Facile deposition of Nb2O5 thin film as an electron-transporting layer

Subscriber access provided by University of Winnipeg Library

Article

Facile deposition of Nb2O5 thin film as an electrontransporting layer for highly efficient perovskite solar cells Deli Shen, Weifeng Zhang, Yafeng Li, Antonio Abate, and Mingdeng Wei ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00859 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Facile deposition of Nb2O5 thin film as an electron-transporting layer for highly efficient perovskite solar cells

Deli shen,a,b Weifeng Zhang, a,b Yafeng Li,*a,b Antonio Abate,b,c Mingdeng Wei,*a,b a

State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou

University, Fuzhou, Fujian 350002, China b

Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002,

China c

Helmholtz-Zentrum Berlin für Materialien und Energie, Kekuléstrasse 5, Berlin

12489, Germany

ABSTRACT: In a typical high-performance perovskite solar cell (PSC), the electron-transporting layer (ETL) plays a critical role in facilitating the transportation of the photo-generated carriers from perovskite to electrode. In the present work, the solution-processed Nb2O5 film is successfully prepared to serve as an alternative for replacing the overwhelmingly dominated TiO2 ETL with a facile deposition approach. By adjusting the spin-coating process of the precursor solution, the best power conversion efficiency of 19.2% is achieved in the Nb2O5-based planar PSC when the thickness of C-Nb2O5 ETL is about 100 nm. The characterization including ultraviolet photoelectron

spectroscopy,

electronic

impedance

spectroscopy

and

photoluminescence spectrum results elucidate that the impressive photovoltaic

1 ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

performances can be attributed to the proper Nb2O5 ETL conductive band, the lower charge recombination rate, the more efficient extraction and transportation of the photo-generated carriers from the perovskite absorber.

KEYWORDS: facile deposition, Nb2O5, electron-transporting layer, perovskite solar cell, photovoltaic property

,

Corresponding authors: [email protected]

[email protected] 2

ACS Paragon Plus Environment

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION

In recent years, many studies are devoted to the development of high performance perovskite solar cells (PSCs). In 2009, Miyasaka’s group firstly employed perovskite as a sensitizer in dye-sensitized solar cells, where the lead halide perovskite has been brought into the solar cell research field, and a power conversion efficiency (PCE) of 3.8% was achieved.1 In just a few years, the PCE of the PSCs has been boosted to be above 20%.2-4 The rapid development of PSCs is probably owned by the unique properties of the perovskite material such as high light-absorption coefficient, low exciton binding energy and long charge-carrier diffusion length.5-7 Currently, the efficiency of PSC with a certified PCE of 22.1% has been obtained, which employ a thin TiO2 layer as an electron-transporting layer (ETL) in combination with a thick solid perovskite absorber over-layer.8

A perovskite solar cell includes five main constituents, namely transparent conductive oxide (TCO), electron-transporting layer, perovskite light absorber, hole-transporting layer (HTL) and metal electrode. Also, it is regularly classified the architecture of PCSs into two basic kinds of types, one is the mesoscopic structure that utilizes the mesoporous semiconducting materials as the ETL such as TiO28, SnO29-11, ZnO12, WO313, ZnSnO414, Zn2Ti3O815, BaTiO316, SrTiO317 or mesoporous insulator like Al2O36 and SiO218 as the scaffold. Another is the planar structure where ultra-thin compact layer such as TiO219, SnO220, ZnO21, CeOx22 can be used for the ETL. No matter the mesoscopic or planar structure, titanium dioxide (TiO2) is the 3 ACS Paragon Plus Environment


Page 4 of 37

most widely used as the ETL for PSCs, and to date the reports about PSCs with PCE of nearly 20% have mainly been with mesoporous or compact TiO2 as the ETL.2, 4 For instance, Sargent et al. demonstrated that a chlorine-capped TiO2 colloidal nanocrystal film could mitigate the interfacial recombination and improve the interface binding in planar PSCs, and a certified efficiency of 20.1% was achieved.23 Yang et al. showed that Yttrium doped TiO2 ETL not only improved the electron transport channel in the device, but also enhanced the carrier concentration and finally these changes produced a PCE of 19.3%.24 However, some studies have confirmed that TiO2 based PSCs are suffered from decreased stability under UV light due to the light-induced desorption of surface-absorbed oxygen.25-28 Thus, it is crucial to exploit novel ETL materials with desirable properties that, in consequence, will be beneficial to the photovoltaic performances of the PSC devices.

In the past several years, ZnO has been widely explored as a promising ETL material. Liu et al. reported solution-processed ZnO nanoparticles for PSCs, and the PCE of 15.7% was obtained.29 Uddin et al. developed a low temperature (＜150 oC), sol-gel processed ZnO thin film for PSCs, and investigated the interfacial charge transfer characteristics between the MAPbI3 film and the low temperature sol-gel based ZnO film.21 However, ZnO has been proved to be susceptible to react with acid and alkaline solution which will accelerate the decomposition of MAPbI3. Accordingly, the chemically unstable properties of ZnO are detrimental to the long-term stability of PSCs.30 Moreover, SnO2 whose electron mobility is about


Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100-200 cm2 V-1 s-1,31 which is one or two orders of magnitude larger than that of TiO2, has also been employed as a promising ETL within the PSCs. Also, You et al. used low-temperature solution-processed SnO2 nanoparticles as an efficient ETL to achieve a certified efficiency of 19.9% and the devices were almost free of hysteresis.20 Hagfeldt et al. proposed a simple technological approach for depositing SnO2 layers, which yielded a highly stabilized PCE close to 21% and exhibited stable performance under real operating condition for over 60 h.32 Additionally, a few other high mobility metal oxides have been also investigated. However, the PCEs of the PSCs based on these novel ETLs like In2O3 (≈ 20 cm2 V-1 s-1)33 and WOx (10-20 cm2 V-1 s-1)34 are still below 15%. Therefore, it is extremely important to explore other potential metal oxides that are suitable for the ETLs in planar PSCs and further improve the performance of the devices.

Compared with the most popular TiO2, Nb2O5 is a promising n-type semiconductor, whose optical band gap and electronic properties are similar to those of TiO2,35 More importantly, Nb2O5 displays improved chemical stability relative to that of conventional TiO2. Furthermore, the conduction band (CB) edge of Nb2O5 is considered to be close to that of TiO2.36 Based on the previous reports of dye-sensitized solar cells, Nb2O5 was demonstrated as an effective blocking layer for decreasing the recombination rate of carriers at the surface.37-41 As for the PSCs, Miyasaka et al.42 and Graeff et al.43 have introduced the Nb2O5 thin film as a hole-blocking layer, independently. Nevertheless, the PCEs of these PSCs did not



Page 6 of 37

reach a satisfying level (less than 13%). Very recently, Liu et al.

44-48

directly

employed the E-beam evaporated Nb2O5 film as the ETL for PSCs without any post-treatment, and the PCE was soared to 18.59%. However, this approach involved the delicate equipment which is probably costly. On the contrary, although the sol-gel process provided a facile way for the preparation of Nb2O5 ETL, the corresponding performance (≈10%) was far away from satisfactory.

42, 43

Therefore, a facile method

for preparing Nb2O5 film with high performance is highly desired.

Herein, a solution-processed approach is developed to deposit Nb2O5 film on rigid substrates with superior uniformity. In addition, the CB of the Nb2O5 film is -4.37 eV, which matches better with the CB of (FAPbI3)1-x(MAPbBr3)x perovskite (-4.36 eV) than TiO2 (-4.21 eV). And the corresponding device achieves PCE up to 19.2%, which is comparable to the PCE of devices based on TiO2 as the ETL. To the best of our knowledge, the PCE achieved in this work is the highest recorded for Nb2O5-based PSCs to date.

2. EXPERIMENTAL SECTION

2.1 The synthesis of the mesoporous Nb2O5

All chemicals were analytical reagent and applied directly without any purification. Nb2O5 nanoparticles were synthesized according to the reported procedures.49 Firstly, 0.27 g C10H5NbO20 (99.5%, Alfa-Aesar) were dissolved in 20 mL deionized water under magnetic stirring to obtain a transparent solution. Secondly, C2H5OH (99.8%, 6 ACS Paragon Plus Environment

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Aladdin) was added to the above solution to obtain 30 mL transparent solution. Then, the obtained solution was transferred into Teflon-lined stainless steel autoclave, and was performed for the sealed autoclave in an electric oven at 180 oC for 240 h. After the autoclave being cool, the product was collected and washed three times by deionized water and anhydrous ethanol. Finally, the powder was dried at 70 oC for 24 h which was ready for preparing the paste later.

2.2 Preparation of the substrate

Firstly, FTO-coated glass (14 Ω/sq2, Nippon sheet glass) was patterned by etching with Zn powder and 2 M HCl. Then, the substrates were cleaned with detergent (diluted to the volume ratio of 5% with deionized water), deionized water, ethanol, acetone and 2-propanol in sequence. Lastly, the substrates were dried with clean dry air.

2.3 Preparation of Nb2O5 blocking layer

Nb2O5 blocking layer was obtained via spin-coating process. A 0.5 mM Nb2O5 sol-gel precursor solution was prepared by dissolution of niobium ethoxide (99.9%, Alfa-Aesar) in ethanol (99.8%, Aladdin). Meanwhile, 20 µl (2 M) hydrochloric acid should be dropped into the sol-gel solution to act as a stabilizer. After 2 h stirring at room temperature, the precursor solution was deposited on clean FTO substrates with 4000 r.p.m spin rate for 30 s with a 500 r.p.m s-1 ramp, followed by pre-drying at 150 o

C for 30 min and then annealed at 550 oC for 30 min. Briefly, spin coating speed of 7 ACS Paragon Plus Environment


Page 8 of 37

the precursor solution under 4000 r.p.m min-1 is denoted as C4-Nb2O5, the speeds of 2000 and 6000 r.p.m min-1 are denoted as C2-Nb2O5 and C6-Nb2O5 as well, respectively.

2.4 Preparation of the Nb2O5 mesoporous layer

The Nb2O5 paste was prepared by our previously described procedures.50 Then, the Nb2O5 paste was spin-coated on the BL layer at 2000 r.p.m for 30 s, where the pristine paste was diluted in ethanol (0.1 g mL-1). After drying at 150 oC for 30 min, the film was annealed at 550 oC for 30 min.

2.5 Preparation of (FAPbI3)1-x(MAPbBr3)x perovskite precursor solution and the deposition of the perovskite film

The perovskite film was deposited in an Argon-filled glovebox, which the precursor solution of FAI (1 M), PbI2 (1.1 M), MABr (0.2 M) and PbBr2 (0.2 M) in anhydrous DMF:DMSO= 4:1(v:v) were contained.51 The spin coating procedure included two steps, that is, at 1000 r.p.m for 10 s, and then at 6000 r.p.m for 20 s, then 15 s after the beginning of the spin-coating program, chlorobenzene was gently dropped on the spinning substrate through a micro pipette. Finally, the substrate was annealed at 100 oC for 60 min on a hot plate to obtain a dense (FAPbI3)1-x(MAPbBr3)x film.

2.6 Preparation of the hole transporting layer and gold layer 8 ACS Paragon Plus Environment

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Spiro-OMeTAD (99.5%, Xi’an Polymer Light Technology Corp.) (60 mM solution in anhydrous chlorobenzene) solution which was mixed with 28.8 µL 4-tert-butylpyridine and 17.5 µL of a stock solution of 520 mg mL-1 lithium bis(trifluoromethylsulphonyl)imide in acetonitrile was deposited by spin coating (4000 r.p.m for 20 s) as the hole transporting layer on top. An 80 nm-thick gold top electrode was thermally evaporated to form the back contact. The active area of devices were 0.12 cm2 determined by the mask with dimensions of 4 mm × 3 mm.

2.7 Characterizations

The phase purity of the samples was characterized by powder X-ray diffraction (XRD) on a Rigaku Ultima IV using the Cu Kα radiation (λ=1.5418 Å). The X-ray photo-electron spectroscopy (XPS) was recorded on a MUTILLAB 2000. The ultraviolet photoelectron spectroscopy (UPS) for C-Nb2O5 was measured using ESCALAB 250Xi (Thermo Fisher) under a background pressure of 5.0 × 10-7 Pa. The linear extrapolation of the curves was used for the band energies position. He I excitation line (~21.20 eV) from a He discharge lamp was used for the UPS measurements. A clean Au film on a FTO substrate was used for the Fermi level (EF) and the binding energy calibrations. UV-vis spectrum was investigated using a Lambda-950 (PerkinElmer). The scanning electron microscopy (SEM) was measured on a Hitachi S4800 instrument. The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) pattern images were recorded on a Tecnai G2 FEI F20S-TWIN instrument. The steady-state photoluminescence (PL) and 9 ACS Paragon Plus Environment


time-resolved PL spectroscopy of (FAPbI3)1-x(MAPbBr3)x coated on Nb2O5 ETL were performed using Edinburgh FLS 980 instrument with the excitation at 406 nm and emission at 750 nm.

The photovoltaic performance in terms of photocurrent density voltage (J-V) characteristics was determined utilizing a PEC-L11 AM 1.5 solar simulator (Peccell Technology Co. Ltd., with a 1000 W Xe lamp and an AM 1.5 filter) and a Keithley 2400 was used to recorded the data. The J-V curves were measured from both forward and reverse scan directions, and the scan rate was 20 mV s-1, the delay time was 50 ms. A mask with a window of 0.09 cm2 was utilized to define the active area of the devices. The incident photon-to-current conversion efficiency (IPCE) spectra were collected using a PEC-S20 (Peccell Technology Co. Ltd.). Electrochemical Impedance Spectroscopy (EIS) was conducted employing an IM6 (Zahner). The impedance parameters were simulated by fitting of impedance spectrum through Z-view software.

3. RESULTS AND DISCUSSION

To denote the promising application of Nb2O5 as an ETL in PSCs, the mesoscopic and the planar PSCs were both constructed. The fabrication procedures for both types of PSCs are described in Scheme 1. And the structure of mesoscopic perovskite solar consists of fluorine doped tin oxide (FTO) /compact Nb2O5/mesoporous Nb2O5/(FAPbI3)1-x(MAPbBr3)x /Spiro-OMeTAD/Au, where the mesoporous Nb2O5 is used as both electron-transporting layer (ETL) and the supporting layer for perovskite. 10 ACS Paragon Plus Environment

Page 10 of 37

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. The fabrication procedure of the (a) mesoscopic and (b) planar Nb2O5 based perovskite solar cell, respectively. The X-ray diffraction (XRD) pattern of the Nb2O5 nanoparticles is presented in Figure S1a, and the entire conspicuous peaks match well with the standard XRD pattern in JCPDC 00-027-1312, thus, confirming that the formation of the mesoporous Nb2O5 nanoparticles is monoclinic structure. The representative transmission electron microscopy (TEM) images of the Nb2O5 nanoparticles shown in Figure S1b-c exhibiting that the prepared samples are highly dispersed and defined nanoparticles. The high-resolution TEM image (HRTEM) shown in Figure S1d depicts that the Nb2O5 nanoparticles are highly crystalline and the lattice fringe is measured to be 0.39 nm, corresponding to d040-spacing in the XRD pattern. Moreover, the ring-like selected area electron diffraction (SAED) reveals the polycrystalline structure of the Nb2O5 nanoparticles. Figure S2 shows the X-ray photoelectron spectroscopy (XPS) spectrum of the compact Nb2O5 film, and all peaks are assigned to three elements: C, Nb and O, without identifiable impurity. The peaks at 209.70 11 ACS Paragon Plus Environment


Page 12 of 37

and 206.95 eV are ascribed to Nb 3d3/2 and Nb 3d5/2, respectively. The spin orbit splitting is measured as 2.75 eV, indicating that the element of Nb is in the +5 oxidation state.52 To compare the photovoltaic performance between the mesoscopic and planar structure of Nb2O5 based PSCs, the planar PSCs employing compact Nb2O5 thin film as the ETL were also fabricated, which can be expressed as FTO/compact Nb2O5/(FAPbI3)1-x(MAPbBr3)x/Spiro-OMeTAD/Au.

Herein,

the

(FAPbI3)1-x

(MAPbBr3)x films prepared through a one-step process according to the literature

44

on compact/mesoporous Nb2O5 and compact Nb2O5 are denoted as CM-Nb2O5 and C-Nb2O5, respectively. The film qualities of the CM-Nb2O5, C-Nb2O5 and the perovskite films based on CM-Nb2O5 and C-Nb2O5 substrates are shown in Figure 1. The corresponding XRD patterns of the (FAPbI3)1-x(MAPbBr3)x films deposited on CM-Nb2O5 and C-Nb2O5 substrates are depicted in Figure 1a. A set of diffraction peaks at 14.12o, 28.52o and 31.97o arisen from the tetragonal crystal structure of (FAPbI3)1-x(MAPbBr3)x crystals are ascribed to the (110), (220) and (310) crystal faces. Furthermore, it is also found from Figure 1a that no obvious change can be discovered from both of the crystalline phase and crystallinity of perovskite based on two kinds of substrates. At the same time, the morphology of the perovskite deposited on two kinds of substrates can be observed in Figure 1b and c. The formed (FAPbI3)1-x(MAPbBr3)x perovskite films grown on CM-Nb2O5 and C-Nb2O5 substrates exhibit full surface coverage without pin-holes and the micrometer-sized grains. Figure 1d and e reveal the morphologies of the CM-Nb2O5 and C-Nb2O5 film. 12 ACS Paragon Plus Environment

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


It is observed that the mesoporous Nb2O5 film is smooth without apparent reunited phenomenon, which is beneficial to getting the flat surface of perovskite film. Simultaneously, the mesoporous film is not completely compact, which is favorable for the infiltration of the perovskite precursor solution and affords enough skeleton to support the perovskite layer. The image in Figure 1e shows that the C-Nb2O5 is quite uniform and displays the excellent surface coverage on the FTO substrate without any visible cracks.

Figure 1. (a) XRD patterns of perovskite layers formed on mesoporous and compact Nb2O5 film, the SEM images of the perovskite layer formed on the (b) mp-Nb2O5 film and (c) compact Nb2O5, the SEM images of the (d) mesoporous Nb2O5 film and (e) compact Nb2O5 film.



Page 14 of 37

Figure 2a and b display the schematic illustration of the perovskite solar cells with two kinds of structures. The mesoscopic PSC is composed of a layer stack of 100 nm compact Nb2O5, 600 nm perovskite film which is completely infiltrated into the mesoporous Nb2O5 film, 100 nm Spiro-OMeTAD and 80 nm gold electrode. Regarding the planar PSC, the thickness of each film except for the mesoporous film should be controlled the same as in the mesoscopic PSC. All individual layers and interfaces are resolved according to the corresponding cross-sectional images of the mesoscopic and planar PSCs, as exhibited in Figure 2c and 2d.

Figure 3a and b give the photocurrent density-voltage (J-V) curves and incident photon-to-electron conversion efficiency (IPCE) change between the Nb2O5 based mesoscopic PSC and planar PSC. At the same time, the related J-V parameters are summarized in Table 1. These data real that the CM-Nb2O5 based mesoscopic PSC reaches the highest value of 17.4% with a Jsc of 23.2 mA/cm2, a Voc of 1.04 V and an FF of 0.72, while the planar PSC based on Nb2O5 compact layer displays a better photovoltaic performance, resulting in a PCE of 18.2%, with a Jsc of 23.9 mA/cm2, a Voc of 1.04 V and an FF of 0.73. Although both structures have a similar Voc and FF, the Jsc of the planar structure is slightly higher than that of the mesoscopic PSC. Figure 3b shows the IPCE spectra of the best performance PSCs based on two kinds of structures. The IPCE value of the Nb2O5 based planar PSC is higher than that of the mesoscopic structure PSC, which is consistent with the higher Jsc of the device based on planar structure. It is noted that the maximum IPCE of the planar structure reaches nearly 90%, which is probably due to the high charge extraction properties of 14 ACS Paragon Plus Environment

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(FAPbI3)1-x(MAPbBr3)x, and in part owning to the light reflection from the back Au electrode.53 As shown in Figure S3, the PL decay lifetime of CM-Nb2O5/perovskite and C-Nb2O5/perovskite is 397 and 355 ns, respectively. The much faster charge transport

through

the

C-Nb2O5/perovskite

interlayer

compared

with

the

CM-Nb2O5/perovskite one imply that the C-Nb2O5 on FTO helps extract photo-generated carriers from perovskite layer more effectively than from that of the CM-Nb2O5. Besides, the PCE distribution histograms of 30 cells based on C-Nb2O5 and CM-Nb2O5 ETLs are shown in Figure S4 to confirm the reproducibility of the PSCs. It validates that the PCEs are highly reproducible and the PSCs using C-Nb2O5 as the ETL exhibit a better performance than those of the CM-Nb2O5 one.

Figure 2. Schematic illustration of the Nb2O5 based mesoscopic (a) and planar (b) perovskite solar cell. The corresponding cross-sectional SEM image of mesoscopic (c) and planar (d) perovskite solar cell. 15 ACS Paragon Plus Environment


Page 16 of 37

Figure 3. (a) J–V curves for PSCs employing CM-Nb2O5 (black) and C-Nb2O5 (red) as ETL, and (b) the incident photon-to-electron conversion efficiency (IPCE) spectra of the PSCs employing CM-Nb2O5 (black) and C-Nb2O5 (red) as ETL.

Table 1. Photovoltaic parameters of the PSCs employing CM-Nb2O5 (black) and C-Nb2O5 (red) as ETL.

Nb2O5

Jsc/mA·cm-2

Voc/V

FF

PCE/%

CM

23.2

1.04

0.72

17.4

C

23.9

1.04

0.73

18.2

Based on the investigations above, the comparable and even better photovoltaic performances can be achieved from Nb2O5 based planar PSCs. To obtain the best performance of PSCs based on Nb2O5, the parameters used in planar PSCs are optimized in the following step. It is well known that the thickness of the ETL has a great influence on the performance of PSCs. Therefore, the thickness of C-Nb2O5 film 16 ACS Paragon Plus Environment

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is optimized first. Herein, the thickness of the C-Nb2O5 film is controlled by the spin-coating speed. The SEM images and the corresponding cross-sectional SEM images of the C-Nb2O5 films obtained under different spin-coating speeds can be observed from the Figure 4, which shows a great change in the film morphology and the film thickness. Figure 4a indicates that the formed C2-Nb2O5 film is inhomogeneous and the cracks can be found on the film when the spin-coating speed is 2000 r.p.m/min, and the thickness of the C2-Nb2O5 film is about 200 nm (Figure 4b). As the spin-coating speed is increased to 4000 r.p.m/ min, it can form a dense, uniform and pinhole-free film without any cracks, and the corresponding thickness of the C4-Nb2O5 film reduces to be around 100 nm (Figure 4d). When the spin-coating speed is further increased to 6000 r.p.m/ min further, the thickness of the C6-Nb2O5 film decreases to be around 50 nm (Figure 4f), and the exposed FTO can be observed, which is unfavorable to avoid the formation of charge shunt pathways by the direct contact of perovskite layers with FTO.54



Page 18 of 37

Figure 4. The SEM images and corresponding cross-sectional SEM images of the C-Nb2O5 film which was spin-coating on the FTO under different speeds: (a, b) 2000, (c, d) 4000 and (e, f) 6000 r.p.m min-1.

To demonstrate that C-Nb2O5 can be used as an effective ETL, UV-vis absorbance spectrometry and ultraviolet photoelectron spectroscopy (UPS) were used to determine the optical band gap and valence band maximum of the C-Nb2O5 film. The direct band gap energy (Eg) for the C-Nb2O5 is confirmed by fitting the absorption data to the direct transition equation (1).55 (αhν)1/2= A(hν- Eg)

(1)

where α is the absorption coefficient, Eg is the direct band gap and A is a constant. The plot of (αhν)1/2 versus hν is given in Figure 5a, and the extrapolation of the linear 18 ACS Paragon Plus Environment

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


portions of the curve towards an absorption equal to zero gives Eg for direct transitions. The Eg for the C-Nb2O5 film is determined to be 3.37 eV, which is close to the reported value for Nb2O5 semiconductor.56 Figure 5b displays the UPS spectrum for the C-Nb2O5 deposited on an FTO substrate. The valance band energy (EVB) for C-Nb2O5 is estimated at -7.74 eV below vacuum level. From the obtained Eg, the conduction band energy (ECB) is determined to be -4.37 eV, which is lower than ECB of (FAPbI3)1-x(MAPbBr3)x.57 Figure 5c depicts the structure of planar PSC with C-Nb2O5 used as the ETL and the energy band diagram of the device is shown in Figure. 5d. The above results indicate that C-Nb2O5 can be employed as a promising ETL for the planar perovskite solar cells.

Figure 5. (a) The UPS spectrum of the Nb2O5 film, (b) plots of (αhν)1/2 versus photon energy (hν) of C-Nb2O5 prepared on FTO directly, (c) schematic diagrams of the Nb2O5 based planar PSC structure and (d) the energy level of the C-Nb2O5 ETL based PSC. 19 ACS Paragon Plus Environment


Page 20 of 37

The photovoltaic performance of the planar PSCs with different thicknesses of C-Nb2O5 as ETLs were measured under AM 1.5G (100 mW cm-2) illumination. The thickness of the ETL has a heavy influence on the photovoltaic performance. The J-V characteristics of the devices are exhibited in Figure 6a and the photovoltaic parameters are summarized in Table 2. The results illustrate that the PSC prepared under a spin-coating speed of 4000 r.p.m./min exhibits the highest efficiency of 19.2% with a Jsc of 24.0 mA cm-2, a Voc of 1.08 V and an FF of 0.74. Thus, it can be clearly concluded that PCE rises to a maximum first and then fall slightly when the thickness of the C-Nb2O5 film decreased. Further investigations reveal that Jsc seem insusceptible to the thickness of the C-Nb2O5 film, but Voc and FF are subject to the C-Nb2O5 ETL thickness. It is speculated that the improvements of the Voc and FF based on device with C4-Nb2O5 as the ETL are mostly attributed to the superior C4-Nb2O5 film quality with smooth and full coverage on the FTO, which can significantly improve the interface contact with the perovskite film and efficiently reduce the trap-assisted charge recombination at the interfaces.58 Meanwhile, there is no apparent change of surface morphology occurring among the perovskite films that were deposited on the C-Nb2O5 ETL with different thicknesses (Figure S5). The IPCE spectra were also recorded, as displayed in Figure 7. The Jsc calculated by integrating the IPCE curves with the AM 1.5 G photon flux for devices using the different thicknesses of the C-Nb2O5 as the ETLs, is within 5% error compared to the corresponding Jsc obtained from the J-V curves. To further confirm the reproducibility of the planar PSCs based on C4-Nb2O5 ETL, 30 individual devices are fabricated and 20 ACS Paragon Plus Environment

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tested. The PCE distribution histograms of the cells are exhibited in Figure S6, which reveal a small discreteness and a good central tendency.

Figure 6. The J-V curves of the PSCs which based on C-Nb2O5 film as the ETL by varying the speed of spin coating to control the thickness of the ETL.

Table 2. Photovoltaic parameters of the PSCs which based on C-Nb2O5 film as the ETL by varying the speed of spin coating to control the thickness of the ETL. Jsc/mA·cm-2

Voc/V

FF

PCE/%

200

23.6

1.04

0.68

16.7

4000

100

24.0

1.08

0.74

19.2

6000

50

24.3

1.06

0.72

18.6

Speed of spin

The thickness

coating/ rpm min-1

of C-Nb2O5/ nm

2000



Page 22 of 37

Figure 7. The incident photon-to-electron conversion efficiency spectra of the PSCs employing different spin-coating speeds: 2000 (black), 4000 (red) and 6000 (blue) r.p.m/ min.

To better understand the interfacial dynamics of the PSCs with different thicknesses of C-Nb2O5 ETLs, electrical impedance spectroscopy (EIS) was measured. Figure S7 gives the Nyquist plots of the devices which were measured at 0.7 bias voltage under dark condition. The traces were fitted with an equivalent circuit model which was composed of the series resistance (Rs) and the recombination resistance (Rrec).19, 59 As the perovskite/HTM interface is identical in all devices, the Rrec is mainly associated with the C-Nb2O5/perovskite interface. Therefore, its value reflects the electron recombination properties at the perovskite/ETL interfaces. The values of the equivalent circuit are listed in Table S1. Apparently, the device based on 100 nm thickness of C4-Nb2O5 ETL exhibits the largest Rrec, indicating that the lowest carrier recombination rate and the electron back flow from the C4-Nb2O5 ETL to Spiro-OMeTAD HTM is retarded, as witnessed by its highest FF and Voc (Figure 6). 22 ACS Paragon Plus Environment

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Moreover, the Rs value of the C6-Nb2O5 based device is the smallest and it is consistent with the slightly higher Jsc value.

To acquire a better comprehension of internal reasons as to why the C4-Nb2O5 ETL based PSC performed better than others, steady-state photoluminescence (PL) and time-resolved PL (TRPL) were measured in order to investigate the charge injection/separation behavior. As shown in Figure 8a, the perovskite film deposited on glass substrate produces the highest PL intensity due to the high non-radiative recombination occurring in the film. In comparison, the PL intensity of the perovskite films on C-Nb2O5 substrates is reduced significantly, indicating that the deposited C-Nb2O5 ETLs help extract carriers from the perovskite layer and the degree of the carriers’ radiative recombination is effectively reduced. Besides, when the C4-Nb2O5 ETL is spin-coated on FTO, the PL intensity is decreased to the lowest level, meaning that the charge transfer at the ETL-perovskite interface is conspicuously promoted. To further examine the subtle difference in charge transfer of the perovskite films on different thicknesses of the C-Nb2O5 ETL, TRPL was also used. The PL decay lifetime of C2-Nb2O5/perovskite, C4-Nb2O5/perovskite and C6- Nb2O5/perovskite is 2.2, 1.2 and 3.6 ns, respectively, comparing to 52.2 ns for perovskite film on bare glass. The faster decay and slightly shorter PL lifetime of the C4-Nb2O5/perovskite sample suggest that the charge extraction from the perovskite is the most efficient for C4-Nb2O5 ETL and is probably ascribed to the smooth and completely compact surface of the C4-Nb2O5 ETL, which lead to the superior interfacial contact and the hole-blocking property of the film. 23 ACS Paragon Plus Environment


Page 24 of 37

Figure 8. (a) Photoluminescence spectra (excitation at 406 nm) and (b) time-resolved photoluminescence (excitation at 406 nm and emission at 760 nm) of the perovskite film with different C-Nb2O5thicknesses.

The

stability

of

PSCs

has

become

a

significant

issue

for

their

commercialization.60,61 Therefore, in order to investigate the air-stability and photocurrent hysteresis of C4-Nb2O5 ETL based device, the performance of the unencapsulated PSCs are characterized as a function of storage time in ambient air (10% relative humidity and 20-25 oC) under forward and reverse scans, as shown in Figure S8 and Figure S9. The initial PCE value of the C4-Nb2O5 ETL based device was 18.96% and the a PCE of 17.77% was retained after storage for 30 days. The PCE decreases slowly about 6.3%. When tested under different scan directions, as shown in Figure S9, the optimal C4-Nb2O5 ETL based device gives FF of 0.74 and PCE of 17.12% at the reverse scan, which is a little higher than the FF of 0.72 and a PCE of 16.50% obtained at the forward scan. In this regard, it is inferred that the little hysteresis of C4-Nb2O5 ETL based device possibly originated from the superior contact between (FAPbI3)1-x(MAPbBr3)x and C4-Nb2O5 film, which leads to the more 24 ACS Paragon Plus Environment

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


efficient electron injection and transportation. Additionally, based on previous reports on the hysteresis in TiO2 ETL based PSCs, the J-V hysteresis in C-Nb2O5 ETL based devices is probably due to the unbalanced electrons and holes flux from perovskite to ETLs and HTLs, respectively.56 Therefore, the J-V hysteresis in different ETL-based PSCs could be alleviated or even eliminated by the interfacial modification approach.63

4. Conclusions

In summary, a facile deposition of Nb2O5 film was obtained by a solution-processed spin-coating procedure, and both mesoscopic and planar types of PSCs based on Nb2O5 film were investigated. The CM-Nb2O5 ETL based mesoscopic PSC achieves a PCE of 17.4%, while a PCE as high as 19.2% is obtained in the planar PSCs by controlling the spin-coating speed of the precursor solution. The impressive device performance is possibly attributed to the suitable thickness of C-Nb2O5 ETL and excellent quality of perovskite film, which resulted in efficient charge transportation and collection. Therefore, according to these findings, Nb2O5 can be utilized as a promising alternative to the currently prevalent TiO2 ETL for efficient and stable PSCs. The procedures presented in this report provide a facile approach for the large-scale fabrication of ETLs for use in PSCs.



Page 26 of 37

ASSOCIATED CONTENT Supporting Information XRD pattern, TEM images and XPS spectrum of the obtained Nb2O5 sample.(Figure S1 and S2). TR-PL spectra of perovskite film with CM-Nb2O5 and C-Nb2O5 as the ETLs, respectively. (Figure S3) Histograms of PCEs determined from 30 PSC devices based on CM-Nb2O5 and C-Nb2O5, respectively. (Figure S4) The SEM surface images of (FAPbI3)1-x(MAPbBr3)x-based layer deposited on (a) C2-Nb2O5, (b) C4-Nb2O5 and (c) C6-Nb2O5 ETL, respectively. (Figure S5) Statistical histogram of PCE for 30 measured devices based on C4-Nb2O5 as ETL. (Figure S6) Nyquist plots of the PSCs based on C-Nb2O5 as the ETL which was spin-coated on the FTO under different speeds. (Figure S7) EIS parameters of PSCs based on C-Nb2O5 as the ETL which was spin-coated on the FTO under different speeds. (Table S1) Long-term stability test of the C4-Nb2O5 ETL based PSC. (Figure S8) J-V characteristics in the forward and reverse scanning directions of the PSCs employing C-Nb2O5 as the ETL which was spin-coated on the FTO under different speeds. ( Figure S9) ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC 91433104 and 21750110442) and Fujian Provincial Department of Science and Technology (2015J05022).


Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050 (2) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional engineering of perovskite materials for high-performance solar cells. Nature 2015, 517, 476 (3) Bi, D. Q.; Tress, W.; Dar, M. I.; Gao, P.; Luo, J. S.; Renevier, C.; Schenk, K.; Abate, A.; Giordano, F.; Baena, J. P. C.; Decoppet, J. D.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, M.; Hagfeldt, A. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Science Advances 2016, 2, 7 (4) Saliba, M.; Matsui, T.; Seo, J. Y.; Domanski, K.; Correa-Baena, J. P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; Gratzel, M. Cesium-containing

triple

cation

perovskite

solar cells:

improved

stability,

reproducibility and high efficiency. Energy & Environmental Science 2016, 9, 1989 (5) Xing, G. C.; Mathews, N.; Sun, S. Y.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344 (6) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643 (7) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nature Photonics 2014, 8, 506 27 ACS Paragon Plus Environment


Page 28 of 37

(8) 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. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376 (9) Roose, B.; Johansen, C. M.; Dupraz, K.; Jaouen, T.; Aebi, P.; Steiner, U.; Abate, A. A Ga-doped SnO2 mesoporous contact for UV stable highly efficient perovskite solar cells. Journal of Materials Chemistry A 2018 (10) Ke, W. J.; Fang, G. J.; Liu, Q.; Xiong, L. B.; Qin, P. L.; Tao, H.; Wang, J.; Lei, H. W.; Li, B. R.; Wan, J. W.; Yang, G.; Yan, Y. F. Low-Temperature Solution-Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 6730 (11) Yang, G.; Chen, C.; Yao, F.; Chen, Z. L.; Zhang, Q.; Zheng, X. L.; Ma, J. J.; Lei, H. W.; Qin, P. L.; Xiong, L. B.; Ke, W. J.; Li, G.; Yan, Y. F.; Fang, G. J. Effective Carrier-Concentration Tuning of SnO2 Quantum Dot Electron-Selective Layers for High-Performance Planar Perovskite Solar Cells. Adv Mater 2018, 30, 9 (12) An, Q. Z.; Fassl, P.; Hofstetter, Y. J.; Becker-Koch, D.; Bausch, A.; Hopkinson, P. E.; Vaynzof, Y. High performance planar perovskite solar cells by ZnO electron transport layer engineering. Nano Energy 2017, 39, 400 (13) Mahmood, K.; Swain, B. S.; Kirmani, A. R.; Amassian, A. Highly efficient perovskite solar cells based on a nanostructured WO3-TiO2 core-shell electron transporting material. Journal of Materials Chemistry A 2015, 3, 9051 (14) Bao, S.; Wu, J. H.; He, X.; Tu, Y. G.; Wang, S. B.; Huang, M. L.; Lan, Z. 28 ACS Paragon Plus Environment

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mesoporous Zn2SnO4 as effective electron transport materials for high-performance perovskite solar cells. Electrochimica Acta 2017, 251, 307 (15) Chen, Z. N.; Pang, A.; Shen, D.; Wei, M. Highly Efficient Perovskite Solar Cells Based on Zn2Ti3O8 Nanoparticles as Electron Transporting Material with Efficiency over 17. ChemSusChem 2017 (16) Okamoto, Y.; Suzuki, Y. Mesoporous BaTiO3/TiO2 Double Layer for Electron Transport in Perovskite Solar Cells. J. Phys. Chem. C 2016, 120, 13995 (17) Wang, C.; Tang, Y.; Hu, Y. J.; Huang, L.; Fu, J. X.; Jin, J.; Shi, W. M.; Wang, L. J.; Yang,

W.

G.

Graphene/SrTiO3

nanocomposites

used

as

an

effective

electron-transporting layer for high-performance perovskite solar cells. Rsc Advances 2015, 5, 52041 (18) Yun, J.; Ryu, J.; Lee, J.; Yu, H.; Jang, J. SiO2/TiO2 based hollow nanostructures as scaffold layers and Al-doping in the electron transfer layer for efficient perovskite solar cells. Journal of Materials Chemistry A 2016, 4, 1306 (19) Yang, D.; Zhou, X.; Yang, R. X.; Yang, Z.; Yu, W.; Wang, X. L.; Li, C.; Liu, S. Z.; Chang, R. P. H. Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells. Energy & Environmental Science 2016, 9, 3071 (20) Jiang, Q.; Zhang, L. Q.; Wang, H. L.; Yang, X. L.; Meng, J. H.; Liu, H.; Yin, Z. G.; Wu, J. L.; Zhang, X. W.; You, J. B. Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)(2)PbI3-based perovskite solar cells. Nature Energy 2017, 2, 1 (21) Mahmud, M. A.; Elumalai, N. K.; Upama, M. B.; Wang, D. A.; Chan, K. H.; 29 ACS Paragon Plus Environment


Page 30 of 37

Wright, M.; Xu, C.; Hague, F.; Uddin, A. Low temperature processed ZnO thin film as electron transport layer for efficient perovskite solar cells. Solar Energy Materials and Solar Cells 2017, 159, 251 (22) Hu, T.; Xiao, S.; Yang, H.; Chen, L.; Chen, Y. Cerium oxide as an efficient electron extraction layer for p-i-n structured perovskite solar cells. Chemical Communications 2018, 54, 471 (23) Tan, H. R.; Jain, A.; Voznyy, O.; Lan, X. Z.; de Arquer, F. P. G.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M. J.; Zhang, B.; Zhao, Y. C.; Fan, F. J.; Li, P. C.; Quan, L. N.; Zhao, Y. B.; Lu, Z. H.; Yang, Z. Y.; Hoogland, S.; Sargent, E. H. Efficient and stable solution-processed planar perovskite solar cells via contact passivation. Science 2017, 355, 722 (24) Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542 (25) Li, W. Z.; Zhang, W.; Van Reenen, S.; Sutton, R. J.; Fan, J. D.; Haghighirad, A. A.; Johnston, M. B.; Wang, L. D.; Snaith, H. J. Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification. Energy & Environmental Science 2016, 9, 490 (26) Wojciechowski, K.; Leijtens, T.; Siprova, S.; Schlueter, C.; Horantner, M. T.; Wang, J. T. W.; Li, C. Z.; Jen, A. K. Y.; Lee, T. L.; Snaith, H. J. C-60 as an Efficient n-Type Compact Layer in Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 2399 (27) Ito, S.; Tanaka, S.; Manabe, K.; Nishino, H. Effects of Surface Blocking Layer of 30 ACS Paragon Plus Environment

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sb2S3 on Nanocrystalline TiO2 for CH3NH3PbI3 Perovskite Solar Cells. J. Phys. Chem. C 2014, 118, 16995 (28) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells. Nature Communications 2013, 4, 8 (29) Liu, D. Y.; Kelly, T. L. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nature Photonics 2014, 8, 133 (30) Cheng, Y. H.; Yang, Q. D.; Xiao, J. Y.; Xue, Q. F.; Li, H. W.; Guan, Z. Q.; Yip, H. L.; Tsang, S. W. Decomposition of Organometal Halide Perovskite Films on Zinc Oxide Nanoparticles. ACS Appl. Mater. Interfaces 2015, 7, 19986 (31) Tiwana, P.; Docampo, P.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Electron Mobility and Injection Dynamics in Mesoporous ZnO, SnO2, and TiO2 Films Used in Dye-Sensitized Solar Cells. Acs Nano 2011, 5, 5158 (32) Anaraki, E. H.; Kermanpur, A.; Steier, L.; Domanski, K.; Matsui, T.; Tress, W.; Saliba, M.; Abate, A.; Gratzel, M.; Hagfeldt, A.; Correa-Baena, J. P. Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide. Energy & Environmental Science 2016, 9, 3128 (33) Qin, M. C.; Ma, J. J.; Ke, W. J.; Qin, P. L.; Lei, H. W.; Tao, H.; Zheng, X. L.; Xiong, L. B.; Liu, Q.; Chen, Z. L.; Lu, J. Z.; Yang, G.; Fang, G. J. Perovskite Solar Cells Based on Low-Temperature Processed Indium Oxide Electron Selective Layers. ACS Appl. Mater. Interfaces 2016, 8, 8460 31 ACS Paragon Plus Environment


Page 32 of 37

(34) Wang, K.; Shi, Y. T.; Dong, Q. S.; Li, Y.; Wang, S. F.; Yu, X. F.; Wu, M. Y.; Ma, T. L. Low-Temperature and Solution-Processed Amorphous WOx as Electron-Selective Layer for Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 755 (35) Rani, R. A.; Zoolfakar, A. S.; O'Mullane, A. P.; Austin, M. W.; Kalantar-Zadeh, K. Thin films and nanostructures of niobium pentoxide: fundamental properties, synthesis methods and applications. Journal of Materials Chemistry A 2014, 2, 15683 (36) Jose, R.; Thavasi, V.; Ramakrishna, S. Metal Oxides for Dye-Sensitized Solar Cells. J. Am. Ceram. Soc. 2009, 92, 289 (37) Luo, H. L.; Song, W. J.; Hoertz, P. G.; Hanson, K.; Ghosh, R.; Rangan, S.; Brennaman, M. K.; Concepcion, J. J.; Binstead, R. A.; Bartynski, R. A.; Lopez, R.; Meyer, T. J. A Sensitized Nb2O5 Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell. Chemistry of Materials 2013, 25, 122 (38) Xia, J. B.; Masaki, N.; Jiang, K. J.; Yanagida, S. Sputtered Nb2O5 as a novel blocking layer at conducting Glass/TiO2 interfaces in dye-sensitized ionic liquid solar cells. J. Phys. Chem. C 2007, 111, 8092 (39) Sayama, K.; Sugihara, H.; Arakawa, H. Photoelectrochemical properties of a porous Nb2O5 electrode sensitized by a ruthenium dye. Chemistry of Materials 1998, 10, 3825 (40) Ou, J. Z.; Rani, R. A.; Ham, M. H.; Field, M. R.; Zhang, Y.; Zheng, H.; Reece, P.; Zhuiykov, S.; Sriram, S.; Bhaskaran, M.; Kanee, R. B.; Kalantar-Zadeh, K. Elevated Temperature Anodized Nb2O5: A Photoanode Material with Exceptionally Large Photoconversion Efficiencies. Acs Nano 2012, 6, 4045 32 ACS Paragon Plus Environment

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Le Viet, A.; Jose, R.; Reddy, M. V.; Chowdari, B. V. R.; Ramakrishna, S. Nb2O5 Photoelectrodes for Dye-Sensitized Solar Cells: Choice of the Polymorph. J. Phys. Chem. C 2010, 114, 21795 (42) Kogo, A.; Numata, Y.; Ikegami, M.; Miyasaka, T. Nb2O5 Blocking Layer for High Open-circuit Voltage Perovskite Solar Cells. Chemistry Letters 2015, 44, 829 (43) Fernandes, S. L.; Veron, A. C.; Neto, N. F. A.; Nuesch, F. A.; da Silva, J. H. D.; Zaghete, M. A.; Graeff, C. F. D. Nb2O5 hole blocking layer for hysteresis-free perovskite solar cells. Materials Letters 2016, 181, 103 (44) Feng, J. S.; Yang, Z.; Yang, D.; Ren, X. D.; Zhu, X. J.; Jin, Z. W.; Zi, W.; Wei, Q. B.; Liu, S. Z. E-beam evaporated Nb2O5 as an effective electron transport layer for large flexible perovskite solar cells. Nano Energy 2017, 36, 1 (45) Yang, D.; Yang, R. X.; Ren, X. D.; Zhu, X. J.; Yang, Z.; Li, C.; Liu, S. Z. Hysteresis-Suppressed High-Efficiency Flexible Perovskite Solar Cells Using Solid-State Ionic-Liquids for Effective Electron Transport. Adv Mater 2016, 28, 5206 (46) Yang, D.; Yang, Z.; Qin, W.; Zhang, Y. L.; Liu, S. Z.; Li, C. Alternating precursor layer deposition for highly stable perovskite films towards efficient solar cells using vacuum deposition. Journal of Materials Chemistry A 2015, 3, 9401 (47) Niu, J. Z.; Yang, D.; Ren, X. D.; Yang, Z.; Liu, Y. C.; Zhu, X. J.; Zhao, W. G.; Liu, S. Z. Graphene-oxide doped PEDOT: PSS as a superior hole transport material for high-efficiency perovskite solar cell. Org. Electron. 2017, 48, 165 (48) Zhu, X. J.; Yang, D.; Yang, R. X.; Yang, B.; Yang, Z.; Ren, X. D.; Zhang, J.; Niu, J. Z.; Feng, J. S.; Liu, S. Z. Superior stability for perovskite solar cells with 20% 33 ACS Paragon Plus Environment


Page 34 of 37

efficiency using vacuum co-evaporation. Nanoscale 2017, 9, 12316 (49) Lu, H. Y.; Xiang, K. X.; Bai, N. B.; Zhou, W.; Wang, S. L.; Chen, H. Urchin-shaped Nb2O5 microspheres synthesized by the facile hydrothermal method and their lithium storage performance. Materials Letters 2016, 167, 106 (50) Dou, J.; Li, Y. F.; Xie, F. Y.; Ding, X. K.; Wei, M. D. Metal-Organic Framework Derived Hierarchical Porous Anatase TiO2 as a Photoanode for Dye-Sensitized Solar Cell. Crystal Growth & Design 2016, 16, 121 (51) Roose, B.; Godel, K. C.; Pathak, S.; Sadhanala, A.; Baena, J. P. C.; Wilts, B. D.; Snaith, H. J.; Wiesner, U.; Gratzel, M.; Steiner, U.; Abate, A. Enhanced Efficiency and Stability of Perovskite Solar Cells Through Nd-Doping of Mesostructured TiO2. Advanced Energy Materials 2016, 6, 7 (52) Ma, Q.; Rosenberg, R. A. Angle-resolved X-ray photoelectron spectroscopy study of the oxides on Nb surfaces for superconducting r.f. cavity applications. Appl. Surf. Sci. 2003, 206, 209 (53) Ahn, N.; Son, D. Y.; Jang, I. H.; Kang, S. M.; Choi, M.; Park, N. G. 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. 2015, 137, 8696 (54) Lan, Z.; Xu, X.; Zhang, X.; Tang, J.; Zhang, L.; He, X.; Wu, J. Low-temperature solution-processed efficient electron-transporting layers based on BF4-capped TiO2 nanorods for high-performance planar perovskite solar cells. Journal of Materials Chemistry C 2018, 6, 334 34 ACS Paragon Plus Environment

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(55) Orel, B.; Macek, M.; Grdadolnik, J.; Meden, A. In situ UV-Vis and ex situ IR spectroelectrochemical investigations of amorphous and crystalline electrochromic Nb2O5 films in charged/discharged states. J. Solid State Electrochem. 1998, 2, 221 (56) Xu, Y.; Schoonen, M. A. A. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am. Mineral. 2000, 85, 543 (57) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 7 (58) Heo, J. H.; Song, D. H.; Han, H. J.; Kim, S. Y.; Kim, J. H.; Kim, D.; Shin, H. W.; Ahn, T. K.; Wolf, C.; Lee, T. W.; Im, S. H. Planar CH3NH3PbI3 Perovskite Solar Cells with Constant 17.2% Average Power Conversion Efficiency Irrespective of the Scan Rate. Adv Mater 2015, 27, 3424 (59) Yang, D.; Yang, R. X.; Zhang, J.; Yang, Z.; Liu, S. Z.; Li, C. High efficiency flexible perovskite solar cells using superior low temperature TiO2. Energy & Environmental Science 2015, 8, 3208 (60) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Advanced Energy Materials 2015, 5, 23 (61) Park, N. G.; Gratzel, M.; Miyasaka, T.; Zhu, K.; Emery, K. Towards stable and commercially available perovskite solar cells. Nature Energy 2016, 1, 8 (62) Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J. Efficient 35 ACS Paragon Plus Environment


Page 36 of 37

organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nature Communications 2013, 4, 6 (63) Zhao, C.; Chen, B. B.; Qiao, X. F.; Luan, L.; Lu, K.; Hu, B. Revealing Underlying Processes Involved in Light Soaking Effects and Hysteresis Phenomena in Perovskite Solar Cells. Advanced Energy Materials 2015, 5, 6


Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Nb2O5 thin film was obtained by a facile deposition approach, and was employed as the electron-transporting layer for perovskite solar cells, resulting in a maximum PCE of 19.2%.


Facile deposition of Nb2O5 thin film as an electron-transporting layer

Recommend Documents