Surface optimization by poly(α-methylstyrene) as additive in the antisolution to enhance lead-free Sn-based perovskite solar cells

Solar Energy 194 (2019) 272–278

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Surface optimization by poly(α-methylstyrene) as additive in the antisolution to enhance lead-free Sn-based perovskite solar cells

T

Wenjin Zeng1, Daiqi Cui1, Zhi Li, Yanan Tang, Xiao Yu, Yinghao Li, Yunkai Deng, Ru Ye, ⁎ Qiaoli Niu, Ruidong Xia, Yong Min Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), School of Materials Science and Engineering, New Energy Technology Engineering Laboratory of Jiangsu Province, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar cells Perovskite Lead-free Poly(α-methylstyrene) Polymer additive Antisolution

Surface quality of the perovskite layer plays an important role in the determination of the device performance. This study presents an effective method to optimize the surface morphology for the lead-free formamidinium tin iodide (FASnI3) perovskite solar cells. Poly(α-methylstyrene) (PAMS) was applied as polymer additive into the diethyl ether at different blend concentration. The addition of PAMS can apparently reduce the pinholes at the surface of the FASnI3 film without ruining the interior crystallinity of the bulk FASnI3 layer. Therefore the nonradiative carrier traps can be effectively suppressed. The surface of the FAsnI3 film becomes more hydrophobic which facilitates the consequent spin-coating of the acceptor material solution. The modification of FASnI3 film’s wettability is favorable of the photo-induced carriers’ extraction and transportation. As a result, the power conversion efficiency of the PAMS-modified devices is greatly improved.

1. Introduction Hybrid organic-inorganic perovskite solar cells (PSCs) have been rapidly developed in the past decades. The power conversion efficiency (PCE) of PSCs increased from 3.9% (Kojima et al., 2009) to beyond 24% (KRICT/MIT, 2019) as for the lead-based perovskite. Many effective strategies have been reported to enhance the device performance of lead-based PSCs, including interface engineering (Zhou et al., 2014), antisolvent mixture (Liu et al., 2018a), solvent additives (Yang et al., 2015), triple cations (Saliba et al., 2016), tandem device architecture (Eperon et al., 2016), etc. However the inherent toxicity of lead may restrain the mass commercialization of lead-based PSCs in the future. Therefore increasing concern has been focused on seeking for lead-free perovskite materials as the light absorbers (Ke et al., 2018). Among the various lead-free potential candidate materials, tin-based perovskites are the most attractive due to their promising device performance (Stoumpos et al., 2013). One of the most popular tin-based perovskite is formamidinium tin iodide (FASnI3) (Kumar et al., 2014; Koh et al., 2015). With the further insight into the mechanism, the PCE of FASnI3based devices have increased gradually, even though there is still large room to catch up with the current level of lead-based PSCs (Liao et al., 2016; Liu et al., 2018b). One of the important research topics to further

improve the FASnI3-based PSCs is to develop appropriate methods for the optimization of the bulk crystallization and surface morphology of the FASnI3 layer. Both Additive technology and solvent engineering are proved to be effective strategies (Nguyen et al., 2019; Lee et al., 2016; Liao et al., 2016). During the typical one-step fabrication process of the conventional planar FASnI3-based PSCs, it is necessary to wash the precursor film by the antisolvent (or antisolution) to induce the formation of the perovskite structure. Therefore the antisolvent (or antisolution) is dominant in determining the film quality of the FASnI3 film (Li et al., 2018). Although single diethyl ether has been widely used as the antisolvent to induce the formation of the perovskite layer in the fabrication of FASnI3-based PSCs, the antisolution with additives to modified the properties of diethyl ether is still worthy of investigation in seeking for further improvement of the device performance. Poly(α-methylstyrene) (PAMS) is a disubstituted symmetric polymer with large steric hindrance group of benzene ring (Osa et al., 1999). It belongs to the same category as poly(methylmethacrylate) (PMMA), which exhibits passivation effect on the perovskite film when PMMA is used as the additive in the precursor solution of the lead-based perovskite layer (Jiang et al., 2018). Due to its large steric hindrance group, PAMS would not easily penetrate into the interior film, unlike those additives of small

⁎

Corresponding author. E-mail address: [email protected] (Y. Min). 1 W. Zeng and D. Cui have equally contributed to this work. https://doi.org/10.1016/j.solener.2019.10.088 Received 2 August 2019; Received in revised form 21 October 2019; Accepted 30 October 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

Solar Energy 194 (2019) 272–278

W. Zeng, et al.

Table 1 Device performance of the perovskite solar cells with different blend concentration of poly(α-methylstyrene) in the antisolution. Blend Ratio

VOC (V)

JSC (mA/cm2)

JSC′ (mA/cm2)*

FF (%)

PCE (%)

Control 0.3 0.5 1.0 2.0 3.0

0.268 0.225 0.314 0.259 0.228 0.230

12.73 15.70 17.64 16.46 13.50 13.99

12.86 15.86 17.82 16.62 13.64 14.13

39.43 39.29 41.09 45.02 44.26 41.49

1.35 1.39 2.28 1.92 1.36 1.33

* JSC′ is the theoretical value of the current density integrated from the IPCE curves in Fig. 2(b).

from China Southern Glass Holding Co Ltd with a surface resistance of 10 Ω/sq. The matrix solvent of the perovskite precursor is DMF and DMSO, with a molar ratio of 4:1. FAI and SnI2 were dissolved in the matrix solvent at the molar ratio of 1:1, with the ultimate concentration kept constant at 1.0 mol/mL. The FAI-SnI2 precursor solution was stored in the glovebox under dry nitrogen atmosphere before spincoating. PAMS was dissolved in diethyl ether at a serial weight concentration of 0.3, 0.5, 1.0, 2.0 and 3.0 mg/mL, used as the antisolution for the modified devices. For simplicity, the devices modified by PAMS antisolution were labeled as Device (0.3), Device (0.5), Device (1.0), Device (2.0) and Device (3.0) corresponding to the blended concentration of PAMS in diethyl ether.

Fig.1. (a) Device architecture, (b) chemical structure of poly(α-methylstyrene) and (c) schematic diagram of the energy level for each functional layer in the device architecture.

molecules (Du et al., 2019; Zhu et al., 2019). Therefore it may be feasible using the antisolution with PAMS as additive to modify the surface property of the FASnI3 film without destroying the bulk crystallinity of the FASnI3. In this study, we are the first to apply PAMS as the polymer additive in the diethyl ether at different blended concentrations to prepare the antisolution for the washing of the precursor layer of FASnI3. Its effect on the interior and surface properties of the FASnI3 layer was investigated, as well as the consequent enhanced electronic performance.

2.2. Device preparation Polished ITO glass was thoroughly cleaned before use in ultrasonic solvent bath of detergent, acetone, isopropanol and deionized water in sequence. Surface treatment of oxygen plasma was also performed on the ITO glass to completely remove the organic residues before the spincoating process. PEDOT: PSS solution was spin-coated on the ITO glass substrate at 3500 rpm and annealed at 130 °C for 30 min to form a solid film of around 50 nm. The precursor solution of FAI: SnI2 was spincoated on top of PEDOT: PSS layer under the washing of the antisolvent or antisolution, diethyl ether blended with PAMS, to prepare the FASnI3 perovskite film. After the thermal annealing of 15 min conducted on the perovskite film of FASnI3, PCBM solution of 30 mg/mL in chlorobenzene was spin-coated in subsequence to prepare a 50-nm electrontransport layer on top. The whole fabrication process was completed after the thermal evaporation of 10 nm BCP and 100 nm Ag on top of the PCBM layer under the high vacuum of 4 × 10−4 Pa.

2. Experimental section 2.1. Materials Tin (II) iodide (SnI2), PAMS, bathocuproine (BCP), [6,6]-phenylC61-butyric acid methyl ester (PCBM), dimethyl sulfoxide (DMSO), N, N-dimethyl formamide (DMF), diethyl ether, chlorobenzene and silver (Ag) pellets were purchased from Sigma-Aldrich Corporation. Formamidinium iodide (FAI) was purchased from Xi’an Polymer Light Technology Corporation, as used without further purification. The holetransporting polymer PEDOT: PSS (CLEVIOS P VP AI 4083), i.e. poly (ethylenedioxythiophene) doped with poly(styrenesulfonate), was purchased from Bayer Corporation and used as received. Indium-tin oxide (ITO) coated glass was used as the device substrate, purchased

2.3. Characterization instruments The characteristic curves of current density vs voltage (J-V) of the

Fig. 2. The characteristic curves of (a) photocurrent density vs voltage (J-V) and (b) incident photo-to-current efficiency (IPCE) of the perovskite solar cells with different blend concentration of poly(α-methylstyrene) in the antisolution. 273

Solar Energy 194 (2019) 272–278

W. Zeng, et al.

Fig. 3. Top-view scanning SEM images of the perovskite layers after the washing by antisolution with different blend concentration of poly(α-methylstyrene), (a) Control (i.e. 0 mg/mL), (b) 0.3 mg/mL, (c) 0.5 mg/mL, (d) 1.0 mg/mL, (e) 2.0 mg/mL and (f) 3.0 mg/mL.

3. Results and discussions

perovskite solar cells were tested under the illumination of 1 sun (100 mW/cm2) on a solar cell testing system consisting of a computerprogrammed sourcemeter (Keithley 2400) and a solar simulator (AM 1.5G, Newport), with the light density of 100 mW/cm2 calibrated by a standard Si photodiode. The monochromatic incident photon-to-electron conversion efficiency (IPCE) was collected on a calibrated testing system (Qtest Station 1000AD). The thickness of the organic layers was measured using a calibrated surface profiler (Alfa Step-500, Tencor). Top-view surface morphology was studied using scanning electron microscope (SEM, Hitachi S-4800) and atomic force microscope (AFM, Bruker FastScan), respectively. X-ray diffraction (XRD) patterns were collected on a X-ray diffractometer (D8 Advance, Bruker Co.) using monochromatic Cu Source (λκα1(Cu) = 0.15418 nm) at 5.0 kV. The equilibrium Photoluminescence (PL) spectra were recorded by a steadystate spectrometer (Edinburgh Instruments F900) under the excitation of 520 nm. Time-resolved photoluminescence (TRPL) spectra were collected on a lifetime and steady-state spectrometer (FLS980, Edinburgh Instruments Ltd.) with the laser wavelength of 479 nm. The contact angles were recorded on a contact angle measurement (KRUSS DSA20). All the performance characterization was carried out in the ambient atmosphere without any encapsulation of the devices.

As presented in Fig. 1a, the device architecture of the lead-free FASnI3 perovskite solar cells consists of ITO/PEDOT:PSS (50 nm)/ FASnI3 layer (300 nm)/PCBM (70 nm)/BCP (10 nm)/Ag (100 nm). Except the control devices using pure diethyl ether as the antisolvent, all the modified devices employed the blended antisolution for the precursor layer’s washing, containing the diethyl ether as the matrix solvent and different blend concentration of PAMS as the additive, at the concentration of 0.3 mg/mL, 0.5 mg/mL, 1.0 mg/mL, 2.0 mg/mL and 3.0 mg/mL, respectively, to promote the formation of the perovskite film. The chemical structure of PAMS is indicated in Fig. 1b. The schematic diagram of the energy level for each functional layer relative to the vacuum level is indicated in Fig. 1c. The characteristic curves of current density versus voltage (J-V) and the incident photo-to-current efficiency (IPCE) were demonstrated in Fig. 2a and b, with detailed device parameters summarized in Table 1 for comparison. Obvious changes appeared in the open circuit voltage (VOC) and the short-circuit current density (JSC) as the blend concentration of PAMS varied. Especially when the blended concentration of PAMS reaches 0.5 mg/mL in Device (0.5), the optimum performance was attained, with the VOC of 0.314 V and the JSC of 17.64 mA/cm2. Compared to that of the control device, the JSC of Device (0.5) increased by more than 38% in the relative value, while the VOC increased by 274

Solar Energy 194 (2019) 272–278

W. Zeng, et al.

Fig. 4. Tapping-mode AFM images of the perovskite layers after the washing by antisolution with different blend concentration of poly(α-methylstyrene), (a) Control (i.e. 0 mg/mL), (b) 0.3 mg/mL, (c) 0.5 mg/mL, (d) 1.0 mg/mL, (e) 2.0 mg/mL and (f) 3.0 mg/mL.

more than 17%, respectively. As a result, a maximum PCE of 2.28% was achieved, significantly increased by about 69% compared to the 1.35% of the control device. But it also indicates that excessive addition of PAMS into the diethyl ether can also lead to degradation of the device performance. As indicated in Table 1, when the concentration of PAMS increases beyond 0.5 mg/mL, the value of VOC and JSC decreased reversely, leading to the reduction of PCE. IPCE curves of Fig. 2(b) present similar trends of photo-induced current. At the concentration of 0.5 mg/mL for PAMS in diethyl ether, the device exhibits the optimum IPCE. As indicated in Table 1, the theoretical value JSC' integrated from IPCE curves are in accordance with JSC collected on our solar cell testing system. To investigate the variation in the surface morphology of FASnI3 layer after the washing of PAMS-modified antisolution, top-view electron scanning microscopy (SEM) images were collected as indicated in Fig. 3a–f. Obvious changes can be observed by comparing the SEM images of FASnI3 surface morphology. As for the control device, small pinholes exist at the perovskite layer surface. When small amount of PAMS added into the diethyl ether, i.e. 0.3 mg/mL or 0.5 mg/mL, the small pinholes disappear from the surface. Few pinholes can lessen the leakage current and enhance the fill factor of the devices, which is in accordance with the results listed in Table 1. However excessive

Fig. 5. XRD patterns of the perovskite layers after the washing by antisolution with different blend concentration of poly(α-methylstyrene), (a) Control (i.e. 0 mg/mL), (b) 0.3 mg/mL, (c) 0.5 mg/mL, (d) 1.0 mg/mL, (e) 2.0 mg/mL and (f) 3.0 mg/mL.

275

Solar Energy 194 (2019) 272–278

W. Zeng, et al.

addition of PAMS in the antisolution can result in the aggregation of PAMS on the perovskite surface. Fig. 3d indicates that when the PAMS concentration reaches 1.0 mg/mL, flowerlike aggregates of PAMS can be seen. As the PAMS concentration continues to increase, serious phase separation between the PAMS aggregates and FASnI3 aggregates appear. Therefore, it indicates the addition of PAMS in the antisolution play an important role in the formation of the FASnI3 layer. The surface roughness of the FASnI3 layer is also significantly influenced corresponding to the changes of the surface morphology due to the addition of PAMS in the antisolution. According to the AFM images indicated in Fig. 4a–f, the smoothest FASnI3 layer can be attained as the PAMS concentration is 0.5 mg/mL. The minimum rootmean-square (RMS) roughness comes out to be 17.8 nm on the basis of the AFM images. At higher concentration, surface becomes rougher due to the formation of phase separation, which can result in the degradation of the device performance. It should be noted that the influence of PAMS remains at the outer surface of the FASnI3 layer. As a polymer additive, with the large steric hindrance group of benzene ring, PAMS does not penetrate into the interior of the precursor film during the washing process using PAMS-

Fig. 6. Photoluminescence (PL) spectra of the perovskite layers washed by pure diethyl ether (control) and the PAMS-modified antisolution (0.5 mg/mL).

Fig. 7. Measurement of the solid-water contact angles of the perovskite layers after the washing by antisolution with different blend concentration of poly(αmethylstyrene), (a) Control (i.e. 0 mg/mL), (b) 0.3 mg/mL, (c) 0.5 mg/mL, (d) 1.0 mg/mL, (e) 2.0 mg/mL and (f) 3.0 mg/mL. 276

Solar Energy 194 (2019) 272–278

W. Zeng, et al.

antisolution can suppress the non-radiative trap-assisted recombination of the carriers. The surface of the FASnI3 film becomes more hydrophobic after the washing of the PAMS-modified antisolution, which can enhance the adhesion of PCBM solution on the FASnI3 film. Therefore the PAMS-modified FASnI3/PCBM film exhibits better property of carrier extraction and transportation, as supported by the TRPL measurement. As a result, great enhancement of the device performance has been attained for the PAMS-modified FASnI3 perovskite solar cells. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

Fig. 8. Time-resolved photoluminescence (TRPL) spectra of the FASnI3 layer/ PCBM layer, with the precursor film of FASnI3 layer washed by pure diethyl ether (control) and the PAMS-modified antisolution (0.5 mg/mL).

The authors would like to thank the financial support of Jiangsu Shuangchuang Innovation Group Project for Innovative Team of Jiangsu Province (Grant No. 090300316001), the National Natural Science Foundation of China (Grants Nos. 61874058, 51861145301, and 61376023), the National Key Basic Research Program of China (973 Program, Grant No. 2015CB932203) and the open project of New Energy Technology Engineering Laboratory of Jiangsu Province (No. KF0104). Wenjin Zeng would like to give sincere appreciation and great respect to Prof. Thomas Russell in University of Massachusetts Amherst for Prof. Russell’s rigorous supervision and constructive advice during Zeng’s visiting as a research scholar.

modified antisolution. As indicated in Fig. 5, there is no obvious difference of XRD patterns between the control device and the PAMSmodified devices at different blend concentration of PAMS in the antisolution. The influence of PAMS on the growth of the interior crystal of FASnI3 perovskite layer is negligible. It acts more like the filler, able to fill up the pinholes in the surface of FASnI3 layer and reduce the interfacial defects during the formation process of the perovskite film if the PAMS concentration is suitable. Therefore, the FASnI3 film washed by the PAMS-modified antisolution possesses a higher PL intensity, as demonstrated in Fig. 6, indicating the suppressed non-radiative trapassisted recombination due to the reduction of defects of the FASnI3 film surface (Chen et al., 2018). The existence of PAMS at the surface of the FASnI3 layer can modify the wettability of the perovskite film. The measurement of the solidwater contact angle reveals that the surface of the FASnI3 film becomes more hydrophobic after the washing of the PAMS-modified antisolution. As calculated from the images in Fig. 7a–f, the control device has a contact angle of 51.68°, while the devices washed by the PAMS- modified antisolution have a larger contact angle of 90.04°, 91.35°, 91.48°, 91.74° and 92.14°, corresponding to the PAMS concentration of 0.3 mg/ mL, 0.5 mg/mL, 1.0 mg/mL, 2.0 mg/mL and 3.0 mg/mL, respectively. The contact angle seems not to be directly related to the PAMS concentration in the antisolution. There is no significant change in the contact angle even though the PAMS concentration varied from 0.3 mg/ mL to 3.0 mg/mL as indicated in Fig. 7b–f. Since PCBM were subsequently spin-coated, which acts as the charge acceptor and the electron transporting material, on top of the FASnI3 layer from the non-polar and hydrophobic solvent of chlorobenzene, a more hydrophobic FASnI3 surface is favorable to the adhesion of PCBM solution to the FASnI3 layer, therefore benefits the extraction and transport of the photo-induced carriers. This speculation is supported by the TRPL spectra shown in Fig. 8. The shorter PL lifetime and enhanced PL quenching of the FASnI3/PCBM film with PAMS modified reveals a better charge extraction of the accepting layer in the perovskite devices (Sun et al., 2016).

References Chen, K., Wu, P., Yang, W.Q., Su, R., Luo, D.Y., Yang, X.Y., Tu, Y.G., Zhu, R., Gong, Q.H., 2018. Low-dimensional perovskite interlayer for highly efficient lead-free formamideinum tin iodide perovskite solar cells. Nano Energy 49, 411–418. Du, X.Z., Qiu, R.Z., Zou, T.Y., Chen, X.X., Chen, H.J., Zhou, H., 2019. Enhanced uniformity and stability of Pb-Sn perovskite solar cells via Me4NBr passivation. Adv. Mater. Interfaces 6, 1900413. Eperon, G.E., Leijtens, T., Bush, K.A., Prasanna, R., Green, T., Wang, J.T.W., McMeekin, D.P., Volonakis, G., Milot, R.L., May, R., Palmstrom, A., Slotcavage, D.J., Belisle, R.A., Patel, J.B., Parrott, E.S., Sutton, R.J., Ma, W., Moghadam, F., Conings, B., Babayigit, A., Boyen, H.G., Bent, S., Giustino, F., Herz, L.M., Johnston, M.B., McGehee, M.D., Snaith, H.J., 2016. Perovskite-perovskite tandem photovoltaics with optimized bandgaps. Science 354, 861–865. Jiang, J.X., Wang, Q., Jin, Z.W., Zhang, X.S., Lei, J., Bin, H.J., Zhang, Z.G., Li, Y.F., Liu, S.Z., 2018. Polymer doping for high-efficiency perovskite solar cells with improved moisture stability. Adv. Energy Mater. 8, 1701757. Ke, W.J., Stoumpos, C.C., Spanopoulos, I., Chen, M., Wasielewski, M.R., Kanatzidis, M.G., 2018. Diammonium cations in the FASnI3 perovskite structure lead to lower dark currents and more efficient solar cells. ACS Energy Lett. 3, 1470–1476. Koh, T.M., Krishnamoorthy, T., Yantara, N., Shi, C., Leong, W.L., Boix, P.P., Grimsdale, A.C., Mhaisalkar, S.G., Mathews, N., 2015. Formamidinium tin-based perovskite with low Eg for photovoltaic applications. J. Mater. Chem. A 3, 14996–15000. Kojima, A., Teshima, K., Shirai, Y., Miyasaka, T., 2009. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051. KRICT/MIT, 2019. NRE Laboratory, Best Research-Cell Efficiencies. < https://www.nrel. gov/pv/assets/pdfs/best-research-cell-efficiencies.20190703.pdf > . Kumar, M.H., Dharani, S., Leong, W.L., Boix, P.P., Prabhakar, R.R., Baikie, T., Shi, C., Ding, H., Ramesh, R., Asta, M., Graetzel, M., Mhaisalkar, S.G., Mathews, N., 2014. Lead-free halide perovskite solar cells with high photocurrents realized through vacancy modulation. Adv. Mater. 26, 7122–7127. Lee, S.J., Shin, S.S., Kim, Y.C., Kim, D., Ahn, T.K., Noh, J.H., Seo, J., Seok, S.I., 2016. Fabrication of efficient formamidinium tin iodide perovskite solar cells through SnF2–pyrazine complex. J. Am. Chem. Soc. 138, 3974–3977. Li, F.C., Yuan, J.Y., Ling, X.F., Zhang, Y.N., Yang, Y.G., Cheung, S.H., Ho, C.H.Y., Gao, X.Y., Ma, W.L., 2018. A universal strategy to utilize polymeric semiconductors for perovskite solar cells with enhanced efficiency and longevity. Adv. Funct. Mater. 28, 1706377. Liao, W., Zhao, D., Yu, Y., Grice, C.R., Wang, C., Cimaroli, A.J., Schulz, P., Meng, W., Zhu, K., Xiong, R.G., Yan, Y.F., 2016. Lead-free inverted planar formamidinium tin triiodide perovskite solar cells achieving power conversion efficiencies up to 6.22%. Adv. Mater. 28, 9333–9340. Liu, J.J., Li, N., Jia, J.B., Dong, J., Qiu, Z.W., Iqbal, S., Cao, B.Q., 2018a. Perovskite films grown with green mixed anti-solvent for highly efficient solar cells with enhanced stability. Sol. Energy 181, 285–292. Liu, X.H., Yan, K., Tan, D.W., Liang, X., Zhang, H.M., Huang, W., 2018b. Solvent

4. Conclusions In summary, we have presented an effective way to improve the lead-free FASnI3-based perovskite solar cells by using PAMS as polymer additive in the antisolution. Investigation of the surface morphology and XRD patterns reveals that the washing process by the PAMS-modified antisolution can reduce the surface defects of the FASnI3 film without destroying its bulk crystallinity if the concentration of PAMS is optimized. Therefore the PAMS-modified devices exhibit a stronger PL intensity which indicates the washing process by the PAMS-modified 277

Solar Energy 194 (2019) 272–278

W. Zeng, et al.

infrared photoluminescent properties. Inorg. Chem. 52, 9019–9038. Sun, C., Wu, Z.H., Yip, H.L., Zhang, H., Jiang, X.F., Xue, Q.F., Hu, Z.C., Hu, Z.H., Shen, Y., Wang, M.K., Huang, F., Cao, Y., 2016. Amino-functionalized conjugated polymer as an efficient electron transport layer for high-performance planar-heterojunction perovskite solar cells. Adv. Energy Mater. 6, 1501534. Yang, W.S., Noh, J.H., Jeon, N.J., Kim, Y.C., Ryu, S.C., Seo, J.W., Sang, I.S., 2015. Highperformance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 348, 1234–1237. Zhou, H.P., Chen, Q., Li, G., Luo, S., Song, T.B., Duan, H.S., Hong, Z.R., You, J.B., Liu, Y.S., Yang, Y., 2014. Interface engineering of highly efficient perovskite solar cells. Science 345, 542–546. Zhu, K.P., Cong, S., Lu, Z., Lou, Y.H., He, L., Li, J.M., Ding, J.N., Yuang, N.Y., Rümmeli, M.H., Zou, G.F., 2019. Enhanced perovskite solar cell performance via defect passivation with ethylamine alcohol chlorides additive. J. Power Sour. 428, 82–87.

engineering improves efficiency of lead-free tin-based hybrid perovskite solar cells beyond 9%. ACS Energy Lett. 3, 2701–2707. Nguyen, B.P., Shin, D.G., Jung, H.R., Kim, J.R., Nguyen, T.T.T., Yoon, S., Yi, Y., Jo, W., 2019. Phase formation and local charge transport of lead-free CH3NH3Sn(I1-xBrx)3 (0≤x≤1) perovskite solar cells fabricated by solvent optimization. Sol. Energy 186, 136–144. Osa, M., Sumida, M., Yoshizaki, T., Yamakawa, H., Ute, K., Kitayama, T., Hatada, K., 1999. Determination of stereochemical compositions of oligo- and poly(α-methylstyrene)s by 1H and 13C NMR. Polym. J. 32, 361–369. Saliba, M., Matsui, T., Seo, J.Y., Domanski, K., Correa-Baena, J.P., Nazeeruddin, M.K., Zakeeruddin, S.M., Tress, W., Abate, A., Hagfeldt, A., Grätzel, M., 2016. Cesiumcontaining triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy Environ. Sci. 9, 1989–1997. Stoumpos, C.C., Malliakas, C.D., Kanatzidis, M.G., 2013. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-

278