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Optimization of the energy level alignment between the photoactive layer and the cathode contact utilizing solution-processed hafnium acetylacetonate as buffer layer for efficient polymer solar cells Lu Yu, Qiuxiang Li, Zhenzhen Shi, Hao Liu, Yaping Wang, Fuzhi Wang, Bing Zhang, Songyuan Dai, Jun Lin, and Zhan'ao Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 18, 2015

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ACS Applied Materials & Interfaces

Optimization of the energy level alignment between the photoactive

layer

and

the

cathode

contact

utilizing

solution-processed hafnium acetylacetonate as buffer layer for efficient polymer solar cells

Lu Yu, Qiuxiang Li, Zhenzhen Shi, Hao Liu, Yaping Wang, Fuzhi Wang*, Bing Zhang, Songyuan Dai, Jun Lin*, Zhan’ao Tan*

State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, Beijing Key Laboratory of Novel Thin Film Solar Cells, North China Electric Power University, Beijing 102206, China.

*Corresponding Authors E-mail: [email protected] (Z. A. Tan); [email protected] (J. Lin); [email protected] (F. Z. Wang)

Abstract Inserting appropriate interfacial buffer layer between the photoactive layer and contact electrodes makes great impact on the performance of polymer solar cells (PSCs). Ideal interfacial buffer layers could minimize the interfacial traps and the interfacial barriers caused by the incompatibility between the photoactive layer and electrodes. In this work, we utilized solution–processed hafnium(IV) acetylacetonate

ACS Paragon Plus Environment


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(Hf(acac)4) as an effective cathode buffer layer (CBL) in PSCs to optimize the energy level alignment between the photoactive layer and the cathode contact, with the short-circuit current density (Jsc), open-circuit voltage (Voc) and fill factor (FF) all simultaneously improved with Hf(acac)4 CBL, leading to enhanced power conversion efficiencies (PCEs). Ultraviolet photoemission spectroscopy (UPS) and scanning Kelvin probe microscopy (SKPM) were performed to confirm that the interfacial dipoles were formed with the same orientation direction as the build-in potential between the photoactive layer and Hf(acac)4 CBL, benefiting the exciton separation and electron transport/extraction. In addition, the optical characteristics and surface morphology of the Hf(acac)4 CBL were also investigated.

Keywords:

energy level alignment; cathode buffer layer; polymer solar cells;

hafnium acetylacetonate; interfacial dipoles

1. Introduction Polymer solar cells (PSCs) have attracted growing interest over the last decades due to their unique advantages of light-weight, low-cost, and flexible manufacturing. Great efforts have been made to break through the power conversion efficiency (PCE) bottleneck. However, there are still challenges to achieve further efficiency improvements in PSCs based on conjugated polymers and fullerene derivatives.1-4 device performance of PSCs is mainly governed by the following four aspects: light absorption proprieties of the photoactive materials, charges separation and transfer


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characteristics between the donor and acceptor, charges transport efficiencies in the photoactive layer, and charges extraction and collection of the electrode. From the perspective of photoactive materials, developing and designing new donors and acceptors for the active layer and optimizing the morphology are undisputed the most original and common driving forces among all of the approaches to improve the PCEs.5-12 Apart from this, from the perspective of photovoltaic devices, applying interfacial layers to optimize the energy level alignment of the device, adjust the light distribution within the photoactive layer and decrease the interfacial defects has also proven to be critical in achieving high performance PSCs.13-17 At the interface between the photoactive layer and the metal electrode, there the interfacial contact resistance, energy band barrier and surface defects due to the incompatibility between the organic semiconductors and electrodes, which are the main factors of PSC energy loss for a given donor-acceptor system.18 Introducing appropriate electrode buffer layers may effectively align the energy levels, minimize the energy band barrier and maximize the charge transport efficiency. Considering the work-function of the metal cathode and the photoactive layer, low work-function cathode buffer layers (CBLs) are generally desired to achieve high performance PSCs with enhanced electron collection, such as thermally evaporated Ca or Ba.19 However, the vacuum deposition technique is not cost-effective for the large-scale fabrication the low work-function metals are vulnerable to be oxidized in ambient air, leading to the instability of the devices.20 Therefore, a series of solution-processed cathode materials such as metal oxides titanium oxide (TiOx),21-22 zinc oxide (ZnO),23-27



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small molecules and polymer materials,28-31 transition metal chelates,32-35 and a of self-assembled materials have been successfully introduced as the CBLs.36 our group used an alcohol-soluble titanium chelate, titanium diisopropoxide bis(2,4-pentanedionate) (TIPD), as the cathode buffer layer to be spin-coated on ITO the inverted PSCs, which benefited the intimate contact with the photoactive layer and achieved better aligned energy levels, leading to higher charge collection efficiencies on both electrodes and the improvement of the device PCEs.37 We also employed the alcohol-soluble zirconium(IV) acetylacetonate (ZrAcac) into the conventional PSCs the CBL to effectively decrease the series resistance (Rs) and enhance light harvest, improving photovoltaic performance.35 Inspired by our previous work on the successful employment of TIPD and as the CBLs, we speculate that the alcohol-soluble hafnium(Ⅳ) acetylacetonate (Hf(acac)4, Figure 1), another kind of titanium group acetylacetonate chelate bearing four delocalized β-diketonate ligands, should also have relatively low work-function work as the CBL in PSCs. Therefore, in this study, we introduced the Hf(acac)4 as the CBL simply by spin-coating its ethanol solution on the photoactive layer, without any thermal annealing or other post-treatment. Obviously, our approach of Hf(acac)4 CBL deposition is environment-friendly and suitable for mass production. To check the suitability of Hf(acac)4 CBL, different kinds of donor-acceptor materials were for device fabrication. After the incorporation of the Hf(acac)4 CBL, the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) were increased simultaneously, resulting in the enhanced overall performance. The PCE of the PSCs


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based

on

poly(((4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl ) bis(trimethyl))-co-(5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c’]dithiophene-4,8-dione)) (PBDTBDD): [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) increased from 5.84% to 7.38% with Hf(acac)4 CBL. For PSCs based on poly(3-hexylthiophene) (P3HT): PC60BM, the PCE was increased by 36.7% with Hf(acac)4 CBL, from 2.94% to 4.02%. In addition, the incorporation of the Hf(acac)4 CBL could also protect the photoactive layer from the damage of the hot metal atoms during cathode evaporation as it has been demonstrated that the hot Al atoms could diffuse into the photoactive layer during thermal evaporation.22, 25, 38

2. Experimental Section 2.1 Materials The ITO glass with a sheet resistance of 10Ω/sq was purchased from the CSG Holding Co., LTD (China). PEDOT:PSS (Clevious P VP AI 4083) was provided by the H. C. Stark company. The P3HT and PC60BM were purchased from the Rieke Metals Inc. and Nano-C Inc. respectively. The PBDTBDD was synthesized in the laboratory according to the previous literature.39 1,8-diiodooctane (DIO) was supplied by Sigma Aldrich. The Hf(acac)4 powder was purchased from Alfa Aesar. All materials were directly used without any purification.

2.2 Device fabrication



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Four kinds of PSCs (device A-D) were designed and fabricated with the device structures

listed

as

follows:

(A)

ITO/PEDOT:PSS/P3HT:PC60BM/Al;

ITO/PEDOT:PSS/P3HT:PC60BM/ ITO/PEDOT:PSS/PBDTBDD:PC60BM/Al;

Hf(acac)4/Al; (D)

(B) (C)

ITO/PEDOT:PSS/

PBDTBDD:PC60BM/Hf(acac)4/Al. Devices A and C are the control devices without Hf(acac)4 CBL. The ITO glass substrates were sequentially washed by ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, and subsequently dried in an oven at 150 °C for 15 minutes. After being treated in ultraviolet-ozone (UVO) chamber (Ultraviolet Ozone Cleaner, Jelight Company, USA) for 15 minutes, the ITO glass substrates were spin-coated with a PEDOT:PSS layer at 2000 rpm for s, and then baked at 150 °C in ambient atmosphere for 15 minutes. Subsequently, the PEDOT:PSS coated ITO substrates were transferred into a glove-box filled with nitrogen to be spin-coated with the photoactive layer. For devices A and B, the photoactive layer was prepared by spin-coating the 1,2-dichlorobenzene solution of P3HT and PC60BM (1:1 w/w, polymer concentration of 20 mg/mL) on the coated ITO substrates at 800 rpm for 20 s followed by solvent annealing in covered glass petri dishes for 2 h. For devices C and D, the PBDTBDD:PC60BM (1:1 w/w, polymer concentration of 12.5 mg/mL) blend solution with 3% volume ratio of DIO additive was spin-coated on the PEDOT:PSS coated ITO substrates at 1200 rpm for s to form the photoactive layer. The Hf(acac)4 ethanol solution was simply made by adding the Hf(acac)4 powder into the anhydrous ethanol solvent to obtain a colorless solution with concentration of 0.5-1.5 mg/mL. Onto the photoactive layer coated


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substrates, the Hf(acac)4 ethanol solution was spin-coated for 40 s to obtain the Hf(acac)4 cathode buffer layer with different thickness, without carrying out any thermal annealing or other post-treatment. Finally, the Hf(acac)4 CBL coated were transferred into the vacuum chamber and the 100 nm metal electrode Al was thermally deposited.

2.3 Measurements The current density-voltage (J-V) measurements of devices were performed inside a nitrogen-filled glove box using Keithley 2400 Source Measure Unit (SMU) under simulated AM1.5G irradiation (100 mW/cm2) using a xenon-lamp-based solar simulator (SAN-EI, AAA grade). Ultraviolet photoelectron spectroscopy (UPS) measurements were conducted on KRATOS Axis Ultra DLD spectrometer with a base pressure of 3×10-8 torr and bias of -9 V, and He I (21.22 eV) was applied as the excitation source. An AC Mode III (Agilent) atomic force microscope (AFM) operated in the tapping mode under ambient atmosphere at room temperature was used to measure the surface morphologies of the photoactive layer and Hf(acac)4 CBLs. scanning Kelvin probe microscopy (SKPM) measurements were also carried out on the same AFM using the standard SKPM mode. The external quantum efficiency (EQE) measurements were conducted on QE-R systems (Enli Tech) with the standard single-crystal Si photovoltaic cell calibrated at each wavelength. The transmittance and absorbance spectra of the devices were measured by LAMBDA 950 UV/Vis/NIR Spectrophotometer.



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3. Results and discussion To verify the suitability of Hf(acac)4 as the CBL in PSCs, devices based on two different types of photoactive materials, PBDTBDD:PC60BM and P3HT:PC60BM, were investigated with a sandwiched structure of ITO/PEDOT:PSS/photoactive layer/Hf(acac)4/Al as shown in Figure 1(a). The molecular structures of the donors (PBDTBDD and P3HT), acceptor (PC60BM) and Hf(acac)4 are given in Figure 1(b). Figure 1(c) shows the UPS spectra of the Hf(acac)4 film spin-coated on ITO, and the inset is the absorption spectra of the Hf(acac)4 film in the visible light region. The absorption peak of the Hf(acac)4 film is located at around 300nm due to the n-π* and π-π* intra-ligand electronic transitions,35, 46-47 and there is no absorption in the visible light region. That means there is no competitive photon absorption with photoactive layer. Based on the UPS spectra, the work-function of Hf(acac)4 was calculated to be 3.86 eV (21.22-17.36=3.86 eV), since the onset of photoemission is 17.36eV. The highest occupied molecular orbital (HOMO) energy level of Hf(acac)4 is 6.64 eV (21.22-(17.36-2.78)=6.64 eV) considering the cutoff of the binding energy (2.84 eV). Combined the HOMO level with the energy band gap (3.80eV), determined from the onset (326.50nm) of the Hf(acac)4 absorption, the lowest unoccupied molecular (LUMO) level could be calculated as 2.84 eV (6.64 eV-3.80eV= 2.84eV). Figure 1(d) shows the LUMO and the HOMO energy levels of materials involved in PSCs, in which the values for the energy levels of P3HT, PBDTBDD and PC60BM were taken from the literatures.39-41 It can be observed that the low Fermi level (-3.86 eV) of


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Hf(acac)4 lies close to the LUMO energy level of PC60BM (-3.90 eV), which is energetically favorable for the electron extraction from the LUMOs of PC60BM to the Al electrode through Hf(acac)4. In addition, the HOMO energy level (-6.64 eV) of Hf(acac)4 is much lower than that of PC60BM (-5.90 eV), which could effectively the holes. According to such energy level diagram, we assume that the electric dipole may be formed at the interface between the Hf(acac)4 CBL and the photoactive layer. Therefore, both electron transport and built-in potential could be enhanced, leading to the increased Voc, Jsc, and FF.

Figure 1. (a) Device configuration; (b) Molecular structures of PBDTBDD, P3HT, PC60BM, and Hf(acac)4; (c) UPS spectra of Hf(acac)4, and the inset refers to the absorption spectra of Hf(acac)4; (d) Schematic energy level diagram of the materials



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involved in the PSCs.

To verify the formation of the interfacial dipoles between the Hf(acac)4 CBL and the photoactive layer, we applied SKPM to probe the surface potential changes before and after depositing the Hf(acac)4 layer on the top of the photoactive layer. Figure 2 and (b) show the surface potential change for PBDTBDD:PC60BM, after spin-coating the Hf(acac)4, the surface potential increased to 510 mV, 310 mV positive than that of the photoactive layer without the Hf(acac)4 CBL. The surface potential change before and after depositing the Hf(acac)4 layer on the top of the P3HT:PC60BM film is also tested, as shown in Figure S1. Compared with the surface potential of 265 mV of the P3HT:PC60BM active layer, after introduction of the Hf(acac)4, the surface potential increased to 301 mV, more positive than that of the photoactive layer without the Hf(acac)4 CBL. Such results confirmed that the microscopic electric dipole was formed after the incorporation of the Hf(acac)4 CBL, and the direction of the dipole moment was from the Hf(acac)4 CBL to the photoactive layer, the same as that of the built-in potential in the PSC device. Thus the actual potential across the whole device was enhanced. In order to further investigate the interfacial dipoles induced by the Hf(acac)4

CBL,

UPS

measurements

were

ITO/PEDOT:PSS/PBDTBDD:PC60BM

conducted

on and

ITO/PEDOT:PSS/PBDTBDD:PC60BM/Hf(acac)4, and the secondary electron cutoff regions are presented in Figure 2 (c). The work-function of the photoactive blend decreased by 0.1 eV with the incorporation of the Hf(acac)4 CBL, suggesting the


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built-in potential has been enhanced. Such finding is in good agreement with SKPM results as shown in Figure 2 (a) and (b).29, 42-43 Combining with the absorption results of the PBDTBDD:PC60BM layer as shown in Figure 2(d), we could obtain the energy level structure of the PBDTBDD:PC60BM layer with and without Hf(acac)4 CBL, as shown in Figure 2(e). Valence orbital labels, including the work function (Φ), ionization energy (IE), and electron affinity (EA), as well as relative energies are given with respect to the Fermi level. The energy bands of the blend shows a very favorable energetic alignment with that of Hf(acac)4, since the work function of the blend is 4.40 eV, and that of the Hf(acac)4 is 3.80 eV, a significant change in the vacuum level of the conductive film could be observed, where the ∆Evac ≈ 0.6 eV. The energy bending at the interface implies the formation of dipoles. The interfacial dipole is oriented with the same direction for electron transport from photoactive layer to the Hf(acac)4, as shown in Figure 2(f), which could benefit the electrons transport and extraction, potentially improved the Voc, FF, and Jsc of PSCs with Hf(acac)4 CBL.


44-45

and


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Figure 2. (a) Surface potential of ITO/PEDOT:PSS/PBDTBDD:PC60BM; (b) surface potential of ITO/PEDOT:PSS/PBDTBDD:PC60BM/Hf(acac)4; (c) UPS spectra of ITO/PEDOT:PSS/BHJ and ITO/PEDOT:PSS/BHJ/Hf(acac)4; (d) absorption spectra of the PBDTBDD:PC60BM layer; (e) energy level alignment of thin films from PBDTBDD:PC60BM

and

Hf(acac)4

and

(f)

a

PBDTBDD:PC60BM/Hf(acac)4 interface.


dipole

schematic

of

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The tapping-mode atomic force microscope (AFM) measurements were conducted to investigate the effect of the Hf(acac)4 deposition on the surface morphology. The AFM images of substrate, photoactive layer and Hf(acac)4 spin-coated on the photoactive layers are shown in Figure S2. The chemical component of Hf(acac)4 powder and as prepared Hf(acac)4 on photoactive layer was analyzed by X-ray photoelectron spectroscopy (XPS) as shown in Figure S3. The background was subtracted from the XPS spectra by using a Shirley-type background subtraction. As shown in Figure S3 (a), the XPS spectrum of the core level of Hf 4f for the powder sample has a strong spin–orbit doublet due to Hf 4f5/2 at 17.8 eV and Hf 4f7/2 at 19.4 eV. The area ratio of the two peaks of each doublet is 3:4. For the spectrum of thin Hf(acac)4 film on the active layer, as shown in Figure S3 (b), The two peaks are unchanged located at 17.8 eV and 19.4 eV, respectively, with a same intensity ratio of 3:4.51 This result give a strong supporting to make the conclusion that the component of the thin film is unchanged to be the Hf(acac)4. To verify the feasibility of the Hf(acac)4 as the CBL in PSCs, we applied the Hf(acac)4 to the devices based on PBDTBDD:PC60BM photoactive materials. The PSCs were fabricated with the sandwiched structures as shown in Figure 1(a). The J-V curves of the devices with and without Hf(acac)4 CBLs in the dark and under 100 mW/cm2 (AM 1.5G) illumination are displayed in Figure 3, and all measured parameters are summarized in Table 1. All the parameters presented in Table 1 are the average values of 20 individual devices. As shown in Figure 3(a), the PSC based on PBDTBDD:PC60BM without Hf(acac)4 CBL exhibited a PCE of 5.84%, with a Jsc



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of 10.50 mA/cm2, a Voc of 0.82 V, and a FF of 0.68. After the insertion of the Hf(acac)4 CBL, significant enhancement in device performance can be achieved. The thickness of Hf(acac)4 layer is also optimized by varying the speed of spin-coating. Ellipsometer measurements revealed that the Hf(acac)4 CBL thickness of 7.19, 6.36, 5.69 and 4.07 nm is obtained when the speed of spin-coating is 1000, 2000, 3000, and 4000 rpm, respectively. The transmittance spectra, as shown in Figure 3(e), confirm that Hf(acac)4 CBL has no competitive photon absorption with the photoactive layer. Simultaneous improvement in Jsc, Voc and FF is obtained upon modification of Hf(acac)4 CBLs, leading to an enhancement in PCE. The Voc (0.86-0.87 V) and FF (71%-73%) of the devices are almost same regardless of the thickness of Hf(acac)4 CBL, while the Jsc is much more sensitive to the Hf(acac)4 thickness. When the thickness of Hf(acac)4 increasing from 4.07 to 5.69 nm, the Jsc improves from 10.85 to 11.89 mA/cm2, while further increasing the Hf(acac)4 thickness to 7.19 nm could result in the drop of the Jsc to 10.42 mA/cm2. When the thickness of Hf(acac)4 CBL is 5.69 nm, the device exhibits the highest PCE of 7.38%, with the Jsc, Voc and FF of 11.89 mA/cm2, 0.87 V and 0.71, respectively. The series resistance of the devices is calculated from the dark current at 1.0 V. The Rs of the device without Hf(acac)4 layer is 6.0 Ωcm2, while that of devices with incorporation of Hf(acac)4 is greatly decreased, which is beneficial to the enhancement of Jsc. In addition, as shown in Figure 3(b), the external quantum efficiency (EQE) spectra of the PSCs exhibit an enhanced photocurrent response in whole photo-sensitive range from 330 to 750 nm with the insertion of Hf(acac)4 CBL. The integrated Jsc (11.39 mA/cm2) from the EQE


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spectra is very close to that (11.89 mA/cm2) derived from J-V curves. To further verify the compatibility of the Hf(acac)4 CBL with different materials, devices based on P3HT:PC60BM photoactive materials were also fabricated with the optimized Hf(acac)4 thickness of 5.69 nm. As shown in Figure 3(c), for the PSCs using P3HT:PC60BM as the photoactive layer, the insertion of the Hf(acac)4 CBL also improved the device PCE, from 2.94% to 4.02%, with the Jsc, Voc and FF all enhanced simultaneously, from 9.06 to 10.17 mA/cm2, from 0.54 to 0.58 V, and from 0.60 to 0.69, respectively. The series resistance of the device decreased from 5.8 to 2.5 Ωcm2 with the incorporation of the Hf(acac)4 CBL. The enhanced photocurrent response was also observed in the EQE spectra of the P3HT:PC60BM-based device with the Hf(acac)4 CBL, as shown in Figure 3(d).



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Figure 3. J-V curves of the PSCs based on (a) PBDTBDD:PC60BM and (c) P3HT:PC60BM with and without Hf(acac)4 CBL under the illumination (AM 1.5 G, 100 mW/cm2), and the insets are dark currents at full bias scanning from -1.5 to 1.5 V; EQE spectra of the PSCs based on (b) PBDTBDD:PC60BM and (d) P3HT:PC60BM with and without Hf(acac)4 CBL; (e) Transmittance spectra of PBDTBDD:PC60BM spin-coated on ITO/PEDOT:PSS substrate with and without


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Hf(acac)4 CBL (3000 rpm); (f) J-V curves of the PSCs based on PBDTBDD:PC60BM with different thickness of Hf(acac)4 CBL under the illumination of AM 1.5 G, 100 mW/cm2.

Table 1. Device parameters of PSCs based on PBDTBDD:PC60BM and P3HT: PC60BM. Photoactive layer

PBDTBDD:PC60BM

Thickness of CBL (nm)

Jsc [mA/cm2]

Voc [V]

FF [%]

PCE RS[a] [%] [Ωcm2]

without

10.50

0.82

68

5.84

6.0

4.07

10.85

0.86

71

6.62

1.4

5.69

11.89

0.87

71

7.38

3.3

6.36

11.20

0.86

70

6.78

3.7

7.19

10.42

0.87

73

6.48

4.0

without

9.06

0.54

0.60

2.94

5.8

5.69

10.17

0.58

0.69

4.02

2.5

P3HT: PC60BM [a]

Series resistance (RS) of the PSCs in the dark are obtained at around 1.0 V.

The tapping-mode atomic force microscope (AFM) was performed to study the effect of the Hf(acac)4 CBL spin-coated on the photoactive layer. The surface morphology evolution of the substrate, photoactive layer and Hf(acac)4 deposited on the photoactive layer is shown in Figure S2 in Supporting Information (SI). The influence of the Hf(acac)4 CBL thickness on the surface morphologies is illustrated in Figure 4. When the Hf(acac)4 CBL thickness is 7.19 nm, there shows relatively large domains of about 300 nm in diameter for the Hf(acac)4 CBL, as illustrated in Figure



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4(b), indicating the aggregation of the Hf(acac)4. These aggregations potentially induce short-circuit and surface traps of the devices. With thinner Hf(acac)4 CBL of 6.36 nm, the aggregation of the Hf(acac)4 becomes less significant (Figure 4(c)). When the thickness is decrease to 5.69 nm, the surface of the Hf(acac)4 CBL is relatively homogeneous without any aggregation (Figure 4(d)). The further increase of the spin-coating speed results in a Hf(acac)4 layer of 4.07 nm, and a non-continuous surface morphology is observed (Figure 4(e)). Therefore, the change of Jsc is closely related with the surface morphology of Hf(acac)4 CBL. For PBDTBDD:PC60BM based photoactive layer, the root-mean-square (rms) roughness is 3.29 nm, as shown in Figure 4(a). Under the optimum condition, the surface becomes more rough as the rms roughness increases to 5.32 nm with the modification of the Hf(acac)4, and the surface-area (SA) effectively increases from 25.06 um2 to 25.43 um2. The incorporation of the Hf(acac)4 layer lead to much tighter contact with the metal electrode, thus decreasing the interfacial barrier. The optimized surface morphology, as well as the energy level alignment between the photoactive layer and the cathode contact with the Hf(acac)4 CBL contribute to the improvement of the photovoltaic performance.


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Figure 4. AFM surface topographic images of photoactive layer (a) without, and with Hf(acac)4 CBL spin-coated with a speed of (b) 1000 (14 nm), (c) 2000 (11 nm), (d) 3000 (8 nm), and (e) 4000 rpm (4 nm). The scan size is 5 µm × 5 µm.

A combined experimental and simulation approach, impedance spectroscopy (IS), was applied to probe the dynamics of charge transfer and recombination at the interface. Devices based on PBDTBDD:PC60BM active system with and without Hf(acac)4 buffer layer were measured under 10mV bias, as shown in Figure 5. It can be seen the impedance responses of these devices are semicircles in the complex plane with lateral axis standing for the real part of the impedance and vertical axis as the negative imaginary part of the impedance. At high frequencies (shown in the inset of Figure 5), the plots of devices with CBL intersect with lateral axis at about 60Ω, while the device without CBL intersect with the lateral axis at about 100Ω, which represents the series resistance of the devices. While at low frequencies, these plots intersect with lateral axis at different points which is related to recombination resistance. The recombination resistance can be obtained from the diameter of the semicircle on the lateral axis, and that of the device with and without Hf(acac)4 CBL is shown as 40 kΩ and 33 kΩ, respectively. Which indicates that the device with the Hf(acac)4 CBL decrease the series resistance and meanwhile increase the recombination resistance, and thus reduce the charge recombination and improve the charge transfer.50-51


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Figure 5. Impedance spectra of devices based on PBDTBDD:PC60BM.

The effect of Hf(acac)4 CBL on the electron mobility was also investigated by measuring the J-V curves of electron-only devices in configurations of (A) ITO/Al/PBDTBDD:PC60BM/Al and (B) ITO/Al/PBDTBDD:PC60BM/Ha(acac)4/Al. The results was fitted by the space-charge-limited current (SCLC) model.46 The results plotted as ln(JL3/V2) vs (V/L)0.5 are shown in Figure S4, where L is the thickness of the PBDTBDD:PC60BM blend layer. The Electron mobility is derived from the intercept of the lines on the axis of ln(JL3/V2), and is calculated to be 4.34×10-3 and 6.41×10-3 cm2/(V·s) for device A and B, respectively. The results indicate that the insertion of Ha(acac)4 CBL can increase the electron transport and collection efficiency, which is beneficial to the enhancement of Jsc and FF.

To examine the use of Hf(acac)4 CBL for enhancing the stability of the devices, the stability of the devices made from PBDTBDD:PC60BM and P3HT:PC60BM was tested in a nitrogen filled glove-box as shown in Figure 6. The photovoltaic



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performance of the devices was tested once a day. The devices with Hf(acac)4 CBL demonstrate better stability for both photoactive layers. For the devices based on PBDTBDD:PC60BM photoactive layer, the PCE of the device with Ca/Al cathode drops to 61.7% of its initial value after stored in nitrogen filled glove-box for 312 hours (13 days). Using Hf(acac)4 instead of Ca improves the stability of the device dramatically, PCE of the device remains 82.3% of its initial value. The stability improvement should be attributed to the barrier effect of Hf(acac)4 which blocks the hot metal atoms penetrating into the photoactive layer during thermal evaporation of the Al or Ca/Al metal cathode. The devices based on P3HT:PC60BM also shows similar stability trend. The results indicates that Hf(acac)4 could be one of the promising cathode buffer materials for enhancing the lifetime of PSCs.


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Figure 6. Long-term PCE stability of the PSCs based on (a) PBDTBDD:PC60BM and (b) P3HT:PC60BM without and with either Ca or Hf(acac)4 CBL. The devices were stored in an N2 filled glove-box.

Conclusions In this study, solution-processed Hf(acac)4 was successfully utilized as a buffer layer to optimize the energy level alignment between the photoactive layer and the cathode contact for efficient polymer solar cells. The insertion of Hf(acac)4 CBL induced an interfacial dipole oriented with the same direction of build-in potential, simultaneously increasing Jsc, Voc and FF. Furthermore, the introduction of the



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Hf(acac)4 CBL effectively optimized the surface morphology, leading to the reduction of the surface defects as well as the decrease of the charge recombination. Our work gives a new option for the selection of the solution-processed CBL for designing high efficiency PSCs.

Acknowledgements The authors thank Prof. Jianhui Hou in Institute of Chemistry, Chinese Academy of Sciences for Ellipsometer measurements. This work was supported by the NSFC (51573042, 51173040, 51303052), SRFDP (20130036110007), The National Key Basic Research Program of China (973 project, 2015CB932201), Program for New Century Excellent Talents in University (NCET-12-0848), The Science and Technology Commission of Beijing Municipality, China (Z141100003314003), Beijing Higher Education Young Elite Program (YETP0713), State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University, 201404) and Fundamental Research Funds for the Central Universities, China (JB2015RCJ02, 2014MS35, 2015ZZD06).

Supporting Information Available: Surface potential of P3HT:PC60BM spin-coated on the ITO/PEDOT:PSS substrates with and without Hf(acac)4 layer; AFM images of substrate, photoactive layer and Hf(acac)4 spin-coated on the photoactive layers; XPS of Hf 4f in Hf(acac)4 powder and Hf(acac)4 film on the active layer; Current-voltage data

from

the

devices

of

ITO/Al/PBDTBDD:PC60BM/Al


and

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ITO/Al/PBDTBDD:PC60BM/Ha(acac)4/Al. This material is available free of charge via the Internet at http://pubs.acs.org.

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Optimization of the Energy Level Alignment ... - ACS Publications

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