Mixed-Organic-Cation (FA)x(MA)1âxPbI3 Planar Perovskite Solar

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Mixed-Organic-Cation (FA)x(MA)1−xPbI3 Planar Perovskite Solar Cells with 16.48% Efficiency via a Low-Pressure Vapor-Assisted Solution Process Jing Chen,†,‡ Jia Xu,†,§ Li Xiao,†,§ Bing Zhang,†,‡ Songyuan Dai,*,†,§ and Jianxi Yao*,†,‡ †

State Key Laboratory of Alternate Electrical Power System With Renewable Energy Sources, North China Electric Power University, Beijing 102206, China ‡ Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing 102206, China § Beijing Key Laboratory of Novel Film Solar Cell, North China Electric Power University, Beijing 102206, China S Supporting Information *

ABSTRACT: Compared to that of methylammonium lead iodide perovskite (MAPbI3), formamidinium lead iodide perovskite (FAPbI3) has a smaller energy band gap and greater potential efficiency. To prevent the transformation of α-FAPbI3 to δ-FAPbI3, preparation of (FA)x(MA)1−xPbI3 was regarded as an effective route. Usually, the planar (FA)x(MA)1−xPbI3 perovskite solar cells are fabricated by a solution process. Herein, we report a low-pressure vaporassisted solution process (LP-VASP) for the growth of (FA)x(MA)1−xPbI 3 perovskite solar cells that features improved electron transportation, uniform morphology, high power conversion efficiency (PCE), and better crystal stability. In LP-VASP, the (FA)x(MA)1−xPbI3 films were formed by the reaction between the PbI2 film with FAI and MAI vapor in a very simple vacuum oven. LP-VASP is an inexpensive way to batch-process solar cells, avoiding the repeated deposition solution process for PbI2 films, and the device had a low cost. We demonstrate that, with an increase in the MAI content, the (101) peak position of FAPbI3 shifts toward the (110) peak position of MAPbI3, the (FA)x(MA)1−xPbI3 perovskites are stable, and no decomposition or phase transition is observed after 14 days. The photovoltaic performance was effectively improved by the introduction of MA+ with the highest efficiency being 16.48% under conditions of 40 wt % MAI. The carrier lifetime of (FA)x(MA)1−xPbI3 perovskite films is approximately three times longer than that of pure FAPbI3. Using this process, solar cells with a large area of 1.00 cm2 were fabricated with the PCE of 8.0%. KEYWORDS: mixed-organic-cation lead halide perovskite, perovskite solar cells, vapor-assisted deposition, planar heterojunction, power conversion efficiency

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Replacing the MA+ with the formamidinium (FA) cation, which has an ionic radius relatively larger than that of MA+ in the organic−inorganic lead iodide perovskite material, can change the metal−halide−metal bond angle and thus leads to a narrower band gap (1.48 eV) compared to that of MAPbI3.10 FAPbI3 has become a promising material applied in PSCs and extensively studied by many groups.10−18 A longer charge diffusion length and superior stability were observed in FAPbI3 films compared to those observed in MAPbI3.18−23 However, FAPbI3 has two crystal structures, i.e., a trigonal structure (perovskite phase, black color, α-FAPbI3) and a hexagonal structure (nonperovskite phase, yellow color δ-FAPbI3), depending on the synthesis temperature.11,24 The α-FAPbI3 is stable at a high temperature (over 160 °C) and can convert into

INTRODUCTION Organic−inorganic halide perovskite solar cells (PSCs) have attracted enormous interest in recent years due to the remarkable progress in their power conversion efficiency (PCE), i.e., from 3.8 to 22.1% in the past six years.1,2 The outstanding performance of PSCs takes advantage of some unique features of the organic−inorganic halide perovskite films such as the excellent light absorption, suitable optical band gap, and long-range charge transportation.3−5 Methylammonium (MA)-based lead iodide (MAPbI3) perovskite materials, which possess a band gap of 1.57 eV, are extensively used as the light harvester in PSCs.6−8 However, it was reported to undergo a reversible phase transition between tetragonal and cubic symmetry with a low phase transition temperature, which strongly influences the device stability.9 Thus, it is meaningful to explore other novel organic−inorganic halide perovskite materials which possess a lower band gap and superior photo and thermal stability to substitute MAPbI3 in efficient PSCs. © 2017 American Chemical Society

Received: October 20, 2016 Accepted: January 5, 2017 Published: January 5, 2017 2449

DOI: 10.1021/acsami.6b13410 ACS Appl. Mater. Interfaces 2017, 9, 2449−2458

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ACS Applied Materials & Interfaces δ-FAPbI3 under an ambient humid atmosphere.25,26 Even the coexistence of these two phases in the as-fabricated FAPbI3 films would degrade the device performance. Thus, obtaining phase-stable FAPbI3 perovskites is important for enhanced PSC performance. Introducing MA+ into the FAPbI3 perovskite structure to fabricate (FA)x(MA)1−xPbI3 has been reported as an effective way to achieve a phase-stable perovskite.27−32 Up to now, many solvent approaches have been explored to synthesize the (FA)x(MA)1−xPbI3 films, which were generally assembled in PSCs with TiO2 mesoscopic architecture.27,29,31,32 Planar-heterojunction architecture, as another important PSC structure, is favored for its potential to fabricate both flexible and multijunction cells.33 However, only two methods, i.e., utilizing a nonstoichiometric intermediate28 and a repeated interdiffusion process,30 have been confirmed effective to prepare smooth (FA)x(MA)1−xPbI3 films on planar substrates. Further, both of these two methods were also in solution process. It is quite possible that the methods for preparing (FA)x(MA)1−xPbI3 films on mesoscopic framework substrates were not applicable to preparing them on planar substrates because the quality and morphology of the perovskite films are highly correlated with the substrates and many experimental parameters.34,35 Therefore, the crucial point of assembling (FA)x(MA)1−xPbI3 films into efficient planar-heterojunctionstructured PSCs is exploiting various simple methods to obtain the perovskite films with high quality on the planar substrates. Besides solution methods, some vapor deposition methods, including dual-source vacuum evaporation,36 low-pressure chemical vapor deposition,37 and vapor-assisted solution process (VASP),33,35 have also been developed to fabricate pinhole-free MA-based perovskite films. Compared with the conventional solution process, vapor deposition can reduce the over-rapid intercalating reaction rate between PbI2 and CH3NH3I and avoid some undesirable structural transitions during solution processing, which could result in optimized surface morphology of perovskite.35,37,38 There is no report about the fabrication of (FA)x(MA)1−xPbI3 films through VASP. Furthermore, fabricating efficient PSCs in a large scale is beginning to be taken into consideration for further development of PSCs. Some film-forming methods have been reported for production of large-scale, flexible PSCs with high PCE and stability.39−44 So far, (FA)x(MA)1−xPbI3 has not been applied in efficient PSCs with large-scale. It is necessary to develop proper synthesis approaches for (FA)x(MA)1−xPbI3 films which are applicable for large-scale fabrication. As we know, introducing more than one additional cation and anion into the perovskite matrix can boost the PCE further. A stabilized PCE of 21.1% was based on a mixture of a triple Cs/MA/FA cation.45 A PCE of 21.6% was obtained based on mixed cation perovskite films in a single step from a solution containing a mixture of FAI, PbI2, MABr, and PbBr2.46 Obviously, the incorporation of multiple cations and anions will bring many additional experimental variables. When a new method was adopted to fabricate mixed cations perovskites, it should be better to introduce only one additional cation first at the beginning of the study. In this work, a kind of VASP to fabricate (FA)x(MA)1−xPbI3 films was investigated. Thus, introducing other additional cations and anions to boost the PCE of PSCs can be investigated further in our future work. Herein, a low-pressure VASP (LP-VASP) method was explored to fabricate smooth (FA)x(MA)1−xPbI3 films on planar substrates. Then, the relevant planar-heterjunction PSCs

were assembled. In the LP-VASP, the (FA)x(MA)1−xPbI3 films were formed by the reaction between the PbI2 film with FAI and MAI vapor in a very simple vacuum oven. The effect of different contents of MAI on the crystalline, photoelectric performance, and carrier lifetime of the (FA)x(MA)1−xPbI3 films and then on the performance of PSCs was investigated. The best-performing PSC possessed a PCE of 16.48% with a fill factor (FF) of 73.56%, a short circuit current (Jsc) of 22.51 mA/ cm2, and an open circuit voltage (Voc) of 1.00 V. The best PCE in our work is very close to the highest PCE (16.5%) obtained from the reported (FA)x(MA)1−xPbI3-based PSCs.30 Using this process, solar cells with a large area of 1.00 cm2 were fabricated with the PCE of 8.0%. Moreover, the X-ray diffraction (XRD) results demonstrated that the (FA)x(MA)1−xPbI3 perovskites were stable and that there was no decomposition or phase transition after two weeks. The device performance was stable for up to 288 h after fabrication.

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EXPERIMENTAL SECTION

Materials. NH2CHNH2I, CH3NH3I and 2,2′,7,7′-tetrakis(N,Ndi-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) (99.7%) were purchased from Borun Chemicals (Ningbo, China). PbI2 (99%) and 1,2-dichlorobenzene (98%) were both purchased from Acros. C60 and N,N-dimethylformamide (DMF) were both purchased from Alfa Aesar. Isopropanol was purchased from J&K Scientific Co., Ltd. Tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III)tris(bis(trifluoromethylsulfonyl)imide) (FK209-cobalt(III)-TFSI) was purchased from MaterWinChemicals (Shanghai, China). Bis(trifluoromethane) sulfonimide lithium salt and tert-butylpyridine (tBP) were purchased from Sigma-Aldrich. All chemicals were used directly without further purification. Glass substrates with a transparent fluorine-doped tin oxide (FTO, thickness 2.2 mm, sheet resistance 15 Ω/square) layer were used for the PSCs. Device Fabrication. The devices with an FTO/compact-TiO2/ C60/(FA)x(MA)1−xPbI3/spiro-OMeTAD/Au structure were prepared as follows. First, the FTO glass substrates were etched using Zn powder and hydrochloric acid. Then, the FTO glass substrates were cleaned sequentially using alkaline liquor, liquid detergent, deionized water, and ethanol. After being dried under clean dry air, the FTO was sintered at 500 °C for 30 min in air to remove residual organic matter. Subsequently, the dense TiO2 underlayer was spin-coated onto the FTO glass at 3000 rpm for 30 s and sintered at 500 °C for 1 h, forming a compact TiO2 layer. Then, the C60 layer was spin-coated on the cTiO2 layer at 1500 rpm for 30 s and annealed at 60 °C for 2 min. After cooling to room temperature, the PbI2 solution in DMF (kept at 70 °C) was spin-coated onto the FTO/c-TiO2/C60 layer at 3000 rpm for 30 s and then heated on a hot plate at 70 °C for 30 min in a nitrogen-filled glovebox. NH2CHNH2I and CH3NH3I powders were uniformly spread around the PbI2 coated substrates in a Petri dish covered with a lid. The petri dish was placed in a vacuum oven (10 kPa) set at 170 °C for 30 min. The total mixture mass of FAI and MAI was constant with varied mass of MAI. After this reaction process, the as-prepared perovskite films were first washed with isopropanol to remove the FAI and MAI residue and subsequently heated at 170 °C for 5 min in a nitrogen-filled glovebox. Several different concentrations of MAI powder, including 10 wt % (MA10), 20 wt % (MA20), 30 wt % (MA30), 40 wt % (MA40), and 50 wt % (MA50), were used to fabricate different mixed PSCs. Additionally, the pure FAPbI3 and pure MAPbI3 were recorded as MA0 and MA100, respectively. Lastly, the spiro-OMeTAD, which served as the hole transport material layer (HTL), was spin-coated on the perovskite layers at 4000 rpm for 30 s. Finally, Au was thermally evaporated on top to form the back electrode (60 nm) at an atmospheric pressure of 4 × 10−4 Pa. Characterization. The current density−voltage (J−V) characteristic was measured with a Keithley 2400 source-meter together with a sunlight simulator (XES-300T1, SAN-EI Electric, AM 1.5), which was calibrated using a standard silicon reference cell. The solar cells were masked with a black aperture to define an active area of 0.09 cm2. The 2450


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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the preparation process. (b) Cross-sectional SEM image and schematic device architecture of the FTO/TiO2/ C60/perovskite/spiro-OMeTAD/Au cell. J−V curves of various PSCs were simulated under AM 1.5G (100 Mw cm−2) solar irradiation in a glovebox. Curves under both forward and reverse voltage scanning directions were recorded. Scanning electron microscopy (SEM) images were taken with a SU8010SEM (Hitachi). XRD was measured with a Bruker X-ray diffractometer with a Cu Kα radiation source. The diffraction angle was scanned from 10° to 80° at a scanning speed of 3.5° per min. Atomic force microscopy (AFM) images were acquired in tapping mode with a 5500 AFM (Agilent Technologies). Incident photon-to-electron conversion efficiency (IPCE) was measured in air using a QE-R measurement system (Enli Technology). Time-resolved photoluminescence (TR-PL) spectra were collected using a transient state spectrophotometer (F900, Edinburgh Instruments). Samples were excited with a 660 nm pulsed diode laser with a repetition rate of 2.5 MHz and an excitation intensity of ∼14 nJ/cm2. The absorption spectra were measured with a UV-2450 spectrophotometer (Shimadzu) from 300 to 900 nm.

Figure 2. fabricated The J−V fabricated

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RESULT AND DISCUSSION The schematic diagram of the preparation process of LP-VASP deposition is shown in Figure 1a. Figure 1b represents a representative SEM cross-sectional image of a PSC in our experiments and a schematic diagram of the device architecture. Compact TiO2, C60, pervoskite, spiro-OMeTAD, and Au were deposited successively on top of the FTO glass substrate. In the PSCs, the compact TiO2 and C60 layers together constituted the electron transport layer (ETL), and the spiro-OMeTAD acted as the hole transport layer (HTL). The total thickness of the cell was about 700 nm, most of which was owing to the perovskite (340 nm) and HTL layers (280 nm). The thicknesses of ETL and Au electrode were 60 and 20 nm, respectively. Two kinds of pure FAPbI3 PSCs were fabricated via a twostep solution process and LP-VASP, respectively. The J−V curves were measured under both forward and reverse voltage scanning (Figure 2), and the corresponding photovoltaic performances extracted from the reverse voltage scanning cases are listed in Table 1. The values of average PCE, which were calculated from 20 devices for each fabrication process case, showed that the performance of the pure FAPbI3 PSCs

J−V curves of the pure FAPbI3-based PSCs. FAPbI3 was via a two-step solution process and LP-VASP, respectively. curves are of PSC based on the MA10 case, which was via LP-VASP.

Table 1. Performance of FAPbI3 and (FA)x(MA)1−xPbI3 with 10 wt % MAI PSCs and the Average PCE Calculated by 20 Devices in Each Fabricated Case process

samples

solution

MA0reverse MA0forward MA0reverse MA0forward MA10reverse MA10forward

LP-VASP

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PCEavg (%)

0.91

17.98

62.14

10.22

10.50

0.90

17.70

44.27

7.07

7.45

0.93

21.55

61.99

12.36

13.10

0.90

21.50

52.68

10.22

11.15

0.98

21.55

70.13

14.79

13.80

0.98

21.50

63.37

13.32

12.45

fabricated via LP-VASP is superior to those prepared through the two-step solution process. In addition, the box plots of PCE, Voc, Jsc, and FF of all the pure FAPbI3 PSCs are shown in 2451


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Figure 3. SEM images of (FA)x(MA)1−xPbI3 perovskite films in (a) MA0, (b) MA10, (c) MA20, (d) MA30, (e) MA40, and (f) MA50.

(FA)x(MA)1−xPbI3 perovskites. The morphology and surface texture of the perovskites were examined by SEM (Figure 3) and AFM (Figure S3). The SEM images reveal that the MA0MA40 samples with different amounts of MAI annealed at 170 °C can form pinhole-free, uniform, dense films that fully cover the TiO2 layer. However, some voids were observed on the surface of the MA50 perovskite films (Figure 3f). This was probably due to the stress caused by the introduction of a larger amount of MA+ during annealing at 170 °C, which led to the shrinkage of the perovskite film. It is clearly observed that the mean grain size decreased from 650 nm (MA0) to 500 nm (MA10). The amount of MAI did not greatly impact the morphology of the perovskite films. The mean grain size was approximately 400−500 nm, as shown in Figures 3b−f. As the mixed concentration of MAI powder changed, the morphology of the (FA)x(MA)1−xPbI3 perovskite films did not obviously change. The relationship of the mean grain size and the concentration of MAI are shown in Figure S4. It has been reported that the mixed organic cation can largely improve the morphology of films by solution process.43 However, the results herein showed that the mixed organic cation has little effect on the morphology of films by LP-VASP. The difference can be attributed to the different growth kinetics processes of the films fabricated through reactions in solution and vapor−solid reactions in VASP. The AFM images (Figure S3) are consistent with the SEM results. From the AFM results, we can observe that the (FA)x(MA)1−xPbI3 perovskite films show much better uniformity compared to that of the FAPbI3 film. The rootmean-square roughnesses of the films are listed in Figure S3

Figure S1. The best performed pure FAPbI3 PSC fabricated via a two-step solution process achieved a PCE of only 10.22% with a Voc of 0.91 V, a Jsc of 17.98 mA/cm2, and an FF of 62.14%. When fabricated through LP-VASP, the best performed pure FAPbI3 obtained a higher PCE of 12.36% with a Voc of 0.93 V, a Jsc of 21.55 mA/cm2, and an FF of 61.99%. Further, it can be seen from Figure 2 that the trend of PCE obtained by forward voltage scanning of the PSCs fabricated through different approaches was the same as that in the reverse voltage scanning situation. The morphology of pure FAPbI3 prepared via LP-VASP is more compact and dense than the film fabricated via the two-step solution process (Figure S2). Moreover, δ-FAPbI3 exists in the as-prepared films when fabricated by a two-step solution process (Figure S2c). Only the pure α-FAPbI3 phase could be observed in the film fabricated by LP-VASP. Thus, the films fabricated via LP-VASP contained a more stable phase and showed better device performance compared to that with the two-step solution process. To optimize the device performance of pure FAPbI3 fabricated by LP-VASP, the (FA)x(MA)1−xPbI3 mixed with 10 wt % MAI was prepared through the LP-VASP method. The PSC mixed with 10 wt % MAI showed the best performance with a PCE of 14.79%. It was demonstrated that the mixture of MAI and FAI is favorable for improving the performance of PSCs. To systematically study the evolution of the morphologies and the crystallinity of perovskite materials and achieve excellent performance of the PSC by the LP-VASP method, different amounts of MAI were used to fabricate 2452


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Figure 4. (a) XRD patterns of the perovskite films fabricated with different MA involved (from bottom to up: MA100−MA0). The black # and * represent PbI2 and the substrate FTO/TiO2, respectively. The black diamond (⧫) signs signify the characteristic peaks of FAPbI3. (b) The enlarged XRD patterns for all the samples illustrated in panel a with the 2θ range from 13.6 to 14.8°. The diffraction peak in the MA0 curve corresponds to the (101) lattice plane for FAPbI3.

toward a higher angle. We used the (101) peak of FAPbI3 as the representative peak to clearly explain the results. With an increase in the MAI content, an obvious shift of 2θ was observed for the FA-based materials (as shown by the peak at ∼14° in Figure 4b). The (101) peak position of FAPbI3 shifts toward the (110) peak position of MAPbI3. The gradual shift in the diffraction angle is a strong indication that a mixed phase of (FA)x(MA)1−xPbI3 is formed. MA+ and FA+ are both effectively introduced into the same lattice. Additionally, the samples were stored in a nitrogen glovebox environment for 14 days, and the XRD characterization was then carried out (Figure S5). The results showed that the XRD peaks of the mixed cases were consistent with Figure 4a. From Figure S5, it could be seen that the (FA)x(MA)1−xPbI3 perovskites were stable and that there was no decomposition or phase transition. Figure 5 shows the UV−vis absorption spectra of (FA)x(MA)1−xPbI3 perovskite films, where the absorption onset gradually blue-shifted from 830 nm for MA0 to 796 nm for MA50. Due to the smaller ionic radius of MA+, the Eg became larger and the absorption edge blue-shifted with the addition of MAI. The Eg of MA0 is approximately 1.49 eV. The Eg values of mixed perovskite films MA10, MA20, MA30, MA40, and MA50 are similar (∼1.55 eV), and these values are listed in Figure 5. This similarity is because the amount of MA+ that was introduced into the crystal lattice is small, so the Eg showed a much smaller change. To accurately study the impact of mixed MAI on the photovoltaic performance of (FA)x(MA)1−xPbI3 solar cells, we fabricated 20 cells of each mixed case. The J−V curves of the best-performing PSCs of each mixed case were measured under the reverse voltage scanning voltages shown in Figure 6, and the corresponding performance parameters are summarized in Table 2. The photovoltaic performance of each cell was enhanced by introducing MAI into the crystal lattice. The PCE of pure FAPbI3 PSC reached only 14.57% with a Voc of 0.94 V, a Jsc of 23.33 mA/cm2, and an FF of 66.49%. Additionally the PCE of mixed cases first increased to a maximum of 16.48% in MA40 with a Voc of 1.00 V, a Jsc of 22.51 mA/cm2, and an FF of 73.56% and then decreased to 15.67% with Voc of 0.98 V, a Jsc of 22.64 mA/cm2, and an FF of 70.30% as the concentration of MAI increased in MA50. The photovoltaic properties of the various cells described above show that the improvements in Voc and FF all contribute to the enhanced PSC performance achieved by the introduction of MAI. The increase of Voc was due to the higher Femi energy level, which is the result of

and remain similar despite changing amounts of MAI. It has been widely accepted that the compactness and uniformity of perovskite films are beneficial for enhancing the structural properties of perovskites for good device performance. XRD and UV−vis absorption were used to investigate the crystal structure and optical absorption of the films, as shown in Figure 4 and Figure 5, respectively. As shown in Figure 4a, a

Figure 5. UV−vis absorption spectrum of all the samples in each mixed case.

peak at 12.6° for PbI2 is observed for all the samples, which indicates that residual PbI2 exists in the perovskite films. Several groups have reported that residual PbI2 between the interfaces of the perovskite thin film and TiO2 layer can effectively passivate the perovskite grain boundaries and thus improve the charge transportation.47 Furthermore, the reflections of pure FAPbI3 perovskite films at 14.02, 19.87, 24.35, 28.17, 31.55, 40.25, and 42.75°, which are marked by black solid diamonds, are indexed to (101), (110), (202), (220), (222), (400), and (330) crystallographic planes, respectively, as the line of MA0 shows in Figure 4. The crystal structure is consistent with αFAPbI3.13 The XRD results of pure FAPbI3 indicate that the perovskite films that were fabricated by LP-VASP have a black polymorph. The high reaction temperature made the gas phase reaction occur sufficiently without phase transition caused by hydrolysis and the ambient humid atmosphere.11,24 In addition, the observed characteristic peaks of pure MAPbI3 films at 14.21, 20.11, 24.59, 28.55, 31.98, 40.77, and 43.20° correspond to the (110), (112), (202), (220), (312), (224), and (314) crystallographic planes shown in the purple line of MA100.48 With an increasing amount of MAI, the peak positions of FAPbI3 move 2453


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Figure 6. J−V curves of the PSCs without and with MAI, which were prepared with different concentrations. Only the curves from the best performing PSCs of each set measured under the reverse voltage scanning are shown.

Figure 7. J−V curves of large area perovskite solar cells with 1.00 cm2 areas in MA40.

to the decay of a normalized Jsc of 94.4% and a normalized FF of 98.3%, which harmed the efficiency of the solar cells. These results demonstrated that the perovskite solar cells were stable and the results were repeatable. Equivalent circuit analysis was carried out to extract more information about the cells from their J−V curves. Solar cells are roughly equivalent to a parallel circuit consisting of a current source and a diode. The output current density (J) of the cells can be described as50

Table 2. Parameters of Best Device Performance of Each Mixed Case Shown in Figure 6 samples

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

MA0 MA10 MA20 MA30 MA40 MA50

0.94 0.97 0.99 0.99 1.00 0.98

23.33 23.14 22.87 22.63 22.51 22.64

66.49 68.02 68.70 70.59 73.56 70.30

14.57 15.27 15.58 15.78 16.48 15.67

J = J0 (eq(V − JR s/ AKT ) − 1) +

V − JR s − Jsc R sh

(1)

where J0 is the reverse saturated current density, Jsc is the measured short-circuit current density, Rs is the series resistance, A is the ideality factor of a heterojunction, K is the Boltzmann constant, T is the absolute temperature, q is the elementary charge, and V is the direct-circuit bias voltage applied to the cell. When Rs ≪ Rsh, eq 1 can be expressed as

relatively large Eg with increasing amounts of MAI. Interfacial recombination loss seriously harms Voc, and the voids of the film in MA50 led to the possibility of increased recombination of electrons and holes; thus, the Voc in MA50 decreased, which in turn damages the device efficiency.49 The Jsc of 23.33 mA/ cm2 in MA0 was the highest value observed, and the Jsc of mixed cases showed no obvious changes. This result is because the Eg of pure FAPbI3 is obviously narrower than that of (FA)x(MA)1−xPbI3 perovskite films, which was consistent with the UV−vis results. The J−V curves of the best PCE cell measured under forward and reverse voltage scanning are shown in Figure S6. Expect for the J−V curves of pure FAPbI3, the mixed devices had a small hysteresis behavior. It can be demonstrated that the introduction of MA+ in FAPbI3 formed (FA)x(MA)1−xPbI3 can reduce the hysteresis behavior. Moreover, the hysteresis of the device in MA40 was minimized. We also fabricated a large area solar cell using MA40 with an area of 1.00 cm2 and obtained a PCE of 8.00% with a Jsc of 17.12 mA/ cm2, a Voc of 0.88 V, and an FF of 52.80% under reverse scanning measurement. The device performance is shown in Figure 7. The box plots of PCE, Voc, Jsc, and FF of all the PSCs with varying MAI concentrations are shown in Figure 8. The average statistical efficiency values of each cell displayed the same tendency as the PCE as the best-performing samples shown in Table 2. The average PCE initially achieved a maximum value in the MA40 sample and then decreased in the MA50 sample. The solar cell in MA40 stored in a nitrogen environment was used to test the stability (Figure S7). We can see that after 168 h, the cell exhibited stable device performance with a normalized PCE of 99.2%, a normalized Jsc of 99.4%, a normalized Voc of 100%, and a normalized FF of 99.9%. At up to 288 h, the cells still maintained good stability with a normalized PCE of 93.5%. The decay in PCE was mainly due

dV AKT 1 ≈ Rs + dJ q J + Jsc

(2) −1

Figure 9a shows the plots of dV/dJ vs (J + Jsc) and the linear fitting curves for the (FA)x(MA)1−xPbI3 PSCs according to eq 2. From the results of linear fitting, Rs and A can be calculated by the slope and intercept, respectively. Figure 9b shows the derived values of A and Rs along with the changed concentration of MAI. In our case, A decreased from 2.92 to 2.60, and Rs decreased from 1.85 Ω cm2 to 1.05 Ω cm2. The ideality factor occurs on behalf of carrier recombination and diffusion mechanisms for the well-behaved single heterojunction solar cell, where the ideality factor is in the range 1 < A < 2. Because the structures of planar (FA)x(MA)1−xPbI3 PSCs are much more complex than those of TiO2 / CH3NH3PbI3/Au,50 the ideality factor in our study is larger than 2, indicating that the carrier transportation is more complex for the planar heterojunction (FA)x(MA)1−xPbI3 solar cells. This may be one of the reasons for the low Voc in our study compared to those of top solar cells.45,46,51,52 Moreover, the ideality factor has a small fluctuation range, and the smallest value of A is 2.60 for the (FA)x(MA)1−xPbI3 in MA40. This demonstrates that all of the devices have similar diode characteristics, and the PSC in MA40 was the best. As we know, a low Rs is a necessity for a high-performance solar cell with a high FF. The Rs derived from the J−V curves under illumination is low. The lowest value is 1.05 Ω cm2 for the (FA)x(MA)1−xPbI3 in MA40, so it has the highest FF, which is 2454


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Figure 8. Statistical results for (a) Jsc, (b) Voc, (c) FF, and (d) PCE values of PSCs with mixed different concentrations of MAI. Twenty samples of each device set were measured.

Figure 9. (a) Plots of (dV/dJ) vs (J + Jsc)−1 for the best performing cells under illumination (linear fitting curves also are shown). (b) Series resistances (Rs) and ideality factors (A) derived from fitting curves in panel a.

consistent with the results of Figure 8c. This also explains the high efficiency of (FA)x(MA)1−xPbI3 in MA40. To study the spectral response of the devices, the IPCE spectra of the best-performing PSC is shown in Figure 10. The integrated photocurrent obtained from the IPCE curve of the cell in MA40 was 20.74 mA/cm2. This value is slightly lower than the Jsc obtained from the J−V curves under both forward

and reverse bias voltage scanning. Chemical and physical changes would affect the performance of PSCs, which are affected by different historical processes in the measurement.53 The high IPCE value of PSCs can be attributed to the strong absorption of visible light by the (FA)x(MA)1−xPbI3 film and effective electron−hole separation and collection. For the high efficiency performance of PSC, less nonradiative recombination, longer carrier lifetime, and effective charge transport are needed. The perovskite films prepared on the quartz substrate without other layers were used for the TR-PL characterization. Usually, TR-PL is used to estimate the carrier’s recombination behavior in perovskite films. The TR-PL results are shown in Figure 11. In Figure 11, biexponential decay functions were used to fit the decay curves, which contain a fast (t2) and a slow (t1) decay component. Rel.% represents the area proportion of the slow (t1) or the fast (t2) decay component in the fitting curves. A calculated average lifetime (tavg), which takes into account both time constants and their weighting, was used to evaluate the whole lifetime of the film. The parameters resulting from the fittings are summarized in Table 3. With the content of MAI increasing, the average carrier lifetime improved from 9.56 to 32.63 ns. By the introduction of MA+ in a pure FAPbI3 perovskite structure, the carrier lifetime of (FA)x(MA)1−xPbI3 perovskite films is approximately three

Figure 10. IPCE spectra of (FA)x(MA)1‑xPbI3 mixed with 40 wt % MAI. 2455



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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13410. SEM image of pure FAPbI3 fabricated by two-step solution process and LP-VASP and XRD patterns of two samples; box plots of PCE, Voc, Jsc, and FF of PSCs fabricated by different process; AFM images and the mean size of (FA)x(MA)1−xPbI3 in each mixed case; cross-sectional SEM images and the thickness of each layer; XRD patterns of (FA)x(MA)1−xPbI3 perovskite films in each mixed case after 14 days; the bestperforming PCE in each mixed case and the PSC in MA40 tested for 288 h (PDF)

Figure 11. TR-PL spectra for (FA)x(MA)1‑xPbI3 samples with different mixed concentrations of MAI on insulating substrates.

Table 3. Time Parameters Derived from the Fitting Results of the Transient TR-PL Decay Curves Shown in Figure 10 T1

T2

■

Tavg

samples

value (ns)

rel. (%)

value (ns)

rel. (%)

value (ns)

MA0 MA10 MA20 MA30 MA40 MA50

19.37 43.09 52.36 59.84 65.44 46.10

46.35 69.93 56.04 45.64 49.24 66.01

0.78 0.83 0.79 0.79 0.81 0.84

53.65 30.07 43.96 54.36 50.76 33.99

9.56 30.38 29.69 27.74 32.63 30.71

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jianxi Yao: 0000-0002-5472-9337 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (863 Program) (Grant 2015AA050602), the National Natural Science Foundation of China (Grant 51372083), the Jiangsu Province Science and Technology Support Program of China (Grant BE2014147-4), and the Fundamental Research Funds for the Central Universities (Grant 2014ZZD07 and 2015ZD11)

times longer than that of pure FAPbI3. Additionally, the (FA)x(MA)1−xPbI3 perovskite film MA40 has the longest carrier lifetime. This most likely contributed to the better carrier-collection efficiency observed in MA40 (Figure 8d), as it enhanced the diffusion length of the material. Moreover, due to a small amount of MA+ being introduced into the crystal lattice, the values of tavg show little variation for the different mixed cases, and the nonradiative recombination is relatively strong compared to those of the top cells.45,46,51,52 It may be another possible reason why the Voc in our study was much lower than those of the top cells. However, this also proves that the mixture of MAI and FAI during the vapor process is beneficial for improving the efficiency of PSC.

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REFERENCES

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CONCLUSIONS In summary, the LP-VASP method was adopted to successfully fabricate smooth (FA)x(MA)1−xPbI3 perovskite films on the planar substrates, and then FTO/TiO2/C60/perovskite/spiroOMeTAD/Au planar-heterojunction PSCs were fabricated. The crystallization, optical, and photovoltaic properties of the (FA)x(MA)1−xPbI3 films were systematically studied. The results show that a mixture of MAI and FAI during the vapor process can improve the crystallization, absorption, and carrier lifetime of perovskite films. The highest PCE of 16.48% with an FF of 73.56%, a Jsc of 22.51 mA/cm2, and a Voc of 1.00 V was obtained under reverse voltage scanning. The highest integrated photocurrent density of 20.74 mA/cm2 was achieved. The 50 wt % MAI mixed with FAI was unfavorable due to crystallization, leading to diminished performance of the PSCs. The (FA)x(MA)1−xPbI3 perovskites remained stable, and there was no decomposition or phase transition. Moreover, the device performance diminished by only 6.5% for a normalized PCE after 288 h under a nitrogen environment. The specific stoichiometric ratio of (FA)x(MA)1−xPbI3 needs to be further investigated. This strategy can be expected to pave the way for the development of mixed-organic-cation perovskite solar cells and other types of devices. 2456


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Mixed-Organic-Cation (FA)x(MA)1âxPbI3 Planar Perovskite Solar

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