INTRODUCTION
Photodetectors (PDs), which are capable of capturing and converting photons to charge carriers and thus generating electrical outputs in sensors1⇓-3, are widely applied in numerous fields such as optical communication, automatic controls, night vision, space exploration, and so on4. High performance heterostructures are vital components of PDs5⇓⇓-8. Most PDs are composed of inorganic semiconductor materials such as GaN, Si and InGaAs9⇓⇓-12. However, the complex fabrication mechanisms and high costs resulting from adopting rigid substrates with thick active materials as PDs13, have severely hindered the development of flexible devices for commercial applications. Exploring the feasible materials is the key for developing wearable optoelectronic devices14.
Organic-inorganic halide perovskites (OIHPs) in optoelectronics exhibit several advantageous properties such as treatability of solution, adjustable band gap, high absorption coefficient, and long carrier life15,16. OIHPs have been widely applied in solar cells17, light emitting diodes (LEDs), lasers, neuromorphic devices18, and PDs19. In recent years, perovskite solar cells have become hot research issues due to their properties of easier manufacturing and high power conversion efficiency. The top electron transport layer in reverse perovskite solar cells passivates the device and also performs other characteristic functions20. Polyvinylidene fluoride (PVDF) is incorporated into the perovskite photodiodes by interface modification strategy, which slows down the optical voltage attenuation of perovskite devices through electric field induced polarization. The attained composition enables the facile detection of both modulated light-emitting diodes and ultrafast pulse lasers21. Switchable spectral response could be generated by the novel class, p-i-n-i-p structured organic-inorganic hybrid perovskite photodetector through modulating the bias voltage,which can assist in developing the spatially efficient and filter-free imaging systems22. Crystallisation kinetics take the key role in the luminescence of perovskite thin films. The crystallinity and crystal orientation of perovskite films are controlled by modifying and adjusting the electron transport layer, which in turn improves the stability of perovskite and enables the high efficiency of perovskite light-emitting diodes23. OIHPs may be developed as neuromorphic device materials because of their advantageous optoelectronic properties and low-temperature processings. Organic floating-gate optical memory, which is endowed with optical wavelength, light intensity and light irradiation time recognition, can be utilized by maneuvering the wide range of light absorptions by perovskites. The resistive state modulation ability of light-driven floating gate memory tends to be stronger than that of the conventional voltage-driven floating gate memory, which can be beneficial for developing the future light-driven neuromorphic devices18. Perovskites based wearable optoelectronic devices have always exhibited the property of being flexible and stretchable. Several methods have been reported to play the function of improving the perovskites performance, including the passivation of interfacial and grain boundary defects for inhibiting the interface recombination and accelerating the hole transfer24. Moreover, the instability that three-dimensional (3D) perovskites could be influenced by the air, moisture, and UV light25,26, has severely restricted its development for commercial applications. The 3D structure is converted to 2D by inserting organic cations. The general formula of 2D OIHPs is (A')m(A)n−1MnX3n+1 (A = Cs+, CH3NH3+, or HC(NH2)2+; M = Ge2+, Sn2+, Pb2+; X = Cl−, Br−, I−) where A' is monovalent (m = 2) or divalent (m = 1) cation, and n is the number of inorganic layers27. The semiconductor inorganic layer and insulating organic layer in perovskite are alternately arranged to form 2D quantum well nanostructure in bulk crystal28. It is due to this specific arrangement that 2D perovskite is endowed with the advantageous properties of stability, structural flexibility and tunable photoelectric characteristics. The layered perovskites can be classified as Ruddlesden-Popper (RP) phases and Dion-Jacobson (DJ) phases based on the interlayer cations. RP phases are of two divalent cations while DJ phases one monovalent cation. The structure resulting from the alternate hydrogen bonding between diamine cation and inorganic layer in DJ phases is much more stable than that from interlayer van der Waals forces in RP phases29. Furthermore, the DJ phases with stable structure can decrease the low-frequency structural vibrations of the lattice, which weakens the thermally induced electron-phonon coupling and thus leads to long carrier life30. With the increase of ‘n’, the optical and electronic properties of 2D DJ perovskites evolve to 3D perovskites. Consequently, the photoelectric properties are improved while the stability degraded. 2D perovskite shows anisotropy in charge transfer, however, the insulation with organic cations blocks charge transfer in vertical direction.
Colloidal all-inorganic halide perovskite (CsPbX3, X = Cl, Br, I) quantum dots contribute to the formation of flexible and transparent electronics31,32, which are tunable and endowed withhigh absorption coefficient, long carrier life and environmental stability33. Recent studies have reported the adoption of modification strategies to significantly enhance the response parameters of photodetectors. Wang et al. proposed a strategy to improve the performance of CsPbBr3 QDs photodetector. The response time of QDs photodetector modified by ligands and embedded in poly(3-hexylthiophene) (P3HT) was shortened nearly ten times than that of the device based on pure QDs. Moreover, the maximum responsivity, specific detectivity, and stability had been improved34. As nanomaterial, quantum dots could form smooth, high-quality and mechanically stable film on flexible substrate, thus producing flexible and wearable device35. In 2016, Song et al. first reported the flexible photodetectors fabricated by atomic thin two-dimensional CsPbBr3 nanosheets through a simple solution process, which were assembled into large-area andcrack-free high-quality films acting as the flexible and ultra-thin optoelectronic devices. The flexible photodetector exhibited excellent electronic transmission, stability, and flexibility36,37. Relying on the high-power output, the quantum dots offered photosensitive layer for the photoelectric equipment. Wang et al. designed the perovskite photodetector based on ZnO nanorods (NRs) as the ETL and CsPbBr3 QDs as the photoabsorber, which obviously enhanced the photodetector response38. With good bending strength, the power conversion efficiency of quantum dots-based photovoltaics was lower than that of hybrid perovskite thin-film cells. This could be ascribed to the poor carrier extraction and charge transfer39. In addition, the sensitivity of colloidal quantum dots was five times lower than that of the crystalline semiconductor photodetectors40.
Herein, a flexible n3/QDs layered heterostructure PDs were designed, in which QDs acted as photosensitive absorption layer for enhancing the light trapping. The isolated water from perovskite layer improved the hydrophobicity and made perovskite a long-range carrier transport layer. The QDs layer in contact with perovskite layer could passivate the defects to form a high-quality film. The optimal QDs ratio is of great significance for the design and implementation of the advanced heterostructured optoelectronic devices. A series of QDs ratios were explored for the optimum performance, in which the photodetector response was improved to 615% for 660 nm wavelength with the optimal QDs concentration. The device maintained the photoelectric performance of up to 5000 bending cycles in different bending states.
RESULTS AND DISCUSSION
Fabrication of n3/QDs and the morphological characterization
The fabrication process of n3/QDs layered heterostructure PDs is shown in Fig. 1a. The fully dissolved n3 solution was spin-coated on interdigital electrode at 4000 rpm for 40 s and annealed at 60 °C for 10 min. Different concentrations of QDs were then spin-coated on n3 thin film at 4000 rpm for 5s and annealed at 60 °C for 10 min. The details are given in the experimental section. The n3/QDs layered heterostructure with different concentrations of QDs were increased from 0 to 1 c (c = 4.16 mmol L−1), including 0 c (n3), 0.1 c (n3-0.1c), 0.2 c (n3-0.2c), 0.5 c (n3-0.5c), and 1 c (n3-1c). The crystal structures of thin films were characterized by X-ray diffractometer (XRD) with the diffraction peaks around 14.03° and 28.37° of n3 corresponding to (110) and (220) planes, respectively (Fig. 1b)41. Thin films showed dominant orientation in the vertical direction which facilitated the carriers to travel through the layers42. The diffraction peaks of QDs around 14.13° and 28.79° are assigned to (100) and (200) planes43. XRD patterns of each part of n3 and QDs heterostructure are given in Fig. S1. XRD signal is weak beause of the micro quantity of QDs in the heterostructure. XRD patterns of films composed of n3 and QDs are consistent with the previous findings. The transmission electron microscopic (TEM) image of QDs is displayed in Fig. 1c. QDs have cubic shape with an average size of 14 nm obtained from particle size distribution. An interplanar distance of 3.4 Å was acquired from HR-TEM image, assigned to (200) planes (Fig. S2). SEM image of the cross-section of layered heterostructure is shown in Fig. 1d, in which n3/QDs is clearly traceable. The photograph between Fig. 1c and 1d is the luminescence of QDs under UV. UV-visible absorption spectra in Fig. 1e show the optical properties of device with different concentrations of QDs. The device with QDs expands spectral response range in contrast to that without QDs. The photo-response is linked to the excellent optical absorption of QDs. Multiple slopes appearing in the absorption spectra indicate the formation of lower layers41.
Fig. 1. Device fabrication and characterization of n3/QDs thin films. a, Schematic illustration of the process of device fabrication. b, Stacked thin film XRD patterns of n3, QDs, and their heterostructure. c, TEM image of QDs (inset: particle size distributions acquired from TEM). d, SEM image of the cross-sectional view of n3/QDs layered heterostructure. The photograph between c and d is the luminescence of QDs under UV. e, The absorption spectra of n3, n3-1c, n3-0.5c, n3-0.2c, n3-0.1c layered heterostructure after annealing. |
The top view SEM images exhibit the surface morphology of thin film layered heterostructure for exploring the influence of QDs concentration (Fig. 2a-e). Some pinholes are observed in the thin films of n3, n3-0.2c, and n3-0.1c. The n3-0.5c shows dense surface with relatively larger crystal grains. There are cracks in the n3 perovskite thin films observed under optical microscope, which are consistent with the SEM. And these cracks might be created by solvent evaporation during the process of annealing44. The cracks can create defects which lead to strain relaxation45. QDs addition brings tunability to passivate the surface defects46,47. The n3-1c displays stacking on the film resulting from excessive amount of QDs, while some pinehole are observed in n3-0.2c and n3-0.1c, indicating the quantities of QDs being not enough. Fig. 2f-j show the diverse morphologies of thin films by fluorescent microscope with different concentrations of QDs. As shown in n3, partial aggregation contributed to the unevenness of thin film. The surfaces of other films show QDs above n3. From n3-0.1c to n3-1c, the QDs are in the state of disordered stacking. The n3-0.2c displays small and broken dendritic, while n3-0.1c exhibits long and sparse. The n3-0.5c is of dense and flat image. The elemental distributions of n3-1c, n3-0.5c, n3-0.2c, and n3-0.1c are given in Fig. S3. The results reveal that QDs are distributed in the perovskite layer.
Fig. 2. Characterization of n3/QDs thin films. a-e, Top view SEM images, f-j, Forward fluorescent microscope images, k-o, AFM images (Numbers in the lower left part are root-mean-square (RMS) roughness) of n3, n3-1c, n3-0.5c, n3-0.2c, n3-0.1c from left to right, respectively. |
The surface roughness was also investigated to test the effect of QDs on thin film layered heterostructure. The atomic force microscopy (AFM) measures the roughness of thin films (Fig. 2k-o). The root-mean-square (RMS) roughness analysis shows that the surface of n3-0.5c tends to be smooth with the RMS roughness of 3.77 nm due to the QDs gradually filling of QDs in the pinholes. RMS roughness of n3 film is 7.12 nm and those corresponding to n3-1c, n3-0.2c, and n3-0.1c are 4.10, 4.65, and 6.16 nm, respectively. The phenomena above are consistent with the SEM. More intra-grain spaces and cracks on the surface resulting from excessive roughness lead to more charge carrier trapping and less charge carrier extraction48. Therefore, decreasing the surface roughness is essential for PDs49.
Photoelectric performance of n3/QDs PDs
The schematic structure of n3/QDs layered heterostructure PDs is given in Fig. 3a. The device was fabricated with different concentrations of QDs by spin coating onto the n3 thin film. QDs concentration was optimized for the optimum photoelectric performance. The Current-Voltage (I-V) logarithm curves of n3-0.5c device were recorded with different light intensities in the dark and under 405 nm (Fig. 3b), 660 nm (Fig. 3c), and 808 nm (Fig. 3d) between −3 V and 3 V bias. The symmetrical distribution of I-V logarithm curves demonstrates that the thin films were distributed on Au electrode with good ohmic contact50,51. The device photocurrent increased with increasing light intensity. Current-time (I-t) curves of n3-0.5c corresponding to I-V curves with 405 nm illumination are exhibited in Fig. 3f, 660 nm in Fig. 3g, and 808 nm in Fig. 3h, operated at 3 V bias. The I-t curves obtained through laser light are consistent with the I-V curves with change in the light intensity. The I-t curves show sharp increase with increasing carrier drift velocity, while drop in current is shown as the capacitive response on PDs film49. The I-V logarithm curves and normalized I-t curves of n3 device are shown in Fig. S4, S5. Fig. S6, S7 show the curves for n3-1c and n3-0.2c, respectively. The I-V logarithm curves and normalized I-t curves of n3-0.1c are given in Fig. S8, S9. The same light illumination and intensity were selected for the best-optimized PD performance. The data obtained for all devices in the horizontal aspect exhibited the best photoresponse at 660 nm when compared to the other laser lights. This is the result of energy drawn from light source matching the band gap (n3 : 1.99 eV, QDs : 1.80 eV). From vertical dimension, the photocurrent of n3-0.5c is dominant, which is 515% more than n3 under 660 nm, and nearly 154% at 405 nm. QDs make significant difference in the layered structure under 660 nm. The devices with QDs are of higher photocurrent in contrast to n3 but less than n3-0.5c.
Fig. 3. Schematic structure and photoelectric performance of n3/QDs PDs. a, The schematic structure of the n3/QDs layered heterostructure photodetector. Current-voltage (I-V) logarithm curves of the n3-0.5c device under the light illumination of b,405 nm c, 660 nm and d, 808 nm between −3 V and 3 V bias. e, 3D responsivity plot of n3, n3-1c, n3-0.5c, n3-0.2c, n3-0.1c-based photodetectors under 405, 660, 808 and 980 nm light illumination at 3 V bias (Incident light power: 10 mW). Current-time (I-t) curves of the n3-0.5c device under the light illumination of f, 405 nm and g, 660 nm and h, 808 nm with different light intensities operating at 3 V bias. |
The responsivity (R), which is a critical parameter for indicating the PDs sensitivity, is calculated by the following formula:
D* is a figure of merit used to characterize the sensitivity of photodetector as defined by the equation53:
where Iph is the photocurrent, Ilight is the current under illumination, Idark is the dark current, P is the incident light intensity, S is the device effective area, and q is the elementary electronic charge. The wavelength-dependent responsivity of PDs with different QDs concentrations at 3 V bias (Incident light power: 10 mW) is shown in Fig. 3e, in which the best-optimized PDs are selected. From 3D responsivity plot, n3-0.5c displays the optimum responsivity among all the light illuminations. R as a function of light intensity is presented in Fig. 4a, b. After the comparison and analysis, R of n3-0.5c PD under 660 nm is increased to 615% compared with n3, while the R is increased to 254% at 405 nm wavelength, indicating the QDs achieved remarkable results under 660 nm light illumination. As shown in Fig. S10, the response speedwas characterized by the rise time (τrise) and decay time (τdecay). The τrise and τdecay for n3/QDs layered heterostructure PDs were measured under 660 nm light illumination at the light intensity of 4.405 mW cm−2 at 3 V bias, respectively. The wavelength-dependent D⁎ of PDs with varying QDs concentrations at 3 V bias (incident light power: 10 mW) is shown in Fig. S11. From 3D D⁎ plot, n3-0.5c displays the optimum D⁎ among all the light illuminations with the incident light power of 10 mW. Fig. 4c shows the photocurrent and R of n3-0.5c under 660 nm at 0 V bias. The corresponding I-t curves are depicted in Fig. S12. The responsivity of n3-0.5c-based photodetectors at the wavelengths of 405 nm and 532 nm at 0 V bias is shown in Fig. S13. The electron-hole pairs by photoexcitation are separated by inner electric fields at the interface of electron or hole extraction layer/perovskite. Due to the type I energy band alignment, electrons move from n3 into the QDs till their Fermi energy levels reach equilibrium. This creates a loss region and a built-in field pointing to QDs side53. Driven by the built-in electric field, the Au electrode collects photogenerated electrons on n3 side. The photogenerated electron-hole moves in the opposite direction under electric field effect. The photogenerated electrons on QDs side are blocked by the conduction band and cannot drift to n3 side for generating the photocurrent53. The recombination of photogenerated electron-hole pairs is suppressed. QDs in the heterostructure can collect photogenerated holes from n3, which inhibits the recombination of photogenerated carriers in n3 and improves the collection efficiency of carriers. The possible mechanism of n3-0.5c can be the self-powered PD. The inner electric field is formed at the interface of n3/QDs heterojunction to separate the electron and hole from n3 layer when device is illuminated with 660 nm light. The energy of 660 nm is close to the bandgap of n3 and QDs. The laser light thus acts as a driving force in generating the electric field for photogenerated carrier transport. The devices of other QDs concentrations have non-radiative recombination centers because of the defects. Tests results for all the n3/QDs heterostructure devices are analyzed, which shows that the photocurrent is small or even none under 405 nm (incident light power: 10 mW), 660 nm (incident light power: 10 mW), and 532 nm (incident light power: 36 mW) light illumination at 0 V bias (Figs. S14, 15).
Fig. 4. Responsivity of n3/QDs PDs. Responsivity of n3, n3-1c, n3-0.5c, n3-0.2c, n3-0.1c-based photodetectors at the wavelength of a, 660 nm b, 405 nm. c, Responsivity and Photocurrent of n3-0.5c as a function of light intensity under 660 nm light illumination at 0 V bias. |
Flexibility and stability of n3/QDs PDs
The flexible attachment of 2D perovskite to substrate makes its layered structure superior to those of other perovskites54. The nanoscale QDs bring mechanical flexibility55. The mechanical flexibility was tested by the bending cycle numbers and bending conditions. The schematic diagram of flexible device is illustrated in Fig. 5a. Inset on the left is the photograph of n3-0.5c in bending state. Flexibility of n3-0.5c PD with the flattest thin film was measured by I-V logarithm curves (Fig. 5b) and I-t curves (Fig. 5c) as the function of bending cycle numbers under 660 nm light illumination at 3 V bias (Intensity: 4.405 mW cm−2). The dark and light currents slightly decrease compared with their initial values. After bending by 3000 cycles, the device photo-response remains at 92% of its initial value. 97% photo-response is intact up to 5000 bending cycles. In order to explore the possible reasons, the relative change in conductivity during bending was also calculated and shown in Fig. S16. The change in conductivity of n3-0.5c during the bending process shows a tendency to decrease, followed by a small increase and then a slight decrease. The phenomenon above is explained as follows: In the process of perovskite film formation, there are weak connections around the crystal which breaks under the stress of 3000 bending cycles, and reduction in conductivity is seen through Fig. S16. Fewer cracks keep the device stable. As shown in Fig. S17, the top view SEM images reveal the surface morphology of n3-0.5c thin films. There exist no significant differences between the cracks of thin films after bending for 3000 and 4000 cycles. Bending leads to small breaks in the crystal and decreased conductivity, however, it also introduces vacancies to provide recombination centers during the 3000-4000 bending cycles. These vacancies may trap photogenerated carriers to compensate for the influence of conductivity, the self-healing recovers light response56. After 4000 bending cycles, the change in conductivity may be ascribed to the defects which lead to the photocarrier recombination during the bending process. The films remain flat at the locations without cracks, which also show excellent device flexibility.
Fig. 5. Flexibility of n3/QDs PDs. a, Schematic diagram of the flexible device(inset: photograph of the flexible photodetector.). b, 3D Current-voltage (I-V) logarithm curves c, Current-time (I-t) curves of the flexible device (n3-0.5c) with various bending cycle numbers. d, The 3D Current−voltage (I-V) logarithm curves of the flexible device (n3-0.5c) with various bending radii. e, Current-time (I-t) curves of the flexible device (n3-0.5c) with various bending radii. The Current−voltage (I-V) tests are measured between -3 V and 3 V bias, the Current−time (I-t) tests are measured at 3 V bias. All tests are under 660 nm light illumination at the light intensity of 4.405 mW cm−2. |
The n3-0.5c device can withstand bending of various radii ranging from 6.08 mm to 11.36 mm. Fig. 5d shows I-V logarithm curves and Fig. 5e presents I-t curves of the device in different bending states. The optical images are shown as inset. The device photocurrent does not change with the increase of the bending degree at 660 nm under 3v bias (Intensity: 4.405 mW cm−2). The results exhibitexcellent mechanical flexibility of n3-0.5c PD.
The environmental stability of the device plays a significant role for its application. The change in photocurrent was recorded after storage for different days in conventional environment. The I-V logarithm curve of n3-0.5c is shown in Fig. 6a, and I-t curve in Fig. 6c, measured in 660 nm light illumination at 3 V bias (Intensity: 4.405 mW cm−2). The photo-response of n3-0.5c remains 50% of the initial value after 90 days. The device photo-response decayed to 35% after 150 days, which is 2.27 times that of n3 under the same test conditions. The contact angles of DI water on thin films were measured and shown in Fig. 6b. The n3 has 38.2° showing hydrophilicity, while n3-0.5c has 97.1° indicating hydrophobicity. The other films with different QDs concentrations are n3-1c with 86.3° (Fig. S18a), n3-0.2c with 73.9° (Fig. S18b), and n3-0.1c with 71.1° (Fig. S18c). This data validates the characterization results. The self-assembly of oleic acid (OA) ligand introduced through quantum dots exposes the hydrophobic chain on interface, thus enhancing the device hydrophobicity57. The heterostructure of n3/QDs exhibits lower trap densities than that of n3 PD, which is resulted from the passivation of QDs. The ligands of capped QDs could protect thin film from moisture absorption, which is also verified by the contact angle.
Fig. 6. Comparison of n3-0.5c photoresponse by day under 660 nm light illumination at 3 V bias at the light intensity of 4.405 mW cm-2 after storage in the ordinary dryer for as-prepared. a, Current-voltage (I-V) logarithm curves c, Current-time (I-t) curves. b, Contact angle measurements of n3 and n3-0.5c thin film. d, Time-dependent response of n3 and n3-0.5c after 3000 s of operation under 660 nm light illumination at 3 V bias at the same light intensity of 4.405 mW cm−2 in ambient conditions. |
The time-dependent photoresponse determines the stability of the device under 660 nm light illumination (Intensity：4.405 mW cm−2) at 3 V bias and irradiated for 3000 s. Fig. 6d shows reduction of photocurrent in n3 with increasing on/off switching times. Compared with the initial value, the photocurrent is reduced by 18.6%, while the photocurrent of n3-0.5c is increased by 17.9%. The time-dependent photo-response curves of n3-1c, n3-0.2c, and n3-0.1c are shown in Fig. S19 a-c. The self-assembly ligand forms a matrix outside QDs and protects PDs surface from irradiation58. Constant light illumination can lead to structural damage. The n3 may have additional defects leading to the photocarrier recombination in constant light illumination. The “photoactivation” behavior may be the reason for the increasing trend of photocurrent during the whole test period, which facilitates the removal of dangling bonds or undesirable surface states by high energy photons59. In summary, the n3-0.5c PD exhibits the optimum photo-response in continuous laser light irradiation.
In addition, some previously reported PDs were compared with the ones in the current workon the aspects of flexibility and stability, as shown in Table S1. PD with the optimal concentration of QDs exhibits excellent flexibility compared with other devices60,61. The comparable or even higher flexibility is attributed to the excellent film-forming quality of the device and the minimal damage of the film during bending. However, the stability of the device is lower than those of other reported62, and the performance of the device also remains to be improved.
Operation mechanisms of n3/QDs PDs
Ultraviolet photoemission spectroscopy (UPS) was employed to investigate the energy levels alignment of layered heterostructure, which determines the positions of valence band minimum (VBM) and conduction band maximum (CBM) for n3 (Fig. S20). The Eonset is 16.10 eV, which is related to secondary electrons. Work function (WF) of n3 is 5.1 eV, which is relative to 0 eV assigned to gold standard, defined as the Fermi level (EF). The difference between EF and EVBM is 0.54 eV (Fig. S20). Based on the bandgap of n3 (1.99 eV), the positions of ECBM (−5.64 eV) and EVBM (−3.65 eV) were determined. The band gap of QDs is 1.80 eV, in which the positions of ECBM and EVBM are −5.50 eV and −3.70 eV, respectively43. Energy levels alignment of n3/QDs with carrier transportation process is shown in Fig. 7a, indicating the formation of type I band alignment. The difference between CBM and VBM of the n3 and QDs is very small, which are 0.05 eV and 0.14 eV, forming a band matching structure that lowers the energy barrier of carrier transport process.
Fig. 7. Operation mechanisms of n3/QDs PDs. a, Energy band diagram for photodetector. b, Time-resolved photoluminescence lifetime (TRPL) of thin films treated with different concentrations of QDs. c, Schematic of the n3/ QDs photodetector. |
The carrier life of thin films was calculated by time-resolved transient photoluminescence (TRPL), as shown in Fig. 7b. Results are listed in Table 1. The biexponential decay model is used to fit the PL decay curves. The n3-0.5c has the longest life of 1.1378 nscompared with n3, n3-1c, n3-0.2c, and n3-0.1c with the life of 0.8388, 1.0014, 1.1106, and 1.1087 ns, respectively. The n3-0.5c has few defects, thereby reducing the non-radiative recombination. The n3-1 has the shortest life with QDs. Non-radiative recombination centers were introduced by the excessive QDs,63. The cracks of n3 couldn't be filled by the n3-0.2c and n3-0.1c (Fig. 2d, e), which increases the time for photocarrier to diffuse from n3 to QDs. Based on rough surface, n3 exhibits more non-radiative recombination centers that provide local sites for fast relaxation of photoexcited excitons13.
Table 1. Comparison of carrier life of n3/QDs heterostructure photodetectors with different QDs concentrations. |
| Empty Cell | A1 | τ1 (ns) | A2 | τ2 (ns) | τave (ns) |
|---|---|---|---|---|---|
| n3 | 18.9195 | 0.7985 | 0.0969 | 2.9593 | 0.8388 |
| n3-1c | 11.9443 | 0.9447 | 0.0797 | 3.3779 | 1.0014 |
| n3-0.5c | 0.0463 | 4.2702 | 9.2731 | 1.0757 | 1.1378 |
| n3-0.2c | 10.2192 | 1.0466 | 0.0995 | 3.2363 | 1.1106 |
| n3-0.1c | 10.6837 | 1.0396 | 0.1109 | 3.2423 | 1.1087 |
The probable operation mechanism of n3/QDs PD with different QDs concentrations is elaborated in Fig. 7c. Photoelectrons and holes generated from perovskite with larger band gap become excitons. The presence of quantum dots improve efficiency through carrier transportation in perovskite and emit bright-light with narrower band gap64. Under light, QDs as photosensitive layer absorb incident light for generating electron-hole pairs. The electrons and holes are transferred to Au electrode for producing photocurrent. The best photoelectric performance was obtained by adjusting the QDs concentration in heterojunction structure. When QDs concentration is c, the radiative recombination is dominant, thereby reducing the carrier lifetime63. The photoresponse of the drivers increases with decreasingquantum dot concentration, indicating that the charge diffusesacross the interface of n3 and QDs. When QDs concentration is insufficient, relatively sparse and rough film is created on the surface of n3 with defects,which causes decrease in conductivity and further leads to poor photoelectric performance. The n3-0.5c is an ideal PD with the optimum QDs concentration, showing excellent photoelectric performance, flexibility and stability.
CONCLUSIONS
In summary, an efficient UV-NIR flexible photodetector based on n3/QDs heterostructure was designed in the current work. Photoresponsivity of the device with the optimal concentration of quantum dots was six times that of the device without QDs. And the device exhibitedexcellent photodetection performance with negligible degradation up to 5000 bending cycles at different bending angles. The device exhibited optical self-healing with increasing number of bending cycles. The optical responsivity of the device was 227% of that of the monolayer perovskite device after 5 months of storage in desiccator, demonstrating its environmental stability. The advantages of perovskite and quantum dots were combined to develop new photodetector materials. These findings provide scope for designing flexible and wearable types of perovskite PDs.
METHODS
Materials
Lead (II) oxide (PbO, 99%, Sigma-Aldrich), 4-(aminomethyl) piperidine (96%), hydroiodic acid (HI, 55.0-58.0%, Macklin), and hypophosphorous acid (H3PO2, 50 wt.% in H2O, Macklin), methylamine (30 wt. % in H2O), lead iodide (PbI2, 99%, Sigma-Aldrich), cesium carbonate (Cs2CO3, 99%, Aldrich), octadecene (ODE, 90%, Aldrich), oleic acid (OA, 90%, Aldrich), oleyla-mine (OLA, 70%, Aldrich), hexane (anhydrous, 95%, Sigma), methylacetate (MeOAc, anhydrous, 99.5%, Sigma), toluene (> 99.5%, Guoyao chemical reagent), diethyl ether (> 99.7%, Guoyao chemical reagent), N,N-dimethylformamide (DMF, 99.8%, with molecular sieves, water ≤ 50 ppm (K. F. Macklin), dimethylsulfoxide (DMSO, > 99.8%, Aldrich).
Synthesis of methylammonium iodide (MAI)
MAI was obtained by adding MA solution in fractions to 2 mol L−1 hydroiodic acid solution and stirred for 2 h at 0 °C in ice bath. MAI was collected by completely evaporating the solution at 80 °C. The product was ultrasonically washed with excess of diethyl ether. The resulting powder was dried at 80 °C and stored for further use.
Synthesis of n3
The n3 was synthesized according to a previous method41. 669 mg (3 mmol) of PbO was dissolved in 6 mL hydroiodic acid and 1 mL hypophosphorous acid solution by stirring for 5 min at ∼130 °C until the solution became clear with bright yellow color. 477 mg of MAI powder was then added and stirred for 5 min at the same temperature. 0.5 mL hydroiodic acid was added to 0.33 mmol 4 AMP for the protonation in separate vial and stirred. The protonated 4 AMP solution was added to the previous solution at 130 °C and stirred for 5 min. Dark red plate-like crystals were obtained during slow cooling to room temperature, which were heated at 60 °C under vacuum for 4 hours after suction filtration before use. For device fabrication, n3 crystals were dissolved in DMF and DMSO (ratio of DMF: DMSO was 9:1) with a concentration of 1 mol L−1. The content was filtered with 0.22 μm filters.
QDs synthesis and purification
QDs were synthesized according to the procedures of the previously reported method43. To prepare Cs-oleate solution, 0.407 g of Cs2CO3, 20 mL ODE, and 1.25 mL OA were added to 250 mL three-neck flask and degassed under vacuum for 0.5 h at 120 °C. 0.5 g of PbI2 and 25 mL ODE were stirred and degassed for 0.5 h at 120 °C. The flask was filled with N2 and kept in constant N2 flow. 2.5 mL OA and 2.5 mL OLA were then added and put under vacuum until PbI2 got dissolved and there was no gas released (about 0.5 h). The N2 gas was made to flow in the flask and heated to 185 °C. After 2 mL Cs-oleate solution was injected, the mixture rapidly became dark red and reaction was quenched in an ice bath. QDs were precipitated by anhydrous MeOAc and centrifuged at 5000 rpm for 5 min (ratio of QDs crude solution: MeOAc was 1:3). The precipitates were dispersed in hexane and MeOAc (ratio of hexane: MeOAc was 1:2) and centrifuged at 5000 rpm for 5 min. QDs were then redispersed in 20 mL hexane and centrifuged at 7500 rpm for 5 min to remove excess PbI2 and Cs-oleate. The supernatant was kept at 4 °C for 12 h in dark to precipitate excess Cs-oleate and Pb-oleate. After cooling, the QDs were centrifuged again at 5000 rpm for 5 min before use. For the device fabrication, hexane was dried under vacuum and dispersed into toluene with the concentration of 4.16 mmol L−1.
Device fabrication
The interdigital electrode with PET substrate was cleaned with ethanol and dried before use. The gap distance was 50 μm. 35 μL of n3 solution was spin-coated on interdigital electrode substrate at 4000 rpm for 40 s and annealed at 60 °C for 10 min. 66 μL of QDs solutions with 0c, 1c, 0.5c, 0.2c, and 0.1c concentrations were spin-coated separately on interdigital electrode, which was previously covered with n3 thin film at 4000 rpm for 5 s and annealed at 60 °C for 10 min. The device with layered heterojunction formed by two dimensional DJ perovskite n3 and QDs was stored in a dryer for 12 h to remove the residual solvent. The prepared photodetector was tested in ambient conditions.
Device characterization and measurements
The X-ray diffraction pattern was measured with a powder X-ray diffractometer (Empyrean, PANalytical B.V.) with Cu Kα1 radiation (λ = 1.5406 Å). The ultra-violet absorption spectrum was performed by the ultraviolet-visible near-infrared spectrophotometer (Lambda 750s, PerkinElmer). The surface morphology was presented on SEM (Zeiss Germany, Merlin). The energy dispersive spectrometry (EDS) mapping was measured by the X-Max N20 of Oxford. Steady-state photoluminescence (PL) was recorded using Fluoromax-4 fluorescence spectroscopy (HORIBA). Time-resolved PL was acquired on ultraviolet photoemission spectra (UPS), which was measured with Thermo Scientific nexsa. The surface topography was obtained by AFM (DIMENSIONICON, Bruker Nano) in tapping mode. The QDs size was characterized by the transmission electron microscope (TEM, JEM2100F). All electrical measurements were conducted with Keithley 2450 connecting with the probe station at room temperature with the relative humidity of 35 ± 5%.
MISCELLANEA
Author contributions The concept of flexible photodetector based on 0D-2D perovskite heterostructure was conceived by Y.L.M. and J.W.; Y.L.M. and Y.Y.L. explored the quantum dot concentration of the device with the best performance, and characterized the related devices; Optical experiments were realized by Y.L.M. and H.W.; Y.L.M. and M.K.W. fabricated the film and device. The manuscript was written by Y.L.M. and J.W. with input from all authors.
Supplementary materials Supplementary materials include: EDS mappings of n3-1c, n3-0.5c, n3-0.2c, n3-0.1c. Contact angle measurements of n3-1c, n3-0.2c, and n3-0.1c thin films. Current-voltage (I-V) logarithm curves of the n3, n3-1c, n3-0.2c and n3-0.1c devices were obtained under the light illumination of 405 nm, 660 nm and 808 nm between −3 V and 3 V bias. Current-time (I-t) curves of the n3, n3-1c, n3-0.2c and n3-0.1c devices under the light illumination of 405 nm, 660 nm and 808 nm with different light intensities operating at 3 V bias. Current-time (I-t) curves of the n3-0.5c device were obtained under the light illumination of 660 nm with the light intensities of 4.405 mW cm−2, 8.811 mW cm−2, and 13.216 mW cm−2 operating at 0 V bias. Relative change in conductivity of the n3-0.5c device was tested during bending. Time-dependent response of n3-1c, n3-0.2c, and n3-0.1c after 3000 s of operation was measured under 660 nm light illumination at 3 V bias at the same light intensity of 4.405 mW cm−2 in ambient conditions. UPS spectrum of n3 thin films. Energy difference between the Fermi level (EF) and valence band maximum (EVBM) obtained from UPS analysis.
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.chip.2022.100032.
Acknowledgements W.J. acknowledges support from the National Natural Science Foundation of China (No. 62204032), the Natural Science Foundation of Liaoning Province of China (2021-MS-08) and the Fundamental Research Funds for the Central Universities, NEU (02060022121002). We thank Testing Center of Northeastern University for the support in PL and XRD test.
Declaration of Competing Interest The authors declare no competing interests.
