Journal of Power Sources 440 (2019) 227157
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Achieving efficient flexible perovskite solar cells with room-temperature processed tungsten oxide electron transport layer Fengyou Wang a, b, d, Yuhong Zhang a, b, Meifang Yang a, b, Jinyue Du a, b, Lili Yang a, b, d, *, Lin Fan a, b, Yingrui Sui a, b, Xiaoyan Liu a, b, Jinghai Yang a, b, c, ** a
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun, 130103, China National Demonstration Center for Experimental Physics Education, Jilin Normal University, Siping, 136000, China Si Ping Hong Zui University Science Park, Siping, 136000, China d Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Changchun, 130103, China b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Room-temperature and large-area aWOx ETL has been developed for flex ible PSCs. � The trade-off between charge recombi nation and transfer is relaxed. � Flexible PSCs based on a-WOx exhibits an excellent mechanical bending stability. � Large area a-WOx ETL achieve high ef ficiency for flexible perovskite solar cells.
A R T I C L E I N F O
A B S T R A C T
Keywords: Low temperature Amorphous tungsten oxides Electron transport layer Flexible solar cells Interface recombination
For flexible perovskite solar cells, achieving high power conversion efficiency by using a room-temperature technology to fabricate a compact electron transport layer is one of the best options. Here, we develop an annealing-free, dopant-free, and amorphous tungsten oxide as electron transport layer by vacuum evaporation for flexible perovskite solar cells. The compact amorphous tungsten oxide electron transport layer with different thicknesses (0–50 nm) was directly deposited on flexible PEN/ITO substrate. A model of the improvement mechanism is proposed to understand how the thickness tailoring simultaneously enhances the crystallization and relaxes the trade-off between interface recombination and charge transfer. By optimizing the amorphous tungsten oxide thickness, the high homogeneous, uniform, and dense electron transport layer with a thickness of 30 nm is found to not only decrease the pinhole of the perovskite layer, but also enhance charge transport with reducing resistance. Furthermore, the mechanical bending stability revealed that, the fabricated perovskite solar cells show stable power conversion efficiency up to more than 1000 bending cycles. The room-temperature processed fabrication enables the amorphous tungsten oxide to become a potential electron transport layer candidate for the large-scale flexible perovskite solar cells, which becomes compatible with practical roll-to-roll solar cells manufacturing.
* Corresponding author. Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Changchun, 130103, China. ** Corresponding author. Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun, 130103, China. E-mail addresses:
[email protected] (L. Yang),
[email protected] (J. Yang). https://doi.org/10.1016/j.jpowsour.2019.227157 Received 12 June 2019; Received in revised form 23 August 2019; Accepted 13 September 2019 Available online 18 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 440 (2019) 227157
highest PCE of 15.65%. However, the solution process limits its appli cation for the large-scale flexible PSCs [36]. In this work, we developed a room-temperature processed amor phous WOx (a-WOx) as the large-area ETL of flexible PSCs. This vacuum evaporated technique facilitates the large-area and compact deposition of a-WOx and provides thickness control at room temperature, which blocks the unwanted back reaction for PSCs. Moreover, the solvent-free manufacture route has the advantage of environmentally friendly and much more convenient. Our results indicate that by varying the a-WOx thickness, perovskite films with significantly increased grain size, declined grain boundaries and decreased pinhole areas are achieved, thus improving overall device performance. The fabrication of flexible PSCs using room-temperature ETL enables a new approach towards efficient flexible PSCs, making the field closer to large-scale industrial manufacturing.
1. Introduction Flexible and lightweight thin film solar cells have drawn extensive interest for their promising development in many aspects, including wearable electronics textiles, portable chargers, remote power supplies, flexible display devices, flying objects, etc. [1,2], due to their outstanding superiorities in lightweight, continuous roll-to-roll pro cessability, easy to process, easy to storage, handling, transport, and installation, etc. [3,4] Outstanding success has been made in the appli cation of inorganic-organic hybrid perovskite material to solar cells, due to its excellent intrinsic performance, such as high charge carrier mobility, long carrier diffusion length, quite less trap density, low exciton-binding energy, etc. [5–13] More importantly, perovskite ma terial has the advantages of low-temperature solution-processed route, high optical absorption coefficient and high power conversion efficiency (PCE), which gives it a great potential to explore perovskite solar cells (PSCs) for lightweight, flexible, and wearable applications [14–18]. To create greater value for excellent characteristics of the inorganic-organic hybrid perovskite materials, it is urgent to take action to manufacture flexible PSCs and the roll-to-roll devices. Flexible PSCs, especially those with conventional architecture (n-ip), ordinarily apply polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) as substrates, instead of rigidity substrates. These flexible plastic substrates can only withstand temperatures below 150 � C [19]. Thus, to make prepare high-performance flexible PSCs, beyond the general considerations of carrier mobility and conductivity, energy levels position, and stability under fabrication process, there are other considerate factors for material selection for different layers of the de vices, such as operating temperatures and bending stability [19]. Therefore, the electron transport layer (ETL) should be cautiously cho sen owing to their strict fabrication procedure. Reports show that due to environmentally friendly, superior carrier mobility and low-cost, the metal oxides can act as a potential ETL application material for high-performance flexible PSCs. The scheme for synthesizing nano structured metal oxide has been adopted for fabricating ETL for high-performance perovskite solar cells. For example, Guo et al. have successfully used Low-temperature solution deposited niobium oxide (Nb2O5) films as efficient ETL for planar perovskite solar cell. The results indicate low-temperature solution deposited Nb2O5 film can be a low cost and easy-prepared ETL material for high performance PSC [20]. SnO2 as ETLs have also used for PSCs. The SnO2 ETL has appealing properties such as excellent charge mobility, low-temperature prepara tion method, and favorable energy level position to enhance the PSCs efficiency [21]. In 2015, Tingli Ma et al. first used SnO2 as the ETL to fabricate a PSCs, which achieved a PCE of 6.4% [22]. From then on, to high performance PSCs, using low-temperature solution-processed SnO2 as ETL was reported by several groups [23–27]. Fang et al. reported that yttrium-doped SnO2 ETL synthesized by an in-situ hydrothermal growth process can significantly improve the photovoltaic performance of PSCs [28]. Liu et al. developed an effective ETL using low-temperature solu tion processed Nb-doped SnO2 to enhance the electron extraction and effectively suppress charge recombination, leading to improved solar cell performance [29]. Besides, several examples of low-temperature ZnO ETL, TiO2 ETL, etc., by solution process have already been re ported, but most of them face with complex synthesis processes or long-term stability challenges, which limits their further application for flexible PSCs [30–33]. Thus, exploring a new process or new functional materials is urgently desired for low-temperature ETL. Very recently, the non-stoichiometric tungsten oxide (WOx) has attracted much attention due to its high electron mobility and conductivity, strong light trans mittance, and suitable bandgap alignment with perovskite absorber, which gives it an enormous potential to substitute the aforementioned materials [34]. Ma and co-workers proposed the usage of solution pro cessed for employing low-temperature WOx ETL into flexible PSCs. They found that the WOx precursor solution etched the ITO layer [35]. Wang et al. spin-coated Nb: WOx as ETL for flexible PSCs, and obtained the
2. Experimental section Fabrication of the a-WOx ETL. The a-WOx films were fabricated on the ITO glass substrates by vacuum thermal evaporation method. Before operating ETL layers, ITO glass substrates were cleaned with deionized water, acetone, and alcohol for 20 min each in series. The cleaned ITO glass substrates were loaded into a custom holder for thermal evapora tion. Thermal evaporation was performed using VZZ-400 thermal evaporation equipment, with a tungsten boat placed 10 cm below and parallel to the sample holder platform. 0.5 g WOx powder (Aladdin, 99.9%) was weighed into an empty tungsten boat prior to evaporation. The WOx powder was evaporated at a rate of 0.5 Å/s onto ITO glass substrates using a pressure of 5 � 10 4 Pa pressure during the deposi tion, and power of 280 W (4 V, 70 A) is employed. Deposition thickness of 15, 30, and 50 nm was dominated, respectively. The thickness of the films was monitored by the in-situ thickness detection system of the equipment. The photograph and schematic diagram of the evaporation system was exhibited in Fig. S1. More details about the PSCs fabrication procedure, materials and characterization are demonstrated in Support information as Supple mentary Note 1 and 2. 3. Results and discussion X-ray photoelectron spectroscopy (XPS) was utilized to study the chemical compositions of a-WOx. The W 4f spectrum can be deconvo luted into four peaks (Fig. 1a). The peaks situated at 34.68 and 36.83 eV are assigned to the W6þ oxidation state, while the ones situated at 35.38 and 37.63 eV are assigned to the W5þ oxidation state [34]. At room-temperature conditions, the XPS O1s peak contains three con stituents (Fig. 1b). The first component situated at 530.33 eV, belonged – O bonds of WOx. The second O 1s peak situated at 531.35 eV to the W– – corresponds to O atoms that participate in the oxide stoichiometry, possibly in the participation of hydroxyl groups attached to the tungsten atoms. The third O 1s peak at 532.78 eV belongs to water in the a-WOx internal composition or probably adsorbed on the a-WOx film surface. Further, the detailed composition analysis of the O and W shows an atomic ratio of 14.3:46.0, which confirms the nonstoichiometric nature of the a-WOx. We performed a transmission electron microscopy (TEM) to study the a-WOx crystallization. Fig. 1c shows that no lattice fringes and diffraction rings can be found, which confirms that the as-prepared a-WOx sample exhibits the amorphous nature. X-ray diffraction (XRD) pattern of a-WOx sample (50-nm-thick) on glass was recorded in 2θ from 10� to 80� . The broad and featureless peak at 24.38� is attributed to the glass substrate (Fig. 1d). The paucity of characteristic peaks shown in the XRD also verifies the amorphous nature of the as-prepared a-WOx. Excellent optical and electrical characteristics are essential factors for high-performance ETL to decide the photovoltaic performance of the devices. The optical transmission of a-WOx with different thicknesses is shown in Fig. 2a. Fig. S2 shows the corresponding statistical histogram 2
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of average transmission with different thicknesses a-WOx films. The thickness of the a-WOx was monitored by an in-situ thickness detection system in the thermal evaporation equipment. As the thickness of a-WOx increases from 15 to 50 nm, the transmission of the a-WOx/ITO/PEN is slightly declined. The inset of Fig. 2a shows the photograph of the a-WOx prepared by thermal evaporation on 100 cm2 flexible ITO/PEN sub strate. Compared with common spin-coated films, the large-area evap orated films without any visible chromatic aberration or cracks. This is preferred for large-scale PSCs application. Except for the optical prop erties, the electrical characteristic also plays a vital role in photovoltaic devices because excellent electrical characteristic is beneficial for extraction and transportation of photo-generated carrier [37]. In this study, electrical current-voltage (I–V) testing is utilized to investigate the electrical characteristic of the a-WOx films with various thicknesses. Fig. 2b indicates that there is only a small decrease in the current density for sample 30-nm-thick a-WOx compared with that of 15-nm-thick a-WOx; thus, 30-nm-thick a-WOx does not influence the conductivity substantially. As film thickness increases further, the negative effect of thickness increased as is shown by the continued decline in electrical conductivity for 50-nm-thick a-WOx. The valence band maximum (VBM), conductive band minimum (CBM), work function (WF), and the bandgap of a-WOx were measured by ultraviolet photoelectron spec troscopy (UPS) and UV–vis absorption (Fig. 2c–e), respectively. The work function, bandgap, and the distance between VBM and Femi level are 5.4 eV (21.22–16.32 eV), 3.25 eV, and 2.35 eV, respectively. Accordingly, the level alignment of the a-WOx and MAPbI3 layer can be illustrated as the inset of Fig. 2e. The formation of cascade energy level sequence between a-WOx and MAPbI3 could achieve better electron transport and holes-blocking effect. Atomic force microscope (AFM) was employed to investigate the surface morphology of the a-WOx films. The AFM patterns of a-WOx film prepared with different thicknesses on the ITO substrate are shown in Fig. 3a-d. The quantitative characterization and average root-meansquare (RMS) roughness of the different thicknesses a-WOx films
surface were investigated. Fig. S3 shows the corresponding RMS roughness distribution of the different thicknesses a-WOx films. The RMS roughness of bare ITO substrate is 9.42 nm. As a-WOx film thick ness increases, the surface becomes denser and smoother, which is manifested by the reduced RMS roughness. The 50-nm-thick a-WOx has the smallest RMS roughness with 3.65 nm as valleys of ITO surface are preferably overfilled by a-WOx film. Generally, the smoother ETL sur face would lead to a denser perovskite film without appreciable pinholes to decrease the leakage current of the solar cell. The top-view scanning electron microscope (SEM) images were employed to investigate the influences of various a-WOx thicknesses on the surface morphology of perovskite films. In Fig. 3e-h, perovskite layers were grown on 0-nmthick or 15-nm-thick a-WOx film are non-uniform with pinholes. Fig. S4 shows the corresponding grain size distribution of the perovskite layers deposited on various a-WOx substrates. With increasing a-WOx thickness from 30 to 50 nm, the perovskite grain size increases largely, and negligible pinholes could be found. Meanwhile, XRD patterns were employed to characterize the influences of a-WOx film thickness on the perovskite crystallinity (Fig. 3i). Three XRD peaks located at 14.08� , 28.26� and 31.68� can be seen for all samples, which are corresponding to the (110), (220) and (310) of MAPbI3, respectively [38]. The in tensities of these peaks enhance with a-WOx films thickness increasing, which indicates the perovskite crystallinity was improved. We also employed the intensity ratios of I110/I310 and I220/I310 to assess the grains orientation of the films, which were estimated by the value of the intensity of (110) and (220) peaks divided by the intensity of (310) peak, respectively. Fig. S5 and Table S1 shows intensity ratios of I110/I310 and I220/I310 of the perovskite layers deposited on various thicknesses of a-WOx substrate. As the thickness of a-WOx increases from 15 to 50 nm, the intensity ratios of I110/I310 and I220/I310 increased. The increased intensity ration of I110/I310 and I220/I310 shows that the perovskite grains have a preferred orientation of (110), which is beneficial for charge transport [39]. Generally, a smooth and clear surface is beneficial to perovskite films growing with large grain size because it could
Fig. 1. XPS spectra of (a) W 4f and (b) O 1s peaks for room-temperature deposited a-WOx. (C) TEM images and SAED images of a-WOx film. (d) XRD spectra of roomtemperature deposited a-WOx samples. 3
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Fig. 2. (a) Optical transmission spectra of a-WOx samples with various thicknesses of: 0 nm, 15 nm, 30 nm, and 50 nm, respectively. Inset: The digital picture of the a-WOx films deposited on 100 cm2 PEN/ITO substrate. (b) I–V characteristics of a-WOx with different thicknesses. Inset: Depiction of mea surement architecture. (c) The secondary-electron cut-off region, and (d) valence band of UPS spectra of a-WOx film. (e) Optical energy bandgap of a-WOx film. Inset: Schematic of the energy band diagram of the MAPbI3 and a-WOx.
uniform nucleation reduces the inter-extrusion between each grain, which is conducive to better crystallization. The mechanism of growing large grain on the smooth surface is illustrated in Fig. 3j, which shows the nucleation and grain growth process [40,41]. In order to obtain the favorable thickness of a-WOx as an ETL for PSCs, systematic characterizations of the a-WOx-based devices were performed. Fig. 4a exhibits the architecture of the flexible PSCs. A room temperature processed a-WOx ETL was deposited onto the ITO/PEN substrate. Then, a perovskite film, MAPbI3, was fabricated on the ETL by employing the anti-solvent engineering. A hole transport layer, SpiroOMeTAD, was deposited on the perovskite, and the Ag contact was thermal evaporated on Spiro-OMeTAD layer. As shown in Fig. 4b, the side-view SEM image of the PSCs clearly shows the layers: a 30-nm-thick a-WOx ETL, a 550-nm-thick perovskite, a 280-nm-thick Spiro-OMeTAD, and a 200-nm-thick Ag contact, respectively. Fig. 4c–d presents the statistical output parameters of the 30 flexible PSCs with each thickness (0–50 nm) of a-WOx ETLs. It is found that the thickness of the a-WOx layer has a crucial influence on device performance. The open-circuit voltage (Voc) of the solar cells increases with the thickness. Notably, the short-circuit current density (Jsc) and fill factor (FF) show a trend of increasing (0–30 nm) and then decreasing (50 nm). Correspondingly, the PCE is improved as increasing the thickness of a-WOx from 15 nm to 30 nm. The PSCs utilizing a 30-nm-thick a-WOx demonstrates the highest PCE of 15.85%, a Jsc of 22.15 mA cm 2, a FF of 75.4%, and a Voc of 0.95 V. However, further increase the thickness declines the perfor mance of the device. According to the working mechanism of the solar cells, we infer that this trend changes may be originated from the bal ance between charge transfer resistance and the interface recombina tion. The thicker a-WOx layer could effectively prohibit the hole backdiffusion at heterointerface thereby suppress the interface recombina tion. The Voc of the PSCs is directly determined by the velocity of interface recombination (Sit) as given by [42,43]:
Fig. 3. (a–d) AFM images of a-WOx films with thicknesses of: (a) 0 nm, (b) 15 nm, (c) 30 nm, and (d) 50 nm, respectively. All the a-WOx films are deposited on PEN/ITO substrate. (e–h) Top-view SEM images of the perovskite layers deposited on a-WOx substrate with thicknesses of: (e) 0 nm, (f) 15 nm, (g) 30 nm, and (h) 50 nm, respectively. The dash circles highlight the pinholes in perovskite films. (i) XRD pattern of perovskite layers based on different thick nesses of a-WOx films. (j) Schematic diagram of perovskite growth on different substrate.
prevent the formation of too dense nuclei for heterogeneous nucleation. Compared with the bare ITO, depositing thicker a-WOx film could fulfill the valleys and make it smoother. Thus, during the films growth, the
Voc ¼
4
φB q
nkT qNv Sit lnð Þ q Jsc
(1)
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Fig. 4. (a) Device architecture of the PSCs. (b) Cross-section SEM of PSCs with structure of PEN/ITO/a-WOx/MAPbI3/Spiro-OMeTAD/Ag. The inset shows the photograph of flexible PSCs. (c–f) Statistical output parameters of the flexible PSCs with various thickness of the a-WOx films.
between interfacial recombination and charge transfer in the devices, electrical impedance spectroscopy (EIS) was tested (Fig. 5c). The lowfrequency reign arc is corresponding to the recombination resistance (Rrec) and high-frequency reign arc the transport resistance (Rtr) [44]. As the thickness of the a-WOx layer increasing (0 → 30 nm), the Rrec elevated due to the minimized shunt path between the ITO and the perovskite absorber layer while no obvious change of Rtr can be found. However, PSCs based on 50-nm-thick a-WOx layer exhibited much larger Rtr than the ones based on 30-nm-thick a-WOx layer. This indicates the inferior photo-generated charge transportation, instead of the interface recombination, maybe the main reason for the performance deteriora tion of the PSCs with 50 nm a-WOx ETL. Accordingly, the correlation between interface recombination and charge transfer at a-WOx/MAPbI3 can be systematically elucidated: The PSCs fabricated without a-WOx layer can be equivalent to a construction possesses enormous pinholes, which causes photo-generated carrier recombination at the hetero-interface (➀ in Fig. 6). The PSCs with 15 or 30-nm-thick WOx ETLs can reduce the shunt path between the absorber layer and the ITO substrate, therefore minimizing the interface recom bination. However, the PSCs based on a 15-nm-thick a-WOx showed a Voc and FF smaller compared with that gained in the PSCs based on a 30nm-thick a-WOx ETL, which demonstrated that a 15-nm-thick a-WOx might not suppress the recombination process sufficiently (➁ in Fig. 6). The PSCs applying a 30-nm-thick a-WOx ETL possesses the highest ef ficiency, which can be ascribed to the enhanced perovskite crystalliza tion and the suppressed interface recombination (➂ in Fig. 6). As the thickness of the a-WOx film is further increased, the charge transfer resistance is increased, although the thickness uniformity of a-WOx film
where the q is the elementary charge, φB is the effective barrier, Nv is the effective density of states, and kT is the thermal energy. It can be concluded that a low interface recombination (low Sit) would improve the Voc of the solar cells. However, further increasing the thickness also increases the resistance of the device, which hinders the charge extraction to external circuit thus deteriorates the FF and the Jsc of the PSCs. To clarify the difference of photovoltaic performances, open-circuit voltage decay (OCVD) was utilized to characterize the recombination dynamics in PSCs. In this work, an illumination pulse supplied by solar irradiation light with 1 sun intensity was used to create photo-generated carrier density embodied by Voc of the PSCs. The short light soaking time precedes to OCVD examination prevents the possible dedication of electrostatics to the photovoltage decay of devices. The photovoltage decay curve as a function of time upon turning off the illumination was demonstrated in Fig. 5a. As the a-WOx thickness increasing, the OCVD of the PSCs declines much slower than the control cells, which indicates that the cells using thick a-WOx have much longer carrier lifetime and lower interface recombination rate than the cells without a-WOx ETL. The recombination behavior is also investigated by dark J-V measure ment. Fig. 5b shows that the PSCs with thicker a-WOx ETL possess smaller leakage current. Therefore, all these results indicate the sup pressing of charge recombination may be the dominant reason for the performance improvement when the thickness of a-WOx increases from 0 to 30 nm. However, it is worth noting that the lower recombination does not correspondingly improve the photovoltaic performance for the PSCs with 50-nm-thick a-WOx ETL. To further investigate the correlative
Fig. 5. (a) The OCVD curves, (b) dark J-V characteristics of the PSCs based on different thicknesses a-WOx. (c) Nyquist plots of PSCs based on different thicknesses aWOx at frequency ranging from 0.1 MHz to 0.1 Hz in dark. 5
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Fig. 6. Schematic illustration of the charge behavior at a-WOx/MAPbI3 hetero-interface for various thickness of a-WOx ETLs.
and crystalline quality of perovskite layer were excellent (➃ in Fig. 6). These results also suggested that in the appropriate thickness a-WOx ETL, along with full coverage of the rough ITO substrate, can accelerate electron transport and suppress interface recombination to improve the efficiency of PSCs. The a-WOx layer with a thickness of 30 nm is more suitable for solar cell application because it relaxes the trade-off be tween interface recombination and carrier transportation. The best performances of the PSCs with 30-nm-thick a-WOx ETL in reverse and forward scan direction are displayed in Fig. 7a. To quanti tatively evaluate the hysteresis effect of the flexible PSCs, a hysteresisfactor (H-factor) can be defined as H-factor ¼ (Jrs(0.8Voc) - Jfs(0.8Voc)/Jrs (0.8Voc)), where the Jfs(0.8Voc) and Jrs(0.8Voc) represent the photocurrent
density at 80% of the Voc for the forward scan and reverse scan, respectively [35]. We found the calculated H-factor of the PSCs based on 30-nm a-WOx ETL was 0.014, which indicates a very weak hysteretic effect. In addition, to our best knowledge, this is a record PCE for flexible solar cells based on WOx ETL, and a relative high PCE for all kinds of flexible MAPbI3 solar cells by now (Fig. S6 and Table S2). The EQE curve of the flexible PSCs with 30-nm-thick a-WOx has been shown in Fig. 7b. In addition, to test flexing tolerance of the flexible PSCs, a bending ex amination was employed considering the various bending curvature radii (r) and after 1000 flexing cycles. We evaluated the effects of me chanical durability on device performance at various bending curvature radii after 1000 flexing cycles (Fig. 7c). The flexible cells at r ¼ 14 mm Fig. 7. (a) J-V curves of the champion cell measured at forward (from 0.2–1.2 V) and reverse (from 1.2 to 0.2 V) scans, respectively. (b) EQE curve along with the champion cell. (c) The PCEs of flexible de vice based on MAPbI3 at different bending curvature radii after 1000 flexing cycles. (d) The stability measurements of PSCs without any encapsulation exposed at ambient environment of 45% humidity. (e) Corresponding steady-state photocurrent and power output at maximum power point.
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after 1000 bending cycles demonstrated no apparent decline in PCE, which is illustrated to be secure in the ITO bending examination. Ac cording to the present literature, when the curvature radius is greater than 14 mm, ITO will not appear any cracks [45]. But, under a sudden curvature radius at r ¼ 4 mm, device efficiency is dropped owing to the conductivity loss in the brittle ITO layer, consistent well with the ex pectations given in other groups [46,47]. Compared to previous reports, the PCEs of our PSCs exhibit lower degradation. To verify the long-term stability of flexible PSCs, the PSCs without encapsulation were placed at room-temperature with ~45% humidity in atmospheric conditions (Fig. 7d). It shows the normalized PCE of flexible PSCs maintains 80% of its beginning performance after 30 days, which represents relative high stability of the non-encapsulation device. Fig. 7e indicates the steady-state photocurrent density and steady-state efficiencies plots as a function of time when the solar cells were maintained at maximum power output point. It is obvious that the PCE of 30-nm-thick a-WOx ETL rapidly stabilizes at 15.32% with steady-state photocurrent density of 20.88 mA cm 2, corresponding well with the J-V result.
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4. Conclusions A facile and room-temperature processed a-WOx is developed as ETL for efficient flexible PSCs, except for additional treatments and dopant. The thickness of the a-WOx has an important influence in dominating the surface morphology of the subsequently fabricated perovskite layer and device performance. We consider that the thickness of the a-WOx should be sophistically modulated to enhance the perovskite crystallization and relax the trade-off between interface recombination and charge trans portation. The PCE of the champion the solar cell with 30-nm-thick aWOx is raised to 15.85%. Moreover, the flexible PSCs showed high bending stability at different bending curvature radii. We believe this work not only explores a room temperature ETL for PSCs application, but also highlights the large-area a-WOx may be one of the potential ETL for further roll-to-roll solar cells manufacturing. Acknowledgements The authors gratefully acknowledge the support from National Na ture Science Foundation of China (Grant Nos. 11904127, 61904066, 61605059, 61775081, and 61705079), Program for the development of Science and Technology of Jilin province (Item No. 20180520182JH, 20180519016JH), and the Thirteenth Five-Year Program for Science and Technology of Education Department of Jilin Province (Item No. JJKH20180759KJ, JJKH20190998KJ), Talent Development Fund Proj ect in Jilin Province, Special Project of Industrial Technology Research and Development in Jilin Province (2019C042-2), and Construction Program for Innovation Research Team of Jilin Normal University (Grant No.201703). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227157. References [1] F.D. Giacomo, A. Fakharuddin, R. J, T.M. Brown, Energy Environ. Sci. 9 (2016) 3007. [2] M. Park, H.J. Kim, I. Jeong, J. Lee, H. Lee, H.J. Son, D.-E. Kim, M.J. Ko, Adv. Energy Mater. 5 (2015), 1501406. [3] F. Wang, M. Yang, S. Ji, L. Yang, J. Zhao, H. Liu, Y. Sui, Y. Sun, J. Yang, X. Zhang, J. Power Sources 395 (2018) 85–91. [4] X. Hu, Z. Huang, X. Zhou, P. Li, Y. Wang, Z. Huang, M. Su, W. Ren, F. Li, M. Li, Y. Chen, Y. Song, Adv. Mater. 29 (2017), 1703236. [5] Y. Ming, M. Xu, S. Liu, D. Li, Q. Wang, X. Hou, Y. Hu, Y. Rong, H. Han, J. Power Sources 424 (2019) 261.
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