Investigating Recombination and Charge Carrier Dynamics in a One

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Investigating Recombination and Charge Carrier Dynamics in One-Dimensional Nanopillared Perovskite Absorber Hyeok-Chan Kwon, Wooseok Yang, Daehee Lee, Jihoon Ahn, Eunsong Lee, Sunihl Ma, Kyungmi Kim, Seong-Cheol Yun, and Jooho Moon ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07559 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Investigating Recombination and Charge Carrier Dynamics in One-Dimensional Nanopillared Perovskite Absorber Hyeok-Chan Kwon, Wooseok Yang, Daehee Lee, Jihoon Ahn, Eunsong Lee, Sunihl Ma, Kyungmi Kim, Seong-Cheol Yun, and Jooho Moon* Department of Materials Science and Engineering, Yonsei University 50 Yonsei–ro, Seodaemun–gu, Seoul 120–749, Republic of Korea

*Corresponding author, e-mail: [email protected]

tel.: +82-2-2123-2855, fax: +82-2-312-5375.

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ABSTRACT

Organometal halide perovskite materials have become an exciting research topic as manifested by intense development of thin film solar cells. While high performance solar cells based planar and mesoscopic configurations have been reported, one-dimensional (1-D) nanostructured perovskite solar cells are rarely investigated despite of their expected promising optoelectrical properties such as enhanced charge transport/extraction. Herein, we have analyzed the 1-D nanostructure effects of organometal halide perovskite (CH3NH3PbI3-XClX) on recombination and charge carrier dynamics by utilizing a nanoporous anodized alumina oxide scaffold to fabricate a vertically aligned 1-D nanopillared array with controllable diameters. It was observed that the one-dimensional perovskite exhibits faster charge transport/extraction characteristics, lower defect density, and lower bulk resistance than the planar counterpart. As the aspect ratio increases in the one-dimensional structures, in addition, the charge transport/extraction rate enhances and the resistance further decreases. However, when aspect ratio reaches 6.67 (diameter ~ 30 nm), the recombination rate is aggravated due to high interface-to-volume ratio induced defect generation. To obtain the full benefits of 1-D perovskite nanostructuring, our study provides a design rule to choose the appropriate aspect ratio of 1-D perovskite structure for the improved photovoltaic (PV) and other optoelectrical applications.

KEYWORDS: Organometal halide perovskite solar cell, one-dimensional perovskite, nanopillared structure, charge carrier dynamics, recombination dynamics


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Organometal halide perovskite (ABX3; A: methylammonium or formamidinium, B: Pb or Sn, X: I, Cl, or Br) solar cells employing a liquid electrolyte was first documented in 2009 with efficiency of 3.8%,1 followed by improved performance in 2011 with power conversion efficiency (PCE) of 6.5%.2 By replacing liquid electrolyte to solid-state hole transport layer (HTL), perovskite solar cells have experienced significant developments as PCE has soared to 22.7% within several years.3,4 Key enabling factors including high electron and hole mobility, long carrier diffusion length, and high absorption coefficient contribute to the arrival of the third generation PV field.5-9 The typical perovskite solar cell structure is composed of a perovskite absorber layer sandwiched between charge transport materials, i.e., an electron transport layer (ETL) and a hole transport layer. The precedent structure of the perovskite solar cells involves the infiltrated absorber into a mesoporous TiO2 scaffold,3 resembling dye-sensitized solar cells. Since that, thin film planar-type absorber as well as the penetrated absorber into mesoporous Al2O3 scaffold has been suggested after the observation that the hybrid perovskite can function as both light absorber and ambipolar charge transporter.8,10,11 These optoelectrical properties accelerate the extensive study on nanostructured perovskite solar cells. Nanostructuring strategies have been successfully implemented to improve the performance of PV devices in terms of enhanced charge transport/extraction and plasmonic light trapping.12-14 One-dimensional (1-D) nanostructured light absorbers of Si, InP, and GaAs have also received significant research attention because the generated photocarriers can be transported in the desired direction by limiting the movement of charge carriers to one dimension.12,15-17 In particular, vertically aligned 1-D nanostructures enable simultaneous improvements in both photocarrier generation and charge collection due to their capabilities to increase light absorption and facilitate rapid charge transport. Furthermore, the one-dimensional


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structure demonstrates different characteristics depending on its shape, thickness, and length. In this regard, in-depth understanding of 1-D structured organometal halide perovskites can provide an opportunity to further improve solar cell performance as well as to explore 1-D nanostructuredependent optoelectronic properties for other applications. Instead of the formation of 1-D perovskite absorber, more easily obtainable 1-D nanostructured ETLs (e.g., TiO2 and ZnO) has been served as a template to define 1-D perovskite structure.18-20 From the viewpoint of contact area with absorber layer, the onedimensional ETL structure lies between the mesoporous and the planar structures. This consideration implies that one-dimensional ETL structure can resolve the drawbacks of long meandering carrier paths associated with the mesoporous structure and relatively slow charge extraction in the planar structure. However, one-dimensional ETL structure-based perovskite structure still demonstrates inferior performance as compared to mesoporous and planar structure counterparts, indicating the lack of fundamental understanding on recombination and charge carrier dynamics in 1-D structured perovskite devices. Although self-assembled randomly grown 1-D perovskite nanowires were implemented into solar cell application,21,22 randomly oriented perovskite nanowires likely impede the charge transport from the absorber to charge extraction layer. Vertically aligned one-dimensional structure better fits analyses of the one-dimensional perovskite characteristics. While synthesis of one-dimensional perovskite nanowire arrays using an anodized aluminium oxide (AAO) has been demonstrated,23,24 but their PV performance was unavailable. Furthermore, whenever either ordered or randomly oriented perovskite nanowires are in contact with charge extraction layer, it is difficult to distinguish between the effect of better charge extraction and the effect of 1-D nanostructured perovskite itself.


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Recently, we successfully fabricated vertically aligned 1-D nanostructured organometal halide perovskite (CH3NH3PbI3-XClX) solar cell by using AAO thin film as a scaffold.25 AAO can well serve as a template to structure the 1-D absorber by filling the precursor solution into the pores of AAO. This methodology allowed us to fabricate vertically aligned 1-D nanopillared perovskite absorbers separately by insulating AAO walls incapable of charge extraction. This architecture is suitable for investigating charge carrier transport and extraction properties. In particular, precise controllability over the pore diameters of AAO thin film templates provides us the opportunity to evaluate charge carrier dynamics as a function of aspect ratios of 1-D nanopillared perovskite structures. Herein we select four different aspect ratios of 6.67, 4.17, 3.03, and 0 (i.e., planar configuration) by varying the AAO pore diameters, leading to the nanopillared perovskites with the varying aspect ratios. Scanning electron microscopy and tunneling electron microscopy analyses were performed to analyze the morphologies and crystal structures of perovskite/AAO. Influences of 1-D nanostructured perovskite on photo-carrier dynamics

were

thoroughly

evaluated

by

impedance

spectroscopy,

time-resolved

photoluminescence, and intensity modulated photocurrent/photovoltage spectroscopy. Our results offer significant insights into photo-carrier dynamics in 1-D perovskite structure, suggesting a design rule to implement nanostructured perovskites for improved PVs, lightemitting diodes, or photodetectors.

RESULTS AND DISCUSSION To fabricate 1-D nanopillared perovskite (CH3NH3PbI3-XClX) array surrounded by an insulating AAO scaffold, compact TiO2 (c-TiO2) was firstly coated on the cleaned fluorine-doped tin oxide (FTO) substrate by conventional sol-gel method and Al film was then deposited thereon by a


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thermal evaporation process. After that, AAO/c-TiO2/FTO substrate was prepared by anodizing and widening process in acid solution, followed by filling the perovskite precursor solution into scaffold.

After

the

perovskite

layer

formation,

2,2′,7,7′-tetrakis(N,N-di(4-

methoxyphenyl)amino)-9,9′-spirobifluorene (spiro-OMeTAD) layer and Au electrode were deposited to complete the devices. Scanning electron microscopy (SEM) images of the full device with and without AAO scaffolds are shown in Figure S1 in Supporting Information. It is well observable that the 1-D nanopillared perovskite array is sandwiched between bottom c-TiO2 ETL and upper spiro-OMeTAD HTL. For the photoluminescence analysis, the same fabrication method was used on a bare glass substrate without Au electrode instead of FTO glass. A more detailed experimental method is described in Experimental Section. Figure 1a-d shows crosssectional SEM images of spiro-OMeTAD/perovskite/glass configuration including 1-D nanopillared perovskite structures filled within three different AAO pore sizes of 66 nm, 48 nm, and 30 nm, as well as planar structured perovskite. Vertically well-aligned 1-D nanopillared perovskite array forms inside AAO scaffolds, indicating a complete filling into all AAO pores, while individual nanopillar is surrounded by AAO walls. SEM images of the AAO scaffold surface with various pore sizes are shown in Figure S2. Based on the average pore sizes (diameter) of AAO determined by image analysis using imageJ software (Table S1 in Supporting Information) and perovskite height obtained from SEM image, the aspect ratio (L/D : L is length and D is diameter of the nanopillar) was calculated to be 3.03, 4.17, 6.67, and 4.76 × 10-7 (for planar type, this value can be approximated to 0). Figure 1e shows a current density (J) - voltage (V) curve for full perovskite cells with four different aspect ratios. The cell performance parameters are listed in Table 1 and the average values and its distribution obtained from 16 devices for each sample are also summarized in


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Table S2. It should be noted here that our samples exhibited relatively low PCE values in the range of 4 - 9 % due to the presence of thin perovskite absorber layers (~ 150 nm). Demonstration of high efficient nanopillared solar cells is beyond the scope of the current study. With the increasing aspect ratio, the short circuit current density (JSC) decreases because the area fraction of perovskite absorber becomes diminished from 100 % to 10.5% as shown in a scheme of Figure 1e (the perovskite phase is colored in brown). The absorbance spectra for each sample are shown in Figure S3, representing decreasing tendency as the aspect ratio increases due to the reduced amount of perovskite absorber. On the other hand, planar perovskite solar cell (aspect ratio ~ 0) reveals lower open circuit voltage (VOC) as compared to 1-D nanopillared devices having similar VOC values regardless of the aspect ratio. This VOC drop observed in the planar structure is ascribed to the pinholes generated during single-step spin-coating of perovskite precursor solution as shown in Figure S4.26 Despite of the variation in PCE depending upon different structures, all samples exhibit a little hysteresis and high fill factors (~70 %), suggesting representative PV behavior, so that our samples deserve further investigation to reveal the effect of 1-D nanopillar structure on recombination and charge carrier dynamics in perovskite. Identifying the electrically active defects in 1-D perovskite is important to understand its recombination mechanism because the recombination of the photogenerated electron-hole pair strongly depends on the defect concentration and its energy level inside the band gap. It is wellknown that temperature dependent admittance spectroscopy (TAS) is a powerful tool for evaluating the defect energy level and density in the absorber layer in the solar cells.27-29 For the TAS analysis, the full device (Au/spiro-OMeTAD/perovskite (+AAO)/c-TiO2/FTO) was used and the capacitance spectra were obtained at the temperatures from 160 K to 300 K in the dark


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condition with a frequency range between 0.3 MHz and 100 Hz (Figure S5). The defect energy level, Ea, can be derived by the following equation: ωo (T) = ξoT2exp(-Ea/kT)

(1)

where ωo is the inflection frequency corresponding to the minima in ω·dC/dω vs. ω graph, ξo is the temperature independent parameter, k is Boltzmann constant, and T is the absolute temperature. In the Arrhenius plot of ln(ωo/T2) vs. 1/T, the slope of the line regression indicates the Ea. Figure S6 shows the defect energy level of both planar and 1-D nanopillared perovskite cells, revealing that all samples have similar defect energy level (0.24 - 0.25 eV) above the valence band maximum. The origin of this shallow defect could be ascribed to iodide ion vacancy acting as a non-radiative recombination center according to the literature.30 Defect density, NT, can be derived by the following equation:

NT = –

Vbi dC ω qW dω kT

(2)

where W is depletion width, Vbi is built in potential, q is an elementary charge, ω is AC voltage frequency, and C is capacitance. The values of Vbi and W are obtained from Mott–Schottky analysis (Figure S7). Note that before calculating (dC/dω), we normalized the measured capacitance by multiplying effective area of the perovskite: the effective area of 100% for the aspect ratio of 0, 32% for the 3.03, 26% for the 4.17, and 11% for the 6.67, respectively. The defect density distributions show broad spectrum in which maximum peaks are located at the defect energy level of 0.24 - 0.25 eV regardless of aspect ratios, indicating the defect density of perovskites (Figure 2a-d). Figure 2e shows the defect density value calculated from TAS measurement as a function of aspect ratio. These results exhibited that the 1-D perovskite nanostructures have explicitly lower defect density as compared to the planar structure.


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According to the previous studies on defect density in perovskite films, the defects such as halide ion vacancies could evolve during crystallization and slow crystallization rate allows for highquality perovskite crystal with lower defect density.31-33 Relatively lower defect density observed in our 1-D nanopillared perovskites as compared to planar structure could be also attributed to slow crystallization rate involved inside AAO scaffold. During one-step spin-coating process, the color of precursor solution gradually changes from yellow to dark brown, indicative of crystallization of perovskite films. The precursor solution deposited on AAO scaffold undergoes slower color change as compared to the one placed on the c-TiO2/FTO planar substrate, indicating the slow crystallization during the 1-D nanopillared perovskite formation. When perovskite crystals are nucleated inside scaffold, the tensile stress can develop, which raises the energy required for the crystal growth, thereby retarding the crystallization rate.34 It is noteworthy that 1-D perovskite with the aspect ratio of 6.67 reveals slightly higher defect density than the perovskite with the aspect ratio of 4.17. This is presumably due to higher interface area between perovskite and AAO scaffold since iodide ion vacancy forms more easily at the surface and interface than inside perovskite lattice, as discussed in a simulation study.35 Despite the difference in defect density among samples, negligible difference in J-V hysteresis was observed (Figure 1e) possibly due to the complicate relationship between the defect density and J-V hysteresis. J-V hysteresis is generally governed by multiple factors, such as ion migration36 ferroelectric polarization,37 and/or recombination at interfaces38. In addition, there are many studies in which the hysteresis strongly depends on the quality of interfaces, rather than the bulk defects.39 Since the typical charge transport materials were used in our study, we believe that small hysteresis of all samples is caused by the high-quality interfaces at ETL/perovskite and perovskite/HTM. Additionally, our results show a relatively lower defect density in 1-D


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nanopillar than thin film counterpart, in which the defect densities maintained within 1018 level. Considering the fact that the defect density of perovskite obtained from TAS analysis can vary from 1016 to 1018 level depending on the fabrication condition,40,28,29 the defect density variation in our study is insignificant to influence the J-V hysteresis. In addition to the identification of defect states, analyzing the electrical properties is crucial to understand the charge transport behavior in 1-D perovskite. The slop of tangential line drawn at VOC (x-intercept) where the current density is zero in the J-V curve allows us to compare the series resistance of each solar cell (Figure S8). Note that the identical c-TiO2 ETL and spiro-OMeTAD HTL were used in all device fabrication, thus the difference in series resistance is solely attributed to the difference in perovskite absorber structures. The resistivities of 1-D perovskite materials need to be compared in terms of area specific resistance by considering the effective area of nanopillared perovskite structures based on the obtained area fraction (Table S1). Since AAO is an insulator, having substantially higher resistance than perovskite materials, the contribution of AAO to the overall conductance is negligible in this parallel resistor structure. As the aspect ratio increases from 0 to 6.67, the area specific resistance significantly decreases from 29.45 to 4.33 Ω·cm2, indicative of lower resistivity of 1-D perovskite as compared to planar structure (Table S3). However, area specific resistance obtained from J-V curves is a mere indicator regarding the overall resistance of the whole device and the resistance contributed by absorber layer is unable to be deconvoluted. We performed impedance spectroscopy (IS) measurement on the full cells with different aspect ratios, which is able to analyze the resistance at each layer or interface, to gain insights into electrical properties such as charge transfer, recombination, and charge accumulation of perovskite solar cells. Figure 3a-d shows the Nyquist plot by measuring the impedance at a range


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between 0.3 MHz and 1 Hz with an AC voltage of 10 mV under varying light intensity conditions. Two distinctive arcs are observed, which are commonly seen in the IS analysis of perovskite solar cell.41-44 Widely used equivalent circuit model (inset of Figure 3d) was employed to fit this Nyquist plot.41,44 Equivalent circuit model consists of apparatus resistance (RS), bulk perovskite resistance (Rbulk), and geometrical capacitance (Cgeo), which are observed at the high-frequency region. The model also includes ion movement resistance (Rion) and surface accumulation capacitance (CS), which are obtained at the low-frequency region. However, the slow components (i.e., Rion and CS) below 1 Hz where reflects ion movement in bulk perovskite can be excluded when optoelectrical properties are only concerned in this study, so that Figure 3e and f show the fitted fast component values as a function of light intensity for the nanopillared perovskites with varying aspect ratios. The slopes of the resistance and capacitance values are similar regardless of aspect ratios, suggesting that the mechanism of charge transfer or recombination is identical for all structures and in turn the IS comparative study for understanding the 1-D nanostructure difference in the perovskite layer is valid.45 The geometrical capacitance (Cgeo) derived from high frequency arc in the Nyquist plots literally means the electronic capacitance resulting from the geometrical factors, such as roughness and device structure. Thus, the Cgeo value could be significantly changed when different geometrical factors are involved. For example, the Cgeo of perovskite film is largely varied depending on the structure of the mesoporous TiO2 layer.46 In our case, all four samples, including thin film and 1D structured, exhibited similar Cgeo values after being normalized with effective area of perovskite layer, indicating that all of our perovskite samples, regardless of the aspect ratio, have similar geometry except for the effective area (Figure 3f). This similar vertical geometry as confirmed by similar Cgeo enabled us to compare the resistance values accurately among the


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samples. The Rbulk values shown in Figure 3e decrease with the increasing aspect ratio, implying that the 1-D nanopillared perovskite structures exhibit better charge transport behavior due to the confined one-dimensional carrier path along the vertical direction. This observation is consistent with the variations in area specific resistances obtained from J-V curve (Table S3). According to the simulation study on perovskite charge transport conducted by Zhao et al., tetragonal perovskite has a mean free path of 67.3 nm for electrons and 284 nm for holes.47 Since the mean free path is larger than the diameter of AAO (30 - 66 nm), the AAO scaffold could restrict carrier path along the vertical direction and in turn photo-generated electrons and holes are readily transported and extracted to ETL and HTL. Time-resolved photoluminescence (TRPL) analysis with/without HTM was performed to observe the dynamics of the photo-excited carriers in the 1-D nanostructured perovskite as shown in Figure 4. Figure 4a shows the TRPL results without HTM in which all spectra were fitted with a bi-exponential function with the fast (τ1) and slow (τ2) lifetime components. One possible explanation of the bi-exponential decay in TRPL results is the mono- and bi-molecular recombination as observed in the previous reports.48 Note that we used excitation fluence of 1 µJ/cm2 corresponding to 4.36 × 1016 carriers/cm3 and the recombination mechanism in TRPL analysis is changed at ~ 1015 carriers/cm3 according to the literature.49 At the excitation fluences lower than 1015 carriers/cm3, monomolecular recombination between photoexcited electrons and unintentionally photo-doped holes predominantly occurs, showing mono-exponential PL decay. When the excitation fluence is higher than 1015 carriers/cm3, the bimolecular recombination between photoexcited electrons and holes begins to appear since the all traps are filled by photoexcited carrier. Beyond that, both mono- and bimolecular recombination coexist in TRPL signal and the bi-exponential PL decay is observed. Because we used excitation fluence higher than


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1015 carriers/cm3, both mono- and bimolecular recombination could coexist in our case regardless of the aspect ratio. In this case, the fast lifetime (τ1) could represent bimolecular recombination, while the slow lifetime (τ2) is possibly related to monomolecular recombination of electrons with photo-doped holes. Dominant recombination process between bimolecular and monomolecular can be discernable by comparing the weight fraction of two lifetime values. As shown in Figure 4b and Table 2, the weight fraction of τ1 is much higher in planar perovskite (65.40%) than 1-D nanopillared perovskite (54.22% for 3.03 aspect ratio, 51.86% for 4.17 aspect ratio, 52.20% for 6.67 aspect ratio). This means that planar structure retains higher bimolecular recombination induced by higher defect density than 1-D perovskites. However, all 1-D structures with three aspect ratios exhibit similar weight fractions with less than 1-2% difference. For a better understanding of the recombination rate, the τ2 values are compared. It was observed in Figure 4c that the slow lifetime (τ2), which represents the photo-carrier lifetime of the perovskite layer, reaches the maximum at the aspect ratios of 4.17, followed by a decrease when the aspect ratio of the perovskite is 6.67. This is presumably because more interfacial defects as compared to bulk defects are generated as the interface-to-volume ratio increases. This observation supports the TAS results (Figure 2e) in which planar perovskite has higher defect density than 1-D nanopillared perovskites. It has been reported that mesoporous Al2O3 scaffold based perovskite showed shorter PL lifetime than planar structure.50,51 This is because the mesoporous structure has extremely large surface area, leading to the formation of polycrystalline perovskites having significantly high defect density. By contrast, when grown in the AAO with a diameter of 30 - 66 nm, this well-controlled scaffold allows the formation of aligned 1-D nanopillared high-quality perovskite crystals, leading to longer carrier lifetime as compared to planar structure.


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In the case of the TRPL with HTM quenching layer (Figure 4d), the fast lifetime (τ1) is related to the time at which the electrons located in the excited conduction band are transported to the HTM, whereas the slow lifetime (τ2) reflects radiative recombination of carriers. When considering the weight fraction of fast time values (Figure 4e), the weight fraction of τ1 tends to increase from 65.44% to 90.99% with the increasing aspect ratio from 0 to 6.67. This means that the charge recombination loss lessens and the charge extraction becomes dominant as the aspect ratio of one-dimensional structure increases. The τ1 values, related to the transport and extraction of the photo-excited carrier, as a function of aspect ratio are presented in Figure 4f. It was found that τ1 decreases to 2.63 ns, 2.25 ns, 2.14 ns, and 1.99 ns as the aspect ratio is increased to 0, 3.03, 4.17, and 6.67, respectively. This implies that carrier extraction rate improves when perovskite is formed into 1-D nanopillar shape, and further enhancement is expected as the aspect ratio increases (i.e., small diameter of AAO pore). Still, the question arises whether or not different crystallinity due to confinement during crystallization may affect to the charge transport behavior. Therefore, we have fabricated mesoporous Al2O3 scaffold based perovskite structure with similar thickness in order to distinguish the 1-D nanostructuring effect from the confinement effect (Figure S9a and b). The mesoporous Al2O3 scaffold can induce similar confinement effect to 1-D AAO template during the crystallization due to small pores of mesoporous Al2O3, but does not yield the 1-D morphology of the perovskite. Thus, if the enhanced charge transport observed in our 1-D nanopillared perovskite is caused by the nanoscale confinement effect during the crystallization, mesoporous Al2O3 scaffold based perovskite should reveal similar enhancement. Conversely, if the enhancement is due to the 1-D morphological effect, slower charge transport is expected to be observed in the mesoporous sample due to the randomly oriented scaffold. We have performed TRPL measurement of


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perovskite in mesoporous Al2O3 with/without spiro-OMeTAD quenching layer to investigate carrier dynamics in this mesoporous structure. Interestingly, perovskite grown in mesoporous Al2O3 pores shows similar PL lifetime to that grown in AAO sample (1-D nanopillar), indicating both perovskites have similar carrier lifetime (Figure S9c). However, when hole transport quenching layer was coated on the perovskite (Figure S9d), mesoporous Al2O3 exhibited longer PL lifetime than the planar or 1-D nanopillar structured perovskite, representing much slower charge transport in randomly oriented scaffold. These results clearly demonstrate that the fast charge transport in our 1-D nanopillared perovskite is not caused by the nanoscale confinement effect, but by the 1-D architecture. Therefore, it can be concluded that 1-D perovskite structures demonstrate better charge transport and extraction capabilities and reduced carrier recombination loss. Since fundamental material properties may not be directly correlated to the characteristics of full devices, the analysis of charge carrier dynamics in a full working device with an external circuit should be obtained. Especially at high light intensities, the positively charged iodide ion vacancies in the perovskite are filled by the photoelectron, leading to different charge transport/extraction or recombination characteristics, which is unobservable under weak intensities.52 Intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) analysis is a powerful technique that can investigate recombination or charge carrier dynamics of the full device by observing the change in voltage or current signal, while sweeping the frequency of intensity modulated light. In the high-frequency light, there is no voltage/current change because the cell does not have enough a time to react, but when the frequency is sufficiently low, there is a time to react, exhibiting the difference in current/voltage. In the current/voltage and frequency graph, the time value of the inflection point represents either


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electronic transport time or electronic lifetime of the photo-carriers. In this regard, IMVS/IMPS analyses were performed under varying light intensity between 20 - 50 mW/cm2. The charge transport rate is represented by the electronic transport time (τt) from τt = 1/(2πfIMPS) where fIMPS is the frequency at the minimum point of the imaginary part of voltage in IMPS measurement (Figure 5).34,44 Note that the term transport time does not mean the average time it takes for electrons to travel a certain distance. Instead, the transport time of IMPS analysis, obtained from the characteristic light modulation frequency, represents how fast the electrons could be injected into the electron transport layer at the interface. On the other hand, the recombination rate can be evaluated in terms of the electronic lifetime (τl) from τl = 1/(2πfIMVS), where fIMVS is the frequency at the minimum point of the imaginary part of current in IMVS analysis (Figure S10). Bode plots of the imaginary part of current as a function of modulated frequency using IMPS measurement for the samples with different aspect ratio under various illumination from 20 to 50 mW/cm2 are shown in Figure 5a, b, c, and d, in which vertical dotted lines in each graph represent fIMPS. The obtained electronic transport time as a function of light intensity are shown in Figure S11. The plots for carrier transport time vs. light intensity reveal similar slopes for all the perovskites with varying aspect ratios. This implies that all structures involve the same charge transport characteristics, confirming that the IMPS analysis based comparison between the samples having different aspect ratio is valid. Note that the time scales of IMPS are order of magnitude different than those of TRPL measurements. In TRPL measurement, luminescence signal resulting from the recombination of photoexcited electrons and holes is measured by the luminescence detector and the luminescence signal is generally observed within nano-second (ns) scale. IMVS/IMPS measurements, on the other hand, the changes in voltage/current signal are measured as a function of the modulation frequency of the light source and the rate constant


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is converted from the characteristic frequencies of the light modulation. Unlike the TRPL analysis that studies the recombination within semiconductor of interest, IMVS gives us information about the total recombination loss in the fully working solar cell. These working principle differences between these analyses lead to the difference in observation timescale. Figure 5e presents the electronic transport time of the photo-excited carrier obtained from IMPS analysis for the perovskites with different aspect ratio. With the increasing aspect ratio of the 1-D perovskite, the electronic transport time becomes diminished, indicative of faster charge transport/extraction property. This observation well matches with the IS and TRPL analyses, allowing us to understand that the 1-D structure indeed enables fast charge transport/extraction even under the working condition of full devices. In order to confirm whether our results of fast carrier transport in 1-D nanostructured perovskite structure can be applied to practical PV devices or not, we have measured the IMPS with thicker film and longer nanopillars. As shown in Figure S12a and b, we have fabricated devices with 1.7 times thicker (longer) perovskite absorber layer with the thickness of about 340 nm for both planar and 1-D nanopillar (while pillar diameter of 66 nm). These samples showed increased JSC than thinner ones shown in Figure 1 because the thicker perovskite could absorb more light, exhibiting about 12% power conversion efficiencies (Figure S12c and Table S4). Figure S13 presents that the 1-D structure shows a response at higher frequency of intensity modulated light than the planar structure, representing faster charge transport in 1-D structure, which is similar to the result in thinner (shorter) perovskite (Figure 5). These results of thicker (longer) perovskite layer clearly demonstrate that the fast charge transport in 1-D perovskite is not limited in short length and that 1-D structure can be applied to the practical PV device.


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Figure S10 shows the IMVS measurement based electronic lifetime of the photo-excited carrier for the perovskites with different aspect ratio. However, there is a significant inconsistency between TRPL and IMVS lifetime. The electronic lifetimes of 1-D nanopillared structures are shorter than that of the planar structure, although the nanopillared perovskites contain lower defect density as determined by both TAS and TRPL results. we hypothesized that this inconsistency is caused by the large difference in measurement conditions, such as device structure and light intensity. Specifically, TRPL analysis uses the pulsed light, while the IMVS measurement is performed under continuous illumination with 10% of intensity modulation. In addition, the light intensity in IMVS is generally much higher than that of TRPL. Also, there is a significant difference in sample configuration in which perovskite+AAO/glass is used for TRPL, while Au/HTM/perovskite+AAO/TiO2/FTO/glass is utilized for IMVS. Therefore, it is important to note that it is quite difficult to specify the recombination mechanism by IMVS due to the complicate device structure (Au/HTM/perovskite+AAO/TiO2/FTO/glass) containing much more interfaces that could be considered as recombination sites. Therefore, further study about the inconsistency between TRPL and IMVS is necessary. However, given that all characterization methods on defect density or lifetime, such as TAS, TRPL and IMVS, indicate the minimum defect density and longest lifetime in the 4.17 sample consistently, we can conclude that the defect density doesn’t follow linear trend. This non-linear behavior is probably influenced by the too many surface defects as above mentioned. Throughout the manuscript, we observed that 1-D structured perovskite with smaller diameter (i.e., higher aspect ratio) exhibits enhanced charge transport behavior as confirmed by impedance spectroscopy, TRPL, and IMPS measurements, showing consistent results. In this respect, the device such as photodetector and phototransistor, whose priority performance factor


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is fast response time related to fast charge transport and extraction time from absorber to charge transport layer, should choose small diameter (in our case, 30 nm) of 1-D structure to fully utilize the enhanced charge transport rate. However, the recombination rate likely increases when too many defects are generated at the AAO/perovskite interface (especially the nanopillar diameter becomes below 48 nm). Therefore, in case of the device that requires faster transport as well as less recombination such as light-emitting diode and solar cell, the optimization of nanopillar diameter in order to limit the formation of interface defects. The performance of 1-D nanopillared perovskite solar cells could be further enhanced if the proper interface modification is employed to reduce the interface recombination while maintaining the enhanced charge transfer in 1-D structure. The morphology and crystal structure of the perovskite could affect the charge carrier dynamics, so it is necessary to observe microstructural features associated with the 1-D nanopillared perovskites. As shown in Figure 6a, the planar perovskite reveals the XRD diffraction peaks at 14.27° and 28.59°, attributed to the (110) and (220) crystal planes, respectively, which well corresponds to the crystal structure of the previously reported tetragonal CH3NH3PbI3-XClX.31 In particular, no other peaks except for (110) plane appear, indicative of (110) preferred orientation as commonly observed in one-step fabrication method using MAI and PbCl255-57 Similar XRD diffraction patterns are also detected for 1-D nanopillared perovskites with three aspect ratios. Interestingly, the (110) peak shift of ~ 0.05° toward lower angle with respect to planar perovskite is observed for all 1-D nanopillared perovskites (Figure 6b). The peak shift is related to the extension in d-spacing from 0.6201 nm to 0.6223 nm. This volume expansion might be related to the tensile stress developed by the attachment of the growing perovskite crystal to the scaffold walls, while constraining the shrinkage during crystallization.34


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Increased energy for crystal growth retards the crystallization rate within AAO scaffold as compared to planar structure, supporting interfacial area-dependent defect density difference between planar and 1-D nanopillared perovskites, as described in Figure 2e. To confirm crystal volume expansion of the 1-D perovskite, steady state photoluminescence (PL) was measured as presented in Figure S14. The PL signal for 1-D nanopillared samples reveals peak position located at smaller wavelength (4 - 6 nm) as compared to thin film counterpart, while all samples show similar FWHM values. This blue-shifting in the peak position with fixed FWHM indicates slightly enlarged band gap in 1-D nanostructures without formation of additional recombination centers because FWHM would get broader if additional recombination centers are generated upon the nanostructuring. This slightly enlarged band gap can be attributed to the crystal volume expansion of 1-D perovskite. When compared to the peak intensity of FTO substrate as an internal reference, on the other hand, the peak intensity of (110) plane increases dramatically as the aspect ratio increases from 0 to 4.17, but decreases as the aspect ratio further increases to 6.67. The peak intensity variation as a function of aspect ratio reflects the perovskite crystal quality. It is believed that 1-D nanopillared perovskites exhibit better crystal quality as manifested by a fewer defect density as compared to planar structure, despite of less absolute perovskite volume involved in each sample. However, the lower peak intensity at the aspect ratio of 6.67 implies the deteriorated crystallinity with large defective states when grown in a very small pore due to the increased interface-to-volume ratio as reported by Im et al.21 Figure 6c shows a low magnification transmission electron microscope (TEM) image of an AAO scaffold based perovskite full cell with the aspect ratio of 3.03 cross-sectioned by focused ion beam (FIB). The perovskite phase grown inside AAO scaffold seems to be polycrystals composed of small granules. However, selected area electron diffraction (SAED)


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obtained from red-circled region reveals sharp diffraction spots as shown in an inset of Figure 6c, indicating that nanopillared perovskite exists as quasi-single crystals rather than polycrystals. Polycrystalline-like morphology appears presumably due to the physical damage by FIB during cross-sectioning the sample. Similar morphological features are observable for the perovskite with the aspect ratio of 6.67 in which the SAED patterns indicate that nanopillared perovskite shows one zone axis even when two or three nanopillars are taken together (Figure S15). Besides the sharp diffraction spot, several extra spots with diminished intensity might be responsible for the FIB induced damage. In particular, since the zone axis represented by the SAED pattern is the [001] direction, the perovskite crystals are aligned to [001] direction (or (110) plane) parallel to the c-TiO2. HRTEM result also reveals that the (220) plane is aligned in parallel with the AAO wall (Figure 6d), which is consistent with the XRD result regarding a preferred orientation along the (110) plane direction. It should be also noted here that the interplanar distance of (220) plane is observed to be ~0.325 nm at the nanopillared perovskite, which is slightly large than reported values of ~0.317 nm.31,57 This could reflect the volume expansion of perovskite crystals when grown inside AAO pore as explained in the XRD diffraction peak shift. Microstructural investigation in conjunction with crystallographic evaluation support that all 1-D nanopillared perovskites including planar structure have the (110) preferred growth direction, implying that crystallographic orientation has insignificant influence on our analysis of the carrier dynamics. Furthermore, one-dimensional perovskites grow into quasi-single crystals, which can serve as a valid model system for analyzing the charge carrier dynamics by excluding the influence of the grain boundaries.

CONCLUSIONS


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The effects of 1-D perovskite structure on recombination and charge carrier dynamics have been carefully investigated by utilizing an AAO thin film scaffold to fabricate a vertically aligned 1-D nanopillared structures with controllable diameters. Both series resistances extracted from the JV curve and IS analysis reveal that the one-dimensional perovskite has lower resistances of the absorber as compared to planar counterpart. TRPL and IMPS analyses demonstrate that onedimensional structure facilitates the charge transport/extraction rate with respect to planar structure, and increased aspect ratio of 1-D perovskite further improves the charge transport/extraction rate. The AAO scaffold likely interrupts the lateral charge transport, whereas carrier transport along the vertical direction is enhanced and in turn photo-generated electrons and holes are readily transported to ETL and HTL. TAS and TRPL analyses also prove better crystal quality associated with one-dimensional perovskite due to slow crystallization. However, the defect density increases due to high interface-to-volume ratio as the pillar diameter reaches 30 nm (aspect ratio of 6.67), and the charge recombination is aggravated. With the considerations of both recombination and charge carrier dynamics, it is crucial to control the interfacial defect states by varying the crystal growth rate and to passivate the surfaces of 1-D structure to impede the recombination. To obtain the full benefits of 1-D perovskite nanostructuring, our study proposes a design rule to choose the proper aspect ratio of 1-D perovskite structure for the improved PV and other optoelectrical applications. Microstructural and crystallographic evaluations confirm that crystal orientation has insignificant influence on our analysis of the carrier dynamics and our AAO scaffold based structure serves as a valid system for analyzing the sole effect of one-dimensional perovskites on the recombination and charge carrier dynamics.


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EXPERIMENTAL METHODS Fabrication of AAO scaffold perovskite solar cells: FTO glass was cleaned using the ultrasonic bath for 30 min in ethanol, followed by UVO treatment for 15 min. After partially taping for bottom electrode contact, compact TiO2 layer was spin-coated using the precursor solution of titanium isopropoxide (0.6816 g, 99.999%, Sigma Aldrich) and HCl (0.084 ml) in ethanol (8 ml) at 4000 rpm for 30 s. After 10 min drying at 120℃, the samples were annealed at 500℃ for 30 min in a box furnace. For fabricating AAO scaffold on the compact TiO2 layer, 270 nm thick aluminium was thermally deposited with a rate of 1 ℃/s on the 2×6.5 cm substrate, followed by an anodizing and widening process. For anodizing Al film, 40 V of DC voltage was applied in the oxalic acid solution (30 g oxalic acid powder in 800 ml of DI water) with carbon cathode. After that, Al was converted to the oxide, followed by rinsing with DI water. The widening process was performed by dipping the whole substrate to the phosphoric acid solution for 10, 20, and 30 min to control the pore diameters of AAO scaffolds. The fabricated substrate was annealed at 500℃ for 30 min to eliminate the residues. After cooled down, four different substrates (compact TiO2 substrate without AAO and with AAO pore diameter of 30, 48, and 66 nm) were UVO treated for 30 min. Then, 23 wt% of perovskite precursor solution with 3:1 molar ratio of methylammonium iodide (CH3NH3I, 99.9%, Dyesol) and lead chloride (PbCl2, 99.999%, Sigma Aldrich) in N, N-Dimethylformamide (DMF, anhydrous 99.9%, Sigma Aldrich) was spin-coated at 4000 rpm for 60 s after the solution was aged more than 5 h. After annealed at 100℃ for 105 min, the HTL solution of 72 mg of 2,2',7,7'-Tetrakis-(N,N-di-4methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD, 99.9%, Borun chemistry) in 1 ml chlorobenzene with addition of 28.8 µL of 4-tert-butylpyridine (tBP) and 17.6 µL of bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) solution (520 mg in acetonitrile) was


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spin-coated at 3000 rpm for 30 s. For the top electrode, 70 nm Au was thermally evaporated on the top of the device with the rate of 1℃/s. For the PL analysis, all processes were identically performed except for the uses of glass substrate instead of FTO glass and compact TiO2. Characterizations: The 1-D nanostructured perovskites were observed by using field emission scanning electron microscope (FE–SEM, JSM–7001F, JEOL Ltd., Tokyo, Japan). The phase evolution of perovskite was determined by X-ray diffractometer (Rigaku Miniflex 600, The Woodlands, USA). High-resolution transmission electron microscopy (HR-TEM) image and selected area electron diffraction (SAED) pattern was obtained using ultra corrected energy filtering TEM (Libra 200 HT Mc Cs, Zeiss, German). All TEM samples were prepared using a dual focused ion beam (FIB) system (Quanta 3D FEG, USA). After depositing platinum protective layer on the top of samples of interest, rough and fine millings using Ga ion with current of 15 nA and 50 pA, respectively, were done. The prepared lamella samples were cut and mounted on the TEM grids using in-situ lift-out technique. The J-V characteristics were determined using a solar simulator (Sol3A Class AAA, Oriel Instrument, Irvine, USA) and digital source meter (Keithley Instruments Inc., Cleveland, USA) under calibrated 1 sun (100 mW/cm2) and 1.5 AM condition with certified Si reference cell (Newport Corporation, Irvine, USA). A metal mask was used for defining the active area (0.06 cm2). Temperature dependent admittance spectroscopy was measured using frequency response analyzer (1252A, Solartron, England) with perturbation of 30 mV AC with a frequency sweep from 0.3 MHz to 0.1 Hz under dark and short circuit condition. For the temperature control of the device, temperature– controlling stage combined with a nitrogen cooling system (LNP95, Linkam Scientific Instruments, UK) was used. Impedance spectroscopy was measured using a frequency response analyzer (1252A, Solartron, England) while applying a perturbation of 10 mV AC with a


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frequency between 0.3 MHz and 0.1 Hz under varying intensities of white light illumination. Time-resolved photoluminescence was also measured using time-correlated single-photon counting (TCSPC) system (MicroTime-200, Picoquant, Germany). A single-mode pulsed diode laser (470 nm with a pulse width of ~100 ps and an average power of < 0.1 µW) was used as an emission source. A dichroic mirror (490 DCXR, AHF), a long-pass filter (HQ500lp, AHF), a 75 µm pinhole, a 700 nm long-pass filter and a single photon avalanche diode (PDM series, MPD) were used to collect emissions from the samples. Intensity modulated photocurrent/photovoltage spectroscopy measurements were performed using electrochemical workstation (Zennium, Zahner, German) and a potentiostat (PP211, Zahner, German) under various white light intensity, and light intensity modulation was applied with 10% of DC voltage.

ACKNOWLEDGMENT This work was supported by a grant from the National Research Foundation of Korea funded by the Korean government (MISP) (No. 2012R1A3A2026417).

SUPPORTING INFORMATION SEM images of cross-sectional view of full cell, top view of AAO, and top view of planar perovskite, tables of AAO pore size and solar cell performance parameter, temperature dependent admittance spectroscopy data, J-V curve and table for series resistance calculation, bode plot and electronic lifetime and electronic transport time from IMVS and IMPS measurements, TEM image and SAED pattern. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR CONTRIBUTIONS J.M. directed and conceived the study and experiments. H.-C. K., E. L., S. M., K. K., and S. Y. fabricated and analysed the cells. H.-C. K., W. Y., D. L., J. A., and J. M. co-wrote the manuscript. All authors contributed to discussion of the results and characterization.

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(a)

(b)

~0 aspect ratio (Planar)

3.03 aspect ratio (66 nm pore)

Spiro-OMeTAD

Spiro-OMeTAD perovskite

AAO+perovskite

glass

glass

100 nm

100 nm

(c)

(d)



Spiro-OMeTAD

Spiro-OMeTAD

AAO+perovskite

AAO+perovskite

glass

glass

100 nm

100 nm

(e) Current density (mA/cm2)

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

Page 32 of 40

18

Aspect ratio

0 3.03 4.17 6.67

15 12 9 6 3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

Figure 1. SEM images of spiro-OMeTAD/perovskite+AAO/glass samples for CH3NH3Pb3-XClX perovskites with aspect ratios of (a) 0 (without AAO), (b) 3.03, (c) 4.17, and (d) 6.67. (e) J-V curves of the cells with four different aspect ratios. Inset schemes represent vertically aligned 1D nanopillared perovskite structures with varying aspect ratios. Solid and open symbols indicate negative (VOC → JSC) and positive (JSC → VOC) voltage scan directions, respectively.


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Table 1. Cell performance parameters of 1-D nanopillared perovskite with various aspect ratios Aspect ratio

Scan direction

VOC (V)

JSC (mA/cm2)

0.00

Negative

0.86

14.31

70.74

8.69

Positive

0.84

14.96

67.29

8.41

Negative

0.99

13.91

67.05

9.22

Positive

0.96

13.95

62.90

8.38

Negative

0.99

10.10

69.75

6.99

Positive

0.97

10.18

63.01

6.23

Negative

0.98

6.83

69.62

4.68

Positive

0.97

6.78

63.01

4.15

3.37

4.17

6.67

Fill factor (%) Efficiency (%)


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ACS Nano

(a)

(b) Aspect ratio : 3.03

)

19

Defect density (cm ev

-1

10

19

10

-3

-3


-1

)

Aspect ratio : 0

18

10

300 K 280 K 260 K 240 K

17

10

220 K 200 K 180 K

16

10

160 K

0.1

0.2

0.3

0.4

18

10

300 K 280 K 260 K 240 K

17

10

220 K 200 K 180 K

16

10

0.5

160 K

0.1

0.2

0.3

(d) )

Aspect ratio : 4.17

-1

19

10

0.5

Aspect ratio : 6.67 19

10

-3

-3


-1

)

(c)

0.4

Ea (eV)

Ea (eV)


18

10

300 K 280 K 260 K 240 K

17

10

220 K 200 K 180 K

16

10

160 K

0.1

0.2

0.3

0.4

18

10

300 K 280 K 260 K 240 K

17

10

220 K 200 K 180 K

16

10

160 K

0.1

0.5

0.2

0.3

0.5

18

3.0x10

-1

)

(e)

0.4

Ea (eV)

Ea (eV)

18

2.5x10

-3


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

Page 34 of 40

18

2.0x10

18

1.5x10

18

1.0x10

17

5.0x10

0

1

2

3

4

5

6

7

Aspect ratio

Figure 2. Defect density distributions converted from TAS measurements. Maximum peaks are located at the defect energy level of 0.24 − 0.25 eV regardless of aspect ratio, indicating the defect density of perovskites with aspect ratios of (a) 0, (b) 3.03, (c) 4.17, and (d) 6.67. (e) Defect density derived from TAS measurements as a function of perovskite aspect ratios.


34

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ACS Nano

Figure 3. Nyquist plots of 1-D perovskite cells with aspect ratios of (a) 0, (b) 3.03, (c) 4.17, and (d) 6.67 obtained from impedance spectroscopy. The equivalent circuit model (inset of (d)) was used for fitting. Various light intensities were used and the samples were under short circuit conditions. The fitted fast component values of (e) perovskite bulk resistance (Rbulk) and (f) geometrical capacitance (Cgeo) are plotted as a function of light intensity for the nanopillared perovskites with varying aspect ratios.


35

ACS Nano

Aspect ratio 0 3.03 4.17 6.67

Without HTM

0

10

-1

10

(d) 10

PL intensity (A.U.)

PL intensity (A.U.)

(a)

Aspect ratio 0 3.03 4.17 6.67

With HTM

0

-1

10

-2

10

0

20

40

60

80

100

120

0

3

6

Time (ns)

(b)

(e) 90 Weight fraction (%)

Weight fraction (%)

9

12

15

Time (ns) τ1 (fast lifetime) τ2 (slow lifetime)

80

60

40

20

τ1 (charge extraction) τ2 (radiative recombination)

75 60 45 30 15 0

-1

0

1

2

3

4

5

6

-1

7

0

1

2

3

4

5

6

7

5

6

7

Aspect ratio

Aspect ratio

(f)

(c)

2.6 90

τ1 (ns)

2.4

τ2 (ns)

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

Page 36 of 40

60

2.2

2.0

30 -1

0

1

2

3

4

5

6

7

-1

0

1

2

3

4

Aspect ratio

Aspect ratio

Figure 4. Time-resolved photoluminescence graph of 1-D perovskite on glass (a) without, and (d) with spiro-OMeTAD quenching layer. The weight faction of time values obtained by fitting using bi-exponential model (b) without and (e) with HTL. (c) PL lifetime values of τ2 as a function of aspect ratio for the perovskites without HTL, and (f) PL lifetime values of τ1 for the perovskites with HTL.


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ACS Nano

Table 2. TRPL fitting data using a bi-exponential model for the 1-D perovskite/glass samples with/without spiro-OMeTAD quenching layer

Aspect ratio Without HTL

With HTL

0

3.03

4.17

6.67

τ1 (ns)

6.67

21.80

22.28

10.84

Intensity (%)

65.40

54.22

51.86

52.20

τ2 (ns)

32.58

101.40

105.72

89.91

Intensity (%)

34.61

45.78

48.15

47.80

τ1 (ns)

2.63

2.25

2.14

1.99

Intensity (%)

65.44

69.02

80.73

90.99

τ2 (ns)

13.67

7.04

8.35

9.10

Intensity (%)

34.56

30.99

19.27

9.01


37

ACS Nano

(a)

(b) 0.000

-0.001

ImCurrent (A)

ImCurrent (A)

0.000

Aspect ratio : 0 2

20 mW/cm 2 30 mW/cm 2 40 mW/cm 2 50 mW/cm -0.002

fIMPS

10k

Aspect ratio : 3.03

-0.002

2

20 mW/cm 2 30 mW/cm 2 40 mW/cm 2 50 mW/cm

100k

10k

Frequency (Hz)

fIMPS 100k

Frequency (Hz)

(d)

0.000

-0.001

-0.001

-0.003

0.000

ImCurrent (A)

(c)

ImCurrent (A)

Aspect ratio : 4.17 2


Aspect ratio : 6.67

-0.001

2

fIMPS

-0.002 10k

100k


fIMPS

10k

100k

Frequency (Hz)

Frequency (Hz)

(e)

2

20 mW/cm

1.8

2

30 mW/cm

2

1.6

40 mW/cm

2

50 mW/cm

τt (µs)

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

Page 38 of 40

1.4 1.2 1.0 0.8 0

1

2

3

4

5

6

7

Aspect ratio

Figure 5. Bode plot of the imaginary current part of samples with aspect ratio of (a) 0, (b) 3.03, (c) 4.17, and (d) 6.67 using IMPS measurement under various illumination intensities. The fIMPS represents the frequency where each imaginary part reaches its minimum value. (e) Calculated τt value as a function of aspect ratio under various illumination intensities.


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ACS Nano

Figure 6. (a) X-ray diffraction patterns of 1-D nanopillared perovskite with various aspect ratios and (b) a magnified view of the peak at 14.2° corresponding to the (110) plane. (c) TEM images of a spiro-OMeTAD/perovskite+AAO/c-TiO2/FTO cell with an aspect ratio of 3.03 together with an SAED pattern obtained from the red circled area. The sample was cross-sectioned by focused ion beam. (d) HRTEM image of the sample shown in (c).


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Page 40 of 40

Graphical Abstract


40

Investigating Recombination and Charge Carrier Dynamics in a One

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