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Sequential solvent processing with hole transport materials for improving efficiency of traditionally-structured perovskite solar cells
Author’s Accepted Manuscript Sequential Solvent Processing with Hole Transport Materials for Improving Efficiency of Traditionally-Structured Perovskite Solar Cells T.T. Tong, X.H. Li, S.H. Guo, J. Han, B.Q. Wei www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30596-7 https://doi.org/10.1016/j.nanoen.2017.09.050 NANOEN2226
To appear in: Nano Energy Received date: 22 July 2017 Revised date: 23 September 2017 Accepted date: 25 September 2017 Cite this article as: T.T. Tong, X.H. Li, S.H. Guo, J. Han and B.Q. Wei, Sequential Solvent Processing with Hole Transport Materials for Improving Efficiency of Traditionally-Structured Perovskite Solar Cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.09.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sequential Solvent Processing with Hole Transport Materials for Improving Efficiency of Traditionally-Structured Perovskite Solar Cells Tengteng Tong, Xuanhua Li, * Shaohui Guo, Jian Han, Bingqing Wei*
T. T. Tong, Prof. X. H. Li, S. H. Guo, J. Han, and Prof. B. Q. Wei, State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi'an, 710072, China. E-mail: [email protected]
Prof. B. Q. Wei, Department of Mechanical Engineering, University of Delaware, Newark, DE19716, USA E-mail: [email protected]
Abstract The solvent treatment of the perovskite surfaces has been popularly applied to enhance the device performance of planar perovskite solar cells (PSCs) and become the most useful method for fully solution-processed devices. However, the simple solvent engineering cannot address all interfacial contact problems in the traditionally-structured perovskite devices, limiting the applicability of solution processing. Here, we introduce a universal solvent engineering technology, termed sequential solvent processing with hole transport materials (HTMs), which can be used to fabricate high efficient traditionally-structured PSCs. Our new approach induces interdiffusion between the perovskite and HTM layers, which improves their interfacial contacts, by dripping a chlorobenzene solution of the HTM onto a perovskite precursor before the perovskite crystallization is complete, thereby enabling the HTM to penetrate into the perovskite layer. Furthermore, the approach enhances the quality of the perovskite crystals, improves interfacial energy band alignment, and densifies the interface, which are advantageous for carrier extraction from the perovskite layer to the HTM. Our simple and effective strategy achieves power conversion efficiency (PCE) of 18.39% with 22.6% increament, compared to the control device (PCE=15.00%). Overall, this approach is a significant improvement over the existing fabrication methods and proves to be an importantly new method for future research on PSCs.
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Keywords: perovskite solar cells; solvent processing; interfacial contact; power conversion efficiencies; electrical effect
1. Introduction Perovskite solar cells (PSCs) have attracted considerable attention over the past few years owing to their ability to achieve high efficiencies with inexpensive materials by using a solution preparation method [1-11]. Planar heterojunction PSCs, developed by Snaith et al., have been widely used as one of the most efficient configurations of PSCs [8, 12]. As shown in Scheme 1(a), a traditionallystructured PSC has a unique sandwich-type structure that contains a hybrid halide perovskite layer [6, 13,14], typically CH3NH3PbI3 (MAPbI3) [15], which serves as a visible-light-absorbing material located between an electron transport material (ETM) and a hole transport material (HTM) [16-18]. TiO2 or SnO2 is commonly used as an ETM [1, 19,20] and is deposited on conductive glass substrates [e.g., indium tin oxide (ITO)] [21-23]. Common HTMs include 2,2′,7,7′-tetrakis(N, N-dip-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), poly(3-hexylthiophene) (P3HT), and
poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]
dithiophene-2,6-diyl][3-fluoro-2-[(2-
ethylhexyl)-carbonyl]-thieno-[3,4-b] thiophenediyl]] (PTB7) [24-27].
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The blending of γ-butyrolactone (GBL) and N,N-dimethylformamide (DMSO) is used as an effective solvent for lead iodide (PbI2) and methylammonium iodide (MAI) [28]. The solvent with a mixture of PbI2 and MAI is dropped onto a TiO2 film through one-step spin coating as a perovskite precursor [29-31], followed by heat treatment to form a perovskite crystal, MAPbI3 [2, 32]. To enhance the device performance, the solvent treatment of the perovskite surfaces is one of the most important steps [11,31,33-36]. For example, when the perovskite precursor was deposited with a mixture of GBL and DMSO, followed by the dripping addition of chlorobenzene (CB) while spinning, relatively uniform and dense layers were formed [34,36]. After the HTM solution, e.g., spiro-OMeTAD [37], is spin-coated on top of the MAPbI3 film, a layer–layer contact is obtained (termed as a conventional device), as shown in Scheme 1(b). The CB drop solvent engineering technology (termed as the CB-treated method) is applied to enhance the device performance and has become the most popular method for fully solution-processed devices [Scheme 1(b)] [35,38]. However, the simple spin-coating method does not completely address the interfacial contact problems in the perovskite layer, limiting the applicability of solution processing [39,40]. Recently, several new solvent and interface engineering technologies have been developed in the invertedstructured PSCs; and significant improvements of device performance have been achieved [2, 3, 18,27,34,36,38]. Unfortunately, the interfacial contact problems in the traditionally-structured PSCs are yet addressed. Herein, we report a new solution engineering technology for efficient traditionally-structured PSCs by employing a sequential CB solvent processing with HTMs (termed as the CB/HTM-treated method) to induce inter-diffusion between the perovskite and the HTM [termed as a layer-evolved device; Scheme 1(c)]. The proposed device structure is realized by dripping a CB solution of the HTM, e.g., spiro-OMeTAD onto the perovskite precursor before the perovskite crystallization is complete, thereby enabling the spiro-OMeTAD to penetrate into the perovskite layer. Consequently, the spiro-OMeTAD premixed in the CB solvent is retained in the perovskite films. Subsequently, a
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mixed layer of perovskite and spiro-OMeTAD is formed on top of the light-absorbing layer [Scheme 1(c)]. This method makes a tight contact between the perovskite film and HTM, which facilitates charge carrier extraction and improves the electrical properties of the PSCs. This enhancement of the original solvent engineering technique allows a fully solution-treated PSC to improve the PCE from 15.00% to 18.39% under the standard testing conditions.
2. Experimental Section Device Fabrication: All materials were purchased from Sigma-Aldrich and used as received without further purification. A perovskite precursor solution could be obtained through the mixture of CH3NH3I and PbI2 (1:1, molar ratio) in the solvent of γ-butyrolactone and DMSO (7:3, v:v). The TiO2 solution was prepared as an ETM according to the previous method [41]. The spiro-OMeTAD, P3HT, or PTB7 was used for a HTM. A solution containing 28 μL of 4-tert-butyl pyridine and 17 μL of Li-TFSI (520 mg Li-TFSI in 1 mL acetonitrile) was added to a CB solution (1 mL) of spiroOMeTAD (80 mg), P3HT(15 mg), or PTB7 (10 mg), respectively. The device structure was ITO/TiO2/MAPbI3/HTM/Au [42,43]. Regarding the typical device fabrication, ITO glass substrates (15 mm × 15 mm, 15 Ω sq−1 resistance) were first cleaned with decontamination powder and then ultrasonicated successively in DI water, acetone, and anhydrous ethanol (each for 10 min). The ITO glasses were then treated with UV ozone for 15 min. To make compact TiO2 layers, the cleaned ITO glasses were coated with a TiO2 solution by spin-coating at 4000 rpm for 60 s. The compact TiO2 coated ITO substrates were dried at 150 °C for 10 min and then allowed to cool down to room temperature slowly. A perovskite precursor solution was spin-coated onto TiO2 layer at 3000 rpm for 40 s with a dripping CB solvent (CB-treated method) or a CB solvent containing HTM with different concentration (e.g., 0-4 mg/mL) (CB/HTM-treated method). The substrates were then heated at 100 °C for 10 min and allowed to cool down to room temperature. Subsequently, a HTM was deposited on the top of the CH3NH3PbI3 perovskite layer by spin coating. Devices were then kept 4
overnight in air. Finally, a top Au electrode (100 nm thick) was thermally evaporated on the top of the HTM. Solar Cell Characterizations: J–V characteristics of devices were measured under AM 1.5G illumination (100 mW cm−2) from a solar simulator. The morphologies of the conventional device and the layer-evolved device without top Au electrodes were characterized by a field-emission scanning electron microscope (SEM, FEI 450). The X-ray diffraction patterns (XRD) were collected using a PANalytical X'Pert XRD system with reference X-ray illumination as Cu Kα radiation at 0.154 nm. The film absorption spectrum was obtained using a UV–Vis–NIR spectrophotometer (Lambda 35, Perkin-Elmer). Photoluminescence (PL) of the films were performed using a fluorescence spectrometer (FLS 980) with an excitation at 470 nm, similar to the one reported previously [44,45].
3. Results and Discussion A basic PSC with the device structure of ITO/TiO2/MAPbI3/HTM/Au was fabricated. Here, spiroOMeTAD is used as the HTM for demonstrating the new method. Devices with other HTMs such as P3HT and PTB7 will be investigated later. The CB-treated method is used as a solvent engineering for conventional devices (as the control devices), whereas the CB/spiro-OMeTAD-treated method is adopted for the new device configuration termed as the layer-evolved devices. To demonstrate the new solvent method, a series of experiments have been performed to investigate the morphology, device performance, and optical and electrical properties. Scanning electron microscopy (SEM) measurements were performed to study the morphology of the two devices. Figure 1(a) and 1(c) show the top-view and cross-sectional SEM images of the MAPbI3 film after application of pure CB solvent, respectively. Small cracks can be observed on the surface of the MAPbI3 film, and the MAPbI3 film has a high roughness after application of the pure CB solvent. However, the cracks in the MAPbI3 film can be filled if the CB solvent premixed with 5
spiro-OMeTAD is used. As shown in Figure 1(b) and 1(d), a thin layer of MAPbI3 and spiroOMeTAD mixture atop the perovskite crystals is observed. In addition, the CB/spiro-OMeTADtreated film is smooth compared with the pure CB-treated film. After a spiro-OMeTAD solution was subsequently spin-coated on top of the CB-treated film [Figure 1(e)], it can be observed that the spiro-OMeTAD film has some pinholes. By contrast, the spiro-OMeTAD film coated on top of the CB/spiro-OMeTAD-treated film of the layer-evolved device is much more uniform than the conventional device with fewer holes and defects [Figure 1(f)]. For the CB-treated case, MAPbI3 is in direct contact with spiro-OMeTAD [Figure 1(g)] and results in a loose interfacial contact with spiro-OMeTAD. However, this interfacial contact is denser when MAPbI3 is premixed with spiroOMeTAD in the layer-evolved device [Figure 1(h)]. In addition, the CB/spiro-OMeTAD treatment improves the quality of perovskite crystallization because it separates the presumably unstable intermediate from the sensitive solvent vapor atmosphere during spin coating [15]. X-ray diffraction (XRD) was performed to study the perovskite crystallization [46,47]. As shown in Figure 2(a) and 2(b), the half peak width of XRD peak for the CB/spiro-OMeTAD-treated MAPbI3 film is smaller than that of the CB-treated MAPbI3 film, which indicates that the crystallinity of the CB/spiroOMeTAD-treated MAPbI3 film is significantly higher than that of the pure CB-treated film. The device performance of both the conventional and layer-evolved devices was investigated, and the typical device characteristics and parameters are summarized in Figure 3 and Table 1, respectively. The conventional device shows only a moderate efficiency of 15.00%, with a shortcircuit photocurrent density (Jsc), of 21.15 mA/cm2, Voc of 1.02 V, and fill factor (FF) of 0.69. These data are averages from 40 devices of the conventional devices, and all of the PCEs are within the range of 13.00–16.00%. The concentration of the spiro-OMeTAD premixed with CB, which is a key parameter to achieve high PCEs, has been optimized. With the increase of the spiro-OMeTAD concentration from 0.5 mg/ml to 1.0 mg/ml, and to 2.0 mg/ml, the PCE increases from 16.32% to 17.51%, and to 18.39%, respectively. When the spiro-OMeTAD concentration is further increased to
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4.0 mg/ml, the PCE drastically declines to 13.63%. Thus, the optimal spiro-OMeTAD concentration is about 2.0 mg/ml, and the optimized average PCE reaches 18.39% (all of the PCEs are within 17.00–19.50%), with Jsc of 23.14 mA/cm2, Voc of 1.06 V, and FF of 0.75, which is 22% higher than that of the pure CB-treated device. The improved PCE mainly results from improvements in Jsc and FF. To understand the reason for the improvement in Jsc, the absorption spectra were measured for the following five samples on TiO2 substrates including one conventional device and four layerevolved devices with different concentrations of spiro-OMeTAD in CB. As shown in Figure 2(c), the absorption spectra of perovskite films are almost unchanged after the devices are treated with the spiro-OMeTAD solution, indicating that the increase in Jsc is not due to the light absorption property. Incident photon-to-current efficiencies (IPCE) of these devices have also been measured, which are comprehensive quantities of both optical absorption and electrical effects [48]. As shown in Figure 2(d), the IPCE increases when the treatment of the perovskite films changes from CB to CB/spiroOMeTAD. With the increase of spiro-OMeTAD concentration from 0.5 to 2.0 mg/ml, the IPCE is gradually improved. When the spiro-OMeTAD concentration reaches 4.0 mg/ml, the IPCE decreases. Jsc can also be calculated from the IPCE spectra. For these five devices, Jsc is calculated to be 21.02, 21.35, 22.54, 23.04, and 19.40 mA/cm2, respectively, which is in good agreement with the Jsc derived from the J–V measurements. Considering the unchanged light absorption properties of the perovskite–spiro-OMeTAD films, the significant improvement in photovoltaic performance is likely due to positive electrical effects rather than improved light harvesting. The electrical effects including exciton separation, carrier transport and extraction, carrier recombination, and interfacial resistance were systematically studied. To better understand the exciton separation efficiency, photoluminescence (PL) characterization was performed. As shown in Figure 4(a), PL occurs at similar wavelengths with a peak around 778 nm for all the samples, which corresponds to the PL spectrum of MAPbI3 [33,49]. However, more quenching is observed for the
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CB/spiro-OMeTAD-treated device compared with the CB-treated device, indicating more efficient exciton separation in the MAPbI3 film. In addition, the MAPbI3 films treated by CB solvent premixed with different concentrations of spiro-OMeTAD show different PL intensities. When the concentration of spiro-OMeTAD is increased from 0.5 mg/ml to 1.0 mg/ml and then to 2.0 mg/ml, the PL intensity of the MAPbI3 film is gradually decreased, indicating that exciton separation in the MAPbI3 film and the mixed layer between the MAPbI3 film and spiro-OMeTAD film is more efficient for a concentration of 2.0 mg/ml, followed by 1.0 mg/ml and then 0.5 mg/ml. The crystal quality is a possible reason for the better exciton separation. As shown in Figure 2(a) and 2(b), the half peak width of XRD peaks gradually decreases when the concentration of spiro-OMeTAD increases from 0.5 mg/ml to 1.0 mg/ml and then to 2.0 mg/ml. Increasing the concentration of spiroOMeTAD to 4.0 mg/ml results in a thicker film with relatively poor crystal quality, which will restrain the exciton generation and separation efficiency and lead to obvious emittance [Figure 4(a)]. To analyze the hole transport and extraction, we fabricated hole-only devices with a structure of ITO/PEDOT:PSS/MAPbI3/spiro-OMeTAD/Au. As shown in Figure 4(b), the hole current density is significantly larger for the layer-evolved device than for the conventional device, indicating better hole extraction properties of the layer-evolved device. Owing to the improved interfacial contact, there are fewer pinholes or defect sites introduced in the perovskite/spiro-OMeTAD interface. As a result, the “trap sites” of the carrier in the interface is effectively reduced [Figure 4(f) and 4(g)]. It should be mentioned that the better energy alignment between the perovskite layer and the spiroOMeTAD layer is an important factor for efficient hole extraction. The introduction of perovskite/spiro-OMeTAD mixed layer can affect the interfacial energy band, which is also related to the charge transport kinetics [50]. Because the valence band (VB) energies of perovskite and spiro-OMeTAD thin films are -5.4 and -5.2 eV, respectively [26, 27], the VB energy of the interdiffusion layer is between -5.4 and -5.2 eV [Figure 4(e))]. Thus, the mixed layer between
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perovskite and spiro-OMeTAD is beneficial to the hole extraction from the perovskite layer to the spiro-OMeTAD. The carrier recombination was further investigated by plotting the J–V curve of the device versus light intensity. Figure 4 (c) presents a power law dependence of Jsc on light intensity (J = Iα). When α = 0.75, a device is space charge limited because of an interfacial barrier or a carrier imbalance. If α value is near 1, a negligible space charge limit could be achieved [51,52]. In the current work, the device with an intermixed layer shows a sharper slope of 0.98 than the conventional device with a slope of 0.89. The slopes of some layer-evolved devices nearly reach 1, signifying negligible interfacial recombination losses in the layer-evolved devices. Furthermore, Figure 4 (d) shows the Voc measured as a function of light intensity. It is well known that when the slope is greater than 1 kT/q, where k is the Boltzmann constant, T is the temperature in Kelvin, and q is the electron charge, Voc becomes strongly dependent on light intensity, and additional interfacial trap-assisted Shockley-Read-Hall (SRH) recombination is involved [3,53]. In the current work, the conventional device has a slope value of 1.96 kT/q, indicating a stronger interfacial trap-assisted recombination than the layer-evolved device (1.34 kT/q). In addition to the better energy alignment between the perovskite layer and spiro-OMeTAD layer for beneficial carrier extraction, the improved interfacial contact and crystal quality of the layer-evolved device are also possible reasons for the reduced interfacial trap-assisted recombination [Figure 4(e)-4(g)]. Consequently, we can conclude that the sequential solvent processing with HTMs reduces the interfacial recombination losses and SRH recombination, which is critical to improving device performance. Therefore, due to the series positive electrical effects including efficient exciton separation, improved carrier transport and extraction between CH3NH3PbI3 and spiro-OMeTAD, and reduced carrier recombination, Jsc is significantly enhanced for the layer-evolved device compared with the conventional device. Regarding the higher FF, it can be attributed to decreased series resistance (Rs) and increased shunt resistance (Rsh), as shown in Figure 3(b) and Table 1. The Voc is slightly
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increased from 1.02 to 1.06 V after the device is treated by the sequential solvent processing with HTMs, which can be attributed to the reduced carrier recombination. For PSCs measured under certain scanning conditions, the hysteresis in the J–V curve is a very important issue [4, 46,54-60]. As shown in Figure 5(a), the hysteresis for the conventional device is very serious. After the CB/spiro-OMeTAD treatment is applied, the hysteresis for the layer-evolved device is greatly improved. In order to quantify the degree of hysteresis, we used a formula (1) to define the hysteresis factor: ysteresis fa tor=
C reverse - C for ard C reverse
(1) The hysteresis factors of PSCs are summarized in Table 2. Compared with the big hysteresis factor of 0.40 for the conventional device, the hysteresis factor for the layer-evolved device is only 0.09. Although there are controversies on the causes of the hysteresis , efficient hole carrier transport and extraction, and reduced carrier recombination seem to be possible reasons for the reduction of hysteresis. The proposed CB/spiro-OMeTAD treatment method is a universal approach. When spiroOMeTAD is changed with other HTMs, such as P3HT or PTB7, which are typical P-type materials, the device efficiency is still increased (device structure: ITO/TiO2/MAPbI3/P3HT or PTB7/Au). As shown in Figure 5(b) and 5(c) and Table 2, the device efficiency is significantly increased from 10.36% to 12.30% using P3HT as the HTM, and from 8.13% to 11.03% using PTB7 as the HTM after CB/P3HT (optimized concentration is 0.6 mg/ml) or CB/PTB7 (optimized concentration is 0.6 mg/ml) treatment is applied, respectively. Compared to a hysteresis factor of 0.46 for the conventional device, the hysteresis factor for the layer-evolved device is only about 0.08 when P3HT is used as the HTM. When PTB7 is used as the HTM, the hysteresis factor is decreased from 0.37 for the conventional device to only 0.08 for the layer-evolved device. Therefore, it is a general method for enhancing the performance of PSCs and reducing recombination and hysteresis (Figure 5). 10
4. Conclusions In summary, we have demonstrated a simple, effective, and universal solvent engineering technology referred as the sequential solvent processing with HTMs for high-performance traditionallystructured PSCs. The layer-evolved devices exhibit a significant enhancement in Jsc, FF, and PCE. As a result, a PCE of 18.39% can be readily achieved, whereas the PCE for reference devices is 15.00%. Furthermore, premixing HTM in CB plays several important roles in enhancing the device performance: (1) improving the crystallinity of the perovskite crystals, (2) allowing better alignment of the interfacial energy bands, and (3) enhancing the interfacial contact and improving carrier extraction from the active layer to the HTM. Most importantly, this approach provides a new universal method for high-performance PCS fabrications.
Acknowledgments This research is supported by the National Natural Science Foundation of China (51221001, 51472204, 51571166, 51521061, and 61505167) and the Program of Introducing Talents of Discipline to Universities (B08040). We also thank the support of the Natural Science Research Project of Shaanxi Province (2016JM5001), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU) (Grant No. 147-QZ-2016), and the Key Scientific and Technological Team from Shaanxi Province (No. 2015KCT-12).
Conflict of Interest The authors declare no conflict of interest.
References
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[1] T. Hu, T. Becker, N. Pourdavoud, J. Zhao, K.O. Brinkmann, R. Heiderhoff, T. Gahlmann, Z. Huang, S. Olthof, K. Meerholz, D. Tobbens, B. Cheng, Y. Chen, T. Riedl, Adv. Mater. (2017) DOI: 10.1002/adma.201606656. [2] W. Chen, L. Xu, X. Feng, J. Jie, Z. He, Adv. Mater. 29 (2017) 1603923. [3] H. Chen, W. Fu, C. Huang, Z. Zhang, S. Li, F. Ding, M. Shi, C. Z. Li, A.K.Y. Jen, H. Chen, Adv. Energy Mater. (2017) DOI: 10.1002/aenm.201700012. [4] J.B. Patel, J. Wong-Leung, S. Van Reenen, N. Sakai, J.T.W. Wang, E.S. Parrott, M. Liu, H.J. Snaith, L.M. Herz, M.B. Johnston, Adv. Electron. Mater. 3 (2017) 1600470. [5] S. Y. Lin, T. S. Su, T. Y. Hsieh, P. C. Lo, T. C. Wei, Adv. Energy Mater. (2017) DOI: 10.1002/aenm.201700169. [6] M. Zhang, J. Wang, L. Li, G. Zheng, K. Liu, M. Qin, H. Zhou, X. Zhan, Adv. Sci. (2017) DOI: 10.1002/advs.201700025. [7] D. Liu, T.L. Kelly, Nat. Photonics 8 (2013) 133-138. [8] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395-398. [9] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gratzel, S. Mhaisalkar, T.C. Sum, Science 342 (2013) 344-347. [10] J.T. Wang, J.M. Ball, E.M. Barea, A. Abate, J.A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H.J. Snaith, R.J. Nicholas, Nano Lett. 14 (2014) 724-730. [11] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S.I. Seok, Nat. Mater. 13 (2014) 897-903. [12] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643-647. [13] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Energy Environ. Sci. 7 (2014) 982. [14] C. Wehrenfennig, G.E. Eperon, M.B. Johnston, H.J. Snaith, L.M. Herz, Adv. Mater. 26 (2014) 1584-1589. [15] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Nature 517 (2015) 476-480. [16] N.J. Jeon, H.G. Lee, Y.C. Kim, J. Seo, J.H. Noh, J. Lee, S.I. Seok, J. Am. Chem. Soc. 136 (2014) 7837-7840. [17] D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P.A. Dowben, O.F. Mohammed, E.H. Sargent, O.M. Bakr, Science 347 (2015) 519-522. [18] Z. Li, J. Tinkham, P. Schulz, M. Yang, D.H. Kim, J. Berry, A. Sellinger, K. Zhu, Adv. Energy Mater. 7 (2016) 1601451.
12
[19] H. Hussain, G. Tocci, T. Woolcot, X. Torrelles, C.L. Pang, D.S. Humphrey, C.M. Yim, D.C. Grinter, G. Cabailh, O. Bikondoa, R. Lindsay, J. Zegenhagen, A. Michaelides, G. Thornton, Nat. Mater. 16 (2016) 461-465. [20] Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nat. Energy 2 (2016) 16177-16182. [21] D. Zhang, W.C.H. Choy, F. Xie, W.E.I. Sha, X. Li, B. Ding, K. Zhang, F. Huang, Y. Cao, Adv. Funct. Mater. 23 (2013) 4255-4261. [22] Y. Tu, J. Wu, M. Zheng, J. Huo, P. Zhou, Z. Lan, J. Lin, M. Huang, Nanoscale 7 (2015) 2053920546. [23] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, F. Giordano, S.M. Zakeeruddin, M. Gratzel, R.H. Friend, J. Phys. Chem. Lett. 7 (2016) 3264-3269. [24] J. Liu, Y. Wu, C. Qin, X. Yang, T. Yasuda, A. Islam, K. Zhang, W. Peng, W. Chen, L. Han, Energy Environ. Sci. 7 (2014) 2963-2970. [25] W.H. Nguyen, C.D. Bailie, E.L. Unger, M.D. McGehee, J. Am. Chem. Soc. 136 (2014) 1099611001. [26] S. Ryu, J.H. Noh, N.J. Jeon, Y. Chan Kim, W.S. Yang, J. Seo, S.I. Seok, Energy Environ. Sci. 7 (2014) 2614-2619. [27] M. Li, Z. K. Wang, Y. G. Yang, Y. Hu, S. L. Feng, J. M. Wang, X. Y. Gao, L. S. Liao, Adv. Energy Mater. 6 (2016) 1601156. [28] H. Zhang, D. Li, J. Cheng, F. Lin, J. Mao, A.K.Y. Jen, M. Grätzel, W.C.H. Choy, J. Mater. Chem. A 5 (2017) 3599-3608. [29] J. Peng, T. Duong, X. Zhou, H. Shen, Y. Wu, H.K. Mulmudi, Y. Wan, D. Zhong, J. Li, T. Tsuzuki, K.J. Weber, K.R. Catchpole, T.P. White, Adv. Energy Mater. 7 (2016) 1601768. [30] A. Huang, L. Lei, J. Zhu, Y. Yu, Y. Liu, S. Yang, S. Bao, X. Cao, P. Jin, ACS Appl. Mater. Inter. 9 (2017) 2016-2022. [31] Y. Dong, W. Li, X. Zhang, Q. Xu, Q. Liu, C. Li, Z. Bo, Small 12 (2016) 1098-1104. [32] W. Q. Wu, F. Huang, D. Chen, Y. B. Cheng, R.A. Caruso, Adv. Energy Mater. 6 (2016) 1502027. [33] X.H. Zhang, J.J. Ye, L.Z. Zhu, H.Y. Zheng, X.P. Liu, X. Pan, S.Y. Dai, ACS Appl. Mater. Inter. 8 (2016) 35440-35446. [34] Q. Wu, W. Zhou, Q. Liu, P. Zhou, T. Chen, Y. Lu, Q. Qiao, S. Yang, ACS Appl. Mater. Inter. 8 (2016) 34464-34473. [35] M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y.B. Cheng, L. Spiccia, Angew. Chem. Int. Edit. 53 (2014) 9898-9903.
13
[36] Y. Wu, X. Yang, W. Chen, Y. Yue, M. Cai, F. Xie, E. Bi, A. Islam, L. Han, Nat. Energy 1 (2016) 16148-16152. [37] J.A. Christians, R.C. Fung, P.V. Kamat, J. Am. Chem. Soc. 136 (2014) 758-764. [38] C. H. Chiang, C. G. Wu, Nat. Photonics 10 (2016) 196-200. [39] X. Fang, Y. Wu, Y. Lu, Y. Sun, S. Zhang, J. Zhang, W. Zhang, N. Yuan, J. Ding, J. Mater. Chem. C 5 (2017) 842-847. [40] T. Liu, C. Wang, J. Hou, C. Zhang, H. Chen, H. He, N. Wang, H. Wu, G. Cao, Small 12 (2016) 5146-5152. [41] X. Li, X. Ren, F. Xie, Y. Zhang, T. Xu, B. Wei, W.C.H. Choy, Adv. Opt. Mater. 3 (2015) 12201231. [42] M. Yang, T. Zhang, P. Schulz, Z. Li, G. Li, D.H. Kim, N. Guo, J.J. Berry, K. Zhu, Y. Zhao, Nat. Commun. 7 (2016) 12305-12309. [43] Y. Zhou, M. Yang, S. Pang, K. Zhu, N.P. Padture, J. Am. Chem. Soc. 138 (2016) 5535-5538. [44] S. Guo, X. Li, J. Zhu, T. Tong, B. Wei, Small 12 (2016) 5692-5701. [45] X. Li, S. Guo, C. Kan, J. Zhu, T. Tong, S. Ke, W.C.H. Choy, B. Wei, Nano Energy 30 (2016) 549-558. [46] X. Zhang, J. Ye, L. Zhu, H. Zheng, G. Liu, X. Liu, B. Duan, X. Pan, S. Dai, Nanoscale 9 (2017) 4691-4699. [47] L. Qian, Y. Sun, M. Wu, D. Xie, L. Ding, G. Shi, Adv. Mater. (2017) DOI: 10.1002/adma.201606175. [48] X. Li, J. Zhu, B. Wei, Chem. Soc. Rev. 45 (2016) 3145-3187. [49] H. Zhang, J. Cheng, F. Lin, H. He, J. Mao, K.S. Wong, A.K. Jen, W.C. Choy, ACS Nano 10 (2016) 1503-1511. [50] M.C. Kim, B.J. Kim, D.Y. Son, N.G. Park, H.S. Jung, M. Choi, Nano Lett. 16 (2016) 5756-5762. [51] D. Yang, X. Zhou, R. Yang, Z. Yang, W. Yu, X. Wang, C. Li, S. Liu, R.P.H. Chang, Energy Environ. Sci. 9 (2016) 3071-3078. [52] J.E. Moser, Nat. Mater. 16 (2016) 4-6. [53] D. Zhao, W. Ke, C.R. Grice, A.J. Cimaroli, X. Tan, M. Yang, R.W. Collins, H. Zhang, K. Zhu, Y. Yan, Nano Energy 19 (2016) 88-97. [54] B. Roose, K.C. Gödel, S. Pathak, A. Sadhanala, J.P.C. Baena, B.D. Wilts, H.J. Snaith, U. Wiesner, M. Grätzel, U. Steiner, A. Abate, Adv. Energy Mater. 6 (2016) 1501868. [55] H.S. Kim, N.G. Park, J. Phys. Chem. Lett. 5 (2014) 2927-2934. [56] H.J. Snaith, A. Abate, J.M. Ball, G.E. Eperon, T. Leijtens, N.K. Noel, S.D. Stranks, J.T. Wang, K. Wojciechowski, W. Zhang, J. Phys. Chem. Lett. 5 (2014) 1511-1515. 14
[57] H.S. Kim, I.H. Jang, N. Ahn, M. Choi, A. Guerrero, J. Bisquert, N.G. Park, J. Phys. Chem. Lett. 6 (2015) 4633-4639. [58] B. Li, Y. Zhang, X. Zhou, Z. Liu, Q. Liu, X. Li, J. Alloys Compd. 673 (2016) 265-271. [59] Q. Tai, P. You, H. Sang, Z. Liu, C. Hu, H. L. Chan, F. Yan, Nat. Commun. 7 (2016) 11105. [60] Z. Liu, P. You, C. Xie, G. Tang, F. Yan, Nano Energy. 28 (2016) 151-157.
Tengteng Tong received his B.S.degree from Anhui University Of Technology in 2014,and he is now an M.S. candidate at Northwestern Polytechnical University under the supervision of Professor Bingqing Wei and Professor Xuanhua Li.. His research focuses on perovskite solar cells.
Dr. Xuanhua Li received his B.S. degree from Wuhan University of Technology in 2007, and M.S. degree from USTC in 2010. After that, he started his doctoral studies and received a Ph.D. degree at Department of Electrical and Electronic Engineering, University of Hong Kong in 2014. After a short research in the Institute of Intelligent Machines, CAS, he began his career at the Center of Nano Energy Materials, Northwestern Polytechnical University as a professor. His research is focused on synthesis of 2D nanomaterials and novel metal NPs. He is also interested in plasmonenhanced solar cells, photocatalytic reaction, and optical sensors.
15
Shaohui Guo received his B.S. degree from Northestern Polytechnical University in 2014, and he is now a Ph.D. student at Northwestern Polytechnical University under the supervision of Professor Bingqing Wei and Professor Xuanhua Li. His current research includes the development of nanostructured materials for photocatalysis and surface enhanced Raman scattering (SERS).
Jian Han received his B.S. degree from Xi`an University of Technology in 2012, and M.S. degree from Xi`an University of Architecture and Technology in 2016. And now, he is pursuing his doctoral degree at Northwestern Polytechnical University under the supervision of Professor Xuanhua Li. His research is focused on solar cells.
Dr. Bingqing Wei is a Professor in the Department of Mechanical Engineering at the University of Delaware, USA. He was an Assistant Professor in the Department of Electrical & Computer Engineering and Center for Computation & Technology at Louisiana State University from 2003 to 2007. From 2000 to 2003, he was a Research Scientist at Rensselaer Polytechnic Institute, Department of Materials Science and Engineering and Rensselaer Nanotechnology Center. Dr. Wei 16
was a visiting scientist at Max-Planck-Institut für Metallforschung, Stuttgart, Germany in 1998 and 1999. From 1992 to 2001, he was a faculty member at Tsinghua University in Beijing.
Figure 1. Top view SEM images of the MAPbI3 film treated with (a) pure CB solvent and (b) CB/spiro-OMeTAD. The inset of (a) is an enlarged region. The arrows indicate some cracks in the 17
film. Cross-sectional SEM images of the MAPbI3 film treated with (c) pure CB solvent and (d) CB/spiro-OMeTAD. Top view SEM images of the spiro-OMeTAD film of (e) the conventional device and (f) the layer-evolved device without a top Au electrode. Cross-sectional SEM images of (g) the conventional device and (h) the layer-evolved device without a top Au electrode.
(b) Conventional
1.0
0.5 mg/ml
0.8
Intensity (a.u.)
1.0 mg/ml
2.0 mg/ml
4.0 mg/ml 20
3.5
14.2
2.0
14.4
Conventional 0.5 mg/ml 2.0 mg/ml
100
1.5
14.6
2 (degree)
(d) 120
IPCE (％)
2.5
0.2 0.0
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
3.0
0.4
40
2 (degree)
(c) 4.0
Absorbance
30
0.6
2
10
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
25
1.0 mg/ml 4.0 mg/ml
20
80
15
60 40
10
20
5
1.0 0.5 0.0 400
500
600
700
800
900
0 300
0 400
500
600
700
Wavelength (nm)
Wavelength (nm)
Integrated Current density (mA/cm )
Intensity (a.u.)
(a)
800
Figure 2. (a) XRD patterns of the CB- and CB/spiro-OMeTAD-treated perovskite films. (b) Enlarged 14°–15° region of (a). (c) UV–Vis absorption spectra and (d) IPCE spectra of the CB- and CB/spiro-OMeTAD-treated perovskite devices with different concentrations of spiro-OMeTAD premixed in the dripping CB solvent.
18
(b) Current density (mA/cm2)
Current Density (mA/cm2)
(a) 25 20
15
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
10
5
0 0.0
0.2
0.4
0.6
0.8
0
-2
-4
-6
-8
-10 -0.2
1.0
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
0.0
12
0.6
12
Conventional
10
8
8
Count
10
Count
0.4
0.8
1.0
1.2
(d)
(c)
6
4
2
2
14.2
14.6
15.0
15.4
15.8
16.2
16.4
Layer-evolved
6
4
0 13.8
0.2
Voltage (V)
Voltage (V)
0 17.2
17.6
18.0
18.4
18.8
19.2
19.6
PCE (％)
PCE (％)
Figure 3. Photovoltaic characteristics of the two types of perovskite devices. (a) Current density– voltage (J–V) curves measured under AM 1.5 simulated sunlight for the conventional and layerevolved devices with spiro-OMeTAD premixed with CB at different concentrations. (b) Dark J–V curves for the conventional and layer-evolved devices. Statistical PCE distributions of the (c) conventional and (d) layer-evolved devices.
19
(b)
Current Density (mA/cm2)
(a)
Intensity
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
700
720
740
760
780
800
500 400
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
300 200 100 0 0
820
1
2
3
Voltage (V)
Wavelength (nm)
(c)25
(d) 1.05
Conventional 2.0 mg/ml
20
Jsc (mA/cm2)
1.00
Voc (V)
15
10
0.95
0.90
Conventional 2.0 mg/ml
5
0.85 0 20
40
60
80
Light Intensity (mW/cm2)
100
20
(e)
CB
CB
(f)
h+
Mixed interlayer
+
80
e_
100
Spiro-OMeTAD MAPbI3
Spiro-OMeTAD
(g) +
+
60
h+ e_
recombination
Perovskite
VB
40
Light Intensity (mW/cm2)
+
VB
+
h+ e_
Spiro-OMeTAD Mixture MAPbI3
Figure 4. (a) PL spectra of perovskite films treated by different concentrations of solvent-deposited spiro-OMeTAD. (b) J–V characteristics of hole-only devices with perovskite films treated by different concentrations of solvent-deposited spiro-OMeTAD. (c) Dependence of Jsc on light intensity. (d) Dependence of Voc on light intensity. Schematic of hole carrier transport from the active layer to the HTM for (e) schematic showing the band shifts of perovskite and spiro-OMeTAD with an interdiffusion layer, (f) the conventional device, and (g) the layer-evolved device.
20
(a) 25
2 Current Density (mA/cm )
(b)
20
15
10
5
0 0.0
Conventional forward Conventional reverse Layer-evolved forward Layer-evolved reverse
0.2
0.4
0.6
0.8
1.0
20
15
10
Conventional forward Conventional reverse Layer-evolved forward Layer-evolved reverse
5
0 0.0
0.2
Voltage(V)
0.4
0.6
0.8
Voltage (V)
Current Density (mA/cm2)
(c) 17.5 15.0 12.5 10.0 7.5 5.0 2.5 0.0
Conventional forward Conventional reverse Layer-evolved forward Layer-evolved reverse 0.2
0.4
0.6
0.8
1.0
Voltage (V)
Figure 5. J–V hysteresis of solar cells under forward and reverse scans with a scan rate of 400 mV/s with different HTMs: (a) spiro-OMeTAD, (b) P3HT, and (c) PTB7. “Forward scan” corresponds to the scan from 0.0 V to 1.0 V and “reverse s an” orresponds to the s an from 1.0 V to 0.0 V.
Table 1. Photovoltaic parameters of devices treated with the CB solvent premixed with spiroOMeTAD at various concentrations. Concentrationa) [mg/mL]
Voc [V]
Jsc [mA cm−2]
FF [%]
PCE [%]
Rs [Ω cm−2]
Rsh [Ω cm−2]
0
1.02 ± 0.02
21.15 ± 0.55
69.45± 0.04
15.00 ± 0.83
11.34 ± 0.75
1276.46 ± 35.00
0.5
1.06 ± 0.02
21.56 ± 0.32
71.42 ± 0.03
16.32 ± 0.62
7.85 ± 0.54
1358 .89± 27.00
1.0
1.06 ± 0.02
22.82 ± 0.33
72.34 ± 0.03
17.51 ± 0.64
7.06 ± 0.62
1534.62 ± 31.00
2.0
1.06 ± 0.02
23.14 ± 0.32
75.02 ± 0.03
18.39 ± 0.72
5.55 ± 0.43
1937.45 ± 29.00
4.0
1.06 ± 0.03
19.57 ± 0.38
66.23 ± 0.04
13.63 ± 1.03
13.57 ± 0.81
1149.00 ± 39.00
a)
Concentration of spiro-OMeTAD in CB solvent. 21
22
Table 2. Comparison of photovoltaic parameters for the conventional and layer-evolved devices. spiro-OMeTAD
Voc [V]
Jsc [mA cm−2]
FF [%]
PCE [%]
Hysteresis factor
Conventional device
Forward Reverse
1.02± 0.02 0.89± 0.03
21.15± 0.55 21.05± 0.82
69.45± 0.04 47.84± 0.05
15.00 ± 0.83 8.92± 1.02
0.40± 0.08
Layer-evolved device
Forward Reverse
1.06 ± 0.02 1.03± 0.02
23.14± 0.32 23.02± 0.63
75.02± 0.03 70.10± 0.04
18.39± 0.72 16.61± 0.83
0.09± 0.02
Conventional device
Forward Reverse
0.85± 0.02 0.67± 0.03
18.56± 0.60 18.43± 0.72
65.93± 0.03 44.62± 0.04
10.36± 0.62 5.50± 0.80
0.46± 0.09
Layer-evolved device
Forward Reverse
0.83± 0.02 0.80± 0.02
19.82± 0.43 19.81± 0.52
75.02± 0.02 71.24± 0.04
12.30± 0.61 11.27± 0.92
0.08± 0.01
Conventional device
Forward Reverse
0.91± 0.02 0.86± 0.03
13.84± 0.73 13.75± 0.83
65.12± 0.04 43.56± 0.04
8.13± 0.90 5.15± 1.02
0.37± 0.05
Layer-evolved device
Forward Reverse
0.93± 0.02 0.87± 0.02
16.72± 0.52 16.70± 0.71
71.24± 0.03 69.82± 0.04
11.03± 0.82 10.12± 0.91
0.08± 0.02
P3HT
PTB7
23
Scheme 1. (a) Traditional structure of a perovskite solar cell (PSC) and molecular structures of holetransport materials (HTMs). Preparation process for the light-absorbing layer of (b) a conventional device and (c) a layer-evolved device with an intermixed layer.
Highlights (1) A simple, effective, and universal solvent engineering technology is proposed. 24
(2) Sequential solvent processing with HTMs improves device efficiency. (3) New approach induces layer inter-diffusion and improves interfacial contacts. (4) PCE of 18.39% with 22.6% increment is achieved.
25
PII: DOI: Reference:
S2211-2855(17)30596-7 https://doi.org/10.1016/j.nanoen.2017.09.050 NANOEN2226
To appear in: Nano Energy Received date: 22 July 2017 Revised date: 23 September 2017 Accepted date: 25 September 2017 Cite this article as: T.T. Tong, X.H. Li, S.H. Guo, J. Han and B.Q. Wei, Sequential Solvent Processing with Hole Transport Materials for Improving Efficiency of Traditionally-Structured Perovskite Solar Cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.09.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sequential Solvent Processing with Hole Transport Materials for Improving Efficiency of Traditionally-Structured Perovskite Solar Cells Tengteng Tong, Xuanhua Li, * Shaohui Guo, Jian Han, Bingqing Wei*
T. T. Tong, Prof. X. H. Li, S. H. Guo, J. Han, and Prof. B. Q. Wei, State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi'an, 710072, China. E-mail: [email protected]
Prof. B. Q. Wei, Department of Mechanical Engineering, University of Delaware, Newark, DE19716, USA E-mail: [email protected]
Abstract The solvent treatment of the perovskite surfaces has been popularly applied to enhance the device performance of planar perovskite solar cells (PSCs) and become the most useful method for fully solution-processed devices. However, the simple solvent engineering cannot address all interfacial contact problems in the traditionally-structured perovskite devices, limiting the applicability of solution processing. Here, we introduce a universal solvent engineering technology, termed sequential solvent processing with hole transport materials (HTMs), which can be used to fabricate high efficient traditionally-structured PSCs. Our new approach induces interdiffusion between the perovskite and HTM layers, which improves their interfacial contacts, by dripping a chlorobenzene solution of the HTM onto a perovskite precursor before the perovskite crystallization is complete, thereby enabling the HTM to penetrate into the perovskite layer. Furthermore, the approach enhances the quality of the perovskite crystals, improves interfacial energy band alignment, and densifies the interface, which are advantageous for carrier extraction from the perovskite layer to the HTM. Our simple and effective strategy achieves power conversion efficiency (PCE) of 18.39% with 22.6% increament, compared to the control device (PCE=15.00%). Overall, this approach is a significant improvement over the existing fabrication methods and proves to be an importantly new method for future research on PSCs.
1
Keywords: perovskite solar cells; solvent processing; interfacial contact; power conversion efficiencies; electrical effect
1. Introduction Perovskite solar cells (PSCs) have attracted considerable attention over the past few years owing to their ability to achieve high efficiencies with inexpensive materials by using a solution preparation method [1-11]. Planar heterojunction PSCs, developed by Snaith et al., have been widely used as one of the most efficient configurations of PSCs [8, 12]. As shown in Scheme 1(a), a traditionallystructured PSC has a unique sandwich-type structure that contains a hybrid halide perovskite layer [6, 13,14], typically CH3NH3PbI3 (MAPbI3) [15], which serves as a visible-light-absorbing material located between an electron transport material (ETM) and a hole transport material (HTM) [16-18]. TiO2 or SnO2 is commonly used as an ETM [1, 19,20] and is deposited on conductive glass substrates [e.g., indium tin oxide (ITO)] [21-23]. Common HTMs include 2,2′,7,7′-tetrakis(N, N-dip-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), poly(3-hexylthiophene) (P3HT), and
poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]
dithiophene-2,6-diyl][3-fluoro-2-[(2-
ethylhexyl)-carbonyl]-thieno-[3,4-b] thiophenediyl]] (PTB7) [24-27].
2
The blending of γ-butyrolactone (GBL) and N,N-dimethylformamide (DMSO) is used as an effective solvent for lead iodide (PbI2) and methylammonium iodide (MAI) [28]. The solvent with a mixture of PbI2 and MAI is dropped onto a TiO2 film through one-step spin coating as a perovskite precursor [29-31], followed by heat treatment to form a perovskite crystal, MAPbI3 [2, 32]. To enhance the device performance, the solvent treatment of the perovskite surfaces is one of the most important steps [11,31,33-36]. For example, when the perovskite precursor was deposited with a mixture of GBL and DMSO, followed by the dripping addition of chlorobenzene (CB) while spinning, relatively uniform and dense layers were formed [34,36]. After the HTM solution, e.g., spiro-OMeTAD [37], is spin-coated on top of the MAPbI3 film, a layer–layer contact is obtained (termed as a conventional device), as shown in Scheme 1(b). The CB drop solvent engineering technology (termed as the CB-treated method) is applied to enhance the device performance and has become the most popular method for fully solution-processed devices [Scheme 1(b)] [35,38]. However, the simple spin-coating method does not completely address the interfacial contact problems in the perovskite layer, limiting the applicability of solution processing [39,40]. Recently, several new solvent and interface engineering technologies have been developed in the invertedstructured PSCs; and significant improvements of device performance have been achieved [2, 3, 18,27,34,36,38]. Unfortunately, the interfacial contact problems in the traditionally-structured PSCs are yet addressed. Herein, we report a new solution engineering technology for efficient traditionally-structured PSCs by employing a sequential CB solvent processing with HTMs (termed as the CB/HTM-treated method) to induce inter-diffusion between the perovskite and the HTM [termed as a layer-evolved device; Scheme 1(c)]. The proposed device structure is realized by dripping a CB solution of the HTM, e.g., spiro-OMeTAD onto the perovskite precursor before the perovskite crystallization is complete, thereby enabling the spiro-OMeTAD to penetrate into the perovskite layer. Consequently, the spiro-OMeTAD premixed in the CB solvent is retained in the perovskite films. Subsequently, a
3
mixed layer of perovskite and spiro-OMeTAD is formed on top of the light-absorbing layer [Scheme 1(c)]. This method makes a tight contact between the perovskite film and HTM, which facilitates charge carrier extraction and improves the electrical properties of the PSCs. This enhancement of the original solvent engineering technique allows a fully solution-treated PSC to improve the PCE from 15.00% to 18.39% under the standard testing conditions.
2. Experimental Section Device Fabrication: All materials were purchased from Sigma-Aldrich and used as received without further purification. A perovskite precursor solution could be obtained through the mixture of CH3NH3I and PbI2 (1:1, molar ratio) in the solvent of γ-butyrolactone and DMSO (7:3, v:v). The TiO2 solution was prepared as an ETM according to the previous method [41]. The spiro-OMeTAD, P3HT, or PTB7 was used for a HTM. A solution containing 28 μL of 4-tert-butyl pyridine and 17 μL of Li-TFSI (520 mg Li-TFSI in 1 mL acetonitrile) was added to a CB solution (1 mL) of spiroOMeTAD (80 mg), P3HT(15 mg), or PTB7 (10 mg), respectively. The device structure was ITO/TiO2/MAPbI3/HTM/Au [42,43]. Regarding the typical device fabrication, ITO glass substrates (15 mm × 15 mm, 15 Ω sq−1 resistance) were first cleaned with decontamination powder and then ultrasonicated successively in DI water, acetone, and anhydrous ethanol (each for 10 min). The ITO glasses were then treated with UV ozone for 15 min. To make compact TiO2 layers, the cleaned ITO glasses were coated with a TiO2 solution by spin-coating at 4000 rpm for 60 s. The compact TiO2 coated ITO substrates were dried at 150 °C for 10 min and then allowed to cool down to room temperature slowly. A perovskite precursor solution was spin-coated onto TiO2 layer at 3000 rpm for 40 s with a dripping CB solvent (CB-treated method) or a CB solvent containing HTM with different concentration (e.g., 0-4 mg/mL) (CB/HTM-treated method). The substrates were then heated at 100 °C for 10 min and allowed to cool down to room temperature. Subsequently, a HTM was deposited on the top of the CH3NH3PbI3 perovskite layer by spin coating. Devices were then kept 4
overnight in air. Finally, a top Au electrode (100 nm thick) was thermally evaporated on the top of the HTM. Solar Cell Characterizations: J–V characteristics of devices were measured under AM 1.5G illumination (100 mW cm−2) from a solar simulator. The morphologies of the conventional device and the layer-evolved device without top Au electrodes were characterized by a field-emission scanning electron microscope (SEM, FEI 450). The X-ray diffraction patterns (XRD) were collected using a PANalytical X'Pert XRD system with reference X-ray illumination as Cu Kα radiation at 0.154 nm. The film absorption spectrum was obtained using a UV–Vis–NIR spectrophotometer (Lambda 35, Perkin-Elmer). Photoluminescence (PL) of the films were performed using a fluorescence spectrometer (FLS 980) with an excitation at 470 nm, similar to the one reported previously [44,45].
3. Results and Discussion A basic PSC with the device structure of ITO/TiO2/MAPbI3/HTM/Au was fabricated. Here, spiroOMeTAD is used as the HTM for demonstrating the new method. Devices with other HTMs such as P3HT and PTB7 will be investigated later. The CB-treated method is used as a solvent engineering for conventional devices (as the control devices), whereas the CB/spiro-OMeTAD-treated method is adopted for the new device configuration termed as the layer-evolved devices. To demonstrate the new solvent method, a series of experiments have been performed to investigate the morphology, device performance, and optical and electrical properties. Scanning electron microscopy (SEM) measurements were performed to study the morphology of the two devices. Figure 1(a) and 1(c) show the top-view and cross-sectional SEM images of the MAPbI3 film after application of pure CB solvent, respectively. Small cracks can be observed on the surface of the MAPbI3 film, and the MAPbI3 film has a high roughness after application of the pure CB solvent. However, the cracks in the MAPbI3 film can be filled if the CB solvent premixed with 5
spiro-OMeTAD is used. As shown in Figure 1(b) and 1(d), a thin layer of MAPbI3 and spiroOMeTAD mixture atop the perovskite crystals is observed. In addition, the CB/spiro-OMeTADtreated film is smooth compared with the pure CB-treated film. After a spiro-OMeTAD solution was subsequently spin-coated on top of the CB-treated film [Figure 1(e)], it can be observed that the spiro-OMeTAD film has some pinholes. By contrast, the spiro-OMeTAD film coated on top of the CB/spiro-OMeTAD-treated film of the layer-evolved device is much more uniform than the conventional device with fewer holes and defects [Figure 1(f)]. For the CB-treated case, MAPbI3 is in direct contact with spiro-OMeTAD [Figure 1(g)] and results in a loose interfacial contact with spiro-OMeTAD. However, this interfacial contact is denser when MAPbI3 is premixed with spiroOMeTAD in the layer-evolved device [Figure 1(h)]. In addition, the CB/spiro-OMeTAD treatment improves the quality of perovskite crystallization because it separates the presumably unstable intermediate from the sensitive solvent vapor atmosphere during spin coating [15]. X-ray diffraction (XRD) was performed to study the perovskite crystallization [46,47]. As shown in Figure 2(a) and 2(b), the half peak width of XRD peak for the CB/spiro-OMeTAD-treated MAPbI3 film is smaller than that of the CB-treated MAPbI3 film, which indicates that the crystallinity of the CB/spiroOMeTAD-treated MAPbI3 film is significantly higher than that of the pure CB-treated film. The device performance of both the conventional and layer-evolved devices was investigated, and the typical device characteristics and parameters are summarized in Figure 3 and Table 1, respectively. The conventional device shows only a moderate efficiency of 15.00%, with a shortcircuit photocurrent density (Jsc), of 21.15 mA/cm2, Voc of 1.02 V, and fill factor (FF) of 0.69. These data are averages from 40 devices of the conventional devices, and all of the PCEs are within the range of 13.00–16.00%. The concentration of the spiro-OMeTAD premixed with CB, which is a key parameter to achieve high PCEs, has been optimized. With the increase of the spiro-OMeTAD concentration from 0.5 mg/ml to 1.0 mg/ml, and to 2.0 mg/ml, the PCE increases from 16.32% to 17.51%, and to 18.39%, respectively. When the spiro-OMeTAD concentration is further increased to
6
4.0 mg/ml, the PCE drastically declines to 13.63%. Thus, the optimal spiro-OMeTAD concentration is about 2.0 mg/ml, and the optimized average PCE reaches 18.39% (all of the PCEs are within 17.00–19.50%), with Jsc of 23.14 mA/cm2, Voc of 1.06 V, and FF of 0.75, which is 22% higher than that of the pure CB-treated device. The improved PCE mainly results from improvements in Jsc and FF. To understand the reason for the improvement in Jsc, the absorption spectra were measured for the following five samples on TiO2 substrates including one conventional device and four layerevolved devices with different concentrations of spiro-OMeTAD in CB. As shown in Figure 2(c), the absorption spectra of perovskite films are almost unchanged after the devices are treated with the spiro-OMeTAD solution, indicating that the increase in Jsc is not due to the light absorption property. Incident photon-to-current efficiencies (IPCE) of these devices have also been measured, which are comprehensive quantities of both optical absorption and electrical effects [48]. As shown in Figure 2(d), the IPCE increases when the treatment of the perovskite films changes from CB to CB/spiroOMeTAD. With the increase of spiro-OMeTAD concentration from 0.5 to 2.0 mg/ml, the IPCE is gradually improved. When the spiro-OMeTAD concentration reaches 4.0 mg/ml, the IPCE decreases. Jsc can also be calculated from the IPCE spectra. For these five devices, Jsc is calculated to be 21.02, 21.35, 22.54, 23.04, and 19.40 mA/cm2, respectively, which is in good agreement with the Jsc derived from the J–V measurements. Considering the unchanged light absorption properties of the perovskite–spiro-OMeTAD films, the significant improvement in photovoltaic performance is likely due to positive electrical effects rather than improved light harvesting. The electrical effects including exciton separation, carrier transport and extraction, carrier recombination, and interfacial resistance were systematically studied. To better understand the exciton separation efficiency, photoluminescence (PL) characterization was performed. As shown in Figure 4(a), PL occurs at similar wavelengths with a peak around 778 nm for all the samples, which corresponds to the PL spectrum of MAPbI3 [33,49]. However, more quenching is observed for the
7
CB/spiro-OMeTAD-treated device compared with the CB-treated device, indicating more efficient exciton separation in the MAPbI3 film. In addition, the MAPbI3 films treated by CB solvent premixed with different concentrations of spiro-OMeTAD show different PL intensities. When the concentration of spiro-OMeTAD is increased from 0.5 mg/ml to 1.0 mg/ml and then to 2.0 mg/ml, the PL intensity of the MAPbI3 film is gradually decreased, indicating that exciton separation in the MAPbI3 film and the mixed layer between the MAPbI3 film and spiro-OMeTAD film is more efficient for a concentration of 2.0 mg/ml, followed by 1.0 mg/ml and then 0.5 mg/ml. The crystal quality is a possible reason for the better exciton separation. As shown in Figure 2(a) and 2(b), the half peak width of XRD peaks gradually decreases when the concentration of spiro-OMeTAD increases from 0.5 mg/ml to 1.0 mg/ml and then to 2.0 mg/ml. Increasing the concentration of spiroOMeTAD to 4.0 mg/ml results in a thicker film with relatively poor crystal quality, which will restrain the exciton generation and separation efficiency and lead to obvious emittance [Figure 4(a)]. To analyze the hole transport and extraction, we fabricated hole-only devices with a structure of ITO/PEDOT:PSS/MAPbI3/spiro-OMeTAD/Au. As shown in Figure 4(b), the hole current density is significantly larger for the layer-evolved device than for the conventional device, indicating better hole extraction properties of the layer-evolved device. Owing to the improved interfacial contact, there are fewer pinholes or defect sites introduced in the perovskite/spiro-OMeTAD interface. As a result, the “trap sites” of the carrier in the interface is effectively reduced [Figure 4(f) and 4(g)]. It should be mentioned that the better energy alignment between the perovskite layer and the spiroOMeTAD layer is an important factor for efficient hole extraction. The introduction of perovskite/spiro-OMeTAD mixed layer can affect the interfacial energy band, which is also related to the charge transport kinetics [50]. Because the valence band (VB) energies of perovskite and spiro-OMeTAD thin films are -5.4 and -5.2 eV, respectively [26, 27], the VB energy of the interdiffusion layer is between -5.4 and -5.2 eV [Figure 4(e))]. Thus, the mixed layer between
8
perovskite and spiro-OMeTAD is beneficial to the hole extraction from the perovskite layer to the spiro-OMeTAD. The carrier recombination was further investigated by plotting the J–V curve of the device versus light intensity. Figure 4 (c) presents a power law dependence of Jsc on light intensity (J = Iα). When α = 0.75, a device is space charge limited because of an interfacial barrier or a carrier imbalance. If α value is near 1, a negligible space charge limit could be achieved [51,52]. In the current work, the device with an intermixed layer shows a sharper slope of 0.98 than the conventional device with a slope of 0.89. The slopes of some layer-evolved devices nearly reach 1, signifying negligible interfacial recombination losses in the layer-evolved devices. Furthermore, Figure 4 (d) shows the Voc measured as a function of light intensity. It is well known that when the slope is greater than 1 kT/q, where k is the Boltzmann constant, T is the temperature in Kelvin, and q is the electron charge, Voc becomes strongly dependent on light intensity, and additional interfacial trap-assisted Shockley-Read-Hall (SRH) recombination is involved [3,53]. In the current work, the conventional device has a slope value of 1.96 kT/q, indicating a stronger interfacial trap-assisted recombination than the layer-evolved device (1.34 kT/q). In addition to the better energy alignment between the perovskite layer and spiro-OMeTAD layer for beneficial carrier extraction, the improved interfacial contact and crystal quality of the layer-evolved device are also possible reasons for the reduced interfacial trap-assisted recombination [Figure 4(e)-4(g)]. Consequently, we can conclude that the sequential solvent processing with HTMs reduces the interfacial recombination losses and SRH recombination, which is critical to improving device performance. Therefore, due to the series positive electrical effects including efficient exciton separation, improved carrier transport and extraction between CH3NH3PbI3 and spiro-OMeTAD, and reduced carrier recombination, Jsc is significantly enhanced for the layer-evolved device compared with the conventional device. Regarding the higher FF, it can be attributed to decreased series resistance (Rs) and increased shunt resistance (Rsh), as shown in Figure 3(b) and Table 1. The Voc is slightly
9
increased from 1.02 to 1.06 V after the device is treated by the sequential solvent processing with HTMs, which can be attributed to the reduced carrier recombination. For PSCs measured under certain scanning conditions, the hysteresis in the J–V curve is a very important issue [4, 46,54-60]. As shown in Figure 5(a), the hysteresis for the conventional device is very serious. After the CB/spiro-OMeTAD treatment is applied, the hysteresis for the layer-evolved device is greatly improved. In order to quantify the degree of hysteresis, we used a formula (1) to define the hysteresis factor: ysteresis fa tor=
C reverse - C for ard C reverse
(1) The hysteresis factors of PSCs are summarized in Table 2. Compared with the big hysteresis factor of 0.40 for the conventional device, the hysteresis factor for the layer-evolved device is only 0.09. Although there are controversies on the causes of the hysteresis , efficient hole carrier transport and extraction, and reduced carrier recombination seem to be possible reasons for the reduction of hysteresis. The proposed CB/spiro-OMeTAD treatment method is a universal approach. When spiroOMeTAD is changed with other HTMs, such as P3HT or PTB7, which are typical P-type materials, the device efficiency is still increased (device structure: ITO/TiO2/MAPbI3/P3HT or PTB7/Au). As shown in Figure 5(b) and 5(c) and Table 2, the device efficiency is significantly increased from 10.36% to 12.30% using P3HT as the HTM, and from 8.13% to 11.03% using PTB7 as the HTM after CB/P3HT (optimized concentration is 0.6 mg/ml) or CB/PTB7 (optimized concentration is 0.6 mg/ml) treatment is applied, respectively. Compared to a hysteresis factor of 0.46 for the conventional device, the hysteresis factor for the layer-evolved device is only about 0.08 when P3HT is used as the HTM. When PTB7 is used as the HTM, the hysteresis factor is decreased from 0.37 for the conventional device to only 0.08 for the layer-evolved device. Therefore, it is a general method for enhancing the performance of PSCs and reducing recombination and hysteresis (Figure 5). 10
4. Conclusions In summary, we have demonstrated a simple, effective, and universal solvent engineering technology referred as the sequential solvent processing with HTMs for high-performance traditionallystructured PSCs. The layer-evolved devices exhibit a significant enhancement in Jsc, FF, and PCE. As a result, a PCE of 18.39% can be readily achieved, whereas the PCE for reference devices is 15.00%. Furthermore, premixing HTM in CB plays several important roles in enhancing the device performance: (1) improving the crystallinity of the perovskite crystals, (2) allowing better alignment of the interfacial energy bands, and (3) enhancing the interfacial contact and improving carrier extraction from the active layer to the HTM. Most importantly, this approach provides a new universal method for high-performance PCS fabrications.
Acknowledgments This research is supported by the National Natural Science Foundation of China (51221001, 51472204, 51571166, 51521061, and 61505167) and the Program of Introducing Talents of Discipline to Universities (B08040). We also thank the support of the Natural Science Research Project of Shaanxi Province (2016JM5001), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU) (Grant No. 147-QZ-2016), and the Key Scientific and Technological Team from Shaanxi Province (No. 2015KCT-12).
Conflict of Interest The authors declare no conflict of interest.
References
11
[1] T. Hu, T. Becker, N. Pourdavoud, J. Zhao, K.O. Brinkmann, R. Heiderhoff, T. Gahlmann, Z. Huang, S. Olthof, K. Meerholz, D. Tobbens, B. Cheng, Y. Chen, T. Riedl, Adv. Mater. (2017) DOI: 10.1002/adma.201606656. [2] W. Chen, L. Xu, X. Feng, J. Jie, Z. He, Adv. Mater. 29 (2017) 1603923. [3] H. Chen, W. Fu, C. Huang, Z. Zhang, S. Li, F. Ding, M. Shi, C. Z. Li, A.K.Y. Jen, H. Chen, Adv. Energy Mater. (2017) DOI: 10.1002/aenm.201700012. [4] J.B. Patel, J. Wong-Leung, S. Van Reenen, N. Sakai, J.T.W. Wang, E.S. Parrott, M. Liu, H.J. Snaith, L.M. Herz, M.B. Johnston, Adv. Electron. Mater. 3 (2017) 1600470. [5] S. Y. Lin, T. S. Su, T. Y. Hsieh, P. C. Lo, T. C. Wei, Adv. Energy Mater. (2017) DOI: 10.1002/aenm.201700169. [6] M. Zhang, J. Wang, L. Li, G. Zheng, K. Liu, M. Qin, H. Zhou, X. Zhan, Adv. Sci. (2017) DOI: 10.1002/advs.201700025. [7] D. Liu, T.L. Kelly, Nat. Photonics 8 (2013) 133-138. [8] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395-398. [9] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gratzel, S. Mhaisalkar, T.C. Sum, Science 342 (2013) 344-347. [10] J.T. Wang, J.M. Ball, E.M. Barea, A. Abate, J.A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H.J. Snaith, R.J. Nicholas, Nano Lett. 14 (2014) 724-730. [11] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S.I. Seok, Nat. Mater. 13 (2014) 897-903. [12] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643-647. [13] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Energy Environ. Sci. 7 (2014) 982. [14] C. Wehrenfennig, G.E. Eperon, M.B. Johnston, H.J. Snaith, L.M. Herz, Adv. Mater. 26 (2014) 1584-1589. [15] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Nature 517 (2015) 476-480. [16] N.J. Jeon, H.G. Lee, Y.C. Kim, J. Seo, J.H. Noh, J. Lee, S.I. Seok, J. Am. Chem. Soc. 136 (2014) 7837-7840. [17] D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P.A. Dowben, O.F. Mohammed, E.H. Sargent, O.M. Bakr, Science 347 (2015) 519-522. [18] Z. Li, J. Tinkham, P. Schulz, M. Yang, D.H. Kim, J. Berry, A. Sellinger, K. Zhu, Adv. Energy Mater. 7 (2016) 1601451.
12
[19] H. Hussain, G. Tocci, T. Woolcot, X. Torrelles, C.L. Pang, D.S. Humphrey, C.M. Yim, D.C. Grinter, G. Cabailh, O. Bikondoa, R. Lindsay, J. Zegenhagen, A. Michaelides, G. Thornton, Nat. Mater. 16 (2016) 461-465. [20] Q. Jiang, L. Zhang, H. Wang, X. Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Nat. Energy 2 (2016) 16177-16182. [21] D. Zhang, W.C.H. Choy, F. Xie, W.E.I. Sha, X. Li, B. Ding, K. Zhang, F. Huang, Y. Cao, Adv. Funct. Mater. 23 (2013) 4255-4261. [22] Y. Tu, J. Wu, M. Zheng, J. Huo, P. Zhou, Z. Lan, J. Lin, M. Huang, Nanoscale 7 (2015) 2053920546. [23] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, F. Giordano, S.M. Zakeeruddin, M. Gratzel, R.H. Friend, J. Phys. Chem. Lett. 7 (2016) 3264-3269. [24] J. Liu, Y. Wu, C. Qin, X. Yang, T. Yasuda, A. Islam, K. Zhang, W. Peng, W. Chen, L. Han, Energy Environ. Sci. 7 (2014) 2963-2970. [25] W.H. Nguyen, C.D. Bailie, E.L. Unger, M.D. McGehee, J. Am. Chem. Soc. 136 (2014) 1099611001. [26] S. Ryu, J.H. Noh, N.J. Jeon, Y. Chan Kim, W.S. Yang, J. Seo, S.I. Seok, Energy Environ. Sci. 7 (2014) 2614-2619. [27] M. Li, Z. K. Wang, Y. G. Yang, Y. Hu, S. L. Feng, J. M. Wang, X. Y. Gao, L. S. Liao, Adv. Energy Mater. 6 (2016) 1601156. [28] H. Zhang, D. Li, J. Cheng, F. Lin, J. Mao, A.K.Y. Jen, M. Grätzel, W.C.H. Choy, J. Mater. Chem. A 5 (2017) 3599-3608. [29] J. Peng, T. Duong, X. Zhou, H. Shen, Y. Wu, H.K. Mulmudi, Y. Wan, D. Zhong, J. Li, T. Tsuzuki, K.J. Weber, K.R. Catchpole, T.P. White, Adv. Energy Mater. 7 (2016) 1601768. [30] A. Huang, L. Lei, J. Zhu, Y. Yu, Y. Liu, S. Yang, S. Bao, X. Cao, P. Jin, ACS Appl. Mater. Inter. 9 (2017) 2016-2022. [31] Y. Dong, W. Li, X. Zhang, Q. Xu, Q. Liu, C. Li, Z. Bo, Small 12 (2016) 1098-1104. [32] W. Q. Wu, F. Huang, D. Chen, Y. B. Cheng, R.A. Caruso, Adv. Energy Mater. 6 (2016) 1502027. [33] X.H. Zhang, J.J. Ye, L.Z. Zhu, H.Y. Zheng, X.P. Liu, X. Pan, S.Y. Dai, ACS Appl. Mater. Inter. 8 (2016) 35440-35446. [34] Q. Wu, W. Zhou, Q. Liu, P. Zhou, T. Chen, Y. Lu, Q. Qiao, S. Yang, ACS Appl. Mater. Inter. 8 (2016) 34464-34473. [35] M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y.B. Cheng, L. Spiccia, Angew. Chem. Int. Edit. 53 (2014) 9898-9903.
13
[36] Y. Wu, X. Yang, W. Chen, Y. Yue, M. Cai, F. Xie, E. Bi, A. Islam, L. Han, Nat. Energy 1 (2016) 16148-16152. [37] J.A. Christians, R.C. Fung, P.V. Kamat, J. Am. Chem. Soc. 136 (2014) 758-764. [38] C. H. Chiang, C. G. Wu, Nat. Photonics 10 (2016) 196-200. [39] X. Fang, Y. Wu, Y. Lu, Y. Sun, S. Zhang, J. Zhang, W. Zhang, N. Yuan, J. Ding, J. Mater. Chem. C 5 (2017) 842-847. [40] T. Liu, C. Wang, J. Hou, C. Zhang, H. Chen, H. He, N. Wang, H. Wu, G. Cao, Small 12 (2016) 5146-5152. [41] X. Li, X. Ren, F. Xie, Y. Zhang, T. Xu, B. Wei, W.C.H. Choy, Adv. Opt. Mater. 3 (2015) 12201231. [42] M. Yang, T. Zhang, P. Schulz, Z. Li, G. Li, D.H. Kim, N. Guo, J.J. Berry, K. Zhu, Y. Zhao, Nat. Commun. 7 (2016) 12305-12309. [43] Y. Zhou, M. Yang, S. Pang, K. Zhu, N.P. Padture, J. Am. Chem. Soc. 138 (2016) 5535-5538. [44] S. Guo, X. Li, J. Zhu, T. Tong, B. Wei, Small 12 (2016) 5692-5701. [45] X. Li, S. Guo, C. Kan, J. Zhu, T. Tong, S. Ke, W.C.H. Choy, B. Wei, Nano Energy 30 (2016) 549-558. [46] X. Zhang, J. Ye, L. Zhu, H. Zheng, G. Liu, X. Liu, B. Duan, X. Pan, S. Dai, Nanoscale 9 (2017) 4691-4699. [47] L. Qian, Y. Sun, M. Wu, D. Xie, L. Ding, G. Shi, Adv. Mater. (2017) DOI: 10.1002/adma.201606175. [48] X. Li, J. Zhu, B. Wei, Chem. Soc. Rev. 45 (2016) 3145-3187. [49] H. Zhang, J. Cheng, F. Lin, H. He, J. Mao, K.S. Wong, A.K. Jen, W.C. Choy, ACS Nano 10 (2016) 1503-1511. [50] M.C. Kim, B.J. Kim, D.Y. Son, N.G. Park, H.S. Jung, M. Choi, Nano Lett. 16 (2016) 5756-5762. [51] D. Yang, X. Zhou, R. Yang, Z. Yang, W. Yu, X. Wang, C. Li, S. Liu, R.P.H. Chang, Energy Environ. Sci. 9 (2016) 3071-3078. [52] J.E. Moser, Nat. Mater. 16 (2016) 4-6. [53] D. Zhao, W. Ke, C.R. Grice, A.J. Cimaroli, X. Tan, M. Yang, R.W. Collins, H. Zhang, K. Zhu, Y. Yan, Nano Energy 19 (2016) 88-97. [54] B. Roose, K.C. Gödel, S. Pathak, A. Sadhanala, J.P.C. Baena, B.D. Wilts, H.J. Snaith, U. Wiesner, M. Grätzel, U. Steiner, A. Abate, Adv. Energy Mater. 6 (2016) 1501868. [55] H.S. Kim, N.G. Park, J. Phys. Chem. Lett. 5 (2014) 2927-2934. [56] H.J. Snaith, A. Abate, J.M. Ball, G.E. Eperon, T. Leijtens, N.K. Noel, S.D. Stranks, J.T. Wang, K. Wojciechowski, W. Zhang, J. Phys. Chem. Lett. 5 (2014) 1511-1515. 14
[57] H.S. Kim, I.H. Jang, N. Ahn, M. Choi, A. Guerrero, J. Bisquert, N.G. Park, J. Phys. Chem. Lett. 6 (2015) 4633-4639. [58] B. Li, Y. Zhang, X. Zhou, Z. Liu, Q. Liu, X. Li, J. Alloys Compd. 673 (2016) 265-271. [59] Q. Tai, P. You, H. Sang, Z. Liu, C. Hu, H. L. Chan, F. Yan, Nat. Commun. 7 (2016) 11105. [60] Z. Liu, P. You, C. Xie, G. Tang, F. Yan, Nano Energy. 28 (2016) 151-157.
Tengteng Tong received his B.S.degree from Anhui University Of Technology in 2014,and he is now an M.S. candidate at Northwestern Polytechnical University under the supervision of Professor Bingqing Wei and Professor Xuanhua Li.. His research focuses on perovskite solar cells.
Dr. Xuanhua Li received his B.S. degree from Wuhan University of Technology in 2007, and M.S. degree from USTC in 2010. After that, he started his doctoral studies and received a Ph.D. degree at Department of Electrical and Electronic Engineering, University of Hong Kong in 2014. After a short research in the Institute of Intelligent Machines, CAS, he began his career at the Center of Nano Energy Materials, Northwestern Polytechnical University as a professor. His research is focused on synthesis of 2D nanomaterials and novel metal NPs. He is also interested in plasmonenhanced solar cells, photocatalytic reaction, and optical sensors.
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Shaohui Guo received his B.S. degree from Northestern Polytechnical University in 2014, and he is now a Ph.D. student at Northwestern Polytechnical University under the supervision of Professor Bingqing Wei and Professor Xuanhua Li. His current research includes the development of nanostructured materials for photocatalysis and surface enhanced Raman scattering (SERS).
Jian Han received his B.S. degree from Xi`an University of Technology in 2012, and M.S. degree from Xi`an University of Architecture and Technology in 2016. And now, he is pursuing his doctoral degree at Northwestern Polytechnical University under the supervision of Professor Xuanhua Li. His research is focused on solar cells.
Dr. Bingqing Wei is a Professor in the Department of Mechanical Engineering at the University of Delaware, USA. He was an Assistant Professor in the Department of Electrical & Computer Engineering and Center for Computation & Technology at Louisiana State University from 2003 to 2007. From 2000 to 2003, he was a Research Scientist at Rensselaer Polytechnic Institute, Department of Materials Science and Engineering and Rensselaer Nanotechnology Center. Dr. Wei 16
was a visiting scientist at Max-Planck-Institut für Metallforschung, Stuttgart, Germany in 1998 and 1999. From 1992 to 2001, he was a faculty member at Tsinghua University in Beijing.
Figure 1. Top view SEM images of the MAPbI3 film treated with (a) pure CB solvent and (b) CB/spiro-OMeTAD. The inset of (a) is an enlarged region. The arrows indicate some cracks in the 17
film. Cross-sectional SEM images of the MAPbI3 film treated with (c) pure CB solvent and (d) CB/spiro-OMeTAD. Top view SEM images of the spiro-OMeTAD film of (e) the conventional device and (f) the layer-evolved device without a top Au electrode. Cross-sectional SEM images of (g) the conventional device and (h) the layer-evolved device without a top Au electrode.
(b) Conventional
1.0
0.5 mg/ml
0.8
Intensity (a.u.)
1.0 mg/ml
2.0 mg/ml
4.0 mg/ml 20
3.5
14.2
2.0
14.4
Conventional 0.5 mg/ml 2.0 mg/ml
100
1.5
14.6
2 (degree)
(d) 120
IPCE (％)
2.5
0.2 0.0
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
3.0
0.4
40
2 (degree)
(c) 4.0
Absorbance
30
0.6
2
10
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
25
1.0 mg/ml 4.0 mg/ml
20
80
15
60 40
10
20
5
1.0 0.5 0.0 400
500
600
700
800
900
0 300
0 400
500
600
700
Wavelength (nm)
Wavelength (nm)
Integrated Current density (mA/cm )
Intensity (a.u.)
(a)
800
Figure 2. (a) XRD patterns of the CB- and CB/spiro-OMeTAD-treated perovskite films. (b) Enlarged 14°–15° region of (a). (c) UV–Vis absorption spectra and (d) IPCE spectra of the CB- and CB/spiro-OMeTAD-treated perovskite devices with different concentrations of spiro-OMeTAD premixed in the dripping CB solvent.
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(b) Current density (mA/cm2)
Current Density (mA/cm2)
(a) 25 20
15
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
10
5
0 0.0
0.2
0.4
0.6
0.8
0
-2
-4
-6
-8
-10 -0.2
1.0
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
0.0
12
0.6
12
Conventional
10
8
8
Count
10
Count
0.4
0.8
1.0
1.2
(d)
(c)
6
4
2
2
14.2
14.6
15.0
15.4
15.8
16.2
16.4
Layer-evolved
6
4
0 13.8
0.2
Voltage (V)
Voltage (V)
0 17.2
17.6
18.0
18.4
18.8
19.2
19.6
PCE (％)
PCE (％)
Figure 3. Photovoltaic characteristics of the two types of perovskite devices. (a) Current density– voltage (J–V) curves measured under AM 1.5 simulated sunlight for the conventional and layerevolved devices with spiro-OMeTAD premixed with CB at different concentrations. (b) Dark J–V curves for the conventional and layer-evolved devices. Statistical PCE distributions of the (c) conventional and (d) layer-evolved devices.
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(b)
Current Density (mA/cm2)
(a)
Intensity
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
700
720
740
760
780
800
500 400
Conventional 0.5 mg/ml 1.0 mg/ml 2.0 mg/ml 4.0 mg/ml
300 200 100 0 0
820
1
2
3
Voltage (V)
Wavelength (nm)
(c)25
(d) 1.05
Conventional 2.0 mg/ml
20
Jsc (mA/cm2)
1.00
Voc (V)
15
10
0.95
0.90
Conventional 2.0 mg/ml
5
0.85 0 20
40
60
80
Light Intensity (mW/cm2)
100
20
(e)
CB
CB
(f)
h+
Mixed interlayer
+
80
e_
100
Spiro-OMeTAD MAPbI3
Spiro-OMeTAD
(g) +
+
60
h+ e_
recombination
Perovskite
VB
40
Light Intensity (mW/cm2)
+
VB
+
h+ e_
Spiro-OMeTAD Mixture MAPbI3
Figure 4. (a) PL spectra of perovskite films treated by different concentrations of solvent-deposited spiro-OMeTAD. (b) J–V characteristics of hole-only devices with perovskite films treated by different concentrations of solvent-deposited spiro-OMeTAD. (c) Dependence of Jsc on light intensity. (d) Dependence of Voc on light intensity. Schematic of hole carrier transport from the active layer to the HTM for (e) schematic showing the band shifts of perovskite and spiro-OMeTAD with an interdiffusion layer, (f) the conventional device, and (g) the layer-evolved device.
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(a) 25
2 Current Density (mA/cm )
(b)
20
15
10
5
0 0.0
Conventional forward Conventional reverse Layer-evolved forward Layer-evolved reverse
0.2
0.4
0.6
0.8
1.0
20
15
10
Conventional forward Conventional reverse Layer-evolved forward Layer-evolved reverse
5
0 0.0
0.2
Voltage(V)
0.4
0.6
0.8
Voltage (V)
Current Density (mA/cm2)
(c) 17.5 15.0 12.5 10.0 7.5 5.0 2.5 0.0
Conventional forward Conventional reverse Layer-evolved forward Layer-evolved reverse 0.2
0.4
0.6
0.8
1.0
Voltage (V)
Figure 5. J–V hysteresis of solar cells under forward and reverse scans with a scan rate of 400 mV/s with different HTMs: (a) spiro-OMeTAD, (b) P3HT, and (c) PTB7. “Forward scan” corresponds to the scan from 0.0 V to 1.0 V and “reverse s an” orresponds to the s an from 1.0 V to 0.0 V.
Table 1. Photovoltaic parameters of devices treated with the CB solvent premixed with spiroOMeTAD at various concentrations. Concentrationa) [mg/mL]
Voc [V]
Jsc [mA cm−2]
FF [%]
PCE [%]
Rs [Ω cm−2]
Rsh [Ω cm−2]
0
1.02 ± 0.02
21.15 ± 0.55
69.45± 0.04
15.00 ± 0.83
11.34 ± 0.75
1276.46 ± 35.00
0.5
1.06 ± 0.02
21.56 ± 0.32
71.42 ± 0.03
16.32 ± 0.62
7.85 ± 0.54
1358 .89± 27.00
1.0
1.06 ± 0.02
22.82 ± 0.33
72.34 ± 0.03
17.51 ± 0.64
7.06 ± 0.62
1534.62 ± 31.00
2.0
1.06 ± 0.02
23.14 ± 0.32
75.02 ± 0.03
18.39 ± 0.72
5.55 ± 0.43
1937.45 ± 29.00
4.0
1.06 ± 0.03
19.57 ± 0.38
66.23 ± 0.04
13.63 ± 1.03
13.57 ± 0.81
1149.00 ± 39.00
a)
Concentration of spiro-OMeTAD in CB solvent. 21
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Table 2. Comparison of photovoltaic parameters for the conventional and layer-evolved devices. spiro-OMeTAD
Voc [V]
Jsc [mA cm−2]
FF [%]
PCE [%]
Hysteresis factor
Conventional device
Forward Reverse
1.02± 0.02 0.89± 0.03
21.15± 0.55 21.05± 0.82
69.45± 0.04 47.84± 0.05
15.00 ± 0.83 8.92± 1.02
0.40± 0.08
Layer-evolved device
Forward Reverse
1.06 ± 0.02 1.03± 0.02
23.14± 0.32 23.02± 0.63
75.02± 0.03 70.10± 0.04
18.39± 0.72 16.61± 0.83
0.09± 0.02
Conventional device
Forward Reverse
0.85± 0.02 0.67± 0.03
18.56± 0.60 18.43± 0.72
65.93± 0.03 44.62± 0.04
10.36± 0.62 5.50± 0.80
0.46± 0.09
Layer-evolved device
Forward Reverse
0.83± 0.02 0.80± 0.02
19.82± 0.43 19.81± 0.52
75.02± 0.02 71.24± 0.04
12.30± 0.61 11.27± 0.92
0.08± 0.01
Conventional device
Forward Reverse
0.91± 0.02 0.86± 0.03
13.84± 0.73 13.75± 0.83
65.12± 0.04 43.56± 0.04
8.13± 0.90 5.15± 1.02
0.37± 0.05
Layer-evolved device
Forward Reverse
0.93± 0.02 0.87± 0.02
16.72± 0.52 16.70± 0.71
71.24± 0.03 69.82± 0.04
11.03± 0.82 10.12± 0.91
0.08± 0.02
P3HT
PTB7
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Scheme 1. (a) Traditional structure of a perovskite solar cell (PSC) and molecular structures of holetransport materials (HTMs). Preparation process for the light-absorbing layer of (b) a conventional device and (c) a layer-evolved device with an intermixed layer.
Highlights (1) A simple, effective, and universal solvent engineering technology is proposed. 24
(2) Sequential solvent processing with HTMs improves device efficiency. (3) New approach induces layer inter-diffusion and improves interfacial contacts. (4) PCE of 18.39% with 22.6% increment is achieved.
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