Reports exponential (12, 20), to biexponential (13, 14), to stretchedexponential (6, 9) functions with varying levels of fidelity. These distributions have in turn been explained in terms of unintenDane W. deQuilettes,1 Sarah M. Vorpahl,1 Samuel D. Stranks,2* tional doping (21) or charge trapping (22). The perovskite Hirokazu Nagaoka,1 Giles E. Eperon,2 Mark E. Ziffer,1 Henry J. 2 1 growth conditions (3, 4, 10) and Snaith, David S. Ginger † post-deposition treatments (12, 1 Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195-1700, USA. 2Clarendon Laboratory, 23) can greatly alter film morUniversity of Oxford, Parks Road, Oxford OX1 3PU, UK. phology, carrier lifetime, and *Present address: Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA device performance, yet the un02139, USA. derlying relations between these †Corresponding author. E-mail: [email protected]
parameters are important open questions. For instance, perovThe remarkable performance of hybrid perovskite photovoltaics is skite films grown from nonattributed to their long carrier lifetimes and high photoluminescence (PL) stoichiometric mixed halide efficiencies. High-quality films are associated with slower PL decays, and it (Cl/I) precursor solutions have has been claimed that grain boundaries have a negligible impact on exhibited lifetimes of hundreds performance. We used confocal fluorescence microscopy correlated with of nanoseconds, but PL lifetimes scanning electron microscopy to spatially resolve the PL decay dynamics in films grown from chloridefrom films of nonstoichiometric organic-inorganic perovskites, free precursors are generally CH3NH3PbI3(Cl). The PL intensities and lifetimes varied between different much shorter (9, 20). grains in the same film, even for films that exhibited long bulk lifetimes. Correlated confocal PL and The grain boundaries were dimmer and exhibited faster nonradiative decay. scanning electron microscopy Energy-dispersive x-ray spectroscopy showed a positive correlation (SEM) have been a powerful tool between chlorine concentration and regions of brighter PL, whereas PL to reveal structure/function relaimaging revealed that chemical treatment with pyridine could activate tionships in biology (24). We previously dark grains. applied similar techniques to study structure/function relaAs active layers in solar cells, organic-inorganic perovskites tionships in perovskite films. We found substantial local PL (1, 2) combine the promise of solution processing (3, 4) with heterogeneity even for CH NH PbI (Cl) films with average 3 3 3 the ability to tailor the band gap through chemical substitu- lifetimes of ~1 μs (comparable to the longest lifetimes retion (5–7), yielding solar cell power conversion efficiencies ported) (9, 10), which suggests that considerable scope reas high as 20.1% (8). Concomitant with their photovoltaic mains for reducing nonradiative recombination in these performance, perovskites also exhibit high fractions of radi- films. In addition to observing entire grains that appear ative recombination, with apparent carrier lifetimes of 250 dark, we also observed that grain boundaries are associated ns or longer (9, 10), and are challenging the dogma that so- with PL quenching, indicating that they are not as benign as lution-processed semiconductors inevitably possess high has been suggested previously (25, 26). We further used PL densities of performance-limiting defects. Ensuring that all microscopy to show that post-deposition chemical treatrecombination is radiative is critical for approaching the ments can activate previously dark regions in the film, and thermodynamic efficiency limits for solar cells and other we correlated local energy-dispersive x-ray spectroscopy optoelectronic devices (11). (EDS) with confocal fluorescence maps, finding that brightCarrier recombination lifetimes measured by photolumi- er grains with longer lifetimes were associated with local nescence (PL) are commonly taken as a hallmark of perov- spikes in Cl concentration. skite film quality, with longer decay lifetimes used as We studied CH3NH3PbI3(Cl) films prepared on glass indicators of better-performing materials (9, 10, 12–14). Car- slides by spin-coating a nonstoichiometric mixed halide prerier recombination kinetics have been described as a com- cursor solution composed of CH NH I and PbCl (3:1) in 3 3 2 bination of trap-assisted, monomolecular (first-order), and N,N-dimethylformamide (19, 27). Films prepared under bimolecular (second-order) recombination (15). Although identical conditions and incorporated into standard solar most studies agree that radiative bimolecular recombination cell device architectures (fig. S1) (19) exhibit power converdominates at high initial carrier densities (n0 > 1017 cm−3) sion efficiencies (η) up to 14.5% (Fig. 1A), which is compara(15–18), reports of kinetics at lower excitation densities (and ble to efficiencies in other reports using this architecture (2, relevant to solar cell operation) (19) range from single- 28). Figure 1B shows that our PL lifetimes are as long as
Impact of microstructure on local carrier lifetime in perovskite solar cells
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those reported for films used in the best devices to date (10). These films exhibited average carrier lifetimes >1000 ns when excited at low intensity (30 nJ/cm2, n0 ~ 1015 cm−3). At short times (Fig. 1B, inset), the PL decay could appear nearly single-exponential, but at longer times, the decay deviated from a single-exponential decay (6, 9, 15). We fit the decay in Fig. 1B with a stretched-exponential function of characteristic lifetime τc = 431 ns and distribution parameter β = 0.57, which we interpret as arising from a superposition of exponential relaxation functions (see below) with an average lifetime of = 1005 ns (19, 29). Green and co-workers recently examined microscopic PL quenching of discontinuous perovskite islands with n- and p-type capping layers (30). Here, we used fluorescence microscopy to probe the inherent decay properties of neat semiconducting films. Figure 1 shows a correlated SEM micrograph (Fig. 1C), confocal PL image (Fig. 1D), and an overlaid SEM/PL microscopy image (Fig. 1E) of a highperforming perovskite film on a glass substrate. Although this film appears contiguous (Fig. 1C) and exhibits = 1005 ns, Fig. 1D shows a large distribution in local PL intensity across the film. We observed these large distributions in films prepared in different research labs (fig. S2D) (19), and we exclude variations in film thickness (fig. S2) (19) and photodegradation during imaging (fig. S3) (19) as primary causes. The PL intensity not only varied from grain to grain, with roughly 30% of grains imaged in Fig. 1C consisting of dark grains (19), but we also observed ~65% lower PL intensity at grain boundaries (fig. S4, A to C) (19), after deconvolution of the microscope point-spread function (fig. S5) (19). These results are surprising because, through considerations of detailed balance (11, 31), one expects high-performance films to have minimal nonradiative decay. Instead, the spatial variations in PL intensity in the polycrystalline perovskite films are suggestive of variations in local nonradiative decay rates. By taking local steady-state and time-resolved PL data, we confirmed that darker regions have greater nonradiative loss. Figure 2 shows a confocal PL image (Fig. 2A) along with local PL spectra (Fig. 2B) and lifetime data (Fig. 2, C to E) from a film with a long average bulk lifetime ( = 1010 ns). Figure 2B shows the steady-state spectra of a bright (red square) and dark (blue circle) region. The PL spectrum collected at the dark region is both red-shifted (~2 nm) and slightly broader than the bright region (fig. S6) (19). These trends suggest a less sharp band edge (32), probably caused by the presence of defect states or shallow trapping levels in the darker regions. In Fig. 2, C to E, we show local PL decays of the indicated dark and bright regions at low (1 μJ/cm2), medium (2.1 μJ/cm2), and high (3.4 μJ/cm2) excitation fluences. Several studies have reported a transition from trap-assisted monomolecular recombination to free-carrier bimolecular recombination over this fluence range (15, 18). Consistent with the picture that bright regions have fewer nonradiative pathways, bright regions show a slower decay, a transition to bimolec-
ular recombination–dominated kinetics at a lower excitation fluence, and more efficient PL quenching when contacted by fullerene (fig. S7) (19) in comparison to dim regions. We modeled the PL dynamics (black lines in Fig. 2, C to E) as a combination of trapping, monomolecular, and bimolecular recombination (19). We report a higher deep trap-state density in the dark region (4 × 1016 cm−3) as compared to the bright region ( 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 347, 967–970 (2015). Medline doi:10.1126/science.aaa5760 36. W. Nie, H. Tsai, R. Asadpour, J. C. Blancon, A. J. Neukirch, G. Gupta, J. J. Crochet, M. Chhowalla, S. Tretiak, M. A. Alam, H. L. Wang, A. D. Mohite, High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science 347, 522–525 (2015). Medline doi:10.1126/science.aaa0472 37. D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent, O. M. Bakr, Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 347, 519–522 (2015). Medline 38. M. Grätzel, The light and shade of perovskite solar cells. Nat. Mater. 13, 838–842 (2014). Medline doi:10.1038/nmat4065 39. G. Grancini, S. Marras, M. Prato, C. Giannini, C. Quarti, F. De Angelis, M. De Bastiani, G. E. Eperon, H. J. Snaith, L. Manna, A. Petrozza, The impact of the crystallization processes on the structural and optical properties of hybrid perovskite films for photovoltaics. J. Phys. Chem. Lett. 5, 3836–3842 (2014). doi:10.1021/jz501877h 40. Y. Tidhar, E. Edri, H. Weissman, D. Zohar, G. Hodes, D. Cahen, B. Rybtchinski, S. Kirmayer, Crystallization of methyl ammonium lead halide perovskites: Implications for photovoltaic applications. J. Am. Chem. Soc. 136, 13249–13256 (2014). Medline doi:10.1021/ja505556s 41. S. T. Williams, F. Zuo, C. C. Chueh, C. Y. Liao, P. W. Liang, A. K. Jen, Role of chloride in the morphological evolution of organo-lead halide perovskite thin films. ACS Nano 8, 10640–10654 (2014). Medline doi:10.1021/nn5041922
supplementary materials (19). SUPPLEMENTARY MATERIALS www.sciencemag.org/cgi/content/full/science.aaa5333/DC1 Materials and Methods Supplementary Text Figs. S1 to S11 19 December 2014; accepted 14 April 2015 Published online 30 April 2015; 10.1126/science.aaa5333
ACKNOWLEDGMENTS This material is based in part on work supported by the State of Washington through the University of Washington Clean Energy Institute. D.W.D. acknowledges support from an NSF Graduate Research Fellowship (DGE-1256082). S.M.V. acknowledges support from a National Defense Science and Engineering Graduate Fellowship. The research leading to these results has received funding from the European Union Seventh Framework Program (FP7/2007-2013) under Grant Agreement No. 604032 of the MESO project. G.E. is supported by the Engineering and Physical Sciences Research Council and Oxford Photovoltaics through a Nanotechnology Knowledge Transfer Network Collaborative Award in Science and Engineering. The authors gratefully acknowledge funding from the National Institute for Biomedical Imaging and Bioengineering (NIH grant EB002027) supporting the National ESCA and Surface Analysis Center for Biomedical Problems and ToF-SIMS instrumentation. D.W.D. thanks I. Braly, S. Braswell, D. Moerman, and B. Miller for valuable assistance. S.M.V. gratefully acknowledges D. Graham for assistance with ToF-SIMS. Additional data, including materials, methods, and key controls, are available online as
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Fig. 1. Solar cell device measurement, bulk PL lifetime measurement, and correlated images from (SEM) and fluorescence microscopy experiments. (A) Light current-voltage (J-V) characteristics of a highperforming mixed halide perovskite solar cell. (B) Bulk time-resolved PL decay trace of CH3NH3PbI3(Cl) perovskite film on glass after excitation at 470 nm, 125 kHz, 30 nJ/cm2 (n0~1015cm−3) and fitted to a stretchedexponential function with = 1005 ns, (τc = 431 ns, β = 0.57), with nearly single-exponential dynamics at short times (inset). (C) Correlated SEM micrograph, (D) fluorescence image, and (E) composite image showing significant variations in PL intensity across different grains and grain boundaries.
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Fig. 2. Fluorescence microscopy of CH 3NH3PbI 3(Cl) film and local PL measurements. (A) A 3 μm–by–3 μm fluorescence image of the perovskite film with bulk lifetime = 1010 ns (τc = 433 ns, β = 0.57). (B) Relative steady state PL spectra of bright (red square) and dark (blue circle) regions. (C) Time-resolved PL decay curves of bright (red square) and dark (blue circle) regions after excitation at 470 nm, 125 kHz, φ = 1 μJ/cm2 (n0 ~ 5 × 1016 cm−3), (D) φ = 2.1 μJ/cm2 (n0 ~ 1 × 1017cm−3), and (E) bright region measured at φ = 2.1 μJ/cm2 versus dark region measured at φ = 3.4 μJ/cm2 (n0 ~ 1.6 × 1017cm−3), showing that dark regions require higher initial carrier densities to exhibit kinetics dominated by bimolecular recombination. Black traces are simulations to the data (19).
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Fig. 3. Fluorescence microscopy of CH 3NH3PbI 3(Cl) film with pyridine vapor treatment. (A) Fluorescence image before and (B) after treatment showing activation of the CH3NH3PbI3(Cl) film. (C) Bulk steady-state PL spectra showing the relative PL intensities before (blue circle) and after (red square) treatment (inset) and normalized spectra showing a slight blue shift and narrowing of full width at half maximum after treatment. (D) Grain boundary PL line scan before [blue line in (A)] and after [red line in (B)] treatment, showing slight relative reduction in PL quenching across the grain boundary after treatment.
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Fig. 4. Correlated images and line scans of CH 3NH3PbI 3(Cl) film using fluorescence microscopy, SEM, and EDS. (A) SEM micrograph overlaid on fluorescence image and (B) EDS line scan showing that the local elemental weight ratio of Cl/(Cl+I) tracks areas of higher integrated PL intensity, indicating that Cl is associated with better-performing grains.
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