Pore Size Dependent Hysteresis Elimination in Perovskite Solar Cells

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Pore Size Dependent Hysteresis Elimination in Perovskite Solar Cells Based on Highly Porous TiO2 Films with Widely Tunable Pores of 15− 34 nm Jun Shao,†,‡ Songwang Yang,*,† Lei Lei,† Qipeng Cao,† Yu Yu,†,‡ and Yan Liu*,† †

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China S Supporting Information *

ABSTRACT: Pore size and porosity of the porous materials play an important role in catalysis, dye-sensitized solar cells and mesoscopic perovskite solar cells (PSC), etc. Increasing pore size and porosity of mesoporous TiO2 is crucial for facilitating porefilling of perovskite, charge extraction on TiO2/CH3NH3PbI3 interface and thus cell performance enhancement. Highly porous TiO2 films (TFs) with a large pore size that extends the limit of particle size have been achieved through a novel TiO2 paste using copolymer P123 as a pore-adjusting agent and 2-butoxyethyl acetate as a solvent. A highly porous structure with the pore size of 34.2 nm and porosity of 73.5% has been obtained, the porosity of which is the largest that has ever been reported in the screenprinted TiO2 thick films. The pore size and porosity of TFs can be successively adjusted in a certain range by tuning the P123 content in the pastes. As particle size and surface area of TFs are kept almost constant, the specific investigation on the effect of varied pore size on the performance of bilayer-structured PSCs becomes possible. The hysteresis phenomenon, the notorious problem of PSCs, is found to depend greatly on pore size and porosity of TFs, that is, pore-filling of perovskite. The suppressing effect of highly porous TFs on hysteresis by avoiding charges accumulation on the interface due to enhanced interfacial contact is proved by the invariable photocurrent response after prebias treatment. A hysteresis-free solar cell with an efficiency of 15.47% was achieved by depositing a 242 nm-thick perovskite capping layer upon 350 nm-thick TF with a pore size of 34.2 nm. This method developed for the preparation of highly porous TFs provides a new way to fabricate hysteresis-free PSCs and is widely applicable for the fabrication of other mesoporous metal oxide films with large pore sizes.

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INTRODUCTION Porous materials are of great interest because of their wide applications in adsorption, separation, catalysis, biotechnology, sensing, energy storage and conversion.1 Design and synthesis of mesoporous TiO2 (mp-TiO2) materials with controllable pore size have extending implications for the particular applications in photocatalysts, lithium-ion batteries, quantum dot-sensitized solar cells, dye-sensitized solar cells (DSSCs) and hybrid lead halide perovskite solar cells (PSCs). PSCs, which employing the new light-harvesting material, e.g., methylammonium lead trihalide (CH3NH3PbX3), have seen tremendous power conversion efficiencies (PCEs) exceeding 20%.2 As CH3NH3PbX3 is found to be good at transporting both electrons and holes,3 PSCs with high performance are also achieved with insulating Al2O3 scaffold4 or with planar structure abandoning the mesoporous layer at all.5,6 Previous studies have pointed out that mp-TiO2 materials are extremely unstable © XXXX American Chemical Society

to UV light irradiation and may cause degradation of perovskite as they are typical photocatalysts.7−9 Fortunately, it appears that the mp-TiO2 films are stable to full-solar-spectrum irradiation and it will be possible for the PSC devices to withstand stressful long-term light irradiation.4 Until now, mesoscopic PSCs, particularly bilayer-structured PSCs with a combination of mesoporous layer and capping layer, are still the most popular choice for highly efficient PSCs.10−12 The mp-TiO2 layer functions as both scaffold for the infiltration of perovskite and the electron injection layer for the effective separation of photoelectron and holes. Properties of mp-TiO2, such as their crystallinity,13 morphologies,14−16 and particle size,17,18 are widely investigated in DSSCs and Received: August 17, 2016 Revised: September 20, 2016

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DOI: 10.1021/acs.chemmater.6b03445 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

porosity of TFs enlarges, perovskite deposited on scaffold exhibits better infiltration by the evidence of thinner capping layer. The effect of varying pore size of TFs on the performance of bilayer-structured PSCs is investigated specifically by excluding other factors such as morphology or particle size of TiO2 nanoparticles. PSC devices fabricated on such highly porous TFs show higher PCE and diminished hysteresis with larger pore size and better pore-filling of perovskite. PSCs are pretreated under forward bias or reverse bias in dark to accelerate ion migration before photocurrent−voltage and steady state photocurrent characterization. PSCs based on highly porous TFs suppress slow photocurrent response after prebias treatment and provide the evidence that the hysteresis problem is eliminated by avoiding charges accumulation on the interface of TiO2/CH3NH3PbI3 due to effective infiltration of perovskite. Hysteresis-free PSC with a PCE of 15.47% was finally achieved by depositing a dense and fully covered perovskite onto the TF with 34.2 nm-pore-size and 73.5% porosity (calculated by the Barrett−Joyner−Halenda (BJH) method). The novel method for preparing screen-printable paste to form highly porous TiO2 films we present here has a bright application prospect and research significance as it can be extended to electronically conductive adhesives, ceramic materials, mesoporous catalyst and other fields.

mesoscopic PSCs. Recent research demonstrates that the infiltration of PbI2 or CH3NH3PbI3 into the TiO2 scaffold is crucial for effective perovskite conversion,12 successful charge separation,19 and thus better cells performance. Besides the improvement of active layer forming procedure,20,21 enlarging the pore size and porosity of TiO2 scaffold is an alternative way to accomplish the target of complete perovskite-infiltration. It has been pointed out that the device performance of PSCs is affected by the particle size of TiO2. TiO2 with larger particle size is favorable in PSCs, as it creating wider tunnels that are beneficial for the perovskite penetration.17 In such a situation, the device performance is essentially affected by the optimization of pore size and porosity of mp-TiO2. However, further increase of the TiO2 particle size reduces the efficiency as a result of decreased porosity.22 Another way to tune the pore size of TiO2 film is by surfactants or polymers. For example, triblock copolymer, HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2−CH2O)20H (Pluronic P123), is commonly used to form highly porous TiO2 films.23−25 However, the average pore size achieved by applying P123 as template in sol−gel, hydrolysis or hydrothermal method is limited to ∼10 nm, which is not large enough for the complete infiltration of perovskite particles. On the other hand, the low crystallinity of TiO2 achieved by using polymer in sol−gel method hinders fast electron injection and efficiency enhancement.26 To the best of our knowledge, TiO2 thick films above 10 μm with high porosity above 70% prepared by printing viscous pastes have not been reported. Therefore, further tuning the pore size and porosity to exceed the limit of particle size is challenging. Despite the pursue of high PCE through the optimization of chemical and physical properties of mp-TiO2, there are more fierce challenges that hinder the real commercialization of PSCs such as degradation of perovskite material itself and hysteresis phenomenon.27 Abnormal hysteresis that strongly depends on the scan direction and rate during J−V measurement affects the accurate measurement of performance and long-term stability of PSC devices. Different magnitudes of hysteresis can be observed for a variety of device architectures28,29 (planar PSCs,30 mesoscopic PSCs on mp-TiO2, superstructured PSCs on Al2O3,31,32 and inverted polymeric PSCs33). Devices without significant hysteresis have been successfully fabricated,34 but understanding the origin of hysteresis is equally important. Several origins of hysteresis have been proposed such as ferroelectric characteristics, trap sites and ion migration.27 Ion migration is confirmed to occur in perovskite material by many direct and indirect evidence and is broadly postulated to be the cause of notorious hysteresis problem in PSCs.35−38 It has been confirmed by many reports that bilayer-structured PSCs employing mp-TiO2 exhibit diminished hysteresis compared to planar PSCs and superstructured PSCs,29,39,40 but sometimes hysteretic behavior is empirically observed even though a mesoporous TiO2 layer is used.29 So, it is of significant importance to study systematically the relationship between pore size and porosity of mp-TiO2 and hysteresis behavior of the corresponding PSCs. In this work, we propose a novel approach to modulate the pore size and porosity of porous TiO2 films consecutively while the specific surface area and particle size of TiO2 films are fixed. Highly porous TiO2 films (TFs) with a large pore size of 50 nm (most probable pore size, calculated from pore size distribution curve) and a high porosity of 73.5% are achieved by applying P123 as pore-adjusting agent and 2-butoxyethyl acetate (BCA) as solvent in the screen-printable paste. As pore size and

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RESULTS AND DISCUSSION Porous TiO2 Film. Porous TiO2 films were prepared from a novel paste, which contains the hydrothermally synthesized TiO2 nanoparticles, ethyl cellulose (EC), P123 and BCA. We first characterized the physical properties of synthesized TiO2 nanoparticles. Particle sizes of the well-crystallized TiO2 nanoparticles are 20.5 nm calculated from the transmission electron microscopy (TEM) image (Figure S1) and 22.4 nm calculated from the powder X-ray diffraction (XRD) pattern (Figure S2), respectively. A pore size of 19.3 nm and a porosity of 54.0% were obtained by the random packing of the raw TiO2 nanoparticles (Figure S3), which was not large enough for the complete penetration of perovskite. So we developed a novel system of TiO2/EC/P123/BCA for screen-printable TiO2 pastes, in which P123 acted as the pore-adjusting agent and BCA as the solvent. It is worth pointing out that the novel TiO2 paste we have developed here has appropriate rheology as seen in Figure S4, permitting application in forming smooth and homogeneous films by a screen-printing process. Highly porous TiO2 films (denoted as TFs) were prepared by those pastes with varied P123 content and sintered at 510 °C. As shown in Table 1, pore size from 16.0 to 34.2 nm and porosity from 57.1 to 73.5% can be tuned successively by adjusting P123 content in the pastes. It is notable that the surface area of TFs is kept constant within the margin of error. The invariable surface area is caused by the constant particle size of the same batch of synthesized TiO2 nanoparticles. In a word, the desired porosimetry properties of TFs are successfully tuned by adjusting the P123 content in the pastes in a certain range. Figure 1a−c shows the top-view scanning electron microscopy (SEM) images of TF-16, TF-25 and TF-34 (−16, −25 and −34 indicating the actual average pore size of TFs as shown in Table 1). Compared to the top morphology of pristine TF-16 without added P123, mesopores in the scale of tens of nanometers and micropores in the scale of hundreds of nanometers exhibited in TF-25 and TF-34 corroborate the fact that the added P123 leaves much larger voids in those films after sintering and thus causes consequential larger pore size B


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Chemistry of Materials Table 1. Structural Characteristics of Sintered TiO2 films As Achieved by the Adjustment of P123 Content in TiO2 Pastes sample TF-16 TF-20 TF-25 TF-28 TF-31 TF-34

P123:TiO2 (weight ratio)

1:3 1:2 1:1 2:1

solvent

surface areaa (m2/g)

pore sizeb (nm)

porosityc (%)

terpineol BCA BCA BCA BCA BCA

66.5 66.8 65.2 64.1 66.9 66.3

16.0 20.4 25.9 28.6 31.9 34.2

57.1 63.2 67.9 69.9 73.1 73.5

slight increase in pore size from 15 to 18 nm as listed in Table S1. Besides, the films are prone to crack during sintering procedure and no further enlargement of pore size is obtained by further increase of the added P123 content as shown in Figure S6. By using BCA instead of terpineol, the pore size and porosity of porous TiO2 films are successively regulated with increasing P123 content and no cracking appears until the weight ratio of P123 to TiO2 is above 2:1. Why can highly porous TiO2 films be successfully achieved by this novel system of TiO2/EC/P123/BCA? It is known that chemicals with −OH group such as butanol, pentanol and hexanol, act as cosurfactants or swelling agents, are located at hydrophilic−hydrophobic interface to swell the block copolymer micelles by reducing interfacial curvatures.41 So the adding of terpineol with −OH group, results in larger swelling micelles as seen in the scheme of Figure 2 based on our experiment

a

Surface area calculated from the linear portion of the Brunauer− Emmett−Teller (BET) equation (P/P0 = 0.05−0.30). bPore size calculated by the BJH method using desorption branches. cPorosity calculated from pore volume by the BJH method using desorption branches.

and higher porosity. The large pores in the scale of hundreds of nanometers shown in the cross section image of Figure 1d also confirm the highly porous property of TFs, which is beneficial for the fully pore-filling of perovskite. According to the pore size distribution curves in Figure 1e, the most probable pore size rises up with respect to the P123 content. The wider pore size distribution (Figure 1e) and sharp increase in the region of higher relative pressures (Figure 1f) with the increase of added P123 content indicate that more large pores exist in those films, which is consistent with the former observations from SEM images. The largest average pore size of the TFs obtained by this method we demonstrate is 34.2 nm and the corresponding porosity is 73.5%, of which the porosity is the highest parameter that has ever been achieved for thick TiO2 films to the best of our knowledge. The thickness of TF with porosity of 73.5% we obtained here can be up to 10 μm without cracking by screen-printing process. Further increase of the added P123 content makes the obtained TFs tend to crack and thus there is no further improvement on the pore size and porosity (Figure S5). For common screen-printable paste, terpineol is usually used as the solvent. However, in the preliminary experiment, films prepared by the system of TiO2/EC/P123/terpineol only show

Figure 2. Schematic representation of the formation mechanism of the highly porous TiO2 films presented in our proposed synthesis strategy.

Figure 1. Top-view SEM images of porous TiO2 films TF-16 (a), TF-25 (b) and TF-34 (c) and the corresponding cross section SEM image of TF34 (d). Pore size distribution curves (e) and nitrogen adsorption−desorption isotherms (f) for sintered TiO2 films. C


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Figure 3. Cross section SEM images of the PSC devices based on 1L TF-16 (a), TF-25 (b) and TF-34 (c) and 2L TF-16 (d), TF-25 (e) and TF-34 (f). Corresponding top-view SEM images of the perovskite films deposited on 1L TF-16 (g), TF-25 (h) and TF-34 (i) and 2L TF-16 (j), TF-25 (k) and TF-34 (l).

increasing pore size and porosity of TFs, which implies a better pore-filling of perovskite into TiO2 scaffold as the precursor content and spin coating speed are kept constant. It should be pointed out that the white dots shown in Figure 3e were the fragments of silver contacts that dropped onto the HTM layer during sample preparation before SEM characterization. The corresponding top-view SEM images of the perovskite capping layers are exhibited in Figures 3g−l, which clearly display the surface morphology and coverage of perovskite films. By enhancement of pore size and porosity of TFs, voids appear on the surface of perovskite capping layer and cause poor coverage of perovskite as seen in Figure 3. As pore size and porosity of TFs increase, more perovskite particles infiltrate into the TiO2 scaffold, which makes it difficult for the less raw material of perovskite left upon the scaffold forming a dense layer with complete coverage. This phenomenon we observe here is also an evidence that the penetration of perovskite into TiO2 scaffold is improved by increasing pore size and porosity of TiO2 films. It is expected that a dense and fully covered capping layer would be achieved by further improvement of deposition method. To confirm the better interconnection between TiO2 and perovskite and therefore the better charges extraction due to better pore-filling by increased pore size and porosity, steadystate photoluminescence (PL) spectra of samples FTO/blTiO2/CH3NH3PbI3, FTO/bl-TiO2/1L-TF-16-CH3NH3PbI3/ CH 3 NH 3 PbI 3 , FTO/bl-TiO 2 /2L-TF-16-CH 3 NH 3 PbI 3 / CH3NH3PbI3 and FTO/bl-TiO2/2L-TF-P34-CH3NH3PbI3/ CH3NH3PbI3 were measured, where bl-TiO2 is a TiO2 blocking layer. As can be seen in Figure 4a, all the samples show typical PL peaks centered at ∼765 nm, corresponding to the excitation

results. As pore-forming agent of EC also exists in this system, the pore volume becomes so large that the scaffold construction collapses during removing organic components by sintering procedure and therefore the films crack and no obvious increase of pore size is observed, which could be approved by the fact that the cracking problem can be solved by washing with ethanol to remove P123 before the adding of terpineol. On the other hand, while using BCA without −OH group instead of terpineol, P123 acts as pore-adjusting agent merely because of its high molecular weight. Due to the steric effect of P123, large voids are formed inside the films after removal of additives by sintering as shown in Figure 2. Adding more EC component is not an alternative choice as high viscous EC makes the films hard to print and inhomogeneous. Finally the successful porous properties adjustment of TiO2 films is accomplished by this novel screen-printable paste containing P123 as pore-adjusting agent and BCA as solvent. Crack-free TiO2 thick films with high porosity of 73.5% that have never been literally reported are achieved by this method. It should be mentioned that this method is also suitable for other highly porous metal oxide films. PSCs Performance. The prepared highly porous TF-25 and TF-34 as well as pristine TF-16 were employed to fabricate mesoscopic PSCs. Figure 3a−f exhibits the cross section architectures of PSCs deposited on one-layer (1L) and twolayer (2L) TFs with varied pore size, respectively. A typical bilayer structure, which combines underneath mesoporous layer infiltrated with perovskite and upper perovskite capping layer is observed. Thickness of 1L TiO2 film is ∼250 nm whereas thickness of 2L TiO2 film is ∼450 nm. For both 1L and 2L TFs, thickness of perovskite capping layer gradually reduces with D


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Figure 4. Steady-state photoluminescence spectra of perovskite films deposited on blocking layer (dash line) and on 1L TF-16 (black line), 2L TF16 (blue line) and 2L TF-34 (red line), respectively (a). Schematic device architecture of bilayer-structured PSC (b). Dependence of JSC, VOC, FF and PCE on the pore size of 1L 250 nm-thick TFs (c, e) and 2L 450 nm-thick TFs (d, f), PSCs fabricated via one-step solution method (c, d) and two-step VASP method (e, f).

TFs but not the forming quality of perovskite layers. Photovoltaic parameters of short-circuit photocurrent-density (JSC), open-circuit voltage (VOC), fill factor (FF) and power conversion efficiency (PCE) as a function of pore size of TFs are plotted in Figure 4c−f, where each data set is carried out from at least 3 devices. Tables S2 and S3 list detailed parameters of the champion devices and calculated average PCEs of PSCs under standard simulated solar radiation (AM 1.5, 100 mW/cm2). The change trends of PCEs on pore sizes are similar for these two different device fabrication processes, which strongly proves that the device performance is mostly affected by the pore-filling of perovskite. It could be found out that PCE is mostly affected by the variation of FF. FF increases first with increasing pore size from 16.0 to 25.9 nm, indicating effective electron injection on the interface because of sufficient pore-filling. However, The average coordination number of TiO2 particles reduces with increasing porosity,43 which causes discontinuity of TiO2 and block charge transport pathway inside TiO2 scaffold. Therefore, FF reduces with further enlarged pore size of 34.2 nm. The JSC increases at first and

wavelength of 457 nm, which is in agreement with the previously reported data.42 For the perovskite films deposited on mp-TiO2, the PL intensity decreases significantly due to the injection of electrons from perovskite to TiO2. The PL intensity further reduces with increasing pore size and film thickness of TF, which indicates more effective electron-injection from perovskite to TiO2 caused by the improved interconnection on the interface of TiO2/CH3NH3PbI3 because of better porefilling. We further investigated the current density−voltage characteristics for PSCs based on TF-16, TF-25 and TF-34, consisting of a device structure of FTO/bl-TiO2/mp-TiO2− CH3NH3PbI3/CH3NH3PbI3/spiro-OMeTAD/Ag, where spiroOMeTAD is a hole transport material (HTM) of (2,2′,7,7′tetrakis-N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene, and Ag is the silver contact (Figure 4b). The deposition of perovskite was carried out by using two methods, one-step solution method and two-step vapor-assisted solution method (VASP), in order to confirm that the photovoltaic performance and hysteresis behavior were related to the porous properties of E


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Figure 5. Scan direction dependent J−V curves for the PSCs based on 1L 250 nm-thick TF-16 (a, d), 2L 450 nm-thick TF-25 (b, e) and 2L 450 nmthick TF-34 (c, f) via one-step solution method (a, b and c) and two-step VASP method (d, e and f).

gradually reduces with further increasing pore size and thickness of mesoporous layer. We propose that the JSC is affected by the combination of electron-extraction of mp-TiO2 and coverage of perovskite capping layer. Better penetration of perovskite into the TiO2 scaffold could be achieved because of wide tunnels in mesoscopic framework with large pore size and high porosity, which leads to the improvement of JSC due to more effective charge injection on the interface of TiO2/ CH3NH3PbI3. However, larger pore size and thicker film reduce the capping layer thickness and cause voids on the surface of perovskite as seen in top-view images of Figure 3, and this ultimately results in reduced JSC due to recombination of photoelectrons and holes because of possible contact of TiO2 and spiro-OMeTAD. No reduction on VOC is observed until the pore size is enlarged to 34.2 nm. As better pore-filling achieved with thick porous film of high porosity, more electrons injected to TiO2 lower the Fermi energy level at equilibrium between EF(TiO2) and EF(CH3NH3PbI3),44 leading to a lower VOC. As a result of a compromise between VOC, JSC and the FF, the champion performances among the fabricated PSCs are 15.77% with a JSC of 22.55 mA/cm2, a VOC of 1.039 V, and an FF of 67.34% via the one-step solution method, and 12.24% with a JSC of 20.34 mA/cm2, a VOC of 0.910 V, and an FF of 66.12% via the two-step VASP method. Dependence of Hysteresis on Pore Size. To explore further the importance of pore-filling of perovskite for the diminished hysteresis in PSCs, we investigated the effect of varying pore size of TFs on the hysteresis extents of J−V curves under reverse scan (from open circuit to short circuit, RS) and the opposite way forward scan (from short circuit to open circuit, FS). Figure 5 demonstrates the typical hysteresis behavior of bilayer-structured PSCs fabricated on 1L TF-16, 2L TF-25 and 2L TF-34 with varied pore size, porosity and film thickness. The more detailed photovoltaic parameters and J−V curves for each cell of 1L (and 2L) TF-16, TF-25 and TF-34 are shown in Table S4, Figures S7 and S8. Here, we qualify the hysteresis extent in J−V curve by defining hysteresis index (HI) as eq 1 according to the literature.45

HI =

JRS (0.6VOC) − JFS(0.6VOC) JRS (0.6VOC)

(1)

It is noteworthy that hysteresis extents of mesoscopic PSCs depend strongly on the porous properties of TFs as seen in Figure 5 and Table S4. According to the estimated HI, the hysteresis extent successively diminishes with increasing pore size and film thickness of TFs. We attribute the reason to the fast charge extraction by improved infiltration of perovskite into mp-TiO2 that suppresses charge accumulation on the interlayer of TiO2/CH3NH3PbI3,46 which will be discussed later. PSCs fabricated by the VASP method show much lower HI due to the fact that the VASP method would further facilitate the pore-filling of perovskite into mp-TiO2 as the CH3NH3I vapor could easily get into the bottom of the scaffold. By using the one-step solution procedure, although forming of intermediate MAI·PbI2·DMSO retards fast-evaporation of DMF, crystallization occurs during diethyl ether dripping and subsequent annealing on hot plate is in the time scale of minutes. Immediate formation of perovskite due to strong ionic interactions between the metal cations and halogen anions hinders excellent infiltration of perovskite into the bottom of TiO2 scaffold. By contrast, the VASP procedure avoids the extremely high reaction rate of perovskite often observed in the solution-processed method.42 A smooth and uniform PbI2 framework is first formed inside the TiO2 scaffold and subsequently exposed to CH3NH3I vapor at 110 °C under a reduced pressure (100 Pa) for several hours to complete perovskite transformation. As diffusion of the raw material CH3NH3I is improved in the case of VASP, a fully pore-filling is yielded. A schematic for comparison on these two diverse methods is shown in Figure S9. Ultimately, a hysteresis-free PSC is achieved by depositing perovskite on 450 nm-thick TF34 via the two-step VASP method. The lower PCEs of hysteresis-less PSCs are caused by the poor coverage of perovskite on the scaffold as shown in Figures 3i,k.l. We expect that PSCs with both high PCE and less hysteresis could be achieved by depositing a fully covered perovskite thin layer over the mp-TiO2 with completely infiltrated perovskite. F


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contacts.36 The photocurrent under a certain bias will not get to a steady state until the migration and redistribution of ions reach equilibrium. As a result, the transient photocurrent that being recorded in J−V characterization is underestimated under FS and overestimated under RS, and the hysteresis is observed. By applying porous TiO2 scaffold as shown in Figure 6b, mpTiO2 is capable of extracting electrons from I− and resulting in I, which would avoid charge accumulation on the interface and the consequent screen field. By improving the pore-filling of perovskite with highly porous TFs, hysteresis would be further diminished and eliminated because of greatly accelerated charge transfer from I− to TiO2 at the interface of TiO2/CH3NH3PbI3. Planar PSCs containing a compact TiO2 layer suffer from fierce hysteresis because of inefficient charge transfer from perovskite to a compact TiO2 layer due to much smaller interfacial contact.39 Severe hysteresis observed in mesosuperstructured PSCs is consistent with the fact that more charges accumulate on the interface due to the insulating characteristics of Al2O3 scaffold.29 And the null hysteresis that always being observed in inverted PSCs with PCBM is caused by the superior charge extraction properties of PCBM with respect to TiO2.39 In a word, enlarging the interfacial contact between perovskite and TiO2 by effective pore-filling suppresses the charge accumulation caused by ion migration and diminishes the possibility of hysteresis. To prove the rationality of our proposed mechanism, we electrically poled PSCs beforehand, which was able to create a large accumulation of mobile ions on the interface to imitate ion migration.46 We electrically poled PSCs based on 250 nmthick TF-16, 250 nm-thick TF-25 and 450 nm-thick TF-34 for 30 s under dark condition with −1 V bias (imitating the ion migration under FS) or 1 V bias (imitating the ion redistribution under RS). The steady state current and J−V curves were recorded afterward under 1 sun illumination. As shown in Figure 7a, compared to the initial cell without prebias treating (no poling samples), the slowly increasing

Suppression of Charge Accumulation by Pore-Filling. The above-mentioned results have clearly demonstrated that hysteresis is strongly influenced by varied pore size and porosity of TiO2 scaffold. In other words, hysteresis largely depends on the pore-filling of perovskite. We attribute such a phenomenon to the suppression of charge accumulation on the TiO2/ CH3NH3PbI3 interface by using highly porous TFs. We propose a plausible mechanism of the suppressive effect on hysteresis by highly porous TFs as illustrated in Figure 6. Ions

Figure 6. Mechanism of ion migration in bulk perovskite (a) and suppressing effect of mp-TiO2 by electron extraction (b).

of I− and CH3NH3+ travel to the anode (electron transport layer, ETL) and the cathode (hole transport material, HTM) under increasing external bias from 0 to VOC of FS, and migrate back into the bulk perovskite under reducing external bias of RS as shown in Figure 6a. The accumulated charges on the interfaces screen the intrinsic build-in electric field and affect the collection of photogenerated charge carriers on both

Figure 7. Steady state photocurrent for PSCs fabricated on 250 nm-thick TF-16 (a), 250 nm-thick TF-25 (b) and 450 nm-thick TF-34 (c). Devices were measured at a bias voltage equal to voltage corresponding to maximum power point. J−V curves for PSCs fabricated on 250 nm-thick TF-16 (d), 250 nm-thick TF-25 (e) and 450 nm-thick TF-34 (f). None poling, −1 V poling and 1 V poling were applied on devices for 30 s under dark conditions beforehand. G


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Chemistry of Materials current after prebias of −1 V could be ascribed to an enhanced build-in field due to ion migration. And the gradually reducing current after prebias of 1 V is caused by the redistribution of accumulated ions. The transient lower current observed under prebiasing of −1 V is consistent with reducing PCE obtained as shown in Figure 7d. On the opposite, a higher transient current under prebiasing of 1 V and corresponding increased PCE are detected. As the response of steady state photocurrent after prebias treatment is in the time scale of tens of seconds shown in Figure 7a, the current recorded during J−V characterization is actually a transient value that underestimated at FS and overestimated at RS. The shaky line observed in Figure 7b was caused by the damaging on perovskite due to bias treatment. But it is worth mentioning that the variation on PCE under prebias treatment can be recovered by storing the same cell in the dark overnight as shown in Figure S10, and thus the varied PCEs after bias treatment are not caused by the degradation of perovskite or the electron extraction from I−. By enhancement of the pore size of TFs and pore-filling of perovskite, such deviations of steady state current and J−V curves are diminished as seen in Figure 7, which implies suppression of charge accumulation and hysteresis by improving infiltration of perovskite into TiO2 scaffold. Hysteresis-Free PSCs. To obtain PSCs with both high PCE and eliminated hysteresis, we deposited a fully covered perovskite layer over the mp-TiO2 with completely infiltrated perovskite by the one-step solution method due to its low-cost and ease of use. We subtly adjusted the film thickness of spincoated TF-34 by careful control of the TiO2 content in the dilution of TiO2 paste. The most probable pore size of TF-34 spin-coated by 3 wt % dilution is 46.3 nm. And it is further expanded up to 54.4 nm by increasing the dilute concentration from 3 to 6 wt % as shown in Figure 8a. The further increase of pore size along with increased film thickness is beneficial for the fully pore-filling of perovskite into the thicker TFs by the onestep solution method. Figure 8b shows the cross-sectional SEM images of devices incorporating TF-34 with different thicknesses of 290, 350 and 559 nm achieved by spin-coating 3, 4 and 6 wt % TiO2 dilutions, respectively. The corresponding thicknesses of perovskite capping layers are 308, 242 and 214 nm, respectively. Decent efficiencies above 15% have been achieved as dense and fully covered perovskite thin layers are formed over the completely infiltrated TiO2/CH3NH3PbI3 as observed in Figure 8c. As shown in Figure 8d, a certain extent of hysteresis still exists in the cell based on 290 nm TF-34, which is caused by the severe ion migration in the thicker perovskite capping layer. However, the lower PCE for the cell with null hysteresis that based on 559 nm TF-34 is caused by the inefficient charge transfer due to the excessively thick scaffold. Finally, the champion cell based on 350 nm TF-34 gives a PCE of 14.85%, a JSC of 21.90 mA/cm2, a VOC of 1.044 V, and a FF of 64.95 at FS, and a PCE of 15.47%, a JSC of 21.97 mA/cm2, a VOC of 1.064 V, and a FF of 66.18 at RS. The steady-state current measurement of the same cell shown in Figure S11 gives a stabilized output power of 15.3% under 0.84 V constant forward bias, which agrees well with the PCE by J− V measurement.

Figure 8. Pore size distribution for the TF-34 films spin-coated by 3 wt % TiO2 dilution (black line), 4 wt % TiO2 dilution (blue line) and 6 wt % TiO2 dilution (red line) (a). Cross section images of perovskite deposited on TiO2 scaffold (b), top-view images of perovskite capping layers (c), and scan direction dependent J−V curves (d) based on the corresponding spin-coated films of 290 nm-thick TF-34 (left), 350 nm-thick TF-34 (middle) and 559 nm-thick TF-34 (right).

films to the best of our knowledge. Pore size and porosity of mp-TiO2 is successively regulated by facilely adjusting the adding amount of P123 in the novel system of TiO2/EC/P123/ BCA for screen-printable paste. The SEM images of cross section architectures and PL measurements confirm better pore-filling of perovskite into the scaffold and thus faster electron extraction on the interface with increasing pore size and improved pore-filling. Bilayer-structured PSCs performance is affected by the compromise of improved charge extraction by better pore-filling with enlarged pore size and increasing charge recombination by diminishing coverage of capping layer. Hysteresis extent is sequentially reduced with increasing pore size and porosity of TFs. Slow recovery of steady state photocurrent and deviation of J−V curve measurements after prebiasing confirm the link between pore-filling and hysteresis. A reliable mechanism has been proposed that improving perovskite infiltration by larger pore size leads to the fast electron injection on the interface, which suppresses charge accumulation and thus diminishes hysteresis. By further achieving both complete infiltration of perovskite into scaffold and fully coverage of perovskite capping layer, hysteresis-free PSC with a PCE of 15.47% is ultimately obtained. This novel method we demonstrate here for the preparation of highly porous TiO2 films provides a new way to fabricate hysteresisfree PSCs. It is also widely applicable in other fields such as electronically conductive adhesives, ceramic materials and mesoporous catalyst, etc.

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CONCLUSIONS In conclusion, we have demonstrated a novel method to obtain highly porous TiO2 films with tunable pore size of 16.0−34.2 nm and porosity of 57.1−73.5%, porosity of which is the highest that has ever been literally reported for the TiO2 thick H


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silver acting as the counter electrode was thermally evaporated on top of the hole-transporting layer. Characterization. The crystalline structure of the hydrothermal synthesized TiO2 nanoparticles was investigated by powder X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer. Data were collected over 2θ values from 20° to 80°, at a scan speed of 4° /min. Transmission electron microscopy (TEM) image of TiO2 nanoparticles was carried out using a FEI Tecnai G2 F20 microscope. Nitrogen adsorption−desorption isotherms were measured with a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The specific surface area was determined from the linear portion of the Brunauer− Emmett−Teller (BET) equation (P/P0 = 0.05−0.30). The pore size distribution was calculated using the Barret−Joyner−Halenda (BJH) model. The scanning electron microscopy (SEM) images of the porous TiO2 films, device architectures were obtained using FEI Magellan 400. The rheological characteristics of the pastes were determined using a DVIII+ Brookfield Programmable Rheometer, equipped with an HB6 RV/H Brookfield spindle, with a disc diameter of 14.62 mm. Viscosity was measured as a function of the shear rate. Steady photoluminescence spectroscopy (PL) was recorded on a Horiba-Ltd. FluoroMax-4 device with an excitation wavelength of 457 nm. All the perovskite samples were deposited onto the films of mpTiO2 and bl-TiO2 on FTO glass as described above. The current density−voltage (J−V) curves were measured in air at room temperature with a solar simulator equipped with a 450 W xenon lamp and a Keithley-2420 source meter (AM 1.5G, 100 mW/cm2). Light intensity of the measurements was determined using a calibrated silicon solar cell (Oriel-91150) as reference for approximating 1 sun light intensity. The J−V curves were obtained by applying an external voltage bias with a scan rate of 40 mV/s without any precondition before characterization. The active area of cells was fixed at 0.07 cm2. To confirm the ion migration, PSCs were kept at a forward bias of 1 V or a reverse bias of −1 V for 30 s in dark before J−V curves and steady state current characterizations. Steady state current of the PSCs was measured for more than 2 min at a bias voltage equal to voltage corresponding to maximum power point (determined from J−V curve).

EXPERIMENTAL SECTION

Preparation of Porous TiO2 Film. Anatase TiO2 nanoparticles were prepared according to the literature.47 After washed with deionized water and ethanol for several times, the suspension of 2 g TiO2 particles in ethanol was stirred overnight to accomplish a complete dispersion. P123 (Aldrich, M = 5800) and 0.8 g of ethyl cellulose (EC, Aldrich, 30−60 mpas) were predissolved in ethanol before adding to the TiO2/EtOH solution, respectively. 7.2 g of terpineol or 2-butoxyethyl acetate (BCA, Aladdin, 98%) was finally added before evaporation. The TiO2/P123 ratio in grams varied from 3:1 to 1:2 to adjust the pore size and porosity of TiO2 film. After evaporation to remove ethanol, the viscous pastes were obtained. The porous TiO2 films were prepared from those TiO2 pastes with varied P123 content by a screen-printing process and sintered at 510 °C to remove the organic additives. Synthesis of CH3NH3I. Methylamine iodide (CH3NH3I) was synthesized and purified on the basis of a reported method.48 30 mL of methylamine (33 wt % in ethanol, Sigma-Aldrich) and 28 mL of hydroiodic acid (57% in water, Sigma-Aldrich) reacted in a 250 mL round-bottomed flask at 0 °C in an ice bath for 2 h with stirring. The solvent was then removed with a rotary evaporator by heating the solution at 60 °C under reduced pressure, and the precipitate was then crystallized. The product was washed with diethyl ether for three times, and then dissolved in ethanol, recrystallized using diethyl ether and finally dried in a vacuum at 60 °C for 24 h to yield CH3NH3I. Fabrication of PSC Devices. FTO substrates were cleaned and UV−O3 treated beforehand. A 40 nm-thick blocking layer was prepared with a sol−gel method. The sol used here was prepared by mixing titanium tetraisopropoxide (TTIP) contained solution A (TTIP, ethanol, acetylacetone) and acid solution B (ethanol, HCl, H2O). The solution was spin coated onto FTO at a spin coating speed of 3000 rpm for 20 s. The substrate was calcined at 510 °C for 30 min in air. After cooling to room temperature, the compact TiO2 films were treated in 40 mM aqueous solution of TiCl4 for 40 min at 70 °C, and then rinsed with deionized water and ethanol. The TiCl4 treated substrates were again calcined at 510 °C for 30 min. TiO2 pastes were diluted in ethanol (1:5, weight ratio) and spin coated onto the substrates at a speed of 3000 rpm for 20 s. A 250 nm-thick mesoporous TiO2 layer was achieved after sintered at 510 °C for 30 min. The procedure was repeated one more time to achieve another 450 nm-thick mesoporous TiO2 layer. Perovskite thin films were prepared with one-step solution and twostep VASP methods, respectively. For the one-step solution method, the perovskite precursor solution was prepared as reported by Ahn et al.49 461 mg of PbI2 (Sigma-Aldrich, 99%), 159 mg of CH3NH3I (TCI, 98%) and 78 mg of DMSO (Sigma-Aldrich, 99.9%) (molar ratio 1:1:1) were mixed in 600 mg of anhydrous DMF solution at room temperature with stirring for 1 h. The perovskite precursor solution was spin-coated on the mp-TiO2 layer at 5000 rpm for 20 s and 0.5 mL of diethyl ether was dripped on the rotating substrate at the sixth second. The perovskite film was then dried at 100 °C on a hot plate for 2 min. For the two-step VASP method,50 PbI2 (Aladdin, 99.8%) was dissolved in DMF at 85 °C (578 mg/mL), and spin coated onto the mp-TiO2 surface at 6500 rpm for 5 s. The substrates were then dried on a 100 °C hot plate for 2 min to remove the remaining solvent. The as-prepared PbI2 infiltrated TiO2 film was faced down at a constant distance of around 3 mm against the CH3NH3I powder. The reaction was conducted in a vacuum oven under the pressure of 100 Pa at 110 °C for 6.5 h. After the formation of perovskite, the film was rinsed with isopropanol for 45 s followed by drying with air. Finally, the perovskite films deposited by different methods were annealed at 100 °C for 45 min to enhance the crystallinity. A hole-transporting layer was deposited onto the perovskite film by spin coating at 4000 rpm for 30 s. The solution contained 72.3 mg of spiro-OMeTAD (Merk), 1 mL of chlorobenzene (Sigma-Aldrich), 17.5 μL of lithiumbis(trifluoromethanesulfonyl)imide (Li-TFSI, J&K scientific Ltd.) solution (520 mg cm−3 Li-TFSI in 1 mL acetonitrile) and 28.5 μL of 4-tert-butylpyridine (TBP, Sigma-Aldrich). Finally, 80 nm-thick

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03445. TEM image, XRD pattern and pore size distribution of synthesized TiO2 nanoparticles, rheology diagram of TiO2 pastes, comparison of the pore size with further enhanced amount of P123, structure characteristics of terpineol-based TiO2 films, detailed photovoltaic parameters, J−V cures under FS and RS, schematic figure for comparison of two deposition methods, J−V curves of the cell prebias treated under varied voltage and time, steady-state output measurement of the hysteresis-free cell (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*S.Y.: e-mail, [email protected]. *Y.L.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National High Technology Research and Development Program of China (Grant No. 2014AA052002), Science and Technology Service I


Article


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Network Initiative of the Chinese Academy of Sciences (KFJSW-STS-152), Shanghai Municipal Natural Science Foundation (Grant No. 16ZR1441000), Shanghai Municipal Sciences and Technology Commission (Grant No. 12DZ1203900) and the Shanghai High & New Technology’s Industrialization Major Program (Grant No. 2013-2).

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Pore Size Dependent Hysteresis Elimination in Perovskite Solar Cells

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