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Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films Philip Schulz, Anne-Marie Dowgiallo, Mengjin Yang, Kai Zhu, Jeffrey L. Blackburn, and Joseph Berry J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02721 • Publication Date (Web): 12 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016

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The Journal of Physical Chemistry Letters

Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films Philip Schulz*, Anne-Marie Dowgiallo, Mengjin Yang, Kai Zhu, Jeffrey L. Blackburn, Joseph J. Berry* National Center for Photovoltaics, National Renewable Energy Laboratory, Golden CO, 80401, USA

ABSTRACT: In spite of the rapid rise of metal organic halide perovskites for next generation solar cells, little quantitative information on electronic structure of interfaces of these materials is available. The present study characterizes the electronic structure of interfaces between semiconducting single walled carbon nanotube (SWCNT) contacts and a prototypical methylammonium lead iodide (MAPbI3) absorber layer. Using photoemission spectroscopy we provide quantitative values for the energy levels at the interface and observe the formation of an interfacial dipole between SWCNTs and perovskite. This process can be ascribed to electron donation from the MAPbI3 to the adjacent SWCNT making the nanotube film n-type at the interface and inducing band bending throughout the SWCNT layer. We then use transient absorbance spectroscopy to correlate this electronic alignment with rapid and efficient

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photoexcited charge transfer. The results indicate that SWCNT transport and contact layers facilitate rapid charge extraction and suggest avenues for enhancing device performance.

TEXT: Photovoltaic devices based on hybrid organic-inorganic perovskite absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiencies of over 20%.1 In the scientific community these technologic advances supply a tremendous driving force to understand the fundamental physical and chemical properties of this material class.2 In particular, the underlying mechanisms with respect to charge separation and collection remain unresolved, and the understanding of electronic coupling to adjacent charge extraction and transport layers is in its infancy. Yet, electronic alignment at the interface between the perovskite absorber and transport layer is likely to become a key ingredient for designing next generation devices which overcome the current performance limits.3 A recent analysis pointed out that better substitutes for current functional layer and contact materials used in perovskite cell architectures could not only lower the overall manufacturing costs but reduce the environmental imprint of this technology.4 Recently we discovered, through direct and inverse photoemission spectroscopy (PES) experiments, that the electronic structure of organic materials employed in charge carrier transport layers directly impacts the device characteristics of perovskite solar cells in both conventional and inverted device geometries.5-6 A mismatch between the transport-relevant energy levels of the charge transport layer and perovskite film


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could introduce a barrier detrimental for carrier extraction.7 Moreover, impeding carrier selectivity at the respective interface could lead to carrier cross-over and undesired recombination. To date the stability of encapsulated devices has raised concerns, although there are demonstrations of robust non-encapsulated perovskite solar cells based on a porous scaffold and a carbon top contact in a “hole-conductor free” device.8 Similarly, the use of single-walled carbon nanotubes (SWCNT) as a viable contact layer has been recently demonstrated to improve carrier collection and also enhance cell durability.9-10 In particular, the stability of more conventional perovkite solar cells (especially with respect to moisture ingress) was dramatically improved by using a SWCNT network embedded in an inert and resilient polymer matrix.11 SWCNTs exhibit unique electronic properties which can be tuned by the inherent design of the individual tubes themselves but also by adjusting covalently and non-covalently bonded atomic, molecular and ionic functionalizations attached to the SWCNT structure.12 These properties range from metallic behavior, to degenerately doped semiconducting SWCNTs with zero band gap, to true semiconducting SWCNTs with diameter-tunable band gaps.12-14 In the last decade new synthesis routes and technological advances in SWCNT separations, doping, and ink formulations have led to high-precision deposition of functional and opto-electronically tailored SWCNT layers for charge extraction and transport.15-20 These advances, in turn, should enable the integration of tailored SWCNT charge transport layers that exhibit a high optical transmission in the visible range and doping characteristics that fit either electron or hole transport for a wide variety of perovskite solar cell architectures. Despite the initial successes of laminated carbon contacts, it remains unclear how the highly customizable electronic properties of the SWCNT layers may be leveraged to optimize charge


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extraction in the device. Beyond the demonstration of efficient and stable perovskite devices with SWCNT hole transport layers, Habisreutinger, et al. clearly demonstrated charge extraction into semiconducting SWCNTs via steady-state photoinduced absorption spectroscopy.10 However, as of yet the ground-state coupling of electronic transport levels of semiconducting SWCNT to the respective transport levels in the perovskite material, crucial for promoting fast charge separation and/or slow charge recombination, have not been documented. Additionally, the details of excited-state charge separation and recombination are presumably in the range of femto- or picoseconds to micro- or milliseconds, respectively, and have not been explored at all for these interfaces. Moreover, there is evidence that SWCNT may have a significant number of traps and tail states in the band gap which can for instance lead to trap-mediated recombination.21 So far the role in charge transfer to an adjacent perovskite layer in unclear due to the lack of a direct experimental assessment of this interface. Thus, first and foremost, direct measurements of the electronic processes occurring at the interface between perovskite absorber and respective SWCNT contact are required. In the present study we report on the direct determination of the energy level alignment at the junction between methylammonium lead iodide (MAPbI3) and a near-monochiral (6,5) SWCNT overlayer by ultraviolet and X-ray photoemission spectroscopy (UPS/XPS) (Figure 1a). In addition we monitor the implications of the electronic coupling on photoinduced charge transfer across this interface by transient absorption spectroscopy (TAS), utilizing both charge-related spectroscopic signatures for the selected semiconducting (6,5) SWCNTs and the band-edge bleach of the MAPbI3 layer. In the selection process the SWCNT are wrapped with the high band gap material polymer PFO Bis-Pyridine (PFO-BPy). Several previous studies suggest that the electronic energy levels of this polymer-wrapping lie well outside the bandgap of the SWCNTs


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(Type I alignment) and are therefore not expected to induce charge transfer between polymer/SWCNT.22 It follows, and recent studies indicate, that the wrapping polymer does not hinder interfacial charge transfer.21,23 Material quality of both compounds, MAPbI3 and SWCNT, and crystal phase purity of the perovskite layer are confirmed by UV-Vis absorption spectroscopy and X-Ray diffraction (Figure 1b, 1c). Our findings indicate a unique interfacial alignment process including the formation of an interface dipole, due to electron donation from the MAPbI3 film to the adjacent SWCNT film. Core-level XPS measurements suggest that this charge transfer appears to occur selectively from the MA compound. Consequently, the SWCNT layer becomes n-type at the interface, inducing band bending within the SWCNT layer that appears to facilitate efficient hole extraction from the MAPbI3 valence band into the region of the highest valence state (V1) of the SWCNT film. The evolution of the electronic structure at the MAPbI3/SWCNT interface is tracked by gradually increasing the SWCNT layer thickness on top of a perovskite absorber layer as used in the conventional solar cell geometry (MAPbI3/TiO2/FTO/glass). The perovskite film is conformally grown on top of a compact titanium oxide layer by using a solvent-engineering approach.24 The films used in this study are highly relevant for device technology and produce power conversion efficiencies on the order of 15% on concurrently produced solar cells with the standard spiro-MeOTAD hole transport layer (see supplement figure S1). The UPS measurements on this sample structure are shown in Figure 2. All energies are given as binding energies with respect to the Fermi level at EF = 0. The work function, i.e. difference between EF and the vacuum level (Evac), is determined from the secondary electron cut-off depicted in figure panel 2a whereas figure 2b shows the valence band region with a semi-logarithmic plot of the gap state region in the inset.


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The work function of the bare MAPbI3 film amounts to 4.2 eV which is similar to values reported earlier for comparable films.5-6 After spray-coating with 2 nm of SWCNT the work function drops by over 0.2 eV down to 4.0 eV which reflects a change in the surface potential. Note that a 2 nm thick porous SWCNT/polymer layer corresponds to roughly half a monolayer equivalent if the SWCNT were considered to be uniformly distributed, so for convenience we use the approximation of 4 nm as roughly one monolayer. For additional details on the determination of SWCNT film thickness, see the detailed discussion in the Methods section. When increasing the SWCNT layer thickness to 4 nm the work function remains between 4.0 and 4.1 eV. From this we can conclude that an interface dipole is formed immediately as we establish roughly one monolayer equivalent (~ 4 nm) of SWCNT coverage on top of the MAPbI3 film. Spray-coating of thicker SWCNT films, 9 nm and 20 nm, leads to an increase in work function up to 4.2 eV and 4.5 eV, respectively. This result can be interpreted as screening of the initial interface dipole as we advance through the SWCNT layer away from the MAPbI3/SWCNT interface. The valence band maximum for the plain MAPbI3 film is found at 1.5 eV below EF derived from a linear fit to the leading edge of the valence band and a lower limit of 1.2 eV below EF for the onset. The latter value leads to an ionization energy (IE) of 5.4 eV which is in good agreement with previous studies.5-6,25 Given a band gap of 1.7 eV, the perovskite is found to be n-type as reported earlier for films grown on titania substrates.5 This single particle band gap has been derived from direct and inverse photoemission measurements and is therefore kept consistent for the value determined by this methodology in this study. Given the measurement uncertainty of 0.1 eV these values are compatible with the commonly reported band gap of 1.65 eV.26


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As the first layer of SWCNT (2 nm) is deposited on top of the perovskite a clear emergence of the SWCNT valence band electronic structure is observed with two distinct features peak features centered at 2.5 eV and 3.8 eV below EF. For reference a 9 nm thick film of SWCNT on top of a gold surface has been measured for correct peak assignments (see supplement figure S2). Note that in the following we label the distinctive van Hove singularities of the valence band features of the SWCNT film as V1, V2, … Vn and the respective features in the conduction band C1, C2, … Cn. V1 and C1 are analogously used for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively. Direct determination of the SWCNT V1 peak center, which would correspond to the valence band maximum, remains ambiguous as a significant amount of tail state DOS is detected down to a binding energy of 0.7 eV below the Fermi edge which potentially obscures the position of the SWCNT V1 level. However, from the semi-log depiction in the inset of figure 2b a broad feature can be distinguished which lines up with the V1 level projected from the V4 and V3 level according to the energetic differences found for the reference SWCNT film (figure S2). With these considerations in mind, the center of the V1 level is located at a binding energy of 1.1 eV with respect to EF. Projecting the optical transition energy of 1.2 eV (Fig. 1b) and the exciton binding energy (0.2-0.3 eV)26-27 on these energy levels yields a position for the conduction band minimum at 0.1 eV above EF, which indicate that this very thin (2 nm) SWCNT layer directly adjacent to the perovskite layer is n-type. The ionization energy (IE) is the distance from the V1 level to the vacuum level, which amounts to 5.1 eV for the 2 nm thick SWCNT film. As the SWCNT layer thickness is increased the position of all valence band features described above undergoes a uniform shift. For a 4 nm thick SWCNT layer the spectra appear shifted by less than 0.1 eV towards higher binding energies, which indicates that at a nominal coverage of one


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monolayer equivalent the SWCNT layer on top of the perovskite is slightly more n-type than at the half monolayer equivalent coverage. As the thickness of the SWCNT film is increased to 9 nm, all valence band features are shifted by 0.2 eV towards lower binding energies. This behavior is concurrent with the previously discussed change in work function and indicates that the SWCNT layer becomes less n-type. Finally, at an overlayer thickness of 20 nm, tail state onset and molecular orbital peaks are shifted by additional 0.3 eV towards lower binding energies which leads to the conclusion that the SWCNT layer returns to a near-intrinsic state further away from the interface to the perovskite film. These results can be summarized in a band diagram depicted in Figure 3. In accordance with earlier studies, the MAPbI3 film on top of TiO2 is n-type.5 By forming the interface to an adjacent (slightly p-type) SWCNT layer an interface dipole amounting to 0.2 eV is formed while at the same time the SWCNT layer becomes n-doped in the interface region. A charge carrier (hole) transiting from the valence band in the MAPbI3 film onto the V1 level of the SWCNT film would encounter no potential barrier if we consider the maximum DOS onset to be relevant for charge transfer across the interface. In contrast, a hole residing on the SWCNT side of this contact would see an injection barrier of about 0.4 eV for being transferred back to the MAPbI3 valence band. However this potential step would be lower or perhaps non-existent if we assume direct transition between the VBM of the MAPbI3 film and the SWCNT V1 level disregarding the tail state DOS in the latter case. This scenario, with respect to band offsets and charge carrier injection barriers, is comparable to previously investigated MAPbI3/spiro-MeOTAD junctions.5 Further away from the interface the SWCNT film becomes intrinsic again by undergoing a total band bending of over 0.5 eV which amounts to nearly half the optical band gap. As the Fermi level position in the gap is changed in this process, the vacuum level tracks with the valence


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band position in the SWCNT film. This scenario is consistent with a concomitant shift of the valence band and change in work function by 0.5 eV when comparing the 2 nm thick SWCNT film to the 20 nm thick SWCNT film on top of MAPbI3. As a consequence, the ionization energy of the SWCNT layer remains at the equilibrium position of 5.2 eV. XPS measurements were performed to further probe band bending effects in both layers, MAPbI3 and SWCNT, as a function of SWCNT overlayer thickness. A summary of XPS core level spectroscopy data for the atomic constituents in all layers are shown in Figure 4. The Pb 4f and I 3d core level (supplementary information, figure S4) originate from the PbI sub-lattice in the MAPbI3 film. Increasing the SWCNT overlayer thickness lead to a uniform attenuation in intensity of these core level signals, which is representative for a conformal coverage as a result of the good control in the SWCNT spray-coating process.16 Moreover the lead and iodine peak positions remain unchanged with increasing SWCNT film coverage. This finding indicates that the band bending of the SWCNT film in the UPS experiments is not mirrored in the PbI sublattice of the MAPbI3 film. Similar results were obtained previously for thin films of the organic hole transport materials spiro-MeOTAD and MePTCDI deposited on top of perovskite absorbers which did not cause any band bending in the underlying perovskite film.5 A more complete picture of the interfacial electronic alignment can be derived from the N 1s and C 1s core level regions in figure 4a and 4b. Starting with the plain MAPbI3 film only one significant N 1s peak is observed at a binding energy of 402.40 eV, which corresponds to the N in the MA sub-lattice. As the SWCNT layer is deposited on top, a second peak appears centered at a binding energy of 398.95 eV, which originates from the wrapping PFO-BPy polymer. As the SWCNT thickness is increased, a continuous shift of this PFO-BPy related N 1s core level towards lower binding energies is observed. Transitioning from a 2 nm to a 20 nm thick SWCNT


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coating this shift amounts to over 0.35 eV, which is in line with the band bending determined from the shift of the valence band observed in the UPS measurements. Since the core levels of the PFO-BPy species (C 1s and N 1s) as well as the core levels in the SWCNT species (C 1s) are referenced to the same Fermi level, band bending (i.e. the shift of EF within the band gap) translates to the same magnitude in peak shifts for both components of the SWCNT/PFO-BPy film. Note that the electron attenuation length in XPS and UPS differ from each other due to differences in the kinetic energy of the emitted electrons. In this sense the UPS result is more representative of the energy level positions at the very surface layer while the XPS result averages over multiple layers. Another subtle but robust observation can be made for the N 1s component stemming from the MA group in the perovskite. Even at a SWCNT overlayer thickness of 9 nm the feature can still be observed and a minimal shift of roughly 0.06 eV towards lower binding energies compared to the peak position for the plain perovskite film is observed. Evaluation of the C 1s core level region yields analogous results. Here, the methylamine related C 1s peak is found at a binding energy 286.40 eV for the plain MAPbI3 film whereas the signal from the SWCNT/PFO-BPy complex is observed at a binding energy of 284.95 eV for a 2 nm SWCNT overlayer. Again, the peak position of the SWCNT/PFO-BPy signal shifts by 0.35 eV over the course of 20 nm SWCNT film thickness in accordance with the band bending. Control experiments on neat PFO-BPy films indicate that the SWCNTs contribute most significantly to this C 1s envelope, although the PFO-BPy carbon atoms contribute some intensity on the low binding energy side of the peak. The C 1s signature of the MA ion undergoes a shift of almost 0.1 eV with the first 2 nm of SWCNT film on top. However, a meaningful evaluation of this component for the thicker SWCNT films is prohibited by the adjacent overlaying C 1s signal of the SWCNT/polymer complex.


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The shift of both the N 1s as well as C 1s, components of the MA moiety are close to the accuracy limit for the respective XPS measurements and peak fitting procedures, but are statistically significant. Importantly, the change in peak position for these MA-related peaks is quantitatively different from the case of the static Pb and I core levels which indicates that charge transfer specifically between the MA group in the perovskite and the SWCNT appears to take place and be the cause for the doping of the first monolayer of the adjacent SWCNT film. Indeed, electron donation from methylamine groups to SWCNT networks, resulting in n-doped nanotubes, has been found in our previous studies of SWCNT thin films.18 In the present case, competing electron donation mechanism from surface MA molecules to either the iodide sublattice in the perovskite or the adjacent SWCNT layer can be envisioned as visualized in figure 4d. With a clear picture of the interfacial band alignment, we turned to transient absorption spectroscopy (TAS) experiments to probe excited state charge transfer across the SWCNT/MAPbI3 interface. TAS experiments were carried out on films consisting of glass with a layer of MAPbI3 (glass/MAPbI3), FTO-coated glass with layers of compact TiO2 and MAPbI3 (TiO2/MAPbI3), and TiO2/MAPbI3 produced with an overlying layer of monochiral (6,5) SWCNTs ~9 nm thick (TiO2/ MAPbI3/(6,5)). Each film was excited using ~120 femtosecond laser pulses centered at an excitation wavelength of 400 nm, where the pulse entered the film architecture through the MAPbI3 or TiO2 layer first. TAS spectra were collected using visible and near-infrared (near-IR) probe wavelengths where MAPbI3 exhibits a strong bleach signal at ~750 nm due to the band edge transition (Fig. 5d inset) and a broad induced absorption (IA) in the near-IR, both of which are due to the presence of photoexcited charge carriers (Fig. 5a, see also Fig. S7).28-32 Upon addition of an overlayer of (6,5) SWCNTs (Fig. 5b), a narrow bleach


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appears at 1008 nm following 400 nm excitation, superimposed on the broad near-IR perovskite IA. This bleach corresponds to the S11 exciton transition of the (6,5) SWCNTs, and suggests that charges (presumably holes)10-11 have been transferred from the MAPbI3 layer to the SWCNTs (vide infra). At long delay times (>1 ns), the broad IA from MAPbI3 has decayed almost completely, and the S11 exciton bleach remains, along with an IA at 1174 nm that has previously been assigned to a trion or charged exciton (X+) on the SWCNT.23 The S11 and X+ transitions can be seen more clearly in the inset in Fig. 5c at a delay time of 3.8 ns. Both the S11 bleach and the trion IA result from charge carriers remaining in the (6,5) SWCNTs at long delays following photoexcitation of the MAPbI3 layer. Specifically, as expected by the band diagram in Fig. 3 and previous studies,10-11 photoinduced hole transfer should occur from MAPbI3 to the SWCNTs. Subsequently, holes in the SWCNT would induce an S11 bleach at 1008 nm due to state filling. At the same time the SWCNT hole can bind to an exciton generated from probe pulses centered at ~1174 nm to form a trion, a three-body quasiparticle consisting of an exciton with an additional hole.23 In contrast to the trion IA that appears solely when charges are present on the SWCNTs, the S11 bleach can be observed when either charges or excitons (or both) are present. However, control experiments performed on neat (6,5) SWCNT films at a photon fluence of ~2.5 × 1011 photons pulse-1 cm-2 (see Methods) showed no appreciable signal at these fluences. Additionally, while the band diagram in Figure 3 does not allow us to exclude exciton or electron transfer, either one of these scenarios would result in rapid recombination since the exciton lifetime in (6,5) s-SWCNT films is very short.33 Importantly, the S11 bleach is long-lived for the TiO2/MAPbI3/(6,5) film compared to the signal at 1008 nm for the bare TiO2/MAPbI3 film (Fig. 5c). All of these considerations imply that the S11 bleach and trion IA observed in the


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TiO2/MAPbI3/(6,5) sample are consistent with hole transfer from the perovskite layer to the (6,5) SWCNTs. In addition, the dynamics of the band edge bleach signal at 750 nm (1.65 eV) were examined for glass/MAPbI3, TiO2/MAPbI3, and TiO2/MAPbI3/(6,5) following 400 nm pump excitation (Fig. 5d). Interestingly, there is little difference between the kinetics at 750 nm for the glass/MAPbI3 and TiO2/MAPbI3 films, suggesting inefficient electron extraction by the compact TiO2 layer over the ~5 ns experimental window probed here. This observation agrees with a recent time resolved microwave conductivity (TRMC) study on similarly prepared TiO2/MAPbI3 samples. Specifically, that study reported an identical yield of charge carriers following the ~4 ns pump pulse when a MAPbI3 layer was grown either directly on quartz or on a compact TiO2 layer.34 Based on supporting evidence that the microwave probe in that experiment primarily probed mobile electrons in the MAPbI3 layer, this observation indicates negligible electron transfer from MAPbI3 to the compact TiO2 layer over the window of our current TAS experiment. In contrast, the transient band edge bleach kinetics of the MAPbI3 layer decay faster in the presence of the (6,5) SWCNT layer. This decay coincides with the rise of the (6,5) S11 bleach, indicating that both signals reflect the efficient hole extraction by SWCNTs (see also Figure S8, which corrects the S11 bleach signal for the overlapping broad IA from the perovskite). Our TAS results suggest that holes are extracted by semiconducting SWCNTs on a much faster time scale than electron extraction by the compact TiO2 layer in these tri-layer films. Wojciechowski, et al. also found that an organic contact (a C60-based monolayer) dramatically enhanced charge extraction (electrons in this case) relative to the compact TiO2 layer.34 The PES measurements in our study suggest that these dramatic differences may result, at least in part,


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from the details of the interfacial ground-state charge transfer reactions that can establish beneficial interfacial band offsets and facilitate charge transfer and separation. Specifically, Wojciechowski, et al. propose that interface states in the compact TiO2 layer create undesirable band-bending in the perovskite layer that ultimately results in an electrostatic barrier for electron transfer across this interface.34-35 In contrast, our PES results on the SWCNT/MAPbI3 interface suggest that no such band bending is observed in the perovskite layer at the MAPbI3/SWCNT contact. Rather our results indicate the presence of a beneficial band bending within in the SWCNT layer, which could help to shuttle holes away from the interface and thus presumably hinder the back-transfer of holes into the MAPbI3 layer, resulting in slow recombination. These results motivate future time-resolved studies at longer time delays to specifically probe the dynamics of recombination. In conclusion, we report on the electronic energy level alignment at the MAPbI3/SWCNT interface, which leads to the formation of a unique charge transfer mechanism. Electron donation appears to be potentially from the MA molecule to the SWCNTs resulting in an interface dipole and n-doped SWCNTs directly adjacent to the perovskite layer. We speculate that this process or similar mechanism might potentially stabilize the MAPbI3 surface and lead to more resilient devices. However, from the data it is clear that thicker SWCNT layers undergo band bending and return to an intrinsic state further away from the perovskite/nanotube interface. This ground state charge transfer at the interface should facilitate photoexcited charge transfer in the vicinity of this junction and hinder recombination. Transient absorbance measurements are consistent with holes being rapidly and efficiently transferred from the MAPbI3 valence band to the adjacent SWCNT layer. These results provide mechanistic information regarding the role of SWCNT layers as efficient hole extraction layers in perovskite-based photovoltaic devices.


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Methods

Perovskite film preparation: Fluorine-doped tin oxide (FTO, TEC15, Hartford, IN) was cleaned by an overnight base bath soaking (5 wt% NaOH in ethanol). A compact TiO2 layer was deposited by spray pyrolysis of 0.2 M titanium diisopropoxide bis(acetylacetonate) in 1-butanol solution at 450 oC. A modified method based on the solvent engineering approach was used to deposit the perovskite film.24 Therein the precursor was made of 44 wt% of 1:1 molar ratio of MAI and PbI2 in γButyrolactone (GBL, Aldrich)/dimethyl sulfoxide (DMSO, Sigma-Aldrich) (7/3 v/v). The substrates were span at 4500 rpm for 50 s with a drop of toluene being casted during the spinning. The perovskite film was fully crystalized by annealing at 85 oC for 10 min. Perovskite films on glass substrates for the TAS control measurements were prepared in the same manner except without the TiO2 spray pyrolysis process.

Carbon nanotube layer preparation: CoMoCat SG65i SWCNTs were purchased from Southwest Nanotechnologies, Inc., PFOBPy was purchased from American Dye Source, and laser vaporization (LV) nanotubes were synthesized in house. SWCNTs were dispersed in ~2.5 mg/mL PFO-BPy in toluene through tip sonication for 30 minutes (Cole-Palmer CPX 750) in a bath of cool (18°C) flowing water, followed by ultracentrifugation for 5 minutes using an SW32Ti motor (Beckman) at 13,200 rpm and 20°C. The supernatant was retained and contained predominantly (6,5) or semiconducting LV SWCNTs. In order to remove excess polymer, the supernatant containing the chirality-sorted


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SWCNTs was centrifuged for 20 hours at 24,100 rpm and 0°C. In this case, the resulting supernatant was discarded and the pellet containing (6,5) or LV SWCNT material was redispersed in toluene. This polymer removal process was repeated until a 1:1 SWCNT:PFOBPy mass ratio was obtained. (6,5) and LV SWCNT thin films ~10 nm thick were prepared through ultrasonic spray deposition onto the TiO2/MAPI films using a dispersion flow rate of 0.25 mL/min and gas flow rate of 7.0 std L/min.16 The nozzle power was fixed at 0.8 W and the substrate was heated to 130°C to allow for evaporation of the solvent. After spraying the films, they were soaked in a hot toluene bath (80°C) to remove some residual polymer and better couple SWCNTs within the film. In order to ensure the films were in an inert atmosphere during the TA experiments, they were introduced into a helium glovebox, and another quartz slide was sealed on top of the sample using a polymer film (Surlyn, Solaronix) heated to 90°C. The polymer was cut into a hollow frame so that it only sealed the outer edges of the slide and did not interfere with the sample. The thickness values given should be viewed as effective thicknesses, as it is technically challenging to directly measure the thickness of the very thin 2 and 4 nm thick films. The methods determining film thickness are provided in detail in our recent study.16 The thickness of the SWCNT film is controlled by the number of spray coats from an ink of a given concentration. The determination of the effective thickness of our SWCNT layers is produced using a calibration curve. To produce this calibration curve, we take a single SWCNT ink and generate a series of films with varying numbers of spray coats (which results in films with varying thickness). After performing the toluene soak on all samples, we measure the thickness of each sample by atomic force microscopy (AFM) and also measure absorbance. We then plot the optical density of the S11 excitonic transition versus the AFM-measured film thickness for the


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series of samples to generate the calibration curve. This calibration curve then lets us determine the “effective thickness” of very thin samples (i.e., thinner than 4 nm). With regards to our delineation of half-monolayer and full monolayer, these are convenient estimates for the degree of SWCNT coverage on the perovskite surface, based on the properties of the films produced by ultrasonic spraying.16 Since the SWCNTs are 0.8 – 1.3 nm in diameter and the polymer:SWCNT ratio is ~1:1, we estimate that the full thickness of the SWCNT/polymer hybrid is on the order of ~2 nm. As the filling fraction of our SWCNT films is approximately 50% (alternatively ~50% void space), a 2 nm film represents roughly a half monolayer of SWCNTs (a full monolayer of SWCNTs being uniformly deposited onto the substrate with no void space). Perovskite/SWCNT interfaces were prepared by spraying different perovskite substrates with varying thicknesses of SWCNT layers. For a given series of measurements, the SWCNT layers were all sprayed from the same SWCNT ink and the SWCNT layer thickness was controlled by the number of spray coats on a particular perovskite substrate. For each SWCNT layer sprayed onto a perovskite substrate, a corresponding witness slide was prepared (beside the perovskite substrate, in the same spray run) on a glass substrate. We then estimated the SWCNT layer effective thickness by measuring the absorbance of the witness slide and using the calibration curve (discussed above) to correlate the optical density of the S11 peak with thickness. Figure S5 demonstrates the reproducibility of this method for producing a series of interfaces with controllable variance of SWCNT thicknesses. Photoemission Spectroscopy: Photoemission Spectroscopy measurements were performed on a Kratos NOVA spectrometer calibrated to the Fermi edge and core level positions of sputter-cleaned metal (Au, Ag, Cu, Mo) surfaces. Ultraviolet photoemission spectra were acquired from the He I (21.22 eV)


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excitation line at a nominal experimental resolution below 150 meV. X-ray photoemission spectra were taken using monochromated Al Kα radiation (1486.7 eV) at a resolution of 400 meV (pass energy 10 eV, step size 0.025 eV, dwell time 120 ms). XPS spectra were fit using Pseudo-Voigt profiles. The acquired spectra were all referenced to the previously determined Fermi level of the sputter-cleaned metal calibration samples. The robustness of the photoemission measurements was verified by repeating the experiment for three different batches of perovskite films as well as for two batches of (6,5) SWCNT and laser vaporized SWCNT.

Transient Absorption Spectroscopy: Steady state absorbance measurements were carried out on a Varian Cary 500 spectrophotometer. Femtosecond pump-probe TAS experiments were performed on a 1-kHz regeneratively amplified Ti:Sapphire laser system that produces 4 mJ laser pulses at 800 nm. The Ti:Sapphire laser pumps an optical parametric amplifier (OPA) to generate 400 nm light, which was chopped at a rate of 500 Hz and used as the excitation pump pulse. The pump fluence was 5 × 1013 and 3 × 1012 photons pulse-1 cm-2 for near-IR and visible probe measurements, respectively. In Figures 5 a-c, a pump fluence of 5 × 1013 photons pulse-1 cm-2 was used where the perovskite layer only transmits ~0.5% of the light at 400 nm; hence the pump fluence reaching the nanotube layer is approximately 2.5 × 1011 photons pulse-1 cm-2. Near-IR (800 nm < λprobe < 1700 nm) and visible (400 nm < λprobe < 800 nm) continuum probe pulses were generated by passing a portion of the amplified 800 nm light through a sapphire plate. The probe pulse was delayed in time with respect to the pump pulse using a motorized translation stage mounted with a retroreflecting mirror. The pump and probe pulse were spatially overlapped at the quartz slide,


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and the sample was excited through the TiO2 layer first. Typical averaging times for data collection were 5-8 seconds, at each pump-probe delay, to achieve high signal-to-noise. The instrument response function (IRF) was 117 ± 5 fs. The measurements were carried out from 5 ps before time zero to 5.6 ns after time zero.

FIGURES:

Figure 1 (a) Photoemission spectroscopy of MAPbI3/SWCNT interface. (b) Absorption spectrum of (6,5) SWCNT and MAPbI3 indicate high material purity. (c) X-ray diffractogram of the fully converted crystalline MAPbI3 film.


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Figure 2 UPS spectra. (a) Secondary electron cut-off for work function (WF) determination (b) Valence band region with inset of the gap state region in semi-logarithmic representation. The values to the right denote the VBM of MAPbI3 and center position of the SWCNT van Hove singularities, respectively, while the values in parenthesis refer to the onset of the tail state DOS.


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Figure 3 Energy level diagram. All energies are referenced to a common Fermi level. Values for the work function (WF) are denoted in blue, electron affinity and ionization energy of the MAPbI3 layer are denoted in black and ionization energies of the SWCNT layer are denoted in green. Dark shaded areas define the quasi-continuum of states in the valence and conduction band in the perovskite whereas light shaded areas depict the region of gap states. The reference SWCNT measurement referes to a 9 nm thick SWCNT film on Au (see supplementary figure S2).


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Figure 4 XPS spectra (a-b). Nitrogen (a) and carbon (b) core level spectra reveal shifts in both methylamine and SWCNT species. (c) Schematic band diagram demonstrating core level shifts in the MA species at the interface and band bending in the SWCNT layer (d) Schematic representation of ground state charge transfer in form of electron donation from the methylamine site in the perovskite unit cell to the SWCNT.


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Figure 5: Differential absorption spectra for (a) TiO2/MAPbI3 and (b) TiO2/ MAPbI3/(6,5) SWCNT films at several delay times following 400 nm pump excitation. (c) Normalized transients of S11 exciton bleach for TiO2/ MAPbI3/(6,5) SWCNT film (red trace) compared to signal at 1008 nm for TiO2/ MAPbI3 film (black trace). Inset shows the difference absorption spectrum for TiO2/ MAPbI3/(6,5) SWCNT at a delay time of 3.8 ns where the S11 exciton bleach at 1008 nm and X+ trion IA at 1174 nm can be clearly seen. (d) Normalized transients of band edge bleach at a probe wavelength of 750 nm following 400 nm pump excitation for films of glass/ MAPbI3 (red trace), TiO2/ MAPbI3 (black trace), and TiO2/ MAPbI3/(6,5) SWCNT (blue trace). Inset shows the band edge bleach of the TiO2/ MAPbI3 film at a delay time of 1 ns.


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AUTHOR INFORMATION Corresponding Author *corresponding authors: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Work was supported by the hybrid perovskite solar cell program of the National Center for Photovoltaics funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Office of Solar Energy Technology under Award Number DE-AC36– 08GO28308DOE with the National Renewable Energy Laboratory (NREL). A.-M. D. and J. L. B. were supported by the Solar Photochemistry Program of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract No. DE-AC36-08GO28308 to NREL.

Supporting Information Available: Material includes j-V curve of a representative device, UPS and XPS control measurements as well as additional transient absorbance.

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