Hydrophobic Organic Hole Transporters for Improved Moisture

Subscriber access provided by ORTA DOGU TEKNIK UNIVERSITESI KUTUPHANESI

Article

Hydrophobic Organic Hole Transporters for Improved Moisture Resistance in Metal Halide Perovskite Solar Cells Tomas Leijtens, Tommaso Giovenzana, Severin N. Habisreutinger, Jonathan S. Tinkham, Nakita K. Noel, Brett A Kamino, Golnaz Sadoughi, Alan Sellinger, and Henry J. Snaith ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10093 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 12, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Hydrophobic Organic Hole Transporters for Improved Moisture Resistance in Metal Halide Perovskite Solar Cells Tomas Leijtens1,2, Tommaso Giovenzana2, Severin Habisreutinger1, Jonathan S. Tinkham3, Nakita K. Noel1, Brett A. Kamino4, Golnaz Sadoughi1, , Alan Sellinger3*, and Henry J. Snaith1* 1

University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom

2

Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305

3

Department of Chemistry and Geochemistry, Colorado School of Mines, 156 Coolbaugh Hall, 1012 14th Street, Golden, CO 80401 4

Oxford Photovoltaics Ltd, Centre for Innovation & Enterprise, Begbroke Science Park, Woodstock Road, Oxford, OX5 1PF *Prof. Alan Sellinger *Prof. Henry J Snaith

[email protected] [email protected]

Abstract Solar cells based on organic-inorganic perovskite semiconductor materials have recently made rapid improvements in performance, with the best cells performing at over 20 % efficiency. With such rapid progress, questions such as cost and solar cell stability are becoming increasingly important to address if this new technology is to reach commercial deployment. The moisture sensitivity of commonly used organic-inorganic metal halide perovskites has especially raised concerns. Here we demonstrate that the hygroscopic lithium salt commonly used as a dopant for the hole transport material in perovskite solar cells makes the top layer of the devices hydrophilic and causes the solar cells to rapidly degrade in the presence of moisture. By using novel, low cost, and hydrophobic hole transporters in conjunction with a doping method

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

incorporating a pre-oxidized salt of the respective hole transporters, we are able to prepare efficient perovskite solar cells with greatly enhanced water resistance.

KEYWORDS Perovskite solar cells; hole transporters; stability; humidity; doping; hydrophobic

Introduction Solar cells based on organic-inorganic perovskites have recently surged to the forefront of photovoltaics research1–4. They can be solution processed, are made of low cost materials, and have tunable electronic properties. Devices based on the most commonly used CH3NH3 or HC(NH2)2PbI3 perovskite have reached impressive certified power conversion efficiencies (PCEs) of over 20 %5–9. Such rapid improvements in device performance, with clear sight of further improvements possible, means that one of the most pressing questions which remains to be answered is whether this class of solar cells can perform in a stable manner under prolonged exposure to harsh real–world conditions. We have previously demonstrated that perovskite solar cells encompassing a mesoporous TiO2 electron collecting layer suffer from an instability to ultraviolet irradiation10. The mesosuperstructured and planar heterojunction solar cells employing only compact TiO2partially overcome this problem and look to be strong candidates as high performing and stable photovoltaic technologies10,11. We have recently demonstrated further improvements in the resilience to long term operation under full spectrum sun light by entirely substituting the compact TiO2 for a solid film of C60 as the n-type collection layer12. Even so, this class of perovskite materials is unstable when exposed to water, which tends to dissolve the ionic crystal, finally degrading it to PbI213–15. While the solar cells will be


Page 2 of 30

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


encapsulated for commercial application, such encapsulation can be prone to cracking and edge leakage, and if the requirement for excessive encapsulation can be relaxed, then it is likely that the perovskite solar modules will both last longer and be less expensive. Therefore, it remains imperative to prepare solar cells with an inherent resistance to moisture ingress. One method could be to make the top layer of the solar cells more resistant to water by using a hydrophobic material. In the most conventional structures, with the electron collecting contact on the glass substrate and the hole collecting contact on top of the perovskite layer, this would mean that hydrophobic hole transport materials (HTMs) could be used to prepare more stable perovskite solar cells. We have recently improved the stability of perovskite solar cells by using a hole transport layer composed of an inert polymer such as polymethylmethacrylate (PMMA) or polycarbonate (PC) infiltrated with hole selective carbon nanotubes. This approach is very promising, but a more simple approach would be to use a hydrophobic and more water impermeable organic hole transporting material. In the most conventional device architecture, a lithium salt (lithium bistrifluoromethanesulfonimide - LiTFSI) doped (N2,N2,N2 ′ ,N2 ′ ,N7,N7,N7 ′ ,N7 ′ -octakis(4methoxyphenyl)-9,9 ′ -spirobi[9H-fluorene]-2,2 ′ ,7,7 ′ -tetramine – (spiro-MeOTAD) small molecule HTM has remained the standard, and achieves as high performance as any other HTM, although several alternative small molecule and polymeric HTMs have been proposed with the advantages of being less expensive to prepare and sometimes offering enhanced stability16 due to the hydrophobic nature of the polymers2,5,17–20.The vast majority of alternative HTMs also make use of LiTFSI as a dopant2,5,19,20. This has proven necessary to reduce series resistance losses due to hole transport in the HTM21–23. As we show here, however, the hygroscopic lithium salt used to dope the HTM makes the HTM layer highly hydrophilic, which negatively influences the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

stability of the solar cells24. A recent report by Nguyen et al25,26 demonstrates that it is possible to dope spiro-OMeTAD directly using a silver salt dopant, to yield a pre-oxidized HTM (in the form of a HTM+ TFSI- salt)26 that can be used to quantitatively dope the final HTM layer without the need for any lithium salts. This also allows for a much more controlled doping process. We have simultaneously developed this same doping technique,26,27 and here extend it to two alternative HTMs that are also simpler and less expensive to produce than spiro-OMeTAD. The simple small molecules, doped by their respective oxidized salts, can perform equally well as the conventional Li-TFSI doped spiro-OMeTAD in perovskite mesosuperstructured solar cells. By omitting lithium salts from the HTM layers we improve their water resistance, while the doping method allows us to retain high photovoltaic performance. We further demonstrate that we can improve the hydrophobic properties of the HTMs by tuning the solubilizing groups, with great benefits to the water resistance of the final solar cells.

Results and Discussion

Figure 1. Conductivity of spiro-OMeTAD doped with the commonly used Li-TFSI salt at 30 mol% (grey line), the spiro+ SbCl6- salt (red circles), and the spiro+ TFSI- salt (black squares) as a function of molar concentration


Page 4 of 30

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nguyen et al25 recently reported a synthetic route to prepare and isolate a purely oxidized salt form of the commonly used HTM, spiro-OMeTAD. They used silver tetrafluorosulfonimide (AgTFSI) as the dopant to yield a spiro2+ 2TFSI- salt which is then purified and added directly to the spiro-OMeTAD solution as a quantitative dopant26. Here, we develop this doping route to yield several different HTM+ A- salts with varying HTMs and anions. We note that we limit our study to the first oxidation state to be able to more effectively compare between the different HTMs. First, we explore the influence of the counter anion on the charge transport characteristics of the doped HTM films. If the negative charge on the counter anion is more delocalized, a higher hole mobility is expected as the holes have a weaker coulomb interaction with the anion; i.e. the more weakly coordinating the anion is, the more stable the cation and the higher the resultant hole mobility in the doped HTM. This has been previously observed for the case of PEDOT:PSS and P3HT, and predicted in the case of spiro-OMeTAD28,29. Here, we prepared spiro+ SbCl6- and spiro+ TFSI- in order to compare their conductivities and understand the influence of the anion on the hole mobility. For this experiment we employed a commonly used p-dopant, (tris(4-bromophenyl)aminium hexachloroantimonate) that contains the SbCl6- anion as the counter anion to form and then isolate spiro+ SbCl6-. We then took some of the resulting powder and performed a simple anion exchange to yield spiro+ TFSI-, as we describe in the experimental section. We plot the conductivities in Figure 1 as a function of doping density with respect to the neutral spiro-OMeTAD content. Both salts are effective p-type dopants, but it is apparent that at an equivalent molar doping fraction, the TFSI- salt results in improved conductivity as compared to both the SbCl6- salt and the optimized lithium salt doping routes, approaching values up to 10-4 S cm-1. Because the negative charge on TFSI is highly delocalized and the molecule is sterically hindered from coming into very close proximity to the cations30,31,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TFSI is known to be one of the most weakly coordinating anions available31. This has made it the anion of choice for electrochemical applications such as batteries32. Figure 1 makes it clear that these properties are also important in terms of its role as a counter anion in the radical HTM+ A- salt. Having found that the TFSI anion yields superior hole conductivities at equivalent molar doping fractions, we turned to the route reported by Nguyen et al25–27, using AgTFSI as the oxidant and source of TFSI anions. We then synthesized the oxidized salts of spiro-OMeTAD as well as two triphenyl amine – based HTMs, which we show in Scheme 1 (synthetic route shown in SI).

Scheme 1. Chemical structures, formulae, and molecular weights of the two alternative triphenyl-amine based HTMs used in this study. The synthesis route can be found in Scheme S1 of the Supporting Information.

We have previously reported that a simple HTM, termed AS44, has a suitable mobility and HOMO level (-4.89 eV vs vacuum) to allow it to function efficiently in solid-state dye sensitized solar cells33. Here, we prepare the oxidized TFSI salt of AS44 as well as a derivative of AS44 (termed EH44) where the hexyl solubilizing group is replaced by an ethyl-hexyl solubilizing group to provide slightly more hydrophobicity and solubility to the HTM. This may allow EH44 to function as a more protective HTM layer on top of the perovskite. EH44 was found to have a matching HOMO level to AS44 as determined by cyclic voltammetry (-4.85 eV)


Page 6 of 30

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shown in Figure S7. The bandgaps are 3 eV, as determined by the UV-vis spectra provided in Figure 2 and Figure S8. Differential scanning calorimetry (DSC) showed a glass-transition temperature relatively unchanged from AS44 (52 °C vs. 59 °C33), but also had no discernible melting point (Figure S9). In Figure 2, we plot the absorption of spiro-OMeTAD, AS44, and EH44 films with and without 10 molar % doping of their respective oxidized salts. All the doped HTMs exhibit the absorption features around 500, 700, and 1500 nm that are characteristic of the first oxidized state of methoxy functionalized triphenyl amines21,34,35. Notably, the oxidized HTM spectra of AS44 and EH44 are somewhat blue shifted, absorbing less in the green part of the spectrum. This will have some benefits in solar cells relying on the light reflected from the back electrode, as AS44 and EH44 will absorb less light in the visible part of the solar spectrum on the back-reflected light path36,37. The legend also denotes the conductivity values of the final doped films. All doped films exhibit high conductivities around or above the 10-5 S cm-1 required to make a perovskite solar cell with minimal series resistance due to hole transport (assuming 300-400 nm HTM capping layers)21,23,37. In fact, the conductivities of over 10-4 S cm-1 suggest that HTM layers of around a micron thick should still function well and result in minimal series resistance losses37. Identifying the oxidized species is difficult; the radical cationic nature prevents identification by NMR, and the ion pair will only show the parent cation mass in MALDI (indistinguishable from the neutral M+ peak). Previously, Nguyen et al25 were able to only report elemental analysis for their oxidized spire-OMeTAD samples. UV-Vis absorbance of the oxidized salts of EH44 and AS44 (Figure 2 and Figure S9) show the appearance of a polaronic band in the near IR along with red-shifting of the lowest energy absorbance band from the neutral species. This is consistent with destabilization of the resulting SOMO after oxidation,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

which will decrease the energy of the associated excitation. Adding another equivalent of AgTFSI increases the polaronic band and completely removes the band around 500 nm, consistent with removal of the SOMO electron to make the dication.

Figure 2. The UV-vis absorption of HTM films on glass is plotted with (red) and without (black) 10 molar % of the respective HTM+ TFSI- salt dopants. The conductivities with and without dopant are given in the legend.

Having prepared several lithium salt–free HTM layers with high conductivities, we examined their performance in mesosuperstructured perovskite solar cells, with a cell structure of fluorine doped tin oxide (FTO)/compact TiO2 (50nm) / mesoporous Al2O3 infiltrated with perovskite (300nm) / Perovskite capping layer (300nm) / HTM(400nm) / Au. The performance parameters (see Figure 3 and Table 1) for the solar cells incorporating all three materials are similar and equivalent to the standard lithium-salt doped spiro-OMeTAD HTM layer with champion JV derived PCEs of over 13 % and average PCEs of 10 %. It is clear that this route towards doping HTMs can be successfully applied to a wide variety of HTMs to prepare high performance lithium salt–free perovskite solar cells. While the optimized HTM layer thicknesses


Page 8 of 30

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were around 400 nm, the high conductivities obtained by the new doping technique enable fabrication of efficient (10 – 13 % PCEs) solar cells with HTM layers 700 - 900 nm (Figure S1) using all three HTMs. The latter finding is important for industrial processing where larger thickness tolerances are desirable. Furthermore, EH44 requires many fewer steps to prepare than spiro-OMeTAD, making EH44 a promising replacement for the commonly used but synthetically costly spiro-OMeTAD. For example, the spiro series of materials requires many additional steps just to form the 2,2’,7,7’-tetrabromospirobifluorene precursor from which the spiro-OMeTAD is prepared via Pd catalyzed Buchwald-Harwig amination chemistry38. We do note however, that the solar cells exhibited strong hysteresis in the J-V curves, as has been often reported for perovskite solar cells of this kind39–41. We have recently established a testing protocol that allows us to find the accurate steady-state performance of such devices by holding the solar cell at its maximum power point and measuring the current once the cell has stabilized39. This yields the “stabilized power output” (SPO) of the device. The grey squares in Figure 3 represent the stabilized power output of the perovskite solar cells, and show that doping spiro-OMeTAD with the spiro-TFSI salt yields comparable or superior SPOs (9.3 vs 8.5 %) compared to the standard LiTFSI doped spiro-OMeTAD. Hence, we can ascertain that this doping technique has no negative influence on the hysteretic behavior of the solar cells. The SPO of the devices employing EH44 as the HTM (again doped with its oxidized TFSI salt) are somewhat lower but similar to that for the Li-TFSI doped spiro-OMeTAD reference devices, at around 8 %. The SPO of AS44 is even lower, at 4 %. The reason for such a low SPO for the AS44 devices is unclear, but may be due to differences in recombination rates across the perovskite – HTM interface. Using a more effective electron extraction layer, such as C60, to improve electron extraction and reduce the electron density in the perovskite layer should allow



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

this deficiency to be overcome12,42. We point out that the stabilized performances of the devices are low, as has been often observed for solar cells using a planar TiO2 electron collecting layer12,43. The low SPO depends on the optoelectronic quality of the perovskite (carrier lifetime, trap densities etc) may depend on the quality of the perovskite precursors and varies across different batches over time. The good performance of the reverse sweep of the JV curves is however still a good indication that the HTMs do not limit the device performance and demonstrates that these HTMs can be readily applied to higher efficiency device structures such as those employing SnO2 electron collection layers43. Most importantly, the new HTM layers are clearly comparable in performance to the standard lithium-doped spiro-OMeTAD.


Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. The solar cell performance parameters are displayed for the different HTMs, where other than the Spiro Li-tbp reference samples, all HTMs were doped with their respective TFSI salts. PCE stands for power conversion efficiency, JSC for short circuit current, VOC for open circuit voltage, and FF for fill factor. The black dots represent the mean, the stars the maximum, and the box lines represent the 25, 0, and 75th percentiles in the distribution. The grey squares represent the stabilized power conversion efficiency (after 60 seconds stabilization).

Interestingly, we can also establish that the addition of tert-butyl pyridine (tBP) is critical to obtain solar cells with high stabilized power outputs in the LiTFSI – free solar cells. Figure S2 demonstrates that devices incorporating tBP have superior SPOs than those without. The SPO appears to be determined by the efficiency of charge extraction, and is hence related to both extraction rates and carrier recombination lifetimes5,40,41,44. We have previously been able to improve carrier lifetimes and hence the SPOs of the solar cells by surface passivation with pyridines and thiophenes45. It is possible that the tBP additive commonly used in the HTM solution performs a similar role, passivating surface defects which limit the intrinsic carrier lifetime and may also act as recombination centers at the interface. A complete analysis of the influence of tBP on SPO is beyond the scope of this work and will be the subject of future research.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

Table 1. Device parameters. The average device parameters are tabulated, while the champion efficiencies are included in italics.

Spiro Li-TFSI Spiro TFSI AS44 TFSI EH44 TFSI

PCE (%) - max 10.2 - 13.3 10.3 - 13.4 8.5 - 10.9 10.2 – 13.2

JSC (mA cm-2) 17.5 18.1 19.9 18.6

VOC (V) 0.99 0.89 0.81 0.94

FF 0.55 0.56 0.51 0.60

SPO (%) 8.4 9.3 4.2 7.9

The motivation to remove the Li-TFSI salt from the HTM layers was to improve the stability of the perovskite solar cells to the presence of water. An HTM layer composed of hydrophilic moieties will be more readily wet by water, and thus more susceptible to moisture ingress than one composed of hydrophobic moieties. Of course, the permeability to liquid water and water vapor will also depend on other factors such as pinhole density, but the hydrophobicity should play a strong role in water resistance for films of similar smoothness, and certainly for films which are well fabricated. With this in mind, we performed water contact angle measurements upon the different HTM layers used for the perovskite solar cells in this work. We present the results in Figure 4, which demonstrate that the hydrophobicity of the HTM films can be strongly affected by the presence of LiTFSI as well as the HTM chemical structure itself. We observe an increase in the contact from 56° to 77° when we replace Li-TFSI with the spiro+ TFSI- salt, demonstrating that the presence of the hygroscopic lithium cations leads to a more hydrophilic HTM layer. We also observe that the doped AS44 HTM exhibits a slightly improved hydrophobicity when compared with spiro-OMeTAD, while the doped EH44 is even more hydrophobic with a contact angle of 96°. As such, we are able to tune the hydrophobic properties of the HTM layer used in high performance perovskite solar cells by relatively minor adjustments to the HTM chemical structure. In addition, the molecular structure of this type of HTM, with the carbazole core unit, makes it especially simple to functionalize the molecule with


Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


alkyl chains which allow for both a high material solubility in useful solvents such as chlorobenzene and toluene (in which the perovskite is stable) as well as a high hydrophobicity.

Figure 4. Contact angle measurements were taken of water droplets on films of a) Spiro-MeOTAD doped with LiTFSI, b) spiro-OMeTAD, c)AS44, d) EH44 all doped with 10 mol % of their respective HTM+ TFSIsalts, and e) PMMA. Drop shape analysis was done using a Kruss Drop Shape Analyzer with 18 Mohm water and a 5uL dispense volume.

We have recently demonstrated that such hydrophobic properties are extremely beneficial to solar cell stability24. We found that moisture ingress at elevated temperatures leads to a loss of methylammonium iodide (CH3NH3+I-) from the perovskite structure, leaving just lead (II) iodide (PbI2) in the final film. This may occur through the loss of CH3NH2 and HI helped by the presence of moisture, as suggested by Walsh et al46. Recent work by ourselves47,48 and others14,15,49,50 has elucidated the mechanism by which the material degrades in the presence of water and how this is accelerated under heating. Water, either in liquid or vapor form, can infiltrate the structure to form a combination of mono- and di-hydrated materials which are in equilibrium with the perovskite phase and from which the organic component can readily escape upon heating. Elevated temperatures drive the reaction further towards the final irreversibly formed PbI2 end product by volatizing the organic cations that are loosely bound in the hydrated phases47. This occurs rapidly depending on the temperature and humidity level and results in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

formation of PbI2. Keeping water out of the perovskite film will make the solar cells both more water and heat resistant (in air). It is possible to prevent this degradation from occurring by encapsulating the samples with inert polymers such as polymethylmethacrylate or polycarbonate, which are more hydrophobic and water - impermeable in nature than the commonly used LiTFSI – doped spiro-OMeTAD HTM. However, the doped EH44 films are significantly more hydrophobic than the PMMA films. As such, the doped EH44 HTM layers should be superb moisture–blocking layers, especially when thick (> 500 nm) layers are used, for stable high performance perovskite solar cells.

Figure 5. a-d) UV-vis absorption spectra before and after 30 hrs exposure to 85 °C on a hotplate in ambient conditions (≈ 50 % humidity), for the different HTMs. e) shows the difference in absorption after the 30 hrs heat treatment for the different HTMs; spiro-OMeTAD doped with Li-TFSI (LiTFSI), spiro-OMeTAD doped with spiro-OMeTAD+-TFSI-(S+), AS44 doped with AS44+TFSI- (44+). EH44 doped with EH44+-TFSI- (EH44+).

To investigate whether the improved hydrophobic properties translate to improved CH3NH3PbI3 perovskite stability, we coated the different HTM layers onto the perovskite films and exposed them to 85°C on a hotplate in ambient conditions (room temperature relative humidity of 50 - 60 %) for 30 hrs. The reason to perform this type of aging test is that the heat


Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


accelerates the effects of water vapor-induced degradation, as we have recently proposed47 and discussed above. The measurement then serves as a quick test of how well the complete solar cell can withstand exposure to water vapor. We show the absorption spectra of the films in Figures 5a-d, and show that the films coated with Li-TFSI doped spiro-OMeTAD start to degrade, the spiro-OMeTAD doped with its TFSI salt (S+) degrades only slightly, while the AS44 and EH44 HTMs doped with their respective TFSI salts do not change over this time scale. By plotting the change in absorption before and after the aging experiment in Figure 5e, we can observe that the change is due to a loss of the characteristic absorption from the perovskite crystal (the negative features for the EH44 sample are likely to be due to slight differences in sample positioning on the different measurement days). This is consistent with our and others’ previous observation that the perovskite films decompose to PbI2 upon heat exposure in the presence of water vapor, and that this is more prominent in samples with hydrophilic top layers such as Li-TFSI doped spiro-OMeTAD14,24,51. This can, however, be slowed by removing LiTFSI from the HTM matrix, and even more so by employing hydrophobic HTM layers such as AS44 and EH44. To verify the resistance to water ingress upon complete immersion, we immersed in water perovskite films coated by Li-TFSI doped spiro-OMeTAD and TFSI salt doped (10 mol %) EH44 and monitored the change in absorption (at 700 nm) over time. We plot the results in Figure 6a and 6b, which demonstrate that while the LiTFSI doped spiro-OMeTAD coated perovskite rapidly degrades to what appear to be large yellow PbI2 crystallites (with strong light scattering properties at 700 nm) the EH44 coated perovskite films only start to show signs of degradation after almost 2 minutes of immersion. As a visual confirmation, we immersed complete solar cells made with the different HTM layers in a beaker of deionized water for 1 minute each. As we show in Figure 6c, all of the films except for that protected by



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

the doped EH44 completely degraded, leaving behind rough opaque yellow films14,15. We observe that the perovskite film protected by a doped EH44 layer however remains unchanged, demonstrating effective encapsulation just from the HTM material itself. This confirms that not only are the EH44 HTM layers more hydrophobic, but that this property translates to an improved resistance to liquid water ingress as well as water vapor ingress (confirmed via the 30 hr aging at 85 C in air). Such resistance is likely to stem from the combination of EH44’s hydrophobicity and its ability to form smooth films with few pinholes. We note that the characteristic roughness that can occur in LiTFSI–doped spiro-OMeTAD films (due to the presence of small pinholes), as seen by eye, can be mitigated just by removal of the LiTFSI and associated acetonitrile which interact with water in the air to cause changes to film wetting, or of course simply by spin coating under drier conditions. This means that the films of the HTMs doped with their own oxidized salts are generally far smoother and pinhole-free than those doped with LiTFSI if spin coated in humid air. This difference in film morphology is mitigated when the films are spin coated in relatively dry (< 30 % RH) conditions, as shown in Figure S7. Hence, films of LiTFSI doped films spin coated in dry conditions (as done throughout our study) are indistinguishable by optical microscopy from films of EH44. Despite similar film morphology, our results clearly show that EH44 forms a far more effective moisture (vapor and liquid) barrier than does LiTFSI doped spiro-OMeTAD. This is strong evidence that the hydrophobicity of the HTMs plays a vital role in determining resistance to moisture ingress when film quality is similar. Our results demonstrate that the moisture sensitivity of perovskite solar cells can be largely minimized simply by careful selection of the organic HTM layer, and that there is not a necessity to move to more complex systems such as carbon nanotubes embedded in an insulating


Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


polymer matrix. There are many factors that may affect long term stability in such solar cells, such as UV light, heat, oxygen, electrical bias and of course water exposure, making long term device stability a true challenge. Because of the many convoluting factors affecting long term device stability, we believe that directly monitoring the absorption of the films gives us the most direct evaluation of the moisture resistance of the various HTM films. As a result, we believe that our results point to an avenue towards at least minimizing the moisture instability. Concerns have also been raised over lead contamination of groundwater by leaching from cracks in eventual commercial encapsulated perovskite solar cells; protective HTMs such as the ones we describe here will help to minimize this problem.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

Figure 6. a) Depicts the change in optical density of perovskite films covered with either Li-TFSI doped spiroOMeTAD or the HTM salt doped EH44 as a function of time immersed in water. Here, samples were placed in a cuvette containing deionized water and the absorption change monitored at 700 nm. The increase in measured absorption, calculated as 1-log(transmission) corresponds to an increase in scattering due to the formation of large PbI2 crystallites, as shown in b) for the case of a Li-TFSI doped spiro-OMeTAD covered sample. c) Demonstrates the degradation that takes place in full solar cells after 1 minute of immersion in deionized water. It is apparent that EH44 can act to protect the solar cells even upon complete immersion in water. The picture is taken of the back

To summarize, we have prepared several lithium salt – free hole transport layers, including two simply synthesized small molecule alternatives to spiro-OMeTAD, for use in high performance perovskite solar cells by doping the HTMs with their pre-oxidized salt. We demonstrate similar solar cell performance for devices containing the HTMs doped with their respective oxidized HTM+ TFSI- salts as for the conventional Li-TFSI doped spiro-OMeTAD. By removing the need for Li-TFSI and by tuning hydrophobic functional groups of the new


Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HTMs, we show that it is possible to use the HTM layer as a highly hydrophobic protective layer in perovskite solar cells, which can even cope with being submersed in water.

Experimental HTM Synthesis The hole transport materials AS44 and EH44 were synthesized as described previously33. Most

materials

and

anhydrous

solvents

were

purchased

from

Sigma-Aldrich.

Dimethoxyphenylamine was purchased from TCI, catalysts were purchased from Strem Chemicals,

and

dibromocarbazole

was

provided

by

St.

Jean

Photochemicals

(http://www.sjpc.com/). 1H and 13C NMR spectra were recorded using a JEOL ECA-500 at room

temperature.

Matrix-Assisted

Laser

Desorption/Ionization

Time-of-Flight

Mass

spectrometry (MALDI-TOF MS) was performed on an Bruker Ultraflextreme MALDI-TOF mass

spectrometer

using

reflector

mode.

Samples

were

prepared

with

a

1,8,9-

trihydroxyanthracene matrix from DCM solution.

N-hexyl-2,7-dibromocarbazole. 2,7-dibromocarbzole (1.39 g, 4.2 mmol) and K2CO3 (1.10 g, 8.0 mmol) were added to a dry schlenk flask and purged with three vacuum to argon cycles to remove any moisture. Anhydrous DMF (25 mL) was added, and the reaction heated at 100 °C for 30 minutes. 1-bromohexane (0.72 mL, 5.1 mmol) was added in one portion, and the reaction heated overnight. The mixture was poured into excess deionized water and extracted with ethyl acetate. The organic portions were collected, dried over MgSO4, and the solvent removed by rotary evaporation. The crude solid was purified by flash chromatography using a hexanes to ethyl acetate gradient to yield a



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

white powder (1.68 g, 96%). 1H NMR (CDCl3, 500 MHz) δ 7.89 (d, 2H, J=8.59), 7.52 (d, 2H, J=1.40), 7.34 (dd, 2H, J=8.41, 1.43), 4.19 (t, 2H, J=7.45), 1.83 (m, 2H), 1.32(m, 6H), 0.88 (t, 3H, J=7.16).

2,7-di(N,N-dimethoxyphenylamino)-N-hexylcarbazole (AS44).

N-hexyl-2,7-dibromocarbazole (1.68 g, 4.4 mmol), dimethoxyphenylamine (2.08 g, 9.0 mmol), sodium t-butoxide (0.87 g, 9.0 mmol), and bis(tri-t-butylphosphine)palladium (10 mg, 20 µmol) were added to a dry schlenk flask and degassed by evacuation and refilling with argon. Anhydrous toluene (25 mL) was added via syringe and the reaction heated at 100 °C for 24 hours. The reaction was poured into excess deionized water and extracted with ethyl acetate. The organic portions were collected and washed three times with 5% aqueous HCl solution. The organic portions were dried over MgSO4, and the solvent removed by rotary evaporation. The crude solid was purified by flash chromatography with a hexanes to ethyl acetate gradient followed by recrystallization from isopropanol to yield a pale yellow solid (1.02 g, 33 %). 1H NMR (DMSO-d6, 500 MHz) δ 7.75 (d, 2H, J=8.59), 6.95 (d, 8H, J=9.16), 6.84 (d, 8H, J=9.16), 6.77 (d, 2H, J=1.43), 6.63 (dd, 2H, J=8.59, 1.43), 3.88 (t, 2H, J=4.00), 3.69 (s, 12H), 1.46 (m, 2H), 1.05 (m, 6H), 0.72 (t, 3H, J=6.87). 13C NMR (CDCl3, 125 MHz) δ 155.61, 141.90, 126.07, 120.52, 118.55, 117.49, 115.29, 102.30, 55.73, 31.23, 28.42, 26.50, 22.39, 14.29. MALDI-TOF MS (m/z) [MH]+ 706.978 , calc 706.364.

N-(2-ethylhexyl)-2,7-dibromocarbazole. 2,7-dibromocarbzole (2.0 g, 6.2 mmol) and K2CO3 (1.6 g, 12.0 mmol) were added to a dry


Page 20 of 30

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


schlenk flask and purged with three vacuum to argon cycles to remove any moisture. Anhydrous DMF (30 mL) was added, and the reaction heated at 100 °C for 30 minutes. 1-bromo-2ethylhexane (1.1 mL, 6.2 mmol) was added in one portion, and the reaction heated overnight. The mixture was poured into excess deionized water and extracted with ethyl acetate. The organic portions were collected, dried over MgSO4, and the solvent removed by rotary evaporation. The crude solid was purified by flash chromatography using a hexanes to ethyl acetate gradient to yield a white powder (2.5 g, 93%). 1H NMR (CDCl3, 500 MHz) δ 7.88 (d, 2H, J=8.59), 7.50 (d, 2H, J=1.20), 7.33 (dd, 2H, J=8.40, 1.15), 4.06 (m, 2H), 2.01 (m, 1H), 1.39-1.24 (m, 8H), 0.91 (t, 3H, J=7.15), 0.87 (t, 3H, J=7.45).

2,7-di(N,N-dimethoxyphenylamino)-N-(2-ethylhexyl)carbazole (EH44) N-(2-ethylhexyl)-2,7-dibromocarbazole (1.68 g, 3.8 mmol), dimethoxyphenylamine (1.93 g, 8.4 mmol), sodium t-butoxide (0.81 g, 8.4 mmol), and bis(tri-t-butylphosphine)palladium (9 mg, 19 µmol) were added to a dry schlenk flask and degassed by evacuation and refilling with argon. Anhydrous toluene (25 mL) was added via syringe and the reaction heated at 100 °C for 24 hours. The reaction was poured into excess deionized water and extracted with ethyl acetate. The organic portions were collected and washed three times with 5% aqueous HCl solution. The organic portions were dried over MgSO4, and the solvent removed by rotary evaporation. The crude solid was purified by flash chromatography with a hexanes to ethyl acetate gradient to yield a pale yellow solid (1.55 g, 52 %). 1H NMR (DMSO-d6, 500 MHz) δ 7.73 (d, 2H, J=8.02), 6.96 (d, 8H, J=8.59), 6.85 (d, 8H, J=8.60), 6.75 (s, 2H), 6.63 (dd, 2H, J=8.31,1.43), 3.72 (d, 2H, J=4.01), 1.50 (m, 1H), 1.02 (m, 8H), 0.68 (t, 3H, J=6.87), 0.59 (t, 3H, J=7.45). 13C NMR (C6D6, 125 MHz) δ 155.81,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

146.87, 142.28, 126.12, 120.24, 118.24, 114.79, 102.78, 54.72, 46.41, 39.27, 30.96, 28.95, 24.39, 22.95, 13.92, 10.64. MALDI-TOF MS (m/z) [MH]+ 734.355, calc 734.395. HTM+TFSI- preparation The HTMs were dissolved at 2 wt % in chlorobenzene. Either tris(4-bromophenyl)aminium hexachloroantimonate or silver trifluoromethylsulfonylamide was added at 95 mol % (from a solution in methanol) to ensure that primarily the first oxidation state was obtained. The mixture was stirred overnight, after which it was filtered through a 200 nm pore PTFE filter. The filtered solution was placed in a rotary evaporator to remove excess solvent and reduce the volume to 5 % of the initial volume. The remaining solution was diluted in Toluene (X 10) and cooled at 3 C overnight. A fine black powder was collected via filtration, and was rinsed with cold toluene. This step ensured that no neutral HTM was collected. The powder was then dissolved in warm methanol at 20 mg/ml. This was placed in the refrigerator overnight and the resulting black precipitate collected via filtration and washed with cold methanol. This step ensured that no unreacted dopant (eg AgTFSI) was present in the final powder. The powder was dried at 75 C overnight. Approximate yields were 50 %. In the case of EH44, hexane was used in the place of toluene and ethylene glycol was used in the place of methanol. For the anion exchange, tris(4-bromophenyl)aminium hexachloroantimonate was the pdopant, resulting in Spiro+ SbCl6-. This was dissolved in ethanol, and Et4N-TFSI added at a 10fold molar excess. The solution began to form a precipitate, and was cooled overnight. The solid was collected and rinsed in cold methanol. Solar cell preparation The solar cells were prepared as reported in the literature52. Fluorine doped tin oxide was patterned and cleaned with halmanex, acetone, and isopropanol. A compact TiO2 layer was


Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


deposited via spin coating (2000 rpm) from an acidic (35 ul of 2M HCL per 5 ml of solution ) solution of titanium isopropoxide in ethanol after which it was sintered at 450 C for 30 minutes. A mesoporous alumina scaffold was deposited via spin coating (2500 rpm) a colloidal dispersion of < 50 nm alumina nanoparticles (Sigma Aldrich, product number 702129) in isopropanol (1:2 by volume). This was dried at 150 C for 10 minutes. After cooling, a 40 wt % solution of PbCl2 and methylammonium iodide (3:1 molar ration of MAI to PbCl2) was deposited by at 2000 rpm. The films were dried in an oven at 100 C for 1 hour. The Spiro-OMeTAD (from Lumtech) hole transporter solutions were spin coated on top at 100 mg / ml and 2000 rpm to yield 200-400 nm thick films. These were either doped by addition of 0.8M tbp and 20mM LiTFSI or by adding in 10 wt % of the pre-oxidized powder with and without 0.8M tbp, as discussed in the text. To make thick films, the films were spin coated at 800 rpm. The new HTM solutions were prepared identically except that the concentration of HTM was 150 mg / ml. Current-voltage characteristics were measured under AM 1.5 100 mWcm2 simulated sunlight (ABET Technologies Sun 2000) with a Keithley 2400. The apparatus for device characterization was calibrated with an NREL certified KG5 filtered Si reference diode. The cell area under test was defined with a square aperture in a metal mask to be 0.09 cm2. Conductivity measurements These were carried out by spin coating films of the various HTMs onto a cleaned and oxygen plasma etched glass substrate. A 50 nm thick gold four-point probe electrode design was deposited via thermal evaporation through a shadow mask. The four-point probe setup was one where current was forced through the outer electrodes and voltage sensed through the inner two. The I-V curves were recorded with a Keithley 2000 sourcemeter. Similar films (without contacts) were used for the absorption and contact angle measurements.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

Contact angle measurements Drop shape analysis was done using a Kruss Drop Shape Analyzer with 18 Mohm water and a 5uL dispense volume. Drops were measured at 6 places across the substrate. Room temperature was 18 C. Cyclic voltammetry, UV-Vis Absorbance and Differential Scanning Calorimetry Cyclic voltammetry was used to characterize the HOMO energy levels of the HTMs using a Princeton Applied Research Versatek3 potentiostat in a three-electrode setup with Ag/Ag+ reference electrode, platinum wire auxiliary electrode, and platinum disk working electrode. Measurements were performed as solutions in dichloromethane with 0.1 M tetrabutylammonium hexafluorophosphate as a supporting electrolyte. Ferrocene was used as an external standard, with the oxidation onset relative to ferrocene/ferrocenium taken as EHOMO. Absorbance spectra were taken with a Beckman Coulter DU 800 spectrophotometer as dilute dichloromethane solutions. Differential Scanning Calorimetry was performed using a TA Instruments DSC Q2000 at a scan rate of 10°C/min under nitrogen flow.

Supporting Information. Reaction schemes, cyclic voltammetry, HNMR and CNRM, MALDI, DSC, and absorption spectra for AS44 and EH44. JV curves for solar cells employing thick (> 700 nm) hole transporter layers. Performance with and without tert-butyl pyridine.

ACKNOWLEDGEMENTS T. Leijtens would like to acknowledge the EU Seventh Framework (FP7/2007-2013) under the project “DESTINY” as well as Horizon 2020 Marie Curie actions under project “Crystal Solar”. We thank Oxford PV ltd for use of their facilities.


Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES

(1)

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050–6051.

(2)

Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science. 2012, 338 (6107), 643–647.

(3)

Im, J.-H.; Lee, C.-R.; Lee, J.-W.; Park, S.-W.; Park, N.-G. 6.5% Efficient Perovskite Quantum-Dot-Sensitized Solar Cell. Nanoscale 2011, 3, 4088–4093.

(4)

Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2.

(5)

Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. Il. Solvent Engineering for High-Performance Inorganic-Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897–903.

(6)

Wojciechowski, K.; Saliba, M.; Leijtens, T.; Abate, A.; Snaith, H. J. Sub-150 °C Processed Meso-Superstructured Perovskite Solar Cells with Enhanced Efficiency. Energy Environ. Sci. 2014, 7, 1142–1147.

(7)

Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. Il. Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13 (4), 1764–1769.

(8)

Leijtens, T.; Stranks, S. D.; Eperon, G. E.; Lindblad, R.; Johansson, E. M. J.; McPherson, I. J.; Rensmo, H.; Ball, J. M.; Lee, M. M.; Snaith, H. J. Electronic Properties of MesoSuperstructured and Planar Organometal Halide Perovskite Films: Charge Trapping, Photodoping, and Carrier Mobility. ACS Nano 2014, 8, 7147–7155.

(9)

NREL Solar Cell Efficiency Chart http://www.nrel.gov/ncpv/. (Accessed Nov. 14, 2016).

(10)

Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming Ultraviolet Light Instability of Sensitized TiO₂ with Meso-Superstructured Organometal Tri-Halide Perovskite Solar Cells. Nat. Commun. 2013, 4, 2885.

(11)

Pathak, S. K.; Abate, A.; Leijtens, T.; Hollman, D. J.; Teuscher, J.; Pazos, L.; Docampo, P.; Steiner, U.; Snaith, H. J. Towards Long-Term Photostability of Solid-State Dye Sensitized Solar Cells. Adv. Energy Mater. 2014, 4 (8).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12)

Wojciechowski, K.; Leijtens, T.; Spirova, S.; Schlueter, C.; Hoerantner, M.; Wang, J. T.W.; Li, C.-Z.; Jen, A. K.-Y.; Lee, T.-L.; Snaith, H. J. C60 as an Efficient and Stable NType Compact Layer in Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6 (12), 2399– 2405.

(13)

Leguy, A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P. R. F. The Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals and Solar Cells. Chem. Mater. 2015, 27 (9), 3397–3407.

(14)

Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L. Investigation of CH3NH3PbI3 Degradation Rates and Mechanisms in Controlled Humidity Environments Using in Situ Techniques. ACS Nano 2015, 9 (2), 1955–1963.

(15)

Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite upon Controlled Exposure to Humidified Air. J. Am. Chem. Soc. 2015, 137 (4), 1530–1538.

(16)

Cai, B.; Xing, Y.; Yang, Z.; Zhang, W.-H.; Qiu, J. High Performance Hybrid Solar Cells Sensitized by Organolead Halide Perovskites. Energy Environ. Sci. 2013, 6 (5), 1480– 1485.

(17)

Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316–319.

(18)

Bi, D.; Yang, L.; Boschloo, G.; Hagfeldt, A.; Johansson, E. M. J. Effect of Different Hole Transport Materials on Recombination in CH 3NH3PbI3 Perovskite-Sensitized Mesoscopic Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1532–1536.

(19)

Xu, B.; Sheibani, E.; Liu, P.; Zhang, J.; Tian, H.; Vlachopoulos, N.; Boschloo, G.; Kloo, L.; Hagfeldt, A.; Sun, L. Carbazole-Based Hole-Transport Materials for Efficient SolidState Dye-Sensitized Solar Cells and Perovskite Solar Cells. Adv. Mater. 2014, 26 (38), 6629–6634.

(20)

Li, H.; Fu, K.; Hagfeldt, A.; Grätzel, M.; Mhaisalkar, S. G.; Grimsdale, A. C. A Simple 3,4-Ethylenedioxythiophene Based Hole-Transporting Material for Perovskite Solar Cells. Angew. Chemie - Int. Ed. 2014, 53, 4085–4088.

(21)

Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.; Errico, M. E.; Kirkpatrik, J.; Ball, J. M.; Docampo, P.; McPherson, I.; Snaith, H. J. Lithium Salts as “Redox Active” PType Dopants for Organic Semiconductors and Their Impact in Solid-State DyeSensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 2572–2579.


Page 26 of 30

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22)

Cappel, U. B.; Daeneke, T.; Bach, U. Oxygen-Induced Doping of Spiro-MeOTAD in Solid-State Dye-Sensitized Solar Cells and Its Impact on Device Performance. Nano Lett. 2012, 12, 4925–4931.

(23)

Leijtens, T.; Lim, J.; Teuscher, J.; Park, T.; Snaith, H. J. Charge Density Dependent Mobility of Organic Hole-Transporters and Mesoporous TiO2 Determined by Transient Mobility Spectroscopy: Implications to Dye-Sensitized and Organic Solar Cells. Adv. Mater. 2013, 25 (23), 3227–3233.

(24)

Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J. Carbon Nanotube/Polymer Composites as a Highly Stable Hole Collection Layer in Perovskite Solar Cells. Nano Lett. 2014, 14 (10), 5561–5568.

(25)

Nguyen, W. H.; Bailie, C. D.; Unger, E. L.; McGehee, M. D. Enhancing the HoleConductivity of Spiro-OMeTAD without Oxygen or Lithium Salts by Using spiro(TFSI)2 in Perovskite and Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 10996–11001.

(26)

Hsieh, B. R.; Li, X.-C.; Sellinger, A.; Jabbour, G. E. Semiconducting Hole Injection Materials for Organic Light Emitting Devices. US Patent US6830830 B2.

(27)

Snaith, H.; LEIJTENS, T.; Abate, A.; Sellinger, A. Organic Semiconductor Doping Process. Brittish Patent Application WO2014191767 A1.

(28)

Xia, J.; Masaki, N.; Lira-Cantu, M.; Kim, Y.; Jiang, K.; Yanagida, S. Influence of Doped Anions on poly(3,4-Ethylenedioxythiophene) as Hole Conductors for Iodine-Free SolidState Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 1258–1263.

(29)

Abate, A.; Staff, D. R.; Hollman, D. J.; Snaith, H. J.; Walker, A. B. Influence of Ionizing Dopants on Charge Transport in Organic Semiconductors. Phys. Chem. Chem. Phys. 2014, 16, 1132–1138.

(30)

Brookhart, M.; Grant, B.; Volpe Jr., A. F. [(3,5-(CF3)2C6H3)4B]-[H(OEt2)2]+: A Convenient Reagent for Generation and Stabilization of Cationic, Highly Electrophilic Organometallic Complexes. Organometallics 1992, 11, 3920–3922.

(31)

Golding, J. J.; MacFarlane, D. R.; Spiccia, L.; Forsyth, M.; Skelton, B. W.; White, A. H. Weak Intermolecular Interactions in Sulfonamide Salts: Structure of 1-Ethyl-2-Methyl-3Benzyl Imidazolium Bis[(trifluoromethyl)sulfonyl]amide. Chem. Commun. 1998, 15, 1593–1594.

(32)

Sakaebe, H.; Matsumoto, H. N-Methyl-N-Propylpiperidinium Bis(trifluoromethanesulfonyl)imide (PP13-TFSI) - Novel Electrolyte Base for Li Battery. Electrochem. Commun. 2003, 5, 594–598.

(33)

Leijtens, T.; Ding, I. K.; Giovenzana, T.; Bloking, J. T.; McGehee, M. D.; Sellinger, A. Hole Transport Materials with Low Glass Transition Temperatures and High Solubility



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for Application in Solid-State Dye-Sensitized Solar Cells. ACS Nano 2012, 6 (2), 1455– 1462. (34)

Fantacci, S.; De Angelis, F.; Nazeeruddin, M. K.; Grätzel, M. Electronic and Optical Properties of the Spiro-MeOTAD Hole Conductor in Its Neutral and Oxidized Forms: A DFT/TDDFT Investigation. J. Phys. Chem. C 2011, 115, 23126–23133.

(35)

Cappel, U. B.; Gibson, E. A.; Hagfeldt, A.; Boschloo, G. Dye Regeneration by SpiroMeOTAD in Solid State Dye-Sensitized Solar Cells Studied by Photoinduced Absorption Spectroscopy and Spectroelectrochemistry. J. Phys. Chem. C 2009, 113, 6275–6281.

(36)

Bailie, C. D.; Christoforo, M. G.; Mailoa, J. P.; Bowring, A. R.; Unger, E. L.; Nguyen, W. H.; Burschka, J.; Pellet, N.; Lee, J. Z.; Gratzel, M.; Noufi, R.; Buonassisi, T.; Salleo, A.; McGehee, M. D. Semi-Transparent Perovskite Solar Cells for Tandems with Silicon and CIGS. Energy Environ. Sci. 2015, 8 (3), 956–963.

(37)

Docampo, P.; Guldin, S.; Leijtens, T.; Noel, N. K.; Steiner, U.; Snaith, H. J. Lessons Learned: From Dye-Sensitized Solar Cells to All-Solid-State Hybrid Devices. Adv. Mater. 2014, 26 (24).

(38)

Pudzich, R.; Fuhrmann-Lieker, T.; Salbeck, J. Spiro Compounds for Organic Electroluminescence and Related Applications. Emiss. Mater. Nanomater. SE - 74 2006, 199, 83–142.

(39)

Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W. Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 1511–1515.

(40)

Zhang, Y.; Liu, M.; Eperon, G. E.; Leijtens, T.; McMeekin, D. P.; Saliba, M.; Zhang, W.; De Bastiani, M.; Petrozza, A.; Herz, L.; Johnston, M. B.; Lin, H.; Snaith, H. Charge Selective Contacts, Mobile Ions and Anomalous Hysteresis in Organic-Inorganic Perovskite Solar Cells. Mater. Horizons 2015, 00, 1–8.

(41)

Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat Mater 2015, 14 (2), 193–198.

(42)

Wojciechowski, K.; Stranks, S. D.; Abate, A.; Sadoughi, G.; Sadhanala, A.; Kopidakis, N.; Rumbles, G.; Li, C.; Friend, R. H.; Jen, A. K.-Y.; Snaith, H. J. Heterojunction Modification for Highly Efficient Organic-Inorganic Perovskite Solar Cells. ACS Nano 2014, 12701–12709.

(43)

Baena, J. P. C.; Steier, L.; Tress, W.; Saliba, M.; Neutzner, S.; Matsui, T.; Giordano, F.; Jacobsson, T. J.; Kandada, A. R. S.; Zakeeruddin, S. M. Highly Efficient Planar Perovskite Solar Cells through Band Alignment Engineering. Energy Environ. Sci. 2015, 8 (10), 2928–2934.


Page 28 of 30

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44)

Wojciechowski, K.; Stranks, S. D.; Abate, A.; Sadoughi, G.; Sadhanala, A.; Kopidakis, N.; Rumbles, G.; Li, C.-Z.; Friend, R. H.; Jen, A. K.-Y. Heterojunction Modification for Highly Efficient Organic–Inorganic Perovskite Solar Cells. ACS Nano 2014, 8 (12), 12701–12709.

(45)

Noel, N. K.; Abate, A.; Stranks, S. D.; Parrott, E. S.; Burlakov, V. M.; Goriely, A.; Snaith, H. J. Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic–Inorganic Lead Halide Perovskites. ACS Nano 2014, 8 (10), 9815– 9821.

(46)

Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; Van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584–2590.

(47)

Leijtens, T.; Grancini, G.; Eperon, G. E.; Ball, J. M.; Bastianni, M. de; Wojciechowski, K.; Martino, N.; Snaith, H. J.; Petrozza, A. Mapping Electric Field – Induced Switchable Polarization and Structural Degradation in Hybrid Lead Halide Perovskite Thin Films. Adv. Energy Mater. 2015, 10.1002/aenm.201500962.

(48)

Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 10.1002/aenm.201500963.

(49)

Vincent, B. R.; Robertson, K. N.; Cameron, T. S.; Knop, O. Alkylammonium Lead Halides. Part 1. Isolated PbI64− Ions in (CH3NH3)4PbI6•2H2O. Can. J. Chem. 1987, 65 (5), 1042–1046.

(50)

Leguy, A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P. R. F. The Reversible Hydration of CH 3 NH 3 PbI 3 in Films, Single Crystals and Solar Cells. Chem. Mater. 2015, 27 (9), 3397–3407.

(51)

Liu, J.; Pathak, S.; Stergiopoulos, T.; Leijtens, T.; Wojciechowski, K.; Schumann, S.; Kausch-Busies, N.; Snaith, H. J. Employing PEDOT as the P-Type Charge Collection Layer in Regular Organic–Inorganic Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 1666–1673.

(52)

Ball, J. M.; Lee, M. M.; Hey, A.; Snaith, H. J. Low-Temperature Processed MesoSuperstructured to Thin-Film Perovskite Solar Cells. Energy Environ. Sci. 2013, 6, 1739– 1743.

TOC



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Page 30 of 30

Hydrophobic Organic Hole Transporters for Improved Moisture

Recommend Documents