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Facilitating electron collection of organic photovoltaics by passivating trap states and tailoring work function
Solar Energy 181 (2019) 9–16
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Facilitating electron collection of organic photovoltaics by passivating trap states and tailoring work function Xinyuan Zhang, Yu Sun, Mei Wang, Houxiao Cui, Wenfa Xie, Liang Shen, Wenbin Guo
T
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State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electron transport Charge recombination Interface engineering Tailoring work function Interface passivation
The low-temperature solution-processed ZnO normally has intrinsic oxygen vacancies and various surface defects such as organic residues and surface adsorbates. When ZnO film is used in organic solar cells as electron selective layer, these defects will serve as bimolecular recombination centers for light induced charge carriers and trap electrons, leading to an unsatisfied device performance. Herein, we demonstrate an effective way to employ a simple combination of UV-ozone (UVO) and dipole treatment to passivate the ZnO layer, and the electronic property of ZnO layer is great improved upon eliminating the residual hydrocarbons and passivating surface traps. The reengineered uniform ZnO films play a major role on the facilitated charge transfer and decreased contact resistance, resulting in an enhanced device efficiency. The optimized ZnO transport layer also regulates the upper active layer depositing and morphology, and consequently increases the dissociation of photo-excitons and suppresses charge recombination.
1. Introduction Bulk heterojunction (BHJ) organic photovoltaics (OPVs) have become popular due to the integrating advantages of cost-effective, flexibility, colorful, transparency, and large-scale fabrication (Kim et al., 2007; Su et al., 2012; Li et al., 2015; Chen et al., 2013). Rapid efficiency progress mainly originates from new materials synthesis, cascade structure design, and ternary blend solar cells (Su et al., 2011; Guo et al., 2014; Kang et al., 2015; Cheng et al., 2016; Lin et al., 2016). OPV devices are based on the similar multilayered frameworks including the photoactive layer and metallic or metal oxide electrodes, thus the interface modification of these layers can’t be overlooked in the device fabrication. Moreover, the exploration of interfacial engineering is equally essential to the active layer morphology, which should not be ignored for pursuing high-efficiency device (Wu et al., 2016; Tran et al., 2017). The inverted device structure has been used in OPVs preparation for its stability by avoiding hygroscopic and corrosive poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate acid) (PEDOT:PSS) to device degradation (Lim et al., 2016; Cheun et al., 2012). Applying a suitable electron transport layer (ETL) between active layer and ITO cathode is an useful approach to obtain the matched energy level alignment and reduce the free carriers recombination probability (Zhu et al., 2014; Zhu et al., 2014; Zhang et al., 2014; Park et al., 2014; Holman et al., 2013). The improved interfacial contact and ⁎
reduced energy barrier facilitate electron transfer and collection of ITO cathode, which can enhance the short-circuit current density (Jsc) and fill factor (FF) of device. For inverted OPVs (i-OPVs), the transparent ZnO ETL has been used due to its solution-processed, excellent optical and electronic properties, and environmental innoxiously. However, the solution-processed ZnO film always suffers from the defect states, such as oxygen-vacancies and hydrocarbon contamination (Sun et al., 2011; Huang et al., 2017; Lin et al., 2016; Stubhan et al., 2012). Furthermore, the defect states will cause the increased recombination of photo-generated charge carriers and hinder the improvement of device efficiency. Meanwhile, the chemisorbed oxygen species in ZnO film surface can capture electrons from the conduction band (Trost et al., 2013), forming a depletion region near the surface of ZnO and leading to high resistivity. At this point, the superior ZnO film with fewer defects is highly desired and critical for highly efficient performance iOPVs. Many efforts have been performed to passivate the defect states of solution-prepared ZnO film. Doped ZnO with a wide range of materials such as fullerene derivatives, aluminum, and Sn, is a good strategy to achieve high charge carrier mobility compared to pristine ZnO (Aprilia et al., 2013; Zhang et al., 2014; Hoye et al., 2014; Ma et al., 2005). However, the doped ZnO films still have excessive surface trap sites such as dangling bonds and surface groups, which would cause a significant photo-current loss. In addition, many studies have been
Corresponding author. E-mail address: [email protected] (W. Guo).
https://doi.org/10.1016/j.solener.2019.01.077 Received 23 November 2018; Received in revised form 16 January 2019; Accepted 24 January 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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reported that surface modification of a sol-gel ZnO film by an ultrathin dipolar interlayer can contribute to a favorable electron transport, resulting in a big impact on the device performance (Nian et al., 2015; Chen et al., 2015; Xiang et al., 2013; Blum and Shaked, 2015; Su et al., 2012; Kosten et al., 2013; Yu et al., 2015; Chien et al., 2012). However, this method can’t completely eliminate the intrinsic defect states of ZnO layer, which will obstruct the electron transport and collection. Qiao’s and Jo’s groups reported that an improved device performance with short UV–ozone treatment of a ZnO film and followed by a decreased efficiency for longer exposure time (Adhikary et al., 2013; Cho et al., 2014). ZnO wurtzite structure can be significantly improved and polymeric surfactant can be efficiently eliminated by the UV–ozone treatment. Unfortunately, p-type defects induced by UV-ozone will push the Fermi-level away from vacuum level, leading to a mismatched energy alignment of device. In this work, the combination of UV-ozone treatment in atmosphere and interface coating process was carried out to reengineer low-temperature prepared ZnO ETL in i-OPVs. UV–ozone irradiation with different time was first used to re-modulate the ZnO film (UVO-ZnO) property, which correlated with the surface electronic structure and chemical composition. Simultaneously, a thin polymer interlayer was incorporated between ZnO and BHJ active layer to form strong interface dipoles in i-OPVs in terms of aligning energy level, and consequently reducing the work function (WF) of transport layers. Two kinds of common polymer materials of poly(vinylpyrrolidone) (PVP) and polyethylene glycol (PEG) were used as interlayer individually. Both PVP and PEG have lone pairs of electrons on O atoms, which will not only passivate the shallow trap state, but also bind strongly to hydroxyl and carboxylate groups. Compared to the devices with pristine ZnO ETL, the optimized UV-ozone treatment of ZnO/polymer cathode buffer layer (ETL) has advantages in accelerating the electrons collection, suppressing charge recombination, and minimizing the exciton quenching in OPVs (Sharma et al., 2017; Cowan et al., 2013). As a result, both Jsc and FF of the reengineered i-OPVs based on PTB7:PC71BM BHJ are considerably enhanced.
PEG1 are presented as PEG-6000 and dissolved in methanol solution with concentration of 0.1, 0.5 and 1 mg/mL. The cathode is indium tin oxide (ITO) glass substrate. Initially, ITO coated substrate was cleaned with detergent, deionized water, acetone, and IPA, and dried in a nitrogen stream. The precursor ZnO solution was filtered by a 0.45 μm filter. The ZnO solution was spin-coated at 3000 rpm for 60 s and annealed at 200 °C for 1 h in oven. The thickness of ZnO film is about 35 nm. In the reengineering process, the prepared ETL was treated by UV-ozone for 2, 5, 10, and 15 min (2′, 5′, 10′, 15′UVO-ZnO), respectively. The UVO cleaner equipped with an Hg lamp (50 Hz, 200 W) was employed for the preparation of UVO exposed ZnO film. PVP or PEG solution was spin-coated onto the UVO-irradiated ETL to get an ultrathin interlayer. PTB7:PC71BM solution with the optimized ratio was dissolved in chlorobenzene with 3.0 vol% 1,8-diiodooctane (DIO), and the blend solution was spin-coated onto the prepared interlayer surface. The resulting thickness of active layer was about 90 nm. At last, MoO3 (10 nm) and Ag (100 nm) were sequentially deposited by thermally evaporating under high vacuum (9.0 × 10-4 Pa). The measurement and characterization are included in Supporting Information. 3. Results and discussion Water-/alcohol-soluble PVP and PEG are low cost non-conjugated polymer, which have been previously used as effective electron transport layer in conventional OPVs to facilitate electron extraction (Zhou et al., 2017; Rao and Vinni, 1993). The illuminated J-V curves of all fabricated devices with untreated ZnO, UVO-ZnO, and reengineered ETL (UVO-ZnO/PVP or PEG) are shown in Fig. 2a. All the related photovoltaic parameters are summarized in Table 1, including Jsc, FF, open-circuit voltage (Voc), and power conversion efficiency (PCE). Obviously, the reference device presented an unsatisfactory PCE of 7.40% with Voc of 0.72 V, a Jsc of 15.58 mA/cm2, and a FF of 65.55%, whereas the champion efficiency of 9.74% and 9.45% for device with UVO-ZnO/PVP and UVO-ZnO/PEG ETL were realized in the optimized condition, respectively. The 5́ UVO-ZnO/PVP1 based device obtained an increased Voc of 0.74 V, a Jsc of 18 mA/cm2, and a FF of 73.12%. The 5́ UVO-ZnO/PEG0.5 based device exhibited an increased Voc of 0.73 V, a Jsc of 17.86 mA/cm2, and a FF of 72.32%. UVO irradiation time of ZnO ETL showed an obvious effect on device performance. After 2 min of UVO dispose, PCE was enhanced to 7.95% with a simultaneous increase of Jsc and FF. After a short irradiation within 5 min, the increased Jsc and decreased FF were observed. However, Jsc and FF both severely decreased when irradiation time was further increased. In addition, UVO treatment showed a negative influence on Voc of i-OPVs. The WF of ZnO film is increased due to the redundant oxygen species, which induces p-type defects by UVO treatment in atmosphere. To further enhance the performance of device, PVP and PEG interface modification of ZnO with different UVO irradiated time were carried out and J-V characteristic of these i-OPVs are presented in Fig. 2b and c. The detailed device parameters are included in Table 2. Jsc and FF are significantly enhanced after introducing PVP and PEG interlayer. It is worth noting that PCE of i-OPVs with 5́ UVO-ZnO/PVP1 is improved by 31.6% and 23.4% compared to those of the reference device respectively. Moreover, PVP and PEG interlayers showed the potential on compensating the loss of Voc caused by UVO irradiation. Meanwhile, the role of reengineered ETL on the performance of i-OPVs was also verified by the corresponding EQE spectra (Fig. 2d). Although a short time UVO can reduce intragap states of ZnO films, the surface dangling bonds and chemical O2 at interface still act as trap centers. The incorporation of dipole interlayer not only relieves energy offset, but also restrains the interfacial bimolecular recombination loss of the captured photogenerated charge carries. Moreover, J-V characteristic of 5′UVO-ZnO devices using different concentration of PVP or PEG for modification are also investigated, and the results are displayed in Fig. S1a and b (Supporting Information). The detailed parameters of
2. Experimental section All materials involved in PTB7:PC71BM based i-OPVs were as follows. Inverted and reengineered OPVs with the structure (Fig. 1) of ITO/ZnO/PTB7:PC71BM/MoO3/Ag and ITO/UVO-ZnO/polymer interlayer/PTB7:PC71BM/MoO3/Ag were fabricated according to previous reports. A sol-gel ZnO solution was prepared by dissolving zinc acetate dihydrate (0.84 mg, 99.999% trace metals basis, Aldrich) in 2-methoxyethanol (5 mL, 99.8%, anhydrous, Aldrich), as well as a small amount of ethanolamine (0.1 mL, Aldrich). PVP was dissolved in methanol solution with different concentrations (0.5, 1 and 1.5 mg/mL) and denoted as PVP0.5, PVP1, and PVP1.5. Similarly, PEG0.1, PEG0.5, and
Fig. 1. Device configuration used in this work and molecule structure of PTB7, PC71BM, PVP, and PEG, respectively. 10
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Fig. 2. (a) The J-V characteristic of the devices with untreated ZnO, UVO-ZnO, and reengineered ETL (UVO-ZnO/PVP or PEG). The J-V curves of i-OPVs based on (b) PVP and (c) PEG interfacial modification of different UVO irradiated time ZnO as ETL. (d) EQE spectra of the devices with ZnO, 5′UVO-ZnO, and 5′UVO-ZnO/PVP or PEG ETLs.
device performance are included in Table S1, and the results indicate the best performance was achieved after incorporating 1 mg/mL PVP or 0.5 mg/mL PEG. A high concentration dipole polymer interlayer will decrease the conductivity, which is disadvantage to device efficiency. X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface chemical composition of the ZnO and UVO-ZnO films. In Fig. S2a–c, the core-level XPS profiles of Zn 2p, O 1s, and C 1s, of ETL were measured as the function of the UVO exposure time. The binding energy of Zn 2p3/2 peak value was 1022.7 eV in the ZnO layer (Fig. S2a). The maximum peak value of Zn 2p3/2 shifts toward lower binding energy after UVO treatment, implying that more Zn atoms are tied to O atoms (Cebulla et al., 1998; Lee et al., 2009). The XPS results of O 1s exhibit asymmetric spectra in Fig. S2b. The UVO irradiation amplified the relative magnitude of the peak at 531.7 eV, which manifests that more Zn-O bonds are consisted in ZnO matrix near the upper surface of the ETL (Hsieh et al., 2007). The relative magnitude of the peak at 530.6 eV was also raised, suggesting that additional O2 molecules and other surface oxygen species (i.e. oxygen containing functional groups) were absorbed (Small et al., 2012). As demonstrated in C1s XPS spectrum (Fig. S2c), ZnO and UVO-ZnO films owning three peaks at 284.7, 286.3, and 288.6 eV are attributed to CeC, CeO, and O]CeOH, respectively. The content of solvent residents is gradually removed by UVO treatment as convinced by lower peak intensity. Obviously, the intensity of peak at 284.7 eV is decreased with the increase of UVO treatment time, resulting from the decomposition of the carbon containing organic elements. However, the same phenomenon could not be observed at peak of 286.3 eV. Therefore, it can be deduced that residual hydrocarbons may react with abound oxygen to produce CeO bonding after the long time UVO irradiation. The X-ray diffraction (XRD) spectrum of ZnO films on glass
Table 1 Photovoltaic parameters of the all fabricated i-OPVs with untreated ZnO, UVOZnO and with reengineered ETL (UVO-ZnO/PVP or PEG). i-OPVs
VOC (V)
JSC (mA cm−2)
FF (%)
PCE (%)
Reference device
0.72
15.58
65.55
7.40
2′ UVO-ZnO 5′ UVO-ZnO 10′ UVO-ZnO 15′ UVO-ZnO
0.71 0.71 0.70 0.65
16.89 16.96 15.81 15.00
66.22 65.50 60.46 55.91
7.95 7.89 6.70 5.49
5′ UVO-ZnO/PVP1 5′ UVO-ZnO/PEG0.5
0.74 0.73
18.00 17.86
73.12 72.32
9.74 9.45
Table 2 Photovoltaic parameters of all i-OPV devices using ZnO/PVP or PEG ETLs under different UVO treatment time. i-OPVs
VOC (V)
JSC (mA cm−2)
FF (%)
PCE (%)
0′ UVO-ZnO/PVP1 2′ UVO-ZnO/PVP1 5′ UVO-ZnO/PVP1 10′ UVO-ZnO/PVP1 15′ UVO-ZnO/PVP1
0.74 0.73 0.74 0.72 0.70
16.25 16.98 18.00 15.24 14.59
67.18 72.13 73.12 68.78 61.30
8.11 8.91 9.74 7.59 6.24
0′ UVO-ZnO/PEG0.5 2′ UVO-ZnO/PEG0.5 5′ UVO-ZnO/PEG0.5 10′ UVO-ZnO/PEG0.5 15′ UVO-ZnO/PEG0.5
0.74 0.73 0.73 0.72 0.69
16.19 16.90 17.86 15.22 14.41
67.15 70.42 72.32 67.94 60.94
8.01 8.66 9.45 7.47 6.03
11
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Fig. 3. (a) XRD patterns of the raw ZnO film with different UVO treatment duration on glass substrate (0, 2, 5, 10, and 15 min UVO exposure from down to top). (b) The light transmission spectra of ITO/ZnO, and ITO/5′UVO-ZnO, and ITO/UVO-ZnO/PVP or PEG. (c) Zoom in the transmission spectra. (d) The diode current-voltage characteristics of ITO/ZnO/Ag as a function of UVO treatment time, and ITO/5′UVO-ZnO/PVP1 or PEG0.5.
substrate was implemented, and representative data of the films without and with UVO treatment are shown in Fig. 3a. Standard PDF JCPDS:36-1451 was used as comparison. It can be seen that broad and weak (1 0 0), (0 0 2), (1 1 0), and (1 1 2) resonances happen in ZnO films because of the low film forming temperature (200 °C). For samples subjected to UVO treatment time of 2, 5, 10, and 15 min, the intensity of diffraction peaks were almost the same. The intensity ratio enhancement demonstrates that ZnO films tend to form a stable crystal structure. The light transmission spectra and the zoom in version of ITO/ZnO, and ITO/5́ UVO-ZnO, and ITO/5́ UVO-ZnO/PVP1 or PEG0.5 composite layers were displayed in Fig. 3b and c. The transmission of UVO treated ZnO is enhanced, while the transmission ratio at the wavelength range of 400–800 nm was slightly decreased after introducing polymer interlayer. All kinds of ETLs indicate inappreciable changes
and allow majority photons utilized by the active layer. (Rafique et al., 2017) To investigate the role of UVO irradiation and dipole polymer surface modification, the current-voltage characteristics of diode are shown in Fig. 3d, which is acquired in the device configuration of ITO/ ZnO/Ag, ITO/UVO-ZnO/Ag, and ITO/5́ UVO-ZnO/PVP1 or PEG0.5/Ag. Additionally, the electrical conductivity can be calculated from the equation of σ = G*d/A, here σ is the electrical conductivity, G is the conductance which can be derived from slope of the above curve, d is thickness of the film, and A is the device area. Compared to the pristine ZnO film, the UVO-ZnO film treated with a short time presented a steeper slope, which means a bigger conductivity. Therefore, it can be concluded that the introduction of PVP and PEG will inevitably decrease the conductivity of ETL. Because PVP or PEG interlayers are ultra-thin, UVO-ZnO/PVP or PEG devices behave a negligible decreased 12
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Fig. 4. (a) Schematic diagram of water contact angle measurement, (b) ZnO, (c) 2′UVO-ZnO, (d) 5′UVO-ZnO, (e) 10′UVO-ZnO, (f) 15′UVO-ZnO, (g) 5′UVO-ZnO/PVP, and (h) 5′UVO-ZnO/PEG films.
shown in Fig. 5a, the work function (WF) of ZnO film without reengineering is 4.4 eV, while WF of 2́ UVO-ZnO film is 4.33 eV. A short UVO irradiation could remove some surface organic residues. The energy band bending induced by the contacting of acceptor with UVOZnO and UVO-ZnO/dipole interlayer are displayed in Fig. 5b and c. However, an increased UV–ozone treatment time (5, 10, and 15 min) of ZnO pushed the Fermi-level away from the vacuum level, thus the WF of films is increased to 4.52, 4.60, and 4.74 eV (Fig. 5b), respectively, which could be attributed to the redundant oxygen species (Olson et al., 2008; Lepage et al., 2012; Ben Khalifa et al., 2004). On the contrary, introducing PVP or PEG interlayer on 5́ UVO-ZnO surface provided a decreased energy level offset between ETL and BHJ (Fig. 5c), leading to a lower WF of 4.18 and 4.23 eV respectively. Such a positive impact of energy potential indicates that an electric dipole moment pointing outward from ETL is obtained at the surface of ZnO. The oxygen atoms of the polymer bulk will strongly bind to defect states and make up for the shallow trap close to the conduction band, which not only
current compared to UVO-ZnO device. The surface wettability of cathode interlayer may have influence on its interface contact to the BHJ. Thus, the wetting properties of ZnO, UVO-ZnO, UVO-ZnO/PVP, and UVO-ZnO/PEG films on ITO was explored by water contact angle (WCA) measurement, and the results are presented in Fig. 4a–h. The unprocessed ZnO substrate displays a WCA of 42° (Fig. 4b), while WCA of ZnO is decreased to 39° after 2 min UVO treatment (Fig. 4c). After the irradiation time was further increased, ZnO indicated the remarkable decreased WCA of 27°, 14°, and 8° under 5, 10, and 15 min UVO process, respectively (Fig. 4d–f). Therefore, UVO process is not conducive to active layer deposition. After introducing PVP or PEG interlayer, WCA demonstrated 43° and 45° respectively, which are the same level to untreated ZnO film (Fig. 4g and h). The performance enhancement mechanism of UVO treatment and PVP or PEG modifier was also investigated from the energy band by Kelvin probe microscopy (KPM), and results are displayed in Fig. S3. As
Fig. 5. (a) Energy levels alignment of i-OPVs with ZnO, UVO-ZnO, and UVO-ZnO/PVP or PEG. The energy band bending of PC71BM contacting with (b) UVO-ZnO and (c) UVO-ZnO/PVP or PEG ETL. 13
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Fig. 6. (a) PL spectra of ITO/ZnO/BHJ, ITO/UVO-ZnO/BHJ, and ITO/UVO-ZnO/PVP or PEG/BHJ films. (b) J–V curves of electron-only devices (ITO/ETL/ PTB7:PC71BM/BCP/Al) with raw ZnO film, 5′UVO-ZnO film, and 5′UVO-ZnO/PVP1 or PEG0.5 ETLs, respectively.
passivates surface traps but also eliminates the interface barrier (Yang and Tsutsui, 2000; Jao et al., 2016; Dualeh et al., 2014), resulting in efficient electron extraction and suppressed interface recombination (Bartesaghi et al., 2015; Wang et al., 2016; Chauhan et al., 2018). To verify the hypothesis that the reengineered ETL can reduce the energy barrier and remove the surface defect states, the PL spectra of ITO/ZnO/BHJ, ITO/UVO-ZnO/BHJ, and ITO/UVO-ZnO/PVP or PEG/ BHJ composite layer were measured (Fig. 6a). An excitation peak is observed on untreated ZnO around 720 nm due to the exciton recombination loss. Moreover, a weakened emission of the reengineered ZnO can be detected, indicating that the cooperation of UVO process and polymer interlayer contributes to eliminate the inherent defect states and improve the connection of composite films. Therefore, the photoexcited electron can transfer to the reengineered ETL more efficiently, resulting in the increased Jsc and PCE. Single-electron devices with configuration of ITO/ETL/PTB7:PC71BM/BCP/Ag were fabricated to investigate the electron extraction of optimized devices (Pockett et al., 2015; Sun et al., 2016). The dark J-V curves and the corresponding electron mobilities are shown in Fig. 6b. The improved current densities of the devices with reengineered ZnO ETLs demonstrate the facilitated electron transport, accounting for the improved device efficiency. The surface morphology of ZnO, UVO-ZnO, and UVO-ZnO/polymer were tested by AFM topography in Fig. S4a–g. The UVO treatment decreased the root-mean-square (RMS) roughness to 2.03 nm for the film comprising 5́ UVO-ZnO from 2.72 nm for the pristine ZnO film. As shown in the AFM images, UVO treatment is helpful to eliminate the carboxyl groups and other organic residents on ZnO surface, resulting in the smooth film. After increasing UVO processed time, the excess atomic oxygen species could diffuse into the ZnO film more easily, which can be supported by the decreased RMS roughness (Fig. S4) and XPS result. Combined with PVP or PEG modification layer, the films demonstrate smoother surface and lower RMS of 1.41 nm and 1.63 nm, respectively. This status would potentially enhance the contact of composite layers, leading to an increased FF along with a reduced contact resistance. Moreover, the roughness of the different sub-layer would also affect the morphology of the PTB7:PC71BM BHJ as confirmed in Fig. 7a–d. Apparently, PTB7:PC71BM film on untreated ZnO displays a relatively rough active layer with large phase domain (RMS of 8.64 nm), which would be unfavorable for device performance. However, the active layer on reengineered ETLs show uniform glossy surface (RMS of 5.94 and 6.16 nm for 5́ UVO-ZnO/PVP and PEG respectively), and smaller scale phase separation indicates the efficient exciton dissociation. Fig. 8 plots the J-V characteristic of the devices with different ETLs
Fig. 7. AFM topography images of active layer surface morphology on different ETLs of (a) ZnO, (b) 5′UVO-ZnO, (c) 5′UVO-ZnO/PVP, and (d) 5′UVO-ZnO/ PEG.
in dark. The dark current density of the UVO-ZnO cell in the reverse bias is lower than that of the untreated ZnO, suggesting a suppressed leakage current at the ZnO/BHJ interface upon UVO treatment. Furthermore, after incorporating PVP or PEG dipole interlayer, the devices show more ideally rectification property (Wu et al., 2016). Based on forward bias region of (> 0.8 V), the injected current density of the reengineered ETL device is increased compared to the untreated device, demonstrating a reduced electron injection energy barrier at cathode (He et al., 2011). Consequently, the electron transfer from acceptor to ETL is promoted, contributing to the improved performance. Additionally, the electrochemical impedance spectroscopy (EIS) of the fabricated i-OPVs was carried out under open-circuit voltage with a frequency ranging from 20 Hz to 2 MHz to probe the resistance property of photovoltaic devices (Zuo and Ding, 2017). Fig. 8b demonstrates the Cole-Cole curve of spectra for devices with different ETLs. The fitted equivalent circuit is shown in Fig. S5 (Ma et al., 2014). Because the OPVs had the identical active layer, the improved charge transportation was only ascribed to the improved interface between BHJ and the cathode. Upon incorporating the reengineered ETL, R1 dramatically reduced from 1472.5 to 502.9 or 570.3 Ω cm2, whereas R2 slightly 14
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Fig. 8. (a) Dark J-V curves of i-OPVs with different ETL of ZnO, 5′UVO-ZnO, ZnO/PVP1 or PEG0.5 and 5′UVO-ZnO/PVP1 or PEG0.5. (b) Nyquist plots of above i-OPVs.
decreased from 47.2 to 20.1 or 18.7 Ω cm2, leading to the improved charge transport in OPVs. The incorporation of polymer interlayer and UVO treatment simultaneously promote the interfacial contact, thus prohibiting the recombination of electron and hole (Xia et al., 2015).
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4. Conclusion In summary, we developed a facile way to improve the performance of OPVs by incorporating a polymer interlayer and using UVO treatment. Among the only UVO treated i-OPVs, 2 min UVO treatment of ZnO ETL was the best, resulting in the highest device performance of 7.95%. After the dipole interlayer was incorporated, the optimal device demonstrates the best efficiency including an improved Voc of 0.74 V, a Jsc of 18.00 mA cm−2, and a FF of 73.12%, yielding the highest efficiency of 9.74%. This work provides a smart strategy of interfacial engineering to achieve high performance OPVs. Acknowledgments The authors are grateful to National Natural Science Foundation of China (61875072), the Special Project of the Province-University Coconstructing Program of Jilin Province (SXGJXX2017-3), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH), and Project of Graduate Innovation Fund of Jilin University (2017175, 2017170) for the support to the work. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.01.077. References Adhikary, P., Venkatesan, S., Adhikari, N., Maharjan, P.P., Adebanjo, O., Chen, J., Qiao, Q., 2013. Enhanced charge transport and photovoltaic performance of PBDTTT-C-T/ PC70BM solar cells via UV–ozone treatment. Nanoscale 5, 10007. Aprilia, A., Wulandari, P., Suendo, V., Hidayat, R., Fujii, A., Ozaki, M., 2013. Influences of dopant concentration in sol-gel derived AZO layer on the performance of P3HT:PCBM based inverted solar cell. Sol. Energy Mater. Sol. Cells 111, 181–188. Bartesaghi, D., Carmen Pérez, I., Kniepert, J., Roland, S., Turbiez, M., Neher, D., Koster, L.J.A., 2015. Competition between recombination and extraction of free charges determines the fill factor of organic solar cells. Nat. Commun. 6, 7083. Ben Khalifa, M., Vaufrey, D., Tardy, J., 2004. Opposing influence of hole blocking layer and a doped transport layer on the performance of heterostructure OLEDs. Org. Electron. 5, 187–198. Blum, O., Shaked, N.T., 2015. Predication of photothermal phase signatures from arbitrary plasmonic nanoparticles and experimental verification. Light Sci. Appl. 4, e322. Cebulla, R., Wendt, R., Ellmer, K., 1998. Al-doped zinc oxide films deposited by simultaneous Rf and Dc excitation of a magnetron plasma: relationships between
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Facilitating electron collection of organic photovoltaics by passivating trap states and tailoring work function Xinyuan Zhang, Yu Sun, Mei Wang, Houxiao Cui, Wenfa Xie, Liang Shen, Wenbin Guo
T
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State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electron transport Charge recombination Interface engineering Tailoring work function Interface passivation
The low-temperature solution-processed ZnO normally has intrinsic oxygen vacancies and various surface defects such as organic residues and surface adsorbates. When ZnO film is used in organic solar cells as electron selective layer, these defects will serve as bimolecular recombination centers for light induced charge carriers and trap electrons, leading to an unsatisfied device performance. Herein, we demonstrate an effective way to employ a simple combination of UV-ozone (UVO) and dipole treatment to passivate the ZnO layer, and the electronic property of ZnO layer is great improved upon eliminating the residual hydrocarbons and passivating surface traps. The reengineered uniform ZnO films play a major role on the facilitated charge transfer and decreased contact resistance, resulting in an enhanced device efficiency. The optimized ZnO transport layer also regulates the upper active layer depositing and morphology, and consequently increases the dissociation of photo-excitons and suppresses charge recombination.
1. Introduction Bulk heterojunction (BHJ) organic photovoltaics (OPVs) have become popular due to the integrating advantages of cost-effective, flexibility, colorful, transparency, and large-scale fabrication (Kim et al., 2007; Su et al., 2012; Li et al., 2015; Chen et al., 2013). Rapid efficiency progress mainly originates from new materials synthesis, cascade structure design, and ternary blend solar cells (Su et al., 2011; Guo et al., 2014; Kang et al., 2015; Cheng et al., 2016; Lin et al., 2016). OPV devices are based on the similar multilayered frameworks including the photoactive layer and metallic or metal oxide electrodes, thus the interface modification of these layers can’t be overlooked in the device fabrication. Moreover, the exploration of interfacial engineering is equally essential to the active layer morphology, which should not be ignored for pursuing high-efficiency device (Wu et al., 2016; Tran et al., 2017). The inverted device structure has been used in OPVs preparation for its stability by avoiding hygroscopic and corrosive poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate acid) (PEDOT:PSS) to device degradation (Lim et al., 2016; Cheun et al., 2012). Applying a suitable electron transport layer (ETL) between active layer and ITO cathode is an useful approach to obtain the matched energy level alignment and reduce the free carriers recombination probability (Zhu et al., 2014; Zhu et al., 2014; Zhang et al., 2014; Park et al., 2014; Holman et al., 2013). The improved interfacial contact and ⁎
reduced energy barrier facilitate electron transfer and collection of ITO cathode, which can enhance the short-circuit current density (Jsc) and fill factor (FF) of device. For inverted OPVs (i-OPVs), the transparent ZnO ETL has been used due to its solution-processed, excellent optical and electronic properties, and environmental innoxiously. However, the solution-processed ZnO film always suffers from the defect states, such as oxygen-vacancies and hydrocarbon contamination (Sun et al., 2011; Huang et al., 2017; Lin et al., 2016; Stubhan et al., 2012). Furthermore, the defect states will cause the increased recombination of photo-generated charge carriers and hinder the improvement of device efficiency. Meanwhile, the chemisorbed oxygen species in ZnO film surface can capture electrons from the conduction band (Trost et al., 2013), forming a depletion region near the surface of ZnO and leading to high resistivity. At this point, the superior ZnO film with fewer defects is highly desired and critical for highly efficient performance iOPVs. Many efforts have been performed to passivate the defect states of solution-prepared ZnO film. Doped ZnO with a wide range of materials such as fullerene derivatives, aluminum, and Sn, is a good strategy to achieve high charge carrier mobility compared to pristine ZnO (Aprilia et al., 2013; Zhang et al., 2014; Hoye et al., 2014; Ma et al., 2005). However, the doped ZnO films still have excessive surface trap sites such as dangling bonds and surface groups, which would cause a significant photo-current loss. In addition, many studies have been
Corresponding author. E-mail address: [email protected] (W. Guo).
https://doi.org/10.1016/j.solener.2019.01.077 Received 23 November 2018; Received in revised form 16 January 2019; Accepted 24 January 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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reported that surface modification of a sol-gel ZnO film by an ultrathin dipolar interlayer can contribute to a favorable electron transport, resulting in a big impact on the device performance (Nian et al., 2015; Chen et al., 2015; Xiang et al., 2013; Blum and Shaked, 2015; Su et al., 2012; Kosten et al., 2013; Yu et al., 2015; Chien et al., 2012). However, this method can’t completely eliminate the intrinsic defect states of ZnO layer, which will obstruct the electron transport and collection. Qiao’s and Jo’s groups reported that an improved device performance with short UV–ozone treatment of a ZnO film and followed by a decreased efficiency for longer exposure time (Adhikary et al., 2013; Cho et al., 2014). ZnO wurtzite structure can be significantly improved and polymeric surfactant can be efficiently eliminated by the UV–ozone treatment. Unfortunately, p-type defects induced by UV-ozone will push the Fermi-level away from vacuum level, leading to a mismatched energy alignment of device. In this work, the combination of UV-ozone treatment in atmosphere and interface coating process was carried out to reengineer low-temperature prepared ZnO ETL in i-OPVs. UV–ozone irradiation with different time was first used to re-modulate the ZnO film (UVO-ZnO) property, which correlated with the surface electronic structure and chemical composition. Simultaneously, a thin polymer interlayer was incorporated between ZnO and BHJ active layer to form strong interface dipoles in i-OPVs in terms of aligning energy level, and consequently reducing the work function (WF) of transport layers. Two kinds of common polymer materials of poly(vinylpyrrolidone) (PVP) and polyethylene glycol (PEG) were used as interlayer individually. Both PVP and PEG have lone pairs of electrons on O atoms, which will not only passivate the shallow trap state, but also bind strongly to hydroxyl and carboxylate groups. Compared to the devices with pristine ZnO ETL, the optimized UV-ozone treatment of ZnO/polymer cathode buffer layer (ETL) has advantages in accelerating the electrons collection, suppressing charge recombination, and minimizing the exciton quenching in OPVs (Sharma et al., 2017; Cowan et al., 2013). As a result, both Jsc and FF of the reengineered i-OPVs based on PTB7:PC71BM BHJ are considerably enhanced.
PEG1 are presented as PEG-6000 and dissolved in methanol solution with concentration of 0.1, 0.5 and 1 mg/mL. The cathode is indium tin oxide (ITO) glass substrate. Initially, ITO coated substrate was cleaned with detergent, deionized water, acetone, and IPA, and dried in a nitrogen stream. The precursor ZnO solution was filtered by a 0.45 μm filter. The ZnO solution was spin-coated at 3000 rpm for 60 s and annealed at 200 °C for 1 h in oven. The thickness of ZnO film is about 35 nm. In the reengineering process, the prepared ETL was treated by UV-ozone for 2, 5, 10, and 15 min (2′, 5′, 10′, 15′UVO-ZnO), respectively. The UVO cleaner equipped with an Hg lamp (50 Hz, 200 W) was employed for the preparation of UVO exposed ZnO film. PVP or PEG solution was spin-coated onto the UVO-irradiated ETL to get an ultrathin interlayer. PTB7:PC71BM solution with the optimized ratio was dissolved in chlorobenzene with 3.0 vol% 1,8-diiodooctane (DIO), and the blend solution was spin-coated onto the prepared interlayer surface. The resulting thickness of active layer was about 90 nm. At last, MoO3 (10 nm) and Ag (100 nm) were sequentially deposited by thermally evaporating under high vacuum (9.0 × 10-4 Pa). The measurement and characterization are included in Supporting Information. 3. Results and discussion Water-/alcohol-soluble PVP and PEG are low cost non-conjugated polymer, which have been previously used as effective electron transport layer in conventional OPVs to facilitate electron extraction (Zhou et al., 2017; Rao and Vinni, 1993). The illuminated J-V curves of all fabricated devices with untreated ZnO, UVO-ZnO, and reengineered ETL (UVO-ZnO/PVP or PEG) are shown in Fig. 2a. All the related photovoltaic parameters are summarized in Table 1, including Jsc, FF, open-circuit voltage (Voc), and power conversion efficiency (PCE). Obviously, the reference device presented an unsatisfactory PCE of 7.40% with Voc of 0.72 V, a Jsc of 15.58 mA/cm2, and a FF of 65.55%, whereas the champion efficiency of 9.74% and 9.45% for device with UVO-ZnO/PVP and UVO-ZnO/PEG ETL were realized in the optimized condition, respectively. The 5́ UVO-ZnO/PVP1 based device obtained an increased Voc of 0.74 V, a Jsc of 18 mA/cm2, and a FF of 73.12%. The 5́ UVO-ZnO/PEG0.5 based device exhibited an increased Voc of 0.73 V, a Jsc of 17.86 mA/cm2, and a FF of 72.32%. UVO irradiation time of ZnO ETL showed an obvious effect on device performance. After 2 min of UVO dispose, PCE was enhanced to 7.95% with a simultaneous increase of Jsc and FF. After a short irradiation within 5 min, the increased Jsc and decreased FF were observed. However, Jsc and FF both severely decreased when irradiation time was further increased. In addition, UVO treatment showed a negative influence on Voc of i-OPVs. The WF of ZnO film is increased due to the redundant oxygen species, which induces p-type defects by UVO treatment in atmosphere. To further enhance the performance of device, PVP and PEG interface modification of ZnO with different UVO irradiated time were carried out and J-V characteristic of these i-OPVs are presented in Fig. 2b and c. The detailed device parameters are included in Table 2. Jsc and FF are significantly enhanced after introducing PVP and PEG interlayer. It is worth noting that PCE of i-OPVs with 5́ UVO-ZnO/PVP1 is improved by 31.6% and 23.4% compared to those of the reference device respectively. Moreover, PVP and PEG interlayers showed the potential on compensating the loss of Voc caused by UVO irradiation. Meanwhile, the role of reengineered ETL on the performance of i-OPVs was also verified by the corresponding EQE spectra (Fig. 2d). Although a short time UVO can reduce intragap states of ZnO films, the surface dangling bonds and chemical O2 at interface still act as trap centers. The incorporation of dipole interlayer not only relieves energy offset, but also restrains the interfacial bimolecular recombination loss of the captured photogenerated charge carries. Moreover, J-V characteristic of 5′UVO-ZnO devices using different concentration of PVP or PEG for modification are also investigated, and the results are displayed in Fig. S1a and b (Supporting Information). The detailed parameters of
2. Experimental section All materials involved in PTB7:PC71BM based i-OPVs were as follows. Inverted and reengineered OPVs with the structure (Fig. 1) of ITO/ZnO/PTB7:PC71BM/MoO3/Ag and ITO/UVO-ZnO/polymer interlayer/PTB7:PC71BM/MoO3/Ag were fabricated according to previous reports. A sol-gel ZnO solution was prepared by dissolving zinc acetate dihydrate (0.84 mg, 99.999% trace metals basis, Aldrich) in 2-methoxyethanol (5 mL, 99.8%, anhydrous, Aldrich), as well as a small amount of ethanolamine (0.1 mL, Aldrich). PVP was dissolved in methanol solution with different concentrations (0.5, 1 and 1.5 mg/mL) and denoted as PVP0.5, PVP1, and PVP1.5. Similarly, PEG0.1, PEG0.5, and
Fig. 1. Device configuration used in this work and molecule structure of PTB7, PC71BM, PVP, and PEG, respectively. 10
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Fig. 2. (a) The J-V characteristic of the devices with untreated ZnO, UVO-ZnO, and reengineered ETL (UVO-ZnO/PVP or PEG). The J-V curves of i-OPVs based on (b) PVP and (c) PEG interfacial modification of different UVO irradiated time ZnO as ETL. (d) EQE spectra of the devices with ZnO, 5′UVO-ZnO, and 5′UVO-ZnO/PVP or PEG ETLs.
device performance are included in Table S1, and the results indicate the best performance was achieved after incorporating 1 mg/mL PVP or 0.5 mg/mL PEG. A high concentration dipole polymer interlayer will decrease the conductivity, which is disadvantage to device efficiency. X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface chemical composition of the ZnO and UVO-ZnO films. In Fig. S2a–c, the core-level XPS profiles of Zn 2p, O 1s, and C 1s, of ETL were measured as the function of the UVO exposure time. The binding energy of Zn 2p3/2 peak value was 1022.7 eV in the ZnO layer (Fig. S2a). The maximum peak value of Zn 2p3/2 shifts toward lower binding energy after UVO treatment, implying that more Zn atoms are tied to O atoms (Cebulla et al., 1998; Lee et al., 2009). The XPS results of O 1s exhibit asymmetric spectra in Fig. S2b. The UVO irradiation amplified the relative magnitude of the peak at 531.7 eV, which manifests that more Zn-O bonds are consisted in ZnO matrix near the upper surface of the ETL (Hsieh et al., 2007). The relative magnitude of the peak at 530.6 eV was also raised, suggesting that additional O2 molecules and other surface oxygen species (i.e. oxygen containing functional groups) were absorbed (Small et al., 2012). As demonstrated in C1s XPS spectrum (Fig. S2c), ZnO and UVO-ZnO films owning three peaks at 284.7, 286.3, and 288.6 eV are attributed to CeC, CeO, and O]CeOH, respectively. The content of solvent residents is gradually removed by UVO treatment as convinced by lower peak intensity. Obviously, the intensity of peak at 284.7 eV is decreased with the increase of UVO treatment time, resulting from the decomposition of the carbon containing organic elements. However, the same phenomenon could not be observed at peak of 286.3 eV. Therefore, it can be deduced that residual hydrocarbons may react with abound oxygen to produce CeO bonding after the long time UVO irradiation. The X-ray diffraction (XRD) spectrum of ZnO films on glass
Table 1 Photovoltaic parameters of the all fabricated i-OPVs with untreated ZnO, UVOZnO and with reengineered ETL (UVO-ZnO/PVP or PEG). i-OPVs
VOC (V)
JSC (mA cm−2)
FF (%)
PCE (%)
Reference device
0.72
15.58
65.55
7.40
2′ UVO-ZnO 5′ UVO-ZnO 10′ UVO-ZnO 15′ UVO-ZnO
0.71 0.71 0.70 0.65
16.89 16.96 15.81 15.00
66.22 65.50 60.46 55.91
7.95 7.89 6.70 5.49
5′ UVO-ZnO/PVP1 5′ UVO-ZnO/PEG0.5
0.74 0.73
18.00 17.86
73.12 72.32
9.74 9.45
Table 2 Photovoltaic parameters of all i-OPV devices using ZnO/PVP or PEG ETLs under different UVO treatment time. i-OPVs
VOC (V)
JSC (mA cm−2)
FF (%)
PCE (%)
0′ UVO-ZnO/PVP1 2′ UVO-ZnO/PVP1 5′ UVO-ZnO/PVP1 10′ UVO-ZnO/PVP1 15′ UVO-ZnO/PVP1
0.74 0.73 0.74 0.72 0.70
16.25 16.98 18.00 15.24 14.59
67.18 72.13 73.12 68.78 61.30
8.11 8.91 9.74 7.59 6.24
0′ UVO-ZnO/PEG0.5 2′ UVO-ZnO/PEG0.5 5′ UVO-ZnO/PEG0.5 10′ UVO-ZnO/PEG0.5 15′ UVO-ZnO/PEG0.5
0.74 0.73 0.73 0.72 0.69
16.19 16.90 17.86 15.22 14.41
67.15 70.42 72.32 67.94 60.94
8.01 8.66 9.45 7.47 6.03
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Fig. 3. (a) XRD patterns of the raw ZnO film with different UVO treatment duration on glass substrate (0, 2, 5, 10, and 15 min UVO exposure from down to top). (b) The light transmission spectra of ITO/ZnO, and ITO/5′UVO-ZnO, and ITO/UVO-ZnO/PVP or PEG. (c) Zoom in the transmission spectra. (d) The diode current-voltage characteristics of ITO/ZnO/Ag as a function of UVO treatment time, and ITO/5′UVO-ZnO/PVP1 or PEG0.5.
substrate was implemented, and representative data of the films without and with UVO treatment are shown in Fig. 3a. Standard PDF JCPDS:36-1451 was used as comparison. It can be seen that broad and weak (1 0 0), (0 0 2), (1 1 0), and (1 1 2) resonances happen in ZnO films because of the low film forming temperature (200 °C). For samples subjected to UVO treatment time of 2, 5, 10, and 15 min, the intensity of diffraction peaks were almost the same. The intensity ratio enhancement demonstrates that ZnO films tend to form a stable crystal structure. The light transmission spectra and the zoom in version of ITO/ZnO, and ITO/5́ UVO-ZnO, and ITO/5́ UVO-ZnO/PVP1 or PEG0.5 composite layers were displayed in Fig. 3b and c. The transmission of UVO treated ZnO is enhanced, while the transmission ratio at the wavelength range of 400–800 nm was slightly decreased after introducing polymer interlayer. All kinds of ETLs indicate inappreciable changes
and allow majority photons utilized by the active layer. (Rafique et al., 2017) To investigate the role of UVO irradiation and dipole polymer surface modification, the current-voltage characteristics of diode are shown in Fig. 3d, which is acquired in the device configuration of ITO/ ZnO/Ag, ITO/UVO-ZnO/Ag, and ITO/5́ UVO-ZnO/PVP1 or PEG0.5/Ag. Additionally, the electrical conductivity can be calculated from the equation of σ = G*d/A, here σ is the electrical conductivity, G is the conductance which can be derived from slope of the above curve, d is thickness of the film, and A is the device area. Compared to the pristine ZnO film, the UVO-ZnO film treated with a short time presented a steeper slope, which means a bigger conductivity. Therefore, it can be concluded that the introduction of PVP and PEG will inevitably decrease the conductivity of ETL. Because PVP or PEG interlayers are ultra-thin, UVO-ZnO/PVP or PEG devices behave a negligible decreased 12
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Fig. 4. (a) Schematic diagram of water contact angle measurement, (b) ZnO, (c) 2′UVO-ZnO, (d) 5′UVO-ZnO, (e) 10′UVO-ZnO, (f) 15′UVO-ZnO, (g) 5′UVO-ZnO/PVP, and (h) 5′UVO-ZnO/PEG films.
shown in Fig. 5a, the work function (WF) of ZnO film without reengineering is 4.4 eV, while WF of 2́ UVO-ZnO film is 4.33 eV. A short UVO irradiation could remove some surface organic residues. The energy band bending induced by the contacting of acceptor with UVOZnO and UVO-ZnO/dipole interlayer are displayed in Fig. 5b and c. However, an increased UV–ozone treatment time (5, 10, and 15 min) of ZnO pushed the Fermi-level away from the vacuum level, thus the WF of films is increased to 4.52, 4.60, and 4.74 eV (Fig. 5b), respectively, which could be attributed to the redundant oxygen species (Olson et al., 2008; Lepage et al., 2012; Ben Khalifa et al., 2004). On the contrary, introducing PVP or PEG interlayer on 5́ UVO-ZnO surface provided a decreased energy level offset between ETL and BHJ (Fig. 5c), leading to a lower WF of 4.18 and 4.23 eV respectively. Such a positive impact of energy potential indicates that an electric dipole moment pointing outward from ETL is obtained at the surface of ZnO. The oxygen atoms of the polymer bulk will strongly bind to defect states and make up for the shallow trap close to the conduction band, which not only
current compared to UVO-ZnO device. The surface wettability of cathode interlayer may have influence on its interface contact to the BHJ. Thus, the wetting properties of ZnO, UVO-ZnO, UVO-ZnO/PVP, and UVO-ZnO/PEG films on ITO was explored by water contact angle (WCA) measurement, and the results are presented in Fig. 4a–h. The unprocessed ZnO substrate displays a WCA of 42° (Fig. 4b), while WCA of ZnO is decreased to 39° after 2 min UVO treatment (Fig. 4c). After the irradiation time was further increased, ZnO indicated the remarkable decreased WCA of 27°, 14°, and 8° under 5, 10, and 15 min UVO process, respectively (Fig. 4d–f). Therefore, UVO process is not conducive to active layer deposition. After introducing PVP or PEG interlayer, WCA demonstrated 43° and 45° respectively, which are the same level to untreated ZnO film (Fig. 4g and h). The performance enhancement mechanism of UVO treatment and PVP or PEG modifier was also investigated from the energy band by Kelvin probe microscopy (KPM), and results are displayed in Fig. S3. As
Fig. 5. (a) Energy levels alignment of i-OPVs with ZnO, UVO-ZnO, and UVO-ZnO/PVP or PEG. The energy band bending of PC71BM contacting with (b) UVO-ZnO and (c) UVO-ZnO/PVP or PEG ETL. 13
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Fig. 6. (a) PL spectra of ITO/ZnO/BHJ, ITO/UVO-ZnO/BHJ, and ITO/UVO-ZnO/PVP or PEG/BHJ films. (b) J–V curves of electron-only devices (ITO/ETL/ PTB7:PC71BM/BCP/Al) with raw ZnO film, 5′UVO-ZnO film, and 5′UVO-ZnO/PVP1 or PEG0.5 ETLs, respectively.
passivates surface traps but also eliminates the interface barrier (Yang and Tsutsui, 2000; Jao et al., 2016; Dualeh et al., 2014), resulting in efficient electron extraction and suppressed interface recombination (Bartesaghi et al., 2015; Wang et al., 2016; Chauhan et al., 2018). To verify the hypothesis that the reengineered ETL can reduce the energy barrier and remove the surface defect states, the PL spectra of ITO/ZnO/BHJ, ITO/UVO-ZnO/BHJ, and ITO/UVO-ZnO/PVP or PEG/ BHJ composite layer were measured (Fig. 6a). An excitation peak is observed on untreated ZnO around 720 nm due to the exciton recombination loss. Moreover, a weakened emission of the reengineered ZnO can be detected, indicating that the cooperation of UVO process and polymer interlayer contributes to eliminate the inherent defect states and improve the connection of composite films. Therefore, the photoexcited electron can transfer to the reengineered ETL more efficiently, resulting in the increased Jsc and PCE. Single-electron devices with configuration of ITO/ETL/PTB7:PC71BM/BCP/Ag were fabricated to investigate the electron extraction of optimized devices (Pockett et al., 2015; Sun et al., 2016). The dark J-V curves and the corresponding electron mobilities are shown in Fig. 6b. The improved current densities of the devices with reengineered ZnO ETLs demonstrate the facilitated electron transport, accounting for the improved device efficiency. The surface morphology of ZnO, UVO-ZnO, and UVO-ZnO/polymer were tested by AFM topography in Fig. S4a–g. The UVO treatment decreased the root-mean-square (RMS) roughness to 2.03 nm for the film comprising 5́ UVO-ZnO from 2.72 nm for the pristine ZnO film. As shown in the AFM images, UVO treatment is helpful to eliminate the carboxyl groups and other organic residents on ZnO surface, resulting in the smooth film. After increasing UVO processed time, the excess atomic oxygen species could diffuse into the ZnO film more easily, which can be supported by the decreased RMS roughness (Fig. S4) and XPS result. Combined with PVP or PEG modification layer, the films demonstrate smoother surface and lower RMS of 1.41 nm and 1.63 nm, respectively. This status would potentially enhance the contact of composite layers, leading to an increased FF along with a reduced contact resistance. Moreover, the roughness of the different sub-layer would also affect the morphology of the PTB7:PC71BM BHJ as confirmed in Fig. 7a–d. Apparently, PTB7:PC71BM film on untreated ZnO displays a relatively rough active layer with large phase domain (RMS of 8.64 nm), which would be unfavorable for device performance. However, the active layer on reengineered ETLs show uniform glossy surface (RMS of 5.94 and 6.16 nm for 5́ UVO-ZnO/PVP and PEG respectively), and smaller scale phase separation indicates the efficient exciton dissociation. Fig. 8 plots the J-V characteristic of the devices with different ETLs
Fig. 7. AFM topography images of active layer surface morphology on different ETLs of (a) ZnO, (b) 5′UVO-ZnO, (c) 5′UVO-ZnO/PVP, and (d) 5′UVO-ZnO/ PEG.
in dark. The dark current density of the UVO-ZnO cell in the reverse bias is lower than that of the untreated ZnO, suggesting a suppressed leakage current at the ZnO/BHJ interface upon UVO treatment. Furthermore, after incorporating PVP or PEG dipole interlayer, the devices show more ideally rectification property (Wu et al., 2016). Based on forward bias region of (> 0.8 V), the injected current density of the reengineered ETL device is increased compared to the untreated device, demonstrating a reduced electron injection energy barrier at cathode (He et al., 2011). Consequently, the electron transfer from acceptor to ETL is promoted, contributing to the improved performance. Additionally, the electrochemical impedance spectroscopy (EIS) of the fabricated i-OPVs was carried out under open-circuit voltage with a frequency ranging from 20 Hz to 2 MHz to probe the resistance property of photovoltaic devices (Zuo and Ding, 2017). Fig. 8b demonstrates the Cole-Cole curve of spectra for devices with different ETLs. The fitted equivalent circuit is shown in Fig. S5 (Ma et al., 2014). Because the OPVs had the identical active layer, the improved charge transportation was only ascribed to the improved interface between BHJ and the cathode. Upon incorporating the reengineered ETL, R1 dramatically reduced from 1472.5 to 502.9 or 570.3 Ω cm2, whereas R2 slightly 14
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Fig. 8. (a) Dark J-V curves of i-OPVs with different ETL of ZnO, 5′UVO-ZnO, ZnO/PVP1 or PEG0.5 and 5′UVO-ZnO/PVP1 or PEG0.5. (b) Nyquist plots of above i-OPVs.
decreased from 47.2 to 20.1 or 18.7 Ω cm2, leading to the improved charge transport in OPVs. The incorporation of polymer interlayer and UVO treatment simultaneously promote the interfacial contact, thus prohibiting the recombination of electron and hole (Xia et al., 2015).
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4. Conclusion In summary, we developed a facile way to improve the performance of OPVs by incorporating a polymer interlayer and using UVO treatment. Among the only UVO treated i-OPVs, 2 min UVO treatment of ZnO ETL was the best, resulting in the highest device performance of 7.95%. After the dipole interlayer was incorporated, the optimal device demonstrates the best efficiency including an improved Voc of 0.74 V, a Jsc of 18.00 mA cm−2, and a FF of 73.12%, yielding the highest efficiency of 9.74%. This work provides a smart strategy of interfacial engineering to achieve high performance OPVs. Acknowledgments The authors are grateful to National Natural Science Foundation of China (61875072), the Special Project of the Province-University Coconstructing Program of Jilin Province (SXGJXX2017-3), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH), and Project of Graduate Innovation Fund of Jilin University (2017175, 2017170) for the support to the work. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.01.077. References Adhikary, P., Venkatesan, S., Adhikari, N., Maharjan, P.P., Adebanjo, O., Chen, J., Qiao, Q., 2013. Enhanced charge transport and photovoltaic performance of PBDTTT-C-T/ PC70BM solar cells via UV–ozone treatment. Nanoscale 5, 10007. Aprilia, A., Wulandari, P., Suendo, V., Hidayat, R., Fujii, A., Ozaki, M., 2013. Influences of dopant concentration in sol-gel derived AZO layer on the performance of P3HT:PCBM based inverted solar cell. Sol. Energy Mater. Sol. Cells 111, 181–188. Bartesaghi, D., Carmen Pérez, I., Kniepert, J., Roland, S., Turbiez, M., Neher, D., Koster, L.J.A., 2015. Competition between recombination and extraction of free charges determines the fill factor of organic solar cells. Nat. Commun. 6, 7083. Ben Khalifa, M., Vaufrey, D., Tardy, J., 2004. Opposing influence of hole blocking layer and a doped transport layer on the performance of heterostructure OLEDs. Org. Electron. 5, 187–198. Blum, O., Shaked, N.T., 2015. Predication of photothermal phase signatures from arbitrary plasmonic nanoparticles and experimental verification. Light Sci. Appl. 4, e322. Cebulla, R., Wendt, R., Ellmer, K., 1998. Al-doped zinc oxide films deposited by simultaneous Rf and Dc excitation of a magnetron plasma: relationships between
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