Lead halide perovskite sensitized WSe₂ photodiodes with ultrahigh open circuit voltages

Abstract

Two-dimensional semiconductors (2DSCs) have attracted considerable interests for optoelectronic devices, but are often plagued by the difficulties in tailoring the charge doping type and poor optical absorption due to their atomically thin geometry. Herein, we report a methylammonium lead iodide perovskite (CH₃NH₃PbI₃)/2DSC heterojunction device, in which the electric-field controllable ion migration in the perovskite layer is exploited to induce reversible electron- and hole-doping effects in the underlying monolayer tungsten diselenide (WSe₂) to form a programmable p–n photodiode. At the same time, the CH₃NH₃PbI₃ layer functions as a highly efficient sensitization layer to greatly boost the optical absorption and external quantum efficiency (EQE) of the resulting photodiode. By asymmetrically poling the perovskite layer, gold-contacted CH₃NH₃PbI₃/WSe₂ devices show a switchable open circuit voltage up to 0.78 V, along with a high EQE of 84.3%. The integration of tunable graphene-contacts further improves the photodiode performance to achieve a highest open circuit voltage of 1.08 V and a maximum EQE of 91.3%, greatly exceeding those achieved previously in 2DSC lateral diodes. Our studies establish a non-invasive approach to switch optoelectronic functions and open up a new avenue toward high-performance reconfigurable optoelectronic devices from 2DSCs.

1 Introduction

Electrostatic doping has been widely used in low-dimensional materials, including carbon nanotube (CNT), and two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) [1,2,3,4,5,6,7,8,9,10]. Unlike conventional lattice doping with impurity atoms [11], it is difficult to achieve doping in nanoscale materials due to the limited physical space. The electrostatic doping opens an effective pathway to tune the charge carriers in nanoscale materials without introducing impurity atoms which can perturb the atomic arrangement and degrade the intrinsic electronic properties of the nanoscale materials [12]. Recently, ionic solids have been explored for creating a p–n junction in monolayer 2D materials in which the frozen mobile ions provide electrostatic fields to modulate the carrier density of underlying 2D semiconducting channel. Due to the well-defined shape of ionic solid, local control of the doping on 2D semiconductors (2DSCs) allows diverse designs for integration of solid-state electronic/optoelectronic devices with minimum crosstalk. For example, the manipulation of silver ions in solid-state superionic silver iodide (AgI) was employed for tailoring the carrier type of 2DSCs to achieve reversibly programmable transistors, diodes, photodiodes and logic gates [13].

To date, the monolayer TMDs have been widely adopted in novel optoelectronic applications such as electrically tunable light-emitting diodes (LEDs) [14], gate-controlled p–n junction diodes [15], and solar cells [16]. However, the monolayer TMDs also exhibit some intrinsic limits for high-performance optoelectronic applications. In particular, incorporation of impurity dopants in the atomically thin 2D lattices has been fundamentally limited by the little physical space in the atomically thin lattices. It has been a persistent challenge to controllably tailor the charge doping type/density in monolayer 2DSCs using selected lattice dopants. Consequently, the p–n photodiodes made from 2DSCs to date are often plagued by non-ideal contacts at either p- or n-side, which limits the achievable open circuit voltage (V_OC). Additionally, total light absorption and spectral sensitivity of 2DSCs are fundamentally limited by their atomically thin geometry [17], compromising the photocarrier generation efficiency and the achievable external quantum efficiency (EQE). Considerable efforts have been devoted to overcoming such intrinsic limitations by heterogeneously integrating with other well-known optoelectronic materials [18,19,20]. For example, interfacing with organic dye molecules has been demonstrated as an effective strategy to control its optoelectrical properties [21].

The hybrid lead halide perovskites (LHPs) have received substantial attention for photovoltaics due to their excellent optoelectronic performance and low fabrication cost [22]. Despite its extraordinary potential, the “soft lattice” ionic LHPs are typically plagued with ion migrations under voltage bias, leading to poor material stability [23, 24] and large hysteresis in the voltage-dependent photocurrents [25, 26]. The migration of positively or negatively charged ions could induce ion accumulation or ionic charge imbalance under applied electric fields [27]. Here, we exploit such ionic charge imbalance in LHPs to induce reversible doping in nearby 2DSCs to create high-performance photodiodes.

Methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI₃) represents the most prominent examples of LHPs with excellent optical absorption and photoresponsive properties [28], but is seriously plagued by ionic motion [29]. Although undesirable for stable operation of solar cell applications [30], the accumulation of ionic charge from the bias-induced ions migration in MAPbI₃ can be exploited for selectively doping nearby 2DSCs to create perovskite-sensitized 2D photodiodes with high optoelectronic performance. In this regard, the atomically thin 2DSCs are ideally suited for efficiently coupling with the ionic solids that serve as a non-covalent doping agent to reversibly induce reconfigurable p-type or n-type doping effect. Such tunable doping effect further offers a new class of 2DSC-based photodiodes with switchable polarities. With van der Waals integration [31] of ionic solids with excellent optoelectronic properties, the 2D diodes formed from ionic-doping effect provide an efficient way to extract photogenerated carriers in MAPbI₃.

2 Results and discussion

2.1 Doping effect of MAPbI₃ perovskite on WSe₂ field-effect transistor

To elucidate the electrostatic doping effect introduced by MAPbI₃ perovskite, the Au-contacted two-terminal monolayer WSe₂ field-effect transistors (FETs) (Additional file 1: Fig. S1) on top of highly doped (p⁺⁺) silicon covered with 290-nm SiO₂ were adopted for their ambipolarity [32]. The ambipolar nature of WSe₂ is amenable for achieving both n-type and p-type doping effects, which is essential for forming photodiode and efficiently extracting the photogenerated electrons and holes (Fig. 1). The MAPbI₃ perovskite was integrated on top of the WSe₂ device through an aligned transfer of lead iodide (PbI₂) flake [33, 34] followed by vapor phase thermal conversion process under methylammonium iodide (CH₃NH₃I) vapor [35,36,37], producing an overall device structure of: MAPbI₃/Au-WSe₂-Au on a SiO₂/Si substrate (Fig. 1a and Additional file 1: Fig. S2). Our previous studies [36, 37] showed that, after the conversion process, a prominent single photoluminescence peak of MAPbI₃ perovskite was located at around ~ 1.63 eV, indicating a successful conversion from PbI₂ to MAPbI₃ perovskite. All the MAPbI₃/WSe₂ FETs were encapsulated within few-layer hexagonal boron nitride (h-BN) to prevent surface degradation of MAPbI₃ perovskite under ambient conditions [23, 38].

Fig. 1

Illustrations of WSe₂ FET integrated with MAPbI₃ perovskite to form a programmable perovskite sensitized photodiode. a Schematic illustration of the MAPbI₃/WSe₂ FET device structure. b Schematic illustration of the extraction of photogenerated carriers from MAPbI₃ into polarized WSe₂ diode channel

As a reference, WSe₂ FET was studied before van der Waals integration with MAPbI₃ perovskite (Fig. 2a). Transfer characteristics (I_DS-V_BG) of the WSe₂ FET showed ambipolar behavior with a slight p-doping effect. After MAPbI₃ integration, the MAPbI₃/WSe₂ FET showed a strong p-type doping effect with the drain-source current (I_DS) (Fig. 2b) increased by about 2–5 orders of magnitude compared to that of WSe₂ FET. The resulting p-type doping effect of the WSe₂ channel can be attributed to the charge transfer in the MAPbI₃/WSe₂ heterojunction [39]. On the other hand, the drain-source current of the MAPbI₃ FET without monolayer WSe₂ was essentially at the baseline of the measurement resolution (Fig. 2c), about 4–5 orders of magnitude smaller than that of the MAPbI₃/WSe₂ FET. This result indicates that the MAPbI₃ perovskite itself does not directly contribute to significant charge transport in the MAPbI₃/WSe₂ device but primarily serves as a doping agent to greatly increase carrier density in monolayer WSe₂.

Fig. 2

Doping effects in MAPbI₃/WSe₂ heterostructure. a–c Transfer curves (I_DS-V_BG) of WSe₂ FET, MAPbI₃/WSe₂ FET and MAPbI₃ FET measured with A-B, A-B and C-B electrodes where A and C were used as drain electrodes and B was grounded. The insets (left) show schematic illustrations of each device structure. The insets (right) show the optical images of the devices, where the white dashed frames mark the position of WSe₂. The white scale bar is 10 μm. Each device was measured at V_DS = 5 V under dark condition. d Photocurrents (I_ph) of WSe₂ FET, MAPbI₃ FET and MAPbI₃/WSe₂ FET. Each device was measured at V_DS = 5 V under white light illumination condition

By comparing the device current measured under light illumination and dark conditions in transfer curves (I_DS-V_BG at V_DS = 5 V), we have determined the photocurrents (I_ph = I_light – I_dark) for WSe₂ FET (I_ph,WSe2), MAPbI₃ FET (I_ph,perov) and MAPbI₃/WSe₂ FET (I_ph,hetero) (Fig. 2d). The photocurrent observed in WSe₂ FET (I_ph,WSe2) is rather low due to low optical absorption in the atomically thin WSe₂. Similarly, the photocurrent observed in MAPbI₃ FET (I_ph,perov) is also rather low, which might be largely attributed to the poor charge transport properties in MAPbI₃. In contrast, the I_ph achieved in MAPbI₃/WSe₂ FET (I_ph,hetero) is more than 4 orders of magnitude larger than the sum of the photocurrent achieved in WSe₂ FET (I_ph,WSe2) and MAPbI₃ FET (I_ph,perov), suggesting significant synergistic effects of MAPbI₃/WSe₂ heterostructure in efficient extracting the photocarriers. Such a synergistic effect can be attributed to efficient photon absorption and photocarrier generation in MAPbI₃ with excellent optical absorption properties, and the rapid separation and efficient transport of photocarriers by the WSe₂ channel with excellent charge transport properties [40].

2.2 Gate-controlled ionic doping effect

The above studies clearly demonstrate the doping effect by MAPbI₃ ionic solid can considerably tailor the charge transport properties of monolayer WSe₂ semiconductor. MAPbI₃ perovskite is also well known for its low activation energy for ion migration and bias-induced ion migration [41]. It is thus possible to create an ionic profile within MAPbI₃ ionic solid by applying external voltages to drive the ion movement to produce a charge imbalance between positively or negatively charged ions in the desired locations of MAPbI₃ perovskite (e.g., at the interface between perovskite and 2D material). These imbalanced ions near the MAPbI₃/WSe₂ interface can induce an electrostatic doping effect to the underlying WSe₂. To this end, we studied the feasibility of switching the majority carrier type in WSe₂ devices by applying vertical bias voltages to back-gate electrode in MAPbI₃/WSe₂ FET (Fig. 3a). WSe₂ devices fabricated on SiO₂/Si substrate typically show dominant p-type behavior that is difficult to reverse to n-type. In this regard, the MAPbI₃/WSe₂ FET was fabricated on a (3-aminopropyl) triethoxysilane (APTES) treated SiO₂/Si substrate, which provides n-doping effect to the WSe₂ channel [42]. By adopting the APTES-treated SiO₂/Si substrate, the p-doping effect in WSe₂ channel originated from the SiO₂/Si substrate can be suppressed. This facilitates a type-switchable doping effect to WSe₂ channel by mobile ions in MAPbI₃ perovskite ionic solid.

Fig. 3

Carrier type-switchable MAPbI₃/WSe₂ FETs. a Schematic illustration of as-fabricated MAPbI₃/WSe₂ FET. b, c Schematic illustrations of negative and positive back-gate poling process that enriches a net positive or negative ionic charge at MAPbI₃/WSe₂ interface. d Transfer curves (I_DS-V_BG) of as-fabricated MAPbI₃/WSe₂ FET. e, f Transfer curves (I_DS-V_BG) of MAPbI₃/WSe₂ FET after negative and positive back-gate poling processes, showing n- and p-channel FET characteristics. The insets show schematic illustrations of resulting ionic charge distribution. Each device was measured at V_DS = 5 V under dark condition

Previous studies indicate the I⁻ show considerable mobility under external bias [43], which allows an external voltage to drive the ion movement and induce the desired ionic doping effect. To examine the doping effect induced by the accumulated mobile ions at the MAPbI₃/WSe₂ interface, a vertical electric field from back-gate electrode was applied against initial non-poled state. All the poling processes in this paper were conducted at 400 K to promote ion migration in MAPbI₃ perovskite. After 5 min of the electrical poling process, the device was cooled down to 79 K under the continuous poling voltage to freeze the ions at the designed location [44, 45]. For example, a negative back-gate electric field (V_BG = − 60 V) pushes the negative ions (I⁻) away from the WSe₂/MAPbI₃ interface, resulting in a net positive charge at the interface. This net positive charge can electrostatically increase electron concentration in WSe₂ channel, resulting in n-type FET. On the other hand, a positive poling gate bias attracts the I⁻ moving towards the MAPbI₃/WSe₂ interface, which increases the hole concentration in WSe₂ channel and produces a p-type FET.

The transfer curve (I_DS-V_DS) of the as-fabricated MAPbI₃/WSe₂ FET (Fig. 3a) showed p-type characteristics under dark condition at 79 K (Fig. 3d). After the negative vertical poling process (Fig. 3b), the transfer curve was shifted from p-type to n-type behavior, confirming that electrons became the dominant carrier type (Fig. 3e). It is important to note that the MAPbI₃ perovskite does not directly contribute to the charge transport of the FET (Fig. 2c) but only provides the electrostatic doping effect to modulate the dominant carrier type and concentration. To highlight the controllability of the doping effect, a positive back-gate bias voltage (V_BG = 60 V) was applied to the identical device against the previously n-programmed state during the poling process (Fig. 3c). The results indicated the dominant carrier type was completely switched back to holes (Fig. 3f). The results suggested the vertical ionic poling process can be used to rationally tailor the dominant charge carrier type in MAPbI₃/WSe₂ heterostructure devices.

2.3 Drain-source-controlled ionic doping effect

The above studies clearly demonstrate reversible MAPbI₃ poling and WSe₂ doping with either electrons or holes. Taking a step further from the uniform poling with the vertical electric field, we have also exploited lateral electric fields for non-uniformly poling the ionic solid for asymmetric doping, and creation of a functional diode (Fig. 4a). In this case, ions in MAPbI₃ perovskite are polarized by the drain-source bias. The migration of ions in MAPbI₃ perovskite is driven by lateral electric field (V_DS), which leads to an ionic charge imbalance along the channel length. The resulting ionic profile induces opposite doping effects at both ends of the underlying WSe₂ channel. In the case of a negative bias lateral poling process, negative ions are accumulated near the grounded electrode (Fig. 4b). After the process, a net negative electrostatic potential is built up in the WSe₂ channel near grounded electrode, inducing p-type doping effect. On the other side of the WSe₂ channel near the biasing electrode, a net positive electrostatic potential is developed, inducing an n-type doping effect. Collectively, a forward biased n-p diode is created. In the same manner, a positive lateral poling process produces a forward biased p–n diode (Fig. 4c).

Fig. 4

Polarity-switchable MAPbI₃/WSe₂ diodes and photodiodes. a Schematic illustrations of as-fabricated MAPbI₃/WSe₂ device. b, c Schematic illustrations of non-uniform poling processes of resulting ionic charge distribution induced by negative and positive V_DS. The electrode on the left-hand side was used as a drain electrode and the other side was grounded. d Output curves (I_DS-V_DS) of the as-fabricated MAPbI₃/WSe₂ device. e, f Output curves (I_DS-V_DS) of the device after sequential lateral poling processes with negative and positive poling V_DS. The insets show schematic illustrations of resulting ionic charge profile. The measurements were conducted in dark (blue curve) and 532-nm laser illumination (Power density: 15 W/m², red curve)

The output characteristics (I_DS-V_DS) of monolayer WSe₂ FET showed no current within V_DS = $\pm$ 1 V at V_BG = 0 V under both dark and light illumination conditions (Additional file 1: Fig. S3). After lamination of MAPbI₃ perovskite and h-BN, the as-fabricated MAPbI₃/WSe₂ FET showed asymmetric output characteristics under dark condition (Fig. 4d). This initial output behavior might be attributed to Schottky barrier between WSe₂ channel and Au electrode at 79 K. After the negative poling process, rectifying output behavior exhibited a negative turn-on voltage and high output current beyond the turn-on voltage (Fig. 4e). To highlight the switchable doping effect, the lateral poling process with positive V_DS was conducted to the identical device. The rectification behavior of the diode was changed to the opposite polarity that showed a positive turn-on voltage and high output current in the positive bias regime (Fig. 4f). Such polarity-switchable rectification behavior demonstrates that the diode behavior is indeed resulted from poling-induced ionic doping effect rather than the non-ideal contact.

These programmed diodes can also function effective photodiodes. Under 532-nm laser excitation (15 W/m²), the output characteristics of the as-fabricated MAPbI₃/WSe₂ FET showed a V_OC close to zero voltage, suggesting there was minimal extrinsic doping effect at this early state (Fig. 4d). The negatively poled photodiode delivered an V_OC of − 0.78 V (Fig. 4e). After the positive lateral poling process, the identical photodiode showed a V_OC of + 0.75 V very close to that of the negatively poled photodiode, demonstrating that the polarity is reversibly programmed by the poling process (Fig. 4f). The obtained V_OCs are comparable to the state-of-the-art results from previous reports (Additional file 1: Table S1).

To examine the optoelectronic performance of the MAPbI₃/WSe₂ photodiodes, a short-circuit photocurrent (I_ph at V_DS = 0 V) of ~ 83.3 pA and a photoresponse time of ~ 450 µs was obtained (Additional file 1: Fig. S4). In addition, the external quantum efficiency (EQE) of the photodiode was extracted from the equation:

$$\mathrm{EQE}=\frac{{I}_{\mathrm{SC}}}{{P}_{\mathrm{laser}}A}\times \frac{\mathrm{hc}}{\mathrm{e}\lambda }$$

(1)

where the short-circuit current is I_SC, the power density of incident photons is P_laser, the effective area is A, the Planck’s constant is h, the speed of light is c, the electron charge is e and the wavelength of the light is λ. The extracted EQE of 84.3% at 532 nm represents the highest value achieved from lateral or sensitized 2D diode at zero bias voltage (V_DS = 0 V) (Additional file 1: Table S1) [46]. Such strong enhancement could be attributed to high optical absorption and long electron-carrier carrier diffusion length in MAPbI₃ perovskite [28, 47,48,49,50]. These excellent optoelectronic properties of the perovskite can ensure the photogenerated carriers in the MAPbI₃ to reach the p–n junction in the monolayer WSe₂ channel, where they are separated by the built-in potential and extracted to external circuit (Fig. 1b).

To elucidate the contribution of the measured photocurrent in the MAPbI₃/WSe₂ photodiode, we have examined the poling effect in both the MAPbI₃ FET (Fig. 2b) and the MAPbI₃/WSe₂ FET (Fig. 2c). The I_DS-V_DS curves were measured after the poling process for both devices (Additional file 1: Fig. S5). It is important to note that the MAPbI₃/WSe₂ device showed prominent rectification ratio under both dark (> 10⁴) and laser illumination (> 10³) conditions; while the MAPbI₃ device exhibited negligible currents or rectification ratio. Together, the rectification behavior and output currents were mainly contributed by the WSe₂ channel rather the MAPbI₃, and the MAPbI₃ mainly served as a tunable doping agent and a sensitizer for significantly enhanced photon absorption efficiency and photocarrier generation.

2.4 Enhanced performance with graphene contact

Our findings show that the integration of the monolayer WSe₂ semiconductor and the MAPbI₃ ionic solid offers a type-switchable FET that can be reversibly programmed to either n-type or p-type characteristics depending on the direction of applied poling potentials. However, a single type of metal contact is not ideal for achieving optimized carrier transport at both n-type and p-type regions of the doped WSe₂ channel. Due to the fixed work-function of metal, one of the contacts could be non-ideal and form a Schottky barrier compromising the p–n diode effect. In this context, it is desirable to adopt a tunable work-function that can make optimal contacts either for electrons or holes. To this end, monolayer graphene was selected as tunable electrical contacts (Fig. 5a, b) for its electrostatically tunable work-functions [51,52,53,54]. Unlike metal electrodes, the work-function of graphene contact can be tuned by back-gate voltage or accumulated ions in the poled MAPbI₃ perovskite [13]. For the poled devices, the graphene contacts are electrostatically doped with electrons or holes by the poled MAPbI₃, with their work-function matching well with conduction or valence band edge of the WSe₂ channel electrostatically doped by the same poled MAPbI₃. The applied gate bias (V_BG) is used to further tune carrier type and concentration in WSe₂ to match with that of the doped graphene. In this case, a net positive charge in the MAPbI₃ perovskite increases the electron doping in graphene to reduce its work-function, and make it an optimum contact for n-type WSe₂ channel. Similarly, a net negative charge increases hole doping in graphene, increasing its work-function and making it a better contact for p-type WSe₂ channel.

Fig. 5

Optoelectronic properties in graphene-contacted programmable MAPbI₃/WSe₂ photodiodes. a Schematic illustration of photogenerated carrier extraction in a graphene-contacted MAPbI₃/WSe₂ photodiode under 532-nm laser illumination. b Optical image of the fabricated graphene-contacted MAPbI₃/WSe₂ FET. The black and red dashed frames mark the position of a pair of graphene and WSe₂. The white scale bar is 10 μm. c, d Gate-tunable output characteristics (I_DS-V_DS) of the graphene-contacted device after negative lateral poling process measured under dark (c) and 532-nm laser illumination (d) conditions. The inset shows a schematic illustration of the ionic charge profile. e Open circuit voltages (V_OC) extracted from the I_DS-V_DS curves after positive (blue) and negative (red) lateral poling processes. f External quantum efficiency (EQE) extracted from the gate-dependent short circuit current (I_SC) measured from negatively poled device under 532-nm laser illumination

Indeed, the graphene-contacted MAPbI₃/WSe₂ FET showed much larger gate-tunability (Fig. 5c) than the Au-contacted devices (Additional file 1: Fig. S6). This can be attributed to the effective modulation of the work-function of the graphene contact. In addition, the I_DS-V_DS curves under 532-nm laser illumination exhibited a wide gate-tunable V_OC from 0.03 to 1.08 V after positive poling process and a tunable V_OC from 0 to -0.96 V after negative poling process (Fig. 5d). The short circuit current measurement also shows a highly tunable EQE from 2.7 to 91.3% (Fig. 5e). It is important to note that, to be best of our knowledge, the V_OC of 1.08 V represents the highest V_OC ever achieved in WSe₂-based diodes in lateral structure, highlighting the excellent photodiode characteristics achieved in ion-doped p–n junctions (Additional file 1: Table S1). It should be noted that our study is fundamentally different from previous studies of photodetection using 2DSC/perovskite heterojunctions, in which the perovskite layer functions as a photogate and does not exhibit power generation [40, 49]. In contrast, the polarized perovskite in our study modifies the carrier type and creates a sensitized p–n photodiode in the 2DSC channel, leading to active power generation and self-powered photodetection with record-high V_OC and zero bias EQE.

3 Conclusion

In summary, we have reported a unique design of MAPbI₃/WSe₂ device as a programmable photodiode. The MAPbI₃ perovskite served as not only programmable ionic dopants but also the excellent optical absorber and highly efficient sensitization layer, resulting in the record-high optoelectronic performance (V_OC = 1.08 V; EQE = 91.3%) among WSe₂-based lateral-structured photodiodes reported to date. Our solid ionic doping approach offers a non-invasive way to tailor the electronic properties of 2DSCs and to reversibly tune the polarities of reconfigurable photodiodes without chemical manipulation. Furthermore, a combination of atomically thin 2D materials and ionic solids enables efficient coupling between electronic transport and ionic transport, which could open a new pathway to unconventional computing, information storage systems and programmable optoelectronic devices.

4 Methods

4.1 Fabrication of MAPbI₃/WSe₂ transistor

The MAPbI₃/WSe₂ devices were fabricated by integrating PbI₂ flakes with prefabricated WSe₂ FETs followed by MAPbI₃ conversion process. Briefly, WSe₂ FETs were contacted with Au (30 nm) through electron-beam lithography defined patterns. The exfoliated PbI₂ flakes from its layered crystal were transferred onto the prefabricated devices through dry transfer approach we have previously reported [36]. Subsequently, the transferred PbI₂ flakes were converted to MAPbI₃ through vapor phase reaction with CH₃NH₃I vapor with the resulting thickness of ~ 50 nm. A few-layer h-BN flakes were transferred on top of the entire devices as encapsulation layers. The graphene-contacted devices were fabricated by transferring two parallel graphene flakes on top of an exfoliated monolayer WSe₂ flake, followed by standard electron-beam lithography and electron-beam evaporation to connect the transferred graphene with Cr/Au (20 nm/80 nm) metal electrodes.

4.2 Device poling process and characterization

To facilitate ion migration in MAPbI₃ perovskite, the devices were placed on the heating stage of the probe station (Lakeshore, TTP4) and heated to 400 K. After 5 min of applying poling voltages either to V_DS or V_BG, the devices were cooled down to 79 K. All the measurements were conducted at 79 K unless otherwise noted. The magnitude of the applied poling voltages for vertical poling by V_BG was 60 V. For lateral poling process by V_DS, 5 V was applied for about 3 µm of channel length. The poling voltages were continuously applied during the cooling process. The electrical measurements were conducted in the probe station equipped with 532-nm wide field laser (Coherent, 532-100). The measurement data were obtained by a precision source/measure unit (Agilent, B2902A) and a computer-controlled analogue-to-digital converter.