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 (VOC). 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
figure 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

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As a reference, WSe2 FET was studied before van der Waals integration with MAPbI3 perovskite (Fig. 2a). Transfer characteristics (IDS-VBG) 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 (IDS) (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
figure 2

Doping effects in MAPbI3/WSe2 heterostructure. a–c Transfer curves (IDS-VBG) 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 VDS = 5 V under dark condition. d Photocurrents (Iph) of WSe2 FET, MAPbI3 FET and MAPbI3/WSe2 FET. Each device was measured at VDS = 5 V under white light illumination condition

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By comparing the device current measured under light illumination and dark conditions in transfer curves (IDS-VBG at VDS = 5 V), we have determined the photocurrents (Iph = Ilight – Idark) for WSe2 FET (Iph,WSe2), MAPbI3 FET (Iph,perov) and MAPbI3/WSe2 FET (Iph,hetero) (Fig. 2d). The photocurrent observed in WSe2 FET (Iph,WSe2) is rather low due to low optical absorption in the atomically thin WSe2. Similarly, the photocurrent observed in MAPbI3 FET (Iph,perov) is also rather low, which might be largely attributed to the poor charge transport properties in MAPbI3. In contrast, the Iph achieved in MAPbI3/WSe2 FET (Iph,hetero) is more than 4 orders of magnitude larger than the sum of the photocurrent achieved in WSe2 FET (Iph,WSe2) and MAPbI3 FET (Iph,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
figure 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 (IDS-VBG) of as-fabricated MAPbI3/WSe2 FET. e, f Transfer curves (IDS-VBG) 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 VDS = 5 V under dark condition

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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 (VBG = − 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 (IDS-VDS) 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 (VBG = 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 (VDS), 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
figure 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 VDS. The electrode on the left-hand side was used as a drain electrode and the other side was grounded. d Output curves (IDS-VDS) of the as-fabricated MAPbI3/WSe2 device. e, f Output curves (IDS-VDS) of the device after sequential lateral poling processes with negative and positive poling VDS. 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)

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The output characteristics (IDS-VDS) of monolayer WSe2 FET showed no current within VDS = \(\pm\) 1 V at VBG = 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 VDS 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 VOC close to zero voltage, suggesting there was minimal extrinsic doping effect at this early state (Fig. 4d). The negatively poled photodiode delivered an VOC of − 0.78 V (Fig. 4e). After the positive lateral poling process, the identical photodiode showed a VOC 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 VOCs 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 (Iph at VDS = 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 ISC, the power density of incident photons is Plaser, 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 (VDS = 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 IDS-VDS 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 (VBG) 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
figure 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 (IDS-VDS) 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 (VOC) extracted from the IDS-VDS curves after positive (blue) and negative (red) lateral poling processes. f External quantum efficiency (EQE) extracted from the gate-dependent short circuit current (ISC) measured from negatively poled device under 532-nm laser illumination

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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 IDS-VDS curves under 532-nm laser illumination exhibited a wide gate-tunable VOC from 0.03 to 1.08 V after positive poling process and a tunable VOC 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 VOC of 1.08 V represents the highest VOC 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 VOC 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 (VOC = 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 VDS or VBG, 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 VBG was 60 V. For lateral poling process by VDS, 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.