Analysis of a transparent organic photoconductive sensor

Organic Electronics 13 (2012) 2250–2256

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Analysis of a transparent organic photoconductive sensor Wouter Woestenborghs a,c,⇑, Patrick De Visschere a, Filip Beunis a,c, Geert Van Steenberge b, Kristiaan Neyts a,c, Arnout Vetsuypens d a

Liquid Crystals & Photonics Group, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Ghent, Belgium CMST, Ghent University, Industriepark Zwijnaarde, 9000 Ghent, Belgium c Center for Nano and Biophotonics, Ghent University, Belgium d Technology & Innovation Group, Healthcare Division, Barco NV, Belgium b

a r t i c l e

i n f o

Article history: Received 10 May 2012 Received in revised form 12 June 2012 Accepted 28 June 2012 Available online 15 July 2012 Keywords: m-MTDAB PTCBI Photoconductive sensor Transparent sensor Opto-electronic sensor

a b s t r a c t The electro-optical performance of transparent photoconductive sensors based on stacks of organic layers is investigated. The photoconductive sensors are composed of interdigitated electrodes covered with a stack of two transparent organic compounds: a hole transport layer 1,3,5-tris[(3-methylphenyl)phenylamino]benzene (m-MTDAB) and an exciton generation layer 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). The photocurrent through the device is measured as a function of the voltage across the electrodes for different illumination levels. Based on the measurements we can explain the working principle of the photoconductive sensor and compare the performance of four different stacks. In order to study the optical sensitivity in more detail, a photoconductive device with two parallel electrodes is manufactured and illuminated by a line-shaped laser beam that covers only a fraction of the gap between the electrodes. The current through the photoconductive sensor is measured as a function of the position of the local illumination for a set of voltages. The experimental results confirm that there is a high-field space charge region near the cathode. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Electronic devices based on organic compounds are often an attractive alternative for existing inorganic semiconductor devices because they are less expensive and can be deposited on a large area. In some cases organic electronic devices can present interesting properties like flexibility and high transparency. Organic electronic devices can be designed using similar models as are used Abbreviations: CT, charge transfer; EGL, exciton generation layer; HTL, hole transport layer; m-MTDAB, 1,3,5-Tris[(3-methylphenyl)phenylamino]benzene; PTCBI, 3,4,9,10-perylenetetracarboxylic bis-benzimidazole; BI, background illumination. ⇑ Corresponding author at: Liquid Crystals & Photonics Group, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Ghent, Belgium. Tel.: +32 479391061; fax: +32 92643594. E-mail address: [email protected] (W. Woestenborghs). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.049

for inorganic semiconductor devices. However, the mobilities of the charge carriers in organic materials are a few orders of magnitude lower than in inorganic semiconductors, and this leads typically to higher driving voltages and/or lower currents. However in organic materials the dissociation probability of photon-induced excitons is small due to the large binding energy (0.5–1.0 eV) [1]. To separate the charge carriers, a weaker bound charge-transfer (CT) state can be regarded as an intermediate state, which typically occurs at the interface of two organic materials. Gao et al. [2] found there exists a critical energy offset between donor–acceptor organic materials for the formation of a charge-transfer (CT) state. In this article we present an organic electronic device where the CT principle is used to achieve photoconductivity. The heterojunction of donor–acceptor materials we use is based on the ‘Chemical Vapor-phase Detector’ published

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2. Experimental procedures

the PTCBI molecules. He shows that the PTCBI orientation depends on the substrate surface treatment and proposed that traps are formed when PTCBI is deposited directly on glass due to the silanol (–OH) groups. In our devices PTCBI is deposited on an m-MTDAB. Since the orientation of the PTCBI molecules in our deposited layers is unknown we estimate the electron mobility to be in a range between 2  102 and 2  104 cm2/V s. The HOMO and LUMO levels are respectively 6.3 eV and 4.6 eV [14]. The absorption coefficient for wavelengths in the range 500–700 nm is significantly higher for PTCBI than for m-MTDAB, and therefore this layer is called the exciton generation layer (EGL). To measure the absorption spectrum of m-MTDAB and PTCBI a deposition, by thermal evaporation, for each compound has been made on thin microscope cover glasses. The thermal evaporation of the organic compounds is performed in a Leybold UNIVEX 450 vacuum system at a pressure of 2  106 mbar. The progress of the deposition is monitored by means of a quartz crystal. The thickness of the deposited layer is measured with a WYKO NT3300 profiling system. The length of the gap between the electrodes determines the total current flowing through the photoconductive sensor. A finger pattern is used to achieve a sufficient gap length within a confined area (Fig. 1b). Fig. 1 a shows a cross section of a single period of the repetitive electrode pattern for a photoconductive sensor with one HTL and one EGL layer. The order of the organic layers is selected to obtain a sufficiently high ON/OFF ratio, it is based on the results obtained by John Ho [3]. The thickness of the EGL is a trade off between transparency and sensitivity. The substrates used for the performance study have a large-area of 20  20 cm2. They contain 36 photoconductive sensors, organized in a matrix of 6 by 6 finger patterns. Each finger pattern (Fig. 1b) consists of two inderdigitated electrodes of which the fingers overlap over a distance of 16 mm (L)

The organic photoconductive sensor consists of two interdigitated ITO electrodes coated with two organic materials all deposited on a glass substrate. The sensor remains transparent if thicknesses of the organic layers are sufficiently thin. The organic materials used are: 1,3,5Tris[(3-methylphenyl)phenylamino]benzene (m-MTDAB, Sigma Aldrich) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI, Sensient). m-MTDAB is a hole transport material with hole mobility lp  3  103 cm2/V s [10]. The molecular structure of m-MTDAB is similar to that of 1,3,5-tris(di-2-pyridylamino)benzene (TDAB) except for the nitrogen atoms in the outer benzene rings and the absence of methyl groups in meta orientation. Since the ionization potential (HOMO) and the electron affinity (LUMO) are determined by the functional groups in the m-MTDAB molecule, similar to TDAB, we believe that the HOMO and LUMO levels deviate not much from respectively 5.09 eV and 1.64 eV [11]. The optical properties of m-MTDAB were studied by A.K. Bansal et al. [12]. As a thin film m-MTDAB is transparent and shows no phosphorescence in the visible spectrum. PTCBI is an electron transport material. Dhagat et al. [13] found that the electron mobility in PTCBI is a function of the orientation of

Fig. 1. Cross section of one period of a photoconductive sensor. On top of the fingers two organic layers, a HTL and an EGL are deposited by thermal evaporation. (a); top view of a two interdigitated electrodes (finger pattern) (b); top view of two parallel electrodes used in the experiment with local illumination. The circular section allows to reduce the length of the photoconductive sensor by making a cut (c).

by Ho et al. [3]. The heterojunction is a stack of an acceptor material forming the exciton generation layer (EGL), and a donor material forming the hole transport layer (HTL). The photoconductive sensor is designed for detection of visible wavelengths. The sensor is almost completely transparent for this wavelength range which is interesting for many applications but limits the sensitivity. To the best of our knowledge no lateral photoconductive sensors with an organic double layer has been published, except by Ho et al. [3]. A photodetector using a blend as photosensitive material was reported previously [4–6], and a similar device again using only a single layer but sensitive in the near-infra red was published by Natali et al. [7]. Our structure is also reminiscent of the organic thin-film transistor photo sensor studied e.g. by Hamilton et al. [8], but also in this case a single organic polymer was used and our device does not include a gate electrode. Finally the double layer used is similar to the heterojunctions used in organic solar cells [9], but in this case a planar structure is used instead of our lateral structure. To study the performance of the photoconductive sensor we have designed four different stacks with the same electrode pattern. The vacuum deposition of the organic layer stack and the encapsulation of the devices has been out-sourced based on our design. An additional test device with only two electrodes dedicated for a local illumination experiment has been produced. The absorption of the photoconductive sensor is derived from the transmission spectrum of the EGL. The characteristics of the different photoconductor devices are measured under different illumination levels. Details on the design of the samples and on the measurements can be found in Section 2. In Section 3 we discuss the measurement results and compare the performance of the different samples.

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Table 1 Four substrates with different organic stacks and one substrate for the study with local illumination. Substrate name

Organic stack

S1 S2 S3 S4 Sl

40 nm HTL/10 nm EGL 40 nm HTL/5 nm EGL 100 nm HTL/10 nm EGL 40 nm HTL/10 nm EGL/40 nm HTL 40 nm HTL/10 nm EGL

and the electrodes are each 15 mm wide (W). Each finger of the electrodes is 80 lm wide and separated from the next by a gap of 20 lm. Since the dimensions in the cross section are three orders of magnitude smaller than the length of the fingers we can consider it as a 2D system. The finger pattern can be seen as 2384 mm long sensor folded up in a confined area of 16 mm by 15 mm. The substrates with patterned ITO electrodes (SiO2 passivation layer, ITO t = 65 nm Rs = 15 X/sq) are purchased from Naranjo Substrates (Groningen, Netherlands). The deposition of the organic compounds on the four substrates, each with a different stack (see Table 1) and the encapsulation under a N2 atmosphere with CaO getters are performed by the Fraunhofer Institute for Photonic Microsystems (IPMS, Dresden, Germany).

For the local illumination experiment a 10  10 cm2 ITO covered glass substrate (sample Sl) has been purchased from Präzisions Glas & Optik (PGO, polished glass substrate 1.1 mm thick, SiO2 passivation layer, ITO Rs = 50 X/sq). The substrate is rinsed to remove possible contamination. Lithography is performed to define an electrode pattern of two single parallel electrodes 80 lm wide and 20 lm apart (Fig. 1c). The length over which the electrodes are parallel to one and other is 10 mm, it can be reduced to 1 mm by cutting the circular electrode. The deposition of 40 nm HTL and 10 nm EGL and encapsulation under a N2 atmosphere with CaO getters are performed by IPMS Fraunhofer. We examine the performance of a photoconductive sensor by measuring the current with a Keithley PicoAmpMeter 6485, as a function of the voltage applied across the electrodes of the device. The voltages are applied with a Keithley SourceMeter 2425 in the range between 0 and 10 V. During the current measurement the photoconductive sensor is illuminated with a white LED backlight powered with a Keithley 220 current source. The backlight consists of a reflective cavity with white LED’s and diffuser foils to obtain a uniform illumination. The illuminance incident on the photoconductive sensor is measured with a Chroma 5 Colorimeter (X-Rite).

Fig. 2. Schematic drawing of the setup to construct a laser line with variable intensity and length between the electrodes of Sl.

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In the experiment with local illumination the current through the photoconductive sensor is measured as a function of the position of the illumination, a laser beam shaped into a narrow line. A schematic drawing of the microscope setup for the local illumination experiment is presented in Fig. 2. The measurements are repeated for an applied voltage of 0.5, 3 and 6 V. The position of the illumination is varied between the electrodes of the photoconductive sensor. The photoconductive sensor with two electrode lines is placed on a Nikon Ti Eclipse microscope with a 20 objective. To visualize the electrodes we use the microscope light with a condenser lens. The homogeneous microscope light is considered as the background illumination (BI). On an optical table next to the microscope the beam from a laser diode (5 mW @ 635 nm, Thorlabs L635P005) is expanded 24 times to obtain a parallel beam with a diameter of approximately 2.5 mm. The expanded beam is passed through a cylindrical lens to obtain a elliptical laser beam that is parallel in one plane and divergent in the orthogonal plane. The length of the laser line is controlled with a diaphragm in the focal plane of a 2 times beam expander. In the second filter block of the microscope a dichroic beam splitter (Semrock) is used to couple the laser beam into the 20 objective. The laser beam is focused on the HTL/EGL stack. Reflected laser light makes it possible to visualize the line-shaped laser beam between the electrodes (Fig. 3). The laser line is parallel to the electrodes and has a width in the order of one micrometer. The intensity of the laser beam can be controlled by rotating the half-wave plate. The position of the laser line with respect to the electrode lines is adjusted by positioning the microscope stage. For each position of the microscope stage the current through the photoconductive sensor is measured with and without the incident laser beam. To switch off the laser illumination the dichroic beam splitter is removed from the optical path. The inserting and removing of the dichroic beamsplitter has no influence on the background illumination of the photoconductive sensor or on the position of the local illumination.

3. Results and discussion To obtain the material absorption coefficients the spectral transmission of the two materials is measured with a Perkin Elmer Lambda 35 photo-spectrometer. The thickness of the organic layers are determined with the WYKO profiling system. The transmission of 2 samples with respectively 20.7 nm PTCBI and 28.8 nm m-MTDAB is shown in Fig. 4. For m-MTDAB (the HTL) we observe a higher transmission for longer wavelengths and a drop towards the UV. The transmission spectrum of PTCBI (the ETL or EGL) shows a stronger absorption around 545 nm (green) and around 650 nm (red). The absorption coefficient of the EGL near 545 nm is about 1.3  107 m1. Exposure of the photoconductive sensor to monochromatic light shows the device responds only to wavelengths absorbed by the PTCBI. To verify the expected symmetry of the photoconductive sensor we measured the I(V) characteristic for a consecutive positive and negative voltage sweep, and the graph is symmetric. The electro-optic performance of the photoconductive sensors is measured for two illumination levels (lambertian emitter with 150 cd/m2 and 300 cd/ m2) and in the dark state (Dark current). The results are shown in Fig. 5 and Fig. 6. For each of the four substrates (S1–S4) there are small variations in the I(V) curves over the 36 photoconductive sensors, probably due to non-uniformities in the organics layer thicknesses. The differences for devices on the same substrate are considerably smaller than the differences for devices on substrates with different stacks. The I(V) curves of the organic photoconductive sensors show two more or less linear regions with different slopes. At low voltages (region 1) the current increases more or less linearly with the voltage and the resistance for the illuminated devices is between 3 and 20 MW. As expected in a photoconductor, the conductivity increases with the illumination, but this relation is not linear. The slope of the IV curve decreases considerably at a certain knee-voltage (or transition voltage Vt) which is in the range 1–2.5 V and for higher voltages, the current

Transmission [%]

100 90 80 70 PTCBI 20.7nm

60

m-MTDAB 29.8nm

50 350

450

550

650

Wavelength [nm] Fig. 3. Line-shaped laser beam focused on the HTL/EGL stack, illuminating a small fraction of the 20 lm gap between the two 80 lm wide ITO electrodes of the photoconductive sensor.

Fig. 4. Transmission measurements of a single layer of m-MTDAB (20.7 nm) and PTCBI (29.8 nm).

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550

S1

550

309 cd/m²

450

167 cd/m² Dark current

350

Current [nA]

Current [nA]

450

250

350 250

321 cd/m²

150

150

50

50

-50 0

5

-50 0

10

550

S2

Dark current

250

Current [nA]

Current [nA]

156 cd/m²

350

Dark current

5

10

S4

450

320 cd/m² 450

157 cd/m²

Voltage [V]

Voltage [V]

550

S3

350 250

323 cd/m²

150

157 cd/m²

150

Dark current

50

50 -50 0

-50 0

5

10

Voltage [V] Fig. 5. Current as a function of voltage measured on a photoconductive sensor sample with 40 nm HTL/10 nm EGL (S1) and 40 nm HTL/5 nm EGL (S2). Each sample is measured at two illumination levels and the dark current.

increases at a slower pace. The On/Off ratio Ri for a given illumination Ev and at a voltage V is given by;

Ri ¼

Ii  IDark IDark

here Ii is the current at illumination Ev and IDark is the dark current both at a voltage V. Ri is a function of the voltage V with a maximum in the vicinity of Vt. For the highest luminance we obtain as maximal ratio for the samples; S1 S2 S3 S1 R 309 ¼ 95; R 320 ¼ 15; R 321 ¼ 57; R 323 ¼ 180. For some devices (S1 and S2) the dark current (and also the current under illumination) increases quadratically at higher voltages. Some characteristics show slightly negative values near the origin, but this is due to non-equilibrium conditions of the measurement. Let us now consider the differences between the four substrates. Substrate S2 has an EGL of only 5 nm instead of 10 nm for the other substrates. The currents for S2 are considerably smaller, which can partly be explained by

5

10

Voltage [V] Fig. 6. Current as a function of voltage measured on a photoconductive sensor sample with 100 nm HTL/10 nm EGL (S3) and 40 nm HTL/10 nm EGL/40 nm HTL (S4). Each samples is measured at two illumination levels and the dark current.

the lower absorption of light (and exciton generation) and partly by the lower electron conductivity. For substrate S3 the HTL that is covering the ITO electrodes is 100 nm (instead of 40 nm for the other substrates). This substrate has the highest Vt (2.5 V) and the IV characteristic below Vt is less straight compared to the other substrates. For the two substrates with the higher total thickness of the HTL (S3 and S4), the currents in region two are considerably larger than for the substrates S1 and S2. The interpretation of these measurements is not straightforward and numerical simulations of the characteristics are underway. However, we can make some estimations concerning the charge transport. In region 1 the devices can be described with a constant conductivity. The current density is the product of the carrier density, the average mobility of the charge carriers (for electrons and holes) and the electric field. The mobility of holes and electrons is a constant, the field is proportional with the applied voltage. The carrier density is obviously a non-linear function of the illumination, which is the result of a balance between generation and recombination,

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probably involving trapped states. The occurrence of traps is confirmed by the hysteresis in the forward–backward voltage sweep and by transient measurements. The carrier density can be larger if the layers are thicker, because the probability for recombination is reduced. If the voltage increases above Vt the simple image outlined above breaks down, because near (one of) the electrodes (one of) the charge carriers is carried away faster than the other and a space charge region develops which takes up an important part of the applied voltage. Trapping of charges can play a very important role in this process. Outside of the space charge region, the organic stack provides the same (high) conductivity as is observed in the region 1, but the space charge region is a region with a much higher impedance. Increasing the voltage further contributes mainly to increasing the space charge region and only marginally to the current. For the substrates S1, S2, S4 the behaviour in region 1 is practically linear. The non-linearity for sample S3 at low voltages may be due to the fact that the electrons in the EGL/ETL cannot easily travel to the anode through the 100 nm thick HTL and some voltage is needed to assist the charge transfer there. The substrate S4 yields the photoconductor with the preferred characteristics because it combines a high contrast between dark and illuminated with a high current in region 2. To study the optical sensitivity of the samples we determine Vt from the I(V) curves at different illuminations (Ev). To determine Vt the intersection between the linear regressions at higher and lower voltages of the I(V) curve is calculated. By plotting the Vt as a function of Ev in a double logarithmical axis system the measurement points line up (Fig. 7). From the curves we find that the relation between Vt and Ev is of the form;

V t ¼ cEav

ð1Þ

Table 2 shows the parameters for the best fit between formula (1) and the measurement data.

Table 2 Parameters for formula (1) to obtain a best fit to each of the Vt(Ev) curves. Sample

c [V/(cd/m2)a]

a

S1 S2 S3 S4

0.1690 0.1793 0.5562 0.1391

0.2571 0.3647 0.3593 0.3279

The sample Sl prepared for the local illumination experiment has the same stack of organic materials as S1 but only two electrodes with a length of 10 mm (about 240 times less than for the finger pattern). Fig. 8 shows the I(V) characteristic measured in a similar way as for previous samples. Compared to S1 the transition between the two regimes is at 2 V instead of 1.5 V. Also we find that the current at a given illumination and voltage in regions 1 and 2 are approximately 240 times smaller than the currents for the same voltage in S1, as expected. Fig. 9 shows the current dependence on the position of the local illumination with respect to the electrodes of the photoconductive sensor. The local illumination is approximately 1 lm wide and stretches over a length of 1 mm. The x-coordinate varies from 0 lm at the anode to 20 mm at the cathode. For the higher voltages applied (6 V) the photoconductive sensor is more sensitive when it is illuminated near the cathodic electrode, in the bulk the device is less sensitive. This can be explained by assuming that the space charge region, which is the region with the highest electric field is located in the vicinity of the cathode. Since the width of the space charge region is small compared with the gap width, the outcome of this experiment will not change for a different gap width, assuming the gap remains wider than the space charge layer width. Additional illumination and generation of electrons and holes in the highfield region leads to a higher current because the charge carriers are separated more effectively. In the low-field region, the large amount of electrons and holes that are gen-

Sµ

1.0

1

10

Current [nA]

Knee voltage [V]

0.8

0

10

0.6

0.4

320 cd/m²

-1

10

1

10

2

10 Illumination [cd/m²]

154 cd/m²

0.2

S1 S2 S3 S4

Dark current 0.0 3

10

Fig. 7. The knee-voltage (Vt) as a function of the illumination (Ev) for sample S& to S4, plotted on a double logarithmical axis system.

0

5

10

Voltage [V] Fig. 8. Current–voltage measurements for sample Sl with two parallel electrodes.

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0.20

@ 6V Laser On @ 3V Laser On @ 0,5V Laser On

of the laser beam leads mainly to more recombination of additional charge carriers, not to additional current.

@ 6V Laser Off @ 3V Laser Off @ 0,5V Laser Off

4. Conclusion In this article we present a transparent organic photoconductive sensor with a lateral electrode configuration. The spectral sensitivity of the photoconductive sensor is investigated by studying the absorption spectra of the organic compounds that build up the device. The exciton generation material, which is the photosensitive layer, is most sensitive to green and red. The I(V) curves of the photoconductor sensors consist of two more or less linear parts with different slopes. The transition from region 1 into region 2 is explained by the occurrence of a space charge region. The slopes in regions 1 and 2 and the knee voltage are functions of the illumination and of the parameters of the organic stack. In a microscope experiment with local illumination a region of increased sensitivity is found near the cathode for voltages above the knee. This indicates that there is a region with positive space charge near the cathode.

Current [nA]

0.15

0.10

0.05

Acknowledgement

0.00 0

5

10

15

20

Position [µm] Fig. 9. Current as a function of the position of the laser line with respect to the anode. The experiment is performed at DC voltages of 0.5, 3 and 6 V between the anode and cathode. At each position the current is measured under BI (Laser Off) and subsequently under BI + laser line (Laser On).

erated locally by the laser beam have a large chance to recombine in the region where they are created and have only a limited effect on the photocurrent. The microscope experiment shows that the space charge is near the cathode and therefore the high-field region must have an excess of holes in the HTL. This may be because holes have a lower mobility than electrons or because holes are trapped more easily in the stack. A similar measurement for 3 V applied over the electrodes shows that the increased sensitivity at the cathode is not as strong as in the measurement for 6 V. This implies that the space charge region is not as pronounced for voltages closer to Vt of the I(V) curve. For voltages below Vt (for example for 0.5 V) the effect of the local illumination on the measured current is very small, also at the cathode. In this case the local illumination and charge generation

This work is a result of the TARDIS research & development project funded by the IWT (Institute for the Promotion of Innovation by Science and Technology in Flanders), it is also the result of collaboration within the IAP (Interuniversity Attraction Poles Programme – Belgian State – Belgian Science Policy). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

[14]

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