Thermal Stability-Enhanced and High-Efficiency Planar Perovskite

Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38467-38476

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*,†,‡ †

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 and ‡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, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: As the electron transport layer (ETL) of perovskite solar cells, oxide semiconductor zinc oxide (ZnO) has been attracting 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 performance 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 remain relatively low in efficiency and stability. Herein, we adopted a novel multistep 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 450 °C) 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, a mesoporous-free “planar” configuration has been developed recently.16−20 The planar PSCs are often fabricated by using the electron transport materials, such as SnO2, TiO2, and ZnO,21−26 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 to have an © 2017 American Chemical Society

Received: July 25, 2017 Accepted: October 13, 2017 Published: October 13, 2017 38467

DOI: 10.1021/acsami.7b10994 ACS Appl. Mater. Interfaces 2017, 9, 38467−38476

Research Article

ACS Applied Materials & Interfaces

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 microscope (SEM) image of ZnO nanoparticles deposited on a glass/FTO substrate. (f) Optical properties of ZnO films with and without PCBM.

high-quality CH3NH3PbI3 film. Compared with the common annealing (CA) method, the PCE of the cells fabricated by the 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 is inserted between the MAPbI3 film and ZnO film, the degradation of the CH3NH3PbI3 film is effectively suppressed. More interestingly, an enhancement of PCE is observed. The efficiency of the device increased from 17.5 (without PCBM) to 19.07% by choosing a suitable thick PCBM. To the best of our knowledge, this is the highest record ever reported for perovskite solar cells based on the ZnO ETL. This implies that ZnO will be a greatly promising ETL material for future applications of PSCs.

ing to remove the residuals on the 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 the perovskite layer.36,37 These methods are effective to a certain extent and indeed have a positive effect on enhancing the performance of the device. However, the efficiency of perovskite solar cells with the ZnO ETL remains inferior compared to the efficiency of those with the TiO2 and SnO2 ETLs. [6,6]-Phenyl-C61-butyric acid methyl ester (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. Nie et al.40 reported MAPbI3/PCBM inverted planar hybrid PSCs, and the high PCE of 17.7% was achieved. 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 that of TiO2. Very recently, 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%. This suggests that it is possible to obtain the high-performance PSCs when PCBM-passivated ZnO is used as the ETL. Herein, we adopt a novel multistep annealing (SA) method to prepare a porous PbI2 film for the formation of a pure CH3NH3PbI3 film. The results reveal that the SA method is helpful to fabricate

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 and 13 mL of methanol, respectively. Before the KOH solution was added, the zinc acetate dihydrate solution was processed with a constant 65 °C water bath for 10 min. Then, the mixed solution was stirred for 2.5 h at 65 °C. Thereafter, 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 38468


Research Article


Figure 2. Schematic illustration of the (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 methods, the inset table lists the detail parameters of the cells. the usage, the ZnO precursor was filtered through a 0.45 μm poly(vinylidene fluoride) (PVDF) syringe filter. 2.2. Perovskite Solar Cell Fabrication. The fluorine-doped tin oxide (FTO) substrate was thoroughly cleaned by acetone, ethanol, and deionized water, respectively. Then, it was treated by an O2 plasma for 3 min to increase the hydrophilic nature of the surface. For the ZnO layer, the prepared ZnO precursor was spin-coated on wellcleaned FTO substrates at 3000 rpm for 30 s and then annealed at 150 °C for 10 min to remove the organic solvent in ambient air. This procedure was repeated several times to obtain a continuous smooth ZnO film with an appropriate thickness. The PCBM film was deposited by spin coating PCBM chlorobenzene solution on the ZnO layer with a rate of 3000 rpm for 40 s and annealed on a hotplate at 85 °C for 5 min in a glovebox. The perovskite layer was deposited by a two-step dipping method. First, the PbI2 layer was deposited by spin coating 1.1 M PbI2, which dissolved in dimethylformamide/dimethyl sulfoxide (DMF/DMSO) (90/10 v/v) at 3000 rpm for 30 s. The asprepared PbI2 film was annealed by a multistep annealing method (60, 80, and 100 °C 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 °C for 5 min to form a well-crystallized CH3NH3PbI3 layer. Subsequently, the spiro-OMeTAD hole transport layer (72.3 mg of spiro-OMeTAD, 28.8 μL of 4-tert-butyl pyridine, and 17.5 μL of lithium-bis (trifluoromethanesulfonyl) imide (Li-TFSI) solution (520 mg Li-TFSI in 1 mL of acetonitrile) all dissolved in 1 mL of chlorobenzene) was prepared by spin coating at 4000 rpm for 30 s. Finally, a 80 nm thick Au layer was deposited by thermal evaporation. The area of the 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 the ZnO layer with and without PCBM was analyzed by an atomic force microscope (AFM) (Ntegra upright, NT-MDT, Russia). A field-emission scanning electron microscope (FESEM, JEOL 7100 F) was used to acquire surface and cross-sectional morphologies. The X-ray diffraction (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 1.5 G one-sun illumination provided by a solar simulator (Newport Oriel Sol 3 A Class AAA, 64023 A). 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. Timeresolved 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 frequencies 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 38469


Research Article


Figure 3. Top viewed SEM images of PbI2 films prepared by the (a) CA method and (b) SA method, and the corresponding SEM images of CH3NH3PbI3 film prepared by the (c) CA method, (d) SA method.

respectively. As shown in Figure 2a, after PbI2 is deposited, the film is directly annealed at a constant temperature of 100 °C for 15 min before it is dipped into CH3NH3I solution. For the SA method, PbI2 film is annealed successively at 60, 80, and 100 °C 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 the perovskite film when the CA method is used, although the 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 in facilitation of the formation of CH3NH3PbI3. The absorbance spectra of the CH3NH3PbI3 film fabricated by different annealing methods are shown in the Supporting Information, Figure S2. It is clear that the film fabricated by the CA method has a lower absorbance than that of the film fabricated by the 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 the 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 the SA method is used.

approximate diameter of 10 nm revealed by the transmission electron microscope (TEM), as shown in Figure 1a. The HRTEM image in Figure 1b indicates that the ZnO NPs are highly crystallized, which is extremely important to reduce the traps in the ZnO ETL and improve the ability of charge transfer. The particles exhibit a hexagonal wurtzite structure and a polycrystalline 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 the 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 the PCBM/ZnO bilayer film just decreases slightly compared to that of the pure ZnO film. 3.2. Multistep Annealing Method. To fabricate a highquality CH3NH3PbI3 film without any PbI2 residual, we adopt a novel multistep annealing (SA) method to prepare a porous PbI2 film, which is helpful in the formation of pure CH3NH3PbI3. Figure 2a,b illustrates the schematic of the common annealing (CA) and SA procedure of the PbI2 films, 38470


Research Article


Figure 4. (a) Photographs of perovskite layers deposited on different substrates annealed for different times at 100 °C in air. XRD patterns of the CH3NH3PbI3 films deposited on the (b) ZnO layer, (c) ZnO/PCBM layer, and (d) PCBM layer.

The morphology control of the PbI2 film is the key factor for forming the high-quality CH3NH3PbI3 film. As shown in Figure 3a, the PbI2 film prepared by the CA method indicates an extremely dense and pinhole-free morphology, whereas a PbI2 film prepared by the 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 rates of the PbI2 crystal. When the SA method was adopted, it seemed impossible to vaporize away all of the DMSO during the first half of the 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 a better quality perovskite film, as reported by Han et al.45 As a consequence, the corresponding CH3NH3PbI3 film shows apparently different morphologies as well. Figure 3c shows an uneven grain size and a rough CH3NH3PbI3 film derived from the CA method. However, a very uniform and relatively smooth perovskite film prepared by the SA method is observed, as shown in Figure 3d. Therefore, the SA method is effective to control the morphology of the PbI2 film and further to control the formation of a better quality CH3NH3PbI3 film. 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 °C. 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 reported by Kelly et al.33,34 To enhance the thermal stability of the device, we fabricated the cells by the 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 direct contact between ZnO and the perovskite film. To examine the effect of PCBM passivation, we systemically investigated the thermal stability of perovskite films deposited on FTO/ZnO, FTO/ZnO/PCBM, and the FTO/PCBM layer, respectively. Figure 4a shows the photographs of the perovskite layers deposited on different substrates with different annealing times varying from 5 to 40 min. Apparently, the perovskite film fabricated on the FTO/ZnO layer is easily decomposed and demonstrates yellow color after annealing for 20 min at 100 °C in air. In contrast, the color of the perovskite layer deposited on FTO/ZnO/PCBM and the FTO/PCBM layer remains dark after 40 min. The XRD patterns in Figure 4b−d show the evolution of crystalline phases of the CH3NH3PbI3 films fabricated on different substrates under the different annealing times. 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 of annealing, it is hard to detect the peak of the CH3NH3PbI3 film, whereas the perovskite film deposited on the FTO/ZnO/PCBM layer remains pure phase until the annealing time is longer than 15 min, as shown in Figure 4c. Besides, the peaks of the CH3NH3PbI3 film are still dominant after annealing for 40 min. For the perovskite film deposited on the FTO/PCBM layer, it seems that it has an excellent thermal 38471


Research Article


Figure 5. (a) PCEs of the devices with different exposure times in an ambient environment (40−50% humidity, T = 23 °C). (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.

stability, as shown in Figure 4d. The appearance of the 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 the PbI2 peak between Figure 4c and Figure 4d is ascribed to the fact that the PCBM layer is too thin to cover the whole area of the ZnO layer, as shown in the Supporting Information, Figure S1d, and the exposed ZnO NPs would deteriorate the quality of the CH3NH3PbI3 film when an annealing process is conducted. The results suggest that CH3NH3PbI3 is indeed sensitive to the ZnO layer and a buffer layer of PCBM has an undeniable positive effect on the enhancement of the thermal stability of the perovskite film. 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 °C). Clearly, the PCEs of the devices without 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 nonpacked devices, and the pinhole perovskite film that allows water to penetrate 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 that without PCBM (only about 40% of the initial efficiency after 10 days), as 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 water because of the fact that PCBM shows a poor hydrophilicity. (2) The CH3NH3PbI3 film deposited on the PCBM-passivated ZnO layer shows a denser morphology with fewer pinholes than that without PCBM passivation, as shown in the Supporting Information, Figure S3. Better morphology and fewer pinholes and voids allow a limited path for water penetration, which is essential for longterm stability, as is reported by Wu et al.51 Figure 5c,d shows the J−V curves of the devices without and with PCBM passivation varying with the exposure time, and the corresponding detailed parameters are summarized in the 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 a PCBM-passivated device shows a better stability, indicating that PCBM passivation plays a positive effect on the long-term stability of the device. 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 the ZnO layer with and without PCBM passivation are shown in the Supporting Information, Figure S3. Clearly, the film deposited on the PCBM-passivated ZnO layer shows a denser and more uniform morphology, which is helpful to enhance the performances of the device, as reported by Petrozza et al.46,47 The energy diagram of the device with a layer of PCBM is shown in Figure 6b. It is clear that the LUMO energy level of PCBM (−3.9 eV) is consistent with the band of the CH3NH3PbI3 film, which 38472


Research Article


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.

OMeTAD interface is identical in both cases, which would certainly enhance the current density of the device. Figure 6d shows the J−V curves of the best performing 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 the ZnO surface, the device shows an outstanding enhancement of Jsc from 21.4 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 the ZnO layer is shown in Figure S5, Supporting Information). The detailed 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 performing 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.

indicates that there is no barrier between the interface for extracting electrons, whereas 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 timeresolved photoluminescence (TRPL) measurement and electrical impedance spectroscopy (EIS) characterization. The TRPL spectra of the FTO/CH 3 NH 3 PbI 3 , 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 AM 1.5G 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 semicircles. 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 the ZnO layer without PCBM passivation, in keeping with the energy level alignment between PCBM and CH3NH3PbI3 film. A lower value of R1 allows a more efficient electron extraction at the PCBM/ CH3NH3PbI3 interface because the CH3NH3PbI3/Spiro-

Table 1. Detail Parameters of the Devices with Different PCBM Concentrations

38473

concentration (mg mL−1)

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

0 1 2 3 4

1.08 1.10 1.08 1.07 1.09

21.4 22.1 22.8 22.3 21.86

75.35 73.35 77.3 76.3 75.8

17.51 18.0 19.07 18.2 18.03


Research Article


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 that enabled the formation of a highly uniform, relatively smooth, and pure perovskite film. The efficiency of the device made by the SA method increased from 15.5 to 17.5%. Furthermore, a thin layer of PCBM was inserted between the ZnO and perovskite film to passivate the 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 the 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 the device can be enhanced. This suggests that the ZnO is a promising candidate of electron transport material for high-performance perovskite solar cells for future applications.

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 the PCBMpassivated device is mainly due to the better performing CH3NH3PbI3 film as well as the band alignment between PCBM and CH3NH3PbI3 film, leading to a nonbarrier environment for electron transportation. 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, which is similar to that of non-PCBM-passivated devices. It implies that PCBM passivation has little influence on the Voc of the device. Interestingly, more than 90% of devices have current density over 20.5 mA cm−2 and average fill factor over 75% when the ZnO surface is passivated by a thin PCBM layer, as shown in Figure 7b,c. Compared with devices without PCBM passivation, the PCBM-passivated devices demonstrate significant enhancements of 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 the perovskite layer.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10994. Additional data for AFM images of the ZnO layer and ZnO/PCBM layer (Figure S1), absorbance spectra of the 38474


Research Article


■

(9) Li, X.; Bi, D. Q.; Yi, C. Y.; Decoppet, J. D.; Luo, J. S.; Zakeeruddin, S. M.; Hagfeldt, A.; Grätzel, M. A Vacuum Flash-Assisted Solution Process for High-Efficiency Large-Area Perovskite Solar Cells. Science 2016, 353, 58−62. (10) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S.Il. High-Performance Photovoltaic Perovskite Layers Fabricated Through Intramolecular Exchange. Science 2015, 348, 1234− 1237. (11) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S.Il. Solvent Engineering for High-Performance Inorganic-Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897−903. (12) Xing, Y.; Sun, C.; Yip, H. L.; Bazan, G. C.; Huang, F.; Cao, Y. New Fullerene Design Enables Efficient Passivation of Surface Traps in High Performance p-i-n Heterojunction Perovskite Solar Cells. Nano Energy 2016, 26, 7−15. (13) Tai, Q.; You, P.; Sang, H. Q.; Liu, Z. K.; Hu, C. L.; Chan, H. L. W.; Yan, F. Efficient and Stable Perovskite Solar Cells Prepared in Ambient Air Irrespective of the Humidity. Nat. Commun. 2016, 7, No. 11105. (14) Yi, C.; Li, X.; Luo, J. S.; Zakeeruddin, S. M.; Grätzel, M. Perovskite Photovoltaics with Outstanding Performance Produced by Chemical Conversion of Bilayer Mesostructured Lead Halide/TiO2 Films. Adv. Mater. 2016, 28, 2964−2970. (15) Wang, Y. K.; Yuan, Z. C.; Shi, G. Z.; Li, Y. X.; Li, Q.; Hui, F.; Sun, B. Q.; Jiang, Z. Q.; Liao, L. S. Dopant-Free Spiro-Triphenylamine/Fluorene as Hole Transporting Material for Perovskite Solar Cells with Enhanced Efficiency and Stability. Adv. Funct. Mater. 2016, 26, 1375−1381. (16) Chen, Y. H.; Chen, T.; Dai, L. M. Layer-by-Layer Growth of CH3NH3PbI3−xClx for Highly Efficient Planar Heterojunction Perovskite Solar Cells. Adv. Mater. 2015, 27, 1053−1059. (17) Wu, C. G.; Chiang, C. H.; Tseng, Z. L.; Nazeeruddin, M. K.; Hagfeldt, A.; Grätzel, M. High Efficiency Stable Inverted Perovskite Solar Cells Without Current Hysteresis. Energy Environ. Sci. 2015, 8, 2725−2733. (18) Zhao, L. C.; Luo, D. Y.; Wu, J.; Hu, Q.; Zhang, W.; Chen, K.; Liu, T. H.; Liu, Y.; Zhang, Y. F.; Liu, F.; Russell, T. P.; Snaith, H. J.; Zhu, R.; Gong, Q. H. High-Performance Inverted Planar Heterojunction Perovskite Solar Cells Based on Lead Acetate Precursor with Effciency Exceeding 18%. Adv. Funct. Mater. 2016, 26, 3508−3514. (19) Heo, J. H.; Han, H. J.; Kim, D.; Ahn, T. K.; Im, S. H. 18.1% Hysteresis-Less Inverted CH3NH3PbI3 Planar Perovskite Hybrid Solar Cells. Energy Environ. Sci. 2015, 8, 1602−1608. (20) Kim, H. B.; Choi, H.; Jeong, J.; Kim, S.; Walker, B.; Song, S.; Kim, J. Y. Mixed Solvents for the Optimization of Morphology in Solution-Processed, Inverted-Type Perovskite/Fullerene Hybrid Solar Cells. Nanoscale 2014, 6, 6679−6683. (21) Ke, W.; Fang, G. J.; Liu, Q.; Xiong, L. B.; Qin, P. L.; Tao, H.; Wang, J.; Lei, H. W.; Li, B. R.; Wan, J. W.; Yang, G.; Yan, Y. F. LowTemperature Solution-Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 6730−6733. (22) Ke, W.; Xiao, C. X.; Wang, C. L.; Saparov, B.; Duan, H. S.; Zhao, D. W.; Xiao, Z. W.; Schulz, P.; Harvey, S. P.; Liao, W. Q.; Meng, W. W.; Yu, Y.; Cimaroli, A. J.; Jiang, C. S.; Zhu, K.; Jassim, M. A.; Fang, G. J.; Mitzi, D. B.; Yan, Y. F. Employing Lead Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells. Adv. Mater. 2016, 28, 5214−5221. (23) Yang, D.; Zhou, X.; Yang, R. X.; Yang, Z.; Yu, W.; Wang, X. L.; Li, C.; Liu, S. Z.; Chang, R. P. H. Surface Optimization to Eliminate Hysteresis for Record Efficiency Planar Perovskite Solar Cells. Energy Environ. Sci. 2016, 9, 3071−3078. (24) Qiu, W.; Merckx, T.; Jaysankar, M.; Masse de la Huerta, C.; Rakocevic, L.; Zhang, W.; Paetzold, U. W.; Gehlhaar, R.; Froyen, L.; Poortmans, J.; Cheyns, D.; Snaith, H. J.; Heremans, P. Pinhole-Free Perovskite Films for Efficient Solar Modules. Energy Environ. Sci. 2016, 9, 484−489.

perovskite layer made by different annealing methods (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) (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Jianying Wang: 0000-0001-9612-7257 Xianbao Wang: 0000-0001-7765-4027 Haoshuang Gu: 0000-0003-1232-2499 Jinhua Li: 0000-0002-5226-0272 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11574075, 61106070, 51673060, 21401049, 51272071, and 11304088), Natural Science Fund for Distinguished Young Scholars of Hubei Province, China (No. 2016CFA036), Hubei Provincial Department of Science & Technology (2015CFB266, 2016CFB199, and 2014CFA096), Hubei Provincial Department of Education (Q2016010 and D201602).

■

REFERENCES

(1) Saliba, M.; Matsui, T.; Seo, J. Y.; Domanski, K.; Baena, J. C.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; Grätzel, M. Cesium-containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ. Sci. 2016, 9, 1989−1997. (2) Son, D. Y.; Lee, J. W.; Choi, Y. J.; Jang, I. H.; Lee, S.; Yoo, P. J.; Shin, H.; Ahn, N.; Choi, M.; Kim, D.; Park, N. G. Self-Formed Grain Boundary Healing Layer for Highly Efficient CH3NH3PbI3 Perovskite Solar Cells. Nat. Energy 2016, 1, No. 16081. (3) Xu, B.; Bi, D. Q.; Hua, Y.; Liu, P.; Cheng, M.; Grätzel, M.; Kioo, L.; Hagfeldt, A.; Sun, L. C. A Low-Cost Spiro[fluorene-9,9′-xanthene]Based Hole Transport Material for Highly Efficient Solid-State DyeSensitized Solar Cells and Perovskite Solar Cells. Energy Environ. Sci. 2016, 9, 873−877. (4) Bi, D.; Tress, W.; Dar, M. I.; Gao, P.; Luo, J.; Renevier, C.; Schenk, K.; Abate, A.; Giordana, F.; Baena, J. C.; Decoppet, J. D.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M.; Hagfeldt, A. Efficient Luminescent Solar Cells Based on Tailored Mixed-Cation Perovskites. Sci. Adv. 2016, 2, No. e1501170. (5) Deng, Y. H.; Dong, Q. F.; Bi, C.; Yuan, Y. B.; Huang, J. S. AirStable, Efficient Mixed-Cation Perovskite Solar Cells with Cu Electrode by Scalable Fabrication of Active Layer. Adv. Energy Mater. 2016, No. 1600372. (6) Mei, A.; Li, X.; Liu, L. F.; Ku, Z. L.; Liu, T. F.; Rong, Y. G.; Xu, M.; Hu, M.; Chen, J. Z.; Yang, Y.; Grätzel, M.; Han, H. W. A HoleConductor−Free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability. Science 2014, 345, 295−298. (7) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (8) Research Cell Efficiency Records. http://www.nrel.gov/ncpv/ images/efficiencychart. 38475


Research Article

ACS Applied Materials & Interfaces (25) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics 2014, 8, 133−138. (26) Zhou, H. W.; Shi, Y. T.; Wang, K.; Dong, Q. S.; Bai, X. G.; Xing, Y. J.; Du, Y.; Ma, T. L. Low-Temperature Processed and Carbon-Based ZnO/CH3NH3PbI3/C Planar Heterojunction Perovskite Solar Cells. J. Phys. Chem. C 2015, 119, 4600−4605. (27) Anaraki, E. H.; Kermanpur, A.; Steier, L.; Domanski, K.; Matsui, T.; Tress, W.; Saliba, M.; Abate, A.; Grätzel, M.; Hagfeldt, A.; CorreaBaena, J. P. Highly Efficient and Stable Planar Perovskite Solar Cells by Solution-Processed Tin Oxide. Energy Environ. Sci. 2016, 9, 3128− 3134. (28) Jiang, Q.; Zhang, L. Q.; Wang, H. L.; Yang, X. L.; Meng, J. H.; Liu, M.; Yin, Z. G.; Wu, J. L.; Zhang, X. W.; You, J. B. Enhanced Electron Extraction Using SnO2 for High-Efficiency Planar-Structure HC(NH2)2PbI3-Based Perovskite Solar Cells. Nat. Energy 2016, 2, No. 16177. (29) Heo, J. H.; Lee, M. H.; Han, H. J.; Patil, B. R.; Yu, J. S.; Im, S. H. Highly Efficient Low Temperature Solution Processable Planar Type CH3NH3PbI3 Perovskite Flexible Solar Cells. J. Mater. Chem. A 2016, 4, 1572−1578. (30) Wang, P.; Zhao, J. J.; Liu, J. X.; Wei, L. Y.; Liu, Z. H.; Guan, L. H.; Cao, G. Z. Stabilization of Organometal Halide Perovskite Films by SnO2 Coating with Inactive Surface Hydroxyl Groups on ZnO Nanorods. J. Power Sources 2017, 339, 51−60. (31) Liu, T. H.; Hu, Q.; Wu, J.; Chen, K.; Zhao, L. C.; Liu, F.; Wang, C.; Lu, H.; Jia, S.; Russell, T.; Zhu, R.; Gong, Q. H. Mesoporous PbI2 Scaffold for High-Performance Planar Heterojunction Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, No. 1501890. (32) Chen, Q.; Zhou, H. P.; Song, T. B.; Luo, S.; Hong, Z. R.; Duan, H. S.; Dou, L. T.; Liu, Y. S.; Yang, Y. Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites Toward High Performance Solar Cells. Nano Lett. 2014, 14, 4158−4163. (33) Liu, D.; Yang, J. L.; Kelly, T. L. Compact Layer Free Perovskite Solar Cells with 13.5% Efficiency. J. Am. Chem. Soc. 2014, 136, 17116− 17122. (34) Yang, J. L.; Siempelkamp, B. D.; Mosconi, E.; Angelis, F. D.; Kelly, T. L. Origin of the Thermal Instability in CH3NH3PbI3 Thin Films Deposited on ZnO. Chem. Mater. 2015, 27, 4229−4236. (35) Tseng, Z. L.; Chiang, C. H.; Chang, S. H.; Wu, C. G. Surface Engineering of ZnO Electron Transporting Layer via Al Doping for High Efficiency Planar Perovskite Solar Cells. Nano Energy 2016, 28, 311−318. (36) Zuo, L.; Gu, Z. W.; Ye, T.; Fu, W. F.; Wu, G.; Li, H. Y.; Chen, H. Z. Enhanced Photovoltaic Performance of CH3NH3PbI3 Perovskite Solar Cells Through Interfacial Engineering Using Self-Assembling Monolayer. J. Am. Chem. Soc. 2015, 137, 2674−2679. (37) Cheng, Y.; Yang, Q. D.; Xiao, J. Y.; Xue, Q. F.; Li, H. W.; Guan, Z. Q.; Yip, H. L.; Tsang, S. W. Decomposition of Organometal Halide Perovskite Films on Zinc Oxide Nanoparticles. ACS Appl. Mater. Interfaces 2015, 7, 19986−19993. (38) Liu, T. H.; Chen, K.; Hu, Q.; Zhu, R.; Gong, Q. H. Inverted Perovskite Solar Cells: Progresses and Perspectives. Adv. Energy Mater. 2016, 6, No. 1600457. (39) Fan, R. D.; Huang, Y.; Wang, L. G.; Li, L.; Zheng, G.; Zhou, H. P. The Progress of Interface Design in Perovskite-Based Solar Cells. Adv. Energy Mater. 2016, 6, No. 1600460. (40) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J. C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H. L.; Mohite, A. D. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains. Science 2015, 347, 522−525. (41) Qiu, W.; Bastos, J. P.; Dasgupta, S.; Merckx, T.; Cardinaletti, I.; Jenart, M. V. C.; Nielsen, C. B.; Gehlhaar, R.; Poortmans, J.; Heremans, P.; McCulloch, I.; Cheyns, D. Highly Efficient Perovskite Solar Cells with Crosslinked PCBM Interlayers. J. Mater. Chem. A 2017, 5, 2466−2472. (42) Zhang, W. H.; Xiong, J.; Wang, S.; Liu, W.; Li, J.; Wang, D. F.; Gu, H. S.; Wang, X. B.; Li, J. H. Highly Conductive and Transparent

Silver Grid/Metal Oxide Hybrid Electrodes for Low-Temperature Planar Perovskite Solar Cells. J. Power Sources 2017, 337, 118−124. (43) Li, W.; Fan, J. D.; Li, J. W.; Mai, Y. H.; Wang, L. D. Controllable Grain Morphology of Perovskite Absorber Film by Molecular SelfAssembly toward Efficient Solar Cell Exceeding 17%. J. Am. Chem. Soc. 2015, 137, 10399−10405. (44) Li, L.; Chen, Y. H.; Liu, Z. H.; Chen, Q.; Wang, X. D.; Zhou, H. P. The Additive Coordination Effect on Hybrids Perovskite Crystallization and High-Performance Solar Cell. Adv. Mater. 2016, 28, 9862−9868. (45) Wu, Y. Z.; Islam, A.; Yang, X. D.; Qin, C. J.; Liu, J.; Zhang, K.; Peng, W. Q.; Han, L. Y. Retarding the Crystallization of PbI2 for Highly Reproducible Planar-Structured Perovskite Solar Cells via Sequential Deposition. Energy Environ. Sci. 2014, 7, 2934−2938. (46) Tao, C.; Velden, J. V. D.; Cabau, L.; Montcada, N. F.; Neutzner, S.; Kandada, A. R. S.; Marras, S.; Brambilla, L.; Tommasini, M.; Xu, W. D.; Sorrentino, R.; Perinot, A.; Caironi, M.; Bertarelli, C.; Palomares, E.; Petrozza, A. Fully Solution-Processed n-i-p-Like Perovskite Solar Cells with Planar Junction: How the Charge Extracting Layer Determines the Open-Circuit Voltage. Adv. Mater. 2017, 29, No. 1604493. (47) Tao, C.; Neutzner, S.; Colella, L.; Marras, S.; Kandada, A. R. S.; Gandini, M.; Bastiani, M. D.; Pace, G.; Manna, L.; Caironi, M.; Bertarelli, C.; Petrozza, A. 17.6% Stabilized Efficiency in LowTemperature Processed Planar Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 2365−2370. (48) Wojciechowski, K.; Ramirez, I.; Gorisse, T.; Dautel, O.; Dasari, R.; Sakai, N.; Hardigree, J. M.; Song, S.; Marder, S.; Riede, M.; Wantz, G.; Snaith, H. J. Cross-linkable Fullerene Derivatives for SolutionProcessed n-i-p Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 648− 653. (49) Ma, T.; Tadaki, D.; Sakuraba, M.; Sato, S.; Iwata, A. H.; Niwano, M. Effects of Interfacial Chemical States on the Performance of Perovskite Solar Cells. J. Mater. Chem. A. 2016, 4, 4392−4397. (50) Giordano, F.; Abate, A.; Baena, J. P. C.; Saliba, M.; Matsui, T.; Im, S. H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Hagfeldt, A.; Graetzel, M. Enhanced Electronic Properties in Mesoporous TiO2 via Lithium Doping for High-Efficiency Perovskite Solar Cells. Nat. Commun. 2016, 7, No. 10379. (51) Chiang, C. H.; Nazeeruddin, M. K.; Graetzel, M.; Wu, C. G. The Synergistic Effect of H2O and DMF Towards Stable and 20% Efficiency Inverted Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 808−817.

38476


Thermal Stability-Enhanced and High-Efficiency Planar Perovskite

Recommend Documents