Spray-coated CsPbBr3 Quantum Dot Films for Perovskite Photodiodes

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Functional Inorganic Materials and Devices

Spray-coated CsPbBr3 Quantum Dot Films for Perovskite Photodiodes Zhi Yang, Minqiang Wang, Li Junjie, Jinjuan Dou, Hengwei Qiu, and Jinyou Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07334 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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ACS Applied Materials & Interfaces

Spray-coated CsPbBr3 Quantum Dot Films for Perovskite Photodiodes Zhi Yang,1,* Minqiang Wang,1,* Junjie Li,1 Jinjuan Dou,1 Hengwei Qiu,1 Jinyou Shao2 1

Electronic Materials Research Laboratory (EMRL), Key Laboratory of Education Ministry;

International Center for Dielectric Research (ICDR), Shaanxi Engineering Research Center of Advanced Energy Materials and Devices, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China. 2

State Key Laboratory of Manufacturing Systems Engineering, School of Mechanical

Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China. Corresponding Author: [email protected]; [email protected] Keywords: Inorganic perovskite, quantum dots, photodiode, spray deposition Abstract: Large-area film deposition and high material utilization ratio are the crucial factors for large-scale application of perovskite optoelectronics. Recently, all-inorganic halide perovskite CsPbBr3 have attracted great attentions because of their high phase stability, thermal stability and photo-stability. However, most of reported perovskite devices were fabricated by spin-coating suffering from a low material utilization ratio of 1 % and a small coverage area. Here we developed a spray-coating technique to fabricate CsPbBr3 quantum dot (QD) film photodiode which had high material utilization ratio of 32 % and deposition rate of 9 nm/s. The film growth process was studied, and substrate temperature and spray time were two key factors for the deposition of uniform and crack-free QD films. The spray-coated photodiode was demonstrated to be more suitable for working in photodetector mode because a low dark current density of 4×10-4 mA cm-2 resulting from extremely low recombination current contributed to a high detectivity of 1×1014 Jones. A high responsivity of 3 A W-1 was obtained at -0.7 V under 365 nm illumination, resulting from low charge transfer resistance

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and high charge recombination resistance. We believe that spray deposition technique will benefit the fabrication of perovskite QD film optoelectronics in large-scale. 1. Introduction All-inorganic halide perovskite CsPbBr3 have attracted growing interests in a variety of optoelectronic

devices

including

solar

cells,

light-emitting

diodes

(LEDs)

and

photodetectors.1-5 Its high absorption coefficient results from direct bandgap nature of the electronic transition. Its cubic phase stability at room temperature (RT), and thermal- and photo-stability are related to reasonable ion radius and inorganic nature, respectively, which surpass its counterparts such as CsPbI3, CH3NH3PbX3 (X=Cl, Br, I) and CH(NH2)2PbX3 (X=Cl, Br, I).6 Furthermore, CsPbBr3 quantum dots (QDs) have a narrow photoluminescence (PL) emission line width and a high PL quantum yield (PLQY) resulting from low density of defects and traps within the bandgap.7,8 A planar PIN photodiode can be operated in photodetector mode at negative voltage and photovoltaic mode at positive voltage. Stimulated with the great success of perovskite solar cells, there is a rapid progress in the performance of perovskite photodetectors. In 2014, Dou et al. reported the first solution-processed CH3NH3PbI3-xClx perovskite photodetector with a large detectivity of 1014 Jones resulting from proper interface design.9 Dong et al. fabricated CH3NH3PbI3 perovskite photodetector with high responsivity of 203 A W-1 at -1 V and photoconductive gain of 489, and high gain was related to hole traps in the perovskite film top surface.10 Then both Domanski et al. and Chen et al. demonstrated a photocurrent amplification effect because of high photoconductive gain in regular planar structure without a hole blocking layer, but increased dark current contributed to a large noise.11,12 However, all these photodetectors were based on the perovskite film by one-step method and people’s interests focus on optimizing device structure. There are little reports on all-inorganic halide perovskite QD film photodiode detectors which have better air stability. More importantly, the popular method employed for fabricating solution-processed perovskite film is spin-coating which is only suitable for lab2 ACS Paragon Plus Environment

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scale fabrication, and it mainly suffers from low material utilization ratio and limited nmscale film thickness.13 By contrast, spray-coating allows to deposit a large-area film on a variety of substrates including planar and curved surfaces and meshes, and its high material utilization ratio is quite beneficial to improve waste management. This up scalable technique has already been used to successfully fabricate perovskite solar cells by one-step or two-step methods, and traditional PbS and CdTe QD film solar cells.14-21 Differently, colloidal perovskite QDs suffer from easy decomposition during purification process, as well as a substantial aggregation during film formation increasing the difficulty of preparing pinholefree films, so there is still no report on spray deposition of perovskite QD films until now. Spray-coating is the technique to deposit films by applying pressurized gas, including three steps: atomization of the liquid, droplet transport and deposition. Atomization of the liquid is controlled by the flow rate of the aerating gas and nozzle size. A rough surface is obtained if the solvent evaporates totally before droplets arrive to the substrate. In contrast, the QDs are mobile on the surface if a solvent shell still surrounds the QDs upon impingement. Therefore, the optimized nozzle-to-sample-distance (NSD) is needed to guarantee that the substrates locate at the dilute droplet density region, where the droplets are just wet enough but not dissolve the underlying layer. Nevertheless, the deposition is the most difficult to control because drying processes are strongly governed by non-equilibrium kinetics, and the system will under the transient states before the final structure is installed.22-24 Fortunately, grazing incidence small-angle x-ray scattering (GISAXS) developed by Müller-Buschbaum’s group allows for gaining deep insight in the film-forming process in situ.25,26 Their results show that the deposition is affected by several factors including substrate temperature, boiling point of the solvent, spray time and solute concentration. Depending on substrate temperature and boiling point of the solvent, the droplets may rebounce, spread or break into smaller one travelling independently. Besides, with increasing spray time, there are evolutionary film surface morphology and increased film thickness. 3 ACS Paragon Plus Environment


In this work, a spray technique has been developed to deposit large-area uniform and compact CsPbBr3 QD films on rigid and flexible substrates. We have fabricated CsPbBr3 photodiode detectors with regular planar structure based on spray-coated QD films, and the impact of substrate temperature and spray time on film structure and the performances of photovoltaic and photodetector have been investigated. Furthermore, we have compared film characterizations and the device performances of spray-coated and spin-coated QD films. Despite higher roughness, spray-coated QD film device has a close power conversion efficiency (PCE) with spin-coated one, indicating the feasibility of perovskite QD film photodiodes fabricated by spray deposition. In particular, electrochemical impedance spectroscopy (EIS) and theoretical simulation of current density-voltage offer an efficacious but facile method to study the interface condition, perovskite film quality, and the relationship between interfacial charge transfer process and recombination current in perovskite photodiodes. Excitingly, a large-area CsPbBr3 QD films of 10×10 cm2 on the flexible substrate and the high material utilization ratio of 32 % open a door to achieve QD film optoelectronics by spray deposition. 2. Experimental Section Spray deposition: A homemade spray platform was designed to fabricate CsPbBr3 QD films on rigid or flexible substrates. The spray deposition was achieved by using an ANEST IWATA HP-BC1 airbrush equipped with a 0.3-mm nozzle. The input air pressure was controlled preciously using a Ultimus™ I dispensing workstation. In order to achieve the fabrication of QD films with a large area, the substrate was installed on a motorized XY stages, and then the spray coordinates could be controlled preciously. Besides, the substrate was placed onto a hotplate to control the deposition temperature. We adjusted the spray parameters based on the spray deposition steps including liquid atomization, droplet transport and deposition of liquid film. The NSD and carrier gas pressure were set to 4 cm and 20 psi respectively to ensure the substrate located at the dilute regime of droplet density, and the 4 ACS Paragon Plus Environment

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droplets’ size decreases with increasing flow rate. Finally, the whole platform was put into an air-filled plexiglass glove box to avoid the spread of spraying aerosol. Device fabrication: First, a TiO2 layer was deposited on FTO glass by spin-coating an acidic solution of tetrabutyl titanate at 3500 rpm followed by an air-anneal at 400 ℃ for 30 mins. CsPbBr3 QDs were synthesized and purified according to previous recipe.7 The QD hexane solution with a concentration of 0.4 mg mL-1 was put into the siphon-feed bottle reservoir. As a contrast, both spray-coating and spin-coating were employed to deposit CsPbBr3 QD films onto the TiO2/FTO glass substrates. The spray deposition was conducted as mentioned above, and different film thicknesses were controlled by adjusting substrate temperature and deposition time. Next, the QD films were post-treated to improve the conductivity. The QD films were washed with ethyl acetate to remove extra organic ligand, followed by air-anneal at corresponding spray substrate temperatures. The QD films were also prepared by spin-coating method at 2500 rpm for 30 s followed by the same post-treatment process, and 500-nm film thickness was achieved by repeated spin-coating. Subsequently, the hole-transporting layer was prepared by spin-coating a 2,2’,7,7’-tetrakis-(N,N-di-pmethoxyphenylamine)9,9’-spirobifluorene (Spiro-OMeTAD) ethyl alcohol solution with the concentration of 40 mg mL-1 at 4000rpm. Finally, an 80-nm-thick Au electrode was deposited via thermal evaporating. Device Characterization: CsPbBr3 QD film morphologies were characterized by FEI Quatan 250 FEG SEM and Vecco Innova AFM. Optical transmittance data were acquired with a Jasco V-570 UV/Visible/NIR spectrophotometer. The current density-voltage curves and time-dependent photoresponse measurements were recorded by Keithley 2400 source meter. For photovoltaic mode measurement, the samples were illuminated under a 150 W xenon lamp with the intensity of 100 mW/cm2. For photodetector mode measurement, the samples were illuminated using 365 and 525 nm LED sources. The photoresponse linearity was obtained by changing LED illumination intensity, and a standard photodetector (Thorlabs, 5 ACS Paragon Plus Environment


PM-100A) was used to quantify irradiance intensity. The incident photon-to-current conversion efficiency (IPCE) was measured by the solar cell quantum efficiency measurement system (SCS10, Zolix), and the wavelength-dependent responsivity spectra was calculated from the expression responsivity=IPCE×λ/1240. EIS was recorded by using an electrochemical workstation (CHI660D, Shanghai Chenhua). The measured frequency ranged from 1 Hz to 100 kHz, and the amplitudes were set at 0 V and open-circuit voltage (Voc) of 1.2 V, respectively. 3. Results and Discussion We developed a spray platform using a commercial airbrush, as shown in Figure 1a. A fine mist containing oleic acid and oleylamine capped CsPbBr3 QDs with size of 20 nm (Figure S1) dispersed in hexane was atomized using pressurized air gas. Then the mixed liquid-gas flow underwent a rapid expansion after it passed the nozzle, and the droplet density gradually decreased from dense region to dilute region. Finally, QD films were deposited on a substrate once the solvent evaporated. Figure 1b shows spray-coated QD films on a 10×10 cm2 flexible PET and a 1.5×0.5 cm2 FTO glass deposited with TiO2 layer, indicating the large-area coating and the flexibility of spray deposition. The film structures of spray-coated QD films are highly sensitive to several spray parameters. Based on carefully controlled gas pressure, solute concentration, solvent and NSD, we investigated the impact of substrate temperature and spray time in detail. Besides QD films, the preparation of hole-transporting layer can also affect the performance of photodiode device. As shown in Figure S2, we can see increased transmittance at 520 nm for QD films after Spiro-OMeTAD deposited using chlorobenzene as solvent, indicating the partial loss of QDs during spin-coating hole-transporting layer, and thus significantly reduced performance was obtained. Fortunately, this could be effectively avoided by using ethyl alcohol as solvent which has been successfully employed as antisolvent in the QDs’ purification.3,7


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The uniform and compact film can only be obtained at low solution surface tension and contact angle, which can be controlled by adjusting substrate temperature.22 Substrate temperature can also affect perovskite crystallization and modify the film morphology for preparing perovskite film by one-step method, but crystallization need not be considered for perovskite QDs because high crystallinity has been guaranteed during QDs’ synthesis process. From Figure 1c, we can see a large number of cracks and obvious clusters for QD film spraycoated at RT, which is caused by that mobile QDs can coagulate with the assistance of excessive solvent due to slow solvent evaporation. The cracked film is also found for 60 oC substrate in Figure S3a, b because it is still lower than the boiling point of hexane. When substrate temperatures are increased to 90 and 120 oC in Figure 1d and Figure S3c, d, we can see dense crack-free QD films, which is a result of QDs’ uniform deposition with the assistance of fast solvent evaporation. However, the partial cracks can be observed when substrate temperature further increases to 150 oC in Figure S3e, f, which may result from too fast solvent evaporation before the droplet impingement. Transmittance spectra can be used as a quantitative tool to monitor the thickness of QD films in Figure 1e. There is an obviously decreased transmittance with increasing substrate temperatures, illustrating increased film thickness in accordance with the cross-section scanning electron microscope (SEM) results in Figure S4. Figure 1f and Figure S7 show that substrate temperatures can mainly affect shortcircuit current (JSC) and dark current. We can see nearly linearly increased thickness in Figure 1g, but the maximum JSC is obtained at 90 oC. The decreased JSC for 120 and 150 oC may result from that too thick film will decrease photo-generated carrier separation efficiency. It is worth noting that RT spray-coated QD film photodiode cannot work normally performing very low JSC and high dark current because of a large number of cracks and thin thickness, illustrating the significant effect of substrate temperature on the performance of spray-coated device. We also calculated the high film deposition rate of 9 nm/s and the large material utilization ratio of 32 % for spray-coated QD films at 90 oC. 7 ACS Paragon Plus Environment


Another parameter spray time was also studied to understand the QD film growth process clearly and we investigated its effect on film morphology and device performance. We characterize the film evolution in Figure 2a according to the SEM images in Figure 2b-e and Figure S5. It is quite difficult to understand the early film growth because of thin thickness and the morphology interference from bottom TiO2 layer. We can observe a uniform and compact QD film spray-coated for 15 s, but increased number of clusters are found when spray time increases to 30 s. With continuously increasing spray time to 45 and 60 s, we can find decreased number but increased size of clusters, indicating the growth of original QD clusters. However, a small number of pinholes are observed for QD film spray-coated for 90 s, and there are increased number and size of pinholes when spray time further increases to 120 and 150 s, which is a signal of film quality deterioration. This structure evolution is quite similar with spray-coated ZnO film monitored by in situ GISAXS.27 The coffee ring structures are quite common for spray-coated film, resulting from rapid solvent drying upon contact with the heated substrate.21 Fortunately, we have not found coffee ring structures by monitoring different film growth stages, illustrating that suitable spray parameters were chosen. Moreover, we can see an obviously decreased transmittance with increasing spray time in Figure 2f, which is also a result of increased film thickness in accordance with the cross-section SEM results in Figure S6. There are nearly complete absorptions for QD films spray-coated for 120 and 150 s when the wavelength is shorter than 520 nm, indicating their enough thickness to capture high-energy light. The current density-voltage curves of photodiodes fabricated with different spray time in dark and under AM1.5 illumination are shown in Figure S8 and Figure 2g, respectively. There are similar dark current curves for photodiodes spray-coated with different time, and the rectification ratio of 10 at ±1 V is small because large turn-on voltage of diffusion region lead to a small current at 1 V, and the current density of 10-3 mA cm-2 at -1 V is small enough compared with other perovskite photodiodes.9,10 Finally, the linearly increased film thickness with increasing spray time in 8 ACS Paragon Plus Environment

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Figure 2h provides a powerful evidence to indicate the substrate locating at the linear growth region, because an irregular spray time-thickness dependence will be obtained for inappropriate NSD. Interestingly, the maximum JSC is obtained at 90 s because the thickness of 780 nm is enough large to absorb light and the photo-generated carrier separation efficiency will be decreased when film thickness further increases far exceeding charge diffusion length. Besides as-mentioned technique-level advantages, spray-coating relies on the solution with a low concentration of 0.4 mg mL-1 while a much higher concentration of 40 mg mL-1 is required for spin-coating (details in Supporting Information). Such high concentration for spin-coating lead to an unstable solution state due to easy-aggregation, increasing the difficulty to prepare compact film. In order to evaluate the quality of spray-coated QD film, we compared its roughness and device performance with spin-coated QD film. From atomic force microscopy (AFM) in Figure 3a, we can see local buckling for spray-coated QD film with an average roughness of 15 nm, which is not as smooth as spin-coated one. Despite this, spray-coated QD film is compact enough to meet the requirement of building planar heterostructure. From Figure 3b, the champion PCE of 2.64 % and 3.19 % are obtained for spray-coated and spin-coated photodiodes working in photovoltaic mode, respectively. Obviously, spray-coated device has a reduced performance, and lower JSC instead of VOC is the main reason, in accordance with lower IPCE in Figure S9. In order to explain these differences, the EIS spectra of devices in dark (Figure 3c) and under illumination (Figure 3d) were used to investigate the interfacial charge transfer process. The high-frequency arc is related to charge transfer resistance (Rct) in the device, while the low-frequency arc is related to charge recombination resistance (Rr) generally taking place at the interfaces or within the perovskite QD film layer,28 as-summarized in Table S1. We can see decreased Rct under increased applied bias voltage or illumination, resulting from the separation and extraction of more carriers, while the similar decreased Rr is caused by significantly increased free charge 9 ACS Paragon Plus Environment


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carriers in the perovskite layer, producing opposing electric field at the interfacial of TiO2 and perovskite.29 Under the same bias voltage of 0 V or VOC and the condition of dark or illumination, spray-coated device has a larger Rct than spin-coated one, which is possibly caused by thicker QD film thickness or increased interface recombination due to large roughness, explaining a lower JSC for spray-coated device. Surprisingly, a larger Rr for spraycoated device indicate low recombination rate, which is also supported by that the low frequency spectrum merges with the high frequency according to Yadav et al.’s analysis.29 Furthermore, there are nearly equal peak frequencies for spray-coated and spin-coated devices (Figure S10), indicating the same charge transfer rates at the transporting layer/perovskite interface. Next, a new double heterojunctions equivalent circuit developed by Liao et al. was employed to simulate the current density-voltage characteristic of planar structure perovskite PIN photodiode.30 We fitted the dark current density-voltage curves of diodes in Figure 4a using equation 1, and the fitting values of Rsh, Jr and Jd are summarized in Table 1. Larger Jr and Jd of spray-coated photodiode suggest its larger charge recombination rate at the interface of perovskite and hole-transporting layer, which may be caused by a larger QD film roughness. qV

qV

V J= + J r (e mr KT − 1) + J d (e md KT − 1) Rsh

(1)

where J is the current density, V is the voltgae, Rsh is the shunt resistance, Jr is the recombination current density, Jd is the diffusion current density, q is the elementary charge, mr and md are the ideal factors, K is the Boltzmann constant, T is the absolute temperature. Regions Ⅰ, Ⅰ and Ⅰ in Figure 4a are related to shunt current (the first term), recombination current (the second term) and diffusion current (the third term), respectively. The four parameters Rsh, Rs, m1+m2 and J0 can be obtained in Figure 4b (plot of equation 2) and c (plot of equation 3) based on photocurrent density-voltage curves in Figure 3b. 10 ACS Paragon Plus Environment

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dV 1 + Rsh−1 dV ( m1 + m2 ) KT dJ ) + R ( = s dJ q J SC − J − V Rsh ln( J SC − J − V

Rsh

)=

q (V + JRs ) + ln J 0 ( m1 + m2 ) KT

(2)

(3)

where m1 and m2 are the ideal factors, JSC is the short-circuit current density, Rs is the series resistance, J0 is the reverse saturation current density. Theoretically, for a single heterojunction model, the smaller value of ideal factor reflects the less carrier recombination at the interface, and it approaches to 2 when the diode current is dominated by carrier indirect recombination in depleted space-charge region.30 Both spray-coated and spin-coated diodes show the carriers’ indirect recombination dominant behavior which is supported by the m1+m2 of 4 using a double heterojunction model. J0 reflects the thermal emission rate of electrons which is directly related to the recombination rate.31 The obtained J0 in Table 1 is at the same order with reported CH3NH3PbI3 perovskite devices,10,30,31 suggesting acceptable recombination rate for both diodes. Based on the CsPbBr3 film’s bandgap of 2.25 eV and VOC of 1.29 eV, the calculated bandgap-to-VOC loss is about 1 eV, which is larger than reported CsPbI3 and CsPb(Br/I)3 devices but remains a typical level for CsPbBr3 device.32-36 The small J0 explains relatively high VOC, approaching to the highest reported value of 1.5 V.37 Overall, very close m1+m2 and J0 of two diodes illustrate the reliable quality of spray-coated QD films. Having established the optimized spray deposition parameters, we carefully evaluated the key performance parameters of photodiode detector: responsivity, gain, linear dynamic range (LDR) and temporal response. First, the current density-voltage curves of photodiode under 365 and 525 nm are shown in Figure 5a and b. The hysteretic behavior strongly depending on voltage sweep rate can be clearly observed for this planar perovskite film photodiode. The faster the scan rate is, the larger the hysteresis becomes, which is in accordance with Domanski et al.’s result.11 Besides, when scanning in backward direction, we can observe larger photocurrent for three scan rates regardless of scanning order and illumination



wavelength. The intersections of current density scanned in backward and forward directions are about -0.7 V for different scan rates and wavelengths, which is suitable as the working voltages of photodiodes because this photocurrent is no depending on scan direction. Next, we can calculate LDR from the dependence of photocurrent on incidence intensity in Figure 5c. There are same LDR of 90 dB for both 365 and 525 nm illumination, which is larger than our reported CsPbBr3 nanosheets photoconductor detector.5 Then, the wavelength-dependent responsivity, gain and photocurrent shown in Figure 5d, e and Figure S11 agree well with the profile derived from the absorption curves, and monotonically increased responsivity and gain with increasing applied voltages can be obtained. At -0.7 V, the responsivities reach 1.2 and 3 A W-1 and gains are 3 and 10 at 525 and 365 nm, respectively. According to the simplified formula of detectivity,38 the calculated detectivity is 1×1014 Jones at 365 nm. The high detectivity of photodetectors mainly depends on extremely low dark current which is ultimately limited by the recombination current, and small recombination current of 1.3×10-7 mA cm-2 is responsible for high detectivity. Although the responsivity of spray-coated CsPbBr3 QD film photodiode detector is smaller than reported CH3NH3PbI3 photodiode because of its lower absorption coefficient and carrier diffusion length,10-12,39 it is still larger than the largest responsivity of 0.53 A W-1 for CsPbBr3 nanosheet photoconductor detector and 0.01 A W-1 for Au-plasma enhanced CsPbBr3 QD photoconductor detector,5,40 as summarized in Table S2. The gain exceeding 1 is a result of photocurrent amplification in photodiode. Finally, fast response and recovery speeds are shown in Figure 5f, resulting from intrinsic built-in electric field of photodiode. Overall, the high gain in photodetector mode and the competitive PCE in photovoltaic mode of spray-coated CsPbBr3 QD film diode demonstrates the great potential of high-performance QD film photoelectronics fabricated via spray deposition method. The device performance can be further improved on basis of perovskite QD films with more low roughness. Future works can focus on minimizing size of clusters by adjusting wetting contact angle such as using mixed solvent or plasma treatment. 12 ACS Paragon Plus Environment

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4. Conclusions In summary, we have fabricated CsPbBr3 QD film photodiodes based on spray-coating technique in air. The dense crack-free QD film was obtained at 90 oC substrate, and there was nearly linearly increased film thickness and JSC with increasing substrate temperatures. We have observed increased number of clusters during the morphology evolution and increased film thickness of spray-coated QD films, and the maximum JSC was obtained at 90 s. Despite higher roughness of 15 nm, when photodiode worked in photovoltaic mode under positive bias voltage, spray-coated device had a close PCE with spin-coated one. The slightly inferior PCE of spray-coated device is caused by a lower JSC resulting from larger Rct and Rsh by EIS analysis. From the physical viewpoint, a higher recombination rate is responsible for the reduced PCE on the basis of larger Jr and J0 from the theoretical simulation of dark and photo current density-voltage curves. When photodiode worked in photodetector mode at -0.7 V, responsivity, detectivity, gain and LDR were 3 A W-1, 1×1014 Jones, 10 and 90 dB at 365 nm LED illumination, respectively. The high detectivity of photodetector mainly depends on extremely low recombination current of 1.3×10-7 mA cm-2. The high material utilization ratio of 32 %, large-area coating on flexible surfaces, high deposition rate of 9 nm/s and low-cost instrument solidify the spray deposition as a highly competitive technique to fabricate perovskite QD film photodiodes with high performance.

Supporting Information Material utilization ratio calculation; SEM images of QD films obtained at different substrate temperatures and time; transmittance spectra; current density-voltage curves in dark and under AM 1.5 illumination; bode phase plots; IPCE curves; wavelength-dependent photocurrent density curves; device performance comparison.

Acknowledgements



This research was financial supported by Natural Science Foundation of China (NSFC Grant Nos. 61604122, 51572216 and 61774124), China Postdoctoral Science Foundation (Grant 2017M613139) and 111 Program (No. B14040).

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(28) Yang, Z.; Wang, M.; Ding, J.; Sun, Z.; Li, L.; Huang, J.; Liu, J.; Shao, J. Semitransparent ZnO-CuI/CuSCN Photodiode Detector with Narrow-band UV Photoresponse. ACS Appl. Mater. Interfaces 2015, 7, 21235-21244. (29) Yadav, P.; Alotaibi, M. H.; Arora, N.; Dar, M. I.; Zakeeruddin, S. M.; Grätzel, M. Influence of the Nature of A Cation on Dynamics of Charge Transfer Processes in Perovskite Solar Cells. Adv. Funct. Mater. 2018, 28, 1706073. (30) Liao, P.; Zhao, X.; Li, G.; Shen, Y.; Wang, M. A New Method for Fitting CurrentVoltage Curves of Planar Heterojunction Perovskite Solar Cells. Nano-Micro Lett. 2018, 10, 5. (31) Li, Y.; Xu, Z.; Zhao, S.; Qiao, B.; Huang, D.; Zhao, L.; Zhao, J.; Wang, P.; Zhu, Y.; Li, X.; Liu, X.; Xu, X. Highly Efficient p-i-n Perovskite Solar Cells Utilizing Novel LowTemperature Solution-Processed Hole Transport Materials with Linear π-Conjugated Structure. Small 2016, 12, 4902-4908. (32) Zeng, Q.; Zhang, X.; Feng, X.; Lu, S.; Chen, Z.; Yong, X.; Redfern, S. A. T.; Wei, H.; Wang, H.; Shen, H.; Zhang, W.; Zheng, W.; Zhang, H.; Tse, J. S.; Yang, B. PolymerPassivated Inorganic Cesium Lead Mixed-Halide Perovskites for Stable and Efficient Solar Cells with High Open-Circuit Voltage over 1.3 V. Adv. Mater. 2018, 30, 1705393. (33) Liang, J.; Wang, C.; Wang, Y.; Xu, Z.; Lu, Z.; Ma, Y.; Zhu, H.; Hu, Y.; Xiao, C.; Yi, X.; Zhu, G.; Lv, H.; Ma, L.; Chen, T.; Tie, Z.; Jin, Z.; Liu, J. All-inorganic Perovskite Solar Cells. J. Am. Chem. Soc. 2016, 138, 15829-15832. (34) Kulbak, M.; Gupta, S.; Kedem, N.; Levine, I.; Bendikov, T.; Hodes, G.; Cahen, D. Cesium Enhances Long-term Stability of Lead Bromide Perovskite-based Solar Cells. J. Phys. Chem. Lett. 2016, 7, 167-172. (350 Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells. J. Phys. Chem. Lett. 2015, 6, 24522456. (36) Hoffman, J. B.; Zaiats, G.; Wappes, I.; Kamat, P. V. CsPbBr3 Solar Cells: Controlled Film Growth through Layer-by-Layer Quantum Dot Deposition. Chem. Mater. 2017, 29, 9767-9774. (37) Akkerman, Q. A.; Gandini, M.; Stasio, F. D.; Rastogi, P.; Palazon, F.; Bertoni, G.; Ball, J. M.; Prato, M.; Petrozza, A. Manna, L. Strongly Emissive Perovskite Nanocrystal Inks for High-voltage Solar Cells. Nat. Energy 2017, 2, 16194. (38) Yang, Z.; Wang, M.; Song, X.; Yan, G.; Ding, Y.; Bai, J. High-performance ZnO/Ag Nanowire/ZnO Composite Film UV Photodetectors with Large Area and Low Operating Voltage. J. Mater. Chem. C 2014, 2, 4312-4319. (39) Lin, Q.; Armin, A.; Lyons, D. M.; Burn, P. L.; Meredith, P. Low Noise, IR-Blind Organohalide Perovskite Photodiodes for Visible Light Detection and Imaging. Adv. Mater. 2015, 27, 2060-2064. (40) Dong, Y.; Gu, Y.; Zou, Y.; Song, J.; Xu, L.; Li, J.; Xue, J.; Li, X.; Zeng, H. Improving All-Inorganic Perovskite Photodetectors by Preferred Orientation and Plasmonic Effect. Small 2016, 12, 5622-5632.


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Figure 1. Characterization of CsPbBr3 QD films spray-coated with different substrate temperatures. (a) Schematic of spray deposition equipment. (b) Photograph of spray-coated QD films on flexible or rigid substrates. Flexible substrate was PET with the size of 10×10 cm2, and rigid substrate was FTO glass deposited with TiO2 layer. The device had an active area of 0.2×0.2 cm2. Plain-view SEM images of spray-coated QD films on rigid substrate fabricated at the substrate temperature of (c) RT and (d) 90 oC. The insets are corresponding high magnification SEM images. (e) Transmittance spectra of QD films spray-coated at different substrate temperatures. (f) Current density-voltage curves of devices under AM 1.5 illumination. (g) Thickness of QD films and Jsc of devices as a function of substrate temperatures, and 5 devices were counted for every temperature. All the QD films have a same spray time of 60 s.



Figure 2. Characterization of CsPbBr3 QD films spray-coated with different time. (a) Schematic for the growth of QD films in spray deposition. SEM images of QD films spraycoated with different time (b) 15 s, (c) 30 s, (d) 60 s and (e) 90 s. (f) Transmittance spectra of spray-coated QD films. (g) Current density-voltage curves of devices under AM 1.5 illumination. (h) Thickness of QD films and Jsc of devices as a function of spray time, and 5 devices were counted for every time. All the QD films have a same spray substrate temperature of 90 oC.


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Figure 3. (a) Top and angled AFM views of spin-coated and spray-coated QD films. (b) Current density-voltage curves of the photodiodes working in photovoltaic mode. Nyquist plots of the photodiode measured at different bias voltages under the condition of (c) dark and (d) AM 1.5 illumination.



Figure 4. (a) Plots of dark current density-voltage curves of diode and the fitting curves. (b) Plots of -dV/dJ vs (1+Rsh-1dV/dJ)/(JSC-J-V/Rsh) and the fitting curves. (c) Plots of ln(JSC-JV/Rsh) vs V+JRs and the fitting curves. All fitting curves are located around the VOC. The inset in (c) is the equivalent circuit based on improved double pn junction model for perovskite diode.


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Figure 5. Photoresponse performance of photodiode working in photodetector mode. The effect of scan rate and direction on the current density-voltage curves of diodes under (a) 365 nm and (b) 525 nm LED illumination with the intensity of 4.3 mW/cm2. The backward scanning is defined as measuring from 1 V to -1 V, while the forward scanning is opposite. (c) The dependence of photocurrent at -0.7 V on incidence intensity. (d) The wavelengthdependent responsivity and (e) the wavelength-dependent gain of diode under different bias voltages. (f) Time-dependent photoresponse curves at -0.7 V. The CsPbBr3 QD film was fabricated with spray substrate temperature of 90 oC and spray time of 90 s.

Table 1. The diode electric parameters of spray-coated and spin-coated photodiodes derived from the fitting curves in Figure 4. Device

Rs

Rsh 2

J0 2

m1+m2 -2

Jr

Jd -2

(mA cm ) (mA cm-2)

(Ω cm ) (Ω cm ) (mA cm ) Spray-dark

-

3.5×105

-

-

1.3×10-7

3.9×10-13

Spin-dark

-

1.7×105

-

-

2.1×10-8

1.7×10-14

Spray-light

3.4

1094

8.7×10-6

4

-

-

4.1

-

-

Spin-light

14.1

628

4.5×10

-6



ToC Figure


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Figure 1. Characterization of CsPbBr3 QD films spray-coated with different substrate temperatures. (a) Schematic of spray deposition equipment. (b) Photograph of spray-coated QD films on flexible or rigid substrates. Flexible substrate was PET with the size of 10×10 cm2, and rigid substrate was FTO glass deposited with TiO2 layer. The device had an active area of 0.2×0.2 cm2. Plain-view SEM images of spraycoated QD films on rigid substrate fabricated at the substrate temperature of (c) RT and (d) 90 oC. The insets are corresponding high magnification SEM images. (e) Transmittance spectra of QD films spraycoated at different substrate temperatures. (f) Current density-voltage curves of devices under AM 1.5 illumination. (g) Thickness of QD films and Jsc of devices as a function of substrate temperatures, and 5 devices were counted for every temperature. All the QD films have a same spray time of 60 s. 145x82mm (300 x 300 DPI)

ACS Paragon Plus Environment


Figure 2. Characterization of CsPbBr3 QD films spray-coated with different time. (a) Schematic for the growth of QD films in spray deposition. SEM images of QD films spray-coated with different time (b) 15 s, (c) 30 s, (d) 60 s and (e) 90 s. (f) Transmittance spectra of spray-coated QD films. (g) Current densityvoltage curves of devices under AM 1.5 illumination. (h) Thickness of QD films and Jsc of devices as a function of spray time, and 5 devices were counted for every time. All the QD films have a same spray substrate temperature of 90 oC.

167x109mm (300 x 300 DPI)


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Figure 3. (a) Top and angled AFM views of spin-coated and spray-coated QD films. (b) Current densityvoltage curves of the photodiodes working in photovoltaic mode. Nyquist plots of the photodiode measured at different bias voltages under the condition of (c) dark and (d) AM 1.5 illumination. 87x70mm (300 x 300 DPI)



Figure 4. (a) Plots of dark current density-voltage curves of diode and the fitting curves. (b) Plots of -dV/dJ vs (1+Rsh-1dV/dJ)/(JSC-J-V/Rsh) and the fitting curves. (c) Plots of ln(JSC-J-V/Rsh) vs V+JRs and the fitting curves. All fitting curves are located around the VOC. The inset in (c) is the equivalent circuit based on improved double pn junction model for perovskite diode. 68x152mm (300 x 300 DPI)


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Figure 5. Photoresponse performance of photodiode working in photodetector mode. The effect of scan rate and direction on the current density-voltage curves of diodes under (a) 365 nm and (b) 525 nm LED illumination with the intensity of 4.3 mW/cm2. The backward scanning is defined as measuring from 1 V to 1 V, while the forward scanning is opposite. (c) The dependence of photocurrent at -0.7 V on incidence intensity. (d) The wavelength-dependent responsivity and (e) the wavelength-dependent gain of diode under different bias voltages. (f) Time-dependent photoresponse curves at -0.7 V. The CsPbBr3 QD film was fabricated with spray substrate temperature of 90 oC and spray time of 90 s. 124x65mm (300 x 300 DPI)



75x50mm (300 x 300 DPI)


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Spray-coated CsPbBr3 Quantum Dot Films for Perovskite Photodiodes

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