Interface engineering by using TiO2 nanocubic modifier in planar heterojunction perovskite solar cells

Journal Pre-proof Interface engineering by using TiO2 nanocubic modifier in planar heterojunction perovskite solar cells Pengyu Su, Li Liu, Huizhen Yao, Tie Liu, Bowen Zhang, Jun Wang, Shuang Feng, A. Runa, Wuyou Fu, Haibin Yang PII:

S1566-1199(19)30517-8

DOI:

https://doi.org/10.1016/j.orgel.2019.105490

Reference:

ORGELE 105490

To appear in:

Organic Electronics

Received Date: 16 May 2019 Revised Date:

10 August 2019

Accepted Date: 6 October 2019

Please cite this article as: P. Su, L. Liu, H. Yao, T. Liu, B. Zhang, J. Wang, S. Feng, A. Runa, W. Fu, H. Yang, Interface engineering by using TiO2 nanocubic modifier in planar heterojunction perovskite solar cells, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105490. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical abstract: Wo and w-1 devices based on wo and w-1 TiO2

Interface Engineering by Using TiO2 Nanocubic Modifier in Planar Heterojunction Perovskite Solar Cells Pengyu Sua, Li Liua, Huizhen Yaob, Tie Liua, Bowen Zhanga, Jun Wanga, Shuang Fenga, Runa Aa, Wuyou Fua,*, Haibin Yanga a

State Key Laboratory of Superhard Materials, Jilin University, Qianjin Street 2699, Changchun,

130012, People’s Republic of China b

SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology,

International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China Author for correspondence: Wuyou Fu E-mail: [email protected] Keywords: TiO2 nanocubic; interfacial modifier; suppress charge recombination; perovskite; solar cells Abstract: In this work we adopt a nanoscale TiO2 nanocubic particle as interfacial modifier between compact TiO2 layer and CH3NH3PbI3 layer. The TiO2 nanocubic modifier can passivate the defect states and suppress charge recombination at the TiO2/MAPbI3 interface. As a result, the photovoltaic performances of planar heterojunction perovskite solar cells were effectively boosted. Under standard air-mass 1.5 global illumination, the open-circuit voltage of solar cells with TiO2 nanocubic modifier was increased from 0.93 V to 1.02 V, and achieved a maximum power conversion efficiency of 14.70% with average value of 13.36%, which is 1.3 times higher than the solar cells based on without modified TiO2 layer (10.44%).

1. Introduction

Organic-inorganic hybrid perovskite semiconductor materials, such as ABX3 (A = methylammonium, formamidinium, Cs or mixed-cation; B = Pb; X = I, Br, Cl or mixed-anion), have become a promising light-harvesting material for perovskite solar cells (PSCs) [1-6]. The power conversion efficiency (PCE) of PSCs have surpassed 23% on laboratory-type cells, which is comparable to conventional silicon-based photovoltaics [7]. Planar heterojunction perovskite solar cells exhibit highly efficient photoelectrical conversion capacity and have aroused great attention due to their low processing costs, simple fabrication, flexibility for device optimization and multijunction construction [8-9]. The configuration of a typical planar heterojunction PSC is constituted by a glass/transparent conductive oxide/electron-transport layer (ETL)/perovskite light absorption layer/hole-transport layer (HTL)/metal electrode [10-20]. In recent years, intense efforts have been devoted to developing planar heterojunction PSCs and provided solar cells with PCE exceeding 21% [21-23]. For planar heterojunction PSCs, TiO2 is commonly employed as an electron-transport layer which has a significant influence on the quality of perovskite layer and the interface between the two layers [24-27]. The compact TiO2 (c-TiO2) layer, which can be easily fabricated by spin-coating an acidic solution of tetrabutyl titanate in ethanol, was used as ETL. The devices with different c-TiO2 films were investigated in the supplementary materials (Fig. S1,2,3 and Table S1). Nevertheless, pinholes exist in the thin c-TiO2 films, resulting in severe charge recombination due to the selective contact of TiO2 ETL and perovskite layer. Furthermore, this phenomenon will lead to huge PCE loss in planar heterojunction PSCs. The nobel laureate professor Herbert Kroemer once said that “the interface is the device” [28]. The interfacial property has been shown to play a

vital role in PSCs with superior photovoltaic performance in the past few years. The defect states present at the interface between TiO2 layer and perovskite absorption layer could potentially be improved by modifying the interface. Various interface modification methods have been employed to further boost the performance of PSCs [18, 24, 29]. Excellent interface property is essential for high-efficiency PSCs. Hence, eliminating interfacial defects has an important significance in boosting the performance of planar heterojunction PSCs. In this study, a simple and effective interfacial modification method was introduced to fabricate efficient planar heterojunction PSCs. A novel nanoscale TiO2 nanocubic film was adopted as the interfacial modifier between c-TiO2 and CH3NH3PbI3 (MAPbI3). The resultant TiO2 film possessed superior electron extraction ability and effectively suppressed charge recombination in the interface of TiO2/MAPbI3. Furthermore, the modified TiO2 nanocubic ETL bonded tightly with perovskite layer and provided a better platform for the growth of high-quality perovskite absorber. Obviously, the enhanced properties of TiO2 film are benefit for the TiO2 nanocubic modifier. The interface modification is responsible for the enhanced photovoltaic performance of PSCs with an average PCE of 13.36%. 2. Experimental 2.1. Experimental The compact TiO2 (c-TiO2) layer and interfacial modification layer were synthesized by simple spin-coating and chemical bath deposition (CBD) method, respectively. The c-TiO2 was deposited on clean SnO2:F (FTO) substrates by spin-coating an acidic solution of butyl titanate in ethanol (the concentrations of tetrabutyl titanate/acetic acid/ethanol = 0.07 mL/0.13 mL/0.78 mL) at 500 rmp for 5 s and 4000 rpm for 30 s followed. After drying at 100 °C for 10 min, they were

sintering at 500 °C for 30 min in the air. For synthesizing TiO2 interface modification layer, the FTO/c-TiO2 substrates were immersed into an aqueous solution containing 25 mM TiCl4 and (NH4)2TiF6 with different concentrations (0 mM, 0.5 mM, 1 mM and 1.5 mM) at 70 °C for 30 min. In addition, the TiO2 films were annealed at 450 °C for 2 h in air atmosphere and cooled down to room temperature. In this hydrolyzation process, TiCl4 was used as Ti source. Another reaction precursor (NH4)2TiF6 played two roles in the process: (1) Ti source; (2) F- as the shape modifier of TiO2 nanoparticles. After added (NH4)2TiF6 into the precursor solution, the shape of TiO2 nanoparticles changed from ellipsoidal to cubical. For clarity of presentation, in the following sections, we designate the c-TiO2 film as wo TiO2 (without modified TiO2), and designate the modified c-TiO2 films with 0 mM (the solution only contains 25 mM TiCl4), 0.5 mM, 1 mM and 1.5 mM (NH4)2TiF6 as w-0, w-0.5, w-1 and w-1.5 TiO2 films. The CH3NH3PbI3 films were fabricated by a one-step solution deposition method in the air within the humidity of 15-25 %. 462 mg of PbI2, 159 mg of CH3NH3I, and 160 mL of DMSO were mixed in 640 mL of DMF solution at room temperature. The completely dissolved solution was spin-coated on the TiO2 films at 500 rpm for 5 sec and 4000 rpm for 20 sec followed. 500 µL of diethyl ether was dropped onto the rotating substrate during the high speed at 10 sec before the end of this procedure. All the films were kept for 10 min and then annealed at 100 °C for 10 min in air. The colour of annealed films changed from transparent to dark brown indicating the crystallization of MAPbI3. 50 µL hole transport layer (HTL) as described in reference was spin-coated

onto

the

MAPbI3

film

at

4000

rmp

for

10

sec

(0.073

g

2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD), 29 µL of 4-tert-butylpyridine, 18 µL of a lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution

(520 mg Li-TFSI/1 mL in acetonitrile) were dissolved in 1 mL chlorobenzene) [30]. Finally, a 80 nm Ag back electrode was deposited on top of HTL under high vacuum by thermal evaporation, and the active area of devices were 0.12 cm2. For clarity of presentation, in the following sections, we designate the PSCs based on wo TiO2 and w-1 TiO2 films as wo and w-1 devices. 2.2. Characterization The morphology and microstructure of films were observed by the FEI MAGELLAN 400 Scanning Electron Microscope (SEM). High-resolution transmission electron microscopy (HR-TEM) was performed by using a JEOL JEM-2200FS at an acceleration voltage of 200 KV. Ultraviolet-visible (UV-vis) absorption spectra were recorded on a UV-3150 double-beam spectrophotometer at room temperature. The composition and crystal structure of products were checked by X-ray diffractometer (XRD, Rigaku D/max-2500, Cu Kα radiation, λ = 1.5418 Å). The steady-state photoluminescence (PL) spectra were obtained through a micro-Raman spectrometer (HR Evolution) with an excitation source of 473 nm laser. The electrochemical impedance spectroscopy (EIS) measurements were conducted on the Solartron 1260 impedance analyser equipped with a Solartron 1296 dielectric interface with the frequency range between 107 Hz and 10-1 Hz. The external quantum efficiency (EQE) curves were recorded on QTest Station 1000A (CROWNTECH, INC.). The J-V curves of devices were measured by a Keithley 2400 source measure unit. 2. Results and discussion

Fig. 1. Top-view SEM images of (a-e) wo TiO2, w-0 TiO2, w-0.5 TiO2, w-1 TiO2 and w-1.5 TiO2. (f-j) The corresponding cross-section SEM images. (k-o) The schematic diagrams, respectively. From Fig. 1, a dual layer film formed on the FTO after introducing interfacial modifier, and the top layer grow with the increasing concentration of (NH4)2TiF6. As shown as Fig. 1a and f, the without modified wo TiO2 film is so thin that FTO substrate can be observed clearly. The thin wo TiO2 film may exist pinholes (the areas marked with red-dotted lines in Fig. 1 and Fig. S1), which

can capture photo-generated carriers and decrease the performance of PSCs. To overcome this limitation, interfacial modification is introduced to wipe out defect states. As shown in Fig. 1b and g, an ultrathin TiO2 layer is formed on the wo TiO2 film when the added (NH4)2TiF6 is 0 mM (only TiCl4 in the solution). The SEM images suggest that the top layer of w-0 TiO2 is composed of nanoparticles. Interestingly, the shape of TiO2 nanoparticles change from ellipsoidal to cubical due to the introduced F-. Fig. 1c and h are the SEM images of w-0.5 TiO2, which own more cubic nanoparticles in the top layer but the same thickness compared with w-0 TiO2. During the CBD process, the added reaction precursor ((NH4)2TiF6) increase the solution concentration, leading to a thicker TiO2 top layer. As shown in Fig. 1d, i, n, it is clear that the w-1 TiO2 film is thicker and rougher than wo, w-0 and w-0.5 TiO2 films. The thicker w-1 TiO2 film with nanocubic modifier may mitigate defect states and suppress the interfacial recombination effectively. The rougher surface is benefit for the interface binding and can enlarge the contact area with MAPbI3, providing more transfer paths for electrons. After modified with 1.5 mM (NH4)2TiF6 and 25 mM TiCl4, some cracks are observed in the SEM image of w-1.5 TiO2 (Fig. 1e). These cracks may lead to current leakage in PSCs. In addition, the thicker w-1.5 TiO2 film decreases the light transmittance, so the number of photons reaching the MAPbI3 layer will be reduced. As a result, the photocurrent of device dropped, and the device performance degraded.

Fig. 2. HR-TEM images of (a) w-0 TiO2 and (b) w-1 TiO2. (c) Enlarged SEM view of w-1 TiO2

film. As shown in Fig. 2a, the w-0 TiO2 nanoparticles are ellipsoidal which with grain sizes of 5-10 nm. Fig. 2b and c are the HR-TEM and enlarged SEM images of w-1 TiO2 film, it is obvious that the small nanoparticles are cubic particles and sizes with ranging from 15 to 25 nm. In addition, the inter-planar distance between the neighbouring lattice fringes of both TiO2 is 0.352 nm which is corresponding to the inter-planar distance from (101) planes of TiO2 [JCPDS no. 21-1272] (Fig. 2a, b). This investigation also implies that no phase transformation taken place during modification, though F- is introduced in the CBD process.

Fig. 3. (a) UV-vis absorption spectra and (b) band gap of wo TiO2 and w-1 TiO2. (c) Energy level diagrams of FTO/wo TiO2/MAPbI3 and FTO/w-1 TiO2/MAPbI3. The ultraviolet-visible (UV-vis) absorption spectra of wo TiO2 and w-1 TiO2 are plotted in Fig. 3a as a function of wavelength. Notably, the modified nanocubic w-1 TiO2 film exhibit superior absorption capacity. This can be attributed to the nanocubic particles in the w-1 TiO2 film, which increase the capture ability to incident light. The optical path of incident light in the w-1 TiO2 film becomes longer, so the absorption of TiO2 film becomes stronger. Both TiO2 films exhibit absorption in the ultraviolet spectral region and have no significant absorption for visible

light due to the wide band gap of TiO2. This result can also be confirmed in Fig. 3b. It is well known that the band gap of materials depends on the absorption coefficient (α) and discontinuous energy of the photons. The relationship between them is as follows: (αhυ)2=c(hυ-Eg)

(1)

c is a constant, υ is frequency and h is the Planck constant. As shown in Fig. 3b, the optical band gap can be obtained by the external tangent of the (αhυ)-photon energy curve at α=0. Through this curve, we can get the band gaps of wo TiO2 and w-1 TiO2 are 3.25 eV and 3.22 eV. The band gap narrows as the particle size increases, so we speculate the change is because the larger particle size of w-1 TiO2. Fig. 3c describes the transport mechanism of photo-generated electrons between energy bands. As shown in Fig. 3c, the energy-level of MAPbI3 matched well with both TiO2. Electrons on the MAPbI3 valence band (VB) were excited to the conduction band (CB) under illumination. Then, they transferred along the purple arrow to the TiO2 CB, and finally collected by the FTO electrode.

Fig. 4. (a) XRD patterns of FTO/wo TiO2/MAPbI3 and FTO/w-1 TiO2/MAPbI3. (b) Schematic of the MAPbI3 XRD structure viewed along c axis. The MAPbI3 films deposited on wo TiO2 and w-1 TiO2 are investigated. From the XRD patterns, which is presented in Fig. 4a, it is evident that neither PbI2 nor other nonperovskite phases in MAPbI3 film, indicating the film fabricated from perovskite precursor solution crystallize in a pure tetragonal phase MAPbI3. However, the full width of half maximum (FWHM) of all the diffraction peaks of FTO/w-1 TiO2/MAPbI3 film are smaller than FTO/wo TiO2/MAPbI3 film (Table S2). It indicates that the MAPbI3 film deposited on w-1 TiO2 has higher crystallinity. Fig. 4b is the schematic of the MAPbI3 XRD structure viewed along c axis. MAPbI3 crystals grow along the [110] direction as indicated by the red arrow.

Fig. 5. Top-view SEM images of (a) FTO/wo TiO2/MAPbI3 and (b) FTO/w-1 TiO2/MAPbI3. Cross-sectional SEM images of (c) FTO/w-1 TiO2/MAPbI3 film, (d) enlarged w-1 device, (e) wo device and (f) w-1 device. Fig. 5a is the top-view SEM image of FTO/wo TiO2/MAPbI3, it is clear that many gaps exist in the perovskite layer, which will lead to the direct contact between wo TiO2 ETL and HTL. As a

result, the carrier recombination rate will be aggravated and the photovoltaic performance will be poor. As shown in Fig. 5b, the MAPbI3 film grown on top of w-1 TiO2 film is smooth and gap-free. Moreover, MAPbI3 film is uniform and with large grains, the size of some grains is even greater than 1 µm. As previous reports, large grain size means less grain boundaries, which can significantly reduce charge-carrier recombination at grain boundaries in perovskite film [31-32]. Fig. 5c shows that high-quality monolayer MAPbI3 film is formed and binding with TiO2 firmly. Compared with multilayer MAPbI3, monolayer MAPbI3 exist less grain boundaries which can trap photon-generated carriers and lead to a low PCE. In devices with monolayer MAPbI3 films, charges do not need to go through any grain boundaries during their transport in the out-of-plane direction before being collected by electrodes [31]. Therefore, the gap-free and monolayer MAPbI3 film with large grain size is beneficial for fabricating high performance PSCs. PSCs based on w-1 TiO2 film were fabricated with the device architecture of FTO/w-1 TiO2/MAPbI3/HTL/Ag as shown in Fig. 5d.

Fig. 6. Contact angles of deionized water on (a) wo TiO2 and (b) w-1 TiO2. Fig. 5e and f depict the cross-sectional SEM images of PSC devices based on wo and w-1 TiO2 films, respectively. As indicated in Fig. 5e, some gaps exist in MAPbI3 film and the interface of wo TiO2/MAPbI3. These gaps will result in defect states and trapped the photo-generated

carriers in the MAPbI3 layer and at the interface with TiO2 ETL. Compared with the device based on wo TiO2 film, no gaps were found in w-1 device. Additionally, MAPbI3 layer is developed with full coverage on w-1 TiO2 (Fig. 5f). In order to gain some insight into the mechanism underlying the performance improvement, we measured the contact angle of deionized water on both TiO2 surface. As shown in Fig. 6, the contact angles decreased from 66.2o to 32.4o, indicating that TiO2 nanocubic modifier can improve the wetting between the perovskite layer and the TiO2 layer. This suggests that the nanocubic modifier can result in stronger binding at the w-1 TiO2/MAPbI3 interface and provide a better platform for the growth of high-quality perovskite absorber.

Fig. 7. (a) Steady-state PL spectra for perovskite films deposited on glass, wo TiO2 and w-1 TiO2. (b) Time-resolved PL of FTO/wo TiO2/MAPbI3 and FTO/w-1 TiO2/MAPbI3 taken at the emission of 700 nm, and the insert table is a summary of lifetime parameters. Steady-state photoluminescence (PL) spectra is an effective characterization method to

explore the transfer efficiency of light-excited charges in PSCs. Fig. 7a shows the steady-state PL spectra of glass/MAPbI3, glass/wo TiO2/MAPbI3 and glass/w-1 TiO2/MAPbI3 samples. All of the PL emission peaks are located at 770 nm. Moreover, the PL intensity of glass/wo TiO2/MAPbI3, which processed with TiO2, is lower than the glass/MAPbI3 sample. The PL peak intensity is further quenched after forming the dual layer w-1 TiO2, indicating a lower photo-generated carrier recombination and prospectively resulting in a superior device performance. These results can be ascribed to the interfacial modification which can improve the film quality of MAPbI3 and help to suppress the photo-generated carrier recombination and it is conducive to electron extraction at the interface between TiO2 and MAPbI3. The electron extraction capability of ETLs was further assessed by the time-resolved photoluminescence (TRPL) decay spectra. Perovskite film processes three dominating charge-carrier recombination: Auger recombination, trap-related non-radiative recombination and radiative recombination [33]. Fig. 7b shows the TRPL spectra of samples depending on different TiO2 films, which were taken at the emission of 700 nm. The carrier lifetimes for both samples were obtained by fitting the decay curves using a three-exponential equation:

f (t ) = A + B1 exp(−t / τ 1 ) + B 2 exp(−t / τ 2 ) + B 3 exp(−t / τ 3 )

(2)

The average decay time (τave) was determined by the equation:

τ ave

B τ + B2τ 2 + B3τ 3 = 11 B1τ 1 + B2τ 2 + B3τ 3 2

2

2

(3)

where τi are the decay time of each decay channel and Bi are the amplitude for each exponential component. The insert table presented the parameters extracted from the fits. Notably, w-1 shows a τave of 74.69 ns, which is faster than 95.3 ns of the wo sample. The faster electron injection process from MAPbI3 layer to w-1 TiO2 ETL is indispensable for PL quenching.

Fig. 8. Nyquist plots obtained from electrochemical impedance spectra (EIS) results for wo and w-1 devices. The inset shows the equivalent circuit used for fitting. Table 1 Fitted parameters from EIS of devices based on wo and w-1 TiO2. TiO2

Rs (ohm)

Rtr (ohm)

CPEtr (F)

Rrec (ohm)

CPErec (F)

wo

4.991×10-2

220

2.4×10-6

920

1.8×10-6

w-1

1.014×10-3

135

3×10-5

1250

3.3×10-6

In order to further clarify the effect of TiO2 nanocubic modifier on carrier recombination, electrochemical impedance spectroscopy (EIS) was conducted to investigate the charge-transfer properties of photovoltaic devices. Two charge-transfer processes are usually investigated in PSCs: transport and recombination [34-35]. Therefore, a equivalent circuit with two series circuits was created. As shown in Fig. 8, the insert picture depicts the equivalent circuit which can be used to the Nyquist plots and extract the impedance parameters. The equivalent circuit composed of Rs (series resistance), Rtr (transport resistance), CPEtr (transport capacitance), Rrec (recombination resistance) and CPErec (recombination capacitance). The Rs is obtained from the intercept in real part at high frequencies. According to previous reports, the Rtr in high-frequency region is associated with the carrier transport in selective layer or at the interface with perovskite. The low

frequency region of impedance plot can reflect the recombination process and the corresponding resistance is Rrec, which is inversely related to the recombination rate of photo-generated electrons [36-37]. We fabricated devices with the architecture of FTO/wo TiO2/MAPbI3/HTL/Ag and FTO/w-1 TiO2/MAPbI3/HTL/Ag. Both devices use the same HTL, so the difference between their Rtr and Rrec values originates from the differences of TiO2 ETL, perovskite layer and their interface. Here, we mainly discuss the transport process from MAPbI3 to TiO2, and recombination processes in TiO2/MAPbI3. The EIS measurements were conducted under 0.8 V applied bias in darkness, and the fitted parameters were listed in Table 1. The Rtr value of w-1 device is 135 ohm, which is smaller than the wo device (220 ohm for Rtr). The lower Rtr value represents that electrons can easily be transferred from perovskite layer to w-1 TiO2 ETL. This result can be attributed to the suitable band gap of w-1 TiO2, and the enhanced adhesion with perovskite layer due to the presence of nanocubic modifier [38]. Rrec reflects the ability to block charge recombination in the interface of TiO2/MAPbI3. The Rrec of w-1 device is 1250 ohm, which is larger than the device based on wo TiO2 film (950 ohm). The smaller Rrec suggests that high recombination rate exists in wo device due to the existed defect states in wo TiO2/MAPbI3, which agrees with its low open-circuit voltage. The larger Rrec in w-1 device suggests that charge recombination can be suppressed more effectively after insert the nanocubic modifier, the open-circuit voltage rises from 0.93 V to 1.02 V as a result. Therefore, the perovskite solar cells containing a TiO2 nanocubic layer possesses low internal resistance and superior charge injection characteristics, and this result is agreed with the PL.

Fig. 9. (a) The schematic diagram of w-1 device. (b) J-V curves of devices based on different TiO2 films. (c) The best cell. (d) EQE spectrum and integrated Jsc of devices based on wo TiO2 and w-1 TiO2 films. (e) Steady state output of wo and w-1 devices at Vmp of 0.7 V and 0.8 V, respectively. (f) Dark storage stability of devices using wo and w-1 TiO2 films.

Table 2 Photovoltaic parameters of devices based on different TiO2 films. Sample

Jsc (mA cm-2)

Voc (V)

FF

PCE (%)

wo

17.51

0.93

0.63

10.24

w-0

17.76

0.95

0.63

10.76

w-0.5

18.52

0.96

0.67

12.28

w-1

19.60

1.02

0.67

13.40

w-1.5

18.80

0.97

0.67

12.31

It is the first time that the nanoscale TiO2 nanocubic film used as interfacial modifier to enhance devices properties. Fig. 9a shows the device architecture of planar heterojunction PSCs which fabricated with TiO2 nanocubic. Devices with different TiO2 films were used to evaluate the photovoltaic metrics of devices by measuring their photocurrent density-voltage (J-V) curves (Fig. 9b). The device based on w-1 TiO2 film gave a PCE of 13.40% with short-circuit photocurrent density (Jsc) of 19.60 mA cm-2, open-circuit voltage (Voc) of 1.02 V, and fill factor (FF) of 0.67 (Table 2). As a comparison, the device assembled by wo TiO2 film exhibited a Jsc of 17.51 mA cm-2, Voc of 0.93 V, and FF of 0.63 to a PCE of 10.24%. The improvement of Jsc mainly resulted from the enhanced electron extraction ability and enlarged contact area of TiO2 nanocubic and MAPbI3. The enhancement of Voc can be attributed to the modified nanocubic w-1 TiO2 film, which possess less defects and can effectively suppresses the charge recombination. The increase of FF can be ascribed to the improved interface performance of TiO2/MAPbI3. However, the photovoltaic performance of w-1.5 device dropped. This phenomenon can be attributed to two reasons: (1) the thicker w-1.5 TiO2 film decreases the light transmittance, (2) the cracks existed in the surface will lead to current leakage. Inevitably, the photovoltaic performance of devices

decreased. Fig. 9c is the J-V curve of the best cell. The device prepared with 1 mM (NH4)2TiF6 achieved the peak PCE (14.70%), with Jsc of 19.15 mA cm-2, Voc of 1.02 V, and FF of 0.74. Fig. 9d illustrates the measured external quantum efficiency (EQE) and corresponding integrated Jsc spectra of devices based on TiO2 films with and without TiO2 nanocubic modifier. The EQE value of w-1 sample is higher than the without (wo) modified one. Furthermore, the w-1 device shows an excellent photocurrent response from 400 nm to 700 nm, with the EQE exceeding 80% in the wavelength range of 500-600 nm. In addition, the integrated photocurrent density (17.19 mA cm−2 of wo device and 19.30 mA cm-2 of w-1 device) from EQE spectra is agreed with the corresponding Jsc (17.51 mA cm−2 of wo device and 19.60 mA cm−2 of w-1 device) obtained from the J-V curves. Fig. 9e is the stabilized maximum power output and short-circuit photocurrent density at maximum power point for wo and w-1 devices biased at their Vmp (voltage at the maximum power point) of 0.7 V and 0.8 V, respectively. The wo device exhibit a maximum photocurrent of 12.17 mA cm-2, corresponding to the maximum power output of 8.52%. After modification, the w-1 device shows a maximum power output of 12.92%, with the photocurrent of 16.16 mA cm-2. The stabilized maximum power output of w-1 device is consistent with the PCE from J-V sweeps. It should be noted that the current density and PCE of w-1 device maintain stability whereas the wo device show a decrease in the first 60 s. This phenomenon can be ascribed to the accumulation of charges at the interface of wo TiO2/MAPbI3. Simultaneously, the stability of devices were also investigated, the results confirm that the high-efficiency w-1 devices with TiO2 nanocubic layer exhibited superior stability and continuous higher PCE than which based on wo TiO2 (Fig. 9f).

Fig. 10. Statistical photovoltaic parameters of PSCs with different TiO2 films: (a) Jsc, (b) Voc, (c) FF, and (d) PCE. Devices were measured at reverse scan and obtained from 25 devices for each TiO2 film are shown. We fabricated 25 devices for each TiO2 film, and all devices were measured at reverse scan. As can be seen from Fig. 10, the photovoltaic performances of devices boost with the increase of (NH4)2TiF6 concentration. Devices incorporating (NH4)2TiF6 at the optimized concentration (1 mM) achieve an average PCE of 13.36%. This significantly improves 1.3 times as compared to the wo devices with an average PCE of 10.44%. It confirms that the TiO2 nanocubic is a simple and effective modifier to produce high-performance devices. 4. Conclusion By employing a TiO2 nanocubic as interfacial modifier between compact TiO2 (c-TiO2) and perovskite layer, defects existed in wo TiO2/MAPbI3 are effectively passivated. The modified w-1

TiO2 binds tightly with perovskite film and provides a better platform for the growth of high-quality perovskite absorber. The w-1 TiO2 film with excellent quality and photoelectronic properties yields significant increase in PCE from 10.24% to 13.40%. The enhanced performance can be attributed to the extraordinary merits of TiO2 nanocubic modifier, which can enhance the electron injection efficiency in the interface of TiO2/MAPbI3. The advent of such unique modifier opens up new avenues for future development of light-emitting diode or photodetector based on

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Highlights • A nanoscale TiO2 nanocubic particle film was adopted as interfacial modifier. • Defects existed in TiO2/CH3NH3PbI3 are effectively passivated. •

The nanocubic modifier can suppress charge recombination and enhance the electron extraction ability of TiO2.

•

The modified w-1 TiO2 binds tightly with high quality perovskite absorber.

Author contributions Pengyu Su and Wuyou Fu conceived and designed the experiments. Pengyu Su, Tie Liu and Shuang Feng measured the SEM images. Pengyu Su and Jun Wang carried out the TEM text. Pengyu Su carried out the Xrd, steady-state PL, electrochemical impedance spectroscopy (EIS) measurements. Pengyu Su, Bowen Zhang and Runa A performed the device fabrication and photovoltaic performance measurements. Li Liu, Huizhen Yao and Haibin Yang helped to revise the manuscript.