Red light sensitive heterojunction organic field-effect transistors based on neodymium phthalocyanine as photosensitive layer

    Red light sensitive heterojunction organic field-effect transistors based on neodymium phthalocyanine as photosensitive layer Wenli Lv, Yu Tang, Bo Yao, Maoqing Zhou, Xiao Luo, Yao Li, Junkang Zhong, Lei Sun, Yingquan Peng PII: DOI: Reference:

S0040-6090(15)00670-7 doi: 10.1016/j.tsf.2015.06.059 TSF 34462

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

11 September 2014 15 June 2015 23 June 2015

Please cite this article as: Wenli Lv, Yu Tang, Bo Yao, Maoqing Zhou, Xiao Luo, Yao Li, Junkang Zhong, Lei Sun, Yingquan Peng, Red light sensitive heterojunction organic field-effect transistors based on neodymium phthalocyanine as photosensitive layer, Thin Solid Films (2015), doi: 10.1016/j.tsf.2015.06.059

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ACCEPTED MANUSCRIPT

Red light sensitive heterojunction organic field-effect transistors based on neodymium

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phthalocyanine as photosensitive layer

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Wenli Lva, Yu Tanga, Bo Yaoa,c, Maoqing Zhoua, Xiao Luoa, Yao Lia, Junkang Zhonga, Lei Suna, and Yingquan Penga,b,* a

Institute of Microelectronics, School of Physical Science and Technology, Lanzhou University, South

b

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Tianshui Road 222#, Lanzhou 730000, People‘s Republic of China

Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou

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University, South Tianshui Road 222#, Lanzhou 730000, People‘s Republic of China Department of Physics, Shaoxing University, Shaoxing 312000, People‘s Republic of China

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Abstract

Compared with organic photodiodes, photoresponsive organic field-effect

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transistors (photOFETs) exhibit higher sensitivity and lower noise. The performance of photOFETs based on conventional single layer structure is generally poor due to the low carrier mobility of the active channel materials. We demonstrate a high

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performance photOFET operating in red light with a structure of C60/neodymium phthalocyanine (NdPc2) planar heterojunction. PhotOFETs based on single-layer NdPc2

and

C60/NdPc2

heterojunction (denoted as

NdPc2-photOFETs and

C60/NdPc2-photOFETs, respectively) were fabricated and characterized. It is concluded that the photOFETs with heterojunction structure showed superior performance compared to that of single layer photOFETs. And for red light with a wavelength of 655 nm, C60/NdPc2-photOFETs exhibited a large photoresponsivity of ~0.8 A/W, which is approximately 62 times larger than that of NdPc2-photOFETs under the same conditions. The high performance of C60/NdPc2-photOFETs is

*

Corresponding author. Tel.: +86 931 8915362; fax: +86 931 8915362. E-mail address: [email protected] (Yingquan Peng) 1

ACCEPTED MANUSCRIPT attributed to its high light absorption coefficient, high exciton dissociation efficiency and high carrier mobility. Photoresponsive

organic

field-effect

transistors

(PhotOFETs),

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Keywords:

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Neodymium phthalocyanine, Heterojunction.

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1. Introduction

Organic field-effect transistors (OFETs) were extensively researched due to their attractive advantages such as large-area [1], flexibility [2] and wide range of potential

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applications like display drivers and sensors. Among them, the photoresponsive organic field-effect transistors (photOFETs) have better application prospects because

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of low noise and high sensitivity compared with photodiodes [3]. PhotOFETs are a kind of three terminal optoelectronic device in which light is used as an additional

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control quantity to create charge carriers in addition to the carriers induced by the gate

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voltage. Photosensitive properties of photOFETs are extensively influenced by the absorption efficiency of light-sensitive materials for the incident light. A large number

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of metal phthalocyanines with Q-band absorption at 600-800 nm [4] were used as photosensitive materials in organic solar cells and photOFETs, such as copper phthalocyanine (CuPc) [5, 6], lead phthalocyanine (PbPc) [7], zinc phthalocyanine

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(ZnPc) [8] and palladium phthalocyanine (PdPc) [9]. In this study, we first fabricated photOFETs utilizing a rare-earth metal phthalocyanine—neodymium phthalocyanine (NdPc2) as photosensitive layer because of its relatively high absorption coefficient under red light. However, the photoresponsivity of single layer metal phthalocyanine photOFETs is generally low due to the relatively low mobility of the active layer and poor exciton dissociation efficiency. Organic heterojunctions have been widely used to improve the performance of devices, such as organic light-emitting diodes [10], OFETs [7, 11] and organic solar cells [12]. PhotOFETs fabricated with a donor material used as photosensitive layer, and an acceptor material with high mobility used as channel layer can be an effective strategy to realize high performance photOFETs, due to the higher exciton dissociation efficiency at the donor/acceptor interface. As an excellent 2

ACCEPTED MANUSCRIPT electron accepter material with high electron mobility [13], C60 was widely used in organic electronic devices [14-17]. It was reported that the single layer pentacene photOFETs showed a photoresponsivity of 0.1-0.45 A/W at red light [6]. Our group

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also reported photoresponsivity improved pentacene based photOFETs via inserting a C60 buffer layer between the source/drain electrodes and pentacene active layer,

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which exhibit a high photoresponsivity of up to 4.27 A/W at zero gate voltage under red light [27].

In this work, we report on photOFETs using NdPc2 as photosensitive layer.

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Single-layer NdPc2 photOFETs and C60/NdPc2 planar-heterojunction photOFETs (denoted as NdPc2-photOFETs and C60/NdPc2-photOFETs, respectively) were

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fabricated and investigated. Compared with the former, the photoresponsivity of C60/NdPc2-photOFETs is obviously enhanced and reaches up to ~0.8 A/W for

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2. Experimental details

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unoptimized device.

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Phthalocyanine neodymium was synthesized through following procedures detailed in the literature [18]. The molecular structure of NdPc2 [19] is shown in the insert of Fig. 1a and it can be seen that an NdPc2 molecule is consisted of a central Nd

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(neodymium) atom and π-electron system with sandwich structure. C60 was purchased from Luminescence Technology Co., Ltd., Taiwan, and used as received. A heavily n-doped Si substrates with a resistivity of 0.03 Ohm·cm acts as the gate electrode with a 1000 nm thermally grown SiO2 layer as the gate dielectric. The substrates were ultrasonically cleaned by acetone, ethanol and de-ionized water, and was dried with N2 gas blowing and baked in an oven with a temperature of 60 °C for 20 minutes. For the device fabrication, 50-nm-thick C60 and 25-nm-thick NdPc2 layer were deposited on the top of Si/SiO2 successively (monitored by a quartz crystal oscillator) under a vacuum of ~2×10-3 Pa. Au source/drain electrodes were vacuum deposited through a shadow mask which defined a channel length (L) / width (W) of

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ACCEPTED MANUSCRIPT 25 μm/3 mm. At the same time, single layer device with NdPc2 (NdPc2-photOFETs) as the active layer were fabricated for comparison.

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In order to study the film-forming property, 25-nm-thick NdPc2 films were

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deposited on SiO2 and then X-ray diffraction (XRD) patterns were taken from a diffractometer (Rigaku D/max-2400 with Cu Kα radiation). For optical absorption

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measurements, 25-nm-thick NdPc2 films and 20-nm-thick C60 films were vacuum deposited on cleaned quartz substrates, respectively. And TU-1901 spectrometer was used to measure the optical absorption of the films. AFM analysis was carried out in

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tapping mode using an Agilent 5500 AFM system. I-V characterization of devices was performed by an organic semiconductor measuring system in a vacuum chamber

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(vacuum level, ~10-2 Pa). Top illumination was implemented using a red laser diode with a power density of 100 mW/cm2 and emission centered at 655 nm, and the

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neutral density filters with various transmittances were used to vary the light power.

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3. Results and discussion

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As shown in Fig. 1a, NdPc2 film deposited on SiO2 exhibits one predominant peak at 6.6°, which is consistent with that reported in literature [20]. It is shown that the film is polycrystalline with an average grain size of 87.55 nm, which is calculated

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from Scherrer formula (the corresponding full-width of half maximum is 0.3°). Fig. 1b shows the absorption spectra of NdPc2 and C60 films. A strong absorption peak at 638 nm is observed. On the contrary, the absorption of C60 film is mainly in the ultraviolet region due to its wide band gap (~2.6 eV) [21]. Therefore, the absorption of C60/NdPc2 bilayer film in the visible region is considered to be equivalent to that of NdPc2 single-layer film. It is worth noting that in comparison with PdPc film (absorption peak at 616nm) [7] and CuPc film (absorption peak at 625 nm) [22], NdPc2 is one favored red light-sensitive organic semiconductor material among metal phthalocyanine compounds, the absorption peak of NdPc2 in visible region is more close to the wavelength commonly used in red light (650-670 nm) [23-26]. Additionally, the maximum absorption coefficient in Q-band of NdPc2 film reaches 4

ACCEPTED MANUSCRIPT up to ~ 1.7 × 105 cm-1, which is greater than that of CuPc (~ 5.6 × 104 cm-1), PbPc (~ 1.1 × 105 cm-1) [7], PdPc (~ 1.0 × 105 cm-1) [9] and pentacene (~ 5.2 × 104 cm-1), [27]

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respectively.

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The surface morphology of NdPc2, C60 and C60/NdPc2 bilayer films is shown in Fig. 2. The root-mean-square (RMS) roughness of NdPc2 film is only 0.84 nm, which

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is far less than the literature report (2.2-2.8 nm) [20]. It is observed that the grain size (in-plane) is about 90 nm, which is consistent with the calculated value from XRD data. The RMS roughness of C60/NdPc2 bilayer film is only 3.27 nm, which is also

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smaller than that of C60 film (5.48 nm, see Fig. 2b). We can infer that NdPc2 with small grain size (~ 90 nm) filled into the surface of C60 film and it makes the surface

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of the C60/NdPc2 film tend to be smoother. Between the interface of NdPc2 and C60, needlelike-C60 morphology significantly results in an increase of the contact area and

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an improvement of the exciton dissociation efficiency [20].

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As can be seen from Fig. 3a, NdPc2-photOFET shows typical p-type field-effect characteristics at negative gate and drain voltage (denoted Vg and Vd respectively).

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With the increase of incident light power density, the device shows obvious response to the red light, and the drain current of the device obviously increases. For Vg = -100 V, the drain current of the device is only -35 nA in the dark while it increases to -81

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nA under red light illumination with a power density of 87.67 mW/cm2, which is ~ 2.5 times of that in the dark. From the saturation region of the transfer characteristic, the field effect mobility (µFET) and the threshold voltage (VT) of the device can be calculated by the following equation:

Id 

2 W Ci FET Vg  VT  2L

(1)

where Id is the drain current, and Ci is the capacitance of the gate dielectric per unit area (3.18 nF/cm2). The µFET of NdPc2-photOFETs is determined to be 7.2 × 10-5 cm2/Vs, which is much smaller than that of the high mobility organic semiconductors, such as C60 [13], pentacene [27], and CuPc [9].

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ACCEPTED MANUSCRIPT From Fig. 3b it can be seen that light illumination results in a positive shift of VT of the device from -50 V (in the dark) to -35 V. This photovoltaic effect results from the transport of photo generated holes and the trapping of photo generated electrons

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near the source electrode [28, 29].

The photosensitivity and photoresponsivity (denoted by P and R respectively) are

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two key performance parameters of photOFETs. The photosensitivity is defined as the ratio of the photocurrent (Iph) to the dark current (Idark) [30] and the photoresponsivity is defined as the ratio of photocurrent to the incident optical power on the channel of

I dark



I ill  I dark I dark

(2)

I ph Popt

(3)

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R

I ph

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P

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device

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where Iill is the drain current under illumination and Popt can be calculated from

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incident light power density (Pin) multiplied by the active area of device (W × L). The P and R of the device were calculated from the data in Fig. 3b. Although NdPc2 film has high absorption efficiency for red light, the hole mobility is relatively low, which

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limits the transport of photo generated holes, resulting in a small photocurrent. Therefore, the maximum photosensitivity (165) and photoresponsivity (13 mA/W) of NdPc2-photOFETs (denoted Pmax and Rmax respectively) are relatively small, as shown in Table 1. To obtain a better performance, C60/NdPc2-photOFETs were fabricated and characterized. Typical output and transfer characteristics of C60/NdPc2-photOFETs in the dark and under illumination are shown in Fig. 4. It can be seen that C60/NdPc2-photOFET shows typical n-type field-effect characteristics. Compared with NdPc2-photOFET, the dark current of C60/NdPc2-photOFET increases significantly from 35 nA to 232 nA at Vg = -100 V. When the power density of the incident light reaches to 87.67 mW/cm2, the Id of C60/NdPc2-photOFET reaches up to 4.4 µA, which is ~ 20 times of that in the dark at Vg = 100 V. However, the Id of 6

ACCEPTED MANUSCRIPT NdPc2-photOFET is only -81 nA under the same conditions. The photovoltaic effect results in a negative shift of VT of C60/NdPc2-photOFET from 64 V (in the dark) to 60

V

(under

87.67

mW/cm2 illumination).

Fig.

4c

shows

the

R

of

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C60/NdPc2-photOFET as a function of the Pin at Vg = 100 V. It can be seen that the R decreases with the increased power density of the incident light, which is consistent

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with that reported in literature [27]. For C60/NdPc2-photOFET, the Rmax reaches up to 806 mA/W at the Pin of 1.41 mW/cm2, which is approximately 60 times larger than that of NdPc2-photOFET (13 mA/W), as shown in the inset of Fig. 4c. The

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photoresponsivity of C60/NdPc2-photOFET is greater than or comparable to that of the photOFETs with pentacene as photosensitive layer [6, 31]. This is attributed to the

planar-heterojunction interface.

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high electron mobility of C60 channel layer and the enhanced exciton efficiency at the

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As shown in Table 1, compared with NdPc2-photOFET, the mobility of

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C60/NdPc2-photOFET is larger by two orders of magnitude in the dark. Moreover, the Pmax and Rmax of NdPc2-photOFET are 2 and 62 times higher than those of

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NdPc2-photOFET, respectively. The results suggest that NdPc2 is an available organic red light-sensitive material and the donor/acceptor heterojunction structure is an

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effective strategy to achieve high performance photOFETs. 4. Conclusions In conclusion, photOFETs based on C60/NdPc2 heterojunction and single layer NdPc2

were

fabricated

and

investigated.

The

results

showed

that

C60/NdPc2-photOFET demonstrates superior performance to the single layer photOFET. Under 655 nm light illumination, C60/NdPc2-photOFET exhibits a large photoresponsivity of ~ 0.8 A/W, which is approximately 62 times larger than that of NdPc2-photOFETs under the same conditions. This suggests that a high performance photOFET can be achieved by using a low mobility donor material as the photosensitive layer and adopting a high mobility acceptor material as the channel layer. 7

ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the National Natural Science Foundation of China

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Grant No. 10974074 and the Research Fund for the Doctoral Program of Higher

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Education of China Grant No. 20110211110005.

[1] H.

Sirringhaus,

M.

Ando,

Materials

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ACCEPTED MANUSCRIPT Table 1. Performance parameters of the devices. µFET, dark Pmax

(mA/W)

7.2×10-5

165

13

C60/NdPc2-photOFETs

7.4×10-3

335

806

a

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NdPc2-photOFETs

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(cm2/Vs)

Rmaxb

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Device (|Vd| = 50 V)

a

At Vg = -28 V for NdPc2-photOFETs or Vg = 34 V for C60/NdPc2-photOFETs under

the illumination with 87.67 mW/cm2;

At |Vg| = 100 V under the illumination with 1.41 mW/cm2.

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b

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Fig. 1. (a) The XRD patterns of NdPc2 film deposited on Si/SiO2. Inset: Molecular structure of NdPc2; (b) UV-Vis absorption spectrums of NdPc2 and C60 films deposited on quartz substrate.

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Fig. 2. AFM images of (a) NdPc2, (b) C60 and (c) C60/NdPc2 thin film; (d) The structure of

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photOFETs and incidence orientation of illumination. Fig. 3. (a) Output characteristic in the dark and under illumination with different intensity at Vg =

illumination.

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-100 V; (b) Transfer characteristics at Vd = -50 V of NdPc2-photOFETs in the dark and under

Fig. 4. (a) Output characteristic under illumination with different intensity at Vg = 100 V. Inset:

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Output characteristic in the dark; (b) Transfer characteristics at Vd = 50 V of C60/NdPc2-photOFETs in the dark and under illumination; (c) Photoresponsivity of C60/NdPc2-photOFETs as a function of the incident light power density at Vg = 100 V and Vd = 50 V. Inset: Plot of R-Pin of NdPc2-photOFETs at Vg = -100 V and Vd = -50 V.

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ACCEPTED MANUSCRIPT Highlights

fabricated

C60/NdPc2

photoresponsive

organic

field-effect

transistors

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We

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The mobility of light-sensitive organic materials is generally low.

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(photOFETs).

The performance of C60/NdPc2 photOFETs is superior than single-layer NdPc2

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photOFETs.

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C60/NdPc2 photOFETs exhibited a large photoresponsivity of ~0.8 A/W for red light.

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