Flexible heterostructures based on metal phthalocyanines thin films obtained by MAPLE
Introduction
The organic materials have been studied for replacement of the inorganic materials in many applications [1], [2]. The great advantage of the organic materials arises from their properties such as: compatibility with different substrates (including flexible), the availability of various deposition methods for the preparation of coatings and low cost processing. Also, the interest was focused on the bulk organic materials [3], [4], [5] and on the organic thin films [6], [7], [8], ones of the most investigated class being the metal phthalocyanines due to their chemical and thermal stability [9]. The phthalocyanines are small organic molecules which are characterized by a large absorption coefficient in the visible part of the light spectrum [10] and a relatively high hole mobility that make them suitable as donor materials in organic photovoltaic applications (OPV) [9], [11], [12]. Zinc phthalocyanine (ZnPc) and magnesium phthalocyanine (MgPc) are frequently used as absorbers in the OPV (e.g. ZnPc absorption coefficient was found to be around 1.5 × 105 cm−1 in the 600–750 nm range [10]). Moreover, it has been proved that this material can converts excitons into the light even under film thicknesses of 100 nm [13]. The phthalocyanines have also applications in OLEDs, gas sensors, medicine, optical communications and due to their large third-order nonlinearity are promising candidates, for the nonlinear optical devices [14], [15].
The properties of the thin films and the performance of devices strongly depend on the deposition method. For phthalocyanine thin film preparation, different methods such as: spin-coating [16], Langmuir–Blodgett [17], thermal vacuum evaporation [18] or pulsed laser deposition in [19], etc., were used. It was found that the materials in the thin films form dictates the devices performances [20].
In the OPV design, a key element is the transparent conductor electrode (TCE). In the last time attempts have been made in order to replace the ITO electrode with other cheaper materials (IZO, AZO, etc.) presenting the same optical (high transparency) and electrical (low resistivity) properties. Due to its properties ITO remains the most used transparent electrode [21], [22]. In the OPV architecture, another component is the acceptor layer. Combined with the phthlocyanines (p-type materials), a non-metallic phorphyrine 5,10,15,20-tetra(4-pyrydil)21H,23H-porphyne (TPyP) that also presents a wide spectral absorption range [23] can be used in order to achieve a donor/acceptor (D/A) heterostructure. By multilayer deposition of ZnPc or MgPc and TPyP, a p–n heterojunction with two stacked organic layers can be prepared. Such structure is favorable from energetically point of view, considering the position of the ionization potential (IP) and electron affinity (EA) levels in the investigated materials (EIP;ZnPc = 5.28 eV and EEA;ZnPc = 3.28 eV [8]; EIP;MgPc = 5.4 eV and EEA;MgPc = 3.9 eV [24]; EIP;TPyP = 6.8 eV, EEA;TPyP = 4.1 eV [23]). Also, the heterostructures can be obtained by mixing materials into donor:acceptor form (D:A), in order to surpass the limit imposed by the exciton length in the organic materials (∼5–30 nm). In this way, the photogenerated excitons can be dissociated instantly into free charge carriers [25].
For the devices technology, the deposition method must assure the preparation of films with reproducible optical and electrical characteristics. Frequently, for the deposition of the above mentioned materials it was used thermal vacuum evaporation (TVE). To obtain high quality thin films with adequate properties, we use the matrix assisted pulsed laser evaporation (MAPLE) method for deposition of functional organic heterostructures. MAPLE is a suitable technique for processing soft materials like organic compounds assuring the preservation of the materials chemical composition. Due to the fact that uses a frozen target formed from a mixture this method does not depend on the melting point of the used materials as is the case in the thermal vacuum deposition. In OPV, the layer thickness is an important parameter to increase the light absorption in the active layer. Using MAPLE, the layers thickness can be controlled by the number of laser pulses and the laser fluence. Organic thin films characterized by good optical and electrical properties can be prepared by MAPLE [26], [27]. Comparatively with the spin-coating method, by MAPLE can be obtained homogeneous layers and can be easily obtained stacked layers (multilayers) without affecting the bottom layer.
In this context, we present a comparative study on organic heterostructures based on ZnPc(MgPc) and TPyP stacked or mixed layers obtained by MAPLE.
Section snippets
Experimental
ITO (Solaronix) on flexible substrate (polyethylene terephthalate) with a 14 Ω/sq resistivity and 25 × 15 mm area were used in the heterostructures fabrication. ZnPc, MgPc, and TPyP thin films as well as (ZnPc(MgPc)/TPyP) stacked films and (ZnPc(MgPc):TPyP) blend films were deposited using a MAPLE set-up by means of an excimer laser source with KrF* (Coherent, CompexPro 205, λ = 248 nm, τFWHM ∼ 25 ns) [27], [28] that operated a laser repetition rate of 5 Hz. The laser beam was focused on the organic
Results and discussion
Generally, the phthalocyanines films obtained by thermal vacuum evaporation on the unheated substrates were found to be amorphous [29]. In the case of the ZnPc films was evidenced that the orientation of the flat ZnPc molecules with stacking axis parallel to the substrate leads to a lower charge carrier transport for OPV applications [13]. In Fig. 1 are presented the diffractograms of the investigated layers. For the single layers, the absence of the diffraction peaks (Fig. 1a) suggests that
Conclusions
Using the MAPLE technique, homogenous layers based on ZnPc, MgPc and TPyP can be successfully deposited on ITO flexible substrates. The organic films have a globular morphology characteristic to this deposition method and to the phthalocyanines compounds. The obtained films preserve the optical properties (absorbance and photoluminescence) of the initial materials. A photogeneration process appears under light excitation in the structures based on ZnPc:TPyP mixture The electrical parameters of
Acknowledgments
This research was financially supported by the Romanian Ministry of Education and Research trough National Core Program, Contract No. 45N/2009, Bilateral Agreement Romania-Franta 783/2014, PN-II-ID-PCE-2012-4-0467 and ANCS STAR_ROSA Contract No. 65/2013.
References (40)
- et al.
Melt growth and characterization of pure and doped meta-dinitrobenzene crystals
J. Cryst. Growth
(1999) - et al.
Electrical transport in crystalline perylene derivatives films for electronic devices
Solid State Sci.
(2008) - et al.
Zinc phthalocyanine—Influence of substrate temperature, film thickness, and kind of substrate on the morphology
Thin Solid Films
(2011) - et al.
Laser prepared organic hetrostructures on glass/AZO substrates
Appl. Surf. Sci.
(2014) - et al.
MAPLE prepared polymeric thin films for non-linear optics applications
Appl. Surf. Sci.
(2009) - et al.
Structural and optical properties of thermal evaporated magnesium phthalocyanine (MgPc) thin films
Appl. Surf. Sci.
(2008) - et al.
Influence of RMS strain on optical band gap of ZincPhthalocyanine (ZnPc) thin films
Sol. Energy
(2006) - et al.
Effect of annealing temperature on the optical properties of thermally evaporated tin phthalocyanine thin films
Appl. Surf. Sci.
(2008) - et al.
FTIR, TGA and DC electrical conductivity studies of phthalocyanine and its complexes
J. Mol. Struct.
(2005) - et al.
Influence of the gate dielectric on the mobility of single rubrene crystal field-effect transistors
Appl. Phys. Lett.
Studies on the growth, optical, thermal and mechanical properties of pure and o-nitroaniline doped Benzil crystals
Cryst. Growth Des.
Investigation of the growth process of organic/inorganic doped aromatic derivatives crystals
J. Optoelectron. Adv. Mater.
Effect of the metallic contact on the electrical properties of organic semiconductor film
J. Optoelectron. Adv. Mater.
High open circuit voltage in efficient thiophene-based small molecule solution processed organic solar cells
Org. Electron.
Efficient organic solar cells based on planar metallophthalocyanines
Chem. Mater.
Quantitative estimation of electronic quality of zinc phthalocyanine thin films
Phys. Rev. B: Condens. Matter
Increasing the efficiency of zinc-phthalocyanine based solar cells through modification of the anchoring ligand
Energy Environ. Sci.
Fabrication characterization of phthalocyanine-based organic solar cells
Mater. Sci. Appl.
Cited by (35)
Enhanced singlet oxygen production over a photocatalytic stable metal organic framework composed of porphyrin and Ag
2021, Journal of Colloid and Interface ScienceCitation Excerpt :To obtain more structural information, FT-IR spectra (Fig. 1c) for AgTPyP and TPyP are further investigated. The peak at 970 cm−1 is attributed to the CN stretching vibration in the porphyrin ring [36], and it does not shift for AgTPyP compared with that for TPyP, which indicates that Ag+ ions are not inserted into the porphyrin ring. The characteristic absorption peak of the pyridine ring moves from 1590 cm−1 for TPyP to 1597 cm−1 for AgTPyP [36], which is caused by the increase of conjugation due to the formation of a 2D structure.
On the Physical Properties PEDOT:PSS Thin Films
2020, Materials Today Communications