Supplementary Material for
Carbon Nanotube Woven Textile Photodetector Ahmed Zubair,1 Xuan Wang,1 Francesca Mirri,2 Dmitri E. Tsentalovich,2 Naoki Fujimura,3 Daichi Suzuki,3 Karuppasamy P. Soundarapandian,4 Yukio Kawano,3 Matteo Pasquali,2, 5, 6, ∗ and Junichiro Kono1, 5, 7, † 1 Department of Electrical and Computer Engineering, Rice University, Houston, Texas, USA Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas, USA 3 Quantum Nano-electronics Research Center, Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan 4 Department of Physics and Nanotechnology, SRM University, Chennai, Tamil Nadu, India 5 Department of Materials Science and NanoEngineering, Rice University, Houston, Texas, USA 6 Department of Chemistry, Rice University, Houston, Texas, USA 7 Department of Physics and Astronomy, Rice University, Houston, Texas, USA (Dated: December 20, 2017) 2
I.
DETECTOR OPERATION MECHANISM
The operation mechanism of our CNT photodetectors is based on the photothermoelectric (PTE) effect. Light (or, more generally, electromagnetic radiation) hits and heats the CNT system, which produces a voltage through the Seebeck effect. The produced open-circuit voltage between two electrodes located at xL and xR is given by Z TL Z xL ∆V = SCNT (T )dT = SCNT (x)∇T dx (1) TR xR h ixL Z xL dSCNT (x) = SCNT (x)T (x) − dx, (2) T (x) dx xR xR where TL and TR are the temperatures of the two electrodes and SCNT is the Seebeck coefficient of CNTs. As the first term on the right-hand side is zero (or small) in most situations, to make this voltage finite, we have to make the second term finite by producing a spatial gradient of S, i.e., dS dx . Specifically, we design the detector structure to maximize the photovoltage by engineering the spatial gradient of the Seebeck coefficient, SCNT (x) through doping. We take advantage of the fact that the Seebeck coefficient of CNTs is strongly dependent on the Fermi energy [1]. In particular, we previously demonstrated that a p-type nanotube and an ntype nanotube have opposite signs of Seebeck coefficient, and therefore, the maximum possible PTE effect appears at a p–n junction [2]. This model is fundamentally different from the situation of a simple thermocouple junction in the sense that the photovoltage signal arises from the presence of the graCNT dient of the Seeback coefficient in the CNT, i.e., dSdx , as opposed to a temperature gradient, ∇T . Usually for a semiconductor, the largest Seebeck coefficient is obtained when the semiconductor is lightly doped. However, for thermoelectric power devices, it is more important to maximize the thermoelectric power factor σS 2 or the thermoelectric figure of merit. The optimum generally occurs at high doping levels.
∗ †
[email protected] [email protected]
To estimate the photovoltage, one needs to know the spatial profile of the temperature, T (x). The temperature profile can be obtained by considering local heating of the CNT by light illumination and heat dissipation into the substrate of the CNT fiber or sheet. In our case, as the CNT fiber/sheet is suspended in air, the heat dissipates into air. For the case of focused optical illumination, the temperature profile can be given by T (x − x0 ) = Tmax e−|x−x0 |/λ ,
(3)
where λ is the thermal decay length, x0 is the focus position of the illumination beam, and Tmax is the maximum temperature in the CNT. The thermal length scale for optical heating is given by r κCNT d , (4) λ= G where κCNT is the thermal conductivity of the CNTs, d is the diameter/width of the fiber, and G is the thermal conductance between the CNT fiber and air. The maximum temperature is p Tmax = √ , d dGκCNT
(5)
where p is the absorbed optical power. The thermal conductance G is proportional to the thermal conductance of the substrate, i.e., air when the CNT is suspended in air. The thermal conductance for air, κair , is ∼0.025 Wm−1 K−1 . We can deduce the dependence of photothermal signal, ∆V on G and κCNT as follow 1 ∆V ≈ √ G
(6)
and ∆V ≈ √
1 κCNT
(7)
Thus, a lower G will result in a greater photothermal signal, but that will increase the thermal length. And, a lower κCNT produces a greater photothermal signal and shorter thermal length. A larger thermal length will put a constraint on the distance between two consecutive junctions in
2
(a)
(c)
(b)
(d) 405 nm 660 nm 1350 nm 4.53 µm 96.5 µm
Voltage (mV)
1.5
1.0
0.5
0.0 0
2
4
6
8
Power (mW)
FIG. S1. I–V characteristics and power dependence of the CNT-fiber detector. I–V characteristics of the photodetector without illumination and under illumination at different wavelengths: (a) Visible (660 nm), (b) far-infrared (119 µm), and (c) far-infrared (215.8 µm). (d) Power dependence of the open-circuit photovoltage at different wavelengths: ultraviolet (405 nm), visible (660 nm), near-infrared (1350 nm), midinfrared (4.53 µm), and far-infrared (96.5 µm). Experimental data are shown by markers and solid lines are straight line fits.
the fiber. The fiber of d = 20 µm used in the work has κCNT fiber = 314 Wm−1 K−1 and G = 379 Wm−2 K−1 [3]. Using these parameter values, the thermal length is estimated to be ∼2.03 mm. At any position, as heat propagates in both directions, the distance between two illumination points has to be greater than 2λ. At a junstion, the Seebeck coefficient will a have maximum gradient, making the junction most sensitive to illumination. So the junction acts as the active area of these devices. Due to this thermal length constraint, a detector with multiple junctions should be designed to make the distance between adjacent junctions around 4 mm.
II.
ULTRABROADBAND RESPONSE
Figures S1(a)-(c) show the I–V characteristics of the CNTfiber detector with 660 nm, 119 µm, and 215.8 µm wave excitation. Without illumination, the I–V curve is linear and passes through the origin. Under photoexcitation, the I–V curve rigidly shifts in the same way as in the case for the other wavelengths described in the main text (Fig. 4). Figure S1(d) shows the open-circuit photovoltage of the detector as a function of incident power at 405 nm, 660 nm, 1350 nm, 4.53 µm, and 96.5 µm.
III.
NOISE PERFORMANCE
We performed noise measurements on the CNT fiber and CNT rope photodetector devices. The noise performance of the devices was comparable to the theoretical value of the ther√ mal (or Johnson) noise limit given by 4kB T R, where kB is the Boltzmann constant, R is the resistance of the device, and T is the temperature. Figure S2 shows the noise voltage (or noise spectral density) as a function of modulation frequency. A description of the noise spectral density measurements can be found in the Experimental Section.
FIGURE S1
TABLE S1. Average noise spectral density (or noise voltage) for different CNT detectors. Device type
Noise voltage, N V/Hz1/2
Single-fiber
8.85×10−10
Multiple-fiber
2.95×10−10
Rope
4.47×10−10
3 -6
10
TABLE S3. Performance of CNT-fiber detector as solar harvester.
CNT Fiber Photodetector Device CNT Rope Photodetector Device )
zH/V( egatloV esioN
2/1
EQE F F
-7
10
Rs
0.8% 25% 46 Ω -8
10
-9
10
V.
FIBER DIAMETER DEPENDENCE OF PHOTORESPONSE
-10
10
To get the diameter dependence of photoresonse, we consider Eq. (1). The first term of the right hand side will be negligible if the length of the photodetector is much larger than the thermal length. Therefore, We will have
-11
10
0
10
1
2
10
10
3
10
4
10
5
10
Frequency (Hz)
FIG. S2. Noise voltage as a function of operating frequency for CNT fiber, sheet and rope photodetectors. The large noise voltages around 50 Hz originate from the power source of the measurement system.
Z
xL
∆V ≈ − xR
= −Tmax ∆SCNT As shown in Table S1, the average noise spectral densities for the single-fiber, multiple-fiber, and rope detectors were found to be 8.85×10−10 , 2.95×10−10 , 6.2×10−10 , and 4.47×10−10 V/Hz1/2 , respectively. The NEP values in the far-infrared regime for our detector are comparable to those of state-of-the-art room-temperature bolometers, pyroelectric detectors, Schottky diodes and Golay cell detectors, as shown in Table S2. The D∗ of the CNT-fiber detectors at 1350 nm wavelength is 2.02×105 cm Hz1/2 /W, which is comparable to that of CNT bolometers [4].
dSCNT (x) dx dx p∆SCNT , = √ d dGκCNT
T (x)
∆V ≈
1 . d3/2
To demonstrate the device as a solar harvester, we evaluated the characteristics performance such as the external quantum efficiency, fill factor, and parasitic series resistance. The external quantum efficiency, EQE is given by
(12)
3.0
Experimental )Vm( egatlovotohP
CHARACTERISTICS AS AN ENERGY HARVESTER
(11)
Assuming that the electrical conductivity, σ, thermal conductivity, κ, and thermal conductance between the CNT-fiber and surroundings, G, remains constant, we have
Fitted
2.5
IV.
(10)
2.0
1.5
@ 660 nm 1.0
10mW
0.5
Electrons/sec EQE = , Incident photons/sec
(8) 0.0 20
and the fill factor, F F , is defined by FF =
Vm Im , VOC ISC
40
60
80
100
120
140
CNT fiber diameter (µm)
(9)
where Im and Vm are the current and voltage, respectively, at the maximum power point. The parasitic series resistance is not negligible in our device. The series resistance, Rs arises from the resistance of the CNT-fiber and contacts to the current flow. We used a two-probe configuration to measure Rs . The power conversion efficiency of our CNT-fiber photodetector is ∼4.4×10−6 . The characteristics performance parameters of a single junction single CNT-fiber detector are listed in Table S3.
FIG. S3. Photovoltage response of CNT-fiber photodetector as a function of diameter. Black filled triangles are experimental data and blue solid line represent fitted line.
We measured the photoresponse of CNT-fiber photodetectors with different fibers. Figure S3 shows the diameter dependence of the photoresponse of CNT-fiber photodetectors at 660 nm. We fitted the experimental data with a power function of d as below ∆V (d) =
a + c, db
(13)
4 TABLE S2. Noise equivalent power of far-infrared detectors. Detector type
NEP
Refs.
W/Hz1/2 Golay cell
10−8 –1.4×10−10
Commercial
Pyroelectric detector
5×10−8 –4×10−10
Commercial
−13
∼10
Cooled bolometer
Commercial
Uncooled bolometer (VOx )
>3×10−10
[5]
Schottky diode
>10−10
[6]
Current work
9×10
where, a, b, and c are the fitting parameters. After fitting, b, which is the exponent of d, was found to be −0.965 ±0.298. This deviation of the value of b from the estimated 1.5 can be explained by taking into consideration the variation of conductivities. For our case, σ, κ, and G varied for different diameters of CNT-fiber photodetector. Also, the optical attenuation length at 660 nm was smaller than the diameters of all the CNT-fiber. VI. PROPERTIES OF CNTS USED TO FABRICATE THE DETECTORS AND TEMPERATURE DEPENDENCE OF ELECTRICAL CONDUCTIVITY OF FIBERS
The wall type, aspect ratio, and average diameter of the CNTs used in this work are summarized in Table S4. Also, Fig. S4 shows the electrical conductivity of the p+ and p− fibers as a function of temperature from 300 K to 400 K. Previously, we reported the temperature dependence of resistivity of fibers manufactured using CNTs purchased from Teijin Aramid BV [7] and Carbon Nanotechnologies, Inc. [8].
−10
-
VII. WASHING, DRYING, AND IRONING OF THE CNT-WEAVED TEXTILE PHOTODETECTOR
To determine the washability of the wearable CNT-weaved textile photodetector incorporated in the shirt, we completed a cycle of washing, drying, and ironing the photodetector incorporated. We used a small compact portable washing machine (Model XPB36, Panda) equipped with a spin dryer to wash and dry the shirt. A standard laundry detergent was used during washing. The washing cycle consisted of the following steps: (i) pouring water in the washing chamber to immerse the shirt completely, (ii) 6 minutes of rinsing, (iii) draining the water, (iv) pouring in water again, (v) rinsing again for 3 minutes to remove the detergent, and (vi) draining the water again. Then, we used the spin dryer to remove the water from the shirt in the washing machine and air-dried it overnight. We repeated the washing and drying cycle using cold and 40◦ C water.
3.5
)m/SM( ytivitcudnoC lacirtcelE
+
p
Fiber
-
p Fiber
3.0
2.5
2.0
1.5
300
320
340
360
380
400
Temperature (K)
FIG. S4. Electrical conductivity as a function of temperature from room temperature to 400 K for the p+ and p− fibers.
FIG. S5. Washability of the CNT-weaved shirt photodetector. The I–V characteristics of the detector with and without illumination by a white light lamp before washing (red solid and dashed line, respectively), after cold water washing (green solid and dashed line, respectively), after 40◦ C water washing (blue solid and dashed line, respectively), and after ironing (dark brown solid and dashed line, respectively) are shown.
5 TABLE S4. Properties of CNTs used to Fabricate the Detectors. CNT manufacturer
Wall Type
Aspect Ratio
Average Diameter nm
Carbon Nanotechnologies, Inc.
Double Wall
2600
2.34
Unidym, Inc.
Double Wall
4010
2.00
Teijin Aramid BV
Double Wall
4400
2.10
Meijo Nano Carbon Co., Ltd
Double Wall
4080
1.5
To measure the response, we globally illuminated this textile photodetector with broadband white light. We obtained the same photovoltage response as before washing (Fig. S5).
Finally, we ironed the shirt using a commercial clothes iron and measured the voltage response after that. The response did not change after ironing, as shown in Fig. S4.
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