D-A-D-typed Hole Transport Materials for Efficient Perovskite Solar

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D-A-D-typed Hole Transport Materials for Efficient Perovskite Solar Cells:Tuning Photovoltaic Properties via the Acceptor Group Peng Xu, Peng Liu, Yuanyuan Li, Bo Xu, Lars Kloo, Licheng Sun, and Yong Hua ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04003 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Materials & Interfaces

D-A-D-typed Hole Transport Materials for Efficient Perovskite Solar Cells:Tuning Photovoltaic Properties via the Acceptor Group Peng Xu, a Peng Liu, b Yuanyuan Li, c Bo Xu, *d Lars Kloo,*b Licheng Sun, *e and Yong Hua *a a

Yunnan Key Laboratory for Micro/Nano Materials & Technology, School of Materials Science and Engineering, Yunnan University, Kunming 650091, Yunnan P. R. China.

b

Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30, SE-100 44, Stockholm , Sweden. c

Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, Chemical Science and Engineering, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden.

d

Physical Chemistry, Department of Chemistry-Ångström Laboratory, Uppsala University, Uppsala, Sweden. e

Organic Chemistry, Department of Chemistry, KTH Royal Institute of Technology, SE-10044, Stockholm, Sweden. Corresponding

author:

[email protected];

[email protected];

[email protected];

[email protected].

ABSTRACT: Two D-A-D structured hole-transport materials (YN1 and YN2) have been synthesized and used in perovskite solar cells. The two HTMs have low-lying HOMO levels and impressive mobility. PSCs fabricated with YN2 showed a PCE value of 19.27% in ambient air, which is significantly higher than that of Spiro-OMeTAD (17.80%). PSCs based on YN1 showed an inferior PCE of 16.03%. We found that the incorporation of the stronger electronwithdrawing group in the HTM YN2 improves the PCE of PSCs. Furthermore, the YN2-based

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PSCs exhibit good long-term stability retaining 91.3% of its initial efficiency, while PSCs based on Spiro-OMeTAD retained only 42.2% after 1000h lifetime (dark conditions). These promising results can provide a new strategy for the design of D-A-D HTMs for PSCs appications in future.

KEYWORDS:

perovskite solar cells, hole-transport material, mobility, spiro-OMeTAD,

acceptor.

Introduction Perovskite-based solar cells (PSCs) offer special opportunities as very promising energy source owing to its low-cost solution-based manufacturing technology and high power conversion efficiencies (PCEs). The reported efficiency of PSCs has shown a remarkable enhancement from 3.8% in 2009 to a certified value of 22.1% in 2016.1-8 The most common studied conventional PSC is of the mesoscopic architecture, which generally constitutes five components: a FTO conductive substrate, a n-type mesoporous semiconductor metal oxide film, the perovskite-based light absorber layer, a p-type HTM layer, and a metal electrode.9-12 Nowadays, the HTMs in high performance PSCs play a key role in efficiently extracting and collecting photo-generated holes from the perovskite absorber in order to minimize undesired charge recombination losses and thus avoid a loss in device performance.13-16 In addition to benefits on the PCE, extremely uniform HTM capping layer on the surface of perovskite layer could increase the long-term stability of PSCs. Up to now, various new classes of organic smallmolecule HTMs based on 3,4-ethylenedioxythiophene, fluorene, carbazole, tetrathiafulvalene, tetraphenyl, triphenylamine, and spirobifluorene cores have been developed for the fabrication of PSCs that have exhibited impressive photovoltaic performance.17-23 Among them, the state-of-


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the-art small molecule Spiro-OMeTAD has been the best performing used HTM for PSCs with the highest PCE reported to over 20%. However, its expensive synthetic cost hampers the potential commercial application of Spiro-OMeTAD in the future. Additionally, SpiroOMeTAD suffers from a relatively low hole-mobility (~10−5 cm2 V−1 s−1) in its pristine form, which results in inferior device performance.24-25 Thus, the development of new high mobility HTM molecular materials through facile synthetic routes to replace Spiro-OMeTAD for PSC applications is highly desirable. Donor (D)−acceptor (A)−donor (D) molecules have been widely investigated in organic solar cells because their facile synthesis, low-lying HOMO energy levels and impressive hole mobility.26-29 However, the D−A−D organic materials-based HTMs has been very rarely reported for PSC applications.30-31 Moreover, the effects of various acceptor groups on the optoelectronic, electronic and photovoltaic properties of D-A-D HTMs have not been fully explored yet. In this work, two novel D-A-D HTMs (YN1 and YN2) featuring triphenylamine as the donor along with benzothiadiazole (BT) or thienopyrazine (TP) units as the acceptor were prepared for comparison, as shown in Figure 1. Benzothiadiazole is a very cheap starting material and has been widely used to construct organic photovoltaic materials owing to its excellent electronwithdrawing ability and good mobility. In comparison with BT, TP is a much stronger electrondeficient unit, which can enhance the intramolecular charge transfer (ICT) within the D-A-D organic molecules, thus improving the mobility. Moreover, two bulky phenyl groups attached to the TP backbone is expected to ensure a strong ability of suppressing HTM intermolecular aggregation, and finally forming ideal HTM film morphology.[32-33] By introducing BT onto HTM YN1, the PCE of PSCs device has amounted to 16.03%. To further understand the effect of stronger electron-deficient acceptor in the HTM on the performance of the corresponding


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PSCs, TP is introduced as the core structure. As a result, YN2-baed PSCs achieves a higher PCE of 19.27%, which is the highest PCE reported for PSC devices employing D-A-D typed HTMs so far. We are encouraged to note that YN2 has the higher PCE value than Spiro-OMeTAD (17.80%) measured under the same working conditions. Moreover, the effect of the two different acceptor cores (BT and TP) onto optical and electronic properties and their solar cell performances were systematically studied. These results reveal that YN2 containing the stronger acceptor TP can enable a low-lying HOMO level, higher mobility, better performance and stability of solar cells than for the corresponding YN1 material. Our studies are indicative for the design of highly efficient and stable D-A-D HTMs for PSCs application in future.

Figure 1. Molecular structures of YN1 and YN2.

Results and Discussions The detailed synthetic routes for the two HTMs YN1 and YN2 are depicted in the Scheme S1. The two HTMs were synthesized through a straightforward condensation reaction with the total yields for the two HTMs exceeding 80%. The new HTMs were confirmed by NMR spectroscopy and EA. The normalized UV/Vis spectra of YN1 and YN2 in CH2Cl2 are shown in Figure 2a and their photophysical properties are listed in Table 1. It can be noted that the two HTMs exhibit two


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distinct major absorption bands from 300nm to 700nm. The maximum absorption peak of YN1 is presented at 480 nm. The peak of YN2 at 586 nm shows a 106 nm bathochromic shift which can be explained by the stronger electron-withdrawing property of TP. As shown in Figure 5b, YN1, YN2 and Spiro-OMeTAD exhibit the nearly identical IPCE action curve from 300nm to 800nm, which indicate that YN1 and YN2 as HTMs used in PSCs have no contribution to light harvesting of the perovskite, although they show a strong absorption at 350 to700 nm. The electronic properties of YN1 and YN2 were investigated by cyclic voltammetry and the redox data are collected in Table 1. Obviously, the two HTMs exhibit two quasi-reversible oxidation waves (Figure 2b). The first redox wave is ascribed to the oxidation of the electron donor triphenylamine and the second wave corresponds to the electron removal from the acceptor unit. The estimated HOMO levels of YN1 and YN2 are around -5.35 and -5.40 eV, respectively. The lower-lying HOMO level of YN2 is mainly ascribed to its greater electronwithdrawing ability of TP which would make it not easy to remove an electron from YN2. As shown in Figure 3, the HOMO levels of these two HTMs are sufficiently positive with respect to that of perovskite (-5.65 eV), indicating that hole injection are energetically feasible. Besides, the LUMO values of YN1 (-3.13 eV) and YN2 (-3.57 eV) are much more negative than the perovskite material (-4.05 eV), which could block the undesired electron flow from the perovskite to Au electrode, thus reducing the charge recombination rate.34-35 YN2 shows a slightly low-lying LUMO level than YN1, which is due to the higher the electron affinity of the electron-withdrawing TP unit.


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Figure 2. (a) Absorption spectra of YN1 and YN2. (b) Cyclic voltammograms of YN1 and YN2.

Figure 3. (a) Device architecture of PSCs. (b) The energy level diagram in PSCs.

Here, the hole-mobility of the HTMs were measured via space-charge-limited currents (SCLCs) according to previous reports,24 as it is key parameter for estimating new HTMs in PSCs application. The corresponding results are listed in Table 1. The hole mobility values of YN1 and YN2 is 7.42 × 10-4 cm2·V−1·s−1 and 9.65 × 10-4 cm2·V−1·s−1, respectively. It can be clearly seen that the higher hole mobility of YN2 could be contributed to the introduction of the stronger acceptor TP unit expected to enhance efficient ICT.36-37 Obviously, both YN1 and YN2 demonstrate five times higher hole-mobility than the HTM Spiro-OMeTAD (1.58 × 10-4


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cm2·V−1·s−1).18 It suggests that D-A-D structured molecule could be a promising hole-transport material candidate for PSCs application.

Figure 4. a) Hole-mobility measurements for YN1 and YN2. b) Steady state PL spectra of perovskite and perovskite/HTMs.

To determine hole extraction ability at the interface between perovskite and HTM, we investigated the steady-state PL spectra of the perovskite/HTMs, in which the PL quenching of pervoskite film indicates efficient extraction at the interface.38-40 As shown in Figure 4b, the perovskite layer exhibits a strong PL peak between 750-780 nm, and then the PL intensity was significantly quenched when the different HTMs was spin-coated onto the perovskite film. This result indicates efficient extraction of holes at the interface between perovskite and HTMs. Especially, the PL quenching efficiency of YN2 (ca. 98%) is higher than Spiro-OMeTAD (ca. 93%), which proves that YN2 has stronger hole extraction capability in PSCs. Furthermore, the transient photocurrent decay measurement (Figure S3) clearly indicated that YN2 have a better hole-extracting ability in comparision with YN1 and Spiro-OMeTAD, leading to better photovoltaic performance.


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Table 1. The physical parameters for YN1, YN2 and Spiro-OMeTAD. HTM

λmaxa [nm]

Eo-ob [eV]

HOMO b [eV]

LUMO b [eV]

Hole Mobility [cm2·V−1·s−1]

YN1

480

2.22

-5.35

-3.13

7.42×10-4

YN2 Spiro-OMeTAD

586 /

1.83 3.16

-5.40 -5.22

-3.57 -2.06

9.65×10-4 1.58×10-4

a)

Measured in CH2Cl2 solution.

b)

Electrolyte: 0.1 M n-Bu4NPF6 in CH2Cl2 solution;

Reference electrode: Ag/0.01 M AgNO3; Working electrode: a glassy carbon disk; Counter electrode: a platinum wire. Scan rate: 0.05 V/s. Each measurement was calibrated with Fc. E1/2 Fc = 0.20 V. EHOMO= −5.22− (E1/2−E1/2 Fc). d) ELUMO = EHOMO + E0-0. Figure 5 depicts the J–V curves of YN1, YN2 and Spiro-OMeTAD HTMs-based PSCs employing the mixed perovskite (FAPbI3)0.85(MAPbBr3)0.15 as the light-absorber, and these key parameters are shown in Table 2. The YN1-based device achieves a PCE of 16.03% with a Voc of 1.07 V, a Jsc of 22.36 mA cm-2 and a FF of 0.67. The YN2-based device showed a Voc of 1.11 V, a Jsc of 23.15 mA cm-2 and a FF of 0.75, yielding a PCE of 19.27%. These parameters of the PSCs-based on YN2 are much higher than that for the Spiro-OMeTAD-based device (a Voc of 1.09 V, a Jsc of 22.69 mA cm-2, a FF of 0.72, and an overall PCE of 17.80%). Obviously, the PSCs performance is strongly linked to the HTMs molecular structure. The Jsc, Voc and FF values of YN2-based device is significantly higher than that of YN1-based device. The enhanced Jsc for YN2-based PSCs is in good consistent with the integral Jsc form the IPCE spectra. The higher Voc and FF value in PSCs can be ascribed to the relatively lower-lying HOMO level and the higher hole-mobility of the YN2, respectively. These photovoltaic results suggest that the incorporation of stronger acceptor into D-A-D typed HTMs can improve the photovoltaic performance of PSCs.


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Figure 5. a) J-V curves of PSCs based on the three HTMs. b) IPCE curves of PSCs based on the three HTMs.

Table 2. PSCs parameters of devices based on the three HTMs. HTMs

YN1

Jsc

Integral Jsc

Voc

(mA cm-2)

(mA cm-2)

(V)

22.36

22.15

1.07

(22.35±0.23) YN2

23.15

(1.06±0.01) 22.87

(23.05±0.27)

a)

Spiro-

22.69

OMeTAD

(22.23±0.28)

22.54

FF

PCE (%)

0.67

16.03 a)

(0.67±0.01) (15.36±0.45) b)

1.11

0.75

19.27

(1.11±0.02)

(0.73±0.02)

(18.57±0.32)

1.09

0.72

17.80

(1.10±0.01)

(0.71±0.02)

(17.23±0.39)

The maximum value; b) The average value were obtained from 20 devices. Top-down SEM images of the HTMs layers on the surface of the (FAPbI3)0.85(MAPbBr3)0.15

layers are exhibited in Figure 6a-6c. Figure 6a shows that the homogeneous perovskite film completely disperses on the TiO2 substrate. When the YN1 is deposited onto the perovskite surface, some large particles and pinholes can be detected on the film, which could enable some direct contact between the Au and perovskite, leading to the lower PCE and instability of the


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YN1-based cells. Noticeably, the YN2 produces extraordinarily homogeneous HTM capping layers on of the perovskite materials without any observable particles or pinholes, which means that the bulky TP unit can present the aggreagtion and crystallization of YN2 and improve its distribution, providing a smoother HTM surface.41 Therefore, the superior photovoltaic performance of the YN2-based cell may be originated from its smoother HTM layer.

Figure 6. The top view of HTM capping layer of Perovskite (a), YN1 (b) and YN2 (c) on the perovskite films. (d) A histogram of PCEs for 36 PSCs devices. The PCEs histograms for the three HTMs-based PSCs devices and the corresponding statistical parameters are exhibited in Figure 6d, which demonstrated an excellent PCEs reproducibility for those D-A-D HTMs-based PSCs devices, in which the average value of the devices fabricated with YN2 as a HTM was higher than that of Spiro-OMeTAD, indicating that D-A-D typed materials are great potential HTMs for highly efficient PSCs application.


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To understand the charge recombination process at the perovskite/HTMs interface in PSCs device, the ESI Nyquist plots of PSCs with various HTMs were measured in the dark (Figure 7a). The figure shows a primay semicircle in the middle frequency region, which is concerned with the charge-transport resistance (Rrec) in the device [42]. In general, a higher Rrec is related to a more efficient charge transfer, leading to lower carrier recombination in the device. Obviously, YN2-based PSCs device exhibited higher Rrec than the YN1 and Spiro-OMeTAD-based devices, which indicates that lower charge recombination rate in the YN2-based device, leading to a higher PSCs performance. Moreover, the PSCs based on YN2 as a HTM shows less carrier recombination because of its more homogeneous HTM film of YN2, which can be able to decrease efficiently the charge recombination.

Figure 7. a) Nyquist ESI spectra of PSCs based on the YN1, YN2 and Spiro-OMeTAD under the dark. b) The stability of the PSCs based on YN1, YN2 and Spiro-OMeTAD. The stability test of PSCs with YN1, YN2 and Spiro-OMeTAD as a HTM without encapsulation was tested under air conditions (temperature: 25 °C and humidity: 40−45%) (Figure 7b). The PCEs of YN1 and YN2-based solar cells are operational with 91.3% and 61.6% performance retention after 1000 hours, respectively. By contrast, the PSCs device based


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on Spiro-OMeTAD maintains only 42.2% of its initial PCE under the same working conditions. It is noted that the YN2-based device exhibited greater stability owing to its smooth HTM film act as an effective moisture barrier.

Conclusion In summary, we have synthesized a new class of D-A-D-based HTMs (YN1 and YN2) with triphenylamine as a donor and benzothiadiazole (BT) or thieno[3,4-b]pyrazine (TP) unit as an acceptor for application in PSCs. The YN2-based PSCs affords an impressive PCE value of 19.27% at air condition, superior to that of the solar cell fabricated with Spiro-OMeTAD (17.80%), while the PSCs with YN1 as a HTM showed a much lower PCE of 16.03%. The influence of the acceptor group of the two HTMs on optical and electronic properties and their solar cell performance was systematically investigated. Our results prompt to the conclusion that the photovoltaic properties of D-A-D typed HTMs can be finely tuned by incorporating various electron withdrawing groups, and that D-A-D small molecule is a potential HTM candidate for highly stable and efficient PSCs.

Experimental Section Materials and Reagents All solvents and other chemicals was purchased from Sigma-Aldrich company and used without further purification. MABr, FAI, PbBr2 (99.99%) and PdI2 (99.99%) were purchased from China Xi’an Polymer Light Technology Company. The detailed synthetic routes for the two new HTMs YN1 and YN2 are depicted in the Scheme S1.


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Fabrication and Characterization of PSCs device The FTO substrates (15 Ω) were etched with 4M HCl acid and zinc powder to form the desired electrode. The substrates were cleaned with deionized water, acetone and ethanol in an ultrasonic bath for 30min. The compact layer of TiO2 was prepared by spray pyrolysis of solution (2M acetylacetoneand 0.2M titanium isopropoxide in isoproponal) on the cleaned FTO substrate. After, a layer of mesoporous TiO2 particles with the thickness of 200 nm were prepared by spincoating on the FTO glass. The perovskite precursor solution containing FAI (1 M), MABr (0.2 M), PbI2 (1.1 M) and PbBr2 (0.2 M) in anhydrous DMF:DMSO=4:1 (v/v) was spin-coated at 1000rpm and 4000 rpm for 10s and 30 s, respectively. During the second step, 100 mL of chlorobenzene were dropped at 15 s prior to the end of the program. Then, the perovskite film was annealed at 100 oC for 1 h. Here the concentration of YN1 or YN2 is 30 mg mL-1 in chlorobenzene. The doped spiro-OMeTAD/chlorobenzene (80 mg/mL) solution was prepared with addition of 20 µL Li-TFSI (520 mg Li-TFSI in 1 mL acetonitrile), and 30 µL tertbutylpyridine (tBP), cobalt dopant FK209. As a last step 80 nm of gold top electrode were thermally evaporated under high vacuum. Current-voltage characteristics were measured under 100 mW/cm2 (AM 1.5G illumination) using a Newport solar simulator (model 91160) and a Keithley 2400 source/meter. Incident photon-tocurrent conversion efficiency (IPCE) spectra were recorded using a computer-controlled setup consisting of a Xenon light source (Spectral Products ASB-XE-175), a monochromator (Spectra Products CM110), and a potentiostat (LabJack U6 DAQ board), calibrated by a certified reference solar cell (Fraunhofer ISE). The photocurrent decay was determined by monitoring photocurrent transients by applying a small square-wave modulation to the base light intensity. The Scanning Electron Microscope (SEM) images were investigated using Zeiss MERLIN Field


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Emission SEM. The cells were masked with a black metal mask limiting the active area to 0.126 cm2 and reducing the influence of the scattered light.

Supporting Information Experimental details on synthesis of YN1 and YN2, Nyquist ESI spectra, the cross-section image of the device architecture, Normalized photocurrent decay, Differential scanning calorimetry (DSC) of YN1 and YN2, A simple analysis of relative costs of YN2, Mobility Measurements and Electrochemical impedance measurements.

Acknowledgments We thank the High-Level Talents Foundation of Yunnan University, the Science and Engineering Foundation of Yunnan University (KC1710160), the Swedish Research Council, the Swedish Energy Agency, the Knut and Alice Wallenberg Foundation, the National Natural Science Foundation of China (21120102036, 91233201), the National Basic Research Program of China (973 program, 2014CB239402 for the financial support.

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D-A-D-typed Hole Transport Materials for Efficient Perovskite Solar

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