High Open-Circuit Voltage: Fabrication of Formamidinium Lead

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High Open-Circuit Voltage: Fabrication of Formamidinium Lead Bromide Perovskite Solar Cells Using Fluorenedithiophene Derivatives as Hole-Transporting Materials Neha Arora, Simonetta Orlandi, M. Ibrahim Dar, Sadig Aghazada, Gwenole Jacopin, Marco Cavazzini, Edoardo Mosconi, Paul Gratia, Filippo De Angelis, Gianluca Pozzi, Michael Grätzel, and Mohammad Khaja Nazeeruddin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00077 • Publication Date (Web): 08 May 2016 Downloaded from http://pubs.acs.org on May 9, 2016

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ACS Energy Letters

High Open-Circuit Voltage: Fabrication of Formamidinium Lead Bromide Perovskite Solar Cells using Fluorene-Dithiophene Derivatives as Hole-Transporting Materials Neha Arora,1 Simonetta Orlandi,2 M. Ibrahim Dar,1,3 Sadig Aghazada,1 Gwénolé Jacopin,4 Marco Cavazzini,2 Edoardo Mosconi,5 Paul Gratia,1 Filippo De Angelis,5 Gianluca Pozzi,2 Michael Graetzel3 and Mohammad Khaja Nazeeruddin*1 1

Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and

Engineering, École Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland 2

Istituto di Scienze e Tecnologie Molecolari del Consiglio Nazionale delle Ricerche, CNR-

ISTM, via Golgi 19, I-20133, Milano, Italy. 3

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École

Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland 4

Laboratory of Quantum Optoelectronics, Institute of Physics, École Polytechnique Fédérale de

Lausanne, Lausanne CH-1015, Switzerland.

ACS Paragon Plus Environment

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Computational Laboratory for Hybrid Organic Photovoltaics (CLHYO), CNR-ISTM, via Elce

di Sotto 8, I-06123, Perugia, Italy. AUTHOR INFORMATION Corresponding Author *[email protected]


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ABSTRACT Four different fluorene-dithiophene derivatives based hole transporting materials (HTMs) (SO7-10) have been synthesized via a facile route, and were successfully used in the fabrication of formamidinium lead bromide perovskite solar cells. Detailed characterization of the new compounds was carried out through 1H/13C NMR spectroscopy, mass spectrometry, UVvisible and photoluminescence spectroscopy, and cyclic voltammetry. Under AM1.5 G illumination, the mesoscopic CH(NH2)2PbBr3 perovskite solar cell employing SO7 as the HTM displayed an outstanding photovoltage (Voc) of 1.5 V with an efficiency (η) of 7.1%. The photovoltaic performance is at par with the device using the state-of-the-art Spiro-OMeTAD as HTM, which delivered a Voc of 1.47 V and a maximum η of 6.9%. Density functional theory (DFT) approach with GW simulations including spin-orbit coupling (SOC-GW) and electrochemical measurements brought out deeper HOMO levels for newly synthesized fluorenedithiophene derivatives, which eventually makes them promising HTMs for perovskite solar cells, especially where high photovoltage is desired.

TOC GRAPHICS


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Fabrication of cost-effective solar cells has become a rapidly expanding field of research and in this direction a large amount of literature on perovskite materials has gushed forth.1 Miyasaka and co-workers pioneered the use of organolead halide perovskite compounds as light-harvester in liquid photoelectrochemical cells.2,3 However, an unavoidable problem associated with liquid solar cells is the instability of perovskite sensitizers in the electrolyte. To combat the stability issue, Kim et al. and Lee et al., reported solid-state heterojunction perovskite solar cells using 2,2,7,7-tetrakis-(N,N-di-p-methoxyphenylamine) 9,9-bifluorene (Spiro-OMeTAD) as HTM.4,5 However, the arduous and expensive synthesis of Spiro-OMeTAD demands to develop costeffective and efficient alternate HTMs which in fact has been a subject of investigation in the recent past. In addition, conducting polymers such as poly(triarylamine) (PTAA) and poly(3hexylthiophene) (P3HT) have been employed for the fabrication of high efficiency devices.6,7 Inorganic p-type semiconductors (CuI, CuSCN) have also been used as HTMs for CH3NH3PbI3 solar cell yielding appreciable performances.8,9 In the last two years, the interest in small molecules as HTMs for perovskite solar cells has also grown considerably.10,11 The wide bandgap of bromide perovskites makes them very attractive to extract high open-circuit voltage (Voc), which in turn could be beneficial for tandem solar cell applications and in driving electrochemical reactions.12,13 Heo et al. realized a high Voc of 1.5 V by controlling the growth of CH3NH3PbBr3 structures.14 Seok and co-workers reported a Voc of 1.4 V based on mesoscopic CH3NH3PbBr3 solar cells employing poly(indenofluoren-8-triarylamine) (PIF8-TAA) as HTM.15 Edri et al. attained a Voc of 1.5 V from chloride substituted CH3NH3PbBr3 solar cell displaying moderate efficiency (η = 2.5%).16 Recently, fluorene-dithiophene derivatives have received attention due to their simple synthesis, ease of modification by molecularly engineering the substitutent electron


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donor/acceptor groups and high photovoltaic performance.17 Herein, we present the design and synthesis of four new fluorene-dithiophene derivative based HTMs and their successful application in mesoscopic CH(NH2)2PbBr3 (FAPbBr3) perovskite solar cells. Due to the stronger hydrogen bonding, enhanced spin−orbit coupling and relatively larger size of FA cation, the bandgap of FAPbBr3 (~2.26 eV) decreases as compared to CH3NH3PbBr3 .18 Such a narrowing of the bandgap extends the absorption onset of FA-based perovskites towards longer wavelength, that will allow to harness relatively more photons of the solar spectrum.19 All the new HTMs based FAPbBr3 devices exhibited impressive photovoltage of > 1.45 V with a PCE of ~7.0%. The results are comparable to a PCE of 6.7 % obtained while using well-studied HTM, Spiro-OMeTAD, under standard illumination of 100 mW·cm

−2

. The deeper

HOMO level of one of the HTMs, SO7 allowed to fabricate FAPbBr3 solar cell yielding a remarkably high Voc of 1.5 V and a PCE exceeding 7% under AM 1.5 G illumination at 100 mW cm-2, thus far the highest values reported for the mesoscopic FAPbBr3 perovskite solar cells. Finally, the alignment of the vertical oxidation potential energies for the newly synthesized SO7SO10 series and Spiro-OMeTAD benchmark was investigated using a hybrid DFT approach with GW simulations including spin-orbit coupling (SOC-GW). Only a few examples of fluorene-dithiophene derivatives in which a fluorene moiety and a cyclopenta[2,1-b:3,4-b']dithiophene unit are connected by a common sp3-hybridized carbon have been reported so far.20,21,22,23 This is easily understood considering the difficulties in the preparation of similar compounds. Under standard acidic conditions the generation of the spiro linkage through intramolecular ring closure of 2,2’-bithiophene derivatives is often hampered by competing intermolecular side reactions leading to the formation of considerable amounts of byproducts. 20 It has been recently shown that the introduction of protecting groups on the electron-


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rich α-positions of the thiophene units in combination with the use of suitable Lewis acids markedly increases the efficiency of the intramolecular cyclization process of 2,2’-bithiophene derivatives.24 This finding allowed us to develop a convenient access to the versatile fluorenedithiophene derivative 1 in which the presence of two bromine atoms on the fluorene moiety opens the way to a variety of promising structural modifications through well-established synthetic methods. In the present case, 1 was used as the starting material for the preparation of fluorene-dithiophene derivatives by palladium catalyzed cross-coupling reaction with suitable diarylamines (Scheme 1). The desired compounds SO7, SO8, SO9, and SO10 were thus obtained in good to excellent yields, ranging from 64 to 90% with respect to 1. The fluorenedithiophene compounds were thoroughly characterized by 1H/13C NMR spectroscopy and high resolution mass spectrometry. The analytical data corroborated the proposed HTM structures.

SO7 Ar = Ph Br

Br

S

1

S

Ar2NH

NAr2

Ar2N

Pd2(dba)3, tBu3P, tBuONa Toluene

S

S

SO8 Ar = p-C6H13C6H4 SO9 Ar = p-MeC6H4 SO10 Ar = p-PhOC6H4

Scheme 1. General conditions for the synthesis of SO7-SO10.

Cyclic voltammetry (CV) in combination with UV-visible and photoluminescence (PL) spectroscopy was used to determine the energy levels of SO7-10, as summarized in Table 1.


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Table 1. Spectroscopic and electrochemical data of HTMs. (a) Oxidation potentials were obtained via CV measurements; (b) E0-0 values were determined from the intersection of normalized absorption and emission spectra; (c) HTMs LUMO levels were estimated as follows: Eox(HTM+/HTM) = Eox(HTM+/HTM) – E0-0

λmax (nm)

(absorption) λmax (nm)

(emission) E0-0 (eV)

HTM

HOMO vs. NHE

LUMO vs. NHE

SpiroOMeTAD

0.72; 0.83; 386 1.03

419

3.05

-2.33

SO7

0.91; 1.2

379

402

3.16

-2.25

SO8

0.81; 1.13

386

413

3.09

-2.28

SO9

0.81; 1.12

386

411

3.10

-2.29

SO10

0.83; 1.09

384

416

3.09

-2.26

Cyclic voltammograms of SO7-10 (Figure 1a) exhibit reversible redox couples indicating a good electrochemical stability. The HOMO energy levels calculated from the first oxidation potential of SO7-10 were -5.41 -5.31, -5.31, and -5.33 eV, respectively, which are slightly deeper as compared to Spiro-OMeTAD (-5.22 eV).25 The SO7 HTM shows the highest oxidation potential, which is due to the absence of any donating substituents on the diphenylamine moieties, whereas SO8, SO9, and SO10 have very close HOMO energy levels which can be attributed to comparable donating power of the hexyl-, methyl- and phenoxy- substituents. The normalized UV-Vis absorbance and photoluminescence (PL) spectra of SO7-10 in dichloromethane are shown in Figure 1b. The absorption maxima (λmax) of SO7, SO8, SO9, and SO10 are centered at 379, 386, 386, and 384 nm, respectively. The photoluminescence spectra corresponding to all the HTM molecules exhibit a Stokes shift


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(∆λ = λem – λabs) of around 30 nm. The new HTMs showed only a slight shift in their absorption and PL maxima compared to those of Spiro-OMeTAD. From the intersection of normalized absorption and emission spectra, the optical bandgap (E0-0) values were calculated and using the ground state oxidation potentials, we estimated the lowest unoccupied molecular orbital (LUMO) of HTMs. Due to the presence of heteroatom, all the four HTMs have more positive LUMO energy levels than Spiro-OMeTAD, which are still significantly more positive than perovskite conduction band thereby acting as efficient electron blocking layers.

Figure 1. (a) Cyclic voltammograms and (b) UV-Vis absorbance (left set of curves) and steadystate photoluminescence spectra (right set of curves) of Spiro-OMeTAD and SO7-10 HTMs. We investigated the time-integrated photoluminescence (TIPL) (Figure 2a) of FAPbBr3 perovskite films to probe the impact of HTM deposition on the intensity of TIPL. FAPbBr3 films were fabricated using the sequential deposition method.26 Briefly describing, a solution of PbBr2 was spin coated onto the substrate and subsequently the resulting films were converted into FAPbBr3 perovskite films by dipping into isopropanol solution of formamidinium bromide.


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Finally, the HTMs were deposited by spin-casting a solution of Spiro-OMeTAD and SO7-SO10 onto the FAPbBr3 films [see experimental details]. Upon exciting the pristine perovskite film at 420 nm, strong and narrow PL spectra with fullwidth half maxima of about 25 nm appear at 545 nm. The presence of HTM quenched the emission of perovskite film, which confirms the injection of holes from the valence band of the FAPbBr3 into the HOMO of HTMs. Furthermore, time-resolved photoluminescence (TRPL) decay measurements were performed to study the charge carrier dynamics in pristine and HTM coated FAPbBr3 perovskite films (Figure 2b). The charge carrier dynamics of pristine perovskite film was fitted using two exponential decay model,27 which brought out lifetimes of 22 ns and 96 ns, respectively, for fast and slow components. The long lasting component is due to the recombination occurring in the bulk of the perovskite structures while as a fast relaxation component could be attributed to the surface recombination.28 In HTM coated FAPbBr3 films, the charge carrier lifetime (Figure 2, inset) decreased considerably, which manifests efficient injection of holes from the valence band of perovskite into the HOMO of the HTM. Comparatively, the FAPbBr3 perovskite films containing Spiro-OMeTAD as HTM showed the fastest charge carrier decay dynamics (τ1spiro< τ1SO9< τ1SO101.45 V


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with the exception of SO8 which shows slightly lower photovoltage (Voc = 1.43 V). Observation of low photovoltage in SO8 is in agreement with hole injection dynamics as revealed by TRPL studies. The photovoltaic performance of the cells employing four new HTMs is comparable to the device using the state-of–the-art Spiro-OMeTAD. In general, Voc corresponds to the difference in the quasi-Fermi levels of the electrons and holes under illumination. The HOMO level of the HTM will only affect the Voc if the recombination of electrons in the FAPbBr3 with holes in the HTM is faster than that of the holes in the perovskite. The higher Voc of the device relative to Spiro-OMeTAD counterpart, can be rationalized by considering the deeper HOMO level of SO7 corroborating the cyclic voltammetry results. Furthermore, to present the statistics in photovoltaic performance data for 25 devices based on each HTM is summarized in Table S1. To further estimate the photocurrents, we measured incident photon-to-current-efficiency (IPCE) of the reference (Spiro-OMeTAD) and the best photovoltaic device (SO7) as a function of wavelength (Figure S2). IPCE spectra show that the generation of photocurrent begins at ~550 nm, which is in accordance with the band gap of pure FAPbBr3.19 The photocurrent density values integrated from the IPCE data are slightly lower than those obtained from current-voltage measurement. Such a variation in the photocurrent could be due to the low illumination intensity employed for the IPCE measurement.29 Additionally, it is well known that perovskite solar cells exhibit scan speed dependent hysteresis in current-voltage curves which has been attributed to the accumulation and migration of ions at various interfaces and within the perovskite material.30 Such a hysteresis phenomenon leads to the overestimating of the photovoltaic parameters obtained from a J-V curve. In our case, at a scanning rate of 0.1 V/s, the devices based on the SO7-10 HTMs showed negligible hysteresis (Figure 4).


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8 6 4 2 0 -2

Current density (mAcm-2)

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SO7 Voc to Jsc Jsc to Voc

200 400 600 800 1000 1200 1400 1600 8 6 4 2 0 -2

SO8 Voc to Jsc Jsc to Voc

0 8 6 4 2 0 -2

200 400 600 800 1000 1200 1400 1600 SO9 Voc to Jsc Jsc to Voc

0 8 6 4 2 0 -2

200 400 600 800 1000 1200 1400 1600 SO10 Voc to Jsc Jsc to Voc

0

200 400 600 800 1000 1200 1400 1600

Potential (mV)

Figure 4. Current−voltage hysteresis of FAPbBr3 perovskite devices containing new HTMs recorded at a scan rate of 0.1 V/s under stimulated AM1.5 100 mWcm-2 photon flux To investigate the electronic properties of the new HTMs against those of the perovskite absorbers we performed first principles simulations, combining a hybrid DFT approach for the molecular HTMs with GW simulations including spin-orbit coupling (SOC-GW)31 for the bromide perovskites, see Supporting Information for computational details. In particular, we investigated the alignment of the vertical oxidation potential energies for the newly synthesized SO7-SO10 series and Spiro-OMeTAD benchmark HTMs with the calculated density of states of FAPbI3 and FAPbBr3 (Figure 5). While the oxidation potentials of the molecular HTMs are calculated on an absolute scale against the vacuum, to align the perovskites band edges on an


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absolute energy scale, we set the valence band maximum (VBM) of FAPbI3 to its experimental value18 and consequently positioned all the other calculated energy levels. Our SOC-GW calculations provided band-gap values in very good agreement with the experiment,32 showing a band-gap increase from ∼1.5 to ∼2.3 eV when replacing iodine with bromine, with a larger valence band than conduction band offset, ∆VB/CB in Figure 5.

Figure 5.

Left: SOC-GW calculated Density of States (DOS) for FAPbI3 and FAPbBr3

perovskites (red and blue lines, respectively), along with the calculated vertical oxidation potentials for the investigated series of HTMs. Right: Isodensity plot of calculated HOMOs for the SO7-SO10 HTMs. The calculated FAPbI3/ FAPbBr3 alignment of energy levels is consistent with photoelectron spectroscopy data, showing a 0.5 eV valence band offset between MAPbI3 and MAPbBr3. 33,34 Notably, a similar agreement with the experimental oxidation potential values is retrieved from our DFT calculations, showing a steady increase of the HTMs oxidation potential when moving


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from Spiro-OMeTAD to SO9 → SO7, with SO10 showing a slightly increased potential compared to SO9 and SO8. Interestingly, all the HTMs feature a similar HOMO spatial distribution, mainly localized on the aromatic triphenylamine-like moieties, as in SpiroOMeTAD.35 The substituents modulate the HTMs energy levels through increased electron donation, which raises the HOMO levels. Expectedly, the unsubstituted SO7 HTM shows the highest oxidation potential. The highest Voc extracted with the SO7 HTM correlates with the best matching energy levels with the FAPbBr3 perovskite, with ∼0.2 eV of driving force for hole injection, though some variation of the Voc with the molecular structure beyond the HTM oxidation potential suggests that specific interactions with the perovskite surface, possibly mediated by the HTM substituents, may modulate the photovoltaic performance. In summary, we have designed and synthesized four new fluorene-dithiophene derivative based HTMs. Electrochemical measurements in combination with theoretical studies revealed a higher oxidation potential for SO7-SO10, which rationalizes the higher Voc obtained using SO7 as compared to its reference counterpart, i.e., Spiro-OMeTAD. To the best of our knowledge, fabrication of mesoscopic FAPbBr3 perovskite solar cells exhibiting power conversion efficiency exceeding 7% and Voc of 1.5 V has not been reported so far. Overall the photovoltaic performance showed good reproducibility and current-voltage curves exhibited negligible hysteresis. TRPL studies brought out that the injection of holes from the valence band of FAPbBr3 perovskite into the HOMO of HTM molecules majorly followed the oxidation potentials of the latter. Our results demonstrate that these easy to synthesize new fluorenedithiophene derivatives are promising HTM candidates for the fabrication of efficient bromidebased perovskite solar cells, especially where high photovoltage is in demand.


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ASSOCIATED CONTENT Supporting Information. Experimental details, cross-sectional SEM images, IPCE spectra, and conductivity data, 1H and 13 C NMR spectra. AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT N.A. gratefully acknowledges financial support from the Swiss confederation under Swiss Government Scholarship programme and QEERI. We thank the European Community’s Seventh Frame- work Programme (FP7/2007-2013) under grant agreement no. 281063 of the Powerweave project, under grant agreement n° 604032 of the MESO project, (FP7/2007-2013) ENERGY.2012.10.2.1; grant agreement no. 308997, NANOMATCELL for financial support. The authors thank Dr. Michael Saliba for providing HTM solutions. M.G. and M.I.D. thank the King Abdulaziz City for Science and Technology (KACST) for financial support.

REFERENCES (1) Gao, P.; Gratzel, M.; Nazeeruddin, M. K. Organohalide Lead Perovskites for Photovoltaic Applications. Energy Environ. Sci 2014, 7, 2448-2463.


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(2) 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. (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.; et al. Lead Iodide Perovskite Sensitized All-SolidState Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (5) 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, 643647. (6) 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, 897903. (7) 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, 1480-1485. (8) Christians, J. A.; Fung, R. C. M.; Kamat, P. V. An Inorganic Hole Conductor for OrganoLead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide. J. Am. Chem. Soc. 2014, 136, 758-764.


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(9) Qin, P.; Tanaka, S.; Ito, S.; Tetreault, N.; Manabe, K.; Nishino, H.; Nazeeruddin, M. K.; Grätzel, M. Inorganic Hole Conductor-Based Lead Halide Perovskite Solar Cells with 12.4% Conversion Efficiency. Nat Commun 2014, 5, 3834. (10) Ganesan, P.; Fu, K.; Gao, P.; Raabe, I.; Schenk, K.; Scopelliti, R.; Luo, J.; Wong, L. H.; Gratzel, M.; Nazeeruddin, M. K. A Simple Spiro-Type Hole Transporting Material for Efficient Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 1986-1991. (11) Qin, P.; Tetreault, N.; Dar, M. I.; Gao, P.; McCall, K. L.; Rutter, S. R.; Ogier, S. D.; Forrest, N. D.; Bissett, J. S.; Simms, M. J.; et al. A Novel Oligomer as a Hole Transporting Material for Efficient Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1400980 (12) Schreier, M.; Curvat, L.; Giordano, F.; Steier, L.; Abate, A.; Zakeeruddin, S. M.; Luo, J.; Mayer, M. T.; Gratzel, M. Efficient Photosynthesis of Carbon Monoxide from CO2 Using Perovskite Photovoltaics. Nat. Commun. 2015, 6, 7326. (13) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Grätzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-Abundant Catalysts. Science 2014, 345, 1593-1596. (14) Heo, J. H.; Song, D. H.; Im, S. H. Planar CH3NH3PbBr3 Hybrid Solar Cells with 10.4% Power Conversion Efficiency, Fabricated by Controlled Crystallization in the Spin-Coating Process. Adv. Mater. 2014, 26, 8179-8183. (15) Ryu, S.; Noh, J. H.; Jeon, N. J.; Chan Kim, Y.; Yang, W. S.; Seo, J.; Seok, S. I. Voltage Output of Efficient Perovskite Solar Cells with High Open-Circuit Voltage and Fill Factor. Energy Environ. Sci. 2014, 7, 2614-2618.


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(16) Edri, E.; Kirmayer, S.; Kulbak, M.; Hodes, G.; Cahen, D. Chloride Inclusion and Hole Transport Material Doping to Improve Methyl Ammonium Lead Bromide Perovskite-Based High Open-Circuit Voltage Solar Cells. J. Phys. Chem. Lett. 2014, 5, 429-433. (17) Saliba, M.; Orlandi, S.; Matsui, T.; Aghazada, S.; Cavazzini, M.; Correa-Baena, J.-P.; Gao, P.; Scopelliti, R.; Mosconi, E.; Dahmen, K.-H.; et al. A molecularly engineered hole-transporting material for efficient perovskite solar cells. Nature Energy 2016, 1, 15017. (18) Amat, A.; Mosconi, E.; Ronca, E.; Quarti, C.; Umari, P.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F. Cation-Induced Band-Gap Tuning in Organohalide Perovskites: Interplay of Spin–Orbit Coupling and Octahedra Tilting. Nano Lett. 2014, 14, 3608-3616. (19) Hanusch, F. C.; Wiesenmayer, E.; Mankel, E.; Binek, A.; Angloher, P.; Fraunhofer, C.; Giesbrecht, N.; Feckl, J. M.; Jaegermann, W.; Johrendt, D.; et al. Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium Lead Bromide. J. Phys. Chem. Lett. 2014, 5, 2791-2795. (20) Mitschke, U.; Bauerle, P. Synthesis, Characterization, and Electrogenerated Chemiluminescence of Phenyl-Substituted, Phenyl-Annulated, and Spirofluorenyl-Bridged Oligothiophenes. J. Chem. Soc., Perkin Trans. 2001, 1, 740-753. (21) Ong, T.-T.; Ng, S.-C.; Chan, H. S. O.; Vardhanan, R. V.; Kumura, K.; Mazaki, Y.; Kobayashi, K. Development of a Novel Isotype Organic Heterojunction Diode Consisting of Poly{7-Spiro(9-Fluorenyl)Cyclopentadithiophene} and Poly(3-Octylthiophene). J. Mater. Chem. 2003, 13, 2185-2188.


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(22) Ting, H.-C.; Tsai, C.-H.; Chen, J.-H.; Lin, L.-Y.; Chou, S.-H.; Wong, K.-T.; Huang, T.-W.; Wu, C.-C. A Novel Amine-Free Dianchoring Organic Dye for Efficient Dye-Sensitized Solar Cells. Org. Lett. 2012, 14, 6338-6341. (23) Jeux, V.; Dalinot, C.; Allain, M.; Sanguinet, L.; Leriche, P. Synthesis of Spiro[Cyclopenta[1,2-b:5,4-b′]DiThiophene-4,9′-Fluorenes] SDTF Dissymmetrically Functionalized. Tetrahedron Lett. 2015, 56, 1383-1387. (24) Pozzi, G.; Orlandi, S.; Cavazzini, M.; Minudri, D.; Macor, L.; Otero, L.; Fungo, F. Synthesis and Photovoltaic Applications of a 4,4′-Spirobi[Cyclopenta[2,1-B;3,4B′]Dithiophene]-Bridged Donor/Acceptor Dye. Org. Lett. 2013, 15, 4642-4645. (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) Arora, N.; Dar, M. I.; Hezam, M.; Tress, W.; Jacopin, G.; Moehl, T.; Gao, P.; Aldwayyan, A. S.; Deveaud, B.; Grätzel, M.; et al. Photovoltaic and Amplified Spontaneous Emission Studies of High-Quality Formamidinium Lead Bromide Perovskite Films. Adv. Funct. Mater. 2016, 26, 2846-2854. (27) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344.


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(28) Dar, M. I.; Abdi-Jalebi, M.; Arora, N.; Moehl, T.; Grätzel, M.; Nazeeruddin, M. K. Understanding the Impact of Bromide on the Photovoltaic Performance of CH3NH3PbI3 Solar Cells. Adv. Mater. 2015, 27, 7221-7228. (29) Sheng, R.; Ho-Baillie, A.; Huang, S.; Chen, S.; Wen, X.; Hao, X.; Green, M. A. Methylammonium Lead Bromide Perovskite-Based Solar Cells by Vapor-Assisted Deposition. J. Phys. Chem. C 2015, 119, 3545-3549. (30) 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, 193-198. (31) Umari, P.; Mosconi, E.; De Angelis, F. Relativistic GW calculations on CH3NH3PbI3 and CH3NH3SnI3 Perovskites for Solar Cell Applications. Sci. Rep. 2014, 4, 4467 (32) Aharon, S.; Dymshits, A.; Rotem, A.; Etgar, L. Temperature Dependence of Hole Conductor Free Formamidinium Lead Iodide Perovskite based Solar Cells. J. Mater. Chem. A 2015, 3, 9171-9178. (33) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764-1769. (34) Schulz, P.; Edri, E.; Kirmayer, S.; Hodes, G.; Cahen, D.; Kahn, A. Interface Energetics in Organo-Metal Halide Perovskite-Based Photovoltaic Cells. Energy Environ. Sci. 2014, 7, 13771381.


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(35) 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.


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High Open-Circuit Voltage: Fabrication of Formamidinium Lead

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