Space environment effects on flexible, low-voltage organic thin film

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Space environment effects on flexible, low-voltage organic thin film transistors Laura Basiricò, Alberto Francesco Basile, Piero Cosseddu, Simone Gerardin, Tobias Cramer, Marta Bagatin, Andrea Ciavatti, Alessandro Paccagnella, Annalisa Bonfiglio, and Beatrice Fraboni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08440 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Applied Materials & Interfaces

Space environment effects on flexible, low-voltage organic thin film transistors Laura Basiricò*1, Alberto Francesco Basile1, Piero Cosseddu2, Simone Gerardin3, Tobias Cramer1, Marta Bagatin3, Andrea Ciavatti1, Alessandro Paccagnella3, Annalisa Bonfiglio2 and Beatrice Fraboni1 1

Department of Physics and Astronomy, University of Bologna, 40127 Bologna, Italy

2

Department of Electrical and Electronic Engineering, University of Cagliari, 09123 Cagliari,

Italy 3

Department of Information Engineering, University of Padova, 35131 Padova, Italy

KEYWORDS: Proton beam irradiation, Organic Electronics, Thin film transistor degradation, Electronic transport properties, Radiation damage, Flexible electronics

ABSTRACT Organic electronic devices fabricated on flexible substrates are promising candidates for applications in environments where flexible, light-weight and radiation hard materials are required. In this work, device parameters as threshold voltage, charge mobility and trap density of 13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) based OTFTs have been monitored performing electrical measurements before and after irradiation by high-energy

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protons. The observed reduction of charge carrier mobility following irradiation can be only partially ascribed to the increased trap density. Indeed, we used other techniques to identify additional effects induced by proton irradiation in such devices. Atomic Force Microscopy reveals morphological defects occurring in the organic dielectric layer induced by the impinging protons, which, in turn, induce a strain on the TIPS-pentacene crystallites lying above. The effects of this strain are investigated by Density–Functional–Theory simulations of two model structures, which describe the TIPS-pentacene crystalline films at equilibrium and under strain. The two different density of states distributions in the valence band have been correlated with the Photo-Current spectra acquired before and after proton irradiation. We conclude that the degradation of the dielectric layer and the organic semiconductor sensitivity to strain are the two main phenomena responsible for the reduction of OTFT mobility after proton irradiation.

1. Introduction Organic materials are being extensively studied worldwide as they possess unique and outstanding features, such as large area processing on flexible substrates, easy and low-cost fabrication techniques and the ability to chemically tailor their properties. In particular, thanks to their low weight and the possibility to be deposited over large areas on highly conformable substrates, their potential employment in space applications, not only as adhesive or insulating layer, but also as active devices such as organic solar cells (OPVs) and Organic Thin Film Transistors (OTFTs) used as chemical, gas, bio and strain sensors, has been explored.1,2 However, during space missions, materials and devices are exposed to extreme stresses such as high vacuum, thermal cycles, and heavy irradiation, including UV, X-rays, and ionizing particles (electrons, protons and heavy ions). Investigations on the effects of space radiation on OTFTs


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and other organic devices, thus, are needed in order to explore the applicability of organic electronics in radiation harsh environments, and, indeed, such effects are under debate in literature. Ab-initio calculations have shown that the energy threshold for hydrogen (H) desorption from aromatic chains in the most common organic semiconductors such as pentacene and rubrene is ~3 eV,3 i.e. much lower than typical X-ray and heavy-ion energies. Therefore, the increase of trap density observed in surface-free rubrene OTFTs after 1MeV proton irradiation was attributed to the formation of dangling bonds due to H removal from the acene rings.4 These traps have deep energy levels in the gap and thus cause only a shift of the rubrene OTFT threshold voltage, whereas the channel mobility remains unchanged.

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On the contrary, the

channel mobility of pentacene OTFTs was found to decrease after UV and α-particle irradiation,6,7 thus suggesting that also the density of traps energetically located near the valenceenergy band (i.e. the Highest Occupied Molecular Orbital (HOMO) of the organic semiconductor) increased during irradiation. However, a recent study on the effects of highenergy proton irradiation on pentacene field-effect transistor, fabricated onto a rigid Si/SiO2 (gate/dielectric) substrate, showed that pentacene itself is not directly degraded by irradiation and attributed the changes in the electrical properties observed in irradiated samples to the protonbeam induced trapped charges at the pentacene/SiO2 interface and in the SiO2 layer.8 We

have

recently

reported

on

how

flexible

devices

based

on

6,13-

Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), a standard solution-grown material for the fabrication of organic devices (2-terminal and OTFTs) onto flexible plastic substrates, can be used as real-time, direct ionizing radiation detectors,9 thus opening unprecedented perspectives for space applications of organic electronics. Therefore, a detailed study on the radiation hardness of these devices is mandatory in order to envisage their exploitation, not only as


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electronic components but also as ionizing radiation sensors (both as X-ray imagers and as wearable dosimeter) during space missions, where payload size, weight, low-power supply and portability are relevant issues.10 In this work, we report on the effects of high-energy proton irradiation on the electrical, morphological and opto-electronic properties of TIPS-pentacene based OTFTs. Namely, we exposed the devices to 3MeV protons at high doses. Aging effects of TIPS-Pentacene OTFTs were first evaluated by means of electrical characterization. In particular, the transfer characteristics have been acquired over time by measuring the current flowing between drain and source electrodes in function of the voltage applied to the gate electrode. For simplicity, we refer to such characteristics as IV. From this characterization. We observe that the sub-threshold slope and the channel mobility strongly depend on the irradiation conditions, indicating an increase of trap density in the semiconductor after proton exposure. In addition, it is noteworthy that the irradiation results in a stabilization of the threshold voltage with respect to aging effect of nonirradiated samples. Atomic Force Microscopy (AFM) measurements attributed the irradiation effect on the OTFTs properties to morphological defects, namely hillocks, formed in the dielectric layer following proton irradiation. These defects affect, in turn, the morphology of the semiconducting organic crystallites, causing the observed electrical transport degradation after proton irradiation. The density of states (DOS) distribution in the valence band calculated by Dispersion-corrected Density-Functional-Theory (DFT-D) can be related to the Photo-Current (PC) spectra of the reference and irradiated samples, which give information on the high-energy density of states in the valence energy-band (HOMO) of the semiconductor. From this comparison we can infer a clear indication of modifications of the energetic structure of the organic semiconductor. Such a modification is in accordance with the strain imposed by


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the irradiation induced hillocks in the underlying organic dielectric layer, revealed by AFM measurements on the organic semiconductor layer, which also justify the observed degradation of transistor electrical performance after proton irradiation. It is worth noting that the proton irradiation damage reported in this work, i.e. structural damage (hillocks formation) of the organic dielectric, is a completely different effect with respect to what has been studied and reported by Kim et al.8, where proton irradiation induced trapped charges inside the SiO2 dielectric layer and at the pentacene/SiO2 interface. Moreover, the work of Kim et al.8 strengthens our interpretation, since also in our work we show how the organic semiconducting layer (TIPS-pentacene) is not directly damaged by proton irradiation. 2. Results and Discussion 2.1 Electrical Characterization and Proton Irradiation Effects The structure of the OTFTs under study is displayed in Figure 1 and described in the Experimental Section. Figure 2 shows the transfer characteristics in the saturation regime (IDVG characteristics measured at the drain-source bias VD = -2V), sweeping the gate-source voltage (VG) from +2V up to -3 V (the IGVG characteristics are reported in Figure S1).

Figure 1. Low-voltage OTFT sketch and a cross-section image by Scanning Electron Microscope of TIPS-pentacene OTFTs fabricated on Kapton flexible substrate plated with Al, for gate contact, and using


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a stack of a few monolayers of Al2O3 and 70-nm thick Parylene C, as gate dielectric. The marker corresponds to 50 nm. Protons with energy of 3 MeV were used to irradiate the samples at doses between 1012 cm–2 and 1013 cm–2.

All the characteristics in Figure 2 show a quadratic increase of the drain current ID with increasing VG, indicating the saturation regime of the device. Therefore, we extracted the channel mobility, µeff, and the threshold voltage, VT, by fitting the IV curves to the equation:11 ID =

Ci W µ eff (VG − VT ) 2 2 L

(1)

Typical aging effects on the OTFT electrical properties are those shown in Figure 2(a), which reports the IDVG characteristics measured on the reference sample, shortly after fabrication (black solid line) and after being exposed to air for 560 h (the total time elapsed between the first measurement and the one after irradiation for irradiated samples, red dashed line). The main difference between the two curves is a positive VT shift in the aged device compared to the reference sample. This shift is due to a higher holes density in the aged semiconductor, which is likely due to an increased acceptor dopant concentration after air exposure. This effect is similar to what is usually observed in evaporated pentacene thin films, where adsorbed O2 molecules capture electrons and thus act as doping impurities for p-type conduction.12 However, the OTFT transport properties remained stable during air exposure, as indicated by the fact that the IV curves in Figure 2(a) have comparable extracted mobility values, i.e. µeff ≈ (5 ± 2) × 10–2 cm2 V-1 s-1, similar to values reported in literature for drop cast TIPS-pentacene OTFTs.13 The effects of 3 MeV proton irradiation with doses of 1012 cm–2, 5×1012 cm–2 and 1013 cm–2 on TIPS-pentacene OTFTs are shown in Figure 2(b), Figure 2(c) and Figure 2(d), respectively. In all these cases it is worth noting, as can be inferred from IDVG characteristics, that the channel


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mobility decreased with increasing dose; moreover, the shift of the threshold voltage toward positive values due to aging, described above for the reference sample, is reduced in the irradiated samples and it becomes negative at the highest dose (Figure 2(d)).

Figure 2. IDVG characteristics measured in saturation at VD = –2 V before (solid black lines) and after irradiation (red dashed lines) for the reference sample without irradiation (560 h was the total time elapsed between the first measurement and the one after irradiation) (a) and for the samples irradiated at doses of 1.0×1012 cm–2 (b), 5.0×1012 cm–2 (c) and 1.0×1013 cm–2 (d).

The results are reported in Figure 3(a) and Figure 3(b) showing the variations

∆VT = VT (0) − VT (rad ) and ∆µ eff / µ eff (0) = [ µ eff (0) − µ eff (rad )] / µ eff (0) , induced by different proton doses, as function of time, where µ eff (rad ) , VT (rad ) and µ eff (0) , VT (0) are the channel mobility and the threshold voltage measured after and before irradiation, respectively. For the


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reference sample µ eff (0) , VT (0) represents the values shortly after fabrication, while µ eff (rad ) and VT (rad ) are the values measured after the total time elapsed between the first measurement and the one after irradiation for irradiated samples. The positive shift of VT, observed in the reference sample after 560 hours, further increases with increasing storage time, as shown in Figure 3(a), up to 1300 hours (solid black squares). In contrast, the irradiated samples show a reduced VT shift and are more stable during the entire storage time. This beneficial effect of irradiation on the stability of the IV characteristics was already reported for pentacene OTFTs exposed to low energy ion irradiation and was attributed to the formation of stable charged species within the hydrocarbon matrix following ion implantation processes.14 On the other hand, radiation-induced defects are also responsible for the decrease of mobility with increasing radiation dose shown in Figure 3(b). The values of ∆µeff/µ eff(0) are independent of the storage time in both the reference and the irradiated samples.


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Figure 3: Variation of the threshold voltage, ∆VT (a), and of the channel mobility relative to µeff(0), ∆µ eff /µeff(0) (b), as a function of time for the reference sample (black squares), and for devices irradiated at doses of 1.0×1012 cm–2 (red circles), 5.0×1012 cm–2 (blue triangles) and 1.0×1013 cm–2 (brown diamonds).

In order to find a possible correlation between the observed mobility variations and the possible creation of interfacial defects due to proton irradiation, interfacial defect densities per unit area and unit energy, NT, were estimated from the sub-threshold slope S of the IDVG characteristics reported in Figure 2 by using the equation:15 NT =

 Ci  S − 1  q  kT ln(10) 

(2)

where kT is the thermal energy expressed in eV and q is the elementary charge. The differences


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∆NT, between the values of NT measured before and after irradiation are reported in Figure 4, together with ∆µeff/µeff(0) values, as function of the irradiation dose. The values for the reference sample, i.e. the difference between the values measured in the as-fabricated device and after 560h, are also shown as red stars. On average, ∆NT increases by about a factor of 2 when the irradiation dose is increased from 1012 cm-2 to 1013 cm-2. This is comparable to the ratio between ∆µeff/µeff(0) at these two doses. A proportionality relationship between ∆µeff/µeff(0) and ∆NT can be obtained from the definition of the channel mobility, µeff, which is related to the intrinsic mobility, µ0, by the expression:16,17

µ eff = µ 0

n

(3)

C i (VG − VT )

Here n is the free-carrier density and Ci (VG –VT) = n + NT is the total charge density in the channel. By taking the difference between the channel mobility after irradiation, µeff(rad) and

µeff(0), we obtain: ∆µ eff

µ eff (0)

=

µ eff (0) − µ eff (rad ) ∆N T = µ eff (0) C i (VG − VT )

(4)

Equation 4 indicates that ∆µeff/µeff(0) is proportional to ∆NT, when VT is a constant independent of irradiation dose. The latter assumption is supported by the values |∆VT| ≈ 0.2V

Space environment effects on flexible, low-voltage organic thin film

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