ChemComm COMMUNICATION. Bifunctional alkyl chain barriers for efficient perovskite solar cells

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1 ChemComm COMMUNICATION View Article Online View Journal View Issue Cite this: Chem. Commun., 2015, 51, 7047 Received 10th January 2015, Accepted 12th March 2015 Bifunctional alkyl chain barriers for efficient perovskite solar cells Jing Zhang,* a Zhelu Hu, a Like Huang, a Guoqiang Yue, a Jinwang Liu, a Xingwei Lu, a Ziyang Hu, a Minghui Shang, b Liyuan Han c and Yuejin Zhu* a DOI: /c5cc00128e Perovskite solar cells as a hot research topic show the necessity of controlling the interface. In this work, an insulating alkyl chain layer is self-assembled at the perovskite/hole transport material interface, which successfully exhibits a dual function: blocking electron recombination and resisting moisture at the same time. Improved solar energy conversion efficiency and stability of the device are both achieved. Solid-state methylammonium lead halide perovskite solar cells are attracting much attention due to their ease of preparation, low cost and high efficiencies. 1 4 To date, a set of diverse and fundamentally very different perovskite solar cell configurations has been developed, 5 for example, planar structured (without mesoporous TiO 2 layer), 6 8 meso-superstructured 9,10 (with inert mesoporous Al 2 O 3 or ZrO 2 scaffolds) and hole transport material-free perovskite solar cells. 11 Though their structures are diverse, all these structures contain interfaces. In general, carriers are created in the perovskite absorber after absorption of incident photons, and travel through transport pathways including the electron or hole transport layer, the electrodes and each interface in between. 12 Compared with other thin film solar cells, the perovskite solar cell is more prone to surface recombination due to imperfect crystal passivation and undesirable interfacial properties, resulting in fill factor (FF), open circuit voltage (V oc ) and short-circuit current ( J sc ) losses. 13 Therefore, how to precisely manipulate the interface of perovskite solar cells to maximize the electron and hole separation/collection and minimize the electron recombination depends mainly on the real cell performance. More recently, efforts related to the interface engineering to change the interfacial photoelectrical properties have been made, a Department of Microelectronic Science and Engineering, Ningbo University, Zhejiang, , China. zhangjing@nbu.edu.cn b School of Materials Science and Engineering, Ningbo University of Technology, Zhejiang, , China c Photovoltaics Materials Unit, National Institute for Materials Science, Tsukuba, Ibaraki, , Japan Electronic supplementary information (ESI) available: Reaction equations S1 and S2; the efficiency distribution of the perovskite photovoltaic cells (Fig. S1), the equivalent circuit of the Nyquist plot (Fig. S2), hysteresis analysis (Fig. S3). DOI: /c5cc00128e such as changing the work function of the electrode to a more suitable value, 12 and using a more efficient electron collection layer 14,15 and blocking layer 5 for efficient charge separation and transportation. Supramolecular halogen bond passivation of surface defect states of perovskites also effectively reduces interface recombination in perovskite solar cells. 16 Despite the evident success of interface engineering to improve the perovskite solar cell performance, the instability problem of perovskite materials remains an open issue for outdoor photovoltaic applications. 17 The instability stems from the CH 3 NH 3 PbI 3 degradation under humid conditions. Replacement of the CH 3 NH 3 group with a more stable unit under moisture 18 or using a hydrophobic hole transport material 19 can effectively alleviate the problem. Here, a layer of long alkyl chains (dodecyl) is judiciously selfassembled on a perovskite/tio 2 nanoparticle surface, which is the first time that such an unreactive molecule is used to perform in a bifunctional manner in perovskite solar cells. On one hand, such alkyl chains function as an electrically insulating barrier, thereby reducing interfacial charge recombination losses in perovskite solar cells, resulting in a much enhanced FF, V oc and J sc. On the other hand, the interface layer with alkyl chains effectively changes the hydrophilic nature of the perovskite surface to a hydrophobic one, thus endowing the device with a higher resistance towards moisture. It is believed that this bifunctional interface modification can be utilized in other structures of perovskite solar cells. The formation process of the perovskite device is shown in Scheme 1. Self-assembly of the monolayer of alkylalkoxysilane moleculesiscarriedoutbydip-coatingtheperovskite/tio 2 film in dodecyltrimethoxysilane (C 12 -silane in short) isopropanol solution (with varying concentrations of 0.2, 0.15, 0.1 and 0.05 M) for 5 min, and baked at 80 1Ctoevaporatethesolvent.AmphiphilicC 12 -silane has a hydrophobic alkyl chain and readily hydrolyzable trimethoxysilane groups, which can either hydrolyze to form silanol, Si OH (further forming Si O Si and Si OHHO Si bonds, the reaction equations are shown in S1 and S2, ESI ), or undergo alcoholysis with the solvent. The hydrolyzed silanol has a Si OH group which is an electrophile (Lewis acid), while the I in the perovskite is rich in electronpairs(lewisbase). 16 Therefore, C 12 -silane can be adsorbed This journal is The Royal Society of Chemistry 2015 Chem. Commun., 2015, 51,

2 View Article Online Communication ChemComm Fig. 2 SEM images of (a) the perovskite film on porous TiO 2 /compact TiO 2 /FTO and (b) the 0.1 M C 12 -silane modified film. The insets show the corresponding higher magnification images. Scheme 1 The formation process of the mesoporous perovskite device with C 12 -silane modification. onto the surface of the perovskite by hydrogen bonding of Si OH with I, leaving the long alkyl chain end group to stick out from the surface. By properly choosing the concentration of C 12 -silane in isopropanol, a self-assembled monolayer can be formed, as shown intheenlargedviewinscheme1. Attenuated total reflectance-fourier transform infrared (ATR- FTIR) measurements (Fig. 1) are performed to check the chemical modification of C 12 -silane on the surface of the perovskite. The Pb I bond is not visible in the IR region; only the organic groups are reflected in the IR spectra. After the interface modification with C 12 -silane, new bands clearly appeared in the corresponding spectrum as indicated by the arrows: the strong g-ch 2 stretching bands at 2920 and 2850 cm 1,andthe(CH 2 ) n (n 4 4) characteristic vibrations at 720 cm 1, which are ascribed to the existence of the C 12 H 25 alkyl chains; 20 the hydrogen bond of Si OHI does not directly correspond to a vibration peak in the spectrum, however, it will affect the Si OH vibration. For freshly alcoholized C 12 -silane (top of Fig. 1), it shows the Si O and Si O CH 3 vibration bands at cm When it is assembled on the perovskite surface, the vibration band moves to a lower frequency region of cm 1, indicating that there is an interaction with Si O. Due to the electrophilic properties of Si OH and the electron pairs on I,Si OHI hydrogen bonds are formed, influencing the Si O vibrations and shifting their peaks to a lower frequency region. The above changes in the ATR-FTIR spectra indicate that C 12 -silane is successfully assembled on the surface of the perovskite through hydrogen bonding. The modified perovskite is further checked by scanning electron microscopy (SEM) to examine the surface morphology. As shown in Fig. 2, both samples are well crystallized in this experiment presenting large crystals, however, there still exist holes which do not cover the TiO 2 nanocrystalline layer. Compared with the original perovskite film, the C 12 -silane modified surface is more compact (inset of Fig. 2b) which is due to the fact that C 12 -silane might influence the crystallization process of the perovskite during baking. A phenomenon that should be addressed is that morphology deformations usually appear in C 12 -silane modified samples during the scanning process. This is actually ascribed to the insulating properties of the alkyl chains. The unsuccessful infiltration of perovskites with pinholes results in low-resistance shunting paths, causing electron recombination from the electron transport material (ETM) to the hole transport material (HTM), which is detrimental to the device performance. 8,22 24 The long alkyl chains form a spacer between the perovskite and the HTM (spiro-meotad), which may retard the electron recombination process especially the recombination from the uncovered TiO 2 in the working device, thus an improved performance can be achieved. Current density versus voltage ( J V) characteristics of the perovskite photovoltaic cells are measured to examine the effects. A batch of more than 35 solar cells is tested and the efficiency distribution is indicated in Fig. S1 (ESI ). Most of the cells with C 12 -silane modification exhibit superior performances than those of the original perovskite solar cells. For each C 12 -silane concentration, a representative cell is chosen with the photovoltaic parameters shown in Table 1. The original perovskite solar cell exhibits a J sc of ma cm 2, V oc of V, FF of and Eff of 9.88%. The original device performs ordinarily. In contrast, after C 12 -silane modification, V oc Table 1 The photovoltaic parameters of the C 12 -silane (different concentrations) modified devices. Light intensity: 95.6 mw cm 2 Concentration [M] J sc [ma cm 2 ] V oc [V] FF Eff [%] Fig. 1 Top: FT-IR plot of freshly prepared C 12 -silane in isopropanol. Bottom: ATR-FTIR spectra of the perovskite CH 3 NH 3 PbI 3 film and the C 12 -silane (0.1 M) modified perovskite film (C 12 -silane/ch 3 NH 3 PbI 3 ) Chem. Commun., 2015, 51, This journal is The Royal Society of Chemistry 2015

3 View Article Online ChemComm Communication Fig. 3 (a) The J V curves of the original perovskite solar cell and the 0.1 M C 12 -silane modified device under 95.6 mw cm 2 (AM1.5) illumination and in the dark; (b) the cell energy level (versus vacuum) diagram of this kind of perovskite solar cell. and FF show a solid enhancement, thus the performance is largely improved. The optimum C 12 -silane modification (0.1 M) results in a performance of 13.74% with a J sc of ma cm 2, V oc of V, and FF of The J V curves of these two samples are shown in Fig. 3(a). The increase in FF is due to the enhanced shunt resistance and the reduced series resistance of the cell. 25 In this work, FF exhibits a great improvement from for the original device to for the 0.1 M C 12 -silane modified solar cell. C 12 -silane with insulating alkyl chains provides spatial separation between the electrons in the perovskite and the holes in the solid-state hole conductor, increasing the shunt resistance of the device, thus the recombination is retarded and FF is largely improved. Such insulating or blocking properties of the C 12 -silane layer in the working cell are illustrated in Fig. 3(b). The photovoltage will be determined by the difference between the Fermi energy of the perovskite and the HOMO (the highest occupied molecular orbital) energy of HTM. 2 A change in V oc correlates with the rate of the charge carrier extraction and recombination. 26 Here, the improvement in V oc after C 12 -silane modification, as shown in Table 1, is due to the reduced interface recombination resulting from the alkyl chain barrier. The higher short-circuit photocurrent densities ( J sc )ofthec 12 -silane modified device mean high charge separation yields, which is ascribed to the reduced recombination. A too high C 12 -silane content (0.2 M) might influence the hole transfer between the perovskite and spiro-meotad, which might lead to the reduced J sc compared with that of a device with a lower C 12 -silane content (Table 1). The blocking effect of the alkyl chains can also be identified in the J V curves measured in the dark, as shown in Fig. 3(a). The onset of the current of the C 12 -silane modified device measured in the dark is moved to a higher bias voltage, which means that recombination is greatly suppressed (hysteresis properties are shown in Fig. S3, ESI. ) Electrochemistry impedance spectroscopy (EIS), which has recently been successfully introduced to perovskite solar cell systems to monitor interfacial changes, 27,28 is carried out for the two corresponding samples. Two arcs appear in the Nyquist plots in Fig. 4a: the high frequency arc (left part) is ascribed to the contact resistance at the HTM/Ag electrode interface; while the lower frequency one (right part) is associated with the recombination resistance (R rec ) and chemical capacitance of the system 29 (equivalent circuit, Fig. S2, ESI ). R rec, which is the diameter of the Fig. 4 Nyquist plots of the original and 0.1 M C 12 -silane modified devices measured in the dark and at a bias of 0.8 V (a); obtained R rec and t versus bias voltage plots (b). The lines with black squares represent the original device; the lineswithredcirclesshowtheresultsofthe0.1mc 12 -silane modified cells. middle frequency arc, is related to the recombination of electrons from perovskite/tio 2 with the hole transport layer (HTM). It is clear that R rec is much larger for the 0.1 M C 12 -silane modified device in the dark at a bias of 0.8 V compared with that of the original device, which means that the charge recombination is blocked. This is consistent with the photovoltaic properties of the devices. The electron lifetime t can be derived from the formula t =(2pf ) 1, where f is the character frequency. The 0.1 M C 12 -silane modified sample shows a longer electron lifetime of 6.13 ms at a 0.8 V bias voltage in the dark, compared with 2.43 ms for the original perovskite solar cell. Different bias voltages were applied for the EIS measurement. The derived R rec and t values plotted against the bias voltage are presented in Fig. 4b. The C 12 -silane modified sample shows higher R rec and t values under all the bias voltages applied, in contrast to those of the unmodified solar cell. The results strongly support the insulating/blocking effect of the alkyl chains in the C 12 -silane interlayer, which thus prolongs the electron lifetime and facilitates the electron collection. In fact, interface engineering to form an insulating layer suppressing electron recombination is not new in dyesensitized solar cells. 30 Here, the same function of the alkyl chains is also achieved in perovskite solar cells. What is more, it is interesting to notice another phenomenon of the long alkyl chain layer: the contact angles of deionized water on the surfaces of the C 12 -silane modified and original perovskite films differ greatly (Fig. 5). ThecontactangleoftheC 12 -silane modified perovskite film is changed to 86.31, nearlytwotimeslargerthanthe44.11 of the original film. It indicates that C 12 -silane with its hydrophobic alkyl chain effectively changes the hydrophilic properties of the perovskite film surface. The modified sample is more hydrophobic than the original one. It is known that CH 3 NH 3 PbI 3 is extremely soluble in water and even under low humidity conditions, which makes this device instable. Even the Li salt in HTM is hygroscopic which Fig. 5 The static contact angles of the perovskite film (left) and the C 12 -silane/perovskite film (right) with deionized water. This journal is The Royal Society of Chemistry 2015 Chem. Commun., 2015, 51,

4 View Article Online Communication Fig. 6 XRD patterns of the perovskite film (a) and the 0.1 M C 12 -silane modified perovskite (b) on glass at 40% relative humidity; normalized efficiency change with time (c), in which the solar cells are unsealed and the humidity of the ambient environment is no more than 45%. will worsen the instability. 19 Covering the hydrophilic perovskite surface with hydrophobic chains might change the situation. Decomposition of CH 3 NH 3 PbI 3 in moisture will convert it to a PbI 2 phase. 17 In Fig. 6(a) and (b), it is clear that a new phase of PbI 2 is present in the original perovskite film after two days as indicated by the star, and the intensity of this peak increases with time. It indicates that decomposition increases with time. In contrast, the C 12 -silane modified perovskite is more stable in the test environment. Such a phenomenon is ascribed to the long alkyl chain layer that is water resistant. The efficiency test results are shown in Fig. 6(c). The efficiency of the original device without C 12 -silane modification shows a steady decrease in J sc and general efficiency; whilethatofthemodifiedoneismorestableoveraprolonged time. The unsealed cell with C 12 -silane modification shows an increase in V oc and FF within the first 100 h. After 600 h in the ambient environment, the efficiency keeps 85% of its initial value with J sc being decreased. In conclusion, by a simple dip-coating method, an insulating and hydrophobic layer of long alkyl (dodecyl) chains is formed at the perovskite/htm interface. On one hand, the insulating alkyl chains prohibit the electron back reaction from the TiO 2 /perovskite to HTM, thus effectively enhancing the performance of the as-fabricated solar cell with a solid increase in FF and V oc. Electrochemical characterizations also verify this effect. On the other hand, the hydrophobic alkyl chain layer makes the surface of the perovskite more resistant to moisture, which renders the device more stable when compared with the unmodified one in an ambient environment. The performance of the C 12 -silane modification thus proves to be judicious and bifunctional. In the future, other long alkyl chain silanes (C 8,C 16 or C 20 ) may be explored to further optimise the electron transport and moisture resistant properties of perovskite solar cells. The authors thank Dr Qidong Tai and Dr Jiangwei Feng for their suggestions on this work. We also acknowledge the financial support from the National Natural Science Foundation of China (Grant No , and ), the Qiangjiang Talented Person Project of Zhengjiang (No. QJD ) and the K.C. Wong Magna Fund at Ningbo University. Notes and references ChemComm 1 M. D. McGehee, Nat. Mater., 2014, 13(9), H. S. Kim, C. R. Lee and J. H. Im, et al., Sci. Rep., 2012, 2, J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Gratzel, Nature, 2013, 499(7458), B. Cai, Y. Xing and Z. Yang, et al., Energy Environ. Sci., 2013, 6(5), W. Chen, W. Yongzhen, J. Liu, C. Qin, X. Yang, A. Islam, Y.-B. Cheng and L. Han, Energy Environ. Sci., 2014, 8, M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501(7467), Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y. Liu, G. Li and Y. Yang, J. Am. Chem. Soc., 2013, 136(2), G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and H. J. Snaith, Adv. Funct. Mater., 2014, 24(1), M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338(6107), D. Bi, S.-J. Moon, L. Haggman, G. Boschloo, L. Yang, E. M. J. Johansson, M. K. Nazeeruddin, M. Gratzel and A. Hagfeldt, RSC Adv., 2013, 3(41), Y. Xu, J. Shi and S. Lv, et al., ACS Appl. Mater. Interfaces, 2014, 6(8), H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu and Y. Yang, Science, 2014, 345(6196), Y. Yamada, T. Nakamura, M. Endo, A. Wakamiya and Y. Kanemitsu, J. Am. Chem. Soc., 2014, 136(33), D. Zhong, B. Cai and X. Wang, et al., Nano Energy, 2015, 11, Z. Zhu, J. Ma and Z. Wang, et al., J. Am. Chem. Soc., 2014, 136(10), A. Abate, M. Saliba and D. J. Hollman, et al., Nano Lett., 2014, 14, M. Gratzel, Nat. Mater., 2014, 13(9), I. C. Smith, E. T. Hoke, D. Solis-Ibarra, M. D. McGehee and H. I. Karunadasa, Angew. Chem., 2014, 126(42), L. Zheng, Y.-H. Chung, Y. Ma, L. Zhang, L. Xiao, Z. Chen, S. Wang, B. Qu and Q. Gong, Chem. Commun., 2014, 50(76), J. Zhang, Y. Yang, S. Wu, S. Xu, C. Zhou, H. Hu, B. Chen, X. Xiong, B. Sebo and H. Han, Nanotechnology, 2008, 19(24), Z. Cheng, B. Shi, B. Gao, M. Pang, S. Wang, Y. Han and J. Lin, Eur. J. Inorg. Chem., 2005, N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and S. I. Seok, Nat. Mater., 2014, 13(9), M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y.-B. Cheng and L. Spiccia, Angew. Chem., 2014, 126(37), Y. Wu, A. Islam, X. Yang, C. Qin, J. Liu, K. Zhang, W. Peng and L. Han, Energy Environ. Sci., 2014, 7(9), J.-W. Lee, D.-J. Seol, A.-N. Cho and N.-G. Park, Adv. Mater., 2014, 26(29), J.-H. Im, I.-H. Jang, N. Pellet, M. Grätzel and N.-G. Park, Nat. Nanotechnol., 2014, 9(11), V. Gonzalez-Pedro, E. J. Juarez-Perez and W.-S. Arsyad, et al., Nano Lett., 2014, 14(2), B. Suarez, V. Gonzalez-Pedro, T. S. Ripolles, R. S. Sanchez, L. Otero and I. Mora-Sero, J. Phys. Chem. Lett., 2014, 5(10), D. Liu, J. Yang and T. L. Kelly, J. Am. Chem. Soc., 2014, 136(49), P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, T. Sekiguchi and M. Grätzel, Nat. Mater., 2003, 2(6), Chem. Commun., 2015, 51, This journal is The Royal Society of Chemistry 2015

5 Electronic Supplementary Material (ESI) for ChemComm. This journal is The Royal Society of Chemistry 2015 The reaction equations of C 12 H 25 Si(OCH 3 ) 3 alcoholysis (S1) and the SiOH condensation process (S2). (C 12 H 25 )Si(OCH 3 ) 3 + (CH 3 ) 2 CHOH (C 12 H 25 )Si(OH) 3 + (CH 3 ) 2 CHOCH 3 SiOH + HOSi Si O Si +H 2 O (S1) (S2) Number of devices Efficiency (%) 0 Figure S1. The efficiency distribution of the perovskite photovoltaic cells with (red bars, red fitting line) or without (dark bars, dark fitting line) C 12 -Silane modification. Figure S2. The equivalent circuit of the Nyquist plot. Rs represents the series resistance, One RC element (R H, C H ) is related to the hole transport in spiro- OMeTAD, and the other one represents the recombination resistance (Rrec) and chemical capacitance (Cμ) on the photoanode side.

6 0.020 Current density (Acm -2 ) origin, reverse origin, forward C 12 -silane, reverse C 12 -silane, forward Voltage (V) Figure S3. The hysteresis analyze of the original and 0.1M C 12 -silane modified perovskite solar cells.