Communication Perovskite Photodetectors
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High-Performance Flexible Photodetectors based on HighQuality Perovskite Thin Films by a Vapor–Solution Method Wei Hu, Wei Huang, Shuzhen Yang, Xiao Wang, Zhenyu Jiang, Xiaoli Zhu, Hong Zhou, Hongjun Liu, Qinglin Zhang, Xiujuan Zhuang, Junliang Yang, Dong Ha Kim, and Anlian Pan* Flexible photodetectors (PDs) have attracted a great deal of attention due to their potential applications in next-generation portable and wearable optoelectronic devices, such as image sensors, optical fiber communications, and environmental monitoring.[1–7] To date, many organic[3,8,9] and inorganic semiconductor[10,11] polycrystalline thin films have been explored as the light absorbers for flexible PDs, however, these films suffer from either relatively low carrier mobility, poor photo responsivity, or complicated preparation procedures, which greatly limit their industry deployment in flexible optoelectronic systems. Organometal halide perovskites have recently been demonstrated to be promising light-harvesting material for optoelectronic devices, owing to the extraordinary properties, like direct bandgap, high light-absorption coefficient, long charge-carrier lifetime, and diffusion length.[12–49] Although significant progress has been made in fabricating perovskite thin films and devices on rigid substrates,[16–22] preparing perovskite thin films on flexible substrates and constructing flexible PDs were reported only very recently. For example, Hu et al. realized flexible PDs with spincoated methylammonium lead triiodide (CH3NH3PbI3) thin films, which show a broadband spectral response and have a responsivity (R) of ≈0.0367 A W−1 at 780 nm at 3 V.[23] Deng et al. reported CH3NH3PbI3-network-based flexible image sensors using the spin-coating technique and achieved an R value of 0.10 A W−1 at 650 nm at 10 V.[24] Flexible PDs based on CH3NH3PbI3 microwire arrays through a doctor-blading method have also been reported by Dang et al.[26] Yang’s group fabricated CH3NH3PbI3 nanowire arrays by a roll-to-roll microgravure printing method and realized flexible PDs with a R of 2.0 mA W−1 at 10 V.[27] However, these aforementioned solution-based methods introduced pinhole and incomplete surface coverage in perovskite thin films, due to their low nucleation rate and slow crystallization speed induced by the slow evaporation of the high-boiling-point solvent, N,N-dimethylformamide (DMF) (153 °C).[23–29] Consequently, the PD devices based on these low-quality perovskite films have very low performance and need relatively high working voltage. Therefore, it
Organometal halide perovskites are new light-harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low-temperature process capability. However, the preparation of high-quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor–solution method is developed to achieve uniform and pinhole-free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as-prepared high-quality perovskite thin films, high-performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W−1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin-film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high-quality perovskite thin films opens up a new avenue to develop high-performance flexible optoelectronic devices.
Dr. W. Hu, W. Huang, S. Yang, Dr. X. Wang, Dr. Z. Jiang, Dr. X. Zhu, Dr. H. Zhou, Dr. H. Liu, Dr. Q. Zhang, Dr. X. Zhuang, Prof. A. Pan Key Laboratory for Micro-Nano Physics and Technology of Hunan Province State Key Laboratory of Chemo/Biosensing and Chemometrics School of Physics and Electronics Hunan University Changsha, Hunan 410082, P. R. China E-mail: [email protected] Dr. W. Hu, W. Huang, S. Yang, Dr. X. Wang, Dr. Z. Jiang, Dr. X. Zhu, Dr. H. Zhou, Dr. H. Liu, Dr. Q. Zhang, Dr. X. Zhuang, Prof. A. Pan Synergetic Innovation Center for Quantum Effects and Application Hunan Normal University Changsha 410081, P. R. China Prof. J. Yang Hunan Key Laboratory for Super-microstructure and Ultrafast Process School of Physics and Electronics Central South University Changsha, Hunan 410083, P. R. China Prof. D. H. Kim Department of Chemistry and Nano Science College of Natural Sciences Ewha Womans University 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea
DOI: 10.1002/adma.201703256
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is of fundamental importance and remains a great challenge to obtain high-quality perovskite thin films with complete surface coverage on flexible substrates. Here, we have successfully prepared high-quality perovskite thin films on flexible poly(ethylene terephthalate) (PET) substrates using an advanced vapor–solution method. Differing from the commonly used high-boiling-point solvents in the allsolution methods, structurally homogeneous PbI2 layers were first prepared using a novel vapor deposition process, and then converted into high-quality perovskite thin films through a fast nucleation and crystallization process using a low-boiling-point solvent, isopropyl alcohol (IPA) (82.5 °C).[30–49] Based on the as-prepared perovskite thin films, we constructed high-performance flexible PDs with the R value reaching 81 A W−1 at a low working voltage of 1 V, which is three orders higher than has ever been reported for flexible perovskite thin-film PDs to date. In addition, our flexible PDs exhibited excellent stability and reliability in situation of bending, which maintained high performance under different radii of curvature (3–10 mm) and after repetitive bending of 200 cycles. This vapor–solution method for high-quality perovskite thin films preparation may open up a new avenue to develop high-performance flexible optoelectronic devices. The vapor–solution growth of a CH3NH3PbI3 film on a flexible PET substrate is schematically shown in Figure 1a. First, a PbI2 film (100 nm) was prepared using thermally physical vapor deposition (PVD) in a vacuum chamber at 10−4 Pa pressure. Three different deposition rates, 2.6, 1.5, and 0.7 nm s−1, were used to explore the morphology of the obtained PbI2 films. At a high deposition rate of 2.6 nm s−1, a PbI2 film with leaf-like structures was obtained as shown in Figure 1c and Figure S1 (Supporting Information).[50–52] With a slower growth rate (1.5 nm s−1), the amount of the leaf-like structures decreased
apparently, and many irregularly shaped microcrystals appeared, as shown in Figure 1d. When the deposition rate was further reduced to 0.7 nm s−1, a homogeneous PbI2 film with relatively high uniformity was achieved, as observed in the topview scanning electron microscopy (SEM) image (Figure 1e). These results indicate that the morphological and structural characteristics of PbI2 films on the flexible substrates are greatly influenced by the PVD deposition rate. A low deposition rate is critical to the formation of homogeneous PbI2 films. Second, a solution of CH3NH3I dissolved in IPA (15 mg mL−1) was spin-coated on the as-prepared structurally homogeneous PbI2 film (with a growth rate of 0.7 nm s−1) at 3000 rpm for 30 s, followed immediately by 10 min annealing at 100 °C in the ambient environment. As a result, a uniform and compact perovskite thin film over the entire PET substrate has been achieved, as shown in Figure 2. A typical cross-sectional scanning electron microscopy (SEM) image (Figure 2a) indicates a pinhole-free perovskite thin film with a thickness of ≈180 nm. The top-view SEM images of the as-prepared perovskite thin film with two different magnifications are shown in Figure 2b,c, respectively. These images demonstrate that the asformed CH3NH3PbI3 thin film possesses full surface coverage on the PET substrate with a grain size of ≈200 nm. Figure 2d shows the corresponding X-ray diffraction (XRD) of the asprepared CH3NH3PbI3 thin film on PET substrate. The strong peaks at 14.00° and 28.36° can be assigned to the (110) and (220) of the CH3NH3PbI3 crystal, indicating a tetragonal crystal structure of perovskite thin film with high crystallinity.[24,30,44] The absence of peak at 12.65°, which is normally attributed to the impurity of PbI2, suggests a complete transformation of PbI2 via the solution process. As shown in Figure 2e, the as-prepared CH3NH3PbI3 thin film has light absorption in the visible range from 400 to 780 nm,[18–24] and the inset is a photograph
Figure 1. Fabrication procedure for perovskite thin film using vapor–solution fabrication method and the PbI2 thin-film morphology. a) Schematic diagram of a patterned ITO/PET substrate. b–d) The top-view SEM images of the PbI2 layers at deposition rate of 2.6, 1.5, and 0.7 nm s−1, respectively (scale bar: 200 nm). Adv. Mater. 2017, 1703256
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Figure 2. Characteristics of the perovskite thin film on PET substrate. a) SEM cross-sectional (scale bar: 100 nm) image. b,c) Low- and highmagnification SEM images of the surface of a prepared perovskite thin film (scale bars: 2 µm in (b) and 200 nm in (c)). d) XRD pattern of the asprepared perovskite thin film. e) UV–vis absorption curve (inset is a photograph of the perovskite thin film on PET substrate). f) PL spectrum of the perovskite thin film (under excitation wavelength and power of 480 nm, 0.15 mW).
of a typical perovskite thin film on PET substrate. Figure 2f shows the steady-state photoluminescence (PL) spectrum of the perovskite thin film. The PL peak is located at ≈770 nm, which is consistent with the previously reported results.[11–13] Comparing with the previously reported perovskite thin films through solution-based routes (Figure S2 and S3, Supporting Information),[24–29] the above results suggest that the perovskite thin film grown by our vapor–solution method clearly possesses the advantages of high compactness and high crystallization in a large area. These overwhelming performances could be due to the combination of the relatively high uniformity of the preformed PbI2 film, the fast nucleation and crystallization using a low-boiling-point IPA solvent. Flexible PDs were further constructed based on the as-prepared high-quality perovskite thin films. As shown in Figure 3a, a CH3NH3PbI3 active layer was deposited on an indium-doped
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tin oxide (ITO) coated PET substrate (as shown in Figure 1b). The interdigital ITO electrodes were patterned using photo lithography and wet etching technique, as shown in Figure S4 (Supporting Information). The fabricated PDs without encapsulation were moved to a probe station for performance measurements at room temperature. Figure 3b shows the current– voltage (I–V) curves of a perovskite thin-film PD under dark and vertically illuminated with a monochromatic light, respectively. It is noted that the dark current density (dark current per unit area, Jdark) is ≈3 × 10−5 A cm−2 at 1 V, which is much lower than those of previously reported PDs.[24–29] The low Jdark indicates that the free-carrier concentration is exceedingly low, and the migration of charge carriers along the two electrodes is suppressed due to high contact barrier at ITO/perovskite interface under dark.[19,20] Under illumination (power intensity remains at 0.42 µW, incident wavelengths vary from 400 to 760 nm), the
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Figure 3. Performance of the flexible perovskite thin-film PD. a) Photograph of a flexible perovskite PD at bending situation (with interdigital electrodes on PET substrate). b) Current–voltage curves of the device as a function of illumination wavelength with fixed incident illumination power. c) Wavelength-dependent EQE. d) R of the flexible PD at different voltages. e) Illumination power-dependent I–V curves of the flexible PD. f) Linear dynamic range of the flexible PD as a function of illumination power (fixed at a bias of 1 V) at the incident light wavelength of 680 nm.
photocurrent density (Jph, defined as the current density difference between illumination and dark) increases dramatically and exhibits a good linear relationship with the bias (Figure 3e). The photon-response property reveals that a large number of charge carriers has been generated in the flexible PD under the incident light. Furthermore, the excellent contact properties under illumination, forming between ITO electrodes and perovskite thin film, are attributed to the carrier-selective trapping at the ITO/perovskite interfaces and the bipolar transport characteristics of the perovskite material.[19,20] The cutoff wavelength of the PD is found to be ≈800 nm, with the Jph dropping dramatically to the level of Jdark at longer wavelengths, which is highly consistent with the absorption spectrum of the perovskite film in Figure 2b. The photo-to-dark current ratio of the PD can reach 100 at 1 V from the I–V data, indicating that the flexible PD has considerable signal-to-noise ratio at a low voltage and is suitable for practical photon-detection applications. To further evaluate
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the performance of the flexible PD, two important figure of merits, external quantum efficiency (EQE) and R, are obtained and discussed in detail. EQE = Iph/qΦ, is defined as the number of carriers produced per photon, and R = Iph/PIn, is defined as the ratio of photocurrent to incident-light intensity, where Iph is the photocurrent, q is the elementary charge, Φ is the photon flux, and PIn is the incident optical power. The EQE and R at different biases are shown in Figure 3c,d, respectively. At 1 V, the flexible PD exhibits impressive EQE and R of 1.5 × 104% and 75 A W−1 within the spectral range from 400 to 700 nm, respectively, which are among the highest EQE and R values reported to date.[23–27] A maximum R of 81 A W−1 is achieved at 680 nm, corresponding to a gain of about 103 electrons/photons. Compared with the reported flexible perovskite PDs,[23–25] a higher photon-induced charge-carrier density could be generated in our device due to more photons being absorbed by the compact active layer under the same incident power density.
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Subsequently, there are more charge carriers going through the flexible perovskite PD, which results in a high photoconductive gain. The detectivity (D*) of the device exceeds 1011 Jones, as shown in Figure S5 (Supporting Information), which is also comparable with most of the reported flexible perovskite PDs.[19–27] Figure 3e shows the illumination-power-dependent I–V curves of the PD under the incident light at 680 nm, which shows that the Iph of our device increases accordingly with the illumination power. The dependence of Iph on illumination power at 1 V is shown in Figure 3f, which exhibits a good linear relationship with the power density varying from 0.3 to 42.5 µw cm−2, indicating that the flexible perovskite thinfilm PD is capable of detecting incident power in a wide range. Bending stability is an important factor affecting the flexible PDs’ suitability for practical applications in portable electronic devices. In this work, bending tests of the flexible PDs without encapsulation were carried out with different bending radii of curvature and numbers of bending cycles. Figure 4b–f shows the photographic images of a PD with different bending radii of 10.0, 7.2, 4.8, 4.2, and 3.3 mm, respectively. Figure 4g shows the corresponding I–V curves of the flexible PD with incident
wavelength at 680 nm and power intensity at 0.12 µW. Accordingly, Figure 4h shows Iph and EQE of the PD under different bending statuses at 1 V with respect to that of the nonbending data. These values remain almost constant at curvature radii varying from 10.0 to 3.3 mm, indicating excellent bending stability of the flexible PD. Furthermore, measurements after the repetitive bending cycles are performed to verify the PDs flexibility endurance (Movie S1, Supporting Information). Figure 4i shows the flexible PD I–V curves measured at flat states after various numbers of bending cycles. The values of Iph and EQE upon bending cycling at 1 V are shown in Figure 4j. The flexible PD shows no significant performance degradation after repetitive bending of 200 cycles, indicating its excellent operational stability during repetitive bending tests. The values of R maintain at ≈70 A W−1 under the two different bending conditions described above (Figure S6, Supporting Information). These results indicate that the fabricated perovskite thin-film PDs own excellent flexibility under the mechanical bending situations. We attribute these extraordinary characteristics of the flexible PDs to the high quality of the perovskite thin films
Figure 4. Performance of the perovskite thin-film PD at different curvature radii and after repetitive bending of cycles (with incident light of 680 nm). a–f) Photographs of the flexible PD at curvature radii from ∞ to 3.3 mm. g,h) I–V curves (g) and Iph and EQE (h) of the flexible PD as the function of bending radii at 1 V. i,j) I–V curves (i) and Iph and EQE (j) of the flexible PD as the function of repetitive bending cycles at 1 V.
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Figure 5. Photoresponse and band diagram of the flexible perovskite PD. a) I–V curve for a hole-only perovskite device. The inset shows the device structure of the hole-only device. b) The device band diagram under illumination at bias. c) The photoresponse curves of the PD after different numbers of repetitive bending cycles. d–f) Dynamic photoresponse (d), fall time (e) and rise time (f) for the flexible PD after repetitive bending of 200 cycles.
constructed by the novel vapor–solution method. In order to evaluate the perovskite thin-film quality, a hole-only device (inset in Figure 5a) has been constructed to estimate hole trap density in this perovskite active layer.[53,54] The device dark current (Id) was measured as shown in Figure 5a. There are three regions that can be identified according to different values of the exponent n. The linear Id–V relation (n = 1, red line) indicates an ohmic response at a low bias (
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High-Performance Flexible Photodetectors based on HighQuality Perovskite Thin Films by a Vapor–Solution Method Wei Hu, Wei Huang, Shuzhen Yang, Xiao Wang, Zhenyu Jiang, Xiaoli Zhu, Hong Zhou, Hongjun Liu, Qinglin Zhang, Xiujuan Zhuang, Junliang Yang, Dong Ha Kim, and Anlian Pan* Flexible photodetectors (PDs) have attracted a great deal of attention due to their potential applications in next-generation portable and wearable optoelectronic devices, such as image sensors, optical fiber communications, and environmental monitoring.[1–7] To date, many organic[3,8,9] and inorganic semiconductor[10,11] polycrystalline thin films have been explored as the light absorbers for flexible PDs, however, these films suffer from either relatively low carrier mobility, poor photo responsivity, or complicated preparation procedures, which greatly limit their industry deployment in flexible optoelectronic systems. Organometal halide perovskites have recently been demonstrated to be promising light-harvesting material for optoelectronic devices, owing to the extraordinary properties, like direct bandgap, high light-absorption coefficient, long charge-carrier lifetime, and diffusion length.[12–49] Although significant progress has been made in fabricating perovskite thin films and devices on rigid substrates,[16–22] preparing perovskite thin films on flexible substrates and constructing flexible PDs were reported only very recently. For example, Hu et al. realized flexible PDs with spincoated methylammonium lead triiodide (CH3NH3PbI3) thin films, which show a broadband spectral response and have a responsivity (R) of ≈0.0367 A W−1 at 780 nm at 3 V.[23] Deng et al. reported CH3NH3PbI3-network-based flexible image sensors using the spin-coating technique and achieved an R value of 0.10 A W−1 at 650 nm at 10 V.[24] Flexible PDs based on CH3NH3PbI3 microwire arrays through a doctor-blading method have also been reported by Dang et al.[26] Yang’s group fabricated CH3NH3PbI3 nanowire arrays by a roll-to-roll microgravure printing method and realized flexible PDs with a R of 2.0 mA W−1 at 10 V.[27] However, these aforementioned solution-based methods introduced pinhole and incomplete surface coverage in perovskite thin films, due to their low nucleation rate and slow crystallization speed induced by the slow evaporation of the high-boiling-point solvent, N,N-dimethylformamide (DMF) (153 °C).[23–29] Consequently, the PD devices based on these low-quality perovskite films have very low performance and need relatively high working voltage. Therefore, it
Organometal halide perovskites are new light-harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low-temperature process capability. However, the preparation of high-quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor–solution method is developed to achieve uniform and pinhole-free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as-prepared high-quality perovskite thin films, high-performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W−1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin-film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high-quality perovskite thin films opens up a new avenue to develop high-performance flexible optoelectronic devices.
Dr. W. Hu, W. Huang, S. Yang, Dr. X. Wang, Dr. Z. Jiang, Dr. X. Zhu, Dr. H. Zhou, Dr. H. Liu, Dr. Q. Zhang, Dr. X. Zhuang, Prof. A. Pan Key Laboratory for Micro-Nano Physics and Technology of Hunan Province State Key Laboratory of Chemo/Biosensing and Chemometrics School of Physics and Electronics Hunan University Changsha, Hunan 410082, P. R. China E-mail: [email protected] Dr. W. Hu, W. Huang, S. Yang, Dr. X. Wang, Dr. Z. Jiang, Dr. X. Zhu, Dr. H. Zhou, Dr. H. Liu, Dr. Q. Zhang, Dr. X. Zhuang, Prof. A. Pan Synergetic Innovation Center for Quantum Effects and Application Hunan Normal University Changsha 410081, P. R. China Prof. J. Yang Hunan Key Laboratory for Super-microstructure and Ultrafast Process School of Physics and Electronics Central South University Changsha, Hunan 410083, P. R. China Prof. D. H. Kim Department of Chemistry and Nano Science College of Natural Sciences Ewha Womans University 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea
DOI: 10.1002/adma.201703256
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is of fundamental importance and remains a great challenge to obtain high-quality perovskite thin films with complete surface coverage on flexible substrates. Here, we have successfully prepared high-quality perovskite thin films on flexible poly(ethylene terephthalate) (PET) substrates using an advanced vapor–solution method. Differing from the commonly used high-boiling-point solvents in the allsolution methods, structurally homogeneous PbI2 layers were first prepared using a novel vapor deposition process, and then converted into high-quality perovskite thin films through a fast nucleation and crystallization process using a low-boiling-point solvent, isopropyl alcohol (IPA) (82.5 °C).[30–49] Based on the as-prepared perovskite thin films, we constructed high-performance flexible PDs with the R value reaching 81 A W−1 at a low working voltage of 1 V, which is three orders higher than has ever been reported for flexible perovskite thin-film PDs to date. In addition, our flexible PDs exhibited excellent stability and reliability in situation of bending, which maintained high performance under different radii of curvature (3–10 mm) and after repetitive bending of 200 cycles. This vapor–solution method for high-quality perovskite thin films preparation may open up a new avenue to develop high-performance flexible optoelectronic devices. The vapor–solution growth of a CH3NH3PbI3 film on a flexible PET substrate is schematically shown in Figure 1a. First, a PbI2 film (100 nm) was prepared using thermally physical vapor deposition (PVD) in a vacuum chamber at 10−4 Pa pressure. Three different deposition rates, 2.6, 1.5, and 0.7 nm s−1, were used to explore the morphology of the obtained PbI2 films. At a high deposition rate of 2.6 nm s−1, a PbI2 film with leaf-like structures was obtained as shown in Figure 1c and Figure S1 (Supporting Information).[50–52] With a slower growth rate (1.5 nm s−1), the amount of the leaf-like structures decreased
apparently, and many irregularly shaped microcrystals appeared, as shown in Figure 1d. When the deposition rate was further reduced to 0.7 nm s−1, a homogeneous PbI2 film with relatively high uniformity was achieved, as observed in the topview scanning electron microscopy (SEM) image (Figure 1e). These results indicate that the morphological and structural characteristics of PbI2 films on the flexible substrates are greatly influenced by the PVD deposition rate. A low deposition rate is critical to the formation of homogeneous PbI2 films. Second, a solution of CH3NH3I dissolved in IPA (15 mg mL−1) was spin-coated on the as-prepared structurally homogeneous PbI2 film (with a growth rate of 0.7 nm s−1) at 3000 rpm for 30 s, followed immediately by 10 min annealing at 100 °C in the ambient environment. As a result, a uniform and compact perovskite thin film over the entire PET substrate has been achieved, as shown in Figure 2. A typical cross-sectional scanning electron microscopy (SEM) image (Figure 2a) indicates a pinhole-free perovskite thin film with a thickness of ≈180 nm. The top-view SEM images of the as-prepared perovskite thin film with two different magnifications are shown in Figure 2b,c, respectively. These images demonstrate that the asformed CH3NH3PbI3 thin film possesses full surface coverage on the PET substrate with a grain size of ≈200 nm. Figure 2d shows the corresponding X-ray diffraction (XRD) of the asprepared CH3NH3PbI3 thin film on PET substrate. The strong peaks at 14.00° and 28.36° can be assigned to the (110) and (220) of the CH3NH3PbI3 crystal, indicating a tetragonal crystal structure of perovskite thin film with high crystallinity.[24,30,44] The absence of peak at 12.65°, which is normally attributed to the impurity of PbI2, suggests a complete transformation of PbI2 via the solution process. As shown in Figure 2e, the as-prepared CH3NH3PbI3 thin film has light absorption in the visible range from 400 to 780 nm,[18–24] and the inset is a photograph
Figure 1. Fabrication procedure for perovskite thin film using vapor–solution fabrication method and the PbI2 thin-film morphology. a) Schematic diagram of a patterned ITO/PET substrate. b–d) The top-view SEM images of the PbI2 layers at deposition rate of 2.6, 1.5, and 0.7 nm s−1, respectively (scale bar: 200 nm). Adv. Mater. 2017, 1703256
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Figure 2. Characteristics of the perovskite thin film on PET substrate. a) SEM cross-sectional (scale bar: 100 nm) image. b,c) Low- and highmagnification SEM images of the surface of a prepared perovskite thin film (scale bars: 2 µm in (b) and 200 nm in (c)). d) XRD pattern of the asprepared perovskite thin film. e) UV–vis absorption curve (inset is a photograph of the perovskite thin film on PET substrate). f) PL spectrum of the perovskite thin film (under excitation wavelength and power of 480 nm, 0.15 mW).
of a typical perovskite thin film on PET substrate. Figure 2f shows the steady-state photoluminescence (PL) spectrum of the perovskite thin film. The PL peak is located at ≈770 nm, which is consistent with the previously reported results.[11–13] Comparing with the previously reported perovskite thin films through solution-based routes (Figure S2 and S3, Supporting Information),[24–29] the above results suggest that the perovskite thin film grown by our vapor–solution method clearly possesses the advantages of high compactness and high crystallization in a large area. These overwhelming performances could be due to the combination of the relatively high uniformity of the preformed PbI2 film, the fast nucleation and crystallization using a low-boiling-point IPA solvent. Flexible PDs were further constructed based on the as-prepared high-quality perovskite thin films. As shown in Figure 3a, a CH3NH3PbI3 active layer was deposited on an indium-doped
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tin oxide (ITO) coated PET substrate (as shown in Figure 1b). The interdigital ITO electrodes were patterned using photo lithography and wet etching technique, as shown in Figure S4 (Supporting Information). The fabricated PDs without encapsulation were moved to a probe station for performance measurements at room temperature. Figure 3b shows the current– voltage (I–V) curves of a perovskite thin-film PD under dark and vertically illuminated with a monochromatic light, respectively. It is noted that the dark current density (dark current per unit area, Jdark) is ≈3 × 10−5 A cm−2 at 1 V, which is much lower than those of previously reported PDs.[24–29] The low Jdark indicates that the free-carrier concentration is exceedingly low, and the migration of charge carriers along the two electrodes is suppressed due to high contact barrier at ITO/perovskite interface under dark.[19,20] Under illumination (power intensity remains at 0.42 µW, incident wavelengths vary from 400 to 760 nm), the
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Figure 3. Performance of the flexible perovskite thin-film PD. a) Photograph of a flexible perovskite PD at bending situation (with interdigital electrodes on PET substrate). b) Current–voltage curves of the device as a function of illumination wavelength with fixed incident illumination power. c) Wavelength-dependent EQE. d) R of the flexible PD at different voltages. e) Illumination power-dependent I–V curves of the flexible PD. f) Linear dynamic range of the flexible PD as a function of illumination power (fixed at a bias of 1 V) at the incident light wavelength of 680 nm.
photocurrent density (Jph, defined as the current density difference between illumination and dark) increases dramatically and exhibits a good linear relationship with the bias (Figure 3e). The photon-response property reveals that a large number of charge carriers has been generated in the flexible PD under the incident light. Furthermore, the excellent contact properties under illumination, forming between ITO electrodes and perovskite thin film, are attributed to the carrier-selective trapping at the ITO/perovskite interfaces and the bipolar transport characteristics of the perovskite material.[19,20] The cutoff wavelength of the PD is found to be ≈800 nm, with the Jph dropping dramatically to the level of Jdark at longer wavelengths, which is highly consistent with the absorption spectrum of the perovskite film in Figure 2b. The photo-to-dark current ratio of the PD can reach 100 at 1 V from the I–V data, indicating that the flexible PD has considerable signal-to-noise ratio at a low voltage and is suitable for practical photon-detection applications. To further evaluate
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the performance of the flexible PD, two important figure of merits, external quantum efficiency (EQE) and R, are obtained and discussed in detail. EQE = Iph/qΦ, is defined as the number of carriers produced per photon, and R = Iph/PIn, is defined as the ratio of photocurrent to incident-light intensity, where Iph is the photocurrent, q is the elementary charge, Φ is the photon flux, and PIn is the incident optical power. The EQE and R at different biases are shown in Figure 3c,d, respectively. At 1 V, the flexible PD exhibits impressive EQE and R of 1.5 × 104% and 75 A W−1 within the spectral range from 400 to 700 nm, respectively, which are among the highest EQE and R values reported to date.[23–27] A maximum R of 81 A W−1 is achieved at 680 nm, corresponding to a gain of about 103 electrons/photons. Compared with the reported flexible perovskite PDs,[23–25] a higher photon-induced charge-carrier density could be generated in our device due to more photons being absorbed by the compact active layer under the same incident power density.
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Subsequently, there are more charge carriers going through the flexible perovskite PD, which results in a high photoconductive gain. The detectivity (D*) of the device exceeds 1011 Jones, as shown in Figure S5 (Supporting Information), which is also comparable with most of the reported flexible perovskite PDs.[19–27] Figure 3e shows the illumination-power-dependent I–V curves of the PD under the incident light at 680 nm, which shows that the Iph of our device increases accordingly with the illumination power. The dependence of Iph on illumination power at 1 V is shown in Figure 3f, which exhibits a good linear relationship with the power density varying from 0.3 to 42.5 µw cm−2, indicating that the flexible perovskite thinfilm PD is capable of detecting incident power in a wide range. Bending stability is an important factor affecting the flexible PDs’ suitability for practical applications in portable electronic devices. In this work, bending tests of the flexible PDs without encapsulation were carried out with different bending radii of curvature and numbers of bending cycles. Figure 4b–f shows the photographic images of a PD with different bending radii of 10.0, 7.2, 4.8, 4.2, and 3.3 mm, respectively. Figure 4g shows the corresponding I–V curves of the flexible PD with incident
wavelength at 680 nm and power intensity at 0.12 µW. Accordingly, Figure 4h shows Iph and EQE of the PD under different bending statuses at 1 V with respect to that of the nonbending data. These values remain almost constant at curvature radii varying from 10.0 to 3.3 mm, indicating excellent bending stability of the flexible PD. Furthermore, measurements after the repetitive bending cycles are performed to verify the PDs flexibility endurance (Movie S1, Supporting Information). Figure 4i shows the flexible PD I–V curves measured at flat states after various numbers of bending cycles. The values of Iph and EQE upon bending cycling at 1 V are shown in Figure 4j. The flexible PD shows no significant performance degradation after repetitive bending of 200 cycles, indicating its excellent operational stability during repetitive bending tests. The values of R maintain at ≈70 A W−1 under the two different bending conditions described above (Figure S6, Supporting Information). These results indicate that the fabricated perovskite thin-film PDs own excellent flexibility under the mechanical bending situations. We attribute these extraordinary characteristics of the flexible PDs to the high quality of the perovskite thin films
Figure 4. Performance of the perovskite thin-film PD at different curvature radii and after repetitive bending of cycles (with incident light of 680 nm). a–f) Photographs of the flexible PD at curvature radii from ∞ to 3.3 mm. g,h) I–V curves (g) and Iph and EQE (h) of the flexible PD as the function of bending radii at 1 V. i,j) I–V curves (i) and Iph and EQE (j) of the flexible PD as the function of repetitive bending cycles at 1 V.
Adv. Mater. 2017, 1703256
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Figure 5. Photoresponse and band diagram of the flexible perovskite PD. a) I–V curve for a hole-only perovskite device. The inset shows the device structure of the hole-only device. b) The device band diagram under illumination at bias. c) The photoresponse curves of the PD after different numbers of repetitive bending cycles. d–f) Dynamic photoresponse (d), fall time (e) and rise time (f) for the flexible PD after repetitive bending of 200 cycles.
constructed by the novel vapor–solution method. In order to evaluate the perovskite thin-film quality, a hole-only device (inset in Figure 5a) has been constructed to estimate hole trap density in this perovskite active layer.[53,54] The device dark current (Id) was measured as shown in Figure 5a. There are three regions that can be identified according to different values of the exponent n. The linear Id–V relation (n = 1, red line) indicates an ohmic response at a low bias (
