Thermal Stability-Enhanced and High-Efficiency Planar Perovskite

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Thermal stability-enhanced and high-efficiency planar perovskite solar cells with interface passivation Weihai Zhang, Juan Xiong, Li Jiang, Jianying Wang, Tao Mei, Xianbao Wang, Haoshuang Gu, Walid A Daoud, and Jinhua Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10994 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Thermal stability-enhanced and high-efficiency planar perovskite solar cells with interface passivation Weihai Zhang,†,‡,§ Juan Xiong,*,‡ Li Jiang,† Jianying Wang,† Tao mei,† Xianbao Wang,† Haoshuang Gu,‡ Walid A. Daoud,§ Jinhua Li *,†,‡ †

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the

Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of

Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, China

‡

Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Sciences,

Hubei University, Wuhan, 430062, China

§

School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Hong Kong.

ABSTRACT: As electron transport layer (ETL) of perovskite solar cells, oxide semiconductor Zinc oxide (ZnO) has been attracted great attention due to its relatively high mobility, optical transparency, low temperature fabrication and good environment stability. However, the nature of ZnO will react with the patron on methylamine, which would deteriorate the performances of cells. Although many methods including high temperature annealing, doping and surface modification have been studied to improve the efficiency and stability of perovskite solar cells with ZnO ETL, devices remains relatively low efficiency and stability. Herein, we adopted a novel multi-step annealing method to deposit a porous PbI2 film and improved the quality and uniformity of perovskite films. The cells with ZnO ETL were fabricated at the temperature of < 150℃ by solution processing. The power conversion efficiency

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(PCE) of the device fabricated by the novel annealing method increased from 15.5% to 17.5%. In order to enhance the thermal stability of CH3NH3PbI3 (MAPbI3) on the ZnO surface, a thin layer of small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was inserted between the ZnO layer and perovskite film. Interestingly, the PCE of PCBM-passivated cells could reach nearly 19.1%. To our best knowledge, this is the highest PCE value of ZnO-based perovskite solar cells until now. More importantly, PCBM modification could effectively suppress the decomposition of MAPbI3 and improve the thermal stability of cells. Therefore, the ZnO is a promising candidate of electron transport material for perovskite solar cells in the future applications. KEYWORDS: perovskite solar cells; crystallinity; zinc oxide; electron transport material; interface passivation 1. INTRODUCTION Organometal halide perovskite solar cells (PSCs) have attracted much attention in recent years due to their high photovoltaic efficiency and low fabrication cost.1-6 Their power conversion efficiency (PCE) jumped from 3.8% in 20097 to a certified 22.1% in 20168 via film crystal growth control, band gap adjustment and interface engineering.9-12 In a typical mesoporous PSCs, a compact TiO2 as well as porous TiO2 film was commonly used as an electron transport layer (ETL).13-15 However, the high sintering temperature (>450℃) was required to form the highly crystallized TiO2 films, which made it impossible to fabricate low cost and flexible solar cells. To fabricate low-temperature and high efficiency PSCs, mesoporous-free ‘planar’


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configuration has been developed recently.16-20 The planar PSCs are often fabricated by using the electron transport materials such as SnO2, TiO2 and ZnO21-26 which can be deposited by low-temperature solution processing. These materials have a relatively high carrier mobility and superior stability, which made them ideal candidates as transport layers. The planar perovskite solar cells based on a SnO2 ETL have been reported with an efficiency over 20%.27,28 When a modified TiO2 film was adopted as the ETL, the efficiency of cells could reach 19.62%.23 Compared to TiO2 and SnO2, the ZnO has the highest carrier mobility. However, the highest efficiency of cell based on ZnO ETL was only 17.7%, reported by S.H. Im et. al.29 The poor performances would be mainly ascribed to several factors. Firstly, the hydroxyl groups, organic residuals and oxygen vacancies on the surface of the ZnO layer can easily lead to thermal decomposition.30 Secondly, without a porous structure, it is always difficult to acquire pure CH3NH3PbI3 film without some PbI2 residuals which play a negative effect on the transportation of the charges.31,32 Thirdly, the ZnO itself causes the deprotonation of the methylammonium cation,which results in the loss of methylamine and the formation of PbI2.33.34 Therefore, it is difficult to obtain high-efficiency PSCs by using the ZnO ETL. Several Strategies were addressed to resolve the above mentioned problems, include high temperature annealing to remove the residuals on ZnO surface,34 adopting aluminum doped ZnO (AZO) as the ETL to reduce the bad effect,35 and depositing an intermediate layer to prevent the direct interaction between ZnO and perovskite layer.36,37 These methods are effective to a certain extend and indeed have a positive effect on enhancing the performance of the



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device. However, the efficiency of perovskite solar cells with ZnO ETL remains inferior compared to those with TiO2 and SnO2 ETL. PCBM is an important organic electron transport material in PSCs.18-20 The lowest unoccupied molecular orbital (LUMO) energy level of PCBM is 3.9 eV that can match the conductive band level of MAPbI3 well.38,39 Electron transport can be very efficient between the interface of MAPbI3 and PCBM. W. Nie, et al.40 reported MAPbI3/PCBM inverted planar hybrid PSCs and the high PCE of 17.7% was achieved. J. H. Heo et al.19 also reported that the PCE of the same structured PSCs could reach 18.1%. The high efficiency was due to the high conductivity of PCBM compared to TiO2. Very recently, W. Qiu, et al.41 used cross linked PCBM as the interlayer between perovskite and TiO2 to fabricate planar PSCs with a high efficiency of 18.4%. It suggest that it is possible to obtain the high-performance PSCs when PCBM-passivated ZnO is used as the ETL. Herein, we adopt a novel multi-step annealing (SA) method to prepare porous PbI2 film for the formation of pure CH3NH3PbI3 film. The results reveal that SA method is helpful to fabricate high quality CH3NH3PbI3 film. Compared to common annealing (CA) method, the PCE of the cells fabricated by SA method increased from 15.5% to 17.5%. Furthermore, a series of perovskite solar cells based on ZnO and PCBM-passivated ZnO ETL were fabricated to investigate the thermal stability of the devices. When a buffer layer of PCBM was inserted between MAPbI3 film and ZnO film, the degradation of the CH3NH3PbI3 film is effectively suppressed. More interesting, an enhancement of PCE is observed. The efficiency of the device increased from 17.5% (without PCBM)


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to 19.07% by choosing a suitable thick PCBM. To our best knowledge, this is the highest record ever reported for perovskite solar cells based on ZnO ETL. It implies that ZnO will be greatly promising ETL material for the future applications of PSCs. 2. EXPERIMENTAL SECTION 2.1. Preparation of ZnO nanoparticles. The ZnO nanoparticles (NPs) were synthesized according to our former work.42 Zinc acetate dihydrate (ZnAc·2H2O) (0.59 g) and potassium hydroxide (KOH) (0.296 g) were dissolved in 25 mL and 13 mL methanol, respectively. Before the KOH solution was added, the zinc acetate dihydrate solution was processed with a constant 65℃ water bath for 10 min. Then the mixed solution was stirred for 2.5 h at 65℃. After that, the solution was cooled down and stored overnight to precipitate the product. Finally, the transparent ZnO precursor was produced by dispersing the precipitate in the n-butanol (14 mL), methanol (1 mL) and chloroform (2 mL) with a concentration of 6 mg ml-1. Before the usage, the ZnO precursor was filtered through a 0.45 µm polyvinylidene fluoride (PVDF) syringe filter. 2.2. Perovskite solar cell fabrication. The FTO substrate was thoroughly cleaned by acetone, ethanol and deionized water, respectively. Then they were treated by an O2 plasma for 3 min to increase the hydrophilic nature of the surface. For ZnO layer, the prepared ZnO precursor was spin-coated on well-cleaned FTO substrates at 3000 r.p.m. for 30 s and then annealed at 150 ℃ for 10 min to remove the organic solvent in ambient air. This procedure was repeated several times to obtain a continuous smooth



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ZnO film with an appropriate thickness. The PCBM film was deposited by spin-coating PCBM chlorobenzene solution on ZnO layer with a rate of 3000 r.p.m for 40 s and annealed on a hotplate at 85 °C for 5 min in a glove box. The perovskite layer was deposited by a two-step dipping method. First, PbI2 layer was deposited by spin coating 1.1M PbI2 which dissolved in DMF/ DMSO (90/10 v/v) at 3000 r.p.m. for 30 s. The as-prepared PbI2 film was annealed by a multi-step annealing method (60 ℃, 80 ℃, and 100 ℃ for 5 min, respectively) to obtain the uniform and porous PbI2 film. Then, the porous PbI2 film was dipped into a solution of CH3NH3I in 2-propanol with a concentration of 10 mg/ml for 5 min. The initial perovskite film was dried under a flow of clean air at 100 ℃ for 5min to form well-crystallized CH3NH3PbI3 layer. Subsequently, the spiro-OMeTAD hole transport layer (HTL) (72.3 mg of spiro-OMeTAD, 28.8 µL of 4-tert-butyl pyridine and 17.5 µL lithium-bis (trifluoromethanesulfonyl) imide (Li-TFSI) solution (520 mg Li-TFSI in 1 mL acetonitrile) all dissolved in 1 mL chlorobenzene) was prepared by spin coating at 4000 r.p.m. for 30 s. Finally, a 80 nm thick Au layer was deposited by thermal evaporation. The area of Au electrode was 6 mm2. 2.3. Device characterization. A transmission electron microscope (TEM) (Tecnai G20, FEI) was used to analyze the ZnO nanoparticles. The optical transmittance and absorbance spectra were obtained by an ultraviolet-visible diffuse reflection spectroscopy (UV-vis-DRS, Shimadzu UV-3600). The roughness of ZnO layer with and without PCBM was analyzed by an atomic force microscope (AFM) (Ntegra upright, NT-MDT, Russia). A field-emission scanning electron microscopy (FESEM,


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JEOL 7100F) was used to acquire surface and cross-sectional morphologies. The XRD patterns (2θ scans) were obtained by Bruker Advanced D8 X-ray diffractometer using Cu Kα (λ = 0.154 nm) radiation. Current density-voltage (J-V) curves of the perovskite solar cells were measured by Keithley 237 under AM 1.5G one-sun illumination provided by a solar simulator (Newport Oriel Sol 3A Class AAA, 64023A). The external quantum efficiency (EQE) spectrum was measured under a constant white light supplied by an array of white light-emitting diodes. The excitation beam coming from a 300 W xenon lamp was focused through a double-monochromator and chopped at 10 Hz. The monochromatic light intensity for the measurement was calibrated by a reference silicon photodiode. Time-resolved photoluminescence (TRPL) spectra were measured by a PL system (DW-PLE03). The electrochemical impedance spectroscopy (EIS) measurements were carried out under 1 Sun light illumination by using a computer-controlled electrochemical workstation (Zennium Zahner, Germany). A bias of the open-circuit voltage and a small voltage perturbation of 10 mV were applied during the measurement at frequency ranging from 1 MHz to 1 Hz. 3. RESULTS AND DISCUSSION 3.1. Characterization of ZnO nanoparticles and thin films. The prepared ZnO-NPs are very uniform and show an approximate diameter of 10 nm revealed by the transmission electron microscopy (TEM) as shown in Figure 1a. HRTEM image in Figure 1b indicates that the ZnO-NPs are highly crystallized, which is extremely important to reduce the traps in ZnO ETL and improve the ability of charge transfer.



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The particles exhibit a hexagonal wurtzite structure and a poly-crystalline structure as confirmed by the electron diffraction pattern in Figure 1c and X-ray diffraction (XRD) pattern in Figure 1d. A dense and pinhole-free ZnO film is observed by a field-emission scanning electron microscopy (FESEM) as shown in Figure 1e. Atomic force microscope images shown in Supporting Information reveal a smooth surface of ZnO film with a root-mean-square (RMS) at 2.31 nm (Figure S1a). Moreover, a smoother surface with RMS=1.35 nm can be observed when a thin layer of PCBM is coated onto the ZnO layer as shown in Figure S1c. The optical transmittance and absorbance of the ZnO layer with and without PCBM are shown in Figure 1f.The transmittance of the layer with and without PCBM are both greater than 90% in the visible region. The absorbance spectra reveal that the absorption of PCBM/ZnO bilayer film just decreases slightly compared to pure ZnO film.


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Figure 1. Properties of ZnO NPs. (a) Transmission electron microscopy (TEM) image of ZnO nanoparticles. (b) High-resolution TEM image of ZnO nanoparticles. (c) Electron diffraction pattern of ZnO nanoparticles. (d) X-ray diffraction pattern of ZnO nanoparticles. (e) Scanning electron microscopy (SEM) image of ZnO nanoparticles deposited on a glass/FTO substrate. (f) Optical properties of ZnO films with and without PCBM. 3.2. Multi-step annealing method. To fabricate high quality CH3NH3PbI3 film without any PbI2 residual, we adopt a novel multi-step annealing (SA) method to prepare porous PbI2 film which is helpful to the formation of pure CH3NH3PbI3. Figure 2a and b illustrate the schematic of the common annealing (CA) and SA procedure of the PbI2 films, respectively. As shown in Figure 2a, after PbI2 is deposited, the film is directly annealed at a constant temperature of 100 ℃ for 15 min before it is dipped into CH3NH3I solution. For SA method, PbI2 film is annealed successively at 60 ℃, 80 ℃, and 100 ℃ for 5 min respectively, as shown in Figure 2b. The XRD patterns of the perovskite films derived from different PbI2 annealing methods are shown in Figure 2c. Clearly, there is some PbI2 residual in perovskite film when CA method is used. While SA method tends to obtain the pure perovskite film. It suggests that the formation of pure perovskite film can be mainly attributed to the porous PbI2 film (shown in Figure 3b) which is beneficial to the easy permeation of the CH3NH3I solution into the PbI2 film and to facilitate the formation of CH3NH3PbI3. The absorbance spectra of the CH3NH3PbI3 film fabricated by different annealing method are shown in Supporting Information Figure S2. It is clear that the



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film fabricated by CA method has a lower absorbance than that fabricated by SA method, which would be mainly due to the PbI2 residual. Figure 2d shows the current density-voltage (J-V) curves of the solar cells fabricated by different annealing methods. The detail parameters are shown in the inset table. Obviously, the device fabricated by CA method shows a relatively low PCE of 15.5%. The poor performances are primarily due to the lower current density (Jsc) and fill factor (FF), which could be caused by the excess PbI2.31,32 However, PCE of cells increases to 17.5% when SA method is used.

Figure 2. Schematic illustration of (a) CA method and (b) SA method. (c) XRD patterns of the CH3NH3PbI3 film derived from different annealing methods. (d) J-V curves of the devices fabricated by different annealing method, the inset table lists the


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detail parameters of the cells. The morphology controlling of PbI2 film is very key factor for forming the high-quality CH3NH3PbI3 film. As shown in Figure 3a, the PbI2 film prepared by CA method indicates an extremely dense and pinhole-free morphology. While a PbI2 film prepared by SA method indicates a uniform and porous morphology, as shown in Figure 3b. The remarkable difference of the morphology would be mainly due to the different growth rate of the PbI2 crystal. When a SA method was adopted, it seems impossible to vapor away all of DMSO during the first half of annealing process at the relatively low temperature which is lower than the boiling point of DMSO. It has been reported that DMSO shows stronger capability than DMF to coordinate with PbI2 to form a complex PbI2-DMSO intermediate, which would retard the crystallization of the PbI2 film and form a porous structure.43,44 Besides, the intermediate phase is proven to be helpful to form better quality perovskite film as reported by L.Y. Han et al.45 As a consequence, the corresponding CH3NH3PbI3 film shows an apparently different morphologies as well. Figure 3c shows an uneven grain size and rough CH3NH3PbI3 film derived from CA method. However, a very uniform and relatively smooth perovskite film prepared by SA method is observed as shown in Figure 3d. Therefore, SA method is effective to control the morphology of PbI2 film and further to control the forming of better quality CH3NH3PbI3 film.



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Figure 3. Top viewed SEM images of PbI2 films prepared by (a) CA method and (b) SA method, and the corresponding SEM images of CH3NH3PbI3 film prepared by (c) CA method, (d) SA method. 3.3. Stability enhancement. Although a relatively high efficiency of 17.5% is achieved by the SA method, we find out that the perovskite film is easily decomposed when the film is annealed at 100 ℃. The main reason may be the deprotonation of the methylammonium cation caused by ZnO which results in the loss of methylamine and the formation of PbI2, as is reported by T.L. Kelly et. al.33,34 To enhance the thermal stability of the device, we fabricated the cells by above-mentioned SA method with a structure of FTO/ ZnO/ PCBM/ CH3NH3PbI3/ Spiro-OMeTAD/ Au. A thin layer of PCBM was inserted as a buffer layer to avoid the


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direct contact between ZnO and perovskite film. In order to examine the effect of PCBM-passivation, we systemically investigated the thermal stability of perovskite films deposited on FTO/ ZnO , FTO/ ZnO/ PCBM and FTO/ PCBM layer, respectively. Figure 4a shows the photographs of the perovskite layers deposited on different substrates with different annealing time varying from 5 to 40 min. Apparently, the perovskite film fabricated on FTO/ ZnO layer is easily decomposed and demonstrates yellow color after annealing 20 min at 100 ℃ in air. In contrast, the color of perovskite layer deposited on FTO/ ZnO/ PCBM and FTO/ PCBM layer remain dark after 40 min. The XRD patterns in Figure 4b, c and d show the evolution of crystalline phases of the CH3NH3PbI3 films fabricated on different substrates under the different annealing time. As shown in Figure 4b (FTO/ ZnO/ perovskite), a PbI2 peak at 12.6° is observed when the film is annealed for 10 min and the peak is stronger with a longer annealing time. After 40 min annealing, it is hardly to detect the peak of CH3NH3PbI3 film. While, the perovskite film deposited on FTO/ ZnO/ PCBM layer remains pure phase until the annealing time is longer than 15 min, shown in Figure 4c. Besides, the peaks of CH3NH3PbI3 film are still dominant after annealing for 40 min. For the perovskite film deposited on FTO/ PCBM layer, it seems that it has an excellent thermal stability as shown in Figure 4d. The appearance of PbI2 peak would be mainly attributed to the effect of water for the fact that the annealing experiment is done in an ambient atmosphere. The different intensity of PbI2 peak between Figure 4c and d is ascribed to the fact that the PCBM layer is too thin to cover the whole area of ZnO layer as shown in Supporting Information, Figure



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S1d, and the exposed ZnO NPs would deteriorate the quality of CH3NH3PbI3 film when an annealing process is conducted. The results suggest that CH3NH3PbI3 is indeed sensitive to ZnO layer, and a buffer layer of PCBM has an undeniable positive effect on the enhancement of the thermal stability of the perovskite film.

Figure 4. (a) The photographs of perovskite layers deposited on different substrates annealing for different time at 100 ℃ in air. XRD patterns of the CH3NH3PbI3 films deposited on (b) ZnO layer, (c) ZnO/ PCBM layer and (d) PCBM layer. In addition to thermal stability, long-term stability is also very important for the application of the perovskite solar cells. Therefore, to ensure whether PCBM passivation has a positive or negative effect on the long-term stability of the device, we conducted a stability test. Figure 5a shows the results of the stability test of the devices with and without PCBM passivation in an ambient environment without encapsulation (40-50% humidity, T = 23 ℃). Clearly, the PCEs of the devices without


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as well as with PCBM passivation decreased with the increase of the exposure time. The instability of the devices would be mainly attributed to the intrinsic properties of the perovskite material (sensitive to water), the non-packed devices and the pinhole perovskite film which allows water penetrated into the film easily, as shown in Figure 3. In spite of the degradation, we can notice that the device with PCBM passivation (maintains over 60% of the initial efficiency after 10 days) shows a higher stability than those without PCBM (only about 40% of the initial efficiency after 10 days), shown in Figure 5b. The better stability performance of the PCBM-passivated device would be mainly ascribed to two factors: (1) The inserting layer of PCBM between ZnO and CH3NH3PbI3 film is helpful to suppress the penetration of the water for the fact that PCBM shows a poor hrdrophilicity. (2) The CH3NH3PbI3 film deposited on PCBM-passivated ZnO layer performs a denser morphology with fewer pinholes than that without PCBM passivation, as is shown in Supporting Information, Figure S3. Better morphology and fewer pinholes and voids allow a limited path for water penetrating, which is essential for long-term stability, as is reported by C.G. Wu et. al.51 Figure 5(c) and (d) show the J-V curves of the devices without and with PCBM passivation varying with the exposure time, and the corresponding detail parameters are summarized in Supporting Information Table S1. It is easily concluded that the degradation of the device performance is determined by the overall deterioration of the Voc, Jsc and FF. Nevertheless, it can be noted that PCBM-passivated device shows a better stability, indicating that PCBM passivation plays a positive effect on the long-term stability of the device.



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Figure 5. (a) PCEs of the devices with different exposure time in an ambient environment (40-50% humidity, T=23 ℃). (b) Long-term stability test of the different types of devices. Corresponding J-V curves of the devices without (c) and with (d) PCBM passivation varying with the exposure time. 3.4. Photovoltaic performance. Figure 6a shows the cross-sectional SEM image of the completed device with a layer of PCBM. It can be seen that the perovskite layer has an approximate thickness of 500 nm. The top viewed SEM images of CH3NH3PbI3 films grown on ZnO layer with and without PCBM passivation are shown in Supporting Information, Figure S3. Clearly, the film deposited on PCBM-passivated ZnO layer shows a denser and more uniform morphology, which is helpful to enhance the performances of the device as is reported by A. Petrozza et. al.46,47 The energy diagram of the device with a layer of PCBM is shown in Figure 6b.


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It is clear that the LUMO energy level of PCBM (-3.9 ev) is consistent with the band of CH3NH3PbI3 film, which indicates that there is no barrier between the interface for extracting electrons, while the holes can be blocked.38,39 Therefore, the recombination of the carriers can be reduced at this interface, which is well demonstrated by the results of time-resolved photoluminescence (TRPL) measurement and electrical impedance spectroscopy (EIS) characterization. The TRPL spectra of the FTO/ CH3NH3PbI3, FTO/ ZnO/ CH3NH3PbI3, FTO/ ZnO/ PCBM/ CH3NH3PbI3 and FTO/ ZnO/ PCBM/ CH3NH3PbI3/ Spiro in Figure 6c indicate that the lifetime of the carriers can be significantly reduced when a layer of PCBM is applied. It implies that an efficient electron transfer occurred from CH3NH3PbI3 to PCBM.47-49 To further study the mechanism of the charge transport process, we carried out EIS characterization. The Nyquist plots of the devices with and without PCBM as measured at open-circuit voltage under 1 sun AM1.5G illumination are shown in Figure S4, Supporting Information. The inset image reveals the corresponding equivalent circuit. It is clear that the data in the plot are separated into two semicircle. The first semicircle refers to the R1 for the charge-transfer resistance and the second semicircle refers to R2 for the charge-recombination resistance.23,33,50 The corresponding fitting values of the Rs, R1 and R2 are summarized in Table S2. As expected, the PCBM-passivated device shows a reduced value of R1 (365.1 Ω), much lower than the resistance (470.3 Ω) obtained using ZnO layer without PCBM passivation, in keeping with the energy level alignment between PCBM and CH3NH3PbI3 film. A lower value R1 allows a more efficient electron extraction at the PCBM/ CH3NH3PbI3 interface since the



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CH3NH3PbI3/ Spiro-OMeTAD interface is identical in both cases, which would certainly enhance the current density of the device.

Figure 6. (a) Cross-sectional SEM image of the perovskite solar cell. (b) Energy level diagram of each layer of the device. (c) Time-resolved photoluminescence (TRPL) spectra of the perovskite based on different substrates. (d) J-V curves and (e) EQE spectra as well as corresponding integrated current density of the PSCs with and without PCBM passivation. Figure 6d shows the J-V curves of the best performed devices with and without PCBM. The device without PCBM has the short circuit current density (Jsc) of 21.4 mA cm-2, Voc of 1.08 V and FF of 75.35%, producing a PCE of 17.5%. Interestingly, when the optimal thickness PCBM is deposited on ZnO surface, the device shows an outstanding enhancement on Jsc from 21.4 mA cm-2 to 22.8 mA cm-2 and the champion PCE of 19.07% is achieved (the performance of the device based on PCBM ETL without ZnO layer is shown in Figure S5, Supporting Information). The detail


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parameters of the devices for different concentrations of PCBM are summarized in Table 1 and the corresponding J-V curves are shown in Figure S6. The external quantum efficiency (EQE) spectra and integrated Jsc of the best-performance devices are shown in Figure 6e. It is clear that the EQE of the device with PCBM is higher than that of the device without PCBM. The corresponding integrated Jsc of the devices with and without PCBM are in good agreement with the J-V measured results. It is considered that the higher Jsc of PCBM-passivated device is mainly due to the better performed CH3NH3PbI3 film as well as the band alignment between PCBM and CH3NH3PbI3 film, leading to a non-barrier environment for electron transportation. Table 1. The detail parameters of the devices with different PCBM concentrations. Concentration (mg ml-1)

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0

1.08

21.4

75.35

17.51

1

1.10

22.1

73.35

18.0

2

1.08

22.8

77.3

19.07

3

1.07

22.3

76.3

18.2

4

1.09

21.86

75.8

18.03

The histograms in Figure 7 show the distribution of Voc, Jsc, FF and PCE for the devices without and with PCBM passivation. As shown in Figure 7a, the average Voc of PCBM-passivated devices is about 1.06 V that is similar to that of no PCBM-passivated devices. It implies that PCBM passivation has little influence on Voc of device. Interestingly, more than 90% devices have current density over 20.5 mA cm-2 and average fill factor over 75% when ZnO surface is passivated by a thin PCBM



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layer, as shown in Figure 7b and c. Compared to devices without PCBM passivation, the PCBM-passivated devices demonstrate significant enhancements on Jsc and FF. Finally, the elevated PCEs of PCBM-passivated devices are achieved. The overall enhancement of the performances of devices can be ascribed to the pure and uniform perovskite film, very smooth interfaces, and band alignment between PCBM and perovskite layer.

Figure 7. Histograms of the parameters measured for 100 separate devices with and without PCBM, respectively. (a) Voc, (b) Jsc, (c) FF and (d) PCE.

4. CONCLUSIONS In summary, we employed a novel SA method to deposit a porous PbI2 film which enabled the formation of highly uniform, relatively smooth and pure perovskite film. The efficiency of the device made by SA method increased from 15.5% to 17.5%.


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Furthermore, a thin layer of PCBM was inserted between the ZnO and perovskite film to passivate ZnO surface. Experimental results demonstrated that a layer of PCBM can not only effectively suppress the thermal decomposition of the perovskite film through insulating the direct contact of the ZnO and CH3NH3PbI3, but also reduce the interface barrier for the band alignment between PCBM and perovskite layer. Combining with the novel SA method and the modification of PCBM, the PCE of device can reach 19.07% and thermal stability of device can be be enhanced. It suggest that the ZnO is a promising candidate of electron transport material for high-performance perovskite solar cells in the future applications.

ASSOCIATED CONTENT Supporting Information Additional data for AFM images of the ZnO layer and ZnO/ PCBM layer (Figure S1), absorbance spectra of the perovskite layer made by different annealing method (Figure S2), SEM images of the CH3NH3PbI3 films (Figure S3), Nyquist plots of the devices (Figure S4), J-V curve and EQE spectrum of the PCBM based device (Figure S5), J-V curves of the devices with different concentrations of PCBM (Figure S6), parameters of the devices varying with the exposure time (Table S1) and the fitting parameters of the EIS (Table S2).

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11574075, 61106070, 51673060, 21401049 51272071, 11304088),



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Natural Science Fund for Distinguished Young Scholars of Hubei Province, China (No. 2016CFA036), Hubei Provincial Department of Science & Technology (2015CFB266, 2016CFB199, 2014CFA096), Hubei Provincial Department of Education (Q2016010 and D201602).

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Energy Environ. Sci. 2017, 10, 808-817.


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TOC Graphic 41x11mm (300 x 300 DPI)


Thermal Stability-Enhanced and High-Efficiency Planar Perovskite

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