Efficient NiOx Hole Transporting Layer Obtained by the Oxidation of

Jun 25, 2019 - Efficient NiOx Hole Transporting Layer Obtained by the Oxidation of Metal Nickel Film for Perovskite Solar Cells ...
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Efficient NiOx Hole Transporting Layer Obtained by the Oxidation of Metal Nickel Film for Perovskite Solar Cells Shangzheng Pang, Chunfu Zhang,* Hang Dong, Dazheng Chen, Weidong Zhu, He Xi, Jingjing Chang, Zhenhua Lin, Jincheng Zhang, and Yue Hao Wide Bandgap Semiconductor Technology Disciplines State Key Laboratory, Shaanxi Joint Key Laboratory of Graphene, School of Microelectronics, Xidian University, Xi’an 710071, China

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S Supporting Information *

ABSTRACT: In the preparation of PSCs, NiOx is considered to be one very promising hole transport layers for its intrinsic p-type doping nature, deep-lying valence band, and high light transmittance. However, the thickness, uniformity, and composition of the NiOx film are not easy to accurately control by the solution spin-coating process. In this work, instead of the solution-processed NiOx, metal Ni is first deposited on the FTO/glass substrate by an e-beam evaporator and then converted to the NiOx by exposing it in the air with the help of annealing, which avoids the usage of a toxic solvent in the solution method. The deposition thickness and composition of the NiOx films could be well controlled. The results demonstrate that the Ni-oxidized NiOx HTL shows wonderful energy alignment, better charge extraction capability, and great efficiency in photoluminescence quenching when in contact with the perovskite film. By combining the two-step solution process method, highperformance PSC has been achieved based on the Ni-oxidized NiOx HTL, which is well above that based on the PEDOT:PSS and the solution-processed NiOx. Also, transient photocurrent (TPC) and transient photovoltage (TPV) measurements demonstrate that the device based on the Ni-oxidized NiOx HTL possesses a suppressed charge recombination and an outstanding ability of charge extraction. Furthermore, the Ni-oxidized NiOx-based devices exhibit better stability compared to the devices based on solution-processed NiOx and PEDOT:PSS HTL. All the results show that the Ni oxidation method is an efficient method to prepare the NiOx HTL in PSCs. KEYWORDS: hole transporting layer, Ni-oxidized NiOx, energy alignment, suppressed charge recombination, two-step deposition method, stability

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INTRODUCTION Perovskite solar cells (PSCs) have received much attention and acquired great development in the recent years for their unique advantages including excellent absorption coefficient, good charge carrier mobility, and so on.1−6 The power conversion efficiency (PCE) increases quickly from 3.8% at the initial stage to over 24% nowadays in only a few years.7 Generally, PSCs have two main types of structures: n−i−p structure and p−i−n structure. The n−i−p PSCs, which are usually based on a mesoporous titanium dioxide (M-TiO2) scaffold electron transport layer (ETL), a methylammonium lead iodide (CH3NH3PbI3) light-absorption layer, and a 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-bifluorene (spiro-OMeTAD) hole transport layer (HTL), take a mainstream position and achieve tremendous progress. In view of reliability, however, the n−i−p mesoporous structured cells suffer from serious current−voltage hysteresis and organic HTL instability when the devices are exposed to the natural environment.7−10 There is also some argument that mesoporous TiO2 films may cause the perovskite layer degradation because of their enhanced properties of photocatalysis.11−13 Last but not the least, spiro-OMeTAD is by no means cheap, which could place © 2019 American Chemical Society

restrictions on the development of perovskite solar cells in cosmic industry production. Compared with the n−i−p structure, due to its easy fabrication, negligible current− voltage hysteresis, and decent PCE, the p−i−n inverted planar structured cell design appears to become more and more popular compared with the forward mesoporous structured cell. For p−i−n inverted PSCs, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PCBM) are usually selected as HTL and ETL, respectively.11−16 However, the intrinsic acidity and hygroscopicity of PEDOT:PSS usually lead to the negative effects on the device long-term stability.17 To solve the stability problems, inorganic HTL is adopted in PSCs. Some inorganic materials, including NiOx,18,19 CuI,20 and CuSCN,21,22 have been used as effective HTLs in previous works. Among them, NiOx is considered as one of the most competitive HTL for its inherent p-type doping nature, simple preparation, high optical transmittance in the visible range, and Received: January 23, 2019 Accepted: June 25, 2019 Published: June 25, 2019 4700

DOI: 10.1021/acsaem.9b00169 ACS Appl. Energy Mater. 2019, 2, 4700−4707

Article

ACS Applied Energy Materials

Figure 1. (a) Device structure of PSCs (FTO/glass/perovskite/NiOx/BCP/PC61BM/Ag) and (b) in accordance with energy band diagrams (for clarity, all the layers thickness and their real thickness are not in proportion). The charge transport process is shown by the corresponding energy band diagrams.

PSCs, and the corresponding devices are compared with the cells based on the PEDOT:PSS and solution-processed NiOx HTLs. The results demonstrate that the Ni-oxidized NiOx HTL shows wonderful energy alignment, better charge extraction capability, and great efficiency in photoluminescence (PL) quenching. Finally, combining with the two-step solution process method, the high PCE exceeding 18.1% has been achieved based on the Ni-oxidized NiOx HTL, which is far above that based on the PEDOT:PSS and the solutionprocessed NiOx.

a deep-lying valence band (VB) which matches well with the VB of perovskite.20−24 The performance of inverted p−i−n PSCs has acquired great promotion which is closer to that of the n−i−p devices attributed to the application of NiOx HTL. To obtain highly efficient PSCs, the deposition of NiOx films has been developed in many techniques, consisting of vacuum deposition,25−27 electrodeposition,28 atomic layer deposition,29 and solution-processed deposition.30−34 Among these methods, solution-processed deposition is widely used, which is a simple and low manufacturing cost technology. However, there are some difficulties in preparing large area and high quality NiOx films, and at the same time the thickness, uniformity, and composition of the NiOx film are not easy to accurately control through the solution spin-coating method. Besides, some of the solvent in the solution-processed NiOx is toxic, which is harmful to the human body and the environment.35,36 The vacuum-processed NiOx is an alternative choice, which could potentially meet the demand of a uniform, large-area, precise thickness deposition technique and at the same time avoid the usage of a toxic solvent. Based on the vacuum process, the directly sputtered NiOx has been used in PSCs;22−24 however, the reported device performance is still relatively unsatisfactory compared to the solution-processed NiOx due to the low quality of the sputtered NiOx. There is another vacuum process way to prepare NiOx HTL by the oxidation of nickel thin film which has been previously reported by Lai and others.45−47 In Lai’s work the concept of Ni-oxidized NiOx was first reported in the preparation of PSCs;47 by the optimization of annealing temperature high quality NiOx HTL was acquired. Also, the deposition thickness and the thickness uniformity and the composition of the NiOx films could be precisely controlled compared to the solution-processed NiOx. While the efficiency of Nioxidized NiOx-based devices is not very high, there is still much room to improve its performance by further investigating the Ni-oxidized NiOx HTL. Great effort is urgently required in the NiOx HTL based on the vacuum process. In this work, instead of the direct deposition of NiOx, metal Ni is first deposited on the FTO/glass substrate by an e-beam evaporator and then converted to NiOx by exposing it in the air with the help of annealing (named Ni-oxidized NiOx). The deposition thickness and quality of the NiOx films could be well controlled. This Ni-oxidized NiOx HTL is used in the

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RESULTS AND DISCUSSION As reported previously, the PEDOT:PSS has acted as HTL for a long time in inverted planar PSCs.39−43 For PEDOT:PSS, the inherent acidity and hygroscopicity would cause some negative effects on the device stability. Because the NiOx exhibits better characteristics, PEDOT:PSS has been replaced with NiOx in this work. The schematic and the corresponding energy band diagrams of inverted planner PSCs are shown in Figure 1. Different parts play different roles in the device structure. Under the condition of the valence band maximum (VBM) of 5.44 eV, the NiOx serves as the HTL and electron block layer. The PCBM plays an electron transport layer and the hole block layer. The BCP acts as the interface modification layer, and the FTO and the Ag film are selected as the top and bottom transparent electrodes. The vacuumprocessed Ni-oxidized NiOx is demonstrated in this work. For comparison, the conventional solution-processed NiOx is also prepared. For the solution-processed NiOx, the precursor solution was spin-coated on the FTO/Glass substrates at 3000 rpm for 45 s and then treated by annealing at 250 °C for 1 h. For Ni-oxidized NiOx, Ni was first evaporated on the FTO/ glass substrate, and then the glass/FTO/Ni triple layers were treated by annealing at 500 °C in air environment for 15 min to convert Ni to NiOx. Our experiment also convinced the conclusion that the Ni-oxidized NiOx shows the best performance based on the 500 °C annealing procession as is shown in Figure S2c,d. We first try to investigate the impacts of Ni-oxidized NiOx thickness on the material properties and device performance, so that we can acquire the perfect thickness of the NiOx HTL layer and maximize the device performance. Different thicknesses of Ni-oxidized NiOx HTLs have been compared 4701

DOI: 10.1021/acsaem.9b00169 ACS Appl. Energy Mater. 2019, 2, 4700−4707

Article

ACS Applied Energy Materials

Figure 2. (a) Optical transmittance (inset: average light transmittance) of the glass/FTO/NiOx samples and (b) transmittance comparison of glass/FTO/HTL samples (all spectra are referenced to air). (c, d) Statistics parameters of PSCs: (c) short-circuit current density and (d) PCE of the PSCs based on the Ni-oxidized NiOx substrate. (e) The IPCE spectra for the champion device based on the Ni-oxidized NiOx HTL with different thicknesses prepared by the one-step deposition method.

Figure 3. (a) XRD patterns of reference pattern, solution-processed, and Ni-oxidized NiOx films. AFM images (5 μm × 5 μm) of (b) solutionprocessed and (c) Ni-oxidized NiOx thin films. SEM images of (d) Ni-oxidized NiOx and (e) solution-processed thin films. (f) Transmission spectra of solution-processed and Ni-oxidized NiOx thin films. Tauc plot for (g) Ni-oxidized NiOx and (h) solution-processed HTL films on sapphire substrates. (i) UPS spectra of Ni-oxidized NiOx solution-processed and PEDOT:PSS HTL films on FTO substrates.

the point where the NiOx thickness is 20 nm and the wavelength ranges from 300 to 800 nm, AVT gets 70.01% as the maximum value. AVT starts to drop with the thickness of NiOx layer further improving. Figure 2b shows comparison between the glass/FTO/NiOx sample and the glass/FTO/ PEDOT:PSS sample, as is shown that the average transmittance of glass/FTO/NiOx sample is close to glass/FTO/

in the contrast experiments. Figure 2a shows the transmittance spectra of glass/FTO/NiOx samples with different Ni-oxidized NiOx thicknesses, and Figure S1 shows the corresponding absorption spectra. It can be observed that with the thickness increasing of NiOx layer the light transmittance expresses an obvious downward trend. The average light transmission (AVT) of glass/FTO/NiOx samples is shown in the inset. At 4702

DOI: 10.1021/acsaem.9b00169 ACS Appl. Energy Mater. 2019, 2, 4700−4707

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ACS Applied Energy Materials

Figure 4. (a, b) Top-view SEM images of the perovskite films grown on the Ni-oxidized (a), solution-processed NiOx (b), and PEDOT:PSS (c) substrates. (d) XRD patterns of perovskite films grown on the solution-processed and Ni-oxidized NiOx and PEDOT:PSS substrate. (e) Steadystate PL spectra of perovskite films grown on the solution-processed NiOx, Ni-oxidized NiOx, and PEDOT:PSS substrate.

oxidized NiOx thin film shows that on the FTO substrate it has similar smooth and compact surface morphology (RMS: 23.6 nm). The surface morphology of PEDOT:PSS is shown in Figure S4, which has a relatively smooth surface with the RMS value about 9.6 nm. As can be seen in Figure 3d,e, the Nioxidized NiOx thin film exhibits larger grain size and better covered surface morphology compared to the solutionprocessed NiOx thin film which may be helpful to the transport of hole. Different approaches were used to check the transmittance spectra so as to decide the NiOx thin film absorption and the optical bandgap. As is expressed in Figure 3f, all NiOx thin films show an acceptable transmittance at visible light range 350−800 nm. The optical bandgap could be further obtained from the transmittance spectra. The Tauc plot for both samples is demonstrated in Figure 3g,h; from this the optical bandgap of different NiOx films can be determined.33 The detailed calculation is shown in the Supporting Information. Compared to the solution-processed NiOx possessing a bandgap of 3.52 eV, the Ni-oxidized NiOx exhibits a slightly larger bandgap of 3.65 eV. Both samples have a wide bandgap, which benefits the PSCs application. The work functions of solution-processed NiOx, Ni-oxidized NiOx, and PEDOT:PSS were examined by UPS spectra. As Figure 3i shows, compared with the Ni-oxidized NiOx, the Fermi level of solution-processed NiOx has a slight decrease with the energy level about 0.11 eV. Therefore, compared with solution-processed NiOx (PEDOT:PSS), which possesses the work function of −5.33 eV (−5.2 eV), the Ni-oxidized NiOx exhibits a work function of −5.44 eV, which is slightly decreased. The declining work function of Ni-oxidized NiOx is nearly equal to the valence band of perovskite (−5.4 eV), which is helpful for better band alignment, electron blocking, and hole transporting. What is more, the potential disparity between valence band (VB) of NiOx and conduction band (CB) of perovskite may rise because of decreased work function and hence may potentially increase the Voc of the devices.39 By the X-ray photoelectron spectroscopy (XPS) spectra analyzing the

PEDOT:PSS, which shows the superior optical properties of the Ni-oxidized NiOx as the HTL in PSCs. We also fabricated the control devices based on the one-step solution process method to investigate the impacts of NiOx thickness on the device performance. The statistic results of the devices are presented in Figure 2c,d, and the other statistic results fill factor (FF) and open circuit voltage (Voc) of the devices are presented in Figure S2. The PCE reaches the highest value for the device with the NiOx thickness of 20 nm and then decreases gradually for the devices with thicker NiOx films. The electrical parameters, especially the PCE, displayed a narrow distribution under the condition of the device parameters based on 20 nm NiOx, indicating it has the advantage of reliability. It is supposed that when the NiOx thickness is too thin (lower than 20 nm), it may be discontinuous, and thus the performance of device is relatively low. When the NiOx thickness is too thick (higher than 20 nm), the Ni-oxidized process may be incomplete or the NiOx is too thick for the charge transport, which leads to the decrease of the device performance. The IPCE spectra for the optimal devices based on different thicknesses of Ni-oxidized NiOx HTLs are expressed in Figure 2e. The device with the 20 nm Nioxidized NiOx shows the highest values, which again confirms that 20 nm is the optimal thickness for the Ni-oxidized NiOx. The qualities of the Ni-oxidized NiOx thin films were further investigated by the XRD technique and compared to the solution-processed NiOx. Figure 3a shows the XRD patterns of reference pattern, solution-processed, and Nioxidized NiOx films. The typical cubic structure of NiOx has the Bragg peaks at 37.3°, 43.3°, and 62.9° which would be assigned to (111), (200), and (220) planes.37 This indicates that both the solution-processed and Ni-oxidized NiOx films have acceptable crystallinity, consistent with previous reports.23,38 The surface morphologies of solution-processed and Ni-oxidized NiOx thin films could be acquired from the AFM image. As is expressed in Figure 3b,c, compared with the solution-processed NiOx thin film (RMS: 21.3 nm), the Ni4703

DOI: 10.1021/acsaem.9b00169 ACS Appl. Energy Mater. 2019, 2, 4700−4707

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ACS Applied Energy Materials

Figure 5. (a) Device parameters of the solution-processed and Ni-oxidized NiOx HTL-based PSCs. (b) The best J−V curve for solution-processed NiOx and Ni-oxidized NiOx HTL-based devices. (c) IPCE spectra and integrated current density for the solution-processed NiOx and Ni-oxidized NiOx HTL-based devices. (d) Steady output current density for the solution-processed NiOx and Ni-oxidized NiOx HTL-based devices measured at short circuit state. (e) TPC and (f) TPV measurements of PSCs based on solution-processed and Ni-oxidized NiOx substrates. (g) Voc depends on light intensity of devices based on solution-processed and Ni-oxidized NiOx substrates. The J−V curve using forward scan (−0.2 to 1.2 V) and reverse scan (1.2 to −0.2 V) measurement Ni-oxidized NiOx-based devices (h) and (i) solution-processed NiOx-based devices. The voltage step is 0.01 V, and delay times are set at 0, 10, and 100 ms.

steady-state PL results of all the three HTLs and perovskite films on top of the three HTLs. The perovskite thin film exhibits much more effective PL quenching on Ni-oxidized NiOx thin film compared to that on solution-processed NiOx and PEDOT:PSS thin films, indicating that the Ni-oxidized NiOx has better charge extraction and transport compared with others. All the results indicated that the charge transport and extraction is more efficient between the perovskite and Ni-oxidized NiOx HTL. This is helpful to boost the performance of PSCs. The above results showed that the Ni-oxidized NiOx has obvious advantages in the above discussion. However, the best PCE (14.85%) of NiOx-based devices is still not very satisfactory. To further improve the device’s performance, we adopt the two-step deposition method to replace the above one-step deposition method as mentioned in the Experimental Section to fabricate PSCs.46 As expressed in Figure S6, through XRD, PL, and SEM tests the two-step solution prepared perovskite layer exhibits a higher diffraction intensity, slightly higher PL intensity, and larger grain size compared with the one-step solution method, indicating the better crystal quality. All the following discussions will be based on the devices prepared by the two-step deposition method. The statistical results of PCE and other parameters for the fabricated devices are exhibited in Figure 5a and Figure S7. The electrical parameters, especially the PCE of the Nioxidized NiOx-based devices, show the same narrow distribution and higher average values compared to the devices based on the solution-processed NiOx, verifying that the conditions are reliable. The champion J−V curve for solutionprocessed NiOx and Ni-oxidized NiOx HTL-based devices are shown in Figure 5b. The PSC based on Ni-oxidized NiOx

element composition of Ni-oxidized and solution-processed NiOx thin films could be acquired. O 1s core levels and the XPS spectra of the Ni 2p are exhibited in Figure S3. The Ni3+/ Ni2+ ratio of Ni-oxidized NiOx is higher than the solutionprocessed NiOx, which indicated that the Ni-oxidized NiOx possessing better film quality and crystallinity.48,17 This is caused by the enhancement of NiOx film conductivity and the improvement of the contact properties in devices.48 As is well known, the formation and quality of the perovskite films are associated with the different HTL substrates. It is important to investigate the perovskite films formed on the solution-processed NiOx, Ni-oxidized NiOx, and PEDOT:PSS thin films. SEM and XRD were respectively used to examine the perovskite film morphology and film crystallinity. As can be seen from Figure 4a−c, the perovskite film based on Nioxidized NiOx exhibits the biggest grain size distribution (500−800 nm) compared to that on solution-processed NiOx (400−700 nm) and PEDOT:PSS films (300−700 nm). As far as we know, the difference is mainly related to the different surface states and surface energies of different HTL substrates. Among the different HTL substrates, the perovskite film based on Ni-oxidized NiOx film exhibited the best covered surface morphology with the densest crystallized grains. Figure 4d shows that intense diffraction peaks at 14.1° and 28.4° respectively assigned to the (110) and (220) crystal planes of tetragonal phase are exhibited in all the perovskite thin films, while the perovskite film exhibits the highest diffraction intensity compared to solution-processed NiOx and PEDOT:PSS-based thin films. The higher intensity is related higher crystal quality, also indicating the better charge transport and extraction. Figure S5 and Figure 4e show the 4704

DOI: 10.1021/acsaem.9b00169 ACS Appl. Energy Mater. 2019, 2, 4700−4707

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ACS Applied Energy Materials exhibits a promising PCE of 18.13% with a Voc = 1.08 V, Jsc = 21.37 mA/cm2, and FF = 0.78, far above the solutionprocessed NiOx-based devices with a PCE = 17.23% (Voc, Jsc, and FF are 1.05 V, 22.09 mA/cm2, and 0.74, respectively). The Voc shows a significant increase which has led to the better work of Ni-oxidized NiOx HTL, and this performance is also much higher than the device based on the PEDOT:PSS HTL with a lower PCE of 14.82% (Voc, Jsc, and FF are 1.00 V, 18.93 mA/cm2, and 0.78, respectively). As Figure 5c shows, the corresponding integrated current density based on IPCE curves of solution-processed and Ni-oxidized NiOx thin films is 21.60 and 21.30 mA/cm2, respectively. The mismatch is in the negligible range of 2% between the IPCE integrated Jsc and J−V measured Jsc which verified the credibility of the J−V result. The IPCE curves and corresponding integrated current density of PEDOT:PSS-based devices shown in Figure S8 indicate the device based on the PEDOT:PSS HTL has a poor performance. The enhanced IPCE value of the device based on the Nioxidized NiOx should be due to the improved charge extraction capability and better energy alignment. This also suggests less recombination in the device based on the Nioxidized NiOx film. Figure 5d shows the steady output current density of solution-processed NiOx and Ni-oxidized NiOxbased devices measured at constant illumination and short circuit state for 120 s. Its steady output voltage at open circuit state is shown in Figure S7d. Both types of devices show a stable output, which confirms the validity of the above measurement. This is further proven by the TPC and TPV measurement of the PSCs. The photocurrent decay of solution-processed and Ni-oxidized NiOx-based devices measured at the condition of short circuit is shown in Figure 5e. It can be seen that the device based on Ni-oxidized NiOx exhibits faster decay (0.86 μs) than the device based on solution-processed NiOx (1.60 μs) and PEDOT:PSS (1.9 μs). This indicated that the charge extraction and transport of the Ni-oxidized NiOx-based device is better than others, which is also consistent with the enhancement of the photocurrent. The charge-recombination lifetime could be measured by the photovoltage technique. As is shown in Figure 5f, the charge recombination lifetime of the device based on Ni-oxidized NiOx (1.24 ms) increases compared to the that based on solution-processed NiOx (0.7 ms) and PEDOT:PSS (0.5 ms) when measured at the condition of open circuit. It can be found that there are less charge recombination and trap density in the Ni-oxidized NiOx-based device. In Figure 5g, to further explore the photogenerated carrier recombination mechanism, Voc is chosen as a function of light intensity under the condition of device operating. From the linear fitting of logarithm of light intensity versus Voc, we can respectively get a slope of 2.33 and 2.43 kBT/q, corresponding to devices that are based on Nioxidized NiOx and solution-processed NiOx (here kB is the Boltzmann constant, T is the absolute temperature, and q is the elementary charge).The slope of the device based on Nioxidized NiOx is smaller than that based on solution-processed NiOx, which shows fewer trap-assisted recombination exists in the device based on Ni-oxidized NiOx substrates. It is believed that the device hysteresis relates to ion migration, interface traps, crystal defects of perovskite, and so on. To investigate the act of photocurrent hysteresis played on NiOx-based PSCs, different scan rates (the delay times are set as 0, 10, and 100 ms) and different scan directions are adopted in PSCs test.

Figures 5h and 5i show almost no photocurrent hysteresis and nearly identical J−V characteristics no matter the scan rate directions for both the Ni-oxidized NiOx-based device and solution-processed NiOx-based device. A comparison of the performance of PSCs based on our Ni-oxidized NiOx and other NiOx-based HTLs has been implemented and enumerated in Table S1. It is shown that our device based on the Ni-oxidized NiOx layer is highest among the devices based on the vacuum-processed NiOx HTL and also among the highest devices based on the other method-processed NiOx HTL. Eventually, we investigated the stability of the devices by keeping the device in the air environment with relative humidity about 30% for 200 h. As expressed in Figure 6, the

Figure 6. Stability of unencapsulated devices based on solutionprocessed and Ni-oxidized NiOx HTLs in air environment.

devices based on Ni-oxidized NiOx exhibit remarkable PCE stability as the PCE almost kept constant after storing the device for about 200 h. The Voc and Jsc are nearly unchanged, and only FF slightly reduced. For solution-processed NiOx, the PCE changes more intense compared with the Ni-oxidized NiOx-based devices especially at the initial measurement. But both the stabilities are much better than the PEDOT:PSSbased device displayed in Figure S9. The results confirm that Ni-oxidized NiOx-based PSCs exhibit satisfactory stability which is favorable for the future commercialization.

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CONCLUSION In conclusion, we have reported a Ni-oxidized NiOx HTL and its utilization in inverted planar heterojunction PSCs with PCE of 18.13% by combining with two-step prepared MA0.7FA0.3PbI3−xClx perovskite. This is one of the most competitive values for NiOx-based devices. Multifaceted characterization results demonstrate that the Ni-oxidized NiOx HTL has the best energy alignment with the perovskite film, the perovskite film based on the Ni-oxidized NiOx HTL has a larger grain size and a better material quality, and the device base on Ni-oxidized NiOx HTL possesses a suppressed charge recombination and an outstanding ability of charge extraction. Ni-oxidized NiOx HTL shows a promising application in highly efficient PSCs.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00169. 4705

DOI: 10.1021/acsaem.9b00169 ACS Appl. Energy Mater. 2019, 2, 4700−4707

Article

ACS Applied Energy Materials

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Optical absorption of glass/FTO/NiOx layer; statistical parameters of PSCs: FF, Voc of the perovskite devices based on the Ni-oxidized NiOx HTLs; XPS spectra of O 1s and Ni 2p for solution-processed and Ni-oxidized NiOx thin films; AFM images of solution-processed PEDOT:PSS; SEM of solution-processed PEDOT:PSS; steady-state PL of three HTLs on FTO/glass substrate; SEM of one-step solution method perovskite layer; SEM of two-step solution method perovskite layer; PL of two methods prepared perovskite layer; XRD of two methods prepared perovskite layer; comparison of Voc for the PSCs based on the two-step process method; comparison of FF for the PSCs; comparison of Voc for the PSCs; comparison of Jsc for the PSCs; The best J−V curve for PSCs with PEDOT:PSS HTL; IPCE spectra and integral current for the PEDOT:PSS HTL-based device; stability in air environment of PEDOT:PSS HTL-based devices (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Chunfu Zhang: 0000-0001-9555-3377 Weidong Zhu: 0000-0002-3872-7323 He Xi: 0000-0003-0684-4979 Jingjing Chang: 0000-0003-3773-182X Zhenhua Lin: 0000-0002-2965-1769 Jincheng Zhang: 0000-0001-7332-6704 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 61704128, 61874083, and 11435010), Natural Science Foundation of Shaanxi, China (Grants 2017JM6049 and 2018JQ6039), and Fundamental Research Funds for the Central Universities (Grant JB181408).

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DOI: 10.1021/acsaem.9b00169 ACS Appl. Energy Mater. 2019, 2, 4700−4707