Perovskite-Based Triboelectric Nanogenerator and Its Application Towards Self-Powered Devices

Abstract

Research on triboelectric nanogenerators (TENGs) and self-powered devices has rapidly grown in recent years since its first report in 2012 by Prof. Wang’s group. Triboelectric polymers have been a frontier of the research, attributed to their high surface potential and consequently high voltage output. To further advance the field, in recent years, photoactive semiconductor materials have been introduced which offer an additional current generation mechanism under light excitation, boosting the output current of the TENG. In addition, the semiconductor-based TENG further provides an ability to detect photo-signals beyond mechanical signals, adding high value towards advanced multi-functional sensor applications. In this regard, this article aims to review the recent progress in semiconductor-based TENGs, particularly on metal-halide perovskites, and their applications to self-powered electronics. Finally, the prospects and challenges of the perovskite-based TENG are discussed.

1. Metal-Halide Perovskites

Metal-halide perovskites (perovskites hereafter) are ideal energy materials with strong light absorption, large charge mobility, and high defect tolerance [1,2,3]. The bandgaps of perovskites can also be readily tuned by changing the halide compositions, enabling customization of absorption and emission spectra for dual-mode energy harvesting or sensing [4,5,6,7]. In addition, perovskites can be dissolved in aprotic polar solvents at room temperature, which facilitates large-scale fabrication at a lower cost [8,9]. As a result, perovskites have attracted worldwide attention over the last decade in various applications such as triboelectric nanogenerators (TENGs), solar cells, LEDs, and photodetectors [10,11,12,13,14,15,16,17,18,19,20].

1.1. Structure and Classification of Perovskites

Perovskite has a general formula of ABX₃, where A represents a monovalent cation such as CH₃NH₃, B is a divalent metallic cation such as Pb²⁺, Sn²⁺ or Cu²⁺, and X refers to a halide anion such as chloride (Cl⁻), bromide (Br⁻), or iodide (I⁻). The schematic of perovskite, a bulk 3D type, is shown in Figure 1a, where corner-sharing BX₆ octahedra form a 3D network, and A cations fill in the space between these octahedra [21]. The crystal structure of perovskites is affected by the size of ions. Generally, symmetry and stability of perovskites are determined by a Goldschmidt tolerance factor (t) and octahedral factor (μ), which are expressed by Equations (1) and (2), where R_A, R_B, and R_X stand for an effective ionic radius of ions A, B, and X, respectively. When 0.81 < t < 1.11 and 0.44 < μ < 0.9, perovskites form a stable cubic phase. When t < 0.81 or t > 1.1, lattice distortion of perovskites results in a tetragonal or orthorhombic structure with poor stability [21].

$$t={\displaystyle \frac{R_A+R_X}{\sqrt{2}\left(R_B+R_X\right)}}$$

(1)

$$\mu ={\displaystyle \frac{R_B}{R_X}}$$

(2)

Quasi-2D perovskites are achieved by introducing large organic cations at A sites, such as butylammonium (BA) and phenethylammonium (PEA). As illustrated in Figure 1b [22], 3D structures are sliced into 2D layers by organic spacer cations. Quasi-2D perovskites can mainly be classified as Ruddlesden-Popper (RP), Dion-Jacobson (DJ), and alternating cation (ACI) phases. Taking lead (Pb) perovskites as an example, 3D perovskites have a cubic lattice structure in which [PbX₆]⁴⁻ octahedra share corners and form a cubic cage to contain an A-site cation in the middle (Figure 1a). In contrast, the organic spacers in quasi-2D perovskites are sufficiently large to confine the A-sites so that they cut the cubic cages into slices of corner-sharing [PbX₆]⁴⁻ octahedra to form 2D structures (Figure 1b) [22]. Depending on the number of [PbX₆]⁴⁻ octahedron layers n, quasi-2D perovskites can form a series of phases with a same organic spacer [23]. Compared with 3D perovskites, quasi-2D perovskites are not constrained by the Goldschmidt tolerance factor due to the incorporation of long-chain organic cations. The parameter n can be adjusted from one to infinity. When n = 1, the structure corresponds to a pure 2D perovskite, while n = ∞ represents a 3D perovskite (Figure 1c). This flexibility allows for the tailored design of perovskite materials to meet specific application requirements [24].

1.2. Properties of Perovskites

Perovskites have superior optoelectronic properties, making them an ideal candidate for multiple energy harvesting or sensing devices.

1.2.1. Strong Light Absorption Capability

Perovskites show a high absorption coefficient of 10⁵ cm⁻¹, which is an order of magnitude higher compared to silicon (Figure 1d). The high absorption coefficient enables an ultrathin perovskite film as the active layer, reducing the material cost and the probability of carrier recombination [25]. For example, Wei et al. introduced halide perovskite CsBi₃I₁₀ (CBI) as a triboelectric material to form a TENG. The authors utilized wide ranges of light absorption of CBI for high current generation attributed to a narrow bandgap (1.77 eV) of CBI [26]. Similarly, Yang et al. proposed a MAPbI_XCl_3−X perovskite-based TENG whose performance can be improved by photo-induced charge carrier generation [27].

1.2.2. Tunable Bandgap

Through adjusting a chemical composition or stoichiometric ratio of perovskites, the bandgap can be tuned continuously [28]. For instance, (PMA)₂PbCl₄, where PMA is benzylammonium, is one of the pioneering ferroelectric 2D perovskites reported. (PMA)₂PbCl₄ exhibits remarkable ferroelectric properties at a bandgap of 3.65 eV. However, at this bandgap, its light sensitivity is limited to wavelengths shorter than 387 nm, which does not align with requirements for ferroelectric materials for optoelectronic applications. As the electronic structure of the inorganic framework predominantly governs the bandgap of the material, incorporation of Br and I into (CHA)₂PbCl₄ could tune the bandgap ranging from 3.05 to 2.74 eV [24].

1.2.3. Long Carrier Diffusion Length

To couple a light excitation effect with triboelectricity, a carrier diffusion length is an important parameter which determines the efficiency of charge collections [29]. Assuming the presence of a Schottky barrier at the electrode interface, photocurrent is governed by the carrier diffusion based on Equation (3). Here, I_ph represents a photocurrent, x a distance from the electrode, and L a diffusion length [30,31].

$$I_{ph}\propto exp\left(-{\displaystyle \frac{x}{L}}\right)$$

(3)

However, as weak ionic semiconductors, perovskites are faced with serious stability issues [32,33,34]. On the one hand, the weak ionic nature results in a poor resistance to moisture and oxygen in the environment [35,36,37,38]. On the other hand, the weak ionic interaction in crystals also indicates a low energy barrier for ion migration, leading to ion accumulation at interfaces in devices [39,40] and photo-induced halide segregation in mixed composition [41,42]. The device instability and inherent material instability hinder the development of perovskites as versatile energy harvesting or sensing materials. In comparison with 3D perovskite, quasi-2D perovskite exhibits advantages of higher stability, constituent flexibility, and stronger tunable bandgap capability.

Figure 1. (a) Lattice structure of 3D perovskites (APbX3). (b) Lattice structure of quasi-2D perovskites where PEA = phenylethyl ammonium is an example of organic spacers. n indicates the number of octahedra layers. Reprinted with permission from Ref. [22]. 2016, American Chemical Society. (c) An illustration of how bandgaps vary with n values in quasi-2D perovskites where BA = butylamine is an example of organic spacers. Reprinted with permission from Ref. [23]. 2022, The American Association for the Advancement of Science. (d) The absorption spectra of perovskite and other materials. Reprinted with permission from Ref. [43]. 2014, Springer Nature.

Figure 1. (a) Lattice structure of 3D perovskites (APbX3). (b) Lattice structure of quasi-2D perovskites where PEA = phenylethyl ammonium is an example of organic spacers. n indicates the number of octahedra layers. Reprinted with permission from Ref. [22]. 2016, American Chemical Society. (c) An illustration of how bandgaps vary with n values in quasi-2D perovskites where BA = butylamine is an example of organic spacers. Reprinted with permission from Ref. [23]. 2022, The American Association for the Advancement of Science. (d) The absorption spectra of perovskite and other materials. Reprinted with permission from Ref. [43]. 2014, Springer Nature.

2. TENG Working Principles

Triboelectricity, which occurs when two different materials including chemically identical materials come into contact, is a common electrical phenomenon in daily life. However, the mechanism behind it has been elusive. Currently, an electron transfer is known as the dominant contact electrification (CE) mechanism [44]. Due to varying atomic, molecular, or material structures, different materials have different electron affinities. As a result, when two materials come into contact, one emits electrons and the other accepts them. Through continuous contact–release processes, triboelectric charging occurs at the contact surface.

According to the nature of gaining or losing electrons in certain materials, a variety of materials can be arranged in triboelectric series. Figure 2a shows a standardized triboelectric series where the reference contact material of this triboelectric series is unified as a liquid metal which has good deformability and provides maximum atomic-scale contact with the counter material [45]. Figure 2b illustrates the triboelectric charge density and qualitative triboelectric series of the perovskite studied [46]. The standardized measurement process and liquid metal with maximum atomic-scale contact as reference materials can provide a universal quantitative standard for relevant works for further studies.

2.1. Four Modes of TENG

To properly induce triboelectrification in different applications, four representative operation modes have been developed, which include the following: (i) vertical contact-Separation mode; (ii) lateral sliding mode; (iii) single-electrode mode; and (iv) freestanding triboelectric layer mode (Figure 2c) [47]. Although suitable for different states of motions, they all present contact-separation motions between triboelectric layers, which can be rationalized by triboelectrification.

The contact-separation mode is the most classic type of TENG. In this mode, one triboelectric material directly contacts a metal electrode, or two different triboelectric layers face each other with electrodes attached to the bottom of each triboelectric layer. Under periodic external mechanical forces, the structure with the triboelectric layer-metal electrode or the triboelectric-triboelectric layer periodically contacts and separates. By the triboelectric principle, alternating voltage and current is generated by the contact-separation mode. The V-Q-x relationship of the contact-separation mode is described by the Equation (4), where σ and ε₀ are the surface charge density of a triboelectric material and vacuum permittivity, respectively. V is an output voltage, R is a load resistance, S is an active area, d₀ is the effective thickness of a triboelectric pair, x is a separation distance, and F is the Fourier-transform method performed [48].

$$V\left(t\right)=\left[{\displaystyle \frac{\sigma }{{\epsilon }_0}}x\left(t\right)-{\displaystyle \frac{\sigma }{{\epsilon }_0}}{\displaystyle \frac{d_0+x\left(t\right)}{RS{\epsilon }_0}}F\left(t\right)\right]$$

(4)

The sliding mode has a similar structure to the contact-separation mode but is mainly based on a horizontal relative displacement. During the working process, the two materials maintain good contact with no movement in a vertical direction. When the relative horizontal sliding occurs between two materials, the surfaces of the triboelectric layer–metal or the triboelectric-triboelectric layer are charged. The output is generated in different directions with respect to the direction of the relative sliding. The basic V-Q-x relationship of the sliding-mode TENG can be described by Equation (5), where x is the sliding distance [49,50].

$$\begin{array}{c}V={\displaystyle \frac{\sigma d_0}{{\epsilon }_0}}[{\displaystyle \frac{l}{l-x\left(t\right)}}exp\left(-{\displaystyle \frac{d_0}{{\epsilon }_{0}RS}}{{\displaystyle \int }}_0^t{\displaystyle \frac{l}{l-x\left(t\right)}}d{t}^{\prime }\right)\hfill \\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}+{\displaystyle \frac{d_0}{{\epsilon }_{0}RS}}{\displaystyle \frac{l}{l-x\left(t\right)}}{{\displaystyle \int }}_0^{t}exp\left({\displaystyle \frac{d_0}{{\epsilon }_{0}RS}}{{\displaystyle \int }}_t^{t^{\prime }}{\displaystyle \frac{l}{l-x\left(\delta \right)}}d\delta \right)d{t}^{\prime }-1]\hfill \end{array}$$

(5)

The freestanding mode is based on the natural friction of independent layers between the surrounding electrodes. The model uses insulating material as a separate layer so that its surface charge can be maintained longer. The independent layer causes natural friction between two electrode materials connected by a load. During a relative motion, an uneven charge distribution occurs between two connected electrodes, causing the external circuit to generate an electric current. The output performance produced by this mode is generally lower than the previous two modes. However, it does not require direct contact between the triboelectric layers; therefore, the mechanical wear of the material is reduced, helping to extend the life of the TENG [51].

As for the single-electrode mode, one electrode is connected to the output node and the other is grounded, providing an electron transfer between the two. The single electrode mode has better flexibility than other modes because there is no reference electrode. However, a single-electrode configuration has the disadvantage of an unstable output. This mode of operation is suitable for applications that require freedom of motion [52].

2.2. Electron Transfer Mechanism

CE is prevalent in all types of materials, presenting a difficulty for explaining charge transfer across different materials. Nowadays, there is still a challenge to establish a unified triboelectric model between different materials. Since an electron transfer is considered the main mechanism for triboelectric charge transfer, this section describes the mechanism of an electron transfer between various TENG materials.

2.2.1. Metal-Dielectric and Metal-Metal

For triboelectrification between a metal–dielectric layer, if the electronic structure of a dielectric layer can be characterized by a band diagram, surface states can be used to explain the charge transfer phenomenon, while metal is described by the Fermi level or a work function [44]. Surface states can be found at the surface of a material where some electrons cannot be fully bound, as they can be inside the material due to incomplete coordination of atoms or molecules, consequently occupying energy states within the bandgap. In addition, defects in the material such as vacancies, impurities, dislocations, etc., can also cause electrons to occupy surface states [53].

When a metal and a dielectric layer are in contact, if the highest occupied surface state level (E₀) of the dielectric is higher than the Fermi level of the metal E_f, some electrons may be transferred from the dielectric to the metal to balance the energy difference until the same energy level is reached, as shown in Figure 3a [44,53]. Vice versa, as shown in Figure 3b, when E₀ is lower than E_f, electrons will flow from the metal to the dielectric material [53]. In addition, when two metals are in contact, the triboelectric properties are determined by Fermi energy level. If there is a difference in the work function, charge transfer also occurs between the two metals during contact, as shown in Figure 3c [54]. Electrons will flow from Metal 2 with lower work function to Metal 1 whose work function is higher [54].

2.2.2. Dielectric-Dielectric

When two different dielectrics are in contact, some electrons may be transferred from the material with higher surface state energy to another one, as shown in Figure 4a [44]. The transferred charge will not be returned even after the two materials are separated. In general, insulators with low electrical conductivity usually retain electrostatic charges easily. For materials whose electronic structure can be characterized by a band diagram, a surface state model is available. However, for other materials that cannot be characterized by a band diagram, a universal model needs to be developed to explain CE [44].

For general materials, electron transfer can be explained in terms of the interfacial barrier [54]. Under external pressure, the distance between the surface atoms gradually reduces and then falls below an equilibrium distance, changing an atomic interaction from mutual attraction to mutual repulsion. In addition, originally separated electron clouds start to overlap each other, forming bonds between atoms, which can be further shortened by external forces. In this process, an initial single potential well becomes a double potential well. Due to the strong overlap between the electron clouds in the repulsion region, the interfacial barrier is reduced, resulting in the transfer of electrons, as shown in Figure 4b [44].

2.2.3. Semiconductor-Semiconductor or Semiconductor-Metal

When a metal and semiconductor are used as a friction material pair, a direct current is generated by sliding the metal over the surface of the semiconductor wafer, which constitutes a Schottky junction. Similarly, when an N- and a P-type semiconductor contact as a friction material pair, direct current flows through the P-type semiconductor sliding on the surface of the N-type semiconductor, which constitutes a PN junction. Upon contact between a metal–semiconductor or two semiconductors, an internal electric field, i.e., built-in potential/electric field, is generated at the junction. When relative sliding occurs, new bonds are formed; consequently, energy is released as electron-hole pairs are excited at the interface between the two materials. Through a Schottky junction (Figure 5a) or PN junction (Figure 5b), the electrons and holes are separated, resulting in direct current [44].

2.3. Energy Generation Mechanism

Direct current (DC) is characterized by a constant direction and magnitude, while alternating current (AC) is defined by periodic changes in both direction and magnitude over time. Both forms of current have distinct applications in various fields [57]. For instance, DC is commonly used to power electronic devices and stabilize the operating voltage of communication equipment. In contrast, AC is widely utilized in high-voltage transmission systems due to its efficiency in long-distance transmission and large-scale applications. Currently, the majority of TENG outputs are in the form of alternating current; thus, there has been extensive research on DC-TENGs over the last decade. However, since most electronic devices require DC for charging, AC must subsequently be converted into DC for practical applications [58,59].

Coupling the photovoltaic effect with the triboelectric effect has been shown to be an effective method to enhance the AC-TENG and DC-TENG outputs. Halide perovskites exhibit excellent photovoltaic conversion capabilities due to their unique octahedral structure and optimal bandgap properties [60]. Therefore, the DC-TENG based on the halide perovskite AC-TENG has been widely investigated.

2.3.1. AC-TENG

Fundamental Mechanism

The energy generation mechanism of TENGs is based on triboelectrification and electrostatic induction [61]. Triboelectrification is the process where two different materials become electrically charged through contact and separation, resulting in a charge imbalance due to the transfer of electrons. Electrostatic induction occurs when the electric field from the charged materials induces a movement of charges in a nearby conductor, generating displacement current without direct contact. Thus, the underlying principle of a TENG is considered as using displacement current [62]. When two different materials come into contact and interact through periodic mechanical force, triboelectric charging occurs. This friction generates charges, leading to the creation of a potential difference. The time-varying electric field resulting from a charge distribution and an induced polarization leads to generation of Maxwell’s displacement current [54,62]. The displacement current density is defined as Equation (6), where the total displacement current density is denoted as J_D. The electric displacement vector is represented by D, the medium polarization vector is denoted as P_S, the permittivity is denoted as ε, and the electric field is represented by E. By integrating the current density, we can obtain the displacement current as shown in Equation (7), where the medium surface is denoted as S, the field point is represented by r, the distribution of free charges is indicated as ρ, and the total free charge on the electrode is represented by Q [58].

$$J_D={\displaystyle \frac{\partial D}{\partial t}}=\epsilon {\displaystyle \frac{\partial E}{\partial t}}+{\displaystyle \frac{\partial P_s}{\partial t}}$$

(6)

$$I_D=\int J_{D}dS=\int {\displaystyle \frac{\partial D}{\partial t}}dS={\displaystyle \frac{\partial }{\partial t}}\int 𝛻·Ddr={\displaystyle \frac{\partial }{\partial t}}\int \rho dr={\displaystyle \frac{\partial Q}{\partial t}}$$

(7)

In the external circuit, the potential difference between the two electrodes creates a conduction current. This current, along with the internal displacement current, forms a complete circuit. When the TENG is in a steady state, the displacement current equals the conduction current, as there is no leakage current at this time. The displacement current aligns with D when $\partial D/\partial t$ > 0, while it opposes D when $\partial D/\partial t$ < 0. Notably, the displacement current field is distributed throughout the volume between the two electrodes [62]. As illustrated in Figure 6a, in a contact–separation-type AC-TENG, electrons are unable to pass through the dielectric material; thus, a conduction current cannot be generated. Instead, electrons periodically transfer between the two electrodes, resulting in the formation of an alternating electric field, which subsequently produces an alternating current. In a single-electrode mode, the ground functions as the second electrode [58].

Capacitor Model

The working principle of a TENG relies on an electrostatic induction, with the fundamental component of this process being a capacitor; therefore, a TENG exhibits capacitive characteristics. The V-Q-x relationship of a TENG is similar to the V-Q relationship of capacitors [49]. The voltage difference in a TENG arises from the effects of polarized triboelectric charges and the transferred charge Q on the potential. For simplification, we treat this structure as a typical capacitor, so the effect of the transferred charge Q on the potential is −$Q/C\left(x\right)$. Based on a superposition principle, the total voltage difference is shown in Equation (8) [63].

$$V=-{\displaystyle \frac{1}{C\left(x\right)}}Q+V_{oc}\left(x\right)$$

(8)

When an external circuit is shorted, Equation (8) can be written as Equations (9) and (10). The lumped parameter equivalent circuit model can be derived from Equation (10), as shown in Figure 6b. An AC-TENG is represented as an ideal power source in conjunction with a capacitor whose capacitance varies. Since the inherent resistance of a TENG is designed to be close to its maximum value, a parasitic resistance from metal electrodes and a decay of surface triboelectric charges can be neglected, resulting in an absence of resistance in this model [63]. As previously discussed, an AC-TENG operates in four distinct modes. These modes can be broadly categorized into two types: a contact–separation type and a sliding type. Liu et al. showed the capacitor model for both types, as shown in Figure 6c. For the contact–separation type, the variation in capacitance is driven by the changing distance between the two electrodes. In contrast, for the sliding type, the capacitance changes due to the varying contact area between the electrodes [58,63].

$$0=-{\displaystyle \frac{1}{C\left(x\right)}}Q+V_{oc}\left(x\right)$$

(9)

$$Q_{sc}\left(x\right)=V_{oc}\left(x\right)C\left(x\right)$$

(10)

Figure of Merits

A TENG operates in four distinct modes, necessitating a standardized approach to quantify performance comparison. Consequently, the concept of figure of merits (FOM) has been introduced [64]. The V-Q-x relationship of TENGs is defined in Equation (10) [63]. To define the detailed distance x, a minimum achievable charge reference state is used which assumes that at x = 0, and the short-circuit transferred charges Q_SC(x) and the open-circuit voltage V_OC(x) are set to zero [64,65]. Meanwhile, when x = x_max, Q_SC(x) and V_OC(x) reach their maximum. The schematic diagram of a sliding mode AC-TENG illustrating the distance x is shown in Figure 7a. Due to a continuous and periodic sliding motion generating a periodic electrical signal, the merits of TENGs can be described using a V-Q diagram with a load resistance. The output energy is expressed in Equation (11) [64].

$$E=\overline{P}T=\underset{0}{\overset{T}{{\displaystyle \int }}}VIdt=\underset{t=0}{\overset{t=T}{{\displaystyle \int }}}VdQ=\oint VdQ$$

(11)

From Figure 7b, it can be observed that the plot reaches a steady state in subsequent cycles, and the total cycling charge Q_C is defined as the difference between the maximum and minimum transferred charges during each cycle of energy output (CEO). Therefore, the steady-state behavior for different external resistances can be simulated as illustrated in Figure 7c. The larger the total cycling charge, the greater the output energy; however, the total cycling charge remains less than Q_SC,max(x). To address this, a switch is implemented in parallel with the external resistance to achieve instantaneous short-circuit conditions, as shown in Figure 7d. The output energy can be limited in the trapezoid shown in Figure 7e, and the area of the trapezoid is denoted as E_m. Then, the structural FOM of the TENG can be derived as in Equation (12), where ${\epsilon }_0$ is the permittivity of the vacuum [64].

$$FOM={\displaystyle \frac{2{\epsilon }_0}{{\sigma }^2}}{\displaystyle \frac{E_m}{A{x}_{max}}}$$

(12)

Based on Equation (12), the performance of different designs can be simulated. For CFT, CS, SFT, LS, and SEC, the relationship is illustrated in Figure 7f, where CFT > CS > SFT > LS > SEC. CFT is a contact freestanding triboelectric layer mode (FT), CS is a contact separation mode, SFT is a sliding FT, LS is a lateral sliding mode, and SEC is a single-electrode contact mode.

Electrostatic Breakdown

As the performance of a triboelectric nanogenerator is largely determined by the triboelectric charge density, there have been enormous research efforts on material modifications, structural optimizations, and other means to enhance this charge density [45]. For example, using corona discharging can result in a high output charge density of 240 μCm⁻² [66]. However, a critical limitation of a TENG’s performance is air breakdown, which caps the maximum achievable charge density [67]. Once air breakdown occurs, a dielectric layer undergoes discharge, leading to a sharp decrease in charge density [68]. The limitation factors of the surface charge density of an AC-TENG are shown in Equation (13), where σ_{triboelectrification} is a TE charge density, σ_{air breakdown} is the charge density after air breakdown, and σ_{dielectric breakdown} is a maximum charge density that the dielectric layer can store.

$${\sigma }_{TENG}=min\left({\sigma }_{triboelectrification},{\sigma }_{air breakdown},{\sigma }_{dielectric breakdown}\right)$$

(13)

As illustrated in Figure 8a, a gap voltage ($V_{GAP}$) exists between the metal electrode and the dielectric layer for the contact-separation mode, leading to air breakdown in the TENG system. In Figure 8b, the ion injection process is captured using in situ measurements which track the charge density from the ground to the bottom electrode, while Figure 8c demonstrates the charge difference between the two electrodes. The decrease from 300 μCm⁻² to 160 μCm⁻² further indicates that the air breakdown restricts the maximum retainable charge density in TENGs. This phenomenon limits the overall energy output and efficiency of TENGs. Thus, Wang et al. used a high-vacuum environment to avoid air breakdown which can make output charge density achieve 660 μCm⁻² [67]. However, a complex structure and stability issue of the method make it difficult for TENGs to receive a high charge density directly in the ambient atmosphere. Zhang et al. analyzed the relationship between dielectric thickness and surface charge density for a contact–separation TENG, and they found that reducing dielectric thickness can achieve higher surface charge density. Fluorinated ethylene propylene (FEP) film with a thickness of 15 μm was employed, and 418 μCm⁻² and 740 μCm⁻² of charge density were achieved for a contact–separation mode and sliding mode, respectively [68]. Various innovative approaches have been developed to prevent the breakdown effect in AC-TENGs. Further details about this approach will be discussed in the following sections.

2.3.2. DC-TENG

Conventional TENGs are based on the coupling of CE and electrostatic induction, which leads to an alternating current (AC) output. Since most electronic devices operate under constant current, an AC output of a TENG usually needs to be converted into DC through auxiliary components, which creates an obstacle to miniaturization and integration of the device [56,69,70]. As a result, DC-TENGs have attracted a wide range of attention. Several DC-TENG implementations have been reported, including mechanical rectification and phase control. In addition, unique effects, such as air breakdown and a tribovoltaic effect, have been used to limit the direction of motion of electrons for DC output, exhibiting a simple structure and high efficiency.

DC-TENG Based on Non-Semiconductors

Based on the conventional AC-TENG, mechanical rectification achieves a rectifier-like effect by appropriately exchanging the positions of the two electrodes, thus outputting a constant current. Such devices usually have a mechanical structure of rings [71], discs [72], and cylinders [73] to facilitate the exchange of electrode positions, as shown in Figure 9 (a,b), (c,d), and (e,f), respectively. A phase difference design is also an effective DC-TENG implementation method. By constructing multiple TENGs that output AC with different phases and subsequently rectifying and stacking the outputs of each phase, AC power generated by a conventional TENG can be converted to DC power with a low crest factor [74,75], as shown in Figure 10a,b.

In 2019, Liu et al. proposed a DC-TENG utilizing the air breakdown effect, which led to a rising interest in the application of the phenomenon [76]. The air breakdown DC-TENG is based on a sliding mode TENG coupling the friction electric effect and the electrostatic breakdown effect.

The device consists of a charge collection electrode (CCE), a friction electrode (FE), and friction electric layers, as shown in Figure 11a. Throughout the process, electrons on the FE were transferred to the PTFE via the friction electric effect, and then to the CCE via electrostatic breakdown, and ultimately back to the FE via the external circuit, producing a continuous DC output, as shown in Figure 11b. The equivalent circuit of the device consisted of a charge source and a breakdown capacitor consisting of a CCE and PTFE film (Figure 11c).

DC-TENG Based on Semiconductors

Recently, a new type of DC-TENG has been developed based on semiconductor materials. The basic mechanism of this DC-TENG is the tribovoltaic effect firstly proposed by Wang in 2019 [77], which is similar to the photovoltaic effect. However, the environmental energy that is converted into electrical energy is obtained through friction rather than light [56,77,78,79]. The two friction materials making up this new DC-TENG can be a metal-semiconductor pair [80,81,82,83] or a P-type–N-type semiconductor pair [84,85,86,87]. By the relative motion between the two friction materials, the DC-TENG can directly output DC without passing through a rectifier.

The low output efficiency of the DC-TENG is mainly due to the inefficient charge generation and transfer process, which limits its further development [79]. To enhance its performance, an effective strategy is to couple optical energy with mechanical energy by leveraging the photovoltaic properties of semiconductor materials. Halide perovskite materials exhibit excellent photovoltaic conversion capabilities due to their unique octahedral structure and optimal bandgap properties [60]. The research on the application of perovskite materials in DC-TENGs is different from AC-TENGs, mainly focusing on optimizing the low-impedance design and boosting the output current to achieve more efficient energy conversion [88].

Therefore, different types of perovskite-based DC-TENGs have been widely developed. For example, Yin et al. developed a light-assisted DC tribovoltaic nanogenerator as shown in Figure 12a, where DC was generated from friction between an N-type perovskite and a P-type perovskite [56]. A design of dual perovskite electrodes maximized the photovoltaic characteristics of the device, facilitating tribo-photovoltaic coupling with output voltage and a current up to around 8.72 V and 30.84 μA under light illumination. Furthermore, Yuan et al. developed a tribo-photovoltaic-coupled TENG based on an Al/CsPbBr₃ Schottky junction, as shown in Figure 12b. Under light illumination, the output voltage and current density reached 3.69 V and 11.46 A/m², respectively [55]. Lee et al. developed a DC-TENG based on an N-type perovskite (CsFAMA) and a P-type conductive polymer (PEDOT:PSS), as shown in Figure 12c, which produced a high direct current and voltage output of about 2.1 μA cm² and 0.33 V, respectively [70].

2.4. Optical Mechanisms

In the presence of light, electron-hole pairs of photoactive semiconductors can be excited [89,90,91]. When relative motion between two frictionally active materials generates frictional charges at an interface, photoexcited carriers can significantly increase the surface charge density of the materials. For example, in a DC-TENG, contact between a perovskite and metal forms a Schottky junction, while contact between an N-type perovskite and a P-type perovskite forms a PN junction [56]. By a built-in electric field, the photogenerated carriers move directionally along with the friction charge, which is extracted by the electrode and generates DC output. Thus, for photoactive perovskites, the charge density at the material surface increases in the presence of light, which can be applied to energy generation and optical sensing [44].

3. Applications

3.1. Triboelectric Nanogenerator

TENGs can capture various forms of mechanical energy such as body movements, ambient vibrations, wind, ocean waves, and so on, making them suitable for both everyday technology and large-scale renewable energy applications. The following sections will explore specific applications of TENGs.

3.1.1. Energy Harvesting from Body Movements

The human body can provide enough biomechanical energy for energy harvesting application. The movement of the fingers is a common example, which can be converted into electrical energy through a TENG. For example, Shaukat et al. demonstrated a TENG application using a multi-layered quasi-2D perovskite with the composition (PEA)₂(MA)_n−1Pb_nI_3n+1. An open-circuit voltage of the TENG varied with respect to the value of <n> = 1–5 of the quasi-2D perovskite layers. For the quasi-2D TENG with <n> = 5 layers, it demonstrated excellent electrical performance, with an open-circuit voltage reaching 306 V. The short-circuit current can reach 4.71 μA (Figure 13a,b). The device can harvest energy from human motion as illustrated in Figure 13c showing the tapping device using one, two, and three fingers [92].

Jiang et al. demonstrated the use of a lead-free perovskite/polymer nanofiber composite (LPPS-NFC) in a hybrid triboelectric and piezoelectric nanogenerator (TPENG) integrated into the sole of a shoe for energy harvesting. This approach contrasts with previous examples where the mechanism of TENGs typically necessitates two distinct materials and relative displacement, thereby constraining device configuration and applications. In this work, a hybrid of nanogenerators with triboelectric and piezoelectric effects was realized, as illustrated in Figure 13d. Compression induced charge flow in the TPENG, enhancing an electrical output through both piezoelectric and triboelectric effects. The device cycled through charging and discharging with applied and released forces, continuously boosting electrical output. Furthermore, incorporating Cs₃Bi₂Br₉ perovskites improved the charge-trapping capability of the nanofiber composite and optimized the polar crystalline phase of PVDF-HFP. The TPENG achieved a peak output voltage of 290 V and demonstrated harvesting body movement energy when attached to various parts of the body, as shown in Figure 13e,f [93].

In another work, Zhi et al. demonstrated a core–shell and biocompatible Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers, in short CIC@HFP NFs, as shown in Figure 13g. The CIC@HFP NFs-based TENG generated an open-circuit voltage ranging from 3 to 8 V when positioned on the wrist and elbow and an open-circuit voltage of 130 V when placed on the sole of the foot, as shown in Figure 13h [94].

3.1.2. Energy Harvesting from Nature

Solar cells can convert solar energy into electrical power; however, in many regions, solar energy is intermittent. Therefore, the ability to harvest various environmental energy sources is essential for future self-powered electronics. In 2023, Yuan et al. demonstrated a rainwater energy-harvesting system based on a perovskite quantum-dots-embedded polydimethylsiloxane composite film (PQDP) TENG. In addition, the TENG was integrated with Si solar cells to enable multiple energy harvesting. As shown in Figure 14a, the device included a Ag₂ electrode deposited on the perovskite quantum-dots-embedded polydimethylsiloxane composite film (PQDP) surface, positioned to coincide with a silver grid electrode on the solar cell to maximize sunlight capture. When positively charged raindrops struck the device, the initial equilibrium was disrupted, causing positive charges to migrate to the top of the device and generating electric current. As the raindrops departed, these positive charges shifted back to the bottom of the device, producing current in the opposite direction, as shown in Figure 14b. V_OC and I_SC produced by raindrops are shown in Figure 14c, and their pulse width was 52 ms and 7 ms, respectively [95]. As solar energy is renewable, there has been considerable research focused on the photo-induced enhancement of TENGs. Su et al. designed a single-structure TENG without additional electronic components. The device uses MAPbI₃ as an election transfer layer to enhance the total output. Under illumination, V_OC, I_SC, and the amount of electric charge (Q) increased by 11%, 11%, and 9%, respectively [96].

Later, Wang et al. investigated the coupling of triboelectric and photovoltaic effects in CsPbBr₃ perovskite. To address the impact of PVDF’s insulating properties on electron transport, they implemented a carbon/perovskite structure, as shown in Figure 14d. When exposed to full-spectrum standard simulated sunlight (AM 1.5 G), a reversed voltage signal of around 1 V was detected, along with an increased current of 270 μA, as demonstrated in Figure 14e. The separation of photo-induced electrons and photo-induced holes resulted in the addition of a photocurrent on top of the triboelectric current, demonstrating the synergistic enhancement by both the triboelectric and photovoltaic effects [97]. Similarly, Ippili et al. combined a MAPbI₃-PVDF composite TENG with a conventional photodetector, utilizing the harvested energy to power the photodetector. The output voltage rose from ~44.7 to ~67.9 V as the light intensity increased from zero to 3.23 mW/cm² [98].

Organic-inorganic hybrid perovskites such as MAPbI₃ face performance degradation due to repeated contact and friction, as their triboelectric surfaces are exposed to the environment. Furthermore, these photo-induced TENGs exhibit a trade-off where an increase in current density is accompanied by a reduction in output voltage, caused by shielding or short-circuit effects. Kim et al. proposed a PVDF-TrFE/PEDOT:PSS/MAPbI₃ composite, as shown in Figure 14f. In this structure, PVDF-TrFE prevented degradation of MAPbI₃, thereby enhancing stability. Meanwhile, PEDOT served as a carrier path, directing the movement of electrons and holes. The current density increased proportionally as the light density increased from 0 to 40 mW/cm², as shown in Figure 14g [12].

3.2. Self-Powered Sensors

3.2.1. Optical and Mechanical Dual-Mode Sensor

In the age of information, sensors have been developed for different types of stimuli, such as pressure, light, temperature, and humidity [99,100,101]. However, these devices can only detect single-mode external stimulus, limiting their versatility [102,103,104]. Therefore, it is necessary to develop a single device capable of simultaneously collecting multiple signals. Perovskite materials, due to their excellent dielectric and optical properties, have great potential for dual-mode mechanical and optical sensing.

For example, methylammonium lead halide perovskite (MAPbX) is one of the widely used perovskites for photodetectors. By combining with TENGs, self-powered dual-mode sensors can be realized [17,98,105]. However, due to the toxicity of lead (Pb), MAPbX perovskites exhibit limited applicability. Antimony (Sb)-based perovskites, having low toxicity and perfect optoelectronic characteristics, can be alternatives to lead-based ones [106,107,108]. For example, Ding et al. developed a photo-enhanced dual-mode self-powered detector with the TENG structure of ITO/Cs₃Sb₂Cl₃I₆/PDMS/ITO. The perovskite was treated by hydriodic acid (HI), providing an iodine-rich environment to prevent lattice defects. The TENG in a contact-separation mode collected mechanical energy from the environment and produced a voltage which was linearly related to the external force. The pressure sensor exhibited sensitivities of 0.989 V/kPa in the range of 6.2–68.2 kPa and 0.141 V/kPa in 68.2–93 kPa, as shown in Figure 15a. In addition, the photogenerated carriers in Cs₃Sb₂Cl₃I₆ were directed to move in the presence of the built-in electric field, and the resulting change in surface charge density enabled optical sensing. The mean maximal optical responsivity value of the device was around 13.78 μA/W (Figure 15b) [109].

As seen in the above example, changes in the density of photogenerated carriers can lead to changes in photocurrent and resistance values. Thus, mechanical and optical sensing can be further characterized by pulses in signal waveforms and baseline changes in resistance values. For example, Wu et al. developed a bimodal self-powered TENG with MAPbI₃-PVA as photoactive and triboelectric films and PTFE as the corresponding negative friction layer. By attaching a three-by-three sensor array to the volunteer’s cervical vertebra (Figure 15c), lighting illumination and motions of cervical vertebra could be continuously monitored. The dual-mode sensor exhibited a mechanical sensitivity of 1.6 V/N and the highest photoelectric responsivity of 64 mA/W. The amplitude and speed of cervical rotation were obtained by calculating the pulse amplitude and inter-pulse delay of the output signal of each sensor. Light intensity, as the most significant factor affecting the impedance value of perovskite materials, was characterized by the drifting of resistance value, as shown in Figure 15d. By decoupling mechanical and optical information from the output, the bimodal e-skin was demonstrated as an effective tool for long-term detection of phototherapy and motion in rehabilitation exercises [110].

3.2.2. Humidity and Temperature Sensor

As temperature and humidity change over time, monitoring ambient weather conditions is crucial for various applications, such as industrial production, agricultural management, weather forecasting, and indoor environmental quality control [111,112,113]. Attributed to the sensitivity of halide perovskites to humidity and temperature, halide perovskite-based TENGs have potential applications in self-powered temperature and humidity sensing. Comparing organic–inorganic hybrid perovskites, inorganic perovskites exhibit higher stability and thus have been widely employed for temperature and humidity sensing [55,114]. In inorganic perovskite-based TENGs, a decrease in temperature and humidity leads to an increase in output voltage, resulting from multiple factors including decreases in thermionic emission, mobility, and a slight shift of the Fermi level toward the center of the bandgap. Similarly, the reduction in humidity leads to an impediment of surface charge storage and transport by the water film adsorbed on the perovskite, resulting in a decrease in output power [55,115]. These fluctuations in signals can be utilized to monitor temperature and humidity changes using the TENG.

For example, Yuan et al. developed a DC-TENG based on an Al/CsPbBr₃ Schottky junction for collecting mechanical and optical energy (Figure 16a). The output voltage was linearly correlated with humidity (30–60%) and temperature (30–80 °C), as shown in Figure 16b [55]. Jiao et al. also reported a decrease in output voltage with increases in temperature (25–65 °C) and humidity (30–70%) in the Cs_0.05FA_0.7MA_0.25PbI₃ perovskite-based TENG, as shown in Figure 16c,d. However, the authors reported slight degradation of output performance when the temperature and humidity returned to their original values due to instability in materials [115].

For humidity sensing, a capacity of moisture absorption of the perovskite material is an important consideration [116,117]. For example, Mondal et al. developed a CsPbI₃-PVDF composite-based hybrid piezo-triboelectric nanogenerator as a humidity sensor. Due to its hydrophobicity attributed to the surface porosity, the CsPbI₃-PVDF film was reported as an attractive option for moisture monitoring. As shown in Figure 16e,f, the output voltage of the hybrid nanogenerator decreased from 207 to 81.3 V when the humidity increased from 34 to 91%. Figure 16g shows the response diagram describing the sensing behavior of this device, which exhibited the high sensitivity of a 2.20 V/unit change in humidity [118].

Figure 16. (a) The structure of a DC-TENG based on Al/CsPbBr₃ Schottky junction for collecting mechanical and optical energy. Reprinted with permission from Ref. [55]. 2022, John Wiley and Sons. (b) The output of DC-TENG based on Al/CsPbBr₃ Schottky junction. Reprinted with permission from Ref. [55]. 2022, John Wiley and Sons. (c) The structure of a Cs_0.05FA_0.7MA_0.25PbI₃ perovskite-based TENG. Reprinted with permission from Ref. [115]. 2024, Elsevier. (d) The temperature- and humidity-related output of the Cs_0.05FA_0.7MA_0.25PbI₃ perovskite-based TENG. Reprinted with permission from Ref. [115]. 2024, Elsevier. (e–g) The temperature- and humidity-related output of a CsPbI₃-PVDF composite-based hybrid piezo-triboelectric nanogenerator. Reprinted with permission from Ref. [118]. 2024, American Chemical Society.

Figure 16. (a) The structure of a DC-TENG based on Al/CsPbBr₃ Schottky junction for collecting mechanical and optical energy. Reprinted with permission from Ref. [55]. 2022, John Wiley and Sons. (b) The output of DC-TENG based on Al/CsPbBr₃ Schottky junction. Reprinted with permission from Ref. [55]. 2022, John Wiley and Sons. (c) The structure of a Cs_0.05FA_0.7MA_0.25PbI₃ perovskite-based TENG. Reprinted with permission from Ref. [115]. 2024, Elsevier. (d) The temperature- and humidity-related output of the Cs_0.05FA_0.7MA_0.25PbI₃ perovskite-based TENG. Reprinted with permission from Ref. [115]. 2024, Elsevier. (e–g) The temperature- and humidity-related output of a CsPbI₃-PVDF composite-based hybrid piezo-triboelectric nanogenerator. Reprinted with permission from Ref. [118]. 2024, American Chemical Society.

3.2.3. Physiology Sensor

In addition to the applications mentioned above, TENGs show great potential for applications in the field of self-powered wearable sensors [119,120,121,122]. Materials such as polydimethylsiloxane (PDMS) [123,124,125] and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) [126,127,128] have become premier materials for the fabrication of wearable TENGs due to their excellent mechanical flexibility and electrical properties. To further enhance the triboelectric properties of these materials and consequently the sensitivity of wearable sensors, the introduction of conductive fillers has become an effective strategy [129,130,131]. Compared to other fillers, perovskite fillers support electrical property modulation and energy level alignment of triboelectric materials, leading to improved charge transfer efficiency and sensitivity [94,132,133].

Perovskite materials have been extensively studied as a reinforcing filler for wearable flexible materials [134,135,136,137]. For example, Chen et al. realized a flexible TENG for real-time biomechanical detection based on hydrochromic CsPbBr₃-KBr (CPB-K) microcrystals with PDMS coating as a friction layer (Figure 17a). The flexible TENG based on the CPB-K/PDMS film was successfully tested on the human body. This sensor was mounted on the neck, hand, abdomen, and leg, enabling the detection of the motion state of each part, and some of examples are shown in Figure 17b. For example, the movement of the legs at various angles resulted in the generation of 2 to 8 V of output potential, which demonstrated a potential application of the device in human health monitoring and human-machine interfaces [133]. These distinct differences in waveforms prove the high sensitivity in the TENG device. Additionally, Jiang et al. developed a kind of self-healable and stretchable gas-solid triboelectric nanogenerator based on a PSDU matrix with ferroelectric (DF-CBA)₂CuCl₄ fillers, in which perovskite fillers enable better output performance through the formation of a hydrogen bonding network [138]. This power output is 1400 times higher than that of a dense PSDU-based TENG.

However, the addition of conductive fillers may lead to changes in the original properties of the flexible material, which affect the sensitivity of the TENG in detecting human motion [139,140]. For example, the incorporation of nanofillers in PVDF-HFP leads to inhomogeneities and discontinuities at the nanofiller-PVDF interface, which can cause inhomogeneous dispersion of the nanocrystals along the uniaxial direction and β-phase instability. This leads to charge loss at the crystalline interface and limited the improvement of TENG performance [94,141]. To resolve this issue, Zhi et al. prepared Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers with a core–shell structure by one-step electrostatic spinning method (Figure 17c) [94]. Hydrogen bonds were formed between Cs₂InCl₅(H₂O), acting as an inducer, and PVDF-HFP, which facilitated the automatic well alignment of the dipoles and stabilized the β-phase. The TENG based on Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers was attached to the major joints of the human body, such as the finger joints, wrist joints, and elbow joints. The output signals exhibited different scales of repetitive waveforms at different bending angles, as shown in Figure 17d [94].

In addition, the luminescent properties of perovskites hold great promise in the field of biosensing [142,143,144]. For example, Chen et al. proposed CsPbBr₃@KBr, exhibiting narrow green photoluminescence and high quantum yield (Figure 17e) [145]. The CsPbBr₃@KBr was employed as a PVA filler for a high-performance positive triboelectric material in TENGs. The prepared TENG enables human body sensing and real-time motion detection of multiple parts of the human body (Figure 17f).

Figure 17. (a) The structure of a flexible TENG based on CsPbBr₃-KBr/PDMS film. Reprinted with permission from Ref. [133]. 2022, Elsevier. (b) Detection of the motion state based on the flexible TENG based on CsPbBr₃-KBr/PDMS film. Reprinted with permission from Ref. [133]. 2022, Elsevier. (c) Structure of Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers. Reprinted with permission from Ref. [94]. 2024, American Chemical Society. (d) Detection of the motion state using TENG based on Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers. Reprinted with permission from Ref. [94]. 2024, American Chemical Society. (e) Narrow green photoluminescence and high quantum yield of CsPbBr₃@KBr. Reprinted with permission from Ref. [145]. 2023, Elsevier. (f) Human body sensing based on TENG with CsPbBr₃@KBr as a PVA filler. Reprinted with permission from Ref. [145]. 2023, Elsevier.

Figure 17. (a) The structure of a flexible TENG based on CsPbBr₃-KBr/PDMS film. Reprinted with permission from Ref. [133]. 2022, Elsevier. (b) Detection of the motion state based on the flexible TENG based on CsPbBr₃-KBr/PDMS film. Reprinted with permission from Ref. [133]. 2022, Elsevier. (c) Structure of Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers. Reprinted with permission from Ref. [94]. 2024, American Chemical Society. (d) Detection of the motion state using TENG based on Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers. Reprinted with permission from Ref. [94]. 2024, American Chemical Society. (e) Narrow green photoluminescence and high quantum yield of CsPbBr₃@KBr. Reprinted with permission from Ref. [145]. 2023, Elsevier. (f) Human body sensing based on TENG with CsPbBr₃@KBr as a PVA filler. Reprinted with permission from Ref. [145]. 2023, Elsevier.

3.2.4. Triboelectric-Induced Photodetector

Photodetectors can convert optical signals into electrical signals, which have a wide range of applications in imaging, optical communication, sensing, and bio-detection [146,147,148]. A perovskite-based TENG can couple two effects, namely, a triboelectric and photovoltaic effect, enabling self-powered optical sensing. In the presence of light, perovskite generates photogenerated carriers, increasing the surface charge density of the material [149,150]. The coupling of electric–photovoltaic and triboelectric effect for perovskite TENGs has demonstrated its application to optical sensing.

For light detection applications, perovskites need to have good light absorption, which requires specific optical properties of the materials [151,152]. Wei et al. fabricated a photo-enhanced TENG based on lead-free bismuth halide perovskite CsBi₃I₁₀ (CBI) film (Figure 18a). CBI exhibited broad absorption capability compared to other photoactive perovskites, attributed to a narrow bandgap of 1.77 eV compared to other counterparts with larger bandgaps (Figure 18b). Under illumination, the output voltage and current of the CBI/PDMS TENG grew by around 21.7% and 15.8%, which was caused by photogenerated carriers, as shown in Figure 18c [26].

To further enhance the unidirectional photocurrent gain, it is desirable to integrate a perovskite solar cell into a TENG. Towards this goal, Guo et al. introduced a triboelectric–photoelectric-coupled hybrid TENG based on CsPbBr₃ and a multi-wall carbon nanotubes (MWCNTs)-PDMS composite film (PC), as shown in Figure 18d. The composite film is both a negative friction layer and a photogenerated hole collector. As conductive fillers and photogenerated hole-harvesting materials, MWCNTs in PDMS reduced the open-circuit voltage to enhance the short-circuit current density. As demonstrated in Figure 18e, the short-circuit current density of the device shows a clear current-dependent variation, which demonstrates its potential for optical sensing [153].

Figure 18. (a) A photo-enhanced TENG based on lead-free bismuth halide perovskite CsBi₃I₁₀ (CBI) film. Reprinted with permission from Ref. [26]. 2023, Elsevier. (b) A narrow bandgap of 1.77 eV of CBI. Reprinted with permission from Ref. [26]. 2023, Elsevier. (c) Output voltage and current of the CBI/PDMS TENG. Reprinted with permission from Ref. [26]. 2023, Elsevier. (d) Triboelectric–photoelectric-coupled hybrid TENG based on CsPbBr₃ and carbon nanotubes-PDMS composite film. Reprinted with permission from Ref. [153]. 2021, John Wiley and Sons. (e) Optical sensing ability of the triboelectric–photoelectric-coupled hybrid TENG. Reprinted with permission from Ref. [153]. 2021, John Wiley and Sons.

Figure 18. (a) A photo-enhanced TENG based on lead-free bismuth halide perovskite CsBi₃I₁₀ (CBI) film. Reprinted with permission from Ref. [26]. 2023, Elsevier. (b) A narrow bandgap of 1.77 eV of CBI. Reprinted with permission from Ref. [26]. 2023, Elsevier. (c) Output voltage and current of the CBI/PDMS TENG. Reprinted with permission from Ref. [26]. 2023, Elsevier. (d) Triboelectric–photoelectric-coupled hybrid TENG based on CsPbBr₃ and carbon nanotubes-PDMS composite film. Reprinted with permission from Ref. [153]. 2021, John Wiley and Sons. (e) Optical sensing ability of the triboelectric–photoelectric-coupled hybrid TENG. Reprinted with permission from Ref. [153]. 2021, John Wiley and Sons.

4. Perspectives

With the rapid development of the Internet of Things (IoT), an increasing number of sensors are required, and the most common method of powering these sensors is through energy storage devices, typically batteries. As a consequence, the demand for energy is becoming ever more urgent. Furthermore, since many batteries contain toxic materials such as lithium and cobalt, their disposal poses significant environmental harm. Therefore, energy harvesters have emerged as a promising solution in this era. One such energy harvester is the TENG, invented by Prof. Zhong Lin Wang’s group in 2012 [154]. TENG’s ability to generate high voltages makes it particularly advantageous in certain applications which need high voltage as input, such as plasma generators [155]. In addition, TENG as an energy harvester offers various advantages over other generators. For instance, attributed to a flexible structure of a TENG, it can be specifically designed to suit different environments, making it highly suitable for harvesting ambient energy [154]. Also, the material selection for TENGs is very diverse, almost encompassing all materials that exhibit triboelectric properties. This allows TENGs to be customized according to different application requirements. Finally, due to the low manufacturing cost of TENGs, they offer good cost-effectiveness when produced on a large scale [24]. Compared to other types of TENGs, perovskite-based TENGs exhibit strong light absorption, high charge mobility, and excellent defect tolerance [1,2,3]. These characteristics enhance their performance in energy harvesting and sensing applications.

At the same time, however, TENGs have faced challenges. The first challenge is that the charge transfer mechanism of triboelectricity has not been defined yet. Diaz and Felix-Navarro suggest that there are three possible species of charge transfer mechanisms, considering the types of transferred charge and the driving forces involved. However, it remains uncertain which species is applicable [156]. This uncertainty makes it challenging to predict the performance of TENGs [54]. For example, ferroelectric perovskite materials typically exhibit high piezoelectric properties due to their significant remanent polarization. However, there is no straightforward correlation between their actual piezoelectricity and remanent polarization due to the surface morphology and electronic properties of perovskites [157]. The second challenge is that power/energy generated is not high enough in practice, i.e., in the actual environment. Currently, it is capable of charging small-scale electronic devices, including sensors and LEDs, with limited power output [11,26,158]. One of the major reasons for this bottleneck is the occurrence of air breakdown, governed by Paschen’s law [67]. Many studies have analyzed the causes of air breakdown and suggested various approaches to address the issue. Some researchers propose conducting experiments in high-vacuum environments, while others recommend reducing the dielectric thickness as a potential solution [67,68]. There are also some studies where TENGs have achieved energy harvesting efficiency rates of up to 80%, provided that the external mechanical force is optimally aligned with the proposed TENG’s design. Therefore, future research should focus more actively on circuit design to enhance the output charge density [54]. For example, Zheng et al. combined TENGs with the power management circuit which includes the passive self-switching circuit and LC filter circuit. The results showed that the system can store energy a hundred times higher than general circuits with a 100 µF capacitor [159]. In order to further improve performance, researchers can focus on coupling effects such as triboelectric–photoelectric-coupled TENGs, since perovskite-based TENGs exhibit strong light absorption. Also, for AC-TENGs, the rectifier wastes certain energy, so it is important to research DC-TENGs in the future.

In addition to functioning as an energy harvester, TENGs can also serve as sensors. However, perovskite-based TENGs exhibit instability under thermal and humid conditions, leading to variations in output for the same mechanical input [54,55,114]. Therefore, a deeper understanding of the triboelectrification mechanism is needed to design methods that can control these variables, ensuring greater stability for TENGs [54]. Although perovskite-based TENGs offer superior performance, they also face numerous challenges. Many perovskites contain lead, which is toxic to humans and presents a significant barrier to the commercialization of TENGs. Additionally, the rigidity and brittleness of perovskites hinder their suitability for use in wearable electronics [157]. Therefore, developing high-performance lead-free perovskites is of great significance. In addition, exploring the various properties of perovskite-based TENGs in the future and integrating these properties is crucial. For example, coupling the semiconductor properties, tunable energy levels and bandgaps, and photovoltaic effects of halide perovskite-based TENGs could enable them to harvest energy from multiple input sources [157].

TENGs have only been in development for a little over a decade, making them a relatively new and innovative approach to energy harvesting. Despite their short history, they have demonstrated significant potential in harnessing energy from a wide range of mechanical sources, such as human motion, ocean waves, and ambient vibrations. Ongoing advancements in materials science, device architecture, and integration techniques suggest that TENGs could play a critical role in the future of sustainable energy solutions.

5. Conclusions

Attributed to their ability to provide additional photocurrent, photosensitive semiconductors provide an effective solution to the unbalanced current-voltage output of the TENG. As a star photoactive semiconductor material, perovskite has gained widespread attention. In this article, we discussed the material properties and energy conversion mechanisms of perovskite-based TENGs and summarized the latest applications of perovskite-based TENGs, mainly in terms of energy harvesting and sensing. On the material side, the outstanding properties of 3D perovskites and quasi-2D perovskites, especially the photoelectric properties, were highlighted. Problems with their stability and solutions were then further discussed. On the mechanism side, the energy transfer mechanism between different media and the energy generation mechanism of DC-TENGs and AC-TENGs were presented, especially for DC-TENGs based on semiconductor materials. Finally, the applications of perovskite-based TENGs were discussed in detail, focusing on the harvesting of energy from the human body, nature, and light and multimodal sensing. The results clearly show that the perovskite-based TENG exhibits excellent output capabilities and has promising prospects.