Based 3D Oligomers for Perovskite Solar Cells

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Article

Tailor-Making Low-Cost Spiro[fluorene-9,90xanthene]-Based 3D Oligomers for Perovskite Solar Cells Bo Xu, Jinbao Zhang, Yong Hua, ..., Anders Hagfeldt, Alex K.-Y. Jen, Licheng Sun [email protected] (E.M.J.J.) [email protected] (L.S.)

HIGHLIGHTS Two SFX-based 3D oligomers were tailor-made by a one-pot synthesis approach One of the oligomers, X55, was successfully applied in highly efficient PSCs High efficiency of 20.8% was achieved with X55 as the hole transport material The low-cost 3D HTMs can render a PCE close to 21% in PSCs

Two low-cost spiro[fluorene-9,90 -xanthene] (SFX)-based 3D organic hole transport materials (HTMs), termed X54 and X55, were tailor-made by a one-pot synthesis approach for perovskite solar cells (PSCs). PSC devices based on X55 as the HTM show a very impressive power-conversion efficiency of 20.8% under 100 mW$cm2 AM1.5G solar illumination, which is much higher than the PCE of the reference devices based on X54 (13.6%) and the standard HTM-Spiro-OMeTAD (18.8%) under the same conditions.

Xu et al., Chem 2, 676–687 May 11, 2017 ª 2017 Elsevier Inc. http://dx.doi.org/10.1016/j.chempr.2017.03.011

Article

Tailor-Making Low-Cost Spiro[fluorene-9,90-xanthene]-Based 3D Oligomers for Perovskite Solar Cells Bo Xu,1,2,9 Jinbao Zhang,3,9 Yong Hua,4 Peng Liu,4 Linqin Wang,1 Changqing Ruan,5 Yuanyuan Li,6 Gerrit Boschloo,3 Erik M.J. Johansson,3,* Lars Kloo,4 Anders Hagfeldt,7 Alex K.-Y. Jen,2 and Licheng Sun1,8,10,*

SUMMARY

The Bigger Picture

The power-conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have increased rapidly from about 4% to 22% during the past few years. One of the major challenges for further improvement of the efficiency of PSCs is the lack of sufficiently good hole transport materials (HTMs) to efficiently scavenge the photogenerated holes and aid the transport of the holes to the counter-electrode in the PSCs. In this study, we tailor-made two low-cost spiro[fluorene-9,90 xanthene] (SFX)-based 3D oligomers, termed X54 and X55, by using a one-pot synthesis approach for PSCs. One of the HTMs, X55, gives a much deeper HOMO level and a higher hole mobility and conductivity than the state-of-theart HTM, Spiro-OMeTAD. PSC devices based on X55 as the HTM show a very impressive PCE of 20.8% under 100 mW$cm2 AM1.5G solar illumination, which is much higher than the PCE of the reference devices based on Spiro-OMeTAD (18.8%) and X54 (13.6%) under the same conditions.

Perovskite solar cells (PSCs) have recently emerged as one of the most promising technologies for solar-energy conversion because of their high efficiency and simple solution processability. The power-conversion efficiency (PCE) of PSCs has been improved from 3.8% to 22.1% during the last 7 years, which can even compete with that of traditional siliconbased solar cells. Organic hole transport materials (HTMs) play an important role in extracting the photogenerated holes and transporting the holes to the counter-electrode in a PSC, thus making them a vital component for efficient photoelectric conversion. Therefore, the development of a new generation of HTMs with facile synthesis and high charge mobility is of high importance for using PSCs for further industrial application. Herein, we have tailor-made a low-cost spiro[fluorene-9,90 xanthene]-based 3D HTM termed X55 by using a one-pot synthesis approach for PSCs and achieved a high PCE of 20.8% under 100 mW$cm2 AM1.5G solar illumination.

INTRODUCTION Organic-inorganic halide perovskite solar cells (PSCs) have emerged as one of the most promising solar technologies for achieving large-scale and low-cost photovoltaic power generation because of their high efficiency, promising stability, and simple solution processability.1–4 During the past few years, the power-conversion efficiencies (PCEs) of PSCs have increased rapidly from about 4% to over 22%, which is comparable with the performance of traditional crystalline silicon-based solar cells.5–8 In a typical PSC device structure, a layer of an n-type and a p-type semiconductor is required at the front or back of the perovskite material to assist charge separation and transport, respectively.9 In this context, organic hole transport materials (HTMs) have been considered to be the ideal p-type semiconductor materials for PSCs because of their solution processability, tunable energy level, good hole mobility, and easy accessibility by chemical synthesis.10–15 Small-molecule 2,20 ,7,70 -tetrakis-(N,N-dip-methoxy-phenyl-amine)9,90 spirobifluorene (Spiro-OMeTAD) (Figure 1A) and polymer-polytriarylamine (PTAA)16–18 are two of the most commonly used organic HTMs for PSCs, in which record-high efficiencies have been demonstrated. Despite the many efforts that have been devoted to the design and synthesis of new small-molecule and polymer-based organic HTMs for PSCs by different research groups during the past 4 years,19–24 small-molecule Spiro-OMeTAD is still considered to be the best HTM.

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The main reason is that the three-dimensional (3D) Spiro-OMeTAD exhibits good solubility in organic solvents, such as toluene and chlorobenzene, which is very beneficial for the formation of the hole transport layer on top of the perovskite crystals. This allows fewer pinholes and better morphology of the hole transport layer, leading to outstanding device performance.25 However, the complex synthetic process of the 3D spiro core unit, 2,20 ,7,70 -tetrabromo-9,90 -spirobi[fluorene] (4Br-SBF), significantly limits the suitability for large-scale industrial application of SpiroOMeTAD in PSCs.23 Therefore, many researchers have tried to develop new 3D spiro-type HTMs based on simple synthesis and high efficiency to replace the conventional Spiro-OMeTAD in PSCs.26,27 In 2016, Ganesan et al.28 reported a costeffective 2H,20 H,4H,40 H-3,30 -spirobi[thieno[3,4-b][1,4]dioxepine]-based 3D spirotype HTM named PST1 for PSCs, and they obtained a PCE of 13.44%. Since then, HTMs based on spirobi[cyclopenta[2,1-b:3,4-b0 ]dithiophene]29 and spiro[acridine9,90 -fluorene] core units have also been reported.30,31 A breakthrough was made by Saliba et al.32 in early 2016; a spiro[cyclopenta[2,1b:3,4-b0 ]dithiophene-4,90 -fluorene]-based HTM, termed FDT, was presented on the basis of molecular engineering, and a high and impressive efficiency of 20.2% was achieved. This result is comparable with those of devices using Spiro-OMeTAD (PCE = 19.7%) under the same conditions. Most recently, two facile spiro[fluorene9,90 -xanthene] (SFX)-based HTMs, X59 and X60, synthesized via a two-step procedure, were reported by our group and offer remarkably high PCEs of 19.8% in PSCs.23,33 Despite the comparable performance in PSCs emerging from alternative 3D spiro-type-HTMs, only a few examples of organic HTMs reported in PSCs can comprehensively outperform the state-of-the-art Spiro-OMeTAD material. A 3D spiro-type HTM that is based on an easy synthetic approach and can offer a PCE close to 21% when applied in PSCs has not yet been suggested. Previous work from our group has shown that the 3D spiro core SFX can be easily synthesized in a one-pot reaction with inexpensive phenol and 9-fluorenone as the starting materials and with an extremely high yield of over 90%.23 More importantly, the estimated synthetic cost of the SFX unit is around 30 times lower than that of SBF, which makes SFX-based molecules highly suitable for large-scale industrial production. In this work, we have extended our previous work by further molecular engineering of two SFX-based 3D oligomers termed, X54 and X55 (Figure 1A), by using a one-pot Buchwald-Hartwig cross-coupling reaction with very cheap starting materials.34 One of the tailored HTMs, X55, shows a much deeper highest occupied molecular orbital (HOMO) level and higher hole mobility and conductivity, as well as better thermal stability, than that of Spiro-OMeTAD. In addition, HTM-X55, constructed of three SFX units, shows an excellent 3D structure and much better film-forming ability than Spiro-OMeTAD and X54. PSC devices based on X55 as the HTM show an impressively high PCE of 20.8% under 100 mW$cm2 AM1.5G solar illumination, which is much higher than the PCEs of 18.8% and 13.6% obtained for the reference devices based on Spiro-OMeTAD and X54 under the same conditions. In addition, the X55-based solar cells exhibited excellent stability with 93% of the initial efficiency retained after long-term aging (6 months) under a controlled atmosphere. To the best of our knowledge, this is the first report of a low-cost 3D spiro-type HTM that can outperform the well-known HTM-Spiro-OMeTAD in PSCs. Clearly, this work demonstrates that the facile one-pot synthesis of HTM X55 holds great potential for the replacement of the commonly used HTM Spiro-OMeTAD in PSCs and also has potential for application in other optoelectronic devices.

1Organic

Chemistry, Center of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden

2Department

of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA

3Physical Chemistry, Center of Molecular Devices,

Department of Chemistry A˚ngstro¨m Laboratory, Uppsala University, 75120 Uppsala, Sweden 4Applied

Physical Chemistry, Center of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden

5Nanotechnology

and Functional Materials, Department of Engineering Science, Uppsala University, 75120 Uppsala, Sweden

6Wallenberg

Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, 10044 Stockholm, Sweden

7Laboratory

of Photomolecular Science, Institute of Chemical Sciences and Engineering, E´cole Polytechnique Fe´de´rale de Lausanne, 1015 Lausanne, Switzerland

8State

Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology, 116024 Dalian, China

9These

authors contributed equally

10Lead

Contact

*Correspondence: [email protected] (E.M.J.J.), [email protected] (L.S.) http://dx.doi.org/10.1016/j.chempr.2017.03.011

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X54

Side view-1

Side view-2

Top view

X55

Side view-1

Side view-2

Top view

Figure 1. Chemical Structure and Molecular Geometry of the HTMs (A) Chemical structures of the HTMs X54, X55, and Spiro-OMeTAD; the molecular weight is given in g/mol. (B) Simulated molecular geometry of X54 and X55.

RESULTS AND DISCUSSION Molecular Design and Synthesis It has been widely demonstrated that HTMs with a molecular structure extending in three dimensions is beneficial for obtaining good films and coverage. These are properties that can significantly reduce pinhole-induced charge losses in a PSC device and thus enhance excellent device performance.20 Therefore, we designed and synthesized two 3D oligomers, X54 and X55, with different numbers of SFX-functionalized triarylamine-based monomers for HTMs to be applied in PSCs. Density functional calculations (DFT) showed that, compared with the molecular geometry of X54 (which is based on only two SFX units), the HTM X55 exhibited a highly distorted spatial structure because of the three SFX units, (Figure 1B). The synthetic route to forming X55 is quite straightforward (Scheme 1), and the product can be obtained by a one-pot approach with 1Br-SFX and 2Br-SFX as the starting materials and a high yield of over 80%. The synthetic route of X54 is analogous and is also displayed in Scheme S1. The detailed synthetic information and material characterization can be found in the Supplemental Information; 1H NMR, 13C NMR, and highresolution mass spectroscopy (HR-MS) are shown in Figures S1–S6. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out for investigating the thermal properties of X54 and X55 (Figures 2A and 2B). The decomposition temperatures (Td) of X54 and X55 were 441 C and 446 C,

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Scheme 1. The One-Pot Synthesis Approach of X55

respectively, which are higher than that of Spiro-OMeTAD (422 C). The glass transition temperatures (Tg) obtained from the DSC curves also show that X54 (134 C) and X55 (174 C) have higher Tg values than Spiro-OMeTAD (121 C). These results clearly indicate that X54 and X55 exhibit significantly better thermal stability than Spiro-OMeTAD. Photophysical Properties To further investigate the electronic properties of the HTMs, we evaluated the molecular orbitals and reorganization energy (ER) of X54 and X55 by using DFT calculations with the Gaussian 09 package. As shown in Figure S7, both the HOMOs and the lowest unoccupied molecular orbitals (LUMOs) displayed an electron density of X54 and X55 fully delocalized over the whole molecule. In addition, X55 displayed a marginally smaller ER (480 meV) than Spiro-OMeTAD (495 meV) and X54 (495 meV). This indicates that X55 might exhibit as fast or even faster hole transport in a solid thin film formed by the compound (Table 1).35 To investigate the charge-carrier mobility of the three HTMs, we analyzed space-charge-limited currents (SCLCs) and two-contact electrical conductivity; the fitted J-V curves for each material (detailed information can be found in the Supplemental Information), as well as the corresponding hole mobility and conductivity data, are depicted in Figures 2C and 2D and Table 1. Clearly, X55 exhibited a good hole mobility of 6.81 3 104 cm2$V1$s1, which is much higher than that of X54 (8.25 3 105 cm2$V1$s1) and Spiro-OMeTAD (1.48 3 104 cm2$V1$s1). In addition, the hole mobility of X54, X55, and Spiro-OMeTAD under doping conditions was also investigated; the curves and corresponding data are displayed in Figure S10. The concentrations of HTMs and dopants used in the measurement are the same as in the photovoltaic devices. The hole mobility values of all HTMs showed slight enhancement after doping but maintained the same trend. This result also coincides with the computational study, which showed that X55 had a smaller calculated ER, indicating faster hole transport rate. We also noted better conductivity for X55 and X54 than for Spiro-OMeTAD under the same experimental conditions. This could be due to a larger degree of conjugation in X55 and X54 than in SpiroOMeTAD. The normalized UV-visible (UV-vis) absorption and photoluminescence spectra of X54, X55, and Spiro-OMeTAD in toluene are shown in Figure S8, and the

Chem 2, 676–687, May 11, 2017

679

0.8

Weight percentage (%)

100

0.7

Heat Flow (W/g)

X54 X55 Spiro-OMeTAD

80 70 60

X54 X55 Spiro-OMeTAD

0.6

90

HTMs

X54

X55

Spiro-OMeTAD

Td (oC)

441.5

446.4

421.9

200

400

0.5 0.4 0.3 0.2 0.1

600

0.0

800

HTMs

X54

X55

Spiro-OMeTAD

Tg (oC)

134.1

174.4

121.1

60

90

Temperature (oC) 1

150

180

210

240

1E-7

0.1

1E-8

Current (A)

Current density (mA/cm2)

120

Temperature (oC)

0.01

X54 X55 Spiro-OMeTAD

1E-3 1E-4 1E-5

1E-9

X54 X55 Spiro-OMeTAD

1E-10

0

1

2

3

Applied Bias (V)

4

5

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Applied Bias (V)

Figure 2. Thermal and Photoelectrical Properties of the HTMs (A) Thermogravimetric analysis (TGA) of X54, X55, and Spiro-OMeTAD. (B) Differential scanning calorimetry results (DSC) of X54, X55, and Spiro-OMeTAD. (C) J-V characteristics of X54-, X55-, and Spiro-OMeTAD-based hole-only devices. (D) Current-voltage characteristics of X54-, X55-, and Spiro-OMeTAD-based solid films.

corresponding data are listed in Table 1. X55 exhibits an absorption band in the visible light region with an absorption maximum around 406 nm, which is red shifted around 20 nm, whereas the maximum absorption peaks of X54 (380 nm) and Spiro-OMeTAD (388 nm) are in the UV light region. The emission bands of X54 and X55 are slightly red shifted as well in comparison with Spiro-OMeTAD (Table 1). This can be explained by the larger conjugated system of X54 and X55 because of the two or three SFX units present. The optical band gap (Egap) of the HTMs can be obtained from the intersection of the emission and absorption spectra (Figure S8 and Table 1). All HTMs can be oxidized by the p-type dopant FK209.36 Figure 3A shows the UV-vis absorption spectra of the HTM solutions (2 3 105 M in toluene) upon stepwise addition of the p-type dopant FK209. We note that the addition of the dopant FK209 to the Spiro-OMeTAD solution caused a significant decrease in the absorption band intensity at 388 nm and an increase in two new absorption bands at 516 and 704 nm, corresponding to the Spiro-OMeTAD+ and Spiro-OMeTAD2+ radical cations, respectively. However, X54 and X55 displayed a moderate decrease in the absorption bands at 380 and 406 nm, respectively, and an increase in new absorption bands at 493 and 525 nm, respectively; no further spectral features were observed, even for high dopant concentrations (Figure 3A). This result demonstrates that X54 and X55 cannot be further oxidized to X542+ and X552+ by FK209 because of the deeper second-redox energy levels of X54 and X55 than of Spiro-OMeTAD (Figure 3C). This is also in good agreement with the results from the DFT calculations (Table S1).

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Table 1. Summary of Optical, Electrochemical, and Photoelectrical Properties and Reorganization Energy of the HTMs Used in This Study HTMs

labs (nm)

lem (nm)

Eox (V)a

Egap (eV)b

HOMO (eV)c

HOMO (eV)d

ER (meV)e

Conductivity (S$cm1)g

Hole Mobility (cm2$V1$s1)f

X54

380

420

0.80

2.95

5.30

2.25

495

3.26 3 104

8.25 3 105

4

6.81 3 104 1.48 3 104

X55

406

425

0.73

2.93

5.23

2.26

480

8.43 3 10

Spiro-OMeTAD

388

414

0.63

3.05

5.13

2.06

495

1.43 3 104

a

Redox potentials were calibrated against the normal hydrogen electrode (NHE) by the addition of ferrocene (E(Fc/Fc+) = 630 mV versus NHE) as a reference. Calculated from the intersection of the normalized absorption and emission spectra (Figure S8). c HOMO = (Eox + 4.5 eV). d LUMO = HOMO + Egap. e ER (reorganization energy) was calculated by the four-point method on the basis of the adiabatic potential energy surface. f Pure HTM without doping. g Doping using 30 mM LiTFSI and 3 mol % FK209. b

Electrochemical Properties To estimate the relative energy levels of the HTMs, we performed cyclic voltammetry (Figure 3B) and differential pulsed voltammetry (DPV) (Figure 3C) in dichloromethane (DCM) solution (see the corresponding data in Table 1). The energy-level diagrams of the related materials used in this study are illustrated in Figure 3E. The results show that the HOMO energy levels of X54 and X55 were 5.30 and 5.23 eV versus vacuum, respectively, which are 0.17 and 0.10 eV deeper, respectively, than that of Spiro-OMeTAD under the same conditions (Figures 3C and 3E). In principle, the photovoltage of a PSC is determined by the electrochemical potential difference between the electron and hole contacts. The hole contact is related to the HOMO level of the HTM layer, suggesting that X54 and X55 could offer higher open-circuit voltages (VOC) in PSC devices. Considering that the valence band of the mixed-ion perovskite used in this study was 5.65 eV versus vacuum,19 all three HTMs have higher HOMO levels than mixed-ion perovskite materials, indicating that they can effectively extract the charge from the latter at the interface. Photovoltaic Properties To evaluate the photovoltaic properties of these two designed HTMs (X54 and X55) in PSC devices, we used the standard mesoporous device configuration (Figure 3D) of FTO/compact-TiO2/meso-TiO2/[HC(NH2)2]0.85[CH3NH3]0.15Pb[I0.85Br0.15]3/HTM/Au; details of the device fabrication are described in the Supplemental Information. SpiroOMeTAD was used as a reference HTM in this study. The J-V curves obtained from the best-performing solar cells are displayed in Figure 4A, and the corresponding photovoltaic parameters are summarized in Table 2. Under AM 1.5G irradiation (100 mW$cm2), the X55-based devices showed a VOC of 1.15 V, a short-circuit photocurrent density (JSC) of 23.4 mA$cm2, and a fill factor (FF) of 0.77, yielding an impressively high PCE of 20.8%. This is higher than that of the traditionally used Spiro-OMeTAD-based solar cells, which showed a PCE of 18.8% under the same conditions. The PCE of PSCs based on X55 as the HTM is also commensurate with the highest reported efficiency to date. The X55-based devices exhibited better photovoltaic performance than those based on Spiro-OMeTAD regarding all three efficiency parameters, i.e., VOC, JSC, and FF. In comparison, the X54-based devices exhibited a much lower PCE of 13.6% with a VOC of 0.95 V, a JSC of 21.3 mA$cm2, and an FF of 0.67. This difference could be explained by the poor solubility of X54 in chlorobenzene, resulting in poor film morphology. Figure S9 shows that the solubility of X54 in chlorobenzene is significantly lower than 70 mg/mL, whereas the solubility of X55 is clearly higher. X55 consists of one more SFX unit between the diarylamine groups, which significantly changes the geometry and spatial structure of the molecule (Figure 1B), obviously leading to better solubility and film-forming ability.

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0% 45% 90% 120% 150% 210% 240% 270% 300%

X54+

0% 15% 30% 60% 120% 150% 210% 240% 300%

X55+

Spiro+ Spiro2+ 300

400

500

600

700

0% 15% 30% 45% 60% 75% 90% 105% 120%

+0.17

X54

X55

+0.10

X55

Spiro-OMeTAD

800 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Wavelength (nm)

X54

Potential (V)

Spiro-OMeTAD

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Potential (V)

Figure 3. Photophysical and Electrochemical Properties of the HTMs (A) UV-vis absorption of X54, X55, and Spiro-OMeTAD solutions (10 5 M in toluene) upon stepwise addition of the p-type dopant FK209. (B) Cyclic voltammetry results of X54, X55, and Spiro-OMeTAD (10 4 M in dichloromethane). (C) Normalized differential pulsed voltammetry (DPV) results of X54, X55, and Spiro-OMeTAD (10 4 M in dichloromethane). (D) Device structure of the mesoscopic PSC used in this study. (E) Energy-level diagram of the related materials used in this study.

It has been widely demonstrated that a uniform surface coverage of the perovskite crystals by the HTM layer is important to reduce the charge recombination losses at the interface between the perovskite and HTMs. This can significantly influence the charge-collection efficiency and device performance of PSCs.20 Therefore, we further investigated the surface morphology of the three HTMs by scanning electron microscopy (SEM), as shown in Figure 5A. X55 clearly had a uniform surface on the top of the perovskite film, validating its excellent film-forming ability. In contrast, a layer of tiny crystals was formed on the surface the perovskite film by X54. For Spiro-OMeTAD, we noticed some small pits on top of the film (Figure 5A). The different film-forming abilities of the three HTMs can significantly affect the overall device performance. In addition, the stability of the PSCs at maximum power-point conditions, the corresponding incident photon-to-electron conversion efficiency (IPCE) spectra, and the related integrated photocurrents of the champion solar cells

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A

D

6

C

E

F

X54 X55 Spiro-OMeTAD

5

Number of Devices

B

4 3 2 1 0

8

10

12

14

16

18

20

22

Efficiency (%)

Figure 4. Photovoltaic Properties of the HTMs (A) J-V characteristics of the champion PSC devices using X54, X55, and Spiro-OMeTAD as the HTM obtained under 100 mW$cm 2 AM1.5G solar illumination. (B) Corresponding IPCE spectra of X54-, X55-, and Spiro-OMeTAD-based PSCs. (C) Stability of the photovoltaic parameters maintaining the PSCs at the maximum power point. (D) Statistical efficiencies for X54-, X55-, and Spiro-OMeTAD-based PSCs (at least 16 individual devices were studied). (E) J-V characteristics of Spiro-OMeTAD-based PSCs under different scan conditions after aging for 6 months. (F) J-V characteristics of X55-based PSCs under different scan conditions after aging for 6 months.

are described in Figures 4C and 4B. The results show that the integrated photocurrents from the IPCE spectra match well with the measured JSC (Figure 4B), and the X55-based devices show very stable photovoltaic performance within 1,000 s exposure under the maximum power-point conditions. In addition, the statistical efficiencies of the three HTM-based devices are given in Figure 4D, which reveals good reproducibility of the fabricated devices. Notably, the X55-based PSCs show an average PCE of 19.4% with a small SD of 0.9%, which is significantly higher than that of the Spiro-OMeTAD-based devices, showing an average PCE of 17.4% and an SD of 1.0%. Device Stability The stability of the devices based on the HTMs is highly important for PSCs to be considered for commercial application, and it has been previously shown that the HTM layer can play an important role in the protection of the perovskite layer against degradation in the presence of moisture.20 Therefore, we further investigated the stability of X55- and Spiro-OMeTAD-based solar cells. Non-encapsulated devices were stored in ambient air with humidity below 20%. The J-V curves of the solar cells studied at different scan conditions and the corresponding photovoltaic data are recorded in Figures 4E and 4F. After aging for 6 months, the efficiency of the X55based devices decreased by only 7% (from 20.8% to 19.3%; Figure 4F), whereas

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Table 2. Photovoltaic Parameters Determined from J-V Measurements of PSCs Based on X54, X55, and Spiro-OMeTAD HTMs

a

VOC (V)

JSC (mA$cm2)

JSCcalc (mA$cm2)a

FF

h (%)

havg G SD (%)b

X54

0.95

21.3

19.8

0.67

13.6

11.9 G 1.1

X55

1.15

23.4

22.8

0.77

20.8

19.4 G 0.9

Spiro-OMeTAD

1.13

23.1

22.3

0.72

18.8

17.4 G 1.0

Calculated JSC from the IPCE spectra. The average efficiency values and SD were obtained from at least 16 individual devices.

b

the efficiency of the Spiro-OMeTAD-based solar cells decreased by 10% (from 18.8% to 16.9%; Figure 4E). These results demonstrate that the X55-based PSCs exhibit good device stability both under dark and working conditions. The high uniformity and homogeneity of the X55 film coverage on top of the perovskite material (Figure 5A) is a possible explanation for the good stability results obtained. Interestingly, we also noticed only very small hysteresis behavior for both X55- and SpiroOMeTAD-based solar cells, even after long-term aging (Figures 4E and 4F). In addition, we further investigated the stability of X55- and Spiro-OMeTAD-based solar cells in ambient air conditions with a humidity of 60% at different temperatures (Figure S13). The non-encapsulated devices were put on a hotplate for 5 min under a controlled temperature, and the range of temperatures was set from 20 C to 100 C. The X55-based solar cell showed better stability than the Spiro-OMeTADbased device from 20 C to 80 C. This might be because the X55-based hole transport layer had better hydrophobicity than that of the Spiro-OMeTAD-based film. From 90 C to 100 C, the efficiencies of both devices decreased rapidly. Fast degradation of the unsealed PSCs at high temperature has also been reported previously; this could be related to thermal degradation of the perovskite crystal. The hydrophobicity of the HTM film is also highly important for the stability of a PSC because it can protect the perovskite layer against degradation under moisture. Therefore, the hydrophobicity of the three hole transport layers was determined, as shown in Figure S12. The derived contact angles between HTM films and water for X54, X55, and Spiro-OMeTAD were 84.2 , 82.7 , and 68.5 , respectively. Obviously, the hydrophobicity of X54- and X55-based films were almost the same, but they were much better than that of the Spiro-OMeTAD-based film. The main reason could be that the hydrophilic methoxy groups on the structures of X54 and X55 (two methoxy groups) are less hydrophilic than those on Spiro-OMeTAD (eight methoxy groups). This is also another possible explanation for the good stability of X55-based devices. Charge Transfer and Transport Properties To determine the origin of the photovoltaic characteristics for the HTMs, X54, X55, and Spiro-OMeTAD, we systematically investigated the interface charge transfer between those HTMs and the perovskite film, as well as their hole transporting properties, by steady-state photoluminescence (PL) and transient photocurrent decay measurements. As shown in Figure 5B, we carried out transient photocurrent decay by applying a small modulation of light on top of a constant light intensity of 100 mW$cm2 to the solar cells under short-circuit conditions. One can extract the charge-collection kinetics by following the photo-induced current decay,19,22 and the difference in current decay kinetics is directly related to the hole transport property of the HTMs. The X54-based devices exhibited a much slower current decay response than the reference HTM-Spiro-OMeTAD. In contrast, the X55-based solar cells demonstrated faster photocurrent decay than the other systems. The faster

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A

B

C

Figure 5. Morphology, Charge Transfer, and Transport Properties of the HTMs (A) Scanning electron microscopy (SEM) images of the HTM layer (top view). (B) Photoluminescence decay of perovskite films with and without HTMs in contact. (C) Normalized transient photocurrent decay under short-circuit conditions.

current decay of X55 could be attributed to its higher hole mobility and conductivity, as shown in Figures 2C and 2D. In addition, the charge-collection efficiency can also be affected by the hole transfer process at the perovskite-HTM interface. This process can be studied by steady-state PL, as displayed in Figure 5C. The X54-based devices demonstrated a much higher PL intensity than the solar cells based on X55. This indicates that the interfacial hole transfer between X55 and perovskite is much more efficient than that between X54 and perovskite. Correspondingly, the efficient charge collection in X55-based devices can be related to fast hole transport and interfacial hole transfer, which also explains their high FF (0.77) and JSC (23.4 mA$cm2). Conclusions In conclusion, two SFX-based 3D oligomers, X54 and X55, have been tailor-made via a simple one-pot reaction and successfully applied as efficient HTMs for high-performance PSCs. X55 shows a much deeper HOMO level and higher hole mobility and conductivity, as well as better thermal stability, than the well-known Spiro-OMeTAD HTM. In addition, X55, constructed with three bulky SFX units, shows a much better film-forming ability than Spiro-OMeTAD and X54. This could be attributed to the excellent 3D structure of X55, which offers higher solubility in the organic solvents used for spin coating. PSC devices with X55 as the HTM show a remarkably high

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PCE of 20.8% under standard solar illumination (100 mW$cm2 AM1.5G), which is much higher than the PCEs of 18.8% and 13.6% from the reference Spiro-OMeTADand X54-based devices, respectively. Furthermore, the results of steady-state PL and transient photocurrent decay measurements clearly indicate that X55 has much better hole extracting and transporting capabilities and thus better photovoltaic performance. Importantly, the X55-based solar cells exhibit excellent stability after long-term aging for 6 months. This work clearly demonstrates that the low-cost material X55 has great potential for replacing the commonly used Spiro-OMeTAD HTM in PSCs. Furthermore, the facile synthesis of X55 via a one-pot approach makes this HTM very promising for upscaled production in large quantities at low cost.

EXPERIMENTAL PROCEDURES Full experimental procedures are provided in the Supplemental Information.

SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, 13 figures, 1 table, and 1 scheme and can be found with this article online at http://dx.doi. org/10.1016/j.chempr.2017.03.011.

AUTHOR CONTRIBUTIONS L.S., E.M.J.J., and B.X. conceived the project and the research plan. B.X. performed syntheses, physical characterizations, and analyses. J.Z. did the device fabrications and analyses. C.R. did the SEM measurements. Y.L. did the DSC and TGA measurements. P.L. did the DFT calculations. All authors participated in discussion and writing the manuscript.

ACKNOWLEDGMENTS We acknowledge financial support from the Swedish Research Council, Swedish Energy Agency, Knut & Alice Wallenberg Foundation, National Natural Science Foundation of China (21120102036 and 91233201), and National Basic Research Program of China (973 program, 2014CB239402). A.K.-Y.J. thanks the Boeing-Johnson Foundation for financial support. Received: January 26, 2017 Revised: March 6, 2017 Accepted: March 17, 2017 Published: May 11, 2017

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