High-efficiency perovskite solar cell based on TiO2 nanorod arrays under natural ambient conditions: Solvent effect

Ceramics International 45 (2019) 12353–12359

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High-efficiency perovskite solar cell based on TiO2 nanorod arrays under natural ambient conditions: Solvent effect

T

Rui Li, Huanyu Zhang, Rongxia Chai, Mei Zhang, Min Guo∗ School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: TiO2 nanorod arrays Perovskite solar cells PbI2 solvent Interfacial contact Recombination resistance

Well oriented TiO2 nanorod arrays (TNRAs), served as an electron transmission carrier, have become the hotspot in perovskite solar cells (PSCs) for providing the direct channel for efficient charge transport and facilitating the pore filling of perovskite. However, the interfacial contact between perovskite film and TNRAs is one of the main factors that restricts the power conversion efficiency (PCE) of TNRAs based PSCs at present. In this paper, by employing a two-step spin-coating method, the influence of PbI2 solvent such as PbI2/DMSO + ACN and PbI2/ DMSO + DMF + HAc solutions, on the preparation and performance of TNRAs based PSCs was intensively investigated. More importantly, the relationship between the micro-characteristics of perovskite film, especially the interfacial contact between perovskite film and TNRAs, and macro-properties of PSCs was elucidated. Experimental results indicate that two compact perovskite films with similar large grain size were synthesized on TNRAs from the two different PbI2 solvents, which leads to similar short circuit current densities (Jsc). However, the PSC from PbI2/DMSO + DMF + HAc solution shows a larger Voc of 1.10 V compared with that from PbI2/ DMSO + ACN solution, and thus a higher PCE of 16.57% was achieved under natural ambient conditions. The key reasons for the enhanced performance may be ascribed to the faster electron extraction and larger electron recombination resistance in the PSC, which mainly depends on the nature of perovskite/TNRAs interfaces.

1. Introduction Owing to the rapid increase of performance (over 22%) [1,2] and low fabrication cost [3,4], the organic-inorganic hybrid perovskite solar cells (PSCs) have drawn tremendous attention over the last decade. In general, PSCs are mainly composed of three parts including perovskite layer, electron and hole transport layers (ETL, HTL), which co-determine the performance of the device [5,6]. Early reported PSCs were dominantly based on mesoporous metal oxide structures [7], especially TiO2-based structures [8,9]. The randomly packed TiO2 nanoparticulate film not only act as an electron transporter but also serve as a scaffold for the mechanical support. However, due to the irregular distribution of nanoparticles, this TiO2 mesoporous structure presents two major problems including lots of grain boundaries existing between particles and poor pore filling of the light harvester, both of which are not conducive to fast carrier transmission [10]. To address these issues, well-oriented TiO2 nanorod/ nanowire arrays have been adopted as ETL to provide electron a direct channel for efficient transmission [11–14] and improve pore filling rate of the perovskite [15–18], thus leading to high performance of PSCs. Yang et al. [19] firstly employed TNRAs as ETL for PSCs, achieving a ∗

PCE of 9.4% with 1.5 μm TNRAs. Xu et al. [20] used TiO2 nanorod arrays (NRs) as photo-anodes and obtained a champion efficiency of 11.7% with 900 nm length of the NRs. Recently, Li et al. [21] utilized UV-ozone on TNRAs for surface treatment to improve the interfacial contact between perovskite film and TNRAs, and achieved a champion PCE of 18.22% from 200 nm length TNRAs based PSCs. Overall, the photoelectric conversion efficiency of devices based on TNRAs is still inferior to PSCs using TiO2 nanoparticles as support. The main reasons may be that well-aligned TNRAs have smooth surface and poor adsorption ability of perovskite grains [22,23]. Hence, improving the interfacial contact between perovskite layer and TNRAs is of great importance to enhance the photoelectric properties of TNRAs based devices. One approach is to tune the parameters of as-prepared TNRAs, including the diameter, length and density, surface treatment, and many works have been done on this issue [19–21]. Another is to change the preparation conditions of perovskite film, especially the effect of different PbI2 solvents on the interface contact between the prepared perovskite and the oriented NRs, finally achieving the enhanced performance of obtained PSCs [24–27]. Liu et al. [26] mixed PbI2, CH3NH3I (MAI) and CH3NH3Cl (MACl) as precursor solution, and obtained high quality perovskite films with enhanced photo-generated

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

https://doi.org/10.1016/j.ceramint.2019.03.159 Received 29 November 2018; Received in revised form 20 February 2019; Accepted 21 March 2019 Available online 23 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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carriers collection efficiency, which was mainly determined by the substantial increased contact area between perovskite/TNRAs. In our previous work [28], five different solvents were utilized to elucidate the relationship between PbI2 solution and perovskite property on mesoporous TiO2. We found that the PbI2/DMSO + ACN solution with smaller radius of the colloid clusters and lower boiling point promoted the formation of porous PbI2 precursor, and then forming an improved perovskite film with fewer bulk defects, finally obtaining a highest PCE of 15.32%. And the PbI2 solvent could affect the micro-characteristics of perovskite, especially the defects existed in perovskite films and at perovskite/TiO2 interfaces. So far, most studies have focused on the former and produced some successful results [19–21]. However, based on the research of the latter, especially the solvent effect on the formation and performance of the NRs based PSCs under air conditions, has not been reported yet. In this paper, well-aligned TNRAs with suitable length and density were synthesized by a hydrothermal method, and subsequently a twostep spin-coating method was employed to fabricate the perovskite films on them under ambient environment (relative humidity (RH) = 50%). The effect of two PbI2 solvents including the champion solvent DMSO + ACN according to our previous work [28] and solvent DMSO + DMF + HAc on the properties of perovskite/TNRAs interface was intensively investigated. Compared with the PbI2/DMSO + ACN one, the TNRAs based PSC prepared by PbI2/DMSO + DMF + HAc solution showed a higher PCE of 16.57%. The relationship between the microstructure and performance of formed PSCs was also discussed.

5 min, 500 °C for 30 min, respectively. The TiO2 precursor was prepared by mixing equimolar titanium butoxide (99.0%, Aladdin) and acetylacetone (AR, Sinopharm chemical reagent Co., Ltd.) in 34 mL ethanol. After stirring for 10 min, another solution with 0.13 mL hydrochloric acid (HCl) (37 wt%, Sinopharm chemical reagent Co., Ltd.) and 1.7 mL distilled water in equal volumes of ethanol was dropped in it and continued stirring for 30 min. TNRAs were synthesized by a hydrothermal method. Typically, to prepare precursor solution, equal volumes of distilled water and HCl (20 ml) were mixed in 100 mL beaker. Then, 0.7 mL titanium butoxide (99.9%, Aladdin) was dropped into the beaker, followed by stirring for 10 min. After stirring, 40 mL above solution was transferred into a Teflon-lined stainless steel autoclave, in which the compact TiO2 coated substrate had been pre-placed. The hydrothermal reaction was finished at 180 °C for 80 min. The substrate was taken out after the autoclave cooling to room temperature and rinsed with deionized water. Finally, the as-formed TNRAs were annealed at 500 °C for 30 min. 2.2. Device fabrication

Fig. 1 gives the general flow diagram for the fabrication of TNRAs based PSCs. Firstly, the TNRAs were prepared using a hydrothermal method. And then the perovskite film was produced on the top of TNRAs by a two-step spin-coating method. The specific operation processes and characterization are as follows:

The schematic flow diagram for the preparation processes of PSCs based on TNRAs was shown in Fig. 1. All experiments were conducted in air conditions (RH = 50%). The PbI2 solutions (1 M) were produced by dissolving PbI2 in DMSO + ACN and DMSO + DMF + HAc solvents. The formed PbI2 solutions were kept heating at 105 °C while the TNRAs coated substrate would be preheated at 105 °C for 100 s before deposition of PbI2 precursor. The deposition process of PbI2 precursor on the TNRAs (∼800 nm) was conducted at 3000 rpm for 30 s. And then, 220 μL MAI solution (200 mg MAI in 8 mL 2-propanol solvent) was dropped onto the sample, followed by spinning at 5000 rpm for 20 s to rid of excess solution and annealing at 105 °C for 10 min. About 45 μL spiro-OMeTAD solution was spin-coated on perovskite layer as HTL at 4000 rpm for 20 s. Finally, a thermal evaporation method was induced to deposit 80 nm of gold on the top of the HTL.

2.1. Synthesis of TNRAs

2.3. Instruments and characterization

Glass substrates coated with patterned FTO were ultrasonic cleaned sequentially in detergent, ethanol (99.8%, Sinopharm chemical reagent Co., Ltd.) and distilled water, each step for 10 min of sonication, finally handled with UV-irradiation for 20 min. The cleaned FTO substrates were covered by TiO2 compact layers (0.1 M TiO2 precursor) with a spin-coating rate at 3000 rpm for 30 s and then annealed at 130 °C for

X-ray diffraction (XRD) patterns were obtained from a Rigaku Dmax-2500 diffractometer, and the scanning electron microscopy (SEM) was performed with field emission scanning electron microscope (FESEM) (Zeiss supra 55) conducted at 10 kV. The J-V curve of the devices (active area ∼0.09 cm2) were recorded under simulated AM 1.5 G irradiation (100 mW cm−2) by using CHIe760C software.

2. Experimental

Fig. 1. General flow diagram for the fabrication process of TNRAs based PSC. 12354

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Fig. 2. (a–b) Top and cross-section view SEM images of TNRAs, (c) XRD pattern of TNRAs, (d) Statistical distribution histogram for diameter of TNRAs.

Dynamic light scattering (DLS) measurement (Laser Light Scattering Spectrometer ALV/DLS/SLS-5022F (ALV/Laser Vertriebsgesellschaft m. b. H)) was employed to measure the cluster particle size in PbI2 solution. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) were carried out at an excitation wavelength of 470 nm by using lifetime and steady state spectrometer FLS920 (Edinburgh Instruments Ltd). Electrochemical impedance spectroscopy (EIS) and external quantum efficiency (EQE) were operated with CHIe760C software in dark with 0.8 V bias voltage and a QE system from Newport, respectively. The test frequency of EIS ranged from 0.1 Hz to 105 Hz. 3. Results and discussion 3.1. Effect of PbI2 solvent on the preparation of perovskite film Fig. 2 illustrated the SEM images and XRD patterns of as-prepared TNRAs. It can be seen from Fig. 2d that the diffraction peaks at 2θ of 25.3°, 37.8° were in accordance with rutile TiO2 (JCPDS 86-0147) [29], confirming the product was TiO2. In addition, as is shown in Fig. 2a, b and d, vertically aligned TRNAs with average diameter of 45 nm, length around 800 nm and the density of ca. 174 rod·μm−2 were synthesized on FTO substrate, so, the area ratio of as-formed TRNAs to original TiO2 compact layer (1 × 2 cm2) was around 35.2%. Li et al. [21] introduced

a value of void-space-fraction (VSF = 1-ρ × Dav2, ρ (rod·μm−2) and Dav (μm) are the average NR density and diameter, respectively) to evaluate the influences of NR density and diameter on the device performance, and an optimal VSF of 0.682 was determined to achieve the top PCE of PSCs. In this study, given that the VSF of as-prepared TRNAs was calculated to be ca. 0.648 from Fig. 2d, very close to the optimal VSF 0.682, so, the density and diameter of as-formed TNRAs were considered to be appropriate for preparing PSCs and no more researches were needed to adjust the growth parameters of TNRAs. According to our previous work [28], the properties of the PbI2 solvent including the colloid size in precursor solution and boiling point played an important role on the morphology of PbI2 precursor layer. Solution with lower boiling point and smaller clusters radius such as PbI2/DMSO + ACN, were easier to form porous PbI2-DMSO on mesoporous TiO2. This porous precursor could promote the formation of a well performance perovskite film with high density, few defects, large grain size and appropriate amount of PbI2 residue, finally improving the device efficiency. So, the optimal solution PbI2/DMSO + ACN [28] and PbI2/DMSO + DMF + HAc solution were chosen as objects to study the influence of solvents on the photoelectric properties of TNRAs based PSCs. Typical XRD patterns of PbI2 adducts and perovskite films were presented in Fig. 3. For both samples, the diffraction peaks appeared at

Fig. 3. XRD patterns of (a) PbI2-DMSO adducts, (b) perovskite films prepared with PbI2/DMSO + ACN and PbI2/DMSO + DMF + HAc solutions. 12355

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Fig. 4. SEM images of (a–b) PbI2 adducts, (c–d) perovskite films and (e–f) solar devices prepared with PbI2/DMSO + ACN and PbI2/DMSO + DMF + HAc solutions, respectively (the inset in Fig. 4b is size distributions of the two complexes in PbI2/DMSO + ACN and PbI2/DMSO + DMF + HAc solutions).

2θ of 9.3° and 9.8° (Fig. 3a) illustrated the main component of PbI2 precursors was PbI2-DMSO [30]. Compared with the intensities of diffraction peaks of formed PbI2-DMSO adduct, more amorphous PbI2DMSO adduct were generated on TNRAs from PbI2/ DMF + DMSO + HAc solution than that from PbI2/DMSO + ACN solution, which resulted in more effective transformation from PbI2DMSO to CH3NH3PbI3 (MAPbI3) [31]. In Fig. 3b, both two perovskite films showed diffraction peaks at 2θ of 32.0°, 28.6° and 14.1°, which confirmed the main component was MAPbI3. Notably, the sample produced using PbI2/DMSO + DMF + HAc solution exhibited lower intensity of peak ascribed to PbI2 at 2θ of 12.7°, indicating few PbI2 remnants in perovskite film. Early reports found that an appropriate amount of PbI2 residue existed in perovskite film could passivate the grain boundary, reduce electron-hole recombination and thus improve the PCE of PSCs, but too much excess PbI2 would do harm to the solar device [32–34]. Fig. 4 gave the influence of two solutions (PbI2/ DMF + DMSO + HAc and PbI2/DMSO + ACN) on the morphology of PbI2 adducts and perovskite films on TNRAs. It can be observed from Fig. 4c that the PbI2/DMSO + ACN based sample showed obvious white phase which had been proven to be PbI2 [35–37]. Because of smaller residual PbI2 existed in PbI2/DMSO + DMF + HAc based perovskite film, it is hardly observed white phase in Fig. 4d and the residual PbI2 might distribute in perovskite grain boundary. The result was in line with reference [36]. Moreover, compact perovskite film with slightly larger average grains was synthesized from PbI2/ DMSO + ACN solution compared with that from PbI2/ DMSO + DMF + HAc solution. This case may be mainly caused by the

different roughness of PbI2-DMSO adduct films during the first-step deposition as shown in Fig. 4a, which could be further elucidated by the size of PbI2 complexes formed in both solvents. The inset in Fig. 4b depicted the size distributions of the two complexes in PbI2/DMSO + ACN and PbI2/DMSO + DMF + HAc solutions by conducting DLS measurement. It is obvious that the size of the colloid in PbI2/DMSO + ACN solution was smaller than that in PbI2/ DMSO + DMF + HAc solution. During the first-step deposition, the formed smaller clusters DMSO-PbI2-ACN and stronger volatility of ACN contributed to the interlaced rod grains in PbI2 adduct films (Fig. 4a), while, the larger clusters and weaker volatility of DMF and HAc resulted in compact PbI2 adduct films (Fig. 4b). During the second-step process, interlaced rod structure with more nucleating sites and growth space promoted forming compact perovskite films with relatively larger grains (Fig. 4c). However, probably due to the existence of HAc, the PbI2/DMF + DMSO + HAc based PbI2 precursor possessed the lower crystallinity (Fig. 3a) and this amorphous structure could accelerate the reaction between PbI2 adduct and MAI, thus contributed to the moderate amount of excess PbI2 in perovskite and the similar perovskite grain size to the PbI2/DMSO + ACN one [38]. Large grain sizes along with reduced grain boundary could bring out few defects existed in perovskite film and reduced charge transfer resistance, thereby enhanced the final performance of PSCs. In addition, as is shown in the cross-section views of PSCs (Fig. 4e and f), the TNRAs were fully filled with perovskites and capping layers were formed on them, which demonstrated different PbI2 solutions had little influence on the filling of perovskite in TNRAs. That is to say, little difference in device performance aroused from pore filling ability.

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Fig. 5. (a) J-V curves of TNRAs based PSCs from PbI2/DMSO + ACN and PbI2/DMSO + DMF + HAc solutions, (b) IPCE of the champion TNRAs device prepared using PbI2/DMF + DMSO + HAc solution (c) UV–visible absorption spectra of TNRAs based perovskite films from PbI2/DMSO + ACN and PbI2/ DMSO + DMF + HAc solutions. Table 1 Photovoltaic parameters of TNRAs based PSCs produced with PbI2/ DMSO + ACN and PbI2/DMSO + DMF + HAc solutions. Solvents

Jsc (mA·cm−2)

Voc (V)

FF

PCE (%)

DMSO + ACN DMSO + DMF + HAc

20.9 19.7

0.96 1.10

0.66 0.76

13.35 16.57

3.2. Effect of PbI2 solvent on performance of TNRAs based PSCs TNRAs PSCs from the two PbI2 solutions were produced to investigate the influence of different PbI2 solvents on device performance. The J-V curves of the two TNRAs based PSCs conducted under standard AM1.5G solar were illustrated in Fig. 5a and Table 1 presented the corresponding photovoltaic parameters. The two TNRAs based PSCs had the similar short-circuit current density (Jsc), e.g., 20.9 mA cm−2 and 19.7 mA cm−2, but different open-circuit voltage (Voc), e.g., 0.96 V and 1.10 V, which resulted in a higher PCE of 16.57% from PbI2/ DMF + DMSO + HAc solution than from PbI2/DMSO + ACN solution (13.35%). The incident photo-to-electron conversion efficiencies (IPCE) were above/nearly 80% from 400 nm to 650 nm, which mainly determined the Jsc (19.7 mA cm−2) of the PSC according to Eq. λmax

Jsc = q ∫ PAM1.5 (λ )IPCE(λ ) dλ , (PAM1.5 (λ ) (Im) and q (1.602 × 10−19 C)

[40]. The ηlh, ηinj and ηec are associated with the quantity and kinds of light absorbing materials, energy level matching between light absorber and semiconductors, as well as the type and structure of semiconductor, respectively. The two devices had the same perovskite absorbers and TRNAs structures, indicating both ηinj and ηec were similar. The ηlh, which can be drawn from absorption spectra (Fig. 5c), is also considered to be same, thus contributing to the final similar Jsc. In addition, based on the calculation formula of Voc (Eq. (1)), its value is related to the Jsc and recombination impedance (Rrec), and larger Jsc and Rrec result in higher Voc.

Voc =

(1)

Where q is the amount of charge that an electron carries, 1.602 × 10−19 C; kB is the boltzmann constant, 1.38 × 10−23 J/K; T is the thermodynamic temperature, K; τe is the electron lifetime, ms; β is the reciprocal of ideal factor, d is the thickness of photo-anode film, μm; and Rrec is the recombination impedance, Ω [41,42]. TRPL spectra of the two TNRAs based PSCs were conducted to compare charge recombination and charge transfer behaviors between perovskite films and TNRAs as indicated in Fig. 6. A two exponential decay function as shown in Eq. (2) was used to fit the measured TRPL spectra for evaluating the decay process [43]. The corresponding fitting results were summarized in Tables 2 and 3.

λmin

represent photon flux from the simulated sunlight and the charge of electron, respectively.) [38]. Considering the fact that the two perovskite films had similar grain size and pore-filling ability according to Fig. 4c–f, so, further investigation on the PSCs should be conducted to clarify the underlying reasons for the different efficiencies of solar cells. Generally, the PCE of the PSC is equal to the ratio of the maximum output power (Pmax) to total incident power (Pin): PCE=Pmax/Pin [39]. The Pmax is proportional to Jsc, Voc and FF. The Pin is a constant and its numerical size depends on intensity of incident light (a simulated sun light in our study). As is shown in Eq. Jsc∝ηlh × ηinj × ηec, the Jsc is directly proportional to the light harvest efficiency (ηlh), the injection efficiency of electrons (ηinj) and the electrodes collection efficiency (ηec)

qβRrec kB T Jsc τe ln( ) × q qd (1 − p) kB T

x x y = y0 + A1 exp(− ) + A2 exp(− ) τ1 τ2

(2)

Where τ1 is the decay time of fast process, τ2 is the decay time of slow process, A1 and A2 are the corresponding relative amplitudes and y0 is the offset [44,45]. According to the relative contribution of fast B1 and slow B2 processes, the mean carrier lifetime (τmean) can be obtained by the Eq. Bi = Aiτi/(A1τ1+A2τ2) (i = 1, 2) and τmean = B1τ1 +B2τ1 [46]. When exciting from perovskite layer, the fitting results can reflect the quality of perovskite films including non-radiative recombination caused by defects near grain boundaries and radiative recombination occurring in bulk perovskite [44,47]. The fast process τ1 reflects the Fig. 6. TRPL spectra of MAPbI3 films from PbI2/ DMSO + ACN and PbI2/DMSO + DMF + HAc solutions were tested at an excitation wavelength of 470 nm and the signals were recorded at 775 nm. (a) excitation from the perovskite side, (b) excitation from the FTO side (the inset in Fig. 6a is PL spectra of as-formed MAPbI3 films).

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Table 2 Two-exponential fitting results of TRPL measurement (excitation from perovskite side). Solvents

τ1 (ns)

τ2 (ns)

B1 (%)

B2 (%)

τmean (ns)

DMSO + ACN DMSO + DMF + HAc

13.45 17.65

30.51 38.68

18.91 51.97

81.09 48.03

27.28 27.75

Table 3 Two-exponential fitting results of TRPL measurement (excitation from FTO side). Solvents

τ1 (ns)

τ2 (ns)

B1 (%)

B2 (%)

DMSO + ACN DMF + DMSO + HAc

0.86 0.56

8.74 3.31

52.02 91.23

47.98 8.77

DMSO + DMF + HAc solution exhibited smaller τ1 and larger B1, suggesting more charge extraction efficiency from perovskite to TNRAs. This case might be attributed to the better interfacial contact between perovskite film and TNRAs including decreased defects and enlarged interfacial area. The faster charge extraction existed in the PbI2/ DMF + DMSO + HAc based sample, may be the key reason for the improved Voc (Fig. 5a). Furthermore, given that the Rrec (Ω) is proportional to Voc according Eq. (1), EIS measurement was conducted at a bias of 0.8 V in the dark to elucidate the recombination behavior in the TNRAs PSCs. Fig. 7 illustrated Nyquist plots of the PSCs in the dark and corresponding equivalent circuit. The fitting data results were also given in Fig. 7. In the dark, the observed semi-arc were closely related to the electron recombination processes including the perovskite phase electron-hole recombination process and perovskite/TNRAs interface electron-hole recombination process [50–52]. Obviously, the TNRAs PSC from PbI2/ DMSO + DMF + HAc solution had a higher Rrec of 301.4 Ω compared with the device from PbI2/DMSO + ACN solution (188.7 Ω), which confirmed that a restrained charge recombination existed in the PbI2/ DMSO + DMF + HAc based PSCs, and thus higher Voc was achieved. According to the analysis from above TRPL (excitation from perovskite side), the recombination process happening in as-prepared perovskite films from two different solvents was similar, so, it is reasonable to assume that the higher Rrec mainly came from better interfacial contact between perovskite and TNRAs in PbI2/DMSO + DMF + HAc based PSC. This result also agreed well with the TRPL analysis (excitation from FTO side). 4. Conclusions

Fig. 7. Electrochemical impedance spectra of the PSCs from PbI2/ DMSO + ACN and PbI2/DMSO + DMF + HAc solutions measured in dark with 0.8 V bias voltage. Inset is the equivalent circuit.

carrier lifetime at perovskite grain boundaries, the slow process τ2 reflects the carrier lifetime in bulk perovskite. The mean carrier lifetime τmean reflects the defect concentration in perovskite film and lower τmean means more trap-assisted non-radiative and radiative recombination at defects. As can be seen from Table 2, the two perovskites samples had close τmean of 27.75 and 27.28 ns, respectively. Due to possessing similar τmean, the two samples might have the same defect density, which contributed to the same similar behavior of charge carriers including exciton dissociation and electron-hole recombination process occurred in perovskite films. The obtained results were also confirmed by PL spectra of the MAPbI3 films as illustrated in inset of Fig. 6. Both samples showed peaks at around 775 nm and the PL intensity were close, demonstrating similar recombination efficiency and higher charge-carrier separation efficiency of the two perovskite films. When exciting from FTO layer, the fitting results can reflect not only the information of perovskite, such as defects from bulk perovskite and grain boundaries, but the property of interfacial contact between perovskite film and TNRAs, for instance, defects and contact area from the interfaces [44,48]. The fast process τ1 reflects the extraction of the excited electron transfer from the perovskite to TNRAs, while the slow process τ2 reflects the recombination (non-radiative and radiative electron-hole recombination) of the excited electron back to the ground state in perovskite film, B represents relative amplitudes and relative contribution (B1+B2 = 1) [49]. Smaller τ1 and larger B1 can be deemed to the fast extraction of the excited electron from perovskite to TNARs and the contribution to total quenching process, respectively. Compared with the two samples on TNRAs, the film from PbI2/

In summary, well aligned TRNAs with suitable average diameter of 45 nm, length around 800 nm and the density of 174·rod·μm−2 were synthesized on FTO substrates by a hydrothermal method. DMSO + ACN and DMSO + DMF + HAc were introduced as solvents to study the influence of PbI2 solution on the preparation and performance of TNRAs based PSCs. SEM results showed two similar compact perovskite films were formed on TNRAs from different PbI2 solvents, which led to similar Jsc. Meanwhile, a larger Voc of 1.10 V was obtained from PbI2/DMSO + DMF + HAc based PSC, which contributed to the higher PCE of 16.57% in air conditions. The reasons for the enhanced efficiency of PSCs were revealed by TRPL and EIS analysis, and the larger electron recombination resistance caused by the decreased defects and enlarged interface contact area between perovskite film and TNRAs may be the most important factors. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51572020, 51772023). References

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