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Temperature and Composition-Dependent Density of States in Organic Small-Molecule/ Polymer Blend Transistors Simon Hunter, Alexander D

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Temperature and Composition-Dependent Density of States in Organic Small-Molecule/ Polymer Blend Transistors Simon Hunter, Alexander D Temperature and composition-dependent density of states in organic small-molecule/ polymer blend transistors Simon Hunter, Alexander D. Mottram, and Thomas D. Anthopoulos Citation: J. Appl. Phys. 120, 025502 (2016); doi: 10.1063/1.4955282 View online: http://dx.doi.org/10.1063/1.4955282 View Table of Contents: http://aip.scitation.org/toc/jap/120/2 Published by the American Institute of Physics JOURNAL OF APPLIED PHYSICS 120, 025502 (2016) Temperature and composition-dependent density of states in organic small-molecule/polymer blend transistors Simon Hunter, Alexander D. Mottram, and Thomas D. Anthopoulosa) Department of Physics and Centre for Plastic Electronics, Imperial College London, South Kensington SW7 2AZ, United Kingdom (Received 9 March 2016; accepted 23 June 2016; published online 14 July 2016) The density of trap states (DOS) in organic p-type transistors based on the small-molecule 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), the polymer poly(triarylamine) and blends thereof are investigated. The DOS in these devices are measured as a function of semiconductor composition and operating temperature. We show that increasing operating temperature causes a broadening of the DOS below 250 K. Characteristic trap depths of 15 meV are measured at 100 K, increasing to between 20 and 50 meV at room-temperature, dependent on the semiconductor composition. Semiconductor films with high concentrations of diF-TES ADT exhibit both a greater density of trap states as well as broader DOS distributions when measured at room-temperature. These results shed light on the underlying charge transport mechanisms in organic blend semiconductors and the apparent freezing-out of hole conduction through the polymer and mixed polymer/small molecule phases at temperatures below 225 K. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4955282] I. INTRODUCTION has been investigated relatively widely, with a few reports looking at the consistency between them.7,11,18 However, to- Organic semiconductors have long been touted as prom- date there have been no reports on the DOS in semiconductor ising candidates for next-generation electronics. Thin-film blend systems and how this is impacted by compositional transistors based on organic semiconductors have achieved and microstructural changes. carrier mobilities greater than that of amorphous silicon (i.e., This work describes the DOS observed in OTFTs >1cm2/V s), and increasingly in the order of 10 cm2/V s.1–4 employing crystalline small molecule, amorphous polymer, Beyond this, organic semiconductors are mechanically flexi- and polymer: small molecule blend semiconductors. We ble and can be deposited over large areas from solution, describe the evolution of the DOS as the morphology and making them potentially suitable for ubiquitous low-cost composition of the semiconductor changes from completely electronics applications. amorphous polymer to polycrystalline small molecule. From One of the most promising materials systems that has this, we are able to directly attribute the changes in device emerged over the past few years is that of polymer: small performance to the changes occurring on a molecular level at molecule semiconductor blends. When deposited from solu- the semiconductor-dielectric interface. tion the two components strongly phase separate, with the A mobility edge model, originally developed to describe small molecule component being expelled to the film surface the DOS of amorphous inorganic semiconductors, is fre- where it crystallises. Thus, the resulting film microstructure quently used to describe band-gap states in OTFTs, regard- can be controlled via the blending ratio and the deposition less of whether the semiconductor is amorphous, or conditions. Such materials systems have demonstrated hole 19,20 mobilities of up to 5 cm2/V s, and the use of newer, ultra- crystalline. The mobility edge model is used to describe high mobility polymer and small molecule semiconductors systems where both order and disorder are present: A rectan- promise device performance even higher than this.5 gular band of mobile states attributed to regions of high mo- While achieving charge carrier mobilities in the order of lecular order turns into a distribution of localized states that 10 cm2/V s is important for the viability of organic thin-film decays into the band gap. The energy dividing these two transistors (OTFTs) in many commercial applications, a regions is named the mobility edge. Conduction in practical greater understanding of the density of states (DOS) in the organic transistors is controlled by these localised states, band gap of blended organic semiconductors is also required. where the approximation is commonly made that there is a The trap states influence not only charge carrier mobility but band in which carriers are able to move freely, and a distri- also the threshold voltage and subthreshold characteristics of bution of trap states in the band-gap where carriers are OTFTs: all critical parameters for the viability of such devi- immobile. The number of free carriers in the semiconductor ces. The DOS in single crystal,6–8 polycrystalline and amor- (and hence the channel conductivity of an OTFT) is a func- phous small molecule,9–13 and polymer14–17 semiconductors tion of the quasi-Fermi level (the point where occupation of states is half due to the application of an external bias) and the DOS (Figure 1). The key parameters of the mobility edge a)Author to whom correspondence should be addressed. Electronic mail: model are the band mobility, l0, the DOS at the mobility [email protected] edge, N0, and the width of the trap distribution E0. 0021-8979/2016/120(2)/025502/7/$30.00 120, 025502-1 Published by AIP Publishing. 025502-2 Hunter, Mottram, and Anthopoulos J. Appl. Phys. 120, 025502 (2016) II. METHODS The gate bias modulation of charge carrier mobility in a TFT is determined by the density of trap states close to the mobility edge. As such, measuring the transconductance of a TFT can be used to estimate the DOS function. The Grunewald€ method is a specific example of this. The method was developed to calculate the distribution of trap states in a-Si TFTs and has been applied to analyses of a range of dis- ordered semiconductors.25–28 The method enables DOS extraction from individual transfer curves, unlike most other methods that require a temperature-dependence to be FIG. 1. Depiction of the DOS function as a function of energy from the mo- 8,27 bility edge. The Fermi function is overlaid to demonstrate how the convolu- recorded: Such requirements are less than ideal given that tion of this with the DOS leads to a calculation of the occupied states. the actual DOS in a material may be temperature dependent. Assumptions that are made in the derivation of the Grunewald€ method are limited but include The semiconductor blend system studied here employs the small molecule p-type semiconductor 2,8-difluoro- (1) Uniform distribution of charge density along the chan- 5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES nel, i.e., the transistor is operating in the linear regime. ADT), and the amorphous polymer p-type semiconductor (2) The semiconductor is homogenous and isotropic within poly(triarylamine) (PTAA). OTFTs are fabricated with dif- the active channel of the transistor. ferent semiconductor blend ratios, resulting in an evolution (3) Interfacial potential, V0, is continuous across the semi- in film morphology and trap state characteristics. An opti- conductor—dielectric interface, i.e., taking into account mised blend ratio of 50 wt. % diF-TES ADT has been gate bias-induced band-bending. shown to demonstrate hole mobilities of >2cm2/V s.21,22 (4) The semiconductor being investigated can be modelled Previous investigations into this materials system have as having a charge carrier transport level and a tail of demonstrated that hole conduction occurs via percolation localised states as described above. through the high mobility, crystalline diF-TES ADT 23 These assumptions and their implications on the present regions. A percolation threshold of 39 wt. % diF-TES study are addressed in turn: ADT component was observed, with the calculated carrier mobility changing by a factor of 500 across this threshold. (1) Transistors were operated in their linear regime during Previous work showed that the optimum blend ratio of data collection in this study. 50 wt. % diF-TES ADT is due to a reduced activation (2) Counter to assumption 2 above, Figure 2 demonstrates energy for charge transport existing at this composition.24 that the semiconductor morphology in these polycrystal- This work describes a more comprehensive approach to line films is not homogeneous. However, even for low DOS analysis in this materials system, placing particular wt. % diF-TES ADT blends, charge transport is domi- emphasis on the role of the small molecule component in nated by the small molecule component. Therefore, we the semiconductor blend. make the simplification that the active channel consists FIG. 2. Polarized optical microscopy images of the semiconductor film microstructure. For low wt. % diF-TES ADT compositions, the film is only partly composed of crystalline mate- rial, whereas for high wt. % diF-TES ADT the film surface is entirely crys- talline but increasingly disordered. 025502-3 Hunter, Mottram, and Anthopoulos J. Appl. Phys. 120, 025502 (2016) of small molecule only. The crystalline diF-TES ADT is Temperature-dependent electrical measurements were con- also polycrystalline in nature, meaning that the extracted ducted down to 90 K under vacuum, using a Keithley 4200 DOS functions are likely to not be representative of the parameter analyser. intrinsic semiconductor under investigation, but rather a function of both semiconductor and morphological III. COMPOSITION AND TEMPERATURE features. DEPENDENCE OF DOS (3) The use of spin casting to form the dielectric layer in the Eq. (1) describes the DOS as a function of Fermi level; devices tested here results in a high-quality semiconduc- therefore, knowledge of the Fermi level relative to the mobil- tor-dielectric interface.
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