High-Stability, Self-Powered Perovskite Photodetector Based on a CH

Sep 18, 2017 - melting point.12 Especially, GaN has good stability, and also, the weak oxidation of N ... grain boundaries of the methylammonium lead ...
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High-Stability, Self-Powered Perovskite Photodetector Based on a CH3NH3PbI3/GaN Heterojunction with C60 as an Electron Transport Layer Hai Zhou, Jun Mei, Mengni Xue, Zehao Song, and Hao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Science, Hubei University, Wuhan 430062, People’s Republic of China ABSTRACT: We reported first photodetectors based on a CH3NH3PbI3/GaN heterojunction with or without an electron transport layer (ETL). Through the investigation, the device without an ETL showed good sensitivity, high stability, and repeatability. Particularly, when C60 was applied as the ETL, the device showed a self-powered characteristic with high stability. The best self-powered device with 6 nm C60 displayed an on/off ratio of greater than 5000, peak responsivity of 0.198 A/W, and detectivity of 7.96 × 1012 cm Hz1/2/W, which is comparable with the other reports. The high performance mentioned above was attributed mainly to the C60 layer, which can reduce the density of trap states and passivate the grain boundaries of the CH3NH3PbI3 absorbing layer to facilitate intermolecular charge transport.

1. INTRODUCTION A photodetector (PD) is a fundamental module device in safety monitoring, optical communication, and biological sensing.1−3 Hybrid organic−inorganic halide perovskite materials used as light absorbers in PDs possess excellent merits such as a direct band gap with a large absorption coefficient and a long charge carrier lifetime and diffusion length.4−6 However, the poor stability in oxygen and moist conditions blocks its application.7−9 Therefore, improving the stability of organic−inorganic halide perovskite materials is an urgent need. Many ways to improve the stability had been reported, such as heavily doped inorganic charge extraction layers (NiMgLiO) as an electron transport layer (ETL),10 solution-processed metal oxide transport layers as an ETL and hole transport layer (HTL),11 and so forth. In these ways, stable ETLs and HTLs are the keys for improving the stability. GaN with a direct band gap of 3.4 eV has received peculiar attention due to its outstanding optical and electrical properties, high carrier mobility, superior chemical stability, as well as high melting point.12 Especially, GaN has good stability, and also, the weak oxidation of N may result in better stability for an organic−inorganic perovskite; therefore, p-type GaN may be a good HTL in perovskite PDs. Furthermore, the wide band gap of GaN may expand the response wavelength of a perovskite PD to the ultraviolet (UV) region, and the PD will be applied in a wide wavelength region. For a self-powered PD, the ETL is also very important. It is known that fullerenes such as PCBM and C60 can reduce the density of trap states and passivate the grain boundaries of the methylammonium lead iodide (CH3NH3PbI3) absorbing layer;13,14 they are very popular as ETLs in perovskite solar cells.15,16 In addition, the C60 can be packed more densely than PCBM to facilitate intermolecular charge transport;17 also, it can be prepared by a vacuum process © 2017 American Chemical Society

instead of the solution-processed method. These advantages will further enhance the stability and photoelectric response performance of the device.18,19 Herein, we reported first an ETL-free perovskite PD based on the CH3NH3PbI3/GaN structure. Our detector showed high sensitivity, good stability, and repeatability with a peak responsivity of 0.16 A/W and peak detectivity of 2.89 × 1012 cm Hz1/2/W at 500 nm and 0.2 V. Furthermore, by inserting a C60 ETL, the device showed self-powered characteristics, and the best self-powered device with 6 nm C60 showed an on/off ratio of greater than 5000, a peak responsivity of 0.198 A/W, and detectivity of 7.96 × 1012 cm Hz1/2/W.

2. EXPERIENTAL SECTION The fluorinated tin oxide (FTO) coated glass substrates with a sheet resistance of 15 Ω/sq were cleaned with deionized water and acetone, followed by ethanol for 15 min by ultrasonic agitation. Then, the samples were treated by UV-ozone for 30 min. For the preparation of self-powered PDs, C60 with different thicknesses was prepared by evaporation on FTO glass. The perovskite active layer (CH3NH3PbI3) was fabricated by a typical two-step dip-coating method.20,21 To prepare PbI2 solution, a 462 mg PbI2 (99.99%) was dissolved in 1 mL of N,N-dimethyl sulfoxide (DMF, Sigma-Aldrich, 99.8%) solution and stirred at 70 °C for 12 h. First, FTO substrates were put on a hot plate heated at 70 °C for 30 min. Then, the prepared hot PbI2 solution was quickly deposited by a spin-coat technique on the FTO substrate at 500 rpm for 1 s and then 3000 rpm for 30 Received: July 30, 2017 Revised: September 13, 2017 Published: September 18, 2017 21541

DOI: 10.1021/acs.jpcc.7b07536 J. Phys. Chem. C 2017, 121, 21541−21545

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The Journal of Physical Chemistry C s. After that, the substrates coated with the PbI2 were annealed at 70 °C for 5 min. Then, the FTO with dried PbI2 films were dipped into a CH3NH3I solution (150 mg of CH3NH3I dissolved in 15 mL of 2-propanol) at 70 °C for 3 min. Subsequently, the sample was rinsed with 2-propanol and then spin-coated at 3000 rpm for 30 s. Finally, the samples were annealed at 70 °C for 30 min. After the perovskite layer was prepared on the FTO substrates, the sample was quickly covered by GaN and then to be pressed and fixed.22 In this study, the commercially available p-type Mg-doped GaN had a thickness of 480 nm, a resistivity of 0.29 Ω cm, and a carrier concentration of 5.50 × 1017 cm−3. Finally, to complete the device preparation, the In electrode was used on the GaN with ohmic contacts. Finally, the effective area of the heterojunction was 0.10 cm2. In this study, the morphology and crystallinity of the samples were characterized by field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F) and X-ray diffraction (XRD, D8 FOCUS X-ray diffraction) with Cu Kα radiation at 40 kV and 40 mA, respectively. The absorption spectrum was measured by a UV−vis−NIR spectrophotometer (MPC-3100 SHIMADZ- U) ranging from 300 to 1000 nm. All of the current−voltage (I−V) and current−time (I−T) curves were measured with a Keithley 4200 electrometer. The photoresponse was measured with a 66984 Xe arc source (300 W Oriel) and an Oriel Coriel cornerstone 260 1/4 m monochromator to get various wavelengths of monochromatic light.

Figure 2. (a) Device configuration diagram. (b) Simplified energy band diagram of the CH3NH3PbI3/GaN structure PD under forward bias. (c) I−V curves of the devices measured under dark and light density of 1.42 mW/cm2. (d) The log plot for the I−V curve of (c).

3. RESULTS AND DISCUSSION Figure 1a shows the top-view SEM image of the perovskite film on a FTO substrate. The perovskite film presents a dense and

Figure 3. (a) I−T (current−time) characteristics of the device with different irradiance at 0.2 V. (b) Enlarged view of a single cycle of the I−T curve. Spectral responsivity (c) and detectivity (d) versus different incident wavelengths of the device at 0.2 V. Stability and durability experiment based on the perovskite PD: (e) time course of the change of the photocurrent of the device with different irradiance at 0.2 V and (f) error analysis diagram of (e).

Figure 1. SEM images of CH3NH3PbI3 films made on FTO substrates: (a) top-view image; (b) cross-section-view image. (c) XRD patterns of CH3NH3PbI3 films on FTO substrates. (d) Optical absorption spectra of the FTO/CH3NH3PbI3 structure.

shows the optical absorption spectra of the CH3NH3PbI3 perovskite film by a UV−vis−NIR spectrophotometer. The CH3NH3PbI3 exhibits a wide spectral absorption range with a high absorption value from 300 to 800 nm, implying that the device has a good photoresponse in a wide region. We can also see that there is an absorbance peak at about 500 nm, which may be more suitable to be absorbed by perovskite due to its photon energy not being too big or small and leads to a highest photoresponse around this wavelength.

smooth surface with large grains and a cubic-like shape. The cross section of the device is shown in Figure 1b, which has a 1200 nm perovskite layer covered on a glass substrate with 300 nm of FTO. The crystallinity of our perovskite film on the FTO substrate was analyzed by XRD and is shown in Figure 1c. From the XRD patterns, the perovskite film shows many strong peaks, indicating that it has good crystalline quality. Figure 1d 21542

DOI: 10.1021/acs.jpcc.7b07536 J. Phys. Chem. C 2017, 121, 21541−21545

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Figure 6. (a) I−T curve of the device with 6 nm C60. (b) Stability of the device tested at zero bias at room temperature with air humidity of 50−60%. Spectral responsivity (c) and detectivity (d) curves of the device with 6 nm C60 measured at 0 V.

Figure 4. (a) Device configuration diagram with C60. (b) XRD patterns of FTO, FTO/C60, FTO/CH3NH3PbI3, and FTO/C60/ CH3NH3PbI3. (c) Transmission spectrum curves of FTO substrates with different thicknesses of the C60 layer. (d) Absorption spectrum curves of CH3NH3PbI3/FTO with different thicknesses of C60.

current under the forward biases, the log plot for the I−V curve is shown in Figure 2d. From the figure, the current increased clearly under illumination, and the value is very big. At 0.2 V, the device shows a dark current as low as 5.99 × 10−10 A and photocurrent of 3.48 × 10−8 A. Figure 3a shows the I−T characteristics with a good cycling photoresponse of the device with different irradiance at 0.2 V, and an enlarged view of a single cycle is shown in Figure 3b. The photocurrent is measured under irradiance of 0.03, 0.09, 0.25, 0.61, and 1.42 mW/cm2 at 0.2 V. The photoresponse curves show that the detector is stable, reliable, and repeatable. Also, the photocurrent increases with the increase of the power density. From the enlarged view of a single cycle shown in Figure 3b, with light intensity of 0.25 mW/cm2, the PD shows a rise time of 0.34 s and a decay time of 0.59 s, demonstrating a fast response characteristic. Spectral responsivity (Rλ) and detectivity (D*) are two important parameters to estimate the performance of the PD.

Figure 5. Photoresponse characteristics of devices measured at 0 V and under illumination with an intensity of 0.04 mW/cm2 and the thickness of the C60 layer at (a) 2, (b) 4, (c) 6, and (d) 8 nm.

Rλ and D* are calculated by the formulas Rλ =

D* = Figure 2a shows the structure of the device, and Figure 2b shows the simplified energy band diagram of the device without an ETL. From the band diagram, when forward bias is applied, the band structure of the device will bend, which benefits the transport of photogenerated carriers, resulting in the large photocurrent and good photoresponse. To investigate the photoresponse of the device, the I−V characteristics are measured in the dark and under light illumination with a power density of 1.42 mW/cm2, shown in Figure 2c. Both red and black curves in the dark and under light illumination present good rectifying behavior. From the band structure of the GaN/perovskite heterojunction, we can see that the conduction band of GaN (∼−4.2 eV) is lower than that of perovskite (∼−3.8 eV), and the electrons can flow easily from perovskite to GaN; therefore, the device shows a good Ilight/Idark ratio at forward bias. On the other hand, it is very difficult for the electrons to overcome the barrier from GaN to perovskite. Therefore, the device shows a small Ilight/Idark ratio at reverse bias. To easily observe the photocurrent increase versus dark

Rλ 2qJd

Iph SL light

and

, where Iph, S, Llight, q, and Jd are the photocurrent

density, the irradiance area, the light intensity, the elementary charge, and the dark current density, respectively, and are shown in Figure 3c,d, respectively. At 0.2 V, the device shows the peak responsivity of 0.027 A/W and the peak detectivity of 4.8 × 1011 cm Hz1/2/W, which are located at around 500 nm, which corresponds to the peak spectral absorption of the CH3NH3PbI3. In addition, the visible/UV rejection ratio (R510/ R365) is approximately 16.7, indicating that our PD shows relatively high visible response and had good spectral selectivity. The small visible/UV ratio may be because for our devices under illumination the light will transmit through the perovskite absorber first, and then, the extra photons, especially UV photons, reach GaN, resulting in UV response. Therefore, although many of UV photons will be absorbed by perovskite, there will always be extra UV photons to reach the GaN side, which has been confirmed by little UV responsivity at 365 nm. The photostability of the CH3NH3PbI3 PD was measured at room temperature in air under a relative humidity of 50−60%. As shown in Figure 3e,f, all of the photocurrent is reduced by 21543

DOI: 10.1021/acs.jpcc.7b07536 J. Phys. Chem. C 2017, 121, 21541−21545

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The Journal of Physical Chemistry C Table 1. Performance Parameters Based on CH3NH3PbX3 Photodetectors in This and Previously Reported Work structure

material structure

responsivity (A/W)

detectivity (Jones)

on/off ratio

refs

6 nm C60/CH3NH3PbI3 /GaN FTO/TiO2−CH3NH3P-bI3/FTO PCBM/CH3NH3PbBr3 /PEDOT:PSS FTO/TiO2 /CH3NH3PbX3/spiro/Au CH3NH3PbI3 single crystal ITO/PEDOT/CH3NH3PbX3/PCB-M/C60/Al

film film film film film film

0.198 4.9 × 10−4 0.321 0.4 0.0115 0.21

7.96 × 1012

>5000 72

this work 21 22 23 24 25

less than 1% during 10 min. Also from Figure 3f, we can find that the device shows more stabilty under illumination with lower light intensity, which will lay the foundation for the study of weak light detection. Figure 4a shows the device configuration diagram with C60, and Figure 4b displays XRD patterns of FTO, FTO/C60, FTO/ CH3NH3PbI3, and FTO/C60/CH3NH3PbI3. By comparing the XRD curves of FTO and FTO/C60, there are no different diffraction peaks, indicating that the C60 prepared by vacuum evaporation is amorphous.14 In addition, from the diffraction curves of FTO/C60 and FTO/C60/CH3NH3PbI3, we can see that the diffraction peaks at 14.2, 2°, 23.6, 24.6, 28.2, 28.5, 31.9, 40.6, and 43.2° respectively correspond to the CH3NH3PbI3 perovskite (110), (112), (211), (202), (004), (220), (114), (224), and (314) lattice planes,23 showing that the crystallinity of the CH3NH3PbI3 perovskite thin film is very good and there are no diffraction peaks of PbI2, illustrating that PbI2 was completely CH3NH3PbI3 perovskite. The band gap of C60 is about 1.7 eV, and it is an effective optical absorption layer. In order to achieve higher detection performance of C60 based PDs, the thickness of the C60 layer needs to be optimized. The transmittance of FTO glass with a different thickness of C60 was measured and is shown in Figure 4c. From the curves, with the increase of the C60 layer thickness, the values of the light transmittance decreased. Also, the C60 layer in the short-wavelength region has obvious absorption. Figure 4d shows absorption spectra of FTO/ CH3NH3PbI3 with different thicknesses of C60 layers. As can be seen from the curves, both CH 3 NH 3 PbI 3 and C 60 / CH3NH3PbI3 have very good absorption at 300−800 nm, especially at 300−550 nm. Compared to the single CH3NH3PbI3 layer, the absorption of the film with a C60 layer increased significantly. Also, with the increase of the C60 layer thickness, the light absorption also increased, which may have resulted in a great photocurrent when the C60 layer was applied. In addition, all of the absorption curves showed an absorption edge at about 760 nm, which is attributed to the CH3NH3PbI3 perovskite film. The photoresponse characteristic of the devices was investigated by optimizing the thickness of the C60, and the results are shown in Figure 5a−d (measured at 0 V and under illumination with an intensity of 0.04 mW/cm2). On the basis of the previous investigation, the CH3NH3PbI3/GaN structure without a C60 layer had no obvious photoresponse ability when the bias was 0 V; therefore, we did not consider it. When the thickness of C60 was about 2 or 4 nm, the C60 film was too thin and very hardly covered the surface of FTO, resulting in an unstable photocurrent (seen from Figure 5a,b). When the thickness of the C60 film reached 6 nm, it could be uniformly covered on the FTO surface and the device exhibited a stable current. With increasing thickness of C60, the number of photons entering the perovskite absorber layer decreased, and subsequently, the photocurrent decreased. Also, the series

resistance of the device increased when the thickness of C60 increased, resulting in a decrease of photocurrent, which can be seen from Figure 5d. Therefore, the optimal thickness of C60 is 6 nm. Figure 6a represented the I−T curve of the device with 6 nm C60 at 0 V and under illumination with 0.04 mW/cm2 light intensity. From the curve, the device showed a rise and fall time of 0.45 and 0.63 s, respectively. Also, we could calculate that the on/off ratio was greater than 5000. To investigate the stability, the devices were measured under a long duration of illumination with an air humidity of 50−60% at 0 V, and the results are shown in Figure 6b. The device was illuminated in 1 h, and there was no obvious change in photocurrent, indicating that our device had good durability and could be used for a long time. Spectral responsivity and detectivity curves are shown in Figure 6c,d at 0 V. From the curves, the device showed two peaks located at about 365 and 500 nm, respectively. At 365 nm, the values of the responsivity and detectivity are 0.037 A/ W and 1.47 × 1012 cm Hz1/2/W, respectively, which might be attributed to GaN. Also, we can see that the biggest values of responsivity and detectivity are about 0.198 A/W and 7.96 × 1012 cm Hz1/2/W, respectively, which were attributed to CH3NH3PbI3. For comparison, some reports based on CH3NH3PbX3 PDs are listed in Table 1, from which we can find that the performance of our devices is comparable with the best of them.24−28 From above, the device with 6 nm C60 shows highly selfpowered performance; this is attributed to the optimal thickness of C60. From our experiments, the devices without C60 cannot show self-powered characteristics; the reason may be attributed to the band structure and the interface between the perovskite and FTO substrate. For perovskite, its photoelectrons may prefer p-GaN rather than FTO due to the poor interface between the perovskite and the FTO substrate, which can result in recombination of carriers and reduce photocurrent. However, C60 with a high electron mobility and conductivity can promote the transmission of electrons and reduce nonradiative recombination. Also, it can improve the perovskite/FTO interface, passivate the grain boundary of perovskite, and reduce the density of the trap state. In addition, the energy gap levels of C60 and CH3NH3PbI3 match well, benefiting the carrier transport. Therefore, the device with C60 can result in the self-powered characteristic and effectively improve the device performance.

4. CONCLUSIONS In summary, we prepared a visible PD based on the CH3NH3PbI3/GaN structure with a two-step method growing high-quality CH3NH3PbI3 crystals. The device without an ETL showed high sensitivity, good stability, and repeatability. Particularly, when C60 was applied as the ETL and the device showed a self-powered characteristic. The best self-powered 21544

DOI: 10.1021/acs.jpcc.7b07536 J. Phys. Chem. C 2017, 121, 21541−21545

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of perovskite solar cells via solution-processed metal oxide transport layers. Nat. Nanotechnol. 2015, 11 (1), 75. (12) Zhang, X.; Liu, Q.; Liu, B.; Yang, W.; Li, J.; Niu, P.; Jiang, X. Giant UV photoresponse of a GaN nanowire photodetector through effective Pt nanoparticle coupling. J. Mater. Chem. C 2017, 5 (17), 4319−4326. (13) Abrusci, A.; Stranks, S. D.; Docampo, P.; Yip, H. L.; Jen, A. K. Y.; Snaith, H. J. High-performance perovskite-polymer hybrid solar cells via electronic coupling with fullerene monolayers. Nano Lett. 2013, 13 (7), 3124−3128. (14) Yoon, H.; Kang, S. M.; Lee, J. K.; Choi, M. Hysteresis-free lowtemperature-processed planar perovskite solar cells with 19.1% efficiency. Energy Environ. Sci. 2016, 9 (7), 2262−2266. (15) Liu, Y.; Bag, M.; Renna, L. A.; Page, Z. A.; Kim, P.; Emrick, T.; Venkataraman, D.; Russell, T. P. Understanding interface engineering for high-performance fullerene/perovskite planar heterojunction solar cells. Adv. Energy Mater. 2016, 6, 1501606. (16) Shao, Y.; Xiao, Z.; Bi, C.; Yuan, Y.; Huang, J. Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells. Nat. Commun. 2014, 5, 5784. (17) Liang, P. W.; Chueh, C. C.; Williams, S. T.; Jen, A. K. Y. Roles of fullerene-based interlayers in enhancing the performance of organometal perovskite thin-film solar cells. Adv. Energy Mater. 2015, 5, 1402321. (18) Bi, C.; Yuan, Y.; Fang, Y.; Huang, J. Low-Temperature Fabrication of Efficient Wide-Bandgap Organolead Trihalide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5 (6), 1401616. (19) Li, Y.; Ye, S.; Sun, W.; Yan, W.; Li, Y.; Bian, Z.; Liu, Z.; Wang, S.; Huang, C. Hole-conductor-free planar perovskite solar cells with 16.0% efficiency. J. Mater. Chem. A 2015, 3, 18389. (20) Wojciechowski, K.; Leijtens, T.; Siprova, S.; Schlueter, C.; Hörantner, M. T.; Wang, J. T. W.; Li, C. Z.; Jen, A. K. Y.; Lee, T. L.; Snaith, H. J. C60 as an efficient n-type compact layer in perovskite solar cells. J. Phys. Chem. Lett. 2015, 6, 2399−2405. (21) Xue, M.; Zhou, H.; Xu, Y.; Mei, J.; Yang, L.; Ye, C.; Zhang, J.; Wang, H. High-performance ultraviolet-visible tunable perovskite photodetector based on solar cell structure. Science China Materials 2017, 60 (5), 407−414. (22) Zhou, H.; Gui, P.; Yu, Q.; Mei, J.; Wang, H.; Fang, G. Selfpowered, visible-blind ultraviolet photodetector based on n-ZnO nanorods/i-MgO/p-GaN structure light-emitting diodes. J. Mater. Chem. C 2015, 3, 990−994. (23) Song, Z.; Watthage, S. C.; Phillips, A. B.; Tompkins, B. L.; Ellingson, R. J.; Heben, M. J. Impact of processing temperature and composition on the formation of methylammonium lead iodide perovskites. Chem. Mater. 2015, 27 (13), 4612−4619. (24) Xia, H. R.; Li, J.; Sun, W. T.; Peng, L. M. Organohalide lead perovskite based photodetectors with much enhanced performance. Chem. Commun. 2014, 50 (89), 13695−13697. (25) Bao, C.; Zhu, W.; Yang, J.; Li, F.; Gu, S.; Wang, Y.; Yu, T.; Zhu, J.; Zhou, Y.; Zou, Z. Highly Flexible Self-Powered Organolead Trihalide Perovskite Photodetectors with Gold Nanowire Networks as Transparent Electrodes. ACS Appl. Mater. Interfaces 2016, 8 (36), 23868−23875. (26) Sutherland, B. R.; Johnston, A. K.; Ip, A. H.; Xu, J.; Adinolfi, V.; Kanjanaboos, P.; Sargent, E. H. Sensitive, fast, and stable perovskite photodetectors exploiting interface engineering. ACS Photonics 2015, 2 (8), 1117−1123. (27) Cao, M.; Tian, J.; Cai, Z.; Peng, L.; Yang, L.; Wei, D. Perovskite heterojunction based on CH3NH3PbBr3 single crystal for highsensitive self-powered photodetector. Appl. Phys. Lett. 2016, 109, 233303. (28) Fang, Y.; Huang, J. Resolving Weak Light of Sub-picowatt per Square Centimeter by Hybrid Perovskite Photodetectors Enabled by Noise Reduction. Adv. Mater. 2015, 27 (17), 2804−2810.

device with 6 nm C60 displayed an on/off ratio of greater than 5000, peak responsivity of 0.198 A/W, and detectivity of 7.96 × 1012 cm Hz1/2/W. The high performance mentioned above was attributed mainly to the C60 layer, which can reduce the density of trap states and passivate the grain boundaries of the CH3NH3PbI3 absorbing layer to facilitate intermolecular charge transport.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hao Wang: 0000-0002-4894-7653 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported in part by the National Nature Science Foundation of China (No. 51372075).

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REFERENCES

(1) Dong, H. L.; Zhu, H. F.; Meng, Q.; Gong, X.; Hu, W. P. Organic photoresponse materials and devices. Chem. Soc. Rev. 2012, 41, 1754− 1808. (2) Di, C. A.; Zhang, F. J.; Zhu, D. B. Multi-functional integration of organic field-effect transistors (OFETs): advances and perspectives. Adv. Mater. 2013, 25, 313−330. (3) Yu, J.; Chen, X.; Wang, Y.; Zhou, H.; Xue, M.; Xu, Y.; Li, Z.; Ye, C.; Zhang, J.; van Aken, P. A.; et al. A high-performance self-powered broadband photodetector based on a CH3NH3PbI3 perovskite/ZnO nanorod array heterostructure. J. Mater. Chem. C 2016, 4 (30), 7302− 7308. (4) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, 341− 344. (5) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-range balanced electron-and holetransport lengths in organic-inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (6) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 2013, 52, 9019. (7) Bryant, D.; Aristidou, N.; Pont, S.; Sanchez-Molina, I.; Chotchunangatchaval, T.; Wheeler, S.; Durrant, J. R.; Haque, S. A. Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells. Energy Environ. Sci. 2016, 9 (5), 1655−1660. (8) Song, Z.; Abate, A.; Watthage, S. C.; Liyanage, G. K.; Phillips, A. B.; Steiner, U.; Graetzel, M.; Heben, M. J. Perovskite Solar Cell Stability in Humid Air: Partially Reversible Phase Transitions in the PbI2-CH3NH3I-H2O System. Adv. Energy Mater. 2016, 6 (19), 1600846. (9) Aristidou, N.; Eames, C.; Sanchez-Molina, I.; Bu, X.; Kosco, J.; Islam, M. S.; Haque, S. A. Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells. Nat. Commun. 2017, 8, 15218. (10) Chen, W.; Wu, Y.; Yue, Y.; Liu, J.; Zhang, W.; Yang, X.; Chen, H.; Bi, E.; Ashraful, I.; Grätzel, M.; et al. Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers. Science 2015, 350 (6263), 944−948. (11) You, J.; Meng, L.; Song, T. B.; Guo, T. F.; Chang, W. H.; Hong, Z.; Chen, H.; Zhou, H.; Chen, Q.; Liu, Y.; et al. Improved air stability 21545

DOI: 10.1021/acs.jpcc.7b07536 J. Phys. Chem. C 2017, 121, 21541−21545