Porphyrin Dimers as Hole Transporting Layers for High-efficiency and

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Letter

Porphyrin Dimers as Hole Transporting Layers for High-efficiency and Stable Perovskite Solar Cells Yu-Hsien Chiang, Hsien-Hsin Chou, Wei-Ting Cheng, Yun-Ru Li, Chen-Yu Yeh, and Peter Chen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00607 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Energy Letters

Porphyrin Dimers as Hole Transporting Layers for High-Efficiency and Stable Perovskite Solar Cells Yu-Hsien Chiang†a, Hsien-Hsin Chou†b, Wei-Ting Chengb, Yun-Ru Lib, Chen-Yu Yeh*b and Peter Chena*

a. Department of Photonics, Hierarchical Green-Energy Materials (Hi-GEM) Research Center and Center for Micro/Nano Science and Technology (CMNST), National

Cheng

Kung

University,

Tainan

701

(Taiwan),

E-mail:

[email protected]

b. Department of Chemistry, Research Center for Sustainable Energy and Nanotechnology (RCSEN), and Innovation and Development Center of Sustainable Agriculture (IDCSA), National Chung Hsing University, Taichung 402 (Taiwan), E-mail: [email protected]

These authors contributed equally to this work.

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ABSTRACT

In this work, we demonstrate the optimum utilization of porphyrin-based hole transporting materials (HTMs), namely WT3 and YR3, for fabricating triple-cation perovskites solar cells. These newly-designed HTMs based on dimeric porphyrin structure exhibit good HOMO level, high hole mobility and great charge extraction ability for perovskite solar cells. Moreover, through proper molecular engineering, dimeric porphyrins WT3 and YR3 are capable of forming films free of pinhole, with more uniform and dense surface leading to enhanced device performance. Perovskite solar cells using WT3 HTM achieves a PCE of 19.44% which is higher than that using YR3 (17.84%) and even Spiro-OMeTAD (18.62%) under 1 Sun AM 1.5G illumination. In addition, WT3-based devices show better stability than Spiro-based counterparts under moisture, light-soaking and thermal testing conditions.


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ACS Energy Letters

TOC GRAPHICS

The generation of electricity from free and inexhaustible sunlight renders solar power to be one of the most promising renewable energy technologies. Among the modern photovoltaic technologies, perovskite solar cells (PSCs) exhibit promising properties such as solution-processable deposition, long carrier lifetime, bipolar diffusion, strong light absorption and tunable band gap, thus attracting much attention.1-4 Numerous studies have been dedicated to optimizing film quality of perovskites and charge transporting layers of PSC devices, leading to a fast improvement of PSCs exceeding 22% power conversion efficiency (PCE).5-16 For example, the anti-solvent method is one of the most successful techniques to deposit high quality thin-film perovskite with large grain size and uniform morphology.17 The tunable compositions of 3D perovskite structure, ABX3, can also be varied deliberately for enhanced crystalline quality and film morphology, as demonstrated by a recent report using triple and quarter-cation with mixed halides perovskite to reach over 20% of PCE.13,

18



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Hole-transporting materials (HTMs), on the other hand, are another key components to achieve high performance of PSCs. A great majority of PSCs employs spiro-type molecules

such

as

2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9-9′-spirobifluorene (Spiro-OMeTAD) as small-molecule HTMs leading to high efficiency despite their costly synthesis and probable usage of harmful reagent.19 Therefore, the need to advance HTM design for facile synthesis, low cost and high performance arises. Many small-molecule organic HTMs with high device performance are designed accordingly

with

structural

modification

spiro(fluorene-9,9'-xanthene)-based bifluorenylidene-based

on

spiro

(19.8%)12

X59

architecture, and

as

(19.84%),20 (17.8%),21

KR216

spiro-phenylpyrazole-9,9'-thioxanthene-based

X60

such

Ppyra-TXA

(18.06%),22

benzotrithiophene-based BTT-3 (18.2%)23and so on. In the meantime, growing attention has focused on new efficient hole-transporters involving porphyrin-related backbone such as natural abundant chlorophylls,24

artificial porphyrins,25-26

phthalocyanines27, and expanded porphyrins.28 In 2016, Li et al. reported the use of 3,5-di(dodecyloxy)phenyl-tethered chlorophyll Chl-1 as HTM for MAPbI3-xClx-based solar cells (MA= methylammonium) and the corresponding devices reached PCE of 11.44% with open-circuit voltage (VOC) of 0.83 V and short-circuit current (JSC) of 4 ACS Paragon Plus Environment

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ACS Energy Letters

20.92 mA cm-2.24 Cho et al. employed symmetric zinc phthalocyanines (HT-ZnPc) bearing four 5-hexylthienyl substituents as HTMs for mixed-ion perovskite solar cells to reach PCE of 17.5%,27 which surpass that for tert-butyl substituted TB-ZnPc with PCE of 13.3%, showing the dramatic impact of peripheryl functional groups. In our previous report, we firstly used porphyrin-ethynylaniline conjugates Y2 and Y2A2 as HTMs for MAPbI3-based solar cells, achieving 16.6% and 10.5% of PCE, respectively.25 Later, Chen and co-workers developed arylamine-substituted porphyrins as HTMs combing mixed-cation perovskite (FAPbI3)0.85(MAPbI3)0.15 (FA= formamidinium) to obtain 17.8% of PCE.26 However, none of these porphyrinoid materials outperform widely used Spiro-OMeTAD.

N

Ar

Ar

N N Zn N N

N N Zn N N

Ar

Ar

N

O

WT3

Ar = O

YR3

Ar N

N N

M

N

N

N Ar

Ar =

O

Y2

Scheme 1. Molecular structures of porphyrin-based HTMs.

Our previous study concluded that the overall efficiency of Y2 is partially affected by considerable loss in open-circuit voltage. A viable approach in improving VOC and 5 ACS Paragon Plus Environment


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film morphology free of pinholes is to optimize the intermolecular interaction of porphyrin molecules for enhanced hole mobility. Therien and co-workers concluded that hole-doped dimeric porphyrin conjugate is capable to span hole polaron delocalization length of 17.9 Å in contrast to 13.9 Å for monomer in solution.29 Furthermore, a suitable HOMO level should be retained to minimize energy loss toward charge injection and transportation.10 To meet both features, we turn out to employ the dimeric porphyrin conjugates structure.30-32 In this work, we synthesized two porphyrin dimers WT3 and YR3 where di-substitution of electron-donating ethynylaniline moieties is attached to two lateral meso-positions of porphyrin cores (Scheme 1). Both exhibit enhanced hole mobility owing to better intermolecular interaction between hole transporting molecules. When combined with the usage of high quality mixed-cation perovskite layer, WT3 device possesses the best PCE of 19.44% surpassing that for Spiro-OMeTAD (18.62%). Of the most important, stability tests demonstrate outstanding stability of WT3 than spiro-based device either under moisture environment, light-soaking or temperature stress.


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ACS Energy Letters

Figure 1. The UV-vis spectra for WT3 and YR3 in THF solution and as doped thin film on TiO2. Table 1. Photophysical and electrochemical data for porphyrin-based HTMs λabs (ε)[a]

λem[b]

νst[c]

λabs[d]

νst[c]

[nm (103 M-1 cm-1)]

[nm]

[cm-1]

[nm]

[cm-1]

788

555

714, 806

284

1.60

804

895

726, 788

253

1.58

710

534

478, 694

325

1.77

Eg[e]

469 (141), 497 (sh), 593 (8), 705 (55), WT3 755 (70) 468 (246), 494 (sh), 593 (14), 704 (94), YR3 750 (84) Y2[f]

472 (326), 684 (97)

[a] Absorption is measured in THF at 25 oC. [b] Emission is measured in THF at 25 oC using the longest wavelength absorption maximum as excitation wavelength. [c] Stokes shift. [d] Absorption of thin-film samples coated on TiO2. [e] Optical bandgap (Eg) is obtained from cross point of normalized λabs and λem measured in solution. [f] Values were obtained from reference.25

Both porphyrin HTMs used in this study were synthesized according to the established procedures (detailed in Supporting Information).25 According to our preliminary studies on porphyrin HTMs, there is no clear connection between the high efficiency of porphyrins and their absorption properties. However, it is anticipated 7 ACS Paragon Plus Environment


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that an optimized intermolecular interaction would lead to better carrier mobility. As shown in Figure 1 and Figure S2, UV-vis absorption spectra in solution show that the absorption for both dimers WT3 (705 and 755 nm) and YR3 (704 and 750 nm) are significantly red-shifted and broadened compared to the reported monomeric porphyrins Y2 (684 nm) and ZnP26 (563 and 610 nm) owing to good electronic communication between the two porphyrin units and elongation of π-conjugation (Table 1). When spin-coated as thin-film, the absorption for both dimers are much broadened covering the ultraviolet-visible-near infrared region (ca. 350–900 nm) which is broader than that observed for MAPbI3 perovskite layer (ca.800 nm).6, 33 This strong intermolecular π-aggregation might be indicative of an enhanced hole mobility of dimeric porphyrin HTMs, as elucidated in the following.


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Figure 2. (a) Cyclic voltammograms of WT3 and YR3 with (lines with open circles) or without (solid or dashed lines) the presence of Fc/Fc+ internal standard; (b) The energy level diagram for device components including varied HTMs (measured from UPS); (c) The steady-state PL spectra of different HTMs coated on perovskite/glass substrate and compare with pristine perovskite layer (the inset shows magnified PL intensity);

(d)

The

n-type

PSCs

with

glass/FTO/cp-TiO2/mp-TiO2/perovskite/HTM/Au structure.

Cyclic voltammograms of these compounds reveal that the oxidation potential (E1/2(ox)) for alkoxyphenyl-substituted dimers WT3 and YR3 are cathodically shifted by 0.11 V and 0.08 V, respectively, compared to monomeric Y2 (Figure 2a and Table 2). This is resulted from elongated π-conjugation and exactly echoes our preliminary



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theoretical calculations for gas-phase HTM single molecule. As shown in Figure S3, the newly designed WT3 reveals good energy lying of frontier orbitals, where the HOMO is slightly up-shifted and the LUMO is retained compared to Y2. The experimentally determined energy levels for each component in PSC including HTMs has the trend of HOMO level as YR3 (-5.11 eV) > WT3 (-5.14 eV) > Spiro-OMeTAD ~ Y2 (-5.22 eV) ~ perovskite (-5.45 eV). The work function and binding energy of doped dimeric porphyrin and Spiro-OMeTAD thin-films are measured by ultraviolet photoemission spectra, shown in Figure S4 which demonstrates the similar trend of HOMO levels as that observed from electrochemical measurements. The HOMOs of dimeric porphyrins, WT3 (-5.2 eV) and YR3 (-5.1 eV) are higher than that of perovskite which would offer suitable driving force for hole extraction from perovskite (Figure 2b). In addition, as observed in monomeric Y2, the LUMOs for both dimers are sufficiently high to impede possible current leakage owing to electron-hole recombination at the interfaces of perovskite and HTM. The hole-extraction ability for different HTMs at the heterojunction interfaces with fixed perovskite thickness of ca. 450 nm are monitored via steady-state photoluminescence (PL). As shown in Figure 2c, the PL signal for WT3/perovskite bilayer is quenched more efficiently than other porphyrin HTMs and Spiro-OMeTAD.


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ACS Energy Letters

Table 2. Electrochemical data for porphyrin-based HTMs.[a]

WT3 YR3 Y2

[e]

Spiro-OMeTAD

E1/2(ox)

E1/2(red)

E0-0*[b]

EHOMO[c]

ELUMO[d]

[V vs Fc]

[V vs Fc]

[V]

[eV]

[eV]

-1.55

-1.56

-5.14

-3.54

-1.59

-1.57

-5.11

-3.53

-1.71

-1.65

-5.22

+0.04, +0.16 +0.01, +0.13, +0.53

+0.41,

+0.12, +0.30, +0.63 [f]

+0.12, +0.23, +0.44

–

–

-5.22

-3.45 [g]

-2.28[f]

[a] Unless stated, redox potentials were measured in CH2Cl2 containing 0.1 M [(n-Bu)4N]PF6 as supporting electrolyte. Potentials are reported versus ferrocene/ferrocenium (Fc/Fc+). [b] Zero-zero excitation energy (E0-0*) is obtained from HOMO and optical bandgap (Eg) using the equation: E0-0* = HOMO – Eg. [c] The HOMO is calculated using the formula: EHOMO = -5.1 – E1/2(ox). [d] The LUMO is calculated similarly as EHOMO. [e] Values were obtained from reference.25 [f] Values were obtained from reference.34 [g] Values were obtained from reference.35

With suitable molecular design of the dimeric porphyrins, we anticipated that they would exert better hole mobility and more favorable HOMO for good carrier transportation and minimized energy loss for high performance PSCs. The resulted space-charge-limited current with fitted J-V curve for different HTMs (Figure S5) are summarized in Table 2 and Table S1. Both WT3 (4.2 × 10- 4 cm2 V-1 s-1) and YR3 (9.3 × 10- 5 cm2 V-1 s-1) have higher hole mobility than monomeric porphyrin Y2 (3.2 × 10- 5 cm2 V-1 s-1), ascribed to better intermolecular electronic coupling. Compared to WT3, the branched alkyl chains in YR3 possibly interfere molecular packing of porphyrins and impede the intermolecular hopping of holes, leading to slightly lower hole mobility. In contrast, the appropriate substitution of peripheral functional groups



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in WT3 provides better intermolecular π-π packing for remarkable hole mobility, which is even better than that of Spiro-OMeTAD (1.4 × 10- 4 cm2 V-1 s-1).

The device architecture of PSCs has an optimized n-type structure with FTO/cp-TiO2/mp-TiO2 (200 nm)/perovskite (450 nm)/HTM/Au (Figure 2d), where cp and mp represent compact and mesoporous layer, respectively, and the perovskite active layer employs the state-of-the-art perovskite composition containing stoichiometric of Cs0.05[(FA0.83MA0.17)PbI0.83Br0.17]0.95 triple-cation perovskite in precursor solution (The fabrication methods are detailed in Supporting Information).13 The high quality of perovskite film was confirmed via X-ray diffraction pattern (XRD) and scanning electron microscopy (SEM) presenting uniform surface with grain size of 200-300 nm (Figure S6 and S7a). The spin-coated HTM films containing Spiro-OMeTAD (200 nm), WT3 (50 nm) or YR3 (70 nm), as shown in Figure S8, were doped with tert-butyl pyridine (tBP), lithium bis(trifluoromethane)sulfonimide (Li-TFSI)

and

FK209

(tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)

tri[bis(trifluoromethane)sulfonimide]) as additives. Both WT3 and YR3 films show uniform, complete coverage and pinhole free surface covering on perovskite layer from top-view SEM images owing to dense molecular packing of the dimers (Figure S7). Again, compared to monomeric porphyrins Y2 and Y2A225 where minor pinholes were observed, dimeric WT3 and YR3 largely reduce possible direct contact 12 ACS Paragon Plus Environment

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ACS Energy Letters

between back electrode and perovskite layer owing to more spread π-conjugation and enhanced packing of molecular backbone, thus lowering the charge recombination.

Figure 3. (a) The J-V curve for best-performing triple-cation PSCs using different HTMs with scan rate of 110 mV s-1 and mask area of 0.15 cm2. (b) IPCE plots for PSCs with WT3 or Spiro-OMeTAD. Dashed lines indicate calculated Jsc from integrated IPCE values.

Table 3. Device performance for perovskite solar cells with different HTMs, measured under one sun AM 1.5G (1000 W m-2) illumination.

VOC

JSC

FF

PCE

µ

[mV]

[mA cm-2]

[%]

[%]

[cm2/Vs]

WT3

1095.6

22.60

78.52

19.44

4.2x10- 4

YR3

1038.5

22.97

74.75

17.84

9.3x10- 5

Y2

1085.6

22.62

73.02

17.93

3.2x10- 5

Spiro-OMeTAD

1086.0

22.64

75.74

18.62

1.4x10- 4



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The current density-voltage (J-V) curves for triple-cation perovskite solar cells having active area of 0.15 cm2 under one sun AM1.5 G illumination with Spiro-OMeTAD, WT3, YR3 or Y2 as HTMs are displayed in Figure 3a and their photovoltaic characteristic is summarized in Table 3. As expected, the WT3 device demonstrates the highest PCE of 19.4% with Voc of 1.096 V, Jc of 22.60 mA cm-2 and FF of 78.52%, outperforming the Spiro-OMeTAD device with PCE of 18.6% (Voc of 1.086 V; Jsc of 22.64 mA cm-2; FF of 75.74%). The WT3-based device also exhibit better efficiency than YR3 device with PCE of 17.84% (Voc of 1.039 V; Jsc of 22.97 mA cm-2; FF of 74.75%) and Y2 device with PCE of 17.93% (Voc of 1.086 V; Jsc of 22.62 mA cm-2; FF of 73.02%), and also outperform those for all reported porphyrinoid materials such as ZnP (17.78%)26, Chl-1 (11.44%),24 and Y2 (16.6%)25 using double-cation or MAPbI3 perovskite. The superior and competitive performance of WT3 device to Spiro-OMeTAD and other porphyrinoid derivatives echoes our aforementioned findings on steady-state PL, hole mobility test as well as surface morphology observations and is in agreement with our molecular design. The incident photon-to-electron conversion efficiency (IPCE) measurements for WT3 reveals comparable coverage from 350 nm to ca. 800 nm as Spiro-OMeTAD with IPCE values >85% in the whole visible region (Figure 3b). The hysteresis effect for porphyrin HTMs are found to be WT3 < Y2 < YR3, as illustrated in Figure S9 14 ACS Paragon Plus Environment

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ACS Energy Letters

showing both forward (from short-circuit current to open-circuit voltage) and reverse scans. We performed a steady-state maximum power point (mpp) tracking measurement to confirm its stabilized PCE. The efficiency of WT3 device received from mpp tracking is around 16.5%, which is approaching to the reverse scan efficiency of 17.42%, not the forward value (14.26%) (Figure S10).

We further proceeded to examine the device performance of PSCs under T5 fluorescent lamp at 200 lux illumination (equivalent to Pin of 65 µW cm-2), for comparison of artificial light environment versus standardized 1 Sun condition. As shown in Figure S11 and in Table S2, the measured PCE values go with the trend Spiro-OMeTAD (25.26%; 16.42 µW cm-2) > WT3 (23.94%; 15.56 µW cm-2) > Y2 (22.40%; 14.56 µW cm-2) > YR3 (20.18%; 13.12 µW cm-2). Despite slightly lower Voc of 0.767 V, the WT3 device exhibits higher Jsc of 27.90 mA cm-2 than Spiro-OMeTAD (Voc of 0.805 V; Jsc of 27.31 mA cm-2). It is found that under low light condition, WT3 possesses slightly higher Jsc and lower Voc values compared to that for Spiro-OMeTAD. The lower Voc of WT3 device under low light implies it linear response of Voc vs light intensity is slightly different from Spiro-OMeTAD. The different distribution of electronic state of these two HTMs may not be similar. It is possibly that WT3 has less density of state near the HOMO level so the available shift of quasi Fermi level under light illumination is more. Under low-light condition, we 15 ACS Paragon Plus Environment


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suggest that quasi Fermi level stays close to the intrinsic position (Spiro-OMeTAD: -5.25 eV and WT3: -5.20 eV), leading to the Voc difference in ~40 mV. Under high light intensity, quasi Fermi of WT3 may shift down more than Spiro-OMeTAD which attributes to the higher performance under high light condition.

Nevertheless, the porphyrin type of hole transporting materials are competitive with Spiro-OMeTAD considering the energy-harvesting of PSCs for in-door applications in Internet of Things (IoTs) and portable electronic devices.36 We next did stability tests for PSCs fabricated with different HTMs under ambient environment in dark with RH (relative humidity) of 40±10%. Figure 4(a) displays normalized PCE for the unencapsulated WT3 device compared to Spiro-OMeTAD as a function of storage time (t). The WT3 device showed excellent stability remaining more than 90% of the initial PCE after 800 hours. On the contrary, a dramatic drop down to 80% of the initial PCE was observed for Spiro-OMeTAD device within 300 hours, which degrades much faster than WT3 device. The hydrophobic property for different HTMs was studied using surface contact angle method (Figure S12). The contact angle between water and Spiro-OMeTAD is 65o, in agreement with previous report,37 whereas those for porphyrin materials Y2, YR3 and WT3 are 81o, 90o and 103o, respectively. It is worth noting that the dimer WT3 material is extremely hydrophobic compared to most of HTMs in the literatures.37-39 Therefore, it would be reasonable to 16 ACS Paragon Plus Environment

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ACS Energy Letters

explain

the

greater

ambient

stability

of

porphyrin-based

devices

than

Spiro-OMeTAD-based devices due to the more effective shielding and protection of the perovskite active layer toward moisture in air.

Figure 4. The device stability test (without encapsulation) for PSCs with HTMs Spiro-OMeTAD or WT3 under different condition. (a) moisture stability test for devices stored in dark environment (RH 40±10%). (b) light-soaking stability test (LED, 1 sun intensity, RH 15-20%) using MoOx/Al as electrode. (c) thermal stability test placing MoOx/Al-capped devices on the hot plate with varied temperature for 30 mins (RH 15-20%). All J-V curve measurements were conducted at ambient environment (RH 60±5%) under 1 sun AM 1.5G illumination. The recorded results are presented with normalized PCE as a function of storage time or temperature.

After initial assessment for stability toward humidity, we further tested the stability under light and thermal stress for our devices. Here MoOx/Al is introduced to device architecture as effective electrode instead of Au for perovskite solar cells to conduct experiments since it is noted that gold could diffuse into the device and damage the performance under heat or light condition.18, 40 Non-encapsulated devices are placed



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under 15-20% RH to exclude different degrade mechanisms such as moisture.41-42 Each of WT3 and Spiro-based cells were tested with three samples for light-soaking under 1 sun intensity with LED under ambient environment at 15-20% RH. The results are present in Figure 4(b) with the normalized PCE as a function of time (hour). The WT3 devices reveal remarkable stability after 20 h of light-soaking condition without significant drop in PCE. On the contrary, the Spiro-based devices show noticeable degradation to 80% of initial PCE after only 20 h of illumination. The reason for degradation is considered mainly from different HTM involved in the test and is triggered by light. In Figure 4(c), the fabricated devices were placed on the hot plate at variant temperature for 30 min under 15-20% RH to conduct the thermal stability test. Again, the WT3 devices demonstrate high thermal stability without significant reduction of PCE even at 120 oC, whereas the Spiro-OMeTAD devices dropped down to around 60%-70% of initial PCE. The poor stability of Spiro-OMeTAD toward elevated temperature could be attributed to the corresponding low glass transition temperature (Tg) around 125 oC.43-44 This phenomenon is similar to that reported by Kim et al. with sizable drop of PCE at above 80 oC.45 We noted that only a few groups have tested and discussed about the thermal stability for Spiro-OMeTAD devices. To the best our knowledge, this is the first result that the perovskite solar cells could work under the temperature of > 100 oC without shrinking 18 ACS Paragon Plus Environment

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ACS Energy Letters

performance. The dimeric porphyrin materials designed in this work thus can be considered robust HTM alternative for perovskite solar cells based on the superior results presented in this work.

In summary, we have designed and synthesized two novel dimeric porphyrin materials, WT3 and YR3. These dimers perform efficiently as hole transporting materials (HTMs) for perovskite solar cells (PSCs) owing to suitable HOMO level, great charge injection ability and hole mobility. With the use of triple-cation perovskite, the fabricated PSCs using HTMs WT3 and YR3 show excellent power conversion efficiency (PCE) of 19.44% and 17.84%, respectively, comparable to Spiro-OMeTAD (18.62%) and the reported HTM Y2 (17.92%) under AM 1.5G 1 Sun illumination. The dimeric structure of porphyrin HTMs enable formation of more uniform thin films to prevent pinhole existing on the surface observed for single porphyrins Y2 and Y2A2 in our previous studies. It is evidenced that the much more robust and hydrophobic WT3 hole transporting layer is capable of protecting the perovskite solar cells effectively leading to improved device stability as compared to Spiro-OMeTAD. The excellent performance of dimeric porphyrin HTMs, especially in the case of WT3, is believed to originate from better intermolecular packing of dimeric porphyrins and is found to outperform Spiro-OMeTAD both in efficiency and stability. 19 ACS Paragon Plus Environment


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website Experimental details, material synthesis, NMR analysis, synthetic routes for porphyrin-based HTMs, the UV-Vis spectra, computational calculations for energy level, UPS measurement, hole mobility measurement, XRD, SEM for perovskite, J-V curve measurement under one sun and dim-light condition, the contact angle measurement.

AUTHOR INFORMATION

Corresponding authors: E-mail: [email protected] E-mail: [email protected] Author contributions: Yu-Hsien Chiang, Hsien-Hsin Chou contributed equally to this work. 20 ACS Paragon Plus Environment

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ACS Energy Letters

Notes

The authors declare no competing financial interest ACKNOWLEDGMENT The authors thank the financial support for this work from the Ministry of Science and Technology (MOST) in Taiwan with Grant No. MOST 104-2119-M-005-005, MOST 105-2119-M-005-001,

MOST

106-2119-M-006-027

and

MOST

106-2119-M-006-017, and the “Innovation and Development Center of Sustainable Agriculture" and "the Hierarchical Green-Energy Materials (Hi-GEM) Research Center" from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. REFERENCES (1) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764-1769. (2) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. (3) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344-347. (4) He, M.; Zheng, D.; Wang, M.; Lin, C.; Lin, Z. High Efficiency Perovskite Solar Cells: from Complex Nanostructure to Planar Heterojunction. J. Mater. Chem. A 2014, 2, 5994-6003. (5) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (6) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (7) 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. 21 ACS Paragon Plus Environment


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