Ultrafast Dynamics of Hole Injection and Recombination in

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Ultrafast Dynamics of Hole Injection and Recombination in Organo Metal Halide Perovskite Using Nickel Oxide as p-Type Contact Elec-Trode Alice Corani, Ming-Hsien Li, Po-Shen Shen, Peter Chen, Tzung-Fang Guo, Amal El Nahhas, Kaibo Zheng, Arkady Yartsev, Villy Sundstrom, and Carlito S. Ponseca J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00238 • Publication Date (Web): 04 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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The Journal of Physical Chemistry Letters

Ultrafast Dynamics of Hole Injection and Recombination in Organo Metal Halide Perovskite using Nickel Oxide as p-Type Contact Electrode Alice Corani†, Ming-Hsien Li∥, Po-Shen Shen∥, Peter Chen∥,‡,§,*, Tzung-Fung Guo∥,‡,§,*, Amal El Nahhas†, Kaibo Zheng†, Arkady Yartsev†, Villy Sundström†, and Carlito S. Ponseca Jr.†,* †

Division of Chemical Physics, Lund University, Box 124, 221 00 Lund, Sweden of Photonics, National Cheng Kung University, Tainan, Taiwan 701 § Research Center for Energy Technology and Strategy (RCETS), Tainan, Taiwan 701 ∥Department

‡

Advanced Optoelectronic Technology Center (AOTC), Tainan, Taiwan 701

Supporting Information Available ABSTRACT: There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organo-metal halide perovskite based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the sub-picosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of ps to few ns. Due to the low conductivity of NiO(np), holes are pinned at the interface and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.

TOC Graphic

Keywords: perovskite solar cells, terahertz spectroscopy, ultrafast dynamics, charge injection Solid-state organometal lead halide perovskites (OMHP), such as CH3NH3PbX3 (X = I, Br, Cl), have recently attracted tremendous research attention due to their power conversion

efficiency (PCE), which has advanced between 15~20% in the last 5 years.1-4 Attractive features of these materials include direct optical bandgap of around 1.5 eV,5 low exciton binding energy,6 long charge diffusion lengths,7 and broad absorption range from visible to near-infrared (~800 nm) with high absorption coefficient (~104 cm-1 at 550 nm). Equally important is the ability to form relatively uniform films under lowtemperature solution processes. OMHPs were first employed in sensitized-type solar cells with iodide liquid electrolyte having a PCE around 3~6%.8,9 However, these devices lack stability due to instant dissolution of the perovskite in the highly polar liquid electrolyte. A remarkable improvement was achieved when the liquid hole-transporting medium was replaced with solid-state organic molecules or polymers as hole transport materials (HTMs). Perovskite-based solar cells (PSCs) then emerged, based on mesoscopic active layers with n-type contact substrate materials as n-i-p heterojunction photovoltaic5,10 with various materials and architectures.11-13 Spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-pmethoxyphenylamine)-9,9’-bifluorene), which has become a commonly employed HTM, gives decent PCE.1,2,4 However, it requires complicated synthesis and its high price hinders the commercialization of low-cost solar cells. Another avenue to explore is the use of inorganic hole conducting semiconductors with high mobility, high transparency in the visible region, and good chemical stability. Recently, inorganic HTMs like NiO, CuI, and CuSCN were used and have demonstrated a PCE of over 15% for both planar and mesoscopic structures.14 In particular, p-type Cu-doped NiO thin film exhibits PCE over 15.4%15, while its mesoscopic counterpart gives over 11%.16,17 Very recently combustion processed Cu:NiOx and pulsed laser deposited NiO films were used as counter electrode and a PCE of 17% was reached.18,19 This demonstrates the potential of using NiO as an effective inorganic hole extractor. In order to understand the mechanisms accounting for the efficiency and timescale of hole injection into NiO(np) in this

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electrode, time-resolved terahertz (THz) spectroscopy (TRTS) is applied. We note that this technique has been a useful tool in measuring photoconductivity and carrier mobility of materials in a non-contact manner, minimizing the errors in measuring its electrical properties. In fact, it has been utilized in studying early-time charge carrier dynamics in several solar cell technologies, namely dye-20 and quantum-dot21 sensitized, organic,22-24 inorganic25 and recently perovskite26-30 based solar cells. On one hand, a planar sample was prepared to measure the photoconductivity of the neat methyl ammonium lead iodide (MAPbI3) perovskite material. On the other hand, ZrO(np) was sensitized to produce its mesoscopic counterpart. Because of the high band gap of ZrO2 (5.34 eV31) there is no expected charge injection into this material. By comparing their charge dynamics one can understand the influence of morphology on the mobility of electrons and holes. Furthermore, to validate if indeed there is a charge transfer, both NiO(np) and TiO2(np) were attached and their photoconductivity compared. A full solar cell device has also been fabricated to verify that such a structure gives photocurrent through a NiO electrode. Four types of substrates were prepared: quartz, quartz/NiO(np), quartz/TiO2(np) and quartz/ZrO2(np). On top of these, a MAPbI3 perovskite film was spin coated at 4000 rpm for 30 seconds. The MAPbI3 perovskite precursor was prepared with the equimolar mixture of (33 wt%) CH3NH3I and PbI2 in N,N-dimethylformamide similar to Ref 18. The pulsed excitation light (λpump = 400 nm) enters the sample from the active material side with penetration depth of 100 nm32 where 100 nm MAPbI3 capping layer is deposited on top of 50 nm mesoporous structure. This ensures that the photoconductivity we obtained is from interface of the perovskite and charge accepting materials. We found that neat thin film of MAPbI3 and MAPbI3/ZrO2(np) have very similar photoconductivity kinetics, i.e. non-decaying up to 1 ns and both have a total mobility of about 11 cm2/Vs. This indicates that ZrO2(np) does not alter the early time dynamics of charge generation in MAPbI3, and only has a scaffolding role, similar to that of nanostructured Al2O3. Upon attaching the MAPbI3 to either TiO2(np) or NiO(np), the mobility decreased to approx. half (~6 cm2/Vs) for both samples. In the case of MAPbI3/TiO2(np), this can be explained as ultrafast injection of electrons to TiO2(np) as reported earlier.26 This result also shows that holes are injected into NiO(np) on a similar timescale as electron injection into TiO2(np). Recombination commences from several hundreds of ps to a few ns due to low mobility in NiO(np), pinning holes at the interface. Moreover, we show the full characterization of a similarly prepared device having a power conversion efficiency of 13.7%, confirming that holes are transferred to the NiO(np) electrode. Device Characterization A full solar cell device was fabricated having a heterojunction structure of FTO/NiOx/NiO(np)/Perovskite/PCBM/Bathocuprione(BCP)/Al. A NiOx compact layer is first formed via a low-temperature sputter process on which a uniform pinhole-free perovskite layer is deposited via a solution process using solvent engineering.3 On top of this a PCBM layer is thermally evaporated to serve as electron extracting electrode. A metal electrode is thermally deposited to complete the cell. The J-V characteristics and IPCE spectra of the resulting device having an area of 0.3 cm2 are presented in Figure 1(a) and 1(b) and measured

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under AM 1.5G illumination. A delivered power conversion efficiency of 13.7 % is obtained with a VOC of 1.01 V, a JSC of 22.1 mA/cm2 and fill factor of 61.6%. Figure 1(c) displays the cross-section SEM image of the mesoporous NiO-based perovskite solar cell wherein the thickness of the perovskite capping layer is estimated to be ~400 nm to saturate light absorption. The XRD diffraction pattern of the THz measurement sample is presented in Figure 1(d) where the characteristic peaks of perovskite are observed. Compared with TiO2-based devices, NiO-based devices exhibit similar performances in open-circuit voltage and short-circuit current density. However, the lower fill factor of the NiO-device as compared to TiO2-based devices is the main reason for its relatively lower PCE. Despite its lower photovoltaic performances than for MAPbI3/TiO2-based devices, the decent PCE strongly indicates that not only the electrons reached the Al electrode, but most importantly, holes have been transferred from MAPbI3 to NiO(np) and eventually to FTO.

Figure 1. (a) J-V curve, and (b) IPCE spectra, and (c) cross section image of the fabricated device, and (d) the XRD diffraction pattern of the THz sample.

Ultrafast dynamics To determine the timescale of hole injection and ensuing dynamics of holes and electrons, ultrafast TRTS is utilized. The main result of the technique is a change in THz photoconductivity (∆σ) of the material, which is a product of quantum yield (ξ) and the sum of the mobility (µ) of electrons and holes as expressed by the following equation:

∆σ = ξ × µtot =

ξ ( µe + µ h ) nexce0

,

where e0 is the elementary charge and nexc is the charge carrier density determined by the pump intensity and optical density of the active material. On the earliest timescale, the quantum yield of charge formation (ξ) is assumed to be 100%. As such, the mobility obtained here represents the lower limit of mobility. At the later timescale, ξ represents the change in charge population which is time varying depending on the fate of the charges.30 In neat MAPbI3, the total mobility is the sum of electron and hole mobility. However, when either of the


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charge carriers is injected to a metal oxide acceptor, it will adopt the property of the accepting material. For example, in the case of neat perovskite attached to TiO2(np), where there is ultrafast injection of electrons from the perovskite, the total mobility will then be µtot = µe,TiO2 + µh,perovskite. As reported earlier, the mobility of electrons in TiO2(np) is 0.1 cm2/Vs.33 Hence, the µtot obtained in a MAPbI3/TiO2(np) sample is dominated by the mobility of the holes left in the perovskite, and only a minor part (0.1 cm2/Vs) comes from the electrons in TiO2(np). Using this approach and by substituting the metal oxide material such that it accepts either of the carriers, one can elucidate the contribution of each to the total mobility.

Figure 2. Early time THz photoconductivity kinetics of neat MAPbI3 (black), attached to nanoparticles of ZrO2(np) (red), TiO2(np) (green) or NiO(np) (blue). λpump = 400 nm, Iexc = 1.7 x 1012 ph/cm2 per pulse. Open symbol are data while solid trace is guide for the eye.

Figure 2 shows the plot of early time dynamics of THz photoconductivity of the four samples studied. For neat MAPbI3, the mobility obtained is 11 cm2/Vs, very similar to that reported by Wehrenfennig, et al.34 When MAPbI3 is attached to ZrO2(np) where the bandgap is too high (5.34 eV) for electron injection to occur, the rise time of the THz photoconductivity kinetics as well as the mobility is very similar to that of neat MAPbI3. This indicates that the mechanism of generation and separation of charge carriers in MAPbI3 is not altered by the presence of ZrO2(np). Hence, ZrO2(np) acts like a scaffolding, similar to the role that Al2O3 plays.26 When MAPbI3 is attached to TiO2(np), the mobility is reduced by about two times, to ~6 cm2/Vs. As explained above and shown in Eq. 1, there are two possibilities for the THz photoconductivity to decrease. One is that the population of mobile charges (ξ) is reduced, or the mobility of either electrons (µe) or holes (µh) decreased. In the case of MAPbI3/TiO2(np), it should be stressed that the reduction in the THz photoconductivity kinetics happens within the instrument response function, that is, as soon as the charges are photogenerated. Similar to previous reports, there is an ultrafast injection of electrons from MAPbI3 to TiO2(np).26 Therefore, this reduction in the photoconductivity is a result of the disappearance of highly mobile electrons from the perovskite. From this, two important conclusions can be drawn. First, that since electrons are already in TiO2(np), the ~6 cm2/Vs obtained from the MAPbI3/TiO2(np) sample originates from the mobility of holes left in the perovskite. Second, the reduction in mobility is almost half the total mobility obtained in neat MAPbI3 (~11 cm2/Vs) where both electrons and holes contribute to the mobility. Therefore, the mobility of electrons in the perovskite material is ~6 cm2/Vs, very similar to the mobility

of holes, leading to the balanced charge transport and the very similar diffusion lengths reported for these materials.7 This is also consistent with calculations, suggesting very similar effective masses of electrons and holes.35 More interesting is the THz photoconductivity kinetics of MAPbI3 attached to NiO(np). As shown in Figure 2, it has identical rise time and mobility as the MAPbI3/TiO2(np) sample, showing that hole injection to NiO is equally fast as electron injection to TiO2(np). The favorable band alignment between the valence bands of MAPbI3 and NiO(np), having a 0.2 eV difference is enough to drive this ultrafast injection. This is the first report of such ultrafast injection of holes from a sensitizer to NiO(np). This ultrafast injection of holes was also recently observed from perovskite to Spiro-OMeTAD and was also suggested to be driven by the favorable band alignment between the interfaces of the two materials28. The intrinsic conductivity of stoichiometric NiO is 10-13 S/cm,36 which is orders of magnitude lower than in an undoped (10-8 S/cm) or doped Spiro-OMeTAD (10-4 S/cm).37 This means that once holes are injected into NiO(np), their mobility is below the detection limit of the instrument. Therefore, the mobility measured (~6 cm2/Vs) in this sample originates from electrons that are left in the perovskite after the ultrafast hole injection. Thus, we conclude that mobilities of electrons and holes in the present preparation of MAPbl3 are the same within experimental error, in agreement with a previous report.26 Steady state spectroscopic techniques were also previously used in order to probe hole injection. For example, the photoluminescence spectra of MAPbl3/NiO(np) show strong quenching of emission compared to the large photoluminescence signal from MAPbl3/Al2O3 (See figure 4a in Ref. 38). In addition, optical transient absorption spectra of MAPbl3/NiO(np) show absorption in the 900-1500 nm region, while it is absent in neat MAPbl3, indicating the presence of holes in the metal oxide phase.39 The relatively high power conversion efficiency measured in the FTO/NiOx/NiO(np)/Perovskite/PCBM/BCP/Al device, Fig 1(a), is another evidence that holes are arriving at the counter electrode. We stress that these steady state measurements complement the results here, but it is the THz photoconductivity data that reveals the timescale of hole injection. Recently, Trifiletti et al. also reported efficient hole injection from perovskite to NiO in agreement with our results presented here.40 To determine the timescale of recombination, we compare the THz photoconductivity kinetics of the neat MAPbl3 with that of MAPbl3/NiO(np) at different excitation intensities and up to 1 ns. In Fig. 3(a), neat MAPbl3 has the fastest decay in its THz photoconductivity kinetics when the excitation is highest (8.9 x 1012 ph/cm2) while it is non-decaying or almost flat at the lowest excitation intensity (8.9 x 1011 ph/cm2). From Eq. 1, decrease of the photoconductivity may be due to either charge population or relaxation of mobility of either electrons or holes or both. For neat MAPbl3, it has been reported that the mobility remains constant for at least 1 ns as probed by complementary transient absorption spectroscopy and THz measurements.26 Moreover, since both electrons and holes stay in the perovskite, and in the absence of acceptor metal oxide, this suggests that the decay can only be due to a decrease in charge population. In fact, excitation intensity dependent decay is reminiscent of our previous results on organic solar cells, as well as perovskite solar cell materials, where second order recombination was found to be the dominating recombination channel at high excitation intensities.24,26 The decay at high



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intensities observed here is therefore assigned to second order non-geminate charge recombination, while at the lowest excitation, this recombination is eliminated. Admittedly, there seems to be a noticeable scattering in the data points for the first 100 ps and apparent increase in the kinetics from about 200 ps. However, we should note that the lowest excitation intensity used here (8.9 × 1011 ph/cm2) is at two to three orders of magnitude lower than that previously used in organic solar cells.24 This results in a rather low signal to noise, but also largely eliminates higher order effects, e.g. non-geminate recombination24 that masks other decay channels. Despite the low fluence used here, it is still two orders of magnitude higher than ambient solar conditions (˜10 × 109 ph/cm2); but since the THz kinetics is flat, we know that both geminate and nongeminate recombination are not present at this time scale (1 ns). The photoluminescence kinetics of Deschler et al.41 and the THz kinetics of Wehrenfennig, et al.34, has a flat trace of emission (Fig. 1b) and photoconductivity (Fig. 2a, 6 uJ/cm2) for at least two ns, respectively, agreeing with our results. However, this does not mean that at the later time scale both recombination channels are also absent. In fact, as we previously reported, this can be observed in the tens of microsecond time scale.26 (a)

(b)

Figure 3. THz photoconductivity kinetics of (a) neat MAPbI3 and (b) MAPbI3/NiO(np) pumped at three different intensities. Open symbol are data while solid trace is guide for the eye.

Shown in Fig. 3(b) is a prominent decay in the THz photoconducvitiy kinetics of MAPbI3/NiO(np) persisting at all excitation intensities. From the discussion above, we concluded that at the lowest excitation intensity, second order recombination is eliminated in neat MAPbI3. In constrast, the decay in MAPbI3/NiO(np) remains even at the lowest excitation indicating a different mechanism of recombination. As dis-

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cussed above, hole injection from MAPbI3 to NiO(np) occurs on the earliest timescale. This means that the NiO(np) are positively charged and holes are navigating within the mesoporous structure at very low mobility. The picture that is emanating from this is that highly mobile electrons that stay in the perovskite meet localized low mobility holes in NiO(np) at the MAPbI3/NiO(np) interface and the rate at which they recombine is reflected by the decay of the THz photoconductivity kinetics, i.e. several hundred ps to a few ns. It is also possible that there is a minute population of mobile holes left in the perovskite that are not injected to NiO(np) as shown by a weak photoluminescence of MAPbI3/NiO(np) (Fig. 4a of Ref 37). This may be coming from some perovskite not directly in contact with NiO(np). However, this contribution should be very small as evidenced by the large decrease of the THz photoconductivity signal (Fig. 2), showing that injection of holes must be efficient. One also cannot discount the influence of the defect density at the surface of NiO(np) that traps the electrons, which could also manifest as a decay. Recently, we reported that charge carrier dynamics in MAPbI3 could be highly influenced by the presence of dark carriers due to defects.28 Nonetheless, the high PCE obtained from the FTO/NiOx/NiO(np)/Perovskite/PCBM/BCP/Al device suggests that this channel should be rather small, otherwise efficiency would suffer. The several hundred ps to few ns recombination time between the injected holes in NiO(np) and electrons in the perovskite, observed here, is at least three orders of magnitude shorter than the few microsecond26 recombination time of injected electrons in TiO2(np) and holes in perovskite. We surmised that due to the very low conductivity of NiO(np), holes are pinned at the interface and the rate at which electrons find holes determines the recombination rate. This means that there is a large potential for improvement of these devices. Recently, several high efficiency NiO(np)/perovskite solar cells have been reported using different synthetic methods. For example, improved electrical conductivity of Cu-doped NiO with higher work function matching well with wide band gap perovskite of Br-doped MAPbI3 led to a remarkable voltage and device performance.15 Moreover, a highly crystalline Cu-doped NiOx formed by combustion process exhibited higher electrical conductivity delivering PCE of efficiency of 17.7%.19 In addition, co-doping Mg2+ and Li+ into NiO was recently proven to further improve conductivity by one order of magnitude as compared to Cu-doped NiO.42 Another very successful case is the use of high quality pulsed laser deposition (PLD) of nanostructured-NiO film, which resulted in very high fill factor.18 These works show the significant effects of NiO quality on the photovoltaic performance of perovskite solar cells. For the perovskite deposition procedure, several improvements have also been considered. A uniform, flat and dense perovskite film with large crystallites was reported to produce very high efficiency devices.3,43,44 A solventengineered perovskite solar cell containing a PLD-NiO film exhibited a PCE of 17.3%.18 By using p-type NiO and n-type ZnO (or TiO2) as electrodes, all metal-oxide charge extraction layers were demonstrated having efficiencies over 15% with long-term stability42,45. All these results indicate the great success and potential of the use of NiO for perovskite solar cells as p-type contact material. In summary, hole injection from perovskite to NiO(np) is shown to occur on the sub-ps timescale driven by the energy difference between the two materials. The recombination


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channel is dominated by electrons in the conduction band meeting holes injected into NiO(np) and proceeds within hundreds of ps to a few ns. This recombination channel represents a loss mechanism and it is therefore desirable for it to be arrested. One approach to achieve this is by NiO(np) doping, similar to strategies employed for other hole transporting materials like Spiro-OMeTAD. Through this, the mobility of holes will be increased and extraction faster, preventing them to recombine with electrons in the perovskite.

AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] [email protected]

Notes The authors declare no competing financial interests.

Supporting Information The full list of the names of authors in the references. This information is available free of charge via the internet at http://pubs.acs.org.

ACKNOWLEDGMENT The Swedish Energy Agency (STEM), the Swedish Research Council, the Knut&Alice Wallenberg foundation and the European Research Council (Advanced Investigator Grant to VS, 226136-VISCHEM) are acknowledged. The nanometer Consortium at Lund University (nmc@LU) is also acknowledged. PC and TFG thank the financial support from the Ministry of Science and Technology (MOST) of Taiwan (103-2221-E-006-029-MY3, 1042119-M-006-004)

REFERENCES (1) 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. (2) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (3) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic−Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897−903. (4) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-B.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (5) 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.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (6) Tanaka, K.; Takahashi, T.; Ban, T.; Kondo, T.; Uchida, K.; Miura, N. Comparative Study on the Excitons in Lead-Halide-Based PerovskiteType Crystals CH3NH3PbBr3 CH3NH3PbI3. Solid State Commun. 2003, 127, 619–623. (7) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and HoleTransport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344–347. (8) 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. (9) Im, J. H.; Lee, C. R.; Lee, J. W.; Park, S. W.; Park, N.-G. Efficient Perovskite Quantum-Dot-Sensitized Solar Cell. Nanoscale 2011, 3, 4088−4093.

(10) 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, 643–647. (11) Kim, H.-S.; Im, S. H.; Park, N.-G. Organolead Halide Perovskite: New Horizons in Solar Cell Research. J. Phys. Chem. C 2014, 118, 56155625. (12) Luo, S.; Daoud, W. A. Recent Progress in Organic-Inorganic Halide Perovskite Solar Cells: Mechanisms and Material Design. J. Mat. Chem. A 2015, 3, 8992-9010. (13) Salim, T.; Sun, S.; Abe, Y.; Krishna, A., Grimsdale, A.; Lam Y. L. Perovskite-based Solar Cells: Impact of Morphology and Device Architecture on Device Performance. J. Mat. Chem. A 2015, 3, 8943-8969. (14) Li, M.-H.; Shen, P.-S.; Wang, K.-C.; Guo, T.-F.; Chen, P. Inorganic p-Type Contact Materials for Perovskite-based Solar Cells. J. Mat. Chem. A 2015, 3, 9318-9319. (15) Kim, J. H.; Liang, P. W.; Williams, S. T.; Cho, M.; Chueh, C.-C.; Glaz, M. S.; Ginger, D. S.; Jen, A. K.-Y. High-Performance and Environmentally Stable Planar Heterojunction Perovskite Solar Cells Based on a Solution-Processed Copper-Doped Nickel Oxide Hole-Transporting Layer. Adv. Mat. 2015, 27, 695-701. (16) Wang, K.-C.; Shen, P.-S.; Li, M.-S.; Chen, S.; Lin, M.-W.; Chen, P.; Guo, T. F. Low-Temperature Sputtered Nickel Oxide Compact Thin Film as Effective Electron Blocking Layer for Mesoscopic NiO/CH3NH3PbI3 Perovskite Heterojunction Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 11851–11858. (17) Chen, W.; Wu, Y.; Liu, J.; Qin, C.; Yang, X.; Islam, A.; Cheng, Y.B.; Han, L. Hybrid Interfacial Layer Leads to Solid Performance Improvement of Inverted Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 629-640. (18) Park, J. H.; Seo, J.; Park, S.; Shin, S. S.; Kim, Y. C.; Jeon, N. J.; Shin, H. W.; Ahn, T. K.; Noh, J. H.; Yoon, S. C.; et al. Efficient CH3NH3PbI3 Perovskite Solar Cells Employing Nanostructured p-Type NiO Electrode Formed by a Pulsed Laser Deposition. Adv. Mater. 2015, 27, 4013-4019. (19) Jung, J. W.; Chueh, C.-C.; Jen, A. K.-Y. A Low-Temperature, Solution-Processable, Cu-Doped Nickel Oxide Hole-Transporting Layer via the Combustion Method for High-Performance Thin-Film Perovskite Solar Cells. Adv. Mater. 2016, 27, 7874-7880. (20) Harlang, T. C. B.; Liu, Y.; Gordivska, O.; Fredin, L. A.; Ponseca, C. Jr. S.; Huang, P.; Chabera, P.; Kjaer, K. S.; Mateos, H.; Uhlig, J.; et al. Iron Sensitizer Converts Light to Electrons with 92% Yield. Nat. Chem. 2015, 7, 883-889. (21) Zidek, K.; Zheng, K.; Ponseca, C. S., Jr.; Messing, M. E.; Wallenberg, L. R.; Chabera, P.; Abdellah, M.; Sundström, V.; Pullerits, T. Electron Transfer in Quatum-dot Sensitized ZnO Nanowires: Ultrafast TimeResolved Absorption and Terahertz study. J. Am. Chem. Soc. 2012, 134, 12110-12117. (22) Ponseca, C. S., Jr.; Němec, H.; Vukmirović, N.; Fusco, S.; Wang, E.; Andersson, M.; Chavera, P.; Yartsev, A.; Sundström, V. Electron and Hole Contributions to the Terahertz Photoconductivity of a Conjugated Polymer:Fullerene Blend Identified. J. Phys. Chem. Lett. 2012, 3, 24422446. (23) Vukmirović, N.; Ponseca, C. Jr. S.; Němec, H.; Yartsev, A.; Sundström, V. Insights into the Charge Carrier Terahertz Mobility in Polyfluorenes from Large-Scale Atomistic Simulations and TimeResolved Terahertz Spectroscopy. J. Phys. Chem. C 2012, 116, 19665−19672. (24) Ponseca, C. S., Jr.; Yartsev, A.; Wang, E.; Andersson, M.; Vithanage, D.; Sundström, V. Ultrafast Terahertz Photoconductivity of Bulk Heterojunction Materials Reveals High Carrier Mobility up to Nanosecond Time Scale. J. Am. Chem. Soc. 2012, 134, 11836. (25) Ponseca, C. Jr. S.; Nemec, H.; Wallentin, J.; Anttu, N.; Beech, J. P.; Iqbal, A.; Borgstrom, M.; Pistol, M.-E.; Samuelson, L.; Yartsev A. Bulklike Transverse Electron Mobility in an Array of Heavily n-Doped InP Nanowires Probed by Terahertz Spectroscopy. Phys. Rev. B 2014, 90, 085405. (26) Ponseca, C. S.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; et al. J Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-Long Balanced Mobilities, and Slow Recombination. J. Am. Chem. Soc. 2014, 136, 5189−5192. (27) Valverde-Chavez, D. A.; Ponseca, C. S., Jr.; Stoumpos, C. C.; Yartsev, A.; Kanatzidis, M. G.; Sundstrom, V.; Cooke, D. G. Intrinsic Femtosecond Charge Generation Dynamics in Single Crystal CH3NH3PbI3. Energy Environ. Sci. 2015, 8, 3700−3707.



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

(28) Ponseca, C. S.; Hutter, E. M.; Piatkowski, P.; Cohen, B.; Pascher, T.; Douhal, A.; Yartsev, A.; Sundstrom, V.; Savenije, T. J. Mechanism of Charge Transfer and Recombination Dynamics in Organo Metal Halide Perovskites and Organic Electrodes, PCBM and Spiro-OMeTAD: Role of Dark Carriers. J. Am. Chem. Soc. 2015, 137, 16043−16048. (29) Piatkowski, P.; Cohen, B.; Ponseca, C. Jr. S.; Salado, M.; Kazim, S.; Ahmad, S.; Sundstrom, V.; Douhal, A. Unraveling Charge Carriers Generation, Diffusion, and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film. J. Phys. Chem. Lett. 2016, 7, 204−210. (30) Ponseca, C. Jr. S.; Sundstrom, V. Revealing the Ultrafast Charge Carrier Dynamics in Organo Metal Halide Perovskite Solar Cell Materials using Time Resolved THz Spectroscopy. Nanoscale 2016. DOI: 10.1039/c5nr08622a (31) Kim, C.-S.; Jeong, H.-D. Bangap Tuning in Nanoprous TiO2-ZrO2 Hybrid Thin Films. Bull. Korean Chem. Soc. 2007, 28, 2333-2337. (32) Karakus, M.; Jensen, S. A.; D’Angelo, F.; Turchinovich, D.; Bonn, M.; Cánovas, E. Phonon−Electron Scattering Limits Free Charge Mobility in Methylammonium Lead Iodide Perovskites. J. Phys. Chem. Lett. 2015, 6, 4991-4996. (33) Hendry, E.; Koeberg, M.; O’Regan, B.; Bonn, M. Local Field Effects on Electron Transport in Nanostructured TiO2 Revealed by Terahertz Spectroscopy. Nano Lett. 2006, 6, 755-759. (34) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, S. J.; Herz, L. M. High Charge Carrier Mobilities and Lifetimes in Organo Lead Trihalide Perovskites. Adv. Mater. 2013, 26, 1584-1589. (35) Quarti, C.; Mosconi, E.; De Angelis, F. Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell Operation. Chem. Mater. 2014, 26, 6557−6569. (36) Wittenauer, M. A.; Van Zandt, L., L.; Surface Conduction versus Bulk Conduction in Pure Stoichiometric NiO crystals. Philos. Mag. B 1982, 46, 659–667. (37) Nguyen, W. H.; Bailie, C. D.; Unger, E. L.; McGehee, M. D. Enhancing the Hole-Conductivity of Spiro-OMeTAD without Oxygen or Lithium

Page 6 of 6

Salts by Using Spiro(TFSI)2 in Perovskite and Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 10996−11001. (38) Wang, K.-C.; Jeng, J.-Y., Shen, P.-S.; Chang, Y.-C.; Diau, E. W.-G.; Tsai, C.-H.; Chao, T.-Y.; Hsu, H.-C.; Lin, P.-Y.; Chen,, P.; et al. p-Type Mesoscopic Nickel Oxide/Organometallic Perovskite Heterojunction Solar Cells. Sci. Rep. 2014, 4, 4756. (39) Hsu, H.-Y., Huang, C.-Y.; Fathi, A.; Shiu, J.-W.; Chung, C.-C.; Shen, P.-S-.; Guo, F.-T.; Chen, P.; Lee, Y.-P.; Diau, E. W.-G. Femtosecond Excitonic Relaxation Dynamics of Perovskite on Mesoporous Films of Al2O3 and NiO Nanoparticles. Angew. Chem. 2014, 126, 9493-9496. (40) Trifiletti, V.; Roiati, V.; Colella, S.; Giannuzzi, R.; De Marco, L.; Rizzo, A.; Manca, M.; Listorti, A.; Gigli, G. NiO/MAPbI3-xClx/PCBM: A Model Case for an Improved Understanding of Inverted Mesoscopic Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 4283–4289. (41) Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D. D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; et al. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421-1226. (42) Che, W.; Wu, Y.; Yue, Y.; Liu, J.; Zhang, W.; Yang, X.; Chen, H.; Bi, E.; Ashraful, I.; Graetzel, M.; et al. Efficient and Stable Large-Area Perovskite Solar Cells with Inorganic Charge Extraction Layers. Science 2015, 350, 944-948. (43) Chiang, C.-H; Tseng, Z.-L.; Wu, C.-G. Planar Heterojunction perovskite/PC71BM Solar Cells with Enhanced Open-Circuit Voltage via a (2/1)-Step Spin-Coating Process. J. Mater. Chem. A 2014, 2, 1589715903. (44) Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Fray-Weale, A.; Bach, U.; Cheng, Y.B. Spiccia, L. A Fast DepositionCrystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem. Int. Ed. 2014, 53, 9898-9903. (45) You, J.; Meng, L. M.; Song, B. S.; Guo, T.-F.; Yang, M. Y.; Chang, W.-H.; Hong, Z.; Chen, H.; Zhou, H.; Chen, Q.; et al. Improved Air Stability of Perovskite Solar Cells via Solution-Processed Metal Oxide Transport Layers. Nat. Nanotechnol. 2016, 11, 75-81.


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