18. F. Humphreys, Scr. Mater. 51, 771–776 (2004).
19. G. Rohrer et al., Mater. Sci. Technol. 26, 661–669 (2010).
20. See the supplementary materials.
21. J. Hernández, J. Solla-Gullón, E. Herrero, J. Electroanal. Chem.
574, 185–196 (2004).
22. J. Cho, H. Ha, K. Oh, Metall. Mater. Trans. A Phys. Metall. Mater.
Sci. 36, 3415–3425 (2005).
23. A. Wuttig, Y. Surendranath, ACS Catal. 5, 4479–4484
24. A. S. Hall, Y. Yoon, A. Wuttig, Y. Surendranath, J. Am. Chem.
Soc. 137, 14834–14837 (2015).
25. C. H. Chen, K. E. Meadows, A. Cuharuc, S. C. Lai, P. R. Unwin,
Phys. Chem. Chem. Phys. 16, 18545–18552 (2014).
26. C. H. Chen et al., Anal. Chem. 87, 5782–5789 (2015).
27. A. Hassel, M. Seo, Electrochim. Acta 44, 3769–3777 (1999).
28. J. Perez, E. Gonzalez, H. Villullas, J. Phys. Chem. B 102,
29. A. Wuttig, M. Yaguchi, K. Motobayashi, M. Osawa,
Y. Surendranath, Proc. Natl. Acad. Sci. U.S.A. 113,
30. C. L. Bentley et al., Chem. Sci. 8, 6583–6593 (2017).
31. M. Tavares, S. Machado, L. Mazo, Electrochim. Acta 46,
32. S. Villert, C. Maurice, C. Wyon, R. Fortunier, J. Microsc. 233,
33. M. D. Vaudin et al., Ultramicroscopy 148, 94–104 (2015).
34. M. Calcagnotto, D. Ponge, E. Demir, D. Raabe, Mater. Sci. Eng.
A 527, 2738–2746 (2010).
35. K. Troost, P. Vandersluis, D. Gravesteijn, Appl. Phys. Lett. 62,
36. Y. Guo, T. Britton, A. Wilkinson, Acta Mater. 76, 1–12
37. A. Wilkinson, D. Randman, Philos. Mag. 90, 1159–1177 (2010).
38. J. Jiang, T. Britton, A. Wilkinson, Acta Mater. 61, 7227–7239
39. P. Littlewood, T. Britton, A. Wilkinson, Acta Mater. 59,
40. S. Sun, B. Adams, W. King, Philos. Mag. A 80, 9–25 (2000).
41. J. Hu, S. Chang, F. Chen, J. Kai, Mater. Chem. Phys. 74,
42. H. Abdolvand, A. J. Wilkinson, Int. J. Plast. 84, 160–182 (2016).
43. E. Nes, Prog. Mater. Sci. 41, 129–193 (1997).
44. T. Radetic, F. Lançon, U. Dahmen, Phys. Rev. Lett. 89, 085502
45. A. Ulvestad, J. N. Clark, R. Harder, I. K. Robinson, O. G. Shpyrko,
Nano Lett. 15, 4066–4070 (2015).
46. R. Valiev, A. Korznikov, R. Mulyukov, Mater. Sci. Eng. A 168,
47. V. Stolyarov, Y. Zhu, I. Alexandrov, T. Lowe, R. Valiev, Mater.
Sci. Eng. A 343, 43–50 (2003).
48. R. Ueji, N. Tsuji, Y. Minamino, Y. Koizumi, Acta Mater. 50,
49. E. J. Gwak, J. Y. Kim, Nano Lett. 16, 2497–2502 (2016).
50. M. Liu et al., Nature 537, 382–386 (2016).
51. M. Gsell, P. Jakob, D. Menzel, Science 280, 717–720 (1998).
52. M. Mavrikakis, B. Hammer, J. Nørskov, Phys. Rev. Lett. 81,
53. J. V. Barth, H. Brune, G. Ertl, R. J. Behm, Phys. Rev. B Condens.
Matter 42, 9307–9318 (1990).
54. H. A. Hansen, J. B. Varley, A. A. Peterson, J. K. Nørskov,
J. Phys. Chem. Lett. 4, 388–392 (2013).
55. S. Kobayashi, S. Tsurekawa, T. Watanabe, Beilstein
J. Nanotechnol. 7, 1829–1849 (2016).
This research was supported by NSF (CHE-1565945) and the Air
Force Office of Scientific Research Multidisciplinary Research Program
of the University Research Initiative (FA9550-14-1-0003). R.M.
gratefully acknowledges NSF for a predoctoral fellowship.
We thank C. Spence (Gatan, Inc.) for preparation of Ar ion milled
samples. EBSD was performed at the Stanford Nano Shared Facilities
(SNSF), which is supported by NSF under award ECCS-1542152. All
data are reported in the main text and the supplementary materials.
Materials and Methods
Figs. S1 to S13
Tables S1 and S2
12 July 2017; accepted 23 October 2017
A generic interface to reduce the
efficiency-stability-cost gap of
perovskite solar cells
Yi Hou,1,2 Xiaoyan Du,3 Simon Scheiner,4 David P. McMeekin,5 Zhiping Wang,5
Ning Li,1 Manuela S. Killian,6 Haiwei Chen,1 Moses Richter,1 Ievgen Levchuk,1
Nadine Schrenker,7 Erdmann Spiecker,7 Tobias Stubhan,1 Norman A. Luechinger,8
Andreas Hirsch,9 Patrik Schmuki,6 Hans-Peter Steinrück,3 Rainer H. Fink,3
Marcus Halik,4 Henry J. Snaith,5 Christoph J. Brabec1,10*
A major bottleneck delaying the further commercialization of thin-film solar cells based on
hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a
generic interface architecture that combines solution-processed, reliable, and cost-efficient
hole-transporting materials without compromising efficiency, stability, or scalability of
perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WOx)/conjugated polymer
multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts
universally with various scalable conjugated polymers. In a simple device with regular planar
architecture and a self-assembled monolayer, Ta-WOx–doped interface–based perovskite
solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light
stability. By eliminating additional ionic dopants, these findings open up the entire class
of organics as scalable hole-transporting materials for perovskite solar cells.
Thin-film solution-processed solar cells based on a hybrid organohalide lead perovskite semiconductor have achieved certified pow- er conversion efficiencies (PCEs) exceeding 22% (1). Early efforts to bring this technology from the lab to the market quickly revealed
certain disadvantages of perovskites, including
the use of toxic lead, the diffusion of ionic defects
causing a hysteresis effect, long-term stability,
water sensitivity, the complexity of the ink formulation, as well as the cost efficiency and compatibility of the interface materials (2–12). Of these, a
critical limitation on commercializing this technology is the absence of suitable hole-transporting
materials (HTMs) that offer low material costs,
printability from green solvents, and full performance without sacrificing long-term stability.
At present, state-of-the-art devices still use HTMs
such as 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)
amino]-9,9′-spirobifluorene (spiro-MeOTAD) and
(PTAA), and additives such as lithium salts, cobalt
complexes, and 4-tert-butyl pyridine (TBP) are
often used to enhance device efficiency. However,
lithium salts and TBP can cause intensive degra-
dation processes within the device. Therefore,
attention has increasingly turned to the develop-
ment of ionic dopant–free HTMs. Several groups
have observed that stability is enhanced when
dopants are either removed from the organic
HTMs or totally avoided. However, all of these
methods compromise device efficiency (13–15).
Alternative bilayer approaches that use thermal-
ly evaporated molybdenum oxides (MoOx) have
produced excellent time-zero performance but
suffer from fast degradation (16, 17). Current mate-
rial engineering strategies, such as adding cross-
linking agents to HTMs, enhance the stability of
the active material (18); however, this occurs at
the cost of more complex processing and a still
unsatisfactory protection of the fairly reactive
MoOx against perovskite’s ionic complexes (17).
In addition to stability, the costs, solubility,
processing properties, and scalability of spiro-MeOTAD and PTAA are limited and do not fulfill
the requirements for large-area module processing.
High web speed is desired in high-throughput–
processing methods such as roll-to-roll coating,
and fabrication complexity is increased because
of the short shelf life of doped HTM inks and
the required slow oxygen-doping process (19).
Carbon-based p-conjugated semiconductors have
been widely investigated over the past 20 years
of organic photovoltaic and organic field-effect
transistor development. Many have exceeded
high efficiency and/or high mobility for solution-processed lab-scale devices. The required balance
between efficiency, cost, stability, and scalability
of synthesis identified several of these polymers
that can be scaled up to industrial volume. These
low-cost and scalable polymers are the most
promising alternatives to spiro-MeOTAD if they
can function as stable HTMs for perovskite solar
cells without ionic doping.
We develop a fully solution-processed bilayer hole-extraction contact consisting of tantalum-doped
tungsten oxide (Ta-WOx) and a polythiophene derivative, poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-
which leads to perovskite solar cells with negligible hysteresis and a maximum power efficiency
of 21.2%, the highest performance observed to
date for perovskite solar cells with ionic dopant–
free HTMs. We experimentally show that the
Ta-WOx interface layer allows for a substantial
reduction in the transport barrier between an