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Self-assembled NiO microspheres for efficient inverted mesoscopic perovskite solar cells
Solar Energy 193 (2019) 111–117
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Self-assembled NiO microspheres for efficient inverted mesoscopic perovskite solar cells Guodong Li, Kaiming Deng, Yanfei Dou, Yinsheng Liao, Deng Wang, Jihuai Wu, Zhang Lan
T ⁎
Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Fujian Key Laboratory of Photoelectric Functional Materials and College of Materials Science & Engineering, Huaqiao University, Xiamen 361021, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inverted mesoscopic perovskite solar cell Self-assembled NiO microspheres Photovoltaic performance
Perovskite solar cells (PSCs) with both normal (n-i-p) and inverted (p-i-n) mesoscopic structures usually exhibit higher efficiency than their planar counterparts because the mesoporous charge transport layers can supply heterogeneous nucleation sites for growing high quality perovskite crystals and enlarged charge separation area for better charge extraction. However, comparing with the achieved extremely high or even the certified world record efficiency of mesoscopic PSCs, the significant improvement of inverted mesoscopic PSCs has yet been made, mainly owing to the lack of suitable p-type semiconductors for preparing mesoporous hole transport layers (HTLs). Here, an emulsion-based bottom-up self-assembly strategy is used to prepare NiO microspheres from well-dispersed NiO nanocrystals. The self-assembled NiO microspheres are further used to fabricate mesoporous NiO HTLs of the inverted mesoscopic PSCs. The as-prepared mesoporous NiO HTL with self-assembled NiO microspheres can provide more suitable graded energy alignment, better charge carrier dynamics and reduced dark recombination in the device comparing with the inverted planar PSC with NiO nanocrystal HTL, contributing to obviously enhanced photovoltaic performance and nearly eliminated photocurrent-voltage hysteresis. Due to the general strategy of emulsion-based bottom-up self-assembly for microspheres synthesis, it will overcome the shortage of p-type materials for preparing efficient inverted mesoscopic PSCs.
1. Introduction Recently, many attentions have been paid to perovskite solar cells (PSCs) because the certified highest power conversion efficiency (PCE) has sky-rocketed to 24.2% during the recent ten years (Jiang et al., 2019; Kojima et al., 2009; pv-efficiency-chart). Owing to the remarkable photophysical properties of organic-inorganic halide perovskite materials, PSCs with normal (n-i-p) or inverted (p-i-n) mesoscopic and planar structures all can work efficiently (Lee et al.,2012; Liu and Kelly, 2013; Shi et al., 2015; Zhang et al., 2019; Zhou et al., 2019). Generally, mesoscopic-structure PSCs with mesoporous electron transport layers (ETLs) usually exhibit higher efficiency than their planar counterparts and achieve most of the highest certified PCEs during the different development periods of the PSCs (Burschka et al., 2013; Jeon et al., 2018, 2015; Li et al., 2016; Yang et al., 2017). The enlarged charge separation area at the mesoporous ETLs/perovskite interfaces benefits for better charge extraction (Cai et al., 2018; Huckaba et al., 2019; Wang et al., 2018). Meanwhile, the mesoporous ETLs are good for formation of high quality perovskite films by suppling heterogeneous nucleation sites during the perovskite crystal growth (Pascoe et al.,
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2016; Tidhar et al., 2014). Owing to the lack of suitable p-type semiconductors for preparing mesoporous hole transport layers (HTLs), the inverted mesoscopic PSCs are not so widely studied as those of normal mesoscopic and inverted planar counterparts. Nevertheless, several typical studies of inverted mesoscopic PSCs demonstrate that mesoporous HTLs can play similar functions in the inverted mesoscopic devices as those mesoporous ETLs in the normal mesoscopic devices. For instances, Chen et al. used p-type CuGaO2 nanocrystals (NCs) to fabricate mesoporous CuGaO2 HTLs on compact NiOx films for preparing inverted mesoscopic PSCs. Compared with the inverted planar counterparts, the mesoporous CuGaO2 HTLs can more effectively extract holes from perovskite and depress charge recombination due to the increased contact area in the mesoporous CuGaO2 HTLs/perovskite interfaces and the formation of graded energy alignment in the inverted mesoscopic devices, and the inverted mesoscopic devices achieve higher efficiency and better stability (Chen et al., 2018); Sun et al. fabricated inverted mesoscopic PSCs with chemical bath deposited mesoporous NiO HTLs. The inverted mesoscopic PSCs achieved fill factors (FFs) as high as 85% and efficiencies up to 16.7% with the aid of mesoporous NiO HTLs to enhance light harvesting and charge
Corresponding author. E-mail address: [email protected] (Z. Lan).
https://doi.org/10.1016/j.solener.2019.09.064 Received 22 June 2019; Received in revised form 1 August 2019; Accepted 17 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Scheme 1. Schematic illustration of the emulsion-based bottom-up self-assembly strategy for preparing NiO microspheres.
ozone for 30 min. For preparing p-NiO HTLs, the as-synhesized NiO NCs dispersed in toluene (10 mg mL−1) was spin-coated on the patterned FTOs at 4000 rpm for 30 s, soon afterwards heated at 500 °C for 30 min. For preparing m-NiO HTLs, the as-synthesized NiO microspheres dispersed in water (20 mg mL−1) was spin-coated on the as-prepared pNiO HTLs at 4000 rpm for 30 s, soon afterwards heated at 500 °C for 30 min. The thickness of the HTLs were measured by a stylus profilometer (D600, KLA Tencor, US). MAPbI3 perovskite layers were deposited on the p-NiO and m-NiO HTLs with the same method as our formerly reported (Tang et al., 2018). The PC61BM ETLs were prepared on the MAPbI3 perovskite layers by spin-coating chlorobenzene solution of PC61BM (20 mg mL−1) at 1500 rpm for 45 s and dried at 70 °C for 10 min; after that the saturated methanol solution of BCP was dripped on top of the PC61BM layers during spin-coating at 6000 rmp and dried at 70 °C for 10 min again. Finally, Au electrodes about 100 nm were thermally evaporated on the PC61BM ETLs under high vacuum through a shadow mask.
transporting efficiencies. These data are much higher than the inverted planar ones (FF of 73% and PCE of 14.5%) (Sun et al., 2018); and Yao et al. developed centimeter-sized inverted mesoscopic PSCs with bilayer structure containing p-type Cu doped NiOx nanoparticle-based mesoporous HTLs and Cu-doped NiOx blocking layers. The large-sized devices achieved a decent efficiency of 18.1%, significantly improved stability, and negligible hysteresis (Yao et al., 2017). Here, we present a novel self-assembled method to prepare NiO microspheres (as shown in Scheme 1) and use them to fabricate inverted mesoscopic PSCs with NiO microspheres HTLs (m-NiO HTLs) and NiO NCs blocking layers (p-NiO HTLs). The well crystalline NiO NCs with oleylamine (OAm) ligands are pre-synthesized with solvothermal method (Tang et al., 2018). The as-synthesized OAm-capped NiO NCs can be well dissolved in toluene or hexane and the solution is used to prepare NiO microspheres by an emulsion-based bottom-up selfassembly strategy (Bai et al., 2007). Furthermore, the self-assembled NiO microspheres are used as building blocks to fabricate m-NiO HTLs on the p-NiO HTLs atop FTOs. Comparing with the inverted planar PSC with p-NiO HTL, the optimized inverted mesoscopic one with m-NiO HTL yields obviously enhanced photovoltaic performance and nearly eliminated photocurrent-voltage (J-V) hysteresis, owing to the more suitable graded energy alignment, better charge carrier dynamics and depressed dark recombination.
2.4. Characterization Morphologies of the samples were observed by a JEM-2100 transmission electron microscopy (TEM) and a SU8000 field-emission scanning electron microscopy (SEM). Ultraviolet photoelectron spectroscopy (UPS) and UV–Vis absorption spectra were measured with a photoelectron spectrometer (ESCALAB 250 Xi, Thermo Fisher Scientific) and a Lamda 950 UV–Vis-NIR spectrophotometer, respectively. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were measured by a fluorescence spectrophotometer (Thermo Scientific Lumina) with the excitation wavelength of 460 nm from a Xenon lamp and an Omin-λ Monochromator/ Spectrograph (Zolix) with the time-correlated singlephoton counting method (Pico harp 300) and excitation wavelength of 507 nm from a pulsed laser diode (120 nJ per cm2 per pulsed) at room temperature, respectively. Photovoltaic performance of the inverted PSCs were evaluated by the J-V curves, stabilized current density and power output curves in an ambient environment. For measuring J-V curves, a computer-controlled Keithley 2400 source meter under simulated AM 1.5 G solar illumination at 100 mW cm−2 with #94043A solar simulator (PVIV-94043A, Newport, USA) was used, the voltage step and delay time were 20 mV and 10 ms, respectively; and the forward and reverse scans started from −0.1 V to 1.2 V and 1.2 V to −0.1 V, respectively. Additionally, the stabilized current density and power output curves were recorded close to the maximum power points, which were extracted from the J-V curves.
2. Experimental section 2.1. Materials All used reagents were purchased from Sigma-Aldrich Corp, unless specifically mentioned. [6, 6]-Phenyl C61 butyric acid methyl ester (PC61BM, 99.5%) was supplied by Luminescence Technology Corp, Taiwan, China. 2, 9-dimethyl-4, 7-diphenyl-1, 10- phenanthroline (BCP) and methylammonium iodide (MAI) were supplied by Xi'an Polymer Light Technology Corp, China. FTO glasses with sheet resistance of 15 Ω square−1 were purchased from Nippon Glass Corp (Japan). 2.2. Preparation of NiO nanocrystals and NiO microspheres NiO NCs were synthesized with the same method as our formerly reported (Tang et al., 2018). The as-synthesized OAm-capped NiO NCs were used to prepare NiO microspheres by the emulsion-based bottomup self-assembly strategy as shown in Scheme 1 (Bai et al., 2007). In details, 1 mL hexane solution of NiO NCs (15 mg mL−1) was injected into 10 mL aqueous solution of sodium dodecyl sulfate (SDS, 150 mg mL−1) under ultrasonic irradiation by an ultrasonic cell grinder with a power of 200 W for 5 min. The NiO microspheres were then obtained by heating the mixture at 70 °C for 2 h to evaporate the hexane and further centrifuging at a rate of 10,000 rpm for 10 min.
3. Results and discussion Fig. 1a and b show the high resolution TEM (HRTEM) image and the selected-area electron diffraction (SAED) patterns of the NiO NCs. The atomic lattice fringes with lattice spacings of 0.209 nm and 0.241 nm can be observed in Fig. 1a, corresponding to (0 1 2) and (1 0 1) planes of hexagonal NiO (PDF standard cards, JCPDS 44-1159, space group R3 m), respectively. The SAED pattern as shown in Fig. 1b further confirms the synthesis of hexagonal NiO NCs (Tang et al., 2018). Fig. 1c and d show the TEM and SEM images of NiO microspheres, one can see
2.3. Device fabrication The p-NiO HTLs and m-NiO HTLs were prepared on the laser-patterned FTOs with size of 1.5 × 1.5 cm2. The impurities on the FTOs were carefully cleaned out with isopropanol, acetone, distilled water and ethanol in order. And then, the cleaned FTOs were treated with UV112
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Fig. 1. HRTEM image (a) and SAED patterns (b) of NiO NCs; TEM image (c), SEM image (d) and diameter distribution histogram (e) of NiO microspheres taken from the SEM image of (d).
spin-coating one time of NiO microspheres water dispersion (20 mg mL−1) on a FTO and the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO, respectively. It is seen that the m-NiO HTL shows special sphere-like feature with a thickness about 210 nm and can be well infiltrated by the perovskite. Fig. 2c and d show the top-view SEM images of perovskite layers prepared on the m-NiO HTL and p-NiO HTL, one can find that the perovskite has larger crystal grains on the m-NiO HTL than on the pNiO HTL, which can be owing to the existed heterogeneous nucleation sites supplied by the mesoscopic scaffold of m-NiO HTL (Tidhar et al., 2014).
that the NiO microspheres are assembled by the NiO NCs and the NiO microspheres are highly dispersed with no obvious agglomeration. The X-ray diffraction (XRD) pattern of NiO microspheres is also measured and presented in Fig. S1 of supporting information. It shows identical diffraction peaks with NiO NCs as our formerly reported (Tang et al., 2018), which verifies again that the NiO microspheres are assemblied from NiO NCs. The diameter distribution histogram (Fig. 1e) of NiO microspheres taken from the SEM image of Fig. 1d reveals an average diameter of 158.63 ± 27.54 nm, suitable for preparing mesoscopic HTLs. Fig. 2a and b show the cross-sectional SEM images of m-NiO HTL by
Fig. 2. Cross-sectional SEM images of NiO microspheres on a FTO (a) and the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO (b); top-view SEM images of perovskite layers prepared on the m-NiO HTL (c) and p-NiO HTL (d). 113
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Fig. 3. UPS spectra (a), the Fermi edge (EF, edge) region (b) and the cut-off energy (Ecut-off) region (c), absorbance (d) and Tauc plots (e) of the p-NiO HTL and m-NiO HTL; energy level alignment of the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO (f).
Fig. 3 presents UPS, UV–Vis absorption spectra, Tauc plots and corresponding energy level alignment of the p-NiO HTL and m-NiO HTL. From UPS spectra, the parameters including Fermi level (EF), valence band maximum (EVB), conduction band minimum (ECB) and energy bandgap (Eg) can be calculated according to the following equations: EF = Ecut-off (cut-off binding energy)-21.2 eV (emission energy from He irradiation), EVB = EF − EF, edge (Fermi edge), ECB = EVB + Eg and Eg is obtained from the Tauc plots (Jung et al., 2017; Song et al., 2019). These energy level data combining with those of perovskite summarized from the reference (Wang et al., 2019), the energy level alignment of the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO can be obtained as shown in Fig. 3f. It is found that the insertion of m-NiO HTL between the p-NiO HTL and perovskite can form a graded energy alignment, more suitable for efficient hole-extraction (Sun et al., 2018). Fig. 4 shows photovoltaic performance of the inverted mesoscopic PSCs with m-NiO HTLs prepared by spin-coating different times of NiO microspheres water dispersion (20 mg mL−1) and the inverted planar one with p-NiO HTL. The J-V curves and the corresponding photovoltaic parameters including short-circuit current density (Jsc), opencircuit voltage (Voc), fill factor (FF) and PCE shown in Fig. 4a and Table 1 reveal that the inverted mesoscopic PSCs with m-NiO HTLs can achieve a champion PCE of 18.17% by spin-coating one time of NiO microspheres water dispersion (20 mg mL−1) on the p-NiO HTLs, other more times of spin-coating lead to quickly decreased photovotaic performance. It is said that the thickness of mesoscopic transport layer has
Table 1 Photovoltaic parameters of the inverted mesoscopic PSCs with m-NiO HTLs prepared by spin-coating different times of NiO microspheres water dispersion (20 mg mL−1) and the inverted planar one with p-NiO HTL by reverse scanning. Spin-coating time
Thickness (nm)
VOC/V
JSC/mA cm−2
FF/%
PCE/%
Control (planar) 1 time 2 times 3 times
35 ± 3 210 ± 25 348 ± 36 475 ± 28
1.083 1.105 1.068 1.036
21.11 22.19 20.70 20.26
69.88 74.07 70.07 65.04
15.98 18.17 15.49 13.65
significant influence on the photovoltaic performance of the device (Yang et al., 2016), due to the relative big size of NiO microspheres, the different spin-coating times will result in obviously changed thickness of m-NiO HTLs and photovoltaic performance of the devices. Although the thickness of m-NiO HTL by one time spin-coating (210 ± 25 nm) is thicker than the optimized value of the traditional mesoscopic charge transport layers (about 150 nm) (Jeon et al., 2014; Trifiletti et al., 2015), the special structure of the emulsion-based bottom-up self-assembled NiO microspheres can guarantee good photovoltaic performance. In order to identify the enhancement, the J-V curves of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL measured by both forward and reverse scans are shown in Fig. 4b, and the corresponding photovoltaic parameters are summarized in Table 2. Comparing with the inverted planar
Fig. 4. J-V curves of the inverted mesoscopic PSCs with m-NiO HTLs prepared by spin-coating different times of NiO microspheres water dispersion (20 mg mL−1) and the inverted planar one with p-NiO HTL by reverse scanning (a), those of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL measured by forward and reverse scans (b); steady-state output of Jsc and PCE of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL (c). 114
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perovskite/p-NiO HTL sample, one can observe that the quenching efficiency can be enhanced after inserting m-NiO HTL between perovskite and p-NiO HTL. That is, if the PL intensity of the perovskite/p-NiO HTL sample is set as 100%, then that of the perovskite/m-NiO HTL/p-NiO HTL sample can be decreased to 58.01%, indicating more efficient holeextraction in the latter one (Deng et al., 2019; Teo et al., 2019). For calculating the TRPL parameters including a fast extraction time (τ1), a slow radiative recombination lifetime (τ2), the corresponding decay amplitudes (A1 and A2), and the average TRPL decay time (τavg) as summarized in Table 3, the TRPL data in Fig. 6b are fitted by the following bi-exponential equiation (Du et al., 2018):
Table 2 Photovoltaic parameters of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL measured by forward and reverse scans. PSC
VOC/V
JSC/mA cm−2
FF/%
PCE/%
Planar
Reverse Forward Average
1.083 1.050 1.067
21.11 21.12 21.12
69.88 68.11 69.00
15.98 15.10 15.54
m-NiO
Reverse Forward Average
1.105 1.104 1.105
22.20 22.30 22.25
74.07 73.27 74.00
18.17 18.04 18.11
I (t ) =
∑ Ai exp (−t /τi) i
one with p-NiO HTL, the optimized inverted mesoscopic PSC with mNiO HTL achieves obviously enhanced photovoltaic parameters, for instance, it has a high reverse-scanned PCE (forward-scanned PCE) of 18.17% (18.04%) and the ratio of forward-scanned PCE versus reversescanned PCE is 99.28%, nearly eliminating J-V hysteresis; nonetheless, these values of the inverted planar one with p-NiO HTL are 15.98% (15.10%) and 94.49%, respectively. The steady-state output of Jsc and PCE of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with pNiO HTL are obtained by tracking maximum power point (MPP) at a bias voltage as shown in Fig. 4c. Under a bias of 0.92 V, the optimized inverted mesoscopic PSC with m-NiO HTL can yield a stabilized PCE of 18.09% with a stabilized Jsc of 19.66 mA cm−2; whereas, the inverted planar one with p-NiO HTL shows lower values of 15.24% and 18.48 mA cm−2, respectively. The J-V curves of one batch of 20 devices of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL, respectively, are measured and the statistical distributions of the photovoltaic parameters as shown in Fig. 5 further demonstrate the enhanced photovoltaic performance of the device with m-NiO HTL. For revealing the difference of charge carrier dynamics in the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL, the PL and TRPL spectra of the samples with structures of perovskite/m-NiO HTL/p-NiO HTL and perovskite/pNiO HTL are measured and shown in Fig. 6a and b. Comparing with the
(1)
Furthermore, the τavg is calculated with Ai and τi values as follows (Lv et al., 2018):
τavg =
∑ Ai τi2 ∑ Ai τi
(2)
It is found that the perovskite/m-NiO HTL/p-NiO HTL sample has a smaller τ1 than that of the perovskite/p-NiO HTL one (4.53 ns versus 7.92 ns). Because the fast extraction time is related with the dynamic of hole-extraction from perovskite to HTL, the smaller value indicates the more efficient hole-extraction in the sample (Baloch et al., 2018), which is according with the PL results as aforementioned. Nonetheless, τ2 of the sample is also greatly dropped from 65.12 ns to 37.32 ns after inserting m-NiO HTL between perovskite and p-NiO HTL. The smaller τ2 indicates the faster radiative recombination in the sample (Byranvand et al., 2018). However, A2 is also dropped from 27.85% to 20.11%, so it might not obviously influence the device performance. Additionally, τavg of the perovskite/m-NiO HTL/p-NiO HTL sample is 11.12 ns, much smaller than that of the perovskite/p-NiO HTL one (23.85 ns). Therefore, it can conclude that the perovskite/m-NiO HTL/p-NiO HTL sample shows a better charge carrier dynamic than that of the perovskite/pNiO HTL one. Dark J-V curves of the optimized inverted mesoscopic PSC with mNiO HTL and the inverted planar one with p-NiO HTL as shown in Fig. 6c are fitted with the following equations based on the equivalent
Fig. 5. Statistical distributions of the photovoltaic parameters for the optimized inverted mesoscopic PSCs with m-NiO HTLs and the inverted planar ones with p-NiO HTLs: distributions of Jsc (a), Voc (b), FF (c) and PCE (d). (All devices are tested with reverse scan and one batch of 20 devices is used for each condition). 115
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Fig. 6. PL (a) and TRPL (b) spectra of the perovskite/m-NiO HTL/p-NiO HTL and perovskite/p-NiO HTL films; dark I-V curves of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL (c).
general method for preparing many kinds of microspheres from welldispersed nanocrystals, so it will overcome the bottleneck of p-type material shortage for preparing efficient inverted mesoscopic PSCs.
Table 3 The TRPL parameters of τ1, τ2, A1, A2 and τavg of the perovskite/m-NiO HTL/pNiO HTL and perovskite/p-NiO HTL films. Sample
τavg/ns
τ1/ns
τ2/ns
A1/%
A2/%
perovskite/p-NiO HTL perovskite/m-NiO HTL/p-NiO HTL
23.85 11.12
7.92 4.53
65.12 37.32
72.15 79.89
27.85 20.11
Acknowledgements The authors would like to acknowledge the supports of the National Natural Science Foundation of China (Nos. 61474047, 51472094 and U1705256), the Natural Science Foundation of Fujian Province (2019J02012), the Fujian Provincial Youth Top-notch Talents Supporting Program, the Graphene Powder & Composite Research Center of Fujian Province (2017H2001) and the Cultivation Program for Postgraduate in Scientific Research Innovation Ability of Huaqiao University (No. 17013081044).
circuit of Shockley iodide in a single junction device to calculate the saturation current density (J0) and ideality factor (n), which are the major parameters contributing to the suppressed recombination (Wetzelaer et al., 2011).
qV J = J0 ⎡exp ⎛ − 1⎞ ⎤ ⎢ nkT ⎝ ⎠⎥ ⎦ ⎣
(3)
lnJ = lnJ0 + qV / nkT
(4)
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.09.064.
where V is the applied bias, q is the electron charge, k is the Boltzmann’s constant and T is the kelvin temperature. The calculated data reveal that the optimized inverted mesoscopic PSC with m-NiO HTL has a lower J0 (2.25 × 10−6 mA cm−2) than that of the inverted planar one with p-NiO HTL (4.31 × 10−5 mA cm−2), demonstrating significantly reduced recombination in the optimized inverted mesoscopic PSC with m-NiO HTL. Additionally, the device with m-NiO HTL also has a smaller n (2.25) compared to the inverted planar one with p-NiO HTL (2.52), indicating the decreased recombination loss (Cai et al., 2018). In addition, the transmission spectra of the p-NiO and m-NiO HTLs are measured and presented in Fig. S2 of Supplementary Information. It is seen that compared with the p-NiO HTL, the m-NiO HTL shows a slight decreased transmission. Because the perovskite layer is well penetrated into m-NiO HTL, so the slight decreased transmission of the neat m-NiO HTL will marginally influence light absorbance of the perovskite layer. Therefore, the improved charge transfer kinetics and the suppressed dark recombination mainly contribute to enhanced photovoltaic performance of the optimized inverted mesoscopic PSC with m-NiO HTL.
References Bai, F., Wang, D., Huo, Z., Chen, W., Liu, L., Liang, X., Chen, C., Wang, X., Peng, Q., Li, Y., 2007. A versatile bottom-up assembly approach to colloidal spheres from nanocrystals. Angew. Chem. Int. Edit. 46, 6650–6653. https://doi.org/10.1002/anie. 200701355. Baloch, A.A.B., Alharbi, F.H., Grancini, G., Hossain, M.I., Nazeeruddin, M.K., Tabet, N., 2018. Analysis of photocarrier dynamics at interfaces in perovskite solar cells by time-resolved photoluminescence. J. Phys. Chem. C 122, 26805–26815. https://doi. org/10.1021/acs.jpcc.8b07069. Burschka, J., Pellet, N., Moon, S.-J., Humphry-Baker, R., Gao, P., Nazeeruddin, M.K., Grätzel, M., 2013. Sequential deposition as a route to high-performance perovskitesensitized solar cells. Nature 499, 316. https://doi.org/10.1038/nature12340. Byranvand, M.M., Kim, T., Song, S., Kang, G., Ryu, S.U., Park, T., 2018. P-type CuI islands on TiO2 electron transport layer for a highly efficient planar-perovskite solar cell with negligible hysteresis. Adv. Energy Mater. 8, 1702235. https://doi.org/10.1002/ aenm.201702235. Cai, M., Ishida, N., Li, X., Yang, X., Noda, T., Wu, Y., Xie, F., Naito, H., Fujita, D., Han, L., 2018. Control of electrical potential distribution for high-performance perovskite solar cells. Joule 2, 296–306. https://doi.org/10.1016/j.joule.2017.11.015. Chen, Y., Yang, Z., Wang, S., Zheng, X., Wu, Y., Yuan, N., Zhang, W.-H., Liu, S., 2018. Design of an inorganic mesoporous hole-transporting layer for highly efficient and stable inverted perovskite solar cells. Adv. Mater. 30, 1805660. https://doi.org/10. 1002/adma.201805660. Deng, K., Tang, J., Lan, Z., 2019. Highly efficient inverted planar perovskite solar cells from TiO2 nanoparticles modified interfaces between NiO hole transport layers and conductive glasses. J. Mater. Sci: Mater. Electron. 30, 529–536. https://doi.org/10. 1007/s10854-018-0319-z. Du, Y., Xin, C., Huang, W., Shi, B., Ding, Y., Wei, C., Zhao, Y., Li, Y., Zhang, X., 2018. Polymeric surface modification of NiOx-based inverted planar perovskite solar cells with enhanced performance. ACS Sustain. Chem. Eng. 6, 16806–16812. https://doi. org/10.1021/acssuschemeng.8b04078. Huckaba, A.J., Lee, Y., Xia, R., Paek, S., Bassetto, V.C., Oveisi, E., Lesch, A., Kinge, S., Dyson, P.J., Girault, H., Nazeeruddin, M.K., 2019. Inkjet-printed mesoporous TiO2 and perovskite layers for high efficiency perovskite solar cells. Energy Tech. 7, 317–324. https://doi.org/10.1002/ente.201800905. Jeon, N.J., Na, H., Jung, E.H., Yang, T.-Y., Lee, Y.G., Kim, G., Shin, H.-W., Il Seok, S., Lee, J., Seo, J., 2018. A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells. Nat. Energy 3, 682–689. https://doi.org/10.1038/ s41560-018-0200-6. Jeon, N.J., Noh, J.H., Kim, Y.C., Yang, W.S., Ryu, S., Seok, S.I., 2014. Solvent engineering
4. Conclusion In summary, we use the emulsion-based bottom-up self-assembly strategy to synthesize NiO microspheres and successfully prepare high efficiency and nearly eliminated J-V hysteresis of the inverted mesoscopic PSCs with m-NiO HTLs. It is found that the m-NiO HTL can help to form larger perovskite crystal grains, more suitable graded energy alignment in the device, better charge carrier dynamics with higher PL quenching efficiency and faster hole-extraction, and reduced dark recombination with substantially lower J0 of the sample than the p-NiO HTL based one. These advantages contribute to obviously enhanced photovoltaic performance of the inverted mesoscopic PSCs compared with the inverted planar ones. The optimized inverted mesoscopic PSC with about 210 nm thick m-NiO HTL can yield a PCE of 18.17%, higher than that of the inverted planar one with p-NiO HTL (15.98%). Moreover, the emulsion-based bottom-up self-assembly strategy is a 116
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lanthanum in a nickel oxide-based inverted perovskite solar cell for efficiency and stability improvement. ChemSusChem 12, 518–526. https://doi.org/10.1002/cssc. 201802231. Tidhar, Y., Edri, E., Weissman, H., Zohar, D., Hodes, G., Cahen, D., Rybtchinski, B., Kirmayer, S., 2014. Crystallization of methyl ammonium lead halide perovskites: implications for photovoltaic applications. J. Am. Chem. Soc. 136, 13249–13256. https://doi.org/10.1021/ja505556s. Trifiletti, V., Roiati, V., Colella, S., Giannuzzi, R., De Marco, L., Rizzo, A., Manca, M., Listorti, A., Gigli, G., 2015. NiO/MAPbI3-xClx/PCBM: a model case for an improved understanding of inverted mesoscopic solar cells. ACS Appl. Mater. Interfaces 7, 4283–4289. https://doi.org/10.1021/am508678p. Wang, T., Ding, D., Zheng, H., Wang, X., Wang, J., Liu, H., Shen, W., 2019. Efficient inverted planar perovskite solar cells using ultraviolet/ozone-treated NiOx as the hole transport layer. Sol. RRL 3, 1900045. https://doi.org/10.1002/solr.201900045. Wang, Y., Li, Y., Guo, Z., Liu, W., Zhang, R., Chu, L., Li, X.A., 2018. Ethanol addition for morphology regulation of TiO2 nanorod arrays towards efficient hole-conductor-free perovskite solar cells. Funct. Mater. Lett. 11, 1850080. https://doi.org/10.1142/ S1793604718500807. Wetzelaer, G.A.H., Kuik, M., Lenes, M., Blom, P.W.M., 2011. Origin of the dark-current ideality factor in polymer:fullerene bulk heterojunction solar cells. Appl. Phys. Lett. 99, 153506. https://doi.org/10.1063/1.3651752. Yang, I.S., You, J.S., Sung, S.D., Chung, C.W., Kim, J., Lee, W.I., 2016. Novel spherical TiO2 aggregates with diameter of 100nm for efficient mesoscopic perovskite solar cells. Nano Energy 20, 272–282. https://doi.org/10.1016/j.nanoen.2015.12.031. Yang, W.S., Park, B.-W., Jung, E.H., Jeon, N.J., Kim, Y.C., Lee, D.U., Shin, S.S., Seo, J., Kim, E.K., Noh, J.H., Seok, S.I., 2017. Iodide management in formamidinium-leadhalide–based perovskite layers for efficient solar cells. Science 356, 1376–1379. https://doi.org/10.1126/science.aan2301. Yao, K., Li, F., He, Q., Wang, X., Jiang, Y., Huang, H., Jen, A.K.Y., 2017. A copper-doped nickel oxide bilayer for enhancing efficiency and stability of hysteresis-free inverted mesoporous perovskite solar cells. Nano Energy 40, 155–162. https://doi.org/10. 1016/j.nanoen.2017.08.014. Zhang, X., Zhang, W., Wu, T., Wu, J., Lan, Z., 2019. High efficiency and negligible hysteresis planar perovskite solar cells based on NiO nanocrystals modified TiO2 electron transport layers. Sol. Energy 181, 293–300. https://doi.org/10.1016/j.solener.2019. 02.011. Zhou, Q., Liang, L., Hu, J., Cao, B., Yang, L., Wu, T., Li, X., Zhang, B., Gao, P., 2019. Highperformance perovskite solar cells with enhanced environmental stability based on a (p-FC6H4C2H4NH3)2[PbI4] capping layer. Adv. Energy Mater. 9, 1802595. https:// doi.org/10.1002/aenm.201802595.
for high-performance inorganic-organic hybrid perovskite solar cells. Nat. Mater. 13, 897. https://doi.org/10.1038/NMAT4014. Jeon, N.J., Noh, J.H., Yang, W.S., Kim, Y.C., Ryu, S., Seo, J., Seok, S.I., 2015. Compositional engineering of perovskite materials for high-performance solar cells. Nature 517, 476. https://doi.org/10.1038/nature14133. Jiang, Q., Zhao, Y., Zhang, X., Yang, X., Chen, Y., Chu, Z., Ye, Q., Li, X., Yin, Z., You, J., 2019. Surface passivation of perovskite film for efficient solar cells. Nat. Photonics. https://doi.org/10.1038/s41566-019-0398-2. Jung, K.H., Seo, J.Y., Lee, S., Shin, H., Park, N.G., 2017. Solution-processed SnO2 thin film for a hysteresis-free planar perovskite solar cell with a power conversion efficiency of 19.2%. J. Mater. Chem. A 5, 24790–24803. https://doi.org/10.1039/C7TA08040A. Kojima, A., Teshima, K., Shirai, Y., Miyasaka, T., 2009. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051. https://doi.org/10.1021/ja809598r. Lee, M.M., Teuscher, J., Miyasaka, T., Murakami, T.N., Snaith, H.J., 2012. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 338, 643–647. https://doi.org/10.1126/science.1228604. Li, X., Bi, D., Yi, C., Décoppet, J.-D., Luo, J., Zakeeruddin, S.M., Hagfeldt, A., Grätzel, M., 2016. A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells. Science 353, 58–62. https://doi.org/10.1126/science.aaf8060. Liu, D., Kelly, T.L., 2013. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat. Photonics 8, 133. https://doi.org/10.1038/nphoton.2013.342. Lv, Y., Cai, B., Wu, Y., Wang, S., Jiang, Q., Ma, Q., Liu, J., Zhang, W.-H., 2018. High performance perovskite solar cells using TiO2 nanospindles as ultrathin mesoporous layer. J. Energy Chem. 27, 951–956. https://doi.org/10.1016/j.jechem.2018.01.020. Pascoe, A.R., Meyer, S., Huang, W., Li, W., Benesperi, I., Duffy, N.W., Spiccia, L., Bach, U., Cheng, Y.-B., 2016. Enhancing the optoelectronic performance of perovskite solar cells via a textured CH3NH3PbI3 morphology. Adv. Funct. Mater. 26, 1278–1285. https://doi.org/10.1002/adfm.201504190. Shi, D., Adinolfi, V., Comin, R., Yuan, M., Alarousu, E., Buin, A., Chen, Y., Hoogland, S., Rothenberger, A., Katsiev, K., Losovyj, Y., Zhang, X., Dowben, P.A., Mohammed, O.F., Sargent, E.H., Bakr, O.M., 2015. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 347, 519–522. https://doi. org/10.1126/science.aaa2725. Song, J., Zhang, W., Wang, D., Deng, K., Wu, J., Lan, Z., 2019. Colloidal synthesis of Ydoped SnO2 nanocrystals for efficient and slight hysteresis planar perovskite solar cells. Sol. Energy 185, 508–515. https://doi.org/10.1016/j.solener.2019.04.084. Sun, J., Lu, J., Li, B., Jiang, L., Chesman, A.S.R., Scully, A.D., Gengenbach, T.R., Cheng, Y.-B., Jasieniak, J.J., 2018. Inverted perovskite solar cells with high fill-factors featuring chemical bath deposited mesoporous NiO hole transporting layers. Nano Energy 49, 163–171. https://doi.org/10.1016/j.nanoen.2018.04.026. Tang, J., Jiao, D., Zhang, L., Zhang, X., Xu, X., Yao, C., Wu, J., Lan, Z., 2018. Highperformance inverted planar perovskite solar cells based on efficient hole-transporting layers from well-crystalline NiO nanocrystals. Sol. Energy 161, 100–108. https://doi.org/10.1016/j.solener.2017.12.045. Teo, S., Guo, Z., Xu, Z., Zhang, C., Kamata, Y., Hayase, S., Ma, T., 2019. The role of
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Self-assembled NiO microspheres for efficient inverted mesoscopic perovskite solar cells Guodong Li, Kaiming Deng, Yanfei Dou, Yinsheng Liao, Deng Wang, Jihuai Wu, Zhang Lan
T ⁎
Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Fujian Key Laboratory of Photoelectric Functional Materials and College of Materials Science & Engineering, Huaqiao University, Xiamen 361021, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inverted mesoscopic perovskite solar cell Self-assembled NiO microspheres Photovoltaic performance
Perovskite solar cells (PSCs) with both normal (n-i-p) and inverted (p-i-n) mesoscopic structures usually exhibit higher efficiency than their planar counterparts because the mesoporous charge transport layers can supply heterogeneous nucleation sites for growing high quality perovskite crystals and enlarged charge separation area for better charge extraction. However, comparing with the achieved extremely high or even the certified world record efficiency of mesoscopic PSCs, the significant improvement of inverted mesoscopic PSCs has yet been made, mainly owing to the lack of suitable p-type semiconductors for preparing mesoporous hole transport layers (HTLs). Here, an emulsion-based bottom-up self-assembly strategy is used to prepare NiO microspheres from well-dispersed NiO nanocrystals. The self-assembled NiO microspheres are further used to fabricate mesoporous NiO HTLs of the inverted mesoscopic PSCs. The as-prepared mesoporous NiO HTL with self-assembled NiO microspheres can provide more suitable graded energy alignment, better charge carrier dynamics and reduced dark recombination in the device comparing with the inverted planar PSC with NiO nanocrystal HTL, contributing to obviously enhanced photovoltaic performance and nearly eliminated photocurrent-voltage hysteresis. Due to the general strategy of emulsion-based bottom-up self-assembly for microspheres synthesis, it will overcome the shortage of p-type materials for preparing efficient inverted mesoscopic PSCs.
1. Introduction Recently, many attentions have been paid to perovskite solar cells (PSCs) because the certified highest power conversion efficiency (PCE) has sky-rocketed to 24.2% during the recent ten years (Jiang et al., 2019; Kojima et al., 2009; pv-efficiency-chart). Owing to the remarkable photophysical properties of organic-inorganic halide perovskite materials, PSCs with normal (n-i-p) or inverted (p-i-n) mesoscopic and planar structures all can work efficiently (Lee et al.,2012; Liu and Kelly, 2013; Shi et al., 2015; Zhang et al., 2019; Zhou et al., 2019). Generally, mesoscopic-structure PSCs with mesoporous electron transport layers (ETLs) usually exhibit higher efficiency than their planar counterparts and achieve most of the highest certified PCEs during the different development periods of the PSCs (Burschka et al., 2013; Jeon et al., 2018, 2015; Li et al., 2016; Yang et al., 2017). The enlarged charge separation area at the mesoporous ETLs/perovskite interfaces benefits for better charge extraction (Cai et al., 2018; Huckaba et al., 2019; Wang et al., 2018). Meanwhile, the mesoporous ETLs are good for formation of high quality perovskite films by suppling heterogeneous nucleation sites during the perovskite crystal growth (Pascoe et al.,
⁎
2016; Tidhar et al., 2014). Owing to the lack of suitable p-type semiconductors for preparing mesoporous hole transport layers (HTLs), the inverted mesoscopic PSCs are not so widely studied as those of normal mesoscopic and inverted planar counterparts. Nevertheless, several typical studies of inverted mesoscopic PSCs demonstrate that mesoporous HTLs can play similar functions in the inverted mesoscopic devices as those mesoporous ETLs in the normal mesoscopic devices. For instances, Chen et al. used p-type CuGaO2 nanocrystals (NCs) to fabricate mesoporous CuGaO2 HTLs on compact NiOx films for preparing inverted mesoscopic PSCs. Compared with the inverted planar counterparts, the mesoporous CuGaO2 HTLs can more effectively extract holes from perovskite and depress charge recombination due to the increased contact area in the mesoporous CuGaO2 HTLs/perovskite interfaces and the formation of graded energy alignment in the inverted mesoscopic devices, and the inverted mesoscopic devices achieve higher efficiency and better stability (Chen et al., 2018); Sun et al. fabricated inverted mesoscopic PSCs with chemical bath deposited mesoporous NiO HTLs. The inverted mesoscopic PSCs achieved fill factors (FFs) as high as 85% and efficiencies up to 16.7% with the aid of mesoporous NiO HTLs to enhance light harvesting and charge
Corresponding author. E-mail address: [email protected] (Z. Lan).
https://doi.org/10.1016/j.solener.2019.09.064 Received 22 June 2019; Received in revised form 1 August 2019; Accepted 17 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Scheme 1. Schematic illustration of the emulsion-based bottom-up self-assembly strategy for preparing NiO microspheres.
ozone for 30 min. For preparing p-NiO HTLs, the as-synhesized NiO NCs dispersed in toluene (10 mg mL−1) was spin-coated on the patterned FTOs at 4000 rpm for 30 s, soon afterwards heated at 500 °C for 30 min. For preparing m-NiO HTLs, the as-synthesized NiO microspheres dispersed in water (20 mg mL−1) was spin-coated on the as-prepared pNiO HTLs at 4000 rpm for 30 s, soon afterwards heated at 500 °C for 30 min. The thickness of the HTLs were measured by a stylus profilometer (D600, KLA Tencor, US). MAPbI3 perovskite layers were deposited on the p-NiO and m-NiO HTLs with the same method as our formerly reported (Tang et al., 2018). The PC61BM ETLs were prepared on the MAPbI3 perovskite layers by spin-coating chlorobenzene solution of PC61BM (20 mg mL−1) at 1500 rpm for 45 s and dried at 70 °C for 10 min; after that the saturated methanol solution of BCP was dripped on top of the PC61BM layers during spin-coating at 6000 rmp and dried at 70 °C for 10 min again. Finally, Au electrodes about 100 nm were thermally evaporated on the PC61BM ETLs under high vacuum through a shadow mask.
transporting efficiencies. These data are much higher than the inverted planar ones (FF of 73% and PCE of 14.5%) (Sun et al., 2018); and Yao et al. developed centimeter-sized inverted mesoscopic PSCs with bilayer structure containing p-type Cu doped NiOx nanoparticle-based mesoporous HTLs and Cu-doped NiOx blocking layers. The large-sized devices achieved a decent efficiency of 18.1%, significantly improved stability, and negligible hysteresis (Yao et al., 2017). Here, we present a novel self-assembled method to prepare NiO microspheres (as shown in Scheme 1) and use them to fabricate inverted mesoscopic PSCs with NiO microspheres HTLs (m-NiO HTLs) and NiO NCs blocking layers (p-NiO HTLs). The well crystalline NiO NCs with oleylamine (OAm) ligands are pre-synthesized with solvothermal method (Tang et al., 2018). The as-synthesized OAm-capped NiO NCs can be well dissolved in toluene or hexane and the solution is used to prepare NiO microspheres by an emulsion-based bottom-up selfassembly strategy (Bai et al., 2007). Furthermore, the self-assembled NiO microspheres are used as building blocks to fabricate m-NiO HTLs on the p-NiO HTLs atop FTOs. Comparing with the inverted planar PSC with p-NiO HTL, the optimized inverted mesoscopic one with m-NiO HTL yields obviously enhanced photovoltaic performance and nearly eliminated photocurrent-voltage (J-V) hysteresis, owing to the more suitable graded energy alignment, better charge carrier dynamics and depressed dark recombination.
2.4. Characterization Morphologies of the samples were observed by a JEM-2100 transmission electron microscopy (TEM) and a SU8000 field-emission scanning electron microscopy (SEM). Ultraviolet photoelectron spectroscopy (UPS) and UV–Vis absorption spectra were measured with a photoelectron spectrometer (ESCALAB 250 Xi, Thermo Fisher Scientific) and a Lamda 950 UV–Vis-NIR spectrophotometer, respectively. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were measured by a fluorescence spectrophotometer (Thermo Scientific Lumina) with the excitation wavelength of 460 nm from a Xenon lamp and an Omin-λ Monochromator/ Spectrograph (Zolix) with the time-correlated singlephoton counting method (Pico harp 300) and excitation wavelength of 507 nm from a pulsed laser diode (120 nJ per cm2 per pulsed) at room temperature, respectively. Photovoltaic performance of the inverted PSCs were evaluated by the J-V curves, stabilized current density and power output curves in an ambient environment. For measuring J-V curves, a computer-controlled Keithley 2400 source meter under simulated AM 1.5 G solar illumination at 100 mW cm−2 with #94043A solar simulator (PVIV-94043A, Newport, USA) was used, the voltage step and delay time were 20 mV and 10 ms, respectively; and the forward and reverse scans started from −0.1 V to 1.2 V and 1.2 V to −0.1 V, respectively. Additionally, the stabilized current density and power output curves were recorded close to the maximum power points, which were extracted from the J-V curves.
2. Experimental section 2.1. Materials All used reagents were purchased from Sigma-Aldrich Corp, unless specifically mentioned. [6, 6]-Phenyl C61 butyric acid methyl ester (PC61BM, 99.5%) was supplied by Luminescence Technology Corp, Taiwan, China. 2, 9-dimethyl-4, 7-diphenyl-1, 10- phenanthroline (BCP) and methylammonium iodide (MAI) were supplied by Xi'an Polymer Light Technology Corp, China. FTO glasses with sheet resistance of 15 Ω square−1 were purchased from Nippon Glass Corp (Japan). 2.2. Preparation of NiO nanocrystals and NiO microspheres NiO NCs were synthesized with the same method as our formerly reported (Tang et al., 2018). The as-synthesized OAm-capped NiO NCs were used to prepare NiO microspheres by the emulsion-based bottomup self-assembly strategy as shown in Scheme 1 (Bai et al., 2007). In details, 1 mL hexane solution of NiO NCs (15 mg mL−1) was injected into 10 mL aqueous solution of sodium dodecyl sulfate (SDS, 150 mg mL−1) under ultrasonic irradiation by an ultrasonic cell grinder with a power of 200 W for 5 min. The NiO microspheres were then obtained by heating the mixture at 70 °C for 2 h to evaporate the hexane and further centrifuging at a rate of 10,000 rpm for 10 min.
3. Results and discussion Fig. 1a and b show the high resolution TEM (HRTEM) image and the selected-area electron diffraction (SAED) patterns of the NiO NCs. The atomic lattice fringes with lattice spacings of 0.209 nm and 0.241 nm can be observed in Fig. 1a, corresponding to (0 1 2) and (1 0 1) planes of hexagonal NiO (PDF standard cards, JCPDS 44-1159, space group R3 m), respectively. The SAED pattern as shown in Fig. 1b further confirms the synthesis of hexagonal NiO NCs (Tang et al., 2018). Fig. 1c and d show the TEM and SEM images of NiO microspheres, one can see
2.3. Device fabrication The p-NiO HTLs and m-NiO HTLs were prepared on the laser-patterned FTOs with size of 1.5 × 1.5 cm2. The impurities on the FTOs were carefully cleaned out with isopropanol, acetone, distilled water and ethanol in order. And then, the cleaned FTOs were treated with UV112
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Fig. 1. HRTEM image (a) and SAED patterns (b) of NiO NCs; TEM image (c), SEM image (d) and diameter distribution histogram (e) of NiO microspheres taken from the SEM image of (d).
spin-coating one time of NiO microspheres water dispersion (20 mg mL−1) on a FTO and the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO, respectively. It is seen that the m-NiO HTL shows special sphere-like feature with a thickness about 210 nm and can be well infiltrated by the perovskite. Fig. 2c and d show the top-view SEM images of perovskite layers prepared on the m-NiO HTL and p-NiO HTL, one can find that the perovskite has larger crystal grains on the m-NiO HTL than on the pNiO HTL, which can be owing to the existed heterogeneous nucleation sites supplied by the mesoscopic scaffold of m-NiO HTL (Tidhar et al., 2014).
that the NiO microspheres are assembled by the NiO NCs and the NiO microspheres are highly dispersed with no obvious agglomeration. The X-ray diffraction (XRD) pattern of NiO microspheres is also measured and presented in Fig. S1 of supporting information. It shows identical diffraction peaks with NiO NCs as our formerly reported (Tang et al., 2018), which verifies again that the NiO microspheres are assemblied from NiO NCs. The diameter distribution histogram (Fig. 1e) of NiO microspheres taken from the SEM image of Fig. 1d reveals an average diameter of 158.63 ± 27.54 nm, suitable for preparing mesoscopic HTLs. Fig. 2a and b show the cross-sectional SEM images of m-NiO HTL by
Fig. 2. Cross-sectional SEM images of NiO microspheres on a FTO (a) and the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO (b); top-view SEM images of perovskite layers prepared on the m-NiO HTL (c) and p-NiO HTL (d). 113
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Fig. 3. UPS spectra (a), the Fermi edge (EF, edge) region (b) and the cut-off energy (Ecut-off) region (c), absorbance (d) and Tauc plots (e) of the p-NiO HTL and m-NiO HTL; energy level alignment of the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO (f).
Fig. 3 presents UPS, UV–Vis absorption spectra, Tauc plots and corresponding energy level alignment of the p-NiO HTL and m-NiO HTL. From UPS spectra, the parameters including Fermi level (EF), valence band maximum (EVB), conduction band minimum (ECB) and energy bandgap (Eg) can be calculated according to the following equations: EF = Ecut-off (cut-off binding energy)-21.2 eV (emission energy from He irradiation), EVB = EF − EF, edge (Fermi edge), ECB = EVB + Eg and Eg is obtained from the Tauc plots (Jung et al., 2017; Song et al., 2019). These energy level data combining with those of perovskite summarized from the reference (Wang et al., 2019), the energy level alignment of the semi-device with a framework of perovskite/NiO mesoscopic layer/NiO NCs planar layer/FTO can be obtained as shown in Fig. 3f. It is found that the insertion of m-NiO HTL between the p-NiO HTL and perovskite can form a graded energy alignment, more suitable for efficient hole-extraction (Sun et al., 2018). Fig. 4 shows photovoltaic performance of the inverted mesoscopic PSCs with m-NiO HTLs prepared by spin-coating different times of NiO microspheres water dispersion (20 mg mL−1) and the inverted planar one with p-NiO HTL. The J-V curves and the corresponding photovoltaic parameters including short-circuit current density (Jsc), opencircuit voltage (Voc), fill factor (FF) and PCE shown in Fig. 4a and Table 1 reveal that the inverted mesoscopic PSCs with m-NiO HTLs can achieve a champion PCE of 18.17% by spin-coating one time of NiO microspheres water dispersion (20 mg mL−1) on the p-NiO HTLs, other more times of spin-coating lead to quickly decreased photovotaic performance. It is said that the thickness of mesoscopic transport layer has
Table 1 Photovoltaic parameters of the inverted mesoscopic PSCs with m-NiO HTLs prepared by spin-coating different times of NiO microspheres water dispersion (20 mg mL−1) and the inverted planar one with p-NiO HTL by reverse scanning. Spin-coating time
Thickness (nm)
VOC/V
JSC/mA cm−2
FF/%
PCE/%
Control (planar) 1 time 2 times 3 times
35 ± 3 210 ± 25 348 ± 36 475 ± 28
1.083 1.105 1.068 1.036
21.11 22.19 20.70 20.26
69.88 74.07 70.07 65.04
15.98 18.17 15.49 13.65
significant influence on the photovoltaic performance of the device (Yang et al., 2016), due to the relative big size of NiO microspheres, the different spin-coating times will result in obviously changed thickness of m-NiO HTLs and photovoltaic performance of the devices. Although the thickness of m-NiO HTL by one time spin-coating (210 ± 25 nm) is thicker than the optimized value of the traditional mesoscopic charge transport layers (about 150 nm) (Jeon et al., 2014; Trifiletti et al., 2015), the special structure of the emulsion-based bottom-up self-assembled NiO microspheres can guarantee good photovoltaic performance. In order to identify the enhancement, the J-V curves of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL measured by both forward and reverse scans are shown in Fig. 4b, and the corresponding photovoltaic parameters are summarized in Table 2. Comparing with the inverted planar
Fig. 4. J-V curves of the inverted mesoscopic PSCs with m-NiO HTLs prepared by spin-coating different times of NiO microspheres water dispersion (20 mg mL−1) and the inverted planar one with p-NiO HTL by reverse scanning (a), those of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL measured by forward and reverse scans (b); steady-state output of Jsc and PCE of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL (c). 114
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perovskite/p-NiO HTL sample, one can observe that the quenching efficiency can be enhanced after inserting m-NiO HTL between perovskite and p-NiO HTL. That is, if the PL intensity of the perovskite/p-NiO HTL sample is set as 100%, then that of the perovskite/m-NiO HTL/p-NiO HTL sample can be decreased to 58.01%, indicating more efficient holeextraction in the latter one (Deng et al., 2019; Teo et al., 2019). For calculating the TRPL parameters including a fast extraction time (τ1), a slow radiative recombination lifetime (τ2), the corresponding decay amplitudes (A1 and A2), and the average TRPL decay time (τavg) as summarized in Table 3, the TRPL data in Fig. 6b are fitted by the following bi-exponential equiation (Du et al., 2018):
Table 2 Photovoltaic parameters of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL measured by forward and reverse scans. PSC
VOC/V
JSC/mA cm−2
FF/%
PCE/%
Planar
Reverse Forward Average
1.083 1.050 1.067
21.11 21.12 21.12
69.88 68.11 69.00
15.98 15.10 15.54
m-NiO
Reverse Forward Average
1.105 1.104 1.105
22.20 22.30 22.25
74.07 73.27 74.00
18.17 18.04 18.11
I (t ) =
∑ Ai exp (−t /τi) i
one with p-NiO HTL, the optimized inverted mesoscopic PSC with mNiO HTL achieves obviously enhanced photovoltaic parameters, for instance, it has a high reverse-scanned PCE (forward-scanned PCE) of 18.17% (18.04%) and the ratio of forward-scanned PCE versus reversescanned PCE is 99.28%, nearly eliminating J-V hysteresis; nonetheless, these values of the inverted planar one with p-NiO HTL are 15.98% (15.10%) and 94.49%, respectively. The steady-state output of Jsc and PCE of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with pNiO HTL are obtained by tracking maximum power point (MPP) at a bias voltage as shown in Fig. 4c. Under a bias of 0.92 V, the optimized inverted mesoscopic PSC with m-NiO HTL can yield a stabilized PCE of 18.09% with a stabilized Jsc of 19.66 mA cm−2; whereas, the inverted planar one with p-NiO HTL shows lower values of 15.24% and 18.48 mA cm−2, respectively. The J-V curves of one batch of 20 devices of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL, respectively, are measured and the statistical distributions of the photovoltaic parameters as shown in Fig. 5 further demonstrate the enhanced photovoltaic performance of the device with m-NiO HTL. For revealing the difference of charge carrier dynamics in the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL, the PL and TRPL spectra of the samples with structures of perovskite/m-NiO HTL/p-NiO HTL and perovskite/pNiO HTL are measured and shown in Fig. 6a and b. Comparing with the
(1)
Furthermore, the τavg is calculated with Ai and τi values as follows (Lv et al., 2018):
τavg =
∑ Ai τi2 ∑ Ai τi
(2)
It is found that the perovskite/m-NiO HTL/p-NiO HTL sample has a smaller τ1 than that of the perovskite/p-NiO HTL one (4.53 ns versus 7.92 ns). Because the fast extraction time is related with the dynamic of hole-extraction from perovskite to HTL, the smaller value indicates the more efficient hole-extraction in the sample (Baloch et al., 2018), which is according with the PL results as aforementioned. Nonetheless, τ2 of the sample is also greatly dropped from 65.12 ns to 37.32 ns after inserting m-NiO HTL between perovskite and p-NiO HTL. The smaller τ2 indicates the faster radiative recombination in the sample (Byranvand et al., 2018). However, A2 is also dropped from 27.85% to 20.11%, so it might not obviously influence the device performance. Additionally, τavg of the perovskite/m-NiO HTL/p-NiO HTL sample is 11.12 ns, much smaller than that of the perovskite/p-NiO HTL one (23.85 ns). Therefore, it can conclude that the perovskite/m-NiO HTL/p-NiO HTL sample shows a better charge carrier dynamic than that of the perovskite/pNiO HTL one. Dark J-V curves of the optimized inverted mesoscopic PSC with mNiO HTL and the inverted planar one with p-NiO HTL as shown in Fig. 6c are fitted with the following equations based on the equivalent
Fig. 5. Statistical distributions of the photovoltaic parameters for the optimized inverted mesoscopic PSCs with m-NiO HTLs and the inverted planar ones with p-NiO HTLs: distributions of Jsc (a), Voc (b), FF (c) and PCE (d). (All devices are tested with reverse scan and one batch of 20 devices is used for each condition). 115
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Fig. 6. PL (a) and TRPL (b) spectra of the perovskite/m-NiO HTL/p-NiO HTL and perovskite/p-NiO HTL films; dark I-V curves of the optimized inverted mesoscopic PSC with m-NiO HTL and the inverted planar one with p-NiO HTL (c).
general method for preparing many kinds of microspheres from welldispersed nanocrystals, so it will overcome the bottleneck of p-type material shortage for preparing efficient inverted mesoscopic PSCs.
Table 3 The TRPL parameters of τ1, τ2, A1, A2 and τavg of the perovskite/m-NiO HTL/pNiO HTL and perovskite/p-NiO HTL films. Sample
τavg/ns
τ1/ns
τ2/ns
A1/%
A2/%
perovskite/p-NiO HTL perovskite/m-NiO HTL/p-NiO HTL
23.85 11.12
7.92 4.53
65.12 37.32
72.15 79.89
27.85 20.11
Acknowledgements The authors would like to acknowledge the supports of the National Natural Science Foundation of China (Nos. 61474047, 51472094 and U1705256), the Natural Science Foundation of Fujian Province (2019J02012), the Fujian Provincial Youth Top-notch Talents Supporting Program, the Graphene Powder & Composite Research Center of Fujian Province (2017H2001) and the Cultivation Program for Postgraduate in Scientific Research Innovation Ability of Huaqiao University (No. 17013081044).
circuit of Shockley iodide in a single junction device to calculate the saturation current density (J0) and ideality factor (n), which are the major parameters contributing to the suppressed recombination (Wetzelaer et al., 2011).
qV J = J0 ⎡exp ⎛ − 1⎞ ⎤ ⎢ nkT ⎝ ⎠⎥ ⎦ ⎣
(3)
lnJ = lnJ0 + qV / nkT
(4)
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.09.064.
where V is the applied bias, q is the electron charge, k is the Boltzmann’s constant and T is the kelvin temperature. The calculated data reveal that the optimized inverted mesoscopic PSC with m-NiO HTL has a lower J0 (2.25 × 10−6 mA cm−2) than that of the inverted planar one with p-NiO HTL (4.31 × 10−5 mA cm−2), demonstrating significantly reduced recombination in the optimized inverted mesoscopic PSC with m-NiO HTL. Additionally, the device with m-NiO HTL also has a smaller n (2.25) compared to the inverted planar one with p-NiO HTL (2.52), indicating the decreased recombination loss (Cai et al., 2018). In addition, the transmission spectra of the p-NiO and m-NiO HTLs are measured and presented in Fig. S2 of Supplementary Information. It is seen that compared with the p-NiO HTL, the m-NiO HTL shows a slight decreased transmission. Because the perovskite layer is well penetrated into m-NiO HTL, so the slight decreased transmission of the neat m-NiO HTL will marginally influence light absorbance of the perovskite layer. Therefore, the improved charge transfer kinetics and the suppressed dark recombination mainly contribute to enhanced photovoltaic performance of the optimized inverted mesoscopic PSC with m-NiO HTL.
References Bai, F., Wang, D., Huo, Z., Chen, W., Liu, L., Liang, X., Chen, C., Wang, X., Peng, Q., Li, Y., 2007. A versatile bottom-up assembly approach to colloidal spheres from nanocrystals. Angew. Chem. Int. Edit. 46, 6650–6653. https://doi.org/10.1002/anie. 200701355. Baloch, A.A.B., Alharbi, F.H., Grancini, G., Hossain, M.I., Nazeeruddin, M.K., Tabet, N., 2018. Analysis of photocarrier dynamics at interfaces in perovskite solar cells by time-resolved photoluminescence. J. Phys. Chem. C 122, 26805–26815. https://doi. org/10.1021/acs.jpcc.8b07069. Burschka, J., Pellet, N., Moon, S.-J., Humphry-Baker, R., Gao, P., Nazeeruddin, M.K., Grätzel, M., 2013. Sequential deposition as a route to high-performance perovskitesensitized solar cells. Nature 499, 316. https://doi.org/10.1038/nature12340. Byranvand, M.M., Kim, T., Song, S., Kang, G., Ryu, S.U., Park, T., 2018. P-type CuI islands on TiO2 electron transport layer for a highly efficient planar-perovskite solar cell with negligible hysteresis. Adv. Energy Mater. 8, 1702235. https://doi.org/10.1002/ aenm.201702235. Cai, M., Ishida, N., Li, X., Yang, X., Noda, T., Wu, Y., Xie, F., Naito, H., Fujita, D., Han, L., 2018. Control of electrical potential distribution for high-performance perovskite solar cells. Joule 2, 296–306. https://doi.org/10.1016/j.joule.2017.11.015. Chen, Y., Yang, Z., Wang, S., Zheng, X., Wu, Y., Yuan, N., Zhang, W.-H., Liu, S., 2018. Design of an inorganic mesoporous hole-transporting layer for highly efficient and stable inverted perovskite solar cells. Adv. Mater. 30, 1805660. https://doi.org/10. 1002/adma.201805660. Deng, K., Tang, J., Lan, Z., 2019. Highly efficient inverted planar perovskite solar cells from TiO2 nanoparticles modified interfaces between NiO hole transport layers and conductive glasses. J. Mater. Sci: Mater. Electron. 30, 529–536. https://doi.org/10. 1007/s10854-018-0319-z. Du, Y., Xin, C., Huang, W., Shi, B., Ding, Y., Wei, C., Zhao, Y., Li, Y., Zhang, X., 2018. Polymeric surface modification of NiOx-based inverted planar perovskite solar cells with enhanced performance. ACS Sustain. Chem. Eng. 6, 16806–16812. https://doi. org/10.1021/acssuschemeng.8b04078. Huckaba, A.J., Lee, Y., Xia, R., Paek, S., Bassetto, V.C., Oveisi, E., Lesch, A., Kinge, S., Dyson, P.J., Girault, H., Nazeeruddin, M.K., 2019. Inkjet-printed mesoporous TiO2 and perovskite layers for high efficiency perovskite solar cells. Energy Tech. 7, 317–324. https://doi.org/10.1002/ente.201800905. Jeon, N.J., Na, H., Jung, E.H., Yang, T.-Y., Lee, Y.G., Kim, G., Shin, H.-W., Il Seok, S., Lee, J., Seo, J., 2018. A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells. Nat. Energy 3, 682–689. https://doi.org/10.1038/ s41560-018-0200-6. Jeon, N.J., Noh, J.H., Kim, Y.C., Yang, W.S., Ryu, S., Seok, S.I., 2014. Solvent engineering
4. Conclusion In summary, we use the emulsion-based bottom-up self-assembly strategy to synthesize NiO microspheres and successfully prepare high efficiency and nearly eliminated J-V hysteresis of the inverted mesoscopic PSCs with m-NiO HTLs. It is found that the m-NiO HTL can help to form larger perovskite crystal grains, more suitable graded energy alignment in the device, better charge carrier dynamics with higher PL quenching efficiency and faster hole-extraction, and reduced dark recombination with substantially lower J0 of the sample than the p-NiO HTL based one. These advantages contribute to obviously enhanced photovoltaic performance of the inverted mesoscopic PSCs compared with the inverted planar ones. The optimized inverted mesoscopic PSC with about 210 nm thick m-NiO HTL can yield a PCE of 18.17%, higher than that of the inverted planar one with p-NiO HTL (15.98%). Moreover, the emulsion-based bottom-up self-assembly strategy is a 116
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G. Li, et al.
lanthanum in a nickel oxide-based inverted perovskite solar cell for efficiency and stability improvement. ChemSusChem 12, 518–526. https://doi.org/10.1002/cssc. 201802231. Tidhar, Y., Edri, E., Weissman, H., Zohar, D., Hodes, G., Cahen, D., Rybtchinski, B., Kirmayer, S., 2014. Crystallization of methyl ammonium lead halide perovskites: implications for photovoltaic applications. J. Am. Chem. Soc. 136, 13249–13256. https://doi.org/10.1021/ja505556s. Trifiletti, V., Roiati, V., Colella, S., Giannuzzi, R., De Marco, L., Rizzo, A., Manca, M., Listorti, A., Gigli, G., 2015. NiO/MAPbI3-xClx/PCBM: a model case for an improved understanding of inverted mesoscopic solar cells. ACS Appl. Mater. Interfaces 7, 4283–4289. https://doi.org/10.1021/am508678p. Wang, T., Ding, D., Zheng, H., Wang, X., Wang, J., Liu, H., Shen, W., 2019. Efficient inverted planar perovskite solar cells using ultraviolet/ozone-treated NiOx as the hole transport layer. Sol. RRL 3, 1900045. https://doi.org/10.1002/solr.201900045. Wang, Y., Li, Y., Guo, Z., Liu, W., Zhang, R., Chu, L., Li, X.A., 2018. Ethanol addition for morphology regulation of TiO2 nanorod arrays towards efficient hole-conductor-free perovskite solar cells. Funct. Mater. Lett. 11, 1850080. https://doi.org/10.1142/ S1793604718500807. Wetzelaer, G.A.H., Kuik, M., Lenes, M., Blom, P.W.M., 2011. Origin of the dark-current ideality factor in polymer:fullerene bulk heterojunction solar cells. Appl. Phys. Lett. 99, 153506. https://doi.org/10.1063/1.3651752. Yang, I.S., You, J.S., Sung, S.D., Chung, C.W., Kim, J., Lee, W.I., 2016. Novel spherical TiO2 aggregates with diameter of 100nm for efficient mesoscopic perovskite solar cells. Nano Energy 20, 272–282. https://doi.org/10.1016/j.nanoen.2015.12.031. Yang, W.S., Park, B.-W., Jung, E.H., Jeon, N.J., Kim, Y.C., Lee, D.U., Shin, S.S., Seo, J., Kim, E.K., Noh, J.H., Seok, S.I., 2017. Iodide management in formamidinium-leadhalide–based perovskite layers for efficient solar cells. Science 356, 1376–1379. https://doi.org/10.1126/science.aan2301. Yao, K., Li, F., He, Q., Wang, X., Jiang, Y., Huang, H., Jen, A.K.Y., 2017. A copper-doped nickel oxide bilayer for enhancing efficiency and stability of hysteresis-free inverted mesoporous perovskite solar cells. Nano Energy 40, 155–162. https://doi.org/10. 1016/j.nanoen.2017.08.014. Zhang, X., Zhang, W., Wu, T., Wu, J., Lan, Z., 2019. High efficiency and negligible hysteresis planar perovskite solar cells based on NiO nanocrystals modified TiO2 electron transport layers. Sol. Energy 181, 293–300. https://doi.org/10.1016/j.solener.2019. 02.011. Zhou, Q., Liang, L., Hu, J., Cao, B., Yang, L., Wu, T., Li, X., Zhang, B., Gao, P., 2019. Highperformance perovskite solar cells with enhanced environmental stability based on a (p-FC6H4C2H4NH3)2[PbI4] capping layer. Adv. Energy Mater. 9, 1802595. https:// doi.org/10.1002/aenm.201802595.
for high-performance inorganic-organic hybrid perovskite solar cells. Nat. Mater. 13, 897. https://doi.org/10.1038/NMAT4014. Jeon, N.J., Noh, J.H., Yang, W.S., Kim, Y.C., Ryu, S., Seo, J., Seok, S.I., 2015. Compositional engineering of perovskite materials for high-performance solar cells. Nature 517, 476. https://doi.org/10.1038/nature14133. Jiang, Q., Zhao, Y., Zhang, X., Yang, X., Chen, Y., Chu, Z., Ye, Q., Li, X., Yin, Z., You, J., 2019. Surface passivation of perovskite film for efficient solar cells. Nat. Photonics. https://doi.org/10.1038/s41566-019-0398-2. Jung, K.H., Seo, J.Y., Lee, S., Shin, H., Park, N.G., 2017. Solution-processed SnO2 thin film for a hysteresis-free planar perovskite solar cell with a power conversion efficiency of 19.2%. J. Mater. Chem. A 5, 24790–24803. https://doi.org/10.1039/C7TA08040A. Kojima, A., Teshima, K., Shirai, Y., Miyasaka, T., 2009. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051. https://doi.org/10.1021/ja809598r. Lee, M.M., Teuscher, J., Miyasaka, T., Murakami, T.N., Snaith, H.J., 2012. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 338, 643–647. https://doi.org/10.1126/science.1228604. Li, X., Bi, D., Yi, C., Décoppet, J.-D., Luo, J., Zakeeruddin, S.M., Hagfeldt, A., Grätzel, M., 2016. A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells. Science 353, 58–62. https://doi.org/10.1126/science.aaf8060. Liu, D., Kelly, T.L., 2013. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat. Photonics 8, 133. https://doi.org/10.1038/nphoton.2013.342. Lv, Y., Cai, B., Wu, Y., Wang, S., Jiang, Q., Ma, Q., Liu, J., Zhang, W.-H., 2018. High performance perovskite solar cells using TiO2 nanospindles as ultrathin mesoporous layer. J. Energy Chem. 27, 951–956. https://doi.org/10.1016/j.jechem.2018.01.020. Pascoe, A.R., Meyer, S., Huang, W., Li, W., Benesperi, I., Duffy, N.W., Spiccia, L., Bach, U., Cheng, Y.-B., 2016. Enhancing the optoelectronic performance of perovskite solar cells via a textured CH3NH3PbI3 morphology. Adv. Funct. Mater. 26, 1278–1285. https://doi.org/10.1002/adfm.201504190. Shi, D., Adinolfi, V., Comin, R., Yuan, M., Alarousu, E., Buin, A., Chen, Y., Hoogland, S., Rothenberger, A., Katsiev, K., Losovyj, Y., Zhang, X., Dowben, P.A., Mohammed, O.F., Sargent, E.H., Bakr, O.M., 2015. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 347, 519–522. https://doi. org/10.1126/science.aaa2725. Song, J., Zhang, W., Wang, D., Deng, K., Wu, J., Lan, Z., 2019. Colloidal synthesis of Ydoped SnO2 nanocrystals for efficient and slight hysteresis planar perovskite solar cells. Sol. Energy 185, 508–515. https://doi.org/10.1016/j.solener.2019.04.084. Sun, J., Lu, J., Li, B., Jiang, L., Chesman, A.S.R., Scully, A.D., Gengenbach, T.R., Cheng, Y.-B., Jasieniak, J.J., 2018. Inverted perovskite solar cells with high fill-factors featuring chemical bath deposited mesoporous NiO hole transporting layers. Nano Energy 49, 163–171. https://doi.org/10.1016/j.nanoen.2018.04.026. Tang, J., Jiao, D., Zhang, L., Zhang, X., Xu, X., Yao, C., Wu, J., Lan, Z., 2018. Highperformance inverted planar perovskite solar cells based on efficient hole-transporting layers from well-crystalline NiO nanocrystals. Sol. Energy 161, 100–108. https://doi.org/10.1016/j.solener.2017.12.045. Teo, S., Guo, Z., Xu, Z., Zhang, C., Kamata, Y., Hayase, S., Ma, T., 2019. The role of
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https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20181214.pdf.
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