Cross-Reference to Related Application

This application claims the benefit of Singapore Patent Application No. 10201906752T, entitled“Triboelectric Cells” and filed on July 22, 2019, which is expressly incorporated by reference herein in its entirety.

Field of the Invention

The present disclosure relates to a new type of mechanical energy harvester and a method thereof. Particularly, this disclosure relates to triboelectric cells, which is a new type of electric generator that could generate a direct current (DC) due to relative movement between two electrodes. More particularly, this disclosure relates to a semiconductor electrode in contact with another semiconductor (or metal) electrode such that relative movement between the two electrodes converts mechanical power to DC power.

Summary of the Prior Art

Mechanical power can be converted into electric power mainly through electromagnetic, electrostatic, piezoelectric and triboelectric processes. While triboelectrification is widely known, harvesting mechanical energy through a triboelectric process only began in 2012. Since then, study of triboelectrification and exploration of the potential applications have been greatly promoted. A typical triboelectric generator has two electrodes with distinct electron affinities (at least one of the electrodes is insulative). When the two electrodes are brought in contact, electrons transfer from the lower to the higher electron affinity electrode. The transferred charges are stuck on the insulative electrode surface and induce a displacement current in between the two electrode surfaces due to relative movements between the two electrodes. One commonly known triboelectric generator comprises two equal sized electrodes that are slid against each other, leading to a variation of the contacted surface area. In the contacted area, the transferred electrons attract the positive charges to the opposite contacted surface so that the electric field is only confined in between the two charged and contacted surfaces. By contrast, in the non-contacted surface area, the transferred electrons either induce positive charges to the backside conductive layer of the electrode (for an insulative electrode) or just simply discharge (for a conductive electrode), causing a current in the circuit connected to the two electrodes. When the sliding motion is periodically repeatable, the ratio of the contacted over non-contacted surface areas also periodically varies. This causes an alternating current (AC) in the circuit.

In terms of the efficiency of utilizing in non-interrupted electronic, signal processing and electricity storage, DC current is more favourable when compared to AC current. Hence, those skilled in the art are striving to provide DC electric generators.

Summary of the Invention

The above and other problems are solved and an advance in the state of the art is made by the triboelectric cells provided by embodiments in accordance with the disclosure. The first advantage of embodiments of the triboelectric cells in accordance with the disclosure is that the triboelectric cells are able to generate a DC power. The second advantage of embodiments of the triboelectric cells in accordance with the disclosure is that the triboelectric cells are able to convert kinetic energy into DC power for small scale devices. The third advantage of embodiments of the triboelectric cells in accordance with the disclosure is that the triboelectric cells make use of semiconductor materials which can be easily integrated into IC chips and other semiconductor devices.

A first aspect of the disclosure relates to a triboelectric cell. The triboelectric cell comprises a first electrode, a second electrode and a load. The first electrode comprises a first material having a first Fermi level and the second electrode comprises a second material having a second Fermi level that is different from the first Fermi level. The load electrically connects the first electrode to the second electrode. A contact surface of the first electrode is continuously in contact with a contact surface of the second electrode during operation of the triboelectric cell, and relative movement between the first electrode and the second electrode generates a direct current flow through the load.

In an embodiment of the first aspect of the disclosure, the second electrode may have a smaller dimension than the first electrode.

In an embodiment of the first aspect of the disclosure, the first electrode further comprises a first metal material arranged beneath the first material, the second electrode further comprises a second metal material arranged over the second material, and the load is electrically connecting the first metal material to the second metal material. In an embodiment of the first aspect of the disclosure, the first material comprises a semiconductor material of a first doping type, and the second material comprises a semiconductor material of a second doping type.

In an embodiment of the first aspect of the disclosure, the first electrode further comprises another semiconductor material of the second doping type arranged between the first material and the first metal material.

In an embodiment of the first aspect of the disclosure, the second electrode further comprises another semiconductor material of the first doping type arranged between the second material and the second metal material.

In an embodiment of the first aspect of the disclosure, the first electrode further comprises a first passivation layer arranged on the first material and a first interlayer arranged between the first material and the first metal material, wherein a surface of the first passivation layer facing away from the first material is configured as the contact surface of the first electrode.

In an embodiment of the first aspect of the disclosure, the second electrode further comprises a second passivation layer arranged on the second material and a second interlayer arranged between the second material and the second metal material, wherein a surface of the second passivation layer facing away from the second material is configured as the contact surface of the second electrode.

In an embodiment of the first aspect of the disclosure, the semiconductor material of the first doping type is an n-type semiconductor and the semiconductor material of the second doping type is a p-type semiconductor.

In an embodiment of the first aspect of the disclosure, the semiconductor material of the first doping type is a p-type semiconductor and the semiconductor material of the second doping type is an n-type semiconductor.

In an embodiment of the first aspect of the disclosure, the first material comprises a first metal material and the second material comprises a second metal material.

In an embodiment of the first aspect of the disclosure, the first electrode further comprises a first functional layer arranged on the first metal material and configured as the contact surface of the first electrode. In an embodiment of the first aspect of the disclosure, the second electrode further comprises a second functional layer arranged on the second metal material and configured as the contact surface of the second electrode.

In an embodiment of the first aspect of the disclosure, the first metal material has a lower work function than the second metal material.

In an embodiment of the first aspect of the disclosure, the first metal material has a higher work function than the second metal material.

In an embodiment of the first aspect of the disclosure, the triboelectric cell further comprises an additive provided between the first electrode and the second electrode.

In an embodiment of the first aspect of the disclosure, the triboelectric cell further comprises a plurality of the first electrodes, wherein the plurality of the first electrodes are adjacently connected to each other in series; and a plurality of the second electrodes, wherein the plurality of the second electrodes are adjacently connected to each other in series, wherein the load is electrically connecting the last first electrode of the plurality of the first electrodes to the last second electrode of the plurality of the second electrodes.

In an embodiment of the first aspect of the disclosure, the triboelectric cell further comprises a plurality of a pair of electrodes, wherein each pair of electrodes comprises the first electrode and the second electrode and the plurality of a pair of electrodes are serially connected such that the first electrode of a pair of electrodes is connected to the second electrode of the adjacent right pair of electrodes, the second electrode of the pair of electrodes is connected to the first electrode of the adjacent left pair of electrodes and the load is electrically connected between the second electrode of a first pair of electrodes and the first electrode of a last pair of electrodes.

A second aspect of the disclosure relates to a method for generating a direct current (DC) power based on the triboelectric cell according to any one or more of the embodiments of the first aspect of the disclosure comprising moving the second electrode against the first electrode such that the contact surface of the first electrode is continuously in contact with the contact surface of the second electrode.

Brief Description of the Drawings

The above advantages and features in accordance with this invention are described in the following detailed description and are shown in the following drawings: Figure 1A illustrates the schematic and the energy band diagrams for a triboelectric cell when a first electrode and a second electrode are apart from each other.

Figure IB illustrates the schematic and the energy band diagrams for a triboelectric cell when a first electrode and a second electrode are in contact with each other;

Figure 1C illustrates the schematic and the energy band diagrams for a triboelectric cell when a first electrode and a second electrode are moving relatively against each other;

Figure 2.1 illustrates a first embodiment;

Figure 2.2 illustrates the first embodiment with a second electrode moving to the right on a first electrode;

Figure 2.3 illustrates the first embodiment with the second electrode moving to the left on the first electrode;

Figure 2.4 illustrates the first embodiment with the second electrode rotating axially on the first electrode;

Figure 3 illustrates the first embodiment with the first and second electrodes having additional passivation layers and interlayers

Figure 4 illustrates a second embodiment;

Figure 5 illustrates the second embodiment with first and second electrodes having additional passivation layers and interlayers;

Figure 6 illustrates a third embodiment;

Figure 7 illustrates the third embodiment with first and second electrodes having additional passivation layers and interlayers;

Figure 8 illustrates a fourth embodiment;

Figure 9 illustrates the fourth embodiment with first and second electrodes having additional passivation layers and interlayers;

Figure 10 illustrates a fifth embodiment;

Figure 11 illustrates a sixth embodiment;

Figure 12 illustrates a seventh embodiment having a plurality of pairs of electrodes in parallel connection;

Figure 13 illustrates a seventh embodiment having a plurality of pairs of electrodes in series connection;

Figure 14 illustrates an eighth embodiment; Figure 15 illustrates a process for generating a DC power;

Figure 16a illustrates a schematic drawing of two electrodes and the load connection according to an experiment;

Figure 16b illustrates a graph of a sliding distance of a second electrode versus time according to the experiment of Figure 16a;

Figure 16c illustrates a graph of a sliding speed of the second electrode versus time according to the experiment of Figure 16a;

Figure 16d illustrates a graph of generated transient current versus time according to the experiment of Figure 16a;

Figure 16e illustrates a graph of collected charge from the transient current, or Q idt, versus time according to the experiment of Figure 16a;

Figure 17 illustrates average current and electric power dissipated to the load resistor as a function of load resistance R;

Figure 18 illustrates short circuit current Isc and open circuit voltage Voc generated under several constant sliding speeds with a weight of 100 g on top of a p-type electrode;

Figure 19a illustrates an experiment carried by rotating a second electrode on a first electrode;

Figure 19b illustrates a graph of Isc and Voc versus force according to the experiment of Figure 19a;

Figure 20 illustrates results generated from electrode pairs of n-type silicon and p-type Gallium Arsenide (GaAs);

Figure 21 illustrates results generated from electrode pairs of p-type silicon and n-type

GaAs;

Figure 22 illustrates results generated from electrode pairs of p-type silicon and aluminium (Al) plate;

Figure 23 illustrates results generated from electrode pairs of p-type silicon and gold (Au) plate;

Figure 24 illustrates results generated from electrode pairs of n-type silicon and Al plate; Figure 25 illustrates results generated from electrode pairs of n-type silicon and Au plate; and Figure 26 illustrates current generated under dry sliding and wet sliding of electrode pairs of oppositely doped silicon at a speed of 20 mm/s.

Detailed Description

The present disclosure relates to a new type of mechanical energy harvester and a method thereof. Particularly, this disclosure relates to triboelectric cells, which is a new type of electric generator that could generate a direct current (DC) due to relative movement between two electrodes. More particularly, this disclosure relates to a semiconductor electrode in contact with another semiconductor (or metal) electrode such that relative movement between the two electrodes converts mechanical power to DC power.

The triboelectric cells according to this disclosure can be used as electric current generators, mechanical energy harvesters, mechanical-electric interfaces, mechanical sensors, etc. The structures and working principle of the devices are fundamentally different from those of well reported electric generators where AC power is generated.

In this disclosure, a DC power is generated when one of the two electrodes is slid on the other. P-type semiconductors, n-type semiconductors and low and high work function metals in various combinations can be utilized as the electrode materials. Briefly, a triboelectric cell comprises a first electrode, a second electrode and a load. The first electrode comprises a first semiconductor material(s) or a first metal material. The second electrode comprises a second semiconductor material(s) or a second metal material. The first electrode has a different Fermi level compared to the second electrode. The two ends of the load are electrically connected to the first electrode and second electrode, respectively. Relative sliding of the second electrode on the first electrode while maintaining continuous contact causes a DC flow through the load. The working principle of the triboelectric cell will be described as follows.

For purposes of this disclosure, the semiconductors mean all types of semiconducting materials, including inorganic semiconductors and organic semiconductors, regardless of their dimensions and crystallinities. Inorganic semiconductors include silicon, III-V semiconductors, metal oxide semiconductors, etc.

For purposes of this disclosure, the metals mean all types of metallic materials. The low work function metals refer to metals having smaller work functions (while they are relatively stable in the ambient environment), typically less than 4.3 eV, such as aluminium (Al), titanium (Ti), silver (Ag), lead (Pb), etc. The high work function metals refer to metals having larger work functions, typically higher than 5.0 eV, such as gold (Au), platinum (Pt), nickel (Ni), etc.

For purposes of this disclosure, the semiconductors and metal materials can be in form of particles (diameter from sub-nanometre up to several millimetres), wires (diameter from several nanometres up to several millimetres), films (thickness from several nanometres up to several millimetres) and in bulk. They can be hard or flexible.

For purposes of this disclosure, passivation layers may be coated to the exterior surface of the semiconductor material of the first electrode or to the exterior surface of the semiconductor material of the second electrode. The passivation layers could reduce the density of surface states of the semiconductors, passivate the surfaces and promote electron transport across the contacted surfaces, etc. Thus, the passivation layers can be dielectric materials, like silicon dioxide (S1O2), silicon nitride (S13N4), aluminium oxide (AI2O3), hafnium dioxide (FlfCh), etc as well as thin metal coating and decoration, like tungsten (W), cobalt (Co), palladium (Pd), Ag, Pt, etc. The passivation layers may also be chemical modification layers where functional groups can be introduced to the surfaces of the semiconductor materials or metallic materials in favour of the performances of the devices described above.

For purposes of this disclosure, the interlayers may be introduced between a semiconductor and a metal material for the purpose of enhancing the electrical contact performance and reducing the electrical contact resistance. The interlayers can also be formed in between a semiconductor and a metal through physical or chemical processes, like thermal annealing, etc. Thus, the interlayers can be metal oxides, highly doped semiconductors, semimetals, etc.

For purposes of this disclosure, the functional layers coated to the exterior of the first metal electrodes or the exterior of the second metal electrodes can be dielectric thin films, including polymer thin films, like polystyrene, polypropylene, polyoxmethylene, etc.

For purposes of this disclosure, an additive may be introduced to the contacted surfaces of the two electrodes for the purposes of lubrication, air-gap filling, electrical contact improvement, heat dissipation, etc. The additive can be solid powders, like molybdenum disulfide (M0S2), graphene, etc. or liquids, like water, etc.

For purposes of this disclosure, the first electrode and second electrode may be provided between two substrates. The objective of adding the substrates is to support the two electrodes. For example, a first substrate may be provided on the exterior of the first electrode and the second substrate may be provided on the exterior of the second electrode. The substrates can be any forms of solid materials.

For purposes of this disclosure, an array of first electrodes may be provided on a first substrate (either a whole substrate or an array of separated substrates) and an array of second electrodes may be provided on a second substrate (either a whole substrate or an array of separated substrates). The array of first electrodes is aligned with the array of second electrodes so that the array of second electrodes can be placed on top of the array of first electrodes.

One skilled in the art will recognise that the listed materials here are not meant to be exhaustive and other materials may be implemented without departing from the disclosure.

Figures 1A, IB and 1C illustrate the schematics and energy band diagrams for a triboelectric cell with a p-type semiconductor as a first electrode 1 and an n-type semiconductor as a second electrode 2 when the first electrode 1 and the second electrode 2 are apart from each other, in contact with each other and moving relatively against each other. The arrows 100 pointing from the electrons to the holes stand for surface electric dipole moment. The arrows 200 pointing from the holes to the electrons stands for built-in field direction. Ec is the bottom of the conduction band, Ev the top of the valence band, E, the intrinsic level and E _F the Fermi level, V _M the built-in potential, yi, y2 are the potentials with respect to the Fermi levels in the p-type and n-type semiconductors.

Figure 1A shows the energy band diagrams when the first electrode 1 and the second electrode 2 are apart from each other. The electrostatic potential of the p-type neutral region with respect to the Fermi level E _F is determined by i _t º— l/q(E _L — E _F ) =—kT/q \n(N _A /ni), where q is the unit charge, N _A is the acceptor concentration, E, is the intrinsic Fermi level, k is the Boltzmann constant, T is the absolute temperature in Kelvin degree and m is the intrinsic carrier concentration. Correspondingly, the electrostatic potential of the n-type neutral region with respect to E _F is xp2 = kT/q In (iV _D / n ), where N _D is the donor concentration.

Figure IB shows the energy band diagrams when the first electrode 1 and the second electrode 2 are in contact with each other. In this contacted state, electrons can diffuse from the n- type semiconductor (second electrode 2) to the p-type semiconductor (first electrode 1 ) (and holes diffuse from the first electrode 1 to the second electrode 2) due to their chemical potential difference, leading to formation of a p-n junction across the two contacted surfaces. In other words, a p-n junction is formed so that positive space charges are in the depletion region in the n-type semiconductor (second electrode 2) and negative space charges are in the depletion region in the p-type semiconductor (first electrode 1). A built-in electric field (pointing from the n- to p-type semiconductor) is established in the p-n junction. Specifically, at the thermal equilibrium, a common flat Fermi level establishes across the two entire electrodes, leading to a built-in potential in the p-n junction and zero current flowing in the external circuit.

V _bi = 2 - i = kT/q In (N _A N _D /n )

Figure 1C shows the energy band diagrams when the first electrode 1 and the second electrode 2 are moving relatively against each other. Specifically, when the second electrode 2 slides on the first electrode 1, electrons and holes are excited at the frictional surfaces due to triboelectrification process. The electrons and holes are then swept out of the p-n junction, forming a DC current flowing along the direction of the built-in electric field, i.e., flowing from the first electrode 1 to the second electrode via the load R. As the current is driven by the built-in electric field, the current flow direction is always consistent with the direction of the built-in electric field, regardless of the relative sliding directions. Similarly, when two metal electrodes are in contact through one or two functional layers (coated onto the metal electrode contacting surfaces), an electric field pointing from the low to the high work function metal is established. Similarly, the building electric field also play a role of sweeping electrons (created by the frictional process) out of the contacted surfaces to form a DC current. The second electrode 2 has a smaller dimension than the first electrode to achieve non-change in the contact surface between the first and second electrodes. This means that the contact surface of the second electrode 2 is always or continuously in contact with the contact surface of the first electrode 1 and moves within a perimeter of the first electrode 1 during operation of the triboelectric cell.

Electrons and holes are generated due to the energy released by breaking of the bonds across the contacted surfaces in the friction sliding motion, and they can be expelled out of the p- n junction due to the built-in electric field, forming a DC current. Due to overall charge neutrality, the space charges in the p-n junction can be regarded as an effective dipole whose dipole moment points from the negatively charged space charges in the p-type electrode (first electrode 1 ) to the positively charged space charges in the n-type electrode (second electrode 2), perpendicular to the contacted surfaces under the thermal equilibrium as shown in figure IB. When the top electrode (second electrode 2) is being slid as shown in figure 1C, the space charge region in the bottom electrode (first electrode 1) would follow the sliding motion, but with a small lagged distance of d ~ nt as shown in figure 1C, where t is the dielectric relaxation time and v is the sliding speed. As a result, the effective dipole moment slightly deviates from that under the thermal equilibrium, causing a change in the potential energy of the dipole. This released energy from the newly formed dipole could excite electrons and holes in the lagged region with an area of wd ~ wv t.

DC generation has been clearly demonstrated by sliding a doped semiconductor electrode against another oppositely doped semiconductor or metal electrode as long as the two electrodes are of distinct Fermi levels. The generated DC current flows in the same direction as that of the built-in electric field in the dynamic p-n junction near the contacted surfaces and it does not depend on the sliding direction. It increases with the sliding speed and acceleration, decreases with increasing operational temperature and does not depend on atmospheric pressure. At a given speed, transient sliding could more efficiently generate the current than constant sliding. These findings suggest that electrons and holes are generated at the contacted surfaces and then swept out of the contacted p-n junction by the built-in electric field to form the DC power.

The triboelectric cells according to this disclosure comprise a first electrode, a second electrode and a load electrically connecting the first electrode to the second electrode. The first electrode comprises a first material having a first Fermi level while the second electrode comprises a second material having a second Fermi level that is different from the first Fermi level. A contact surface of the first electrode is continuously in contact with a contact surface of the second electrode and relative movement between the first electrode and the second electrode generates a direct current flow through the load. In one embodiment, the second electrode may have a smaller dimension than the first electrode so that the contact surface between the first and second electrodes during the relative movement may not change. Specifically, the contact surface of the second electrode 2 is always or continuously in contact with the contact surface of the first electrode 1 and moves within a perimeter of the first electrode 1 during operation of the triboelectric cell. One skilled in the art will recognised that other configurations can be implemented without departing from the invention. For example, the first electrode 1 may have a smaller dimension than the second electrode 2 so that during relative movement between the first electrode 1 and the second electrode 2, the contact surface of the second electrode 2 is always or continuously in contact with the contact surface of the first electrode 1. In another example, the first electrode 1 and the second electrode 2 may share the same dimension. For example, the contact surface of the first electrode 1 and the contact surface of the second electrode 2 may be circular in shape. When the contact surface of the second electrode 2 rotates axially on the contact surface of the first electrode 1 on a common axis, the contact surface of the second electrode 2 is always or continuously in contact with the contact surface of the first electrode 1.

The exact details of the two electrodes pairing will now be described as follows.

First embodiment

According to a first embodiment of this disclosure, and with reference to figure 2.1, the triboelectric cell comprises a first electrode 1 , a second electrode 2 and a load R.

In this embodiment, the first electrode 1 comprises a first metal material 103 arranged beneath the first material 101 and the second electrode 2 comprises a second metal material 104 arranged over the second material 102. In other words, the first metal material 103 is coated on the first material 101 and the second metal material 104 is coated on the second material 102 such that the first material 101 and the second material 102 are sandwiched between the first metal material

103 and the second metal material 104.

In one embodiment, a contact surface of the first material 101 acts as the contact surface of the first electrode 1 and a contact surface of the second material 102 acts as the contact surface of the second electrode 2. The contact surface of the first electrode 1 is in contact with the contact surface of the second electrode 2 during operation of the triboelectric cell. Specifically, the contact surface of the first material 101 of the first electrode 1 is distal from the first metal material 103 and the contact surface of the second material 102 of the second electrode 2 is distal from the second metal material 104. In more detail, the contact surface of the first material 101 of the first electrode 1 is the first surface of the first material 101 and the first metal material 103 is coated on the second surface of the first material 101. The contact surface of the second material 102 of the second electrode 2 is the first surface of the second material 102 and the second metal material

104 is coated on the second surface of the second material 102.

The two ends of the electric load R are connected to the first metal 103 and the second metal 104, respectively, to form the external circuit 105.

In this embodiment, the first material 101 comprises a semiconductor material of a first doping type, and the second material 102 comprises a semiconductor material of a second doping type. Different configurations may be provided as follows:

1. The semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material; and 2. The semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material.

Essentially, the first material 101 has a different Fermi level compared to the second material 102.

We now refer to figures 2.2-2.4 where the semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material (i.e. the first material 101 comprises the n-type semiconductor material and the second material 102 comprises the p-type semiconductor material). When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, positive space charges are in the depletion region 106 in the first material 101 and negative space charges are in the depletion region 107 in the second material 102. A built-in electric field (pointing from the n-type to p-type semiconductor, i.e. from the first material 101 to the second material 102) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 105. When the contact surface of the second electrode 2 (in this case, the contact surface of the p- type semiconductor material) is slid against the contact surface of the first electrode 1 (in this case the contact surface of the n-type semiconductor material) towards the right as shown in figure 2.2, slid against the contact surface of the first electrode 1 towards the left as shown in figure 2.3 or is rotated on the contact surface of the first electrode 1 as shown in figure 2.4, electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 105, forming a DC current flowing (as indicated by arrows in figures 2.2 to 2.4) along the direction of the built-in electric field. Specifically, a DC current flows from the second material 102 to the second metal 104 to the load R to the first metal 103 and then to the first material 101.

In the embodiment where the semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material (i.e. the first material 101 comprises the p-type semiconductor material and the second material 102 comprises the n-type semiconductor material), a DC current generated flows from the first material 101 to the second material 102. Specifically, when the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, negative space charges are in the depletion region in the first material 101 and positive space charges are in the depletion region in the second material 102. A built-in electric field (pointing from the n-type to p-type semiconductor, i.e. from the second material 102 to the first material 101) is established. When there is no any relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 105. When the contact surface of the second electrode 2 (in this case, the contact surface of the n-type semiconductor material) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the contact surface of the p-type semiconductor material), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 105, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the first material 101 to the first metal 103 to the load R to the second metal 104 and then to the second material 102.

In the first embodiment and referring to figure 3, the first electrode 1 may further comprise a first passivation layer 117 arranged on the first material and a first interlayer 125 arranged between the first material 101 and the first metal material 103. In this configuration, a surface of the first passivation layer 117 facing away from the first material 101 is configured as the contact surface of the first electrode 1. Specifically, the first passivation layer 117 is provided on a surface of the first material 101 distal from the first metal material 103 and acts as the contact surface of the first electrode 1.

In this embodiment, the second electrode 2 may further comprise a second passivation layer 118 arranged on the second material 102 and a second interlayer 124 arranged between the second material 102 and the second metal material 104. In this configuration, a surface of the second passivation layer 118 facing away from the second material 102 is configured as the contact surface of the second electrode 2. Specifically, the second passivation layer 118 is provided on a surface of the second material 102 distal from the second metal material 104 and acts as the contact surface of the second electrode 2.

Second embodiment

According to a second embodiment of this disclosure, and with reference to figure 4, the triboelectric cell comprises a first electrode 1, a second electrode 2 and a load R. The second embodiment may be considered as an extension of the first embodiment where the first electrode 1 further comprises another semiconductor material 208A arranged between the first material 201A and the first metal material 203A. The doping type of the semiconductor material 208A is different from the doping type of the first material 201 A. Instead, the semiconductor material 208A shares the same doping type as the second material 202A.

In this embodiment, the first electrode 1 comprises a first material 201 A, a first metal material 203 A and another semiconductor material 208 A arranged between the first material 201 A and the first metal material 203A. The second electrode 2 comprises a second metal material 204A arranged over the second material 202A. In other words, the first metal material 203A is coated on the semiconductor material 208A and the second metal material 204A is coated on the second material 202A such that the first material 201 A together with the semiconductor material 208 A and the second material 202A are sandwiched between the first metal material 203A and the second metal material 204A when the first electrode 1 is in contact with the second electrode 2.

In one embodiment, a contact surface of the first material 201 A acts as the contact surface of the first electrode 1 and a contact surface of the second material 202A acts as the contact surface of the second electrode 2. The contact surface of the first electrode 1 is in contact with the contact surface of the second electrode 2 during operation of the triboelectric cell. Specifically, the contact surface of the first material 201 A of the first electrode 1 is distal from the first metal material 203 A and the contact surface of the second material 202A of the second electrode 2 is distal from the second metal material 204A. In more detail, the contact surface of the first material 201 A of the first electrode 1 is the first surface of the first material 201 A and the semiconductor material 208 A is arranged on the second surface of the first material 201 A. The contact surface of the second material 202A of the second electrode 2 is the first surface of the second material 202A and the second metal material 204A is coated on the second surface of the second material 202A.

The two ends of the electric load R are connected to the first metal 203A and the second metal 204 A, respectively, to form the external circuit 205.

In this embodiment, the first material 201 A comprises a semiconductor material of a first doping type, and the second material 202A comprises a semiconductor material of a second doping type. As mentioned above, the semiconductor material 208A shares the same doping type as the second material 202A. Hence, the semiconductor material 208A is also a semiconductor material of the second doping type. Different configurations may be provided as follows:

1. The semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material; and 2. The semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material.

Essentially, the first material 201 A has a different Fermi level compared to the second material 202A.

In the configuration where the semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material (i.e. the first material 201 A comprises the n-type semiconductor material and the second material 202A and the semiconductor material 208A comprise the p-type semiconductor material), a p-n junction is formed across the contacted surfaces of the first material 201 A and second material 202A. Another p-n junction is formed between the first material and the semiconductor material 208A in the first electrode 1. This p-n junction may enhance electron transport.

When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, positive space charges are in the depletion region in the first material (or n-type semiconductor) 201 A and negative space charges are in the depletion region in the second material (or p-type semiconductor) 202A. A built-in electric field (pointing from the n-type to p-type semiconductor, i.e. from the first material 201 A to the second material 202A) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 205. When the contact surface of the second electrode 2 (in this case, the contact surface of the p-type semiconductor material) is slid against or is rotated on the contact surface of the first electrode 1 (in this case, the contact surface of the n-type semiconductor material), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 205, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the second material 202A of the second electrode 2 to the load R and to the first material 201 A of the first electrode 1.

In the configuration where the semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material (i.e. the first material 201 A comprises the p-type semiconductor material and the second material 202A and the semiconductor material 208A comprise the n-type semiconductor material), a p-n junction is formed across the contacted surfaces of the first material 201 A and second material 202A. Another p-n junction is formed between the first material and the semiconductor material 208A in the first electrode 1. This p-n junction may enhance electron transport.

When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, negative space charges are in the depletion region in the first material 201 A and positive space charges are in the depletion region in the second material 202A. A built- in electric field (pointing from the n-type to p-type semiconductor, i.e. from the second material 202 A to the first material 201 A) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 205. When the contact surface of the second electrode 2 (in this case, the contact surface of the n-type semiconductor material) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the contact surface of the p-type semiconductor material), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 205, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the first material 201 A of the first electrode 1 to the load R and to the second material 202A of the second electrode 2.

In the second embodiment and referring to figure 5, the first electrode 1 may further comprise a first passivation layer 217 A arranged on the first material 201 A and a first interlayer 225A arranged between the semiconductor material 208A and the first metal material 203A. In this configuration, a surface of the first passivation layer 217A facing away from the first material 201 A is configured as the contact surface of the first electrode 1. Specifically, the first passivation layer 217A is provided on a surface of the first material 201 A distal from the first metal material 203 A and acts as the contact surface of the first electrode 1.

In this embodiment, the second electrode 2 may further comprise a second passivation layer 218A arranged on the second material 202A and a second interlayer 224 A arranged between the second material 202A and the second metal material 204A. In this configuration, a surface of the second passivation layer 218 A facing away from the second material 202 A is configured as the contact surface of the second electrode 2. Specifically, the second passivation layer 218 A is provided on a surface of the second material 202A distal from the second metal material 204A and acts as the contact surface of the second electrode 2.

Third embodiment According to a third embodiment of this disclosure, and with reference to figure 6, the triboelectric cell comprises a first electrode 1, a second electrode 2 and a load R. Essentially, the third embodiment may be considered as the reverse configuration of the second embodiment where the first electrode 1 and the second electrode 2 are interchanged. Specifically, instead of the first electrode 1 , the second electrode 2 now comprises another semiconductor material 208C arranged between the second material 202C and the second metal material 204C. The doping type of the semiconductor material 208C is different from the doping type of the second material 202C. Specifically, the semiconductor material 208C shares the same doping type as the first material 201C.

In this embodiment, the second electrode 2 comprises a second material 202C, a second metal material 204C and another semiconductor material 208C arranged between the second material 202C and the second metal material 204C. The first electrode 1 comprises a first metal material 203C arranged beneath the first material 201C. In other words, the second metal material 204C is coated on the semiconductor material 208C and the first metal material 203C is coated on the first material 201C such that the second material 202C together with the semiconductor material 208C and the first material 201C are sandwiched between the first metal material 203C and the second metal material 204C when the first electrode 1 is in contact with the second electrode 2.

In one embodiment, a contact surface of the first material 201C acts as the contact surface of the first electrode 1 and a contact surface of the second material 202C acts as the contact surface of the second electrode 2. The contact surface of the first electrode 1 is in contact with the contact surface of the second electrode 2 during operation of the triboelectric cell. Specifically, the contact surface of the first material 201C of the first electrode 1 is distal from the first metal material 203C and the contact surface of the second material 202C of the second electrode 2 is distal from the second metal material 204C. In more detail, the contact surface of the first material 201C of the first electrode 1 is the first surface of the first material 201C and the first metal material 203C is arranged on the second surface of the first material 201C. The contact surface of the second material 202C of the second electrode 2 is the first surface of the second material 202C and the semiconductor material 208C is coated on the second surface of the second material 202C.

The two ends of the electric load R are connected to the first metal 203C and the second metal 204C, respectively, to form the external circuit 205. In this embodiment, the first material 201C comprises a semiconductor material of a first doping type, and the second material 202C comprises a semiconductor material of a second doping type. As mentioned above, the semiconductor material 208C shares the same doping type as the first material 201C. Hence, the semiconductor material 208C is also a semiconductor material of the first doping type. Different configurations may be provided as follows:

1. The semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material; and

2. The semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material.

Essentially, the first material 201C has a different Fermi level compared to the second material 202C.

In the configuration where the semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material (i.e. the first material 201C and the semiconductor material 208C comprise the n-type semiconductor material and the second material 202C comprises the p-type semiconductor material), a p-n junction is formed across the contacted surfaces of the first material 201C and the second material 202C. Another p-n junction is formed between the second material 202C and the semiconductor material 208C in the second electrode 2. This p-n junction may enhance electron transport.

When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, positive space charges are in the depletion region in the first material 201C and negative space charges are in the depletion region in the second material 202C. A built- in electric field (pointing from the n-type to p-type semiconductor, i.e. from the first material 201C to the second material 202C) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 205. When the contact surface of the second electrode 2 (in this case, the contact surface of the p-type semiconductor material) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the contact surface of the n-type semiconductor material), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 205, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the second material 202C of the second electrode 2 to the load R and to the first material 201C of the first electrode 1.

In the configuration where the semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material (i.e. the first material 201C and the semiconductor material 208C comprise the p-type semiconductor material and the second material 202C comprises the n-type semiconductor material), a p-n junction is formed across the contacted surfaces of the first material 201C and the second material 202C. Another p-n junction is formed between the second material and the semiconductor material 208C in the second electrode 2. This p-n junction may enhance electron transport.

When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, negative space charges are in the depletion region in the first material 201C and positive space charges are in the depletion region in the second material 202C. A built- in electric field (pointing from the n-type to p-type semiconductor, i.e. from the second material 202C to the first material 201C) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 205. When the contact surface of the second electrode 2 (in this case, the contact surface of the n-type semiconductor material) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the contact surface of the p-type semiconductor material), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 205, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the first material 201C of the first electrode 1 to the load R and to the second material 202C of the second electrode 2.

In the third embodiment and referring to figure 7, the first electrode 1 may further comprise a first passivation layer 217C arranged on the first material 201C and a first interlayer 225C arranged between the first material 201C and the first metal material 203C. In this configuration, a surface of the first passivation layer 217C facing away from the first material 201C is configured as the contact surface of the first electrode 1. Specifically, the first passivation layer 217C is provided on a surface of the first material 201C distal from the first metal material 203C and acts as the contact surface of the first electrode 1. In this embodiment, the second electrode 2 may further comprise a second passivation layer 218C arranged on the second material 202C and a second interlayer 224C arranged between the semiconductor material 208C and the second metal material 204C. In this configuration, a surface of the second passivation layer 218C facing away from the second material 202C is configured as the contact surface of the second electrode 2. Specifically, the second passivation layer 218C is provided on a surface of the second material 202C distal from the second metal material 204C and acts as the contact surface of the second electrode 2.

Fourth embodiment

According to a fourth embodiment of this disclosure, and with reference to figure 8, the triboelectric cell comprises a first electrode 1, a second electrode 2 and a load R. Essentially, the fourth embodiment may be considered as an extension of the first embodiment where the first electrode 1 further comprises another semiconductor material 308A arranged between the first material 301 A and the first metal material 303A and the second electrode 2 further comprises another semiconductor material 311A arranged between the second material 302 A and the second metal material 304A. The doping type of the semiconductor material 308A is different from the doping type of the first material 301A. Specifically, the semiconductor material 308A shares the same doping type as the second material 302A. Further, the doping type of the semiconductor material 311A is different from the doping type of the second material 302A. Specifically, the semiconductor material 311A shares the same doping type as the first material 301 A.

In this embodiment, the first electrode 1 comprises a first material 301 A, a first metal material 303A and another semiconductor material 308A arranged between the first material 301A and the first metal material 303 A. The second electrode 2 comprises a second material 302A, a second metal material 304A and another semiconductor material 311A arranged between the second material 302A and the second metal material 304A.

In other words, the first metal material 303A is coated on the semiconductor material 308A and the second metal material 304A is coated on the semiconductor material 311A such that the first material 301 A together with the semiconductor material 308 A and the second material 302A together with the semiconductor material 311 A are sandwiched between the first metal material 303A and the second metal material 304A when the first electrode 1 is in contact with the second electrode 2. A contact surface of the first material 301A acts as the contact surface of the first electrode 1 and a contact surface of the second material 302A acts as the contact surface of the second electrode 2. The contact surface of the first electrode 1 is in contact with the contact surface of the second electrode 2 during operation of the triboelectric cell. Specifically, the contact surface of the first material 301A of the first electrode 1 is distal from the first metal material 303A and the contact surface of the second material 302A of the second electrode 2 is distal from the second metal material 304A. In more detail, the contact surface of the first material 301 A of the first electrode 1 is the first surface of the first material 301 A and the semiconductor material 308 A is coated on the second surface of the first material 301 A. The contact surface of the second material 302A of the second electrode 2 is the first surface of the second material 302A and the semiconductor material 311A is coated on the second surface of the second material 302 A.

The two ends of the electric load R are connected to the first metal 303A and the second metal 304 A, respectively, to form the external circuit 305.

In this embodiment, the first material 301 A comprises a semiconductor material of a first doping type, and the second material 302A comprises a semiconductor material of a second doping type. As mentioned above, the semiconductor material 308A shares the same doping type as the second material 302 A while the semiconductor material 311A shares the same doping type as the first material 301 A. Hence, the semiconductor material 308A is also a semiconductor material of the second doping type while the semiconductor material 311A is also a semiconductor material of the first doping type. Different configurations may be provided as follows:

1. The semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material; and

2. The semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material.

Essentially, the first material 301 A has a different Fermi level compared to the second material 302A.

In the configuration where the semiconductor material of the first doping type is an n-type semiconductor material and the semiconductor material of the second doping type is a p-type semiconductor material (i.e. the first material 301 A and the semiconductor material 311A comprise the n-type semiconductor material and the second material 302A and the semiconductor material 308A comprise the p-type semiconductor material), a p-n junction is formed across the contacted surfaces of the first material 301 A and the second material 302A. Another p-n junction is formed between the first material 301 A and the semiconductor material 308A in the first electrode 1. This p-n junction may enhance electron transport. Yet another p-n junction is formed between the second material 302A and the semiconductor material 311A in the second electrode 2. This p-n junction may enhance electron transport.

When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, positive space charges are in the depletion region in the first material 301A and negative space charges are in the depletion region in the second material 302A. A built- in electric field (pointing from the n-type to p-type semiconductor, i.e. from the first material 301A to the second material 302A) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 305. When the contact surface of the second electrode 2 (in this case, the contact surface of the p-type semiconductor material) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the contact surface of the n-type semiconductor material), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 305, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the second material 302A of the second electrode 2 to the load R and to the first material 301 A of the first electrode 1.

In the configuration where the semiconductor material of the first doping type is a p-type semiconductor material and the semiconductor material of the second doping type is an n-type semiconductor material (i.e. the first material 301 A and the semiconductor material 311A comprise the p-type semiconductor material and the second material 302A and the semiconductor material 308A comprise the n-type semiconductor material), a p-n junction is formed across the contacted surfaces of the first material 301 A and the second material 302A. Another p-n junction is formed between the first material 301 A and the semiconductor material 308A in the first electrode 1. This p-n junction may enhance electron transport. Yet another p-n junction is formed between the second material 302A and the semiconductor material 311A in the second electrode 2. This p-n junction may enhance electron transport.

When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact, negative space charges are in the depletion region in the first material 301 A and positive space charges are in the depletion region in the second material 302A. A built- in electric field (pointing from the n-type to p-type semiconductor, i.e. from the second material 302A to the first material 301A) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 305. When the contact surface of the second electrode 2 (in this case, the contact surface of the n-type semiconductor material) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the contact surface of the p-type semiconductor material), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 305, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the first material 301A of the first electrode 1 to the load R and to the second material 302A of the second electrode 2.

In the third embodiment and referring to figure 9, the first electrode 1 may further comprise a first passivation layer 317A arranged on the first material 301 A and a first interlayer 325A arranged between the semiconductor material 308A and the first metal material 303A. In this configuration, a surface of the first passivation layer 317A facing away from the first material 301 A is configured as the contact surface of the first electrode 1. Specifically, the first passivation layer 317A is provided on a surface of the first material 301 A distal from the first metal material 303 A and acts as the contact surface of the first electrode 1.

In this embodiment, the second electrode 2 may further comprise a second passivation layer 318A arranged on the second material 302A and a second interlayer 324A arranged between the semiconductor material 311 A and the second metal material 304A. In this configuration, a surface of the second passivation layer 318A facing away from the second material 302A is configured as the contact surface of the second electrode 2. Specifically, the second passivation layer 318A is provided on a surface of the second material 302A distal from the second metal material 304A and acts as the contact surface of the second electrode 2.

Fifth embodiment

According to a fifth embodiment of this disclosure and with reference to figure 10, the triboelectric cell comprises a first electrode 1 , a second electrode 2 and a load R.

In this embodiment, the first electrode 1 comprises a first metal material 401 A. The first electrode 1 may further comprise a first functional layer 419 A arranged on the first metal material 410A and configured as the contact surface of the first electrode 1. The inner surface of the first metal material 401 A may be coated with the first functional layer 419A. The second electrode 2 comprises a second metal material 402A. The second electrode 2 may further comprise a second functional layer 420A arranged on the second metal material 402A and configured as the contact surface of the second electrode 2. The inner surface of the second metal material 402A may be coated with the second functional layer 420A.

The first metal material 401 A and the second metal material 402 A are different work function metals, i.e. have different work functions. Essentially, the first metal material 401A has a different Fermi level compared to the second metal material 402A. Different configurations may be provided as follows:

1. The first metal material 401 A is a high work function metal and the second metal material 402A is a low work function metal; and

2. The first metal material 401 A is a low work function metal and the second metal material 402A is a high work function metal.

The two ends of the electric load R are connected to the first metal material 401 A and the second metal material 402 A, respectively, to form the external circuit 405.

In this embodiment, one of the two functional layers 419A and 420 A is provided so that the contact surfaces of the first metal material 401 A and the second metal material 402A are in contact through at least one of the functional layers 419A and 420A or both. In one configuration, the first electrode 1 comprises the first metal material 401 A, and the second electrode 2 comprises the second metal material 402A and the second functional layer 420A arranged on the second metal material 402 A. The first metal material 401 A is configured as or provides the contact surface of the first electrode 1 , and a surface of the second functional layer 420A facing away from the second metal material 402 A is configured as the contact surface of the second electrode 2. In another configuration, the first electrode 1 comprises the first metal material 401 A and the first functional layer 419 A arranged on the first metal material 401 A, and the second electrode 2 comprises the second metal material 402 A. A surface of the first functional layer 419A facing away from the first metal material 401 A is configured as the contact surface of the second electrode 1, and the second metal material 402A is configured as or provides the contact surface of the second electrode 2. In a further configuration, the first electrode 1 comprises the first metal material 401 A and the first functional layer 419A arranged on the first metal material 401 A, and the second electrode 2 comprises the second metal material 402A and the second functional layer 420 A arranged on the second metal material 402A. A surface of the first functional layer 419A facing away from the first metal material 401 A is configured as the contact surface of the second electrode 1 , and a surface of the second functional layer 420A facing away from the second metal material 402A is configured as the contact surface of the second electrode 2.

The two ends of an electric load R are connected to the first metal material 401 A and the second metal material 402 A, respectively, to form the external circuit 405.

We now refer to the configuration where the first metal material 401 A is a high work function metal and the second metal material 402A is a low work function metal. When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact and moving relative to each other, a built-in electric field (pointing from the second metal material (low work function metal) 402 A to first metal material (high work function metal) 401 A) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 405. When the contact surface of the second electrode 2 (in this case, the inner surface of the second metal material 402A or the second functional layer 420A) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the inner surface of the first metal material 401 A or the first functional layer 419A), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 405, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the first metal material 401 A of the first electrode 1 to the load R and to the second metal material 402 A of the second electrode 2.

We now refer to the configuration where the first metal material 401 A is a low work function metal and the second metal material 402A is a high work function metal. When the contact surface of the first electrode 1 and the contact surface of the second electrode 2 are in contact and moving relative to each other, a built-in electric field (pointing from the first metal material (low work function metal) 401 A to second metal material (high work function metal) 402A) is established. When there is no relative motion between the two electrodes, the contacted electrodes are in thermal equilibrium and no current flows through the external circuit 405. When the contact surface of the second electrode 2 (in this case, the inner surface of the second metal material 402A or the second functional layer 420A) is slid against or is rotated on the contact surface of the first electrode 1 (in this case the inner surface of the first metal material 401 A or the first functional layer 419A), electrons and holes created due to triboelectrification are then swept out of the contacted surfaces through the external circuit 405, forming a DC current flowing along the direction of the built-in electric field. Specifically, a DC current flows from the second metal material 402 A of the second electrode 2 to the load R and to the first metal material 401 A of the first electrode 1.

Sixth embodiment

According to a sixth embodiment of this disclosure and with reference to figure 11 , the triboelectric cell comprises a first electrode 1 , a second electrode 2, a first substrate 511 , a second substrate 512 and a load R.

The first electrode 1 and the second electrode 2 pairing is in accordance with any of the above embodiments.

The first substrate 511 is provided on the first electrode 1. The first substrate 511 is provided on a surface of the first electrode distal or facing away from the contact surface of the first electrode 1 (“exterior or outer surface of the first electrode 1”). The purpose of the first substrate 511 is for supporting the first electrode 1.

The second substrate 512 is provided on the second electrode 2. The second substrate 512 is provided on a surface of the first electrode distal or facing away from the contact surface of the second electrode 2 (“exterior or outer surface of the second electrode 2”). The purpose of the second substrate 512 is for supporting the second electrode 2.

The two ends of an electric load R are connected to the first metal material (not shown) of the first electrode 1 and the second metal material (not shown) of the second electrode 2, respectively, to form the external circuit 505. The second electrode 2 and the second substrate 512 may have a smaller dimension than the first electrode 1 and the first substrate 511 to achieve non change in the contact surface between the first and second electrodes.

Seventh embodiment

According to a seventh embodiment of this disclosure and with reference to figure 12, the triboelectric cell comprises a number of pairs of electrodes, a first substrate 511, a second substrate 512 and a load R.

Each pair of electrodes comprises a first electrode 1 and a second electrode 2. The first electrode 1 and the second electrode 2 pairing is in accordance with any of the above embodiments.

The first substrate 511 is provided on the exterior surface of all the first electrodes 1. The purpose of the first substrate 511 is for supporting the first electrodes 1. While figure 12 shows a first substrate 511 for each first electrode 1 , one skilled in the art will recognise that a single first substrate may be provided on the outer surface of all the first electrodes 1 instead where the first electrodes 1 are arranged apart from each other.

The second substrate 512 is provided on the outer surface of all the second electrode 2. The purpose of the second substrate 512 is for supporting the second electrode 2. Similarly, while figure 12 shows a second substrate 512 for each second electrode 2, one skilled in the art will recognise that a single second substrate 512 may be provided on the outer surface of all the second electrodes 2 instead where the second electrodes 2 are arranged apart from each other.

In this embodiment, each pair of electrodes can be electrically connected in parallel or in series. Figure 12 shows the parallel connection where all first electrodes are adjacently connected to each other in series and all second electrodes are adjacently connected to each other in series. The load is then electrically connecting the last of the first electrode to the last of the second electrode to form a parallel connection.

Figure 13 shows the series connection of multiple pairs of electrodes, where each pair of electrodes comprises the first electrode and the second electrode. The multiple pairs of electrodes are serially connected such that the first electrode of a pair of electrodes is connected to the second electrode of the adjacent right pair of electrodes, the second electrode of the pair of electrodes is connected to the first electrode of the adjacent left pair of electrodes and the load is electrically connected between the second electrode of a first pair of electrodes and the first electrode of a last pair of electrode. As shown in figure 13, the first electrode 1 of the first (1st) pair of electrodes is connected to the second electrode 2 of the second (2nd) pair of electrodes, the first electrode 1 of the second (2nd) pair of electrodes is connected to the second electrode 2 of the third pair of electrodes (3rd) and so on. The first electrode 1 of the last (n-th) pair of electrodes is connected to one end of the load R of the external circuit 505C and the second electrode 2 of the first (1st) pair of electrodes is connected to the other end of the load R. Alternatively, the second electrode 2 of the first (1st) pair of electrodes is connected to the first electrode 1 of the second (2nd) pair of electrodes, the second electrode 2 of the second (2nd) pair of electrodes is connected to the first electrode 1 of the third (3rd) pair of electrodes and so on. The second electrode 2 of the last (n-th) pair of electrodes is connected to one end of the load R of the external circuit 505C and the first (1st) electrode 1 of the first pair of electrodes is connected to the other end of the load R.

Eighth embodiment According to an eighth embodiment of this disclosure and with reference to figure 14, the triboelectric cell comprises a first electrode 1, a second electrode 2, an additive 615 and a load R.

The first electrode 1 and the second electrode 2 pairing is in accordance with any of the above embodiments.

The additive 615 is introduced between the first electrode 1 and the second electrode 2. This means that the additive 615 is provided between the two contacted surfaces of the first electrode 1 and second electrode 2. The two ends of an electric load R are connected to the first electrode 1 and the second electrode 2 to form the external circuit 605. The additive comprises deionized (DI) water or water with a small concentration of additives. The additive can also comprise solid powders including but not limited to M0S2 and graphene.

A method for generating a DC power based on any of the above embodiments can be described in the following manner and with reference to figure 15. The process 1500 begins with step 1505 by providing the first electrode 1 and the second electrode 2 according to any of the above embodiments.

In step 1515, process 1500 connects the first electrode 1 and the second electrode 2 to the external circuit 105 with a load R. In this step, a weight may be provided on the second electrode 2 so that the contact surface of the second electrode 2 maintains continuous contact with the contact surface of the first electrode 1.

In step 1520, process 1500 moves the second electrode 2 against the first electrode 1 such that the contact surface of the first electrode 1 is continuously in contact with the contact surface of the second electrode 2 to generate a DC flow between the first electrode 1 and the second electrode 2.

Experimental Findings

The main experimental results together with supporting information have been published in Ran Xu, Qing Zhang, Jing Yuan Wang, Di Liu, Jie Wang, Zhong Lin Wang, “Direct current triboelectric cell by sliding an n-type semiconductor on a p-type semiconductor” , Nano Energy 66 (2019) 104185. Some selected experimental results are shown below for a better understanding of the devices.

For example, the first electrode 1 was a 4 inch highly doped n-type silicon wafer with a metal pad on its bottom end. The second electrode 2 was a 1 x 1 cm ² lowly doped p-type silicon wafer with a metal pad on its top end. The second electrode 2 was placed on the first electrode 1 so that the two silicon surfaces were facing and in contact with each other. A small weight was sometimes placed on the second electrode 2 to make a better contact between the first and second electrodes. The first electrode 1 was fixed onto a stationary stage. The second electrode 2 was driven by a motional stage whose motion direction, speed and acceleration could be well controlled.

Figure 16(a) shows the schematic drawing of two electrodes and the load connection. For the results shown in figures 16(d) and 16(e), the resistance of the load R is taken to be 0 ohm. In other words, the first electrode 1 was directly connected to the second electrode 2 through an ammeter. A weight of lOOg was placed on top of the second electrode 2. Figure 16(b) shows the sliding distance of the second electrode 2 measured against time. The second electrode 2 was slid forward (up to 4 cm) and then backward periodically on the first electrode 1. Figure 16(c) shows the sliding speed of the second electrode 2 measured against time. The speed was controlled at 50 mm/s for both forward and backward sliding. Figure 16(d) shows the generated transient current measured against time. Once the second electrode 2 was slid against the first electrode 1, the DC current is generated. As shown in figure 16(d), the current flow direction does not depend on the sliding direction. Figure 16(e) shows the collected charge from the transient current, or Q = J r _Q t idt, measured against time.

Figure 17 shows the average current and the electric power dissipated to the load resistor as a function of the load resistance R. A weight of lOOg was placed on top of the second electrode 2. The speed was kept at 20 mm/s for both forward and backward sliding. The second electrode 2 was slid forward (up to 4 cm) and then backward periodically. The average current I maintained unchanged till R is about 10 ³ ohm. Further increasing R leads to a significant decrease in the current. In contrast, the converted electric power P=Ri ² dissipated to the load resistor R showed a peak at R~10 Mohm.

The sliding speed v is directly related to the friction power as P = /r _s v, where F is the normal force and /./ , refers to the sliding friction coefficient which is independent of the apparent contact area and sliding velocity in dry friction conditions. With the experiment conditions in this work, m is estimated to be 0.2. Thus, the number of electron-hole pairs is expected to be linearly proportional to the friction power or the sliding speed in the triboelectrification process.

Figures 18A and 18B show short circuit current Isc and open circuit voltage Voc under several constant sliding speeds with a weight of lOOg on top of the p-type electrode (second electrode 2). Isc increased by 10 times and Voc was more than tripled for constant speed increasing from 10 mm/s to 200 mm/s, as shown in Figure 18(B). A higher sliding speed contributes a higher friction power, resulting in more electron and hole excitation at the contact surfaces, which are subsequently separated by the built-in electric field in the p-n junction, generating a larger DC current. It is worth noting that for the speed higher than 100 mm/s, Voc saturated at about 0.31V, which corresponded to the chemical potential difference of 0.35 V between the two electrodes. Figure 18C shows the average current and the electric power dissipated to the load resistor as a function of the load resistance R, which shows similar results as Figure 17.

The influences of transient sliding speeds on Voc and Isc were studied using several accelerations from 0.05m/s ² to lm/s ² (See figure 18D). The p-type electrode with a weight of lOOg on top was accelerated from the rest stage for a distance of 8cm. It is seen from figure 18(E) that the value of Isc at 1 m/s ² increased from about 0.25mA for acceleration of 0 mm/s (the constant speeds as discussed above) to 0.58 mA under 100 mm/s. This suggests that the acceleration process could more efficiently excite electrons and holes at the contacted surfaces than the constant speed process. This reflects that the mechanical power could be dissipated to the surfaces more easily, efficiently generating electrons and holes, in the former process. By contrast, the mechanical power dissipated to the surface regions may form a temperature profile through which heat could transport into the bulk of the doped semiconductor electrodes, leading to weak electron-hole generation in the latter process. From figure 18F, Voc was found to be saturated around 0.3 V at a lower transient speed under smaller accelerations. Under an acceleration of 1 m/s ² , Voc did not show apparent saturation till the speed was increased up to 120mm/s.

In addition to straight back-and-forth sliding, rotation sliding was also carried out as shown in figure 19. A 4-inch lightly boron doped p-type Si wafer 2820 (resistivity p~5 W-cm) (second electrode) was spun on top of another 4-inch heavily phosphorus doped n-type Si wafer 2810 (resistivity p~5x l0 ³ W-cm) (first electrode) with a common central axis. The contact force was exerted onto the top p-type wafer 2820 and measured using a force gauge installed underneath the n-type silicon wafer 2810, as shown in figure 19a. An adjustable sampling resistor with resistance R was connected to the Al/Au pads 2811 and 2821 on the backside of the two wafers through an ammeter. The upper p-type silicon wafer was spun under a normal force changed from 0.3N (weight of the upper wafer and its sample holder) up to 10 N at a rotation speed maintained at 30 rpm. By spinning clockwise and counter-clockwise, a DC current was observed flowing from the p-type to n-type Si electrode, regardless of the rotation direction. Figure 19b shows Isc and the open circuit voltage (Voc) measured under R=¥ as the function of the forces. It can be clearly seen that Isc increased from 19 nA under 0.3 N up to 250 nA under 10 N. Correspondingly, Voc increased from 0.2 V to 0.55 V and showed an apparent saturation when force was larger than 5 N.

Other than sliding a silicon electrode on another silicon electrode, experiments have also been conducted for sliding a doped semiconductor electrode on another doped semiconductors or metals. The results show DC current being generated as long as the two electrode materials have distinct chemical potentials (or work functions). The current flows from the lower chemical potential (or higher work functions) electrode through the load resistor to the other electrode. The generated results from the electrode pairs of oppositely doped GaAs and silicon, p-type silicon and Al, and n-type silicon and Au are shown in figures 20-25. Specifically, figure 20 shows results generated from an electrode pair of n-type silicon and p-type GaAs, where figure 20a shows the open circuit voltage, Voc; figure 20b shows the short circuit current, Isc, when sliding under IN normal force at lOOmm/s; figure 20c shows the chemical potential difference between the two electrodes, V _bi measured using the variable capacitance method; and figure 20d shows the current and average power generated over varying loading resistance. Figure 21 shows results generated from an electrode pair of p-type silicon and n-type GaAs, where figure 21a shows Voc; figure 21b shows Isc when sliding under IN normal force at lOOmm/s; figure 21c shows V _bi measured using the variable capacitance method; and figure 2 Id shows the current and average power generated over varying loading resistance. Figure 22 shows results generated from an electrode pair of p- type silicon and Al metal, where figure 22a shows Voc; figure 22b shows Isc when sliding under IN normal force at lOOmm/s; figure 22c shows V _bi measured using the variable capacitance method; and figure 22d shows the current and average power generated over varying loading resistance. Figure 23 shows results generated from an electrode pair of p-type silicon and Au metal, where figure 23a shows Voc; figure 23b shows Isc when sliding under IN normal force at lOOmm/s; figure 23c shows V _bi measured using the variable capacitance method; and figure 23d shows the current and average power generated over varying loading resistance. Figure 24 shows results generated from an electrode pair of n-type silicon and Al metal, where figure 24a shows Voc; figure 24b shows Isc when sliding under IN normal force at lOOmm/s; figure 24c shows V _bi measured using the variable capacitance method; and figure 24d shows the current and average power generated over varying loading resistance. Figure 25 shows results generated from an electrode pair of n-type silicon and Au metal, where figure 25a shows Voc; figure 25b shows Isc when sliding under IN normal force at lOOmm/s; figure 25c shows V _bi measured using the variable capacitance method; and figure 25d shows the current and average power generated over varying loading resistance.

Figure 26 shows the generated current Isc under dry sliding and wet sliding at a speed of 20 mm/s. No weight was introduced and R = 0 ohm. Clearly, in the first 10 min, the second electrode was slid on the first electrode under dry condition and the average generated current was about 10 nA. In contrast, from 10 min onwards where 0.1 ml DI water was added to the contacted surfaces, the average generated current was found to increase by at least one order of magnitude.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention.