ultrahigh-quality crystalline silicon, grown at high
temperatures, offers comparable or better deep
trap densities (108 < ntraps < 1015 cm–3) (21, 22). The
exceptionally low trap density found experimentally can be explained with the aid of density functional theory (DFT) calculations performed on
MAPbBr3, which predict a high formation energy
for deep trap defects when MAPbBr3 is synthesized under Br-rich conditions (e.g., from PbBr2
and MABr), such as is the case in this study (9).
REFERENCES AND NOTES
1. National Renewable Energy Laboratory, Best Research-Cell Efficiencies; www.nrel.gov/ncpv/images/
efficiency_chart.jpg.
2. A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem.
Soc. 131, 6050–6051 (2009).
3. T. C. Sum, N. Mathews, Energy Environ. Sci. 7, 2518–2534 (2014).
4. C. Wehrenfennig, M. Liu, H. J. Snaith, M. B. Johnston,
L. M. Herz, Energy Environ. Sci. 7, 2269–2275 (2014).
5. S. D. Stranks et al., Science 342, 341–344 (2013).
6. D. B. Mitzi, Prog. Inorg. Chem. 48, 1–121 (1999).
7. Y. Tidhar et al., J. Am. Chem. Soc. 136, 13249–13256 (2014).
8. M. Xiao et al., Angew. Chem. Int. Ed. 53, 9898–9903 (2014).
9. See supplementary materials on Science Online.
10. J. J. Choi, X. Yang, Z. M. Norman, S. J. L. Billinge, J. S. Owen,
Nano Lett. 14, 127–133 (2014).
11. J. R. Haynes, W. Shockley, Phys. Rev. 81, 835–843 (1951).
12. G. Giorgi, K. Yamashita, J. Mater. Chem. A 10.1039/
C4TA05046K (2015).
13. E. Edri, S. Kirmayer, D. Cahen, G. Hodes, J. Phys. Chem. Lett. 4,
897–902 (2013).
14. M. A. Lampert, P. Mark, Current Injection in Solids (Academic
Press, New York, 1970).
15. J. R. Ayres, J. Appl. Phys. 74, 1787–1792 (1993).
16. I. Capan, V. Borjanović, B. Pivac, Sol. Energy Mater. Sol. Cells
91, 931–937 (2007).
17. A. Balcioglu, R. K. Ahrenkiel, F. Hasoon, J. Appl. Phys. 88,
7175–7178 (2000).
18. L. L. Kerr et al., Solid-State Electron. 48, 1579–1586 (2004).
19. C. Goldmann et al., J. Appl. Phys. 99, 034507 (2006).
20. Y. S. Yang et al., Appl. Phys. Lett. 80, 1595–1597 (2002).
21. J. R. Haynes, J. A. Hornbeck, Phys. Rev. 100, 606–615
(1955).
22. J. A. Hornbeck, J. R. Haynes, Phys. Rev. 97, 311–321
(1955).
ACKNOWLEDGMENTS
We thank N. Kherani, B. Ramautarsingh, A. Flood, and P. O’Brien
for the use of the Hall setup. Supported by KAUST (O.M.B.) and by
KAUST award KUS-11-009-21, the Ontario Research Fund Research
Excellence Program, and the Natural Sciences and Engineering
Research Council of Canada (E.H.S.).
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/347/6221/519/suppl/DC1
Materials and Methods
Figs. S1 to S12
References (23–44)
12 November 2014; accepted 19 December 2014
10.1126/science.aaa2725
SOLAR CELLS
High-efficiency solution-processed
perovskite solar cells with
millimeter-scale grains
Wanyi Nie,1 Hsinhan Tsai,2 Reza Asadpour,3† Jean-Christophe Blancon,2†
Amanda J. Neukirch,4,5 Gautam Gupta,1 Jared J. Crochet,2 Manish Chhowalla,6
Sergei Tretiak,4 Muhammad A. Alam,3 Hsing-Lin Wang,2‡ Aditya D. Mohite1‡
State-of-the-art photovoltaics use high-purity, large-area, wafer-scale single-crystalline
semiconductors grown by sophisticated, high-temperature crystal growth processes. We
demonstrate a solution-based hot-casting technique to grow continuous, pinhole-free
thin films of organometallic perovskites with millimeter-scale crystalline grains. We
fabricated planar solar cells with efficiencies approaching 18%, with little cell-to-cell
variability. The devices show hysteresis-free photovoltaic response, which had been a
fundamental bottleneck for the stable operation of perovskite devices. Characterization
and modeling attribute the improved performance to reduced bulk defects and improved
charge carrier mobility in large-grain devices. We anticipate that this technique will lead
the field toward synthesis of wafer-scale crystalline perovskites, necessary for the
fabrication of high-efficiency solar cells, and will be applicable to several other material
systems plagued by polydispersity, defects, and grain boundary recombination in
solution-processed thin films.
The recent discovery of organic-inorganic pe- rovskites offers promising routes for the de- velopment of low-cost, solar-based clean global energy solutions for the future (1–4). Solution-processed organic-inorganic hybrid perovskite planar solar cells, such as CH3NH3PbX3 (X = Cl, Br, I), have achieved high average power conversion efficiency (PCE) values of ~16% using a titania-based planar structure (1–7) or ~10 to 13% in the phenyl-C61-butyric acid methyl ester (PCBM)–
based architecture (8–10). Such high PCE values
have been attributed to strong light absorption
and weakly bound excitons that easily dissociate
into free carriers with large diffusion length (11–13).
The average efficiency is typically lower by 4 to
10% relative to the reported most efficient device,
indicating persistent challenges of stability and re-
producibility. Moreover, hysteresis during device
operation, possibly due to defect-assisted trapping,
has been identified as a critical roadblock to the
commercial viability of perovskite photovoltaic
technology (14–17). Therefore, recent efforts in the
field have focused on improving film surface cov-
erage (18) by increasing the crystal size and im-
proving the crystalline quality of the grains (19),
which is expected to reduce the overall bulk defect
density and mitigate hysteresis by suppressing
charge trapping during solar cell operation. Al-
though approaches such as thermal annealing
(20, 21), varying of precursor concentrations and
carrier solvents (22), and using mixed solvents
(23) have been investigated, control over structure,
grain size, and degree of crystallinity remain key
scientific challenges for the realization of high-
performance devices.
Here, we report a solution-based hot-casting
technique to achieve ~18% solar cell efficiency
based on millimeter-scale crystalline grains, with
relatively small variability (~2%) in the overall PCE
from one solar cell to another. Figure 1A schematically describes our hot-casting process for deposition of organometallic perovskite thin films. Our
approach involves casting a hot (~70°C) mixture of
lead iodide (PbI2) and methylamine hydrochloride
(MACl) solution onto a substrate maintained at a
temperature of up to 180°C and subsequently spin-coated (15 s) to obtain a uniform film (Fig. 1A). In
the conventional scheme, the mixture of PbI2 and
MACl is spin-coated at room temperature and then
post-annealed for 20 min on a hot plate maintained at 100°C. Figure 1, B to D, illustrates the
obtained crystal grain structures using this hot-casting technique for various substrate temperatures and processing solvents. We chose a
PbI2/MACl molar ratio of 1:1 in all experiments
described in this Report to achieve the basic perovskite composition and the best morphology
(see fig. S1 for images). We observed large, millimeter-scale crystalline grains with a unique
leaf-like pattern radiating from the center of the
grain [see scanning electron microscopy (SEM)
image of microstructure in fig. S2]. The x-ray diffraction (XRD) pattern shows sharp and strong
perovskite (110) and (220) peaks for the hot-casted
film, indicating a highly oriented crystal structure
(fig. S3). The grain size increases markedly (Fig. 1,
B and D) as the substrate temperature increases
from room temperature up to 190°C or when
using solvents with a higher boiling point, such
1Materials Physics and Application Division, Los Alamos
National Laboratory, Los Alamos, NM 87545, USA. 2Physical
Chemistry and Applied Spectroscopy Division, Los Alamos
National Laboratory, Los Alamos, NM 87545, USA. 3School of
Electrical and Computer Engineering, Purdue University,
West Lafayette, IN 47907, USA. 4Theoretical Chemistry and
Molecular Physics Division, Los Alamos National Laboratory,
Los Alamos, NM 87545, USA. 5Center for Nonlinear Studies,
Los Alamos National Laboratory, Los Alamos, NM 87545,
USA. 6Materials Science and Engineering, Rutgers University,
Piscataway, NJ 08854, USA. *These authors contributed equally
to this work. †These authors contributed equally to this work.
‡Corresponding author. E-mail: amohite@lanl.gov (A.D.M.);
hwang@lanl.gov (H.-L. W.)
