WOx@PEDOT CoreâShell Nanorods: Hybrid Hole-Transporting

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WOx@PEDOT Core-Shell Nanorods: Hybrid Hole-Transporting Materials for Efficient and Stable Perovskite Solar Cells Ping Liu, Chen Wang, Dong-Ying Zhou, Quan Yuan, Yu Wang, Yao Hu, Dongwei Han, and Lai Feng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00277 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Energy Materials

WOx@PEDOT Core–Shell Nanorods: Hybrid Hole–Transporting Materials for Efficient and Stable Perovskite Solar Cells Ping Liu,a,b Chen Wang,a,b Dongying Zhou,a,b Quan Yuan,a,b Yu Wang,a,b Yao Hu,a,b Dongwei Han,a,b Lai Fenga,b,*

a

Soochow Institute for Energy and Materials InnovationS (SIEMIS), College of Physics,

Optoelectronics and Energy, Soochow University, Suzhou 215006, China b

Jiangsu Key Laboratory of Advanced Carbon Materials and Wearable Energy

Technologies, Soochow University, Suzhou 215006, China

KEYWORDS: core–shell nanostructures, p−i−n perovskite solar cell, interface engineering, hole transporting layer, device stability

1

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ABSTRACT PEDOT–coated WOX nanorodes (NRs) were prepared for the first time by simply stirring WOX nanowires (NWs) with 3,4–ethylenedioxythiophene (EDOT) in aqueous solution. A series of spectroscopic characterizations indicate that the polymerization of EDOT occurrs not only on the surface but also along the [010] planes of WOX NW, resulting in the truncation of long WOX NW to produce WOX@PEDOT NRs with abundant oxygen vacancies. Furthermore, WOX@PEDOT NRs were used to prepare hole transport layer (HTL) in planar p–i–n perovskite solar cells (PeSCs). The WOX@PEDOT–based devices yielded a comparable average power conversion efficiency (PCE) of 12.89% with improved open–circuit voltage (VOC), fill factor (FF) but lower short–circuit current density (JSC), as compared to the devices with conventional PEDOT:PSS (12.88%). The observed device performance is mainly attributed to the better perovskite texture on the WOX@PEDOT HTL, improved energy alignment and suppressed charge recombination at the WOX@PEDOT/perovskite interface as well as lower charge conductivity of the WOX@PEDOT HTL. In addition, the PeSCs based on WOX@PEDOT–doped PEDOT:PSS showed remarkably improved PCEs up to 14.73%, which may be ascrible to the combined merit of WOX@PEDOT NRs and PEDOT:PSS.

More impressively, benefiting from the inherent neutral nature

of WOX@PEDOT NRs, WOX@PEDOT–based devices exhibited obviously improved stability compared to that with PEDOT:PSS HTL. These results thus demonstrate a path towards the development of new hybrid nanostructures for efficient and stable PeSCs.

2


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1. INTRODUCTION In recent years, planar p–i–n type perovskite solar cells (PeSCs) have attracted great interests due to their high record power conversion efficiency (PCE) exceeding 22%1,2 as well as the possibility to be processed at low–temperature,3–5 which allow cost–effective and scalable fabrication of high–performance and flexible PeSCs. However, similar to meso–structured n–i–p PeSCs, p–i–n planar PeSCs also suffer from significant stability issues,6,7 which are induced by not only perovskite layer but also electrode buffer layers. Owing to the low stability of PeSCs, the real applications of PeSCs has been seriously hindered. Thus, currently, numerous efforts have been devoted to improving the stability of PeSCs.8–16 Poly(3,4–ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) is the most frequently used anode buffer layer or hole–transport layer (HTL) in p–i– n PeSCs.17–19 However, the strong acidic nature of PSS component may cause severe corrosion on ITO electrode and result in rapid degradation of device performance.20,21 Alternatively, a variety of transition metal oxides and metal salts, such as NiOx,22–24 Cu–doped NiOx,25,26 CuOx (and Cu2O),27,28 WOx,29 V2O530 and CuSCN,31,32 have been introduced and used as HTL in p–i–n PeSCs, resulting in significantly enhanced device stability, though lower PCEs were obtained sometimes as compared to the PEDOT:PSS–based devices. Among them, WOX is a well–studied functional metal oxide,33 which features widely tunable bandgap (i.e., 0 eV for WO2 and 2.6–3.1 eV for WO3),34−36 high electron 3



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mobility (10−20 cm2·V−1·s−1) and excellent chemical stability.37,38 Benefiting from these advantages, WOX nanostructures have been well developed as HTL in polymer solar cells39,40 and/or as ETL in PeSCs.41–43 However, WOX–based HTL has been rarely reported for PeSCs. In a very recent report, WOX layer prepared using thermal evaporation method in vacuum was used as HTL in PeSC for the first time. Solution processable WOX–HTL is still missing to date, though it would be more compatible with low–cost and large–scale manufacturing process. To seek for possible problems related to the solution processed WOX–HTL, the literature concerning WOX–based ETLs was revisited. According to the literature, the pristine WOX–based ETL might suffer from the following issues: (i) unfavored energy alignment and (ii) inherent charge recombination at the WOX/perovskite interface.44 To circumvent these problems, WO3@TiO2 core– shell nanostructures have been alternatively applied as ETL in n–i–p PeSCs, which results in a better energy alignment as well as suppressed charge recombination at the ETL/perovskite interface.44 It is reasonably to speculate that similar issues may be encountered when using WOX as HTL in PeSCs. To design and prepare a well–performed HTL for p–i–n PeSCs, novel WOX nanostructures with suitable electronic structures and modified surface properties are highly desired. Recently, there are particular interests in organic/inorganic hybrid nanostructures owing to their combined merits such as tunable electrical properties inherited from inorganic nanostructures, modified surface structures and solution–processability 4


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enabled by conducting polymers. Motivated by these advantages, in this work, we present a novel hybrid nanostructure WOX@PEDOT (x=2.5) nanorods (NRs), which can be facilely prepared through a simple reaction of WOX (x=2.72) nanowires (NWs) and 3,4–ethylenedioxythiophene (EDOT) in aqueous solution. Particularly, with the presence of W6+, EDOT undergoes oxidative polymerization, yielding PEDOT not only on the surface but also along the [010] plane of WOX NWs. Meanwhile, the long WOX NW is truncated into short pieces with abundant oxygen vacancies during the polymerization to produce WOX@PEDOT NRs. To demonstrate their structural and compositional advantages, solution–processable WOX@PEDOT NRs were used as HTL and dopants in PEDOT:PSS HTL for p–i–n PeSCs. The device tests showed that comparable PCEs were achieved for WOX@PEDOT and PEDOT:PSS–based PeSCs, respectively. The PeSCs with WOX@PEDOT:PEDOT:PSS yielded even higher PCEs. More importantly, benefiting from the neutral nature of WOX@PEDOT NRs, WOX@PEDOT–based PeSCs exhibited significantly improved stability as compared to those with PEDOT:PSS.

2. EXPERIMENTAL 2.1 Materials Tungsten hexachloride (WCl6, 99.9%) was purchased from Aladdin. Ethanol (>99.7%) was purchased from Sinopharm Chemical Reagent Co. , Ltd. 3,4–ethoxylene dioxy thiophene (EDOT) was purchased from TCI. Methylammonium iodide (MAI, 99.5%) was purchased from Shanghai MaterWin New Materials Co. , Ltd. PbCl2 (99.999%), PbI2 (99.999%) and bathocuproine (BCP) were purchased from Sigma Aldrich. 5



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Anhydrous N,N–Dimethylformamide (99.9%), and chlorobenzene (99.9%) were purchased from Alfa Aesar. PC61BM was manufactured by Solarmer Materials Inc. 2.2 Synthesis of WOX@PEDOT NRs WOX nanowires (NWs) were prepared using a previously reported solvothermal method.45 Briefly, tungsten hexachloride (WCl6, 250 mg) was dissolved in 20 mL ethanol and stirred at room temperature for 20 minutes to obtain a pale yellow solution. Then, the resulting solution was transferred to a Teflon–lined stainless–steel autoclave and heated at 160 ℃ for 24 h. After cooling the reaction mixture to room temperature, the product was collected by centrifugation and washed repeatedly with water and ethanol, followed by vacuum drying at 45 ℃ overnight. To prepare WOX@PEDOT NRs, 35 mg WOX NWs were dispersed in 1 mL deionized water under vigorous stirring at room temperature, and 16 µL 3,4– ethylenedioxythiophene (EDOT) was added dropwise to this solution. The mixture was stirred continuously for ca. 30 days and the solution color gradually changed from blue to dark blue (see Fig. S1 in SI). The reaction mixture was allowed to stand overnight and a small amount of precipitate could be observed. Then, the dark–blue and homogeneous upper layer containing WOX@PEDOT NRs was carefully collected by decanting. Finally, WOX@PEDOT NRs were obtained by a freeze–dry processing. 2.3 Physical characterizations

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Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images were obtained on a Hitachi SU8010 and a Hitachi HT7700 instrument, respectively. High–resolution TEM (HR–TEM) image, high–angle annular dark–field scanning TEM (HAADF–STEM) image, HAADF–STEM energy–dispersive

X–ray spectroscopy (HAADF–STEM–EDS)

maps and

selective area electron diffraction (SAED) data were collected on a FEI Tecnai F20 instrument at an acceleration voltage of 200 kV. UV–Vis absorption and transmission

spectra

were

recorded

using

a

Shimadzu

UV2600

spectrophotometer. Steady–state photoluminescence (PL) spectra were obtained with fluorescence spectrophotometer (FLS980) using an excitation wavelength of 468 nm. Time–resolved PL (TRPL) spectra were measured with Lifespec II (Edinburgh Instrument, UK) with picosecond light pulser (Hamamatsu) using a pump light wavelength of 477 nm and probe wavelength of 760 nm. X–ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements were performed on an Escalab 250Xi (Thermo Fisher) spectrometer with a source of fixed–energy radiation (i.e., a monochromated Al Kα excitation source with a photon energy of 2000 eV for XPS and a He I radiation (21.22 eV) from an unfiltered gas discharge lamp for UPS). X–ray diffraction (XRD) and Fourier transform infrared (FT–IR) spectra were recorded using a Bruker D8 Advance instrument with a Cu Kα source (λ = 0.154 nm) and a Bruker Tensor 27 instrument, respectively. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were conducted on a SII Nano 7



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Technology Inc TG–DTA instrument (TG/DTA 7300) at a heating rate of 10 o

C/min from room temperature to 800 oC under air atmosphere. Atomic force

microscopy (AFM) images were obtained on a Dimension Icon AFM (Bruker) in tapping mode. 2.4 Device fabrication and characterizations P–i–n planar perovskite solar cells were fabricated on patterned ITO glass substrates with a sheet resistance of ~15 Ω/sq. The ITO glass was cleaned according to previously reported method.46 To prepare the hole–transport layer (HTL), poly(3,4–ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) solution was spin–coated at 2000 rpm for 40 s on the ITO substrate, followed by a thermal annealing at 150 ℃ for 15 min. Alternatively, the as–prepared WOX@PEDOT aqueous solution (10 mg/mL) was spin–coated on ITO substrate at 2000 rpm for 40 s, followed by a similar thermal annealing process. The thickness of WOX@PEDOT layer was controlled by single, double and quadruple spin–coating. To prepare WOX@PEDOT–doped PEDOT:PSS HTL, the aqueous solution of WOX@PEDOT NRs was blended into the PEDOT:PSS solution with a volume ratio of 1:1, and then the mixture was spin–coated on ITO substrate and thermally annealed in the same way. The doping content of WOX@PEDOT NRs in PEDOT:PSS was controlled by varying the concentration of WOX@PEDOT aqueous solution (i.e., 1, 5, 10 mg/mL). After the deposition of HTL, the ITO glass with HTL was transferred into N2–filled glove box. A CH3NH3PbI3–XClX perovskite layer was prepared by spin–coating (4000 rpm for 40 s) a 8


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dimethylformamide (DMF) solution (500 µL) containing 100.6 mg CH3NH3I, 50.6 mg PbCl2, 50.2 mg PbI2 (in a molar ratio of 5.75:1.6:1) on the HTL, followed by a thermal annealing at 80 ℃ for 120 min. After that, electron transporting layers (ETL) were deposited on the perovskite layer by spin–coating a chlorobenzene solution of PC61BM (20 mg/mL) at 2000 rpm for 40 s and an ethanol solution of BCP (0.5 mg/mL) at 2000 rpm for 40 s, subsequently. Finally, Ag electrode (100 nm) was thermally deposited onto the ETL under the vacuum of 1 × 10–4 Torr using a shadow mask, defining a device area of 4 mm2. The space charge limited current (SCLC) device was fabricated with a configuration of ITO/HTL/perovskite/MoO3 (10 nm)/Ag. The current density versus voltage (J–V) characteristics of the devices were measured using a Keithley 2400 SourceMeter under illumination with a light intensity of 100 mW cm–2 provided by an Oriel 150 W solar simulator with an AM 1.5G filter. The light source was calibrated using a standard silicon solar cell before test. The external quantum efficiency (EQE) was measured by a solar cell spectral response measurement system QE–R3011 (Enli Technology Co., Ltd.).

3. RESULTS AND DISCUSSION WOX NWs were synthesized according to the literature reported surfactant– free alcohothermal method.45 As shown in SEM images (Fig. 1a, S2), sea urchin– like nanostructures were obtained, which are composed by a large amount of WOX NWs entangled with each other. The length of WOX NWs is about several hundreds of nanometers and their diameter is ca. 3–10 nm. The blue color of the 9



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as–synthesized WOX NWs suggests their oxygen–deficient nature. All these characteristics are fully consistent with the literature report.45 Core–shell WOX@PEDOT NRs were prepared by mixing the as–synthesized WOX NWs and EDOT in an aqueous solution. After constantly stirring for ca. 30 days at room temperature, the solution color changed from blue to dark blue, indicating the oxidative polymerization of EDOT to PEDOT. As shown in the SEM and TEM images (Fig. 1b, 1c), polymer coated rode–like nanostructures can be observed. Their length is ca. 10–20 nm, much shorter than that of WOX NWs. These results indicate that, the long WOX NWs were truncated into short NRs along with the polymerization of EDOT. As reference, WOX NWs stirred under the same conditions but without EDOT gave rise to slightly less entangled NWs and no short NR can be observed (see Fig. S3). Thus, it appears that the polymerization of EDOT plays an important role in truncating the long WOX NWs. The structural feature of WOX@PEDOT NRs was further characterized using high–resolution TEM. As shown in Fig. 1d, the core–shell structure of WOX@PEDOT NR can be clearly seen. The core is identified as a WOX NR with a length of ca. ~15 nm and a diameter of 3–4 nm, which shows lattice fringes with a d–spacing of 0.38 nm, matching well with the [010] planes of monoclinic phase of WO2.72.45 The SEAD patterns of WOX NRs are shown in the inset of Fig. 1d. The spot–like patterns reveal that the encapsulated WOX NR is crystalline in nature, in good agreement with the HR– TEM observations. The amorphous shell component can be assigned to PEDOT. EDS element mapping confirms the presence of C, S species, which are major compositions 10


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of PEDOT, over the WOX NR surface, indicative of the PEDOT coating. These observations demonstrate that the formation of WOX@PEDOT NRs starts from the polymerization of EDOT on the surface and also along the [010] planes of WOX NWs, which result in not only PEDOT coating but also truncation of the WOX NWs to produce WOX@PEDOT NRs (see Fig. S4 for schematic illustration of the formation process).

Figure 1. SEM images of (a) pristine WOX and (b) WOX@PEDOT NRs. (c) TEM and (d) HR–TEM images of WOX@PEDOT NRs. Inset shows selected area electron diffraction patterns (SAED). (e) STEM–HAADF and corresponding EDS elemental mapping images.

11



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The hybrid compositions of WOX@PEDOT NRs were further investigated by means of XPS. Fig. 2 presents the survey XPS spectra of pristine WOX NWs and WOX@PEDOT NRs along with the corresponding HR W 4f spectra. As expected, in the spectrum of pristine WOX NWs, there are only two peaks corresponding to W and O elements, respectively. The W4f spectrum can be divided into four peaks. The major and mirror peaks (i.e., 36.0/38.1 eV and 36.8/34.8 eV) with a ratio of ca. 4.2:1 can be assigned to the W6+ and W5+oxidation states, respectively,39 indicating a formula of WO2.90 rather than WO2.72 reported by literature,45 which may be attributed to the partial surface oxidation of WOX to WO3 during the XPS sample preparation.34 For comparison, the survey XPS spectrum of WOX@PEDOT NRs displays multiple peaks corresponding to W, O, C and S elements, in good agreement with the EDS results. The W4f spectrum can be well fitted by only two peaks at 34.7 and 36.7 eV, respectively, corresponding to solely W5+ oxidation state. The predominant W5+ species should result from the reduction of W6+ during the oxidative polymerization of EDOT, which also yields a large amount of oxygen vacancies in the encapsulated WOX NRs. Additionally, XRD patterns of the pristine WOX NWs and WOX@PEDOT NRs are provided in Fig. S5. Both samples similarly show a predominant diffraction peak at 23.4°, corresponding to a d–spacing of 3.80 Å. This value is very close to the [010] plane spacing (i.e., 3.7–3.8 Å) in monoclinic phase of WO2.7245,47 and also agrees well with the HR–TEM observations. The similar XRD patterns suggest that the crystalline phase of WOX is maintained despite of the significant reduction of W6+ to W5+. This result demonstrates

12


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again the nonstoichiometric property of WOX, which refers to the fact that the lattice of WOX can withstand a considerable amount of oxygen deficiency.

Figure 2. XPS survey and W4f spectra of (a,b) WOX and (c,d) WOX@PEDOT. (Note: * in (a) is originated from the carbon that was added as internal reference.)

FT–IR spectrum of WOX@PEDOT NRs is shown in Fig. S6, and compared with that of pristine WOX. It is clearly seen that the pristine WOX NWs display only a few absorption bands in the region of 1000–500 cm–1, which correspond to the W–O (848 cm–1) species and the stretching vibrations of the bridging oxygen atoms O–W–O (780 cm–1).45 For comparison, WOX@PEDOT NRs show multiple additional absorption bands over the range of 2000~500 cm–1, which are typical for PEDOT. Particularly, the bands around 1515, 1478 and 1359 cm–1 can be assigned to conjugative C=C/C–C vibrations. The weak bands around 950 cm–1 and those at 1220, 1145, 1092, 1065 cm–1 can be attributed to the stretching vibrations of the C–S–C bond in thiophene ring and 13



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C–O–C bending vibration in ethylenedioxy group, respectively.48,49 These results also confirm the formation of PEDOT conformal coating on the surface of WOX NRs. To estimate the contents of the core and shell components, TGA measurement was performed under air atmosphere. As shown in Fig. S7, the initial weight loss below 200 o

C (ca. 4.6%) can be attributed to the removal of absorbed water and solvent. Then, the

PEDOT shell undergoes degradation in the range of 200–350 oC, resulting in a major weight loss of 16.8%. Thus, a weight ratio of ca. 1:5 is calculated for PEDOT/WOX. The slight weight increase (ca. 3.8%) in the temperature range of 350–800 oC might be ascribed to the oxidation of WO2.5 to WO3 in the air, which provides another evidence for the proposed formation of oxygen vacancies in the encapsulated WOX NRs. It has been well established that appropriate reduction of the oxygen content in WO3 results in varied electronic band structure and significantly increased conductivity.50 To estimate the electronic band structure of WOX@PEDOT NRs, UPS studies were performed. The secondary electron cut−off and valance region photoemission spectra of WOX@PEDOT NRs are shown in Fig. 3, and compared with those of pristine WOX NWs and PEDOT:PSS. Accordingly, the work function (WF) of WOX@PEDOT NRs is calculated to be 5.2 eV, higher than those of WOX NWs (4.78 eV) and PEDOT:PSS (5.1 eV). As a result, an improved energy level alignment is achieved at the HTL/perovskite interface, indicating a reduced energy loss during the hole–transport at this interface. The improved WF might be ascribed to the combined effect of PEDOT coating and the presence of rich oxygen vacancies in the encapsulated WOX NRs. 14


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Additionally, the VB edge level of WOX@PEDOT NRs is calculated to be –7.3 eV according to the valance region photoemission spectrum, in good agreement with that reported for WO3 in the literature.39

Figure 3. (a) Secondary electron cutoff and valance band regions in UPS spectra of PEDOT:PSS, WOX and WOX@PEDOT. (b) Schematic energy level diagram of different HTLs in perovskite solar cell relative to vacuum. Based on the above results, we fabricated planar p−i−n PeSCs with a configuration of ITO/HTL/CH3NH3PbI3−xClx/PC61BM/BCP/Ag (See Fig. S8) to study the hole– transporting or collection performance of HTL, in which four different HTLs (i.e., WOX@PEDOT NRs and WOX@PEDOT–doped PEDOT:PSS, PEDOT:PSS and pristine WOX) were employed under the optimized conditions (See Fig. S9, S10 and Table S1, S2 for optimization details). Typically, the cross–sectional SEM image of the WOX@PEDOT–based PeSC is shown Fig. S11. It is clearly seen that the WOX@PEDOT HTL was deposited to around 20 nm in thickness, which was confirmed by AFM and ellipsometric measurements (see Fig. S12, S13). Fig. 4a displays the typical J–V characteristics for the devices with different HTLs. The extracted 15



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photovoltaic parameters are summarized in Table 1. The PEDOT:PSS–based devices yielded an average PCE of 12.88% with a VOC of 0.94 V, a FF of 0.69 and a JSC of 19.68 mA cm–2, which are reasonable values compared with literature using similar fabrication methods.51 Replacing PEDOT:PSS with WOX@PEDOT as HTL resulted in a comparable PCE of 12.89% with remarkably improved VOC (0.97 V) and FF (0.75) but lower JSC (17.25 mA cm–2). Using WOX@PEDOT:PEDOT:PSS as HTL afforded a higher average PCE of 14.30% with all parameters slightly improved. For the best– performing device, a PCE of 14.73% was achieved. In comparison, the devices based on pristine WOX NWs led to a much lower PCE (1.81%). These results indicate that WOX@PEDOT NRs are much better hole–transporting materials than pristine WOX NWs, which can be used as HTL to replace conventional PEDOT:PSS or as dopants to enhance the performance of PEDOT:PSS. To verify the JSC obtained in J–V tests, EQE measurements were performed for the PeSCs with different HTLs. It is clearly seen that all EQE spectra feature a board band in the spectral region of 330−800 nm (see Fig. 4b). The

JSC

is

calculated

to

be

18.96

mA

cm−2

for

the

device

with

WOX@PEDOT:PEDOT:PSS, 16.89 mA cm−2 for that with WOX@PEDOT NRs and 18.60 mA cm−2 for the reference device with PEDOT:PSS, all consistent with the results of J−V tests within 10% error. To

investigate

the

reproducibility

of

the

WOX@PEDOT

and

WOX@PEDOT:PEDOT:PSS–based devices, 20–25 individual devices were fabricated for each type of PeSC. The histograms of their PCEs as well as all parameters are plotted and presented in Fig. S14, S15. The devices with WOX@PEDOT NRs and 16


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WOX@PEDOT:PEDOT:PSS both display smaller distributions of PCEs and parameters, as compared to those of PEDOT:PSS–based devices. These results indicate that the solution–processed WOX@PEDOT layer and WOX@PEDO–T:PEDOT:PSS layer could be potential candidates as the HTL for planar p–i–n PeSC. Additionally，to evaluate the authentic performance of the PeCSs with different HTLs, steady–state efficiencies corresponding to the point of maximum power output were measured and illustrated in Figure 4c. For the device with WOX@PEDOT:PEDOT:PSS HTL, efficiency of 14.57% was achieved, while efficiencies of 13.26% and 13.34% were obtained for the devices with WOX@PEDOT and PEDOT:PSS HTLs, respectively. All these results are in good agreement with those indicated by J–V tests.

Figure 4. (a) J−V curves and (b) EQE spectra of PeSCs with different HTLs. (c) Steady–state photocurrent and efficiency measured at the maximum power point (0.86 V, 0.84 V and 0.86 V for PeSCs with different HTLs, respectively). Table 1. Photovoltaic parametersa of PeSCs with different HTLs. VOC (V)

JSC (mA cm–2 )

FF (%)

PCE (%) b

PEDOT:PSS

0.94

19.68

69.5

12.88 (13.26)

3.3

380

WOX NWs

0.57

6.57

48.9

1.81 (2.02)

40.9

110

HTL

Rs (Ω cm–2 ) R sh (Ω cm–2)

17



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WOX @PEDOT NRs

0.97

17.58

75.4

12.89 (13.24)

4.0

1707

WOX @PEDOT: PEDOT:PSS

0.97

20.76

70.9

14.30 (14.73)

3.2

406

a

Averaged over 20–25 individual devices. b The best PCE value is given in bracket.

To better understand the photovoltaic performances of the WOX@PEDOT based HTLs, a series of characterizations were performed. Firstly, morphologies of these HTLs were studied using AFM technique. As shown in Fig. 5a–d, the pristine ITO shows a uniform surface with a root–mean–square (RMS) roughness of 2.92 nm. After deposition of a PEDOT:PSS layer, RMS decreased to 1.41 nm, in good agreement with the reference report.52 Doping the PEDOT:PSS layer with WOX@PEDOT NRs slightly increases the RMS to 3.46 nm. However, WOX@PEDOT layer exhibits a much rougher surface with a RMS up to 12.90 nm, which might result in an enlarged interfacial contact between the HTL and perovskite layer and hence facilitate the hole transport between them.53,54 The transparency of these HTLs was examined with transmittance spectra. As shown in Fig. 5e, ITO/PEDOT:PSS film present higher transparency with an average transmittance up to 87% owing to lower roughness relative to others.55 For comparison,

ITO/WOX@PEDOT:PEDOT:PSS

and

pristine

ITO

show

lower

transparency due to the slightly higher surface roughness, while ITO/WOX@PEDOT film displays further reduced transparency with an average transmittance of 79%, which might be attributed to its rough surface. The electrical conductivities of these HTL were estimated by measuring dark J–V curves for the devices with a structure of ITO/HTL/MoO3/Ag (Fig. 5f). WOX@PEDOT doped PEDOT:PSS shows the highest 18


Page 19 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conductivity, followed by PEDOT:PSS and WOX@PEDOT. These results agree well with the Rs values calculated from the light J–V.

Figure

5

(a,b,c,d)

AFM

images

of

pristine

ITO,

ITO/PEDOT:PSS,

ITO/WOX@PEDOT and ITO/WOX@PEDOT:PEDOT:PSS. (Note: A scale bar of 20 nm for a,b,d and a scale bar of 100 nm for c.) (e) The transmittance spectra of different HTLs on ITO substrate. (f) Dark J–V curves for ITO/HTL/MoO3/ Ag devices.

19



Page 20 of 40

Moreover, the morphologies of the perovskite layers grown on different HTLs were studied by means of SEM. As shown in Fig. 6a, obvious pine holes can be observed for the perovskite layer grown on PEDOT:PSS HTL. In contrast, compact perovskite layers with higher coverage were obtained on WOX@PEDOT and WOX@PEDOT:PEDOT:PSS–based

HTLs

(see

Fig.

6b,c),

indicating

that

WOX@PEDOT NRs have certain effect in guiding the growth of perovskite layer. These pin–hole free perovskite layers are believed to be advantageous for mitigating short– circuiting, charge leaking, and large series resistance.56 The quality of the perovskite layers was examined by XRD measurements. As shown in Fig. 6d, the perovskite layers grown on different HTLs display similar XRD patterns, revealing diffraction peaks corresponding to the [110], and [220] crystallographic planes of the MAPbI3−XClX perovskite.51 Nevertheless, the perovskite layer grown on WOX@PEDOT or WOX@PEDOT:PEDOT:PSS HTL shows more evident [110] peak, suggesting that this perovskite layer grows preferentially along the [110] plane with improved crystallinity.29,57 Fig. 6e illustrates the UV–Vis spectra of the perovskite layers on different HTLs. All HTL/perovskite films show identical feature absorptions. Among them, WOX@PEDOT/perovskite and WOX@PEDOT:PEDOT:PSS/perovskite films show higher absorbance than PEDOT:PSS/perovskite film in the spectral range of 350– 750 nm, though the transmittance of the underlayer (i.e., WOX@PEDOT and WOX@PEDOT:PEDOT:PSS) is lower than that of PEDOT:PSS. The higher absorbance might be attributed to the better film texture of the perovskite layers on WOX@PEDOT

20


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and WOX@PEDOT:PEDOT:PSS HTLs (i.e., being more compact and better crystalized),58,59 respectively, as suggested above.

Figure 6. (a,b,c) SEM images for the perovskite layers grown on different HTLs. (d) XRD patterns of the perovskite layers grown on different HTLs. (e) UV–Vis absorption spectra

of

the

HTL/perovskite

films.

(f)

Dark

J–V

curves

for

ITO/HTL/perovskite/MoO3(10 nm)/Ag devices. Furthermore, the role of WOX@PEDOT–based HTLs in facilitating the hole transport from perovskite layer to ITO was evaluated using the space charge limited current (SCLC) method.60–62 The dark J–V curves measured for the hole–only devices with a configuration of ITO/HTL/perovskite/MoO3/Ag are provided in Fig. 6f and fitted using the Mott–Gurney law.63,64 The vertical hole mobility (µo) is calculated to be 2.66×10–4 cm2 V–1 s–1 for WOX@PEDOT:PEDOT:PSS–based device, higher than that (1.80×10–4 cm2 V–1 s–1) of PEDOT:PSS–based device, while a lower value of 1.14×10–4 21



Page 22 of 40

cm2 V–1 s–1 is for WOX@PEDOT–based device. The lower µo of the WOX@PEDOT– based device might be due to the lower conductivity of WOX@PEDOT HTL relative to others, as above mentioned. To evaluate the charge transfer between perovskite layer and different HTLs, steady–state photoluminescence (PL) spectra were measured by using samples with a configuration of glass/with or without HTL/perovskite. As shown in Fig. 7a, significant quenching effect was observed for all HTLs. This indicates that all these HTLs can efficiently facilitate the charge separation at the HTL/perovskite interface. To confirm the charge transport processes, time–resolved (TR) PL measurements were conducted. The PL decay curves consisting of fast and slow decay processes are fitted with a biexpotenential decay function (see Fig. 7b) and the results are summarized in Table 2. It has been well established that the fast decay process is related to the quenching of free carriers in the perovskite layer through transport to ETL or HTL,65,66 and can be used to identify

the

quenching

effect

of

the

interfacial

layer.

In

the

case

of

WOX@PEDOT/perovskite film, the fast decay time (τ1) of 3.36 with a fraction (f1) of 17.3%

was

detected.

In

comparison,

PEDOT:PSS/perovskite

and

WOX@PEDOT:PEDOT: PSS/perovskite films exhibited shorter τ1 of 2.53 and 1.77 ns with higher f1 of 29% and 41.1%, respectively. These results can be interpreted that using WOX@PEDOT NRs to replace PEDOT:PSS induces slightly lower charge collection efficiency probably owing to the slightly lower conductivity of WOX@PEDOT NRs, despite of the more favorable energy alignment at the WOX@PEDOT/perovskite interface. As for WOX@PEDOT doped PEDOT:PSS HTL, 22


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the shorter τ1 and higher f1 indicate enhanced charge collection, which might be attributed to the improved conductivity as well as the more favorable energy alignment at the HTL/perovskite interface.

Figure 7. (a) Steady–state and (b) time–resolved photoluminescence spectra of WOX@PEDOT/perovskite,

WOX@PEDOT:PEDOT:PSS/perovskite

and

PEDOT:PSS/perovskite. Table

2. Lifetime and

weight fractions obtained from the

time–resolved

photoluminescence spectra by fitting with bi–exponential decay function. HTL

τ1(ns)

f1(%)

τ2(ns)

f2(%)

PEDOT:PSS

2.53

29.0

12.02

71.0

WOX@PEDOT

3.36

17.3

8.82

82.7

WOX@PEDOT:PEDOT:PSS 1.77

41.1

9.22

58.9

It is noteworthy that the VOC is evidently improved from 0.94 to 0.97 V, when employing WOX@PEDOT NRs as HTL or dopants in PEDOT:PSS HTL. The improved VOC

may

be

readily

attributed

to

the

better

energy

alignment

at

the 23



Page 24 of 40

WOX@PEDOT/perovskite interface as above mentioned, which not only minimizes the energy loss during hole–transport but also enlarges the built–in potential across the device. Additionally, as the role of shunt–induced charge recombination in limiting VOC has been well identified in the previous works,67,68 the shunt recombination in the devices with different HTLs was evaluated using dark J–V. As shown in Fig. 8a, the current density at reversed bias is evidently reduced when WOX@PEDOT NRs and WOX@PEDOT:PEDOT:PSS are used as HTL to replace PEDOT:PSS, suggesting smaller leakage current or reduced shunt recombination current in the WOX@PEDOT and WOX@PEDOT:PEDOT:PSS–based devices. These results are in good agreement with the shunt resistance (Rsh) calculated from the light J–V curves, which suggests higher Rsh for the WOX@PEDOT and WOX@PEDOT:PEDOT:PSS–based devices whereas lower Rsh for PEDOT:PSS–based devices. The smaller leakage current as well as the higher Rsh in WOX@PEDOT and WOX@PEDOT:PEDOT:PSS–based devices can be readily ascribed to the more compact perovskite layers grown on the WOX@PEDOT HTL and doped HTL, as discussed above, which results in reduced short–circuit contact between HTL and ETL. To further probe the charge recombination events in these PeSCs, the dependence of VOC on the light intensity was measured and plotted in Fig. 8b. The WOX@PEDOT– based device exhibits a weak dependence of VOC on the light intensity with a slope of 1.01 kT/q, indicating that the radiative recombination of free carriers is predominate.69 In comparison, PEDOT:PSS–based device shows a stronger dependence with a slightly higher slope of 1.22 kT/q, indicating more significant non–radiative recombination at 24


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the HTL/perovskite interface.69 These results hence suggest that WOX@PEDOT HTL can more efficiently reduce interfacial recombination than PEDOT:PSS, which obviously contributes to the improvement in VOC. In addition, to evaluate the J–V hysteresis of the WOX@PEDOT and WOX@PEDOT:PEDOT:PSS–based devices, the J–V curves of typical devices were recorded using both the forward and reverse scan mode (with a scan rate of 0.1 V s–1), and compared with that of PEDOT:PSS–based device. As shown in Fig. S16, all devices demonstrate negligible J–V hysteresis. This result reveals the critical role of WOX@PEDOT–based HTLs in suppressing the J–V hysteresis, which validates the aforementioned advantages of efficient charge–transfer and suppressed charge recombination at the HTL/perovskite interface.

Figure 8. (a) Dark J–V curves of the devices with different HTLs. (b) Illumination dependence of open circuit voltage (VOC) of WOX@PEDOT and PEDOT:PSS–based devices. Benefiting from the neutral nature of WOX@PEDOT NRs (see Fig. S17), it is predictable that WOX@PEDOT–based HTL gives rise to negligible corrosion on ITO electrode and therefore enhanced device stability. To verify this prediction, the 25



Page 26 of 40

stabilities of the devices with different HTL were tested under N2 atmosphere. As shown in Fig. 9a, WOX@PEDOT–based devices show remarkably improved stability as compared to the PEDOT:PSS and WOX@PED–OT:PEDOT:PSS–based devices. Particularly, the PCE of the WOX@PEDOT–based device slightly increases during the beginning 10 d of storage in N2 and then slightly decreases to ca. 95% of the initial value after 35 d of storage in N2. This trend is similar to that observed for the devices with V2O5/PEDOT and Al:ZnO–based interfacial layers.70,71 For comparison, PEDOT:PSS and WOX@PEDOT:PEDOT:PSS–based devices decayed to 77% and 85% of their initial PCEs, respectively, after the same storage period, which is mainly induced by the gradual decrease in JSC and FF (see Fig. 9b–d). However, it is clearly seen that VOC is nearly constant for all devices during the storage time. These results are similar to those reported for Cu:NiOX–based devices.26 The faster PCE decay has been attributed to the acidic characteristic of PEDOT:PSS, as above mentioned, which causes corrosion on the ITO electrode and leads to decreased conductivity of ITO/HTL.

26


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Figure 9. (a) Normalized PCE along with (b,c,d) normalized parameters including JSC, VOC and FF of the devices with different HTLs as a function of storage time in N2 atmosphere. 4. CONCLUSIONS In summary, we report a facile and scalable method to prepare hybrid WOX@PEDOT NRs by mixing WOX NWs and EDOT in aqueous solution under constant stirring at room temperature. It was observed for the first time that the long WOX NW was truncated owing to the polymerization of EDOT occurring not only on the surface but also along the [010] planes of WOX NW to produce WOX@PEDOT NRs with abundant oxygen vacancies. When employing WOX@PEDOT NRs as HTL in solution–processed planar p–i–n PeSC, the devices exhibited superior stability as well as a comparable PCE of 12.89% with improved VOC, FF but lower JSC, as compared to those of conventional PEDOT:PSS. The observed device performance 27



Page 28 of 40

can be accounted for by the better perovskite texture on the WOX@PEDOT HTL, improved energy alignment and suppressed charge recombination at the WOX@PEDOT interface, inherent neutral nature and lower charge conductivity of the WOX@PEDOT HTL.

Additionally, using WOX@PEDOT NRs doped

PEDOT:PSS as HTL led to higher PCEs up to 14.73% with all parameters improved, which might be attributed to the combined merits of WOX@PEDOT NRs and PEDOT:PSS. This work thus demonstrates the path towards the development of new hybrid nanostructures for efficient and stable PeSCs.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: ******.

Photos of the samples in water solution and their PH values, SEM of as–prepared WOX NWs and those obtained after constant stirring, Schematic illustration of synthesis mechanism and device architecture, XRD patterns, FT–IR and TGA spectra of the samples, J–V curves and photovoltaic parameters of PeSCs including WOX@PEDOT HTL with different thickness and WOX@PEDOT:PEDOT:PSS with different doping content, Cross–sectional SEM image of typical device, Ellipsometric raw data and AFM height image for ITO/WOX@PEDOT film, PCE distributions for PeSCs with different HTLs and histogram of PCE, VOC, JSC, FF, J−V curves of PeSCs with both forward and reverse scan modes. 28


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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported in part by the Natural Science Foundation of China (51372158, 51772195 and 61705150), the Natural Science Foundation of Jiangsu Province (BK20160325) and Suzhou Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies.

29



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WOx@PEDOT CoreâShell Nanorods: Hybrid Hole-Transporting

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