Molecular Engineering of Simple BenzeneâArylamine Hole

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Molecular engineering of simple benzene-arylamine hole transporting materials for perovskite solar cells Xuepeng Liu, Fantai Kong, Shengli Jin, Wangchao Chen, Ting Yu, Tasawar Hayat, Ahmed Alsaedi, Hongxia Wang, Zhan'ao Tan, Jian Chen, and Songyuan Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06193 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Applied Materials & Interfaces

Molecular

engineering

of

simple

benzene-arylamine

hole

transporting materials for perovskite solar cells Xuepeng Liu,a,b Fantai Kong,a* Shengli Jin,d Wangchao Chen,a,b Ting Yu,a,b Tasawar Hayat,c Ahmed Alsaedi,c Hongxia Wang,e Zhan’ao Tan,c Jian Chen,a Songyuan Daid,c,a* a

Key Laboratory of Photovolatic and Energy Conservation Materials, Institute of

Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui, 230088, P. R. China b c

University of Science and Technology of China, Hefei, 230026, P. R. China NAAM Research Group, Department of Mathematics, Faculty of Science, King

Abdulaziz University, Jeddah, 21589, Saudi Arabia d

Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power

University, Beijing, 102206, P.R. China e

School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Brisbane QLD 4001, Australia

Abstract: Three benzene-arylamine hole transporting materials with different number of terminal groups were prepared. It is noted that that the molecule with three arms (H-Tri) shows lower HOMO level and better film morphology on perovskite layer than the molecules with two or four arms (H-Di, H-Tetra). When these molecules were applied to perovskite solar cells, H-Tri-based one shows better performance compared to H-Di- or H-Tetra-based one. Photoluminescence and impedance spectroscopy study demonstrate that H-Tri can improve hole-electron separation efficiency and decrease charge recombination, thus leading to better performance. Moreover, the H-Tri-based device shows comparable performance and much less materials cost than conventional spiro-OMeTAD. Therefore, we have presented a new low-cost and high-performance hole transporting material through simple molecular engineering. Keywords: perovskite solar cell, molecular engineering, hole transporting materials, three arms, low-cost

1 ACS Paragon Plus Environment


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Introduction In recent years, perovskite solar cells (PSCs) have drawn global attention because of its many advantages, such as long hole and electron diffusion lengths, high charge carrier mobility, and low-temperature solution-fabrication process, et al.1-6 Since the solid-state hole transporting material (HTM) was used in PSCs in 2012,2 the remarkable photoelectric conversion efficiency (PCE) of PSCs exceeding 22% has been obtained till now.7 In PSC devices, HTMs play a crucial role in collecting and transferring photogenerated hole, suppressing internal charge recombination, and enhancing stability.8 Many new HTMs have been developed to replace expensive 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spiro-bifluorene

(spiro-

OMeTAD). Among them, multi-armed arylamine HTMs have proved their potential in the application for PSCs. The type of HTMs are usually prepared by varying different core and connecting different types of arylamine terminal groups (or arms).811

In these HTMs, the type of terminal arms or cores greatly affect the property of

HTM, subsequently affect the performance of PSCs.12-19 However, developing new HTM by varying the number of terminal groups were much not sufficient until now. Therefore, we synthesized three simple arylamine molecules (H-Di, H-Tri, H-Tetra) with a benzene core through simple molecular engineering to obtain efficient HTM. The molecular structures of investigated HTMs are shown in Figure 1. We further studied the influence of different number of arms on HTMs on absorption spectra, electrochemical, hole mobility, film-forming ability and photovoltaic properties in PSCs. It is noted that the molecule with three arms show obviously better film morphology and lower highest occupied molecular orbital (HOMO) level than the other two molecules. When used as HTM in PSCs, H-Tri-based devices show better performance than that of H-Di and H-Tetra. Photoluminescence measurements and impedance spectroscopy study demonstrate that H-Tri can improve hole-electron separation efficiency and decrease charge recombination, thus leading to better performance. Moreover, the H-Tri-based PSCs shows a promising PCE of 16.31%.


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Figure 1. Molecular structures of new HTMs containing different number of terminal arms incorporated into a benzene core. Results and discussion Photophysical, electrochemical and hole mobility properties The UV-vis absorption and fluorescence emission spectrum of H-Di, H-Tri and HTetra in chlorobenzene solution are shown in Figure 2a. The corresponding data are summarized in Table 1. The maximum absorption peaks of H-Di, H-Tri and H-Tetra is 362 nm, 342 nm and 339 nm, respectively. Obviously, the absorption capacity of all compounds is weak in the visible region, thus they do not compete for absorption with perovskite in PSC devices. The compounds show a blue-shift spectrum responding along with the increasing number of OMeTPA arms, which can result from that increasing terminal arms enhance electron delocalization over the molecule. The optical band gap (Eg) of H-Di, H-Tri and H-Tetra calculated from normalized UVvis absorption and fluorescence emission spectrum are 3.05, 3.11 and 3.12 eV, respectively. In addition, all compounds show large stokes shifts (H-Di: 103 nm, HTri: 119 nm and H-Tetra: 121 nm), which implies that they will undergo large change in structure in excited state.20



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Figure 2. (a) The UV-vis absorption and fluorescence emission spectra in chlorobenzene

solution.

(b)

Cyclic

voltammogram.

(c)

J-V

curves

for

ITO/PEDOT:PSS/HTM(H-Di, H-Tri or H-Tetra)/Au devices.

Table 1 Photophysical, electrochemical and hole mobility properties of H-Di, H-Tri and H-Tetra. 4 ACS Paragon Plus Environment

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λmax[a]/

λPL[a,b]/

Eg/

HOMO[c]

LUMO[d]

μ/

nm

nm

eV

/eV

/eV

cm2V-1s-1

H-Di

362

465

3.05

-5.27

-2.22

2.13×10-8

H-Tri

342

461

3.11

-5.29

-2.18

1.93×10-7

H-Tetra

339

460

3.12

-5.27

-2.15

9.56×10-7

Compound

[a] UV-vis absorption and emission spectrum were recorded in chlorobenzene solution. [b] Excitation at λmax. [c, d] HOMO and LUMO level were estimated according to previous reports.20 The electrochemical properties of the compounds are carried out by cyclic voltammograms (CV) measurements, as shown in Figure 2b. The redox peaks of all compounds are highly reversible, indicating their good electrochemical stability.21,22 The HOMO levels of H-Di, H-Tri and H-Tetra calculated from CV data are -5.27, 5.29 and -5.27 eV, respectively. As the HOMO level of MAPbI3 is -5.43 eV,2 which suggests that all the compounds have favorable driving force for electron-hole extraction at MAPbI3/HTMs interface. The lowest unoccupied molecular orbital (LUMO) levels calculated from photophysical and electrochemical tests are -2.22, 2.18 and -2.15 eV for H-Di, H-Tri and H-Tetra, respectively. As shown in Figure 3b, the significantly higher LUMO value of new compounds than that of MAPbI3 can prevent electrons from MAPbI3 to Au and ensure hole transfer at the HTMs/MAPbI3 interface when HTMs (H-Di, H-Tri or H-Tetra) are excited. To clear the effect of different number of arms on hole mobility property of these HTMs, the hole-only devices were fabricated and measured according to previous report,20 as shown in Figure 2c. The thickness of H-Di, H-Tri and H-Tetra are 25, 43 and 43 nm, respectively. The estimated hole mobilities of H-Di, H-Tri and H-Tetra calculated by Mott-Gurney law are 2.13×10-8, 1.93×10-7 and 9.56×10-7 cm2 Vs , respectively. Obviously, the more terminal arms on HTM structure can improve

1 -1

the hole transporting properties.13 Scanning electron microscopy and atomic force microscopy To study the film-forming ability of these HTMs, we study the film morphology using scanning electron microscopy (SEM) and atomic force microscopy (AFM) 5 ACS Paragon Plus Environment


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measurements. The below three picture in Figure 3 are the SEM images of MAPbI3 layers covered by different HTMs, and the other three are the corresponding AFM surface topographies of H-Di, H-Tri and H-Tetra. The film-forming ability of the HTM on perovskite or glass surface is an important factor to be considered to obtain a good reproducibility of device performance. According to the AFM results, the rootmean-square (RMS) roughness of H-Di, H-Tri and H-Tetra is 3.5 nm, 2.3 nm and 11.2 nm, respectively. Taking SEM result into account, we can find that though HTetra owns most OMeTPA arms and highest hole mobility, it shows significantly rough surface.

Figure 3. SEM of MAPbI3 layers covered by H-Di, H-Tri and H-Tetra and AFM topographical images of H-Di, H-Tri and H-Tetra. Devices performance and costs To preliminary prove the potential, we fabricate the MAPbI3-based mesoporous PSCs devices incorporating new compounds as HTM layer by conventional spin-coating method. Schematic device structure of PSCs is shown in Figure 4a. The energy level of each component in MAPbI3-based PSC devices (including new-developed compounds) is shown in Figure 4b, which can clearly illustrate that these compounds have potential to be HTM for PSCs.


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Figure 4. (a) Schematic device structure of the MAPbI3-based mesoscopic PSC device, (b) energy level of each component.

Figure 5. (a) J-V characterization under 1.5G illuminations, (b) IPCE spectra of the MAPbI3-based PSCs employing H-Di, H-Tri and H-Tetra. Table 2 Photovoltaic properties of MAPbI3-based PSCs based on H-Di, H-Tri and H-Tetra. HTM

Jsc (mA cm-2)

Voc (V)

FF (%)

PCE (%) [a]

H-Di

16.76

0.93

62

9.61 (8.28±0.75)

H-Tri

20.04

1.02

72

14.88 (13.51±0.57)

H-Tetra

19.28

0.97

73

13.61 (11.11±1.17)

[a] Average PCE is obtained from 10 PSC devices, and the detailed parameters are listed in Table S1. The current density-voltage (J-V) characteristics of the PSC devices employing developed compounds as HTMs are shown in Figure 5a, which are recorded under AM 1.5G illuminations (100 mW cm-2). The corresponding photovoltaic parameters (open-circuit voltage (Voc), short-circuit current density (Jsc), 7 ACS Paragon Plus Environment


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fill factor (FF) and PCE) are listed in Table 2. The PSC device fabricated with H-Tri yields a high PCE of 14.88%, with a Jsc of 20.04 mA cm-2, a Voc of 1.02 V and a FF of 0.72. Under the same condition, the devices with H-Di and H-Tetra show a lower PCE of 9.61% and 13.61%, respectively. To get a comprehensive investigation, the mean values with standard deviation (STDEV) of each group are listed in Table 2. The detailed device parameters of each device are shown in Table S1. H-Tri-based devices exhibit relatively lower STDEV, which indicates that H-Tri-based device shows a better reproducibility, followed by H-Di, and H-Tetra-based devices. Obviously, the surface coverage of H-Tetra on perovskite in PSC device is pretty rough (Figure 3), which leads to insufficient contact of perovskite and HTM layer. Therefore, the PCE of H-Tetra-based device show a higher STDEV value than that of H-Di and H-Tri. Moreover, though the film-forming ability of H-Tetra is worse than H-Di, the devices H-Tetra-based devices show better performance, which could be resulted from significantly better hole mobility of H-Tetra. Therefore, improving the film-forming ability of H-Tetra by introducing long chain alkyl or inserting π-linker between the benzene core and terminal arms could also be promising due to the better hole mobility. From overall J-V data, we noted that the performances of the PSCs employing H-Tri and H-Tetra are better than that of another molecule due to the increased Jsc and FF, which can be resulted from improving hole mobility with the increasing number of arms in the HTM structure.23 On the other hand, the H-Tribased devices show higher Voc than H-Di and H-Tetra, which can be explained by the lower HOMO level and good film-forming ability of H-Tri, so the charge extraction at perovskite/H-Tri interface is more efficiency than perovskite/H-Di and perovskite/H-Tetra

interface

(Figure

7,

photoluminescence

and

impedance

spectroscopy measurements). Figure 5b shows the incident photon-to-current conversion efficiency (IPCE) spectrum of the PSC employing H-Di, H-Tri or HTetra. The IPCE spectra of H-Di and H-Tetra-based devices are deteriorated in 400800 nm compared to H-Tri-based one, which may result from inferior driving force for hole transfer efficiency in MAPbI3/H-Di (or H-Tetra) interface than that of H-Tri due to different hole mobility and film-forming ability (Figure 3). As shown in Figure S1 and Table S2, the hysteresis behavior of the PSC devices with H-Di, H-Tri or HTetra was evaluated from reverse and forward scan direction. As higher hole mobility of HTM can transfer the photogenerated hole effectively, therefore, the devices with H-Di show a obvious hysteresis behavior than H-Tri and H-Tetra. 8 ACS Paragon Plus Environment

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Figure 6. The J-V characteristics of the (FAPbI3)0.85(MAPbBr3)0.15-based PSCs based on H-Tri and spiro-OMeTAD. As shown in previous results, H-Tri show best performance than the other two molecules in MAPbI3-based devices. To compare the performance of H-Tri-based PSC devices with that of spiro-OMeTAD, we further optimized the devices through compositional engineering of perovskite materials ((FAPbI3)0.85(MAPbBr3)0.15). The corresponding J-V characteristics are shown in Figure 6 and Table S3. The bestperforming of H-Tri-based PSC shows a promising PCE of 16.31%, with a Voc of 1.05 V, a Jsc of 20.43 mA cm-2, and a FF of 0.76. Under the same conditions, the device employing spiro-OMeTAD shows a PCE of 16.76%. Therefore, we believe that through extensive optimization of perovskite preparation and the device fabrication, PCE value of the PSC employing H-Tri close to 20% can be attainable. As the stability is important for PSCs, we perform aging tests of the devices with different HTMs. The devices were kept under same condition (35% relative humidity at room temperature). As shown in Figure S2, the PSCs with H-Tri show best stability than that of H-Di or H-Tetra, which is also similar with spiro-OMeTADbased one whether the perovskite is CH3NH3PbI3 or (FAPbI3)0.85(MAPbBr3)0.15. As expected, the devices with H-Di or H-Tetra show inferior stability, which is is consistent with that of SEM and AFM study. Since the production costs is a evaluation criterion for application prospect of HTM, we estimate the material cost of H-Tri following previous report.24 As shown in Table S4 and Table S5, the calculated cost of H-Tri is around 44 $/g, which is



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much less than spiro-OMeTAD, indicating that H-Tri could be promising HTM for PSCs. Photoluminescence measurements and impedance spectroscopy study of MAPbI3based PSCs. To determine the capability of new compounds in acting as HTMs in PSCs, we measured steady-state photoluminescence (PL) of the MAPbI3/HTM bilayer films, which is shown in Figure 7a. To accurately reflect the MAPbI3/HTM interfaces in PSC devices, we doped these HTMs in accordance with the solar fabrication. Upon exciting the pristine MAPbI3 film at 473 nm, strong emission spectra appears in 700850 nm. The PL spectra of all the bilayers show a blue shift, which may be due to chemical interaction between MAPbI3 and HTM.25 We found that all MAPbI3/HTM bilayer show a dramatic quenching compared to MAPbI3 film. And H-Di, H-Tri and H-Tetra with MAPbI3 bilayer show the quenching values of 83%, 91% and 96%, respectively. The result indicates that the hole-electron separation efficiency in MAPbI3/H-Tri interface is the highest, which can be resulted from the good filmforming ability and comparable hole mobility. We further employ impedance spectroscopy (IS) to investigate the influence of HTM on the performance of PSCs. Figure 7b shows Nyquist plots of PSCs with different HTM measured at 0.9 V under dark condition. The recombination resistance (Rrec) could be estimated from impedance spectra. In IS spectra, the first arc is resulted from charge transfer behavior at cathode/HTM.26 The arc in lower frequency is attributed to the recombination between HTM and TiO2,27,28 which is the focus here. The fitting results are obtained using a equivalent circuit, which is shown in the inset of Figure 7b. It is noted that the recombination resistance (Rrec) of the H-Tri-based devices is higher than H-Di and H-Tetra-based one, indicating that the recombination in H-Tri-based devices is slower than another two devices. Thus, the solar cells with H-Di or H-Tetra as HTM show a lower Voc due to higher recombination. Likewise, the PSCs with H-Tri shows lower recombination because the film-forming ability of H-Tri is better and forms a smooth surface to prevent it infiltration and decreases the charge recombination, leading to a better performance.


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Figure 7. (a) PL spectra of MAPbI3/HTM bilayer and MAPbI3 film, excitation at 473 nm. (b) Nyquist plots of the PSC with different HTMs. Conclusions In summary, three simple symmetric benzene-arylamine hole transporting materials by varying the number of terminal arms were introduced to PSCs. It is found that the molecule with three arms owns best film morphology on perovskite layer and lower HOMO level, which brought a great effect on PSCs performance. Therefore, the PSC fabricated with H-Tri exhibits better photovoltaic performance than that of H-Di and T-Tetra. After optimization, the H-Tri-based device shows a promising PCE of 16.31%. Therefore, the low-cost and efficient H-Tri is also a promising substitute for conventional spiro-OMeTAD. Experimental section Synthesis The synthetic routine of H-Di, H-Tri and H-Tetra is illustrated in Scheme 1. All chemicals and solvents were pure grade and were used without further processing. compound 1 was synthesized as has been reported.29,30 The other two final compounds were prepared via simple Suzuki-Miyaura cross-coupling reaction. Molecular structures of them are determined by

C NMR, 1H NMR and MALDI-TOF-MS, as

13

shown in Figure S3-11.



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Scheme 1. Synthetic routes to H-Di, H-Tri and H-Tetra. Synthesis of compound H-Di Compound 1 (960 mg, 2.2 mmol), 1,4-dibromobenzene (compound 2: 0.24 g, 1 mmol), DMF (20 mL), Pd(PPh3)4 (81 mg, 0.7 mmol), and 2 M K2CO3 (3 mL) are added into a 50 mL flask, then degassed using Ar. The reaction mixture is stirred at 95 C for 1 day. After cooling down, the reaction mixture is poured into cold Na2SO4

o

aqueous

solution.

Precipitated

solid

is

purified

by

conventional

column

chromatography (CH2Cl2/petroleum ether = 2/1, v/v) to get pure H-Di (0.56 g, 82%). H NMR (400 MHz, DMSO-d6) δ 7.64 (m, 4H), 7.54 (d, J = 6.5 Hz, 4H), 7.06 (d, J =

1

4.0 Hz, 8H), 6.94 (d, J = 6.6 Hz, 8H), 6.85 (d, J = 8.5 Hz, 4H), 3.76 (s, 12H).

C

13

NMR (101 MHz, CDCl3) δ 155.94, 148.13, 140.97, 138.93, 132.69, 127.34, 126.72, 126.63, 120.87, 114.77, 55.52. HRMS (MALDI-TOF) m/z: [M+] calcd, 684.30; found, 684.29. Synthesis of compound H-Tri The synthesis procedure of H-Tri was same as H-Di. 1,3,5-tribromobenzene (compound 4: 0.31 g, 1 mmol) was used instead of compound 3. The amount of other chemicals and solvents are increase simultaneously. Yield (0.79 g, 80%). 1H NMR (400 MHz, DMSO-d6) δ 7.72 - 7.57 (m, 9H), 7.07 (d, J = 4.8 Hz, 12H), 6.94 (d, J = 6.1 Hz, 12H), 6.86 (d, J = 7.0 Hz, 6H), 3.76 (s, 18H). 13C NMR (101 MHz, CDCl3) δ 155.86, 148.22, 141.82, 140.94, 133.24, 127.76, 126.65, 123.27, 120.82, 114.77, 55.40. HRMS (MALDI-TOF) m/z: [M+] calcd, 987.42; found, 987.39. Synthesis of compound H-Tetra 12 ACS Paragon Plus Environment

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The synthesis procedure of H-Tetra was same as H-Di. 1,2,4,5-tetrabromobenzene (compound 4: 0.39 g, 1 mmol) was used instead of compound 3. The amount of other chemicals and solvents are increase simultaneously. Yield (0.95 g, 74%). 1H NMR (400 MHz, CHCl3) δ 7.49 (s, 2H), 7.08 (s, 24H), 6.87 (s, 24H), 3.83 (s, 24H).

C

13

NMR (101 MHz, CDCl3) δ 155.78, 146.98, 141.19, 138.87, 133.72, 132.22, 130.45, 126.47, 120.14, 114.63, 55.50. HRMS (MALDI-TOF) m/z: [M+] calcd, 1290.55; found, 1290.59. Device fabrication and Instrumentation The PSC devices were assembled according to the reported method (including the preparation of perovskite and optimization process of HTM solution).14,30 The used instrumentation in the work also refer to our previous reports.14,20,30 Acknowledgements This work is supported by the National Basic Research Program of China (No. 2015CB932200), CAS-Iranian Vice Presidency for Science and Technology Joint Research Project (No. 116134KYSB20160130), Natural Science Foundation of Anhui Province (No. 1508085SMF224), National Natural Science Foundation of China (No. 51474201) and the External Cooperation Program of BIC, Chinese Academy of Sciences (No. GJHZ1607). Supporting Information Detailed photovoltaic parameters of PSCs employing H-Di, H-Tri and H-Tetra, stability data, hysteresis behavior, materials costs, 1H NMR,

C NMR and HRMS

13

(MALDI-TOF) figures Corresponding Authors *E-mail: [email protected]. Phone: +86 55165593222. *E-mail: [email protected]. Phone: +86 1061772268. References (1)

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