Helmut Sitter - Academia.edu (original) (raw)
Papers by Helmut Sitter
Chemical Physics, 2009
A comparative study of steady-state and time-resolved photoluminescence of para-sexiphenyl ͑PSP͒ ... more A comparative study of steady-state and time-resolved photoluminescence of para-sexiphenyl ͑PSP͒ films grown by organic molecular beam epitaxy ͑OMBE͒ and hot wall epitaxy ͑HWE͒ under comparable conditions is presented. Using different template substrates ͓mica͑001͒ and KCl͑001͒ surfaces͔ as well as different OMBE growth conditions has enabled us to vary greatly the morphology of the PSP crystallites while keeping their chemical structure virtually untouched. We prove that the broad redshifted emission band has a structure-related origin rather than being due to monomolecular oxidative defects. We conclude that the growth conditions and type of template substrate impacts substantially on the film morphology ͑measured by atomic force microscopy͒ and emission properties of the PSP films. The relative intensity of the defect emission band observed in the delayed spectra was found to correlate with the structural quality of PSP crystallites. In particular, the defect emission has been found to be drastically suppressed when ͑i͒ a KCl template substrate was used instead of mica in HWE-grown films, and ͑ii͒ in the OMBE-grown films dominated by growth mounds composed of upright standing molecules as opposed to the films consisting of crystallites formed by molecules lying parallel to the substrate.
Journal of Crystal Growth, 1992
ABSTRACT
Journal of the American Chemical Society, 2011
During the last years, self-assembled organic nanostructures have been recognized as a proper fun... more During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule-surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model.
Applied Physics Letters, 2011
The electric field and temperature dependence of the electron mobility is studied comparatively i... more The electric field and temperature dependence of the electron mobility is studied comparatively in the bulk of fullerene ͑C 60 ͒ diodes and at the interface with dielectric of organic field effect transistors ͑OFETs͒. Electron mobility values follow a Poole-Frenkel-type electric field dependence in both types of devices. The activation energy for electron transport is electric field dependent and follows the square root law of field in both devices as predicted by Gill's law. The same Gill's energy E Gill = 34 meV is measured in diodes and OFETs, which corresponds well to Meyer-Neldel energy ͑E MN = 35 meV͒. It is shown that both the electric field and charge carrier concentration must be accounted for the description of disordered charge transport.
Molecular Crystals and Liquid Crystals, 2002
In this work we use atomic force microscopy to study the early growth stages of para-sexiphenyl (... more In this work we use atomic force microscopy to study the early growth stages of para-sexiphenyl (PSP) films grown on mica by Hot Wall Epitaxy. It is shown that self-organization of PSP molecules occurs during the growth controlled mainly by the substrate temperature and deposition time. In addition X-ray diffraction measurements were performed using synchrotron radiation. These measurements confirmed very high crystalline quality of the grown films.
Organic Electronics, 2011
Charge transport is comparatively studied in the bulk and at the interface of disordered fulleren... more Charge transport is comparatively studied in the bulk and at the interface of disordered fullerene films fabricated using physical vapour deposition. Charge carrier concentration and temperature dependent electron mobilities are comparatively studied using charge extraction by linearly increasing voltage (CELIV) technique and organic field-effect transistors (OFET) measurements. Electron mobility is at least two orders of magnitude higher than hole mobility in the fullerene films. Lower mobility values and stronger concentration dependence in diodes is observed. Carrier concentration dependent activation energy is experimentally measured in both types of devices. Larger activation energy for electron transport is required at lower carrier concentrations. Meyer-Neldel rule (MNR) for electron mobility is observed in both the bulk of the fullerene films and in the transistor channel at the interface. Meyer-Neldel energy (E MN = 35 meV), which is interpreted as disorder parameter, is the same in both device geometries, which suggest that the level of disorder and energetical landscape for charge transport is similar in the bulk of fullerene films and at the interface with the insulator. Disorder formalism is used to qualitatively explain that either carrier concentration or the nature of non-equilibrium charge carrier transport in CELIV compared to steady-state OFET measurements is responsible for observed transport properties.
Physical Review B, 2011
We present a systematic study on the epitaxial growth of α-sexithiophene (6T) on two different ty... more We present a systematic study on the epitaxial growth of α-sexithiophene (6T) on two different types of substrates, phlogopite and muscovite mica. The study is based on a structural and morphological analysis using specular x-ray diffraction, x-ray diffraction pole figure technique, transmission electron microscopy, atomic force microscopy, and optical microscopy. It is shown, that 6T molecules on mica substrates crystallize in two different configurations, characterized either by the (411) or (100) contact plane. Whereas the first orientation consists of needle-like structures, the latter one forms an island-like morphology, and both crystal orientations exhibit a well-defined relationship with respect to each other. We demonstrate that this finding can be explained by directed epitaxy of islands along the sidewalls of the needle-like structures.
We extended our analytical effective medium theory [ Phys. Rev. B 81 045202 (2010)] to describe t... more We extended our analytical effective medium theory [ Phys. Rev. B 81 045202 (2010)] to describe the temperature-dependent hopping charge carrier mobility at arbitrary electric fields in the large carrier density regime. Special emphasis was made to analyze the influence of the lateral electric field on the {Meyer–Neldel} {(MN)} phenomenon observed when studying the charge mobilities in thin-film organic field-effect
Thin Solid Films, 1989
ABSTRACT
MRS Online Proceeding Library
The Review of scientific instruments, 2014
In this paper, a measurement system is presented which enables fast and accurate determination of... more In this paper, a measurement system is presented which enables fast and accurate determination of the relative angular emission intensity of light emitting devices, taking into account their specific features such as low light output, narrow spectral distribution, high spatial luminous intensity ratios, and small dimensions. Application of logarithmic sensors allows for fast measurement of relative emission by simple analog circuitry, while locating 18 of them on a fixed ring around the emitter allows a motionless measurement system. As a result, the proposed system can determine the relative angular emission in less than 100 ms with a resolution of 5° for symmetric emitters.
Physical Review B, 1994
ABSTRACT The dielectric function of cubic and hexagonal CdSe has been measured by spectroscopic e... more ABSTRACT The dielectric function of cubic and hexagonal CdSe has been measured by spectroscopic ellipsometry in the 2-25-eV range. The results are compared with similar data for CdS and other II-VI compounds and also with band-structure calculations. Relevant critical points are listed. Transitions originating in the Cd 4d levels are identified and an estimate of the corresponding exciton binding energies is made.
Springer Series in Materials Science, 2013
In this chapter the potential of Organic-Organic heteroepitaxy is discussed concerning the abilit... more In this chapter the potential of Organic-Organic heteroepitaxy is discussed concerning the ability to provide efficient color tuning of organic nano-fibers deposited on muscovite mica substrates. The first part is focused on the epitaxial growth of rod-like molecules on sheet silicates which has been analyzed by de-
ACS applied materials & interfaces, Jan 10, 2014
The influence of the nature of interface between organic semiconductor and gate dielectric on bia... more The influence of the nature of interface between organic semiconductor and gate dielectric on bias stress electrical stability of n-type C60-based organic field effect transistors (OFETs) was studied. The bias stress induced threshold voltage (Vth) shift was found to depend critically on the OFET device structure: the direction of V(th) shift in top-gate OFETs was opposite to that in bottom-gate OFETs, while the use of the dual-gate OFET structure resulted in just very small variations in V(th). The opposite direction of Vth shift is attributed to the different nature of interfaces between C60 semiconductor and Parylene dielectric in these devices. The V(th) shift to more positive voltages upon bias stress in bottom-gate C60-OFET was similar to that observed for other n-type semiconductors and rationalized by electron trapping in the dielectric or at the gate dielectric/C60 interface. The opposite direction of Vth shift in top-gate C60-OFETs is attributed to free radical species cre...
Langmuir : the ACS journal of surfaces and colloids, Jan 26, 2013
It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T... more It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite mica exhibit the intriguing possibility to prepare lasing nanofibers of tunable emission wavelength. For p-6P/6T heterostructures, two different types of 6T emission have been observed, namely, the well-known red emission of bulk 6T crystals and additionally a green emission connected to the interface between p-6P and 6T. In this study, the origin of the green fluorescence is investigated by photoelectron spectroscopy (PES). As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate. The similarity between p-6P/6T heterostructures on muscovite mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning force microscopy (SFM), and optical spectroscopy. PES measurements show that the interface between p-6P and 6T crystals is...
Applied Physics A, 2014
We are presenting a long-time bias stress stability of C 60 -based n-type organic field effect tr... more We are presenting a long-time bias stress stability of C 60 -based n-type organic field effect transistors (OFETs), in bottom gate, top contacts configuration, with aluminium (Al), silver (Ag) and gold (Au) source-drain contacts. The results clearly shows that the bias stress effects in C 60 -based n-type OFETs is similar to p-type OFETs and it can be reduced by using an appropriate metal for the source-drain contacts. During the bias stress time, the threshold voltage shift and an increase in the contacts resistance have also been measured. On the basis of the stability of the device parameters, it is proposed that the Al source-drain contact-based devices gives better stability as compared to the devices with Ag and Au source-drain contacts. Our results show that the bias stress-induced threshold voltage shift is due to the trapping of charges in the channel region and in the vicinity of the source-drain contacts.
Synthetic Metals, 2014
We are presenting the long time operational stability of C 60 based n-type organic field effect t... more We are presenting the long time operational stability of C 60 based n-type organic field effect transistors (OFETs). Behavior of the device characteristics has been monitored under different conditions of bias stress up to 120 days. By measuring several cycles of measurements of transfer and output characteristics, the long time stability of C 60 based OFETs and their reproducibility have been documented. The major instability of the threshold voltage is discussed by considering the effects of light and darkness on de-trapping of the trapped charges during the bias stress. A shift in the threshold voltage during the continuous bias stress occurs, which recovers back to the initial value during the storage time in a glove box. By comparing the device characteristics, the fluctuations in the device parameters are presented for the entire period of measurements. Furthermore, no appreciable change in the relative source-drain current has been measured during the entire period of investigations, thus the C 60 based low voltage n-type OFETs proves satisfactory stability for an electronic circuit design.
Organic Electronics, 2014
The bias stress effect on C 60 based n-type OFETs was studied comprehensively. The role of dielec... more The bias stress effect on C 60 based n-type OFETs was studied comprehensively. The role of dielectric layers and the active layers on bias stress effect was quantified by choosing three different dielectric layers with three different morphologies of active layers. It was found, that the bias stress induced charges can be trapped in active layer as well as in dielectric layers. The charge trapping in active layers happened ten times faster as compared to the trapping of charges in dielectric layers. It was proved, that the use of appropriate dielectric layers increases the bias stress stability by decreasing bias stress effects up to 55%. It was experimentally proven that the fabrication of electrically stress stable devices is possible by using C 60 layers grown at higher substrate temperature or with large grain sizes. The OFETs fabricated with larger grain sizes also fulfil the criterion of Guard bands for modelling of electronic circuits, which is a first step of organic electronics towards the industrialization. On the basis of experimental evidences, the criterion of choosing appropriate dielectric layers for OFETs was also proposed.
Synthetic Metals, 2014
Air stability of C 60 based n-type OFETs was investigated. At ambient conditions, the unencapsula... more Air stability of C 60 based n-type OFETs was investigated. At ambient conditions, the unencapsulated OFETs show rapid degradation in source-drain currents. In order to study the effects of encapsulation layers on air stability, the OFETs were encapsulated with single layers and bilayers. The OFETs protected by bilayer encapsulation show better air stability as compared to a single layer encapsulation. It has been found, that the barrier performance of the encapsulation layer can be improved by decreasing the surface roughness of the encapsulation layer. Our proposed encapsulation layers for n-type OFETs are transparent and flexible. Therefore, it can be used to encapsulate all type of organic semiconductor based devices also on plastic substrates for flexible devices.
Chemical Physics, 2009
A comparative study of steady-state and time-resolved photoluminescence of para-sexiphenyl ͑PSP͒ ... more A comparative study of steady-state and time-resolved photoluminescence of para-sexiphenyl ͑PSP͒ films grown by organic molecular beam epitaxy ͑OMBE͒ and hot wall epitaxy ͑HWE͒ under comparable conditions is presented. Using different template substrates ͓mica͑001͒ and KCl͑001͒ surfaces͔ as well as different OMBE growth conditions has enabled us to vary greatly the morphology of the PSP crystallites while keeping their chemical structure virtually untouched. We prove that the broad redshifted emission band has a structure-related origin rather than being due to monomolecular oxidative defects. We conclude that the growth conditions and type of template substrate impacts substantially on the film morphology ͑measured by atomic force microscopy͒ and emission properties of the PSP films. The relative intensity of the defect emission band observed in the delayed spectra was found to correlate with the structural quality of PSP crystallites. In particular, the defect emission has been found to be drastically suppressed when ͑i͒ a KCl template substrate was used instead of mica in HWE-grown films, and ͑ii͒ in the OMBE-grown films dominated by growth mounds composed of upright standing molecules as opposed to the films consisting of crystallites formed by molecules lying parallel to the substrate.
Journal of Crystal Growth, 1992
ABSTRACT
Journal of the American Chemical Society, 2011
During the last years, self-assembled organic nanostructures have been recognized as a proper fun... more During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule-surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model.
Applied Physics Letters, 2011
The electric field and temperature dependence of the electron mobility is studied comparatively i... more The electric field and temperature dependence of the electron mobility is studied comparatively in the bulk of fullerene ͑C 60 ͒ diodes and at the interface with dielectric of organic field effect transistors ͑OFETs͒. Electron mobility values follow a Poole-Frenkel-type electric field dependence in both types of devices. The activation energy for electron transport is electric field dependent and follows the square root law of field in both devices as predicted by Gill's law. The same Gill's energy E Gill = 34 meV is measured in diodes and OFETs, which corresponds well to Meyer-Neldel energy ͑E MN = 35 meV͒. It is shown that both the electric field and charge carrier concentration must be accounted for the description of disordered charge transport.
Molecular Crystals and Liquid Crystals, 2002
In this work we use atomic force microscopy to study the early growth stages of para-sexiphenyl (... more In this work we use atomic force microscopy to study the early growth stages of para-sexiphenyl (PSP) films grown on mica by Hot Wall Epitaxy. It is shown that self-organization of PSP molecules occurs during the growth controlled mainly by the substrate temperature and deposition time. In addition X-ray diffraction measurements were performed using synchrotron radiation. These measurements confirmed very high crystalline quality of the grown films.
Organic Electronics, 2011
Charge transport is comparatively studied in the bulk and at the interface of disordered fulleren... more Charge transport is comparatively studied in the bulk and at the interface of disordered fullerene films fabricated using physical vapour deposition. Charge carrier concentration and temperature dependent electron mobilities are comparatively studied using charge extraction by linearly increasing voltage (CELIV) technique and organic field-effect transistors (OFET) measurements. Electron mobility is at least two orders of magnitude higher than hole mobility in the fullerene films. Lower mobility values and stronger concentration dependence in diodes is observed. Carrier concentration dependent activation energy is experimentally measured in both types of devices. Larger activation energy for electron transport is required at lower carrier concentrations. Meyer-Neldel rule (MNR) for electron mobility is observed in both the bulk of the fullerene films and in the transistor channel at the interface. Meyer-Neldel energy (E MN = 35 meV), which is interpreted as disorder parameter, is the same in both device geometries, which suggest that the level of disorder and energetical landscape for charge transport is similar in the bulk of fullerene films and at the interface with the insulator. Disorder formalism is used to qualitatively explain that either carrier concentration or the nature of non-equilibrium charge carrier transport in CELIV compared to steady-state OFET measurements is responsible for observed transport properties.
Physical Review B, 2011
We present a systematic study on the epitaxial growth of α-sexithiophene (6T) on two different ty... more We present a systematic study on the epitaxial growth of α-sexithiophene (6T) on two different types of substrates, phlogopite and muscovite mica. The study is based on a structural and morphological analysis using specular x-ray diffraction, x-ray diffraction pole figure technique, transmission electron microscopy, atomic force microscopy, and optical microscopy. It is shown, that 6T molecules on mica substrates crystallize in two different configurations, characterized either by the (411) or (100) contact plane. Whereas the first orientation consists of needle-like structures, the latter one forms an island-like morphology, and both crystal orientations exhibit a well-defined relationship with respect to each other. We demonstrate that this finding can be explained by directed epitaxy of islands along the sidewalls of the needle-like structures.
We extended our analytical effective medium theory [ Phys. Rev. B 81 045202 (2010)] to describe t... more We extended our analytical effective medium theory [ Phys. Rev. B 81 045202 (2010)] to describe the temperature-dependent hopping charge carrier mobility at arbitrary electric fields in the large carrier density regime. Special emphasis was made to analyze the influence of the lateral electric field on the {Meyer–Neldel} {(MN)} phenomenon observed when studying the charge mobilities in thin-film organic field-effect
Thin Solid Films, 1989
ABSTRACT
MRS Online Proceeding Library
The Review of scientific instruments, 2014
In this paper, a measurement system is presented which enables fast and accurate determination of... more In this paper, a measurement system is presented which enables fast and accurate determination of the relative angular emission intensity of light emitting devices, taking into account their specific features such as low light output, narrow spectral distribution, high spatial luminous intensity ratios, and small dimensions. Application of logarithmic sensors allows for fast measurement of relative emission by simple analog circuitry, while locating 18 of them on a fixed ring around the emitter allows a motionless measurement system. As a result, the proposed system can determine the relative angular emission in less than 100 ms with a resolution of 5° for symmetric emitters.
Physical Review B, 1994
ABSTRACT The dielectric function of cubic and hexagonal CdSe has been measured by spectroscopic e... more ABSTRACT The dielectric function of cubic and hexagonal CdSe has been measured by spectroscopic ellipsometry in the 2-25-eV range. The results are compared with similar data for CdS and other II-VI compounds and also with band-structure calculations. Relevant critical points are listed. Transitions originating in the Cd 4d levels are identified and an estimate of the corresponding exciton binding energies is made.
Springer Series in Materials Science, 2013
In this chapter the potential of Organic-Organic heteroepitaxy is discussed concerning the abilit... more In this chapter the potential of Organic-Organic heteroepitaxy is discussed concerning the ability to provide efficient color tuning of organic nano-fibers deposited on muscovite mica substrates. The first part is focused on the epitaxial growth of rod-like molecules on sheet silicates which has been analyzed by de-
ACS applied materials & interfaces, Jan 10, 2014
The influence of the nature of interface between organic semiconductor and gate dielectric on bia... more The influence of the nature of interface between organic semiconductor and gate dielectric on bias stress electrical stability of n-type C60-based organic field effect transistors (OFETs) was studied. The bias stress induced threshold voltage (Vth) shift was found to depend critically on the OFET device structure: the direction of V(th) shift in top-gate OFETs was opposite to that in bottom-gate OFETs, while the use of the dual-gate OFET structure resulted in just very small variations in V(th). The opposite direction of Vth shift is attributed to the different nature of interfaces between C60 semiconductor and Parylene dielectric in these devices. The V(th) shift to more positive voltages upon bias stress in bottom-gate C60-OFET was similar to that observed for other n-type semiconductors and rationalized by electron trapping in the dielectric or at the gate dielectric/C60 interface. The opposite direction of Vth shift in top-gate C60-OFETs is attributed to free radical species cre...
Langmuir : the ACS journal of surfaces and colloids, Jan 26, 2013
It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T... more It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite mica exhibit the intriguing possibility to prepare lasing nanofibers of tunable emission wavelength. For p-6P/6T heterostructures, two different types of 6T emission have been observed, namely, the well-known red emission of bulk 6T crystals and additionally a green emission connected to the interface between p-6P and 6T. In this study, the origin of the green fluorescence is investigated by photoelectron spectroscopy (PES). As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate. The similarity between p-6P/6T heterostructures on muscovite mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning force microscopy (SFM), and optical spectroscopy. PES measurements show that the interface between p-6P and 6T crystals is...
Applied Physics A, 2014
We are presenting a long-time bias stress stability of C 60 -based n-type organic field effect tr... more We are presenting a long-time bias stress stability of C 60 -based n-type organic field effect transistors (OFETs), in bottom gate, top contacts configuration, with aluminium (Al), silver (Ag) and gold (Au) source-drain contacts. The results clearly shows that the bias stress effects in C 60 -based n-type OFETs is similar to p-type OFETs and it can be reduced by using an appropriate metal for the source-drain contacts. During the bias stress time, the threshold voltage shift and an increase in the contacts resistance have also been measured. On the basis of the stability of the device parameters, it is proposed that the Al source-drain contact-based devices gives better stability as compared to the devices with Ag and Au source-drain contacts. Our results show that the bias stress-induced threshold voltage shift is due to the trapping of charges in the channel region and in the vicinity of the source-drain contacts.
Synthetic Metals, 2014
We are presenting the long time operational stability of C 60 based n-type organic field effect t... more We are presenting the long time operational stability of C 60 based n-type organic field effect transistors (OFETs). Behavior of the device characteristics has been monitored under different conditions of bias stress up to 120 days. By measuring several cycles of measurements of transfer and output characteristics, the long time stability of C 60 based OFETs and their reproducibility have been documented. The major instability of the threshold voltage is discussed by considering the effects of light and darkness on de-trapping of the trapped charges during the bias stress. A shift in the threshold voltage during the continuous bias stress occurs, which recovers back to the initial value during the storage time in a glove box. By comparing the device characteristics, the fluctuations in the device parameters are presented for the entire period of measurements. Furthermore, no appreciable change in the relative source-drain current has been measured during the entire period of investigations, thus the C 60 based low voltage n-type OFETs proves satisfactory stability for an electronic circuit design.
Organic Electronics, 2014
The bias stress effect on C 60 based n-type OFETs was studied comprehensively. The role of dielec... more The bias stress effect on C 60 based n-type OFETs was studied comprehensively. The role of dielectric layers and the active layers on bias stress effect was quantified by choosing three different dielectric layers with three different morphologies of active layers. It was found, that the bias stress induced charges can be trapped in active layer as well as in dielectric layers. The charge trapping in active layers happened ten times faster as compared to the trapping of charges in dielectric layers. It was proved, that the use of appropriate dielectric layers increases the bias stress stability by decreasing bias stress effects up to 55%. It was experimentally proven that the fabrication of electrically stress stable devices is possible by using C 60 layers grown at higher substrate temperature or with large grain sizes. The OFETs fabricated with larger grain sizes also fulfil the criterion of Guard bands for modelling of electronic circuits, which is a first step of organic electronics towards the industrialization. On the basis of experimental evidences, the criterion of choosing appropriate dielectric layers for OFETs was also proposed.
Synthetic Metals, 2014
Air stability of C 60 based n-type OFETs was investigated. At ambient conditions, the unencapsula... more Air stability of C 60 based n-type OFETs was investigated. At ambient conditions, the unencapsulated OFETs show rapid degradation in source-drain currents. In order to study the effects of encapsulation layers on air stability, the OFETs were encapsulated with single layers and bilayers. The OFETs protected by bilayer encapsulation show better air stability as compared to a single layer encapsulation. It has been found, that the barrier performance of the encapsulation layer can be improved by decreasing the surface roughness of the encapsulation layer. Our proposed encapsulation layers for n-type OFETs are transparent and flexible. Therefore, it can be used to encapsulate all type of organic semiconductor based devices also on plastic substrates for flexible devices.