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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Efficient Planar Heterojunction FA1−xCsxPbI3 Perovskite Solar Cells with Suppressed Carrier Recombination and Enhanced Open Circuit Voltage via Anion-Exchange Process Jing Chen,†,‡ Jia Xu,‡,§ Chenxu Zhao,†,‡ Bing Zhang,‡,§ Xiaolong Liu,†,‡ Songyuan Dai,†,§ and Jianxi Yao*,†,‡ Beijing Key Laboratory of Energy Safety and Clean Utilization, ‡State Key Laboratory of Alternate Electrical Power System With Renewable Energy Sources, and §Beijing Key Laboratory of Novel Film Solar Cell, North China Electric Power University, Beijing 102206, China
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ABSTRACT: Introduction of Cs into FAPbI3 displayed great potential to stabilize the black perovskite phase by forming FA1−xCsxPbI3, which has been investigated widely based on solution process. During solution processing, the over-rapid intercalating reaction rate between PbI2 and A cations (FA+ and Cs+) can bring some undesirable structural transitions. However, in vapor-assisted solution process (VASP), the overrapid intercalating reaction rate can be reduced effectively. In addition, the formation process can be regulated significantly by the intermediate perovskite phase. In this study, FACl was employed together with FAI to improve the FA0.9Cs0.1PbI3 films by VASP. In the vapor deposition process, the FACl and FAI vapor coreacted with the PbI2 solid films, preferentially forming the intermediate perovskite phase FA0.9Cs0.1PbIxCly. The intermediate perovskite phase FA0.9Cs0.1PbIxCly supplied a plenty of seeds for rapid nucleation of perovskite, which prolonged the crystallization time of FA0.9Cs0.1PbI3, and thus, a smooth FA0.9Cs0.1PbI3 film with suppressed nonradiative recombination, prolonged carrier lifetime and decreased trap state density was acquired. Corresponding planar heterojunction perovskite solar cells achieved a champion power conversion efficiency (PCE) of 16.39% with a Voc of 0.99 V, Jsc of 22.87 mA/cm2, and fill factor of 74.82% under reverse scanning. Meanwhile, a hysteresis index of the FACl-10 device was decreased to 0.024 compared with 0.075 of the control device. Moreover, under the condition of nitrogen atmosphere, the normalized PCE of FACl-10 device diminished only 4.9% which was more stable comparing with 31.88% diminishing of the control device after 30 days. KEYWORDS: vapor assisted solution process, FA1−xCsxPbI3 perovskite solar cells, anion-exchange process, enhanced open circuit voltage, suppressed carrier recombination
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bond angles leading to an optimum band gap (∼1.45 eV).6 The superior optoelectronic property, as well as thermal stability of black perovskite phase (α-FAPbI3) renders the attractiveness for PSCs. However, the α-FAPbI3 is very sensitive to the temperature and humidity.7 It was reported to encounter free transition to yellow nonperovskite hexagon structure phase (δ-FAPbI3) at room temperature, which hinders its stable photovoltaic performance for practical application.8−10 To overcome the instability issues, mixed cation perovskites have been regarded as one of the effective strategies and the photoelectric properties of the devices have also been improved significantly.11−15 As is well known, the perovskite ABX3 structure can be formed by using A cations with different sizes, which are given
INTRODUCTION Hybrid organic−inorganic perovskite have been regarded as one of the most promising photovoltaic materials because of its extremely low cost and scalable processibility. With their advantages of appropriate band gap, high absorption coefficient, and long carrier diffusion length, 23.7% power conversion efficiency (PCE) has been realized by the perovskite solar cells (PSCs) fabricated by a solution process.1−4 Generally, perovskites share a chemical formula of ABX3, where A is the organic−inorganic cation [A = methylammonium (MA+), formamidinium (FA+) cesium (Cs+) or rubidium (Rb+)], B is the divalent metal (B = Pb2+ or Sn2+), and X is the halide anion (X = Cl−, Br−, or I−).5 In recent years, the natural volatilization disadvantage of MA always results in the strong instability of the devices. Therefore, the FA system has been intensively studied. It has been proved that FA could effectively modify Pb(B)−I(X) bonding length and/or corresponding © XXXX American Chemical Society
Received: October 27, 2018 Accepted: January 3, 2019 Published: January 3, 2019 A
DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces by Goldsmith tolerance factor (t-factor).16 Generally, materials with t-factor in the 0.8−1.0 range can be attributed to the regular cubic black phase perovskite structure. For FAPbI3, more than one structure usually is found depending on the temperature and preparation methods. In addition, it is more stable in hexagonal δ-phase with a t-factor larger than 1.17,18 The t-factor can be reduced from t > 1 to 0.8 < t < 1.0 by introducing a slightly smaller Cs cation into the FAPbI3 lattice, and the equilibrium temperature and formation energy of the desired black perovskite phase could be lowered effectively. This alloying resulted in the improved stability and device performance.17 Besides, on the basis of the one-step solution process, Park and co-workers have primarily demonstrated that the solar cells based on FA1−xCsxPbI3 showed an enhanced moisture and photo stability compared qith pure FAPbI3, ascribing to the enhanced FA and iodide interaction caused by the contraction of cubo-octahedral volume.19 However, with Cs concentration varied when x > 0.1, a yellow phase CsPbI3 and the irregular shifting of X-ray diffraction (XRD) peaks of (101) plane was observed, which indicated the disadvantage of one-step solution process.19 This phenomenon might be resulted from the over-rapid reaction rate in the solution process which leads to the inconsistency between the composition of the final prepared films and the precursor solution.20 Substantial efforts have to be devoted to optimize the fabrication process to obtain the stable phase. On the basis of the two-step solution process, Huang et al. employed Nmethylimidazole (NMI) as an additive in FAI solution. Because of the reduced symmetry of the NMI molecule caused by the interaction between NMI and PbI2-based precursor, the reaction rate between FAI and PbI2 has been slowed down and thus retarded the fast crystallization of perovskites.21 In the presence of NMI, the micrometer grain size film which facilitates the charge dissociation and transportation was achieved. The photovoltaic performance has been enhanced from 12.69% of pure FAPbI3 to 15.38% of Cs0.15FA0.85PbI3.21 Besides, the intermediate perovskite phase has also been demonstrated to play a key role in the growth of perovskite films.22 By incorporating chloride into the precursor solution, the nucleation of perovskites from the amorphous phase was circumvented via creation of kinetically accessible and structurally coherent intermediates.23,24 A two-step solution method with post-interdiffusion of CsI and FAI/ MACl on the substrate has been developed by Liu et al. During the reaction process, an intermediate perovskite phase FAxMA1−xPbIyCl3−y was first formed, which assisted the crystal growth of perovskite film and thermally activated MACl sublimated in the final products. The PCE as high as 20.43% based on Cs-containing FAPbI3 PSC was achieved.25 However, how the intermediate perovskite phase affects the crystal growth process has not been discussed in detail. In the general solution process, it is really difficult to precisely control the final FA1−xCsxPbI3 film composition on account of the volatility of FAI during annealing process.26 Meanwhile, during solution process, the over-rapid intercalating reaction rate between PbI2 and A cations such as MA+, FA+, and Cs+ can bring some undesirable structural transitions.19,25,27−33 However, the over-rapid intercalating reaction rate can be effectively reduced in the vapor-assisted solution process (VASP).34 In vapor deposition, the precursor solution of PbI2 was precoated on the surface of the substrate and then reacted with the saturated ammonium salt vapor. Therefore, the crystal
composition and structure would not be changed by the volatilization of FA.20 To our best knowledge, only one research has been reported about the preparation of FA1−xCsxPbI3 PSCs based on VASP by Luo et al. In their study, the phase-segregation phenomenon was observed in the XRD results which might cause high defects in as-prepared perovskite films. A highest PCE of 14.45% with a Voc of 0.86 V was obtained. The Voc in their study was much lower than that of FA1−xCsxPbI3 PSCs prepared by solution process.26 In this study, the improved FA0.9Cs0.1PbI3 films were prepared by low-pressure VASP (LP-VASP) using the mixture of FACl and FAI as the reaction vapor provider. In the vapor deposition process, FACl and FAI vapor coreacted with PbI2 solid films preferentially forming the intermediate perovskite phase FA0.9Cs0.1PbIxCly. The intermediate perovskite phase FA0.9Cs0.1PbIxCly supplied a plenty of seeds for rapid nucleation of perovskite. Moreover, the intermediate perovskite phase formation process prolonged the crystallization time of FA0.9Cs0.1PbI3, and thus, a smooth FA0.9Cs0.1PbI3 film with large grain size could be acquired. The as-prepared FA0.9Cs0.1PbI3 films exhibited suppressed nonradiative recombination, prolonged carrier lifetime, and decreased trap state density. The film growth processes have been discussed in detail. Corresponding planar heterojunction PSCs were assembled. The Voc enhanced significantly by the mixture of FACl and FAI. In addition, the fill factor (FF) was also improved. The enhancement of Voc and FF resulted in the improved photovoltaic performance, and the champion PCE reached 16.39% with a Voc of 0.99 V, short circuit current density (Jsc) of 22.87 mA/cm2 and FF of 74.82% under reverse scanning. Meanwhile, the hysteresis index (HI) of FACl-10 device (0.024) was smaller than that of the control device (0.075), the hysteresis effect of the corresponding devices alleviated with the introduction of FACl. Furthermore, stored under nitrogen atmosphere for 30 days, the normalized PCE of FACl-10 device diminished only 4.9% which was more stable comparing with 31.88% of the control device.
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RESULTS AND DISCUSSIONS The perovskite FA1−xCsxPbI3 (x = 0, 0.05, 0.1 and 0.15) have been prepared by LP-VASP, and the best condition was x = 0.1 (FA0.9Cs0.1PbI3) according to the results of Figure S1. Besides, the absorption intensity of the FA0.9Cs0.1PbI3 film reduced (Figure S2a) and the crystallinity became poor (Figure S2b) when the vapor-treated time was longer than 30 min. In addition, the corresponding PCEs also deteriorated after longer vapor deposition (Figure S2c,d). It can be demonstrated that although the perovskite films did not decompose, the quality of perovskite film was degraded. Therefore, the FA0.9Cs0.1PbI3 perovskite material with vapor deposition 30 min would be used in the later discussion. Deposited by different FACl content, the morphologies of FA0.9Cs0.1PbI3 films were characterized by scanning electron microscopy (SEM) (Figure 1a−d). The SEM results showed that the morphologies of all the films prepared by VASP were pinhole-free, uniform, dense, and fully covering the substrate. The corresponding Gaussian distributions of the grain size in each case are shown in Figure 1e−h. Because of the introduction of FACl, the mean grain size gradually enlarged from 450 nm of control film to 780 nm of FACl-5 film and 810 nm of FACl-10 film. However, the mean grain size of FACl-15 decreased to 400 nm drastically. The influence of different FACl concentrations on morphology would be discussed later B
DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
position. Additionally, the absorption edge onset at ∼812 nm also did not shift obviously of control, FACl-5, FACl-10, and FACl-15 films (Figure 1j). The band gap obtaining from the Tauc plot was inserted in Figure 1j. The value of band gap was regarded as the intercept of (αhν)2 against light energy (hν) plotting, which was established to be 1.53 eV. The band gap of all the films remained unchanged. Moreover, X-ray photoelectron spectroscopy (XPS) was carried out to prove no residual Cl element in the final films accurately (Figure S3). From Figure S3b, it can be seen that no Cl element was detected in FACl-5, FACl-10, and FACl-15 films. And the corresponding high-resolution XPS spectra of Pb 4f (Figure S3c) and I 3d (Figure S3d) did not move. These results demonstrated that the composition of all final films was FA0.9Cs0.1PbI3.35 To further explore the effect of Cl on the growth of FA0.9Cs0.1PbI3, the growth kinetics was monitored by XRD for understanding the growth process of the films. Because high FACl content would degenerate the crystallinity of the FACl15 film, the comparative study was conducted between the control and the FACl-10 film. XRD patterns from 11° to 15° region of the control and FACl-10 films with the time evolution (0−30 min) are shown in Figure 2. Corresponding XRD patterns over the entire range of angles are shown in Figure S4. The measured reaction time was 0, 3, 5, 7, 10, 15, 20, 25, and 30 min, respectively. For the XRD patterns of the control film (Figure 2a), the characteristic diffraction peak at 14.06° which was assigned to the (101) crystallographic plane of FA0.9Cs0.1PbI3 appeared at 7 min. When the reaction time proceeded from 7 to 30 min, the peak intensity increased without peak position shifting. As the reaction goes on, the peak intensity of 12.7° belonging to PbI2 (001) crystallographic plane decreased. It can be concluded that the lead source precursor directly reacted with FAI vapor, forming the FA0.9Cs0.1PbI3 gradually. However, different from the control film, after lead source precursor-coated substrates put into the FAI and FACl vapor atmosphere only for 3 min, a characteristic peak at 14.07°, that is, FA0.9Cs0.1PbIxCly (101) plane appeared in the XRD pattern of the FACl-10 film (Figure 2b). The corresponding (101) peak position shifted from 14.07° to 14.09° at 5 min, 14.11° at 7 min, 14.10° at 10 min, and then moved back to 14.06° at 15 min. In the whole reaction process, the peak intensity of the PbI2 (001) turned weaker and disappeared at 20 min. Therefore, it can be concluded that an intermediate perovskite phase FA0.9Cs0.1PbIxCly was first formed in the film. Then, as the reaction time increases, much more Cl ions intercalated into the FA0.9Cs0.1PbIxCly lattice accompanied by the movement of the diffraction peak at 14.07° to 14.11° at 7 min.23,36 As the reaction proceeds, the characteristic peak of FA0.9Cs0.1PbIxCly moved back to 14.06° which was the position of FA0.9Cs0.1PbI3, indicating the sublimation of Cl anions in the final prepared film. Generally, at the beginning of this reaction, I ions and Cl ions entered the lattice simultaneously. Afterward, Cl ions were replaced by I ions gradually. The Cl ions sublimated in the form of FACl at high temperature. The exchange process between anion I ions and Cl ions induced by FACl was defined as anion-exchange process (AIE process). To illustrate the beneficial influences of the AIE process accurately, the evolution of relative intensity of the characteristic peak at 14.06° to that of the fluorine doped tin oxide (FTO) at 37.87° is shown in Figure 2c. The FACl-10 film nucleated earlier (3 min) than the control film (7 min).
Figure 1. Morphological, XRD and UV analysis of FA0.9Cs0.1PbI3 films prepared at 30 min. (a−d) Top-view SEM images of perovskite films (a) without and with (b) 5, (c) 10, and (d) 15 mg FACl doped, respectively. (e−h) Gaussian distribution of the grain size measured from the corresponding SEM images by nano measure. (i) XRD patterns of the perovskite films fabricated with different FACl involved. The black “*” and black diamond “⧫” represent the substrate FTO/TiO2 and characteristic peaks of FA0.9Cs0.1PbI3. (j) UV−vis absorption of the control, FACl-5, FACl-10, and FACl-15 perovskite films.
combined with the analysis of the XRD and SEM evolution on growth kinetics. The usage of FACl could properly enlarge the grain size to some extent. Whether the large grain size is beneficial for the carrier transportation and collection remains to be determined. To reveal the inherent influence mechanism, different measurements were employed for further verification. First, the crystal structure of the ultimate films prepared with and without FACl was investigated (Figure 1i). The characteristic peaks of FA0.9Cs0.1PbI3 were labeled by the black solid diamonds. In addition, all the perovskite films displayed the same peak position at 2θ angle of 14.06°, 19.92°, 24.44°, 28.25°, 31.67°, 40.37°, and 42.92° which were indexed to (101), (110), (202), (220), (222), (400), and (330) crystallographic planes of pure FA0.9Cs0.1PbI3, respectively. The enlarged view of XRD patterns for all the films at 14.06° is inserted into Figure 1i, clearly showing that no shifting of peak C
DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. XRD diffraction patterns of (a) the control and (b) FACl-10 perovskite films with the evolution of reaction process. The reaction time was assigned 0, 3, 5, 7, 10, 15, 20, 25, and 30 min, respectively. (c) The variation of relative intensity of perovskite (110) peak (2θ = 14.09°) and substrate peak (2θ = 37.9°) in the whole reaction time with and without FACl doped, respectively.
During the whole reaction process, the relative intensity of FACl-10 film was still stronger than that of control film. The formation of both perovskite films kept going on until the end of the reaction. Furthermore, the SEM images of control and FACl-10 films at 0, 3, 7, 20, 25, and 30 min are shown in Figure S5. It can be seen that dark perovskite grains appeared until 7 min of the control film (Figure S5c). For the FACl-10 film, the dark perovskite grains emerged at 3 min (Figure S5h). With the reaction time proceeding, only a small amount of unreacted PbI2 remaining at the grain boundaries was left (Figure S5j). However, for the control film, the PbI2 still remained at 25 min. Until the end of the reaction at 30 min, the finally perovskite films were formed. Besides, the corresponding UV− vis absorption spectra (Figure S6) were also consistent with the results of XRD and SEM. These results demonstrated that the exchange of I ions and Cl ions was completed on the base of the FA0.9Cs0.1PbIxCly perovskites. Therefore, these prenucleation grains were defined as the seeds of FA0.9Cs0.1PbI3 formation. Besides, it has been widely demonstrated that rapid nucleation and slow crystal growth were favorable for the perovskite film with a large grain size.21,37 From the XRD results of the obtained film with FACl (Figure 2b), it could be observed that the characteristic peak of the intermediate perovskite phase FA0.9Cs0.1PbIxCly appeared at 3 min. With the process proceeding, I ions gradually substituted the Cl ions to form FA0.9Cs0.1PbI3. From 3 to 30 min, it took about 27 min for the crystal growth with FACl treatment. However, without FACl (Figure 2a), the characteristic peak of FA0.9Cs0.1PbI3 appeared at 7 min, indicating a relative slower nucleation, and the crystal growth time was shorter than the time of FACl system shown in Figure 2b. With the increase of FACl, the alteration of the nucleation and crystal growth may be positive to the large grain size to some extent. Therefore, the grain size of the film increased because of the introduction of FACl. With the FACl concentration increasing, the number of the nucleation centers might be increased. As we know, crystal growth was the process of dissolution and recrystallization. Excessive nucleation might be one of the main reasons for the decreased grain size in FACl-15 film. XPS analysis was performed to identify the compositions of the samples for further verification. Survey XPS spectra of FA0.9Cs0.1PbIxCly films prepared at 0, 3, 5, 7, 10, 15, 20, 25, and 30 min are shown in Figure 3a. Corresponding high-resolution
Figure 3. (a) Survey XPS spectra of FA0.9Cs0.1PbIxCly films prepared at 0, 3, 5, 7, 10, 15, 20, 25, and 30 min, respectively. (b) Corresponding high-resolution XPS spectra of the FA0.9Cs0.1PbIxCly films at Cl 2p regions.
XPS spectra of the FA0.9Cs0.1PbIxCly films at Cl 2p regions are shown in Figure 3b. The peak of Cl 2p was detected when the reaction process at 3, 5, 7, 10, and 15 min. During the reaction process, the peak position of Cl 2p shifted to higher binding energy from 198.09 eV at 3 min to 198.38 eV at 7 min. The peak shifts ascribed to the intercalation of Cl ions into the lattice and the improvement of chemical bonding, which is consistent with the compressed lattice confirmed by XRD (Figure 2b). As the reaction goes on, the peak position of Cl 2p shifted back from 198.38 eV at 7 min to 198.26 eV at 15 min. The shifts in binding energy were clear evidence for I ions and Cl ions exchanging after the long-time reaction. When the reaction time was longer than 15 min, the peak of Cl 2p disappeared, indicating that the Cl ions have been sublimated in the form of FACl. Thus, the final component of perovskite films was FA0.9Cs0.1PbI3. Besides, to verify when the Cl disappeared for other two cases, XPS of FACl-5 and FACl-15 film was carried out (Figure S7). The Cl signal disappeared after 5 min in the reaction process of the FACl-5 film (Figure S7a). For the FACl-15 film, the Cl element signal vanished after 15 min (Figure S7b). Although the Cl element disappeared at the same time during the FACl-10 and FACl15 reaction process, the binding energy of Cl reached maximum at 5 min of FACl-15 film which was earlier than 7 min of FACl-10 film. This might be due to the difference in the amount of gas molecules (FAI and FACl vapor) in vacuum. To demonstrate the Cl ions supplied by FACl were enough to D
DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces exchange with I ions during the reaction process,energydispersive X-ray spectroscopy (EDS) spectral characterizations of the remained powder with different FACl contents (5, 10 and 15 mg) was executed after VASP (Figure S8). After VASP, Cl element could be detected, indicating the existence of FACl in the remained powder. These results confirm that there was sufficient FACl involved in the reaction process. On the basis of the above investigations, a reasonable quality optimization mechanism for perovskite films was proposed. The detailed formation processes of the FA0.9Cs0.1PbI3 perovskite with and without FACl are shown in Figure 4.
Figure 5. (a) Steady-state PL spectra and (b) TRPL decay of FA0.9Cs0.1PbI3 films of control and FACl-10 films, respectively.
components, a fast (τ1) decay ascribed to the surface recombination and a slow (τ2) decay related to the recombination occurring in bulk perovskite.38,39 To evaluate the whole lifetime of the perovskite films, the average lifetime was calculated (tave), which took into account both time constants and their weighting. The corresponding parameters derived from the fitted curves are summarized in Table 1. For Table 1. Time Parameters Derived from the Fitting Results of the Transient TR-PL Decay Curves Shown in Figure 5b τ1
Figure 4. Detailed formation processes of FA0.9Cs0.1PbI3 perovskite. The formation processes for the perovskite (a) without and (b) with FACl.
τ2
τave
samples
value (ns)
rel. (%)
value (ns)
rel. (%)
value (ns)
controlled FACl-10
1.659 1.857
79.59 72.52
15.17 24.64
20.41 27.48
11.50 21.18
the two components, the calculated carrier lifetime was increased from 1.66 ns (τ1), 20.41 ns (τ2) of control film to 1.86 ns (τ1), 27.48 ns (τ2) of FACl-10 film, respectively. Both the interface and the bulk perovskite were improved by the formation of the intermediate perovskite phase. Moreover, the tave of the control film (11.50 ns) was prolonged to 21.18 ns of the FACl-10 film. By the AIE process, the carrier lifetime of the FACl-10 perovskite film was approximately 2 times longer than that of the control one. It can be concluded that the perovskite film deposited by AIE effectively suppressed the nonrecombination and enhanced the carrier lifetime. Hence, the quality of FA0.9Cs0.1PbI3 films was improved significantly. However, because of the low trap state density in perovskite film, some measurement conditions such as the films thickness would affect the carrier lifetime obtained from the TRPL results.40,41 Although the composition of control film and FACl-10 films was FA0.9Cs0.1PbI3, the carrier lifetime exhibited distinct difference. The trap state might impact the quality of perovskite film significantly. For quantification analysis, the space charge limited current (SCLC) method was used to study the trap states density. The dark J−V curves were measured from an electron-injecting device. Figure 6a,b describes the dark J−V curves of devices based on the control film and FACl-10 film, respectively. The voltage (VTFL) which marked the transition from the ohmic region (n = 1) to the trap-filled limit (TFL) region (n > 3) was used to calculate the trap state density by eq 1.42−45
Initially, the lead source precursor solution contained PbI2 and CsI. Without FACl, FAI gas molecule and inorganic CsI inserted into the inorganic octahedral framework [PbI6]4− layer to form FA0.9Cs0.1PbI3 perovskite in vapor deposition (Figure 4a). However, with FACl, there was an intermediate transition process (AIE process) (Figure 4b). Cl ions with a small ionic radius (181 pm) preferentially intercalated into the [PbI6]4− together with I ions (ionic radius 220 pm). The formation of FA0.9Cs0.1PbIxCly intermediate perovskite phase supplied the nucleation center sites rapidly. In the subsequent reaction, because the Cl has much weaker bond affinity to Pb than I in lattice under the 170 °C reaction temperature, the I ions and Cl ions exchanged gradually and the Cl ions sublimated in the form of FACl.23,24 Through AIE process, the high-quality perovskite film with the enlarged grain size was prepared, which would possess the lowered nonrecombination and prolonged carrier lifetime. To explore the charge separation and recombination dynamics in perovskite without and with the AIE process, the PL spectra were characterized. A comparative study was conducted on the control film and FACl-10 film. The steadystate PL intensity of the FACl-10 film was an order of magnitude higher than that of the control film, indicating that the FACl-10 film had a relatively low nonradiative recombination loss (Figure 5a). For further verification, time-resolved photoluminescence (TRPL) measurement (Figure 5b) was also carried out. Perovskite films prepared on the quartz substrate without other layers were used for the characterization. Biexponential decay function was used to fit the decay curves. Generally, the function contained two decay
ntrap = 2ε0εrVTFL /eL2
(1)
where the L is the thickness of the perovskite films, e is the elementary charge, and ε0 and εr are the relative dielectric constant of vacuum and FA0.9Cs0.1PbI3 film permittivity, respectively. From the J−V curves (Figure 6a,b), it can be E
DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 6. Dark J−V measurement of the electron-only devices displaying VTFL bend point behavior for perovskite films prepared without (a) and with (b) FACl doped. (c) Statistical results of corresponding electron mobilities.
seen that the VTFL of control device and FACl-10 devices was 0.772 and 0.575 V, respectively. In addition, the corresponding trap states density decreased from 4.17 × 1015 cm−3 of the control film to 2.07 × 1015 cm−3 of the FACl-10 film, which was consistent with the results of TRPL measurement. Low trap-state density of perovskite film might associate with the enhanced FF and V oc of the corresponding devices. Furthermore, the intrinsic mobility distribution estimated by SCLC is shown in Figure 6c.44−46 In addition, the average electron mobility was estimated by SCLC increased from 0.11 cm2 V−1 s−1 of control film to 0.27 cm2 V−1 s−1 of FACl-10 film. For further verification, the trap state density of FACl-5 and FACl-15 film was also measured (Figure S9). The average electron mobility was 0.22 and 0.21 cm2 V−1 s−1 for FACl-5 and FACl-15, respectively. Perovskite films prepared by AIE process possessed the decreased trap state density. The devices were equipped with the typical configuration of FTO/TiO2/C60/FA0.9Cs0.1PbI3/sprio/Au, and corresponding cross-section images are shown in Figure 7b. To accurately study the impact of AIE process on the photovoltaic performance, 20 devices under each condition were fabricated. J−V curves of the optimal performance device under each condition under forward and reverse scanning are shown in Figure 7a, respectively. Corresponding performance parameters are summarized in Table 2. PSCs fabricated without the AIE process performed the best PCE of 14.27% with a Voc of 0.89 V, Jsc of 23.01 mA/cm2, and FF of 69.90% under reserve scanning. Among all the cases, devices based on FACl-10 film possessed the optimtimal PCE of 16.39% compared with other devices, with a Voc of 0.99 V, Jsc of 22.87 mA/cm2, and FF of 74.82% under reverse scanning. The photovoltaic performance was superior to the PCE of 14.45% and Voc of 0.86 V reported by Luo et al.’s,26 especially the Voc enhanced significantly. With the excessive FACl, the device efficiency deteriorated to 15.16% with a Voc of 0.93 V, Jsc of 22.19 mA/cm2, and FF of 73.28% under reverse scanning. Besides, the hysteresis effect
Figure 7. (a) J−V curves of best-performing PSCs based on control, FACl-5, FACl-10, and FACl-15 under reverse and forward scanning. (b) Cross section of the devices fabricated by VASP. Statistical results for (c) Voc, (d) Jsc, (e) FF, and (f) PCE values of PSCs based on perovskite films without and with different content of FACl. Twenty samples of each device set were measured.
has been reported widely as one of typical characteristics of PSCs performance. To date, HI was considered to be a reasonable parameter for quantifying hysteresis effect.35 The calculated HIs were 0.075, 0.033, 0.024, and 0.051 for control, FACl-5, FACl-10, and FACl-15 devices, respectively. The minimized HI of FACl-10 device might be attributed to the improved quality of the perovskite film with lowered trap states density. Additionally, statistical results of Jsc, Voc, FF, and PCE of all the PSCs without and with AIE process are shown in Figure 7c,d. Different from the slight change in Jsc, the Voc and FF were improved significantly, especially for Voc. Besides, the values of average statistical efficiency (PCEave) under each condition showed the same trend with the efficiency of the optimal device shown in Table 2. Considering all the cases, the FACl-10 devices achieved the highest PCEave of 15.56%. From the statistical results, it can be seen that the increased PCEs were attributed to the improvement of Voc and FF. The high quality of perovskite films prepared by the AIE process with suppressed nonradiative recombination, prolonged carrier lifetime, and lowered trap state density might be the main reason of the elevated Voc and FF. The IPCE spectrum of the optimal PSCs was carried out (Figure S10). The integrated photocurrent calculated from IPCE spectrum of the PSC based on FACl-10 film was 20.97 mA/cm2, which was slightly lower than the measured Jsc under reverse and forward scanning. This might be attributed to the physical and chemical changes in the measurement, which would influence the photovoltaic performance of PSCs. The strong absorption in visible light F
DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Table 2. Parameters of Best Device Performance of Each Case under Reverse and Forward Scanning in Figure 7a samples control FACl-5 FACl-10 FACl-15
reverse forward reverse forward reverse forward reverse forward
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
HI
PCEave (%)
0.89 0.88 0.95 0.95 0.99 0.98 0.93 0.94
23.01 22.98 22.70 22.87 22.32 22.24 22.19 22.06
69.90 59.57 71.41 63.43 74.82 70.54 73.28 66.19
14.27 12.08 15.35 13.90 16.39 15.50 15.16 13.76
0.075
13.59 ± 0.44
0.033
14.71 ± 0.48
0.024
15.56 ± 0.49
0.051
13.85 ± 0.61
Because the recombination rate of the control device was higher comparing with that of the FACl-10 device, it was significant to quantify the electron lifetime in devices. Voc decay was one of the effective strategies to investigate the electron recombination process in the perovskite film and anode.50,51 Figure 8c shows the Voc decay curves of PSCs based on control film and FACl-10 film. The Voc decay curves can be explained effectively with three electron transport processes in different voltage regions. Under illumination, the high voltage region was attributed to free electrons and at low voltage, the inverted parabola region was mainly related to the electrodes interface.50,51 The internal trapping and de-trapping of electrons in bulk perovskite materials reflected by exponential increase voltage region was the critical consideration of same composition photoactive layer (FA0.9Cs0.1PbI3) prepared through different reaction process. The electron lifetime could be obtained by eq 2
range and effective electron−hole separation and collection of FA0.9Cs0.1PbI3 film resulted in the relative high IPCE. Electrochemical impedance spectroscopy (EIS) was employed to extract more information about the transportation and recombination mechanism of PSCs. The Nyquist plots of cells (Figure 8a) based on control and FACl-10 films were
τn = −kBTe−1(dVoc/dt )−1
(2)
where T was the temperature, kB was the Boltzman constant, and e was the elementary charge. Figure 8d shows the relationship between the calculated τn of injected carriers in cells and the Voc of control and FACl-10 devices. The τn of FACl-10 device was longer than that of control device. For example, for the PSCs based on control and FACl-10 films at the voltage of 0.4 V, the τn were 0.29 and 0.91 s. It can be concluded that prepared by AIE process the electron lifetime increased and the charge recombination rate of perovskite absorbing layer reduced consisting with the EIS results (Figure 8b). Furthermore, the stabilized output at the maximum power point (Vmax) of the best performing control and FACl-10 devices and the corresponding performance measured for 30 days stored in N2 atmosphere were carried out (Figure 9). Under Vmax, the output of both devices was relatively stable.
Figure 8. (a) Nyquist plots of devices based on control and FACl-10 perovskite films at dc bias of 0.8 V under 1 sun illumination. (b) Fitted parameters recombination resistance (Rrec) and series resistance (Rs) at different applied voltage from 0 to 0.8 V derived from the fit model in Figure S6. (c) Transient photovoltage decay curves and (d) the relationship between extracted lifetime of the injected carriers in the devices and Voc of the PSCs based on control and FACl-10.
measured under AM 1.5 at dc bias voltage of 0.8 V. The semicircle that appeared in the low-frequency range was ascribed to the recombination resistance (Rrec) between the electron transport layer and the perovskite layer. The recombination rate of photo-generated carriers was inversely related to the Rrec.47,48 Rs was regarded as the series resistance of the external circuit and electrode.47−49 In our study, the composition of perovskite layer and the contact layer were unchanged, so the recombination process was the main consideration. Rs and Rrec derived from the equivalent circuit diagram (Figure S11) at different applied voltage (0−0.8 V) are shown in Figure 8b. It can be observed that the Rs of FACl10 device was smaller than that of the control device, which resulted in higher FF. The lower Rrec of the control device resulted in the higher recombination rate of photo-generated carriers. This result was identical with the results of photovoltaic performance in Figure 7.
Figure 9. (a) Stabilized output holding the voltage at the maximum power point for the best performing PSCs based on control and FACl-10. (b) Normalized PCE of devices based on the control and FACl-10 films in 30 days. G
DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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perovskite film resulted in the significantly enhanced Voc. FF was also increased and the change tendency consisted with Voc. The champion PCE among all the cases was 16.39% with a Voc of 0.99 V, Jsc of 22.87 mA/cm2, and FF of 74.82%, and the control device possessed a PCE of 14.27% with a Voc of 0.89 V, Jsc of 23.01 mA/cm2, and FF of 69.90% under reverse scanning. Meanwhile, the HI of FACl-10 device was decreased to 0.024 compared with 0.075 of control device. Moreover, the normalized PCE of FACl-10 device diminished only 4.9% which was more stable comparing with 31.88% of the control device after 30 days put in nitrogen atmosphere.
However, the PCE of control device possessed a significant attenuation compared to FACl-10 device. It can be seen within 30 days that the FACl-10 devices fabricated by AIE process exhibited stable performance with only 4.9% declined. However, the PCE of control sample reduced 31.88%. It might correlate to the high quality of perovskite film with reduced defects, which would decrease the probability of the decomposition caused by undesirable water penetration.
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EXPERIMENTAL SECTION
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Materials. HC(NH2)2I (FAI) and HC(NH2)2Cl (FACl), FK209cobalt(III)-TFSI, Li-TFSI, and spiro-OMeTAD were purchased from Borun Chemicals (Ningbo, China). CsI (99.9%), PbI2 (99%), 1,2dichlorobenzene (98%), chlorobenzene, and titanium(IV) isopropoxide (98+%) were purchased from Acros. N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and C60 were purchased from Alfa Aesar. Anhydrous isopropanol was purchased from J&K Scientific Co., Ltd. tBP was purchased from Sigma-Aldrich. All chemicals were used directly without further purification. Glass substrates with a transparent FTO (thickness 2.2 mm, 15 Ω/square) layer were used for the PSCs. Device Fabrication. The detailed preparation process of the planar structure of FTO/TiO2/C60/FA0.9Cs0.1PbI3/spiro/Au devices was described below. The cleaning of the conductive substrate (FTO) and the preparation of the dense layer (TiO2/C60) were the same as our provisos reports.35 The procedure of the spin-coated lead source was prepared as follows. The 1 M PbI2 and 0.1 M CsI in DMF/ DMSO solution (kept at 70 °C) was spin-coated onto the FTO/cTiO2/C60 layer at 3000 rpm for 30 s and then the lead source-coated substrates annealed at 70 °C for 15 min in a N2-filled glovebox.20,35 In the vapor deposition process, the gas-phase reaction powders were the mixture of FAI and FACl. The total mixture mass of FAI and FACl was constant (0.2 g) with varied mass of FACl. Several different concentrations of FACl powder, including 2.5 wt % (5 mg FACl), 5 wt % (10 mg FACl), and 7.5 wt % (15 mg FACl) labeled as FACl-5, FACl-10, and FACl-15 were used to fabricate different mixed PSCs. In addition, FA0.9Cs0.1PbI3 films not deposited with FACl was recorded as control. The other specific detailed processes were consistent with our previous study.20 Then, the hole transportation layer (spiro-OMeTAD) was spin-coated on the perovskite layer. Finally, the back electrode (60 nm) Au was thermally evaporated on top of spiro-OMeTAD.20,35 Characterization. The characterizations of measurements in this study, including the current density−voltage (J−V) characteristic, SEM, XRD, UV−vis, X-ray photoemission spectra (XPS), steady-state fluorescence spectra (PL) and TR-PL spectra, impedance spectroscopy measurement, and Voc decay (on the period of 10 s), were consistent with the previous study.35 Besides, the EDS spectra measurement was carried out with a SU8010SEM (Hitachi) equipped with an EDS detector. The electron-only devices with the FTO/cTiO2/perovskite/PCBM/Ag structure were fabricated to measure the dark J−V curves, and the electron mobility was derived from the fitted J−V curves by the Mott−Gurney equation. The trap state density was determined by the TFL voltage. The best-performing PSCs were stored in nitrogen atmosphere at 22 °C and ∼10% humidity for 30 days, and the J−V characteristics were measured every day.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18807.
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Histograms of PCEs of CsxFA1−xPbI3 with different concentrations; UV−vis absorption and XRD of FA0.9Cs0.1PbI3 films after 30, 40, and 120 min vapor deposition and corresponding histograms of PCEs, XPS characterization of the control, FACl-5, FACl-10, and FACl-15 perovskite films and the corresponding highresolution XPS spectra of the FA0.9Cs0.1PbIxCly films at Cl 2p regions; XRD diffraction patterns of the control and FACl-10 perovskite films with the evolution of reaction process from 11° to 51°; SEM images of the control film and FACl-10 film with the evolution of reaction time; EDS spectral characterizations of FACl-5, FACl-10 and FACl-15 films; calculated trap density and electron mobility of perovskite films with different FACl concentration; and IPCE spectra of FA0.9Cs0.1PbI3 film based on FACl-10 and the equivalent circuit diagram of the Nyquist plots (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Songyuan Dai: 0000-0001-5710-9208 Jianxi Yao: 0000-0002-5472-9337 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (no. 51772095), the Fundamental Research Funds for the Central Universities (nos. 2018MS040 and 2018ZD07)
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CONCLUSIONS In summary, we employed FACl to optimize the perovskite crystallization from the nucleation regulation and crystal growth in vapor deposition. The pure FA0.9Cs0.1PbI3 film with lowered nonradiative recombination, prolonged carrier lifetime, and decreased trap state density was formed by the AIE process. Experiment results have demonstrated the existence of FA0.9Cs0.1PbIxCly intermediate phase, as well as its great contribution to the optimized perovskite crystallization and the photovoltaic performance. The high-quality
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REFERENCES
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DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.8b18807 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
