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Kai Zhang, Zhicheng Hu, Rongguo Xu, Xiao-Fang Jiang, Hin-Lap Yip,* Fei Huang,* and Yong Cao Bulk-heterojunction polymer solar cells (BHJ-PSCs) have drawn significant attention in both industrial and academic fields due to their potential for low cost and high-throughput solution processing.[1–3] Over the past few years, much progress had been made in improving the performance of PSCs, with power conversion efficiencies (PCEs) of over 10% and 11% for small-area single junctions and tandem PSCs, respectively.[4–7] As a PCE of 10% is often viewed as the threshold performance for commercial applications,[8] these achievements have triggered the development of low cost, high-throughput roll-to-roll (R2R) coating processes for manufacturing high-performance large-area PSC modules. However, it has been shown that the materials and device processes optimized for spin-cast, small-area devices may not be applicable to the production of high-performance R2R-coated PSC modules, as they lead to a very poor translation efficiency from champion cell to module.[9] One of the identified reasons for this low translation efficiency is that the relatively thin BHJ films (≈100 nm) and interlayers (≈5–50 nm) used in the champion devices are incompatible with the R2R coating process, for which thicker films are required to ensure high surface coverage and film homogeneity.[10] One way to overcome this problem is to develop new materials that can work efficiently under thick film conditions or new device processes that can homogeneously deposit the materials with controllable thickness and properties. To achieve the former goal, several classes of novel active layer materials have recently been developed and shown to produce very high efficiencies, even with thick films of 300–1000 nm.[4,11–13] In terms of interfacial materials, although some advances have been made in enabling interlayer work efficiently up to less than 100 nm,[14–17] it is still far behind the demand for roll-to-roll processing and remains a major challenge. One alternative strategy to overcome the coating problem of a thin interlayer film is to develop a new deposition technique that allows the formation of uniform and pinhole-free thin films.
K. Zhang, Z. Hu, R. Xu, Dr. X.-F. Jiang, Prof. H.-L. Yip, Prof. F. Huang, Prof. Y. Cao Institute of Polymer Optoelectronic Materials and Devices State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou 510640, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201500972
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High-Performance Polymer Solar Cells with Electrostatic Layer-by-Layer Self-Assembled Conjugated Polyelectrolytes as the Cathode Interlayer
Electrostatic layer-by-layer (eLbL) self-assembly is usually conducted through alternating adsorption of oppositely charged substances on precharged substrates, an easy and efficient approach to produce controlled film composition, architecture, and properties at the nanometer scale.[18,19] This technique is particularly suitable for the preparation of conjugated polyelectrolyte (CPE) films, which are polymers that contain π-conjugated backbones with ionized side chains. The ionic nature of CPEs endows them with the possibility of being processed by eLbL assembly. The introduction of eLbL-deposited CPE film into organic electronic devices was first demonstrated in organic light-emitting diodes (OLED).[20,21] However, there are only a few reports of CPEs eLbL film use for PSCs, and those devices typically exhibited relatively poor device performance.[22,23] In fact, CPEs are one of the most effective cathode interlayer materials.[24–26] However, due to their relatively low electron mobility, one drawback of CPEs is that they must be kept extremely thin (≈5 nm) to achieve optimal device performance,[14] which is not compatible with the roll-to-roll coating process. To solve this problem, we designed and synthesized a complimentary pair of anionic and cationic CPEs and used the eLbL self-assembly process to prepare a cathode interlayer with tunable thickness and surface functionality optimized for highly efficient PSCs. As shown in Figure 1, the anionic polyelectrolyte, poly[9,9-bis(4′-butanoatel)fluorene-co-alt-2,7-(9,9-dioctylfluorene)] sodium salt (PFCOO−Na+) with weakly acidic carboxylic acid groups electrostatically interacts with the cationic polyelectrolyte, poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN+Br−), forming a self-assembled eLbL film on indium tin oxide (ITO) substrates and acting as the cathode interlayer. Poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) were used as the BHJ layer, and inverted device based on the optimized eLbL interlayer showing a very high average PCE of 9.41%, which is much better than that of the reference PSC without the cathode interlayer. We also showed the potential application of the eLbL cathode interlayer in a large-area device by replacing the 1.5 × 1.5 cm2 substrate with a 7.5 cm × 7.5 cm one. The eLbL films deposited on the large-area substrate were of equal quality to that of the small-area substrate, suggesting that this technique could be applied to large-area processing for PSC modules. To prepare the eLbL films of PFN+/PFCOO−, the precleaned ITO substrate was first treated with oxygen plasma under a low
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Figure 1. Illustration of the eLbL interlayers in inverted polymer solar cells. a) Chemical structures of polymer donor, fullerene derivative acceptor, and polyelectrolyte materials that were used; b) architecture of inverted PSC structure with eLbL cathode interlayer.
vacuum to make it negatively charged.[27] It was then immersed in the cationic polyelectrolyte PFN+Br− solution to accomplish the electrostatic absorption of the first monolayer (denoted as n = 0.5), and then in the anionic polyelectrolyte PFCOO−Na+ solution to form the first bilayer (denoted as n = 1). As PFN+ had a net positive charge and PFCOO− had a net negative charge, alternation of the immersion steps between the cationic and anionic materials led to the formation of the PFN+/ PFCOO− multilayers with a controllable number of layers. During the eLbL deposition procedure, the by-product NaBr was removed in the rinsing steps. UV–vis absorption spectra of the eLbL films are shown in Figure 2a. An absorption band in the 320–430 nm range and an absorption peak at 384 nm are ascribed to the absorption of the polyfluorene backbone.[28] This increased with the number of multilayers, which means the polyelectrolyte pairs were deposited on each other alternatively. More precisely, a plot of absorbance at 384 nm versus layer number is depicted in Figure 2b. (a) 0.5 1 2 4 6 8 10 12 14 16 18 20
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A linear relationship between absorbance and layer number is shown, indicating a well-controlled accumulation of the eLbL films. The thickness of the 20 layer eLbL film measured by a surface profiler was 19 ± 2 nm, therefore the average thickness of one bilayer was ≈1 nm, which is in agreement with other eLbL films reported elsewhere.[22,23] X-ray photoelectron spectroscopy (XPS) was used to examine the surface composition of the PFN+/PFCOO− films on ITO substrates. The C(1s), N(1s), O(1s), Na(1s), and Br(3d) peaks are shown in Figure 3a–e, respectively. In Figure 3a,b, the C(1s) and N(1s) peak intensity at 284.9 and 402.7 eV increased with the layer numbers, indicating the formation of thicker PFN+/ PFCOO− films. In Figure 3c, the O(1s) peak intensity at 530.1 eV exhibited a reverse trend compared to those of C(1s) and N(1s) and the peak intensity decreased when the layer number increased. Theoretically, the content of O atoms should be twice that of the N atoms, but because there is a dominated O(1s) signal from the bottom ITO substrate, when the
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Figure 2. UV–vis absorption spectra of eLbL films. a) Absorption spectra of PFN+/PFCOO− bilayers with layer number up to 20. b) The absorbance of PFN+/PFCOO− bilayers at 384 nm versus layer number.
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Figure 3. X-ray photoelectron spectra of eLbL films. a) C(1s), b) N(1s), c) O(1s), d) Br(3d), and e) Na(1s) regions for PFN+/PFCOO− eLbL films.
thickness of the eLbL films increased, less signal from the ITO substrate was detected and therefore the O(1s) peak decreased. It is worth noting that deconvolution of the O(1s) peak for the PFCOO− and ITO background is challenging in this case due to the weak signal of O(1s) from PFCOO− and the low resolution of the equipment. Nevertheless, the success of the preparation of the multilayer eLbL film was revealed by the subsequent increase in the C(1s) and N(1s) signals. In Figure 3d,e, the small Na(1s) and Br(3d) signals suggested that the NaBr by-product was largely washed away, leaving only tiny amounts of Na+ and Br− adsorbed on the eLbL films (shown in Table S1, Supporting Information). This finding is important as it has been reported that ion migration in the interfacial materials may affect the long-term stability of PSC devices;[22,29] thus, the removal of free ions in our case may partly overcome this problem. On the other hand, it was reported that the free anions in polyelectrolytes can result in interfacial doping with improved charge extraction property.[30] The residual Br− ions in the n = x.5 eLbL films may therefore serve as a dopant to PCBM, which improved charge extraction and device performance. However, there was a competition between ion migration and doping, the optimized eLbL films may reach to a balance between the ion migration and doping by partly removing free ions.
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Scanning Kelvin probe microscopy (SKPM) was used to detect the work function (WF) variation by increasing the layer number of eLbL films (summarized in Figure S1a and Table S2, Supporting Information). First, the WF of bare ITO (n = 0) was measured to be 4.62 eV, similar to the commonly reported value (4.6–4.7 eV). After eLbL self-assembly of the first monolayer (n = 0.5) of the cationic polyelectrolyte PFN+, the WF decreased to 4.35 eV, and then increased to 4.41 eV after absorbing a layer of anionic polyelectrolyte PFCOO− (n = 1). Alternatively depositing these cationic/anionic polyelectrolytes on ITO further decreased the WF to a saturated value between 4.1 and 4.2 eV. The WFs of these multilayers are found to exhibit an odd–even effect related to the sequence of the polyelectrolytes. When the first PFN+ layer was absorbed on the ITO surface, PFN+ interacted with the OH− on ITO surface, forming dipoles pointing outward from the ITO surface to PFN+ (illustrated in Figure 1b).[31–34] The first monolayer (donated as n = 0.5) is highly ordered because of the selective orientation of OH−. As dipole is a vector, the highly ordered first monolayer created an ensemble large dipole which significantly decreased the WF of ITO to 4.35 eV. Compared with the highly ordered OH− groups on ITO, the N+ on PFN+ were randomly distributed due to the flexible polymer backbone and side chains. When the following anionic PFCOO−Na+ absorbed on PFN+, dipoles were formed
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between PFCOO− and PFN+ with direction pointing toward the substrate. However, the overall dipole was comparatively small due to the tilted dipoles. Therefore, the total effective dipole of n = 1.0 layer was smaller than the highly ordered n = 0.5 layer, thus the WF of n = 1.0 layer coated ITO just increased slightly to 4.41 eV, but still lower than that of the bare ITO. During the self-assembly process of the second PFN+ layer, the majority of the PFN+ polymer will interact with the PFCOO− layer, however, some of the uncovered area of the n = 1 bilayer coated ITO will fill up by the PFN+ polymer, leading to a further reduction of the WF. Subsequently, deposition of the second PFCOO− will raise up again the WF. Similar cycles continue for the subsequent layers and eventually the ITO WF started to saturate when n > 3.5, similar phenomenon was also reported in the literature.[35] The WF of the eLbL films saturated at 4.1–4.2 when n > 3.5 and matched well with the lowest unoccupied molecular orbital (LUMO) level of PCBM (normally 4.0–4.2 eV), making them a suitable interlayer to achieve Ohmic contact at the cathode. The uniformity and surface roughness of the eLbL films were studied using an atomic force microscope (AFM). The ITO substrates used in our study have a smooth surface with a surface roughness of 1.25 nm (Figure S1b, Supporting Information). After the deposition of four bilayers of eLbL film, a small roughness change from 1.25 to 1.81 nm was observed, and there were no apparent pin holes detected. The quantitative hill-valley data of n = 4.0 layer is shown in Figure S2 (Supporting Information). The deepest dip on n = 4.0 layer was 3.6 nm, while the thickness of n = 4.0 layer was around 4.0 nm estimated from the absorption spectrum and surface profile measurement, suggesting that the eLbL film with high surface coverage on ITO were achieved when n ≥ 4.0. Further deposition of the PFN+/PFCOO− film to n = 9 showed no change in the surface morphology and the surface roughness remained less than 2 nm. Thus, the uniformity of the eLbL films together with the WF tuning ability ensure that the eLbL films can be used as efficient cathode interlayers in inverted PSCs. This result suggests that a layer-by-layer self-assembly process is
none PFCOONa PFNBr eLbL (4.5)
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very efficient and the self-limiting growth mechanism is important to achieve smooth films with a thickness of nanometers. This film deposition technique is also compatible with the R2R coating process as the film can be prepared by alternatively immersing in different chemical bath solutions to complete the deposition and rinsing processes, providing a potential strategy to overcome the non-homogeneous film formation problem in large-area coating of ultrathin interlayers. Inverted polymer solar cells with device structures of ITO/ eLbL/PTB7-Th:PC71BM/MoO3/Al were fabricated. As shown in Figure 4 and Table 1, devices with bare ITO as a cathode exhibited quite a poor device performance, with a PCE of 3.11%. In the case of the eLbL film modified devices (shown in Figure S3 and Table S2, Supporting Information), the performance of the OPV devices was strongly dependent on the thickness and sequence of the eLbL interlayer. After the deposition of the first layer of PFN+, the open-circuit voltage (Voc) improved from 0.40 to 0.46 V together with a PCE of 3.89%. The performance of the PSCs continued to increase with the layer number of the film until it reached a maximum at n = 4.5, showing a maximum PCE of 9.41%. When n < 4.5, devices showed moderate performance due to the low coverage of the films, which allowed the BHJ partially in contact with the bottom ITO substrate. This created leakage paths with reducing shunt resistance, therefore reducing the fill factor (FF) and Voc. When n equal to 4.5 and 5.5, high FF of 0.69 and 0.67 and PCE of 9.4% and 9.1% were achieved, respectively. This could be attributed to the improved coverage of the LbL films, in addition, the films are relatively thin so the resistance was small in those cases. These combined factors led to the high FF and performance of the corresponding devices. However, when the thickness further increased, despite better LbL film coverage could be achieved, the series resistance in the device increased because of the relatively poor electron transport property of the polyfluorene-based polyelectrolyte, which led to reduced FF and PCE. Our results are in good agreement with other reports which suggested that the optimal thickness of the polyfluorene-based interlayer for high-performance OPVs is about 4–5 nm.[14] Thus, we can
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Figure 4. Inverted PSC device performances with eLbL films as cathode interlayers. a) J–V curves of inverted PSCs with PFNBr, PFCOONa and eLbL bilayer as cathode interlayer. Bare ITO without interlayer was also included; b) PCE value versus layer number.
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Active layer PTB7-Th:PC71BM
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Statistic data achieved from eight independent devices, the device area was 16 mm2.
conclude that the Voc not only depends on the WF of the interfacial layer, but also affects other factors such as the film coverage and resistance within the device. For comparison, devices with interlayers prepared using spin-cast of ≈5 nm of pure cationic PFN+Br− and pure anionic PFCOO−Na+ films were also fabricated. PFN+Br− showed a great enhancement in performance when compared with devices fabricated on bare ITO, showing a PCE of 8.38%. This result is also in good agreement with other reports based on pure CPE interlayers.[36] Devices based on a PFCOO−Na+ interlayer showed relatively poor performance, with a PCE of only 3.41%, suggesting that anionic polyelectrolyte alone is not suitable for cathode modification. The better performance of the eLbL films is attributable to its high coverage of the ITO substrate, as shown in the AFM images (Figure S2, Supporting Information), while the pure cationic or anionic polyelectrolyte films may exhibit low coverage of ITO as polyelectrolytes often form aggregates in solution and develop an island-like morphology, resulting in poor coverage after spin-coating.[26,37] Transient photocurrent (TPC) measurements are often used to study the competition between carrier sweep-out by the internal field and recombination during the operation of BHJ solar cells.[38] As shown in Figure 5, the TPC of PSCs with and without the eLbL cathode interlayer were measured at 0 bias. The device without the cathode interlayer had a charge
1 0.8 0.6
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Figure 5. Transient photocurrent of inverted PSC devices with 4.5 and 5.0 layer eLbL films as cathode interlayers. ITO without interlayer was included as a comparison.
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Table 1. Photovoltaic performance of PTB7-Th/PC71BM based inverted PSCs with PFNBr, PFCOONa and eLbL bilayer as cathode interlayer. A device without any interlayer is also included as comparison.a)
extraction time of 0.81 µs, while the one with n = 4.5 layers had a largely reduced charge extraction time of 0.44 µs, consistent with reported values when efficient charge extraction layers were applied.[39–41] The increase in charge extraction rate is one of the major factors contributing to the improvement of device performance. The PSC with the n = 5.0 layers eLbL cathode interlayer had a slightly increased charge extraction time of 0.52 µs compared to the one with the n = 4.5 layers, suggesting that the surface composition of the eLbL layer also plays a role in determining the electron extraction property of the interlayer. To fully explore their compatibility for large-area coating processes, we deposited eLbL films with five and ten bilayers (n = 5 and 10) onto ITO substrates with a size of 7.5 cm × 7.5 cm. Absorption study on nine different positions of the substrate (Figure S4, Supporting Information) had nearly identical absorption intensity, suggesting that the film was uniform in thickness over the whole substrate. To test the potential use of the eLbL cathode interlayer in larger area PSCs, devices with an active area of 96 mm2 were prepared and the corresponding J–V curve is shown in Figure S5 (Supporting Information). The Voc of the PSC remained at 0.79 V, but the short-circuit current density (Jsc), FF, and PCE decreased to 16.1 mA cm−2, 60%, and 7.6%, respectively. We consider the decrease in PCE to be mainly due to the Ohmic loss from the transparent ITO electrode. It has been well reported that the relatively high sheet resistance of ITO is a limiting factor for maintaining good performance PSC when the device size is increased.[42,43] This problem remains to be solved, and will probably require new transparent electrodes with higher conductivity and advanced module design.[9,44] Nevertheless, work on achieving high-efficiency large-area PSC modules using eLbL interlayers and new transparent electrodes is ongoing in our laboratory. In conclusion, an easy and efficient approach to preparing a cathode interlayer with controlled film composition, uniformity, and thickness at the nanometer scale is reported, using an electrostatic layer-by-layer self-assembly process. Inverted PSCs with the optimized eLbL layer show a PCE as high as 9.41%. The advantages of the eLbL process include the following: i) it uses an environment friendly aqueous solution under an ambient atmosphere, ii) it is suitable for achieving uniform, ultrathin, and large-area cathode interlayers, and iii) it can be used for the R2R coating process using a simple dipping and rinsing procedure in chemical bath solutions. We believe that these unique properties of the eLbL films provide a new solution for the preparation of efficient interlayers for PSC and are
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a potential candidate for use in high-performance large-area PSC modules in the future.
Experimental Section eLbL Assembly of PFN+Br− with PFCOO−Na+: ITO substrates were rinsed with standard procedure. After drying in oven overnight, the ITO substrates were plasma treated to make it negative charged, and then immersed in a 1 mg mL−1 PFCOO−Na+ aqueous solution for 15 min. Later on, the substrate was rinsed thoroughly with DI water and dried by a stream of pure nitrogen gas, then immersed in 1 mg mL−1 PFN+Br− solution with mixed solvent H2O:DMF (95:5 v/v) for 15 min, later on rinsed with DI water and dried by a stream of pure nitrogen gas to accomplish a bilayer. The process was repeated to obtain desired layer number of eLbL films on ITO substrates. Devices Fabrication and Characterization: The eLbL layer coated ITO substrates were transferred into N2 protected glove box. As a contrast, PFNBr and PFCOONa were spin-coated onto ITO from their methanol solution with a concentration of 0.5 mg mL−1 to achieve ≈5 nm interlayer. Then, the PTB7-Th:PC71BM (1:1.5 w/w) active layer was prepared by spin-coating a mixed solvent of chlorobenzene/1,8-diiodoctane (100:3 v/v) solution with PTB7-Th concentration of 10 mg mL−1 onto eLbL layer, PFNBr and PFCOONa coated ITO substrates, the thickness was 100 nm. Finally, 10 nm MoO3 and 100 nm Al were thermally evaporated as anode through a shadow mask (active area was defined) in a vacuum chamber with a base pressure of 2 × 10−6 mbar. The current-density–voltage (J–V) curves were measured on a computer-controlled Keithley 2400 source meter under 1 sun, AM 1.5 G spectrum from a class solar simulator (Japan, SAN-EI, XES-40S1), the light intensity was 100 mW cm−2 as calibrated by an NREL certified reference monocrystal silicon cell (Hamamatsu). The IPCE spectra were recorded using a commercial IPCE measurement system (Beijing, Zolix, DSR 100UV-B).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The work was financially supported by the Ministry of Science and Technology (No. 2014CB643500), the Natural Science Foundation of China (Nos. 21125419, 51323003, and 51361165301), and Guangdong Natural Science Foundation (Grant No. S2012030006232). Received: February 26, 2015 Revised: April 14, 2015 Published online: May 12, 2015
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[7] C.-C. Chen, W.-H. Chang, K. Yoshimura, K. Ohya, J. You, J. Gao, Z. Hong, Y. Yang, Adv. Mater. 2014, 26, 5670. [8] C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia, S. P. Williams, Adv. Mater. 2010, 22, 3839. [9] A. Armin, M. Hambsch, P. Wolfer, H. Jin, J. Li, Z. Shi, P. L. Burn, P. Meredith, Adv. Energy Mater. 2014, 5, DOI: 10.1002/ aenm.201401221. [10] F. C. Krebs, Sol. Energy Mater. Sol. Cells 2009, 93, 465. [11] X. Hu, C. Yi, M. Wang, C.-H. Hsu, S. Liu, K. Zhang, C. Zhong, F. Huang, X. Gong, Y. Cao, Adv. Energy Mater. 2014, 4, DOI: 10.1002/aenm.201400378. [12] S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W. You, J. Am. Chem. Soc. 2011, 133, 4625. [13] W. Li, K. H. Hendriks, W. S. C. Roelofs, Y. Kim, M. M. Wienk, R. A. J. Janssen, Adv. Mater. 2013, 25, 3182. [14] S. Liu, K. Zhang, J. Lu, J. Zhang, H. L. Yip, F. Huang, J. Am. Chem. Soc. 2013, 135, 15326. [15] Z.-G. Zhang, B. Qi, Z. Jin, D. Chi, Z. Qi, Y. Li, J. Wang, Energy Environ. Sci. 2014, 7, 1966. [16] Z. A. Page, Y. Liu, V. V. Duzhko, T. P. Russell, T. Emrick, Science 2014, 346, 441. [17] J. Liu, J. Wu, S. Shao, Y. Deng, B. Meng, Z. Xie, Y. Geng, L. Wang, F. Zhang, ACS Appl. Mater. Interfaces 2014, 6, 8237. [18] G. Decher, Science 1997, 277, 1232. [19] P. Bertrand, A. Jonas, A. Laschewsky, R. Legras, Macromol. Rapid Commun. 2000, 21, 319. [20] J. Tian, C.-C. Wu, M. E. Thompson, J. C. Sturm, R. A. Register, M. J. Marsella, T. M. Swager, Adv. Mater. 1995, 7, 395. [21] J. W. Baur, S. Kim, P. B. Balanda, J. R. Reynolds, M. F. Rubner, Adv. Mater. 1998, 10, 1452. [22] D. A. Rider, B. J. Worfolk, K. D. Harris, A. Lalany, K. Shahbazi, M. D. Fleischauer, M. J. Brett, J. M. Buriak, Adv. Funct. Mater. 2010, 20, 2404. [23] B. J. Worfolk, T. C. Hauger, K. D. Harris, D. A. Rider, J. A. M. Fordyce, S. Beaupré, M. Leclerc, J. M. Buriak, Adv. Energy Mater. 2012, 2, 361. [24] S. Liu, Z. Zhang, D. Chen, C. Duan, J. Lu, J. Zhang, F. Huang, S. Su, J. Chen, Y. Cao, Sci. China Chem. 2013, 56, 1119. [25] C. Duan, K. Zhang, C. Zhong, F. Huang, Y. Cao, Chem. Soc. Rev. 2013, 42, 9071. [26] C. V. Hoven, A. Garcia, G. C. Bazan, T.-Q. Nguyen, Adv. Mater. 2008, 20, 3793. [27] X. Wu, L. Liu, T. Yu, L. Yu, Z. Xie, Y. Mo, S. Xu, Y. Ma, J. Mater. Chem. C 2013, 1, 7020. [28] F. Huang, H. Wu, D. Wang, W. Yang, Y. Cao, Chem. Mater. 2004, 16, 708. [29] Y. M. Chang, R. Zhu, E. Richard, C. C. Chen, G. Li, Y. Yang, Adv. Funct. Mater. 2012, 22, 3284. [30] C.-Z. Li, C.-C. Chueh, F. Ding, H.-L. Yip, P.-W. Liang, X. Li, A. K.-Y. Jen, Adv. Mater. 2013, 25, 4425. [31] B. H. Lee, I. H. Jung, H. Y. Woo, H.-K. Shim, G. Kim, K. Lee, Adv. Funct. Mater. 2014, 24, 1100. [32] Y. Zhou, C. Fuentes-Hernandez, J. Shim, J. Meyer, A. J. Giordano, H. Li, P. Winget, T. Papadopoulos, H. Cheun, J. Kim, M. Fenoll, A. Dindar, W. Haske, E. Najafabadi, T. M. Khan, H. Sojoudi, S. Barlow, S. Graham, J.-L. Brédas, S. R. Marder, A. Kahn, B. Kippelen, Science 2012, 336, 327. [33] F. Liu, Z. A. Page, V. V. Duzhko, T. P. Russell, T. Emrick, Adv. Mater. 2013, 25, 6868. [34] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 1999, 11, 605. [35] Q. Chen, B. J. Worfolk, T. C. Hauger, U. Al-Atar, K. D. Harris, J. M. Buriak, ACS Appl. Mater. Interfaces 2011, 3, 3962. [36] S.-I. Na, T.-S. Kim, S.-H. Oh, J. Kim, S.-S. Kim, D.-Y. Kim, Appl. Phys. Lett. 2010, 97, 223305. [37] J. H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, U. Scherf, A. J. Heeger, G. C. Bazan, J. Am. Chem. Soc. 2011, 133, 8416.
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[42] S. Choi, W. J. Potscavage, B. Kippelen, J. Appl. Phys. 2009, 106, 054507. [43] W.-I. Jeong, J. Lee, S.-Y. Park, J.-W. Kang, J.-J. Kim, Adv. Funct. Mater. 2011, 21, 343. [44] F. Guo, X. Zhu, K. Forberich, J. Krantz, T. Stubhan, M. Salinas, M. Halik, S. Spallek, B. Butz, E. Spiecker, T. Ameri, N. Li, P. Kubis, D. M. Guldi, G. J. Matt, C. J. Brabec, Adv. Energy Mater. 2013, 3, 1062.
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[38] S. R. Cowan, R. A. Street, S. Cho, A. J. Heeger, Phys. Rev. B 2011, 83, 035205. [39] A. Kumar, G. Lakhwani, E. Elmalem, W. T. S. Huck, A. Rao, N. C. Greenham, R. H. Friend, Energy Environ. Sci. 2014, 7, 2227. [40] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Commun. 2014, 5, 5784. [41] Z. Tang, W. Tress, Q. Bao, M. J. Jafari, J. Bergqvist, T. Ederth, M. R. Andersson, O. Inganas, Adv. Energy Mater. 2014, 4, DOI: 10.1002/aenm.201400643.
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Kai Zhang, Zhicheng Hu, Rongguo Xu, Xiao-Fang Jiang, Hin-Lap Yip,* Fei Huang,* and Yong Cao Bulk-heterojunction polymer solar cells (BHJ-PSCs) have drawn significant attention in both industrial and academic fields due to their potential for low cost and high-throughput solution processing.[1–3] Over the past few years, much progress had been made in improving the performance of PSCs, with power conversion efficiencies (PCEs) of over 10% and 11% for small-area single junctions and tandem PSCs, respectively.[4–7] As a PCE of 10% is often viewed as the threshold performance for commercial applications,[8] these achievements have triggered the development of low cost, high-throughput roll-to-roll (R2R) coating processes for manufacturing high-performance large-area PSC modules. However, it has been shown that the materials and device processes optimized for spin-cast, small-area devices may not be applicable to the production of high-performance R2R-coated PSC modules, as they lead to a very poor translation efficiency from champion cell to module.[9] One of the identified reasons for this low translation efficiency is that the relatively thin BHJ films (≈100 nm) and interlayers (≈5–50 nm) used in the champion devices are incompatible with the R2R coating process, for which thicker films are required to ensure high surface coverage and film homogeneity.[10] One way to overcome this problem is to develop new materials that can work efficiently under thick film conditions or new device processes that can homogeneously deposit the materials with controllable thickness and properties. To achieve the former goal, several classes of novel active layer materials have recently been developed and shown to produce very high efficiencies, even with thick films of 300–1000 nm.[4,11–13] In terms of interfacial materials, although some advances have been made in enabling interlayer work efficiently up to less than 100 nm,[14–17] it is still far behind the demand for roll-to-roll processing and remains a major challenge. One alternative strategy to overcome the coating problem of a thin interlayer film is to develop a new deposition technique that allows the formation of uniform and pinhole-free thin films.
K. Zhang, Z. Hu, R. Xu, Dr. X.-F. Jiang, Prof. H.-L. Yip, Prof. F. Huang, Prof. Y. Cao Institute of Polymer Optoelectronic Materials and Devices State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou 510640, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201500972
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High-Performance Polymer Solar Cells with Electrostatic Layer-by-Layer Self-Assembled Conjugated Polyelectrolytes as the Cathode Interlayer
Electrostatic layer-by-layer (eLbL) self-assembly is usually conducted through alternating adsorption of oppositely charged substances on precharged substrates, an easy and efficient approach to produce controlled film composition, architecture, and properties at the nanometer scale.[18,19] This technique is particularly suitable for the preparation of conjugated polyelectrolyte (CPE) films, which are polymers that contain π-conjugated backbones with ionized side chains. The ionic nature of CPEs endows them with the possibility of being processed by eLbL assembly. The introduction of eLbL-deposited CPE film into organic electronic devices was first demonstrated in organic light-emitting diodes (OLED).[20,21] However, there are only a few reports of CPEs eLbL film use for PSCs, and those devices typically exhibited relatively poor device performance.[22,23] In fact, CPEs are one of the most effective cathode interlayer materials.[24–26] However, due to their relatively low electron mobility, one drawback of CPEs is that they must be kept extremely thin (≈5 nm) to achieve optimal device performance,[14] which is not compatible with the roll-to-roll coating process. To solve this problem, we designed and synthesized a complimentary pair of anionic and cationic CPEs and used the eLbL self-assembly process to prepare a cathode interlayer with tunable thickness and surface functionality optimized for highly efficient PSCs. As shown in Figure 1, the anionic polyelectrolyte, poly[9,9-bis(4′-butanoatel)fluorene-co-alt-2,7-(9,9-dioctylfluorene)] sodium salt (PFCOO−Na+) with weakly acidic carboxylic acid groups electrostatically interacts with the cationic polyelectrolyte, poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN+Br−), forming a self-assembled eLbL film on indium tin oxide (ITO) substrates and acting as the cathode interlayer. Poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) were used as the BHJ layer, and inverted device based on the optimized eLbL interlayer showing a very high average PCE of 9.41%, which is much better than that of the reference PSC without the cathode interlayer. We also showed the potential application of the eLbL cathode interlayer in a large-area device by replacing the 1.5 × 1.5 cm2 substrate with a 7.5 cm × 7.5 cm one. The eLbL films deposited on the large-area substrate were of equal quality to that of the small-area substrate, suggesting that this technique could be applied to large-area processing for PSC modules. To prepare the eLbL films of PFN+/PFCOO−, the precleaned ITO substrate was first treated with oxygen plasma under a low
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www.MaterialsViews.com (a)
(b) O
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Figure 1. Illustration of the eLbL interlayers in inverted polymer solar cells. a) Chemical structures of polymer donor, fullerene derivative acceptor, and polyelectrolyte materials that were used; b) architecture of inverted PSC structure with eLbL cathode interlayer.
vacuum to make it negatively charged.[27] It was then immersed in the cationic polyelectrolyte PFN+Br− solution to accomplish the electrostatic absorption of the first monolayer (denoted as n = 0.5), and then in the anionic polyelectrolyte PFCOO−Na+ solution to form the first bilayer (denoted as n = 1). As PFN+ had a net positive charge and PFCOO− had a net negative charge, alternation of the immersion steps between the cationic and anionic materials led to the formation of the PFN+/ PFCOO− multilayers with a controllable number of layers. During the eLbL deposition procedure, the by-product NaBr was removed in the rinsing steps. UV–vis absorption spectra of the eLbL films are shown in Figure 2a. An absorption band in the 320–430 nm range and an absorption peak at 384 nm are ascribed to the absorption of the polyfluorene backbone.[28] This increased with the number of multilayers, which means the polyelectrolyte pairs were deposited on each other alternatively. More precisely, a plot of absorbance at 384 nm versus layer number is depicted in Figure 2b. (a) 0.5 1 2 4 6 8 10 12 14 16 18 20
0.3 0.2 0.1
Absorbance @ 384 nm (a. u.)
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A linear relationship between absorbance and layer number is shown, indicating a well-controlled accumulation of the eLbL films. The thickness of the 20 layer eLbL film measured by a surface profiler was 19 ± 2 nm, therefore the average thickness of one bilayer was ≈1 nm, which is in agreement with other eLbL films reported elsewhere.[22,23] X-ray photoelectron spectroscopy (XPS) was used to examine the surface composition of the PFN+/PFCOO− films on ITO substrates. The C(1s), N(1s), O(1s), Na(1s), and Br(3d) peaks are shown in Figure 3a–e, respectively. In Figure 3a,b, the C(1s) and N(1s) peak intensity at 284.9 and 402.7 eV increased with the layer numbers, indicating the formation of thicker PFN+/ PFCOO− films. In Figure 3c, the O(1s) peak intensity at 530.1 eV exhibited a reverse trend compared to those of C(1s) and N(1s) and the peak intensity decreased when the layer number increased. Theoretically, the content of O atoms should be twice that of the N atoms, but because there is a dominated O(1s) signal from the bottom ITO substrate, when the
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Figure 2. UV–vis absorption spectra of eLbL films. a) Absorption spectra of PFN+/PFCOO− bilayers with layer number up to 20. b) The absorbance of PFN+/PFCOO− bilayers at 384 nm versus layer number.
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Figure 3. X-ray photoelectron spectra of eLbL films. a) C(1s), b) N(1s), c) O(1s), d) Br(3d), and e) Na(1s) regions for PFN+/PFCOO− eLbL films.
thickness of the eLbL films increased, less signal from the ITO substrate was detected and therefore the O(1s) peak decreased. It is worth noting that deconvolution of the O(1s) peak for the PFCOO− and ITO background is challenging in this case due to the weak signal of O(1s) from PFCOO− and the low resolution of the equipment. Nevertheless, the success of the preparation of the multilayer eLbL film was revealed by the subsequent increase in the C(1s) and N(1s) signals. In Figure 3d,e, the small Na(1s) and Br(3d) signals suggested that the NaBr by-product was largely washed away, leaving only tiny amounts of Na+ and Br− adsorbed on the eLbL films (shown in Table S1, Supporting Information). This finding is important as it has been reported that ion migration in the interfacial materials may affect the long-term stability of PSC devices;[22,29] thus, the removal of free ions in our case may partly overcome this problem. On the other hand, it was reported that the free anions in polyelectrolytes can result in interfacial doping with improved charge extraction property.[30] The residual Br− ions in the n = x.5 eLbL films may therefore serve as a dopant to PCBM, which improved charge extraction and device performance. However, there was a competition between ion migration and doping, the optimized eLbL films may reach to a balance between the ion migration and doping by partly removing free ions.
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Scanning Kelvin probe microscopy (SKPM) was used to detect the work function (WF) variation by increasing the layer number of eLbL films (summarized in Figure S1a and Table S2, Supporting Information). First, the WF of bare ITO (n = 0) was measured to be 4.62 eV, similar to the commonly reported value (4.6–4.7 eV). After eLbL self-assembly of the first monolayer (n = 0.5) of the cationic polyelectrolyte PFN+, the WF decreased to 4.35 eV, and then increased to 4.41 eV after absorbing a layer of anionic polyelectrolyte PFCOO− (n = 1). Alternatively depositing these cationic/anionic polyelectrolytes on ITO further decreased the WF to a saturated value between 4.1 and 4.2 eV. The WFs of these multilayers are found to exhibit an odd–even effect related to the sequence of the polyelectrolytes. When the first PFN+ layer was absorbed on the ITO surface, PFN+ interacted with the OH− on ITO surface, forming dipoles pointing outward from the ITO surface to PFN+ (illustrated in Figure 1b).[31–34] The first monolayer (donated as n = 0.5) is highly ordered because of the selective orientation of OH−. As dipole is a vector, the highly ordered first monolayer created an ensemble large dipole which significantly decreased the WF of ITO to 4.35 eV. Compared with the highly ordered OH− groups on ITO, the N+ on PFN+ were randomly distributed due to the flexible polymer backbone and side chains. When the following anionic PFCOO−Na+ absorbed on PFN+, dipoles were formed
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between PFCOO− and PFN+ with direction pointing toward the substrate. However, the overall dipole was comparatively small due to the tilted dipoles. Therefore, the total effective dipole of n = 1.0 layer was smaller than the highly ordered n = 0.5 layer, thus the WF of n = 1.0 layer coated ITO just increased slightly to 4.41 eV, but still lower than that of the bare ITO. During the self-assembly process of the second PFN+ layer, the majority of the PFN+ polymer will interact with the PFCOO− layer, however, some of the uncovered area of the n = 1 bilayer coated ITO will fill up by the PFN+ polymer, leading to a further reduction of the WF. Subsequently, deposition of the second PFCOO− will raise up again the WF. Similar cycles continue for the subsequent layers and eventually the ITO WF started to saturate when n > 3.5, similar phenomenon was also reported in the literature.[35] The WF of the eLbL films saturated at 4.1–4.2 when n > 3.5 and matched well with the lowest unoccupied molecular orbital (LUMO) level of PCBM (normally 4.0–4.2 eV), making them a suitable interlayer to achieve Ohmic contact at the cathode. The uniformity and surface roughness of the eLbL films were studied using an atomic force microscope (AFM). The ITO substrates used in our study have a smooth surface with a surface roughness of 1.25 nm (Figure S1b, Supporting Information). After the deposition of four bilayers of eLbL film, a small roughness change from 1.25 to 1.81 nm was observed, and there were no apparent pin holes detected. The quantitative hill-valley data of n = 4.0 layer is shown in Figure S2 (Supporting Information). The deepest dip on n = 4.0 layer was 3.6 nm, while the thickness of n = 4.0 layer was around 4.0 nm estimated from the absorption spectrum and surface profile measurement, suggesting that the eLbL film with high surface coverage on ITO were achieved when n ≥ 4.0. Further deposition of the PFN+/PFCOO− film to n = 9 showed no change in the surface morphology and the surface roughness remained less than 2 nm. Thus, the uniformity of the eLbL films together with the WF tuning ability ensure that the eLbL films can be used as efficient cathode interlayers in inverted PSCs. This result suggests that a layer-by-layer self-assembly process is
none PFCOONa PFNBr eLbL (4.5)
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very efficient and the self-limiting growth mechanism is important to achieve smooth films with a thickness of nanometers. This film deposition technique is also compatible with the R2R coating process as the film can be prepared by alternatively immersing in different chemical bath solutions to complete the deposition and rinsing processes, providing a potential strategy to overcome the non-homogeneous film formation problem in large-area coating of ultrathin interlayers. Inverted polymer solar cells with device structures of ITO/ eLbL/PTB7-Th:PC71BM/MoO3/Al were fabricated. As shown in Figure 4 and Table 1, devices with bare ITO as a cathode exhibited quite a poor device performance, with a PCE of 3.11%. In the case of the eLbL film modified devices (shown in Figure S3 and Table S2, Supporting Information), the performance of the OPV devices was strongly dependent on the thickness and sequence of the eLbL interlayer. After the deposition of the first layer of PFN+, the open-circuit voltage (Voc) improved from 0.40 to 0.46 V together with a PCE of 3.89%. The performance of the PSCs continued to increase with the layer number of the film until it reached a maximum at n = 4.5, showing a maximum PCE of 9.41%. When n < 4.5, devices showed moderate performance due to the low coverage of the films, which allowed the BHJ partially in contact with the bottom ITO substrate. This created leakage paths with reducing shunt resistance, therefore reducing the fill factor (FF) and Voc. When n equal to 4.5 and 5.5, high FF of 0.69 and 0.67 and PCE of 9.4% and 9.1% were achieved, respectively. This could be attributed to the improved coverage of the LbL films, in addition, the films are relatively thin so the resistance was small in those cases. These combined factors led to the high FF and performance of the corresponding devices. However, when the thickness further increased, despite better LbL film coverage could be achieved, the series resistance in the device increased because of the relatively poor electron transport property of the polyfluorene-based polyelectrolyte, which led to reduced FF and PCE. Our results are in good agreement with other reports which suggested that the optimal thickness of the polyfluorene-based interlayer for high-performance OPVs is about 4–5 nm.[14] Thus, we can
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6
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Figure 4. Inverted PSC device performances with eLbL films as cathode interlayers. a) J–V curves of inverted PSCs with PFNBr, PFCOONa and eLbL bilayer as cathode interlayer. Bare ITO without interlayer was also included; b) PCE value versus layer number.
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Active layer PTB7-Th:PC71BM
Interlayer
Jsc [mA cm−2]
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None
16.20 ± 0.21
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PFNBr
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PFCOONa
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40 ± 2.8
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eLbL (n = 4.5)
17.04 ± 0.25
0.80 ± 0.00
69 ± 2.1
9.41 ± 0.14
a)
Statistic data achieved from eight independent devices, the device area was 16 mm2.
conclude that the Voc not only depends on the WF of the interfacial layer, but also affects other factors such as the film coverage and resistance within the device. For comparison, devices with interlayers prepared using spin-cast of ≈5 nm of pure cationic PFN+Br− and pure anionic PFCOO−Na+ films were also fabricated. PFN+Br− showed a great enhancement in performance when compared with devices fabricated on bare ITO, showing a PCE of 8.38%. This result is also in good agreement with other reports based on pure CPE interlayers.[36] Devices based on a PFCOO−Na+ interlayer showed relatively poor performance, with a PCE of only 3.41%, suggesting that anionic polyelectrolyte alone is not suitable for cathode modification. The better performance of the eLbL films is attributable to its high coverage of the ITO substrate, as shown in the AFM images (Figure S2, Supporting Information), while the pure cationic or anionic polyelectrolyte films may exhibit low coverage of ITO as polyelectrolytes often form aggregates in solution and develop an island-like morphology, resulting in poor coverage after spin-coating.[26,37] Transient photocurrent (TPC) measurements are often used to study the competition between carrier sweep-out by the internal field and recombination during the operation of BHJ solar cells.[38] As shown in Figure 5, the TPC of PSCs with and without the eLbL cathode interlayer were measured at 0 bias. The device without the cathode interlayer had a charge
1 0.8 0.6
ITO 5.0
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4
Figure 5. Transient photocurrent of inverted PSC devices with 4.5 and 5.0 layer eLbL films as cathode interlayers. ITO without interlayer was included as a comparison.
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Table 1. Photovoltaic performance of PTB7-Th/PC71BM based inverted PSCs with PFNBr, PFCOONa and eLbL bilayer as cathode interlayer. A device without any interlayer is also included as comparison.a)
extraction time of 0.81 µs, while the one with n = 4.5 layers had a largely reduced charge extraction time of 0.44 µs, consistent with reported values when efficient charge extraction layers were applied.[39–41] The increase in charge extraction rate is one of the major factors contributing to the improvement of device performance. The PSC with the n = 5.0 layers eLbL cathode interlayer had a slightly increased charge extraction time of 0.52 µs compared to the one with the n = 4.5 layers, suggesting that the surface composition of the eLbL layer also plays a role in determining the electron extraction property of the interlayer. To fully explore their compatibility for large-area coating processes, we deposited eLbL films with five and ten bilayers (n = 5 and 10) onto ITO substrates with a size of 7.5 cm × 7.5 cm. Absorption study on nine different positions of the substrate (Figure S4, Supporting Information) had nearly identical absorption intensity, suggesting that the film was uniform in thickness over the whole substrate. To test the potential use of the eLbL cathode interlayer in larger area PSCs, devices with an active area of 96 mm2 were prepared and the corresponding J–V curve is shown in Figure S5 (Supporting Information). The Voc of the PSC remained at 0.79 V, but the short-circuit current density (Jsc), FF, and PCE decreased to 16.1 mA cm−2, 60%, and 7.6%, respectively. We consider the decrease in PCE to be mainly due to the Ohmic loss from the transparent ITO electrode. It has been well reported that the relatively high sheet resistance of ITO is a limiting factor for maintaining good performance PSC when the device size is increased.[42,43] This problem remains to be solved, and will probably require new transparent electrodes with higher conductivity and advanced module design.[9,44] Nevertheless, work on achieving high-efficiency large-area PSC modules using eLbL interlayers and new transparent electrodes is ongoing in our laboratory. In conclusion, an easy and efficient approach to preparing a cathode interlayer with controlled film composition, uniformity, and thickness at the nanometer scale is reported, using an electrostatic layer-by-layer self-assembly process. Inverted PSCs with the optimized eLbL layer show a PCE as high as 9.41%. The advantages of the eLbL process include the following: i) it uses an environment friendly aqueous solution under an ambient atmosphere, ii) it is suitable for achieving uniform, ultrathin, and large-area cathode interlayers, and iii) it can be used for the R2R coating process using a simple dipping and rinsing procedure in chemical bath solutions. We believe that these unique properties of the eLbL films provide a new solution for the preparation of efficient interlayers for PSC and are
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a potential candidate for use in high-performance large-area PSC modules in the future.
Experimental Section eLbL Assembly of PFN+Br− with PFCOO−Na+: ITO substrates were rinsed with standard procedure. After drying in oven overnight, the ITO substrates were plasma treated to make it negative charged, and then immersed in a 1 mg mL−1 PFCOO−Na+ aqueous solution for 15 min. Later on, the substrate was rinsed thoroughly with DI water and dried by a stream of pure nitrogen gas, then immersed in 1 mg mL−1 PFN+Br− solution with mixed solvent H2O:DMF (95:5 v/v) for 15 min, later on rinsed with DI water and dried by a stream of pure nitrogen gas to accomplish a bilayer. The process was repeated to obtain desired layer number of eLbL films on ITO substrates. Devices Fabrication and Characterization: The eLbL layer coated ITO substrates were transferred into N2 protected glove box. As a contrast, PFNBr and PFCOONa were spin-coated onto ITO from their methanol solution with a concentration of 0.5 mg mL−1 to achieve ≈5 nm interlayer. Then, the PTB7-Th:PC71BM (1:1.5 w/w) active layer was prepared by spin-coating a mixed solvent of chlorobenzene/1,8-diiodoctane (100:3 v/v) solution with PTB7-Th concentration of 10 mg mL−1 onto eLbL layer, PFNBr and PFCOONa coated ITO substrates, the thickness was 100 nm. Finally, 10 nm MoO3 and 100 nm Al were thermally evaporated as anode through a shadow mask (active area was defined) in a vacuum chamber with a base pressure of 2 × 10−6 mbar. The current-density–voltage (J–V) curves were measured on a computer-controlled Keithley 2400 source meter under 1 sun, AM 1.5 G spectrum from a class solar simulator (Japan, SAN-EI, XES-40S1), the light intensity was 100 mW cm−2 as calibrated by an NREL certified reference monocrystal silicon cell (Hamamatsu). The IPCE spectra were recorded using a commercial IPCE measurement system (Beijing, Zolix, DSR 100UV-B).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The work was financially supported by the Ministry of Science and Technology (No. 2014CB643500), the Natural Science Foundation of China (Nos. 21125419, 51323003, and 51361165301), and Guangdong Natural Science Foundation (Grant No. S2012030006232). Received: February 26, 2015 Revised: April 14, 2015 Published online: May 12, 2015
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