Improving Efficiency and Light Stability of Perovskite Solar Cells by

Bella et al. used the fluorescent organic dye V570 as a photoluminescent converter on the front side of the PSC device to prevent the UV portion of th...
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Improving Efficiency and Light Stability of Perovskite Solar Cells by Incorporating YVO4:Eu3+, Bi3+ Nanophosphor into the Mesoporous TiO2 Layer Junjie Jin, Hao Li, Cong Chen, Boxue Zhang, Wenbo Bi, Zonglong Song, Lin Xu, Biao Dong, Hongwei Song, and Qilin Dai ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00192 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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Improving Efficiency and Light Stability of Perovskite Solar Cells by Incorporating YVO4:Eu3+, Bi3+ Nanophosphor into the Mesoporous TiO2 Layer Junjie Jin1, Hao Li1, Cong Chen1, Boxue Zhang1,Wenbo Bi1, Zonglong Song1, Lin Xu1, Biao Dong1, Hongwei Song*1and Qilin Dai*2 1

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin

University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China 2

Department of Chemistry, Physics and, Atmospheric Sciences, Jackson State University, Jackson,

Mississippi 39217, United States

*E-mail: [email protected]

*E-mail: [email protected]

Abstract

Even though a high power conversion efficiency (PCE) of 22.1% is achieved in the perovskite solar cells (PSCs), the ultraviolet (UV) light stability in PSCs is still a challenge that needs to be overcome before their actual application in the outdoor environment. In this work, we incorporated the YVO4:Eu3+, Bi3+ luminescent down-shifting (LDS) material into the mesoporous TiO2. The YVO4:Eu3+, Bi3+ would convert the high UV photons into the low energy visible photons (red light) and improves the utilization of UV light in PSCs, which results in PCE enhancing and UV light stability improving in PSCs. By controlling the YVO4:Eu3+, Bi3+ concentrations, an optimized PCE of 17.9% is obtained, which shows an improvement of 9.8% compared with the bare device. More importantly, the device based on m-TiO2/YVO4:Eu3+, Bi3+ layer 1 ACS Paragon Plus Environment

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has distinguished stability under UV light irradiation, which could maintain 70% initial efficiency after 60 h UV irradiation. Keywords: perovskite solar cells, YVO4:Eu3+, Bi3+, luminescent down-shifting, photovoltaic performance, ultraviolet light stability 1. Introduction

PSCs have obtained increasing attention in recent years as a new generation photovoltaic devices due to its low cost, easy fabrication technique and high PCE.1-6 Even though the PSCs have achieved a considerable PCE with 22.1%, a few limitations still need to be addressed before their practical application in the outdoor environment, such as the stability of PSCs.7-9 Compared to the traditional silicon cells with a slightly decrease in PCE (less than 0.5% after 25-years stored), several factors such as moisture and ultraviolet (UV) light irradiation can cause the perovskite material decomposition, leading to the decreased device performance.10-12 The moisture stability in PSCs can be improved in several ways, but the UV light stability in PSCs still remains challenging up to now.13-15 In the present research, TiO2 is usually used as electron transfer material in PSCs.16-18 However, it is known that TiO2 is susceptible to the UV light. Exposing TiO2 to UV light for a long time, deep traps at TiO2 surface that oxygen vacancies induced would lead to the recombination loss of photo-generated carriers.

19-22

Furthermore, photo-generated holes in TiO2

with strong oxidative can react with I ions, which results in degradation of CH3NH3PbI3 and decline of performance.23-24 Hence, reducing UV irradiation of the incident solar spectrum on the TiO2 layer can improve the UV light stability of PSCs. Bella et al. used the fluorescent organic dye V570 as a photo-luminescent converter on the front side of the PSC device to prevent the UV portion of the incident solar spectrum, leading to increased PCE by 8% and improved UV light stability over 500 h under continuous illumination.25 We incorporated a photo downshifting layer-SrAl2O4:Eu2+, Dy3+ into the PSCs to convert UV light to visible light that can be used by PSCs and to suppress the device degradation which 2 ACS Paragon Plus Environment

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affected by the UV light, resulting in the improvement of PCE from 16.7% to 17.8% and a considerably improved UV light stability after 100 h continuous illumination.26 These methods proved that the luminescent down-shifting (LDS), which downshifts a part of incident high energy UV photons to low energy visible photons, is an effective approach to improve the device stability and PCE. Among these LDS materials, Europium (Eu3+) doped yttrium vanadate (YVO4) reveals the outstanding UV degradation resistant due to its high absorption coefficient in UV range of 220-350 nm, YVO4 will transfer the absorbed UV photon energy to the Eu3+ ions, emitting an intense red light, and the absolute luminescence quantum efficiency can reach 80% or more.27-30 Notably, the emission of YVO4:Eu3+ falls in the absorption range of the perovskite materials. In addition, the Bi3+ ion was demonstrated to be an excellent sensitizer for YVO4:Eu3+, which could improve the intensity of luminescence and make excitation spectrum broader.31-33 Hence, the YVO4:Eu3+, Bi3+ LDS material is promising UV absorbing converters for the PSCs, which could not only improve the UV light stability of PSCs, but could also enhance the light harvesting. Here, we incorporated the YVO4:Eu3+, Bi3+ into the mesoporous TiO2 to effectively prevent the device degradation caused by the UV light and to enhance the light harvesting, leading to an improved PCE of 17.9% compared that of the control device (16.3%), meanwhile, 70% initial efficiency kept after 60 h UV irradiation for the YVO4:Eu3+, Bi3+ modified device compared to 20% for the control device. 2. Experimental Section Synthesis of YVO4:Eu3+, Bi3+ LDS material. By using hydrothermal method, YVO4:Eu3+, Bi3+ nanophosphor has been prepared.32 Firstly, a certain amount of NH4VO3 was dissolved in deionized water. Then, the solution was added into nitric acid aqueous solution which contains the Y(NO3)3·6H2O, Eu(NO3)3·6H2O, and Bi(NO3)3·5H2O. Stirred the solution for 1 3 ACS Paragon Plus Environment

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h and then transferred into Teflon-lined autoclave (50 ml) and kept at 180 ℃ for 24 h. The resultant product was collected by centrifugation, washed with ethanol and deionized for several times and finally dried in air at 80 ℃ for 24 h. Device fabrication.

The detailed procedures in the device fabrication refer to our previous work except for the mesoporous TiO2 and perovskite layer.19 The TiO2 pastes (Dyesol DSL 18NR-T) with a series amount of YVO4:Eu3+, Bi3+ nanophosphors (0, 2.5, 5.0, 7.5, and 10.0 wt%) were diluted with ethanol (1:4 mass ration). After that, the solution was spin-coated onto the compact TiO2 layer for 30 s at 4000 rpm. Heating them at 500 ℃ for 30 min and cooling to room temperature. Subsequently, MAI and PbI2 (molar ratio is 1:1) dissolved into the mixed solution of DMSO and DMF (1 ml, volume ratio is 3:7), which was spin-coated onto the mesoporous TiO2 layer for 10 s at 1000 rpm and further 40 s at 4000 rpm, respectively. Moreover, 100 µL chlorobenzene was dropped onto the film at the end of spinning process. Finally, the film was annealed for 1 h at 100 ℃ in glovebox. 3. Result and discussion

The schematic configuration of PSC device is given in Figure 1a, which is made up of glass/FTO, a compact TiO2 (c-TiO2) layer and a modified mesoporous layer (m-TiO2 with YVO4:Eu3+, Bi3+ nanophosphors), a CH3NH3PbI3 perovskite layer, a Spiro-OMeTAD layer, and an Au contact layer. When incident sunlight passed PSCs at FTO side, majority of visible photons has been absorbed by the layer of perovskite, as well as partial UV photons can be converted into visible photons with lower energy by using YVO4:Eu3+, Bi3+, which could utilize sunlight more efficiently in PSCs. Therefore, the PCE and UV light stability of PSCs can be enhanced. Figure 1b and Figure S1 show the energy dispersion spectrum (EDS) mapping date of the m-TiO2/YVO4:Eu3+, Bi3+ film, which are used to confirm the presence of Y, V, Eu, and 4 ACS Paragon Plus Environment

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Bi elements in the mesoporous TiO2 layer. Figure S2 shows the XRD pattern of the YVO4:Eu3+, Bi3+ nanophosphors and the standard card. The diffraction peaks at 18.8°, 25.0°, 33.6°, and 49.8° which is corresponding to (101), (200), (112) and (312) planes of YVO4, indicating the formation of clean phase YVO4. The phase is very critical to the luminescence quantum efficiency. Figure S3 gives the SEM image of the YVO4:Eu3+, Bi3+ nanophosphors. It can be seen that the morphology of YVO4:Eu3+, Bi3+ nanophosphors is similar to the m-TiO2, leading to the incorporation of YVO4:Eu3+, Bi3+ has little influence on the m-TiO2 structure, as shown in Figure S4. However, the root-mean-square (RMS) roughness reduces from 8.8 nm to 6.0 nm as the YVO4:Eu3+, Bi3+ incorporated into m-TiO2. The smaller RMS roughness means that the m-TiO2/YVO4:Eu3+, Bi3+ film is much smoother, which results in the improved interface contact between the m-TiO2 and perovskite layer. This indicates the recombination at mesoporous film/perovskite interface decreases as YVO4:Eu3+, Bi3+ incorporated into the m-TiO2, leading to improve the device performance.19, 34 Figure 1(c) presents the excitation and emission spectrum of Y0.97-xVO4:0.03Eu3+, xBi3+ (x=0, 0.05) nanophosphors. It can be seen that the excitation band of the Y0.92VO4:0.03Eu3+, 0.05Bi3+ monitored at 621 nm ranges from 250 nm to 380 nm with a peak at 343 nm. The excitation band, is attributed to the 6s2-6s6p transition of Bi3+. In addition, the excitation of Y0.92VO4:0.03Eu3+, 0.05Bi3+ is much broader compared that of Y0.97VO4:0.03Eu3+, indicating that more UV photons can be converted into the visible photons as Bi3+ ions are introduced into the YVO4:Eu3+ to be utilized by the perovskite layer. For the emission spectra in Figure 1(c), the emission peak at 621 nm is assigned to 5D0-7F2 electric-dipole transition of Eu3+.31 To obtain the optimized samples for photo-conversion, Y0.97-xVO4:0.03Eu3+, xBi3+ nanophosphors with different Bi concentrations (x= 0, 0.10, 0.15, and 0.20), were prepared and their emission spectrum are shown in Figure S5. Notably, the emission intensity becomes stronger after the Bi3+ incorporated into the Y0.97VO4:0.03Eu3+, and the emission intensity shows that the maximum value as x=0.05. Figure 1d shows the corresponding film thicknesses of c-TiO2, m-TiO2/YVO4:Eu3+, Bi3+, 5 ACS Paragon Plus Environment

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CH3NH3PbI3, Spiro-OMeTAD and Au are 60 nm, 180 nm, 350 nm, 240 nm, 100 nm, respectively in this work.

Figure 1. (a) The device structure of PSCs. (b) Energy dispersive spectroscopy (EDS) images of the m-TiO2/YVO4:Eu3+, Bi3+ film. (c) The excitation and emission spectrum of YVO4:Eu3+ and YVO4:Eu3+, Bi3+ (λem=343 nm, λex=621 nm). (d) Cross-sectional view of the PSC device in this work. To confirm the possibility of using their photoluminescence (PL) effect in devices, the emission spectra of the m-TiO2 with different YVO4:Eu3+, Bi3+ concentrations as shown in Figure 2a. It can be observed that the m-TiO2 containing YVO4:Eu3+, Bi3+ shows red emission, whereas the bare m-TiO2 has no emission under 343 nm excitation. In addition, the fluorescence intensity is getting stronger with increasing the 6 ACS Paragon Plus Environment

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concentration of YVO4:Eu3+, Bi3+. Figure 2b shows the UV-Vis absorption of perovskite film deposited on m-TiO2 with different YVO4:Eu3+, Bi3+ concentrations. The intensity of UV-Vis absorption ranging from 300 to 400 nm increases after the YVO4:Eu3+, Bi3+ incorporated into the m-TiO2. This increased absorbance in this region indicates more UV light can be used. Furthermore, we also measured the UV-Vis absorption of perovskite deposited on m-TiO2/YVO4:Eu3+ and m-TiO2/YVO4:Eu3+, Bi3+, respectively, as shown in Figure S6. The introduced concentrations of YVO4:Eu3+ and YVO4:Eu3+, Bi3+ are equal to each other. The absorption intensity of perovskite with m-TiO2/YVO4:Eu3+, Bi3+ is stronger than that of the perovskite with m-TiO2/YVO4:Eu3+, which indicates that there are more light can be used by perovskite layer. Therefore, the YVO4:Eu3+, Bi3+ can convert more UV light into the visible light than YVO4:Eu3+.

Figure 2. (a) The emission spectrum of the m-TiO2 with different YVO4:Eu3+, Bi3+ concentrations. (b) The UV-Vis absorption spectrum of perovskite deposited on m-TiO2 with different YVO4:Eu3+, Bi3+ concentrations. Figure 3a presents the current density-voltage (J-V) curves of the PSCs based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+, and the relevant photovoltaic parameters are listed in Table 1. It can be seen that the control device based on m-TiO2 layer has a short-circuit current density (JSC) of 20.62 mA/cm2, an open-circuit voltage (VOC) of 1.07 V and a fill factor (FF) of 73.8%, resulting in a PCE of 16.3%. On the contrary, the PSCs based on the 5 wt% YVO4:Eu3+, Bi3+ doped m-TiO2 layer exhibits a PCE of 17.9% with a 7 ACS Paragon Plus Environment

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JSC of 21.73 mA/cm2, an VOC of 1.10 V and a FF of 74.9%. The statistical PCE histograms of 40 PSC devices based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ layer prepared by the same condition are presented in Figure 3b.6 For the devices based on m-TiO2/YVO4:Eu3+, Bi3+ layer, the average PCE is ~17.5% over 40 devices, which demonstrates its outstanding reproducibility. To determine advantageous influences of YVO4:Eu3+, Bi3+ on photovoltaic, measurements of photoelectric and electrochemical were tested. Figure 3c exhibits the incident photo-to-current conversion efficiency (IPCE) spectrum in region of 300-800 nm of the device with and without YVO4:Eu3+, Bi3+. Obvious enhancement in the region of 300-400 nm are shown in Figure 3d which is contributed to the incorporated YVO4:Eu3+, Bi3+. This suggests that UV light are used in the modified PSC due to the light conversion of nanophosphor.35 Hence, the enhanced IPCE is beneficial for the improved photocurrent. Furthermore, the integrated JSC values calculated from the IPCE spectrum are 20.59 mA/cm2 and 21.70 mA/cm2 for the devices based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ layer, which are close to the values obtained from the J-V curves. The electrochemical impedance spectroscopy (EIS) of the devices based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ are shown in Figure 3e, and the inset is the corresponding equivalent circuit. The fitting parameters are given in Table S1. The charge transfer resistance (RS) is the resistance at the Spiro-OMeTAD/Au electrode interface, and the carrier recombination resistance (Rreb) is the resistance at the mesoporous film/perovskite interface.36-37 As the YVO4:Eu3+, Bi3+ is incorporated into the m-TiO2 layer, Rreb exhibits a significant increase from 567.1 Ω cm2 to 749.7 Ω cm2, indicating that the YVO4:Eu3+, Bi3+ doped in the m-TiO2 layer reduced the loss of interfacial charge recombination, which leads to the improvement of charge transfer from perovskite to m-TiO2. The photoluminescence (PL) spectrum of the perovskite films based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ were also measured to analyze the charge recombination processes. As shown in Figure 3f, the PL intensity of perovskite based on m-TiO2/YVO4:Eu3+, Bi3+ decreases compared with the control film, indicating decreased carried recombination at the interface and improved electron extraction from the perovskite film.38 8 ACS Paragon Plus Environment

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In addition, the influence of YVO4:Eu3+, Bi3+ concentration on PSC device performance was also studied (Figure S7 and Table 1). Noticeably, as the YVO4:Eu3+, Bi3+ concentration increases from 2.5 wt% to 5.0 wt%, the PCE improves, but further increase the concentration to 10.0 wt%, the device performance decreases. This is owing to the fact that defect increases arising from the excessive nanophosphor incorporated into the m-TiO2.

39

In addition, the Rreb decreased as the increase of the incorporating

concentration from 7.5 wt% to 10 wt%, as shown in Figure S8 and Table S1. The decreased Rreb indicates that the charge recombination at the mesoporous film/perovskite interface increases, leading to the decreased PCE.34-35

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Figure 3. (a) J-V curves. (b) The corresponding histograms of PCE. (c) IPCE. (d) Detail of IPCE curves below 400 nm. (e) Fitting curves from Nyquist plots at a frequency ranging from 1 Hz to 100 kHz with an applied bias of 0.8 V. (f) PL spectra for the devices based on the m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+. Table 1. The photovoltaic parameters for the PSCs based on the m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+. 10 ACS Paragon Plus Environment

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Device

PCE (%)

JSC (mA/cm2)

m-TiO2

16.3

20.62

1.07

73.8

with 2.5 wt% YVO4:Eu3+, Bi3+

17.1

21.26

1.09

73.8

with 5.0 wt% YVO4:Eu3+, Bi3+

17.9

21.73

1.10

74.9

with 7.5 wt% YVO4:Eu3+, Bi3+

16.6

20.83

1.08

73.7

with 10.0 wt% YVO4:Eu3+, Bi3+

15.6

20.02

1.07

72.8

VOC (V)

FF (%)

In order to study how the YVO4:Eu3+, Bi3+ affects the UV light stability of the PSCs, we exposed the two types of devices to 365 nm 25 mW cm-2 UV light illumination in the ambient environment, and the device performance was measured every 5 h. Figure 4a provides the relationship of normalized PCE-testing time for the devices based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+. Control device reveals a dramatically decrease in PCE, where only 20% of initial PCE has been retained after 60 h UV irradiation. However, the device based on the m-TiO2/YVO4:Eu3+, Bi3+ exhibits significantly improved stability under the same testing conditions, the PCE remains nearly 70% of its initial PCE. Notably, eight devices were performed the testing of light stability at each data point, which ensured a high reliability (Table S2). In addition, the other normalized photovoltaic parameters (JSC, VOC, and FF) as functions of irradiation time under UV light for the devices based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ are presented in Figure S9, which shows the similar trends with PCE values. The improvement of UV light stability of PSCs is because of the effective LDS process of the YVO4:Eu3+, Bi3+. According to the previous studies, the oxygen vacancies would formation at the surface of the TiO2 under long time UV light exposure. The oxygen vacancies capture electrons from I-, to form I2, resulting in the chemical imbalance between CH3NH3+, CH3NH2 and H+.40-41 Hence, UV-induced catalytic reaction of TiO2 leads to the decomposition of perovskite film. This is also confirmed in the XRD measurement, as shown in Figure 4b. When control device exposed in the UV light

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for 60 h, a new diffraction peak at 12.49° is observed in the XRD pattern, which is ascribed to (001) diffraction peak of PbI2, indicating the decomposition of perovskite material.42-44 The PbI2 would block the charge transport, resulting in the decrease of PCE. In comparison, there are no PbI2 diffraction peaks in the device based on the m-TiO2/YVO4:Eu3+, Bi3+ after during the UV-aging. In addition, we also studied the long-time stability of the PSCs based on the m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ layer, respectively. As shown in Figure S10, the PCE of the PSCs based on the m-TiO2/YVO4:Eu3+, Bi3+ layer shows similar downward trends compared with the control device.

Figure 4. (a) Normalized stability with error lines of the PSCs based on the m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ after UV light irradiation for 60 h. (b) XRD data of perovskites before and after UV light irradiation. 4. Conclusions In summary, we have successfully incorporated the YVO4:Eu3+, Bi3+ LDS material into the mesoporous TiO2, which can improve the photovoltaic performance and UV light stability of the PSCs. Consequently, the PCE increases from 16.3% to 17.9%, which is attributed to the enhanced light harvesting in the UV region of the device. Meanwhile, the introduction of YVO4:Eu3+, Bi3+ prevents the decomposition of perovskite through the LDS process. Therefore, the UV light stability of the PSC is improved significantly after UV aging for 60 h. Our results prove that incorporating an ideal LDS material into PSCs is an effective 12 ACS Paragon Plus Environment

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approach to enhance the photovoltaic performance and UV light stability of devices. Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental characterization. SEM and EDS images for the m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+ film. The photovoltaic performance for the device based on m-TiO2 and m-TiO2/YVO4:Eu3+, Bi3+. Author Information Corresponding Authors

*E-mail: [email protected] (Hongwei Song)

*E-mail: [email protected] (Qilin Dai)

Notes

The authors declare no competing financial interest.

Acknowledgement This work was supported by the Major State Basic Research Development Program of China (973 Program) (No. 2014CB643506), the National Natural Science Foundation of China (Grant nos. 61674067, 11504131, 11674126, 11374127, 11674127, 81201738), Graduate Innovation Fund of Jilin University (No. 2017174), the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3),the Jilin Province Natural Science Foundation of China (NO. 20150520090JH, 20160418055FG, 20170101170JC), Jilin Provincial Economic Structure Strategic Adjustment Fund Special Projects (No. 2014Y082) and the Jilin Province Science Fund for Excellent Young Scholars (No. 20170520129JH, 20170520111JH). 13 ACS Paragon Plus Environment

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