Introduction

Solar energy, mostly spread over visible (Vis) to near-infrared (NIR) region, shows enormous potential in the energy market because of its essentially unlimited supply and renewable, cheap and clean nature.1, 2 Currently, light detecting and harvesting are two noteworthy photoelectric-conversion processes for exploiting the encouraging solar energy. Here, photodetectors that capture light signals and convert them into electrical signals are important functional units in various energy systems.3,4,5,6 Among them, broadband photodetectors that can detect light from ultraviolet (UV) to Vis to NIR are critical for color imaging, optical communication, environmental monitoring, and day/night-time surveillance,3, 7,8,9,10 becoming a hot topic recently. However, today commercial inorganic photodetectors have some disadvantages with respect to relatively narrow response spectra and high cost because of the elaborate production process, strongly limiting their broad applications. For instance, silicon photodetectors are mainly used to sense the Vis/NIR light, while silicon carbide-based devices are ordinarily selected for UV sensing.11, 12 Especially, some IR photodetectors based on narrow-bandgap semiconductors (such as InGaAs)13 must be cooled to utilize due to the abundant intrinsic thermal charge carriers (i.e., larger dark current), complicating the routine use of devices. Therefore, it is urgently desired to obtain a facile and cost-effective UV–Vis–NIR photodetector, achieving room-temperature operational photoelectric-conversion.

Fortunately, organolead halide perovskites with a MAPbX3 (MA = CH3NH3 +; X = Cl−/I−/Br−) structure have been proven to be a promising class of materials for optoelectronic devices, including solar cells,14 light-emitting diodes,15, 16 photodetectors,17,18,19 and lasers,20 owing to their excellent optical and electrical properties with large absorption cross-section, long photocarrier diffusion length, low trapping density, and high charge mobility.21,22,23,24 Meanwhile, they also remain effective solution processability and compatibility with flexible substrates.18 However, the spotlight perovskites, MAPbX3, can absorb only the photons in the UV–Vis region caused by their wide bandgaps (larger than 1.6 eV)24, 25, restricting some long-wave applications.26 The further extension or modulation of the absorption spectrum (in particular toward the NIR) is of interest for the implementation of organohalide perovskites in photoelectric devices.8, 27, 28 Therefore, an alternative method should be proposed to achieve broader coverage for UV–Vis–NIR region by combining MAPbX3 with other narrow-bandgap materials. Metal phthalocyanine is a class of organic dyes which can be easily obtained to satisfy this condition in practical applications.29 Lead phthalocyanine (PbPc) is a shuttlecock-shaped molecule with a high absorption coefficient in the NIR region, which can be further enhanced by means of triclinic absorption induced by selected molecule template or improved substrate temperature.30,31,32 We have successfully demonstrated highly efficient NIR absorption in organic phototransistors32 and upconversion devices33 based on the proposed strategies above.

Given the above-mentioned advantages of the material’s properties, we fabricate and characterize a lateral photoconductor made of heterostructure of organolead perovskite (CH3NH3PbI3 − x Cl x ) and phthalocyanine films, which combine the synergetic properties of high UV–Vis absorbance of perovskite with enhanced NIR light-absorbing for triclinic PbPc. The as-prepared device shows remarkable photosensitivities from the UV to NIR range with ultralow dark current. Note that this ordinary lateral design not only simplifies the use of transparent electrodes as incident window and interface modification materials in vertical sandwich structure,5, 26 but it is also not restricted by the polarity of applied voltage, which is commonly reverse-biased in vertical-structure photodetectors3, 5, 26, in spite of the fact that the vertical structure can achieve a faster response time owing to its shorter transport path for charge carriers. This methodology, in view of materials and architectural design, provides a reference to achieve facile, cost-effective and flexible broadband photodetectors with superior photosensitive performance.

Results

Device characterizations

Figure 1a displays the three-dimensional scheme of PbPc/MAPbI3 − x Cl x heterostructured photodetector and the corresponding molecular structure of as-adopted materials, respectively. A simple two-step process is adapted here to form PbPc/MAPbI3 − x Cl x thin film, as illustrated in Fig. 1b. First, FESEM images are obtained to observe the morphology and microstructure of the heterostructured PbPc/MAPbI3 − x Cl x as shown in Figs 2a, b. The thermally evaporated PbPc film is identified to have a biggish crystal grain size (~100 nm) with dense and uniform shape, which indicates that a better crystallinity with less grain boundary is obtained here.32 It has been established that the average grain size of organic film increases with substrate temperature and the charge mobility is higher inside grain.32, 34, 35 In contrast, the surface of perovskite films shows micrometer-sized porous structure instead of compact film (Supplementary Fig. S3b versus 3a) to form random networks (Fig. 2b). This is an optimized design for controlling perovskite morphology by using an analogous dilute-solution-phase method.36 It has been reasonably revealed that the porous structure of perovskite layer has positive impacts on the device broadband photosensitivity. This is different from those design requirements of solar cells or LEDs based on compact perovskite films.37, 38 On one hand, it is useful to achieve a larger surface-to-volume ratio and an easier light penetration from upper perovskite to PbPc layer, thereby enhancing photo-absorbing of the heterostructured films with limited thickness. On the other hand, physical boundaries among the porous structure can impede carriers’ effective transport in darkness. Thus, a relatively low dark current is expected to achieve in the architecture.36 A control device with compact perovskite films is fabricated simultaneously to verify this analysis. As shown in Supplementary Fig. S4a, the heterostructured device with compact perovskite films shows a ~20 times higher dark current (~1 nA) than the experimental device with porous perovskite films. Although the control device achieves a relatively improved photocurrent (Supplementary Fig. S4b), the photosensitivity (P) will be far less than that of experimental device (Supplementary Fig. S4c), which is disadvantageous for weak light detection (section 5 of Electronic Supplementary Information, ESI). The result of energy dispersive analysis system of X-ray confirms the element distribution of CH3NH3PbI3 − x Cl x (Supplementary Fig. S3c) and no other foreign elements are presented. X ray diffraction (XRD) patterns of the PbPc powder and films prepared at different substrate temperatures (T sub) are depicted in Fig. 2c, which indicates that PbPc raw powder and film (deposited at T sub = 25 °C) is mainly monoclinic phase (200);31, 39 whereas, a triclinic phase is observed by depositing PbPc at T sub = 140 °C, assigned to triclinic (100).31, 32 Previous publications have manifested that triclinic PbPc has a strong absorption band in NIR region, suggesting PbPc can be used as NIR photosensitizer.31, 32, 39 In addition, XRD patterns of both MAPbI3 − x Cl x and PbPc/MAPbI3 − x Cl x films have similar features, showing two main peaks at 2θ = 14.18° and 28.51° associated with the (110) and (220) planes of perovskite crystals, respectively.14 This result reflects that MAPbI3 − x Cl x film is polycrystalline, and PbPc molecule has no significant impact on the crystalline structure of subsequently deposited perovskite. The absorption spectra of as-prepared PbPc, MAPbI3 − x Cl x and PbPc/MAPbI3 − x Cl x films are displayed in Fig. 2d. It is clearly observed that an enhanced NIR absorption is achieved for PbPc film deposited at T sub = 140 °C. Generally, PbPc film will achieve B band π→π* and Q band n→π* transitions as two main absorption bands upon UV and NIR illuminations, respectively (section 4 of ESI), and the peak at 436 nm is attributed to the characteristic absorption peak of PbPc.32 The absorption spectrum of MAPbI3 − x Cl x is from 250 to 780 nm since the bandgap energy of ~1.6 eV, which is also in good agreement with the reported results.16, 17 It is noteworthy that there exists complementary spectra for PbPc and MAPbI3 − x Cl x photoactive materials, resulting in a broader coverage of UV–Vis–NIR photoabsorption region. As expected, the absorption spectrum of PbPc/MAPbI3 − x Cl x film further corroborates the result of broadband absorption above, coming to be promising for the fabrication of a UV–Vis–NIR photodetector.

Fig. 1
figure 1

Schematic configuration and fabrication for achieving an organolead halide perovskite-phthalocyanine heterostructured photodetector. a Schematic diagram of PbPc/MAPbI3 − x Cl x photodetector. b Fabrication scheme for the formation of PbPc/MAPbI3 − x Cl x films

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Fig. 2
figure 2

Properties of organolead perovskite and phthalocyanine. a Representative SEM image of PbPc film evaporated at 140 °C (substrate temperature). b SEM image of MAPbI3 − x Cl x perovskite film with porous structure. c XRD patterns of the PbPc powder and the 50 nm-thick PbPc films evaporated at 25 and 140 °C (upper half-plane), MAPbI3 − x Cl x and PbPc/MAPbI3 − x Cl x thin films (lower half-plane). d Absorption spectra of the 50 nm-thick PbPc films evaporated at 25 and 140 °C (upper half-plane), MAPbI3 − x Cl x and PbPc/MAPbI3 − x Cl x thin films (lower half-plane)

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Optoelectrical properties of as-prepared devices

The PbPc/MAPbI3 − x Cl x -based photodetectors are further studied by I–V characteristics in the dark and under the UV–Vis–NIR monochromatic illuminations, measured at room temperature. Figure 3a shows the typical I–V curve of the photodetector in the dark, manifesting a sub-nA (~60 pA at V = 50 V) scale dark-current even at the applied voltage V = ±50 V. This result is in good agreement with the morphology and microstructure analysis described above. Furthermore, we select eight specific lasers (wavelength: 405, 450, 532, 655, 780, 808, 830 and 850 nm) to characterize the broadband photosensitive performances of as-fabricated devices. As it is exposed to light with different-wavelength lasers at various powers, the photocurrents enhance drastically with similar behavior (Fig. 3 and Supplementary Fig. S5), and a microampere scale photocurrent is recorded under illumination of 405 nm (UV), 532 nm (Vis) and 780 nm (NIR) lasers at a relatively low power intensity, demonstrating its broadband photodetection characteristics with high sensitivity. It is obvious that the as-prepared photodetectors show significant responsivity for UV–Vis illumination, contributed to an excellent light harvest of MAPbI3 − x Cl x films in this range;5, 24 meanwhile, a non-negligible photocurrent enhancement in the NIR region is mainly originated from triclinic PbPc absorption.31, 32, 39 The results above are further corroborated by a detailed performance comparison among single PbPc, single perovskite and PbPc/Perovskite heterostructured devices, as shown in Supplementary Fig. S6, from which it can be clearly found that the PbPc/Perovskite heterostructure with complementary absorption spectra covering UV–Vis–NIR region will obtain an improved photocurrent and broadband photoresponse. In addition, the thicknesses and deposition sequence of perovskite and PbPc layer can be important variables for this device due to its photoconductive property and different charge-injection. By changing the thicknesses of perovskite (Supplementary Fig. S7a) or PbPc (Supplementary Fig. S7b), the relative intensity and distribution of photoresponsivity will get changed correspondingly. However, it should be noted that a markedly enhanced dark current will be obtained in the case of 100 nm PbPc layer due to enhanced bulk conductivity (Supplementary Fig. S7c), which depresses the photosensitivity of the corresponding devices. Likewise, a relatively improved dark current will be achieved when changing the deposition sequence (i.e., Perovskite/PbPc) as a result of an enhanced hole-injection for an ohmic contact (Au/PbPc, Supplementary Fig. S8a). Thus, the device photosensitivity will also be depressed in this framework (Supplementary Fig. S8b).

Fig. 3
figure 3

I–V characteristics of perovskite-PbPc photodetectors under dark and different monochromatic light illumination over UV–Vis–NIR range. a in darkness; b under Near-UV illumination of 405 nm; c under visual illumination of 532 nm; and d under NIR illumination of 780 nm

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Another important technological aspect for optoelectronic devices is their time-response stability and speed. The time-dependent photoresponse over 120 sec with a 5-period on-off operation is measured under different excitation wavelengths. The heterostructured photodetector shows a stable and repeatable on-off switching in UV–Vis–NIR range (Fig. 4a, c, e and Supplementary Fig. S9), indicating the excellent photocurrent stability at room temperature. For the application of photodetectors, a fast response and recovery speed is commonly desired character.4, 5 Herein, the temporal response of our devices is characterized using chopper-generated light pulses (section 7 and Supplementary Fig. S10 in ESI). Figure 4b, d, e show the representative transient photocurrents of this device during the low-frequency (~150 and 300 Hz) switching of selected incident light. The dynamic response is noticeable and the rise and fall time are both shorter than 0.6 ms (Table S1), which can meet lots of application needs.3, 4 It is noted that the response speed of the PbPc/MAPbI3 − x Cl x device is also comparable or even superior to those newly developed broadband photodetectors as listed in Table S2. It should be mentioned that response time of ~0.1 ms is the present experimental setup limits (mechanical chopper). In order to evaluate the actual response time of as-prepared photodetectors, we used a high-frequency red light-emitting diode (~300 kHz) as excitation source achieved by combining a signal generator (Supplementary Fig. S11a). Thus, an ultrafast response time (~600 ns) is obtained here (Supplementary Fig. S11b, c), which is comparable or even superior to some recent published results based on perovskite absorbing layer.5, 8, 18, 19, 25, 36 The high repeatability and fast response of the device are promising for large-area photodetector applications.

Fig. 4
figure 4

Time-dependent photocurrent of perovskite-PbPc photodetectors at different wavelengths and photo-switching frequencies with a constant light power and voltage (V = −50 V). a, b, c, d and e, f are corresponding to Near-UV illumination of 405 nm, visual illumination of 532 nm and NIR illumination of 780 nm, respectively

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Time stability and device reproducibility

In order to apply for practical applications, time-stability is a major issue for the as-prepared PbPc/MAPbI3 − x Cl x heterostructured photodetector, which is measured by recording the photocurrent changes during storage under ambient conditions for 7 days. The time dependence of R and P for the photodetectors upon illumination with a 655 nm light at 50 V is shown in Fig. 5a. The photocurrent almost remains the same as the initial value (Supplementary Fig. S12a). Thus, it can be found that the photoresponsivity almost keeps constant within 7 days, and the fluctuation amplitude is within 1.3%, which approaches the measurement error. Whereas, the P has a relatively obvious decrease for initial 16 h, and then keeps constant. This behavior possibly results from the moisture and oxygen corrosion, leading to leakage current occurring at those induced defect-sites to enhance dark current (Supplementary Fig. S12a).8, 17 Therefore, the timely encapsulation of devices may be essential to enhance the stability of the device for future application. Furthermore, a comparison of environmental stability of single PbPc, single perovskite, and PbPc/Perovskite heterstructured devices are investigated in detail. One can clearly find that the relative responsivity (R) of the three groups of devices displays a good time stability with a fluctuation of less than 10% (Supplementary Fig. S12b). In addition, the relative sensitivity variation shows a similar trend to the analysis results above, as shown in Supplementary Fig. S12c. The results above should be attributed to good environmental stability of metal phthalocyanine29 and MAPbI3 − x Cl x mixed halide perovskite (Supplementary Fig. S12d)40, 41. Device reproducibility is another key factor for future commercialization. Figure 5b gives the distribution of normalized R for as-prepared three groups of devices from two batches of samples. These devices show high reproducibility and uniformity with a ~5% deviation from the average normalized R, and showing 100% yield.

Fig. 5
figure 5

Device stability, reproducibility, uniformity and flexibility measurements of the perovskite-PbPc series photodetectors. a The photosensitivity and photoresponsivity versus time of exposure in ambient condition for PbPc/Perovskite heterostructured devices. b The distribution of normalized photoresponsivity R for single PbPc, single perovskite and PbPc/Perovskite heterostructured devices in two batches of samples (a batch of samples have 8 devices). c I–V curves of the flexible device with PbPc/MAPbI3 − x Clx heterostructure fabricated on transparent PET substrate for 655 nm illumination; the inset is time-dependent photocurrent measurement at P inc = 31.66 mW cm−2. d I–V curves of the flexible photodetector under dark and illumination without bending and after 500 bend cycles; the inset is the photograph of the flexible photodetector. e Plot of photoresponsivity R versus bending angle (corresponding to curvature radius r) of the flexible photodetector under broadband light excitation. The insets show corresponding photographs of the device under the different bending states. f Photoresponsivity variations with increasing number of bending cycles at different curvature radius under 655 nm light excitation

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Physical flexibility

A flexible photodetector is also fabricated on poly(ethylene terephthalate) (PET) substrate based on the PbPc/MAPbI3 − x Cl x heterostructure because the structural frameworks of the organolead perovskite and phthalocyanine are both flexible and deformable. The morphology of PbPc and perovskite are displayed in Supplementary Fig. S13 after bending manipulation, which indicates that the films remain nearly non-destructive with little cracks. As shown in Fig. 5c, the flexible device clearly illustrates high photosensitivity and the inset further shows a repeatable and stable on-off switching for as-adopted 655 nm illumination. The device flexibility is studied by bending to a fixed bending angle of about 80° and repeated bending/recovery manipulation as a bend cycle (Supplementary Fig. S14). After 500 bend cycles, the photodetector still shows high photosensitivity features (~104 at ± 50 V), as evidenced in Fig. 5d. Compared to the device in its initial state before bending, the photocurrent and dark current remain nearly unchanged, revealing that the photodetector is not influenced by the external bending stress. Furthermore, the broadband responsivity stability of the flexible photodetector at various bending curvatures is shown in Fig. 5e. The responsivity is almost constant for three different bending states under different light excitation, indicating enormous potential for flexible broadband photodetection. Notably, the flexible device shows no significant decrease in photoresponsivity under continuous bending up to 1000 cycles of different bending curvature radius (Fig. 5f). This simple device structure on flexible substrate suggests its compatibility for scale-up fabrication, which paves the way for commercialization of broadband photodetection technology.

Discussion

To quantitatively evaluate the performances of as-prepared photodetectors, the photocurrent (I ph) and dark current (I dark) are first extracted from I–V curves (Fig. 3, Supplementary Fig. S5 and section 6 of ESI), and used to calculate photoresponsivity (R), photosensitivity (P), external quantum efficiency (EQE) and detectivity (D*). Table S1 summarizes all key metrics for the selected UV–Vis–NIR wavelength illuminations on the PbPc/MAPbI3 − x Cl x heterostrctured photodetectors. The as-prepared devices illustrate comparable performance parameters to the state-of-the-art broadband detectors (more details in Table S2).8, 9, 19, 26, 30, 42, 43 For example, R was calculated to be 554 mA W−1 at 405 nm (UV), 419 mA W−1 at 532 nm (Vis), 130 mA W−1 at 780 nm (NIR) at a low power intensity, manifesting its broadband weak-light detection. We further characterize the heterostructured photodetector by varying the excitation power at a fixed excitation wavelength (450 nm, Fig. 6a). The photocurrent increases superlinearly with increasing incident optical intensity (P inc), and it reaches several μA at high excitation powers (30 mW cm−2). Whereas, the R versus P inc relationship displays a superlinear decrease behavior, which is also commonly observed in some organic and inorganic photodetectors.10, 17, 44 This effect can be associated with a reduction of the number of photogenerated carriers available for extraction under high light intensity.45 Further, the intensity dependent photocurrent and photoresponsivity can be fitted by the power laws I ph ∝P inc γ and R ∝ P inc α−1,10, 44 and the γ and α are estimated to be 0.80 and 0.79, which are obviously superior to those values based on previous published semiconductors.10, 18 It has been confirmed that the non-unity exponentials are the result of the recombination kinetics of photocarriers relating to trap states and coupling effect.6, 46 Thus, the higher exponentials mean lower charge recombination or trapping ratios, beneficial to achieve high-performance photodetectors. As expected, it’s clearly found that the PbPc/MAPbI3 − x Cl x -based device is UV–Vis–NIR broad spectral response or detection, even at a low light intensity P inc = 0.01 mW cm−2 (Supplementary Fig. S15). Based on the pA-scale dark current (Fig. 3a), another remarkable feature of this device is the excellent photosensitivity to broadband illuminations. As shown in Fig. 6b, a minimum P value (>10) can be obtained for full spectrum, significantly up to 104 for 450 nm excitation, which is the one of the maximum values currently published.10, 36, 42, 47 Furthermore, the color map of P can more clearly and intuitively reveal excellent broadband sensitivity (Fig. 6c). All results above indicate that the PbPc/MAPbI3 − x Cl x -based photodetectors achieve a good photodetection for UV–Vis–NIR region (Table S2). The potential mechanism for the photodetection can be explained by a simplified energy band diagram (Fig. 6d). Firstly, a recent publication has clearly demonstrated that the as-prepared MAPbI3 − x Cl x perovskite is a strong N-type characteristics of films with a small exciton binding energies of 37–50 meV.22 Thus, a P–N heterostructure will form when the perovskite is deposited onto P-type PbPc films,32 which may greatly enhance the dissociation of photoexcitons along with lower charge recombination.10 The charge transfer via Femi level tuning between the interfaces of Au/MAPbI3 − x Cl x and PbPc/MAPbI3 − x Cl x , resulting in the band bending, and therefore forming Schottky type barrier (Φ B) as well as depletion layers (Fig. 6e), suppressing the dark injection in the heterostructured bilayers. Thus, both the bulk-limited current induced by the porous perovskite/phthalocyanine films and the injection-limited current induced by interfacial barrier lead to ultralow pA-scale dark current,18 which is a global device design by considering heterojunction effect and charge-injection interface. Under illumination with the photon energy greater than the energy gap of the semiconductors (Fig. 6f), excitons are generated in MAPbI3 − x Cl x (UV–Vis) and PbPc (NIR) films, and laterally separated by the applied bias or depletion layers. Electrons are transferred to N-type perovskite, leading to an increase in free carrier density. The increased carrier density in MAPbI3 − x Cl x film lowers the effective barrier height Φ B and fills trap states. As a result, bulk-limited effect will disappear and injection-limited dominates charge conduction, leading to the easier carriers tunneling and transport than that of devices in darkness, thus, to a significant enhancement in film conductivity and photocurrent. This proposed mechanism has also been successfully used to interpret the performance of other porous-structured photodetectors.9, 36, 48

Fig. 6
figure 6

Photosensitive properties and mechanisms of the perovskite-PbPc photodetectors. a Incident power dependence of photoresponsivity and photocurrent under 405 nm illumination. b The dependence of photosensitivity on the incident wavelength. c Color plot of the maximum photosensitivity of as-prepared device as a function of wavelength and applied bias; the data is extracted from Fig. 3 and Supplementary Fig. S5. d Schematic energy level diagram of the adopted materials; these related values are based on refs. 16, 31. e Band diagram of the PbPc/MAPbI3 − x Cl x photodetector in darkness. f Band diagram and charge transport of the PbPc/MAPbI3 − x Cl x photodetector under bias and illumination

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In summary, a facile lateral photodetector with bilayer PbPc/MAPbI3 − x Cl x heterostructrued films was demonstrated. The as-prepared broadband photodetector displayed ultrahigh room-temperature photosensitivity with pA-scale dark current in UV–Vis region and extended its photoresponse to the NIR region due to enhanced triclinic absorption of PbPc molecule. More significantly, it also demonstrated moderate environmental stability, reproducibility and favorable mechanical flexibility. We believe that there are no fundamental obstacles to extending the design to other perovskite-organic semiconductors energy conversion systems, which would lead to low-cost, large-area, high-performance broadband photodetectors.

Methods

Device fabrication

The broadband photodetector architecture consisted of the following stack of layers: a transparent substrate, p-type photoactive layer, n-type photoactive layer and lateral gold electrodes. Here, commercial Corning Glass (or flexible PET) was employed as a transparent substrate. 50 nm PbPc was thermally evaporated onto the substrates with a substrate-temperature (T sub) of 140 °C in a high vacuum chamber (less than 1.0 × 10−3 Pa), on which the organolead halide perovskite (MAPbI3 − x Cl x ) was prepared by a solution-processed spin-coating method, the detailed fabrication see the Supporting Information. The gold electrodes were thermally evaporated onto the perovskite layer through the shadowed masks.

Device measurements

I–V measurements were carried out in the dark and illumination condition with commercial UV–Vis–NIR laser diodes. Electrical data were recorded using an organic semiconductor characterization system operated at room temperature, and the intensity of the incident light was monitored by a power meter. More details see the Supporting Information.