Donor-Acceptor-Donor-Type Cyclopenta[2,1-b;3,4-b']dithiophene

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Donor-Acceptor-Donor-Type Cyclopenta[2,1-b;3,4-b’]dithiophene Derivatives as A New Class of Hole Transporting Materials for Highly Efficient and Stable Perovskite Solar Cells Yan-Duo Lin, Seid Yimer Abate, Hsin-Cheng Chung, KangLing Liau, Yu-Tai Tao, Tahsin J. Chow, and Shih-Sheng Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00859 • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 5, 2019

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ACS Applied Energy Materials

Donor-Acceptor-Donor-Type

Cyclopenta[2,1-

b;3,4-b’]dithiophene Derivatives as A New Class of Hole Transporting Materials for Highly Efficient and Stable Perovskite Solar Cells Yan-Duo Lin†,* Seid Yimer Abate,‡ Hsin-Cheng Chung,† Kang-Ling Liau,¶ Yu-Tai Tao,‡,* Tahsin J. Chow,‡,§,* and Shih-Sheng Sun‡,* †Department

of Applied Chemistry, National Chiayi University, No. 300 Syuefu Rd., Chiayi

City 600, Taiwan, ROC. ‡Institute

of Chemistry, Academia Sinica, No. 128, Sec. 2, Academia Rd., Nankang, Taipei

115, Taiwan, ROC. ¶Department

of Chemistry, National Central University, No. 300, Zhongda Rd., Zhongli

District, Taoyuan City 320, Taiwan, ROC. Department of Chemistry, Tung Hai University, No.1727, Sec.4, Taiwan Boulevard, Xitun

§

District, Taichung 407, Taiwan, ROC KEYWORDS: Donor-Acceptor-Donor, cyclopenta[2,1-b;3,4-b’]dithiophene, perovskite solar cell, hole-transporting materials, n-i-p configuration ABSTRACT: Three new donor-acceptor-donor-type (D-A-D) hole-transporting materials (HTMs), YC-1–YC-3, based on 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b’]dithiophene (DiCN-CPDT) core structure endowed with two arylamino-based units as peripheral groups were designed, synthesized and applied in perovskite solar cells (PSCs). Hole mobility, steady-

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state photoluminescence, thin-film surface morphology on top of perovskite layer, and photovoltaic performance for YC series were systematically investigated and compared with that of Spiro-OMeTAD. It was found that YC-1 exhibited more efficient hole transport and extraction characteristics at the perovskite/HTM interface. Meanwhile, the film of YC-1 showed a homogeneous and dense capping layer coverage on perovskite layer without any pinholes, leading to the improvement of the fill factor and open circuit voltage. PSC device based on YC-1 as HTM exhibited a high power conversion efficiency (PCE) of 18.03%, which is comparable to that of the device based on the benchmark Spiro-OMeTAD (18.14%), and also a better long-term stability with 85% of the initial efficiency retained in excess of 500 hours under the condition of 30% relative humidity, presumably due to the hydrophobic nature of the material. This work demonstrates that the dicyanomethylene-CPDT-based derivatives are promising HTMs for efficient and stable PSCs.

INTRODUCTION Metal-halide perovskite solar cells (PSCs) are a promising sustainable solar energy converter, because of the unique advantages of high charge carrier mobility, panchromatic light-harvesting capability with high extinction coefficients, and long exciton diffusion length for perovskite. Owing to their potentially low manufacturing cost, light weight, and high power conversion efficiency (PCE), the PSC is a promising future alternative to traditional siliconbased solar cells.1-10 The PCE of PSC has risen from 3.8%, reported by Miyasaka group in 2009, to more than 23%, reported by You’s group 9 years later. 11-14 In typical PSC devices, hole-transporting materials (HTMs) play an essential role in achieving high performance. To date,

2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene

(Spiro-

OMeTAD) has been widely used as the HTM in PSCs despite its low charge carrier mobility, complicated and low-yield synthesis, which seriously hinder its wide application. 15-16


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Therefore, it is crucial to pursue new HTMs with improved characteristics through costeffective synthetic strategies. Various kinds of small molecular,17-25 polymeric,26-28 and inorganic HTMs

29-33

have been synthesized and used as the HTM in PSCs. Compared with

polymeric and inorganic HTMs, small-molecule-based organic HTMs have the advantages of handy batch-to-batch reproduction, well-defined chemical structure and a precise molecular weight.34-36 As a result, many efforts have been devoted to and great achievements have been made in small molecular HTMs designs, making them promising alternatives to SpiroOMeTAD. Among various types of small molecule-based organic HTMs, donor-π-donor (Dπ-D) type HTMs have been studied extensively and used in PSCs such as truxenes, anthanthrone, fluorene, and triphenylamine-based derivatives.37-41 In particular, carbazole42-44 and phenothiazine45-46 derivatives have been developed as low-cost and highly efficient alternatives to Spiro-OMeTAD in PSCs. In contrast, there are relatively fewer reports on the device performance of donor-acceptor-donor type (D-A-D) HTMs in PSCs.17, 47-52 The optical and electronic properties of small-molecule HTMs with D-A-D structure can be easily tuned by varying the electron-withdrawing cores or end-capping electron-donating groups. Moreover, D-A-D type HTMs often show great promise in hole mobility and conductivity, which facilitate the extraction of holes from perovskite layer as well as transporting of holes toward the metal electrode. Therefore, D-A-D type organic molecules may pose as good candidates as efficient HTMs in PSCs. Oligothiophenes have been extensively studied owning to their promising potential applications in organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), organic solar cells (OSCs) and perovskite solar cells (PSCs).53-61 In general, a longer conjugation is expected to improve the electronic coupling between molecules and thus may improve the device performances. However, because of free rotation around the single bonds in thiophene oligomers, they display a shorter conjugation with poorer device characteristics



as compared to those with fused thiophenes. Fused thiophenes are well-known to give coplanar conformations through enhanced π…π overlap, which facilitates intra- and interchain charge transport.62 Therefore, fused thiophenes have been incorporated in the π-spacer of many molecules for various photovoltaic applications.63 Recently, many research groups devote their efforts to develop new fused thiophene molecules as core building blocks for HTMs in PSCs, such as thiophene-vinylene-thiophene, 64 indacenodithiophene,65 naphthodithiophene,66 benzotrithiophene,67 benzodithiophene, 68 tetrathienoanthracene, 69 and these have been shown to achieve high PCE for PSCs. Among fused-thiophene-incorporated HTMs, cyclopenta[2,1b;3,4-b’]dithiophene (CPDT) derivatives have also been explored as HTMs in PSCs with good photovoltaic performance because its rigid and planar structure is beneficial to the intermolecular π−π interaction, as well as hole-transporting ability. 22, 70-74 From the structural point of view, the CPDT-based HTMs have so far been molecularly engineered with the D-πD configuration. To the best of our knowledge, CPDT-based HTMs with D-A-D structure have not been reported. Therefore, it is highly important to gain a deep understanding of the possible impact of CPDT-based HTMs with the D-A-D structural component on charge carrier collection and extraction, and photovoltaic performance in the PSCs. The elucidated structure/property relationship will provide guidelines for future design of materials for PSC applications. Herein, we prepared a series of D-A-D type HTMs, where the molecular design has 4dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b’]dithiophene (DiCN-CPDT) as the central core linked with two-armed arylamine moieties, which are denoted as YC-1, YC-2, and YC-3 as shown in Figure 1. We have also systematically investigated the impact of the different electron-donating groups on the film formation on the perovskite layer, hole mobility, photoluminescence spectra as well as photovoltaic performance of this YC series of HTMs. The devices were fabricated with a configuration of FTO/compact TiO2/meso-porous


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TiO2/CH3NH3PbI3/YC/Ag. Impressively, the devices based on YC-1 achieved a maximum PCE of 18.03%, which is comparable to that of the devices employing the state-of-the-art Spiro-OMeTAD as HTM (18.14%) under the same experimental conditions. However, the PSCs prepared with YC-2 showed a significantly low PCE of 4.05%, probably due to its poor solubility in the spreading solvent and thus a poor uniformity of the YC-2 film. In contrast, the YC-3, which has alkyl chains on the thiophene units and thus improved solubility, exhibited a smoother surface morphology and better coverage on top of the perovskite layer, giving an improved PCE of 15.78%. The superior performance of YC-1 compared to that of YC-2 and YC-3 in the PSCs is suggested to be due to the higher hole mobility of YC-1, better hole collection capability at the perovskite/YC-1 interface, and the smoother film without pin-holes on the perovskite surface. Furthermore, YC-1-based devices showed good long-term stability in maintaining 85% of initial performance after 500 hours of storage, which is a better durability than that of Spiro-OMeTAD-based devices (64%). It is believed that the DiCNCPDT-based HTMs are promising candidates for HTM in PSCs.

Figure 1. Chemical structures of YC-1, YC-2, and YC-3.

RESULTS AND DISCUSSION YC series was prepared by the synthetic route shown in Scheme 1, and the experimental details are provided in the Supporting Information. Compound 1 reacted with a commercially available 2-bromo-3-hexylthiophene in a Suzuki coupling reaction to provide the target



molecule 2. Then compound 2 was converted to 3 by lithiation and subsequent reaction with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The Suzuki coupling between 2-(2,5dibromo-3,4-dihydrocyclopenta[a]pentalen-7-ylidene)malononitrile (4)75 with various pinacol boronates 1,23 3, and 5,23 respectively in the presence of palladium catalyst readily generated the three YC derivatives, with yields ranging from 75 to 82%. The details of the synthesis and characterization of the target molecules are given in Supporting Information. Compared to the multistep synthesis of Spiro-OMeTAD, 76 YC series were facilely synthesized via crosscoupling reaction. The thermal stability of the YC derivatives was investigated with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (data shown in Figure S1). The TGA data showed that the decomposition temperatures (Td, at 5% weight loss) for YC-1, YC-2, and YC-3 are 290, 421, and 397 ºC, respectively. The relatively lower Td for YC-1 could be ascribed to its smaller molecular weight. However, the Td’s of both YC-2 and YC-3 are comparable to that of Spiro-OMeTAD (424 ºC). These results revealed that YC-based HTMs were thermally stable and suitable for application in PSCs. According to DSC scans, the glass transition temperatures (Tg) were observed at 118 and 134 ºC for YC-1 and YC-3, respectively. However, a glass transition endotherm was not detected for YC-2 in the explored temperature range. Notably, the Tg of YC-3 is higher than that of Spiro-OMeTAD (120 ºC). These are important factors affecting the stability of the devices.


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Scheme 1. Synthetic route of YC series.

The normalized UV-vis absorptions of YC series and Spiro-OMeTAD in chlorobenzene solutions and as thin solid films are shown in Figure 2a; with the corresponding data summarized in Table 1. The three YC derivatives all show three distinctive absorption peaks in the 300-500 nm region. The absorption maxima λmax’s of YC-1, YC-2 and YC-3 are located at 410, 447, and 422 nm, with corresponding molar extinction coefficients of 5.54 x 104, 6.21 x 104, and 5.56 x 104, respectively. The absorption maxima of YC-2 and YC-3 are red-shifted compared with that of YC-1 due to the incorporation of an additional thiophenylene group, which extends the π-conjugation length accordingly. Moreover, the main absorption bands of YC-3 are blue-shifted by 25 nm with respect to that of YC-2. This may be attributed to the increased steric hindrance in YC-3 between the thiophene ring and the triphenylamine moiety due to the hexyl substitution. In addition, it is worth noting that the absorption spectra of YC series feature a broad and weak absorption peak at around 850 nm, which is ascribed to the



low-oscillator n-π* transition due to lone pairs of the nitrile groups. Moreover, the UV-vis spectra of YC-1, YC-2, and YC-3 in the thin film states exhibited slightly red-shifts with respect to that in solutions, giving absorption peaks at 413, 453, and 439 nm, respectively, indicating that a weak intermolecular π-π interaction in the solid state. To get an insight into the optical properties of the YC series, the absorption spectra and vertical energies of YC series are calculated according to time-dependent density functional theory (TDDFT) with B3LYP functional together with the standard 6-31G(d,p) basis set. Their simulated spectra are depicted in Figure S2. The TDDFT excited states calculations were performed on the lowest 10 singletsinglet excitations of YC derivatives, neglecting the influence of solvent. The results of transitions with oscillator strength above 0.1 are summarized in Table S1, except the lowest transition for YC derivatives in order for comparisons. The calculated maximum absorption peaks of YC-1, YC-2 and YC-3 locate at 432, 511, and 473 nm, respectively. YC-2 and YC3 are found to be red-shifted in comparison with that of YC-1, indicating that an additional thiophene ring can result in a larger red-shift of absorption. In addition, YC-2 absorption shows a bathochromic shift compared to that of YC-3. Furthermore, the simulated spectra of YC series also show one board and weak absorption band with relatively low oscillator strength at the long wavelength, which comply remarkably well with the observed absorption band.


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Figure 2. (a) Normalized UV/vis absorption spectra of the YC series and Spiro-OMeTAD in chlorobenzene solutions and thin films. (b) Differential pulse voltammetry of YC series measured at a sweep rate of 100mV/s. (c) Energy level alignment of our PSCs with YC series and Spiro-OMeTAD as HTMs. (d) Photoemission yield plots of YC series in air.

Theoretical molecular orbital calculation was carried out using Gaussian 03 at B3LYP/631g (d,p) level, to characterize optimized geometries, orbital energies, and electron density distributions of the HOMO and LUMO states of YC series. The optimized molecular geometries of YC series are shown in Figure 3. It is worth noting that YC-3 shows a larger dihedral angle of 47o between the thiophene unit and triphenylamine moiety as compared to that of YC-2 (22o), presumably due to the hexyl substituent on the thiophene ring. The higher twist of the π-system results in a blue shift in the absorption maximum of YC-3 compared with



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that of YC-2. The electron density of the HOMO of YC series extends over the entire molecule while the electron density of the LUMO mainly centers on the central DiCN-CPDT core (Figure 4). Moreover, the calculated HOMO and LUMO energy levels were -4.54/-3.07, -4.52/3.21, and -4.57/-3.18 eV for YC-1, YC-2, and YC-3, respectively. Consequently, the energy gaps of YC-1, YC-2, and YC-3 are calculated to be 1.47, 1.31 and 1.39 eV, respectively.

25.79o 25.19o

22.64o 22.65o

47.37o 47.37o

Figure 3. The optimized structures of YC series calculated with DFT on B3LYP-6-31G (d,p).


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Figure 4. Frontier orbitals and energy level of HOMO and LUMO orbital surfaces of YC series calculated with DFT at the B3LYP/6-31G(d,p) level.

Table 1. Electrochemical and photophysical data of YC series and Spiro-OMeTAD HTM YC-1 YC-2 YC-3 SpiroOMeTAD aMaximum

λabsa(nm) (ε 10-4/M-1 cm-1) 410 nm (5.54) 447 nm (6.21) 422 nm (5.56) 389 nm (18.37)

λabsb(nm) 413 453 439 375

λfa (nm)

EHOMOc (eV)

E0-0d (eV)

ELUMOe (eV)

Ip f (eV)

EHOMOg (eV)

ELUMOg (eV)

501

-5.37

2.71

-2.66

-5.28

-4.54

-3.07

581

-5.36

2.43

-2.93

-5.21

-4.52

-3.21

580

-5.44

2.53

-2.91

-5.32

-4.57

-3.18

428

-5.22

3.03

-2.19

-5.14

-4.24

-2.23

of the absorption and fluorescence bands in chlorobenzene solutions. b Absorption spectra measured in thin film

state cMeasured in THF/tetra-n-butylammonium hexafluorophosphate (0.1 M) solution by using the differential pulse voltammetry. dThe value of E0-0 was determined from the intersection of normalized absorption and fluorescence spectra. e

Calculated from EHOMO － E0-0. fIonization potential was measured by the photoemission in air method from films.

gDFT/B3LYP/6-31G(d,p)

level calculated values.

The electrochemical behaviour of the YC series was investigated with differential pulse voltammetry (DPV) (Figure 2b) in THF solution and the data are collected in Table 1. The ferrocene/ferrocenium (Fc/Fc+) reference was used as a standard reference, which was assigned the energy level of −4.8 eV versus vacuum level. The energy level alignments of the devices with different HTMs are illustrated in Figure 2c. The HOMO energy levels of YC-1, YC-2, and YC-3 measured by DPV are -5.37, -5.36, and -5.44 eV, respectively, indicating that the HOMO levels of YC series are more positive than that of perovskite, ensuring sufficient driving force for hole transfer from perovskite to the YC HTMs. As can be seen from Figure 2c, when the hexyl-substituted thiophene unit was introduced in YC-3, there was a decrease in the oxidation potential and the corresponding HOMO energy level. The optical band gap energies (Eg) determined from the intersection of normalized absorption and fluorescence spectra are 2.71, 2.43, and 2.53 eV for YC-1, YC-2, and YC-3, respectively. The LUMO energy levels



were calculated to be -2.66, -2.93, and -2.91 eV, respectively. These are also much higher than the conduction band of perovskite, so that electron transfer from perovskite to the Ag counter electrode was blocked. Under parallel conditions, the HOMO value of Spiro-OMeTAD calculated from DPV is -5.22 eV, which is lower than that of YC series. In order to further investigate the potential use of YC series as HTM in perovskite-based solar cells, the solid-state ionization potential (Ip) values of the YC films were determined by using photoelectron spectroscopy in air (PESA), as shown in Figure 2d. The optical data are also collected in Table 1. The Ip values calculated for YC-1, YC-2, and YC-3 from PESA measurement are 5.28, 5.21, and 5.32 eV, respectively. This indicates that HOMO levels of the YC series are higher than the perovskite valence band (−5.43 eV), which is favored for the hole injection from the perovskite layer to HTMs. The HOMO values obtained from the PESA measurements showed the same trends obtained by DFT calculations and DPV measurements. The hole mobility of YC series and Spiro-OMeTAD, another key parameter in the photovoltaic performance, was also measured using the hole-only devices with a device structure of ITO glasses/PEDOT:PSS/HTM/Ag. From the current plot (Figure S3) and spacecharge-limited current model, the hole mobility of YC-1, YC-2, YC-3, and Spiro-OMeTAD were determined to be 1.92 x 10-5, 4.53 x 10-7, 1.84 x 10-5, and 6.12 x 10-5 cm2V-1s-1, respectively. The hole mobility of YC-1 is slightly higher than that of YC-3. Furthermore, YC1 and YC-3 showed two orders of magnitude higher hole-mobility than that of YC-2, indicating better charge transport properties and higher Jsc may be expected in PSCs. The crystal property of the DiCN-CPDT core-based HTMs was analysed by powder X-ray diffraction patterns (PXRD) as shown in Figure S4. The powder patterns for YC series suggested similar amorphous properties in films. These results indicate that difference in hole mobilities of the HTMs should not have originated from the stacks in the solid state.


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To understand the hole extraction and transfer at the perovskite/HTMs interface, we investigated the steady-state photoluminescence (PL) spectra of the perovskite/HTM bilayers. As shown in Figure 5, the pristine perovskite layer exhibited an intense PL emission at ~774 nm. When YC-1 and YC-3 were deposited on top of the perovskite film, a lower PL intensity was obtained in comparison with the perovskite/YC-2 bilayer. The above results demonstrate that both of YC-1 and YC-3 exhibit better hole extraction ability than that of YC-2.

Figure 5. Steady-state photoluminescence spectra of YC series and Spiro-OMeTAD deposited on perovskite.

The morphology of the carrier transporting layer is also critical to the performance of PSCs. The formation of a dense and uniform coverage of HTM film on the perovskite layer is of vital importance for achieving highly efficient PSCs. The morphologies of YC series and SpiroOMeTAD films were characterized by using scanning electron microscopy (SEM). As shown in Figure 6, the films of YC-1, YC-3, and Spiro-OMeTAD exhibited an extraordinarily homogeneous and dense capping layer on perovskite layer without any observable particles or pinholes. In contrast, the film of YC-2 revealed a poor morphology with pinholes and island-



like structures, which might be due to the poor solubility. This non-uniformity of YC-2 layer will inevitably lead to direct contact between perovskite and metal electrode, leading to decreased Voc.77 Moreover, the atomic force microscopy (AFM) was employed to scrutinize the film morphologies of YC series and Spiro-OMeTAD deposited on the perovskite layer. As shown in Figure 7, the root-mean-square (RMS) roughness of YC-1, YC-2, YC-3, and SpiroOMeTAD films are 1.50, 8.02, 2.13, and 2.36 nm, respectively. AFM images confirmed that the YC-1 and YC-3 films were smoother than that of YC-2. Such uniform surface is beneficial to the contacts between perovskite/HTM/electrode interfaces and thus the hole extraction from the perovskite layer. Therefore, it is expected that YC-1 and YC-3 films would lead to less charge recombination and enhanced Jsc and Voc values.78

Figure 6. Top view SEM morphology of YC series and Spiro-OMeTAD capping layers on the perovskite films.


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Figure 7. AFM microgram and 3D images of YC-1 (a), YC-2 (b), YC-3 (c) and SpiroOMeTAD (d) films deposited on perovskite/mp-TiO2/c-TiO2/FTO. All images were taken in 5µm*5µm area.

To evaluate the effect of YC series as HTMs on the photovoltaic performance, we fabricated PSCs

with

a

conventional

n-i-p

device

configuration

of

FTO/c-TiO2/mp-

TiO2/CH3NH3PbI3/HTM/Ag (Figure 8a) and the details of device preparation can be found in the Supporting Information. The cross-sectional SEM images of the PSC devices employing YC series and Spiro-OMeTAD as the HTM are shown in Figure 8b and Figure S5. A clear layer-by-layer structure of the device can be seen from the SEM images. The thicknesses of cTiO2, perovskite/mp-TiO2, the HTMs layer and Ag electrode were determined to be about 45 nm, 400 nm, 205 nm, and 80 nm, respectively. The current–voltage (J-V) characteristics with reverse scans of PSCs based on different HTMs under 100 mW cm-2 AM 1.5G illumination are presented in Figure 8c and their corresponding photovoltaic parameters are summarized in Table 2, together with that of the control PSCs fabricated with Spiro-OMeTAD as HTM. All the HTMs were doped with Li-TFSI and 4-tert-butylpyridine (t-BP) additives for better conductivity. Moreover, these perovskite solar cells using YC series were left in dry box for 12 hours before measurement to give the best performance.



Figure 8. (a) Device architecture, and (b) Cross-sectional SEM image of PSCs with the device structure of FTO/c-TiO2/mp-TiO2/perovskite/YC-1/Ag. (c) J-V curves of champion cells based on YC series and Spiro-OMeTAD with reverse scan. (d) IPCE plots and integrated current densities of perovskite solar cells based on YC series and Spiro-OMeTAD.


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Table 2. Device parameters obtained from J-V curves using YC series and Spiro-OMeTAD as HTMs under different scan directiona

HTM

Scan Direction Reverse scan

YC-1 Forward scan Reverse scan YC-2

1.03 23.67 averagec 1.00±0.02 23.73±0.53 maxb max

Reverse scan

max

average

Forward scan

maxb

Reverse scan

maxb

0.74 c

0.99

maxb

1.01

Performances of devices measured using a 0.04

cm2

73.94

18.03

17.75

59.16 4.05 53.77±9.25 3.13±0.64 61.42

2.29

70.04 15.78 73.52±3.85 14.76±0.81

21.07

1.01 22.77 averagec 0.99±0.02 22.99±0.59

PCE(%)

78.45

5.04

0.99 22.76 0.97±0.02 20.89±1.53

FF (%)

71.07±1.73 17.07±0.49

22.62

0.74 9.25 averagec 0.76±0.02 7.86±2.05

b

Forward scan

1.00

b

maxb

Spiro-OMeTAD

Jsc (mA cm-2)

maxb

Forward scan YC-3

a

Voc (V)

72.05

15.03

78.87 18.14 75.66±2.30 17.30±0.68

22.85

77.65

17.92

b

working area. Values refer to the cell

with the highest PCE. c Calculated based on 15 measurements. The best performance obtained with YC-1-based PSCs had an impressive PCE of 18.03%, with an open circuit voltage (Voc) of 1.03 V, a short-circuit current density (Jsc) of 23.67 mA cm-2 and a fill factor (FF) of 0.739, which are on par with the control device using SpiroOMeTAD (PCE of 18.14%, Voc of 1.01 V, Jsc of 22.77 mA cm-2 and FF of 0.789). In contrast, YC-2-based PSCs achieved a much lower efficiency of 4.05% with a Voc of 0.74 V, a Jsc of 9.25 mA cm-2 and an FF of 0.592, which could be ascribed to its poor surface coverage and film morphology on the perovskite layer as analysed above. A higher series resistance and slow hole extraction between the perovskite layer and YC-2 layer (Figure 5) are suggested. In YC3, an additional n-hexyl chain was attached to the thiophene unit to improve the solubility and thus film morphology (Figures 6 and 7). Indeed, the YC-3-based device delivered a dramatically improved PCE of 15.78%, with a Voc of 0.99 V, a Jsc of 22.76 mA cm-2 and an FF



of 0.70, over those of YC-2-based device. The Voc, Jsc and FF values of YC-1–based device are higher than that of the other two DiCN-CPDT-based (YC-2 and YC-3) devices presumably because more efficient charge extraction at the perovskite/HTM interface. In addition, the Voc value for YC-1 is slightly higher than that of YC-3 in spite of the slightly higher HOMO level of the former material (Figure 2c), indicating that YC-1 exhibits more efficient charge collection, which is in agreement with the hole mobility and PL measurements. It might be ascribed to a relatively homogenous film with good morphology and better coverage on top of perovskite layer for YC-1.78 Moreover, the steric hindrance from the hexyl groups in YC-3 may influence the intermolecular packing of YC-3 in the solid state and reduce its hole mobility in comparison with YC-1. Furthermore, it is clear that the trend of FF values of PSC devices employing YC series and Spiro-OMeTAD matched well the trend obtained from the hole mobility measurements of these molecules. It is noteworthy that YC-1 showed comparable Voc value and even higher Jsc values than those obtained for Spiro-OMeTAD. Moreover, YC series exhibited small hysteresis except for YC-2, as shown in Figure S6 and Table 2. The incident photon to current conversion efficiency (IPCE) spectra of devices based on YC series and Spiro-OMeTAD are shown in Figure 8d. The integrated photocurrents of the PSCs using YC-1, YC-2, YC-3, and Spiro-OMeTAD are 21.37, 8.82, 20.19, and 20.49 mA cm-2, respectively, which is in good agreement with the current density measured from the J-V curves. Moreover, the PSCs based on YC-1 and YC-3 showed higher IPCE in the whole range of UV-vis region than that of YC-2. The stabilized PCE of the devices of YC series near the maximum power point were tested to evaluate the efficacy of the J-V curves (Figure 9), which exhibited highly stable steady-state PCE of 17.87%, 4.01%, and 15.39%, respectively. Figure 10 shows the statistical box charts of PCE of devices with the four HTMs for at least 15 devices, and the average PCE of the devices based on YC1, YC-2, YC-3, and Spiro-OMeTAD were 17.07 ± 0.49%, 3.13±0.64%, 14.76±0.81%, and


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17.30±0.68%, respectively, which further confirm that the DiCN-CPDT-based PSCs exhibited good reproducibility. These photovoltaic results suggest that incorporation of dicyanosubstituent CPDT unit as the central core of the HTMs is a good strategy for PSCs fabrication.

Figure 9. The stabilized power output of PSCs devices for (a) YC-1, (b) YC-2, (c) YC-3, and (d) Spiro-OMeTAD at their maximum power point.



Figure 10. Box charts of (a) Voc, (b) Jsc, (c) FF, and (d) PCE measured using reverse scans for 15 devices based on YC series and Spiro-OMeTAD.

Finally, we investigated the long-term stability of the fabricated solar cells with YC series and Spiro-OMeTAD as HTMs under a 30% relative humidity without encapsulation and stored in air at room temperature. As shown in Figure 11, the devices with YC-1 retained more than 85% of its original performance after 500 hours, while devices with YC-2, YC-3 and SpiroOMeTAD as the HTM retained 32, 82, and 64% after 500 hours. It is clear that solar cells with YC-1 as the HTM exhibited a relatively higher stability. The excellent stability of the YC-1based device may result from the higher hydrophobicity of YC-1 film as well as the more uniform capping layer on top of perovskite layer (Figures 6 and 7), which prevented water penetration into the perovskite layer. The hydrophobic character of different HTMs were also investigated with water contact angle measurements. Contact angle of 96o, 77o, 90o, and 80o were obtained on films with YC-1, YC-2, YC-3, and Spiro-OMeTAD, respectively (shown in


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Figure S7). Thus a higher contact angle and higher hydrophobicity of YC-1 film may prevent the invasion of moisture into perovskite layer and contribute to the excellent stability of the YC-1-based solar cells.

Figure 11. Stability test for devices based on YC and Spiro-OMeTAD under a 30% humidity atmosphere. Perovskite solar cell performance parameters (a) Voc, (b) Jsc, (c) FF, and (d) PCE versus time (h) is presented.

CONCLUSIONS In conclusion, three new D-A-D type hole-transporting materials, coded as YC-1, YC-2, and YC-3 with 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b’]dithiophene (DiCN-CPDT) cores as the electron acceptor and two bis(p-methoxyphenylaminophenyl) groups as electron donor have been designed and synthesized for PSCs application. This YC series shows a



compatible energy level alignment with perovskites. Comparatively, YC-1 exhibited higher hole mobility, a more homogeneous film morphology on perovskite layer than the other DiCNCPDT-based HTMs. Moreover, photoluminescence spectra showed that YC-1 film extracted holes more efficiently. As a consequence, the YC-1-based PSCs exhibited a high PCE of 18.03% with small hysteresis, which is comparable with that of the devices using SpiroOMeTAD (18.14%). In addition, the devices based on YC-1 showed significantly improved long-term stability compared with the cells employing Spiro-OMeTAD, presumably due to the hydrophobic nature and homogeneous film. Our work demonstrated that the introduction of the 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b’]dithiophene unit as the central core is a promising strategy for designing efficient HTMs. EXPERIMENTAL SECTION Characterization. Nuclear magnetic resonance (NMR, 1H and

13C)

spectra were recorded on a Bruker 400

spectrometer in CDCl3 solution with the chloroform peaks (1H δ = 7.26 ppm and 13C δ = 77.00 ppm) as internal standard. High-resolution fast atom bombardment (FAB) mass spectra were obtained with a Jeol JMS 700 double-focusing spectrometer. UV-vis spectra were obtained on a Jasco V-530 double beam spectrophotometer. Photoluminescence spectra were obtained on a Hitachi F-4500 fluorescence spectrophotometer. Oxidation and reduction potentials of 1.0 × 10-3 M samples and 0.1 M supporting electrolyte of tetrabutylammonium hexafluorophosphate in dry THF solution were measured by cyclic voltammetry with a scan rate of 100 mV/s. The Ag/AgNO3 electrode, a platinum disk, and a platinum wire were used as reference, working, and auxiliary electrodes respectively. The SEM images were collected on a JEOL-7401 microscope. The surface morphologies and thicknesses of various thin films were measured with a Nano-Scope NS3A system (Digital Instrument).


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Materials and Reagents. All reagents were obtained from Aldrich and used without further purification if not mentioned specifically. THF was dried by sodium metal. The syntheses of compound 1 and 5 have been reported.23 Details of synthetic procedures and structural characterization of compounds 2, 3, YC-1, YC-2, and YC-3 by using 1H NMR, 13C NMR spectroscopy, and mass spectrometry are provided in Supporting Information. Solar Cell Fabrication. FTO glass was cut into an area of 1.5 cm*1.8 cm and patterned by etching part of the FTO(~4cm) using zinc powder and drops of 2N HCl. The patterned FTO substrates were cleaned by sonication in series in detergent (2%) solution, DI water, acetone, and 2-propanol, respectively for 20 min each. The patterned FTO substrates were dried with N2 gas, followed by UV-ozone treatment for 15 min. A solution of titanium diisopropoxidebis(acetylacetonate) (75 % in 2-propanol) in ethanol (1:10 v/v) was sprayed over the pre-annealed FTO substrate (450 °C for 30 min) to prepare c-TiO2 and then the substrate was annealed at 500 °C for 45 min – 60 min. Then the substrate was left to cool naturally. Subsequently, 30 NRT mp-TiO2 was spin-coated at 5000 rpm for 30 s from the solution of 150 mg of mp-TiO 2 paste in 1ml of ethanol. The solution was mixed by orbital shaker at least for 4 hrs. After the spin-coating, the film was initially annealed at 120 °C for 10 min, and then at 500 °C for 30 min. The annealing procedure for both c-TiO2 and mp-TiO2 was done step by step. Initially, the temperature was increased to 225 °C for 5 min, then 325 °C for 5min, 375 °C for 10 min and finally 500 °C for the required time. Once the mp-TiO2 substrate was cooled naturally to room temperature, a 30 min UV-Ozone treatment was applied to remove the residual organics on the mp-TiO2. This substrate was used for the next process immediately. The perovskite precursor, methylammonium lead iodide (CH3NH3PbI3) solution was prepared by mixing equimolar PbI2



and CH3NH3I (1M,1M) with the addition of 1M DMSO (ca. 73µL) in 0.63 mL of DMF in a glove box at a controlled level of O2 and H2O. The solution was stirred at room temperature and filtered with a 0.22 µm PVDF filter. The clear solution was spin-coated at a speed of 4000 rpm for 25s. About 10-12s from the start of the spinning process diethyl ether was dripped as anti-solvent to slow down the crystallization process. Transparent film was formed after the spinning process, and smooth black perovskite layer was achieved after the film was annealed at 70oC for 1 min, and 100oC for 5min.79-80 The HTMs were prepared by mixing 40 mg YC derivatives or Spiro-OMeTAD in 0.5ml chlorobenzene. The solution was doped with 7.5 µl LiTFSI, which was separately prepared and taken from a stock solution of 0.05 mg Li-TFSI in 100 µl acetonitrile and 14.5 µl TPB. About 35-40 µL of the doped YC derivatives or SpiroOMeTAD solution was spin-coated at 4000 rpm for 30s. For back electrode, 70 nm-80 nm Ag was deposited through a mask by thermal evaporation at low pressure of 8*10-7 Torr at a controlled rate of ca. 1Å/s and 0.3Å/s for Ag and Au respectively. Solar Cell Performance Measurement. The J-V curves were recorded by using Keithley 2400 source measurement unit under simulated AM 1.5G illumination at an intensity of 100 mW/cm2. The intensity of the simulated sunlight was calibrated to 100 mW/cm2. An IPCE spectrometer (Enlitech, QE‐R) was used for the incident monochromatic photon-to-current conversion efficiency (IPCE) measurements. General procedures for Suzuki reaction. To a mixture of pinacol ester derivatives (2.1 mmol), Pd(PPh3)4 (4 mol%), and aryl bromide (1.00 mmol) was added 2 M K2CO3 (aq.) (5 mL) and THF (7 mL) in a one-necked flask equipped with a magnetic stirrer under argon. The reaction mixture was then heated at 80 °C under argon for 18 h. After the reaction mixture had cooled to room temperature, water was


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added. The product was extracted with CH2Cl2, and the combined organic layer was dried (MgSO4) and concentrated in vacuo. Synthetic Procedures. Synthesis of 2. A one-necked flask containing 2 M K2CO3 (12 mL), THF (15 mL), 2-Bromo-3-hexylthiophene (1.15 g, 4.64 mmol), 1 (2 g, 4.64 mmol), and Pd(PPh3)4 (0.110 g, 2 mol %) equipped with a magnetic stirrer under argon was heated at 80 oC

for 18 h. The reaction mixture was then poured into water and the product was extracted

with CH2Cl2. The combined organic layer was dried with anhydrous MgSO 4 and evaporated to dryness. The crude product was purified by column chromatography over silica gel with CH2Cl2:hexane (1:10, v:v) as eluent, affording the desired product as a light yellow liquid in 85% yield. 1H NMR 400 MHz, CDCl3): δ 7.36 (d, J = 4.0 Hz, 1H), 7.20 (d, J = 8.0 Hz, 2H), 7.06 (d, J = 8.4 Hz, 4H), 6.99 (d, J = 4.0 Hz, 1H), 6.92 (d, J = 8.4 Hz, 4H), 6.77 (d, J = 8.0 Hz, 2H) ppm; 13C NMR 100 MHz, CDCl3): δ 156.4, 148.2, 140.2, 137.8, 137.6, 130.2, 130.0, 127.5, 125.8, 123.8, 119.1, 115.5, 55.7, 31.4, 30.7, 28.9, 28.5, 22.5, 14.4 ppm; HRMS (FAB) m/z [M+] calcd for C30H33O2NS: 471.2232; found: 471.2235. Synthesis of 3. Compound 2 (2 g, 4.2 mmol) was dissolved in dry THF (65 mL) under N2. After the mixture was cooled to -78 °C, n-BuLi (1.6 M in hexane, 3.2 mL, 5.1 mmol) was added dropwise, and the mixture was stirred at -78 °C for 1 h. 2-Isopropoxy-4,4,5,5tetramethyl-1,3,2-dioxaborolane (1.00 mL, 5.09 mmol) was added dropwise. The solution was stirred at -78 °C for 1 h and then at room temperature overnight. After addition of water (45 mL), the extraction was carried out with CH2Cl2. The organic phase of the extracts was dried over anhydrous MgSO4, and the solvents were removed by evaporation. Column chromatography on a silica gel stationary phase with CH2Cl2:hexane (1:3, v:v) as eluent affording the product as a light yellow liquid in 75% yield. 1H NMR 400 MHz, CDCl3): δ 7.40 (s, 1H), 7.22 (d, J = 8.0 Hz, 2H), 7.08 (d, J = 8.4 Hz, 4H), 6.92 (d, J = 8.4 Hz, 4H), 6.74 (d, J = 8.0 Hz, 2H) ppm; 13C NMR 100 MHz, CDCl3): δ 156.6, 148.6, 145.4, 140.5, 140.4, 140.0,



139.4, 129.9, 127.7, 125.2, 118.6, 115.5, 84.3, 55.7, 31.4, 30.7, 28.9, 28.3, 25.0, 25.0, 22.5, 14.4 ppm; HRMS (FAB) m/z [M+] calcd for C36H44BO4NS: 597.3083; found: 597.3084. Synthesis of YC-1. Compound YC-1 was obtained according to the standard Suzuki reaction as described above. Further purification was performed by flash chromatography using a mixed solvent (CH2Cl2:hexane =1:1) as eluent to provide the desired product as a black solid in 82% yield, mp 130-131 °C; 1H NMR 400 MHz, CDCl3): δ 7.26 (d, J = 7.8 Hz, 4H), 7.14 (s, 2H), 7.01 (d, J = 8.4 Hz, 8H), 6.91 (d, J = 8.4 Hz, 4H), 6.73 (d, J = 7.8 Hz, 8H), 3.74 (s, 12H) ppm; 13C

NMR 100 MHz, CDCl3): δ 156.5, 153.8, 148.9, 147.8, 143.4, 140.9, 139.8, 127.4, 126.1,

124.4, 119.1, 116.2, 115.4, 113.2, 76.1, 55.7 ppm; HRMS (FAB) m/z [M+] calcd for C52H38O4N4S2: 846.2334; found: 846.2331. Synthesis of YC-2. Compound YC-2 was obtained according to the standard Suzuki reaction as described above. Further purification was performed by flash chromatography using a mixed solvent (CH2Cl2:hexane =1:1) as eluent to give the desired product as a black solid in 81% yield, mp 268-269 °C; 1H NMR 400 MHz, CDCl3): δ 7.38 (s, 4H), 7.36 (s, 2H), 7.11-7.06 (m, 12H), 6.90 (d, J = 8.8 Hz, 4H), 6.87-6.83 (m, 8H), 3.81 (s, 12H) ppm; 13C NMR 100 MHz, CDCl3): δ 156.2, 148.7, 144.8, 143.2, 141.1, 140.9, 140.4, 133.5, 126.9, 126.4, 125.4, 125.1, 122.2, 120.1, 118.4, 114.8, 112.5, 77.0, 55.5 ppm; HRMS (FAB) m/z [M+] calcd for C60H42O4N4S4: 1010.2089; found: 1010.2095. Synthesis of YC-3. Compound YC-3 was obtained according to the standard Suzuki reaction as described above. Further purification was performed by flash chromatography using a mixed solvent (CH2Cl2:hexane =1:1) as eluent to provide the desired product as a black solid in 75% yield, mp 220-221 °C; 1H NMR 400 MHz, CDCl3): δ 7.35 (s, 2H), 7.20 (d, J = 8.4 Hz, 4H), 7.10 (d, J = 8.8 Hz, 8H), 7.03 (s, 2H), 6.92 (d, J = 8.4 Hz, 4H), 6.85 (d, J = 8.8 Hz, 8H), 3.80 (s, 12H), 2.61 (t, J = 7.6, 4H), 1.63 (t, J = 7.6 Hz, 4H), 1.35—1.29 (m, 12H), 0.88 (t, J = 6.4


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Hz, 6H) ppm; 13C NMR 100 MHz, CDCl3): δ 156.2, 154.3, 148.3, 143.0, 141.1, 141.1, 140.5, 138.9, 138.5, 132.6, 129.6, 126.9, 126.6, 125.3, 119.7, 118.0, 114.8, 112.5, 77.0, 55.5, 31.6, 30.9, 29.3, 28.9, 22.6, 14.1 ppm; HRMS (FAB) m/z [M+] calcd for C72H66O4N4S4: 1178.3967; found: 1178.4008. ASSOCIATED CONTENT Supporting Information. Calculated UV−vis spectra, computational results for energy levels, hole mobility measurements, TGA curves, DSC scans, PXRD, J−V curve measurements, detailed characterization data and 1H, 13C NMR, and mass spectra of new compounds, and water contact angle measurements. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Y.-D. Lin) *Email: [email protected] (Y.-T. Tao) *Email: [email protected] (S.-S. Sun) *Email: [email protected] (Tahsin J. Chow) Author Contributions Y. D. Lin and S. Y. Abate contributed equally to this paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

We are grateful to Academia Sinica for support of this research. Y.-D. Lin thanks the support from Ministry of Science and Technology (Grant No MOST 107-2113-M-415006-).



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Table of Contents (TOC)



Figure 1. Chemical structures of YC-1, YC-2, and YC-3. 395x59mm (300 x 300 DPI)


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Figure 2. (a) Normalized UV/vis absorption spectra of the YC series and Spiro-OMeTAD in chlorobenzene solutions and thin films. (b) Differential pulse voltammetry of YC series measured at a sweep rate of 100mV/s. (c) Energy level alignment of our PSCs with YC series and Spiro-OMeTAD as HTMs. (d) Photoemission yield plots of YC series in air. 429x328mm (96 x 96 DPI)



Figure 3. The optimized structures of YC series calculated with DFT on B3LYP-6-31G (d,p). 454x139mm (300 x 300 DPI)


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Figure 4. Frontier orbitals and energy level of HOMO and LUMO orbital surfaces of YC series calculated with DFT at the B3LYP/6-31G(d,p) level. 306x239mm (120 x 120 DPI)



Figure 5. Steady-state photoluminescence spectra of YC series and Spiro-OMeTAD deposited on perovskite. 214x184mm (300 x 300 DPI)


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Figure 6. Top view SEM morphology of YC series and Spiro-OMeTAD capping layers on the perovskite films. 431x330mm (96 x 96 DPI)



Figure 7. AFM microgram and 3D images of YC-1 (a), YC-2 (b), YC-3 (c) and Spiro-OMeTAD (d) films deposited on perovskite/mp-TiO2/c-TiO2/FTO. All images were taken in 5µm*5µm area. 254x190mm (96 x 96 DPI)


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Figure 8. (a) Device architecture, and (b) Cross-sectional SEM image of PSCs with the device structure of FTO/c-TiO2/mp-TiO2/perovskite/YC-1/Ag. (c) J-V curves of champion cells based on YC series and SpiroOMeTAD with reverse scan. (d) IPCE plots and integrated current densities of perovskite solar cells based on YC series and Spiro-OMeTAD. 431x306mm (96 x 96 DPI)



Figure 9. The stabilized power output of PSCs devices for (a) YC-1, (b) YC-2, (c) YC-3, and (d) SpiroOMeTAD at their maximum power point. 427x313mm (96 x 96 DPI)


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Figure 10. Box charts of (a) Voc, (b) Jsc, (c) FF, and (d) PCE measured using reverse scans for 15 devices based on YC series and Spiro-OMeTAD. 430x297mm (96 x 96 DPI)



Figure 11. Stability test for devices based on YC and Spiro-OMeTAD under a 30% humidity atmosphere. Perovskite solar cell performance parameters (a) Voc, (b) Jsc, (c) FF, and (d) PCE versus time (h) is presented. 413x320mm (96 x 96 DPI)


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Scheme 1. Synthetic route of YC series. 363x244mm (300 x 300 DPI)


Donor-Acceptor-Donor-Type Cyclopenta[2,1-b;3,4-b']dithiophene

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