Triboelectric generator

Abstract

The invention relates to a method comprising applying a mechanical force to a device, the device comprising a first stack comprising a first electrode, a charge transport layer in electrical contact with the first electrode, and a first triboelectric layer in electrical contact with the charge transport layer and comprising a first triboelectric material, wherein photoinduced electron-hole pairs are generated in the first triboelectric material when said first triboelectric material is exposed to incident light, and a second stack comprising a second electrode, and a second triboelectric layer in contact with the second electrode and comprising a second triboelectric material. The mechanical force causes the device to transition between a compressed state in which the first triboelectric layer is in physical contact with the second triboelectric layer and a released state in which there is a separation between the first and second triboelectric layers. The method further comprises exposing the device to incident light such that photoinduced electron-hole pairs are generated in the first triboelectric layer.

Description

[0001] Triboelectric Generator

[0002] Field

[0003] This specification relates to a device and method for amplification in triboelectric generators.

[0004] Background

[0005] An electronic device may be powered by an electromechanical energy harvester such as an electromagnetic, piezo-electric or triboelectric nanogenerator. A triboelectric nanogenerator utilises the triboelectric effect. The triboelectric effect is based on the principle of contact electrification and electrostatic induction that occurs between the surfaces of two different triboelectric materials, when they come into contact with each other, under an applied mechanical force. The mechanical contact harvests the electrical energy due to the difference of electrostatic charges on the surfaces of the contacting materials.

[0006] Typically, triboelectric nanogenerators are formed from two different dielectrically opposite materials that can create friction, leading to the charge transfer which in turn generates an electrical output. There is a need for further developments in this field.

[0007] Summary of Invention

[0008] The specification provides for a method comprising applying a mechanical force to a device, the device comprising a first stack comprising a first electrode, a charge transport layer in electrical contact with the first electrode, and a first triboelectric layer in electrical contact with the charge transport layer and comprising a first triboelectric material, wherein photoinduced electron-hole pairs are generated in the first triboelectric material when said first triboelectric material is exposed to incident light, and a second stack comprising a second electrode, and a second triboelectric layer in contact with the second electrode and comprising a second triboelectric material. The mechanical force causes the device to transition between a compressed state in which the first triboelectric layer is in physical contact with the second triboelectric layer and a released state in which there is a separation between the first and second triboelectric layers. The method further comprises exposing the device to incident light such that photoinduced electron-hole pairs are generated in the first triboelectric layer.

[0009] In an embodiment, the first triboelectric material is a metal halide. In an embodiment, the first triboelectric material is a halide perovskite. In an embodiment, the device is exposed to incident light simultaneous to applying the mechanical force to the device.

[0010] In some embodiments, applying the mechanical force generates a potential between the first and second triboelectric layers. Further, in some embodiments the photoinduced holes are transported via the charge transport layer to the first electrode, and / or the photoinduced electrons move to a first surface of the first triboelectric layer to increase the surface charge density.

[0011] The specification further provides for a device, the device comprising a first stack comprising a first electrode, a charge transport layer in electrical contact with the first electrode, and a first triboelectric layer in electrical contact with the charge transport layer and comprising a first triboelectric material, wherein photoinduced electron-hole pairs are generated in the first triboelectric material when said first triboelectric material is exposed to incident light. The device further comprises a second stack comprising a second electrode, and a second triboelectric layer in electrical contact with the second electrode and comprising a second triboelectric material. In a compressed state the first triboelectric layer is in physical contact with the second triboelectric layer, and in a released state there is a separation between the first and second triboelectric layers.

[0012] In some embodiments, the device further comprises an output terminal connected to the first and second electrodes. For example, the output terminal may be configured to provide a voltage pulse in response to a transition from the released state to the compressed state and / or vice versa.

[0013] In an embodiment, the first triboelectric material has a negative charge type and the second triboelectric material has a positive charge type. In an embodiment, the first triboelectric material is a metal halide. In an embodiment, the first triboelectric material is a halide perovskite. In an embodiment, the second triboelectric material is a thermoplastic polyurethane (TPU).

[0014] In some examples, the charge transport layer comprises poly[bis(4-phenyl)(2,4,6- trimethylphenyl)amine] (PTAA). In some examples, the charge transport layer comprises nickel oxide (NiOx).

[0015] The first electrode may comprise a glass substrate coated with indium tin oxide (ITO). In an embodiment, the device is a triboelectric nanogenerator.

[0016] Brief Description of the Drawings

[0017] Figure 1 shows a device according to the present specification;

[0018] Figure 2 shows a device transitioning between a fully compressed state and a fully released state under dark conditions, in accordance with an example embodiment;

[0019] Figure 3 shows a device transitioning between a fully compressed state and a fully released state when exposed to incident light, in accordance with an example embodiment;

[0020] Figure 4 depicts a method according to the present specification;

[0021] Figure 5 is a graph showing the voltage stored in a capacitor over time, in accordance with an example embodiment;

[0022] Figure 6 is a graph showing the output signal under light and dark conditions, in accordance with an example embodiment;

[0023] Figure 7 is a graph showing the output signal when exposed to incident light, in accordance with an example embodiment;

[0024] Figure 8 is a graph showing the effect of light intensity on the voltage, charge and current generated by a device according to the present specification;

[0025] Figure 9 is a graph showing the output current generated by a device according to the present specification at different intensities of incident light.

[0026] Detailed Description

[0027] Figure 1 shows a device 100 according to the present specification. The device 100 may be described as a triboelectric nanogenerator.

[0028] The device 100 comprises a first stack 101 and a second stack 102.

[0029] The first stack 101 comprises a first electrode 103. The first electrode 103 may comprise any suitable material. In an embodiment, the first electrode 103 comprises a glass substrate coated with a transparent electrode material, to allow light to pass through the glass substrate. In an embodiment the glass substrate is coated with indium tin oxide (ITO), wherein the ITO acts as the electrode. In an embodiment, the glass substrate is coated with fluorine tin oxide (FTO), wherein the FTO acts as the electrode.

[0030] The first stack 101 also comprises a first triboelectric layer 104. The first triboelectric layer 104 comprises a first triboelectric material 105, wherein photoinduced electronhole pairs are generated in the first triboelectric material 105 when said first triboelectric material 105 is exposed to incident light. In other words, the first triboelectric material 105 is both triboelectric and photosensitive.

[0031] In an embodiment, the first triboelectric material 105 comprises a metal halide material. More specifically, the first triboelectric material 105 may comprise a perovskite material, such as a halide perovskite. In this specification, the term perovskite refers to any material having a crystalline structure following the formula ABX3, where A and B are positively charged ions ('cations') and X is a negatively charged ion ('anion'). For example, the first triboelectric material 105 may comprise a triple-cation lead binary halide perovskite (TCHP) having the chemical formula Cso.o5FAo.8oMAo.i5Pb(Io.85Bro.i5)3, where Cs, FA, and MA are caesium, formamidinium, and methylammonium cations respectively.

[0032] The first stack 101 further comprises a charge transport layer 106. The purpose of the charge transport layer 106 is to extract and transport the charge carriers efficiently into an electrode. The charge transport layer 106 can facilitate hole extraction or transportation, and may alternatively be referred to as a hole transport layer. The charge transport layer 106 may also limit charge recombination and ion migration.

[0033] Suitable materials for the charge transport layer 106 include poly[bis(4-phenyl)(2,4,6- trimethylphenyl)amine] (PTAA) or nickel oxide (NiOx).

[0034] The first stack 101 is arranged such that the charge transport layer 106 is in electrical contact with the first electrode 103, and the first triboelectric layer 104 is in electrical contact with the charge transport layer 106. In this way, an effective charge transport mechanism is set up across the first stack 101. The respective layers of the first stack 101 may also be in physical contact with one another. For example, the charge transport layer 106 may be sandwiched between the first electrode 103 and the first triboelectric layer 104, as depicted in Figure 1. In some embodiments, the first stack 101 is produced by depositing a layer of the charge transport material onto the first electrode 103, and subsequently depositing the first triboelectric material 105 onto the charge transport layer 106. In such cases, deposition may comprise spin coating, followed by curing and a UV-ozone treatment.

[0035] The second stack 102 comprises a second electrode 107. The second electrode 107 may comprise any suitable material. For example, the second electrode 107 may comprise a copper electrode. In an embodiment, the second electrode 107 may comprise a glass substrate coated with a transparent electrode material in the same manner as the first electrode 103. This increases the amount of incident light reaching the first triboelectric layer 104 and thus enhances the photoinduced current.

[0036] The second stack 102 also comprises a second triboelectric layer 108 in electrical contact with the second electrode 107. The second triboelectric layer 108 may also be in physical contact with the second electrode 107, as shown in Figure 1.

[0037] The second triboelectric layer 108 comprises a second triboelectric material 109 which is different from the first triboelectric material 105. Any suitable triboelectric material may be used as the second triboelectric material 109. For example, the second triboelectric material 109 may comprise thermoplastic polyurethane (TPU). Thus, in an embodiment the second stack 102 may be produced by depositing a TPU layer onto a copper electrode.

[0038] The first triboelectric material 105 may act a neutral triboelectric material. In other words, the first triboelectric material 105 may be employed as either a positive or negative triboelectric material, depending on the polarity of the counter triboelectric material (i.e. the second triboelectric material 109). In an embodiment, the first triboelectric material 105 has a negative charge type and the second triboelectric material 109 has a positive charge type. However, it will be appreciated that if a second triboelectric material 109 is employed which has a negative charge type, the first triboelectric material 105 will have a positive charge type.

[0039] The device 100 may be configured such that the first triboelectric layer 104 and the second triboelectric layer 108 are facing each other, as shown in Figure 1. More specifically, the device 100 may be configured such that in a compressed state the first triboelectric layer 104 is in physical contact with the second triboelectric layer 108, and in a released state there is a separation between the first and second triboelectric layers 104, 108.

[0040] The device 100 may additionally comprise a spacer (not depicted), configured to maintain the separation between the first and second triboelectric layers 104, 108 when in the released state. In some embodiments, the spacer may maintain a separation of at least 2mm, such as at least 5mm, such as at least 1cm.

[0041] In some embodiments, the device 100 may additionally comprises an output terminal 110 connected to the first and second electrodes 103, 107. The output terminal 110 is configured to provide an electrical output signal in response to a flow of current across the first and second electrodes 103, 107. For example, the output terminal 110 is configured to provide a voltage pulse in response to a transition from the released state to the compressed state and / or vice versa.

[0042] The first and second electrodes 103, 107 may each be connected to the output terminal 110 via one or more cables.

[0043] In an embodiment, the output terminal 110 may be connected to a capacitor for storing the charge generated by the device 100.

[0044] In the embodiment of Figure 1 and as described above, the second stack 102 has a different composition to the first stack 101. However, the skilled person will appreciate that in some embodiments not shown in the Figures the second stack 102 may have the same composition as the first stack 101. In other words, the second stack 102 may additionally comprise a further charge transport layer in electrical contact with the second triboelectric layer 108 and the second electrode 107. In such an embodiment, the charge transport layers on the first and second stacks respectively must have opposite polarity (i.e., one must be an electron transport layer and the other must be a hole transport layer).

[0045] The device 100 may additionally comprise a protective coating and / or casing to prevent damage to the electrodes and / or the first and second triboelectric layers 104, 108.

[0046] Figure 2 illustrates the device 100 transitioning between a fully compressed state and a fully released state in dark conditions i.e., with no optical illumination of the device 100. As described above, the device 100 comprises first and second electrodes 103, 107 connected to an output terminal 110; first and second triboelectric layers 104, 108; and a charge transport layer 106. In this embodiment, the first triboelectric layer 104 comprises a negative triboelectric material and the second triboelectric layer 205 comprises a positive triboelectric material.

[0047] At step 201, the device 100 is in a fully compressed state. In the fully compressed state, a mechanical compression force is applied to an outer surface of the device 100. Note that in Figure 2, the force is depicted as being applied to the outer surface of the second stack 102. However, the force could additionally or alternatively be applied to the first stack 101, or to both stacks simultaneously. In some embodiments the mechanical compression force may be generated by vibrating the device 100.

[0048] In the fully compressed state, the first triboelectric layer 104 is at maximum contact (or friction) with the second triboelectric layer 108. This results in the generation of equal and opposite triboelectric charges across the surfaces of the first and second triboelectric layers 104, 108, owing to triboelectrification. As shown in Figure 2, negative charges are generated on the surface of the first triboelectric layer 104, and positive charges on the surface of the second triboelectric layer 108. The surface charges are in equilibrium, such that there is no resulting current flow across the electrodes.

[0049] At step 202, the mechanical compression force is released such that the first triboelectric layer 104 begins to separate from the second triboelectric layer 108. The separation between the first and second triboelectric layers 104, 108 disturbs the equilibrium of charges on their respective surfaces. As the first and second triboelectric layers 104, 108 are in electrical contact with the first and second electrode 103, 107 respectively, these disturbed charges induce the opposite charges on their respective electrodes. Hence, a positive polarity is induced on the first electrode 103 and a negative polarity on the second electrode 107. This results in a flow of current from the negative first electrode 103 to the positive second electrode 107, generating a positive output pulse at the output terminal 110.

[0050] At step 203, the device 100 is in a fully released state. The first and second triboelectric layers 104, 108 are fully separated and the surface charges on the first and second triboelectric layers 104, 108 are again in equilibrium. Hence, there is no flow of current between the electrodes. At step 204, the mechanical compression force is again applied to the device 100 as described above. The first and second triboelectric layers 104, 108 move towards each other, and the equilibrium of the surface charges is again disturbed, inducing charges on the first and second electrodes 103, 107. The induced charges flow in the opposite direction to step 202, resulting in a negative output pulse at the output terminal 110.

[0051] Once the device 100 returns to a fully compressed state, the surface charge distribution of the first and second triboelectric layers 104, 108 are once again in equilibrium.

[0052] This process repeats in a cyclic manner as shown in Figure 2. Continuous cyclic mechanical compression forces applied and released on the device 100 result in the generation of alternating current (AC) from the electrodes to the output terminal 110 as the electronic device 100 transitions from a compressed state to a released state and vice versa.

[0053] In an embodiment, the first triboelectric material 105 comprises a halide perovskite, and the charge transport layer 106 comprises PTAA. The inventors have identified this combination as having the most significant triboelectric charging effect.

[0054] Figure 3 shows the same device 100 transitioning between a fully compressed state (301) and a fully released state (303) when exposed to incident light. In this way, the device 100 may be used as a multi-energy harvester.

[0055] With regard to the application of the mechanical compression force and the resulting triboelectric effects, steps 301, 302, and 303 may be regarded as corresponding to steps 201, 202, and 203 of Figure 2 respectively.

[0056] At the same time as applying the cyclic mechanical compression forces, the device 100 is exposed to incident light such that photoinduced electron-hole pairs are generated in the first triboelectric layer 104. These electron-hole pairs are generated in addition to the triboelectric charges generated as the device 100 transitions between a compressed and released state.

[0057] At step 301, the device is in a fully compressed state. The photoinduced electrons (negative charge carriers) move to the surface of the first triboelectric layer 104, where they supplement the negative triboelectric charges. This results in additional positive triboelectric charges being generated on the surface of the second triboelectric layer 108.

[0058] At the same time, the photoinduced holes (positive charge carriers) are transported via the charge transport layer 106 to the first electrode 103. As noted above, the charge transport layer 106 facilitates hole transportation, and may also limit charge recombination and ion migration.

[0059] At step 302, the mechanical compression force is released. The additional positive triboelectric charges generated on the second triboelectric layer 108 induce additional negative charges on the second electrode 107. At the same time, the photoinduced holes increase the positive charge on the first electrode 103. As a result of these additional charges, the positive output pulse generated at the output terminal 110 is amplified compared to when the device 100 is operating under mechanical stimulation alone.

[0060] At step 303, the device is in a fully released state. The incident light continues to generate photoinduced electron-hole pairs in the first triboelectric layer 104. The photoinduced electrons move to the surface of the first triboelectric layer 104, where they supplement the negative triboelectric charges. At the same time, the photoinduced holes are transported via the charge transport layer to the first electrode 103.

[0061] In this manner, the effects of the triboelectrification are enhanced by the additional generation of photoinduced electron-hole pairs.

[0062] In some embodiments, the device 100 may be exposed to incident light without applying a mechanical compression force. In this case, only the photoinduced electron-hole pairs contribute to the output current.

[0063] Figure 4 depicts a method according to the present invention. At 401 a mechanical force is applied to a device, such as the device 100 described above. The mechanical force causes the device to transition between a compressed state in which a first triboelectric layer is in physical contact with a second triboelectric layer and a released state in which there is a separation between the first and second triboelectric layers.

[0064] At 402 the device is exposed to incident light such that photoinduced electron-hole pairs are generated in the first triboelectric layer. Steps 401 and 402 may be performed in any order, or simultaneously. The combination of the two method steps results in an electrical output as described above.

[0065] By exposing the device 100 to incident light at the same time as applying a mechanical compression force, the energy conversion efficiency of the triboelectric nanogenerator is improved, such that the electrical output of the device 100 is significantly increased. Figure 5 shows the voltage stored in a capacitor connected to the device 100 via the output terminal 110, as a function of time. Line 501 shows the performance of the device 100 when a mechanical compression force is applied under dark conditions. Line 502 shows the performance of the same device 100 when simultaneously exposed to incident light. It is immediately apparent that significantly more voltage is stored in the capacitor when the device is subject to both mechanical force and incident light (i.e., both triboelectric and photoelectric effects are present), compared to the triboelectric effects alone.

[0066] Because the device 100 generates an electrical output signal upon application of a mechanical force, it can be used as a movement sensor. For example, if located on a water pipe, the device 100 can be used as a flow detector due to the mechanical vibration caused by the flowing water. Furthermore, as described above, this electrical output can be amplified by exposing the device 100 to incident light, increasing the sensitivity of the device to mechanical vibrations.

[0067] Figure 6 shows the effect of exposing the device 100 to incident light on the electrical output signal of the device 100. The top graph (i) shows the electrical output signal under dark conditions (left) and when exposed to incident light (right). The high intensity peaks 601 correspond to the application of the mechanical compression force, and represent the signal generated through the triboelectric effect as described in relation to Figure 2. It is immediately apparent that, due to the additional photoinduced charges, the amplitude of these peaks is significantly increased when the device 100 is exposed to incident light, as described in relation to Figure 3.

[0068] As illustrated in Figure 6, not only are the peaks 601 amplified when the device 100 is exposed to incident light, but, unexpectedly, the background (or baseline) signal 602 is also amplified. (ii) and (iii) of Figure 6 show an enlarged view of the background signal under dark (ii) and light (iii) conditions respectively. Under dark conditions, the background signal is negligible and random. On the other hand, when exposed to incident light, the background signal forms a series of low intensity peaks. These peaks form due to an additional current generated solely by the photoinduced charges. The frequency of the peaks reflects the frequency of the incident light. In Figure 6, the incident light has a frequency of 50 Hz, and hence the intensity of the incident light switches from positive to negative at a rate of 50 Hz (i.e., a duration of 0.02s per cycle). This generates a background signal comprising low intensity peaks having a frequency of 50 Hz.

[0069] Thus, not only is the output signal generated by the triboelectric effect amplified compared to a standard triboelectric nanogenerator when the device is exposed to incident light, but the background signal is also amplified. This can be seen in Figure 7, which again shows the electrical output signal of the device when exposed to incident light. In particular, enlarged view (iii) shows the effect of the photoinduced current on the high intensity peaks 601, while (iv) shows the low intensity peaks which form the background signal 602.

[0070] The incident light may be ambient light, such as natural light. For example, the incident light may have an intensity of less than 1000 lux, such as less than 500 lux, such as less than 200 lux. Alternatively, the incident light may have an intensity of more than 1000 lux, such as more than 5000 lux, such as more than 7500 lux, such as more than 10,000 lux.

[0071] In an embodiment the increase in the background signal may be dependent on the intensity of the incident light, because increasing the light intensity increases the number of photoinduced electron-hole pairs generated in the first triboelectric material 105.

[0072] Figure 8 show the enhancement percentages of the electrical output voltage (801), charge (802), and current (803) of the device 100 as a function of the intensity of the incident light. In addition to the incident light, the device 100 is also subject to an applied mechanical force. The enhancement percentage is the percentage increase in the output signal (voltage, charge, or current) when the device 100 is subject to incident light of a given intensity, compared to when the device 100 is operating in dark conditions (i.e., only under mechanical stimulation). Thus, for the current 803, the enhancement percentage is calculated as Ienhancement% — 100 X (Ilight”Idark) / Idark / Where Light is the output current when the device 100 is subject to both mechanical force and incident light, and Idark is the output current when the device is operating under dark conditions (i.e., subject to mechanical force only). Figure 8 shows that the performance of the device 100 is linearly enhanced at low light intensities, and reaches saturation at high light intensities, such as above approx. 1000-2000 lux.

[0073] Owing to this sensitivity to light intensity, the device 100 could also be used as a photosensor.

[0074] Figure 9 shows the output current of the device 100 at different intensities of the incident light, under both dark conditions and when subject to incident light. 901 shows the output current under dark conditions i.e. under mechanical stimulation only. 902 shows the output current when the device 100 is subject to incident light at an intensity of 0.2k lux. It is clear that the output current is enhanced when subject to incident light (the 'ON' state; where the device is subject to both mechanical force and incident light) compared to operating under dark conditions. 903-906 show the output current of the device 100 when subject to increasing light intensities of 500 lux, 1000 lux, 5000 lux, and 10,000 lux respectively. The output current is further enhanced by increasing the intensity of the incident light.

[0075] Various modifications and variations will be apparent to those skilled in the art which fall within the scope of the following claims.

Claims

Claims1. A method comprising : applying a mechanical force to a device, the device comprising : a first stack comprising : a first electrode, a charge transport layer in electrical contact with the first electrode, and a first triboelectric layer in electrical contact with the charge transport layer and comprising a first triboelectric material, wherein photoinduced electron-hole pairs are generated in the first triboelectric material when said first triboelectric material is exposed to incident light; and a second stack comprising : a second electrode, and a second triboelectric layer in contact with the second electrode and comprising a second triboelectric material; wherein the mechanical force causes the device to transition between a compressed state in which the first triboelectric layer is in physical contact with the second triboelectric layer and a released state in which there is a separation between the first and second triboelectric layers, and exposing the device to incident light such that photoinduced electron-hole pairs are generated in the first triboelectric layer.

2. The method of claim 1, wherein the first triboelectric material is a metal halide.

3. The method of claim 1 or 2, wherein the first triboelectric material is a halide perovskite.

4. The method of any preceding claim, wherein the device is exposed to incident light simultaneous to applying the mechanical force to the device.

5. The method of any preceding claim, wherein applying the mechanical force generates a potential between the first and second triboelectric layers.

6. The method of any preceding claim, wherein the photoinduced holes are transported via the charge transport layer to the first electrode.

7. The method of any preceding claim, wherein the photoinduced electrons move to a first surface of the first triboelectric layer to increase the surface charge density.

8. A device comprising : a first stack comprising : a first electrode, a charge transport layer in electrical contact with the first electrode, and a first triboelectric layer in electrical contact with the charge transport layer and comprising a first triboelectric material, wherein photoinduced electron-hole pairs are generated in the first triboelectric material when said first triboelectric material is exposed to incident light; a second stack comprising : a second electrode, and a second triboelectric layer in electrical contact with the second electrode and comprising a second triboelectric material; wherein in a compressed state the first triboelectric layer is in physical contact with the second triboelectric layer, and wherein in a released state there is a separation between the first and second triboelectric layers.

9. The device of claim 8, wherein the first triboelectric material is a metal halide.

10. The device of claim 8 or 9, wherein the first triboelectric material is a halide perovskite.

11. The device of any of claims 8 to 10, further comprising an output terminal connected to the first and second electrodes.

12. The device of claim 11, wherein the output terminal is configured to provide a voltage pulse in response to a transition from the released state to the compressed state and / or vice versa.

13. The device of any of claims 8 to 12, wherein the first triboelectric material has a negative charge type and the second triboelectric material has a positive charge type.

14. The device of any of claims 8 to 13, wherein the charge transport layer comprises poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA).

15. The device of any of claims 8 to 14, wherein the charge transport layer comprises nickel oxide (NiOx).

16. The device of any of claims 8 to 15, wherein the first electrode comprises a glass substrate coated with indium tin oxide (ITO).

17. The device of any of claims 8 to 16, wherein the second triboelectric material comprises a thermoplastic polyurethane (TPU).

18. The device of any of claims 8 to 17, wherein the device is a triboelectric nanogenerator.

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