Fabrication of Perovskite Films with Long Carrier Lifetime for Efficient

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Fabrication of Perovskite Films with Long Carrier Lifetime for Efficient Perovskite Solar Cells from Low Toxic 1-Ethyl-2-Pyrrolidone Lili Zhi, Yanqing Li, Xiaobing Cao, Yahui Li, Xian Cui, Daming Zhuang, Lijie Ci, and Jinquan Wei ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01327 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Energy Materials

Fabrication of Perovskite Films with Long Carrier Lifetime for Efficient Perovskite Solar Cells from Low Toxic 1-Ethyl-2-Pyrrolidone Lili Zhi 1, 2, Yanqing Li 2,3, Xiaobing Cao 4, Yahui Li 4, Xian Cui 4, Daming Zhuang4, Lijie Ci 1*, Jinquan Wei 4* 1. SDU & Rice Joint Center for Carbon Nanomaterials, Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, China 2. Department of Physics, Changji College, Changji 831100, Xinjiang, China 3. State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China 4. State Key Lab of New Ceramic and Fine Processing, Education Ministry Key Laboratory for Advanced Materials Processing Technology, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

*Corresponding authors: [email protected] (L. C.); [email protected] (J. W.)

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Abstract High quality perovskite films with large grains and long carrier lifetime are crucial for efficient solar cells. It is a simple and effective way to increase the perovskite grain size by adding some additives in the precursor solution. Here, we use a low toxic and low volatile organic compound of 1-ethyl-2-pyrrolidone (NEP) as an additive in the PbI2(PbBr2)/Dimethylformamide precursor solution to form a Lewis adduct of PbI2·NEP, which facilitate forming perovskite intermediate embedded with NEP molecules. The residual NEP molecules are kept in the narrow space between the intermediate film and substrate during annealing by using a closed-steam annealing method, which prolong the dissolution-recrystallization process. With assistance of the residual NEP, high quality perovskite films with smooth surface, PbI2 free, high crystallinity, large columnar grains, and long carrier lifetime are fabricated. Correspondingly, the power conversion efficiency of the best perovskite solar cells is enhanced significantly from 10.8% to 17.0 %.

Keywords: Perovskite solar cell; Lewis adduct; additive; 1-ethyl-2-pyrrolidone (NEP); dissolution-recrystallization


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1. Introduction Hybrid organic-inorganic halide perovskite solar cells (PSCs) have promising applications in photovoltaics due to their rapid growth of power conversion efficiency (PCE) and low processing cost.[1-6] The quality of perovskite films are crucial for high performance solar cells. Perovskite films with large columnar grains, low trap density, and long carrier lifetime are beneficial to the overall photovoltaic performance.[7-9] Several approaches have been proposed to fabricate smooth perovskite films with large grains and good crystallinity, such as substrate pre-heating,[10] blade coating,[11] solution-induced Ostwald ripening,[12] solvent annealing,[13, 14] additive engineering,[1518]

and so on. Among these methods, the additive engineering, which can control the growing

procedure of perovskite, is a simple and effective method to improve the quality of the perovskite films. For example, Liang et al.[15] modified the crystallization rate by employing 1,8-diiodooctane (DIO) to coordinate with Pb2+ and inducing the formation of a more thermodynamically stable intermediate. Zou et al.[16] added NH4Cl additive as a chloride source to produce perovskite films with smooth morphology. Heo et al.[17] used CH3NH3PbI3/DMF solution adding with HI additive to fabricate solar cells with high PCE. He found that HI not only prevents the decomposition of CH3NH3PbI3, but also improves the solubility of PbI2, which is beneficial to form a dense perovskite film with a long diffusion length and low trap density. Han et al.[18] also reported an additive engineering to realize high quality perovskite films with large grains (>2 μm) and long carrier lifetime (~930 ns). It is noted that the carrier lifetime of the perovskite can be as long as microsecond scale in a mono-crystal,[19, 20] but it reduces significantly to only ~10 to ~100 ns for the perovskite films containing nanocrystals due to high trap density and grain boundaries.[21, 22] Although there are some reports on the positive roles of grain boundaries in the perovskite films,[2326]

it is still a trend to prepare perovskite films with large columnar grains, which can prolong the

carrier lifetime and improve the performance of solar cells.[27]


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At the same time, many researchers focused on the roles of aprotic polar solvents, e.g. dimethylformamide (DMF), dimethyl sulphoxide (DMSO), N-methyl pyrrolidone (NMP), on improving the quality of the perovskite films. Yang et al. [1] fabricated high efficiency perovskite solar cells (>20%) by adding some DMSO in the precursor solution through an intramolecular exchange between formamidinium iodide (FAI) and the DMSO molecules, where DMSO intercalated into PbI2 to form an intermediate phase of PbI2·DMSO. Lee et al.[28] pointed out that the aprotic polar solvents, containing oxygen, sulfur, or nitrogen atoms that are able to donate a pair of electrons, were Lewis bases, which can form Lewis acid-base adducts of PbI2·Sol through dative Pb-O, Pb-S, or Pb-N bonds. High quality perovskite films were thus easily fabricated through the Lewis adducts. We also fabricated smooth perovskite films from the Lewis adducts by an improved two-step method.[29-30] It is noted that most of the solvents, used for additives, are toxic and might be harmful for health and environments. Furthermore, it was reported that the residual solvent embedded in precursor films have great impact on morphology of the perovskite films after annealing.[31-34] Very recently, we used a closed-steam annealing (CSA) method to prepare perovskite films with large columnar grains and improve the efficiency of the solar cells.[35] The NMP steams were kept at the narrow space between the perovskite films and substrate during annealing, which play an active role on inducing dissolution-recrystallization of the perovskite grains. 1-ethyl-2-pyrrolidone (NEP) is also an appropriate Lewis base because it contains an oxygen atom that is capable to donate a pair of electrons, and has a lower saturated vapor pressure than that of DMF (see Figure S1). More importantly, NEP is a low toxic organic solvent in comparison with those commonly used in fabricating perovskite films, which is safe for both humanity and environments. It provides more environmental benefits when compares to the toxic DMF, DMSO,


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NMP and pyridine in the future scalable production.[36] Here, we investigate the roles of NEP solvent on improving the quality of perovskite films by adding into the PbI2(PbBr2)/DMF precursor solution.

2. Experiment Fabrication of devices A compact and a mesoporous TiO2 layer were deposited on cleaned FTO glass substrate (TEC7, Pilkington) by spin coating in sequence.[30] Perovskite films were fabricated according to a modified sequential deposition technology.[4] In brief, 1.3 M PbI2/DMF solution added with 5 % PbBr2 and different volume concentration of NEP was deposited onto the TiO2/FTO substrate by spin-coating at 3000 rpm for 30 s to form a wet PbI2/PbBr2 precursor film. Then, 350 μL NH2CHNH2I (FAI) (80 mg/mL) and CH3NH3Br (MABr) (10 mg/mL) mixed 2-propanol (IPA) solution was spin-coated onto the wet PbI2/PbBr2 precursor film drop by drop at 5500 rpm for 30 s to form a perovskite precursor film. The obtained perovskite precursor films were separately annealed at 100°C for 30 min by two different annealing methods: conventional-direct annealing (CDA) and closed-steam annealing (CSA).[35] For the CDA method, the FTO/TiO2/perovskite precursor film was placed on a hot plate with the glass side down, where the residual solvent evaporates quickly from the precursor film. For the CSA method, the precursor film was placed on a SiO2/Si wafer mounted on a hot plate with the glass side up. The residual solvent steams were confined in the narrow space between the precursor film and SiO2/Si wafer for some times, which enhance the dissolution-recrystallization effect through Oswald ripening during annealing. After cooling down, a spiro-OMeTAD film was deposited from72.3 mg spiro-OMeTAD dissolved in 1 mL chlorobenzene by spin-coating using as a hole transport layer (HTM).[30] Finally, a 60 nmthick gold film was deposited onto the HTM as a back electrode. The active area of each cell was defined as 6 mm2, controlled by a patterned shadow mask.


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Characterization The PbI2-based Lewis adducts were characterized by the thermal gravimetric analysis (TGA/Q5000IR) and Fourier transform infrared (FTIR) spectroscopy (VERTEX 70V). The perovskite films were characterized by scanning electron microscopy (SEM, MERLIN VP Compact) and X-ray diffraction (XRD, Smartlab, Rigaku), respectively. The UV/Vis absorbance was measured by a spectrophotometer (Cary 5000, Agilent Technologies). Steady-state and timeresolved photoluminescence (PL) decay spectra were recorded by a transient optical spectrometer (FLS920). The steady PL spectra were measured by exciting with a monochromatic xenon lamp source (central wavelength λ exc= 460 nm). The time-resolved PL spectra were obtained by exciting with a laser beam at 405 nm (power: 20 μW, pulse width: 52.9 ps, pulse period: 50 ns, beam size: 1.2 mm) in a time-correlated single photon counting (TCSPC) system. Light currentvoltage (J-V) curves were recorded by a Keithley 4200-SCS at a scan rate of 10 mV/s under illustration with a solar simulator (AM 1.5G, 100 mW/ cm2, 91195, Newport). Incident photon-tocurrent efficiency (IPCE) was performed with a solar cell quantum efficiency measurement system (QEX10; PV Measurements).

3. Results and Discussion Figure 1a and b are Fourier transform infrared (FTIR) transmittance spectra of two kinds of solvents (DMF and NEP) and their corresponding Lewis acid-base adducts of PbI2‧Sol (Sol=DMF, NEP). The peaks locating at 1670 and 1650 cm−1 are referred to the stretching vibration of C=O bond for the DMF and NEP solvents, respectively. Both of the NEP and DMF are polar aprotic solvents containing C=O bonds (see Figure S1). After forming Lewis adducts of PbI2·Sol, the C=O peaks downshift to 1658 and 1633 cm-1, respectively. It shows that there are interaction between the solvents and PbI2, forming dative Pb-O bonds in the PbI2·Sol.[37-39] This interaction between Pb2+ and solvent molecule is beneficial for promoting the uniform growth of the


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perovskite layer.[40] Figure 1c shows TGA curves of PbI2 powder and PbI2·Sol. The PbI2·DMF begins to decompose at ~140 °C, while the PbI2·NEP begins at ~210 °C. It implies that the PbI2·NEP has higher thermal stability than PbI2·DMF due to stronger interaction between NEP and PbI2. The Lewis adducts of PbI2∙NEP and PbI2∙DMF are further characterized by XRD (see Figure 1d). There are two characteristic diffraction peaks at 8.25°and 9.02°in the XRD curve of PbI2·NEP, which is similar to that of PbI2∙DMF (9.06°and 9.62°). It indicates that both of DMF and NEP can intercalate into the PbI2 lattice, resulting in lattice expansion. Since the molecular size of NEP is larger than DMF (see Figure S1), the PbI2·NEP has larger lattice distance than the PbI2∙DMF. At the same time, the ionic radius of FA+ (0.253 nm), MA+ (0.217 nm), Br– (0.195 nm) and I– (0.216 nm) are smaller than those of the DMF (0.427 nm) and NEP (0.467 nm).[41] Therefore, the ions of FA+, MA+, Br– and I– can insert in the PbI2·Sol lattice easily. After spinning with the mixed solution of FAI/MABr/IPA, the PbI2∙Sol convert to perovskite intermediates of FAMAPbI3Sol xBrx·

quickly.[28,42] The intermediates then transform to perovskite by annealing.

The residual solvents embedded in the perovskite intermediate films have active roles in dissolution-recrystallization process of the perovskite grains during annealing, which essentially enhances the Ostwald ripening effect.[34, 42] During the annealing process, the perovskite precursor films release the residual solvents and then convert to perovskite. The small perovskite grains are dissolved by the solvent steam, and then recrystallize to large grains, leading to grain coarsening through Ostwald ripening. Figure 2 shows SEM images of the perovskite films prepared from the PbI2(PbBr2)/DMF solutions added with different volume concentration of NEP and annealed by the CDA method. As expected, the surface of perovskite films become uniform and smooth by adding some NEP in the solution. The average grain size of the perovskite films prepared from the PbI2(PbBr2)/DMF solutions adding with 5, 10, and 20% NEP are 247, 304 and 323 nm (see Figure


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S2), which are larger than that without adding with NEP (205 nm). It indicates that grain size is increased by introducing NEP. In order to extend the recrystallization process, we use CSA method to anneal the perovskite intermediate films by putting its face down on a flat SiO2/Si wafer, which can keep the solvent steam in the narrow space between the intermediate film and SiO2/Si longer. The average grain size reaches to 412 nm for the film prepared from CSA method without adding NEP (Figure 3a and S3a), which is about twice larger than the sample prepared from the CDA method (205 nm, Figure S2a). It shows the great effect of annealing method on the quality of the perovskite films. This is due to that residual DMF solvents extend the recrystallization process by the CSA method. The low toxic NEP has lower saturated vapor pressure than DMF, which implies that it has stronger effect in Ostwald ripening process. Figure 3b-d show SEM images of the perovskite films prepared from CSA method by adding different concentration of NEP in the PbI2(PbBr2)/DMF solutions. The average grain sizes are 462, 718, and 1027 nm for the samples prepared by adding 5%, 10%, and 20% NEP (see Figure 3b-d and S3). It shows great effects of the NEP on fabricating high quality perovskite films with large grains by the CSA method. However, there are some holes in the perovskite films at high concentration of NEP due to excessive Ostwald ripen (Figure 3d). In comparison with the films prepared from the CDA method, the perovskite films prepared from the CSA method exhibit smooth surface with large columnar grains across total film in the thickness direction (see Figure S4), resulting from the sufficient Ostwald ripen. The perovskite films consisting of columnar grains are beneficial for high efficient PSCs due to low trap density.[43] Figure 4a and b show XRD curves of the perovskite films prepared from different annealing methods and by adding with different amount of NEP in the PbI2(PbBr2)/DMF solution. There are several characteristic peaks centered at 14.2º, 28.4º, and 32.0º, corresponding to (110), (220) and


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(310) planes of the cubic perovskite.[44] There are also some residual PbI2 in the sample prepared from the CDA method. The XRD peak of the PbI2 (12.7°) is getting stronger as the increase of NEP amount (see Figure 4a). However, the XRD peak of the residual PbI2 almost disappears in the perovskite films prepared from the CSA method (Figure 4b). It implies that the residual PbI2 result from evaporation of MAI with the residual solvent in dissolution-recrystallization process in open circumstance. Here, we use an intensity ratio of I110/I310 to evaluate the grain orientation, which is calculated by the intensity of (110) peak divided by the intensity of (310) peak.[32] The I110/I310 is 2.57 for the film fabricated with 10% NEP additive via the CSA method, while it is only 1.90 for the film fabricated without NEP additives via the CDA method. The increase of I110/I310 indicates that the perovskite films have a preferred orientation of (110) plane in the dissolutionrecrystallization process, which is conducive to charge effective transmission.[32] Figure 4c illustrates steady-state photoluminescence (PL) spectra of the perovskite films deposited on the glass substrates. The PL intensity of the perovskite films prepared from CSA method was enhanced notably. It indicates that the quality of the perovskite film is getting better, resulting in large columnar grains and low trap density (Figure 3 and Figure S4). It is clear from Figure S4 that the grain boundaries decrease significantly when adding some NEP in the precursor solution and using the closed-steam annealing method. Although there are some reports on the positive roles on the optoelectronic properties,[23-26] the carrier lifetime in the perovskite films with large grains is much longer than that with small grains.[26] It implies that the recombination of charge carrier in the perovskite film with large columnar grains is suppressed significantly. Therefore, the PL emission in the perovskite film prepared from NEP and CSA increases evidently due to low non-radiative recombination. It is noted that there is a slight red shift (∼9 nm) in PL spectra for the perovskite films prepared from CSA method compared with the CDA method (see


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Figure S5). The slight shift may be caused by a slight inner filter effect in perovskite films, where the emission is partially re-absorbed by perovskite.[45] Figure 4d shows time-resolved PL spectra of the perovskite films, which are well fitted by a biexponential decay model (Table S1). It shows that the carrier lifetime of the perovskite films adding with 10% NEP is longer than those without NEP. For the films fabricated from the CAS method by adding with 10% NEP, the average carrier lifetime (τaverage) reaches 561.19 ns, 3.6 times that of the films fabricated from the CDA method without adding NEP additive, which result from large grain size in the perovskite films. Importantly, the τaverage is in several hundred ns scale, which are much longer than those in previous reports.[33-34] It suggests that the perovskite films have good crystallinity and low defect density. It was reported that both of the PL emission and the carrier lifetime was increased significantly by Lewis base passivation, where the perovskite films were covered with a very thin layer of thiophene and pyridine.[46] Here, we believe that the enhanced PL emission and slow time resolved PL dynamics derive mainly from the high quality perovskite film, rather than the Lewis base passivation. The residual solvent embedded in the perovskite precursor film are removed completely after long time annealing at 100 °C according to XRD curves in Figure 4. The perovskite films consisted of large columnar grains which fully across the films can suppress nonradiative recombination significantly. Therefore, the perovskite films fabricated from solution adding with NEP exhibit long carrier lifetime. Figure 5a shows UV-vis absorbance spectra of four different perovskite films. The sample fabricated from 10% NEP solution via the CSA method has the highest absorbance intensity in the visible region (inset of Figure 5a), showing the best quality of the perovskite film. The high absorbance intensity is conductive to high emission, which is consistent with the result of the PL spectra (see Figure 4c ).


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We also fabricate PSCs from different perovskite films. Figure 5b is a cross-sectional SEM image of a completed PSC prepared by adding 10% NEP and using CSA method. It demonstrates an architecture of the PSC with a uniform perovskite film containing large columnar grains. Figure 5c shows the reverse scanning J-V curves of the best PSCs adding with and without NEP additives fabricated by the CDA and CSA methods, respectively. The photovoltaic parameters of the corresponding solar cells are summarized in Table 1. By adding with 10% NEP, the efficiency increases from 10.85% to 12.91% for the solar cells fabricated by the CDA method, which mainly derives from the enhancement of current density (from 18.55 to 21.23 mA/cm2). The efficiencies are further improved by using the CSA method, which are 17.07% and 14.47% for the solar cells fabricated from the PbI2(PbBr2)/DMF solution with and without 10% NEP. The enhancement of the photovoltaic performance is contributed to high quality perovskite films with large columnar gains. The statistical results of the photovoltaic parameters by counting 20 cells in Figure S6 shows the similar tendency of the best PSC. It shows that high quality perovskite films as well as high performance PSCs are fabricated by adding some low toxic NEP in PbI2(PbBr2)/DMF solution, and by using CSA method. A steady-state current output and efficiency of the PSCs at the maximum power point are shown in Figure 5d. The solar cell fabricated via the CSA method adding with 10% NEP exhibits a rapid increase of current density (J) to 18.50 mA/cm2 and a high stabilized PCE of around 16.00%. In contrast, the solar cell fabricated from the CDA method without adding NEP has relative low J of 13.65 mA/cm2 and a PCE of only 9.51%. All the cells have evident hysteresis in J-V curves (Figure S7). But, the J-V curve hysteresis reduces evidently by adding NEP and using the CSA method. The solar cells fabricated from the CSA method adding with 10% NEP also show higher external quantum yield ability than those from the CDA method without adding NEP (Figure 5e). It is


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worth noting that the integrated Jsc from IPCE is lower than the value obtained from the J-V reverse scan, which might be attributed to some defect states in the electron transport layer of TiO2 and interfaces, or mismatch of light spectra.[47] In addition, the J-V curves measured at different scan rates almost overlap with each other (see Figure 5f). The corresponding photovoltaic parameters hardly change (see Table S2), which further confirms the reliability and reproducibility of the performance of solar cells.

4. Conclusions In summary, we introduce a low toxic NEP additive in the PbI2(PbBr2)/DMF precursor solution to fabricate high quality perovskite films with large columnar grains for efficient PSCs through a closed-steam annealing method. It forms a Lewis adduct of PbI2·NEP, when NEP is added in the precursor solution. The lattice of PbI2 are expanded by solvent molecules, which facilitate ionic transportation of FA+, MA+, Br– and I– in the Lewis adducts and formation of perovskite intermediates. The residual NEP in the intermediates strengthen the Ostwald ripening effect during the closed-steam annealing treatment. As a result, high quality perovskite films with smooth surface and large columnar grains, high crystallinity and long carrier lifetime, are fabricated. The photovoltaic performance of the perovskite solar cells are thus enhanced significantly by adding some NEP in the precursor solution and using closed-steam annealing method. ASSOCIATED CONTENT Supporting Information The Supporting information are available free of charge on the ACS publication website. Molecular structures of DMF and NEP; Distribution diagram of grain size for perovskite films from different annealing method; Cross-sectional SEM images of the perovskite films; Normalized PL spectra of perovskite films from different conditions; Box chart of the photovoltaic parameters


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of PSCs; Forward and reverse scan J-V curves of the PSCs; Charge carrier lifetime of the four different perovskite films; Photovoltaic parameters of PSCs under different scan rates. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; *E-mail: [email protected]. Phone: +86-10-62781065. Notes The authors declare no competing financial interest. Acknowledgments This work was mainly supported by the Startup Funding of Distinguished Professorship of “Thousand Talents Program” (31370086963030), Shenzhen Jiawei Photovoltaic Lighting Co. Ltd., Tsinghua University Initiative Scientific Research Program (20161080165). This work was also supported by the Natural Science Foundation of Xinjiang Uygur Autonomous Region (No. 2016D01C008), Opening Project of State Key laboratory of Crystal Material (No. KF1610), and the Scientific Research Program of the Higher Education Institution of Xinjiang (XJEDU2017M038).


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Figure and captions

Figure 1. Fourier transform infrared transmittance spectra of DMF, NEP and their corresponding PbI2 Lewis acid-base adducts. (a) DMF and PbI2∙DMF, (b) NEP and PbI2∙NEP. (c) Thermal gravimetric analysis of the Lewis adducts. (d) XRD spectra of the Lewis adducts.


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Figure 2. SEM images of the perovskite films prepared by the conventional-direct annealing (CDA) method from the PbI2/PbBr2/DMF solution adding with different amount of NEP. (a) without NEP, (b) 5 vol.%, (c) 10 vol.%, (d) 20 vol.%.


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Figure 3. SEM images of the perovskite films prepared by the closed-steam annealing (CSA) method from the PbI2/PbBr2/DMF solution adding with different amount of NEP additives. (a) without NEP, (b) 5 vol.%, (c) 10 vol.%, (d) 20 vol.%.


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Figure 4. XRD curves of the perovskite films prepared from PbI2/PbBr2/DMF solution adding with different amount of NEP. (a) Annealed by the CDA method, (b) Annealed by the CSA method. (c) Steady-state photoluminescence spectra. (d) Time-resolved photoluminescence decays spectra fabricated from different conditions.


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Figure 5. Characterization of perovskite films and their corresponding photovoltaic properties. (a) UV-vis absorption spectra. (b) Cross-sectional SEM image of a complete solar cell. (c) JV curves of the best PSCs fabricated from different conditions. (d) Steady-state current output and efficiency at the maximum power point for the best PSCs. (e) IPCE spectra of the best PSCs fabricated from the PbI2/PbBr2/DMF solution adding with 10% and without NEP. (f) JV curves of the best PSC measured at different scan rates.


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Table Table 1. Photovoltaic parameters of the best PSCs fabricated from four different conditions.

Samples

Jsc (mA/cm2)

Voc (V)

FF

PCE

W/O NEP, CDA

18.55

0.88

0.66

10.85

10% NEP, CDA

21.23

0.95

0.64

12.91

W/O NEP, CSA

21.32

0.97

0.70

14.47

10% NEP, CSA

23.02

0.98

0.76

17.07


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Table of Contents


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Fabrication of Perovskite Films with Long Carrier Lifetime for Efficient

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