Dalton Transactions View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. PERSPECTIVE Cite this: DOI: 10.1039/c7dt02421e View Journal A chemical approach to perovskite solar cells: control of electron-transporting mesoporous TiO2 and utilization of nanocarbon materials Tomokazu Umeyama *a and Hiroshi Imahori *a,b Over the past several years, organometal halide perovskite solar cells (PSCs) have attracted considerable interest from the scientiﬁc research community because of their potential as promising photovoltaic devices for use in renewable energy production. To date, high power conversion eﬃciencies (PCEs) of more than 20% have been primarily achieved with mesoscopic-structured PSCs, where a mesoporous TiO2 (mTiO2) layer is incorporated as an electron-transporting mesoporous scaﬀold into the perovskite crystal, in addition to a compact TiO2 (cTiO2) as an electron-transporting layer (ETL). In this Perspective, we ﬁrst summarize recent research on the preparation strategies of the mTiO2 layer with a high electron transport capability by facile sol–gel methods instead of the conventional nanoparticle approach. The importance of the control of the pore size and grain boundaries of the mTiO2 in achieving high PCEs for PSCs is discussed. In addition, an alternative method to improve the electron transport in the mTiO2 layer Received 5th July 2017, Accepted 11th October 2017 via the incorporation of highly conductive nanocarbon materials, such as two-dimensional (2D) graphene DOI: 10.1039/c7dt02421e and one-dimensional (1D) carbon nanotubes, is also summarized. Finally, we highlight the utilization of zero-dimensional (0D) nanocarbon, i.e., fullerenes, as an n-type semiconducting material in mesostruc- rsc.li/dalton ture-free planar PSCs, which avoids high-temperature sintering during the fabrication of an ETL. 1. a Introduction Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail: firstname.lastname@example.org, email@example.com b Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Photovoltaic devices directly convert unlimited solar power into electricity and so are regarded as a promising means to provide a sustainable source of electric power. Since the pioneering work on photovoltaic devices based on organic– Tomokazu Umeyama was born in 1976 and studied chemistry at Kyoto University. He received his BS (1999), MS (2001), and PhD (2004) in polymer chemistry under the guidance of Prof. Y. Chujo. He was also a fellow of the Japan Society for the Promotion of Science (JSPS) in 2003–2004. He moved to the Department of Molecular Engineering in the same institute as an Assistant Professor Tomokazu Umeyama (2004–2013) and is presently an Associate Professor (2013–to date) in the group of Prof. Imahori. His current interests involve development of photofunctional organic and nanocarbon materials. Hiroshi Imahori completed his doctorate in organic chemistry at Kyoto University. In 1992, he became an Assistant Professor, ISIR, Osaka University and then moved to the Graduate School of Engineering, Osaka University, as an Associate Professor. Since 2002, he has been a Professor of Chemistry, Graduate School of Engineering, Kyoto University. He has received the JSPS Prize (2006), CSJ Award for Creative Hiroshi Imahori Work (2006), Osaka Science Prize (2007), NISTEP Researcher Award (2007), and the ECS Fellow (2016). His current interests involve artificial photosynthesis, organic solar cells, and organic functional materials. This journal is © The Royal Society of Chemistry 2017 Dalton Trans. View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Perspective inorganic halide perovskites (CH3NH3PbX3, X = I, Br, or Cl) reported by Miyasaka and coworkers in 2009,1 perovskite solar cells (PSCs) have stimulated worldwide attention as a nextgeneration solar cell because of their multiple advantages over inorganic-based cells, including light weight, low cost, and ease of fabrication.2–4 The increasing attention on PSCs is also due to unprecedented rapid progress in improving their power conversion eﬃciencies (PCEs). Within less than 7 years, PCEs have been increased from 3.8%1 to 22%.5–11 In PSCs, perovskite layers are typically sandwiched between two electrodes, with charge collection facilitated by respective intermediate layers, one being an electron-transporting layer (ETL), which transports electrons but blocks holes, while the other is a holetransporting layer (HTL), which transfers holes but blocks electrons. Planar PSCs can be divided into n–i–p (conventional) and p–i–n (inverted) structures, in which the ETL and HTL are respectively adjacent to bottom transparent conducting electrodes such as fluorine-doped tin oxide (FTO) and indium-tin oxide (ITO) (Fig. 1a and b). Conversion of light energy to electricity generally involves the following fundamental steps: (i) absorption of incident photons by the perovskite, (ii) rapid charge dissociation to form free carriers,12 (iii) electron and hole diﬀusion in the perovskite toward the ETL and HTL, (iv) their diﬀusion in the ETL and HTL toward the respective electrodes, and (v) charge collection at the electrodes. Ideal materials for the ETL and HTL require high charge mobilities and suitable energy levels of the conduction and valence bands (CB and VB) that are wellmatched to those of the perovskite to facilitate charge injection and reduce energy loss. Compact structures that are free from defects and pinholes are also essential for the ETL and HTL to suppress charge recombination (CR). Fig. 1 Schematic illustrations of typical (a) planar n–i–p (conventional), (b) planar p–i–n (inverted), and (c) mesoscopic structures for PSCs. ETL: electron-transporting layer, HTL: hole-transporting layer, ETMS: electron-transporting mesoporous scaﬀold. Dalton Trans. Dalton Transactions The diﬀusion length for holes is longer than that for electrons in a typical perovskite material, CH3NH3PbI3.13 To compensate for the shorter electron diﬀusion length, an electrontransporting mesoporous scaﬀold (ETMS) for the perovskite crystalline is often employed (Fig. 1c). Mesoporous TiO2 (mTiO2) is widely employed as the ETMS in combination with compact TiO2 (cTiO2) as an ETL, where electrons are transported not only by the perovskite but also through the mTiO2 to the FTO/cTiO2 electrode, leading to better charge collection.14 Crystalline TiO2 has a suitable CB edge position to extract a photogenerated electron in a perovskite material, a long electron lifetime, and a high transparency.15,16 Although other metal oxide semiconductors including SnO2 and ZnO with relatively high electron mobilities have also been investigated as materials for use as an ETL and ETMS and have achieved high PCE values,17–25 most of the PSCs with PCEs of more than 20% have utilized compact and mesoscopic TiO2 materials so far.10,26–29 Spin-coating a paste containing TiO2 nanoparticles with sizes of 20–50 nm onto the cTiO2 layer followed by subsequent sintering is a typical procedure to prepare a uniform mTiO2 layer with a three-dimensional (3D) mesoporous network. However, the preparation of the TiO2 nanoparticle paste is time-consuming and tedious, thereby impeding the advantages of cost-eﬀective PSCs. More importantly, a large number of grain boundaries in the mesoporous network of nanoparticles lead to rapid CR. The rather homogeneous network of nanoparticles also involves random electron transit paths and thus limits the net electron transport rate, deteriorating the device performance. It should be emphasized that the uniform and complete infiltration of the perovskite7 into a 3D mesoporous structure is desirable for the formation of a highly crystalline perovskite structure exhibiting excellent device performance.30–32 The morphology, pore size, and electron transport ability of mTiO2 also play important roles in determining the device performance of mesoscopic PSCs. TiO2 nanorods and nanowires with a one-dimensional (1D) alignment have been utilized as an ETMS to improve charge transport properties and suppress CR in PSCs,33–37 but device performance has been generally inferior to those using TiO2 nanoparticles, as a consequence of the loss of a large surface area and incomplete formation of perovskite crystals.38 In addition, poor adhesion of nanowires to the electrode often causes a problem for device fabrication, and a good electrical contact between nanowires and electrodes should be carefully addressed.39–45 As a result, some recent research has focused on developing facile strategies to fabricate an ETMS layer with superior charge transport properties and controlled mesoscopic structures for the PSC applications. In this Perspective, focusing on our work46–48 and related studies, we highlight recent strategies for the facile preparation of high-performance electron-transporting mTiO2 layers. First, polymer material-assisted sol–gel methods are systematically introduced. The importance of controlling pore size (Fig. 2a) and reducing grain boundaries (Fig. 2b) in the mTiO2 layer by judicious selection of the employed polymer materials for high-performance PSCs is described in detail. In response to This journal is © The Royal Society of Chemistry 2017 View Article Online Dalton Transactions Perspective Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Fig. 4 Schematic illustration of a planar n–i–p PSC device with a fullerene-based ETL. However, such values frequently depend in a large part on device fabrication techniques and conditions that are oﬀ-topic for this manuscript, and therefore direct comparisons of the PCE values from diﬀerent reports are not scientifically meaningful in many cases. Therefore, we emphasise the comparisons of PCE values from reference devices described in each paper. Fig. 2 (a) Eﬀect of pore size in mTiO2 on perovskite formation. (b) Eﬀect of TiO2 nanoparticle boundaries on the electron transportation ability. 2. Polymer-assisted structure control of mTiO2 prepared by sol–gel reactions 2.1. the explosive growth of PSC research, several excellent reviews discussing the design of compact and mesoporous electron-transporting materials in PSCs have recently appeared,15,16,49–52 but a comprehensive review on this topic has not been published. Then, we summarize the incorporation of nanocarbon materials such as two-dimensional (2D) graphene and 1D carbon nanotubes (CNTs), as a method to improve electron transport in a mTiO2 layer (Fig. 3). The highly conductive nature and unique structure of these nanocarbon materials can enhance electron transport in mTiO2 by bridging the TiO2 nanoparticle boundaries, as has been observed in organic–inorganic hybrids with nanocarbon.53–56 Furthermore, we also focus on the utilization of zero-dimensional (0D) nanocarbon, i.e., fullerenes, as an n-type semiconducting material in mesostructure-free planar n–i–p PSCs, to explore the feasibility of relatively low PCE, but low-cost and flexible PSC devices with low hysteresis that can be processed at low temperature (Fig. 4). Finally, solubility control of fullerene molecules for stacked-layer device fabrication is presented in detail. There are significant diﬀerences in the PCE values of PSC devices with respect to each report in this Perspective. Fig. 3 Smooth electron transport by graphene in an mTiO2 layer. This journal is © The Royal Society of Chemistry 2017 Block copolymer template To fabricate a mTiO2 film with a high specific surface area by the standard nanoparticle method, particle size should be reduced. In such a case, the electron transport is undesirably slowed down owing to the increase in the particle boundaries (Fig. 2b). In addition, the pore size in mTiO2 films would become small, preventing the infiltration of the perovskite into them. Li and coworkers utilized a mesoporous film made from a block copolymer, polystyrene-block-poly(2-vinyl pyridine), as a template for producing a boundary-less mTiO2 layer by combining atomic layer deposition (ALD) and calcination.57 The mesoscopic structure and thickness of the mTiO2 layer were tunable by changing the conditions of film formation of the block copolymer. The PSC device based on the optimized mTiO2 layer with a configuration of FTO/mTiO2CH3NH3PbI3−xClx/P3HT/Ag (P3HT; poly(3-hexylthiophene)) showed a higher PCE (12.5%) than a planar PSC device without the mTiO2 layer (9.8%). However, this method requires the use of a costly ALD system as well as high-temperature calcination, and so other cost-eﬀective techniques are desirable. A simple approach to obtain boundary-less mesoporous structures of metal oxides is based on sol–gel reactions assisted by self-assembling amphiphilic block copolymers.58–63 Metal oxide precursors are usually embedded in a hydrophilic domain of a phase-separated block copolymer, and then sintered to remove the organic materials and form the mesoporous metal oxides, reflecting the phase separation architecture. The normal nanoparticle approach yields mTiO2 with an “inverse mesospace”, but such self-assembly methods generally provide a “mesospace” structure (Fig. 5a). As sacrificial templates, poly(alkylene oxide)-based block copolymers, e.g., poly(ethylene glycol)-block-poly( propylene glycol)-block-poly Dalton Trans. View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Perspective Dalton Transactions diameter could be tuned by varying the weight ratio of TMB/ F127. On the other hand, the TiO2 film prepared in the same manner without TMB exhibited high-density crystalline TiO2 nanopillars surrounded by the inverse mesospace,64,65 indicating that the presence of the swelling agent played a pivotal role in directing the mesophase of the TiO2 films as well as in increasing the pore size. The mTiO2 layers obtained by the sol– gel reaction were used as the ETMS in PSC devices with a configuration of FTO/cTiO2/mTiO2-CH3NH3Pb(I0.9Br0.1)3/PTAA/Au (PTAA; poly(triarylamine)).65 The device based on the mTiO2 with an average pore size of 15 nm showed a higher PCE of 12.8% (short-circuit current density ( JSC) = 18.8 mA cm−2, open-circuit voltage (VOC) = 1.04 V, and fill factor (FF) = 0.660) than that with the average pore size of 10 nm (PCE = 11.7%, JSC = 18.6 mA cm−2, VOC = 1.00 V, and FF = 0.626). The enlargement of the mesospace by TMB exerted a positive eﬀect on device performance, which may be attributed to the improved infiltration of the perovskite formed throughout the mTiO2 layer (Fig. 2a). However, the PCE was still comparable to that of the device based on mTiO2 with an average pore size of 15 nm prepared by the conventional nanoparticle method (PCE = 12.7%, JSC = 19.6 mA cm−2, VOC = 0.987 V, and FF = 0.658). This result indicates that further optimization of sol– gel reaction conditions using block copolymer templates to make suitable pore sizes is necessary to take full advantage of the mesospace-structured mTiO2. Fig. 5 (a) Structures of inverse mesospace and mesospace in mesoporous ﬁlms. (b) Preparation of an mTiO2 layer with a mesospace structure by an F127-assisted sol–gel reaction with a swelling agent TMB. (c) Structure of amphiphilic graft copolymer, PVC-g-POEM. (ethylene glycol) (named Pluronic P123, F127 etc., depending on the molecular weight and composition ratio), are often used to define the phase separation structure.58–64 mTiO2 films prepared with such poly(alkylene oxide)-based block copolymers typically have seamless mesospace structures, but their pore sizes are generally lower than 10 nm, which would inhibit the desirable infiltration of the perovskite into the mTiO2 films. To overcome the pore size limitation of mesospace TiO2 films, Seok et al. utilized a hydrophobic solvent, 1,3,5-trimethylbenzene (TMB), as a swelling agent in combination with a sacrificial block copolymer template, F127.65 A mixed solution of F127 and TMB in ethanol was stirred to form template micelles with expanded sizes, and then a titania precursor (Ti(O-Bu)4 and HCl) for the sol–gel reaction was added and stirred for 24 h (Fig. 5b). TMB is miscible with the hydrophobic part of the surfactant micelle and is therefore able to increase the size of the hydrophobic core. After spin-coating onto a cTiO2 layer on a FTO substrate (denoted as FTO/cTiO2), the as-prepared thin film was aged for 3 days at room temperature and then sintered (Fig. 5b). The TiO2 film obtained possessed well-ordered body-centered cubic mesospace with diameters of 10–15 nm throughout the entire film thickness. The Dalton Trans. 2.2. Graft copolymer template Amphiphilic graft copolymers, an alternative to block copolymers, are also available as a template for the mTiO2 structures. Graft copolymers are more attractive than block copolymers due to their low cost and ease of synthesis.66 A mTiO2 layer with a uniform and interconnected mesospace structure was prepared by a sol–gel process with Ti(O-iPr)4 and a graft copolymer, PVC-g-POEM. The copolymer consisted of hydrophilic poly(oxyethylene methacrylate) (POEM) side chains that could interact with inorganic TiO2 precursors and a hydrophobic poly(vinyl chloride) (PVC) backbone that could produce a mesopore upon calcination (Fig. 5c).66,67 In contrast to the block-copolymer-based sol–gel reaction, the pore size was increased up to 70 nm, the pore becoming larger with an increase in the ratio of the PVC hydrophobic domain in the copolymer composition. Moreover, the thickness of the mesoporous layer was adjusted to be 300–800 nm by varying the concentration of the solution. The PSC device with a configuration of FTO/cTiO2/mTiO2-CH3NH3PbI3/spiro-OMeTAD/Ag (spiro-OMeTAD; 2,2′,7,7′-tetrakis(N,N-bis(p-methoxyphenyl) amino)-9,9′-spirobifluorene) exhibited the highest PCE of 11.9% ( JSC = 20.21 mA cm−2, VOC = 0.982 V, and FF = 0.599) when the pore size and the thickness of the mTiO2 were set to be 70 nm and 300 nm, respectively.67 The pore size also aﬀected the photovoltaic properties, with both the JSC and VOC values increasing when the pore size of the mTiO2 increased from 30 to 70 nm, probably because of the more eﬃcient formation of large and defect-free perovskite crystals in the pore as the pore size increased (Fig. 2a). Additionally, its photovol- This journal is © The Royal Society of Chemistry 2017 View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Dalton Transactions taic performance was superior to that of the PSC device based on the mTiO2 prepared by the conventional nanoparticle ( particle size: ca. 50 nm) method (PCE = 8.39%, JSC = 16.54 mA cm−2, VOC = 0.822 V, and FF = 0.617). Unfortunately, the electron transport abilities of the mTiO2 layers prepared by the graft copolymer-assisted sol–gel reaction and the nanoparticle method in this study were not compared. Nevertheless, these results demonstrate the potential of the graft copolymer-templated mTiO2 nanostructures as an ETMS to fabricate costeﬀective and highly eﬃcient mesoscopic PSCs. 2.3. Polymer sphere template Amphiphilic block and graft copolymers can be regarded as “soft templates” that have flexible structures, but they cause self-assembly under specific conditions to form nanostructures through hydrophilic and hydrophobic interactions. H. G. Yang and coworkers have used “hard templated” polymer materials, such as polystyrene (PS) spheres, to direct the porous structures into mTiO2 scaﬀolds.68 As illustrated in Fig. 6a, two kinds of titanium precursor solutions, aqueous Ti(SO4)2 solution and Ti(SO4)2 in PS (diameter: ∼100 nm) emulsion, were spin-coated on an FTO substrate in this order. After sintering and rinsing to remove the templates, a uniform inverse opal-like TiO2 (ioTiO2) film was obtained. The diameter of the pore was ∼100 nm, reflecting well the sizes of the PS sphere templates. The pores were connected to each other by the TiO2 wall with a thickness of 6 nm. Interestingly, the FTO/cTiO2/ioTiO2 substrate showed an excellent transmittance of visible light in comparison with the bare FTO, while the FTO/cTiO2/mTiO2 substrate that was prepared Perspective with P25 (TiO2 nanoparticles with diameters of 20–30 nm) exhibited lower transmittance than the bare FTO in the wavelength region from 300 to 600 nm.68 This antireflection property of the ioTiO2 film enabled the incident light to arrive at the perovskite layer more eﬃciently. The best performance PSC with a configuration of FTO/cTiO2/ioTiO2-CH3NH3PbI3/ spiro-OMeTAD/Ag demonstrated a PCE value of 13% ( JSC = 21.93 mA cm−2, VOC = 0.973 V, and FF = 0.61). These JSC and PCE values are significantly higher than those of the device with the conventional P25 mesoporous layer ( JSC = 19.90 mA cm−2 and PCE = 11%) as a consequence of the excellent light manipulation ability of the ioTiO2 layer. Therefore, the hard template methodology could pave the way for introducing photonic structures into a TiO2 scaﬀold for perovskite crystals for use in high-performance and low-cost mesoscopic PSCs. S. Yang et al. have investigated the size eﬀect of PS spheres on PSC device performance with an HTL-free structure of FTO/ cTiO2/ioTiO2-CH3NH3PbI3/carbon paste.69 The ioTiO2 layers with pore diameters of 160 nm, 200 nm, and 470 nm and wall thicknesses of 25–50 nm were prepared by a PS-sphere templated sol–gel reaction. The antireflection property was most enhanced in the ioTiO2 layer with a 200 nm pore, and therefore the PCE (12.02%) and JSC (22.67 mA cm−2) of the device based on the ioTiO2 layer with the 200 nm pores were higher than those of the others. The authors also demonstrated by photoluminescence decay and electrical impedance spectroscopy (EIS) measurements that the charge extraction and charge transport capabilities of the ioTiO2-based PSC devices were superior to those of the planar n–i–p structure-based ones. 2.4. Fig. 6 Preparation of (a) an ioTiO2 layer with a mesospace structure by a PS sphere-assisted sol–gel reaction and (b) an mTiO2 layer with an inverse mesospace structure by a PMMA-assisted sol–gel reaction. This journal is © The Royal Society of Chemistry 2017 Mesoscopic phase separation We have recently established a methodology to obtain a mTiO2 layer by a facile sol–gel technique assisted by a general copolymer, poly(methyl methacrylate) (PMMA).46 In contrast to the amphiphilic block and graft copolymers, PMMA does not form nanometer-sized structures on its own. Nevertheless, mesoscopic phase separation was caused during the sol–gel reaction of titanium precursors in the presence of PMMA. For instance, by spin-coating a precursor solution containing 3.1% Ti(OiPr)4, 1.6% TiCl4, and 1.0% PMMA in chloroform onto an FTO/ cTiO2 substrate and with subsequent calcination, a TiO2 film with a crack-free inverse mesoscopic structure was obtained, where nanoparticles with a size of ∼30 nm were interconnected seamlessly (Fig. 6b). The pores were suﬃciently large and the TiO2 domains were well-connected, seamless structures in comparison with the mTiO2 film made by using the conventional TiO2 nanoparticle (diameter: ∼20 nm) paste. Furthermore, control of the mesoporous structure of TiO2 was attainable by changing the loaded PMMA amount.46 The higher loading of up to 4.0% PMMA rendered the TiO2 particle sizes smaller (diameter: ∼20 nm) and the grain boundary more distinct. This point is highly beneficial because changing the loading amount of PMMA is much easier than varying the structures and component ratios of block and graft copolymers. Conversely, the same sol–gel method with 1% PS instead Dalton Trans. View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Perspective of PMMA gave a compact TiO2 film without forming the porous structures.46 This result corroborates the importance of additive polymer structures in assisting mesopore formation in TiO2. PMMA has polar ester groups on the side chains, whereas PS consists only of nonpolar hydrocarbons. The high polarity of the esters may improve their miscibility with titanium reagents, inducing meso-sized phase separation. We constructed and evaluated a PSC device based on mTiO2 prepared by the sol–gel reaction with 1% PMMA with a configuration of FTO/cTiO2/mTiO2-CH3NH3PbI3/spiroOMeTAD/Au (PCE = 14.0%, JSC = 18.7 mA cm−2, VOC = 0.979 V, and FF = 0.763). This device outperformed the reference devices with the mTiO2 prepared by the conventional nanoparticle (diameter: ∼20 nm) approach (PCE = 13.1%, JSC = 18.2 mA cm−2, VOC = 0.961 V, and FF = 0.749).46 The electron transport behavior of the PSC devices based on the mTiO2 layers prepared by the PMMA-assisted sol–gel method and the nanoparticle approach was examined by measuring transient photocurrent in a short circuit, revealing that the electron diﬀusion coeﬃcient (De) of the former was much higher than the latter. In mesoscopic PSCs, the generated electrons travel across the mTiO2 layers before reaching the FTO/cTiO2 electrode.14 Fast electron transport in the mTiO2 layer in the former device is attributed to the lack of an obvious boundary between the TiO2 particles (Fig. 2b). This enhanced De value resulted in the superior photovoltaic parameters in the former. Consistently, the PSC devices based on mTiO2 with a higher PMMA loading (e.g., the device based on mTiO2 prepared by the 4% PMMA-assisted sol–gel method; PCE = 8.59%, JSC = 13.4 mA cm−2, VOC = 0.900 V, and FF = 0.712) showed lower device performance, reflecting the smaller sizes and more distinct grain boundary of the TiO2 particles (Fig. 2b). These results exemplify the great potential of the facile sol–gel technique with a simple polymer additive to provide mesostructure-tuned TiO2 layers with no clear boundaries and thereby contribute significantly toward the development of low-cost mesoscopic PSCs. Dalton Transactions eﬃciencies and thereby PCEs of dye-sensitized solar cells.76–79 In 2014, Snaith and coworkers reported the utilization of graphene in PSCs based on cTiO2 and mesoporous Al2O3 layers, suggesting that graphene incorporation into the cTiO2 layer lowered interfacial resistance between the cTiO2 layer and the FTO, resulting in the promotion of electron transport.80 Meanwhile, Yang et al. inserted an ultrathin layer of graphene quantum dots (GQDs) between CH3NH3PbI3 and mTiO2.81 GQDs have a small size of only several nanometers with special quantum-confinement eﬀects and edge eﬀects, making them distinct from conventional large graphene.82 They found that the GQDs facilitated the electron injection from CH3NH3PbI3 to mTiO2 rather than improving the electron transportation, boosting the JSC and PCE values of the PSC devices from 15.43 mA cm−2 and 8.81% to 17.06 mA cm−2 and 10.2%, respectively.81 The first example of incorporating large graphene materials into mTiO2 and cTiO2 layers was reported by our group in 2015 (Fig. 7a).47 Compact TiO2 was deposited on an FTO substrate by spin-coating an ethanol solution of titanium diisopropoxide bis(acetylacetonate) with 0.15 wt% GO under an ambient atmosphere and with subsequent thermal treatment at 500 °C under a nitrogen flow to cause simultaneous transformation of amorphous TiO2 to crystal, and facile GO reduction to RGO, providing FTO/cTiO2(RGO) (cTiO2(RGO); an RGO-embedded cTiO2 layer). The RGO-incorporated mTiO2 (mTiO2(RGO)) was then fabricated on the cTiO2(RGO) layer by spin-coating a TiO2 nanoparticle paste with 0.015 wt% GO and after subsequent calcination at 500 °C under nitrogen to form FTO/cTiO2(RGO)/ mTiO2(RGO). Both the cTiO2 and mTiO2 layers were crack-free even with the inclusion of RGO. We fabricated and evaluated the PSCs with a configuration of FTO/cTiO2(RGO)/ mTiO2(RGO)-CH3NH3PbI3/spiro-OMeTAD/Au, establishing the cascading energetic sequences (Fig. 7b).47 The cross-sectional view of the device obtained using scanning electron microscopy (SEM) revealed the uniformity of the cTiO2(RGO) 3. Nanocarbon embedment into mTiO2 3.1. Graphene embedment Graphene, a 2D nanocarbon material, possesses remarkable properties including excellent electron mobility, good durability, and high transparency.70 Reduced graphene oxide (RGO), which is obtained by the intensive oxidation of graphite to yield graphene oxide (GO) and the subsequent reduction of GO to remove the oxygen-containing functional groups, has some defects, but is functionally similar to graphene and easier to be processed.71,72 Therefore, as a strategy to facilitate electron transport in TiO2 materials, graphene and RGO have been widely embedded to improve the performance of TiO2based devices such as photocatalysts and solar cells.73–80 Several studies have demonstrated that the incorporation of RGO into nanostructured TiO2 can boost charge collection Dalton Trans. Fig. 7 (a) Schematic illustration and (b) energy diagram of an RGOembedded PSC device with a mesoscopic structure. This journal is © The Royal Society of Chemistry 2017 View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Dalton Transactions and mTiO2(RGO)-CH3NH3PbI3 layers, indicating that RGO was seamlessly and homogeneously integrated into the composite. Averaged JSC, VOC, and FF values of the RGO-embedded PSC device were 16.5 mA cm−2, 0.835 V, and 0.674, respectively, yielding a PCE value of 9.29%. The photovoltaic parameters were higher than those of the device with the RGO-embedded cTiO2 and RGO-free mTiO2 layers (FTO/cTiO2(RGO)/mTiO2CH3NH3PbI3/spiro-OMeTAD/Au; PCE = 8.14%, JSC = 16.1 mA cm−2, VOC = 0.799 V, and FF = 0.633) and RGO-free TiO2 layers (FTO/cTiO2/mTiO2-CH3NH3PbI3/spiro-OMeTAD/Au; PCE = 6.61%, JSC = 14.9 mA cm−2, VOC = 0.761 V, and FF = 0.583). These results unambiguously corroborate the positive eﬀect of RGO-incorporation into the mTiO2 and cTiO2 layers on photovoltaic properties. EIS measurements revealed the remarkably lower series resistance (Rs) of the PSC with RGO-embedded TiO2 layers than that with RGO-free TiO2 layers.47 The addition of RGO facilitated electron transport in the TiO2 layers by a bridging eﬀect and reduced the contact resistance at the perovskite/mTiO2 and cTiO2/FTO interfaces (Fig. 3). Furthermore, most of the electron density is on RGO, and the resultant TiO2 in contact with the perovskite accelerated the charge injection from the perovskite to the TiO2 layers. Following our work, several groups reported the utilization of RGO– or graphene–mTiO2 nanocomposites as an ETMS in PSCs.83–87 Jung and coworkers fabricated a mTiO2(RGO) layer on the FTO/cTiO2 substrate by spin-coating a slurry containing TiO2 nanoparticles and RGO following sintering.83 Note here that GO was chemically reduced by hydrazine before mixing with the TiO2 nanoparticles, whereas the crystallization of TiO2 and the reduction of GO occurred simultaneously during sintering in our method.47 The optimized PSC device with a configuration of FTO/cTiO2/mTiO2(RGO)-CH3NH3PbI3/spiroOMeTAD/Ag achieved a PCE of 15% ( JSC = 22.0 mA cm−2, VOC = 0.93 V, and FF = 0.707), which was higher than the RGO-free mesoscopic PSC device (PCE = 12%, JSC = 19.6 mA cm−2, VOC = 0.86 V, and FF = 0.668). An EIS analysis was conducted, revealing that the mTiO2(RGO) film reduced the internal resistance and enhanced the charge collection eﬃciency relative to RGOfree mTiO2 (Fig. 3). Despite the absence of RGO in the cTiO2 layer, the electron flow from the RGO in the mTiO2(RGO) to the cTiO2 remained suﬃciently eﬃcient. On the one hand, Nazeeruddin et al. attained an excellent PCE value of 20% ( JSC = 21.98 mA cm−2, VOC = 1.11 V, and FF = 0.80) for a PSC device with a configuration of FTO/cTiO2/mTiO2(RGO)(FAPbI3)0.85(CH3NH3PbI3)0.15/spiro-OMeTAD/Au (FA; formamidinium), where the mTiO2 surface was treated with bistrifluoromethanesulfonimidate (Li-TFSI) to facilitate interfacial electron injection.84 They also incorporated RGO into the perovskite and spiro-OMeTAD layers, but unwanted shunt resistance pathways were generated, deteriorating the overall device performance. Carlo and coworkers embedded graphene prepared by ultrasonic-assisted liquid-phase exfoliation of graphite into the mTiO2 layer.86 The maximum PCE reached up to 16.3% ( JSC = 22.95 mA cm−2, VOC = 1.03 V, and FF = 0.689) in a PSC with a configuration of FTO/cTiO2/mTiO2(graphene)CH3NH3PbI3/spiro-OMeTAD/Au. More importantly, the This journal is © The Royal Society of Chemistry 2017 Perspective addition of graphene enhanced the stability, with the graphene-embedded PSC device retaining more than 88% of its initial PCE after 16 h of prolonged 1-sun illumination at the maximum power point, while the PCE of the graphene-free PSC decreased by ∼40% under the same conditions. The results listed here suggest that the RGO- or grapheneembedded TiO2 layers can be considered as cost-eﬀective, critical components in achieving a significant improvement in the PCEs and stabilities of PSCs. 3.2. Carbon nanotube embedment CNTs, a 1D nanocarbon material, also exhibit excellent conductivity originating from their unique structure.88 As a result, the integration of CNTs into an ETMS is also expected to provide an ultrafast electron transport pathway to enhance photovoltaic performance.89–92 Recently, Shapter et al. embedded single-walled carbon nanotubes (SWNT) into mTiO2 for PSC applications for the first time by adding surfactant-assisted SWNT aqueous dispersion into a TiO2 nanoparticle paste, spin-coating the paste onto an FTO/cTiO2 substrate, and sintering at 450 °C.93 As expected, the PSC device based on the SWNT-embedded mTiO2 (denoted as mTiO2(SWNT)) with a configuration of FTO/cTiO2/ mTiO2(SWNT)-CH3NH3PbI3/spiro-OMeTAD/Au outperformed (PCE = 16%, JSC = 21.964 mA cm−2, VOC = 1.041 V, and FF = 0.70) the PSC with SWNT-free mTiO2 (PCE = 14%, JSC = 19.485 mA cm−2, VOC = 0.988 V, and FF = 0.70). Theoretical studies showed that the interaction between SWNT and TiO2 increases the CB minimum of TiO2, leading to an improvement in the VOC value. The EIS and dark J–V measurements revealed reduced CR and low series resistance compared to a SWNT-free device. Furthermore, the SWNT-embedded PSC showed enhanced resistance to light and long-term storage stabilities, demonstrating the excellent potential of the SWNT as an additive into the mTiO2. 4. Planar n–i–p PSCs with solution processable fullerene-based ETL 4.1. Fullerene-based ETL Despite the high performance of mesoscopic PSCs based on mTiO2 and cTiO2, the preparation of highly crystalline TiO2 layers requires high-temperature sintering (>450 °C). This requirement causes a rise in production cost and precludes its application to flexible plastic substrates. Although low-temperature-processed TiO2, SnO2, and ZnO compact layers have been applied to planar n–i–p PSCs to circumvent this problem,23,25,80,94 synthesis of these materials often involves long reaction times and a solvent washing process. Moreover, a hysteresis in the J–V curve measured in the forward and reverse bias scans is more severe in the planar PSCs than in the mesoscopic PSCs.95 Compared to metal oxide ETLs, the facile, low-cost solution-based processes of soluble n-type organic semiconductors are advantageous.16,52 Phenyl-C61-butyric acid methyl ester Dalton Trans. View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Perspective (PCBM, Fig. 4) and other fullerene derivatives have been recognized as promising candidates for the n-type material in ETLs and have been widely used in planar p–i–n PSCs.96–106 Fullerene-based ETLs reduce the density of trap states and passivate the grain boundaries of the perovskite-absorbing layer, suppressing hysteresis in the J–V curves.107,108 In such planar p–i–n PSCs, however, poly(3,4-ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) is predominantly used as the HTL material on a transparent electrode, and its acidic and hygroscopic nature impairs the long-term stability and PCE of the device.109–111 Therefore, placing a fullerene-based material on the transparent electrode as an ETL in planar n–i–p PSCs is highly desirable. It is noteworthy that the attachment of solubilizing groups, as seen in PCBM, is necessary for good solution processability, but it simultaneously deteriorates the resistance to attack by N,N-dimethylformamide (DMF), which is commonly used in fabricating the perovskite layers on the ETL.2–4 Partial dissolution of the ETL would create shunting paths and thereby impair the device performance. In the following sections, recent examples of achieving orthogonal solubility to fabricate planar n–i–p PSCs with fullerene-based ETLs are introduced. 4.2. Dalton Transactions method was extended to a flexible ITO/PEN (PEN; poly(ethylene naphthalate)) substrate and achieved a PCE of 11.1%, suggesting its potential for flexible photovoltaic devices that can be processed at low temperatures. 4.3. Self-assembled monolayer (SAM) A SAM of fullerene molecules on a transparent electrode has been demonstrated to act as an eﬃcient and reliable ETL in planar n–i–p PSCs.113 First, a bare FTO surface was activated by oxygen plasma to increase the amount of hydroxyl surfaceterminated groups (Fig. 8a). Then, the substrate was dipped into a solution of N-[3-(triethoxysilyl)propyl]-2-carbomethoxy3,4-fulleropyrrolidine (Sil-C60) in anhydrous toluene to form a covalent bond between the FTO and Sil-C60 (Fig. 8a). Dry conditions and the activated FTO promoted the self-assembly and Orthogonal solvent processing Seok et al. successfully utilized PCBM as an ETL material in a planar n–i–p PSC in a configuration of FTO/PEI/PCBM/ CH3NH3PbI3/PTAA/Au (PEI; polyethyleneimine) using an orthogonal solvent process.112 PEI was used here as an interfacial material that modifies the work function of an FTO and facilitates electron extraction. For orthogonal solvent processing, robust solvent resistance of a bottom layer against the spin-coating solvent of an upper layer is necessary. The poor solubility of PEI in chlorobenzene enabled the deposition of the PCBM layer onto the FTO/PEI substrate. In addition, dimethyl sulfoxide (DMSO)/γ-butyrolactone (GBL) (3 : 7, v/v) was used instead of DMF as a spin-coating solvent to form the perovskite layer because GBL is a marginal solvent for PCBM. However, perovskite crystallization was retarded by the interaction with solvent molecules and thereby the perovskite layer prepared from the mixed solvent formed a textile-like inhomogeneous structure that did not fully cover the substrate.95,112 To quickly remove the residual solvent and make the perovskite film smoother, diethyl ether was dropped onto the substrate during the spinning process. Diethyl ether was selected as the dripping solvent because it causes no damage to the underside PEI and PCBM layers due to its low solubility. Then, a PTAA HTL was prepared by spin-coating from toluene, a poor solvent for CH3NH3PbI3, and the Au electrode was deposited by thermal evaporation. The PSC device showed a high PCE value of 15% ( JSC = 21.8 mA cm−2, VOC = 0.98 V, and FF = 0.72) in the reverse bias scan and a steady-state PCE of 14% by measuring the stabilized photocurrent held at a forward bias of 0.82 V.112 The diﬀerence in the PCE values between the reverse bias scan and the steady-state was significantly small when compared to other planar n–i–p PSCs with the cTiO2 as the ETL. Furthermore, this orthogonal solvent processing Dalton Trans. Fig. 8 Preparation of (a) FTO/Sil-C60-SAM, (b) FTO/Sil-C60-crosslink, (c) FTO/PCBCB-crosslink, and (d) FTO/C60 by post-thermal treatment of FTO/C60(9MA). This journal is © The Royal Society of Chemistry 2017 View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Dalton Transactions avoided the cross-linking of Sil-C60, resulting in a dense fullerene monolayer on the FTO (denoted as FTO/Sil-C60-SAM). As a result of the stiﬀ covalent bonding, the fullerene monolayer was tolerant to the subsequent perovskite and HTL formations by solution processes. The fabricated PSC device with a configuration of FTO/Sil-C60-SAM/(FAPbBr3)0.1(FAPbI3)0.65(CsPbI3)0.05 (CH3NH3PbI3)0.2/spiro-OMeTAD/Au attained a PCE of 15% ( JSC = 19.4 mA cm−2, VOC = 1.04 V, and FF = 0.74) in the reverse bias scan and a comparable steady-state PCE of 15%.113 In contrast to the spin-coating method, this solution deposition technique minimizes the consumption of fullerene materials and thereby reduces the overall cost of large-scale device fabrication. 4.4. Post-treatment Another strategy to achieve orthogonal solubilities for the fabrication of stacked-layer devices is a post-treatment approach.114,115 Snaith and coworkers employed two soluble fullerene-based precursor molecules to generate insolubilized films as ETLs in planar n–i–p PSCs.116 One precursor molecule is Sil-C60 that can not only form a SAM on the activated FTO (Fig. 8a), but also can cause cross-linking by a sol–gel reaction with acid-treatment (Fig. 8b). The other is phenyl-C61-butyric acid benzocyclobutene ester (PCBCB), which has a structure similar to PCBM, but possesses a benzocyclobutene group that can cause thermally induced ring-opening reactions (Fig. 8c). By using post-treatments, namely trifluoroacetic acid (TFA) vapor exposure and heating at 200 °C, Sil-C60 and PCBCB yielded insoluble films on the FTO, which can function as the ETLs. These substrates are denoted as FTO/Sil-C60-crosslink and FTO/PCBCB-crosslink, respectively (Fig. 8b and c). In contrast to the SAM method, the sol–gel reaction of Sil-C60 on the non-activated FTO yielded the robust fullerene-based film with a thickness of a few tens of nanometers. The optimized PSC devices with the architectures of both FTO/Sil-C60-crosslink/ CH3NH3PbI3/spiro-OMeTAD/Au and FTO/PCBCB-crosslink/ CH3NH3PbI3/spiro-OMeTAD/Au exhibited high PCE values of 18% ( JSC = 23.0 mA cm−2, VOC = 1.07 V, and FF = 0.73 for the former and JSC = 22.4 mA cm−2, VOC = 1.11 V, and FF = 0.73 for the latter) in the reverse bias scans and steady-state PCEs of 17% and 15%, respectively.116 These results demonstrate that highly soluble Sil-C60 and PCBCB can act as precursors for producing eﬃcient and reliable ETL in the planar n–i–p PSCs. Although the attachment of a solubilizing group on a fullerene core is necessary for the deposition of compact fullerenebased layers with controlled film thickness, pristine C60 with no bulky substituents is expected to be packed more densely to facilitate intermolecular charge transport.117–122 Indeed, planar p–i–n PSC devices (ITO/PEDOT:PSS/CH3NH3PbI3/fullerene/Ag) with a C60 ETL outperforms devices with the PCBM ETL.101 To use pristine C60 molecules as an ETL in planar n–i– p PSCs with suﬃcient solution processability, we recently applied the post-thermal-treatment methodology.48 A film of a highly soluble precursor compound, i.e., C60–9-methylanthracene adduct (C60(9MA)), was formed on an FTO substrate by spin-coating, and then the C60(9MA) film was heated at 140 °C to cause the retro-Diels–Alder reaction, yielding a This journal is © The Royal Society of Chemistry 2017 Perspective pristine C60 film with a thickness less than 10 nm (Fig. 8d). C60(9MA) possesses a superior film-forming property relative to pristine fullerene C60.123 In addition, because pristine C60 is poorly soluble in DMF, the formation of a CH3NH3PbI3 layer on the FTO/C60 substrate caused little damage to the underlying C60 film. The best-performing PSC device with a configuration of FTO/C60/CH3NH3PbI3/spiro-OMeTAD/Au showed the PCE values of 15.6% ( JSC = 21.1 mA cm−2, VOC = 0.988 V, and FF = 0.748) in the reverse bias scan and 14.5% in the forward bias scan. These values are higher than those of the planar n–i–p PSC device with the cTiO2 as the ETL (FTO/C60/ CH3NH3PbI3/spiro-OMeTAD/Au, PCE = 14.2%, JSC = 19.6 mA cm−2, VOC = 0.974 V, and FF = 0.745 in the reverse bias scan and PCE = 11.6% in the forward bias scan).48 In addition, the hysteresis behavior, i.e., the diﬀerence in the PCE values between the reverse and forward scans of the C60-based device was impressively suppressed compared to a cTiO2-based device owing to the enhanced electron-selective collection ability of the pristine fullerene. These results suggest that the post-treatment concept is of significant importance for the solutionprocessed, low-cost, and flexible PSC device fabrication where thin ETLs with high uniformity and well-controlled thickness are required. 5. Summary and outlook This Perspective has provided an overview of recent notable strategies for the fabrication of mTiO2 layers with high electron-transport properties, for applications as ETMSs in PSCs; particle boundary-less mesoporous structures by various polymer material-assisted sol–gel reactions and nanoparticlebased mesoporous structures bridged by 1D and 2D nanocarbons. A quick charge transportation in such PSC devices is of pivotal importance in maximizing their performance. Thus, these strategies have contributed to improvements of the PCEs. Furthermore, perovskite crystallization is known to be aﬀected by the pore size in ETMSs. Thus, especially in the former strategy, the control and optimization of mesoporous structures by varying the structures and amounts of templating polymers also attained an enhancement of device performances. In addition, we have also focused on recent attempts to utilize fullerene-based materials as compact ETLs in planar n– i–p PSCs. Various methodologies to enable the construction of the stacked-layer structures under low-temperature conditions were summarized. The topics in this article are not a comprehensive list of ETMSs and ETLs in PSC research, but include the exploration of new design and concepts for ETMSs and ETLs to inspire further studies on PSCs. PSC research has continued to evolve and impressively high PSC performance has been realized, allowing their commercialization in the near future. There are two options to take advantage of PSCs. One is based on hard glass substrates and high-temperature-sintering processes to enable TiO2 crystallization, aiming to achieve higher PCEs than already commercialized multicrystalline silicon solar cells. Although the high- Dalton Trans. View Article Online Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40. Perspective temperature processes raise the production cost to some extent and close the door to the utilization of flexible plastic substrates, PSCs with crystalline TiO2 materials still have the advantages of cost reduction and mass production over crystalline silicon solar cells. As discussed in this Perspective, developing new fabrication methodologies for mTiO2 with precise control over the morphology, porosity, thickness, and crystallinity will further facilitate favorable perovskite crystallization and long-range electron transport. Additional functionalities, such as antireflection properties, will also be realized in order to improve the photovoltaic performance. Therefore, the optimization of the mTiO2 structure in PSC devices will provide a path toward high PCE values close to the theoretical limit of 26%. Another option is based on plastic substrates and low-temperature processes, accomplishing the flexibility and low cost, that is inaccessible to crystalline silicon solar cells. As also shown in this Perspective, ETLs composed of organic fullerene molecules are suitable for this purpose. Recently, a SnO2 layer has been fabricated by a simple, lowtemperature, solution process.17–22 The planar PSC device with the SnO2 layer as an ETL showed excellent device performance, with a PCE of 20.7%. The eﬃcient, cost-eﬀective, and flexible characteristics22 of such PSC devices will make them economically viable for commercialization. However, there are still significant obstacles to overcome before PSC technologies warrant commercialization. Longterm stability, reproducibility, and large-scale fabrication are the most desirable characteristics in addition to the high eﬃciency and flexibility.9,124–127 The degradation mechanism of perovskites need to be investigated in detail both experimentally and theoretically. The stability of all components in the PSC devices other than the perovskite layer should also be improved.128 In order to scale up device size with high throughput and material saving, perovskite deposition should move beyond spin-coating-based methods. Nucleation and crystal growth may be significantly diﬀerent depending on the procedure used for perovskite film formation. In addition, engineering of perovskite–ETMS/ETL and ETL–electrode interfaces requires a deep understanding of relevant charge separation and collection mechanisms. The diﬀerence in the mesoscopic structure of mTiO2 and the embedding of a dopant such as a nanocarbon material into mTiO2 may also exert a significant influence on the interaction of such a perovskite– ETMS/ETL interface. Theoretical calculations based on firstprinciples density functional theory and Car–Parrinello molecular dynamics can contribute to a precise understanding of the electron injection and carrier recombination at such interface,129–131 which has been already studied intensively in dye-sensitized solar cells.132–134 This issue also overlaps with the hysteresis behavior of the PSCs. Further design and elaborated synthesis of new electron-transporting materials that allow precise control of morphology, fine-tuning of energy levels, and provide high charge mobilities with controlled surface engineering will play a core role in the market development of large-scale perovskite-based photovoltaic devices. Dalton Trans. 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