Cesium-Doped Vanadium Oxide as the Hole Extraction Layer for

Jan 26, 2018 - (39, 40) Thus, the reduced VOC in the PSCs VOX:0.3Cs is attributed to the increased IPs of the VOX:0.3Cs thin film compared to that of ...
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Article Cite This: ACS Omega 2018, 3, 1117−1125

Cesium-Doped Vanadium Oxide as the Hole Extraction Layer for Efficient Perovskite Solar Cells Xiang Yao,† Jun Qi,† Wenzhan Xu,† Xiaofang Jiang,† Xiong Gong,*,†,‡ and Yong Cao† †

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡ Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: In this study, we report the utilization of lowtemperature solution-processed Cs-doped VOX thin films as the hole extraction layers (HELs) in perovskite solar cells (PSCs). It is found that the VOX:yCs (where y is the mole ratio of Cs versus V and y = 0.1, 0.3, and 0.5) thin films possess better electrical conductivities than that of the pristine VOX thin film. As a result, the PSCs incorporated with the VOX:yCs HEL exhibit large fill factors and high short-circuit currents, with consequently high power conversion efficiencies, which is more than 30% enhancement as compared with pristine VOX HEL. Our studies provide a facial way to enhance the electrical conductivity of the hole extraction layer for boosting device performance of perovskite solar cells. MoO3,25 and VOX26 have also been utilized as the HEL in the PHJ PSCs. Among them, VOX has been extensively studied because it possesses good thermal and chemical stability and it can be prepared with low cost as well.27−29 Snaith et al. first applied solution-processed VOX thin films as the HEL in the PHJ PSCs, but the PHJ PSCs with the VOX HEL exhibited low device performance due to the poor coverage of CH3NH3PbI3 layer on the VOX HEL.26 Recently, we reported lowtemperature solution-processed VOX HEL for the PHJ PSCs, however an interfacial layer is required to modify the VOX HEL for approaching high PCEs.30 In this study, we report solution-processed Cs-doped VOX thin film as the HEL for the PHJ PSCs. A combination of characterizations indicate that the VOX:yCs (where y is the mole ratio of Cs versus V, and y = 0.1, 0.3, and 0.5) thin films possess better electrical conductivities than that of the pristine VOX thin film. As a result, the PHJ PSCs incorporated with the VOX:yCs HEL exhibit large fill factors (FFs), high short-circuit currents, and consequently high PCEs compared to that with pristine VOX HEL.

1. INTRODUCTION Perovskite solar cells (PSCs) have attracted great attention in both academic and industrial sectors due to the superior optoelectronic properties of hybrid perovskite materials and cost-effective manufacturing processes of PSCs.1−4 Power conversion efficiencies (PCEs) of PSCs have been boosted from 3.8 to 22.1% in the past 8 years.5−9 In PSCs, a typical device structure was inherited from dye-sensitized solar cells, where the mesoscopic TiO2 layer was used as the electron extraction layer (EEL) and the bis(trifluoromethane)sulfonimide lithium salt (LiTFSI)-doped 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiroOMeTAD) was used as the hole extraction layer (HEL), respectively.10,11 However, high-temperature (500 °C) sintering was required to deposit the mesoscopic TiO2 EEL. Moreover, the deteriorative stability of the LiTFSI-doped spiro-OMeTAD HEL significantly impeded the manufacturing applications of PSCs.12,13 To circumvent these problems, a planar heterojunction (PHJ) device was developed, where the perovskite active layer was sandwiched between the anode and cathode electrodes. 1 4 − 1 7 Typically, the poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HEL and the phenyl-C61-butyric acid methyl ester (PC61BM) EEL are used in the PHJ PSCs for boosting device performance.18,19 However, the acidity and hydrophilicity of the PEDOT:PSS layer have been demonstrated to degrade the device performance.20 Rather than the PEDOT:PSS HEL, wide band gap transparent metal oxides, such as NiO,21,22 CuOX,23,24 © 2018 American Chemical Society

2. RESULTS AND DISCUSSION X-ray photoelectron spectroscopy (XPS) was first carried out to study the composition of the pristine VOX thin film and VOX:yCs thin films. All XPS spectra were calibrated by the C 1s Received: December 6, 2017 Accepted: January 17, 2018 Published: January 26, 2018 1117

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Figure 1. XPS spectrum of (a) V 3d and O 1s and (b) Cs 3d of pristine VOX thin film and the VOX:yCs thin films.

Figure 2. (a) Transmittance spectrum of pristine VOX and the VOX:yCs thin films; (b) the plot of (αhν)1/2 against hν for pristine VOX and the VOX:yCs thin films; (c) the I−V characteristics of a diode with the device architecture of indium tin oxide (ITO)/ VOX:yCs/Ag in dark.

the VOX thin film after the Cs-doping. The O 1s peak located at 531 eV shows a shift (Figure 1a) as the Cs-doped molar ratio is increased, which confirms that ammonium metavanadate was completely transformed into the VOX thin film after the Csdoping. Figure 2a depicts the transmittance spectra of pristine VOX thin film and the VOX:yCs thin films. It is found that the VOX:yCs thin films possess slightly varied transmittances ranging from 75 to 91% from wavelengths of 375−520 nm, as the doping concentrations of Cs are changed. In addition, both VOX and VOX:yCs thin films present over 80% transmittance at wavelengths ranging from 450 to 800 nm. Such high transmittance enables to ensure sun light to be absorbed by CH3NH3PbI3 layer.33 The optical band gap is estimated from the plot as the intercept of the linear fit at the onset of thin-film absorption with χ-axis.34 Figure 2b presents the (βE)1/2 versus hν of pristine VOX thin film and the

core level located at 284.6 eV. Figure 1a presents V 3d and O 1s peaks from pristine VOX and the VOX:yCs thin films. The binding energy observed from these thin films are in good agreement with those reported values.31,32 As shown in Figure 1b, peaks located at 737 and 722 eV from the VOX:yCs thin films are observed. These peaks are assigned to Cs 3d3/2 and Cs 3d5/2, correspondingly. Moreover, the intensity of these peaks are gradually enhanced with the increase of the Cs-doped mole ratio in the VOX:yCs thin films. These results indicate the Cs element is indeed doped in the VOX:yCs thin films. Figure S1 in the Supporting Information (SI) displays the binding energy of 400 eV corresponding to N 1s peak. The VOX film displays a relatively high N 1s peak, indicating residual N of ammonium metavanadate existed in the VOX thin film. The presence of a decreased amount of nitrogen along with the increased Csdoped molar ratio in Figure S1 virtually suggests that ammonium metavanadate was completely transformed into 1118

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Figure 3. Atomic force microscopy (AFM) height images of (a) VOX, (c) VOX:0.1Cs, (e) VOX:0.3Cs, and (g) VOX:0.5Cs thin films; AFM phase images of (b) VOX, (d) VOX:0.1Cs, (f) VOX:0.3Cs, and (h) VOX:0.5Cs thin films.

VOX:0.5Cs thin film are estimated to be 7.93 × 10−4, 8.78 × 10−4, 1.05 × 10−3, and 1.08 × 10−3 S m−1, respectively. As the Cs-doped mole ratios increase from 0 to 0.3, the electrical conductivities of thin films show a simultaneous increase, whereas, as the Cs-doped mole ratio is further increased to 0.5, the electrical conductivity doesn’t rise higher. Figure 3 and Table 1 show the surface film morphologies of pristine VOX thin film and the VOX:yCs thin films. The root mean square (RMS), which is used to describe the surface

VOX:yCs thin films, where β is the optical absorption coefficient, which is calculated using the relation βd = ln(1/ T), where T is the transmittance and d is the film thickness, and E is the photon energy. Thus, both pristine VOX and the VOX:yCs thin films exhibit a similar optical band gap of 3.5 eV, indicating that Cs-doping has a negligible influence on the optical band gaps of the VOX:yCs thin films. The electrical conductivities of pristine VOX and the VOX:yCs thin films were further investigated by evaluation of the current versus voltage (I−V) characteristics of a diode with a device architecture of ITO/VOX (or VOX:yCs)/Ag, where ITO is indium tin oxide and Ag is silver. The results are shown in Figure 2c. The electrical conductivity (σ) is described as I = σSd−1V, where d is the film thickness and S is the device area.35 The electrical conductivities of pristine VOX thin film, the VOX:0.1Cs thin film, the VOX:0.3Cs thin film, and the

Table 1. RMS Roughness of Pristine VOX and the VOX:yCs Thin Films items root-mean-square (RMS) roughness 1119

VOX

VOX:0.1Cs

VOX:0.3Cs

VOX:0.5Cs

0.32

1.27

1.51

0.84

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Figure 4. (a) J−V characteristics of the PSCs incorporated with pristine VOX and the VOX:yCs HELs; (b) external quantum efficiency (EQE) spectrum of the PSCs incorporated with pristine VOX and the VOX:yCs HELs.

Table 2. Photovoltaic Parameters of PSCs with Pristine VOX HEL and the VOX:yCs HELs HELs VOX VOX:0.1Cs VOX:0.3Cs VOX:0.5Cs

VOC (V) 0.95 0.93 0.92 0.92

(±0.03) (±0.05) (±0.03) (±0.03)

JSC (mA cm−2) 16.83 20.14 20.67 19.69

(±1.75) (±0.76) (±0.55) (±0.87)

FF (%) 70.17 74.91 76.46 65.10

(±3.75) (±3.40) (±1.95) (±3.84)

roughness of 0.32 nm, is obtained from pristine VOX thin film, whereas the enlarged RMS roughness of 1.27 and 1.51 nm are observed from the VOX:0.1Cs and VOX:0.3Cs thin films, respectively. The enlarged RMS roughness of VOX:0.3Cs thin film indicates that the interfacial adhesion between the VOX:0.3Cs thin film and the CH3NH3PbI3 layer would be enhanced. The improved interfacial adhesion is helpful to provide an additional charge pathway from the CH3NH3PbI3 layer to the HELs.36,37 In Figure 3d,f, an inhomogeneous phase separation pattern and the small island microstructural film morphology are observed from the VOX:0.1Cs and VOX:0.3Cs thin films. Such film morphologies are consistent with the nanodomain structure in the VOX thin films reported by others.31 In addition, as the Cs-doped mole ratio is further increased to 0.5, the RMS roughness draws back to 0.84 nm. The smooth film morphology from the VOX:0.5Cs thin film might be attributed to the Cs enrichment with high Cs-doped concentration. The PSCs with a device structure of ITO/HELs/ CH 3 NH3 PbI 3/PC 61 BM/bathocuproine (BCP)/Ag, where either pristine VOX thin film or the VOX:yCs thin films are used as the HELs, are fabricated for evaluating the functionality of the VOX:yCs HELs. Figure 4a presents the current density versus voltage (J−V) characteristics of the PSCs incorporated with pristine VOX HEL and the VOX:yCs HELs (where PSCs are represented as PSCs VOX and PSCs VOX:yCs, respectively). Table 2 summarizes the device performance parameters of the PSCs with different HELs. The PSCs VOX exhibits a VOC of 0.95 V, a short-circuit current (JSC) of 16.83 mA cm−2, a fill factor (FF) of 70.17%, and a PCE of 11.15%, whereas the PSCs VOX:0.1Cs exhibits a VOC of 0.93 V, a JSC of 20.14 mA cm−2, a FF of 74.91%, and a PCE of 13.91%. The PSCs VOX:0.3Cs exhibits a VOC of 0.92 V, a JSC of 20.67 mA cm−2, a FF of 76.46%, and a PCE of 14.48%. The PSCs VOX:0.5Cs shows a VOC of 0.92 V, a JSC of 19.69 mA cm−2, a FF of 65.10%, and a reduced PCE of 11.77%. Approximately 30% PCE enhancement is achieved from the PSCs VOX:0.3Cs in contrast to that of the PSCs VOX. The improved PCEs from the PSCs

PCE (%) 11.15 13.91 14.48 11.77

(±1.26) (±0.53) (±0.41) (±0.75)

RS (Ω cm2)

RSH (Ω cm2)

5.07 3.37 3.25 6.58

946.49 2510.16 3275.86 635.61

VOX:0.1Cs and PSCs VOX:0.3Cs are mainly originated from the significantly enhanced JSC and FF, implying the charge collection efficiencies are improved by the highly electrical conductive VOX:0.1Cs and VOX:0.3Cs HELs. A reduced PCE from the PSCs VOX:0.5Cs is mainly attributed to the decreased FF, which is probably due to the reduced interfacial adhesion between the VOX:0.5Cs layer and the CH3NH3PbI3 layer, giving rise to the low charge collection efficiency in PSCs. Table S1 in SI summarizes the photovoltaic parameters of the PSCs with VOX:0.3Cs HELs processed from different metavanadate concentrations. It is found that the best device performance is observed from the PSCs with VOX:0.3Cs HEL, where the VOX:0.3Cs HEL is processed from metavanadate with a concentration of 13.3 mg mL−1. To investigate the reduced VOC in PSCs with VOX:yCs HELs, Figure S2a,b in the SI presents the ultraviolet photoelectron spectroscopy (UPS) of pristine VOX thin film and the VOX:0.3Cs thin film. The secondary electron cutoff and the Fermi level (EF) are used to determine the ionization potentials (IPs).38 The IPs are estimated to be −4.9 and −4.7 eV for pristine VOX thin film and the VOX:0.3Cs thin film, respectively. A good match of the IP values with the highest occupied molecular orbital energy level (−5.4 eV) of the CH3NH3PbI3 layer would result in a large VOC in PSCs.39,40 Thus, the reduced VOC in the PSCs VOX:0.3Cs is attributed to the increased IPs of the VOX:0.3Cs thin film compared to that of the pristine VOX thin film. Figure 4b presents the external quantum efficiency (EQE) spectra of PSCs. The PSCs VOX:yCs displays an enhanced EQE response from 420 to 790 nm as compared to that of the PSCs VOX. The JSC values from integration of the EQE spectra are 17.29, 19.80, 20.00, and 19.38 mA cm−2 for the PSCs VOX, PSCs VOX:0.1Cs, PSCs VOX:0.3Cs, and PSCs VOX:0.5Cs, respectively. These JSC values are in good agreement with those obtained from the J−V characteristics. To further understand the underlying physics of enhanced JSC and FF of the PSCs incorporated in the VOX:yCs HELs, the series resistance (RS) and the shunt resistance (RSH) are 1120

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Figure 5. (a) JSC dependence upon different light intensities for the PSCs with different HELs; (b) VOC dependence upon different light intensities for the PSCs with different HELs.

estimated from J−V characteristics and the results are summarized in Table 2. The RS value from the PSCs VOX:0.3Cs is 3.25 Ω cm2, which is smaller than that for the PSCs VOX:0.1Cs (3.37 Ω cm2) and PSCs VOX (5.07 Ω cm2). The smaller RS can reduce the contact resistance and/or bulk resistance, resulting in high photocurrent flow through PSCs, and consequently high JSC.41 Because the PSCs VOX:0.5Cs has a dramatically large RS value of 6.58 Ω cm2, a reduced FF is obtained from the PSCs VOX:0.5Cs. For solar cells, it is of great importance to keep the RSH as large as possible because the large RSH value can avoid the current loss and leakage current.42 The RSH value from the PSCs VOX:0.3Cs is 3275.86 Ω cm2, which is the largest one compared to that for the PSCs VOX:0.1Cs (2510.16 Ω cm2), PSCs VOX (1583.33 Ω cm2), and PSCs VOX:0.5Cs (635.61 Ω cm2). The enlarged RSH values for the PSCs VOX:0.3Cs are beneficial for suppressing the leakage current, resulting in a larger FF and JSC. The enlarged RSH values are in good agreement with enhanced FF from the devices with corresponding HELs. To understand charge carrier recombination kinetics of the PSCs incorporated with the VOX:yCs HELs, the light-intensity dependence on JSC and VOC is further investigated and the results are shown in Figure 5a. According to the power-law fit, the JSC can be correlated with the light intensity by JSC ∼ Iα, where I is the light intensity. The extent of the deviation of α from 1 is closely related with nongerminate recombination loss in PSCs. A larger deviation implies more pronounced nongerminate recombination.42 As shown in Figure 5a, under the circuit-current condition, the PSCs VOX displays a large deviation of α = 0.91 from 1, implying more pronounced nongerminate recombination, whereas the PSCs VOX:0.1Cs and PSCs VOX:0.3Cs show smaller deviations of α = 0.94 and α = 0.96 from 1, respectively. The decreased deviations indicate a minimized nongerminate recombination, which is well consistent with the observed increase in the RSH and high hole collection efficiency. Moreover, as the Cs-doped mole ratio is increased, the PSCs VOX:0.5Cs shows an enlarged deviation of α = 0.93 from 1, implying an enhanced nongerminate recombination in PSCs VOX:0.5Cs. At the open-circuit condition, the relation between light intensity and VOC can be described as VOC ∼ S ln(I), where I is the light intensity and S is the slope.35 Figure 5b depicts the VOC dependence of I for the PSCs incorporated with different HELs. The VOC of the PSCs VOX is dependent on the I, with a slope, S, of 2.08 kT/q (ca. 0.052), where T is the absolute temperature, q is the elementary charge, and k is the Boltzmann

constant. The PSCs VOX:0.1Cs and PSCs VOX:0.3Cs exhibit an ideal slope of 1.04 kT/q (ca. 0.026) and kT/q (ca. 0.025), respectively. The ideal slope of the PSCs VOX:0.1Cs and PSCs VOX:0.3Cs indicates trap-assisted charge recombination can almost be suppressed in the PSCs VOX:0.1Cs and PSCs VOX:0.3Cs.35,42 Meanwhile, the slope S of 1.36 kT/q (ca. 0.034) is observed from the PSCs VOX:0.5Cs, suggesting more serious trap-assisted charge recombination in contrast to those of the PSCs VOX:0.1Cs and PSCs VOX:0.3Cs. The results are consistent with the photovoltaic performances of the devices with the VOX:yCs HELs. To understand the effect of different HELs on the chargeextraction process in PSCs, transient photocurrent (TPC) measurement was conducted to monitor the transient chargeextraction lifetime. The TPC measurement can provide direct information on charge extraction, transport, and trapping in PSCs.43,44 Figure 6 presents the normalized TPC of the PSCs

Figure 6. Transient photocurrent measurement for PSCs with different HELs.

with different HELs. The smaller charge-extraction lifetime of 0.22 μs is observed from the PSCs VOX:0.3Cs as compared to that of the PSCs VOX (0.39 μs) and PSCs VOX:0.1Cs (0.30 μs). The smaller charge-extraction lifetime indicates the PSCs VOX:0.3Cs exhibits reduced trap-assisted charge recombination and higher charge-extraction efficiency.45 In addition, the PSCs VOX:0.5Cs possesses a dramatically larger charge-extraction lifetime of 0.65 μs, indicating less charge carriers to be extracted 1121

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Figure 7. J−V characteristics of PSCs with different HELs measured at forward scan (from −0.2 to 1.0 V) and reversed scan (from 0.2 to −1.0 V).

20% and the ambient temperature varying in the range from 25 to 30 °C even without encapsulation. The devices were measured periodically, with the relative humidity varying in the range from 50 to 70%. Figure S3 in SI displays the PCE, JSC, VOC, and FF changes of the devices with stored time. The PCEs of the PSCs retained 83.28, 94.04, 92.54, and 48.16% of their initial PCEs after 30 days when incorporated with VOX, VOX:0.1Cs, VOX:0.3Cs, and VOX:0.5Cs HELs, respectively. The moderated reduction of PCE is associated with slow decline of the JSC, VOC, and FF. Although the devices were stored in the atmosphere conditions without the high humidity, the moderated reduction also confirms that both the devices incorporated with VOX:0.1Cs and VOX:0.3Cs HELs display excellent stability in ambient conditions.

by VOX:0.5Cs HEL. These results are in good agreement with the results from the light-intensity dependence on JSC and VOC curves. Figure 7 shows the hysteresis behaviors of the PSCs with pristine VOX and VOX:yCs HELs. In our study, under the forward scan, the PCE of the PSCs VOX is 12.32%; when the PSCs VOX is under the reversed scan, the PCE is 13.07%. These results indicate a 6.1% deviation under different scan directions. The PSCs VOX:0.1Cs shows a PCE of 13.93% at the forward scan direction and a PCE of 14.52% at the reversed scan direction. These results indicate a 4.2% deviation under different scan directions. For the PSCs VOX:0.3Cs, the PCE of 14.42% is observed under the forward direction and a PCE of 14.03% under the reversed direction, indicating a 2.7% deviation under different scan directions. The degree of photocurrent hysteresis in the PSCs VOX:0.3Cs is dramatically reduced in contrast to that of the PSCs VOX and the PSCs VOX:0.1Cs. The decreased photocurrent hysteresis is probably originated from the highly electrically conductive VOX:0.3Cs HEL and the highly efficient charge extraction process, where suppressed charge recombination takes place,35 whereas, the PSCs VOX:0.5Cs exhibits a PCE of 11.58% at the forward scan direction. Under the reversed scan, the PSCs VOX:0.5Cs exhibits a PCE of 12.34%. A 6.6% deviation is observed from the PSCs VOX:0.5Cs under different scan directions. The enlarged hysteresis in the PSCs VOX:0.5Cs might be due to the deteriorative charge collection efficiency in the PSCs VOX:0.5Cs. As we all know, the stability of the PSCs in ambient conditions is one of the most difficult problems before the PSCs move toward the market. We have examined the storage stability of the devices incorporated with pristine VOX and VOX:yCs HELs. The measured devices were stored in the dry box with the relative humidity varying in the range from 10 to

3. CONCLUSIONS In summary, we fabricated the VOX:yCs thin films by adding cesium hydroxide monohydrate into metavanadate precursor solution. The VOX:yCs thin films exhibit high electrical conductivity in contrast to that of the VOX thin film. According to the AFM results, the addition of Cs element in the VOX thin films can change their phase separation pattern and microstructural film morphology. Moreover, the VOX:0.3Cs thin film exhibits enlarged surface roughness, resulting in enhanced interfacial adhesion between the HEL layer and the CH3NH3PbI3 layer. The light-intensity dependence on JSC and VOC and TPC results indicate that the VOX:0.3Cs HELs can effectively suppress the charge recombination process and enhance the hole extraction efficiency in PSCs. As a results, the PSCs VOX:0.3Cs exhibits better device performance than that of the PSCs VOX, PSCs VOX:0.1Cs, and PSCs VOX:0.5Cs. All of these results demonstrate that our findings provide a protocol for preparing HEL toward high-performance PSCs. 1122

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4. EXPERIMENTAL SECTION 4.1. Materials. Bathocuproine (BCP), ammonium metavanadate, cesium hydroxide monohydrate, anhydrous N,Ndimethylformamide (DMF), chlorobenzene, and ethanol were purchased from Sigma-Aldrich. Lead(II) iodide (PbI2) was purchased from Alfa-Aesar. Phenyl-C61-butyric acid methyl ester (PC61BM) was purchased from 1-Material Inc. Methylammonium iodide (CH3NH3I) was purchased from Xi’an Polymer Light Technology Corp. All materials are used as received without further purification. 4.2. Fabrication of VOX:yCs Thin Films. The composite of 53.2 mg of metavanadate powder mixed with cesium hydroxide monohydrate with different molar ratios (where y is the mole ratio of Cs versus V, y = 0.1, 0.3, and 0.5) were dissolved into 4 mL of deionized water followed with being magnetically stirred at 60 °C till the solution was completely changed to a transparent solution. Afterward, the solution was filtered by poly(tetrafluoroethylene) filters (2 μm). VOX:yCs thin films were spun-cast on the top of precleaned indium tin oxide (ITO) glass substrates from above solutions, followed with thermal annealing at 210 °C for 8 min in the ambient atmosphere. 4.3. Characterization of VOX:yCs Thin Films. Transmittance spectra of the VOX:yCs thin films were measured on a HP 8453 spectrophotometer. Atomic force microscopy (AFM) was performed on Bruker multimode 8. X-ray photoelectron spectroscopy (XPS) was conducted on an Axis Ultra DLD spectrometer, with Al Kα excitation radiation. Ultraviolet photoelectron spectroscopy (UPS) was obtained by using PHI 5000 VersaProbe III from He I lamp with energy of 21.22 eV and a −5 V bias for enhancing signals. 4.4. Fabrication of PSCs. The CH3NH3PbI3 thin films were fabricated by a two-step method. An ∼150 nm PbI2 layer was first spin-coated on the top of the VOX:yCs thin films precoated ITO substrates, followed with thermal annealing at 80 °C for 10 min in the glovebox with nitrogen atmosphere and then cooled down to room temperature. The CH3NH3I thin film was then deposited on top of the PbI2 thin film from CH3NH3I ethanol solution, with a concentration of 35 mg mL−1, followed by placing the prepared thin film in the petri dish. Afterward, the prepared thin film was thermally annealed at 100 °C for 2 h to convert PbI2 and CH3NH3I into CH3NH3PbI3 thin film. It should be noted during the thermal annealing process, ∼7 μL of DMF solvent was added at the center of the petri dish for tuning the film morphology of CH3NH3PbI3 thin film.46 A PC61BM thin layer (∼60 nm) was then deposited on top of the CH3NH3PbI3 thin film from chlorobenzene solution (20 mg mL−1) at 1000 rpm for 36 s. Thereafter, ∼5 nm BCP was then spin-casted on top of the PC61BM layer from ethanol solution (0.5 mg mL−1) at 4000 rpm for 30 s. Finally, an ∼100 nm silver electrode was deposited on the top of the BCP layer by thermal evaporation in vacuum at a base pressure of 2 × 10−6 mbar. The device area was measured to be 0.057 cm2. 4.5. Characterizations of PSCs. The current density versus voltage (J−V) characteristics of PSCs were recorded with a Keithley 2400 source meter under AM 1.5G illumination from a solar simulator (Japan, SAN-EI, XES-40S1), with a light intensity of 100 mW cm−2. The light intensity was calibrated by using a certified reference silicon cell (PV measurement, with a KG5 filter). The external quantum efficiency (EQE) spectrum of PSCs was conducted on a DSR100UV-B spectrometer with a

bromine tungsten light source and an SR830 lock-in amplifier. The light intensity was calibrated with a Si detector. Transient photocurrent (TPC) measurement: the solar cells was serially connected to a digital oscilloscope (Tektronix TDS 3052C) and the input impedance of the oscilloscope set to 1 MΩ and 50 Ω, respectively, for monitoring the charge extraction time. When pumped by a mode-locked Ti:sapphire oscillator-seeded regenerative amplifier with a pulse energy of 1.3 mJ at 800 nm (Spectra Physics Spitfire Ace), the pulsed laser provided 120 fs wide λ ≈ 500 nm pulses, with a repetition rate of 1 kHz. The 500 nm laser pulses were generated from an optical parametric amplifier with TOPAS-Prime. The TPC was measured by applying 500 nm laser pulses with a low pulse energy to the short-circuited devices in the dark.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01944. Photovoltaic parameters of PSCs with the VOX:0.3Cs HELs, where the VOX:0.3Cs HELs are processed from different metavanadate concentrations (Table S1); XPS spectrum of N 1s of pristine VOX thin film and the VOX:yCs thin films (Figure S1); UPS data of pristine VOX thin film and the VOX:0.3Cs thin films (Figure S2a,b); the stability of PSCs with different HELs over 30 days of storage in air (Figure S3a−d) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (330) 972 3406. ORCID

Xiong Gong: 0000-0001-6525-3824 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors at the South China University of Technology acknowledge NSFC (51329301) for financial support.

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REFERENCES

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DOI: 10.1021/acsomega.7b01944 ACS Omega 2018, 3, 1117−1125