Organolead halide perovskites beyond solar cells: self-powered devices and the associated progress and challenges

Conventional electronic devices powered by lithium-ion batteries or supercapacitors face the challenge of oﬀering long-term and self-sustaining operations. Self-powered devices based on emerging energy harvesting technologies can help of the development of long-lasting transducers with a small form factor. Organolead halide perovskites due to their excellent electro-optical properties are promising photosensitive materials for the development of such self-powered devices. The following review aims at summarizing recent developments made in the field of organolead halide perovskite based self-powered applications. The underlying mechanism driving the self-powered operation in electronic devices such as photodetectors, tactile sensors, and gas sensors is discussed in detail. Finally, the current challenges and prospects of these perovskite based optoelectronic applications are highlighted.


Introduction
With the ongoing continuous advancement in the fields of the Internet of Things (IoT), 1-3 portable and wearable electronic devices, 4,5 structural monitoring, 6,7 and implantable medical devices, 8,9 the indispensable dependence of electronic devices and sensors on an external source of power poses a significant challenge. As it stands, electronic devices and sensors, especially those located in remote places, are powered solely by batteries. The limited lifetime of batteries mandates frequent replacement, and, more importantly, batteries also pose a risk of leaking toxic hazardous substances into the environment. 10,11 Therefore, concurrently with the current efforts for the miniaturization of sensors and integrated circuits, 12 there is an equally critical need for transducers that are capable of operating by harvesting power ) leads to the formation of a stable tetragonal phase at room temperature. 23 The 3-D framework of MABX 3 is devoid of distortion in the MA-X interaction, as otherwise observed in systems involving ethyl ammonium (CH 3 CH 2 NH 3 + ) and formamidinium cation (HC(NH 2 ) 2+ ). 24 In the context of occupying the B site in the ABX 3 system, lead (Pb) has been reported as the superior constituent in comparison to its IV A metal counterparts such as tin (Sn), both in terms of stability and performance and consequentially has been the most widely used metal ion. Ideally, utilization of those with lower atomic numbers (such as Ge or Sn) can be used to lower the energy bandgap of the corresponding perovskite and in addressing the toxicity of Pb, but their utilization is limited by the low intrinsic ionic stability of their divalent oxidation state. 25 The halide anion X provides much more freedom of variation in its composition. Iodide (I), lying closest to Pb in the periodic table, shares a similar covalent character and leads to a stable perovskite structure. Upon progressing down the VII A group elements (Cl -I), with an increase in the atomic size of the anion, the absorption spectra shift towards a longer wavelength (redshift) which is attributed to a decrease in electronegativity. 24 Hence, chloride and bromide incorporation along with the iodide anion at the X site offers a facile way to tune the bandgap of the halide perovskites. Owing to the exhibition of remarkable properties such as broad absorption spectrum, high absorption coefficient, long carrier lifetime, low trap density, and large diffusion length, organolead halide perovskites (OLHPs) are ideally suited as a material of choice for photovoltaic devices and light energy harvesting. 26 For instance, the MAPbI 3 tetragonal phase exhibits a suitable bandgap in the range of 1.51-1.55 eV, with an 820 nm absorption edge, exceeding the optimal 1.1-1.4 eV bandgap range governed by the Shockley-Queisser limit for a single-junction solar cell. 27 Hence, MAPbI 3 is the most typically employed OLHP in photovoltaic applications.
While significant research efforts have been devoted to enhancing the PCE of perovskite solar cells (PSC), new applications of these materials are being researched with increasing efforts, for instance, lasers, light-emitting diodes (LEDs), photodetectors (PDs), gas sensors and tactile sensors. [28][29][30][31][32][33] In the past few years, OLHP based optoelectronic devices and sensors, capable of operating without an external power supply and exhibiting excellent performance, have been actively researched and reported, continuously advancing the field of self-powered devices. 34 Therefore, a review focusing on OLHP based selfpowered devices will be useful for researchers in this field and also the broader research community. This review provides a detailed discussion of the mechanisms driving self-powered operation in OHLP based electronic devices of varying architecture and their associated figures of merit. Understanding these ongoing developments of perovskite-based self-driven devices will pave the way for further optimization and advancements in this area for future commercialization. A wide array of applications beyond solar cells are discussed where OLHP harvests energy and simultaneously serves as an active layer in an integrated device. A limited discussion focuses on cases where OHLP harvests energy and drives an independent device. Primarily, the latest advancements in PDs, IoTs, transistors, photovoltachromic cells, and sensors are reviewed. Finally, the current challenges of the perovskite-based self-Vivek Maheshwari Vivek Maheshwari is an associate professor of chemistry at the University of Waterloo. He is also a member of the Waterloo Institute for Nanotechnology.
His group's research is on lead halide perovskite materials for devices and sensors, 1D materials for wearable devices, and nanomaterials for electrocatalysis. He completed his bachelor's degree from the Indian Institute of Technology, Delhi, and his doctorate from Virginia Tech. powered devices are presented along with a discussion on future research prospects.

Photodetector (PD)
A PD converts an optical input such as visible light photons into an electrical signal 35 and has wide-ranging applications in the detection of light intensity, spectral range detection, 36 thermal imaging, 37 remote imaging, 38 and so forth. The significant figures of merit for PDs include responsivity, detectivity, light switching ratio (on/off ratio), spectral selectivity, linear dynamic range, and response time. 39 Currently, most of the commercial photodetectors are based on crystalline GaN, Si, and InGaAs. 40 The underlying mechanism driving the PD operation is either a p-n (p-i-n) junction, Schottky based, or a photoconductive effect, where the conductivity of the active material increases on interaction with the incident photons. Broad spectral range (190-1100 nm) commercial photodetectors based on the Si p-n junction (photodiodes) (e.g., Hamamatsu S1336) exhibit peak responsivities of 0.12 and 0.5 A W À1 at 200 and 960 nm, respectively, and those designed especially for the visible range (340-720 nm) photometry (e.g., Hamamatsu S8265) exhibit a responsivity of 0.3 A W À1 at the peak sensitivity wavelength of 540 nm, with a dark current of B20 pA. These commercial PDs usually require an external power supply and hence need an integrated assembly in the device which can make the device bulky and serve as a bottleneck for utilization in a remote location. Furthermore, these commercial photodiodes need stringent manufacturing controls which limit them to low-volume, high-value markets. To overcome the existing trade-offs between performance, form factor, cost, and most importantly, power consumption, self-powered PDs are being investigated with intense interest. [41][42][43] Self-driven PDs can be obtained when photodiodes work at zero bias, similar to solar cells working under short-circuit conditions. Owing to their excellent electro-optical characteristics, the OLHP based selfpowered PDs can offer the benefit of being lightweight with a small device size, without compromising on device performance. 44 The early reports on perovskite-based PDs with self-powered characteristics (but did not focus on it exclusively) appeared in 2014 when Dou et al. demonstrated a solution processed 'inverted' device (solar cell) configuration PD based on organic-inorganic hybrid CH 3 NH 3 PbI 3Àx Cl x . The device had a high detectivity of up to 10 14 Jones and was capable of operating at 0 V. 45 Based on the charge separation mechanism, OLHP PDs can generally be classified into the following categories: p-n (or p-i-n) junction, Schottky junction, polarization effects based, and photoelectrochemical-type PDs. Besides these, an integrated system of energy harvesting units (such as TENG) that drives the light sensitive OLHP to achieve detection can be considered as a distinct type of self-driven PD. The following section summarizes recent advances in PDs driven by these operating mechanisms. Some common strategies that are being employed for enhancing the figures of merit for perovskite PDs have also been discussed. The last subsection is dedicated to flexible OLHP PDs that are now receiving substantial attention as promising candidates for next-generation portable and wearable electronics.

p-n (or p-i-n) junction-based PDs
The p-n junction-based PDs are the most common type reported so far in the literature. A typical PD with a vertical architecture consists of a perovskite layer sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL) (p-i-n type). It is widely accepted that the interfacial contact properties between the perovskite and transport layers affect the efficiency of photovoltaic devices. Hence, by careful selection of the transport layers, PDs with effective dissociation and transport of photogenerated charge carriers to their respective counter electrode contacts can be fabricated. 46 ETLs not only influence the electron transfer and collection but also behave as the hole blocking layer to suppress the electron-hole recombination at the interface. 47,48 Significant attention has been paid to ZnO as an electron transport material (ETL) in the early research works reporting OLHP based self-powered PDs. Specifically, morphological tuning of ZnO material was used for the performance optimization of OLHP PDs. Initially, Wang et al. coupled the pyroelectric effect generated in wurtzite ZnO nanowires (NWs) with the photoexcitation ability of MAPbI 3 . 49 The assembly with well-aligned energy band levels led to enhanced charge-carrier separation at the ZnO/perovskite interface. The perovskite also contributed to low dark currents and low background temperature. In the self-powered mode, this UV sensitive heterostructure assembly exhibited 322% enhancement in specific detectivity and responsivity, complemented with five orders of magnitude improvement in the rise time (5.4 s to 53 ms) and fall time (8.9 s to 63 ms) in comparison to devices operating at an applied bias of 0.3 V. The decline in device performance due to applied bias was attributed to the reduced pyro-potential of ZnO NWs owing to the high dark currents and high background temperature. Another report on well-aligned energy levels in the ZnO nanorod (NR)/MAPbI 3 heterostructure appeared a year later. Here, Yu et al. used an inorganic metal oxide layer of MoO 3 as a hole transport layer to enhance exciton transport while simultaneously protecting the organic perovskite layer. 50 The corresponding broadband PD with an appropriate thickness of MoO 3 (12 nm) yielded a high detectivity value of 3.56 Â 10 14 Jones and 24.3 A W À1 photo-responsivity under 500 nm excitation and no applied bias. Other than preferable band alignment, Zhu et al. proposed the hypothesis that a uniformly distributed blend of ZnO microspheres and nanosheets can effectively utilize the high surface area of microspheres while decreasing series resistance in a vertically configured glass/fluorine doped tin oxide (FTO)/ZnO/MAPbI 3 / MoO 3 /Au device. 51 Under illumination, the self-driven PD exhibited spectral response from 300 to 800 nm with responsivity and detectivity values reported as high as 48 mA W À1 and 4.5 Â 10 11 Jones, respectively. The on/off ratio was up to 1400, while the rise time and fall time of the photoresponse were recorded to be 14 and 12 ms, respectively. Despite the facile synthesis process and high electron mobility with increased area of contact of the ZnO NR layer, the photoelectric performance of PDs is generally limited. This is due to the presence of defect states, especially oxygen vacancy which may lead to enhanced charge recombination at the interface between perovskite and ZnO NRs. To primarily reduce the defect density, Zhou et al. doped ZnO with a slightly smaller Ga 3+ . 52 The Ga doped ZnO NR layer (as ETL) served as an effective scaffold with enhanced electron mobility. The resultant HTL-free PD device prepared using graphite electrode had an on/off ratio, a responsivity, and a detectivity of B2.5 Â 10 3 ,0 . 3AW À1 , and 1.3 Â 10 12 Jones, respectively. The work by Yang et al. involved the coupling of a MgO/ZnO bilayer with the perovskite layer. 53 The ZnO microsphere array scaffold facilitated the penetration of the perovskite into the array and resulted in improved light-harvesting, while MgO passivated the interface between ZnO and perovskite, enhancing stability and carrier transport. Recently, Liu et al. proposed a Cu ion-induced p-type doping in perovskite films by the addition of CuSCN. 54 58 T h ea i r -s t a b l ei n o r g a n i cH T Le n a b l e dt h eP Dt or e t a i n9 2 % performance despite storing the device in ambient air for a month. The template of quasi-identical micron-sized Si pillars offered a large interfacial contact area for incident light while simultaneously reducing the reflection. The authors proposed that the Si-NPA template also effectively enhanced carrier transport and reduced the collection path of the photogenerated carriers leading the self-driven PDs to exhibit a high on/off ratio of 8.2 Â 10 4 , a photoresponsivity of 8.13 mA W À1 , a specific detectivity of 9.74 Â 10 12 Jones, and fast response speeds of 253.3/230.4 ms at zero bias under 780 nm light illumination. Towards realizing the commercial applications of self-driven PD based on OLHP, the work of Shen et al. can be considered as one of the most significant contributions. The authors demonstrated a homemade time-resolved photoluminescence (TRPL) system ( Fig. 1a) facilitated by an ultrafast OLHP PD with the configuration of indium tin oxide (ITO)/poly(bis(4-phenyl) (2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI 3 /C 60 /2,9-dimethyl-4, 7-diphenyl-1,10-phenanthroline (BCP)/copper (Cu). 59 The device area dependent sub-nanosecond response time (0.95 ns for 0.04 mm 2 device area) of the PD enabled the detection of the photoluminescence lifetime for typical organic and hybrid materials, ranging from several nanoseconds to microseconds ( Fig.1b-d).Inarecentwork,Shanet al. presented a bifunctional light-emitting/detecting LiFi (light fidelity) fiber utilizing perovskite QDs. 60 The authors capitalized on the small exciton binding energy and high carrier mobility of the all-inorganic perovskite QDs (CsPbBr 3 ) to successfully integrate electroluminescence with narrow emission (B19 nm) and photodetection in a single fiber. The flexible assembly exhibited green colored luminance of B100 cd m À2 at 7 V after being subjected to multiple bending cycles and the corresponding current efficiency was 1.67 cd A À1 . The band bending in the space charge region at the interface between the perovskite QDs and the transport layers (due to differences in the Fermi levels) enabled a zero-bias operation of the PD with an on/off ratio of 1.5. OLHP QDs can potentially be utilized based on a similar principle for bifunctional devices. A major challenge that hinders the commercial viability of OLHP PDs (and other devices) is their instability in air and under ambient conditions. OLHP material rapidly degrades and decomposes in the presence of oxygen and moisture. Furthermore, electric field induced ion migration primarily across the perovskite grain boundaries further enhances material degradation, eventually worsening the optoelectronic properties. 61 H o w e v e r ,a tt h es a m et i m e ,t h eC -A F Ma n d PC-AFM studies carried out by Li et al. in MAPbI 3 /CdS heterojunction PD indicated that a short transport distance at the grain boundaries acted as preferential spots of charge carrier transportation and exciton separation. 62 The study of the photoelectric mechanism at the nanoscale showed that higher photocurrent values are observed at the grain edges. This offers the potential of grain boundary engineering towards fine-tuning the overall device performance. The fabricated PD exhibited a high current on/off ratio of B1.13 Â 10 5 , detectivity of B9.79 Â 10 10 Jones, and PCE of up to B10.05%. Table 1 summarizes the heterojunction based self-powered OLHP PDs and their associated key figures of merit.
Although the p-n junctions have been widely applied in the fabrication of OLHP based self-driven PDs, little attention has been paid to the formation of the p-n homojunction within OLHP. Pang et al. stressed that the transport layers are optional in perovskite-based optoelectronic devices and claimed that the intrinsic light-induced self-poling ability in the perovskite could drive a Pt/CH 3 NH 3 PbI 3 /SiO 2 /Si/Al configured PD. 63 The researchers through series of experiments ruled out the possibility of the observed self-powered characteristics being driven by the Schottky junction or poling of the device (by applying an external bias for a short duration before recording the device characteristics at 0 V). The authors hypothesized that under illumination, a built-in electric field was generated as positive ions/vacancies accumulated at the perovskite/SiO 2 interface and negative ions/vacancies accumulated at the perovskite/Pt interface, owing to band bending in the asymmetric device. Under the influence of the  built-in electric field, the photogenerated charge carriers drifted to their respective counter electrodes. While the holes were collected by the Pt electrode, the electrons traversed through the SiO 2 film and were collected by the Al electrode by the ion trap-assisted emission mechanism (Frenkel-Poole emission). By contrast, under the dark conditions, SiO 2 effectively acted as an insulating layer and functioned as a barrier layer for carriers, leading to ultralow dark currents. The detectivity of the fabricated self-powered PD was measured to be 8.8 Â 10 10 Jones with a high on/off ratio of 10 5 at0V.Inaseparatework,Renet al. deliberately transformed the top surface of the p-type perovskite (MA + rich and Pb 2+ deficient stoichiometry) to n-type via bombarding with Ar + ions leading to the formation of a p-n homojunction within the perovskite layer. 64 The resultant built-in field in the perovskite promoted the separation and transport of photo-induced carriers without any external bias. The Ar + ion bombarding facilitated MAPbI 3 p-n homojunction PD exhibited an increase by one order of magnitude in the on/off ratio at 532 nm in comparison to the pristine MAPbI 3 device.

Schottky junction-based PDs
A typical PD assembly incorporating a photosensitive semiconducting material with two ohmic metal contacts requires an external bias to drive the separation of the photogenerated electron-hole pairs. However, if one of the Ohmic contacts is replaced by a Schottky barrier contact, the built-in electrical field at that interface can lead to the separation of the photogenerated charge carriers. Unlike PDs with Ohmic contacts, the Schottky junction PDs, therefore, offer the ability to detect light irradiation without an external power supply and with high sensitivity and response speed complemented by the low dark currents. Multiple reports demonstrating the Schottky junction facilitated self-driven PDs are found in the literature. As a seminal work, the Schottky junction photodiode of the ITO/ MAPbI 3 /Au vertical heterostructure assembly presented by Pandey et al. exhibited a photosensitivity of 1.33 Â 10 2 with the rise time and decay time of 91 and 101 ms, respectively. 65 Subsequently, several important works on high-detectivity and low-noise perovskite photodiodes were reported by different research groups. The ultrahigh mobility and conductivity of multilayer graphene and its ability to form a Schottky barrier with TiO 2 were combined with the high yield photocarrier transport and strong broadband light absorption capability (260-900 nm) of CH 3 NH 3 PbI 3 ,b yL iet al. 66 Both graphene and perovskite act as ambipolar materials and therefore, depending on the inherent Schottky field, electrons could be transferred vertically from perovskite to TiO 2 via the underlying graphene while the remnant holes remaining in the graphene were transferred horizontally and collected in a separate FTO electrode. The self-driven PD (FTO/TiO 2 -Graphene/MAPbI 3 / PTAA) demonstrated a high responsivity (B0.375 A W À1 ) and specific detectivity (B10 11 Jones) and an excellent on/off ratio of 4 Â 10 6 in comparison to the commonly used Au-perovskite-Au configuration. The protection by the top PTAA layer enabled negligible decay in the performance of the PD despite storage under ambient conditions for 20 days and stability over 1000 cycles of operation. Other than graphene, 2D transition-metal dichalcogenides (TMDs) are being extensively researched for a wide array of potential applications, including optoelectronics, biosensors, and piezoelectricity. [67][68][69][70][71][72] As a typical TMD material, MoS 2 has suitable band levels and excellent charge transport properties, thus, is well suited to construct PDs based on the Schottky effect. 73 Bai et al. were amongst the first ones to use single-layer MoS 2 and MAPbI 3 perovskite heterojunction in a vertically configured PD. 74 At a light intensity of 0.7 mW cm À2 , the selfpowered PD exhibits a responsivity of 60 mA W À1 with an inverse relation to power density. Zeng et al. combined multilayered PdSe 2 , another TMD material, with FA 1Àx Cs x PbI 3 perovskite to achieve a Schottky junction empowered fast selfdriven PD with broad spectral sensitivity (200-1550 nm). 75 Fig. 2a provides the schematic illustration and Fig. 2b-l represents the optoelectronic response of the PdSe 2 /perovskite Schottky junction PD device. The high carrier mobility of PdSe 2 and reduced surface defects at the TMD/perovskite interface enabled the Schottky junction-based PdSe 2 /perovskite PD to exhibit a large on/off ratio of B10 4 , a high responsivity of 313 mA W À1 ( Fig. 2j and l), a specific detectivity of B10 13 Jones (Fig. 2l), and rapid response speeds of 3.5/4 ms. Moreover, the device exhibited sensitivity to polarized light, with a polarization sensitivity of 6.04 (Fig. 2m). To explore the infrared image sensing ability of the fabricated PD, homemade masks in the shape of alphabets such as P, O, L, Y, and U were placed between the laser illumination and the device as seen in Fig. 2n. The authors were then able to generate a 2D contrast mapping profile by plotting the dark current and photocurrent of the PD, at each pixel. Since the pixel illuminated by an 808 nm light demonstrated a considerable photocurrent of B0.05 mA, while the rest of the area displayed negligible dark currents, the authors were able to establish that the Cs-doped FAPbI 3 PD could effectively resolve images under infrared illumination (Fig. 2o).

Ferroelectric and polarization effect based PDs
Ferroelectrics present a promising alternative for generating power in single-phase homogeneous materials by using polarization to trigger light detection. However, the presence of the ferroelectric polarization effect in OLHP is still debatable. [76][77][78][79][80][81] Saraf et al. discussed the interplay between the polarization effect and ion migration as a function of poling conditions in MAPbI 3 based PD (Fig. 3a). 82 T h ew o r ks u p p o r t e dt h ep r e s e n c eo f ferroelectric effect in MAPbI 3 as the PD poled under air and illumination lead to a ferroelectric dominated response, while poling under N 2 and dark conditions leads to ion migration effects resulting in the formation of a p-n homojunction which is signified by a reversal in signs for open-circuit voltage (V oc )and short circuit current (I sc ) as observed in Fig. 3b-e. To obtain a stable response, poling was done under a small external bias applied across the Au/MAPbI 3 /Au configuration in air and under illumination for a short duration of 10 minutes. The poled device could then operate without requiring an external power supply for the next 2 days. Besides, the authors adopted an additive engineering strategy of integrating polystyrene (PS) in the MAPbI 3 films to enhance their stability. The PS-MAPbI 3 based planar configuration PD device illustrated an impressive performance stability of 480% post 20 days. On the other hand, t h ep r i s t i n eM A P b I 3 counterpart device suffered from a 70-85% performance decay within 4 days of operation under similar Irrespective of the debatable existence of ferroelectric polarization in perovskites, incorporation of an additional layer of a ferroelectric material in the perovskite PD architecture can enhance the built-in electric field leading to improved charge extraction. A report demonstrating polarization induced internal electric field in a perovskite-based device appeared in 2018 where Cao et al. integrated ferroelectric SrTiO 3 (STO) to serve as an interlayer in a sandwich structure of FTO/STO/ MAPbI 3 /Spiro-OMeTAD/Ag. 84 The polarized STO layer provides a built-in potential and promotes downward band bending, helping to separate and transport charge carriers generated in MAPbI 3 upon illumination. Upon optimization of the STO layer density, the photocurrent was enhanced up to 0.956 mA with response speeds of 0.3 s (rise time) and o0.1 s (decay time) at 0 V in a PD positively poled at 1 V for 5 min. However, unlike the report of Saraf et al., there was an obvious decrease in photoresponse upon exposure to air for 24 h. The substantial performance decay can be attributed to the decomposition and degradation of the OLHP active layer which is indicative of the limited stability of the fabricated PD. The authors later demonstrated a poling induced self-driven bulk heterojunction PD. This research involved the integration of another ferroelectric material P(VDF-TrFE) to enhance the strength of the ferroelectric field of the perovskite layer, facilitating better separation of the photogenerated charge carriers. 85 The corresponding PD exhibited high responsivity (20 mA W À1 ), large detectivity (1.4 Â 10 13 Jones), and fast response speed (92/193 ms) at the wavelength of 650 nm. Later, the group extended the same composite assembly in the form of a nanowire array via an imprinting method assisted by commercially available digital versatile discs (DVD). 86 The resultant flexible hybrid device on the PEN substrate exhibited high detectivity (7.3 Â 10 12 Jones), fast response time (88/154 ms) at zero bias, and impressive mechanical stability through bending up to 180 degrees.

Triboelectric nanogenerator (TENG) based PDs
Triboelectric nanogenerators (TENGs) can act as an efficient driving source for harvesting mechanical energy to power devices such as PDs. When a physical contact occurs at the interface of two dissimilar materials, typically opposite triboelectric charges are created on the two surfaces. A mechanical motion can then induce an inherent electric potential difference capable of driving electrons back and forth between the electrodes. Su et al. fabricated a UV-Visible broad range PD based on the dual properties of MAPbI 3 : photoconductivity and surface triboelectric charge density. 87 An oscillating V oc was generated upon the top copper electrode periodically getting into contact with the underlying perovskite layer. Upon photoexcitation, electrons from the photogenerated electron-hole pair got quickly captured by the mesoporous TiO 2 layer, while holes tend to partially neutralize the negative triboelectric charges present on the surface leading to an instantaneous decrease in surface charge density. The corresponding device demonstrated a responsivity of 7.5 V W À1 with a response time of o80 ms. Later, Hsiao et al. demonstrated a similar TENG assisted perovskite PD that could generate photo response upon bending of the flexible assembly. 88 Guo et al. designed a selfpowered organic optical communication system (SOCS). 89 This system was composed of organic light-emitting diode (OLED) driven by triboelectric nanogenerators (TENGs) and perovskite PD. In this SOCS, the mechanical signals were converted into light by the OLED and then transferred to voltage signals by the perovskite PD. The combination of TENG with OLED exhibiting an emission peak at B524 nm served as a light emitter in the robotic hand, while PD served as the output voltage of SOCS with a maximum current transfer ratio of 30%. The output voltage of SOCS was modified by the different mechanical actions of the robotic hand which could be used to transmit the information on human-machine interaction.
One of the biggest limitations of conventional TENGpowered devices is their dependency on motion actuators as a power source which are prone to fluctuations due to variation in the surrounding conditions, severely hindering the accuracy of photodetection. The self-powered PD reported in work by Leung et al. aimed at mitigating this issue by excluding mechanical pressure being applied to the TENG (Fig. 4a-c). 90 The PD instead utilized the voltage regulated circuit containing a resistor and a Zener diode, which enabled uniform photodetection characteristics irrespective of irregular motion, such as human finger tapping (Fig. 4d). The fundamental principle comprised perovskite acting as a variable resistor dependent on the incident light intensity. The voltage in the photosensitive layer followed an inverse relation with light intensity owing to a decrease in resistivity by the photogenerated carriers ( Fig. 4e and f). The flexible and transparent polymer architecture of the PD empowered a stable device performance after 1000 bending cycles and photosensitivity at 360 degrees of illumination (Fig. 4j). The self-powered PD exhibited an impressive maximum voltage responsivity of 79.4 V mW À1 cm À2 and a B90% change in voltage at an incident light intensity of 100 mW cm À2 compared to the voltage obtained in dark. Table 2 summarizes a representative list of OLHP based PDs where various strategies such as crystal structure engineering, modulation of the synthesis procedure, and broadening of the spectral range were employed to enhance device characteristics.

Common strategies to enhance the figures of merit of OLHP PDs
1.5.1 Crystallite structure engineering. Several attempts have been made to improve the figures of merit for the perovskite based self-powered PDs by engineering the crystallite structure and composition. Perovskite single-crystal materials exhibit high carrier mobility (4100 cm 2 V À1 s À1 ), long diffusion lengths, low trap-state densities (B10 10 cm À3 ), and high absorption coefficients as compared to their polycrystalline counterparts which are beneficial for the realization of highperformance PDs. 91 Fang et al. published a pioneering report on single crystal based planar PD driven by a drum shaped assembly of two DVDs functioning as a triboelectric nanogenerator. 92 The nano-patterned polycarbonate substrate face of DVD was used as one of the friction layers of the TENG. An Al conductive tape was a p p l i e do nt h er e v e r s es i d eo ft h eD V Dt os e r v ea sa ne l e c t r o d e . A sanded Cu conductive tape pasted on another DVD acted as the opposite friction layer. Multiple pieces of polyethylene terephthalate (PET) were used to connect the two friction layers face-to-face, resulting in a drum shaped TENG with the single crystal perovskite sandwiched in between. Fig. 5a represents the equivalent circuit of the self-powered PD system in which OLHP single crystal served as a TENG. The self-powered device yielded u pt o2 0 0Va n d5 5m Au p o nb e i n gs u b j e c t e dt of i n g e rt a p p i n g ( F i g .5 ba n dc ) .T h ec r u d ey e ts i m p l i s t i ca p p r o a c he n a b l e d large responsivity of 196 V (mW cm À2 ) À1 with a wide detection range from 10 to 100 mW cm À2 ( F i g .5 d -f ) .C a p i t a l i z i n go nt h e advantage of higher current saturation density offered by asymmetric metal contacts (Au-Al), Ding et al. fabricated a facile Schottky junction (Au/CH 3 NH 3 PbI 3 /Al) enabled PD with an inherent built-in field. 93 T h es i n g l ec r y s t a lM A P b I 3 device exhibited responsivity as high as 0.24 A W À1 and a fast rise time and decay time of 71 ms and 112 ms, respectively. Cao et al. fabricated a MAPbBr 3 /MAPbI x Br 3Àx core-shell heterojunction-based PD. 94 The strong local electric field arising from band bending at the MAPbBr 3 /MAPbI x Br 3Àx heterojunction facilitated efficient exciton dissociation and suppressed charge recombination. Under the influence of the inherent field, the electrons were injected into the CH 3 NH 3 PbI x Br 3Àx shell while the holes were transferred to the MAPbBr 3 single crystal core, enabling photoresponse at zero bias. The self-powered PD exhibited a responsivity of 11.5 mA W À1 and a quantum efficiency of 3.17%, both being higher than the respective values observed for the MAPbBr 3 single crystal counterpart. Despite the advantages offered by single crystals, fabricating solution processed large-area single crystal films with controllable thickness is still a major challenge. Pan et al. instead proposed utilization of perovskite microcrystals (MCs) that possess advantages similar to single crystal perovskite (low defect state density and high carrier mobility) with the ability of tuning film thickness by tweaking precursor solution concentration and antisolvent content. 95 The authors fabricated a HTL-free ITO/SnO 2 /MAPb(I x Br 1Àx ) 3 MC film/ carbon heterojunction assembly with adjustable band gap and the ability to operate without external bias. With the decrease in t h ei o d i d ec o n t e n t( x) in the perovskite precursor solution, the absorption edge of the MAPb(I x Br 1Àx ) 3 MC film gradually shifted to the blue region with the corresponding bandgap increasing from 1.53 eV of MAPbI 3 to 2.22 eV of MAPbBr 3 .TheperovskiteMCfilms with optimized thickness led to an on/off ratio of 2 Â 10 5 ,a responsivity of 0.26 A W À1 , a specific detectivity of 7.01 Â 10 11 Jones, a rise/fall time of 80/580 ms, and a linear dynamic range of 107 dB. Furthermore, the MC based PDs show negligible attenuation upon continuous exposure to light and air for 30 minutes and an overall 16% decrease in performance when stored in air for a month. In a recent work, Perumal et al. presented an ultrasonication based ligand-assisted reprecipitation (LARP) technique to produce air-stable MAPbI 3 nanocrystals. The MAPbI 3 nanocrystals were spray-casted or drop-casted over patterned asymmetric ITO-Ag electrodes to obtain a self-powered PD. 96 The optimized PD showed a responsivity of 1.42 A W À1 and a specific detectivity of 1.77 Â 10 13 Jones under an 808 nm light illumination at zero bias, and maintained 90% performance over 1 month.
1.5.2 Synthesis strategy. The quality and compactness of the perovskite film strongly determine its inherent stability and  97 The expansive gas-solid reaction of perovskite crystallites with optimal annealing at 120 1C for 30 min led to the restriction of pinholes to grain boundaries while maintaining the integrity of the thin film. Simultaneously,  the accumulated strain at the colliding grain boundary interfaces led to selective evaporation of MAI leaving behind a thin layer of PbI 2 at the grain boundaries. The authors claimed that the wider bandgap of PbI 2 in comparison to perovskite provided good passivation at the grain boundaries and helped reduce local recombination centres and block local leakage current. The corresponding self-driven vertically configured FTO/TiO 2 / perovskite/Spiro-OMeTAD/Ag PD device exhibited a detectivity of B10 12 Jones, a responsivity of 0.55 A W À1 with fast rise time and fall time of 460 ns and 640 ns, respectively (Fig. 6). The two-month air-stable PD also showcased a wide 3 dB bandwidth up to 0.9 MHz. Adams et al. proposed to alleviate the repeatability and scalability challenges associated with the typical anti-solvent drip method used in the synthesis of perovskite films by using the anti-solvent bath method with improvised annealing. 98 101 The corresponding PD exhibited a responsivity of 10 À4 AW À1 at 0 V.

1.5.3
Broadening of the PD spectral range. One of the limitations associated with perovskite-based PDs is their attenuated absorption capacity in the NIR range. To broaden the spectral range of the perovskite-based PDs, two approaches have been commonly adopted: (1) lowering the bandgap of perovskite by substituting cation or/and a halide moiety 102 or (2) combining the perovskite layer with another narrow bandgap semiconductor. Wang et al. followed the first strategy of cation and halide substitution to fabricate a low bandgap (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 perovskite-based PD with a broadband response extending from UV (300 nm) to NIR (1000 nm). 103 The asfabricated vertically configured ITO/PEDOT:PSS/(FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 /C 60 /BCP/Ag device exhibited almost identical EQE spectra under 0 and À0.2 V, indicating its self-powered ability.
The authors used C 60 as a hole blocking layer and to passivate the surface charge traps, leading to a reduced leakage current under reverse bias. The thin BCP layer served as a cathode buffer layer to enhance the contact between the C 60 layer and the Ag cathode. Zhu et al. illustrated a room-temperature strategy to manipulate the crystallization kinetics of Sn-rich binary perovskite films fabricated over a complementary metal-oxidesemiconductor (CMOS) compatible silicon substrate. 104 The Pb-Sn mixed perovskites are considered promising narrow-bandgap materials offering the advantage of expanding the spectral response to the NIR region. However, compared to their Pb analogues, Sn-based perovskite precursors have a greater tendency to react and crystallize at room temperature, resulting in high surface roughness and severe pinholes in the corresponding films. By carrying out careful experimental and theoretical studies focused primarily on the timespan between the antisolvent washing process and the post-annealing treatment, the authors were able to control the density and location of the compact nanocrystals formed in the precursor films. Upon flash annealing, these compact nanocrystals coalesced into a smooth pin-hole free MASn x Pb 1Àx I 3 film with preferred orientation and reduced trap density. The corresponding selfpowered devices with a carrier extraction layer/metal/Si substrate structure, achieved a high responsivity of 0.2 A W À1 at 940 nm, a large linear dynamic range of 100 dB, and a fast fall time of 2.27 ms. The authors also demonstrated a 6 Â 6-pixel array based on the Sn rich binary perovskite films with excellent photocurrent uniformity. In another interesting work aimed at producing filterfree/spectrum sensitive PD, as illustrated in Fig. 7a, Sun et al. demonstrated in situ bandgap gradient within an array of KMAPbCl x Br 3Àx absorber films when subjected to a temperature bridge across the integrated device. 105 The integrated self-driven PD with a high spectral resolution (B80 nm) (Fig. 7b and c) exhibited a rapid response time (B71 ns) due to the influence of carrier concentration (Cl À vacancies) over interfacial capacitance while maintaining a maximum external quantum efficiency (EQE) of over 90%. Fig. 7d represents the schematic layout of the KMAPbCl x Br 3Àx film-based spectrum detection system. On the other hand, following the second approach for enhancing the spectral response of perovskite-based PDs, Cao et al. combined a narrow bandgap n-Si wafer with an active layer of MAPbI 3 ,spincoated over a TiO 2 interlayer to fabricate a trilayer hybrid PD. 106 The TiO 2 thickness dependent I-V rectification characteristics suggested the presence of a built-in electric field in the asprepared PD and its ability to work under zero bias. Showcasing a broad spectral response up to 1150 nm, the PD exhibited a rise time and decay time of 50 and 150 ms, respectively.

Flexible PDs
Compared with the traditional devices based on a rigid silicon substrate, flexible PDs have wider applications in the field of wearable and portable devices owing to their reduced weight and promising applications in healthcare, robotics, epidermal sensing, and so forth. Due to their low-temperature solutionprocessing and light absorption characteristics, perovskite thin films can serve as an optimal light-harvesting material in such applications. Among the initial works based on OLHP flexible PDs, Bao et al. demonstrated a flexible PD with poly(ethylene 2,6-naphthalate) (PEN)/Au NW serving as a transparent electrode. 107 The peak EQE and responsivity of the device reached 60% and 314 mA W À1 , respectively, with a 4.0 ms rise time and a 3.3 ms fall time. In another work, Sun et al. utilized a blend of CH 3 NH 3 PbI 3 and Spiro-OMeTAD as a photosensitive material over low-cost carbon cloth, serving both as a substrate and a conducting electrode to fabricate a flexible PD. 108 The solution processed device exhibited high detectivity and light responsivity, a large on/off ratio, and a broad-spectrum response to light ranging from UV (300 nm) to NIR (820 nm) at zero bias. The flexible PD exhibited a fast response speed and reproducible characteristics under white light irradiation after 80 bending cycles and showed photoresponse even at high bending angles such as 1801. The same group later reported a flexible PD based on a single fibre. Here, perovskite microcrystals were solution coated over a commercial carbon fibre with an intermediated thin layer of TiO 2 . 109 A CuO nanowire-Cu 2 O compact layer grown over a flexible Cu fibre was double twisted over the perovskite-coated carbon fibre to function as a cathode while the carbon tape end of the parent fibre was utilized as an anode Fig. 8a represents the complete fabrication process and energy level diagram of the as-fabricated fibre PD. Under the selfpowered mode, the flexible PD exhibited ultralow dark current (10 À11 A) empowering high detectivity of up to 10 13 Jones while the narrow bandgap of CuO enabled the broadening of the spectral response range from UV (350 nm) to NIR (1050 nm) (Fig. 8b-e). The device demonstrated a rapid response speed (o200 ms) and reproducible photoresponse characteristics after being subjected to 60 bending cycles. Another similar work reporting a flexible wire-shaped PD is from Adams et al. 110 The research emphasized the predominance of Joule heating (direct conversion of electric energy to heat) over the conventional hot-plate heating method by allowing rapid heating or cooling of a substrate and thereby enabling a higher degree of control. The ohmic heating method allowed uniform temperature distribution across the thread-like carbon nanotube yarn (CNY) which, in turn, facilitated the deposition of a compact polycrystalline perovskite layer. The asymmetric top InGa and bottom CNY electrode contacts created a built-in potential sufficient to overcome the binding energy of photogenerated electron-hole pairs. The resultant PD had a responsivity of 10.  effectively serve as the electron transport and hole blocking layer over the ITO/PEN substrate with MAPbI 3 serving as a photosensitive layer. 112 The resultant PD exhibitedabroadbandresponse up to the NIR range with responsivity up to 451 mA W À1 at 720 nm without any external power supply. The flexible device showed only a 15% loss of current with a bending radius up to 2.  116 The formation of the metal/perovskite Schottky barrier at the interface of asymmetric Al (4.3 eV) and Ni (5.1 eV) electrodes led to a built-in electric field in the device which could effectively separate charge carriers. The corresponding self-powered device exhibited a responsivity of 0.227 A cm À2 at an incident light power of 1 Â 10 À8 Wcm À2 and an on/off ratio of 147. Although appreciable developments have been made in the field of flexible self-powered PDs, it is worth considering that most of the reported PDs have small active areas and their optimum performance lasts only for few thousand cycles. Hence, besides optimizing the photosensitive perovskite layer to mitigate material degradation issues, a careful selection of suitable electrodes and substrates to facilitate large-area self-powered PDs with a stable long-term device operation is equally critical. In a recent work, Saraf et. al utilized Au-Pt gold nanoparticle chain-based electrodes on a flexible filter membrane to fabricate a porous Au-Pt/PS-MAPbI 3 /Au-Pt PD assembly. 117 The flexible PD maintained 85% of its initial performance after 10 000 bending cycles at 1201. The highly flexible nanoparticle chain electrodes can potentially be expanded to fabricate asymmetric contact to induce a built in electric field within the perovskite and obtain a self-powered PD. The key figures of merit and device configuration of OLHP based flexible PDs are presented in Table 3.

Supercapacitors
Photovoltaic cells have the limitation of unstable power output due to fluctuations in sunlight. Integration of energy storage devices with solar cells to store energy during sunlight and provide it under low light conditions can be used to power wearable electronics. [118][119][120][121][122][123][124][125][126] This provides a stable output power compared with that of a solar cell alone. Li et al. reported a flexible PSC driven photo-rechargeable lithium-ion capacitor (LIC) for strain sensors. 127 It achieved an overall efficiency of 8.41% with a high output voltage of 3 V at a discharge current density of 0.1 A g À1 . The photovoltaic unit was made by the deposition of OLHP on an ITO coated PET sheet in Ag/BCP/ PCBM/MA 1Ày FA y PbI 3Àx Cl x /NiO x /ITO/PET configuration. A single cell reached a corresponding PCE of 14.01%, V oc of 1.05 V, shortcircuit current density ( J sc ) of 18.71 mA cm À2 , and fill factor (FF) of 0.71. Researchers combined four of these PSCs to provide 3.95 V with 10.2% efficiency under AM 1.5G solar irradiation. The LIC could be charged to 3 V in 20 min with a charge current of 1.22 mA. As observed in Fig. 9a-c, the wearable device was able to monitor finger motion and pulsation continuously and precisely, without an external power source. In a separate work, Du et al. integrated a flexible graphene-based supercapacitor and a perovskite based solar cell to fabricate a self-powered device. 128 The configuration of the perovskite hybrid solar cell (pero-HSC) was ITO/PEDOT:PSS/CH 3 NH 3 PbI 3Àx Cl x /PC 61 BM/Al. Under AM 1.5G illumination, the pero-HSC exhibited a PCE of 14.13%, a V oc of 0.90 V, a J sc of 22.59 mA cm À2 , and an FF of 0.695. The supercapacitor was charged to 0.75 V via the solar cell and with an overall energy storage efficiency of 70.9% by using carbon nanotubes (CNTs) as a bridge between self-powered organometal halide PSCs and supercapacitors to improve the overall performance. 129 As observed in Fig. 9d, the CNTs acted as a pathway for holes and electrons resulting in a smaller resistance. Under AM 1.5G simulated sunlight, the PSC of MAPbI 3 based solar cell achieved a PCE of 2.47%, a V oc of 0.7 V, a J sc of 9.2 mA cm À2 , and an FF of 38%. The photocapacitor could be charged to 0.7 V in about 80 s. Following this, it slowly discharged from 0.7 V to 0.3 V, lasting for more than 300 s in dark, representing a more stable output than bare solar cells as illustrated in Fig. 9e.

Transistors and photocatalysis
Photovoltaic devices have also been coupled with transistors 74,135-139 and electrochemical cells for degradation of organic pollutants, 140,141 due to their high V oc (0.9 V to 1.5 V) and power generation capacity. Jeong et al. reported a MAPbI 3 film coupled with PTAA, exhibiting a J sc and V oc of B21 mA cm À2 and B1.08 V, respectively, with 18.3% PCE. 135 The V oc under red-LED could effectively gate the MoTe 2 field-effect transistors (FET) modulating the drain current (I D ), which could turn on/off an OLED pixel (Fig. 10). Chen et al. reported that multi-walled CNTs and a methylamine (CH 3 NH 2 ) modified perovskite achieved a PCE of 10.39%. 140 It was demonstrated that the CNT-incorporated MAPbI 3 perovskite thin film formed a smooth heterojunction structure, which facilitated the separation and transfer of charges. Also, CH 3 NH 2 could self-heal the perovskite surface while maintaining high crystallinity. This inhibited the charge recombination and improved the efficiency of the device. Thus, the CNT/CH 3 NH 2 perovskite exhibited a high J sc value of 22.02 mA cm À2 . Combined with the CNT/TiO 2 photoanodes and the perovskite PSC (as power source), the system exhibited an efficient degradation of rhodamine B to almost 100% within 80 min.

Photovoltachromic cells
Photovoltachromic cells (PVCCs) can adjust the optical transmittance and coloration by their photovoltaic behaviors. [142][143][144] They are of interest for applications in smart windows in buildings and vehicles to manage daylight admittance. A photovoltachromic device integrated with a perovskite based photovoltaic cell enables their operation without an external bias. 145 Owing to their reduced size, the MHP NCs exhibited strong luminescence and high quantum yield. The blue, green, and red MHP NCs were utilized to fabricate a panchromatic photodetector cell that showed a full-colored emission spectrum spanning 435-631 nm ( Fig. 11a and b). The effective panchromatic photon harvesting was attributed to the individual absorption of blue, green, and red light by the corresponding MHP NCs and the independent transfer of the photogenerated electrons into the mesoporous TiO 2 electrode. Cannavale et al. were amongst the first to report a self-powered photovoltachromic device integrated using a perovskite and an electrochromic supercapacitor (ECS). 149 Here, the ECS included a layer of WO 3 which could store charges and change the color at the same time owing to the change of the chemical state in W. As reported, WO 3 is transparent, and WO 3 À is colored. 146 As seen in Fig. 11c-e, once the device was exposed to solar illumination, the perovskite was capable of generating enough power to activate the coloring of WO 3 without requiring any external bias. Therefore, the device processed a chromic transition from semi-transparent to dark blue when irradiated by solar light. The perovskite layer presented a V oc of 0.68 V, a J sc of 12.5 mA cm À2 ,a nF Fo f0 . 6 3 ,a n daP C Eo f5 .  150 This design not only provided a seamless integration of energy harvesting and storage device but also improved photostability. The photocharging process changed the color of the PVCSs from transparent to dark blue. During the fully charged state, the colored PVCSs blocked off most of the light automatically turning the PSC into the low power operating state and preventing the cell from long-time exposure, thereby prolonging the device lifetime. The transparent electrodes facilitated the possibility of PSC to be illuminatedfrombothsides.Intheco-cathodestructure,theFTO glass served as the co-cathode for both PSC and ECS. While for the co-anode structure, the MAM electrode served as a co-anode for With the color change, the PCEs of the co-anode and co-cathode PVCSs decreased to 3.73% and 2.26%, respectively.

Pressure sensors
Flexible pressure sensors have numerous applications in areas such as artificial electronic skins, 145,151,152 health monitoring devices, [153][154][155][156] soft robotics, [157][158][159] and wearable technology. [160][161][162][163][164][165] It also improves the seamless interfacing of such devices with the environment to act as a medium for information collection to drive robotics and human-artificial intelligence interactions. 166 The OLHP based self-powered pressure sensors and the corresponding device characteristics are listed in Table 4.

Mechanoluminescent device
Shohag et al. reported a flexible pressure sensor by integrating a mechanoluminescent device with a B500 nm thick organometal halide perovskite. 162 In this work, the authors attempted at reducing the defect density of the perovskite by incorporating bromine which led to reduced trapping of charge carriers at the grain boundaries and hence, enhanced the efficiency. An applied mechanical force led the ZnS:Cu crystals to emit mechanoluminescence at B543 nm, which could be then absorbed by the ultrathin layer of perovskite (MAPb(Br 0.1 I 0.9 ) 3 )t og e n e r a t ea photocurrent. The photocurrent generated in the perovskite layer increased with the increase in the intensity of light emitted from the ZnS:Cu layer, in direct proportion to the applied pressure. The sensor was able to detect pressure from 11 kPa to 460 kPa with a sensitivity of 0.095 kPa À1 . The sensor also exhibited a steady response over 1000 bending cycles, which is promising for monitoring health signals.

Piezoelectric effect assisted sensing
MAPbI 3 is a semi-conductor and poly(vinylidene fluoride) (PVDF) is a piezoelectric material. Researchers have combined these two materials to develop self-powered pressure sensors based on their combined properties. For example, Jella et al. constructed a flexible and wearable piezoelectric generator (PEG) by using the MAPbI 3 perovskite and PVDF as a piezoelectric flexible polymer matrix. 167 The strong dipolar interactions between MA + and the fluorine groups of the PVDF (-CF 2 -) result in a high dielectric constant, low leakage current, and high stability. As observed in Fig. 12, under an applied force, macrodipoles are formed resulting in charge accumulation at the MAPbI 3 -PVDF interface with opposite polarity, so the charge density of the electrode decreases generating a positive signal while the releasing process produced a negative signal. The device showed a V oc of 45.6 V and a J sc of 4.6 mAc m À2 with high mechanical stability. The generated output can power commercial LEDs under a 50 N applied force by finger tapping. Sultana et al. produced a flexible self-powered perovskite based piezoelectric nanogenerator (P-PNG) by electrospinning PVDF nanofibers and MAPbI 3 . 158 The mechanical vibration led to a change of output power density with a piezoelectric coefficient of B19.7 pC N À1 which shows appreciable potential for a pressure sensor. The V oc increased with the frequency of applied stress as more charges are generated. As illustrated in Fig. 13a-d, at 4 Hz, the V oc reached a maximum of 220 mV while gradually decreasing to 100 mV at 15 Hz. The authors also measured the stability of the device, which showed a stable response for 60 000 cycles at 4 Hz presented a flexible and highly porous FAPbBr 2 I-PVDF P-PNG assembly. 159 The high strain of the self-assembled highly ordered polymer scaffold and enhanced relative permittivity of the perovskite led to a 5 fold improvement in the strain-induced  piezo potential (B85 V) and 15 times higher current (B30 mA) in the perovskite-polymer composite P-PNG relative to those of the pristine PVDF PNG, at an applied load of 138 g. Utilizing the P-PNG, the authors further demonstrated a self-powered universal wireless electronics node (SIWEN) capable of communicating remotely with personal electronics. Energy harvesting from a real-life scenario such as automobile vibrations and biomechanical motion was also demonstrated. Eom et al. used the CVD method to fabricate a 500 nm thin film of MAPbI 3 . 169 The film was stable for a month under ambient conditions. Owning to an intrinsic piezoelectric property of MAPbI 3 , the device showed sensitivity to both pressure and light. The MAPbI 3 film had a linear sensitivity to a pressure of 8.34 mV kPa À1 and 0.02 nA kPa À1 within a pressure range of 40 kPa, generated by finger touch. There was a time delay of 0.072 and 0.087 s in detection on applying and removing the pressure. The delay was attributed to the domain switching due to applied pressure on the film. In an interesting work, Saraf et al. combined a layer of ZnO nanosheets as a pressure-sensitive electron extraction layer on the MAPbI 3 film ( Fig. 13e and f). 166 A small external bias was applied for few minutes to the MAPbI 3 film to induce polarization effects capable of generating spontaneous voltage and current response upon light illumination once the external bias was removed. The pressure applied on the top ZnO layer increased the interfacing area between the MAPbI 3 film and the ZnO layer, leading to a higher collection of electrons. This action increased the value of J sc (or V oc ) in proportion to the applied pressure, as shown in Fig. 13f. The sensor exhibited a linear response from 0.5 kPa to 76 kPa with a sensitivity of 0  170 Also, the inclusion of the polymer led to a softer film with a lower modulus which dissipated the force stimuli leading to a wide operating range. Once poled, the film was functional as a self-driven light-powered tactile sensor for 120 h and could be re-poled for continued operational performance. The light-powered tactile sensor fabricated by the assembly of 1w t %P S -M A P b I 3 film and ZnO exhibited a wide operating range from 4 Pa up to 333 kPa with linear response and a sensitivity of 19.77 kPa À1 .

Gas sensors
Today, there is an increasing need for portable self-powered gas sensing devices to monitor personal health, toxic gas release, and safety in public space. 141  fabricated a photovoltaic cell with the vertical configuration of FTO/ TiO 2 /FMCPIB/carbon. The authors believed that NO 2 can undergo interaction with the amine cation of FMCPIB. This interaction was attributed to reducing the defects in the perovskite film which decreases the charge recombination and hence increasing the density of free charge carriers. Fig. 14 represents the device performance of NO 2 detection at room temperature under O 2 rich conditions. A linear relationship up to 8 ppm with a 0.2 ppm detection limit under ambient conditions was observed. A higher NO 2 concentration led to a higher output current due to the interaction between NO 2 and FMCPIB. Owing to the functionality of the FMCPIB to act as a gas sensor, light harvester, and capacitive energy storage material (due to mobile ions), the self-powered device stayed in operation for 1.7 h in dark without requiring an external power supply. The measured response and recovery times were 17 s and 126 s, respectively. Such gas sensing ability banking on the inherent material properties of OLHP may pave new ways to monitor environmental safety using self-powered sensors.

Challenges and future prospects
Continuous research has led to significant advances in the OLHP based self-powered devices both in terms of their performance and expanding their applications. Various devices besides solar cells, such as PDs, supercapacitors, and TENGs, are being fabricated primarily using OLHP as a photosensitive layer. The development of self-powered electroluminescent devices using OLHP can also benefit from an increased research focus. The challenge lies in the typical high voltage requirement for generating appreciable luminescence. The use of polarization effects in OLHP is a potential route for such devices, with the research required to understand and enhance these characteristics for the generation of greater energy and electric potential. The inherent stability issue of the perovskite material is a major challenge that persists due to the inherent structure of the material (high ion mobilities) and its sensitivity to multiple external factors (such as humidity, oxygen, illumination, and temperature). Strategies such as interfacial engineering, additive integration, doping, and encapsulation are being widely adopted to reduce perovskite degradation. A significant scope also exists for the improvement of the inherent perovskite structure and composition. These combined efforts result in improving the lifetime of the OLHP based devices and should continue to progress them towards commercial viability. With the concerns of toxicity surrounding lead, there are ongoing efforts of substituting it with other divalent ions such as Sn, Ge, Bi, and Sb, but the resulting reduction in performance needs to be addressed in order to develop lead-free perovskites. 177,178 At a macroscale, as it stands, the majority of OLHP based self-powered devices and sensors are rigid. Although, in the last few years flexible transducers are being researched and reported with greater intensity, they experience performance degradation during long-term operations. Most wearable and portable electronic devices and sensors mandate the ability to be stretched and bent over a couple of thousand cycles without incurring significant decay in output over time. Alternatives for 3D OLHP such as 2D, 1D, and 0D OLHP may offer enhanced flexibility and might provide viable options. [179][180][181][182] Equally critical is the need for developing appropriate substrates and electrodes for long-term device operation while ensuring wearability comfort. Furthermore, it is worth considering that almost all OLHP self-powered portable and wearable devices are tested at 1 sun. However, the indoor performance of these devices with limited illumination is rarely reported. Therefore, developing standard protocols for testing devices under such limited light conditions need to be developed as this will allow a more accurate comparison of devices.
Although low levels of the output signals and noncontinuous operation seem to impede the overall capability of the OLHP based self-driven devices and sensors, several other major factors contribute to hinder the commercial viability of such perovskite-based transducers. Currently, there is a lack of a standardized large-scale manufacturing process that could ensure performance consistency amongst batches of the same device. Besides, there is limited research available over performance-cost analysis specific to the OLHP self-powered devices that could drive them from a lab prototype to a commercial finished product. OLHP is considered a low-cost material compared to other semiconductors (such as Si and organic compounds); however, a more detailed cost analysis is required for the quantification of its cost advantage (including Pb recycling needs). Techniques such as printing need to be promoted to drive up large scale cost-effective production of the OLHP based self-powered devices. The use of advanced microfabrication and nanofabrication techniques can further expand the integration of other flexible electronic, photonic, and optoelectronic devices with OLHP as an energy harvester.
In summary, significant advancements have been made in OLHP based self-powered applications, but still there are critical challenges that need to be addressed before they can be considered commercially viable products. The rapid development of materials science, processing, and manufacturing technology can enable the progress of the OLHP based self-powered system in our day-to-day life at a rapid pace and the commercialization of OLHP based photovoltaic cells provides important lessons for this road map.

Conflicts of interest
There are no conflicts to declare.