Highly Efficient Phenoxazine Core Unit Based Hole Transport

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Highly Efficient Phenoxazine Core Unit Based Hole Transport Materials for Hysteresis-Free Perovskite Solar Cells Cheng Chen, Xingdong Ding, Hong-Ping Li, Ming Cheng, Henan Li, Li Xu, Fen Qiao, and Huaming Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12678 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

Highly Efficient Phenoxazine Core Unit Based Hole Transport Materials for Hysteresis-Free Perovskite Solar Cells Cheng Chen, a Xingdong Ding, a Hongping Li, a Ming Cheng, a * Henan Li, b Li Xu, a Fen Qiao, c Huaming Li a a

Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China

*E-mail: [email protected], b

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

212013, P. R. China c

School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, P.

R. China

Keywords: phenoxazine, hole transport materials, perovskite solar cells, low-cost, mesoporous structure

Abstract Two novel simple-constructed and low-cost hole transport materials (HTMs) POZ9 and POZ10, incorporating phenoxazine (POZ) core unit, were designed and synthesized for application in perovskite solar cells (PSCs). The typically semblable molecular structure of POZ9 and POZ10 cause them to possess similar energy levels. However, their photovoltaic performances are quite different from each other due to the small variations of N-substitution on POZ ring. The PSCs based on POZ10, which contains three N, N-di-4-methoxyphenylamino units, achieved a power conversion efficiency (PCE) of 19.4%, while the PSC adopting POZ9 as HTM obtained a lower PCE of 17.1%. Moreover, the light intensity dependence research showed that POZ10 has a better hole transporting ability and can efficiently resist the charge recombination.

Main Text During the past few years, as one of the most promising photovoltaic technologies, the organic-inorganic perovskite solar cells (PSCs) attracted great attentions and ACS Paragon Plus Environment

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experienced an enormous upswing, achieving a remarkable power conversion efficiency (PCE) of 23.7%. 1-2 The remarkable photovoltaic performance of PSCs can be attributed to the broad and intensive light absorption, low exciton binding energy, high charge carrier mobility and long charge diffusion length of the perovskite materials.

3-6

Once excited by sunlight, excitons will generate in perovskite material.

The generated excitons will diffuse to perovskite/hole transport material (HTM) and perovskite/electron transport material (ETM) interfaces, and separated into electrons and holes there. Then the electrons and holes are transported through ETM and HTM, and collected by the corresponding electrodes. Therefore, during the exciton separation and charge transport process, HTM and ETM play important roles in facilitating exciton separation and preventing charge recombination. To date, in highly efficient PSCs, the most commonly used ETM is TiO2, which is low-cost and stable. In

HTM

aspect,

2,

2’,

7,

7’-Tetrakis-(N,

N-di-4-methoxyphenylamino)-9,

9’-spirobifluorene (Spiro-OMeTAD) is undoubtedly the most recognized small molecular HTM. 7-9 However, the cost of Spiro-OMeTAD is prohibitively high due to its challenging synthesis process, such as extremely low reaction temperature, harsh acidic and basicity condition, complicated routes, low yield and so on. Moreover, to achieve high PCEs, high-purity Spiro-OMeTAD is commonly required. Due to these reasons, alternative HTMs with different core building blocks, such as triphenylamine, pyrene, silolothiophene, carbazole, phenothiazine and S, N-heteropentacene, have been reported. 10-27 Phenoxazine (POZ) is a low-cost electron-rich heteroaromatic unit, and can be easily modified through relatively easy synthetic procedure in mild conditions. Herein, by employing POZ core unit, two new materials POZ9 and POZ10 were designed and synthesized. The research results manifest that the number of the donor group N, N-di-4-methoxyphenylamino has big effects on the properties of HTMs. The optimized device based on POZ10 containing three N, N-di-4-methoxyphenylamino units achieved a higher power conversion efficiency (PCE) of 19.4% without hysteresis.

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Figure 1 The structures of HTMs POZ9 and POZ10

Scheme 1 The synthetic routes of HTMs POZ9 and POZ10 The structures of HTMs POZ9 and POZ10 are shown in Figure 1. The detailed synthetic routes and characterizations of POZ9 and POZ10 are depicted in Scheme 1 and Supporting Information. The target HTMs POZ9 and POZ10 can be easily synthesized through palladium-catalyzed cross-coupling reaction (see Scheme 1) with total yield of 66.9% and 68.1%, respectively. The introduction of 4-methoxyphenyl group or N, N-bis(4-methoxyphenyl) aniline-4-yl group on the phenoxazine core were intend to improve the thermal stability and the spatial configuration of target materials. According to reported models, the synthesis cost of HTMs POZ9 and POZ10 were



calculated to be 121 $/g and 126 $/g, respectively, which are lower than the synthesis cost of classical HTM Spiro-OMeTAD (around 200 $/g). 10-11

Figure 2 Quantum chemical calculations of HTMs POZ9 and POZ10 Quantum chemical calculations were conducted to gain insight into the electronic properties of POZ9 and POZ10. As shown in Figure 2, POZ10 shows more three-dimensional spatial configuration than POZ9. For POZ9, the highest occupied molecular orbital (HOMO) is delocalized throughout the entire main molecular backbone, while the lowest unoccupied molecular orbital (LUMO) transfer to the molecule central 4-methoxyphenyl substituted POZ unit. Similarly, the HOMO and LUMO orbitals of POZ10 is localized on the main backbone and N, N-bis(4-methoxyphenyl) aniline-4-yl substituted POZ core unit, respectively. The overlaps of the molecular orbitals in both POZ9 and POZ10 indicates the formation of neutral excitons and hole-transfer transition. In addition, the reorganization energy is calculated to investigate the hole-transport efficiency. In comparison with 172 meV for HTM POZ9, the lower reorganization energy of 143 meV for POZ10 suggests higher hole mobility. The hole mobility was further measured by using space charge limited current (SCLC) method. The hole mobilities of POZ9 and POZ10 were extracted to be 2.18×10-4 and 2.46×10-4cm2·V-1·s-1 (see Figure S1a), respectively, matching well with the DFT calculation results. For doped POZ9 and POZ10, POZ10 comes out with the higher conductivity (2.09×10-4 S·cm-1) than POZ9 (1.01×10-4 S·cm-1) (see Figure S1b), which is benefit for improving device fill factor (FF). ACS Paragon Plus Environment

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Figure 3 a) UV-Vis absorption spectra of HTMs POZ9 and POZ10 in dichloromethane solution (1.0 × 10−5 M), b) Cyclic voltammograms of HTMs POZ9 and POZ10 in dichloromethane solution (1.0 × 10−4 M)

Figure 4 a) Schematic structure of perovskite solar cell, b) energy levels diagram of perovskite solar cell, c) cross-section SEM of perovskite solar cell, d) top-view SEM of perovskite film, e) tope-view SEM of POZ9 film on perovskite surface, f) tope-view SEM of POZ10 film on perovskite surface Both POZ9 and POZ10 show two absorption peaks in UV region (see Figure 3a). The slight blue-shift of POZ10 as compared to POZ9 can be attributed to the higher degree of spatial configuration. The optical band gaps (Eg) of POZ9 and POZ10 are estimated from the absorption onset wavelength, and the Eg were calculated to be 2.81 eV and 2.90 eV for POZ9 and POZ10, respectively. Cyclic voltammograms of HTMs POZ9 and POZ10 in dichloromethane solution are shown in Figure 3b. The redox peaks of POZ9 and POZ10 are both reversible, indicating



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that these two HTMs have good electrochemical stability. Furthermore, the HOMO energy levels of HTMs POZ9 and POZ10 were calculated to be -5.20 eV and -5.18 eV,

respectively.

The

valence

band

(VB)

of

the

mixed

perovskite

(FAPbI3)0.85(MAPbBr3)0.15 used here is -5.65 eV, hole transfer from the perovskite to HTMs POZ9 or POZ10 is favorable. The LUMO levels of POZ9 and POZ10 were estimated to be -2.39 eV and -2.28 eV, respectively. The conduction band (CB) of (FAPbI3)0.85(MAPbBr3)0.15 is -4.05 eV, indicating that electron could be efficiently blocked by the HTM, and therefore suppress the charge recombination (see Figure 4b).

Figure 5 a) Current density-voltage properties of PSCs employing POZ9 and POZ10 as HTMs, b) IPCE spectra of PSCs employing POZ9 and POZ10 as HTMs, c) steady-state current density and PCE of PSCs employing POZ9 (bias: 0.85 V) and POZ10 (bias: 0.89V) as HTMs at the maximum power output point The photovoltaic properties of POZ9 and POZ10 were evaluated by fabricating PSCs with the following configuration: FTO/c-TiO2/m-TiO2/Perovskite/HTM/Au (see Figure 4a). The mixed perovskite (FAPbI3)0.85(MAPbBr3)0.15 was utilized as light


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harvesting material in this work due to its excellent photovoltaic properties. As

shown in the SEM images Figure 4c and Figure 4d, a flat and compact perovskite film with the thickness of 550 nm can be detected. The optimized PSCs exhibit good layer-by-layer structure. The thickness of HTL is around 120 nm. The perovskite layer can be fully covered by both POZ9 and POZ10, while the POZ9 film was inhomogeneous and small, aggregated dots can be obviously observed (Figure 4e). The POZ10 film is more uniformed and smoother (Figure 4f), which can be attributed to the better spatial configuration of POZ10. Figure 5a presents the current density–voltage (J-V) scans for PSCs

employing POZ9 and POZ10 as HTMs, respectively, and the corresponding photovoltaic parameters are listed in Table 1. The POZ9 based PSCs show slightly high hysteresis with PCEs of 15.9% and 15.6% for scan from open-circuit (OC) to short-circuit (SC) and reverse scan, respectively. The hysteresis index (HI) was calculated to be 0.05. In contrast, the POZ10 based PSCs exhibit higher PCE of 19.4% with dramatically improved short-circuit current density (Jsc), open-circuit voltage (Voc) and fill factor (FF). Moreover, the hysteresis (HI = 0.01) of the POZ10 based PSCs is negligible. From the series resistance (Rs) values for the POZ9 and POZ10 based PSCs (see Table 1), we deduced that the increasement of FF can be attributed to the higher hole conductivity of POZ10 film. To better evaluate these two novel HTM POZ9 and POZ10, reference devices with Spiro-OMeTAD as HTM were also fabricated for comparation. Through optimization, the Spiro-OMeTAD based PSC obtained the highest PCE of 19.7% with the HI value of 0.04 (see Figure S3 and Table S7). Therefore, POZ10 is a promising alternative in terms of PCE and material cost. As shown in the incident photon-to-electron conversion efficiency (IPCE) spectra (see Figure 5b), the POZ9 and POZ10 based PSCs showed identical light absorption range (from to 350-800 nm), therefore, the higher Jsc of PSC employing POZ10 as HTM is mainly attributed to its higher hole mobility and transport efficiency, which is confirmed by the higher IPCE values for POZ10 based PSC obtained in the ACS Paragon Plus Environment


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whole response range and the steady state PL decay measurements (see Figure S2). The steady-state PL quenching behavior indicates the hole transfer efficiency from the perovskite into the HTMs, and the time-resolved PL decay can clearly show the hole extraction rate. The higher quenching yield of POZ10 (92%) than that of POZ9 (86%) and the shorter PL lifetime of perovskite/POZ10 system (4.6 ns) indicate that POZ10 was more efficient at hole extraction, which is consistent with the higher hole mobility of HTM POZ10 mentioned above. The integrated current

densities for POZ9 and POZ10 based PSCs are 21.4 and 22.9 mA·cm−2, respectively, matching well with the J-V measurements. In addition, POZ9 and POZ10 based PSCs show good reproducibility with the average PCEs of

15.86±0.891% and 17.92±0.891%, respectively (see Figure S4, Table S8 and Table S9). To confirm the accuracy of the J-V measurements, steady-state current densities and PCEs at the maximum power output points were further tested and shown in Figure 5c. The POZ9 and POZ10 based devices showed the steady-state current densities of 20.0 mA·cm-2 and 21.6 mA·cm-2, and the steady-state PCEs of 17.0% and 19.2%, respectively, which is also agree well with the J-V measurements. Table 1. The best photovoltaic performances of PSCs employing POZ9 and POZ10 as HTMs hyster Scan HTM

Voc / V direction

Jsc /

FF

PCE

esis

mA·cm-2

/%

/%

index

Rsr /

Rsh /

Ω·cm2 Ω·cm2

/% From OC 1.06

22.1

72.8

17.0

to SC POZ9

0.05

9.8

4578

0.01

4.9

7421

From SC 1.05

22.2

72.6

16.9

1.1

23.1

76.2

19.4

to OC From OC POZ10 to SC


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From SC 1.1

23.2

75.8

19.3

to OC

Note: scan forward: from open circuit (OC) to short circuit (SC); scan backward: from short circuit (SC) to open circuit (OC).

Figure 6 Light intensity dependence of a) Jsc and b) Voc for PSCs with POZ9 and POZ10 as HTMs Considering the same components except HTM used in POZ9 and POZ10 based PSCs, the higher Voc of POZ10 based device should be ascribed to the lower recombination kinetics at the perovskite/HTL interface. To further confirm our deduction, we investigated the light intensity dependence of the Jsc and Voc. 28 Figure 6a shows the power law dependence of the Jsc with light intensities (J = Iα). For POZ9 and POZ10 based PSCs, the slope values (α) were fitted to be 0.914 and 0.951 respectively. The higher α for POZ10 based PSCs indicates that the recombination loss at the perovskite/HTL interface is better restricted. Moreover, as shown in Figure 6b, the POZ9 and POZ10 based PSCs present slope values of 1.68kT/q and 1.17kT/q

respectively,

suggesting

interfacial

trap-assisted

Shockley−Read−Hall (SRH) recombination is involved in both PSCs. The weaker Voc dependence on the light intensity of POZ10 based PSC indicates reduced SRH recombination, therefore leading to higher photovoltaic performance. The stability of PSC is another crucial consideration for the future commercial application. Devices without encapsulation were kept in a desiccator under a relative



humidity around 40%. The device that incorporating POZ9 and POZ10 as HTMs maintain almost 86.5% and 93% of its initial performance, respectively (see Figure 7). The improved stability of the POZ10 based device was attributed to the more homogeneous nature of the film as discussed above, which in turn prevent the perovskite film from being destroyed by water vapor. Under the same conditions, for the reference device with Spiro-OMeTAD as HTM, the PCE dropped from 19.7% to 15.7% (dropped 20.3%) after 20 days. This is good evidence that our POZ10 could be a low-cost alternative of Spiro-OMeTAD in PSCs.

Figure 7 a) Voc, b) Jsc, c) FF and d) PCE variation tendency of POZ9, POZ10 and Spiro-OMeTAD based PSCs during aging tests In summary, we reported two low cost, POZ core unit based HTMs POZ9 and POZ10. The only difference is the POZ10 possesses a bigger substituent group on the N position of phenoxazine core to compose a more stereo molecular structure. Applied in PSCs, POZ10 based device showed a higher PCE of 19.4% due to efficiently resisted SRH recombination during working process. Furthermore, the device based on HTM POZ10 showed better stability than the device with POZ9 as HTM,


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maintaining almost ~93% of its initial performance after 480 h aging. We firmly believe that our POZ core unit based HTM adjusted via molecular engineering are promising HTM candidates for the fabrication of low cost and highly efficient PSCs. Acknowledgements This work was financially supported by the China Natural Science Foundation (Grants 21805114), the Jiangsu natural science foundation (BK20180867, BK20180869), the Jiangsu University Foundation (17JDG032, 17JDG031), the high-performance computing platform of Jiangsu University, the Priority Academic Program Development of Jiangsu Higher Education Institutions Supporting Information The experimental details, characterization of HTMs, statistics of device performance, hole mobility and conductivity of HTMs and PL decay results supplied as Supporting Information. Reference 1.

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Highly Efficient Phenoxazine Core Unit Based Hole Transport

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