Journal of Energy Chemistry 44 (2020) 115–120
Contents lists available at ScienceDirect
Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Benzothiadiazole-based hole transport materials for high-efficiency dopant-free perovskite solar cells: Molecular planarity effect Xiang Zhou a,b, Fantai Kong a,∗, Yuan Sun a, Yin Huang a,b, Xianxi Zhang c, Rahim Ghadari d a
Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230088, Anhui, China b University of Science and Technology of China, Hefei 230026, Anhui, China c Shandong Provincial Collaborative Innovation Center of Chemical Energy Storage & Novel Cell Technology, Liaocheng University, Liaocheng 252000, Shandong, China d Computational Chemistry Laboratory, Department of Organic and Biochemistry, Faculty of Chemistry, University of Tabriz, 5166616471 Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 25 July 2019 Revised 29 August 2019 Accepted 16 September 2019 Available online 2 October 2019 Keywords: Hole transport materials Planarity Perovskite Solar cells
a b s t r a c t A new benzothiadiazole-based D-A-D hole transport material (DTBT) has been designed and synthesized with a more planar structure by introducing of thiophene bridges. The results indicate a lower band gap and quite higher hole mobility for the DTBT. Furthermore, the enhancement in molecular planarity with simple thiophene unit increases the hole mobility of DTBT (8.77 × 10−4 cm2 V−1 s−1 ) by about 40%. And when DTBT is used as hole transport material in perovskite solar cells, the photoelectric conversion efficiency of the corresponding dopant-free devices is also significantly improved compared with that of the conventional BT model molecule without thiophene. In terms of device stability, DTBT-based devices show a favorable long-term stability, which keep 83% initial efficiency after 15 days. Therefore, the introducing of thiophene bridges in D-A-D typed HTMs can improve the molecular planarity effectively, thereby increasing the hole mobility and improving device performance. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
1. Introduction Over the past decade, perovskite solar cells (PSCs) have obtained worldwide attention for their rapidly growing performance, which has been increased from 3% of its initial efficiency to 23.7% [1–4]. Moreover, hole transport materials (HTMs) play a key role in extracting and transmitting holes from the perovskite materials to the counter electrode [5], and 2,2 ,7,7 -Tetrakis-(N,Ndi-4-methoxyphenylamino)−9,9 -spirobifluorene (spiro-OMeTAD) is the most popular choice of HTMs for its high efficiency [6,7]. However, the complicated synthesis process, high cost, relatively low hole mobility limit its commercial application. To improve the hole mobility, it must be used with additives such as lithium-bis (trifluoromethanesulfonyl)-imide (Li-TFSI), tert-butylpyridine (tBP). Unfortunately, these additives may result in serious damage to device stability with drawing water vapor into the perovskite layer [8–10]. Therefore, a dopant-free substitute of spiroOMeTAD is a key issue for the commercialization of perovskite solar cells.
∗
Corresponding author. E-mail address:
[email protected] (F. Kong).
Till now, D-A-D typed small organic molecule HTMs have been applied in PSCs [11–14] for their simple structure, easy to synthesize and almost no difference between batches compared to polymers [15]. Although the D-A-D structure make HTMs with a lower HOMO energy level and higher hole mobility [16–20], there is evidence that the large dihedral angle between the arm group and the core group will obstruct the π -π stacking, thereby affect the charge transfer of the material [21,22]. Most recently, the researchers became to realize the importance of the planarity of hole transport molecule (as summarized in Table S1) [23–28]. In these studies about molecular planarity of hole transport materials, the researchers changed the planarity of the molecules by replacing different central and arm group. Therefore, it is not clear whether the improvement of device performance is due to the influence of molecular planarity or the introduction of different central or arm groups. In our D-A-D system, electron-withdrawing group plays a key role in the hole transport property of the material. Hence, we introduce two thiophene units into the D-A-D molecular backbone to investigate the effect of the molecular planarity for hole transport material on the device performance in Perovskite solar cells in this work. DFT calculation proves that DTBT (as shown in Fig. 1) with a smaller dihedral angle between the triphenylamine arms and core acceptor. And hole mobility test
https://doi.org/10.1016/j.jechem.2019.09.020 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
116
X. Zhou, F. Kong and Y. Sun et al. / Journal of Energy Chemistry 44 (2020) 115–120
Fig. 1. Molecular structures of BT and DTBT.
shows that DTBT owns a higher hole mobility, 8.77 × 10−4 cm2 V−1 s−1 , which contributed to the planar structure and strength π -π stacking. Moreover, PSCs with dopant-free DTBT as HTM get a higher photoelectric conversion efficiency (PCE) of 13.5%, and the obtained PSCs efficiency is among the highest as compared to that of other D-A-D typed HTMs-based dopant-free devices [17,19,29,30]. 2. Experimental
hydroiodide (1.15 M) dissolved in DMF/DMSO (1/4 v/v)). It is a twostep program with 1100 and 4600 rpm for 10 and 30 s, respectively. At the last 15 s of the second step, 100 mL chlorobenzene was spin coated onto the substrate and the substrate was transferred to a 100 °C hotplate for 1 h. After cooled to room temperature, the solution of pristine BT and DTBT in chlorobenzene (25 mg mL−1 for BT and 20 mg mL−1 for DTBT) was spin coated onto the substrate at 30 0 0 rpm for 20 s. Finally, about 60 nm of gold was deposited onto the substrate by thermally evaporated to form a back electrode.
2.1. Synthesis
3. Results and discussion
All solvents and chemicals were purchased and without further purification. Two materials were prepared via simple Suzuki cross-coupling reaction. The detailed experimental methods can be found in the Supplementary Information.
To investigate the difference between the photophysical properties of the two materials, the UV–Vis absorption and fluorescence emission spectra of BT and DTBT were examined in dichloromethane solution and the results are shown in Fig. 2(a), the related data are listed in Table 1. Both materials show two absorption peaks in the UV–visible region and the peaks at the larger wavelength position are related to charge transfer from the electron rich donor groups to the electron deficient acceptor group [31]. Compared to the absorption peak of BT (485 nm), DTBT (548 nm) is 63 nm bathochromic shift which demonstrates that it owns a higher intra-molecular charge transport ability [27]. As two materials with the same electron rich donor groups (triphenylamine) and electron deficient acceptor group (benzothiadiazole), the difference can be attributed to the introduction of the two thiophene bridges, which lead to changes in molecular structure and intra-molecular charge transport (ICT). Furthermore, the large stokes shifts (180 nm for BT and 165 nm for DTBT) prove that the two materials undergo large structural changes in the excited state, which is conducive to the pore-filling of HTMs [32]. We also found
2.2. Device fabrication The FTO glass substrates were etched by hydrochloric acid and zinc powder. Then the substrates were cleaned by ultrasonic bath for 30 min, rinsed with deionized water and ethanol, and annealed at 510 °C for 30 min. A TiO2 dense blocking layer was deposited on the cleaned FTO substrate by spray pyrolysis method from a solution of bis (acetylacetonate) and titanium diisopropoxide in isopropanol at 450 °C. After that, the mesoporous TiO2 layer was deposited by spin coating a diluted TiO2 slurry (dilute Deysol 30NR-T in absolute ethanol at a 1:5.5 mass ratio), spin coating set to 40 0 0 rpm for 20 s. Then the substrate was sintering at 510 °C for 30 min. The perovskite light absorbing layer was obtained by spin coating a precursor solution (PbI2 (1.2 M) and methylamine
Fig. 2. (a) Absorption and emission spectra of BT and DTBT in dichloromethane solution, (b) cyclic voltammogram.
X. Zhou, F. Kong and Y. Sun et al. / Journal of Energy Chemistry 44 (2020) 115–120
117
Table 1. Photophysical and electrochemical properties of BT and DTBT. HTM BT DTBT a b c d e
λmax a (nm) 485 548
λPL a (nm) 665 712
Eg b (eV) 2.23 1.96
HOMOc (eV) −5.26 −5.18
LUMOd (eV) −3.03 −3.22
μ (cm2 V−1 s−1 ) −4
6.27 × 10 8.77 × 10−4
HOMOe (eV)
LUMOe (eV)
−4.92 −4.83
−2.42 −2.85
Absorption and emission spectra were measured in dichloromethane solution. Optical band gap is calculated from the intersection of absorption and emission spectra. HOMO level is obtained from CV with the calibrated of ferrocene, and calculated as E1/2 ox vs. Fc/Fc+ + 0.67 vs. NHE + 4.44 vs. Vacuum. ELUMO = Eg +EHOMO. Calculated from the DFT theory.
Fig. 3. (a) Energy level diagram of the BT and DTBT from CV. (b) HOMO and LUMO orbitals calculated from DFT theory. Dihedral angle of optimized BT (c) and DTBT (d) molecular structure.
that the absorption peak of thin films of BT (493 nm) and DTBT (560 nm) show a bathochromic shift compared to the solution as depict in Fig. S2, it demonstrates the strong intermolecular π -π stacking of benzothiadiazole-based HTMs in the thin film state, which expecting a higher hole mobility [24,33]. The optical band gap (Eg ) of the material can reflect the interaction between the electron donating group and the electron withdrawing group on the molecular structure, in general, the smaller the band gap, the stronger the interaction [34]. And for BT and DTBT, their Eg values are 2.23 eV and 1.96 eV, respectively, which indicates that the insertion of thiophene units in DTBT can enhance the intra-molecular interaction. The electrochemical properties of the two materials are shown in Fig. 2(b), the derived HOMO energies (Fig. 3(a)) of BT and DTBT are −5.26 eV and −5.18 eV, and the corresponding LUMO energies are −3.03 eV and −3.22 eV, which are both much higher than the conduction band of perovskite material, and can effective
block the undesired charge transfer from the perovskite layer to the Au electrode [35,36]. The energy level trend obtained from the experiments is also consistent with the theoretical calculation. We performed density functional theory (DFT) calculations on BT and DTBT based on the Gaussian 16 program package. As shown in Fig. 3(b), the electron distributions in the HOMO levels of the optimized ground state geometry of BT and DTBT are throughout the whole molecular structure, while LUMO levels predominantly concentrate on the core acceptor. This kind of electron distribution is favorable to facilitate the formation of excitons and holes migration [37,38]. In addition, the LUMO level of DTBT has extend to the thiophene units and that means the introducing of thiophene bridges change the energy distribution of the material. To further investigate the effect of the insertion of thiophene bridges on molecular structure, we also calculate the dihedral angle between the triphenylamine
118
X. Zhou, F. Kong and Y. Sun et al. / Journal of Energy Chemistry 44 (2020) 115–120
Fig. 4. (a) Square root of current density-Voltage curves from SCLC measurements. (b) The steady-state PL spectra of perovskite and with the HTMs.
arms and the core acceptor. As shown in Fig. 3(c,d), the dihedral angle is −35.6° for BT molecule, and the values between triphenylamine and thiophene, thiophene and benzothiadiazole in the DTBT molecule are 13.5° and −26° after thiophene units were introduced, respectively. The results reveal that the DTBT molecule is more planar than that of BT by the connection of thiophene units. In order to clarify the effect of molecular planarity on hole mobility which is a key properties for estimate a new HTM in the application of the PSCs, we developed the hole only devices based BT and DTBT (FTO/PEDOT:PSS/HTMs/Au), and test the J–V characteristics of the devices in the SCLC region according to previous report [39]. The corresponding J–V curves are depicted on Fig. 4(a). The test hole mobility values of BT and DTBT are 6.27 × 10−4 cm2 V−1 s−1 and 8.77 × 10−4 cm2 V−1 s−1 by taking advantage of Mott– Gurney law [40]. It is apparent that DTBT exhibits a higher hole mobility with a more planar structure. The underlying reason is that the more planar molecular structure enhances the π -π stacking and facilitates charge transfer. It suggests that the cooperation of thiophene bridges in D-A-D typed HTMs can planarize the molecular structure and thus improve the hole mobility. To verify the more efficient hole extraction and transmission ability of DTBT compared to BT, the steady-state photoluminescence (PL) was tested on the perovskite/HTM interface. As shown in Fig. 4(b), compared to the pristine perovskite film, the perovskite/BT and perovskite/DTBT interface exhibit a 77% and 70% quenching efficiency, respectively. As previous studies have reported that quenching efficiency can represent the hole extraction and transmission ability of HTMs to a certain extent [41], it is obvious that DTBT have a stronger hole extraction and transmission ability. And this increment can be related to the existence of thiophene bridges, which make the molecular structure more planar. Moreover, the blue shift of steady-state PL spectra of perovskite/BT (766 nm) and perovskite/DTBT (754 nm) compared to pristine perovskite film (770 nm) means that the traps inside the perovskite films were filled by HTMs [42,43]. The most straightforward way to evaluate a new HTM is to apply it to devices, so we developed the PSCs employing the MAPbI3 as the light absorber with dopant-free BT and DTBT as the HTMs. The current density-voltage (J–V) curves are exhibited in Fig. 5(a), and the detail key parameters are summarized in the Table 2. Compared to the BT-based PSCs (a PCE of 9.89% with a Jsc of 19.6 mA
Table 2. Photovoltaic performance of PSCs with pristine BT, DTBT and spiroOMeTAD. HTM
Jsc (mA cm−2 )
Voc (V)
FF (%)
PCE (%)
BT DTBT Spiro-OMeTAD
19.6 21.5 20.8
0.926 0.906 0.893
54.5 69.3 42.1
9.89 13.50 7.80
cm−2 , a Voc of 0.92 V and a fill factor (FF) of 0.54), DTBT shows a superior device performance with a PCE of 13.5% (a Jsc of 20.8 mA cm−2 , a Voc of 0.90 V and a FF of 0.69), which is the highest efficiency as compared to that of other D-A-D typed HTMs based dopant-free devices. The reference devices using pristine spiroOMeTAD as the HTM under the similar conditions display a PCE of 7.80% with a Jsc of 20.8 mA cm−2 , a Voc of 0.89 V and a FF of 0.42, respectively. The enhanced Jsc and FF of DTBT-based PSCs can be attributed to the higher hole mobility of DTBT [44]. While the lower Voc can be explained that the HOMO level of DTBT is a little higher than that of BT [45]. These results indicate that the incorporation of thiophene bridges into D-A-D-typed HTMs which make the molecular structure more planar will improve the device performance effectively. In order to further study the charge recombination in the PSCs devices, we test the electrochemical impedance spectroscopy (EIS) at 0.9 V in a dark condition. Fig. 6(a) displays two semicircular in the middle frequency region, which is related to the recombination resistance (Rrec ) in the PSCs [46]. In general, the bigger Rrec of the device means the lower charge recombination, and in this EIS spectrum we clearly see that DTBT-based PSCs show a higher Rrec than BT-based devices, which thanks to the higher hole mobility of DTBT. The lower carrier recombination rate leads to a higher performance in the devices that with DTBT as HTM. Series resistances (RS ) of the devices with BT (123 ) and DTBT (73 ) as HTM were obtained from the corresponding slop of J–V curves directly [47]. It shows an obvious reduction of RS with the cooperation of thiophene units in DTBT. While the lower RS has a good compliance with the higher FF of DTBT-based devices [48]. As the devices are totally same except for HTMs, this difference in RS is mainly ascribed to the influence of HTMs, and DTBT can be inferred have a lower resistance than BT, which also give a strong evidence to the better performance DTBT-based PSCs.
X. Zhou, F. Kong and Y. Sun et al. / Journal of Energy Chemistry 44 (2020) 115–120
119
Fig. 5. (a) J–V curves of the based-BT, DTBT and spiro-OMeTAD dopant-free PSCs. (b) Corresponding IPCE spectrum of the PSC devices.
Fig. 6. (a) Nyquist spectra of the PSCs with BT and DTBT under dark condition. (b) Long-term stability of the PSCs based on the BT and DTBT.
To study the effect of different HTMs on the long-term stability of the devices, the corresponding PSCs are unsealed and placed in air, with a relative humidity of 30% and a temperature of 20 °C. After 15 days, we found that the devices with BT, DTBT and spire-OMeTAD as HTMs maintained 76%, 83% and 72% of initial efficiency (Fig. 6(b)). Though all three typed devices show a certain decrease, DTBT-based devices demonstrate a best long-term stability, which indicate that the stability of PSCs is closely related to HTMs. In order to better understand the enhanced stability of PSCs with DTBT as hole transport material, we tested the contact angle of water on the different HTMs and the results are shown in Fig. S3. The water contact angle of DTBT (91.7°) is higher than that of BT (81.6°) and Spiro-OMeTAD (67.5°), It demonstrates that DTBT with a superior hydrophobic properties, which improves the stability of the corresponding devices. In addition, thermal stability
is a key factor for the durability and stability of HTM, so we test the thermal properties of new compounds by differential scanning calorimetry (DSC), as shown in Fig. S4. DSC data show that the melting point of BT and DTBT are 119 and 223 °C, respectively. Obviously, the increasing melting point of DTBT can improve the thermochemical stability of the compounds, which is good for the application in the PSC devices. 4. Conclusions In conclusion, we have successfully improved the planarity of molecule by introducing thiophene bridges into the D-A-D molecular backbone. And it was found that the insertion of thiophene bridges can effectively reduce the dihedral angle between triphenylamine arms and the core acceptor which makes the molecule
120
X. Zhou, F. Kong and Y. Sun et al. / Journal of Energy Chemistry 44 (2020) 115–120
more planar, and the hole mobility thus increases from 6.27 × 10−4 to 8.77 × 10−4 cm2 V − 1 s − 1 , up nearly 40%. The corresponding dopant-free DTBT-based PSCs get a PCE of 13.5% which is the highest efficiency among the reported dopant-free D-A-D typed HTMs. Therefore, the incorporation of thiophene bridges between the arms and core group, especially for a D-A-D molecule that with a larger core group, may be an effective method to make the molecular structure more planar and improve the hole mobility of HTMs. Declaration of Competing Interest There are no conflicts to declare. Acknowledgments This work was financially supported by the National Key R&D Program of China (2018YFB1500101), National Basic Research Program of China (No. 2015CB932200), CAS-Iranian Vice Presidency for Science and Technology Joint Research Project (No. 116134KYSB20160130). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.09.020. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] H.S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, S.J. Moon, R. Humphry-Baker, J.H. Yum, J.E. Moser, M. Grätzel, N.G. Park, Sci. Rep. 2 (2012) 591. [3] M. Saliba, T. Matsui, J.Y. Seo, K. Domanski, J.P. Correa-Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Energy Environ. Sci. 9 (2016) 1989–1997. [4] NREL Best Research-Cell Efficiencies; Available from: https://www.nrel.gov/pv/ assets/pdfs/pv- efficiency- chart.20190103.pdf (accessed Jan 2019). [5] J.P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T.J. Jacobsson, M. Grätzel, A. Hagfeldt, Energy Environ. Sci. 10 (2017) 710–727. [6] M. Saliba, T. Matsui, K. Domanski, J.Y. Seo, A. Ummadisingu, S.M. Zakeeruddin, J.P. Correa-Baena, W.R. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Science 354 (2016) 206–209. [7] K.T. Cho, S. Paek, G. Grancini, C. Roldan-Carmona, P. Gao, Y.H. Lee, M.K. Nazeeruddin, Energy Environ. Sci. 10 (2017) 621–627. [8] T. Leijtens, T. Giovenzana, S.N. Habisreutinger, J.S. Tinkham, N.K. Noel, B.A. Kamino, G. Sadoughi, A. Sellinger, H.J. Snaith, ACS Appl. Mater. Interfaces 8 (2016) 5981–5989. [9] B. Xu, Z.L. Zhu, J.B. Zhang, H.B. Liu, C.C. Chueh, X.S. Li, A.K.Y. Jen, Adv. Energy Mater. 7 (2017) 1700683. [10] P. Liu, B. Xu, Y. Hua, M. Cheng, K. Aitola, K. Sveinbjornsson, J. Zhang, G. Boschloo, L. Sun, L. Kloo, J. Power Sources 344 (2017) 11–14. [11] H.R. Li, K.W. Fu, P.P. Boix, L.H. Wong, A. Hagfeldt, M. Grätzel, S.G. Mhaisalkar, A.C. Grimsdale, ChemSusChem 7 (2014) 3420–3425. [12] P. Gratia, A. Magomedov, T. Malinauskas, M. Daskeviciene, A. Abate, S. Ahmad, M. Grätzel, V. Getautis, M.K. Nazeeruddin, Angew. Chem. Int. Ed. 54 (2015) 11409–11413. [13] I. Petrikyte, I. Zimmermann, K. Rakstys, M. Daskeviciene, T. Malinauskas, V. Jankauskas, V. Getautis, M.K. Nazeeruddin, Nanoscale 8 (2016) 8530–8535. [14] W.Q. Zhou, Z.H. Wen, P. Gao, Adv. Energy Mater. 8 (2018) 1702512.
[15] L. Calio, S. Kazim, M. Grätzel, S. Ahmad, Angew. Chem. Int. Ed. 55 (2016) 14522–14545. [16] P. Xu, P. Liu, Y.Y. Li, B. Xu, L. Kloo, L.C. Sun, Y. Hua, ACS Appl. Mater. Interfaces 10 (2018) 19697–19703. [17] G.H. Wu, Y.H. Zhang, R. Kaneko, Y. Kojima, Q. Shen, A. Islam, K. Sugawa, J. Otsuki, J. Phys. Chem. C 121 (2017) 17617–17624. [18] F. Wu, Y. Ji, C. Zhong, Y. Liu, L.X. Tan, L.N. Zhu, Chem. Commun. 53 (2017) 8719–8722. [19] H.D. Pham, K. Hayasake, J. Kim, T.T. Do, H. Matsui, S. Manzhos, K. Feron, S. Tokito, T. Watson, W.C. Tsoi, N. Motta, J.R. Durrant, S.M. Jain, P. Sonar, J. Mater. Chem. C 6 (2018) 3699–3708. [20] H. Zhang, Y.Z. Wu, W.W. Zhang, E.P. Li, C. Shen, H.Y. Jiang, H. Tian, W.H. Zhu, Chem. Sci. 9 (2018) 5919–5928. [21] S.H. Kang, M.J. Jeong, Y.K. Eom, I.T. Choi, S.M. Kwon, Y. Yoo, J. Kim, J. Kwon, J.H. Park, H.K. Kim, Adv. Energy Mater. 7 (2017) 1602117. [22] K. Rakstys, M. Saliba, P. Gao, P. Gratia, E. Kamarauskas, S. Paek, V. Jankauskas, M.K. Nazeeruddin, Angew. Chem. Int. Ed. 55 (2016) 7464–7468. [23] X.P. Zong, W.H. Qiao, Y. Chen, H. Wang, X. Liu, Z. Sun, S. Xue, ChemistrySelect 2 (2017) 4392–4397. [24] S.H. Kang, C.Y. Lu, H.R. Zhou, S. Choi, J. Kim, H.K. Kim, Dyes Pigment. 163 (2019) 734–739. [25] S. Lee, S. Kwak, K. Lee, B.G. Kim, M. Kim, D.H. Wang, W.S. Han, Acta Crystallogr. Sect. C Struct. Chem. 75 (2019) 919–926. [26] F. Wu, Y.H. Shan, J.H. Qiao, C. Zhong, R. Wang, Q.L. Song, L.N. Zhu, ChemSusChem 10 (2017) 3833–3838. [27] S. Mabrouk, M.M. Zhang, Z.H. Wang, M. Liang, B. Bahrami, Y.G. Wu, J.H. Wu, Q.Q. Qiao, S.F. Yang, J. Mater. Chem. A 6 (2018) 7950–7958. [28] J.Y. Cui, W. Rao, W.X. Hu, Z.M. Zhang, W. Shen, M. Li, R.X. He, J. Mater. Sci. 53 (2018) 6626–6636. [29] S. Carli, J.P.C. Baena, G. Marianetti, N. Marchetti, M. Lessi, A. Abate, S. Caramori, M. Grätzel, F. Bellina, C.A. Bignozzi, A. Hagfeldt, ChemSusChem 9 (2016) 657–661. [30] X.X. Ye, X.J. Zhao, Q.Y. Li, Y.F. Ma, W.W. Song, Y.Y. Quan, Z.C. Wang, M.K. Wang, Z.S. Huang, Dyes Pigment. 164 (2019) 407–416. [31] S. Paek, M.A. Rub, H. Choi, S.A. Kosa, K.A. Alamry, J.W. Cho, P. Gao, J. Ko, A.M. Asiri, M.K. Nazeeruddin, Nanoscale 8 (2016) 6335–6340. [32] H. Li, K. Fu, A. Hagfeldt, M. Grätzel, S.G. Mhaisalkar, A.C. Grimsdale, Angew. Chem. Int. Ed. Engl. 53 (2014) 4085–4088. [33] I. Cho, N.J. Jeon, O.K. Kwon, D.W. Kim, E.H. Jung, J.H. Noh, J. Seo, S.I. Seok, S.Y. Park, Chem. Sci. 8 (2017) 734–741. [34] J.B. Giguere, Q. Verolet, J.F. Morin, Chem. Eur. J. 19 (2013) 372–381. [35] Z.H. Bakr, Q. Wali, A. Fakharuddin, L. Schmidt-Mende, T.M. Brown, R. Jose, Nano Energy 34 (2017) 271–305. [36] T.S. Qin, W.C. Huang, J.E. Kim, D.J. Vak, C. Forsyth, C.R. McNeill, Y.B. Cheng, Nano Energy 31 (2017) 210–217. [37] A. Krishna, D. Sabba, H.R. Li, J. Yin, P.P. Boix, C. Soci, S.G. Mhaisalkar, A.C. Grimsdale, Chem. Sci. 5 (2014) 2702–2709. [38] Z.S. Huang, H.L. Feng, X.F. Zang, Z. Iqbal, H.P. Zeng, D.B. Kuang, L.Y. Wang, H. Meier, D.R. Cao, J. Mater. Chem. A 2 (2014) 15365–15376. [39] Y. Wang, Z.L. Zhu, C.C. Chueh, A.K.Y. Jen, Y. Chi, Adv. Energy Mater. 7 (2017) 1700823. [40] H.J. Snaith, M. Grätzel, Appl. Phys. Lett. 89 (2006) 262114. [41] X.P. Liu, X.Q. Shi, C. Liu, Y.K. Ren, Y.Z. Wu, W. Yang, A. Alsaedi, T. Hayat, F.T. Kong, X.L. Liu, Y. Ding, J.X. Yao, S.Y. Dai, J. Phys. Chem. C 122 (2018) 26337–26343. [42] Xiaojuan Zhao, Yunyun Quan, Han Pan, Qingyun Li, Yan Shen, Zu-Sheng Huang, Mingkui Wang, J. Energy Chem. 32 (2019) 85–92. [43] C. Sun, Z.H. Wu, H.L. Yip, H. Zhang, X.F. Jiang, Q.F. Xue, Z.C. Hu, Z.H. Hu, Y. Shen, M.K. Wang, F. Huang, Y. Cao, Adv. Energy Mater. 6 (2016) 1501534. [44] H. Nishimura, N. Ishida, A. Shimazaki, A. Wakamiya, A. Saeki, L.T. Scott, Y. Murata, J. Am. Chem. Soc. 137 (2015) 15656–15659. [45] R. Azmi, S.Y. Nam, S. Sinaga, Z.A. Akbar, C.L. Lee, S.C. Yoon, I.H. Jung, S.Y. Jang, Nano Energy 44 (2018) 191–198. [46] A. Dualeh, T. Moehl, N. Tetreault, J. Teuscher, P. Gao, M.K. Nazeeruddin, M. Grätzel, ACS Nano 8 (2014) 4053–4053. [47] S.T. Lv, Y.K. Song, J.Y. Xiao, L.F. Zhu, J.J. Shi, H.Y. Wei, Y.Z. Xu, J. Dong, X. Xu, S.R. Wang, Y. Xiao, Y.H. Luo, D.M. Li, X.G. Li, Q.B. Meng, Electrochim. Acta 182 (2015) 733–741. [48] M. Nazim, S. Ameen, M.S. Akhtar, M.K. Nazeeruddin, H.S. Shin, Sol. Energy Mater. Sol. Cells 180 (2018) 334–342.