An amorphous precursor route to the conformable

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Cite this: J. Mater. Chem. A, 2016, 4, 12897

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An amorphous precursor route to the conformable oriented crystallization of CH3NH3PbBr3 in mesoporous scaffolds: toward efficient and thermally stable carbon-based perovskite solar cells† Haining Chen,a Xiaoli Zheng,a Qiang Li,b Yinglong Yang,a Shuang Xiao,a Chen Hu,a Yang Bai,a Teng Zhang,a Kam Sing Wongb and Shihe Yang*a CH3NH3PbBr3 (MAPbBr3)-perovskite solar cells (Br-PSCs) have attracted much attention due to their green-long wavelength transparency and high open-circuit voltage originating from their large bandgap (2.2 eV). However, the efficiency of carbon-based Br-PSCs without organic hole transport materials (HTM) is still low due to the inappropriate quality of MAPbBr3 deposited in a relatively thick porous scaffold. Herein, an amorphous precursor route based on a two-step sequential method is exploited to conformably and seamlessly grow MAPbBr3 in the TiO2 porous scaffold. In the first step, the amorphous Pb–Br precursor containing a large amount of DMF molecules was prepared by lowering the post-treatment temperature to 25 C, affording full pore filling and smooth surface capping. The conversion to MAPbBr3 in the second step was accelerated by the molecular exchange between DMF and MABr in IPA solution. Moreover, by solvent engineering through the addition of non-polar cyclohexane into the MABr IPA solution, the molecular exchange process was tuned in such a way to separate the nucleation and growth of MAPbBr3 crystals, leading to the preferential [001] orientation with an even surface finishing and the subsequent light absorption enhancement and trap state reduction.

Received 19th July 2016 Accepted 25th July 2016

Using MAPbBr3 films in carbon-based PSCs has boosted their efficiency to 8.09% (Voc ¼ 1.35 V), a record value for HTM-free Br-PSCs, also comparable to that of the best HTM-based Br-PSCs. Significantly,

DOI: 10.1039/c6ta06115j

non-encapsulated devices showed no efficiency decay after storage in dry air (25–30 C and 10–20% humidity) for 90 days. What is more, the efficiency was retained up to about 90% after storage for

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15 days under high heat stress (air, 80 C and 50–85% humidity).

1. Introduction The rise of hybrid organic/inorganic perovskite solar cells (PSCs) is mainly accredited to the exceptional optoelectronic properties of the materials (e.g. MAPbX3, MA ¼ CH3NH3, X ¼ Cl, Br or I), including a high absorption coefficient,1,2 high mobility,3–6 a long balanced carrier diffusion length4,5,7,8 and low exciton binding energy.9 So far, PSCs with the highest PCE were commonly fabricated from I-containing perovskites. Meanwhile, the unique properties of other perovskite materials have also attracted much attention.2,10–20 For example, the MAPbBr3 perovskite (Eg ¼ 2.2 eV), albeit not an optimal absorber for

a

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. E-mail: [email protected]

b

Department of Physics, The Hong Kong University of Science and Technology, Kowloon, Hong Kong † Electronic supplementary information (ESI) available: UV-vis spectra, XRD patterns and SEM images. See DOI: 10.1039/c6ta06115j

This journal is © The Royal Society of Chemistry 2016

single junction solar cells, could be used in the front cell of multi-junction solar cells,21–23 and could provide a rather high Voc for some special applications such as solar driven electrochemical energy storage and water splitting.22,24–27 Furthermore, MAPbBr3 is less sensitive to humidity and heat than its MAPbI3 counterparts thanks to its stable cubic phase structure.28 Until now, Br-PSCs based on the typical structure of FTO/ TiO2/MAPbBr3/hole transport material (HTM)/metal electrode have obtained a PCE of 10.4% and a Voc of 1.51 V,25 using a home-made HTM (polyindenouoren-8-triarylamine (PIF8-TAA)) prepared by Seok et al.29 and Im et al.25 When commercially available HTMs were used, both PCE and Voc were decreased to, for example, 7.3% and 1.09 V for P3HT,25 9.1% and 1.45 V for Spiro-MeOTAD,27 and 9.3% and 1.35 V for PTAA.25 Although the organic HTM-based Br-PSCs have achieved impressive PCE and Voc, the reported efficient organic HTMs were usually expensive, especially non-commercial ones, and operationally unstable, which will limit the practical application of PSCs.

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Journal of Materials Chemistry A

Fortunately, HTM-free PSCs have come to the fore and have been shown to operate efficiently as well, owing to the unique ambipolar properties of the perovskite that allows it to serve not only as a light harvester but also as a hole conductor.7,8,30–33 The rst HTM-free Br-PSCs reported by Dymshits et al. using Au as the hole extraction electrode achieved a PCE of 2.02% with a Voc of 1.35 V,24 followed by a large improvement to 6.53% with a Voc of 1.36 V achieved by depositing a MAPbBr3 monocrystalline layer on a planar substrate.34 However, these PCE values are still signicantly lower than those of the corresponding HTM-based devices. The most important reason behind this has been the inadequate quality of the MAPbBr3 lm deposited into the relatively thick TiO2 mesoporous scaffold (above 500 nm), an essential component in HTM-free PSCs for efficient charge separation.32,33,35 Although the commonly used one-step solution method has been proved successful in depositing MAPbBr3 on planar substrates,25,36 it always leads to only partial surface coverage and poor pore lling on mesoporous substrates due to the more inhomogeneous crystallization kinetics.22,37,38 On the other hand, the conventional two-step sequential solution method oen led to an incomplete conversion of the strongly bound PbBr2 to perovskite and a rough surface of the grown MAPbBr3 with a poor coverage arising from the anisotropic growth.24,39–41 Herein, we report an amorphous precursor route based on a two-step sequential solution method to considerably improve the MAPbBr3 lm quality in the TiO2 mesoporous scaffold. The amorphous precursor route is prevalent in biomineralization in highly heterogeneous biological milieus, and could be adapted to crystallization in the mesoporous TiO2 substrate for uniformly lling the pore volume and seamlessly capping the mesoporous layer, as illustrated in Fig. 1. To implement this strategy, we lowered the post-treatment temperature to get an amorphous precursor consisting of Pb–Br and a large amount of DMF molecules (DMF/Pb–Br), intended to enhance pore lling and full coverage. Then, a non-polar solvent cyclohexane

Schematic illustrating the advantages of growing a MAPbBr3 layer in a TiO2 mesoporous scaffold by the amorphous precursor route: seamless pore filling, smooth coverage and uniform crystalline orientation. Fig. 1

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(CYHEX) was added into the conventional MABr IPA solution to tune the molecular exchange process between DMF and MABr in the second step, which not only adjusts the preferential crystal orientation to the [001] direction to get an even surface but also enhances light absorption and reduces trap states. As a result, the application of the MAPbBr3 in paintable carbonbased HTM-free PSCs boosted the PCE to 8.09% (Voc ¼ 1.35 V), a record value for HTM-free Br-PSCs and among the highest PCE levels of HTM-based Br-PSCs. Signicantly, our nonencapsulated device did not show any PCE decay aer storage in dry air (25–30 C and 10–20% humidity) for 90 days. Furthermore, their PCE was retained up to about 90% aer storage under high heat stress conditions (air atmosphere, 80 C and 50–85% humidity) for 15 days.

2.

Results and discussion

2.1 Deposition of amorphous Pb–Br precursors in the porous scaffold Pb–Br precursors were deposited in the TiO2 scaffold by spin coating with 1.4 M PbBr2 in DMF. Aer spin coating, the samples were heated at different temperatures for 5 min, followed by cooling to room temperature. The inset in Fig. 2(a) is the photograph of the obtained Pb–Br precursors, showing that the precursors are more transparent at lower temperature, demonstrating the suppressed light scattering capacity (see Fig. S1†). The XRD results in Fig. 2(a) indicate that the precursors obtained at 25 C (25-Pb–Br) and 50 C (50-Pb–Br) have similar patterns and besides the peaks of FTO glass, only one obvious diffraction peak at 10.1 is observed, which could not be indexed to any compound in the PDF card or reported literature. When the temperature is increased to 70 C (70-Pb–Br), several new diffraction peaks appeared and the relatively weak peaks at around 23.6 , 35.9 and 37.7 could be indexed to those of PbBr2. But other additional peaks as well as that at 10.1 could not be indexed to any known compounds. As the temperature is further increased to 100 C (100-Pb–Br), many intense peaks, corresponding to those of PbBr2, are observed and the unknown peak at 11.8 that appears in the pattern of 70-Pb–Br is still present. To further determine the precursor composition, Fourier transform infrared (FT-IR) spectroscopy spectra and Raman spectra were measured. As shown in Fig. 2(b), the absorption peaks at 668 cm1, 110 cm1, 1248 cm1, 1375 cm1, 1429 cm1 and 1627 cm1 correspond to the in-plane bending vibration of the O]C–N group, the rocking mode of the CH3 group, asymmetric stretching of C–N, stretching of C–N, the bending umbrella mode of the CH3 group and stretching of C]O in DMF, respectively.42,43 Clearly, there is a large amount of DMF in 25-Pb–Br, 50-Pb–Br and 70-Pb–Br, while all absorption peaks show very weak intensity for 100-Pb–Br, indicating that most of the DMF has evaporated away. The Raman spectrum of 100-Pb–Br shows two strong peaks at 109 cm1 and 120 cm1, both corresponding to the B1g symmetry of PbB2 crystals,44 which weaken, broaden and slightly redshi for the low-temperature precursors (25-Pb–Br, 50-Pb–Br and 70-Pb–Br), as shown in Fig. 2(c). This phenomenon conrms the reduced bond strength and bond energy due to the interference of DMF


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Fig. 2 Effect of post-treatment temperature on the compositions and morphologies of Pb–Br precursors in the TiO2 mesoporous scaffold. (a) XRD patterns, (b) FT-IR spectra, (c) Raman spectra, (d) top-view and (e) cross-sectional SEM images.

molecules. In addition, no peak relative to the characteristic peaks of other lead-based compounds, e.g. PbO, is observed, excluding the generation of lead oxide-based compounds for explaining the additional peaks in the XRD patterns. Combining XRD, FT-IR and Raman results, it can be concluded that the 25-Pb–Br and 50-Pb–Br precursors are nearly amorphous, composed of DMF and Pb–Br (DMF/Pb–Br), perhaps with a rather small portion of DMF$Pb–Br crystallites, just like the widely reported PbI2(DMSO)x crystals,11,14,45,46 whose crystallinity tends to be improved aer the partial evaporation of DMF at 70 C. Post-treatment at 100 C would evaporate most of the DMF molecules to generate highly crystalline PbBr2 with a little DMF$Pb–Br residue. SEM images were then collected to evaluate the morphology change with post-treatment temperatures. As shown in Fig. 2(d), for the 100-Pb–Br precursor, large crystals ranging from hundreds of nanometers to several micrometers in size are


sparsely covered on the scaffold and a large portion of the scaffold is exposed. The coverage is improved for the 70-Pb–Br precursor, but there is still a considerably large exposed area of the scaffold. The scaffold is then fully covered with the precursor at the post-treatment temperature of 50 C with an observable texture pattern on the surface, indicating a slightly rough surface. Signicantly, a full coverage with an ultrasmooth surface is obtained by the 25-Pb–Br precursor. Fig. 2(e) vividly presents the partial coverage of the 100-Pb–Br precursor on the TiO2 porous scaffold (500–600 nm) with a poor pore lling, but a full coverage of the 25-Pb–Br precursor with an ultra-smooth morphology. On the basis of the above results and discussion, we propose the following precursor formation mechanism at different posttreatment temperatures (Fig. 3). At low temperature, Tlow (e.g. 25 and 50 C), most of the DMF molecules are present in the precursor lm, which prevents the crystallization of PbBr2 or

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Schematics illustrating the main composition and structure of the Pb–Br precursors obtained at different post-treatment temperatures.

Fig. 3

DMF$Pb–Br. This nearly amorphous feature of the DMF/Pb–Br precursor affords it good exibility, which hence facilitates a full coverage and good pore lling in the porous scaffold. As the post-treatment temperature is increased, Tmid (e.g. 70 C), the partial evaporation of DMF molecules leads to the crystallization of DMF$Pb–Br, which tends to induce a local nucleation and as a result, exposes the scaffold at a later growth stage. Aer the post-treatment temperature is increased to 100 C (Thigh), the further evaporation of DMF molecules leads to the nucleation and growth of PbBr2 crystals. The volume contraction from DMF$Pb–Br crystals to PbBr2 crystals would expose more scaffold surface and hence lead to poorer coverage. Therefore, our present work suggests the growth of a good pore lling and full coverage of the Pb–Br precursor in the porous scaffold at low temperature from 25 to 50 C, which exhibits an amorphous feature. 2.2 Conversion of the amorphous precursors to MAPbBr3 in MABr IPA solution First, we evaluated the conversion behavior of different Pb–Br precursors to MAPbBr3 in the conventional MABr IPA solution with a MABr concentration of 10 mg ml1. The conversion temperature and duration were xed at room temperature and 30 min. All MAPbBr3 samples were sequentially annealed at 100 C for 20 min. Fig. 4(a) presents the XRD patterns of the resulting MAPbBr3, indicating a considerably different phase composition. The MAPbBr3 from 100-Pb–Br (100-IPA MAPbBr3) still exhibits the strong peaks of PbBr2, e.g. (020) plane, more intense than those of MAPbBr3, e.g. (001) plane. In comparison, the MAPbBr3 from 70-Pb–Br (70-IPA MAPbBr3) presents obviously intense peaks of MAPbBr3 with signicantly weak peaks of PbBr2. For the MAPbBr3 from 50-Pb–Br (50-IPA MAPbBr3), no peak of PbBr2 is observed and all the peaks could be indexed to the cubic phase MAPbBr3 except the peaks from FTO and the TiO2 scaffold. Similarly, the MAPbBr3 from 25-Pb–Br (25-IPA MAPbBr3) exhibits no peaks of PbBr2 and in addition, the diffraction peaks of MAPbBr3 become stronger compared with those of 50-IPA MAPbBr3. XRD results indicate that inserting MABr molecules into high-crystallinity PbBr2 crystals faces serious difficulty and even extending the reaction duration to 8 h could not afford

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complete conversion (Fig. S2†), which is signicantly harder than that faced when inserting MAI molecules into the layered PbI2 crystals, as reported in the literature.40,47,48 The accelerated conversion of DMF$Pb–Br to MAPbBr3 should be attributed to the favorable intramolecular exchange between crystallized DMF and MABr molecules.11,45 However, this intramolecular exchange behavior still could not afford a complete conversion, possibly a predominant problem for MAPbBr3 deposition by the conventional two-step solution method, successfully adopted for MAPbI3 and FAPbI3 deposition.11,45 Interestingly, our amorphous Pb–Br precursors (DMF/Pb–Br) with an ultra-smooth surface have signicantly promoted the conversion, which should be beneted from the much easier molecular exchange between non-crystallized DMF and MABr molecules and the more exible amorphous structure for efficient MABr penetration.49 Light absorption properties of different MAPbBr3 are studied by recording UV-vis absorption spectra. As exhibited in Fig. 4(b), obvious absorption onsets could be easily observed at around 540 nm, corresponding to the characteristic absorption limit of MAPbBr3 whose bandgap was reported to be around 2.2 eV. The absorption enhancement at the onset is very small for 100-IPA MAPbBr3 due to the low conversion of PbBr2. In comparison, 70-IPA MAPbBr3 exhibits a signicant increase in the absorption enhancement at the onset, resulting from the increased conversion of PbBr2. And the absorption enhancement at the onset further increases for 25-IPA and 50-IPA MAPbBr3, attributed to the complete conversion of PbBr2 to MAPbBr3. In the wavelength range above 540 nm, there are obvious differences in the UV-vis spectra among different MAPbBr3 lm samples, which contributed to their light scattering capacity differences and could partially reect their roughness differences in the following order: 25-IPA < 50-IPA < 70-IPA < 100-IPA, which will be further examined by SEM. SEM images of different MAPbBr3 are presented in Fig. 4(c) and (d). As indicated, 100-IPA MAPbBr3 shows an obviously different morphology compared with 100-Pb–Br. Aer the chemical conversion in MABr IPA solution, small rod-shaped crystals with a square cross-section stands out from the large island crystals, suggesting the cubic symmetry of the MAPbBr3 unit cell, as indicated in the XRD pattern. However, the MAPbBr3 still exhibits very poor coverage on the scaffold stemming from the original PbBr2 morphology. Cubic phase MAPbBr3 crystals could be also observed for the 70-IPA MAPbBr3, but their coverage on the scaffold is well improved, beneting from the improved morphology of the 70-Pb–Br precursor. Further, full coverage of 50-IPA MAPbBr3 on the scaffold is obtained, but the lm structure becomes very rough and relatively small crystals with sizes of 100–300 nm are found to construct the whole lm. More obviously, 25-IPA MAPbBr3 becomes even rougher with a full coverage on the scaffold and similar grain sizes. The cross-sectional SEM image in Fig. 4(d) demonstrates a partial coverage and poor pore lling of 100-IPA MAPbBr3 on the scaffold with a disconnected capping layer at a thickness of 600–700 nm. In contrast, 25-IPA MAPbBr3 shows a full coverage and good pore lling. The capping layer with a thickness of 1.5–2.2 mm could be divided into two parts. The


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Fig. 4 Conversion of Pb–Br precursors to MAPbBr3 in MABr IPA solution. (a) XRD patterns, (b) UV-vis spectra, (c) top-view and (d) cross-sectional SEM images of the MAPbBr3 obtained from different Pb–Br precursors.

bottom part exhibits a good connection with a thickness of 600–700 nm, while the top part is very rough and exhibits a poor connection with a thickness of about 1–1.5 mm. The SEM results have pointed out different growth mechanisms of MAPbBr3 crystals for different Pb–Br precursors in MABr IPA solution (10 mg ml1). For the high-crystallinity precursors (100-Pb–Br and 70-Pb–Br), local nucleation leads to the growth of large-sized and rod-shaped crystals. In a clear comparison, homogeneous nucleation behavior has occurred for the amorphous precursors (50-Pb–Br and 25-Pb–Br) and the growth process should have terminated shortly, leading to small sized crystals with an irregular shape and pure phase. And the severe molecular exchange and rapid volume expansion during the conversion process caused the crystallites squeeze into each other out of the lm and form the rough surface.


2.3 Addition of non-polar CYHEX to MABr IPA solution for solvent engineering of perovskite crystallization As stated above, the nearly amorphous DMF/Pb–Br precursor has led to a pure and full coverage MAPbBr3 layer in the scaffold, but the severe molecular exchange and rapid volume expansion in the conventional MABr IPA solution (polar solvent with high concentration) result in a fairly rough surface, which is not suitable for the following deposition of high quality hole extraction materials (HTM, Au or carbon electrodes). In order to ease off the molecular exchange and reduce the volume expansion rate, simply reducing the MABr concentration in IPA is not proved to be an effective way to get even MAPbBr3 layers, as presented in Fig. S4,† which prompted us to modify the conventional IPA solvent. In our previous work, we have succeeded in evening out the MAPbI3 layers by lowering the solvent

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polarity with the addition of non-polar CYHEX into MAI IPA solution,50 which beneted from the slowed intercalation of MAI with PbI2 at the initial stage (or nucleation) and the suppressed dissolution–recrystallization of MAPbI3 at the later conversion stage. Herein, we also adopted this strategy to manipulate the conversion of Pb–Br precursors to MAPbBr3. Fig. 5(a) presents the photographs of the MABr IPA solution before and aer the addition of CYHEX, demonstrating that MABr precipitates out aer the addition. We have also got the saturated concentration (Csat) of MABr at different CYHEX contents, accompanied by the solvent polarity change that is simply calculated according to Psystem ¼ Fvol(IPA) P(IPA) + Fvol(CYHEX) Pvol(CYHEX), where Psystem, P(IPA) and Pvol(CYHEX) represent polarity values of the mixed solvent, IPA and CYHEX, respectively; Fvol(IPA) and Fvol(CYHEX) represent the volume fractions of IPA and CYHEX in the mixed solvent, respectively. As indicated in Fig. 5(b), both Psystem and Csat decrease with the increase of CYHEX content, with a Csat value of about 20.52 mg ml1 for pure IPA and only about 0.0067 mg ml1 for the mixed solvent containing 95% CYHEX. To investigate the effects of CYHEX content on the properties of MAPbBr3, we chose the 10 mg ml1 MABr as the basic concentration with the conversion duration xed at 30 min. In pure IPA solvent, as presented above, the conversion is completed without any peak of PbBr2. When the CYHEX content is gradually increased to 50%, as shown in Fig. 5(c), XRD patterns do not show any obvious change, even Csat at 50% CYHEX is lowered to only 2.59 mg ml1. And as presented in Fig. 5(d), the peak intensity of MAPbBr3 (001) planes is also very close and no peak of PbBr2 is found. When the CYHEX content is increased to 80%, Csat is lowered to about 0.67 mg ml1 and the peaks of PbBr2 are readily observed, indicating a slower conversion rate at so low MABr concentration. As the CYHEX content is further increased to 90%, the peaks of MAPbBr3 and PbBr2 decrease and increase, respectively. Although no conversion happens at 0.04 mg ml1 MABr in IPA (Fig. S4†), it still can be concluded that lowering the solvent polarity helps to promote the conversion process, similar to the MAPbI3 system. When the CYHEX content is further increased to 95%, no obvious conversion is observed as Csat is signicantly lowered to only about 0.0067 mg ml1. Besides the obvious change in the composition of MAPbBr3, it is surprising to nd that the relative intensity of the MAPbBr3 (001) plane with other planes tends to increase with the CYHEX content between 20% and 90%, see the peak intensity ratios of (001)/(011) and (001)/(012) in Fig. 5(e), especially when the CYHEX content was above 80%. This indicates that the preferential orientation of cubic MAPbBr3 is gradually adjusted to the [001] direction with increasing CYHEX content, as highlighted in Fig. 5(f) and (g). Again, SEM images were taken to evaluate the morphologies of the MAPbBr3 from different MABr solutions. At 20% and 50% CYHEX contents (see Fig. S5† and 5(h), respectively), their morphologies are similar to that from a pure IPA solvent (Fig. 4(c) and (d)), which is consistent with the XRD results, and the total lm thickness is 2.5 mm. An obvious change is observed when the CYHEX content is increased to 80%. The

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MAPbBr3 crystals become large and compactly stack with each other. And the cross-sectional SEM images indicate that the MAPbBr3 with a total lm thickness of 1.7 mm becomes obvious even in comparison with those from the MABr solutions with 0%, 20% and 50% CYHEX. As the CYHEX content is further increased to 90%, extremely smooth MAPbBr3 is obtained and the crystals stack with each other very intimately and compactly. And the whole active MAPbBr3 layer is determined to be about 1.2 mm thick and the scaffold is well lled. On further increasing the CYHEX content to 95%, a large amount of PbBr2 is present and as a result, the nal lm shows a similar morphology to that of 100-Pb–Br (Fig. 2(d)) with smooth and large crystals but poor coverage. Therefore, the addition of CYHEX into MABr IPA solution has successfully achieved an even MAPbBr3 with good pore lling and evenness. Since the evenness of MAPbBr3 exhibits a close relationship with the [001] orientation ratio, the higher [001] orientation achieving the more even MAPbBr3, we believe that the obvious [001] orientation plays an important role in evening out the MAPbBr3 layer. At this stage, there is still PbBr2 residue in MAPbBr3, which needs to be further converted by extending the reaction time. The reaction time in the MABr IPA/CYHEX solution with 90% CYHEX was further extended from 0.5 h to 8 h to increase conversion. As the conversion duration prolongs, the peaks of MAPbBr3 become more and more intense with the complete disappearance of the peaks of PbBr2 at 8 h (see Fig. 6(a)). Fig. 6(b) summarizes the change in the trend of the peak intensity of the MAPbBr3 (001) plane with conversion time, accompanied by those of other two typical process-based MAPbBr3. The (001) peak intensity of 25-IPA/CYHEX MAPbBr3 is signicantly higher than those of 100-IPA and 25-IPA MAPbBr3, suggesting an obviously high crystallinity induced by the mixed solvent. As a side note, the (001) peak intensity increases gradually with reaction time from 0.5 h to 8 h for 100-IPA MAPbBr3, clearly attributed to the increased conversion of PbBr2 to MAPbBr3. However, its conversion is still not complete aer 8 h, demonstrating the considerably higher difficulty for converting PbBr2 crystals to MAPbBr3 in MABr IPA solution. For 25-IPA MAPbBr3, the (001) peak intensity is higher than that of 100-IPA MAPbBr3 due to its complete conversion, and it tends to keep steady during the rst 4 h. However, it starts to decrease aer 8 h, implying the possible dissolution of MAPbBr3 in IPA solvent at the later stage, which is well conrmed in Fig. S3.† So far, XRD results have well suggested the obviously higher MAPbBr3 crystallinity produced from MABr IPA/CYHEX solution. In addition to the enhanced peak intensity (higher crystallinity) with reaction time for 25-IPA/CYHEX MAPbBr3, the peak intensity ratios of (001)/(011) and (001)/(012) are considerably increased as the reaction time is prolonged from 0.5 h to 2 h (Fig. 6(c)). This phenomenon demonstrates that the MAPbBr3 crystals with a poor [001] vertical alignment tend to terminate their growth, or they preferentially rotate to align the [001] direction perpendicular to the substrate at their latter growth stage. It appears that the vertically orientated [001] is a preferred orientation that can ensure compact lm and smooth propagation of the crystalline growth. As the reaction time is


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Fig. 5 Conversion of 25-Pb–Br precursors (DMF/Pb–Br) to MAPbBr3 in MABr IPA/CYHEX solution. (a) Photograph of the MABr IPA solutions before and after the addition of CYHEX, (b) relationships of polarity and Csat of the MABr solutions to different CYHEX contents, (c) XRD patterns of the MAPbBr3 from the MABr solutions with different CYHEX contents but a fixed reaction time at 0.5 h, (d) change of MAPbBr3 (001) plane density and PbBr2 (020) plane intensity with the CYHEX content, (e) relationship of the plane intensity ratio of (001)/(011) and (001)/(012) to the CYHEX content, (f) MAPbBr3 crystal structure with highlighted typical different planes, (g) schematic illustrating the crystal orientation before and after the addition of CYHEX and (h) SEM images of the MAPbBr3 from the MABr solutions with different CYHEX contents.


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Fig. 6 Effects of reaction time in MAI IPA/CYHEX solution (90% CYHEX) on the crystalline orientation and morphology of MAPbBr3 films. (a) XRD patterns and (b) the (001) diffraction intensity as a function of reaction time for three preparation processes of MAPbBr3 (25-IPA/CYHEX, 25-IPA and 100-IPA). (c) Diffraction intensity ratios of (001)/(011) and (001)/(012) as a function of reaction time in IPA/CYHEX solution. (d) Top-view and (e) cross-sectional SEM images of the 25-IPA/CYHEX samples.

increased from 2 h to 8 h, both these peak intensity ratios remain almost steady (about 11–12 and 6.5–7.5 for (001)/(011) and (001)/(012), respectively). For 100-IPA MAPbBr3, the peak intensity ratios of (001)/(011) and (001)/(012) only increase slightly with reaction time and their values are signicantly lower than those of 25-IPA/CYHEX MAPbBr3, about 1.8–2.6 and 1.0–1.3, respectively. For 25-IPA MAPbBr3 both the peak intensity ratios of (001)/(011) and (001)/(012), in the ranges of 2.6–3.6 and 1.2–2.5, respectively, increase gradually at the initial 4 h, but they slightly decrease as the reaction time extends to 8 h, which is possibly due to the fact that MAPbBr3 preferentially dissolves from the (001) planes, a similar phenomenon for MAPbI3.51

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SEM images (Fig. 6(d) and (e) and S6†) were further recorded to evaluate the morphology change of MAPbBr3 with reaction time in MABr IPA/CYHEX solution. Similar to the 25-IPA/CYHEX MAPbBr3 at 0.5 h (Fig. 5(h)), the 25-IPA/CYHEX MAPbBr3 at 2 h also exhibits a compact morphology with a slight increase in the crystal size about 200–400 nm that seem to fuse together. Prolonging the reaction time to 4 h, the MAPbBr3 crystals present distinct and sharp grain edges, exhibiting vivid cubic shaped crystals compactly stacked, which implies a good crystallinity with the compact morphology. Further increasing the reaction time to 8 h, cubic shaped crystals become sharper, suggesting higher crystallinity, which agrees well with the XRD results. The cross-sectional SEM image in Fig. 6(e) indicates an even and


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compact MAPbBr3 capping layer with a thickness of about 900 nm fully covered on the scaffold. More importantly, the MAPbBr3 is thoroughly lled into the scaffold (500–600 nm). The cross-sectional SEM image of 25-IPA/CYHEX MAPbBr3 at 8 h shows much larger crystal sizes compared with the observed crystal size on the top-view images (Fig. 6(d)). This implies possible coalescence of the neighboring small crystals with similar sizes and orientations (e.g. [001] direction) by eliminating a common boundary, which is essentially an oriented attachment growth behavior. Additionally, we have tested other nonpolar solvents, such as hexane (HEX). A similar morphology was obtained (Fig. S7†), which evidences the generality of our method. We believe that the growth mechanism of even MAPbBr3 should result from the modied reaction thermodynamics with the change of solvent polarity. At a higher MABr concentration (10 mg ml1) in IPA (moderately polar), reaction (1) happens vigorously, leading to a lot of small crystals and subsequent aggregation, forming a rough surface (Fig. 7(a)). Pb–Br (s) + CH3NH3+ (sol) + 2Br (sol) / CH3NH3PbBr3 (s)

(1)

characterized by recording UV-vis spectra and time-resolved photoluminescence (TRPL) spectra. As shown in Fig. 8(a), all spectra show clear absorption onsets at around 540 nm, corresponding to the characteristic absorption of MAPbBr3. Compared with 100-IPA and 25-IPA MAPbBr3, 25-IPA/CYHEX MAPbBr3 exhibits a much more obvious absorption enhancement at the onset, which should be attributed to the complete conversion of PbBr2 to high-quality MAPbBr3 crystals. In the wavelength range above 540 nm, the light absorption intensity of 25-IPA/CYHEX MAPbBr3 is signicantly lower than those of 100-IPA and 25-IPA MAPbBr3, suggesting that the lowest light scattering capacity originated from its more even and better pore lling features. Trap states were typically observed in perovskite,52,53 which still have important impacts on photovoltaic performance and the less trap states get the better performance.54 These trap states were suggested to be closely related to deposition methods, which led to different morphologies (such as crystallinity and grain size) of perovskite layers.8,9,55 The TRPL technique is useful to check for differences in the trap state concentration from the three different processes. As shown in Fig. 8(b), the PL signals decay at signicantly different rates (called as quenching rate) among the three MAPbBr3 samples in

The aim to reduce the nucleation number and smoothen the reaction by simply decreasing the MABr concentration did not work well because reaction (1) is not thermodynamically favorable at low MABr concentration in IPA (Fig. 7(b)). Instead, reducing the solvent polarity by adding a nonpolar solvent would push reaction (1) forward at low MABr concentration due to the reduction in the saturated concentration (Csat) of MABr. As a result, the nucleation number would be signicantly reduced and the conversion reaction would be smoothened, which would favour the crystalline orientation and the growth of even MAPbBr3 lms (Fig. 7(c)). Besides comparing the perovskite crystallization and morphology of the perovskite lms grown from the typical three processes (100-IPA, 25-IPA and 25-IPA/CYHEX), the light absorption properties and trap state density of the lms were also

Photoelectronic property comparison among the MAPbBr3 prepared by different processes. (a) UV-vis spectra of the MAPbBr3 deposited on the scaffold and (b) TRPL spectra of the MAPbBr3 deposited on insulating glass. The gray solid lines are the corresponding fitting results using the biexponential model. Fig. 8

Fig. 7 Growth mechanism of MAPbBr3 in different MABr solutions: (a)

IPA solution with high concentration (e.g. 10 mg ml1), (b) IPA solution with low concentration (e.g. 0.04 mg ml1) and (c) IPA/CYHEX (e.g. 1/ 9) solution with low concentration (e.g. 0.04 mg ml1).


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the following order, 100-IPA > 25-IPA > 25-IPA/CYHEX. The TRPLs are tted by the biexponential model:56 y ¼ A1 exp(x/s1) + A2 exp(x/s2), where A1 and A2 are the prefactors, s1 and s2 are the lifetimes of the fast and slow decay components, respectively. For simplicity, weighted average values of s1 and s2 are utilized to represent the PL lifetime (s) according to the equation: s ¼ A1/(A1 + A2) s1 + A2/(A1 + A2) s2. And the PL lifetimes (s) of 100-IPA, 25-IPA and 25-IPA/CYHEX MAPbBr3 are calculated to be about 1.10 ns, 2.32 ns and 6.47 ns, respectively. The highest quenching rate of the conventional 100-IPA MAPbBr3 demonstrates the lowest crystal quality, affording the most recombination pathways, while the signicantly slowed quenching rate of 25-IPA MAPbBr3 suggests the reduced trap states by the DMF/Pb–Br precursor. The slowest quenching by

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25-IPA/CYHEX implies that the addition of nonpolar CYHEX in IPA not only smoothens the MAPbBr3 layer but also well suppresses the formation of trap states. Therefore, the TRPL results conrm the higher MAPbBr3 crystallinity from IPA/CYHEX, consistent with the XRD results. 2.4 Construction and performance evaluation of paintable carbon-based Br-PSCs To evaluate photovoltaic properties of the various perovskite layers prepared, paintable carbon-based PSCs were fabricated by directly printing a commercial carbon paste on perovskite layers,35,50,57 according to the procedure illustrated in Fig. 9(a), followed by annealing at 100 C for 30 min. Fig. 9(b) presents a typical cross-sectional SEM image of the carbon-based PSCs

Fig. 9 Device architecture and performance of the paintable carbon-based PSCs with different MAPbBr3 films. (a) Deposition process of the carbon electrode; (b) cross-sectional SEM image; (c) working principle; (d) J–V curves with forward and reverse scans; (e) steady photocurrent density at a voltage close to the maximum output point; (f) IPCE spectra; variation of (g) Jsc and (h) Voc with light intensity; and (i) J–V curve of the best-performing 25-IPA/CYHEX device.

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using 25-IPA/CYHEX MAPbBr3, indicating an intimate contact at the perovskite/carbon interface, which mainly benets from the even surface of this MAPbBr3. The working principle is briey shown in Fig. 9(c). The photogenerated electrons in the conduction band (CB) of MAPbBr3 are injected into the CB of TiO2 and then transported to a FTO substrate, while the photogenerated holes in the valence band (VB) of MAPbBr3 would be extracted by using a carbon electrode. Current density–voltage (J–V) curves with forward and reverse scans were then recorded for PSCs with the MAPbBr3 prepared by the three typical processes (100-IPA, 25-IPA and 25-IPA/CYHEX) and the results are displayed in Fig. 9(d) and Table 1. For the 100-IPA device, the forward scan affords an open-circuit voltage (Voc) of 0.86 V, a short-circuit current density (Jsc) of 0.74 mA cm2, a ll factor (FF) of 35% and a PCE of 0.22%, while the reverse scan exhibits a slightly higher performance with a Voc of 1.02 V, a Jsc of 0.75 mA cm2, a FF of 44% and a PCE of 0.34%, which has a hysteresis index (HI) of 35%, HI ¼ |(PCE(reverse) PCE(forward))|/PCE(reverse).58 Signicantly, performance is obviously improved by the 25-IPA device, with a Voc of 1.27 V, a Jsc of 5.20, a FF of 42%, and a PCE of 2.76% for the forward scan, and a Voc of 1.31 V, a Jsc of 5.15 mA cm2, a FF of 33%, and a PCE of 2.25% for the reverse scan. And compared with the 100-IPA device, the hysteresis phenomenon is well suppressed with a reduced HI of 23%. Impressively, the performance is considerably improved by using the 25-IPA/CYHEX device and all photovoltaic parameters (Voc, Jsc, FF and PCE are 1.34 V, 8.08 mA cm2, 68% and 7.41% for the forward scan, respectively, and 1.35 V, 8.22 mA cm2, 70% and 7.79% for the reverse scan, respectively) are largely increased in comparison with the former two devices, with much less hysteresis (HI ¼ 5%). Photovoltaic parameter distributions of these three devices are presented in Fig. S9,† which also provides a good demonstration of the higher performance reproducibility of the 25-IPA/CYHEX device. To further authenticate the veracity of the measured efficiency, the steady-state photocurrent outputs at the voltage close to the maximum power points (0.8 V, 1.02 V and 1.05 V for 100-IPA, 25-IPA and 25-IPA/CYHEX devices, respectively) were measured and monitored as a function of time. As shown in Fig. 9(e), the photocurrent densities of all three devices rise quickly to the maximum values aer light is turned on. However, both the maximum photocurrent densities of 100-IPA

Photovoltaic parameters obtained from the paintable carbonbased PSCs fabricated using different MAPbBr3 films

Table 1

Devices 100-IPA 25-IPA 25-IPA/CYHEX

Scanning direction

Voc (V)

Jsc (mA cm2)

FF (%)

PCE (%)

HI (%)

Forward Reverse Forward Reverse Forward Reverse Best

0.86 1.02 1.27 1.31 1.34 1.35 1.35

0.74 0.75 5.20 5.15 8.08 8.22 8.35

35 44 42 33 68 70 72

0.22 0.34 2.76 2.25 7.41 7.79 8.09

35


23 5

and 25-IPA devices decrease from their initial values of 0.57 mA cm2 and 3.68 mA cm2 to the steady values of 0.27 mA cm2 (53% decrease) and 2.51 mA cm2 (32% decrease), respectively, which further reects the large hysteresis effect in these two devices. In comparison, the maximum photocurrent density of the 25-IPA/CYHEX device is almost stabilized with little change (initial and steady values are 7.11 mA cm2 and 7.07 mA cm2, respectively, corresponding to only 0.6% decrease) over the time period of our experiment (150 s). The steady-state photocurrent output measurement, which exhibits steady PCEs at 0.22%, 2.56% and 7.42% for 100-IPA, 25-IPA and 25-IPA/CYHEX devices, respectively, are consistent with the J–V curve results. Many factors could contribute to photovoltaic device performance differences. However, since the substrate, TiO2 scaffold and carbon electrode were the same in our experiment, the main effect should be related to the differences in the MAPbBr3 lms. The light absorption capacity is closely related to Jsc: a higher absorption capacity tends to afford a higher Jsc, which could well explain the differences in Jsc for different devices. A poor perovskite coverage would easily lead to a direct contact between TiO2 and the carbon electrode, resulting in more charge recombination loss and, mainly, a reduction in Voc. As a result, the 25-IPA and 25-IPA/CYHEX devices registered obviously higher Voc than the 100-IPA device. In addition, the trap states in MAPbBr3 would also induce charge recombination. Hence, the lower trap state density of 25-IPA/CYHEX MAPbBr3 than that of 25-IPA MAPbBr3 can be explained by the slightly higher Voc for the 25-IPA/CYHEX device than that for the 25-IPA device. Furthermore, the contact at the perovskite/ carbon interface (compare Fig. 9(b) and S8†) greatly inuences the charge transfer and thus FF. The best interface contact in the 25-IPA/CYHEX device due to the most even perovskite surface would signicantly boost FF. As a result, the 25-IPA/ CYHEX device achieved the best photovoltaic performance. IPCE spectra were then recorded to examine the photovoltaic response in the wavelength range of the solar spectrum. As indicated in Fig. 9(f), all IPCE spectra exhibit similar onsets at around 540 nm, corresponding to the characteristic absorption limit of MAPbBr3. The 100-IPA device presents a relatively low IPCE value at a wavelength shorter that 400 nm, smaller than 10%, while the IPCE value of the 25-IPA device is signicantly increased to around 50%. Furthermore, the IPCE value around 90% is obtained by the 25-IPA/CYHEX device. The order of IPCE values among these three devices has a similar order with their Jsc from the J–V curves, which agrees well with their different absorption intensities below their absorption limit. Besides, we believe that the better interface contact and the elimination of recombination pathways at the perovskite/carbon interface of the 25-IPA/CYHEX device, in comparison with those of the 25-IPA and 100-IPA devices, respectively, would also improve carrier collection efficiency and hence IPCE. The light intensity dependence of Jsc and Voc was also investigated in our carbon-based PSCs; see Fig. 9(h) and (g), respectively. A linear dependence of Jsc on incident light intensity (I) is found for 25-IPA and 25-IPA/CYHEX devices with a a similar value of 0.98 that is calculated according to Jsc f Ia, J. Mater. Chem. A, 2016, 4, 12897–12912 | 12907

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which is typical for high-performance photovoltaic devices, demonstrating the independence of charge collection efficiency on light intensity and non space-charge limited photocurrents.59,60 The a value of the 100-IPA device is about 0.82, deviating largely from 1. Hole and electron mobilities are expected to be comparable according to previous reports and should not lead to carrier imbalance in PSCs.7,8 However, the large amount of residual PbBr2 in 100-IPA perovskite would block the electron extraction and hence increase the photocurrent.61 The Voc–I curve shows that the Voc increases with I and the slope in Voc vs. I varies with different MAPbBr3: 329 mV per decade for the 100IPA device, 141 mV per decade for the 25-IPA device and 129 mV per decade for the 25-IPA/CYHEX device. The diode ideality factors m, dened using the following equation,17 are calculated to be 12.80, 5.48 and 5.02, respectively: vV mkB T ¼ vln I0 q

Br-PSCs

HEM

With HTM

PCBTDPP CBP PF8-TAA P3HT P-TAA PIF8-TAA PEDOT:PSS Spiro-MeOTAD Au Au Au Carbon nanotube Graphite/ carbon black

Without HTM

(2)

where kB is Boltzmann's constant, q is the elementary charge, and T is temperature in Kelvin (298 K in our case). These m values are larger than those of HTM-based PSCs reported previously (ranging from 1 to 4),17,60–63 probably because the carbon paste could not well passivate the surface states of perovskite to suppress trap-assisted recombination in the devices compared with the commonly used HTM layers. However, the signicantly decreased m for the 25-IPA and 25-IPA/CYHEX devices indicates that the trap-assisted recombination has been reduced substantially, as well supported by the TRPL results. The other phenomenon that the 25-IPA/ CYHEX devices exhibit signicantly higher Jsc, FF and PCE than 25-IPA devices though their m values are similar, suggests that more serious non trap-assisted recombination, such as radiative recombination, should have happened in 25-IPA devices because of the worse hole extraction by the carbon electrode resulting from the poorer interface contact (see Fig. S8(b)†). Aer preliminary optimization, the best-performing 25-IPA/ CYHEX device yields a PCE of 8.09%, resulting from a Voc of 1.35 V, a Jsc of 8.35 mA cm2, and a FF of 72% (Fig. 9(h)). So far, this is the highest reported PCE value for MAPbBr3-based HTM-free PSCs24,26,34 and is higher than those achieved by the HTM-based PSCs using P3HT,25 PCBTDPP,64 CBP,65 PF8-TAA29 and PEDOT:PSS,37 as listed in Table 2. And it is also comparable to the PCE using the most popular HTMs, such as Spiro-MeOTAD,27 PTAA25 and modied PTAA (PIF8-TAA).25 Considering that photovoltaic stability is now one of the most important issues for PSCs, we have also evaluated this aspect for our carbon-based PSCs using 25-IPA/CYHEX MAPbBr3 as the prototype. For organic photovoltaic devices, there are several categories of test protocols, such as dark, outdoor, simulated light and stress testing and thermal cycling,66 each could be subdivided into three levels: basic (level 1), intermediate (level 2) and advanced (level 3). For most of the stability tests on PSCs conducted so far, the testing conditions (storage in the glovebox and room temperature) have been well below the basic level (level) of dark test protocols, too far from practical applications. Here, we took a big step forward by testing our PV device stability following some of the standard

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Table 2 Summarized best photovoltaic parameters of MAPbBr3based PSCs with different HTMs or without HTMa

a

Voc (V)

Jsc (mA cm2)

FF (%)

PCE (%)

Ref.

1.16 1.38 1.36 1.09 1.35 1.51 1.04 1.45 1.35 1.37 1.36 1.31

4.47 0.7 6.3 8.5 8.4 8.4 7.2 9.2 2.7 5.41 6.79 5.86

59 40 70 79 82 82 57 68 55 75 69 75

3.04 0.37 6.0 7.3 9.3 10.4 4.3 9.1 2.02 5.57 6.53 5.76

64 65 29 25 25 25 37 27 24 26 34 22

1.35

8.35

72

8.09

This work

Note: HEM ¼ hole extraction materials.

test protocols under the conditions closer to practical applications. First, our non-encapsulated devices were stored in a dry air atmosphere (25–30 C and 10–20% humidity). As presented in Fig. 10, essentially no PCE degradation was observed aer storage for 90 days, suggesting their obviously high stability. Then, as high temperature and humidity have to be tolerated by photovoltaic devices, we have also investigated the stability of our non-encapsulated devices under heat stress test conditions (air, 80 C and 50–85% humidity), close to the ISOS-D-2 protocol (dark, intermediate level (level 2)).66–68 Impressively, the PCE of our device retains its 90% initial value aer 15 days and aer 30 days storage, higher than 80% initial PCE is still retained, which attests to the high thermal stability of our PV devices. This excellent stability especially the high thermal stability

Fig. 10 Stability evaluation of the paintable carbon-based PSCs using the 25-IPA/CYHEX MAPbBr3 film without encapsulation. Condition 1: air atmosphere, 25–30 C and about 10–20% humidity, and condition 2 (inset): air atmosphere, 80 C and about 50–85% humidity. J–V curves were measured periodically in ambient air (about 50–85% humidity) to get the photovoltaic parameters.


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should be attributed to the use of the carbon electrode with high water resistance and thermal stability,35,69 and the high-quality MAPbBr3 crystals obtained by our novel strategy (25-IPA/CYHEX). Undoubtedly, encapsulation would even more drastically improve the PV device stability to a higher level, which is surely one of the ongoing research directions in our laboratory.

3.

Conclusions

We have demonstrated an amorphous precursor route based on a two-step sequential solution method for growing high-quality MAPbBr3 in the relatively thick TiO2 porous scaffold. In the rst step, by lowering the post-treatment temperature to 50 C or 25 C, an amorphous Pb–Br precursor containing a large amount of DMF molecules (DMF/Pb–Br) with good pore lling and full coverage was obtained, which successfully accelerated the following conversion to pure MAPbBr3 in MABr IPA solution in comparison with the high-crystallinity Pb–Br precursors with poor pore lling and partial coverage obtained at high temperature (70 C or 100 C). In order to reduce the roughness of MAPbBr3 obtained in MABr IPA solution, non-polar CYHEX was added to lower the solvent polarity, which manipulated the conversion process (mainly molecular exchange between DMF and MABr) to control the nucleation and growth of MAPbBr3 crystals. This solvent engineering strategy signicantly adjusted the preferential crystal orientation to the [001] direction, affording the MAPbBr3 layer with an even surface. In addition, light absorption capacity was well improved accompanied by trap state reduction. Aer applying the MAPbBr3 in the paintable carbon-based HTM-free PSCs, the PCE was boosted to 8.09% (Voc ¼ 1.35 V), a record value for MAPbBr3-based HTM-free PSCs and even among the highest PCE levels of the MAPbBr3-based PSCs with HTMs. Signicantly, our nonencapsulated device did not show any PCE decay aer storage in a dry air atmosphere (10–20% humidity and 25–30 C) for 90 days. And furthermore, its PCE was retained up to about 90% aer storage under high heat stress conditions (air atmosphere, 50–85% humidity and 80 C) for 15 days, suggesting a considerably high thermal stability. Therefore, our amorphous precursor route in combination with the unique solvent engineering has successfully obtained high-quality MAPbBr3 in the TiO2 scaffold to afford HTM-free carbon-based PSCs with high PCE and stability.

4. Methods 4.1

Preparation of TiO2 porous scaffolds

First, a TiO2 blocking layer was spin-coated on cleaned FTO glass at 2000 rpm for 20 s using 0.15 M titanium diisopropoxide bis(acetylacetonate) in 1-butanol solution, followed by heating at 125 C for 5 min. Then, TiO2 scaffolds were deposited by spin coating at 5000 rpm for 30 s using a commercial TiO2 paste (Dyesol 30 NRD, Dyesol) diluted in ethanol at a ratio of 1 : 2.5. Aer drying at 100 C for 5 min, the TiO2 scaffolds were gradually heated to 550 C, baked at this temperature for 30 min and cooled to room temperature.


4.2

Preparation of CH3NH3Br

CH3NH3Br was prepared by reacting 24 ml of methylamine (40 wt% in water, J&K) and 12 ml of hydrobromic acid in an ice bath for 2 h with stirring. The precipitate was recovered by rotary evaporation at 50 C and carefully removing the solvents with the help of a low temperature circulator. To improve the purity, the as-prepared CH3NH3Br was re-dissolved in ethanol and precipitated with diethyl ether, which was repeated twice. Finally, the white solid was collected by ltration and dried at 70 C in a vacuum. 4.3

Deposition of the Pb–Br precursor

The Pb–Br precursor solution was prepared by dissolving 1.4 M PbBr2 in DMF at 70 C. Then, the precursor lms were deposited by spin coating the precursor solution at a speed of 2000 rpm for 20 s. The as-deposited samples were kept at room temperature or annealed at different temperatures (25 C, 50 C, 70 C and 100 C) for 5 min, followed by cooling to room temperature. 4.4

Conversion of the PbBr2 precursors to CH3NH3PbBr3

Firstly, CH3NH3Br was prepared by dissolving 10 mg ml1 CH3NH3Br in IPA or different concentrations of CH3NH3Br in IPA/CYHEX (or IPA/HEX) solvents with different CYHEX contents (90% HEX). And then, the PbBr2 samples were immersed in the CH3NH3Br solution for different times to be converted to CH3NH3PbBr3. Finally, the as-prepared CH3NH3PbBr3 samples were heated at 100 C for 15 min. 4.5

Deposition of the carbon electrode

A commercial carbon paste purchased from Guangzhou Seaside Technology Co., Ltd was used as carbon paint for the carbon electrode. The carbon extraction layer was simply deposited by printing the carbon paint on the perovskite layer at room temperature followed by drying at room temperature for 30 min and then heating at 100 C for 30 min. 4.6

Material characterization

X-ray diffraction (XRD) patterns were recorded on a Philips PW-1830 X-ray diffractometer with Cu Ka radiation (K ¼ 0.15418 nm). The morphology was evaluated on a JEOL 6700F or 7100F SEM at an accelerating voltage of 5 kV. UV-vis absorption spectra were recorded on a Perkin-Elmer UV-vis spectrophotometer (model Lambda 20). FT-IR spectra were measured on a Bruker Vertex 70 FTIR spectrometer, while the Raman spectra were acquired on a Renishaw RM 3000 Micro-Raman system. 4.7

Photovoltaic characterization

The solar light simulator (Newport solar simulator, model number 6255, 150 W Xe lamp, AM 1.5 global lter) was calibrated to 1 sun (100 mW cm2) using a silicon reference solar cell equipped with a KG-5 lter. The active cell area was xed at about 20 mm2 and J–V curves were recorded on an IM6x electrochemical workstation (ZAHNER-Elektrik GmbH & Co., KG, Germany). Since there was no obvious PCE difference for our

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masked or non-masked devices, no mask was used in our further measurements for convenience. IPCE spectra were recorded using an IPCE kit developed by ZAHNER-Elektrik in the AC mode with a frequency of 1 Hz. And Voc–I and Jsc–I curves were also measured on the ZAHNER-Elektrik and the wavelength of the illumination light was 458 nm. For steady state PL and time-resolved PL measurements, a tunable Ti:sapphire femtosecond-pulsed laser was used as the excitation light source, with an excitation wavelength of 400 nm and an incident light intensity of 1 W cm2. A Hamamatsu C5680-04 streak camera was used for the TRPL experiments.

Paper

7

8

9 4.8

Stability evaluation

The device stability was studied by storing the device under different conditions. And J–V curves were measured periodically in ambient air (about 50–85% humidity) to get the photovoltaic parameters.

10

Acknowledgements This work was supported by the HK-RGC General Research Funds (GRF No. 16300915), the HK Innovation and Technology Fund (ITS/004/14) and the RGC Areas of Excellence Scheme (AoE/P-02/12).

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