High-Performance Perovskite Solar Cells with Large

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High-Performance Perovskite Solar Cells with Large Grain-Size obtained by using the Lewis Acid-Base Adduct of Thiourea Shuo Wang, Zirui Ma, Beibei Liu, Wenchao Wu, Yu Zhu, Ruixin Ma,* and Chengyan Wang* solar cells for obtaining devices with both superior performance and high reproducibility.[6–9] Several different strategies have been recently developed to increase the crystallinity and surface morphology of the perovskite layer. In terms of the deposition technique, a series of methods, involving typical solution based methods,[10] vapor based deposition methods,[11] and pressure application methods,[12] has been used to fabricate uniform and dense perovskite solar cells (PSCs). For selecting perovskite precursor species, the lead precursor PbI2 is generally replaced with Pb(NO3)2,[13] Pb(SCN)2,[14] and/or Pb(CH3COO)2[15] to decelerate crystallization kinetics and select crystallographic orientation. This aims to improve the morphology of perovskite film and the device stability. To improve its light harvesting efficiency, band-gap, hole, and electron diffusion length and the crystallization of the perovskite films, MAI (CH3NH3I) is generally substituted by FAI (HC(NH2)2I),[16] Rb,[17] and/or Cs[18] NH4Cl, as an ideal additive, is able to delay the CH3NH3PbI3 crystallization and serve as a binder to interconnect separated CH3NH3PbI3 crystals.[19] The devices with NH4SCN and moisture-assisted deposition method provides a facile way to obtain the device with high stability under humidity.[20] In addition to the variation of lead and organic precursors, some ligand compounds that serving as electron donor (Lewisbases) could also greatly impact the perovskite films properties. Three types of Lewis-bases can coordinate with PbI2, that is, Odonor (DMSO, NMP), N-donor (Prydine, Aniline), and S-donor (Thiourea, Thioacetamide).[21,22] The strong coordination molecule dimethyl sulfoxide (DMSO) has been adopted to fabricate uniform and dense perovskite films as immediate phase PbI2 DMSO and MAI PbI2 DMSO complexes in the precursor solution.[23] It has been used in one-step[24] and two-step[25] process. Given the high boiling point of DMSO, the introduction of anti-solvent is required (e.g., toluene,[26] chlorobenzene,[27] and ether[28]) in order to remove the extra DMSO via one-step process as film is growing. Yet, DMSO is easy to remove from the PbI2 framework, impacting the perovskite grain growth apparently.[21] To achieve effective photon capture, fast carrier transport, and suppressed ion migration, smooth surfaces, and

Recently, perovskite solar cells (PSCs) have been rapidly developed, counting as the most promising alternative to the Si solar cells. The fabrication of perovskite films with controlled crystallinity and grain size is critical for highly efficient and stable solar cells. In this work, thiourea (TU) serving as a Lewis acid-base adduct is introduced into the CH3NH3PbI3 precursor. A smooth and large grained perovskite crystal is obtained without the intermediate phase 5C(NH2)2 using the ideal thiourea amount. Thiourea, through forming PbI2 S5 MAI PbI2 DMSO thiourea in perovskite precursor solution, significantly impacts the perovskite crystallinity and morphology, as proved using X-ray diffraction patterns and infrared spectroscopy. Light harvesting, suppressed defect state, and enhanced charge separation and transport of the perovskite absorber layer are improved. The optimum performance of perovskite solar cells with TU demonstrated a power conversion efficiency (PCE) of 19.80%, an average steady-state PCE of 18.60% and potent stability under ambient air.

1. Introduction Organic–inorganic hybrid perovskite solar cells are broadly applied, especially for photovoltaic applications, in which perovskite materials are served as the direct band-gap semiconductors with a series of unique properties. These properties contain remarkable optical absorption from the visible to the near infrared (NIR) region, long electron-hole diffusion lengths, long carrier life and high dielectric constant.[1–3] Device power conversion efficiency (PCE over 20%) has been substantially improved for recent 6 years as devices fabrication and materials understanding were advanced rapidly. The solar-to-electrical energy conversion efficiency of the perovskite solar cells has increased from 3.8% by Miyasaka and co-workers[4] to 22.1% by Kim and co-workers under 1 sun illumination in the efforts of numerous researchers.[5] A delicate is critical on the the crystallization behavior, the deposition method, interfacial engineering, and the reactant components of the perovskite S. Wang, Z. Ma, B. Liu, W. Wu, Y. Zhu, Prof. R. Ma, Prof. C. Wang School of Metallurgy and Ecological Engineering University of Science and Technology Beijing Beijing 100083, P.R. China E-mail: [email protected]; [email protected]

DOI: 10.1002/solr.201800034

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large grains are often required in the perovskite films. Thus, more Lewis-bases ligands need to be exploited to improve the performance of the perovskite solar cells. The mixed Lewis-bases of Thiourea1-xDMSOx have been used as the additive in formamidinium lead iodide perovskite (FAPbI3) to achieve high quality perovskite films with high photovoltaic performance.[29] However, the role of thiourea in the methylamine lead iodide perovskite precursor soltion (MAI/ PbI2/DMF/DMSO) is still unclear. Herein, the Lewis-bases ligand (e.g., Thiourea) was introduced as the additive into the perovskite precursor (PbI2/MAI/DMF/DMSO) solution to attain large grains and smooth surfaces of the perovskite films. A proper ratio of TU was found exerting a very weak or negligible impact on the bonding of PbI2-DMF and PbI2-DMSO complexes. A possible mechanism has been preseted concerning the perovskite grain growth as TU is added. The proper ratio precursor complexes effectively impact the grain size and roughness of the fabricated perovskite thin films. The asresulted perovskite films, with the aid of TU additive, exhibited enhanced light absorption, suppressed surface defect, and decreased series resistance. Moreover, the meso-structure of FTO/TiO2/perovskite /Spiro-OMeTAD/Au was employed to study the the proposed film growth mechanism. The power conversion efficiency of the TU-aided perovskite thin films was 19.80% with evidently increased stability and repeatability.

2. Experimental Section Device Fabrication: FTO glass substrates, with a sheet resistance of 8 Ω/& (Pilkington), were cleaned with alkaline detergent, acetone, absolute ethanol, deionized water for 15 min, respectively, and then dried with a nitrogen flow and further cleaned by UV-ozone for 10 min before they were used for spin-coating. The TiO2 compact layers were prepared by dissolving titanium diisopropoxide bis(acetylacetonate) solution in ethanol with a 1:10 volume ratio. The TiO2 compact films were obtained by spin-coating of precursor solution on the UV-ozone treated substrates at 3000 rpm for 30 s by annealing at 150 C for 5 min and 500 C for 30 min. Mesoporous TiO2 layer (mass ratio 1:7) was spin-coated on the TiO2 compact films at 5000 rpm for 30 s and then annealed in air at 500 C for 30 min and cooled down to room temperature. The perovskite CH3NH3PbI3 films were prepared through the following method: first, the additive thiourea (TU) was dissolved into DMF and introduced by means of introducing various amounts of TU (0–1.0 M, we refer to as TU-0, . . .,TU-1.0) into the perovskite precursor solution containing MAI (1.0 M) and PbI2 (1.0 M) in anhydrous DMF: DMSO (4:1, v/v) at the ambient temperature, then, filtered with 0.22 mm nylon filter to obtain a clear solution. The perovskite film was fabricated on the as-prepared ETL in nitrogen atmosphere. The precursor solution was spin-coated on the TiO2 coated substrates at 3000 rpm for 30 s and 150 μl chlorobenzene was slowly dripped on the surface of the film 18 s after the beginning of spin-coating. The as-obtained films were annealed at 100 C for 0.5 h. 72.3 mg Spiro-OMeTAD, 28.8 μl 4-tert-butylpyridine and 17.5 μl of Li-bis(trifluoromethanesulfonyl)imide solution (520 mg mL1 in acetonitrile) were dissolved in 1 mL chlorobenzene to form Spiro-OMeTAD

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solution, Spiro-OMeTAD layer was sequentially spin-coated at 3000 rpm for 30 s to form the hole transfer layer. Finally, Au electrode was deposited using thermal evaporation at a constant evaporation rate of 0.1 nm s1. Except for the fabrication of TiO2 layer, the whole process is carried out in glove-box under Ar condition at home temperature. The active area was 0.1 cm2.

2.1. Characterization The field emission scanning electron microscope (FESEM) images were obtained on a ZEISS SUPRA55. Energy dispersive spectrometry (EDS, Thermo-NS7, manufacturer) was used to determine the elemental composition. X-ray diffraction (XRD) patterns were collected with a SmartLab from Rigaku at 40 Kv and 150 mA by using Cu-Ka radiation (λ ¼ 0.15405 nm). AFM figures were measured using 300 HV scanning force microscope (SEIKO). The photovoltaic performance of PSCs was recorded using a Keithley 4200 source meter under one-sun AM 1.5G (100 mW cm2) illumination with a solar light simulator (Newport Oriel Sol3A Class AAA, 64023A Simulator), which was calibrated using a NREL standard Si solar cell. EIS measurements were conducted by an electrochemical workstation (CHI660d) (1 MHz–100 Hz) and the fitting software was Zview software. The UV–Vis light absorption measurement was performed by using an ultraviolet-visible (UV–Vis) spectrophotometer (Shimadzu UV-3101 PC). The external quantum efficiency (EQE) measurements were obtained on a Keithley 2000 multimeter as a function of the wavelength from 350 to 800 nm on the basis of a Spectral Products DK240 monochromator. The infrared spectroscopy was measured by Fourier transform infrared spectrometry (VERTEX80v, Bruker). The PL spectra and fluorescence decay curves were taken out with combined steady state (FLS980, Edinburgh). The active area of the cell is 0.1 cm2. All samples were measured in air (25 C).

3. Results and Discussion Figure 1 is a schematic diagram of the one-step deposition of perovskite films by an anti-solvent precipitation. MAI and PbI2 mixed with various amount of thiourea were first spread over the substrate covered with mp-TiO2/c-TiO2/FTO. The antisolvent chlorobenzene was then dripped onto the substrate to extract the original solution dimethylformamide (DMF). Finally, perovskite films with different grain sizes and morphology were obtained via heating at 100 C for 30 min. With the new perovskite precursor, highly crystallized and large grained MAPbI3 films were fabricated. Figure 2a–f presented the top-view scanning electron microscopy (SEM) images of the perovskite films prepared from different amount of thiourea (TU). The crystal grain size increases substantially from 200 nm to over 2 μm, which confirms that thiourea in the precursor has exerted a crucial impact on the nucleation and subsequent crystal growth processes. The optimum performance of the perovskite films is presented in Figure 2d, The crystal grain size is approximately 1.5 mm without any hole, being 6–7 times larger than that of the pristine crystal grain size (as presented in Figure 2a). Interestingly, the holes, defects and

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Figure 1. Preparation processes of the one-step deposition of perovskite films with different amount of thiourea.

impurities were observed on the grain boundaries with the increase of TU concentration (Figure 2e,f), which primarily arose from the excess amount of thiourea in the perovskite films, The “black-color” impurities on the grain boundaries are PbI2 S5 5C(NH2)2 according to the results of XRD and UV (shown later). The opened grain boundary and impurities might lead to charge leakage between the infiltrated hole transfer layer and the electron transfer layer. Given that the rough surface and intensive grain boundaries were reported as the most efficient routes for ion migration,[5] the smooth and large grained perovskite obtained from the TU-0.5 was promising to provide high efficiency perovskite solar cells with less hysteresis. The interface between the active layer and the hole transfer layer significantly impacts the device performance. Accordingly, it is critical to investigate the perovskite layer roughness. Figure 3 shows the AFM images for the surface of perovskite films prepared with different amount of TU, the images show dense and compact grains in Figure 3a (TU-0) and Figure 3b (TU-0.5). The holes and rod-like impurities are clearly observed in Figure 3c (TU-1.0), which complies well with the results of SEM images. The RMS roughness of the films is 22.70 nm (TU-0), 31.30 nm (TU-0.5), and 25.6 nm (TU-1.0). The RMS roughness could be crucial for hole-transporting as impacting the contact area between the perovskite active layers and the hole transfer

layer. Higher roughness could lead to an increase in the contact area between the active layer and the hole transfer layer which consequently improves charge transportation. However, low open-circuit voltage (Voc) and fill factor (FF) may be caused due to the over-high RMS roughness.[30] The perovskite film with thiourea containing precursor exhibited typical diffraction peaks of solution-processed perovskite MAPbI3, inclusive of several weak peaks and two notable characteristic peaks at 14.2 and 28.4 corresponding to (110) and (220), respectively, without any shift, as presented in Figure 4a.[31] The FWHM and intensity of the (110) peak are showed in Figure 4b, the crystallinity of the perovskite films was evidently increased by introducing thiourea into the perovskite precursor, and the intensity of the (110) peak in TU-1.0 sample is nearly five orders of magnitude higher than that of the pristine film. The FWHM of the (110) plane is reduced from 0.17 to 0.06 , which indicates the increased crystallite size. The weak peaks of (112), (211), (202), (310), and (312) are almost invisible in Figure 4a, indicating that the crystallographic orientation, especially the (110) and (220) plane, was strongly induced as thiourea is added. According to the results of the XRD pattern and SEM images, there was no variation in the angle and FWHM of diffraction peaks of perovskite with various amount of thiourea, the thiourea likely existed mainly at the surface and/or

Figure 2. SEM images of perovskite films prepared from different conditions: (a) TU-0, (b) TU-0.1, (c) TU-0.3, (d) TU-0.5, (e) TU-0.7, and (f) TU-1.0. The insets in (d), (e), and (f) show high-resolution images of the perovskite films.

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Figure 3. AFM images of (a) TU-0, (b) TU-0.5, (c) TU-1.0.

grain boundaries instead of being inserted into the crystal structure. Such an oriented crystal growth is conducive to the formation of large-sized perovskite grains with limited thickness and critical for increasing the device performance.[32] The XRD patterns of TU-0, TU-0.5, TU-0.7, and TU-1.0 from 10 to 40 were presented in Figure 4c with higher recognition, to analyze the phase transformation of the perovskite films with the thiourea. As for the TU-0 and TU-0.5, the peak of impurities is invisible. However, several distinct peaks were found in the X-ray diffraction (XRD) pattern of the TU-0.7 and TU-1.0. There were identified to be the PbI2 and intermediate phase PbI2 SC(NH2)2 as thiourea was excessively added in the perovskite films. The intermediate phase PbI2 SC(NH2)2 could greatly impact the performance of the devices. To examine the impact exerted by thiourea on the photovoltaic performance of PSCs, Figure 5a demonstrates the photovoltaic performance of the perovskite solar cells prepared with various thiourea concentration. The detailed performance parameters are summarized and listed in Table 1. With thiourea-containing precursor, short-circuit current density (JSC) was obtained

higher, which increased from 22.38 to 23.97 mA/cm2. This was primarily stemmed from better light absorption and charge transportation as the grain size increased. Open-circuit voltage (VOC) was greatly impacted by the relative position of quasiFermi level in the contacted perovskite and electron transfer material (ETM), and the defect-induced recombination in the electron transport channels.[33] The conduction band was lower positioned in perovskite films with large sized grains and fewer surface defects. With the enlarged grains and reduced surface defects, the VOC remarkably increased from 1.03 to 1.09 V in the sequentially processed perovskite solar cells. The fill factor (FF) was the third parameter determining the power conversion efficiency of perovskite solar cells, which was restricted by the complex energy losses from the electrode surface reflection, series resistance, and shunt resistance.[34] FF was improved from 0.72 to 0.75 through introducing TU into the perovskite precursor. However, All the parameters of perovskite precursor with 0.7 M and 1.0 M TU were diminished. Especially, the TU-1.0 sample showed an S-shaped J–V curve and relatively low efficiency of 8.50%, it is probably because of the formation of a

Figure 4. a) XRD patterns of perovskite films fabricated under different conditions. b) Full-width at half maximum (FWHM) and intensity of the (110) peak of all the samples. c) The XRD patterns of TU-0, TU-0.5, TU-0.7, and TU-1.0 from 10 to 40 . The intensity of the vertical axis is indexed to give the weak peaks higher recognition. All the samples were prepared on the mp-TiO2/c-TiO2/FTO substrates.

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Figure 5. a) The current density–voltage curves of perovskite solar cells fabricated under different conditions. b) Cross-section image of the perovskite 5C(NH2)2 film. The FTO solar cells fabricated with thiourea containing precursor. c) UV–Vis measurement for the perovskite films, PbI2 film and PbI2 S5 substrate was used as the baseline. d) Nyquist plot of the perovskite solar cells measured with the frequency range from 100 Hz to 1 MHz under AM 1.5 G irradiation at a direct current bias of 0 V. Symbols are experimental data and solid lines correspond to the fits using the equivalent circuit (inset).

non-ohmic contact impeding the extraction or injection of holes and electron at the perovskite/spiro-OMeTAD or the mp-TiO2/ perovskite interface, which is stemmed from the blocked charge transport and serious charge recombination produced by the intermediate phase PbI2 S5 5C(NH2)2 on the grain boundaries. In general, the photovoltaic parameters of all the devices increased, and the PSCs employing 0.50 M TU, compared with the control device (PCE ¼ 16.60 %), exhibited the highest PCE value of up to 19.80%. A typical cross-sectional SEM image of PSCs is depicted in Figure 5b. The solar cell was stacked in the structural: 400 nm FTO, 50 nm compact TiO2, 300 nm mesoporous TiO2 and perovskite, 200 nm Spiro-OMeTAD, 80 nm Au electrode. All the layers are visible in the SEM image. More importantly, highly uniform, and monolithic perovskite grains were observed in the absorption layer, which was contributed to the longitudinal electron and hole transport.[35]

Table 1. Parameters of current density–voltage curves obtained from perovskite solar cells fabricated under different conditions. Jsc [mA cm2]

Voc [V]

FF

PCE [%]

TU-0

22.38

1.03

0.72

16.60

TU-0.1

22.08

1.08

0.73

17.40

TU-0.3

23.41

1.09

0.74

18.89

TU-0.5

23.97

1.09

0.75

19.80

TU-0.7

19.38

1.04

0.65

13.10

TU-1.0

11.97

1.00

0.71

8.50

Sample

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To understand the impact exerted by thiourea on the optical properties of perovskite films, the ultraviolet–visible (UV–vis) spectrum was measured in Figure 5c. The film, compared with the pristine film, presented stronger absorption at the full light absorption range as the increase of TU content and the absorption edge was slightly red-shifted to ensure that the low energy photons can be captured by perovskite. This was attributed to the increased crystallinity, enlarged grain size, and possibly reduced reflection.[36] The UV–Vis spectrum of PbI2 and PbI2 S5 5C(NH2)2 was also measured to analyze the decreased absorption of TU-1.0 from 450nm to 500nm. The absorption was decreased due to the surface defects caused by the intermediate phase PbI2 S5 5C(NH2)2, as indicated in the spectra results. EIS was conducted to describe the charge transfer process and the Nyquest plot is shown in Figure 5d. Results of the fitting results are shown in Table 2. The Nyquist plot can be used to distinguish the charge transfer/transport and the charge recombination at interfaces between different films. The EIS was measured at a voltage bias of 0 V under one-sun light intensity with frequency ranging from 100 Hz to 1 MHz, according to Garcia-Belmonte’s new equivalent circuit (inset Figure 5d).[37,38] The symbols Rseries, R1, R2, R3, C1, C2, and Cbulk are shown in the circuit model. Rseries is related to the series resistance originating from the electrodes, Cbulk is mainly associated with the bulk perovskite layer, R3 is heavily influenced by the contact transport resistance, R1 and C1 represent the parameters associated with the contacts to the perovskite, additional kinetic and charge storage processes occurring at the interface are represented by elements R2 and C2. Rseries and R3 are basically the same in all the samples, R1 and R2 in TU-0.5

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Table 2. Fitting results of the Nyquist plot. Rseries [Ω]

R1 [Ω]

R2 [Ω]

R3 [ Ω ]

TU-0

22.08

23.30

5.27

112.02

TU-0.1

27.50

91.00

47.60

130.05

TU-0.3

34.15

287.50

152.20

124.90

TU-0.5

30.05

373.50

221.30

154.82

TU-0.7

27.21

73.29

31.04

157.10

TU-1.0

23.77

146.1

118.50

171.41

Sample

sample, arising from the enlarged grain size, surmount the TU-0 sample (TU-0: R1 ¼ 23.30 Ω, R2 ¼ 5.27 Ω, TU-0.5: R1 ¼ 373.50 Ω, R2 ¼ 221.30 Ω), and the larger R1 and R2 value would inhibit carrier recombination to facilitates the injection and storage of charge. The R1 and R2 value in TU-0.7 and TU-1.0 are much lower than that in TU-0.5, the intermediate phase PbI2 S5 5C(NH2)2 on the grain boundaries would break the contacts to the perovskite, it matches well with the results of J-V measurement. As accordingly illustrated, the interface properties and device performance were increased by using thiourea containing precursor. An infrared (IR) spectroscopic study was performed to ascertain how thiourea works in the perovskite formation process. Figure 6a shows IR spectra for the DMF/DMSO solution with thiourea, PbI2 thiourea and MAI PbI2, MAI PbI2 0.5M thiourea, and MAI PbI2 1.0M thiourea. The C5 50, S5 5O, and C5 5S vibration was observed in full scanning range. Enlarged spectra behave differently, as shown in Figure 6b–d.

Figure 6. Infrared spectroscopy of the perovskite precursors in (a) full scanning range, and expanded fingerprint region for (b) 1800–1575 cm1, (c) 1050–950 cm1, and (d) 780–725 cm1.

C5 5O and S5 5O vibration (Figure 6b) have been used to expound the coordination ability of DMF and DMSO toward PbI2 species. The positions of C5 5O peaks (Figure 6b) are located at 1670 cm1 in all samples, which indicates that TU exerts no impact on the

Figure 7. Schematic diagram and schematic reaction process of the deposition of perovskite films.

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Figure 8. a) The J–V curves of the forward and reverse direction scans of TU-0. b) The stable output current density and PCE at the max power point (0.8 V) of the TU-0 sample. c) The J–V curves of the forward and reverse direction scans for the PSCs of TU-0.5. d) The stable output current density and PCE at the max power point (0.85 V) of TU-0.5.

coordination ability of DMF toward PbI2.[23] The positions of S5 5O peaks (Figure 6c) are different, The S5 5O stretching frequencies of PbI2 DMSO thiourea and MAI PbI2 DMSO thiourea are detected in lower wavenumber than that of DMSO thiourea. The decreased S5 5O stretching frequency arises from decrease in bond strength between sulfur and oxygen as a result of the adduct formation. The S5 5O stretching frequencies of perovskite precursor containing 0, 0.5 M and 1.0 M thiourea are 1015 cm1. the positions of DMSO and PbI2 DMSO are 1045 cm1 and 1020 cm1, which complied well with the Choi and co-workers.[24] Thus, it is noteworthy that the additive of thiourea exerts no effect on the coordination ability of DMSO toward PbI2. The C5 5S stretching vibration was observed at 743 cm1 for pure thiourea, which was shifted to 725 cm1 for the compound phase PbI2 thiourea. This was further shifted to 732 cm1 for the 1.0 M thiourea-containing precursor (see Figure 6d). We found that the wavenumber of C5 5S peaks in the thiourea-containing precursor was higher than that of the PbI2 thiourea, which revealed the variation of electron density with the competitive effect of MAI Pb and Pb S5 5C(NH2)2. Thus, thiourea is verified in the formation of MAI PbI2 DMSO TU in the perovskite formation process. With the above phase, optical, IR spectra and morphology analyses, the main schematic reaction process is presented in Figure 7. The reactants PbI2, MAI, DMSO, and thiourea were

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first dissolved in DMF solution, PbI2 was known to be a Lewis acid,[29] which would combine with the Lewis bases thiourea and DMSO in the perovskite precursor solution. After spin-coating, intermediate phase PbI2 DMSO Thiourea DMF was obtained, which then transferred into PbI2 DMSO Thiourea after traditional anti-solvent treatment. During the annealing process, DMSO was easy to be removed from the PbI2 framework, and thiourea would still combaine with PbI2 in the films to slow down the reaction between PbI2 and MAI, then the large perovskite grain was obtained. However, once the excess amount of thiourea the reaction was introduced into the precursor solution, the intermediate phase PbI2 S5 5C(NH2)2 phase would be observed on the grain boundaries of the perovskite films due to the stronger coordination between thiourea and PbI2. Finally, monolithic perovskite grains were formed without intermediate phase, which were believed to benefit the performance of perovskite solar cells. The PSCs employing 0.50 M TU exhibited the highest PCE value of up to 19.80% (see Figure 5a). Figure 8a,c presents the hysteresis of TU-0 and TU-0.5. The TU-0.5 has a PCE of 19.80% in the reverse J–V curve and 18.56% in the forward J–V curve. And The TU-0 only has a PCE of 16.60% in the reverse J–V curve and 14.43% in the forward J–V curve. As indicated, the I–V curves of the TU-0.5 with forward and reverse direction scans almost overlap each other, which indicates negligible hysteresis,

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Figure 9. a) EQE spectra of TU-0 and TU-0.5 based PSCs. b) The steady-state photoluminescence curves emitted at 760 nm and were obtained for the perovskite films upon excitation at 445 nm. c) Recombination resistance of the solar cells under different biases. d) J-V curves for devices based on TU-0 and TU-0.5 under dark condition.

compared with that of the TU-0. The stable PCE of TU-0.5 (see Figure 8d) that measured at the maximum power point is 18.60%. It is approaching to the PCE extracted from the J–V curve. The stable PCE of TU-0 (see Figure 8b) that measured at the maximum power point is only 15.76%. The significant differences are mainly due to the difference in FF. Figure 9a shows that the external quantum efficiency (EQE) of TU-0.5 along the entire range of wavelengths from 350 to 800 nm surmounts that of TU-0, this is primarily because of the enhanced the charge collection and light harvesting capability by introducing TU. Photocurrent is generated at 780 nm in accordance with the UV–Vis spectra results (see Figure 5c). Integration of the EQE spectra (350–800 nm) over the AM 1.5G solar emission spectrum yielded short-circuit photocurrent densities of 21.37 and 20.43 mA cm2 for TU-0.5 and TU-0, respectively. To investigate the emission properties of the MAPbI3 perovskite crystal, a quenched steady-state PL spectroscopy study was performed (Figure 9b). All samples were prepared on m-TiO2/cTiO2/FTO substrates. The perovskite emission peak intensities were found increasing evidently significantly with the addition of thiourea precursor, while the FWHM was slightly narrowed for the final product, which indicated the reduced surface defect and enhanced crystallinity. The peak was slightly red-shifted from 761 to 765 nm, which revealed the adjustment of energy level dispersion. It complies well with Tian and co-workers.[21]

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Figure 9c shows the recombination resistances of TU-0 and TU-0.5 tested under different biases. The recombination resistance (Rrec) consisted of the interfacial resistances of perovskite/ETL and perovskite/HTM. These mechanisms can be impacted by The contact materials, surface morphology and device configuration. As reported in the literature dlg ðRrec Þ ¼ 2:303qn kB T ,[39] lg(Rrec) was inversely proportional to the dV applied voltage. The recombination resistance (Rrec) of TU-0.5 surmounted that of TU-0 under different biases. The enhanced recombination resistance of TU-0.5 revealed reduced surface defect states and improved FF and Voc. As shown in Figure 9d, the dark current of the devices (Figure 9d) reveals the other function of the TU, that is, leakage current reduction. TU-0.5 has smaller dark current than the controlled one. The smaller dark current indicated a reduced series resistance in the TU-0.5 device, which could prevent the current leakage and improve the Jsc and FF. The use of TU reduces the current leakage and increases the diode rectification ratio, which may be explicated by the higher work function, reduced surface defect states and larger grain size in TU-0.5.[30,40] The statistic PCE histogram of the solar cells along with the Gaussian fitting was also illustrated in Figure 10a. For the TU-0.5 solar cells, the average PCE shows a significant improvement in PCE, from 15.50% to 18.00% upon introduction of the TU additive. The long-term stability of the TU-0 and the TU-0.5 solar

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Figure 10. a) Statistical PCE histogram for the perovskite solar cells (from 40 samples for each sample). b) Long-term stability of solar cells for 30 d. All devices were stored in the dark in ambient air (humidity: 10–20%, temperature: 10–20 C).

cells in ambient environment (10–20% humidity, 10–20 C temperature) were also studied and the results are shown in Figure 10b. Clearly, the TU-0.5 solar cells exhibited excellent stability against humidity and temperature in the ambient air. After 30 d of storage, the TU-0.5 solar cells retained over 75% of the initial PCE, whereas the pristine solar cells lowered to less than 10% in 30 d. Such superb ambient air stability could be attributed to the large grain size, high crystallinity, and reduced surface defect states.[41] The fast degradation of the TU-0 solar cell was stemmed from the numerous surface defects introduced by the small grain size.

4. Conclusion A novel perovskite precursor was developed to fabricate CH3NH3PbI3 films. Thiourea was verified in the formation of MAI PbI2 DMSO Thiourea in the perovskite formation process. The crystallization process was effectively controlled by introducing varied thiourea into the perovskite precursor. An intermediate phase PbI2 S5 5C(NH2)2 was formed by introducing over 0.7 M thiourea. This intermediate phase increased the interface recombination and further decreased the photovoltaic performance. Compact, smooth, and large grained perovskite films were fabricated with 0.5 M TU to faster charge transport and separation at the interface. The film presented stronger absorption at the full light absorption range as TU content increased and the absorption edge was slightly red-shifted. The best performance of perovskite solar cells with TU demonstrated a PCE of 19.80% and an average steady-state PCE of 18.60%. The TU-0.5 solar cells exhibited excellent stability against humidity and temperature under the ambient air.

Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (230201606500078) and the National Natural Science Foundation of China (NO. U1302274 and 51674026).

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Keywords intermediate phase, perovskite solar cells, large grains, thiourea Received: February 13, 2018 Revised: March 12, 2018 Published online:

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