Letter Cite This: ACS Appl. Electron. Mater. 2019, 1, 1054−1058
pubs.acs.org/acsaelm
Suppressing Dark Current in Organic Phototransistors through Modulating Electron Injection via a Deep Work Function Electrode Ren Shidachi, Hiroaki Jinno, Sunghoon Lee, Tomoyuki Yokota, and Takao Someya* Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Downloaded via UNIV OF SOUTHERN INDIANA on July 24, 2019 at 03:47:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Organic phototransistors (OPTs) have been extensively investigated as photodetectors because of their internal photocurrent amplification. However, OPTs tend to show high dark current because they are derived at its ON state. Herein, we developed an OPT with low dark current by forming an injection barrier and suppressing the carrier injection in the dark. The dark current was reduced by over 4 orders of magnitude. The light-induced charge traps have maintained the responsivity at 42 mA W−1, consistent with the conventional OPTs. This low dark current OPT should expand the scope of OPT usage in practical electronic applications. KEYWORDS: organic semiconductors, photodetectors, organic phototransistor, charge traps, bulk heterojunction, dark current
S
emiconductor photodetectors are essential for optoelectronic applications such as image sensing or optical communications. Recent progress in image recognition technology has led to increased demand for lightweight and highly sensitive photodetectors. Compared to conventional inorganic semiconductor-based photodetectors, photodetectors using organic semiconductors have attracted significant interest for their effective light absorption, solution processability, and flexibility.1 Organic photodiodes (OPDs) and organic phototransistors (OPTs) are the most commonly used organic photodetectors. OPDs are widely used due to their simplicity and the intensively studied properties of organic photovoltaics. Many interesting applications have been reported, including flexible image sensor array and pulse oximetry using OPDs.2−4 In contrast, OPTs exhibit unique characteristics, including large photocurrents and the ability to form an active circuit by themselves.1 The internal amplification of photocurrent allows for the efficient detection of weak light signals, including nearinfrared.5 Another advantage of OPTs is that they are three terminal devices, facilitating the simplification of designing active circuits.6 It is well-known that OPTs featuring organic semiconductors forming a bulk heterojunctions (BHJs) exhibit relatively fast switching time on the order of a few milliseconds due to fast charge separation in BHJs.7,8 Recently, more applications using OPTs have been developed.5,6 Typical figures of merit used in the field of OPTs include responsivity (R) and photosensitivity (P) which can be defined using the following equations: R=
Id_light − Id_dark ΔId = Pin Pin © 2019 American Chemical Society
P=
Id_light Id_dark
(2)
where Id_dark and Id_light refer to the dark and light currents, respectively. Large photocurrent results in large responsivity, and the wide dynamic range of current leads to high photosensitivity.9 OPTs have shown photosensitivity values of ≥104, and though these numbers are impressive, it is difficult to achieve both high responsivity and photosensitivity simultaneously.9−14 An OPT that achieved both high responsivity and photosensitivity was reported by Chu et al., who introduced charge traps on a surface of dielectric layer and deposited a high mobility organic semiconductor on top of the device. Their OPT yielded a photosensitivity of >102 with a responsivity of >100 mA W−1.15 However, maintaining high photosensitivity and responsivity is still difficult because OPTs act as transistors even under dark conditions. The maximum responsivity is achieved when the drain and gate bias are kept high, but the dark current gets amplified in this condition. A typical OPT exhibits a dark current between 100 nA and a few mA depending on its mobility and channel length/width. Every OPT reported to date could not achieve maximum photosensitivity and responsivity simultaneously. Herein, we present an OPT that simultaneously exhibits high photosensitivity and responsivity. We suppressed the charge injection into the semiconductor layer by modulating the source/drain contact. By selecting an n-type semiReceived: March 4, 2019 Accepted: June 24, 2019 Published: June 24, 2019
(1) 1054
DOI: 10.1021/acsaelm.9b00136 ACS Appl. Electron. Mater. 2019, 1, 1054−1058
Letter
ACS Applied Electronic Materials
metal where the work function is deep and has been widely used in organic electronics for the hole injection layer.16,17 A 30 nm thick blend film of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (OC1C10-PPV) and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) was formed by spin-casting and used as an active layer. MoOx and Au were deposited by thermal evaporation, and the channel width and length were 700 and 38 μm, respectively. We exposed the source/contact side of the OPT to light in the experiments described below. To investigate the work function and the injection barrier of the MoOx layer, we performed ultraviolet photoelectron spectroscopy (UPS) on each layer of the MoOx and OC1C10-PPV:PCBM. The samples were prepared on an ndoped Si wafer with a thickness of 30 nm. Figure S1a shows the UPS result of MoOx. The evaluation of the ionization energy (IE) was determined to be −5.8 eV. The UPS results of OC1C10-PPV:PCBM are shown in Figure S1b. Two IE values were obtained from the curve, corresponding to the IE of OC1C10-PPV and PCBM. OC1C10-PPV theoretically should have shallower IE; thus, we concluded that the IE for OC1C10PPV and PCBM was −5.2 and −5.4 eV, respectively. The results are illustrated in the energy diagram shown in Figure 1b. The electron affinities and work functions of OC1C10-PPV, PCBM, and Au were brought from previous reports which were −2.8, −3.8, and −5.1 eV, respectively.18,19 Figures 2a and 2b compare the transfer curves of the OPT by using the Au and MoOx contacts. The transfer curves were measured in the dark sweeping the gate voltage (Vgs) from −10 to 15 V with a drain voltage (Vds) of 15 V. The drain current (Id) under dark at Vgs = 15 V for the OPT with MoOx was
conductor with an electrode with deep work function, we suppressed the dark current by over 4 orders of magnitude. The dark current was suppressed to 105. The responsivity of this OPT was 5.9 × 10−3 A W−1 while the optical output power was 42 mW cm−2. The OPT structure is shown in Figure 1a. The MoOx layer was selected as source/drain contact. MoOx is a transition
Figure 1. (a) Schematics of the cross-sectional view of an OPT and the structure of OC1C10-PPV and PCBM. (b) Energy diagram of OC1C10-PPV, PCBM, MoOx, and Au.
Figure 2. Transfer curves measured when exposed to light of various output powers ranging from dark to 42 mW cm−2 for Au contact (a) and MoOx contact (b). Gate voltage dependency on responsivity and photosensitivity for Au contact (c) and MoOx contact (d). 1055
DOI: 10.1021/acsaelm.9b00136 ACS Appl. Electron. Mater. 2019, 1, 1054−1058
Letter
ACS Applied Electronic Materials
Figure 3. (a) Output power of the light source dependency on responsivity. (b) Time response of the OPT with MoOx and without MoOx (Au).
5.9 × 10−3 A W−1 under illumination with a power density of 42 mW cm−2, which is consistent with OPTs with similar structure.22,23 The gate voltage dependence on responsivity and photosensitivity is plotted in Figure 3a,b. Both OPTs with and without MoOx showed similar responsivity while their photosensitivity differs. The maximum responsivity was achieved at Vgs = 15 V, which was 5.9 × 10−3 A W−1 for the OPT with MoOx and 4.6 × 10−3 A W−1 for that without MoOx. The maximum photosensitivity without the MoOx was achieved at Vgs = 5 V, which was 33. The photosensitivity starts decreasing with higher gate voltage, and at Vgs = 15 V, where the responsivity maximizes, it reduces down to 1.2. The photosensitivity for the MoOx contact kept increasing with increasing gate voltage and reached its maximum at Vgs = 15 V and was 6.9 × 104, and the maximum responsivity and photosensitivity were achieved simultaneously. The linear response of the responsivity as a function of the light output power is shown in Figure 3a. The OPT with MoOx showed a linear response of photocurrent as a function of light, as shown in Figure S4. This resulted in constant responsivity with any output power. This is a characteristic most often seen in photodiodes and arises from the dark current being nearly zero. The OPT prepared herein showed a larger dynamic range in terms of signal while maintaining its high response compared to OPDs. Another important characteristic for OPTs using BHJ is their time response. Figure 3b shows the time response comparison of the OPT with MoOx and without MoOx (Au).
The output power of light was 42 mW cm−2, and an optical chopper was used to chop the light at 4 Hz. The time for the photocurrent to reach 90% of the ON current was
