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Anthradithiophene based hole-transport material for efficient and stable perovskite solar cells
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Anthradithiophene based hole-transport material for efficient and stable perovskite solar cells Guohua Wu , Yaohong Zhang , Ryuji Kaneko , Yoshiyuki Kojima , Ashraful Islam , Kosuke Sugawa , Joe Otsuki , Shengzhong Liu PII: DOI: Reference:
S2095-4956(20)30080-2 https://doi.org/10.1016/j.jechem.2020.02.021 JECHEM 1111
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Journal of Energy Chemistry
Received date: Revised date: Accepted date:
16 January 2020 17 February 2020 19 February 2020
Please cite this article as: Guohua Wu , Yaohong Zhang , Ryuji Kaneko , Yoshiyuki Kojima , Ashraful Islam , Kosuke Sugawa , Joe Otsuki , Shengzhong Liu , Anthradithiophene based holetransport material for efficient and stable perovskite solar cells, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.02.021
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Highlights
A novel anthradithiophene based hole-transport material is designed.
BTPA-7 exhibits higher hole mobility and Tg than spiro-OMeTAD.
BTPA-7 based PSC shows comparable device performance to that of spiro-OMeTAD.
More hydrophobic BTPA-7 than spiro-OMeTAD is beneficial for its PSC stability.
Anthradithiophene based hole-transport material for efficient and stable perovskite solar cells Guohua Wua,b, Yaohong Zhangc,*, Ryuji Kanekob,d, Yoshiyuki Kojimab, Ashraful Islamd,*, Kosuke Sugawab, Joe Otsukib,*, Shengzhong Liua,* a
Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi
Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China b
College of Science and Technology, Nihon University, Chiyoda-ku 101-8308, Tokyo, Japan
c
Department of Engineering Science, Faculty of Informatics and Engineering, The University of Electro-
Communications, Chofu 182-8585, Tokyo, Japan d
Photovoltaic Materials Group, National Institute for Materials Science (NIMS), Tsukuba 305-0047,
Japan *Corresponding authors. Email address: [email protected] (Y. Zhang), [email protected] (A. Islam), [email protected] (J. Otsuki), [email protected] (S. Liu). Abstract A novel hole-transport material (HTM) based on an anthradithiophene central bridge named BTPA-7 is developed. In comparison to spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene), the synthetic steps of BTPA-7 are greatly reduced from 6 to 3 and the synthetic cost of BTPA-7 is nearly a half that of spiro-OMeTAD. Moreover, BTPA-7 exhibits a relatively lower conductivity but higher hole mobility and higher glass transition temperature (Tg) than spiro-OMeTAD. Compared with the photovolatic performance for spiro-OMeTAD, FA0.85MA0.15PbI3 and MAPbI3 PSC devices based on BTPA-7 exhibit slightly lower PCEs with the values of 17.58% (18.88% for spiroOMeTAD) and 11.90% (13.25% for spiro-OMeTAD), respectively. Nevertheless, a dramatically higher Jsc of PSC based on BTPA-7 is achieved, which arises from the higher hole mobility of BTPA-7. In addition, the relatively hydrophobic character of BTPA-7 eventually enhances the PSC device stability. Lower cost, higher hole mobility, higher Tg, satisfactory photovoltaic performance, and superior device stability of BTPA-7 can be utilized as a substitute for spiro-OMeTAD in PSCs. Keywords: Anthradithiophene; Hole-transport material; Stability; Synthetic cost 1. Introduction Solar cells based on organic-inorganic halide perovskites have achieved a certified power conversion
efficiency (PCE) of 25.2% [1] due to their ease of fabrication and attractive optoelectronic properties, which is comparable to that of the commercialized crystalline silicon [2‒5]. Compared with the initial low PCE of 3.8% [6], the impressive progress of perovskite solar cells (PSCs) is mainly ascribed to the comprehensive optimization of device components including perovskite materials, hole transport layer, and electron transport layer [7‒9]. It is apparent that hole transport materials (HTMs) play an essential role as mentioned below [10,11]. First, the primary function of a HTM is to extract the holes from the photoexcited perovskite materials and immediately to transport the holes toward the counter electrode. Second, the HTM can be utilized as a blocking layer to prevent the excited electrons in perovskite materials from transferring to the counter electrode. Third, the HTM layer is also introduced as a blocking layer to protect the perovskite materials against the water and oxygen and eventually affects the long-term stability of PSCs. Now, various HTMs have emerged including inorganic compounds [12‒15], polymers [16‒18], and small molecules. spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene) is the typical utilized small molecule HTM for PSCs [19]. However, high synthetic cost and the device stability induced by its relatively hydrophilic character have restricted its further application in PSCs.[19] It is required to develop alternative small molecule HTMs to spiro-OMeTAD with low-cost production, suitable energy levels matching the perovskite, a high hole mobility, a high conductivity, and an elevated glass transition temperature [20,21]. A variety of novel HTMs has gradually emerged in recent years, such as truxene-core derivatives [22,23], triazatruxene derivatives [24], azomethine derivatives [25,26], pentacene derivatives [27‒29], fluoranthene-core derivatives [30], a fullerene-core derivative [31], star-shaped molecules [32‒35], spiro[fluorene-9,9'-xanthene]derivatives [36‒38]. Anthradithiophene (ADT) with two thiophene units fused to the ends of the anthracene core, has been utilized as a key molecular structural component for application in organic electronic devices.[39] The ADT unit displays extended conjugation and a high hole mobility, which would be beneficial to ameliorate the hole mobility and elevate the glass transition temperature of molecules based on coplanar linear ADT unit. So far, there is scarce report on ADT based HTMs for PSCs. We herein synthesized a novel ADT based HTM, BTPA-7, to expand its application in PSCs. In order to further extend the conjugation and enhance the solubility in organic solvents, four triphenylamine (TPA) branches as the four leaflets in a lucky clover are artfully attached to the ADT unit (shown in Fig. 1). The thermal, optical, and electronic properties as well as the conductivity and hole mobility of BTPA-7 were initially investigated.
In order to reveal the relationship between the properties and PSC performance, the
identical FA0.85MA0.15PbI3 or MAPbI3 (FA= HC(NH2)2, MA=CH3NH3) PSC devices for BTPA-7 and the reference spiro-OMeTAD were then studied in detail.
Fig. 1. Molecular structure of BTPA-7 (syn/anti).
2. Experimental 2.1. Synthetic procedures
Scheme 1. Synthetic route for BTPA-7. (a) 15% KOH solution, EtOH, r.t., 2 h; (b) CBr4, PPh3, toluene, 80 °C, 24 h; (c) N,N-bis(4-methoxyphenyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine, THF, Pd(PPh3)4, 85 °C, 48 h. The synthetic route for BTPA-7 is illustrated in Scheme 1. The intermediate 1, 2, and BTPA-7 have been synthesized as follows. 2.1.1. Synthesis of 1 To a EtOH (100 mL) solution of thiophene-2,3-dicarboxaldehyde (1.0 g, 7.1 mmol) and 1,4cyclohexanedione (0.38 g, 3.4 mmol), 15 % KOH solution (4 mL) was added. The mixture was stirred for 2 h, then filtered to yield 0.97 g (3.0 mmol, 88%) of compound 1 as yellow powder. 2.1.2. Synthesis of 2
An anhydrous toluene solution (25 mL) of compound 1 (0.39 g, 1.22 mmol), carbon tetrabromide (1.83 g, 3.64 mmol), and triphenyl phosphine (1.93 g, 7.35 mmol) were heated at 80 °C for 24 h. After cooling, a residue was obtained by filteration and then washed with toluene. Compound 2 was finally isolated by column chromatography (hexane/CHCl3 = 10/1) as an off-white solid (0.33 g, 0.52 mmol, 42%). 1H NMR (400 MHz, CDCl3) δ/ppm: 8.27 (s, 2H), 8.22 (s, 2H), 7.50 (d, 2H, J= 5.6 Hz), 7.33 (d, 2H, J= 5.6 Hz).
13
C NMR (100 MHz, CDCl3) δ/ppm: 139.6, 139.8, 138.4, 138.1, 132.6, 132.5, 132.3,
132.2, 128.5, 128.3 124.1, 123.7, 122.7, 122.4, 121.8, 121.5, 89.9. 2.1.3. Synthesis of BTPA-7 A THF solution (30 mL) of compound 2 (376 mg, 0.60 mmol), (4-(4,4,5,5-tetramethyl-(1,3,2)dioxaborolan-2-yl)-phenyl)-di-(4-methoxyphenyl)-amine (1.14 g, 4.80 mmol), K2CO3 (2 M aqueous solution, 2 mL), and Pd(PPh3)4 (120 mg, 1.32 mmol) were first degassed and then heated at 90 °C for 48 h. After cooling, THF was evaporated and the resulted residue was extracted with CHCl3. BTPA-7 (740 mg, 0.48 mmol) was isolated by column chromatography (hexane/ethyl acetate = 3:1) as a light yellow solid in 81% yield. 1H NMR (400 MHz, CDCl3): δ/ppm: 7.53 (d, 4 H, J = 8.8 Hz), 7.29 (d, 2 H, J = 5.6 Hz), 7.17 (m, 8 H), 7.03 (d, 2 H, J = 5.6 Hz), 6.96 (t, 16 H, J = 9.2 Hz), 6.84 (t, 8 H, J = 7.8 Hz), 6.76 (m, 16 H), 3.76 (s, 24 H).
13
C NMR (100 MHz, CDCl3): δ/ppm: 155.5, 147.3, 147.2, 141.4, 141.3, 139.5,
137.2, 137.1, 135.4, 135.2, 135.1, 130.6, 126.0, 123.7, 123.2, 121.9, 121.6, 121.6, 114.6, 55.5. APCIHRMS m/z ([M+2H]+): calcd: 1530.5574, Found: 1530.5480. Anal. calcd for C100H80N4O8S2∙0.3 CHCl3: C, 76.94; N, 3.58; H, 5.17. Found: C, 76.80; N, 3.41; H, 5.08. 2.2. PSC device fabrication procedure Briefly, a glass/FTO substrate was first cleaned and then prepared TiO2 film on it by TiCl4 hydrolyzed reaction. The obtained glass/FTO/TiO2 substrate was annealed and then treated by UV-ozone for 10 min. Then a FA0.85MA0.15PbI3 or MAPbI3 perovskite precursor solution was immediately spincoated on the above glass/FTO/TiO2 substrate. An antisolvent chlorobenzene (CB) solution was dripped during the second step of the spin-coated process. The glass/FTO/TiO2/FA0.85MA0.15PbI3 or MAPbI3 substrate was subsequently annealed at 150 °C or 100 °C for 30 min. Then a HTM layer solution was spin-coated onto the cooled glass/FTO/TiO2/FA0.85MA0.15PbI3 or MAPbI3 substrate. At last, a 800 nm gold electrode was thermally evaporated onto the above glass/FTO/TiO2/FA0.85MA0.15PbI3 or MAPbI3/HTM substrate. 3. Results and discussion
Compound 1 was synthesized by using the Claisen-Schmidt condensation as syn-anti isomeric mixtures (drawn as anti isomer) due to a difficulty with isomer separation. Then compound 2 was synthesized by using the Corey-Fuchs reaction from compound 1. Subsequently compound 2 and N,Nbis(4-methoxyphenyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine were used to form the target BTPA-7 via a palladium-catalyzed Suzuki coupling reaction. Therefore, 3 reaction steps were required for the synthesis of BTPA-7, which showed less reaction steps than that for spiro-OMeTAD. The synthesis of BTPA-7 costs probably half that for spiro-OMeTAD. The synthesis cost data for them are summarized in Table S1. The facile synthesis and low synthesis cost of BTPA-7 could make it a competitive alternative to spiro-OMeTAD in PSCs.
Fig. 2. (a) Absorption spectra in CH2Cl2 (0.01 mM); (b) cyclic voltammograms for BTPA-7; (c) energy diagram of PSCs with BTPA-7; (d) steady-state photoluminescence spectra of BTPA-7 and spiroOMeTAD on perovskite films with respect to pristine FA0.85MA0.15PbI3 perovskite film. The optical spectrum of BTPA-7 in the ultraviolet and visible region is shown in Fig. 2(a). The maximum absorption of BTPA-7 locates at 299 nm with a molar extinction coefficient of 7.69 × 10 4 M−1
cm−1, which is attributed to the π-π* transition of triphenylamine and anthradithiophene unit. There is also a weak absorption band for BTPA-7 at 330-450 nm, which is attributed to the n-π* transition of triphenylamine and anthradithiophene unit. Electrochemical property of BTPA-7 was studied by the cyclic voltammogram as shown in Fig. 2(b). The highest occupied molecular orbital (HOMO) level can be derived from the equation (EHOMO (in eV) = – 4.5 – Eox (in V vs. NHE)), where Eox is the first oxidation potential. Then BTPA-7 exhibits a HOMO energy level of −5.29 eV, which is positive than the valence band of MAPbI3 (−5.43 eV) [40] or FA0.85MA0.15PbI3 (−5.63 eV) [41] but negative than the work function of gold as illustrated in the energy diagram of PSC in Fig. 2(c). This result indicates that BTPA7 can be utilized as a HTM in MAPbI3 or FA0.85MA0.15PbI3 solar cells due to its matched energy levels. Besides, BTPA-7 can act as an electron blocking layer to prohibit the transfer of electrons in the perovskite to the gold due to its more positive lowest unoccupied molecular orbital (LUMO) level compared with the conduction band of perovskite. The BTPA-7 structure is optimized by using density functional theory (DFT) with the B3LYP functional and the 6-311G (d,p) basis set. The frontier molecular orbitals of optimized BTPA-7 structure are shown in the inset of Fig. 2c. The electron density of the HOMO in BTPA-7 is mainly located at two of the four triphenylamine units, while the electron density of the LUMO is delocalized at the anthrathiophene central bridge. The HOMO to LUMO transition of BTPA-7 is the main singlet optical transition. The steady-state photoluminescence measurement of BTPA-7 and spiro-OMeTAD spin-coated on MAPbI3 or FA0.85MA0.15PbI3 are shown in Figs. S6 and 2(d). The photoluminescence peaks for MAPbI3 (766 nm) or FA0.85MA0.15PbI3 (798 nm) showed almost no change upon the addition of BTPA-7 or spiro-OMeTAD. Similar but a little smaller quenching trends of photoluminescence for MAPbI3 or FA0.85MA0.15PbI3 films upon the addition of BTPA-7 were observed compared to the spiro-OMeTAD film, which suggests that BTPA-7 has a little smaller hole extraction ability compared with that of spiro-OMeTAD. The conductivity of BTPA-7 was measured with a Au/HTM/Au device after doping with LiTFSI and TBP (Fig. S7). Under the same condition, BTPA-7 revealed a lower conductivity as 9.32×10−5 S cm−1 than that of the standard spiroOMeTAD (2.82×10−4 S cm−1) [40]. The hole mobility of BTPA-7 was measured using space-chargelimited-current methods (Fig. S7). The hole mobility of BTPA-7 and the standard spiro-OMeTAD were 7.79×10−5 cm2 V−1 s−1 and 1.95×10−5 cm2 V−1 s−1, respectively. The higher hole mobility of BTPA-7 may be attributed to the planar anthradithiophene unit. Table 1. Optical and electrochemical properties of BTPA-7. Materials
BTPA-7 a
Abs. λamax (nm)
HOMOb (eV vs.
(ε/M−1 cm−1)
vacuum)
299 (7.69 × 104)
−5.29
E0-0c (eV)
LUMOd (eV vs. vacuum)
3.11
−2.18
Conductivity
Hole mobility
(S cm−1)
(cm2 V−1 s−1)
9.32× 10−5
7.79×10−5
e
Measured in CH2Cl2 solution; b Eox measured by CV measurement in CH2Cl2. EHOMO =−4.5−Eox; c Energy gap (E0-0) calculated from the
intersection of the normalized absorption and photoluminescence spectra;
d
LUMO=HOMO−E0-0 ; e Measured with a Au/HTM/Au device
after doping with LiTFSI and TBP
Fig. 3. (a) The current density–voltage curve of MAPbI3 (area 1.02 cm2) and FA0.85MA0.15PbI3 (area 0.09 cm2) PSCs based on BTPA-7 compared with the standard spiro-OMeTAD; (b) IPCE spectra of FA0.85MA0.15PbI3 (area 0.09 cm2) PSCs based on BTPA-7 and spiro-OMeTAD. Table 2. Summary of photovoltaic parameters of BTPA-7 and spiro-OMeTAD based PSCs. HTMs BTPA-7 spiro-OMeTAD BTPA-7 spiro-OMeTAD
PVSK MAPbI3 FA0.85MA0.15PbI3
Jsc (mA cm−2) 20.04 19.50 24.74 24.08
Voc (V) 0.96 1.01 1.01 1.07
FF 0.62 0.67 0.71 0.73
PCE (%) 11.90 13.25 17.58 18.88
Area (cm2) 1.02 1.02 0.09 0.09
Scan direction Forward scan Reverse scan
Then we fabricated a batch of 10 PSC devices with a structure of FTO/TiO2/ MAPbI3/HTM/Au (area 1.02 cm2) and a batch of 15 PSC devices with a structure of FTO/TiO2/FA0.85MA0.15PbI3/HTM/Au (area 0.09 cm2) to investigate the photovoltaic performance of BTPA-7 as the HTM layer in comparison with the standard spiro-OMeTAD. Fig. 3(a) shows the current density-voltage (J-V) curves of the best FA0.85MA0.15PbI3 (reverse scan, RS) and MAPbI3 (forward scan, FS) device performance based on BTPA7 and spiro-OMeTAD under AM1.5G 1 sun (100 mW cm−2) intensity. The photovoltaic data are listed in Table 2. The corresponding statistical distributions of the photovoltaic parameters including PCE, Jsc, Voc, and FF for BTPA-7 and spiro-OMeTAD based FA0.85MA0.15PbI3 PSC devices are shown in Fig. S8. The best PCE of 17.58% was achieved for FA0.85MA0.15PbI3 PSC based on the BTPA-7 HTM with a shortcircuit current density (Jsc) of 24.74 mA cm−2, an open-circuit photovoltage (Voc) of 1.01V, a fill factor (FF) of 0.71. The standard spiro-OMeTAD exhibited a PCE of 18.88% with a Jsc of 24.08 mA cm−2, a Voc of 1.07 V, a FF of 0.71 under the same condition. BTPA-7 based MAPbI3 PSC exhibited the best PCE of 11.90%, while the standard spiro-OMeTAD presented the best PCE of 13.25%. By comparison with the spiro-OMeTAD whether in the MAPbI3 or in the FA0.85MA0.15PbI3 device, BTPA-7 exhibits a
competitive photovoltaic performance with a superior Jsc value but with inferior Voc and FF values. The higher Jsc of PSC based on BTPA-7 than spiro-OMeTAD is in good agreement with the integrated Jsc value from Figs. 3(b) and S9, which is probably induced by the higher hole mobility of BTPA-7. The lower Voc values of PSC based on BTPA-7 than that of the standard spiro-OMeTAD may be caused by an easier charge recombination process as demonstrated below in the impedance part. We also carried out the BTPA-7 and spiro-OMeTAD based PSC hysteresis investigation. The corresponding J-V curves are shown in Fig. S10. PCEs of 17.35% and 16.30% were achieved for BTPA-7 based PSC under RS and FS conditions with a hysteresis index of 6.4%. Higher PCEs of 18.47% (RS) and 17.00% (FS) were obtained for spiro-OMeTAD but with a larger hysteresis index of 8.6%.
Fig. 4. (a) Nyquist plots of devices based on BTPA-7 and spiro-OMeTAD under 0.95 V bias; (b) differential scanning calorimetry curve for BTPA-7; (c) thermal gravimetric measurement for BTPA-7; (d) long-term stability of the PSCs based on BTPA-7 and spiro-OMeTAD under the humidity of ~40% RH. The electrochemical impedance spectra (EIS) were then measured with a bias of 0.95 V under dark to understand the different photovoltaic performance of BTPA-7 in comparison to a spiro-OMeTAD reference device [42,43]. Fig. 4(a) compares the Nyquist plots of BTPA-7 and spiro-OMeTAD based PSC devices. Only one semicircle is observed which indicates that both of BTPA-7 and spiro-OMeTAD show a main recombination process. Thus a simple equivalent circuit model can be employed including a series resistance (Rs) and a recombination resistance (Rrec) paralleled with a capacitance (Cμ) [44,45]. After fitting the Nyquist plot using the above equivalent circuit model, spiro-OMeTAD based PSC device
exhibits a Rs and a Rrec with values of 19 and 1742 Ω, respectively. The Nyquist plot fitting results of BTPA-7 based PSC device exhibit both a larger Rs (30 Ω) and a lower Rrec (220 Ω), which may arise from the lower conductivity of the BTPA-7 layer. These results corroborate the lower FF and Voc values of BTPA-7 based PSC as shown in the J-V curves compared with the spiro-OMeTAD reference PSC device [46]. Stability is the challenge for the PSC commercialization [47,48]. Various parameters related to PSC stability need to be investigated in detail. The glass transition temperature (Tg) property of BTPA-7 relating to its noncrystalline phase stability was performed as shown in Fig. 4(b). BTPA-7 shows a higher Tg value of 136 ℃ than spiro-OMeTAD (125 ℃). The thermogravimetric analysis (TGA) on BTPA-7 was then conducted to analyze its intrinsic thermal stability. As shown in Fig. 4(c), BTPA-7 presents the decomposed temperature above 300 ℃ in N2 atmosphere indicating its excellent thermal stability. The water contact angle of BTPA-7 on a MAPbI3 film is 85°, higher than that of spiro-OMeTAD (70°) [49]. The hydrophobicity of BTPA-7 would improve the long-term stability of PSCs. The PSC long-term stabilities of BTPA-7 and the standard spiro-OMeTAD were investigated under a continuous humidity stress (~40% RH). After 192 h, the device performance of BTPA-7 maintained 86% of its initial PCE, while the spiro-OMeTAD based PSC maintained only 74% of its initial PCE. These results are most likely owing to the higher hydrophobicity of BTPA-7. 4. Conclusions A novel HTM named BTPA-7 with an anthradithiophene central bridge and four methoxysubstituted triphenylamine units as branches is facilely synthesized via 3 reaction steps. Due to the conjugation of the anthradithiophene unit, a higher hole mobility and a higher Tg are observed for BTPA7 even though it exhibits a lower conductivity compared with the reference spiro-OMeTAD. The best PCE of 17.58% is achieved for a BTPA-7 based FA0.85MA0.15PbI3 PSC. Under the same condition, the standard spiro-OMeTAD exhibits a PCE of 18.88%. By comparison with the spiro-OMeTAD whether in the MAPbI3 or the FA0.85MA0.15PbI3 device, BTPA-7 exhibits a competitive photovoltaic performance with a superior Jsc value resulted from its higher hole mobility. The BTPA-7 based PSC device shows better long-term stability than that for spiro-OMeTAD, which is attributed to the relatively hydrophobic character of BTPA-7. The present results demonstrate a potential anthradithiophene based alternative to spiro-OMeTAD for efficient PSCs with enhanced stability. Declaration of competing interests The authors declare that they have no conflict of interest. Acknowledgments
This work was financially supported by the National Key Research and Development Program of China (2016YFA0202403), the National University Research Fund (GK261001009), the Changjiang Scholar and Innovative Research Team (IRT_14R33), the Overseas Talent Recruitment Project (B14041), the Chinese National 1000-talent plan program (Grant No. 111001034). A.I. and J.O. thank the JSPS Kakenhi grants (No. 26288113 and 15K05486). J.O. also acknowledges the support from the Strategic Research Foundation at Private Universities (Nihon University and the MEXT, Japan). G.W. acknowledges the Natural Science Foundation of Shaanxi Province (2019JQ-423), the Fundamental Research Funds for the Central Universities (GK201903053), and Key Lab of photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences (No. PECL2019KF019). References [1] NREL chart, https://www.nrel.gov/pv/assets/images/efficiency-chart.png. [2] L. Zhang, J. Wu, D. Li, W. Li, Q. Meng, Z. Bo, J. Mater. Chem. A 7 (2019) 14473-14477. [3] J. Wu, C. Liu, B. Li, F. Gu, L. Zhang, M. Hu, X. Deng, Y. Qiao, Y. Mao, W. Tan, Y. Tian, B. Xu, ACS Appl. Mater. Inter. 11 (2019) 26928-26937. [4] Y. Wang, W. Chen, L. Wang, B. Tu, T. Chen, B. Liu, K. Yang, C.W. Koh, X. Zhang, H. Sun, G. Chen, X. Feng, H.Y. Woo, A.B. Djurišić, Z. He, X. Guo, Adv. Mater. 31 (2019) 1902781. [5] Y. Hou, Z.R. Zhou, T.Y. Wen, H.W. Qiao, Z.Q. Lin, B. Ge, H.G. Yang, Nanoscale Horiz. 4 (2019) 208-213. [6] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050-6051. [7] H. Li, G. Wu, W. Li, Y. Zhang, Z. Liu, D. Wang, S. Liu, Adv. Sci. 6 (2019) 1901241. [8] X. Lai, F. Meng, Q.-Q. Zhang, K. Wang, G. Li, Y. Wen, H. Ma, W. Li, X. Li, A.K.K. Kyaw, X.W. Sun, M. Du, X. Guo, J. Wang, W. Huang, Sol. RRL 3 (2019) 1900011. [9] S. Liu, S. Li, J. Wu, Q. Wang, Y. Ming, D. Zhang, Y. Sheng, Y. Hu, Y. Rong, A. Mei, H. Han, J. Phys. Chem. Lett. 10 (2019) 6865-6872. [10] F. Liu, F. Wu, W. Ling, Z. Tu, J. Zhang, Z. Wei, L. Zhu, Q. Li, Z. Li, ACS Energy Lett. 4 (2019) 2514-2521. [11] J. Urieta-Mora, I. Garcia-Benito, A. Molina-Ontoria, N. Martin, Chem. Soc. Rev. 47 (2018) 8541-8571. [12] Q. Zeng, Y. Di, C. Huang, K. Sun, Y. Zhao, H. Xie, D. Niu, L. Jiang, X. Hao, Y. Lai, F. Liu, J. Mater. Chem. C 6 (2018) 7989-7993. [13] X. Jin, Y. Yuan, C. Jiang, H. Ju, G. Jiang, W. Liu, C. Zhu, T. Chen, Sol. Energy Mater. Sol. Cells 185 (2018) 542-548. [14] J. Chen, N.-G. Park, J. Phys. Chem. C 122 (2018) 14039-14063. [15] X. Liu, Y. Wang, E. Rezaee, Q. Chen, Y. Feng, X. Sun, L. Dong, Q. Hu, C. Li, Z.X. Xu, Sol. RRL 2 (2018) 1800050. [16] S. Yang, C. Yang, X. Zhang, Z. Zheng, S. Bi, Y. Zhang, H. Zhou, J. Mater. Chem. C 6 (2018) 9044-9048. [17] G.W. Kim, J. Lee, G. Kang, T. Kim, T. Park, Adv. Energy Mater. 8 (2018) 1701935.
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Graphical abstract
A novel anthradithiophene based hole-transport material named BTPA-7 is designed. Compared with spiro-OMeTAD, BTPA-7 based PSC device exhibits comparable photovolatic performance with a dramatically higher Jsc due to its higher hole mobility.
Anthradithiophene based hole-transport material for efficient and stable perovskite solar cells Guohua Wu , Yaohong Zhang , Ryuji Kaneko , Yoshiyuki Kojima , Ashraful Islam , Kosuke Sugawa , Joe Otsuki , Shengzhong Liu PII: DOI: Reference:
S2095-4956(20)30080-2 https://doi.org/10.1016/j.jechem.2020.02.021 JECHEM 1111
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
16 January 2020 17 February 2020 19 February 2020
Please cite this article as: Guohua Wu , Yaohong Zhang , Ryuji Kaneko , Yoshiyuki Kojima , Ashraful Islam , Kosuke Sugawa , Joe Otsuki , Shengzhong Liu , Anthradithiophene based holetransport material for efficient and stable perovskite solar cells, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.02.021
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Highlights
A novel anthradithiophene based hole-transport material is designed.
BTPA-7 exhibits higher hole mobility and Tg than spiro-OMeTAD.
BTPA-7 based PSC shows comparable device performance to that of spiro-OMeTAD.
More hydrophobic BTPA-7 than spiro-OMeTAD is beneficial for its PSC stability.
Anthradithiophene based hole-transport material for efficient and stable perovskite solar cells Guohua Wua,b, Yaohong Zhangc,*, Ryuji Kanekob,d, Yoshiyuki Kojimab, Ashraful Islamd,*, Kosuke Sugawab, Joe Otsukib,*, Shengzhong Liua,* a
Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi
Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China b
College of Science and Technology, Nihon University, Chiyoda-ku 101-8308, Tokyo, Japan
c
Department of Engineering Science, Faculty of Informatics and Engineering, The University of Electro-
Communications, Chofu 182-8585, Tokyo, Japan d
Photovoltaic Materials Group, National Institute for Materials Science (NIMS), Tsukuba 305-0047,
Japan *Corresponding authors. Email address: [email protected] (Y. Zhang), [email protected] (A. Islam), [email protected] (J. Otsuki), [email protected] (S. Liu). Abstract A novel hole-transport material (HTM) based on an anthradithiophene central bridge named BTPA-7 is developed. In comparison to spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene), the synthetic steps of BTPA-7 are greatly reduced from 6 to 3 and the synthetic cost of BTPA-7 is nearly a half that of spiro-OMeTAD. Moreover, BTPA-7 exhibits a relatively lower conductivity but higher hole mobility and higher glass transition temperature (Tg) than spiro-OMeTAD. Compared with the photovolatic performance for spiro-OMeTAD, FA0.85MA0.15PbI3 and MAPbI3 PSC devices based on BTPA-7 exhibit slightly lower PCEs with the values of 17.58% (18.88% for spiroOMeTAD) and 11.90% (13.25% for spiro-OMeTAD), respectively. Nevertheless, a dramatically higher Jsc of PSC based on BTPA-7 is achieved, which arises from the higher hole mobility of BTPA-7. In addition, the relatively hydrophobic character of BTPA-7 eventually enhances the PSC device stability. Lower cost, higher hole mobility, higher Tg, satisfactory photovoltaic performance, and superior device stability of BTPA-7 can be utilized as a substitute for spiro-OMeTAD in PSCs. Keywords: Anthradithiophene; Hole-transport material; Stability; Synthetic cost 1. Introduction Solar cells based on organic-inorganic halide perovskites have achieved a certified power conversion
efficiency (PCE) of 25.2% [1] due to their ease of fabrication and attractive optoelectronic properties, which is comparable to that of the commercialized crystalline silicon [2‒5]. Compared with the initial low PCE of 3.8% [6], the impressive progress of perovskite solar cells (PSCs) is mainly ascribed to the comprehensive optimization of device components including perovskite materials, hole transport layer, and electron transport layer [7‒9]. It is apparent that hole transport materials (HTMs) play an essential role as mentioned below [10,11]. First, the primary function of a HTM is to extract the holes from the photoexcited perovskite materials and immediately to transport the holes toward the counter electrode. Second, the HTM can be utilized as a blocking layer to prevent the excited electrons in perovskite materials from transferring to the counter electrode. Third, the HTM layer is also introduced as a blocking layer to protect the perovskite materials against the water and oxygen and eventually affects the long-term stability of PSCs. Now, various HTMs have emerged including inorganic compounds [12‒15], polymers [16‒18], and small molecules. spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene) is the typical utilized small molecule HTM for PSCs [19]. However, high synthetic cost and the device stability induced by its relatively hydrophilic character have restricted its further application in PSCs.[19] It is required to develop alternative small molecule HTMs to spiro-OMeTAD with low-cost production, suitable energy levels matching the perovskite, a high hole mobility, a high conductivity, and an elevated glass transition temperature [20,21]. A variety of novel HTMs has gradually emerged in recent years, such as truxene-core derivatives [22,23], triazatruxene derivatives [24], azomethine derivatives [25,26], pentacene derivatives [27‒29], fluoranthene-core derivatives [30], a fullerene-core derivative [31], star-shaped molecules [32‒35], spiro[fluorene-9,9'-xanthene]derivatives [36‒38]. Anthradithiophene (ADT) with two thiophene units fused to the ends of the anthracene core, has been utilized as a key molecular structural component for application in organic electronic devices.[39] The ADT unit displays extended conjugation and a high hole mobility, which would be beneficial to ameliorate the hole mobility and elevate the glass transition temperature of molecules based on coplanar linear ADT unit. So far, there is scarce report on ADT based HTMs for PSCs. We herein synthesized a novel ADT based HTM, BTPA-7, to expand its application in PSCs. In order to further extend the conjugation and enhance the solubility in organic solvents, four triphenylamine (TPA) branches as the four leaflets in a lucky clover are artfully attached to the ADT unit (shown in Fig. 1). The thermal, optical, and electronic properties as well as the conductivity and hole mobility of BTPA-7 were initially investigated.
In order to reveal the relationship between the properties and PSC performance, the
identical FA0.85MA0.15PbI3 or MAPbI3 (FA= HC(NH2)2, MA=CH3NH3) PSC devices for BTPA-7 and the reference spiro-OMeTAD were then studied in detail.
Fig. 1. Molecular structure of BTPA-7 (syn/anti).
2. Experimental 2.1. Synthetic procedures
Scheme 1. Synthetic route for BTPA-7. (a) 15% KOH solution, EtOH, r.t., 2 h; (b) CBr4, PPh3, toluene, 80 °C, 24 h; (c) N,N-bis(4-methoxyphenyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine, THF, Pd(PPh3)4, 85 °C, 48 h. The synthetic route for BTPA-7 is illustrated in Scheme 1. The intermediate 1, 2, and BTPA-7 have been synthesized as follows. 2.1.1. Synthesis of 1 To a EtOH (100 mL) solution of thiophene-2,3-dicarboxaldehyde (1.0 g, 7.1 mmol) and 1,4cyclohexanedione (0.38 g, 3.4 mmol), 15 % KOH solution (4 mL) was added. The mixture was stirred for 2 h, then filtered to yield 0.97 g (3.0 mmol, 88%) of compound 1 as yellow powder. 2.1.2. Synthesis of 2
An anhydrous toluene solution (25 mL) of compound 1 (0.39 g, 1.22 mmol), carbon tetrabromide (1.83 g, 3.64 mmol), and triphenyl phosphine (1.93 g, 7.35 mmol) were heated at 80 °C for 24 h. After cooling, a residue was obtained by filteration and then washed with toluene. Compound 2 was finally isolated by column chromatography (hexane/CHCl3 = 10/1) as an off-white solid (0.33 g, 0.52 mmol, 42%). 1H NMR (400 MHz, CDCl3) δ/ppm: 8.27 (s, 2H), 8.22 (s, 2H), 7.50 (d, 2H, J= 5.6 Hz), 7.33 (d, 2H, J= 5.6 Hz).
13
C NMR (100 MHz, CDCl3) δ/ppm: 139.6, 139.8, 138.4, 138.1, 132.6, 132.5, 132.3,
132.2, 128.5, 128.3 124.1, 123.7, 122.7, 122.4, 121.8, 121.5, 89.9. 2.1.3. Synthesis of BTPA-7 A THF solution (30 mL) of compound 2 (376 mg, 0.60 mmol), (4-(4,4,5,5-tetramethyl-(1,3,2)dioxaborolan-2-yl)-phenyl)-di-(4-methoxyphenyl)-amine (1.14 g, 4.80 mmol), K2CO3 (2 M aqueous solution, 2 mL), and Pd(PPh3)4 (120 mg, 1.32 mmol) were first degassed and then heated at 90 °C for 48 h. After cooling, THF was evaporated and the resulted residue was extracted with CHCl3. BTPA-7 (740 mg, 0.48 mmol) was isolated by column chromatography (hexane/ethyl acetate = 3:1) as a light yellow solid in 81% yield. 1H NMR (400 MHz, CDCl3): δ/ppm: 7.53 (d, 4 H, J = 8.8 Hz), 7.29 (d, 2 H, J = 5.6 Hz), 7.17 (m, 8 H), 7.03 (d, 2 H, J = 5.6 Hz), 6.96 (t, 16 H, J = 9.2 Hz), 6.84 (t, 8 H, J = 7.8 Hz), 6.76 (m, 16 H), 3.76 (s, 24 H).
13
C NMR (100 MHz, CDCl3): δ/ppm: 155.5, 147.3, 147.2, 141.4, 141.3, 139.5,
137.2, 137.1, 135.4, 135.2, 135.1, 130.6, 126.0, 123.7, 123.2, 121.9, 121.6, 121.6, 114.6, 55.5. APCIHRMS m/z ([M+2H]+): calcd: 1530.5574, Found: 1530.5480. Anal. calcd for C100H80N4O8S2∙0.3 CHCl3: C, 76.94; N, 3.58; H, 5.17. Found: C, 76.80; N, 3.41; H, 5.08. 2.2. PSC device fabrication procedure Briefly, a glass/FTO substrate was first cleaned and then prepared TiO2 film on it by TiCl4 hydrolyzed reaction. The obtained glass/FTO/TiO2 substrate was annealed and then treated by UV-ozone for 10 min. Then a FA0.85MA0.15PbI3 or MAPbI3 perovskite precursor solution was immediately spincoated on the above glass/FTO/TiO2 substrate. An antisolvent chlorobenzene (CB) solution was dripped during the second step of the spin-coated process. The glass/FTO/TiO2/FA0.85MA0.15PbI3 or MAPbI3 substrate was subsequently annealed at 150 °C or 100 °C for 30 min. Then a HTM layer solution was spin-coated onto the cooled glass/FTO/TiO2/FA0.85MA0.15PbI3 or MAPbI3 substrate. At last, a 800 nm gold electrode was thermally evaporated onto the above glass/FTO/TiO2/FA0.85MA0.15PbI3 or MAPbI3/HTM substrate. 3. Results and discussion
Compound 1 was synthesized by using the Claisen-Schmidt condensation as syn-anti isomeric mixtures (drawn as anti isomer) due to a difficulty with isomer separation. Then compound 2 was synthesized by using the Corey-Fuchs reaction from compound 1. Subsequently compound 2 and N,Nbis(4-methoxyphenyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzenamine were used to form the target BTPA-7 via a palladium-catalyzed Suzuki coupling reaction. Therefore, 3 reaction steps were required for the synthesis of BTPA-7, which showed less reaction steps than that for spiro-OMeTAD. The synthesis of BTPA-7 costs probably half that for spiro-OMeTAD. The synthesis cost data for them are summarized in Table S1. The facile synthesis and low synthesis cost of BTPA-7 could make it a competitive alternative to spiro-OMeTAD in PSCs.
Fig. 2. (a) Absorption spectra in CH2Cl2 (0.01 mM); (b) cyclic voltammograms for BTPA-7; (c) energy diagram of PSCs with BTPA-7; (d) steady-state photoluminescence spectra of BTPA-7 and spiroOMeTAD on perovskite films with respect to pristine FA0.85MA0.15PbI3 perovskite film. The optical spectrum of BTPA-7 in the ultraviolet and visible region is shown in Fig. 2(a). The maximum absorption of BTPA-7 locates at 299 nm with a molar extinction coefficient of 7.69 × 10 4 M−1
cm−1, which is attributed to the π-π* transition of triphenylamine and anthradithiophene unit. There is also a weak absorption band for BTPA-7 at 330-450 nm, which is attributed to the n-π* transition of triphenylamine and anthradithiophene unit. Electrochemical property of BTPA-7 was studied by the cyclic voltammogram as shown in Fig. 2(b). The highest occupied molecular orbital (HOMO) level can be derived from the equation (EHOMO (in eV) = – 4.5 – Eox (in V vs. NHE)), where Eox is the first oxidation potential. Then BTPA-7 exhibits a HOMO energy level of −5.29 eV, which is positive than the valence band of MAPbI3 (−5.43 eV) [40] or FA0.85MA0.15PbI3 (−5.63 eV) [41] but negative than the work function of gold as illustrated in the energy diagram of PSC in Fig. 2(c). This result indicates that BTPA7 can be utilized as a HTM in MAPbI3 or FA0.85MA0.15PbI3 solar cells due to its matched energy levels. Besides, BTPA-7 can act as an electron blocking layer to prohibit the transfer of electrons in the perovskite to the gold due to its more positive lowest unoccupied molecular orbital (LUMO) level compared with the conduction band of perovskite. The BTPA-7 structure is optimized by using density functional theory (DFT) with the B3LYP functional and the 6-311G (d,p) basis set. The frontier molecular orbitals of optimized BTPA-7 structure are shown in the inset of Fig. 2c. The electron density of the HOMO in BTPA-7 is mainly located at two of the four triphenylamine units, while the electron density of the LUMO is delocalized at the anthrathiophene central bridge. The HOMO to LUMO transition of BTPA-7 is the main singlet optical transition. The steady-state photoluminescence measurement of BTPA-7 and spiro-OMeTAD spin-coated on MAPbI3 or FA0.85MA0.15PbI3 are shown in Figs. S6 and 2(d). The photoluminescence peaks for MAPbI3 (766 nm) or FA0.85MA0.15PbI3 (798 nm) showed almost no change upon the addition of BTPA-7 or spiro-OMeTAD. Similar but a little smaller quenching trends of photoluminescence for MAPbI3 or FA0.85MA0.15PbI3 films upon the addition of BTPA-7 were observed compared to the spiro-OMeTAD film, which suggests that BTPA-7 has a little smaller hole extraction ability compared with that of spiro-OMeTAD. The conductivity of BTPA-7 was measured with a Au/HTM/Au device after doping with LiTFSI and TBP (Fig. S7). Under the same condition, BTPA-7 revealed a lower conductivity as 9.32×10−5 S cm−1 than that of the standard spiroOMeTAD (2.82×10−4 S cm−1) [40]. The hole mobility of BTPA-7 was measured using space-chargelimited-current methods (Fig. S7). The hole mobility of BTPA-7 and the standard spiro-OMeTAD were 7.79×10−5 cm2 V−1 s−1 and 1.95×10−5 cm2 V−1 s−1, respectively. The higher hole mobility of BTPA-7 may be attributed to the planar anthradithiophene unit. Table 1. Optical and electrochemical properties of BTPA-7. Materials
BTPA-7 a
Abs. λamax (nm)
HOMOb (eV vs.
(ε/M−1 cm−1)
vacuum)
299 (7.69 × 104)
−5.29
E0-0c (eV)
LUMOd (eV vs. vacuum)
3.11
−2.18
Conductivity
Hole mobility
(S cm−1)
(cm2 V−1 s−1)
9.32× 10−5
7.79×10−5
e
Measured in CH2Cl2 solution; b Eox measured by CV measurement in CH2Cl2. EHOMO =−4.5−Eox; c Energy gap (E0-0) calculated from the
intersection of the normalized absorption and photoluminescence spectra;
d
LUMO=HOMO−E0-0 ; e Measured with a Au/HTM/Au device
after doping with LiTFSI and TBP
Fig. 3. (a) The current density–voltage curve of MAPbI3 (area 1.02 cm2) and FA0.85MA0.15PbI3 (area 0.09 cm2) PSCs based on BTPA-7 compared with the standard spiro-OMeTAD; (b) IPCE spectra of FA0.85MA0.15PbI3 (area 0.09 cm2) PSCs based on BTPA-7 and spiro-OMeTAD. Table 2. Summary of photovoltaic parameters of BTPA-7 and spiro-OMeTAD based PSCs. HTMs BTPA-7 spiro-OMeTAD BTPA-7 spiro-OMeTAD
PVSK MAPbI3 FA0.85MA0.15PbI3
Jsc (mA cm−2) 20.04 19.50 24.74 24.08
Voc (V) 0.96 1.01 1.01 1.07
FF 0.62 0.67 0.71 0.73
PCE (%) 11.90 13.25 17.58 18.88
Area (cm2) 1.02 1.02 0.09 0.09
Scan direction Forward scan Reverse scan
Then we fabricated a batch of 10 PSC devices with a structure of FTO/TiO2/ MAPbI3/HTM/Au (area 1.02 cm2) and a batch of 15 PSC devices with a structure of FTO/TiO2/FA0.85MA0.15PbI3/HTM/Au (area 0.09 cm2) to investigate the photovoltaic performance of BTPA-7 as the HTM layer in comparison with the standard spiro-OMeTAD. Fig. 3(a) shows the current density-voltage (J-V) curves of the best FA0.85MA0.15PbI3 (reverse scan, RS) and MAPbI3 (forward scan, FS) device performance based on BTPA7 and spiro-OMeTAD under AM1.5G 1 sun (100 mW cm−2) intensity. The photovoltaic data are listed in Table 2. The corresponding statistical distributions of the photovoltaic parameters including PCE, Jsc, Voc, and FF for BTPA-7 and spiro-OMeTAD based FA0.85MA0.15PbI3 PSC devices are shown in Fig. S8. The best PCE of 17.58% was achieved for FA0.85MA0.15PbI3 PSC based on the BTPA-7 HTM with a shortcircuit current density (Jsc) of 24.74 mA cm−2, an open-circuit photovoltage (Voc) of 1.01V, a fill factor (FF) of 0.71. The standard spiro-OMeTAD exhibited a PCE of 18.88% with a Jsc of 24.08 mA cm−2, a Voc of 1.07 V, a FF of 0.71 under the same condition. BTPA-7 based MAPbI3 PSC exhibited the best PCE of 11.90%, while the standard spiro-OMeTAD presented the best PCE of 13.25%. By comparison with the spiro-OMeTAD whether in the MAPbI3 or in the FA0.85MA0.15PbI3 device, BTPA-7 exhibits a
competitive photovoltaic performance with a superior Jsc value but with inferior Voc and FF values. The higher Jsc of PSC based on BTPA-7 than spiro-OMeTAD is in good agreement with the integrated Jsc value from Figs. 3(b) and S9, which is probably induced by the higher hole mobility of BTPA-7. The lower Voc values of PSC based on BTPA-7 than that of the standard spiro-OMeTAD may be caused by an easier charge recombination process as demonstrated below in the impedance part. We also carried out the BTPA-7 and spiro-OMeTAD based PSC hysteresis investigation. The corresponding J-V curves are shown in Fig. S10. PCEs of 17.35% and 16.30% were achieved for BTPA-7 based PSC under RS and FS conditions with a hysteresis index of 6.4%. Higher PCEs of 18.47% (RS) and 17.00% (FS) were obtained for spiro-OMeTAD but with a larger hysteresis index of 8.6%.
Fig. 4. (a) Nyquist plots of devices based on BTPA-7 and spiro-OMeTAD under 0.95 V bias; (b) differential scanning calorimetry curve for BTPA-7; (c) thermal gravimetric measurement for BTPA-7; (d) long-term stability of the PSCs based on BTPA-7 and spiro-OMeTAD under the humidity of ~40% RH. The electrochemical impedance spectra (EIS) were then measured with a bias of 0.95 V under dark to understand the different photovoltaic performance of BTPA-7 in comparison to a spiro-OMeTAD reference device [42,43]. Fig. 4(a) compares the Nyquist plots of BTPA-7 and spiro-OMeTAD based PSC devices. Only one semicircle is observed which indicates that both of BTPA-7 and spiro-OMeTAD show a main recombination process. Thus a simple equivalent circuit model can be employed including a series resistance (Rs) and a recombination resistance (Rrec) paralleled with a capacitance (Cμ) [44,45]. After fitting the Nyquist plot using the above equivalent circuit model, spiro-OMeTAD based PSC device
exhibits a Rs and a Rrec with values of 19 and 1742 Ω, respectively. The Nyquist plot fitting results of BTPA-7 based PSC device exhibit both a larger Rs (30 Ω) and a lower Rrec (220 Ω), which may arise from the lower conductivity of the BTPA-7 layer. These results corroborate the lower FF and Voc values of BTPA-7 based PSC as shown in the J-V curves compared with the spiro-OMeTAD reference PSC device [46]. Stability is the challenge for the PSC commercialization [47,48]. Various parameters related to PSC stability need to be investigated in detail. The glass transition temperature (Tg) property of BTPA-7 relating to its noncrystalline phase stability was performed as shown in Fig. 4(b). BTPA-7 shows a higher Tg value of 136 ℃ than spiro-OMeTAD (125 ℃). The thermogravimetric analysis (TGA) on BTPA-7 was then conducted to analyze its intrinsic thermal stability. As shown in Fig. 4(c), BTPA-7 presents the decomposed temperature above 300 ℃ in N2 atmosphere indicating its excellent thermal stability. The water contact angle of BTPA-7 on a MAPbI3 film is 85°, higher than that of spiro-OMeTAD (70°) [49]. The hydrophobicity of BTPA-7 would improve the long-term stability of PSCs. The PSC long-term stabilities of BTPA-7 and the standard spiro-OMeTAD were investigated under a continuous humidity stress (~40% RH). After 192 h, the device performance of BTPA-7 maintained 86% of its initial PCE, while the spiro-OMeTAD based PSC maintained only 74% of its initial PCE. These results are most likely owing to the higher hydrophobicity of BTPA-7. 4. Conclusions A novel HTM named BTPA-7 with an anthradithiophene central bridge and four methoxysubstituted triphenylamine units as branches is facilely synthesized via 3 reaction steps. Due to the conjugation of the anthradithiophene unit, a higher hole mobility and a higher Tg are observed for BTPA7 even though it exhibits a lower conductivity compared with the reference spiro-OMeTAD. The best PCE of 17.58% is achieved for a BTPA-7 based FA0.85MA0.15PbI3 PSC. Under the same condition, the standard spiro-OMeTAD exhibits a PCE of 18.88%. By comparison with the spiro-OMeTAD whether in the MAPbI3 or the FA0.85MA0.15PbI3 device, BTPA-7 exhibits a competitive photovoltaic performance with a superior Jsc value resulted from its higher hole mobility. The BTPA-7 based PSC device shows better long-term stability than that for spiro-OMeTAD, which is attributed to the relatively hydrophobic character of BTPA-7. The present results demonstrate a potential anthradithiophene based alternative to spiro-OMeTAD for efficient PSCs with enhanced stability. Declaration of competing interests The authors declare that they have no conflict of interest. Acknowledgments
This work was financially supported by the National Key Research and Development Program of China (2016YFA0202403), the National University Research Fund (GK261001009), the Changjiang Scholar and Innovative Research Team (IRT_14R33), the Overseas Talent Recruitment Project (B14041), the Chinese National 1000-talent plan program (Grant No. 111001034). A.I. and J.O. thank the JSPS Kakenhi grants (No. 26288113 and 15K05486). J.O. also acknowledges the support from the Strategic Research Foundation at Private Universities (Nihon University and the MEXT, Japan). G.W. acknowledges the Natural Science Foundation of Shaanxi Province (2019JQ-423), the Fundamental Research Funds for the Central Universities (GK201903053), and Key Lab of photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences (No. PECL2019KF019). References [1] NREL chart, https://www.nrel.gov/pv/assets/images/efficiency-chart.png. [2] L. Zhang, J. Wu, D. Li, W. Li, Q. Meng, Z. Bo, J. Mater. Chem. A 7 (2019) 14473-14477. [3] J. Wu, C. Liu, B. Li, F. Gu, L. Zhang, M. Hu, X. Deng, Y. Qiao, Y. Mao, W. Tan, Y. Tian, B. Xu, ACS Appl. Mater. Inter. 11 (2019) 26928-26937. [4] Y. Wang, W. Chen, L. Wang, B. Tu, T. Chen, B. Liu, K. Yang, C.W. Koh, X. Zhang, H. Sun, G. Chen, X. Feng, H.Y. Woo, A.B. Djurišić, Z. He, X. Guo, Adv. Mater. 31 (2019) 1902781. [5] Y. Hou, Z.R. Zhou, T.Y. Wen, H.W. Qiao, Z.Q. Lin, B. Ge, H.G. Yang, Nanoscale Horiz. 4 (2019) 208-213. [6] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050-6051. [7] H. Li, G. Wu, W. Li, Y. Zhang, Z. Liu, D. Wang, S. Liu, Adv. Sci. 6 (2019) 1901241. [8] X. Lai, F. Meng, Q.-Q. Zhang, K. Wang, G. Li, Y. Wen, H. Ma, W. Li, X. Li, A.K.K. Kyaw, X.W. Sun, M. Du, X. Guo, J. Wang, W. Huang, Sol. RRL 3 (2019) 1900011. [9] S. Liu, S. Li, J. Wu, Q. Wang, Y. Ming, D. Zhang, Y. Sheng, Y. Hu, Y. Rong, A. Mei, H. Han, J. Phys. Chem. Lett. 10 (2019) 6865-6872. [10] F. Liu, F. Wu, W. Ling, Z. Tu, J. Zhang, Z. Wei, L. Zhu, Q. Li, Z. Li, ACS Energy Lett. 4 (2019) 2514-2521. [11] J. Urieta-Mora, I. Garcia-Benito, A. Molina-Ontoria, N. Martin, Chem. Soc. Rev. 47 (2018) 8541-8571. [12] Q. Zeng, Y. Di, C. Huang, K. Sun, Y. Zhao, H. Xie, D. Niu, L. Jiang, X. Hao, Y. Lai, F. Liu, J. Mater. Chem. C 6 (2018) 7989-7993. [13] X. Jin, Y. Yuan, C. Jiang, H. Ju, G. Jiang, W. Liu, C. Zhu, T. Chen, Sol. Energy Mater. Sol. Cells 185 (2018) 542-548. [14] J. Chen, N.-G. Park, J. Phys. Chem. C 122 (2018) 14039-14063. [15] X. Liu, Y. Wang, E. Rezaee, Q. Chen, Y. Feng, X. Sun, L. Dong, Q. Hu, C. Li, Z.X. Xu, Sol. RRL 2 (2018) 1800050. [16] S. Yang, C. Yang, X. Zhang, Z. Zheng, S. Bi, Y. Zhang, H. Zhou, J. Mater. Chem. C 6 (2018) 9044-9048. [17] G.W. Kim, J. Lee, G. Kang, T. Kim, T. Park, Adv. Energy Mater. 8 (2018) 1701935.
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Graphical abstract
A novel anthradithiophene based hole-transport material named BTPA-7 is designed. Compared with spiro-OMeTAD, BTPA-7 based PSC device exhibits comparable photovolatic performance with a dramatically higher Jsc due to its higher hole mobility.