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High Open-Circuit Voltage: Fabrication of Formamidinium Lead Bromide Perovskite Solar Cells Using Fluorene−Dithiophene Derivatives as Hole-Transporting Materials Neha Arora,† Simonetta Orlandi,‡ M. Ibrahim Dar,†,§ Sadig Aghazada,† Gwénolé Jacopin,∥ Marco Cavazzini,‡ Edoardo Mosconi,⊥ Paul Gratia,† Filippo De Angelis,⊥ Gianluca Pozzi,‡ Michael Graetzel,§ and Mohammad Khaja Nazeeruddin*,† Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland ‡ Istituto di Scienze e Tecnologie Molecolari del Consiglio Nazionale delle Ricerche, CNR-ISTM, via Golgi 19, I-20133 Milano, Italy § Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland ∥ Laboratory of Quantum Optoelectronics, Institute of Physics, École Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland ⊥ Computational Laboratory for Hybrid Organic Photovoltaics (CLHYO), CNR-ISTM, via Elce di Sotto 8, I-06123 Perugia, Italy †
S Supporting Information *
ABSTRACT: Four different fluorene−dithiophene derivative-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, ultraviolet−visible 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 on par with the device using the state-of-the-art SpiroOMeTAD as HTM, which delivered a Voc of 1.47 V and a maximum η of 6.9%. A density functional theory approach with GW simulations including spin−orbit coupling and electrochemical measurements revealed deeper highest occupied molecular orbital levels for newly synthesized fluorene−dithiophene derivatives, which eventually makes them promising HTMs for perovskite solar cells, especially when high photovoltage is desired.
F
demands the development of cost-effective 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 cells yielding appreciable performances.8,9 In the last two years, the interest in small
abrication 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-p dimethoxyphenylamino)-9,9′- spirobifluorene (SpiroOMeTAD) as hole-transporting material (HTM).4,5 However, the arduous and expensive synthesis of Spiro-OMeTAD © 2016 American Chemical Society
Received: April 20, 2016 Accepted: May 7, 2016 Published: May 8, 2016 107
DOI: 10.1021/acsenergylett.6b00077 ACS Energy Lett. 2016, 1, 107−112
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http://pubs.acs.org/journal/aelccp
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ACS Energy Letters Scheme 1. General Conditions for the Synthesis of SO7−SO10
of 2,2′-bithiophene derivatives.24 This finding allowed us to develop convenient access to the versatile fluorene−dithiophene 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 fluorene−dithiophene compounds were thoroughly characterized by 1H/13C NMR spectroscopy and high-resolution mass spectrometry. The analytical data corroborated the proposed HTM structures. Cyclic voltammetry (CV) in combination with ultraviolet (UV)−visible and photoluminescence (PL) spectroscopy was used to determine the energy levels of SO7−10, as summarized in Table 1.
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 because of their simple synthesis, ease of modification by molecularly engineering the substitutent electron 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 that of CH3NH3PbBr3.18 Such a narrowing of the bandgap extends the absorption onset of FA-based perovskites toward longer wavelength, which will allow it to harness relatively more photons of the solar spectrum.19 All the new HTM-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 the fabrication of FAPbBr3 solar cells 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 SO7−SO10 series and Spiro-OMeTAD benchmark was investigated using a hybrid density functional theory (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 to date.20−23 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-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
Table 1. Spectroscopic and Electrochemical Data of HTMsa HTM SpiroOMeTAD SO7 SO8 SO9 SO10
HOMO vs NHE 0.72; 0.83; 1.03 0.91; 1.2 0.81; 1.13 0.81; 1.12 0.83; 1.09
λmax (absorption) (nm)
λmax (emission) (nm)
E0−0 (eV)
LUMO vs NHE
386
419
3.05
−2.33
379 386 386 384
402 413 411 416
3.16 3.09 3.10 3.09
−2.25 −2.28 −2.29 −2.26
a
Oxidation potentials were obtained via CV measurements. E0−0 values were determined from the intersection of normalized absorption and emission spectra. HTMs LUMO levels were estimated as follows: Eox(HTM+/HTM) = Eox(HTM+/HTM) − E0−0.
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, SO8, SO9, and SO10 were −5.41, −5.31, −5.31, and −5.33 eV, respectively, which are slightly deeper as compared to that of 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 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 (Δλ = 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 108
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Figure 1. (a) Cyclic voltammograms and (b) UV−vis absorbance (left set of curves) and steady-state photoluminescence spectra (right set of curves) of Spiro-OMeTAD and SO7−10 HTMs.
Figure 2. Spectroscopic studies of different FAPbBr3 perovskite samples: (a) time-integrated photoluminescence spectra and (b) timeresolved photoluminescence spectra measured at a wavelength near band gap that yields maximum PL signal upon exciting pristine and HTMcoated perovskite samples at 420 nm. (Inset: summarized charge carrier lifetime and corresponding intensities obtained by using two exponential model.)
pristine perovskite film was fitted using two exponential decay model,27 which revealed lifetimes of 22 and 96 ns, for fast and slow components, respectively. The long-lasting component is due to the recombination occurring in the bulk of the perovskite structures, whereas a fast relaxation component could be attributed to the surface recombination.28 In HTMcoated FAPbBr3 films, the charge carrier lifetime decreased considerably (Figure 2, inset), 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 < τ1SO10 < τ1SO7) which infers faster extraction of holes at the perovskite/spiro-OMeTAD interface. The trend in charge carrier lifetime follows the order of oxidation potentials of the HTMs. The larger the offset between the VB of FAPbBr3 perovskite and the HOMO of the HTM, the faster the PL decay. Astoundingly, S08 does not follow the trend that could be linked to other factors such as chemical interaction of HTM molecules with the FAPbBr3 perovskite surface due to the presence of long alkyl groups. From the contemplation based on PL and TRPL studies, we contend not merely the position of HOMO but also the chemical interaction between HTM and perovskite could influence the lifetime of charge carriers. Therefore, such a critical dependence of charge carrier dynamic properties on the band alignment and chemical interaction demands meticulous engineering of chemical structures of HTMs.
normalized absorption and emission spectra, the optical bandgap (E0−0) values were calculated, and using the groundstate oxidation potentials, we estimated the lowest unoccupied molecular orbital (LUMO) of HTMs. Due to the presence of heteroatom, all four HTMs have LUMO energy levels that are more positive than that of Spiro-OMeTAD, which are still significantly more positive than perovskite conduction band, thereby acting as efficient electron-blocking layers. 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, 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. Finally, the HTMs were deposited by spin-casting a solution of Spiro-OMeTAD and SO7−SO10 onto the FAPbBr3 films (see Experimental Methods in the Supporting Information). Upon exciting the pristine perovskite film at 420 nm, strong and narrow PL spectra with full-width 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 109
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Figure 3. (a) Device architecture based on FTO/compactTiO2/meso-TiO2/FAPbBr3/HTM/Au and (b) photovoltaic performance of FAPbBr3 perovskite devices with Spiro-OMeTAD and SO7−10 as HTMs recorded at a scan rate of 0.1 V/s under simulated AM1.5 100 mW cm−2 photon flux. Open symbols represent dark current. (Inset: summarized photovoltaic parameters derived from current−voltage curves.)
New fluorene−dithiophene derivative-based HTMs showed higher oxidation potentials (Figure 1a) as compared to SpiroOMeTAD; therefore, we expect a higher photovoltage from SO7−10-based bromide perovskite solar cells. We explored the photovoltaic performance of FAPbBr3 perovskite solar cells fabricated using the four different fluorene−dithiophene derivatives (SO7, SO8, SO9, and SO10; Figure 3a) The current density−voltage (J−V) characteristics of the FAPbBr3 perovskite solar cells prepared from different HTMs under simulated air mass 1.5 global (AM 1.5 G) solar irradiation are shown in Figure 3b, and the extracted photovoltaic parameters are summarized in the inset. The J−V curves obtained from the best-performing solar cells among the 25 devices prepared using each HTM are displayed. The highest energy conversion efficiency of 7.1% (Voc = 1.50 V, short-circuit current density (Jsc) = 6.9 mA·cm−2, and fill factor (FF) = 0.69) was achieved by employing SO7 as the HTM for the device based on FTO/compactTiO 2 /meso-TiO 2 / FAPbBr3/HTM/Au architecture. For comparative analysis, the device based on Spiro-OMeTAD was also fabricated, and the obtained maximum η value is 6.9% (Voc = 1.47 V, Jsc = 6.7 mA·cm−2, and FF = 0.70). All the new HTMs displayed strikingly high Voc values of >1.45 V 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 affect the Voc only 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 SpiroOMeTAD 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 (SpiroOMeTAD) 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). To investigate the electronic properties of the new HTMs against those of the perovskite absorbers, we performed first-
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 mW cm−2 photon flux. 110
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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 to date. Overall, the photovoltaic performance showed good reproducibility, and current−voltage curves exhibited negligible hysteresis. TRPL studies revealed 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 fluorene−dithiophene derivatives are promising HTM candidates for the fabrication of efficient bromide-based perovskite solar cells, especially when high photovoltage is in demand.
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
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00077. Experimental details, cross-sectional SEM images, IPCE spectra, conductivity data, and 1H and 13C NMR spectra (PDF)
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.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: mdkhaja.nazeeruddin@epfl.ch.
oxidation potentials of the molecular HTMs are calculated on an absolute scale against the vacuum, to align the perovskites band edges on an 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, ΔVB/CB in Figure 5. 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 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 Spiro-OMeTAD.35 The substituents modulate the HTM 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
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS N.A. gratefully acknowledges financial support from the Swiss confederation under Swiss Government Scholarship Programme and QEERI. We thank the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement 281063 of the Powerweave project, under Grant Agreement 604032 of the MESO project, (FP7/20072013) ENERGY.2012.10.2.1; Grant Agreement 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.
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REFERENCES
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DOI: 10.1021/acsenergylett.6b00077 ACS Energy Lett. 2016, 1, 107−112
