Nickel oxide and polytetrafluoroethylene stacked structure as an interfacial layer for efficient polymer solar cells

Accepted Manuscript Nickel Oxide and Polytetrafluoroethylene Stacked Structure as an Interfacial Layer for Efficient Polymer Solar Cells

Shuai Huang, Ancan Yu, Yunhe Wang, Yuting Tang, Si Shen, Bonan Kang, S. Ravi P. Silva, Geyu Lu PII:

S0013-4686(19)30037-4

DOI:

10.1016/j.electacta.2019.01.028

Reference:

EA 33431

To appear in:

Electrochimica Acta

Received Date:

24 July 2018

Accepted Date:

06 January 2019

Please cite this article as: Shuai Huang, Ancan Yu, Yunhe Wang, Yuting Tang, Si Shen, Bonan Kang, S. Ravi P. Silva, Geyu Lu, Nickel Oxide and Polytetrafluoroethylene Stacked Structure as an Interfacial Layer for Efficient Polymer Solar Cells, Electrochimica Acta (2019), doi: 10.1016/j. electacta.2019.01.028

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ACCEPTED MANUSCRIPT

Nickel Oxide and Polytetrafluoroethylene Stacked Structure as an Interfacial Layer for Efficient Polymer Solar Cells Shuai Huanga, Ancan Yua, Yunhe Wanga, Yuting Tanga, Si Shena, Bonan Kanga,*, S. Ravi P. Silvab, and Geyu Lua a

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and

Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China b

Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford,

Surrey GU2 7XH, United Kingdom

E-mail address: [email protected] *Corresponding author: Bonan Kang E-mail address: [email protected] Tel.: +86 431 85168270; Fax: +86 431 85168270.

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ABSTRACT: An efficient polymer solar cell (PSC) has been demonstrated by incorporating an ultrathin interfacial insulating organic layer of polytetrafluoroethylene (PTFE) between a photoactive layer and hole-collecting buffer layer (NiOx). The photoactive layer is made with bulk heterojunction composites of poly[N-9’’-hepta-decanyl-2,7-carbazolealt-5,5-(4’,7’-di-2-thienyl2’,1’,3’-ben-zothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM). The PTFE layer not only improves the energy level alignment by forming an interfacial dipole at the interface but also reduces the inherent incompatibility between the hydrophobic active layer and hydrophilic NiOx layer, thereby benefiting to the charge extraction and transport in the solar cells. As a result, with the NiOx/PTFE stacked structure, all the photovoltaic performance parameters are significantly improved, leading to a higher power conversion efficiency (PCE) of up to 7.11% compared to the control device without PTFE layer (PCE 5.50%). The PTFE layer provides a superior alternative to interfacial engineering of the metal oxide/organic semiconductor interface in polymer solar cells and other organic electronic devices.

KEYWORDS: NiOx, PTFE, work function, charge extraction, inherent incompatibility, polymer solar cell

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1. INTRODUCTON Polymer solar cells (PSCs), as a viable source of renewable energy, have received widespread research attention on account of their unique merits of lightness, mechanical flexible, cost-effective, and large-area fabrication.1-7 The most widely used PSCs are based on the bulk-heterojunction (BHJ) configuration, which establishes the nanoscale networks in bicontinuous electron-donors and fullerene acceptors.8-12 Numerous efforts have been made to improve the photovoltaic performance of PSCs.13,14 Recently, the PCE of the BHJ PSCs has exceeded 11%.15-19 Considering the tremendous progress in device efficiency of PSCs, we have noticed that the improvements in photovoltaic performance are generally based on the innovative materials and the advanced device structure. In particular, the interface modification is an effective approach for improving the cell efficiency and stability among these breakthroughs. The typical BHJ PSCs are designed with a sandwich architecture that consists of a photoactive layer between a hole-conducting PEDOT:PSS film coated ITO substrate and a metal cathode with low work function.20 In this conventional structure, PEDOT:PSS has been shown to be a promising anode interfacial layer, which exhibits high optical transparency and improves the energy level alignment with polymer donors.21 However, the long-standing acidic and hygroscopic nature of PEDOT:PSS interlayer leads to the rapid physical and chemical degradation of solar cells.22-24 In order to overcome these disadvantages, new alternatives to traditional PEDOT:PSS buffer layer have been developed and widely used as the anode interfacial layers, including some solution-processed transition metal oxides, usually NiOx, V2O5, WO3 and MoO3.25,26 In comparison with the n-type MoO3 and V2O5, wide band-gap NiOx is considered an excellent interface material in PSC applications because of its wide band gap (>3.0 eV), small electron affinity (1.8-2.1 eV) and good charge extraction properties.27-29 However, the

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device performance is far from satisfied by inserting the metal oxide alone because of the limited hole extraction and transport capacity as well as the inherent incompatibility between the hydrophobic photoactive layer and hydrophilic inorganic metal oxide. Polytetrafluoroethylene (PTFE), as a fluorocarbon based material, has various superiorities including light weight, low cost, moisture resistance, flexibility and low chemical reactivity.30 At present, PTFE is one of the most stable polymer materials with good chemical stability and thermal stability. It has been previously reported in our works that the rich, negatively charged fluorine aligns on the surface of bottom electrode and forms an oriented dipole at the ITO/ active layer interface with the dipole moment directed inward the ITO, thus increasing the effective work function of ITO and reducing the energy barrier between the photoactive layer and ITO anode.31,32 However, the reported efficiency of the PSCs with the pristine PTFE-modified ITO anodes was only 2.27% and 5.42% for the P3HT:PC61BM and PCDTBT:PC71BM system, respectively.32,33 Herein, we attempt to achieve higher efficiency by introducing the NiOx/PTFE stacked structure. Our proposed PTFE layer on the NiOx film reduces the inherent incompatibility between the hydrophobic BHJ layer and hydrophilic NiOx film. More importantly, the PTFE layer reduces the interfacial energy barrier by generating an interfacial dipole moment between NiOx and the photoactive layer, thus facilitating hole extraction and suppressing charge recombination. As expected, by incorporating the NiOx/PTFE stacked structure, all the photovoltaic performance parameters of PSCs based on the PCDTBT:PC71BM system were simultaneously improved, yielding higher PCE of 7.11% compared to the PSCs buffered by the pristine NiOx layers (PCE 5.50%). Therefore, the introduction of NiOx/PTFE stacked layer is an efficient and simple approach for improving the photovoltaic performance of the PSCs. This

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current work represents an important step towards developing simple and efficient interfacial layers for the high performance BHJ PSCs without resorting to the complex, cumbersome, and expensive organic interlayers. Moreover, the insertion of PTFE layer on the NiOx film provides an excellent strategy for further studying the PSCs or other electrochemical devices to improve their performance in the near future. 2. EXPERIMENTAL SECTION 2.1. Materials and Solutions. FTO substrates were obtained from Dongguan Xiangcheng Group. PCDTBT was supplied from 1-material Chemscitech, while PC71BM was supplied from Lumtek Corp. The polymer donor and fullerene derivative (1:4 by weight) were predissolved in 1, 2-dichlorobenzene and stirred at 60 °C for about 14 h. The NiOx precursor solution was obtained by dissolving Ni(CH3COO)2·4H2O into the mixed solution of monoethanolamine and ethanol under sufficient stirring. 2.2. Cell Fabrication. The FTO substrates were ultrasonicated in acetone, deionized water and ethanol for 15 min every time, and dried at 100°C for 10 min. The FTO glasses were exposed to oxygen plasma for 60 s. The NiOx precursor solution was spun coated at 1000 rpm for 30 s, followed by annealing at 300°C for 1 h in air. The PTFE layers with different thickness of 0, 0.5, 1.0, 1.5, and 2.0 nm were thermally evaporated on the NiOx films in a vaccum of 7.0 × 10-4 Pa. The deposition rate of PTFE layers is about 0.02 Å/s. After that, the absorber layers were spin-coated at 1500 rpm for 30 s and annealed at 70°C for 30 min. Subsequently, 1 nm tris(8-hydroxy quinoline) aluminum (Alq3) and 100 nm Al electrode were evaporated on the BHJ layers. The working area of the finished PSC was about 0.05 cm2. 2.3. Characterization. The photovoltaic characteristics of the PSCs were conducted on a Keithley 2400 Source Meter using a solar simulator of Newport 9225 1A-1000. Incident

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photo-electron conversion efficiency (IPCE) was tested using Pharos Technology QEM1000. Atomic force microscopy (AFM) images were taken using SPM-9500j3. Photoluminescence (PL) measurements were performed using Edinburgh Instruments LTD (FLSP920). Impedance spectroscopy measurements were conducted on a model CHI630E electrochemical analyzer (Shanghai ChenHua Instrument Co. Ltd.). The absorption spectra were obtained using UV-3600 spectrometer. The contact potential differences (CPD) of the films were carried out using the SKP5050 Kelvin Probe Force Microscope (KPFM) in air. 3. RESULTS AND DISCUSSION Figure 1 presents the cell structure adopted in our study and the energy band diagram. To explore the impact of a thin PTFE layer on the work function (WF) of NiOx, a Kelvin probe technique was performed to obtain the information on the WF or partial surface potential with high resolution.34 The KPFM measures the CPD caused by the different WFs of the sample and probe tip. Once the work function of the cantilever is known, it is possible to determine the actual WF of the sample according to the calculating formula, ∆𝑊𝐴𝑢

𝑊sample = 5.1 ― 1000 +

∆𝑊𝑠𝑎𝑚𝑝𝑙𝑒 1000

(1)

where ΔWAu is the error value calibrated by standard Au sample, Wsample is the WF of the sample, and ΔWsample is the CPD value measured by KPFM. The WF of the samples is the average value of 100 points over the scan area of 0.9  0.9 μm. As presented in Figure 2a, the WF of FTO is able to be tuned by coating FTO surface with an inorganic buffer layer. It should be noted that the WF was enhanced from 5.15 eV to 5.45 eV after coating the PTFE layer on the NiOx film. According to the energy level diagrams in Figure 2b, the rich, negatively charged fluorine aligns on the NiOx film and forms an oriented dipole at the NiOx/active layer interface with the dipole moment directed inward the NiOx. The PTFE dipole layer makes a vacuum level shift, leading to

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an enhancement in the work function of NiOx film. The work function shift is benefited to match well with the HOMO of donor material PCDTBT. Consequently, the NiOx/PTFE interlayer facilitates charge injection and transport by reducing the interface energy barrier between the photoactive layer and anode. In order to optimize the photovoltaic performance of PSCs, we investigate the impact of different thickness (0, 0.5, 1.0, 1.5, 2.0 nm) of the PTFE layer on the cell efficiency and the corresponding J-V curves are presented in Figure 3a. Table 1 summarizes the detailed photovoltaic parameters of the studied cell. The control cell prepared with the single NiOx interlayer exhibits a PCE of 5.50% with a photocurrent density (Jsc) of 10.81 mA cm-2, an open circuit potential (Voc) of 0.85 V, and a fill factor (FF) of 59.86%. Interestingly, by introducing the PTFE layer on the NiOx film, the photovoltaic performance of PSC begins to improve slightly. When the thickness of PTFE is 1.5 nm, it reaches a higher efficiency value of 7.11% with Jsc of 12.55 mA cm-2, Voc of 0.89 V, and FF of 63.89%. As the thickness of PTFE is further increased to 2.0 nm, the PCE of the NiOx/PTFE-based PSC drops to 5.64%. These results denote that the photovoltaic performance of the cells are remarkably improved by depositing the PTFE layer on NiOx film and the optimal thickness is 1.5 nm. Compared to the control device with NiOx alone, both the Jsc and Voc are enhanced remarkably, leading to an improved fill factor and efficiency for the device based on the NiOx/PTFE stacked layer. The improved photovoltaic performance of the cell inserting a PTFE layer is mainly attributed to the reduced the interfacial energy barrier by the formation of a strong interfacial dipole layer, which is created by the negatively charged fluorine rich PTFE layer. However, it is noted that the thickness of PTFE is very critical to optimize the device performance. Considering that PTFE is a good electrical insulating material, with a high ionization potential (9.8 eV) and a large resistivity (1018 Ω/cm)

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for its bulk properties,35 a thicker PTFE layer between FTO and NiOx will block the charge collection and finally decrease the short circuit current of PSCs, while a very thin layer of PTFE is usually required to act as effective interfacial material for better energy level alignment. As shown in the IPCE spectra (Figure 3b), the device with the NiOx/PTFE interfacial layer presents an enhanced light response, especially in the region of 350-620 nm. The device with the NiOx/PTFE layer shows a maximum IPCE value of 79%, whereas the control cell exhibits a lower light response of 67%. These data indicate the improved interface characteristics and the superior charge transfer capability in the device based on the NiOx/PTFE layer, thus leading to an increase in PCE. Figure 3c shows the dark J-V characteristics of the PSCs with the different interfacial layers. It can be observed that the NiOx/PTFE-based cell has a lower dark current density and superior diode characteristic compared to the cell with the NiOx film only. Thereby, the improved Jsc for the cell prepared with NiOx/PTFE layer is expected. Figure 4 compares the UV-visible absorption spectra of the different films. Both the NiOx/PCDTBT:PC71BM and the NiOx/PTFE/PCDTBT:PC71BM films show almost identical or slightly different absorbance in the visible-light region, which suggests that the light absorption or attenuation effect is ignorable for the enhanced performance by inserting a thin layer of PTFE on the NiOx film. Furthermore, considering the almost no or very small absorptions for the NiOx/PTFE films in the wavelength range of 300-800 nm, the NiOx/PTFE interlayer allows the visible light to the photoactive layer. From the above analysis, we can conclude that the light harvesting of devices are not responsible for the enhancement in device efficiency and there must be an inherent reason behind the improved performance. In addition, the surface morphology and roughness of the NiOx films with and without PTFE layer are determined by the AFM measurement in a tapping mode. As shown in Figure 5a and 5b, the introduction of PTFE layer

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does not affect the morphology of the NiOx films and both the films present identical root-mean-square (RMS) of 19.2 nm. For the ultra-thin layer of PTFE used in our solar cell fabrication, it follows the morphology of the underneath NiOx surface and thus the roughness of the NiOx/PTFE film is predominantly influenced by the underlayer. These results confirm that the existence of the PTFE layer did not interfere with the BHJ layer coating. Next, we measure the water contact angles of the NiOx films without and with PTFE layer, as shown in Figure 5c and 5d. It can be observed that the PTFE layer changes the surface energy of the NiOx film, thus leading to a noticeable change in the water contact angle from 32.1° (NiOx) to 56.3° (NiOx/PTFE). This data implies that the surface of NiOx/PTFE is more hydrophobic than that of the NiOx. The enhanced hydrophobicity of the NiOx/PTFE interlayer improves the compatibility between the BHJ layer and the NiOx film and thus helps to form better interface contact with the photoactive layer, which is in accordance with the impedance spectroscopy results (Figure 6). To study the charge transport properties at the interface between the FTO electrode and the photoactive layer, the electrochemical impedance measurements were conducted on the completed devices with different interlayers. Electrochemical impedance spectroscopy (EIS) is a powerful technique to study the charge extraction and transport on electrode surfaces in polymer solar cell devices.36 Figure 6 presents the representative Nyquist plots for the PCDTBT:PC71BM based PSCs at DC bias of 0.8 V under dark and illumination conditions, respectively. These plots are complex plane representations of the real part of the impedance, resistance or Z′, and the imaginary part of the impedance response, reactance or Z″. As shown in the inset of Figure 6, the impedance data were fitted to an equivalent circuit model composing of a series resistance (Rs) and a parallel circuit of a charge transfer resistance (R1) and a constant phase element (CPE).37-41

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In a typical Nyquist plot, the resistance Rs is mainly caused by the sheet resistance of electrodes and the electrical contacts. The resistance R1 is primarily originated from the charge transport resistance at the electrode/photoactive layer interfaces and inside of the BHJ layer.42,43 The CPE indicates a non-ideal behavior of the capacitor. Table 2 summarizes the parameters determined by the fitting of the experimental data. Apparently, upon deposition of the PTFE layer, the charge transfer resistance (R1) between the BHJ layer and NiOx film is notably reduced owing to the improved compatibility between them. Figure 7 presents the Nyquist plots for the PSCs under simulated illumination and dark conditions at DC bias of -0.2 V, 0.0 V, 0.2 V, 0.4 V, and 0.6 V, respectively. Notably, each of these devices presents a single semicircle in the complex plane, and the diameter size of the semicircles in impedance spectra depends strongly on both DC applied bias and illumination. The radius of the semicircle for the PSC decreases dramatically with increasing the bias potential from 0.0 V to 0.6 V under both darkness and illumination, indicating the decline of the corresponding charge-transfer resistance. Furthermore, it can be observed that the devices with NiOx/PTFE stacked interlayers show smaller diameters at the same applied bias potential under dark or light conditions compared to the pristine NiOx-based devices. Since all the fabrication parameters of the solar cells in this work are identical except for the insertion of the PTFE interlayer, the difference in resistance is thus benefitted from the PTFE modification effect. These results indicate that the ultrathin PTFE layer reduces the interface resistance between FTO anode and the photoactive layer and facilitates the hole transport from the BHJ active layer to FTO anode, which is consistent with J-V curve data above. The decreased resistance in the NiOx/PTFE-based PSCs can be attributed to the improved compatibility between the BHJ layer

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and the NiOx film. The EIS results have further validated the important function of inserting an ultrathin PTFE layer between the BHJ layer and the NiOx film. Then, the conductive properties are qualitatively compared by linear sweep voltammetry (LSV). As shown in Figure 8a, the LSV curves of the devices with FTO/NiOx (or NiOx/PTFE)/Al structures are studied to estimate the conductivity of NiOx and NiOx/PTFE films. Both J-V curves are linearly correlated, indicating a good ohmic contact between the electrode and the studied film. The slope of each LSV curves corresponds to the conductivity of the device. It is apparent that the slope of LSV curve for the NiOx/PTFE stacked film is larger than that of the reference NiOx film, which suggests a higher electrical conductivity. The enhancement of conductivity will reduce the series resistance of the device and promote the charge transport, resulting in an enhancement in Jsc. For better insight into the process of the electron-hole’s actions, we measured the steady-state PL spectra of NiOx/PCDTBT and NiOx/PTFE/PCDTBT films deposited on the glass. As shown in Figure 8b, the sharp luminescence peak centered at about 700 nm is ascribed to the fluorescence emission of PCDTBT film. It is apparent that the PCDTBT film coated on NiOx/PTFE shows the more pronounced fluorescence quenching behavior as compared to the NiOx/PCDTBT sample. These experimental data prove that more efficient photoinduced charge transfer from photoactive layer to NiOx film after incorporating the thin PTFE layer, thus resulting in the reduced carrier recombination loss.44,45 This may also help to increase the photocurrent and PCE value of the solar cells.46 Next, the hole-only devices (FTO/NiOx/PTFE/PCDTBT:PC71BM/MoO3/Al) were fabricated and tested. The effect of PTFE on charge transport in the solar cell can be reflected by the current density. As expected, the hole-only device based on the NiOx/PTFE stacked structure shows the larger current density as compared to the NiOx-based devices under the same

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conditions, as seen clearly in the Figure 9. The enhanced current density for the NiOx/PTFE-based device proves the improved hole transport capability cause by the reduced energy barrier between the NiOx film and photoactive layer, which is in agreement with the above discussion. According to the Mott-Gurney SCLC equation, the corresponding hole mobilities were calculated to be 1.20×10−5 cm2/(V s) and 2.79×10−5 cm2/(V s) for the control device and NiOx/PTFE-based device, respectively. In order to explore the underlying mechanism for improving performance by inserting the PTFE layer, the carrier transport and collection characteristics are investigated by estimating the maximum exciton generation rate (Gmax) in the studied solar cells. Figure 10 displays the plots of photocurrent density (Jph) versus effective potential (Veff). Veff is obtained by the expression of Veff = V0-V, where V and V0 are the applied bias potential and compensation potential, respectively. Jph is given by Jph = JL-JD, where JL and JD are the current density under illumination and in dark.47 In the ideal situation, the drift contribution is dominant at a large effective potential, and the Jph tends to be saturated. Hypothesizing that all the photoinduced excitons can be dissociated into free charge carriers and collected by the electrodes. The saturation photocurrent density (Jsat) is limited only by the Gmax in the saturation region.48 In this case, the Gmax can be calculated by Jsat = eGmaxL, here, L is the thickness of BHJ layer and e is the elementary charge. The values of Gmax for the cells based on the NiOx/PTFE and NiOx are 8.23 × 1027 m−3 s−1 (Jsat = 131.58 A m−2) and 6.95 × 1027 m−3 s−1 (Jsat = 111.23 A m−2), respectively. The enhanced value of Gmax suggests the improved charge extraction and transport capacity in the NiOx/PTFE-based devices. 4. CONCLUSION

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In this work, we have successfully demonstrated improved photovoltaic performance parameters in the NiOx-based PSCs through inserting an insulating organic layer of PTFE on the NiOx surface. The improved device performance is attributed to the reduced energy barrier for hole injection and transport by the formation of a strong interfacial dipole layer between the NiOx surface and active layer. Furthermore, the PTFE layer coating improves the wettability between the hydrophilic NiOx layer and hydrophobic photoactive layer. With the NiOx/PTFE stacked layer, a high PCE value of 7.11% is achieved from the PCDTBT:PC71BM BHJ solar cell, exhibiting a ~30% enhancement over the reference cell. This facile and low-cost NiOx/PTFE stacked film proved to be an efficient anode buffer layer material for PSCs and could also be used as an ideal interface modification layer for CdTe, CuInGaSe, dye-sensitized solar cells or other related electrochemical devices where functional charge-transport layers are required. ACKNOWLEDGMENT The financial support was provided by the National Key Basic Research and Development Program of China (Grant 2016YFB0401001), and the Royal Society International programme. We also thank Open Project of the State Key Laboratory of Supramolecular Structure and Materials of Jilin University (sklssm201809).

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REFERENCE [1] M. Li, W. Zhang, X. Tang, J. Jin, H. Wang, L. Chen, W. Lv, R. Chen, W. Huang, Bromine-terminated additives for phase-separated morphology control of PTB7:PC71BMbased polymer solar cells, ACS Sustainable Chem. Eng. 5 (2017) 11668-11675. [2] D. Yang, P. Fu, F. Zhang, N. Wang, J. Zhang, C. Li, High efficiency inverted polymer solar cells with room-temperature titanium oxide/polyethylenimine films as electron transport layers, J. Mater. Chem. A 2 (2014) 17281-17285. [3] L. Lu, T. Xu, W. Chen, E. S. Landry, L. Yu, Ternary blend polymer solar cells with enhanced power conversion efficiency, Nat. Photonics 8 (2014) 716-722. [4] K. I. Jayawardena, L. J. Rozanski, C. A. Mills, M. J. Beliatis, N. A. Nismy, S. R. P. Silva, ‘Inorganics-in-organics’: Recent developments and outlook for 4G polymer solar cells, Nanoscale 5 (2013) 8411-8427. [5] S. Nam, M. Song, H. Kim, D. D. C. Bradley, Y. Kim, Thickness effect of bulk heterojunction layers on the performance and stability of polymer:Fullerene solar cells with alkylthiothiophene-containing polymer, ACS Sustainable Chem. Eng. 5 (2017) 9263-9270. [6] S. Huang, Y. Wang, S. Shen, Y. Tang, A. Yu, B. Kang, S. R. P. Silva, G. Lu, Enhancing the performance of polymer solar cells using solution-processed copper doped nickel oxide nanoparticles as hole transport layer, J. Colloid Interface Sci. 535 (2019) 308-317. [7] S. Nam, S. Lee, J. Jeong, J. Seo, H. Kim, D.-I. Song, Y. Kim, Light-induced open circuit voltage increase in polymer solar cells with ternary bulk heterojunction nanolayers, ACS Sustainable Chem. Eng. 3 (2014) 55-62. [8] D. I. Kutsarov, I. Rašović, A. Zachariadis, A. Laskarakis, M. A. Lebedeva, K. Porfyrakis, C. A. Mills, M. J. Beliatis, B. Fisher, K. Bruchlos, S. Ludwigs, S. Logothetidis, S. R. P. Silva,

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ACCEPTED MANUSCRIPT

Achieving 6.7% efficiency in P3HT/indene-C70 bisadduct solar cells through the control of vertical volume fraction distribution and optimized regio-isomer ratios, Adv. Electron. Mater. 2 (2016)1600362. [9] Y. Li, Y. Zou, Conjugated polymer photovoltaic materials with broad absorption band and high charge carrier mobility, Adv. Mater. 20 (2008) 2952-2958. [10]N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Photoinduced electron transfer from a conducting polymer to buckminsterfullerene, Science (1992) 1474-1476. [11]F. Wang, Z. a. Tan, Y. Li, Solution-processable metal oxides/chelates as electrode buffer layers for efficient and stable polymer solar cells, Energy Environ. Sci. 8 (2015) 1059-1091. [12]G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Polymer photovoltiac cells: Enhanced efficiencies via a network of internal donor-acceptor heterojunctions, Science 270 (1995) 1789. [13]E. Wang, L. Wang, L. Lan, C. Luo, W. Zhuang, J. Peng, Y. Cao, High-performance polymer heterojunction solar cells of a polysilafluorene derivative, Appl. Phys. Lett. 92 (2008) 033307. [14]C. Han, Y. Cheng, L. Chen, L. Qian, Z. Yang, W. Xue, T. Zhang, Y. Yang, W. Cao, Enhanced performance of inverted polymer solar cells by combining ZnO nanoparticles and poly[(9,9-bis(3'-(n,n-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyfluorene)]

as

electron transport layer, ACS Appl. Mater. Interfaces 8 (2016) 3301-3307. [15]J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Efficient organic solar cells processed from hydrocarbon solvents, Nat. Energy 1 (2016) 1-7. [16]A. R. b. M. Yusoff, D. Kim, H. P. Kim, F. K. Shneider, W. J. da Silva, J. Jang, A high efficiency solution processed polymer inverted triple-junction solar cell exhibiting a power

15

ACCEPTED MANUSCRIPT

conversion efficiency of 11.83%, Energy Environ. Sci. 8 (2015) 303-316. [17]V. Gupta, V. Bharti, M. Kumar, S. Chand, A. J. Heeger, Polymer-polymer forster resonance energy transfer significantly boosts the power conversion efficiency of bulk-heterojunction solar cells, Adv. Mater. 27 (2015) 4398-4404. [18]W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganas, F. Gao, J. Hou, Fullerene-free polymer solar cells with over 11% efficiency and excellent thermal stability, Adv. Mater. 28 (2016), 4734-4739. [19]C.-C. Chen, W.-H. Chang, K. Yoshimura, K. Ohya, J. You, J. Gao, Z. Hong, Y. Yang, An efficient triple-junction polymer solar cell having a power conversion efficiency exceeding 11%, Adv. Mater. 26 (2014) 5670-5677. [20]S. Huang, Y. Tang, Y. Dang, X. Xu, Q. Dong, B. Kang, S. R. P. P. Silva, Low-temperature solution-processed Mg:SnO2 nanoparticles as an effective cathode interfacial layer for inverted polymer solar cell, ACS Sustainable Chem. Eng. 6 (2018) 6702-6710. [21]Y.-H. Kim, S.-H. Lee, J. Noh, S.-H. Han, Performance and stability of electroluminescent device

with

self-assembled

layers

of

poly(3,4-ethylenedioxythiophene)-poly

(styrenesulfonate) and polyelectrolytes, Thin Solid Films 510 (2006) 305-310. [22]F. Jiang, W. C. Choy, X. Li, D. Zhang, J. Cheng, Post-treatment-free solution-processed non-stoichiometric NiOx nanoparticles for efficient hole-transport layers of organic optoelectronic devices, Adv. Mater. 27 (2015) 2930-2937. [23]A. Singh, S. K. Gupta, A. Garg, Inkjet printing of NiO films and integration as hole transporting layers in polymer solar cells, Sci. Rep. 7 (2017) 1775. [24]S. Huang, Y. Tang, A. Yu, Y. Wang, S. Shen, B. Kang, S. R. P. Silva, G. Lu, Solution-processed SnO2 nanoparticle interfacial layers for efficient electron transport in

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ZnO-based polymer solar cells, Org. Electron. 62 (2018) 373-381. [25]A. Garcia, G. C. Welch, E. L. Ratcliff, D. S. Ginley, G. C. Bazan, D. C. Olson, Improvement of interfacial contacts for new small-molecule bulk-heterojunction organic photovoltaics, Adv. Mater. 24 (2012) 5368-5373. [26]C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, B. P. Rand, Solution-processed MoO3 thin films as a hole-injection layer for organic solar cells, ACS Appl. Mater. Interfaces 3 (2011) 3244-3247. [27]J. Zhang, J. Wang, Y. Fu, B. Zhang, Z. Xie, Efficient and stable polymer solar cells with annealing-free solution-processible NiO nanoparticles as anode buffer layers, J. Mater. Chem. C 2 (2014) 8295-8302. [28]P. F. Ndione, A. Garcia, N. E. Widjonarko, A. K. Sigdel, K. X. Steirer, D. C. Olson, P. A. Parilla, D. S. Ginley, N. R. Armstong, R. E. Richards, E. L. Ratcliff, J. J. Berry, Highly-tunable nickel cobalt oxide as a low-temperature p-type contact in organic photovoltaic devices, Adv. Energy Mater. 3 (2013) 524-531. [29]E. L. Ratcliff, J. Meyer, K. X. Steirer, N. R. Armstrong, D. Olson, A. Kahn, Energy level alignment in PCDTBT:PC70BM solar cells: Solution processed NiOx for improved hole collection and efficiency, Org. Electron. 13 (2012) 744-749. [30]H. Ågren, V. Carravetta, O. Vahtras, L. G. M. Pettersson, Orientational probing of polymeric thin films by nexafs: Calculations on polytetrafluoroethylene, Phys. Rev. B. 51 (1995) 17848-17855. [31]B. Kang, L. W. Tan, S. R. P. Silva, Ultraviolet-illuminated fluoropolymer indium-tin-oxide buffer layers for improved power conversion in organic photovoltaics, Org. Electron. 10 (2009) 1178-1181.

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ACCEPTED MANUSCRIPT

[32]B. Kang, L. W. Tan, S. R. P. Silva, Fluoropolymer indium-tin-oxide buffer layers for improved power conversion in organic photovoltaics, Appl. Phys. Lett. 93 (2008) 133302. [33]P. Zhang, X. Xu, Y. Dang, S. Huang, X. Chen, B. Kang, S. R. P. Silva, PTFE/MoO3 anode bilayer buffer layers for improved performance in PCDTBT:PC71BM blend organic solar cells, ACS Sustainable Chem. Eng. 4 (2016) 6473-6479. [34]T. Glatzel, H. Hoppe, N. S. Sariciftci, M. C. Lux-Steiner, M. Komiyama, Kelvin probe force microscopy study of conjugated polymer/fullerene organic solar cells, Jpn. J. Appl. Phys. 44 (2005) 5370-5373. [35]Y. Gao, L. Wang, D. Zhang, L. Duan, G. Dong, Y. Qiu, Bright single-active layer small-molecular organic light-emitting diodes with a polytetrafluoroethylene barrier, Appl. Phys. Lett. 82 (2003) 155-157. [36]F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Sero, J. Bisquert, Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy, Phys. Chem. Chem. Phys. 13 (2011) 9083-9118. [37]Q. Zhang, L. Bai, Z. Bai, P. Hu, C. Liu, Theoretical analysis and design of a near-infrared broadband absorber based on EC model, Opt. Express 23 (2015) 8910-8917. [38]Y. Zhang, L. Li, S. Yuan, G. Li, W. Zhang, Electrical properties of the interfaces in bulk heterojunction organic solar cells investigated by electrochemical impedance spectroscopy, Electrochim. Acta 109 (2013) 221-225. [39]Q. Wan, X. Guo, Z. Wang, W. Li, B. Guo, W. Ma, M. Zhang, Y. Li, 10.8% efficiency polymer solar cells based on PTB7-Th and PC71BM via binary solvent additives treatment, Adv. Funct. Mater. 26 (2016) 6635-6640. [40]S. S. Reddy, K. Gunasekar, J. H. Heo, S. H. Im, C. S. Kim, D. H. Kim, J. H. Moon, J. Y.

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Lee, M. Song, S. H. Jin, Highly efficient organic hole transporting materials for perovskite and organic solar cells with long-term stability, Adv. Mater. 28 (2016) 686-693. [41]P. Fu, X. Guo, B. Zhang, T. Chen, W. Qin, Y. Ye, J. Hou, J. Zhang, C. Li, Achieving 10.5% efficiency for inverted polymer solar cells by modifying the ZnO cathode interlayer with phenols, J. Mater. Chem. A 4 (2016) 16824-16829. [42]J. You, C. C. Chen, L. Dou, S. Murase, H. S. Duan, S. A. Hawks, T. Xu, H. J. Son, L. Yu, G. Li, Y. Yang, Metal oxide nanoparticles as an electron-transport layer in high-performance and stable inverted polymer solar cells, Adv. Mater. 24 (2012) 5267-5272. [43]W. L. Leong, S. R. Cowan, A. J. Heeger, Differential resistance analysis of charge carrier losses in organic bulk heterojunction solar cells: Observing the transition from bimolecular to trap-assisted recombination and quantifying the order of recombination, Adv. Energy Mater. 1 (2011) 517-522. [44]Y. Xiao, H. Wang, S. Zhou, K. Yan, W. Xie, Z. Guan, S.-W. Tsang, J.-B. Xu, Efficient ternary bulk heterojunction solar cells with PCDTBT as hole-cascade material, Nano Energy 19 (2016) 476-485. [45]X. Sun, J. Ni, C. Li, L. Huang, R. Xu, Z. Li, H. Cai, J. Li, J. Zhang, Air-processed high performance ternary blend solar cell based on PTB7-Th:PCDTBT:PC70BM, Org. Electron. 2016, 37, 222-227. [46]H. Choi, J. S. Park, E. Jeong, G. H. Kim, B. R. Lee, S. O. Kim, M. H. Song, H. Y. Woo, J. Y. Kim, Combination of titanium oxide and a conjugated polyelectrolyte for high-performance inverted-type organic optoelectronic devices, Adv. Mater. 23 (2011) 2759-2763. [47]S. R. Cowan, A. Roy, A. J. Heeger, Recombination in polymer-fullerene bulk heterojunction

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solar cells, Phys. Rev. B 82 (2010) 245207. [48]L. Lu, Z. Luo, T. Xu, L. Yu, Cooperative plasmonic effect of Ag and Au nanoparticles on enhancing performance of polymer solar cells, Nano Lett. 13 (2013) 59-64.

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Figure captions Figure 1. (a) Schematic illustration of the PSCs with the NiOx/PTFE interfacial layer. (b) Energy band diagram of each component material in the PSCs. Figure 2. (a) Work function images of FTO, FTO/NiOx, and FTO/NiOx/PTFE samples. (b) Illustration of decreased WF and vacuum level shift. Figure 3. (a) J-V curves of PSCs with various thickness of PTFE layer. (b) IPCE spectra and (c) dark J-V characteristics of the cells with NiOx and NiOx/PTFE interlayer. Figure 4. Optical absorption spectra of the NiOx and NiOx/PTFE films. Figure 5. AFM images for the surface of (a) NiOx and (b) NiOx/PTFE film. Water contact angle images of (c) NiOx and (d) NiOx/PTFE film surface. Figure 6. Nyquist plots of the devices based on the different interlayers at DC bias of 0.8 V under (a) dark and (b) illumination conditions. Figure 7. Bias dependence of Nyquist plots of the device with the NiOx interlayers under (a) dark ambient conditions and (b) AM 1.5 G illumination at 100 mW cm−2. Nyquist plots of the NiOx/PTFE-based devices at varied applied bias under (c) dark and (d) light irradiation. Figure 8. (a) Linear sweep voltammetry (LSV) curves of the different films. (b) PL spectra of NiOx/PCDTBT and NiOx/PTFE/PCDTBT composite films. Figure 9. (a) J-V characteristics and J0.5-V characteristics of the hole-only devices in the dark. Figure 10. Plot of Jph versus Veff in the PSCs with NiOx and NiOx/PTFE film samples.

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Figure 1. (a) Schematic illustration of the PSCs with the NiOx/PTFE interfacial layer. (b) Energy band diagram of each component material in the PSCs.

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Figure 2. (a) Work function images of FTO, FTO/NiOx, and FTO/NiOx/PTFE samples. (b) Illustration of decreased WF and vacuum level shift.

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Figure 3. (a) J-V curves of PSCs with various thickness of PTFE layer. (b) IPCE spectra and (c) dark J-V characteristics of the cells with NiOx and NiOx/PTFE interlayer.

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Figure 4. Optical absorption spectra of the NiOx and NiOx/PTFE films.

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Figure 5. AFM images for the surface of (a) NiOx and (b) NiOx/PTFE film. Water contact angle images of (c) NiOx and (d) NiOx/PTFE film surface.

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Figure 6. Nyquist plots of the devices based on the different interlayers at DC bias of 0.8 V under (a) dark ambient conditions and (b) illumination conditions. The insets show the equivalent circuits.

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Figure 7. Bias dependence of Nyquist plots for the device with the NiOx interlayers under (a) dark ambient conditions and (b) AM 1.5 G illumination at 100 mW cm−2. Nyquist plots of the NiOx/PTFE-based devices at varied applied bias under (c) dark and (d) light irradiation.

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Figure 8. (a) Linear sweep voltammetry (LSV) curves of the different films. (b) PL spectra of NiOx/PCDTBT and NiOx/PTFE/PCDTBT composite films.

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Figure 9. (a) J-V characteristics and J0.5-V characteristics of the hole-only devices in the dark.

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Figure 10. Plot of Jph versus Veff in the PSCs with NiOx and NiOx/PTFE film samples.

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Table captions Table 1. Photovoltaic performance parameters of the cells. Table 2. Fitting parameters from Nyquist plots calculated by an equivalent circuit at the bias voltage of 0.8 V.

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Table 1. Photovoltaic performance parameters of the cells.

Interfacial Layer

Voc/V

Jsc/mA cm-2

FF/%

PCE/%

NiOx

0.85±0.02

10.81±0.03

59.86±0.02

5.50±0.03

NiOx/PTFE

0.89±0.01

12.55±0.02

63.89±0.03

7.11±0.02

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Table 2. Fitting parameters from Nyquist plots calculated by an equivalent circuit at the bias voltage of 0.8 V.

Interfacial Layer

Conditions

Rs/Ω·cm2

R1/Ω·cm2

CPE-T/μF·cm-2

CPE-P

bare NiOx

dark

0.9

90.3

0.4

0.93

NiOx/PTFE

dark

0.6

55.1

0.3

0.96

bare NiOx

illumination

0.8

9.3

1.0

0.95

NiOx/PTFE

illumination

0.3

5.7

0.5

0.93

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