Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted

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Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted Planar FA-Cs Mixed-Cation Perovskite Solar Cells Xiuwen Xu, Menglin Li, Yue-Min Xie, Yuhui Ma, Chunqing Ma, Yuanhang Cheng, Chun-Sing Lee, and Sai Wing Tsang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20933 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted Planar FA-Cs Mixed-Cation Perovskite Solar Cells Xiuwen Xu,†,‡,§ Menglin Li,†,‡,§ Yue-Min Xie,†,‡,§ Yuhui Ma,†,‡,§ Chunqing Ma,‡ Yuanhang Cheng,†,‡,§ Chun-Sing Lee,‡ Sai-Wing Tsang†,‡,§,*

†

Department of Materials Science and Engineering, City University of Hong Kong, Hong

Kong SAR, P. R. China ‡

Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong,

Hong Kong SAR, P. R. China § City

University of Hong Kong Shenzhen Research Institute, Shenzhen, 518057, P. R. China

KEYWORDS: inverted planar perovskite solar cells, two-step spin-coating method, morphology engineering, nanorod, photoluminescence spectroscopy

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Depth-dependent growth of perovskite crystals remains challenging for highperformance perovskite solar cells made by a two-step spin-coating method. Effective morphology engineering approaches that enable depth-independent perovskite crystals growth and facile characterization technique to monitor subtle yet influential changes accompanied are urgently required. Here, a porous and intercrossed PbI2-(CsI)0.15 nanorods scaffold is prepared by integrating CsI incorporation with toluene dripping in ambient air, and the mechanism underlying is uncovered. With this porous scaffold and moisture-assisted thermal annealing, depth-independent growth of FA0.85Cs0.15PbI3 is achieved, as evidenced in the photoluminescent (PL) spectra acquired by exciting the perovskite film from the top and bottom side, individually. It is of broad interest that PL spectroscopy is demonstrated as a sensitive technique to monitor the depth-dependent growth of perovskite. Moreover, the resulting inverted planar FA0.85Cs0.15PbI3 perovskite solar cells deliver an efficiency of 16.85%, along with superior thermal and photo stability. By incorporating 2% large-sized diammonium cation, propane-1,3-diammonium, the efficiency is further increased to 17.74%. Our work not only proposes a unique porous PbI2-(CsI)0.15 nanorods scaffold to achieve highquality perovskite films in a two-step method, but also highlights the distinctive advantage of PL spectroscopy in monitoring the depth-dependent quality of perovskite films.

1. Introduction Since the pioneering work reported in 2013, the inverted planar perovskite solar cell (PSC) has become a mainstream device architecture adopted by researchers in the community.1 Recently, the record PCE of the inverted planar PSC has been pushed up to 21.5%.2 Although it is still inferior to that (> 23%) of the regular-structure devices, the planar inverted PSCs are advantageous in terms of simple fabrication, negligible hysteresis, and 2 ACS Paragon Plus Environment

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mechanical flexibility, therefore holding great prospect for commercialization.3-6 In this context, researches that aim to developing highly efficient and stable inverted planar PSCs are flourishing. Among all factors that determine the long-term performance of PSCs, the intrinsic stability of the perovskite is considered as the highest priority. Compared to the perovskites containing volatile methylammonium (MA) cation, the formamidinium (FA)-cesium (Cs) mixed cation perovskite are emerging as a promising alternative for high-efficiency PSCs with excellent thermal and photo stability.7-10 Very recently, Snaith et al reported a record PCE of 21.1% with FA0.83Cs0.17PbI2.7Br0.3 as light harvester in a regular-structure PSC. Impressively, the authors showed that compared to the MA and MA-FA-Cs perovskites, the FA-Cs counterpart had superior thermal and photo stability particularly when exposed to high intensity of solar irradiation.11 In spite of the encouraging intrinsic crystal stability and high PCE, the FA-Cs perovskites reported so far are almost based on regular structure (Table S1), where high temperature treatment for metal oxide (e.g., TiO2, SnO2, etc.) interlayer is required, which is incompatible with light-weight and flexible devices. Moreover, these FA-Cs based regular PSCs usually suffer from obvious current density-voltage (J-V) hysteresis, which deteriorates the long-term stability.12-13 In contrast, implement of FA-Cs mixed perovskite in inverted planar PSCs remains scarce, as summarized in Table S1. In 2017, J. Huang et al first reported an inverted planar FA-Cs PSCs with NiO (500 oC sintered) as the hole transport material, showing a PCE of 16.0% and negligible J-V hysteresis.9 As far as it is known, it is still the highest efficiency for inverted FA-Cs PSCs, which lags significantly behind that of the regular ones (21.1%). Therefore, it is imperative to develop strategies to boost the PCE of inverted planar FA-Cs PSCs, especially processable at low temperature. It is well known that the growth of high-quality perovskite is indispensable to achieve high-performance PSCs.14 Among various perovskite fabrication approaches, the perovskite crystal growth is more controllable and reproduceable by using a two-step spin-coating 3 ACS Paragon Plus Environment


method.15-16 In this method, a lead iodide (PbI2) layer is firstly deposited, and then the FAI or MAI is spin-coated onto the PbI2, followed by thermal annealing to drive the conversion reaction of the perovskite. However, due to the small activation energy for perovskite crystallization, tightly packed perovskite crystals are formed immediately once the FAI or MAI is dropped on the PbI2 surface, and it is associated with a significant crystal volume expansion. As a result, the tight perovskite top layer blocks the FAI or MAI penetrating into the deeper layer and leaves substantial PbI2 unreacted.14, 17-18 Therefore, the perovskite growth in a two-step method tends to be depth-dependent. To overcome this issue, morphology engineering including aging,19-20 solvent treatment16,

21-22

and additive control,23 has been

proposed to construct porous PbI2 scaffold layers. These porous scaffold layers increase the accessibility of PbI2, facilitating the depth-independent growth of high-quality perovskite layer.16,

19-20, 23-24

Despite the variety of morphology engineering techniques, an effective

approach to control the morphology and porosity of the precursor layers with nano-structured PbI2 has not been reported. Moreover, investigations on the facilitated perovskite growth by using these porous PbI2 scaffold layers are mainly limited to MAPbI3 (Table S2). It is questionable whether and to what extent it can promote the conversion of perovskite with FA cation or other long-alkyl ammonium cations, such as n-butylammonium (BA) and propane1,3-diammonium (PDA). Theoretically, the growth of high-quality large cation based perovskite is more challenging, which simultaneously requires high porosity of PbI2 for efficient cation diffusion and dense PbI2 layer to ensure full film coverage.19 Therefore, developing novel strategies to further engineering the porosity of the dense precursor layer is of great importance to achieve depth-independent growth of high-quality perovskite with large cations. Here, we report a novel porous PbI2-(CsI)0.15 scaffold consisting of uniformly distributed and intercrossed nanorods, which achieves a fine balance between high-porosity and dense precursor layers. Interestingly, such a distinct porous structure can only be obtained when 4 ACS Paragon Plus Environment

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toluene dripping is integrated with CsI incorporation in ambient air, and the formation mechanism is uncovered. With the non-porous and porous PbI2-(CsI)0.15, inverted planar FA0.85Cs0.15PbI3 PSCs are fabricated via a two-step method with thermal annealing in N2 and humid air, respectively. By comparing the PL spectra of the top and the bottom layer of the FA0.85Cs0.15PbI3 films, it is confirmed that only with a combined assistance of moisture and porous scaffold, depth-independent growth of high-quality FA0.85Cs0.15PbI3 can be achieved. The corresponding inverted planar PSCs offers a PCE of 16.85%, which is significantly higher than that of the counterparts fabricated without thermal annealing in air (12.62%) or porous scaffold (14.69%). Importantly, by incorporating a small amount of large-sized PDA cation, the PCE of the Cs0.15PDA0.03FA0.82PbI3 PSCs is further increased to 17.74%.

2. Experimental Section 2.1 Chemicals Formamidinium iodide (FAI), propane-1,3-diammonium iodide (PDAI) and lead iodide (PbI2, 99%) were purchased from Greatcell Solar and Acros Organics, respectively. Cesium iodide (CsI), anhydrous ethanol, isopropanol (IPA), toluene, chlorobenzene (CB), N, Ndimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) were bought from SigmaAldrich. Poly-(4-butylphenyldiphenylamine) (PTPD) were supplied by Xi’an Polymer Light Technology Corp. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 2,9-dimethyl-4,7diphenyl-1,10-phenanthroline (BCP) were provided by Nano-C and Lumitech, respectively. All these materials were used as received. 2.2 Perovskite film preparation 461 mg PbI2 and 39 mg CsI were dissolved in DMSO and DMF (79 µl/1 ml) mixture solution, and stirred at 70 oC overnight to form a homogenous PbI2-(CsI)0.15 soution. Meanwhile, a FAI solution is prepared by dissolving 45 mg FAI in IPA and stirring at room 5 ACS Paragon Plus Environment


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temperature overnight. To prepared non-porous PbI2-(CsI)0.15 layer, the hot PbI2-(CsI)0.15 solution (80 oC) was spin-coated onto the preheated PTPD/ITO (80 oC) at a speed of 3500 rmp for 40s in ambient air with a relative humidity (RH) of 60±10%, followed by thermal annealing at 80 oC for 5 min . Differently, to prepare porous PbI2-(CsI)0.15 nanorod network, the antisolvent toluene was poured at the 10 s since the spin-coating starts, other conditions are identical to that of non-porous PbI2-(CsI)0.15 layer. Imediatedly, the samples were transferred to a glovebox and cool down to room temperature. Then FAI was spin-coated onto PbI2-(CsI)0.15 layer, followed by thermal annealing at 140 oC for 1 h in glovebox and in humid air (RH: 60±10%), respectively. The relative humdity is the typical humdity level in the laboratory. 2.3 Device fabrication The

device

adopts

a

inverted

planar

confirguration

of

ITO/PTPD/FA0.85Cs0.15PbI3/PCBM/BCP/Ag. The cleaned indium tin oxide (ITO) glasses were first treated by a ultraviolet-ozone (UVO) irradiation for 20 min. Then poly-TPD (4 mg ml-1in CB) was spin-coated on the ITO at 6000 rmp for 40 s and annealed at 120 oC for 10 min. After that, a 20 s UVO exposure was employed to tune the hydrophilicity of the PTPD layer.25 The perovskite was then deposited onto the PTPD according to the aforementioned methods. Subsequently, a 20 nm-thick PCBM layer and a 5 nm-thick BCP layer was deposited onto perovskite by spin-coating 25 mg ml-1 PCBM (in CB) and 2 mg ml-1 BCP (in ethanol) solution at a speed of 2000 rmp for 40 s, respectively. Finally, a Ag electrode (80 nm) was deposited by thermal evaporation. The active area of the device was 0.07 cm2. 2.4 Characterizations Scanning electron microscopy (SEM) images were acquired by a Philips XL30 FEG scanning electron microscopy. X-ray diffraction patterns (XRD) were characterized by a D2 Phaser instrument with Cu Kα radiation. UV-Vis absorption spectra were monitored by a UVVIS spectrometer (PerkinElmer, Lambda 2S). Steady photoluminescence (PL) spectra were 6 ACS Paragon Plus Environment

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recorded on a home-built equipment with a 315 nm laser.26 Time-resolved photoluminescence (TRPL) spectra were measured by a time correlated single photon counting spectrometer with a 485 nm laser (Edinburgh Instruments, LifeSpec II). To investigate the depth-dependent perovskite film quality, the samples (ITO/perovskite) were excited from the top (perovskite) and bottom (ITO) side, respectively, and the corresponding PL and TRPL profiles were compared. The current density-voltage (J-V) curves were acquired by a Keithley 2400 source meter with a solar simulator (AM 1.5G, 100 mW cm-2), and external quantum efficiency (EQE) measurements were carried on home-built set-up.27 The stabilized PCE was tested by tracking the current density of an unencapsulated device exposed to one sun illumination, and a voltage bias corresponding to the maximum power point was applied to the device. The long-term stability of the encapsulated devices was evaluated by measuring the J-V characteristics every 30 min, and the devices were exposed to continuous light soaking and heated to 60 oC in ambient air (RH: 60±10%).

3. Results and Discussions First, we sought to investigate the impact of different processing conditions on the PbI2(CsI)0.15 morphology. Among the processing conditions, toluene dripping effectively regulates the precipitation of precursor salts during the spin-coating process, thus greatly affecting the morphology of the deposited film.28-29 Therefore, we compared the morphology of the PbI2(CsI)0.15 layers spin-coated with and without toluene dripping in air, the detailed processing procedures are described in the experimental section. Interestingly, the PbI2-(CsI)0.15 with toluene dripping exhibits a morphology of porous and intercrossed PbI2-(CsI)0.15 nanorod scaffold (Figure 1a), whereas the one without toluene dripping shows a closely packed film morphology with a few pinholes and nanorods (Figure 1b). It should be noted that the porous scaffold obtained here is different from all previously reported porous PbI2, where the 7 ACS Paragon Plus Environment


scaffold has a unique intercrossed PbI2-(CsI)0.15 nanorod morphology. It simultaneously features with high porosity and dense precursor layer, which is favorable for the depthindependent growth of high-quality perovskite film. In addition, we have also found that both moisture and CsI incorporation are simultaneously required for the formation of the nanorod scaffold. As shown in Figure 1c, by applying the same spin-coating method inside a moisture-free N2 filled glovebox, only a closely packed PbI2-(CsI)0.15 layer similar to that in Figure 1b can be obtained. In case of the PbI2 solution without CsI added, the morphology of the coated film is totally different, which consists of a lot of irregular nanoparticles atop, as shown in Figure 1d. The above results indicate that the successful construction of this unique nanorod scaffold is a combined effect from 1) CsI incorporation, 2) moisture, and 3) toluene dripping.

Figure 1. SEM images of PbI2-(CsI)0.15: (a) with toluene dripping in air; (b) without toluene dripping in air; (c) with toluene dripping in N2; (d) A SEM image of PbI2 prepared by toluene dripping in air.


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In order to disentangle the effects of the above three factors on the formation of porous PbI2-(CsI)0.15 nanorod scaffold, in-depth investigations were carried out. Regarding to the effect of CsI incorporation, the PbI2-(CsI)x (x=0-1) layers with different CsI contents were prepared with toluene dripping in air, as shown in Figure S1. It is found that when x< 0.1, the PbI2-(CsI)x layers show similar film morphologies to that of the PbI2 layer (Figure S1a-b). In contrast, the nanorod structure starts to appear when x=0.1. Further increasing x from 0.125 to 0.15, the porous nanorod scaffold morphology becomes more and more evident (Figure S1de). The corresponding XRD patterns show that when x increases to 0.125, a weak yet obvious XRD peak at 10o is identified, indicating the formation of low-crystalline CsPbI3 (Figure S2).30 More interestingly, in the case of x>0.1, although the film morphologies are vastly different, the nanorod structure is consistently observed in all cases (Figure S1f-i). In particular, when x increases to 0.8 and 1, a more pronounced nanorod morphology is observed, meanwhile, the characteristic peak of CsPbI3 at around 10o becomes the most dominating peak (Figure S2). This phenomenon unambiguously demonstrates that the nanorod is associated with the formation of low-crystalline CsPbI3. In addition, the crystallinity of PbI2 gradually decreases with the increasing CsI content, suggesting suppressed PbI2 crystallization due to CsI incorporation (Figure S2). To investigate the moisture effect, the PbI2-(CsI)0.15 and PbI2 prepared with toluene dripping in humid air (RH:60±10%) and N2 were compared (Figure S3). Evidently, without moisture in N2 filled glovebox, both PbI2-(CsI)0.15 and PbI2 layers show smooth and uniform film morphology, whereas their counterparts prepared in humid air (RH:60±10%) tend to be more porous with intercrossed nanorod or randomly isolated nanoparticle (Figure S3a-d). This can be partially explained by the fact that the moisture considerably affects PbI2 nucleation during the spin-coating process, resulting in films with different amount of holes or voids.31 The detailed effect of vastly different moisture levels (RH70%) on the PbI2-(CsI)0.15 formation requires further investigations. In addition, Figure S3e-f compares 9 ACS Paragon Plus Environment


the XRD patterns of the PbI2 and PbI2-(CsI)0.15 prepared in N2 and in humid air, respectively. Both PbI2 and PbI2-(CsI)0.15 prepared in humid air exhibit much weaker XRD peak at around 12.7o as compared to that prepared in N2. Considering the only difference is the presence of moisture in the former case, it is reasonable to conclude that the moisture (RH: 60±10%) suppresses the high-crystalline PbI2 formation. As for the toluene dripping effect, the PbI2-(CsI)0.15 prepared in air with varied toluene dripping time during the spin-coating process were studied (Figure S4). Interestingly, if toluene is dripped too early (less than 4 s), the precursor solution is directly washed away. It is because in that time window, the precursor solution cannot reached sufficiently high supersaturation for PbI2 or CsI precipitation even with toluene dripping.32 Only after spincoating for 6 s, the PbI2-(CsI)0.15 can be deposited, but the morphology is highly dependent on the dripping time. Specifically, with the dripping time increases, the porous nanorod morphology first becomes evident and then gradually becomes unobvious (Figure S4b-i), while their XRD patterns show that the PbI2 crstallinity first decreases and then increases (Figure S5). It is worth meantioning that the porous nanorod morphology seems to be accompanied with the low-crystalline PbI2, which has also been found during the investigation of the CsI incorporation and mositure effects as presented above. With such a high consistency between the porous nanorod morphology and low-crystalline PbI2, we reason that the low-crystalline PbI2 is favorable for the porous nanorod scaffold formation, although whether the low-crystalline PbI2 can facilitate the subsequent perovskite growth remains to be an open question.19,

33-34

In addition, the origin of the toluene dirrping time

dependent PbI2-(CsI)0.15 morphology is uncovered. By gradually adding CsI or PbI2 into an 80 oC

DMF/DMSO mixture solution, the solubility limit of CsI or PbI2 is roughly determined to

be 165 and 1075 mg ml-1, which means that the saturation ratio of CsI and PbI2 is 23.6 and 42.9%, respectively (Figure S6 and Table S3). Undoubtedly, the different saturation ratio of CsI and PbI2 increases during the spin-coating process, and toluene dripping at different 10 ACS Paragon Plus Environment

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timeslot will immediately induce the CsI and PbI2 supersaturation at different levels. As supersaturation is the driving force for PbI2 and CsI precipitation and crystallization, different supersaturation levels of PbI2 and CsI leads to different relative precipitation rate of PbI2 and CsI, thus resulting in slightly different morphology of the PbI2-(CsI)0.15.29,

32, 35

This

hypothesis is strongly supported by the observation that the PbI2-(CsI)0.15 prepared by 1.5 M precursor solution with toluene dripping at 10 s shows similar morphology with that prepared by 1 M precursor solution with toluene dripping at 25 s (Figure S6 c-d). Based on the above results, it is validated that the formation of low-crystalline CsPbI3 is required for the nanorod morphology. Meanwhile, the low-crystalline PbI2, fast PbI2 nucleation and properly tuned relative precipitation rate of PbI2 and CsI profoundly affect the porous scaffold structure. More specifically, among the three factors, CsI incorporation leads to the low-crystalline CsPbI3 formation and inhibits the growth of high-crystalline PbI2, the moisture causes fast PbI2 nucleation while suppressing the growth of high-crystalline PbI2, and toluene dripping tunes the difference between PbI2 and CsI precipitation rate by changing their supersaturation level during the spin-coating process. Aiming to get further insight into the impact of the PbI2-(CsI)0.15 morphology on the resulting FA0.85Cs0.15PbI3 perovskite films quality in a two-step method, we first prepared the porous PbI2-(CsI)0.15 nanorod scaffold with toluene dripping and non-porous PbI2-(CsI)0.15 layer without toluene dripping in air. After the spin-coating of FAI, the films were annealed in N2 or in air for comparison. Table S4 summarizes the preparation conditions of different samples for this study. As shown in Figure 2 a-d, all the as-prepared perovskite films show uniform and pinhole-free film morphology with densely packed crystal grains. The average grain size of NP-N, P-N, NP-A and P-A is 377, 418, 420 and 496 nm, respectively (Figure 2e). Notably, the grain size of the perovskite annealed in air is larger than the one annealed in N2. It is because that the moisture in air can partially dissolve the perovskite precursors and enlarge the crystal grain, and annealing perovskite in air with a wide range of RH is generally 11 ACS Paragon Plus Environment


beneficial.36-37 Similarly, compared to the perovskite derived from the non-porous PbI2(CsI)0.15, the one derived from the porous PbI2-(CsI)0.15 has larger grain size. It is probably because the perovskite nucleation in non-porous PbI2-(CsI)0.15 mainly occurs on the surface leading to high density of nucleation sites, whereas for porous PbI2-(CsI)0.15 scaffold, the nucleation process is more evenly initiated throughout the film. This hypothesis is further confirmed by the slightly decreased grain size of the perovskite films derived from the less porous PbI2-(CsI)0.15 scaffolds prepared with toluene dripping at 20 s and 30 s (Figure S7). Figure 2f shows the X-ray diffraction patterns (XRD) of the NP-N, P-N, NP-A and P-A samples. It is found that the all the XRD peaks of the NP-A and P-A sample are attributed to either perovskite or ITO, and the P-A shows the best crystallinity among the four samples. Differently, besides the perovskite characteristic peak, a minor peak at 12.7o is observed in both NP-N and P-N samples, indicating the existence of unreacted PbI2. This can be explained by the fact that during the annealing process, moisture in air provides an aqueous atmosphere to facilitate the diffusion of the precursor ions, thereby promoting the conversion of PbI2 to perovskite.38 Furthermore, the absorbance spectra of the four samples were acquired, as shown in Figure 2g, and they show similar absorbance across all wavelengths. The above results demonstrate that both the porous scaffold and moisture are beneficial for the growth of perovskite with large grain size, high crystallinity and strong light absorbance. However, these features are not the sufficient conditions that directly correlated to the photovoltaic device performance. Given that SEM is a surface characterization technique, which is incapable of gaining information of the deeper layer of the perovskite. Although X-ray can penetrate throughout the entire perovskite layer, as evidenced by the observed ITO peaks (Figure 2f), XRD is limited to monitor the crystalline phase, while the amorphous phase existing in the perovskite layer cannot be overlooked. Moreover, the UV-vis absorption spectrum can only give a generic overview of the excited state and bandgap energies of the perovskite, while the transport and extraction behaviors of the excited charges 12 ACS Paragon Plus Environment

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cannot be manifested, which are critical for the photovoltaic device operation. In this sense, invasive steady and time-resolved photoluminescence (PL) measurements were performed to gain insights into the unidentified amorphous phases and photoelectrical properties of the perovskite films.39 Considering the laser excitation with wavelength below 500 nm is mostly absorbed at the interface,40 it allows to differentiate the photoelectrical properties of the perovskite layer close to the top and bottom side, individually.

Figure 2. SEM images (a-d), grain size distribution (e), XRD patterns (f) and absorbance spectra (g) of the NP-N, P-N, NP-A and P-A perovskite, where the prefixes NP and P denotes non-porous and porous PbI2-(CsI)0.15, respectively, and the postfixes N and A stands for annealing in N2 and air, respectively.

As displayed in Figure 3a, the steady PL spectra acquired by exciting different samples from the top side all show a single symmetrical peak at 798 nm. The PL intensity gradually increases from NP-N, P-N, NP-A to P-A, indicating a suppressed nonradiative recombination probably due to reduced traps and defects density. Interestingly, when the samples are excited from the (ITO) bottom side, both the intensity and shape of the PL peak in each case are different. The NP-N sample exhibits two obvious PL peaks: one is a weak and broad peak centralized at 700 nm, and another is a relatively strong peak located at 792 nm. Since the 13 ACS Paragon Plus Environment


broad peak at 700 nm is not associated to neither the crystalline PbI2 nor perovskite phases, it can only be attributed the perovskite intermediates in amorphous phase.8, 41 Meanwhile, the blue-shifted peak from 798 nm to 792 nm suggests that the chemical composition of the perovskite close to the bottom side slightly differs from that of the top layer. This result strongly verifies that even after 1 h thermal annealing in N2, the growth of the NP-N sample is still highly depth-dependent, with substantial amorphous perovskite intermediates remained in the bottom layer. Differently, for the P-N sample, the broad peak at 700 nm is disappeared, and an asymmetric peak at 798 nm is observed. It indicates that with the porous PbI2-(CsI)0.15 scaffold, the depth-dependent growth of perovskite has been mitigated to a large extent. Moreover, with the assistant of moisture, the NP-A and P-A samples only show a strong and systematical PL peak at 798 nm, further demonstrating the positive effect of moisture upon perovskite conversion. In particular, the PL peak intensity of the P-A sample observed from the bottom side is almost identical to that observed from its top side. It implies that the depthindependent growth of perovskite has been achieved with the combined assistance of moisture and porous scaffold. Figure 3b compares the time-resolved photoluminescence (TRPL) spectra obtained by exciting the samples from the top and bottom sides. The TRPL curves are bi-exponential in nature and fitted according to the equation: y=A1exp(−t/t1)+A2exp(−t/t2), where the fast decay is due to traps-induced nonradiative recombination and the slow decay is attributed to the radiative recombination of electrons and holes.42 The detailed fitting parameters are summarized in Table S5, and the PL lifetime was calculated by a weighted average method.43 It is found that when the NP-N, P-N, NP-A and P-A are excited from the top side, the average lifetime (τavg,

top)

is 37.7, 54.9, 67.4 and 114.9 ns, respectively. For comparison, when the

samples are excited from the bottom side, the average lifetime (τavg, bottom) is 12.0, 40.4, 52.3 and 104.6 ns for NP-N, P-N, NP-A and P-A, respectively. Evidently, except for P-A, the τavg, bottom

of NP-N, P-N and NP-A is much shorter than its corresponding τavg, top. As summarized 14 ACS Paragon Plus Environment

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in Figure 3c, by either changing the non-porous PbI2-(CsI)0.15 to porous PbI2-(CsI)0.15 scaffold or using air annealing to replace N2 annealing, the τavg, top and the ratio of τavg, bottom / τavg, top both increase, which agrees well with the above steady PL results. To gain more information on the depth-dependent perovskite growth, the cross-sectional SEM images of the non-porous PbI2-(CsI)0.15, porous PbI2-(CsI)0.15 scaffold, NP-A and P-A were obtained. As expected, compared to the non-porous PbI2-(CsI)0.15, the porous PbI2-(CsI)0.15 scaffold shows obvious nanorod morphologies with more open space for FAI ingress and perovskite conversion (Figure S8a-b). As a result, the P-A exhibits more uniform morphology and larger grain size with more crystals vertically orientated (Figure S8d), whereas in the case of NP-A, a lot of small crystals at its bottom layer are observed (Figure S8c). This phenomenon further demonstrates the facilitated depth-independent growth of P-A.

Figure 3. Steady PL spectra (a) and TRPL spectra (b) of the NP-N, P-N, NP-A and P-A perovskite excited from top and bottom side; (c) Comparison of PL lifetime of the top layer and PL lifetime ratio of the top and bottom layer of the NP-N, P-N, NP-A and P-A perovskite.



Through the above investigations, it is evidenced that moisture and porous scaffold are both favorable for depth-independent growth of high-quality perovskite. As illustrated in Figure 4, in the case of the NP-N sample, once FAI is dropped onto the non-porous PbI2-CsI, the perovskite conversion takes place along with significant crystal volume expansion, thus forming a tight perovskite layer atop.17 This tight perovskite layer makes the FAI difficult to diffuse into deep layer. Consequently, even after thermal annealing in N2 for 1 h, amorphous perovskite intermediates still exist in the bottom layer of the perovskite film (Figure 3a). On the other hand, in the case of P-A, FAI can directly penetrate into the entire porous PbI2-CsI scaffold, and during the thermal annealing in air, moisture provides aqueous environment for efficient precursor ion diffusion.36, 38 As a result, depth-independent growth of high-quality and amorphous intermediates free perovskite has been achieved.

Figure 4. Schematic illustration of the perovskite formation based on the non-porous and porous PbI2-(CsI)0.15.


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Figure 5. J-V characteristics (a), EQE (b) and integrated Jsc (c) of the champion device based on the NP-N, P-N, NP-A and P-A perovskite; (d) PCE distribution of devices based on the NP-N, P-N, NP-A and P-A perovskite.

To identify to what extent the porous scaffold and air annealing affect the photovoltaic device performance, inverted planar PSCs based on NP-N, P-N, NP-A and P-A were fabricated. As shown in Figure 5a and Table 1, the NP-N PSC exhibits a low PCE of 8.74%, with a current density (Jsc) of 16.00 mA cm-2, an open-circuit voltage (Voc) of 0.92 V and a fill factor (FF) of 59.35%. In comparison, the P-N PSC achieves remarkably enhanced photovoltaic parameters: a Jsc of 19.30 mA cm-2, a Voc of 0.96 V and a FF of 68.11%, offering a PCE of 12.62%. Undoubtedly, such a substantial increase of PCE is attributed to the porous scaffold induced improvement of perovskite quality, particularly mitigated amorphous perovskite intermediates in the bottom layer. Moreover, with combined assistance from moisture and porous scaffold, the as-formed P-A perovskite features with large-sized grain 17 ACS Paragon Plus Environment


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(~496 nm), high crystallinity and depth-independent crystal growth. As a result, the corresponding device delivers a further enhanced PCE of 16.85%, with a Jsc of 22.96 mA cm2,

a Voc of 1.02 V and a FF of 71.96%. Besides, the PCE of the P-A device is also obvious

higher than that of the NP-A device (14.69%), further demonstrating the high quality of P-A perovskite film. In addition, both the NP-A and P-A shows no obvious J-V hysteresis (Figure S9), which is a distinct advantage of inverted planar PSC containing PCBM against the regular PSCs.44

Table 1. Summary of photovoltaic parameters of PSCs based on the NP-N, P-N, NP-A and PA perovskite. Jsc

Voc

FF

PCE

[mA cm-2]

[V]

[%]

[%]

NP-N (Champion)

15.32±2.32 (16.00)

0.88±0.04 (0.92)

49.76±6.53 (59.35)

6.77±1.34 (8.74)

P-N (Champion)

18.52±0.41 (19.30)

0.96±0.02 (0.96)

59.19±4.69 (68.11)

10.47±1.02 (12.62)

NP-A (Champion)

21.77±0.77 (22.06)

0.96±0.02 (0.96)

63.69±3.78 (69.35)

13.26±0.79 (14.69)

P-A (Champion)

22.82±0.50 (22.96)

1.01±0.02 (1.02)

65.78±4.67 (71.96)

15.09±0.91 (16.85)

Sample

For further demonstration, external quantum efficiency (EQE) measurements were conducted, as shown in Figure 5b. Evidently, the EQE of NP-N and P-N PSC is much lower than that of the NP-A and P-A device across all wavelengths, which is consistent with the differences in their photovoltaic performance. Moreover, compared to the NP-A PSC, the P-A device shows an obviously higher EQE values in longer wavelength (690 to 780 nm), which indicates the reduced carrier recombination due to the depth-independent growth of highquality perovskite.45 The integrated Jsc of the NP-N, P-N, NP-A and P-A devices is 15.93, 19.08, 21.65 and 22.42 mA cm-2, respectively (Figure 5c), which shows less than 5% mismatch with that determined from the J-V curves. Figure 5d shows the histograms of PCE 18 ACS Paragon Plus Environment

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for the devices in each case, and the NP-N, P-N, NP-A and P-A PSCs achieve an average PCE of 6.77%, 10.47%, 13.26% and 15.09%, respectively.

Figure 6. (a) The maximum power point tracking of the devices (without encapsulation) based on the NP-A and P-A perovskite, the devices are exposed to one sun illumination in air (RH: 60±10%); (b) The long-term stability of the NP-A and P-A PSCs exposed to continuous one sun light soaking at 60 oC in air (RH: 60±10%), and the initial photovoltaic parameters are : NP-A (Jsc=21.83 mA cm-2, Voc=0.96 V, FF=0.67 and PCE=14.04%) and P-A (Jsc=22.73 mA cm-2, Voc=1.02 V, FF=0.70 and PCE=16.23%).

Device stability, particularly when the device is operating under continuous light soaking and heat stress, is a critical issue that must be concerned. Figure 6a shows the maximum power point tracking of unencapsulated devices based on NP-A and P-A perovskite, and the devices were exposed to one sun illumination in air (RH: 60±10%). Benefiting from the superior intrinsic photo-stability of FA0.85Cs0.15PbI3, the PCEs of NP-A and P-A show almost no performance decay after 2 h continuous operation at their maximum power point. 19 ACS Paragon Plus Environment


Moreover, the long-term stability test indicates that after 300 h light soaking and heat stress, the PCE of the encapsulated NP-A and P-A device retains 80% and 85% of their initial value, respectively. The superior stability can be theoretically attributed to the following features: i) It is difficult for FA+ to release protons upon light illumination, which would combine with Ito form HI; ii) The incorporation of Cs+ further enhances the interaction between FA+ and I-; iii) The volatile MA-free characteristic renders the perovskite with superior thermal stability.10, 46 To validate whether even higher efficiency could be achieved by using this porous PbI2(CsI)0.15 nanorod scaffold, propane-1,3-diammonium (PDA) cation is intentionally incorporated in the FA-Cs perovskite. It is because that PDA cation containing two ammonium groups might also function as an efficient defect passivators, similar to the previously reported tetra-ammonium or diammonium cations.47-48 Besides, with the ammonium group at both end, PDA cations can electrostatically connect the adjacent inorganic frameworks,49 rendering the perovskite with excellent stability.50-51 Our primary study shows that by incorporating 2 % PDA cations, the Cs0.15PDA0.02FA0.83PbI3 PSCs can achieve an obviously improved average and champion PCE of 16.31% and 17.74%, respectively, along with a J-V hysteresis free characteristic (Figure S10a-c). Figure S10d compares the TRPL spectra of the FA-Cs perovskite with and without PDA incorporation. Evidently, with 2% PDA incorporated, the PL lifetime is increased from 114.9 ns to 149.3 ns, which suggests the suppressed nonradiative recombination contributed by the defect passivation effect of PDA. Further optimization, device stability tests and detailed investigations on the effect of PDA incorporation are still ongoing.

4. Conclusion


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In summary, a porous and intercrossed PbI2-(CsI)0.15 nanorods scaffold is prepared by integrating toluene dripping with CsI incorporation in air. This unique porous scaffold simultaneously features with high porosity and dense precursor layer, and the mechanism underlying is proposed. With combined assistance of the porous scaffold and moisture, inverted planar FA0.85Cs0.15PbI3 PSCs achieves an efficiency of 16.85% with J-V hysteresis free characteristic and superior thermal and photo stability. In addition, by incorporating 2% of PDA cation, the efficiency can be further improved to 17.74%. More importantly, compared to SEM, XRD and absorbance spectroscopy, PL spectroscopy by exciting the top and bottom layer of perovskite is demonstrated to be a sensitive and indicative technique to evaluate the depth-dependent quality of perovskite. Our work provides a practical approach to achieve high-quality perovskite films in a two-step method and highlights the distinct advantage of PL spectroscopy in monitoring the depth-dependent quality of perovskite films. In addition, the as-proposed PbI2-(CsI)0.15 nanorod scaffold with high porosity for perovskite conversion and CsPbI3 embedded acting as perovskite seed are of particular scientific interest and great potential in the fabrication of all-inorganic perovskite.

ASSOCIATED CONTENT

Supporting Information Table S1. Summary of PSCs based on FA-Cs mixed perovskite reported to date. Table S2. Summary of porous PbI2 based PSCs. Table S3. The concentration, solubility limit and satration ratio of precursor salts in a hot DMF/DMSO mixture solution (1000/79, v/v, 80 oC). Table S4. Detailed processing conditions of the NP-N, NP-A, P-N and P-A perovskite.



Table S5. The fitting data deduced from the TRPL of the NP-N, P-N, NP-A and P-A perovskite. Figure S1. SEM images of PbI2-(CsI)x prepared with toluene dripping in air: (a) x=0, (b) x=0.05, (c) x=0.10, (d) x=0.125, (e) x=0.15, (f) x=0.30, (g) x=0.50, (h) x=0.80 and (i) x=1.00. Figure S2. XRD patterns of PbI2-(CsI)x prepared with toluene dripping in air (x=0 to 1). Figure S3. SEM images of PbI2-(CsI)0.15 (a) and PbI2 (b) prepared with toluene dripping in air (RH: 60%); SEM images of PbI2-(CsI)0.15 (c) and PbI2 (d) with toluene dripping in N2; XRD patterns of PbI2-(CsI)0.15 (e) and PbI2 (f) prepared with toluene dripping in air (RH: 60%) and N2. Figure S4. (a) Digital photograph of PbI2-(CsI)0.15 prepared in ambient air with toluene dripped at different time during the spin-coating process; SEM images of PbI2-(CsI)0.15 prepared in air with toluene dripping at different time during the spin-coating process: (b) 6 s, (c) 8s, (d) 10s, (e) 12 s, (f) 15 s, (g) 20s, (h) 25 s and (i) 30 s. Figure S5. (a) XRD patterns and (b) the intensity of PbI2 (001) crystal plane of PbI2-(CsI)0.15 prepared in air with toluene dripping at different time during the spin-coating process. Figure S6. (a) Photographs of different amount of CsI and PbI2 dissolved in hot DMF/DMSO mixture solutions (1000/79, v/v, 80 oC); SEM images of PbI2-(CsI)0.15 layers prepared by 1 M precusor solution in air with toluene dripping at 10 s (b) and 25 s (c); (d) A SEM image of PbI2-(CsI)0.15 layers prepared by 1.5 M precusor solution in air with toluene dripping at 10 s. Figure S7. SEM images of the perovskite derived from the prous PbI2-(CsI)0.15 prepared with different toluene tripping time: (a) 10 s, (b) 20 s and (c) 30 s; (d) is their grain size distribution. Figure S8. Cross-sectional SEM images of non-porous PbI2-(CsI)0.15 (a), porous PbI2-(CsI)0.15 (b), the NP-A perovskite (c) and P-A perovskite (d) on P-TPD/ITO. Figure S9. J-V characteristics of the champion device based on the NP-A (a) and P-A (b) perovskite; (c) Comparison of the photovoltaic parameters of devices based on the NP-A and P-A perovskite. 22 ACS Paragon Plus Environment

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Figure S10. J-V characteristics (a), EQE and intergrated Jsc (b) of the champion device based on Cs0.15PDA0.02FA0.83PbI3 derived from the porous PbI2-(CsI)0.15 scaffold; (c) The PCE distribution of devices based on Cs0.15PDA0.02FA0.83PbI3 derived from the porous PbI2(CsI)0.15 scaffold; (d) The TRPL spectra of the Cs0.15FA0.85PbI3 and Cs0.15PDA0.02FA0.83PbI3 derived from the porous PbI2-(CsI)0.15 scaffold.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] (Sai-Wing Tsang) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We gratefully thank the Hong Kong Innovation and Technology Commission (Project No: ITS/186/16), the National Natural Science Foundation of China (Project No: 61574120), the Natural Science Foundation of Guangdong Province (Project No. 2015A030313001), and CityU Applied Research Grant (Project No. 9667127).

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Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted

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