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Novel antisolvent-washing strategy for highly efficient carbon-based planar CsPbBr3 perovskite solar cells
Journal of Power Sources 439 (2019) 227092
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Novel antisolvent-washing strategy for highly efficient carbon-based planar CsPbBr3 perovskite solar cells Xingyue Liu a, Zhiyong Liu a, Xianhua Tan a, Haibo Ye a, Bo Sun a, Shuang Xi b, Tielin Shi a, Zirong Tang a, Guanglan Liao a, c, * a b c
State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, 430074, China School of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing, 210037, China Shenzhen Huazhong University of Science and Technology Research Institute, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Antisolvent washing strategy is created to prepare high-quality CsPbBr3 films. � An excellent PCE of 8.55% is achieved for the carbon-based CsPbBr3 PSCs. � Superb moisture and thermal stabilities are obtained for our devices. � The use of CuPc HTL and carbon elec trode helps to reduce the production costs.
A R T I C L E I N F O
A B S T R A C T
Keywords: Antisolvent-washing CsPbBr3 Carbon-based Planar heterojunction High efficiency and stability Low costs
All-inorganic CsPbBr3 is attracting tremendous attentions in photovoltaic field due to its superior stability. However, CsPbBr3 perovskite always suffers from a poor crystallinity and film morphology. Many efforts have been paid on the CsBr deposition process to improve the film quality, while few attentions are paid on the crystallization kinetics of the PbBr2 framework film. Here, we demonstrate a novel antisolvent-washing strategy for the PbBr2 film for the first time to fabricate high-quality CsPbBr3 film. This technique has a significant impact on the nucleation and growth of PbBr2 crystals. As-prepared CsPbBr3 films exhibit more homogeneous with higher crystallinity and coverage as well as larger grain sizes compared to those untreated ones. The bestperforming antisolvent-treated perovskite solar cell achieves a scanned power conversion efficiency of 8.55%, which is an excellent efficiency for planar CsPbBr3 cells reported yet. This enhancement can be mainly attributed to the more effective charge transport and suppressed non-radiative recombination caused by the reduced defect densities. Moreover, our devices show superb stability when stored in air for 1000 h and upon persistent thermal attack at 80 � C. Our work provides a new train of thought for controlling the growth dynamics and film morphology of CsPbBr3 films.
* Corresponding author. State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, 430074, China. E-mail address: [email protected] (G. Liao). https://doi.org/10.1016/j.jpowsour.2019.227092 Received 25 February 2019; Received in revised form 19 August 2019; Accepted 31 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
performance of all-inorganic CsPbBr3 PSCs was also boosted by inter facial passivation for the CsPbBr3 layers using quantum dots and charge transport materials, etc [22–25]. Most of the researches on CsPbBr3 PSCs were focused on the modification of the CsBr deposition process and the passivation of the already formed CsPbBr3 films, while few attentions had been paid on the crystallization dynamics of the pre-formed PbBr2 films. Yan’s group used pyridine-vapor to treat PbBr2 film during its annealing process [26]. The as-formed intermediate phase could reduce the formation energy of the final CsPbBr3 perovskite and the post-annealing temperature was down to 160 � C. However, both the phase-purity and morphology of the CsPbBr3 films became even worse and only a low champion PCE of 6.05% was achieved for the as-prepared devices. Various processing treatments, including antisolvent washing (or solvent engineering), vapor-assisted annealing, vacuum flash annealing, delayed-annealing and gas blowing have been reported to significantly ameliorate the crystallization process and fabricate high-quality perov skite films [27–30]. Among them, antisolvent washing is considered as the most effective and easily operated approach. Herein, we develop an elegant antisolvent-washing treatment for the PbBr2 film preparation for the first time. The delay time of dropping CB antisolvent is investigated and optimized. It is found that the CB droplet could remove the excess N, N-Dimethylformamide (DMF) solvent in the wet PbBr2 film immediately during the spin-coating process, inducing a fast nucleation and modified crystallization of PbBr2, and thus a highly covered PbBr2 film [31]. Consequently, the resultant CsPbBr3 film shows a more homogeneous morphology with higher crystallinity and coverage as well as larger average grain size, compared to the counterparts prepared without the antisolvent treatment. The best-performing antisolvent-treated PSC achieves a superior PCE of 8.55% under reverse scan directions with all the photovoltaic parameters get augmented. By contrast, the untreated counterparts only gain a champion PCE of 6.94%. This enhancement can be mainly attributed to the more effective charge transport and sup pressed non-radiative recombination rate caused by the improved film quality with decreased current shunting pathways and trap states in the CsPbBr3 film. Moreover, the unencapsulated all-inorganic CsPbBr3 de vice exhibits no performance degradation when stored in ambient con dition for over 1000 h at room temperature (~25 � C). Upon persistent thermal attack at 80 � C in air for one month, the as-prepared device still retains 95.2% of its initial PCE. Our work paves the way for the practical application of cost-effective, highly efficient and stable all-inorganic CsPbBr3 PSCs.
Organic-inorganic hybrid perovskites have shown great potential to rival multicrystalline silicon for photovoltaic applications due to their unique photovoltaic properties, such as tunable bandgap, high light absorption coefficient, low exciton binding energy, benign defect properties as well as superior tolerance to defects [1–5]. Although a power conversion efficiency (PCE) over 23% has been achieved for perovskite solar cells (PSCs) [6], the stability issue, especially for the thermal stability under operational condition, still set a big barrier for the commercialization of PSCs. Replacing the organic components (MAþ, FAþ, etc) with inorganic ions (such as Csþ, Rbþ, etc) to form pure inorganic perovskites should be a feasible strategy to ameliorate this situation [7]. Among all the inorganic perovskites, CsPbI3 and CsPbIxBr3-x are the most widely researched materials these days owing to their good thermal stability and suitable bandgap, based on which the PCEs over 17% and 14.8% have been obtained, respectively [8,9]. However, both the CsPbI3 and CsPbIxBr3-x can easily convert into non-perovskite phase in air at room temperature, hampering their practical application. All-inorganic CsPbBr3, with outstanding moisture, oxygen and thermal stability under harsh conditions, emerges as a competitive candidate of light absorbers that is attracting widespread attentions. CsPbBr3 PSC was firstly proposed in 2015 and delivered a PCE of 5.95% with poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport layer (HTL) and Au electrode [10]. Though satisfactory thermal stability was obtained, the high costs of state-of-the-art hole transport material PTAA and noble metal Au set another obstacle for its mass production. Liu’s group devised a carbon-based CsPbBr3 PSC with a structure of compact TiO2/mesoporous TiO2/CsPbBr3/Carbon for the first time and got a PCE of 6.7% [11]. The abandon of the expensive HTL and noble metal electrode was beneficial to low down the production costs. This structure was also regarded as a classical structure for CsPbBr3 devices and widely adopted by other groups [12–14]. Apart from the intrinsic large bandgap of CsPbBr3 perovskite (2.3 eV), the PCEs of CsPbBr3 PSCs were also limited by the low crystallinities and poor morphologies of the CsPbBr3 active layers fabricated in conven tional two-step solution route [10,15]. This could be mainly ascribed to the quite low solubility of the CsBr precursor in currently available solvents, resulting in a lack of sufficient raw materials in the formation process of CsPbBr3 and thus a discontinuous film morphology [16]. Many researches have already been carried out to improve the crystal lization process of CsPbPb3 films. Chang et al. successfully prepared highly crystalline CsPbBr3 films by optimizing the reaction time and temperature during the CsBr-immersion process [13]. But the PSCs only gained a champion PCE of 5.0%. Teng et al. developed a facile face-down liquid-space-restricted deposition approach to prevent the decomposition of CsPbBr3 in the CsBr methanol solution in 2018 [17]. The resultant film morphology and light harvesting ability of CsPbBr3 were greatly enhanced. The as-prepared devices yielded a PCE of 5.86%, which was an excellent efficiency for planar CsPbBr3 PSC. Recently, Tang’s group creatively proposed a multistep spin-coating method to fabricate high-quality CsPbBr3 films [15]. Both the crystallinity and phase-purity were pronouncedly augmented and the best-performing device demonstrated a high PCE of 9.72%. The same group further enhanced the PCE of CsPbBr3 PSCs to over 10% through metal ion doping, surface modification and spectra engineering [14,18–20]. Zeng et al. presented a space-confined growth strategy for the controllable growth of the polycrystalline CsPbBr3 perovskites by freezing the pre cursor solution within the gaps of ordered polystyrene sphere templates [16]. The fabrication of dense CsPbBr3 films with relatively low trap density and high carrier mobility were realized. Our group also intro duced a modified multistep deposition technique on the basis of the traditional two-step deposition route [21]. The improved phase-purity and film morphology of the CsPbBr3 perovskites led to a much enhanced PCE for the carbon-based planar PSCs. Besides, the
2. Experimental section 2.1. Materials Lead bromide (PbBr2, �99.99%) and cesium bromide (CsBr, �99.99%) are bought from Xi’an p-OLED. Titanium tetrachloride (TiCl4, �99.5%), N, N-Dimethylformamide (DMF, �99.9%), dimethylsulfoxide (DMSO, �99.9%), and copper (II) phthalocyanine (CuPc) are purchased from Aladdin. Chlorobenzene (�99.8%) is from Alfar Aesar. NiCl2⋅6H2O (�98.0%) is bought from Sinopharm Chemical Reagent Co., Ltd and the commercial carbon paste is from Shenzhen Dongdalai Chemical Co., Ltd. All the chemicals and reagents are directly used without any further purification. 2.2. Device fabrication The conductive fluorine-doped tin oxide (FTO) glass substrates (NSG-10) were chemically etched with 2 M hydrochloric acid and zinc powder. Then, the FTO substrates were sequentially cleaned with detergent, acetone, anhydrous alcohol and deionized water in an ul trasonic bath each for 15 min, followed by an O3/ultraviolet treatment for 30 min to remove the organic residues. The compact TiO2 electron transport layers (ETLs) were prepared through the hydrolysis of the 2
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Journal of Power Sources 439 (2019) 227092
TiCl4 aqueous solution as we reported before [32]. The TiCl4 (stored in the freezer at about 0 � C) was diluted to 200 mM with 0.01 M NiCl2⋅6H2O being added. The cleaned FTO substrates were then placed vertically in a glass container filled with TiCl4 precursor solution and kept at 70 � C for 3 h in a thermostat water bath. Subsequently, the substrates were washed with deionized water in a sonication bath for 5 min to remove any loosely bound materials, and then annealed at 200 � C on a hotplate for 1 h. The CsPbBr3 perovskites were deposited on the ETL by a multistep spin-coating method. 1.0 M PbBr2 in DMF solu tion was kept at 75 � C and the substrate was also pre-heated at 75 � C before the PbBr2 spin-coating process. Then, the PbBr2 solution was spin-coated on the compact TiO2 ETLs at a speed of 2000 rpm for 30 s. For the antisolvent-treated PSCs, 115 μL chlorobenzene was dropped onto the substrates at delay times of 2, 5 and 8 s from the starts of the PbBr2 spin-coating process. Afterwards, the substrates were heated at 90 � C for 1 h. The CsBr was deposited via a multistep solution-processing solution method reported by Tang’s group [15]. A 0.07 M CsBr methanol solution was spin-coated onto the PbBr2 films at a speed of 2000 rpm for 30 s, followed by being annealed at 250 � C for 5 min. After repeating this process for five times, the films were cleaned with isopropanol (IPA) at a speed of 2000 rpm for 30 s. Then, the substrates were annealed at 250 � C for another 5 min. For the preparation of the hole transport layer (HTL), 35 nm CuPc was thermally evaporated on the CsPbBr3 films under a base pressure of less than 3 � 10 8 Torr at a speed of ~0.5 Å/s determined by a quartz crystal monitor. Eventually, the commercial carbon paste was deposited on the prepared film by doctor-blading technique and then dried at 100 � C for 15 min to form the counter electrodes of the devices.
Kα radiation (λ ¼ 1.5418 Å) at 25� and the data were collected with a 0.013� step size (2θ). The absorbance spectra of the CsPbBr3 films were obtained by a UV–visible spectrophotometer (UV 2600, Shimadzu). The atomic force microscopy (AFM) images of the untreated and antisolventtreated PbBr2 films were obtained by an Innova SPM 9700 (Shimadzu, Japan) in a tapping mode. The surface morphologies of the PbBr2 and CsPbBr3 films and the cross-sectional images of the whole devices were examined by the scanning electron microscopy (SEM, Sirion 200, FEI, Heland and GeminiSEM 300, Carl Zeiss, German). The current densityvoltage (J-V) characteristics were recorded at a sweep rate of 10 mV/s by using a computer-controlled electrochemical station (Autolab PGSTA302 N, Netherlands) under simulated AM 1.5G (100 mW/cm2) one sunlight illumination generated by a solar simulator (Oriel 94043A, Newport Corporation, Irvine, CA, USA), which was calibrated by a NREL-traceable KG5 filtered silicon reference cell. The active area of the cells in the measurements were defined by a metal mask with an aper ture area of 0.071 cm2. The open-circuit photovoltage decay (OCVD), capacitance-voltage as well as the electrochemical impedance spec troscopy (EIS) measurements were also performed on this equipment. The EIS spectra were fitted via the Nova software. Moreover, the steadystate photoluminescence (PL) and time-resolved photoluminescence (TR-PL) decay spectroscopy were measured via FluoTime300 (Pico Quant, German). 3. Results and discussion Fig. 1a shows the schematic illustration of the antisolvent-assisted CsPbBr3 fabrication process. A PbBr2 precursor in DMF (1 M) is firstly spin-coated onto the compact TiO2 ETLs with the CB antisolvent drop ped onto the wet film at varied delay times from the start of the spincoating process. The role of CB droplet is to rapidly remove the excess DMF solvent in the wet PbBr2 film during the PbBr2 spin-coating process,
2.3. Device characterization The X-ray diffraction (XRD) measurements were carried out on a x’pert3 powder X-ray diffractometer (PANalytical, Netherland) with Cu
Fig. 1. (a) Schematic illustration of the fabrication process of the CsPbBr3 films with the PbBr2 films treated by CB antisolvent. (b) XRD patterns of the FTO substrate and CsPbBr3 films with CB dropped at different delay times from the start of the spin-coating process. (c) UV–vis absorption spectra of the CsPbBr3 films treated by CB antisolvent at varied delay times. The bandgap calculation of the 5s-treated CsPbBr3 film is shown in the inset. 3
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thus reducing the solubility of PbBr2 in the mixed solvent. The residual vacancy caused by the evaporation of DMF can promote the crystal growth of the PbBr2 crystals [31]. Therefore, this treatment indeed contributes to a more uniform nucleation and a modified crystallization of the PbBr2 crystal compared to the untreated counterparts. Then the CsBr methanol solution is deposited by a multistep spin-coating tech nique reported previously to form CsPbBr3 films [15]. It is observed that the color of the PbBr2 films changes from white to orange-yellow after the intercalation reaction with CsBr, suggesting the formation of CsPbBr3 perovskites. The arrow at the bottom of Fig. 1a indicates the phase transition in the whole spin-coating process, which can be usually described into the following reactions [15]: 2PbBr2 þ CsBr → CsPb2Br5 (with excess amount of PbBr2)
(1)
CsPb2Br5 þ CsBr → 2CsPbBr3
(2)
CsPbBr3 þ 3CsBr → Cs4PbBr6 (when CsBr is excessive)
(3)
impact the crystallization of PbBr2 film a lot, in line with the XRD re sults. The light absorption properties of the CsPbBr3 films with their PbBr2 precursor films treated by antisolvent washing at varied delay times are further studied (Fig. 1c). All the films exhibit an approximate absorption band edge of ~535 nm, corresponding to a calculated bandgap of 2.32 eV as depicted in the inset. It is clear that the 5s-treated CsPbBr3 film possesses the strongest light harvesting capability, which can be mainly attributed to the higher crystallinity and phase-purity as well as higher film coverage (will be discussed later) of the film than those of other films. This is beneficial to yield a higher photocurrent output. The 2s-treated CsPbBr3 film even shows a weaker light absorp tion due to its lower crystallinity and film coverage (Fig. S2b) than that of the untreated counterpart (Fig. 2b). As one of the reactants of CsPbBr3, PbBr2 film not only provides the kinetically favorable nucleation centers for the formation of the CsPbBr3 perovskite, but also acts as a framework for the crystal growth [35]. Thus, both the film crystallization process and morphology will be greatly affected by the pre-deposited PbBr2 layer. The deposition of a crystalline PbBr2 film usually contains two main processes: nucleation and crystal growth. In order to fabricate homogeneous and highly covered PbBr2 films, it is necessary to obtain numerous and uniformly-distributed nucleus before crystal growth [36]. Increasing the supersaturation of the precursor solution during the spin-coating process by antisolvent washing is an effective method to achieve this. The scanning electron microscopy (SEM) images of the PbBr2 films treated by CB at varied delay times and the corresponding CsPbBr3 films based on them are given in Fig. 2. There exists many micro-scale holes in the PbBr2 film deposited without antisolvent treatment (Fig. 2a) due to the nonuniform nucleation. Consequently, the resultant CsPbBr3 film ex hibits a poor morphology and coverage with a small average grain size of 593 nm and a great number of grain boundaries (Fig. 2b). The pinholes existing in the CsPbBr3 film tend to serve as the current shunting path ways, leading to a severe leakage current and thus a low charge collection efficiency [32]. The cross-sectional SEM image of the CsPbBr3 film (Fig. 2c) also reveals the existence of lots of grain boundaries inside the film, which is consisting of many small disorderly arranged grains. Note that grain boundaries in the polycrystalline perovskites are widely considered to be responsible for causing non-radiative recombination and trapping of charge carriers [37]. When CB antisolvent is introduced at a delay time of 2s, the PbBr2 film with a much rougher surface with a root-mean-square roughness (RMS) of 98.8 nm (Fig. S3b) is obtained after spin-coating. The final CsPbBr3 film based on it even gets a lower coverage than the untreated film (Fig. S2b). This indicates the prema ture antisolvent washing won’t be favorable for the crystallization of both the PbBr2 and the resultant CsPbBr3 films, in agreement with above XRD analysis. When CB treatment is introduced within stage 2 (5s), the PbBr2 precursor solution gets highly concentrated and becomes super saturated rapidly. This provokes a faster and more uniform nucleation of PbBr2 compared to the untreated counterpart. The as-prepared PbBr2 film achieves a much higher coverage with a smoother surface (Fig. S3c) as well as decreased hole numbers and sizes (Fig. 2d). As a result, there is no any pinholes can be observed in the final CsPbBr3 film as shown in Fig. 2e. The average grain size of the homogeneous CsPbBr3 film is also boosted to 1.08 μm with much reduced grain boundaries. The increased grain sizes could affect both the surface and bulk properties of the CsPbBr3 perovskites, including enhancing the crystallinity, reducing the electronic trap states at the surface and in the bulk as well as suppressing the structural defects associated with the pinholes [38]. This is condu cive to the charge transport and suppression of carrier recombination, contributing to a higher short-circuit current density (Jsc) and open-circuit voltage (Voc). The cross-sectional SEM image of the 5s-treated CsPbBr3 film (Fig. 2f) further confirms the close packing of the vertical- and monolayer-aligned perovskite grains. This enables the photoinduced carriers to transport across the ETL/HTL towards the electrode in the out-of-plane direction without passing through grain boundaries [21]. Therefore, the 5s-treated CsPbBr3-based devices are
The crystallization kinetics of the resultant CsPbBr3 films based on the PbBr2 films treated by CB antisolvent at delay times of 2, 5 and 8 s (denoted as 2s-, 5s- and 8s-treated films hereafter) are detailedly investigated by X-ray diffraction (XRD) measurements, as depicted in Fig. 1b. All the as-prepared perovskites present diffraction peaks at 15.19� , 21.58� , 30.69� , 34.48� , 44.10� and 49.56� , which can be assigned to the (100), (110), (200), (210), (220), and (310) lattice planes of the CsPbBr3 phase (PDF#54–0752), respectively. Peaks at 11.60� , 24.03� , 29.38� and 30.29� can also be obviously observed for these films, corresponding to the (002), (202), (213), (221) lattice planes of the CsPb2Br5 phase (PDF#25–0211). Apart from the incomplete reac tion with CsBr, the existence of the PbBr2-rich CsPb2Br5 phase may be ascribed to the partial phase transition from CsPbBr3 to tetragonal CsPb2Br5 under thermal attack (beyond 150 � C) in the post-annealing process [33]. The small diffraction peaks at 12.68� , 12.89� , 25.55� , 27.56� and 28.69� also identify the formation of the CsBr-rich Cs4PbBr6 phase (PDF#54–0750), coinciding well with the previous report [18]. For the 2s-treated CsPbBr3 film, all the peaks corresponding to the CsPbBr3 phase even become weaker compared to the untreated film, suggesting a lower crystallinity. When CB antisolvent is dropped at a delay time of 5s, the peaks of CsPbBr3 phase get much stronger and sharper while those of the CsPb2Br5 and Cs4PbBr6 phase become weaker. This reflects that the antisolvent washing at 5s can promote the crys tallization process of CsPbBr3 perovskite, leading to a higher crystal linity and phase-purity. Further increasing the delay time to 8s, the crystallinity of the 8s-treated CsPbBr3 film only shows a slight improvement while no distinct change is observed in the XRD charac teristics compared to the untreated film. The different crystalline properties of these CsPbBr3 films are closely related to the PbBr2 seed layers treated by antisolvent washing at varied delay times. The XRD patterns of the corresponding PbBr2 films (Fig. S1) also shown that the 5s-treated PbBr2 film delivers a much higher peak intensity than the other samples, especially in (130) and (140) orientations, reflecting a higher crystallinity of the film. By contrast, the antisolvent-washing at delay times of 2 and 5 s hasn’t exhibited an obvious impact on the crystallization of the PbBr2 films. Generally, in the first several seconds (0–3s, stage 1) of the PbBr2 spin-coating process, the dominant action is removing the excess DMF solvent. Introduction of CB antisolvent washing at this stage won’t promote the nucleation rate of PbBr2 since the precursor solution is far from supersaturation [34]. On the contrary, premature CB treatment is even harmful for the CsPbBr3 growth as revealed by the XRD patterns. In 4–6 s after spin-coating (stage 2), CB antisolvent washing can significantly facilitate the evaporation of the residue DMF solvent, inducing a faster and more uniform nucleation and better crystal growth. This may in turn promotes the crystallinity of the final CsPbBr3 films. After spin-coating for over 7s (stage 3), the wet film starts to dry with the nucleation process finished and heterogeneous crystallization occurring. Dropping of antisolvent at this stage may not 4
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Journal of Power Sources 439 (2019) 227092
Fig. 2. (a) Top-view SEM images of the PbBr2 film prepared without antisolvent treatment. The corresponding (b) top-view and (c) cross-sectional SEM images of the untreated CsPbBr3 film. Top-view SEM images of the (d) PbBr2 and (e) CsPbBr3 films, and (f) the resultant cross-sectional SEM images of the CsPbBr3 film treated by CB antisolvent at a delay time of 5s.
expected to deliver a much enhanced charge extraction efficiency and reduced recombination rates compared with the untreated counterparts. The film quality of the 8s-treated PbBr2 and CsPbBr3 only gets a slight improvement since the CB dropped at stage 3 won’t promote the nucleation process effectively as discussed above. The steady-state photoluminescence (PL) measurements are per formed to detect the trap states in the as-prepared CsPbBr3 light absorber layers [39]. Since the CsPbBr3 perovskites are directly depos ited on FTO substrates without charge transport layer, the photoinduced carriers in the excited state cannot be extracted out quickly and result in
a radiative recombination. A higher PL intensity is always related to fewer traps or defects [40,41]. The 5s-treated sample shows the highest PL intensity at around 530 nm (Fig. 3a), indicating the fewest trap states in the film. This enhancement in PL intensity can be attributed to the modification effect of CB antisolvent washing on the crystallinity and grain boundaries, resulting in much reduced trapped states. The 2s-treated CsPbBr3 film exhibits the strongest PL quenching for there are most defects existing in the grain boundaries and pinholes of the film as evidenced by SEM images (Figs. S2a–b). Besides, the slight blue-shift in the PL peak of the 5s-treated film further confirms the decreased defect
Fig. 3. (a) Steady-state PL and (b) TR-PL spectra of the CsPbBr3 films deposited on FTO substrates with CB antisolvent dropped at different delay times. (c) Crosssectional SEM image and (d) the corresponding energy diagram of the whole device. 5
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densities compared to the untreated counterpart [42–44]. The corre sponding time-resolved photoluminescence (TR-PL) measurements of the same samples are also carried out as presented in Fig. 3b, from which the carrier transport dynamics and carrier lifetimes can be investigated. The intralayer carrier transport process in the perovskites is closely related to the impurity level at the grain boundaries [45]. In specific, a longer carrier lifetime reflects a slower intralayer recombination rate caused by a lower impurity level in the CsPbBr3 film. The PL lifetime can be well fitted by a bi-exponential decay function [21]:
result, the average PCE of the 5s-treated devices is increased from 6.23% to 7.99% compared to the untreated counterparts. This enhancement can be mainly attributed to the faster charge transfer and slower carrier recombination rates caused by less trap states in the 5s-treated CsPbBr3 films as discussed above. The narrower PCE distribution of the 5streated devices, with a smaller standard deviation of 0.33 than of the untreated PSCs (0.45), also suggests a higher reproducibility of them. It is expected to be related to the much improved crystallization properties and film qualities of the 5s-treated CsPbBr3 perovskites. Fig. 5a depicts the current density-voltage (J-V) characteristics of the best-performing untreated and 5s-treated PSCs. The untreated devices only gain a highest PCE of 6.94%, with a Jsc of 7.36 mA/cm2, a Voc of 1.292 V and a FF of 0.730. After treated by CB washing at a delay time of 5s, the champion PCE of the PSCs is boosted to 8.55% measured under reverse scan directions, with a Jsc of 7.92 mA/cm2, a Voc of 1.362 V and an excellent FF of 0.793. The incident-photo-conversion-efficiency (IPCE) measurements of them (Fig. S5) further reveal the enhance ment in Jsc for the 5s-treated devices. Since the hysteresis behaviors are existing in both the untreated and 5s-treated PSCs (Fig. S6), the steadystate photocurrent and PCE outputs at the maximum power point are recorded to estimate the efficiency of the devices accurately [41,46,47]. As shown in Fig. 5b, the current and PCE outputs of the best-performing untreated and 5s-treated cells rise up quickly to the maximum once the light is turned on. A stabilized PCE of 6.16% with a current density of 5.70 mA/cm2 is obtained at an applied voltage bias of 1.08 V for the untreated devices. After treated by CB, the 5s-treated device delivers a higher stabilized PCE of 7.95% corresponding to a photocurrent density of 6.74 mA/cm2 at a voltage bias of 1.18 V. In addition, the ratio of the stabilized to the scanned efficiencies for the untreated and 5s-treated devices are about 0.89 and 0.93, respectively. The larger ratio of the 5s-treated device reveals a smaller hysteresis in the device [48]. This can be attributed to a faster charge extraction efficiency and more sup pressed interfacial charge accumulation [49], originating from the less trap states in the 5s-treated CsPbBr3 films. The dark J-V measurements of the untreated and 5s-treated devices are further performed and the results are shown in Fig. 5c. Both of the PSCs exhibit a typical diode behavior with almost no dark current is detected under negative voltage bias. The 5s-treated device shows a smaller leakage current than the untreated counterpart, suggesting a higher shunt resistance of the former device. This can be ascribed to the better film quality of the 5s-treated CsPbBr3 perovskite with less pin holes and grain boundaries as certified by SEM images (Fig. 2). The suppressed dark current is favorable for the realization of a higher Jsc and FF [50]. It has been reported that the intercept of the linear portion of the dark J-V curve to the voltage axis corresponds to the Voc of a PSC [40]. The Voc extracted from the dark J-V curves of the best-performing untreated and 5s-treated devices are about 1.32 and 1.40 V, respec tively, close to the J-V measurements obtained under one sunlight illumination. The fragile stability of PSCs under working conditions is still a major impediments for the practical application of PSCs. Here, the moisture and thermal stability tests of the optimized 5s-treated devices without any encapsulation are conducted in ambient air. The as-prepared PSC exhibits a superb stability with no decline in PCE being observed when stored in air (~40% relative humidity, 25 � C) for over 1000 h (Fig. 5d). This can be mainly attributed to the introduction of the chemically stable CuPc HTL and carbon counter electrode (CE). Besides, the highly hydrophobic and dense CuPc and commercial carbon layer can effec tively protect the perovskite from moisture and oxygen damage [32,51]. Upon persistent thermal attack at 80 � C in air for one month, the device only shows a slight performance degradation, still retaining 95.2% of its initial PCE. The excellent thermal stability can be explained from two aspects. On one hand, the CuPc hole transport materials possesses higher intrinsic thermal stability than the widely-used polymer and small molecule hole conductors like Spiro-OMeTAD and PTAA [52]. On the other hand, all-inorganic CsPbBr3 itself is highly thermal-stable with a
(4)
f(t) ¼ A1exp(-t/τ1) þ A2exp(-t/τ1)þB
where τ1 and τ2 are the slow and fast decay time constants, whilst A1, A2 are the corresponding fractional amplitudes of τ1 and τ2, respectively. The fast decay is generally relevant to the carrier quenching process at the interface while the slow decay component can reflect the trapassisted radiative recombination in the bulk perovskite phase, respec tively [21]. The lifetime parameters of carriers in these films extracted from the TR-PL decay curves are summarized in Table 1. The τ1 and τ2 of the CsPbBr3 perovskite prepared without CB treatment are 19.75 and 6.17 ns, respectively, yielding an average carrier lifetime of 11.86 ns. The 2s-treated CsPbBr3 delivers the lowest average lifetime of merely 11.14 ns due to the heavy trap-assisted recombination, in line with the quickest PL quenching (Fig. 3a). When CB antisolvent is dropped at a delay time of 5s, the average carrier lifetime of the perovskite increases to 16.88 ns with a τ1 and τ2 of 25.32 and 6.93 ns, respectively. The average carrier lifetime of the 8s-treated perovskite also gets a slight augmentation to 13.45 ns compared to that of the untreated CsPbBr3. The largest τ1 value of the 5s-treated film reveals the effective sup pression of the trap-assisted recombination rates within the bulk perovskite, suggesting the lowest defect densities in the film. This is in favor of improving the charge extraction and collection efficiency, leading to a higher Jsc and Voc. Fig. 3c illustrates the cross-sectional SEM images of the as-fabricated planar heterojunction PSC with a structure of FTO/Ni–TiO2/CsPbBr3/CuPc/carbon. The Ni-doped compact TiO2 ETL was prepared by a chemical bath deposition method as we previously reported [32]. The relatively rougher surface of the TiO2 film than that of the widely used spin-coated TiO2 conduces to the deposition of CsPbBr3 since CsPbBr3 film is easy to fall off from a low-roughness surface (certified by our experiments as shown in Fig. S4). The thick ness of the CsPbBr3 absorber layer is about 450 nm while the CuPc HTL is too thin to be observed. The energy diagram of the whole device is drawn in Fig. 3d. The cascade-like conduction band (CB) alignment between the FTO, Ni–TiO2 and CsPbBr3 is beneficial for the electron transportation and collection, whilst the higher CB and valence band (VB) of CuPc than that of CsPbBr3 is conducive to blocking electrons and extracting holes. The key photovoltaic parameters, including Jsc, Voc, fill factor (FF) and PCE, of the untreated and CB-treated PSCs are recorded under simulated AM 1.5G (100 mW/cm2) illumination as listed in Table 2. It appears that the 5s-treated devices possess the best performance while the 2s-treated ones exhibit the lowest efficiency. To highlight the pro motion effect of antisolvent washing on the device performance, the box charts exhibiting the statistical features of the key parameters for the untreated and 5s-treated PSCs are given in Fig. 4. The introduction of CB treatment at a delay time of 5 s brings about a comprehensive improvement of all the photovoltaic parameters for the devices. As a Table 1 Lifetime parameters extracted from the TR-PL spectroscopy for the CsPbBr3 perovskites with CB antisolvent dropped at different delay times. Sample
τave [ns]
τ1 [ns]
A1
τ2 [ns]
A2
FTO/untreated CsPbBr3 FTO/2s-treated CsPbBr3 FTO/5s-treated CsPbBr3 FTO/8s-treated CsPbBr3
11.86 11.14 16.88 13.45
19.75 19.28 25.32 21.84
41.9% 39.0% 54.1% 44.3%
6.17 5.94 6.93 6.65
58.1% 61.0% 45.9% 56.7%
6
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Table 2 Photovoltaic parameters of the PSCs with CB antisolvent treated at varied delay times. These data were extracted from 25 devices prepared in a batch. Device Untreated 2s-treated 5s-treated 8s-treated
average champion stabilized average champion stabilized average champion stabilized average champion stabilized
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
7.02 � 0.59 7.36 5.70 6.81 � 0.72 7.25 5.37 7.79 � 0.42 7.92 6.74 7.17 � 0.48 7.48 5.87
1.255 � 0.049 1.292 1.08 1.246 � 0.057 1.268 1.06 1.325 � 0.041 1.362 1.18 1.276 � 0.045 1.315 1.12
0.707 � 0.048 0.730
6.23 � 0.83 6.94 6.16 6.02 � 0.95 6.68 5.69 7.99 � 0.64 8.55 7.95% 6.52 � 0.73 7.29 6.57
0.710 � 0.041 0.727 0.774 � 0.023 0.793 0.713 � 0.037 0.741
Fig. 4. Box-charts of the (a) Jsc, (b) Voc, (c) FF and (d) PCE of 25 untreated and 5s-treated cells, respectively.
high decomposition temperature over 467 � C [21]. Since the CuPc and commercial carbon used are inexpensive, our devices show great po tentials in practical application with the merits of cost-effective, easy- to-preparation as well as highly efficient and robust. Electrochemical impedance spectroscopy (EIS) measurements are employed to get an insight into the interfacial charge transfer dynamics in the untreated and 5s-treated PSCs. Fig. 6a presents the Nyquist plots recorded at a bias potential of 0.8 V under one sunlight illumination with a frequency range from 1 MHz to 0.01 Hz. It is clear that there exists two main arcs in each Nyquist plot. Generally, the small arc at high frequency is considered to relate to the hole transport at the interface of perovskite/HTL or HTL/electrode, reflecting the charge transfer resis tance (Rct) in parallel with a HTM capacitance (CPE1). The large arc at low frequency is relevant to the electron recombination process at the TiO2/perovskite interface, corresponding to a recombination resistance (Rrec) in parallel with a chemical capacitance (CPE2) [39,43,53]. Based on the analysis, the simplified equivalent circuit is drawn in the inset of Fig. 6a. As fitted by this model, the 5s-treated device gets a lower Rct of 87.5 Ω than that of the untreated counterpart (143 Ω), indicating a faster charge extraction. The Rrec of the 5s-treated PSC is promoted from 1.21
to 1.68 kΩ after CB treatment, suggesting to a lower recombination rate. The enhanced charge transport and suppressed carrier recombination contribute to a higher Jsc and Voc. The Rs evaluated from the starting point in the real part of the Nyquist plots for the 5s-treated and untreated devices is around 36.4 and 38.5 Ω, respectively. The lower Rs is conducive to the achievement of a higher FF. In addition, the smaller chemical capacitance of the 5s-treated device (194 nF/cm2) than that of the untreated device (312 nF/cm2) further reveals the less interfacial charge accumulation exists in the former device. This can be mainly attributed to the improved charge transfer capability and reduced trap states of the CsPbBr3 film after treated by CB antisolvent. The overall enhancement in Jsc, Voc and FF yield a higher PCE for the 5s-treated devices, which is in good accordance with the J-V measurements. The open-circuit photovoltage decay (OCVD) measurements are conducted for both the untreated and 5s-treated device and the results are plotted in Fig. 6b. It appears that the 5s-treated cell possesses a higher intrinsic Voc and a slower photovoltage decay than the untreated one. The carrier lifetimes (τn) of the devices can be roughly evaluated according to the following formula [54]:
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Journal of Power Sources 439 (2019) 227092
Fig. 5. (a) J-V curves and (b) steady-state photocurrent and PCE outputs of the best-performing untreated and 5s-treated PSCs recorded under AM 1.5G illumination. (c) Dark J-V characteristics of the untreated and 5s-treated PSCs. (d) PCE variations of the 5s-treated PSCs when stored in ambient air with a relative humidity of ~40% at room temperature (25 � C) and 80 � C, respectively. Fig. 6. (a) EIS measurements of the untreated and 5s-treated PSCs measured under light illumination with the simplified equivalent circuit depicted in the inset. (b) The OCVD measurements of the untreated and 5s-treated devices. Inset shows the calculated carrier lifetimes under varied voltage of them. (c) Voc variations of the untreated and 5s-treated PSCs recorded under different light intensity. (d) MottSchottky plots at different applied bias voltages extracted from the impedance analysis of the un treated and 5s-treated devices, respectively.
τn ¼
kB T dVoc ð Þ q dt
1
and positive elementary charge, respectively. The calculated carrier lifetimes of the untreated and 5s-treated devices under varied voltage are presented in the inset of Fig. 6b. Obviously, the 5s-treated device
(6)
where kB is the Bolzman constant, T and q are the absolute temperature 8
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Journal of Power Sources 439 (2019) 227092
possesses a much longer carrier lifetime with a lower interface recom bination rate, corresponding to a higher Voc. The interface charge recombination process in the PSC can also be assessed by ideality factor (n). It can be reflected by plotting the Voc as a function of the light in tensity [25]. The Voc of PSC usually decreases rapidly at a low light intensity (ψ ) of less than 0.1 sun (10 mW/cm2), due to the current shunting and trap-assisted recombination [55]. When the ψ is larger than 0.1 sun, a reduction in Voc is related to the interfacial recombina tion. The Voc vs. light intensity for the untreated and 5s-treated devices is illustrated in Fig. 6c. The n can be calculated by the following equation [56]: n¼
q dVoc kB T dInðψ Þ
Conflicts of interest There are no conflicts of interest to declare. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant nos. 51675210, 51675209 and 51805195), the China Postdoctoral Science Foundation (Grant no. 2018M640691) and Fund from Science, Technology and Innovation Commission of Shenzhen Municipality (Grant no. JCYJ20170818165724025). We also appreciate the Analytical and Testing Center and Flexible Electronics Research Center of Huazhong University of Science and Technology for the SEM, XRD and steady-state PL measurements.
(7)
The ideality factor of the 5s-treated is 1.31 while that of the un treated counterpart is 2.13. The decreased n value of the 5s-treated device demonstrates the lower interface charge recombination in the PSC. This may be attributed to the fewer defects existing in the interface of TiO2/CsPbBr3 and CsPbBr3/HTL for the 5s-treated device. Further more, capacitance-voltage measurements are performed under dark condition to investigate the properties of the interface of the as-prepared PSCs. The built-in potential of the 5s-treated device (1.38 V) is higher than that of the untreated counterpart (1.31 V), as evaluated from the intercept of the linear portion of the Mott-Schottky plots (Fig. 6d) to the voltage axis [21]. This enhancement in built-in potential may be ascribed to the decreased defects in the interface of the 5s-treated de vices. A larger built-in potential is expected to facilitate the separation of photoinduced carriers and thus decrease the interfacial charge accu mulation and carrier recombination [57]. The enlarged built-in poten tial also provides an extended depletion region to hamper the back transfer of electrons from the TiO2 ETL to the CsPbBr3 light absorber layer. Therefore, the antisolvent treated devices tend to deliver a higher Voc compared to the untreated counterparts, in good agreement with the J-V characteristics recorded under one sunlight illumination.
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4. Conclusions In this study, we propose an elegant antisolvent-washing strategy for high-quality PbBr2 film preparation for the first time. The delay time of dropping CB antisolvent is detailedly investigated and the optimized value is 5s. The CB droplet can remove the excess DMF solvent in the wet PbBr2 film immediately, resulting in a fast nucleation of PbBr2 crystal and thus a highly covered PbBr2 film. The resultant CsPbBr3 film based on the CB-treated PbBr2 exhibits a more homogeneous morphology with higher crystallinity and coverage as well as larger average grain size (1.08 μm) than the untreated film. As a consequence, all the photovol taic parameters of the devices based on the 5s-treated CsPbBr3 perov skite get augmented. The best-performing 5s-treated PSC demonstrates an excellent scanned PCE of 8.55% with a Jsc of 7.92 mA/cm2, a Voc of 1.362 V and a high FF of 0.793. By contrast, the untreated counterparts only obtains a champion PCE of 6.94% with a Jsc of 7.36 mA/cm2, a Voc of 1.292 V and a FF of 0.730. The 5s-treated PSC also achieves a higher stabilized PCE output of 7.95% than that of the untreated one (6.16%), getting an increase by 29.1%. This enhancement can be mainly attrib uted to the more effective charge transport and suppressed non-radiative recombination rates caused by the improved film quality with decreased trap states of the CsPbBr3 perovskites. Moreover, the unencapsulated CsPbBr3 device shows no decline in PCE when stored in air for over 1000 h at room temperature (~25 � C). Upon persistent thermal attack at 80 � C in air, the as-prepared device still retains 95.2% of its initial PCE for one month. Our work provides an effective processing approach for fabricating high-quality CsPbBr3 films and accelerate the practical application of cost-effective, highly efficient and stable all-inorganic CsPbBr3 PSCs. Besides, the as-fabricated high-quality and robust CsPbBr3 films also show a great potential for their application in highperformance tandem solar cells and photodetectors. 9
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Novel antisolvent-washing strategy for highly efficient carbon-based planar CsPbBr3 perovskite solar cells Xingyue Liu a, Zhiyong Liu a, Xianhua Tan a, Haibo Ye a, Bo Sun a, Shuang Xi b, Tielin Shi a, Zirong Tang a, Guanglan Liao a, c, * a b c
State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, 430074, China School of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing, 210037, China Shenzhen Huazhong University of Science and Technology Research Institute, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Antisolvent washing strategy is created to prepare high-quality CsPbBr3 films. � An excellent PCE of 8.55% is achieved for the carbon-based CsPbBr3 PSCs. � Superb moisture and thermal stabilities are obtained for our devices. � The use of CuPc HTL and carbon elec trode helps to reduce the production costs.
A R T I C L E I N F O
A B S T R A C T
Keywords: Antisolvent-washing CsPbBr3 Carbon-based Planar heterojunction High efficiency and stability Low costs
All-inorganic CsPbBr3 is attracting tremendous attentions in photovoltaic field due to its superior stability. However, CsPbBr3 perovskite always suffers from a poor crystallinity and film morphology. Many efforts have been paid on the CsBr deposition process to improve the film quality, while few attentions are paid on the crystallization kinetics of the PbBr2 framework film. Here, we demonstrate a novel antisolvent-washing strategy for the PbBr2 film for the first time to fabricate high-quality CsPbBr3 film. This technique has a significant impact on the nucleation and growth of PbBr2 crystals. As-prepared CsPbBr3 films exhibit more homogeneous with higher crystallinity and coverage as well as larger grain sizes compared to those untreated ones. The bestperforming antisolvent-treated perovskite solar cell achieves a scanned power conversion efficiency of 8.55%, which is an excellent efficiency for planar CsPbBr3 cells reported yet. This enhancement can be mainly attributed to the more effective charge transport and suppressed non-radiative recombination caused by the reduced defect densities. Moreover, our devices show superb stability when stored in air for 1000 h and upon persistent thermal attack at 80 � C. Our work provides a new train of thought for controlling the growth dynamics and film morphology of CsPbBr3 films.
* Corresponding author. State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, 430074, China. E-mail address: [email protected] (G. Liao). https://doi.org/10.1016/j.jpowsour.2019.227092 Received 25 February 2019; Received in revised form 19 August 2019; Accepted 31 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
performance of all-inorganic CsPbBr3 PSCs was also boosted by inter facial passivation for the CsPbBr3 layers using quantum dots and charge transport materials, etc [22–25]. Most of the researches on CsPbBr3 PSCs were focused on the modification of the CsBr deposition process and the passivation of the already formed CsPbBr3 films, while few attentions had been paid on the crystallization dynamics of the pre-formed PbBr2 films. Yan’s group used pyridine-vapor to treat PbBr2 film during its annealing process [26]. The as-formed intermediate phase could reduce the formation energy of the final CsPbBr3 perovskite and the post-annealing temperature was down to 160 � C. However, both the phase-purity and morphology of the CsPbBr3 films became even worse and only a low champion PCE of 6.05% was achieved for the as-prepared devices. Various processing treatments, including antisolvent washing (or solvent engineering), vapor-assisted annealing, vacuum flash annealing, delayed-annealing and gas blowing have been reported to significantly ameliorate the crystallization process and fabricate high-quality perov skite films [27–30]. Among them, antisolvent washing is considered as the most effective and easily operated approach. Herein, we develop an elegant antisolvent-washing treatment for the PbBr2 film preparation for the first time. The delay time of dropping CB antisolvent is investigated and optimized. It is found that the CB droplet could remove the excess N, N-Dimethylformamide (DMF) solvent in the wet PbBr2 film immediately during the spin-coating process, inducing a fast nucleation and modified crystallization of PbBr2, and thus a highly covered PbBr2 film [31]. Consequently, the resultant CsPbBr3 film shows a more homogeneous morphology with higher crystallinity and coverage as well as larger average grain size, compared to the counterparts prepared without the antisolvent treatment. The best-performing antisolvent-treated PSC achieves a superior PCE of 8.55% under reverse scan directions with all the photovoltaic parameters get augmented. By contrast, the untreated counterparts only gain a champion PCE of 6.94%. This enhancement can be mainly attributed to the more effective charge transport and sup pressed non-radiative recombination rate caused by the improved film quality with decreased current shunting pathways and trap states in the CsPbBr3 film. Moreover, the unencapsulated all-inorganic CsPbBr3 de vice exhibits no performance degradation when stored in ambient con dition for over 1000 h at room temperature (~25 � C). Upon persistent thermal attack at 80 � C in air for one month, the as-prepared device still retains 95.2% of its initial PCE. Our work paves the way for the practical application of cost-effective, highly efficient and stable all-inorganic CsPbBr3 PSCs.
Organic-inorganic hybrid perovskites have shown great potential to rival multicrystalline silicon for photovoltaic applications due to their unique photovoltaic properties, such as tunable bandgap, high light absorption coefficient, low exciton binding energy, benign defect properties as well as superior tolerance to defects [1–5]. Although a power conversion efficiency (PCE) over 23% has been achieved for perovskite solar cells (PSCs) [6], the stability issue, especially for the thermal stability under operational condition, still set a big barrier for the commercialization of PSCs. Replacing the organic components (MAþ, FAþ, etc) with inorganic ions (such as Csþ, Rbþ, etc) to form pure inorganic perovskites should be a feasible strategy to ameliorate this situation [7]. Among all the inorganic perovskites, CsPbI3 and CsPbIxBr3-x are the most widely researched materials these days owing to their good thermal stability and suitable bandgap, based on which the PCEs over 17% and 14.8% have been obtained, respectively [8,9]. However, both the CsPbI3 and CsPbIxBr3-x can easily convert into non-perovskite phase in air at room temperature, hampering their practical application. All-inorganic CsPbBr3, with outstanding moisture, oxygen and thermal stability under harsh conditions, emerges as a competitive candidate of light absorbers that is attracting widespread attentions. CsPbBr3 PSC was firstly proposed in 2015 and delivered a PCE of 5.95% with poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport layer (HTL) and Au electrode [10]. Though satisfactory thermal stability was obtained, the high costs of state-of-the-art hole transport material PTAA and noble metal Au set another obstacle for its mass production. Liu’s group devised a carbon-based CsPbBr3 PSC with a structure of compact TiO2/mesoporous TiO2/CsPbBr3/Carbon for the first time and got a PCE of 6.7% [11]. The abandon of the expensive HTL and noble metal electrode was beneficial to low down the production costs. This structure was also regarded as a classical structure for CsPbBr3 devices and widely adopted by other groups [12–14]. Apart from the intrinsic large bandgap of CsPbBr3 perovskite (2.3 eV), the PCEs of CsPbBr3 PSCs were also limited by the low crystallinities and poor morphologies of the CsPbBr3 active layers fabricated in conven tional two-step solution route [10,15]. This could be mainly ascribed to the quite low solubility of the CsBr precursor in currently available solvents, resulting in a lack of sufficient raw materials in the formation process of CsPbBr3 and thus a discontinuous film morphology [16]. Many researches have already been carried out to improve the crystal lization process of CsPbPb3 films. Chang et al. successfully prepared highly crystalline CsPbBr3 films by optimizing the reaction time and temperature during the CsBr-immersion process [13]. But the PSCs only gained a champion PCE of 5.0%. Teng et al. developed a facile face-down liquid-space-restricted deposition approach to prevent the decomposition of CsPbBr3 in the CsBr methanol solution in 2018 [17]. The resultant film morphology and light harvesting ability of CsPbBr3 were greatly enhanced. The as-prepared devices yielded a PCE of 5.86%, which was an excellent efficiency for planar CsPbBr3 PSC. Recently, Tang’s group creatively proposed a multistep spin-coating method to fabricate high-quality CsPbBr3 films [15]. Both the crystallinity and phase-purity were pronouncedly augmented and the best-performing device demonstrated a high PCE of 9.72%. The same group further enhanced the PCE of CsPbBr3 PSCs to over 10% through metal ion doping, surface modification and spectra engineering [14,18–20]. Zeng et al. presented a space-confined growth strategy for the controllable growth of the polycrystalline CsPbBr3 perovskites by freezing the pre cursor solution within the gaps of ordered polystyrene sphere templates [16]. The fabrication of dense CsPbBr3 films with relatively low trap density and high carrier mobility were realized. Our group also intro duced a modified multistep deposition technique on the basis of the traditional two-step deposition route [21]. The improved phase-purity and film morphology of the CsPbBr3 perovskites led to a much enhanced PCE for the carbon-based planar PSCs. Besides, the
2. Experimental section 2.1. Materials Lead bromide (PbBr2, �99.99%) and cesium bromide (CsBr, �99.99%) are bought from Xi’an p-OLED. Titanium tetrachloride (TiCl4, �99.5%), N, N-Dimethylformamide (DMF, �99.9%), dimethylsulfoxide (DMSO, �99.9%), and copper (II) phthalocyanine (CuPc) are purchased from Aladdin. Chlorobenzene (�99.8%) is from Alfar Aesar. NiCl2⋅6H2O (�98.0%) is bought from Sinopharm Chemical Reagent Co., Ltd and the commercial carbon paste is from Shenzhen Dongdalai Chemical Co., Ltd. All the chemicals and reagents are directly used without any further purification. 2.2. Device fabrication The conductive fluorine-doped tin oxide (FTO) glass substrates (NSG-10) were chemically etched with 2 M hydrochloric acid and zinc powder. Then, the FTO substrates were sequentially cleaned with detergent, acetone, anhydrous alcohol and deionized water in an ul trasonic bath each for 15 min, followed by an O3/ultraviolet treatment for 30 min to remove the organic residues. The compact TiO2 electron transport layers (ETLs) were prepared through the hydrolysis of the 2
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TiCl4 aqueous solution as we reported before [32]. The TiCl4 (stored in the freezer at about 0 � C) was diluted to 200 mM with 0.01 M NiCl2⋅6H2O being added. The cleaned FTO substrates were then placed vertically in a glass container filled with TiCl4 precursor solution and kept at 70 � C for 3 h in a thermostat water bath. Subsequently, the substrates were washed with deionized water in a sonication bath for 5 min to remove any loosely bound materials, and then annealed at 200 � C on a hotplate for 1 h. The CsPbBr3 perovskites were deposited on the ETL by a multistep spin-coating method. 1.0 M PbBr2 in DMF solu tion was kept at 75 � C and the substrate was also pre-heated at 75 � C before the PbBr2 spin-coating process. Then, the PbBr2 solution was spin-coated on the compact TiO2 ETLs at a speed of 2000 rpm for 30 s. For the antisolvent-treated PSCs, 115 μL chlorobenzene was dropped onto the substrates at delay times of 2, 5 and 8 s from the starts of the PbBr2 spin-coating process. Afterwards, the substrates were heated at 90 � C for 1 h. The CsBr was deposited via a multistep solution-processing solution method reported by Tang’s group [15]. A 0.07 M CsBr methanol solution was spin-coated onto the PbBr2 films at a speed of 2000 rpm for 30 s, followed by being annealed at 250 � C for 5 min. After repeating this process for five times, the films were cleaned with isopropanol (IPA) at a speed of 2000 rpm for 30 s. Then, the substrates were annealed at 250 � C for another 5 min. For the preparation of the hole transport layer (HTL), 35 nm CuPc was thermally evaporated on the CsPbBr3 films under a base pressure of less than 3 � 10 8 Torr at a speed of ~0.5 Å/s determined by a quartz crystal monitor. Eventually, the commercial carbon paste was deposited on the prepared film by doctor-blading technique and then dried at 100 � C for 15 min to form the counter electrodes of the devices.
Kα radiation (λ ¼ 1.5418 Å) at 25� and the data were collected with a 0.013� step size (2θ). The absorbance spectra of the CsPbBr3 films were obtained by a UV–visible spectrophotometer (UV 2600, Shimadzu). The atomic force microscopy (AFM) images of the untreated and antisolventtreated PbBr2 films were obtained by an Innova SPM 9700 (Shimadzu, Japan) in a tapping mode. The surface morphologies of the PbBr2 and CsPbBr3 films and the cross-sectional images of the whole devices were examined by the scanning electron microscopy (SEM, Sirion 200, FEI, Heland and GeminiSEM 300, Carl Zeiss, German). The current densityvoltage (J-V) characteristics were recorded at a sweep rate of 10 mV/s by using a computer-controlled electrochemical station (Autolab PGSTA302 N, Netherlands) under simulated AM 1.5G (100 mW/cm2) one sunlight illumination generated by a solar simulator (Oriel 94043A, Newport Corporation, Irvine, CA, USA), which was calibrated by a NREL-traceable KG5 filtered silicon reference cell. The active area of the cells in the measurements were defined by a metal mask with an aper ture area of 0.071 cm2. The open-circuit photovoltage decay (OCVD), capacitance-voltage as well as the electrochemical impedance spec troscopy (EIS) measurements were also performed on this equipment. The EIS spectra were fitted via the Nova software. Moreover, the steadystate photoluminescence (PL) and time-resolved photoluminescence (TR-PL) decay spectroscopy were measured via FluoTime300 (Pico Quant, German). 3. Results and discussion Fig. 1a shows the schematic illustration of the antisolvent-assisted CsPbBr3 fabrication process. A PbBr2 precursor in DMF (1 M) is firstly spin-coated onto the compact TiO2 ETLs with the CB antisolvent drop ped onto the wet film at varied delay times from the start of the spincoating process. The role of CB droplet is to rapidly remove the excess DMF solvent in the wet PbBr2 film during the PbBr2 spin-coating process,
2.3. Device characterization The X-ray diffraction (XRD) measurements were carried out on a x’pert3 powder X-ray diffractometer (PANalytical, Netherland) with Cu
Fig. 1. (a) Schematic illustration of the fabrication process of the CsPbBr3 films with the PbBr2 films treated by CB antisolvent. (b) XRD patterns of the FTO substrate and CsPbBr3 films with CB dropped at different delay times from the start of the spin-coating process. (c) UV–vis absorption spectra of the CsPbBr3 films treated by CB antisolvent at varied delay times. The bandgap calculation of the 5s-treated CsPbBr3 film is shown in the inset. 3
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thus reducing the solubility of PbBr2 in the mixed solvent. The residual vacancy caused by the evaporation of DMF can promote the crystal growth of the PbBr2 crystals [31]. Therefore, this treatment indeed contributes to a more uniform nucleation and a modified crystallization of the PbBr2 crystal compared to the untreated counterparts. Then the CsBr methanol solution is deposited by a multistep spin-coating tech nique reported previously to form CsPbBr3 films [15]. It is observed that the color of the PbBr2 films changes from white to orange-yellow after the intercalation reaction with CsBr, suggesting the formation of CsPbBr3 perovskites. The arrow at the bottom of Fig. 1a indicates the phase transition in the whole spin-coating process, which can be usually described into the following reactions [15]: 2PbBr2 þ CsBr → CsPb2Br5 (with excess amount of PbBr2)
(1)
CsPb2Br5 þ CsBr → 2CsPbBr3
(2)
CsPbBr3 þ 3CsBr → Cs4PbBr6 (when CsBr is excessive)
(3)
impact the crystallization of PbBr2 film a lot, in line with the XRD re sults. The light absorption properties of the CsPbBr3 films with their PbBr2 precursor films treated by antisolvent washing at varied delay times are further studied (Fig. 1c). All the films exhibit an approximate absorption band edge of ~535 nm, corresponding to a calculated bandgap of 2.32 eV as depicted in the inset. It is clear that the 5s-treated CsPbBr3 film possesses the strongest light harvesting capability, which can be mainly attributed to the higher crystallinity and phase-purity as well as higher film coverage (will be discussed later) of the film than those of other films. This is beneficial to yield a higher photocurrent output. The 2s-treated CsPbBr3 film even shows a weaker light absorp tion due to its lower crystallinity and film coverage (Fig. S2b) than that of the untreated counterpart (Fig. 2b). As one of the reactants of CsPbBr3, PbBr2 film not only provides the kinetically favorable nucleation centers for the formation of the CsPbBr3 perovskite, but also acts as a framework for the crystal growth [35]. Thus, both the film crystallization process and morphology will be greatly affected by the pre-deposited PbBr2 layer. The deposition of a crystalline PbBr2 film usually contains two main processes: nucleation and crystal growth. In order to fabricate homogeneous and highly covered PbBr2 films, it is necessary to obtain numerous and uniformly-distributed nucleus before crystal growth [36]. Increasing the supersaturation of the precursor solution during the spin-coating process by antisolvent washing is an effective method to achieve this. The scanning electron microscopy (SEM) images of the PbBr2 films treated by CB at varied delay times and the corresponding CsPbBr3 films based on them are given in Fig. 2. There exists many micro-scale holes in the PbBr2 film deposited without antisolvent treatment (Fig. 2a) due to the nonuniform nucleation. Consequently, the resultant CsPbBr3 film ex hibits a poor morphology and coverage with a small average grain size of 593 nm and a great number of grain boundaries (Fig. 2b). The pinholes existing in the CsPbBr3 film tend to serve as the current shunting path ways, leading to a severe leakage current and thus a low charge collection efficiency [32]. The cross-sectional SEM image of the CsPbBr3 film (Fig. 2c) also reveals the existence of lots of grain boundaries inside the film, which is consisting of many small disorderly arranged grains. Note that grain boundaries in the polycrystalline perovskites are widely considered to be responsible for causing non-radiative recombination and trapping of charge carriers [37]. When CB antisolvent is introduced at a delay time of 2s, the PbBr2 film with a much rougher surface with a root-mean-square roughness (RMS) of 98.8 nm (Fig. S3b) is obtained after spin-coating. The final CsPbBr3 film based on it even gets a lower coverage than the untreated film (Fig. S2b). This indicates the prema ture antisolvent washing won’t be favorable for the crystallization of both the PbBr2 and the resultant CsPbBr3 films, in agreement with above XRD analysis. When CB treatment is introduced within stage 2 (5s), the PbBr2 precursor solution gets highly concentrated and becomes super saturated rapidly. This provokes a faster and more uniform nucleation of PbBr2 compared to the untreated counterpart. The as-prepared PbBr2 film achieves a much higher coverage with a smoother surface (Fig. S3c) as well as decreased hole numbers and sizes (Fig. 2d). As a result, there is no any pinholes can be observed in the final CsPbBr3 film as shown in Fig. 2e. The average grain size of the homogeneous CsPbBr3 film is also boosted to 1.08 μm with much reduced grain boundaries. The increased grain sizes could affect both the surface and bulk properties of the CsPbBr3 perovskites, including enhancing the crystallinity, reducing the electronic trap states at the surface and in the bulk as well as suppressing the structural defects associated with the pinholes [38]. This is condu cive to the charge transport and suppression of carrier recombination, contributing to a higher short-circuit current density (Jsc) and open-circuit voltage (Voc). The cross-sectional SEM image of the 5s-treated CsPbBr3 film (Fig. 2f) further confirms the close packing of the vertical- and monolayer-aligned perovskite grains. This enables the photoinduced carriers to transport across the ETL/HTL towards the electrode in the out-of-plane direction without passing through grain boundaries [21]. Therefore, the 5s-treated CsPbBr3-based devices are
The crystallization kinetics of the resultant CsPbBr3 films based on the PbBr2 films treated by CB antisolvent at delay times of 2, 5 and 8 s (denoted as 2s-, 5s- and 8s-treated films hereafter) are detailedly investigated by X-ray diffraction (XRD) measurements, as depicted in Fig. 1b. All the as-prepared perovskites present diffraction peaks at 15.19� , 21.58� , 30.69� , 34.48� , 44.10� and 49.56� , which can be assigned to the (100), (110), (200), (210), (220), and (310) lattice planes of the CsPbBr3 phase (PDF#54–0752), respectively. Peaks at 11.60� , 24.03� , 29.38� and 30.29� can also be obviously observed for these films, corresponding to the (002), (202), (213), (221) lattice planes of the CsPb2Br5 phase (PDF#25–0211). Apart from the incomplete reac tion with CsBr, the existence of the PbBr2-rich CsPb2Br5 phase may be ascribed to the partial phase transition from CsPbBr3 to tetragonal CsPb2Br5 under thermal attack (beyond 150 � C) in the post-annealing process [33]. The small diffraction peaks at 12.68� , 12.89� , 25.55� , 27.56� and 28.69� also identify the formation of the CsBr-rich Cs4PbBr6 phase (PDF#54–0750), coinciding well with the previous report [18]. For the 2s-treated CsPbBr3 film, all the peaks corresponding to the CsPbBr3 phase even become weaker compared to the untreated film, suggesting a lower crystallinity. When CB antisolvent is dropped at a delay time of 5s, the peaks of CsPbBr3 phase get much stronger and sharper while those of the CsPb2Br5 and Cs4PbBr6 phase become weaker. This reflects that the antisolvent washing at 5s can promote the crys tallization process of CsPbBr3 perovskite, leading to a higher crystal linity and phase-purity. Further increasing the delay time to 8s, the crystallinity of the 8s-treated CsPbBr3 film only shows a slight improvement while no distinct change is observed in the XRD charac teristics compared to the untreated film. The different crystalline properties of these CsPbBr3 films are closely related to the PbBr2 seed layers treated by antisolvent washing at varied delay times. The XRD patterns of the corresponding PbBr2 films (Fig. S1) also shown that the 5s-treated PbBr2 film delivers a much higher peak intensity than the other samples, especially in (130) and (140) orientations, reflecting a higher crystallinity of the film. By contrast, the antisolvent-washing at delay times of 2 and 5 s hasn’t exhibited an obvious impact on the crystallization of the PbBr2 films. Generally, in the first several seconds (0–3s, stage 1) of the PbBr2 spin-coating process, the dominant action is removing the excess DMF solvent. Introduction of CB antisolvent washing at this stage won’t promote the nucleation rate of PbBr2 since the precursor solution is far from supersaturation [34]. On the contrary, premature CB treatment is even harmful for the CsPbBr3 growth as revealed by the XRD patterns. In 4–6 s after spin-coating (stage 2), CB antisolvent washing can significantly facilitate the evaporation of the residue DMF solvent, inducing a faster and more uniform nucleation and better crystal growth. This may in turn promotes the crystallinity of the final CsPbBr3 films. After spin-coating for over 7s (stage 3), the wet film starts to dry with the nucleation process finished and heterogeneous crystallization occurring. Dropping of antisolvent at this stage may not 4
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Fig. 2. (a) Top-view SEM images of the PbBr2 film prepared without antisolvent treatment. The corresponding (b) top-view and (c) cross-sectional SEM images of the untreated CsPbBr3 film. Top-view SEM images of the (d) PbBr2 and (e) CsPbBr3 films, and (f) the resultant cross-sectional SEM images of the CsPbBr3 film treated by CB antisolvent at a delay time of 5s.
expected to deliver a much enhanced charge extraction efficiency and reduced recombination rates compared with the untreated counterparts. The film quality of the 8s-treated PbBr2 and CsPbBr3 only gets a slight improvement since the CB dropped at stage 3 won’t promote the nucleation process effectively as discussed above. The steady-state photoluminescence (PL) measurements are per formed to detect the trap states in the as-prepared CsPbBr3 light absorber layers [39]. Since the CsPbBr3 perovskites are directly depos ited on FTO substrates without charge transport layer, the photoinduced carriers in the excited state cannot be extracted out quickly and result in
a radiative recombination. A higher PL intensity is always related to fewer traps or defects [40,41]. The 5s-treated sample shows the highest PL intensity at around 530 nm (Fig. 3a), indicating the fewest trap states in the film. This enhancement in PL intensity can be attributed to the modification effect of CB antisolvent washing on the crystallinity and grain boundaries, resulting in much reduced trapped states. The 2s-treated CsPbBr3 film exhibits the strongest PL quenching for there are most defects existing in the grain boundaries and pinholes of the film as evidenced by SEM images (Figs. S2a–b). Besides, the slight blue-shift in the PL peak of the 5s-treated film further confirms the decreased defect
Fig. 3. (a) Steady-state PL and (b) TR-PL spectra of the CsPbBr3 films deposited on FTO substrates with CB antisolvent dropped at different delay times. (c) Crosssectional SEM image and (d) the corresponding energy diagram of the whole device. 5
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densities compared to the untreated counterpart [42–44]. The corre sponding time-resolved photoluminescence (TR-PL) measurements of the same samples are also carried out as presented in Fig. 3b, from which the carrier transport dynamics and carrier lifetimes can be investigated. The intralayer carrier transport process in the perovskites is closely related to the impurity level at the grain boundaries [45]. In specific, a longer carrier lifetime reflects a slower intralayer recombination rate caused by a lower impurity level in the CsPbBr3 film. The PL lifetime can be well fitted by a bi-exponential decay function [21]:
result, the average PCE of the 5s-treated devices is increased from 6.23% to 7.99% compared to the untreated counterparts. This enhancement can be mainly attributed to the faster charge transfer and slower carrier recombination rates caused by less trap states in the 5s-treated CsPbBr3 films as discussed above. The narrower PCE distribution of the 5streated devices, with a smaller standard deviation of 0.33 than of the untreated PSCs (0.45), also suggests a higher reproducibility of them. It is expected to be related to the much improved crystallization properties and film qualities of the 5s-treated CsPbBr3 perovskites. Fig. 5a depicts the current density-voltage (J-V) characteristics of the best-performing untreated and 5s-treated PSCs. The untreated devices only gain a highest PCE of 6.94%, with a Jsc of 7.36 mA/cm2, a Voc of 1.292 V and a FF of 0.730. After treated by CB washing at a delay time of 5s, the champion PCE of the PSCs is boosted to 8.55% measured under reverse scan directions, with a Jsc of 7.92 mA/cm2, a Voc of 1.362 V and an excellent FF of 0.793. The incident-photo-conversion-efficiency (IPCE) measurements of them (Fig. S5) further reveal the enhance ment in Jsc for the 5s-treated devices. Since the hysteresis behaviors are existing in both the untreated and 5s-treated PSCs (Fig. S6), the steadystate photocurrent and PCE outputs at the maximum power point are recorded to estimate the efficiency of the devices accurately [41,46,47]. As shown in Fig. 5b, the current and PCE outputs of the best-performing untreated and 5s-treated cells rise up quickly to the maximum once the light is turned on. A stabilized PCE of 6.16% with a current density of 5.70 mA/cm2 is obtained at an applied voltage bias of 1.08 V for the untreated devices. After treated by CB, the 5s-treated device delivers a higher stabilized PCE of 7.95% corresponding to a photocurrent density of 6.74 mA/cm2 at a voltage bias of 1.18 V. In addition, the ratio of the stabilized to the scanned efficiencies for the untreated and 5s-treated devices are about 0.89 and 0.93, respectively. The larger ratio of the 5s-treated device reveals a smaller hysteresis in the device [48]. This can be attributed to a faster charge extraction efficiency and more sup pressed interfacial charge accumulation [49], originating from the less trap states in the 5s-treated CsPbBr3 films. The dark J-V measurements of the untreated and 5s-treated devices are further performed and the results are shown in Fig. 5c. Both of the PSCs exhibit a typical diode behavior with almost no dark current is detected under negative voltage bias. The 5s-treated device shows a smaller leakage current than the untreated counterpart, suggesting a higher shunt resistance of the former device. This can be ascribed to the better film quality of the 5s-treated CsPbBr3 perovskite with less pin holes and grain boundaries as certified by SEM images (Fig. 2). The suppressed dark current is favorable for the realization of a higher Jsc and FF [50]. It has been reported that the intercept of the linear portion of the dark J-V curve to the voltage axis corresponds to the Voc of a PSC [40]. The Voc extracted from the dark J-V curves of the best-performing untreated and 5s-treated devices are about 1.32 and 1.40 V, respec tively, close to the J-V measurements obtained under one sunlight illumination. The fragile stability of PSCs under working conditions is still a major impediments for the practical application of PSCs. Here, the moisture and thermal stability tests of the optimized 5s-treated devices without any encapsulation are conducted in ambient air. The as-prepared PSC exhibits a superb stability with no decline in PCE being observed when stored in air (~40% relative humidity, 25 � C) for over 1000 h (Fig. 5d). This can be mainly attributed to the introduction of the chemically stable CuPc HTL and carbon counter electrode (CE). Besides, the highly hydrophobic and dense CuPc and commercial carbon layer can effec tively protect the perovskite from moisture and oxygen damage [32,51]. Upon persistent thermal attack at 80 � C in air for one month, the device only shows a slight performance degradation, still retaining 95.2% of its initial PCE. The excellent thermal stability can be explained from two aspects. On one hand, the CuPc hole transport materials possesses higher intrinsic thermal stability than the widely-used polymer and small molecule hole conductors like Spiro-OMeTAD and PTAA [52]. On the other hand, all-inorganic CsPbBr3 itself is highly thermal-stable with a
(4)
f(t) ¼ A1exp(-t/τ1) þ A2exp(-t/τ1)þB
where τ1 and τ2 are the slow and fast decay time constants, whilst A1, A2 are the corresponding fractional amplitudes of τ1 and τ2, respectively. The fast decay is generally relevant to the carrier quenching process at the interface while the slow decay component can reflect the trapassisted radiative recombination in the bulk perovskite phase, respec tively [21]. The lifetime parameters of carriers in these films extracted from the TR-PL decay curves are summarized in Table 1. The τ1 and τ2 of the CsPbBr3 perovskite prepared without CB treatment are 19.75 and 6.17 ns, respectively, yielding an average carrier lifetime of 11.86 ns. The 2s-treated CsPbBr3 delivers the lowest average lifetime of merely 11.14 ns due to the heavy trap-assisted recombination, in line with the quickest PL quenching (Fig. 3a). When CB antisolvent is dropped at a delay time of 5s, the average carrier lifetime of the perovskite increases to 16.88 ns with a τ1 and τ2 of 25.32 and 6.93 ns, respectively. The average carrier lifetime of the 8s-treated perovskite also gets a slight augmentation to 13.45 ns compared to that of the untreated CsPbBr3. The largest τ1 value of the 5s-treated film reveals the effective sup pression of the trap-assisted recombination rates within the bulk perovskite, suggesting the lowest defect densities in the film. This is in favor of improving the charge extraction and collection efficiency, leading to a higher Jsc and Voc. Fig. 3c illustrates the cross-sectional SEM images of the as-fabricated planar heterojunction PSC with a structure of FTO/Ni–TiO2/CsPbBr3/CuPc/carbon. The Ni-doped compact TiO2 ETL was prepared by a chemical bath deposition method as we previously reported [32]. The relatively rougher surface of the TiO2 film than that of the widely used spin-coated TiO2 conduces to the deposition of CsPbBr3 since CsPbBr3 film is easy to fall off from a low-roughness surface (certified by our experiments as shown in Fig. S4). The thick ness of the CsPbBr3 absorber layer is about 450 nm while the CuPc HTL is too thin to be observed. The energy diagram of the whole device is drawn in Fig. 3d. The cascade-like conduction band (CB) alignment between the FTO, Ni–TiO2 and CsPbBr3 is beneficial for the electron transportation and collection, whilst the higher CB and valence band (VB) of CuPc than that of CsPbBr3 is conducive to blocking electrons and extracting holes. The key photovoltaic parameters, including Jsc, Voc, fill factor (FF) and PCE, of the untreated and CB-treated PSCs are recorded under simulated AM 1.5G (100 mW/cm2) illumination as listed in Table 2. It appears that the 5s-treated devices possess the best performance while the 2s-treated ones exhibit the lowest efficiency. To highlight the pro motion effect of antisolvent washing on the device performance, the box charts exhibiting the statistical features of the key parameters for the untreated and 5s-treated PSCs are given in Fig. 4. The introduction of CB treatment at a delay time of 5 s brings about a comprehensive improvement of all the photovoltaic parameters for the devices. As a Table 1 Lifetime parameters extracted from the TR-PL spectroscopy for the CsPbBr3 perovskites with CB antisolvent dropped at different delay times. Sample
τave [ns]
τ1 [ns]
A1
τ2 [ns]
A2
FTO/untreated CsPbBr3 FTO/2s-treated CsPbBr3 FTO/5s-treated CsPbBr3 FTO/8s-treated CsPbBr3
11.86 11.14 16.88 13.45
19.75 19.28 25.32 21.84
41.9% 39.0% 54.1% 44.3%
6.17 5.94 6.93 6.65
58.1% 61.0% 45.9% 56.7%
6
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Table 2 Photovoltaic parameters of the PSCs with CB antisolvent treated at varied delay times. These data were extracted from 25 devices prepared in a batch. Device Untreated 2s-treated 5s-treated 8s-treated
average champion stabilized average champion stabilized average champion stabilized average champion stabilized
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
7.02 � 0.59 7.36 5.70 6.81 � 0.72 7.25 5.37 7.79 � 0.42 7.92 6.74 7.17 � 0.48 7.48 5.87
1.255 � 0.049 1.292 1.08 1.246 � 0.057 1.268 1.06 1.325 � 0.041 1.362 1.18 1.276 � 0.045 1.315 1.12
0.707 � 0.048 0.730
6.23 � 0.83 6.94 6.16 6.02 � 0.95 6.68 5.69 7.99 � 0.64 8.55 7.95% 6.52 � 0.73 7.29 6.57
0.710 � 0.041 0.727 0.774 � 0.023 0.793 0.713 � 0.037 0.741
Fig. 4. Box-charts of the (a) Jsc, (b) Voc, (c) FF and (d) PCE of 25 untreated and 5s-treated cells, respectively.
high decomposition temperature over 467 � C [21]. Since the CuPc and commercial carbon used are inexpensive, our devices show great po tentials in practical application with the merits of cost-effective, easy- to-preparation as well as highly efficient and robust. Electrochemical impedance spectroscopy (EIS) measurements are employed to get an insight into the interfacial charge transfer dynamics in the untreated and 5s-treated PSCs. Fig. 6a presents the Nyquist plots recorded at a bias potential of 0.8 V under one sunlight illumination with a frequency range from 1 MHz to 0.01 Hz. It is clear that there exists two main arcs in each Nyquist plot. Generally, the small arc at high frequency is considered to relate to the hole transport at the interface of perovskite/HTL or HTL/electrode, reflecting the charge transfer resis tance (Rct) in parallel with a HTM capacitance (CPE1). The large arc at low frequency is relevant to the electron recombination process at the TiO2/perovskite interface, corresponding to a recombination resistance (Rrec) in parallel with a chemical capacitance (CPE2) [39,43,53]. Based on the analysis, the simplified equivalent circuit is drawn in the inset of Fig. 6a. As fitted by this model, the 5s-treated device gets a lower Rct of 87.5 Ω than that of the untreated counterpart (143 Ω), indicating a faster charge extraction. The Rrec of the 5s-treated PSC is promoted from 1.21
to 1.68 kΩ after CB treatment, suggesting to a lower recombination rate. The enhanced charge transport and suppressed carrier recombination contribute to a higher Jsc and Voc. The Rs evaluated from the starting point in the real part of the Nyquist plots for the 5s-treated and untreated devices is around 36.4 and 38.5 Ω, respectively. The lower Rs is conducive to the achievement of a higher FF. In addition, the smaller chemical capacitance of the 5s-treated device (194 nF/cm2) than that of the untreated device (312 nF/cm2) further reveals the less interfacial charge accumulation exists in the former device. This can be mainly attributed to the improved charge transfer capability and reduced trap states of the CsPbBr3 film after treated by CB antisolvent. The overall enhancement in Jsc, Voc and FF yield a higher PCE for the 5s-treated devices, which is in good accordance with the J-V measurements. The open-circuit photovoltage decay (OCVD) measurements are conducted for both the untreated and 5s-treated device and the results are plotted in Fig. 6b. It appears that the 5s-treated cell possesses a higher intrinsic Voc and a slower photovoltage decay than the untreated one. The carrier lifetimes (τn) of the devices can be roughly evaluated according to the following formula [54]:
7
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Journal of Power Sources 439 (2019) 227092
Fig. 5. (a) J-V curves and (b) steady-state photocurrent and PCE outputs of the best-performing untreated and 5s-treated PSCs recorded under AM 1.5G illumination. (c) Dark J-V characteristics of the untreated and 5s-treated PSCs. (d) PCE variations of the 5s-treated PSCs when stored in ambient air with a relative humidity of ~40% at room temperature (25 � C) and 80 � C, respectively. Fig. 6. (a) EIS measurements of the untreated and 5s-treated PSCs measured under light illumination with the simplified equivalent circuit depicted in the inset. (b) The OCVD measurements of the untreated and 5s-treated devices. Inset shows the calculated carrier lifetimes under varied voltage of them. (c) Voc variations of the untreated and 5s-treated PSCs recorded under different light intensity. (d) MottSchottky plots at different applied bias voltages extracted from the impedance analysis of the un treated and 5s-treated devices, respectively.
τn ¼
kB T dVoc ð Þ q dt
1
and positive elementary charge, respectively. The calculated carrier lifetimes of the untreated and 5s-treated devices under varied voltage are presented in the inset of Fig. 6b. Obviously, the 5s-treated device
(6)
where kB is the Bolzman constant, T and q are the absolute temperature 8
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Journal of Power Sources 439 (2019) 227092
possesses a much longer carrier lifetime with a lower interface recom bination rate, corresponding to a higher Voc. The interface charge recombination process in the PSC can also be assessed by ideality factor (n). It can be reflected by plotting the Voc as a function of the light in tensity [25]. The Voc of PSC usually decreases rapidly at a low light intensity (ψ ) of less than 0.1 sun (10 mW/cm2), due to the current shunting and trap-assisted recombination [55]. When the ψ is larger than 0.1 sun, a reduction in Voc is related to the interfacial recombina tion. The Voc vs. light intensity for the untreated and 5s-treated devices is illustrated in Fig. 6c. The n can be calculated by the following equation [56]: n¼
q dVoc kB T dInðψ Þ
Conflicts of interest There are no conflicts of interest to declare. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant nos. 51675210, 51675209 and 51805195), the China Postdoctoral Science Foundation (Grant no. 2018M640691) and Fund from Science, Technology and Innovation Commission of Shenzhen Municipality (Grant no. JCYJ20170818165724025). We also appreciate the Analytical and Testing Center and Flexible Electronics Research Center of Huazhong University of Science and Technology for the SEM, XRD and steady-state PL measurements.
(7)
The ideality factor of the 5s-treated is 1.31 while that of the un treated counterpart is 2.13. The decreased n value of the 5s-treated device demonstrates the lower interface charge recombination in the PSC. This may be attributed to the fewer defects existing in the interface of TiO2/CsPbBr3 and CsPbBr3/HTL for the 5s-treated device. Further more, capacitance-voltage measurements are performed under dark condition to investigate the properties of the interface of the as-prepared PSCs. The built-in potential of the 5s-treated device (1.38 V) is higher than that of the untreated counterpart (1.31 V), as evaluated from the intercept of the linear portion of the Mott-Schottky plots (Fig. 6d) to the voltage axis [21]. This enhancement in built-in potential may be ascribed to the decreased defects in the interface of the 5s-treated de vices. A larger built-in potential is expected to facilitate the separation of photoinduced carriers and thus decrease the interfacial charge accu mulation and carrier recombination [57]. The enlarged built-in poten tial also provides an extended depletion region to hamper the back transfer of electrons from the TiO2 ETL to the CsPbBr3 light absorber layer. Therefore, the antisolvent treated devices tend to deliver a higher Voc compared to the untreated counterparts, in good agreement with the J-V characteristics recorded under one sunlight illumination.
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4. Conclusions In this study, we propose an elegant antisolvent-washing strategy for high-quality PbBr2 film preparation for the first time. The delay time of dropping CB antisolvent is detailedly investigated and the optimized value is 5s. The CB droplet can remove the excess DMF solvent in the wet PbBr2 film immediately, resulting in a fast nucleation of PbBr2 crystal and thus a highly covered PbBr2 film. The resultant CsPbBr3 film based on the CB-treated PbBr2 exhibits a more homogeneous morphology with higher crystallinity and coverage as well as larger average grain size (1.08 μm) than the untreated film. As a consequence, all the photovol taic parameters of the devices based on the 5s-treated CsPbBr3 perov skite get augmented. The best-performing 5s-treated PSC demonstrates an excellent scanned PCE of 8.55% with a Jsc of 7.92 mA/cm2, a Voc of 1.362 V and a high FF of 0.793. By contrast, the untreated counterparts only obtains a champion PCE of 6.94% with a Jsc of 7.36 mA/cm2, a Voc of 1.292 V and a FF of 0.730. The 5s-treated PSC also achieves a higher stabilized PCE output of 7.95% than that of the untreated one (6.16%), getting an increase by 29.1%. This enhancement can be mainly attrib uted to the more effective charge transport and suppressed non-radiative recombination rates caused by the improved film quality with decreased trap states of the CsPbBr3 perovskites. Moreover, the unencapsulated CsPbBr3 device shows no decline in PCE when stored in air for over 1000 h at room temperature (~25 � C). Upon persistent thermal attack at 80 � C in air, the as-prepared device still retains 95.2% of its initial PCE for one month. Our work provides an effective processing approach for fabricating high-quality CsPbBr3 films and accelerate the practical application of cost-effective, highly efficient and stable all-inorganic CsPbBr3 PSCs. Besides, the as-fabricated high-quality and robust CsPbBr3 films also show a great potential for their application in highperformance tandem solar cells and photodetectors. 9
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