High Efficiency Inverted Planar Perovskite Solar Cells with Solution

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High efficiency inverted planar perovskite solar cells with solution-processed NiO hole contact x

Xuewen Yin, Zhibo Yao, Qiang Luo, Xuezeng Dai, Yu Zhou, Ye Zhang, Yangying Zhou, Songping Luo, Jianbao Li, Ning Wang, and Hong Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13372 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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ACS Applied Materials & Interfaces

High Efficiency Inverted Planar Perovskite Solar Cells with Solution-Processed NiOx Hole Contact

Xuewen Yina, Zhibo Yaoa, Qiang Luoa, Xuezeng Daia, Yu Zhoua, Ye Zhanga, Yangying Zhoua, Songping Luoa, Jianbao Lia b, Ning Wangc*, Hong Lina* a

State Key Laboratory of New Ceramics & Fine Processing, School of Materials

Science and Engineering, Tsinghua University, Beijing 100084, P. R. China. b

Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island

Resources, Materials and Chemical Engineering Institute, Hainan University, Haikou 570228, China c

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of

Electronic Science and Technology of China, Chengdu 610054, P. R. China.

*Corresponding E-mail: [email protected]; [email protected]

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■ABSTRACT NiOx is a promising hole transporting material for perovskite solar cells due to its high hole mobility, good stability and easy processibility. In this work, we employed a simple solution-processed NiOx film as the hole-transporting layer in perovskite solar cells. When the thickness of perovskite layer increased from 270 nm to 380 nm, the light absorption and photogenerated carrier density were enhanced and the transporting distance of electron and hole would also increase at the same time, resulting in a large charge transfer resistance and a long hole-extracted process in the device, characterized by the UV-Vis, photoluminescence and electrochemical impedance spectroscopy spectrum. Combining both of these two factors, an optimal thickness of 334.2 nm was prepared with the perovskite precursor concentration of 1.35 M. Moreover, the optimal device fabrication conditions were further achieved by optimizing the thickness of NiOx hole-transporting layer and PCBM electron selective layer. As a result, a best power conversion efficiency of 15.71% was obtained with a Jsc of 20.51 mA·cm-2, a Voc of 988 mV and a FF of 77.51% with almost no hysteresis. A stable efficiency of 15.10% was caught at the maximum power point. This work provides a promising route to achieve higher efficiency perovskite solar cells based on NiO or other inorganic hole-transporting materials.

Keywords: un-doped nickel oxide; hole transporting layer; inverted planar structure; perovskite solar cells; high efficiency.


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■ INTRODUCTION Methylammonium lead halide perovskites have been widely investigated as light absorption materials for efficient solution-processed solar cells due to their superior optoelectronic properties, such as suitable bandgap, ambipolar transport property, small exciton binding energy, broad range for light absorption with high extinction coefficient, long charge-carrier diffusion length and lifetime.1-4 Nowadays, the certified efficiency of perovskite solar cells (PSCs) has already reached up to 22%, which proved to be a viable candidate for silicon solar cells and organic photovoltaics owing to its lower cost and simpler fabrication process.5-7 In the conventional normal n-i-p type PSCs, researchers have found that a high power conversion efficiency largely depends on the effectiveness of the hole-transporting material

(HTM).8,

9

Organic

semiconducting

HTMs

such

as

2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD),10 poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)

11, 12

or

poly(3-hexylthiophene-2,5-diyl) (P3HT)13 could be very expensive, which could largely hamper the development of the perovskite photovoltaic technology in industry. Meanwhile, doping hygroscopic lithium salts or corrosive pyridine additives into HTMs may lead to instability and inferior performance of PSCs as the dopants are highly sensitive to moisture.14-16 There has been a growing interest in inverted planar device architectures typically employing a MAPbI3-PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) bilayer junction because of the simple fabrication and relatively small hysteresis. The most 3 ACS Paragon Plus Environment


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popular

p-type

HTM

used

in

inverted

planar

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devices

is

poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS),

usually which,

however, is not suitable for long-term stability due to its high acidity and hygroscopicity.17 Comparing to organic HTMs, p-type inorganic HTMs are usually chemically stable and cost-effective.18,

19

There have been several reports on

employing p-type inorganic materials as HTMs in PSCs, such as CuSCN,20-22 PbS quantum qots,23 CuI,24,

25

(reduced) graphene oxide,14,

26-28

Cu2O

29

and NiO.30,

31

Among these materials, one of the most investigated inorganic candidates is nickel oxide. This is benefited from its wide band gap and high conduction band edge, which is crucial to be served as electron blocking layer. To date, studies on PSCs using NiOx as HTM have made great progresses. Seok et al 32 employed nanostructured NiOx by pulse laser deposition (PLD) as HTM in PSCs and achieved a very promising PCE of 17.3%. Seongrok Seo et al

33

prepared ultra-thin and undoped NiO films by atomic

layer deposition (ALD) and obtained a PCE of 16.4%. However, their NiOx films applied were deposited by an expensive method, which is not suitable for large scale fabrication. Therefore, developing a less costly solution-processed NiOx HTM for PSCs is highly desirable. Yang et al

30

adopted the sol-gel method to synthesize a

30-40 nm thick NiOx HTM layer and obtained a relatively low PCE of 9.11% due to low short-circuit current density (Jsc) and fill factor (FF), which can be attributed to the rough surface and relatively thin thickness of perovskite. Chen et al

34

prepared

NiOx films by spray pyrolysis and achieved a relatively low PCE of 10.2% when with no doping and interfacial modification due to low FF. As listed in table S3, at present, 4 ACS Paragon Plus Environment

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the photovoltaic performance of PSCs using solution-processed NiOx as HTMs is still not satisfactory because of the low FF or Jsc. In

this

work,

we

focus

on

PSCs

with

an

inverted

structure

of

FTO/NiOx/MAPbI3/PCBM/Ag, in which NiOx films were prepared by a simple solution-procsessed method. A detailed investigation on the effect of the thickness of NiOx, MAPbI3, and PCBM on photovoltaic performance of PSCs were given, during which the interfacial charge transfer properties of NiOx/perovskite were analyzed by using electrochemical impedance spectroscopy (EIS) and photoluminescence measurements. By optimizing NiOx contact and the thickness of perovskite absorption layer and PCBM electron transporting layer, a promising PCE of 15.71% was obtained, which is comparable to the most state-of the-art high-performance PSCs using NiOx films as HTMs prepared by ALD33 or spray pyrolysis.34 This work provides a reasonable reference to fabricate high efficiency inverted PSCs based on solution-processed NiOx or other inorganic HTMs.

■ RESULTS AND DISCUSSION 1 The effect of the thickness of perovskite layer. The device configuration and energy level diagram of the fabricated PSCs with an inverted planar structure were presented in figure 1a-b. The commercially available FTO glass substrate was firstly covered with a thin NiOx layer by spin-coating NiOx precursor solution in ethanol. The light harvester, MAPbI3, was then prepared by a chlorobenzene assisted fast-crystalline method, which was widely employed to 5 ACS Paragon Plus Environment


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achieve a continuous, full coverage, and flat film. (Figure 1c) After annealing, PCBM was used as ETM by spin- coating on perovskite in chlorobenzene. A typical cross-sectional SEM image of the device (MAPbI3 precursor solution concentration: 1.35 M) was demonstrated in figure 1d, showing that the thickness of NiOx, MAPbI3, PCBM, and silver were approximately 49.4 nm, 334.2 nm, 68.5 nm, and 93.2 nm, respectively. The photovoltaic performance of the device was largely influenced by the thickness of light-absorption layer, which was carefully tuned by varying the concentration of perovskite precursors in this work. As depicted in figure 2a, the strong Bragg peaks at 14.26°, 20.03°, 23.61°, 24.62°, 28.53°, 31.91°, 35.14°, 40.66° and 43.48° could be ascribed to (110), (200), (211), (220), (320), (310), (321), (400) and (411) planes of the perovskite MAPbI3 with an orthorhombic Pnma crystal structure, respectively. The peak intensity of perovskite behaved much stronger with higher concentration of precursor due to the increase of perovskite layer’s thickness, which could be seen in figure S1; The thickness of MAPbI3 were 269.4 nm, 334.2 nm, 354.9 nm and 379.7 nm for MAPbI3 solution with concentration of 1.25 M, 1.35 M, 1.45 M and 1.50 M, respectively. The light absorption from 400 nm to 750 nm also increased when the thickness of as-made perovskite films was increased, as shown in figure 2b.

To investigate the effect of perovskite thickness on the photovoltaic performance, figure 3 showed photovoltaic performance distribution of PSCs based on different MAPbI3 thickness measured under AM 1.5 simulated sun light (100 mW·cm−2). The 6 ACS Paragon Plus Environment

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curves were scanned from the open circuit voltage (Voc) to the short circuit current density (Jsc) at a scan rate of 0.076 V·s-1. As shown in figure 3, PSCs fabricated with 269.4 nm thick MAPbI3 with concentration of 1.25 M displayed an average Jsc of 17.91 mA·cm−2, a Voc of 0.953 V, and a FF of 57.33%, resulting in a relatively low PCE of 9.83%. When increasing the thickness of MAPbI3 to 334.2 nm with concentration of 1.35 M, the average Jsc and FF of the device were increased to 20.23 mA·cm−2 and 66.09%, respectively, both of which were much higher than those of the devices with other three concentration perovskite precursor. Moreover, the Voc was stabilized at 0.98 V, which was a little lower than the device with the perovskite concentration of 1.45 M. As a consequence, a highest PCE was achieved in the device with the perovskite precursor concentration of 1.35 M. The effect of perovskite thickness on the photovoltaic performance was characterized and analyzed by EIS and photoluminescence spectra, shown in the following chapter. IPCE spectra of PSCs with different MAPbI3 thickness were tested from 300 nm to 800 nm as shown in figure 4. It was worth noting that the IPCE was similar to those previously reported for inverted PSCs.35 The perovskite film showed a highest IPCE value of 82.78% at 550 nm, which was consistent with the highest transmittance of FTO at 550 nm, showing the maximum photon-to-electron conversion efficiency. The loss from 300 nm to 450 nm could be ascribed to the absorption of NiOx films. However, the increase from the 650 nm to 800 nm could be ascribed to the enhancement of perovskite thickness, which prevented the transmittance of long wavelength light. The integrated current density derived from the IPCE spectra 7 ACS Paragon Plus Environment


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corresponded well to the measured value from I-V measurement under simulated sunlight. The steady-state photoluminescence spectroscopy of MAPbI3 interfaced with NiOx HTM were tested to assess the effect of thickness of MAPbI3 on the charge transfer of NiOx/MAPbI3. As illustrated in figure 5a, the perovskite MAPbI3 showed a luminescence peak at 772 nm, and the MAPbI3 film with thickness of 269.4 nm on NiOx had the lowest photoluminescence intensity among those investigated films. The photoluminescence intensity was enhanced with the increased concentration of MAPbI3, which should be attributed to the increased thickness of absorption layer MAPbI3.We fitted the dynamic PL decay time curve to a biexponential decay function:

y = A1 exp(-t / τ 1 ) + A2 exp(-t / τ 2 )

(1)

The fitting results were shown in Table S1. In this function, the small decay time constant τ1 reflects the diffusion of the photogenerated excitons into defects. The large time constant τ2 is associated with the radiative exciton lifetime of MAPbI3 on HTM layer.30 A smaller value of τ2 indicates a faster hole-extract process from perovskite to NiOx. In this work, the device with perovskite precursor concentration of 1.25 M had a minimum hole-extraction lifetime τ2 (30.35 ns) because of the thinnest perovskite thickness. EIS measurements were also performed in the dark to investigate the internal charge transporting and recombination through the devices. Figure 5b showed the Nyquist plots of PSCs at a bias of 0.7 V in the dark, the inset was the simulative equivalent 8 ACS Paragon Plus Environment

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circuit model. Only one semicircle could be distinguished from the Nyquist plots, corresponding to the interfacial transfer between NiOx and perovskite, which delivers information about the charge transfer resistance (Rct). It is obvious that an efficient charge transfer process will lead to a small Rct, which gives a small semicircle as observed. 36 By fitting the Nyquist plots using the simulative equivalent circuit model, the total series resistances Rs of the devices were 10.64 Ω, 10.03 Ω, 11.75 Ω, 9.68 Ω for 1.25 M, 1.35 M, 1.45 M and 1.50 M, respectively. The closeness in Rs values in those devices was due to the similar device structure. The obviously different fitted Rct values of 153.9 Ω, 3126 Ω, 4575 Ω, 12752 Ω corresponded to the devices with different perovskite thicknesses of 1.25 M, 1.35 M, 1.45 M and 1.50 M, respectively. The smaller Rct implicates a faster charge transportation at the interface of NiOx/MAPbI3. In general, the Rct only depends on the perovskite thickness. Here, Rct as a function of perovskite thickness was plotted in figure S2, which clearly showed that the increase of Rct was almost linearly dependent on the film thickness from thickness of 270 to 350 nm. The reason of the misleading results of figure 5b was that the perovskite film thickness increase from 1.25 to 1.35 M was greatly larger than that from 1.35 to 1.45M. Furthermore, the difference of Rct varied from 350 nm to 380 nm was obviously larger that than at thinner thickness before the critical point (thickness about 350 nm). As perovskite thickness further increased, the slope of Rct about thickness took a break, which means a slight change in thickness will lead to a huge increase in Rct. This trend was also detected in PL decay measurement. As the absorption, PL spectra and EIS results indicted, increasing the perovskite 9 ACS Paragon Plus Environment


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concentration would lead to the increased thickness of perovskite layer, thus enhance the light absorption and photogenerated carriers for the device. On the other hand, when increasing the thickness of MAPbI3, the electron and hole generated in MAPbI3 would come across a long road to transfer, resulting in a large charge transfer resistance and a long hole- extracted process in the device. Both of two factors combined to result the best photovoltaic performance device with 334.2 nm thick perovskite layer prepared with perovskite precursor concentration of 1.35 M.

2 The effect of the thickness of NiOx layer. The photovoltaic performance of the device is also largely affected by the thickness of hole transporting layer. In this study, we prepared the HTM layer by spin-coating method. The film thickness could be controllable tuned by varying the concentration of precursor or the spin-coating times, which could induce the change of photovoltaic properties. As shown in figure S3, the thicknesses of NiOx films with different spin coating times were 40 nm, 49nm, 59nm and 71nm for one, two, three and four times, respectively. As displayed in the XRD spectra in figure 6a , the Bragg peaks at 37.24°, 43.28°and 62.86° could be assigned to the typical diffractions of (111) , (002) and (220) planes of the NiOx with a cubic Fm3m crystal structure (PDF: 47-1049), respectively. The optical properties of the NiOx films on the different thick samples were determined with the UV-vis absorption spectra, as shown in figure 6b. It is clearly seen that NiOx film showed high transmittance of over 70% from 400 nm to 800 nm, only a ~3 % 10 ACS Paragon Plus Environment

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decrease comparing to bare FTO glass, which was highly desirable for high performance PSCs because it ensured the incoming photons arriving to the neighboring absorber layer. The morphology and microstructure of NiOx films were observed by SEM (Figure 6c), showing a continuous and full-covered NiOx film formed by many small nanoparticles with size of several nanometers. The energy dispersive spectroscopy (EDS) spectrum of the NiOx deposited on top of FTO was shown in figure 6d, where nickel and oxygen elements could be obviously observed. To quantitatively characterize the surface roughness of NiOx films with different thickness, AFM was employed under tapping mode. Figure 7a-c showed comparative two-dimension AFM images of the NiOx films with different thickness on FTO. Figure 7d-e showed corresponding three-dimension plots. The surface roughnesses (Ra) and its root-mean-square (RMS) for NiOx films were listed in figure 7. RMS values were 34.73 nm, 30.98 nm and 29.32 nm for NiOx films with thickness of 0 nm, 59 nm and 71 nm, respectively. Compared with that on bare FTO, the Ra and RMS values decreased with the increase of spin-coating times, indicating that the films turned to be flatter with the increase of spin-coating times, which could also be verified in SEM images of NiOx films with different thickness in figure S4. We also tested the steady-state photoluminescence of MAPbI3 on the NiOx films with different thickness. As illustrated in figure S5, the quenching effect was observed on interfaces of NiOx/perovskite with different NiOx films thickness. In detail, the glass/FTO/NiOx/CH3NH3PbI3 samples exhibited 56.6%, 45.3%, 49.7% and 57.4% of the PL intensity of glass/CH3NH3PbI3 for 40 nm, 49 nm, 59 nm and 71 nm thick NiOx 11 ACS Paragon Plus Environment


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films, respectively. To further confirm the charge transport processes, the time-resolved PL was also conducted and the corresponding results were summarized in table S2. The PL lifetime was fitted with a biexponential decay function containing a fast decay and a slow decay process explained as above mentioned. The sample with 49 nm thick NiOx (spin-coating two times precursor) had a minimum hole-extraction lifetime τ2 (32.85 ns). As the thickness of NiOx layer increasing from 0 nm to 49 nm, both the PL intensity and τ2 decreased due to the enhancement of hole-extraction capability. When the thickness of NiOx layer increased from 49 nm to 71 nm, however, both the PL intensity and τ2 increased due to the reduction of charge transfer from CH3NH3PbI3 to NiOx, which could be seen in table 1. Based on static and decay PL spectra, the sample with 49 nm thick NiOx film had a fastest hole-extraction effect. The photovoltaic performances with different NiOx thickness -based PSCs were summarized in figure 8 and table 1. When increasing the thickness of NiOx from 40 nm to 49 nm, the Jsc, Voc and FF of the device increased to 20.48mA·cm−2, 0.978 V and 69.83%, resulting to an improvement in PCE, appropriately due to the suitable HTL thickness and modified surface roughness. As listed in table 1, further increase in the HTL thickness would lead to the increase of series resistance Rs from 3.31 ohm·cm2 (the device with 49 nm thick NiOx) to 4.44 ohm·cm2 (the device with 71 nm thick NiOx), which could largely hamper Jsc and Voc. This was probably due to the increased recombination and a higher resultant series resistance when dissociated carriers travel a longer distance to reach the FTO electrode.


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Table 1. Device performances of NiOx films with different thickness Thickness Time

Jsc(mA·cm-2)

Voc(V)

FF(%)

PCE(%)

Rs(ohm·cm2)

17.33

0.968

63.42

10.64

4.06

0.978

69.83

13.99

3.31

(nm) 1

40

2

49

3

59

18.35

0.968

65.57

11.64

4.08

4

71

18.05

0.897

64.61

10.46

4.44

20.48

3 The effect of PCBM concentration. We also investigated the effect of the thickness of PCBM layer on the photovoltaic performance of PSCs. As shown in figure 9a, PSCs fabricated with 16 mg·mL-1 PCBM as ETM displayed a Jsc of 19.03 mA·cm−2, resulting in a relatively low PCE of 13.50%. When the PCBM concentration increase to 18 mg·mL-1, the device showed a best PCE of 14.71% with a Jsc of 19.67 mA·cm−2. Further increasing the PCBM concentration to 20 mg·mL-1, the Jsc decreased to 17.38 mA·cm−2, thereby obtaining a lower PCE of 11.77%. When PCBM concentration was relatively low (16 mg·mL-1), 13 ACS Paragon Plus Environment


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too thin ETL makes leakage current occurred due to the direct contact between perovskite layer and Ag back electrode. Furthermore, the direct contact between Ag and perovskite also led to the formation of the AgI compound, which would worsen the device performance.37,

38

The thickness of PCBM layer increased with the

concentration of PCBM and thus avoid the contact between perovskite and Ag back contact as well as the formation of AgI compound. However, relatively thick PCBM layer (20 mg·mL-1) also resulted in the decrease in Jsc and PCE. This was probably due to the increased recombination and a higher resultant series resistance when dissociated carriers travel a longer distance to reach the Ag electrode.39 In order to further investigate the effect of the series resistance Rs on the photovoltaic characteristics, the J-V characteristics were analyzed by using the following expression: = − exp q + ⁄ − 1

(2)

where J is the density of the current passing through the external circuit, Jph is the light induced current density, which is considered equal to the Jsc. J0 is the dark saturate current density, q is the elementary charge, V is the applied voltage, Rs is the series resistance, A is the ideality factor (typically 1＜A＜2), kB is the Boltzmann constant, and T is the absolute temperature.40, 41 According to Eq. (2), Shockley equation is deduced when the J is zero, which could be used to predict the variation of Voc.

(

)

Voc = Ak BT ln ( J ph / J 0 ) + 1 q

(3) 14 ACS Paragon Plus Environment

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Eq. (3) could be transformed into the following expressions

−

dV AK BT = + Rs dJ q( J ph − J )

(4)

The data from J-V curves shown in figure 9a were used for analysis by combining Eq. (4).The plots of -

dV versus (Jph-J)-1 were exhibited in figure 9b. It could be found dJ

that there is a good linear relationship between -

dV and (Jph-J)-1. Linear plot fitting dJ

of figure 9b could gave Rs values of 0.28 Ω·cm2, 0.15 Ω·cm2 and 0.31 Ω·cm2 for PCBM concentration of 16 mg·mL-1, 18 mg·mL-1 and 20 mg·mL-1, respectively. The series resistance at illumination was very small, which could be comparable to the other references.42 From Eq. (2), a small series resistance is necessary for a high-performance PSC accompanied with a high FF. Moreover, Jsc (a), Voc (b), FF (c) and PCE(d) distributions as the concentration of PCBM were displayed in figure S6.

4 Photovoltaic properties and stability of the device. Finally, photo-stability, IPCE spectrum, and reverse and forward scanning tests were performed on the top-performing cell. As shown in figure 10a, the device displayed insignificant hysteresis with reverse scanning PCE of 14.59% and forward scanning PCE of 15.71% at a scan rate of 0.076 V·s-1.The IPCE spectrum was shown in figure 10b, the integrated current density was 19.52 mA·cm-2, which was well consistent with the measured Jsc of 20.51 mA·cm-2, obtained by the I-V curve (Table 2). In addition, the steady-state photocurrent measurement shown in figure 10c demonstrated that the cell yields a stable photocurrent density of 19.10 mA·cm-2 at the 15 ACS Paragon Plus Environment


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maximum power point (applied voltage at 0.845 V) and consequently results in a stable efficiency of 15.10%, which was close to the PCE value obtained by reverse scan of the PSCs. The air stability of devices without encapsulation (humidity < 30%) was characterized and presented in figure 10d. The PCE of the device remained above 61.63% of the initial value after 1500 h of atmospheric storage due to the decreased FF, which could be ascribed to the formation of a thin insulating intermediate layer caused by halide ionic migration,43 possibly AgI compound, between PCBM and Ag electrode.37 XRD of solar cell stored in air for 1500 h was shown in figure S7 in supporting information. The Bragg peaks at 23.68°, 25.32°and 32.78° could be assigned to the typical diffractions of (002), (011) and (012) planes of the AgI, respectively. (Red dots in figure S7)44 Moreover, in figure S8, secondary electron images of solar cell stored in air for 1500 h were shown superimposed with the Auger electron signals along a line scan across the solar cell. The results clearly showed that I element came cross PCBM layer to silver layer. Elemental mapping by SEM, which is helpful in examining the quantitative composition and distribution of constituents throughout the device was shown in figure S9. The results also proved that a distribution of I was found in the silver layer for solar cell stored in air for 1500h and almost not found for fresh solar cell. Combining with those results, AgI compound were formed after solar cell stored in air due to halide ionic migration Chen et al

34

used TiOx intermediate layer to insert into PCBM and Ag electrode to hinder the halide ionic-migration-induced degradation, and obtained a PCE > 90% of the initial PCE remained after 1000 hours light soaking with encapsulation. Thus, introducing a 16 ACS Paragon Plus Environment

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buffer layer between PCBM and Ag electrode could improve the device stability, which would be what we need to do next.

Table 2. Photovoltaic parameters of the best device Jsc(mA··cm-2) Scan direction

Voc(V)

PCE(%) Steady

FF(%) I-V

EQE

I-V state

Forward: Voc →Jsc

0.988

77.51

20.51

Reverse: Jsc →Voc

0.988

73.31

20.15

19.52

15.71

15.10

14.59

■CONCLUSION In summary, we have developed a simple NiOx preparation method and fabricated a device with an inverted structure of FTO/NiOx/MAPbI3/PCBM/Ag. After optimizing the thickness of NiOx HTM layers, and the solution concentration of perovskite light absorber as well as PCBM electron transporting material layer, we obtained an optimized efficient planar PSCs fabricated with spin-coating 59 nm thick NiOx, 334 nm thick MAPbI3 and 69 nm thick PCBM on FTO in sequence. A decent PCE of 15.71% with a Jsc of 20.51 mA·cm-2, a Voc of 988 mV and a FF of 77.51% and a stabilized efficiency of 15.10% were achieved. This work provides a promising route 17 ACS Paragon Plus Environment


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to achieve a more highly efficient stable inverted PSCs modified by improving the conductivity of NiOx layer and adding intermediate layer such as TiOx between PCBM and silver to improve the device stability.

■EXPERIMENTAL SECTION Materials synthesis All the reagents used in this study were of analytical grade. The ethanol solution containing 0.1 M nickel nitrate hexahydrate (Aladdin) was stirred overnight and filtered with 0.45 µm nylon filters before use. Subsequently, the NiOx precursor solution obtained was spun on the cleaned fluorine-doped tin oxide -coated glass substrate (FTO) at 4000 rpm for 30 s and this was repeated by several times, followed by annealing at 350 °C for 60 min with a ramping rate of 3 °C·min-1. CH3NH3I (MAI) was first synthesized by reacting 24 mL CH3NH2 (30-33 wt.% in ethanol) and 10 mL hydroiodic acid (45 wt.% in water) in a 250 mL round-bottom flask with 100 mL ethanol at 0 °C for 2 h with continuous stirring. The precipitates were recovered by evaporating the solution at 100 °C for 1 h. The product was dissolved in ethanol, recrystallized with diethyl ether, and finally dried at 60 °C in a vacuum oven for 24 h. Fabrication of solar cells Patterned FTO substrates were cleaned in the solvents of deionized water, ethanol, acetone and isopropyl alcohol subsequently. Then conductive glasses were treated with ultraviolet light for 15 minutes to make it well hydrophilic. The perovskite 18 ACS Paragon Plus Environment

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precursor was prepared by mixing MAI with PbI2 (Sigma Aldrich) in molar ratio 1:1 in anhydrous N, N-dimethylformamide (Alfa Aesar) with concentration varying from 1.35 M to 1.50 M. The perovskite precursor solution was then spun on NiOx coated substrates at 5000 rpm and after 5 s 180 µL anhydrous chlorobenzene (Alfa Aesar) was quickly dropped onto the center of the substrate, then the substrates were heated on a 100 °C hotplate for 15 min. Subsequently, the electron transporting material (ETM) PC61BM (Luminescence Technology Corp.) in anhydrous chlorobenzene was deposited on perovskite films by spin coating at 1000 rpm for 40 s. Finally, silver electrodes were deposited by thermal evaporation. Characterization X-ray diffraction (XRD) pattern data were recorded using a Bruker D8 Advance diffractometer (40 kV, 40 mA, Cu Kα1, λ= 1.5406 Å.). The morphology of samples was characterized with scanning electron microscope (Merlin, Zeiss, Germany). The surface morphologies were investigated using an atomic force microscopy (AFM) unit in noncontact mode (Nanonavi SPA400, SEIKO, Japan). Static emission spectra and dynamic emission decay spectra were measured by an Edinburgh FLS 920 instrument (Livingston, WL, UK). A picosecond pulsed diode lasers with excitation wavelength of 405 nm was available to record the emission decay curves for lifetimes in the range of 50 ns -1000 ns. Ultraviolet-visible absorption spectra were recorded on a Lambda 950 spectrophotometer in the 300-800 nm wavelength range at room temperature (Perkinelmer, USA). A 450 W ozone-free xenon lamp was used for steady-state measurements. The photocurrent-voltage (I-V) characteristics of PSCs 19 ACS Paragon Plus Environment


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were measured with a digital source meter (2400, Keithley Instruments, USA) under AM 1.5G illumination (100 mW·cm−2), which was realized by a solar simulator (91192, Oriel, USA, calibrated with a standard crystalline silicon solar cell). The incident photon to current efficiency spectra (IPCE) were characterized by using a QEX10 solar cell quantum efficiency measurement system (QEX10, PV measurements, USA). Prior to the measurement, a standard silicon solar cell was used as the reference (012-2013, Pharos Technology). EIS were measured by using the Zahner system (Zahner, Zahner-Electrik GmbH&Co.KG, Germany). The Z-view software was used to analyze the impedance data. Electrochemical impedance spectroscopy (EIS) were recorded at potentials of 0.7 V in the dark at frequencies ranging from 0. 1 Hz to 100 kHz; the oscillation potential amplitudes were adjusted to 10 mV. The complete analysis and accurate distribution of the elements were characterized by Auger electron spectroscopy (PHI 710, ULVAC, Japan).

■ ASSOCIATED CONTENT Supporting Information

Cross-sectional SEM images and elemental mapping of the devices and NiOx films, SEM images of NiOx ﬁlms fabricated by spin-coating different times, normalized PL spectra of the devices with different NiOx precursor spin-coating times, summary of PL decay time and the performances and secondary electron image and Auger analysis of NiOx-based organic-inorganic hybrid perovskite solar cells. This material is available free of charge via the Internet at http://pubs.acs.org. 20 ACS Paragon Plus Environment

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■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected]. NOTES The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This work was supported by the Joint NSFC-ISF Research Program, jointly funded by the National Natural Science Foundation of China and the Israel Science Foundation (2015DFG52690) and the National Natural Science Foundation of China (NSFC, 51272126).

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■ FIGURE CAPTIONS

Figure 1. a) The schematic of NiOx-based perovskite solar cell; b) the energy band level of solar cell; c) the SEM image of perovskite layer; d) the cross-sectional SEM 28 ACS Paragon Plus Environment

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image of the device.

Figure 2. The XRD patterns (a) and absorption spectra (b) of different concentration perovskite precursors.

Figure 3. Voc (a), Jsc (b), FF (c) and PCE(d) distributions with different concentration perovskite precursors.



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Figure 4. IPCE spectra with different concentration perovskite precursors.

Figure 5. The photoluminescence spectra (λex = 460 nm) (a) and EIS Nyquist plots obtained under dark condition at -0.7 V bias voltages (Inset: shows equivalent circuit employed to Nyquist fitting.) (b) of perovskite films with different perovskite 30 ACS Paragon Plus Environment

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concentration precursors.

Figure 6. a) XRD patterns of nickel oxide on slide glass (blue) and standard cubic Fm3m nickel oxide (red); b) transmittance spectra of different nickel oxide layers; c) SEM image of 49 nm thick nickel oxide (Inset: EDS test area); d) EDS spectra of 49 nm thick nickel oxide.



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Figure 7. AFM images of NiOx ﬁlms fabricated by spin-coating different times on FTO: bare FTO (a, d), 2 times (b, e), and 4 times (c, f). (Inset: Ra: surface roughness; RMS: root mean square roughness)


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Figure 8. I-V curves of the devices with different NiOx precursor spin-coating times

Figure 9. I-V curves (a) and plots of -

dV and (Jph-J)-1 (b) of the device with dJ

different concentration PCBM. 33 ACS Paragon Plus Environment


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Figure 10. I-V curves (a), IPCE spectrum (b), steady-state photocurrent measurement (c), and I-V curves stored in air (d) of the best performance device.


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Graphical Abstract


High Efficiency Inverted Planar Perovskite Solar Cells with Solution

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