Efficient and Stable Perovskite Solar Cells Using Molybdenum Tris

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, U...
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Efficient and Stable Perovskite Solar Cells Using Molybdenum Tris(dithiolene)s as p‑Dopants for Spiro-OMeTAD Alba Pellaroque,† Nakita K. Noel,† Severin N. Habisreutinger,† Yadong Zhang,‡ Stephen Barlow,‡ Seth R. Marder,‡ and Henry J. Snaith*,† †

Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States

‡

S Supporting Information *

ABSTRACT: Metal halide perovskite solar cells have now reached efficiencies of over 22%. To date, the most efficient perovskite solar cells have the n-i-p device architecture and use 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene or poly(triarylamine) as the hole transport material (HTM), which are typically doped with lithium bis((trifluomethyl)sulfonyl)amide (Li-TFSI). Li-TFSI is hygroscopic and detrimental to the long-term performance of the solar cells, limiting its practical use. In this work, we successfully replace Li-TFSI by molybdenum tris(1-(methoxycarbonyl)-2-(trifluoromethyl)ethane-1,2-dithiolene), Mo(tfd-CO2Me)3, or molybdenum tris(1-(trifluoroacetyl)-2-(trifluoromethyl)ethane-1,2-dithiolene), Mo(tfd-COCF3)3. With these two dopants, we achieve stabilized power conversion efficiencies up to 16.7% and 15.7% with average efficiencies of 14.8% ± 1.1% and 14.4% ± 1.2%, respectively. Moreover, we observe a significant enhancement of the long-term stability of perovskite solar cells under 85 °C thermal stressing in air.

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with an additional p-dopant, typically a tris(pyrazolylpyridine)cobalt(III) complex.6,10 For both spiro-OMeTAD and PTAA, the presence of the Li-TFSI has thus far been a prerequisite to obtaining high efficiency. However, the use of Li-TFSI has been found to be detrimental to the long-term performance of the solar cells because of its hygroscopic nature.11,12 Furthermore, Li-TFSI does not directly oxidize spiro-OMeTAD, but rather promotes oxidation of the HTM by molecular oxygen to reach appropriate conductivities; the requirement for oxygen exposure thus makes it difficult to precisely control the exact fraction of spiro-OMeTAD that is oxidized.13 Because both device stability and reliability are crucial factors for a photovoltaic technology, it is necessary to achieve highly efficient operation without relying on the use of hygroscopic dopants such as Li-TFSI. Excluding Li-TFSI to improve stability has been achieved to recent years through several approaches.6 However, this has generally resulted in a significant drop in performance, for example, Shin et al. replaced PTAA with NiO, resulting in much improved stability but a concurrent efficiency drop from 21 to 14%.7 Another

ecently, organic−inorganic halide perovskite solar cells have emerged as a very promising alternative to existing photovoltaic technologies and have reached certified power conversion efficiencies of over 22%.1 However, one of the hurdles which can potentially prevent the commercialization of this technology is the stability of the devices under realworld conditions. One of the critical concerns is the instability of iodoplumbate-based perovskite materials themselves, which quickly degrade to PbI2 when exposed to moisture and heat.2−4 In their review, Habisreutinger et al. present strategies to avoid the most common degradation pathways in perovskite devices and stress the importance of preparing solar cells which have an inherent resistance to moisture ingress.5 To date, the most efficient perovskite solar cells are fabricated with the n-i-p device architecture, i.e., with the perovskite deposited on the electron-transporting layer, and employ 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′spirobifluorene (spiro-OMeTAD) or a triarylamine polymer (PTAA) as the hole-transporting material (HTM).6−8 However, neat organic HTMs suffer from low hole mobility and low conductivity,9 and therefore require doping to perform efficiently. This is generally accomplished by addition of both the ionic dopant lithium bis((trifluoromethyl)sulfonyl)amide (Li-TFSI) and 4-tert-butylpyridine (tBP), often in combination © 2017 American Chemical Society

Received: July 11, 2017 Accepted: August 15, 2017 Published: August 15, 2017 2044

DOI: 10.1021/acsenergylett.7b00614 ACS Energy Lett. 2017, 2, 2044−2050

Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters

To confirm that Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 can effectively dope spiro-OMeTAD, we examine absorption spectra of doped and undoped films, as we show in panels a and b of Figure 1, respectively. For the neat films, we observe

strategy has been to use nonhygroscopic, chemically inert conductive additives such as carbon nanotubes or graphene in place of Li-TFSI.11,14 These additives serve to increase the conductivity of the HTM layer, while reducing its vulnerability to moisture ingress. The addition of salts of preoxidized spiroOMeTAD and related compounds can increase the conductivity of the HTM and delivers comparable performance to Li-TFSI,15,16 while doped HTMs have also been obtained from solutions of spiro-OMeTAD that are partially oxidized indirectly using protic species.17 Recently, Bolink and coworkers have demonstrated some extremely efficient fully vapor-deposited perovskite solar cells employing dopants developed for organic light-emitting diode applications, using 1,3,4,5,7,8-hexafluoro-9,9,10,10-tetracyano-2,6-naphthoquinodimethane (F6-TCNNQ) as a p-dopant for a bis(diarylamino)terphenyl HTM.18 Various cobalt(III)-complexes19,20 and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ)21 have also been successfully used as sole dopant for solution-processed spiro-OMeTAD films. Nonetheless, to achieve record efficiencies, the former still requires the addition of lithium salts, and the latter is highly volatile, poorly soluble, and prone to diffusion in some host materials.22,23 Molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd)3) is less volatile, more soluble, and less prone to diffusion than F4-TCNQ and has been used to p-dope a variety of materials. For example, it p-dopes N,N′-di[(1-naphthyl)-N,N′diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD) when coevaporated,24 and the Mo(tfd)3− anion does not diffuse significantly through α-NPD at temperatures of up to 110 °C. Although Mo(tfd)3 has reasonable solubility in organic solvents by itself, its doping products are not always very soluble in solvents suitable for spin-coating semiconductor films; hence, the more soluble analogues molybdenum tris(1-(methoxycarbonyl)-2(trifluoromethyl)ethane-1,2-dithiolene), Mo(tfd-CO2Me)3, and molybdenum tris(1-(trifluoroacetyl)-2-(trifluoromethyl)ethane1,2-dithiolene), Mo(tfd-COCF3)3, have been developed for solution processing.25,26 Mo(tfd-CO2Me)3 has been used to dope P3HT to afford an effective hole-transporting material for organic photovoltaics27 and has been found to p-dope spiroOMeTAD in a study of electric-field-induced dopant drift.28 Mo(tfd-COCF3)3 has been used in solution-processed fieldeffect transistors based on p-doped 6,13-bis(triisopropylsilylethynyl)pentacene/polymer blends.29 Very recently, Chang et al. reported a 18.5% efficient “inverted” p-i-n perovskite solar cell employing a solution-processed Mo(tfdCOCF3)3-doped poly(3,4-ethylenedioxythiophene (PEDOT) HTM.30 All three of these molybdenum tris(dithiolene) (Mo(dt)3) derivatives are air-stable in the solid state, and all exhibit reversible reductions to stable monoanions at potentials (+0.12 to +0.39 V vs ferrocene in dichloromethane) sufficient to oxidize spiro-OMeTAD26 to its monocation (+0.05 V vs ferrocene in dichloromethane), and both Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 are soluble in solvents commonly used for solution processing, making them good candidates to replace Li-TFSI as a dopant for spiro-OMeTAD in perovskite-based solar cells. In this study, we successfully use molybdenum tris(dithiolene)s to p-dope spiro-OMeTAD, replacing the conventional and hygroscopic Li-TFSI/FK-209/tBP doping cocktail. Through the use of these dopants, we achieve stabilized power conversion efficiencies greater than 16% and significantly enhance the long-term stability of perovskite solar-cells under thermal stressing.

Figure 1. Changes in the ultraviolet−visible (UV−vis) absorption spectra of films of spiro-OMeTAD with the addition of (a) Mo(tfdCOCF3)3 and (b) Mo(tfd-CO2Me)3 to the casting solution, with their respective chemical structures shown as insets. Peaks at 520 and 690 nm can be assigned to oxidized spiro-OMeTAD (likely spiro-OMeTAD+, but spiro-OMeTAD2+ has an essentially identical spectrum),31 with minor contributions from Mo(dt)3− absorptions. (c) Absorption spectra of pristine spiro-OMeTAD compared to the standard Li-TFSI/FK209/tBP-doped and molybdenum-doped spiro-OMeTAD. For both Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 a molar concentration of 5% relative to spiro-OMeTAD was chosen. The molar concentrations of additives for the Li-doped reference with respect to spiro-OMeTAD are 50%, 3%, and 330% for Li-TFSI, FK209, and tBP, respectively.

the characteristic absorption peak of neutral spiro-OMeTAD at 390 nm. With the addition of 1 mol % of either molybdenum dopant to the solution used to cast the film, a decrease is seen in the absorption peak at 390 nm and the appearance of two new absorption features, at ca. 500 and 700 nm, further decreases at 390 nm peak, coupled with increases at 500 and 700 nm being seen with increased dopant concentration. The mono- and dication of spiro-OMeTAD both show absorptions in these regions,31 although presumably it is the monocation that is formed in the presence of excess neutral spiroOMeTAD; the monoanions of the Mo(dt)3 dopants also absorb at similar wavelengths (ca. 490 and 670 nm in CH2Cl226), albeit weakly. The spectra, therefore, suggest that, consistent with solution electrochemical data15,26 and with a previous study of dopant drift that included Mo(tfd-Co2Me)3doped spiro-OMeTAD,28 Mo(tfd-COCF3)3 and Mo(tfdCO2Me)3 are indeed capable of oxidizing spiro-OMeTAD. In Figure 1c, we compare the absorption of neat to oxidized spiroOMeTAD, where the spiro-OMeTAD has been oxidized using either the standard concentration of Li-TFSI (50 mol %) with tBP (330 mol %) additive and the cobalt-complex dopant FK209 (3 mol %), or 5 mol % of either of the molybdenum 2045

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Figure 2. (a) Conductivity of spiro-OMeTAD doped with the commonly used Li-TFSI/FK209/tBP after exposure to dry air for 24 h (gray line), and with Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 as a function of molar doping content relative to spiro-OMeTAD. The molar % of additives for the Li-doped reference with respect to spiro-OMeTAD is 50%, 3%, and 330% for Li-TFSI, FK209, and tBP, respectively. (b) UV−vis absorbance spectra of Li-TFSI/FK209/tBP-doped spiro-OMeTAD taken immediately after spin-coating in nitrogen and after air exposure for 24 h in a desiccator with corresponding film conductivities.

Figure 3. Current density−voltage characteristics of the champion devices using spiro-OMeTAD doped with (a) Li-TFSI/FK209/tBP, (c) Mo(tfd-COCF3)3, and (e) Mo(tfd-CO2Me)3. Panels b, d, and f show the stabilized photocurrent densities and efficiencies of the devices presented in panels a, c, and e, respectively, measured by holding the devices at their J−V determined maximum power point for 50 s.

To correlate the evolution of absorption features with the increase of conductivity due to the introduction of holes on pdoping, we performed four-point probe conductivity measurements. Figure 2a shows the conductivity of spiro-OMeTAD as a function of doping concentration. The measurements were made immediately after spin-coating the films in a nitrogen atmosphere. Both Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 enhance the conductivity by almost 4 orders of magnitude

tris(dithiolene) dopants. While the absorption features of oxidized spiro-OMeTAD are present for all the dopants, the LiTFSI/FK209/tBP-doped spiro-OMeTAD exhibits weaker features in the 500 and 700 nm regions, perhaps suggesting a higher efficiency of doping by the Mo dopants, although contributions by Mo(dt)3− absorptions in the same wavelength range preclude precise quantification. 2046

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ACS Energy Letters Table 1. Device Performance Parameters of Champion Devicesa JSC [mA·cm−2] Li-TFSI/FK-209/tBP Mo(tfd-COCF3)3 Mo(tfd-CO2Me)3 a

VOC [V]

FF

PCE [%]

Rs [Ω·cm−2]

FB-SC

SC-FB

FB-SC

SC-FB

FB-SC

FB-SC

FB-SC

SC-FB

FB-SC

SC-FB

SPO [%]

SPO/PCE

21.97 22.11 21.98

21.96 22.12 21.98

1.11 1.07 1.06

1.11 1.04 1.02

0.73 0.76 0.74

0.68 0.68 0.69

18.2 17.8 17.2

16.8 15.6 15.4

4.33 2.47 3.78

6.42 4.77 6.01

18.1 16.7 15.7

0.99 0.94 0.92

FB-SC: Forward-Bias to Short-Circuit; SC-FB: Short-Circuit to Forward-Bias.

the best performing device. Losses in voltage are compensated to some degree by an increase in fill factor (FF), which we attribute to the lower series resistance of the Mo(dt)3-doped HTMs, due to their increased hole conductivities (Figure 2a). As a result, Mo(dt)3-doped devices achieve J−V determined power conversion efficiencies (PCE) surpassing 17% for our champion devices. In a statistical analysis across four batches of devices (Figure S3) comparing Li-TFSI/FK209/tBP to Mo(tfdCOCF3)3 and Mo(tfd-CO2Me)3, we observe that Mo(dt)3doped devices lead to similar short-circuit current (JSC), lower VOC, and higher FF compared to the Li-TFSI/FK209/tBP reference devices. We also find that the ratio of steady-state performance to scanned performance is on average 10% lower for Mo(dt)3-doped spiro-OMeTAD compared to Li-based HTM. In order to improve the stabilized power output (SPO) of devices using Mo(dt)3 p-dopants, we studied the influence of tBP on scanned and steady-state performances (Figures S4 and S5). At very low concentrations, tBP has a beneficial effect on device performance for Mo(tfd-COCF3)3-doped spiro-OMeTAD (Figure S4). The introduction of tBP leads to an increase in VOC and stabilized power output and improves the ratio of steady-state performance to scanned performance. However, with the addition of tBP at greater than 2 μL per mL of spiroOMeTAD solution (equivalently 20 mol %), the device performance plummets because of an abrupt drop in FF. We observe a similar effect for Mo(tfd-CO2Me)3-doped spiroOMeTAD (Figure S5). However, in the latter case tBP does not lead to any clear improvement in device performance. We attribute this decrease in device performance upon tBP addition to the “de-doping” of the spiro-OMeTAD (Figure S6). The absorption spectra of Mo-doped spiro-OMeTAD taken before and after the addition of tBP shows an increase in the strength of the ground-state spiro-OMeTAD absorption at 390 nm and a decrease in the absorption features at both 520 and 690 nm. This clearly indicates a reduction of oxidized spiro-OMeTAD within the films. Because Mo(dt)3-based devices without the use of tBP still deliver SPOs of 16.7% for Mo(tfd-COCF3)3 and 15.7% for Mo(tfd-CO2Me)3, we do not add tBP to the HTM for the following stability studies. Notably, our study highlights that tBP, which is commonly employed in the highest efficiency perovskite solar cells, plays a dual role. It is in part responsible for increasing the VOC and the stabilized power output of the solar cells,34 but it also prevents phase segregation of Li-TFSI and spiro-OMeTAD35 (Figure S7). However, because the basic nature of tBP results in dedoping of organic HTMs, exploring the root cause of the beneficial effect of tBP and discovering other means to circumvent its use are key challenges to enable higher efficiencies from alternative dopant compositions, such as the Mo-dopants which we are studying here. To investigate whether the replacement of lithium salts with Mo(dt)3-based complexes has an impact upon device stability, we subjected the complete devices to a thermal stress test by heating the nonencapsulated devices to 85 °C on a hot plate in the dark, under ambient atmosphere (relative humidity of 30−

relative to that of undoped spiro-OMeTAD, resulting in conductivity higher than that obtained with Li-TFSI/FK209/ tBP. The conductivity increases exponentially with the dopant concentration. We also show, as a straight line, the conductivity of spiro-OMeTAD doped with Li-TFSI/FK209/tBP at the standard concentration after leaving the film in dry air for 24 h. Our results are consistent with effective p-doping of spiroOMeTAD by both Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3. Furthermore, the Mo(dt)3 complexes react instantly with the spiro-OMeTAD host material, consistent with a straightforward exergonic electron-transfer mechanism, in contrast to Li-TFSI doping which requires oxygen, as we have previously reported.32 Figure 2b compares the absorption spectra of spiro-OMeTAD doped with Li-TFSI/FK209/tBP immediately after spin-coating in inert atmosphere, and after keeping the sample in dry air for 24 h; with time, features attributed to oxidized spiro-OMeTAD increase in strength consistent with the creation of additional charges carriers (holes) leading to higher conductivity (denoted in the legend). In contrast, upon air exposure Mo(dt)3-doped films remain stable over time, indicating that oxidation of spiro-OMeTAD in ambient conditions is not contributing to changes in conductivity in the Mo(dt)3-doped films (Figure S8). Notably, our results reveal that spiro-OMeTAD films doped with only 5 mol % Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 exhibit higher conductivities than the Li-TFSI/FK209/tBP doped films, and at 30 mol % doping are around 1 order of magnitude more conductive. This is consistent with much higher doping efficacy of the Mo(dt)3 dopants versus Li-TFSI, and in addition less of a negative impact upon mobility at high concentrations. Next, we investigate the performance of Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 doped spiro-OMeTAD in perovskite solar cells using the device architecture FTO/SnO2/PC60BM/ FA0.85MA0.15Pb(I0.85Br0.15)3/Spiro-OMeTAD (with dopants)/ Au, where PC60BM is [6,6]-phenyl-C60-butyric acid methyl ester. For our control devices, we use the commonly employed additives Li-TFSI, FK209, and tBP.33 To keep the conductivity to within the same order of magnitude across the various dopants, we fixed the doping level to 5 mol % of Mo(tfdCOCF3)3 and Mo(tfd-CO2Me)3 with respect to the molar concentration of spiro-OMeTAD, corresponding to a conductivity of ∼5 × 10−5 S·cm−1 (Figure 2a). Furthermore, we find that this doping concentration gives the best trade-off between scanned and steady-state performances for both Mo(dt)3 dopants, which we show in Figures S1 and S2. In Figure 3, we show the current density−voltage (J−V) characteristics and steady-state efficiency of the best performing devices doped with Li-TFSI/FK209/tBP and both Mo(dt)3 pdopants, notably without the addition of tBP, recorded under simulated AM1.5, 1 sun irradiance. We present the performance parameters of these champion devices in Table 1. With the use of the molybdenum complexes in the absence of tBP, the opencircuit voltage (VOC) decreases from 1.13 to 1.07 V and 1.06 V for Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3, respectively, for 2047

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expect further improvements to be achievable by moving to FA/Cs and butylammonium/FA/Cs mixed cation compositions. In summary, we show that Mo(dt)3 complexes can be used to efficiently p-dope spiro-OMeTAD. Unlike Li-TFSI, the Mo(dt)3 dopants react instantly with spiro-OMeTAD to increase the conductivity of the p-type layer by up to 4 orders of magnitude, compared to undoped neat spiro-OMeTAD. Our champion devices deliver stabilized power conversion efficiencies of 16.7% for Mo(tfd-COCF3)3 and 15.7% for Mo(tfdCO2Me)3 and significantly enhance the long-term stability of perovskite solar-cells. After exposing nonencapsulated devices to 85 °C in air and dark for 500 h, Mo(dt)3-doped spiroOMeTAD devices maintain up to 70% of their initial performance while steady-state efficiencies of the solar cells fabricated using lithium salts quickly decline from 17.1% to 5.1%. Our stability tests reveal that Mo(dt)3 complexes have a remarkable positive impact on the long-term stability of perovskite solar cells. In addition, our study highlights the beneficial impact of the additive tBP upon performance but its incompatibility with these new p-dopants. Therefore, focusing on elucidating new routes to improve the efficiency with “pdopant compatible treatments” is required. In addition, investigating this p-dopant with more thermally stable perovskites, such a FA/Cs mixed cation compositions, should deliver further improvements in overall stability.

40%) for 500 h. The cells were periodically removed from the hot plate and tested at room temperature under simulated AM1.5 (100 mW·cm2) sunlight. The cells therefore also underwent 10 thermal cycles from room temperature to 85 °C. We present the average stabilized efficiencies of a batch of eight devices per parameter measured by holding the devices at their J−V determined maximum power point for 60 s, and their corresponding standard deviations are shown in Figure 4a.

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EXPERIMENTAL METHODS Molybednum Tris(dithiolene) Dopant Synthesis. The p-dopants Mo(tfd-CO2Me)325 and Mo(tfd-COCF3)337 were synthesized according to the literature. Solar Cell Fabrication. Fluorine-doped tin oxide (FTO) coated glass sheets (7 Ω·cm−2 Hartford Glass) were etched with zinc powder and HCl (3 M) to obtain the required electrode pattern. The sheets were first cleaned with detergent (2% Hellmanex in water) and then rinsed sequentially with deionized water, acetone, and isopropanol. The last traces of organic residues were removed by oxygen plasma cleaning for 10 min. The electron-transporting layer was prepared by first spin-coating a solution of SnCl4·5H2O in isopropanol38,39 (17.5 mg·mL−1). The substrates were then dried in air at 100 °C for 10 min then annealed at 180 °C for 60 min. Once the substrates were cooled to room temperature, a solution of [6,6]-phenyl-C 60-butyric acid methyl ester (PCBM) in dichlorobenzene (DCB) (5 mg·mL−1) was dynamically spincoated onto the substrate at 4000 rpm and dried at 100 °C for 5 min. A mixed-cation lead mixed-halide perovskite solution was prepared from a precursor solution made of formamidinium iodide (1.25 M, Dyesol), lead iodide (1.38 M, TCI), methylammonium bromide (0.25 M, Dyesol), and lead(II) bromide (0.25 M, Sigma-Aldrich) in a 4:1 (v:v) mixture of anhydrous DMF:DMSO (Sigma-Aldrich) and stirred at 65 °C for 15 min. After being cooled to room temperature, the FA0.85MA0.15Pb(I0.85Br0.15)3 perovskite solution was spin-coated onto the substrate in dry air. The perovskite was deposited by first spin coating the solution at 1000 rpm for 10 s, followed by 6000 rpm for 35 s. Anisole (100 μL) was dropped onto the spinning substrate 5 s before the end of the program. Films were then annealed in an oven at 100 °C for 1 h. The spiroOMeTAD (Lumtec) was then dissolved in chlorobenzene (Sigma-Aldrich, 85 mg·mL−1), and solutions were spin coated onto the films in a nitrogen-filled glovebox at 2000 rpm to yield 300−400 nm thick films. For the reference cells, Li-TFSI (500

Figure 4. Aging for 500 h at 85 °C in the dark at 30% humidity of nonencapsulated high-performance perovskite solar cells comparing Li-TFSI/FK209/tBP and Mo(dt)3-doped spiro-OMeTAD. (a) Average stabilized efficiencies of eight devices measured by holding the devices at their J−V determined maximum power point for 60 s with the corresponding standard deviations. (b) Average stabilized efficiencies over the initial stabilized power output. Within 24 h, lithium-doped devices deteriorate by 50% while both Mo(dt)3doped devices sustain 70% of their initial performance after 500 h of aging.

Notably, devices fabricated with Mo(dt)3-doped HTM layers show significantly slower deterioration than devices made using the typically employed Li-TFSI/FK209/tBP additives. After 500 h of aging, devices employing HTMs doped with both Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 still yield SPOs greater than 10%, while the SPOs of their Li-TFSI/FK209/ tBP doped counterparts plummet from 17.1% to 5.1% on average (Table S2). Mo(dt)3-doped spiro-OMeTAD film, being more hydrophobic (Figure S9), protects the perovskite layer from water ingress and slows the degradation of the perovskite layer into PbI2, as we show in Figure S10. As previously reported,36 all cells suffer from a fast deterioration or “burn-in” over the first 40 h and thereafter degrade at a much slower, linear rate. Within 24 h, devices with lithium-doped HTMs degrade by 50% while both Mo(dt)3-doped HTMs retain 65− 70% of their initial device performance after 500 h of aging, as we show in Figure 4b. In Figure S11 we show the evolution of photovoltaic performance parameters determined from the current−voltage curves. We reveal that these Mo(dt)3 complexes are extremely promising candidates for replacing the current state-of-the-art dopants employed to p-dope the hole-transporting layer. We finally note that, throughout this study, we employ FA0.85MA0.15Pb(I0.85Br0.15)3 as the perovskite layer, which is not our most thermally stable perovskite absorber layer. We 2048

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ACS Energy Letters mg·mL−1 in 1-butanol), FK-209 (230 mg·mL−1in acetonitrile), and tBP were added at 20 μL·mL−1, 10 μL·mL−1, and 35 μL· mL−1, respectively. Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 were dissolved in chlorobenzene (15 mg·mL−1), then added to the HTM solution at the molar concentrations discussed in the text. All doped solutions were stirred at 70 °C for an hour. Devices containing a Li-doped HTM were left overnight in a desiccator in air, while Mo(dt)3-based devices were left in a nitrogen-filled glovebox. Lastly, 80 nm thick gold electrodes were evaporated onto the devices through a shadow mask, using a thermal evaporator. Current Density−Voltage Characteristics. The current density− voltage (J−V) curves were measured (2400 Series SourceMeter, Keithley Instruments) under simulated AM1.5 sunlight at (100 mW·cm−2) irradiance generated by an Abet Class AAB sun 2000 simulator, with the intensity calibrated with an NREL calibrated KG5 filtered Si reference cell. The mismatch factor was calculated to be less than 1%. The solar cells were masked with a metal aperture to define the active area to be 0.0919 cm2. UV−Vis Absorption Spectroscopy. Absorbance spectra were measured with a Lambda 1050 UV−vis/NIR spectrometer (PerkinElmer). The samples were prepared by simply spincoating the various HTM on glass substrates. Conductivity Measurements. The samples were prepared by simply spin-coating the differently doped HTMs on glass substrates. Resistivity values were calculated by measuring the HTM film thickness with a Dektak profilometer and multiplying it by the film’s sheet resistance. The HTM sheet resistance was measured using an in-line four-point probe with 0.635 mm tip spacing from Jandel Engineering, which was connected to a Keithley 2450 sourcemeter. Then the conductivity was calculated as the inverse of the resistivity.

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rative Energy Technology R&D Program of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) with financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20148520011250).

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(1) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide management in formamidinium-lead-halide−based perovskite layers for efficient solar cells. Science 2017, 356, 1376−1379. (2) Leguy, A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; et al. The reversible hydration of CH3NH3PbI3 in films, single crystals and solar cells. Chem. Mater. 2015, 27, 3397−3407. (3) Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L. Investigation of CH3NH3PbI3 degradation rates and mechanisms in controlled humidity environments using in situ techniques. ACS Nano 2015, 9, 1955−1963. (4) Christians, J. a; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air. J. Am. Chem. Soc. 2015, 137, 1530−1538. (5) Habisreutinger, S. N.; McMeekin, D. P.; Snaith, H. J.; Nicholas, R. J. Research update: Strategies for improving the stability of perovskite solar cells. APL Mater. 2016, 4, 091503. (6) Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; et al. Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science 2016, 354, 206−209. (7) Shin, S. S.; Yeom, E. J.; Yang, W. S.; Hur, S.; Kim, M. G.; Im, J.; Seo, J.; Noh, J. H.; Seok, S. Il. Colloidally prepared La-doped BaSnO3 electrodes for efficient, photostable perovskite solar cells. Science 2017, 356, 167−171. (8) Tan, H.; Jain, A.; Voznyy, O.; Lan, X.; García de Arquer, F. P.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M.; Zhang, B.; Zhao, Y.; et al. Efficient and stable solution-processed planar perovskite solar cells via contact passivation. Science 2017, 355, 722−726. (9) Snaith, H. J.; Grätzel, M. Enhanced charge mobility in a molecular hole transporter via addition of redox inactive ionic dopant: Implication to dye-sensitized solar cells. Appl. Phys. Lett. 2006, 89, 262114. (10) Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.-P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; et al. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy Environ. Sci. 2016, 9, 1989−1997. (11) Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J. Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells. Nano Lett. 2014, 14, 5561−5568. (12) Liu, J.; Pathak, S.; Stergiopoulos, T.; Leijtens, T.; Wojciechowski, K.; Schumann, S.; Kausch-Busies, N.; Snaith, H. J. Employing PEDOT as the p-type charge collection layer in regular organic−inorganic perovskite solar cells. J. Phys. Chem. Lett. 2015, 6, 1666−1673. (13) Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.; Errico, M. E.; Kirkpatrik, J.; Ball, J. M.; Docampo, P.; McPherson, I.; et al. Lithium salts as “redox active” p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells. Phys. Chem. Chem. Phys. 2013, 15, 2572. (14) Xiao, J.; Shi, J.; Liu, H.; Xu, Y.; Lv, S.; Luo, Y.; Li, D.; Meng, Q.; Li, Y. Efficient CH3NH3PbI3 perovskite solar cells based on graphdiyne (GD)-modified P3HT hole-transporting material. Adv. Energy Mater. 2015, 5, 1401943. (15) Nguyen, W. H.; Bailie, C. D.; Unger, E. L.; McGehee, M. D. Enhancing the hole-conductivity of spiro-OMeTAD without oxygen or

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00614. Mo(tfd-COCF3)3 and Mo(tfd-CO2Me)3 concentration dependence, device statistics, tBP concentration dependence and its influence on Mo(td)3 -doped spiroOMeTAD, and stability (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nakita K. Noel: 0000-0002-8570-479X Stephen Barlow: 0000-0001-9059-9974 Seth R. Marder: 0000-0001-6921-2536 Henry J. Snaith: 0000-0001-8511-790X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly funded by the engineering and physical sciences research council (EPSRC) U.K.; the European Seventh Framework Programme [FP7/2007-2013] (DESTINY MarieCurie project) under Grant Agreement 316494; the AFOSR under Agreement Number FA9550-15-1-0115; the ONR through N00014-14-1-0126; and the international Collabo2049

DOI: 10.1021/acsenergylett.7b00614 ACS Energy Lett. 2017, 2, 2044−2050

Letter

ACS Energy Letters lithium salts by using spiro(TFSI)2 in perovskite and dye-sensitized solar cells. J. Am. Chem. Soc. 2014, 136, 10996−11001. (16) Leijtens, T.; Giovenzana, T.; Habisreutinger, S. N.; Tinkham, J. S.; Noel, N. K.; Kamino, B. A.; Sadoughi, G.; Sellinger, A.; Snaith, H. J. Hydrophobic organic hole transporters for improved moisture resistance in metal halide perovskite solar cells. ACS Appl. Mater. Interfaces 2016, 8, 5981−5989. (17) Abate, A.; Hollman, D. J.; Teuscher, J.; Pathak, S.; Avolio, R.; D’Errico, G.; Vitiello, G.; Fantacci, S.; Snaith, H. J. Protic ionic liquids as p-dopant for organic hole transporting materials and their application in high efficiency hybrid solar cells. J. Am. Chem. Soc. 2013, 135, 13538−13548. (18) Momblona, C.; Gil-Escrig, L.; Bandiello, E.; Hutter, E. M.; Sessolo, M.; Lederer, K.; Blochwitz-Nimoth, J.; Bolink, H. J. Efficient vacuum deposited p-i-n and n-i-p perovskite solar cells employing doped charge transport layers. Energy Environ. Sci. 2016, 9, 3456− 3463. (19) Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N.L.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. Tris(2-(1H-pyrazol-1yl)pyridine)cobalt(III) as p-type dopant for organic semiconductors and its application in highly efficient solid-state dye-sensitized solar cells. J. Am. Chem. Soc. 2011, 133, 18042−18045. (20) Burschka, J.; Kessler, F.; Nazeeruddin, M. K.; Grätzel, M. Co(III) Complexes as p-dopants in solid-state dye-sensitized solar cells. Chem. Mater. 2013, 25, 2986−2990. (21) Luo, J.; Jia, C.; Wan, Z.; Han, F.; Zhao, B.; Wang, R. The novel dopant for hole-transporting material opens a new processing route to efficiently reduce hysteresis and improve stability of planar perovskite solar cells. J. Power Sources 2017, 342, 886−895. (22) Tyagi, P.; Dalai, M. K.; Suman, C. K.; Tuli, S.; Srivastava, R. Study of 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane diffusion in organic light emitting diodes using secondary ion mass spectroscopy. RSC Adv. 2013, 3, 24553. (23) Li, J.; Rochester, C. W.; Jacobs, I. E.; Friedrich, S.; Stroeve, P.; Riede, M.; Moulé, A. J. Measurement of small molecular dopant F4TCNQ and C60F36 diffusion in organic bilayer architectures. ACS Appl. Mater. Interfaces 2015, 7, 28420−28428. (24) Qi, Y.; Sajoto, T.; Kröger, M.; Kandabarow, A. M.; Park, W.; Barlow, S.; Kim, E.-G.; Wielunski, L.; Feldman, L. C.; Bartynski, R. A.; et al. A molybdenum dithiolene complex as p-dopant for holetransport materials: a multitechnique experimental and theoretical investigation. Chem. Mater. 2010, 22, 524−531. (25) Paniagua, S. A.; Baltazar, J.; Sojoudi, H.; Mohapatra, S. K.; Zhang, S.; Henderson, C. L.; Graham, S.; Barlow, S.; Marder, S. R. Production of heavily n- and p-doped CVD graphene with solutionprocessed redox-active metal−organic species. Mater. Horiz. 2014, 1, 111−115. (26) Mohapatra, S. K.; Zhang, Y.; Sandhu, B.; Fonari, M. S.; Timofeeva, T. V.; Marder, S. R.; Barlow, S. Synthesis, characterization, and crystal structures of molybdenum complexes of unsymmetrical electron-poor dithiolene ligands. Polyhedron 2016, 116, 88−95. (27) Said, M. M.; Zhang, Y.; Dasari, R. R.; Anjum, D. H.; Munir, R.; Hu, H.; Amassian, A.; Barlow, S.; Marder, S. R. Ultra-low p-doping of poly(3-hexylthiophene) and its impact on polymer aggregation and photovoltaic performance. Org. Photonics Photovoltaics 2016, 4, 1−16. (28) Müller, L.; Rhim, S.; Sivanesan, V.; Wang, D.; Hietzschold, S.; Reiser, P.; Mankel, E.; Beck, S.; Barlow, S.; Marder, S. R.; et al. Electric-field-controlled dopant distribution in organic semiconductors. Adv. Mater. 2017, 29, 1701466. (29) Belasco, J.; Mohapatra, S. K.; Zhang, Y.; Barlow, S.; Marder, S. R.; Kahn, A. Molecular doping and tuning threshold voltage in 6,13bis(triisopropylsilylethynyl)pentacene/polymer blend transistors. Appl. Phys. Lett. 2014, 105, 063301. (30) Chang, C.-Y.; Tsai, B.-C.; Hsiao, Y.-C. Efficient and stable vacuum-free-processed perovskite solar cells enabled by a robust solution-processed hole transport layer. ChemSusChem 2017, 10, 1981−1988. (31) 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. (32) Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.; Errico, M. E.; Kirkpatrik, J.; Ball, J. M.; Docampo, P.; McPherson, I.; et al. Lithium salts as “redox active” p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells. Phys. Chem. Chem. Phys. 2013, 15, 2572. (33) 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. (34) Habisreutinger, S. N.; Noel, N. K.; Snaith, H. J.; Nicholas, R. J. Investigating the role of 4-tert-butylpyridine in perovskite solar cells. Adv. Energy Mater. 2017, 7, 1601079. (35) Juarez-Perez, E. J.; Leyden, M. R.; Wang, S.; Ono, L. K.; Hawash, Z.; Qi, Y. Role of the dopants on the morphological and transport properties of spiro-MeOTAD hole transport Layer. Chem. Mater. 2016, 28, 5702−5709. (36) Wang, Z.; McMeekin, D. P.; Sakai, N.; van Reenen, S.; Wojciechowski, K.; Patel, J. B.; Johnston, M. B.; Snaith, H. J. Efficient and air-stable mixed-cation lead mixed-halide perovskite solar cells with n-doped organic electron extraction layers. Adv. Mater. 2017, 29, 1604186. (37) Dai, A.; Zhou, Y.; Shu, A. L.; Mohapatra, S. K.; Wang, H.; Fuentes-Hernandez, C.; Zhang, Y.; Barlow, S.; Loo, Y.-L.; Marder, S. R.; et al. Enhanced charge-carrier injection and collection via lamination of doped polymer layers p-doped with a solutionprocessible molybdenum complex. Adv. Funct. Mater. 2014, 24, 2197−2204. (38) Ke, W.; Fang, G.; Liu, Q.; Xiong, L.; Qin, P.; Tao, H.; Wang, J.; Lei, H.; Li, B.; Wan, J.; et al. Low-Temperature solution-processed tin oxide as an alternative electron transporting layer for efficient perovskite solar cells. J. Am. Chem. Soc. 2015, 137, 6730−6733. (39) Anaraki, E. H.; Kermanpur, A.; Steier, L.; Domanski, K.; Matsui, T.; Tress, W.; Saliba, M.; Abate, A.; Grätzel, M.; Hagfeldt, A.; et al. Highly efficient and stable planar perovskite solar cells by solutionprocessed tin oxide. Energy Environ. Sci. 2016, 9, 3128−3134.

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DOI: 10.1021/acsenergylett.7b00614 ACS Energy Lett. 2017, 2, 2044−2050