VOC as a function of light intensity for large- and
small-grain devices (Fig. 3D). By linearly fitting VOC
versus log-scaled light intensity [ln(I)], we obtained
a slope of ~1.0 kBT/q (where kB is the Boltzmann
constant, T is absolute temperature, and q is
elementary charge) for a large-grain device (
hot-cast at 180°C), which suggests that a bimolecular
recombination process dominates during device
operation (29–31), similar to that observed in high-quality semiconductors such as silicon and GaAs.
In contrast, for the small-grain device (hot-cast at
100°C), a slope of 1.64 kBT/q is measured, that
is an indicator of trap-assisted recombination
(Fitting details can be found in (24)).
Finally, direct optical characterization of the
material characteristics also supports the hypoth-
esis that material crystalline quality is correlated
to grain size. Specifically, we evaluated the overall
crystalline quality of large-area grains (24) by per-
forming microphotoluminescence, absorption spec-
troscopy, and time-resolved photoluminescence
measurements on large and small grains. The
band-edge emission and absorption for large
grains (>103 mm2) were observed at 1.627 eV and
1.653 eV, respectively (Fig. 4A). As the grain size
decreased, two concomitant effects were observed:
(i) a blue shift of the band-edge photoluminescence
by ~25 meV (Fig. 4B), and (ii) linewidth broadening
of ~20 meV (Fig. 4C). Such blue shifts were predicted
by our density functional theory (DFT) simula-
tions (24) (fig. S12) and are possibly attributed to
the composition change at the grain boundaries.
The increase of emission line width at grain
boundaries can be attributed to disorder and de-
fects. We observed a bimolecular recombination
process of free electrons and holes for the large-
grain crystals by means of time-resolved photo-
luminescence spectroscopy (Fig. 4D), which is
a strong indicator of good crystalline quality
(24) (fig. S13). This is in contrast to a mono-
exponential decay observed in previous reports
for small grain size or mesoporous structures
(11, 12, 32) and with our measurements on small
grains, and is representative of nonradiative de-
cay due to trap states. These results are also con-
sistent with our earlier measurements of VOC
as a function of light intensity, which suggest
reduced trap-assisted recombination in large-
Beyond our results described above, further
enhancements in efficiency can be expected by
improving the interface between perovskite and
PCBM, obtaining better band alignment, and
using an inverted structure. From the perspective of the global photovoltaics community, these
results are expected to lead the field toward the
reproducible synthesis of wafer-scale crystalline
perovskites, which are necessary for the fabrication of high-efficiency single-junction and hybrid
(semiconductor and perovskite) planar cells.
REFERENCES AND NOTES
1. J. Burschka et al., Nature 499, 316–319 (2013).
2. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami,
H. J. Snaith, Science 338, 643–647 (2012).
3. A. Mei et al., Science 345, 295–298 (2014).
4. H. Zhou et al., Science 345, 542–546 (2014).
5. H.-S. Kim et al., Sci. Rep. 2, 591 (2012).
6. M. Liu, M. B. Johnston, H. J. Snaith, Nature 501, 395–398 (2013).
7. N. J. Jeon et al., Nat. Mater. 13, 897–903 (2014).
8. P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon, H. J. Snaith,
Nat. Commun. 4, 2761 (2013).
9. O. Malinkiewicz et al., Nat. Photonics 8, 128–132 (2014).
10. J. Seo et al., Energy Environ. Sci. 7, 2642–2646 (2014).
11. S. D. Stranks et al., Science 342, 341–344 (2013).
12. G. Xing et al., Science 342, 344–347 (2013).
13. J. S. Manser, P. V. Kamat, Nat. Photonics 8, 737–743 (2014).
14. M. Grätzel, Nat. Mater. 13, 838–842 (2014).
15. M. D. McGehee, Nat. Mater. 13, 845–846 (2014).
16. R. S. Sanchez et al., J. Phys. Chem. Lett. 5, 2357–2363 (2014).
17. H. J. Snaith et al., J. Phys. Chem. Lett 5, 1511–1515 (2014).
18. G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely,
H. J. Snaith, Adv. Funct. Mater. 24, 151–157 (2014).
19. M. Xiao et al., Angew. Chem. Int. Ed. 53, 9898–9903 (2014).
20. B. Conings et al., Adv. Mater. 26, 2041–2046 (2014).
21. A. Dualeh et al., Adv. Funct. Mater. 24, 3250–3258 (2014).
22. Q. Wang et al., Energy Environ. Sci. 7, 2359–2365 (2014).
23. H.-B. Kim et al., Nanoscale 6, 6679–6683 (2014).
24. See supplementary materials on Science Online.
25. [Editorial] Nat. Photonics 8, 665 (2014).
26. Q. Chen et al., J. Am. Chem. Soc. 136, 622–625 (2014).
27. G. Grancini et al., J. Phys. Chem. Lett. 5, 3836–3842 (2014).
28. Z. Xiao et al., Adv. Mater. 26, 6503–6509 (2014).
29. S. R. Cowan, A. Roy, A. J. Heeger, Phys. Rev. B 82, 245207 (2010).
30. C. M. Proctor, C. Kim, D. Neher, T.-Q. Nguyen, Adv. Funct.
Mater. 23, 3584–3594 (2013).
31. C. M. Proctor, M. Kuik, T.-Q. Nguyen, Prog. Polym. Sci. 38,
32. C. Wehrenfennig et al., Adv. Mater. 26, 1584–1589 (2014).
Work at Los Alamos National Laboratory (LANL) was supported
by the U.S. Department of Energy, Office of Basic Energy Sciences,
Work Proposal 08SPCE973 (W.N., G.G., and A.D.M.) and by
the LANL LDRD program XW11 (A.D.M., H.-L. W., and S. T.).
This work was done in part at the Center for Integrated
Nanotechnologies, an Office of Science User Facility. Work at
Purdue University was supported by the U.S. Department of
Energy under DOE Cooperative Agreement no. DE-EE0004946
(“PVMI Bay Area PV Consortium”). We thank C. Sheehan
for the high-resolution cross-sectional SEM images. A.J.N. and
S. T. thank C. Katan, J. Even, L. Pedesseau, and M. Kepenekian
for useful discussions as well as starting coordinates for bulk
perovskites. Author contributions: A.D.M. conceived the idea,
designed and supervised experiments, analyzed data, and
wrote the manuscript. H.-L. W. and H. T. designed the synthesis
chemistry for perovskite thin-film growth and analyzed data.
W.N. developed the hot-casting, slow-quenching method for
large-area crystal growth along with H. T. and also performed
device fabrication and solar cell testing, x-ray diffraction and
analyzed the data. J.-C.B. performed optical spectroscopy
measurements, analyzed the data under the supervision of
J.J.C. R.A. performed device modeling simulations. M.A.A.
conceived the device modeling, supervised the device modeling,
analyzed crystal growth mechanisms, and co-wrote the paper.
A.J.N. performed DFT calculations under the guidance of S. T., who
designed the DFT calculations, analyzed the data, and provided
guidance to the project. G.G. and M.C. conceived the XRD
measurements and analyzed the data, co-designed the
experiments, and contributed to the organization of the
manuscript. All authors have read the manuscript and agree
to its contents.
Materials and Methods
Figs. S1 to S21
Tables S1 to S5
9 October 2014; accepted 23 December 2014
Fig. 4. Spectrally, spatially, and temporally resolved microphotoluminescence spectroscopy.
(A) Normalized absorbance (black) and microscopically resolved PL emission spectra (red) obtained
for a large-grain sample. (B) Normalized, microscopically resolved emission spectra for different
grain sizes. (C) Relative shift and linewidth broadening of the band-edge emission as a function of
grain area (with respect to the largest grain). (D) Normalized, microscopically resolved time-correlated single-photon histograms of both a large and a small grain (black). The red and blue lines
are fits to the intensity decay considering interband relaxation, radiative bimolecular recombination,
and nonradiative decay into states below the gap (see figs. S13 and S14).