Insulated Interlayer for Efficient and Photostable Electron-Transport

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Insulated Interlayer for Efficient, and Photostable Electron-Transport-Layer-Free Perovskite Solar Cells Pengjun Zhao, Manhyung Han, Wenping Yin, Xing Zhao, Seul-Gi Kim, Yaping Yan, Minwoo Kim, Young Jae Song, Nam-Gyu Park, and Hyun Suk Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00021 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Materials & Interfaces

Insulated Interlayer for Efficient, and Photostable Electron-Transport-Layer-Free Perovskite Solar Cells Pengjun Zhao †,, Manhyung Han †,, Wenping Yin ‡,, Xing Zhao §,, Seul Gi Kim§,, Yaping Yan ∥, , Minwoo Kim ⊥,, Young Jae Song ⊥,, Nam Gyu Park §,, Hyun Suk Jung †,* †

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, 440-746, KOREA ‡ Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, KOREA §

Department of Chemical Engineering, Sungkyunkwan University, Suwon, 440-746, KOREA

∥

Department of Interdisciplinary of Physics and Chemistry, Sungkyunkwan University, Suwon 440-746, KOREA ⊥

SKKU Advanced Institute of Nanotechnology (SAINT) & Department of Physics, Sungkyunkwan University, Suwon, 440-746, KOREA *Corresponding Author: E-mail: [email protected]

KEYWORDS Bathocuproine, photo stability, perovskite solar cell, electron-transfer-layer-free, modulation doping Abstract Currently, the most efficient perovskite solar cells (PSCs) mainly use planar and mesoporous titanium dioxide (TiO2) as an electron-transporting-layer (ETL). However, due to its intrinsic photocatalytic properties, TiO2 can decompose perovskite absorber and lead to poor stability under solar illumination (ultraviolet light). Herein, simplified architectural ETL-free PSC with enhanced efficiency and outstanding photostability is produced by the facile deposition of Bathocuproine (BCP) interlayer. Power conversion efficiency (PCE) of ETL-free PSC improves from 15.56% to 19.07% after inserting the BCP layer, which is the highest efﬁciency reported for perovskite solar cells involving ETL-free architecture, versus 19.03% for the n-i-p full device using TiO2 as ETL. The BCP interlayer has been demonstrated to be with several positive effects on the devices photovoltaic performances such as “modulation doping” of perovskite layer, surface modification of FTO work function, and enhancing the 1 ACS Paragon Plus Environment

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carrier charge efficiency between FTO and perovskite. Moreover, the BCP- based ETL-free devices exhibits outstanding photo stability: the unencapsulated BCP-based ETL-free PSCs sustain over 90% of their initial efficiencies after 1000 h storage in the air and maintain 92.2% after 450 h exposure to full solar irradiation (without UV filter), compared to only 14.1% in the n-i-p full cells under the same condition. 1. INTRODUCTION Rapid development of hybrid perovskite solar cells (PSCs) has led the photovoltaic area to new horizons in recent years. The dramatic growing of certified efficiency, low cost and solution processable thin film makes the PSCs be regarded as the most prospective candidate to supersede silicon materials and serve as promising third-generation photovoltaic devices.1-5 Sustaining efforts are ongoing to carry forward the industrialization of PSCs.6 Unfortunately, there are still several nonnegligible obstacles impeding realistic application. The primary one relates to the stability of PSCs devices. Generally, three aspects are usually utilized to evaluate the PSCs’ stability: thermal, humidity and the light stability.7-11 The first two are easily alleviated by appropriate encapsulation technology. However, it is always embarrassing for a solar cell device to be labile under long term illumination. The illumination degradation of PSCs devices is raised from not only the perovskite material itself, but also other substrates in contact with the perovskite thin film. Currently, the most commonly used and efficient architecture for perovskite solar cells is the n-i-p structure composed of several functional layers: inorganic n-type oxide, (mainly TiO2) planar with mesoporous layer12-15 as the electron transport layer (ETL) on the surface of a transparent conductive oxide substrate (TCO), perovskite light absorber layer and the organic polymer donor

material

((2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene

(Spiro-OMeTAD) or polytriarylamine (PTAA))16,17 as the hole transport layer (HTL). In this kind of structure, TiO2, especially nanoparticles in the mesoporous layer, has been proven to be an excellent photo catalyst under UV radiation, that can decompose organic groups, which 2 ACS Paragon Plus Environment

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is the dominant factor responsible for the photo instability of PSCs. On account of this, plenty of strategies have been taken to reduce the negative effectiveness of TiO2 such as interface passivation, introduction of another n-type oxide between TiO2 and the perovskite layer, using a UV-filter, and so on.13,15,18 The best way to eliminate the effect of TiO2 is to remove this layer from the PSCs, without sacrificing photovoltaic performance, namely, ETL-free devices.19,20 Simplified ETL-free PSCs have several asvantages compared to the regular PSCs, such as reduced cost, low temperature processing, and ease to fabricate devices on flexible substrates. However, to date, the reported ETL-free PSCs usually have much lower efficiencies than those of TiO2 ETL devices.19-28 In this report, we demonstrate a novel efficient and photostable electron-transport-layer-free perovskite solar cell with a photovoltaic conversion efficiency over 19.0%, which is equivalent to that of PSCs with TiO2 mesoporous layer, by introducing an insulated interlayer of Bathocuproine (2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline, C26H20N2, BCP hereafter) between FTO substrate andthe perovskite layer. More conspicuously, the BCP based ETLfree PSCs exhibited favorable photostability when the unencapsulated devices were exposed to AM 1.5 illumination, even when using Ag as the electrode. 2. EXPERIMENTAL SECTION 2.1 Materials All chemical reagents are commercial available. Lead iodide (PbI2, 99.995%), lead bromide (PbBr2, 99.999%), Cesium iodide (CsI, 99.999%) and Bathocuproine was purchased from Sigma-Aldrich (U.S.A) and Methanaminium bromide (CH3NH3Br, MABr) and Formamidine iodide (CH(NH2)2I, FAI) from Xi’An P-OLED Co. (China). 2.2 Device fabrication FTO-coated glass substrates were etched by laser into the target patterns, and then rinsed with deionized water, ethanol and acetone. For BCP based ETL-free devices, BCP (0.5 3 ACS Paragon Plus Environment


mg/mL) was dissolved into isopropyl alcohol (IPA) and then spin-coated on the FTO substrate at different speeds. For n-i-p full cells, a TiO2 compact layer was deposited on the substrates by spin-coating at 4000 rpm. for 30s, using a titanium diisopropoxide bis(acetylacetonate) solution (75 % in 2-propanol, Sigma-Aldrich) in ethanol (1:19, volume ratio) as precursor and baking for 5 min at 120 oC, the compact TiO2 layer was annealed at 500 °C for 60 min. After that, TiCl4 water solution (0.04M) was used to treat the TiO2 surface at 70 °C for 20 min followed by heat treatment at 500 °C for 60 min. The mesoporous TiO2 layer composing 18 nm sized particles was deposited by spin-coating at 5000 rpm for 20 s using a TiO2 paste diluted in ethanol (1:5.5, weight ratio). After drying at 100 °C for 10 min, the TiO2 films were heated to 500 °C, and annealed at this temperature for 60 min. The 1.3 M [CsPbI3]0.05[(FAPbI3)0.85(MAPbBr3)0.15]0.95 perovskite precursor solutions were prepared by dissolving corresponding amounts of FAI, MABr, PbI2 in DMSO/DMF (1:9 v/v) mixed solvent (5% molar excess of PbI2), The perovskite precursor solution was coated onto the BCP or mp-TiO2/bl-TiO2/FTO substrate by an adduct method. Briefly, after exposed the substrates under UV-ozone irradiation for 15 min, the dissolved solution was spin-coated on the substrates at 4000 rpm for 25 s and 0.5 mL of antisolvent was slowly dripped on the rotating substrate 5 s after the start of the spin-procedure. The semi-transparent perovskite•DMSO adduct film was heated at 150 oC for 10 min in order to obtain a dense perovskite film. After cooling to room temperature, 30 µL HTM was deposited by spin coating at 4000 rpm for 30 s. The HTM solution consists of spiro-OMeTAD (36 mg), 4-tertbutylpyridine (14.4 µL) and lithium bis(trifluoromethylsulphonyl)imide acetonitrile solution (8.8 µl, 520 mg mL-1) dissolved in chlorobenzene (0.5 mL). Finally, 120 nm of silver metal was thermally evaporated on top of the device to form the back contact. 2.3 Characterizations

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The current-voltage characteristics of the solar cells were determined using a solar simulator (Newport Oriel Solar 3A Class AAA, 64023A) and a potentiostat (CHI 660D, CH instruments); the measurements were conducted under AM 1.5G sun (100 mA cm−2) illumination. The potentiostat was calibrated using a standard Si-solar cell (Oriel, VLSI standards) and a light sensor current controller (Newport Oriel digital exposure controller, Model 68945). All devices were measured by masking the active area with a thin mask (0.14 cm2). J–V characteristics of all devices were measured at a voltage scan rate of 0.1 V s−1. Unless stated otherwise, the scan direction is reverse scan (from V=1.2 V to V=0 V). Incident photon-to-current efficiency (IPCE) profiles were measured by a specially designed EQE system (PV measurement Inc.). Monochromatic beam in the range of 300-850 nm was generated by a 75 W Xenon source lamp (USHIO, Japan) while IPCE data were collected in DC mode without bias light. SEM images were obtained from a JSM-7600F hot field emission scanning electron microscope (JEOL, Japan). The steady-state photoluminescence (PL) spectra were measured with a fluorescence spectrophotometer (HITACHI F-7000) equipped with a standard light source (P/N 250-0123), whose entire wavelength range was 200 nm to 900 nm, combined with an instrumental response. Time-resolved photoluminescence (TRPL) curves were recorded using a commercial Time Correlated Single Photon Counting (TCSPC) system (FluoTim 200, PicoQuant). Samples were photoexcited using a 670 nm picosecond diode laser (LDH-P-C-670, PicoQuant) with a variable repetition rate (800 kHz). The emitted PL was spectrally dispersed with monochromater (ScienceTech 9030) for each PL signal, and was collected by a fast photon multiplier tube (PMT) detector (PMA 182, PicoQuant GmbH) with a magic angle (54.7°) arrangement. The incident angle of excitation pulse was set to be about 30° with respect to the sample. The resulting instrumental response function was about 160ps in full-width-half5 ACS Paragon Plus Environment


maximum. All the signals were measured at the emission peak (770±5nm) for perovskite. In addition, a cut-off filter (GG-692nm, Semrock) was applied to block the stray scatter.

3. Results and Discussion BCP is a commonly used buffer layer between the metal electrode and electron collection layer in organic light-emitting diodes (OLEDs) or organic photovoltaic cells (OPVCs), to control the electrical properties at the organic/metal interface, although the specific mechanism of BCP has not been understood fully.29,30 The architecture diagram of our BCPbased ETL-free PSC is presented in Figure 1 (a). A BCP thin layer was spin-coated on the surface of FTO glass substrate without coating with an organic or inorganic ETL, and its thickness was tuned by adjusting the spin-coating speed. Films of mixed cation perovskite [CsPbI3]0.05[(FAPbI3)0.85(MAPbBr3)0.15]0.95 with the 5% molar percentage excess of PbI2 were produced fby a one-step spin-coating method from a solution of cesium iodide (CsI), formamidinium iodide (FAI), PbI2, methylammonium bromide (MABr), and PbBr2 in a mixed solvent of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO), as the light absorber. Spiro-MeOTAD and silver electrode were deposited on the perovskite layer sequentially. A typical cross-sectional scanning electron microscopy (SEM) image of this PSC is shown in Figure 1 (b); the PSC contained a thin layer BCP (invisible), a mix cation perovskite film with the thickness of 350 nm, 200 nm spiro-OMeTAD and a 120 nm Ag electrode. From Figure 1 (c), we can investigate the transfer directions of carriers (holes and electrons) from the schematic of energy levels.31 The highest occupied molecular orbital (HOMO) of BCP is lower than the valence band (VB) edge of perovskite, while the lowest unoccupied molecular orbital (LOMO) of BCP is higher than the conduction band (CB) bottom of perovskite. It seems that neither electrons nor holes can transfer from perovskite to the BCP layer from the perspective of energy levels.


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In Figure 2 (a), the efficiency of ETL-free PSC without BCP layer was 15.56%, which is close to the value reported earlier by Cui et al.25 Usually, regular ETL-free PSCs have poor fill factors (FF), lower than 0.70 (0.69 in this work), which is mainly caused by significant interfacial recombination between the FTO electron contact substrate and the perovskite layer.32 After insetting the BCP interlayer into the ETL-free PSC, the photovoltaic parameters of the J-V curve were enhanced, and the efficiency dramatically increased to 19.07%, which is even higher than that of a TiO2-based n-i-p structured full cell (19.03%). To the best of our knowledge, this PCE is higher than most of reported values in efficient ETL-free structured PSC to date (a literature summary of the PCEs of ETL-free PSCs is presented in Table S1), which is equivalent to TiO2 n-i-p full cells. Compared to the cells without BCP layer, the BCP based ETL-free PSCs shows much reduced hysteresis, but still quite large (Figure S1 (a)). So the real power output of the devices were estimated by measuring the steady-state photocurrent output at the maximum power point under simulated solar illumination33, which is shown in Figure S1 (b). The BCP based cell yeilds a steady state output PCE of 18.5 %, which is a little bit lower than the reverse scan efficiency. We also calculated the series resistance (Rs) and shunt resistance (Rsh) for each cell, derived from the slopes of the J–V characteristic curve close to V=0V and V= Voc.34 Results in Table 1 show that the Rs were approximately equal, i.e., 1.25, 1.03, and 1.82 Ω cm2 for ETL-free without BCP, ETL-free with BCP and n-i-p full devices, respectively. On the other hand the BCP interlayer leads to a substantial increase of Rsh, the value of Rsh rose from 572.5 to 3560.0 Ω cm2 after BCP interposition, and this value was larger than 3450.7 Ω cm2 in the n-i-p full cell, implying that BCP is capable of increasing PSC Rsh and leading to enhancements of open voltage (Voc) and FF.35,36 Consistent with the Jsc displayed from the J-V curves, incident photon-to-current efficiency (IPCE) measurements in Figure 2 (b) demonstrate that the BCP-based ETL-free cell had 7 ACS Paragon Plus Environment


higher Jsc than the n-i-p full structured cell. Integral current densities were 21.8, 21.1 and 20.29 mA cm-2 for BCP based ETL-free cell, n-i-p full structured cell, and ETL-free without BCP interlayer cell, respectively, compared to the measured values of 22.35, 21.62, and 21.10 mA cm-2. According to earlier reports, the TiO2 nanolayer has adverse effects on the charge transfer in PSC devices: trap states on the surface as well as in bulk TiO2 can be filled with excitated electrons under illumination. This non-radiative recombination process is capable of causing a leakage current inside solar cells, and the leakage current neutralizes part of the photo-generated current.37 Considering that BCP is more “insulated” than TiO2, this kind of energy loss can be avoided when substituting mesoporous TiO2 with a BCP layer, resulting in the improvement of Jsc. In order to clearly demonstrate that the that the BCP layer has fully covered the FTO surface, the top-view SEM image, AFM morphology as well as the current mapping of FTO and BCP layer on FTO have been measured, which are displayed in Figure S2. The surface morphology of FTO has almost no change after BCP coating, furthermore, the surface roughness of bare FTO is 43.8 nm and slightly decreased to 38.0 nm after BCP layer depositing, from the SEM and AFM height images in Figure S2 (a-d), imply that the coated BCP layer forms homogenous and continuous layer on FTO surface. More convincing, both the current mapping images (Figure S2 (e, f)) show uniform current distribution and the geometry of current domains is match with that of the crystal grains. In addition, the average current value in the BCP/FTO samples (ca. 12.0nA) is several times large than the FTO sample (ca. 3.0nA), indicating that the BCP coating (1000 rpm) can increasing the conductivity of FTO, which is consistent with the solar cell J-V curves. To further investigate the functional mechanism and impact of BCP layer on the photovoltaic performance of ETL-free PSCs, we fabricated various ETL-free PSCs with different BCP thicknesses by tuning the spin speed (1000, 2000, 3000 and 4000 rpm), and the obtained J-V curves are summarized in Figure 3 (a). The thicknesses of BCP layers were detected by 8 ACS Paragon Plus Environment

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atomic force microscope (AFM) profiles (Figure S3): 40, 25, 10, 5 nm for 1000, 2000, 3000 and 4000 rpm, respectively. It should note that the thickness here is a nominal value, rather than an accurate value (see discussion in Supporting Information). All of the BCP based cells exhibits higher PCEs than bare ETL-free cells. Meanwhile, there was an inverse correlation between PCEs and BCP-thickness. The champion cell’s PCE decreased from 19.07% to 18.47% when the BCP coating speed increased from 1000 to 2000 rpm, and further reduced to 18.07% and 17.20% for 3000 and 4000 rpm, respectively. Specific to each parameters, we find that Jsc is relatively constant with BCP thickness; on the other hand, Voc, and FF, present downward trends when BCP decreases, thus resulting in the reduction of PCEs. The function of the BCP interlayer can also be reflected from the evolution of Rsh (Table 1). Rsh of all the BCP-based PSCs were several times larger than those without BCP and the Rsh values roughly stepped down with increasing the BCP coating speed. In addition, we checked the surface and cross-sectional morphologies of each FTO/BCP/perovskite thin film by scanning electron microscopy (SEM), to identify if the BCP layer exerts any impact on the grain growth behavior of perovskite thin films (Figure S4 and Figure S5). Firstly, all samples were 350 nm thick, proving that the BCP interlayer had almost no influence on perovskite thickness. Meanwhile, all the perovskite films show dense and pinhole free crystal grain arrangement, with crystallite dimension of about 150 nm. However, the 5 nm BCP layer, which presents the most efficient PCEs (according to previous data), is the only exception. In Figure S5 (b), the average grain size was more than 200 nm. The increased crystal size might result from the modification of substrate surface roughness by the BCP layer. SEM images demonstrate that the BCP interlayer had tiny influence on the grain growth process of perovskite layers. To further elucidate the properties of the BCP/perovskite interface, we measure the charge transfer performance of each thin film by the steady-state photoluminescence (STPL) and time correlated single photon counting (TCSPC) technology. 9 ACS Paragon Plus Environment


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The normalized STPL and time-resolved photoluminescence (TRPL) spectra are exhibited in Figure. 4. It can be seen from Figure 4 (a) that the perovskite thin films had mono emission peak at 772 nm, and no peak shifts or other shoulder peak appeared with different coating thicknesses, suggesting the perovskite layer’s excitation mode remained unchanged before and after the BCP inset. Perovskite thin films’ PL intensities on different substrates (Glass and FTO) in Figure S6 reveals that BCP interlayer deposited at 1000 rpm shows best preventive effect on exciton quenching at the interface of the perovskite/FTO,38,39 which might lead to outstanding photovoltaic performance.40 From the TRPL spectra in Figure 4 (b), it can be seen that the photoluminescence decay on the glass/BCP/perovskite was slower than on glass/perovskite surface, indicating that the BCP layer presents no quenching effect to the perovskite thin film. On the contrary, the quenching process was much faster in FTO/BCP/perovskite than the film without BCP. This phenomenon demonstrates that the BCP interlayer adjusts the electron carrier injection behavior between perovskite and FTO substrate. In addition, the TRPL profiles were fitted according to a dual-exponential function,

41,42

and

the fitted parameters (τ1, τ2, A1 and A2) are listed in Table 2. Charge transfer efficiency (CTE) was also calculated according to Equation 1, to determine the electron transportation capacity with and without a BCP interlayer. CTE is the proportion of charge transfer relative to the summation of rates in all other processes, calculated using the average lifetime of each sample as: CTE = 1 −

τୈ୅ ൗτୈ

(1)

where τDA and τD are the donor (perovskite) average fluorescence lifetimes in the presence and absence of the acceptor (FTO substrate), respectively.43 The CTE obtained for film without BCP layer was 60.98%, while this value dramaticlly increased to 74.18% after BCP layer insertion, exhibiting an enhancement of over 20%. This issue plays the core influencing factor that most responsible for the increasing PCE of ETL-free PSCs after inset of the BCP layer. 10 ACS Paragon Plus Environment

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An exploratory hypothetical mechanism of the BCP interlayer to enhance electron transfer effects between FTO and perovskite can be briefly explained in Figure 4 (c). There are two beneficial effects at the heterointerface between FTO, or BCP and the perovskite layer. The first factor can be attributed to the socalled “modulation doping”. Modulation doping is an important strategy to significantly increase the free carriers’ concentration without compromising mobility, or the introduction of dopant impurities in heterostructure semiconductor device engineering. The most commonly used materials system for modulation doping is the GaAs/AlGaAs heterostructure interface.44-48 Earlier literature has proven that modulation doping can passivate interfacial trap states.44 In this article, the energy band alignment at the BCP/perovskite interface is similar to that of the GaAs/AlGaAs heterostructure: BCP (AlGaAs) has a more positive CB position than that of perovskite (GaAs); simultaneously, the VB position of BCP (AlGaAs) is more negative than perovskite (GaAs), making it possible to realize n-type doping of perovskite and thus increase the concentration as well as lifetime of free carriers. The modulation doping effect can be usually detected from the instinctive extension of carrier lifetime. In this work, an experimental phenomenon can imply the modulation doping effect from the carrier lifetime variation of perovskite thin film after interoperable by BCP. From Figure 4 (b) and Table 2, the slow portion (τ2) of perovskite thin film was prolonged from 779.69 to 903. 84 ns before and after BCP layer inseting, demonstrating improved photon-excited carrier transfer performance. The other factor is the band bending effect. As shown in Figure 4 (c), there is a band barrier at the BCP/FTO surface, electron density tends to redistribute and generate an equilibrium energy level (Eequilibrium).49 Eequilibrium would be located a little higher than the FTO CB (−4.4 eV) when considering the relative thickness of BCP and FTO, and will reduce the energy barrier at the perovskite/FTO interface, to enhance electron extraction of the solar cell device. This standpoint can be verified by the surface work function change from Kelvin probe force microscope (KPFM) profiles. As discussed in the Supporting Information (Figure S7), it can 11 ACS Paragon Plus Environment


be inferred that the conduction band bottom of BCP/FTO was raised by 160 mV compared to bare FTO, bringing it closer to that of the perovskite layer. The most important advantage of the BCP-based ETL-free PSC is its improved photo stability under illumination. To clarify this point, we tested the stability of the PSCs under different conditions, including a comparison between the BCP ETL-free and n-i-p full devices, and the results are exhibited in Figure 5 (a). All the devices were unencapsulated and used silver as the electrode. The light-soaking test was carried out under AM 1.5 illuminant without a UV filter. In the dark condition, the two types of PSC presented quite good stability. After 1000 h storage in air, the average residual efficiency was 93.7% for n-i-p full devices and 92.6% for BCP-based ETL-free devices, indicating the interface between BCP and perovskite is inactive. Next, an extremely harsh condition was applied to test the light-soaking stability of when maximum current flow passing through the devices, i.e., short circuit condition in air. Unlike the maximum power point, there is no external loading under short circuit condition and the entire current is applied to the solar cell device itself. Figure 5 (b) displays the variation tendency profiles for 3000 s of the output Jsc. Both cells displayed a ‘burn-in period’ for the first 100-150 s. It is encouraging that the attenuation percentages were quite different. The BCP ETL-free device exerted a tremendous jarless tendency. There was only a slight decline from 22.2 to 21.5 mA cm-2 and the output Jsc still maintained at 92.1 %; on the other hand, the n-i-p full cell presented a sharp falloff during the burn-in period and kept dropping off to only 52.34 % until the end of the test. Moreover, we examined the long term photo-stability by exposing the devices to AM 1.5 illumination in an Ar glovebox (Figure 5 (c)). As expected, BCP cells present stronger resistance to photodegradation than the n-i-p full devices. The average PCE retained 92.2% of the initial value compared to 14.1% in the n-i-p full cells. We particular point out that hereby the solar cell used for photo-stability test is unsealed ones and employed Ag as the electrode, rather than Au electrode utilized in many previous reports.4,7,9,13,15,50 12 ACS Paragon Plus Environment

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Normalized photovoltaic parameters in Figure S8 show that PCE photodegradation is mainly caused by the declines of Jsc and FF, while Voc did not change much with illumination time. We speculate that the severe attenuation of n-i-p full cells resulted from perovskite photocatalytic decomposition caused by the TiO2 layer. IPCE profiles (Figure 5 (d)) of the cells after 450 h illumination were plotted to check if the perovskite layer still functioned normally. Compared to the initial situation, the BCP-based ETL-free cell retained high conversion efficiencies of 70-85% (between 400-700 nm), yet only 15-40% retained for the ni-p full cell. A peak around 350 nm was noticed in the full cell’s curve, which should correspond to the characteristic absorption of TiO2. The above residual IPCE spectra demonstrate that TiO2 is one of the key factors affecting the photo instability of perovskite photovoltaic devices. 4. CONCLUSIONS To conclude, we improved the ETL-free structured perovskite solar cell to an efficiency of 19.07 % which is approximately equivalent to conventional n-i-p full-structured devices using mp-TiO2 as ETL (19.03%), by insetting a BCP insulting layer between the FTO substrate and perovskite thin film. The BCP interlayer is capable of modulating and optimizing the electron carrier injection from perovskite to the FTO substrate: the carrier charge efficiency was improved from 60.98% to 74.1 %. Most importantly, the BCP-based ETL-free PSCs exhibited outstanding stability under full illumination. After 450 h continuous AM 1.5 soaking, the BCP-based cells retained 92.2 % initial performance while the n-i-p full cell degraded almost completely. This report provides a new strategy guidance for designing novel simplified architectural ETL-free perovskite solar cells.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website



Acknowledgements This work was financially supported by Technology Development Program to Solve Climate Changes (No.2015M1A2A2056827), the National Research Foundation of Korea (NRF), funded by Ministry of Science, ICT and Future Planning (MSIP) of Korea under contracts (NRF-2012M3A7B4049986 and NRF-2014R1A2A2A01007722), and the Global Frontier R&D Program of the Center for Multiscale Energy System (2012M3A6A7054855).

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Figure Caption: Figure 1. (a) Architecture diagram, (b) cross-sectional SEM image, and (c) energy levels (relative to vacuum) for each functional layer of BCP-based ETL-free perovskite solar cells (Energy level position data is from Reference 31). Figure 2. (a) J-V curves of the with and without BCP-based ETP-free, and n-i-p full structured perovskite solar cells. (b) IPCE spectrum of each kind of perovskite solar cell. Figure 3. (a) J-V curves of the BCP-based ETL-free perovskite solar cells depending on BCP interlayer thickness. (b) – (e) Statistical profile of each photovoltaic parameter. For each parameter, J-V data from 10 different cells were collected for the statistic process. Figure 4. STPL (a) /TRPL (b) spectra of different thin films. (c) A schematic diagram of energy band bending at the interface of FTO and BCP interlayer. Figure 5. Stability test of the BCP based ETP-free and n-i-p full structured perovskite solar cells. (a) Storage in the air (b) short-circuit under full illumination in the air (20±3 oC , Humility < 20 %), (c) under continuous one sun illumination (AM1.5, 100mWcm-2, without UV-filter) in Ar glovebox at 20±3 oC ( All of the data was collected from 5 different devices for each condition, un-encapsulated devices) and (d) Representative IPCE of the BCP based ETL-free and n-i-p full cell after 450 hours’ illumination. Table Caption: Table 1. J-V parameters statistics of the perovskite solar cells Table 2. Fitted TRPL parameters and calculated CTE values



Figure 1. (a) Architecture diagram, (b) cross-sectional SEM image, and (c) energy levels (relative to vacuum) for each functional layer of BCP-based ETL-free perovskite solar cells (Energy level position data is from Reference 31).


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Figure 2. (a) J-V curves of the with and without BCP-based ETP-free, and n-i-p full structured perovskite solar cells. (b) IPCE spectrum of each kind of perovskite solar cell.



Figure 3. (a) J-V curves of the BCP-based ETL-free perovskite solar cells depending on BCP interlayer thickness. (b)-(e) Statistical profile of each photovoltaic parameter. For each parameter, J-V data from 10 different cells were collected for the statistic process.


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Figure 4. STPL (a) /TRPL (b) spectra of different thin films. (c) A schematic diagram of energy band bending at the interface of FTO and BCP interlayer.



Figure 5. Stability test of the BCP based ETP-free and n-i-p full structured perovskite solar cells. (a) Storage in the air (b) short-circuit under full illumination in the air (20±3 oC , Humility < 20 %), (c) under continuous one sun illumination (AM1.5, 100mWcm-2, without UV-filter) in Ar glovebox at 20±3 oC ( All of the data was collected from 5 different devices for each condition, un-encapsulated devices) and (d) Representative IPCE of the BCP based ETL-free and n-i-p full cell after 450 hours’ illumination.


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Table 1. J-V parameters statistics of the perovskite solar cells J Parameters

1000 rpm (ca. 40 nm)

2000 rpm (ca. 25 nm)

3000 rpm (ca. 10 nm)

4000 rpm (ca. 5 nm)

w/o BCP (Reference)

V

sc

-2

FF

oc

R

R

PCE (%)

(Ω cm )

(Ω cm )

s

2

sh

2

(mA cm )

(V)

Max.

22.35

1.10

0.776

19.07

1.03

3560.0

Ave.

22.18

1.07

0.757

18.05

--

--

Max.

22.14

1.09

0.762

18.47

1.13

2905.5

Ave.

22.13

1.08

0.736

17.51

--

--

Max.

22.10

1.09

0.748

18.07

1.26

2042.0

Ave.

22.10

1.07

0.727

17.17

--

--

Max.

22.11

1.08

0.724

Ave.

22.13

1.03

0.650

14.91

--

--

Max.

21.10

1.07

0.690

15.56

1.25

572.5

Ave.

21.15

1.01

0.645

13.81

--

--


17.20

1.44

2541.6


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Table 2. Fitted TRPL parameters and calculated CTE values

Sample

τ1 (ns)

τ2 (ns)

Glass-perov.

5.46

779.69

Glass-BCP-perov.

6.27

903.84

FTO-perov.

4.40

311.64

FTO-BCP-perov.

4.54

242.67

A1

A2

94.00

99.40

(48.6%)

(51.4%)

94.94

99.48

(48.8%)

(51.2%)

99.60

98.88

(50.2%)

(49.8%)

104.2

98.4

(51.4%)

(48.6%)

Ave.(ns)

CTE

403.41

465.82 157.40

60.98%

120.27

74.18%

Where, I(t) = A1exp(−t/τ1) + A2exp(−t/τ2), CTE=1-τDA/τD (the values of the goodness-of-fit parameter (r2) are all close to 1.0) .


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Table of Contents (TOC):


Insulated Interlayer for Efficient and Photostable Electron-Transport

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