Supporting Information Transparent Triboelectric Nanogenerators and Self-powered Pressure Sensors Based on Micro-patterned Plastic Films Feng-Ru Fan,,, Long Lin,, Guang Zhu,, Wenzhuo Wu, Rui Zhang, Zhong Lin Wang *,, School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China * To whom correspondence should be addressed, zhong.wang@mse.gatech.edu
Figure S1. Principle and strategy from TEG to FTNG: (i) replacing Kapton film with transparent PDMS film, (ii) replacing Au electrodes with transparent ITO electrodes, then the entire structure is flexible and transparent, (iii) fabricating various PDMS patterned arrays to enhance the friction effect, resulting in a high-output generator.
Figure S2. A detailed description of the entire power generation process. (a) The initial state of the device. (b) With a bending force and deformation of the structure, electrostatic triboelectric charges with opposite signs are generated and distributed on the two internal surfaces of the polymer films through a friction process. (c) During the release of the deformation, the opposite triboelectric charges become separated with an air gap and a dipole moment forms, and the induced electric potential difference will drive the electrons flow across the external load. (d) The electric potential between the planar electrodes reaches equilibrium and electrons accumulate on the one side of the electrodes, leaving the other side positively charged. (e) When another bending process starts, the dipole moment disappears or reduced and the accumulated electrons will be driven to flow in the opposite direction. Therefore, an alternating electrons flow in the external load will be observed with cycled bending and releasing process.
Figure S3 Switching-polarity test of the output voltage of the triboelectric nanogenerator. Open circuit voltage when (a, b) forward-connected and (c, d) reverse-connected to measurement system.
Figure S4. SEM images of the patterned PDMS thin film with 5 µm cubic features (A) and 5 µm pyramid features (B). The insets are 45 -tilted high magnification images of each structure. The results clearly show that we can fabricate the uniform patterned features in a large area.
Figure S5. Performance characterization of different FTNGs shows the important role of PDMS layer. The output voltages of FTNGs made of PET&PET and PET-PDMS&PDMS-PET are far lower than that of PET&PDMS-PET. It clearly indicates that we should use materials with distinct triboelectric characteristics to get high output.
Triboelectric Series 1 Figure S6. The triboelectric series is a list that ranks various materials according to their tendency to gain (negative) or lose electrons (positive) in contact charging and frictional charging process. PDMS (negative) is far away from PET (positive) in the list. Using the combination of PDMS and PET as the friction interface, we can achieve high-output power generation. (1) Diaz A. F. ; Felix-Navarro R. M. J. Electrostat. 2004, 62, 277-290.
Calculation of the Detection Limit of Pressure Sensor The calculation of the contact pressure induced by a falling object (a droplet of water and a falling feather) was based on a simple physical model, combining the gravity term and the pulse term. The detailed calculations are shown below. When the object falls on the surface of the pressure-sensor device, it encounters two processes: 1) falls through the air and touches the surface of the device, 2) completely falls on the device. The descending velocity of the object increases to maximum value in the first process and decreases to zero in the second one. Here, we set m as the mass of the object, h as the falling height, v as the maximum falling velocity, F as the contact force, p as the contact pressure, t as the time span during the second process, and S as the effective area of the device. Based on the kinetic energy theorem and momentum theorem, we have m 1 2 2 g h= m v (1) ( F mg) t= m v (2) F = p S (3) The next step is the estimation of the t value. For the droplet of water, it could be calculated as 2r t=, here r is radius of the water droplet (assuming that the droplet is spherical shape), and it could v be estimated by the weight of the droplet. For the case of the feather, t could be estimated as the time variation between the two consecutive voltage peaks induced by the feather, which is shown in Fig. R1. So we get m droplet =8 mg, m feather =20 mg, h =5 cm, g =9.8 N/kg, S =9 cm 2, r droplet =1.24 mm. Based on Equation (1), we can calculate v =0.99 m/s for both cases. For the droplet case, 2r t= =2.5 ms. Then we can get p droplet =3.6 Pa by Equation (2) and (3). For the feather case, v t =0.12 s based on Fig. R1, then we can get p feather =0.4 Pa. The low end detection limit was calculated by dividing the contact pressure of the feather by the signal-to-noise ratio in Fig. 4B. It could be observed that the maximum induced voltage by the feather
was ~0.3 V, and the noise level was below 10 mv. So the signal-to-noise ratio was estimated as 30 (or even more). Then we can calculate that the low end detection limit is 0.4/30 Pa=13 mpa. Figure S7 Voltage response of the feather induced pressure sensor device showing the time span during the falling process. The inset is a magnified voltage-time relationship. It could be clearly observed that the time difference between the two processes is 0.12 s. Video Video S1. A real-time live view of the pressure sensor used for monitoring the slight pressure generated by a droplet of distilled water, a piece of bird feather and even a small piece of paper.