graphs of Fig. 4. The values of the E_g^onset and E_g^opt for Na₂Pc + TCNQ film are presented in Fig. 4b, and due to the presence of TCNQ they are slightly higher than those reported for pristine Na₂Pc by Alosabi et al. [41] (E_g^onset =1.31 eV and E_g^opt =2.25 eV). In other hand, Fig. 4a and c shows the E_g^opt for the triphenylamine film (E_g = 2.93 eV) and for the nylon 11-ZnO film (E_g = 2.08 eV).

Fig. 4 [Images not available. See PDF.]

Variation of (αhν)^1/2 with hν for a triphenylamine, b Na₂Pc + TCNQ and c nylon 11-ZnO films

The electronic transition explains the optical bandgap is a consequence of defects, structure disorder and traps. The Urbach energy (E_U) can be used to determine the defects localized in the bandgap and can be calculated according [55]:$$\alpha =A_{a}exp\left(\frac{\mathit{hv}}{E_U}\right)$$where in addition to the parameters defined above, A_a is a constant of the material that conforms to α at the energy gap. Figure 5 displays the linear relationship between ln(α) and hν for each of the films that make up the heterostructures. The values of the E_U were determined from the reciprocal of the slope from this linear relation. The Urbach energy corresponds to the width of the band tail, which is related to localized states within the energy gap, possibly caused by structural defects [55]. The Urbach energies for the films of triphenylamine, Na₂Pc + TCNQ and nylon 11-ZnO are 0.12 eV, 0.25 eV and 0.97 eV respectively, which is an indication of the greater number of defects in the nylon11-ZnO film. The E_U obtained have values consistent with the type of film: its lowest value is for the pristine triphenylamine film, while the highest E_U is for the hybrid film.

Fig. 5 [Images not available. See PDF.]

ln(α) versus hν for a triphenylamine, b Na₂Pc + TCNQ and c nylon 11-ZnO films

In order to complement the study on the optical properties of the nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine heterostructure, the PL spectrum is investigated. Figure 6a shows the PL spectrum in the wavelength range 400–1000 nm where a He–Cd laser with 325 nm wavelength is used for the exciting light at room temperature and is evident that this heterostructure exhibits luminescence in the visible region. The PL emission spectra shows two broad PL emission peaks related to the heterostructure compositional films. A strong emission characteristic of the excitonic band is observed in the visible region from 400 nm with a band tail up to 750 nm and centered at 459 nm, resulting in a blue light emission. Also, an infrared broad emission band from 800 to 1000 nm and centered at 890 nm, is observed. The intense PL emission in the visible range was observed with naked eye and it varies in intensity and slightly in color while changing the laser spot position on the sample. These variations may be related to the heterostructure composition, homogeneity, films thicknesses and roughness, consequence of the fabrication process. It is observed that the PL intensity is larger for the heterostructure center, but while varying the position to the sample edge the PL intensity is decreased. The visible band intensity decreases approximately 80%, at the center/edge position and approximately 86% at the edge position. But for the infrared band, the intensity decreases approximately 79%, at the center/edge position and approximately 87% at the edge position. A possible variation in the heterostructure composition, homogeneity, roughness, and films thicknesses increases the intensity of PL emission intensity with the position, indicating a direct relation with the device absorption. While changing the laser spot position the observed emission bands were maintained, except for a slight variation in shape for the visible band approximately at 470 nm, but the emission intensities were larger toward the film center. The observed emission is the contribution of the heterostructure individual layers, thus a deeper analysis was conducted and shown on Fig. 6b and c, and Table 1.

Fig. 6 [Images not available. See PDF.]

Nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine heterostructure a PL spectra, and PL spectra deconvolution for the b 400–800 nm range and c 750–1050 nm range

Table 1. Figure 4 peak deconvolution results

Peak Index	Area fit	Area contribution (%)	Center (nm)	Height (a.u.)	FWHM (nm)
1 (Fig. 4b)	3.02 × 106	3.50	444.72	1.41 × 105	20.10
2 (Fig. 4b)	5.81 × 107	67.28	465.30	8.43 × 105	65.00
3 (Fig. 4b)	1.42 × 107	16.40	528.03	2.18 × 105	61.17
4 (Fig. 4b)	9.94 × 106	11.51	584.35	9.67 × 104	96.62
5 (Fig. 4b)	1.13 × 106	1.31	698.17	1.28 × 104	83.39
1 (Fig. 4c)	1.43 × 106	6.05	824.27	3.35 × 104	40.21
2 (Fig. 4c)	1.13 × 107	48.04	882.40	1.42 × 105	75.15
3 (Fig. 4c)	1.08 × 107	45.90	953.08	7.56 × 104	142.75

The PL visible band peak was fitted for deconvolution with five peaks related to each film emission that conform the Nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine structure. For the PL infrared band peak was fitted for deconvolution with three peaks. It is possible to observe and compare the difference in the peaks total area and height. The peaks at 444 nm, 584 nm is related to the Pc singlet exciton [56], while the 698 nm peak is related to the Na₂Pc π–π relaxing mode with the TCNQ doping [56, 57]. On the other hand, the 824 nm peak to the Pc, the 882 nm peak to Pc vibrating mode [56–58] while the peaks at 465 and 953 nm are related to the nylon 11 doublet [57, 59]. The nylon 11 emission at 465 nm is red shifted compared to literature [60], consequence of the ZnO particles. Finally, the 528 nm peak is related to ZnO particles [61, 62]. The largest emission contribution is for the 465 nm peak, followed by the 528 nm peak and the least for the 444 nm peak, for the visible band. However, the largest contribution is for the 882 nm peak, followed by the 953 nm peak and by the 824 nm, for the infrared band. From previous results it is observed that all the species in the heterostructure contribute to its photoluminescence. These results are important because, although the top layer of the heterostructure and showing the strongest emission is made up of nylon and ZnO particles, which are components with low optical properties, the photoluminescent effect of Na₂Pc prevails in the nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine heterostructure. Therefore, this photoluminescence would also be evident in the Na₂Pc + TCNQ/triphenylamine heterostructure.

Electrical characterization of the heterostructures

To evaluate the electrical properties of the heterostructures, the organic devices shown in Fig. 1 were fabricated. J–V characteristic curves were obtained for Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al devices, under darkness and illuminated conditions, as shown in Fig. 7. For the illuminated conditions, a white LED was used. The J–V plots show a Schottky type curve behaviour for the forward bias, typical for a solar cell. However, for the reverse bias no constant current saturation, as commonly for conventional solar cells, is observed but an inverse Schottky type curve behaviour is observed. Figure 7 curve is not symmetrical, indicating a change in the conduction mechanisms due to the formed barrier and is voltage dependent. Under darkness condition, a marked difference of the curve shape and values with the heterostructure can be observed. It is interesting to note that by adding the dielectric layer, the current density at 1.5 V drops to its half in magnitude. But for − 1.5 a decrease of the current density of 21% is observed. These observations indicate that the current limitation could be related to a variation in the total barrier by the introduction of the dielectric film, and therefore affecting the device electrical response, thus its electrical parameters. On the other hand, for light conditions the J–V curves for both heterostructures change their behaviour and the current density values were increased, even at 0 V, resulting in a photovoltaic effect. The latter observation is with no AM1.5 standardized parameters and despite the low power LED used for the characterization setup. This effect varies upon the heterostructure and polarization which is related to the device architecture.

Fig. 7 [Images not available. See PDF.]

 J–V characteristic curves of the heterostructures for darkness and illuminated conditions

For the forward bias region, an increase of as much as 15% (2.96 × 10⁻⁶ A/cm²) and 40% (3.51 × 10⁻⁶ A/cm²) in current density for Na₂Pc + TCNQ/triphenylamine/p-Si and nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si heterostructures is observed, respectively. For the reverse bias region, an increase of as much of a 25% and a 6% in current density is observed, for the film without and with dielectric film respectively. These results indicate that the introduction of the dielectric film in the heterostructure plays an important role in the device conductivity and the charge carrier’s transport.

Additional electrical parameters were calculated from the J–V characteristics for darkness conditions and shown on Table 2. Among these parameters Table 2 presents the threshold voltage (V_th), ideality factor, saturation current (I_s), carrier mobility, conductivity, and short circuit current density (J_sc). The mobility was calculated using the following equation for the space charge limited current (SCLC) [39]:$${\text{J}}_{\text{SCLC}}=\frac{9{{\mathrm{ϵ}}_{\text{r}}{\mathrm{ϵ}}_0\mathrm{\mu }\text{V}}^2}{8{\text{L}}^3}$$where J_SCLC is the current density for the space charge limited current, V the voltage, ε_r and ε₀ are the relative material permittivity (3.6 for the CuPc and 6.2 for nylon 11 [39, 63]) and vacuum permittivity (8.85 × 10⁻¹⁴ Fcm⁻¹), respectively. L is the conduction length and µ the carrier mobility. It can be observed that the incorporation of the dielectric film reduces the V_th from 0.62 to 0.18 V while the ideality factor is drastically decreased from 3.13 to 0.73. However, the ideal value for a diode is close to 1 indicating that the curve presents a change in the shape toward an ideal diode. On the other hand, the saturation current value is slightly decreased which is the opposite observation to the conductivity 30% increase. It is interesting to note that the electrical current change also can be observed in Fig. 7 at very small positive voltages, while for higher voltages the current density magnitude is higher for the heterostructure without dielectric, maybe related to the variation in RMS. The resulting Jsc values, for both heterostructures, are indicative of light absorption, despite the 10⁻⁷ A/cm² values, confirming their feasible application for solar cell devices. Also, an important effect of the Nylon 11-ZnO can be observed in the Jsc, where almost three times the current density value was obtained at 0 V suggesting an increase of the light absorption and an induced electric field within the heterostructure. The latter may explain the larger photovoltaic effect of this structure, and the ~ 2 times mobility increase to 3.10 × 10⁻⁵ cm²/Vs, values that are close to previous works results [39]. Also, the embedded ZnO particles in nylon 11 increase the conductivity of the heterostructure functioning as an electron conduction path through nylon 11, but likewise the device resistances change affecting the J–V curve shape. The relatively high mobility obtained for both structures may be related to the Na₂Pc, their structural self-organization characteristics [23] and high carrier mobility [10] which are reflected in an energy migration in the form of charge transport.

Table 2. Device electrical parameters for both heterostructures

Device	Vth (V)	Ideality factor	Is (A)	Mobility (cm2/Vs)	Conductivity (Scm− 1)	Jsc @ 0 V (A/cm2)
Ag/Na2Pc + TCNQ/triphenylamine/p-Si/Al	0.62	3.13	2.74 × 10− 11	1.57 × 10− 5	1.16 × 106	1.02 × 10− 7
Ag/nylon 11-ZnO/Na2Pc + TCNQ/triphenylamine/p-Si/Al	0.18	0.73	2.25 × 10− 11	3.10 × 10− 5	1.51 × 106	3.13 × 10− 7

Figure 8 shows the J–V characteristic curves of both heterostructures under different incident light color. For the illuminated J–V characteristics of the devices, the incident light was generated by a diodes array, performed at 25 °C and the light enters the device through the top layer (Fig. 1). It can be observed that there is a small variation of the curve by changing the incident light color. Also, a change in the curve behaviour is observed for the blue light of the Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al device. These observations are related to the previous PL discussed results, where the maximum emission is for this wavelength. Figure 8b blue curve at 1.5 V presents a photocurrent density of 0.75 × 10^− 5 A/cm² and is the highest observed, however it can be compared to the photocurrent at 0 V is of 9.32 × 10⁻⁸ A/cm², which is quite low. Nevertheless, the highest photoconductivity obtained by this device was for the green incident light (3.08 × 10⁻⁷ S/cm) for the orange incident light (4.55 × 10⁻⁷ S/cm).

Fig. 8 [Images not available. See PDF.]

 J–V characteristic curves of the a Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and b Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al cells under different incident light color

Figure 9 plots the incident photon energy dependent Jsc, photocurrent, conductivity and photo-sensitivity for the Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al heterostructures, and obtained from Fig. 8 curves analysis. The latter allows the comparison as a first approach of both devices’ parameters, which vary due to the incident photon energy and can be related to the optoelectronic properties mainly related to the light absorption and carrier extraction efficiency of a solar cell. Comparing Fig. 9a and b deepens the analysis of the nylon 11-ZnO film introduction consequence, where a marked variation is shown. In general, with its introduction, the Jsc and conductivity values are larger due to the change in charge carrier mobility, while the photocurrent values variation depends on the photon energy, and the photo-sensitivity values are smaller for most of the photon energies but with an increasing tendency. Figure 9a shows a decreasing trend of the device conductivity and photo-sensitivity, with the incident photon energy. But this is the opposite when adding the dielectric film (Fig. 9b). On the other hand, the Jsc and photocurrent do not show any specific trend in Fig. 9a but a decreasing trend is observed for the cell with dielectric film. The photosensitivity values for the heterostructure device were almost 40% while by adding the dielectric film decreases to barely 25%. But an important increase of the Jsc, conductivities and photocurrent values are indicative that a better mobility and more carriers are generated for the solar cell heterostructure that includes the dielectric film. These results indicate that these heterostructures can be used for solar cell applications and remark the large effect of the nylon 11-ZnO dielectric film on their performance. Complementary experiments were conducted to evaluate their performance toward solar cell applications, using a LED base solar simulator, where the EQE and Jsc spectra measurements were carried out for the heterostructure with nylon 11-ZnO dielectric film (Fig. 9c). The EQE spectrum is related to the response of each layer of the heterostructure and to the device optical and electrical losses, which directly affect the power conversion efficiency of a solar cell. To improve the device performance light management strategies, thickness, texturing, and anti-reflective layers optimization are commonly conducted. The largest losses may be attributed to optical losses, where the surface reflection, interface reflection and active layer thickness limit the light absorption, as shown in the Fig. 9c UV and IR range. The maximum EQE values were approximately within the 600–700 nm range which is related to the reduction of the transmittance observed in Fig. 3b for this range. Also, significant charge collection losses may be due to a reduction in charge carrier lifetime and diffusion lengths caused by defects, however the obtained mobilities are comparable to previous results [39]. Hence, for a solar cell device, these losses impact the maximum operating power point output and power conversion efficiency, where the reduction in the Jsc can be related to a reduction in the charge carrier extraction efficiency. From Fig. 9c, a 79.7% EQE mean value was obtained, consequence of the combination of the heterostructure topography, and optoelectronic properties previously mentioned. It exhibits a maximum of 98.2% at ~ 686 nm which can be attributed to excitons and is within the absorption band observed on Fig. 3a. The EQE decreases, for longer wavelengths can be related to an absorption decrease of the heterostructure. It is interesting to note that a steep decrease is shown for longer wavelengths ( > ~ 1050 nm), indicating that the heterostructure efficiently absorbs photons and generates current over an extended wavelength window. But the sudden decrease of almost 70% is related to high charge recombination and transmittance. On the other hand, for shorter wavelengths, an intense increase from 58 to 85% is observed and correlated to the high absorption of Na₂Pc + TCNQ/triphenylamine for these wavelengths. Moreover, for energies equal to E_g an 88% EQE peak is observed, but for higher energies a decrease of the EQE may be indicative of short charge carrier diffusion length due to surface and interface recombination’s, and non-radiative recombination’s [64] due to the nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine heterostructure. Nevertheless, for higher energies than E_g^opt a relatively high EQE is maintained caused by high mobility and longer diffusion lengths [64], but with various oscillations due the absorption and conduction mechanisms involved. Active layer-dielectric interface energy offset. For instance, an 93% peak is observed at the E_g^opt energy. Also Fig. 7c shows the Jsc spectrum, which has a 0.48 mA/cm² and 0.72 mA/cm² mean and maximum values, respectively. These values are almost 10 times lower than other reported for different device technologies [36, 37, 42]. The highest Jsc range values coincide with the absorption and EQE previous observations. On the other hand, to explain the intense Jsc reduction band between 700 and 780 nm further analysis should be conducted. Nevertheless, a film thickness optimization study should be conducted to improve the external quantum efficiency, and so the power conversion efficiency. A future evaluation of the photovoltaic parameters under standardized AM1.5 conditions, such as the power conversion efficiency and fill factor, among others, will give a deeper analysis to improve the device performance. By comparing Fig. 9b and c a similar behavior is shown, the Jsc decreasing tendency with the energy is also observed, the steep decrease at 2 eV and the oscillation to a valley at 2.62 eV are also present. The Jsc reduction at approximately 820 and 960 nm may be related to the PL emission peaks corresponding to the Pc and nylon (Table 1) that could increase the recombination rate. The EQE and Jsc spectra demonstrate, as first approach for the manufactured devices, that the heterostructure is photo-active and their performance for solar cell applications. However, it should be optically and electrically improved for a better use of all wavelengths, fulfilling the optical and collection losses, even with a more advance contact design to maximize charge extraction and diminish shading.

Fig. 9 [Images not available. See PDF.]

Incident photon energy dependent Jsc, photocurrent, conductivity and photo-sensitivity for a Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and b Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al cells. c Solar simulator EQE and Jsc spectra

Conclusions

Two heterostructure devices of Ag/Na₂Pc + TCNQ/triphenylamine/p-Si/Al and Ag/nylon 11-ZnO/Na₂Pc + TCNQ/triphenylamine/p-Si/Al were fabricated. The topographical study of the two heterostructures revealed grains distributed throughout the surface with small roughness. The heterostructures low roughness value (29.704 and 69.225 nm respectively) can contribute to the charge transport. In other hand, the films that integrate the heterostructures were characterized in their optical behavior and the onset and optical gap were estimated to be 1.53 and 2.86 eV, respectively for Na₂Pc + TCNQ film. For the triphenylamine film and for the nylon 11-ZnO film the optical gap was 2.93 and 2.08 eV respectively. Regarding the Urbach energy, the highest value was obtained for the nylon 11-ZnO film. The optical study was complemented with photoluminescence studies, and a strong emission in the visible range was obtained. However, the UV-vis spectroscopy of two heterostructures exhibited broad absorption in the visible spectrum, which suggests that Na₂Pc is one of the candidates for solar cell applications. A solar cell type curve characteristic was obtained for both structures, and an important increase of photocurrent generation of 40% was observed for the heterostructure with dielectric film compared to a 15% without this film. Besides the change in the optical parameters, the addition of the dielectric layer improves the heterostructure Jsc by 3 times, the conductivity, and the carrier mobility, which are parameters that directly impact the photovoltaic response. A photosensitivity value between 15% and 25%, and a 79.7% EQE mean value was obtained for the heterostructure with the nylon 11-ZnO layer, indicating that the heterostructure is photo-active and may be used toward solar cell applications.

Author contributions

All authors contributed to the study conception and design. LH, MESV and MRS did the experiments. Material preparation, data collection and analysis were performed by LH, MESV and MRS. The first draft of the manuscript was written by LH, MESV and MRS and all authors commented on previous versions of the manuscript. LH, MESV and MRS read and approved the final manuscript.

Funding

Leon Hamui and María Elena Sánchez-Vergara acknowledge the financial support from Anahuac México University, Project number INNDIAHABL170215171 and PI0000067.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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