Multi layer structure for encapsulation of organic transistors

Organic Electronics 10 (2009) 692–695

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Multi layer structure for encapsulation of organic transistors Luca Fumagalli a,*, Maddalena Binda a, Inma Suarez Lopez b,1, Dario Natali a, Marco Sampietro a, Sandro Ferrari b,2, Luca Lamagna b, Marco Fanciulli b a b

Politecnico di Milano, Dipartimento di Elettronica e Informazione, Unità IIT, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Laboratorio Nazionale MDM CNR-INFM, Via Olivetti 2, 20041 Agrate Brianza, Italy

a r t i c l e

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Article history: Received 12 February 2009 Received in revised form 13 March 2009 Accepted 15 March 2009 Available online 24 March 2009

PACS: 68.35.bm 68.35.Ct 68.35.Fx 73.50. h 73.61.Ph

a b s t r a c t A novel encapsulation structure to protect organic thin film transistors against oxygen and moisture contaminations is presented. The sealing architecture is comprised of three-layers: aluminum oxide deposited by means of Atomic Layer Deposition is the actual capping layer, while cross-linked poly-vinylphenol and poly-vinylphenol prevent the contamination/damage of the underlying organic semiconductor during the oxide growth. The process has negligible impact on device mobility but it enables poly-3-hexylthiophene based transistors to operate with an on/off ratio in excess of 103 even after 100 days of continuous ambient air exposure. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Organic transistors Encapsulation Aluminum oxide Atomic layer deposition

1. Introduction Many organic semiconductors are prone to react with ambient moisture and oxygen, with detrimental effects for applications: for example p-type thin film transistors (TFTs) have the tendency to p doping, therefore experiencing a dramatic lowering of the on/off ratio when operating in air [1–3]. To solve this problem, one can either chemically tailor the organic semiconductor to fine tune its HOMO and LUMO energy levels positioning with respect to vacuum [4], but this can also affect other properties such as charge injection or mobility. Another approach relies on * Corresponding author. Tel.: +39 0223993773; fax: +39 022367604. E-mail address: [email protected] (L. Fumagalli). 1 Present address: Istituto Italiano di Tecnologia, Via Morengo 30, 16163 Genova, Italy. 2 Present address: Petroceramics S.r.l.,Via Pasubio 3/5, 24044 Dalmine, Italy. 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.03.003

the encapsulation of the device with a suitable permeation barrier such as polymeric layers [5,6], hybrid organic– metallic layers [7], nitrides [8], or inorganic oxides [9–13]. Recently we showed that Atomic Layer Deposited (ALD) Al2O3 can be used to encapsulate poly-3-hexylthiophene (P3HT) based TFT [14]. In the proposed prototype, the encapsulating structure was comprised of two-layers: poly(4-vinylphenol) (PVP) on top of which Al2O3 was deposited. The former creates a surface rich in OHA groups, thus acting as stopping layer against diffusion of trimethyalluminum (TMA), which is one of Al2O3 precursors. This is achieved thanks to the high reactivity of TMA towards OH functionality. This strategy yielded air stable devices, with mobility comparable to that of uncapped devices, but with a non optimal on/off ratio in the range of many tens. We ascribe this unintentional doping to the second precursor of Al2O3, water [1]. In fact, in the two-layers structure: (i) no water stopping layer is

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Glassware for solutions containing the cross-linking agent have been derivatized by exposure to dimethyldichlorosilane vapors prior to use, since 1,6-bis(trichlorosilyl)-hexane is quite moisture sensitive. As OH groups are consumed during the cross-linking process, the final layer is apolar, and therefore can act as water stopping layer, and is insoluble and therefore can easily sustain the subsequent deposition of PVP from ethyl acetate (MW = 20 KDa, solution 1 mg/ml spun at 6000 rpm). Solvents, dimethyl-dichlorosilane, PVP, 1,6-bis(trichlorosilyl)-hexane (Aldrich) were used as received without further purification. Finally, the capping structure was completed by the deposition of Al2O3 by ALD according to the experimental details reported in our previous paper [14]. To summarize, the capping layer is now composed of three thin films: (i) cross-linked PVP, which is deposited directly on the P3HT and acts as water stopping layer; (ii) PVP which provides the AOH groups to act as TMA stopping layer; (iii) atomic layer deposited Al2O3 which is the actual capping layer (Fig. 1).

Al2O3 PVP X-linked PVP

2. Experimental details

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Fig. 1. Cross section of the device capped with the three-layer structure.

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Devices are based on a bottom contact structure using highly p-doped Si as common gate electrode and 130 nm of thermally grown SiO2 as gate insulator. Platinum source and drain contacts have been realized by means of a standard photolithographic process on the oxide surface (Microfab, Bremen). Devices with channel lengths ranging from 3 to 18 lm have been used (channel width 15 mm). The oxide surface has been carefully cleaned with chloroform and acetone and then plasma polished for 10 min. Immediately after the cleaning process, the SiO2 surface has been functionalized by exposure to dimethyl-dichlorosilane vapors, in order to maximize the mobility and reduce the hysteresis [16]. Solution of Regio-Regular poly-(3hexyltiophene) (MW = 33 kDa) in toluene (5 mg ml 1) was filtered through a 0.45 lm pore size PTFE membrane and finally spin coated at 4000 rpm on the substrate, achieving a thickness of about 30 nm. Samples were then cured at 110 °C for 20 min in high vacuum (p = 10 5 mbar) in order to remove any trace of solvents. According to the process reported in [15] the crosslinked layer was obtained as follows: a solution of PVP (MW = 11 kD) in anhydrous tetrahydrofuran (THF) (4 mg ml 1) was mixed with 1,6-bis(trichlorosilyl)-hexane (9 ll ml 1 in anhydrous THF) in a 1:1 by volume solution. The blend was then spun (5000 rpm) on the P3HT film. Even though THF is not a very good solvent for P3HT, in order to minimize its interaction with the P3HT film, we put in rotation the spin coater before dropping the THF-based solution. When P3HT films were compact and smooth, they withstood this step without experiencing any sizeable dissolution. A thermal curing treatment was finally performed (110–120 °C, 20 min in high vacuum). At this stage the cross-linking agent reacts with OH groups on the PVP so that PVP chains are r-bonded to a siloxane network [15].

X

present to prevent the excess water from ALD process to penetrate into PVP which (ii) being highly hygroscopic can be loaded with significant amount of water which can then be released into P3HT. To solve this problem, here we propose an enhanced sealing architecture by adding a stopping layer against water penetration under the PVP/Al2O3 stack. The additional layer has to satisfy the following requirements: it has to be compact and non polar, in order to avoid water diffusion, and it has to be integrated in the existing process, without destroying the previously deposited layer (P3HT) and without being destroyed by the deposition of the subsequent layers. To address these constraints we adopt a PVP-based cross-linkable layer. Such material has been previously used as gate insulator for all-organic field effect transistors by Facchetti and coworkers [15]. They showed that crosslinked PVP is insoluble in common organic solvents and it has better insulating properties than normal PVP, suggesting that the cross-linked polymer is more compact and pin-hole free.

Fig. 2. Trend of the mobility and ION/IOFF ratio during the process (channel length 18 lm). The first three points are measured in vacuum, the last point in ambient atmosphere. The effect of the process on the mobility is marginal and the final value is close to the value measured in the uncapped devices (as deposited). On the contrary, the ION/IOFF ratio drops after the deposition of the cross-linked PVP before improving after the ALD growth and it finally stabilizes around 104. Note that the ION/IOFF ratio has been calculated as the ratio between the current driven at VG = 30 V and the current at VG = 0 V.

L. Fumagalli et al. / Organic Electronics 10 (2009) 692–695

3. Results and discussion

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We monitored the electrical characteristics of the devices step by step throughout the whole capping process, in order to evaluate the impact of each single stage on the performance of the devices. In high vacuum the uncapped devices showed mobilities between 0.5  10 2 and 0.8  10 2 cm2/Vs with on/off ratio close to 106. After the deposition and the cross-linking of PVP, the ION/IOFF ratio drops between 102 and 103 while the mobility is less affected (Fig. 2). The reduction of the ION/IOFF ratio is probably due to residual unreacted AOH groups on PVP chains. The subsequent deposition of PVP does not modify the mobility or the on/off ratio. Finally the sample is capped by Al2O3. From now on, all electrical measurements are performed in ambient air. While the mobility decreases slightly, the on/off ratio reaches values around 104, with an improvement of more than one order of magnitude with respect to the previous stage (Fig. 2). It is likely that during the ALD growth, some TMA molecules reach the cross-linked PVP layer, reacting

and consuming the residual AOH groups responsible for the P3HT doping. We realized, capped and tested a total of 33 devices on different substrates in the same ALD run. We found that for all the tested devices the mobility was marginally affected by the capping process. Regarding the ION/IOFF ratio, a certain dispersion was observed, ranging from less than 102 to 104, with a majority of devices (55%) in the range 102– 103. From now on, we focus on the best performing samples with on/off ratio greater than 103, which represent about the 30% of the devices tested. Fig. 3 compares transfer characteristics (right) and external characteristics (left) measured in air in samples capped with PVP + Al2O3 and capped with cross-linkedPVP + PVP + Al2O3. In order to better compare the off current in the two structures, transfer characteristics are shown in a logarithmic scale. It is clear that the off current is reduced by about two orders of magnitude in the ‘‘threelayers” structure, if compared to the PVP + Al2O3 structure. This demonstrates that the cross-linked layer is really effective as barrier against water diffusion.

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Fig. 3. Comparison between the performances obtained from the different capping structures (measurements performed in air, channel length 3 lm). The sample with the cross-linked PVP layer shows an improvement of the on/off ratio of about two orders of magnitude and a better saturation of the drain current with respect to the sample capped with PVP/Al2O3.

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Fig. 4. Left, trend of the mobility and ION/IOFF ratio as a function of time for the three-layer capped device (channel length 18 lm). Right, evolution of transfer characteristics curves of the same device.

L. Fumagalli et al. / Organic Electronics 10 (2009) 692–695

By comparing the external characteristics we realize that the saturation of the drain current is improved in the sample with the cross-linked PVP with respect to the sample capped with PVP/Al2O3. This is due to the reduced doping which improves the channel pinch-off at the drain side. Finally, we tested the performances of the capped devices over some months, in order to prove the robustness and the long term stability of the capping layer. Graph in Fig. 4 (left) shows the trend of the mobility and ION/IOFF ratio measured in ambient atmosphere as a function of time. After almost two months from the deposition the ION/IOFF ratio is still very high in the range of 104, comparable to the value measured just after the ALD growth. In the following 60 days the on/off ratio diminishes but maintains a value larger than 103. The mobility does not change significantly over the whole period that we monitored, remaining between 0.6  10 2 and 0.8  10 2 cm2/Vs. Fig. 4 (right) shows the evolution of transfer characteristics measured on the same device (L = 18 lm) in ambient atmosphere. The curves measured in the device just after the ALD growth (0 days) and after 50 days are practically coincident, while after 120 days a marginal increase of the off current can be noticed. 4. Conclusions To conclude, we have developed a three-layers structure for the effective encapsulation of P3HT based TFT. The structure is comprised of an ALD deposited Al2O3 layer, and of two spin-cast films acting as stopping layers for Al2O3 precursors: PVP against TMA and cross-linked PVP against water. With respect to our previous two-layer structure [14], this latter layer provides a dramatic improvement of the ION/IOFF ratio, which reaches values between 103 and 104 in ambient atmosphere. Encapsulated devices have shown ambient stability in excess of a few months.

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Acknowledgements The authors are grateful to G. Tallarida and M. Alia (MDM CNR-INFM, Italy) for collaboration in devices realization, to S. Masci for careful bonding of devices and to P. Trigilio for collaboration in devices characterization. The financial support of Project ‘‘Proteo” (Fondazione Cariplo) is also gratefully acknowledged. References [1] S. Hoshino, M. Yoshida, S. Uemura, T. Kodzasa, N. Takada, T. Kamata, K. Yase, Journal of Applied Physics 95 (2004) 5088. [2] D. Li, E.J. Borkent, R. Nortrup, H. Moon, H. Katz, Z. Bao, Applied Physics Letters 86 (2005) 042105. [3] C.R. Kagan, A. Afzali, T.O. Graham, Applied Physics Letters 86 (2005) 193505. [4] B.S. Ong, Y. Wu, Y. Li, P. Liu, H. Pan, Chemistry – A European Journal 14 (2008) 4766. [5] S.H. Han, J.H. Kim, J. Jang, S.M. Cho, M.H. Oh, S.H. Lee, D.J. Choo, Applied Physics Letters 88 (2006) 073519. [6] H. Jung, T. Lim, Y. Choi, M. Yi, J. Won, S. Pyo, Applied Physics Letters 92 (2008) 163504. [7] T. Sekitani, T. Someya, Japanese Journal of Applied Physics 46 (2007) 4300. [8] S. Koul, Y. Vygranenko, F. Li, A. Sazonov, A. Nathan, Material Research Society Symposium Proceedings 871E (2005) 19.4.1. [9] H. Jeon, K. Shin, C. Yang, C.E. Park, S.-H.K. Park, Applied Physics Letters 93 (2008) 163304. [10] S.-H.K. Park, J. Oh, C. Hwang, J. Lee, Y.S. Yang, H.Y. Chu, Electrochemical and Solid-State Letters 8 (2005) H21. [11] W. Kim, W. Koo, S. Jo, C. Kim, H. Baik, J. Lee, S. Im, Applied Surface Science 252 (2005) 1332. [12] S. Meyer, S. Sellner, F. Schreiber, H. Dosch, G. Ulbricht, M. Fischer, B. Gompf, J. Pflaum, Material Research Society Symposium Proceedings (2007) 965 E. [13] S. Sellner, A. Gerlach, F. Schreiber, M. Kelsch, N. Kasper, H. Dosch, S. Meyer, J. Pflaum, M. Fischer, B. Gompf, Advanced Materials 16 (2004) 1750. [14] S. Ferrari, L. Fumagalli, D. Natali, F. Perissinotti, E. Peron, M. Sampietro, Organic Electronics 8 (2007) 407. [15] M.-H. Yoon, H. Yan, A. Facchetti, T.J. Marks, Journal of the American Chemical Society 127 (2005) 10388. [16] J. Veres, S. Ogier, G. Lloyd, D. De Leeuw, Chemistry of Materials 16 (2004) 4543.