Correlating Crystal Thickness, Surface Morphology, and Charge

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Correlating Crystal Thickness, Surface Morphology, and Charge Transport in Pristine and Doped Rubrene Single Crystals Jae Joon Kim, Stefan Bachevillier, D. Leonardo Gonzalez Arellano, Benjamin P. Cherniawski, Edmund K. Burnett, Natalie Stingelin, Cédric Ayela, Özlem Usluer, Stefan C. B. Mannsfeld, Guillaume Wantz, and Alejandro L. Briseno ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04451 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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ACS Applied Materials & Interfaces

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Correlating Crystal Thickness, Surface Morphology, and

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Charge Transport in Pristine and Doped Rubrene Single Crystals

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Jae Joon Kim,1 Stefan Bachevillier,2 D. Leonardo Gonzalez Arellano,1 Benjamin P.

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Cherniawski,1 Edmund K. Burnett,1 Natalie Stingelin,3,4 Cédric Ayela,2 Özlem Usluer,1,5 Stefan

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C. B. Mannsfeld,6 Guillaume Wantz,2* Alejandro L. Briseno1,7*

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Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA 01003, United States.

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10 11

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School of Materials Science and Engineering and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, GA 30332, United States.

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01062 Dresden, Germany.

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IMS Laboratory, University of Bordeaux, F-33400 Talence, France.

Laboratoire de Chimie des Polymeres Organiques (LCPO), University of Bordeaux, 33615 Pessac Cedex, France. Department of Energy Systems Engineering, Necmettin Erbakan University, 42140 Konya, Turkey. Center for Advancing Electronics Dresden, Dresden University of Technology,

Department of Chemistry, Pennsylvania State University, University Park, PA 16802, United States.

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KEYWORDS

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Rubrene single crystal, molecular steps, surface-doping, surface charge traps, air-gap transistors

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ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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The relationship between charge transport and surface morphology is investigated utilizing

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rubrene single crystals of varying thicknesses. In the case of pristine crystals, the surface

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conductivities decrease exponentially as the crystal thickness increases until ~4 µm, beyond

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which the surface conductivity saturates. Investigation of the surface morphology using optical

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and atomic force microscopy (OM, AFM) reveals that thicker crystals have a higher number of

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molecular steps, increasing the overall surface roughness compared to thin crystals. The density

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of molecular steps as a surface trap is further quantified with the subthreshold slope of rubrene

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air-gap transistors. This thickness-dependent surface conductivity is rationalized by a shift from

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in-plane to out-of-plane transport governed by surface roughness. The surface transport is

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disrupted by roughening of the crystal surface and becomes limited by the slower vertical

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crystallographic axis on molecular step edges. Separately, we investigate surface doping of

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rubrene crystals by using fluoroalkyltrichrolosilane (FTS) and observe a different mechanism for

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charge transport which is independent of surface roughness. This work demonstrates that the

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correlation between crystal thickness, surface morphology, and charge transport must be taken

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into account when measuring organic single crystals. Considering the fact that these molecular

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steps are universally observed on organic/inorganic and single/polycrystals, we believe that our

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findings can be widely applied to improve charge transport understanding.

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1. INTRODUCTION

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Single crystals provide a powerful platform to study intrinsic material properties and the

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effects of external stimuli in organic semiconductors. The advantage of crystal studies lies in

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their low defect concentrations and well-defined structure as compared to their amorphous or

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thin film counterparts. With reproducible material properties, crystals also make ideal candidates


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for studies of external stimuli such as strain,1 light absorption,2 Hall effect,3 and chemical

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doping.4

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Despite the large body of literature on the physical and electrical properties of organic single

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crystals,5-7 the effects of crystal thickness and surface morphology on charge transport have been

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underreported. Previous studies typically employ crystals with thicknesses greater than several

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micrometers due to ease of device fabrication. However, it has been reported that crystal

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thickness influences charge transport as both crystal size and surface quality (roughness)

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increases.8-10 Thus, understanding the correlation between crystal thickness, surface morphology,

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and charge transport is essential for furthering the field of organic electronics in specialized areas

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such as electronic skins, wearable electronics, and for understanding the performance limitations

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of new and existing materials.

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In this work, rubrene single crystals of varying thicknesses were employed to investigate the

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influence of morphology, conductivity, and doping at the crystal surface. Using optical and

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atomic force microscopy (OM, AFM), it was found that the density of molecular steps and

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roughness increases as a function of crystal thickness. As the surface roughens, the surface

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conductivity exponentially decreases until a critical thickness of ~4 µm where conductivity

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becomes constant. We contend that surface transport is disrupted by roughening in thick crystals

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and becomes limited by the slower vertical crystallographic axis present at molecular terrace

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steps.

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fluoroalkyltrichrolosilane (FTS), a different mechanism for charge transport is observed which is

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independent of surface roughness.

Separately,

by

investigating

surface

doping

of

rubrene

crystals

using

22 23

2. EXPERIMENTAL SECTION


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2.1 Rubrene crystal growth

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Rubrene single crystals were grown by a horizontal physical vapor transport method. Rubrene

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powder (Acros Organics) was placed on a cleaned glass sleeve. The source temperature was

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~300 °C (Figure 1c) and Argon gas was used with a flow rate of 100 ml min-1. A single batch of

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crystal growth resulted in a distribution of sizes and thicknesses that can be tuned by changing

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crystallization time, source temperature, or Argon flow rate.

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The optical microscope images were obtained with a Zeiss Axio Scope A1 equipped with Zeiss

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AxioCam ICc1 camera. Crystal thickness and topographic data were obtained using an optical

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profilometer (Zygo NewView 7300, Veeco DekTak 150, and Veeco NT9080), and AFM (Veeco

10

Dimension 3100).

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2.2 Electronic Characterization

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The electrical characteristics were measured with Keithley 4200-SCS probe station unit in

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ambient conditions at room temperature. The surface conductivity of crystals with different

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thicknesses were measured by applying graphite ink (Ted Pella, PELCO conductive graphite) on

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the crystals. The prepared graphite electrodes showed Ohmic contact with rubrene crystals

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irrespective of their surface roughness 5. For the investigation of surface doping effect, The FTS

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(Alfa Aesar) was deposited on rubrene single crystals by adding FTS liquid in a vacuum

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chamber (~ 10-2 Torr) and keeping it overnight. The deposition time and an amount of FTS were

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controlled until the conductivity of rubrene is saturated.

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The air-gap transistors were fabricated by depositing rubrene crystals on Cr/Au (3/17 nm)

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coated polydimethylsiloxane (PDMS). The channel length and dielectric thicknesses were 45 µm

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and 5 µm respectively. The channel width is defined by the crystal size. We calculated the field-

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effect mobilities, subthreshold slopes, and trap densities using following equations.


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2.3 Crystal packing Molecular packing diagram of rubrene crystal was generated by Mercury software.

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3. RESULTS AND DISCUSSION

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Rubrene single crystals were grown via physical vapor transport (PVT), as shown in Figure

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1. The crystals are classified as either thin (0.1-1 µm) or thick (> 4 µm) and identified as

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translucent yellow/orange or red in color, respectively8 (Figure 1a). PVT yields many crystals

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with varying size and thickness, schematically represented in Figure 1b.11 To understand the

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relationship between thickness and roughness in rubrene single crystals, surface topologies were

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characterized (Figure 2) using optical and atomic force microscopy (AFM). Thin crystals have a

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smooth and uniform surface, exhibiting a low monomolecular step12 (~1.3 nm) density less than

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0.01 µm-2 (Figure 2c). Thick crystals have a high density of monomolecular steps along the

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basal plane (Figure 2d) and non-uniform macrosteps (Figure 2b).13,14 The height analysis of thin

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and thick crystals confirming the different distribution of molecular steps are shown in Figure

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S1, and similar analysis was previously reported in literature.15 Macrosteps are large height

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variations composed of densely packed monomolecular steps. The formation of macrosteps can

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be attributed to random crystal growth on molecular step edges16 and imperfections.11,17 Due to

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the vertical operational restriction of the AFM, optical profilometry was employed to analyze the

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crystals’ surface roughness. Optical profilometry reveals an increase in the root-mean-square

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surface roughness as a function of crystal thickness (Figure 2e). Figure S2 shows that both

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molecular steps and macrosteps exhibit no preferred directional orientation and large density

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variations.


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The roughening of surface morphology as the thickness increases can be explained by

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employing crystal growth theory. Roughening in rubrene crystals can either be a true

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thermodynamic phase transition, i.e. there exists a temperature below which the growth is

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smooth and above which the growth becomes rough, or caused by the kinetics or dynamics of the

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growth process itself.18 This latter type of transition has been previously observed in various

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organic

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perylenetetracarboxyl dianhydride (PTCDA) on silver, and copper(II) phthalocyanine (CuPc) on

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graphene substrates.19,20 In all these cases, the growth transitions from an initial layer-by-layer

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growth (up to 4 monolayers for pentacene on SiO2 and up to seven monolayers for DIP on SiO2)

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to island growth, leading to the observed formation of molecular steps and the increase of the

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surface roughness. The physical origin behind such transitions is still not completely understood.

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The most frequently given explanations are of kinetic nature and center on an evolving

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imbalance in the interlayer mass transport as the film grows. For instance, the dynamic

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thickness-dependent Ehrlich-Schwoebel barrier limits the vertical monomer diffusion from

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island edges down to the next-lowest layer,21 reducing on-island diffusivity with increasing

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thickness.19 Also, this formation of molecular steps and subsequent surface roughening has been

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frequently shown in organic single crystals10,22,23 and is also found in inorganic24 and

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polycrystalline films.25

thin

film

systems

such

as

pentacene,

diindenoperylene

(DIP)

on

SiO2,

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To understand the effect of crystal thickness and topography change on the surface electronic

20

properties, we characterized the surface conductivities of rubrene single crystals (Figure 3a).

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Two-probe devices were fabricated by painting colloidal-graphite ink electrodes on rubrene

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crystals. The electrodes conformally coat both smooth and rough crystals forming Ohmic


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contacts5,26 and the influence of contact resistance is further minimized by utilizing long channel

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length devices (> 10 µm).27,28 The surface conductivity, σt of rubrene crystals was determined by

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σt =

L I × W V

(1)

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where L and W are the channel length and width as determined by the crystal dimensions

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between electrodes, I is the current, and V is the voltage. Surface conductivity is terminology

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used to explain the dominant change of conductivity after surface-doping.4,29 Figure 3a shows

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that the surface conductivity of pristine rubrene single crystals (red circles) decreases

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exponentially from 10-8 S/square in the thinnest crystals (~200 nm) to 10-10 S/square in crystals

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≥4 µm. Above this thickness, the surface conductivity plateaus and becomes independent of

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crystal thickness.

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We hypothesize that the observed thickness-dependent surface conductivity manifests from

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the anisotropy of charge transport through different crystallographic axes as illustrated in Figure

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4. The (100) plane between the two graphite electrodes represents the highest conductivity

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direction in orthorhombic rubrene.30 In thin crystals, the smooth surface enables efficient in-

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plane charge transport unperturbed by morphologic defects. Conversely, charge transport is

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restricted in the less conductive vertical (001) direction.31,32 As the crystal thickness increases, a

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higher density of molecular steps disrupts the in-plane charge transport, resulting in an

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exponential decrease in conductivity. In crystals > 4 µm thick, conductivity becomes

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independent of surface morphology as charge transport is dominated by the slower vertical

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direction33; this results in the plateau observed in Figure 3a. While roughness is not limited by

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crystal thickness, there exists a critical height variation where conductivity fully transitions to a

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vertically-limited transport. Further roughening does not impact the conductivity as the surface

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morphology does not influence the vertical transport. A similar behavior is observed in other


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crystalline materials where the out-of-plane conductivity decreases and then saturates with

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increasing layer number and thickness (supporting information, S1).31,34,35 Investigation utilizing

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channel lengths on the order of the step density could isolate and quantify the effect of molecular

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steps between electrodes, yet remains outside the scope of this paper and will be investigated in

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future work.

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To further evaluate the surface quality and electronic landscape we characterize surface trap

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density in rubrene single crystals of varying thickness. Surface trap densities of rubrene single

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crystals were quantified from the subthreshold slope of air-gap transistors. Due to the impact of

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the semiconductor-dielectric interface on transistor performance,36,37 the air-gap transistor

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architecture was utilized, as the dielectric is independent of the surface morphology effects.38,39

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Paralleling the condition of our surface conductivity measurements, i.e., the top surface is in

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contact with air. The air-gap transistor is also ideal for investigating surface trap density due to

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its pristine rubrene-air interface, thus reducing additional interfacial or contact trap sites.

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Figure 5a shows typical transfer characteristics of thin and thick crystals (output

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characteristics are shown in Figure S3). The field effect mobility, µ, was extracted from the

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gate-voltage independent region using the equation

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µ=

∂ ID 2L ×( ) WC i ∂VG

(2)

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where Ci is the capacitance per unit area of the air dielectric, ID is the drain current, VG is the

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gate voltage. The subthreshold slope S is calculated using the equation

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S=

∂VG ∂(log I D )

(3)

and the surface trap density, Nit, was calculated from the equation:


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N it =

Ci Sq ×( − 1) 2 q k BT ln(10)

(4)

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where q is the elementary charge, kB is the Boltzmann constant and T is temperature. A

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representative thin crystal (740 nm) exhibited a hole mobility of 10.6 cm2V-1s-1, current on/off

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ratio of 107, threshold voltage of -8.4 V, subthreshold slope of 5.1 V/decade, and a corresponding

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surface trap density of 9.7 × 1010 cm-2. Whereas, a thick crystal (~4 µm) had a mobility of 6.8

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cm2V-1s-1, current on/off ratio of 107, threshold voltage of -0.3 V, a significantly larger

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subthreshold slope of 27.6 V/decade, and a corresponding surface trap density of 5.3 × 1011 cm-2.

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The average values calculated from more than five devices showed parameter reproducibility

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(Table 1 and Table S1). The output curves (Figure S3) and low leakage currents (Figure S4)

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confirm that air-gap transistors are a reliable method to observe surface-topography changes in

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charge transport. Unlike different surface-conductivities of thin and thick crystals, similar off-

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currents were observed in the transfer curve (Figure 5a). Yet, this tendency can be explained by

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the larger applied voltage between drain and source to achieve mobility measurement in the

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saturation region. In addition, the devices did not show a kink in the ID1/2-VG curve after device

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turn-on, demonstrating that contact resistance is minimized, and the calculated mobility is not

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overestimated.40,41 In Figure 5a, acceptor-like traps appear with increasing crystal thickness as a

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shoulder around 10 to 20 V, as observed in other systems via experiment and simulation.42,43 It

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should be noted that surface trap density vs crystal thickness data follows a similar trend to the

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conductivity measurements, initially increasing with crystal thickness, then plateauing at ~5 µm

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(Figure 5b). Despite the different trap densities, the mobilities of thin and thick crystals show

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similar values without any thickness dependence (Table S1). This suggests that the traps on the

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surface of rubrene are deep traps as previously reported by Podzorov and coworkers44, and these

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deep acceptor-like traps do not affect charge transport behavior45,46 as recently confirmed by


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Frisbie and coworkers.10 We attribute these traps to the presence of rubrene oxide on the surface.

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Rubrene oxide is known to accumulate on step edges47 and form trap sites ~0.3 eV above the

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valance band of rubrene as an acceptor.48,49 The effect of these acceptor-like traps is also

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exhibited in the shift of threshold voltage from -12.5 V to -1.8 V as the thickness increases, as

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rubrene oxide acts as a dopant more readily present on the thick crystals, which have a higher

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density of molecular steps.50 Recently, Frisbie and coworkers demonstrated that a higher density

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of molecular traps and lower charge transport properties were found upon increasing crystal

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thickness. They attributed this phenomenon to the molecular step edge acting as a charge trap

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site and noting that step oxidization being a major contributor, similar conclusions to our study.10

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Onto further understand the effect of surface environment on electronic organic crystals, we

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investigated the impact of surface doping on conductivity of rubrene with varying thicknesses.

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To explore if the thickness-dependent transport is conserved after chemical surface modification,

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we measured the surface conductivity after fluroalkyltrichlorosilane (FTS) self-assembled

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monolayer (SAM) growth. FTS is known to chemically bond to molecular step edges and creates

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a strong dipole interaction and the subsequent charge transfer creates mobile carriers at the doped

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rubrene surface (supporting information, S2).51 Previously, Podzorov and coworkers reported an

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increase in surface conductivity in thick rubrene crystals, however, we sought to understand if

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doped surface conductivities are also subject to roughness dependent behavior as observed in

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pristine crystals.4 We found that after FTS modification, the surface conductivities of all crystals

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(thin and thick) increased to ~5 × 10-7 S/square, independent of crystal thickness (Figure 3a,

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blue squares). From the observed relationship between thickness and surface morphology, we

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conclude that the charge transport is independent from the surface morphology after FTS

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doping.29 Likewise, the independence of gate voltage is observed on the FTS-modified rubrene


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transistor with confirming our assumption (Figure S5). Although an increase in conductivity is

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observed by both of the thickness decrease and surface doping, there is still a measurable

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increase in surface conductivity upon doping thin crystals. In the case of FTS doping, the large

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dipole changes the surface potential, regardless of the surface morphology.51

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4. CONCLUSIONS

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We investigated the thickness-dependent surface conductivity on pristine and FTS-doped

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rubrene crystals. The surface conductivity of pristine crystals exhibited an exponential decrease

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with increasing crystal thickness until plateauing at ~4 µm. This thickness-dependent surface

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conductivity is rationalized by a shift from in-plane to out-of-plane transport mediated by surface

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roughness. FTS doping universally increases the surface conductivity and is shown to be

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independent of surface morphology. This work demonstrates the correlation between crystal

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thickness, surface morphology, and charge transport in rubrene single crystals; an often-

15

overlooked variable when investigating the “intrinsic” properties of organic semiconductors.

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Supporting Information

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The relationship between thickness and vertical charge transport (Section S1); Field-effect

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transistor characteristics after FTS doping (Section S2); The height analysis of the thin and thick

20

crystals (Figure S1); The surface image of a thick rubrene thick crystal (Figure S2); Output

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characteristics of rubrene air-gap transistor (Figure S3); Comparison of drain and gate leakage

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currents in rubrene air-gap transistors (Figure S4); Transfer and output characteristics of rubrene


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field-effect transistors before and after FTS doping (Figure S5); Comparison of device

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properties between thin and thick rubrene air-gap transistors. (Table S1)

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AUTHOR INFORMATION

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Corresponding Author

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*Email: [email protected] (G.W.), [email protected] (A.B.).

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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Acknowledgments: The authors thank C. Daniel Frisbie and Xinglong Ren for providing with

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air-gap transistors. The authors thank Sema Demirci Uzun for providing valuable discussions on

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this project. J.K., D.A., B.C., E.B., and A.B. acknowledge the National Science Foundation

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(DMR-1508627) and the Office of Naval Research (ONR N000147-14-1-0053 and N00014-16-

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1-2612). G.W. and A.B. are thankful to the LabEx AMADEus (ANR-10-LABX-42) in the

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framework of IdEx Bordeaux (ANR-10-IDEX-03-02) i.e. the Investissements d’Avenir

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programme of the French government managed by the Agence Nationale de la Recherche. N.S.

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gratefully acknowledges the support of the IdEx Bordeaux (ANR-10-IDEX-03-02).


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(50) Lüssem, B.; Tietze, M. L.; Kleemann, H.; Hoßbach, C.; Bartha, J. W.; Zakhidov, A.; Leo, K., Doped Organic Transistors Operating in the Inversion and Depletion Regime. Nature communications 2013, 4, 2775. (51) Ellison, D. J.; Lee, B.; Podzorov, V.; Frisbie, C. D., Surface Potential Mapping of Sam‐ Functionalized Organic Semiconductors by Kelvin Probe Force Microscopy. Adv. Mater. 2011, 23, 502-507.

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FIGURES AND TABLES

Figure 1. Rubrene crystals with different thicknesses. (a) Digital photograph of thin and thick rubrene single crystals. (b) Schematic image of the PVT rubrene growth procedure. (c) A distance-temperature profile within PVT tube and chemical structure of rubrene.


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Figure 2. Surface analysis of thin and thick rubrene single crystals. (a, b) Optical microscope images of crystals. (c, d) 2D (upper) and 3D (lower) AFM images of crystals. The inset shows the cross-section of the yellow line corresponds to a monomolecular step. (e) The root mean square surface roughness of rubrene single crystals as a function of the crystal thickness. The surface roughness is measured using optical profilometry from 167 µm × 167 µm area and error bars were calculated from more than five different locations for each crystal.


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Figure 3. Surface conductivity of pristine and FTS-doped rubrene single crystals. (a) Surface conductivities and corresponding plots as a function of crystal thickness. (b) Surface conductivity as a function of the FTS-doping time on a rubrene single crystal. The inset shows a schematic of FTS-doping.


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Figure 4. Schematic of charge transport of rubrene crystal with respect to molecular packing. Only conjugated carbon atoms participating charge transport are marked as red color. Large yellow and small white arrows correspond to in- and out-of- plane charge transport respectively.


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Figure 5. Charge transport behavior on rubrene air-gap transistors. (a) Transfer/square root curve of rubrene air-gap transistors fabricated with thin (red) and thick (blue) crystals. The trap densities were calculated from the subthreshold slope of firstly appeared transfer curve (off to on). The typical rubrene air-gap transistor is shown in the inset. (b) Calculated surface trap densities as a function of crystal thickness.


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Table 1. Performance of the thin and thick rubrene air-gap transistors. The field-effect mobilities, Ion/Ioff ratios, threshold voltages, subthreshold slopes, and trap densities of the thin and thick air-gap transistors were shown respectively. All characteristics were determined from the average of more than five devices.

Thin

Thickness Mobility 2 -1 -1 (µm) (cm V s ) 0.5 ± 0.2 7.1 ± 2.6

Ion/Ioff (ratio) 6 5×10

Thick

7.0 ± 4.1

3×10

Device

6.3 ± 3.0

6

Vth Subthreshold Slope (V/decade) (V) -12.5 ± 3.6 5.1 ± 1.0 -1.8 ± 1.2

30.5 ± 4.4

Trap Density -2 (cm ) 10 9.6×10 5.8×10

11

6


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TABLE OF CONTENTS

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Figure 1. Rubrene crystals with different thicknesses. (a) Digital photograph of thin and thick rubrene single crystals. (b) Schematic image of the PVT rubrene growth procedure. (c) A distance-temperature profile within PVT tube and chemical structure of rubrene. 173x75mm (300 x 300 DPI)


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Figure 2. Surface analysis of thin and thick rubrene single crystals. (a, b) Optical microscope images of crystals. (c, d) 2D (upper) and 3D (lower) AFM images of crystals. The inset shows the cross-section of the yellow line corresponds to a monomolecular step. (e) The root mean square surface roughness of rubrene single crystals as a function of the crystal thickness. The surface roughness is measured using optical profilometry from 167 µm × 167 µm area and error bars were calculated from more than five different locations for each crystal. 85x180mm (300 x 300 DPI)



Figure 3. Surface conductivity of pristine and FTS-doped rubrene single crystals. (a) Surface conductivities and corresponding plots as a function of crystal thickness. (b) Surface conductivity as a function of the FTSdoping time on a rubrene single crystal. The inset shows a schematic of FTS-doping. 86x137mm (300 x 300 DPI)


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Figure 4. Schematic of charge transport of rubrene crystal with respect to molecular packing. Only conjugated carbon atoms participating charge transport are marked as red color. Large yellow and small white arrows correspond to in- and out-of- plane charge transport respectively. 85x113mm (300 x 300 DPI)



Figure 5. Charge transport behavior on rubrene air-gap transistors. (a) Transfer/square root curve of rubrene air-gap transistors fabricated with thin (red) and thick (blue) crystals. The trap densities were calculated from the subthreshold slope of firstly appeared transfer curve (off to on). The typical rubrene airgap transistor is shown in the inset. (b) Calculated surface trap densities as a function of crystal thickness. 84x147mm (300 x 300 DPI)


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83x35mm (300 x 300 DPI)


Correlating Crystal Thickness, Surface Morphology, and Charge

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