- Email: [email protected]
γ-MPTS-SAM modified meso-TiO2 surface to enhance performance in perovskite solar cell
Materials Science in Semiconductor Processing 97 (2019) 21–28
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
γ-MPTS-SAM modified meso-TiO2 surface to enhance performance in perovskite solar cell ⁎
T
⁎
Honglin Lu, Jia Zhuang , Zhu Ma , Yaping Deng, Qintao Wang, Zhongli Guo, Shuangshuang Zhao, Haimin Li The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Self-assembled monolayer Interface modification γ-MPTS Perovskite solar cell
In this work, a self-assembled monolayer (SAM) of γ-Mercaptopropyltrimethoxysilane (γ-MPTS) based a simple and easy-proceeding soaking method is prepared on the interface of the mesoporous TiO2 (meso-TiO2)/perovskite. The meso-TiO2 surface of perovskite solar cells modified with the γ-MPTS-SAM method were characterize by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), contact angle and photoluminescence, it was found that the introduced γ-MPTS can reduce carrier recombination and optimize MAPbI3 crystal growth, which could significantly increase the PCE performance of the cell. The champion PCE is boosted to 16.50% compared with that of the unmodified device (14.36%). Finally, this interface modification method can be adopted for the application of perovskite solar cells in the foreseeable future.
1. Introduction Organic-inorganic halide perovskite solar cells (PSCs) have showed great promise for use in photovoltaic field due to their rapidly improved power conversion efficiency (PCE) from 3.8% to 23.7% in just a few years [1–5]. A classic perovskite solar cell with or without mesoporous scaffold is embedded between the electron transporting layer (ETL) and hole transporting layer (HTL) [6–8]. In general, through the absorption of incident photons, carriers are produced in the perovskite absorber layer that transfers through a transport path including the ETL or HTL, the metal electrodes, and between each interface [9,10]. Therefore, it is critical to improve carriers transport at each interface for preparing high efficiency solar cells. However, PSCs with unsatisfied interface performance probably lead to the decreased cell PCE due to the existence of trap states and defects on the interface [11–13]. Interface modification is considered as an efficient method for enhancing the cell PCE by obviously improving electron transport and reducing the electron recombination, in addition, improving the surface properties for ETL is also important to obtaining high-class film quality for MAPbI3 layer [14–17]. Many efforts have been made about the interface engineering, which majorly focus on the interface of electron/perovskite transport layer [18–21]. For instance, Li et al. reported an efficient approach for enhancing the interfacial properties by using a SAM of 4aminobenzoic acid between the TiO2 layer and the perovskite absorber
⁎
film, which obtained high PCE cell due to fewer traps state density, better electron transfer and film quality of perovskite layer [22]. Zheng et al. reported a simple method that modifying by aminocaproic acid and caproic acid between meso-TiO2 and perovskite layer obtained significant PCE improvement, which could contribute to the accelerated electron extraction and transfer by interface engineering [23]. Additionally, Guojia Fang's group used a 3-aminopropyltriethoxysilane as self-assembled monolayer (SAM) to improve interface between the SnO2 ESL and perovskite film, the result improve cell PCE due to the obtained high quality perovskite layer, the decreased work function of SnO2, the passivated trap states [24]. Therefore, interface modification as a convenient and effective method was implemented to optimize the meso-TiO2/MAPbI3 interface for improved device performance, reduced hysteresis for PSCs. In this study, a self-assembled monolayer (SAM) of γMercaptopropyltrimethoxysilane (γ-MPTS) is prepared on the interface of the meso-TiO2/perovskite layer. By optimizing the concentration of SAM (γ-MPTS) prepared on the meso-TiO2 layer surface, we achieve 16.50% cell PCE versus unmodified-device (14.36%). Through rich characterization means based γ-MPTS-SAM modified and unmodified perovskite solar cells with X-ray diffraction (XRD) scanning electron microscope (SEM), Atomic force microscope (AFM), contact angle and photoluminescence, which show that the introduced SAM can reduce carrier recombination and further optimize MAPbI3 film quality.
Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Zhuang), [email protected], [email protected] (Z. Ma).
https://doi.org/10.1016/j.mssp.2019.02.018 Received 22 October 2018; Received in revised form 12 February 2019; Accepted 15 February 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 1. (a) flow-process diagram of the deposition procedure of SAM based meso-TiO2 substrates. (b) and (c) SEM pictures of FTO/c-TiO2/meso-TiO2 and FTO/cTiO2/meso-TiO2/SAM, (d) FTIR spectra based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2, EDX elemental pictures of (e) FTO/c-TiO2/meso-TiO2, (f) FTO/cTiO2/ meso-TiO2/SAM.
reactants, and finally dried with a nitrogen gun, and then annealed at 500 ℃ for 30 min in a muffle furnace. The as-prepared meso-TiO2 devices were immersed in a 5, 10, 15 mM solution of γ-MPTS in isopropanol for three hours, respectively, thus self-assembling into a monomolecular membrane at the meso-TiO2 surface. After that, the substrates were rinsed by isopropanol and then dried under a flow of N2. The perovskite layer was spin-coating on the FTO substrate/c-TiO2/ meso-TiO2 at 3000 rpm for 55 s. CB (80 µL) was dropped onto the substrate during the spin-coating step at 10 s before begin of the procedure, and then the film was heated at 100 °C for 20 min. An HTL film was prepared by spin-coating spiro-OMeTAD solution on MAPbI3 film at 3000 rpm for 30 s. Finally, Ag electrode with a thickness of ≈100 nm was deposited onto the spiro-OMeTAD coated film by sputtering technique to finish the whole device fabrication process.
2. Experimental section 2.1. Materials and reagents Titanium isopropoxide (99.999%), chlorobenzene (99.9%), N,N-dimethylformamide (DMF, 99.9%), and acetonitrile (99.9%) were purchased from Sigma-Aldrich. γ-Mercaptopropyltrimethoxysilane (99.9%) was achieved from Aldrich. 2,2′,7,7′-Tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiroOMeTAD, 99.5%), CH3NH3I (99.5%), PbI2 (99.99%), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI; 99.9%) and 4-tertbutylpyridine (tBP; 96%) were obtained from Xi’an Polymer Light Technology Company, Fluorine-doped tin oxide (FTO) coated glass substrates (15 Ω/sq) were bought from OPV Tech New Energy Co. tetrabutyl titanate (98.3%) and hydrochloric acid were obtained from Kelong Company. All the materials and reagents were used without further purification.
2.3. Characterization The current-voltage characteristics of perovskite solar cells were recorded using an electrochemical workstation (CHI660D, Chenhua) under AM 1.5 simulated illumination (CEL-S500, Beijing, China). Contact angle were recorded using a contact angle measuring instrument (OCA25, GmbH). Electrochemical impedance spectroscopy (EIS) was measured by electrochemical workstation (CHI660D, Chenhua), with amplitude of 7.5 mV and a measurement frequency from 1 to 100 kHz. Mott-Schottky plots were measured by using an electrochemical workstation (CHI660D, Chenhua) with a standard threeelectrode configuration, which the Ag/AgCl as reference electrode in saturated Na2SO4 and Pt sheet as counter electrode in deionized water. FTIR spectra were measured by Fourier transform infrared spectroscopy (Nicolet 6700). The morphology and composition of meso-TiO2 film was investigated using a scanning electron microscopy (SEM, ZEISS EV0MA15) equipped with energy dispersive X-ray spectroscopy (EDX) detector. X-ray diffraction (XRD, DX-2700, Dandong) measurements were carried out from 22 to 58 with Cu Ka radiation (λ = 0.154 06 nm) at a scanning rate of 4 deg/min. Atomic force microscope (AFM) measurements were measured by an Atomic force microscope spectrometer (Agilent7500). The UV–vis absorption spectra were measured by
2.2. Solar cell fabrication FTO substrates were ultrasonically cleaned with abstergent, acetone, deionized water, and ethanol for 15 min, respectively. The conductive substrates were dried with a nitrogen gun and post-treated with UV-ozone for 15 min. The compact TiO2 (c-TiO2) layer was coating on cleaned FTO glass by spin-coating the titanium precursor solution at 2000 rpm for 30 s. The as-prepared FTO glass was post annealed at 150 ℃ for 15 min, and annealed at 500 ℃ in a muffle furnace for 30 min. Then we started to prepare meso-TiO2 precursor solution, 15 ml hydrochloric acid was mixed with 15 ml deionized water in the situation of magnetic stirring for 10 min 0.75 ml Tetra-n-butyl Titivate was added to the mentioned mixed solution, and then the solution need to vigorously stir for 30 min to obtain a clarified solution. Afterwards, the clear solution was transferred into the reaction kettle equipped with cTiO2/FTO substrates. The sealed reaction kettle was placed in a laboratory oven at 170 ℃ for 1 h. After the autoclave was cooled to room temperature naturally and the substrates were taken out, rinsed with deionized water and ethanol for two times to get rid of the residual 22
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 1. (continued)
Fig. 1. (continued)
Fig. 2. (a) AFM of meso-TiO2 and (b) meso-TiO2/SAM, water contact angles of meso-TiO2 (c), and (d) meso-TiO2/SAM.
TiO2. As shown in Fig. 1(d), the existence of -CH2, Si-O-Si, –SH peaks confirmed the γ-MPTS modification onto the m-TiO2 surface compared with unmodified TiO2. The elemental mapping measured by EDX (Fig. 1(e), (f)) was also used to obtain the chemical element composition of meso-TiO2 and meso-TiO2/SAM [25], the tin elements and most silicon elements originate from the FTO substrate and titanium element come from the compact TiO2 and meso-TiO2. The existence of sulfur elements, carbon elements demonstrates that the γ-MPTS has successfully been modified onto meso-TiO2 by soaking self-assembling method. Therefore, it is very likely to improve PSCs performance. Fig. S1 demonstrates the XRD patterns for meso-TiO2 and meso-TiO2/SAM, which did not discover obvious peak changes that show meso-TiO2 crystal structure was formed and the SAM modification did not influence the crystal structure of ETLs. The atomic force microscopic (AFM) picture of the meso-TiO2 surface based SAM modification showed a smoother surface with a roommean-square (rms) surface roughness of 15.9 nm compared to 17.6 nm for meso-TiO2 without γ-MPTS-SAM modification (Fig. 2(a) and (b)). The smoother surface can provide meso-TiO2 ESL better interface contact with MAPbI3 film [26], the results lead to an improved VOC and an increased fill factor (FF), and also demonstrate that γ-MPTS-SAM
UV–vis spectrometer (UV-2600). In the time-resolved photoluminescence spectra (FLS 980 Edinburgh Instruments), the measurement was conducted at 760 nm with a pulsed diode laser at 510 nm (with an intensity of 0.12 mW cm−2) in a pulse frequency of 1 MHz. The incident photo-current conversion efficiency (IPCE) spectra were recorded using an IPCE system (PVE 300, Bentham, Inc.) as a function of wavelength from 300 to 800 nm. By using a mask, the active area of perovskite solar cells was controlled to 0.16 cm2. 3. Results and discussion Fig. 1(a) shows the deposition procedure of SAM based meso-TiO2 substrates, compact TiO2 was fabricated by spin coating, meso-TiO2 was prepared by hydrothermal approach and SAM was manufactured by soaking method in γ-MPTS solution. The surface SEM images of mesoTiO2 and meso-TiO2/SAM are illustrated in Fig. 1(b) and (c), respectively. γ-MPTS-SAM is used as an interfacial modification layer on the surface of meso-TiO2 ETL, there was no visible morphological changes compared to pure meso-TiO2. The reason was attributed to too thin SAM modification film to be seen. The FTIR (Fig. 1(d)) spectra are used to measure the chemical absorption of γ-MPTS on the surface of meso23
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 3. (a) Top-view SEM images of CH3NH3PbI3 film based on meso-TiO2 and (b) γ-MPTS-SAM modified meso-TiO2 ETLs, (c) XRD patterns of CH3NH3PbI3 films (d) UV–vis absorption spectra of CH3NH3PbI3 based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2 ETLs.
without γ-MPTS-SAM-modified meso-TiO2, which further increase the visible light absorptive characteristic for CH3NH3PbI3 layer. The result could be attributed to achieve better perovskite film quality by γ-MPTSSAM modification onto meso-TiO2 surface. The device model diagram of mesoporous perovskite solar cell based TiO2 as ETL is shown in Fig. 4(a), in which γ-MPTS-SAM is used as a thin γ-MPTS-SAM interface layer to modify the surface of mesoTiO2. As shown in Fig. 4(b), the trimethoxy silane was chemically bonded to the meso-TiO2 surface through hydrogen bond between the hydrolytic alkoxy group and the hydroxyl groups (on the surface of meso-TiO2) [27,28]. For terminal functional groups, firstly, the sulfhydryl group with lone pair electrons in the γ-MPTS could provide passivation effect by donating lone pair electrons to the Pb2+, and form a coordinate bond with the edge of MAPbI3 layer [29,30]. Secondly, this sulfhydryl functional group can form hydrogen bonds with iodine ions (S-H…I) [31,32]. Surface defects for MAPbI3 layer are mainly caused by halogen vacancies [33,34]. By fixing the hydrogen bond (SH…I), the vacancy of iodine ion is reduced, thus the surface defect state for MAPbI3 layer is reduced. This result will reduce carrier recombination and further increase cells PCE [35,36]. In addition, γMPTS-SAM can induce the formation of dipoles moment to enhance the force for the carrier separation and carrier transport [37], which be propitious to increase cell PCE. Fig. 4(c) shows that the reverse and
was successfully modified onto the meso-TiO2 surface. The picture of water contact angles are shown in Fig. 2(c) and Fig. 2(d). After γ-MPTSSAM modification, the contact angle on meso-TiO2 is 28.7 °compared to 37.3° without γ-MPTS-SAM modification. The result shows that mesoTiO2 with γ-MPTS-SAM modification become more hydrophilic, which due to the enhanced connectivity induced by the hydrogen-bonding between the hydrolytic alkoxy group from γ-MPTS-SAM and the hydroxyl groups from meso-TiO2 surface [27]. SEM imagines (Fig. 3(a) and (b)) are carried to study the surface morphologies of the perovskite films with or without γ-MPTS-SAM as interface modification layer. After γ-MPTS-SAM interface modification, the quality of MAPbI3 film become more compact, which in favor the decreased leakage current that lead to higher performance for cell. The positive phenomenon could be ascribed to the improved connectivity between MAPbI3 layer and SAM-modified meso-TiO2 ESL caused by hydrogen bonds formed from sulfhydryl group (in γ-MPTS) with the iodide anions of the MAPbI3 film. Fig. 3(c) shows the XRD patterns MAPbI3 film fabricated on meso-TiO2 and meso-TiO2/γ-MPTS-SAM, which show that perovskite structure was formed and the modification of γ-MPTS-SAM did not influence the crystal structure of MAPbI3 film. The UV–vis absorption spectra (Fig. 3(d)) demonstrates that the CH3NH3PbI3 film based SAM-modified meso-TiO2 ETLs has higher absorption peak within the 400–500 nm than the CH3NH3PbI3 film based 24
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 4. (a) device model diagram, (b) theoretical model of γ-MPTS-SAM modified meso-TiO2, (c) J-V curves, (d) dark current (e) histogram of PCE, (f) IPCE.
produces JSC of 22.43 mA cm−2, VOC of 1.008 V, and FF of 63.50%, exhibiting a PCE of 14.36% under reverse scanning (a PCE of 8.65% under forward scanning), whereas the γ-MPTS-SAMs modification PSCs has a PCE of 16.50% (VOC = 1.028 V, JSC = 23.09 mAcm−2, FF = 69.50%) under reverse scanning and a PCE of 13.17% under forward scanning. The decreased hysteresis index for cell based γ-MPTS-SAM modification meso-TiO2 (0.20) as ETLs is demonstrated (0.38 hysteresis index for without modified device), which can be attributed to the improved perovskite morphology and decreased trap states. By Table S1, we can show that the value of JSC, VOC, and FF of the γ-MPTS-SAM modification are increased when the γ-MPTS-SAM concentration is 5 mmol and 10 mmol compared to the unmodification meso-TiO2. When the modification concentration is 15 mmol, the values of JSC, Voc and FF for cell dropped rapidly. γ-MPTS-SAM has an insulating alkyl chain, too high concentration can cause poor electrical conductivity,
Table 1 Photovoltaic performance of PSCs based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2. ETLs
VOC (V)
JSC (mA/ cm2)
FF (%)
PCE (%)
Hysteresis Index
TiO2 reverse TiO2 forward TiO2 /SAM reverse TiO2 /SAM forward
1.008 0.912 1.028
22.43 22.12 23.09
63.50 43.53 69.50
14.36 8.65 16.50
0.39
0.994
22.39
60.10
13.17
0.20
forward J-V curves current-voltage (J-V) characteristics of the solar cell with and without γ-MPTS-SAM modification, the corresponding results summary is shown in the Table 1. The unmodified meso-TiO2 cell 25
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 5. (a) UV–vis absorption spectra of meso-TiO2 and γ-MPTS-SAMs meso-TiO2 ETLs, (b) indirect band gap model, (c) Mott-Schottky curve of TiO2 and TiO2/SAM, (d)The energy band diagram of cell, (e) PL and (f) TRPL decay curves.
leading to the decreased PCE for cell. The 15 mM concentration of γMPTS-SAM with insulating alkyl chains provides worse spatial separation of carriers between perovskite layer and TiO2 layer, increase the carrier recombination, and further decrease cells efficiency [38,39]. The result can be ascribed to the fact that too many γ-MPTS cause poor electrical conductivity, which further lead to decreased cell PCE. The statistical distribution of the PCE for unmodified and 10 mmol modification γ-MPTS-SAM cells is shown in Fig. 4(e), which the PCE of cell based γ-MPTS-SAM modification meso-TiO2 concentrate in 15%, 16%
compared to 12%, 13% for unmodified device. Error bar (Fig. S2) show that SAM modified meso-TiO2 cell improve VOC, JSC, and FF than unmodified cell, which further demonstrates that SAMs modification is effective method for improving cell PCE. Fig. 4(d) shows the dark current of perovskite solar cells with or without γ-MPTS-SAM modification. The value of dark current reflects the situation for reverse flow of carrier. As shown in Fig. 4(d), the decreased dark current of cell with SAMs modification show the suppressed reverse movement for the carrier, leading to the improved carrier transport from perovskite film 26
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
gap can be attributed to the formation of dipoles moment induced by γMPTS-SAM, which improve the Voc for perovskite solar cells. MottSchottky curve (Fig. 5(c)) was explored to show the changes for conduction band energy [19]. The value of flat band potential (Efb) corresponds to the intersection of the linear part of the curve with the Xaxis. An Efb shift represents the change of relative position of the conduction band. Fig. 5(c) shows that Efb is negatively shifted from −0.584 V to −0.602 V through γ-MPTS-SAM modification, which leads to a higher VOC. Fig. 5(d) shows the energy band diagram of cell. The position of conduction band of Pure TiO2 is obtained from references [4,19]. The analysis of Mott-Schottky curve shows that the conduction band position of γ-MPTS modified TiO2 increases from −4.2 eV to −4.182 eV. The decreased energy barrier between the perovskite layer and TiO2 layer will facilitate carriers transportation, which is also beneficial for achieving an improved JSC. The PL (Fig. 5(e)) and TRPL (Fig. 5(f)) measurements were used to assess the ability of efficient electrons extraction from perovskite layer to meso-TiO2 layer. As shown in Fig. 5(e), the steady-state PL spectra of PSCs based the structure FTO/c-TiO2/meso-TiO2/perovskite film and FTO/c-TiO2/γ-MPTS-SAM modified meso-TiO2/perovskite film, respectively. The MAPbI3 film deposited on γ-MPTS-SAM modified meso-TiO2 shows more evident PL quenching than the pristine meso-TiO2 films, which further demonstrate the improved carrier extraction ability. Moreover, the TRPL result shown in Fig. 5(f) that confirms faster photo-induced carrier transfer from MAPbI3 film to meso-TiO2 after γ-MPTS-SAM modification. The result could be attributed to the decreased trap states originated from hydrogen-bonding interactions (S-H…I). To further investigate the charge transport and recombination properties of cells, electrochemical impedance spectroscopy (EIS) measurement was measured at 0.7 V bias under dark condition (Fig. 6). The inset model in Fig. 6 is the equivalent circuit of devices and Table 2 shows the fitted EIS data. After γ-MPTS-SAMs modification, the cells exhibit smaller contact resistance (Rs) and larger charge recombination resistance (Rct) than the unmodified cells, meaning better chargetransfer ability and less carrier recombination [41]. Besides the enhanced PCE, the stability of cells based γ-MPTS-SAM modification was also improved. The devices are unsealed and are put in ambient environment under the humidity level of 40 ± 10%. The PCEs of cells based meso-TiO2 and γ-MPTS-SAM modification mesoTiO2 were tracked for 180 h (Fig. 7). The γ-MPTS modified cells are more stable than the unmodified cells. The γ-MPTS modified one retained ≈ 80% of its original efficiency, whereas the unmodified cell only had ≈64% of its original efficiency. The enhanced stability is attributed to the improved perovskite film quality by γ-MPTS-SAM modification, which can retard the decomposition of perovskite films.
Fig. 6. EIS of the PSCs based on meso-TiO2 and γ-MPTS-SAM modified mesoTiO2 at 0.7 V bias under dark condition. Table 2 The fitted EIS data of the PSCs based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2. Sample
Contact resistance (RS/ ohm)
recombination resistance (Rct/ ohm)
TiO2 TiO2/SAM
21.48 12.01
6460 11,080
4. Conclusion Fig. 7. Normalized PCE of cells based meso-TiO2 and γ-MPTS-SAM modification meso-TiO2 with the change of time, the cells are unsealed and are put in an ambient environment under the humidity level of 40 ± 10%.
In summary, we have successfully prepared a self-assembled monolayer γ-MPTS on the interface of the meso-TiO2/perovskite film, which lead to a result of the reduced trap states and enhanced interface carrier transport performance. The process achieved the highest PCE (16.50%) with an average efficiency of 15.17% for mesoporous geometry. This strategy based a simple and low-cost method should provide insight into commercialization of PSCs in the near future.
to ETL. Incident photo-to-electron conversion efficiency (IPCE) of these mesoporous perovskite solar cells based on unmodified TiO2 and γMPTS-SAM modified meso-TiO2 layers exhibit wavelength onset at 780 nm that keep in reasonable agreement with the bandgap of the CH3NH3PbI3 (Fig. 4(e)). The results that SAM modified meso-TiO2 cell exhibited higher IPCE value compared the control cell, which is in good accordance with the current density trend. Fig. 5(a) shows the UV–vis absorption spectra of the unmodified and γ-MPTS-SAM modified meso-TiO2 cells. The absorption spectrum of the γ-MPTS-SAM modified meso-TiO2 shows a blue shift about band gap (small image in Fig. 5(a)). The transformed Kubelka–Munk spectrum (Fig. 5(b)) of the unmodified and γ-MPTS-SAM modified meso-TiO2 is tested to decide the optical band gap [40]. The decreased optical band
Acknowledgements The authors gratefully acknowledge the financial support from Sichuan Province Science and Technology Support Program (Grant no. 2018JY0015), Young Scholars’ Development Fund of SWPU (Grant no. 201699010017) and scientific research starting project of SWPU (Grant no. 2017QHZ021).
27
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Appendix A. Supporting information [21]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2019.02.018.
[22]
References [23] [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] H.S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, S.J. Moon, R. Humphry-Baker, J.H. Yum, J.E. Moser, M. Gratzel, N.G. Park, Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Rep. 2 (2012) 591. [3] H. Zhou, Q. Chen, G. Li, S. Luo, T.B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345 (2014) 542–546. [4] N.J. Jeon, H. Na, E.H. Jung, T.Y. Yang, Y.G. Lee, G. Kim, H.W. Shin, Seok Sang II., J. Lee, J. Seo, A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells, Nat. Energy 3 (2018) 682–689. [5] 〈https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20181214.pdf〉. [6] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [7] I.C. Smith, E.T. Hoke, D.S. Ibarra, M.D. Mcgehee, H.I. Karunadasa, A layered hybrid perovskite solar-cell absorber with enhanced moisture stability, Angew. Chem. Int. Ed. 53 (2014) 11232–11235. [8] S.S. Mali, J.V. Patil, H. Kim, C.K. Hong, Synthesis of SnO2 nanofibers and nanobelts electron transporting layer for efficient perovskite solar cells, Nanoscale 10 (2018) 8275–8284. [9] L. Zhu, Z. Shao, J. Ye, X. Zhang, X. Pan, S. Dai, Mesoporous BaSnO3 layer based perovskite solar cells, Chem. Commun. 52 (2015) 970–973. [10] X. Ling, J. Yuan, D. Liu, Y. Wang, Y. Zhang, S. Chen, H. Wu, F. Jin, F. Wu, G. Shi, X. Tang, J. Zheng, S.F. Liu, Z. Liu, W. Ma, Room-temperature processed Nb2O5 as the electron transporting layer for efficient planar perovskite solar cells, Acs. Appl. Mater. Interfaces 9 (2017) 23181–23188. [11] S. Zhang, M. Stolterfoht, A. Armin, Q. Lin, F. Zu, Interface engineering of solution processed hybrid organohalide perovskite solar cells, ACS Appl. Mater. Interfaces 10 (2018) 21681–21687. [12] M. Shahiduzzaman, M. Karakawa, K. Yamamoto, T. Kusumi, K. Yonezawa, Interface engineering of compact-TiOx in planar perovskite solar cells using low-temperature processable high-mobility fullerene derivative, Sol. Energy Mater. Sol. Cells 178 (2018) 1–7. [13] Z.X. Juan, L. Tao, H. Li, W. Huang, P. Sun, Efficient planar perovskite solar cells with improved fill factor via interface engineering with graphene, Nano Lett. 18 (2018) 2442–2449. [14] C. Ran, J. Xi, W. Gao, F. Yuan, T. Lei, Bilateral interface engineering toward efficient 2D–3D bulk heterojunction tin halide lead-free perovskite solar cells, ACS Energy Lett. 3 (2018) 713–721. [15] Y. Hou, J. Yang, Q. Jiang, W. Li, Z. Zhou, X. Li, S. Zhou, Enhancement of photovoltaic performance of perovskite solar cells by modification of the interface between the perovskite and mesoporous TiO2 film, Sol. Energy Mater. Sol. Cells 155 (2016) 101–107. [16] N. Kumari, J.V. Gohel, S.R. Patel, Optimization of TiO2 /ZnO bilayer electron transport layer to enhance efficiency of perovskite solar cell, Mater. Sci. Semicond. Proc. 75 (2018) 149–156. [17] W. Li, W. Zhang, S.V. Reenen, R.J. Sutton, J. Fan, A.A. Haghighirad, M.B. Johnston, L. Wang, H.J. Snaith, Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification, Energy Environ. Sci. 9 (2016) 490–498. [18] S.F. Shaikh, H.C. Kwon, W. Yang, H. Hwang, H. Lee, La2O3 interface modification of mesoporous TiO2 nanostructures enabling highly efficient perovskite solar cells, J. Mater. Chem. A 4 (2016) 15478–15485. [19] Y. Xiang, Z. Ma, J. Zhuang, H. Lu, C. Jia, H. Li, Enhanced performance for planar perovskite solar cells with samarium-doped TiO2 compact electron transport layers, J. Phys. Chem. C. 121 (2017) 20150–20157. [20] Y. Ma, J. Lee, Y. Liu, P.M. Hangoma, W.I. Park, Synchronized-pressing fabrication
[24]
[25] [26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
28
of cost-efficient crystalline perovskite solar cells via intermediate engineering, Nanoscale 10 (2018) 9628–9633. C. Gao, H. Dong, X. Bao, Y. Zhang, A. Saparbaev, Additive engineering to improve efficiency and stability of inverted planar perovskite solar cells, J. Mater. Chem. C. 6 (2018) 8234–8241. B. Li, Y. Chen, Z. Liang, D. Gao, W. Huang, Interfacial engineering by using selfassembled monolayer in mesoporous perovskite solar cell, Rsc. Adv. 5 (2015) 94290–94295. R. Chen, J. Cao, Y. Wu, X. Jing, B. Wu, N. Zheng, Improving efficiency and stability of perovskite solar cells by modifying mesoporous TiO2-Perovskite interfaces with both aminocaproic and caproic acids, Adv. Mater. Inter. 4 (2017) 1700897. G. Yang, C. Wang, H. Lei, X. Zheng, P. Qin, L. Xiong, X. Zhao, Y. Yan, Interface engineering in planar perovskite solar cells: energy level alignment, perovskite morphology control and high performance achievement, J. Mater. Chem. A. 5 (2016) 1658–1666. Y. Li, Y. Guo, Y. Li, X. Zhou, Fabrication of Cd-doped TiO2 nanorod arrays and photovoltaic property in perovskite solar cell, Electrochim. A. 10 (2016) 29–36. Y. Yan, F. Cai, L. Yang, J. Li, Y. Zhang, F. Qin, C. Xiong, Y. Zhou, D.G. Lidzey, T. Wang, Light-Soaking-Free inverted polymer solar cells with an effciency of 10.5% by compositional and surface modifcations to a low-temperature-processed TiO2 electron-transport layer, Adv. Mater. 29 (2017) 1604044. K.A. Luck, T.A. Shastry, S. Loser, G. Ogien, T.J. Marks, Improved uniformity in highperformance organic photovoltaics enabled by (3-aminopropyl) triethoxysilane cathode functionalization, Phys. Chem. Chem. Phys. 15 (2013) 20966–20972. R. Cisneros, M. Beley, F. Lapicque, P.C. Gros, Hydrophilic ethylene‐glycol‐based ruthenium sensitizers for aqueous dye‐Sensitized solar cells, Eur. J. Inorgan. Chem. 1 (2016) 33–39. L. Zuo, Q. Chen, N.D. Marco, Y.T. Hsieh, H. Chen, P. Sun, S.Y. Chang, H. Zhao, S. Dong, Y. Yang, Tailoring the interfacial chemical interaction for high-efficiency perovskite solar cells, Nano. Lett. 17 (2017) 269–275. Y.C. Shih, L.Y. Wang, H.C. Hsieh, K.F. Lin, Enhancing photocurrent of perovskite solarcells via modification of TiO2/CH3NH3PbI3 heterojunction interface with amino acid, J. Mater. Chem. A. 00 (2012) 1–3. Q. Xue, Z. Hu, J. Liu, J. Lin, C. Sun, Z. Chen, C. Duan, J. Wang, C. Liao, W.M. Lau, F. Huang, H.L. Yip, Y. Cao, Highly efficient fullerene/perovskite planar heterojunction solar cells via cathode modification with an amino-functionalized polymer interlayer, J. Mater. Chem. A. 2 (2014) 19598–19603. C. Sun, Z. Wu, H.L. Yip, H. Zhang, X.F. Jiang, Q. Xue, Z. Hu, Z. Hu, Y. Shen, M. Wang, F. Huang, Y. Cao, Amino-Functionalized conjugated polymer as an efficient electron transport layer for high-performance planar-heterojunction perovskite solar cells, Adv. Energy Mater. 6 (2016) 1501534. N.K. Noel, A. Abate, S.D. Stranks, E.S. Parrott, V.M. Burlakov, A. Goriely, H.J. Snaith, Enhanced photoluminescence and solar cell performance via Lewis base passivation of organic-inorganic lead halide perovskites, ACS Nano 8 (2014) 9815–9821. F. Gao, Z. Li, J. Wang, A. Rao, I.A. Howard, A. Abrusci, S. Massip, C.R. McNeill, N.C. Greenham, Trap-induced losses in hybrid photovoltaics, ACS Nano 8 (2014) 3213–3221. Y. Hou, C. Quiroz, S. Scheiner, W. Chen, T. Stubhan, Low‐Temperature and hysteresis‐free electron‐transporting layers for efficient, regular, and planar structure perovskite solar cells, Adv. Energy Mater. 5 (2015) 1501056. L. Liu, A. Mei, T. Liu, P. Jiang, Y. Sheng, Fully printable mesoscopic perovskite solar cells with organic silane self-assembled monolayer, J. Am. Chem. Soc. 137 (2015) 1790–1793. X. Bulliard, S.G. Ihn, S. Yun, Y. Kim, D. Choi, Enhanced performance in polymer solar cells by surface energy control, Adv. Func. Mater. 20 (2010) 4381–4387. J. Zhang, Z. Hu, L. Huang, G. Yue, J. Liu, X. Lu, Z. Hu, M. Shang, L. Han, Y. Zhu, Bifunctional alkyl chain barriers for efficient perovskite solar cells, Chem. Comm. 32 (2015) 7047–7050. M.M. Byranvand, T. Kim, S. Song, G. Kang, S.U. Ryu, T. Park, p-Type CuI Islands on TiO2 electron transport layer for a highly efficient planar-perovskite solar cell with negligible hysteresis, Adv. Energy Mater. 8 (2017) 1702235. H. Zhang, Y. Lv, Y. Guo, X. Tao, C. Yang, Fully-air processed Al-doped TiO2 nanorods perovskite solar cell using commercial available carbon instead of hole transport materials and noble metal electrode, J. Mater. Sci. Mater. Electron. 29 (2018) 3759–3766. J. Li, T. Jiu, B. Li, C. Kuang, Q. Chen, S. Ma, J. Shu, J. Fang, Inverted polymer solar cells with enhanced fill factor by inserting the potassium stearate interfacial modification layer, Appl. Phys. Lett. 108 (2016) 4636–4643.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
γ-MPTS-SAM modified meso-TiO2 surface to enhance performance in perovskite solar cell ⁎
T
⁎
Honglin Lu, Jia Zhuang , Zhu Ma , Yaping Deng, Qintao Wang, Zhongli Guo, Shuangshuang Zhao, Haimin Li The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Self-assembled monolayer Interface modification γ-MPTS Perovskite solar cell
In this work, a self-assembled monolayer (SAM) of γ-Mercaptopropyltrimethoxysilane (γ-MPTS) based a simple and easy-proceeding soaking method is prepared on the interface of the mesoporous TiO2 (meso-TiO2)/perovskite. The meso-TiO2 surface of perovskite solar cells modified with the γ-MPTS-SAM method were characterize by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), contact angle and photoluminescence, it was found that the introduced γ-MPTS can reduce carrier recombination and optimize MAPbI3 crystal growth, which could significantly increase the PCE performance of the cell. The champion PCE is boosted to 16.50% compared with that of the unmodified device (14.36%). Finally, this interface modification method can be adopted for the application of perovskite solar cells in the foreseeable future.
1. Introduction Organic-inorganic halide perovskite solar cells (PSCs) have showed great promise for use in photovoltaic field due to their rapidly improved power conversion efficiency (PCE) from 3.8% to 23.7% in just a few years [1–5]. A classic perovskite solar cell with or without mesoporous scaffold is embedded between the electron transporting layer (ETL) and hole transporting layer (HTL) [6–8]. In general, through the absorption of incident photons, carriers are produced in the perovskite absorber layer that transfers through a transport path including the ETL or HTL, the metal electrodes, and between each interface [9,10]. Therefore, it is critical to improve carriers transport at each interface for preparing high efficiency solar cells. However, PSCs with unsatisfied interface performance probably lead to the decreased cell PCE due to the existence of trap states and defects on the interface [11–13]. Interface modification is considered as an efficient method for enhancing the cell PCE by obviously improving electron transport and reducing the electron recombination, in addition, improving the surface properties for ETL is also important to obtaining high-class film quality for MAPbI3 layer [14–17]. Many efforts have been made about the interface engineering, which majorly focus on the interface of electron/perovskite transport layer [18–21]. For instance, Li et al. reported an efficient approach for enhancing the interfacial properties by using a SAM of 4aminobenzoic acid between the TiO2 layer and the perovskite absorber
⁎
film, which obtained high PCE cell due to fewer traps state density, better electron transfer and film quality of perovskite layer [22]. Zheng et al. reported a simple method that modifying by aminocaproic acid and caproic acid between meso-TiO2 and perovskite layer obtained significant PCE improvement, which could contribute to the accelerated electron extraction and transfer by interface engineering [23]. Additionally, Guojia Fang's group used a 3-aminopropyltriethoxysilane as self-assembled monolayer (SAM) to improve interface between the SnO2 ESL and perovskite film, the result improve cell PCE due to the obtained high quality perovskite layer, the decreased work function of SnO2, the passivated trap states [24]. Therefore, interface modification as a convenient and effective method was implemented to optimize the meso-TiO2/MAPbI3 interface for improved device performance, reduced hysteresis for PSCs. In this study, a self-assembled monolayer (SAM) of γMercaptopropyltrimethoxysilane (γ-MPTS) is prepared on the interface of the meso-TiO2/perovskite layer. By optimizing the concentration of SAM (γ-MPTS) prepared on the meso-TiO2 layer surface, we achieve 16.50% cell PCE versus unmodified-device (14.36%). Through rich characterization means based γ-MPTS-SAM modified and unmodified perovskite solar cells with X-ray diffraction (XRD) scanning electron microscope (SEM), Atomic force microscope (AFM), contact angle and photoluminescence, which show that the introduced SAM can reduce carrier recombination and further optimize MAPbI3 film quality.
Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Zhuang), [email protected], [email protected] (Z. Ma).
https://doi.org/10.1016/j.mssp.2019.02.018 Received 22 October 2018; Received in revised form 12 February 2019; Accepted 15 February 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 1. (a) flow-process diagram of the deposition procedure of SAM based meso-TiO2 substrates. (b) and (c) SEM pictures of FTO/c-TiO2/meso-TiO2 and FTO/cTiO2/meso-TiO2/SAM, (d) FTIR spectra based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2, EDX elemental pictures of (e) FTO/c-TiO2/meso-TiO2, (f) FTO/cTiO2/ meso-TiO2/SAM.
reactants, and finally dried with a nitrogen gun, and then annealed at 500 ℃ for 30 min in a muffle furnace. The as-prepared meso-TiO2 devices were immersed in a 5, 10, 15 mM solution of γ-MPTS in isopropanol for three hours, respectively, thus self-assembling into a monomolecular membrane at the meso-TiO2 surface. After that, the substrates were rinsed by isopropanol and then dried under a flow of N2. The perovskite layer was spin-coating on the FTO substrate/c-TiO2/ meso-TiO2 at 3000 rpm for 55 s. CB (80 µL) was dropped onto the substrate during the spin-coating step at 10 s before begin of the procedure, and then the film was heated at 100 °C for 20 min. An HTL film was prepared by spin-coating spiro-OMeTAD solution on MAPbI3 film at 3000 rpm for 30 s. Finally, Ag electrode with a thickness of ≈100 nm was deposited onto the spiro-OMeTAD coated film by sputtering technique to finish the whole device fabrication process.
2. Experimental section 2.1. Materials and reagents Titanium isopropoxide (99.999%), chlorobenzene (99.9%), N,N-dimethylformamide (DMF, 99.9%), and acetonitrile (99.9%) were purchased from Sigma-Aldrich. γ-Mercaptopropyltrimethoxysilane (99.9%) was achieved from Aldrich. 2,2′,7,7′-Tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiroOMeTAD, 99.5%), CH3NH3I (99.5%), PbI2 (99.99%), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI; 99.9%) and 4-tertbutylpyridine (tBP; 96%) were obtained from Xi’an Polymer Light Technology Company, Fluorine-doped tin oxide (FTO) coated glass substrates (15 Ω/sq) were bought from OPV Tech New Energy Co. tetrabutyl titanate (98.3%) and hydrochloric acid were obtained from Kelong Company. All the materials and reagents were used without further purification.
2.3. Characterization The current-voltage characteristics of perovskite solar cells were recorded using an electrochemical workstation (CHI660D, Chenhua) under AM 1.5 simulated illumination (CEL-S500, Beijing, China). Contact angle were recorded using a contact angle measuring instrument (OCA25, GmbH). Electrochemical impedance spectroscopy (EIS) was measured by electrochemical workstation (CHI660D, Chenhua), with amplitude of 7.5 mV and a measurement frequency from 1 to 100 kHz. Mott-Schottky plots were measured by using an electrochemical workstation (CHI660D, Chenhua) with a standard threeelectrode configuration, which the Ag/AgCl as reference electrode in saturated Na2SO4 and Pt sheet as counter electrode in deionized water. FTIR spectra were measured by Fourier transform infrared spectroscopy (Nicolet 6700). The morphology and composition of meso-TiO2 film was investigated using a scanning electron microscopy (SEM, ZEISS EV0MA15) equipped with energy dispersive X-ray spectroscopy (EDX) detector. X-ray diffraction (XRD, DX-2700, Dandong) measurements were carried out from 22 to 58 with Cu Ka radiation (λ = 0.154 06 nm) at a scanning rate of 4 deg/min. Atomic force microscope (AFM) measurements were measured by an Atomic force microscope spectrometer (Agilent7500). The UV–vis absorption spectra were measured by
2.2. Solar cell fabrication FTO substrates were ultrasonically cleaned with abstergent, acetone, deionized water, and ethanol for 15 min, respectively. The conductive substrates were dried with a nitrogen gun and post-treated with UV-ozone for 15 min. The compact TiO2 (c-TiO2) layer was coating on cleaned FTO glass by spin-coating the titanium precursor solution at 2000 rpm for 30 s. The as-prepared FTO glass was post annealed at 150 ℃ for 15 min, and annealed at 500 ℃ in a muffle furnace for 30 min. Then we started to prepare meso-TiO2 precursor solution, 15 ml hydrochloric acid was mixed with 15 ml deionized water in the situation of magnetic stirring for 10 min 0.75 ml Tetra-n-butyl Titivate was added to the mentioned mixed solution, and then the solution need to vigorously stir for 30 min to obtain a clarified solution. Afterwards, the clear solution was transferred into the reaction kettle equipped with cTiO2/FTO substrates. The sealed reaction kettle was placed in a laboratory oven at 170 ℃ for 1 h. After the autoclave was cooled to room temperature naturally and the substrates were taken out, rinsed with deionized water and ethanol for two times to get rid of the residual 22
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 1. (continued)
Fig. 1. (continued)
Fig. 2. (a) AFM of meso-TiO2 and (b) meso-TiO2/SAM, water contact angles of meso-TiO2 (c), and (d) meso-TiO2/SAM.
TiO2. As shown in Fig. 1(d), the existence of -CH2, Si-O-Si, –SH peaks confirmed the γ-MPTS modification onto the m-TiO2 surface compared with unmodified TiO2. The elemental mapping measured by EDX (Fig. 1(e), (f)) was also used to obtain the chemical element composition of meso-TiO2 and meso-TiO2/SAM [25], the tin elements and most silicon elements originate from the FTO substrate and titanium element come from the compact TiO2 and meso-TiO2. The existence of sulfur elements, carbon elements demonstrates that the γ-MPTS has successfully been modified onto meso-TiO2 by soaking self-assembling method. Therefore, it is very likely to improve PSCs performance. Fig. S1 demonstrates the XRD patterns for meso-TiO2 and meso-TiO2/SAM, which did not discover obvious peak changes that show meso-TiO2 crystal structure was formed and the SAM modification did not influence the crystal structure of ETLs. The atomic force microscopic (AFM) picture of the meso-TiO2 surface based SAM modification showed a smoother surface with a roommean-square (rms) surface roughness of 15.9 nm compared to 17.6 nm for meso-TiO2 without γ-MPTS-SAM modification (Fig. 2(a) and (b)). The smoother surface can provide meso-TiO2 ESL better interface contact with MAPbI3 film [26], the results lead to an improved VOC and an increased fill factor (FF), and also demonstrate that γ-MPTS-SAM
UV–vis spectrometer (UV-2600). In the time-resolved photoluminescence spectra (FLS 980 Edinburgh Instruments), the measurement was conducted at 760 nm with a pulsed diode laser at 510 nm (with an intensity of 0.12 mW cm−2) in a pulse frequency of 1 MHz. The incident photo-current conversion efficiency (IPCE) spectra were recorded using an IPCE system (PVE 300, Bentham, Inc.) as a function of wavelength from 300 to 800 nm. By using a mask, the active area of perovskite solar cells was controlled to 0.16 cm2. 3. Results and discussion Fig. 1(a) shows the deposition procedure of SAM based meso-TiO2 substrates, compact TiO2 was fabricated by spin coating, meso-TiO2 was prepared by hydrothermal approach and SAM was manufactured by soaking method in γ-MPTS solution. The surface SEM images of mesoTiO2 and meso-TiO2/SAM are illustrated in Fig. 1(b) and (c), respectively. γ-MPTS-SAM is used as an interfacial modification layer on the surface of meso-TiO2 ETL, there was no visible morphological changes compared to pure meso-TiO2. The reason was attributed to too thin SAM modification film to be seen. The FTIR (Fig. 1(d)) spectra are used to measure the chemical absorption of γ-MPTS on the surface of meso23
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 3. (a) Top-view SEM images of CH3NH3PbI3 film based on meso-TiO2 and (b) γ-MPTS-SAM modified meso-TiO2 ETLs, (c) XRD patterns of CH3NH3PbI3 films (d) UV–vis absorption spectra of CH3NH3PbI3 based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2 ETLs.
without γ-MPTS-SAM-modified meso-TiO2, which further increase the visible light absorptive characteristic for CH3NH3PbI3 layer. The result could be attributed to achieve better perovskite film quality by γ-MPTSSAM modification onto meso-TiO2 surface. The device model diagram of mesoporous perovskite solar cell based TiO2 as ETL is shown in Fig. 4(a), in which γ-MPTS-SAM is used as a thin γ-MPTS-SAM interface layer to modify the surface of mesoTiO2. As shown in Fig. 4(b), the trimethoxy silane was chemically bonded to the meso-TiO2 surface through hydrogen bond between the hydrolytic alkoxy group and the hydroxyl groups (on the surface of meso-TiO2) [27,28]. For terminal functional groups, firstly, the sulfhydryl group with lone pair electrons in the γ-MPTS could provide passivation effect by donating lone pair electrons to the Pb2+, and form a coordinate bond with the edge of MAPbI3 layer [29,30]. Secondly, this sulfhydryl functional group can form hydrogen bonds with iodine ions (S-H…I) [31,32]. Surface defects for MAPbI3 layer are mainly caused by halogen vacancies [33,34]. By fixing the hydrogen bond (SH…I), the vacancy of iodine ion is reduced, thus the surface defect state for MAPbI3 layer is reduced. This result will reduce carrier recombination and further increase cells PCE [35,36]. In addition, γMPTS-SAM can induce the formation of dipoles moment to enhance the force for the carrier separation and carrier transport [37], which be propitious to increase cell PCE. Fig. 4(c) shows that the reverse and
was successfully modified onto the meso-TiO2 surface. The picture of water contact angles are shown in Fig. 2(c) and Fig. 2(d). After γ-MPTSSAM modification, the contact angle on meso-TiO2 is 28.7 °compared to 37.3° without γ-MPTS-SAM modification. The result shows that mesoTiO2 with γ-MPTS-SAM modification become more hydrophilic, which due to the enhanced connectivity induced by the hydrogen-bonding between the hydrolytic alkoxy group from γ-MPTS-SAM and the hydroxyl groups from meso-TiO2 surface [27]. SEM imagines (Fig. 3(a) and (b)) are carried to study the surface morphologies of the perovskite films with or without γ-MPTS-SAM as interface modification layer. After γ-MPTS-SAM interface modification, the quality of MAPbI3 film become more compact, which in favor the decreased leakage current that lead to higher performance for cell. The positive phenomenon could be ascribed to the improved connectivity between MAPbI3 layer and SAM-modified meso-TiO2 ESL caused by hydrogen bonds formed from sulfhydryl group (in γ-MPTS) with the iodide anions of the MAPbI3 film. Fig. 3(c) shows the XRD patterns MAPbI3 film fabricated on meso-TiO2 and meso-TiO2/γ-MPTS-SAM, which show that perovskite structure was formed and the modification of γ-MPTS-SAM did not influence the crystal structure of MAPbI3 film. The UV–vis absorption spectra (Fig. 3(d)) demonstrates that the CH3NH3PbI3 film based SAM-modified meso-TiO2 ETLs has higher absorption peak within the 400–500 nm than the CH3NH3PbI3 film based 24
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 4. (a) device model diagram, (b) theoretical model of γ-MPTS-SAM modified meso-TiO2, (c) J-V curves, (d) dark current (e) histogram of PCE, (f) IPCE.
produces JSC of 22.43 mA cm−2, VOC of 1.008 V, and FF of 63.50%, exhibiting a PCE of 14.36% under reverse scanning (a PCE of 8.65% under forward scanning), whereas the γ-MPTS-SAMs modification PSCs has a PCE of 16.50% (VOC = 1.028 V, JSC = 23.09 mAcm−2, FF = 69.50%) under reverse scanning and a PCE of 13.17% under forward scanning. The decreased hysteresis index for cell based γ-MPTS-SAM modification meso-TiO2 (0.20) as ETLs is demonstrated (0.38 hysteresis index for without modified device), which can be attributed to the improved perovskite morphology and decreased trap states. By Table S1, we can show that the value of JSC, VOC, and FF of the γ-MPTS-SAM modification are increased when the γ-MPTS-SAM concentration is 5 mmol and 10 mmol compared to the unmodification meso-TiO2. When the modification concentration is 15 mmol, the values of JSC, Voc and FF for cell dropped rapidly. γ-MPTS-SAM has an insulating alkyl chain, too high concentration can cause poor electrical conductivity,
Table 1 Photovoltaic performance of PSCs based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2. ETLs
VOC (V)
JSC (mA/ cm2)
FF (%)
PCE (%)
Hysteresis Index
TiO2 reverse TiO2 forward TiO2 /SAM reverse TiO2 /SAM forward
1.008 0.912 1.028
22.43 22.12 23.09
63.50 43.53 69.50
14.36 8.65 16.50
0.39
0.994
22.39
60.10
13.17
0.20
forward J-V curves current-voltage (J-V) characteristics of the solar cell with and without γ-MPTS-SAM modification, the corresponding results summary is shown in the Table 1. The unmodified meso-TiO2 cell 25
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Fig. 5. (a) UV–vis absorption spectra of meso-TiO2 and γ-MPTS-SAMs meso-TiO2 ETLs, (b) indirect band gap model, (c) Mott-Schottky curve of TiO2 and TiO2/SAM, (d)The energy band diagram of cell, (e) PL and (f) TRPL decay curves.
leading to the decreased PCE for cell. The 15 mM concentration of γMPTS-SAM with insulating alkyl chains provides worse spatial separation of carriers between perovskite layer and TiO2 layer, increase the carrier recombination, and further decrease cells efficiency [38,39]. The result can be ascribed to the fact that too many γ-MPTS cause poor electrical conductivity, which further lead to decreased cell PCE. The statistical distribution of the PCE for unmodified and 10 mmol modification γ-MPTS-SAM cells is shown in Fig. 4(e), which the PCE of cell based γ-MPTS-SAM modification meso-TiO2 concentrate in 15%, 16%
compared to 12%, 13% for unmodified device. Error bar (Fig. S2) show that SAM modified meso-TiO2 cell improve VOC, JSC, and FF than unmodified cell, which further demonstrates that SAMs modification is effective method for improving cell PCE. Fig. 4(d) shows the dark current of perovskite solar cells with or without γ-MPTS-SAM modification. The value of dark current reflects the situation for reverse flow of carrier. As shown in Fig. 4(d), the decreased dark current of cell with SAMs modification show the suppressed reverse movement for the carrier, leading to the improved carrier transport from perovskite film 26
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
gap can be attributed to the formation of dipoles moment induced by γMPTS-SAM, which improve the Voc for perovskite solar cells. MottSchottky curve (Fig. 5(c)) was explored to show the changes for conduction band energy [19]. The value of flat band potential (Efb) corresponds to the intersection of the linear part of the curve with the Xaxis. An Efb shift represents the change of relative position of the conduction band. Fig. 5(c) shows that Efb is negatively shifted from −0.584 V to −0.602 V through γ-MPTS-SAM modification, which leads to a higher VOC. Fig. 5(d) shows the energy band diagram of cell. The position of conduction band of Pure TiO2 is obtained from references [4,19]. The analysis of Mott-Schottky curve shows that the conduction band position of γ-MPTS modified TiO2 increases from −4.2 eV to −4.182 eV. The decreased energy barrier between the perovskite layer and TiO2 layer will facilitate carriers transportation, which is also beneficial for achieving an improved JSC. The PL (Fig. 5(e)) and TRPL (Fig. 5(f)) measurements were used to assess the ability of efficient electrons extraction from perovskite layer to meso-TiO2 layer. As shown in Fig. 5(e), the steady-state PL spectra of PSCs based the structure FTO/c-TiO2/meso-TiO2/perovskite film and FTO/c-TiO2/γ-MPTS-SAM modified meso-TiO2/perovskite film, respectively. The MAPbI3 film deposited on γ-MPTS-SAM modified meso-TiO2 shows more evident PL quenching than the pristine meso-TiO2 films, which further demonstrate the improved carrier extraction ability. Moreover, the TRPL result shown in Fig. 5(f) that confirms faster photo-induced carrier transfer from MAPbI3 film to meso-TiO2 after γ-MPTS-SAM modification. The result could be attributed to the decreased trap states originated from hydrogen-bonding interactions (S-H…I). To further investigate the charge transport and recombination properties of cells, electrochemical impedance spectroscopy (EIS) measurement was measured at 0.7 V bias under dark condition (Fig. 6). The inset model in Fig. 6 is the equivalent circuit of devices and Table 2 shows the fitted EIS data. After γ-MPTS-SAMs modification, the cells exhibit smaller contact resistance (Rs) and larger charge recombination resistance (Rct) than the unmodified cells, meaning better chargetransfer ability and less carrier recombination [41]. Besides the enhanced PCE, the stability of cells based γ-MPTS-SAM modification was also improved. The devices are unsealed and are put in ambient environment under the humidity level of 40 ± 10%. The PCEs of cells based meso-TiO2 and γ-MPTS-SAM modification mesoTiO2 were tracked for 180 h (Fig. 7). The γ-MPTS modified cells are more stable than the unmodified cells. The γ-MPTS modified one retained ≈ 80% of its original efficiency, whereas the unmodified cell only had ≈64% of its original efficiency. The enhanced stability is attributed to the improved perovskite film quality by γ-MPTS-SAM modification, which can retard the decomposition of perovskite films.
Fig. 6. EIS of the PSCs based on meso-TiO2 and γ-MPTS-SAM modified mesoTiO2 at 0.7 V bias under dark condition. Table 2 The fitted EIS data of the PSCs based on meso-TiO2 and γ-MPTS-SAM modified meso-TiO2. Sample
Contact resistance (RS/ ohm)
recombination resistance (Rct/ ohm)
TiO2 TiO2/SAM
21.48 12.01
6460 11,080
4. Conclusion Fig. 7. Normalized PCE of cells based meso-TiO2 and γ-MPTS-SAM modification meso-TiO2 with the change of time, the cells are unsealed and are put in an ambient environment under the humidity level of 40 ± 10%.
In summary, we have successfully prepared a self-assembled monolayer γ-MPTS on the interface of the meso-TiO2/perovskite film, which lead to a result of the reduced trap states and enhanced interface carrier transport performance. The process achieved the highest PCE (16.50%) with an average efficiency of 15.17% for mesoporous geometry. This strategy based a simple and low-cost method should provide insight into commercialization of PSCs in the near future.
to ETL. Incident photo-to-electron conversion efficiency (IPCE) of these mesoporous perovskite solar cells based on unmodified TiO2 and γMPTS-SAM modified meso-TiO2 layers exhibit wavelength onset at 780 nm that keep in reasonable agreement with the bandgap of the CH3NH3PbI3 (Fig. 4(e)). The results that SAM modified meso-TiO2 cell exhibited higher IPCE value compared the control cell, which is in good accordance with the current density trend. Fig. 5(a) shows the UV–vis absorption spectra of the unmodified and γ-MPTS-SAM modified meso-TiO2 cells. The absorption spectrum of the γ-MPTS-SAM modified meso-TiO2 shows a blue shift about band gap (small image in Fig. 5(a)). The transformed Kubelka–Munk spectrum (Fig. 5(b)) of the unmodified and γ-MPTS-SAM modified meso-TiO2 is tested to decide the optical band gap [40]. The decreased optical band
Acknowledgements The authors gratefully acknowledge the financial support from Sichuan Province Science and Technology Support Program (Grant no. 2018JY0015), Young Scholars’ Development Fund of SWPU (Grant no. 201699010017) and scientific research starting project of SWPU (Grant no. 2017QHZ021).
27
Materials Science in Semiconductor Processing 97 (2019) 21–28
H. Lu, et al.
Appendix A. Supporting information [21]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2019.02.018.
[22]
References [23] [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] H.S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, S.J. Moon, R. Humphry-Baker, J.H. Yum, J.E. Moser, M. Gratzel, N.G. Park, Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Rep. 2 (2012) 591. [3] H. Zhou, Q. Chen, G. Li, S. Luo, T.B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345 (2014) 542–546. [4] N.J. Jeon, H. Na, E.H. Jung, T.Y. Yang, Y.G. Lee, G. Kim, H.W. Shin, Seok Sang II., J. Lee, J. Seo, A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells, Nat. Energy 3 (2018) 682–689. [5] 〈https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20181214.pdf〉. [6] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [7] I.C. Smith, E.T. Hoke, D.S. Ibarra, M.D. Mcgehee, H.I. Karunadasa, A layered hybrid perovskite solar-cell absorber with enhanced moisture stability, Angew. Chem. Int. Ed. 53 (2014) 11232–11235. [8] S.S. Mali, J.V. Patil, H. Kim, C.K. Hong, Synthesis of SnO2 nanofibers and nanobelts electron transporting layer for efficient perovskite solar cells, Nanoscale 10 (2018) 8275–8284. [9] L. Zhu, Z. Shao, J. Ye, X. Zhang, X. Pan, S. Dai, Mesoporous BaSnO3 layer based perovskite solar cells, Chem. Commun. 52 (2015) 970–973. [10] X. Ling, J. Yuan, D. Liu, Y. Wang, Y. Zhang, S. Chen, H. Wu, F. Jin, F. Wu, G. Shi, X. Tang, J. Zheng, S.F. Liu, Z. Liu, W. Ma, Room-temperature processed Nb2O5 as the electron transporting layer for efficient planar perovskite solar cells, Acs. Appl. Mater. Interfaces 9 (2017) 23181–23188. [11] S. Zhang, M. Stolterfoht, A. Armin, Q. Lin, F. Zu, Interface engineering of solution processed hybrid organohalide perovskite solar cells, ACS Appl. Mater. Interfaces 10 (2018) 21681–21687. [12] M. Shahiduzzaman, M. Karakawa, K. Yamamoto, T. Kusumi, K. Yonezawa, Interface engineering of compact-TiOx in planar perovskite solar cells using low-temperature processable high-mobility fullerene derivative, Sol. Energy Mater. Sol. Cells 178 (2018) 1–7. [13] Z.X. Juan, L. Tao, H. Li, W. Huang, P. Sun, Efficient planar perovskite solar cells with improved fill factor via interface engineering with graphene, Nano Lett. 18 (2018) 2442–2449. [14] C. Ran, J. Xi, W. Gao, F. Yuan, T. Lei, Bilateral interface engineering toward efficient 2D–3D bulk heterojunction tin halide lead-free perovskite solar cells, ACS Energy Lett. 3 (2018) 713–721. [15] Y. Hou, J. Yang, Q. Jiang, W. Li, Z. Zhou, X. Li, S. Zhou, Enhancement of photovoltaic performance of perovskite solar cells by modification of the interface between the perovskite and mesoporous TiO2 film, Sol. Energy Mater. Sol. Cells 155 (2016) 101–107. [16] N. Kumari, J.V. Gohel, S.R. Patel, Optimization of TiO2 /ZnO bilayer electron transport layer to enhance efficiency of perovskite solar cell, Mater. Sci. Semicond. Proc. 75 (2018) 149–156. [17] W. Li, W. Zhang, S.V. Reenen, R.J. Sutton, J. Fan, A.A. Haghighirad, M.B. Johnston, L. Wang, H.J. Snaith, Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification, Energy Environ. Sci. 9 (2016) 490–498. [18] S.F. Shaikh, H.C. Kwon, W. Yang, H. Hwang, H. Lee, La2O3 interface modification of mesoporous TiO2 nanostructures enabling highly efficient perovskite solar cells, J. Mater. Chem. A 4 (2016) 15478–15485. [19] Y. Xiang, Z. Ma, J. Zhuang, H. Lu, C. Jia, H. Li, Enhanced performance for planar perovskite solar cells with samarium-doped TiO2 compact electron transport layers, J. Phys. Chem. C. 121 (2017) 20150–20157. [20] Y. Ma, J. Lee, Y. Liu, P.M. Hangoma, W.I. Park, Synchronized-pressing fabrication
[24]
[25] [26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
28
of cost-efficient crystalline perovskite solar cells via intermediate engineering, Nanoscale 10 (2018) 9628–9633. C. Gao, H. Dong, X. Bao, Y. Zhang, A. Saparbaev, Additive engineering to improve efficiency and stability of inverted planar perovskite solar cells, J. Mater. Chem. C. 6 (2018) 8234–8241. B. Li, Y. Chen, Z. Liang, D. Gao, W. Huang, Interfacial engineering by using selfassembled monolayer in mesoporous perovskite solar cell, Rsc. Adv. 5 (2015) 94290–94295. R. Chen, J. Cao, Y. Wu, X. Jing, B. Wu, N. Zheng, Improving efficiency and stability of perovskite solar cells by modifying mesoporous TiO2-Perovskite interfaces with both aminocaproic and caproic acids, Adv. Mater. Inter. 4 (2017) 1700897. G. Yang, C. Wang, H. Lei, X. Zheng, P. Qin, L. Xiong, X. Zhao, Y. Yan, Interface engineering in planar perovskite solar cells: energy level alignment, perovskite morphology control and high performance achievement, J. Mater. Chem. A. 5 (2016) 1658–1666. Y. Li, Y. Guo, Y. Li, X. Zhou, Fabrication of Cd-doped TiO2 nanorod arrays and photovoltaic property in perovskite solar cell, Electrochim. A. 10 (2016) 29–36. Y. Yan, F. Cai, L. Yang, J. Li, Y. Zhang, F. Qin, C. Xiong, Y. Zhou, D.G. Lidzey, T. Wang, Light-Soaking-Free inverted polymer solar cells with an effciency of 10.5% by compositional and surface modifcations to a low-temperature-processed TiO2 electron-transport layer, Adv. Mater. 29 (2017) 1604044. K.A. Luck, T.A. Shastry, S. Loser, G. Ogien, T.J. Marks, Improved uniformity in highperformance organic photovoltaics enabled by (3-aminopropyl) triethoxysilane cathode functionalization, Phys. Chem. Chem. Phys. 15 (2013) 20966–20972. R. Cisneros, M. Beley, F. Lapicque, P.C. Gros, Hydrophilic ethylene‐glycol‐based ruthenium sensitizers for aqueous dye‐Sensitized solar cells, Eur. J. Inorgan. Chem. 1 (2016) 33–39. L. Zuo, Q. Chen, N.D. Marco, Y.T. Hsieh, H. Chen, P. Sun, S.Y. Chang, H. Zhao, S. Dong, Y. Yang, Tailoring the interfacial chemical interaction for high-efficiency perovskite solar cells, Nano. Lett. 17 (2017) 269–275. Y.C. Shih, L.Y. Wang, H.C. Hsieh, K.F. Lin, Enhancing photocurrent of perovskite solarcells via modification of TiO2/CH3NH3PbI3 heterojunction interface with amino acid, J. Mater. Chem. A. 00 (2012) 1–3. Q. Xue, Z. Hu, J. Liu, J. Lin, C. Sun, Z. Chen, C. Duan, J. Wang, C. Liao, W.M. Lau, F. Huang, H.L. Yip, Y. Cao, Highly efficient fullerene/perovskite planar heterojunction solar cells via cathode modification with an amino-functionalized polymer interlayer, J. Mater. Chem. A. 2 (2014) 19598–19603. C. Sun, Z. Wu, H.L. Yip, H. Zhang, X.F. Jiang, Q. Xue, Z. Hu, Z. Hu, Y. Shen, M. Wang, F. Huang, Y. Cao, Amino-Functionalized conjugated polymer as an efficient electron transport layer for high-performance planar-heterojunction perovskite solar cells, Adv. Energy Mater. 6 (2016) 1501534. N.K. Noel, A. Abate, S.D. Stranks, E.S. Parrott, V.M. Burlakov, A. Goriely, H.J. Snaith, Enhanced photoluminescence and solar cell performance via Lewis base passivation of organic-inorganic lead halide perovskites, ACS Nano 8 (2014) 9815–9821. F. Gao, Z. Li, J. Wang, A. Rao, I.A. Howard, A. Abrusci, S. Massip, C.R. McNeill, N.C. Greenham, Trap-induced losses in hybrid photovoltaics, ACS Nano 8 (2014) 3213–3221. Y. Hou, C. Quiroz, S. Scheiner, W. Chen, T. Stubhan, Low‐Temperature and hysteresis‐free electron‐transporting layers for efficient, regular, and planar structure perovskite solar cells, Adv. Energy Mater. 5 (2015) 1501056. L. Liu, A. Mei, T. Liu, P. Jiang, Y. Sheng, Fully printable mesoscopic perovskite solar cells with organic silane self-assembled monolayer, J. Am. Chem. Soc. 137 (2015) 1790–1793. X. Bulliard, S.G. Ihn, S. Yun, Y. Kim, D. Choi, Enhanced performance in polymer solar cells by surface energy control, Adv. Func. Mater. 20 (2010) 4381–4387. J. Zhang, Z. Hu, L. Huang, G. Yue, J. Liu, X. Lu, Z. Hu, M. Shang, L. Han, Y. Zhu, Bifunctional alkyl chain barriers for efficient perovskite solar cells, Chem. Comm. 32 (2015) 7047–7050. M.M. Byranvand, T. Kim, S. Song, G. Kang, S.U. Ryu, T. Park, p-Type CuI Islands on TiO2 electron transport layer for a highly efficient planar-perovskite solar cell with negligible hysteresis, Adv. Energy Mater. 8 (2017) 1702235. H. Zhang, Y. Lv, Y. Guo, X. Tao, C. Yang, Fully-air processed Al-doped TiO2 nanorods perovskite solar cell using commercial available carbon instead of hole transport materials and noble metal electrode, J. Mater. Sci. Mater. Electron. 29 (2018) 3759–3766. J. Li, T. Jiu, B. Li, C. Kuang, Q. Chen, S. Ma, J. Shu, J. Fang, Inverted polymer solar cells with enhanced fill factor by inserting the potassium stearate interfacial modification layer, Appl. Phys. Lett. 108 (2016) 4636–4643.