In Situ-Formed and Low-Temperature-Deposited Nb:TiO2 Compact-Mesoporous Layer for Hysteresis-Less Perovskite Solar Cells with High Performance

Recently, reported perovskite solar cells (PSCs) with high power conversion efficiency (PCE) are mostly based on mesoporous structures containing mesoporous titanium oxide (TiO2) which is the main factor to reduce the overall hysteresis. However, existing fabrication approaches for mesoporous TiO2 generally require a high-temperature annealing process. Moreover, there is still a long way to go for improvement in terms of increasing the electron conductivity and reducing the carrier recombination. Herein, a facile one-step, in situ, and low-temperature method was developed to prepare an Nb:TiO2 compact-mesoporous layer which served as both scaffold and electron transport layer (ETL) for PSCs. The Nb:TiO2 compact-mesoporous ETL-based PSCs exhibit suppressed hysteresis, which is attributed to the synergistic effect of the increased interface surface area caused by nano-pin morphology and the improved carrier transportation caused by Nb doping. Such a high-quality compact-mesoporous layer allows the PSCs assembled using optimized 2% Nb-doped TiO2 to achieve a remarkable PCE of 19.74%. This work promises an effective approach for creating hysteresis-less and high-efficiency PSCs based on compact-mesoporous structures with lower energy consumption and cost.

To date, the reported PSCs with high power conversion efficiency (PCE) are typically based on a mesoporous structure containing an indispensable scaffold layer of metal oxide [16]. Titanium oxide (TiO 2 ) has been commonly used as an electron transport layer. The typical mesoporous-type PSC presented by Seok has a structure of FTO/compact TiO 2 /mesoporous TiO 2 and perovskite composite layer/perovskite upper layer/PTAA/Au [17]. It is generally known that the mesoporous TiO 2 contributes most to reduce the overall hysteresis for mesoporous-type PSCs [18]. However, the fabrication of a mesoporous TiO 2 layer often requires a high-temperature (> 450°C) annealing treatment, leading to large energy consumption and limiting its application in flexible devices [19][20][21]. Compared with the mesoporous-type PSCs, planar-type PSCs can be fabricated using a low-temperature and low-cost process [22]. However, planar-type PSCs usually suffer from poor electron conductivity, severe charge recombination, and relatively lower crystallinity, resulting in low PCE with severe hysteresis behavior [23,24].
Extensive efforts have been made to develop highquality TiO 2 electron transport layers (ETLs) with high electron mobility, such as through morphology optimization, surface modifications, and doping. In particular, a wide range of elements have been chosen to prepare TiO 2 doping layers in PSCs, including Lithium (Li) [25,26], Niobium (Nb) [27,28], Platinum (Pt) [29], Sodium (Na) [30], Neodymium (Nd) [31], and Aluminum (Al) [32]. For instance, Liu et al. reported that the Lidoped TiO 2 ETL was beneficial to the performance of the mesoporous-structure PSCs, especially for alleviating the hysteresis effect [26]. Liao et al. reported that the Ptdoped TiO 2 ETL could improve the charge carrier extraction and injection efficiency in n-i-p PSCs [29]. Other ions such as Na, Nb, and transition metal ions [30,31,[33][34][35] were used to modify surface or passivate defect of TiO 2 , contributing to reducing non-radiative recombination. Among these elements, Niobium metal (Nb) is a good candidate as a doping material for titanium oxide electron transport materials due to its similar radius with that of titanium. The results shown by Yin et al. demonstrated that Nb doping could make an improvement in both conductivity and mobility, simultaneously decrease the trap-state density of TiO 2 ETLs for PSCs [27]. Despite these progresses, a relatively high-temperature (150°C) treatment was mandatory and large hysteresis was still observed in PSCs based on Nb-doped TiO 2 . As is well known, current density-voltage (J-V) hysteresis is a critical issue that occurs frequently, especially in planar-structure PSC devices. Severe hysteresis can lead to instability of PSCs and degradation of PCE. For this reason, it is highly desired to develop a hysteresis-less PSC utilizing a simple and lowtemperature method.
Here, we propose a facile one-step, in situ, and lowtemperature (70°C) strategy to develop hysteresis-less PSCs which contain a single Nb:TiO 2 compactmesoporous layer serving as both scaffold and ETL. The Nb:TiO 2 layer contains a compact TiO 2 bottom with nano-pin morphology on the surface, which can be utilized as a scaffold. The hysteresis index decreased significantly from 24.39% for the PSC based on bare TiO 2 to 3.19% for that based on 2% Nb:TiO 2 layer due to the collaborative effect of the increased interface surface area caused by nano-pin morphology on the surface and the improved carrier transportation rate because of the presence of Nb. The high-quality mesoporous layer allowed the PSCs to achieve remarkable PCE of 19.7%. This work promises an effective approach for achieving hysteresis-less and high-efficiency PSCs through scalable and inexpensive methods at low temperature.

Sample Preparation
First, the FTO substrates were successively put into acetone, alcohol, and deionized water to be ultrasonic cleaned of 30 min each. After that, the cleaned substrates were treated by a UV-ozone cleaner for 20 min and then placed in a petri dish. Second, liquid TiCl 4 was dropped into deionized water under the temperature of 0°C to prepare 0.1 M TiCl 4 aqueous solution. Third, NbCl 5 powder was put into the ethanol near the temperature of 0°C to obtain 0.1 M NbCl 5 ethanol solution. Then, X vol.% NbCl 5 ethanol solution and (100-X) vol.% TiCl 4 aqueous solution were dropped onto the surface of FTO substrates sequentially inside the petri dish. After hydrothermal reacting at 70°C for 60 min, the Nb:TiO 2 nanopin feature was formed on the FTO substrates.
The perovskite absorption layer was deposited with the dynamic two-step spin-coating method [36]. First, the PbI 2 precursor solution was obtained by adding 0.462 g PbI 2 into 1 mL DMF. Meanwhile, the CH 3 NH 3 I (MAI) precursor solution was obtained by adding 0.1 g MAI into 2 mL isopropanol (99.5%, Aladdin). Second, 55 μL PbI 2 precursor solution was spun onto the asprepared Nb:TiO 2 ETL film at 3000 rpm for 10 s. At this moment, 55 μL MAI precursor solution was dropped onto the sample immediately, and spinning was continued for 20 s. Finally, the whole film was annealed at 150°C for 15 min.

Characterization Methods
A field-emission scanning electron microscope (FE-SEM, SU8010, Hitachi) was carried out to study the morphologies of the samples. The absorption spectra were recorded with a UV-vis spectrophotometer (Shimadzu, UV-3600). Electrochemical impedance spectroscopy (EIS) was employed to understand the carrier transportation process by an electrochemical workstation (Autolab, PGSTAT 302 N). The current density-voltage (J-V) measurement was recorded using a digital source (Keithley 2400) with the assistance of the solar simulator (ABET Technologies, SUN 3000).

Results and Discussion
A schematic of the PSC structure and the Nb:TiO 2 synthesis procedure is shown in Fig. 1. First, the cleaned FTO substrates were faced up placed in a petri dish. Second, 1 mL NbCl 5 ethanol solution and 49 mL TiCl 4 aqueous solution were poured onto the FTO substrates in the dish sequentially. Third, the dish was transferred into an oven and hydrothermal reacted at 70°C for 1 h. Finally, the TiO 2 layer with nano-pin morphology and 2% Nb doping ratio was formed on the FTO substrates. For the preparation of the control TiO 2 layer, only TiCl 4 aqueous solution (without NbCl 5 ethanol solution) was dropped into the dish containing FTO substrates.
To understand the effect of Nb doping on the evolution of the TiO 2 layer, the morphologies of the control TiO2 and Nb-doped TiO 2 were investigated using scanning electron microscopy (SEM) which is shown in Fig. 2. The bare TiO 2 exhibits a much smoother surface, which is a typical morphology of compact TiO 2 layers in planar PSCs. However, 2% Nb-doped TiO 2 shows a nano-pin texture distributed on the compact bottom. The length of the nano-pin was determined to be 50 ± 20 nm. This indicates that the Nb:TiO 2 layer contains a compact TiO 2 layer with a nano-pin morphology on the surface, which is regarded as a mesoporous layer. Therefore, this in situ formed Nb:TiO 2 compact-mesoporous layer, which was obtained by a one-step process, actually serves as both a scaffold and an ETL in the PSC. The formation of nanopin morphology resulted from the hydrothermal reacting with the assistance of NbCl 5 ethanol solution.
The XPS spectra of 2% Nb:TiO 2 film is shown in Fig. 3. Figure 3a shows the full scan spectra of the 2% Nb:TiO 2 film. It is found that the atom ratio of Nb/Ti (1.3%) is closed to the element doping ratio of 2% in the precursor mixture. As shown in Fig. 3b, the Gaussian peaks located at 458 eV and 464 eV are corresponding to the binding energy of Ti 2p 3/2 and Ti 2p 1/2 . Similarly, the Gaussian fitted lines of Nb 5+ can be deconvolved into two individual peaks which are associated with Nb 3d 5/2 and Nb 3d 3/2 , respectively, at the binding energy of 207 eV and 209 eV (Fig. 3c). The XPS spectra demonstrate the successful doping of Nb in the TiO 2 film. Figure 4a shows the absorption spectra of FTO, bare TiO 2 /FTO, and Nb-doped TiO 2 /FTO. Both bare TiO 2 and Nb-doped TiO 2 exhibit main absorption edge at the wavelength of 300-350 nm. The absorption curve of Nbdoped TiO 2 almost overlaps that of bare TiO 2 . The energy bandgap (E g ) can be calculated based on the absorption spectra using the Tauc equation, which is shown in Fig. 4b. The E g is 4.05 eV for FTO and 3.5 eV for both bare TiO 2 and Nb-doped TiO 2 . Therefore, it can be concluded that Nb doping has little influence on the absorption of TiO 2 . The transmittance is also not shifted during the Nb doping process as shown in Fig. S1.  Thanks to our previously developed non-substrateselective dynamic two-step spin-coating strategy [36], the film uniformity and coverage can be better controlled. Besides, the average crystalline grain sizes of the perovskite films are very similar. Fig. S3 presents the absorption spectra of the perovskite films deposited on the bare TiO 2 and Nb-doped TiO 2 films. No obvious difference in absorption peak is observed between the perovskite films. These results suggest that the nano-pin morphology formation on the Nb-doped TiO 2 compactmesoporous layer could have little effect on the perovskite crystallization by dynamic two-step spin-coating strategy.
To understand the carrier transportation crossing the ETL/perovskite interfaces, the electrical impedance spectroscopy (EIS) was employed. PSCs were fabricated with the structure of FTO/TiO 2 /perovskite film/Spiro-OMe-TAD/Au. Figure 5 shows the Nyquist plots of PSCs   Table S1. It is known that the EIS contains two circular arcs [37]. The high-frequency component is attributed to the charge transport resistance (R ct ), and the low-frequency component is mainly related to the recombination resistance (R rec ) [38]. In this comparison, everything but the perovskite/ETL interface was identical. Thus, only the Nb doping process should be responsible for the resistance (R ct and R rec ) variation. Compared to the bare TiO 2 device, the Nb:TiO 2 device exhibits smaller R ct and larger R rec . The small R ct contributes to more efficient electron extraction, and the large R rec proves lower charge recombination. These results confirm that the Nb:TiO 2 -based compact- mesoporous layer is an effective ETL for both charge transportation improving and carrier recombination rate reducing.
As shown in Fig. 6, the dependence of the PCE of PSCs on the Nb doping contents was investigated. The detail parameters for PSCs with different Nb doping concentrations varying from 0 to 8% was shown in Table 1. It is found that the doping ratio affects the open-circuit voltage (V oc ) and fill factor (FF), which were first increased and then decreased with increasing Nb doping. The device with a 2% Nb-doped TiO 2 layer exhibits the highest V oc of 1.19 eV, J sc of 23.52 mA/cm 2 , and FF of 70.74%, leading to a PCE as high as 19.74% for the champion devices. Thanks to better carrier transportation, all parameters show notable improvement. However, superfluous doping would strengthen the carrier scattering and lead to poor mobility. The incremental recombination will weaken the carrier transport improvement and eventually harm the PCE.
The measured J-V curves of the control and champion device are shown in Fig. 7. It is well known that J-V hysteresis behavior often occurs, especially in planarstructure PSC devices. In this work, the hysteresis of J-V curves of bare compact TiO 2 -based PSC and 2% Nb: TiO 2 compact-mesoporous layer-based PSC were examined. The hysteresis index, (PCE of reverse scan − PCE of forward scan)/PCE of reverse scan [30], reduced markedly from 24.39% for the PSC based on bare compact TiO 2 to 3.19% for the PSC based on 2% Nb-doped TiO 2 layer. It is well known that PSCs based on a mesoporous TiO 2 layer can collect electrons and effectively achieve a balance between the hole flux and electron flux due to its larger surface area, thereby exhibiting less hysteresis [17]. The hysteresis suppression of the Nbdoped TiO 2 -based device is motivated by the conductance increasing and the nano-pin morphology forming. Charge accumulation caused by interfacial capacitance at the ETL/perovskite interface would be reduced and result in hysteresis-less character.

Conclusion
We have developed a facile one-step, in situ, and lowtemperature approach to achieve an Nb:TiO 2 compactmesoporous layer that serves as both scaffold and ETL for PSCs. As a result, PSCs based on 2% Nb-doped TiO 2 can exhibit a remarkable PCE of 19.74%, which is dramatically higher than that of the controlled TiO 2 -based device. The Nb:TiO 2 layer contains a compact TiO 2 bottom with nano-pin morphology on the surface, which can be utilized as a mesoporous layer. Due to the collaborative effect of a large interface surface area and improved carrier transportation rate, the hysteresis of the J-V curve is markedly reduced, with the hysteresis index decreasing significantly from 24.39 to 3.19%. This work promises an effective approach for achieving hysteresis-  Fig. 7 The J-V hysteresis behavior of the PSCs based on bare TiO 2 and 2% Nb:TiO 2 layer under AM 1.5 illumination less and high-efficiency PSCs through a well-designed scalable and cost-efficiency hydrothermal method at low temperature.

Additional File
Additional file 1: Figure S1. The light permeation comparison of TiO 2 and Nb:TiO 2 based ETL. Figure S2. SEM images of (a) perovskite deposited on the pure TiO 2 and (b) perovskite deposited on the TiO 2 layer doped with 2% Nb. Figure S3. Absorbance spectra of perovskite film deposited on pure TiO 2 and 2%Nb:TiO 2 layer.