High-performance inverted planar perovskite solar cells based on efficient hole-transporting layers from well-crystalline NiO nanocrystals

Solar Energy 161 (2018) 100–108

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High-performance inverted planar perovskite solar cells based on efficient hole-transporting layers from well-crystalline NiO nanocrystals

T

⁎

Jie Tanga, Dian Jiaoa, Lei Zhanga, Xuezhen Zhanga, Xiaoxia Xua, Cong Yaob, , Jihuai Wua, ⁎ Zhang Lana, a b

College of Materials Science & Engineering, Institute of Materials Physical Chemistry, Huaqiao University, Xiamen 361021, China China Electronics Technology Group Corporation No. 18TH Research Institute, Tianjin 300384, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Inverted planar perovskite solar cells NiO nanocrystals Solution-processed method Solvothermal synthesis

NiO is an important hole-transporting material for preparing inverted planar perovskite solar cells (PSCs). Apart from some complicated preparation methodologies such as doping routes, pulsed laser deposition, atomic layer deposition and high temperature spray pyrolysis, the simpler solution-processed method can also achieve highquality NiO hole-transporting layers for efficient inverted planar PSCs. One of the prerequisites for solutionprocessed high-quality NiO hole-transporting layers is pre-synthesized highly crystalline NiO nanocrystals (NCs). Here, we use the sophisticated solvothermal method to synthesize highly crystalline NiO NCs. The oleylamine ligands are used to well control the nucleation and growth of NiO NCs and achieve good colloidal stability in toluene, which make for high-quality NiO hole-transporting layers by solution process. The related measurements and analysises reveal that the as-prepared NiO hole-transporting layer shows faster hole extraction, more effectively suppressed recombination and fewer surface trap states than the typical poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) hole-transporting layer, finally contributing to superior photovoltaic performance of the corresponding device. This work highlights the feasibility of colloidal chemical synthetic route for pre-synthesizing highly crystalline and well dispersed semiconductor NCs suitable for preparing efficient PSCs by the simple solution-processed protocol.

1. Introduction Perovskite solar cells (PSCs) have attracted enormous attention in the photovoltaic community due to the demonstrated high efficiency exceeding 20% and low-cost fabrication (Shin et al., 2017; Ye et al., 2017; Boix et al., 2015). PSCs with different structures including mesosuper structure, planar structure and inverted planar structure all can work efficiently if the photon-generated carriers can be successfully separated and collected by the charge (electron or hole)-transporting layers (Kim et al., 2012; Jiang et al., 2017; Wu et al., 2017; Wu et al., 2016a,b; Krishna et al., 2015). So the charge-transporting layers play key roles in determining the photovoltaic performance of PSCs. Scheme 1 shows the general structure of inverted planar PSCs. From front-side to back-side of the device, it consists of fluorine doped tin oxide conductive glass (FTO) substrate, hole-transporting layer (HTL), perovskite layer, electron-transporting layer and Au/Ag layer. (Heo et al., 2015) [6,6]-Phenyl C61 butyric acid methyl ester (PC61BM) is usually used to fabricate electron-transporting layer and 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP) is used as interface modified layer. (Shao

⁎

et al., 2016; Rao et al., 2016) Poly(3,4-ethylenedioxythiophene)/poly (styrenesulfonic acid) (PEDOT:PSS) is one of the most widely used materials for preparing HTL (Adam et al., 2016; Shahbazi et al., 2016; Wang et al., 2014). Yet due to the acidic and hygroscopic characteristics, PEDOT:PSS is not an ideal material for preparing long-term stable inverted planar PSCs (Choi et al., 2015). Therefore, many inorganic materials such as CuSCN (Ye et al., 2015; Xi et al., 2017), NiO (Yin et al., 2017; Liu et al., 2017c; Kwon et al., 2016), CuOx (Sun et al., 2016) have been utilized as substitutes for PEDOT:PSS. Among them NiO is the most successful one because it has a wide band gap (Eg > 3.50 eV), good optical transparency, appropriate energy levels for efficient hole transporting and electron blocking and good chemical stability (Zhu et al., 2014; Chen et al., 2015a; Yin et al., 2016). However, due to the critical shortcoming of high resistivity of NiO, it usually needs to be doped with other elements such as copper (Kim et al., 2015), magnesium and lithium (Chen et al., 2015b). The thickness of NiO HTLs should also be strictly controlled, which further needs some complicated preparation methodologies like pulsed laser deposition, atomic layer deposition and high temperature spray pyrolysis (Park

Corresponding authors. E-mail addresses: [email protected] (C. Yao), [email protected] (Z. Lan).

https://doi.org/10.1016/j.solener.2017.12.045 Received 19 July 2017; Received in revised form 11 November 2017; Accepted 22 December 2017 0038-092X/ © 2017 Elsevier Ltd. All rights reserved.

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because of the obvious advantages of NiO HTL in hole extraction, suppressing recombination and the intrinsic chemical and physical stability. 2. Experimental section 2.1. Materials All used reagents were purchased from Sigma-Aldrich Corp, unless specifically mentioned. PC61BM (99.5%) was supplied by Luminescence Technology Corp, Taiwan, China. BCP and methylammonium iodide (MAI) were supplied by Xi'an Polymer Light Technology Corp, China. PEDOT:PSS (P VP AI 4083) was bought from Clevios Corp. FTO glasses with sheet resistance of 15 Ω □−1 were purchased from Nippon Glass Corp (Japan) and used as substrates for preparing PSCs. 2.2. Preparation of NiO nanocrystals and inverted planar perovskite solar cells Scheme 1. Schematic illustration of the general structure of inverted planar PSCs.

NiO NCs were synthesized by a solvothermal method. Firstly, a solution containing 0.257 g nickel (II) acetylacetonate, 6 mL OAm and 10 mL toluene was prepared. Secondly, the solution was poured into a 50 mL autoclave and heated at 180 °C for 24 h. After natural cooling to room temperature, the solution was poured out and mixed with 20 mL ethanol to precipitate NiO NCs. The NiO NCs were collected after being centrifuged at a rate of 12,000 rpm for 10 min and then re-dispersed in toluene with concentration of 10 mg mL−1. FTOs (1.5 × 1.5 cm2) were etched by Zn powder and 2 M HCl solution to form the designed pattern and then consecutively washed with isopropanol, acetone, distilled water and ethanol. Before preparing inverted planar PSCs, these FTOs were treated with UV-ozone for 30 min. The as-prepared NiO NCs dispersed in toluene (10 mg mL−1) was used for preparing NiO HTLs on the patterned FTOs by spin-coating the dispersion at 4000 rpm for 30 s, soon afterwards heated at 500 °C for 30 min. For comparison, the PEDOT:PSS HTL was also prepared on the patterned FTO. The PEDOT:PSS solution was spin-coated on the patterned FTO at 4000 rpm for 30 s and dried at 150 °C for 10 min to form 40 nm thick PEDOT:PSS HTL (Kim et al., 2017). The thickness of the NiO NCs and PEDOT:PSS films were identified by the cross-sectional SEM images. MAPbI3 perovskite layers were deposited on the NiO and PEDOT:PSS HTLs with the typical anti-solvent methodology (Ahn et al., 2015). A 1.2 mol L−1 MAPbI3 precursor solution made of 1.66 g PbI2, 0.58 g MAI, 500 μL dimethylsulfoxide and 2500 μL N,N-dimethylformamide was prepared firstly. Then, 80 μL precursor solution was dripped on top of the NiO or PEDOT:PSS HTL and spin-coated at 1000 rpm for 10 s and then at 6500 rpm for 20 s. When the second-step spin-coating at 6500 rpm lasted for 5 s, the anti-solvent of chlorobenzene (500 μL) was dripped on the rotating substrate to rinse out residual DMSO and DMF in the precursor film. After thermal treatment at 100 °C for 10 min, the crystalline MAPbI3 perovskite layer was formed. A thin layer of PC61BM was spin-coated onto the MAPbI3 perovskite layer from a 20 mg mL−1 chlorobenzene solution at 1500 rpm for 45 s and dried at 70 °C for 10 min. After that, 100 μL saturated methanol solution of BCP was dripped on top of the PC61BM layer during spin-coating at 6000 rmp and then dried at 70 °C for 10 min again. Finally, an Au electrode about 100 nm was thermally evaporated on the BCP layer under high vacuum through a shadow mask.

et al., 2015; Seo et al., 2016; Wu et al., 2016a,b). Meanwhile, some researchers have been trying to use the simple solution-processed method to fabricate high-quality NiO HTLs, even at low temperature. (Lin et al., 2016; Bai et al., 2016) The typical examples reported by both Yin et al. and Zhang et al. confirmed that the pre-synthesized highly crystalline NiOx nanocrystals (NCs) were suitable for solution-processed high-quality NiOx HTLs for efficient inverted planar PSCs (Yin et al., 2016; Zhang et al., 2016a). They used a chemical precipitation method combining with high-temperature annealing process to synthesize crystalline NiOx NCs. Namely, the nickel salts were firstly reacted with NaOH and then the precipitates were annealed at 270 °C to improve crystallinity. The drawbacks of the preparing method are obvious. It is hard to exactly control the composition, size and shape of the NiOx precipitates by the chemical precipitation method. Furthermore, it is also difficult to avoid aggregation of NiOx NCs during 270 °C annealing process. Thanks to the great advancement of colloidal chemistry, NCs with highly crystalline, uniform shape, controlled size and composition can be synthesized through colloidal routes (Kwon and Hyeon, 2008; Zhang et al., 2016b; Zhou et al., 2016). One of the representative colloidal routes is solvothermal method, which stands among the most reliable, reproducible and simplest protocols available to form well-designed NCs (Lai et al., 2015). For example, Rajendran and Anandan used the simple solvothermal method for the preparation of large scale growth of spherical, hexagonal and rod-like NiO nanostructures, by using nickel acetate tetrahydrate and different ionic and non-ionic surfactants such as cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and poly ethylene glycol (PEG) in methanol (Rajendran and Anandan, 2015). Although the morphologies of NiO nanomaterials can be well controlled by the surfactants assisted solvothermal method, the sizes of NiO nanomaterials are too big to be suitable for preparing HTLs for inverted planar PSCs. Previously, we successfully synthesized wellcrystalline TiO2 quantum dots with size as small as 3.6 nm by using oleic acid assisted solvothermal method. The as-prepared TiO2 quantum dots can be well dispersed in toluene for preparing high quality blocking layers for dye and quantum dot sensitized solar cells (Que et al., 2014; Zhang et al., 2016b). Inspired by these works and taking into account of the actual experimental results, here, we report the synthesis of highly crystalline NiO NCs by the facile oleylamine (OAm) assisted solvothermal method. The OAm molecules are used as coordinating ligands not only for well controlling the nucleation and growth of NCs but also for achieving good colloidal stability in toluene solvent, making for solution-processed fabrication of high-quality NiO HTLs. The device based on NiO HTL shows superior photovoltaic performance and stability to the one based on the typical PEDOT:PSS HTL

2.3. Characterization The morphologies were observed by a JEM-2100 transmission electron microscopy (TEM) and a SU8000 field-emission scanning electron microscopy (SEM). The X-ray diffraction (XRD) patterns were recorded with a Bruker D8 Advance X-ray diffractometer using Cu Kα 101

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radiation (λ = 1.5418 Å). The transmittance spectra were measured by a Lamda 950 UV–Vis-NIR spectrophotometer. The steady-state photoluminescence (PL) spectra were acquired using a fluorescence spectrophotometer (Thermo Scientific Lumina). The time-resolved photoluminescence (TRPL) spectra were acquired using an Omin-λ Monochromator/Spectrograph with the time-correlated single-photon counting method (Zolix). The photovoltaic performance of PSCs were evaluated by the current density-voltage (J-V) characteristic curves, incident-photo-to-current conversion efficiency (IPCE) curves, stabilized current density and power output curves. Namely, the J-V curves of PSCs were recorded with 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) in air. The voltage step and delay time were 20 mV and 10 ms, respectively. The forward and reverse scans started from −0.1 V to 1.2 V and 1.2 V to −0.1 V, respectively. The stabilized current density and power output curves were recorded close to the maximum power point, which was extracted from the J-V curves. The IPCE curves were measured as a function of wavelength from 300 nm to 850 nm using the Newport IPCE system (Newport, USA). The PSCs with the active area of 0.12 cm2 (0.3 × 0.4 cm2) and without any encapsulation were prepared for measurements. The electrochemical impedance spectroscopy (EIS) measurements were conducted on a Zennium electrochemical workstation (IM6) in dark conditions with the frequencies from 100 mHz to 2 M Hz, the bias of 0 V and the amplitude of 20 mV.

NiO NCs before and after being annealed at 500 °C. As shown in Fig. 1a, the as-synthesized NiO NCs without thermal annealing treatment still can exhibit clear and distinct diffraction peaks at 37.25°, 43.29°, 62.85°, 75.40° and 79.37°, which can be indexed to the (1 0 1), (0 1 2), (1 1 0), (1 1 3) and (2 0 2) planes of hexagonal NiO (PDF standard cards, JCPDS 44-1159, space group R-3 m), respectively. After being annealed at 500 °C, the corresponding diffraction peaks become much stronger and sharper compared with the raw NiO NCs, indicating the increased crystallinity of the NiO NCs annealed at high temperature. The typical TEM and HRTEM images shown in Fig. 1b and c reveal that the particle size of NiO NCs is about 5 nm with clearly visible atomic lattice fringes, suggesting their high crystallinity feature. The SAED patterns shown in Fig. 1d further confirm the formation of well crystalline NiO NCs. Therefore, we can conclude that the well crystalline NiO NCs with definite size and shape can be synthesized by the solvothermal method. Fig. 2a shows the TEM image of as-prepared NiO NCs without annealing treatment under low magnification. It is clearly seen that the NiO NCs are well dispersed without any aggregations. Moreover, to investigate the influence of high temperature (500 °C) treatment on the dispersion of NiO NCs, the SEM image of NiO NCs coated FTO with 500 °C annealing treatment was measured and presented in Fig. 2b. It is also seen that the NiO NCs can homogeneously disperse in the film. So it can verify that the as-prepared NiO NCs with solvothermal method can maintain homogeneous dispersion without any aggregations before and after high temperature annealing treatment. Fig. 3a presents the optical transmittance spectra of FTO, different thickness of NiO HTLs coated FTOs and 40 nm thick PEDOT:PSS coated FTO in the visible light region from 400 to 800 nm. It is seen that the NiO HTLs coated FTOs show decreased transmittance compared with that of bare FTO, whereas the variations of transmission are very marginal (within 5%). The transmittance of NiO films is comparable with that of 40 nm thick PEDOT:PSS film in the wavelength region from

3. Results and discussion The crystallinity and morphology of NiO NCs were characterized with XRD, TEM, high resolution TEM (HRTEM) and selected-area electron diffraction (SAED) patterns. Fig. 1a shows the XRD patterns of

Fig. 1. (a) XRD patterns of NiO NCs before and after being annealed at 500 °C; (b) TEM image, (c) HRTEM image and (d) SAED patterns of as-synthesized NiO NCs without annealing treatment dispersed in toluene.

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Fig. 2. (a) TEM image of as-prepared NiO NCs without annealing treatment; (b) SEM image of NiO NCs coated FTO with 500 °C annealing treatment.

400 to 600 nm; and that the NiO films show better transparency than the PEDOT:PSS film in the longer wavelength region from 600 to 800 nm. In order to discriminate the transmittance differences more clearly, the normalized data with the baseline of bare FTO are shown in Fig. 3b. One can observe that by going with the increased thickness of NiO HTLs from 30 to 100 nm, the transmittance of the samples decreases slightly; and further increasing the thickness of NiO HTLs from 100 to 170 nm results in a little larger decreased margin over 1.2%. Nevertheless, the 170 nm NiO HTL coated FTO still has high transmittance over 95% versus the bare FTO, whereas, the 40 nm thick

PEDOT:PSS HTL coated FTO shows obviously decreased transmittance (lower than 95% versus the bare FTO) in the long wavelength range from 635 to 800 nm. The excellent transparency of the as-prepared NiO HTLs is highly desirable for high-performance inverted planar PSCs because it allows maximum photon flux to arrive to the perovskite absorber layer. Fig. 3c shows the typical J-V curves of corresponding devices with different thickness of NiO HTLs. The photovoltaic parameters including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), PCE, series resistance (Rs) and shunt resistance (Rsh) are

Fig. 3. (a) Transmittance of FTO, different thickness of NiO HTLs coated FTOs and 40 nm thick PEDOT:PSS coated FTO; (b) Normalized transmittance of different thickness of NiO HTLs coated FTOs and 40 nm thick PEDOT:PSS coated FTO with the baseline of bare FTO; (c) J-V curves of the inverted planar PSCs with different thickness of NiO HTLs; (d) Cross-sectional SEM image of the as-prepared optimized inverted planar PSCs.

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photovoltaic performance of our device with the ones based one the same commercial PEDOT:PSS (P VP AI 4083) (Table S1, supporting information) reported by other research groups. It is confirmed that the performance of our PEDOT:PSS based device is comparable with the those reported in the references (Huang et al., 2017; Liu et al., 2017a, 2017b; Sun et al., 2017). The corresponding IPCE spectra are shown in Fig. 4b, where integral current densities as a function of wavelength are also presented. Obviously, the IPCE of the device based on NiO HTL is higher than that of the one based on PEDOT:PSS HTL in the whole photoelectric response wavelength region. Accordingly, the integral Jsc of the device based on NiO HTL is higher than its counterpart, which is also consistent with the values from J-V curves (within a 5% deviation). To estimate the photovoltaic performance of the devices more accurately, the steady-state power output measurements were done by prebiasing the devices at their maximum power point under AM 1.5 G illumination (Snaith et al., 2014). As shown in Fig. 4c, the steady-state current density and PCE of the device based on NiO HTL can stabilize at 16.87 mA cm−2 and 15.14% for 300 s, respectively, indicating well stable performance; and these parameters of the device based on PEDOT:PSS HTL can also stabilize at lower values of 13.16 mA cm−2 and 10.69% for 300 s, respectively. Moreover, the PCE histogram along with the Gaussian fitting curves for one batch of 30 devices based on NiO and PEDOT:PSS HTLs, respectively, are displayed in Fig. 4d in order to compare the reproducibility of the devices. It is seen that the PCE distribution range of the devices based on NiO HTLs is smaller than that of the devices based on PEDOT:PSS HTLs. The average PCE of the former devices is 13.75 ± 1.02% whereas that of the latter ones is 8.51 ± 1.51%. So the reproducibility of the devices based on NiO HTLs is better than the ones based on PEDOT:PSS HTLs. To reveal the hole extraction mechanism, steady-state PL and transient TRPL measurements were performed for MAPbI3 films on different substrates including bare FTO, PEDOT:PSS HTL coated FTO and NiO HTL coated FTO. From the steady-state PL spectra shown in Fig. 5a, one can observe that the insertion of both PEDOT and NiO HTLs can induce obviously decreased PL intensity compared to the bare FTO sample, suggesting that the PEDOT:PSS and NiO HTLs are more efficient in hole extraction than the bare FTO. Furthermore, it also can be seen that the PL quenching efficiency of the NiO HTL is higher than PEDOT:PSS HTL, demonstrating that the as-prepared NiO HTL is an effective hole extractor. The transient TRPL spectra shown in Fig. 5b further verify the superior hole extraction property of the NiO HTL. The TRPL decay curves are fitted by a bi-exponential equation [Eq. (1)] (Yang et al., 2016a):

Table 1 The J-V key parameters of inverted planar PSCs with different thickness of NiO HTLs. NiO thickness/ nm

VOC/V

JSC/mA·cm−2

FF/%

PCE/%

Rs/Ω cm2

Rsh/Ω cm2

30 55 70 100 170

1.02 1.06 1.03 1.00 0.96

17.23 19.41 18.61 18.66 18.05

66.96 75.02 70.03 66.22 52.69

11.82 15.47 13.47 12.35 9.13

22.3 28.3 34.1 36.8 87.5

2228.4 5634.2 4537.9 3786.2 2447.2

summarized in Table 1. It is found that the thickness of NiO HTLs has significant influence on the photovoltaic performance of the devices. The best-performance device can be fabricated by adjusting the thickness of NiO HTL to 55 nm, which is different to the ones reported in the references because of the different size, shape and crystallinity of NiO NCs and the used different fabricating protocols of NiO HTLs (Bai et al., 2016; Chen et al., 2015a; Chen et al., 2015b; Kim et al., 2015; Kwon et al., 2016; Liu et al., 2017c; Park et al., 2015; Seo et al., 2016.) This champion device exhibits a high PCE of 15.47%, with a Jsc of 19.41 mA cm−2, a FF of 75.02% and a Voc of 1.06 V. Other devices with thinner or thicker NiO HTLs all show poorer performance. As shown in Fig. 3a and b, the as-prepared NiO HTLs have good transparency, which will feebly affect the performance of PSCs. Due to the critical shortcoming of high resistivity of NiO (Manders et al., 2013), the changed thickness of NiO HTLs will influence the hole extracting and transporting speeds and charge recombination rate (Li et al., 2015a). However, a suitable thickness of NiO HTL is also needed to fully cover the rough surface of FTO for efficient electron blocking and reducing leakage current (Zhang et al., 2016b). The changed Rs and Rsh values well reflect these features. The Rs, which expresses the integral conductivity of the device and directly relates to internal carrier mobility in the device (Wolf and Rauschenbach, 1963), increases gradually with the increased thickness of NiO HTLs. Whereas, the Rsh, which refers to the loss of photocurrent through carrier recombination within the interfaces of each layer in the device (Bouzidi et al., 2007), has a maximum value in the champion device, indicating the lowest loss of photocurrent by the balance of the aforementioned factors. To well illustrate the structure of as-prepared inverted planar PSC, a representative cross-sectional SEM image of the device is shown in Fig. 3d. The main composed layers of NiO, perovskite, PC61BM/BCP and Au can be distinctly observed from the figure and the thicknesses of these layers are about 55 nm, 370 nm, 80 nm and 100 nm, respectively. For comparison, the device based on PEDOT:PSS HTL was also prepared with the same method as the devices based on NiO HTLs; and the photovoltaic properties of the devices based on NiO and PEDOT:PSS HTLs are shown in Fig. 4. The J-V curves measured by forward (from Jsc to Voc) and reverse (from Voc to Jsc) scans are shown in Fig. 4a and the photovoltaic parameters are listed in Table 2. It is seen that the device based on NiO HTL achieves a little higher PCE of 15.47% with a Voc of 1.06 V, a Jsc of 19.41 mA cm−2 and a FF of 75.02% by forward scan than that of the value of 14.72% with a Voc of 1.06 V, a Jsc of 19.44 mA cm−2 and a FF of 71.74% by reverse scan. And the device based on PEDOT:PSS HTL also achieves a little higher PCE of 11.42% with a Voc of 0.97 V, a Jsc of 16.47 mA cm−2 and a FF of 71.83% by forward scan than that of the value of 10.22% with a Voc of 0.96 V, a Jsc of 16.00 mA cm−2 and a FF of 66.41% by reverse scan. The poorer photovoltaic performance of the devices by reverse scan versus forward scan is mainly originated from the lower FF values caused by the lower Rsh values, which result in more serious charge recombination in the devices (Wu et al., 2015). Furthermore, the data also reveal that the main photovoltaic parameters including PCE, Voc, Jsc and FF of the device based on NiO HTL are all higher than its counterpart based on PEDOT:PSS HTL, indicating superior performance of the as-prepared NiO HTL to the traditionally used PEDOT:PSS HTL. To identify the quality of as-prepared PEDOT:PSS based device, we also compare the

I (t ) =

∑

Ai exp(−t / τi )

i

(1)

where Ai is the decay amplitude and τi is the decay time. The fitted results are listed in Table 3. For the FTO/MAPbI3 sample, the PL decay times are τ1 = 87.57 ns with amplitude of 26.52% and τ2 = 8.51 ns with amplitude of 73.48%. After inserting PEDOT:PSS HTL, the FTO/ PEDOT:PSS/MAPbI3 sample shows lower decay times of τ1 = 81.21 ns with amplitude of 24.83% and τ2 = 8.73 ns with amplitude of 75.17%; and that substituting PEDOT:PSS HTL with NiO HTL, the FTO/NiO/ MAPbI3 sample presents the lowest decay times of τ1 = 68.51 ns with amplitude of 20.94% and τ2 = 7.54 ns with amplitude of 79.06%. The average PL decay times (τave) can be calculated with the fitted Ai and τi values according to the following equation [Eq. (2)] (Yang et al., 2016b):

τave =

∑ Ai τi2 ∑ Ai τi

(2)

The calculated data show that the FTO/NiO/MAPbI3 sample has the lowest τave of 20.31 ns versus those of FTO/MAPbI3 (τave = 29.48 ns) and FTO/PEDOT:PSS/MAPbI3 (τave = 26.73 ns) samples, which are consistent with the steady-state PL results. 104

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Fig. 4. Photovoltaic performance of the optimized inverted planar PSCs with NiO and PEDOT HTLs: (a) J-V curves measured by forward and reverse scans, (b) IPCE and the integrated Jsc curves, (c) stabilized current density and power output (PCE) measured with a bias close to the maximum power point of the corresponding J-V curves, (d) PCE histograms along with the Gaussian fitting curves (from one batch of 30 devices).

the fitted slope of the devices based on NiO and PEDOT:PSS HTLs is 1.47 kT/q and 1.83 kT/q, respectively, which indicates that the interfacial trap-assisted SRH recombination is involved in the devices based on both NiO and PEDOT:PSS HTLs; and that the NiO HTL can more effectively reduce SRH recombination than the PEDOT:PSS HTL. In order to further demonstrate the superior effect of the NiO HTL for suppressing recombination in the device, the dark J-V curves of the devices based on NiO and PEDOT:PSS HTLs are shown in Fig. 6b for comparison. Clearly, the device based on NiO HTL shows a relatively lower leakage current than the device based on PEDOT:PSS HTL. Because of the diode-like behavior of the devices, the saturation current density (J0) and ideality factor (n), which are the major parameters contributing to the suppressed recombination, can be extracted from the dark J-V curves based on the equivalent circuit of Shockley iodide in a single junction device and by the following equations [Eqs. (3) and (4)] (Wetzelaer et al., 2011):

Table 2 The J-V key parameters of the inverted planar PSCs with 55 nm thick NiO HTL and 40 nm thick PEDOT HTL. Inverted planar PSCs

VOC/V

JSC/mA cm−2

FF/%

PCE/%

Rs/Ω cm2

Rsh/Ω cm2

NiO (Forward Scan) NiO (Reverse Scan) PEDOT:PSS (Forward Scan) PEDOT:PSS (Reverse Scan)

1.06

19.41

75.02

15.47

28.3

5634.2

1.06

19.44

71.74

14.72

27.9

2692.8

0.97

16.47

71.83

11.42

22.9

3030.9

0.96

16.00

66.41

10.22

25.8

2485.7

To understand the recombination mechanisms in the devices based on NiO and PEDOT:PSS HTLs, the dependence of Voc upon light intensity is studied because all of the photogenerated charge carriers in the MAPbI3 layer will eventually recombine within the device under open-circuit conditions (Zhang et al., 2016a; He et al., 2015). By linearly fitting Voc data versus natural log-scaled light intensity, one can get a slope of the fitting curve. Through comparing the fitted slope with kT/ q (where k is the Boltzmann’s constant, T is the kelvin temperature, and q is the electron charge), the dominant recombination mechanism in the device can be probed. Namely, if the slope is close to kT/q, the bimolecular recombination (the recombination of free electrons and holes in the photoactive layer) is dominant; whereas if the slope is higher than kT/q, the interfacial trap-assisted Shockly-Read-Hall (SRH) recombination will involved (Zhang et al., 2017). As shown in Fig. 6a,

qV J = J0 ⎡exp ⎛ −1⎞ ⎤ ⎢ ⎝ nkT ⎠ ⎥ ⎦ ⎣

(3)

lnJ = lnJ0 + qV / nkT

(4)

where V is the applied bias, J0 is the saturation current density, n is the ideality factor, q is the electron charge, k is the Boltzmann’s constant and T is the kelvin temperature. The lower J0 (3.95 × 10−6 mA cm−2) of the device based on NiO HTL than that of the one based on PEDOT:PSS HTL (J0 = 8.03 × 10−5 mA cm−2) verifies reduced recombination in the former device. Moreover, the smaller n (1.11) of the device based on NiO HTL compared to its counterpart based on PEDOT:PSS HTL (n = 1.61) also conforms the superior holeselectivity of the as-prepared NiO HTL, which also contributes to the 105

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Fig. 5. (a) Steady-state PL spectra and (b) transient TRPL decay curves of FTO/MAPbI3, FTO/ PEDOT:PSS/MAPbI3 and FTO/NiO/MAPbI3.

Table 3 The TRPL decay time, average decay time and the decay amplitude of the FTO/MAPbI3, FTO/PEDOT:PSS/MAPbI3 and FTO/NiO/MAPbI3 samples. Samples

τave/ns

τ1/ns

τ2/ns

A1/%

A2/%

FTO/MAPbI3 FTO/PEDOT:PSS/MAPbI3 FTO/NiO/MAPbI3

29.48 26.73 20.31

87.57 81.21 68.51

8.51 8.73 7.54

26.52 24.83 20.94

73.48 75.17 79.06

decreased charge recombination loss at the interface of NiO/MAPbI3. The surface trap states of hole transport layers also have important influences on the photovoltaic performance of PSCs because they can delocalize charge carriers to induce high capacitance at the interface (Li et al., 2015b). The EIS measurements were performed on the relevant devices at 1 kHZ in dark, and then the capacitance-voltage characteristics were extracted and shown in Fig. 6c. Under this test conditions, obviously changed capacitance occurs with increasing bias voltage, indicating charge accumulation at the interface (Wang et al., 2017). Compared with the data of the device based on PEDOT:PSS HTL, the capacitances of the device based on NiO HTL are clearly smaller, indicating the fewer surface trap states in NiO HTL than PEDOT:PSS HTL. Therefore, as expected, the advantages of faster hole extraction, more effectively suppressed recombination and fewer surface trap states of the NiO HTL contribute to superior photovoltaic performance of the corresponding device to the one based on PEDOT:PSS HTL. Additionally, the stability of the devices based on NiO and PEDOT:PSS HTLs were also checked through recording the changed PCE values of the devices stored in a high humidity (with relative humidity about 60–80%) ambient environment at 25 °C without any encapsulation. As shown in Fig. 7, the device based on NiO HTL shows better stability than the device based on PEDOT:PSS HTL. For details, the former one can maintain 83% of the initial PCE after 72 h, whereas the latter one

Fig. 7. Normalized PCE values of the devices (versus the original ones at 0 h) based on NiO and PEDOT:PSS HTLs stored in a high humidity (with relative humidity about 60–80%) ambient environment at 25 °C without any encapsulation.

suffers serious decline of the PCE to 15% of the initial value in the same duration. Further extending the storing time to 96 h, the former device still keeps 65% of the initial PCE, nevertheless, the PCE of the latter one nearly drops to zero. These results are comparable to the results reported by Zhang et al. (2016a). Their NiO based device also achieves good stability with 80% of initial PCE after 100 h storing time, whereas their PEDOT:PSS based device shows obviously decreased PCE and after 70 h storing time the value drops to zero. As already demonstrated, the acidic and hygroscopic natures of PEDOT:PSS are the key factors inducing poor stability of the corresponding device (Choi et al., 2015), whereas the intrinsic good chemical and physical stability of NiO HTL can result in preferable stability of the relevant device.

Fig. 6. (a) Voc dependence upon light intensity (dots represent experimental data and dashed lines represent fitting results); (b) Dark J-V curves and (c) capacitance-voltage characteristics of the devices based on NiO and PEDOT:PSS HTLs.

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4. Conclusion

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In conclusion, we have demonstrated that high-quality NiO HTLs can be prepared with solution-processed method using well dispersed toluene solution of highly crystalline NiO NCs synthesized by solvothermal method. The NiO HTL shows good transparency, allowing minimum loss of photon flux. Importantly, compared with the typical PEDOT:PSS HTL, the as-prepared NiO HTL shows faster hole extraction, more effectively suppressed recombination and fewer surface trap states. Moreover, the chemical and physical stability of NiO is much better than that of PEDOT:PSS, which has harmful defects of acidity and hygroscopicity. Therefore, the device based on NiO HTL shows superior photovoltaic performance, reproducibility and long-time stability to its counterpart based on PEDOT:PSS HTL. The study opens a feasible way of solution-processed high quality inorganic charge-transporting layers for efficient PSCs through colloidal chemical synthetic route for presynthesizing highly crystalline and well dispersed semiconductor NCs. Acknowledgment The authors would like to acknowledge the supports of the National Natural Science Foundation of China (Nos. 61474047, 51002053, and 51472094), the Fujian Provincial Science Foundation for Distinguished Young Scholars (2015J06011), the Fujian Provincial Youth Top-notch Talents Supporting Program, the Prominent Young Talents and New Century Excellent Talents Supporting Programs in Fujian Provincial University, and the Promotion Program for Yong and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-YX102). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.solener.2017.12.045. References Adam, G., Kaltenbrunner, M., Glowacki, E.D., Apaydin, D.H., White, M.S., Heilbrunner, H., Tombe, S., Stadler, P., Ernecker, B., Klampfl, C.W., Sariciftci, N.S., Scharber, M.C., 2016. Solution processed perovskite solar cells using highly conductive PEDOT:PSS interfacial layer. Sol. Energy Mater. Sol. Cells 157, 318–325. . Ahn, N., Son, D.Y., Jang, I.H., Kang, S.M., Choi, M., Park, N.G., 2015. Highly reproducible perovskite solar cells with average efficiency of 18.3% and best efficiency of 19.7% fabricated via Lewis base adduct of lead(II) iodide. J. Am. Chem. Soc. 137, 8696–8699. . Bai, Y., Chen, H.N., Xiao, S., Xue, Q.F., Zhang, T., Zhu, Z.L., Li, Q., Hu, C., Yang, Y., Hu, Z.C., Huang, F., Wong, K.S., Yip, H.L., Yang, S.H., 2016. Effects of a molecular monolayer modification of NiO nanocrystal layer surfaces on perovskite crystallization and interface contact toward faster hole extraction and higher photovoltaic performance. Adv. Funct. Mater. 26, 2950–2958. . Boix, P.P., Agarwala, S., Koh, T.M., Mathews, N., Mhaisalkar, S.G., 2015. Perovskite solar cells: beyond methylammonium lead iodide. J. Phys. Chem. Lett. 6, 898–907. . Bouzidi, K., Chegaar, M., Bouhemadou, A., 2007. Solar cells parameters evaluation considering the series and shunt resistance. Sol. Energy Mater. Sol. Cells 91, 1647–1651. . Chen, W., Wu, Y.Z., Liu, J., Qin, C.J., Yang, X.D., Islam, A., Cheng, Y.B., Han, L.Y., 2015a. Hybrid interfacial layer leads to solid performance improvement of inverted perovskite solar cells. Energy Environ. Sci. 8, 629–640. . Chen, W., Wu, Y.Z., Yue, Y.F., Liu, J., Zhang, W.J., Yang, X.D., Chen, H., Bi, E.B., Ashraful, I., Gratzel, M., Han, L.Y., 2015b. Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers. Science 350, 944–948. . Choi, H., Mai, C.K., Kim, H.B., Jeong, J., Song, S., Bazan, G.C., Kim, J.Y., Heeger, A.J., 2015. Conjugated polyelectrolyte hole transport layer for inverted-type perovskite solar cells. Nat. Commun. 6, 7348. . He, Z.C., Xiao, B., Liu, F., Wu, H.B., Yang, Y.L., Xiao, S., Wang, C., Russell, T.P., Cao, Y., 2015. Single-junction polymer solar cells with high efficiency and photovoltage. Nat. Photon. 9, 174–179.
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