Organic field effect transistors (OFETs) of poly(p-phenylenevinylene) fabricated by chemical vapor deposition (CVD) with improved hole mobility

Highlights

• Poly(p-phenylenevinylene) (PPV) transistors are fabricated by CVD process.

• The hole mobility is 100 times higher than that of spin coated PPV devices.

• Surface treatment of the transistor dielectric further enhances the hole mobility.

Abstract

This study demonstrates organic field effect transistors (OFETs) of chemical vapor deposited poly(p-phenylenevinylene) (PPV). The optical and electrical properties of the films are reported. It is found that the grain sizes of the film can be engineered by surface treatment of the oxide of the Si/SiO₂ substrate in the transistor with a self assembled monolayer (SAM) of octadecyltrichlorosilane (OTS). The enhancement of domain sizes of the grains with surface treatment leads to an increase in hole mobility to 1.4 × 10⁻⁴ cm²/V s, which is 1000 times higher than that reported for solution processed PPV field effect transistors. A comparison of the hole mobility between CVD grown PPV films and those reported for PPV and its derivatives, grown by spin coating or drop casting, shows that CVD provides an optimized alternative for growth of devices. It is shown that a low annealing temperature of 180 °C is enough to achieve polymerization. Film deposition by CVD has the advantage that up-scaling, with a fine control on the film thickness, is possible without compromising on the electrical/morphological properties. A vacuum based method of polymer deposition opens up the prospect of hybrid devices based on small molecules and polymers.

Introduction

Organic semiconductors have attracted attention due to their applications in solar cells [1], thin film transistors [2], light emitting diodes [3] and sensors [4]. Organic semiconductors can be broadly classified as small molecules and polymers [5]. Small molecule semiconductor devices are usually fabricated by a vacuum evaporation process while polymer devices are usually spin coated, screen printed or inkjet-printed from a solution [6], [7]. The solvent used in these solution processed methods can cause contamination of the film which is undesirable for electronic performance of the devices [8]. Evaporation based methods offer the advantage of reduced impurities (and thereby low trap densities) and the ease of up-scaling for production [7], [9]. Logic circuits fabricated with vacuum evaporated organic semiconductors have superior performance compared to those processed from solution [7]. Polymer devices, dominantly fabricated by low cost solution based processes, have the following disadvantages: (i) spin coating, which leads to uniform films with good electrical and morphological properties, is not feasible to up-scale [6], (ii) printing is a drop casting based process and the films obtained by such a method does not show good electrical/morphological properties as compared to spin coated films, and (iii) screen printing can be an alternative for up-scaling but impurities/residual solvent still remains an issue. Impurities affect the carrier recombination dynamics and degrades the performance of optoelectronic devices [10]. This work demonstrates that growth of polymer films by CVD can be an attractive alternative to solution based processes for device fabrication.

CVD has been widely used in the semiconductor industry for deposition of materials such as polysilicon, silicon dioxide, silicon, silicon nitride, graphene, etc. [11], [12]. CVD of polymer films for semiconductor device applications has been demonstrated [8], [13], [14], [15], [16], [17]. CVD offers a good control over the deposition of the film since there are several growth parameters which can be optimized. The scalability of oxidative CVD for poly(3,4-ethylenedioxythiophene) (PEDOT) films has been demonstrated for large area substrates [18]. CVD of polymer films such as polythiophene, which is not soluble and hence cannot be deposited by spin coating, has been demonstrated [16]. Hence, CVD opens up the prospect of using polymer materials which are insoluble. Since it is possible to deposit films on a wide variety of substrates including flexible substrates by CVD, the process can be scaled up to batch to batch processing and roll to roll processing. New hybrid device architectures with evaporation of molecular semiconductors and CVD of polymers can be achieved [19]. This is advantageous compared to solution processing techniques which need orthogonal solvents for hybrid/tandem device structures [20].

Poly(p-phenylenevinylene) (PPV) is a conjugated polymer and its luminescent property was used for the first polymer LED [21]. Many derivatives of PPV were developed which improved its stability and electronic performance. Since PPV is not soluble in any known solvent, a soluble precursor polymer is spin coated to form a film and then thermally annealed to form PPV [22], [23]. CVD has been shown as an alternate method for deposition of PPV films [24], [25], [26], [27], [28], [29], [30]. CVD deposited PPV has been used as the emissive material in LEDs [24]. There are very few reports on transistors fabricated by CVD deposited polymer films [16], [31] and no reports, according to the authors knowledge, of transistor with CVD deposited PPV as the active material. In this study, OFETs are fabricated with CVD deposited PPV as the active material to compare the electrical characteristics between different deposition techniques. Understanding the transport properties of CVD deposited PPV will enable to develop newer derivatives of PPV for CVD in the future.

Alkylsilanes are used to modify the properties of various surfaces by forming self-assembled monolayers (SAMs) and have been widely used for modification of SiO₂ dielectric surface [32], [33], [34], [35], [36]. The formation of SAM layer on SiO₂ and subsequent deposition of organic semiconductor results in improved mobility in OFETs [33], [37]. In this study octadecyltrichlorosilane [OTS, CH₃(CH₂)₁₇SiCl₃] is used for surface treatment of SiO₂. The functional head group of OTS has chlorine which hydrolyses and interlinks to form siloxane (Si–O–Si). This gets adsorbed on the surface hydroxyl groups of SiO₂ which act as the reaction sites and forms ordered and oriented structures of OTS. This, in turn, helps to form ordered structures of the material grown on it.