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Abstract
With the continual increase of the carrier mobility of organic semiconductors, there is a great need for optimizing the contact between source/drain electrode and organics in order to further improve the performance of organic field-effect transistors. The effects of Au source/drain electrode contact length on the photosensitivity in pentacene-based organic field-effect transistors were systematically investigated. The results show that at a given gate voltage and drain voltage, the drain current increases with the contact length at first and then tends to saturate at a contact length of 0.7 mm. It is observed that the effective mobility under illumination, both in the linear region and the saturation region, as well as the photoresponsivity and the external quantum efficiency, increase with contact length. All of these can be attributed to the reduction of contact resistance with the increase of contact length. Moreover, analytical expressions were derived and successfully describe the measured dependence of the drain current on the contact length.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic electronics is a rapidly emerging field from intense academic to commercial interest over the past two decades. Organic field-effect transistors (OFETs) as the key element have been extensively researched in organic electronic devices, usually fabricated on top source/drain contact and bottom source/drain contact configuration. The transistors with top source/drain contact have smaller contact resistance than the bottom ones [1–3], and the contact resistance can be determined by transfer line measurements (TLM), scanned Kelvin probe microscopy and gated four probe measurements [4–7]. A great need of optimising the contact length of source/drain electrode and organic is required as the continually increasing of carrier mobility [8].
The contact length of an OFET refers to the length of the overlay region of source/drain electrode and organic paralleling to the channel, as shown in Fig. 1. It was reported that source and drain contact resistance account for over 50% of the total device resistance with Au-top contacted devices [4], and the value of contact resistance in the linear regime of the transistor is dependent on its contact length [9,10]. The effect of source/drain contact length on the characteristics of the organic field effect transistor based on pentacene as channel layer have been modeled by Wang et al. [10].
Fig. 1 Schematic cross section of a pentacene thin film transistor in the top contact configuration and an illustration of the current density distribution in pentacene films.
Furthermore, in 2016, Hou and associates investigated the interface dopant insertion layers for further modifications of the contact resistance. The pentacene OFETs with different thicknesses of the p-dopant under the Au electrodes show a significant decrease in threshold voltage from −2.2V to −0.8V and in contact resistance from 55 kΩcm to 10 kΩcm by adding a 1 nm thin dopant interlayer [11]. In 2017, Bhargava and associates demonstrated an order of magnitude lower contact resistance and higher photoresponse values in P3HT OFETs [6]. Although much research work has been conducted on properties of OFETs [6,11–17], the effects of the source/drain electrode contact length are still rarely involved. In this paper, we reported on the effect of source/drain electrode contact length on the photoresponsive performance of pentacene based OFETs. Device samples with different source/drain contact lengths were fabricated and the influences of contact length on the electrical and optical characteristics of OFETs have been investigated.
2. Experiment
The bottom-gate top contact structure of OFETs was adopted in this work as shown in Fig. 2(a). In order to investigate the effects of the contact length, device samples with different length of source/drain contact electrodes ranging from 0.30 mm to 0.90 mm were fabricated. These devices have the identical organic active layer and gate dielectric. For the devices fabrication, a heavily n-doped Si with a resistivity of 0.03 Ω·cm and 1000-nm-thick SiO2 layer was used as the substrate serving as the gate and the gate dielectric, respectively. The substrates were ultrasonically cleaned with acetone, alcohol, deionized water and then blow dried with pure N2. Thereafter, a 50-nm-thick pentacene films were vacuum deposited on the top of SiO2 by thermal evaporation at a rate of 0.5 nm/min. The vacuum during the deposition was less than 10−3 Pa. Finally, Ausource/drain electrodes were vacuum deposited through a shadow mask which defined a channel length (L)/width (W) of 150 μm/2 mm.
Fig. 2 (a) Schematically diagram of the bottom-gate top contact structure of OFET. (b) Five different devices variation of contact length on a substrate.
In the measurement, devices were bottom-up loaded in a vacuum chamber (vacuum level ~10 Pa) with a top glass window for viewing and light incoming. Currente-voltage (I-V) characteristics of devices were measured using an organic semiconductor characterization system. A commercially available NIR laser with a power density of 0.04 mW/cm2 and emission centered at 655 nm was used as the light source. For the measurement of absorption spectrum, a 50 nm thick pentacene film was deposited on quartz glass.
3. Discussion
For top contact devices, the length of injection is not the geometric overlap of the source/drain and gate. Charge injection from metal-semiconductor surface and transport through semiconductor bulk reach to channel edge which formalism current by applying gate voltage and drain voltage. This process must be affected by channel resistance per unit channel length rch and the contact resistivity rc (in Ω/cm2). The charges injected from the contact and spreading in the bulk of semiconductor is not uniformly distributed over whole source/drain contact area [9]. The electrostatic potential distribution in the organic semiconductor leads to the charge transfer between the contact and the channel in a nonperpendicular direction [18]. Only the conductivities of the individual regions were considered when the current density distribution in the different regions of the device, in fact current accumulating under source/drain effective contact region [2,9]
Assuming the source/drain electrodes be symmetric, that is, the source and drain contact resistance are nearly the same, then the resistance to the current flow in the OFET can be expressed as the serial connection of three resistances: two contact resistance Rc and channel resistance Rch, as schematically shown in Fig. 3. When a bias voltage Vd is applied at the drain electrode, one has
where Id is the current flow through the channel. Rch represents the channel resistance per unit channel length, and is define as [9]Here μch is the intrinsic field-effective mobility of the channel, Ci is capacitance per unit area, Vg is applied gate bias voltage, and Vt is the threshold voltage of the transistor. The contact resistance Rc is given by following Eq. (2) [9].
Here d is the source/drain contact length. LT is the characteristic length of the source (drain) series resistance at a fixed Vg, and is expressed as
LT is also known as transfer length, which defines the critical distance over which most of charges are transferred from the metal contacts to the organic semiconductor [19]. Rc is the contact resistivity which comprise of injection resistivity rinj that produce due to metal-semiconductor interface and the bulk resistivity rbulk because of semiconductor bulk access resistance, that is rc = rinj + rbulk [20].
The bulk access resistance which represents the dominants components of the contact resistance, strongly depends on the gate voltage, due to charges must move though the semiconductor bulk to reach the channel from staggered contacts [20–22] Thus the larger the bulk resistivity rbulk, the larger the contact resistance rc (at source/drain terminal).
Based on Eq. (5), when rc<<Wrch, the injection will occur over a narrow length near the edge of source/drain contact, in fact, narrower transfer length in the range of several nanometers to hundreds of nanometers were reported for low contact resistances [23,24]. While for rc>>Wrch, the injection length will be much larger. The valid large rc or small rch ensure that there is large LT and even reach to submillimeter.
3.1 Contact length dependent drain current
The current-voltage characteristics in the dark and under illumination of the devices with different contact lengths are shown in Fig. 4(a) and Fig. 4(b), respectively. As expected, we observed that as the contact length increases from 0.30 mm to 0.90 mm, the drain current sharply increases at first and then saturates at large contact lengths at around 0.75 mm. This is resulted from the decrease of the contact resistance with the contact length.
Fig. 4 Output current-voltage characterization curves of OFETs with different source/drain contact lengths ranging from 0.3 mm to 0.9 mm at a gate voltage of Vg = −80 V. (a) In the dark; (b) Under illumination of 655 nm wavelength and 0.04 mW/cm2 light intensity.
In the presence of contact resistance, let Vch be the effective voltage across the channel, we have Vch = Vd-2RcId from the circuit diagram as shown in Fig. 3. Replacing Vd by Vch in Eq. (1), we have:
Solving above equation for Id, we obtain:
Substitute Eq. (4) into above equation, and Eq. (3) into Eq. (5), one has
As shown in Fig. 5, the measured drain current in the linear region in dependence of the contact length in the dark and under illumination were fitted with Eq. (8), and satisfactory agreement is demonstrated between experiment and theory. The device parameters used for the calculation are W = 2000 μm, L = 150μm, Ci = 3.18 nF/cm2, Vg = −80 V, Vd = −50V, Vt = −21.2 V, with μ and rc as the fitting parameters. The value for rc was determined to be 0.38 × 107 and 0.35 × 107 Ω·cm2 in the dark and under illumination by fitting, respectively. The contact resistance per unit channel width in the dark is calculated to be 1.9 × 107 Ω·cm, which is comparable to that reported in [7], but around 2 orders of magnitude higher than that reported in [2] and [24]. For the fitting, the mobility was taken to be 0.026 cm2/Vs, extracted from the contact length dependent mobility measurement, as shown in Fig. 6a.
Fig. 5 The comparison between measured (symbols) and simulated (solid line) drain currents by Eq. (8) in the dark (a) and under illumination (b) in dependence of contact length in the linear region, at Vg = −90 V, and Vd = −2.9 V.
Fig. 6 The dependence of the linear region and saturation region effective field-effect mobility on the contact length in the dark (filled symbols) and under illumination (open symbols). (a) The linear region effective filed-effect mobility, μeff,lin, extracted from Eq. (10); (b) The saturation region effective filed-effect mobility, μeff,sat, extracted from Eq. (11).
3.2 The dependence of mobility on the contact length
For inorganic FETs, the mobility extracted in the lineal region, μlin, and that in the saturation region, μsat, are equal, while for OFETs, they are unequal. In addition, as the contact resistance is generally large, the mobility extracted directly from the measured output and transfer characteristics including the effect of contact resistance, and thus generally smaller than the intrinsic channel mobility, μch, and gate voltage dependent, therefore we refer them as the effective FET mobility. From the measured output and transfer characteristics, we extracted the effective mobility μeff,lin and μeff,sat by using Eq. (10) and (11), respectively.
It can be seen from Fig. 6 that the effective mobility under illumination, both in the linear region and saturation region, increases with the contact length and the photo illumination, due to decreased contact resistance. μeff,lin saturates at contact lengths over 0.6 mm, may be due to the saturation of contact resistance. The values of μeff,sat lie in the region of 0.0016~0.0026 cm2/ Vs, are comparable to those reported in [27] and [28], but around 2 orders of magnitude smaller than that in [29], and this may be resulted from different film growth conditions, or gate dielectrics and well as their surface treatments.
3.3 The dependence of photoresponsive properties on the contact length
The photoresponsivity (R), photo-to-dark current ratio (PDR), and external quantum efficiency (EQE) are the main parameters of a photodetector. The responsivity R is defined as the ratio of output photocurrent Iph, to the incident optical power, Popt, that is, R = Iph/Popt [25]; EQE represents the number of electrons that generated by one incident photon, and can be expressed as [26]
where h, c, q, λ are Planck constant, light speed, electron charge and optical wavelength, respectively. PDR = Iph/Idark, represents the signal to noise ratio.As shown in Fig. 7, both R and EQE increase with the contact length, which can be understood that the drain current both in the dark and under illumination increase with contact length due to the reduction of contact resistance. The photoresponsivity of ~1 A/W is comparable to that reported in [13], and much larger than that reported for UV light in [14] and [30]. It is worth noting that the EQE exceeds 100%, which implies that photocurrent amplification occurs in the device.
4. Conclusions
The effects of source/drain electrode contact length on the photoresponsive performance of OFETs based on pentacene have been investigated. It is demonstrated that at a given gate voltage and drain voltage, the drain current increases with the contact length at first and then tends to saturate at a contact length of 0.7 mm. It is observed that, the effective mobility under illumination, both in the linear region and saturation region, as well as photoresponsivity and external quantum efficiency increase with contact length. All these can be attributed to the reduction of contact resistance with increasing contact length. Moreover, analytical expressions were derived and successfully describe the measured dependence of drain current on the contact length.
Funding
National Key R&D Program of China (2016YFF0203605).
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