Overcoming Bulk Recombination Limits of Layered Perovskite Solar

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Overcoming Bulk Recombination Limits of Layered Perovskite Solar Cells with Mesoporous Substrates Qiong Wang, Chuanpeng Jiang, Pengpeng Zhang, and Thomas W. Hamann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01592 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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The Journal of Physical Chemistry

Overcoming Bulk Recombination Limits of Layered Perovskite Solar Cells with Mesoporous Substrates Qiong Wang1, Chuanpeng Jiang2, Pengpeng Zhang2 and Thomas W. Hamann1* 1

Department of Chemistry, Michigan State University, 578 S Shaw Lane, East Lansing, USA,

48824. 2

Department of Physics and Astronomy, Michigan State University, 4213 Biomedical Physical

Sciences, East Lansing, USA, 48824. E-mail: [email protected] Abstract: The synthesis and characterization of a lead iodide layered perovskite, [NH2C(I)=NH2]2(CH3NH3)2Pb2I8, is described. The combination of an ideal bandgap of 1.61 eV and excellent compositional stability at ambient conditions make it a promising candidate for integration in solar cells. Planar solar cells utilizing [NH2C(I)=NH2]2(CH3NH3)2Pb2I8 exhibit two interesting phenomena in the photovoltaic performance: an exponential dependence of Jsc on incident light intensity and abnormal J-V response. To investigate the photo-physical properties of

[NH2C(I)=NH2]2(CH3NH3)2Pb2I8

perovskite

planar

solar

cells,

intensity-modulated

photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) were conducted. It is found that the planar structured solar cells of the layered perovskite suffer from bulk recombination which limits the charge collection and photocurrent. Use of a mesoporous TiO2 scaffold layer largely overcomes the recombination limitations of the layered perovskite and significantly improves the photovoltaic performance.

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Introduction Organic-inorganic lead halide perovskite solar cells using CH3NH3PbI3 (MAPbI3) as the light absorber have emerged as one of the most promising photovoltaic devices.1-2 The record NREL certified power conversion efficiency of this type of solar cell has risen to 22%, making it highly competitive to silicon solar cells.3 However, it is broadly recognized that the MAPbI3 perovskite can easily decompose to PbI2, thereby losing its advantageous optical and photo-physical properties,4-8 when it is exposed to moisture.9-10 As a result, the stability of MAPbI3 perovskite solar cells is one of the bottle necks that hampers its entry into the market. To improve its stability, three strategies have been developed. One way is to encapsulate the devices under inert atmosphere before exposure to ambient atmosphere.11-15 This allows the devices to last for a much longer time than devices without encapsulation, however, it also increases the costs and manufacture complexity of perovskite solar cells. The second way is to use some hydrophobic materials deposited on top of/beneath the perovskite layer or on top of the hole transport layer (HTM) to slow/hinder the moisture penetration to the perovskite film.16-23 Significant enhancement in stability of perovskite solar cells has been achieved by this methodology. For example, Snaith’s group used polymer-functionalized single-walled carbon nanotubes (SWNTs) embedded in an insulating polymer matrix (poly(methyl methacrylate), PMMA) as a HTM.23 No sign of degradation of the device PV performance was observed upon water exposure. The third strategy is to develop new perovskite materials that have intrinsically better moisture stability than MAPbI3. For example, it was found that doping MAPbI3 with Cs, Br or Cl can highly improve its stability.24-28 More recently, use of layered perovskites, such as (C6H5(CH2)2NH3)2(CH3NH3)2Pb3I1029 and (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n=2, 3, and 4),30 has been shown to be another viable route to improve device stability since such layered perovskite materials generally demonstrate a robust tolerance against moisture. However, the efficiency of layered perovskite solar cells is generally low compared to their three-dimensional counterparts. On one hand, layered perovskite materials generally have relatively large optical band gap depending on the number of the second cations in the crystal structure, thus limiting the light harvesting of visible spectrum.29-32 On the other hand, the reported layered perovskites exhibit a preferential out-of-plane alignment (along (110) facet) with respect to the substrates.3031

Therefore, long organic cations introduced in layered perovskites block the electron-hole

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transport through the perovskite layer. Recently, a hot-casting method was used to grow layered perovskite films of near-single-crystalline quality which helped improve the efficiency from 4.6% to 12.52%.31 Nevertheless, there is still a lack of understanding of the behavior of this interesting class of layered perovskite materials needed to simultaneously optimize the efficiency and stability of perovskite PVs. In this work, we introduce a layered perovskite, [NH2C(I)=NH2]2(CH3NH3)2Pb2I8 (referred to as Pb2I8), which presents two advantages over previously reported layered perovskites. First, it has an optical band gap of around 1.6 eV, very close to that of MAPbI3 perovskite, thus it is nearly ideal for application as a light absorber in solar cells. Secondly, it exhibits a preferred growth orientation along (002) facet, which indicates that the Pb2I8 layered perovskite grows vertically towards a substrate, suggesting photo-generated carriers may be transported along the inorganic back bone composed of lead iodide inside the Pb2I8 layered perovskite. To test the eligibility of the Pb2I8 layered perovskite as light absorber, planar perovskite solar cells using TiO2 as electron collection layer and spiro-OMeTAD as the hole collection layer were fabricated. Light intensitymodulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) were conducted to investigate the physical processes controlling device performance. The Pb2I8 layered perovskite was further integrated with a mesoporous TiO2 scaffold layer, which greatly improved the performance. Experimental Section Materials The hole transport material used in this study, 2,2’,7,7’-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene

(spiro-OMeTAD)

was

purchased

from

Lumtec.

Methylammonium iodide (MAI) was purchased from Dyesol. Titania (TiO2) paste (Ti-Nanoxide T/SP) was purchased from Solaronix. Pre-patterned fluorine-doped tin oxide (FTO) glass was purchased from Xinyan Technology LTD. All other chemicals were purchased from SigmaAldrich. Unless otherwise stated, all chemicals were used as received. Synthesis Iodoformamidinium iodide (IFAI) was synthesized by adding cyanamide (NH2CN, 0.53g, 12.457 mmol) into 50 ml of 57 wt.% aqueous hydriodic acid (HI) solution at 80°C. After cooling down 3



to room temperature, the white precipitate was collected by filtering the solution in a fume hood, and then washed with diethyl ether before drying in a vacuum oven overnight. As a result, 2.2 grams of IFAI was obtained, representing a yield of 58.5%. A single crystal of IFAI was grown by allowing the reaction solution to cool down slowly without disturbing. The single crystal structure of IFAI is confirmed by comparison with reported data.33 A procedure34 adapted from a report on the synthesis of [NH2C(I)=NH2]2(CH3NH3)2Sn2I8 was used to synthesize the [NH2C(I)=NH2]2(CH3NH3)2Pb2I8 perovskite material. Briefly, MAI (1.386g, 8.72mmol), cyanamide (NH2CN, 0.3737g, 8.72mmol), and lead iodide (PbI2, 4.02g, 8.72mmol) were added in 35 ml of 57 wt.% aqueous HI solution at 85°C under N2. After reacting at 85°C for 20 to 24 hours, the solution was allowed to cool to room temperature, then immersed in a cold bath to cool to -10 °C. After sitting at -10°C for two to three hours, the product was filtered under a N2 atmosphere, dried under flowing N2 at 85°C overnight, then stored in a N2-filled glove box. A product of 0.28 grams was obtained, representing a yield of 4.85%. For the growth of single crystal Pb2I8, the reaction solution was allowed to sit in the three-neck round bottom bottle without stirring/disturbing; a crystal appeared after approximately 2 days. [NH2C(I)=NH2]2(CH3NH3)2Pb2I8 perovskite films were deposited from a mixture of IFAI, MAI, and PbI2 in a molar ratio of 1:1:1 in anhydrous dimethylformamide (DMF), followed by spincoating and annealing; the detailed conditions are provided in the device fabrication section below. The elemental analysis of the as-deposited perovskite film produced fractions of C (2.63%), H(0.88%), N(4.57%) that matched with calculated C (2.617%), H(1.100%), N(4.579%) values from the [NH2C(I)=NH2]2(CH3NH3)2Pb2I8 formula. Device Fabrication Pre-patterned FTO glass was cleaned by ultrasonicating in soap water, acetone, and isopropanol respectively for 10 min before use. A TiO2 compact layer of ~30 nm was spin-coated from a solgel solution prepared by diluting titannium isopropoxide (97%, Sigma Aldrich) in isopropanol, and then calcined at 450°C for 30 min. TiO2 mesoporous layer of 200 nm was spin-coated from diluted commercial TiO2 paste (Ti-Nanoxide HT/SP, Solarnoix), and then annealed at 450°C for 30 min. Solutions of 1M of the MAPbI2.85Br0.15 and MAPbI3 perovskites were prepared by adding 170 mg MAI, 415 mg (461.01mg for MAPbI3) PbI2, and 36.7 mg PbBr2 in 1 ml 4


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anhydrous DMF, and then stirred at 100°C for 10 min until a transparent yellowish solution was formed. Before use, the room temperature perovskite solution was filtered (0.45 µm filter, Sigma). Perovskite films of 153 nm were deposited by spin-coating the above solution at 6 krpm for 30 s, while starting N2 blowing at 5 s.35 The reference perovskite films were then annealed at 100°C for 10 min on a hot plate, calibrated using an infrared temperature gun. 0.25 M solutions of the Pb2I8 layered perovskite were prepared by dissolving 297.86 mg IFAI, 158.97 mg MAI, and 461.01 mg PbI2 in 1 ml anhydrous DMF. Without heating, a transparent, light reddish solution was formed by shaking the vial. Before filtering through a 0.45 µm filter, the solution was allowed to stir for 5 min at room temperature. A Pb2I8 layered perovskite film of 147 nm was deposited by spin-coating the above solution at 2 krpm for 30 s, with N2 blowing introduced at 5 s, followed by annealing at 100°C for 10 min on the hot plate. It should be noted that the film thickness of TiO2 compact layer, TiO2 mesoporous layer, MAPbI2.85Br0.15 and MAPbI3 reference perovskite films, and Pb2I8 layered perovskite film were all measured using Atomic Force Microscopy (experimental details are provided in the Characterization section below). After the deposition of the perovskite layer in ambient atmosphere, all samples were transferred into a N2-filled glove box, where spiro-OMeTAD solution was spin-coated at 4 krpm for 30s. 1 ml spiro-OMeTAD solution was prepared by adding 79.65 mg spiro-MeTAD in anhydrous chlorobenzene (CBN) with 16.5 µl LiTFSI (Bis(trifluoromethane)sulfonimide lithium salt, 99.95%, Sigma) out of 520 mg/ml stock solution in acetonitrile, and 29.3 µl tBP (4-tertButylpyridine, 96%, Sigma). Then all samples were taken out of the glove box and kept inside a desiccator overnight in the dark. The dark oxygen-soaking time is controlled to be around 20 hours. Then Au of 80 nm was thermally deposited on top as the metal contact. Characterization The single crystal measurements were conducted using a Bruker APEX-II CCD diffractometer, under MoKα radiation, and operated at T = 173(2) K. Powder X-ray diffraction measurements were performed using a Bruker Davinci Diffractometer with a Cu Kα1 as the target. The film thickness of TiO2 compact layer, TiO2 mesoporous layer and perovskite layer were measured using an Asylum MFP- 3D-Bio Atomic Force Microscope (AFM). The surface morphology was characterized using scanning electron microscopy (Hitachi S-4700 II FESEM). Optical transmission and reflectance of films were measured using UV-Vis spectrometer (PerkinElmer

5



Lambda 35) with a Labsphere integrating sphere. Photoelectrochemical measurements were performed with a potentiostat (Autolab PGSTAT 128N) interfaced with a xenon arc lamp. An AM1.5 global filter was used to simulate sunlight at 100 mW/cm2. The light intensity was calibrated with a certified reference cell system (Oriel Reference Solar Cell & Meter). The active area of the devices is 0.36 cm2 and a mask with an aperture area of 0.21 cm2 was applied during the measurements. Light intensity-modulated photocurrent spectroscopy (IMPS) was recorded using a 470 nm LED with an LED driver from Metrohm Autolab. The photon flux was calibrated using the photodiode integrated with the LED setup. The IMPS response of devices was measured by applying 10% of the DC LED driving current as the AC perturbation of the incident light intensity in the frequency ranging from 20 kHz to 1 Hz. Electrochemical impedance spectroscopy (EIS) measurements were performed both in the dark and under 0.1 Sun using an FRA2 integrated with the PGSTAT 128N. The impedance spectra were recorded at forward bias ranging from -0.1 V to 1.0 V, with a 20 mV amplitude. Each impedance measurement was recorded at frequency ranging from 1 MHz to 0.1 Hz in equally spaced logarithmic steps. Neutral density filters were used to tune the incident light intensity from 100 mW/cm2 to 0.1 mW/cm2. Steady-state photoluminescence was measured using fluorescence spectrophotometer (Hitachi F-4500). The emission spectra were recorded in wavelength ranging from 600 to 820 nm with the incident light wavelength at 420 nm. Time resolved PL decay were measured using a laser at 405 nm from Picoquant (LDH-D-C-405M, CW-80 MHz). Thermogravimetric analysis (TGA) was conducted using Pyris TGA thermogravimetric analyzer (PerkinElmer) at a heating rate of 2 °C/min. Results and Discussion Figure 1 shows the crystal structure of the Pb2I8 perovskite refined from the XRD measurements on Pb2I8 single crystals. A clear layered structure is exhibited with a layer of organic cation, [NH2C(I)=NH2]+, inserted between every two inorganic layers composed of two connected lead iodide octahedra. From the crystal structure, powder X-ray diffraction of the layered perovskite was calculated, and compared with the measured XRD of Pb2I8 powder, which is given in Figure 2. The powder sample exhibits most of the peaks that are present in the calculated powder XRD. To prepare the layered perovskite film, the powder sample was dissolved in dimethylformamide (DMF) and then spin-coated on TiO2 compact layer covered FTO glass, followed by annealing at

6


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100°C for 15 min on a hot plate in the ambient atmosphere. Figure 2 shows that the layered perovskite film on planar substrates presents a preferred growth orientation along the (002) facet. Based on the crystal structure information of Pb2I8 perovskite in Figure 1b, the lead iodide octahedra grows along [001] facet perpendicular to the surface of planar substrates. As a result, it is expected that when an electron-hole pair is photo-generated in the layered perovskite film, the charge carriers can be efficiently transported along the inorganic backbone with minimal hindrance from the insulating organic cations.

a)

b)

Figure 1. Schematic crystal structure of [NH2C(I)=NH2]2(CH3NH3)2Pb2I8 (noted as Pb2I8) viewed a) from (100) facet and b) from (001) facet. The structure is refined from the XRD measurements on Pb2I8 single crystals collected using a Bruker APEX-II CCD diffractometer, under Mo Kα radiation, and operated at T = 173(2) K. Purple sphere stands for iodide, dark grey sphere stands for lead, light grey sphere stands for carton, blue sphere stands for nitrogen, and white sphere stands for hydrogen.

7


5

15

260 004

033

film

033

powder

110 10

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004

002 120 002

*

120 002 140

020 020 020

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (Counts)


20

cal.

25

30

35

2Theta (Degree) Figure 2. Powder X-ray diffraction (XRD) of Pb2I8 perovskite: calculated data (black) from the single crystal structure presented in Figure 1 as reference, measured data of the powder sample (red) and the film (blue) deposited from solution. The vertical pink dashed line is included to facilitate comparisons between patterns.

The optical properties of the layered perovskite films were examined in comparison with two reference perovskite films, MAPbI3 and MAPbI2.85Br0.15. To eliminate reflection or competitive absorption by the substrates, quartz was used in place of FTO as the substrate for this measurement. Figure 3 shows the superimposed absorption spectra (left axis) and normalized photoluminescence (PL) emission spectra (right axis) of MAPbI3, MAPbI2.85Br0.15 and Pb2I8 films. The absorption onset of Pb2I8 is 783 nm, which is similar to that of MAPbI3 at 781 nm, while MAPbI2.85Br0.15 film shows a small blue shift to 767 nm. (Figure S1a) The absorption coefficient, α is calculated using the equation ߙ = ݀ ିଵ ݈݊

ሺଵ଴଴ିோ%ሻ ்%

, where d is the film thickness

of perovskite layer, R% and T% are the reflectance and transmittance (Figure S1b). It is found that the three samples have similar absorption coefficients over the visible spectrum. Furthermore, the optical bandgaps of the three materials is calculated using the direct bandgap form of Tauc plots (Figure S1c). The optical bandgap of Pb2I8 is found to be 1.61 eV, which is similar to MAPbI3 (1.62 eV) and MAPbI2.85Br0.15 (1.64 eV). Bandgaps ranging from 1.57 eV to 1.61 eV have been reported previously for MAPbI3, where the small difference can be caused by 8


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differences in crystallinity.36-38 The normalized PL spectra in Figure 3 (right axis) shows that one emission peak at 733 nm, 758 nm and 770 nm is observed for Pb2I8, MAPbI2.85Br0.15 and MAPbI3 samples, respectively. Therefore, in agreement with reported results of MAPbI3 and MAPbI2.85Br0.15,39-40 all three perovskite materials discussed in this work have direct bandgaps. The emission peak position of two reference perovskite materials coincides with their absorption onset, however, the emission peak position of Pb2I8 is at a higher energy than its absorption onset. Such anti-Stokes shifts have been reported for (C6H5C2H4NH3)2PbI4 perovskite previously, however the cause is still under investigation.41 It is also worth noting that the PL intensity of layered perovskite films is more than three times that of the reference perovskite samples (Figure S1d). Mitzi et al.42 attributed the strong photoluminescence of Sn analogs to a quantum effect where photo-generated electrons and holes are confined inside the quantum well by the organic

80

733/758/770 nm

60

40

20

0 2.4

2.2

2.0

1.8

1.6

Normalized PL (Counts)

cations.

Absorptance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.4

Photon energy (eV) Figure 3 Absorptance spectra (left, solid lines) and normalized PL emission spectra (right, dashed lines) of MAPbI3 (blue), MAPbI2.85Br0.15 (red) and Pb2I8 (black) films.

Figure 4 presents the XRD patterns of aged samples of Pb2I8, MAPbI2.85Br0.15, and MAPbI3 perovskites at 2-theta of 12° to 15° (Full images of XRD patterns of the samples are given in Figure S2). MAPbI3 and MAPbI2.85Br0.15 perovskite films after aging for 3 days and 72 days in 9



ambient conditions show a strong peak at 2theta of 12.6°, indicating the decomposition of MAPbI3 and MAPbI2.85Br0.15 into PbI2. However, the 72 days aged layered perovskite Pb2I8 exhibits a barely noticeable peak at 2-theta of 12.6°. Hence the layered perovskite clearly exhibits increased compositional stability under ambient conditions compared to the MAPbI3 and MAPbI2.85Br0.15 reference samples. The appearance of peaks at 2-theta of 25°, 26°, 33°, 37° in aged Pb2I8 perovskite samples (Figure S2a) originate from the TiO2 compact layer covered FTO substrates. (XRD spectra of TiO2 compact layer covered FTO substrates can be found in Figure S3) It is also noted that these peaks of the substrates are absent in Pb2I8 fresh samples but appear in Pb2I8 aged samples. This could be due to morphology changes in the perovskite films. This hypothesis is supported by SEM images (Figure S4), which show that for fresh samples, Pb2I8 perovskite forms a compact and smooth surface, but for aged samples, Pb2I8 perovskite changes to islands, leaving the substrates exposed, which are therefore detected by XRD measurements.

Intensity (Counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

12

(002)2D/(110)3D

* PbI2

13

14

15

2Theta (Degree) Figure 4. XRD charcterization of 72 days aged Pb2I8 perovskite film (black line) in comparison with 72 days aged MAPbI2.85Br0.15 film (red line) and 3 days aged MAPbI3 film (blue line). The intensity is normalized at the feature peak of (002) facet for layered perovskite and (110) facet for three-dimensional perovskites.

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Planar structured solar cells with TiO2 as an electron selective layer and spiro-OMeTAD as a hole selective layer were adopted in this work since this simplified structure can be used to understand the photo-physical properties of the layered perovskite material. Figure 5 shows the photovoltaic performance of the layered perovskite planar solar cells in comparison with MAPbI2.85Br0.15 perovskite planar solar cells. MAPbI3 perovskite solar cells are not used as reference cells because it is found that MAPbI3 perovskite films deposited on TiO2 compact layer readily decompose to PbI2 when the relative humidity is beyond 40%. Since all the measurements are conducted in ambient atmosphere, and no special treatment is used to control the humidity, the instability of MAPbI3 perovskite makes the PV results very poor and irreproducible. The film thickness of the layered perovskite and the reference perovskite are both ~150 nm. Plots of the J-V curves as a function of scan rate, and separate dark current plots, can be found in the supporting information (Figure S5).

Current Density (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 4 2 0 -2 -4 0.0

0.4

0.8

1.2

Voltage (V) Figure 5. Plots of J-V curves measured under 1 Sun illumination (symbols) and in the dark (lines) for forward scan (filled symbols or solid line) and reverse scan (open symbols or dashed line) for MAPbI2.85Br0.15 (black) and Pb2I8 (red) planar perovskite solar cells. Scan rate: 5 mVs-1.

There is a large hysteresis in the J-V curves of the planar devices, consistent with prior reports.4345

This behavior has been attributed to ion migration, surface trap states and carrier accumulation

at the interfaces.46-48 The layered perovskite cells also show a hysteresis in the J-V curves, but to 11



a much smaller extent. The performance of the planar layered perovskite cells is quite a bit worse than the reference perovskite, however, with lower current densities, open-circuit voltage (Voc) and fill factors; the J-V curve also has an unusual shape. The low current density in particular is somewhat surprising given the comparable optical properties to the MAPbI2.85Br0.15 perovskite and the preferred growth orientation along (002) facet. Since the short-circuit current density (Jsc) is highly dependent on scan rate, the steady-state Jsc taken from current transient measurements (Figure S6) was compared with Jsc obtained from J-V curve at a slow scan rate of 5 mV/s. Figure 6 shows the Jsc dependence on incident light intensity for reference and layered perovskite solar cells. Both MAPbI2.85Br0.15 perovskite and Pb2I8 perovskite planar solar cells present similar Jsc for the current transient measurement to the J-V curve under forward scan. The Jsc of the MAPbI2.85Br0.15 perovskite solar cells show a linear dependence on light intensity. However, the Jsc of the layered perovskite solar cells exhibit a very unusual exponential dependence on light intensity. We therefore further investigated the underlying cause of the strange shape of the J-V curve and exponential dependence of Jsc on light intensity of the layered perovskite solar cells with IMPS and EIS as described below.

8

3 Current Transient Re-scan (5 mV/s) Fw-scan (5 mV/s)

6

Jsc (mAcm-2)

Jsc (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

Current Transient Re-scan (5 mV/s) Fw-scan (5 mV/s)

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20

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60

80

Light intensity (mWcm

0

100 -2

)

b)

20

40

60

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100 -2

Light intensity (mWcm

)

Figure 6. Jsc depedance on incident light intensity for MAPbI2.85Br0.15 (left) and Pb2I8 (right) planar solar cells measured from current transient (filled circle) and J-V curve at scan rate of 5 mV/s for reverse scan (Re-scan) : scanning from open circuit to short circuit (filled square) and forward scan (Fw-scan): scanning from short circuit to open circuit (open square). Incident light 12


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intensity was tuned by inserting neutral density filters between the sample and AM1.5G 100mWcm-2 light source.

IMPS was used to investigate charge transfer and recombination processes inside the devices at short-circuit. According to the model developed by Peter et al.49, the low-frequency IMPS response in quadrant I can be attributed to the relaxation of photogenerated carriers due to interfacial recombination and transfer. Generally, three important pieces of information can be extracted from the IMPS data. The first is the high frequency intercept in quadrant I, which is interpreted as the instantaneous photocurrent when the recombination processes are assumed to be “frozen out”. The second is the low-frequency intercept which is interpreted as the steady state photocurrent. The third is the ratio of low to high frequency intercepts, which is related to the charge collection efficiency controlled by kinetics charge transfer and recombination processes at the interfaces. 100

100

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LF/HF (%)

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b)

10

20

30

Light intensity (mWcm-2)

Figure 7. The low frequency (LF, filled red circle) and high frequency (HF, open red circle) intercepts in the IMPS complex plane and ratio of LF to HF intercept (black square) as a function of incident light intensity of a) MAPbI2.85Br0.15 perovskite and b) Pb2I8 perovskite planar solar cells.

Figures S7a and S7b present the IMPS response of MAPbI2.85Br0.15 perovskite and Pb2I8 layered perovskite solar cells measured at short-circuit under a range of light intensities. The LF and HF

13



intercepts (right axis) as well as the ratio of LF/HF intercepts (left axis) are plotted in Figures 7a (MAPbI2.85Br0.15 perovskite solar cells) and 7b (Pb2I8 layered perovskite solar cells). The behavior of the layered perovskite solar cells is clearly different from that of reference perovskite solar cells. The HF and LF intercepts of the MAPbI2.85Br0.15 perovskite solar cells both increase linearly with light intensity. The values of the HF and LF are similar, thus the ratio suggests nearly quantitative charge collection. Figure 7b shows that for the layered perovskite solar cells, however, the HF intercept has a very quick rise at very low intensities then increases approximately linearly. The magnitude of the HF response is significantly lower for the Pb2I8 layered perovskite solar cells, suggesting the photocurrent is limited by bulk recombination / transport dynamics. In contrast, the LF intercept grows exponentially with light intensity. The ratio of the HF and LF intercepts is low at low intensities and grows up exponentially, following the same trend as the LF intercept as well as the Jsc results above. As mentioned above, the HF intercept presents the instantaneous photocurrent generated in the system. Therefore, the difference in the dependence function of LF and HF intercepts on light intensity in layered perovskite solar cells means that after charge carriers are generated in the bulk Pb2I8 perovskite layer, the recombination and/or transfer kinetics of charge carriers at the contacts do not vary linearly with light intensity. Thus, even though the electron and hole selective contacts are the same for both perovskites, charge collection is suboptimal for the Pb2I8 layered perovskite solar cells which limits the performance of the planar cells. The reason for this is not yet clear and is the subject of ongoing work. To better understand the PV behavior of Pb2I8 layered perovskite solar cells, impedance spectroscopy measurements were conducted both in the dark and at 0.1 Sun under a range of bias. A low light intensity is used because it is found that extensive measurements under 1 Sun illumination results in a significant drop in the PV performance, whereas the PV performance was stable under 0.1 Sun for the duration of the measurements reported. An example of a Nyquist plot of perovskite solar cells measured under 0.1 Sun illumination at a forward bias of 0.5 V is shown in Figure 8a. Additional Nyquist plots of layered perovskite and reference perovskite solar cells measured in the dark and under 0.1 Sun are provided in the supporting information.

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8

10-4

600

6

300

0

4

0

500

τ1 (s)

-Z'' (ohm)

-Z'' (103 ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

Z' (ohm)

2

0 0

a)

10-5

5

10

Z' (103 ohm)

10-6

15

0.0

b)

0.4

0.8

1.2

bias (V)

Figure 8. Results from electrochemical impedance spectroscopy (EIS) measurements of MAPbI2.85Br0.15 perovskite solar cells (black, squares) and Pb2I8 perovskite planar solar cells (red, circles) under 0.1 Sun illumination. a) Plots of Nyquist plots of impedance data for MAPbI2.85Br0.15 and Pb2I8 planar solar cells at 0.5 V (inset shows magnified high frequency response); b) Plots of τ1 obtained from the high frequency semicircle.

Two semicircles are generally observed in the Nyquist plots, which were fit to the equivalent circuit adopted from recent papers published by Bisquert and others and displayed in the supporting information.50-53 A detailed analysis of EIS response for layered perovskite solar cells has not been established, and is beyond the scope of this paper, however the high frequency arc has been shown to be describe bulk recombination kinetics for MAPbI3 and related perovskite solar cells.50-53 Figure 8b shows a plot of lifetime, τrec, values extracted from fitting the high frequency arc as a function of applied potential under illumination for MAPbI2.85Br0.15 and Pb2I8 layered perovskite solar cells. Taking the assumption that τrec also describes the bulk recombination kinetics for the Pb2I8 layered perovskite cells, these results show that the carrier lifetime is over an order of magnitude lower for Pb2I8 compared to MAPbI2.85Br0.15. Recently, results from time-resolved microwave conductivity measurements showed significantly lower carrier lifetimes of layered perovskites compared to the three dimensional CH3NH3PbI3 material, in excellent agreement with our interpretation.54 This result is also consistent with the low HF photocurrent measured with IMPS described above for the Pb2I8 layered perovskite solar cells. Time-resolved PL decay spectra of Pb2I8 layered perovskite, MAPbI2.85Br0.15 perovskite, and 15



MAPbI3 perovskite films, given in the supporting information, also suggest that Pb2I8 perovskite films have a short PL lifetime than that of reference perovskite films. Such increased bulk recombination would result in significantly lower charge carrier diffusion lengths which explains the reduced photocurrent density and photovoltage as observed in Figure 5. Additional parameters extracted from fitting EIS spectra are provided in the SI. Since bulk recombination of the Pb2I8 layered perovskite results in suboptimal photocurrents compared to the reference perovskite, a TiO2 mesoporous layer was introduced. In this case, the photogenerated carrier diffusion distance is substantially decreased which should help alleviate recombination limitations occurring in the bulk of the perovskite layer. The J-V curves of perovskite cells employing mesoporous TiO2 substrates, referred to as mesoPVs, are displayed in Figure 9a. Table 1 summarizes the photovoltaic parameters of MAPbI2.85Br0.15 and Pb2I8 perovskite mesoPVs measured at AM1.5G (100 mWcm-2). It can be seen that the introduction of 200 nm TiO2 mesoporous layer significantly reduces the hysteresis in both reference perovskite and the layered perovskite devices, consistent with previous reports.55-57 The TiO2 mesoporous layer produces a somewhat higher Jsc and fill factor for the reference perovskite mesoPVs. Strikingly, the use of the mesoporous TiO2 layer significantly changes the photovoltaic behavior of the layered perovskite, although the photovoltaic performance of the layered perovskite is still lower than the reference samples. The J-V curves of layered perovskite mesoPVs in Figure 9a indicate that the perovskite-sensitized TiO2 nanoparticles structure helps to eliminate the influence of the bulk recombination of the layered perovskite. The external quantum efficiency (EQE) of MAPbI2.85Br0.15 and Pb2I8 mesoPVs are shown in Figure 9b. The integrated Jsc from the EQE measurement is close to the value obstained from the J-V curves given in Figure 9a. The internal quantum efficiency (IQE) was calculated from the EQE by deviding by the absorptance of the samples. (Figure S9c) The absorption spectra of samples are given in Figure S33. Figure S9c shows that the internal quantum efficiency of the layered perovskite mesoPVs is smaller than the reference perovksite mesoPVs in a broad spectral wavelength range of 450 nm to 720 nm. If the lower IQE for the layered perovskite was due to diffusion length, it would be reflected in a wavelength dependence. Thus, the charge collection efficiency through the electron/hole selective materials appears to be sub-optimal for the layered perovskite with the mesoporous substrate, consistent with the IMPS results on planar cells.

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Finally, the dependence of Jsc on light intensity for mesoPV is examined as well. (Figure S34). It shows that Jsc exhibits an ideal linear dependence on light intensity for layered perovskite mesoPV.

15

60

10

5

10

40 5 20

0 0.0

0.2

0.4

0.6

0.8

1.0

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a)

0 300

400

500

600

Jsc (mAcm-2)

IPCE (%)

Current Density (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 800

700

Wavelength (nm)

b)

Figure 9. a) J-V curves of MAPbI2.85Br0.15 (black) and Pb2I8 (red) solar cells with a TiO2 mesoporous layer in both reverse scan direction (solid lines) and forward scan direction (dash lines). b) External quantum efficiency, EQE (left axis) and integrated Jsc (right axis) as a function of wavelength for MAPbI2.85Br0.15 (black) and Pb2I8 (red) meso PVs.

Table 1. Photovoltaic parameters of champion solar cells measured under AM1.5G (100mWcm-2) at the scan rate of 10 mV/s with a delay time of 5 s. The average value of photovoltaic parameters of three devices are given in parentheses (). Samples

PCE (%)

Jsc (mAcm-2)

Voc (V)

FF

MAPbI2.85Br0.15 cell-Re

9.56 (8.99)

12.74 (11.93)

1.00 (1.00)

0.75 (0.75)

MAPbI2.85Br0.15 cell-Fw

9.23 (8.75)

12.94 (11.97)

1.00 (1.00)

0.71 (0.73)

Pb2I8 cell-Re

5.10 (4.08)

10.71 (8.77)

0.85 (0.82)

0.56 (0.57)

Pb2I8 cell-Fw

4.66 (3.87)

10.59 (8.73)

0.80 (0.80)

0.55 (0.55)

Conclusion

17



A Pb2I8 layered perovskite was synthesized which exhibits preferred growth orientation along the (002) facet. This layered perovskite has excellent compositional stability in ambient conditions and an optical bandgap around 1.61eV, which is nearly ideal for use in a solar cell. However, use of Pb2I8 perovskite in a planar solar cell configuration results in poor performance with unusual behavior. Results from IMPS measurements showed that the charge collection efficiency is poor for the layered perovskite, and increases exponentially with light intensity. A low HF response also suggests poor bulk (charge transport / recombination) properties. Results from EIS measurements clearly showed that the charge separation efficiency is limited by bulk recombination, confirming poor bulk properties. Thus, a TiO2 mesoporous film was utilized to reduce the diffusion distance of photogenerated charge carriers and thus increase the charge collection efficiency. Indeed, a significant improvement in the performance, as well as the behavior, or the layered perovskite was produced through use of the mesoporous substrate. Thus, we have identified the rate limiting step in photocurrent production of a promising layered perovskite materials and demonstrated the path to push the performance up. The charge collection is still sub-optimal for the Pb2I8 layered perovskite solar cells using TiO2 and spiroOMeTAD as electron and hole selective contacts, respectively. Thus, further improvements in device architecture engineering to match the charge carrier diffusion length with diffusion distance for all photogenerated carriers and optimization of the electron and hole selective contacts offers the opportunity to realize a very stable and efficient perovskite photovoltaic. Supporting information Absorption, steady-state PL emission spectra, time-resolved PL decay, Nyquist plot of IMPS and IS, JV curves of fresh and aged samples, and other information. Acknowledgements The authors acknowledge the Michigan State University Strategic Partnership Grant (SPG) for support for this research.

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Overcoming Bulk Recombination Limits of Layered Perovskite Solar

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