Uniform Permutation of Quasi-2D Perovskites by Vacuum Poling for Efficient, High-Fill-Factor Solar Cells

•The uniform dispersion of different-n-value nanoplates is realized by vacuum poling•Tape-peeling-based optical studies confirm the uniform dispersion•Uniform dispersion enables highly efficient isotropic carrier collection•Uniform dispersion leads to high efficiency (18.04%) and champion FF (82.4%) 2D nanoplates are normally vertically arranged from small to large n values in quasi-2D perovskite films, leading to ordered dispersion of different-n-value nanoplates to demonstrate efficient solar cells based on directional charge extraction. Here, we found a better choice that uniformly arranging different n-value nanoplates can be realized by using vacuum poling method to enable isotropic charge transfer from all small-n-value nanoplates directly to largest-n-value nanoplates. Essentially, this uniform dispersion is formed by mechanically enforcing nucleation during crystallization upon our vacuum poling method. Consequently, record-high fill factor (FF) of 82.4% with maximal power conversion efficiency of 18.04% (Voc = 1.223 V, Jsc = 17.91 mA/cm2) is achieved with excellent stabilities. This work shows that uniformly arranging different-n-value nanoplates offers a new materials processing strategy for developing high-performance quasi-2D perovskite optoelectronic devices. The vertically ordered (small-to-large n) quasi-2D perovskite films serve as common approaches to facilitate directional charge transfer. Here, we report a different strategy of uniformly arranging different-n-value nanoplates (PEA2MAn-1PbnI3n+1) by introducing vacuum poling treatment to enforce nucleation during crystallization. This uniform distribution is verified by delicate mechanical tape-peeling method while monitoring optical absorption, photoluminescence (PL), and energy-dispersive X-ray spectroscopy (EDS). With uniform distribution, efficient carrier transfer within 10 ps is revealed by transient absorption. Moreover, record-high fill factor (FF) of 82.4% with power conversion efficiency (PCE) of 18.04% (Voc = 1.223 V, Jsc = 17.91 mA/cm2) was demonstrated. Superior stability is achieved with retaining 96.1% of initial efficiency after 8-month storage and maintaining 97.7% at 80°C for over 180 h. This uniformly arranging different-n-value nanoplates offers a new material engineering strategy to enhance carrier transfer and extraction for developing high-efficiency and stable quasi-2D perovskite solar cells. The vertically ordered (small-to-large n) quasi-2D perovskite films serve as common approaches to facilitate directional charge transfer. Here, we report a different strategy of uniformly arranging different-n-value nanoplates (PEA2MAn-1PbnI3n+1) by introducing vacuum poling treatment to enforce nucleation during crystallization. This uniform distribution is verified by delicate mechanical tape-peeling method while monitoring optical absorption, photoluminescence (PL), and energy-dispersive X-ray spectroscopy (EDS). With uniform distribution, efficient carrier transfer within 10 ps is revealed by transient absorption. Moreover, record-high fill factor (FF) of 82.4% with power conversion efficiency (PCE) of 18.04% (Voc = 1.223 V, Jsc = 17.91 mA/cm2) was demonstrated. Superior stability is achieved with retaining 96.1% of initial efficiency after 8-month storage and maintaining 97.7% at 80°C for over 180 h. This uniformly arranging different-n-value nanoplates offers a new material engineering strategy to enhance carrier transfer and extraction for developing high-efficiency and stable quasi-2D perovskite solar cells. Incorporation of long-chain organic ligands A′ has improved the optoelectronic performance1Yuan M. Quan L.N. Comin R. Walters G. Sabatini R. Voznyy O. Hoogland S. Zhao Y. Beauregard E.M. 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In the present work, we demonstrate a uniform dispersion of different-n-value nanoplates as a new strategy to enhance the optoelectronic properties of quasi-2D perovskites by using our vacuum poling treatment. Different from reported vacuum-assisted annealing method,23Xie F.X. Zhang D. Su H. Ren X. Wong K.S. Grätzel M. Choy W.C.H. Vacuum-assisted thermal annealing of CH3NH3PbI3 for highly stable and efficient perovskite solar cells.ACS Nano. 2015; 9: 639-646Crossref PubMed Scopus (296) Google Scholar, 24Li X. Bi D. Yi C. Décoppet J.D. Luo J. Zakeeruddin S.M. Hagfeldt A. Grätzel M. A vacuum flash–assisted solution process for high-efficiency large-area perovskite solar cells.Science. 2016; 353: 58-62Crossref PubMed Scopus (1519) Google Scholar we developed our vacuum poling treatment to enforce nucleation during crystallization, leading to uniform dispersion of different-n-value nanoplates. With this new design, the charge transfer and carrier extraction are further improved toward developing high-performance quasi-2D perovskite solar cells. As schematically shown in Figure 1A, the precursor solutions with the stoichiometry designed to = 5 are first spin-cast on preheated substrates to form a high density of nucleus. Then the films are immediately placed in a chamber for 8 min with the gage pressure quickly decreasing to −1.0 bar. Here, applying vacuum poling treatment can simultaneously increase the nucleation and limit the grain sizes by quickly removing residual solvent toward realizing uniform dispersion of different-n-value nanoplates. Afterward, thermal annealing is followed to further crystallize the 2D nanoplates at 100°C for 10 min in N2 atmosphere. Consequently, high-quality quasi-2D perovskite films are formed with a uniform dispersion of different-n-value nanoplates. The competition between the crystallization and nucleation process during vacuum poling treatment is verified by absorption and photoluminescence (PL) results (Figure S1). In contrast, without vacuum poling treatment, the unaccelerated nucleation leads to the crystallization to form nanoplates with small n values on the bottom surface and large n values toward the top surface, creating vertically ordered (small-to-large n) dispersion of nanoplates in quasi-2D perovskite films under thermal annealing. Here, nuclear magnetic resonance (NMR) experiments25Yan L. Hu J. Guo Z. Chen H. Toney M.F. Moran A.M. You W. General post-annealing method enables high-efficiency two-dimensional perovskite solar cells.ACS Appl. Mater. Interfaces. 2018; 10: 33187-33197Crossref PubMed Scopus (59) Google Scholar (Figures S2 and S3) show that vacuum poling treatment does not remove the ligands from the spin-cast film. X-ray diffraction (XRD) patterns for quasi-2D perovskite films with uniform and ordered dispersions prepared with and without vacuum poling treatment are shown in Figure S4. With vacuum poling treatment, the XRD peaks around 14° and 28° are broader than that of the film without vacuum poling. Here, the lineshape broadening reflects the decreased grain sizes (evidenced by scanning electron microscopy [SEM] and atomic force microscopy [AFM] results in Figures S5 and S6) under uniform dispersion of different-n-value nanoplates. Especially, decreasing grain sizes can increase the probabilities to realize the uniform dispersion of different-n-value nanoplates. To reveal the uniform dispersion of different-n-value nanoplates between bottom and top surfaces in quasi-2D perovskite films induced by vacuum poling treatment, we developed a delicate tape-peeling method (Figure 1B; the thicknesses for unpeeled and peeled films are ∼300 and ∼25 nm, respectively), to mechanically detach the multiple monolayers of nanoplates from top surface subsequently toward bottom surface while monitoring energy-dispersive X-ray spectroscopy (EDS), PL, and optical absorption, as shown in Figures S7, 1C, and S8. Interestingly, the atomic ratio of Pb:C:I remains nearly the same after peeling off multiple monolayers for quasi-2D perovskite films prepared with vacuum poling treatment (Figure S7A). It indicates that the uniform dispersion of different-n-value nanoplates is realized between the bottom and top surfaces by using vacuum poling treatment. Moreover, with the mechanical tape-peeling method, very similar PL spectra with dominated peak at 740 nm (see upper figure of Figure 1C) and uniformly decreased absorption in the entire spectral range between 400 and 800 nm (Figures S8A and S8B) are observed after peeling off multiple monolayers of nanoplates from top surface toward bottom surface. These provide additional evidence of uniform distribution of nanoplates within quasi-2D perovskite films prepared with vacuum poling treatment. In contrast, without vacuum poling treatment, EDS results in Figure S7B show that the ratio of C atoms increases after tape peeling, indicating more PEAI ligands (containing more C than MAI) on the bottom. Thus, the formation of an ordered distribution of nanoplates with small n value toward the bottom and large n value toward the top are suggested. Meanwhile, with mechanically tape-peeling method, very different PL spectra (see bottom figure of Figure 1C) are observed: the dominant PL peak locates at 778 nm related to large-n-value nanoplates before peeling and 519 nm related to small-n-value nanoplates after peeling, further proving the existence of ordered distribution of nanoplates. Moreover, after peeling off the multiple monolayers of film prepared without vacuum poling treatment, the absorption spectrum shows two dominant peaks at around 516 nm and 566 nm related to n = 1 and n = 2 nanoplates (Figure S8B), providing further evidence of the vertically ordered distribution of nanoplates. In addition, it should be noted that the coexistence of multiple PL peaks indicates that the charge transfer is not efficient between nanoplates with ordered-n-value dispersion. The spatial distribution of different-n-value nanoplates are illustrated in Figures 1D and 1E. To discuss the dynamics of charge transfer in quasi-2D perovskite films, transient absorption (TA) measurements were carried out. With the vacuum poling treatment, the TA measured through top and bottom shows similar characteristics, as shown in Figure 2A, further verifying the uniform dispersion of different-n-value nanoplates. Without vacuum poling treatment, the TA measured through top and bottom show very different characteristics (Figure 2B), verifying that smaller-n-value and larger-n-value nanoplates are distributed toward bottom and top, respectively. This is consistent with the publication of other ordered quasi-2D perovskite films.7Liu J. Leng J. Wu K. Zhang J. Jin S. Observation of internal photoinduced electron and hole separation in hybrid two-dimentional perovskite films.J. Am. Chem. Soc. 2017; 139: 1432-1435Crossref PubMed Scopus (390) Google Scholar Moreover, TA dynamics at wavelengths at 534, 567, 609, and 724 nm (or 750 nm) corresponding to signals from small-n-value to largest-n-value nanoplates are plotted in Figures 2C and 2D. It is shown that vacuum poling treatment leads to a fast increment within 10 ps on the negative ΔA/A signal at 724 nm in Figure 2C (corresponding to largest-n-value nanoplates). In the meantime, positive ΔA/A signals at other wavelengths are observed, indicating that the charge transfer from small-n-value nanoplates to largest-n-value nanoplates is very efficient without much accumulations between nanoplates. In contrast, without vacuum poling treatment, the negative ΔA/A signal (750 nm in Figure 2D, corresponding to largest-n-value nanoplates) shows a much slower increment within 600 ps (Figure 2D), while negative ΔA/A signals also appear at 567 and 609 nm. It implies that accumulations of charge carriers occur at corresponding small-n-value nanoplates (567 and 609 nm), leading to less efficient charge transfer in the ordered dispersion of different-n-value nanoplates prepared without vacuum poling treatment. It should be noted that vacuum poling method can also be applied to other quasi-2D perovskite films prepared with average n values such as = 4 and 3 for obtaining uniform distribution of nanoplates, as evidenced by TA spectra (Figure S9) exciting through different sides and tape-peeling based PL results (Figure S10). To further understand the dynamics of photoexcited states under uniform and ordered dispersions of different-n-value nanoplates, PL characteristics were analyzed for quasi-2D perovskite films prepared with and without vacuum poling treatment. Under both conditions, the PL spectra consist of various peaks originating from different n-value nanoplates (n = 1, 2, 3, 4, n > 4) peaked at around 518, 570, 612, 710, and 750 nm, as shown in Figures 3A and 3B . However, with vacuum poling, increasing the photoexcitation intensity at 405 nm significantly increases the light emission from the largest-n-value nanoplates (n > 4), while the light emission from small n values (n ≤ 4) are slightly increased (see the inset of Figure 3A). This intensity-dependent PL20Chen P. Meng Y. Ahmadi M. Peng Q. Gao C. Xu L. Shao M. Xiong Z. Hu B. Charge-transfer versus energy-transfer in quasi-2D perovskite light-emitting diodes.Nano Energy. 2018; 50: 615-622Crossref Scopus (81) Google Scholar, 21Quan L.N. Zhao Y. Garcia de Arquer F.P. Sabatini R. Walters G. Voznyy O. Comin R. Li Y. Fan J.Z. Tan H. et al.Tailoring the energy landscape in quasi-2D halide perovskites enables efficient green-light emission.Nano Lett. 2017; 17: 3701-3709Crossref PubMed Scopus (329) Google Scholar indicates that the uniform dispersion of different-n-value nanoplates generates efficient transport of photogenerated carriers (can also be evidenced by power-dependent PL and EL; Figure S11) from small to large n-value nanoplates in quasi-2D perovskite films prepared with vacuum poling treatment. Contrarily, without vacuum poling, increasing excitation intensity mainly increases the light emission from small-n-value nanoplates dominated by n = 1 (Figure 3B). This implies that the transport of photogenerated carriers is less efficient in the ordered dispersion of different-n-value nanoplates. Moreover, we notice that the vacuum poling treatment increases the slope of PL-excitation intensity dependence from 1.61 to 1.95 toward ideal bimolecular recombination in the large-n-value nanoplates with emission at 750 nm, but the slope almost keeps unchanged in small-n-value nanoplates, with emission peaking at around 518, 570, 612, and 710 nm (shown in Figures 3C and 3D). This phenomenon may imply that small-n-value nanoplates can function as passivation agents and largest-n-value nanoplates as the emitting centers, which is similar to our recent observation that small grains can passivate the defects on the surfaces of large grains in hybrid perovskites.26Qin J. Zhang J. Bai Y. Ma S. Wang M. Xu H. et al.Enabling self-passivation by attaching small grains on surfaces of large grains toward high-performance perovskite LEDs.iScience. 2019; 19: 378-387Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar, 27Bai Y. Qin J. Shi L. Zhang J. Wang M. Zhan Y. Zou H. Haacke S. Hou X. Zi J. et al.Amplified spontaneous emission realized by cogrowing large/small grains with self-passivating defects and aligning transition dipoles.Adv. Opt. Mater. 2019; 7: 1900345Crossref Scopus (15) Google Scholar In quasi-2D perovskite films with uniform distribution of nanoplates, large-n-value nanoplates are surrounded by small-n-value nanoplates, which enables more contacts for passivation than ordered dispersion of nanoplates (the films without vacuum poling treatment). As a consequence, the quasi-2D perovskite films with uniform dispersion of different-n-value nanoplates demonstrate longer PL lifetime, as compared to the ordered dispersion (Figure S12). TRPL data detected with the Streak camera in Figures 3E and 3F further indicate the prolonged PL lifetime under vacuum poling treatment. Clearly, the uniform dispersion of nanoplates shows one major emission peak at 750 nm with a longer lifetime, while the ordered nanoplates show multiple emission peaks with shorter PL lifetimes. To understand the carrier extraction mechanism under uniform dispersion of different-n-value nanoplates based on vacuum poling treatment, the photovoltaic performance was characterized based on the device architecture of ITO/PTAA/PEA2MAn-1PbnI3n+1/PC61BM/PEI/Ag. As shown in Figure 4A, the vacuum poling treatment leads to the record-high FF of 82.4% with the PCE reaching 18.04% (Voc = 1.223 V, Jsc = 17.91 mA/cm2), indicating extremely efficient carrier extraction under the uniform dispersion of different-n-value nanoplates. For comparison, we also fabricated devices without vacuum poling treatment using PEDOT: PSS as hole transport layer (HTL) (short circuit occurs in without-vacuum-treated devices due to pinholes when using PTAA as HTL). The I-V curves of best-performing devices with and without vacuum treatment (PEDOT: PSS as HTL) are compared in Figure S13. When replacing PEDOT: PPS with PTAA in vacuum-treated devices, the decreased dark current and increased PL lifetime (Figure S14; Table S1) account for the better photovoltaic performance of PTAA-based devices. External quantum efficiency (EQE) results are shown in Figure S15, and the corresponding integrated Jsc of devices with and without vacuum poling treatment is 18.09 and 17.79 mA/cm2, respectively. It is noted that the quasi-2D perovskite films prepared with vacuum poling treatment shows the bandgap of 1.60 eV, slightly wider than the band gap of 1.57 eV prepared without vacuum poling treatment (Figure S8C). Clearly, the photovoltaic enhancement enabled by vacuum poling method does not result from absorption, which further verifies more efficient transport of photogenerated carriers responsible for the record-high fill factor (FF) and efficiency. 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Moreover, the upper curve of Figure 4C shows the output efficiency at maximum power point (MPP) monitored continuously without device encapsulation. The PCE remains as high as 95.2% of the maximum point after more than 9.5 h, indicating considerably improved operational stability at continuous working conditions, as compared with 3D-only and without-vacuum-poling-treated control devices (Figures S17 and S18). More surprisingly, after heating the device at 80°C for over 180 h, the PCE maintains 97.7% (see the middle curve of Figure 4C) of its original value for quasi-2D perovskite devices prepared with vacuum poling treatment, which indicates extremely good thermal stability. For comparison, the 3D-only devices and quasi-2D devices without vacuum poling treatment drastically drop their PCE within 1 h at 80°C (Figure S19). In addition, the device efficiency retains 96.1% of original value after storing in glove box for 8 months without encapsulation (see down curve of Figure 4C). The statistics data in Figures 4D, S20, and S21 show that vacuum poling treatment significantly improves the reproducibility of device performance. In contrast, devices fabricated without vacuum poling treatment suffer from poorer performance and lower reproducibility (Figure S22). As schematically shown in Figure 4E, the uniform dispersion increases the contact probabilities between different-n-value nanoplates to generate isotropic transport from all small-n-value nanoplates directly to largest-n-value nanoplates. Consequently, photogenerated carriers are isotropically transported from small-n-value nanoplates to largest-n-value nanoplates in all directions in quasi-2D perovskite films prepared with uniform dispersion of nanoplates. It should be emphasized that the uniform dispersion can decrease the transport time to collect photogenerated carriers because all photogenerated carriers in small-n-value nanoplates have an opportunity to directly transport to largest-n-value nanoplates (functioning as a transport-highway network) due to the increased contact probabilities between different-n-value nanoplates. Therefore, a record-high FF (82.4%) is demonstrated in our work. In contrast, in ordered dispersion, the photogenerated carriers are cascaded from n = 1 to n = 2, n = 3, n = 4, n = 5 nanoplates, leading to a prolonged time to collect photogenerated carriers through such cascade process. In ordered dispersion (Figure 4F), large-n-value nanoplates do not form a continuous network vertically, consequently lacking the transport highway to more effectively collect photogenerated carriers. In summary, we explored the uniform dispersion of different-n-value nanoplates homogenously located between bottom and top surfaces in quasi-2D perovskite films [(PEA)2(MA)n-1PbnI3n+1] by introducing vacuum poling treatment. In contrast, without vacuum poling treatment, the different-n-value nanoplates are formed with vertically ordered dispersion from small to large n values between the bottom and top surfaces in quasi-2D perovskite films. The uniform and ordered dispersions of different-n-value nanoplates between the bottom and top surfaces were verified by our tape-peeling method to mechanically detach monolayers of nanoplates from top surface subsequently toward the bottom surface while monitoring the EDS, PL spectra, and optical absorption. The TA results showed that the uniform dispersion of different-n-value nanoplates leads to a fast charge transfer toward the largest-n-value nanoplates within 10 ps. The PL dynamics studies implied that with vacuum poling-enabled uniform dispersion, small-n-value and large-n-value nanoplates may function as passivation agents and light-emitting centers, respectively, in quasi-2D perovskite films with enhanced PL intensity and lifetime. Furthermore, with uniform dispersion of different-n-value nanoplates, the quasi-2D perovskite solar cells demonstrate a record-high FF (82.4%) with maximum PCE of 18.04% (Jsc = 17.91 mA/cm2 and Voc = 1.223 V) based on the device structure of ITO/PTAA/PEA2MAn-1PbnI3n+1/PC61BM/PEI/Ag. Surprisingly, superior stabilities are achieved with retaining over 96% of their initial efficiency after storing for 8 months and 97.7% of the original value at 80°C for over 180 h. Therefore, uniformly arranging different-n-value nanoplates through vacuum poling treatment provides a new strategy to enhance carrier extraction toward developing high-performance quasi-2D perovskite optoelectronics.