Highly Crystalline Perovskite-Based Photovoltaics via Two

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Highly Crystalline Perovskite-based Photovoltaics via Two-dimensional Liquid Cage Annealing Strategy Jong Woo Lee, Haejun Yu, Kisu Lee, Sohyeon Bae, Jungwon Kim, Gi Rim Han, Doyk Hwang, Seong Keun Kim, and Jyongsik Jang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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Journal of the American Chemical Society

Highly Crystalline Perovskite-based Photovoltaics via Twodimensional Liquid Cage Annealing Strategy Jong Woo Lee†, , Haejun Yu‡, , Kisu Lee‡, Sohyeon Bae†, Jungwon Kim‡, Gi Rim Han†, Doyk Hwang†, Seong Keun Kim*,† and Jyongsik Jang*,‡ † Department

of Chemistry, Seoul National University, Seoul 08826, Republic of Korea

‡ World

Class University (WCU) Program of Chemical Convergence for Energy & Environment (C2E2), School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea KEYWORDS: “perovskite solar cell”, “two-dimensional cage annealing”, “grain size”, “perfluorodecalin”

ABSTRACT: Rendering a high crystalline perovskite film is integral to achieve superior performance of PSCs. Here, we

established two-dimensional liquid cage annealing system, a unique methodology for remarkable enhancement in perovskite crystallinity. During thermal annealing for crystallization, wet-perovskite films were suffocated by perfluorodecalin with distinctively low polarity, non-toxic and chemically inert characteristics. This annealing strategy facilitated enlargement of perovskite grain and diminution in the number of trap states. The simulation results, annealing time and temperature experiments supported that the prolonged diffusion length of precursor ions attributed to increase of perovskite grains. Consequently, without any complicated handling, performance of perovskite photovoltaics was remarkably improved and the monolithic grains which directly connected the lower and upper electrode attenuated hysteresis.

INTRODUCTION Development of perovskite solar cells (PSCs) has proceeded at an unprecedented rate; the certified power conversion efficiency (PCE) is currently over 23 %, achieved by exploiting the unique photoelectric properties of perovskite.1,2 Formation of high crystalline perovskite is essential to guarantee superior performance of PSCs; perovskite not only generates charge carriers but also allows these to migrate toward adjacent charge-selective layers. In terms of solutionbased perovskite deposition, numerous instructive strategies, such as solvent-engineering (SE),3 sequential-deposition,4 spray-coating,5 and a blade method,6 have all been introduced to render a highquality perovskite. Particularly, the SE was a breakthrough that led to skyrocketing efficiency of PSCs by yielding the dense perovskite films,3 and a Lewis base-adduct approach was then announced in a more complementary form of SE, raising the efficiency to ca. 20 %.7,8 It is still being emulated in slightly modified ways; organic compounds are blended with anti-solvents9,10 or different anti-solvents are mixed.11 However, the perovskite crystallized only by the conventional SE has a limitations including the small grain size and high defect density, which debases lightcapturing ability and the charge transfer rate. Augmenting perovskite grain size is an appropriate strategy to resolve the above-mentioned drawbacks of perovskite and enhance the performance of PSCs with

relieved hysteresis by suppressing the charge accumulation.12 Generally, to enlarge crystals and foster the morphological traits, it is central to elicit rapid homogeneous nucleation and impede crystallization reaction via chemical or physical handling.13,14 Chemically engineered precursors derived by addition of various enhancers, e.g. CH3NH3Cl,15 Pb(SCN)2,16 CH4N2S,17 and polymers,18 have yielded monolithically grained perovskite with improved photo-physical properties. However, precursor engineering cannot regulate indiscriminate nucleation sites and requires pinpoint control of composition; this is fastidious to reproduce other than in the hands of skilled technicians. Alternatively, various postprocessing methods,19 e.g. solvent-annealing (SA),20 Ostwald-ripening,21 and chemical vapor recrystallization,22 were often employed to compensate shortcomings of as-prepared films by reorganizing the crystal structure and healing defects. These measures resulted in the drastically enlarged perovskite grains, which delivers performance enhancement because the charge carrier diffusion length is proportional to grain size and non-radiative recombination at shallow defect states which are concentrated at grain boundaries (GBs) can be mitigated, simultaneously.23,24 However, the post-treatments are both complex and variable, therefore a straightforward approach is demanded from the viewpoint of production efficiency and high reproducibility. Herein, we report a novel post-treatment method producing highly crystalline CH3NH3PbI3 by

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was much thicker (> 500 nm) than the exciton diffusion length of CH3NH3PbI3 (> 100 nm),38,39 several radiative recombination processes occurred at the bulk region; the associated fluorescence was reflected in the TCSPC results. Therefore, it was suggested that the actual quenching rate in the interfacial region of PFDcaged CH3NH3PbI3 was faster than that of the control, which implies efficient charge carrier separation. Furthermore, fluorescence lifetime imaging microscopy (FLIM) of CH3NH3PbI3 coated onto cover glass was conducted to substantiate the remarkable improvements in efficiency of 2D-PFDCA-based PSCs. The cover glass bearing CH3NH3PbI3 that were thinner than usual was used, to enhance spatial resolution. The control yielded blurred fluorescence images due to the Abbe’s diffraction limit (Figures 3c), but the images of PFD-caged CH3NH3PbI3 displayed distinct grains by virtue of the high-dimensional domain growth (Figure 3d). Furthermore, the overall average fluorescence lifetime of PFD-caged CH3NH3PbI3 was over twice that of control. Although there was the discrepancy between average lifetimes of FLIM and TCSPC, induced from different glass type and thickness of perovskite, the trends were consistent. The average decay time constants of grain interiors were longer than those of GBs due to trap-assisted non-radiative recombination in defect states of the GBs.40,41 The GBs bordering distinctly oriented crystals crucially affects charge carrier dynamics, facilitating non-radiative recombination at both deep and shallow trap sites.40 Thus, control CH3NH3PbI3 containing many defective GBs deteriorate performance of PSCs. Conversely, the monolithic grains formed via 2D-PFDCA possesses a lower density of GBs and directly connected the lower and upper electrodes (Figure 4a), delivering the increase in open-circuit voltage (Voc) of PSCs.42

Device Performance Based on the 2D-PFDCA, PSCs were fabricated employing TiO2 and Spiro-OMeTAD as the electron and hole transport layer (ETL and HTL), respectively. Figure 4a displays a cross-sectional view of the 2DPFDCA-based PSC, revealing the formation of a monolithic CH3NH3PbI3 grain between the chargeselective layers. Typical J-V curves were collected from the two types of PSCs (Figure 4b); the control PSC exhibited a PCE of 13.97 % and a short-circuit current (Jsc) of 20.02 mA/cm2, a Voc of 1.084 V, and a fill factor (FF) of 0.644. On the other hand, use of 2D-PFDCA improved the PCE up to 18.59 % with a Jsc of 21.47 mA/cm2, a Voc of 1.110 V, and an FF of 0.780. The performance gap was well-identified by histogram acquired from 30 devices in Figure 4c; the average PCE rose from ca. 14.10 % up to ca. 17.81 % with employment of the 2D-PFDCA (Figure S6, Table S3). Furthermore, a severe hysteresis in J-V curve of control

PSC was clearly relieved in the 2D-PFDCA-based PSCs (Figure S7). The noticeable increase in Jsc was confirmed by incident photon-to-current efficiency (IPCE) measurement (Figure 4d); the 2D-PFDA-based PSC yielded better IPCE result over the entire waveband. This demonstrated that PFD-caged CH3NH3PbI3 facilitated efficient charge collection toward both electrodes, by reducing defective sites, and allowing strong absorption of incident light. To clarify Voc enhancement, electrochemical impedance spectroscopy was conducted under dark condition (Figure S8). Previous reports interpreted that the semicircle in the low-frequency domain reflected recombination resistance at the TiO2/perovskite interface.43,44 The 2D-PFDCA-based PSCs exhibited much larger recombination resistance than the control, attributed to that the PFD-caged CH3NH3PbI3 formed adjacent to TiO2 ETL had high crystallinity and low defects density as well as the internal bulk phase. Therefore, the long lifetimes of charge carriers and low defect levels were beneficial for the improvement of Voc. Also, PFD-caged CH3NH3PbI3 have a lower areal GB density than the control, leading to relaxation of charge recombination. To confirm performance reliability, the stabilized efficiency was measured of the 2D-PFDCA-based PSC (Figure 4e); Jsc and PCE stabilized to 18.92 mA/cm2 and 17.69 % at a Vmax of 0.935 V, respectively. The PFDCA-based device showed a champion PCE of 19.0 % from associated with a Jsc of 22.07 mA/cm2, a Voc of 1.108 V and an FF of 0.775 (Figure 4f).

CONCLUSION In summary, the 2D-PFDCA process confines the perovskite substrate to the PFD medium, isolating the film surface from the ambient environment and delaying the evaporation of residual solvent from wetperovskite. In turn, this enables to prolong the diffusion length of perovskite precursor ions and formulates a large-grained CH3NH3PbI3 exhibiting high crystallinity, low trap density, and a long exciton lifetime. Consequently, the 2D-PFDCA technique remarkably ameliorated device performance compared to that of a CA-based PSC; Jsc, Voc, and FF were all enhanced. Furthermore, the improved mobility and enhanced charge extraction into adjacent ETL and HTL attenuated hysteresis. We fabricated high-quality perovskites by merely placing the substrate in a bath containing a hydrophobic organic solvent without any complicated handling. Thus, proper utilization of certain liquid chemicals during annealing process can be a good initiator in producing highly crystalline perovskite.

ASSOCIATED CONTENT Supporting Information.


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Journal of the American Chemical Society Experimental details for the device fabrication, characterization methods, all supplementary figures (Figure S1-S8) and table (T1-T3) mentioned in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Author Contributions These authors contributed equally to this paper.

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (NRF2012M3A6A7054855, NRF-2012M3A6A7054861). We thank Kiwan Jeong and Prof. Mansoo Choi of the Multiscale Energy System Laboratory affiliated in Seoul National University for assistance with operation of vacuum thermal evaporation equipment.

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Highly Crystalline Perovskite-Based Photovoltaics via Two

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