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Luminescence-based oxygen sensor structurally integrated with an organic light-emitting device excitation source and an amorphous Si-based photodetector
Journal of Non-Crystalline Solids 352 (2006) 1995–1998 www.elsevier.com/locate/jnoncrysol
Luminescence-based oxygen sensor structurally integrated with an organic light-emitting device excitation source and an amorphous Si-based photodetector Ruth Shinar
a,*
, Debju Ghosh b, Bhaskar Choudhury b, Max Noack a, Vikram L. Dalal a,b, Joseph Shinar c
a Microelectronics Research Center, Iowa State University, Ames, IA 50011, USA Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA Ames Laboratory – USDOE and Department of Physics, Iowa State University, Ames, IA 50011, USA b
c
Available online 13 March 2006
Abstract Structurally integrated photoluminescence-based O2 sensors are described. The structures are based on (1) integrating the excitation source, which is an array of organic light emitting device (OLED) pixels, with a thin-film sensing element, and (2) integrating the sensing element with the photodetector (PD), which is a p–i–n structure based on a-(Si,Ge):H. These components are fabricated on separate glass substrates that are attached back-to-back, resulting in devices with a thickness that is determined by the substrates. Initial design, testing, and issues in the OLED/sensing film/PD three-component structural integration are also reported. 2006 Elsevier B.V. All rights reserved. PACS: 85.60.Jb; 85.60.Dw; 78.60.Fi; 81.05.Gc Keywords: Sensors; Luminescence; Organic light emitting devices; Optical properties
1. Introduction There is a growing need for low-cost, compact bio(chemical) sensor platforms for commercial, including biomedical, applications. This has resulted in efforts to develop low cost, structurally integrated sensors for efficient multianalyte detection [1–3]. Oxygen sensing has been studied extensively [1–3]. Yet efforts are continuing to enhance sensor performance, reduce sensor cost and size, simplify fabrication, and achieve compatibility with in vivo monitoring. Development of field-deployable, compact sensors will benefit gas phase and dissolved O2 monitoring. We are therefore developing a compact photoluminescence (PL)-based O2 sensor
*
Corresponding author. Tel.: +1 515 294 5898; fax: +1 515 294 9584. E-mail address: [email protected] (R. Shinar).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.12.029
to evaluate a new platform, where the PL excitation source is an organic light emitting device (OLED) and the photodetector (PD) is an a-Si:H p–i–n-based structure. A well-known approach for O2 sensing is based on the dynamic quenching of the PL of oxygen-sensitive dyes such as Pt octaethylporphyrin (PtOEP) [2,3]. Collisions with O2 decrease the PL intensity I and lifetime s. Ideally, in a homogeneous matrix, the O2 concentration can be determined by monitoring s or the steady-state I using the Stern–Volmer (SV) equation I 0 =I ¼ s0 =s ¼ 1 þ K SV ½O2 ;
ð1Þ
where I0 and s0 are the values in the absence of oxygen and KSV is a constant. The potential viability of structurally integrated OLEDbased sensors results from the advantages of OLEDs as miniaturizable, flexible, and efficient light sources, and the dramatic improvements in OLEDs [4], which has led
R. Shinar et al. / Journal of Non-Crystalline Solids 352 (2006) 1995–1998
to their emergence in commercial products [5]. OLEDs (100 lm2 to >100 mm2) are easily fabricated on glass or plastic substrates. They consist of an anode, organic layers, and a cathode; their thickness is typically <0.5 lm. a-(Si,Ge):H-based PDs can also be easily fabricated on glass or plastic substrates. Modifying the layer thickness and composition of the p–i–n device enables tuning it to detect the dye emission efficiently. This paper describes developments towards integrated O2 sensors using an OLED excitation source, a PtOEPdoped polystyrene (PS:PtOEP) sensor film, and an a(Si,Ge):H PD. Such sensors could be a basis for compact multianalyte (micro)sensor arrays.
40
30
τ0 / τ
1996
20
10
0 0
20
The structural integration of the components was assessed in several steps. First, integrated OLED/sensing films were evaluated with a PMT. Fig. 1 shows an SV plot of s0/s vs O2 concentration for Alq3/PS:PtOEP. The
80
100
Fig. 1. Stern–Volmer plot of a structurally integrated Alq3 OLED/ PS:PtOEP gas-phase O2 sensor. The photodetector was a PMT. The line is a quadratic fit.
530 nm EL peak of Alq3 matches a PtOEP absorption band. The data shown in Fig. 1 demonstrate the viability of the OLED as an excitation source for 0–100% gas phase O2 detection. The sensitivity S I0/I (100% O2) or S s0/s (100% O2) of Alq3/PS:PtOEP was 37, which is comparable to that obtained using non-integrated excitation sources such as lasers and lamps. Monitoring the sensor performance through s is advantageous as the need for frequent sensor calibration is eliminated. In addition, since s is determined after the end of the EL pulse, the use of filters is less important than when monitoring I. Second, a-Si:H and a-(Si,Ge):H PDs were evaluated by measuring their quantum efficiency (QE) vs wavelength. Fig. 2 shows the results for selected PDs. The goal was to fabricate a PD with a maximal response at the 645 nm
Quantum Efficiency
0.5
a-Si:H
0.4 0.3
a-(Si,Ge):H
0.2 0.1 0.0
a-(Si,Ge):H
500
3. Results and discussion
60
% O2
2. Experimental OLED arrays were fabricated by spin coating or thermally evaporating organic layers on indium tin oxide (ITO)-coated glass [3,4]. The organic layers consisted of hole- and electron- transport layers, and an emitting layer. UV-violet (410 nm peak electroluminescence (EL)) and green (530 nm peak EL) OLEDs were based on poly(Nvinyl carbazole) (PVK) and tris(8-hydroxy quinoline) Al (Alq3), respectively [3,4]. The OLEDs were prepared as encapsulated arrays of 2 · 2 mm2 pixels resulting from mutually perpendicular stripes of the etched ITO anode and evaporated Al cathode [3]. The OLEDs were operated in a DC or pulsed mode at a forward bias of 15–30 V; pulse widths were 50–300 ls and repetition rates were 20–300 Hz. In these ranges, the sensor response was independent of the OLED voltage, pulse width, or repetition rate. PS:PtOEP sensor films were prepared by drop casting 50 lL of a toluene solution with 1 mg/mL dye and 50 mg/mL PS. The films were dried in the dark at ambient temperature. p–i–n structures were fabricated using an ECR reactor [6], either directly on an ITO-covered glass substrate, or on such a substrate coated with a thin protective ZnO layer. The composition and thickness of the p- and i-layers were tuned to detect the 645 nm emission band of PtOEP and to be insensitive to the OLED background. Integrated OLED/sensing film structures were evaluated using a Hamamatsu R6060 photomultiplier tube (PMT). I and s (experimental error <3%) were monitored using DC or pulsed excitation, respectively. The integrated sensor film/PD structures were evaluated using a tungsten– halogen lamp/monochromator excitation source. Gas phase measurements were performed in air or in mixtures of oxygen/nitrogen.
40
600
700
800
Wavelength (nm) Fig. 2. Quantum efficiency vs wavelength of selected PDs with different p- and i-layer compositions. circles: a-Si:H (0.5 lm thick device, 0.15 lm p-layer); squares: a-(Si,Ge):H (0.7 lm thick device, 0.3 lm p-layer, 10% Ge in i-layer); triangles: a-(Si,Ge):H (0.7 lm thick device, 0.3 lm p-layer, 1.6% Ge in p-layer, 13% Ge in i-layer).
R. Shinar et al. / Journal of Non-Crystalline Solids 352 (2006) 1995–1998
25
N2
20 I 0 /I
Response (μV)
150
100
15 10 5 0
air
0
20
40 60 % O2
80
100
50 O2
background
520
540 560 580 Excitation Wavelength (nm)
600
Fig. 3. The effect of O2 concentration on the PD response using a tungsten–halogen lamp/monochromator excitation source and a 600 nm long-pass filter with the integrated PS:PtOEP/a-(Si,Ge):H PD. The inset shows I0/I vs O2 level using a 630 nm long-pass filter.
peak emission of PtOEP and a minimal response at the shorter wavelengths of the OLED EL. As seen in Fig. 2, a-(Si,Ge):H PDs with 10–13% Ge in the i-layer are suitable; however, their dark current is higher than that of PDs with no Ge. long-pass filter
PD
gas flow Sensor
OLED light
Fig. 4. Schematic of the OLED/sensing film/thin film PD set-up in a front detection geometry. The drawing is not to scale.
band-pass filter
1997
Next, integration of the sensing film/a-(Si,Ge):H PD, using the tungsten–halogen lamp/monochromator (set at 535 nm), was tested. Fig. 3 shows the PD response to the PL of the sensor film exposed to O2, air, and N2, as well as to the background light; a 600 nm long-pass filter was placed in front of the PD to reduce the background. Without the sensing film, but with a 600 nm long-pass filter, the background response at an excitation wavelength of 535 nm was 15 lV. However, with the addition of the sensing film, the excitation light at 535 nm is partially absorbed, resulting in PtOEP emission at 645 nm. This emission varies with the O2 level; as expected, it is quenched by oxygen and increases in an N2 atmosphere. The inset of Fig. 3 shows I0/I for the three O2 levels, using a 630 nm long-pass filter that further suppressed the background light. As seen, when using the 600 nm filter, S was 10; it increased to 24 when using the 630 nm filter. We note that S 24 is comparable to values obtained using non-integrated commercial PDs. Optimization of the PD and the integrated structure should increase S further. As a first step towards integration of the three sensor components, an OLED/sensing film/PD structure was assembled in a front detection mode, where the PD is in front of the sensor film (see Fig. 4). Initial results showed a small but reproducible response to O2 with the PVK UV-violet OLED, whose EL overlaps the strong PtOEP absorption band at 380 nm; the PD response changed by 20% when switched between O2 and N2 atmospheres. The small response is suspected to be due to a weak, broad EL band at 610 nm, a relatively large PD dark current (1.5 · 108 A/cm2), and unoptimized device design. Current studies are focusing on improving the design of the sensor and the PD, as well as devising structurally integrated a-Si:H-based filters to block the EL at k > 600 nm. The goal is to reduce the dark current by a factor >100 and the EL at the PL emission band by a factor >4. Fig. 5 shows the envisioned fully integrated OLED/sensing elements/PD array in a back detection geometry, where the PD is co-planar with the OLED pixels; thin isolated Al layers between the OLEDs and PDs will block the edge EL of the OLED. Such an array would be miniaturizable and
sensing element
glass glass PD
OLED
a-Si:H long-pass filter
PD
OLED
PD
a-(Si,Ge):H PD
Fig. 5. Envisioned fully integrated OLED/sensing film/thin film PD array in a back detection configuration. Thin isolated Al layers between the OLEDs and PDs will block the edge EL.
1998
R. Shinar et al. / Journal of Non-Crystalline Solids 352 (2006) 1995–1998
should enable realization of multianalyte (micro)sensor arrays. 4. Concluding remarks The foregoing evaluation of the integrated OLED/sensing film and sensing film/thin film PD for O2 sensing demonstrates the viability of the approach. The integrated modules exhibited large sensitivities for O2 detection via s or I. Current work is focused on improving the OLED and thin film PD compatibility to enable OLED/sensing film/PD integration. Following this step, an array of OLED pixels and PDs will be fabricated on a single substrate, towards the development of compact multianalyte (micro)sensor arrays.
Acknowledgements This work was partially supported by NSF and DOE. Ames Laboratory is operated by ISU for USDOE under Contract W-7405-Eng-82. References [1] [2] [3] [4]
Z. Rosenzweig, R. Kopelman, Anal. Chem. 67 (1995) 2650. P. Douglas, K. Eaton, Sensors Actuators B 82 (2002) 200. B. Choudhury, R. Shinar, J. Shinar, J. Appl. Phys. 96 (2004) 2949. J. Shinar (Ed.), Organic Light Emitting Devices: A Survey, SpringerVerlag, NY, 2003. [5] www.sony.com, www.toshiba.co.jp, www.pioneer.co.jp, www. samsung.co.kr. [6] S. Kaushal, V. Dalal, J. Xu, J. Non-Cryst. Solids 198–200 (1996) 563.
Luminescence-based oxygen sensor structurally integrated with an organic light-emitting device excitation source and an amorphous Si-based photodetector Ruth Shinar
a,*
, Debju Ghosh b, Bhaskar Choudhury b, Max Noack a, Vikram L. Dalal a,b, Joseph Shinar c
a Microelectronics Research Center, Iowa State University, Ames, IA 50011, USA Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA Ames Laboratory – USDOE and Department of Physics, Iowa State University, Ames, IA 50011, USA b
c
Available online 13 March 2006
Abstract Structurally integrated photoluminescence-based O2 sensors are described. The structures are based on (1) integrating the excitation source, which is an array of organic light emitting device (OLED) pixels, with a thin-film sensing element, and (2) integrating the sensing element with the photodetector (PD), which is a p–i–n structure based on a-(Si,Ge):H. These components are fabricated on separate glass substrates that are attached back-to-back, resulting in devices with a thickness that is determined by the substrates. Initial design, testing, and issues in the OLED/sensing film/PD three-component structural integration are also reported. 2006 Elsevier B.V. All rights reserved. PACS: 85.60.Jb; 85.60.Dw; 78.60.Fi; 81.05.Gc Keywords: Sensors; Luminescence; Organic light emitting devices; Optical properties
1. Introduction There is a growing need for low-cost, compact bio(chemical) sensor platforms for commercial, including biomedical, applications. This has resulted in efforts to develop low cost, structurally integrated sensors for efficient multianalyte detection [1–3]. Oxygen sensing has been studied extensively [1–3]. Yet efforts are continuing to enhance sensor performance, reduce sensor cost and size, simplify fabrication, and achieve compatibility with in vivo monitoring. Development of field-deployable, compact sensors will benefit gas phase and dissolved O2 monitoring. We are therefore developing a compact photoluminescence (PL)-based O2 sensor
*
Corresponding author. Tel.: +1 515 294 5898; fax: +1 515 294 9584. E-mail address: [email protected] (R. Shinar).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.12.029
to evaluate a new platform, where the PL excitation source is an organic light emitting device (OLED) and the photodetector (PD) is an a-Si:H p–i–n-based structure. A well-known approach for O2 sensing is based on the dynamic quenching of the PL of oxygen-sensitive dyes such as Pt octaethylporphyrin (PtOEP) [2,3]. Collisions with O2 decrease the PL intensity I and lifetime s. Ideally, in a homogeneous matrix, the O2 concentration can be determined by monitoring s or the steady-state I using the Stern–Volmer (SV) equation I 0 =I ¼ s0 =s ¼ 1 þ K SV ½O2 ;
ð1Þ
where I0 and s0 are the values in the absence of oxygen and KSV is a constant. The potential viability of structurally integrated OLEDbased sensors results from the advantages of OLEDs as miniaturizable, flexible, and efficient light sources, and the dramatic improvements in OLEDs [4], which has led
R. Shinar et al. / Journal of Non-Crystalline Solids 352 (2006) 1995–1998
to their emergence in commercial products [5]. OLEDs (100 lm2 to >100 mm2) are easily fabricated on glass or plastic substrates. They consist of an anode, organic layers, and a cathode; their thickness is typically <0.5 lm. a-(Si,Ge):H-based PDs can also be easily fabricated on glass or plastic substrates. Modifying the layer thickness and composition of the p–i–n device enables tuning it to detect the dye emission efficiently. This paper describes developments towards integrated O2 sensors using an OLED excitation source, a PtOEPdoped polystyrene (PS:PtOEP) sensor film, and an a(Si,Ge):H PD. Such sensors could be a basis for compact multianalyte (micro)sensor arrays.
40
30
τ0 / τ
1996
20
10
0 0
20
The structural integration of the components was assessed in several steps. First, integrated OLED/sensing films were evaluated with a PMT. Fig. 1 shows an SV plot of s0/s vs O2 concentration for Alq3/PS:PtOEP. The
80
100
Fig. 1. Stern–Volmer plot of a structurally integrated Alq3 OLED/ PS:PtOEP gas-phase O2 sensor. The photodetector was a PMT. The line is a quadratic fit.
530 nm EL peak of Alq3 matches a PtOEP absorption band. The data shown in Fig. 1 demonstrate the viability of the OLED as an excitation source for 0–100% gas phase O2 detection. The sensitivity S I0/I (100% O2) or S s0/s (100% O2) of Alq3/PS:PtOEP was 37, which is comparable to that obtained using non-integrated excitation sources such as lasers and lamps. Monitoring the sensor performance through s is advantageous as the need for frequent sensor calibration is eliminated. In addition, since s is determined after the end of the EL pulse, the use of filters is less important than when monitoring I. Second, a-Si:H and a-(Si,Ge):H PDs were evaluated by measuring their quantum efficiency (QE) vs wavelength. Fig. 2 shows the results for selected PDs. The goal was to fabricate a PD with a maximal response at the 645 nm
Quantum Efficiency
0.5
a-Si:H
0.4 0.3
a-(Si,Ge):H
0.2 0.1 0.0
a-(Si,Ge):H
500
3. Results and discussion
60
% O2
2. Experimental OLED arrays were fabricated by spin coating or thermally evaporating organic layers on indium tin oxide (ITO)-coated glass [3,4]. The organic layers consisted of hole- and electron- transport layers, and an emitting layer. UV-violet (410 nm peak electroluminescence (EL)) and green (530 nm peak EL) OLEDs were based on poly(Nvinyl carbazole) (PVK) and tris(8-hydroxy quinoline) Al (Alq3), respectively [3,4]. The OLEDs were prepared as encapsulated arrays of 2 · 2 mm2 pixels resulting from mutually perpendicular stripes of the etched ITO anode and evaporated Al cathode [3]. The OLEDs were operated in a DC or pulsed mode at a forward bias of 15–30 V; pulse widths were 50–300 ls and repetition rates were 20–300 Hz. In these ranges, the sensor response was independent of the OLED voltage, pulse width, or repetition rate. PS:PtOEP sensor films were prepared by drop casting 50 lL of a toluene solution with 1 mg/mL dye and 50 mg/mL PS. The films were dried in the dark at ambient temperature. p–i–n structures were fabricated using an ECR reactor [6], either directly on an ITO-covered glass substrate, or on such a substrate coated with a thin protective ZnO layer. The composition and thickness of the p- and i-layers were tuned to detect the 645 nm emission band of PtOEP and to be insensitive to the OLED background. Integrated OLED/sensing film structures were evaluated using a Hamamatsu R6060 photomultiplier tube (PMT). I and s (experimental error <3%) were monitored using DC or pulsed excitation, respectively. The integrated sensor film/PD structures were evaluated using a tungsten– halogen lamp/monochromator excitation source. Gas phase measurements were performed in air or in mixtures of oxygen/nitrogen.
40
600
700
800
Wavelength (nm) Fig. 2. Quantum efficiency vs wavelength of selected PDs with different p- and i-layer compositions. circles: a-Si:H (0.5 lm thick device, 0.15 lm p-layer); squares: a-(Si,Ge):H (0.7 lm thick device, 0.3 lm p-layer, 10% Ge in i-layer); triangles: a-(Si,Ge):H (0.7 lm thick device, 0.3 lm p-layer, 1.6% Ge in p-layer, 13% Ge in i-layer).
R. Shinar et al. / Journal of Non-Crystalline Solids 352 (2006) 1995–1998
25
N2
20 I 0 /I
Response (μV)
150
100
15 10 5 0
air
0
20
40 60 % O2
80
100
50 O2
background
520
540 560 580 Excitation Wavelength (nm)
600
Fig. 3. The effect of O2 concentration on the PD response using a tungsten–halogen lamp/monochromator excitation source and a 600 nm long-pass filter with the integrated PS:PtOEP/a-(Si,Ge):H PD. The inset shows I0/I vs O2 level using a 630 nm long-pass filter.
peak emission of PtOEP and a minimal response at the shorter wavelengths of the OLED EL. As seen in Fig. 2, a-(Si,Ge):H PDs with 10–13% Ge in the i-layer are suitable; however, their dark current is higher than that of PDs with no Ge. long-pass filter
PD
gas flow Sensor
OLED light
Fig. 4. Schematic of the OLED/sensing film/thin film PD set-up in a front detection geometry. The drawing is not to scale.
band-pass filter
1997
Next, integration of the sensing film/a-(Si,Ge):H PD, using the tungsten–halogen lamp/monochromator (set at 535 nm), was tested. Fig. 3 shows the PD response to the PL of the sensor film exposed to O2, air, and N2, as well as to the background light; a 600 nm long-pass filter was placed in front of the PD to reduce the background. Without the sensing film, but with a 600 nm long-pass filter, the background response at an excitation wavelength of 535 nm was 15 lV. However, with the addition of the sensing film, the excitation light at 535 nm is partially absorbed, resulting in PtOEP emission at 645 nm. This emission varies with the O2 level; as expected, it is quenched by oxygen and increases in an N2 atmosphere. The inset of Fig. 3 shows I0/I for the three O2 levels, using a 630 nm long-pass filter that further suppressed the background light. As seen, when using the 600 nm filter, S was 10; it increased to 24 when using the 630 nm filter. We note that S 24 is comparable to values obtained using non-integrated commercial PDs. Optimization of the PD and the integrated structure should increase S further. As a first step towards integration of the three sensor components, an OLED/sensing film/PD structure was assembled in a front detection mode, where the PD is in front of the sensor film (see Fig. 4). Initial results showed a small but reproducible response to O2 with the PVK UV-violet OLED, whose EL overlaps the strong PtOEP absorption band at 380 nm; the PD response changed by 20% when switched between O2 and N2 atmospheres. The small response is suspected to be due to a weak, broad EL band at 610 nm, a relatively large PD dark current (1.5 · 108 A/cm2), and unoptimized device design. Current studies are focusing on improving the design of the sensor and the PD, as well as devising structurally integrated a-Si:H-based filters to block the EL at k > 600 nm. The goal is to reduce the dark current by a factor >100 and the EL at the PL emission band by a factor >4. Fig. 5 shows the envisioned fully integrated OLED/sensing elements/PD array in a back detection geometry, where the PD is co-planar with the OLED pixels; thin isolated Al layers between the OLEDs and PDs will block the edge EL of the OLED. Such an array would be miniaturizable and
sensing element
glass glass PD
OLED
a-Si:H long-pass filter
PD
OLED
PD
a-(Si,Ge):H PD
Fig. 5. Envisioned fully integrated OLED/sensing film/thin film PD array in a back detection configuration. Thin isolated Al layers between the OLEDs and PDs will block the edge EL.
1998
R. Shinar et al. / Journal of Non-Crystalline Solids 352 (2006) 1995–1998
should enable realization of multianalyte (micro)sensor arrays. 4. Concluding remarks The foregoing evaluation of the integrated OLED/sensing film and sensing film/thin film PD for O2 sensing demonstrates the viability of the approach. The integrated modules exhibited large sensitivities for O2 detection via s or I. Current work is focused on improving the OLED and thin film PD compatibility to enable OLED/sensing film/PD integration. Following this step, an array of OLED pixels and PDs will be fabricated on a single substrate, towards the development of compact multianalyte (micro)sensor arrays.
Acknowledgements This work was partially supported by NSF and DOE. Ames Laboratory is operated by ISU for USDOE under Contract W-7405-Eng-82. References [1] [2] [3] [4]
Z. Rosenzweig, R. Kopelman, Anal. Chem. 67 (1995) 2650. P. Douglas, K. Eaton, Sensors Actuators B 82 (2002) 200. B. Choudhury, R. Shinar, J. Shinar, J. Appl. Phys. 96 (2004) 2949. J. Shinar (Ed.), Organic Light Emitting Devices: A Survey, SpringerVerlag, NY, 2003. [5] www.sony.com, www.toshiba.co.jp, www.pioneer.co.jp, www. samsung.co.kr. [6] S. Kaushal, V. Dalal, J. Xu, J. Non-Cryst. Solids 198–200 (1996) 563.