Efficient and stable perovskite solar cells through e-beam preparation of cerium doped TiO2 electron transport layer, ultraviolet conversion layer CsPbBr3 and the encapsulation layer Al2O3

Solar Energy 198 (2020) 187–193

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Efficient and stable perovskite solar cells through e-beam preparation of cerium doped TiO2 electron transport layer, ultraviolet conversion layer CsPbBr3 and the encapsulation layer Al2O3

T

Junjie Jina, Hao Lia, Wenbo Bia, Cong Chena, Boxue Zhanga, Lin Xua, Biao Donga, ⁎ ⁎ Hongwei Songa, , Qilin Daib, a

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China Department of Chemistry, Physics, and Atmospheric Sciences, Jackson State University, Jackson, MS 39217, United States

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Perovskite solar cells e-beam evaporation Cerium ions Light stability Encapsulation

The electron transport layer (ETL) as an important part for high performing perovskite solar cells (PSCs), can transport photogenerated electrons while block holes. TiO2, as the usual ETL in the PSCs, may cause charge recombination in the device due to its poor electron conductivity. In order to improve the conductivity, we doped the Ce ions into TiO2 and used the e-beam method to fabricate the ETL layer. The doping of Ce could facilitate the charge transport and reduce the charge recombination, resulting in the increased power conversion efficiency (PCE) from 17.98% to 19.34%. And more, we also enhanced the stability of PSCs through the introduction of CsPbBr3 quantum dots (QDs) and Al2O3. As the ultraviolet (UV) to visible conversion layer, CsPbBr3 QDs were coated on the incident light side of the device to enhance the light stability of PSCs, while, as an encapsulation layer, the Al2O3 was put on the back side of the device to enhance the long-term stability of PSCs. As a result, the modified device can maintain 90% of initial efficiency after aging 4000 h in the ambient air.

1. Introduction

transparency, excellent carrier separation ability, and environmental stability (Yang et al., 2015; Lee et al., 2014; Huang et al., 2016). However, its electron conductivity is poor, which may cause the charge recombination in the device (Lü et al., 2010). Ion doping is an effective method to improve the conductivity of the TiO2, for instance, aluminum doping has a striking impact upon the density of sub-gap states and enhancing the conductivity, thus leading to improvement of the device efficiency. Niobium doping can also increase the photogenerated electron injection and extraction from the perovskite layer to the ETL (Yin et al., 2017; Pathak et al., 2014). Besides the above mentioned dopants, the rare earth ions, which provide unique optical and electronic properties because of their unfilled 4f shell, have also proven to be a candidate dopants for modifying the performance of TiO2 (Chen et al., 2018; Giordano et al., 2016; Xiang et al., 2017). Cerium ion, which has a partially occupied 4f and 5d (4f15d16s2) orbital, has been successfully doped in the TiO2 in the DSSCs to increase PCE due to reduced charge recombination in the devices (Zhang et al., 2012; Khannam et al., 2017). Moreover, the preparation of TiO2 ETL is mainly concentrated on the spin-coating method, which required to be annealed at a high

The perovskite solar cells (PSCs) have been recognized as the next generation photovoltaic device because of the excellent properties including suitable band gap, high absorption coefficient, high carrier mobility, long carrier lifetime, and easy for preparation (Turren-Cruz et al., 2018; Saliba et al., 2018; Chen et al., 2015; Zhang et al., 2016; Green et al., 2014). Based on these properties, the PSCs have achieved a certified power conversion efficiency (PCE) of as high as 23.2% at present (Jeon et al., 2018). The typical configuration of PSCs consist of five parts, which contains two electrodes, an electron transport layer (ETL), a perovskite absorb layer, and a hole transport layer (HTL) (Wang et al., 2017; Snaith, 2013). Among them, the ETL acts as a critical role in transporting photogenerated electrons while blocking holes (Yella et al., 2014; Gao et al., 2016). Several materials are chosen as the ETL in the PSCs, such as TiO2, SnO2, and Nb2O5 (Jin et al., 2017; Xiong et al., 2018; Feng et al., 2017). Among these electron transport materials, TiO2 has been wildly applied in the PSCs owing to its high

⁎

Corresponding authors. E-mail addresses: [email protected] (H. Song), [email protected] (Q. Dai).

https://doi.org/10.1016/j.solener.2020.01.048 Received 4 August 2019; Received in revised form 24 December 2019; Accepted 17 January 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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(4:1, v:v)) on the ETL and performed for 15 s at 1000 rpm. Then, 120 μL chlorobenzene was dropped onto it and performed for 35 s at 4000 rpm. After spin-coating process, the perovskite film was annealed at 150 °C for 10 min. Then, spin-coated the Spiro-OMeTAD solution (50 mg of 2,2′,7,7′-Tetrakis (N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD), 22.5 µL of 4-tert-butylpyridine and 22.5 µL of acetonitrile solution containing 170 mg/mL lithium bis(trifluoromethylsulfonyl)imide in 1 ml of chlorobenzene) on the perovskite layer with 1500 rpm for 30 s as the HTL. After that, the Au electrode with a thickness 80 nm was deposited by thermal evaporation in a vacuum chamber (9 *10−4 Pa). Finally, the solution of CsPbBr3 QDs was prepared by spin coating on the incident light side of the device as the UV convert layer, and 60 nm Al2O3 encapsulation layer was prepared by e-beam evaporation method.

temperature (> 450 °C) (Yan et al., 2015; Bao et al., 2014). This method increases the appearances of pinholes in the TiO2 film, which are detrimental to the devices. E-beam evaporation technology has many advantages over spin-coating method, such as low temperature process, uniform film structure, high technique (Qiu et al., 2015; Meng et al., 2016). A PCE of 19.35% is obtained by optimizing film thickness and Ce doping concentration, which is 8% enhancement than that of the device based on undoped TiO2. The improved efficiency is mainly attributed to the increased charge transport and reduced charge recombination by Ce doping in the TiO2. In addition to modifying the ETL of PSCs, we also spin-coated the CsPbBr3 quantum dots (QDs) on the incident light side of the device as the ultraviolet (UV) to visible conversion layer. The application of the UV convert layer was proven to be an effective approach to enhance the photocurrent of the PSCs, while improve the UV light stability (Wang et al., 2017; Li et al., 2018). CsPbBr3 QDs as a new kind of QDs, have attracted much attention due to their superior quality in high photoluminescence quantum yields (PLQYs), excellent photostability, and high sensitivity to UV radiation (Zhou et al., 2017; Swarnkar et al., 2015; Kang and Wang, 2017). It can absorb the UV light and transform it to the visible light which can be absorbed by the perovskite material. It is benefit to the utilization of the UV region of sunlight, thus leading to the PCE improvement from 19.34% to 20.02% while reducing the damage of UV light to perovskite layer. At the same time, we used the ebeam evaporation method to fabricate the Al2O3 as the encapsulation layer of the device to enhance the long-term stability of PSCs. Although encapsulation would decrease the PCE of device to 17.60%, but it can reveal excellent long time stability. As a result, our modified device can maintain 90% of initial efficiency after aging 4000 h in the ambient air.

2.4. Device characterization The surface morphology of films and cross sectional view of device were studied by the SIRION field-emission scanning electron microscope. Atomic force microscopy (AFM; 5500, Agilent, Santa Clara, CA) which operated in contact mode was used to characterized for the local roughness of the thin films. EDS spectra of Ce doped TiO2 films were collected by Nova_NanoSEM430. XRD for films was carried out on a Rigaku D/max 2550 X-ray diffractometer, using a monochromatized Cu target radiation source at a scanning rate of 4°/minute. FluoroSENS from Zolix was used to record for the excitation and emission spectrum. The J-V characteristics of the devices were measured under simulated 100 mW/cm2 AM 1.5G irradiation using an ABET Sun 2000 solar simulator calibrated with a reference Silicon cell (RERA Solutions RR1002), and Keithley Model 2400 as a digital source meter. IPCE spectrum were recorded by using SolarCellScan100. EIS was performed by CHI630E Electrochemical Analyzer (ChenHua, China).

2. Experimental section 2.1. Synthesis of Ce doped TiO2 powders

3. Result and discussion The preparation of the Ce doped TiO2 powders through the sol-gel method. According to the stoichiometric ratio, the isopropyl titanate and Ce(NO3)3 were dissolved in a solution of 1 ml hydrochloric acid, 1 ml acetic acid and 10 ml ethanol. Then the above mixing solution was dried under 160 °C for 6 h to obtain the Ce doped TiO2 powders.

In order to improve the performance and stability of the PSCs, a modified device configuration can be designed as CsPbBr3/FTO/CeTiO2/perovskite/Spiro-OMeTAD/Au/Al2O3, as shown in Fig. 1a. Here, the CsPbBr3 QDs layer was first spin-coated on the incident light side of the FTO to enhance the performance of the PSC while reduce the damage of UV light to perovskite layer. Then, Ce doped TiO2 ETLs with uniform structure were prepared e-beam evaporated method. After that, perovskite absorb layer CsFAMA and HTL Spiro-OMeTAD were fabricated in sequence by spin coating method. Then, Au electrode was evaporated on top of HTL. Finally, the Al2O3 film was evaporated on the surface of the Au electrode as the encapsulation layer of the device, which is beneficial to improve the long-term stability of the PSCs. Fig. 1b is the cross-sectional SEM image of the device, and the corresponding films thicknesses of the Ce-TiO2 ETL, perovskite layer, HTL layer, Au electrode, and the Al2O3 encapsulation layer are determined to be 60, 330, 220, 80, and 60 nm, respectively. Firstly, we explore the effect of the ETL on the device performance. Fig. 2a shows the X-ray diffraction (XRD) patterns of the Ce doped TiO2 films. All diffraction peaks of the Ce doped TiO2 are corresponding to the anatase phase (Zhang et al., 2012). From the enlarged region of (1 0 1) peak shown in Fig. S1, the position of the diffraction peaks shifts to lower angle when the Ce content increases. In addition, the intensity of the (1 0 1) diffraction peaks decreases after doped more content of Ce ions. This may be due to the ionic radius of Ce ions is larger than the Ti ions, which make it incline to stay at the surface or the grain boundaries (Asemi and Ghanaatshoar, 2017). Energy-dispersive spectroscopy (EDS) was used to characterize the doping of Ce in the ETL layer (Fig. S2). It is clear that the Ce elements homogenously distributed in the entire film, indicating the successful doping. Fig. 2b–c show the surface morphologies of the e-beam evaporated un-doped TiO2 and Ce-TiO2 films. From the SEM images, there is no apparent difference after the Ce

2.2. Synthesis of CsPbBr3 perovskite quantum dots The mixing solution (solution A) which contain 0.8 g Cs2CO3, 2 ml oleic acid, and 20 ml 1-octadecene were added in the 3-neck roundbottom flask, and the resulting solution was stirred and degassed with N2. Finally the temperature of the solution was kept at 150 °C for 30 min. After that, the 0.2 g PbBr2, 15 ml 1-octadecene, 1.5 ml oleic acid, and 1.5 ml oleylamine were added to the other 3-neck roundbottom flask followed by heating the solution to 120 °C for 20 min (solution B). After completely dissolved, the solution B was increased to 160 °C, then the solution A (1 ml) was swiftly injected to keep 1 min. After that, the resulting solution was cooled with an ice bath. The CsPbBr3 QDs were precipitated with acetone and the centrifuged followed by dissolution in toluene. 2.3. Device fabrication Fluorine-doped Tin Oxide (FTO) glass substrates were etched with zinc powder and 30% HCl, then ultrasonicated in deionized water, acetone and ethanol successively for 15 min, followed by oxygen plasma treatment for 30 min. After that, Ce doped TiO2 ETL films were prepared by e-beam evaporation method with the evaporation rates of 0.2 nm/s. Then the perovskite absorb layer was prepared on the asprepared ETL under nitrogen atmosphere through one-step spin-coating method. Drop the perovskite precursor solution (1.0 M FAI, 1.1 M PbI2, 0.2 M MABr, 0.2 M PbBr2, and 0.05 M CsI dissolved in DMF: DMSO 188

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Fig. 1. (a) Schematic architecture of the modified PSCs. (b) The cross-sectional SEM image of the PSCs.

doping into TiO2, and both of the two films are uniformity compared to the rough surface of FTO (Fig. S3). Similar results are also observed from atomic force microscopy (AFM) images in Fig. 2d–e. The values of root mean-square (RMS) roughness of the TiO2 films without and with Ce doing are 4.93 and 4.87 nm, respectively. However, the thickness of the layer has a significant influence on the film and the performance of the device. As shown in Fig. S4, we prepared four films with different thicknesses, 40, 60, 80, and 100 nm, respectively. When the 40-nm TiO2 was evaporated on the FTO substrate, there exist many holes, which

cause the poor surface coverage, and the RMS roughness is 8.64 nm. RMS value is 4.93 nm as the TiO2 layer thickness is increased from 40 to 60 nm, indicating that the film becomes more smooth. The prepared thin film fully covers the surface of the FTO, which is beneficial to interface contact and electrons transfer, leading to increased performance of the device as shown in Fig. S5 and Table S1. After continually increasing the thickness of the TiO2 layer to 80 and 100 nm, the RMS roughness is 5.01 and 5.24 nm, respectively, which is a bit larger than that of the 60-nm TiO2. Moreover, the transmittance of these two

Fig. 2. (a) The X-ray diffraction pattern of undoped, 0.1%, 0.3%, 0.5%, and 1.0% Ce-doped TiO2 films prepared by e-beam method. (b) and (c) The SEM images of ebeam evaporated TiO2 films without and with 0.3% Ce doping. (d) and (e) The AFM images of e-beam evaporated TiO2 films without and with 0.3% Ce doping. (f) The corresponding plots of (ahv)2 vs. hv for the determination of the band gap of TiO2 films with Ce doping. (g) and (h) The UPS measurements of pure and Ce doped TiO2 films on Si substrate. 189

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thickness films decrease gradually compared that with the thin TiO2 film, as shown in Fig. S6. The larger RMS roughness and lower transmittance can be the reason for the decline in the device performance as the thickness of the TiO2 layer is over 60 nm. Therefore, the thickness of 60 nm is selected for TiO2 ETL in the device fabrication. Fig. S7 shows the SEM images of the perovskite films on the top of doped and undoped TiO2. It can be seen that the perovskite layer has full coverage onto these two ETL, and there is no effect of Ce doping on the crystallinity of perovskite film. Fig. S8 shows the UV–vis absorption spectra of the TiO2 film doped with different concentration of Ce. Red shift can be observed of the doped TiO2 absorption edge compared to that of the undoped TiO2, which is consistent with the previous work (Zhang et al., 2012). Fig. 2f shows the corresponding plots of (ahv)2 vs. hv to determine the band gap of the films. The optical band gap decreases from 3.60 eV to 3.58 eV after doping. To study the influence of Ce doping on energy level alignment of TiO2 film in the devices, the ultraviolet photoelectron spectroscopy (UPS) measurements were performed, shown in Fig. 2g and h. From the calculation, the valence band minimum (VBM) energy levels of TiO2, Ce doped TiO2 are −7.74 eV and −7.76 eV, respectively. Combine with the optical band gap, the conduction band minimum (CBM) energy levels of Ce doped TiO2 are −4.18 eV, which is a bit lower than the pure TiO2 (−4.14 eV). Previous studies revealed that the downward shifting in the CBM position would suppress the recombination at the interface during electron transfer, while the downward shifting in the VBM position would inhibit the injected electrons from moving back to the perovskite from the ETL (Shaikh et al., 2016; Huang et al., 2016). Moreover, the open-circuit voltage (VOC) under 1 sun was related to the quality of perovskite films, irrespective of the ETL (Gouda et al., 2015; Chang et al., 2016). Therefore, the downshifting CBM position would not affect the VOC in our experiment. In order to investigate the impact of Ce doping on the conductivity (σ) of TiO2 film, the device based on the structure shown in inset of Fig. S9 were fabricated and measured. The conductivity can be evaluated using the following equation (Obrzut and Page, 2009):

TiO2 based PSCs, electrochemical impedance spectroscopy (EIS), which is a very useful technique to analyze the carrier recombination resistance in the device, have been used to study the un-doped TiO2 and Ce-TiO2 based PSCs. The Nyquist plots (as shown in Fig. 3d) were measured in the dark condition with an applied bias voltage of 0.8 V, and the inset is the simulated equivalent circuit, where RS is the series resistance, R1 is the charge transfer resistance and CPE1 is its capacitance, R2 is the recombination resistance and CPE2 is its capacitance (Hwang et al., 2015). R1 decreases from 376.5 Ω to 265.9 Ω as 0.3% Ce is doped in TiO2, while R2 increases from 804.8 Ω to 955.7 Ω, which means that the Ce doping can improve the charge extraction process and reduce charge recombination in the PSCs. To further reveal the impact of Ce doping on charge transfer process at the interface of TiO2 and perovskite layer, the steady state photoluminescence (PL) and time resolved PL (TRPL) measurements were carried out on the perovskite film based on the un-doped TiO2 and Ce-TiO2, respectively. Fig. 3e shows the PL spectra of the FTO/perovskite, FTO/TiO2/perovskite, and FTO/0.3% Ce-TiO2/perovskite samples. Compared to the FTO/perovskite sample, the PL intensities of the other two samples are significantly quenched, and the FTO/0.3% Ce-TiO2/perovskite sample exhibits the lowest PL intensity, which indicates the better charge transport from the perovskite layer to the TiO2 ETL. Fig. 3f is the TRPL spectra for the FTO/TiO2/perovskite and FTO/0.3% Ce-TiO2/perovskite samples, and the decay time constants are obtained from a biexponential decay function below,

F(t ) = A1exp(−t / τ1) + A2exp(−t / τ2) where A1 and A2 are the time-independent coefficients of amplitude fraction, respectively. τ1 and τ2 are the fast decay time constant and slow decay time constant (Fan et al., 2016). The average decay time constants were calculated using the following equation:

τave =

∑ Aiτi2/ ∑ Aiτi

the τave of the FTO/TiO2/perovskite is 61.19 ns, and significantly reduces to 37.33 ns after Ce doping into TiO2, demonstrating that the charge transport from the perovskite layer to TiO2 ETL is improve by Ce doping. The fast charge transport was beneficial to the improvement of the JSC and FF in the device. In addition, the photovoltaic performance of the PSCs based on the different Ce doping concentration are shown in Fig. S10. It noticed that the efficiency decreases as the content of Ce increases to 1.0%, which is attributed to the decreased electron conductivity. Moreover, the photogenerated electrons would be trapped by the Ce4+ states as the concentration of Ce doping is higher, leading to the decreased JSC and PCE (Zhang et al., 2012). In order to extend the working wavelength of PSCs in UV region, the CsPbBr3 QDs as the UV light convert layer was spin-coated on the incident light side of the FTO, as shown in Fig. 1a. From the TEM image (Fig. S11a), the size of synthetized CsPbBr3 QDs is about 8.7 nm, and the XRD pattern (Fig. S11b) shows that the QDs have the cubic phase (PDF#54-0752). It can be seen from the excitation and emission spectra (Fig. S11c, d), the QDs have a broad excitation band in the UV region, and can emit green light (510 nm) with the PLQYs of 86%, which matches well with the absorption range of the light absorber of PSCs. When the sunlight is irradiated on the device, some UV photons will be converted to the visible photons by the down-conversion effect of CsPbBr3 QDs coated on the front of the FTO, and then converted visible light will be absorbed by the perovskite layer to contribute PCE. This is helpful for improving the utilization of UV light by PSCs, resulting in the improved PCE and stability of the PSCs. Fig. 4a shows the transmittance spectra as a function of spin-coating layers for the CsPbBr3 QDs coated on the PSCs. The transmittance in the UV region decreased as the number of coated layers increases due to the UV light absorption of CsPbBr3 QDs. It is also can be seen that the range of 500–550 nm which is the absorb range of the perovskite layer decreased after coating 5 layer QDs, which may causes a little influence on the light

I = σ AD−1V, where A is the contact area (10 mm2), D is the thickness (60 nm) of the TiO2 films, and the σ is the conductivity of the corresponding film. The fitting vales are listed in the Table S2. It can be seen that the conductivity of the 0.3% Ce doped TiO2 is 2 times higher than that of the undoped TiO2, which indicates that Ce doping is an effective way to enhance the conductivity of the TiO2 film. The improved conductivity is mainly due to the decreased formation energy of oxygen vacancies in TiO2 caused by Ce doping according to literature (Asemi and Ghanaatshoar, 2017). However, the conductivity will decrease with further increasing the concentration of Ce doping due to the TiO2 crystallinity deterioration induced by Ce doping. Fig. 3a shows the current density-voltage (J-V) curves of the champion device based on un-doped and 0.3% Ce-TiO2. The device based on the un-doped TiO2 exhibits a PCE of 17.98% with a shortcircuit current density (JSC) of 21.19 mA/cm2, a VOC of 1.111 V and a fill factor (FF) of 76.37%. When the 0.3 mol % Ce was doped into the TiO2 ETL, the PCE was improved significantly to 19.34%, and the corresponding JSC, VOC and FF were 22.26 mA/cm2, 1.112 V and 78.13%, respectively. The statistic PCE of the 55 PSCs based on the undoped TiO2 and 0.3% Ce-TiO2 are investigated in Fig. 3b. Ce-TiO2 based PSCs present an impressive average PCE of 19.1% and excellent reproducibility. It can be observed that the VOC values are not influenced by Ce doping. The improved PCE is mainly due to the enhancement of the JSC and FF, which is explained by increased charge transfer efficiency caused by Ce doping. In order to confirm this, several photoelectric and electrochemistry experiments were performed. As shown in Fig. 3c, incident photon-to-electron conversion efficiency (IPCE) of the Ce-TiO2 based PSCs displays higher intensity than the un-doped TiO2 based PSCs in the whole UV–visible light range, indicating improved charge collection. To further understand the improved efficiency of Ce190

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Fig. 3. (a) J-V curves for the champion devices based on un-doped TiO2 and 0.3% Ce-TiO2. (b) The statistic PCE plot of un-doped TiO2, 0.3% Ce-TiO2 based PSCs. (c) IPCE spectra of un-doped TiO2 and 0.3% Ce-TiO2 based PSCs. (d) Fitting curves from Nyquist plots at a frequency range from 1 Hz to 100 kHz with an applied bias of 0.8 V of un-doped TiO2 and 0.3% Ce-TiO2 based PSCs. (e) The steady state PL spectra and (f) The TRPL spectra of perovskite films based on the un-doped TiO2 and CeTiO2.

Fig. 4. (a) Transmittance spectra of the FTO coated with different layers of CsPbBr3 QDs. (b) J-V curves for the PSCs coated with different layers of CsPbBr3 QDs. (c) IPCE spectra of the PSCs coated by different layers of CsPbBr3 QDs. (d) UV light stability of the PSCs with and without CsPbBr3 QDs coating. 191

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PCE after 4000 h aging.

harvesting of PSCs. Fig. 4b shows the J-V curves of the PSCs coated by the CsPbBr3 QDs with different layers, and the corresponding photovoltaic parameters are shown in the Table S3. It can be seen that the VOC and FF values of the CsPbBr3 QDs coated device are the same compared to the uncoated PSCs, which indicates that the effect of QDs is negligible on the VOC and FF of the device. However, the JSC changes after coating with the QDs. JSC increases as 1–3 QDs layers are coated, and then decrease after coating over 5 layers. The PCE shows the same the tendency with the JSC, and the highest PCE 20.02% is obtained as 3 QD layer are coated on devices, which increases 3.46% compared to the uncoated PSCs. The IPCE increase in the UV region for all samples, benefiting from the effective down-conversion of the UV light. In the visible wavelength region, the IPCE is nearly the same for the case of less than 3 QD layers, and it decreases with further increasing QDs coating layer. The UV light stability of the device was also studied in Fig. 4d. Uncoated and coated devices were illuminated by 365 nm UV light (10 mW cm−2) in ambient environment continuously for 200 h. The PSCs coated with QDs exhibit excellent stability compared to that of the uncoated device. It can maintain 90% of its initial efficiency, while the uncoated device just remains 48% of its initial value only. These results show that the coated QDs on the devices can effectivity improve the efficiency and UV light stability of the PSCs due to the down-conversion effect of CsPbBr3 QDs. In addition to modifying the UV light stability, we also used the ebeam method to prepare the Al2O3 encapsulation layer to improve the moisture stability of the devices. Al2O3 has been proven that it is an effective moisture barrier for devices (Ramos et al., 2018; Meyer et al., 2009; Seo et al., 2012). Fig. S12 shows the SEM image of the surface of the Al2O3 encapsulation layer. The films fabricated through e-beam method are more compact, which can improve the moisture barrier function. The J-V curves of the PSCs with Al2O3 encapsulation layer are shown in Fig. 5a. The results demonstrate that the PCE of encapsulation device decreases to 17.60% compared to the reference device, which may be due to the impact of the Al2O3 evaporation process on the perovskite film. In order to verify it, two perovskite films were prepared with the same condition, and UV–vis absorption spectra of the two perovskite films with and without Al2O3 evaporation layer were collected as shown in Fig. S13. It is noticed that the absorption intensity of perovskite film with Al2O3 evaporation layer decreases compared to that of the uncoated film. It is possible that the generated temperature during the Al2O3 evaporation process destroys the quality of the perovskite film, leading to the decreased absorption. Although the PCE of the encapsulation device decreases, the stability of device is greatly increased, as shown in Fig. 5b. Both the encapsulation and bare devices were aged in the ambient environment of 25–28 °C with 30%-55% humidity. The bare device decreases to below 10% after aged 1000 h, however, the encapsulation device always maintained 90% of the initial

4. Conclusions In this work, several modifications were performed in contrast to the traditional PSCs device in order to improve the PCE and the stability. First of all, we prepared the TiO2 ETL layer by the beam evaporation doped the Ce ions into TiO2 to improve the electron conductivity of TiO2, which is beneficial to increase the charge transport and reduce the charge recombination. The doping of Ce ions into TiO2 leads to an increased PCE from 17.98% to 19.34%. Second, we also coated the photoluminescent CsPbBr3 QDs as the ultraviolet (UV) convert layer on the incident light side of the device to enhance the performance of PSCs. By the introduction of the CsPbBr3 QDs, the PCE relatively increased 3.46% compared to the uncoated PSCs and approached to 20.02%, while the ultraviolet light stability of the device was highly enhanced. Finally, we used the e-beam method to fabricate the Al2O3 as an encapsulation layer of the device to enhance the longterm stability of PSCs. As a result, the modified device can maintain 90% of initial efficiency after aging 4000 h in the ambient air. This work demonstrates that the doping of rare ions into TiO2 ETL layer, the introduction of ultraviolet to visible photon conversion layer in front of the device, and the encapsulation on the back side are of great importance for improving the PCE and stability of the PSCs. 5. Notes The authors declare no competing financial interest. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Key Research and Development Program (2016YFC0207101), the National Natural Science Foundation of China (Grant Nos. 11874181, 61874049, 61775080, 11674126, 11674127, 61674067), the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3), the Jilin Province Natural Science Foundation of China (No. 20180101210JC, 20170101170JC, 20160418055FG), and the Jilin Province Science Fund for Excellent Young Scholars (No. 20170520129JH, 20170520111JH).

Fig. 5. (a) J-V curves for the device with and without Al2O3 encapsulation layer. (b) The stability of the device with and without Al2O3 encapsulation layer. 192

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Appendix A. Supplementary material

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