A Facile Two-Step Interface Engineering Strategy To Boost the

Sep 10, 2017 - A facile two-step strategy was proposed to modify ZnO nanoparticle electron transfer layer in inverted ternary-blend polymer solar cell...
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Research Article pubs.acs.org/journal/ascecg

A Facile Two-Step Interface Engineering Strategy To Boost the Efficiency of Inverted Ternary-Blend Polymer Solar Cells over 10% Xiaoxiang Sun,† Chang Li,† Jian Ni,*,† Like Huang,† Rui Xu,† Zhenglong Li,† Hongkun Cai,† Juan Li,† Yaofang Zhang,‡ and Jianjun Zhang† †

College of Electronic Information and Optical Engineering, The Tianjin Key Laboratory for Optical-Electronic Thin Film Devices and Technology, Nankai University, No. 38 Tongyan Road, Tianjin 300350, China ‡ College of Science, Tianjin Polytechnic University, No. 399 Binshui West Road, Tianjin 300387, China ABSTRACT: A facile two-step strategy was proposed to modify ZnO nanoparticle electron transfer layer in inverted ternary-blend polymer solar cells (TPSCs) by irradiating the ZnO with UV-ozone (UVO) and then spin-coating on it a poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)alt-2,7-(9,9-dioctylfluorene)] (PFN) interfacial modification layer. The combination of UVO and PFN showed the advantage of simultaneously passivating the defect states of ZnO to reduce the defect-assisted electron−hole recombination, modifying the surface morphology and hydrophilicity of ZnO to improve the interfacial contact with the active layer, and optimizing the electron transfer characteristic of ZnO to enhance the electron extraction from the active layer to the cathode. As a result, the champion power conversion efficiency of TPSCs with PTB7-Th:PCDTBT:PC70BM as the active layer was increased from 6.80% to 9.61% by 20 min of UVO irradiation and then further increased to 10.87% after the incorporation of an ultrathin PFN interfacial layer. Because of the ease of fabrication and remarkable boost in efficiency, our results indicate that this two-step strategy provides a simple and effective way to fabricate highly efficient inverted PSCs. KEYWORDS: Two-step strategy, Interfacial modification, Inverted devices, Ternary-blend polymer solar cells

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INTRODUCTION Polymer solar cells (PSCs) have attracted growing attention over the past decades because of their ability to generate electricity in a clean and renewable way.1,2 Thanks to their advantages, such as low cost, light weight, flexibility, and solution processing, extensive research on PSCs has been carried out.3−6 In recent years, tremendous efforts have been made to enhance the power conversion efficiency (PCE) of PSCs, wherein the ternary-blend PSCs (TPSCs) as one of the promising candidates have exhibited great potential to obtain high performance.7−10 TPSCs enjoy both enhanced photon harvesting as a result of incorporating the third component in the active layer and simplicity of fabrication conditions that is used in single bulk-heterojunction (BHJ) solar cells.11−14 More inspiringly, the PCE of TPSCs have exceeded 12%, showing the great potential of the ternary-blend strategy to boost the efficiency of PSCs.15,16 We have previously reported efficient TPSCs based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th):poly[N-9′-heptadecanyl2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) with good air stability.17−19 Hence, further enhancement of the efficiency is conductive to the possibility of largescale application of TPSCs based on PTB7Th:PCDTBT:PC70BM. © 2017 American Chemical Society

To date, two main device architectures have been employed to fabricate PSCs, and the inverted structure has been widely adopted because of its superiority in the performance and stability of PSCs.11,20−22 In the inverted structure of PSCs, a solution-processed ZnO nanoparticle (NP) film is commonly used as an effective electron transfer layer (ETL) because of its high electron mobility, optical transparency, low work function, and environmental stability.23−25 However, further improvement in the PCE is hindered by the presence of defect states on ZnO.26,27 The defect states allow increased recombination of electrons and holes generated in the active layer.23 It is therefore necessary to prepare ZnO NP films with fewer defects in order to further enhance the PCE of inverted PSCs. Previous work has demonstrated that the defect states of ZnO NP films are sensitive to light and that UV-ozone (UVO) treatment can effectively passivate the defect states.23,28 However, defects still exist in ZnO NP films treated only with UVO, and there is still much room for improvement in reducing the number of defect states and boosting the device performance of inverted PSCs. Meanwhile, the presence of surface voids in the ZnO NPs film is inevitable, which would hamper the intimate contact between the ZnO and the active layer.29,30 In this sense, optimizing the Received: June 5, 2017 Revised: August 21, 2017 Published: September 10, 2017 8997

DOI: 10.1021/acssuschemeng.7b01792 ACS Sustainable Chem. Eng. 2017, 5, 8997−9005

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Figure 1. (a) Inverted structure of the TPSCs (ITO/ZnO NPs/PFN/PTB7-Th:PCDTBT:PC70BM/MoO3/Ag). (b) The schematic diagram of the ETL fabrication process.

Figure 2. (a) J−V characteristic curves of TPSCs without and with different UVO irradiation times under AM1.5G illumination at an intensity of 100 mW/cm2. (b) EQE spectra of the corresponding devices.

surface morphology of the ZnO NP film is also necessary to enhance the PCE of inverted PSCs. In addition, the inherent incompatibility between hydrophilic ZnO and the hydrophobic active layer should not be overlooked.31,32 On the other hand, the interfacial modification layer has drawn increasing attention in PSCs, wherein conjugated polyelectrolytes (CPEs) are considered as promising candidates as ZnO interfacial modification materials.33−40 The insertion of CPEs not only reduces the surface energy and surface defects of the ZnO layer but also can effectively improve the interfacial contact between the ZnO and the active layer to reduce the series resistance of the devices.41−43 The device PCE therefore can be significantly improved by introducing a thin CPE layer between the ZnO and the active layer. Inspired by the above-mentioned research works, a two-step interface engineering strategy was proposed to modify the ETL by irradiating the ZnO NP film with UVO and then spincoating on it a poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) interfacial modification layer. In this work, the positive effect of UVO irradiation and PFN interfacial modification on the performance of TPSCs with PTB7-Th:PCDTBT:PC70BM as the active layer was confirmed. The TPSCs with 20 min UVO-irradiated ZnO showed a champion PCE of 9.61%. After incorporation of an ultrathin PFN, a champion PCE of 10.87% was obtained in the TPSCs with PFN-covered 20 min UVO-irradiated ZnO. The synergistic effect of UVO irradiation and PFN interfacial modification was comprehensively investigated by X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy

(TEM), and contact angle, transmittance, and space-chargelimited current (SCLC) measurements. These results demonstrated that the two-step strategy provides a simple and effective way to fabricate highly efficient PSCs.

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EXPERIMENTAL SECTION

Inverted Device Fabrication. The architecture of inverted TPSCs (ITO/ZnO NPs/PFN/PTB7-Th:PCDTBT:PC70BM/MoO3/ Ag) and the fabrication process of the ETL are shown in Figure 1. The ZnO NPs were synthesized according to a literature procedure.44 Mixed PTB7-Th:PCDTBT:PC70BM (0.8:0.2:1.5) was selected as the active layer material, and the mixing ratio optimization process was described in detail elsewhere.17 Before fabrication of TPSCs, the indium tin oxide (ITO) glass substrate was cleaned by ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol sequentially. The ZnO NPs were spin-coated onto the cleaned ITO-coated glass substrate at 3000 rpm for 40 s in air to reach a film with a thickness of ∼30 nm. For the preparation of UVO-irradiated ZnO, the UVO cleaner (YZUV-22C, Beijing Kenuo Instrument Co., Ltd.) equipped with a Hg lamp was used as the platform for irradiation of the ZnO film. The UV lamp can provide a power of 200 W with an irradiation area of about 400 cm2. The UVO treatment unit can produce UV light at two wavelengths (253.7 and 184.9 nm) with high energy simultaneously. The ZnO-coated substrate was placed in the UV processor drawer at a distance of about 40 mm from the UV lamp and irradiated for different times. After that, the substrate was transferred into a nitrogen-filled glovebox, and PFN was spin-coated onto the UVO-irradiated ZnO film from its methanol/acetic acid (100:2 v/v) solution at 5000 rpm for 40 s to achieve a thin layer (∼5 nm). Then the blend active layer was deposited by spin-coating from the preprepared PTB7Th:PCDTBT:PC70BM blend solution (experimental details can be 8998

DOI: 10.1021/acssuschemeng.7b01792 ACS Sustainable Chem. Eng. 2017, 5, 8997−9005

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ACS Sustainable Chemistry & Engineering Table 1. Photovoltaic Parameters for TPSCs Using Different ETLs ETLs

Jsc (mA/cm2)

Voc (V)

FF (%)

best PCE (%)

pristine ZnO 10 min UVO-ZnO 20 min UVO-ZnO 30 min UVO-ZnO 20 min UVO-ZnO/PFN

14.39 16.60 17.66 17.17 19.12

0.79 0.79 0.79 0.80 0.80

59.84 67.60 68.91 68.53 71.04

6.80 8.87 9.61 9.41 10.87

average PCE (%) 6.23 8.31 9.23 9.02 10.44

± ± ± ± ±

0.40 0.34 0.31 0.35 0.25

Figure 3. (a) J−V characteristic curves of TPSCs with UVO-ZnO and PFN-covered UVO-ZnO under AM1.5G illumination at an intensity of 100 mW/cm2. (b) EQE spectra of the corresponding devices. The results for TPSCs with pristine ZnO are also given as a reference.

Figure 4. (a) Dark J−V characteristic curves of the TPSCs with pristine ZnO, UVO-ZnO, and PFN-covered UVO-ZnO. (b) Efficiency distributions of the corresponding devices. The squares represent the average PCEs of 32 devices.

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found in ref 32). Finally, a 7 nm thick MoO3 layer and a 100 nm thick Ag layer were subsequently evaporated through a shadow mask (active area of 6 mm2) under a pressure of 7.0 × 10−4 Pa. For comparison, TPSCs with pristine ZnO were also fabricated. Characterization. The current density−voltage (J−V) measurements on the TPSCs were conducted in the dark or under simulated sunlight at 100 mW/cm2 using AM1.5G-type filter. External quantum efficiency (EQE) spectra were tested using a Solar Cell Quantum Efficiency Measurement System (QEX10) from PV Measurement, Inc. The binding energies in ETLs were determined using XPS (Thermo Scientific, ESCALAB 250Xi). Steady-state PL spectra were characterized at room temperature using a fluorescence spectrophotometer (FLS 920P, Edinburgh, U.K.) with an excitation wavelength of 320 nm. SEM (JEOL JSM-7800F), AFM (SPA 400, Seiko Instruments, Japan), and TEM (Philips Tecnai G2F20) were used to investigate the morphology and roughness. A Kruss DSA 100 goniometer was employed to measure the contact angles of distilled water on the ZnO films. Optical transmittance spectra were measured using a Cary 5000 UV−vis spectrophotometer. The electron mobility in the electron-only devices was assessed using the SCLC method.

RESULTS AND DISCUSSION To optimize the quality of the ZnO ETL and improve the device performance, a two-step interface engineering strategy was carried out by irradiating the ZnO film with UVO and then spin-coating on it a PFN interfacial modification layer, as displayed in Figure 1b. First, the effect of UVO irradiation on the performance of TPSCs was investigated by adjusting the UVO irradiation time. The irradiation time was adjusted from 0 to 30 min in 10 min increments. The J−V curves of TPSCs with different UVO irradiation times are shown in Figure 2a. The detailed device parameters are summarized in Table 1. The TPSCs without UVO irradiation (pristine ZnO) showed a PCE of 6.80% with an open-circuit voltage (Voc) of 0.79 V, a shortcircuit current density (Jsc) of 14.39 mA/cm2, and a fill factor (FF) of 59.84%. After 10 min of UVO irradiation, the PCE improved to 8.87% with Voc = 0.79 V, Jsc = 16.60 mA/cm2, and FF = 67.60%. When the UVO irradiation time was increased to 20 min, the TPSCs showed a best PCE of 9.61% with Voc = 0.79 V, Jsc = 17.66 mA/cm2, and FF = 68.91%. However, with a further increase in UVO irradiation time, mild degradation of 8999

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Figure 5. High-resolution XPS spectra of (a) Zn 2p3/2 and (b) O 1s on the surface of pristine ZnO, UVO-ZnO, and PFN-covered UVO-ZnO on the ITO substrate.

ogy and roughness, hydrophilicity, and optical and electrical properties of the ZnO film were comprehensively studied. XPS was applied to examine the chemical bonding and composition of the ZnO films. Figure 5a shows the binding energies of Zn 2p3/2 for ZnO films. The peak is located at 1022.84, 1022.14, and 1021.74 eV for the pristine ZnO, UVO-ZnO, and PFNcovered UVO-ZnO films, respectively. The downshifted binding energy implies that more Zn atoms are bonded to O atoms in the PFN-covered UVO-ZnO over the UVO-ZnO and pristine ZnO.43,47 The binding energies of O 1s for ZnO films are displayed in Figure 5b. The O 1s spectra display two peaks at binding energies of 530.61 and 531.45 eV, wherein the peak with lower binding energy corresponds to O atoms in a ZnO matrix and the peak with higher binding energy corresponds to an oxygen-deficient component.48−50 It can be noticed that the UVO irradiation increases the relative intensity of the peak at 530.61 eV and decreases the relative intensity of the peak at 531.45 eV. Furthermore, the relative intensity of the peak at 530.61 eV is further increased in the PFN-covered UVO-ZnO. These results imply that the UVO irradiation and PFN interfacial modification could modify the surface chemical bonding and composition of ZnO by decreasing the oxygendeficient component in the ZnO matrix.28,50 In addition, the downshifted binding energy of O 1s is also observed in Figure 5b, which may be attributed to the higher negative electric charge density on oxygen atoms due to the electrostatic interaction.43 To further verify the effect of UVO irradiation and PFN interfacial modification on oxygen deficiency of ZnO, the defect-related luminescence in the ZnO films was investigated by PL measurements, as shown in Figure 6. The broad-band defect emission at 555 nm in the UVO-ZnO film is significantly quenched compared with that in pristine ZnO. The quenched PL indicates a significant reduction in the number of defect states in UVO-ZnO, which may be due to the reduction of oxygen vacancies by penetration of oxygen into the ZnO during the UVO irradiation.23 As a result, the carrier recombination at the ZnO−active layer interface was reduced, and the PCE of the UVO-irradiated TPSCs was obviously enhanced, as discussed in Figure 2. Moreover, the PL is further quenched in PFN-covered UVO-ZnO, indicating that the defect states in the ZnO were further passivated by PFN. It has been reported that the passivation of PFN is due to the bonding between amine molecules and the zinc core.43 Benefiting from the better defect passivation, a further improvement in PCE up to 10.87% was achieved in the TPSCs with PFN-covered UVO-ZnO, as

the PCE was observed. Meanwhile, the effect of UVO irradiation on the performance of TPSCs was also confirmed by the corresponding EQE spectra as presented in Figure 2b. The EQE spectra of the TPSCs show a distinct increase and then decrease over the wavelength range of 450−700 nm with increasing UVO irradiation time. It is apparent that the performance of the TPSCs is drastically enhanced by an appropriate UVO irradiation time in comparison with that of the TPSCs with pristine ZnO. After the first step of the UVO irradiation, an optimum PCE of 9.61% was achieved in the TPSCs with 20 min UVOirradiated ZnO. In the subsequent studies, therefore, the UVO irradiation time for the TPSCs was set to 20 min. To further improve the PCE of TPSCs, the second step of PFN interfacial modification was carried out. The J−V curves of UVOirradiated TPSCs with and without PFN are presented in Figure 3a. The detailed device parameters are also summarized in Table 1. Because of the significant increase in Jsc (19.12 mA/ cm2) and FF (71.04%), a champion PCE of 10.87% is obtained. It is noted that the PCE of the TPSCs with PFN-covered UVOZnO is increased by 59.9% and 13.1% relative to those of the TPSCs with pristine ZnO and UVO-ZnO, respectively. The EQE spectra (Figure 3b) indicate that the increase in Jsc is mainly due to an enhancement of the utilization of sunlight in the wavelength ranges of 350−400 nm and 500−750 nm. Moreover, the EQE curve of the TPSCs with PFN-covered UVO-ZnO shows a red shift, which should be related to the changes in bulk morphology of the blend active layer promoted by PFN (as discussed later). In addition, the positive effect of UVO irradiation and PFN interfacial modification on the performance of TPSCs was also confirmed by the dark J−V characteristic curves of the TPSCs with pristine ZnO, UVO-ZnO, and PFN-covered UVO-ZnO, as shown in Figure 4a. Evidently, the TPSCs with PFN-covered UVO-ZnO show a lower leakage and higher forward bias current, indicating better charge selectivity and suppressed recombination using PFN-covered UVO-ZnO over pristine ZnO and UVO-ZnO.45,46 Figure 4b shows the statistical performance analysis of 32 devices. The PCE of the devices with UVO and PFN treatment exhibits a narrow distribution with an average PCE of 10.44%, suggesting that the two-step strategy provides a simple and effective way to fabricate highly efficient PSCs. To clarify the synergistic effect of the UVO irradiation and PFN interfacial modification on the performance of TPSCs, the binding energies, defect-related luminescence, surface morphol9000

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μm × 10 μm. On the other hand, the RMS roughness is reduced to 3.08 nm by UVO irradiation and then further reduced to 2.45 nm after spin-casting of PFN. The changes in surface morphology and roughness indicate that the voids on the ZnO surface could be partially filled by PFN. The smooth film has fewer traps, which is helpful in reducing the probability of interface-trap-assisted recombination of the electrons and suppressing the leakage current. This result can be used to explain why the TPSCs with PFN-covered UVO-ZnO show the highest FF, as discussed in Figure 3. It has been reported that ZnO is a hydrophilic inorganic material, which can result in poor interfacial contact with the hydrophobic organic active layer.53,54 It is apparent that the synergistic effect of UVO irradiation and PFN interfacial modification can effectively suppress the hydrophilicity of ZnO, as displayed in Figure 8. The water contact angle increases from 54.3° to 63.2° after UVO irradiation and further increases to 75.7° after spin-coating of PFN. This result indicates that the UVO irradiation and PFN interfacial modification are capable of changing the surface characteristic of ZnO from hydrophilic to hydrophobic. The hydrophobic surface is more likely to be in close contact with the hydrophobic active layer, thereby improving the interface quality.43 The changes in surface morphology, roughness, and hydrophilicity of ZnO are likely to affect the BHJ phase morphology of the active layer.30,55 Therefore, the surface and bulk morphologies of the PTB7-Th:PCDTBT:PC70BM-blend active layers deposited on top of the different ZnO films were investigated by AFM and TEM. As shown in Figure 9a−c, the active layer deposited on PFN-covered UVO-ZnO film shows the lowest surface roughness (RMS roughness = 2.32 nm), which should be attributed to the fact that the PFN-covered UVO-ZnO film is smoother than the UVO-ZnO and pristine ZnO films, as shown in Figure 7. It has been reported that a rougher active layer is unfavorable for the device performance because it increases the contact resistance between the active layer and the cathode.56 That is, the insertion of the PFN

Figure 6. PL spectra of the pristine ZnO, UVO-ZnO, and PFNcovered UVO-ZnO films.

shown in Figure 3. It is noted that a reduction in the number of defect states can suppress the recombination of carriers and reduce the leakage current of the devices, which is consistent with the results of dark J−V characteristics presented in Figure 4. The changes in surface morphology and roughness of ZnO were investigated by SEM and AFM, and the results are presented in Figure 7. The presence of voids can be observed in the pristine ZnO film, as shown in Figure 7a. Similar voids were also reported in other literature studies.29,51,52 These voids would hinder the intimate contact between the ZnO and the active layer, leading to an increase in surface contact resistance. This situation was not improved in the UVO-ZnO film, as a considerable number of voids were also observed, as shown in Figure 7b. However, a continuous and uniform surface is obtained after the spin-casting of PFN (Figure 7c). The AFM images reveal the morphology of ZnO in more detail. In Figure 7d, the AFM image shows that the pristine ZnO exhibits a rootmean-square (RMS) roughness of 3.19 nm at a scan scale of 10

Figure 7. (a−c) SEM images of (a) pristine ZnO, (b) UVO-ZnO, and (c) PFN-covered UVO-ZnO films. (d−f) AFM images of (d) pristine ZnO, (e) UVO-ZnO, and (f) PFN-covered UVO-ZnO films with a scan size of 10 μm × 10 μm. 9001

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Figure 8. Water contact angle images of (a) pristine ZnO, (b) UVO-ZnO, and (c) PFN-covered UVO-ZnO films.

Figure 9. (a−c) AFM and (d−f) TEM images of PTB7-Th:PCDTBT:PC70BM-blend active layers on the (a, d) pristine ZnO, (b, e) UVO-ZnO, and (c, f) PFN-covered UVO-ZnO films.

Figure 10. (a) Optical transmittance spectra of the ETLs on the ITO substrate. (b) J−V characteristics of electron-only devices with the configuration of ITO/ETL/active layer/Al. The ETLs include pristine ZnO, UVO-ZnO, and PFN-covered UVO-ZnO films.

enhances the interfacial contact between the active layer and the ETL. Figure 9d−f shows the variation of the bulk morphology of the ternary-blend active layers. The insertion of PFN results in a significant change in the bulk morphology of the blend active layer. As shown in Figure 9f, greater homogeneity and phase separation can be observed in the active layer deposited on the PFN-covered UVO-ZnO film. The optimized bulk morphology of the active layer promoted by PFN favors exciton dissociation and charge transport in the active layer,43,57 which is helpful in improving the conversion efficiency of absorbed photons into charge carriers, thereby contributing to the improvement of Jsc of the TPSCs with PFNcovered UVO-ZnO. Finally, the positive effect of UVO irradiation and PFN interfacial modification on the optical and electrical properties of ZnO was investigated. Figure 10a presents the optical transmittance spectra of the pristine ZnO, UVO-ZnO, and PFN-covered UVO-ZnO films. The UVO and PFN treatment

can significantly enhance the optical transmittance in the wavelength range of 375−475 nm. Although the introduction of PFN reduces the transmittance in the wavelength range of 480−600 nm, the transmittance in the wavelength range of 650−750 nm is enhanced. The optical transmittance results demonstrate that the UVO and PFN treatment facilitates the absorption of light by the active layer. In addition, it is interesting to note that the pristine ZnO-, UVO-ZnO-, and PFN-covered UVO-ZnO-coated ITO substrates present transmittance at some specific wavelength range higher than that of the bare ITO substrate. Similar results have also been reported in other references and are due to the fact that a smoother surface could be beneficial to the light transmittance.32,58 Here the UVO-ZnO and PFN-covered UVO-ZnO films show smoother surfaces compared with the pristine ZnO film, as revealed by the AFM images. The electron transfer characteristic of ZnO films was investigated using the Mott−Gurney space-charge-limited current (SCLC) model.59 Electron-only 9002

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power conversion efficiency of 7.4%. Adv. Mater. 2010, 22, E135− E138. (6) Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. Improved high-efficiency organic solar cells via incorporation of a conjugated polyelectrolyte interlayer. J. Am. Chem. Soc. 2011, 133, 8416−8419. (7) Ferenczi, T. A.; Müller, C.; Bradley, D. D.; Smith, P.; Nelson, J.; Stingelin, N. Organic semiconductor: insulator polymer ternary blends for photovoltaics. Adv. Mater. 2011, 23, 4093−4097. (8) Yang, L.; Yan, L.; You, W. Organic solar cells beyond one pair of donor-acceptor:ternary blends and more. J. Phys. Chem. Lett. 2013, 4, 1802−1810. (9) An, Q.; Zhang, F.; Sun, Q.; Zhang, M.; Zhang, J.; Tang, W.; Yin, X.; Deng, Z. Efficient organic ternary solar cells with the third component as energy acceptor. Nano Energy 2016, 26, 180−191. (10) Zhang, M.; Zhang, F.; An, Q.; Sun, Q.; Wang, W.; Zhang, J.; Tang, W. Highly efficient ternary polymer solar cells by optimizing photon harvesting and charge carrier transport. Nano Energy 2016, 22, 241−254. (11) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent advances in bulk heterojunction polymer solar cells. Chem. Rev. 2015, 115, 12666−12731. (12) An, Q.; Zhang, F.; Zhang, J.; Tang, W.; Deng, Z.; Hu, B. Versatile ternary organic solar cells: a critical review. Energy Environ. Sci. 2016, 9, 281−322. (13) Nam, S.; Lee, S.; Jeong, J.; Seo, J.; Kim, H.; Song, D.-I.; Kim, Y. Light-induced open circuit voltage increase in polymer solar cells with ternary bulk heterojunction nanolayers. ACS Sustainable Chem. Eng. 2015, 3, 55−62. (14) Zhang, X.; Li, Z.; Zhang, Z.; Liu, C.; Li, J.; Guo, W.; Qu, S. Employing easily prepared carbon nanoparticles to improve performance of inverted organic solar cells. ACS Sustainable Chem. Eng. 2016, 4, 2359−2365. (15) Kumari, T.; Lee, S. M.; Kang, S.-H.; Chen, S.; Yang, C. Ternary solar cells with a mixed face-on and edge-on orientation enable an unprecedented efficiency of 12.1%. Energy Environ. Sci. 2017, 10, 258− 265. (16) Zhao, W.; Li, S.; Zhang, S.; Liu, X.; Hou, J. Ternary polymer solar cells based on two acceptors and one donor for achieving 12.2% efficiency. Adv. Mater. 2017, 29, 1604059. (17) Sun, X.; Ni, J.; Li, C.; Huang, L.; Xu, R.; Li, Z.; Cai, H.; Li, J.; Zhang, J. Air-processed high performance ternary blend solar cell based on PTB7-Th:PCDTBT:PC70BM. Org. Electron. 2016, 37, 222− 227. (18) Li, C.; Sun, X.; Ni, J.; Huang, L.; Xu, R.; Li, Z.; Cai, H.; Li, J.; Zhang, Y.; Zhang, J. Methanol solvent treatment: A simple strategy to significantly boost efficiency and stability of air-processed ternary organic solar cells based on PTB7-Th:PCDTBT:PC70BM. Org. Electron. 2017, 50, 63−69. (19) Li, C.; Sun, X.; Ni, J.; Huang, L.; Xu, R.; Li, Z.; Cai, H.; Li, J.; Zhang, Y.; Zhang, J. Ternary organic solar cells based on ZnO-Ge double electron transport layer with enhanced power conversion efficiency. Sol. Energy 2017, 155, 1052−1058. (20) Liao, S. H.; Jhuo, H. J.; Cheng, Y. S.; Chen, S. A. Fullerene derivative-doped zinc oxide nanofilm as the cathode of inverted polymer solar cells with low-bandgap polymer (PTB7-Th) for high performance. Adv. Mater. 2013, 25, 4766−4771. (21) Wang, K.; Liu, C.; Meng, T.; Yi, C.; Gong, X. Inverted organic photovoltaic cells. Chem. Soc. Rev. 2016, 45, 2937−2975. (22) Hu, Z.; Zhang, J.; Xiong, S.; Zhao, Y. Annealing-free, airprocessed and high-efficiency polymer solar cells fabricated by a dip coating process. Org. Electron. 2012, 13, 142−146. (23) Chen, S.; Small, C. E.; Amb, C. M.; Subbiah, J.; Lai, T. h.; Tsang, S. W.; Manders, J. R.; Reynolds, J. R.; So, F. Inverted polymer solar cells with reduced interface recombination. Adv. Energy. Mater. 2012, 2, 1333−1337. (24) Jo, S. B.; Lee, J. H.; Sim, M.; Kim, M.; Park, J. H.; Choi, Y. S.; Kim, Y.; Ihn, S. G.; Cho, K. High performance organic photovoltaic

devices (ITO/pristine ZnO or UVO-ZnO or PFN-covered UVO-ZnO/active layer/Al) were fabricated. As shown in Figure 10b, the electron mobility increased from 1.33 × 10−4 to 8.01 × 10−4 cm2 V−1 s−1 after the two-step treatment. The enhancement of the electron mobility can be attributed to the reduced trap-assisted recombination of electrons and the good interfacial contact between the ZnO and the active layer. Benefiting from the enhanced extraction of electrons from the active layer to the cathode, the Jsc and FF are significantly improved, resulting in the champion PCE of 10.87% in the TPSCs with PFN-covered UVO-ZnO. Therefore, the two-step strategy has been shown to be simple and applicable to the fabrication of highly efficient PSCs.

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CONCLUSIONS A facile two-step strategy with UVO irradiation and PFN interfacial modification was proposed to simultaneously reduce the defect-assisted electron−hole recombination and improve the interfacial contact between the ETL and the active layer as well as to enhance the electron extraction from the active layer to the cathode in the inverted TPSCs. As a result, a champion PCE of 9.61% was achieved by 20 min of UVO irradiation, representing a 41.3% improvement relative to TPSCs without UVO irradiation. After the incorporation of an ultrathin PFN interfacial layer, the champion PCE of TPSCs was further increased to 10.87%, representing an increase by 13.1% relative to that of the TPSCs with 20 min of UVO irradiation. Our results indicate that the two-step strategy provides a simple and effective way to overcome the drawbacks of the inferior quality of solution-processed ZnO NP films and fabricate highly efficient inverted PSCs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jian Ni: 0000-0002-7169-3989 Jianjun Zhang: 0000-0003-2115-5661 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (61377031 and 61404073) and the Natural Science Foundation of Tianjin (17JCYBJC21200).

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DOI: 10.1021/acssuschemeng.7b01792 ACS Sustainable Chem. Eng. 2017, 5, 8997−9005