Compound Quantum Dot–Perovskite Optical Absorbers on Graphene

Compound Quantum Dot−Perovskite Optical Absorbers on Graphene Enhancing ShortWave Infrared Photodetection Alexander A. Bessonov,*,† Mark Allen,† Yinglin Liu,† Surama Malik,† Joseph Bottomley,† Ashley Rushton,† Ivonne Medina-Salazar,† Martti Voutilainen,‡ Sami Kallioinen,‡ Alan Colli,† Chris Bower,† Piers Andrew,† and Tapani Ryhan̈ en‡ †

Emberion Limited, Sheraton House, Castle Park, Cambridge CB3 0AX, United Kingdom Emberion Oy, Metsänneidonkuja 8, Espoo 02130, Finland

‡

S Supporting Information *

ABSTRACT: Colloidal quantum dots (QDs) combined with a graphene charge transducer promise to provide a photoconducting platform with high quantum efficiency and large intrinsic gain, yet compatible with cost-efficient polymer substrates. The response time in these devices is limited, however, and fast switching is only possible by sacrificing the high sensitivity. Furthermore, tuning the QD size toward infrared absorption using conventional organic capping ligands progressively reduces the device performance characteristics. Here we demonstrate methods to couple large QDs (>6 nm in diameter) with organometal halide perovskites, enabling hybrid graphene phototransistor arrays on plastic foils that simultaneously exhibit a specific detectivity of 5 × 1012 Jones and high video-frame-rate performance. PbI2 and CH3NH3I co-mediated ligand exchange in PbS QDs improves surface passivation and facilitates electronic transport, yielding faster charge recovery, whereas PbS QDs embedded into a CH3NH3PbI3 matrix produce spatially separated photocarriers leading to large gain. KEYWORDS: quantum dots, organometal halide perovskite, infrared absorber, graphene phototransistor, sensitivity−response speed trade-off semiconducting QD film strongly absorbing photons up to the short-wave infrared (SWIR) waveband and a two-dimensional (2D) channel providing near ideal transport conditions.3,6,7 The amplification mechanism of sensitized phototransistors demonstrating giant gains up to 108 is achieved through trapping of minority charge carriers in an electrically passive light absorber, while majority carriers, induced by electrostatic coupling, recirculate in the transistor channel many times before recombining.5 Not readily possible in photodiodes, save for devices exploiting avalanche mechanisms, this multiplication process makes low-dark-current phototransistors an attractive platform to attain superior sensitivity raising the signal far above the noise floor. Yet, large gain, a prerequisite of high responsivity and thus detectivity, inevitably comes at the expense of response time, since the long carrier lifetimes

P

hotodetectors capable of converting low-energy infrared photons into an electrical signal are highly desired for numerous rapidly growing applications including optical communication, remote sensing, spectroscopy, infrared cameras, and biomedical and thermal imaging.1 Although compatibility with widely used complementary metal-oxidesemiconductor (CMOS) platforms is still required for certain applications, emerging photodetection technologies, which offer cost-efficient manufacture and mechanical flexibility, alongside tunable and broadband spectral coverage are currently in the spotlight.2−4 The key figures-of-merit are the noise performance (characterized by responsivity (9 ), noise equivalent power (NEP), and specific detectivity (D*)) and the response time (characterized by the frequency response of the device). Graphene-based photoconducting detectors coupled with colloidal quantum dots (QDs) seem to fulfill the current demands of optoelectronics as they have the potential to enable high quantum efficiency (collected charges per photon), maximum gain (detected charges per photon), and low noise.5 The optical absorption and electrical amplification processes are elegantly decoupled in such configuration, with a © 2017 American Chemical Society

Received: February 3, 2017 Accepted: May 30, 2017 Published: May 30, 2017 5547

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Figure 1. Hybrid quantum dot−graphene photodetector on a plastic foil. (a) Photograph of photodetector on a polymer substrate supported by a silicon wafer. (b) Top view optical image of a GFET array device without (left) and with the QD absorber (right). Each pixel has an individual graphene channel of 18 μm in length and 90-μm-wide (two 45-μm-wide channels in parallel), identified by the white dashed-line blocks. The drain and back-gate electrodes are shared between pixels. (c) Schematic cross-section view of device structure with specified materials.

Figure 2. Electrical characteristics of the graphene transducer. (a) Transfer characteristics of unencapsulated GFET on Al2O3 versus GFET on alkyl-terminated SAM, dehydrated and encapsulated. The hydrophobic buffer layer and encapsulation effectively eliminate hysteresis and reduce water-induced p-doping. Scan rate is 1 V/s. (b) Graphene is sensitive to external doping from absorber and its ligands. A shift toward negative gate voltages (Vg) is observed for graphene in contact with PbSMAPbI3. The devices are fabricated on ODPA SAM and encapsulated after QD deposition. The inset cones show the doping state of graphene at Vg = 0 V. (c) Hybrid MAPbI3·PbS results in moderate n-type doping of graphene, slowly turning to heavily p-doped over time or upon MAI treatment and finally reaching the level of the perovskite alone. We propose this effect to be due to the combined effect of chemical transformations shifting the energy bands of QD−perovskite hybrid together with the inward diffusion of mobile [MA]+ ions from the topmost layers toward the graphene. All data are recorded under dark conditions.

largely determine the electronic transport figures as they dope QDs and form an interdot medium through which the carriers diffuse. PbS QDs used in hybrid phototransistors reported in the literature are usually passivated by organic thiol-based ligands and such ligands are far inferior to inorganic ligands in terms of charge transport efficiency and ambient stability.35,36 In addition, the light sensitivity of devices based on PbS QDs capped with organic molecules drops dramatically with increasing the QD size toward SWIR absorption.5,34 Developed for photovoltaic devices, alternative QD electronic passivation strategies, including solution-phase ligand-exchange mechanisms, have provided lower trap densities and depths, leading to long-lived photocarriers with higher electronic diffusivity and faster detrapping.37−40 These properties are believed to improve the sensitivity and response speed of hybrid photodetectors, respectively. An optimized trade-off between the lifetime of photocarriers following excitation and their recombination rate in the dark can yield high-gain, high-speed detectors. Furthermore, we assumed that hybridizing large-size QDs with emerging organometal halide perovskites, showing high electronic mobility and long carrier lifetimes,41 may facilitate charge collection from thicker absorber films while preserving benefits of strong interaction with infrared photons,42,43 breaking the absorption−extraction compromise.

needed to support multiplication inhibit fast switching, limiting operational speed. To address this gain versus response-time trade-off and further extend spectral responsivity in sensitized graphene phototransistors, various absorbers have been investigated to date, including semiconducting quantum dots (PbS,5,8−10 PbSe,11 CdS,12 Cu2Se,13 ZnO,14,15 TiO216,17), 2D crystals (graphene,18 MoS2,19−21 CdSe,22 GaSe23), carbon nanotubes,24 perovskites,25−27 and organic dyes.28−30 Attempts to replace graphene with other high-mobility materials such as IGZO,31 MoS2,32 or SnS233 have been also documented. Nikitskiy et al. reported a hybrid photodiode−phototransistor architecture where the charges in PbS QDs are efficiently directed toward graphene by applying an electric field across the absorber layer.34 This approach, transforming a passive QD stack into an active photodiode, allowed an increase in the optical cutoff frequency up to 1.5 kHz, preserving a relatively high gain of 105. However, the complexity of a device having two electrical gates and associated manufacturing challenges may limit its scalability. There is still plenty of scope for further improvement of the temporal response of passive absorbers by engineering the QDs; tailoring their size, shape, and surface chemistry all greatly affect the nanocrystals’ optoelectronic properties. QD ligands 5548

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Figure 3. Quantum dot surface tuning via ligand exchange reactions. (a) Schematics of the ligand replacement mechanisms using solid-state and solution-phase processes. In the solid-state ligand exchange, oleic acid (OA) ligands are stripped off PbS QDs and replaced by fragments of PbI2 and MAI, whereas formation of a perovskite matrix occurs following the solution-phase process and subsequent film deposition. (b) SWIR absorption spectra of PbS-based solids with process-specific characteristic excitonic peak. The absorption maximum shifts from 1320 to 1540 nm after solid-state ligand replacement. (c) The completeness of ligand exchange in PbS QDs is confirmed by the absence of oleate C− H peaks in the FTIR spectra. Methylammonium signatures including C−H (stretch 2860−2960 cm−1, bend 1465 cm−1) and N−H (stretch 3120−3170 cm−1, bend 1565 cm−1) are clearly visible for MAPbI3·PbS and faintly noticeable for PbSMAPbI3 (magnified 30× in the insets), suggesting predominantly inorganic passivation in the solid-state process.

drain electrodes are biased at a constant 0.1 V relative to the source. The hysteresis in GFETs is largely suppressed by introducing a hydrophobic alkylphosphonic acid self-assembled monolayer (SAM), which masks oxide surface charges and prevents molecular adsorption on graphene due to the low surface energy (Figure 2a). Further vacuum dehydration and encapsulation eliminates hysteretic behavior completely. We find that reproducible hysteresis-free transfer curves in the array are vitally important for stable and uniform device operation (Supporting Figure S3). The light absorber is a key constituent of graphene-based phototransistors as its purpose is to capture incident photons and convert them into electric charges. A PbS QD layer is uniformly deposited atop the GFETs so that it provides a similar level of photogating modulation across the array (Supporting Figure S4). The initial p-type doping of graphene, usually ascribed to external adsorbates such as physisorbed water and organics,44 is altered following the deposition of semiconducting material, as shown in Figure 2. Absorber Chemistry. As synthesized QDs typically require post-treatment to remove stabilizing long-chain alkyl ligands and promote electrical coupling between nanocrystals in the film. This ligand replacement process can be done either in the solution phase prior to deposition or in the solid state rearranging the crystals in the film (Figure 3). Early solid-state passivation approaches exploiting short organic ligands such as EDT demonstrated promising performance of PbS QDs in solar cells and photodetectors; however, later it was realized that their oxygen and moisture sensitivity, poor interdot communication, and slow charge dynamics limited by electrically active deep traps (0.3−0.5 eV) needed to be addressed.35,45 A class of inorganic surface ligands, suggested by Dirin et al.,46 is based on metal halides and perovskite complexes, which exhibit reduced density of midgap trap states, stronger near-field coupling with graphene,47 and redox stability of QDs.43,48,49 In QD-based photoconductors, subgap states play a crucial role in providing high gain, acting as sensitizing

We extend the range of sensitizing absorbers for graphene field-effect transistors (GFETs) by introducing compound QD−perovskite systems, the two-component chemistry of which is translated into the optimized sensitivity and response time. Enhanced response speed is achieved with PbS QD solids passivated with mixed PbI2 and MAI (MA = CH3NH3), here denoted as PbSMAPbI3. Hybrids of PbS QDs and MAPbI3 with increased perovskite loading (denoted as MAPbI3·PbS), where nanoscale heterojunctions with favorable localization of one type of charge carrier are created, result in reversed photogating polarity and unusually large gain (∼107, translated to 33% relative channel resistance change at irradiances 6 nm in diameter) PbS QDs, leading to improvements in both sensitivity and response speed. Replacement of bulky oleate ligands by shorter molecules, naturally maintaining the charge-neutral stoichiometry of the PbS surface, results in a red-shift and broadening of the exciton absorption peak (Figure 3b), typically explained by enhanced coupling between the QDs.51,52 We stress that solid-state ligand exchange using premixed MAI and PbI2 solutions (MAPbI3 precursors) yields PbS QDs capped predominantly by iodine ions and [PbIx] species (x ≤ 3), washing most [MA]+ counterions away with rinsing solvent (Figure 3c). The process leads to nearly organic-free PbS solids, which are content-wise similar to earlier reported MAI- and PbI2-treated PbS QDs,53−55 benefiting from both methods. It was reported in the literature that iodide ligands, in contrast to most organic capping molecules, form n-type PbS QDs with relatively low dark charge density,53,56 being particularly beneficial for stronger depletion of the QD film in contact with graphene (discussed below). Since the chemical state of as-synthesized QD surfaces is often unknown, it is essential to have a variety of highly reactive mobile species, including reducing agents (as in

the perovskite precursor solution) to address all the offstoichiometric sites of different binding strength and polarity. High surface coverage, maximized with atomic-scale ligands, is particularly important for reduced bandtailing.57 Highly diffusive [MA]+ ions, owing to their acidity,54 replace oleate ligands from the QD surface, while reactive [I]− and [PbI3]− species bond strongly to electron-deficient Pb sites.36 In addition, larger PbS QDs have more sulfur-rich facets, which can be stabilized by interaction with electrophilic [PbI]+ and [PbI2]. In this way, MAPbI3-mediated ligand exchange provides complete passivation with electrically conductive ligands, which have long-term stability. Another way to manage the QD surface is the solution-phase ligand exchange, followed by single-step deposition of resulting MAPbI3·PbS hybrids (Figure 3a). The apparent difference from the solid-state process is that the dominant material in the film interfacing graphene is the perovskite,42,58 with sparse inclusion of quantum dots forming the nanojunctions between two semiconductors otherwise absent in PbSMAPbI3. Being isolated and passivated, QDs retain their original exciton absorption peak (Figure 3b). In the first instance, QD-in-perovskite hybrids promise to serve as a sensitizer where electronic transport and SWIR absorption are similarly optimized.59 Low electron−hole recombination rates and long carrier lifetimes in MAPbI3,60 combined with relatively low exciton binding energy of both QDs and perovskites, could allow photoexcited charges to be collected from thicker films thus increasing quantum efficiency. 5550

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below 2 W·m−2 (Supporting Figure S8), far exceeding typical 1−5% reported for QD-GFETs.5,34 Notably, MAPbI3 deposited alone in the same conditions does not give negative photocurrent on p-type graphene and its photoresponse resembles the behavior of PbS QDs (Figure 4), despite the fact that in the dark it is a strong p-dopant for graphene26 (Figure 2c) and expected to provide electron injection following excitation. This unusual energy band movement and electronic transformation of perovskites upon exposure to light requires explanation. Hybrid organic−inorganic perovskites are known to undergo strong self-doping induced by photolysis, which triggers dissociation and deprotonation of the organic cations yielding volatile molecules such as H2, I2, and CH3NH2.63,64 Water and oxygen absorbed from the atmosphere can play a key role in these processes.65 It is therefore possible that initially p-type MA+-rich perovskite adjacent to the graphene is converted into n-type Pb2+-rich material under even relatively low-level illumination, chemically changing the net doping type and density. This photoinduced self-doping may change the interfacial band alignment and direct the photogenerated charges in the opposite direction to that suggested by the built-in electric field observed under dark conditions. Photochemical reactions in MAPbI3 can continuously tune the Fermi level and energy band positions relative to vacuum. Moreover, the band energies of PbS QDs can shift by up to 0.9 eV upon changes in surface chemistry.66 Light-driven electronic transformation in MAPbI3·PbS solids is evidenced by varying the intensity of illumination (λ = 520 nm) under constant GFET bias. We observe a reversal of the photocurrent polarity at approximately 11 mW·m−2, indicating a change of the majority carrier type in the absorber from electrons to holes. At low light intensities, the holes are no longer localized in the QDs and can be injected into the graphene channel through the perovskite matrix. Similar unsynchronized energy band shifts depending on the photon flux level were seen in type-II core-crown absorbers.22 We can expect however that low-energy radiation will be less prone to move the energy levels in the composite owing to the infrared transparency of perovskites, and the effect can be practically avoided by tuning the Fermi level of graphene with respect to the absorber. The observed phenomenon of photogenerated charge flow against the dark built-in field may have the same origin in the perovskite and perovskite-QD hybrid, even though of different polarity. It should be noted that in contrast to initially p-doped MAPbI3, freshly deposited MAPbI3·PbS may exhibit gradient doping with heavily n-type bottommost layers because of limited MAI interdiffusion through the QDs during posttreatment and thus undergo a different self-doping mechanism. Slow solid-state [MA]+ diffusion may be responsible for gradually reversing the doping of graphene-adjacent layers from n-type to p-type,67 ultimately reaching the doping level of MAPbI3 alone (Figure 2c). An optimum MAI/PbI2 ratio must be then found for persistent more advantageous n-type doping of the absorber. Overall, it is imperative to optimize the energy band alignment between QDs and the perovskite such that they form a stable type-II heterojunction, thermodynamically favoring charge separation. This leads to an increase in gain but is unlikely to improve temporal response. Facilitated by the presence of light and moisture, irreversible gradual changes occur over time, often seen in QD-in-perovskite solids;43 these can be addressed using proper dehydration and encapsulation. Devices with solid-state treated PbSMAPbI3, in contrast,

Device Physics. Brought into contact with initially p-doped graphene, semiconducting absorbers of opposite doping polarity create a vertical heterojunction, the built-in field of which promotes charge carrier separation and injection into the graphene channel. The mechanism is absorber specific and depends on the exact doping and electron affinity/work function potential figures. A signature of the charge flow at the interface is a shift in the Dirac point of transfer curves measured in the dark, up to the point where the equilibrium alignment of Fermi energy levels is reached (Figure 2). For PbS QDs inducing n-type doping of graphene, in the operational regime under illumination, it is favorable for electrons to be trapped while the holes are easily injected to the channel (Figure 4). This means that when the device is biased to Vg < VDirac (hole carrier majority) the light irradiation causes an increase in Ids, whereas when Vg > VDirac (electron carrier majority) Ids decreases on illumination. Although both EDTand MAPbI3-treated large QDs behave similarly on graphene, the photogating appears to be stronger in PbSMAPbI3, which is reflected in higher ΔR/R and absolute photocurrent values (Supporting Figure S5). This suggests that despite expected shallower energy bands of PbS QDs capped with organic ligands and consequently a greater Fermi energy mismatch with graphene, the built-in field in inorganically passivated QDs is as strong, maintained by the Fermi level shift toward the conduction band. However, further reduction of the bandgap through increasing the QD size up to 8−10 nm may weaken the effect. In the case of MAPbI3·PbS solids, however, the device physics are drastically different. As-deposited hybrids induce a negative shift of the Dirac point indicating an electron flow to the graphene and suggesting n-type depletion, similar to PbSMAPbI3 (Figure 2c). Upon illumination (wavelength λ = 520 nm), the system transforms such that the absorber now traps predominantly the holes and the charge transfer happens against the built-in field formed in the dark (Figure 4). The flow of photoexcited electrons changes the carrier density of ptype graphene in the opposite way, decreasing the current. In other words, the polarity of photocurrent is reversed solely by tuning the QD−perovskite composition, not by crossing the Dirac point of graphene. In the dark, a type-I junction is likely to form between PbS QDs and large-bandgap perovskites,42 which is even more probable for larger size nanocrystals with bandgap 107 through engineering nanoheterojunctions in MAPbI3·PbS.

Figure 7. Absorber-limited trade-off detectivity versus optical cutoff frequency for hybrid graphene photodetectors. The maximum documented values of each individual parameter are taken from the literature. The detectivity is reported for the following illumination conditions (irradiance, W·m−2; wavelength, nm): PbSEDT (1 × 10−8; 532),5 PbSEDT top-gated (5 × 10−1; 635),34 PbSMAPbI3 (3.3 × 10−3; 520), GaSe (5.7 × 10−2; 532),23 organic dye (3.2 × 10−1; 503, 577, 689),29 MAPbI3 (3.5 × 104; 520),25 CdS (1.5 × 101; 349),12 CdSe/CdS (1 × 102; 532).22 The optical cutoff frequency is the measurement frequency naturally limited by the relaxation time constant. The dotted line is a theoretical model of the trade-off for electrically passive absorbers, thus excluding the top-gated PbSEDT data (see Supporting Information).

multiplication in hybrid photodetectors. Minimized hysteresis in GFETs is equally important for response speed, which is achievable by depositing the graphene channel onto hydrophobic SAMs and further benefits from high-vacuum dehydration and encapsulation. Scaling of high-performance

CONCLUSION Low intrinsic charge density and high carrier mobility of graphene are among the major factors leading to large carrier 5553

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optimizing the radiation absorber with respect to the trade-off between sensitivity and response time. In general, PbS QDs passivated with MAI/PbI2 are attractive for video-rate applications, whereas MAPbI3·PbS hybrids can be considered when ultrahigh gain is needed.

graphene is possible, as we demonstrate here with 16 devices in an array with excellent pixel-to-pixel reproducibility. Wet graphene transfer, solution-processed absorbers, and costefficient polymer foils have been combined providing a path toward eventually fully flexible sensitive photodetectors, a building block of large-area SWIR imagers. Colloidal quantum dots with their tunable properties and breadth of surface chemistry have great potential. The ability to tune PbS QD bandgaps into the SWIR via the quantum size effect and improve interdot coupling using short inorganic ligands means that they demonstrate nearly optimal performance in sensitized GFET photodetectors, fulfilling the fundamental requirements generally imposed on light absorbers. First, PbS QDs have a relatively low work function and, when iodide ligands are employed, exhibit n-doping created by the high ligand density. In contact with p-doped graphene, ntype QDs provide a large built-in field and the deeper depletion needed to drive charges from thicker films. Second, the low exciton binding energy and extended carrier lifetimes of PbS QDs are key to separate excited charges with minimum recombination losses and allow them to drift long distances until injected to the graphene. High gain performance largely relies on long-lived carriers. Third, strongly binding inorganic ligands (such as [I]−, [PbI3]−, and [PbI]+) retard redox degradation, passivate surfaces deeply leaving little room for midgap trap states with energies above 0.2 eV, and provide an electrically conductive interdot medium for fast charge recovery. Larger PbS QDs (>8 nm in diameter), however, progressively lose the influence of barrier material and begin to suffer from oxidation and poor charge transport. Finally, unusually for an inorganic semiconductor, PbS QDs can be processed from solution, which makes them compatible with rapidly growing cost-efficient plastic technologies as well as conventional CMOS platforms. Development of more compact wet-processed QD films with a highly ordered superlattice should give another way to improve the photoresponse. Another dimension in manipulating charge transport within QD-based absorbers lies in introducing the bulk of interdot medium, ideally one that is chemically stabilizing, electrically conductive, and photosensitive, here realized by a matrix of crystalline perovskite. Reduced overlap of the electron and hole wave functions in nanoscale QD−perovskite junctions facilitates exciton dissociation and delocalization of one of the carrier types at a level not easily achievable in pure QDs. Although heterojunctions spatially separating photoexcited carriers lead to superior gain, they appear detrimental to response time, in accord with the discussed trade-off. For this reason, ligand-passivated QDs with shallow traps look more appealing for speed-sensitive applications. Interestingly, by playing with the size-dependent bandgaps of PbS QDs and compositional self-doping of perovskites, it is feasible to tune the carrier type in a passive absorber without applying an external bias. As shown, MAPbI3·PbS provides n-type photogating of GFET, whereas PbS QDs and MAPbI3 alone induce p-type electrostatic coupling, more typical of QD-on-graphene systems. Despite appealing opportunities, the benefits of switching the photogating polarity simply by tailoring the QD−perovskite chemistry warrants further investigation. Compositionally stable QD-in-perovskites are highly desirable. To summarize, we report the fabrication and performance of hybrid SWIR-sensitive QD-GFET photodetector arrays on polymer with detectivity reaching 5 × 1012 Jones while maintaining high signal reading rates, achieved through

METHODS GFET Fabrication. CVD-grown graphene was wet-transferred using the PMMA technique onto a planarized 125-μm-thick poly(ethylene naphthalate) (PEN) substrate (DuPont Teijin Films), which was prepatterned with a gate electrode and dielectric layer and temporarily bonded to a 1-in. silicon wafer carrier. Metal electrodes (gate, routings, and top contacts) were fabricated using standard lift-off patterning of thermally evaporated Cr 2 nm/Au 20−180 nm. Conventional photolithography combined with oxygen plasma etching was used to define graphene channels. The gate dielectrics were 25−40 nm thick Al2O3 films deposited by ALD (Beneq TFS 200) at low temperatures (120 °C) for compatibility with the polymer substrate. SAM treatment of dielectrics was carried out on oxygen plasma preconditioned substrates using n-octadecylphosphonic acid (ODPA) by a 20 h reaction in isopropanol alcohol (IPA) solution (1 mM), followed by IPA rinse and mild hot plate annealing 60 °C, 0.5 h. Each fabricated sample contained 4 linear arrays comprising 16 pixels. The electrodes were routed to the edge of substrate and were compatible with FPC-type connectors. Absorber Layer Deposition. PbS QDs in toluene, λem ≈ 1400 nm (6.3 nm diameter, 0.94 eV optical bandgap), were purchased from Sigma-Aldrich. For the solid-state ligand exchange process, the QDs were deposited on to the substrate using a polydimethylsiloxane (PDMS) stamp transfer method with a QD pattern of predetermined shape obtained by inkjet printing (Fujifilm Dimatix DMP-2831). Based on equimolar amounts of PbI2 and MAI, a saturated solution of MAPbI3 (methylammonium lead iodide) in acetonitrile was used for oleic acid ligand replacement. EDT ligands were also mixed with acetonitrile to obtain a 2 vol % solution. Solid-state ligand exchange was performed in an Ar-filled glovebox by reaction in a puddle for 30 s followed by spin-coating at 2500 rpm for 20 s, and rinsing subsequently in acetonitrile, IPA, toluene, and octane at the same spinning condition, thereby gradually lowering the polarity index. Multiple deposition/ligand exchange steps were used when required for thicker films. QD film thicknesses of 100−150 nm were found to be optimum (λ = 520 nm). Where needed, the QD-GFET samples were dehydrated in high vacuum (10−6 mbar) and encapsulated using epoxy adhesive (Delo LP655) and glass lid atop. Solution-phase ligand exchange was performed in accord with refs 42, 43, and 46. A toluene−octane (1:2) dispersion of PbS QDs (7.5 mg/mL) was added to equimolar PbI2 and MAI mixture (0.5 M) in dimethylformamide (DMF) in equal parts. After stirring vigorously for 1 h, full migration of the QDs to the polar phase was observed. The transparent nonpolar phase was decanted, and the solution was washed stirring with octane three times until all oleic acid was removed. The perovskite−QD solid was precipitated from DMF by adding acetone, followed by centrifugation at 6000 rpm for 2 min, drying under vacuum, and redispersion in n-butylamine (50−100 mg/ mL). MAPbI3·PbS was deposited by spin-coating at 2500 rpm for 20 s, annealed at 70 °C for 10 min, and then post-treated with MAI in IPA (10 mg/mL), rinsed in IPA by spinning, and annealed for a second time at 70 °C for 10 min. The time of soaking the film in MAI solution was critical for device performance and was varied from 5 to 30 s depending on the film thickness. Control MAPbI3 films were prepared the same way using 70 mg/mL solution in n-butylamine, resulting in MAI-rich perovskite. All deposition and annealing manipulations were performed in a glovebox. ATR-FTIR (PerkinElmer Spectrum Two) and Raman (Renishaw inVia) spectra were recorded on thin films in ambient conditions. Electrical and Optoelectronic Characterization. The transfer curves and optoelectronic measurements were carried out in a shielded room using a probe station equipped with Keysight B1500A semiconductor analyzer and B2200A switch-matrix tool. The devices 5554

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ACS Nano were biased at 0.1 V. Contact resistance was measured using a transfer length method (TLM). The active area was illuminated using a 520 nm wavelength laser diode fiber-coupled to a reflective collimator with output beam radius of 2 mm. The incident optical power was controlled with optical attenuators of which the transmittance is known for the optical excitation wavelength. The optical power was measured with a calibrated optical power meter (Newport 2936-R) with either a Si (Newport 918D-SL-OD3R) or Ge (Newport 918DIR-OD3R) photodetector sensor head to cover the visible or infrared wavelength range. The optical power on the device is calculated as the portion of the collimated optical beam that falls on the active area of the device (W × L). The photosignal (the difference of current in the dark and under illumination) was read out using either direct probing or via an FPC-connected ADC board. Spectral responsivity is recorded using a supercontinuum laser source (NKT Photonics SuperK Extreme) with Acousto-Optical Tunable filter system (NKT Select +) on a device fabricated on a silicon substrate suitable for wirebonding integration. The noise was measured in the dark using a Keysight E4727A circuit analyzer, with the same gate voltages and drain−source voltages as the other measurements. The RMS noise of the graphene transducer was integrated over the noise bandwidth. The lowest frequency was determined by the exposure time or the chopping frequency, and the highest frequency was set by the electronics cutoff frequency (see Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00760. Hybrid photodetector operation principles; optical microscopy and AFM images; optoelectronic performance of multipixel devices; electrical stability and hysteresis-free operation; photodetectors with EDTpassivated PbS QDs; FTIR and Raman spectra of MAPbI3·PbS; photoresponse of devices with MAPbI3· PbS; responsivity data and models for different absorbers, 1/f noise performance and detectivity calculations; detectivity-response speed trade-off model (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail address: [email protected]. ORCID

Alexander A. Bessonov: 0000-0002-1551-9011 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS A.A.B. thanks A. Robinson, D. Cotton, E. Spigone, and R. White for their inputs and stimulating discussions. This work was supported by the Finnish Funding Agency for Innovation (Tekes; Grant 4763/31/2014). REFERENCES (1) Rogalski, A. Infrared Detectors: Status and Trends. Prog. Quantum Electron. 2003, 27, 59−210. (2) Saran, R.; Curry, R. J. Lead Sulphide Nanocrystal Photodetector Technologies. Nat. Photonics 2016, 10, 81−92. (3) Kufer, D.; Konstantatos, G. Photo-FETs: Phototransistors Enabled by 2D and 0D Nanomaterials. ACS Photonics 2016, 3, 2197−2210. (4) García de Arquer, F. P.; Armin, A.; Meredith, P.; Sargent, E. H. Solution-Processed Semiconductors for Next-Generation Photodetectors. Nat. Rev. Mater. 2017, 2, 16100. 5555

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