Comprehensive understanding of TiCl4 treatment on the compact TiO2 layer in planar perovskite solar cells with efficiencies over 20%

Journal of Alloys and Compounds 787 (2019) 1082e1088

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Comprehensive understanding of TiCl4 treatment on the compact TiO2 layer in planar perovskite solar cells with efficiencies over 20% Yehui Xu a, Can Gao a, Shiwei Tang a, Jing Zhang a, Yongqi Chen b, Yuejin Zhu a, Ziyang Hu a, * a b

Department of Microelectronic Science and Engineering, Ningbo University, Ningbo, 315211, China College of Science and Technology, Ningbo University, Ningbo, 315211, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 August 2018 Received in revised form 18 January 2019 Accepted 3 February 2019 Available online 4 February 2019

A uniform and compact hole blocking layer is necessary for high-performance perovskite solar cells, since it not only serves as an electron collector but also suppresses the carrier recombination. So far, high efficient perovskite solar cells have been obtained by using a blocking layer TiO2 that requires an additional TiCl4 treatment. In this paper, a comprehensive understanding of TiCl4 treatment on the TiO2 layer was investigated in planar perovskite solar cells. Scanning electron microscope, atomic force microscope, and Kelvin probe force microscopy were applied to investigate the morphology of the TiCl4 treated TiO2, and steady-state photo luminescence spectroscopy and electrical impedance spectroscopy were conducted to study charge carrier dynamics. The TiCl4 treated TiO2 layer can result in high efficiency over 20% in planar perovskite solar cells. Our results demonstrate that the TiCl4 treatment on the compact TiO2 prior to the perovskite deposition was necessary for achieving high-performance solar cells. © 2019 Published by Elsevier B.V.

Keywords: Perovskite solar cells Titanium dioxide Power conversion efficiency Kelvin probe force microscopy Hole blocking layer

1. Introduction Solar cells employing hybrid organic/inorganic perovskite materials instead of dye in dye sensitized solar cells (DSSCs) have been recently considered as one of the promising next generation photovoltaics [1,2]. Since the perovskite materials possess excellent optoelectronic properties including high absorption coefficient, long exciton diffusion lengths and superior charge transport properties [3,4], the power conversion efficiency (PCE) of such perovskite solar cells (PeSCs) have rocketed from 3.8% to over 23% within several years [5e7]. By optimizing perovskite composition and interface engineering [8e12], device stability has also been improved greatly. The perovskite materials were first exploited accompanying with mesoporous TiO2, similar to that of DSSC structure, where an absorber layer is sandwiched between the electron transport layer (ETL) and hole transport layer (HTL) [13]. In previously reported DSSCs, due to the physicochemical instability of molecular dyes, a barrier layer was usually coated on the TiO2 electrode prior to its sensitization by dye molecules [14,15]. The

* Corresponding author. E-mail address: [email protected] (Z. Hu). https://doi.org/10.1016/j.jallcom.2019.02.027 0925-8388/© 2019 Published by Elsevier B.V.

classical method for this barrier layer deposition is derived from TiCl4 hydrolysis followed by sintering at high temperature [16,17]. It has been demonstrated that this treatment can significantly improve the photocurrent of the resulting DSSCs due to the suppression of charge carrier recombination [18,19] Since the organometal perovskite light absorber replaced with dye in DSSCs, this traditional treatment was also inherited in PeSCs. The deep understanding of operational fundamental of PeSCs suggests that this type of PeSCs is totally different from the original DSSCs. The PeSCs with mesoporous TiO2 serving as the scaffold for perovskite growth, have achieved a relatively high PCE, but require high-temperature process and complicated fabrication. By comparison, the planar PeSCs without mesoporous TiO2 endow a simple structure and ease fabrication process. Due to the superior bipolar transport capability and large carrier diffusion length of the perovskite materials [20,21], the perovskite absorber is capable of operating in a simple planar heterojunction architecture [22,23], where a perovskite layer is sandwiched between a HTL and a compact TiO2 film which is coated over fluorine doped tin (FTO) glass substrate [24]. Upon absorption of photons the perovskite generates charge carriers, the electron is transferred to the ETL TiO2 while the hole is transferred to the HTM. The compact layer TiO2 plays two major functions. First, it blocks the direct contact

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between the perovskite and FTO, which otherwise could lead to carrier recombination [25,26]. Second, it serves as an electron selective contact that only collects the photogenerated electrons [27,28]. Therefore, the TiO2 layer should be uniform without cracks and pinholes, otherwise these two functions will be strongly discounted [29]. Chemical bath deposition of TiO2 from TiCl4 hydrolysis is a facile method to deposit TiO2 film or modify the surface of the mesoporous TiO2 layer in DSSCs, resulting in high quality TiO2 film via retarding charge recombination [30e32]. The real role of TiCl4 treatment in planar PeSCs still needs to be consolidated [33e37]. The main motivation of this work is to thoroughly understand the effect of TiCl4 treatment on the compact TiO2 layer and the resulting device performance. In this paper, we intricately study the effect of TiCl4 treatment on the compact TiO2 in planar PeSCs. Comprehensive measurements including SEM, AFM, and KPFM were applied to investigate the surface morphology, and photo luminescence spectroscopy and electrical impedance spectroscopy were applied to investigate the interface charge transfer properties. The device with the TiCl4 treated TiO2 film results in high efficiency up to 20%, and a decent efficiency up to 15% was obtained in a HTLfree device. Our results confirm that the better interface contact is crucial for achieving high-performance solar cells.

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illumination of simulated sunlight (100 mW/cm2) provided by an Oriel solar simulator with an AM 1.5 filter. An aperture area of 0.1 cm2 was maintained using a black metal mask. The top view images of the TiO2/FTO substrates and the perovskite films were studied by a scanning electron microscope (SEM, Hitachi, SU-70, Japan). X-ray diffraction patterns (XRD) were collected using a Bruker instrument (D8 advance, Bruker, German) using Cu Ka radiation at scan rate of 4 min 1. Atomic force microscope (AFM) images were taken using a Bruker Dimension 5000 Scanning Probe Microscope (SPM) in tapping mode. Kelvin probe force microscopy (KPFM) was carried out with Asylum Research Cypher atomic force microscope using a silicon probe coated with Ti/Ir (ASYELEC-01-R2) with a force constant of ~0.5e4.4 N/m and a tip radius of ~28 nm was used for contact potential difference measurement with the lift mode. Steady-state photo-luminescence spectroscopy (PL) measurements were acquired using an Edinburgh Instruments FLS 920 fluorescence spectrometer. Electrochemical impedance spectroscopy was measured with an electrochemical workstation (Zennium, Germany). The scanning frequency was set between 106 Hz and 10 3 Hz and the amplitude of 5 mV as the sine perturbation bias. 3. Results and discussion

2. Experimental 2.1. Device fabrication FTO glass substrates were selectively etched with Zn powder and 4 M HCl. After that, the substrates were cleaned in an ultrasonic bath with ethanol and 2-propanol and were dried at 150  C for 10 min. The substrates were then treated by UV-O3 to remove any organic contaminants. The followed sol-gel method was used to synthesize the pristine titanium dioxide. 0.25 g diethanolamine was added to a solution consisting of 675 ml of titanium isopropoxide and 17.75 ml isopropyl alcohol. After stirring for 5 min, 17.5 ml deionized water was added to the solution. The pristine titanium dioxide precursor was ready after stirring for 40 min. A compact TiO2 films about ~40 nm were deposited onto the FTO substrates by spin coating. The coated substrates were heated at 120  C for 5 min, and then sintered at 450  C for 30 min. For a TiCl4 treatment, the compact TiO2 substrates were immersed in an aqueous solution of TiCl4 (0.04 M) at 70  C for different time (0 min, 30 min, 120 min), then rinsed with deionized water and dried, finally sintered at 450  C for 30 min. To prepare the perovskite precursor solutions, 461 mg of PbI2, 159 mg of CH3NH3I, and 300 ml DMSO were mixed in 700 ml DMF, which was stirred at room temperature for 20 min before use. The perovskite films were deposited onto the pristine TiO2 with two-step spin coating procedures. The first step was 1000 rpm for 5 s with an acceleration of 200 rpm/s. The second step was 3000 rpm for 30 s. Chlorobenzene (~300 mL) was dropped on the spinning substrate during the second spin-coating step at 12 s before the end of the procedure. Subsequently, the films were annealed using a hot plate at 100  C for 10 min in a N2 box. The films were let to cool for 5 min and then 20 mL of a spiro-OMeTAD solution in chlorobenzene (68 mM spiro-OMeTAD, 150 mM tertbutylpyridine, and 25 mM lithium bis (trifluoromethanesulphonyl) imide) was spin coated at 3000 rpm for 30 s. After the spiro-OMeTAD material deposition, a silver electrode (80 nm) was evaporated under high vacuum to complete the device. 2.2. Measurements and characterizations Current-voltage (I-V) curves of the completed PeSCs were measured using a Keithley 2400 source meter under the

The morphologies of the pristine TiO2 films treated with TiCl4 at different times were analyzed using SEM and the images are presented in Fig. 1. In Fig. 1(a), the TiO2/FTO surface with a ~40 nm TiO2 deposition appeared smooth morphology, indicating that TiO2 was enough to cover over the FTO. However, it was found that the pristine TiO2 film still has pinholes with sizes of ~5e10 nm and some cracks across the surface, as shown in Fig. 1(b). After a 30 min TiCl4 treatment these pinholes and cracks were filled with small particles, yielding a smoother surface. However, after a 120 min treatment, some threadlike TiO2 are presented, which indicates the growth model of TiO2 by TiCl4 hydrolysis in Fig. 1(c). This was further demonstrated by the TiCl4 hydrolysis at different times deposited on the naked FTO substrate. Fig. S1 (a, b) compares the SEM images of FTO surfaces after being treated by low concentration TiCl4 solution for 2 h and 10 h. For a 2 h, lots of particles were distributed on the FTO surface in Fig. S1(a). In Fig. S1(b), the small TiO2 particles aggregate into flocculent TiO2 with the extended time of 10 h. This confirms that TiCl4 hydrolysis does not induce a continuous compact film TiO2 but tend to form small TiO2 particles [34]. These small TiO2 particles preferentially accumulate in the local coarse areas and repair the pristine c-TiO2, resulting in the hole- and crack-free TiO2 film. X-ray diffraction patterns (XRD) were used to investigate the structure of the TiCl4 treated TiO2. As shown in Fig. S2, a weak and broad peak at ~25 can still be distinguished, which is typically indexed to the (101) plane of anatase TiO2 (JCPDS 21-1272) even the TiO2 films are very thin [36]. The surface morphologies were further investigated by atomic force microscopy (AFM). Fig. 2(a, c) shows the surface topographical images before and after TiCl4 treatment for 30 min. For the untreated TiO2 surface, the surface roughness value of the root mean square (RMS) is 9.05 nm for the whole scanned area. After TiCl4 treatment for 0.5 h, the RMS of TiCl4-TiO2 film was slightly decreased to 6.74 nm. This contributes to the repair of holes or cracks by small TiO2 particles. These results were consistent with the SEM images in Fig. 1. We further measured at different locations across these samples using Kelvin probe force microscopy (KPFM). KPFM is a surface potential detection method that determines the contact potential difference (CPD) during scanning by compensating the electrostatic forces between the probe and the sample [37,38]. The average surface potential values of the TiCl4-TiO2 film is 776 mV, which is obviously higher than that (281 mV) of the

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Fig. 1. SEM images of the TiO2/FTO films treated with TiCl4 at different times (a) 0 min, (b) 30 min, (c) 120 min.

pristine TiO2 film, as shown in Fig. 2(b, d). This reveals a clear increase of potential variation (positive shift). The high CPD value corresponds to the low work function of film surface. Therefore, the TiCl4 treatment induces the Femi energy band shift down, facilitating the electron transfer from perovskite film to TiCl4-TiO2 layer. It has been found that the hydrophobic has significantly effect on the formation of perovskite thin films [39]. Here, we did not find the large variation of static contact angles. In Fig. S3, the static contact angles of deionized water is only ~80 on the TiCl4-TiO2 substrate, which is comparable to the value of the pristine TiO2 substrate. Fig. 3 shows the surface morphology of perovskite films deposited on these two substrates. From the Top-view SEM images of MAPbI3 films in Fig. 3(a and b), the grain size of the two perovskite layers is uniform and condense. Most of the crystals seem to cross through the whole film without few horizontal grain boundaries, as determined by the cross-sectional SEM images in

Fig. 3(c and d). Note that all diffraction peaks of these perovskite films are same and the intensities are similar as shown in Fig. S4. Therefore, we speculate that the TiO2 substrates have little influence on the perovskite quality. As expected, the UVeVis absorption spectra of these perovskite films are also similar in Fig. S5. To compare the device performance, the currentevoltage (J-V) curves of corresponding PeSCs based on the different TiO2 substrates were characterized. In Fig. 4, the J-V curves were recorded and listed in Table 1. The pristine device exhibits a poor performance with an open-circuit voltage (Voc) of 1.08 V, a short circuit current density (Jsc) of 21.4 mA cm 2, a fill factor (FF) of 0.72 and a PCE of 16.6%, which are improved to 1.11 V, 23.3 mA cm 2, 0.78 and 20.2% for the SVA device, respectively. Because the film quality is comparable, we put the emphasis on the TiO2 quality and the pervoskite/TiO2 interface. Since the device structure and perovskite quality are the same, instead of the slow migration of mobile ions,

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Fig. 2. Atomic force microscopy images of the pristine TiO2 substrate (a) and TiCl4-TiO2 substrate (c), surface potential images of the pristine TiO2 substrate (b) and TiCl4-TiO2 substrate (d).

the formation and the release of interfacial charges is the dominating factor for current-voltage hysteresis [40]. For the treated device, the better interface contact contributes to faster charge transfer and less carrier recombination [41], resulting in an excellent device performance and a reduce current hysteresis. Since the two devices have both the high quality perovskite films, we attribute the improve device performance to the interface properties, as also demonstrated by previous reports in PeSCs [42,43]. In the present work, the better device performance was achieved by further modifying the TiO2 surface with TiCl4 treatment. TiCl4-TiO2/FTO could tune the surface work function and establish better interface contact. The schematic energy level alignment between the perovskite layer and TiO2 with and without TiCl4 treatment is shown in Fig. 5 (a). The impact of TiCl4 treatment on charge transfer properties of the perovskite film was further discussed by steady-state photo-luminescence (PL) spectroscopy. Fig. 5(b) shows the PL spectra of the FTO/TiO2/perovskite and FTO/ TiCl4-TiO2/perovskite films. Compared with the FTO/TiO2/perovskite interface, substantial PL quenching effect occurred within FTO/TiCl4-TiO24/perovskite interface. This means that the charge transfer between perovskite and TiCl4-TiO2 film is more efficient, indicating the suppression of charge recombination in the perovskite film [34]. Meanwhile, the suitable energy level alignment between the perovskite absorber and TiCl4-TiO2 accelerates electron extraction from the absorber layer. To obtain a deeper insight of the role of the TiCl4 treatment in the performance of PeSCs, electrical impedance spectroscopy (EIS) was used to study the interface charge transport [44e46]. The inset of Fig. 5 (c) shows the equivalent circuit used for the fitting. The internal series resistance

(RS) is attributed to the FTO anode and counter electrode, and RCT refers to the charge transfer resistance within the bulk perovskite layer and at the perovskite/carrier transporting layer interface. RCT of the TiCl4-TiO2 device was 3.5 kU, which was lower than the values of 4.5 kU of the pristine devices. The low RCT value implies smooth carrier transport at the interface between the perovskite and the TiCl4-TiO2 layer. The PL results suggest that the low charge transfer efficiency is always existed due to the improper energy level alignment and high carrier recombination between perovskite and FTO. Hence, the better interface contact between the perovskite and the TiCl4-TiO2 layer results in increased Jsc and FF for the TiCl4 treated solar cells. In order to prove the excellent interface property of perovskite/ TiCl4-TiO2, no HTL sprio-OMeAD solar cells with Au electrode were fabricated. In Fig. 4, an efficiency ~15% was achieved, which is comparable to the best value of the HTL-free PeSCs [47,48]. We think that the p-n junction located at TiCl4-TiO2/perovskite is strictly parallel with the substrate. This is especially crucial for planar device structure. Many researchers have demonstrated a sharp drop of potential across the TiO2 interface [49,50]. Therefore, the building of valid p-n region at the TiO2/perovskite interface is the key to facilitate the carrier separation and reduce the carrier recombination [51]. Since the capability of bipolar transporter perovskites enable to eliminate the HTL or ETL, the lack of HTL or ETL may lead to large charge carrier recombination. In these devices, yet whether the ETL is dispensable for acceptable stabilized power output of PeSCs is still in debate. In our opinion, the simplification of device structures is achievable, when the carrier recombination is suppressed. With advancement in the deposition techniques of

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Fig. 3. Top-view SEM images of the CH3NH3PbI3 films deposited on the pristine TiO2 substrate (a) and TiCl4-TiO2 substrate (b), cross-sectional SEM images of the CH3NH3PbI3 films deposited on the pristine TiO2 substrate (c) and TiCl4-TiO2 substrate (d).

Fig. 4. (a) J-V curves of the perovskite solar cells based on the pristine TiO2 layer, TiCl4-TiO2 layer and without HTL, (b) IPCE spectra of the corresponding devices.

Table 1 Photovoltaic parameters for the PeSCs with different substrates derived from the J-V curves. Substrates

Jsc (mAcm

Pristine TiO2 layer TiCl4-TiO2 layer No HTL

21.4 23.3 20.8

2

)

Voc (V)

FF

PCE (%)

1.08 1.11 1.04

0.72 0.78 0.68

16.6 20.2 14.7

perovskites, the employment of cheaper functional layers and the engineering of interfaces [52], the commercialization of HTL-free

PeSCs may eventually come into reality. 4. Conclusion In summary, the compact TiO2 surface modified by TiCl4 solution to achieve the efficient electron-selective contacts for efficient PeSCs was realized. Small TiO2 particles tend to fill within the surface crack and depress the surface work function of the compact TiO2, which promises the enhanced photovoltaic performance of PeSCs with efficiencies over 20%. PL and EIS measurements confirm that the TiCl4 treatment improves the energy alignment between

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Fig. 5. (a) Schematic energy level alignment between the perovskite layer and TiO2 with and without TiCl4 treatment, (b) steady-state PL spectroscopy of the CH3NH3PbI3 films deposited on the TiCl4-TiO2/FTO and pristine TiO2/FTO substrates, (c) electrical impedance spectroscopy (EIS) of the PeSCs based on two different substrates.

the TiO2/perovskite interface, facilitating charge extraction and suppressing charge recombination. Such excellent interface contact also enables to realize the HTL-free PeSCs with high efficiency. Our results imply that elaborate interface engineering provides a promising approach to obtain high performance PeSCs. Acknowledgements This work was supported by the Natural Science Foundation of Zhejiang Province (Grant No. LY18F040003), the National Science Foundation of China (Grant No. 11304170), the Foundation of Zhejiang Educational Commission (Grant No. Y201737090), and the Natural Science Foundation of Ningbo City (Grant No. 2017A610018). The author Z. Hu would like to thank the sponsored by K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.02.027. References [1] O. Malinkiewicz, A. Yella, Y.H. Lee, G.M. Espallargas, M. Graetzel, M.K. Nazeeruddin, H.J. Bolink, Perovskite solar cells employing organic charge transport layers, Nat. Photon. 8 (2014) 128e132. [2] J.A. Christians, P. Schulz, J.S. Tinkham, T.H. Schloemer, S.P. Harvey, B.J. Tremolet de Villers, A. Sellinger, J.J. Berry, J.M. Luther, Tailored interfaces of unencapsulated perovskite solar cells for >1,000 hour operational stability, Nat. Energy 3 (2018) 68e74. [3] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gr€ atzel, S. Mhaisalkar, T.C. Sum, Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3, Science 342 (2013) 344e347. [4] T.J. Jacobsson, J.P. Correa-Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gratzel, A. Hagfeldt, Exploration of the compositional space for mixed lead halogen perovskite for high efficiency solar cells, Energy Environ. Sci. 9 (2016) 1706e1724. [5] W.S. Yang, B.W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S. Il Seok, Iodide management in formamidinium-leadhalideebased perovskite layers for efficient solar cells, Science 356 (2017) 1376e1379. [6] W.Q. Wu, Q. Wang, Y. Fang, Y. Shao, S. Tang, Y. Deng, H. Lu, Y. Liu, T. Li, Z. Yang, A. Gruverman, J. Huang, Molecular doping enabled scalable blading of efficient hole-transport-layer-free perovskite solar cells, Nat. Commun. 9 (2018) 1625e1632. [7] N.J. Jeon, H. Na, E.H. Jung, T.Y. Yang, Y.G. Lee, G. Kim, H.W. Shin, S.I. Seok, J. Lee, and J, Seo, A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells, Nat. Energy https://doi.org/10.1038/ s41560-018-0200-6. [8] Z. Li, M. Yang, J.S. Park, S.H. Wei, J.J. Berry, K. Zhu, Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide solid-state alloys, Chem. Mater. 28 (2016) 284e292.

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