Elucidating the Roles of TiCl4 and PCBM Fullerene Treatment on TiO2

Dec 20, 2017 - A Design Based on a Charge-Transfer Bilayer as an Electron Transport Layer for Improving the Performance and Stability in Planar Perovs...
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Elucidating the Roles of TiCl4 and PCBM Fullerene Treatment on TiO2 Electron Transporting Layer for Highly Efficient Planar Perovskite Solar Cells Jing Ma, Jingjing Chang,* Zhenhua Lin, Xing Guo, Long Zhou, Ziye Liu, He Xi, Dazheng Chen, Chunfu Zhang,* and Yue Hao State Key Discipline Laboratory of Wide Band Gap Semiconductor Tecchnology, Shaanxi Joint Key Laboratory of Graphene, School of Microelectronics, Xidian University, 2 South Taibai Road, Xi’an, 710071, China S Supporting Information *

ABSTRACT: With the fast development of the perovskite solar cell, its energy conversion efficiency has improved rapidly. In this paper, a CH3NH3PbI3−xClx organic−inorganic perovskite solar cell (PSC) was successfully prepared by the one-step solvent engineering method, and the planar heterojunction perovskite solar cell was optimized by interface engineering of the cathode interlayers. It was found that TiCl4 treatment could effectively reduce the TiO2 surface roughness, decrease the surface defects, improve the perovskite thin film crystallinity, and reduce the charge carrier recombination; hence, the short-circuit current density of PSCs and charge extraction and collection efficiencies were enhanced. Furthermore, PC60BM fullerene treatment could optimize the contact condition and reduce the interfacial potential loss. The device hysteresis effects after PCBM treatment were also reduced. The combination of TiCl4 and PC60BM treatments could largely improve the PSC device performance with a power conversion efficiency of 16.4% due to the synergetic effect.

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INTRODUCTION

A cathode interlayer such as TiO2 has been frequently used in planar perovskite solar cells because of the good ohmic contact formed with the perovskite film, which can improve the device performance. To date, various TiO2 interlayers have been used in the devices.19−22 The interface between TiO2 and perovskite plays a critical role in determining the final photovoltaic performance of the device. Generally, a good metal oxide interlayer should have a high film quality with few pinholes, smooth surface morphology, good film conductivity, suitable energy level, and intimate contact with perovskite to ensure a better charge transport and extraction/collection.18−24 However, the low conductivity of TiO2 results in poor electron extraction, leading to charge accumulation in the device. Moreover, the exposure of TiO2 to UV light induces abundant oxygen vacancies on the surface which are deep trap levels

Perovskite solar cells (PSCs) with alkylammonium lead halides such as CH3NH3PbI3 for light absorption have attracted considerable attention because of their low fabrication cost, high light absorption coefficient, long charge diffusion length, and high power conversion efficiency (PCE).1−12 Since the introduction of perovskite as a photosensitive material into dyesensitized solar cells for the first time in 2009, PSCs have developed rapidly; the energy conversion efficiency of small area devices (1 cm2) has exceeded 15%.15 Two types of device structures, one with a scaffold layer and the other with a planar structure, have been reported in the literature. Compared to PSCs with a scaffold layer, planar PSCs have a simpler cell architecture that can avoid the infiltration problems of the sensitizer and hole transport materials into the scaffold layer, thus reducing the charge recombination and improving the performance reproducibility.16−18 © XXXX American Chemical Society

Received: September 26, 2017 Revised: December 6, 2017 Published: December 20, 2017 A

DOI: 10.1021/acs.jpcc.7b09537 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

70 °C for 45 min and then heat treated at 450 °C for 15 min. After that, the samples were transferred into a nitrogen-filled glovebox, and spin-coated with a PC60BM solution (10 mg/mL in chlorobenzene) at 6000 rpm for 45 s, followed by heating at 100 °C for 5 min. To make a uniform perovskite layer, a perovskite precursor solution consisting of 1.26 M PbI2, 0.14 M PbCl2, and 1.35 M MAI in the cosolvent of DMSO:GBL (3:7 volume ratio) was stirred at 60 °C for 2 h,29 and then deposited onto the PC60BM layer at 1000 rpm for 20 s and 4000 rpm for 60 s. It is notable that 320 μL of anhydrous toluene was injected onto the spinning film after 25 s at 4000 rpm. Subsequently, the as-prepared samples were annealed at 100 °C for 15 min with the thickness of the perovskite layer of about 300 nm. Next, the hole transport material (HTM) was spincoated on FTO/TiO2−TiCl4/PC60BM/CH3NH3PbI3−xClx at 1000 rpm for 10 s and 4000 rpm for 45 s using the prepared HTM solution. The HTM solution contained 90 mg of spiroOMeTAD in 1 mL of chlorobenzene with 45 μL of LiTFSI/ acetonitrile solution (170 mg/mL), 10 μL of TBP, and 75 μL of Co(III) complex FK209/acetonitrile solution (7.5 mM). Before the final thermal evaporation, all the prepared samples were exposed to a controlled environment with a relative humidity of 15% for 2 h to facilitate the oxidation of spiro-OMeTAD. After that, a gold electrode of 100 nm was deposited through a shadow mask to create a device area of 0.125 cm2. Device and Material Characterization. All current density−voltage (J−V) curves were recorded using a Keithley 2400 source meter unit under simulated AM 1.5G illumination at an intensity of 100 mW/cm2 with a XES-70S1 solar simulator. The system was calibrated using an NREL-certified monocrystal Si photodiode detector before device testing. All of the measurements were carried out in air at room temperature without encapsulation. The UV−visible absorption spectra were measured on a PerkinElmer Lambda 950 spectrophotometer. Photoluminescence spectra were collected on an Edinburgh Instruments FLS920 spectrofluorometer with an excitation wavelength of 450 nm. Scanning electron microscopy (SEM) images were obtained on a JSM-7800F SEM. Thin film X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance XRD instrument. The surface morphology of TiO2 was measured with a Dimension Icon atomic force microscope (AFM). X-ray photoelectron spectroscopy (XPS) experiments were carried out on an Escalab 250Xi using monochromatic Al Kα (1486.6 eV) as the radiation source. Ultraviolet photoelectron spectroscopy (UPS) experiments were carried out at the Escalab 250Xi using He I (21.2 eV) as the excitation source. Time-resolved photoluminescence (TR-PL) spectra were measured using the Pico Quant Fluotime 300 by using a 510 nm picosecond pulsed laser. Transient photocurrent (TPC) measurement was performed with a system excited by a 532 nm (1000 Hz, 3.2 ns) pulse laser. Transient photovoltage (TPV) measurement was performed with the same system excited by a 405 nm (50 Hz, 20 ms) pulse laser. A digital oscilloscope (Tektronix, D4105) was used to record the photocurrent or photovoltage decay process with a sampling resistor of 50 Ω or 1 MΩ, respectively.

responsible for severe charge accumulation and recombination.24 Hence, various methods have been investigated to modify the TiO2 layer to improve the film quality. Fullerene interlayers, such as PC60BM and C60-SAM, have been used to treat the TiO2 surface to passivate the defects of the bottom TiO2 layer and diminish the current density−voltage (J−V) hysteresis.25,26 Yang et al. applied a TiCl4 treatment on the TiO2 layer to improve the contact of the TiO2/perovskite interface, facilitating charge extraction and suppressing charge recombination.27 However, so far only a few reports have studied the synergetic effect of multitreatment at this interface. It was thought that devices with dual interfacial layer treatments could reveal better performance due to more efficient charge extraction or trap-state passivation and better energy level alignment.28 In this study, a combination of TiCl4 and PC60BM treatments has been applied on the TiO2 surface. It was found that the TiCl4 treatment could effectively reduce the TiO2 surface roughness, decrease the surface defects, and improve the perovskite thin film crystallinity; hence, the short-circuit current density (Jsc) of PSCs and charge extraction and collection efficiencies were enhanced. Furthermore, it was shown that the PC60BM fullerene treatment could optimize the contact condition and reduce the interfacial potential loss. By combining the TiCl4 and PC60BM treatments, the efficiency of PSCs could be largely improved with a power conversion efficiency (PCE) of 16.4%, which is much higher than those of the pristine TiO2 interlayer device (10.8%), and also higher than those of the TiCl4 treated only devices (12.1%) and PC60BM treated only devices (12.2%).

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EXPERIMENTAL SECTION Materials. All chemicals and reagents were used as received without further purification. Methylammonium iodide (MAI, 99.8% purity), and tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine] cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (FK209) were obtained from Dyesol Ltd. Spiro-OMeTAD was supplied by Merck Inc. γ-Butyrolactone (GBL, anhydrous, >99.9% purity) was bought from Aladdin Ltd. Other materials, including lead(II) iodide (PbI2, 99.999% purity), lead(II) chloride (PbCl2, 99.999% purity), [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM, >99.9% purity), dimethyl sulfoxide (DMSO, ≥99.8% purity), toluene (anhydrous, ≥99.8% purity), chlorobenzene (anhydrous, 99.8% purity), acetonitrile (anhydrous, 99.8% purity), 1-butanol (anhydrous, 99.8% purity), 4-tert-butylpyridine (TBP, 96% purity), titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol), titanium(IV) chloride (TiCl4, ≥98.0% purity), and bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, 96% purity), were purchased from Sigma-Aldrich. Fabrication and Characterization of Perovskite Solar Cells. Fluorine-doped tin oxide glass substrates (FTO, Pilkington, TEC8, 8 Ω/sq) were patterned by etching with Zn powder and 2 M HCl diluted in deionized water. The patterned FTO substrates were cleaned sequentially by ultrasonication in detergent, deionized water, acetone, and alcohol for 15 min each. After drying with a N2 stream, a 15 min UV-ozone treatment was used to further clean the substrates. A thin compact TiO2 layer was deposited according to the following procedures: A 0.15 M titanium diisopropoxide bis(acetylacetonate) solution in 1-butanol was spin-coated onto FTO substrates at 4000 rpm for 45 s and annealed at 125 °C for 5 min, and the same process was taken twice with 0.3 M titanium diisopropoxide bis(acetylacetonate) solution. The as-prepared films were sintered at 450 °C for 15 min. After cooling to room temperature, the coated TiO2 films were immersed into 40 mM TiCl4 aqueous solution at

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RESULTS AND DISCUSSION In order to explore the TiO2 surface treatment effects on the perovskite solar cell performance, planar PSC devices with the architecture of FTO/TiO2/PC60BM/CH3NH3PbI3−xClx/spiroOMeTAD/Au were fabricated (Figure 1a). A CH3NH3PbI3−xClx layer is sandwiched between TiO2 and spiro-OMeTAD, which has been widely used in perovskite solar cells. In this structure, B

DOI: 10.1021/acs.jpcc.7b09537 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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open-circuit voltage (Voc), Jsc, fill factor (FF), and PCE are summarized in Table 1. From Figure 2 and Table 1, it can be seen that pristine TiO2 based devices showed a PCE of 10.8% with a Voc of 1.0 V, Jsc of 17.9 mA/cm2, and FF of 0.62. After PC60BM-only treatment, the devices have a higher Voc of 1.04 V and Jsc of 19.1 mA/cm2, resulting in an improved PCE of 12.2%. For TiCl4-only treated devices, the devices exhibited much higher Jsc of 20.84 mA/cm2 and unchanged Voc; hence, the PCE also showed slight improvement. It is needed to mention that the PC60BM and TiCl4 dual treatment enhanced the Voc, Jsc, and FF simultaneously, and the TiO2−TiCl4/ PC60BM devices revealed an average performance with Voc, Jsc, FF, and PCE of 1.08 V, 21.36 mA/cm2, 0.65, and 14.8%,

the CH3NH3PbI3−xClx layer absorbs light, while the spiroOMeTAD and TiO2 layers act as the hole transport and electron transport, respectively. Figure 1b schematically presents the energy band diagram of the electronic materials in the perovksite solar cells. Free electrons and holes are readily generated in the perovskite layer upon light illumination. Due to the deep-lying valence band (VB) of TiO2, holes are blocked by the TiO2 layer and collected by the spiro-OMeTAD modified Au electrode. The electrons generated in the perovskite layer can be transferred to TiO2 and collected by the FTO electrode. The J−V characteristics of PSCs without and with TiO2 surface treatment were recorded under light conditions (Figure 2a). The photovoltaic performance parameters, including the

Figure 1. (a) Schematic structure of devices fabricated in this experiment. (b) Energy band diagram of corresponding materials used in the device.

Figure 2. (a) J−V characteristic curves of CH3NH3PbI3−xClx perovskite device based TiO2, TiO2/PC60BM, TiO2−TiCl4, and TiO2−TiCl4/PC60BM cathode interlayers. (b) J−V characteristic curve of the best device. (c) IPCE spectrum of the best device based on TiO2−TiCl4/PC60BM cathode interlayer. (d) Jph−Veff curves for perovskite solar cells based on different cathode interlayers. C

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The Journal of Physical Chemistry C respectively. The Voc, Jsc, and FF of the TiO2−TiCl4/PC60BM devices all exhibited significant enhancement compared to control devices. Meanwhile, the device series resistance (Rs) decreased to 4.2 Ω cm2, and shunt resistance (Rsh) increased to 6.4 kΩ cm2 compared to control devices. These Rs and Rsh values are consistent with the device performance of the PSCs. The best device exhibited a PCE of 16.4% with a Voc of 1.08 V, a Jsc of 21.8 mA/cm2, and a FF of 0.70. Since the PC60BM-only treated devices showed slightly decreased Jsc and increased Rs values compared to TiCl4-only treated devices, a lower concentration of PC60BM solution was also used to further reduce the PC60BM thickness and corresponding Rs. However, when the PC60BM concentration was reduced to 5 mg/mL, the Jsc and Voc of the devices all decreased (Figure S1), indicating a continuous PC60BM layer was not formed after thermal annealing or perovskite deposition due to the low concentration. Hence, the 10 mg/mL concentration provided the best performance.

The statistical analysis is shown in Figure 3, and it can be seen that the PC60BM and TiCl4 dual treatment significantly improved the device uniformity and reproducibility. The device stability regarding the interlayer treatment was also studied. The unencapsulated devices were stored in air condition with relative humidity less than 30%. Figure S2 presents the normalized PCEs over time. The perovskite devices with TiCl4/PCBM dual treatments exhibited better device stability (72% of initial value) compared to pristine TiO2 based devices (43% of initial value) after 216 h due to the improved film quality and interface contact which will be discussed later. The incident photon-to-current conversion efficiency (IPCE) of the device based on the best condition was characterized and is shown in Figure 2c. The IPCE spectrum showed high values of over 90% from 450 to 650 nm, which give an integrated current density of around 21.0 mA/cm2. This value is consistent with the J−V measurement. It is seen clearly that, for TiCl4

Table 1. Photovoltaic Parameters of PSCs Prepared Using TiO2, TiO2−TiCl4, TiO2/PC60BM, and TiO2−TiCl4/PC60BM Cathode Interlayers substrate TiO2 TiO2/PC60BM TiO2−TiCl4 TiO2−TiCl4/PC60BM

average best average best average best average best

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

Rs (Ω cm2)

Rsh (kΩ cm2)

0.98 1.00 1.00 1.04 0.98 1.00 1.08 1.08

16.99 17.90 18.24 19.10 18.76 20.84 21.36 21.80

0.58 0.62 0.60 0.64 0.56 0.60 0.65 0.70

9.7 10.8 10.8 12.2 10.2 12.1 14.8 16.4

7.3

1.5

6.4

3.6

6.1

1.6

4.2

6.4

Figure 3. Statistic parameters of Jsc, Voc, FF, and PCE for PSCs prepared under different cathode interlayer conditions: TiO2, TiO2/PC60BM, TiO2−TiCl4, and TiO2−TiCl4/PC60BM. D

DOI: 10.1021/acs.jpcc.7b09537 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C treated devices, the Jsc values were all significantly enhanced, indicating an efficient charge extraction/collection and suppressed charge recombination, which is consistent with the literature reported results.27 This is also consistent with what Abdi-Jalebi et al. reported, that the TiCl4 treatment could reduce the subband gap absorption, induce the passivation of trap states at the interface, and improve the charge transport of metal oxide.30 Hence, the charge carrier recombination is minimized, and the Jsc and PCE are enhanced. In order to gain additional insights into the improved Jsc and FF for the TiCl4 treated device, the photocurrent (Jph) is plotted as a function of the effective voltage (Veff). Jph is obtained by subtracting the current density in the dark from the current density under illumination. The Veff, which reflects the internal field in the device, is defined as Veff = V0 − Va, where V0 is the compensation voltage defined as the voltage where Jph = 0 and Va is the applied bias.31 As shown in Figure 2d, the saturation photocurrent at high Veff is much higher for the TiCl4 treated devices; moreover, the corresponding devices exhibited higher Jph from the open-circuit condition to the short-circuit condition compared to control and PC60BM treated perovskite devices, indicating that TiCl4- and PC60BM treated devices reveal higher charge collection efficiency and less recombination loss. It should be mentioned that the open-circuit voltage was enhanced after PC60BM treatment, indicating that the potential loss (Eg − qVoc) is reduced after PC60BM fullerene treatment.16 It can be explained that the valence band and conduction band of PC60BM are around −5.9 and −4.1 eV, respectively, which

can upshift the energy level and increase the potential difference between two electrode contacts (TiO2/PC60BM and spiro-MeOTAD), resulting in significantly enhanced Voc.32 This can be further confirmed by the UPS spectra of TiO2 film before and after PC60BM treatment. As shown in Figure 4, the work function of TiO2 decreased from −4.36 eV (control) to −4.15 eV (TiO2/PC60BM) and −4.06 eV (TiO2−TiCl4/ PC60BM) after PC60BM treatment. TiCl4 treatment has less effect on the work function of the control film. The decreased work function of treated TiO2 facilitates a larger build-in field for charge extraction and collection.33 Moreover, the energy level of perovskite film deposited on the electron transporting layer could be determined by the UPS spectrum (Figure S3). According to the optical band gap of 1.6 eV, conduction band of 4.1 eV, and valence band of 5.7 eV could be obtained. Hence, it can be seen that the energy offset between the TiO2 electron transporting layer and the conduction band of perovskite was reduced (Figure 1b), which is beneficial for better energy level alignment. In order to further investigate the deep mechanism for performance enhancement, transient photocurrent and photovoltage measurements of perovskite solar cells were performed. Figure 5a shows the transient photocurrent decay of perovskite devices measured at the short-circuit condition. It can be seen that, after TiCl4/PCBM dual surface treatment, the devices exhibited faster decay with shorter lifetime (1.08 μs) compared to the pristine TiO2 based device (2.40 μs), and also shorter than PCBM-only and TiCl4-only treated devices. This indicates that the device with dual treatments possessed a more efficient charge transfer process. Hence, the Jsc and FF exhibited significant enhancement. The transient photovoltage test was used to determine the charge-recombination lifetime (Figure 5b) for devices with different surface treatments. It is clear to see that the device with TiCl4/PCBM dual treatment exhibited a much longer lifetime (2.74 ms) compared to PCBM treated TiO2 (1.97 ms), TiCl4 treated TiO2 (2.02 ms), and pristine TiO2 based devices (1.52 ms). This indicated that the charge recombination process was efficiently suppressed and the trap density was reduced after PCBM and TiCl4 dual treatment. The device hysteresis behavior is checked by measuring the J−V curve with different scan directions. As can be seen from Figure 6, the control device based on pristine TiO2 exhibited an obvious hysteresis for reverse and forward scans. This behavior is frequently observed in this structure, and is possibly caused

Figure 4. UPS spectra of TiO2 interlayers with different treatments.

Figure 5. (a) Transient photocurrent and (b) photovoltage decay characteristics of perovskite solar cells based on different electron transporting interlayers. E

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Figure 6. Hysteresis curves of perovskite devices based on TiO2, TiO2/PC60BM, TiO2−TiCl4, and TiO2−TiCl4/PC60BM cathode interlayers.

by perovskite crystal defect assisted traps, interface traps, ion migration, and so on.34−38 After PC60BM treatment, the hysteresis behavior between different scan directions was reduced, indicating PC60BM treatment could reduce the surface-related defects which are responsible for the observed hysteresis effect. The TiCl4 treatment could reduce the leakage current of the forward scan, but it has little effect on the hysteresis behavior. In order to investigate the performance enhancement, X-ray diffraction (XRD) measurement was performed. The thin film crystallinity of the CH3NH3PbI3 film could affect the charge generation efficiency, charge transport, and exciton diffusion length.39 Figure 7 represents the characteristic peak of perovskite, and the CH3NH3PbI3−xClx thin film has strong diffraction patterns at 14.2, 28.5, and 31.9°. These XRD patterns are assigned to ⟨110⟩, ⟨220⟩, and ⟨310⟩ of CH3NH3PbI3−xClx, respectively. It also means that the prepared CH3NH3PbI3−xClx material is the tetragonal phase.16 The diffraction pattern at 12.7° originates from PbI2 because of the excess PbI2 over CH3NH3I (PbI2:CH3NH3I = 1.4:1.35).29 It can be found that the TiCl4 treated device exhibited higher diffraction intensity compared to control and PC60BM treated devices, indicating that TiCl4 treatment could efficiently enhance the perovskite film crystallinity, which is beneficial for the light absorption and charge transport. Hence, the current density is enhanced. The results could be further confirmed by perovskite film morphology and UV−vis light absorption. The perovskite thin film morphology was studied by field-emission scanning electron microscopy (FESEM). From the SEM results as shown in Figure 8, it can be seen that the prepared thin films with different TiO2 treatments showed many more larger crystallites with better surface coverage compared to pristine TiO2 based film. Especially the average crystal size of TiCl4 treated thin film showed an increase compared to pristine thin film, which is consistent with XRD results.

Figure 7. XRD patterns of CH3NH3PbI3−xClx perovskite thin films on TiO2, TiO2/PC60BM, TiO2−TiCl4, and TiO2−TiCl4/PC60BM cathode interlayers. “#” and “&” represent the characteristic peaks of FTO and TiO2, respectively.

The light absorption of the perovskite thin films with different TiO2 surface treatments were also studied by UV−vis spectroscopy (Figure 9a). The thin film has strong absorption from UV down to the near-infrared range with the absorption onset at ca. 770−780 nm (optical band gap ≈ 1.61 eV). Compared to the pristine TiO2 film, the TiCl4 treated film shows much higher absorption. This can be attributed to the higher crystallinity of the latter. It is unexpected that the light absorption of the perovskite films based on TiO2 treated with PCBM decreased, which should be due to the light absorption of PC60BM shown in Figure S4. This is consistent with the F

DOI: 10.1021/acs.jpcc.7b09537 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 8. Top-view SEM images of perovskite films grown on (a) TiO2, (b) PC60BM/TiO2, (c) TiCl4−TiO2, and (d) PC60BM/TiCl4−TiO2 cathode interlayers.

Figure 9. (a) UV−vis absorption and (b) PL spectra of perovskite thin films based on different TiO2 surface treatments: TiO2, TiO2/PC60BM, TiO2−TiCl4, and TiO2−TiCl4/PC60BM.

lower Jsc achieved by PC60BM treatment. The transmittance of TiO2 layers was measured to investigate the effect of surface treatment condition on the perovskite thin film absorption. As shown in Figure S4, the PC60BM modification obviously decreases the film transmittance, which could lower the perovskite film absorption to some extent. This is consistent with the results shown in Figure 8a. The photoluminescence (PL) spectra (Figure 9b) of four different samples were measured, and the emission wavelength is around 758 nm. Compared to pristine TiO2 based perovskite films, either PC60BM or TiCl4 treatment decreases the PL intensity, indicating an efficient PL quenching due to charge

transfer between the electron transporting layer and the perovskite film.16 It should be noted that the peak of TiO2−TiCl4/ PC60BM/CH3NH3PbI3−xClx film has the lowest intensity due to more efficient charge transfer induced quenching, and this is consistent with the above discussed results.16 Time-resolved PL was also performed to study the charge transfer process based on different electron transporting layers (Figure 10). The carrier lifetime was obtained by fitting the PL curves with a double exponential decay model. The data are summarized in Table S1. For the perovskite film deposited on TiO2 layer with TiCl4/PCBM dual treatments, the lifetime was significantly reduced to 16.04 ns compared to 45.85 ns of the film deposited G

DOI: 10.1021/acs.jpcc.7b09537 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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force microscopy (AFM). As can be seen from Figure 11, the root-mean-square (RMS) roughness can be measured by AFM, and high RMS roughness is not conducive to the improvement of device performance generally. The RMS roughness of film treated by TiCl4 decreased from 18.0 to 13.9 nm compared to pristine thin film, while the RMS roughness of film treated by PC60BM decreased to 16.4 nm, which is consistent with XRD results. After both treatments, the surface roughness further decreased to 13.0 nm, indicating that the TiCl4 treatment could effectively smooth the TiO2 surface and passivate the surface defects. The surface defects of TiO2 without and with different treatment conditions were also checked by PL spectra. Figure S5 shows the PL spectra of TiO2 films deposited on glass substrates with/without surface treatment. The emission peak is commonly referred to as a shallow or deep trap level emission (or trap-state emission) which is related to the oxygen vacancies.40 The emission results from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy. For the film with TiCl4 treatment, the PL peak intensity significantly reduced, indicating that the TiCl4 treatment could efficiently reduce the defect of pristine TiO2. For the film with PC60BM treatment, the PL intensity only showed a slight decrement, indicating that the dominant role for PC60BM treatment is tuning the energy level alignment between TiO2

on pristine TiO2. This indicated that the efficient charge transfer induced PL quenching occurred after surface treatment of TiO2 interlayers. This is also consistent with other results, showing that the dual treatments enhance the charge transfer efficiency. Since the perovskite thin film properties are critically related to the TiO2 treatment, we studied the TiO2 thin film properties without and with different surface treatment conditions. The surface morphology of TiO2 thin films was investigated by atomic

Figure 10. Time-resolved photoluminescence spectra of perovskite thin films deposited on different electron transporting layers.

Figure 11. Top-view AFM images of perovskite films grown on (a) TiO2, (b) PC60BM/TiO2, (c) TiCl4−TiO2, and (d) PC60BM/TiCl4−TiO2 cathode interlayers. H

DOI: 10.1021/acs.jpcc.7b09537 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C and perovskite, and it has less effect on the film defect passivition. Hence, the device showed less Jsc improvement.

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CONCLUSION In this study, the processing of a planar heterojunction perovskite solar cell was realized by a one-step method, and the treatments of PC60BM, TiCl4, and their combination were discussed. The PC60BM treatment can reduce the energy mismatch between TiO2 and the perovskite layer, so Voc could be improved. Otherwise, owing to the hydrophobic nature, the absorption of the perovskite films treated by PC60BM decreased. Meanwhile, the TiCl4 treatment efficiently enhances the perovskite film’s crystallinity to enhance charge extraction/collection and surpress charge recombination, so Jsc was enhanced. Ultimately, the devices treated by the combination of TiCl4 and PC60BM reached a PCE of 16.4%. The Voc, Jsc, and FF of the TiO2−TiCl4/PC60BM devices all exhibited significant enhancement compared to control devices due to the synergetic effect of TiCl4 and PC60BM treatments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09537. J−V characteristics, stability of devices, UPS spectrum, transmittance spectra of interlayers, fitted decay times of perovskite films, PL spectra of TiO2 thin films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Jingjing Chang: 0000-0003-3773-182X He Xi: 0000-0003-0684-4979 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (61604119, 61704131), the Natural Science Foundation of Shaanxi Province (2017JQ6002, 2017JQ6031), the Young Talent fund of China Association for Science and Technology, and the Fundamental Research Funds for the Central Universities.

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DOI: 10.1021/acs.jpcc.7b09537 J. Phys. Chem. C XXXX, XXX, XXX−XXX