Activating Old Materials with New Architecture: Boosting Performance of Perovskite Solar Cells with H2O‐Assisted Hierarchical Electron Transporting Layers

Abstract The breakthrough of organometal halide perovskite solar cells (PSCs) based on mesostructured composites is regarded as a viable member of next generation photovoltaics. In high efficiency PSCs, it is crucial to finely optimize the charge dynamics and optical properties matching between the perovskites and electron transporting materials to relax the trade‐off between the optical and electrical requirements. Here, a simple antipolar route with H2O as the additive is proposed to prepare hierarchical electron transporting layers to boost the efficiency of dopant‐free PSCs. The photovoltaic performance of the PSCs is enhanced owing to increased light‐scattering, improved Ostwald ripening, and photo‐generated electron extraction. Optimization of the H2O addition enables a valid power conversion efficiency of 19.9% (reverse scan: 20.02%) to be achieved. The device can retain more than 90% of its initial performance after storage in air more than 30 days. These results are inspiring in that they present that a mesoporous transporting layer could be easily re‐constructed to hierarchical architecture by the antipolar method to further improve the performance of PSCs.


Introduction
Inorganic-organic hybrid perovskite solar cells (PSCs) have attracted considerable attention owing to their unique characteristics such as the broad range in which they absorb sunlight, low exciton binding energy, long electron-hole diffusion length, tunable direct band gaps, and high extinction coefficient. [1][2][3][4] These cells, which have an n-i-p architecture, can be fabricated by using different hole transporting layers (HTLs) The breakthrough of organometal halide perovskite solar cells (PSCs) based on mesostructured composites is regarded as a viable member of next generation photovoltaics. In high efficiency PSCs, it is crucial to finely optimize the charge dynamics and optical properties matching between the perovskites and electron transporting materials to relax the trade-off between the optical and electrical requirements. Here, a simple antipolar route with H 2 O as the additive is proposed to prepare hierarchical electron transporting layers to boost the efficiency of dopant-free PSCs. The photovoltaic performance of the PSCs is enhanced owing to increased light-scattering, improved Ostwald ripening, and photo-generated electron extraction. Optimization of the H 2 O addition enables a valid power conversion efficiency of 19.9% (reverse scan: 20.02%) to be achieved. The device can retain more than 90% of its initial performance after storage in air more than 30 days. These results are inspiring in that they present that a mesoporous transporting layer could be easily re-constructed to hierarchical architecture by the antipolar method to further improve the performance of PSCs.
mesoporous TiO 2 (m-TiO 2 ) is the most commonly used n-type semiconductor and acts as scaffold layer for a large portion of PSCs, owing to its favorable band-edge positions, superior chemical stability, and low cost. However, a major drawback of planar m-TiO 2 ETLs, which renders them unsuitable for application as PSCs, is that their interfacial charge extraction is insufficiently rapid. The charge accumulation at the TiO 2 /perovskite interface induced by ion migration in the perovskite layer is known to cause hysteresis, thereby causing the performance of the device to deteriorate. [7,8] To solve this problem, we previously modified planar TiO 2 ETLs with a thin PC 61 BM buffer layer, and found that the elastic nature of the PC 61 BM could facilitate the formation of high-quality perovskite films and improve the TiO 2 /perovskite interfacial properties. [9] Considering the modest efficiency of these solar cells and the high cost of the PC 61 BM, it is still essential as well as vital to explore a more comprehensive way to boost the charge extraction and performance of these solar cells. Enlarging the TiO 2 /perovskite interfacial area may effectively release the accumulated electrons at the heterointerface because this would increase the probability of electron extraction by the ETL. [10,11] This suggests that suitably shaping the architecture of ETLs may be an alternative approach to optimize the charge dynamics at the TiO 2 /perovskite hetero-interface to improve the performance of the PSCs.
Compared with traditional planar m-TiO 2 ETLs, the hierarchical structure of TiO 2 consists of particles of different sizes that could provide a larger interfacial area for charge extraction and more flexible architecture for light-scattering. In addition, as the substrate of perovskite layer, morphological properties of hierarchical structure TiO 2 (size, distribution, and structure, etc.) can be modulated efficiently; hence, conceptually TiO 2 is a more likely candidate for controlling the nucleation of the perovskite. Thus, in principle, hierarchical TiO 2 could be an effective solution for improving the performance of the device. [12][13][14] Previously, hierarchically structured TiO 2 , which is synthesized by a two-step hydrothermal procedure, was used as the ETL in liquid state dye-sensitized solar cells to enhance the performance of the device. [15] Nevertheless, for PSCs, owing to the lack of appropriate architectural modulation, introducing a substrate with hierarchical architecture usually reduces the crystallinity of the perovskite films, which increases the density of defects and TiO 2 /perovskite interfacial recombination. Therefore, the use of hierarchically structured TiO 2 to enhance the performance of PSCs beyond that of T-m-TiO 2 -based PSCs has not yet been reported.
Accordingly, in this work, we propose a tri-functional hierarchical m-TiO 2 (H-m-TiO 2 ) ETLs, which is first fabricated using H 2 O as an additive to shape the architecture and balance the light-trapping, crystallization, and charge extraction properties. The MAPbI 3 solar cells with the n-i-p structure and H-m-TiO 2 ETLs show a high efficiency of 20.02% (reverse scan) and are almost free of hysteresis. The enhanced performance is attributable to the unique H-m-TiO 2 ETLs, which improves the light-scattering behavior and crystallization of the perovskite layer, and accelerates charge extraction. This work not only demonstrates a simple route for preparing hierarchical carrier transporting layers, but also paves the way for the possible application of hierarchical ETLs in extensive optoelectronic devices.

Results and Discussion
First, we examine the morphological changes upon H 2 O addition (Figure 1a-d). Top-view scanning electron microscopy (SEM) images of m-TiO 2 films with 0, 5, 15, and 25% v/v of added H 2 O are shown in Figure 1a-d, respectively. The traditional m-TiO 2 (T-m-TiO 2 ) film without H 2 O addition shows a flat porous structure, which coincides with the state-of-theart results that were previously reported. [16][17][18] As the addition of H 2 O increase to 15%, the roughness of the surfaces of the m-TiO 2 films increases and some microscale pits on the original m-TiO 2 are created (H-m-TiO 2 ). This can be ascribed to the large differences in polarity between the H 2 O and the solvent of the TiO 2 colloid (ethanol), which provides sufficient surface tension to allow assembling of the TiO 2 particles. Large amounts of TiO 2 nanoparticles are accumulated to form the clusters, whereas particles in other areas become thinly spread as a consequence. Increasing the addition of H 2 O even further  to 25% causes too many TiO 2 particles to wash out during the spin-coating process, resulting in the exposure of large areas of the compact TiO 2 films.
Empirically, different morphological characteristics are likely to affect the optical properties of the ETLs. As the window layer of n-i-p type PSCs, surface texturization is the preferable configuration to ensure effective light-trapping over a wide wavelength range. [19][20][21] We examined the optical properties of the ETLs, including their vertical transmittance (VT) and total transmittance (TT) by UV-Vis-NIR spectrophotometry ( Figure S1, Supporting Information). Increasing of H 2 O addition from 0 to 15% causes the VT to reduce, whereas the TT almost remains unchanged ( Figure S2a,b, Supporting Information), indicating that the surface light-scattering properties of the surface are promoted. However, when the addition of H 2 O is promoted to 25%, the VT only increases slightly because the TiO 2 films are partially washed out (as shown in Figure 1d). The haze factor is one of the major roles to evaluate the lightscattering ability of the substrate. We calculated the haze factor from the variation between VT and TT (Figure 2a) as follows: [22] ( ) Without the addition of H 2 O, T-m-TiO 2 films could not produce an adequate light-scattering effect in the wavelength region of 400-800 nm owing to the small feature size of mesoporous structure. This means that much of the light is transmitted directly by the substrate without changing its orientation. After constructing the hierarchical structure, large TiO 2 clusters were able to effectively scatter the incident light, thereby resulting in an increased haze factor (Figure 1c). Optical simulations of the PSCs prepared on T-m-TiO 2 and H-m-TiO 2 ETLs were carried out to ascertain their light harvesting capability. The shape parameters that were used for the optical simulation are exhibited in Figure S3 in the Supporting Information. Figure 2b shows the 2D optical intensity distribution of electric fields in the perovskite layer. Detailed information of the simulation is included in Note S1 in the Supporting Information. Incident light with a wavelength of 600 nm incident from the fluorinedoped tin oxide (FTO) side was used for the simulation, and the color scale bars illustrating the absorption intensity were confined to the same range. It shows that for perovskite films on T-m-TiO 2 ETLs, a hierarchical electric field which originated from the forward-and reverse-propagating light waves exists in perovskite absorber layer, indicating moderate light-scattering. In comparison, strong light-scattering, owing to the larger surface roughness of the H-m-TiO 2 ETLs, is found in perovskite films, which results in more intense absorption. Figure 2c shows images of the perovskite thin films grown on T-m-TiO 2 ETLs and H-m-TiO 2 ETLs, respectively. Obviously, the H-m-TiO 2 ETLs sample exhibits a darker color, indicating stronger absorption than the T-m-TiO 2 ETLs sample. The light paths are schematically depicted in Figure 2d to visually illustrate benefits of this light-trapping effect for PSCs. Once the incident light approaches the H-m-TiO 2 ETLs, the stronger light-scattering effect (higher haze factor) between the H-m-TiO 2 and MAPbI 3 films prolongs the light path. Consequently, compared with T-m-TiO 2 ETLs, MAPbI 3 films based on H-m-TiO 2 are expected to achieve stronger light absorption ( Figure S4   Except for the optical behavior, the crystallization of perovskite films based on various substrates was investigated. Cuboid MAPbI 3 crystals between 210 and 500 nm are formed on the pristine T-m-TiO 2 ETLs (Figure 3a). For the substrates evolving from T-m-TiO 2 to H-m-TiO 2 , as the addition of H 2 O increases from 0% to 15%, this also causes the grain size of MAPbI 3 films to increase (Figure 2b,c). However, some pinholes and small grains are formed in the MAPbI 3 films, thereby further escalating the H 2 O addition of the m-TiO 2 substrates. The X-ray diffraction (XRD) patterns of the MAPbI 3 films also verify the crystallization features (Figure 3e,f). All compositions exhibit a typical perovskite peak at ≈14°, which corresponds to the (110) orientation of the photoactive black phase of MAPbI 3 . Compared with the other samples, the films grown the on the H-m-TiO 2 achieve the highest diffraction intensity and lowest full width at half maximum (FWHM), which also indicates improved crystallization. All these results imply that the crystallization characteristics of perovskite films seem to be greatly affected by the architecture of the substrates.
Here, we introduce the Ostwald ripening model to unravel the coarsening of the perovskite particles. During the nucleation process, precursors with different particle sizes are initially formed on the H-m-TiO 2 surface. The relationship between the chemical potential and particles radius can be depicted as follows: [23] V r µ µ β = + 2 0 where μ is the chemical potential of the surface, β is the surface energy, μ 0 is the chemical potential for a flat surface, r is the radius of a particle, and V is mole volume of a particle. Hence, a smaller particle is energetically less stable than a larger particle owing to higher chemical potentials. This means small grains are easily dissolved than large grains if there is a sufficient amount of solvent present. The dissolved components between large grain and small grain will lead to a concentration gradient, causing the mass transportation from small grain to large grain according to Fick's first law. [24] Therefore, as the period of annealing is elongated, the small grains vanish and the size of the large grains increases because of the mass transportation of the dissolved component. In this case, compared with the flat T-m-TiO 2 layer, the H-m-TiO 2 layer, which consists of small nanoscale mesoporous and large microscale holes, would require less solvent (DMF and DMSO) after the spin coating process. During the heating procedure, the H-m-TiO 2 layer would inevitably delay the extraction of solvent, and prolong the ripening of the MAPbI 3 precursor to form larger columnar crystals. Hence, the large perovskite grains finally coarsen by absorbing surrounding small grains via mass transportation (Figure 3g). In contrast, for the T-m-TiO 2 layer or the non-optimized layers (with 5% or 25% H 2 O added), due to its flatter architecture, the solvent molecules are extracted more easily from the precursor films, which leave less time for Ostwald ripening process. Considering that crystallization always determines the defects in perovskite films, we also investigated the trap state density and band tail state density in T-m-TiO 2 -based and  H-m-TiO 2 -based perovskite films. We studied the dark currentvoltage characteristics and light-absorption characteristics for electron-only devices. Figure 4a,b illustrates the dark currentvoltage characteristics of the solar cells, indicating linear ohmic response at low bias, a trap-filling regime, and a trap-free space charge limit current (SCLC) regime. The trap state density was determined by the trap-filled voltage as follows: [25] N V qL where ε r is the relative dielectric constant, ε 0 is the vacuum permittivity, V TFL is the onset voltage of the trap-filled limit region, L is the thickness of the film, and q is the elemental charge. We found that the trap densities remarkably decrease from 1.5 × 10 16 cm −3 to 9.2 × 10 15 cm −3 for T-m-TiO 2 ETLs and H-m-TiO 2 ETLs, respectively. We also examined the Urbach energy (E u ) of the films, which probe the sharp onset of absorption at the direct bandgap and deliver significant details regarding disorders in shallow energy level (Figure 3b). The E u for perovskite films are calculated as follows: [26] e E E α α = 0 u where α 0 is constant, α is the absorption coefficient, and E is the photonic energy. We observed lower E u values for the H-m-TiO 2 sample, which indicates a smaller degree of electronic disorder at the band-edge. These results imply that inducing the H-m-TiO 2 ETLs could effectively suppress the defects in the MAPbI 3 films due to the enhanced crystallization. The dependence of the performance of the PSCs on the T-m-TiO 2 and H-m-TiO 2 ETLs were studied in detail to further examine the application potential and develop a comprehensive understanding of the merits of H-m-TiO 2 in PSCs. The fabrication process of the PSCs is demonstrated in Figure S5 in the Supporting Information. Figure 5a, Figure S6 in the Supporting Information. The average PCE of 19.4% for these 20 H-m-TiO 2 devices can be achieved, which is also higher than the devices based on T-m-TiO 2 ETLs.
These results indicate that the performance enhancements of the solar cells can be ascribed to the synergistic effect among different properties. First, the improved lightscattering characteristics via the hierarchical architecture could elevate the light absorption of the MAPbI 3 films by extending the effective light path. Therefore, the spectra response of the solar cells based on H-m-TiO 2 would be better in a large wavelength region (Figure 5d). Second, as shown in Figures 3a-f and 4, introducing the H-m-TiO 2 ETLs could enhance the crystallization of the MAPbI 3 films and reduce the grain boundaries and trap-states density. To obtain deeper insights into the impact of different ETLs on the trapstates and charge-dynamics of the device, the influence of the logarithm of light intensity P on the V oc characteristics was investigated (Figure 5e). This logarithmic relationship is consistent with the fundamental relationship V oc = nk B T/q Adv. Sci. 2019, 6, 1801170 ln(P), where n is a constant referred as the ideality factor, k B is Boltzmann's constant, T is the temperature, and q is the elementary charge. [27,28] According to the literature, the P dependence of the V oc can provide insights into the role of trap-assisted recombination versus bimolecular recombination at open circuit. [29,30] The recombination processes always strongly determine the properties of solar cells at open circuit condition because there is no current extraction and all the photo-generated carriers recombine. It is generally recognized that the ideality factor should be equal to 1 if Langevin recombination dominates, whereas other involvements of the interfacial trap-assisted Shockley-Read-Hall recombination would result in n being larger than 1. In this case, the value of n obtained for T-m-TiO 2 and H-m-TiO 2 is 1.69 and 1.52, respectively. This implies that the trap-assisted Shockley-Read-Hall recombination is present in both devices. In addition, the smaller n of the H-m-TiO 2 -based solar cells also suggests that improved crystallization reduces recombination and consequently leads to the improvement of the photovoltaic performance.
Moreover, as previously reported, a good electronic contact with faster extraction could reduce charge accumulation at the MAPbI 3 /contact layer interface induced by ion migration, and could promote carrier transportation. [9,[31][32][33] Here, we suppose that the vertical architecture (with a large surface area) of the H-m-TiO 2 may form an additional radial collection path for photo-generated charges (Figure 6a,b), which may accelerate the extraction of photo-generated charges (Figure 6c,d).
To confirm the enhanced charge extraction by H-m-TiO 2 ETLs, we characterized the devices by steady photoluminescence (PL) and time-resolved photoluminescence (TRPL) (Figure 6e  extraction could effectively reduce the accumulation of charge at the hetero-interface, separate the holes and electrons, eliminate the recombination, and suppress the hysteresis behavior of the device. To gain more insight on the charge transport and recombination at the interface, electrical impedance spectroscopy (EIS) measurements were also performed in the as-fabricated PSCs (Figure 6g). Nyquist plot of EIS spectra were measured under dark and open circuit conditions, in the frequency range from 0.1 MHz to 10 Hz. As shown in Figure 6e, the fitted equivalent circuit model is composed of a series resistance (R s ), charge transfer resistance (R tr ) at the ETLs/MAPbI 3 and the MAPbI 3 /spiro-OMeTAD interfaces, and recombination resistance (R rec ) forming a parallel circuit with capacitors (C tr and C rec ). The fitted parameters of R s , R tr , and R rec are exhibited in Table S1 in the Supporting Information. The R s is obtained from the x-axis intercept of the highfrequency curve. R tr is ascribed to the high-frequency arc and R rec is assigned to low-frequency arc. The values of R s show little variation for these two devices, indicating that the hierarchical TiO 2 ETLs have no significant effect on the series resistance. The H-m-TiO 2 -based device exhibits a lower value of R tr , probably because the hierarchical architecture could extract the charge effectively. These results could be further verified by applying 0.8 V bias voltage to the device ( Figure S7, Supporting Information). Thus, n carriers could be more easily extracted to the external circuit because charges are not trapped in the trapping centers in the perovskite films. Therefore, the H-m-TiO 2 layer could not only improve the light-scattering behavior and crystallization of the MAPbI 3 's crystallization, but also enhance the extraction of photogenerated charges.
In addition, as an emerging technique for PSCs, it is necessary to evaluate the device stability to assess the application potential. PSCs are known to be sensitive to humid conditions. Furthermore, the heat produced by light illumination cannot be quickly and effectively spread out generally in the encapsulated PSCs, which could be an alternative reason for the instability of inorganic-organic perovskite materials. We, therefore, examined the stability of the PSCs (30 days, under ambient conditions, 50% humidity) based on both T-m-TiO 2 ETLs and H-m-TiO 2 ESLs (Figure 7a). The performance (90% of the pristine PCE) of the devices based on H-m-TiO 2 ETLs was superior. We attribute this to the enhanced crystallization of the MAPbI 3 films, which resist the moisture penetration during the aging process. Figure 7b shows the stabilized outputs of PSCs at the voltage of maximum power point (V mpp ) under continuous irradiation. Both current and PCE of the champion cell change very little after soaking under one-sun for 250 s, which demonstrates the good irradiation stability of the devices.

Conclusion
In summary, we successfully demonstrated solution-processed H-m-TiO 2 ETLs for high-performance MAPbI 3 solar cells. The resultant H-m-TiO 2 ETLs, which were prepared by a H 2 Oassisted method, are highly textured, and confer efficient lighttrapping properties upon the PSCs. Benefiting from its unique architecture, the crystallization of MAPbI 3 is improved owing to the enhanced Ostwald ripening process. Photo-generated charge extraction from MAPbI 3 is also promoted due to the additional radial collection induced by H-m-TiO 2 ETLs. Therefore, the tri-functional H-m-TiO 2 ETLs can provide respectable photoelectrical enhancement to yield a high valid PCE of 19.9% (R-S: 20.02%, F-S: 19.74%) in the derived devices. In addition, a compact and stable protecting layer for device is spontaneously formed to enhance the ambient stability of the derived PSCs. About 90% of its initial PCE can be retained after 30-day exposure in ambient conditions with ≈50% relative humidity. These H-m-TiO 2 ETLs with hierarchical architecture not only provide a simple way to improve the performance and stability of PSCs, but also shows the great advantages in further development of efficient optoelectronic devices.

Experimental Section
Fabrication of H-m-TiO 2 ETLs: FTO glass substrates in the dimension of 2 × 2 cm 2 were patterned by etching with zinc powder and 2 m hydrochloric acid. The substrates were in sequence washed by ultrasonication with soap (5% Hellmanex in water), absolute alcohol, acetone, and deionized water for 20 min, and then cleaned by UV-ozone for 20 min. A 30 nm-thick layer of dense compact TiO 2 was coated onto the FTO-glass by spin-coating at 2000 rpm for 30 s and then annealed at 135 °C for 10 min. The TiO 2 paste (Dyesol 18NR-T) was diluted with ethanol with weight ratio of 1:7. Then, different volume ratios (0%-25%) of deionized water were dripped in the mixed solution. The solution was stirred in room temperature for 1 h. The as-prepared solution was spincoated onto the TiO 2 compact film at 5000 rpm for 30 s to fabricate H-m-TiO 2 layers. These H-m-TiO 2 layers were first dried at 135 °C for 10 min and then sintered at 500 °C for 30 min in air atmosphere.
More details about the materials and PSCs fabrication procedure are demonstrated in Notes S2-S4, Supporting Information.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.