Effect of double bond conjugation on hole mobility of thiophene-based hole transport materials in perovskite solar cells

Materials Chemistry and Physics 240 (2020) 122058

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Effect of double bond conjugation on hole mobility of thiophene-based hole transport materials in perovskite solar cells Jiaren Guo, Yan Zhang, Wanlin Cai, Zemin Zhang, Rongxing He, Wei Shen, Ming Li * Key Laboratory of Luminescence and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Design three hole transporting materials by inserting double bonds. � Influence of introducing double bonds on the hole transport properties. � Suitable π-bridge size is vital to improve the hole mobility. � Planarity has a greater effect on the effective face to face stacking.

A R T I C L E I N F O

A B S T R A C T

Keywords: Perovskite solar cells Hole transport materials Density functional theory Intermolecular packing π-bridge size

It is of great significance to fully explore the relationship between π-bridge size and intermolecular packing for improving the hole mobility of hole transport materials in perovskite solar cells. We performed the density functional theory (DFT) computations of a series of Z26 derivatives (Z26-2, Z26-3, and Z26-4) and investigated the effect of the size of π-bridge and intermolecular packing on hole mobility of hole transport materials. The calculated results show that Z26-2 (7.7 � 10 4 cm2 V 1 s 1) and Z26-3 (1.3 � 10 3 cm2 V 1 s 1) have larger hole mobility than Z26 (5.60 � 10 4 cm2 V 1 s 1) due to their appropriate conjugated length leading to effective face to face packing. The smallest hole mobility of Z26-4 (4.20 � 10 5 cm2 V 1 s 1) is attributed to its overlong conjugation with four double bonds on each side, which produces a long centroid-to-centroid distance and small electronic coupling. The present theoretical study on the relationship between the size of π-bridge and inter­ molecular packing provides insight for the future design of thiophene-based hole transport materials.

1. Introduction Increasing attention has been paid to perovskite solar cells (PSCs) for their record-breaking high-performance [1–4]. Rapid progress has been made with a certified power conversion efficiency (PCE) over 23% [5]. A typical PSC device is composed of a hole transport layer (HTL), a

perovskite active layer, and an electron transport layer (ETL) [6]. Hole transport materials (HTMs) are indispensable for to competitive PCE, and play an important role in promoting hole transfer, inhibiting the recombination of holes and electrons, stabilizing the perovskite layer and reducing the cost of device manufacturing [7]. Organic small molecule HTMs have the advantages of handy

* Corresponding author. E-mail address: [email protected] (M. Li). https://doi.org/10.1016/j.matchemphys.2019.122058 Received 24 June 2019; Received in revised form 25 July 2019; Accepted 23 August 2019 Available online 27 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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Materials Chemistry and Physics 240 (2020) 122058

Fig. 1. Chemical structures of the studied molecules.

purification and abundant raw materials [8]. At present, the HTM widely used in the PSCs is 2,20 ,7,70 -tetrakis(N, 0 0 N -di-p-methoxyphenylamine)-9,9 -spirobifluorene (spiro-OMeTAD) [9]. However, its commercial application is greatly limited due to the complex synthesis process, high production costs and poor stability [10]. Therefore, it is very urgent to find a non-spiro type HTM which is highly efficient, low cost, and has good stability for PSCs. In recent years, non-spiro type HTMs have been applied in PSCs, such as carbazole derivatives [11], thiophene derivatives [12], pyrene de­ rivatives [13], and triphenylamines derivatives [14]. Two hole transport materials (HTMs) containing biphenyl core (HL-1) and carbazole core (HL-2), respectively, have been designed and synthesized [15]. A higher PCE for the HL-2-based PSCs (up to 18.34%) has been obtained compared with that of HL-1-based PSCs (up to 16.14%). Nam et al. re­ ported three pyrene-based HTMs, namely, Py-A, Py-B, and Py-C [16]. Comparable to PCE of spiro-OMeTAD (12.7%), the PSCs based on Py-C achieve a PCE of 12.3%. Zhang et al. synthesized two triphenylamines-based HTMs (Z1012 and Z1013) and the Z1013-based PSCs without any dopant achieve a PCE of 15.4% [17]. These non-spiro type HTMs show efficiency in the range of 12–18%, despite their low hole mobilities. Thiophene derivatives have been widely used as HTMs due to their outstanding photoelectronic properties and high hole mobility. Recently, a thiophene-cored molecule Z26 with an impressive PCE of 20.1% was synthesized facilely by introducing double bonds [18]. Z26 was formed by adding only two double bonds into Z25, which leads to an increase of PCE from 16.9% to 20.1%. How does the increase in the number of double bonds affect hole mobility and further improve PCE? Herein, we designed a series of HTMs molecules by introducing different numbers of double bonds on both sides of the Z25 molecule; Z26-2, for example, was produced by inserting two double bonds between the molecular core and phenyl. Similarly, Z26-3 and Z26-4 were formed by introducing three and four double bonds in the same position, respec­ tively (see Fig. 1). In this work, we studied the effect of increasing the size of π-bridge on geometric structures, frontier molecular orbitals, and hole mobility of HTMs using density functional theory (DFT) and Marcus theory. The results demonstrate that introducing double bonds to the molecule can achieve better planarity. Theoretical analysis demon­ strates that Z26-3 has higher mobility and better stability compared with Z26, where the hole mobility of Z26-3 is up to 1.3 � 10 3 cm2 V 1 s 1. This work provides significant insight into improving the hole mobility of HTMs.

also agrees well with the experimental value ( 5.18 eV). Therefore, all individual HTM molecules were optimized employing this theoretical level. All the optimized geometric structures of molecules had no virtual frequency. In this work, we considered the solvent effect; the dichloro­ methane solvent was employed during the calculations with the conductor-like polarizable continuum model (C-PCM) [19]. The above calculations were implemented by using the Gaussian 09 program [20]. The Polymorph module of the Materials Studio package was used to predict the molecular crystal structure [21]. The ten most common space groups (P21/c, P212121, P1, P21, C2/c, C2, Pna21, Pbca, Cc, and Pbcn) were confined and their total energies were obtained [22]. Finally, the crystal structure with the lowest energy was selected for further calculation. In this work, the charge transfer rate (k) between the neighboring molecules was described as [23]: � � 4π 2 2 1 λ k¼ V pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp : (1) 4kB T h 4πλkB T where kB, T, and h denote the Boltzmann constant, Kelvin temperature, and the Planck constant, respectively. λ denotes the inner reorganization energy which was calculated as [24,25]: � � (2) λ ¼ E*0 E0 þ E*þ Eþ where E0 and E0* represent the energy of the optimized neutral geom­ etry at neutral and charged state, respectively. Eþ and Eþ* represent the energy of the optimized positive-charged geometry at neutral and charged state, respectively [26]. V is the electronic coupling [27]: V¼

JRP

SRP ðHRR þ HPP Þ=2 1 S2RP

(3)

where HPP and HRR are site energies, JRP and SRP denote the charge transfer integral and the spatial overlap, respectively. The hole mobility can be calculated by the Einstein relation [28]:

μ¼

1 e X 2 r ki Pi 2n kB T i i

(4)

where e represents the elementary charge, i represents the chosen transfer pathway, n denotes the spatial dimensionality, and ri denotes the hopping centroid-to-centroid distance. Pi is the relative probability of the i path. Therefore, the angular resolution anisotropic mobility, μΦ, was calculated using the orientation function [29]: e X 2 μΦ ¼ ki ri Pi cos2 γi cos2 ðθi ΦÞ (5) 2kB T i

2. Computational details In order to obtain reliable calculated results, four functionals, including B3LYP, PBE0, M06-2X, and BMK, combined with 6-31G (d,p) were employed to optimize the ground state of Z26. It was found that the highest occupied molecular orbital (HOMO) energy level (EHOMO) of Z26 calculated at the BMK/6-31G (d,p) level ( 5.08 eV) was in good agreement with the experimental value ( 5.16 eV) (see Table S1) [18]. Moreover, the EHOMO of Z25 at the BMK/6-31G (d,p) level ( 5.20 eV)

where γi is the angle between the transport pathway and the chosen crystal plane [30]. Φ is the orientation angle between the transfer channel and the chosen crystal axis; θi is defined as the angle of different transport paths relative to the reference axis [26].

2

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Fig. 2. Frontier molecular orbitals (FMOs) for all investigated molecules at the BMK/6-31G (d,p) level.

3. Results and discussion 3.1. Geometric and electronic structures Molecular geometry has obvious effects on the reorganization en­ ergy, stacking model and hole mobility of organic materials. The opti­ mized ground-state structures of all molecules are displayed in Fig. S1. It is found that the geometry of Z25 is more distorted than those of other molecules. The angles between the thiophene core and the connected benzene ring are 22.20� (Z25), 13.76� (Z26), 3.99� (Z26-2), 0.22� (Z263), and 3.31� (Z26-4), respectively. The angles of Z25 and Z26 agree well with the experimental values of 22.27� (Z25) and 12.09� (Z26). In addition, the angles between the benzene groups of the terminal tri­ phenylamine groups are 65.84� (Z25) and 64.19� (Z26). This is also close to the experimental values of 68.42� (Z25) and 68.62� (Z26). Therefore, the theoretical method used in this work is reliable. Thus, Z26-2, Z26-3, and Z26-4 have better planarity and are expected to have higher hole transport performance than Z25 and Z26. Calculated fron­ tier molecular orbitals of all molecules are displayed in Fig. 2. The HOMOs of all investigated molecules are distributed over the entire molecular skeleton, while the lowest unoccupied molecular orbitals (LUMOs) are mainly located on the molecular core, indicating a CT state was formed, which is beneficial to hole transport [31]. The role of HTMs is to promote hole transport from perovskites to HTMs and prevent electron transfer to the metal electrodes [32]. In order to facilitate effective hole transfer to HTMs, the EHOMO needs to be higher than the valence band of perovskites. Beyond that, the LUMO energy level (ELUMO) should be higher than the conduction band of pe­ rovskites to inhibit the electron transfer to the metal electrodes. E rep­ resents the first excitation energy (see Table S2), and the ELUMO was estimated based on the following definition: LUMO ¼ HOMO þ E. In addition, the range-separated or tuned-range-separated methods also have been proved to give accurate HOMO/LUMO levels for similar systems [33–35]. The EHOMO and ELUMO of all investigated molecules are

Fig. 3. Energy diagram displaying the frontier molecular orbitals computed at the BMK/6-31G** level for all investigated molecules.

shown in Fig. 3 and Table S3. Accordingly, the calculated EHOMO of these molecules (Z25, Z26, Z26-2, Z26-3, and Z26-4) are 5.196, 5.083, 5.064, 5.045, and 5.025 eV, respectively. It is clear that Z26, Z26-2, Z26-3, and Z26-4 possess similar EHOMO, which are higher than that of Z25. This reveals that the EHOMO of all molecules are much higher than the valence band of CH3NH3PbI3 ( 5.43 eV), and thus ensure effective hole transfer from perovskites to HTMs [36]. Calculated ELUMO of these molecules (Z25, Z26, Z26-2, Z26-3, and Z26-4) are 2.02, 2.145, 2.574, 2.705, and 2.794 eV, respectively. The corresponding ELUMO match well with the conduction band of CH3NH3PbI3 ( 3.93 eV), which can block the electron transfer to the metal electrodes [37]. 3.2. Reorganization energy and hole mobility The reorganization energy (λ) has a significant effect on the mobility 3

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Fig. 4. The main hole-hopping pathways selected based on the crystalline structures for all investigated molecules. Table 1 The centroid-to-centroid distances (ri, Å), electronic coupling (V, eV), charge transfer rate (kij, s selected based on the crystalline structures.

1

), and hole mobility (μΦ, cm2 V

1

s 1) of the main hopping pathways

compounds

pathways

ri

V

kij

θi

μΦ

Z25

P T1 T2 T3 P T1 T2 T3 P T1 T2 T3 P T1 T2 T3 P T1 T2 T3

12.948 17.824 8.528 12.770 16.686 17.131 19.156 31.580 17.977 18.780 15.298 25.125 25.692 19.124 18.033 40.060 29.445 51.765 27.831 24.565

2.36 � 10 4 0 2.10 � 10 3 3.50 � 10 1.54 � 10 2.00 � 10 5 3.68 � 10 4 1.00 � 10 5 1.60 � 10 3 2.68 � 10 3 0 0 5.39 � 10 4 0 3.54 � 10 2.71 � 10 5 4.52 � 10 4 0 0 0

1.72 � 107 0 1.36 � 109 3.78 � 105 1.07 � 109 1.80 � 105 6.08 � 107 4.50 � 104 5.20 � 108 1.45 � 109 0 0 5.01 � 107 0 2.17 � 109 1.27 � 105 2.49 � 107 0 0 0

0 26.629 69.509 141.273 0 68.992 123.398 149.574 0 51.367 106.465 144.274 0 44.523 132.372 160.443 0 24.585 50.699 118.753

1.90 � 10

4

5.60 � 10

4

7.70 � 10

4

1.30 � 10

3

4.20 � 10

5

Z26

Z26-2

Z26-3

Z26-4

of HTMs. Generally, a smaller λ means a faster carrier transfer and it is composed of the inner reorganization energy (λin) and outer reorgani­ zation energy (λout) [38]. For most organic molecules, λin is dominant, while λout is often ignored (λ ¼ λin) [39]. In this work, the inner reor­ ganization energies (λin) of all molecules were calculated using the adiabatic potential energy surface method and the corresponding computed results are shown in Fig. S2. Calculated λ values of all mole­ cules (Z25, Z26, Z26-2, Z26-3, and Z26-4) are 0.454, 0.420, 0.493, 0.508, and 0.541 eV, respectively. As can be observed in Fig. S2, the λ

5 3

3

values increase slightly with the extension of double bonds. In addition, the intermolecular electronic coupling plays a decisive role in hole mobility. Higher hole mobility will be obtained if there is a larger electronic coupling. Recently, the PW91 functional was verified to be reliable in calculating intermolecular electronic coupling [40]. In this work, the electronic coupling of the dimer was calculated by the ADF program using the VWN/PW91/TZP method. Based on the predicted crystal structures (see Fig. S3), we took one molecule in a crystal as the center, and its adjacent dimers can be 4

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EAa)/2. EAa represents the adiabatic electron affinity, and IPa is the adiabatic ionization potential. The larger the η value is, the more stable of the material is. As shown in Table 2, calculated η values of all investigated molecules are 2.76 eV (Z25), 2.94 eV (Z26), 3.04 eV (Z26-2), 3.12 eV (Z26-3), and 3.18 eV (Z26-4), respectively. The η values of Z26-3 and Z26-4 are larger than other designed molecules, indicating that Z26-3 and Z26–4 may be more stable. Furthermore, the η values of designed Z26 derivatives (3.04–3.18 eV) are larger than that of Z25 (2.76 eV), indicating that introducing double bonds can improve stability.

Table 2 The adiabatic ionization potential (IPa, eV), and electron affinities (EAa, eV) and the absolute hardness (η, eV) calculated at the BMK/6-31G** level. Compounds

IPa

Z25 Z26 Z26-2 Z26-3 Z26-4

5.35 5.08 5.12 5.06 5.02

EAa 0.18 0.81 0.97 1.19 1.35

η 2.76 2.94 3.04 3.12 3.19

represented as the parallel (P) dimer, the transverse (T) dimer, and the longitudinal (L) dimer. Secondly, we defined the angle between different transport paths and the reference axis as θP, θT1, θT2, and θT3. As can be seen in Fig. 4, the four dimers (P, T1, T2, and T3) are in the same plane, which indicates that the values of γi are 0� . The calculated centroid-tocentroid distance (ri), electronic coupling (V), charge transfer rate (kij), and hole mobility (μΦ) of all molecules are listed in Table 1. The computed μΦ value of Z26 (5.6 � 10 4 cm2 V 1 s 1) agrees well with its experimental value (1.34 � 10 4 cm2 V 1 s 1). The order of μΦ value is Z26-3 (1.3 � 10 3 cm2 V 1 s 1) > Z26-2 (7.7 � 10 4 cm2 V 1 s 1) > Z26 (5.6 � 10 4 cm2 V 1 s 1) > Z25 (1.9 � 10 4 cm2 V 1 s 1) > Z26-4 (4.2 � 10 5 cm2 V 1 s 1). The hole transport properties are not only related to ri, but also affected by intermolecular packing. There are three stacking models of dimers: edge to edge stacking, herringbone to edge stacking and face to face stacking. According to the calculated results, the face to face packing model in all dimers obtains the largest electronic coupling, and the corresponding values of Z25, Z26, Z26-2, Z26-3 and Z26-4 are 2.10 � 10 3 eV (pathway T2), 1.54 � 10 3 eV (pathway P), 2.68 � 10 3 eV (pathway T1), 3.54 � 10 3 eV (pathway T2), and 4.52 � 10 4 eV (pathway P), respectively. From Table 1, we can see that ri corresponding to maximum of electronic coupling follows the order of 29.445 Å (Z26-4) > 18.780 Å (Z26-2) > 18.033 Å (Z26-3) > 16.686 Å (Z26) > 8.528 Å (Z25). It can be clearly seen that ri increases with the increase of π-bridge size. In general, smaller ri results in a larger elec­ tronic coupling. Although Z25 has the smallest ri, the geometry of Z25 is more distorted than that of other molecules, which results in a slightly worse face to face stacking model, thus the electronic coupling value of Z25 is small and similar to that of Z26. In addition, the λ value of Z25 is larger than that of Z26, which leads to smaller hole mobility compared with Z26. Furthermore, Z26-3 has the best planarity (the angle between the thiophene core and the connected benzene ring is only 0.22� ) among all molecules, thus it obtains the largest hole mobility owing to the effective face to face stacking. On the contrary, the electronic coupling of Z26-4 is the smallest, which can be attributed to two items: one is the larger ri aroused by the large π-bridge size, the other is the large value of λ, which ultimately led to the smallest hole mobility. In addition, we find that even though Z26-3 has higher mobility, it also has a larger λ value. The results indicate that electronic coupling plays a more important role in determining hole mobility compared with λ. We speculate that the optimal π-bridge size of thiophene derivatives (with diphenylamine as side chain) is the same as that of Z26-3. To verify this point, a series of molecules (such as Y26-0, Y26-1, and Y26-2) were designed (see Fig. S4) and their hole mobilities were calculated. The order of hole mobility is Y26-3 (5.10 � 10 5 cm2 V 1 s 1) > Y26-4 (2.10 � 10 5 cm2 V 1 s 1) > Y26-2 (9.20 � 10 6 cm2 V 1 s 1) > Y26-1 (7.00 � 10 6 cm2 V 1 s 1) > Y26-0 (1.20 � 10 6 cm2 V 1 s 1) (See Ta­ ble S4). As can be observed in Table S4, the hole mobility of molecule Y26-3 (with three double bonds added to each side) is the largest.

4. Conclusion In summary, we have systematically studied the geometries, elec­ tronic properties, reorganization energies, and hole mobilities of Z26 and its derivatives (Z26-2, Z26-3, and Z26-4), aiming to clarify the changing trend of hole mobility with the extension of the double bonds. In addition, the orientation function was employed to quantitatively assess the hole mobilities of these newly designed molecules as HTMs. The results indicate that the designed molecules (Z26-2, Z26-3, and Z264) possess better planarity, appropriate HOMO/LUMO energy levels and better stability compared with Z26. Among them, Z26-3 has the largest hole mobility owing to the effective face to face packing by inserting three double bonds into each side of Z26. The results presented in this work offer a useful strategy for the design of non-spiro type molecules as efficient HTMs. Acknowledgements We acknowledge generous financial support from the National Nat­ ural Science Foundation of China (91741105, 21173169), the Chongq­ ing Municipal Natural Science Foundation (cstc2018jcyjAX0625), and the Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122058. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] C. Liang, D. Zhao, P. Li, B. Wu, H. Gu, J. Zhang, T.W. Goh, S. Chen, Y. Chen, Z. Sha, G. Shao, T.C. Sum, G. Xing, Simultaneously boost diffusion length and stability of perovskite for high performance solar cells, Nano Energy 59 (2019) 721–729. [3] K. Sivashanmugan, C.-H. Lin, S.-H. Hsu, T.-F. Guo, T.-C. Wen, Interfacial engineering of ZnO surface modified with poly-vinylpyrrolidone and paminobenzoic acid for high-performance perovskite solar cells, Mater. Chem. Phys. 219 (2018) 90–95. [4] H. Li, L. Guo, C.-N. Li, C. Wang, G. Wang, S. Wen, J. Wu, W. Dong, Z.-J. Li, S. Ruan, Enhanced electronic quality of perovskite via a novel C60 o-quinodimethane bisadducts toward efficient and stable perovskite solar cells, ACS Sustain. Chem. Eng. 7 (2019) 8579–8586. [5] N.J. Jeon, H. Na, E.H. Jung, T.-Y. Yang, Y.G. Lee, G. Kim, H.-W. Shin, S. Il Seok, J. Lee, J. Seo, A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells, Nature Energy 3 (2018) 682–689. [6] S.S. Mali, C.K. Hong, p-i-n/n-i-p type planar hybrid structure of highly efficient perovskite solar cells towards improved air stability: synthetic strategies and the role of p-type hole transport layer (HTL) and n-type electron transport layer (ETL) metal oxides, Nanoscale 8 (2016) 10528–10540. [7] J.-J. Liang, M. Li, J.-Y. Zhu, H. Zong, Y. Zhang, S.M. Jain, Z.-K. Wang, Detrimental effect of silver doping in spiro-MeOTAD on the device performance of perovskite solar cells, Org. Electron. 69 (2019) 343–347. [8] R. Azmi, S.Y. Nam, S. Sinaga, Z.A. Akbar, C.-L. Lee, S.C. Yoon, I.H. Jung, S.-Y. Jang, High-performance dopant-free conjugated small molecule-based hole-transport materials for perovskite solar cells, Nano Energy 44 (2018) 191–198. [9] S.-H. Peng, T.-W. Huang, G. Gollavelli, C.-S. Hsu, Thiophene and diketopyrrolopyrrole based conjugated polymers as efficient alternatives to spiroOMeTAD in perovskite solar cells as hole transporting layers, J. Mater. Chem. C 5 (2017) 5193–5198.

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