Application of Perovskite as Solution‐Processed Solid Electrolyte in Polymer Electrochemical Transistor

The electrolyte serves as a key component in organic electrochemical transistors (OECTs) and allows for the modulation of the conductivity of the polymer channel by applying a voltage to the gate electrode. Different from liquid electrolytes in OECTs, the solid electrolyte is beneficial for the integration in the circuit and shows great stability. And considering the fabrication of the device, developing a new solution‐processed solid electrolyte for OECTs is a critical area of research. In this work, the organic–inorganic perovskite is used as the electrolyte to fabricate polymer electrochemical transistors due to its solid‐phase ion migration kinetics. To realize this goal, a separate polyethylene oxide (PEO) layer on top of the poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) layer plays a crucial role since it can affect the electric field and the ion motion. More importantly, based on this device structure, both a polymer light‐emitting electrochemical transistor and the polymer electrochemical phototransistor are realized. The solution‐processed solid organic–inorganic perovskite electrolytes can enable the development of integration in OECTs, which have significant potential for multifunctional applications.


Introduction
Electrolytes significantly affect the operating mechanism and application of organic electrochemical transistors (OECTs) and they can exist in various states of matter: liquid, ionic liquid, and solid.Aqueous solutions have been studied in OECTs due to their good biocompatibility and environmental friendliness.Among them, the aqueous solution of salt is widely used, and the resulted OECTs using a variety of polymers as functional layers DOI: 10.1002/aelm.202300337 can show sub-millisecond response time and high transconductance after modifying the polymer branched chain. [1]he OECTs based on aqueous electrolytes have been widely used in biological signal detection due to their environment-friendly and biocompatible characteristics. [2]However, they still face great challenges in circuit integration mainly due to solution fluidity and unstable performance. [3]The ionic liquid is a molten salt as the new type of electrolyte material, which is liquid at room temperature and composed entirely of ions.Due to its solvent-free characteristics, it shows environmental friendliness. [4]ecause of the high ionic conductivity and stable electrical and thermal properties of ionic liquids, the electrochemical transistors based on the ionic liquids as electrolytes exhibit a faster response speed. [5]However, the humid environment has a serious impact on these OECTs and they are not conducive to circuit integration due to their size and solution fluidity.Compared with the above-mentioned electrolytes, solid polymer electrolytes are easier to integrate into electronics since they are flexible and their preparation technology is mature. [6]6b,7] The most common solid electrolyte is the blend of PEO and alkali salt, which is also widely used in lithium batteries.To prepare PEO composite electrolytes with high ionic conductivity and stability, a variety of organic and inorganic additives are introduced. [8]However, the salt-PEO composite electrolytes have limited electrochemical stability, [9] which can result in the formation of unwanted products or side reactions that can decrease the performance and stability of the devices. [10]In addition, the alkali salt electrolytes can be prone to dendrite formation, which are needle-like structures that can cause short circuits. [11]Recently, ion gels and polyelectrolyte materials have been applied to use as the electrolyte in the OECTs, however, their preparation process is more complicated, which limits their application in electronics manufacturing. [12]o far, most OECTs use liquid/ionic liquid electrolytes, which can limit their performance and stability since liquid electrolytes are prone to leakage, evaporation, and chemical degradation.Moreover, the use of liquid electrolytes limits the application of OECTs in flexible and wearable electronics.Therefore, developing a new solid electrolyte is indeed urgent for OECTs because the performance and stability of the devices on the quality and properties of the electrolyte.
Inorganic perovskite materials have been extensively studied as electrolytes in lithium batteries. [13]The material exhibits high lithium-ion conductivity as well as capacitance and improves device stability.The structure of perovskite materials is ABX 3 , where A is a monovalent cation (both organic and inorganic), B is a bivalent metal cation Pb 2+ or Sn 2+ , and C is a halogen ion (such as Cl − , Br − ).The B-site and C-site ions will form the regular octahedral lattice skeleton of BX 6 2− , and the A-site ions will be located near the lattice sites.Different from inorganic semiconductors, this lattice is flexible and has a weak ionic bond, resulting that both electrons and ions can transport in perovskite. [14]However, inorganic perovskites are usually prepared by thermal deposition to obtain the multiple-function film, which impedes their application in OECTs.In the last decade, one solution-processed perovskite, organic-inorganic hybrid perovskite, has been widely used in optoelectronic devices.The key advantage of the solutionprocessed perovskite layers is the fabrication simplicity. [15]n recent years, the ion migration in organic-inorganic hybrid perovskite has been widely investigated. [16]For traditional optoelectronic devices, the ion migration of perovskite materials can lead to the change of crystal phase and even cause a large hysteresis characteristic.Alwin Daus et al. demonstrated the thin-film transistors (TFTs) with methyl-ammonium lead iodide (MAPbI 3 ) gate dielectrics (the active layer is indium gallium zinc oxide) and Kihyon Hong et al. realized the electrolyte-gated transistors (EGTs) using perovskite materials as the gate insulator layer (the active layer is zinc oxide). [17]In these works, the ion migrations inside the perovskite gate layer were used to form the electric double-layer (EDL).Therefore, the dielectric characteristic of the perovskite layer is utilized to achieve the great performance of transistors.In this work, perovskites, as the electrolyte, provide ion migration into the channel to electrochemical dope the polymer layer, realizing the good-performance electrochemical transistors (OECTs).More importantly, the light-emission and ultraviolet light detection can be achieved by the perovskite electrolyte-gated devices.In this condition, under a sufficient voltage bias, electrons and holes are injected into the polymer film from the cathode and anode, respectively and the polymer is reduced/oxidized.Therefore, perovskite might be applied as a new electrolyte in polymer electrochemical transistors.
In this work, the solution-processed organic-inorganic hybrid perovskite was used as the electrolyte to fabricate a polymer electrochemical transistor.More interestingly, based on this device structure, both polymer light-emitting electrochemical transistors and polymer electrochemical phototransistors can be demonstrated.The perovskite electrolytes can be used as one kind of new solid electrolyte in the electrochemical transistors due to their potential application in the integrated circuit technology.Different from the liquid or gel electrolyte, the solid electrolyte is beneficial for the integration in the circuit. [18]19a,20]

Results and Discussion
To form a perovskite electrolyte-gated polymer electrochemical transistor and guarantee efficient transport of the carriers in the active layer from the source electrode to the drain electrode, the quasi-two-dimensional perovskite (PEA 2 MA 4 Pb 5 Br 16 ) film was applied as an electrolyte layer, providing ions for the polymer layer to participate in the electrochemical doping process.The Scanning Electron Microscope (SEM) image of surface morphology and X-ray Diffraction (XRD) pattern of the perovskite film are shown in Figure S1 (Supporting Information).
The polyethylene oxide (PEO) film was coated as a dielectric layer and the poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV) film was used as the active layer, respectively.The device structure and the transfer characteristic of the device are shown in Figure 1a.1b,c] Because the solvent is not infiltrated for the polymer, the perovskite layer cannot be prepared on the MEH-PPV film.Since the key for the perovskite electrolyte layer is to provide the ion migration into the channel with the gate electric field, the perovskite electrolyte was put under the MEH-PPV layer and the results show that the perovskite layer can also provide the ions migration for the polymer electrochemical doping under the gate electric field.When the sweep voltage was applied to the gate electrode from 0 to −40 V with a step of −0.05 V, the source and drain current (I DS ) at −20 V was measured and implying that the conductivity of the MEH-PPV layer greatly improves with increasing gate voltages.The output curves in Figure 1b show that the gate voltage can regulate the source-drain current and all the output curves reach saturation at large V DS .
During the voltage operation process, the device was imaged under UV illumination and the images before and after the operation are shown in Figure 1c.Before applying voltage, the whole film exhibits strong orange-red photoluminescence due to MEH-PPV and after the operation, highly visible PL quenching could be observed in the whole channel between the drain and source electrodes.It has been reported that electrochemical doping can create additional energy states within the bandgap of the polymer, leading to PL quenching, [21] so that there exists electrochemical doping in the PEA 2 MA 4 Pb 5 Br 16 gated MEH-PPV transistor.The ion migration in perovskite could be related to the doping process and there are four kinds of ions in the perovskite: PEA + , MA + , Pb 2+ , and Br − .Compared with the easy migration for MA + and Br − ions, which have been reported, [22] the PEA + , and Pb 2+ ions are relatively difficult to migrate due to their big migration barrier. [23]To quantify the ions involved in electrochemical doping, the Quartz Crystal Microbalance (QCM) was utilized, which can calculate the material resonance frequency to monitor the mass change of the polymer film during the electrochemical doping process. [24]The results show that 7.7 ng ions participate in doping after applying the negative gate voltage (−1 V for 30 s) between the Au and Au electrode.The structure used for QCM measurement is shown in Figure 1d and more details are shown in the Experimental Section.The quality change of the rigid perovskite layer is calculated through the resonance frequency change of the quartz crystal while the flexible polymer blend layer (MEH-PPV:PEO) does not affect the resonance frequency.Therefore, the ions migration from the perovskite layer to the polymer layer (involving the electrochemical doping) is tested quantitatively.
To further verify the effect of the perovskite layer, the device with only a PEO dielectric layer on the top of the MEH-PPV film was fabricated and tested, as shown in Figure 1e.The constant voltage of −20 V was applied to the source-drain electrodes and the scan voltage was applied to the gate electrode from 0 to −30 V.With the increase of the gate voltage, the source-drain current increased slightly till it reached around 6 nA as the saturation current.The result shows that without the help of the perovskite layer, there are no transistor characteristics in the device based on the PEO/MEH-PPV bilayer.Therefore, the perovskite layer is crucial for the device's operation.The device based on the perovskite layer as the bottom gate layer with the MEH-PPV active layer was prepared, which showed no transistor characteristics due to the larger gate current, as shown in Figure 1f.Therefore, the PEO dielectric layer plays a key role to form the perovskite electrolyte-based electrochemical transistors.
To investigate the function of the PEO layer, various devices have been made and tested.As shown in Figure 2a, the PEO layer was replaced by the polymethyl methacrylate (PMMA) layer (the thickness is around 500 nm), and the transfer curve shows no regulation characteristic.And there was no PL quenching in the MEH-PPV film during this process.The low gate current implies that the PMMA layer has a good dielectric property and compared with the PEO-based device, the poor device performance seems to result from the poor ion migration in the PMMA-based device.It has been demonstrated that PEO can improve the ion motion in MEH-PPV when it is mixed with salt into the MEH-PPV film. [25]o explore the effect of the PEO layer on ion migration, as shown in Figure 2b,c  property.More importantly, in the device without PEO, the electrical conductivity under dark almost showed a constant with increasing temperature while in the device with PEO, the conductivity increased.This clearly implies that the MEH-PPV containing PEO can significantly enhance the ion motion in the device.According to Equation (1), the natural logarithm of conductivity (ln) is linearly dependent on the inverse of the temperature (1/T).
According to the formula, the ion activation energy (E a ) under dark could be extracted.The activation energy of perovskite ions in the PEO device under dark conditions is 1546 meV.Although the dielectric property of PEO led to a decrease in the conductivity, the PEO introduction can be beneficial to the perovskite ion migration in polymer under the gate electric field.Therefore, PEO was added into the MEH-PPV layer to promote the ion's conductivity for the PMMA dielectric layer-based device.The result is shown in Figure 2d and exhibits no big difference compared with Figure 2a.The comparison between the PMMA dielectric device and the PEO dielectric device indicates that the gate current of the former one is three orders in magnitude lower than that of the latter one.This is most likely caused by that the PMMA layer's better dielectric characteristic impedes the potential drop on the MEH-PPV/Perovskite layers and the ion motion is significantly limited.Therefore, the PMMA dielectric layer is not suitable for the device based on the perovskite electrolyte.
In terms of fabrication of the PEO layer, the PEO aqueous solution was dropped on top of the MEH-PPV film and followed by a high temperature (110 °C) annealing.Considering the PEO melting temperature is around 70 °C. [26]During the annealing process, it is reasonable that a mixture of PEO and MEH-PPV can be formed.To study the effect of aqueous solvent on the interface of the two layers when preparing PEO layers on MEH-PPV films.The MEH-PPV films were annealed with and without water on top at 110 °C for various durations and the contact angles of them were tested, as shown in Figure 2e.The results show that the contact angle has no obvious relationship with the annealing time for the film without water.However, with increasing annealing time, the contact angle of the film covered with water decreases.The surface converted by the contact angle of the film increases with the increase of annealing time, which facilitates the penetration of the PEO layer.The change of the film surface energy implies the possibility of a mixed layer formed between MEH-PPV and PEO, which plays a key role to promote perovskite ion motion in the MEH-PPV film.And then we applied the Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) to characterize the morphology change of MEH-PPV film.The results show that after preparing the PEO layer on the MEH-PPV film, the surface of the MEH-PPV film is blended with the PEO.It is beneficial to perovskite ion migration.More details are shown in Figure S3 (Supporting Information).
According to the free volume theory of ion transport in polymer, the ion transport rate is proportional to exp( − B/(T − T C ). [27] As shown in Figure 2f, the transfer curves of the device at various temperatures were characterized and the currents drop from 10 −5 A at 290 K to 10 −9 A at 150 K.In the temperature range from 190 to 290 K, the switch ratio of the device continuously decreases as the temperature decreases.When the temperatures are equal to or below 180 K, the source-drain currents show no significant change with gate voltage.Compared with the temperature effect on the electrical conductivity of the MEH-PPV and perovskite film reported previously, [28] this dramatic current change from 290 to 150 K for this device proves that the ion migration plays the key for the significant conductivity enhancement in MEH-PPV during the device operation at room temperature.There are the ether bonds in PEO that can promote ion motion by vibrating molecular chains. [29]When the temperature falls, the vibration of PEO chains is restrained, so the transport of ions is restrained.
Based on the above experimental analysis, there exists the migration of ions from the perovskite layer into the MEH-PPV with the help of PEO and this is the key for the electrochemical doping of MEH-PPV polymer during the device operation.When applying drain-source voltage, electrons, and holes are injected into the device and both n-doping and p-doping can occur near the drain electrode and the source electrode, respectively.The doping level should be very shallow under low gate voltage since only a few ions can be driven to migrate into the MEH-PPV film.With the gate voltage increasing, the doping process is more rapid due to that more ions can penetrate into the polymer film and spread over the whole channel, leading to the opening state of the OECTs.
Usually, the electrochemical transistors can be the bipolar transistor, in which both electrons and holes can carry current across the transistor, [30] similar to a bipolar junction transistor (BJT). [31]t is known that a bipolar transistor can emit light by adding a light-emitting layer to the transistor, which is also known as a bipolar light-emitting transistor (BLET). [32]They offer several advantages over traditional LEDs, including higher efficiency, faster switching speeds, and greater compatibility with microelectronic circuits.Unlike the conventional BLET, which needs an additional emitting layer, in the perovskite electrolyte-based polymer electrochemical transistor, the MEH-PPV layer itself is also the emitting layer.Actually, the device can achieve electroluminescence controlled by the gate voltage.
The gate-regulated light-emitting characteristic of the device is shown in Figure 3a.With increasing the gate voltage, the electroluminescence (EL) of the device started to appear at the gate voltage of about −40 V and kept increasing with the gate voltage.Same to the reported light-emitting transistors (LETs), [30] the emission zone of the device is a line emission, as shown in the inset of Figure 3a.The device has a 50 μm conducting channel and the maximum line light emission intensity reaches 0.2 Cd m −2 .As shown in Figure 3b, the EL spectrum peak of this electrochemical transistor is 592 nm, which is consistent with the typical EL spectrum of the MEH-PPV organic light-emitting diode (OLED). [33]The LETs generally need a balance of the electron and hole mobilities to achieve highly efficient light emission, [34] which could be achieved in the OECTs due to electrochemical doping.
The time-lapse fluorescence images of the device are shown in Figure 3c.Before the application of a voltage bias, the polymer film exhibited strong orange-red photoluminescence.When the device was operated, the visible PL quenching can be observed and it is strongly dependent on the gate voltage magnitude.The higher gate voltage can introduce more serious PL quenching, which indicates more ions can involve in the electrochemical doping. [35]This is a manifestation that the gate voltage can drive the ions to migrate from the perovskite layer into the MEH-PPV layer.Eventually, when the gate voltage was high enough, an electroluminescence zone can be observed along the continuous doping frontier in the whole channel.To further verify the effect of the gate voltage, the gate electrode was removed from the device.The device did not emit light under the same test conditions, as shown in Figure 3d.This indicates that the gate voltage can significantly adjust the polymer doping level and therefore affects the charge recombination.
The perovskite electrolyte can also be used to realize the "light-controlled" polymer electrochemical transistor, in which the drain-source output signal is controlled by a light signal rather than a voltage signal (the gate electrode was removed).This can be called the perovskite electrolyte-based polymer phototransistor, which is suitable for circuit integration.When the voltage was swept from 0 to 20 V between the source and drain electrodes, the source-drain current can significantly and continuously increase with increasing the power of the UV (365 nm) light illuminations, as shown in Figure 4a.The maximum current on/off ratio reaches around 10 3 at the maximum optical power of 291.67 μW cm −2 .
To clarify the functions of each layer in the polymer phototransistor, various devices were prepared and tested.In the device with polymer and perovskite bilayer structure, the switching ratio was less than 10, as shown in Figure 4b.This result again shows that PEO plays a key role in the device's operation.As mentioned above, there are two functions for the PEO layer: the dielectric characteristic and ion migration assistance.When adding the PMMA layer on top of the MEH-PPV layer as the dielectric layer, the device current on/off ratio increased a little bit but not much at low operation voltage, as shown in Figure 4c.This implies that the dielectric layer plays a role in enhancing the performance of phototransistors since the dielectric layer can help to create a strong electric field across the polymer layer, which enhances the separation and collection of photogenerated carriers. [36]Furthermore, the dielectric layer can improve the stability of the perovskite and MEH-PPV-based device.To verify the ion solvation effect of PEO on the device performance, the device based on the mixture of PEO and MEH-PPV was fabricated and the result is shown in Figure 4d.Adding PEO into the MEH-PPV layer can dramatically increase the device detection capability.The comparison of Figure 4a and Figure 4d indicates that the PEO layer covering on top of the MEH-PPV also can improve the device performance.This may be related to the floating gate effect introduced by the PEO layer, which changes the electric field and thus affects the doping process. [37]o investigate the effect of UV illumination on the polymer electrochemical transistors, an electrochemical cell was fabricated and tested as shown in Figure 4e.The results of cyclic voltammetry show that the oxidation potential of MEH-PPV decreased from 0.404 to 0.366 V under UV illumination (Figure 4f).The MEH-PPV films can be more easily oxidized with the help of UV light, which promotes the electrochemical doping process.Thus, the planer UV device shows the significant response because of the increased ion conductivity and the decreased polymer oxidation potential under UV illumination.

Conclusion
In this work, a novel polymer electrochemical transistor has been demonstrated using organic-inorganic hybrid perovskite as the electrolyte layer.With the help of a separate PEO layer on top of the MEH-PPV layer, the device achieves gate-regulated output characteristics.The PEO layer not only plays the role of the dielectric layer but also promotes ion migration in the MEH-PPV layer.The fluorescence imaging was used to study the perovskite ionsinduced PL quenching of the conjugated polymer and the QCM test quantifies the perovskite ion migration into the polymer, which demonstrate the perovskite ions-induced electrochemical doping of the MEH-PPV film.
In addition, this novel polymer electrochemical transistor could realize both gate regulation electroluminescence and photo-response.Overall, the solution-processed organicinorganic hybrid perovskite electrolytes hold great promise for solid electrolyte polymer electrochemical transistors and advance their applications in the field of flexible and stretchable electronics.

Experimental Section
Poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV, Mn: 150 000-250 000) and polyethylene oxide (PEO, Mn: 100 000) were purchased from Alfa-Aesar and Aldrich, respectively.Deionized Water (H 2 O, for spectroscopy ACS) was purchased from Acros Organics, and PEABr and MABr were purchased from Dyesol.PbBr 2 was purchased from Alfa Aesar.The PEABr, MABr, and PbBr 2 solutions were prepared in DMF solvent with an overall concentration of 192 mg mL −1 and were stirred overnight before use.MEH-PPV and PEO were dissolved in the methylbenzene and deionized water with the concentrations of 10 and 24.4 mg mL −1 , The drain-source current was measured with KEITHLEY 4200-SCS in the probe station using the 100 μm probes, and the fluorescence spectrum and electroluminescence spectrum were measured with 0.150 m FULLY integrated imaging CCD Spectrometer (Excitation light source is IK Series He-Cd Laser).The voltage scan speeds used for the transfer curves and output characteristics were around 2 and 1 V s −1 respectively.The luminous power density and photocurrent were measured with KEITH-LEY 2450 Sourcemeter, KEITHLEY 2010 Multimeter, and Photodiode.The temperature-dependent current was measured with Lake Shore Model 336 Temperature Controller in the Lake Shore Cryotronics under a 10 −6 kPa atmosphere.The UV light source was a 365 nm LED.The fluorescence images were captured utilizing a microscope with a five-megapixel CCD system and TCapture software.CHI660E Electrochemical Station was used to test the impedance characteristics of the device.The film substrate (fully coated ITO) was used as the source/working electrode, a platinum wire was used as the counter electrode, and a leak-free Ag/AgCl electrode was used as the reference electrode.The electrolyte solution was a 0.05 m TBAPF 6 acetonitrile solution.
Electrogravimetric measurements were performed using a QCM200 (Stanford Research Systems) on the 10 MHz gold-coated AT quartz crystal.The voltage biases were applied with a potentiostat (MetroOhm Autolab PGSTAT204) in a two-electrode cell, with Au as the working electrode and counter electrode.The rigid perovskite layer was prepared on a quartz wafer on which the flexible polymer blend layer (MEHPPV:PEO) was spin-coated.The change in frequency was converted to a change in mass using the Sauerbrey equation as follows: where quartz density ( q ) is 2.648 g cm −3 , quartz shear modulus ( q ) is 2.947 × 10 11 g cm s −2 , quartz resonant frequency (f 0 ) is 10 MHz, and active crystal area (A) is 0.2827 cm 2 .

Figure 1 .
Figure 1.a) The transfer characteristic curve of the OECTs; b) The output characteristic curve of the OECTs; c) The fluorescent images of the MEH-PPV film before and after doping; d) The structure used for the Quartz Crystal Microbalance (QCM) test; e) The transfer characteristic curve of the OECTs (the perovskite layer is removed); f) The transfer characteristic curve of the device (the perovskite layer is on the bottom).
, two sandwich devices based on MEH-PPV (with and without PEO)/Perovskite bilayer structure were prepared for AC impedance test to analyze ionic conductivity characteristics.The detail of the (Electrochemical Impedance Spectroscopy) EIS test is shown in FigureS2(Supporting Information).The conductivity of the device was significantly decreased with mixing PEO into the MEH-PPV layer due to its dielectric

Figure 2 .
Figure 2. a) The transfer characteristic curve of the transistors (MEH-PPV as the active layer); b,c) The conductivity and temperature curves of the various devices under a dark environment; d) The transfer characteristic curve of the transistors (MEH-PPV:PEO as the active layer); e) The water contact angle (solid point) and surface energy (hollow point) of the MEH-PPV film under different annealing time with/without water coated; f) The transfer characteristic curve of OECTs at different temperatures.

Figure 3 .
Figure 3. a) Luminance as a function of the gate voltage or the operation time (the light-emitting image is inserted); b) Electroluminescence spectrum of the LECTs; c) Fluorescence images of the device at the various gate voltage; d) Luminance as a function of the operation time when the gate voltage was removed.

Figure 4 .
Figure 4. a-d) The current response of the photo-gated transistor under different 365 nm LED power densities (device structure is inserted, and identical devices are tested under the same structure); e) Schematic diagram of electrochemical cell cyclic voltammetry test; f) Cyclic voltammetry current and voltage curve.