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High-performance mixed-cation mixed-halide perovskite solar cells enabled by a facile intermediate engineering technique
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High-performance mixed-cation mixed-halide perovskite solar cells enabled by a facile intermediate engineering technique Muhammad Mateen a, b, Zulqarnain Arain a, c, Xuepeng Liu a, b, **, Cheng Liu a, b, Yi Yang a, b, Yong Ding a, b, Shuang Ma a, b, Yingke Ren d, Yunzhao Wu a, b, Ye Tao a, b, Pengju Shi a, b, Songyuan Dai a, b, * a
Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, PR China State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing, 102206, PR China Energy Systems Engineering Department, Sukkur IBA University, Sukkur, Pakistan d College of Science, Hebei University of Science and Technology, Shijiazhuang, 050018, China b c
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
� Mixed perovskite can be fabricated by facile intermediate engineering technique. � Developed perovskite exhibits improved quality by proposed method. � Optimized devices show an efficiency of 20.08% with superior stability.
A R T I C L E I N F O
A B S T R A C T
Keywords: FABr Intermediate engineering Mixed-cation mixed-halide perovskite High-quality Stability
Perovskite solar cells (PSCs) have attracted considerable attention in the photovoltaic field. However, most efficient mixed-cation mixed-halide perovskite in PSCs suffers from phase instability and large flux of trap states by existing methods, which limits the device performance. Herein, a facile intermediate engineering technique of the MAI-PbI2 intermediate layer via FABr solution is employed to get high-quality mixed perovskite films. The results illustrate that the proposed approach can improve the grain size, morphology, crystallization, conse quently reducing defect density of the perovskite layer. The devices which treated with FABr exhibit muchenhanced performance in comparison to the pristine and traditional mixed-cation mixed-halide devices. Consequently, a champion PSC with the best power conversion efficiency of 20.08% is obtained. Moreover, the devices based on the developed FABr-treatment technique also shows much-improved stability than the mixed perovskite-based devices fabricated from the traditional method. Therefore, this approach provides a simple technique to produce high-quality mixed perovskite film and subsequently may facilitate the commercialized production of high-performance PSCs in the future.
* Corresponding author. Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, PR China. ** Corresponding author. Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, PR China. E-mail addresses: [email protected] (X. Liu), [email protected] (S. Dai). https://doi.org/10.1016/j.jpowsour.2019.227386 Received 29 August 2019; Received in revised form 15 October 2019; Accepted 30 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Muhammad Mateen, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227386
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traditional compositional engineering approaches for mixed-cation mixed-halide perovskite also induce severe changing in the morphology and crystal structure of the film, associated with halides phase exclusion and light-field-induce ions movement altering the de vice function, despite the high efficiency [24–26]. Therefore, producing phase stable and high-quality perovskite film with minimum trap-density is crucial to obtain high-performance PSCs [27,28]. Studies conveyed that cation or anion intermixing for the perovskite lattice network can be obtained via ions exchange strategies [29]. The progress in post-growth passivation of the underdeveloped film is a more prospective technique to tune the perovskite properties. As an important breakthrough, Seok and coworkers produced a mixed perovskite through incorporation with the MAPbBr3 into FAPbI3, resulting in a black α-phase perovskite at low annealing temperature [30]. Afterward, Xu and co-workers reported the preparation of high-crystallinity thin layers of (FAPbI3)1-x(MAPbBr3)x perovskite via dual ion-exchange method [22]. Zhao et al. demonstrated that the thin film of MAPbI3 treated by a dilute FABr would effectually improve the crystallinity of the final film [31]. The MA-rich perovskite delivers a relatively higher efficiency, which is because highly concentrated MAþ is required to guarantee the formation of the perovskite phase. Recently, dual doping of MAþ and Br was reported to stabilize the FA-rich perovskite phase [30]. Sanith et al. presented the solid-liquid cation-exchange technique to directly transform the MAI perovskite to α-FAI based perovskite [32]. However, the added complexity of the material structure, coexistence of mixed perovskite with undesired yellow phase FAPbI3, uncontrolled ion trapping and the complications in reproducing the performances cast the doubts on the scaling-up of these routes [33–35]. In this work, we demonstrate a simplistic, novel and reproducible technique involving FABr-treatment of underdeveloped MAI perovskite intermediate film with FABr solution to achieve stable mixed perovskite with large grain size, highly-crystalline orientation and decreased trapdensity. By systematically adjusting the FABr concentrations, the highquality FAxMA1-xPbI3-yBry film is directly formed according to the developed method. It is noted that the average PCE was boosted from 17.25% to 19.00% after FABr-treatment, and the best PCE reached 20.08%. Moreover, the long-term stability of PSC is also boosted, which exhibits only 20% efficiency loss after storage for 1248 h in ambient condition.
1. Introduction 2þ þ Perovskite materials (ABX3, A ¼ CH3NHþ or 3 or HC(NH2)2 , B ¼ Pb Sn , X ¼ Cl , Br , or I ) have become the center of attention after their effective adaption in recently emerged applications like perovskite solar cells (PSCs). The exceptional photovoltaic performance of PSCs takes benefits of some exclusive semiconductor properties of the perovskite films such as the strong absorption profile, decent direct optical band gap, and long carrier diffusion length [1–5]. The power conversion ef ficiency (PCE) of PSCs has increased rapidly from the initial 3.8%– 25.2% within a few years [6–10]. Till now, prominent aspects, such as lattice structure, deposition methodology, compositional and stoichio metric engineering of the perovskite material have been investigated and manipulated in efforts to produce efficient and stable perovskite layer [11–14]. Generally, pure methylammonium (CH3NHþ 3 , MA) or for mamidinium (HC(NH2)þ 2 , FA) based perovskite materials (MAPbI3 or FAPbI3) are two most commonly used materials as the active layer in PSCs [15,16]. However, the desired perovskite phases (tetragonal MAPbI3 and trigonal FAPbI3) are sensitive to the transition temperature, humid environment and experience a reversible phase transition be tween their respective perovskite phases and non-perovskite phases (cubic MAPbI3 and hexagonal FAPbI3) [15,17]. Even, the coexistence of both symmetries (perovskite and non-perovskite) for these two mate rials would significantly harm the performance of corresponding PSCs [18]. Mixed-cation mixed-halide PSCs are attested to have some ad vantages over pure perovskite-based devices. Firstly, the inclusion of a minute amount of bromine (Br) and a large amount of iodine (I) within perovskite lattice could significantly lift the open-circuit voltage (Voc) while maintaining the high current density-voltage (Jsc) at the same time [19]. Furthermore, more effective light harvesting with minimum hys teresis in perovskite devices was obtained by embedding MA ions with symmetric FA ions in a balanced stoichiometric proportion [20–22]. However, through orthodox directly mixed precursor methods, the perovskite solution enriched with bromide and iodide tend to nucleate independently during the spin-coating stage because of their phase in-compatibility [12,22]. Thus, many researchers are dedicated to the compositional engineering of mixed-cation mixed-halide perovskite structure for stable and efficient PSCs. Mixed-cation mixed-halide perovskites are usually produced by directly spin-coating blended precursors, including MABr, FAI, and PbI2, PbBr2, etc, in a preferred ratio. Though, in this directly mixing method, iodide and bromide-rich precursor colloidal tend to precipitate inde pendently during the spin-coating stage because of their phase in compatibility. Consequent annealing process further intensifies the crystallization struggle phenomenon and thus low crystallinity, which also results in lots of internal and surface defects [18,20–23]. Moreover, 2þ
2. Experimental section The detail of characterizations and device fabrication are explained in supporting information. In the preparation of perovskite film, lead iodide (PbI2, 461 mg) and methylammonium iodide (MAI, 159 mg) in a mixed solution of DMSO and DMF (4:1 vol ratio) were stirred at 70 � C for 1 h. Above mixed solution were spin-coated on the substrates of (FTO/
Fig. 1. Schematic illustration of the fabrication method for large grain, less defective, highly crystalline mixed perovskite layer via FABr-treatment method.
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SnO2) at 4200 rpm for 30 s, then 110 μL of chlorobenzene (CB) was slowly dropped at the centre of the spinning film in last 10 s. Afterward, 40 μL FABr in isopropanol (IPA) solution were dripped onto the imma ture film with different FABr concentration. Once the FABr solution was dripped over the underdeveloped film, the transparent thin layer sud denly turned into blackish-red color. Finally, FABr-treated film was annealed at a temperature of 100 � C for 15 min and 140 � C for 1 h.
FABr-20 treated film. Furthermore, AFM image (Fig. S1) shows that the root means square roughness of the films are 17.1, 15.5, 14.0, and 14.7 nm for FABr-0, FABr-15, FABr-20, and FABr-30, respectively and supporting that FABr-20 film stands more homogeneous among all the tested films. Morphological smoothness and uniformity at microscale confirmed that intermediate engineering with an optimum amount of FABr in the IPA solution significantly affects the morphology of the final perovskite active layer [18,36]. As shown in Fig. S2, substantial growth in grain dimensions was witnessed as mean grain size surged from ~200 nm for without treated film to ~550 nm for FABr-15 and exceeded ~750 nm for FABr-20 and then declined a bit to ~715 nm for FABr-30 film. After forming the MAIPbI2-DMSO phase, the FAxMA1-xPbI3-yBry intermediate phase further suppresses the nucleation rate and allows grains more time to grow, which leads to a final homogeneous perovskite film with large grain [37, 38]. Generally, the perovskite films with larger grains were demon strated to experience lower trap density and longer carrier lifetimes, certainly as a result of reduced charge accumulation and non-radiative recombination caused by reduced grain boundaries [27,39,40]. There fore, it is admitted that the FABr-treatment has a strong effect on the crystallization and morphology of final mixed-cation mixed-halide perovskite film. To dig deep into the FABr-treatment effect on the final phase of perovskite films, the crystal structure properties of the film are exam ined through X-rays diffraction (XRD, Fig. 3a). It is clearly witnessed that the crystallinity of perovskite films after FABr-treatment is strongly improved. The XRD peak intensity of the treated film rises about fivetimes without any observed peak shift. Therefore, the FABr-treated films have significantly enhanced crystallinity, and the peak intensity rises up to highest at FABr-20 condition. It can also be found in Fig. 3b that the (110) face shows a little shift towards the lower degree. Therefore, the FAxMA1-xPbI3-yBry fabricated with the proposed method is stable, and no phase transition or decomposition was observed during fabrication [39]. The significant improvement in film crystallinity after the FABr-treatment is totally in agreement with the morphological evolution, as discussed earlier [38].
3. Results and discussion 3.1. FABr-treatment process Till now, the directly blended precursors with the desired ratio is a conventional way to prepare mixed perovskite films, which is termed as Trad-Mixed-PSC in this work. It has been proved that the bromide-rich or iodide-rich perovskite colloids easily and leads to phase in compatibility [34,35]. Herein, our proposed technique avoids the blending of mixed precursors. Fig. 1 shows the step-wise method for the preparation of mixed-cation mixed-halide perovskite films via the FABr-treatment method. For FABr-treatment, the varied concentration in IPA solution were 0, 15, 20 and 30 mg mL 1, respectively. Herein, for convenience in the discussion of results, the samples are labeled as FABr-0, FABr-15, FABr-20, and FABr-30. 3.2. Film characterization To study the evolved morphologies and crystal structure of the FABrtreated perovskite films, the film texture was examined by scanning electron microscopy (SEM, Fig. 2) and atomic force microscopy (AFM, Fig. S1). Fig. 2 reveals the noticeable morphological differences among final perovskite films with or without FABr treatment. It is noted that the film without FABr-treatment shows branch crystals and surface layer consisting of small grain distribution, indicating a non-uniform growth of grains. The films treated with FABr exhibit gradually increased grain size and better coverage along with increasing FABr concentration, particularly for FABr-20. Comparatively, large and closely packed grain morphology without any pinholes or non-uniformity is observed in
Fig. 2. Top scanning electron microscopy (SEM) images of the perovskite intermediate engineering with different FABr concentration, (a) FABr-0, (b) FABr-15, (c) FABr-20 and (d) FABr-30 film. 3
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Fig. 3. (a) XRD patterns of the mixed perovskite films FABr-treated with different concentrations of FABr. (b) Extended XRD region for peak positions of perovskite films. (c) UV–vis absorption spectra of the mixed perovskite films FABr-treated with different concentrations of FABr. (d) Finger-print region of UV–vis absorption spectra for FABr-0 and FABr-20 films.
Fig. 4. The proposed graphic process for the formation of black α-phase FAxMA1-xPbI3-yBry perovskite via FABr-treatment.
3.3. Photophysical and proposed transitional mechanism
FAxMA1-xPbI3-yBry with large grain size and highly crystalline film, as illustrated in Fig. 4. MAI-PbI2 are firstly solubilized in the mixed DMSO and DMF solvent system, and the limpid MAI-PbI2 solution is spincoated on the FTO substrate. In the intermediate step, the deposited solution is concentrated by fading of solvent, and spatial steric hin drance of FABr and DMSO delays the conversion from layer phase PbI2 to tetragonal perovskite phase. Once washed with anti-solvent (CB), the chlorobenzene accelerates the perovskite film crystallization, nucleation via quick strain extraction, the MAI-PbI2-DMSO intermediate phase formed, which serves as the platform for further molecular substitution. Then, FABr dripping over MAI-PbI2-DMSO results in the rapid trans formation to the MAIxFABr1-xPbI2-DMSO phase through ion exchange, directly leaving a final black perovskite structure after annealing. Here,
UV–vis absorption spectroscopy measurement (Fig. 3c) was per formed to recognize the impact of FABr-treatment on the photo-physical response of the prepared films. Compared with the FABr-0 film, the treated film shows a stronger absorption, indicating a solid light man aging capability of the perovskite layer, FABr-20 film in particular. The comparative magnified fingerprint region of the FABr-0 and FABr-20 films are presented in Fig. 3d. It should be noted that FABr-20 film ex hibits a slight shift in the absorption edge, suggesting an improved en ergy level compared with non-treated perovskite film [41]. On the basis of the above results, we come forward with a credible transitional mechanism for the formation of black α-phase mixed 4
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Fig. 5. (a) Layers arrangement, (b) cross-sectional SEM image of the fabricated device, (c) J-V curves, and (d) the respective IPCE spectra of non-treated and FABrtreated based PSCs.
FABr intercalates into the MAI-PbI2-DMSO adduct, which would trans form into black α-phase FAxMA1-xPb3-yBry perovskite with large grain and highly crystalline film [27].
17.55% to remarkable 20.08% for the FABr-20 treated device. More over, for additional comparative results, the statistical box plot of FF, Voc, Jsc, and PCE of the devices based on FABr-0 and FABr-20 films are presented in Fig. S3. It is worth noticing that the treated films have higher FF, Voc, and Jsc, which verifies FABr-treatment as an effective approach to transforming pure MAI-based perovskite into mixed-cation mixed-halide perovskite due to enhanced photophysical properties from the better cation halide inter-mixing. When the FABr concentration in creases to 30 mg mL 1, the perovskite film undergoes deterioration probably due to the disruption of the crystals, probably due to the excess FABr concentration [27]. Therefore, the FABr-30-based device exhibits an inferior performance. The difference in device photocurrents is determined by the incident photo-to-electron conversion efficiency (IPCE) spectra (Fig. 5d). It can be found that both FABr-treated and controlled devices show a broad reaction in the spectra window of 300–850 nm. The FABr-20-based de vices have a reasonable enhanced photon-to-electron conversion po tency in most of the reactive spectral window compared with the normal device, which is steady with the trend witnessed in J-V curves. The improved IPCE of the device for FABr-20 might be due to two key facts. Firstly, the even and continuous compact film with homogeneous perovskite is achieved via the introduced intermediate engineering method. Secondly, even film coverage causes highly absorptions of visible-light through the active layer, as proved by the UV–vis absorp tion spectra, which also bring operational, mobility and efficient electron-hole in respective layers, lifting the Jsc and Voc [41,43]. The steady-state PCEs were determined by applying a constant bias equal to the voltage at their respective maximum power-point. Fig. S4 displays bias equal to the curves for the reference and improved devices as a function of the time. The PCEs quickly stabilize at 19.51% and 16.03% for FABr-20 and FABr-0 based devices, well-matched with the J-V measurement [39]. These morphological and optoelectronic improve ment is indispensable for the efficient PSCs, lesser non-radiative recombination, efficient charge carrier transport and lengthier elec tron lifetime. The great inconsistency for the PSC without treatment is perhaps due to the sluggish reaction of Jsc, triggered by non-uniformity of the deposited active layer with small grain size and trap states [44]. The statistical distribution of PCEs for 40 individual Trad-Mixed-
3.4. Photovoltaic performance of PSCs To confirm the feasibility of the method in improving the perfor mance of PSCs, we fabricated the corresponding devices. Fig. 5a depicts a schematic diagram of the device configuration with the structure of glass/FTO/SnO2/perovskite/spiro-OMeTAD/gold. The corresponding cross-section SEM image is shown in Fig. 5b. Fig. 5c shows the photo voltaic performance of J-V characteristics of the PSCs measured under an AM 1.5G solar simulator in dry air. The photovoltaic performance of each cell was verified after the intermediate MAI-PbI2-DMSO interme diate treated with varied FABr concentrations in the IPA solution. The performance parameters are summarized in Table 1. In contrast to FABr0 based device, the FABr treated cells exhibit a higher Voc, increasing from the value of 1.05 V for FABr-0 to 1.10 V for FABr-20, probably due to the reduced defects and less carrier recombination as a result of enhanced quality of FABr-20 FABr-treated perovskite. The Jsc is also marginally improved because of the strong light absorption ability. Another additional parameter that determines the efficiency is the fill factor (FF), which is usually affected by the complex mix energy losses, such as electrode surface reflection, shunt and series resistance [42]. In FABr-20 treated based device, a significant improved FF from 72.71% (non-treated) to 77.47% (FABr-20) is observed. The improved FF might be caused by the decreased series and shunt resistance as a result of better crystallinity and enlarged grains with reduced bulk and surface defects [5,42]. Subsequently, the PCE is impressively raised from Table 1 Parameters of the best-performing PSCs based on films treated with different FABr concentration. Device
Jsc (mA cm
FABr-0 FABr-15 FABr-20 FABr-30
22.93 23.15 23.51 23.24
2
)
Voc (V)
FF (%)
PCE (%)
1.05 1.06 1.10 1.08
72.71 74.39 77.47 75.05
17.55 18.33 20.08 19.00
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Fig. 6. Statistical PCEs of (a) Trad-Mixed-PSCs and (b) FABr-20 based on 30 devices. (c) J-V curves of the best-performing devices based on parameters of the TradMixed-PSCs, FABr-20, and FABr-0 based devices.
PSCs and FABr-20 absorber based devices is present in Fig. 6a,b. Seemingly, the statistical constrictions of FABr-20-based PSCs show the much narrower spreading in the boosted efficiency range compared to the Trad-Mixed-PSCs devices. The contempt with a similar ingredient but with the superior performance of FABr-20-based device verify that the intermediate engineering method is highly effective route to assemble scalable and highly efficient mixed cation mixed halide PSCs [37]. As shown in Fig. 6c, the highest PCE of the mixed PSCs from the introduced method reaches 20.08% while, Trad-Mixed-PSCs and FABr-0 based devices show a PCE of 19.09% and 17.55% under similar condi tions. Fig. S6 compares the hysterical profiles of the devices prepared via FABr-treatment and traditional approaches. Interestingly, the device based on FABr-20 absorber shows a small hysteresis (3%) behavior than its both counterparts (15% and 7%), potentially due to fewer trap-states and resultant reduced interfacial ion trapping [37,45,46]. Such extraordinary leaps in FABr-20 outcomes are perks of the developed FABr-treatment technique, which makes precisely embedding of FABr into the MAPbI3 lattice network easy and effective that could be difficult to achieve via traditional approach with similar enhanced performance.
recombination in the perovskite bulk [15,47]. The parameters resulting from the fitting are present in Table S1. As compared to the τ2 of 82.31 ns of the without treated film, the FABr-20 film had extended carrier life time τ2 of 93 ns. Therefore, the proposed method is proved to be an effective technique to reduce the recombination rate in the perovskite layer. The τave value of the FABr-0 film is 76.19 ns, interestingly, the τave value considerably increased to 103 ns for FABr-20-treated perovskite film, signifying an enhanced crystallinity due to the intermixing of cation, halides assistant with the lower energetic disorders with respect to the FABr-0. This boost in carrier lifetime illustrates an enhanced carrier dynamic of the film in terms of the decreased trap states [37,48]. By introducing FABr in the MAPbI3 lattice, the lifetime of FAxMA1-xP bI3-yBry perovskite is improved nearly five times longer than that of without FABr-treated film. Steady-state photoluminescence (PL) spectra of FABr-0 and FABr-20 films are shown in Fig. 7b. It is found that the FAB-20 film exhibits comparatively sharper and improved PL spectrum in comparison to FABr-0 film. This solid PL spectrum is ascribed to the drop in impulsive non-radiative recombination, which might be the reason to boost per formance. The PL intensity image (Fig. 7c,d of FABr-20 film further verify the accuracy for above single point PL measurement (Fig. 7b), indicating a high quality of perovskite film with less defect, consistent with the [47,49]. Space-charge-limited-current (SCLC) technique was implied to investigate the density of the trap state. The J-V dark curves were determined from an electron-injected PSC. Figs. S5(a and b) defines the J-V dark curves of PSCs base on the FABr-0 and FABr-20, respectively. The voltage trap-filled limit (VTFL), which is the conversion from the ohmic region (n ¼ 1) to the trap-filled limit (TFL) region (n > 3), was
3.5. Carrier transport performance and long-term stability To further dig into the optoelectronic changes of FABr-treatment on the films, time-resolved photoluminescence (TR-PL) decay for FABr-20 treated and controlled films was characterized (Fig. 7a). Both films were deposited on the glass side substrate of the FTO [15,45]. The decay curves were distributed into two decay parts, comprising a fast decay (τ1) and slow decay (τ2). The τ1 was attributed to the non-radiative interfacial recombination, and the τ2 was associated with the trap 6
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Fig. 7. (a) Time-resolved photoluminescence (TRPL), (b) Steady-state photoluminescence (PL) for the FABr-0 and FABr-20 films on the substrates of glass, and corresponding mapping intensity image of (c) FABr-0, and (d) FABr-20 films (integrated from 745 nm to 785).
used to determine the trap states density [50,51], as follows: ntrap ¼ ε0 εr VTFL ∕eL
2
measured to be ~7 times longer than that of FABr-0 based device and ~4 times longer than that of Trad-Mixed-PSCs. The results support that the charge carrier recombination through-out the entire device has been suppressed in the FABr-20 based cell. Witnessed decreased trap densities and resultant suppressed charge carrier recombination advocates that the film passivation effect of FABr-0 is effectively boosted the photo voltaic performance, largely reflected in the higher Jsc, Voc and FF values [53]. Our observations of the reduced trap densities and repressed car rier recombination support that the passivation effect of FABr-0 could efficiently enhance the photovoltaic performance, primarily reflected in the higher Jsc, Voc and FF values [27]. At last, the inverted parabola at the low photo-voltage region is mainly attributed to the compact-SnO2/perovskite interface of elec trodes, which shows an obvious difference in without and with FABr devices (Fig. 8c). Furthermore, electron lifetime (τ) could be calculated through the following equation [7]:
(1)
where e is an elementary charge, L represents the active perovskite layer thickness, and ε0 and εr are the comparative di-electric constant of va cuity and FAxMA1-xPbI3-Bry device permittivity, respectively. From the J-V curve in Figs. S5(a and b), it realized that the VTFL of FABr-0 and FABr-20-based devices are 0.671 V and 0.407 V, respectively. Further more, the trap state density reduced from 6.01 � 1015 cm 3 of FABr-0 to 2.10 � 1015 cm 3 of the FABr-20, which was in agreement with the result of TRPL results. The low trap-state density of devices might contribute to the improved Voc and FF of devices [51,52]. Electrochemical impedance spectra (EIS) are measured to explore the dynamics of charge carrier recombination at the interface between perovskite and electron transport layer. As shown in Fig. 8a, recombi nation resistance (Rrec) is fitted by Nyquist plots with a simulated equivalent circuit, under 1 sun illumination. The Rrec of the FABr-20based device is higher than that of FABr-0 and Trad-Mixed-basedPSCs. The large (Rrec) suggests significantly repressed recombination in the FABr-20-based PSC, which possibly caused by the enhanced car rier flow between active perovskite layer and carrier selective layer.34 Open circuit voltage decays measurement can be an effective way to analyses the relationship between charge carrier recombination process and electron lifetime in the anode. It can reveal the connection between the interface recombination rate and the electron lifetime. Fig. 8b shows the Voc decay curves of FABr-0, FABr-20, and Trad-Mixed-PSCs. There are two stages of the curves of electron transport processes in different voltage regions as described in the decay curves effectively. Firstly, the rapid lifetime constant at high voltage region under illumination attributed to free electrons. Secondly, an exponential decrease or in crease at the mean voltage region can be attributed to the internal trapping and de-trapping of electrons due to bulk trap-states. It can be found that the FABr-20 based device displays much slower photovoltage decay, revealing a slow recombination rate of injected elec trons. The charge carrier lifetime in the FABr-treated based PSC was
τ ¼ KB Te 1 ðdVoc ∕dtÞ
1
(2)
where KB denotes the Boltzmann factor, e is the elementary charge, and T stands for the absolute temperature. The electron lifetime (τ) slowly increases with each case at a low voltage region, mainly the FABr treated-based devices, which is consistent with the Voc decay curve. FABr-20 treated device shows a longer electron lifetime than that of FABr-0 and Trad-Mixed-PSCs at the same voltage region, therefore, carrier recombination through-out the whole PSC devices has been repressed in the former device. In the perovskite films with large size of the grain, the photo-generated carriers can simply be collected with-out encountering bulk defects at grain boundaries, where the charge recombination and current leakage could be significantly reduced [27, 53–56]. Brief stability of the PSCs due to the hygroscopic nature of the perovskite film is a known reality, making it difficult for perovskite devices to transit from laboratory scale to industrial scale. Herein, the normalized efficiency of the un-encapsulated devices as a function of exposure time is shown in Fig. 8d. The prepared devices were aged at 7
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Fig. 8. (a) The electrochemical impedance spectroscopy (EIS) of fabricated PSCs at the voltage of 0.90 V, (b) decay Voc curve, (c) the correlation occupation between electron lifetime and Voc of the prepared device, (d) Normalize PCE progression of the prepared devices aged under 25% humid environment for a time period of 1248 h.
25 � C under the dark for a for a period of 1248 h in the air glove box~25% humidity for the long-term stability measurements. The PSC based on FABr-20 retains around 80% of the initial value, whereas the FABr-0 device lost �90% and Trad-Mixed-PSCs degrades by � 55% of their initial PCEs under the same condition. Such resilient stability of FABr-20 based device could be credited to enhanced crystallinity and reduced defect sites of the perovskite films, which did not let alien species intercalate into the perovskite film to trigger structural decom position [27,37,57]. These outcomes further proved that the introduced intermediate engineering technique by FABr possesses great potential to obtain high-performance mixed PSCs and it can be a breakthrough in the near future.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment We acknowledge support from the National Key Research and Development Program of China (No. 2016YFA0202400), the National Natural Science Foundation of China (No. 51572080, U1702096, and 51702096), the 111 Project (No. B16016), and the Fundamental Research Funds for the Central Universities (No. 2017MS021 and 2019MS027). Appendix A. Supplementary data
4. Conclusions
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227386.
In summary, a facile and reproducible method based on intermediate engineering method by FABr is effectively to produce large-grain and less defect active layer. The quality of mixed perovskite film is charac terized qualitatively by PL, TR-PL, SCLC measurements. The synergistic influence of crystals growth regulation and defect passivation by FABr effectively reduce the defect density and increased the carrier lifetime. The resultant FABr-20 based devices demonstrated an improved PCE of 20.08%, which is higher than the reference FABr-0 and the mixed-cation mixed-halide devices from the conventional way and also environmen tally stable than its counterparts. It is expected that the FABr-treatment engineered perovskites can further boost the progress of efficient and less defect PSCs devices for industrial-scale production.
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Declaration of competing interest The authors declare that they have no known competing financial 8
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High-performance mixed-cation mixed-halide perovskite solar cells enabled by a facile intermediate engineering technique Muhammad Mateen a, b, Zulqarnain Arain a, c, Xuepeng Liu a, b, **, Cheng Liu a, b, Yi Yang a, b, Yong Ding a, b, Shuang Ma a, b, Yingke Ren d, Yunzhao Wu a, b, Ye Tao a, b, Pengju Shi a, b, Songyuan Dai a, b, * a
Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, PR China State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing, 102206, PR China Energy Systems Engineering Department, Sukkur IBA University, Sukkur, Pakistan d College of Science, Hebei University of Science and Technology, Shijiazhuang, 050018, China b c
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
� Mixed perovskite can be fabricated by facile intermediate engineering technique. � Developed perovskite exhibits improved quality by proposed method. � Optimized devices show an efficiency of 20.08% with superior stability.
A R T I C L E I N F O
A B S T R A C T
Keywords: FABr Intermediate engineering Mixed-cation mixed-halide perovskite High-quality Stability
Perovskite solar cells (PSCs) have attracted considerable attention in the photovoltaic field. However, most efficient mixed-cation mixed-halide perovskite in PSCs suffers from phase instability and large flux of trap states by existing methods, which limits the device performance. Herein, a facile intermediate engineering technique of the MAI-PbI2 intermediate layer via FABr solution is employed to get high-quality mixed perovskite films. The results illustrate that the proposed approach can improve the grain size, morphology, crystallization, conse quently reducing defect density of the perovskite layer. The devices which treated with FABr exhibit muchenhanced performance in comparison to the pristine and traditional mixed-cation mixed-halide devices. Consequently, a champion PSC with the best power conversion efficiency of 20.08% is obtained. Moreover, the devices based on the developed FABr-treatment technique also shows much-improved stability than the mixed perovskite-based devices fabricated from the traditional method. Therefore, this approach provides a simple technique to produce high-quality mixed perovskite film and subsequently may facilitate the commercialized production of high-performance PSCs in the future.
* Corresponding author. Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, PR China. ** Corresponding author. Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, PR China. E-mail addresses: [email protected] (X. Liu), [email protected] (S. Dai). https://doi.org/10.1016/j.jpowsour.2019.227386 Received 29 August 2019; Received in revised form 15 October 2019; Accepted 30 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Muhammad Mateen, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227386
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traditional compositional engineering approaches for mixed-cation mixed-halide perovskite also induce severe changing in the morphology and crystal structure of the film, associated with halides phase exclusion and light-field-induce ions movement altering the de vice function, despite the high efficiency [24–26]. Therefore, producing phase stable and high-quality perovskite film with minimum trap-density is crucial to obtain high-performance PSCs [27,28]. Studies conveyed that cation or anion intermixing for the perovskite lattice network can be obtained via ions exchange strategies [29]. The progress in post-growth passivation of the underdeveloped film is a more prospective technique to tune the perovskite properties. As an important breakthrough, Seok and coworkers produced a mixed perovskite through incorporation with the MAPbBr3 into FAPbI3, resulting in a black α-phase perovskite at low annealing temperature [30]. Afterward, Xu and co-workers reported the preparation of high-crystallinity thin layers of (FAPbI3)1-x(MAPbBr3)x perovskite via dual ion-exchange method [22]. Zhao et al. demonstrated that the thin film of MAPbI3 treated by a dilute FABr would effectually improve the crystallinity of the final film [31]. The MA-rich perovskite delivers a relatively higher efficiency, which is because highly concentrated MAþ is required to guarantee the formation of the perovskite phase. Recently, dual doping of MAþ and Br was reported to stabilize the FA-rich perovskite phase [30]. Sanith et al. presented the solid-liquid cation-exchange technique to directly transform the MAI perovskite to α-FAI based perovskite [32]. However, the added complexity of the material structure, coexistence of mixed perovskite with undesired yellow phase FAPbI3, uncontrolled ion trapping and the complications in reproducing the performances cast the doubts on the scaling-up of these routes [33–35]. In this work, we demonstrate a simplistic, novel and reproducible technique involving FABr-treatment of underdeveloped MAI perovskite intermediate film with FABr solution to achieve stable mixed perovskite with large grain size, highly-crystalline orientation and decreased trapdensity. By systematically adjusting the FABr concentrations, the highquality FAxMA1-xPbI3-yBry film is directly formed according to the developed method. It is noted that the average PCE was boosted from 17.25% to 19.00% after FABr-treatment, and the best PCE reached 20.08%. Moreover, the long-term stability of PSC is also boosted, which exhibits only 20% efficiency loss after storage for 1248 h in ambient condition.
1. Introduction 2þ þ Perovskite materials (ABX3, A ¼ CH3NHþ or 3 or HC(NH2)2 , B ¼ Pb Sn , X ¼ Cl , Br , or I ) have become the center of attention after their effective adaption in recently emerged applications like perovskite solar cells (PSCs). The exceptional photovoltaic performance of PSCs takes benefits of some exclusive semiconductor properties of the perovskite films such as the strong absorption profile, decent direct optical band gap, and long carrier diffusion length [1–5]. The power conversion ef ficiency (PCE) of PSCs has increased rapidly from the initial 3.8%– 25.2% within a few years [6–10]. Till now, prominent aspects, such as lattice structure, deposition methodology, compositional and stoichio metric engineering of the perovskite material have been investigated and manipulated in efforts to produce efficient and stable perovskite layer [11–14]. Generally, pure methylammonium (CH3NHþ 3 , MA) or for mamidinium (HC(NH2)þ 2 , FA) based perovskite materials (MAPbI3 or FAPbI3) are two most commonly used materials as the active layer in PSCs [15,16]. However, the desired perovskite phases (tetragonal MAPbI3 and trigonal FAPbI3) are sensitive to the transition temperature, humid environment and experience a reversible phase transition be tween their respective perovskite phases and non-perovskite phases (cubic MAPbI3 and hexagonal FAPbI3) [15,17]. Even, the coexistence of both symmetries (perovskite and non-perovskite) for these two mate rials would significantly harm the performance of corresponding PSCs [18]. Mixed-cation mixed-halide PSCs are attested to have some ad vantages over pure perovskite-based devices. Firstly, the inclusion of a minute amount of bromine (Br) and a large amount of iodine (I) within perovskite lattice could significantly lift the open-circuit voltage (Voc) while maintaining the high current density-voltage (Jsc) at the same time [19]. Furthermore, more effective light harvesting with minimum hys teresis in perovskite devices was obtained by embedding MA ions with symmetric FA ions in a balanced stoichiometric proportion [20–22]. However, through orthodox directly mixed precursor methods, the perovskite solution enriched with bromide and iodide tend to nucleate independently during the spin-coating stage because of their phase in-compatibility [12,22]. Thus, many researchers are dedicated to the compositional engineering of mixed-cation mixed-halide perovskite structure for stable and efficient PSCs. Mixed-cation mixed-halide perovskites are usually produced by directly spin-coating blended precursors, including MABr, FAI, and PbI2, PbBr2, etc, in a preferred ratio. Though, in this directly mixing method, iodide and bromide-rich precursor colloidal tend to precipitate inde pendently during the spin-coating stage because of their phase in compatibility. Consequent annealing process further intensifies the crystallization struggle phenomenon and thus low crystallinity, which also results in lots of internal and surface defects [18,20–23]. Moreover, 2þ
2. Experimental section The detail of characterizations and device fabrication are explained in supporting information. In the preparation of perovskite film, lead iodide (PbI2, 461 mg) and methylammonium iodide (MAI, 159 mg) in a mixed solution of DMSO and DMF (4:1 vol ratio) were stirred at 70 � C for 1 h. Above mixed solution were spin-coated on the substrates of (FTO/
Fig. 1. Schematic illustration of the fabrication method for large grain, less defective, highly crystalline mixed perovskite layer via FABr-treatment method.
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SnO2) at 4200 rpm for 30 s, then 110 μL of chlorobenzene (CB) was slowly dropped at the centre of the spinning film in last 10 s. Afterward, 40 μL FABr in isopropanol (IPA) solution were dripped onto the imma ture film with different FABr concentration. Once the FABr solution was dripped over the underdeveloped film, the transparent thin layer sud denly turned into blackish-red color. Finally, FABr-treated film was annealed at a temperature of 100 � C for 15 min and 140 � C for 1 h.
FABr-20 treated film. Furthermore, AFM image (Fig. S1) shows that the root means square roughness of the films are 17.1, 15.5, 14.0, and 14.7 nm for FABr-0, FABr-15, FABr-20, and FABr-30, respectively and supporting that FABr-20 film stands more homogeneous among all the tested films. Morphological smoothness and uniformity at microscale confirmed that intermediate engineering with an optimum amount of FABr in the IPA solution significantly affects the morphology of the final perovskite active layer [18,36]. As shown in Fig. S2, substantial growth in grain dimensions was witnessed as mean grain size surged from ~200 nm for without treated film to ~550 nm for FABr-15 and exceeded ~750 nm for FABr-20 and then declined a bit to ~715 nm for FABr-30 film. After forming the MAIPbI2-DMSO phase, the FAxMA1-xPbI3-yBry intermediate phase further suppresses the nucleation rate and allows grains more time to grow, which leads to a final homogeneous perovskite film with large grain [37, 38]. Generally, the perovskite films with larger grains were demon strated to experience lower trap density and longer carrier lifetimes, certainly as a result of reduced charge accumulation and non-radiative recombination caused by reduced grain boundaries [27,39,40]. There fore, it is admitted that the FABr-treatment has a strong effect on the crystallization and morphology of final mixed-cation mixed-halide perovskite film. To dig deep into the FABr-treatment effect on the final phase of perovskite films, the crystal structure properties of the film are exam ined through X-rays diffraction (XRD, Fig. 3a). It is clearly witnessed that the crystallinity of perovskite films after FABr-treatment is strongly improved. The XRD peak intensity of the treated film rises about fivetimes without any observed peak shift. Therefore, the FABr-treated films have significantly enhanced crystallinity, and the peak intensity rises up to highest at FABr-20 condition. It can also be found in Fig. 3b that the (110) face shows a little shift towards the lower degree. Therefore, the FAxMA1-xPbI3-yBry fabricated with the proposed method is stable, and no phase transition or decomposition was observed during fabrication [39]. The significant improvement in film crystallinity after the FABr-treatment is totally in agreement with the morphological evolution, as discussed earlier [38].
3. Results and discussion 3.1. FABr-treatment process Till now, the directly blended precursors with the desired ratio is a conventional way to prepare mixed perovskite films, which is termed as Trad-Mixed-PSC in this work. It has been proved that the bromide-rich or iodide-rich perovskite colloids easily and leads to phase in compatibility [34,35]. Herein, our proposed technique avoids the blending of mixed precursors. Fig. 1 shows the step-wise method for the preparation of mixed-cation mixed-halide perovskite films via the FABr-treatment method. For FABr-treatment, the varied concentration in IPA solution were 0, 15, 20 and 30 mg mL 1, respectively. Herein, for convenience in the discussion of results, the samples are labeled as FABr-0, FABr-15, FABr-20, and FABr-30. 3.2. Film characterization To study the evolved morphologies and crystal structure of the FABrtreated perovskite films, the film texture was examined by scanning electron microscopy (SEM, Fig. 2) and atomic force microscopy (AFM, Fig. S1). Fig. 2 reveals the noticeable morphological differences among final perovskite films with or without FABr treatment. It is noted that the film without FABr-treatment shows branch crystals and surface layer consisting of small grain distribution, indicating a non-uniform growth of grains. The films treated with FABr exhibit gradually increased grain size and better coverage along with increasing FABr concentration, particularly for FABr-20. Comparatively, large and closely packed grain morphology without any pinholes or non-uniformity is observed in
Fig. 2. Top scanning electron microscopy (SEM) images of the perovskite intermediate engineering with different FABr concentration, (a) FABr-0, (b) FABr-15, (c) FABr-20 and (d) FABr-30 film. 3
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Fig. 3. (a) XRD patterns of the mixed perovskite films FABr-treated with different concentrations of FABr. (b) Extended XRD region for peak positions of perovskite films. (c) UV–vis absorption spectra of the mixed perovskite films FABr-treated with different concentrations of FABr. (d) Finger-print region of UV–vis absorption spectra for FABr-0 and FABr-20 films.
Fig. 4. The proposed graphic process for the formation of black α-phase FAxMA1-xPbI3-yBry perovskite via FABr-treatment.
3.3. Photophysical and proposed transitional mechanism
FAxMA1-xPbI3-yBry with large grain size and highly crystalline film, as illustrated in Fig. 4. MAI-PbI2 are firstly solubilized in the mixed DMSO and DMF solvent system, and the limpid MAI-PbI2 solution is spincoated on the FTO substrate. In the intermediate step, the deposited solution is concentrated by fading of solvent, and spatial steric hin drance of FABr and DMSO delays the conversion from layer phase PbI2 to tetragonal perovskite phase. Once washed with anti-solvent (CB), the chlorobenzene accelerates the perovskite film crystallization, nucleation via quick strain extraction, the MAI-PbI2-DMSO intermediate phase formed, which serves as the platform for further molecular substitution. Then, FABr dripping over MAI-PbI2-DMSO results in the rapid trans formation to the MAIxFABr1-xPbI2-DMSO phase through ion exchange, directly leaving a final black perovskite structure after annealing. Here,
UV–vis absorption spectroscopy measurement (Fig. 3c) was per formed to recognize the impact of FABr-treatment on the photo-physical response of the prepared films. Compared with the FABr-0 film, the treated film shows a stronger absorption, indicating a solid light man aging capability of the perovskite layer, FABr-20 film in particular. The comparative magnified fingerprint region of the FABr-0 and FABr-20 films are presented in Fig. 3d. It should be noted that FABr-20 film ex hibits a slight shift in the absorption edge, suggesting an improved en ergy level compared with non-treated perovskite film [41]. On the basis of the above results, we come forward with a credible transitional mechanism for the formation of black α-phase mixed 4
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Fig. 5. (a) Layers arrangement, (b) cross-sectional SEM image of the fabricated device, (c) J-V curves, and (d) the respective IPCE spectra of non-treated and FABrtreated based PSCs.
FABr intercalates into the MAI-PbI2-DMSO adduct, which would trans form into black α-phase FAxMA1-xPb3-yBry perovskite with large grain and highly crystalline film [27].
17.55% to remarkable 20.08% for the FABr-20 treated device. More over, for additional comparative results, the statistical box plot of FF, Voc, Jsc, and PCE of the devices based on FABr-0 and FABr-20 films are presented in Fig. S3. It is worth noticing that the treated films have higher FF, Voc, and Jsc, which verifies FABr-treatment as an effective approach to transforming pure MAI-based perovskite into mixed-cation mixed-halide perovskite due to enhanced photophysical properties from the better cation halide inter-mixing. When the FABr concentration in creases to 30 mg mL 1, the perovskite film undergoes deterioration probably due to the disruption of the crystals, probably due to the excess FABr concentration [27]. Therefore, the FABr-30-based device exhibits an inferior performance. The difference in device photocurrents is determined by the incident photo-to-electron conversion efficiency (IPCE) spectra (Fig. 5d). It can be found that both FABr-treated and controlled devices show a broad reaction in the spectra window of 300–850 nm. The FABr-20-based de vices have a reasonable enhanced photon-to-electron conversion po tency in most of the reactive spectral window compared with the normal device, which is steady with the trend witnessed in J-V curves. The improved IPCE of the device for FABr-20 might be due to two key facts. Firstly, the even and continuous compact film with homogeneous perovskite is achieved via the introduced intermediate engineering method. Secondly, even film coverage causes highly absorptions of visible-light through the active layer, as proved by the UV–vis absorp tion spectra, which also bring operational, mobility and efficient electron-hole in respective layers, lifting the Jsc and Voc [41,43]. The steady-state PCEs were determined by applying a constant bias equal to the voltage at their respective maximum power-point. Fig. S4 displays bias equal to the curves for the reference and improved devices as a function of the time. The PCEs quickly stabilize at 19.51% and 16.03% for FABr-20 and FABr-0 based devices, well-matched with the J-V measurement [39]. These morphological and optoelectronic improve ment is indispensable for the efficient PSCs, lesser non-radiative recombination, efficient charge carrier transport and lengthier elec tron lifetime. The great inconsistency for the PSC without treatment is perhaps due to the sluggish reaction of Jsc, triggered by non-uniformity of the deposited active layer with small grain size and trap states [44]. The statistical distribution of PCEs for 40 individual Trad-Mixed-
3.4. Photovoltaic performance of PSCs To confirm the feasibility of the method in improving the perfor mance of PSCs, we fabricated the corresponding devices. Fig. 5a depicts a schematic diagram of the device configuration with the structure of glass/FTO/SnO2/perovskite/spiro-OMeTAD/gold. The corresponding cross-section SEM image is shown in Fig. 5b. Fig. 5c shows the photo voltaic performance of J-V characteristics of the PSCs measured under an AM 1.5G solar simulator in dry air. The photovoltaic performance of each cell was verified after the intermediate MAI-PbI2-DMSO interme diate treated with varied FABr concentrations in the IPA solution. The performance parameters are summarized in Table 1. In contrast to FABr0 based device, the FABr treated cells exhibit a higher Voc, increasing from the value of 1.05 V for FABr-0 to 1.10 V for FABr-20, probably due to the reduced defects and less carrier recombination as a result of enhanced quality of FABr-20 FABr-treated perovskite. The Jsc is also marginally improved because of the strong light absorption ability. Another additional parameter that determines the efficiency is the fill factor (FF), which is usually affected by the complex mix energy losses, such as electrode surface reflection, shunt and series resistance [42]. In FABr-20 treated based device, a significant improved FF from 72.71% (non-treated) to 77.47% (FABr-20) is observed. The improved FF might be caused by the decreased series and shunt resistance as a result of better crystallinity and enlarged grains with reduced bulk and surface defects [5,42]. Subsequently, the PCE is impressively raised from Table 1 Parameters of the best-performing PSCs based on films treated with different FABr concentration. Device
Jsc (mA cm
FABr-0 FABr-15 FABr-20 FABr-30
22.93 23.15 23.51 23.24
2
)
Voc (V)
FF (%)
PCE (%)
1.05 1.06 1.10 1.08
72.71 74.39 77.47 75.05
17.55 18.33 20.08 19.00
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Fig. 6. Statistical PCEs of (a) Trad-Mixed-PSCs and (b) FABr-20 based on 30 devices. (c) J-V curves of the best-performing devices based on parameters of the TradMixed-PSCs, FABr-20, and FABr-0 based devices.
PSCs and FABr-20 absorber based devices is present in Fig. 6a,b. Seemingly, the statistical constrictions of FABr-20-based PSCs show the much narrower spreading in the boosted efficiency range compared to the Trad-Mixed-PSCs devices. The contempt with a similar ingredient but with the superior performance of FABr-20-based device verify that the intermediate engineering method is highly effective route to assemble scalable and highly efficient mixed cation mixed halide PSCs [37]. As shown in Fig. 6c, the highest PCE of the mixed PSCs from the introduced method reaches 20.08% while, Trad-Mixed-PSCs and FABr-0 based devices show a PCE of 19.09% and 17.55% under similar condi tions. Fig. S6 compares the hysterical profiles of the devices prepared via FABr-treatment and traditional approaches. Interestingly, the device based on FABr-20 absorber shows a small hysteresis (3%) behavior than its both counterparts (15% and 7%), potentially due to fewer trap-states and resultant reduced interfacial ion trapping [37,45,46]. Such extraordinary leaps in FABr-20 outcomes are perks of the developed FABr-treatment technique, which makes precisely embedding of FABr into the MAPbI3 lattice network easy and effective that could be difficult to achieve via traditional approach with similar enhanced performance.
recombination in the perovskite bulk [15,47]. The parameters resulting from the fitting are present in Table S1. As compared to the τ2 of 82.31 ns of the without treated film, the FABr-20 film had extended carrier life time τ2 of 93 ns. Therefore, the proposed method is proved to be an effective technique to reduce the recombination rate in the perovskite layer. The τave value of the FABr-0 film is 76.19 ns, interestingly, the τave value considerably increased to 103 ns for FABr-20-treated perovskite film, signifying an enhanced crystallinity due to the intermixing of cation, halides assistant with the lower energetic disorders with respect to the FABr-0. This boost in carrier lifetime illustrates an enhanced carrier dynamic of the film in terms of the decreased trap states [37,48]. By introducing FABr in the MAPbI3 lattice, the lifetime of FAxMA1-xP bI3-yBry perovskite is improved nearly five times longer than that of without FABr-treated film. Steady-state photoluminescence (PL) spectra of FABr-0 and FABr-20 films are shown in Fig. 7b. It is found that the FAB-20 film exhibits comparatively sharper and improved PL spectrum in comparison to FABr-0 film. This solid PL spectrum is ascribed to the drop in impulsive non-radiative recombination, which might be the reason to boost per formance. The PL intensity image (Fig. 7c,d of FABr-20 film further verify the accuracy for above single point PL measurement (Fig. 7b), indicating a high quality of perovskite film with less defect, consistent with the [47,49]. Space-charge-limited-current (SCLC) technique was implied to investigate the density of the trap state. The J-V dark curves were determined from an electron-injected PSC. Figs. S5(a and b) defines the J-V dark curves of PSCs base on the FABr-0 and FABr-20, respectively. The voltage trap-filled limit (VTFL), which is the conversion from the ohmic region (n ¼ 1) to the trap-filled limit (TFL) region (n > 3), was
3.5. Carrier transport performance and long-term stability To further dig into the optoelectronic changes of FABr-treatment on the films, time-resolved photoluminescence (TR-PL) decay for FABr-20 treated and controlled films was characterized (Fig. 7a). Both films were deposited on the glass side substrate of the FTO [15,45]. The decay curves were distributed into two decay parts, comprising a fast decay (τ1) and slow decay (τ2). The τ1 was attributed to the non-radiative interfacial recombination, and the τ2 was associated with the trap 6
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Fig. 7. (a) Time-resolved photoluminescence (TRPL), (b) Steady-state photoluminescence (PL) for the FABr-0 and FABr-20 films on the substrates of glass, and corresponding mapping intensity image of (c) FABr-0, and (d) FABr-20 films (integrated from 745 nm to 785).
used to determine the trap states density [50,51], as follows: ntrap ¼ ε0 εr VTFL ∕eL
2
measured to be ~7 times longer than that of FABr-0 based device and ~4 times longer than that of Trad-Mixed-PSCs. The results support that the charge carrier recombination through-out the entire device has been suppressed in the FABr-20 based cell. Witnessed decreased trap densities and resultant suppressed charge carrier recombination advocates that the film passivation effect of FABr-0 is effectively boosted the photo voltaic performance, largely reflected in the higher Jsc, Voc and FF values [53]. Our observations of the reduced trap densities and repressed car rier recombination support that the passivation effect of FABr-0 could efficiently enhance the photovoltaic performance, primarily reflected in the higher Jsc, Voc and FF values [27]. At last, the inverted parabola at the low photo-voltage region is mainly attributed to the compact-SnO2/perovskite interface of elec trodes, which shows an obvious difference in without and with FABr devices (Fig. 8c). Furthermore, electron lifetime (τ) could be calculated through the following equation [7]:
(1)
where e is an elementary charge, L represents the active perovskite layer thickness, and ε0 and εr are the comparative di-electric constant of va cuity and FAxMA1-xPbI3-Bry device permittivity, respectively. From the J-V curve in Figs. S5(a and b), it realized that the VTFL of FABr-0 and FABr-20-based devices are 0.671 V and 0.407 V, respectively. Further more, the trap state density reduced from 6.01 � 1015 cm 3 of FABr-0 to 2.10 � 1015 cm 3 of the FABr-20, which was in agreement with the result of TRPL results. The low trap-state density of devices might contribute to the improved Voc and FF of devices [51,52]. Electrochemical impedance spectra (EIS) are measured to explore the dynamics of charge carrier recombination at the interface between perovskite and electron transport layer. As shown in Fig. 8a, recombi nation resistance (Rrec) is fitted by Nyquist plots with a simulated equivalent circuit, under 1 sun illumination. The Rrec of the FABr-20based device is higher than that of FABr-0 and Trad-Mixed-basedPSCs. The large (Rrec) suggests significantly repressed recombination in the FABr-20-based PSC, which possibly caused by the enhanced car rier flow between active perovskite layer and carrier selective layer.34 Open circuit voltage decays measurement can be an effective way to analyses the relationship between charge carrier recombination process and electron lifetime in the anode. It can reveal the connection between the interface recombination rate and the electron lifetime. Fig. 8b shows the Voc decay curves of FABr-0, FABr-20, and Trad-Mixed-PSCs. There are two stages of the curves of electron transport processes in different voltage regions as described in the decay curves effectively. Firstly, the rapid lifetime constant at high voltage region under illumination attributed to free electrons. Secondly, an exponential decrease or in crease at the mean voltage region can be attributed to the internal trapping and de-trapping of electrons due to bulk trap-states. It can be found that the FABr-20 based device displays much slower photovoltage decay, revealing a slow recombination rate of injected elec trons. The charge carrier lifetime in the FABr-treated based PSC was
τ ¼ KB Te 1 ðdVoc ∕dtÞ
1
(2)
where KB denotes the Boltzmann factor, e is the elementary charge, and T stands for the absolute temperature. The electron lifetime (τ) slowly increases with each case at a low voltage region, mainly the FABr treated-based devices, which is consistent with the Voc decay curve. FABr-20 treated device shows a longer electron lifetime than that of FABr-0 and Trad-Mixed-PSCs at the same voltage region, therefore, carrier recombination through-out the whole PSC devices has been repressed in the former device. In the perovskite films with large size of the grain, the photo-generated carriers can simply be collected with-out encountering bulk defects at grain boundaries, where the charge recombination and current leakage could be significantly reduced [27, 53–56]. Brief stability of the PSCs due to the hygroscopic nature of the perovskite film is a known reality, making it difficult for perovskite devices to transit from laboratory scale to industrial scale. Herein, the normalized efficiency of the un-encapsulated devices as a function of exposure time is shown in Fig. 8d. The prepared devices were aged at 7
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Fig. 8. (a) The electrochemical impedance spectroscopy (EIS) of fabricated PSCs at the voltage of 0.90 V, (b) decay Voc curve, (c) the correlation occupation between electron lifetime and Voc of the prepared device, (d) Normalize PCE progression of the prepared devices aged under 25% humid environment for a time period of 1248 h.
25 � C under the dark for a for a period of 1248 h in the air glove box~25% humidity for the long-term stability measurements. The PSC based on FABr-20 retains around 80% of the initial value, whereas the FABr-0 device lost �90% and Trad-Mixed-PSCs degrades by � 55% of their initial PCEs under the same condition. Such resilient stability of FABr-20 based device could be credited to enhanced crystallinity and reduced defect sites of the perovskite films, which did not let alien species intercalate into the perovskite film to trigger structural decom position [27,37,57]. These outcomes further proved that the introduced intermediate engineering technique by FABr possesses great potential to obtain high-performance mixed PSCs and it can be a breakthrough in the near future.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment We acknowledge support from the National Key Research and Development Program of China (No. 2016YFA0202400), the National Natural Science Foundation of China (No. 51572080, U1702096, and 51702096), the 111 Project (No. B16016), and the Fundamental Research Funds for the Central Universities (No. 2017MS021 and 2019MS027). Appendix A. Supplementary data
4. Conclusions
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227386.
In summary, a facile and reproducible method based on intermediate engineering method by FABr is effectively to produce large-grain and less defect active layer. The quality of mixed perovskite film is charac terized qualitatively by PL, TR-PL, SCLC measurements. The synergistic influence of crystals growth regulation and defect passivation by FABr effectively reduce the defect density and increased the carrier lifetime. The resultant FABr-20 based devices demonstrated an improved PCE of 20.08%, which is higher than the reference FABr-0 and the mixed-cation mixed-halide devices from the conventional way and also environmen tally stable than its counterparts. It is expected that the FABr-treatment engineered perovskites can further boost the progress of efficient and less defect PSCs devices for industrial-scale production.
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