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The effect of helium plasma etching on polymer-based optoelectronic devices
Thin Solid Films 517 (2009) 5743–5746
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
The effect of helium plasma etching on polymer-based optoelectronic devices David Keith Chambers, Brijesh Raut, Difei Qi, Chad B. O'Neal, Sandra Zivanovic Selmic ⁎ Institute for Micromanufacturing, Louisiana Tech University, 911 Hergot Avenue, Ruston, LA 71272, USA
a r t i c l e
i n f o
Article history: Received 10 March 2008 Received in revised form 23 March 2009 Accepted 25 March 2009 Available online 5 April 2009 Keywords: Organic semiconductors Polymers Etching Optoelectronic devices
a b s t r a c t In this paper the optoelectronic performance of selectively patterned conjugated polymers in light emitting diodes (LEDs) and photodetectors was examined. Polymers were patterned via a dry, non-reactive ion etching process using helium plasma. The polymers studied were the light-emitting poly[2-methoxy-5-(2′ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl], and the conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). The electroluminescent spectra of etched and unetched LEDs are almost identical. There is no correlation between He-ion etching times and LED emission spectra changes. The MEH-PPV-based photodetectors show no decrease in external quantum efficiencies due to increased etch times. Results show that using helium plasma is effective at etching these polymers at predictable rates from selected areas without damaging the working device. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polymer optoelectronics hold enormous potential because of their relative inexpense and ease of processing compared to silicon and GaAs-based technologies. While advances in organic/inorganic thin film materials will continue, complementary improvements in processing tools and techniques are also needed for these devices to realize their promise. Most processing techniques used for inorganic (e.g. silicon) device fabrication do not port well for use in organic device fabrication. A major obstacle to the use of optoelectronic polymer nanofilms is the accurate patterning of these soft and easily degraded materials. This investigation was started because of our need to accurately pattern polymer photodiodes, without damaging device performance. The diversity of materials (metals, ceramics and organics), the difficulties in finding appropriate etchants, the introduction of water, the stringent controls needed for precise wet etching and its inherent variability, and the similarities of the polymers being studied to those in photoresist, make traditional wet etching techniques incompatible with this type of fabrication. The use of reactive ion etching (RIE) has become commonplace in the traditional microfabrication industry [1,2], but its efficacy in patterning semiconducting polymers, and effect on polymer optoelectronic device performance has not been studied. Oxygen O2 and tetrafluoromethane CF4 have been used for etching photoresist [3] or dielectric polyimide [4]. Oxygen is one of the major causes of semiconductive polymer degradation [5–9], thus oxygen RIE must be avoided ⁎ Corresponding author. Tel.: +1 318 257 5145; fax +1 318 257 5104. E-mail addresses: [email protected] (D.K. Chambers), [email protected] (C.B. O'Neal), [email protected] (S.Z. Selmic). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.199
to mitigate oxidative degradation. Dry etching with physical mechanism only, typically utilizes a nonreacting gas such as argon [10]. Instead of RIE, we proposed that a helium plasma excited inside an RIE chamber might be used to pattern semiconductor polymers [11]. The main physical mechanism is bombarding the surface with inert helium ions and sputter etching [12]. We selected helium ion etching because it is an inert gas that will not react with the polymer or other material and since it has low atomic weight, it loses its kinetic energy quickly and thus is a gently etchant suitable for low modulus materials such as polymer films. To our best knowledge inert helium in RIE chamber was never used alone by other researchers to pattern the semiconducting polymers. However, helium together with reactive gases such as oxygen (He–O2) [13,14], carbon dioxide (He–CO2) [13], or He–CF3Br:Ti [12] were used in the past by other researchers. Mixing inert gas (He or Ar) with reactive gas stabilizes plasma, dilutes etchant, and improves heat transfer [12]. In this paper we use only helium plasma to pattern semiconducting polymers. In this paper, the effects of ion etching on the optoelectronic performance of these polymers, namely poly [2-methoxy-5-(2′-ethylhexyloxy)1, 4-phenylenevinylene] (MEH-PPV), poly [9,9-di-(2′-ethylhexyl) fluorenyl-2,7-diyl] (polyfluorene) and the conducting polymer poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), chemical structures of which are given in Fig. 1, are investigated. There was concern that such a high powered technique, especially using reactive species, would damage the performance of these devices. Using O2 for patterning would likely leave residual oxides or other sources of oxygen that would be available as a degradative species to MEH-PPV, and a detrimental layer of aluminum oxide might be formed in our cathode. To avoid these concerns, helium plasma was used. This showed very good results in the etching of both polymers,
5744
D.K. Chambers et al. / Thin Solid Films 517 (2009) 5743–5746 Table 1 Summary of helium ion etch rates for various polymers. Polymer
Power (W)
Etch rate (nm/min)
Polyfluorene PEDOT:PSS MEH-PPV MEH-PPV
25 25 25 100
~ 12 ~ 15 ~ 22 ~ 35
PlasmaTherm SLR 720/740 RIE chamber with the following settings: power 25–100 W, pressure 26.66 Pa, and helium gas flow rate of 50 standard cm3/min. The He ion energy is about 3–12 eV. The resulting profiles were measured KLA Tencor™ profilometer with 1 nm resolution. Low power and short etch times allowed partial etches, which allowed us to measure and compare the profiles of unetched regions with partially etched ones. Scratches were made in the films to expose regions of glass for reference points used to calculate the etch rates that are summarized in Table 1. These data were used in the next fabrication for calculating minimum etch times to fully remove films from unwanted areas of functioning devices. The fabrication procedure was the same for the light emitting diodes (LEDs) and the photodiodes. The ITO coated glass substrates with a surface resistance of 40 Ω/□ were cleaned and etched using diluted HCl. Pattern transfer defining the anode was achieved with standard photolithography. PEDOT:PSS, polyfluorene, and MEH-PPV
Fig. 1. The chemical structures of the MEH-PPV, polyfluorene and PEDOT:PSS polymers.
and no effect or apparent damage was seen on the aluminum layer. Several experiments were performed that demonstrate, firstly, that ion etching is an effective method for the removal of the polymer(s) from the substrate in the selected etch areas, and secondly, that ion etching is not contraindicated because of adverse effects on the active properties of these polymers. It is amenable to application in polymer optoelectronic fabrication such as patterning photonic crystalline structures, complex multi-layer devices and to facilitate packaging. 2. Experimental details Thin films of 150 nm of MEH-PPV were prepared by spin coating in air. Powdered MEH-PPV (Sigma Aldrich™) from a single batch with molecular weight 86,000 g/mol was dissolved in chlorobenzene. These solutions were heated to 80 °C and stirred for 8 h, then filtered and spincast onto indium tin oxide (ITO) coated substrates (Colorado Concepts). The MEH-PPV film thickness was measured to be 150 nm with good uniformity (±10%). PEDOT:PSS was purchased in a prepared aqueous solution (Sigma Aldrich™) and spincast onto ITO substrates in the same manner as MEH-PPV. These films were also measured to be 150 nm with good uniformity (±5%). Polyfluorene (American Dye Source™) was dissolved in chlorobenzene in a manner similar to that of MEH-PPV. Spincast polyfluorene films with thicknesses of 120 nm were measured with uniformity of ±15%; i.e. the MEH-PPV films were generally smoother than the polyfluorene films. All of these samples were then dried in vacuo for 8 h at room temperature. These films were then partially covered with stainless steel shadow masks, and etched in a helium plasma for various times in a
Fig. 2. Overview of fabrication procedure of MEH-PPV based photodiode.
D.K. Chambers et al. / Thin Solid Films 517 (2009) 5743–5746
5745
Fig. 3. Encapsulated MEH-PPV based photodiode after ion etch, as seen through ITO coated glass substrate.
were spincoated onto these ITO substrates in the manner described above. Due to the irregularity in polyfluorene film thickness, PEDOT: PSS/polyfluorene devices were also prepared. Aluminum was used as a shadow mask to define and protect the polymer device area during ion etching. This aluminum layer also served as the device cathode. To achieve this, a steel shadow mask with the inverse of our desired aluminum pattern was fabricated. Using this shadow mask, 300 nm of aluminum was evaporated. Ion etching for polymer patterning was then performed for various times (see Fig. 2 for an overview of the fabrication procedure, using MEH-PPV as an example). Controls were fabricated in exactly the same manner, except for the omission of the reactive ion etching step. The active area of all devices was ~3 mm × 3 mm. These devices were then tested to gauge the effects of etching on device performance.
Fig. 5. EL spectra for unetched (heavy grey line) and He-etched (all other lines) polymer for 1 min (dashed line), 4 min (thin gray line), 6 min (thin black line), 11 min (thick black line) MEH-PPV based devices. The y-axis offset is used for clarity.
matic light of each wavelength in the range of 400 nm to 800 nm (in 10 nm steps), and the resulting current–voltage (I–V) curves were measured using a Keithley 6487 picoammeter. An Oriel lamp and a Spectra Physics Cornerstone 260 1/4 meter monochromator were used as the excitation source.
3. Testing
4. Results and discussion
The LEDs and photodetectors were characterized in air. Electroluminescent (EL) emission spectra for MEH-PPV and polyfluorene based devices were taken using an LDP-3811 (ILX Lightwave) precision current source for electrical excitation and an Ocean Optics USB2000 spectrometer. The emission spectra were collected from LEDs that were unetched or etched for various amounts of time during fabrication. Note that the spectral intensities are in arbitrary units and do not represent absolute irradiance values. The results from the polyfluorene only devices were erratic, probably due to shorts resulting from the roughness of these films. However, the polyfluorene LEDs that included PEDOT:PSS, which had been spincast as a hole injection layer between ITO and polyfluorene, performed much better. Therefore, for the remainder of this paper, polyfluorene devices refer to polyfluorene/PEDOT:PSS bilayer LEDs. For the MEHPPV-based photodetectors a voltage sweep was done from + 1 V to − 3 V, while the active film was excited by monochro-
The MEH-PPV based photodiode with patterned polymer is seen with the optical camera and a micro lens with magnification up to 10× (Fig. 3). The physical removal of the polymer is clearly visible. The EL spectra of the polyfluorene and MEH-PPV based LEDs show no meaningful shift in peak emission wavelengths or other changes correlated with etching (Figs. 4 and 5, respectively). Such a shift would indicate chemical or structural changes in the polymer films. Chain scission because of high impact ionization would cause a reduction in the mean polymer chain length and would be expected to result in a widening of the highest occupied molecular orbital–lowest unoccupied molecular orbital gap and a blue-shift of peak emission in the EL spectra. The blue-shift of the emission spectra peak was not observed in our LEDs patterned by He etching. Possible damage from ion etching was also investigated by comparing the external quantum efficiencies (EQE) of MEH-PPV based photodetectors etched for various amounts of time. The I–V curves
Fig. 4. EL spectra for unetched (solid black line) and etched (solid grey line) Al/ polyfluorene/PEDOT:PSS/ITO light emitting devices.
Fig. 6. External quantum efficiencies of two MEH-PPV based devices versus monochromatic light wavelength with the etch-time as a parameter.
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D.K. Chambers et al. / Thin Solid Films 517 (2009) 5743–5746
were used to calculate the EQEs of these devices. The EQE is defined as the ratio of the number of current carriers obtained to the number of photons incident on the device:
viable alternative for integrated polymer electronics where traditional wet-etching methods are not feasible. Acknowledgments
J ðλÞ hc ; EQ E = sc ΦðλÞ qλ
ð1Þ
Where Jsc is the photodiode current density in photoconductive mode, Φ is the irradiance (light power incident on a surface per unit area), h is the Planck's constant, q is the electron charge, c is the velocity of light in air, and λ is the wavelength of the incident monochromatic light in air. Some variations in EQE values between measurements are seen (Fig. 6), but these are consistent with normal variations in EQEs for these types of devices. As in the emission spectra, there is no correlation between EQE performance and ion etching times. 5. Conclusion Electroluminescent emission tests of LEDs and quantum efficiency tests of photodetectors made from the most commonly studied lightemitting polymer (MEH-PPV) and its oft used hole-injecting adjunct (PEDOT:PSS) showed no degradation of the optoelectrical properties due to helium plasma etching, when directly etched or protected by a thin film of aluminum. The results from the polyfluorene-based LEDs support this conclusion. We see the EQEs of photodetectors patterned by selective Al evaporation prior to etching remain consistent with unetched controls. These results strongly suggest helium ion etching as a general, effective and relatively benign method for patterning of these sensitive polymer nanofilms. This technique is compatible with the more stringent processing needs for fabricating hybrid device structures that contain both organic and inorganic materials. It is a
This material is based upon work in part supported by the DARPA, NAVY SPAWAR SC under award # N66001-05-1-8903, and Louisiana Board of Regents through the Board of Regents Support Fund under contract No. NSF/LEQSF(2005)-Pfund-18. References [1] B. Neppert, B. Heise, H. Kilian, Colloid Polym. Sci. 261 (1983) 577. [2] H. Jansen, H. Gardeniers, M. de Boer, M. Elwenspoek, J. Fluitman, J. Micromechanics Microengineering 6 (1996) 14. [3] W. Pilz, J. Janes, K. Mueller, J. Pelka, Proc. SPIE 1392 (1991) 84. [4] J. Seale, T. Minton, J. Perry, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 38 (1997) 1089. [5] T. Zyung, J. Kim, Appl. Phys. Lett. 67 (1995) 3420. [6] H. Aziz, G. Xu, Synth. Met. 80 (1996) 7. [7] J. Scott, J. Kaufman, P. Brock, R. DiPietro, J. Salem, J. Goitia, J. Appl. Phys. 79 (1996) 2745. [8] D. Sutherland, J. Carlisle, P. Elliker, G. Fox, T. Hagler, I. Jimenez, H. Lee, K. Pakbaz, L. Terminello, S. Williams, F. Himpsel, D. Shuh, W. Tong, J. Jia, T. Callcott, D. Ederer, Appl. Phys. Lett. 68 (1996) 2046. [9] B. Cumpston, I. Parker, K. Jensen, J. Appl. Phys. 81 (1997) 3716. [10] M. Quirk, J. Serda, Semiconductor Manufacturing Processes, Prentice Hall, Upper Saddle River, NJ, 2001. [11] B. Raut, D.K. Chambers, C. O'Neal, S. Selmic, in: J. Qu, G. Kowalski, S.B. Park (Eds.), Electronic and Photonics Packaging, Chicago, U.S.A., November 5–10, 2006, Proceedings of 2006 ASME International Mechanical Engineering Congress and Exposition, 2006, p. 14997. [12] Marc Madou, Fundamentals of Microfabrication, CRC Press, Boca Raton FL, 1997. [13] J. Cardenas, G. Nordin, Micromachining Technology for Micro-Optics and NanoOptics III, San Jose, U.S.A., January 25–27, 2005, Progress in Biomedical Optics and Imaging, Proc. SPIE 5720 (2005) 130. [14] M. Jung, S.P. Beaudoin, H. Choi, J. Electrochem. Soc. 154 (2007) H422.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
The effect of helium plasma etching on polymer-based optoelectronic devices David Keith Chambers, Brijesh Raut, Difei Qi, Chad B. O'Neal, Sandra Zivanovic Selmic ⁎ Institute for Micromanufacturing, Louisiana Tech University, 911 Hergot Avenue, Ruston, LA 71272, USA
a r t i c l e
i n f o
Article history: Received 10 March 2008 Received in revised form 23 March 2009 Accepted 25 March 2009 Available online 5 April 2009 Keywords: Organic semiconductors Polymers Etching Optoelectronic devices
a b s t r a c t In this paper the optoelectronic performance of selectively patterned conjugated polymers in light emitting diodes (LEDs) and photodetectors was examined. Polymers were patterned via a dry, non-reactive ion etching process using helium plasma. The polymers studied were the light-emitting poly[2-methoxy-5-(2′ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl], and the conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). The electroluminescent spectra of etched and unetched LEDs are almost identical. There is no correlation between He-ion etching times and LED emission spectra changes. The MEH-PPV-based photodetectors show no decrease in external quantum efficiencies due to increased etch times. Results show that using helium plasma is effective at etching these polymers at predictable rates from selected areas without damaging the working device. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polymer optoelectronics hold enormous potential because of their relative inexpense and ease of processing compared to silicon and GaAs-based technologies. While advances in organic/inorganic thin film materials will continue, complementary improvements in processing tools and techniques are also needed for these devices to realize their promise. Most processing techniques used for inorganic (e.g. silicon) device fabrication do not port well for use in organic device fabrication. A major obstacle to the use of optoelectronic polymer nanofilms is the accurate patterning of these soft and easily degraded materials. This investigation was started because of our need to accurately pattern polymer photodiodes, without damaging device performance. The diversity of materials (metals, ceramics and organics), the difficulties in finding appropriate etchants, the introduction of water, the stringent controls needed for precise wet etching and its inherent variability, and the similarities of the polymers being studied to those in photoresist, make traditional wet etching techniques incompatible with this type of fabrication. The use of reactive ion etching (RIE) has become commonplace in the traditional microfabrication industry [1,2], but its efficacy in patterning semiconducting polymers, and effect on polymer optoelectronic device performance has not been studied. Oxygen O2 and tetrafluoromethane CF4 have been used for etching photoresist [3] or dielectric polyimide [4]. Oxygen is one of the major causes of semiconductive polymer degradation [5–9], thus oxygen RIE must be avoided ⁎ Corresponding author. Tel.: +1 318 257 5145; fax +1 318 257 5104. E-mail addresses: [email protected] (D.K. Chambers), [email protected] (C.B. O'Neal), [email protected] (S.Z. Selmic). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.199
to mitigate oxidative degradation. Dry etching with physical mechanism only, typically utilizes a nonreacting gas such as argon [10]. Instead of RIE, we proposed that a helium plasma excited inside an RIE chamber might be used to pattern semiconductor polymers [11]. The main physical mechanism is bombarding the surface with inert helium ions and sputter etching [12]. We selected helium ion etching because it is an inert gas that will not react with the polymer or other material and since it has low atomic weight, it loses its kinetic energy quickly and thus is a gently etchant suitable for low modulus materials such as polymer films. To our best knowledge inert helium in RIE chamber was never used alone by other researchers to pattern the semiconducting polymers. However, helium together with reactive gases such as oxygen (He–O2) [13,14], carbon dioxide (He–CO2) [13], or He–CF3Br:Ti [12] were used in the past by other researchers. Mixing inert gas (He or Ar) with reactive gas stabilizes plasma, dilutes etchant, and improves heat transfer [12]. In this paper we use only helium plasma to pattern semiconducting polymers. In this paper, the effects of ion etching on the optoelectronic performance of these polymers, namely poly [2-methoxy-5-(2′-ethylhexyloxy)1, 4-phenylenevinylene] (MEH-PPV), poly [9,9-di-(2′-ethylhexyl) fluorenyl-2,7-diyl] (polyfluorene) and the conducting polymer poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), chemical structures of which are given in Fig. 1, are investigated. There was concern that such a high powered technique, especially using reactive species, would damage the performance of these devices. Using O2 for patterning would likely leave residual oxides or other sources of oxygen that would be available as a degradative species to MEH-PPV, and a detrimental layer of aluminum oxide might be formed in our cathode. To avoid these concerns, helium plasma was used. This showed very good results in the etching of both polymers,
5744
D.K. Chambers et al. / Thin Solid Films 517 (2009) 5743–5746 Table 1 Summary of helium ion etch rates for various polymers. Polymer
Power (W)
Etch rate (nm/min)
Polyfluorene PEDOT:PSS MEH-PPV MEH-PPV
25 25 25 100
~ 12 ~ 15 ~ 22 ~ 35
PlasmaTherm SLR 720/740 RIE chamber with the following settings: power 25–100 W, pressure 26.66 Pa, and helium gas flow rate of 50 standard cm3/min. The He ion energy is about 3–12 eV. The resulting profiles were measured KLA Tencor™ profilometer with 1 nm resolution. Low power and short etch times allowed partial etches, which allowed us to measure and compare the profiles of unetched regions with partially etched ones. Scratches were made in the films to expose regions of glass for reference points used to calculate the etch rates that are summarized in Table 1. These data were used in the next fabrication for calculating minimum etch times to fully remove films from unwanted areas of functioning devices. The fabrication procedure was the same for the light emitting diodes (LEDs) and the photodiodes. The ITO coated glass substrates with a surface resistance of 40 Ω/□ were cleaned and etched using diluted HCl. Pattern transfer defining the anode was achieved with standard photolithography. PEDOT:PSS, polyfluorene, and MEH-PPV
Fig. 1. The chemical structures of the MEH-PPV, polyfluorene and PEDOT:PSS polymers.
and no effect or apparent damage was seen on the aluminum layer. Several experiments were performed that demonstrate, firstly, that ion etching is an effective method for the removal of the polymer(s) from the substrate in the selected etch areas, and secondly, that ion etching is not contraindicated because of adverse effects on the active properties of these polymers. It is amenable to application in polymer optoelectronic fabrication such as patterning photonic crystalline structures, complex multi-layer devices and to facilitate packaging. 2. Experimental details Thin films of 150 nm of MEH-PPV were prepared by spin coating in air. Powdered MEH-PPV (Sigma Aldrich™) from a single batch with molecular weight 86,000 g/mol was dissolved in chlorobenzene. These solutions were heated to 80 °C and stirred for 8 h, then filtered and spincast onto indium tin oxide (ITO) coated substrates (Colorado Concepts). The MEH-PPV film thickness was measured to be 150 nm with good uniformity (±10%). PEDOT:PSS was purchased in a prepared aqueous solution (Sigma Aldrich™) and spincast onto ITO substrates in the same manner as MEH-PPV. These films were also measured to be 150 nm with good uniformity (±5%). Polyfluorene (American Dye Source™) was dissolved in chlorobenzene in a manner similar to that of MEH-PPV. Spincast polyfluorene films with thicknesses of 120 nm were measured with uniformity of ±15%; i.e. the MEH-PPV films were generally smoother than the polyfluorene films. All of these samples were then dried in vacuo for 8 h at room temperature. These films were then partially covered with stainless steel shadow masks, and etched in a helium plasma for various times in a
Fig. 2. Overview of fabrication procedure of MEH-PPV based photodiode.
D.K. Chambers et al. / Thin Solid Films 517 (2009) 5743–5746
5745
Fig. 3. Encapsulated MEH-PPV based photodiode after ion etch, as seen through ITO coated glass substrate.
were spincoated onto these ITO substrates in the manner described above. Due to the irregularity in polyfluorene film thickness, PEDOT: PSS/polyfluorene devices were also prepared. Aluminum was used as a shadow mask to define and protect the polymer device area during ion etching. This aluminum layer also served as the device cathode. To achieve this, a steel shadow mask with the inverse of our desired aluminum pattern was fabricated. Using this shadow mask, 300 nm of aluminum was evaporated. Ion etching for polymer patterning was then performed for various times (see Fig. 2 for an overview of the fabrication procedure, using MEH-PPV as an example). Controls were fabricated in exactly the same manner, except for the omission of the reactive ion etching step. The active area of all devices was ~3 mm × 3 mm. These devices were then tested to gauge the effects of etching on device performance.
Fig. 5. EL spectra for unetched (heavy grey line) and He-etched (all other lines) polymer for 1 min (dashed line), 4 min (thin gray line), 6 min (thin black line), 11 min (thick black line) MEH-PPV based devices. The y-axis offset is used for clarity.
matic light of each wavelength in the range of 400 nm to 800 nm (in 10 nm steps), and the resulting current–voltage (I–V) curves were measured using a Keithley 6487 picoammeter. An Oriel lamp and a Spectra Physics Cornerstone 260 1/4 meter monochromator were used as the excitation source.
3. Testing
4. Results and discussion
The LEDs and photodetectors were characterized in air. Electroluminescent (EL) emission spectra for MEH-PPV and polyfluorene based devices were taken using an LDP-3811 (ILX Lightwave) precision current source for electrical excitation and an Ocean Optics USB2000 spectrometer. The emission spectra were collected from LEDs that were unetched or etched for various amounts of time during fabrication. Note that the spectral intensities are in arbitrary units and do not represent absolute irradiance values. The results from the polyfluorene only devices were erratic, probably due to shorts resulting from the roughness of these films. However, the polyfluorene LEDs that included PEDOT:PSS, which had been spincast as a hole injection layer between ITO and polyfluorene, performed much better. Therefore, for the remainder of this paper, polyfluorene devices refer to polyfluorene/PEDOT:PSS bilayer LEDs. For the MEHPPV-based photodetectors a voltage sweep was done from + 1 V to − 3 V, while the active film was excited by monochro-
The MEH-PPV based photodiode with patterned polymer is seen with the optical camera and a micro lens with magnification up to 10× (Fig. 3). The physical removal of the polymer is clearly visible. The EL spectra of the polyfluorene and MEH-PPV based LEDs show no meaningful shift in peak emission wavelengths or other changes correlated with etching (Figs. 4 and 5, respectively). Such a shift would indicate chemical or structural changes in the polymer films. Chain scission because of high impact ionization would cause a reduction in the mean polymer chain length and would be expected to result in a widening of the highest occupied molecular orbital–lowest unoccupied molecular orbital gap and a blue-shift of peak emission in the EL spectra. The blue-shift of the emission spectra peak was not observed in our LEDs patterned by He etching. Possible damage from ion etching was also investigated by comparing the external quantum efficiencies (EQE) of MEH-PPV based photodetectors etched for various amounts of time. The I–V curves
Fig. 4. EL spectra for unetched (solid black line) and etched (solid grey line) Al/ polyfluorene/PEDOT:PSS/ITO light emitting devices.
Fig. 6. External quantum efficiencies of two MEH-PPV based devices versus monochromatic light wavelength with the etch-time as a parameter.
5746
D.K. Chambers et al. / Thin Solid Films 517 (2009) 5743–5746
were used to calculate the EQEs of these devices. The EQE is defined as the ratio of the number of current carriers obtained to the number of photons incident on the device:
viable alternative for integrated polymer electronics where traditional wet-etching methods are not feasible. Acknowledgments
J ðλÞ hc ; EQ E = sc ΦðλÞ qλ
ð1Þ
Where Jsc is the photodiode current density in photoconductive mode, Φ is the irradiance (light power incident on a surface per unit area), h is the Planck's constant, q is the electron charge, c is the velocity of light in air, and λ is the wavelength of the incident monochromatic light in air. Some variations in EQE values between measurements are seen (Fig. 6), but these are consistent with normal variations in EQEs for these types of devices. As in the emission spectra, there is no correlation between EQE performance and ion etching times. 5. Conclusion Electroluminescent emission tests of LEDs and quantum efficiency tests of photodetectors made from the most commonly studied lightemitting polymer (MEH-PPV) and its oft used hole-injecting adjunct (PEDOT:PSS) showed no degradation of the optoelectrical properties due to helium plasma etching, when directly etched or protected by a thin film of aluminum. The results from the polyfluorene-based LEDs support this conclusion. We see the EQEs of photodetectors patterned by selective Al evaporation prior to etching remain consistent with unetched controls. These results strongly suggest helium ion etching as a general, effective and relatively benign method for patterning of these sensitive polymer nanofilms. This technique is compatible with the more stringent processing needs for fabricating hybrid device structures that contain both organic and inorganic materials. It is a
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