Insulated Interlayer for Efficient and Photostable Electron-Transport

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10132−10140

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Insulated Interlayer for Efficient and Photostable Electron-TransportLayer-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*,† †

School of Advanced Materials Science & Engineering, ‡Department of Energy Science, §Department of Chemical Engineering, Department of Interdisciplinary of Physics and Chemistry, and ⊥SKKU Advanced Institute of Nanotechnology (SAINT) & Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea ∥

S Supporting Information *

ABSTRACT: Currently, the most efficient perovskite solar cells (PSCs) mainly use planar and mesoporous titanium dioxide (TiO2) as an electron-transport layer (ETL). However, because of its intrinsic photocatalytic properties, TiO2 can decompose perovskite absorber and lead to poor stability under solar illumination (ultraviolet light). Herein, a simplified architectural ETLfree PSC with enhanced efficiency and outstanding photostability is produced by the facile deposition of a bathocuproine (BCP) interlayer. Power conversion efficiency of the ETL-free PSC improves from 15.56 to 19.07% after inserting the BCP layer, which is the highest efficiency reported for PSCs involving an ETL-free architecture, versus 19.03% for the n−i−p full device using TiO2 as an ETL. The BCP interlayer has been demonstrated to have several positive effects on the photovoltaic performances of devices, such as “modulation doping” of the perovskite layer, modification of FTO surface work function, and enhancing the charge-transfer efficiency between FTO and perovskite. Moreover, the BCP-based ETL-free devices exhibit outstanding photostability: the unencapsulated BCP-based ETL-free PSCs retain over 90% of their initial efficiencies after 1000 h of storage in air and maintain 92.2% after 450 h of exposure to full solar irradiation (without a UV filter), compared to only 14.1% in the n− i−p full cells under the same condition. KEYWORDS: bathocuproine, photostability, perovskite solar cell, electron-transfer-layer-free, modulation doping

1. INTRODUCTION Rapid development of hybrid perovskite solar cells (PSCs) has led the photovoltaic area to new horizons in recent years. The dramatic growth of certified efficiency, low cost, and solutionprocessable thin film make the PSCs to 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 non-negligible obstacles impeding their realistic application. The primary one is related to the stability of PSC devices. Generally, three aspects are usually utilized to evaluate the PSC stability: thermal, humidity, and light stability.7−11 The first two are easily alleviated by appropriate encapsulation technology. However, it is always difficult for a solar cell device to be stable under long-term illumination. The illumination degradation of PSC devices arises from not only the perovskite material but also other substrates in contact with the perovskite thin film. Currently, the most commonly © 2018 American Chemical Society

used and efficient architecture for PSCs is the n−i−p structure composed of several functional layers: planar and mesoporous inorganic n-type oxide (mainly TiO2) layer12−15 as the electron-transport layer (ETL) on the surface of a transparent conductive oxide substrate, perovskite light absorber layer, and the organic polymer donor material [2,2′,7,7′-tetrakis(N,N-dip-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) or polytriarylamine]16,17 as the hole-transport layer. In this kind of structure, TiO2, especially nanoparticles in the mesoporous layer, has been proven to be an excellent photocatalyst under UV radiation, which can decompose organic groups, which is the dominant factor responsible for the photoinstability of PSCs. On account of this, plenty of strategies have been taken to reduce the negative effect of TiO2, such as interface passivation, introduction of another n-type Received: January 3, 2018 Accepted: March 6, 2018 Published: March 6, 2018 10132

DOI: 10.1021/acsami.8b00021 ACS Appl. Mater. Interfaces 2018, 10, 10132−10140

Research Article

ACS Applied Materials & Interfaces

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 photonto-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), and IPCE data were collected in the dc mode without bias light. Scanning electron microscopy (SEM) images were obtained from a JSM-7600F hot field emission scanning electron microscope (JEOL, Japan). The steady-state photoluminescence (STPL) 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 200−900 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) at a variable repetition rate (800 kHz). The emitted PL was spectrally dispersed with a monochromator (Sciencetech 9030) for each PL signal and was collected by a fast photomultiplier tube 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 160 ps in full width at half-maximum. All signals were measured at the emission peak (770 ± 5 nm) for perovskite. In addition, a cutoff filter (GG-692 nm, Semrock) was applied to block the stray scatter.

oxide between TiO2 and the perovskite layer, and use of a UV filter.13,15,18 The best way to eliminate the effect of TiO2 is to remove this layer from PSCs, without sacrificing photovoltaic performance, namely, ETL-free devices.19,20 Simplified ETL-free PSCs have several advantages compared to the regular PSCs, such as reduced cost, low-temperature processing, and ease of fabricating devices on flexible substrates. However, to date, the reported ETL-free PSCs usually have much lower efficiencies than TiO2-ETL devices.19−28 In this report, we demonstrate a novel efficient and photostable ETL-free PSC with a photovoltaic conversion efficiency over 19.0%, which is equivalent to that of PSCs with a TiO2 mesoporous layer, by introducing an insulated interlayer of bathocuproine (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, C26H20N2, BCP hereafter) between the FTO substrate and the 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 commercially available. Lead iodide (PbI2, 99.995%), lead bromide (PbBr2, 99.999%), cesium iodide (CsI, 99.999%), and BCP were purchased from Sigma-Aldrich (USA), and methanaminium bromide (CH3NH3Br, MABr) and formamidinium iodide (CH(NH2)2I, FAI) were purchased from Xi’An P-OLED Co. (China). 2.2. Device Fabrication. FTO-coated glass substrates were etched by a laser into the target patterns and then rinsed with deionized water, ethanol, and acetone. For BCP-based ETL-free devices, BCP (0.5 mg mL−1) was dissolved into isopropyl alcohol 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 30 s, using a titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol, Sigma-Aldrich) in ethanol (1:19, volume ratio) as a precursor, and baking for 5 min at 120 °C, and the compact TiO2 layer was annealed at 500 °C for 60 min. After that, TiCl4 water solution (0.04 M) 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 comprising 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, and PbI2 in dimethyl sulfoxide (DMSO)/dimethyl formamide (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 exposing 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-coating procedure. The semitransparent perovskite·DMSO adduct film was heated at 150 °C for 10 min to obtain a dense perovskite film. After cooling to room temperature, 30 μL of hole transport material (HTM) was deposited by spin-coating at 4000 rpm for 30 s. The HTM solution consists of spiro-OMeTAD (36 mg), 4-tert-butylpyridine (14.4 μL), and lithium bis(trifluoromethylsulfonyl)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. 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

3. RESULTS AND DISCUSSION BCP is a commonly used buffer layer between the metal electrode and the electron collection layer in organic lightemitting diodes or organic photovoltaic cells, 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 BCP-based ETL-free PSC is presented in Figure 1a. 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 a 5% molar percentage excess of PbI2 were produced by a onestep spin-coating method from a solution of cesium iodide (CsI), FAI, PbI2, methylammonium bromide (MABr), and PbBr2 in a mixed solvent of DMF and DMSO, as the light absorber. Spiro-OMeTAD and silver electrode were deposited on the perovskite layer sequentially. A typical cross-sectional SEM image of this PSC is shown in Figure 1b; the PSC contained a thin-layer BCP (invisible), a mixed cation perovskite film with a thickness of 350, 200 nm spiroOMeTAD, and a 120 nm Ag electrode. From Figure 1c, we can investigate the transfer directions of carriers (holes and electrons) from the schematic of energy levels.31 The highest occupied molecular orbital of BCP is lower than the valence band (VB) edge of perovskite, whereas the lowest unoccupied molecular orbital of BCP is higher than the conduction band (CB) bottom of perovskite. It seems that neither electrons nor holes can transfer from the perovskite to the BCP layer from the perspective of energy levels. 10133


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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 = 0 V and V = Voc.34 Results in Table 1 show that Rs values were approximately equal, that is, 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 Jsc displayed from the J−V curves, IPCE measurements in Figure 2b demonstrate that the BCP-based ETL-free cell had 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 excited electrons under illumination. This nonradiative recombination process is capable of causing a leakage current inside solar cells, and the leakage current neutralizes part of the photogenerated 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 an improvement of Jsc. To clearly demonstrate that the BCP layer has fully covered the FTO surface, the top-view SEM image, atomic force microscopy (AFM) morphology, and 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 deposition, from the SEM and AFM height images in Figure S2a−d, implying that the coated BCP layer forms a homogenous and continuous layer on the FTO surface. More convincingly, both the current mapping images (Figure S2e,f) show uniform current distribution and the geometry of current domains matches with that of the crystal grains. In addition, the average current value in the BCP/FTO samples (ca. 12.0 nA) is several times larger than that in the FTO sample (ca. 3.0 nA), indicating that the BCP coating (1000 rpm) can increasing the conductivity of FTO, which is consistent with the solar cell J−V curves.

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 PSCs (energy level position data are from ref 31).

In Figure 2a, the efficiency of an ETL-free PSC without the 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 (FFs), 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 power conversion efficiency (PCE) is higher than most of reported values in efficient ETL-free-structured PSCs to date (a literature summary of the PCEs of ETL-free PSCs is presented in Table S1), which is equivalent to those of TiO2 n−i−p full cells. Compared to the cells without the BCP layer, the BCPbased ETL-free PSCs show much reduced, but still quite large, hysteresis (Figure S1a). Therefore, the real power output of the devices was estimated by measuring the steady-state photocurrent output at a maximum power point under simulated solar illumination,33 which is shown in Figure S1b. The BCPbased cell yields a steady-state output PCE of 18.5%, which is

Figure 2. (a) J−V curves of ETL-free PSCs with and without BCP and n−i−p full-structured PSCs. (b) IPCE spectrum of each kind of PSCs. 10134


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ACS Applied Materials & Interfaces Table 1. J−V Parameter Statistics of the PSCs 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)

max. ave. max. ave. max. ave. max. ave. max. ave.

Jsc (mA cm−2)

Voc (V)

FF

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

22.35 22.18 22.14 22.13 22.10 22.10 22.11 22.13 21.10 21.15

1.10 1.07 1.09 1.08 1.09 1.07 1.08 1.03 1.07 1.01

0.776 0.757 0.762 0.736 0.748 0.727 0.724 0.650 0.690 0.645

19.07 18.05 18.47 17.51 18.07 17.17 17.20 14.91 15.56 13.81

1.03

3560.0

1.13

2905.5

1.26

2042.0

1.44

2541.6

1.25

572.5

Figure 3. (a) J−V curves of the BCP-based ETL-free PSC 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.

All of the BCP-based cells exhibit higher PCEs than bare ETL-free cells. Meanwhile, there was an inverse correlation between PCE and BCP thickness. The champion cell PCE decreased from 19.07 to 18.47% when the BCP coating speed was 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 thickness 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). The

To further investigate the functional mechanism and impact of the 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 3a. The thicknesses of BCP layers were detected by AFM profiles (Figure S3): 40, 25, 10, and 5 nm for 1000, 2000, 3000, and 4000 rpm, respectively. It should be noted that the thickness here is a nominal value, rather than an accurate value (see the discussion in the Supporting Information). 10135


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

Table 2. Fitted TRPL Parameters and Calculated CTE Valuesa

a

sample

τ1 (ns)

τ2 (ns)

glass−perov. glass−BCP−perov. FTO−perov. FTO−BCP−perov.

5.46 6.27 4.40 4.54

779.69 903.84 311.64 242.67

A1 94.00 94.94 99.60 104.2

(48.6%) (48.8%) (50.2%) (51.4%)

A2

ave. (ns)

CTE (%)

99.40 (51.4%) 99.48 (51.2%) 98.88 (49.8%) 98.4 (48.6%)

403.41 465.82 157.40 120.27

60.98 74.18

I(t) = A1 exp(−t/τ1) + A2 exp(−t/τ2) and CTE = 1 − τDA/τD (the values of the goodness-of-fit parameter (r2) are all close to 1.0).

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 eq 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, which is calculated using the average lifetime of each sample as τ CTE = 1 − DA τD (1)

Rsh values of all BCP-based PSCs were several times larger than those without BCP and 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 SEM to identify if the BCP layer exerts any impact on the grain growth behavior of perovskite thin films (Figures S4 and S5). First, all samples were 350 nm thick, proving that the BCP interlayer had almost no influence on the perovskite thickness. Meanwhile, all perovskite films show dense and pin-hole-free crystal grain arrangement, with a 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 S5b, 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. The 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 STPL and TCSPC technology. The normalized STPL and TRPL spectra are shown in Figure 4. It can be seen from Figure 4a that the perovskite thin films had a monoemission peak at 772 nm, and no peak shifts or other shoulder peak appeared with different coating thicknesses, suggesting that the excitation mode of the perovskite layer remained unchanged before and after the BCP insertion. PL intensities of perovskite thin films on different substrates (glass and FTO) in Figure S6 reveal that the BCP interlayer deposited at 1000 rpm shows the 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 4b, it can be seen that the PL decay on the glass/BCP/perovskite was slower than that on the 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

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 the film without the BCP layer was 60.98%, whereas this value dramatically increased to 74.18% after BCP layer insertion, exhibiting an enhancement of over 20%. It is the core influencing factor that is mostly responsible for the increasing PCE of ETL-free PSCs after the insertion of the BCP layer. An exploratory hypothetical mechanism of the BCP interlayer to enhance electron-transfer effects between FTO and perovskite can be briefly explained in Figure 4c. There are two beneficial effects at the heterointerface between FTO, or BCP, and the perovskite layer. The first factor can be attributed to the so-called “modulation doping”. Modulation doping is an important strategy to significantly increase the free carrier concentration without compromising mobility or the introduction of dopant impurities in heterostructure semiconductor device engineering. The most commonly used material 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 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 the lifetime of free carriers. The modulation doping effect can be usually detected from the 10136


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Figure 5. Stability test of the BCP-based ETL-free and n−i−p full-structured PSC. (a) Storage in air, (b) short circuit under full illumination in air (20 ± 3 °C, Humility < 20%), and (c) under continuous 1 sun illumination (AM1.5, 100 mW cm−2, without UV-filter) in an Ar glovebox at 20 ± 3 °C (all of the data were collected from five different devices for each condition, unencapsulated devices) and (d) representative IPCE of the BCPbased ETL-free and n−i−p full cell after 450 h of illumination.

indicating that the interface between BCP and perovskite is inactive. Next, an extremely harsh condition was applied to test the light-soaking stability when maximum current flow passing through the devices, that is, 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 5b 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 photostability by exposing the devices to AM 1.5 illumination in an Ar glovebox (Figure 5c). As expected, the 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 particularly point out that hereby the solar cells used for the photostability test are unsealed ones and employed Ag as the electrode, rather than a Au electrode, which is utilized in many previous reports.4,7,9,13,15,50 Normalized photovoltaic parameters in Figure S8 show that photodegradation of PCE is mainly caused by the declines of Jsc and FF, whereas Voc did not change much with illumination time. We speculate that the severe attenuation of the stability of n−i−p full cells resulted from perovskite photocatalytic decomposition caused by the TiO2 layer. IPCE profiles (Figure 5d) of the cells after 450 h of illumination were plotted to check if the perovskite layer still functions normally. Compared

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 being interoperated by BCP. From Figure 4b and Table 2, τ2 of the perovskite thin film was prolonged from 779.69 to 903.84 ns before and after BCP layer insertion, respectively, demonstrating the improved photoexcited carrier transfer performance. The other factor is the band bending effect. As shown in Figure 4c, there is a band barrier at the BCP/FTO surface; the electron density tends to redistribute and generate an equilibrium energy level (Eequilibrium).49 Eequilibrium would be located at 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 microscopy profiles. As discussed in the Supporting Information (Figure S7), it can be inferred that the CB bottom of BCP/FTO was raised by 160 mV compared to that of 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 photostability 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 shown in Figure 5a. All 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 PSCs presented quite good stability. After 1000 h of 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, 10137


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through the National Research Foundation of Korea (NRF2014R1A4A1008474), The National Research Foundation of Korea grant funded by the Korea government (MSIP) (No. 2017R1A2B3010927), and Future Materials Discovery Program (NRF-2016M3D1A1027664).

to the initial situation, the BCP-based ETL-free cell retained high conversion efficiencies of 70−85% (between 400 and 700 nm), yet only 15−40% was retained for the n−i−p full cell. A peak at around 350 nm was noticed in the full cell 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 photostability of perovskite photovoltaic devices.

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4. CONCLUSIONS We improved the efficiency of ETL-free-structured PSC to 19.07%, which is approximately equivalent to those of conventional n−i−p full-structured devices using mp-TiO2 as an ETL (19.03%), by insetting a BCP insulating layer between the FTO substrate and the perovskite thin film. The BCP interlayer is capable of modulating and optimizing the electron carrier injection from perovskite to the FTO substrate: the CTE 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 of continuous AM 1.5 soaking, the BCP-based cells retained 92.2% initial performance, whereas the n−i−p full cell degraded almost completely. This report provides a new strategy guidance for designing novel simplified architectural ETL-free PSCs.

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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/acsami.8b00021. Literature statistics of the ETL-free PSC photovoltaic parameters; hysteresis loops of ETL-free PSCs with and without the BCP layer and power output under maximum power point for 200 s; SEM images, AFM morphology, and c-AFM current mapping images of FTO and BCP/FTO samples; 3D surface morphology and height profile for BCP with different spin coating speeds; cross-sectional SEM images of the perovskite thin film spin-coated on FTO glass without/with a BCP interlayer; surface SEM images of the perovskite thin film spin-coated on FTO glass without/with a BCP interlayer; STPL spectra of perovskite thin films on different substrates; 3D height profile and 3D contact potential difference profile of the BCP layer on FTO substrate and contact potential difference curve; characterization and discussion of the AFM data; and normalized photovoltaic parameters as a function of illumination time (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Young Jae Song: 0000-0001-6172-3817 Nam Gyu Park: 0000-0003-2368-6300 Hyun Suk Jung: 0000-0002-7803-6930 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Global Frontier R&D Program of the Center for Multiscale Energy System (2012M3A6A7054855), Basic Science Research Program 10138


Research Article


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Insulated Interlayer for Efficient and Photostable Electron-Transport

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