Effect of rubrene:P3HT bilayer on photovoltaic performance of perovskite solar cells with electrodeposited ZnO nanorods

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Effect of rubrene:P3HT bilayer on photovoltaic performance of perovskite solar cells with electrodeposited ZnO nanorods Q1

Christian Mark Pelicano∗, Hisao Yanagi

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Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan

a r t i c l e

i n f o

Article history: Received 23 September 2017 Revised 21 November 2017 Accepted 21 November 2017 Available online xxx Keywords: Rubrene:P3HT bilayer Electrodeposited ZnO nanorods Perovskite solar cells

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

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a b s t r a c t Improved photovoltaic performance of perovskite solar cells is demonstrated through the synergistic effect of electrodeposited ZnO nanorods and rubrene:P3HT bilayer as electron and hole-transporting layers, respectively. Highly crystalline ZnO nanorods were obtained by electrochemical deposition in a chloride medium. Additionally, rubrene interlayer was able to passivate or cover the grain boundaries of perovskite film effectively that led to reduced leakage current. A perovskite solar cell optimized with ZnO nanorods and rubrene:P3HT bilayer achieved a maximum efficiency of 4.9% showing reduced hysteresis behavior compared with the device having P3HT as the only hole-transporting layer. The application of longer nanorods led to better perovskite infiltration and shorter charge carrier path length. These results highlight the potential of electrodeposited ZnO nanorods and rubrene:P3HT bilayer as charge selective layers for efficient perovskite solar cells. © 2017 Published by Elsevier B.V. and Science Press.

1. Introduction Organic–inorganic metal halide perovskite compounds have recently drawn tremendous research attention for their potential application in optoelectronics, specifically in the field of photovoltaics. In less than a decade of research, perovskite solar cells have demonstrated a remarkable increase in power conversion efficiency (PCE) that exceeded 22% through crystal growth control and interface engineering [1, 2]. These superior recorded efficiencies can be mainly associated with the unique chemical and physical properties of lead-halide perovskite materials including high light absorption coefficient [3], high charge carrier mobilities [4], small excitonic binding energy [5], long charge diffusion length [6], and tunable band gap [7]. In addition, this type of solar cells can be fabricated at low temperature by simple solution processing which makes it suitable for large-scale production. Most highly efficient perovskite solar cells employ nanostructured or mesoporous TiO2 scaffold as the electron selective layer [8,9]. The main reasons for the utilization of TiO2 are good band alignment with the perovskite materials, wide band gap of 3.2 eV and excellent hole blocking capability due to its deep valence band position [10,11]. However, the low electron mobility of TiO2 triggers insufficient charge separation at the electron-transporting layer and perovskite interface [12]. High temperature sintering in the range of 450−550°C is also

∗

Corresponding author. E-mail address: [email protected] (C.M. Pelicano).

necessary to synthesize such complex TiO2 structures, which render them incompatible for production of flexible solar cell devices [13]. Therefore, it is necessary to explore new alternative materials and structures that are appropriate for high performance flexible solar cells. In this respect, ZnO seems to be a potential replacement having a similar wide band gap of 3.37 eV suitable for transparent electron transport layer and higher electron mobility than TiO2 [14,15]. High quality ZnO nanostructures can also be fabricated at low temperatures using various solution techniques [16–20]. Balela et al. have reported a highly efficient perovskite solar cell with a PCE of 15.7% based on ZnO nanoparticles prepared by spin coating [20]. Another study demonstrated that high aspect ratio N-doped ZnO nanorods produced by hydrothermal method could be utilized as electron-transporting layer in perovskite solar cells, which resulted in a PCE of 16.1% [21]. On the other hand, several materials have been designed as candidates for hole-transporting layer in perovskite solar cells such as NiO, CuSCN [22,23], polymers (P3HT, PEDOT:PSS) [24,25], and small molecules (Spiro-MeOTAD, TPB) [26,27]. Most efficient perovskite solar cells are based on Spiro-MeOTAD owing to its matched energy level alignment with perovskite interface, minimizing the energy loss from charge carrier recombination [28,29]. However, the complex and costly production of Spiro-MeOTAD, as well as, its instability in the presence of additives have driven researchers to explore more stable, cheaper but viable hole-transporting materials [30]. Herein, ZnO nanorods were grown by electrochemical deposition in a chloride medium. This technique is very attractive for solar cell application due to its low temperature operation, simplicity,

https://doi.org/10.1016/j.jechem.2017.11.018 2095-4956/© 2017 Published by Elsevier B.V. and Science Press.

Please cite this article as: C.M. Pelicano, H. Yanagi, Effect of rubrene:P3HT bilayer on photovoltaic performance of perovskite solar cells with electrodeposited ZnO nanorods, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.018

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cost-effectiveness and potential for large-scale production. Moreover, we introduced rubrene in a regular type perovskite solar cell as an interlayer between CH3 NH3 PbI3 and P3HT films. Rubrene is a classical p-type organic semiconductor with high hole mobility and chemical stability. It has been widely used in organic light emitting diodes and thin film transistors [31,32]. In this study, we adopted the rubrene interlayer for the first time in a perovskite solar cell based on electrodeposited ZnO nanorods.

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2. Experimental

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2.1. Electrochemical deposition of ZnO nanorods

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The indium tin oxide (ITO)-coated glass substrates (15  sq−1 , Luminescence tech. Corp.) were cleaned through sequential ultrasonic treatments with detergent, ultrapure water, acetone and methanol for 15 min each. Prior to electrodeposition, a 0.25 M zinc acetate (Zn(CH3 COO)2 , Wako) in methanol:water (10:1) solution was spin-coated onto UV–O3 -treated ITO-glass substrate at 1500 rpm for 30 s. After drying at 100°C for 15 min, the films were then annealed at 350°C for 20 min to form ZnO compact layer [34]. A three-electrode electrochemical configuration was employed for the synthesis of ZnO nanorods [35]. The seeded ITO-glass substrate acted as the working electrode, a zinc wire operated as the counter electrode, and saturated calomel electrode (SCE) worked as the reference electrode. The substrates were fixed to a rotating electrode with a constant rotation speed of 100 rotations per minute (rpm). The deposition bath was maintained at 70°C containing 0.05 M KCl (99.5%, Nacalai Tesque) and 5 mM ZnCl2 (98%, Nacalai Tesque). The electrolyte was saturated with pure O2 by bubbling through a glass frit before and during the growth process. The effective area of the electrodeposited surface was about 1.54 cm2 . Electrodeposition was performed at a constant applied voltage of −1.0 V/SCE for 10−20 min using a Hokuto Denko HSV110 potentiostat. After deposition, the samples were immediately rinsed with deionized water and annealed at 350°C for 1 h. Finally, the samples were transferred to a nitrogen-filled glove box for perovskite deposition.

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2.2. Fabrication of perovskite solar cells

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The CH3 NH3 PbI3 layer was deposited on top of ZnO nanostructures using fast deposition crystallization as reported in the literature [36]. The perovskite precursor solution (45 wt%) was prepared by mixing 1:1 mol ratio of PbI2 (99.999%, Wako) and CH3 NH3 I (98.0%, Wako), respectively, in 1 mL of anhydrous N,Ndimethylformamide (DMF, Wako). Then, a 60 μL of this perovskite precursor solution was spin coated on top of electrodeposited ZnO nanorods for 30 s. The substrates were spun at 50 0 0 rpm and after 8 s, 50 μL of toluene was quickly dropped onto the center of the substrate and followed by thermal annealing at 70°C for 10 min. After cooling at room temperature, 60 μL of rubrene in toluene solution (5 mg mL−1 ) was spin-coated on top of the perovskite layer and the films were placed in a vacuum chamber for 15 min. Subsequently, P3HT (Sigma Aldrich) in chlorobenzene solution (15 mg mL−1 , Wako) was spin-coated at 1500 rpm for 120 s.For complete drying of P3HT layer, the samples were then stored in nitrogen-filled glove box for 12 h in dark. Finally, 50 nm Ag electrodes were deposited by thermal evaporation under a base pressure of 2 × 10−4 Pa. The active area of the devices is 0.09 cm2 . The solar cell devices were completed by putting silver paste in the electrode areas and stored again in vacuum to dry up before measurements.

Fig. 1. Variation of current density (lines) as a function of electrodeposition time and total electrical charge density exchanged (square dots) for two deposition potentials.

2.3. Solar cell and materials characterization

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The top-view and cross-sectional images were taken by a lowvacuum scanning electron microscope (SEM, Hitachi SU6600). The focused ion beam-assisted (Hitachi FB2200) cross-sectional SEM image and elemental EDX mapping analysis were obtained through ultra-high-resolution field emission scanning electron microscope (UHR FE-SEM, Hitachi SU9900). Atomic force microscopy (AFM) images were taken using a scanning probe microscope (Seiko SPA-400). Raman scattering measurement was performed at room temperature using a JASCO NRS-4100 Raman Spectrometer. The XRD patterns were characterized by a Rigaku X-ray diffractome˚ The optical ter (RINT-TTR III) with CuKα radiation (λ = 1.542 A). transmission and absorption spectra of the films were obtained on a UV–vis spectrophotometer (JASCO V-530). Photoluminescence spectra were measured under ultraviolet excitation (λex = 365 nm) using a high-pressure mercury-vapour light source (Olympus BH2RFL-T3) coupled with a microscope (Olympus BX51) and a CCD spectrometer (Hamamatsu PMA-12). Current-voltage (J-V) curves were recorded from a Keithley 2611B System Source Meter unit under AM 1.5 G illumination (100 mW cm−2 , Bunko–Keiki, CEP20 0 0RP). The external quantum efficiency (EQE) spectra were obtained under illumination of monochromatic light using the same system at an intensity of 1.25 mW cm−2 .

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3. Results and discussion 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110

O2(g) + 2H2 O(l) + 4e → 4OH

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First, a compact layer of ZnO nanocrystals with a mean thickness of 30 nm was deposited by spin coating on cleaned indium tin oxide (ITO)-coated glass substrates. These nanocrystals serve as a compact hole-blocking layer and at the same time as nucleation sites for nanorod growth. The ZnO nanorods for electron extraction were grown on the seeded substrates by electrochemical deposition for 10−20 min using 0.05 M KCl and 5 mM ZnCl2 with an applied potential of −1.0 V vs. SCE at a temperature of 70°C. Through application of a cathodic potential, OH− ions (Eq. 1) are electrochemically generated which increases the local pH near the seed layer-ITO electrode surface. At the same time, these OH− ions react with Zn2+ ions, which eventually result in the precipitation of ZnO crystals, as shown in Eq. (2): −

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−

135 136 137 138 139 140 141 142 143 144 145 146 147

(1) 148

− Zn2+ (aq ) + 2OH(aq ) → ZnO + H2 O

(2)

Fig. 1 represents the variation of current density with time of the electrodeposited ZnO nanorods. The curves have similar features observed from the electrodeposition of ZnO using nitrate and

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Fig. 2. SEM images of electrodeposited ZnO nanorods for (a) 10 min and (b) 20 min. Images (c and e) and (d and f) are their corresponding cross-sectional and top views with perovskite film on top of each sample.

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chloride media [37,38]. The measured current is due to the reduction reaction of molecular O2 in the electrolyte. The application of the negative potential to the system triggered a sudden increase in current. At this moment, nucleation instantaneously starts and the active surface area rapidly increases due to the three-dimensional growth of each newly formed ZnO crystal [39]. Moreover, the current density reached a maximum value of 0.95 mA cm−2 at around 1.5 s, which can be associated with the coalescence of nuclei. After 1 min, the deposition current reached a plateau at the range of 0.55−0.60 mA cm−2 indicating that the electrodeposited ZnO film now fully covers the existing seed layer. The total electrical charge density exchanged at the end of deposition process for each sample is also plotted in Fig. 1. Subsequently, the theoretical thickness of the electrodeposited ZnO film was estimated from the total electrical charge density exchanged through Eq. (3) [37]:

T = (Q.MW )/(nF .ρ ) 168 169 170 171 172 173 174 175 176 177

(3)

where, T is the thickness, Q is the total electrical charge density exchanged, MW is the molecular weight of ZnO (81.41 g mol−1 ), n is the number of electrons consumed in the electrochemical reactions Eqs. (1) and (2), F is the Faraday’s constant (96, 495 C mol−1 ), and ρ is the density of ZnO (5.61 g cm−3 ). The calculated thicknesses are then compared with actual measured thicknesses based on scanning electron microscopy (SEM) images (Fig. 2). As shown in Table 1, the actual values are slightly higher than the calculated ones and their difference increases with deposition time. Fig. 2 shows the surface morphologies of the electrodeposited ZnO

films. Images (Fig. 2c) are their corresponding cross-sectional images with perovskite film on top of each sample. Hexagonal ZnO nanorods with a mean diameter of about 52 nm and thickness of ∼260 nm were obtained, after 10 min deposition (Fig. 2a). Increasing the deposition time to 20 min initiated the evolution of clustered flat-topped ZnO nanorods, possibly through Ostwald ripening. These nanorods have larger mean diameter of ∼ 90 nm and thickness of ∼ 580 nm, as seen in Fig. 2b. Large spaces can be seen between the clustered nanorods due to their high aspect ratio. These areas are crucial for better penetration of perovskite resulting in higher light absorption. A close inspection of the crosssectional view of the 20 min sample reveals the presence of some voids along the nanorods length. These bulk surface defects caused the underestimation of the calculated thicknesses when compared to the measured values. Fig. 3a shows Raman spectra of the electrodeposited ZnO nanorods under excitation of a 532 nm laser line. The dominant peaks at 99 cm−1 and 438 cm−1 are attributed to the E2 (low) and E2 (high) modes of non-polar optical phonons, respectively. In addition, the small peak at around 333 cm−1 can be related to the E2 (high)-E2 (low) multiphonon mode. The A1 (low) phonon mode at 569 cm−1 is typically induced by defects such as oxygen vacancies and zinc interstitials [40]. The intense peaks and narrow line widths of the E2 -mode peaks verify the high crystallinity of wurtzite ZnO nanorods formed by electrodeposition. This result is further confirmed by X-ray diffraction (XRD) patterns shown in Fig. 6a. Both patterns exhibit a dominant peak at 2θ = 34.45° that can be indexed to the (002) plane of hexagonal wurtzite crystal

Please cite this article as: C.M. Pelicano, H. Yanagi, Effect of rubrene:P3HT bilayer on photovoltaic performance of perovskite solar cells with electrodeposited ZnO nanorods, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.018

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Table 1. Electrodeposited ZnO nanorods parameters as a function of deposition time. Deposition time (min)

J (mA cm−2 )

Q (C cm−2 )

Calculated thickness (nm)

Measured thickness (nm)

Diameter (nm)

Eg (eV)

10 20

0.572 0.596

0.3432 0.7152

258 538

260 580

52 90

3.24 3.22

Fig. 3. (a) Raman spectra, (b) optical transmittance and (c) room temperature photoluminescence spectra of electrodeposited ZnO nanorods for 10 and 20 min. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

206 207 208 209 210 211

structure of ZnO. It should be noted that ZnO peaks on both Raman and XRD patterns have intensified at longer reaction time signifying the increase in ZnO concentration on the surface of the substrate. The optical transmission spectra of electrodeposited ZnO nanorods are presented in Fig. 3b. The 10-min-ZnO sample dis-

played a higher transmittance of 86% at 600 nm compared with 64% of the 20-min-ZnO sample. The lower transmittance of the 20 min-ZnO sample is due to its higher thickness and relatively tilted orientation of nanorods resulting in the Rayleigh scattering of light [41]. Additionally, the ZnO band gap was estimated using the absorption edge of each sample and tabulated in Table 1. The band gap value is slightly reduced for the 20-min-ZnO sample due to its larger dimensions. Fig. 3c shows room temperature photoluminescence spectra of ZnO nanorods. Under excitation wavelength of 365 nm, both samples exhibited a major orange-red emission peak centered at 650 nm (1.90 eV) which originated from deep level oxygen interstitials (Oi ) [42]. The focused ion beam (FIB)-assisted cross-sectional SEM image and the energy level diagram of the perovskite solar cell utilized in this study are shown in Fig. 4a and c, respectively. The device has a configuration of glass/ITO/ZnO seed layer/20-min-ZnO nanorods/CH3 NH3 PbI3 /rubrene:P3HT/Ag, where the nanostructured ZnO and rubrene:P3HT bilayer function as the electron and holetransporting layers, respectively. Fig. 4b illustrates the molecular structure of rubrene. It can be noted that rubrene has the same HOMO level (−5.4 eV) with CH3 NH3 PbI3 and lower HOMO level (−5.0 eV) than P3HT. This band alignment suggests that holes can be transferred more easily at the rubrene/CH3 NH3 PbI3 interface than the P3HT/CH3 NH3 PbI3 interface. To confirm the synergistic effect of electrodeposited ZnO nanorods and rubrene:P3HT bilayer on the charge transfer process of perovskite solar cells, we performed steady-state photoluminescence of glass/ITO/perovskite, glass/ITO/ZnO nanorods/perovskite, glass/ITO/perovskite/P3HT, and glass/ITO/perovskite/rubrene:P3HT, as shown in Fig. 4d. The strong photoluminescence peak at 770 nm reveals that the perovskite film formed using one-step solution process has a high structural quality. Furthermore, about ∼45% PL quenching was observed when perovskite was deposited on top of ZnO nanorods, which validates the electron extraction capability of electrodeposited ZnO. On the other hand, CH3 NH3 PbI3 emission peak was quenched by almost 90% upon contact with P3HT which is in agreement with the literature [43]. Interestingly, the PL intensity was further reduced up to 97% when rubrene was deposited between perovskite and P3HT. This signifies that a more efficient hole-transport process occurs between CH3 NH3 PbI3 and P3HT with rubrene as interlayer. The cross-sectional elemental mapping acquired by energy dispersive X-ray (EDX) analysis is presented in Fig. 5. The Zn signal matches well with the seed layer and nanorod morphology of ZnO layer. The I and Pb signals indicate that the perovskite film is uniformly distributed and has infiltrated the voids between each nanorod. Furthermore, the strong S and C signals follow the shape of the P3HT layer. Some C signals below the P3HT layer could be linked with the perovskite material and rubrene interlayer. Finally, the Ag signal corresponds with the silver electrode. These results demonstrate that there is no evidence of layer-by-layer mixing and all the elemental mappings are in good agreement with the device structure as shown in Fig. 4a. Fig. 2(c−f) shows the cross-sectional and top view SEM images of the as-prepared CH3 NH3 PbI3 film on top of the ZnO nanorods. Porous and coarse perovskite film was formed on top of the 10min-ZnO sample, whereas a smoother film with larger crystallite size was obtained on top of the 20-min-ZnO nanorods (Fig. S1b). Lesser number of pinholes can also be seen for the latter sample. Fig. 6a and b exhibits the XRD pattern and absorption spectra of

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Fig. 4. (a) FIB-assisted cross-sectional SEM image of perovskite solar cell based on electrodeposited ZnO and rubrene:P3HT bilayer. (b) Molecular structure of a rubrene molecule. (c) Energy band diagram and (d) steady-state photoluminescence spectra of perovskite films: glass/ITO/CH3 NH3 PbI3 (black), glass/ITO/ZnO nanorods/CH3 NH3 PbI3 (blue), glass/ITO/CH3 NH3 PbI3 /P3HT (yellow), and glass/ITO/CH3 NH3 PbI3 /rubrene/P3HT (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Cross-sectional elemental mapping of electrodeposited ZnO nanorods-based perovskite solar cell with rubrene:P3HT hole transporting layers.

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the perovskite films. Major peaks (with asterisk) can be indexed to the tetragonal crystal structure of CH3 NH3 PbI3 with a (110) plane growth orientation. It should be noted that a stronger peak intensity at 2θ = 12.6° was detected for the 10-min ZnO-perovskite bilayer, denoting a higher amount of unreacted PbI2 precursor. Additionally, ZnO/perovskite bilayers exhibit panchromatic absorption of light. It can be seen that the 20-min-ZnO/perovskite sample absorbs more light than the 10-min-ZnO/perovskite sample. This could be due to higher perovskite loading in the clustered nanorods formed after 20 min deposition. The band edge analysis shows that the perovskite films has a band gap of Eg = 1.59 eV which is suitable for solar cell application.

Fig. 7 presents the SEM and atomic force microscopy (AFM) images of the surface morphology evolution of hole-transporting layers on top of electrodeposited ZnO nanorods/perovskite heterostructure. Fiber-like microstructures were obtained when P3HT solution is directly spin-coated on perovskite film, as shown in Fig. 7a. This P3HT layer shows a root-mean-square (rms) roughness of 3.71 nm (Fig. 7d) with some pinholes in the range of 20 0−50 0 nm. On the other hand, rubrene layer covered the pinholes of the perovskite film and penetrated its grain boundaries effectively. The rubrene layer is composed of smooth grains with a mean diameter of ∼100 nm and smaller rms roughness of 2.95 nm (Fig. 7e). Fig. 7c shows that the addition of P3HT on top of rubrene layer fur-

Please cite this article as: C.M. Pelicano, H. Yanagi, Effect of rubrene:P3HT bilayer on photovoltaic performance of perovskite solar cells with electrodeposited ZnO nanorods, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.018

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Fig. 6. (a) XRD patterns and (b) optical absorption spectra of perovskite films on top of electrodeposited ZnO nanorods.

Fig. 7. SEM images of (a) CH3 NH3 PbI3 /P3HT (b) CH3 NH3 PbI3 /rubrene and (c) CH3 NH3 PbI3 /rubrene/P3HT spin-coated on top of electrodeposited ZnO nanorods. (d−f) Images are their corresponding AFM images. Table 2. Summary of photovoltaic parameters recorded for perovskite solar cells based on electroeposited ZnO nanorods and rubrene:P3HT bilayer, under 1 sun illumination (AM 1.5 G, 100 mW cm−2 ). ETL/HTL

Scan direction

Jsc (mA cm−2 )

Voc (eV)

FF

PCEave (%)

PCEmax (%)

Hysteresis index

20 min ZnO/Ru:P3HT

Forward Reverse Forward Reverse Forward Reverse

19.0 ± 3.7 19.0 ± 3.8 16.0 ± 2.2 17.5 ± 1.4 13.7 ± 1.0 15.4 ± 0.7

0.50 ± 0.11 0.51 ± 0.12 0.52 ± 0.02 0.52 ± 0.04 0.56 ± 0.05 0.58 ± 0.03

0.38 ± 0.07 0.40 ± 0.02 0.40 ± 0.03 0.40 ± 0.03 0.36 ± 0.01 0.37 ± 0.00

3.62 ± 1.1 3.82 ± 0.9 3.36 ± 0.8 3.61 ± 0.3 2.77 ± 0.4 3.33 ± 0.1

4.87 4.84 4.29 3.86 3.18 3.43

0.020

10 min ZnO/Ru:P3HT 10 min ZnO/P3HT

295 296 297 298 299 300 301

ther filled the grain boundaries of perovskite film with a smoother surface roughness of 1.92 nm. These results show that the passivation of perovskite film by rubrene clearly improved the interfacial area between the hole-selective materials and the absorber layer. In turn, this could reduce the leakage current leading to a higher photocurrent generation and better photovoltaic performance.

0.086 0.141

Fig. 8a and Table 2 present the photocurrent (J)-voltage (V) curves in reverse scan under simulated AM1.5 G illumination (100 mW cm−2 ) and the complete photovoltaic parameters with statistical results in different scan directions, respectively. We first determine the effect of rubrene interlayer by fabricating perovskite solar cells with and without rubrene using the 10 minZnO nanorods as electron selective layer. As shown in Table 2, the

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Fig. 8. (a) J-V curves and (b) EQE spectra for perovskite solar cells based on electrodeposited ZnO nanorods and rubrene:P3HT bilayer.

Fig. 9. Schematic illustration of the synergistic effect of electrodeposited ZnO nanorods and rubrene:P3HT bilayer to suppress hysteresis in perovskite solar cell.

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device without rubrene exhibited an average short circuit current density (Jsc ) of 15.4 mA cm−2 , open circuit voltage (Voc ) of 0.58 V, fill factor (FF) of 0.37, maximum PCE (PCEmax ) of 3.4% and average PCE (PCEave ) of 3.3%. On the other hand, upon addition of rubrene interlayer between the perovskite and P3HT, most photovoltaic parameters increased including average Jsc (17.5 mA cm−2 ), FF (0.40), PCEmax (4.3%) and PCEave (3.6%). This increase could be attributed to the filling effect of rubrene:P3HT bilayer, as discussed earlier, resulting in reduced leakage current and better interfacial area between the active layer and hole-transporting layers. However, the relatively low Voc values for both devices could be related to the utilization of P3HT as main hole-selective layer. It is well known to have a low hole mobility, low charge carrier density and higher recombination rate than spiro-OMeTAD [44,45]. Another possible reason for these relatively low Voc values could be due to the presence of pinholes in perovskite layer leading to the direct contact between the nanorods and hole-transporting materials. Fig. 8b presents the external quantum efficiencies (EQE) of the 10-min-ZnO nanorod based devices with and without rubrene interlayer. As expected, the solar cell device with rubrene:P3HT bilayer displayed a higher EQE over the entire wavelength range than the P3HT only-device, which is in agreement with the Jsc trend in Table 2. The current generation starts at around λ = 800 nm for both devices, which is compatible with the band gap of perovskite layer. Additionally, the EQE peak at λ = 680 nm, which is followed by a dip at λ = 615 nm, is possibly triggered by the absorption edge of P3HT [46,47]. Moreover, the 20-min-ZnO nanorods based device with rubrene:P3HT bilayer exhibited the highest average Jsc of 19 mA cm−2 , PCEave of 3.8% and respectable PCEmax of 4.9%. This enhancement is possibly due to the suppression of recombination losses from the enhanced electron transport within the longer nanorods and shorter path length for electron transfer [48,49]. The increase in Jsc values (from 16−17.5 to 19 mA cm−2 ) could also be associated to the clustered morphology of the 20 min-ZnO

nanorods which led to improved perovskite infiltration and light absorption, as seen in Fig. 6b [50]. As shown in Table 2, the 10-min-ZnO nanorods based peroskite solar cell without rubrene exhibits stronger hysteresis compared with the devices having rubrene interlayer, as specified by the difference of their efficiencies measured in forward (FS) and reverse (RS) scans. Herein, we quantify the hysteresis extent on perovskite solar cells by defining hysteresis index (HI) as Eq. 4 according to the literature [51]:

HI =

JRS (0.6VOC ) − JFS (0.6VOC ) JRS (0.6VOC )

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(4)

According to the estimated HI values, the 20 min-ZnO nanorods based perovskite solar cell with rubrene passivation demonstrate the smallest HI value of 0.02. This means that the hysteresis behavior is synergistically diminished through the addition of rubrene interlayer and the utilization of longer nanorods. Recently, most researchers associate the hysteresis effect through the ionic vacancy defect migration within the perovskite film which include Pb2+ , I− and CH3 NH3 + (MA+ ) ions [52]. Under increasing applied bias from 0 to Voc of FS, these charged defects migrate towards the anode and cathode electrodes, and subsequently travel back into the bulk perovskite film under reducing applied potential of RS. This process initiates an accumulation of charges on the interface between the carrier extraction layers and the active layer and subsequently disrupts the collection of charge carriers’ efficiency on both contacts [53]. Hence, through this mechanism, underestimation under FS and overestimation under RS usually occur, and the hysteresis behavior becomes more evident. Herein, we propose a mechanism for the suppression of hysteresis through the application of ZnO nanorods and rubrene:P3HT bilayer as charge carrier selective contacts. As shown in Fig. 9, the ZnO nanorods interact with I− ions by converting them to I atoms. This process inhibits charge accumu-

Please cite this article as: C.M. Pelicano, H. Yanagi, Effect of rubrene:P3HT bilayer on photovoltaic performance of perovskite solar cells with electrodeposited ZnO nanorods, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.018

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lation on the interface resulting in the stabilization of photocurrent generation at that part of the solar cell. In addition, the filling effect of rubrene layer on the grain boundaries of perovskite film greatly enhances the hole extraction process even in the presence of ionic defects. For the P3HT-only-device, the fiber-like microstructures of P3HT could not cover the grain boundaries of perovskite film effectively. This leads to the entrapment of generated holes simultaneously with the ionic defects along the grain boundaries of the active layer. These results indicate that rubrene is a promising passivation layer for efficient perovskite solar cells.

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We have demonstrated the synergistic effect of electrodeposited ZnO nanorods and rubrene interlayer to enhance the photovoltaic performance of perovskite solar cells and suppress its hysteresis behavior. Through electrodeposition process, highly crystalline ZnO nanorods were produced as shown in XRD, photoluminescence and Raman spectroscopy analyses. Moreover, rubrene layer covered the pinholes of the perovskite film and penetrated its grain boundaries effectively. A fabricated perovskite solar cell based on 20-min-ZnO nanorods and rubrene:P3HT bilayer exhibited the highest PCE of 4.9% and average PCE of 3.8%. The devices with rubrene interlayer also displayed higher EQE over the whole wavelength range and reduced hysteresis behavior compared with the device having only P3HT as the hole-transporting layer. The utilization of longer ZnO nanorods resulted in better perovskite infiltration, reduced charge carrier path length and at the same time improved photovoltaic parameters. Based on the results, electrodeposited ZnO nanorods and rubrene interlayer can work synergistically as electron and hole-transporting layers for efficient perovskite solar cells.

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The authors gratefully acknowledge Mr. Kazuhiro Miyake, Mr. Shohei Katao, Mr. Yasuo Okajima, Nara Institute of Science and Technology for their experimental supports. The authors also thank Dr. Jennifer Damasco Ty for her valuable suggestions relating to the ZnO electrodeposition process.

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2017.10.020.

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Please cite this article as: C.M. Pelicano, H. Yanagi, Effect of rubrene:P3HT bilayer on photovoltaic performance of perovskite solar cells with electrodeposited ZnO nanorods, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.018

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