Facile Sol-Gel Derived Crater-like Dual-functioning TiO2 Electron

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Facile Sol-Gel Derived Crater-like Dual-functioning TiO2 Electron Transport Layer for High Efficiency Perovskite Solar Cells Sunihl Ma, Jihoon Ahn, Yunjung Oh, Hyeok-Chan Kwon, Eunsong Lee, Kyungmi Kim, Seong-Cheol Yun, and Jooho Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

Facile Sol-Gel Derived Crater-like Dual-functioning TiO2 Electron Transport Layer for High Efficiency Perovskite Solar Cells Sunihl Ma, Jihoon Ahn, Yunjung Oh, Hyeok-Chan Kwon, Eunsong Lee, Kyungmi Kim, SeongCheol Yun, and Jooho Moon*

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro Seodaemungu, Seoul 03722, Republic of Korea

KEYWORDS: perovskite solar cells, electron transport layer, titanium dioxide, sol-gel chemistry, crater-like pore formation

ACS Paragon Plus Environment

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ABSTRACT

Organic-inorganic hybrid perovskite solar cells (PSCs) are considered promising materials for low-cost solar energy harvesting technology. An electron transport layer (ETL), which facilitates the extraction of photo-generated electrons and their transport to the electrodes, is a key component in planar perovskite solar cells. In this study, a new strategy to concurrently manipulate the electrical and optical properties of ETLs to improve the performance of PSCs is demonstrated. A careful control over the Ti alkoxide-based sol-gel chemistry leads to a craterlike porous/blocking bilayer TiO2 ETL with relatively uniform surface pores of 220 nm diameter. Additionally, the phase separation promoter added to the precursor solution enables nitrogen doping in the TiO2 lattice, thus generating oxygen vacancies. The crater-like surface morphology allows for better light transmission due to reduced reflection, while the electrically conductive crater-like bilayer ETL enhances charge extraction and transport. Through these synergetic improvements in both optical and electrical properties, the power conversion efficiency of craterlike bilayer TiO2 ETL-based PSCs could be increased from 13.7% to 16.0% as compared to conventional dense TiO2-based PSCs.


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INTRODUCTION Astoundingly high power conversion efficiencies (PCEs) of over 21% have been realized in perovskite solar cells (PSCs) based on mesoporous structures,1 such high efficiencies resulted from a combination of desirable inherent optoelectrical properties of the hybrid perovskite material itself2-6 and developments in structural design, film crystallinity control, and interface engineering.7-9 In the current state-of-the-art, PSCs based on TiO2-based mesoporous structures are known to exhibit high efficiency and stable power output.10 However, for further improvement in the PCE, optical engineering can be employed to boost solar energy absorption.11 A number of approaches have been attempted to harvest greater amounts of light, including embedding light scattering particles inside mesoporous (mp) TiO2 layers12 and incorporating plasmonic nanoparticles into scaffold layers.13-14 However, embedding additional particulate materials has a detrimental effect on electron transport properties; further, it leads to only a marginal improvement in the PCE owing to a concurrent drop in the open-circuit voltage (VOC).12 Kang et al. successfully applied conical-shaped moth-eye 500 nm-dorm structures into mp-TiO2 layers for enhancing their light harvesting efficiency.15 However, additional steps, such as nanoimprint lithography and stamping, are required to fabricate ordered surface nanostructures. Planar-structured PSCs, which have a simpler architecture compared to mesostructured PSCs, have been developed after it was found that hybrid perovskite materials have micrometer range charge carrier diffusion lengths and ambipolar properties.5,

16

Compact TiO2 is also

commonly used as an electron transport layer (ETL) in planar structures, due to its excellent ability to prevent shunting and leakage currents under reverse bias.17 However, unlike several hundred nanometers-thick mp TiO2, the application of an anti-reflection coating on 50–60 nm


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thick TiO2 ETL poses practical limitations. Therefore, recent studies have been rather focused on the improvement of the electrical properties of TiO2 to boost PCE. In this context, doping, a strategy that leads to enhanced charge extraction, is often implemented.18-19 Liu et al. reported that Li doping in a planar-structured TiO2 layer can reduce the number of electron traps and enhance the conductivity of TiO2, thereby leading to higher PCEs as compared to undoped planar TiO2-based PSCs. Recently, Atikur et al. reported that densely packed silicon nano-textures with features smaller than 50 nm enhance broadband anti-reflection in thin film Si solar cells20 without adversely affecting their charge extraction and recombination properties. This observation suggests that nanoscale surface structuring can possibly be used to modulate the optical properties of compact TiO2 ETLs in planar-structured PSCs. It has been reported that TiO2 causes a reduction in transmittance due to its high reflectance, which suggests that ETLs with lower refractive index values are necessary in planar PSCs for efficient light transmission without reflection. In this regard, it is expected that ETL optical engineering along with doping would enhance charge extraction in planar PSCs. Herein, we present a facile sol-gel chemistrybased method to simultaneously tune the optical and electrical properties of TiO2 ETLs without additional complex processes. A TiO2 sol-gel ink containing titanium isopropoxide (Ti(OPr)4), 2methoxyethanol(2ME), acetylacetone (AcAc), and ammonium nitrate (AN), was utilized to deposit TiO2 films. The resulting TiO2 layers exhibit a crater-like porous structure with pores of 100−300 nm diameter and ~50 nm depth. This crater-like porous structure can decrease the difference between the refractive indices of fluorine-doped tin oxide (FTO) and TiO2, thus leading to a reduction in reflectance and increasing light transmission. In addition, it was found that the crater-like TiO2 structure includes interstitially doped nitrogen in its lattice, which


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enhances the conductivity of the ETL. In this manner, the optical and electrical properties of TiO2 layers could be simultaneously improved. Compared to dense TiO2-based planar devices, the crater-like porous/blocking bilayer structured (denoted as crater-like bilayer) TiO2-based PSCs exhibit 14.5% improvement in photocurrent density and 19.5% enhancement in PCE.

RESULTS AND DISCUSSION Figure 1a shows the top view of a scanning electron microscopy (SEM) image of a conventional dense TiO2 thin film commonly used as an ETL in PSCs; the ETL was fabricated using a conventional method based on a titanium isopropoxide (Ti(OPr)4) precursor stabilized by hydrochloric acid in anhydrous ethanol.21 A detailed description is included in the Experimental Section. A dense film with a very smooth surface finish without any pinholes was observed, resembling the microstructures of TiO2 ETLs reported in other studies.22 Rough structures seen in the magnified image of the cross-section reflect the surface structure of the underlying FTO glass substrate. In contrast, when the precursor solution designed for porous TiO2 was used, an entirely different microstructure with crater-like surface pores could be observed (Figure 1b). Circular pores with diameters in the range of 100−300 nm could be observed on the surface and these crater-like pores had a vertical depth of ~50 nm, as seen in the cross-sectional image. Further, the crater-like pores were distributed uniformly throughout the film, and no cracks or particulate agglomerates could be observed. This observation clearly indicates that a uniform porous film on top of a blocking TiO2 layer is readily obtainable as a bilayered ETL by simple sol-gel coating.


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Figure 1. Top view and cross-sectional SEM images of a) dense TiO2 film derived from the conventional sol-gel solution and b) crater-like bilayer TiO2 film derived from the AN- and AcAc- added sol-gel solution, in which the AcAc:AN ratio was 1:1. To examine the distribution of the crater-shaped pores and the surface morphology, atomic-force microscopy (AFM) analysis was conducted. Crater-shaped pores could be clearly observed on the AcAc/AN modified sol-gel derived TiO2, while a dense and smooth surface finish was evident for the conventional sol-gel derived TiO2 in Figure 2a. The average diameter of the crater-like pores was determined to be (219.5 ± 65.8) nm, while their depth was measured to be (42.8 ± 7.7) nm, both of which agree well with the SEM results. The root mean square (RMS) roughness value measured from the AFM surface profile significantly increased from 6.3 nm for dense TiO2 to 22.9 nm for the crater-like bilayer TiO2. Owing to this difference in the


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surface roughness, the crater-like bilayer TiO2 exhibits a surface area ~8.0% greater than that of dense TiO2. In addition, the areal pore fraction (%) of the crater-like bilayer TiO2 surface was calculated to be 50.4% ± 5.8% by image analysis and the pore volume fraction (%) of the craterlike bilayer TiO2 film was determined to be 23.8% ± 6.0% (see Supporting Information for calculation details).

Figure 2. Topographic AFM images and the average height profiles determined in an area of 2.5 × 2.5 µm2 for a) dense TiO2 and b) crater-like bilayer TiO2.

Although both precursor solutions involve Ti(OPr)4 as the main component, different solgel chemistries, owing to the modification of the surface tension of the solvent system by chemical additives, lead to considerably different films with unique surface morphologies. Scheme 1 illustrates the formation of the unique crater-like morphology of the TiO2 bilayer. In sol-gel chemistry, the selection of metal alkoxides, additives, stabilizers, and solvents predominantly affects the rates of hydrolysis and condensation as well as the phase stability, due to which the as-deposited precursor evolves into a solid film with a specific morphology.


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Specifically, Ti(OPr)4 is hydrophobic and hence is hardly miscible with polar solvents unless hydrolyzed. In the case of dense TiO2 films, hydrochloric acid in the solution rapidly hydrolyzes the Ti(OPr)4 precursor, while suppressing condensation. Thus, the HCl-catalyzed metal precursor can be homogeneously mixed with polar solvents during the drying/gelation step, resulting in a flat and dense thin film. Without HCl, it is necessary to retard the hydrolysis and condensation reactions by stabilization; if not controlled, hydrolysis and condensation can result in precipitation. We used acetylacetone as the stabilizer; it acts as a chelating agent and forms a highly stabilized molecular complex, (Ti(AcAc)2(OPr)2), with sufficient hydrophobicity.23 This precursor complex is stable in polar solvents without any phase separation when dispersed at low concentrations. However, it is considered that the addition of AN to the precursor solution (AcAc:AN = 1:1) facilitates self-assembling phase separation during gelation because of the increased surface tension of the polar solvent. AN, which is ionic compound in nature, can be dissociated in polar solvents, which increases its surface tension as well as ionic strength; this in turn induces phase separation. During the spin-coating process, the solvent rapidly evaporates and the concentration of the precursor complex abruptly increases, leading to phase separation between the precursor complex and high-surface tension polar solvent. Upon subsequent heat treatment, the chelated AcAc is decomposed from the Ti precursor and hydrolysis and condensation reactions occur.24 The remnant solvent droplets aggregate to reduce surface energy and move towards the surface of the film, leaving behind crater-shaped pores as the solvent droplets evaporate. These crater-like pores can be controlled by the amount of AN added with respect to AcAc. The pore size decreased to less than ~100 nm when the proportion of AN was low (AcAc:AN = 3:1, Figure S1a), whereas a large quantity of AN resulted in fewer and enlarged pores (AcAc:AN = 1:2, Figure S1c).


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Scheme 1. Proposed formation mechanism of crater-bilayer TiO2. a) Initial stage before spreading the precursor solution, b) molecular structure of Ti alkoxide stabilized by AcAc; the precursor becomes concentrated as the solvent dries during the spin coating process, c) phase separation occurs between the precursor and polar solvent, d) solvent clustering during the gelation process, e) solvent droplet evaporation, and f) 3-dimensional illustration of the craterlike bilayer TiO2.


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The photovoltaic performance of a device (Au/spiro-OMeTAD/perovskite/ETL/FTO configuration) with the crater-like TiO2 as the ETL was investigated. For comparison, a reference cell based on planar dense TiO2 was also fabricated. In addition, to select the optimal morphology of the porous layer on the blocking layer, different bilayered ETL samples were also prepared by controlling the ratio of additives as shown in Figure S2 (AcAc:AN = 3:1 denoted as crater 1, 1:1 denoted as crater 2, and 1:2 denoted as crater 3). When the sol-gel precursor solution contained the same ammount of AN with respect to AcAc (i.e., crater 2 sample), the crater 2 based device showed the highest PCE compared to other porous TiO2 bilayered based cells (Table S1). Based on these observations, the crater 2 sample was selected as the sol-gel precursor condition leading to the optimal morphology of TiO2, and this porous layer was coated on the blocking TiO2 to fabricate crater-like bilayer ETL. Figure 3a and b show the crosssectional SEM images of the PSCs based on the two different types of TiO2 ETL layers. It could be clearly seen that the thickness of each TiO2 layer was approximately the same; care was taken to deposit perovskite layers of similar thickness as well. In addition, the top view of the SEM images reveals that there was no noticeable difference in the perovskite grain size and almost identical crystallinities were confirmed by the X-ray diffraction (XRD) patterns (Figure S3). This observation suggests that the crater-like bilayer TiO2 ETL has no influence on the film growth and coverage of the perovskite phase with respect to the dense TiO2 counterpart. The discrepancy in the performance of these two PSCs is solely attributable to the unique properties of the crater-like bilayer TiO2 ETL and not to the quality of the absorber layer and/or its thickness.


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Figure 3. Cross sectional SEM images of perovskite solar cells with a) dense TiO2 and b) craterlike bilayer TiO2 ETLs. c) J-V curves and d) external quantum efficiency of champion cells based on either a dense TiO2 ETL or a crater-like bilayer TiO2 ETL under 1 sun illumination condition.


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Table 1. Photovoltaic performance parameters of the PSCs based on dense TiO2 and crater-like bilayer TiO2 ETLs Device type

VOC [V]

JSC [mA cm-2]

FF [%]

PCE [%]

Dense

1.05

18.44

70.4

13.7

Average

1.04 ± 0.03

18.73 ± 0.41

68.5 ± 2.7

13.3 ± 0.7

Crater-like bilayer

1.05

20.82

73.0

16.0

Average

1.03 ± 0.02

20.19 ± 0.63

72.2 ± 3.9

15.1 ± 0.9

The crater-like bilayer TiO2-based champion cell exhibited a PCE of 16.0%, with JSC, VOC, and fill factor (FF) values of 20.82 mA cm–2, 1.05 V, and 73.0%, respectively (Figure 3c and Table 1). These device parameters clearly indicate a significant improvement as compared to the dense TiO2-based reference cell, which exhibited a PCE of 13.7%, JSC of 18.44 mA cm–2, VOC of 1.05 V, and FF of 70.4%. The JSC values calculated from the external quantum efficiency (EQE) spectra for the crater-like bilayer TiO2 and dense TiO2 devices (Figure 3d) were 19.52 mA cm–2 and 18.20 mA cm–2, respectively; these values are consistent with the results of the J-V curves. The device statistics calculated using 15 cells for each type of ETL are shown in Figure S4 and Table 1. It should be noted that the devices based on the two types of ETLs exhibit apparent different average JSC values (20.19 mA cm–2 for crater-like bilayer TiO2 vs. 18.73 mA cm–2 for dense TiO2). While a very small drop was observed in the VOC, an increase in the FF value was observed in crater-like bilayer-based cells; this is attributed to the significant reduction in series resistance (RS) from 5.14 Ω·cm2 (dense TiO2 device) to 3.36 Ω·cm2 (crater-like bilayer TiO2 device) as shown in Figure S5.25 Consequently, the average PCE increased from 13.3% to 15.1% when the dense ETL was replaced by the crater-like bilayer TiO2.


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Figure 4. a) Reflectance spectra of the dense TiO2/FTO/glass and crater-bilayer TiO2/FTO/glass substrates. b) Transmittance spectra of dense TiO2/FTO/glass and crater-bilayer TiO2/FTO/glass as well as the spectral irradiance at AM 1.5G. c) Reflectance and d) absorbance of perovskite/dense TiO2/FTO/glass and perovskite/crater-like bilayer TiO2/FTO/glass.

In order to investigate the origins of performance improvement, the optical properties of the TiO2 layers were studied by measuring the reflectance values of the TiO2 layers deposited on FTO/glass; both dense and crater-like bilayer TiO2 films exhibited similar thickness values (~80 nm). The thickness of the films could be controlled by controlling the spin-coating rotation speed. The average reflectance of the crater-like bilayer TiO2 (21.1%) is lower than that of dense TiO2 (21.6%) in the wavelength range of 350 nm to 800 nm, as shown in Figure 4a. This can be


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explained by the fact that the crater-like pores lowered the refractive index of the TiO2 film. Unlike dense TiO2, which has a large refractive index (~2.6), crater-like porous TiO2 can be considered as a mixture of air and dense TiO2. In this regard, the effective refractive index (n) of porous films can be calculated using the Maxwell Garnett model26-27 (n2 -n21 ) (n2 +2n21 )

=(1-f1 )

(n22 -n21 ) (n22 +2n21 )

(1)

where n1 and n2 are the refractive indices of the two constituent phases and f1 and f2 = (1 – f1) are their corresponding volume fractions. The effective refractive index of the crater-like bilayer TiO2 was estimated using the pore volume fraction (f1 = 0.238) obtained from AFM analysis and the refractive indices of air (n1 = 1.0) and TiO2 (n2 = 2.6). The estimated refractive index of the crater-like bilayer TiO2 (n) was 2.3; this resulted in a smaller difference between the refractive indices of the crater-like bilayer TiO2 and FTO (nFTO = 1.9) compared to the corresponding difference between dense TiO2 and FTO. A large difference in the refractive index value at the heterointerface gives rise to significant reflection; therefore, the introduction of a crater-like morphology can allow more light tranmission as the refractive indice difference between TiO2 ETL and FTO becomes narrow. The TiO2 layers with different porous morphologies exhibited different transmittance in which the transmittance in the range of wavelength from 450 nm to 800 nm showed a decreasing tendency with the reducing pore volume fraction (crater 3 > crater 2 > crater 1 in Figure S6). Correspondingly, the crater-like bilayer TiO2 exhibited a higher total transmittance than dense TiO2 due to the reduced reflection (Figure 4b). Dense TiO2/FTO enabled the transmission of more light at wavelengths 450 nm, because the crater-like bilayer is capable of transmitting more photons to the perovskite layer in this region. It can thus be concluded that crater-like TiO2 possesses better light harvesting efficiency than dense TiO2, likely leading to the observed improvement in the PSC performance. The optical improvement associated with the crater-like morphology cannot fully explain the enhancement in the PCE (~19.5%) with respect to the increased transmitted photon energy (~4%). Furthermore, it is difficult to rationalize that the crater-like bilayer TiO2-based cell has higher EQE values (Figure 3d), even at wavelengths below 450 nm, where it suffers from lower transmittance than dense TiO2 (Figure 4b). These considerations led us to investigate the differences in the charge extraction capability of both the ETLs. To quantitatively examine the charge extraction dynamics of the photo-excited carriers generated inside PSCs with a PMMA/perovskite/ETLs/glass configuration, we measured the steady state and time-resolved photoluminescence (PL). Weak light of ~0.1 µW was irradiated onto the glass side to precisely analyze the extraction dynamics of the two types of TiO2 ETLs. Figure 5a and b display the steady state and time-resolved PL spectra, respectively. As shown in Figure 5a, noticeably


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reduced PL intensities were observed in crater-like bilayer sample. This indicated that faster quenching process occurs at the perovskite/crater-like bilayer TiO2 interface than the perovskite/dense TiO2 interface due to more efficient charge extraction of crater-like TiO2. The time-resolved PL curves were fitted with a bi-exponential decay function consisting of a fast decay (τ1) component and a slow decay component (τ2). The fast decay component in a relatively short time scale of a few tens of nanoseconds represents carrier transport from the perovskite to the ETL, whereas the slow decay component at a scale of a few hundreds of nanoseconds results from radiative decay in the perovskite layer.29 As shown in Figure 5b and Table S2, the fast decay lifetime of crater-like bilayer TiO2/perovskite slightly decreased from 57.3 ns to 54.3 ns when compared to dense TiO2/perovskite. Further, the weight fraction of the fast decay component increased from 33.7% to 34.8% upon the introduction of a crater-like bilayer, suggesting better charge extraction through the crater-like bilayer. Smaller τ1 and its larger weight fraction can be interpreted as efficient charge extraction of photo-excited electron from perovskite layer, contributing to fast quenching process as identified in Figure 5a.


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a) dense crater-like bilayer

Perovskite

Intensity (a.u.)

TiO2 glass

650

700

750

800

850

Wavelength (nm) Normalized PL intensity (a.u)

b) 1 Perovskite TiO2 glass

0.1

dense crater-like bilayer

0.01 0.0

0.1

0.2

0.3

0.4

0.5

Time (µs )

-2

)

c) Current density (mA cm

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


0.02 dense crater-like bilayer

0.01

0.00 Au TiO2 FTO

-0.01

-0.02

-0.4

-0.2

0.0

0.2

0.4

Potential (V)

Figure 5. a) Steady-state PL spectra, b) Time-resolved PL spectra of PMMA/perovskite/dense TiO2/glass and PMMA/crater-like bilayer TiO2/glass and c) J-V curves of two different ETL layers embedded between Au and FTO layers.


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To understand the enhanced charge extraction efficiency of crater-like TiO2, the electrical properties of TiO2 ETLs in an Au/TiO2/FTO configuration were analyzed. As shown in Figure 5c, J-V measurements in the region of ohmic response revealed that the conductivity of the crater-like bilayer TiO2 (3.28 ൈ 10–10 S cm–1) was higher than that of dense TiO2 (2.56 ൈ 10–10 S cm–1). The more conductive crater-like bilayer TiO2 allows for faster electron transfer, thus preventing charge accumulation at the perovskite/ETL interface and improving charge transport.30 This observation explains why the crater-like bilayer TiO2 ETL-based PSCs exhibit lower RS and higher FF values (Figure 3c).

Figure 6. XPS spectra of a) N 1s and b) O 1s corresponding to the dense and crater-like bilayer TiO2 layers.


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To clarify the atomic origins of the improved conductivity of the crater-like bilayer TiO2, the crystallinity and electronic state of the two types of TiO2 ETLs were investigated. Figure S7 shows the XRD patterns of the two types of ETLs annealed at 500 °C. Both samples exhibited nearly identical diffraction profiles, in which the main peaks at 2θ values of 25.28°, 37.80°, and 55.06° were assigned to the (101), (004), and (211) planes of anatase TiO2, respectively (JCPDS 21-1272). However, after calibration to the C 1s core level at 284.8 eV, the X-ray photoelectron spectroscopy (XPS) patterns confirmed the presence of nitrogen by the peak at 401.6 eV as well as Ti and O in the crater-like bilayer ETL, whereas only Ti and O were present in the dense ETL (Figure S8a). The Ti 2p core level spectra of both types of ETLs nearly superimposed with each other; the two peaks located at binding energies of 458.7 and 464.4 eV indicate the presence of Ti4+ in TiO2 lattice (Figure S8b).31 Figure 6a shows the XPS spectra of N 1s corresponding to the crater-like bilayer TiO2, in which the peak located at 399.8 eV is attributed to N-O bonding when the interstitial nitrogen is located near the lattice oxygen.32 The invariance in Ti valence also implies the presence of interstitial nitrogen instead of substitutional defects (Figure S8b). The O 1s spectra in Figure 6b revealed two peaks in which the first one located at 529.9 eV undoubtedly corresponded to the O-Ti bond, while the second peak at 531.3 eV was ascribed to non-lattice oxygen species, such as oxygen vacancies.33 In the case of the crater-like bilayer, the peak corresponding to oxygen vacancies was more intense compared to its counterpart corresponding to dense ETL, whose position slightly shifted in the direction of higher energies.34 It has been reported that nitrogen doping promotes oxygen vacancy formation.35 This observation implies that the incorporation of interstitial nitrogen into the TiO2 lattice can be ascribed to the addition of ammonium nitrate into


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the sol-gel precursor. As well-known n-type semiconductor, the conductivity of TiO2 is determined by carrier concentration and can be improved by generating the appropriate oxygen vacancy. Because the nitrogen interstitial doping can induce the formation of the oxygen vacancy in the TiO2 lattice,35 the conductivity can be enhanced by the increased carrier concentration; in turn improving the charge extraction efficiency of the crater-like bilayer TiO2. It is well known that oxygen vacancies generated as a result of interstitial nitrogen doping increase the carrier concentration; this in turn improves the charge extraction efficiency of the crater-like bilayer TiO2.36 It should be noted here that the nitrogen-doping-induced oxygen vacancy might lead to the formation of detrimental surface states located in the range of 0.4–1.18 eV below the conduction band edge,37-38 which makes the oxygen vacancy act as a recombination site. Moreover, the surface trap state induced by excessive oxygen vacancies may cause the trapping of injected electrons from the perovskite layer, which promotes recombination with the holes remaining at the perovskite/TiO2 interface and in turn to a VOC drop.39 The surface trap density of each type of TiO2 ETL was determined using the space charge limiting current (SCLC) model.40 The trap charge threshold voltage (VTFL) at the interface between the trap-filling region and Child-region can be calculated; these two regions are depicted by the green- and red-colored regions, respectively (Figure S9a and b). The VTFL values were determined to be 2.12 V and 1.91 V for the dense TiO2 and crater-like bilayer TiO2 ETL devices, respectively. Subsequently, the trap density (Nt) was calculated to be 1.19 × 1016 cm–2 for the crater-like bilayer-based cells and 1.31 × 1016 cm–2 for the dense TiO2-based cells. There was a negligible difference in the electron trap densities of the cells based on different types of ETLs. This observation allows us to conclude that the concentration of the oxygen vacancies is controlled at an appropriate level, so


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as not to and does not result in a noticeable increase in the surface trap density; it can only be observed in terms of a subtle VOC drop (~0.01 V), as seen in the J-V results (Figure 3c). According to previous research, nitrogen doping could promote higher photocatalytic activity of TiO2.41 Since the high photocatalytic behavior affect the instability of PSCs,42 we need to conduct long-term operational stability tests of both TiO2 PSCs samples under 1 sun illumination. After 1000 seconds, the dense TiO2 cell retained 92% of the maximum PCE whereas the crater-like bilayer based device exhibited a comparable stability behavior maintaining 90% performance as shown in Figure S10. Similar long-term stability between crater-like bilayer and dense TiO2 based devices can be explained by the fact that nitrogen is introduced into the interstitial site rather than the substitutional site, as described in Figure 6a. Because the improved photocatalytic activity in the visible-light region induced by the bandgap narrowing is reported in substitutional nitrogen doping, 41 high photocatalytic activity does not occur in crater-like bilayer sample. Therefore, it is concluded that interstitial nitrogen doping in the crater-like bilayer only enhances the conductivity of TiO2, but does not alter the photocatalytic properties, so that there is no significant difference in the stability between craterlike and dense TiO2 based devices under illumination.


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Figure 7. Nyquist plots of PSCs based on either a dense TiO2 ETL or a crater-like bilayer TiO2 ETL under AM 1.5G 1 sun illumination with a 0.8 V forward bias condition. Impedance spectroscopy can be used to analyze the enhanced charge transfer kinetics associated with the crater-like bilayer-based device.43 Figure 7 shows the Nyquist plots of PSCs based on dense TiO2 and crater-like bilayer TiO2. Two distinguishable arcs are evident, similar to those of typical perovskite-based cells. Table S3 lists the fitting parameters. The arc in the high frequency range represents contact resistance (Rsc) at the interface between the perovskite layer and ETL when the same HTM was utilized. The crater-like bilayer TiO2-based cell exhibited much smaller resistance (8.496 Ω·cm2 ) as compared to the dense TiO2 device (14.19 Ω·cm2 ሻ. This can be explained by the nitrogen doping-driven increased conductivity and the phase separation-induced enlarged interfacial contact area of the ETL. On the other hand, the arc in the low frequency region depends on the recombination resistance (Rrec) of the devices. Because bulk perovskites have large diffusion lengths and small nonradiative recombination rates,44-45 carrier recombination likely occurs predominantly at the interface between the perovskite and ETL. The Rrec of the crater-like bilayer TiO2 device (9.32 Ω·cm2 ) is slightly lower than that of its dense TiO2 counterpart (8.60 Ω·cm2 ). This result correlates well with the slight differences in


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surface trap density and VOC between the two types of PSCs. We also conducted hysteresis evaluation by measuring the reverse and forward scans as shown in Figure S11. To obtain more quantitative information on J-V hysteresis, we estimated a hysteresis index (HI) by using the equation (2):47 Hysteresis index =

JRS ሺ0.8VOC ሻ ି JFS ሺ0.8VOC ሻ JRS ሺ0.8VOC ሻ

(2)

where JRS (0.8VOC) and JFS (0.8VOC) represent the photocurrent density at 80% of VOC for the reverse scan (RS) and forward scan (FS), respectively. The PSCs containing crater-like bilayer TiO2 exhibited less hysteresis (HI=0.220) as compared to dense TiO2-based devices (HI=0.418) owing to their enhanced charge extraction efficiency and increased surface area by crater texturing. The hysteresis behavior difference is revealed again by observing the response of each device after light illumination. As shown in Figure S10, crater-like bilayer based PSCs showed a rapid increase to the peak value of the PCE. On the other hand, in the case of dense TiO2 based PSCs, there was the delay until reaching the maximum PCE after illumination. This observation is consistent with the previous studies in which the perovskite solar cells with higher J-V hysteresis exhibit slower maximum PCE saturation time.48 The stabilized PCE of the cell with a crater-like bilayer TiO2 ETL was ~14.2% (Figure S11c). However, As an inevitable limitation related to TiO2-based planar PSCs, our devices also exhibit hysteresis to some extent due to charge transport imbalance-induced accumulation originating from the huge difference in the conductivities of the TiO2-based ETL and the spiro-OMeTAD hole selective transport layer.46 As a results, the stabilized PCE of the cell with a crater-like bilayer TiO2 ETL was ~14.2% (Figure S11c), which represents the mean value of reverse and forward scans. Further optimization is necessary for balancing the carrier transport properties of the ETL and HTL in order to suppress the hysteresis of planar PSCs.


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CONCLUSIONS In summary, we have developed a new strategy to simultaneously manipulate the electrical and optical properties of ETLs to enhance the performance of planar PSCs. A crater-like porous/blocking bilayer TiO2 ETL was fabricated by carefully manipulating Ti alkoxide-based sol-gel chemistry to induce self-assembling phase separation during the gelation step. Structural analysis revealed that the crater-like bilayer contained uniformly distributed surface pores of 220 nm diameter; these pores lead to less reflection than that observed in conventional dense TiO2 films. When the crater-like bilayer TiO2 was used as an ETL, light absorption into the perovskite layer improved remarkably, because more photon energy could be transmitted through the ETL to reach the perovskite layer. Additionally, an increase in the EQE in the 350–750 nm wavelength range could be attributed to the improved charge extraction efficiency of the craterlike bilayer ETL. XPS analysis in conjunction with impedance spectroscopy suggested that nitrogen was interstitially doped into the crater-like bilayer TiO2 due to the addition of ammonium nitrate into the precursor solution. This N-doping-induced oxygen vacancies, which provided additional electron carriers; these, in turn, led to better conductivity compared to undoped dense TiO2 and enabled effective charge extraction and transport to the electrode. Owing to the synergistic effects of the simultaneous modification of the optical and electrical properties of ETLs, the PSCs based on crater-like bilayer TiO2 exhibited an outstanding performance with a maximum PCE of 16.0%. Our findings suggest that a combined optical and electrical tuning of ETLs is a promising strategy for achieving highly efficient PSCs.


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EXPERIMENTAL SECTION Preparation of TiO2 Precursor Solutions. To prepare the crater TiO2 precursor solutions, initially, titanium isopropoxide (Ti[OCH(CH3)2]4, 99.999%, Sigma Aldrich, St. Louis, MO, USA) was dissolved in 2-methoxyethanol (anhydrous, 99.8%, Sigma Aldrich). After vigorously stirring the solution at room temperature for 10 min, 0.15 M of acetylacetone (≥99%, Sigma Aldrich) was added as a stabilizing agent. Ammonium nitrate (NH4(NO3)2, 99.999%, Aldrich) at a molar ratio of 1:1 to acetylacetone was used to modify the surface tension of the sol-gel solution used for crater TiO2 fabrication. The solution was stirred at room temperature for 12 h. To prepare the conventional sol-gel solution for dense TiO2, titanium isopropoxide and concentrated HCl was dissolved in ethanol. The solution was stirred at room temperature for 12 h in a nitrogen atmosphere in a glove box. Fabrication of Perovskite Solar Cells. FTO glass was cleaned using an ultrasonic bath for 30 min in ethanol. After partially taping for the bottom electrode contact, followed by UV-treatment for 15min. To fabricate the blocking layer, a compact TiO2 layer was spin-coated using the conventional sol-gel solution at 3000 rpm for 30 s. After 15 min of drying at 125 °C, the samples were subjected to UV treatment for 10 min again. To fabricate a thin porous TiO2 layer, the TiO2 precursor solution developed for the crater-like morphology was spin-coated at 2000 rpm for 30 s. After 15 min of drying at 125 °C, the samples were annealed at 500 °C for 30 min in a box furnace. Subsequently, TiCl4 post treatment was conducted. Both the TiO2 thin films were immersed in a 0.02 M aqueous TiCl4 (≥99%, Sigma Aldrich) solution at 90 °C for 10 min and then annealed at 500 °C for 30 min in a box furnace again. To fabricate the perovskite absorber layer, a 53 wt.% mixture of PbI2, CH3NH3I, and dimethyl sulfoxide (Sigma Aldrich) (1:1:1 molar ratio) was dissolved in dimethylformamide (Sigma Aldrich) and then spin-coated on the TiO2


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coated substrates at 4000 rpm for 23 s. Diethyl ether (0.5 mL) was cast onto the rotating substrate over 6 s prior to modifying the film surface in order to remove only dimethylformamide, followed by drying at 50 °C for 3 min and annealing at 100 °C for 10 min. The HTM precursor solution was prepared by dissolving spiro-OMeTAD (99.7%, Borun Molecular, China, 72 mg) in chlorobenzene (1 mL) followed by the addition of 4-t-butylpyridine (28.8 µL) and Li salt solution (520 mg mL–1 lithium bis(trifluoromethylsulfonyl)imide in acetonitrile, 17.6 µL). The HTM precursor solution was spin-coated on the perovskite layer at 3000 rpm for 30 s without any additional annealing. Finally, a 70 nm-thick gold electrode layer was thermally evaporated onto the spiro-OMeTAD film. Optical, Structural, and Electrical Characterization. The optical transmittance and reflectance of the TiO2 thin films as well as the absorption of the perovskite layer were measured using a UV-vis spectrophotometer (V-670, Jasco, Tokyo, Japan). The surfaces, cross sections, and surface profiles of the samples were analyzed by field emission SEM (FE-SEM, JSM-7001F, JEOL Ltd, Tokyo, Japan) and AFM (SPA 400, Seiko Instruments Inc., Chiba, Japan). The crystallinities of the films were determined using a high-resolution XRD (HR-XRD) instrument (Rigaku Smartlab, TX, USA). Impedance spectroscopy was conducted using a frequency response analyzer (1252A, Solartron, England) while applying a perturbation of 10 mV AC at a frequency in the range of 0.3 MHz and 0.1 Hz under white light illumination. The fitting parameter external series resistance (Rext) accounted for the ohmic contribution of contacts and wires, whereas Rsc for selective contact resistance and Rrec related to recombination resistance in the operation of PSCs. The capacitive elements were labeled according to the established interpretation, associated with the dielectric response of the perovskite layer as Cbulk in the highfrequency part and with the surface charge accumulation at the interfaces as C1.


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Steady-state PL decay transients were collected at 770 nm for all films after excitation at 470 nm. Time-resolved photoluminescence measurements were also conducted using an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 40× objective. A single-mode pulsed diode laser (470 nm with ~30 ps pulse width, ~0.1 µW average power, and 0.5 MHz repetition rate) was used as the excitation source. A dichroic mirror (490 DCXR, AHF), a long-pass filter (HQ500lp, AHF), a 100 µm pinhole, a band-pass filter (FB760, Thorlabs), and a single photon avalanche diode (PDM series, MPD) were used to collect emissions from the samples. Characterization of the Photovoltaic Properties. The photovoltaic performance of the perovskite solar cells was evaluated in terms of their current density-voltage (J-V) characteristics using a solar simulator (Sol3A Class AAA, Oriel Instruments, Stratford, CT, USA) and a Keithley 2400 source measurement unit (Keithley Instruments Inc., Cleveland, OH, USA) under air mass (AM) 1.5 and 1 sun (100 mW cm–2) conditions. The 1 sun intensity level was calibrated using a standard Si reference cell certified by the Newport Corporation (Irvine, CA, USA). The active area of the perovskite solar cells was 0.06 cm2, as determined by the aperture mask. Scanning was carried out over a range of –0.1 V to 1.2 V at a rate of 0.52 V s–1 with a dwell time of 50 ms at each point and at a rate of 0.048 V s–1 with a dwell time of 500 ms for the J-V hysteresis measurement. The EQE spectra were recorded using a quantum efficiency measurement system (QEX10, PV Measurements, Inc.).

ASSOCIATED CONTENT Supporting Information.


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Calculation method of pore volume fraction of crater-like bilayer thin film and SEM image of thin films with different ratio of additives, XRD patterns of perovskite/TiO2/FTO with different TiO2 ETL, device performance statistics, open circuit voltage versus short circuit current plot at different irradiance, TRPL fitting data, XPS survey and Ti 2p core level spectra, log J versus log V plot using SCLC model, impedance fitting parameters of PSCs based on different TiO2 ETL. (PDF). The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2012R1A3A2026417).

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(48) Hou, Y.; Quiroz, C. O. R.; Scheiner, S.; Chen, W.; Stubhan, T.; Hirsch, A.; Halik, M.; Brabec, C. J., Low-Temperature and Hysteresis-Free Electron-Transporting Layers for Efficient, Regular, and Planar Structure Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1501056.


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Facile Sol-Gel Derived Crater-like Dual-functioning TiO2 Electron

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