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Planar MgxZn1-xO-based perovskite solar cell with superior ultraviolet light stability
Solar Energy Materials & Solar Cells 208 (2020) 110417
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Planar MgxZn1-xO-based perovskite solar cell with superior ultraviolet light stability Fei Han, Zhongquan Wan, Junsheng Luo, Jianxing Xia, Hongyu Shu, Chunyang Jia * State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Perovskite solar cell MgxZn1-xO Electron-transporting material Ultraviolet light stability
Generally, TiO2-based perovskite solar cell (PSC) is beneficial to high efficiency but poor ultraviolet (UV) light stability. Here, we report that a highly efficient MgxZn1-xO-based (MZO-based) PSC with excellent UV light stability. MZO has a higher electron mobility and deeper conduction band than traditional TiO2, which can reduce the charge accumulation at the MZO/perovskite interface and enhance the charge transfer from perov skite to MZO. Furthermore, the reduced interface loss and energy barrier are beneficial to obtain high opencircuit voltage (Voc). By optimizing, the MZO-based device shows a high Voc of 1.11 V, yielding a promising efficiency of 19.57%. The MZO-based device (unencapsulated) retains 76% of its initial short-circuit current density (Jsc) after 1 year air aging (room temperature, relative humidity: 40–80%) and 8 h UV irradiation (365 nm, 35 W), versus only 12% under the same condition for TiO2-based device. The good UV light stability of MZObased device can be attributed to the reduced electron trap-state density in MZO electron-transporting layer (ETL). Specifically, zinc interstitial and oxygen vacancy mediated defect sites of MZO ETL are effectively passivated, which tightly protects the perovskite layer from degradation under UV light. Our results show a great potential for MZO ETL application in UV-stable PSC.
1. Introduction Perovskite solar cells (PSCs) have got a lot of attention due to the recent surges in device performances via process control and optimiza tion [1–8]. The recording power conversion efficiency (PCE) is as high as 25.2% (certified) [9], which shows great potential for its industrializa tion. Despite the high efficiency has been achieved in PSC, the in stabilities of the perovskite material and the corresponding device need to be addressed for further application. It is well-known that the ma jority of TiO2-based PSCs show excellent PCEs exceeding 20%. Unfor tunately, exploying TiO2 as electron-transporting layer (ETL) material in planar n-i-p structure PSC will lead to a rapid decay of short-circuit current density (Jsc) under ultraviolet (UV) irradiation [10]. Many ef forts including employing an interface layer at the TiO2/perovskite interface [11–14], and further doping of TiO2 [15–18], have been made to protect perovskite from UV light-induced degradation. However, the above methods inevitably damage its performance. Hence, it is extremely imperative and valuable to fabricate stable PSC without damaging its performance. Based on these, exploring new ETLs stable to UV light has attracted
considerable research interests, which is conducive to preparing highly effcient and highly stable PSCs [10,19–22]. Among them, PSCs using SnO2 [21] and Nb2O5 [10,22] as ETLs have shown excellent stabilities under light illumination. Besides, MgxZn1-xO (MZO) has been proved to be an effective ETL material in the solar cells [23,24], but fewer re searches were focused on UV light stability. Hence, we report an oversimplified route for low-temperature, so lution-processed and stable MZO nanocrystals (NCs), and further utilize them to fabricate MZO-based PSC. To the best of our knowledge (sum marized in Table S1), the bare MZO-based device shows an amazing PCE of 19.57% comparing with that of SnO2-based n-i-p PSC by You’s paper [21]. Encouragingly, MZO with tunable properties can form matched band alignment between perovskite and ETL, improving the open-circuit voltage (Voc) of the device. Furthermore, the MZO-based device shows superior UV light stability.
* Corresponding author. E-mail address: [email protected] (C. Jia). https://doi.org/10.1016/j.solmat.2020.110417 Received 1 October 2019; Received in revised form 12 January 2020; Accepted 15 January 2020 Available online 24 January 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (a) XRD and inserted TEM image, high-resolution XPS scans of MZO film (b) Mg 1s scan, (c) Mg 2p scan and (d) Zn 2p scan, respectively. Pink asterisk in XRD represents the diffraction peaks of FTO. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2. Experimental section
determined with PERKIN ELMER Lambda 950 Spectrometer. UPS and XPS were performed with Kratos X-ray Photoelectron Spectrometer (Axis Ultra DLD). Steady-state PL spectraspectra were recorded by the FLS980 Fluorescence Spectrometer (Edinburgh Instruments Spectros copy System). Time-resolved PL (TRPL) were measured on a Fluorolog-3 Spectrofluorometer (Horiba JobinYvon) with a DeltaDiode (406 nm, DD-405L) as the excitation source and a picosecond photon detection module (PPD-850) as the detector (λem ¼ 750 nm). Current-voltage (J-V) performance of the devices were recorded on a CHI660E electro chemical workstation under illumination at AM 1.5 G (100 mW cm-2) provided by a CEL-S500L Xenon light source system (CEAU LIGHT). J-V curves of all devices were measured at a scanning speed of 0.2 V s-1 by controlling the fixed areas of 0.09 and 1 cm2. During the J-V measure ment, we calibrated the light source with a standard Si solar cell. The incident photon-to-current efficiency (IPCE) was obtained by a CIMPSQE/IPCE System (Zahner, Germany). Raman spectroscope was per formed on a Renishaw Micro-Raman Spectroscopy System. We used the electron-only devices to investigate electron-transporting properties and the electron trap-state densities of the MZO and TiO2 ETLs. The electron mobilities of them were calculated according to the Mott-Gurney law.
2.1. Materials and methods Li-TFSI, PTAA (Mr, 15000–25000), tBP, super PbI2, PbBr2, CsI, FAI and MABr were purchased from Xi’an Polymer Light Technology Corp. Acetonitrile, toluene (TL), chlorobenzene (CB), DMF and DMSO were purchased from Sigma Aldrich. Ethanol, NH2(CH2)2OH, Mg (NO3)2⋅6H2O and Zn(Ac)2⋅2H2O were purchased from Aladdin. MZONCs solution was made by dissolving 1 mM Mg(NO3)2⋅6H2O and 9 mM Zn(Ac)2⋅2H2O in 25 mL ethanol with continuous stirring at 80 � C for 0.5 h, after that, 0.01 M NH2(CH2)2OH was added into the above solu tion dropwise, and then the mixture was continued to stir at 80 � C for 3 h. CsFAMA-based perovskite and PTAA solutions were made by the previous reports [1,15,25,26]. The deposition of TiO2 followed our previous works [8,27]. MZO-NCs solution was spin-coated at a speed of 3000 r.p.m. for 30 s and then annealed at 140 � C for 0.5 h. The CsFAMA-based perovskite solution was spin-coated in a two-stage pro gram at 1000 r.p.m. for 10 s and 5000 r.p.m. for 45 s, respectively. During the second stage program, 200 μL CB was immediately poured onto the spin-coating substrate 13 s prior to the end of the program. After that, the substrate was annealed at 110 � C for 90 min. Finally, the PTAA layer was prepared and then 60 nm gold was thermally evaporated.
3. Results and discussion Originally, we come up with a wet-chemical synthesis of MZO-NCs including two processes: crystallization and stabilization, as shown in Scheme S1. Fig. 1a displays the XRD and TEM characterizations of MZO film. According to the experimental procedure, the molar ratio of magnesium to zinc is 1: 9. Based on these, we use the XRD result to monitor the characteristic diffraction peaks of ZnO, whose results are consistent with the preceding literatures [18,28]. XRD of MZO-based perovskite is shown in Fig. S1, which shows tetragonal perovskite has been formed. As can be seen from the inserted TEM image, MZO-NCs are
2.2. Characterization TEM was performed using a JSM 2100 transmission electron mi croscope. SEM was performed using a Hitachi S-520 Scanning Electron Microscope & Inspection System. XRD was characterized using a D8 Discover (Bruker AXS) X-ray diffractometer with a Cu target X-ray tube voltage < 50 kV, the current < 40 mA. UV–vis absorption spectra were 2
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Fig. 2. (a) UV–vis transmission spectra of FTO, FTO/TiO2 and FTO/MZO, respectively. (b) UV–vis absorption spectra of TiO2- and MZO-based perovskite films, respectively. (c) Plot of (Ahv)2 vs hv and (d) UPS of TiO2 and MZO, respectively. (e) Possible band alignment in MZO-based PSC. (f) J1/2-V characteristics of TiO2 (L ¼ 100 nm) and MZO (L ¼ 85 nm) with a ITO/ZnO/ETL/Al structure. (g) Steady-state PL spectra and (h) TRPL spectra for the bare, TiO2- and MZO-based perovskite samples, respectively.
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Fig. 3. Surface SEM of (a) FTO, (b) MZO and (c) the corresponding perovskite film on MZO, respectively. The (d) cross-sectional SEM and (e) device structure (FTO/ MZO/perovskite/PTAA/Au) of MZO-based PSC, respectively.
well dispersed showing a narrow size distribution of 3~5 nm. Refer to the literature [18], the interplanar distance of high-resolution TEM result and the diffraction rings of SAED patterns (Fig. S2) indicate that the hexagon-shaped ZnO have been synthesized. Further, we used the STEM elemental mapping (Fig. S3) of MZO-NCs and the corresponding EDX spectroscopy (Fig. S4) to prove the as-synthesized MZO-NCs. High-resolution XPS scans of Mg 1s, Mg 2p and Zn 2p are shown in
Fig. 1b–d, respectively. The binding energies (BEs) of Mg 1s and Mg 2p located at 1303.87 eV and 50.52 eV, respectively, match well with the previous report [29] of the Mg2þ oxidation state. The BEs of Zn 2p1/2 (1044.83 eV) and Zn 2p3/2 (1021.92 eV) of MZO film suggest that the Zn2þ oxidation state remains unchanged. An overview of XPS spectra contains the core level of O 1s (Fig. S5), hence, the existence of O is proved. Besides, Raman (Fig. S6) result further indicates the formation
Fig. 4. (a) J-V and (b) IPCE curves of the best MZO- and TiO2-based PSCs; The J-V curves were measured at a scanning speed of 0.2 V s-1. (c) Steady-state J-t and stabilized PCE curves of the best MZO-based PSC (VMPP ¼ 0.854 V). (d) Statistical distribution of PCEs of the TiO2- and MZO-based PSCs (each 80 devices). 4
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of 0.75. Besides, this is an important report that low-temprature MZObased PSC can practically outperform the state-of-the-art TiO2-based PSC (PCE of 17.22%, Jsc of 22.16 mA cm-2, Voc of 1.05 V, and FF of 0.74). Based on the energy band results, the higher Voc for MZO-based PSC results from the downward shift in the ECB than TiO2-based device, which can also be demonstrated by the dark J-V curves (Fig. S14). The IPCE (Fig. 4b) results show that both the devices obtain wide platforms in the visible range. The integrated Jsc are 20.89 mA cm-2 and 22.51 mA cm-2 for the best TiO2- and MZO-based PSCs, which exactly matches the corresponding J-V data. The high Jsc of MZO-based PSC can be ascribed to the better coverage and fewer interface defects. Next, we calculated the voltage of the maximum power point (VMPP) using the corresponding J-V data and obtained the steady-state J-t curves of TiO2and MZO-based PSCs, respectively. The stabilized PCE is the product of the steady-state Jsc of device and VMPP of the corresponding device. As shown in Fig. 4c, the steady-state Jsc is 22.63 mA cm-2 and the stabilized PCE is 19.33% (VMPP ¼ 0.854 V) for MZO-based device. The steady-state Jsc and stabilized PCE curves of the best TiO2-based device with a VMPP of 0.802 V are shown in Fig. S15. To check the corresponding reproduc ibility of the TiO2- and MZO-based PSCs, we collected statistical distri bution (Fig. 4d) of PCEs of the two-type PSCs (each 80 devices). The MZO-based devices show superior PV performances than those of the TiO2-based devices. In addition, we tested the J-V curves of MZO-based device with a fixed area of 1 cm2, getting a PCE of 14.33% vs 14.64% for SnO2-based device under the same condition (Fig. S16), the corre sponding PV performances of the devices under reverse scanning are listed in Table S3. Finally, the stabilities of the two-type PSCs without encapsulation were tested under air exposure (room temperature, relative humidity: 40–80%) for 1 year and 8 h UV-irradiation (365 nm, 35 W), as shown in Fig. 5a, Fig. 5b and Fig. S17. The MZO-based device retains 76% of its initial Jsc after 1 year of air aging and 8 h UV-irradiation, versus only 12% under the same condition for TiO2-based device. Besides, we measured the XRD of the corresponding perovskite films before and after UV-irradiation, as shown in Fig. 5c and 5d. Fig. 5e shows the ratio of diffraction peak heights of PbI2 (001) at 12.74� and perovskite (110) at 14.22� , which are used to evaluate the decomposition behavior of perovskite under UV light. Further analysis, the ratio of diffraction peak heights of PbI2 (001) and perovskite (110) calculated from the XRD results vary from 0.14 to 0.18 for MZO-based perovskite film. As a contrast, the ratio of diffraction peak heights of PbI2 (001) and perov skite (110) is increased from 0.15 to 1.15 for TiO2-based perovskite film. Under UV-irradiation, the TiO2 generates O-2, which accelerates the decomposition of perovskite [34–36]. By replacing TiO2 with MZO, the light-induced O-2 are greatly suppressed. The good UV light stability of MZO-based device can be attributed to the lower trap-state density of MZO. The trap-state density was measured via the trap-filled limit voltage (VTFL) using the FTO/ETL/perovskite/PCBM/Au structure. Dark J-V characteristics of the two-type electron-only devices show the VTFL kink point behaviour, as shown in Fig. 5f. The VTFL are 0.84 V and 0.26 V for TiO2- and MZO-based electron-only devices, respectively. The VTFL is 0.37 V for ZnO-based electron-only device (Fig. S18). The relevant calculation process is described in Supporting Information. With regard to these characterization results, the trap-state mitigation of MZO ETL can be illustrated with a schematic (Fig. 5g). The schematic (left) shows the ZnO lattice structure, depicting the zinc interstitial as well as oxygen vacancy acted as trapping or recombination centers. Under UV-irradiation, oxygen vancancy and zinc interstitial leads to the instability at the ZnO/perovskite interface. As a contrast, the zinc interstitial and oxygen vacancy mediated defect sites of MZO ETL are effectively passivated in the schematic (right), which tightly protects perovskite layer from degradation under UV light. Encouragingly, we have found that the MZO-based device can effectively overcome the instability (oxygen vancancy and zinc interstitial) in ZnO-based PSC, particularly the instability at the ZnO/perovskite interface. Meanwhile, the MZO ETL has high chemical stability (Fig. S19). In a word, the
Table 1 The PV parameters of the best TiO2- and MZO-based PSCs, respectively. PSCs
scanning direction
Jsc(mA cm-2)
Voc(V)
FF
PCE(%)
MZO-based
reverse forward reverse forward
23.51 23.72 22.16 22.32
1.11 1.02 1.05 0.98
0.75 0.69 0.74 0.63
19.57 16.71 17.22 13.78
TiO2-based
of the MZO film, which is in accordance with the XPS measurements. Further characterization, as-prepared MZO film shows better trans parency than TiO2 film (Fig. 2a), which can significantly reduce the optical loss in the MZO layer. The UV–vis absorption (Fig. 2b) of MZO/ perovskite substrate shows a slight increase than the TiO2/perovskite substrate. The matched band alignment between perovskite and ETL is crucial to PSC performance, which can be affected by energy gap (Eg), work function (WS) and maximum valance band energy (EVB). Here, the Eg were obtained by the fundamental equation in Tauc relation [8,30], as shown in Fig. 2c. UPS measurement (Fig. 2d) was used to estimate the WS and EVB. The WS, valance band maximum (VBM) and Eg of MZO are 4.19 eV, 3.19 eV and 3.34 eV, respectively. The conduction band edge (ECB) of MZO is 4.04 eV (Table S2) calculated by the band structure (Fig. S7), which is much deeper than that (3.87 eV) of TiO2. The possible band alignment in MZO-based PSC is shown in Fig. 2e, which can affect the charge transfer. Besides, low mobility ETL with more traps would induce an energy barrier further affecting the performance of the cor responding device [31,32]. Based on these, the mobilities of TiO2 and MZO films (Fig. 2f) were measured using a ITO/ZnO/ETL/Al device structure, the detailed calculation procedure is shown in Supporting Information. According to the literatures [17,33], the εr of MZO and TiO2 are assumed to be 10 and 50, respectively. It was found that the mobilities of TiO2 and MZO films are 7.73 � 10-5 cm2 V-1 s and 2.20 � 10-4 cm2 V-1 s, respectively. The facilitated charges transfer and reduced energy barrier further enhance the performance of the MZO-based de vice. To investigate the electron transport and extraction properties of ETL/perovskite interface, we measured the steady-state PL spectra (Fig. 2g) and the TRPL (Fig. 2h) spectra for the three perovskite samples. The steady-state PL result suggests a better electron extraction for the MZO-based perovskite sample. The fitted average PL lifetime (τPL) are 265.71 ns, 118.10 ns and 66.04 ns for the bare, TiO2- and MZO-based perovskite samples, respectively. The faster photo-induced electron transfer from perovskite to MZO indicates that the energy barrier has been reduced. Thus, it can be anticipated that the MZO-based device delivers a prominent PCE with increased Jsc and Voc. SEM of FTO, MZO and the corresponding perovskite film on MZO are shown in Fig. 3a–c. The SEM results show that the MZO-based perovskite film with compact and flat surface morphology was prepared. As a contrast, SEM of TiO2-based perovskite film was shown in Fig. S8. We further investigated the perovskite coverage and interface defects by cross-sectional SEM of MZO-based PSC (Fig. 3d) with a device structure of FTO/MZO/perovskite/PTAA/Au (Fig. 3e). These results indicate the MZO ETL is a prospective candidate for preparing perovskite film with perfect coverage as well as fewer interface defects, which are conducive to improving the photovoltaic (PV) performances of PSCs. According to the preceding characterizations, MZO was beneficial to fabricate high-performance PSC. Extensive experiments including different scanning speeds (Fig. S9), and MZO spin-coating layers (140 � C, 0.5 h) (Fig. S10) were used to optimize the performances of the MZObased PSCs. J-V curves of the optimal TiO2-based PSC under different scanning speeds are shown in Fig. S11. J-V curves of the champion TiO2-, and MZO-based PSCs under forward/reverse scannings are shown in Fig. S12 and Fig. S13, respectively. J-V curves of the best TiO2-, and MZO-based PSCs under reverse scanning are shown in Fig. 4a and the corresponding PV performances of the two-type devices under reverse scanning are listed in Table 1. Encouragingly, the MZO-based PSC shows a striking PCE of 19.57%, a Jsc of 23.51 mA cm-2, a Voc of 1.11 V, and a FF 5
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Fig. 5. (a) J-V curves of the best MZO-based PSC vis 1 year air aging and 8 h UV-irradiation (365 nm, 35 W). (b) Normalized Jsc of the best MZOand TiO2-based PSCs under UV-irradiation after 1 year air aging. XRD of the (c) MZO- and (d) TiO2-based perovskite films before and after 8 h UV-irradiation. (e) The ratio of diffraction peak heights of PbI2 (001) and perovskite (110) for the MZO- and TiO2-based perovskite films before and after 8 h UV-irradiation. (f) Dark J-V character istics of the electron-only devices (FTO/ETL/ perovskite/PCBM/Au). (g) The schematic of trapstate mitigation of MZO.
MZO-based PSC with excellent UV light stability.
excellent UV light stability. The MZO-based device (unencapsulated) retains 76% of its initial Jsc after 1 year air aging (room temperature, relative humidity: 40%~80%) and 8 h UV-irradiation (365 nm, 35 W), versus only 12% of its initial Jsc under the same condition for TiO2-based device. The good UV light stability of MZO-based device can be due to
4. Conclusions To sum up, we report that a highly efficient MZO-based PSC with
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that the zinc interstitial and oxygen vacancy mediated defect sites of MZO are effectively passivated, which tightly protects perovskite from degradation.
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Author contribution F. Han and C. Jia conceived the idea and designed the experiments. F. Han carried out the synthesis and material characterizations. Z. Wan, J. Luo, J. Xia, H. Shu helped with the photovoltaic tests, data analysis and discussion. F. Han and C. Jia wrote the manuscript. Acknowledgments This work was supported by the National Key R@D Program of China (No. 2017YFB0702802) and National Natural Science Foundation of China (Grant Nos. 21572030 and 51773027). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110417. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] E.H. Jung, N.J. Jeon, E.Y. Park, C.S. Moon, T.J. Shin, T.-Y. Yang, J.H. Noh, J. Seo, Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene), Nature 567 (2019) 511–515. [3] T. Liu, Z. Wang, L. Lou, S. Xiao, S. Zheng, S. Yang, Interfacial Post-treatment for Enhancing the Performance of Printable Carbon-Based Perovskite Solar Cells, Solar RRL, 2019, p. 1900278. [4] H. Li, G. Wu, W. Li, Y. Zhang, Z. Liu, D. Wang, S. Liu, Additive engineering to grow micron-sized grains for stable high efficiency perovskite solar cells, Adv. Sci. 6 (2019) 1901241. [5] H.-J. Lee, J.-W. Kang, S.-H. Hong, S.-H. Song, S.-J. Park, MgxZn1-xO/Ag/MgxZn1–xO multilayers as high-performance transparent conductive electrodes, ACS Appl. Mater. Interfaces 8 (2016) 1565–1570. [6] D. Luo, W. Yang, Z. Wang, A. Sadhanala, Q. Hu, R. Su, R. Shivanna, G.F. Trindade, J.F. Watts, Z. Xu, T. Liu, K. Chen, F. Ye, P. Wu, L. Zhao, J. Wu, Y. Tu, Y. Zhang, X. Yang, W. Zhang, R.H. Friend, Q. Gong, H.J. Snaith, R. Zhu, Enhanced photovoltage for inverted planar heterojunction perovskite solar cells, Science 360 (2018) 1442–1446. [7] X. Cao, L. Zhi, Y. Jia, Y. Li, K. Zhao, X. Cui, L. Ci, D. Zhuang, J. Wei, A review of the role of solvents in formation of high-quality solution-processed perovskite films, ACS Appl. Mater. Interfaces 11 (2019) 7639–7654. [8] F. Han, G. Hao, Z. Wan, J. Luo, J. Xia, C. Jia, Bifunctional electron transporting layer/perovskite interface linker for highly efficient perovskite solar cells, Electrochim. Acta 296 (2019) 75–81. [9] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190802. pdf. [10] Z. Wang, J. Lou, X. Zheng, W.-H. Zhang, Y. Qin, Solution processed Nb2O5 electrodes for high efficient ultraviolet light stable planar perovskite solar cells, ACS Sustain. Chem. Eng. 7 (2019) 7421–7429. [11] W. Li, W. Zhang, S. Van Reenen, R.J. Sutton, A.A. Haghighirad, M.B. Johnston, H. J. Snaith, Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification, Energy Environ. Sci. 9 (2016) 490–498. [12] Z. Yuan, Y. Miao, Z. Hu, W. Xu, C. Kuang, K. Pan, P. Liu, J. Lai, B. Sun, J. Wang, S. Bai, F. Gao, Unveiling the synergistic effect of precursor stoichiometry and interfacial reactions for perovskite light-emitting diodes, Nat. Commun. 10 (2019) 2818. [13] J.J. Yoo, S. Wieghold, M.C. Sponseller, M.R. Chua, S.N. Bertram, N.T.P. Hartono, J. S. Tresback, E.C. Hansen, J.-P. Correa-Baena, V. Bulovi�c, T. Buonassisi, S.S. Shin, M.G. Bawendi, An interface stabilized perovskite solar cell with high stabilized efficiency and low voltage loss, Energy Environ. Sci. 12 (2019) 2192–2199.
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Planar MgxZn1-xO-based perovskite solar cell with superior ultraviolet light stability Fei Han, Zhongquan Wan, Junsheng Luo, Jianxing Xia, Hongyu Shu, Chunyang Jia * State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Perovskite solar cell MgxZn1-xO Electron-transporting material Ultraviolet light stability
Generally, TiO2-based perovskite solar cell (PSC) is beneficial to high efficiency but poor ultraviolet (UV) light stability. Here, we report that a highly efficient MgxZn1-xO-based (MZO-based) PSC with excellent UV light stability. MZO has a higher electron mobility and deeper conduction band than traditional TiO2, which can reduce the charge accumulation at the MZO/perovskite interface and enhance the charge transfer from perov skite to MZO. Furthermore, the reduced interface loss and energy barrier are beneficial to obtain high opencircuit voltage (Voc). By optimizing, the MZO-based device shows a high Voc of 1.11 V, yielding a promising efficiency of 19.57%. The MZO-based device (unencapsulated) retains 76% of its initial short-circuit current density (Jsc) after 1 year air aging (room temperature, relative humidity: 40–80%) and 8 h UV irradiation (365 nm, 35 W), versus only 12% under the same condition for TiO2-based device. The good UV light stability of MZObased device can be attributed to the reduced electron trap-state density in MZO electron-transporting layer (ETL). Specifically, zinc interstitial and oxygen vacancy mediated defect sites of MZO ETL are effectively passivated, which tightly protects the perovskite layer from degradation under UV light. Our results show a great potential for MZO ETL application in UV-stable PSC.
1. Introduction Perovskite solar cells (PSCs) have got a lot of attention due to the recent surges in device performances via process control and optimiza tion [1–8]. The recording power conversion efficiency (PCE) is as high as 25.2% (certified) [9], which shows great potential for its industrializa tion. Despite the high efficiency has been achieved in PSC, the in stabilities of the perovskite material and the corresponding device need to be addressed for further application. It is well-known that the ma jority of TiO2-based PSCs show excellent PCEs exceeding 20%. Unfor tunately, exploying TiO2 as electron-transporting layer (ETL) material in planar n-i-p structure PSC will lead to a rapid decay of short-circuit current density (Jsc) under ultraviolet (UV) irradiation [10]. Many ef forts including employing an interface layer at the TiO2/perovskite interface [11–14], and further doping of TiO2 [15–18], have been made to protect perovskite from UV light-induced degradation. However, the above methods inevitably damage its performance. Hence, it is extremely imperative and valuable to fabricate stable PSC without damaging its performance. Based on these, exploring new ETLs stable to UV light has attracted
considerable research interests, which is conducive to preparing highly effcient and highly stable PSCs [10,19–22]. Among them, PSCs using SnO2 [21] and Nb2O5 [10,22] as ETLs have shown excellent stabilities under light illumination. Besides, MgxZn1-xO (MZO) has been proved to be an effective ETL material in the solar cells [23,24], but fewer re searches were focused on UV light stability. Hence, we report an oversimplified route for low-temperature, so lution-processed and stable MZO nanocrystals (NCs), and further utilize them to fabricate MZO-based PSC. To the best of our knowledge (sum marized in Table S1), the bare MZO-based device shows an amazing PCE of 19.57% comparing with that of SnO2-based n-i-p PSC by You’s paper [21]. Encouragingly, MZO with tunable properties can form matched band alignment between perovskite and ETL, improving the open-circuit voltage (Voc) of the device. Furthermore, the MZO-based device shows superior UV light stability.
* Corresponding author. E-mail address: [email protected] (C. Jia). https://doi.org/10.1016/j.solmat.2020.110417 Received 1 October 2019; Received in revised form 12 January 2020; Accepted 15 January 2020 Available online 24 January 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (a) XRD and inserted TEM image, high-resolution XPS scans of MZO film (b) Mg 1s scan, (c) Mg 2p scan and (d) Zn 2p scan, respectively. Pink asterisk in XRD represents the diffraction peaks of FTO. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2. Experimental section
determined with PERKIN ELMER Lambda 950 Spectrometer. UPS and XPS were performed with Kratos X-ray Photoelectron Spectrometer (Axis Ultra DLD). Steady-state PL spectraspectra were recorded by the FLS980 Fluorescence Spectrometer (Edinburgh Instruments Spectros copy System). Time-resolved PL (TRPL) were measured on a Fluorolog-3 Spectrofluorometer (Horiba JobinYvon) with a DeltaDiode (406 nm, DD-405L) as the excitation source and a picosecond photon detection module (PPD-850) as the detector (λem ¼ 750 nm). Current-voltage (J-V) performance of the devices were recorded on a CHI660E electro chemical workstation under illumination at AM 1.5 G (100 mW cm-2) provided by a CEL-S500L Xenon light source system (CEAU LIGHT). J-V curves of all devices were measured at a scanning speed of 0.2 V s-1 by controlling the fixed areas of 0.09 and 1 cm2. During the J-V measure ment, we calibrated the light source with a standard Si solar cell. The incident photon-to-current efficiency (IPCE) was obtained by a CIMPSQE/IPCE System (Zahner, Germany). Raman spectroscope was per formed on a Renishaw Micro-Raman Spectroscopy System. We used the electron-only devices to investigate electron-transporting properties and the electron trap-state densities of the MZO and TiO2 ETLs. The electron mobilities of them were calculated according to the Mott-Gurney law.
2.1. Materials and methods Li-TFSI, PTAA (Mr, 15000–25000), tBP, super PbI2, PbBr2, CsI, FAI and MABr were purchased from Xi’an Polymer Light Technology Corp. Acetonitrile, toluene (TL), chlorobenzene (CB), DMF and DMSO were purchased from Sigma Aldrich. Ethanol, NH2(CH2)2OH, Mg (NO3)2⋅6H2O and Zn(Ac)2⋅2H2O were purchased from Aladdin. MZONCs solution was made by dissolving 1 mM Mg(NO3)2⋅6H2O and 9 mM Zn(Ac)2⋅2H2O in 25 mL ethanol with continuous stirring at 80 � C for 0.5 h, after that, 0.01 M NH2(CH2)2OH was added into the above solu tion dropwise, and then the mixture was continued to stir at 80 � C for 3 h. CsFAMA-based perovskite and PTAA solutions were made by the previous reports [1,15,25,26]. The deposition of TiO2 followed our previous works [8,27]. MZO-NCs solution was spin-coated at a speed of 3000 r.p.m. for 30 s and then annealed at 140 � C for 0.5 h. The CsFAMA-based perovskite solution was spin-coated in a two-stage pro gram at 1000 r.p.m. for 10 s and 5000 r.p.m. for 45 s, respectively. During the second stage program, 200 μL CB was immediately poured onto the spin-coating substrate 13 s prior to the end of the program. After that, the substrate was annealed at 110 � C for 90 min. Finally, the PTAA layer was prepared and then 60 nm gold was thermally evaporated.
3. Results and discussion Originally, we come up with a wet-chemical synthesis of MZO-NCs including two processes: crystallization and stabilization, as shown in Scheme S1. Fig. 1a displays the XRD and TEM characterizations of MZO film. According to the experimental procedure, the molar ratio of magnesium to zinc is 1: 9. Based on these, we use the XRD result to monitor the characteristic diffraction peaks of ZnO, whose results are consistent with the preceding literatures [18,28]. XRD of MZO-based perovskite is shown in Fig. S1, which shows tetragonal perovskite has been formed. As can be seen from the inserted TEM image, MZO-NCs are
2.2. Characterization TEM was performed using a JSM 2100 transmission electron mi croscope. SEM was performed using a Hitachi S-520 Scanning Electron Microscope & Inspection System. XRD was characterized using a D8 Discover (Bruker AXS) X-ray diffractometer with a Cu target X-ray tube voltage < 50 kV, the current < 40 mA. UV–vis absorption spectra were 2
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Fig. 2. (a) UV–vis transmission spectra of FTO, FTO/TiO2 and FTO/MZO, respectively. (b) UV–vis absorption spectra of TiO2- and MZO-based perovskite films, respectively. (c) Plot of (Ahv)2 vs hv and (d) UPS of TiO2 and MZO, respectively. (e) Possible band alignment in MZO-based PSC. (f) J1/2-V characteristics of TiO2 (L ¼ 100 nm) and MZO (L ¼ 85 nm) with a ITO/ZnO/ETL/Al structure. (g) Steady-state PL spectra and (h) TRPL spectra for the bare, TiO2- and MZO-based perovskite samples, respectively.
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Fig. 3. Surface SEM of (a) FTO, (b) MZO and (c) the corresponding perovskite film on MZO, respectively. The (d) cross-sectional SEM and (e) device structure (FTO/ MZO/perovskite/PTAA/Au) of MZO-based PSC, respectively.
well dispersed showing a narrow size distribution of 3~5 nm. Refer to the literature [18], the interplanar distance of high-resolution TEM result and the diffraction rings of SAED patterns (Fig. S2) indicate that the hexagon-shaped ZnO have been synthesized. Further, we used the STEM elemental mapping (Fig. S3) of MZO-NCs and the corresponding EDX spectroscopy (Fig. S4) to prove the as-synthesized MZO-NCs. High-resolution XPS scans of Mg 1s, Mg 2p and Zn 2p are shown in
Fig. 1b–d, respectively. The binding energies (BEs) of Mg 1s and Mg 2p located at 1303.87 eV and 50.52 eV, respectively, match well with the previous report [29] of the Mg2þ oxidation state. The BEs of Zn 2p1/2 (1044.83 eV) and Zn 2p3/2 (1021.92 eV) of MZO film suggest that the Zn2þ oxidation state remains unchanged. An overview of XPS spectra contains the core level of O 1s (Fig. S5), hence, the existence of O is proved. Besides, Raman (Fig. S6) result further indicates the formation
Fig. 4. (a) J-V and (b) IPCE curves of the best MZO- and TiO2-based PSCs; The J-V curves were measured at a scanning speed of 0.2 V s-1. (c) Steady-state J-t and stabilized PCE curves of the best MZO-based PSC (VMPP ¼ 0.854 V). (d) Statistical distribution of PCEs of the TiO2- and MZO-based PSCs (each 80 devices). 4
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of 0.75. Besides, this is an important report that low-temprature MZObased PSC can practically outperform the state-of-the-art TiO2-based PSC (PCE of 17.22%, Jsc of 22.16 mA cm-2, Voc of 1.05 V, and FF of 0.74). Based on the energy band results, the higher Voc for MZO-based PSC results from the downward shift in the ECB than TiO2-based device, which can also be demonstrated by the dark J-V curves (Fig. S14). The IPCE (Fig. 4b) results show that both the devices obtain wide platforms in the visible range. The integrated Jsc are 20.89 mA cm-2 and 22.51 mA cm-2 for the best TiO2- and MZO-based PSCs, which exactly matches the corresponding J-V data. The high Jsc of MZO-based PSC can be ascribed to the better coverage and fewer interface defects. Next, we calculated the voltage of the maximum power point (VMPP) using the corresponding J-V data and obtained the steady-state J-t curves of TiO2and MZO-based PSCs, respectively. The stabilized PCE is the product of the steady-state Jsc of device and VMPP of the corresponding device. As shown in Fig. 4c, the steady-state Jsc is 22.63 mA cm-2 and the stabilized PCE is 19.33% (VMPP ¼ 0.854 V) for MZO-based device. The steady-state Jsc and stabilized PCE curves of the best TiO2-based device with a VMPP of 0.802 V are shown in Fig. S15. To check the corresponding reproduc ibility of the TiO2- and MZO-based PSCs, we collected statistical distri bution (Fig. 4d) of PCEs of the two-type PSCs (each 80 devices). The MZO-based devices show superior PV performances than those of the TiO2-based devices. In addition, we tested the J-V curves of MZO-based device with a fixed area of 1 cm2, getting a PCE of 14.33% vs 14.64% for SnO2-based device under the same condition (Fig. S16), the corre sponding PV performances of the devices under reverse scanning are listed in Table S3. Finally, the stabilities of the two-type PSCs without encapsulation were tested under air exposure (room temperature, relative humidity: 40–80%) for 1 year and 8 h UV-irradiation (365 nm, 35 W), as shown in Fig. 5a, Fig. 5b and Fig. S17. The MZO-based device retains 76% of its initial Jsc after 1 year of air aging and 8 h UV-irradiation, versus only 12% under the same condition for TiO2-based device. Besides, we measured the XRD of the corresponding perovskite films before and after UV-irradiation, as shown in Fig. 5c and 5d. Fig. 5e shows the ratio of diffraction peak heights of PbI2 (001) at 12.74� and perovskite (110) at 14.22� , which are used to evaluate the decomposition behavior of perovskite under UV light. Further analysis, the ratio of diffraction peak heights of PbI2 (001) and perovskite (110) calculated from the XRD results vary from 0.14 to 0.18 for MZO-based perovskite film. As a contrast, the ratio of diffraction peak heights of PbI2 (001) and perov skite (110) is increased from 0.15 to 1.15 for TiO2-based perovskite film. Under UV-irradiation, the TiO2 generates O-2, which accelerates the decomposition of perovskite [34–36]. By replacing TiO2 with MZO, the light-induced O-2 are greatly suppressed. The good UV light stability of MZO-based device can be attributed to the lower trap-state density of MZO. The trap-state density was measured via the trap-filled limit voltage (VTFL) using the FTO/ETL/perovskite/PCBM/Au structure. Dark J-V characteristics of the two-type electron-only devices show the VTFL kink point behaviour, as shown in Fig. 5f. The VTFL are 0.84 V and 0.26 V for TiO2- and MZO-based electron-only devices, respectively. The VTFL is 0.37 V for ZnO-based electron-only device (Fig. S18). The relevant calculation process is described in Supporting Information. With regard to these characterization results, the trap-state mitigation of MZO ETL can be illustrated with a schematic (Fig. 5g). The schematic (left) shows the ZnO lattice structure, depicting the zinc interstitial as well as oxygen vacancy acted as trapping or recombination centers. Under UV-irradiation, oxygen vancancy and zinc interstitial leads to the instability at the ZnO/perovskite interface. As a contrast, the zinc interstitial and oxygen vacancy mediated defect sites of MZO ETL are effectively passivated in the schematic (right), which tightly protects perovskite layer from degradation under UV light. Encouragingly, we have found that the MZO-based device can effectively overcome the instability (oxygen vancancy and zinc interstitial) in ZnO-based PSC, particularly the instability at the ZnO/perovskite interface. Meanwhile, the MZO ETL has high chemical stability (Fig. S19). In a word, the
Table 1 The PV parameters of the best TiO2- and MZO-based PSCs, respectively. PSCs
scanning direction
Jsc(mA cm-2)
Voc(V)
FF
PCE(%)
MZO-based
reverse forward reverse forward
23.51 23.72 22.16 22.32
1.11 1.02 1.05 0.98
0.75 0.69 0.74 0.63
19.57 16.71 17.22 13.78
TiO2-based
of the MZO film, which is in accordance with the XPS measurements. Further characterization, as-prepared MZO film shows better trans parency than TiO2 film (Fig. 2a), which can significantly reduce the optical loss in the MZO layer. The UV–vis absorption (Fig. 2b) of MZO/ perovskite substrate shows a slight increase than the TiO2/perovskite substrate. The matched band alignment between perovskite and ETL is crucial to PSC performance, which can be affected by energy gap (Eg), work function (WS) and maximum valance band energy (EVB). Here, the Eg were obtained by the fundamental equation in Tauc relation [8,30], as shown in Fig. 2c. UPS measurement (Fig. 2d) was used to estimate the WS and EVB. The WS, valance band maximum (VBM) and Eg of MZO are 4.19 eV, 3.19 eV and 3.34 eV, respectively. The conduction band edge (ECB) of MZO is 4.04 eV (Table S2) calculated by the band structure (Fig. S7), which is much deeper than that (3.87 eV) of TiO2. The possible band alignment in MZO-based PSC is shown in Fig. 2e, which can affect the charge transfer. Besides, low mobility ETL with more traps would induce an energy barrier further affecting the performance of the cor responding device [31,32]. Based on these, the mobilities of TiO2 and MZO films (Fig. 2f) were measured using a ITO/ZnO/ETL/Al device structure, the detailed calculation procedure is shown in Supporting Information. According to the literatures [17,33], the εr of MZO and TiO2 are assumed to be 10 and 50, respectively. It was found that the mobilities of TiO2 and MZO films are 7.73 � 10-5 cm2 V-1 s and 2.20 � 10-4 cm2 V-1 s, respectively. The facilitated charges transfer and reduced energy barrier further enhance the performance of the MZO-based de vice. To investigate the electron transport and extraction properties of ETL/perovskite interface, we measured the steady-state PL spectra (Fig. 2g) and the TRPL (Fig. 2h) spectra for the three perovskite samples. The steady-state PL result suggests a better electron extraction for the MZO-based perovskite sample. The fitted average PL lifetime (τPL) are 265.71 ns, 118.10 ns and 66.04 ns for the bare, TiO2- and MZO-based perovskite samples, respectively. The faster photo-induced electron transfer from perovskite to MZO indicates that the energy barrier has been reduced. Thus, it can be anticipated that the MZO-based device delivers a prominent PCE with increased Jsc and Voc. SEM of FTO, MZO and the corresponding perovskite film on MZO are shown in Fig. 3a–c. The SEM results show that the MZO-based perovskite film with compact and flat surface morphology was prepared. As a contrast, SEM of TiO2-based perovskite film was shown in Fig. S8. We further investigated the perovskite coverage and interface defects by cross-sectional SEM of MZO-based PSC (Fig. 3d) with a device structure of FTO/MZO/perovskite/PTAA/Au (Fig. 3e). These results indicate the MZO ETL is a prospective candidate for preparing perovskite film with perfect coverage as well as fewer interface defects, which are conducive to improving the photovoltaic (PV) performances of PSCs. According to the preceding characterizations, MZO was beneficial to fabricate high-performance PSC. Extensive experiments including different scanning speeds (Fig. S9), and MZO spin-coating layers (140 � C, 0.5 h) (Fig. S10) were used to optimize the performances of the MZObased PSCs. J-V curves of the optimal TiO2-based PSC under different scanning speeds are shown in Fig. S11. J-V curves of the champion TiO2-, and MZO-based PSCs under forward/reverse scannings are shown in Fig. S12 and Fig. S13, respectively. J-V curves of the best TiO2-, and MZO-based PSCs under reverse scanning are shown in Fig. 4a and the corresponding PV performances of the two-type devices under reverse scanning are listed in Table 1. Encouragingly, the MZO-based PSC shows a striking PCE of 19.57%, a Jsc of 23.51 mA cm-2, a Voc of 1.11 V, and a FF 5
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Fig. 5. (a) J-V curves of the best MZO-based PSC vis 1 year air aging and 8 h UV-irradiation (365 nm, 35 W). (b) Normalized Jsc of the best MZOand TiO2-based PSCs under UV-irradiation after 1 year air aging. XRD of the (c) MZO- and (d) TiO2-based perovskite films before and after 8 h UV-irradiation. (e) The ratio of diffraction peak heights of PbI2 (001) and perovskite (110) for the MZO- and TiO2-based perovskite films before and after 8 h UV-irradiation. (f) Dark J-V character istics of the electron-only devices (FTO/ETL/ perovskite/PCBM/Au). (g) The schematic of trapstate mitigation of MZO.
MZO-based PSC with excellent UV light stability.
excellent UV light stability. The MZO-based device (unencapsulated) retains 76% of its initial Jsc after 1 year air aging (room temperature, relative humidity: 40%~80%) and 8 h UV-irradiation (365 nm, 35 W), versus only 12% of its initial Jsc under the same condition for TiO2-based device. The good UV light stability of MZO-based device can be due to
4. Conclusions To sum up, we report that a highly efficient MZO-based PSC with
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that the zinc interstitial and oxygen vacancy mediated defect sites of MZO are effectively passivated, which tightly protects perovskite from degradation.
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Author contribution F. Han and C. Jia conceived the idea and designed the experiments. F. Han carried out the synthesis and material characterizations. Z. Wan, J. Luo, J. Xia, H. Shu helped with the photovoltaic tests, data analysis and discussion. F. Han and C. Jia wrote the manuscript. Acknowledgments This work was supported by the National Key R@D Program of China (No. 2017YFB0702802) and National Natural Science Foundation of China (Grant Nos. 21572030 and 51773027). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110417. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] E.H. Jung, N.J. Jeon, E.Y. Park, C.S. Moon, T.J. Shin, T.-Y. Yang, J.H. Noh, J. Seo, Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene), Nature 567 (2019) 511–515. [3] T. Liu, Z. Wang, L. Lou, S. Xiao, S. Zheng, S. Yang, Interfacial Post-treatment for Enhancing the Performance of Printable Carbon-Based Perovskite Solar Cells, Solar RRL, 2019, p. 1900278. [4] H. Li, G. Wu, W. Li, Y. Zhang, Z. Liu, D. Wang, S. Liu, Additive engineering to grow micron-sized grains for stable high efficiency perovskite solar cells, Adv. Sci. 6 (2019) 1901241. [5] H.-J. Lee, J.-W. Kang, S.-H. Hong, S.-H. Song, S.-J. Park, MgxZn1-xO/Ag/MgxZn1–xO multilayers as high-performance transparent conductive electrodes, ACS Appl. Mater. Interfaces 8 (2016) 1565–1570. [6] D. Luo, W. Yang, Z. Wang, A. Sadhanala, Q. Hu, R. Su, R. Shivanna, G.F. Trindade, J.F. Watts, Z. Xu, T. Liu, K. Chen, F. Ye, P. Wu, L. Zhao, J. Wu, Y. Tu, Y. Zhang, X. Yang, W. Zhang, R.H. Friend, Q. Gong, H.J. Snaith, R. Zhu, Enhanced photovoltage for inverted planar heterojunction perovskite solar cells, Science 360 (2018) 1442–1446. [7] X. Cao, L. Zhi, Y. Jia, Y. Li, K. Zhao, X. Cui, L. Ci, D. Zhuang, J. Wei, A review of the role of solvents in formation of high-quality solution-processed perovskite films, ACS Appl. Mater. Interfaces 11 (2019) 7639–7654. [8] F. Han, G. Hao, Z. Wan, J. Luo, J. Xia, C. Jia, Bifunctional electron transporting layer/perovskite interface linker for highly efficient perovskite solar cells, Electrochim. Acta 296 (2019) 75–81. [9] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190802. pdf. [10] Z. Wang, J. Lou, X. Zheng, W.-H. Zhang, Y. Qin, Solution processed Nb2O5 electrodes for high efficient ultraviolet light stable planar perovskite solar cells, ACS Sustain. Chem. Eng. 7 (2019) 7421–7429. [11] W. Li, W. Zhang, S. Van Reenen, R.J. Sutton, A.A. Haghighirad, M.B. Johnston, H. J. Snaith, Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification, Energy Environ. Sci. 9 (2016) 490–498. [12] Z. Yuan, Y. Miao, Z. Hu, W. Xu, C. Kuang, K. Pan, P. Liu, J. Lai, B. Sun, J. Wang, S. Bai, F. Gao, Unveiling the synergistic effect of precursor stoichiometry and interfacial reactions for perovskite light-emitting diodes, Nat. Commun. 10 (2019) 2818. [13] J.J. Yoo, S. Wieghold, M.C. Sponseller, M.R. Chua, S.N. Bertram, N.T.P. Hartono, J. S. Tresback, E.C. Hansen, J.-P. Correa-Baena, V. Bulovi�c, T. Buonassisi, S.S. Shin, M.G. Bawendi, An interface stabilized perovskite solar cell with high stabilized efficiency and low voltage loss, Energy Environ. Sci. 12 (2019) 2192–2199.
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