The Impact of Peripheral Groups on Phenothiazine-based Hole

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The Impact of Peripheral Groups on Phenothiazine-based Hole- Transporting Materials for Perovskite Solar Cells Fei Zhang, Shirong Wang, Hongwei Zhu, Xicheng Liu, Hongli Liu, Xianggao Li, Yin Xiao, Shaik M. Zakeeruddin, and Michael Grätzel ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00395 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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Figure 1 (a) Chemical structures of three HTMs; (b) Synthetic routes for three HTMs 178x123mm (299 x 299 DPI)


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Figure 2 (a)Normalized UV-Vis absorption spectra and photoluminescence spectra of three HTMs in THF solution (c = 1.0×10-5 mol L-1) ;(b) The calculated frontier molecular orbitals of three HTMs ;(c) Energy level diagram of the corresponding materials used in perovskite solar cells; (d) Cross-sectional SEM image of the representative Z30 based device, the scale bar is 200 nm. 177x115mm (300 x 300 DPI)



Figure 3 (a-d) Current-voltage hysteresis curves of perovskite solar cells comprising champion devices with HTMs measured starting with backward scan and continuing with forward scan. The inset is the stabilized power output of three HTMs-based devices; (e) The stabilized power output of the corresponding devices;(f) IPCE spectra and integrated current curves of the corresponding devices. 177x98mm (300 x 300 DPI)


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Figure 4 The reproducibility of 20 devices using different HTMs, (a)Jsc ; (b)Voc; (c)FF; (d)PCE. 178x127mm (299 x 299 DPI)



Figure 5 (a) The stability of corresponding devices in ambient air of 40 % relatively humidity without encapsulation; (b) The stability of corresponding devices during stability test under continuous light irradiation in N2 without encapsulation at their maximum power point and ambient temperature; (c) The contact angles between perovskite/HTMs and water; (d) Surface view SEM images of corresponding perovskite /HTM films. Scale bar is 2µm. 178x161mm (299 x 299 DPI)


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ACS Energy Letters

The Impact of Peripheral Groups on Phenothiazinebased Hole- Transporting Materials for Perovskite Solar Cells

Fei Zhang§ ǂ † +*, Shirong Wang§ ǂ*,Hongwei Zhu§ ǂ, Xicheng Liu§ ǂ, Hongli Liu§ ǂ, Xianggao Li§ ǂ , Yin Xiao§ ǂ, Shaik Mohammed Zakeeruddin†*, Michael Grätzel†* §

School of Chemical Engineering and Technology, Tianjin University, 300072 Tianjin, China

ǂ

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering,

École Polytechnique Fédérale de Lausanne (EPFL), Station 6 CH-1015, Lausanne, Switzerland †

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 300072 Tianjin, China +

Current Address: Chemistry and Nanoscience Center, National Renewable Energy Laboratory,

Golden, Colorado 80401, United States


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ABSTRACT

Three hole transporting materials (HTMs) based on the phenothiazine core containing 4,4dimethyltriphenylamine (Z28), N-ethylcarbazole (Z29) and 4,4-dimethoxytriphenylamine (Z30) as the peripheral groups connected by double bonds were designed and synthesized. The HTMs were tested in mixed cation/anion perovskite solar cells (PSCs) of the composition [(FAPbI3)0.85(MAPbBr3)0.15]. A power conversion efficiency (PCE) of 19.17 % under 100 Mw cm−2 standard AM 1.5G solar illumination is obtained using Z30. Importantly, the devices basedon Z30 show better stability compared to those using Z28, Z29 when aged under ambient air of 40 % relative humidity in the dark for 1008 h and under continuous sunlight soaking without encapsulation for 600 h. These results indicate that the 4,4-dimethoxytriphenylamine is a promising peripheral group in combination with the phenothiazine core providing an alternative to develop small molecular HTMs for efficient and stable PSCs.

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The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased within a few years from 3.81% to over 22%1-4 due to the favorable optoelectronic features of metal halide perovskites. PSCs comprise apart from the perovskite light harvester, electron and hole- transporting materials (HTM), and a metallic counter electrode. The HTM plays an important role in facilitating hole migration from the perovskite layer to back contact and preventing charge recombination.5 The frequently used HTM, spiro-OMeTAD, has several caveats, in particular its tendency to crystallize at 85 °C. There is a need to develop alternative HTMs, which should have suitable energy levels, good solubility in organic solvents, smooth and stable films formation, low-cost production, high hole mobility and elevated glass transition temperature .6-9 Various small molecules, inorganic and polymer HTMs have been developed to replace spiro-OMeTAD 10-19 and several materials have emerged that approach the performance of spiroOMeTAD, especially in terms of PCE, cost, and stability.

20-25

Small molecules have the

advantage of ease of purification and smaller batch-to-batch variation.26-32 Phenothiazine is a low-cost and versatile electron-rich heteroaromatic unit, which provides access to a wide library of HTMs by relatively cheap synthetic procedures. Phenothiazine-based materials have been reported in organic photovoltaics as well as organic transistors.33-35 S-based heterocycles have been found to strengthen the interaction between the perovskite and HTM. 36-38 Herein, we report the facile synthesis of three phenothiazine-based hole transport materials, shown in Figure 1a. Phenothiazine has a butterfly symmetrical nonplanar structure.

39

We

substitute its 3-, and 7- positions by 4,4-dimehtyltriphenylamine, N-ethylcarbazole and 4,4dimethoxytriphenylamine as the peripheral groups affording HTMs coded Z28, Z29 and Z30,


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respectively. These peripheral groups are introduced to tune the energy levels, hole mobility and thermal properties. We investigate here the impact of these peripheral groups on the physicochemical properties and PV performance of phenothiazine-based HTMs. These three HTMs show high glass transition temperatures, hole mobility and suitable valence band levels. The cost of the new HTMs is estimated to be ca 60 US$/g. The devices employing Z30 show power conversion efficiency up to 19.17%. Furthermore, the devices based-on the three HTMs show long shelf life when aged in ambient air of 40 % relative humidity in the dark for 1008 h and good operational stability when subjected continuous light soaking under N2 for 600 h at their maximum power point and ambient temperature.

Figure 1 (a) Chemical structures of three HTMs; (b) Synthetic routes for three HTMs


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The three HTMs were synthesized in three steps involving electrophilic substitution, followed by the Vilsmeier and Wittig reactions. Synthetic routes for Z28, Z29 and Z30 are depicted in Figure 1b and experimental details are given in the Experimental Section of Supporting Information. The new HTMs were characterized by nuclear magnetic resonance (NMR) and mass spectrum (MS) technologies. All the analytical data are consistent with the proposed structures (Figure S1-S9). It is worth noting that the synthesis cost of Z28, Z29 and Z30 are 51.50 $/g, 48.54 $/g and 45.44 $/g respectively.

Figure 2 (a)Normalized UV-Vis absorption spectra and photoluminescence spectra of three HTMs in THF solution (c = 1.0×10-5 mol L-1) ;(b) The calculated frontier molecular orbitals of three HTMs ;(c) Energy level diagram of the corresponding materials used in perovskite solar


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cells; (d) Cross-sectional SEM image of the representative Z30 based device, the scale bar is 200 nm.

To investigate the potential application of these new HTMs in perovskite solar cells, the optimized molecular geometries, the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated by density functional theory (DFT) calculations and results are shown in Figure 2b. The optimized molecular structures of the compounds in Figure 2b reveal that after introducing the double bonds and different peripheral groups, the angles between the phenothiazine core ring and the attached benzene ring of peripheral groups are 13.81°, 14.29° and 3.83° for Z28, Z29 and Z30, respectively. Also, the length of conjugation in the compounds increases by the introduction of 4,4-dimethoxytriphenylamine over that using 4,4-dimehtyltriphenylamine or N-ethylcarbazole as substituents. We can conclude that the Z30 molecule shows more extended π-conjugation than Z28 and Z29 assisting in the hole transport which follows a hopping mechanism.

24,40

The

LUMO of three HTMs is located mainly on central core part while the HOMO orbitals spread over the entire molecular skeleton. This substantial overlapping between HOMO and LUMO orbitals benefits the formation of excitons and hole migration.41 The calculated HOMO levels of Z28, Z29 and Z30 are -4.40 eV, -4.45 eV and -4.27 eV, while the calculated LUMO levels are 1.31 eV, -1.16 eV and -1.20 eV, respectively. With the increase of conjugation area, the HOMO energy level will also increase.42 The wider band gap of Z29 (2.62 eV) compared to Z28 (2.56 eV) and Z30 (2.54 eV) is also ascribed to the more extended π- conjugation system for the latter two HTMs which decrease the energy gap between the HOMO and LUMO.43 Thus, the three HTMs act as a hole selective charge transport layers. We confirmed their energy levels were


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experimentally determined by performing cyclic voltammetry (CV) measurements (Figure S10a). The data are shown in Figure 2c and are summarized in Table 1. The HOMO energy levels of Z28, Z29 and Z30 are -5.39 eV, -5.44 eV and -5.27 eV, which are slightly higher than that of double cation-perovskite, [(FAI)0.85(PbI2)0.85(MABr)0.15(PbBr2)0.15], (-5.5 eV).18 The LUMO levels of HTMs are calculated to be -2.83 eV, -2.82 eV and -2.73 eV, which are above that of double cation-perovskite (-3.9 eV).24 These results agree well with the trend derived from DFT calculations.

Table 1 Photophysical, electrochemical data and thermal characteristics of three HTMs HTM

Eg(eV)

HOMO(eV)

LUMO(eV)

Td(ºC)

Tg(ºC) µ(cm2 V-1 s-1)

-5.39a -2.83a 2.56a 413.1 113.8 6.18×10-5 -1.31b 3.09b -4.40b 2.62a -5.44a -2.82a Z29 406.9 111.4 6.82×10-6 b b -1.16b 3.29 -4.45 2.54a -5.27a -2.73a Z30 421.1 125.7 6.70×10-5 -1.20b 3.07b -4.27b a) : Values determined experimentally from CV measurements, b): theoretically derived values. Z28

Figure 2a showed the normalized UV-vis absorption and photoluminescence (PL) spectra of three HTMs in THF solution (1.0×10-5 mol L-1). The absorption band at 400-450 nm is attributed to the intramolecular charge transfer (ICT) of π-π* transition.44 Due to the smaller degree of conjugation45 , the ICT peak (λabs/max) of Z29 shows a blue shift compared with Z28 and Z30. In addition, the PL spectra of Z28, Z29 and Z30 show maximum emission at 495, 484 and 497 nm respectively.


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Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) show high decomposition temperatures, i.e. Td, 413.1 ºC, 406.9 ºC and 421.1 ºC and glass transition temperatures (Tg,) of 113.8 ºC, 111.4 ºC and 125.7 ºC for Z28, Z29 and Z30 respectively (Figure S10b-c). The higher Tg of Z30 is due to the larger molecular weight and lager molecular stiffness.

46,47

We determined the hole mobilities of three HTMs by using the space charge-

limited current (SCLCs) method. The obtained intrinsic hole mobility (Figure S10d) of Z28, Z29 and Z30 are 6.18×10-5 cm2 V-1 s-1, 6.82×10-6 cm2 V-1 s-1 and 6.70×10-5 cm2 V-1 s-1, respectively, which are comparable with that of spiro-OMeTAD (8.09×10-5 cm2 V-1 s-1). 22 The trend of hole mobility is consistent with extend of π-conjugation after introduction of different peripheral groups as seen from Figure 2b. As shown in Table S3, Z30 has the lower calculated reorganization energy than Z28 and Z29, which agrees with the result of the TOF measurement. 26

The steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) decay spectra are shown in Figure S11. After the HTM materials were coated on top of perovskite films, the PL intensity (Figure S11a) was reduced to roughly 11%, 18%, 6% and 3% of that obtained from pristine films for Z28, Z29, Z30 and spiro-OMeTAD, respectively, suggesting that fast interface charge separation in Z30-based devices can contribute to a better Jsc and FF in the PSCs compared to Z28 and Z29-based devices but lacks behind the hole capture rate by spiro-OMeTAD.24 Figure S11b presents the measured PL decay spectra. The PL decay lifetimes for the devices with Z28, Z29, Z30 and spiro-OMeTAD are significantly shorter than the devices without HTM layer confirming the trends that emerge from the steady state luminescence measurements shown in Figure S11a. Clearly spiro-OMeTAD shows the fastest extraction time followed by Z30.


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Table 2 J-V curves of HTMs based devices under different scan directions with bias step of 5 mV

HTM spiro-OMeTAD (backward) spiro-OMeTAD (forward) Z28 (backward) Z28 (forward) Z29 (backward) Z29 (forward) Z30 (backward) Z30 (forward)

Jsc(mA cm-2)

Voc(V)

FF

PCE (%)

23.76 23.74 23.01 23.04 22.35 22.39 23.53 23.49

1.108 1.104 1.124 1.123 1.087 1.076 1.114 1.105

0.74 0.73 0.69 0.65 0.60 0.56 0.73 0.73

19.66 19.21 17.77 16.81 14.65 13.42 19.17 19.02

Figure 3 (a-d) Current-voltage hysteresis curves of perovskite solar cells comprising champion devices with HTMs measured starting with backward scan and continuing with forward scan. The inset is the stabilized power output of three HTMs-based devices; (e) The stabilized power


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output of the corresponding devices;(f) IPCE spectra and integrated current curves of the corresponding devices.

Figure 4 The reproducibility of 20 devices using different HTMs, (a)Jsc ; (b)Voc; (c)FF; (d)PCE. We fabricate triple cation PSCs to investigate the performance of doped Z28, Z29 and Z30 as HTMs in state of the art devices. Figure 2d shows a typical cross section SEM image of the PSC. Figure 3a-d illustrates the current–voltage (J–V) curves collected under simulated solar illumination (AM 1.5, 100 mW cm-2) for the best PSC among 20 devices and the photovoltaic parameters are summarized in Table 2. The best devices based on Z28, Z29 and Z30 produce open-circuit voltages(Voc) of 1.124 V, 1.087 V and 1.114 V, short-circuit current density (Jsc) of 23.01mA cm-2, 22.35 mA cm-2 and 23.53 mA cm-2 , and a fill factor (FF) of 0.69, 0.60 and 0.73,


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leading to a PCE of 17.77 %, 14.65 % and 19.17 %, respectively, while the spiro-OMeTAD based perovskite solar cells present the best performance with PCE of 19.66 %, Jsc of 23.76 mA cm-2, Voc of 1.108 V, and FF of 0.74 under AM 1.5G (100 mW cm-2) illumination. The lower performance of Z29 is mainly related to an insufficient driving force for hole injections related to its deeper HOMO energy level (–5.44 eV) as compared to the valence band of perovskite (–5.49 eV) and lower hole mobility.19,20 Batches of 20 cells each using these HTMs were fabricated and excellent reproducibility is demonstrated as shown in Figure 4. The trend of Jsc and FF are consistent with that of hole mobility from other reports.24,48,49 This is likely the reason why Z30based devices obtain a better Jsc and FF compared to Z28 and Z29- and a smaller Jsc and FF compared to spiro-OMeTAD. A small hysteresis was observed in the J–V curves. According to previous report50, charge carrier trapping at the interfaces with the electron and hole-selective contact materials is a likely reason for the occurrence of hysteresis. The hysteresis of the device based on Z30 is less pronounced, which we attribute to the faster hole extraction by this material compared to the other phenothiazine derivatives. The stabilized power outputs from devices based on Z28, Z29, Z30 and spiro-OMeTAD are 17.29 %, 13.74 %, 19.12 % and 19.44 % respectively (Figure 3d, 3e), consistent with the obtained PCE. The integrated current densities estimated from the IPCE spectra (Figure 3f) are 22.18 mA cm-2, 21.37 mA cm-2, 22.61 mA cm2

and 23.02 mA cm-2 for Z28, Z29, Z30 and spiro-OMeTAD, respectively, which are in good

agreement with the Jsc values obtained from the J-V curves. Moreover, the similar shape of IPCE spectra for all devices indicates that light absorption by the HTM does not compete with the light harvesting by the perovskite.


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Figure 5 (a) The stability of corresponding devices in ambient air of 40 % relatively humidity without encapsulation; (b) The stability of corresponding devices during stability test under continuous light irradiation in N2 without encapsulation at their maximum power point and ambient temperature; (c) The contact angles between perovskite/HTMs and water; (d) Surface view SEM images of corresponding perovskite /HTM films. Scale bar is 2µm.


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Figure 5a shows the stability tests of corresponding perovskite solar cells in ambient environment of 40 % relatively humidity without encapsulation. As shown in Figure 5a, the PCE maintained 67.1 %, 50.1 % and 85.1 % of the initial value in the Z28, Z29 and Z30-based perovskite solar cell after 1008 hours. The photo stability of corresponding perovskite solar cells was also studied by keeping the devices under continuous simulated AM 1.5 sunlight soaking and under N2 flow without encapsulation at their maximum power point and at ambient temperature. As shown in Figure 5b, the PCE maintained 59.1 %, 45.2 %, 80.2 % of the initial value in the Z28, Z29 and Z30 perovskite solar cell after 600 hours of full sunlight light exposure. We derived further information on the surface energetics of the hole conductors by measuring the contact angle formed by a water droplet. This depends on the surface tension of the 3 interface according to Young’s equation, Figure S12, where the γLV, γSV and γSL are the molar free energies (surface tensions) of the liquid and solid against their saturated vapor and of the interface between liquid and solid, respectively.

51-54

According to the Young equation the

observed 7 ° difference in the contact angles of Z30 and Z28 could arise from either an increase in the difference between the surface tension prevailing at the HTM/gas and HTM/water interface or by an increase in the surface tension between the water droplet and the gas phase or by both. Given the small magnitude of the effect, it is difficult to attribute the observed trend with certainty. Nevertheless we speculate that the molar interfacial entropy of the Z30 is larger at the HTM/water interface than that of Z28 due to decreased ordering by hydrogen bonding with water via the oxygen atoms of the Z30 alkoxy groups. This would lead to the observed increase in the contact angle.


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From the SEM images in Figure 5d, the film based on Z30 is more uniform and of less pinholes than that based on Z28 and Z29, which is consistent with results of the contact angle test (Figure 5c). We ascribed the improvement of devices’ stability mainly due to the smaller number of pinholes in the HTM layer and its increased hydrophobicity.17-20,23,55 As a result, the Z30-based perovskite device showed a much stronger resistance to degradation under ambient condition over longer time periods than the other two corresponding HTM –based devices. In conclusion, we synthesized three novel phenothiazine -based HTMs using a simple lowcost

process.

4,4-dimethyltriphenylamine

(Z28),

N-ethylcarbazole

(Z29)

and

4,4-

dimethoxytriphenylamine (Z30) were introduced into phenothiazine core structure as the peripheral groups connected by double bonds. They are successfully applied in perovskite solar cells based on their high solubility, sufficiently high hole mobility and appropriate HOMO energy level alignment. The perovskite solar cell based on Z30 as HTM affords an impressive PCE of 19.17 %. The devices based on Z30 obtained a higher stability than that of Z28 and Z29 at room temperature aged under ambient air of 40 % relative humidity in the dark without encapsulation after 1008 h and under continuous sunlight soaking without encapsulation after 600 h. These results open a new direction for designing HTMs by using 4,4dimethoxytriphenylamine as the peripheral groups in the future deployment of highly efficient and stable perovskite solar cells. ASSOCIATED CONTENT Supporting Information. Materials measurements, synthesis of HTMs and solar cell fabrication are shown in the Supporting Information. All the analytical data of the structures are shown in Figure S1-S9. CV curves, DSC, TGA curves and current density versus voltage for the HTMs


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are shown in Figure S10. PL spectra and TRPL spectra are shown in Figure S11. Figure S12 shows the contact angle of a liquid droplet wetted to a solid surface and the Young’s equation. Synthesis cost estimation of 1gram Z28, Z29 and Z30 are listed in Table S1-S3. Table S4 lists the reorganization energies of Z28, Z29 and Z30. AUTHOR INFORMATION Corresponding Author *Email: [email protected](F.Z); * Email: [email protected] (S.R.W.); * Email: [email protected](S.M.Z); * Email: [email protected] (M.G.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key R&D Program of China (2016YFB0401303), the National Natural Science Foundation of China (21676188) and Key Projects in Natural Science Foundation of Tianjin (16JCZDJC37100). FZ thanks the Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) for funding. MG. and SMZ thank the King Abdulaziz City for Science and Technology (KACST) for financial support.

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