Effect of ITO electrode patterning on the properties of organic heterostructures based on non-fullerene acceptor prepared by MAPLE

Highlights

• Nano-patterned mixed layer with star-shaped oligomers prepared by MAPLE.

• Shape of nanostructures changed from cylindrical to cone trunk after ITO deposition.

• The nano-patterning is preserved after the deposition of organic layer(s).

• Nano-patterning determined decrease in transmittance between 625 and 800 nm.

• The heterostructures shown good ohmic contact behavior.

Abstract

This paper presents some studies on the organic heterostructures realized by Matrix Assisted Pulsed Laser Evaporation in both bi-layer and mixed layer configurations on glass substrates covered by flat or nano-patterned ITO. The donor, a star-shaped arylenevinylene compound, 4,4′,4″-tris[(4′-diphenylamino) styryl] triphenylamine, and acceptor, a non-fullerene compound, N,N′-bis-(1-dodecyl)perylene-3,4,9,10 tetracarboxylic diimide, were blended in three weight ratios: 1:2, 1:3 and 1:4.

A grating of cylindrical pillars with a periodicity of 1.1 μm has been developed by UV-Nanoimprint Lithography in a polymer layer. The shape of the nanostructures changed to cone trunk by the Pulsed Laser Deposition of ITO on this nanostructured surface.

The effect of the nanostructures and composition on the optical and electrical properties of the heterostructures was analyzed.

The nano-patterning affected both the UV–Vis transmission and photoluminescence through the multiple reflections inside the cavities and at interfaces and the particularities of the molecular arrangement. The patterning was preserved independently of composition, but the roughness increased with increasing acceptor amount.

The I-V characteristics drawn at room temperature in dark revealed an ohmic contact behavior for all heterostructures. The nano-patterning had a similar effect on the current in the heterostructures with mixed layer (1:2) and stacked bi-layer.

Introduction

Considering the growth of energy consumption worldwide, the domain of energy has received an increased attention in the last decades. Starting from the fact that one very important factor which determines the low conversion efficiency of the solar cells based on thin films is the low absorption, many research have been devoted to the identification of different ways for improving the absorption, including the light trapping mechanism. Recently, new methods were proposed in the literature such as: the use of ZnO/a-Si distributed Bragg reflectors in the visible-infrared range [1] or of an array of TiO₂ nanorods [2], the insertion of periodic arrays of nano-scatters [3]. Also, the development of nanostructures, nanomeshes, on the surface of the transparent conductor electrode has improved the charge transport and increased the electrical conductivity in quantum dot sensitized solar cells [4]. Other ways for improving the performances are related to device architecture [5] and use of buffer layers with controlled properties [5], [6]. A special attention is paid not only to energy conversion devices but also to improve the performances and predict the remaining useful life of energy storage devices, supercapacitors [7], [8].

In the last decades, the study of organic semiconductors based devices has attracted the research interest. The main limitations of the organic devices’ performances come from the low mobility of the charge carriers, reduced diffusion length of the exciton and limited interfacial contact area between the donor and acceptor compound. The synthesis of new organic semiconductors and development of new device configurations represent alternatives for obtaining devices with improved performances.

Lately a special attention was paid to investigate the use of nanostructured electrode or insertion of electrode in the active layer as a way to increase the efficiency of charge carrier collection in organic semiconductor [9], [10]. The nanostructured interfaces between the electrode(s) and active layer can improve both the optical and electrical properties of the heterostructures [11], [12]. The nanostructures created inside the organic active layer by the nano-patterning of the electrode can increase the absorption by light trapping. Thus is enhanced the number of excitons and in consequence the number of the charge carriers generated through the dissociation of excitons, increasing the current passing through the heterostructure. Supplementary, the nanostructured electrode can generate an intense electric field and modify the contact area between the electrode and organic layer. This way it is favored the collection of the charge carriers by reducing the transit time between the active layer and electrode and the recombination of the charge carriers in the layer [13], [14].

The charge carriers transport in the organic heterostructures with nanostructured electrode is influenced by the dimensions of patterning [11], [15], [16], [17]. The distribution of the electric field inside the heterostructure is modified by the electrode nano-patterning determining either an improvement or degradation of the electrical properties, depending on the geometrical parameters of the patterning and active layer morphology [15]. Recent results present the effect of nano-patterning on the properties of aluminum layer deposited by sputtering [18] and of the metallic electrode nano-patterning on the properties of the organic heterostructures made on this electrode [19].

The nanostructuring of the transparent conductor electrode layer is a way to control the optical properties by reducing the physical thickness of the absorbent layer and improving the optical absorption of the radiation involved in the charge carriers generation [20], [21]. The electrode with three-dimensional structures such as vertical pillars (columns) assures high interfacial area and enables more direct pathways for the charge transport, shortening the collection length of the free charge carriers [9], [22].

A major drawback in the electrical properties of the organic devices is imposed by the short (3–10 nm) exciton’s diffusion length. Therefore, it is necessary to assure a balance between the short diffusion length of the exciton before dissociation and the thickness of the active layer (>100 nm) necessary to absorb a large fraction from the incident light that will be involved in the generation of excitons. The limited interfacial contact area between the donor and acceptor layer in stacked bi-layer organic heterostructures can be surpassed by blending the donor and acceptor components. This mixture named “bulk heterojunction” (BHJ) [23] favors a fast and long-range charge carriers separation. The morphology of the mixed layer at the nanometer scale can favor the dissociation of excitons and the transfer of the generated electrons and holes from one molecule to another to reach the electrodes before the recombination process takes place. So far, hexylthiophene (P3HT) and a derivative of fullerene, [6,6]-phenyl C61 butyric acid methyl ester (PCMB) are the most studied donor and acceptor materials [24], [25], [26] for BHJ.

Hitherto, organic mixed (bulk heterojunction) active layer and light trapping schemes have been used especially for improving the performances of organic photovoltaic devices [9], [10], [12], [13], [14], [15], [22], [23], [24], [25], [26].

Lately, attempts to increase the performances of other organic devices have been made. Taking into account the main advantages offered by the mixed layer and nanostructuring, the investigation of their effects on the performances of other devices such Organic Field Effect Transistors (OFETs) [27], Organic Phototransistors (OPTs) [28], [29] and Organic Light Emitting Devices (OLEDs) [30], [31] is of great interest. For example, it is expected that the nanostructured transparent electrode will increase the absorption of the light by a trapping mechanism in OPTs or enhance the efficiency through the internal outcoupling of the trapped photons in OLEDs. It is also expected that the mixed layer will increase the current through ambipolar conduction in OFETs. Thus it is important to investigate the effect of nanostructuring on the properties of heterostructures based on mixed layers and analyze if by nanostructuring could be obtained improvement both in optical absorption and charge carriers transport.

The proposed donor for the mixed layer is an oligomer, with star-shaped molecular structure, 4,4′,4″-tris[(4′-diphenylamino) styryl] triphenylamine (IT77,) forming films of high quality because of the extended molecular ordering induced by the specific interactions. It shows an isotropic conduction determined by the intermolecular overlap favoring an efficient transport of the charge carriers and an increased 3D electrical conductivity [32], [33].

The main problems related to fullerene based acceptors are coming from their reduced solubility in common solvents, which affects the homogeneity of the active layer at the nanometer scale. This problem can be surpassed by using non-fullerene acceptors. The proposed acceptor for the mixed layer is a compound from the family of perylene diimide derivatives, N,N′-bis-(1-dodecyl)perylene-3,4,9,10 tetracarboxylic diimide (AMC14), which is characterized by a good solubility favoring a good quality of the layers [34], [35] and a strong electron-deficient character favoring the electron acceptance.

For a good charge transfer from donor to acceptor it is necessary that the donor and acceptor satisfy the previous experimentally deduced condition [36], [37] which establishes an optimal value around 0.3 eV for the offset between the lowest unoccupied molecular orbital (LUMO) of donor and LUMO of acceptor. The selected donor (IT77) and acceptor (AMC14) are characterized by E_LUMO;IT77 = 3.21 eV (deduced by cyclic voltammetry [33]) and E_LUMO;AMC14 = 3.44 eV [38], what corresponds to ΔE_LUMO = 0.23 eV.

The investigated mixed layers consist of a disordered interpenetrating network of electron donating star-shaped oligomer, IT77, and electron-accepting perylene diimide derivative, AMC14, blended in different weight ratio: 1:2; 1:3 and 1:4.

Both Indium Tin Oxide (ITO) and mixed layer IT77:AMC14 have been prepared by laser techniques: ITO by Pulsed Laser Deposition (PLD) and IT77:AMC14 by Matrix Assisted Pulsed Laser Evaporation (MAPLE).

In PLD method, a pulsed laser beam ablates a solid bulk target and the ablated material deposits onto a nearby substrate, the deposition rate depending on the intensity of the laser beam incident on the target and spot size, target-substrate distance, atmosphere, substrate temperature, optical and mechanical properties of the target and the geometrical configuration used for deposition [39], [40], [41].

The difference between PLD and MAPLE is given by the way the target is prepared, MAPLE being adapted for the deposition of soft materials. Since 1990 when this method was used for the first time [42], MAPLE was utilized for the deposition of small molecule organic compounds, oligomers, polymers, biomaterials and nanomaterials [43], [44], [45], [46], [47], [48]. A small quantity of organic compound is dissolved in a volatile solvent matrix characterized by a good absorption of the laser radiation and a molecular weight between 100 Da and 300 Da for minimizing the photochemical damage that would result from the interaction of the laser light with organic target. After that, the solution is frozen in liquid nitrogen and the resulted solid target is fixed in the sample holder inside the deposition chamber. This method is suited for the deposition of mixed layers because it offers a better control of the film thickness, composition and morphology. This is possible because the solution that will be frozen to form the target contains the both components in the desired ratio.

In previous papers we have already studied the star shaped arylenevinylene compound (IT77) as donor compound in BHJ with fullerene derivative as acceptor [49] and the deposition by PLD of ITO thin films on nano-patterned glass substrate [50].

To our knowledge, the preparation by MAPLE of mixed layers containing oligomers with triphenylamine core as donor and a non-fullerene compound as acceptor was not yet investigated, despite their very good donor and acceptor character. In addition, the effect of ITO nano-patterning on the properties of organic heterostructures made on this electrode it is not completely understood and explained.

In this paper, mixed layers containing star-shaped triphenylamine oligomer (IT77) and perylene diimine derivative (AMC14) in different ratio are prepared by MAPLE on flat ITO (ITO_flat) and nano-patterned ITO (ITO_nano). The effect of IT77:AMC14 semiconducting blends composition and ITO/IT77:AMC14 interface nanostructuring on the properties of these organic heterostructures has been studied. The properties of the heterostructures with stacked bi-layer (sandwich configuration) and mixed layer (BHJ configuration) realized on glass/ITO_flat and glass/ITO_nano were comparatively analyzed.