Organic Light-Emitting and Photodetector Devices for Flexible

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REVIEW PAPER IEICE Electronics Express, Vol.14, No.20, 1–16 Organic light-emitting and photodetector devices for ﬂexible optical link and sensor devices: Fundamentals and future prospects in printed optoelectronic devices for high-speed modulation

Hirotake Kajiia) Graduate School of Engineering, Osaka University, 2–1 Yamada-oka, Suita, Osaka 565–0871, Japan a) [email protected]

Abstract: This paper describes the application of organic photonic devices including organic light-emitting and photodetector devices to integrated photonic devices for the realization of ﬂexible optical link and sensor devices. Fundamentals and future prospects in printed optoelectronic devices for high-speed modulation are discussed and reviewed. Keywords: organic light-emitting diodes, organic photodetectors, organic light-emitting transistor, high speed, printed electrodes, sensor Classiﬁcation: Electron devices, circuits and modules

References

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1 Introduction Organic semiconductors have attracted considerable attention owing to their simple and low-cost processing, and high potential for optoelectronic applications. In particular, they have demonstrated the possibility of application in optoelectronic devices fabricated by solution processing for large-area and flexible devices. Organic light-emitting diodes (OLEDs) utilizing fluorescent dyes or conjugated polymers are capable of emission over a wide visible range, highly efficient, and require only a low driving voltage. OLEDs have been realized which have a long lifetime and excellent durability for flat-panel display applications. Employing phosphorescent and thermal active delay fluorescent (TADF) materials yields high efficiencies because breaking the spin conservation rule allows both singlet and triplet excitons to contribute to emission. A specific class of OLEDs has achieved high luminescence efficiency using the phosphorescent emission from a triplet state of phosphorescent materials such as Ir complexes, and the delay fluorescent emission from a singlet state of TADF materials [1, 2]. There are some requirements for OLEDs when they are used not only in display applications but also as various solid state lighting sources. On the other hand, organic photovoltaic including an organic photodetector (OPD) is an organic device with photoelectric conversion characteristics. OPDs with various device structures using donor (D) and accepter (A) materials have been developed. Internet of Things (IoT) and ICT society are supported by various sensor and optical communication devices. Flexible polymer substrates such as polyimide (PI), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), and polyester fi fl © IEICE 2017 (PEs) lms etc. are required to be used for realizing exible optical link and sensor DOI: 10.1587/elex.14.20172002 Received August 31, 2017 devices. These organic devices are driven under the modulation speed from Hz to Accepted September 8, 2017 Published October 25, 2017 MHz regime as shown in Table I. The combination of polymeric waveguides and

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organic optical devices can be realized flexible integrated photonic devices [3]. OLEDs and OPDs also show high potential for use in future information technol- ogy systems, especially for signal processing and optical sensing systems with flexible devices [4, 5, 6]. Compact, integrated sensors utilizing an OLED as the light source and an OPD as the light detection unit have been realized. Semi- transparent optoelectronic devices are one of the key factors for fabricating the stacked OLED and OPDs [7, 8]. Flexibility in optoelectronic devices is attractive for medical and bioengineering. Flexible electronics can be installed outside the human body as the diagnostic and monitoring tools. By integrating two types of polymer LEDs (PLEDs) with OPDs, the reflective pulse oximeter has been demonstrated. Lochner et al. has reported that the all-organic optoelectronic oximeter sensor fabricated on glass and thick PEN substrate is interfaced with conventional electronics at 1 kHz and the acquired pulse rate and oxygenation are calibrated [9]. Ultraflexible reflective oximeter reported by Yokota et al. detects the oxygen concentration of blood when laminated on a figure [10]. Wearable organic optoelectronic sensors including a tissue-oxygenation sensor and a muscle-contraction sensor for medicine have been also reported [11].

Table I. Properties of OLEDs and OPDs in the flexible optical link and sensor devices Device Substrate Application Modulation speed Ref. Polymer waveguide Transmission of moving OLEDs ∼20 MHz (PFM) [3] : d-PMMA, epoxy resin picture images PI Transmission of audio ∼MHz (PCM: pulse OLEDs [4] (thickness: 16 µm) signals code modulation) OPDs PEN Image scanner – [24] OPDs PEN Image sensor 500 Hz [25] PI Visible light and X-ray OPDs ∼kHz [30] (thickness: 24 µm) imagers PEN OPDs X-ray imager – [31] (thickness: 25 µm) OLEDs/ Glass/ Reflective oximeter ∼kHz [9] OPDs PEN OLEDs, Adhensive tape Reflective oximeter ∼Hz [10] OPDs (thickness: 6 µm) OLEDs, Muscle-contraction PET ∼Hz [11] OPDs sensor

The fundamental characteristics of OLEDs and organic solar cells are covered in many reviews and will not be discussed. In this review, we focus on the fundamental properties of OLEDs and OPDs for ﬂexible optical link and sensor applications. In addition, fundamentals and future prospects in printed optoelectronic devices for high-speed modulation are discussed.

2 OLEDs as electro-optical conversion devices

© IEICE 2017 There are two types of LEDs and laser devices for transmitting optical energy. DOI: 10.1587/elex.14.20172002 Received August 31, 2017 LEDs have lower power, but are much less expensive, and used for short distances Accepted September 8, 2017 Published October 25, 2017 and multimode paths. Vacuum-processed OLEDs directly fabricated on the poly-

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mer waveguide and PI substrate, which are covered by thin gas barrier ﬁlms of

SiO2 and Si3N4 formed by a sputtering technique, can be applied in the ﬁeld of optical communication as electro-optical conversion devices for transmitting signals of optical moving images and digital audio [3, 4]. For video signal transmission, OLEDs as shown in Fig. 1(a) are driven by voltages of pulse frequency modulation (PFM), in which the pulse width is kept constant and the pulse generation cycle is modulated, in order to ensure error-free transmission. The typical PFM modulation frequency of carrier wave is about 20 5 MHz. Fig. 1(b) shows that clear audio signals are reproduced using the OLED optical transmission system. A yellow-emitting OLED with rubrene as the emitting dopant doped

in tris(8-hydroxyquinoline)aluminum (Alq3) emissive layer was used as the light source for polymer optical fiber (POF), which has the advantage of low propagation loss. OLEDs are fabricated on a high transparent 16-µm-thick PI film which is resistance to thermal treatment and chemical solvents compared with other polymer films such as PET. The transient phenomenon is related to the charge and discharge of initial capacitance in the OLED. In order to enhance the response speed of the OLEDs, it is effective to apply a positive base voltage to the OLEDs in addition to the pulsed voltage in many cases. OLEDs are driven by lower pulsed voltage with a base voltage that is less than the turn-on voltage of the OLEDs as shown in Fig. 1(b).

(a) (c)

(b)

Fig. 1. (a) Schematic diagrams of cross-sectional view of OLEDs © IEICE 2017 fabricated on polymer waveguide, (b) optical transmission DOI: 10.1587/elex.14.20172002 using OLEDs, and (c) impact testing using OLEDs with ITO Received August 31, 2017 Accepted September 8, 2017 and high conductive PEDOT:PSS anodes. Published October 25, 2017

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Indium-tin oxide (ITO) is suitable for the electrode in organic photonic devices such as OLEDs and OPDs due to its optical transparency. However, indium, which is an element in ITO, is a rare metal and there are concerns about its depletion and rise in its price. Therefore, alternative transparent electrodes are needed. The ﬂexibility of printed devices is one of the requirements with regard to role-to-role processing. Superior mechanical reliability is of high practical importance to ensure reliability during processing and applications. Conductive polymers, silver nanowire (AgNW), carbon nanotube (CNT) and graphene are promising as transparent conductive electrodes for ﬂexible optoelectronic devices. The stress tolerance of the OLEDs with ITO or highly conductive polymer electrodes [poly(ethylenedioxythiophene):poly(styrene sulfonic acid), PEDOT:PSS] as an anode has been reported [12]. Pushing tolerance tests were performed for OLEDs fabricated on polymeric substrates as shown in Fig. 1(c). The stress applied in these pushing tolerance tests was comparable to that applied when human beings touched the sheet by pushing the rod, and the frequency of iterations was about 3 times per second. The devices on an aluminum plate were

pushed from the anode side. For the Alq3-based devices fabricated on ITO-coated PEN substrates, the emission intensity decreased with increasing number of impacts. One of the reasons for the decrease in emission intensity is the destruction of the ITO electrode due to impact. For an ITO electrode, the star-like dark part

increased with the number of impacts. On the other hand, for the Alq3-based device fabricated on PEN substrate, the impact testing of the OLEDs with highly conductive PEDOT:PSS anode revealed that the emission lasted for more than several ten thousand steps. A highly conductive polymer electrode had a sufficient tolerance to mechanical stress. In particular, the development of stretchable electrodes has seen significant progress for use in flexible stretchable devices [13]. Recent example is the demonstration of highly flexible and stretchable PLEDs with PEDOT:PSS anode, which are fabricated on 1.4 µm PET foil substrate and shown to be stretch- compatible to 100% tensile strain [14]. The typical transient properties of a vacuum-processed phosphorescent OLED (PhOLED) and a solution-processed PLED based on poly(9,9-dioctylfluorene-co- benzothiadiazole), F8BT, which were driven at a 0.1 or 1 ms period and duty ratio of 1/10 pulse voltage, are shown in Fig. 2. The decay (rise) time is defined as the time required to change the optical response from 90 (10) to 10% (90%) of its total intensity change. For a PhOLED based on tris(2-phenylpyridine)iridium(III),

Ir(ppy)3, the decay time of electroluminescence (EL) is almost the same as the

that of photoluminescence (PL) in Ir(ppy)3 at low current densities. At above 0.1 A/cm2, the rise time was almost the same as the decay time. The rate equation [15] of the phosphorescence OLEDs is indicated as follows: d½3M ½3M 1 J ¼ k ½3M2 þ ; ð1Þ dt 2 TT qd where τ is the phosphorescent recombination lifetime, [3M] is the concentration of © IEICE 2017 triplet excitons, k is the trip-let-triplet annihilation constant, J is the current DOI: 10.1587/elex.14.20172002 TT Received August 31, 2017 density, d is the length of the exciton formation zone, and q is the electron charge. Accepted September 8, 2017 Published October 25, 2017

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Fig. 2. Typical transient properties of OLEDs: Transient voltage, current, and EL signal for (a) rise and (b) decay parts. (c) Current density dependence of the rise and decay times of a PhOLED based on Ir(ppy)3 and a PLED of ITO/PEDOT:PSS/ poly(9,9-dioctylﬂuorene-co-N-(4-butylphenyl)-diphenylamine), TFB/F8BT/Al/CsF/Ag upon applying rectangular voltage. (d) Frequency dependence of EL intensity in both devices. The insets of (c) and (d) show the time dependence of PL intensity of an F8BT ﬁlm under a 373 nm violet laser pulse and the EL output signal of 100 MHz obtained by directly modulated PLED.

From this rate equation, the transient properties are strongly affected by phosphorescent recombination lifetime. Owing to the decrease of the rise time, the cut-off frequency of the device was improved by increasing the applied current density. The cut-off frequency of the device is mainly limited by the phosphorescent recombination lifetime. PhOLEDs can be expected to be utilized as one of the light sources driven at less than 1 MHz [16]. PLEDs based on fluorene derivatives with short fluorescence lifetime, which exhibit high output power of above 30 mW/cm2 can be expected to be applied to the electro-optical conversion devices [17]. For a PLED based on poly(9,9- dioctylfluorene-co-benzothiadiazole), F8BT, the rise time of EL was almost the

same as the decay time (90-10) of PL in F8BT at higher current densities. For this F8BT device, the ﬂuorescent lifetime of F8BT is the limiting factor of rise time. On the other hand, the decay time of EL was slightly larger than that of PL in F8BT due to the delayed EL from residual carriers in the emissive layer. The rate-limiting factor is the decay time for obtaining high-speed response. Due to the decrease of rise and decay times, the cut-off frequency of more than 50 MHz was achieved. Optical pulses of 100 MHz have been obtained from the PLED at high current densities [18]. PLEDs are expected to be applicable to the electro-optical con- © IEICE 2017 DOI: 10.1587/elex.14.20172002 version devices for high-speed switching in the ﬁeld of optical link and optical Received August 31, 2017 Accepted September 8, 2017 sensor devices. Published October 25, 2017

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The response speed is markedly affected by contact formation between electrodes and organic layer. Fig. 3(a) shows the applied voltage dependence of the rise and decay times of F8BT PLED with LiF/Al cathode. The rise time of emission from the PLED with LiF/Al was decreasing at low voltages, but at high voltages (above 6 V), the rise time rapidly increased. At high voltages, the clear waveform rounding of transient EL was observed as shown in Fig. 3(b). The transient EL signal was almost synchronized with the current waveform. This behavior of transient properties is analyzed using impedance spectroscopy measurement. The complex ac impedance (Z) is expressed as Z ¼ Z0 jZ00, where Z0 is the real part and Z00 is the imaginary part. Fig. 3(c) shows the Cole-Cole plots (impedance) of the device at various applied voltage and the estimated equivalent circuit. In

general, the equivalent circuit consists of parallel resistiveity (Rc)-capacitance (C) circuits, which are mainly related with the F8BT bulk resistance and the

geometrical capacitance, together with the contact resistance (RS) in series. It is noted that negative capacitance related with inductive behaviour of equivalent

circuit was clearly observed. Then, resistance (RL) and inductance (L) series branch

is attached parallel to Rc C parallel circuit. That is, negative capacitance results in waveform rounding of current. Even in the high frequency region, the inﬂuence of negative capacitance occurred with increasing voltages. Therefore, the rapid increase of rise time results from the waveform rounding of transient current density owing to negative capacitance. The waveform rounding of transient EL was improved when PLED was driven by lower pulsed voltages with reverse base voltages as shown in Fig. 3(d). This result suggests that the negative capacitance is related with the residual and trapped carriers. For generating high speed optical pulses, the rise and decay times of F8BT

RL L

RS Rc C

Fig. 3. (a) Applied voltage dependence of rise and decay times in F8BT PLED with LiF/Al by applying pulsed voltages without and with a reverse base voltage of −10 V to the OLED, (b) typical transient current and EL at 10 V, (c) Cole-Cole plots © IEICE 2017 (impedance) of the device at various applied voltage and the DOI: 10.1587/elex.14.20172002 Received August 31, 2017 estimated equivalent circuit, and (d) typical transient EL under Accepted September 8, 2017 various base voltages. Published October 25, 2017

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device with LiF/Al are improved by applying the modulation added to the reverse base voltage as shown in Fig. 3(a). In order to produce organic devices at low cost and in large quantity, shortening the process becomes necessary. In many cases, the cathode electrode of organic optoelectronic devices is vacuum-deposited thorough a metal mask. Metal nanoparticles are also expected to be applied as electrodes in all-solution processed PLEDs. Low curing temperature of the nanoparticle ink used is important in plastics electronic applications. In particular, silver (Ag) nano-ink is promising for low-temperature solution-processed electrodes in PLEDs. Therefore, it is important to develop more effective electron interfacial materials to enhance

electron injection for PLEDs with Ag cathodes. Cesium carbonate, Cs2CO3 has been shown to be a very efﬁcient electron-injection material in PLEDs [19, 20].

Cs2CO3 has a high solubility in polar solvents such as water and alcohol, and is almost completely insoluble in most other organic solvents such as toluene and xylene. Conjugated polyelectrolytes, which are conjugated polymers with pendant groups bearing ionic functionalities and with high solubility in polar solvents, are also promising interfacial layer materials in organic optoelectronic devices [21, 22].

ACs2CO3:conjugated polyelectrolyte blended interfacial layer plays a signiﬁcant role in increasing the injection of electrons from an Ag cathode. The short response times of EL, which operate into the MHz regime, are achieved for this PLED. Since the interfacial layer prevented Ag nanoparticles in Ag nano-ink from penetrating

into the emissive layer, F8BT device with the 10-nm-thick Cs2CO3:conjugated polyelectrolyte interfacial layer and solution-processed Ag cathode exhibits a maximum luminance of approximately 10,000 cd/m2 [23]. The possibility of all-solution processed PLEDs using a solution-processed electron injection layer and Ag nano-ink has been demonstrated.

3 OPDs as opto-electrical conversion devices Someya et al. has reported the fabrication of large-area flexible scanner using organic photoactive layer on a flexible substrate, and indicating the possibility of the flexible organic electronic devices [24]. To demonstrate an flexible image sensor array, the 4-µm thick organic sensor layer was integrated onto an active matrix backplane with a-Si:H thin film transistors fabricated on PEN [25]. These organic image sensors have the potential advantages of considerably flexible and large-area size and wavelength selectivity. Important factors for organic solar cells are absorption wavelength, filling factor, open circuit voltage, and short-circuit current. On the other hand, the electrical characteristics under reverse bias, frequency response, on/off ratio, and color sensitivity must also be evaluated for OPDs. Typical device structures and schematic energy diagrams of various OPDs, and the fundamental characteristics of polymer PDs are shown in Figs. 4 and 5. Frequency response Peumans et al. demonstrated that copper phthalocyanine (CuPc) based OPDs incorporating an ultrathin (0.5 nm) donor–acceptor alternating multilayer stack © IEICE 2017 have a high-speed temporal response of above 400 MHz under irradiation of a DOI: 10.1587/elex.14.20172002 Received August 31, 2017 red single-pulsed laser light [26]. Under repetition pulse light, the frequency Accepted September 8, 2017 Published October 25, 2017

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response speed in multi-layered heterostructure (MLH) OPD based on vacuum- deposited CuPc is lower than that in single-layered heterostructure (SLH) device since some of the generated carriers in the MLH cannot be removed at the high modulation speed [27]. Frequency response of the device with optimized D:A mixed layer is similar to that of SLH [6]. That is, frequency response is influenced by the D-A interface. For SLH device, clear response pulses at 100 MHz are observed using a sinusoidally modulated red laser [28]. Printable photodetectors have huge potential in applications such as large-area photodetector arrays, scanners, etc. Fullerene derivatives doped in several con- ducting polymers act as effective quenchers and electron acceptors, and this photophysics is known as ultrafast photoinduced charge transfer. Poly(3-hexyllthi- ophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends have been studied for organic bulk heterojunction (BHJ) photovoltaic cells because of high hole mobility and a broad absorption spectrum, and high photoelectric conversion efficiencies have consequently been achieved [29]. Visible light and X-ray imagers using P3HT:PCBM devices and CNT (or organic) transistors fabricated on polymer substrates have been demonstrated [30, 31]. For P3HT:PCBM PDs with high sensitivity, cut-off frequency (10–100 kHz order) is relatively low [32, 33]. On the other hand, the cut-off frequency increased with the bias because the dissociation of photogenerated excitons at the hetero-junction is accelerated with increasing external electric field. A fluorene-type polymer, poly(9,9-dioctylfluorene-co-bithiophene), F8T2 based PDs can operate at more than 10 MHz at −4 V under the sinusoidally modulated violet light [34]. The response of 10 MHz order under repetition pulse light indicates a practical possibility for the printed OPD to be used in high-speed optoelectrical conversion applications. The response speed is limited by the effect of carrier traps and the lifetime of charge-transfer state, and the charge carrier mobility. Pulsed signals of approximately 100 MHz have been received by the polymer PDs fabricated by the solution process at −10 V [35]. For the application of high sensitive printed PDs, the achievement of higher speed operation is a problem at low voltages.

© IEICE 2017 DOI: 10.1587/elex.14.20172002 Received August 31, 2017 Fig. 4. Typical device structures of various conventional OPDs and Accepted September 8, 2017 schematic energy diagrams at reverse bias. Published October 25, 2017

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On/off ratio Under reverse voltages, the OPDs need to achieve both low power consumption and high sensitivity. The improvement of photodetector detectivity is required to improve the on/off ratio. The photodetector detectivity is defined as D ¼ ðAfÞ1=2=NEP, where A is the detector area, f is the bandwidth, and NEP is the noise equivalent power. This factor produces a figure of merit which is area independent. The NEP, which is dominated by shot noise, is defined as NEP ¼ 1=2 ð2eIdÞ =S, where e is the electronic charge, Id is the dark current and S is the detector responsibility. The series resistance originates from the bulk of organic layer, electrodes and the contact resistance between the active layer and the electrode. Under illuminated light, the bulk resistance markedly decreases. Therefore, it is important to achieve the improved interface between the organic layer and electrodes. Contact formation between an anode and the organic layer is one of the key issues in the development of organic devices. At present, a PEDOT:PSS layer on the ITO electrode is the most widely used configuration to improve the interface between ITO and an organic layer. However, under reverse voltages, the dark current of organic devices is increased owing to the high conductivity of PEDOT. It is well known that PEDOT:PSS layer etches ITO, resulting ionic dopants generation that can migrate to the active layer.

Fig. 5. Typical properties of OPDs: (a) Frequency response properties of a F8T2:PCBM conventional device with FOPA at sinusoidal modulated laser light ( ¼ 408 nm) illumination under various reverse biases. (b) Current density-voltage characteristics of a P3HT:PCBM conventional device with FOPA and the inverted device with 11-AUPA/Cs2CO3 under the dark condition and light irradiation ( ¼ 500 nm, 5.6 mW/cm2), and (c) applied voltage dependence of IPCE ( ¼ 500 nm) and (d) IPCE spectra of both devices. The inset of (c) shows irradiation time dependence of the normalized photocurrents at 0 V under blue © IEICE 2017 light irradiation ( ¼ 470 nm, 9 mW/cm2). DOI: 10.1587/elex.14.20172002 Received August 31, 2017 Accepted September 8, 2017 Published October 25, 2017

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On the other hand, surface treatment of electrodes using self-assembled mono- layers (SAMs) could be an effective method to improve the performance of organic devices [34, 36, 37]. For example, the work function of ITO modified with fluoroalkyl phosphonic-based SAM, 1H,1H,2H,2H-Perfluorooctanephosphonic acid (FOPA) increased to about −5.3 eV, compared with that of ITO (−4.8 eV). FOPA treatment results not only in lowering of the injection barrier at the ITO/ organic layer interface but also in the lowering of the contact resistance between ITO and the organic layer, which is estimated by impedance spectroscopy [34]. A device with FOPA showed both good rectification characteristic in the dark state and photosensitive characteristic under illumination owing to the improved hole extraction between ITO and P3HT. The conventional device with FOPA, which has a hole-collecting electrode on substrate, exhibited the detector responsibility as approximately 0.23 A/W and the photodetector detectivity D of 1012 cm Hz1=2 W−1 at low voltages under light irradiation ( ¼ 500 nm). ITO electrode with FOPA exhibited hydrophobic surfaces, and the surface free energy of a ITO with FOPA (22 mJ/m2) consisting of fluoroalkyl groups was lowest than those of neat ITO (53 mJ/m2). Therefore, wettability of organic solvent was poor on ITO modified by FOPA and thin films containing of some polymers with low molecular weight and dyes cannot be formed by solution-processing on transparent electrodes with surface treatment of FOPA. Inverted device structure 2 using ITO modified with 11-AUPA (46 mJ/m ) and Cs2CO3, which acts as an electron-collecting cathode, is suitable for the solution-based fabrication [38]. The

inverted device with 11-AUPA/Cs2CO3 exhibited higher incident-photon-to-current conversion efﬁciency (IPCE) and durability under illuminated light, compared to the conventional device with FOPA. To improve the detector responsibility of polymer PDs at low voltages, the fabrication of BHJ is one of key factors. An optimized P3HT:PCBM conventional device with both solvent-vapor treatment of active layer and postproduction thermal annealing treatment reported by Liu et al. [39] reaches D of about 1013 cm Hz1=2 W−1. In addition, the key to obtaining high detectivity is to reduce the dark current. To minimize the dark current, the fabrication of multilayer device with hole and electron blocking layers is one of approaches for improving the detectivity. The detectivity of about 1013 cm Hz1=2 W−1 in multilayer polymer PDs

based on bulk heterojunction and vacuumed-deposited C60 are comparable to or even better than inorganic PDs [40]. By optimizing the additional fabrication conditions, the photodetector detectivity is expected to be improved. Color sensitivity Narrowband applications such as cameras, industrial colorimetric measurements, spectral biological imaging, etc. typically use broadband materials with additional color ﬁlters which cause light waste. The absorption wavelength of OPDs can be selected by choosing organic materials with suitable absorption wavelengths. Seo et al. has reported the fabrication of high sensitive stacked organic CMOS sensor that has a higher aperture ratio using blue green, and red organic photoactive layers [41, 42]. Color-selective OPDs for multispectral imaging are useful for sensor © IEICE 2017 DOI: 10.1587/elex.14.20172002 applications. The development of new acceptor with narrow band spectrum is Received August 31, 2017 Accepted September 8, 2017 required as fullerene derivatives possess blue-green absorption. Published October 25, 2017

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4 Future prospects The device fabrication using organic materials with high mobility is required since RC time constant of devices is one of the determining factors for obtaining the high response speed. The organic light-emitting devices with high output power are essential for optical link and sensor applications to improve the signal to noise ratio. Organic light-emitting transistors (OLETs) fabricated using the crystalline organic materials are multifunctional devices that combine the light emission property of an OLED with the switching property of a field-effect transistor in a single device architecture [43]. Single-layer ambipolar OLETs allow for the efficient formation of excitons as provided by effective pn junctions. In such devices, a saturated electron channel and a saturated hole channel are positioned in series to form p=n junction within the OLET channel, resulting in charge recombination via excitons, resulting in efficient light emission. Top-gate-type devices based on crystalline fluorene-type polymers exhibit both ambipolar and light-emitting properties. As the mobility of crystalized film is higher than that of amorphous film, the OLETs can be driven at the high current density. As the emission occurs in the channel between source/drain (S/D) electrodes, OLETs can be used as the micro-light source by patterning of S/D electrodes. Flexible F8BT single-layer OLET fabricated on PEs film with AgNW S/D electrodes exhibited typical line-shaped emission [44]. Metal-free F8BT OLET using semitransparent CNT S/D and gate electrodes [17] also exhibits ambipolar characteristic and a line-shaped emission pattern as shown in Fig. 6(a).

(a) (b) Fig. 6. Typical device structures and emission patterns of (a) a single layer OLET and (b) a heterostructure OLET.

A multilayer approach to fabricating active layers helps to improve OLET performance. Furthermore, OLETs with multilayers combined with hole and electron transport materials along the carrier channel have been reported [45]. The concept of using an ambipolar bilayer semiconducting heterostructure in OLETs is introduced to provide a new approach to achieve surface emission [46]. In-plane light emission pattern in the heterostructure device based on poly(9,9-dioctylﬂuorene), F8 upper and F8BT lower layers has been achieved. Surface-like light-emitting pattern is clearly observed, as shown in Fig. 6(b), with hole transport dominant in the upper layer and electrons been injected into the lower layer. The optical pulses of more than 0.1 kHz frequency are generated by © IEICE 2017 DOI: 10.1587/elex.14.20172002 directly modulating a bilayer device with an in-plane emission pattern [47]. For Received August 31, 2017 Accepted September 8, 2017 heterostructure OLETs having an oriented bilayer with the channel direction Published October 25, 2017

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parallel to orientation of the polymer chains, a polarized surface-like emission pattern can be also achieved [48]. At this stage, the typical output power of in-plane emission was on the order of 10 µW/cm2 in a heterostructure device with an in-plane emission pattern of 100 µm width, hole and electron mobilities of approximately 103 cm2 V−1 s−1, and an EQE of 1.5%. Given a higher mobility (>101 cm2 V−1 s−1), a shorter channel length (<10 µm) and higher EQE (>10%), a high-power (>0:5 W/cm2) in-plane light emission should to be achievable in a polarized OLET based on crystalline polymer films. The properties of heterostructure OLETs show the possibility of fabricating the micro-light source for an organic integrated optical sensor on a polymeric waveguide. However, the development of printed p-channel and n-channel materials with both high-efficiency and mobility has been far more difficult than those with high-mobility. High speed response is also a problem. To improve S, the OPDs using exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100% have been reported [49]. Printable materials with singlet fission remain challenging. Material techniques for printing and roll-to-roll processing for large-area flexible electronics become important regarding the device integration on flexible substrates. Novel materials for solution-processable and low-temperature processing are necessary for the preparation of devices on plastics. In addition, for optical communication and biomedical sensor applications, more research and development activity is required to develop new materials with inorganic nanoparticles such as quantum dots mixed with organic materials [50] in order to extend the infrared spectrum operation.

5 Conclusion In this paper, fundamentals and future prospects in organic optoelectronic devices for high speed modulation are discussed and reviewed. Polymer LEDs and PDs for ﬂexible electronics can operate into MHz modulation. From the viewpoints of interfacial engineering such as organic/organic, organic/inorganic, organic/electrode interfaces, the research and development of the fabrication in organic photonic devices are essential for obtaining high speed modulation.

Acknowledgments The author would like to thank Emeritus Prof. Y. Ohmori (Osaka Univ.) for valuable discussion and support. The author would like to thank T. Taneda, Y. Sekimoto, N. Takahota, T. Kojima, Y. Kusumoto, K. Hiraoka, Y. Sato, T. Yamamoto, H. Tanaka and Y. Mohri for research work. The author would like to also thank Prof. M. Kondow, Prof. M. Ozaki, and Emeritus Prof. K. Yoshino (Osaka Univ.) for their support. Part of this work was supported by JSPS KAKENHI Grant Number JP26289087.

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Hirotake Kajii received the B.S., M.S., and Ph.D. degrees in electronic engineering from the Faculty of Engineering, Osaka University, Osaka, Japan, in 1996, 1998, and 2000, respectively. In 2000, he joined Osaka University, where he is currently an Associate Professor. His research interests are in the electronic and optical properties of organic and inorganic materials and application of them to optoelectronic devices for optical communication and sensor devices. Dr. Kajii is a member of the Institute of Electronics, Information and Communication Engineers in Japan, the Institute of Electrical Engineers of Japan and the Japan Society of Applied Physics.